Views: 5 Author: Brian Wu Publish Time: 2020-11-03 Origin: Jiangmen Great Fluid Seals Co., Ltd
The polyurethane polymer industry has enormous categories of products for a wide variety of applications. Polyurethane used in the seal industry is a thermoplastic elastomer (TPU). As the name suggests, it behaves like an elastomer but the chemistry is of a thermoplastic. The elasticity of a TPU is brought about through polymer morphology phase changes as in thermoplastics not through vulcanization as seen in other elastomers. Because of its thermoplastic nature, TPU has excellent tensile strength and abrasion resistance that other elastomers are unable to match. Meanwhile, TPUs also have good flexibility and shock absorbing performance. An additional advantage of TPUs is that they can be molded using conventional thermoplastic processes.
Nitrile elastomer NBR is an amorphous random copolymer of butadiene and acrylonitrile. There are numerous NBR copolymers available globally. As a thermoset elastomer, an NBR compound consists of NBR copolymer, carbon black reinforcement fillers, curing agents, molding process aids and specialty additives. NBR articles are molded by injection, transfer, compression or extrusion processes. NBR lends itself to a virtually infinite number of compounded materials and versatile in applications. The essential feature of NBR elastomer is the presence of Nitrile, -C=N, functional group. This polar group is responsible for its significantly increased chemical resistance.
The first commercialization of hydrogenated nitrile elastomer HNBR copolymer was in 1984, almost 50 years after the commercialization of NBR. To obtain HNBR, NBR is hydrogenated during the polymer synthesis process. Hydrogen is selectively added to the unsaturated carbon-carbon double bonds, -C=C-, of butadiene in the NBR polymer to form saturated carbon-carbon single bonds -C-C-. Thus HNBR emphasizes two essential features: nitrile, -C=N, functional groups as in NBR and a hydrogenated backbone. The nitrile polar group is responsible for HNBR's excellent oil and fuel resistance. The hydrogenated backbone is responsible for HNBR's significantly increased high-temperature properties compared to NBR. HNBR has very good ozone and weather resistance thanks to its saturated backbone.
If any hydrogen atoms are replaced by fluorine atoms in an elastomer carbon backbone, this elastomer is typically referred to as a fluoroelastomer. A carbon-fluorine bond C-F is stronger than a carbon-hydrogen bond C-H. The result of replacing a hydrogen with a fluorine atom makes fluoroelastomers the best option for chemical resistance at high temperatures for a long usage life. The compression set resistance of fluoroelastomers is very high, which is valuable for sealing applications. Fluoroelastomers are a general category that includes polymers from various monomer combinations of two or more monomers. Post-curing is common for fluoroelastomers since this will maximize elastomer's tensile strength and compression set resistance. There are three subcategories of fluoroelastomers: FKM, FFKM and FEPM.
Polytetrafluoroethylene (PTFE) has exceedingly strong carbon-fluoride bonds (C-F). PTFE has a simple, linear, flexible and regular molecular structure, which makes it highly crystalline. Commercial PTFE is a high molecular weight polymer. Fluorine atoms form a tight sheath of protection providing PTFE with extreme molecular and physical properties. The sheath prevents PTFE from external influences upon the carbon-carbon backbone. It also results in weak interactions/bindings between polymer chains. These molecular structure properties make PTFE extremely resistant to chemicals or solvents even at very high temperatures and high pressures. PTFE also has very low friction and good anti-stick characteristics. PTFE is tough and flexible even at very low temperatures. However the same molecular structure properties result in mediocre mechanical properties with low stiffness and strength among thermoplastics. PTFE articles cannot be formed with conventional processes for thermoplastics because it does not flow above its crystalline melting point. Parts can be formed by a sintering process under high temperatures.
Ultrahigh Molecular Weight Polyethylene (UHMWPE) has simple and linear carbon-carbon polymer backbone but with molecular weight reaching several millions. This chemical structure makes UHMWPE highly crystalline, thus it offers high tensile strength and dimensional stability even at high pressures. The most outstanding known properties of UHMWPE are wear/abrasion resistance along with chemical resistance to aqueous and hydrocarbon solvents. UHMWPE has a very low coefficient of friction (much lower than nylon and acetal), good toughness and fatigue resistance.
Acetal or Polyoxymethylene (POM) belongs to the polyether family which contains carbon-oxygen-carbon (-C-O-C-) ether linkages in the polymer backbone. Acetal or POM refers to the polyether with only one carbon (methylene) in between ether linkages. To improve its low thermal stability for commercial use, POM has to be chemically modified by one of two means. The first is to modify the ends of polymer chains to yield the corresponding POM homopolymer (POM-H). Its major trade name is Delrin® from Du Pont. The second method is to add 1%-2% ethylene oxide to the polymer chain that results in POM copolymer (POM-C). POM possesses a simple regular backbone, thus it is a highly crystalline polymer. This chemical and morphological structure leads to high mechanical strength, low moisture absorption, high dimensional stability, and good chemical resistance.
FFKM is referred to as perfluoroelastomers, in which ALL hydrogen atoms are replaced by fluorine atoms in the polymer. FFKM has better fluid resistance and base resistance at much higher temperatures than FKM. Raw materials for producing FFKM are very expensive. For this reason, FFKM is considered to be a high cost specialty elastomer. The major FFKM trade names are Kalrez and Technoflon. In order to take advantage of high temperature resistance from FFKM, all other ingredients, especially fillers and cure systems in the formulations have to withstand the temperature at least as much FFKM. The mechanical property loss and thermal expansion of FFKM needs to be carefully considered if seals are used over a wide temperature cycle. FFKM is usually used in stringent applications that other elastomers are unable to match. Typical application environments are hydrocarbon liquids and gases, water and steam, solvents, amines, brake fluids, many acids and alkalis, air or ozone. FFKM is not suited for molten and gaseous metals such as sodium or potassium, nor fluorinated solvents or refrigerants, nor chlorine compounds. FFKM is not suited for steam over 150ºC.
Polyetheretherketone (PEEK) belongs to ketone polymer family. It has a highly conjugated molecular structure with aromatic, ketone and ether linkages. The double ether linkages in PEEK make it more flexible and capable of crystalizing than other members in the ketone polymer family. This chemical structure provides PEEK with exceptional physical and chemical stability at very high temperatures and in aggressive chemical environments. PEEK has much greater mechanical properties and dimensional integrity at high temperatures than other polymers thus it is regarded as the most advanced high performance polymer in demanding applications. Due to the nature of crystallinity of PEEK, its properties can be affected by process temperature controls. Fillers improve PEEK's performance. Glass or carbon fiber can increase the mechanical properties and dimensional stability of PEEK. PTFE, graphite or carbon powder can reduce friction or increase wear life. PEEK articles can be molded by injection or compression process. PEEK is relatively new and it was commercialized only in the late 1970s.