摘要
硬弹性膜材料是制备微孔膜的基础。本文采用熔融挤出—高倍拉伸—退火处理法制备硬弹性等规聚丙烯(iPP)薄膜。研究了聚丙烯分子量及其分布、熔体挤出温度、拉伸比、退火温度、退火时间、不同成核剂的含量等因素对聚丙烯膜的结晶行为与结晶结构、拉伸性能、弹性回复率的影响，成功制备了弹性回复率大于95％的硬弹性iPP薄膜。研究结果表明：
(1)两种等规聚丙烯iPP-A及iPP-B均为假塑性流体，随着剪切速率的增加，二者的表观剪切粘度下降，呈现出假塑性流体典型的“剪切变稀”行为。在相同的温度和剪切速率下，平均分子量较小、分布较宽的iPP-B的表观剪切粘度小于iPP-A的表观剪切粘度，拉伸粘度低于iPP-A的拉伸粘度，表明对于剪切或拉伸流动占主导地位的加工过程，iPP-B熔体更容易发生流动。相同温度时，分子量分布较宽的iPP-B熔体的拉伸破裂强度大于iPP-A熔体的拉伸破裂强度，表明iPP-B熔体具有较高的可拉伸性，可以达到较大的拉伸比。
(2)影响iPP薄膜硬弹性的主要因素有熔体挤出温度、拉伸比、退火温度、退火时间等。在相同的条件下，分子量较高、分子量分布较窄的iPP-A薄膜的弹性回复率、拉伸性能优于iPP-B薄膜。随着挤出温度升高，iPP薄膜的弹性回复率ER_(50)值减小，拉伸强度和断裂伸长率减小。随着拉伸比增加，iPP-A薄膜的熔点升高、片晶厚度和结晶度增加；随着拉伸比增加，iPP膜的弹性回复率增加，拉伸强度增加，断裂伸长率下降。随着退火温度的升高和退火时间的延长，iPP薄膜的熔点升高、片晶厚度和结晶度逐渐增加，弹性回复率逐渐增大，拉伸强度增大，断裂伸长率减小。
(3)加入α成核剂后，iPP树脂的熔点与结晶温度升高，结晶诱导期变短，结晶速率增大，结晶度增加。随着α成核剂质量分数的增加，iPP的拉伸性能和弯曲性能得到了一定程度的提高。加入β成核剂后，iPP由原来的均相结晶转变为异相结晶，其结晶温度提高，结晶速率加快，球晶尺寸减小，结晶度提高，力学性能也得到提高。β成核剂的加入还有利于iPP中β晶型的形成，诱导了相当一部分α晶型向β晶型转变。随着β成核剂质量分数的增加，iPP结晶中β晶相对质量分数增加。
(4)α和β成核剂的加入使得iPP薄膜的熔点升高、片晶厚度和结晶度增加，弹性回复率增大。AFM图表明，硬弹性iPP-A薄膜中含有垂直于薄膜挤出方向且平行排列的片晶结构。
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 ER_(50) 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.
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