Ahmady A. Yassin

DEGRADATION AND STABILIZATION OF POLY(VINYL CHLORIDE)

Abstract Poly(viny1 chloride) (PVC) has many desirable characteristics that have allowed it to achieve its present status as one of the most important commercial polymers. In spite of its enormous technical and economic importance, PVC still possesses many problems. Its rather low stability to the influence of heat and light results in discoloration, hydrogen chloride loss, and serious corrosion phenomena [1], accompanied by changes in the mechanical properties of the article together with a decrease or an increase in molecular weight as a result of chain sassion or crosslinking of the polymer molecules, respectively [2].

Organic thermal stabilizers for rigid poly(vinyl chloride) I. Barbituric and thiobarbituric acids

Abstract Barbituric acid (BA) and thiobarbituric acid (TBA) have been investigated as thermal stabilizers for rigid poly(vinyl chloride) (PVC) at 180°C, in air. The efficiency of these compounds as thermal stabilizers was evaluated by measuring the length of the induction period ( T s ), during which no detectable amount of hydrogen chloride gas could be observed, and also from the rate of dehydrochlorination as measured by continuous potentiometric titration on one hand, and the extent of discoloration of the degraded polymer on the other. The stabilizing efficiencies were compared with that of dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), barium–cadmium–zinc stearate (Ba–Cd–Zn stearate), n -octyl tin mercaptide ( n -OTM) and dibutyl tin maleate (DBTM). With the exception of DBTM reference stabilizer which shows a comparable stabilizing efficiency with BA and TBA, the investigated organic stabilizers exhibit a greater efficiency than those of all other reference stabilizers. Moreover, they impart better color stability to the degraded samples. A radical mechanism for the stabilizing action is suggested. The stabilizing efficiency is attributed to the replacement of the labile chlorine atoms on the PVC chains by a relatively more stable moiety of the stabilizer. This disrupts the formation of conjugated double bonds, which are responsible for the discoloration.

Organic thermal stabilizers for rigid poly(vinyl chloride) II. Benzal thiobarbituric acid and some of its derivatives

Abstract Benzal thiobarbituric acid (BTBA) and two of its derivatives: para -methoxy benzal thiobarbituric acid ( p -MeO-BTBA) and para -chloro benzal thiobarbituric acid ( p -Cl-BTBA) have been investigated as thermal stabilizers for rigid PVC at 180°C, in air. Their stabilizing efficiency is based on measuring the rate of dehydrochlorination and the extent of discoloration of the degraded polymer. The results reveal the higher stabilizing efficiency of the investigated materials as thermal stabilizers for rigid PVC as compared with the industrially used thermal stabilizers: dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), n -octyl tin mercaptide ( n -OTM), dibulyl tin maleate (DBTM) and barium–cadmium–zinc stearate (Ba–Cd–Zn stearate). This is well illustrated by the longer induction period ( T s ) values and by the lower rate of dehydrochlorination. The stabilization efficiency is affected by the nature of the substituent found in the phenyl ring of the BTBA derivatives. Moreover, it was found that these materials lower the extent of discoloration of the polymer during degradation as compared with the reference stabilizers. A radical mechanism for the stabilizing effect of BTBA and its derivatives is proposed. The effect of blending p -Cl-BTBA with either of the reference stabilizers on the stabilizing efficiency of the thermally degraded rigid PVC has been also investigated. The results reveal that mixing of the stabilizers improves both the T s values, the rate of dehydrochloration and the extent of discoloration. This improvement attains its maximum when both the investigated and the reference stabilizers are taken in equivalent weight ratio.

Cyanoguanidine and its complexes as thermal stabilizers for rigid poly(vinyl chloride)

Abstract Cyanoguanidine and its complexes with cadmium and zinc have been investigated as thermal stabilizers for rigid PVC, at 180°C, in air. The results reveal the higher stabilizing efficiency of the investigated materials as shown by the longer induction periods obtained in their presence relative to those using dibutyl tin maleate, cadmium-barium stearate and basic lead carbonate, which are commonly used industrial stabilizers. The stabilizing efficiencies of the complex derivatives are higher than those of the parent cyanoguanidine. A synergistic effect is observed when cyanoguanidine is blended in various molar ratios with either of the soap stabilizers. The synergistic effect is probably due to the ability of cyanoguanidine for in-situ complexation with the metal chlorides which result as stabilization by-products, thus protecting the polymer from their deleterious accelerating degradative effect. An ionic mechanism for the stabilizing effect of cyanoguanidine and its derivatives is proposed.

N-substituted maleimides as thermal stabilizers for plasticized poly(vinyl chloride)

Abstract N -Substituted maleimides have been investigated as thermal stabilizers for plasticized PVC at 180°C in air by measuring the rate of dehydrochlorination and the extent of discoloration of the degraded polymer. The results reveal a higher stabilizing efficiency of the products investigated compared with basic lead carbonate, dibutyl tin maleate, and barium-cadmium stearate stabilizers which are commonly used industrially. The induction period (Ts) is influenced by the nature and position of substituents in the aryl ring together with the nature of the plasticizer used. Moreover, it has been found that the stabilizers investigated impart better color stability to the degraded samples compared with the stearate stabilizer. A synergistic effect was achieved when the products investigated were used as mixtures with the reference stabilizer in equimolar ratios. A radical mechanism is proposed which illustrates the stabilizing action of the products investigated.

Organic thermal stabilizers for rigid poly(vinyl chloride). III. Crotonal and cinnamal thiobarbituric acids

Abstract Crotonal thiobarbituric acid (CTBA) and cinnamal thiobarbituric acid (CiTBA) have been investigated as thermal stabilizers for rigid poly (vinyl chloride) (PVC) at 180°C in air. Their stabilizing efficiency is evaluated by measuring the length of the induction period ( T s ), the period during which no detectable amounts of hydrogen chloride gas could be observed, and also from the rate of dehydrochlorination as measured by continuous potentiometric titration on one hand and the extent of discoloration of the degraded polymer on the other. The results reveal the higher stabilizing efficiency of the investigated materials as compared with industrially used thermal stabilizers such as dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), n -octyl tin mercaptide ( n -OTM), dibutyl tin maleate (DBTM) and barium–cadmium–zinc stearate (Ba–Cd–Zn stearate). This is well illustrated by the longer T s values, and by the lower rates both of dehydrochlorination and discoloration of the polymer during degradation relative to those of the reference stabilizers. A radical mechanism for the stabilizing effect of the investigated materials is suggested. The results indicate the important role played by the extent of conjugation in thiobarbituric acid derivatives on the stabilizing efficiency of the stabilizers. The stabilizing potency of the stabilizer increases with the extent of conjugation. The effect of blending CiTBA with either of the reference stabilizers on the stabilizing efficiency of the thermally degraded rigid PVC has been also investigated. The results reveal that mixing of the stabilizers improve both the T s values, the rate of dehydrochlorination and the extent of discoloration. This improvement attains its maximum when both the investigated and the reference stabilizers are taken in equivalent weight ratios.

Organic thermal stabilizers for rigid poly(vinyl chloride) IV. N-Arylphthalimides

N-Arylphthalimide derivatives have been investigated as thermal stabilizers for rigid poly(vinyl chloride) (PVC). Their stabilizing efficiencies compared with some conventional thermal stabilizers have been measured by the continuous potentiometric determination of the evolved hydrogen chloride gas from the degradation, and by the extent of discoloration of the degraded samples. The results reveal the greater stabilizing efficiencies of most of the investigated derivatives as compared with reference stabilizers. A mechanism for the stabilizing action of the investigated stabilizers is proposed.

Organic thermal stabilizers for rigid poly(vinyl chloride) V. Benzimidazolylacetonitrile and some of its derivatives

Abstract 2-Benzimidazolylacetonitrile (BAN) and some of its conjugated unsaturated derivatives namely: 2-benzimidazolylbenzylidineacetonitrile (BBeAN), 2-benzimidazolyl-ω-phenylpropenylidineacetonitrile (BPAN) and 2-benzimidazolylbut-2-enylidineacetonitrile (BBuAN) have been investigated as organic thermal stabilizers for rigid PVC at 180 °C in air. Their stabilizing efficiencies are based on measuring the length of the induction period ( T s ), the period during which no detectable amounts of HCl gas could be observed, and also from the rate of dehydrochlorination as measured by the continuous potentiometric titration on the one hand, and the extent of discoloration of the degraded polymer samples on the other. The stabilizing efficiencies of the investigated stabilizers were compared with that of dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), barium–cadmium–zinc stearate (Ba–Cd–Zn stearate) and dibutyltin maleate (DBTM) which are commonly used industrial stabilizers. The results reveal the higher stabilizing efficiency of the investigated materials relative to the reference stabilizers concerning the T s values. However, the rate of dehydrochlorination at the later stages of degradation for PVC stabilized with the investigated stabilizers is found to be relatively higher than that in the presence of reference stabilizers. These experimental findings indicate that almost all the stabilizers actions are exhausted during the induction period. The stabilizing efficiencies of the investigated stabilizers can be attributed to the replacement of the labile chlorine atoms on the polymer chains by a relatively more thermally stable groups derived from the stabilizer. This substitution reaction proceeds most probably through a radical mechanism. Moreover, the results also reveal the effect of the extent of conjugated unsaturates in the organic stabilizer molecule. The stabilizing potency of the investigated materials increases with the extent of their conjugated unsaturation sites. On the other hand, the results also reveal that while the extent of discoloration of thermally degraded PVC in the presence of BPAN stabilizer is comparable with most of the investigated reference stabilizers, the other derivatives showed a relatively higher extent of discoloration.

Glucoside derivatives as novel photostabilizers for rigid PVC

Glucoside derivatives of some UV absorbers have been prepared and investigated as photostabilizers for PVC in absence and in presence of Ca-Zn stearate. The results indicate a reasonable stabilizing effect of the glucosides compared with the parent UV absorbers. The prepared glucosides were also investigated as thermal stabilizers for PVC and exhibited good thermal stability compared with that of commercial Ca-Zn stearate heat stabilizer.

Polymerization products of p-benzoquinone as thermal stabilizers for rigid poly(vinyl chloride). Part II—Evaluation of the stabilizing efficiency

Abstract Quinone-tin polymers prepared by the cationic polymerization of p -benzoquinone with tin(II)chloride in the absence of solvent have been investigated as thermal stabilizers for rigid PVC at 180°C in air by measuring the rate of dehydrochlorination. The results reveal the higher stabilizing efficiency of these products relative to dibutyltin maleate, basic lead carbonate and barium-cadmium stearate stabilizers commonly used in industry. The induction period in the early stages of the dehydrochlorination process increases as a function of the metal content in the stabilizer molecule. The evidence indicates that the quinone and the metal part ( ) of the stabilizer participate in the stabilization process by trapping the radical intermediates, as well as blocking the odd electron sites formed on the PVC chains. The mechanism of stabilization suggested to account for the results obtained supports a radical mechanism for the dehydrochlorination reaction.