The Gloss of Injection-Moulded Recovered Polypropylene

Abstract

In our recent work, it was shown that the rate of filling during injection moulding did not significantly affect the tensile and impact properties of recovered and virgin polypropylene, whereas significant changes in the properties were observed after ageing at 100 °C. The purpose of the present work was to extend the former study to include the effects of processing and ageing on surface-appearance-related properties, in particular the gloss. In-mould rheology and gate-seal analyses were performed in order to select the filling and post-filling processing windows, respectively. The influences of filling rate, holding and cooling times on the gloss measured in the gate area and at the far end of the moulded specimens were evaluated. The mould used had an embossed surface giving test plaques with a matte appearance. The results showed that the gloss varied by about 30 to 40% within the filling window obtained by in-mould rheology. The slight gloss difference between the two PP-grades noted at the lowest injection speed vanished with increasing filling rate. Gloss differences were observed between the gate area and the far end of each processed moulding, again at the lowest injection speed. The materials processed at a high filling rate exhibited a fairly constant gloss as the holding time increased regardless of the position of the analysed region, whereas a more significant decrease was noted in the case of the mouldings processed at the low filling rate. The cooling time had a negligible effect on the gloss in comparison to the effect of filling rate. Ageing at 100 °C for 240 h increased the gloss level, especially at the far end of the moulding, but only for specimens manufactured at the lowest filling rate. Roughness measurements with a profilometer showed that the gloss development during processing and ageing was related to the topographical matching between the developing polymer surface and the mould topography.

Effects of injection-molding conditions on the gloss and color of pigmented polypropylene

“…A better replication is achieved with a lower melt viscosity at higher shear rates (higher injection speeds or lower injection times) and higher melt and mold temperatures. This gives a higher gloss in smooth surface regions and a lower gloss in textured regions F and C. This is in agreement with the results reported by Edwards and Choudhury [17] for "polished" surfaces, as well as with those reported by Pisciotti et al [18] for visibly rough surfaces. An increase in the holding pressure has a similar effect, i.e., it improves the ability to replicate the mold surface.…”

“…A positive correlation between filling rate and gloss (negative correlation with the injection time) was reported for the rubber-modified thermoplastics in Refs. 15-17, whereas a negative correlation was recently observed for virgin and recovered polypropylene [18]. It should be pointed out that in most of these works reviewed, the topography of the mold surface or that of the moldings was not reported making difficult to judge the general validity of the results.…”

“…The effect of the mould temperature was typically much higher than that of the melt temperature, the effect of the latter being negligible for acrylonitrile-butadiene-styrene (ABS) [16]. The gloss was reported to increase with increasing flow length for ABS [15] and for virgin and recovered polypropylene [18], although other studies showed the negative correlation between gloss and flow length for some rubber-modified thermoplastics [17]. An increasing holding time leveled off the gloss for ABS after an initial increase [16], whereas an increasing holding/packing pressure typically increased the gloss for the same rubber-modified thermoplastics discussed so far [15][16][17].…”

“…An increasing holding time leveled off the gloss for ABS after an initial increase [16], whereas an increasing holding/packing pressure typically increased the gloss for the same rubber-modified thermoplastics discussed so far [15][16][17]. All the studies examined point to that the cooling time has a negligible effect on the gloss [15,16,18]. A positive correlation between filling rate and gloss (negative correlation with the injection time) was reported for the rubber-modified thermoplastics in Refs.…”

“…During the filling stage, in-mold rheology [25,26] was used to obtain the filling window of PP-1 and PP-2 with regard to the mold cavity used with the processing conditions given in Table 2. Figure 3 shows the influence of the injection speed on the apparent shot resistance to fill (ASRF) of molten PP-1 and PP-2 required to fill 95% of the mold cavity volume (squares).…”