Polyisobutylene Compressibility

There has been an enormous technological interest in tertfa/j-butanol (tBA) dehydration during the past thirty years, first as a primary route to methyl te/f-butyl ether (MTBE) (1) and more recently for the production of isooctane and polyisobutylene (2). A number of commercializable processes have been developed for isobutylene manufacture (eq 1) in both the USA and Japan (3,4). These processes typically involve either vapor-phase tBA dehydration over a silica-alumina catalyst at 260-370°C, or liquid-phase processing utilizing either homogenous (sulfonic acid), or solid acid catalysis (e.g. acidic cationic resins). More recently, tBA dehydration has been examined using silica-supported heteropoly acids (5), montmorillonite clays (6), titanosilicates (7), as well as the use of compressed liquid water (8). 

The calculations indicate that the internal pressure rises by 1970 atm or from 3940 to 5910 atm as the polyisobutylene passes from the glassy state to the liquid state. Some of the data for Table 7 were taken from the compilations of Hirai and Eyring (1959), and Wunderlich (1960) and from the measurements of Ferry and Parks (1936). The absolute value of ft may be considerably in error because the data for the compressibility had to be extrapolated from room temperature to —76° C. Nevertheless, there is no doubt that the internal pressure must be greater above Tg than below Tg inasmuch as a 50% error in ft makes only a 15 % error in A P as long as A ft is known accurately. 

A few plastics which tend to be naturally brittle require an improvement in both their drop (impact) strength and their top loading (compression) strength. In the case of polystyrene, rubber is widely used as an impact modifier. Rigid PVC, particularly when used as a container, may suffer weakness when subjected to, say, a 3 4 foot drop test. Up to 15% of methyl methacrylate butadiene styrene (MBS) copolymer is usually added to improve impact strength. Chlorinated polyethylene has more recently been introduced as a PVC impact modifier. Vinyl acetate is frequently used as a modifier for PVC film. Polythene, LDPE-HOPE can have resistance to stress (environmental stress cracking), improved by the use either of rubber or polyisobutylene. These modifications have not as yet had any pharmaceutical applications. 

Figure 7.4.4 shows data from the rotating clamp device for the transient equibiaxial viscosity at three different extension rates. For comparison, the linear viscoelastic viscosity and the uniaxial viscosity are shown. Results for the biaxial viscosity compare well to those measured in lubricated compression on the same polyisobutylene sample as in Figure 7.4.4 (Chatraei et al., 1981). So far, only results with the rotating clamp method have been reported for this sample. Maximum strains were 2.5 in the biaxial and multiax-ial tests and k < 0.1 s. Friction on the talcum powder may limit the total strain and the detectable stress values. Much larger, more homogeneous samples are required than were used in the lubricated squeezing experiments. However, because the rotating clamps can...

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