Polyolefin classes

In this section, we are concerned with a polyolefin class of polymers, which includes polyethylene, polypropylene, polybutene, polybutadiene resins, and their various copolymers. 

Polyolefins are manufactured and used in much greater quantity than any other class of plastics. The principal polyolefins are polyethylenes of various densities (LDPE, LLDPE, HDPE) and polypropylene (PP) (see Olefin polymers). 

A classification by chemical type is given ia Table 1. It does not attempt to be either rigorous or complete. Clearly, some materials could appear ia more than one of these classifications, eg, polyethylene waxes [9002-88 ] can be classified ia both synthetic waxes and polyolefins, and fiuorosihcones ia sihcones and fiuoropolymers. The broad classes of release materials available are given ia the chemical class column, the principal types ia the chemical subdivision column, and one or two important selections ia the specific examples column. Many commercial products are difficult to place ia any classification scheme. Some are of proprietary composition and many are mixtures. For example, metallic soaps are often used ia combination with hydrocarbon waxes to produce finely dispersed suspensions. Many products also contain formulating aids such as solvents, emulsifiers, and biocides. 

In the same class of polymers, an antistat can exhibit different degrees of effectiveness. As seen in Table 6 the performance of ethoxylated oleyl amine varies among polyolefins. The data for polypropylene (PP) also shows the concentration dependence of antistats. 

Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers The major additional classes of additive are ... 

This difference in reactivity between the different classes of amines explains the difference in the primer performance on polyolefin substrates with ethyl cyanoacrylate-based adhesives [37J. Since primary and secondary amines form low molecular weight species, a weak boundary layer would form first, instead of high molecular weight polymer. Also, the polymer, which does ultimately form, has a lower molecular weight, which would lower adhesives strength [8,9]. 

The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. Most commercial titanium-based PE catalysts are supported on MgCl2. 

Demand for the different classes of polymer additives varies by resin. Modifiers and processing aids rely heavily on PVC while the property extenders are primarily used in non-PVC resins. PVC is by far the largest consuming resin for polymer additives (excluding fillers), accounting for some 80% of the world-wide volume or 60 % in total value. Polyolefins are a distant second accounting for 8% and 17%, respectively [36]. 

Alkyl derivatives of [6]radialene (4) were the first members of this class of polyolefinic compounds ever to be reported (see below). 

Fig. 7.10. (a) Ionization energies of certain classes of organic molecules as a function of their molecular weight, and (b) relative sensitivities for (O) alkylbenzenes, ( ) polyolefines, and ( ) substituted naphthalenes in chlorobenzene CE-CI. Adapted from Ref [51] by permission. American Chemical Society, 1983. 

Olefins or alkenes are defined as unsaturated aliphatic hydrocarbons. Ethylene and propylene are the main monomers for polyolefin foams, but dienes such as polyisoprene should also be included. The copolymers of ethylene and propylene (PP) will be included, but not polyvinyl chloride (PVC), which is usually treated as a separate polymer class. The majority of these foams have densities <100 kg m, and their microstructure consists of closed, polygonal cells with thin faces (Figure la). The review will not consider structural foam injection mouldings of PP, which have solid skins and cores of density in the range 400 to 700 kg m, and have distinct production methods and properties (456). The microstructure of these foams consists of isolated gas bubbles, often elongated by the flow of thermoplastic. However, elastomeric and microcellular foams of relative density in the range 0.3 to 0.5, which also have isolated spherical bubbles (Figure lb), will be included. The relative density of a foam is defined as the foam density divided by the polymer density. It is the inverse of the expansion ratio . 

Indeed, compared with the chemical behavior of other classes of compounds the reactivity of Cgg is that of a fairly localized and electron-deficient polyolefin. The electrophilicity per se is especially reflected by the ease of electrochemical and chemical reductions as well as by nucleophilic additions (Scheme 14.1). In reactions with nucleophiles, the initially formed intermediates Nu Cgg" can be stabilized by... 

---