Elastomers water absorption

However, one of the main points to be considered in the making of a polyurethane elastomer (apart from the reactivities of the isocyanate and curative) is the water absorption. Special precautions must be taken to keep a diol curative (such as butanediol) sufficiently dry for use. Diols have to be kept dry with molecular sieves and dry nitrogen blanketing. 

Electrical properties — dielectric constant (e), representing polarization dissipation factor (tan 8), representing relaxation phenomena dielectric strength (EB), representing breakdown phenomena and resistivity (pv), an inverse of conductivity — are compared with other polymers in Table 5.14.74 The low dielectric loss and high electrical resistivity coupled with low water absorption and retention of these properties in harsh environments are major advantages of fluorosilicone elastomers over other polymeric materials.74... 

Physical properties such as adhesion to metals tear resistance, abrasion resistance, resistance to diffusion of gas as well as resistance to dilute and concentrated acids, aliphatic and aromatic hydrocarbons, ketones, oil and gasoline, water absorption, oxidation, ozone, sunlight, heat aging, low temperature and flame of the common elastomers are documented in the literature.114 Rating of elastomers with respect to resistance to the factors cited above are in terms of outstanding, excellent, very good, good, fair and poor. 

As compared with their low water absorption, silicone elastomers show an extremely high rate of water vapour transmission. The positive aspect of this property permits one to apply sihcone systems over concrete masonry and other substrates upon which other polymeric coatings blister, crack and peel (Cash, 1970). The negative aspect is that silicone rubber cannot be used as a vapour barrier. The permeabihty to liquids is less compared with that to vapours. 

Moisture Absorption. The water vapor permeability of silicone gels, like silicone elastomers, is rather high. However, their moisture absorption is quite low compared to many other polymeric materials. We have measured water absorptions of 0.01 to 0.05% after 100 hours at 65°C and 100% relative humidity. After 16 hours at 15 psi steam while immersed in water, the up-take was found to be only 0.3 to 0.5% again, low compared to other materials. 

The hydroxyl-terminated homo- and copolymers of butadiene have been utilized in the preparation or modification of urethane elastomers and sealants (102). Owing to the presence of unsaturation in these polymers, reinforcement with certain fillers such as carbon black can be achieved. Hydroxyl-terminated polyisobutylenes have also been used for the preparation of NCO-terminated prepolymers (103). Very recently (104). a,u-di-(hydroxy)polyisobutylenes have been prepared by a different route. Polyurethane films prepared from these polyisobutylene glycols have exhibited excellent hydrolytic stability and very low water absorption (105). However, these materials are not yet commercially available. 

To toughen PA, 2-5 wt% of either PO, elastomer, ionomer, acidified or epoxidized copolymer may be added. PA/PO blends of type (2) were developed to improve dimensional stability and to reduce water absorbency of PA. Alloying PA with PO reduces the rate of water migration to and from the blend, but not the inherent water absorption of PA [Utracki and Sammut, 1991, 1992]. The alloying is either a two- or three-step reactive process (1°) acidification of PO, (2°) preparation of a compatibilizer, and (3°) compounding PP, PA, and the compatibilizer. Usually, the reactive blending is carried out in a twin screw extruder [Nishio et al., 1990 Hu and Cartier, 1998], Since it may cause reduction of the blend crystallinity (thus performance), the extend must be optimized. The rigid PA/PP blends usually comprise PA PP =... 

In 1985 General Electric launched two new resins for car exteriors—Noryl GTX, a polyphenylene oxide (PPO)/nylon blend for fenders and other panels, and Lomod polyether ester elastomers for bumper systems. Noryl GTX combines the heat and dimensional stability and the low water absorption of PPO with the chemical resistance of semicrystalline nylons. 

Other elastomers tested for toughening epoxies include polymethylmethacrylate grafted natural rubber (33). Chloroprene rubber and epoxidized vegetable oils. The latter have been found to have better toughness, lower water absorption and a potentially lower price than the unmodified epoxies. 

D 395 Test Method for Rubber Property - Compression Set D 412 Test Method for Vulcanized Rubber, Thermoplastic Rubbers and Thermoplastic Elastomer - Tension D 471 Test Method for Rubber Property - Effect of Liquids D 570 Test Method for Water Absorption of Plastics D 624 Test Method for Tear Strength of Conventional Vulcanized Rubber and Thermoplastic Elastomer D 638 Test method for Tensile Properties of Plastics D 792 Test Method for Specific Gravity (Relative Density) and Density of Plastics by Displacement... 

Effect of Water Absorption on Physical Properties of Vulcanized Elastomers... 

Later, Lake" investigated the kinetics of water (tap water) absorption in elastomers and their effects on properties. He reported that when the absorption... 

Hydroxy terminated polybutadiene (PBU) [2] has also been used for special types of PUs elastomers where products of low water absorption and sensitivity are required. In general the mechanical strength of this class of PUs is lower than that of their polyester and polyether analogues with the advantage of giving PUs of very low glass transition temperatures and hence temperature flexibility. 

WATER ABSORPTION EFFECTS ON POLYURETHANE ELASTOMER CHEMICAL STRUCTURE AND ELECTRICAL PROPERTIES... 

Properties constant with temperature, excellent electrical properties, available from soft gels to strong elastomers, good release properties Brittle, high-temperature stability, electrical properties excellent, low tensile and impact strength Excellent electrical properties and low water absorption, lower strength than other materials... 

Elastomer UV degradation Resistance to water absorption Pollutant Weathering Ozone SO2 NO, H2S... 

VOLUME resistivity The electrical resistance between opposite faces of a 1-cm cube of insulating materials, commonly expressed in ohm-centimeters. The recommended test is ASTMD 256 51T. Also called the specific insulation. vulcanization A chemical reaction in which the physical properties of an elastomer are changed by causing the elastomer to react with sulfur or some other cross-linking agent. WATER absorption The ratio of the weight of water absorbed by a material to the weight of the dry material. 

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