Unstabilized PVC

On heating PVC, as the glass transition temperature is approached, the tensions within the concentrations of like-poles may be released by atoms being pushed apart to an extent that some break from the polymer backbone, resulting in the initiation of dehydrochlorination of the polymer. Unstabilized PVC is known to start degrading at approximately its glass transition temperature [135]. 

All copolymers were purified carefully from unreacted lead undecylenate by extraction with suitable solvents. Figure 6 shows the results of dehydrochlorination of some samples at 170 °C. As can be seen, the rate of dehydrochlorination is not only distinctly lower compared with that of PVC, but there is, initially, also a certain period during which no hydrogen chloride is evolved. Although the sample, prepared with 6 wt. % lead salt in the monomer mixture, contains only about 0.3 mole % lead salt in the polymer, it exhibits a remarkable thermal stability at 170 °C. almost no dehydrochlorination occurs within the first 2 hours. This is, therefore, a genuine case of internal stabilization, which finds expression also in the color of the polymer. Discoloration of the samples is substantially less than that of unstabilized PVC the polymer containing 1 mole % lead salt is still nearly colorless after 2 hours of heat treatment. 

Thermal Stability. The ABS-type polymers containing sufficient DBPF to pass the Underwriter s test were notably less prone to discoloration during processing than unstabilized PVC. On the other hand, they discolored when subjected to processing conditions normally used for conventional ABS polymers. Tribasic lead silicate was as effective as any of the common PVC stabilizers in preventing this discoloration during milling. 

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. 

Thermal degradation of stabilized and unstabilized PVC/CPE was studied by Stipanelov Vrandecic et al. (Stipanelov Vrandecic et al. 2001). Dynamic TGA was performed for the thermal degradation of PVC/CPE blends between 50 °C and 600 °C at a heating rate 2.5 °C min. It was found that the thermal degradation of the pure unstabiUzed and stabilized polymers, as well as the PVC/CPE blends, occurs in two degradation steps dehydroclorination of PVC or/and CPE and the degradation of the dehydrochlorinated residues. 

Fig. 96. Dependence of the stabilizing activity of polyphenylene A (1), polyphenylene B (2), and polyphenylene C (3) on their concentration in PVC and the temperature, v) Amount of HCl (mg) split out from PVC (1 g) in 180 min in a stream of air c) parts by weight of stabilizers (1-3) per 100 parts by weight PVC 4) values of the quantity V for unstabilized PVC.

Index of additive Additive Ratio of rates of dehydro-chlorination of stabilized and unstabilized PVC in 180 min at 175 C, ... 

Figure 5. Echo -stabilization of PVC. Ebmination of HCl during thermo- (argon) (1,2) and thermo-oxidative (air) (3 - 5) destruction of PVC in solution of dioctyl sebacinate 1-4 unstabilized PVC, 5 - PVC, stabilized with diphenylpropane (0.02 wt. %) - echo stabilization 2,4 - PVC with no solvent 448 K.

Fig.9.4. Schematic illustration of synergism and antagonism of PVC thermal stabilizer. Curve 1 stabilizing effect of component A curve 2 stabilizing effect of component B curve 3 stabilizing effect of component C curve 4 stabilizing effect of synergetic mixture of A + curve 5 stabilizing effect of antagonistic mixture of A + C curve 6 unstabilized PVC.

Rigid PVC has been available commercially for over half a century, but the unstabilized polymer could not be molded or extruded until plasticizers were incorporated into it in the 1930s. The solubility parameters of commercial PVC plasticizers such as dioctylphthalate (DOP) are similar to those of PVC... 

It was observed that these transparent 1.5/1.0-PVC/acrylic copolymer (1.5/ 1.0-2EHA/AN)-graft/blend films were somewhat more resistant to degradation from UV exposure than unstabilized homopolymer PVC (see Table VI). The graft/blend films retained clarity and were noticeably less prone to shrinkage in the Fadeometer where the service life was extended from 500 hrs for the control to 2000 hrs for the graft/blend films. 

Fig. 9. Kinetics of yellowing of unstabilized rigid PVC formulations exposed behind different cut-on filters to filtered xenon source radiation [42]...

Pumps cannot develop pressure without imparting some energy or heat. The melt heat increase depends on melt viscosity and the pressure differential between the inlet and the outlet (or AP). The rise can be S F at low viscosity and low AP, and up to 30°F when both these factors are higher. By lowering the melt heat in the extruder, there is practically no heat increase in the pump when AP is low. The result is a more stable process and a higher output rate. This approach can produce precision profiles with a 50 percent closer tolerance and boost output rates 40 percent. Better control of PVC melt heat could increase the output up to 100 percent. In one case, the output of totally unstabilized, clear PVC meat wrap blown film went from 600 to over 1,000 Ib/h with the use of the gear pump. 

In the first degradation step of the stabilized PVC/CPE blends, there are two maxima in the TGA curves for 90/10 and 80/20 blends, while for other blends those maxima become one. By comparing the DTG curves of the stabilized and unstabilized blends, it can be concluded that the Ca/Zn stabilizer... 

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. 

Fig. 98. Rate of dehydrochlorination of PVC at 175°C in a stream of air in the presence of (1) polyphenyl-acetylene up to the temperature of treatment (2) polyphenylacetylene after exposure for 6 hr at 300°C (3) polyphenylacetylene after exposure for 6 hr at 400°C. Vstab/vunstab is the ratio of the rates of dehydrochlorina-tion of the stabilized and unstabilized polymer c is the concentration of polyphenylacetylene.

Figure 15. Torque vs time for unstabilized and stabilized samples of PVC from bottles...

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