The physical properties of polyethylene terephthalate are such that this material lends itself well to the injection stretch blow molding process.
In contrast, the physical properties of polyethylene are considered much less well-suited for injection stretch blow molding. JP-A-2000/086722, published on Mar. 28, 2000, discloses a high density polyethylene resin which is subjected to injection stretch blow molding. The polyethylene resin has a melt flow index of from 1 to 15 g/10 minutes; a flow ratio of from 10 to 14.5; and a density of from 0.961 to 0.973 g/cm3.
Plastic parts stress crack when they are under tensile stress and in contact with liquids either by an oxidative or lubrication mechanism. This process is known as Environmental Stress Cracking. The oxidative mechanism (i.e., cleavage of polymer molecules) is present in liquids that contain oxidants (e.g. hypochlorite, and hydrogen peroxide) and the lubrication mechanism is present in liquids that contain surfactants. With either mechanism, the number density and length of the tie molecules (i.e., molecules that connect the various crystallites) is the controlling parameter for the environmental stress crack resistance (ESCR). Note that stress cracking occurs only in tensile loading, not compressive. In other words, in a bottle, stress cracking can occur only in the regions that are under tensile deformation and in contact with the liquid. As the polyethylene material is under tensile load in the trade or in the lab, the crystallites are under stress and they start moving away from each other as the tie molecules are stretched. In the brittle mode of failure, the tie molecules completely pull away from the crystallites causing them to separate. In the ductile failure, the tie molecules cause the crystallites to fracture and create fibrils. The oxidants in the liquid (e.g. bleach) cleave the tie molecules causing earlier failure than when the material is exposed to water or air. Furthermore, surfactants in the liquid lubricate the disentanglement of the tie molecules and their separation from the crystallites. In terms of measurable properties, ESCR increases with the average molecular weight (as the number of tie molecules increases with molecular weight), and decreases with increasing breadth of the molecular weight distribution, crystallinity and spherulitic size.
Limitations including manufacturing cost and manufacturing speed, and environmental stress crack resistance properties have prevented injection stretch blow molded polyethylene bottles from achieving commercial success. The present invention aims to overcome the limitations of the prior art.