Rotational molding polyethylene

Table 5.3 Properties of Rotational-Molding Polyethylene Resins...

As a result of this extensive experimental program, it has been found that in rotational molding, polyethylene and polypropylene polymers show different degradation behavior. While in polypropylene the thermo-oxidative degradation causes mainly chain scission, in polyethylene crosslinking dominates. The use of increased amounts of antioxidant in the... 

Rotational Molding Polyethylene Processing Guide, Nova Chemicals, Alberta, Canada, 1999. 

TABLE 3-80. EFFECT OF 1/B in. GLASS CONTENT ON VAHIDUS ROTATIONAL MOLDED POLYETHYLENE PROPERTIES... 

For powders, the low-shear melt viscosity must be low enough to allow the material both to wet the internal surface of the mold and to fuse into itself in a homogeneous mass. The common measure used as a guide to the melt flow properties of materials is the Melt Index (MI) measured in g/10 min. Most commercially available rotational molding polyethylenes have an MI between 2 g/10 min and 8 g/10 min (2.16 kg/190°C). Higher flow rates may be required for parts with complex molding details. 

The good impact resistance of rotationally molded polyethylene is exploited in chemical shipment containers, commercial trash cans, and dumpster lids. Rotational molding is used extensively to manufacture playground equipment and garden furniture, where toughness is also at a premium. Aquatic applications include kayaks, buoys, and the hulls of small boats, any of which may be cross-linked for additional impact resistance. 

Rotational Molding. Hodow articles and large, complex shapes are made by rotational mol ding, usuady from polyethylene powder of relatively low viscosity (57—59). The resin is in the form of a fine powder. A measured quantity is placed inside an aluminum mold and the mold is heated in an oven and rotated at low speed. The resin sinters and fuses, coating the inside of the mold. The mold is then cooled by water spray and the part solidifies, dupHcating the inside of the mold. 

Polyethylene is, by far, the most commonly used resin in rotational molding processes. Low density, high density, and linear low density polyethylene are all used extensively. Additionally, crosslinked polyethylene is used for parts that require high chemical or heat resistance or enhanced impact resistance. 

Rotational molding, of linear low density polyethylene, 20 200. See also Rotomolding... 

Brookfield, Ct., SPE, 2002, Paper 192, Session M38-Rotational Molding Materials, pp.5, CD-ROM, 012 INVESTIGATION OF THE PROCESSING CHARACTERISTICS AND MECHANICAL PROPERTIES OF METALLOCENE POLYETHYLENE FOAMS FOR ROTATIONAL MOULDING... 

Orlando, El., 7th-11th May, 2000, paper 268 ROTATIONAL MOLDING OF POLYETHYLENE FOAM BY PELLETS... 

Focusing collectors are usually cast acrylic Fresnel lenses, or mirrors of aluminized polyester film in frames of aluminum. These reflectors are either enclosed in a bubble of poly(vinyl fluoride) film, or under polycarbonate glazing, which may be covered with a fluorocarbon film to reduce the reflectivity. The absorbers for active systems are copper or aluminum since the temperatures are too high (325—370°C) for plastics. The frames, however, can be molded ABS, high density polyethylene or polyurethane, either solid or structural foam. Polybutylene or chlorinated PVC can be used for piping hot water, and tanks can be made of either reinforced polyester or blow- or rotational-molded, high density polyethylene (12—15). 

Rotational molding is used to make hollow objects. In rotational molding, a carefully measured amount of powdered polymer, typically polyethylene, is placed in a mold. The mold is then closed and placed in an oven where the mold turns about two axes as the polymer melts, as depicted in Fig. 3.73. 

Figure 3.74 presents the air temperature inside the mold in a typical rotational molding cycle for polyethylene powders [7],... 

Fig. 8 Effect of the particle packing arrangement on the relative density of a rotational molding grade polyethylene with heating...

The solidification of the polymer melt in rotational molding is relatively slow, in comparison to other processes, and is estimated to be in the range of 10-30°C/min. Moreover, the melt solidification is gradual and nonuniform across the molded part thickness, leading to important variations in the morphological features, as illustrated in Fig. 9, and dictating the properties and overall performance of the final product. The effects are more dramatic for resins with slower crystallization rates, such as polypropylene, compared to that observed with polyethylene. 

Throne, J.L. Sohn, M.-S. Characterization of rotational molding grade polyethylene powders. Adv. Polym. Technol. 1989, 9 (3), 181-192. 

McDaid, J. Crawford, R.J. The grinding of polyethylene powders for use in rotational molding. Proceedings of the Society of Plastics Engineers Annual Technical Conference, Atlanta, GA, Apr 26-30, 1998 The Society of Plastics Engineers Brookfield, 1998. 

Kontopoulou, M. Bisaria, M. Vlachopoulos, J. An experimental study of rotational molding of polypropylene/polyethylene copolymer. Int. Polym. Process. 1997,12, 165-173. 

Olinek, J. Anand, C. Bellehumeur, C.T. Experimental study on the flow of polyethylene powder particles in rotational molding. Polym. Eng. Sci. 2005, 45 (1), 62-73. 

Fillers are not used to any extent in products made by the rotational molding process. Rotational molding is dominated by polyethylene (close to 90% volume) to which even small addition of pigments or fillers (less than 2 wt%) causes a decrease in tensile and impact properties of the products manufactured in this process. Polyethylene is vulnerable to environmental stress cracking which is made worse if fillers are present. 

Polyolefin - Polyolefins are a large class of carbon-chain elastomeric and thermoplastic polymers usually prepared by addition (co)polymerization of olefins or alkenes such as ethylene. The most important representatives of this class are polyethylene and polypropylene. There are branched and linear polyolefins and some contain polar pendant groups or are halogenated. Unmodified polyolefins are characterized by relatively low thermal stability and a nonporous, nonpolar surface with poor adhesive properties. Processed by extrusion, injection molding, blow molding, and rotational molding. Other thermoplastic processes are used less frequently. This class of plastics is used more and has more applications than any other. Also called olefinic resin, olefinic plastic. 

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