Preparation And Study Of Polycarbonate With Stress Cracking Resistance

Environmental stress cracking（ESC）is the phenomenon that plastic products crack or break when subjected to the simultaneous action of stress and chemical agents.The main reasons for ESC of polycarbonate（PC）products are mainly the following two points:1)PC contains a large number of benzene rings in the main chain,and the molecular chain of PC is rigid,the melt viscosity of PC is large,resulting in large internal stress of the PC product after molding.When PC is affected by the external environment,it is easy to form cracks in the internal stress concentration area,resulting in macroscopic cracking or even fracture;2)The rigid main chain of PC will lead to a loose stack of molecular chains with a large number of internal gaps.Therefore,solvent molecules are easy to enter the product and cause PC swelling or dissolution,and ultimately accelerate the formation and development of cracks.The modification methods reported so far can not only optimize the stress cracking resistance of PC,but also significantly reduce the mechanical properties of PC.In order to solve the above problems,this paper effectively improves the ESCR of PC by adding polyvinyl butyral（PVB）with both high mechanical properties and flexible main chain,and maintains the original excellent mechanical properties of PC.Subsequently,by introducing high-hydroxyl content of silica（Si O₂）and organic montmoronite（OMMT）nanoparticles and regulating their distribution,a controllable ternary blend system was constructed,which significantly improved the mechanical properties of PC while significantly improving the ESCR.The specific research contents are as follows:（1）The PVB material with multi-hydroxyl group and compliant backbone was introduced into PC by melt blending,which improved the ESCR of PC and maintained the excellent mechanical properties of PC.The microstructure analysis showed that PVB is dispersed in PC with typical island structure,and the size of dispersed phase is 500～800 nm.The results of FTIR showed that the hydroxyl group in PVB formed hydrogen bond with the carbonate group in PC,which enhanced the interfacial interaction force between PC and PVB and made PVB disperse well in PC.Compared with neat PC,the ESCR of PC/PVB blends was significantly improved and the tensile properties remained basically unchanged.The initial fracture time of neat PC was 3 min,and the total fracture time was 5 min.The initial fracture time of PC/PVB-10 was10 min,and some samples still did not fracture after 20 min immersion.The performance of ESCR was significantly improved.In addition,compared with neat PC,the yield strength and elongation at break of PC/PVB-10 decreased by about 6%,and the excellent mechanical properties of PC were basically maintained.The results of DSC and melt index showed that the internal stress of PC/PVB blends decreased and the fluidity increased with the increase of PVB content.This is because the low T_g and main chain flexibility of PVB itself can promote the movement of PC chain segment,improve the processing flow of PC,reduce the internal stress of products in the cooling process,and thus improve the ESCR of PC.（2）The PC/（PVB/Si O₂）blends with high strength,high toughness and high ESCR were prepared by introducing Si O₂ into PC/PVB-10 and controlling the selective distribution of Si O₂in the PVB phase through the combination of solution premixing and melt blending.By calculating the surface energies of PC and PVB and the interface forces and wetting parameters between PC,PVB and Si O₂,it is found that Si O₂ is more likely to be distributed in the PVB phase in PC/PVB.SEM and TEM results also showed that Si O₂ was selectively distributed in the PVB phase.The addition of Si O₂ can further improve the stress cracking resistance of PC,and the ESCR and mechanical properties of PC/（PVB/Si O₂）blends have been significantly improved.When the contents of Si O₂ is 2 phr,the initial fracture time of PC/（PVB-10/Si O₂-2）is 20 min,which is 400%higher than that of neat PC.The tensile test results showed that the yield strength,elongation at break and toughness of PC/（PVB-10/Si O₂-2）were 115%,111%and 130%of PC.The mechanism analysis showed that the selective dispersion of Si O₂ in PVB inhibited the hydrogen bonding between PVB molecular chains,reduced the T_gof PVB phase,enhanced the movement ability of its chain segment,and then reduced the melt viscosity and internal stress of the blend system,and finally significantly increased the ESCR of PC.At the same time,Si O₂ is a rigid particle,selective distribution in PVB can greatly improve the strength and modulus of PVB phase,and then improve the strength and modulus of the blend.The DMA results can well prove that Si O₂ improves the modulus of PVB phase and blends.（3）PC/PVB/OMMT blends with high strength and high ESCR were prepared by introducing OMMT into PC/PVB blends with melt blending.The microstructure analysis showed that OMMT is mainly dispersed at the interface of PC and PVB,which can refine the size of PVB phase and make the interface of PC and PVB blurred,so as to improve the compatibility of PC and PVB.The reason of compatibility between PC and PVB had been revealed by dissolution experiment and TGA:PVB can react with OMMT,so that PVB can be grafted to OMMT,thus reducing the size of PVB phase and optimizing the compatibility between PC and PVB.The results of CCl₄ immersion test and tensile test showed that the introduction of OMMT can significantly improve the ESCR and strength of PC and PC/PVB.With the addition of OMMT,the toughness of the blend will decrease slightly,but the decrease is less than the increase of modulus and strength.In conclusion,the addition of OMMT and PVB can greatly improve the ESCR of PC,and the tensile property is slightly better than that of PC.