PVC dehydrochlorination rate

Table 10.10 shows the inflnence of the alcohol part of plasticizer degradation on plasticized PVC dehydrochlorination rate. Here thermal stabihty of films (PVC 100 phr, DOP 60 phr, and alcohol 2 phr) was also measured using kinetic determination of HCl evolution and expressed by thermostability nnmber A. It is evident that alcohols of higher molecular weight (above six carbon atoms) have very little inflnence on PVC thermal degradation rate. Only volatile xylene and isobutanol increase degradation with a rate comparable to the weakest acids. 

Figure 11.64. PVC dehydrochlorination rate vs. plasticizer basicity. [Data from Minsker K S, Inti. J. Polym. Mater., 33, Nos.3-4, 1996, p.189-97.]...

When polymer s concentration in a solution increases, the rate of PVC s dehydrochlorination reaction changes as well, and various character of influence of the solvent on a PVC disintegration rate in solution is observed depending on a numerical value of basieity parameter B/em [5-10]. If the relative basicity of employed solvents was B > 50/em, the polymer s degradation rate decreases when its eoncentration increases. If a basicity of the employed solvents was B < 50/cm, the polymer s degradation rate increases with increased concentration of a polymer. In all cases the rate of HCl elimination from a pol5mier has a trend in a limit to reach values of PVC dehydrochlorination rate in absence of the solvent =5 10- (mol HCL/mol PVC)/s. (Fig. 2). 

As already indicated, the measurement of dehydrochlorination rates is not a practical way of assessing the effect of a stabiliser. Thus the congo red test sometimes specified in standards, in which a piece of congo red paper is held in a test tube above a quantity of heated PVC and the time taken for the paper to turn blue due to the evolution of a certain amount of hydrogen chloride, cannot be considered as being of much value. 

Hjertberg and coworkers [38-41] were able to correlate the amount of labile chlorine, tertiary and internal allylic chlorine, to the dehydrochlorination rate. They studied PVC samples with increased contents of labile chlorine, which were obtained by polymerization at reduced monomer concentration. According to their results, tertiary chlorine was the most important defect in PVC. In agreement with other reports [42,43], the results also indicated that secondary chlorine was unstable at the temperatures in question, i.e., random initiation would also occur. 

Jamieson and McNeill [142] studied the degradation of poIy(vinyI acetate) and poly(vinyI chloride) and compared it with the degradation of PVC/PVAc blend. For the unmixed situation, hydrogen chloride evolution from PVC started at a lower temperature and a faster rate than acetic acid from PVAc. For the blend, acetic acid production began concurrently with dehydrochlorination. But the dehydrochlorination rate maximum occurred earlier than in the previous case indicating that both polymers were destabilized. This is a direct proof of the intermolecular nature of the destabilizing effect of acetate groups on chlorine atoms in PVC. The effects observed by Jamieson and McNeill were explained in terms of acid catalysis. Hydrochloric acid produced in the PVC phase diffused into the PVAc phase to catalyze the loss of acetic acid and vice-versa. 

Figure 6. Dehydrochlorination rate vs. temperature for a crude FVC/EPR and a PVC homopolymer...

Figure 11.12. Dehydrochlorination rate of PVC with and without 10% calcium carbonate. [Adapted, by permission, from Braun D, Kraemer K, Recycling of PVC Mixed Plastic Waste, La Manta F P, Ed., ChemTec Publishing, Toronto, 1996.]...

Figure 10.52 shows that addition of the plasticizer decreased dehydrochlorination rate of PVC. Crosslinking rate was reduced with the presence of plasticizers. Also, the carbonyl group formation was slower in the presence of a plasticizer. Comparison of data presented by the same authors for the effect of radiation above and below 290 nm stresses importance of wavelength selection on the results obtained. When studies were conducted with an unfiltered mercury lamp, presence of phthalates increased the carbonyl group formation. 

At PVC s degradation in solution, one of the basic reasons of change of the process kinetic parameters is the nueleophilic activation of PVC s dehydrochlorination reaction. The process is described by mechanism [1-3]. Thus, there is a linear dependence between PVC s thermal dehydrochlorination rate and parameter of solvent s relative basicity BIcm (Fig. 1) [1-3]. (The value B/cm is evaluated by shift of a characteristic band OH of phenol at A, = 3600/cm in an IR-spectrum at interaction with the solvent [4]). It is essentially important that the rate of PVC s dehydrochlorination in the solvents with relative basicity B > 50/cm was always above, than the rate of PVC s dehydrochlorination without the solvent, while when B < 50/cm, PVC s desintegration rate was always less, than at it s destruction without the solvent. The revealed dependence = (B) is described by the equation ... 

Equation (2) well describes a change of PVC s thermal dehydrochlorination rate of its concentration in a solution in view of parameter of relative basicity of the solvent B, irrespective of the chosen solvent (Fig. 3). 

A detailed elucidation of dehydrochlorination rates of PVC blends with HIPS containing 16 % non-grafted PS, poly(styrene-co-acrylonitrile) (SAN), and acrylonitrile-butadiene-styrene terpolymer (ABS) containing 27 % non-grafted SAN in inert atmosphere at 180 °C revealed accelerated degradaon of the PVC component (Braun et al. 1994). 

The maximum rate of autocatalytic PVC dehydrochlorination is considerably higher than the maximum rate of statistical dehydrochlorination of CPE, and it decreases as the ratio of CPE increases from 0 % to 50 %. In the PVC/CPE 30/70 blend, the maximum dehydrochlorination rate of CPE is higher than that of PVC. In the second degradation step, the dehydrochlorination rate of CPE is the highest, but it is lowered by increasing the ratio of PVC in the blends. 

The conversion value ai(m), at the maximum dehydrochlorination rate of PVC, is 19-20 % for 100/0, 90/0, and 80/20 blends. With an additional increase in the CPE content, ai(m) decreases, and for the 30/70 blend it amounts to 9 %, though it is 25 % for the pure CPE. The maximum dehydrochlorination rates of stabilized PVC/CPE blends are achieved at higher conversion in comparison with unstabilized blends, with the exception of the 0/100 blend. 

The dehydrohalogenation of poly(vinylhalides) is important because of its presumed relationship to thermal stability. Most reports agree that PVC and PVDC dehydro-chlorinate by a Zipper Mechanism but kinetic studies have not followed the implications of that mechanism. The rate of PVC dehydrochlorination has been reported to increase, decrease, and remain constant with time and the catalytic effect of hydrogen chloride has not always been observed. PVDC has been reported to follow first order kinetics, but the acceleratory phase of the dehydrochlorination was not adequately accounted for. 

Derivatives of benzoxazolone may provide useful heat stabilizers for PVC. During the thermal breakdown of PVC, the stabilizing effect of benzoxazolone can be observed in a reduction in an overall concentration of polyenes in the macromolecules, an increase in the induction period of formation of hydrogen chloride, and a reduction in the dehydrochlorination rate of the PVC. 

Figure 10.2 Dehydrochlorination rate versus tertiary chlorine atoms + internal double bonds [72] fractionated commercial suspension PVC, polymerization temperature 55°C [73,74], OPVC polymerized at reduced monomer pressure, polymerization temperature 55 C, monomer pressure between 59-92% of the saturation pressure of vinyl chloride at 55 °C [73,74], bulk and suspension polymerization by lUPAC Sub-Group on Defects in the Molecular Structure of PVC and their relation to thermal stability. [Modified from [72].]...

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