Polyethylene additives, effect

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

When the source of initiation is altered from ionising radiation to UV, analogous additive effects to those previously discussed have been found. For reasonable rates of reaction, sensitisers such as benzoin ethyl ether (B) are required in these UV processes. Thus inclusion of mineral acid or lithium perchlorate in the monomer solution leads to enhancement in the photografting of styrene in methanol to polyethylene or cellulose (Table V). Lithium nitrate is almost as effective as lithium perchlorate as salt additive in these reactions (Table VI), hence the salt additive effect is independent of the anion in this instance. When TMPTA is included with mineral acid in the monomer solution, synergistic effects with the photografting of styrene in methanol to polyethylene are observed (Table VII) consistent with the analogous ionising radiation system. 

When ionising radiation is used as source of initiation for grafting instead of UV (10-14), analogous additive effects to those previously discussed, have been found. Thus Inclusion of sulfuric acid in methanollc solutions of styrene leads to an enhancement in copolymerisation to a polyolefin, such as polyethylene, when Irradiated by cobalt - 60 gamma rays (Table VI). 

When MFAs such as TMPTA are Included In the monomer solution enhancement in grafting to polyethylene Is observed at certain styrene concentrations (Table IX). In the presence of both acid and TMPTA as additives, a synergistic effect In the same grafting reaction Is observed (Table IX), consistent with the UV data, thus these additive effects appear to be a general phenomenon In UV and radiation grafting processes. 

Pablos JL, Abrusci C, Marm I, L6pez-Marfn J, Catalina F, Espf E, Corrales T. Photodegradation of polyethylenes comparative effect of Fe and Ca-stearates as pro-oxidant additives. Polym... 

When cellulose replaced polyethylene as backbone polymer, similar acid and lithium perchlorate additive effects are observed for the grafting of styrene in methanol under both UV and ionizing radiation conditions (Table 2). Cellulose is less reactive than polyethylene in these copolymerization reactions, thus higher UV and radiation doses are required with cellulose to achieve yields comparable to those observed with polyethylene. The fact that both cellulose and polyethylene exhibit these additive effects, as do other backbone polymers, indicates that the phenomenon is of general application in these grafting processes. 

Heat stabilizers protect polymers from the chemical degrading effects of heat or uv irradiation. These additives include a wide variety of chemical substances, ranging from purely organic chemicals to metallic soaps to complex organometaUic compounds. By far the most common polymer requiring the use of heat stabilizers is poly(vinyl chloride) (PVC). However, copolymers of PVC, chlorinated poly(vinyl chloride) (CPVC), poly(vinyhdene chloride) (PVDC), and chlorinated polyethylene (CPE), also benefit from this technology. Without the use of heat stabilizers, PVC could not be the widely used polymer that it is, with worldwide production of nearly 16 million metric tons in 1991 alone (see Vinyl polymers). 

PL can be used as a sensitive probe of oxidative photodegradation in polymers. After exposure to UV irradiation, materials such as polystyrene, polyethylene, polypropylene, and PTFE exhibit PL emission characteristic of oxidation products in these hosts. The effectiveness of stabilizer additives can be monitored by their effect on PL efficiency. 

Further variations in the properties of polyethylenes may be achieved by incorporating additives. These include rubber, antioxidants and glass fibres and their effects will be discussed further in Section 11.1.4. 

Polypropylene differs from polyethylene in its chemical reactivity because of the presence of tertiary carbon atoms occurring alternately on the chain backbone. Of particular significance is the susceptibility of the polymer to oxidation at elevated temperatures. Some estimate of the difference between the two polymers can be obtained from Figure 1J.7, which compares- the rates of oxygen uptake of eaeh polymer at 93°C. Substantial improvements can be made by the inclusion of antioxidants and such additives are used in all commercial compounds. Whereas polyethylene cross-links on oxidation, polypropylene degrades to form lower molecular weight products. Similar effects are noted... 

Since acetal resins are degraded by ultra violet light, additives may be included to improve the resistance of the polymer. Carbon black is effective but as in the case of polyethylene it must be well dispersed in the polymer. The finer the particle size the better the ultra violet stability of the polymer but the poorer the heat stability. About 1.5% is generally recommended. For white compounds and those with pastel colours titanium dioxide is as good in polyacetals as most transparent ultraviolet absorbers, such as the benzophenone derivatives and other materials discussed in Chapter 7. Such ultraviolet absorbers may be used for compounds that are neither black, white nor pastel shade in colour. 

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