Elastomers continued

Samples prepared with stirring and poured into test tubes at different times (stopping the stirring) showed the sequence illustrated schematically in Fig. 2. The two layers were distinguishable because of dullness and hardness differences. At a reaction temperature of 80°C, the volume of the upper layer (elastomer continuous) decreases slowly and finally disappears at about 90 min. Samples of both top and bottom layers were studied by transmission electron microscopy techniques, and micrographs for a 10/90 COPE/PSN are shown in Fig. 3. Up to 90 min, samples exhibit elastomer continuous top and plastic continuous bottoms. 

Although the technological basis of all fluorine-containing plastics and most elastomers continues to be the free radical polymerization of fluoroolefins, which themselves are based on the vastly greater fluorocarbon refrigerant industry, important advances have been made in the past two decades These include primarily the production of polymers that are more resistant to degradation by heat, oxidation, bases, and solvents, as well as polymers that are more easily processable, that is, able to be converted mto their final forms for use, whether by thermoplastic or thermoset processes [1,2,3,4]... 

TABLE 4.4 Compilation of ASTM Tests Used for Elastomers (Continued)... 

Finally, although conventional urethane elastomers continue to generate much academic interest, - the number of industrial patents and papers has declined. The field has been covered by a very authoritative review. ... 

Fig. 9 The influence of reinforcing flllers on the properties of an elastomer Continuous line active flllers, dotted line non-active flllers)...

Cured fluoroelastomers can withstand prolonged exposure to high temperatures and retain their elastomeric, mechanical, chemical, and electrical properties better than other elastomers. Continuous service at 450 F is common intermittent exposure at 600 F is possible. Although resistance to compression set is excellent at 400-450 F, low temperature properties (including compression set) are poor, compared to silicones and fluorosilicones. 

A unique process for chemical stabili2ation of a ceUular elastomer upon extmsion has been shown for ethylene—propylene mbber the expanded mbber obtained by extmsion is exposed to high energy radiation to cross-link or vulcani2e the mbber and give dimensional stabUity (9). EPDM is also made continuously through extmsion and a combination of hot air and microwaves or radio frequency waves which both activate the blow and accelerate the cure. 

With a batch process, such as hot isostatic compaction (HIP), heat exchange as used in a continuous reactor is not possible, and it is common practice to provide a furnace within the pressure vessel which is thermally insulated to ensure that the temperature of the vessel does not rise above 300°C. Most HIP operations involve gas pressures in the range 70—200 MPa (10—29,000 psi) and temperatures of 1250—2000°C, occasionally 2250°C (74). The pressure vessel may have a bore diameter from 27 to 1524 mm (75) and is nearly always provided with threaded closures sealed with O-rings made of elastomer provided the temperature is low enough. 

Polypropylene. PP is a versatile polymer, use of which continues to grow rapidly because of its excellent performance characteristics and improvements in its production economics, eg, through new high efficiency catalysts for gas-phase processes. New PP-blend formulations exhibit improved toughness, particularly at low temperatures. PP has been blended mechanically with various elastomers from a time early in its commercialisation to reduce low temperature brittleness. 

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

Polypropylene sheet has been used most extensively however, thermoplastic polyester, polycarbonate, and nylon versions are available (see Elastomers, synthetic Polycarbonates). Continuous strand glass fiber mat is the typical reinforcement. The limited number of sheet suppHers reduces potential for competitive pricing. 

The Hquid monomers are suitable for bulk polymerization processes. The reaction can be conducted in a mold (casting, reaction injection mol ding), continuously on a conveyor (block and panel foam production), or in an extmder (thermoplastic polyurethane elastomers and engineering thermoplastics). Also, spraying of the monomers onto the surface of suitable substrates provides insulation barriers or cross-linked coatings. 

The amine groups thus formed can also react vigorously with the isocyanate groups to continue the chain extension and cross-linking reactions. Hence, ia the systems there are simultaneous foaming, polymerization, and cross-linking reactions, which produce foam elastomers (or plastics). 

Because of the different vulcanization chemistry involved in each commercial ACM, a vulcanization system specific to the cure site present has to be adopted. Many cure systems for labile chlorine containing ACM have been proposed (45). Among these the alkali metal carboxylate—sulfur cure system, or soap—sulfur as it is called in the United States, became the mainstay of acryflc elastomer technology in the early 1960s (46), and continues to be widely used. 

As shown, ethylene—acrylic elastomers will function for greater than 24 months at 121°C, or 6 weeks at 170°C continuous service. Exposures up to 190—200°C can be tolerated, although service life at these temperatures are measured ia days rather than weeks. 

Fig. 2. Tensile strength retention, continuous service, for fluorocarbon elastomers. Compound I (see Table 4).

Certain grades and formulations of the fluorocarbon elastomers are qualified under the code of Eederal Regulations, 21, Eood and Dmgs, Part 177.2600 for use as mbber articles whose intended appHcations requite repeated or continuous contact with food. Elastomer suppHers will provide assistance in formulating for specified uses. 

Epichlorohydrin Elastomers without AGE. Polymerization on a commercial scale is done as either a solution or slurry process at 40—130°C in an aromatic, ahphatic, or ether solvent. Typical solvents are toluene, benzene, heptane, and diethyl ether. Trialkylaluniinum-water and triaLkylaluminum—water—acetylacetone catalysts are employed. A cationic, coordination mechanism is proposed for chain propagation. The product is isolated by steam coagulation. Polymerization is done as a continuous process in which the solvent, catalyst, and monomer are fed to a back-mixed reactor. Pinal product composition of ECH—EO is determined by careful control of the unreacted, or background, monomer in the reactor. In the manufacture of copolymers, the relative reactivity ratios must be considered. The reactivity ratio of EO to ECH has been estimated to be approximately 7 (35—37). 

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