Aliphatic polyolefins polypropylene

Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers 11.1.4 Additives for isotactic polypropylene... 

This polymer is typical of the aliphatic polyolefins in its good electrical insulation and chemical resistance. It has a melting point and stiffness intermediate between high-density and low-density polyethylene and a thermal stability intermediate between polyethylene and polypropylene. 

Low surface energy substrates, such as polyethylene or polypropylene, are generally difficult to bond with adhesives. However, cyanoacrylate-based adhesives can be effectively utilized to bond polyolefins with the use of the proper primer/activa-tor on the surface. Primer materials include tertiary aliphatic and aromatic amines, trialkyl ammonium carboxylate salts, tetraalkyl ammonium salts, phosphines, and organometallic compounds, which are initiators for alkyl cyanoacrylate polymerization [33-36]. The primer is applied as a dilute solution to the polyolefin surface, solvent is allowed to evaporate, and the specimens are assembled with a small amount of the adhesive. With the use of primers, adhesive strength can be so strong that substrate failure occurs during the course of the shear tests, as shown in Fig. 11. 

These are the polymers which are based on unseparated aliphatic hydrocarbons containing are double bond in molecule. The important polyolefins are polyethylene, polypropylene, poly (isobutene) and poly (4-methyl 1-pentene). 

The principal polyolefins are low-density polyethylene (ldpe), high-density polyethylene (hope), linear low-density polyethylene (lldpe), polypropylene (PP), polyisobutylene (PIB), poly-1-butene (PB), copolymers of ethylene and propylene (EP), and proprietary copolymers of ethylene and alpha olefins. Since all these polymers are aliphatic hydrocarbons, the amorphous polymers are soluble in aliphatic hydrocarbon solvents with similar solubility parameters. Like other alkanes, they are resistant to attack by most ionic and most polar chemicals their usual reactions are limited to combustion, chemical oxidation, chlorination, nitration, and free-radical reactions. 

The pyrolysis of polypropylene gives similar results to the pyrolysis of polyethylene (Table 17.3). The amount of methane and oil is slightly higher, the amount of aliphatics is lower. The feedstock recycling of polyolefins is easy. Up to 50 wt% can be obtained as aromatics if the pyrolysis gas is cycled and used as fluidizing gas. The other 50% are gas components. Benzene and toluene reach 25 wt%. 

In the class of quenchers, the nickel chelates are the most well known. For additives that are known to stabilize polyolefins, various attempts have been made to demonstrate energy-transfer to this type of molecules in liquid as well as in solid phase. For example, Briggs and McKellar [21] have shown that nickel (Ni(II)) chelates that are effective UV stabilizers for PP, are efficient acceptors for the excitation energy of triplet anthracene too. From this result it was concluded that the Ni chelates act as quencher in PP. However, this conclusion can be argued because anthracene cannot be expected to be an adequate model for the polypropylene carbonyl chromophores, because the excitation energy of triplet anthracene is 42 kcal/mole, whereas the value for an aliphatic ketone is —74kcal/mole [22]. Efficient energy transfer usually... 

Synthetic polymers are an integral part of everyday life. Synthetic polymers in the form of polyolefins such as polypropylene and polyethylene and other synthetic polymers such as poly(acrylate)s and poly(styrene)s (PSs) are predominantly used in packaging materials. Condensation polymers are produced in smaller amounts than addition polymers. Nevertheless, some condensation polymers such as polyesters with specific reference to poly(ethylene terephthalate) (PET) and aliphatic polyethers, aliphatic polyamides (i.e., nylons), and polysiloxanes are quite important. 

For the purposes of this chapter, polyolefins are defined as polymers based on unsaturated aliphatic hydrocarbons containing one double bond per molecule. Polymers derived from unsaturated aromatic hydrocarbons and dienes are considered in later chapters. At the present time the principal commercial polyolefins are polyethylene (polythene), polypropylene, polyisobutene, polybut-l-ene and poly-4-methylpent-l-ene together with related copolymers. These polymers are considered individually in subsequent sections after a brief account of relevant raw materials. 

The solubility of crystalline polymers is rather less than for amorphous polymers. Polyethylene and isotactic polypropylene have much less miscibility in potential solvents than that exhibited by amorphous aliphatic polymers such as polyisobutylene or ethylene-propylene copolymer. This is associated with the necessity of the heat of mixing overcoming the heat of fusion of the crystalline lattice. The aliphatic hydrocarbon solvents, which dissolve amorphous polymers such as polyisobutylene, have small heats of mixing which carmot disrupt the crystalline lattices of polyolefins. The amorphous regions of the semi-crystaUine polyolefins are swollen by aliphatic hydrocarbon solvents and their mechanical properties are lowered, but they do not dissolve. It is only when the temperature approaches the polymer s melting point that dissolution occurs. 

In the case of the polyolefins, random chain scission is initially the dominant process. This is shown typically for polypropylene in Scheme 2. However some low molar mass oxidation products are formed via vicinal hydroperoxides in both PP and PE [20]. The alkoxyl radicals formed by decomposition of the hydroperoxides contain weak carbon-carbon bonds in the a positions to the hydroperoxide groups, which lead to the formation of low molecular weight aldehydes and alcohols that rapidly oxidise further to carboxylic acids. These are biodegradable species, similar to products formed by hydrolysis of aliphatic polyesters and, as in the case of cis-PI, they are rapidly bioassimilated to give cell biomass (see below). 

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