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• Crosslinkable Polyethylene (XLPE) contains a crosslinking agent that reacts with the resin during the molding cycle, forming a crosslinked molecule similar to a thermoset plastic. This reaction improves the stress crack resist- ance of the rotomolded part. A newer method is called electron- beam crosslinking. This process is a post-molding operation that can be done on any type of poly- ethylene. Many physical proper- ties can be enhanced while the regulatory status, such as FDA approval, can be maintained. The use of post-consumer resin (PCR) and post-industrial regrind (PIR) has become an important factor in rotomolding because of increasing concern about solid waste. LyondellBasell has performed many studies in this area and can assist you in your application. New single-site-catalyst technology allows for the development of new polyethylene grades to meet specific needs. Other Rotomolding Resins Some other resins used in roto- molding are: polyvinyl chloride (PVC), nylon, polycarbonate, polyester, polypropylene, flame- retardant polyethylene, chemically modified polyethylene, such as LyondellBasell Microthene rotolining (RL) resin, and a system called one-step foam, which incorporates a blowing agent (see Table 4). FLAME RETARDANT POLYOLEFIN LyondellBasell has produced a resin certi-fied by Underwriters Laboratory (UL) as having a V-0 or non-burning rat-ing at 1/8-inch thickness. This resin allows rotomolders to choose from regular grades of HDPE or LLDPE rated UL 94 HB or they can use the flame-retardant resin for more strin-gent flammability requirements. POLYPROPYLENE LyondellBasell polypropylene resins are used in applications where higher resistance to heat distortion is required for parts requiring autoclaving or sterilization. Polypropylene requires cryogenic grinding to get a powder adequate for rotational molding. ONE-STEP FOAM One-step foam (OSF) is a system using several powders and blowing agent pellets to allow rotomolders to produce a part with a solid outer skin and filled with either 2, 4, 7 or 8-lbs/cf of foam. The part, molded in one step, can range from being very soft to very rigid. Rotolining and Co-Rotational Molding LyondellBasell Microthene RL resins are produced by a proprietary process that incorporates chemically reac-tive, functional groups into the polymer structure. This chemical modification turns conventional polyolefins into Microthene RL adhesive resins that form strong bonds to a variety of substrates, such as metals, non-metals includ- ing other polymers. Microthene RL resins provide resist- ance to a wide range of chemicals and are designed for applications requiring insulation or resistance to corrosion, abrasion or impact. Typical applications include linings, coating or tie-layers for pipes, drums, tanks or irregularly shaped parts. Microthene RL resins are normally shipped as powders for application methods such as rota- tional lining, co-rotational lining, powder spray and fluidized bed coating. Since Microthene RL resins bond to metal substrates, surface preparation affects adhesion in a manner similar to painted systems and the mold then becomes the finished part. Table 4. Rotational molding resins: advantages and disadvantages RESIN Polyethylene Polypropylene Microthene RL resins Polycarbonate Nylon Polyvinyl Chloride Polyester Flame Retardant Polyolefin One-Step Foam ADVANTAGES Low cost Easy to mold Excellent ESCR High heat distortion temperature Autoclavable Chemical bonding between resin and other material including metal Clarity Toughness Excellent impact strength High heat resistance Flexible Easily painted Excellent impact strength in thin-walled applications V-0 rating Foam-filled parts DISADVANTAGES Lower impact strength than other resins Low impact strength at cold temperatures Higher cost than polyethylene Higher cost than polyethylene Absorbs moisture Harder to mold than polyethylene Expensive Harder to mold than polyethylene Costs more than polyethylene Low stiffness Expensive Harder to mold than polyethylene Lower impact strength Longer molding cycles 7PDF Image | A Guide to Rotational Molding Lyondellbasell
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