Study On The Influence Of Injection Molding Process On The Structure And Properties Of Transparent Polypropylene

Transparent polypropylene(transparent PP)not only has the advantages of corrosion resistance,high temperature resistance,low density and good mechanical properties ordinary PP has,but also has high transparency and high gloss,therefore it is widely used in applications like medical supplies,transparent packaging,automotive industry and daily necessities.As a result of high transparency and its competitive against traditional transparent materials when it comes to low costs,transparent PP sees a growing demand with the rapid expansion of its application range.Transparent PP materials are generally processed and finally used by customers in the form of applied products.Injection molding as one of the most important processing techniques,the changes in process conditions will directly affect the crystal morphology of the products and determine the performance of the products.Compared with ordinary PP,there are relatively few studies on the structural evolution mechanism and the relationship between structure and properties of transparent PP in the injection molding process.The existing research results about the structure and properties of the ordinary PP in injection molding process can not be directly used to guide the processing and production of transparent PP products,due to the special characteristic structure of transparent PP.Therefore,the study of the interaction between the injection molding process-structure-properties of transparent PP not only has a certain academic value,but also has important guiding significance for the development and processing of new transparent PP products.In this paper,the effects of different injection molding parameters on the optical properties and mechanical properties of transparent PP(RP340R)produced by PetroChina were studied by orthogonal test.The effects of key injection molding parameters affecting the optical properties on morphological structure of the products were investigated by means of wide-angle X-ray diffractometry(WAXD),differential scanning calorimetry(DSC),scanning electron microscopy(SEM)and dynamic thermomechanical analyzer(DMA).Further,the relationship between the structure and optical properties of transparent PP was studied.Some meaningful results have been achieved and are summarized as follows:1)Based on the relevant literature,the etching method of the RP340R sample was determined by selecting an appropriate etchant and investigating the etching time.It was clear that the RP340R sample containing the fourth generation sorbitol clarifier exhibits the crystalline morphology of the fiber network structure under certain injection molding conditions.This result provides technical support for the in-depth study of the relationship between the injection molding process-morphological structure-macro performance of transparent PP.2)Through the study of the influence of injection molding process on the crystal morphology of RP340R products,it was found that the crystallization of fiber network exhibited by the transparent PP containing the fourth generation sorbitol clarifier have a significant effect on the haze and light transmittance of the product.In general,when the fiber network crystal is obvious and its fiber structure is small,the transparency of the product is high.However,when the fiber network is not crystallized and the fiber structure size is lage,the transparency of the product is lowered.3)The injection molding process has a significant effect on the formation of the crystallization of fiber network.When the injection pressure is low,the fiber network crystallizes more obviously and the fiber structure size is smaller.As the injection pressure increases,the fiber network crystallization gradually disappears.The effect of injection rate on the crystallization of fiber network has the same trend as the influence of injection pressure on the crystallization of fiber network.The increase of injection rate will lead to the gradual disappearance of fiber network crystallization.And at lower injection pressures,the injection rate has a more pronounced effect on the crystallization of the fiber network.The increase in melt temperature also causes the crystallization of the fiber network to gradually disappear,but too high a temperature may cause thermal degradation of the melt,and the network structure will gradually appear after etching.4)Through orthogonal experiments and the influence of key process parameters on the structure and properties of RP340R,it was found that the most important factor affecting the fiber network crystallization of product is injection pressure.The most important factors affecting the transparency of the prod uct is the injection pressure,followed by the injection rate and the melt temperature,and as these three parameters increase,the haze of the product increases and the transparency decreases.And in the range of orthogonal experiments,the best optical conditions for RP340R products were obtained for the main factors:injection pressure is 25 kg/cm~2,injection speed is30%,melt temperature is 180~oC,and the product molded under this condition had a haze of 6.6%and a light transmittance of 88.1%.The main factors affecting the mechanical properties of the product are melt temperature,injection rate,dwell time,mold temperature,etc.,and their effects on the flexural modulus,yield strength and strength are different.Through the comprehensive balance me thod,a combination of process parameters with better comprehensive performance was obtained,that is,the melt temperature is 220~oC,the injection pressure is 20 kg/cm~2,the mold temperature is 80~oC,the holding pressure is 20 kg/cm~2,and the dwell time is 30 s,injection rate is20%,cooling time is 20 s,and the product molded under this condition had a bending strength of 955.91 MPa,a tensile strength of 27.51 MPa,and an impact strength of 6.4614 KJ/m~2.