Melting vinyl polymers

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

The inability to process PTFE by conventional thermoplastics techniques has nevertheless led to an extensive search for a melt-processable polymer but with similar chemical, electrical, non-stick and low-friction properties. This has resulted in several useful materials being marketed, including tetrafluoro-ethylene-hexafluoropropylene copolymer, poly(vinylidene fluoride) (Figure 13.1(d)), and, most promisingly, the copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether. Other fluorine-containing plastics include poly(vinyl fluoride) and polymers and copolymers based on CTFE. 

Natta, a consultant for the Montecatini company of Milan, Italy, applied the Zeigler catalysts to other vinyl monomers such as propylene and found that the polymers were of higher density, higher melting, and more linear than those produced by the then classical techniques such as free-radical-initiated polymerization. Ziegler and Natta shared the Nobel Prize in 1963 for their efforts in the production of vinyl polymers using what we know today as solid state stereoregulating catalysts. 

Because of its irregular structure, ar-PP is an amorphous polymer with a softening point lower than that of it-PP. In contrast, because of its regular structure, commercial it-PP is a higher-melting crystalline solid. It is important to note that similar stereochemical concepts apply to other vinyl polymers with pendant groups, such as polyvinyl chloride (PVC) and polystyrene (PS). 

The solubility of the vinyl polymer in the epoxy resins will depend on the molecular weight and grade of vinyl used. Polyvinyl esters can be dissolved in liquid epoxy resins that are heated to about 100°C or in melted solid epoxy resins. However, most epoxy-vinyl blends are normally prepared via solvent solution. 

The 2,2,4- (or 2,4,4)-trimethylhexamethylenediamine has a head and a tail. Formally it can be incorporated into the chain according to principles known from vinyl polymers—e.g., in a head-to-head arrangement to the dicarboxylic acid or head-to-tail arrangement. It is quite probable that our melt condensates have a statistical distribution of structure. The different reactivities of the two ends of the diamine may suggest that certain conditions could be visualized under which identical monomers can arrange to macromolecules of different structures. In addition to the modifications by the head-tail principle, the asymmetric carbon atom creates optical isomers, such as the l and the d form or a mixture of both. 

Substituted polyacetylenes usually show a softening point in the range of ca. 200 to 400 °C (Table 27). It should be noted that these softening points are higher than those of usual vinyl polymers. Poly(l-chloro-2-phenylacetylene) does not melt but only decomposes (dehydrochlorinate) at high temperature. It is known that polyacetylene isomerizes at 145 °C from cis to trans and undergoes an exothermic reaction at 325 °C, but does not melt below 400 °C during its differential thermal analysis 9S). 

Long, V. C., G. C. Berry, and L. M. Hobbs Solution and bulk properties of branched poly(vinyl acetates). IV. Melt viscosity. Polymer 5, 517 (1964). 

Since the plastics are produced from petrochemicals derived from hydrocarbons, the motivation to reuse, recycle, or reprocess for energy recovery is primarily driven by an interest in conservation of petroleum resources. Economic factors are also important, but the potential saving of landfill space is more a perception rather than a reality [9]. Most of the categories of vinylic polymers discussed in this chapter are melt-formable, that is, they are thermoplastic materials, rather than nonmelting or thermosetting as are several of the condensation polymers discussed in Chapters 20 and 21. Thus,... 

Problem 2.33 A certain extruder for plastics was found to work best at a melt viscosity of about 20,000 poises. The vinyl polymer of DP , = 750 and Tg — 80°C usually used with this extruder had this viscosity at 150°C. However, a batch of the same polymer received subsequently has DP, = 500. At what temperature should the extruder now be run so that the viscosity remains at optimum conditions ... 

A vinyl polymer with a Z value of 200 was found to have a melt viscosity of 100 poises. What is the viscosity of this polymer when Z = 800 ... 

According to the chemical structure of the hot-melt adhesive polymers (polyamide resins, saturated polyester, ethylene vinyl acetate copolymers, polyurethanes), the processing temperatures range between 120 and 240 °C. 

All isotactic vinyl polymers can be arranged in four ways in the usual crystals. Polypropylene has as stable conformation a 2 3/1 helix that consists of successive tg(—) or tg rotational isomeric states (left-handed or right-handed helices). In addition, it was possible for the helices to have the —CHj groups point up or down , relative to the crystallographic c-axis. In the ideal, monoclinic crystal form I (P2j /c) all four types of helices occur only at specific, symmetry-related positions The equilibrium melting temperature and entropy are 460.7 K and 15.1 J/(K mol), respectively. 

The second example is the stereoregularity displayed by monosubstituted vinyl polymers of olefins. As we saw earlier, these types of polymers can occur in three forms of tacticity isotactic, syndiotactic, and atactic. Isotactic and syndiotactic polymers possess stereoregular structures. Generally these polymers are rigid, crystallizable, high melting, and relatively insoluble. On the other hand, atactic polymers are soft, low melting, easily soluble, and amorphous. 

Vinyl polymers are particularly susceptible to thermal degradation. A typical example is rigid PVC, which is impossible to process under commercially acceptable conditions without the use of thermal stabilizers. Unstabilized PVC imdeigoes dehydrochlorination near the melt processing temperature. This involves liberation of hydrochloric acid and the formation of conjugated double bonds (polyene formation). The intense coloration of the degradation products is due to polyene formation. A second example of a polymer that undergoes nonchain-scission reaction is poly(viriyl acetate) or PVAc. When heated at elevated temperatures, PVAc can liberate acetic acid, which is followed by polyene formation. 

Vinyl polymers and condensation polymers were studied for the production of synthetic fiber. In 1932, Carothers and Hill of Du Pont studied linear aliphatic polyester and showed that fibers of sufficiently good mechanical properties are obtained by melt-spinning and colddrawing [8]. Polyester fiber was considered unsuitable as a commercial fiber because it has a low melting point and hydrolyzes easily with water. Carothers therefore turned his investigation from polyester to polyamide, and, in 1938, Du Pont announced the success of a new fiber called nylon. In Japan, studies to produce textile fibers from PVA began in 1938 and were intensively promoted. 

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