Study On The Effect Of Peroxide Crosslinked LLDPE/HDPE Blends Structure On Mechanical Properties

Linear low density polyethylene has good compatibility with high density polyethylene.Both polyethylenes and blends can be crosslinked with an oxidizing agent.The blended ratio directly affects the change of mechanical properties of polyethylene blends.However,the research on the influence of peroxide crosslinked polyethylene blend structure on mechanical properties is not perfect。It is significant to study this modification method systematically.In this paper,LLDPE/HDPE and mLLDPE/HDPE with different blend ratios were prepared by melt blended.The rheological behavior,crosslinking process and the crystallization kinetics of the blend after crosslinking were analyzed.The effect of the structure of the blend on the mechanical properties was further studied.The main sork and conclusions are as follows:(1)The effects of blended ratio on the molecular weight distribution and branching degree of the blends were analyzed by gel permeation chromatography(GPC)and infrared spectroscopy(FTIR).The FTIR test showed that the degree of branching(N)of high density polyethylene(HDPE)was 9.5 CH3/1000 C,the number average molecular weight Mn measured by GPC was 15,629,and the weight average molecular weight/number average molecular weight(Mw/Mn)ratio was 5.16.As the content of linear low-density polyethylene increases,N of the polyethylene blend increases,Mn increases,and Mw/Mn decreases.The molecular weight distribution of the mLLDPE/HDPE blend is narrower than the LLDPE/HDPE blend with the same blend ratio.The effects of blended ratio on rheological properties were investigated by using a rotary rheometer and a capillary rheometer.The rheological data were fitted using the Carreau model and the Cross model.The results show that the addition of low-density polyethylene increases the zero-cut viscosity(η0)of the blend.Under the same blended ratio,the η0 of the mLLDPE/HDPE blend is smaller than that of the LLDPE/HDPE.The viscous activation energy(Ea)of the blend was calculated by the Arrhenius equation from the fitted zero-cut viscosity.Ea characterizes the extent to which temperature changes affect viscosity.The results show that Ea increases with the increase of linear low-density polyethylene content,and the Ea of the mLLDPE/HDPE blend is larger than LLDPE/HDPE blend under the same mixing ratio.(2)The cross-linking process of polyethylene was measured by a rotational rheometer and a differential scanning calorimeter(DSC),and the cross-linking process was analyzed by reaction kinetics.The results show that the crosslinking reaction of the polyethylene blend is a first-order reaction,and the blended with LLDPE does not affect the reaction order.As the content of mLLDPE increases,the semi-crosslinking time becomes longer,and the activation energy of the crosslinking reaction increases.The cross-linking process of LLDPE/HDPE also has a similar pattern.When the content is low,the effect of LLDPE on the activation energy of the reaction is less.The increase in the activation energy of the reaction is caused by a decrease in the mobility of the polyethylene molecular segment.Gel testing showed that as the content of low density polyethylene increased,the crosslink density of the blend increased.When the low density polyethylene content is higher than about 43%,the effect of mLLDPE on the crosslinked mesh density of the blend is more obvious.(3)The crystallization properties of crosslinked polyethylene were investigated by DSC test.The crystallization process was analyzed by Avrami equation and Mo method.The results showed that the cross-linking HDPE showed an increase in folding free energy,a decrease in crystallization ability and a decrease in crystallinity with the increase of mLLDPE content.The crystal structure was analyzed by polarized light microscopy,SALS,and XRD.The results show that increasing the content of mLLDPE reduces the spherulitic grain size of the crosslinked polyethylene and increases the crystal non-uniformity.The kinetic analysis and crystal morphology analysis of crosslinked LLDPE/HDPE also have similar variation rules.Compared with the cross-linked LLDPE/HDPE by same blended ratio,the cross-linked mLLDPE/HDPE has weaker crystallization ability,smaller spherulite particle size,poor crystal uniformity and lower crystallinity.(4)The creep properties in 0.5 h of cross-linked polyethylene were tested by DMA,and the TTSP and TSSP were verified to predict the feasibility of long-term performance of cross-linked polycondensation.Creep test shows that the creep resistance of cross-linked polyethylene decreases with the increase of mLLDPE content,and the cross-linked LLDPE/HDPE system has similar rules.The short-term creep test at 60℃ showed that the creep resistance of the mLLDPE/HDPE blend was better.The static mechanical results show that the yield strength and tensile modulus of cross-linked polyethylene decrease with the increase of low-density polyethylene content,and the yield strength and tensile modulus have a good linear relationship with the crystallinity of the blend.As the content of low-density polyethylene increases,the elongation at break of the cross-linked blend increases first and then decreases,which is the result of a combination of decreased crystallinity and increased crosslink density.The yield strength and tensile modulus of the mLLDPE/HDPE blends were lower at the same blended ratio.The pendulum notched impact test results show that the impact strength of the crosslinked blend increases with the increase of the linear low density polyethylene content.When the linear low-density polyethylene content was increased from 0 wt%to 43 wt%,the room temperature impact strength of the crosslinked LLDPE/HDPE and the crosslinked mLPE/HDPE was increased from 42 kJ/m2 to 66 kJ/m2 and 71 kJ/m2.When the linear low-density polyethylene content is increased from Owt%to 85wt%,the low-temperature impact strength(-30℃)of the crosslinked LLDPE/HDPE and crosslinked mLPE/HDPE is increased from 9kJ/m2 to 94 kJ/m2 and 107 kJ/m2.