Plastics

Polymers continue to advance into engineering applications that range from automotive and aerospace to medical and electrical.

Plastics begin as a gas (monomer), approach the liquid state for forming, and end up as a solid in their finished form. They can be formed by heat, pressure, or a combination of the two. Adding reinforcements to plastics creates a different class of materials known as composites.

Greater understanding of the fundamental chemistry of polymer formation has led to development of plastics with properties tailored to specific applications. Rather than refining materials found in nature, chemists can now design materials with the same elements nature uses. In fact, polymer chemists have built thousands of plastics based on only eight elements: carbon, hydrogen, nitrogen, oxygen, fluorine, silicon, sulfur, and chlorine.

Chapters on Plastics:

Thermoplastics

Starting with billions of molecules of monomer in a reactor, heat and pressure are applied in the presence of catalysts, causing one of the monomer double bonds to rearrange into two "half-bonds," one at each end. These half-bonds combine with half bonds of other rearranged monomer molecules, forming stable "whole bonds" between them. As each monomer joins with others, the chain length grows until it meets a stray hydrogen, which combines with the reactive end, stopping chain growth at that point.

During the polymerization reaction, millions of separate polymer chains grow in length simultaneously, until all of the monomer is exhausted. By adding predetermined amounts of hydrogen (or other chain-stoppers), chemists can produce polymers having a fairly consistent average chain length. Chain length (molecular weight) is important because it determines many properties of a plastic, and it also affects its processing characteristics. The major effects of increasing chain length are increased toughness, creep resistance, stress-crack resistance, melt temperature, melt viscosity, and processing difficulty.

However, not all polymer molecules can be manufactured to an exact, specified length, so each batch will have an average molecular-weight distribution. There can be either a broad or a narrow spread between molecular weights of the largest and smallest molecules, and the polymer still could have the same average. A narrow distribution provides more uniform properties; a broad distribution makes a plastic easier to process.

After polymerization is completed, the finished polymer chains resemble long, intertwined bundles of spaghetti, with no physical connections between chains. Such a polymer is called a thermoplastic (heat-moldable) polymer.

Intermolecular forces

Although there is no direct physical connection between individual thermoplastic chains, there is a weak electrostatic attraction (van der Waals force) between polymer chains that lie very close together. This intermolecular force, which tends to prevent chain movement, is heat sensitive, becoming stronger when the plastic is cold and weaker when it is hot. Heating a thermoplastic, therefore, weakens the intermolecular forces, allowing the polymer molecules to slide over each other freely during the molding process. Upon cooling, the forces become strong again and "freeze" the molecules together in the new shape.

Molding a thermoplastic is similar to molding candlewax in this respect. But if too much heat is applied or if the plastic is heated for too long a time, the molecular chains break, causing permanent property degradation -- particularly toughness. Continuous pressure (bending or deforming) on a molded part also causes the chains to slide over each other, resulting in creep, or cold flow, which can seriously affect part dimensions.

Strength of the intermolecular attractive force varies inversely with the sixth power of the distance between chains. Thus, as the distance is halved, the attractive force increases by a factor of 64. For this reason, chain shape is as important as chain length. If a polymer molecule has a symmetrical shape that can pack closely, the intermolecular forces are very large compared to those of a nonsymmetrical shape. Therefore, two kinds of polyethylene can have different physical properties because of the difference in their density, which depends on their ability to pack together. The molecules of high-density polyethylene have very few side branches to upset their symmetry, so they can approach adjacent molecules quite closely, resulting in high intermolecular attractive forces. Low-density polyethylene, on the other hand, contains many more side branches, which create nonsymmetrical areas of low density and, therefore, low intermolecular attraction.

Another consequence of denser molecular packing is higher crystallinity. As symmetrical molecules approach within a critical distance, crystals begin to form in the areas of densest packing. A crystallized area is stiffer and stronger; a noncrystallized (amorphous) area is tougher and more flexible. Other effects of increased crystallinity in a polyethylene polymer are increased resistance to creep, heat, and stress cracking, and increased mold shrinkage.

In general, crystalline polymers are more difficult to process, have higher melt temperatures and melt viscosities, and tend to shrink and warp more than amorphous polymers. They have a relatively sharp melting point; that is, they do not soften gradually with an increase in temperature. Rather, they remain hard until a given quantity of heat is absorbed, then change rapidly into a low-viscosity liquid. Reinforcement of crystalline polymers with fibers of glass or other materials improves their load-bearing capabilities significantly.

Amorphous polymers, on the other hand, soften gradually as they are heated, but they do not flow as easily (in a molding process) as do crystalline materials. Reinforcing fibers do not significantly improve the strength of amorphous materials at higher temperatures. Examples of amorphous thermoplastics are ABS, polystyrene, polycarbonate, polysulfone, and polyetherimide. Crystalline plastics include polyethylene, polypropylene, nylon, acetal, polyethersulfone, and polyetheretherketone.

Copolymer structures

Another method for altering molecular symmetry is to combine two different monomers in the polymerization reaction so that each polymer chain is composed partly of monomer A and partly of monomer B. A polymer made from two different monomers is called a copolymer; one made from three different monomers is called a terpolymer. All long, repeating chains are polymers, regardless of how many monomers are used. But when a polymer family includes copolymers, the term "homopolymer" is used to identify the single monomer type. An example is the acetal family; acetal resins are available both in homopolymer and copolymer types.

Final properties of a copolymer depend on the percentage of monomer A to monomer B, the properties of each, and on how they are arranged along the chain. The arrangement may alternate equally between the two monomers, producing a symmetrical shape capable of a high degree of crystallization. Or the arrangement may be random, creating areas of high crystallinity separated by flexible, amorphous areas. Such a copolymer usually has good rigidity and impact strength. Block copolymers have large areas of polymerized monomer A alternating with large areas of polymerized monomer B. In general, a block copolymer is similar to an alternating copolymer except that is has stronger crystalline areas and tougher amorphous areas. If both types of blocks are crystalline, or both amorphous, a wide variety of end products is possible, having characteristics ranging from hard, brittle plastics to soft, flexible elastomers. A graft copolymer is made by attaching side groups of monomer B to a main chain of monomer A. A copolymer having a flexible polymer for the main chain and grafted rigid side chains is very stiff, yet has excellent resistance to impact -- a combination of properties not usually found in the same plastic. Copolymers always have different properties from those of a homopolymer made from either monomer.

Compounders of plastics modify properties of a thermoplastic material by many other methods as well. For example, fibers are added to increase strength and stiffness, plasticizers for flexibility, lubricants for easier molding or for increasing lubricity of the molded parts, antioxidants for higher temperature stability, UV stabilizers for resistance to sunlight, and fillers for economy. Other additives such as flame retardants, smoke suppressants, and conductive fibers or flakes provide special properties for certain applications. Plastic compounds can be varied widely as to type and amount of these additives, and every modification produces a compound with different properties. Examples of thermoplastic products are polyethylene squeeze bottles, nylon gears and rollers, acrylic lenses, ABS (acrylonitrile-butadiene-styrene) business-machine and appliance housings, polystyrene-foam cups, polycarbonate safety helmets and glazing sheet for bus windows, and polyphenylene sulfide chemical pumps and automotive underhood components

Thermosets

Thermoset plastics are made quite differently from thermoplastics. Polymerization (curing) of thermoset plastics is done in two stages, partly by the material supplier and partly by the molder. For example, phenolic (a typical thermoset plastic) is first partially polymerized by reacting phenol with formaldehyde under heat and pressure. The reaction is stopped at the point where mostly linear chains have been formed. The linear chains still contain unreacted portions, which are capable of flowing under heat and pressure.

The final stage of polymerization is completed in the molding press, where the partially reacted phenolic is liquefied under pressure, producing a crosslinking reaction between molecular chains. Unlike a thermoplastic monomer, which has only two reactive ends for linear chain growth, a thermoset monomer must have three or more reactive ends so that its molecular chains crosslink in three dimensions. Rigid thermosets have short chains with many crosslinks; flexible thermosets have longer chains with fewer crosslinks.

After it has been molded, a thermoset plastic has virtually all of its molecules interconnected with strong, permanent, physical bonds, which are not heat reversible. Theoretically, the entire molded thermoset part could be a single giant molecule. In a sense, curing a thermoset is like cooking an egg. Once it is cooked, reheating does not cause remelting, so it cannot be remolded. But if a thermoset is heated too much or too long, the chains break and properties are degraded.

Phenolic, urea, and melamine thermoset plastics are polymerized by a "condensation" reaction, wherein a by-product (water, for example) is created during the reaction in the mold. Such volatile by-products cause dimensional instability and low part strength unless they are removed during molding.

Other thermoset plastics, such as epoxy and polyester, cure by an "addition" reaction, resulting in no volatile by-products and fewer molding problems. Most addition-cured thermoset plastics are liquid at room temperature; the two ingredients can simply be mixed and poured into molds where they crosslink (cure) at room temperature into permanent form -- much like casting concrete. Molds are often heated, however, to speed the curing process.

In general, thermoset plastics, because of their tightly crosslinked structure, resist higher temperatures and provide greater dimensional stability than do most thermoplastics. Examples of thermoset plastic products include glass-reinforced-polyester boat hulls and circuit-breaker components, epoxy printed-circuit boards, and melamine dinnerware.

Alloys and blends

Yet another way to create more variations in plastics is alloying -- an effective and economical method to improve a weak property in a base resin. Plastic alloys, also called blends or polyblends, are usually designed to retain the best characteristics of each material. Properties that have been found to be most responsive to improvement are impact strength, heat-deflection temperature, flame retardance, chemical and weather resistance, and processibility. A study by Battelle reports that research on polymer blending generates about 1,000 patents per year.

Although no totally accepted definition exists, most engineers and chemists in the plastics industry agree that a plastic alloy is identified by most of these characteristics:

• The combination of polymers does not depend on chemical bonds; the mixture is entirely mechanical. Thus, copolymers (some acetals and polyolefins) and terpolymers are not alloys. Nor are epoxy compounds that copolymerize with hardeners that contribute to the properties of the cured resins.
• The mixture has a single melt-transition temperature.
• At least one property or characteristic of the base polymer is improved synergistically by the addition of the other polymer(s). The property may be physical or mechanical, or the improvement may be in processibility or cost. If synergistic improvement is not achieved, at least the best properties of all constituents are retained.
• Each minor component of a plastic alloy constitutes at least 5% of the mix. Many are nearer the 50:50 range. This "requirement" differs considerably from those involving metal alloys. There, only enough of an alloying element need be present to effect a change in a mechanical or physical property. The magnitude of such a change is not important.

IPNs

A new technology that combines incompatible polymers to form blends called interpenetrating polymer networks (IPNs) promises to provide cost/performance benefits not previously available in engineering plastics. Several companies are working on IPN development, but only a few have made developmental IPN materials available.

IPNs consist of an interwoven matrix of two polymers. A typical method for producing these alloys involves crosslinking one of the monomers in the presence of the other polymer. The need for chemical similarity between the two types of molecules is thus reduced because the crosslinking physically traps one phase within the other. The result is a structure composed of two different materials intertwined together, each retaining its own physical characteristics. The relationship is similar to that between small blood vessels and the surrounding tissue in the human body.

Patented IPN technology by Shell Chemical Co. is based on the capability of the company's Kraton G thermoplastic elastomer (styrene-butadiene-block copolymer) to form stable and reproducible structures when properly mixed in the melt stage. The blends provide properties of the individual phases of the mixture and have few or no property losses that might be expected from combining incompatible materials. The results are materials having the best performance features of both an engineering thermoplastic and a thermoplastic rubber.

Although Shell was the first to come out with injection-moldable IPNs, Allied and Du Pont are also working on similar materials. Research at Allied-Signal Corp., for example, is focused on combining the toughness of thermoplastics with the solvent resistance, heat resistance, and dimensional stability of thermoset resins. Allied researchers feel that, when the IPNs become fully commercial, they will likely be processed by reaction-injection molding (RIM), by injection molding with a postmolding oven cure, or by a sheet-molding process followed by a cure cycle.

Other current thermoplastic IPN technology is based on crystalline resins such as nylon 6, 6/6, and 6/10, PBT, acetal, and polypropylene, with silicone as the IPN. These IPNs can be further modified with reinforcements and lubricants such as glass or carbon fibers and PTFE. The silicone IPN functions as a nonmigratory silicone lubricant, release agent, and flow modifier. It also plays an important role in controlling shrinkage and warpage of resins and composites. IPNs provide excellent wear performance in gears and bearings and superior dimensional control in parts such as keyboard frames for electronic typewriters.

Grafting

Another way to improve the compatibility of two dissimilar polymers often involved a third material. The "compatibilizer" material is a grafted copolymer consisting of one of the principal components and a material similar to the other principal component. The functional groups in segment A of the third material will have an affinity for the polymer produced from monomer A, and the functional groups in segment B an affinity for polymer B. The mechanism is similar to that of soap improving the solubility of a greasy substance in water. The soap contains components that are compatible with both substances.

Some degree of compromise is almost always necessary in designing plastic parts. Arriving at the best compromise usually requires satisfying the mechanical, thermal, and electrical requirements of the part, utilizing the most economical resin or compound that will perform satisfactorily and be attractive, and choosing a manufacturing process compatible with the part design and material choice. Setting realistic requirements for each of these areas is of utmost importance.

Probably no plastic will provide 100% of the requirements for the desired performance, appearance, processibility, and price. Selecting the best qualified material is not based simply on comparing numbers on published data sheets; such values can be grossly misleading. For example, choosing the most economical material for a part by comparing the cost per pound of various plastics is a mistake. Some plastics weigh twice as much per cubic inch as others and so would require twice as much to fill a given cavity and cost twice as much to ship.

A more meaningful comparison is cost per cubic inch. But since most expensive plastics are far stronger than the cheaper ones, cost/strength values should be analyzed as well. Paying more per pound or per cubic inch is often more economical if less material can do the job.

Standard test data

There is an attractive simplicity in deciding on the material with the highest ASTM test values as listed in manufacturers' data sheets. Unfortunately, this method seldom results in the best choice. First, the choice of any material should be based on the best combination of properties, not only on one property. An ideal material will have a value for each strength property just sufficient to perform properly and safely in a given application and no higher. The best material choice usually represents a trade-off among satisfactory properties, ease of processing, and cost. It is seldom the plastic with highest values in any single category.

Second, values in published data sheets represent laboratory tests, which do not duplicate real-life molding conditions. Strength of a molded plastic part is significantly affected by such processing factors as direction of flow, pressure during molding, melt temperature, thermal degradation, cooling rate, and stress concentrations. As a result, a high value listed in the data sheets for a given property can be reduced considerably by processing conditions.

Because the molding variables are beyond their control, material suppliers have chosen to make simple, standardized ASTM laboratory tests that are easily reproduced. The tests are not material performance tests for predicting real-life results over a period of time, but are actually material quality tests. They are made for the purpose of assuring a buyer that the batch he buys today is the same formulation and quality he bought last month or last year. The ASTM tests were not intended to compare one material with another on the basis of strength after molding; different configurations and different molding processes can change the values significantly.

Third, ASTM tests are essentially short-term tests with all variables fixed except the one being measured. For example, testing for tensile, flexural, or impact strength is often done at a standard rate of loading. A short-time tensile test may show a high strength value, but if the rate of loading is reduced by 100 or 1,000 times, tensile strength can drop to as little as 20% of the short-time strength. If the loading rate is increased, the tested tensile-strength value may double or triple. The data sheets do not show this.

Plastics are also temperature sensitive; strength properties may vary by a factor of 10 over a temperature difference as little as 200°F. Since most ASTM strength tests are made at room temperature, behavior at other temperatures cannot be reliably predicted from these data. Nor can the behavior be predicted under combinations of temperature, varying loads, and changing environments outside the narrow test conditions.

Flammability tests

The Federal Aviation Administration, the Department of Transportation, and other regulatory agencies have mandated that design materials comply with specific flammability test requirements. Flame-retardancy requirements generally include limits on flame spread, burning time, dripping, and smoke emission.

By far the most stringent and also the most widely accepted of such tests is the Underwriters' Laboratory Standard for safety, UL 94, for electrical devices. The test, involving the burning of a specimen in the vertical position, is the one by which most flame-retardant (FR) plastics are rated. In this test, the best rating is UL 94V-0, which defines a flame duration of 0 to 5 sec, an afterglow of 0 to 25 sec, and the presence of no flaming drips that ignite the dry, absorbent cotton located below the test specimen.

Flame spread and dripping tendencies of test materials are also characterized in the ASTM D635 Standard, a horizontal test that reports average time of burning (ATB) and average extent of burning (AEB). In both the UL and the ASTM tests, the presence of short reinforcing-glass fibers has been found to inhibit dripping in test compounds.

A more quantitative measure of a material's resistance to burning is determined from ASTM D2863, which measures the minimum concentration of oxygen in an oxygen/nitrogen mixture that will support candlelike burning for 3 min or longer. Results are reported as a Limiting Oxygen Index (LOIs). Composites with LOIs over 28% are usually listed as 94V-0.

Smoke emission is measured in the air column above a burning specimen in a National Bureau of Standards smoke chamber. In the test, a specified area of plastic is exposed to heat under flaming conditions. Smoke measurements are reported as "specific optical density." This is a dimensionless value because it represents the optical density measured over unit path length within a chamber of unit volume produced from a test specimen of unit surface area. The optical density measurement, (Dmax), is based on the attenuation of a light beam by smoke accumulating within the closed chamber during flaming combustion. (For a reference, the Dmax for red oak is 76.)

Smoke generated during combustion consists of suspended soot particles that are formed between the pyrolysis zone and the flame front. These particles are molecules of highly condensed ring structures that are most readily formed by aromatic polymers (SAN, SMA, and polyphenylene ether). Polymers having aliphatic carbon backbones, such as polypropylene and nylon, tend to generate less smoke, but this effect is offset in the FR compounds by the increase in smoke caused by halogenated flame-retardant additives. Resins of higher thermal stability (PC, PSF, PES, PEEK, and PPS produce the least smoke of the available UL 94V-0 thermoplastics.

Experience needed

There is no simple procedure for selecting the best plastic for a new application. It must be done with direct experience and knowledge of the behavior of various plastics under the real-life conditions to be encountered by a particular part after it is molded.

Until this experience is acquired, a designer has little choice but to seek the advice of a reliable molder, materials manufacturer, or compounder. Even here, there is the danger that these sources may not be aware of the many compromises a company must make internally among production, engineering, purchasing, and marketing considerations to produce a product that will sell at a profit. Also, a molder might be inclined to recommend the material that works best in his equipment, rather than the best for the application. Thus, the successful design of plastic parts that have the optimum cost/performance characteristics require learning as much as possible about many different plastics and the peculiarities of their processing.

Designers used to take little interest in the molding of parts they designed. They sent the drawings to the molder in another department or another company and expected perfect parts to emerge. But design and processing have become so interrelated that this separation can no longer exist if products are to be consistently successful.

Molders can usually be relied upon to detect and correct visible problems or readily measured factors such as color, surface condition, and dimensions. However, less apparent property changes are another matter. These may not show up until the moldings are in service, unless extensive testing and quality control are used. Such properties as impact strength, toughness, and chemical resistance can be diminished by improper control of processing parameters. Close cooperation between designers and molders can eliminate disappointment and help ensure successful products.

After candidate materials are selected, the design should be tested under real-life conditions involving the temperatures, loading, and environment of the anticipated service. Ideally, the test part should be molded in the shape and from the material to be used in production. In the beginning, this is costly and time consuming, but as experience is acquired, accelerated tests can be developed on simpler shapes; testing will then be more economical but just as reliable.

Understanding the molding process that will produce the part is also necessary. The process directly affects material choice, shape, tolerances, and properties of the part. For example, a container or housing can be made by injection molding, blow molding, thermoforming, or rotational molding. But each process requires a markedly different design, would use a different grade of plastic or a different plastic entirely, and would produce a component with significantly different properties.

All molding processes alter the published data-sheet properties, reducing most strengths and often creating areas of stress concentrations. But each process may create stresses in different areas. Sometimes, processing conditions are so severe that there is no choice but to redesign the shape and change to a different plastic. Unfortunately, reliable data on molded strength properties may never be available because of the basic nature of plastics. Their characteristics are partly those of solids and partly those of viscous liquids, preventing the use of classical Hookean engineering formulas for calculating load distributions accurately.

A starting point

One way to begin is to study similar existing applications to learn which materials, processes, and designs have worked successfully. Next, discuss the application with experienced molders, mold builders, and materials manufacturers and compounders to get their recommendations. Finally, try to select the best plastic for the application by comparing the relevant properties of each material that has been recommended.

The brief summaries presented here indicate property highlights and characteristics of the most common families of thermoplastics and thermosets used in industrial and consumer products. Only general family characteristics are given; many properties can be changed significantly by compounding the base resins with fillers, plasticizers, or reinforcements, or by copolymerizing them with other monomers.

Fillers usually decrease cost, increase stiffness, improve dimensional stability and reduce shrinkage. Plasticizers increase flexibility and reduce most strength properties. Reinforcements (usually glass, carbon, or mineral fibers) improve strength, dimensional stability, and thermal endurance, but increase cost. Copolymers can have either higher or lower properties and cost depending on the monomers used and their proportions.

Prototypes

A prototype plastic part cannot duplicate exactly the performance of an injection-molded production part unless it is molded in a production environment in a production mold. This is normally impractical, so the best that can be expected is an approximation of a production part.

The fastest and most economical way to produce plastic prototypes is to machine them from slab or bar stock. But stock forms of plastics are made by extrusion, not injection molding. Besides not having the same flow orientation of a molded part, extrusions of a given material are usually made from a higher molecular-weight grade than that used for injection molding. Consequently, properties such as impact strength, creep resistance, and chemical resistance tend to be higher in an extruded material. These differences are particularly significant in the crystalline plastics such as nylon, acetal, and polyethylene. The variations are usually smaller in amorphous materials such as ABS, polycarbonate, and polystyrene, but even minor differences can be critical in some applications.

Molding, rather than machining, of prototypes generally provides a better approximation of a production part, but here too, a number of differences in conditions can cause misleading results. For example, if the prototype mold is made from epoxy resin, the molded part will cool at a much slower rate than it would in a production (steel) mold. And cooling rate can affect tensile and impact strength as well as heat and chemical resistance, elongation, and stiffness -- particularly in crystalline plastics.

Making prototypes in an aluminum mold improves their similarity to production parts, but this method also has drawbacks. Here, because of the high thermal conductivity of aluminum, faster cooling is the problem that alters properties from what they would be in a part made in a steel mold. Also, there is difficulty in getting the resin to flow into a mold that cools rapidly. This problem can be offset by higher injection pressure, but the greater density that results causes other variations.

The closest duplication of a production part is produced by injection-molding prototype parts in steel molds. A relatively soft steel can be used for prototypes, so that machining is not difficult. But even here, because certain shortcuts are usually made (in polishing surfaces or in simplifying cooling passages, for example), the quality and accuracy of the resultant moldings are something less than what would be expected in production moldings. Nevertheless, steel prototype molds produce parts that most nearly duplicate production parts. Although this is the most expensive prototyping method, it may be the most economical in that it provides the surest way to avoid expensive changes in production molds.

ABS resins are hard, rigid, and tough, even at low temperatures. They consist of particles of a rubberlike toughener suspended in a continuous phase of styrene-acrylonitrile (SAN) copolymer. Various grades of these amorphous, medium-priced thermoplastics are available offering different levels of impact strength, heat resistance, flame retardance, and platability.

Most natural ABS resins are translucent to opaque, and they can be pigmented to almost any color. Grades are available for injection molding, extrusion, blow molding, foam molding, and thermoforming. Molding and extrusion grades provide surface finishes ranging from satin to high gloss. Some ABS grades are designed specifically for electroplating. Their molecular structure is such that the plating process is rapid, easily controlled, and economical.

Compounding of some ABS grades with other resins produces special properties. For example, ABS is alloyed with polycarbonate to provide a better balance of heat resistance and impact properties at an intermediate cost. Deflection temperature is improved by the polycarbonate; molding ease, by the ABS. Other ABS resins are used to modify rigid PVC for use in pipe, sheeting, and molded parts. Reinforced grades containing glass fibers, to 40%, are also available.

Related to ABS is SAN, a copolymer of styrene and acrylonitrile (no butadiene) that is hard, rigid, transparent, and characterized by excellent chemical resistance, dimensional stability, and ease of processing. SAN resins are usually processed by injection molding, but extrusion, injection-blow molding, and compression molding are also used. They can also be thermoformed, provided that no postmold trimming is necessary. (Because the material is not toughened, thermoformed shapes may crack during conventional trimming operations.)

Properties:

ABS plastics offer a good balance of tensile strength, impact and abrasion resistance, dimensional stability, surface hardness, rigidity, heat resistance, low-temperature properties, chemical resistance, and electrical characteristics. These materials yield plastically at high stresses, so ultimate elongation is seldom significant in design; a part usually can be bent beyond its elastic limit without breaking, although it does stress-whiten. While not generally considered flexible, ABS parts have enough spring to accommodate snap-fit assembly requirements.

Impact properties of ABS are exceptionally good at room temperature and, with special grades, at temperatures as low as -40°F. Because of its plastic yield at high strain rates, impact failure of ABS is ductile rather than brittle. Also, the skin effect which, in other thermoplastics, accounts for a lower impact resistance in thick sections than in thin ones, is not pronounced in ABS materials. A long-term tensile design stress of 1,000 to 1,500 psi (at 73°F) is recommended for most grades.

General-purpose ABS grades may be adequate for some outdoor applications, but prolonged exposure to sunlight causes color change and reduces surface gloss, impact strength, and ductility. Less affected are tensile strength, flexural strength, hardness, and elastic modulus. Pigmenting the resins black, laminating with opaque acrylic sheet, and applying certain coating systems provide weathering resistance. For maximum color and gloss retention, a compatible coating of opaque, weather-resistant polyurethane can be used on molded parts. For weatherable sheet applications, ABS resins can be coextruded with a compatible weather-resistant polymer on the outside surface.

ABS resins are stable in warm environments and can be decorated with durable coatings that require baking at temperatures to 160°F for 30 to 60 min. Heat-resistant grades can be used for short periods at temperatures to 230°F in light load applications. Low moisture absorption contributes to the dimensional stability of molded ABS parts.

Molded ABS parts are almost completely unaffected by water, salts, most inorganic acids, food acids, and alkalies, but much depends on time, temperature, and especially stress level. FDA acceptance depends to some extent on the pigmentation system used. The resins are soluble in esters and ketones, and they soften or swell in some chlorinated hydrocarbons, aromatics, and aldehydes.

Properties of SAN resins are controlled primarily through acrylonitrile content and molecular weight of the copolymer. Increasing both improves physical properties, at a slight penalty in processing ease. Properties of the resins can also be enhanced by controlling orientation during molding. Tensile and impact strength, barrier properties, and solvent resistance are improved by this control.

Special grades of SAN are available with improved UV stability, vapor-barrier characteristics, and weatherability. The barrier resins -- designed for the blown-bottle market -- are also tougher and have greater solvent resistance than the standard grades.

Applications:

Molded ABS products are used in both protective and decorative applications. Examples include safety helmets, camper tops, automotive instrument panels, and other interior components, pipe fittings, home-security devices and housings for small appliances, communications equipment, and business machines. Chrome-plated ABS has replaced die-cast metals in plumbing hardware and automobile grilles, wheel covers, and mirror housings.

Typical products vacuum-formed from extruded ABS sheet are refrigerator liners, luggage shells, tote trays, mower shrouds, boat hulls, and large components for recreational vehicles. Extruded shapes include weather seals, glass beading, refrigerator breaker strips, conduit, and pipe for drain-waste-vent (DWV) systems. Pipe and fittings comprise one of the largest single application areas for ABS.

Typical applications for molded SAN copolymers include instrument lenses, vacuum-cleaner and humidifier parts, medical syringes, battery cases, refrigerator compartments, food-mixer bowls, computer reels, chair shells, and dishwasher-safe houseware products. Because of their compatibility with many higher-priced resins, SAN resins are also used as color-concentrate carriers for some engineering resins.

Acrylic-styrene-acrylonitrile (ASA) polymers are amorphous plastics which have mechanical properties similar to those of ABS resins. However, the ASA properties are far less affected by outdoor weathering.

ASA is a terpolymer that can be produced by either a patented, proprietary reaction process or by a graft process. In the reaction method, ASA is made by introducing a grafted acrylic ester elastomer during a copolymerization of styrene and acrylonitrile (SAN). The finely divided powder is uniformly distributed in and grafted to the SAN molecular chains. The outstanding weatherability of ASA is due to the acrylic ester elastomer.

ASA resins are available in natural, off white, and a broad range of standard and custom-matched colors. Base ASA resins are sold under the trade names Luran S (BASF Plastic Materials), Geloy (General Electric Plastics) and Centrex (Monsanto Chemical Co.). ASA resins can be compounded with other polymers to make alloys and compounds that benefit from ASA's weather resistance. Also, ASA sheet is used as a capstock over other plastics.

Properties:

ASA parts have high gloss, good chemical and heat resistance, and high impact strength, even at low temperatures. Typical heat-deflection temperatures for ASA are 180 to 220 °F (82 to 104 °C) at 264 psi. Tensile strengths are 4,000 to 7,000 psi; elongation at break, 25 to 40%, flexural modulus, 220,000 to 250,000 psi; and notched Izod impact strengths are 9.0 to 11.0 ft-lb/in. (at 73 °F) and 4.0 to 6.0 ft-lb/in. (at -40 °F).

ASA is resistant to saturated hydrocarbons, low-aromatic gasoline and lubricating oil, vegetable and animal oils, water, aqueous solutions of salts, and dilute acids and alkalis. However, it is attacked by concentrated inorganic acids, aromatic and chlorinated hydrocarbons, esters, ethers, ketones, and some alcohols. ASA offers better resistance to environmental stress cracking than ABS. With respect to flame resistance, ASA is available with UL 94-HB classification.

Processing:

ASA resins can be processed by most conventional methods. These include profile and sheet extrusion and coextrusion, injection molding, structural foam molding, and extrusion-blow molding. Extruded sheet can be thermoformed. ASA should be blow molded using extruders with grooved, cooled, and thermally insulated feed sections. Screws should have somewhat deeper flights in order to reduce frictional heat. Optimum results are obtained with machines having accumulators.

ASA parts can be welded using thermal and spin techniques. In some cases, ultrasonic welding is possible. ASA parts also can be solvent welded using 2-butanone, dichloroethylene, or cyclohexane. ASA parts readily accept and retain print and coatings without prior surface treatment. Vacuum metallizing by conventional methods is also possible.

Acetal resins are highly crystalline plastics based on formaldehyde polymerization technology. These engineering resins are strong, rigid, and have good moisture, heat, and solvent resistance. Acetals produced in the U.S. include homopolymers by Du Pont Co. (Delrin) and copolymers by Celanese Engineering Plastics Div., Hoechst Celanese Corp. (Celcon) and copolymers by BASF Corporation Plastic Materials (Ultraform).

Melting points of the homopolymers are higher, and they are harder, have higher resistance to fatigue, are more rigid, and have higher tensile and flexural strength with generally lower elongation. Some high-molecular-weight homopolymer grades are extremely tough and have higher elongation than the copolymers. Homopolymer grades are available that are modified for improved hydrolysis resistance to 180 °F, similar to copolymer materials.

The copolymers remain stable in long-term, high-temperature service and offer exceptional resistance to the effects of immersion in water at high temperatures. Neither type resists strong acids, and the copolymer is virtually unaffected by strong bases. Both types are available in a wide range of melt-flow grades.

The copolymers process easier and faster than the conventional homopolymer grades. However, with the introduction in 1987 of Delrin II homopolymer resins, Du Pont claims to have eliminated that difference. The improve moldability is attributed to the use of a new thermal stabilizer, which gives Delrin II the thermal stability of copolymers while retaining the higher properties of the homopolymer.

Both the homopolymers and copolymers are available in several unmodified and glass-fiber-reinforced injection-molding grades. Both are available in PTFE or silicone-filled grades, and the homopolymer is available in chemically lubricated low-friction formulations. A blow-molding grade (actually, a terpolymer) of Celcon is also produced, and special, high-productivity molding grades of Delrin are available. Several grades of both types comply with Food and Drug Administration regulations for repeated contact with food at temperatures to 250 °F. The National Sanitation Foundation has approved several homopolymer and copolymer resins for use in potable water to 180 °F.

The acetals are also available in extruded rod and slab form for machined parts. These materials are readily machined with standard brass-cutting tools. Property data listed in the table apply to the general-purpose injection-molding and extrusion grade of Delrin 500 and to Celcon M90.

Acetal homopolymers

The homopolymers are available in several viscosity ranges that meet a variety of processing and end-use needs. The higher viscosity materials are generally used for extrusions and for molded parts requiring maximum toughness; the lower viscosity grades are used for injection molding. Elastomer-modified grades offer greatly improved toughness.

Properties:

Acetal homopolymer resins have high tensile strength, stiffness, resilience, fatigue endurance, and moderate toughness under repeated impact. Delrin 100, a high-impact grade, is the toughest acetal. Delrin 100 ST is a super tough grade, delivering up to seven times greater toughness than unmodified acetal in Izod impact tests and up to 30 times greater toughness as measured by Gardner impact tests.

Homopolymer acetals have high resistance to organic solvents, excellent dimensional stability, a low coefficient of friction, and outstanding abrasion resistance among thermoplastics. The general-purpose resins can be used over a wide range of environmental conditions; special, UV-stabilized grades are recommended for applications requiring long-term exposure to weathering. However, prolonged exposure to strong acids and bases outside the range of pH 4 to 9 is not recommended.

Acetal homopolymer has the highest fatigue endurance of any unfilled commercial thermoplastic. Under completely reversed tensile and compressive stress, and with 100% relative humidity (at 73 °F), fatigue endurance limit is 4,500 psi at 10 (to the sixth power) cycles. Resistance to creep is excellent. Moisture, lubricants, and solvents including gasoline and gasohol have little effect on this property, which is important in parts incorporating self-threading screws or interference fits.

The low friction and good wear resistance of acetals against metals makes these resins suitable for use in cams and gears having internal bearings. The coefficient of friction (nonlubricated) on steel, in a rotating thrust washer test, is 0.1 to 0.3, depending on pressure; little variation occurs from 73 to 250 °F. For even lower friction and wear, PTFE-fiber-filled and chemically lubricated formulations are available.

Properties of low moisture absorption, excellent creep resistance, and high deflection temperature suit acetal homopolymer for close-tolerance, high-performance parts. Long-term behavior in various environments can be predicted from design handbook data.

Applications:

Automotive applications of acetal homopolymer resins include fuel-system and seat-belt components, steering columns, window-support brackets, and handles. Typical plumbing applications that have replaced brass or zinc components are shower heads, ballcocks, faucet cartridges, and various fittings. Consumer items include quality toys, garden sprayers, stereo cassette parts, butane lighter bodies, zippers, and telephone components. Industrial applications of acetal homopolymer include couplings, pump impellers, conveyor plates, gears, sprockets, and springs.

Acetal copolymers

The copolymers have an excellent balance of properties and processing characteristics. Melt temperature can range from 360 to 450 °F with little effect on part strength. The resin is available in natural (translucent white) and in a wide range of colors. UV-resistant grades (also available in colors), glass-reinforced grades, low-wear grades, and impact-modified grades are standard. Also available are electroplatable and dimensionally stable, low-warpage grades.

Properties:

Acetal copolymers have high tensile and flexural strength, fatigue resistance, and hardness. Lubricity is excellent. They retain much of their toughness through a broad temperature range and are among the most creep resistant of the crystalline thermoplastics. Moisture absorption is low, permitting molded parts to serve reliably in environments involving humidity changes.

Good electrical properties, combined with high mechanical strength and a UL electrical rating of 100 °C, qualify these materials for electrical applications requiring long-term stability.

A new acetal copolymer resin, Ultraform S 1320X-003, is available from BASF Corporation Plastic Materials. It has a combination of high mechanical and heat deflection properties (close to those of acetal homopolymers) and the good thermal stability and processing properties of acetal copolymer resins. The tensile strength, modulus of elasticity, impact strength, heat-deflection temperature, and surface hardness are about 10% higher than those of general-purpose acetal copolymers.

Impact-modified grades of acetal copolymers have notched Izod impact strengths up to nearly 3 ft-lb/in. at room temperature. The impact-modified acetal copolymer resins have a good balance of toughness and rigidity, with modulus of elasticity as high as 305,000 psi. This compares to 435,000 psi for general-purpose acetal copolymer resin with 1.3 ft-lb/in. notched Izod.

Acetal copolymers have excellent resistance to chemicals and solvents. For example, specimens immersed for 12 months at room temperature in various inorganic solutions were unaffected except by strong mineral acids -- sulfuric, nitric, and hydrochloric. Continuous contact is not recommended with strong oxidizing agents such as aqueous solutions containing high concentrations of hypochlorite ions. Solutions of 10% ammonium hydroxide and 10% sodium chloride discolor samples in prolonged immersion, but physical and mechanical properties are not significantly changed. Most organic reagents tested have no effect, nor do mineral oil, motor oil, or brake fluids. Resistance to strong alkalies is exceptionally good; specimens immersed in boiling 50% sodium hydroxide solution and other strong bases for many months show no property changes.

Strength of acetal copolymer is only slightly reduced after aging for one year in air at 240 °F. Impact strength holds constant for the first six months, and falls off about one-third during the next six-month period. Aging in air at 180 °F for two years has little or no effect on properties, and immersion for one year in 180 °F water leaves most properties virtually unchanged. Samples tested in boiling water retain nearly original tensile strength after nine months.

Applications:

Industrial and automotive applications of Celcon acetal copolymer include gears, cams, bushings, clips, lugs, door handles, window cranks, housings, and seat-belt components. Plumbing products such as valves, valve stems, pumps, faucets, and impellers utilize the lubricity and corrosion and hot-water resistance of the copolymer. Mechanical components that require dimensional stability, such as watch gears, conveyor links, aerosols, and mechanical pen and pencil parts, are other uses. Applications for the FDA-approved grades include milk pumps, coffee spigots, filter housings, and food conveyors. Parts that require greater load-bearing stability at elevated temperatures, such as cams, gears, TV tuner arms, and automotive underhood components are molded from glass-fiber-reinforced grades.

Acrylic thermoplastics are known for their crystal clarity and outstanding weatherability. They are available in cast sheet, rod, and tube; extruded sheet and film; and compounds for injection molding and extrusion.

Cell-cast sheet is produced in several sizes and thicknesses. The largest sheets available are 120 × 144 in., in thicknesses from 0.030 to 4.25 in. Continuous cast material is supplied as flat sheet to ½ in. thick, in widths to 9 ft. Acrylic sheet cast by the continuous process (between stainless-steel belts) is more uniform in thickness than cell-cast sheet. Cell-cast sheet, on the other hand, which is cast between glass plates, has superior optical properties and surface quality. Also, cell-cast sheet is available in a greater variety of colors and compositions. Cast acrylic sheet is supplied in general-purpose grades and in ultraviolet-absorbing, mirrored, super-thermoformable and cementable grades, and with various surface finishes. Sheets are available in transparent, translucent, and opaque colors.

Acrylic film is available in 2, 3, and 6-mil thicknesses, in clear form and in colors. It is supplied in rolls to 60 in. wide, principally for use as a protective laminated cover over other plastic materials.

Injection-molding and extrusion compounds are available in both standard and high-molecular-weight grades. Property differences between the two formulations are principally in flow and heat resistance. Higher molecular-weight resins have lower melt-flow rates and greater hot strength during processing. Lower molecular-weight grades flow more readily and are designed for making complex parts in hard-to-fill molds. Also available are high-impact molding grades, which provide the same transparency and weatherability as the conventional acrylics.

Properties:

Acrylic plastics transmit and control light, resist weather, are stable against discoloration, and have superior dimensional stability and an excellent combination of structural and thermal properties. Clear acrylic plastic is as transparent as the finest optical glass. It has a light transmittance of 92%, exceptionally low haze level of approximately 1%, and an index of refraction of 1.49 -- high enough for use in lenses and other optical applications.

Colorants produce a full spectrum of transparent, translucent, or opaque colors. Most colors can be formulated for long-term outdoor durability. Acrylics are normally formulated to filter ultraviolet energy in the 360-nm and lower band. Other formulations are opaque to UV light or provide reduced UV transmission.

Mechanical properties of acrylics are high for short-term loading. However, for long-term service, tensile stresses must be limited to 1,500 psi to avoid crazing or surface cracking.

The moderate impact resistance of standard formulations is maintained even under conditions of extreme cold. High-impact grades have considerably higher impact strength than standard grades at room temperature, but impact strength decreases as temperature drops. Special formulations ensure compliance with Underwriters' Laboratories standards for bullet resistance.

Although acrylic plastics are among the most scratch resistant of the thermoplastics, normal maintenance and cleaning operations can scratch and abrade them. Special abrasion-resistant sheet is available that has the same optical and impact properties as standard grades.

Toughness of acrylic sheet, as measured by resistance to crack propagation, can be improved severalfold by inducing molecular orientation during forming. Jet-aircraft cabin windows, for example, are made from oriented acrylic sheet.

Acrylic sheet and moldings resist solutions of inorganic acids and alkalies and aliphatic hydrocarbons such as VM&P naphtha, as well as most detergent solutions and cleaning agents. They are attacked, however, by chlorinated and aromatic hydrocarbons, esters, and ketones.

Transparency, gloss, and dimensional stability of acrylics are virtually unaffected by years of exposure to the elements, salt spray, or corrosive atmospheres. These materials withstand exposure to light from fluorescent lamps without darkening or deteriorating. They ultimately discolor, however, when exposed to high-intensity UV light below 265 nm. Special formulations resist UV emission from light sources such as mercury-vapor and sodium-vapor lamps.

Applications:

Cast acrylic sheet is used in aircraft, boat, mass transit, architectural, and protective glazing; internally illuminated outdoor signs, lighting diffusers, and skylights; and product prototypes and demonstration models. Special ultraviolet-absorbing grades are used for document preservation in museums and for various photographic applications.

Acrylic film is used as a laminated protective surface on ABS, PVC, or other plastic sheet that is thermoformed into parts requiring resistance to outdoor weathering. Examples include motorcycle shrouds, recreational-vehicle panels, residential siding, and transformer housings.

Acrylic moldings are used for light-control lenses in lighting fixtures, camera lenses, vending-machine parts, and appliance panels, knobs, and housings. Automotive applications include lenses for taillights and parking lights, instrument panels, nameplates, medallions, and dials. A modified molding compound contains an impact modifier to increase toughness. Parts molded from these durable transparent materials include automotive dials, housewares, piano keys, medical instruments, and toys.

Specially formulated acrylic sheet is available for deeply formed components such as tub-shower units, which are subsequently backed with glass-fiber-reinforced polyester. Chemical resistance of this sheet is superior to that of conventional sheet.

Sheet extruded from the high-impact molding grade is used for signs, thermoformed products, toys, and glass-fiber-polyester-backed components such as camper tops, furniture, and recreational-vehicle bodies.

Alkyd molding compounds are based on unsaturated polyester-type resins, which are combined with crosslinking monomers, catalysts, reinforcements, lubricants, and fillers. The formulations are similar to those of thermosetting polyesters but with lower amounts of monomers.

Alkyds are part of the group of materials that includes bulk-molding compounds (BMCs) and sheet-molding compounds (SMCs). They are processed by compression, transfer, or injection molding. Fast molding cycles at low pressure make alkyds easier to mold than many other thermosets.

Alkyds are furnished in granular compounds, extruded ropes or logs, bulk-molding compound, flake, and puttylike sheets. Except for the putty grades, which may be used for encapsulation, these compounds contain fibrous reinforcement. Generally, the fiber reinforcement in rope and logs is longer than that in granular compounds and shorter than that in flake compounds. Thus, strength of these materials is between those of granular and flake compounds. Because the fillers are opaque and the resins are amber, translucent colors are not possible. Opaque, light shades can be produced in most colors, however.

Properties:

Low-moisture absorption and excellent dimensional stability and electrical properties are the outstanding characteristics of most alkyd molding compounds. For electrical-grade materials, absorption can be as low as 0.5%. Alkyds are relatively low-loss materials, especially the mineral-filled and glass-filled grades. Those containing cellulose may have higher loss characteristics and "drift" with temperature and humidity changes.

Molded alkyd parts resist weak acids, organic solvents, and hydrocarbons such as alcohol and fatty acids; they are attacked by alkalies. Glass and asbestos-filled compounds have better heat resistance than the cellulose-modified types. Depending on type, alkyds can be used continuously to 350 °F and, for short periods, to 450 °F. Alkyd molding compounds retain their dimensional stability and electrical and mechanical properties over a wide temperature range.

Halogen and/or phosphorus-bearing alkyd molding compounds with antimony trioxide added provide improved flame resistance. Other flame-resistant compounds are available that do not contain halogenated resins. Many grades are UL-rated at 94V-0 in sections under 1/16 in. Flammability ratings depend on specific formulations, however, and can vary from 94HB to V-0. Flammability ratings also vary with section thickness.

Applications:

High-impact grades of alkyd compounds (with high-glass content) are used in military switchgear, electrical terminal strips, and relay and transformer housings and bases. Mineral-filled grades, which can be modified with cellulose to reduce specific gravity and cost, are used in automotive ignition parts, radio and TV components, switchgear, and small appliance housings. Alkyds with all-mineral fillers have high moisture resistance and are particularly suited for electronic components. Grades are available that can withstand the temperatures of vapor-phase soldering.

Esters in the allyl family consist principally of diallyl phthalate (DAP) and diallyl isophthalate (DAIP). Both are used as monomers and as prepolymers, which are readily converted to thermoset molding compounds and resins for preimpregnated glass cloth and paper. Allyls are also used as crosslinking agents for unsaturated polyesters.

Compounds based on allyl prepolymers are reinforced with fibers (glass, polyester, or acrylic) and filled with particulate materials to improve properties. Glass fiber imparts maximum mechanical properties, acrylic fiber provides the best electrical properties, and polyester fiber improves impact resistance and strength in thin sections. Compounds can be made in a wide range of colors because the resin is essentially colorless.

Prepregs (preimpregnated glass cloth) based on allyl prepolymers can be formulated for short cure cycles. They contain no toxic additives, and they offer long storage stability and ease of handling and fabrication. Properties such as flame resistance can be incorporated. The allyl prepolymers contribute excellent chemical resistance and good electrical properties.

Properties:

Allyl molding compounds do not corrode copper or plated inserts or contacts, even in hot, humid environments. Molded parts maintain their high electrical properties at high temperature (to 370 °F for DAP and 400 °F for DAIP) and humidity levels. Allyl materials are also characterized by excellent dimensional stability and resistance to moisture, chemicals, and liquid oxygen. They withstand strong and weak acids, alkalies, and organic solvents, even at elevated temperatures.

Allyl moldings have low mold shrinkage and postmold shrinkage -- attributed to their nearly complete addition reaction in the mold -- and have excellent stability under prolonged or cyclic heat exposure. Advantages of allyl systems over polyesters are freedom from styrene odor, low toxicity, low-evaporation losses during evacuation cycles, no subsequent oozing or bleed-out, and long-term retention of electrical-insulation characteristics.

Applications:

Diallyl phthalate monomer is used as a nonvolatile crosslinker in polyester compounds to improve properties and handling characteristics. Deflection temperature is raised to 400 °F or higher, dimensional stability and electrical properties are upgraded, and flexural properties are retained for long periods at elevated temperatures. DAP is also used in combination with polyester resin systems for low-pressure decorative laminates. Allylic resins in powder and liquid form are used for coatings and for impregnating materials.

Allyl prepolymers are particularly suited for critical electronic components that serve in severe environmental conditions. Chemical inertness qualifies the resins for molded pump impellers and other chemical-processing equipment. Their ability to withstand steam environments permits uses in sterilizing and hot-water equipment. Because of their excellent flow characteristics, diallyl-phthalate compounds are used for parts requiring extreme dimensional accuracy. Modified resin systems are used for encapsulation of electronic devices such as semiconductors and as sealants for metal castings.

A major application area for allyl compounds is electrical connectors, used in communications, computer, and aerospace systems. The high thermal resistance of these materials permits their use in vapor-phase soldering operations. Uses for prepolymers include arc-track-resistant compounds for switchgear and TV components. Other representative uses are insulators, encapsulating shells, potentiometer components, circuit boards, and housings.

Allyl-based prepregs are used to make lightweight, intricate parts such as radomes, printed-circuit boards, tubing, ducting, and aircraft parts. Another use is in copper-clad laminates for high-performance printed-circuit boards.

Melamine and urea are the principal commercial thermosetting polymers called aminos. The amino resins are formed by an addition reaction of formaldehyde and compounds containing NH 2 amino groups. They are supplied as liquid or dry resins and filled molding compounds. Applying heat in the presence of a catalyst converts the materials into strong, hard products. Aminos are used as molding compounds, laminating resins, wood adhesives, coatings, wet-strength paper resins, and textile-treating resins.

The base resin used in urea and melamine molding compounds is water-white and transparent. Translucent and opaque colors are produced by adding pigments and opacifying agents. Cellulose fibers improve strength and dimensional stability and reduce light transmission.

Properties:

Moldings made from amino compounds are hard, rigid, abrasion resistant, and have high resistance to deformation under load. They do not impart taste or odor to foods, and they have excellent electrical insulation characteristics. Melamines are superior to ureas in resistance to acids, alkalies, heat, boiling water, and for applications involving wet/dry cycling.

Urea and some melamine molding compounds have flammability ratings of 94V-0. Melamines containing alpha cellulose, mineral, or glass fibers have the greatest flame resistance. These materials can be used for appliance parts classified by UL as indirect supports of current-carrying parts.

Below -70 °F, urea moldings become brittle, but electrical properties are not affected. Extended exposure above 170 °F is not recommended for ureas because of the effect on color. At 300 °F, color change and blistering may occur after exposure of less than one hour.

Melamine compounds containing cellulose or flock fillers are stable in the range of -70 to 250 °F. Asbestos or glass-filled formulations are stable to 400 °F. Above 210 °F, however, color changes may occur.

Melamines and ureas are not resistant to strong oxidizing acids or strong alkalies, but they can be used safely with conventional household chemicals such as naphtha and detergents. They are unaffected by organic solvents such as acetone, carbon tetrachloride, ethyl alcohol, heptane, and isopropyl alcohol. Petroleum, paraffin hydrocarbons, gasoline, kerosene, motor oil, aromatic hydrocarbons, and fluorinated hydrocarbons (Freon) have no apparent effect on urea and melamine moldings. Dimensional stability is good, but moldings do swell and shrink slightly in varying moisture conditions. Baking of molded parts accelerates postmold shrinkage and improves dimensional stability, dielectric strength, and dissipation factor.

Applications:

Typical applications for cellulose-filled urea resins include circuit breakers, receptacles, and other electrical wiring devices, toaster and other appliance bases, pushbuttons, knobs, handles, piano keys, and camera parts.

Cellulose-filled melamine is used principally for dinnerware. Other applications include utensil handles, food-service trays, and housings for electric shavers and mixers. Industrial melamine compounds are used for meter blocks, connector plugs, automotive and aircraft ignition parts, standoff terminals, coil forms, and switch housings.

In liquid form, both urea and melamine resins are used as baking-enamel coatings, particleboard binders, and paper and textile treatment materials. Both resins are also used in compounding adhesives. Melamine, the more durable of the two, is waterproof, which qualifies melamine-based adhesives for exterior use.

Cellulosics are synthetic plastics, but they are not synthetic polymers; they are made from a naturally occurring polymer, cellulose, which is obtained from wood pulp and cotton linters. Cellulose can be made into a film (cellophane) or into a fiber (rayon), but is must be chemically modified to produce a thermoplastic material.

The resins used for plastics production are cellulose acetate, cellulose acetate butyrate, and cellulose propionate, all of which are cellulose esters, and ethyl cellulose, which is a cellulose ether. The plastics produced from them are commonly referred to as "acetate," "butyrate," "propionate," and "ethyl cellulose." They are processed by conventional thermoplastic processes.

The cellulosic plastics are supplied in a wide range of transparent, translucent, and opaque colors, including pearlescents and mottles. Clear resins are available in all except ethyl cellulose, which is light amber in uncolored formulations.

Properties:

Because the cellulosics can be compounded with many different plasticizers in widely varying concentrations, property ranges are broad. These materials are normally specified by flow (defined in ASTM D569), which is controlled by plasticizer content. Hard flows of (low plasticizer content) are relatively hard, rigid, and strong. Soft flows (higher plasticizer content) are tough, but less hard, less rigid, and less strong. They also process at lower temperatures. Thus, within available property ranges listed, no one formulation can provide all properties to the maximum degree. Most commonly used formulations are in the middle flow ranges.

Molded cellulosic parts can be used in service over broad temperature ranges and are particularly tough at very low temperatures. Ethyl cellulose is outstanding in this respect. These materials have low specific heat and low thermal conductivity -- characteristics that give them a pleasant feel.

Dimensional stability of butyrate, propionate, and ethyl cellulose is excellent. Plasticizers used in these materials do not evaporate significantly and are virtually immune to extraction by water. Water absorption (which causes dimensional change) is also low, that of ethyl cellulose being lowest. The plasticizers in acetate are not as permanent as those in other plastics, however, and water absorption of this material is slightly higher.

Butyrate and propionate are highly resistant to water and most aqueous solutions except strong acids and strong bases. They resist nonpolar materials such as aliphatic hydrocarbons and ethers, but they swell or dissolve in low-molecular-weight polar compounds such as alcohols, esters, and ketones, as well as in aromatic and chlorinated hydrocarbons. Acetate is slightly less resistant than butyrate and propionate to water and aqueous solutions, and slightly more resistant to organic materials. Ethyl cellulose dissolves in all the common solvents for this polymer, as well as in such solvents as cyclohexane and diethyl ether. Like the cellulose esters, ethyl cellulose is highly resistant to water.

Although unprotected cellulosics are generally not suitable for continuous outdoor use, special formulations of butyrate and propionate are available for such service. Acetate and ethyl cellulose are not recommended for outdoor use. Most cellulosic formulations are available in 94HB flammability classification. Also available are formulations that can be used in contact with food.

Applications:

Acetate applications include extruded and cast film and sheet for packaging and thermoforming, and extruded rod for tool and appliance handles and machine parts, eyeglass frames, pen barrels and caps, and quality toys.
Propionate molding applications include automotive and marine steering wheels, fuel-filter bowls, toothbrush handles, containers for cosmetics, face shields, safety goggles, and machine components that require transparency and toughness.
Transparent sheets are used in heavy-duty blister packaging and transparent packaging for food contact (FDA-accepted formulations) applications.
Butyrate is used for data-processor and cash-register keys, transparent dial covers, tool handles, and covers for instrument-panel lights. Opaque formulations are used for pens, switch covers, and knobs.
Typical applications for weathering-grade formulations are outdoor signs, window-well covers, skylights, and sprinklers. Extruded sheet is used for windshields, industrial shields, and thermoformed signs. Extruded-profile applications include table edging, tool handles, and trim strips.
Ethyl cellulose is used in helmets, gears, rollers, slides, flashlight housings, and tool handles. Military applications utilize its toughness and its resistance to nitroglycerine and black powder.

Specific properties that separate engineering films from their commodity counterparts include greater tensile and impact strength; improved moisture and gas barrier characteristics; good heat resistance and weatherability; better bonding and lamination; and improved electrical ratings. One or more of these properties can be obtained by choosing from a number of different polymer films.

Several melt-processible engineering thermoplastic films such as oriented polyester, oriented nylon, and unoriented nylon, exhibit high strength especially at high temperatures. In addition, they provide toughness at low temperatures, stiffness and abrasion resistance, and good chemical resistance. Polyester film is made from the PET polymer, principally by Du Pont (Mylar) and ICI (Melinex). The monomer is polymerized, extruded, cast into a web, and biaxially oriented, forming a drawn polyester film.

Oriented polypropylene,

A thermoplastic with low specific gravity, has excellent resistance, relatively high melting point, and good strength. Polycarbonate film is specified for its toughness, clarity, and high heat-deflection temperature.

Polyimides,

Both thermoplastics and thermosets, retain their principal properties over a wide temperature range. Polyimide films are available from ICI (Upilex) and Du Pont (Kapton). They have useful mechanical properties, even at cryogenic temperatures. At -453 °F, the film can be bent around a ¼-in. mandrel without breaking and, at 932 °F, its tensile strength is 4,500 psi. Room-temperature mechanical properties are comparable to those of polyester film.

To minimize transmission of moisture vapors, fluoropolymers are the best choice. This family of materials has a general paraffinic structure with some or all of the hydrogen atoms replaced by fluorine.

Polyetheretherketone

(PEEK), a high-performance thermoplastic, offers outstanding thermal properties as well as resistance to many solvents and proprietary fluids. This film can be used for interbonding or cladding in PEEK structural components. Thermoplastic and thermoset acrylics are noted for exceptional clarity and weatherability, and also offer favorable stiffness, density, chemical resistance, and toughness.

Film produced from PEEK resins (ICI's Stabar) can be laminated to itself or to other substrates. Bond strength depends on surface preparation and adhesive type. The film is available in a transparent, thermoformable grade and in a higher-temperature, heat-stabilized version, which is more crystalline and less transparent (also thermoformable).

Plastic film can be manufactured from almost any resin, but not every resin produces an engineering film. Generally, a resin-based film's properties are related to the chemistry of the basic polymer; however, properties may be further affected by subsequent processing techniques. Manufacturers can choose from, and end users can specify, a range of process treatments that significantly enhance heat stability, mechanical properties, electrical characteristics, barrier properties, and bondability.

Coatings are typically applied by emulsion, solvent, and dry methods. Results vary according to the formula used: PVDC coatings improve barrier properties; polyurethane improves abrasion resistance; and aluminum coatings alter electrical characteristics.

Some processors have developed proprietary antistatic coatings that are cured by electron-beam radiation. Metal coatings produce conductive capabilities and also enhance barrier properties. Often, metallization is used to improve moisture-barrier properties for biaxially oriented nylon and polypropylene.

Surface treatment, which removes low-molecular-weight residue, improves adhesion and appearance. Several methods may be used. Corona discharge techniques position the film between an electrically grounded roller and a high-voltage electrode. A continuous-arc discharge (corona) is generated to clean and activate the film surface.

In gas-plasma surface treatments, film is placed in a reaction chamber. After evacuation, the chamber is charged with oxygen, argon, helium, or nitrogen while a radio-frequency field ionizes the gas. A resultant glow discharge creates free radicals on the surface, improving adhesion.

Film can be passed over a bank of flame jets to activate the surface and burn off contaminants. Other surface preparation techniques include polishing and embossing by roller.

Additionally, performance films combine base-resin properties with advantages unique to the film form. A film may be selected both for its dielectric properties and for its usefulness as a bonding material, thus serving two needs. Fabrication techniques such as coextrusion and lamination produce a film with the best properties of two or more individual films -- strength combined with barrier properties and optical clarity, for example.

The addition of various modifiers and pigments comprise another method for improving film performance. These are blended into the resin melt or introduced at the extruder, supplying the end film with specific characteristics. UV and heat stabilizers are among the most useful additives.

Applications:

For engineering films span a broad spectrum of industrial uses. Film-based products appear in photographic, business graphic, reprographic, and electronic imaging systems. Magnetic-media manufacturers also rely on high-quality flexible films to produce their information-handling products. Packaging end users receive product protection, extended shelf life, and visual impact from a variety of films. In the electronics industry, films are used to construct components such as membrane switches.

Transparent PEEK film is used for a missile nose cone because of its microwave transmittance and resistance to rain erosion. Potential applications for the crystalline grade include helicopter-rotor blade cladding, fine-line microwave circuits, and aircraft interior panels.

Epoxy polymers are cured to form thermoset resins by either homopolymerization of epoxy groups with themselves, or reaction with curing agents such as anhydrides, amines, and novolacs. Because the curing agent contributes significantly to the cured properties of the resin, this is called an addition reaction. Shrinkage during polymerization of epoxy resins is extremely low.

The most widely used epoxy resins are based on the reaction of epichlorohydrin with bisphenol-A. The reaction ratios of these two constituents can be varied to produce products ranging from low-viscosity liquids to high-molecular-weight solids.

The novolacs are another important class of epoxy resins, particularly the ECNs (epoxy cresol novolacs) and EPNs (epoxy phenol novolacs). These polyfunctional resins offer higher thermal properties and improved chemical resistance over bis-A derivatives. The cycloaliphatics comprise a third group of epoxy resins that are particularly important for applications requiring resistance to arc tracking and weathering. A fourth class of epoxy resins is based on the epoxidation of aromatic amines. These resins have good mechanical properties and high thermal capabilities and fatigue resistance.

Properties:

Epoxies have excellent electrical, thermal, and chemical resistance. Their strength can be further increased with fibrous reinforcement or mineral fillers. The variety of combinations of epoxy resins and reinforcements provides a wide latitude in properties obtainable in molded parts.

Molded epoxy parts are hard, rigid, relatively brittle, and have excellent dimensional stability over a broad temperature range. Some fiber-reinforced formulations can withstand service temperatures above 500 °F for brief periods. Their excellent electrical properties, in combination with high mechanical strength, qualify them for electrostructural applications. Resins based on bisphenol-A are adequate for most services. However, cycloaliphatics are recommended for parts subjected to arcing conditions or those requiring outdoor weatherability.

Excellent adhesion in structural applications is another outstanding property of epoxy systems. Epoxy adhesives for bonding many dissimilar materials can be supplied either as one or two-part systems. One-part systems require heat for curing; two-part systems usually cure at room temperature, but properties are improved when the materials are heat cured. Some epoxy adhesive systems can withstand temperatures to 450 °F, although properties at such temperatures are considerably lower than at room temperature.

Applications:

Epoxy resin systems are used with various reinforcements: glass, graphite and aramid fibers, asbestos, cotton, and metal foils. Epoxy laminates are important because of their excellent electrical properties over a wide temperature range, as well as their dimensional stability and chemical resistance. The amine-based resins are used in conjunction with graphite fiber to make structural composites for commercial and military aircraft and components for space equipment. These composites offer significant advantages over metals in the areas of weight savings and corrosion resistance. Printed-circuit boards, consisting of a glass-fabric-reinforced flame-retardant epoxy resin are another major use.

Filled and unfilled liquid systems are used for potting and encapsulating electrical/electronic components ranging from semiconductor devices and miniature coils and switches to large motors and generators. The epoxy resins provide excellent adhesion, and low shrinkage (volume shrinkage is only 2 to 3%) needed for such applications. The compounds cure in 1 to 4 hr, and they do not outgas during the cure or in service. The casting cycle can be significantly accelerated by using one of the newer processes: liquid injection molding or pressure gelation. The latter method can reduce the molding cycle of a 15-lb casting from 4 hr to 10 min. Applications being evaluated for pressure-gelation casting include high-voltage electrical components and pump and valve housings.

Another important use for epoxy resins is in coatings, both as liquids and powders. These finishes have excellent flexibility, impact and abrasion resistance, and adhesion to most substrates. They can be formulated to resist many industrial chemicals and other corrosive materials. Epoxy coatings are typically used as automotive and appliance primers, in industrial maintenance paints, marine finishes, pipe coatings, and decorative topcoats for various products. Epoxy resins are also furnished as molding powders, usually incorporating glass or mineral-fiber reinforcement. These compounds can be molded by conventional thermoset methods. The outstanding electrical and thermal properties qualify the epoxies for industrial switchgear applications and for encapsulating semiconductor devices.

The family of thermoplastics are extremely inert, paraffinic thermoplastic polymers that have some or all of the hydrogen replaced with fluorine. The family of materials includes polytetrafluoroethylene (PTFE, commonly called TFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), polychlorotrifluoroethylene (CTFE), poly (ethylene-chlorotrifluoroethylene (ECTFE) copolymer, ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), polyvinylfluoride (PVF), and copolymers of halogenated and fluorinated ethylenes.

PTFE, FEP, and PFA:

Their high melt viscosity prevents PTFE resins from being processed by conventional extrusion and molding techniques. Instead, molding resins are processed by press-and-sinter methods similar to those of powder metallurgy or by lubricated extrusion and sintering. All other fluoroplastics are melt processible by techniques commonly used with other thermoplastics.

PTFE resins are opaque, crystalline, and malleable. When heated above 648 °F, however, they are transparent, amorphous, relatively intractable, and they fracture if severely deformed. They return to their original state when cooled.

FEP resins offer nearly all of the desirable properties of PTFE, except thermal stability. Maximum recommended service temperature for these resins is lower by about 100 °F. PFA fluorocarbon resins are easier to process than FEP and have higher mechanical properties at elevated temperatures. Service temperature capabilities are the same as those of PTFE.

PTFE resins are supplied as granular molding powders for compression molding or ram extrusion, as powders for lubricated extrusion, and as aqueous dispersions for dip coating and impregnating. FEP and PFA resins are supplied in pellet form for melt extrusion and molding. FEP resin is also available as an aqueous dispersion.

Properties:

Outstanding characteristics of the fluoroplastics are chemical inertness, high and low-temperature stability, excellent electrical properties, and low friction. However, the resins are fairly soft and resistance to wear and creep is low. These characteristics are improved by compounding the resins with inorganic fibers or particulate materials. For example, the poor wear resistance of PTFE as a bearing material is overcome by adding glass fiber, carbon, bronze, or metallic oxide. Wear resistance is improved by as much as 1,000 times, and the friction coefficient increases only slightly. As a result, the wear resistance of filled PTFE is superior, in its operating range, to that of any other plastic bearing material and is equalled only by some forms of carbon.

The static coefficient of friction for PTFE resins decreases with increasing load. Thus, PTFE bearing surfaces do not seize, even under extremely high loads. Sliding speed has a marked effect on friction characteristics of unreinforced PTFE resins; temperature has very little effect.

PTFE resins have an unusual thermal expansion characteristic. A transition at 65 °F produces a volume increase of over 1%. Thus, a machined part, produced within tolerances at a temperature on either side of this transition zone, will change dimensionally if heated or cooled through the zone.

Electrical properties of PTFE, FEP, and FPA are excellent, and they remain stable over a wide range of frequency and environmental conditions. Dielectric constant, for example, is 2.1 from 60 to 10(to the 9th power) Hz. Heat-aging tests at 572 °F for six months show no change in this value. Dissipation factor of PTFE remains below 0.0003 up to 10(to the 8th power) Hz. The factor for FEP and PFA resins is below 0.001 over the same range. Dielectric strength and surface arc resistance of fluorocarbon resins are high and do not vary with temperature or thermal aging.

Applications:

PTFE resin applications can be classified in five categories:
1. Fluid conveying systems -- gaskets, molded packings and seals, piston rings, and bellows.
2. Static and dynamic load supports -- bearings, ball and roller-bearing components, and sliding bearing pads.
3. Release surfaces -- sheet for preventing adhesion, pressure-sensitive tapes, and heat-shrinkable roll covers.
4. Electrical and electronic -- insulation for coaxial cable, fixture and motor lead wire, hookup and panel wiring, industrial signal and control cable, and for standoff and feedthrough components.
5. Thermal system components -- ablative shields.

FEP resin applications include wire and cable insulation for computer and electronic systems, telephone and alarm systems, and business-machine interconnects, FEP resin is also supplied as extruded sheet and film for release surfaces, roll covers, linings for chemical-processing tanks, and piping. A concentrate is available for Freon-blown-foam wire coating.

PFA resins are used for high-temperature wire and cable insulation, heat-shrinkable tubing and roll covers, chemical-resistant linings for process-equipment components, and in semiconductor processing equipment.

Materials do not adhere readily to the slippery surface of FEP, PFA, and PTFE parts. Surfaces can be chemically etched, however, to permit bonding with adhesives. Thus, low-friction surfaces of fluorocarbon tape or film can be bonded to steel, aluminum, rubber, or other materials. FEP and PFA parts can be heat sealed to themselves, to PTFE parts, or to metals at low pressure and temperatures above 590 °F.

CTFE:

Sensitivity to processing conditions is greater in CTFE resins than in most polymers. Molding and extruding operations require accurate temperature control, flow channel streamlining, and high pressure because of the high melt viscosity of these materials. With too little heat, the plastic is unworkable; too much heat degrades the polymer. Degradation begins at about 525 °F. Because of the lower temperatures involved in compression molding, this process produces CTFE parts with the best properties.

Thin parts such as films and coil forms must be made from partially degraded resin. Degree of degradation is directly related to the reduction in viscosity necessary to process a part. Although normal, partial degradation does not greatly affect properties, seriously degraded CTFE becomes highly crystalline, and physical properties are reduced. Extended usage above 250 °F also increases crystallinity.

CTFE plastic is often compounded with various fillers. When plasticized with low-molecular-weight CTFE oils, it becomes a soft, extensible, easily shaped material. Filled with glass fiber, CTFE is harder, more brittle, and has better high-temperature properties.

Properties:

CTFE plastics are characterized by chemical inertness, thermal stability, and good electrical properties, and are usable from 400 to -400 °F. Nothing adheres readily to these materials, and they absorb practically no moisture. CTFE components do not carbonize or support combustion. Up to thicknesses of about 1/8 -in., CTFE plastics can be made optically clear. Ultraviolet absorption is very low, which contributes to its good weatherability.

Compared with PTFE, FEP, and PFA fluorocarbon resins, CTFE materials are harder, more resistant to creep, and less permeable; they have lower melting points, higher coefficients of friction, and are less resistant to swelling by solvents than the other fluorocarbons.

Tensile strength of CTFE moldings is moderate, compressive strength is high, and the material has good resistance to abrasion and cold flow. CTFE plastic has the lowest permeability to moisture vapor of any plastic. It is also impermeable to many liquids and gases, particularly in thin sections.

Applications:

Molded and extruded CTFE resin applications include components for handling and containing corrosive liquids (diaphragms, valves, sight glasses); seals, gaskets, O-rings, valve seats, and packings for liquid-oxygen and hydrogen equipment; and flexible-circuit laminations, wire insulation, jacketed cable, coil bobbins, and other electrical components. CTFE materials are FDA-approved for use in food-handling equipment. Thin, optically clear CTFE moldings are used for infrared windows in missiles, radome covers, oil-reservoir covers, and gage faces.

PVDF:

Polyvinylidene fluoride, the toughest of the fluoroplastic resins, is available as pellets for extrusion and molding and as powders and dispersions for corrosion-resistant coatings. This high-molecular-weight homopolymer has excellent resistance to stress fatigue, abrasion, and to cold flow. Although insulating properties and chemical inertness of PVDF are not as good as those of the fully fluorinated polymers. PTFE and FEP, the balance of properties available in PVDF qualifies this resin for many engineering applications. It can be used over the temperature range from -100 to 300 °F and has excellent resistance to abrasion.

PVDF can be used with halogens, acids, bases, and strong oxidizing agents, but it is not recommended for use in contact with ketones, esters, amines, and some organic acids. Oxygen index is 44.

Although electrical properties of PVDF are not as good as those of other fluoroplastics, it is widely used to insulate wire and cable in computer and other electrical and electronic equipment. Heat-shrinkable tubing of PVDF is used as a protective cover on resistors and diodes, as an encapsulant over soldered joints.

Valves, piping, and other solid and lined components are typical applications of PVDF in chemical-processing equipment. It is the only fluoroplastic available in rigid pipe form. Woven cloth made from PVDF monofilament is used for chemical filtration applications.

A significant application area for PVDF materials is as a protective coating for metal panels used in outdoor service. Blended with pigments, the resin is applied, usually by coil-coating equipment, to aluminum or galvanized steel. The coil is subsequently formed into panels for industrial and commercial buildings.

A recently developed capability of PVDF film is based on the unique piezoelectric characteristics of the film in its so-called beta phase. Beta-phase PVDF is produced from ultrapure film by stretching it as it emerges from the extruder. Both surfaces are then metallized, and the material is subjected to a high voltage to polarize the atomic structure.

When compressed or stretched, polarized PVDF generates a voltage from one metallized surface to the other, proportional to the induced strain. Infrared light on one of the surfaces has the same effect. Conversely, a voltage applied between metallized surfaces expands or contracts the material, depending on the polarity of the voltage.

Representative commercial fluoroplastics:

PTFE, Du Pont (Teflon TFE), Allied-Signal (Halon, TFE), ICI Americas (Fluon) FEP, Du Pont (Teflon FEP) PFA, Du Pont (Teflon PFA) CTFE, 3M (Kel-F), Allied-Signal (Plaskon CTFE) PVDF, Pennwalt (Kynar), Atochem (Foraflon) PVF, Du Pont (Tedlar) ETFE, Du Pont (Tefzel) ECTFE, Allied-Signal (Halar)