| Ethylene-vinyl acetate copolymer(EVA)is a kind of non-toxic,odorless,transparent thermoplastic materials that similar to rubber elastomers.It shows good flexibility,impact resistance,transparency,adhesion and good processing properties.However,the mechanical strength of EVA material is not high which greatly limits its application in many fields.Therefore,the enhanced modification of EVA attracted much attention.Blend modification is the most commonly used method for EVA modification,but blends are not uniformly mixed,resulting in non-uniform material properties.The insitu microfiber reinforcement technology is a new method developed on the basis of common blending to preparing high-performance composite materials.This method utilizes the orientation microfibers formed by the polymer dispersed phase “in situ” during processing to improve mechanical strength of the matrix.The occurence of micro-laminate co-extrusion technology provides a new platform for the preparation of in-situ microfiber composites.Based on this background,micro-nano laminating coextrusion technology is used in this dissertation to make iPP microfibrillate in situ in EVA to achieve effective control of the mechanical properties of EVA.The influences of disperse phase content,processing methods,viscosity ratio,traction rate,and screw speed on the microfibril morphology,crystallization behavior,rheological properties,and mechanical properties of in-situ microfiber composites were studied.The relationship between microfibre morphology and mechanical properties was established,which provides a new way for the development of high-performance EVA materials.Experimental results show:(1)The dispersed phase iPP forms microfibers in the extrusion direction in the matrix EVA after micro-lamina stack co-extrusion,which is different from iPP/EVA BC(common blended composite)sea-island structure.The content of dispersed phase is a key factor affecting the morphology and mechanical properties of the microfibers.The rod or short fibers with low degree of orientation are formed when the content of iPP is low.The number,diameter and degree of entanglement of the microfibers will increase with the increasing content of iPP,and form a similar network structure.The impediment of iPP microfibers on the movement of EVA molecular chains is greater than the interfacial effect on the promotion of heterogeneous nucleation.The crystallinity of MFCs(microfiber composites)with reduced crystallinity is lower than that of BC.The G'(storage modulus),G''(loss modulus),and ?*(complex viscosity)of MFCs have increased and are higher than those of BC.The phases of MFCs will be separated when the iPP content exceeds 15 wt%.The tensile strength and elongation at break of the MFCs sheet gradually increase.When the iPP content is 20wt%,the tensile strength reaches a maximum of 16.71 MPa,which is 42.9% higher than the pure EVA,and the tensile strength and elongation at break of MFCs are higher than that of BC.Therefore,iPP microfibers have significant effects on the enhancement and toughening of EVA.(2)The higher viscosity ratio(The viscosity ratio is the ratio of the apparent viscosity of the dispersed phase to the apparent viscosity of the matrix at the same temperature,the higher viscosity ratio of 0.65 and 0.66)is favorable for the disperse phase to fiber.When the viscosity is relatively low(0.49 and 0.53),the length-to-diameter ratio of iPP microfibers is low,and the degree of microfibril tangles is low.With the increase of viscosity ratio,the diameter of microfibers decreased significantly,the length increased,the distribution of diameter size became more uniform,and the degree of entanglement increased.The impeded degree of movement of EVA molecular chains was enhanced by iPP microfibers and interface,and the interfacial effect on the promotion of heterogeneous nucleation decreased.Therefore,when the viscosity is higher,the crystallinity decreases more significantly,and G',G'',and ?* increase.Because of the maximum G?,G??,and ?* in MFCs with a viscosity ratio of 0.65,the microfibril tangles are high and the complex viscosity is the largest.The network structure of microfibers plays a good role in supporting the matrix,so the mechanical properties are best..(3)The proper traction rate is conducive to the formation of microfibre network structure.Too low traction speed(50r/min)will make the dispersed phase not reach the fiber or form short rod microfibers;too high traction rate(>70r/min)will cause the PP microfibers to break in the process of stretch extrusion,microfiber length Diameter ratio decreases.With the increase of traction speed,the diameter of microfibers,the crystallinity of MFCs,G',G",and ?* all decreased first and then increased,and the tensile strength and elongation at break first increased and then decreased.When the traction rate was 60 r/min,microfibers The diameter,crystallinity,G?,G”,and ?* are the smallest,but the network structure of the microfibers is perfect,which contributes to the improvement of tensile strength and elongation at break.(4)Screw speed has a great influence on the morphology of microfibers.As the screw speed increases,the microfiber diameter increases first and then decreases.When the screw rotation speed is 250 rpm,the diameter of the microfiber is the largest,and the long diameter is relatively small,which has little effect on the EVA molecular chain and can induce the disentanglement of the EVA molecular chain.And the microfiber with the lowest G',G'' and ?* had good processing fluidity.The maximum tensile strength and elongation at break are in the extrusion direction.When the screw speed is 300 rpm,the microfiber diameter is the smallest,the size distribution is the most uniform,the degree of entanglement is the greatest,and the hindrance to the movement of EVA molecular chains is the strongest.The G',G'',and ?* of the composite material are the highest,but the microfibers with small diameter are easy to be broken,so the tensile strength and elongation at break decreased. |
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