| The resource of Polypropylene is quite rich and its price is fairly low.The economic value and develpoment prospects will be greatly improved by thermal conductivity modification of Polypropylene which are applied to a cooling device.In this paper,graphite/magnesium oxide being modified by coupling agent,expanded graphite/magnesium oxide and PP as the main raw material with a total mass of 60 g in which substrate was 60%(W)and packing was 40%(W),thermal conductive composite material was prepared by mixing with mechanical activation and hot pressing mold with plate vulcanizing machine.The thermal conductivity of composite material was tested in order to learn about its thermal performance.The thermal conductive composite materials of PP/graphite/magnesium oxide and PP/expanded graphite/magnesium oxide were prepared respectively,and the simulated LED lighting heat shell application experiments were adopted to measure heat dissipation effect.The results showed that:(1)For the PP/graphite/magnesium oxide polymer materials,the optimum conditions were determined through single factor experiments,and results were as follows:titanate as coupling agent whose dosage was 2%of total mass of graphite and magnesium oxide,the conditions were milling time of 1 h,graphite/MgO mass ratio of 2:1,agitating speed of 150 r·min-1,milling temperature of 50?;and the hot-pressing temperature of 210?,hot-pressing pressure of 5 MPa,hot pressing time of 5 minutes.The thermal conductivity of the composite sheet reached 0.90 W·m-1·K-1,which was 5.98 times of pure PP and 2.1 times of composite by simple mixing without milling.The tensile and flexural strength of composite materials were 12.74 MPa and 7.43 MPa,respectively.They were smaller than pure polypropylene,but still met the requirements of certain industrial products.(2)For the PP/expanded graphite/magnesium oxide,the optimum conditions were determined through single factor experiments,and results were as follows:KH570 as coupling agent whose dosage was 1.5%of total mass of expanded graphite and magnesium oxide,the conditions were milling time of 1 h,graphite/MgO mass ratio of 2:1,agitating speed of 200 r·min-1,milling temperature of 50?;and the hot-pressing temperature of 220?,hot-pressing pressure of 5 MPa,hot pressing time of 5 minutes.The thermal conductivity of the composite sheet reached 0.76 W·m-1·K-1,which was 5.1times of pure PP and 2.1 times of the composite panels by simple mixing without milling.The tensile and flexural strength of composite materials were 17.83 MPa and 9.13 MPa,respectively,1.7 MPa and 7.09 MPa higher than that of PP/MgO style/graphite composite sheets,respectively.Although its thermal conductivity slightly decreased,the mechanical properties increased relatively more.(3)It was showed from SEM that mechanical milling could make graphite/magnesium oxide,expanded graphite/magnesium oxide highly dispersed in the PP matrix,like wound package interconnect with each other,then formed more conduction path effectively,thereby generating thermal network,and gaining the point of improving the thermal conductivity of the material.(4)It was showed from TGA and DSC that pure PP extrapolated onset weight loss temperature was 380 0C,PP/MgO/graphite composite was 398 ?,PP/MgO/EG composite was 419?.Compared with pure PP,the onset weight loss temperature of PP/MgO/graphite and PP/MgO/EG composites extrapolated increased by 18? and 39?,resceptively;The maximum weight loss rate of pure PP,PP/MgO/graphite,PP/MgO/EG composites were 437?,442? and 478?,resceptively;and compared to pure PP,the maximum weight loss rate of composites increased by 5?,41?.This showed that pretreatment before mechanical activation with matrix PP was good for the improving of the thermal stability;After modified,the sheet had bigger melting point and softening point,indicating that modification effect was good.(5)It was showed from XRD that the absorption peaks of 20=15.8°correspongding to ? crystal of PP appeared in the composite of PP/graphite/MgO and PP/EG/MgO.So the crystal type of PP changed from monoclinic system of a crystal which existed before the modification into hexagonal system of ? crystal for the two kinds of composite materials system.The tensile strength,elongation at break and impact toughness of the ? crystal were better than that of the a crystal form.(6)It was showed from FT-IR that the peak at 2920 cm-1 and 2850 cm-1 conrresponding to asymmetric stretching vibration absorption peak of methyl and methylene symmetric stretching vibration absorption peak appered in magnesium oxide modified by titanate,which indicated that surface modification of the thermally conductive filler particles by coupling agent was effective,and had a positive role of improving the thermal conductivity.(7)It was showed from Rheological properties analysis that joining of a thermally conductive filler particles and the process pressing results in a dramatically increase in its viscosity and decreased the liquidity,led to molding difficult.(8)It was showed from the heat application of PP/MgO/graphite and PP/MgO/EG composite sheets that:the higher the thermal conductivity,the lower the increase of the sheet temperature during the experiment,and the time of reaching the final temperature was longer,also the final temperature was lower.This appearance indicated that it had better thermal performance as an approximate LED floor,thus enabled the LED lamp longer life.Owing to the limited to thermal conductivity of the thermally conductive material,so this thermally conductive material could only be applied in low-power LED lighting cooling parts. |
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