Applications, Homopolymers

Daikin Industries, Ltd. Neoflon CTFE CTFE homopolymer Applications Valve components for cryogenic fluids, piping connectors, gear pump, solenoid valve components. Sheet, rod, tube, small mouldings. ... 

The tightrope situation that arises from balancing high mobility, low crystallinity, and optimum crosslinking is often dealt with by using copolymers rather than homopolymers. With chain composition as an additional variable, molecules can be tailored better for specific application situations. 

In any application of a copolymer the rate of formation of the product, its molecular weight, and the uniformity of its composition during manufacture are also important considerations. While the composition of a copolymer depends only on the relative rates of the various propagation steps, the rate of formation and the molecular weight depend on the initiation and termination rates as well. We shall not discuss these points in any detail, but merely indicate that the situation parallels the presentation of these items for homopolymers as given in Chap. 6. The following can be shown ... 

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

One unfortunate characteristic property of polypropylene is the dominating transition point which occurs at about 0°C with the result that the polymer becomes brittle as this temperature is approached. Even at room temperature the impact strength of some grades leaves something to be desired. Products of improved strength and lower brittle points may be obtained by block copolymerisation of propylene with small amounts (4-15%) of ethylene. Such materials are widely used (known variously as polyallomers or just as propylene copolymers) and are often preferred to the homopolymer in injection moulding and bottle blowing applications. 

The higher transparency coupled with the rigidity expected of a conventional polypropylene homopolymer is of particular interest in thin-wall moulding applications. 

The Phillips process for the manufacture of high-density polyethylene may be adapted to produce copolymers of ethylene with small amounts of propylene or but-l-ene and copolymers of this type have been available since 1958. These soon found application in blown containers and for injection moulding. Properties of two grades of such copolymers are compared with two grades of Phillips-type homopolymer in Table 11.11. 

Vinyl chloride-vinyl acetate copolymers may be processed at lower temperatures than those used for the homopolymer. Their main applications were in gramophone records and flooring. Gramophone record compositions are unfilled... 

Copolymers of vinyl chloride with 2-10% propylene became available in the USA in 1973 (Sta-Flow by Air Products and Chemicals Inc.). Compared with the vinyl chloride homopolymer these copolymers have a reduced tendency to dehydrochlorination and thus improved heat stability. This is of importance both in application and during processing, and one particular claim made for these products is their ease of moulding. 

Of the higher vinyl ester homopolymers only poly(vinyl propionate) is currently believed to be of commercial value, being marketed as Propiofan (BASF) for surface coating application where greater alkali resistance is possible than with the normal vinyl acetate based copolymers. 

As mentioned earlier, unmodified polystyrene first found application where rigidity and low cost were important prerequisites. Other useful properties were the transparency and high refractive index, freedom from taste, odour and toxicity, good electrical insulation characteristics, low water absorption and comparatively easy processability. Carefully designed and well-made articles from polystyrene were often found to be perfectly suitable for the end-use intended. On the other hand the extensive use of the polymers in badly designed and badly made products which broke only too easily caused a reaction away from the homopolymer. This resulted, first of all, in the development of the high-impact polystyrene and today this is more important than the unmodified polymer (60% of Western European market). 

The homopolymers, which are formed from alkyl cyanoacrylate monomers, are inherently brittle. For applications which require a toughened adhesive, rubbers or elastomers can be added to improve toughness, without a substantial loss of adhesion. The rubbers and elastomers which have been used for toughening, include ethylene/acrylate copolymers, acrylonitrile/butadiene/styrene (ABS) copolymers, and methacrylate/butadiene/styrene (MBS) copolymers. In general, the toughening agents are incorporated into the adhesive at 5-20 wt.% of the monomer. 

Copolymerization of monomer mixtures often leads lo materials with properties quite different from those of either corresponding homopolymer, giving the polymer chemist a vast amount of flexibility for devising new materials. Table 31.1 lists some common copolymers and their commercial applications. 

For chromatographic sorbents it is necessary that the polymeric cover be uniformly distributed over the silica surface and chemically coupled to it. The appropriate introduction of the initiator is one of the decisive steps of this task. The first method (binding to the surface) increases the yield of grafted polymer. However in this case a large amount of homopolymer is formed. This disadvantage could be prevented by the application of hydroperoxide initiators in combination with the proper redox-agents [78-81],... 

Recently, unique vesicle-forming (spherical bUayers that offer a hydrophilic reservoir, suitable for incorporation of water-soluble molecules, as well as hydrophobic wall that protects the loaded molecules from the external solution) setf-assembUng peptide-based amphiphilic block copolymers that mimic biological membranes have attracted great interest as polymersomes or functional polymersomes due to their new and promising applications in dmg delivery and artificial cells [ 122]. However, in all the cases the block copolymers formed are chemically dispersed and are often contaminated with homopolymer. 

Conventional polymer and phosphonate scale inhibitors may not be appropriate for application in high-pressure and high-temperature reservoirs. Only a limited range of commercially available oil field scale inhibitor chemicals are sufficiently thermally stable at temperatures above 150° C. These chemicals are homopolymers of vinyl sulfonate and copolymers of acrylic acid... 

---