PP Matrices Research Articles

The Effect of Injection Molding Conditions on the Mechanical Properties of an In Situ Composite

Abstract The effect of processing conditions on the mechanical properties of an injection molded polypropylene/liquid crystalline polymer (PP/LCP) blend was studied. The LCP consisted of a copolymer based on poly(ethylene terephthalate) and poly-(hydroxybenzoic acid). Two PP matrices were used to vary the viscosity ratio of the blend. It was found that the highest flexural moduli were obtained for blends with the PP matrix that provided the highest viscosity ratios (closest to unity). These optimal properties were obtained only at one processing temperature, 250°C. At this temperature, the storage (G′) and loss moduli (G″) of the neat LCP were approximately equivalent. The optimal properties at temperatures where G′ ∼ G″ is in agreement with a prior study of another PP/co-polyester LCP blend [15]. Futhermore, the optimal properties were sensitive to the filling time. This was related to heat transfer via the Graetz number. For thinner molds, higher properties are obtained over a broad range of Graetz numbers (temperatures) while for thicker molds higher properties are obtained only at lower Graetz numbers (temperatures). The dependence of the flexural modulus on the Graetz number was related to the ability to deform the LCP at the advancing front and the ability to freeze the morphology possibly even before cessation of flow.

Phase structure and viscoelastic properties

The phase structure and dynamic mechanical properties of three polypropylene/polystyrene (PP/PS) systems of similar composition but various dispersion of the minor PS component have been examined. Two different PP/PS systems were prepared by polymerization of styrene (ST) molecularly dispersed in PP matrices (with the same initial structure) under the conditions leading to a linear or crosslinked PS component. The third PP/PS system has been prepared blending the homopolymers in the molten state. Studies of materials containingin situ polymerized PS revealed nanoscale phase separation of PS (atomic force microscopy) and pointed to the presence of physical entanglements between PS and non-crystalline phase of PP (DSC, dynamic mechanical analysis). The PS component in material prepared by melt mixing appeared to be completely phase-separated into micron-sized domains. Dynamic mechanical analysis revealed also the dependence of viscoelastic behavior of the PP/PS systems on dispersion of the PS inclusions and on the nature of the interface.

Oriented PP–PS composites obtained by polymerization of styrene inside oriented PP matrices. II. Shrinkage behavior and mechanical properties

AbstractThermal shrinkage and the mechanical properties of oriented polypropylene‐polystyrene (PP‐PS) composites obtained by in situ polymerization of styrene in the oriented PP matrices were examined by the analysis of shrinkage stress curves and dynamic mechanical data. It has been shown that by changing the structure of the oriented matrices it is possible to control the properties of the obtained oriented composites. Some composites reveal an increased stability of the obtained oriented composites. Some composites reveal an increased stability of the molecular network at higher temperatures and the modulus of the material obtained from the matrix drawn at 90°C to the natural draw ratio is significantly increased. It is also shown that the differences between the untreated matrices and the PP component of the respective composites observed in the relaxation of the shrinkage stresses correspond very well with the differences seen in the mechanical loss in the frequency range of about 0.02 Hz. Such behavior suggests that similar molecular motions are involved both in the relaxation of shrinkage stresses and in this part of the mechanical loss. © 1993 John Wiley & Sons, Inc.

Oriented PP‐PS composites obtained by polymerization of styrene inside oriented PP matrices. I. Dispersion of PS and the composites' orientation

AbstractDispersion of polystyrene (PS) and the molecular orientation of the polypropylene–polystyrene (PP–PS) composites acquired by the in situ polymerization of styrene in the oriented PP matrices were characterized by microscopic observations, birefringence measurements, and differential scanning calorimetry. It has been shown that applied processing of the “envelope” polymerization leads to obtaining oriented composites with large contents and the specific dispersion of PS. Some composites exhibit a substantial gradient of PS concentration and a gradient of molecular orientation, both strongly dependent on the initial structure of the oriented PP matrix. The modification of the PP structure taking place during processing does not disturb the orientation of the matrices deformed to the higher draw ratios. The structural changes of the oriented PP induced by processing are reflected in the shrinkage behavior and mechanical properties discussed in the second part of this paper. © 1993 John Wiley & Sons, Inc.

Tensile properties of injection molded long fiber thermoplastic composites

AbstractThis paper deals with the mechanical performances of a new class of injection molded long fiber composites based on PP and PBT matrices. Effects of material parameters such as fiber concentration, breakage, orientation, and matrix composition are analyzed. The critical fiber length, l, of the PP long fiber composite, evaluated from the pull‐out length of the tensile fracture surface, was found to be much higher than those previously reported. Tensile strength calculated from the measured ll and fiber length distribution in the molded samples was found to be in agreement with the measured values. From this work it is concluded that higher mechanical performances of the long fiber reinforced thermoplastics will be attained by the injection molding process to further reduce fiber breakage.