Mechanical properties and foaming behavior of injection molded cellulose fiber reinforced polypropylene composite foams

This paper investigates the effects of cellulose content and processing conditions on the mechanical properties and foaming behavior of injection molded cellulose fiber reinforced polypropylene composite foams. Composite foams were injection molded using an advanced structural foam molding machine with N2 as a physical blowing agent. Foamed specimens were prepared at different injection speeds and void fractions. The mechanical properties and foam morphologies of the specimens were evaluated. The results suggested that the specific flexural modulus was increased and the specific flexural strength and Izod impact strength were maintained by foaming, and that the strength, modulus, and notched Izod impact strength increased with the increase of cellulose content. Additionally, the results suggested that the cell morphology of the composite foams was improved by the increase of void fraction and the addition of cellulose fiber.

Processing and characterization of solid and foamed injection-molded polylactide with talc

Polylactide (PLA) suffers from poor processability due to its low melt strength and slow crystallization kinetics and it is thus very challenging to achieve uniformly distributed fine-celled PLA foams with high void fractions in injection molding process. In this work, the low-pressure structural foam molding of linear PLA with a relatively high void fraction of ∼30% was conducted and by fine tuning the talc content and the foaming and processing parameters, a relatively uniform fine-celled structure with improved cell size and cell density was successfully produced. The effects of twin-screw compounding and the addition of talc on the foaming behavior, structural uniformity, crystallinity, and mechanical properties of the solid and foamed PLA samples were investigated. The results showed that the addition of 5 wt.% talc significantly improved the foaming properties such as cell density, cell size, structural uniformity, and consequently improved the mechanical properties of foams. The twin-screw compounding before injection molding did not significantly change the foaming behavior, but adversely affected the mechanical properties of the solid and foamed PLA samples due to mechanical and thermal degradation. The changes in the mechanical properties were discussed in terms of the crystallinity, talc toughening effect, and foam quality.

A Design-of-Experiment Study on the Microcellular Extrusion of Sub-critical CO2 Saturated PLA Pellets

In this study we explore the use of solid-state nucleation in pellets as a means to decouple cell nucleation from cell growth in extrusion. This is achieved by feeding the extruder with gas-saturated PLA pellets. Cell nucleation occurs, similar to that of the well-studied solid-state batch foaming process, when gas-saturated pellets are crushed and heated in the feed section of the extruder. The polymer melt consequently contains the nucleated cells. Further, the gas content in the pellets provides a means to deliver a controlled amount of blowing agent for cell growth at the die exit. It was found that gas concentrations in partially desorbed pellets offers greater control of part density and foam morphology compared to that of fully saturated pellets at 2 MPa. Using Design-of-Experiment statistical methods it was found that gas concentration in the polymer was the most significant variable influencing foam density, whereas the extrudate cooling method was found to control cell size.

Cell morphology of extrusion foamed poly(lactic acid) using endothermic chemical foaming agent

Poly(lactic acid) (PLA) was foamed with an endothermic chemical foaming agent (CFA) through an extrusion process. The effects of polymer melt flow index, CFA content, and processing speed on the cellular structures, void fraction, and cell-population density of foamed PLA were investigated. The apparent melt viscosity of PLA was measured to understand the effect of melt index on the cell morphology of foamed PLA samples. The void fraction was strongly dependent on the PLA melt index. It increased with increasing melt index, reaching a maximum value, after which it decreased. Melt index showed no significant effect on the cell-population density of foamed samples within the narrow range studied. A gas containment limit was observed in PLA foamed with CFA. Both the void fraction and cell-population density increased with an initial increase in CFA content, reached a maximum value, and then decreased as CFA content continued to increase. The processing speed also affected the morphology of PLA foams. The void fraction reached a maximum value as the extruder's screw speed increased to 40 rpm and a further increase in the processing speed tended to reduce the void fraction of foamed samples. By contrast, cell-population density increased one order of magnitude by increasing the screw speed from 20 to 120 rpm. The experimental results indicate that a homogeneous and finer cellular morphology could be successfully achieved in PLA foamed in an extrusion process with a proper combination of polymer melt flow index, CFA content, and processing speed.