Polyvinyl chloride, mechanisms

The properties of a polymer depend not only on its gross chemical composition but also on its molecular weight distribution, copolymer composition distribution, branch length distribution, and so on. The same monomer(s) can be converted to widely differing polymers depending on the polymerization mechanism and reactor type. This is an example of product by process, and no single product is best for all applications. Thus, there are several commercial varieties each of polyethylene, polystyrene, and polyvinyl chloride that are made by distinctly different processes. 

The ductility of GRT-polyethylene blends drastically decreases at ground rubber concentration in excess of 5%. The inclusion of hnely ground nitrile rubber from waste printing rollers into polyvinyl chloride (PVC) caused an increase in the impact properties of the thermoplastic matrix [76]. Addition of rubber powder that is physically modihed by ultrasonic treatment leads to PP-waste ethylene-propylene-diene monomer (EPDM) powder blends with improved morphology and mechanical properties [77]. 

When many molecules combine the macromolecule is termed a polymer. Polymerization can be initiated by ionic or free-radical mechanisms to produce molecules of very high molecular weight. Examples are the formation of PVC (polyvinyl chloride) from vinyl chloride (the monomer), polyethylene from ethylene, or SBR synthetic rubber from styrene and butadiene. 

Liquid membranes consist of an organic phase, which by its hydrophobic nature is relatively impermeable to ions. Originally organic solvents such as decanol were used in conjunction with a porous hydrophobic membrane. These have been replaced by plasticized polyvinyl chloride membranes which behave like liquids yet have improved mechanical properties Other polymers such as silicone, polyurethane and ururshi, a... 

Another reaction mechanism that occurs in some chain-growth polymers is solvolysis. In this type of reaction, a species reacts with a C-X bond, where X represents a halogen, and breaks it. Specifically, this becomes important when describing the degradation of polyvinyl chloride. Acidic species act to remove the chlorine atom, forming hydrochloric acid. 

Figure 22.4 Radical chain mechanism for the degradation of polyvinyl chloride a) initiation, b propagation and c) termination...

Before the mechanism of vinyl polymerization was understood, the question of the structure of vinyl polymers was of considerable interest. Staudinger had written these polymers as having a head-to-tail arrangement of recurring units, but he had not really furnished evidence of the structure. As Carothers once said, Staudinger had assigned the structure by pronouncement. He was as usual correct, and chemical evidence was developed to establish such structures. For example, when monovinyl methyl ketone polymerized, it could produce by head-to-head, tail-to-tail reaction a 1,4-diketone. By head-to-tail polymerization it would give a 1,5-diketone. These two types have different reactions. The study of the polymer proper showed that the polymer was a 1,5-diketone. In the case of polyvinyl chloride, a head-to-head, tail-to-tail polymerization would lead to a 1,2-dihalide compound, and a head-to-tail polymerization would lead to a 1,3-dihalide. 

For many years, it has been known that a small quantity of plasticizer acts as an anti plasticizer for polyvinyl chloride (PVC). During a recent search for effective plasticizers for polycarbonate, W. J. Jackson and J. R. Caldwell found several groups of compounds which acted as antiplasticizers. They increased the tensile modulus and strength and reduced the elongation of polycarbonate films. In contrast to plasticizers, these antiplasticizers affected glass transition temperature quite differently. Their mechanism is explained by the fact that they either increase crystallinity or reduce the mobility of the polymer chain through the bulkiness of their molecules. 

The curves of the raw graft copolymer and of the poly(ethylene-g-vinyl chloride) are rather close to that of the low-density polyethylene. The outstanding fact is the absence of the PVC transition peak (between 60° and 100°C) in the mechanical loss curves of these two products. This means that they contain no rigid PVC phase in spite of the presence of about 25 weight % of ungrafted polyvinyl chloride in the raw graft copolymer. This PVC seems thus to be strongly compatibilized with the other constituents by the poly(ethylene-g-vinyl chloride). 

The mechanical admixture of low molecular weight monomers into polymers normally in the glassy state at room temperature in order to increase the flexibility and softness of the polymer has great technical importance. Thus, such plasticizers as di-2 ethyl n-hexyl phthalate are frequently incorporated into polyvinyl chloride, homopolymer or copolymer, to increase the flexibility and commercial value of this resin. Cast (1953) as well as Hellwege, Knappe and Semjonow (1959) have... 

Woodward and Sauer (1958), have reviewed the many studies of polyvinyl chloride. The only correlation that can be seen between the mechanical and calorimetric data is related to the glass transition which occurs at 92, and 107° C as detected by mechanical damping peaks using 0.67 and 500 cps respectively. These temperatures are to be compared... 

In those days, the mechanisms of carbonization of polyvinyl chloride (PVC) were also under study in our laboratory. We found that PVC transformed to a beautiful lustrous pitch upon heating to 400°C under nitrogen. This PVC pitch could be spun quite easily, by comparison with molten lignin, and thus the pitch-based carbon fiber was first prepared in essentially the same way as the lignin-based carbon fiber. We recognize now that we were fortunate in first using PVC pitch. We later tried many other pitches as raw materials for carbon fiber, but PVC pitch was the only one that could be spun without any pretreatment. This was in 1963. We immediately applied for a patent (2 ), and the fundamentals of pitch preparation and spinning as well as the structure of the finished fibers were published in 1965 (3 ). 

On the other hand, some mechanically compatible blends as well as some dispersed two-phase systems have made respectable inroads into the commercial scene. Many of these are blends of low-impact resins with high-impact elastomeric polymers examples are polystyrene/rubber, poly (styrene-co-acrylonitrile) /rubber, poly (methyl methacrylate) /rubber, poly (ethylene propylene)/propylene rubber, and bis-A polycarbonate/ ABS as well as blends of polyvinyl chloride with ABS or PMMA or chlorinated polyethylene. 

Rubber as the Disperse Phase. In polyblend systems, a rubber is masticated mechanically with a polymer or dissolved in a polymer solution. At the conclusion of blending, a rubber is dispersed in a resin as particles of spherical or irregular shape. We can further subdivide this system into three classes according to the major intermolecular forces governing adhesion (a) by dispersion forces—e.g., the polyblend of two incompatible polymers, (b) by dipole interaction—e.g., the polyblend of polyvinyl chloride and an acrylonitrile rubber (56), and (c) by covalent bond—e.g., an epoxy resin reinforced with an acid-containing elastomer reported by McGarry (43). 

EFFECT OF INDOOR CLIMATE ON THE RATE AND DEGRADATION MECHANISM OF PLASTICIZED POLYVINYL CHLORIDE... 

The paper consists of a series of slides illustrating the mechanisms responsible for degradation in polyvinyl chloride (PVC), and the compositions of heat stabilisers... 

New York City, 2nd-6th May 1999, p.3569-73. 012 MECHANISM OF ORGANOTIN STABILISATION OF POLYVINYL CHLORIDE. IV. PVC STABILISATION BY ALKYLTIN ALKYL MERCAPTOPROPIONATES Fisch M H Bacaloglu R Dooley T Witco Technical Center (SPE)... 

New York City, 2nd-6th May 1999, p.3564-8. 012 MECHANISM OF ORGANOTIN STABILISATION OF POLYVINYL CHLORIDE. 

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