Preformed polymers

Squeezing Flow A disk-shaped 5-cm-diameter and 1-cm-thick molten polymer preform with in 5 x 104Ns /m2, n 0.5 at 200°C is placed between two plates. A 10 kg weight is placed on the upper plate. Calculate the preform thickness and diameter as a function of time. 

This technique is probably one of the simplest ways to form graft copolymers. It consists of carrying out free-radical polymerizations of monomers in the presence of polymers preformed from different monomers. A prerequisite for this synthesis is that the active sites must form on the polymeric backbones during the course of the reactions. Ideally, this occurs if the steps of initiations consist only of attacks by the initiating radicals on the backbones ... 

This technique is probably one of the simplest ways to form graft copolymers. It consists of carrying out firee-radical polymerizations of monomers in the presence of polymers preformed firom different... 

Orientation processes also take place in blow-molding operations. A particularly significant aspect is biaxial stretch blow molding, in which the polymer preform is stretched both in the hoop and axial directions. 

Injection molding is used to obtain objects with a complex shape. The polymer preformed mold is delivered to a mold with the desired final shape under pressure by an injector. An injector is a screw rotating in a heated barrel. The screw is operating axially and acts as a piston. The machine works in cycles. First, the feed is melted and compacted in front of the rotating screw. In the second phase, the melt is injected through a gate into the colded mold, where it solidifies. Then the molded article is ejected, and the cycle is repeated. This machine is used to prepare preforms for PET bottles for mineral water and carbonated drinks. 

Tredgold R H, Young M C J, Hodge P and Khoshdel E 1987 Lightguiding in Langmuir-Blodgett-films of preformed polymers Thin Solid Films 441-9... 

The paste-extmsion process includes the incorporation of ca 16—25 wt % of the lubricant (usually a petroleum fraction) the mixture is roUed to obtain uniform lubricant distribution. This wetted powder is shaped into a preform at low pressure (2.0—7.8 MPa or 19—77 atm) which is pushed through a die mounted in the extmder at ambient temperature. The shear stress exerted on the powder during extmsion confers longitudinal strength to the polymer by fibrillation. The lubricant is evaporated and the extmdate is sintered at ca 380°C. 

Copolymers. There are two forms of copolymers, block and random. A nylon block copolymer can be made by combining two or more homopolymers in the melt, by reaction of a preformed polymer with diacid or diamine monomer by reaction of a complex molecule, eg, a bisoxazolone, with a diamine to produce a wide range of multiple amide sequences along the chain and by reaction of a diisocyanate and a dicarboxybc acid (193). In all routes, the composition of the melt is a function of temperature and more so of time. Two homopolyamides in a moisture-equiUbrated molten state undergo amide interchange where amine ends react with the amide groups. 

Another use is of minor proportions of polyfunctional aHyl esters, eg, diaHyl maleate, ttiaHyl cyanurate, and ttiaHyl isocyanurate, for cross-linking or curing preformed vinyl-type polymers such as polyethylene and vinyl chlotide copolymers. These reactions ate examples of graft copolymerization in which specific added peroxides or high energy radiation achieve optimum cross-linking (see Copolymers). 

Among the preformed polymers cured by minor additions of aHyl ester monomers and catalysts followed by heat or irradiation are PVC cured by diallyl fumarate (82), PVC cured by diallyl sebacate (83), fluoropolymers cured by triaHyl trimeUitate (84), and ABS copolymers cured by triaUyl trimeUitate (85). 

TriaUyl Gyanurate Gure of Preformed Polymers. TAC and TAIC are often used in smaU amounts with vinyl-type and condensation polymers for cured plastics, mbber and adhesive products of high strength, and heat and solvent resistance. In some cases, chemical stabUity is also... 

Fig. 1. (a) Interaction of high energy electromagnetic radiation with a preformed thermoplastic polymer to develop (b) cross-linked network polymer... 

Radiation-induced degradation reactions are in direct opposition to cross-linking or curing processes, in that the average molecular weight of the preformed polymer decreases because of chain scission and without any subsequent... 

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