Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
1999-09-14
2001-07-24
Berman, Susan W. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S164000, C522S111000, C522S112000, C528S059000, C528S061000, C528S065000
Reexamination Certificate
active
06265464
ABSTRACT:
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.
The present invention offers such a method, and further offers unexpected and surprising polymer properties improvements as well. It is a method of treating reaction injection molded polyurethane, polyurethane/urea and polyurea polymers comprising exposing a reaction injection molded polyurethane, polyurethane/urea or polyurea polymer to an amount of infrared energy sufficient to increase the temperature of the polymer to at least 180° C., and then maintaining the temperature of the polymer at or above that temperature, for a time sufficient to increase the Gardner impact property, as measured using ASTMD-3029, when compared to the same polymer which has been heated to the same temperature and maintained thereat for the same time in a convection oven. Preferably the Gardner impact property is increased by at least 25 percent when compared to the same polymer which has been heated to the same temperature and maintained thereat for the same time in a convection oven. More preferably the Gardner impact property is increased by at least 35 percent when compared to the same polymer which has been heated to the same temperature and maintained thereat for the same time in a convection oven. Preferably the polymer is maintained at the desired target temperature (at least 180° C.) from 20 to 35 minutes, and also preferably the temperature of the polymer is increased such that it reaches a level at least −10° C. greater than the highest temperature to which the polymer will be exposed in any subsequent processing, such as the E-coat process. The preferred wavelength is from 0.76 to 2 microns.
Use of the present invention improves important mechanical properties such as impact, heat sag and heat distortion temperature. It also is significantly faster and less expensive when compared with convection heating. Finally, it does not result in degradation of the part surface, since the entire polymer mass is heated essentially simultaneously.
The present invention surprisingly utilizes infrared radiation, which is the form of electromagnetic radiation falling between visible light and radio waves in the electromagnetic spectrum, for its postcure treatment. Infrared radiation is known to provide the highest heat transfer profile, in general, of all types of electromagnetic radiation. Its character is divided by wavelengths, designated as short, medium, and long, and the wavelength for maximum intensity in the “short” wavelength area (known as “high intensity”) is from 0.76 to 2 microns.
The mechanism of operation in this new approach is the absorption of the radiation within this limited wavelength range. This absorption by an organic molecule resulted in the excitation of the molecule to a higher energy state, and return of the molecule to its ground energy state resulted in the release of the energy, primarily as heat. While the authors also experimented with broad-band IR exposures (wavelengths concentrated principally in the range of 2.5-15 microns, but with overtone bands ranging from 1-100 microns), performance suggested a strong preference for the wavelengths from 0.76 to 2 microns. The effect of this IR radiation is that the temperature of the polymer is raised much more rapidly than is possible using a convection oven alone, and this rapid temperature rise, followed by maintenance of the temperature thereafter for a brief time, generally 20 to 35 minutes, results in a fully-cured polymer exhibiting superior properties, for example impact resistance and dimensional stability as measured by heat sag, when compared to an identical polymer which has been postcured without the IR treatment, but which instead has been placed in a convection oven for a time sufficient to reach the target temperature, and then maintained at that same temperature for the same amount of time as the IR-treated polymer. In most applications the IR source is able to raise the temperature of the polymer to the desired postcure temperature in 2½ minutes, compared to 15 to 30 minutes using a convection oven. This reduces the size of the oven required to process body panel parts, which is extremely important for use with conveyor lines, and allows use of either the IR source or conventional convection means to carry out the temperature maintenance portion of the process which follows initial heat-up. Typically the IR source and convection oven are combined in one apparatus to perform
McLaren John W.
Rettmann Kenneth J.
Berman Susan W.
The Dow Chemical Company
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