J. Kubát

The Efficiency of Cellulosic Fillers in Common Thermoplastics. Part II. Filling with Processing Aids and Coupling Agents

Abstract This paper reports on the effect of additives on the properties of injection moulded composites based on polypropylene (PP) and high density polyethylene (HDPE) filled with wood and cellulose flour. Three types of additives were studied: dispersion aids, elastomeric additives, and adhesion promoting agents. Some of the dispersion aids were found to improve the filler dispersion and the impact strength. The modulus remained relatively unaffected while the strength was lowered. The high molecular weight elastomeric additives increased the impact strength and ductility, while the stiffness was reduced. An adhesion promotor based on a maleic anhydride modified polypropylene was found to behave as a true coupling agent, i.e. the strength and ductility increased, the melt strength was improved, and also the hot water resistance was raised. The experimentally determined composite moduli are found to agree relatively well with theoretical predictions. The effect of processing on the shape and size of the...

Prehydrolyzed Cellulose as Reinforcing Filler for Thermoplastics

Abstract This paper presents the results of an experimental study of the mechanical properties of composites consisting of prehydrolyzed cellulose and thermoplastic matrices. The main feature of prehydrolyzed cellulose fibres is a high degree of brittleness, permitting the fibres to be finely comminuted in the shear field of normal compounding and processing machinery. Such an effect can be anticipated to improve the homogeneity and the mechanical parameters of the moulded samples. In the present work, this has been demonstrated with PP, HDPE and PS containing varying amounts of prehydrolyzed cellulose of different origin (bleached pulps). The tests were done on injection moulded samples. The compounding method (Buss-Kneader vs. twin-screw extruder) had only minor influence on the results. Theoretical assessment of the modulus values using the Tsai–Halpin equation gave somewhat lower values than those recorded on experimental samples. This was interpreted in terms of the disintegration of the cellulose co...

Fibre degradation during processing of short fibre reinforced thermoplastics

Abstract Measurements of the fibre degradation process in reinforced thermoplastics subjected to various compounding methods prior to injection moulding are reported. The bulk of the degradation process appeared to occur within a relatively short period after introducing the blend into the compounding extruder. This was true of both feeding methods used, dry-blend feeding into the hopper, and feeding the fibres into the melt at some distance after the hopper. Additional fibre damage was noted in the exit orifice of the compounders and also in the injection moulding step. The value of the moment ratio Lw/Ln of the length distribution appears to support the notion of the fibres being broken in the middle. The materials used were based on carbon, glass, aluminium and brass fibres. The metal fibres suffered more severe degradation, probably due to grinding effects in the screw/barrel clearance.

Activation volumes for flow processes in solids

Abstract Available literature data on creep and stress relaxation in solids of varying structure and composition have been analyzed in terms of an activation volume. In all cases it was found that the activation volume, v, was related to the effective stress σ ∗ (i.e. the difference between the applied and internal stress) through the relation νσ ∗ = ακT , where α is a constant, the value of which is between 6 and 12 depending on the type of flow and the material under study; κ is the Boltzmann constant and T the absolute temperature. This relation is a substantial sharpening and generalisation of the inverse trend between ν and σ as reported by Balasubramanian and Li for secondary creep. A remarkably constant value of α, i.e. 10 ± 1, was found for the exponential law-region of stress relaxation curves obtained with both metallic and polymeric solids.

Electrical properties of polyaniline suspensions

Abstract The ordering of electrically conducting polyaniline particles dispersed in a non-conducting liquid, 1,2,4-trichlorobenzene, in a weak electric field (of the order V mm −1 ) has been studied. The time needed for the formation of conducting chains, depending on the applied voltage, the polyaniline concentration and the viscosity of the suspension medium, was used to calculate the effective relative electric permittivity for polyaniline, ɛ p = 4.8, responsible for the dipole-dipole interaction leading to the structure formation. Nonlinear limit current-voltage characteristics typical of electrorheological suspensions were observed. The formation of the conducting chains could be followed in an optical microscope.

Internal stresses in polyethylene as related to its structure

Abstract When plotting the slope of a relaxation curve (stress versus log time) against the stress, curves are obtained intersecting the stress axis at a stress value which appears to be associated with the internal stress level of the sample. Another method for determining the internal stresses is based on plots of the maximum slope of the relaxation curves against initial stress. The intersection between the straight line obtained and the stress axis determines in this case the internal stress level. The two methods are shown to give similar results when applied to cold-drawn polyethylene samples, the internal stress level amounting to about 10% of the breaking stress. The internal stresses disappear on annealing; they appear to be associated with the occurrence of stretched tie molecules between the crystal lamellae. The longitudinal shrinkage of the cold-drawn samples is shown to be proportional to the corresponding internal stress level.

Creep and stress relaxation in a longitudinal polymer liquid crystal: Prediction of the temperature shift factor

The polymer liquid crystal PLC is the PET/0.6PHB copolymer; PET=poly(ethylene terephthalate), PHB=ρ-hydroxybenzoic acid (LC): 0.6=the mole fraction of PHB. This is a multiphase system with PHB-rich islands in a PET-rich matrix. Tensile creep compliance was measured isothermally from 20 °C to 160 °C in 10 °C intervals. Master curves were determined using the time–temperature superposition for 20 °C and for the glass transition temperature of the PET-rich phase TgPET=62 °C. Experimental values of the temperature shift factor aT as a function of temperature T agree in the entire T range with those from Eq. (7) relating aT to the reduced volume ṽ and the Hartmann equation of state Eq. (10). Values of aT(T) calculated from the Williams–Landel–Ferry (WLF) formula give very large errors below Tg. A control 14 months creep experiment agrees with the theoretical predictions from Eq. (7). Stress relaxation experiments were performed under the constant strain of 0.5% from 20 °C to 120 °C, again master curves were de...

Influence of high injection pressures on the internal stress level in injection moulded specimens

Abstract Internal stresses in high and low density polyethylene specimens, injection moulded at pressures ranging from 100 MPa to 450 MPa have been measured by a stress relaxation method. The internal stress parameter ( σ i ), which is an average value of the internal stress distribution in the samples, changes from a negative value (compressive stresses) at normal injection pressures to a small positive value (frozen-in tensile stresses) at the highest pressures used. The yield stress increases in approximately the same way with the pressure, while the mould shrinkage in the flow direction decreases. It is suggested that the decrease in the absolute value of the internal stress parameter originates from an increase of the melting temperature with pressure, resulting in a more homogeneous solidification.

Influence of high injection moulding pressures on the engineering properties of linear polyethylene

Abstract Two grades of high density polyethylene, one injection moulding grade and another with a substantially higher molecular weight (melt index 0.1 g 10 min ) were injection moulded at pressures ranging from 100 to 500 MPa using a modified conventional injection moulding machine. For the high molecular weight grade, improvements were observed in the elastic modulus, the tensile strength at rupture measured in the flow direction, and the unnotched impact strength. These improvements were accompanied by a second high temperature (137°C) melting peak in d.s.c. diagrams. For both grades it was also found that the mould shrinkage decreased and the crystallinity increased with injection pressure.

The Assessment of Internal Stresses in Plastics by a Stress Relaxation Method

Abstract When plotting the slope of stress relaxation curves (stress vs. log time) against the initial stress, straight lines are obtained intersecting the axis at a stress value which appears to be associated with the internal stress level of the sample. For injection molded samples, the internal stresses are negative, i.e. frozen-in compressive stresses. They disappear on annealing. Both tensile and compressive stresses can be introduced into stress-free samples by suitable thermal treatment when in a state of compression or tensile strain.