Rubber vinyl content

Rubbers containing traces of vinyl groups can be cross-linked by weaker peroxide catalysts, the reaction involving a vinyl group. It is, however, unlikely that vinyl-to-vinyl linking occurs. Where there is a high vinyl content (4-5% molar) it is possible to vulcanise with sulphur. 

Ba-Li and Ba-Mg-Al, which provide unique steric control of the trans-1,4/cis-l,4 ratio of polybutadiene segments ( 6). Although Alfin rubbers ( 7) contain trans contents at 70-757., their vinyl contents are much greater (20-257.) than is characteristic of the new rubbers reported here. 

The polybutadienes prepared with these barium t-butoxide-hydroxide/BuLi catalysts are sufficiently stereoregular to undergo crystallization, as measured by DTA ( 8). Since these polymers have a low vinyl content (7%), they also have a low gl ass transition temperature. At a trans-1,4 content of 79%, the Tg is -91°C and multiple endothermic transitions occur at 4°, 20°, and 35°C. However, in copolymers of butadiene (equivalent trans content) and styrene (9 wt.7. styrene), the endothermic transitions are decreased to -4° and 25°C. Relative to the polybutadiene, the glass transition temperature for the copolymer is increased to -82°C. The strain induced crystallization behavior for a SBR of similar structure will be discussed after the introduction of the following new and advanced synthetic rubber. 

Polybutadiene is produced by solution polymerisation, and one important feature governing the performance of the resultant polymer is the cis 1,4, and 1, 2 vinyl contents. High cis 1,4 polymers (>90%) have a Tg around -90 °C, and hence exhibit excellent low temperature flexibility only exceeded by the phenyl silicones. They also exhibit excellent resilience and abrasion resistance unfortunately the high resilience gives poor wet grip in tyre treads, and hence this rubber finds limited use as the sole base for such compounds. 

The last property is related to the processing of the rubber in the tire making equipment. By using organo-lithium compound in this case, it was possible to maintain a vinyl content not greater than 18, but to produce a polybutadiene styrene copolymer that has random block styrene and without the use of polar modifiers, which normally will increase the 1,2 content. This copolymer, when compounded in the tread recipe, as shown in the Table XVI, gave properties that are actually equivalent to that of emulsion SBR and in some cases even better. This is particularly true in the properties of the Young modulus index, which showed between -38 to -54 C the Stanley London Skid Resistant, in which the control is 100, shows that 110-115 was obtained. 

Another example of rubber analysis is the determination of vinyl-butadiene in SBR. A PA-FTIR spectrum of a high-vinyl content SBR is shown in Figure 2.10. This determination by mid-IR methods invariably involves the use of the 910 cm"1 band due to the wagging motion of the pendent vinyl group of 1,2-butadiene. 

Styrene is frequently used as part of some terpolymers with large practical utilization. One such copolymer is acrylonitrile-butadiene-styrene terpolymer (ABS). Usually it is made as poly(l-butenylene-graft-l-phenylethylene-co-cyanoethylene). This form of the copolymer can be made by grafting styrene and acrylonitrile directly on to the polybutadiene latex in a batch or continuous emulsion polymerization process. Grafting is achieved by the free-radical copolymerization of styrene and acrylonitrile monomers in the presence of polybutadiene. The degree of grafting is a function of the 1,2-vinyl content of the polybutadiene, monomer concentration, extent of conversion, temperature and mercaptan concentration (used for crosslinking). The emulsion polymerization process involves two steps production of a rubber latex and subsequent polymerization of styrene and acrylonitrile in the presence of the rubber latex to produce an ABS latex. 

Medium-c/5 lithium-polybutadiene was first developed by Firestone Tire and Rubber Company in 1955 [86]. Solution polymerization using anionic catalysts is usually based on butyllithium. Alkyllithium initiation does not have the high stereospecificity of the coordination catalysts based on titanium, cobalt, nickel, or neodymium compounds. Polymerization in aliphatic hydrocarbon solvents such as hexane or cyclohexane yields a polymer of about 40 % cis, 50 % trans structure with 10 % 1,2-addition. However, there is no need for higher cis content because a completely amorphous structure is desired for mbber applications the glass transition temperature is determined by the vinyl content. The vinyl content of the polybutadiene can be increased up to 90 % by addition of small amounts of polar substances such as ethers. 

Ungrafted SAN is formed concurrently with grafted SAN, with the ratio controlled by factors that include temperature, chain-transfer agent, pendent vinyl content of rubber, initiator level, and initiator type (69-77). As previously described, occlusions of SAN can also form within the rubber particles with the mass process leading to significantly higher occlusion levels than the emulsion... 

Regarding ABS terpolymers [29], NMR is capable of determining ungrafted butadiene rubber in solvent extracts of these polymers. No aromatic protons of styrene or acrylonitrile protons are seen in the NMR spectra. The vinyl content of the polybutadiene is about 20%. 

The conclusion based on the above data is the following In the low and medium vinyl polybutadiene range, the tan 6 related to rolling resistance remains constant as vinyl content increases, while Tg of these rubbers related to skid resistance increases. Thus there is a region where traction (Tg) can be increased without changing rolling resistance (tan 6). 

The influence of the vinyl content on the viscoelastic behaviour of polybutadienes is shown in Figure 4. Measurements of tan S, the phase angle between stress and strain under sinusoidal deformation, have been performed by Dynamic Mechanical Spectrometry (Rheometrics). Looking at the shift along the temperature axis due to the different vinyl content, a maximum vinyl content of 72% has been chosen, since beyond this limit the polymer can hardly be regarded as a rubber. 

Fig. 5.2. Relationship between Tg and vinyl content of polybutadienes. Data for two sets of work reported by Bahary et a/., 1%7. (Reprinted with permission from Rubber Chem. Technol, 40, 1531 (1967). Copyright by the American Chemical Society.)...

Some of the recent work in the authors laboratory has shown that catalyst compositions of barium alkoxide salts combined with BuLi as well as with complexes of Mg-Al alkyls also result in low-vinyl solution SBRs (LVSBRs) in hydrocarbon solvent. " Such elastomers have a butadiene portion of ran5-l,4-content as high as 90% and a vinyl content as low as 2%. The comonomer placement in the copolymers is predominantly random. The development and properties of these rubbers will be described more fully in Sections 3 and 4, respectively. 

Random copolymers are commercially produced by both emulsion and solution polymerizations. It was discovered in the author s laboratory that the use of barium alkoxides in combination with organometalhc compounds is yet another method to prepare random styrene/butadiene solution rubbers. With Ba/Mg/Al catalyst systems, random SBRs are obtained with no more than 4% vinyl unsaturation. The principal effect of lower vinyl content is a lower glass transition temperature than that of a corresponding, conventional solution SBR. 

The genered types of solution rubbers which are currently available either commercially or in trial quantities can be described broadly in terms of their styrene and vinyl contents and, consequently, Tg, as given in Table 9. Also shown are the various types of MA s and... 

Retrospective View of Vinyl-BR Properties In the 1950s, the Phillips Petroleum Company and the Firestone Tire and Rubber Company started commercial production of polybutadienes by organolithium polymerization for use in tyres. These solution BRs, having low vinyl contents (8-10%), were used in blends with emulsion SBR in tyre treads for balancing traction and wear performance properties. In the early 1970s when styrene monomer was in short supply, developments from Phillips Petroleum Company and EniChem (formerly the International Synthetic Rubber Company) showed that vinyl-BRs with 50-55% vinyl content behaved like emulsion polymerized SBR in tyre tread formulations and exhibited very similar tread wear and wet skid resistance. Tread compounds containing 45%-vinyl polybutadiene showed lower heat build-up and better blow-out resistance than E-SBR and blends of E-SBR with cw-BR. EniChem introduced trial quantities of a medium-vinyl butadiene rubber (MVBR) under the name Intolene 50 in 1973. 

A major development in solution SBRs was first disclosed at the International Rubber Conference in Kiev by R. A. Livigni et and in subsequent publications. " As described in Section 3 of this chapter, these solution SBRs have a butadiene portion of trans-1,4-content 80-90%, vinyl content 2-4%, and a random comonomer sequence distribution. Additionally, the polymerization process, which uses a new catalyst of Mg-Al alkyls complexed with barium alkoxide salts, has all the distinguishing characteristics of organoUthium-initiated polymerization (Tables 13 and 14). 

These rubbers are generally produced in aliphatic solvents [6, 157-159] in a CSTR or a series of CSTRs. The vinyl content, which is usually about 10% in aliphatic solvents, can be varied by the addition of polar compounds (see Table 7.3) over a wide range. 

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