Unvulcanized mbber

Retarders. The purpose of vulcanization retarders is to delay the initial onset of cure in order to guarantee sufficient time to process the unvulcanized mbber. Three main classes of materials are used commercially, including organic acids and anhydrides, cyclohexylthiophthalimide (Santogard PVI or CTP), and a sulfenamide material (Vulkalent E). 

High stmcture blacks ia unvulcanized mbber give higher Mooney viscosities, lower die swell, faster extmsion rates, and better and more rapid dispersion after iacorporation. In vulcanized mbber higher modulus is obtained. High stmcture blacks give lower bulk densities and high vehicle demand ia paint systems. 

Elastomers are often blended with plastics either to improve the impact resistance or to develop new materials having both plastic and elastic behavior. When the elastomer in the blend is dynamically vulcanized, the product is called a thermoplastics vulcanizate (TPV). Blends with unvulcanized mbber phase are usually known as thermoplastic elastomers. TPVs are discussed in another section of this book. This section will deal with recent developments in rubber-plastic blends. 

Incidentally, as is well known, in the unvulcanized state of the filled mbber, the stress upturn does not appear even in the carbon content of = 0.2-0.25. For understanding this phenomenon, we must consider the discontinuity between the SH layers, in addition to the very low modulus of the unvulcanized mbber. Actually, as we discussed earlier, carbon blacks disperse as an aggregate of carbon particles, then the continuity of carbon gels consisting of aggregates must be much poorer than the theoretical calculation based on the perfect dispersion of carbon particles. In this case, the stress cannot be transmitted from carbon gel to carbon gel through such a very soft medium, and as a result, the stress-strain curve of the system is rather similar to the characteristics of the... 

Adhesion of mbber is limited because of its inherent nonpolar nature and the presence of additives in formulation (processing oils, moulding agents, antiozonant waxes, vulcanization aids). Although, unvulcanized mbbers are somewhat less difficult to bond, most of the mbbers used in industry are vulcanized mbbers. To improve their adhesion, a surface treatment is always necessary. 

A large proportion of the circa 3.7 x 106 t/yr of natural mbber used in tires is consumed in tmck tires, off-the-road tires, and aircraft tires, all of which demand a low heat buildup performance. The retreading of tmck tires was also the province of natural mbber until the precured process was developed. Prior to the advent of this technique, the tack of natural mbber was essential for the unvulcanized mbber to adhere onto the buffed carcass, and the thickness of the shoulder region was such that the low heat buildup of natural mbber was essential to ensure that no failure occurred in service. With the precured tread process neither of these attributes were necessary and, synthetic mbber, which was and continues to be used, was found to give particularly good wear performance under low severity conditions, especially in the United States. However, work (56) has shown that natural mbber-based formulations can be developed which give a similar order of wear performance to the all-synthetic mbber tread, but with the additional benefit of lower rolling resistance, and hence better fuel economy. 

X-ray diffraction photographs for unstrained, unvulcanized mbber and a sample stretched up to 650% extension are shown in Figure 14.11 (Treloar, 1970). Comment on those sample features that are responsible for the differences between the two photographs. 

Various properties of the obtained alkyds are displayed in Table 3. Basically, the finished alkyds have three fundamental functional groups, that is hydroxyl, carboxylic and olefinic groups. It was postulated that oidy the hydroxyl and carboxylic groups in alkyds might react with the epoxy groups of ENR at ambient temperature. Hence, it was important to determine the free hydroxyl and carboxylic groups remained in those alkyds prior to the manufacture of unvulcanized mbber composition. Both the free hydroxyl and carboxylic groups could be determined from the hydroxyl number and acid number titrations, respectively. 

These are used in bonding metal or reinforced plastic facings to paper (resin impregnated) honeycomb structures, cork and rubber compositions, cychzed and unvulcanized mbbers, steel to vulcanized rubber, and electrical applications. They are also used as primer for metals to be bonded to wood with phenolics. Polyvinyl butyral-phenolics lack the shear strength and toughness of the polyvinyl formal-phenolic type [8]. 

Unvulcanized Latex and Latex Compounds. A prime consideration has to be the fluid-state stabihty of the raw latex concentrate and hquid compound made from it. For many years, the mechanical stabihty of latex has been the fundamental test of this aspect. In testing, the raw latex mbber content is adjusted to 55% and an 80 g sample placed in the test vessel. The sample is then mechanically stirred at ultrahigh speed (ca 14,000 rpm) by a rotating disk, causing shear and particle cohision. The time taken to cause creation of mbber particle agglomerates is measured, and expressed as the mechanical stabihty time (MST). 

The original SBR process is carried out at. 50° C and is referred to as hot polymerization. It accounts for only about 5% of aU the mbber produced today. The dominant cold polymerization technology today employs more active initiators to effect polymerization at about 5°C. It accounts for about 85% of the products manufactured. Typical emulsion polymerization processes incorporate about 75% butadiene. The initiators are based on persulfate in conjunction with mercaptans (197), or organic hydroperoxide in conjunction with ferrous ion (198). The rest of SBR is produced by anionic solution polymerization. The density of unvulcanized SBR is 0.933 (199). The T ranges from —59" C to —64 C (199). 

Block copolymers with stmctures such as A—B or B—A—B ate not thermoplastic elastomers, because for a continuous network to exist both ends of the elastomer segment must be immobilized in the hard domains. Instead, they are much weaker materials resembling conventional unvulcanized synthetic mbbers (4). 

Dynamic mechanical measurements for elastomers that cover wide ranges of frequency and temperature are rather scarce. Payne and Scott [12] carried out extensive measurements of /a and /x" for unvulcanized natural mbber as a function of test frequency (Figure 1.8). He showed that the experimental relations at different temperatures could be superposed to yield master curves, as shown in Figure 1.9, using the WLF frequency-temperature equivalence, Equation 1.11. The same shift factors, log Ox. were used for both experimental quantities, /x and /x". Successful superposition in both cases confirms that the dependence of the viscoelastic properties of rubber on frequency and temperature arises from changes in the rate of Brownian motion of molecular segments with temperature. 

Probably an important pro-oxidant for all mbbers is ultraviolet (UV) hght. Blake and Bmce [ 17] performed a study of the oxygen absorption rates of unvulcanized NR mbbers under exposure to UV light. It was observed that exposure to light caused dramatic increases in the oxygen absorption... 

Matsushita et al. [127] prepared compositions showing good vulcanizability containing EPDM reclaim. Weather-strip wastes comprising of sulfur-cured EPDM mbber (containing 50% carbon black) were kneaded at 300°C and 3 MPa and extruded to give a rubber (Mw 200,000 gel content 65%), 25 parts of which was mixed with unvulcanized SBR 75, carbon black 37.5, and sulfur 1.3... 

The extrusion process takes unvulcanized rubber and forces it trough a die, which results in long lengths of rubber of a dehnite cross-section. There are two general subdivisions of this technique one extrudes simple products and the other builds products by extruding the mbber onto metal or fabric reinforcement. Products from these techniques include tire tread, cable coating, and rubber hose. 

The writer complains that the brake sleeves [ Manchette ] of the combined Oil and Westing-house brakes was broken in several places. According to an information provided by the company Steyr-Daimler-Puch, successor of Saurer Company, the mentioned brake sleeves were rubber-made cup packings of the vacuum power-steering device which broke frequently. The described pattern was not used to cast the sleeves but to vulcanize them.44 Consequently, Becker would not have been able to produce his own sleeves, since casting air tight, vacuum proof mbber sleeves in patterns behind the Russian front is nearly impossible, but had to order them in an unvulcanized form from the manufacturer in order to vulcanize them in his self-made patterns (if this was possible at all, has not... 

Particulate filled EPDM mbber can be produced via conventional mixing procedures as described in [141]. Pure precipitated silica (Ultrasil VN3) and silica modified with a commerically available organosilane (Si 69), as well as carbon black (N 550) are employed as fillers.7 Unvulcanized samples for AFM observation are prepared by cutting small pieces of the compounds using a scalpel knife [(cryo)... 

Cure-rate mismatch is extreme when the blends constitute high unsaturated diene mbbers like NR, and low unsaturation rubbers like EPDM. It involves the migration of polar curatives from the low unsaturation phase to a more polar high unsaturation phase, further undercuring the low unsaturation phase (11-13,34) and it has been shown that unvulcanized EPDM exists in the vulcanized blend with NR (35). Several approaches have been made to obtain a cocured blend vulcanizate of NR-EPDM without sacrificing the physical properties by (1) increasing the unsaturation of EPDM elastomer so that the cure rate becomes at par with NR or other diene rubbers... 

Chaffer Fabric mA fabric, coated with unvulcanized mb-ber, that is wrapped around the bead section of the tire before vulcanization of the complete tire. The purpose of the chafer fabric is to maintain an abrasion-resistant later of mbber in contact with the wheel on which the tire is mounted. 

---