Plastics are commonly added with certain additives for developing its better characteristics. Recent decades, some kinds of additives which are known as prodegradant additives are being developed for plastics film. Prodegradant additives can result in accelerating plastics degradation. It is important to develop prodegradant additives since plastic waste is viewed as a serious worldwide environmental and health concern as its character of being non-degradable. Many pro-degradant additives, composed of transmetal-organic acid salts, have been investigated in many researches as photodegradable additives for plastics. Cobalt carboxylates, namely cobalt laurate, cobalt palmitate, cobalt stearate, are kinds of which have been applied for LDPE films. In the previous method, cobalt carboxylates were synthesized by reacting sodium carboxylates and cobalt acetate. This paper reports a new method of cobalt carboxylates synthesis. The new method involve reaction between molten carboxylic acid with sodium hydroxide solution to produce sodium carboxylate, and continued by reacting sodium carboxylate with chloride salt of cobalt. First reaction conducted at 80°C and under perfect agitation. Second reaction took place well in the low concentration of cobalt chloride, about 0.2 M or less and temperature 80°C. Cobalt carboxylates (cobalt laurate, cobalt palmitate, cobalt stearate) densities are 0.615, 0.391, 0.364 g/cm3 respectively. Their melting points are 107.83, 109.10, 114.40 °C respectively, obtained by DSC test. Thermogravimetric Analyzer (TGA) test have been done on cobalt carboxylates to investigate their thermal stabilities. All cobalt carboxylates start to degrade over 300°C, as shown in TGA test result. TGA studies indicate that cobalt carboxylates have stability at compounding and film blowing/casting temperature of polyethylene, 180-200°C.
Keywords :Prodegradant additives, cobalt carboxylates, characteristics
Plastics production systematically increases, thus, also plastics waste amount grows. People rely to plastics in everyday activities, such as jugs, clothes, computer, etc [1,2].
Recently, the usage of plastics increases significantly. US’s plastics production in 2000 amounted to more 45,000,000 metric tons . Total resin consumption in Malaysia increased by 8% from 1.6 million MT in 2004 to 1.72 million MT in 2005, of which about 65% were polyolefins (PE & PP) . The municipal solid waste stream in the U.S. totals nearly 160 metric tons per year and consists of about 7-11% by weight of post-consumer plastics .
Plastics is now being viewed as a serious worldwide environmental and health concern, especially for disposable application such as carrier and garbage bags . Its character of being non-degradable, is resulting in river pollution, choking in landfill . The growing environmental concern has made plastics a target of much criticism due to their lack of degradability .
Therefore degradable plastics will be important issue to reduce plastics waste amount. Many researches have been done to obtain methods which can improve degradability of plastics.
Some methods are used to accelerate the degradation process are the addition of transition metal pro-oxidants or carbon monoxide polymer, both of which are designed to catalyse photodegradation and thermal degradation .
The studies on utilizations of some additives have been performed by several researchers to obtain photodegradable plastics. The effect of a series transition metal (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) stearates on the photodegradation of a high-density polyethylene have been examined. The role of metallic compounds on the photodegradation of polyethylene has been extensively studied by several authors .
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The effect of cobalt stearate on accelerated aging of LDPE has been studied. Cobalt stearate has been proven to have significant effect on accelerated aging of LDPE . The effect of cobalt carboxylates namely cobalt laurate, cobalt palmitate, cobalt stearate, on the photo-oxidative degradation of low density polyethylene also have been investigated . Cobalt carboxylates have been synthesized through double decomposition by reacting sodium carboxylates and cobalt acetate [9,10].
This paper report the study of new synthesis method of cobalt carboxyates through double decomposition method by reacting sodium carboxylates and cobalt chloride. This paper also attempt to investigate characteristics of cobalt carboxylates.
Cobalt chloride hexahydrate (Fluka), sodium hydroxide, lauric acid, palmitic acid, and stearic acid (Merck) were used without any treatments. Deionized water was used for all processes.
2.2 Synthesis of cobalt carboxylates
Cobalt carboxylates, namely cobalt laurate, cobalt palmitate, and cobalt stearate, were synthesized through two steps of reactions. First step, carboxylic acids (lauric acid, palmitic acid, and stearic acid) were reacted with sodium hydroxide to produce sodium carboxylates (sodium laurate, sodium palmitate, and sodium stearate).
The second step, sodium carboxylates (sodium laurate, sodium palmitate, and sodium stearate) were reacted with cobalt chloride to produce cobalt carboxylates (cobalt laurate, cobalt palmitate, and cobalt stearate) in solid phase .
The solids were filtered and washed with hot water to separate sodium chloride. Finally, the solids were dried in the oven at 60 °C for 2 hours.
2.3 Characterization of cobalt carboxylates
The characterization comprises density, melting point, and degradation temperature. Melting points were investigated using Differential Scanning Calorimeter (DSC) Q1000 from TA Instruments, in N2 atmosphere, in temperature range 30-200 °C, at heating rate 10 °C/min. Evaluation of degradation temperatures were done using Thermogravimetric Analyzer (TGA) Q500 from TA Instruments, in N2 atmosphere, ramp method, heating rate 20 °C/min, final temperature 1000 °C.
3. Result and Discussion
3.1 Synthesis of cobalt carboxylates
Synthesis of cobalt carboxylates comprises two steps of reactions. The first step, producing sodium carboxylates through reaction below.
These reactions took place in liquid phase at 80 °C. Carboxylic acids were melted and added gradually with sodium hydroxide 0.25 M under agitation for one hour. The amount of carboxylic acids and sodium hydroxide were reacted in stoichiometric ratio. Perfect agitation was needed to reach a complete reaction.
The second step, synthesis of cobalt carboxylates, was conducted by adding cobalt chloride hexahydrate solution 0.20 M into the product of first step reaction which contain sodium carboxylates. Cobalt chloride solution was added gradually. The reaction temperature was maintained at 80 °C, with continuous stirring.
The second reaction could be written as follow:
In the second reaction, cobalt carboxylates, which were produced in solid phase, would form suspension and became hard to be stirred. The low concentration of cobalt chloride was favored since reaction would take place well in low concentration. The low concentration of cobalt carboxylates would produce few solids of cobalt carboxylates. It mean that mixing process would run well and complete reaction could be reached.
3.2 Characterization of cobalt carboxylates
Densities of cobalt carboxylates can be seen in Table 1. All of cobalt carboxylates are in the form of powder and have low density.
Tests have been done using Differential Scanning Calorimeters (DSC) to determine melting point of cobalt carboxylates. The DSC test results can be seen in Fig. 1, Fig. 2, and Fig. 3.
The melting point of cobalt laurate, cobalt palmitate, and cobalt stearate was determined from the peak of endothermic melting transition. Melting points of cobalt carboxylates can be summarized as listed in Table 2 below.
There are two endothermic peaks in Fig. 1 and Fig. 2. Two endothermic peaks are sometimes found in DSC test result. First peak indicate unstable melting process. The second peak represent stable melting process .
Melting points of cobalt carboxylates increase with increasing carbon chain length of carboxylates. Melting point of cobalt stearate which has 18 C is higher than melting point of cobalt palmitate which has 16 C. Melting point of cobalt palmitate is higher than melting point of cobalt laurate which has 12 C.
Thermal stability was investigated using Thermogravimetric Analyzers (TGA). Degradation temperature (initial and final temperature of degradation) was determined from high decrease of weight during heating. It was shown by steep slope of the curve in TGA test result. Cobalt carboxylates lost more than 80% of their weight during degradation process. The final weight remained was around 10 % of initial weight.
The degradation temperature increase from cobalt laurate, cobalt palmitate, and cobalt stearate respectively. All of cobalt carboxylates start to degrade at temperature over 300 °C.
It means, cobalt carboxylates have good thermal stability since the temperature of blending/compounding of polyethylene is 180-200 °C. Cobalt carboxylates will not degrade at temperature of compounding and film blowing/casting of polyethylene.
Synthesis of cobalt carboxylates could be conducted through reaction of sodium carboxylates and cobalt chloride in liquid phase. The melting point of cobalt carboxylates increased with increasing the length of carbon chain. Thermal stability also increased with increasing the length of carbon chain, as indicated by temperature of degradation. Cobalt carboxylates also had thermal stability at temperature of polyethylene processing.
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