Material PropertiesOptimization of tensile properties of injection molded α-nucleated polypropylene using response surface methodology

- α-nucleating agent HPN-20E was added in iPP using co-rotating twin screw extruder.
- All experiments were designed according to response surface method (RSM).
- Tensile samples were prepared using micro-injection molding machine.
- In the presence of α-nucleating agent, crystallinity and crystal size of isotactic polypropylene was improved significantly.

Tensile properties are among the significant properties of isotactic polypropylene (iPP). The mechanical properties including the tensile properties are fairly dependent on the overall crystallinity and crystallite size and their distribution in molded product, type of crystal structure and testing conditions. In presence of α-nucleating agents, the crystallization rate and onset temperature of isotactic PP increase. In this paper, the role of externally added commercial α-nucleating agent HPN-20E (Milliken Inc.) on tensile properties was investigated with respect to tensile properties of pure iPP. The experimental part includes the use of design of experiment (DoE) - response surface methodology (RSM) with central composite design (CCD) having three factors namely mold temperature, melt temperature and injection rate. Two levels of each factor with six centre points and five numbers of replicates were selected. The nucleating agent, HPN-20E, was added 1.0% by wt. in iPP (mfr 11.0 g/10min) using a lab scale co-rotating twin screw extruder. The compounded pellets were dried at 85 °C in a circulating hot air oven for 24 h. The tensile samples (ASTM-638D, type-I) were molded on a micro-injection molding machine (make BabyPlast, Italy). The samples were tested for tensile properties on a universal testing machine (make Lloyds, USA). The measured responses were tensile strength (MPa), Young's modulus (MPa) and work to break (N.mm). The same experimental procedure was also followed for pure iPP and same responses were measured to set the baseline of experiment. The analysis of variance (ANOVA) tests unearth that mold and melt temperatures are highly interacting in nature. That is why previous attempts based on traditional way of varying one parameter at a time were not so successful to relate tensile properties with injection molding variables. The RSM tests resulted into useful quantitative relationship between the tensile properties and injection molding process variables.

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