Postcure treatment for reaction injection molded polyurethanes

#The present invention relates to the field of reaction injection molded polyurethanes, and particularly to a postcure treatment thereof which provides properties improvements.

#It is generally known that reaction injection molded polymers, which are generally denominated as polyurethane-related (polyurea and polyurethane/polyurea) polymers, benefit from a postcure treatment after removal from the mold. Postcure treatments generally serve to react residual isocyanate with unreacted polyol and polyamines to complete the polyurethane and polyurea reaction, permit the evolution of entrapped gases for improved paintability, and allow the formation of a lower energy hydrogen bonded network than that which is found in the uncured polymer. This improves the heat distortion properties resulting in better end use performance. The postcure also reduces the amount of time a part needs to react and degas from 3 days to a few hours. In most of this industry, and particularly in the automotive parts portion thereof, this postcuring has been typically achieved by an extended (normally one hour) exposure to heat in a convection oven. Oven temperatures of 140° C. are currently used for curing the polymers selected to prepare automotive fascia. This is obviously time-consuming and relatively energy-intensive, but the significant improvements in the properties noted hereinabove have strongly supported continuation of methods of manufacture incorporating this postcure treatment.

#However, one application for these polymers which is currently and rapidly expanding is in the area of vertical body panels. Body panels, which have heretofore been made primarily of steel, must generally be subjected to a widely-used, automotive online prepaint process known as “E-coat”, in which an anticorrosion epoxy resin is applied to the steel surfaces and then subjected to oven cure at a much higher temperature, generally in the range of 180-200° C. Use of polymeric body panels presents new challenges, because unless the body panel is sufficiently cured prior to this E-coat process, the higher temperature required for this E-coat process may detrimentally affect the dimensional stability of the panels. However, subjecting the panels to two separate, sequential high temperature processes—a postcure treatment after molding, and a heat treatment as part of the E-coat process—is extremely expensive. This is because the convection ovens required on the conveyor lines in such an approach have to be extremely large and actually have to be heated to temperatures even higher than 200° C. due to the inherent problems associated with convection heating. For example, as the substrate temperature begins to approach the air temperature, heat transfer to the part falls dramatically. This, however, can lead to another problem, which is that using such higher air temperature to assure that the core of the part reaches the desired temperature can result in degradation of the surface of the part. Thus, it is difficult to adequately postcure using such high temperature conveyor lines without encountering problems in polymer quality and without risking poor cure which leads to loss of dimensional stability in subsequent E-coat processing. The alternative, which is to place parts into small, nonconveyor ovens, is also not efficient. The industrial manufacturer therefore needs a method of accomplishing the postcure without relying on high temperature convection ovens, which method can be carried out on conveyorized parts, and which results in a postcure such that subsequent exposure to high temperature convection ovens will not result in unacceptable dimensional changes in the part.

#A panel was molded in a plaque mold using a Cincinnatti-Milcron HT Reaction Injection Molding (R.l.M.) machine. The polymer formulation was SPECTRIM HH* with 20 percent RRIMGLOS1**, wollastonite. [*SPECTRIM HH is a trademark of The Dow Chemical Company. **RRIMGLOS1 is a trademark of Nyco Co.] Following demold, one of the panels was then postcured by heating it to 190° C. and holding the temperature at 190° C. for 1 minute using a ITW BGK, Model #2516-48HP infrared oven. The wavelength of the IR energy varied in the range of 0.76 to 2.0 microns. The temperature was monitored by an internal optical pyrometer with an internal feedback control loop. Afterwards, the panel was placed in a convection oven for 30 minutes at 190° C. and then allowed to cool. The resulting properties of the cooled panel are: Specific gravity 1.25, percent filler 20.23, flexural modulus (parallel) 241,679 psi, flexural modulus (perpendicular) 117,522 psi, heat sag 1.0 mm, Gardner impact 46 in-lb.

#Another panel prepared via the same procedure and using the same formulation as in Example 1 was postcured in a convection oven. After 30 minutes in the oven, the panel reached a temperature of 190° C. and was then allowed to remain in the oven for 30 minutes at 190° C. The resulting properties of the cooled comparative panel were: Specific gravity 1.25, percent filler 20.23, flexural modulus (parallel) 255,923 psi, flexural modulus (perpendicular) 122,148 psi, heat sag 1.3 mm, Gardner impact 32 in-lb.

#A panel was molded on a Cincinnati Milcron HT reaction injection molding (R.I.M.) machine using a plaque mold using the procedure used in Example 1 and Example 2 (Comparative). The panel was postcured in a ITW BGK Model #2516-48HP infrared postcure oven. The panel reached the postcure temperature of 190° C. in 2 minutes and 20 seconds and was then held at that temperature for 30 minutes. The panel was removed from the oven, cooled and the properties were determined. They were: Specific gravity 1.21, percent filler 19.9, flexural modulus (parallel) 221,942 psi, heat sag 9.3 mm, Gardner impact 46 in-lb.