“High performance” is a relative term often used to describe materials and/or attributes of materials. Often over-used and misapplied by zealous marketing types, consumers are often misled by the term and usually are given no point of reference from which to begin material comparisons. So let’s start at the beginning and define High Performance as it pertains to plastics.
The two main sub-Groups are: Thermosets / Thermoplastics
To better understand the family tree of plastics; it is helpful to review their history. (See “The History of Plastics”) However, for this discussion, and as a point of reference, let’s divide the entire family of polymers into two main groups; “Thermosets” and “Thermoplastics”. (We’ll further divide them into sub groups later.)
“Thermosets are like eggs”
Thermosets are those materials which when polymerized take an irreversible set. You can liken thermosets to concrete (or an egg if you prefer) where once subjected to a catalyst and/or elevated temperature undertake changes as the molecular level. After this change, they cannot be returned to their original raw state. Some examples of thermosets would be materials like epoxies, the composites family of Poly-Texx HPV, and Canvas or Linen phenolics, (better known as Bakelite and Micarta). Because they cannot be easily reformed or melted back down, these materials do not lend themselves easily to recycling, scraps and off-cuts are usually discarded to land fills.
“Thermoplastics are like ice”
Thermoplastics are those materials that even after an initial polymerization can be reconstituted to their original state usually through heat and/or a chopping process. At that point they can be reprocessed and reused (Commonly known as recycling). Thermoplastics can be thought of like ice, they can be melted, down and re-polymerized so that they can be used again. Examples of thermoplastics are PETG (2-liter soda bottles), polyethylene and polypropylene (Squeezable ketchup bottles). It should be noted however that once a plastic has been recycled, some of its original properties are diminished. This is primarily due to degradation caused by subsequent heating cycles, as well as the introduction of debris.
Though it could be argued that there are some thermosets that could be described as high performance, we will concentrate on the thermoplastic side of the field. For this discussion, it is convenient to divide thermoplastics into three main sub-groups: standard, engineering, and high performance plastics. We will spend our time today on the last group, but let’s first define the standard and engineering materials.
Standard plastics are generally those materials that have a maximum operating temperature below 180 degrees F. Low electrical properties, and/or chemical resistance. They are usually low in cost and easily available from a large number of producers. These materials are by far the most widely used on a volume-only basis. Essentially, they are useful due to one or two main attributes that specifically match the needs of an application. As a result many standard plastics find their way into consumer products.
- ABS (Computer housings)
- NYLON (Power tool casings)
- ACRYLIC (Point of purchase displays)
- POLYSTYRENE (Credit cards)
- POLYETHYLENE (Milk containers)
- POLYPROPYLENE (Food containers)
- POLYVINYLCHLORIDE (PVC / Water pipe)
From an engineering standpoint, the most useful member of this group may be Ultra High Molecular Weight Polyethylene (UHMW-PE). A member of the polyolefin family, It is a highly abrasion resistant, tough, low cost plastic. Commonly used for bearing and wear components such as chain guides, rollers and pulleys.
Engineering plastics form the largest group of plastics in regard to the number of materials from which to choose. They generally handle temperatures up to 350 degrees F. The physical properties are good while chemical and electrical properties vary. The costs are moderate to high.
- ACETAL (HOMOPOLYMER) – Delrin
- ACETAL (COPOLYMER) – Ensital, Tecaform, Celcon
- FILLED NYLONS – Vekton, Nylatron
- PHENOLICS – Micarta, Bakelite
- PTFE – Halon, Hostaflon
- PVDF – Ensikem, Kynar & Chemfluor
- ETFE – Tefzel, Hostaflon
- POLYETHYLENE TEREPHTHALATE – Ensitep BT, (PET)
- POLYBUTYLENE TEREPHTHATE – Hydex 4101(PBT)
- PPO – Noryl
- POLYCARBONATE – Ensicar, Lexan, Tuffack
The key to choosing the right engineering grade plastic for an application is to first understand the application as completely as you can. Everything from loading, stresses, wear properties, etc. Then the trick is to match the materials capabilities to the application, thereby minimizing the shortcomings of the plastic. For example, the Acetal family (Delrin, Celcon, and Tecaform) is a crystalline material with good physical properties, good chemical resistance and is easily machined. At one time, acetal was only available in a homopolymer (Single resin system). Unfortunately homopolymer acetal extruded shapes such as sheet and rod exhibit a porosity line created when bubbles form and converge during cooling. This line is actually an area, which is more porous than the rest of the cross-section. Though homopolymer acetal is still commonly used, resin suppliers have since corrected this problem with the introduction of the copolymer version (Multi resin system), which minimizes porosity, and eliminates the centerline porosity and discoloration.
The Nylon family is also highly useful for many applications including pulleys, bearings, bearing and wear components. It is strong, tough, resistant to abrasion, and has a low coefficient of friction. Nylon’s weak point is moisture absorption. Some types of Nylon can absorb up to 8% moisture at saturation. The result is not only swelling, but at saturation, water acts to lower its glass transition temperature to 120 degrees F. This means that fully saturated Nylon will tend to soften as temperatures rise over 100F.
Engineering plastics are used for many close tolerance machined parts. They are available in similar materials that are injection moldable, and therefore used as prototypes before injection molding is considered. Thus, engineering plastics, while among the most useful and common, each have attributes and detrimental qualities that require that we understand the application as completely as possible before deciding upon a material.
“High Performance Plastics”
Now that we have defined “Standard” and Engineering” plastics and understand that these classes of materials are limited by a particular attribute like temperature, chemical resistance or moisture absorption. Then, by definition, “High Performance Plastics” are those materials that maintain their physical properties under thermal, chemical or electrical stress. These materials are relatively high in cost, and in some cases only available from a single source. The unique characteristics of each of these often allow them to solve problems not possible with other materials and therefore have earned the right to be called “High Performance”.
For this discussion we will review nine materials. We will subdivide them into three groups; amorphous, crystalline, and imidized.
Amorphous materials begin to soften as soon as heat is applied and physical properties are diminished even at slightly elevated temperatures. It is that characteristic that makes them thermo-formable. (Formable with Heat) The heat resistance of amorphous materials allows them to be steam sterilized, some of them many times over. There are versions that are FDA, USDA, 3A compliant and get extensive use in the medical industry.
The following are not ultra high temperature materials but all three can function above 300 degrees F.:
Polysulfone was introduced by Union Carbide in 1965 as a lower cost heat resistant plastic. Its initial use was to replace Polycarbonate at elevated temperatures and/or in the presence of chemicals. It is the least expensive of the high performance plastics. Polysulfone is available as extruded sheet up to 2” thick and as cast rod and tubing. It can be cut, machined, and molded into many shapes and configurations.
Polyphenylenesulfone (PAES) has the impact resistance of a Polycarbonate and yet is a mid-to-high temperature material. It can be used for sight glasses in hot environments. It is available as extruded sheet up to 3” thick. It too can be cut, machined, and molded into many shapes and configurations.
Polyetherimide (Ultem 1000) was introduced by General Electric in 1982 and they remain the only source of the base resin. However, this popular material is available as extruded rod and plate in a wide range of sizes. On a custom basis, it can also be made in some colors. In it unfilled form its trade name is “Ultem 1000”. It is USDA, FDA, and USP Class IV compliant. Further, repeated steam autoclaving tests indicate that Ultem maintains a high level of properties after 2,000 cycles. Testing at 10 M-rads of Gamma radiation and up to 500 M-rads cumulative exposure showed little effect.
Ultem is easily compounded with glass fibers to increase strength and dimensional stability. Ultem 2300 (30% glass filled PEI) is available in sheet and rod stock and/or injection molded parts. It boasts an increase from 475,000 psi to 800,000 psi in Tensile Modulus and from 500,000 psi to 900,000 psi in Flexural Modulus. Adding 30% glass also increases the dimensional stability of the material at elevated temperatures by lowering the coefficient of thermal expansion from 3.1 to 1.1 x 10 e5 in./in./oF. By comparison Aluminum is about 1.2 x 10 e7 in./in./oF. Ultem can be machined or molded into parts for reusable medical devices, electrical and electronic insulators (particularly as connector components), and a wide range of structural components.
As an amorphous material Ultem is affected by some aromatic chemicals which can cause problems such as crazing, and premature stress cracking. In addition, some adhesives can attack the resin and cause failures along stress lines. Press fits, machined holes, and threaded sections must be designed carefully to avoid stress cracking.
Although these three materials are considered to be High Performance and can handle heat and stress, they are not considered to be a bearing and wear material.
The term crystalline, when used in connection with polymers is really a misnomer; these materials are really semi-crystalline, and the ratio of crystallinity has an effect on their associated properties. Unlike amorphous materials (whose properties are instantly affected by heat), all crystalline materials are characterized by maintaining their physical properties as temperature increases until they reach their Glass Transition Temperature (Tg). At which point their properties fall off precipitously.
As a result, these materials are not thermo-formable but are best used for their resistance to thermal, chemical and low friction. For the most part, they are good candidates for bearing and wear materials and can likely be used where moving parts are involved. All three of the materials we will consider here exhibit Coefficients Of Thermal Expansion (CoTE) that are not only high, but they are anything but linear. Thus making some applications “interesting” from a design standpoint.
PTFE was discovered by Dr. Roy Pluncket of DuPont in the late 1930’s. Reportedly, while working with liquid Freon, he discovered a fine white powder suspended in the beaker. He was unable to break the The Powder back down with any chemistry in the lab, and was only able to effect it with temperatures above 600 degrees F. It was also discovered that it was an excellent insulator, and that it could be processed into shapes by a long process involving pressure and heat. The discovery was kept secret during WWII until DuPont introduced it commercially.
At that time, Dixon Industries in Bristol, RI made saddle bearings out of oil impregnated oak and maple wood and porous cast iron. Bob Rulon-Miller, who owned Dixon, experimented with PTFE bearings but could not overcome the lack of creep strength. Finally, with the help of a PhD, by the name of Saul Ricklin, Bob Rulon-Miller developed the process for blending reinforcing agents with PTFE. Using glass and other fillers, Dixon created the Rulon family of filled PTFE materials.
Over the years Dixon Industries (now part of St. Gobain) and several others have developed many filled PTFE’s. There are versions compounded to reduce creep and wear; there are FDA, USDA, 3A compliant versions; and there are compounds with fillers that harden with heat as the base PTFE softens. The latter group is often used in sealing applications.
The family of filled PTFE materials are almost exclusively used in bearing and wear applications. Their low coefficient of friction and resistance to heat up to 500 degrees F. make them an excellent choice for many demanding seal, bearing and wear applications.
Polyphenylene Sulfide (PPS)
Polyphenylene Sulfide was introduced by Phillips Petroleum in 1973 and found extensive use as machined components in the chemical industry. The Ryton family of PPS was one of the first to be introduced. It had excellent chemical resistance, but was always brittle. As a result it was never produced in an unfilled version. However, 40% glass filled PPS (Ryton R4) and lubricant filled grades for bearing and wear applications are still cost effective alternatives to PEEK.
DSM (Now Quadrant) introduced an unfilled version of PPS called Techtron after Phillip’s rights to the chemistry expired. This is an unfilled version without the chipping problems of earlier formulations. As a result, Techtron is easily machined into components for use in corrosive atmospheres. Techtron is easily machined with experience, even to tight tolerances when required.
Polyetheretherketone is produced under a number of trade names but is generally referred to as PEEK. It was initially synthesized by DuPont in 1962 but was not introduced commercially until ICI did so in 1979.
Since then, PEEK has grown quickly in applications where exposure to harsh chemicals at elevated temperatures is required. In addition, PEEK is easily compounded with a number of fillers. These fillers allow materials with high strength and/or outstanding bearing characteristics to be produced. Bearing grade versions of PEEK are being rated as high as 100,000 PV. As with the other crystalline materials we have reviewed the coefficient of thermal expansion of PEEK compounds is not linear. At about 200 degrees F the rate of expansion begins to accelerate. This attribute and the change in physical properties at nearly the same temperature can sometimes challenge a designer.
Imidized materials is a term used to characterize a family of materials that combine features of both amorphous and crystalline materials. In fact, in some ways they are similar to thermoset plastics in that there are chemical changes that take place as they are processed.
As a result, all imidized materials are difficult to process. In some cases only compression molding is available to make basic shapes.
Polyamideimide was developed by Amoco in 1964 but processing problems limited the initial uses. Even now, while Torlon can be injection molded and extruded, special equipment and special techniques and tools are needed.
The resulting product, however, has many unique characteristics. It is extremely dimensionally stable at high temperatures with nearly linear coefficients of thermal expansion ranging from 0.9 to 1.7 in./in./oF x 10-5. PAI is resistant to radiation, has good electrical properties, and is resistant to chemicals except for strong bases, steam, and some high temperature acids.
Applications for Torlon are in the electrical and electronics industry, as seals and bearings, and as wear surfaces in office equipment. In fact, anywhere that temperature, electrical characteristics, or wear is a requirement, Polyamideimide is a candidate.
Polyamideimide (PAI) is used extensively as a plane bearing material. The ability to handle PV’s as high as 50,000 and maximum no load velocities approaching 1,000 feet/minute and still function at elevated temperatures means that Torlon, particularly Torlon 4301, can be used where few other non-metallics can function.
Torlon 4301 will function against relatively soft shaft materials with minimal increases in wear. Against C1018 steel (Rockwell C6), the increase in wear is 20 to 40% over hardened steel. Wear against 316 SS, however, can be substantial.
Adding lubricants to Torlon 4301 has produced some remarkable results. Thrust washers immersed in hydrocarbon fluid have performed at 1,300,000 PV. In a water lubricated hydraulic motor, vanes have attained 2,000,000 PV.
Polyimides were first introduced by DuPont with the Vespel product lines. DuPont maintained control of the chemistry until a few years ago. Since then several competitors (Furon with the Meldin line and DSM Polymer with their Duratrons, for example) have introduced Polyimides. The result has been the introduction of new compounds, increased availability, and reductions in prices.
Polyimides are almost exclusively used in high temperature applications. The continuous use temperature rating is 580oF and the heat deflection temperature at 264 psi is 592oF. It is not uncommon for Polyimides to be operating well above 600 degrees.
In deep vacuums, Polyimides can outgas small amounts of moisture due to their slightly hygroscopic nature; however few other gasses are emitted and as a result, they find a lot of applications in vacuum deposition chambers.
Polybenzimidazole (PBI) was developed by Hoechst Celanese. It is currently available only as compression molded shapes that can then be machined into components. The size and variety of available shapes is limited, but growing, as tooling and equipment is added and as molding techniques improve. Because PBI is extremely hard, machining it can be a challenge. Diamond tools are recommended. It is also a notch sensitive material, meaning that sharp corners should be avoided. Celazole is also susceptible to moisture absorption (0.40% in 24 hours) and should be dried prior to being exposed to sudden increases in temperatures in the 400 degrees F range.
Because of its cost (100 times the cost of Nylon) and the difficulty in processing, PBI applications remain limited. However, there are times when PBI is the only solution to a difficult design problem.
As a general rule, plastic materials have a higher thermal expansion rate than metals. Further, we must remember that many expansion rates for plastics are non-linear, and of course, the introduction of reinforcing agents and fillers can change the expansion characteristics of any material. All in all, thermal expansion is always a consideration when elevated temperatures are involved. Chemical resistance of plastics varies significantly, not only from material to material but is dependent upon concentrations and temperatures. The best way to insure compatibility is to contact a knowledgeable supplier. Companies like PolyTech keep extensive lists of plastic/chemical compatibility and should, if needed, have contacts at resin suppliers that can assist you.
Electrical properties of these materials vary, but all are insulators and most can, and do, find applications in the electrical and electronics industries in everything from connectors to chip handling systems. Other plastics can be made to be static dissipative or conductive. Always contact your plastics engineer for assistance in choosing a material.
Gamma Rays are used in repeated sterilization: thus, Gamma radiation values are used in this chart. Beta Radiation is also used for one time sterilization of disposable items but causes surface temperatures to rise and may cause some melting.
Nylon 66 104