Preparation And Properties Of PE/PEG Phase Change Materials

With the further development of modern industry and technologies,novel clean energy and energy storage have become the focus of recent researches.Phase-change material(PCM)is of great significance to the conservation of energy.In recent years,studies about PCM focus on single PCM,such as,paraffin,stearic acid and polyols,which have shortcomings of low thermal conductivity and leakage.In view of this,the present work proposed a new type of high and low temperature PCM(HLT-PCM),which could further expand the field of application of PCM.In addition,the thermal conductivity and leakage of the composites were effectively enhanced.In this work,polyethylene glycol(PEG)is used as the PCM at low temperature region,and high-density polyethylene(HDPE)is the counterpart at high temperature region with graphene nanosheet(GNP)as thermal conductive filler.HLT-PCMs were prepared by three methods,e.g.,solution blending,chemical reaction modification and liquid rubber encapsulation,respectively.The summary of the findings in this study are listed as below:The crystallization behavior of three kinds of PCM was analyzed by differential scanning calorimetry(DSC).The order of phase transition enthalpy in high and low temperature regions was ?HS>?HCS>?HRS,and the maximum enthalpy of the PEG phase transition in low temperature region was about 76.8 kJ/kg,while the maximum one of the HDPE phase transition in high temperature region was 105.4 kJ/kg.In addition,the PEG portion crystallinity and crystallization rate of PCM were affected not only by GNPs,but also because HDPE was in solid state at low temperature.In the HDPE phase,low content of GNPs could promote the crystallization of HDPE via heterogeneous nucleation,and the crystallization of HDPE would be limited when the GNPs content was elevated.The thermogravimetric analysis(TGA)showed that the PCM of the CS system had better thermal stability.In the S system,when the temperature increased,it was easy to leak and decompose.In the CS system,it was difficult to decompose due to the graft reaction.The existence of rubber in the RS system could prevent the decomposition process.The thermal conductivity testing results showed that with the increase of GNP content the thermal conductivity of S system was the highest(1.22 W/m-K)and the maximum thermal conductivity of CS system was 1.09 W/m-K while that of RS system(prepared from liquid rubber encapsulation procedure)was only 0.27 W/m-K.The reason should be that the thermal conductivity of rubber was relatively low and it was very difficult to induce the formation of a thermal network structure,which caused the failure of improvement on the thermal conductivity.Through dynamic rheological testing,the phase structure and compatibility of three PCMs were studied and compared.The results showed that with increasing GNP content,the complex viscosity(?*),storage modulus(G')and loss modulus(G")of S-PCM and CS-PCM gradually increased.RS-PCM showed that with the increase of GNP content,?*decreased initially and increased subsequently.The G' and G" of RS-PCM displayed a platform area during an initial stage,and both values increased significantly after the cross-linking.In the study of complex rheological behavior,the relationship between phase structure and compatibility of PCMs was analyzed by Cole-Cole and Han plots.It was found that the compatibility of CS-PCM was better than that of S-PCM.What's more,the Han plots were not suitable for RS-PCM.The durability of three kinds of PCMs was analyzed by the freezing-thawing cycle experiment,and the morphologies of the composites were also observed using SEM.The results showed that the RS-PCM had better durability,and it can effectively prevent the leakage of PEG.After modification of PEG chemical reaction in CS-PCM,the relative mass loss rate can be controlled below 6%,and it can solve the leakage of PEG as well.SEM observations showed that the compatibility of the materials in the S-PCM was poor,and the situations in CS-PCM and RS-PCM were much better.With increase GNP loading,the agglomeration in RS-PCM gradually appeared,and the network structure was readily formed in S-PCM and CS-PCM.In general,S-PCM showed higher phase transition enthalpy than CS-PCM but its compatibility,thermal stability and durability were poorer than CS-PCM.RS-PCM had good compatibility and stability and durability,but its phase transition enthalpy and thermal conductivity were very low in comparison to S-PCM and CS-PCM.