Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1998-04-22
2003-12-09
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...
C521S131000, C521S137000, C521S170000, C521S172000, C521S174000
Reexamination Certificate
active
06660782
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to rigid polyurethane foams and methods for making rigid polyurethane foams.
BACKGROUND OF THE INVENTION
Polyurethane foams are formed by the reaction of a polyisocyanate compound, such as toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI) with a polyhydroxyl compound, such as a polyol. Generally, streams of equal volume of the polyol (i.e., polyol side) and polyisocyanate (isocyanate) are intermixed in a mixing head and then injected into a mold where they react to form the polyurethane foam. Generally, the polyol side also contains water, surfactant, catalysts and added blowing agents.
Generally, there are two types of polyurethane foams: flexible and rigid. In general, flexible foams have open cellular structures and a flexible polyurethane (e.g., uses a low functionality; high molecular weight polyol) which allows them to be elastically deformed. Generally, when making a flexible polyurethane foam, water in the polyol side is used as the blowing agent. The water reacts with the isocyanate producing carbon dioxide that foams the polyurethane as the isocyanate and polyol react.
Rigid foams, on the other hand, generally have a substantially closed cellular structure which essentially fails to elastically deform (i.e., when a rigid foam deforms, it deforms permanently). To provide rigidity, rigid polyurethane foams, typically, are formed using a lower molecular weight polyol than used to make a flexible foam and also a cross-linking polyol. Generally, the cross-linking polyol has (1) a hydroxyl functionality of greater than 3 to 8 (i.e., typically greater than 3 to 8 hydroxyl groups/molecule that can react with the isocyanate), (2) a mean molecular weight of 300 to 800 and high viscosity of 3000 to 20,000 centipoise. The cross-linking polyols are typically added to increase the cross-linking density to form a rigid foam of adequate strength and rigidity.
Unfortunately, the use of high viscosity cross-linking polyols generally raises the viscosity of the polyol side substantially. The increased viscosity of the polyol side typically makes it difficult to achieve efficient mixing with the low viscosity isocyanate side, resulting in inhomogeneous rigid foams. Historically, low viscosity, liquid volatile organic compounds (i.e., added liquid blowing agents) have been used to lower the viscosity. However, this results in volatile organic compound (VOC) emissions when making the foam.
The cross-linking polyols also make it difficult to balance the volumes of the isocyanate side and polyol side due to the high equivalent weight of the cross-linking polyol. This is especially true when the polyol side contains water due to its low equivalent weight of 9. Again, the aforementioned volatile organic compounds are generally added to balance the volume of the polyol and isocyanate side and to blow the foam in the absence of water.
In addition, the cross-linking polyols cause the foam to achieve “gel point” sooner than a foam formed without them. Gel point is when the viscosity of the foaming mass begins to rise exponentially due to link up of polymer domains. Thus, rigid foams made with cross-linking polyols tend to split when made with water because of internal gas pressure from the continued evolution of CO
2
after the foam has gelled.
Consequently, the blowing agent for a rigid foam generally is either (1) a liquid volatile organic compound, such as chloromethane (e.g., CFM-11), that volatizes during the forming of the polyurethane causing the polyurethane to foam or (2) an gaseous organic compound, such as chloromethane (e.g., CFM-12), that is injected into the streams causing the streams to froth and consequently form the rigid foam. These blowing agents have generally been used to avoid one or more of the problems described above. However, they raise environmental and safety concerns.
Thus, it would be desirable to provide a rigid polyurethane foam that avoids one or more of the problems of the prior art, such as one or more of those described above.
SUMMARY OF THE INVENTION
A first aspect of the present invention is a method for forming a polyurethane foam comprising: contacting a first reactant comprised of a polyisocyanate having an average isocyanate functionality of at least 2 and a second reactant comprised of a low molecular weight compound that has at least two to, at most, three groups containing an active hydrogen in the presence of water for a time sufficient to form a substantially rigid foam, provided the foam is formed essentially in the absence of a cross-linking polyol.
A second aspect of the invention is a polyurethane foam comprising the reaction product of a first reactant comprised of a polyisocyanate having an average isocyanate functionality of at least 2 and a second reactant comprised of a low molecular weight compound that has at least two to, at most, three groups containing an active hydrogen and water, wherein the reaction product is formed essentially in the absence of a cross-linking polyol and the polyurethane foam is substantially rigid. A substantially rigid foam, herein, is a rigid foam as understood in the art. For example, the substantially rigid foam generally has a closed cellular structure which essentially fails to elastically deform (i.e., any deformation of the foam tends to be permanent).
Herein, the cross-linking polyol has a hydroxyl functionality of greater than 3 (i.e., greater than 3 hydroxyl groups/molecule that can react with the isocyanate) and a molecular weight of about 300 to 800. Generally, the cross-linking polyol has a viscosity of 3000 to 20,000 centipoise. The foam formed essentially in the absence of the cross-linking polyol means that only trace amounts are present in the reaction mixture that forms the foam. Preferably there is no cross-linking polyol.
By using a low molecular weight compound, such as propylene glycol, a substantially rigid polyurethane foam may surprisingly be formed in the absence of a cross-linking polyol. The foam may also be formed in the absence of a blowing agent other than CO
2
produced from the water-polyisocyanate reaction. It is believed that the low molecular weight compound slows the cross-linking and, consequently, the onset of rigidity of the foam being formed. This slowing is thought to provide a sufficient time for essentially complete evolution of CO
2
from the water isocyanate reaction to allow the foam to form without splitting, as occurs, for example, when using the cross-linking polyol described above. In addition, it is also believed that the use of the low molecular weight compound more completely reacts with the isocyanate groups, resulting in foams generally having higher compressive moduli than those made with cross-linking polyols.
In addition, because of the low equivalent weight of the low molecular weight compound, the first aspect of the invention may also be advantageously performed using volumes of the first and second reactants that are similar, even when the second reactant contains an auxiliary polyol, such as a polyether polyol described later while maintaining the isocyanate index near one. Consequently, the method of the first aspect may be performed using standard polyurethane process equipment. The use of the low molecular weight compound having a low viscosity also results in the second reactant (i.e., the polyol side) to have a low viscosity similar to known polyisocyanates. The viscosity similarity allows the two reactants to be easily mixed and reacted to form a more uniform and homogeneous foam.
The method and foams produced according to the present invention may be used in any suitable application, such as those known in the art, including applications involving, for example, automotive applications requiring stiffening, reinforcing, NVH (noise, vibration and harshness) abatement in a vehicle.
DETAILED DESCRIPTION OF THE INVENTION
The method according to this invention contacts a first reactant comprised of a polyisocyanate having a functionality of at least 2 and a second reactant, comprised
Cooney Jr. John M.
Essex Specialty Products Inc.
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