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Thermal Decomposition Behavior and Kinetics of Composites from Coal and Polyethylene Abstract:A thermogravimetric analysis(TG)was conducted to study the thermal decomposition behavior and kinetics of composites from coal and high density polyethylene(HDPE),linear low density polyethylene(LLDPE)or low density polyethylene(LDPE).The results show that coal facilitates melting of the polyethylene before temperatures reach 700 K in nitrogen due to the exothermic effect of coal.Above 700 K,adding coal into the polyethylene will result in smaller maximum rates of mass loss and higher initial mass loss temperatures of the composites.Hence,some chemical interactions,occurring between liquid compounds released in the pyrolysis of the coal and polymer,depend on several factors,such as coal rank and the molecular structure of polymers.Synergetic effects in coal and polymers were also found.Both chemical interactions and synergetic effects control the entire thermal decomposition behavior of composites.The larger the amount of coal in the composites,the greater the decomposition temperature spans and the higher the maximum decomposition temperature,the smaller the devolatilization rates.The effect of coal on the thermal stability of composites lies in the hydrogen acceptor effect of the coals.Thermal decomposition of the coals,the polymers and related composites can be modelled via first order parallel reactions between 563 K and 763 K. Key words:coal;polyethylene;composite;thermal;kinetics 1 Introduction In the last few years,coal-based composites have attracted much attention because of the excellent macromolecular structural characteristics of coal.In expanding fields of application,attempts have been made to prepare functional composites from coal and polyolefins,such as conductive composites and controlled degradable film.Unfortunately,currently available literature on thermal decomposition of coal-based composites is rare.All the same,thermal decompositions of coal and polymers are intrinsic chemical reaction steps for the preparation of the composite.Investigation into the kinetics of the components is important for process optimization.To achieve good material properties,it is also necessary to conduct numerical simulation of the decomposition process. In order to improve our understanding of the stabilization or degradation control of the composite,it is essential to discover the reactivity of the components and the thermal decomposition kinetics of composite. Thus,in our present study,thermal decomposition behavior of the composites from two different types of coal and industrial polyethylene were investigated by means of TG measurements. 2 Experimental 2.1 Materials For our analysis we have used Chinese Shenfu (SF)and Huayingshan(HYS)coal.The results of the ultimate and approximate analyses are summarized in Table 1.Coal samples were pulverized under 4μm in particle size.HDPE,LLDPE and LDPE used in the experiment are all commercial products and listed in Table 2.Composites from the coal and the polymer were prepared in co-rotation twin-screw extruder model SJSH30,with a rotor speed of 120 r/min andfeeding speed 20 r/min.SF/1F7B,with different blending ratios,was used as an example to show some primary properties of the composites(Table 3). 2.2 Thermogravimetric analysis(TGA) TG analyses were conducted on a NETZSCH TG 209C analyzer,using highly purified nitrogen as the purge gas.Samples of approximately 15 mg were heated to 1 173 K at a heating rate of 10 K/min. 2.3 Thermal decomposition kinetics Coats and Redfern developed an integral method, Eq.(1),which can be applied to TG data.The correct reaction order n is presumed to lead to the best linear plot. (1) Assuming that 2RT/Ea<<1,Eq.(1)can be described as follows. (2) 3 Results and Discussion 3.1 Thermal decomposition reaction order n TG and differential thermogravimetry(DTG) curves of coals were recorded as shown in Fig.1.Fig. 2 shows TG and DTG curves of polymers 6098,7042 and 1F7B in nitrogen. A partial overlap in mass loss temperature ranges of the coals and polymers can be easily observed from Figs.1 and 2.Moreover,the same thermal decomposition mechanism,i.e.,a radical chain scission mechanism,is proffered to explain the thermal decomposition behavior of polymers and coal. Therefore,there might be interactions during thermal decomposition of the composites to some extent. In our work,different reaction orders(n=0,1,2,3) were considered.In each case,the variations of kinetics parameters with reciprocal temperature were plotted(Fig.3). The main temperature range of the volatile release (613–763 K)was taken into account in the plots in Fig.3.It can be observed that the linear range in the plots changed slightly from one reaction order to another.Straight lines with excellent correlation coefficients r were obtained for n=1 in all cases.Hence,theassumption of a first-order reaction appears reasonable for the coals,polymers and composites. 3.2 Thermal decomposition mechanism and kinetics TG plots of the polymers and composites are presented in Fig.4. Clearly,before temperatures of 700 K were reached, mass losses of the composites were higher in the presence of the coal when compared with that of individual polymer matrices.Furthermore,the greater the fraction of coal in the composites,the greater the mass loss obtained.Therefore,the coal improves an endothermic decomposition of the polymers by an exothermic effect before 700 K. Table 4 shows the thermal decomposition characteristics of the coals,polymers and composites,based on TG measurements. As the results in Table 4 show,the wider the decomposition temperature span(Tend–Tonset),the higher the maximum decomposition temperature(Tmax).Decreased devolatilization rates occur with increasing amounts of coal in the composites. It is also clear that the presence of coal in the polymers produces a retarded onset temperature of consequential composites compared to the parent components.Therefore,chemical interactions occur between liquid compounds evolved in the pyrolysis of the coal and polymer,which can postpone this thermal decomposition process and elevate the softening temperature of the composites. The decreases in the devolatilization rates indicate that polymers can play an important role as hydrogen donors,leading to effect on the formation of tar and volatile gas. On the other hand,mass losses measured by the TG analyzer(MLexp)versus time are,for some of the composites,greater than the corresponding theoretical mass losses(MLcal),calculated by the sum of each individual coal and polymer decomposition(Eq.(3)). This may be attributed to the synergetic effect occurring in thermal decomposition of the coal and polyethylene .Moreover,HDPE 6098 poses a highly ordered molecular structure.Generally,the number of reactive sites formed in its thermal process is relatively smaller than those of LLDPE 7042 and LDPE 1F7B.The effect gives rise to a quicker emission of volatile gaseous products and greater apparent mass losses.In contrast,for most of the composites,MLexp is smaller,such as for SF/7042,HYS/7042 and SF/1F7B with different blend ratios,it accounts for the presence of some chemical interactions,which prevail over the synergetic effect. (3) As mentioned earlier,the thermal decomposition reaction order n of the coals,polymers and composites are estimated to be 1.Thus,the apparent thermal decomposition kinetic parameters can be calculated as in Table 5,where SF/1F7B was selected as an example. It can be concluded from Table 5,that the composites have higher apparent activation energy compared to 1F7B alone in 705–763 K and in the temperature range of 563–705 K,SF/1F7B(20/80)represents the highest apparent activation energy. The activation energy confirms the chemical interactions and may imply that the coal participates in chain initiation,transfer and termination of the polymers.Its effect on the thermal stability of the composites lies in the hydrogen acceptor effect of thecoal. 4 Conclusions Coal facilitates melting of the HDPE,LLDPE and LDPE before 700 K and is attributed to the exothermic effect of coal.Above 700 K,the coals play different roles,because addition of polyethylene into the coals results in smaller maximum rates of mass loss and higher initial mass loss temperatures of the composites,suggesting some chemical interactions during the decomposition.The interaction depends on several factors,such as type of coal and molecular structure of the polymers.At the same time,a synergetic effect in the coal and polymer is also found.Both chemical interaction and synergetic effect control the whole thermal decomposition behavior of the composites.On the other hand,the larger the amount of coal in the composites,the greater the decomposition temperature spans and the higher the maximum decomposition temperature the smaller the devolatilization rates.The effect of coals on the thermal stability of composites lies in the hydrogen acceptor effect of the coals.Thermal decomposition of the coals,the polymers and their related composites can be modeled via first order parallel reactions models between 563 and 763 K. 说明，1、2、3那是公式，公式贴不上去，麻烦各位了！没有那么多金币，只有50个！ |
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