The Polymer Crystallization In Extrusion-Cast Field And Annealing Process Study

Extrusion-cast method is an important film manufacturing method. During the melt-stretching process, the polymer melt is stretched between the extrude die and the chill roll, and the precursor film with row-nucleated crystalline structure is obtained. The hard elastic materials with high crystallinity, high elastic modulus and high elastic recovery properties can be fabricated using this method. Meanwhile, the annealing process, an important segment for fabricating hard elastic materials, can thicken the lamellae and improve lamellae orientation and uniformity. The microporous structure can be obtained by cold and hot stretching the hard elastic films. It is important for us to insight the crystallization process during the extrusion and annealing process.In this paper, three types of poylmer raw materials, polypropylene, polyethylene and polylactic, with different crystallization rate were used to fabricate the cast film with lamellae structure and the crystallization processes during the cast and annealing process were followed. The main experimental work and results were listed as follows:1. For polypropylene, the orientation and crystallization during melt stretching were characterized, and their influence on the lamellar morphology was clarified. During melt stretching, the MDR range from 40-200 could be divided into two regions. In region I, MDR below 120, the crystalline morphology transformed from ellipsoid spherulites to lamellae structure, and the orientation, elastic recovery, and lamellar lateral dimension were enhanced. In region Ⅱ, with the MDR increasing to 200, the orientation and lamellae lateral dimension were increased, but the elastic recovery did not change much. The long period, crystalline phase thickness, crystallinity, and lamellae cluster size were kept constant within the whole MDR range, but the orientation was improved from 0.23± 0.02 to 0.41±0.03. Apparently, the orientation induced the increase of lamellar lateral dimension, and it was the main factor deciding the properties of precursor film. Meanwhile the tie molecules volume fraction was increased with the MDR increasing, corresponding with the mechanical properties linear incrasing. It was demonstrated that the contents of tie molecules decided the mechanical properties of PP hard elastic films.2. The precursor PP film was annealed under 105℃ to 145℃ and the corresponding properties of stretched microporous membrane were improved obviously. After annealing, an apparent melt plateau appeared on DSC curve. Where was it from? During the in-situ SAXS experiment, we found that the long period and crystalline thickness were linear increasing with the logarithm of time, but an induction period was observed on amorphous layer thickening. The TMDSC results showed that the melting and recrystallization occurred during annealing, and some new formed spherulites were observed by POM and SEM. The tie molecules volume fraction decreased with the annealing temperature improving, but the orientation parameter of (110) plane was linear increasing. This indicated that the tie molecules were secondary crystallized along the (110) plane during the annealing process.3. For polyethylene, the orientation and crystallization during melt stretching were characterized, and their influence on the lamellar morphology was clarified. During melt stretching, the long period and cryatlline phase thickness were increased with the MDR incrasing from 40 to 212, but the lamellae lateral size turned small. The crystalline orientation along a axis and c axis were increased, whereas the orientation dgree along b axis was kept consitant. The tie molecules were synchronizedly oriented with the a axis. The tie molecule volume fracrion was increased accompaning with MDR increasing, inducing the yield stress linear increasing. But the PE cast film with highest elastic recovery of 61.2% was obtained at MDR of 177 and then reduced to 48.3%. At the same time the lamellae lateral size reduced to 59nm at MDR of 211. The lamellae lateral size was the main reason determining the elastic recovery for PE hard elastic film.4. In-situ annealing process of polyethylene cast film with row-nucleated crystalline structure was followed using SAXS setup equipped with a temperature-controlling unit. The annealing temperatures were set at 105℃,115℃ and 125℃, respectively. It was found that after annealing for 7200s, the long period, crystalline thickness, the real amorphous layer thickness and diffuse transition layer thickness were increased, whereas the linear crystallinity didn’t show pronounced change. Compared with those at 105℃ and 115℃, annealing at 125℃ resulted in apparent increase of long period and crystalline thickness, as well as crystalline thickening rate. During the initial annealing stage, the crystalline thickness was increased apparently. After annealing for 1800s, the crystalline thickness versus annealing time curve showed wave-like shape, due to the occurrence of melting and recrystallization behavior, which could not be observed during early annealing stage. The initial crystalline thickness increase was mainly due to the crystallization of some tie chains in the amorphous region, as well as small transformation of diffuse transition layer into crystalline region. The crystalline orientation degree based on the width of SAXS patterns was improved apparently when the annealing temperature was around maximum crystalline temperature. This work clarified the increase of crystalline thickness induced by annealing for polyethylene film with row-nucleated crystalline structure.5. The polylactic acid films were prepared with different melt-draw ratios by extrusion casting process. The results showed that with increasing the melt-draw ratios (MDRs), the oriented structure appeared at first and then shish-kebab crystals appeared at higher MDR. The vertical shish crystals showed higher melting point than that of oriented structure. The stretching-induced crystallization occurred during the melt-stretching process, but the formed crystals were not strict α form or α’ form crystals. At lower MDR, mainly oriented structure was formed, and with the increase of MDR, the meridional streaks in SAXS patterns appeared accompanying with the shish formation. The kebab and oriented structure signals superimposed together, leading to the irregular equatorial scattering patterns.