Crystal orientation in injection molding of talc‐filled polypropylene

Abstract

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...

Optimization of the crystallinity of polypropylene/submicronic-talc composites: The role of filler ratio and cooling rate

“…Only the main reflections of the monoclinic "-form of iPP are observed for both the pure sample and the composite. This is consistent with previous reports that talc is an "-nucleating agent for iPP [19][20][21][22][23][24][25][26][27]. The 3 intensity profiles of the pure iPP display a great similarity relevant of a structural isotropy ( Figure 5).…”

“…Indeed, the (002) planes characteristic of the silicate sheets of talc stand along the larger dimensions of the talc platelets. The combination of this crystallographic characteristic of the talc platelets and their nucleating effect results in an orientation of the iPP crystals nearly normal to the talc platelets [23][24][25][26][27]. This concomitance of orientation of the chains and the talc platelets is a common feature of injectionmolded composites reinforced with filler of high aspect ratio which tend to align along the flow direction [27,33,[55][56][57].…”

“…In contrast the strongly different profiles along for the iPP matrix in the iPP/7% µ-talc composite are indicative of a strong crystalline texturing due to both the injection-molding and nucleating effect ( Figure 6). Particularly, the very weak intensity of the (040) reflection of iPP in the WAXS profile through the thickness is a clear indication that the iPP chains are lying in the sample plane [23][24][25][26][27]. Moreover, the total absence of the (002) reflection of talc on the WAXS profile through the sample thickness ( Figure 6) reveals that the talc platelets are also lying within the sample plane.…”

“…Among mineral fillers, talc proved to have a good potential of development for iPP-based composites, noticeably in the automotive industry [13][14][15][16][17]. It appeared very soon that talc displayed a strong nucleating effect for iPP [14,[19][20][21][22][23] due to specific physico-chemical interactions between the filler and the polymer matrix that promote epitaxial crystallization [14,[24][25][26][27]. This effect was significantly more pronounced than for other common fillers such as CaCO 3 or kaolin and therefore give rise to better improvement of mechanical properties [14,18].…”

Polymer micro and nanocomposites: Structure, interactions, properties

“…Automatic image analysis greatly facilitates quantitative characterization. Although the same techniques can be used also for particulate fillers [116], their planar geometry and the optical characteristics of the composites make the determination difficult. The average orientation of particles relative to the direction of the external load significantly influences properties.…”