Mechanical stability of PVC

Figure 3. Mechanical stability of PVC latices. Effect of particle size and emulsifier level. Polymer 45% by weight. Na 0.01 mol/l.

Poljmierisation to high solids content has been described in the patent literature (12). However for emulsion polymers a serious problem is one of mechanical stability during polymerisation. Palmgren (22) has studied the mechanical stability of monodisperse PVC latices as a function of particle size, emulsifier concentration, electrolyte concentration and residual monomer content. The mechanical stability of PVC latices of different particle size containing different surface concentrations of SDS emulsifier is shown in Fig 4. At a given emulsifier coverage on the latex, these data indicate that the latex stability decreases with increasing particle size. Latex... 

Whereas other metal salts, especially lead stearates and srdfates, or mixtures of Groups 2 and 12 carboxylates (Ba—Cd, Ba—Zn, Ca—Zn) ate also used to stabilize PVC, the tin mercaptides are some of the most efficient materials. This increased efficiency is largely owing to the mercaptans. The principal mechanism of stabilization of PVC, in which all types of stabilizers participate, is the adsorption of HCl, which is released by the PVC during degradation. This is important because the acid is a catalyst for the degradation, thus, without neutralization the process is autocatalytic. 

In the stabilization of PVC, the principal mode of action of the various stabilizer systems has been explained in terms of the Frye and Horst mechanism, i.e., substitution of labile chlorines by more stable groups. Evidence for other actions, such as HCl neutralization, addition to polyene sequences, and bimetallic complex formation have also been given. Despite the wide acceptance of the Frye and Horst mechanism, researchers have frequently contended that this could not be the dominant mechanism in the stabilization of PVC. 

Horst mechanism of PVC stabilization. In fact, based on his observations Naqvi [134] has proposed an alternative model for the degradation and stabilization of PVC based on polar interactions within the polymer matrix, which will be discussed in the following section (Section VI). 

In conclusion, it may be said that a lot of literature has been published that favors the Frye and Horst mechanism of stabilization. Most of this is based on studies done on low-molecular weight model compound for al-lylicchlorines in PVC, i.e., 4-chloro-2-hexene. Although the large contribution of these studies toward understanding the mechanism of stabilization of PVC cannot be denied, the extrapolation of these results to the processes involved in the actual stabilization of the polymer should be done with extreme care. The polymer represents a complex mixture of macromolecules, which in the melt is not only physically a very different system compared to the low-molecular weight model compound, but invariably contains, apart from stabilizers, other additives, such as plasticizers, lubricants, processing aids, etc., that further complicate the situation. The criticism of the Frye and Horst mechanism is also based on solid experimental evidence, and hence, the controversy is still very much alive. 

Naqvi [134] has proposed an alternative model to the Frye and Horst mechanism for the degradation and stabilization of PVC. At room temperature, PVC is well below its glass transition temperature (about 81°C). The low thermal stability of the polymer may be due to the presence of undesirable concentrations of like-poles in the more or less frozen matrix with strong dipoles. Such concentrations, randomly distributed in the polymer matrix, may be considered to constitute weak or high energy spots in the polymer, the possible sites of initiation of thermal dehydrochlorination. 

Mechanisms of thermal degradation of PVC, the structure of PVC and the stabilization of PVC have been the subject of many reviews. Those by Starnes,44 Endo45 and Ivan46 are some of the more recent. Defect structures in PVC arise during the propagation and chain transfer steps. As with PMMA, PVC formed by... 

Ever since an ISFET that was chemically modified by a valinomycin-containing PVC membrane was reported [141], there has been general consensus on the advantages of this type of microsensor over conventional ISEs. Some serious problems have also been acknowledged, though e.g. the low mechanical stability of the membranes, the interference of COj in the potentiometric response, the lack of a stable micro-reference electrode and the relatively high drift rate of ISFETs). Attachment of the membrane can... 

Composites, in the usual sense, were proposed to improve the mechanical stability of the electrode. An example is 80 wt% coke powder in PVC [192]. Of greater interest are carbons with nanodispersed boron (via BCI3, 900 °C). The compound B0.17C0.83 has a reversible specific capacity of 437 Ah/kg [193]. An even stronger enhancement of ilTs, up to 500 Ah/kg could be achieved by silicon (via SiCLj/via Me2Cl2Si, 950 °C) for the composition Si0.nC0.g9 [194,195]. Structural aspects are compiled in Fig. 22. The improved stoichiometry is attributed to the electron-acceptor capability of boron or silicon. Doping of petroleum-fuel green coke by phosphorus elevates by 20% [191]. 

This section deals with the stabilization of PVC by organotin stabilizers having different alkyl groups (methyl, ethyl, n-butyl, n-octyl, etc.) and different ligands (carboxylates, mercaptides, etc.), the mechanism of reaction and the evaluation of stability by yellowness index and conductivity. 

It is this mechanism which forms the primary focus of the present study, and we shall be particularly concerned with the effects of unsaturation on the electronic structures, equilibrium geometries, and energetics of the polyenyl cations and the polyenes derived therefrom. Some of the more practical aspects of the degradation and stabilization of PVC will then be discussed in terms of our results. The question of initiation by defect structures will be considered elsewhere, and we shall be mainly concerned in this work with the propagative or "unzipping phase of the mechanism. 

Chaochanchaikul K, Narongrit S. Stabilizations of molecular structures and mechanical properties of PVC and wood/PVC composites by Tinuvin and TiO stabilizers. Polym Eng Sci 2011 51 (7) 1354-1365. 

The low thermal stability of PVC originates from the presence of labile structures and of the autocatalytic deleterious effect of the hydrochloric acid evolved. Thermal stabilizers for PVC consist principally of metal carboxylates and organotin compounds (primary stabilizers), used in combination for preventive and curative functions. As with degradation, uncertainties continue to exist in the exact stabilizing mechanisms of these additives. There is evidence that organotin derivatives stabilize PVC by substituting the labile allylic chorine with a more thermally stable thioether group [Eq. (96)]. 

A thermal stabilizer is used to improve the chemical stability of PVC at processing temperatures. Many highly effective PVC thermal stabilizers are already known [16,23-29], but their action mechanism is not yet completely established [13,30-33]. Frye and Horst [34,35] suggest the following mechanism for an organic soap type heat stabilizer where chlorine atoms in labile positions are replaced by the alkyl carboxylate group as follows ... 

Figure 4 Mechanical stability of 45% w/w PVC latlces as a function of particle size and emulsifier level. Reproduced from 0 Palmgren ACS S3rmposlum Series, Emulsion Pol3nnerisatlon 258-275 by permission of the publishers, American Chemical Society (C).

Decomposition to HCl is a characteristic of the pyrolysis of chloroalkanes, but PVC liberates HCl at a much lower temperature than would be expected from studies of 1,3-chlorosubstituted model compounds. The reasons for the unexpectedly low stability of PVC and mechanism of breakdown of the polymer continue to be subjects of argument. Several useful general views of the subject have appeared recently. PVC has become a high tonnage plastic because of the success of PVC stabilization technology. The mechanism of action of PVC stabilizers is also a controversial field. Stabilization aspects lie outwith the scope of this section and are dealt with elsewhere (see Volume 6, Chapter 19). 

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