Dehydrochlorination measurements

Poly(vinyl halides) - The photocatalysed oxidation of PVC has been undertaken in the presence of titanium dioxide and zinc oxide pigments and the extent of dehydrochlorination measured. Acetic and formic acids were major products along with carbon dioxide. Copper(II) dialkyldithiocarbamate complexes are also sensitisers. Photodegradable PVC has also been developed by grafting with benzophenone chromophores. Plasticised PVC also degrades and discolours on irradiation but this is due mainly to the plasticiser. ... 

The CPE material used in this study is CM 0342 supplied by The Dow Chemical Ccxnpany. This CPE grade has a specific gravity of 1.25, a chlorine content of 42 percent, an average Mooney Viscosity [HLl-i-4 (lOO C) ] of 65 eind a residual crystallinity of less than 2 percent. Molded slabs of the pure and compounded polymer were used. Molding was conducted at 163 C for ten minutes which for most compounds is an optimum cure condition. Samples molded under other conditions will be so noted in the text. For dehydrochlorination measurements the samples were cut into ediout 2 mm cubes. 

Dehydrochlorination measurements of these compounds were conducted at 180 c in oxygen atmosphere (Table IV). The rates of dehydrochlorlnation of the unstabilized crossllnked polymers are similar to those of the uncrosslinked polymer regardless of dose level or whether the Irreidiation was conducted in nitrogen or air. Addition of TMPT to the polymer enhances, but only slightly, the overall dehydrochlorlnation rate from 0.070 for the pure uncrosslinked polymer to 0.14 - 0.16 mg. eq. HCl/g. polymer-hour for the radiation crossllnked polymer containing TMPT. 

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. 

Thermal stabilities of modified PVC samples acet-oxylated to varying degrees (reaction temperature 46°C) were determined [45]. Rate of thermal dehydrochlorination at 1% degradation was taken as a measure of thermal stability. The log of the degradation rate is plotted against the acetate content of the polymer in Fig. 2. 

Measuring Methods. Chlorine content was determined by the oxygen flask method (2) on a polymer purified by precipitation from the solution in cyclohexanone. Thermal stability, as HC1 evolution, was determined according to ASTM method D-793-49, determining the quantity of HC1 evolved by the polymer maintained at 180 °C in a nitrogen atmosphere. From the slope of the straight line for the amount of HC1 evolved with time, the constant K for the dehydrochlorination rate (DHC) is deduced. 

Differential thermal analysis was used as a further measure of thermal stability. The polymer powder was pressed at 200°C in air to form a film. The film sample was heated at 10°C/minute from 25°-500°C in a nitrogen atmosphere. The the temperature at the onset of decomposition— i.e.y dehydrochlorination—as well as the peak endothermic temperature of the polymer which had already been subjected to a temperature of 200°C during film formation, were obtained from the DTA plot (Figures 3 and 4). 

Extremely favorable physical and chemical characteristics made PCBs ideal vehicles for widely diverse industrial uses and formulation (see section-Uses). Soon after their industrial production by Monsanto and others (1929) their wide commercial application in products and processes resulted in extensive environmental contamination throughout the world—in air, soil and water. Virtually no living creature is free of PCB contamination, since it has been incorporated by now into all levels of the food chain from the lowliest unicellular creature to man (ref. 142). Table 8 gives PCB concentrations measured in blood sera of 6l6 residents of urban and rural areas of South Carolina in 1972 (ref. 136, p. 36). The GLC technique used (e-capture with Ni-63, after dehydrochlorination) was of limited sensitivity, hence the poor detection of lower PCB levels. Data correlated with race and residence and appeared to be independent of sex and age. Mean concentrations of PCBs found in maternal and cord blood samples (Tokyo, Japan, Dec. 1973 through Feb. 1974) were 2.8 and 1.1 ppb, respectively, calculated on a whole blood basis (Akiyama et al., 1975). Transfer of PCB from the mother to the fetus appeared to be non-selective (ref. l43f). Table 9 gives PCB concentrations found in seven second- and third-trimester fetuses, calculated on both whole tissue and fat bases. Concentrations reported in 19 first trimester fetuses (5-8 weeks) were less than 2 ppb (ref. 136, p. 37). 

The induction period - the axial section on the time coordinate of the dehydrochlorination curve -is a common criterion of all heat stabilizers. Normally, within this period, the processing takes place. The length of the induction period may be considered simply as a measure of the heat stability of PVC. However, this does not - and this must be emphasized - allow any definitive conclusions to be drawn concerning the initial color, which is of the utmost importance from the practical standpoint. From the shape of the dehydrochlorination curve, especially from its gradient, very important conclusions can be drawn, particularly concerning the interactions of stabilizer conversion products with PVC, and also with each other. 

In Figure 3.3.5, the dehydrochlorination curves for a di-n-octyltin mercaptide stabilizer are shown. It can be seen that the induction period increases almost linearly with the stabilizer concentration. As a first approximation, the length of the induction period may also be considered as a measure of the binding capacity of the stabiizer for hydrogen chloride. Furthermore, the gradient of the curves decreases with increasing concentration of the stabilizer, and this can be seen as an indication that the conversion products have a favorable effect on the heat stability of the PVC. 

Dehydrochlorination of the films at 140 and 160 °C can be determined in aMetrohm 763 PVC thermomat. In this instrument HCl evolved by heating PVC is carried by N2 gas through a fixed amount of water. The conductivity change of water vapor versus time is measured. The conductivity is proportional to the amount of HCl evolved. Figure 3.3.9 shows HCl evolution from films of PVC, and of PVC with... 

Recently, Cataldo reported on NMR spectra of the dehydrochlorination products of chlorinated docosane, which were measured in solution... 

Both p,p -DDT and methoxychlor are rapidly deprotonated by HO in aprotic solvents with subsequent elimination of Cl to form the dehydrochlorination products that is, DDT forms DDE (Scheme 7-6). The same products are formed in their reactions with 02 -. Because the reaction rates that are measured by the rotated ring-disk electrode method are fairly rapid, the primary step must be a direct reaction with 02 - and not with HO [from the reaction of 02 - with trace water in the solvent (2 02 - + H2O - 02 + HOO + HO )]. Hence, the initial reaction with 02 - is deprotonation followed by elimination of Cl to form DDE. 

The appeiratus used for mecisuring the rates of dehydrochlorination of CPE has been described elsewhere [ ]. It allows for the volvimetric measurement of HCl evolved from the polymer with time at a constant temperature. 

Dehydrochlorination of CPE Crosslinked with Peroxides and 2-Mercaptoimidazoline, Measured at 180°C in Oxygen... 

The beneficial effects of these systems were not observed for the NA-22 crosslinked CPE. Without the stabilizers and antioxidants a high maximum rate of dehydrochlorination of 140x10 mg. eq. HCl/g. polymer-hour was measured cund with these stabilizing additives the maximum rat of dehydrochlorination rose to a higher value namely 150x10. It is apparent that this... 

Thermal Stability. Thermal stability was measured by dehydrochlorination. Experimental details have been given earlier (20). Bulk samples (100 mg) were treated at 190 C in pure nitrogen atmosphere and HC1 was measured by conductometry. The rate of dehydrochlorination is expresed as evolved HC1 (in percent of the theoretical amount) per minute. The linear part of the conversion curve between 0.1 and 0.3% was used. 

In all samples the amount of tertiary is considerably higher than that of internal allylic chlorine. This is also valid for ordinary PVC where typical values are 0-0.5 internal allylic and 1-2 tertiary chlorine per 1000 VC. We have, therefore, suggested that the latter structure is the most important labile structure in PVC (8). In a recent paper, Ivan et. al. ( 2.) have instead claimed internal allylic chlorine to be most important. For a commercial sample, the content of this structure was given to 0.1 per 10(J0 VC and the rate constant of the dehydrochlorination to about 10 min They did not measure the amount of tertiary chlorine but suggested the presence of an unidentified labile structure. Its rate constant of degradation should be only somewhat less than that of internal allylic chlorine. Furthermore, the content of the unknown structure was claimed to be about four times higher than the concent of... 

The content of labile chlorine can be calculated as the sum of tertiary and internal allylic chlorine. The relation between this measure and the rate of dehydrochlorination is very good. Extrapolation -to zero content indicates the presence of random dehydrochlorination. The total contribution from this type of initiation is of, the same order as that from internal allylic chlorine. However, tertiary chlorine must be considered as the most important labile structure in PVC. 

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