Temperature-resistant elastomers

Temperature reaction rate profiles, 70 86 Temperature-resistant elastomers, 9 562-563... 

Trifluoronitrosomethane (b.p. 86.0°C, 767 mm Hg) is of considerable interest as a component of high temperature-resistant elastomers. This compound has been prepared by treatment of trifluoroiodomethane, in the presence of mercury, with nitric oxide in a photochemical reactor whose mercury lamp emitted radiation at 253.7 mp.. The preparation is particularly sensitive to the initial pressure of the gases, reactant ratio, irradiation time, intensity of the ultraviolet radiation, reaction temperature, and method of removal of nitric oxide from the product [60]. 

MAJOR APPLICATIONS Coatings, textiles finishing, paper saturants, leather finishing, and oil-resistant and high-temperature-resistant elastomers. 

The increasingly demanding requirements for applications under the hood extend beyond the physical service limits of conventional oll-resistant elastomers, such as nitrile rubber (NBR) and halogenated rubbers. Therefore, they are replaced by temperature resistant elastomers such as hydrated nitrile rubbers (HNBR), Figure 5.239. In addition to the required temperature resistance, FINBR exhibits the combination required for hose materials such as high strength and strain, constant modulus, dynamic resistance, abrasion resistance and chemical resistance, see Section 5.5 [796],... 

Properties. Polyurethane elastomers generally exhibit good resiHence and low temperature properties, excellent abrasion resistance, moderate solvent resistance, and poor hydrolytic stabiHty and poor high temperature resistance. As castable mbber, polyurethanes enjoy a variety of uses, eg, footwear, toys, soHd tires, and foam mbber. 

Selection of the right elastomer has to take iato account not only the low temperature resistance, the elastic properties, and mechanical properties, but also price, which can vary widely ia specialty elastomers. 

Mihtary interest in the development of fuel and thermal resistant elastomers for low temperature service created a need for fluorinated elastomers. In the early 1950s, the M. W. Kellogg Co. in a joint project with the U.S. Army Quartermaster Corps, and 3M in a joint project with the U.S. Air Force, developed two commercial fluorocarbon elastomers. The copolymers of vinyUdene fluoride, CF2=CH2, and chlorotrifluoroethylene, CF2=CFC1, became available from Kellogg in 1955 under the trademark of Kel-F (1-3) (see Fluorine compounds, ORGANic-POLYcm.OROTRiFLUOROETHYLENE Poly(vinylidene) fluoride). In 1956, 3M introduced a polymer based on poly(l,l-dihydroperfluorobutyl acrylate) trademarked 3M Brand Fluorombber 1F4 (4). The poor balance of acid, steam, and heat resistance of the latter elastomer limited its commercial use. 

Fluorine-containing rubbers were originally developed during the search for fluid-resisting elastomers which could be used over a wide temperature range. Much of the initial developmental work was a result of contracts placed by the US Army and Air Force. Whilst the eurrent commercial materials are very expensive compared with general purpose rubbers they find a number of both military and non-military applications, particularly in the area of seals and 0-rings. 

During World War II, several new synthetic elastomers were produced and new types of adhesives (mainly styrene-butadiene and acrylonitrile copolymers) were manufactured to produce adequate performance in joints produced with new difficult-to-bond substrates. Furthermore, formulations to work under extreme environmental conditions (high temperature, resistance to chemicals, improved resistance to ageing) were obtained using polychloroprene (Neoprene) adhesives. Most of those adhesives need vulcanization to perform properly. 

As detailed in Table 26.1, various gasket elastomers are available which have chemical and temperature resistance coupled with good sealing properties. The temperatures shown are maximum therefore possible simultaneous chemical action must be taken into account. 

Recently polycyclotrimerization of isocyanates has found successful applications in the preparation of high temperature resistant cellular plastics, coatings and elastomers. 

Structural adhesives are formulated from epoxy resins, phenolic resins, acrylic monomers and resins, high temperature-resistant resins (e.g., polyimidcs), and urethanes. Structural adhesive resins arc often modified by elastomers. 

Polysulfide resins combine with epoxy resins to provide adhesives and sealants with excellent flexibility and chemical resistance. These adhesives bond well to many different substrates. Tensile shear strength and elevated-temperature properties are low. However, resistance to peel forces and low temperatures is very good. Epoxy polysulfides have good adhesive properties down to -100°C, and they stay flexible to -65°C. The maximum service temperature is about 50 to 85°C depending on the epoxy concentration in the formulation. Temperature resistance increases with the epoxy content of the system. Resistance to solvents, oil and grease, and exterior weathering and aging is superior to that of most thermoplastic elastomers. 

However, newer adhesives systems having moderate temperature resistance have been developed with improved toughness but without sacrificing other properties. When cured, these structural adhesives have discrete elastomeric particles embedded in the matrix. The most common toughened hybrids using this concept are acrylic and epoxy systems. The elastomer is generally a amine- or carboxyl-terminated acrylonitrile butadiene copolymer (ATBN and CTBN). 

High-strength and temperature-resistant polymers Styrene-butadiene-styrene, triblock polymer, thermoplastic elastomer Crystalline domains with rigid chains between them and cross-finking chains Rigid-chain domains in a flexible-chain matrix... 

The crystalline aggregates, probably the fringed micelle crystallites, act as physical crosslinks and decrystallize at elevated temperatures. RIM elastomers containing a higher content of rigid crystallizing segments show more resistance to thermal decay. 

Ecdel [Eastman], TM for an elastomer which imparts excellent low-temperature resistance that significantly reduces product loss due to flex cracking and shattering during cold weather distribution. 

---