Polyisobutylene

Polyisobutylene (3.15) is much more resistant towards UV degradation that polypropylene (cf. section 3.1.3). The following mechanism, for photodegradation has been proposed, based on data from ESR spectroscopy [393]  

Carbon-carbon bond scission in the main chain, giving two end radicals  

Scission of a carbon-carbon bond in a side group giving a polymer alkyl radical and a methyl radical  

Methyl radicals abstract hydrogen from the polymer molecules producing new polymer alkyl radicals and methane  

Disproportionation reactions are further responsible for chain scission 

Depending on molecular weight, polyisobutylene, PIB, can be liquid or solid. Liquid grades of PIB are frequently used as plasticizers. Solid grades are chemically inert elastomers which have resistance to the oxidative and thermal degradation and a low gas permeability. Because of these properties they find many applications. Some of these applications require plasticizers. 

A low molecular weight PIB is used in many adhesive compositions. It plays role of a plasticizer and/or a tackifier. Adhesive composition based on EPDM rubber, useful in water-tight joints of roofing membranes, contains 5-9 wt% A pressure sensitive 

The formirlations based on EPDM and brominated butyl rubber are plasticized with PIB to obtain water tight joints of rubber membranes. Peroxide curing causes some degradation of brominated butyl rabber which tackifies product 

An antioxidant was chemically attached to a low molecular weight PIB and used in natural mbber. Improved aging and ozone resistance resulted fiom a better retention of antioxidant. 

Solid PIBs are used in plasticized formulations. In the drag containing adhesive of transdermal patch, 10-40 wt% plasticizer (mineral oil, silicone oil, octyl palmitate, etc.) is added to plasticize material and impart hydrophobic properties. Petrolatum or mineral oil was used in adhesive skin barrier which is a pressure sensitive adhesive based on 

The first wide angle scattering measurements by means of synchrotron radiation were performed on polyisobutylene (Oppanol B) by Koch, Bordas, Schola and Broecker The samples were stretched in a fraction of a second to an elongation between 300 and 800 %. The temperature was 30 °C. One observes that — immediately after stretching — the 020 and 113 reflections gradually appear (Fig. 50). The 

Considering again two adjacent carbons in the main chain of the polymer, six conformations are now possible because of the presence of an asymmetrically substituted carbon atom, as shown in Fig. 2.6. Forms 1 and 6 can be neglected for steric reasons so four different conformations are still possible for the polymer. Atactic polypropylene (see Stereoisomerism) has two trans forms (2 and 5) in the fully extended state (IH) and so, unlike polyisobutylene, is incapable of crystallizing upon being stretched. Isotactic polypropylene, however, having all the methyl groups on one side, crystallizes easily. 

Earlier work had indicated that depropagation accompanied by random scission were the principle primary decomposition mechanisms. 

Lehrle and Pattenden [4] were able to estimate rate constants for thermal degradation before and after irradiation. These results implied that irradiation had a stabilising effect on the stability of PIB. 

The molecular mass of the wax-like compounds, which are volatile at the decomposition temperatures and comprise the basic mass (82-97%) of the products of the thermal degradation of polybutadiene is 739. The average yield of monomer on degradation of poly butadiene does not exceed 1.5% mass of the total quantity of volatile compounds, a result which may be associated with both the partial polymerisation of the monomer produced, which is kept at room temperature, and the formation of vinylcyclohexane  

It is possible to associate the small yield of monomer with the fact that most C-C bonds scissions are accompanied by the transfer of hydrogen atoms. The formation of monomer is possible via cleavage of the carbon-carbon bonds located at the P-position relative to the double bond, which are the weakest, without transfer of a hydrogen atom  

The homopolymer finds a variety of uses, as an adhesive component, as a base for chewing gum, in caulking compounds, as a tackifier for greases, in tank linings, as a motor oil additive to provide suitable viscosity characteristics and to improve the environmental stress-cracking resistance of polyethylene. It has been incorporated in quantities of up to 30% in high-density polyethylene to improve the impact strength of heavy duty sacks. 

The main interest in polybut-1 -ene is in its use as a piping material, where the ability to use a lower wall thickness for a given pressure requirement than necessary with other polyolefins, together with the low density, can lead in some cases to economic use. The principal application is for small-bore cold and hot water piping (up to 95°C) for domestic plumbing. Current world-wide sales are of the order of 16-20X10 tonnes per annum. 

Since only a small amount of atactic material is available as a by-product from the manufacture of isotactic polybut-l-ene, atactic polybut-l-ene is normally produced directly. 

Compared with atactic polypropylene it has a lower softening point (less than 100°C compared with 154°C when assessed by ball and ring methods), has better resistance to subzero temperatures and is completely soluble in aliphatic hydrocarbons. The molecular mass of atactic polybut-l-ene is about twice that of an atactic polypropylene of similar melt viscosity. 

270 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers 

Major polymer applications sealants, roofing membranes 

Important processing methods compounding, vulcanization, coating, sheeting 

Typical fillers carbon black, calcium carbonate, kaolin, zinc oxide, clay 

Typical concentration range carbon black - 20-30 wt%, calcium carbonate - 30-50 wt%, zinc oxide 2-3 wt% 

In the second process, the polymerization is carried out in multiple kneaders or mixers. These are arranged in a series of descending steps. Here, the reaction mixture is carried from one kneader to another with the temperature being raised at each station and completed at the last one. 

The differences lie in molecular weights. They range from 2000 to 20,000 for viscous liquids, to between 100,000 and 400,000 for high molecular weight elastomers that resemble unmilled crepe rubber. The polymers degrade readily from thermal abuse. They can be stabilized effectively, however, by adding small quantities (0.1-1.0%) of such stabilizers as aromatic amines, phenols, or sulfur compounds. Polyisobutylenes are soluble in many hydrocarbons and are resistant to attacks by many chemicals. 

Coordinated anionic polymerizations with Ziegler-Natta catalysts yield similar polymers that range from viscous liquids to rubbery solids. At 0 °C, a catalyst with a 1 16 Ti to A1 molar ratio yields a polymer with a molecular weight of 5000-6000. The molecular weight, however, is dependent upon the reaction time. This contrasts with polymerizations of ethylene, propylene, and 1-butene by such catalysts, where the molecular weights of the products are independent of the reaction time. In addition, there are some questions about the exact molecular structures of the products.  

High molecular weight polyisobutylene has fair tensile strength, but suffers from the disadvantage of considerable cold flow. A copolymer of butylene with some isoprene for crosslinking is therefore used as a commercial elastomer and called butyl rubber. The isoprene is present in the copolymer in only minor proportions (1.4-4.5%). The uncrosslinked material is very similar to polyisobutylene. Copolymers of isobutylene with other dienes are also called butyl rubbers. They can also be terpolymers, where the third component may be cyclopentadiene for improved ozone resistance. 

The molecular weights of the copolymers vary inversely with the quantities of isoprene incorporated, the polymerization temperatures and amount of impurities present during polymerization. Impurities like n-butene or water act as chain-transferring agents. To maintain uniform molecular weights, the conversions are usually kept from exceeding 60%. 

More recent catalysts for syndiotactic polypropylene are complexes, like -propyl(cyclopen-tadienyl-l-fluorenyl)haihium dichloride with methyl aluminoxane [70]. Another, similar catalyst is i-propyl(ri -cyclopentadienyl-r -fluorenyl)zirconium dichloride with methyl aluminoxane. These catalysts yield polymers that are high in syndiotactic material (the zirconium-based compound yields 86% of racemic pentads) [70, 71]. Commercial production of syndiotactic polypropylene is in the early stages. What catalytic system is used, however, is not disclosed at this time. Some of the properties of the two isomers, isotactic and syndiotactic polypropylenes, are compared in Table 6.5. 

The molecular weights of syndiotactic polypropylenes can vary from a number average molecular weight of 25,000-60,000, depending upon reaction conditions [70]. Also, in isotactic polypropylene there is less than one double bond per 1,000 carbon atoms [72]. A typical Mw/M = 5-12. 

The original commercial methods for preparing high molecular weight polyisobutylene by cationic polymerization in good yields were reported in 1940. The reaction was carried out at 40 to 80°C in a diluent with BF3 catalysis [72]. This developed into current commercial practices of polymerizing isobutylene at 80 to 100°C, using liquid ethylene or methyl chloride as a diluent [73,74]. Even at these low temperatures the reaction is quite violent. Methods were developed, therefore, to dissipate 

All commercially important polyisobutylenes are linear, head to tail polymers, with tertiary butyl groups at one end of the chains and vinylidene groups at the other  

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Derivatives of polyisobutylene (6. in Figure 9.1) offer the advantage of control over the molecular weight of the polyisobutylene obtained by cationic polymerization of isobutylene. Condensation on maleic anhydride can be done directly either by thermal activation ( ene-synthesis reaction) (2.1), or by chlorinated polyisobutylene intermediates (2.2). The condensation of the PIBSA on polyethylene polyamines leads to succinimides. Note that one can obtain mono- or disuccinimides. The mono-succinimides are used as... 

The polyispbptylenes (PIB) having molecular weights ranging from 1000 to 2000 are substituted by maleic anhydride, and the polyisobutylene succinic anhydride (PIBSA) formed is neutralized by a polyethylene-polyamine as indicated in Figure 9.10. 

Figure 3.4 Uj versus M" for polyisobutylene samples with three different degrees of crosslinking (T and a constant). [Reprinted with permission from P, J. Flory, Ind. Eng. Chem. 38 417 (1946). Copyright 1946, American Chemical Society.]...

Figure 3.9 Log-log plots of modulus versus time for polyisobutylene at 25 C and polystyrene at 135°C. Note the different units of time for the two substances. (From data of A. V. Tobolsky and E. Catsiff and of H. Fujita and K. Ninomiya. From Ref. 4.)...

Inspection of Fig. 3.9 suggests that for polyisobutylene at 25°C, Ti is about lO hr. Use Eq. (3.101) to estimate the viscosity of this polymer, remembering that M = 1.56 X 10. As a check on the value obtained, use the Debye viscosity equation, as modified here, to evaluate M., the threshold for entanglements, if it is known that f = 4.47 X 10 kg sec at this temperature. Both the Debye theory and the Rouse theory assume the absence of entanglements. As a semi-empirical correction, multiply f by (M/M. ) to account for entanglements. Since the Debye equation predicts a first-power dependence of r) on M, inclusion of this factor brings the total dependence of 77 on M to the 3.4 power as observed. 

Figure 8.13 shows the reduced osmotic pressure for solutions of polyisobutylene in benzene plotted against C2 at several different temperatures. The... 

Use the graphical method outlined above to evaluate and and, from these, X for polyisobutylene in diisobutylketone. 

Polybutenes. Polybutenes are produced by controlled polymerization of butenes and isobutene (isobutylene) (see Butylenes). A typical polyisobutylene stmcture is... 

The low molecular weight materials produced by this process are used as lubricants, whereas the high molecular weight materials, the polyisobutylenes, are used as VI improvers and thickeners. Polybutenes that are used as lubricating oils have viscosity indexes of 70—110, fair lubricating properties, and can be manufactured to have excellent dielectric properties. Above their decomposition temperature (ca 288°C) the products decompose completely to gaseous materials. 

Other Uses. Large quantities of hydrocarbon resins are used in mastics, caulks, and sealants (qv). Polymers for these adhesive products include neoprene, butyl mbber, polyisoprene, NR, SBR, polyisobutylene, acryHcs, polyesters, polyamides, amorphous polypropylene, and block copolymers. These adhesives may be solvent or water-borne and usually contain inorganic fillers. 

Synthetic oils have been classified by ASTM into synthetic hydrocarbons, organic esters, others, and blends. Synthetic oils may contain the following compounds diaLkylben2enes, poly(a-olefins) polyisobutylene, cycloaUphatics, dibasic acid esters, polyol esters, phosphate esters, siUcate esters, polyglycols, polyphenyl ethers, siUcones, chlorofluorocarbon polymers, and perfluoroalkyl polyethers. 

Fig. 1. Polymerization of isobutyiene with various initiating ions in conjunction with (C2H )2A1C1 (3). PIB =polyisobutylene.

Viscosity (Viscosity-Index) Improvers. Oils of high viscosity index (VI) can be attained by adding a few percent of ahnear polymer similar to those used for pour-point depressants. The most common are polyisobutylenes, polymethacrylates, and polyalkylstyrenes they are used in the molecular weight range of about 10,000 to 100,000 (18). A convenient measure for the viscosity-increasing efficiency of various polymers is the intrinsic viscosity Tj, as given by the function... 

Lubrication oil additives represent another important market segment for maleic anhydride derivatives. The molecular stmctures of importance are adducts of polyalkenyl succinic anhydrides (see Lubrication and lubricants). These materials act as dispersants and corrosion inhibitors (see Dispersants Corrosion and corrosion control). One particularly important polyalkenyl succinic anhydride molecule in this market is polyisobutylene succinic anhydride (PIBSA) where the polyisobutylene group has a molecular weight of 900 to 1500. Other polyalkenes are also used. Polyalkenyl succinic anhydride is further derivatized with various amines to produce both dispersants and corrosion inhibitors. Another type of dispersant is a polyester produced from a polyalkenyl succinic anhydride and pentaerythritol [115-77-5]. 

The susceptibihty of dialkyl peroxides to acids and bases depends on peroxide stmcture and the type and strength of the acid or base. In dilute aqueous sulfuric acid (<50%) di-Z fZ-butyl peroxide is resistant to reaction whereas in concentrated sulfuric acid this peroxide gradually forms polyisobutylene. In 50 wt % methanolic sulfuric acid, Z fZ-butyl methyl ether is produced in high yield (66). In acidic environments, unsymmetrical acychc alkyl aralkyl peroxides undergo carbon—oxygen fission, forming acychc alkyl hydroperoxides and aralkyl carbonium ions. The latter react with nucleophiles,... 

Fig. 49. Illustration of the time—temperature superposition principle as based on stress—relaxation data for polyisobutylene (299,300). To convert Pa to...

Thermoplastics. There are five elastomeric membranes that are thermoplastic. Two materials, chlorinated polyethylene (CPE) and polyisobutylene (PIB), are relatively obscure. Thermoplastic materials can be either heat-fused or solvent-welded. In contrast to Hypalon and uncured EPDM, this abiHty to fuse the membranes together remains throughout the life of the material. However, cleaning of the membrane surface after exposure to weather is required. Correct cleaning procedures for specific membranes are available from the individual manufacturer. 

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