SEAL CONNECTEverywhere you look, you likely see many types of industrial plastic. Plastic is a lightweight, durable, inexpensive, and easily modified material. This is part of the reason its usage has increased rapidly and is still growing.
All plastic is industrialized, and different plastic products have distinct properties. These properties can be seen primarily in their:
- Thermal-resistance
- Physical and mechanical properties
- Density
- Structure
Plastic consists of polymers, which are large organic molecules. The molecules are composed of repeating carbon-based units or chains. Polymers are produced when molecules called monomers to form long chains in polymerization.
High-Performance Plastics for Critical Applications
This article focuses on engineered, high-performance plastics often used for or with critical sealing technology. Five base materials are used 99% of the time. They are often used to design compounds by adding fillers to suit specific applications, such as the following:
- A throttle in an air seal
- A lantern ring for a stuffing box
- Bushings or bearings for shaft stabilization
The plastics used are important. These components cannot be constructed from any plastic on the shelf. These five are the primary bases for most industrial, high-performance plastics. Each has unique characteristics, strengths, and limitations. Plastic should be selected carefully to match the parameters of each application.
1. Polytetrafluoroethylene
Polytetrafluoroethylene (PTFE), a universal material, performs well in many applications. It is the most commoditized engineered industrial plastic because of its dependable nature for many purposes.
PTFE has a wide temperature range and broad chemical resistance. An inexpensive option, it is compatible with most other materials. Table 1 details the material’s strengths and limitations.
| Strengths | Limitations |
| Broad chemical resistance | Creep and cold flow |
| Nonflammable | Relatively soft |
| Flexible at low temperatures | Low thermal dissipation |
| Low coefficient of friction (COF) | Poor dimensional stability |
| Stable at high temperatures | High material shrinkage |
| Relatively easy to process | Low rigidity |
| HF resistance | Low electrical conductivity |
| Table 1. PTFE strengths and limitations | |
2. Polyethylene Terephthalate
Polyethylene terephthalate (PET) is a very hard, dense material. It is primarily used for bearings and bushings. With a relatively low service temperature, it is only good to about 210℉.
PET is primarily used in conjunction with proprietary fillers for vertical pump bushings. It is very dimensionally stable, fairly inexpensive, and easy to machine. Table 2 details its strengths and limitations.
| Strengths | Limitations |
| Very rigid and strong | Low melting point |
| Good dimensional stability | Narrow chemical resistance |
| Low COF | |
| Hydrophobic | |
| Easy to machine | |
| FDA-approved for food and liquid contact | |
| Table 2. PET strengths and limitations | |
3. Polyphenylene Sulfide
Polyphenylene Sulfide (PPS) is a higher-cost, very hard material. It has a broad temperature range and chemical resistance. Sometimes, it is referred to as PET’s older, more established brother. PPS has about the same hardness as PET but with broader chemical and temperature resistance. Its range is 500℉ to 550℉.
PPS primarily appears as shaft stabilizing bushings. Anytime a shaft has run out or wobbles, a PPS bushing stabilizes the shaft. It works in conjunction with the whole system:
- The PPS bushing
- A lantern ring
- The packing
These all make up a stuffing box.
Two types of PPS are available: linear and branched. Table 3 details the characteristics of each, and Table 4 lists the strengths and limitations.
| Linear | Branched |
| Long linear molecular chains similar to PTFE | High rigidity |
| Higher tensile strength and elongation | Increased hardness |
| Better melt stability | Increased dimensional stability |
| Less prone to absorbing moisture | Increased creep resistance |
| Better impact resistance | More difficult to process and generally requires pre-curing/cross-linking |
| Table 3. Linear and branched PPS characteristics | |
| Strengths | Limitations |
| Excellent chemical and radiation resistance | It can be very brittle |
| Excellent water absorption | Poor resistance to chlorinated hydrocarbons (For example vinyl chloride and chloromethane) |
| Dimensionally stable | High cost |
| Superior abrasion resistance | Challenging to process |
| Continuous use up to 450℉ | |
| Table 4. PPS strengths and limitations | |
4. Polybenzimidazole
Polybenzimidazole (PBI) is a high-temperature material. It withstands temperatures up to almost 900℉. This makes it an ideal plastic when dealing with difficult, high-temperature chemical applications. It is chemically resistant and dimensionally stable. Many experts consider it the Lamborghini of materials because it can do almost anything.
| Strengths | Limitations |
| Highest compressive strength of any unfilled resin | Can be brittle |
| Excellent tensile and flexural strength | Expensive |
| Dimensionally stable | |
| Very low creep | |
| Low COF | |
| Continuous use up to more than 800℉ | |
| Table 5. PBI strengths and limitations | |
5. Polyether Ether Ketone
Polyether ether ketone (PEEK) is most similar to PPS, with the main difference being that PEEK is not brittle. It is a flexible version of PPS with the same temperature range. The concern with PPS is that it might break or fracture. With PEEK, that is not a concern. Therefore, this makes it the plastic of choice in many applications.
However, PEEK is more expensive partly because of this flexibility. It is often chosen for high-pressure applications. For example, PEEK might make a lantern ring for a high-temperature and high-pressure application.
| Strengths | Limitations |
| Excellent tensile strength, elongation, and wear properties | Not suitable for nitric or sulfuric acid |
| Superior chemical resistance | Expensive |
| Withstands long exposure to high-pressure water and steam | Difficult to process |
| Excellent chemical resistance | |
| Very low flammability | |
| Resistant to Gamma radiation | |
| Continuous use up to 500℉ | |
| Table 6. PEEK strengths and limitations | |
High-Performance Industrial Plastic Trends
The mass production of industrial plastics began in the 1950s, and since then, the industry has grown exponentially. Why? According to the Plastics Industry Association, the diversity of plastic applications makes the industry unique and able to expand continually. It has grown to include new materials and markets that the industry’s earliest practitioners may never have expected or predicted.
Plastic technology continues to increase in terms of what plastics can do. They replace metal components in industrial applications because they are versatile, lighter, and stronger. They also do not corrode. For example, lantern rings were once almost always made out of brass. Today, they are most likely made from PTFE or PEEK. Similarly, some materials that were once bronze are plastic now.
While these trends in the sealing industry occur, uses for plastics are expanding. Plastics are taking center stage in the automotive industry, particularly PBI, PPS, and PEEK. These plastics are noncorrosive and lighter than automotive components previously made of metal.
Another area of expansion is 3D printing plastic components for many different applications. Previously, machining an industrial plastic part for a specific purpose was impossible. Now, 3D printing may make it possible.
Once a plastic component is installed, it may have a long life. The technology continues to improve, and plastics will be seen in more applications over time. Plastics are the future of many industrial applications. High-performance industrial plastics for the sealing industry will grow. They may also be used in previously unheard-of ways.
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