Research On Microcellular Chemical Foaming Behavior Of PP-Based Polypropylene/Polyethylene Terephthalate Microfibrillar Composites

As one of the general resins with the smallest density at present,polypropylene（PP）has become the first choice of lightweight materials for energy saving and material saving in many fields due to its abundant sources,low price,easy processing,and excellent comprehensive mechanical properties.Microcellular Injection Molding（MIM）technology is a novel porous material molding method,which has the advantages of lightweight,high specific strength and good impact performance.However,PP has the structural characteristics of linear polymer chain,with low relative molecular weight and branching degree,and it is easy to disentangle between molecular chains,resulting in poor melt strength.During the foaming process,it is easy to break,merge and collapse bubbles,which seriously affects the product performance.In view of the above technical problems,this paper combines in-situ fibrillation technology and microcellular injection foaming technology,with the help of self-developed high-efficiency endothermic and exothermic composite chemical foaming agent,studies the lightweight of plastic parts from two aspects of material and molding process.The microcellular injection foaming behavior of PP in-situ Fabrillated Composite（IFC）was mainly studied,and the strengthening mechanism of microfibrillation process on PP was discussed.The formation rule of IFC bubbles in the microcellular injection foaming process was revealed,and the material preparation and molding process system of light and high performance plastic parts was constructed.The specific research contents of this paper are as follows:（1）Preparation of PP-based microfiber compositesBased on the"melt blending-hot drawing"microfibrillation process,homopolymeric PP is used as the matrix and PET is used as the dispersed phase,and PP/PET and PP/PET/Talc microfiber composite systems with PET fiber diameters within 400 nm have been successfully prepared..Morphological characterization found that with the increase of drawing rate,the diameter of PET fibers first decreased and then increased;the increase of PET phase domain size and Talc filler will lead to the increase of PET fiber diameter and the decrease of aspect ratio.Dynamic viscoelasticity test further confirmed that PET microfibrillation formed fiber reinforced network in PP matrix,which significantly improved the melt strength of PP.Nonisothermal crystallization results showed that the formation of PET fiber network did not significantly promote the crystallization of PP.The thermal stability results showed that the thermal stability of IFC materials was significantly improved compared with pure PP after one processing thermal history.The tensile strength of IFC injection samples can be increased by nearly 40%compared with pure PP.（2）Research and development of high-efficiency,low-cost absorption-exothermic balance foaming agentUsing anhydrous citric acid as modifier,the solution coating method was used to modify industrial grade sodium bicarbonate（Na HCO₃）,which significantly increased the initial decomposition temperature of Na HCO₃ and shortened the decomposition temperature range.When the ratio is 1:1,the initial thermal decomposition temperature of Na HCO₃ is increased by 40℃,and the temperature decomposition range is shortened by 60%,and the modification effect is remarkable.With modified Na HCO₃as the main foaming agent,AC as the co-foaming agent,nano-zinc oxide as the catalyst,ultrafine talc as the nucleating agent,and thermoplastic elastic material POE as the carrier resin,the composite chemical foaming agent with heat absorption-desorption equilibrium was successfully prepared by optimizing the compound proportion..Compared with the commercial foaming agent,the internal pores of the injection foaming sample were more dense and uniform,and the material cost was also greatly reduced.（3）Cell structure and mechanical properties of IFC material microcellular injection molding chemical foam productsBased on the low-pressure injection molding foaming process,IFC foaming products of PP/PET and PP/PET/Talc systems were prepared respectively.The pure PP sample is extremely difficult to foam,and the minimum cell size is 250μm.The two types of IFC foamed samples have finer cells and more uniform distribution;The distribution is more uniform,and the cell density is not much different.The cell size of the IFC foamed sample with 15%Talc is reduced to 60μm,and the cell density is increased to 8×10⁵ cells/cm³.Except for pure PP,the weight loss rate of all samples before and after foaming exceeded 8%,and the maximum weight loss rate of the PP/PET/Talc system IFC foaming samples reached 10.2%.Compared with the pure PP solid sample,the yield strength of the IFC foamed sample can be increased by a maximum of 17%,which achieves the improvement of the mechanical properties while reducing the weight of the product.