Heat-aging

The correlation between dry heat aging and field aging results is poor. The tests are carried out to obtain some indication of the effect of heat on the 

Standard test method for rubber deterioration by heat and air pressure 

Accelerated aging or heat resistance tests Heat resistance and accelerated air aging test 

In these tests, the samples are placed in an oven at the set temperature (70°C or 100°C) for the required period of time. A control set of samples is kept in the dark at ambient temperature. The volume of the samples in the oven should not exceed 15% of the oven s working volume. Depending on the expected results, the samples are exposed for 1,3, 7,10, or multiples of 7 days. 

On completion of the aging tests, physical tests such as tensile, modulus at various strains, elongation at break, and hardness tests may be carried out. Modern testing such as DMA tests also can be performed. The results are often plotted as a graph against time for the properties examined. 

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Long-term heat-aging tests on Tefzel 280 are in progress. It is expected that its continuous-use temperature will be above 150°C. 

Foams prepared from phenol—formaldehyde and urea—formaldehyde resins are the only commercial foams that are significantly affected by water (22). Polyurethane foams exhibit a deterioration of properties when subjected to a combination of light, moisture, and heat aging polyester-based foam shows much less hydrolytic stabUity than polyether-based foam (50,199). 

Nickel dialkyldithiocarbamates stabili2e vulcani2ates of epichlorhydrinethylene oxide against heat aging (178). Nickel dibutyldithiocarbamate [56377-13-0] is used as an oxidation inhibitor in synthetic elastomers. Nickel chelates of substituted acetylacetonates are flame retardants for epoxy resins (179). Nickel dicycloalkyldithiophosphinates have been proposed as flame-retardant additives for polystyrene (180—182) (see Flame retardants Heat stabilizers). 

Mechanical properties of plastics can be determined by short, single-point quaUty control tests and longer, generally multipoint or multiple condition procedures that relate to fundamental polymer properties. Single-point tests iaclude tensile, compressive, flexural, shear, and impact properties of plastics creep, heat aging, creep mpture, and environmental stress-crackiag tests usually result ia multipoint curves or tables for comparison of the original response to post-exposure response. 

At strains over 300% the stress occurs mostiy in the amorphous regions up to the point where the sample breaks. AH of the grades exhibit permanent set, and the curves of grades with a Shore Hardness of 55 and higher exhibit a yield point. This means that parts have to be designed for low strains to stay within the area of elastic recovery. Special grades of elastomer are available to provide hydrolysis resistance (194), improved heat aging (195), and improved uv-stabihty (196). 

SiHcone elastomers possess outstanding resistance to heat aging. The Si—O—Si backbone imparts resistance to oxygen, o2one, uv, and to some polar fluids. However, the strength of these elastomers is usually just adequate. They have low abrasion resistance and tear strength (see Silicon compounds, silicones). 

Conventional cure systems use relatively high levels (2.5 + phr) of sulfur combkied with lower levels of accelerator(s). These typically provide high initial physical properties, tensile and tear strengths, and good initial fatigue life, but with a greater tendency to lose these properties after heat aging. 

If natural mbber compounds are subjected to thermal aging plus fatigue, the conventional systems perform no better than EV systems. The compromise obtained by usiag semi-EV systems iavolves the balance between heat aging and flex life. 

Fig. 3. Elastomer properties as a function of monomer composition, butyl acrylate (BA), ethyl acrylate (FA), and methoxyethyl acrylate (MEA). (a), (—) glass-transition temperature (------------) swelling in ASTM No. 3 oil (b) (-) residual elongation at break, %, after heat aging.

Antioxidants (qv) usually provide only a marginal improvement ia heat aging resistance because of the inherent oxidation resistance of the saturated backbone of ACM. The antioxidants most frequently used are nonvolatile amine compounds. 

The best heat resistance is obtained when nickel dibutyldithiocarbamate [13927-77-0] (NBC) is incorporated into the compound. NBC contributes to the heat resistance by causing the elimination of unused sulfonyl chloride groups which are then unavailable for additional cross-linking during heat aging. The presence of large amounts of Htharge probably also result in some ionic cross-link formation. 

The electrical-insulating and dielectric properties of the pure EPM/EPDM are excellent, but in compounds they are also strongly dependent on the proper choice of fillers. The electrical properties of vulcanizates are also good at high temperatures and after heat-aging. Because EPM/EPDM vulcanizates absorb Htde moisture, their good electrical properties suffer minimally when they are submerged in water. 

Heat aging Solvents Good Very good Good Excellent Excellent Excellent Good Fair Excellent... 

Amines Ketone-amines Acetone-diphenylainine and acetone-aniline (I) Yes Main use in rubber compounds for good heat aging. 

Since impurities can affect both the polymerisation reaction and the properties of the finished product (particularly electrical insulation properties and resistance to heat aging) they must be rigorously removed. In particular, carbon monoxide, acetylene, oxygen and moisture must be at a very low level. A number of patents require that the carbon monoxide content be less than 0.02%. 

Polymers with very good heat resistance (both in terms of deformation and of heat aging resistance) but which may be processed by conventional techniques, e.g. polysulphones and poly(phenylene oxides). 

Heat aging resistance does not appear to be as good as with the bis-phenol A epoxide but outdoor weathering is said to be superior. 

Heat aging characteristics may be improved by the addition of a few per cent of ferric oxide and barium zirconate to name but two materials mentioned in the literature. 

There is a frequent dememd for materials suitable for housings for computer, domestic, electrical and automotive equipment. In these applications toughness, excellent surface finish, surface durability, reasonable rigidity, dimensional stability, good weathering behaviour and low water absorption are common requirements. In these applications ABS is frequently considered first but its limited deflection temperature and heat aging resistance may lead to other... 

Fig. 10. Tensile shear strength of ECA homopolymer after heat aging at IITC with and without cyclic sulfate, 9, on steel lapshears.

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