Journal of Applied Sciences & Environmental Management, Vol. 9, No. 1, 2005, pp. 127-130                 

Evaluation of the Effect of Temperature on the Stability of Metal   Soaps of Dicarboxylic Acids

IKHUORIA, E U^a*^,; OKIEIMEN, F E^a; AIGBODION, A I^b 

^aIndustrial Agriculture Products Research Laboratory, Department of Chemistry, University of Benin, Benin City, Nigeria.^bEnd-use Division, Rubber Research Institute of Nigeria, Benin City, Nigeria

Code Number: ja05023

ABSTRACT: The thermal stability of calcium and zinc dicarboxylates was studied. The kinetics of the decomposition was studied thermogravimetrically at various temperatures. The rates of the first stage decomposition were used to assess the effect of temperature on the susceptibility of the metal soaps of dicarboxylic acids to decomposition.  Although the values of the rate constant for the decomposition of the soaps were about the same order of magnitude (10^-1s^-1), the soaps of glutaric acid were relatively more stable than the soaps of succinic acid. The values of the rate constant were also observed to be temperature dependent. The enthalpy, entropy and free energy of activation for the decomposition of the metal dicarboxylates were also determined. The thermodynamic values obtained showed that the system is exothermic and that the reaction process is not a spontaneous one. @JASEM

Polyvinyl chloride (PVC) is an important thermoplastic material on account of its versatility and low cost. It is estimated that over 15 million tons are consumed annually worldwide in the manufacture of domestic and industrial products (Whelan and Craft, 1977, Okieimen and Egboaye, 1995). One major drawback in the manufacture of polyvinyl chloride and its products is the inherent low thermal stability of the polymer.  At elevated temperatures well below its decomposition temperature, PVC losses HCl and becomes discoloured leading to changes in the physical and chemical properties of the polymer (Ikhuoria et al, 2000,

Okieimen and Eromosele, 2000). The poor thermal stability of PVC requires the use of stabilisers in the processing of the polymer. Additives of practical application as thermal stabilisers for PVC include metal carboxylates (Bacologlu and Fisch, 1993), metal soaps and epoxies of vegetable oils (Okieimen, 1999, Okieimen and Eromosele, 1995, Okieimen and Sogbaike, 1995).  Although the use of metal carboxylates as stabilisers for PVC, has been extensively studied, reports on the utilisation of derivatives of dicarboxylic acids as stabilisers for PVC are scanty.  In this study, we evaluate the effect of temperature on the stability of metal soaps of some dicarboxylic acids.

EXPERIMENTAL

Preparation of metal dicarboxylates: Metal dicarboxylates of succinic and glutaric acids were prepared by metathesis in alcohol solution following the method described by Burrows et al, 1981. The metal soaps were first prepared by dissolving the dicarboxylic acid (9.2g) in 50ml of hot ethanol, followed by treatment with 20ml of 20wt% potassium hydroxide solution.  To this mixture, 100ml of 30% w/v solution of the appropriate salts were added slowly with continuous stirring.  The precipitated soap of the dicarboxylic acid was filtered off, washed with hot water and air-dried.

Thermal stability studies on the metal soaps of dicarboxylic acids: The thermal stability of zinc succinate, zinc glutarate, calcium succinate and calcium glutarate was examined at 443,453 and 463K.  In a typical experiment, 0.5g of the metal soap was accurately weighed into a previously weighed tube and heated in an oven at the appropriate temperature for a definite period of time (10 – 60min).  At the end of the heating period, the sample was withdrawn from the oven and cooled down under running water.  The residual weight of the sample was determined using the expression:

Where, W_O is the initial weight of the metal dicarboxylate and wt is the residual, Weight of metal dicarboxylate after heating time (t)

RESULTS AND DISCUSSION

The thermal behaviour of calcium and zinc soaps of dicarboxylic acids at 443,453 and 463K was investigated and the data given in Tables 1, 2, 3. Fig.1 show the variation in percentage weight loss with time at 443K for the metal dicarboxylates.

Similar plots as shown in Fig.1 were observed at 453K and 463K respectively.  The plots show an initial increase in the %weight loss with increase in time. It was found that at temperatures up to 463K, the metal soaps of dicarboxylic acids are relatively stable with maximum weight loss of about 7.1%, 12.2%, 9.5% and 14.4% for calcium glutarate, calcium succinate, zinc glutarate and zinc succinate respectively.  The maximum weight loss for the metal dicaboxylates with time at 443, 453 and 463K is shown in Table 1. 

The result show that the soaps of glutaric acid are relatively more stable than the soaps of succinic acid and is found to be of the order: Calcium glutarate > zinc glutarate > calcium succinate > zinc succinate. Fig. 2 shows the rate of decomposition at different intervals of time for zinc glutarate at 443, 453 and 463K as plots of percentage residual wt against time. 

Similar plots as shown in Fig.2 were obtained for zinc succinate, calcium glutarate and calcium succinate respectively. The extent of decomposition was determined by the rate of decomposition within the 1^st 10 minutes (Ikhoria et al, 2000).  Generally, the rate of decomposition is observed to increase with increase in temperature.

The rate of thermal decomposition of metal dicarboxylates may be expressed as follows: (Chemizinott, 1989, James and Prichard, 1974).   

The values of the rate constant for the decomposition of the metal dicarboxylates, k, may be  obtained from semi-logarithmic  plots of weight loss versus time. The values obtained from such plots are given in Table 2.

The values of the rate constant are of the order of 10^-1s^-1 and are somewhat lower for the glutarates than the succinates. This is probably due to the additional methylene group in the carbon chain length of glutaric acid.  The values are also observed to be temperature dependent with increase of 14.5%, 23.36%, 17.56% and 58.91% within the temperature range for calcium glutarate, calcium succinate, zinc glutarate and zinc succinate respectively.  From the dependence of the values of the rate constant on the temperature, value for the activation energy of decomposition of the metal dicarboxylates were found to be 75.6, 59.03, 70.01 and 50.63Kjmol^-1 for calcium glutarate, calcium succinate, zinc glutarate and zinc succinate respectively. These values obtained for the activation energy further supports the stability trend observed with the metal dicarboxylates. The values of the enthalpy of activation, ΔH^≠, for the decomposition of the metal dicarboxylates was calculated using the relationship:

 The entropy of activation, and free energy of activation ,, were also calculated using the expression:

The results of these thermodynamic properties are given in Table 3.

The same value was obtained for the enthalpy change, ,for all for metal dicarboxylates. The negative values of and the positive values of E_a show that the system is an exothermic one. The values obtained for the and also show that the reaction is not spontaneous process.

Conclusion: The results from this study show that metal soaps of dicarboxylic acids are fairly heat stable, with the glutarate being more stable, than the succinates. The reaction was observed to be an exothermic process. Based on heat stability, it therefore appears that the metal dicarboxylates may be suitable as stabiliser of PVC.  It should be noted however, that other properties of additives such as miscibility and compatibility with the polymer and a consideration of the intended end – use of the polymer are essential in choosing additives in polymer formulation.