Rigid polyurethane foams

Details Rigid polyurethane foams Rigid polyurethane foams

2002-05-09

2003-04-01

Cooney, Jr., John M. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C252S182240, C264S045100, C521S111000, C521S112000, C521S130000, C521S131000, C521S155000, C521S170000, C521S172000

Reexamination Certificate

active

06541534

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to rigid polyurethane foams. More specifically, the present invention relates to novel rigid syntactic polyurethane foams that are particularly useful as reinforcement materials, especially in the auto industry.
Rigid foams have been used in the auto and other industries for a number of purposes. For example, rigid foams have been used in the auto and other industries for structural reinforcement, preventing corrosion and damping sound and vibration.
Generally, to be useful as reinforcing foams in automotive applications, it is desirable for rigid foams to have a good balance between density and physical properties such as compression strength. One way that has been tried to accomplish this is to use so-called syntactic foam. Syntactic foams are composites consisting of hollow microspheres (minute hollow bubbles, microbubbles, or microballoons) that are dispersed in a resinous matrix. These microspheres are commonly made from inorganic materials such as glass and silica; and polymeric materials such as epoxy resin, unsaturated polyester resin, silicone resin, phenolics, polyvinyl alcohol, polyvinyl chloride, polypropylene, and polystyrene. One example of syntactic foam known in the art to be used as structural foam is sold by Novamax industries under the tradename “Novacore”. This product uses an epoxy as the continuous resinous matrix.
In these syntactic foams, the resinous matrix is typically substantially non-cellular. However, because the microspheres are hollow, their inclusion reduces the density of the syntactic foam. Thus, essentially all of the reduction in density (relative to that of the unfoamed epoxy matrix) is attributable to the gas contained in the microspheres. In some instances the epoxy matrix is expanded slightly by incorporating into it expandable plastic spheres and/or thermally decomposable blowing agents such as azodicarbonamide or p,p-oxybis(benzene sulphonyl hydrazide). However, the limit of expansion of these materials is usually 80% or less, and applied heat is required in order to obtain even this small amount of expansion.
Although one and two part epoxy-based syntactic foams have enjoyed some success as reinforcing foam in the auto industry, they suffer from some deficiencies. First, epoxy-based syntactic foams are cured by applying heat. In automotive applications, it is frequently difficult to supply sufficient heat to get the epoxy-based syntactic foams to cure throughout the part. Consequently, portions of the foam may be cured while other portions, especially the center of the foam, may be left uncured. Moreover, curing is often done in E-coat and paint cure ovens, which often do not maintain close control over curing temperatures. This can lead to incomplete fills or undercuring when oven temperatures are too low. Undercuring can lead to the foam having a low T
g
, so that it becomes soft and loses its reinforcing effect when warm, such as under summertime conditions. Conversely, oven temperatures that are too high often lead to chemical reaction exotherms resulting in foam scorching, charring, overexpansion or even paint blistering if the exotherm is too high.
Second, epoxy-based syntactic foams are generally very brittle and thus lack fracture toughness. Therefore, these foams tend to shatter on impact (such as in a vehicle collision) or crack easily under stress.
Third, it is difficult to make a suitable epoxy-based syntactic foam at a density lower than about 27-35 pcf. At lower densities, those foams become extremely brittle. However, having a lower density is very important to automobile manufacturers, particularly when the vehicle contains a large amount of the reinforcing foam. The lower density translates into lower foam weight, thereby decreasing the overall weight of the vehicle. Reduced weight often correlates to lower fuel consumption and therefore, lower vehicle operating cost.
Rigid, non-syntactic polyurethane foams have also been used as reinforcing foams in automotive applications. These polyurethane foams are formed by the reaction of a polyisocyanate compound such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) or polymeric MDI with an isocyanate-reactive component, such as a polyol or water. Generally, streams of the isocyanate-reactive component and polyisocyanate are intermixed in a mixing head (together with a blowing agent, if water is not included in the formulation) and then dispensed into a cavity or mold. In the mold or cavity, the isocyanate-reactive component and the polyisocyanate react to form the polyurethane. Any water present in the formulation will react with the polyisocyanate to form carbon dioxide gas. The carbon dioxide gas causes the foaming mass to expand, resulting in a non-syntactic cellular structure.
These non-syntactic polyurethane foams have the advantage that they can be formed at very low densities, thereby decreasing the overall weight of the foam. However, the raw materials used to make these foams are typically liquids having a low viscosity, typically about 1000 cps or less. This causes a problem in automotive applications, because the reaction mixture is usually applied to structural members of the vehicle that are not primarily designed as molds for the foam. These structural members include vertical surfaces or cavities that are not completely sealed. For example, these members may contain openings such as trim attachment holes, unsealed seams, drain flutes and the like. Consequently, the low-viscosity reaction mixture readily leaks through any small openings in the cavity, or flows away from non-horizontal surfaces. In addition, it is often desired to reinforce only certain portions of a particular part. In order to accomplish this with these polyurethane foam formulations, it is necessary to install baffles or dams inside of the part or cavity to insure that the mixture is confined in the desired area. This adds considerable cost to the process.
One approach to overcoming these problems with polyurethane foams is to froth the reaction mixture. Frothing is typically done by mechanically whipping air or other gas into the polyurethane mixture using a high shear mixer. When a frothing method is utilized, leakage is reduced because the reaction mixture leaving the mix head has a creamy, more flow resistant consistency. However, a large enough quantity of gas must be whipped into the reaction mixture to create a flow resistant consistency, and this can ultimately result in a foam having too low a density to provide adequate reinforcement.
Another approach to solving these problems of polyurethane foams is to select highly reactive components so that the reaction mixture has extremely short gel time. However, these formulations tend to be highly exothermic, so precautions must be taken to prevent the heat generated during the reaction from producing temperatures high enough to cause foam scorching or even a foam fire. In addition, these highly reactive mixtures tend to split when used to make larger volume foams. Therefore, these highly reactive mixtures are often used to successively apply thin layers of polyurethane foam or in smaller amounts than are needed to fill the cavity. This greatly limits the range of applications for which these highly reactive mixtures are useful.
SUMMARY OF THE INVENTION
In one aspect, this invention is a polyurethane foam having a bulk density from about 7 to 35 pounds per cubic foot and a compressive modulus of at least 5,000 psi as measured by ASTM D 1621 for a 2″×2″×1″ skinless core foam sample at a deflection rate of 0.1 inch/minute, said foam comprising a non-syntactic cellular polyurethane matrix having dispersed therein from about 5 to about 35 weight percent, based on the weight of the foam, of a syntactic phase comprising hollow microspheres. The foam of this aspect of the invention provides a unique combination of moderately low density and good physical characteristics, particularly compressive modulus. The foam is especially suitable for

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