SynopsisThe structure of skin layer in injection-molded polypropylene which displayed a clear two-phase structure of skin and core has been studied by means of wide-angle x-ray diffraction, small-angle x-ray scattering, melting behavior, density, dynamic viscoelasticity, and tensile test. In skin layer, the c-axis and a*-axis were hig+ly oriented to the machine direction (MD), and the plane of the lamellar structure of about 160 A in thickness was in normal to MD. The density was about 0.907 g/cm:', which was nearly the same as that of core layer. Although the majority of crystallites melted in the same temperature range as in that of the core layer, there was about 5.3% higher temperature melting structure (T, = 182°C). The dynamic tensile modulus E' in MD decreased more slowly with increasing temperature than that of the core layer and held high modulus in the range of ca. 30"C, just above the temperature at which E' of the core layer suddenly dropped. E' in MD was higher than that in TD in the temperature range below 33"C, which was slightly higher than the primary absorption temperature, and the order reversed above 33°C. The tensile yield stress in MD was 1.5 times higher than that of the core layer. The skin layer in MD ruptured just after yielding and did not show necking. The tensile yield stress in T D was about half of that in MD and about 0.7 times that of the core layer. The necking stress in T D was about 0.6 times that of the core layer. In general, a polypropylene melt crystallizes under a high shear stress in injection molding. From these facts, it was concluded that the skin layer is composed of so-called "shishkebab"-like main skeleton structures, whose axis is parallel to MD, piled epitaxially with a*-axis-oriented imperfect lamellar substructure.

SYNOPSISFlexural test specimens were injection-molded from polypropylenes added with 0.5 wt % of calcium carbonate, talc, p-tert-dibutyl-benzoic acid monohydroxy aluminum, or p -dimethyl-benzylidene sorbitol under cylinder temperatures of 200-320°C. Properties such as flexural modulus, flexural strength, heat distortion temperature, Izod impact strength, hardness, and mold shrinkage and higher-order structures such as crystalline texture, crystallinity, a*-axis-oriented component fraction, and degree of crystalline orientation were measured and structure-property relationships were studied. By the addition of crystallization nucleators, the flexural modulus, flexural strength, heat distortion temperature, hardness, and mold shrinkage were increased and Izod impact strength was decreased. The degrees of crystalline orientation such as the orientation fraction OF and c-axis orientation function fe were increased by the addition of nucleators. The degree of the increase was higher as the crystallization temperature was higher. Close relationships were observed between some properties and the degrees of crystalline orientation. I NTRODU CTlO NWhen crystallization nucleators are added to crystalline polymers, the following effects are exp e~t e d l -~: ( i ) improvement of transparency of molded articles, (ii) improvement of processing cycle or spinnability, (iii) improvement of rigidity of molded articles or filaments, (iv) improvement of dimensional precision, ( v ) improvement of gloss, and (vi) prevention of sink. As crystallization nucleators for polypropylene, talc, 4-6 aluminum salts of benzoic acids, 69 and dibenzylidene sorbitols 39697s0-l2 are especially effective and studies have been concentrated on them. This paper is concerned with studies on structures and properties and structure-property relationships of injection moldings of crystallization nucleatoradded polypropylenes. Polypropylene was fixed and effects of addition of talc, p-tert-dibutyl benzoic acid monohydroxy aluminum, or p -dimethylbenzylidene sorbitol, which are widely and frequently used and of calcium carbonate, which shows almost no nucleating effect and hence was used for comparison were studied. Nucleator content was fixed at 0.5 wt %. Flexural test specimens were injection-molded by varying cylinder temperature at 200-320°C.Properties such as flexural modulus, flexural strength, heat distortion temperature, Izod impact strength, hardness, and mold shrinkage and higherorder structures such as crystalline texture, crystallinity, a*-axis-oriented component fraction, and degrees of crystalline orientation were measured and relationships between flexural modulus, flexural strength, and mold shrinkage and degrees of crystalline orientation were studied.

SYNOPSISThe crystal orientation in injection molding of talc-filled polypropylene has been studied by means of polarizing microscopy and X-ray diffraction measurements. The plate planes of talc particles are aligned parallel to the surface of injection molding. The c-and a*-axes of polypropylene crystals are bimodally oriented to the flow direction, and the b-axes are oriented to the thickness direction. Namely, the crystals in the injection molding of talcfilled polypropylene show a kind of uniplanar-axial orientation according to the classification of Heffelfinger and Burton. The degree of the bimodal orientation of c-and a*-axes decreases toward the interior of the injection molding. The b-axis orientation is strong throughout the thickness direction, although it is a little weaker at the surface skin and central regions. This peculiar crystal orientation originates from the flaky shape and crystallization nucleation ability of talc particles. In a mold cavity, talc particles are aligned parallel to the cavity wall by the action of flow, and crystallization progresses, with the (040) plane of polypropylene crystal piling on the plate planes of talc particles and the b-axis, which is the growth direction of polypropylene crystal, being aligned to the thickness and cooling direction of injection molding. The crystal orientation in injection molding of unfilled polypropylene is basically similar to that of talc-filled polypropylene, although the b-axis orientation to the thickness direction is much weaker. INTRODUCTIONIn injection molding of plastics, since a molten resin flows into a cold mold cavity at high speed, where it is solidified, orientation of molecular chains occurs, significantly affecting product properties such as mechanical and optical properties. In the case of injection molding of semicrystalline polymers, since the molecular chains are crystallized under a stress, orientation of crystallites occurs. Several studies have so far been done on the orientation state of crystals in injection-molded semicrystalline polymers.l-12Clark2 found from X-ray diffraction measurement that the skin layer in injection-molded polypropylene has a mixed c-axis and a*-axis orientation, which he called bimodal; he explained its formation * To whom correspondence should be addressed.Journal of Applied Polymer Science, Vol. 42,9-20 (1991) by assuming that the c -axis-oriented component was formed first, and that the a*-axis component was formed by epitaxial overgrowth on the c -axis-oriented substrate. Kantz et a12 made X-ray diffraction measurements on the skin layer in injection-molded polypropylene from various directions and found that crystallites are biaxially oriented to MD and that the tensile yield strength and mold shrinkage in MD were higher as the skin layer was thicker.Mencik and Fitchmun5 found, from polarizing microscopy and X-ray diffraction measurement, that injection-molded polypropylene is composed of five layers, whose thickness changed according to molding conditions. They conjectured that the f...

SYNOPSISFlexural test specimens were injection-molded from six homoisotactic polypropylenes with MFI = 0.49-25.1 dg/min under cylinder temperatures of 200-320°C. Distributions in the flow direction of higher-order structures such as crystallinity X , , thickness of skin layer, a*-axis-oriented component fraction [A* 1, and crystalline orientation functions and distributions in the thickness direction of higher-order structures such as X,, P-crystal contents, [A* 1, and crystalline orientation functions were studied. These higher-order structures are inhomogeneous in the flow and thickness directions, which strongly influences the product properties such as mechanical and thermal properties. Molecular orientation process in injection molding was theoretically analyzed from a viewpoint of growth of recoverable shear strain at the gate and its relaxation in the cavity, which could considerably well explain the variations in the flow and thickness directions of the quantities such as thickness of the skin layer and crystalline orientation functions which express the degree of molecular orientation. INTRODUCTIONIn injection molding of thermoplastics, since molten resin solidifies under inhomogeneous stress and cooling conditions, the inner structures of the molded article are inhomogeneous, influencing the product properties. Consequently, it is important in injection molding technology of thermoplastics to clarify the influences of the primary structures of resin and molding conditions on the inhomogeneous structure of the molded article.Many studies have so far been carried out on the distribution of higher-order structures in injectionmolded thermoplastics. Table I summarizes previous studies on distribution of higher-order structures in injection-molded polypropylenes. The shapes of moldings studied are mainly simple shape test specimens. The influences of resin characteristics such as molecular weight, molecular weight distribution, glass fiber content, and talc content and molding conditions such as cavity thickness, cylinder temperature ( resin temperature ) , mold temperature, injection speed, injection pressure, holding pressure, and cooling time are studied. Higher-order structures studied are crystalline texture (morphology), crystallinity, 6-crystal content, crystal orientation state, degrees of crystalline and amorphous orientations, thickness of skin layer, glass fiber concentration, and orientation of talc. Directions studied are flow direction (MD) and thickness direction (ND).In the present paper, we will report studies on the influences of molecular weight and cylinder temperature on distributions in the flow direction of higher-order structures such as crystallinity X , , thickness of skin layer, a*-axis-oriented component fraction [A* 1, and crystalline orientation functions and distributions in the thickness direction of higher-order structures such as X,, @-crystal content, [ A* ], and crystalline orientation functions in injection-molded polypropylenes. In addition, a theoretical analysis of mol...

SynopsisBy use of a mod with a film gate, two straight polypropylenes (PP) with different melt flow index (MFZ) and a glass fiber-reinforced polypropylene (FRPP) were injection molded at various temperatures into square plates with orientational anisotropy. The anisotropies of tensile property, tensile impact strength, and flexural property were studied on the molded sample cut mainly in the machine direction (MD), 45"-direction (45"), and transverse direction (TD). Both the orders of the yield strength and tensile impact strength of the FRPP, and those of the necking stress and tensile impact strength of the straight PP, were MD >45" >TD, which are reasonable tendencies. The orders of the yield strength and flexural modulus of the straight PP were MD > TD > 45O, which suggests the presence of shear deformation between the lamellae in the skin layer. The variation of the flexural modulus with the angle to the MD fitted well to Hearmon's equation. Generally, for straight PP, the anisotropy of various properties increased as the MFI and cylinder temperature became lower, or as the skin layer became thicker. For the FRPP, the anisotropy increased as the cylinder temperature became higher, or as the degree of the orientation of glass fibers became higher.