RESEARCHERS DEVELOP PROCESSING TECHNOLOGIES, METHODS FOR HIGH-SPEED PRODUCTION OF HIGH-QUALITY POLYURETHANE PULTRUSIONS
17 June 2004 - Bayer MaterialScience AG

The two major hurdles that must be overcome to successfully pultrude polyurethane resins are the fast gel times of the raw materials when mixed and the tendency of the chemistry to produce internal voids in the material during gel and cure.

In a paper titled 'Pultrusion of Fast-Gel Thermoset Polyurethanes: Processing Considerations and Mechanical Properties,' researchers describe how to overcome these hurdles, resulting in a commercially viable means of producing polyurethane pultrusions.

Authors of the paper are Nigel Barksby and James L. Lambach, both of Bayer MaterialScience LLC; Dr. James G. Vaughan and Dr. Ellen Lackey, both of the University of Mississippi; and Harry D. Coffee, Nautilus Composites LLC.

Overcoming Hurdles

The pultrusion processing experiments were conducted at the University of Mississippi using commercial pultrusion equipment. The first phase of experimentation identified workable resin chemistries and process parameters. Mixes of polyols and isocyanates were selected to optimize the processing characteristics for pultrusion. These characteristics included:

low viscosity, to ensure good fiber wetting;
long relative gel time to make set-up and shut down easy and forgiving;
rapid polymerization to allow for high process speeds and a good surface finish; and competitive cost.

In the next phase of experiments, researchers explored the effects of varying temperature and processing speeds. Experiments were also conducted to evaluate the effects of different ratios of polyol and isocyanate for the resin mix, and the effects of other types of internal mold releases on properties and processing. Longer pultrusion run times also were examined to determine the effects of heat build-up, to sample for environmental concerns and to conduct more extensive mechanical property evaluations.

Out Of The Box Thinking Yields Results

To overcome the processing problems associated with the reactivity of the polyurethane resin system, researchers thought 'out of the box,' replacing a conventional open wet-resin bath system with a resin injection system, according to Bayer's Lambach. 'The two components of the resin were pumped at a rate that matched the rate of resin consumption and maintained sufficient pressure in the injector box to ensure adequate wet out,' he explained.

Polyurethane pultrusions ran at pull speeds of 48 to 60 inches per minute, comparing very favorably with product manufactured using conventional pultrusion resin chemistries. The average flexural strength of the high-modulus resin was 232 ksi, with average short-beam strength of 11.5 ksi. The average flexural strength of the low-modulus resin was 219 ksi, with average short-beam strength of 9.9 ksi.

'There are a number of benefits to utilizing polyurethanes in this process. First and foremost, pultrusions made with polyurethanes are more resistant to brittle fracture than pultrusions using conventional resins, such as polyester and epoxy,' said Lambach. 'The toughness of polyurethane allows pultruded materials to be joined with self-tapping screws, without the necessity of pre-drilled pilot holes as is necessary with other pultrusion resin chemistries. This feature can offer significant benefits in assembly operations.

'Furthermore, polyurethane pultrusions exhibit superior impact resistance and adhere exceptionally well to most materials, including the commonly used fibers in composites,' he continued.

'Bayer believes that once the composites industry becomes more aware of the benefits polyurethanes offer and the feasibility of utilizing them in pultrusion, this versatile resin will make great inroads in products produced through this manufacturing process,' concluded Lambach.

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