Morphology And Structure Of High Density Polyethylene (HDPE) Molded By Water-assisted Injection Molding

Water-assisted injection molding(WAIM), which isdeveloped by the conventional injection molding(CIM) and gas-assisted injection molding(GAIM),is an innovative polymer process technologyfor the preparationof hollow parts. The main feature is that the mold cavity is partially or full filled with the polymer melt,following by the injection of pressurized water into the core of polymer melt for assist the cavity to filling and packing. In this thesis, thedifferential scanning calorimetry(DSC),two-dimensional small-angle X-ray scattering(2D SAXS),two-dimensional wide-angle X-ray diffraction(2D WAXD) were used to systematic investigate the WAIM parts. The main results were the following:1) A distinct skin-core-water channel structure was observed across the residual thickness of WAIM moldedhigh density polyethylene(HDPE) part, while the CIM part shows a skin-core structure. In the skin layer, a similar oriented structure(shish kebab) appears for both parts due to the identical shear stress brought by melt filling.Because the oriented structures of the CIM part induced by the melt filling have longer time to relax compared to that of WAIM part, the lamellar orientation parameter of WAIM part is larger than that of CIM part. Due to the low cooling rate and feeble shear strain, the random lamellae are dominant at the core layers of both parts. Furthermore, the shish structure companying the lamellae with low level of orientation emerges in the water channel layer of WAIM part. The shish structure is attributed to the shear stress brought by water penetration, while the low level of lamellae orientation is attributed to the so high cooling capacity of the injected water.Therefore, the lamellar orientation parameter in the water channel layer is smaller than that of the skin layer. In addition, the long period of WAIM part first increases and then decreases with the elevating distance from the skin surface, while that of CIM part tends to increases monotonously. The DSC results show that the crystallinity of the skin layer and water channel layer at different melt temperature is lower than that of the core layer. But the crystallinity has a little increase in the skin layer and steady decline in the water channel layer with increasing melt temperature.The different crystallization behavior could be attributed to the effect of temperature field. At 205°C, the cooling rate in the skin layer is higher than that of the water channel layer; at 235°C, the cooling rate is opposite; at 215°C and 225°C, the cooling rate is nearly equal. In addition, there is a shoulder melting peak in the skin layer,which indicates that there might be another crystalline structure existing, that is, shish kebab structure.2) Compared with the HDPEwith a high molecular weight(HMW), shish kebabstructurewith higher orientation was formed in low molecular weight(LMW)one, especially for water channel layer. The growth of oriented molecular chain is mainly dependent on shear rate, because higher shear rate promotes the higher orientation. Due to the lower viscosity of melt, the larger shear rate was achieved in LMW polymer during WAIM process, which can be used to explain the higher orientation formed in LMW sample. In turn, HMW WAIM sample unexpectedly has the lower orientation.3) Based on banded spherulites appear in the HDPE 2911 and linear low-density polyethylene(LLDPE)7042 parts, we are molded WAIM parts using these two polymers to study the banded spherulites. For WAIM molded HDPE 2911,unexpectly, there are twisted shish kebabs in the skin and water channel zones, and banded spherulites in the core zone. Due to the higher cooling rate and larger shear stress in the skin zone, the lamellae orientation was smaller in the water channel zone,butthe long period and lateral dimension of crystalline lamellaewere larger than that of skin zone.However, for LLDPE parts,the twisted shish kebabs were only found in the skin zone of CIM molded LLDPE, but not appears in the WAIM part. In addition,the representative morphologies of crystal change with gradual increment of instantaneous flow field in crystallization from banded spherulite to normal shish kebab structure and then to shish kebabstructurewith twisted kebab.Thoseresults indicate that the cooling rate is an important factor for twisted lamellae formation. In addition, the result supports the idea that kebabs grow into twisted lamellae at the late stages of crystallization.4) Much more broad distribution of highly oriented shish kebabs was successfully achieved in injection molded HMW HDPE, for the first time, which wasprovided by a novel injection molding technology of WAIM. Our work could provide a promising doorway for this novel technology to achieve high performance products with some desirable physical properties in the light of manipulating the internal morphology and structure by selecting appropriate polymer and adjusting the processing parameters and conditions during the practical molding process.In summary, the rapid cooling rate and shear brought by the water penetration are two principal factors in the oriented structures formation, and water penetration has a dramatic effect on the morphological development through the thickness of the WAIM part.