Properties compression set

Compression set and durometer hardness are also important mechanical properties. Compression set is defined as the amount by which an elastomer fails to return to its original thickness after being subjected to a standard compressive load or deflection for a specified time at a specified temperature. A low percent compression set typifies a more compression resistant elastomeric formulation. Compression set of a closure on a sealed vial is a factor in maintaining the sterility and potency of the drug itself. 

ASTM D1229. 2003. Rubber property - Compression set at low temperatures. 

ASTM D395 method B Standard test methods for rubber property Compression set... 

ASTM D 1229-87 Standard Test Method for Rubber Property — Compression Set at Low Temperatures, 3pp (DOD Adopted) (FSC 0320) (MR) (Comm D-11)... 

American Society for Testing Materials (ASTM) Standard Test Methods for Rubber Property—Compression Set, D395-03. See www.ASTM.org for more details. 

Abrasion resistance Electrical properties Compression set resistance Tear resistance Tensile strength Adhesion to metals Adhesion to fabrics Rebound, cold and hot Resistance to heat aging and flame... 

Standard Test Methods for Rubber Property-Compression Set. 

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... 

Compression set D395, Method A, Method B Rubber property—compression set 9.01... 

Workers at the Chinese university of Yangzhou [35] have used microwaves to modify the surface of waste rubber crumb by devulcanising it and then blending it with NR in various proportions. These mixtures were then vulcanised and the mechanical properties, compression set, swelling behaviour and crosslink density investigated. The results obtained were compared with those of blends that had been prepared using crumb that had not been treated with microwaves. 

Copolymers of tetrafluoroethylene with perfluoro-alkylvinyl ethers were the ultimate elastomers for fluid, chemical, thermal and oxidative resistance. Cure chemistry mainly determined tensile properties, compression set resistance and upper use-temp. The chemistry was described for a perfluoroelastomer using a nitrile-substituted perfluoroalkylvinyl ether as cure-site monomer. It cured by trimerisation of the nitriles to triazine functionality and was devoid of any C-H bonds. Physical properties and thermal resistance in air over 300C were excellent. Peroxide-curable perfluoroelastomers, using hydrocarbon crosslinkers, were limited in performance. 3 refs. 

During the vulcanization, the volatile species formed are by-products of the peroxide. Typical cure cycles are 3—8 min at 115—170°C, depending on the choice of peroxide. With most fluorosihcones (as well as other fluoroelastomers), a postcure of 4—24 h at 150—200°C is recommended to maximize long-term aging properties. This post-cure completes reactions of the side groups and results in an increased tensile strength, a higher cross-link density, and much lower compression set. 

Compression Set. The compression set is an important property ki cushioning appHcations. It has been studied for polyurethane foams (185,186), and has been discussed ki reviews (32,60,156). Compression set has been described as flex fatigue and creep as weU. 

Butadiene—Methacrylic Acid Ionomers. Carboxyl groups can readily be introduced into butadiene elastomers by copolymerization, and the effects of partial neutralization have been reported (63—66). The ionized polymers exhibit some degree of fluidity at elevated temperatures, but are not thermoplastic elastomers, and are very deficient in key elastomer properties such as compression set resistance. 

These new synthetic mbbers were accessible from potentially low cost raw materials and generated considerable woddwide interest. For a time, it was hoped that the polysulftde mbbers could substitute for natural mbber in automobile tires. Unfortunately, these original polymers were difficult to process, evolved irritating fumes during compounding, and properties such as compression set, extension, and abrasion characteristics were not suitable for this apphcation. 

Nitrile mbber compounds have good abrasion and water resistance. They can have compression set properties as low as 25% with the selection of a proper cure system. The temperature range for the elastomers is from —30 to 125°C. The compounds are also plasticized using polar ester plasticizers. The main dilemma is the selection of a heat-stable, nonfugitive plasticizer that also gives good low temperature properties. 

Vinyl Acetate—Ethylene Copolymers. In these random copolymers, the ratio of ethylene to vinyl acetate (EVA) is varied from 30—60%. As the vinyl acetate content increases, the oil and heat resistance increases. With higher ethylene content the physical strength, tensile, and tear increases. The polymers are cured with peroxide. The main properties of these elastomers include heat resistance, moderate oil and solvent resistance, low compression set, good weather resistance, high damping, exceUent o2one resistance, and they can be easily colored (see Vinyl polymers, poly(VINYL acetate)). 

Permanent set and low hysteresis properties depend on minimizing the viscous or plastic component of modulus. Because cross-linking increases elasticity, a high state of cure typically provides the best compression set and heat buildup properties. 

The compression set of sihcone mbber is similar to organic types of mbber at low (0—50°C) temperatures, ranging from 5 to 15% (380). Above 50°C, sihcone mbber is superior, but compression set increases with time and temperature. Sihcone mbber is more tear-sensitive than butyl mbber, and the degree of sensitivity is a function of filler size and dispersion, cross-link density, and curing conditions. The electrical properties of sihcone mbber are generally superior to organic mbbers and are retained over a temperature range from —50 to 250°C (51). Typical electrical values for a heat-cured sihcone mbber are shown in Table 9. 

Cured sihcone LIM mbber can be fabricated with physical properties equivalent to heat-cured mbber (385). Shore A hardness can range from 30 to 70, depending on formulations. Typical physical properties include tensile strengths as high as 9.7 MPa (1400 psi), 500—775% elongation at break, and tear strength of >30 N/mm (180 Ib/in.). Compression sets of less than 10% can be achieved if the material is baked after processing. 

A new family of peroxide-cured dipolymers was introduced in 1991. The peroxide cure provides copolymers that cure faster and exhibit good compression set properties without a postcure. The removal of the cure-site has also made the polymer less susceptible to attack from amine-based additives. By varying the methyl acrylate level in the dipolymer, two offerings in this family have been synthesized, VAMAC D and its more oil-resistant... 

Post-Curing. Whenever production techniques or economics permit, it is recommended that compounds based on terpolymer grades be post-cured. Relatively short press cures can be continued with an oven cure in order to develop full physical properties and maximum resistance to compression set. Various combinations of time and temperature may be used, but a cycle of 4 h at 175°C is the most common. The post-cure increases modulus, gready improves compresson set performance, and stabilizes the initial stress/strain properties, as chemically the polymer goes from an amide formation to a more stable imide formation. Peroxide-cured dipolymer compounds need not be post-cured. 

Mechanical properties depend considerably on the stmctural characteristics of the EPM/EPDM and the type and amount of fillers in the compound. A wide range of hardnesses can be obtained with EPM/EPDM vulcanisates. The elastic properties are by far superior to those of many other synthetic mbber vulcanizates, particularly of butyl mbber, but they do not reach the level obtained with NR or SBR vulcanizates. The resistance to compression set is surprisingly good, in particular for EPDM with a high ENB content. 

Compression Set Resistant. One property of fluorocarbon elastomers that makes them uniquely valuable to the sealing industry is their extreme resistance to compression set. Figure 4 plots compression set vs time for compounds prepared especially for compression set resistance (O-ring grades). 

Post-Curing. Post-curing at elevated temperatures develops maximum physical properties (tensile strength and compression-set resistance) in fluorocarbon elastomers. General post-cure conditions are 16 to 24 h at 200 to 260°C. 

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