Aging of natural rubber

P 4. — — The effect of network structure on aging of natural rubber vul-canizates. Ind. Engng. Chem., Product Research and Development 2, (4), 279 (1963). 

Reversion (when defined as the loss of cross-Unks during nonoxidative thermal vulcanizate aging) is a problem associated mainly with natural rubber or synthetic isoprene polymers. It can occur only under severe conditions in butadiene rubber in SBR, instead of the softening associated with the nonoxidative aging of natural rubber, one can observe... 

The oxidation processes which occur during normal aging of natural rubber (Section 18.2.5) involve the up-take of less than 1% by weight of oxygen and are relatively slow. In the presence of catalysts, materials with oxygen contents of about 5—15% can be obtained such products are known as oxidized rubber . 

EFFECTS OF ANTIOXIDANTS ON THE AGEING OF NATURAL RUBBER LATEX THREAD... 

Autoxidation was first investigated in the aging of natural rubber, although it plays a rather general role. Autoxidation starts with the formation of free radicals delivered from the backbone of the polymer. 

Le Beau, D.S., Basic reactions occurring during rubber reclaiming. II. The influence of solvent naphtha, storage of reclaim and aging of scrap prior to reclaiming, and infrared spectra of natural rubber reclaim. Rubber Chem. TechnoL, 22, 560, 1949. 

Al-Malaika, S. and Amir, E.J., Thermoplastic elastomers Part III—Ageing and mechanical properties of natural rubber-reclaimed rubber/polypropylene systems and their role as solid phase dispersants in polypropylene/polyethylene blends, Polym. Degrad. Stab., 26, 31, 1989. 

Ethylene/propylene products reign supreme among the copolymers. They are elastomers. Plastics containing about 20% or more propylene perform like natural rubber and can be cured by peroxide cross-linking. They are faster to chemical and to ageing than other types of natural rubber. 

Types of Latex Compounds. For comparison with dry-rubber compounds, some examples of various latex compounds and the physical properties of their vulcanizates are given in Table 23. Recipes of natural rubber latex compounds, including one without antioxidant, and data on tensile strength and elongation of sheets made from those, both before and after accelerated aging, are also listed. The effects of curing ingredients, accelerator, and antioxidant are also listed. Table 24 also includes similar data for an SBR latex compound. A phenolic antioxidant was used in all cases. 

Butyl rubber is produced by a process in which isobutylene is copolymerized with a small amount of isoprene using aluminum chloride catalyst at temperatures around — 150° F. (20). The isoprene is used to provide some unsaturation, yielding a product that can be vulcanized (43). Vulcanized Butyl rubber is characterized by high tensile strength and excellent flex resistance furthermore, as a result of its low residual unsaturation (only 1 to 2% of that of natural rubber) it has outstanding resistance to oxidative aging and low air permeability. These properties combine to make it an ideal material for automobile inner tubes (3), and Butyl rubber has continued to be preferred over natural rubber for this application, even when the latter has been available in adequate supply. 

Most vinyls are naturally clear, with an unlimited color range for most forms of the materials. They generally have in common excellent strength, abrasion resistance, and self-extinguishable. In their elastomeric form vinyls usually exhibit properties superior to those of natural rubber in their flexural life, resistance to acids, alcohols, sunlight, wear, and aging. 

Properties of GPO. GPO has excellent low-temperature properties, excellent dynamic properties ressembling that of natural rubber, a good ozone resistance and good heat aging resistance [116]. 

Also, antioxidants were found to decrease the aging rate of natural rubber. Among the antioxidants more frequently used are N-phenyl-p-naphthylamine, 2,6-di-tert-butyl-4-methyl-1-hydroxybenzene (BHT), N-isopropyl-N -phenyl-p-phenylenediamine, N-(1,3-dimethylbutyl)-N -p-phenylenediamine, and poly-(2,2,4-trimethyl-1,2-dihydroquinoline). 

Another problem is that some persons are allergic to the proteins often found in natural rubber. This can be avoided by the use of a polyurethane condom. Unfortunately, as now made, these are more likely to tear (7-9% of the time) than the rubber latex condoms (2% of the time).65 It has been suggested that they be made of guayule rubber latex, which is free of the allergens.66 Thermoplastic elastomers, such as polystyrene-6-(ethylene-co-butene)-Apolystyr ene have been used to make gloves that are more resistant to ozone than those made of natural rubber.67 These should resist aging better than those made of natural rubber. It should be possible to mold a condom without holes from a thermoplastic rubber. 

Table 1 UV stability of Natural Rubber Adhesive After Accelerated Aging in a Carbon-Arc Weatherometer (1.0 Mil Thick)... 

Chloroprene rubber (CR) is perhaps the most natural rubberlike of all synthetic plastics or elastomers, particularly with regard to its dynamic response. CRs are a family of elastomers with a property profile that approaches that of natural rubber (NR) but that has better resistance to oils, ozone, oxidation, flame, aging, and heat. CRs are about 25% heavier than NR and do not have the low temperature flexibiUty of NRs. See dynamic elongation polychloroprene rubber/elastomer rabber. 

Parth, M. Aust, N. Lederer, K. Distribution of molar mass and branching index of natural rubber from Hevea brasiliensis trees of different ages by SEC coupled with online viscometry. Macromol. Symp. 

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