Preparation And Study Of Properties Of Polyethylene Glycol/Silicon Dioxide Composite Phase Change Materials

With the gradual depletion of non-renewable energy throughout the world,improving energy efficiency becomes a hot spot of the scientific research step by step.Phase change materials （PCM） release/absorb large amounts of thermal energy duringthe phase transition process, which is conducive to the recovery and renewable ofwaste heat, thermal energy storage and energy transfer in space or time. It hasimportant application value to improve energy efficiency and relieve the discrepancyof energy requirement in spatial and temporal. The organic solid-liquid PCM iswidely used, numerous in variety, and have a wide range of phase transitiontemperature for choice, whereas have many shortcomings, such as leakage during thephase transition process, poor compatibility, corrosion and low thermal conductivity.The main goal of our researches are to prepare macroscopic solid-solid PCM withhigh thermal conductivity, high latent heat and no leakage during the phase transitionprocess. In this work, we choosed non-corrosive polyethylene glycol（PEG） polymeror oligomer used as phase changing function component, and three differentPEG/SiO₂composite PCM had been prepared through capillary forces of porousstructure in fumed silica, chemical grafted method and Sol-Gel process, respectively.The effect of composite approach, loading of inorganic component SiO₂, molecularweight of PEG on the thermal storage capacity and thermal conductivity of PEG/SiO₂composites had been discussed and analyzed. The PEG/SiO₂composite PCM weprepared have great potential of application value in solar thermal energy storage andbuilding energy conservation field. The main research contents and achievements arelisted as following:Firstly, the influence of molecular weight on thermal storage properties of PEG,the cocrystallization behavior of PEG-1000and PEG-2000, phase separation behaviorand crystallization behavior of physical blending of PEG-1000and fumed silicaHL-200were studied by using Differential scanning calorimeter（DSC）. It’sdemonstrated that phase transition temperature and latent heat of PEG decrease withthe increasing of molecular weight of PEG. When the molecular weight of PEG is 2000or above, the deviation between melt point and crystal point is more than20℃,while the counterpart value of PEG-1000or below is less than15℃indicate that itsphase transition behavior is more trend towards that of small molecular. It is revealedthat an obvious cocrystallization behavior come out when the mass ratio ofPEG-1000/PEG-2000is1:9, the crystallization and melt temperature, latent heat aremore high than that of PEG-2000, as well as melting temperature range becomesnarrow. The phase separation behavior of solution blends of PEG-1000and HL-200exist in the range of weight percentage of PEG-1000between58.5%and76.7%, onephase structure is HL-200acted as skeleton, PEG-1000was impregnated into pores ofHL-200by Capillary force.; the other phase is PEG-1000as continuous phase,HL-200dispersed therein. The crystalline degree of the later phase is more than thatof the former phase.Secondly,3-aminopropyl trimethoxy silane was used to modify HL-200toprepare amino-modified HL-200with NH2group grafted on the surface. Isocyanate（NCO） group ended PEG prepolymer was synthesized by the reaction betweenIsophorone diisocyanate （IPDI） and PEG. Then, the reaction between NCO-endedPEG prepolymer and amino-modified HL-200was used to prepare composites ofPEG grafted HL-200, which are good solid-solid PCM. The influence of reaction time,water content and dosage of silane coupling agent on grafting ratio, particle size anddistribution of amino-modified HL-200was characterized by Fourier transforminfrared absorption spectrometer（FT-IR）, thermal gravimetric analyzer（TG） laserparticle diameter analyzer and physical property measurement system（PPMS）. Theoptimal preparation condition of amino-modified HL-200is that reaction time is4h,mass ratio of silane coupling agent to HL-200is25%, the molar amount of waterequal to silane coupling agent. The particle with maximum grafting ratio of0.830mmol/g, and minimum particle bimodal diameter of91.7and245.6nm wasobtained. The influence of loading of amino-modified HL-200on thermal storageproperties, thermal conductivity and crystal morphology of PEG-6000grafted HL-200composites, was characterized by DSC, PPMS thermal properties measurementsystem and hot stage polarizing microscope（POM）, we found that when adding6g of PEG-6000, isocyanate index R=2.1, amino-modified HL-200loading is1.5g, therelative crystalline degree of PEG-6000/amino-modified HL-200composite is largestat90.99%, melt enthalpy is128.7J/g. Thermal conductivity ofPEG-6000/amino-modified HL-200is increasing with the loading of amino-modifiedHL-200, when loading is23.61%, thermal conductivity reach0.307W/（m·K）, whichis26.86%higher than that of pure PEG-6000. The shackle effect exert on PEG-1000by grafting bond is more larger than that of PEG-6000, PEG-1000/amino-modifiedHL-200composites exhibit less phase change latent heat, when adding10g ofPEG-1000,2g of amino-modified HL-200, isocyanate index R=1.2, melt point of theresult composite is28.8℃, melt enthalpy is80.85J/g. We also found cocrystallizationbehavior exist in PEG-1000/PEG-2000blend grafted HL-200composite with themass ratio of PEG-1000/PEG-2000is1:9.Thirdly, the reaction between PEG-800and γ-（2,3-epoxypropoxy） propyltrimethoxysilane （KH-560） was catalyzed by boron trifluoride etherate, and a siloxanegroups-ended PEG prepolymer was synthesized. Then, this prepolymer washomogenized with ethyl orthosilicate （TEOS）, through hydrolysis and condensationreaction catalyzed by aqueous hydrochloric acid solution, PEG-800/nano-SiO₂composite PCM was prepared via Sol-Gel process, which exhibited a significantsolid-solid phase transition behavior even high at110℃. The thermal storageproperties and thermal conductivity of PEG-800/nano-SiO₂composites were studiedby using DSC, FT-IR and PPMS thermal properties measurement system. It is shownthat the feeding mass fraction of KH-560and TEOS are strong effect on the phasetransition temperature and enthalpy, thermal conductivity of composites is increasingwith the feeding mass fraction of TEOS, which is in direct proportion to the content ofSiO₂component in composties. When adding8g of PEG-800,1.65g of KH-560and3.57g TEOS, the phase transition termperature and enthalpy of the result compositeprepared by Sol-Gel process is24.7℃and88.2J/g. The thermal conductivity is0.295W/（m K） increased by22.92%than that of pure PEG-800when SiO₂massfraction is12%.