Studies On Crystallization Behavior And Mechanical Properties Of Polypropylene Film Prepared In Elongational Stress Field

The hard elastic PP film was prepared through melting extrusion-high ratio stretching-annealing method. The effects of molecular weight and its distribution, the melting extrusion temperature, draw down ratio, annealing temperature, annealing time and the additions of different type of nucleator on the crystallization behavior, morphology, tensile properties and elastic recovery of PP film were studied in this thesis, and the experimental results show that:1) With an increasing shear rate, the shear viscosity of isotactic PP (iPP) decreases, and displays the behavior of shear thinning. At the same temperature and same shear rate or elongational rate, compared with iPP-A, the shear viscosity or elongational viscosity of iPP-B is lower, which has lower average molecular weight and wider distribution, moreover, the tensile strength of iPP-B melt is higher. These indicate that in case of shear flow or extensional flow, iPP-B is easier to flow. During the predominantly extensional flow, iPP-B could be stretched more and have higher drawability.2) The hard elasticity of PP film is controlled by melting extrusion temperature, draw down ratio, annealing temperature and annealing time, etc. If other processing parameters are same, compared with iPP-B, the elastic recovery of iPP-A film is higher and tensile properties are better, which has higher molecular weight and narrower distribution. Improving melting extrusion temperature, the elastic recovery ER₅₀ is decreased, the tensile strength and elongation at break also go down. With the increase of draw down ratio, the melting point of iPP-A goes up, the lamellar thickness and crystallinity increase. The elastic recovery and tensile strength of PP film increases and elongation at break decreases with the increase of draw down ratio. For the annealed film, with the rise of annealing temperature and prolonging of annealing time, the melting point mount up, the lamellar thickness and crystallinity are increased little by little, the elastic recovery and tensile strength are improved and elongation at break decreases.3) With the addition of a nucleator, the melting point and crystallizing temperature of iPP rise, the induction period of crystallization is shortened, the crystallizing gets faster and the crystallinity increases. With the increase of a nucleator amount, the tensile and flexural properties of iPP are improved. Through the addition ofβnucleator, the crystallization process of iPP is transferred from homogeneous nucleation to heterogeneous nucleation. The crystallization temperature goes up, crystallization gets faster, the spherulite size becomes smaller and crystallinity increases. Moreover, the mechanical properties of iPP are improved. The addition ofβnucleator is favorable to formβcrystal and cause more a crystal transform toβcrystal. With the increase ofβnucleator amount, theβcrystal content is increased.4) The addition of a andβnucleator make the melting point of iPP film ascend, lamellar thickness and crystallinity increase, and elastic recovery is improved. AFM of hard elastic iPP film indicates that the existence of stacked lamellae rows which are parallel to each other and perpendicular to the extrusion direction.