Energy Absorbing, Water Blown, Rigid Polyurethane Foam

The invention relates to energy absorbing rigid polyurethane foam compositions. Specifically these are low density, predominantly open celled, water blown rigid polyurethane foams which exhibit minimal spring back or hysteresis characteristics. Such foams are suitable as lightweight alternatives for traditional energy absorbing applications, such as side impact bolsters in automobile doors. The foams of the present invention exhibit properties comparable to energy absorbing foams using chlorinated fluorocarbons as the blowing agent.

U.S. Pat. No. 4,866,102 describes moldable energy absorbing rigid polyurethane foam compositions which are prepared by the reaction of a graft polymer dispersion in a polyoxyalkylene polyether polyol with an alkylene oxide adduct of toluenediamine or diaminodiphenylmethane with an organic polyisocyanate in the presence of a crosslinking agent. Similarily, U.S. Pat. Nos. 4,116,893; 4,190,712; 4,212,954; and 4,282,330 also describe energy absorbing foams utilizing graft polymer dispersions. U.S. Pat. No. 4,722,946 describes the production of energy attenuating viscoelastic polyurethane elastomers and foams, comprising mixtures of linear and branched polyol intermediates, polyisocyanates, and optionally, extenders, blowing agents, and the like, in the presence of a catalyst whereby the isocyanate index is varied from about 65 to about 85. U.S. Pat. No. 4,644,563 describes a method of shoring a geological formation which comprises preparing a rigid polyurethane foam which exhibits nearly constant strain with increasing stress in compression. U.S. Pat. No. 4,696,954 describes the preparation of molded polyurethane foams characterized by high impact strength and good thermal stability. U.S. Pat. No. 4,614,754 describes a high density rigid polyurethane foam which exhibits nearly constant strain with increasing stress in compression. U.S. Pat. No. 4,585,807 describes rigid polyurethane foams employing oxyalkylated

The present invention relates to energy absorbing rigid polyurethane foam compositions, the process for preparing said foams and, the resultant articles of manufacture, e.g. side impact bolsters for automotive applications. The foams of the present invention are predominantly open celled, energy absorbing, rigid polyurethane foams essentially free of chlorinated fluorocarbons and volatile organic carbon blowing agents. Yet the foams of the present invention have characteristics, such as constant crush strength and minimal spring back or hysteresis, comparable to present-day energy absorbing rigid polyurethane foams containing fluorocarbons and volatile organic carbons.

Under ever increasing governmental regulations addressing both personal safety and environmental concerns auto manufacturers have been put in a position where they must meet stringent impact requirements, maintain vehicle weight and reduce the use of materials having a detrimental effect on the environment. Energy absorbing rigid polyurethane foams have provided a partial solution in some impact performance requirements areas; e.g. energy absorbing fillers which can be used to stiffen door support frames, thus aiding in preserving the structural integrity of the passenger compartment of an automobile or as bolsters which can reduce effects of secondary collision (occupant-to-interior). However, the foams exhibiting the desired impact characteristics utilize chlorinated fluorocarbons as the foaming agent. That fact alone reduces their desirability in light of mandates to reduce and eventually eliminate the use of CFC's.

In the instant invention it has been found that certain water blown rigid polyurethane foams can be produced which exhibit energy absorbing characteristics comparable to the CFC blown rigid polyurethane foams. These foams may be employed in energy absorbing applications, such as side impact bolsters in automobiles.

These foams are predominantly open celled; having molded densities ranging from 2.0 pcf to about 4.5 pcf, and a crush strength which remains constant from about 10% to about 70% in loading of up to about 70 psi.

Previously, the foams exhibiting the desired impact characteristics utilized chlorinated fluorocarbons as the foaming agent. Attempts to substitute water into these formulations failed to produce foams exhibiting the same impact characteristics. Some success has been had with certain narrowly defined formulations using water as the blowing agent, and containing a polymer polyol (graft polyol) as necessary elements of the invention. (See co-pending U.S. application Ser. No. 634,643 filed Dec. 27, 1990, also U.S. Pat. Nos. 4,190,712; 4,212,954 and 4,116,893).

Surprisingly, it has now been found that foams exhibiting the desired energy absorbing characteristics may be produced from active hydrogen containing compounds or mixtures thereof, organic isocyanates, water, and additives with the proviso that such compounds are generally capable of forming a rigid polyurethane structure, the critical nature of the invention being the amount of water used in conjunction with the isocyanate and resin portions of a given rigid polyurethane foam formulation. The water must be present in amounts sufficient to act as a blowing agent and a cell opening agent promoting a predominantly open cell structure, yet not be present in amounts great enough to cause the foam to collapse. The amount of water used is highly dependent upon the other formulation components but it has been found that from about 2 weight percent to about 30 weight percent is an effective range. A preferred range of water being from about 6 weight percent to about 15 weight percent.

The polyisocyanates which may be used in the present invention are modified and unmodified polyisocyanates which are well known to those skilled in the art. For the purposes of this invention the term polyisocyanate is used to describe compounds containing, at least two isocyanate groups. Unmodified polyisocyanates include aliphatic or cycloaliphatic and aromatic polyisocyanates. Examples include 2,4- and 2,6-methylcyclohexylenediisocyanate; tetramethylene diisocyanate, cyclohexane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, mixtures of 2,4- and 2,6-toluene diisocyanate, naphthalene-1,5-diisocyanate, 1-methoxyphenyl-2,4-diisocyanate. Preferred isocyanates include 4,4'-diphenylmethane diisocyanate (MDI), mixtures of 4,4'- and 2,4-diphenylmethane diisocyanate, and polymeric polyisocyanates such as polymethylene polyphenylenes polyisocyanates (polymeric MDI). Included within the preferred isocyanates are those modified MDI's containing carbodiimide, allophanate, urethane or isocyanurate structures. The more preferred isocyanates are polymeric MDI and mixtures of polymeric MDI and pure 2,4 and 4,4' MDI. These polyisocyanates are prepared by conventional methods known in the art, e.g. phosgenation of the corresponding organic amine.

For purposes of the present invention isocyanates other than the preferred isocyanates may be present in minor amounts.

In the preparation of the polyurethanes of the present invention the isocyanate is reacted with active hydrogen-containing compounds (polyols are preferred). Hydroxyl group-containing compounds (polyols) useful in the preparation of polyurethanes are described in the Polyurethane Handbook in chapter 3, .sctn.3.1 pages 42-61; and in Polyurethanes: Chemistry and Technology in Chapter II, .sctn..sctn.III and IV, pages 32-47. Many hydroxyl-group containing compounds may be used, including simple aliphatic glycols, dihydroxy aromatics, bisphenols, and hydroxyl-terminated polyethers, polyesters, and polyacetals, among others. Extensive lists of suitable polyols may be found in the above references and in many patents, for example in columns 2 and 3 of U.S. Pat. No, 3,652,639; columns 2-6 of U.S. Pat. No. 4,421,872; and columns 4-6 of U.S. Pat. No. 4,310,632; these three patents being hereby incorporated by reference.

Preferably used are hydroxyl-terminated polyoxyalkylene polyols. The former are generally prepared by well known methods, for example by the base catalyzed addition of an alkylene oxide, preferably ethylene oxide (oxirane), propylene oxide (methyloxirane) or butylene oxide (ethyloxirane) to an initiator molecule containing on the average two or more active hydrogens. Examples of preferred initiator molecules are dihydric initiators such as ethylene glycol, propylene glycol, butylene glycol, neopentyl glycol, 1,6-hexanediol, hydroquinone, resorcinol, the bisphenols, aniline and other aromatic monoamines, aliphatic monoamines, and monoesters of glycerine; trihydric initiators such as glycerine, trimethylolpropane, trimethylolethane, N-alkylphenylenediamines, mono-, di, and trialkanolamines; tetrahydric initiators such as ethylene diamine, propylene diamine, 2,4'-, 2,2'-, and 4,4'- methylenedianiline, toluenediamine, and pentaerythritol; pentahydric initiators such as diethylenetriamine; and hexahydric and octahydric initiators such as sorbitol and sucrose.

Addition of alkylene oxide to the initiator molecules may take place simultaneously or sequentially when more than one alkylene oxide is used, resulting in block, heteric, and block-heteric polyoxyalkylene polyethers. The number of hydroxyl groups will generally equal the number of active hydrogens in the initiator molecule. Processes for preparing such polyethers are described both in the Polyurethane Handbook and Polyurethanes: Chemistry and Technology as well as in many patents, for example U.S. Pat. Nos. 1,922,451; 2,674,619; 1,922,459; 3,190,927; and 3,346,557.

Polyester polyols also represent polyurethane-forming reactants. Such polyesters are well known in the art and are prepared simply by polymerizing polycarboxylic acids or their derivatives, for example their acid chlorides or anhydrides, with a polyol. Numerous polycarboxylic acids are suitable, for example malonic acid, citric acid, succinic acid, glutaric acid, adipic acid, pimelic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid, terephthalic acid, and phthalic acid. Numerous polyols are suitable, for example the various aliphatic glycols, trimethylolpropane and trimethylolethane, a-methylglucoside, and sorbitol. Also suitable are low molecular weight polyoxyalkylene glycols such as polyoxyethylene glycol, polyoxypropylene glycol, and block and heteric polyoxyethylene-polyoxypropylene glycols. These lists of dicarboxylic acids and polyols are illustrative only, and not limiting. An excess of polyol should be used to ensure hydroxyl termination. Although carboxyl groups are also reactive with isocyanates. Methods of preparation of such polyester polyols are given in the Polyurethane Handbook and in Polyurethanes: Chemistry and Technology.

These polyols may be used alone or in combination. However, the use of graft polymer dispersions either alone or in combination is not contemplated by this invention. Those polyols or mixtures of polyols selected should promote a high crosslink density for optimal results.

Any suitable catalyst or mixture of catalysts may be used including tertiary amines such as, for example, triethylenediamine, N-methylmorpholine, N-ethylmorpholine, diethylethanolamine, N-cocomorpholine, 1-methyl-4-dimethylaminoethylpiperazine, 3-methoxypropyldimethylamine, N,N,N'-trimethylisopropyl propylenediamine, 3-diethylaminopropyldiethylamine, dimethylbenzylamine, and the like. Other suitable catalysts are, for example, stannous chloride, dibutyltin-di-2-ethyl hexonate, potassium hexanoate, stannous oxide, as well as other organometallic compounds such as are disclosed in U.S. Pat. No. 2,846,408.

A surface-active agent is generally necessary for production of high grade polyurethane foam according to the present invention, since in the absence of same, the foams collapse or contain very large uneven cells. Numerous surface-active agents have been found satisfactory. Nonionic surface active agents are preferred. Of these, the nonionic surface-active agents such as the well-known silicones have been found particularly desirable. Other surface-active agents which are operative, although not preferred, include polyethylene glycol ethers of long chain alcohols, tertiary amine or alkanol amine salts of long chain alkyl acid sulfate esters, alkyl sulfonic esters, and alkyl arylsulfonic acids.

A chain extender and/or crosslinker is used as well in the present invention. These include those compounds having at least two functional groups bearing active hydrogen atoms such as, hydrazine, primary and secondary diamines, amino alcohols, amine acids, hydroxy acids, glycols or mixtures thereof. Glycerin is an example of a preferred compound used as a crosslinker.

Other optional additives which fall within the spirit of the present invention include known pigments, such as carbon black, dyes, stabilizers against aging and weathering, fungistats, bacteriostats, fillers, or flame retarding agents.

If desired, flame retardants may be incorporated in the foams. Among the flame retardants which may be employed are: pentabromodiphenyl oxide, dibromopropanol, tris(.beta.-chloropropyl)-phosphate, 2,2-bis(bromoethyl) 1,3-propanediol, tetrakis(2-chloroethyl-)ethylene diphosphate, tris(2,3-dibromopropyl)phosphate, tris(.beta.-chloroethyl)phosphate, tris(1,2-dichloropropyl)phosphate, bis-(2-chloroethyl) 2-chloroethylphosphonate, molybdenum trioxide, ammonium molybdate, ammonium phosphate, pentabromodiphenyl oxide, tricresylphosphate, hexabromocyclododecane and dibromoethyl dibromocyclohexane.

The following illustrate the nature of the invention. All parts are weight % unless otherwise indicated.

Examples 1-3 and comparative examples A and B were prepared using the following conditions:

Examples 1-3 and Comparative example A and B were injected into the center of a preheated 10.times.10.times.2.5" mold. The mold was closed. The parts were demolded, cured at 50% humidity and 73.degree. F. prior to testing. Foam compression testing (ASTM D-1621) at 10 percent intervals was carried out on all examples at varying full weights, to determine foam performance characteristics. Comparative example A is a currently used energy absorbing foam composition using Freon F-11a (a chlorinated fluorocarbon). Comparative example B is a formulation similar to comparative example A however water is substituted for F-11a.

Although these parts were made using an open mold pour technique, the system is equally amenable to other fabrication methods known to those skilled in the art, e.g. RIM (reaction injection molding).

All reported test values are averages of multiple testing on each sample.

Examples 1, 2 and 3 illustrate the present invention. Examples 1 and 2 are prepared using the same formulation with different amounts of water present. The resulting foams exhibit the desired characteristics of the invention, i.e. a relatively constant consistency of compressive strength or crush strength, over deflection (ASTM D-1621). This consistency of compressive strength is carried over a range of densities as shown in tables 1 and 2. Example 3 illustrates that energy absorbing rigid foams are possible by adjusting the amount of water necessary to produce foams having the desired physical characteristics.

Table 3 shows the results of ASTMD-1621 foam compression tests for foams produced from example 3.

Table 4 shows the results of ASTM D-1621 foam compression test for comparative examples A and B. As is evident from the data merely substituting water (Ex. B) for a chlorinated fluorocarbon (Ex. B) in a formulation produces a foam which exhibits a significant loss in efficiency, i.e. consistency of compressive strength over deflection.

Table 5 shows test data from a prototype hip bolster part, covering the range of 20 to 50 psi by varying the fill weight. The hip bolster was prepared using the formulation from example 1. All conditions were the same as those for examples 1 through 3 with the exception that a hip bolster mold was used.