| During the forming process,the biaxially oriented polypropylene(BOPP)film experienced higher drawing ratio in both vertical and horizontal directions so that the molecular chains are highly oriented.It tends to shrink when being heated,which results in a change in performances.The effects of annealing at different duration/temperature and the addition of β nucleating agent on the crystal structure and orientation of BOPP films were studied.The results showed that the dimensional stability,mechanical properties and static viscoelasticity were improved.The main work and conclusions are as follows:(1)The biaxially oriented thin films prepared by stretching method were annealed at different temperatures for different times.The crystal structure of BOPP films was measured by differential scanning calorimeter(DSC).It was found that annealing increased the melting point and crystallinity of the films,and an obvious annealing peak was observed in the melting curve after annealing at 135℃,corresponding to the secondary crystallization formed during annealing.The change of orientation degree between crystalline region(fc)and amorphous region(fam)of BOPP films was analyzed by wide-angle Xray diffraction(WAXD)and Fourier transform infrared spectroscopy(FTIR).It was found that the orientation of BOPP films showed obvious anisotropy,and obvious orientation was observed along MD and TD,and FM(orientation along MD)>FT(orientation along TD).With the increase of annealing time/temperature,the fc increases while the fam decreases at first and then increases.Through tensile test,it is found that the breaking strength,elongation at break and elastic modulus all decrease after annealing,and the tensile properties deteriorate.The change of viscoelasticity of BOPP film was characterized by Dynamic thermomechanical analysis(DMA).The results showed that the storage modulus decreased after annealing,but the creep resistance was improved and the recovery ability after creep was enhanced.The tensile creep curve is successfully fitted by Burger model and Weibull distribution function,and the change of its parameters can reflect the influence of temperature and time on the annealing effect.(2)Biaxially oriented polypropylene films were prepared by blow molding,and the effects of different β-nucleating agent contents on the structure and properties of the films were studied.The β-BOPP film was obtained by adding P-nucleating agent,and the characteristic peak of β-crystal appeared in DSC and WAXD curves,and the crystallinity and β-crystal content of the film increased with the increase of its content.FTIR characterization showed that the degree of molecular orientation increased,and the tensile curve showed that the elastic modulus,yield stress and elongation at break were significantly increased,indicating that β-nucleating agent had obvious toughening effect.The creeprecovery curve showed that β-nucleating agent improved the ability of anticreep and recovery.(3)The effect of annealing on the structure and properties of β-BOPP was studied.After annealing at 80℃,the melting peaks of the films move to high temperature,indicating that the structures of both a and β crystals are more perfect,while the summary crystallinity of the samples increases with the increase of annealing time,while the content of β crystals decreases and the length period of a crystals increases.The fam decreases,while the fc increases.After annealing,the modulus decreases obviously,the elongation at break decreases greatly,the necking becomes slower,and the tensile deformation is more uniform,but the creep resistance and recovery ability after creep are improved.(4)The creep and stress relaxation behavior of BOPP film are simulated and analyzed by using ABAQUS finite element software.In the process of creep and stress relaxation,there is obvious stress concentration at the fixed end and uniform stress distribution at the free end.The simulation can reflect the effect of annealing and β nucleating agent. |
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