Stabilizing Effect of Metal Carboxylates of Balanites aegyptiaca Seed Oil ( BSO ) on Poly ( vinyl chloride )

Thermal stabilization studies of poly(vinyl chloride) (PVC) in the presence of lead, cadmium, barium, calcium and zinc carboxylates of Balanites aegyptiaca seed oil (BSO) were carried out in air and under nitrogen atmosphere in the temperature range 170-190 oC. The stabilizing effects of the metal carboxylates were assessed by measurements of the time for degradation to attain 1% conversion, tDH and the corresponding rate of degradation, RDH. Kinetic parameters determined under nitrogen were higher than those determined in air indicating that PVC degraded less in air within the temperature range employed. All metal carboxylates stabilized PVC under the conditions employed for the study and showed a relative order of stabilization of Cd-BSO > Ba-BSO > Pb-BSO > Ca-BSO > Zn-BSO. At higher temperature (190 oC) deleterious effect of Zn-BSO manifested after prolonged heating. This is corroborated by data obtained from measurements of intrinsic viscosity of degraded PVC samples and thermogravimetric analysis. Synergistic effect was not exhibited by the compositions of binary mixtures of Ca/Zn carboxylates investigated while Ba/Cd carboxylates at 1: 9 ratio exhibited synergistic effect on PVC stabilization.


Introduction
Improvement in chemical and fabrication technologies has led to production of many useful domestic, industrial products and materials from poly(vinyl chloride) (PVC) with consequent increase in tonnage use.There is virtually no aspect of world economy PVC has not found application, however, this could not have been possible without the use of thermal stabilizers due to intrinsic thermal instability of the polymer.PVC is processed to useful materials at temperature above 160 ºC (Folarin et al., 2012) although it has glass transition temperature of 70 ºC (Folarin & Sadiku, 2011).At elevated temperature, it degrades via thermal dehydrochlorination that is auto-accelerated by the evolved HCl which results in severe discolouration arising from the formation of conjugated double bonds in the polymer chain and consequent loss of some properties (Tong et al., 2011;Toliwal & Patel, 2009;Liu et al., 2008;Steenwijk et al., 2006).Autocatalytic nature of thermal degradation of PVC and its poor thermal stability make the use of thermal stabilizers during processing imperative (Boussoum et al., 2006).Many compounds (organic and inorganic) have been used as PVC thermal stabilizers (Folarin & Sadiku, 2011) however, metal carboxylates stand out and many had been prepared from pure fatty acids and seed oils (Essien et al., 2012;Folarin et al., 2011a;Folarin & Enikanoselu, 2010;Fang et al, 2009;Egbuchunam et al., 2007;Okieimen et al., 2006;Egbuchunam et al., 2005;Gonzalez-Ortiz et al., 2005;Balkose et al., 2001).Seed oils will remain viable source of PVC thermal stabilizers as a result of their fatty acid contents and availability from different plant sources.Our earlier report (Folarin et al., 2011a) revealed that metal carboxylates of Ximenia americana seed oil (XSO) and Balanites aegyptiaca seed oil (BSO) are thermally stable within the temperature range for processing PVC.This work aims at investigating thermal stabilizing effects of some metal carboxylates of Balanites aegyptiaca seed oil (BSO) on PVC as a way of harnessing its potential in Nigeria.

Materials and Methods
The poly(vinyl chloride) obtained from Sigma-Aldrich Co (USA) (Mn, 0.9 x 10 5 ) was purified and the metal carboxylates were prepared as earlier reported (Folarin et al., 2011a;Folarin et al., 2011b).Thermal degradation of PVC was carried out in the presence of the carboxylates in the temperature range 170 ºC-190 ºC under nitrogen and in air as earlier described (Folarin et al., 2011b).
For viscosity measurements, the degraded PVC samples were purified by solution in cyclohexanone and precipitation in methanol.The precipitated polymers were filtered off and air-dried.Dilute solution viscosity measurements of undegraded and degraded samples were carried out in cyclohexanone using Ubbelohde capillary viscometer mounted in a thermo stated water bath at 30±0.1 ºC.The intrinsic viscosity, [η] was determined from Huggins equation: The stabilizing effects of the metal carboxylates were evaluated from the ratios of intrinsic viscosity of PVC samples degraded in the presence of additives, [η] to viscosity of undegraded sample, [η] 0 i.e., [η]/[η] 0 .Thermogravimetric study of PVC degradation in the presence of the metal carboxylates was conducted using Perkin-Elmer Pyris 6 thermal gravimetric analyzer programmed at a heating rate of 10 o C min -1 up to a temperature of 550 ºC under nitrogen.

Results and Discussion
The conversions in PVC degradation under nitrogen at 190 ºC in the presence of 3 wt% metal carboxylates of BSO is shown in Figure 1.Time for degradation to attain 1% conversion, t DH and corresponding rate of degradation at 1% conversion, R DH were used to evaluate effect of the carboxylates, the results are presented in Table 1.In the absence of additives, t DH and R DH values for PVC degradation increased with temperature in the range 170 -190 ºC.t DH and R DH are 34 min and 3.0 x 10 -2 % min -1 respectively at 190 ºC.Metal carboxylates can interact with evolved HCl especially at early stage of dehydrochlorination and this is one of the basis of their application as PVC thermal stabilizers.Mechanism of stabilization of PVC consists of HCl trapping by the unsaturated fatty acid moieties and substitution of labile chlorine atoms by carboxylate groups (Folarin et al., 2012).All metal carboxylates of BSO, including Zn-BSO, exhibited stabilizing effect on PVC.In the presence of 3 wt% Zn-BSO, degradation of PVC at 180 and 190 ºC is lower having t DH and R DH values of 40 min and 8.15 x 10 -2 %min -1 respectively at the latter temperature.This is in contrast to accentuation of degradation by Zn-XSO earlier reported (Folarin et al., 2012).However, it is in agreement with reported stabilizing effect of zinc soap of khaya seed oil at this temperature (Okieimen & Eromosele, 2000).
Under the same experimental conditions, all metal carboxylates of BSO exhibited higher stabilizing effect than those of XSO earlier reported (Folarin et al., 2012).Specifically, at 190 ºC, Cd-BSO is more active by as much as 69% than Cd-XSO based on t DH values.Our earlier report (Folarin et al., 2011a) showed that metal carboxylates of BSO are more stable thermally than those of XSO at temperature range normally employed in processing PVC.It was also reported much earlier (Okieimen & Ebhoaye, 1992) that unsaturation confers instability on metal soaps of unsaturated acids.Thus, the observed differences in stability of PVC in the presence of metal carboxylates of XSO and BSO may be due to differences in levels of unsaturation in the fatty acid moiety of the oils.Based on iodine value of 158.3 gI 2 100g -1 , XSO is a drying oil while BSO is a semi-drying oil with iodine value of 102.6 gI 2 100g -1 (Folarin et al., 2011b).The order of stabilization by metal carboxylates of BSO is: Cd-BSO > Ba-BSO > Pb-BSO > Ca-BSO > Zn-BSO.
The order of stabilization shown above is supported partly by studies on the metal soaps of khaya seed oil, jatropha seed oil (Okieimen & Eromosele, 2000) and wild olive seed oil (Ximenia americana) (Folarin et al., 2012).Thus, it may be deduced that for different seed oils, the order of stabilization of PVC by the metal carboxylates is a function of intrinsic aggregate chemical properties of their fatty acid constituents.The corresponding R DH values are 1.7 x 10 -2 %min -1 for Cd carboxylate and 4.0 x 10 -2 %min -1 for Zn carboxylate.The order of stabilization by metal carboxylates of BSO is: Cd-BSO > Ba-BSO > Pb-BSO > Ca-BSO > Zn-BSO based on t DH values.This order is consistent with the one observed for the carboxylates under nitrogen.A comparison of Tables 1 and 2 shows that PVC degraded less in air than under nitrogen at the temperature range investigated.All kinetic parameters determined are higher in nitrogen than in air.This could be due to some other reactions such as oxidation of polyene occurring along with dehydrochlorination in air.The oxidation of polyene interrupts elimination of HCl from the polymer backbone thereby hindering degradation.Figures 3 and  4 show the plots of relative intrinsic viscosity [η]/[η] 0 versus degradation time for PVC stabilized by 3 wt% metal carboxylates of BSO at 190 ºC under nitrogen and in air respectively.3 and 4).To further establish the stabilizing effects of individual carboxylates on PVC and to determine the synergistic effects of mixtures of the carboxylates on PVC thermal stabilization, thermogravimetric analysis was performed.The thermogram of PVC samples containing the metal carboxylates is shown in Figure 5.
The temperatures at which maximum degradation, t max and various extents of degradation occurred were used to evaluate the stabilizing effects of the additives.The result of the analysis is presented in Table 5.It shows that all the carboxylates, including Zn-BSO stabilized PVC, however, Cd-BSO is the most effective.This result is consistent with the results of kinetics of degradation and viscosity measurements.Mixed metal carboxylates, usually in the form of Ca/Zn and Ba/Cd carboxylates had been reported to exhibit synergistic effect.Synergism had been attributed to fast exchange reactions between the chlorides of Zn or Cd and carboxylates of Ca and Ba (Balkose et al., 2001).Synergistic effects of Ca/Zn and Ba/Cd mixed carboxylates of the metal carboxylates were investigated using different compositions and the result is presented in Table 6.All compositions of binary mixtures of Ca/Zn carboxylates investigated did not exhibit synergistic effect since all the t max values obtained were lower than that of Ca-BSO (Table 6) and this means that effect of the compositions is addition.For the mixture of Ba/Cd carboxylates, 10 wt% Ba/Cd mixtures exhibited synergistic effect with the highest t max value of 325.9 ºC.However, this is contrary to 90 wt% Ba/Cd mixtures earlier reported for carboxylates of XSO (Folarin et al., 2012).This observed difference could be attributed to difference in fatty acids composition of the two oils.BSO is a semi-drying oil while XSO is a drying oil.The level of unsaturation in the fatty acids appears to have a great effect on the chemical reactivity of the carboxylates and this requires further investigation.
Many researchers have reported on the enhancement of stabilizing effect of metal carboxylates in the presence of epoxidized oils (Egbuchunam et al., 2007;Boussoum et al., 2006;Gonzalez-Ortiz et al., 2005).The effect of the native and epoxidized oils on zinc carboxylate of the oil was investigated and the result is presented in Table 7.
The compositions of the mixture investigated were 10 wt% and 20 wt% oil/zinc carboxylate.The results show that epoxidized oil was more effective than the native in enhancing the stabilizing effect of zinc carboxylate based on t max and temperatures at which various extents of degradation were attained.The effect is concentration-dependent.

Conclusions
Metal carboxylates of BSO stabilized PVC in air and under nitrogen atmosphere in the temperature range 170 -190 ºC.Based on the parameters of kinetic of degradation determined, PVC degraded less is air compare to nitrogen atmosphere.This may be attributed to oxidation of polyene that occurred along side dehydrochlorination in air.The order of stabilization was, Cd-BSO > Ba-BSO > Pb-BSO > Ca-BSO > Zn-BSO.This is corroborated by results of intrinsic viscosity measurements of degraded PVC samples and thermogravimetric analysis.The relative intrinsic viscosity of degraded PVC decreased linearly with increase in degradation time and suggested absence of secondary reactions of polyene.Of all compositions of mixed metal carboxylates of Ca/Zn and Ba/Cd investigated for synergism, only Ba/Cd at 1:9 ratio exerted synergistic effect on PVC stabilization.This is however contrary to 9:1 ratio of Ba/Cd of XSO earlier reported.This suggests that the fatty acid composition of oils is of great importance in determining reactivity of metal carboxylates and this requires further investigations.The epoxidized oil is more effective in enhancing the stabilizing effect of Zn carboxylate and the effectiveness is concentration-dependent.

Figure 1 .
Figure 1.Plots of dehydrochlorination of PVC in the presence of 3 wt % metal carboxylates of BSO under N 2 at 190 ºC

Figure 2 .
Figure 2. Plots of dehydrochlorination of PVC in the presence of 3 wt % metal carboxylates of BSO in air at 190 ºC

Figure 3 .Figure 4 .
Figure 3. Plots of relative intrinsic viscosity vs degradation time for PVC stabilized by 3 wt % metal carboxylates of BSO at 190 ºC under N 2

Figure 5 .
Figure 5. Thermogram of PVC samples containing metal carboxylates of BSO

Table 1 .
Kinetic parameters for dehydrochlorination of PVC under nitrogen in the presence of metal carboxylates of BSO Additive (3 wt%) Temperature ( o C) t DH (min) 10 2 R DH (%min -1 ) Rate Constant k (min -1 )

Table 2
. At 190 ºC degradation temperature, t DH values range from 55 min for sample containing Zn carboxylate to 107 min for sample containing Cd carboxylate.

Table 2 .
Kinetic parameters for dehydrochlorination of PVC in air in the presence of metal carboxylates of BSO

Table 3 .
Intrinsic viscosity of PVC after degradation at 190 ºC under nitrogen in the presence of barium and calcium carboxylates of BSO

Table 4 .
Intrinsic viscosity of PVC after degradation at 190 ºC in air in the presence of barium and calcium carboxylates of BSO

Table 5 .
Evaluation of relative thermal stability of PVC stabilized with metal carboxylates of BSO AdditiveTemperatures at which various extents of degradation were attained ( o C)

Table 6 .
Evaluation of relative thermal stability of PVC stabilized (3 %w/w) with binary mixtures of metal carboxylates of BSO

Table 7 .
Evaluation of relative thermal stability of PVC stabilized with mixtures of oil and Zn-BSO