An experimental study of sandwich injection molding is reported which involves sequential injection of polymer melts with differing melt viscosity into a mold. In isothermal injection molding the relative viscosity of the two melts is the primary variable determining the phase distribution in the mold. Generally the most uniform skin‐core structure occurs when the second melt entering the mold has a slightly higher viscosity than the first melt injected. Large viscosity inequalities lead to nonuniform skin thicknesses. The influence of blowing agents and non‐uniform temperature fields on the extent of encapsulation is described. Temperature fields are very important especially if the first polymer melt injected has a greater activation energy of viscous flow (or a greater temperature dependence of the viscosity function).

The effects of vitamin E, Irganox 1010, and BHT as antioxidants for reducing the release of off-flavor (both off-odor and off-taste) from blow- molded HDPE bottles were investigated. A sensory study was conducted and showed that bottles containing vitamin E had a lower odor intensity and a higher acceptability for storage of drinking water compared to bottles contain ing Irganox 1010 or BHT. Forty-seven volatile compounds were identified to release from the bottles using the purge-and-trap GC/MS analysis. Those vola tile compounds, ranging from C 6 to C18, belonged to the groups of n-alkane, 1- alkene, aldehyde, ketone, phenolic, olefin, and paraffin; among them aldehydes and ketones were the most important due to their very low odor thresholds. The bottles containing vitamin E were found to release the least amount of alde hydes and ketones compared to bottles containing Irganox 1010 or BHT. The acceptability of drinking water is correlated inversely to the intensity of odor and taste as well as to the aldehyde/ketone concentration. Therefore vitamin E, although used at about 20% the concentration of Irganox 1010 or BHT, is a superior antioxidant for reducing off-odor and off-taste from HDPE bottles.

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The effects of individual components in a vitamin E based antioxidant formulation (CF‐120) on off‐odor release, melt flow property and yellow color property of HDPE resin pellets were investigated using a three‐phase study. The formulation consists of vitamin E, glycerol, polyethylene glycol‐400 (PEG‐400), and glyceryl monocaprylate/caprate (GMC). The resin pellets were extruded from HDPE resin powder using a laboratory scale single screw extruder. Off‐odor release from the resin pellets was evaluated using a sensory panel and GC/MS analysis. Melt flow indices and yellowness indices of the resin pellets were also measured. Screening experiment (Phase I) showed that GMC greatly contributed to off‐odor and MFI reduction; vitamin E greatly contributed to yellow color formation. Factorial experiment (Phase II) confirmed that GMC was the major off‐odor contributor. Response surface experiment (Phase III) showed that yellowness index and odor score reasonably correlated to vitamin E and GMC concentrations; MFI and nonanal concentration did not correlate well with vitamin E and GMC concentrations. The identification of octanoic acid by GC/MS from HDPE resin pellets suggested that possible thermal/mechanical degradation of GMC might occur in extrusion. The results are useful for designing a better vitamin E formulation for reducing the off‐odor, without significantly enhancing yellow color formation.

Deficiencies of present anti-oxidant (AO) systems led to our initiating a series of studies with 'phosphite-free' AO systems based on catocopherol (vitamin E) formulations. This study evaluated the effects of several AO systems on the yellowness index (YI), melt flow rate (MFR) and on gel formation after five to eight passes of two types of high density polyethylene (HDPE) and four types of polypropylene (PP) resins through an extruder. It was demonstrated that the a-tocopherol can replace phenolic/phosphite combination as the primary AO system; or the a-tocopherol formulations can simply replace the phosphite portion to achieve better performance. In either case, the problems associated with phosphites can be eliminated or minimized by the ca-tocopherol AO; and usually at a lower cost.