Urethane Catalysts

1. Field of the Invention

This invention pertains to the field of urethane catalysts. More particularly, this invention relates to the use of certain amines as urethane catalysts.

2. Description of the Prior Art

The use of a catalyst in preparing polyurethanes by the reaction of a polyisocyanate, a polyol and perhaps other ingredients is known. The catalyst is employed to promote at least two, and sometimes three major reactions that must proceed simultaneously and competitively at balanced rates during the process in order to provide polyurethanes with the desired physical characteristics. One reaction is a chain extending isocyanate-hydroxyl reaction by which a hydroxyl-containing molecule is reacted with an isocyanate-containing molecule to form a urethane. This increases the viscosity of the mixture and provides a polyurethane containing secondary nitrogen atom in the urethane groups. A second reaction is a cross-linking isocyanate urethane reaction by which an isocyanate-containing molecule reacts with a urethane group containing a secondary nitrogen atom. The third reaction which may be involved is an isocyanate-water reaction by which an isocyanate-terminated molecule is extended and by which carbon dioxide is generated to blow or assist in the blowing of the foam. This third reaction is not essential if an extraneous blowing agent, such as a halogenated, normally liquid hydrocarbon, carbon dioxide, etc., is employed, but is essential if all or even a part of the gas for foam generation is to be generated by this in situ reaction (e.g. in the preparation of "one-shot" flexible polyurethane foams.)

The reactions must proceed simultaneously at optimum balanced rates relative to each other in order to obtain a good foam structure. If carbon dioxide evolution is too rapid in comparison with chain extension, the foam will collapse. If the chain extension is too rapid in comparison with carbon dioxide evolution, foam rise will be restricted, resulting in a high density foam with a high percentage of poorly defined cells. The foam will not be stable in the absence of adequate crosslinking.

It has long been known that tertiary amines, such as trimethylamine, triethylamine, etc., are effective for catalyzing the second crosslinking reaction. Other typical tertiary amines are set forth in U.S. Pat. Nos. 3,925,368; 3,127,436; and 3,243,387 and German OLS Nos. 2,354,952 and 2,259,980. Some of the tertiary amines are effective for catalyzing the third water-isocyanate reaction for carbon dioxide evolution. However, tertiary amines are only partially effective as catalysts for the first chain extension reaction. To overcome this problem, the so-called "prepolymer" technique has been developed wherein a hydroxy-containing polyol component is partially reacted with the isocyanate component in order to obtain a liquid prepolymer containing free isocyanate groups. This prepolymer is then reacted with additional polyol in the presence of a tertiary amine to provide a foam. This method is still commonly employed in preparing rigid urethane foams, but has proven less satisfactory for the production of flexible urethane foams.

For flexible foams, a one-step or "one-shot" process has been developed wherein a tertiary amine, such as triethylenediamine, is employed in conjunction with an organic tin compound. Triethylenediamine is particularly active for promoting the water-isocyanate reaction and the tin compound is particularly active in synergistic combination with the triethylenediamine for promoting the chain extension reaction. However, even here, the results obtained leave much to be desired. Triethylenediamine is a solid and must be dissolved prior to use to avoid processing difficulties. Also, triethylenediamine and other of the prior art amines can impart a strong amine odor to the polyurethane foam.

In addition to problems of odor and handling due to solid character other tertiary amines suffer still further deficiencies. For example, in some instances the compounds are relatively high in volatility leading to obvious safety problems. In addition, some catalysts of this type do no provide sufficient delay in foaming, which dealy is particularly desirable in molding applications to allow sufficient time to situate the preform mix in the mold. Yet other catalysts, while meeting specifications in this area do not yield foams with a desirable tack-free time.

Lastly, while certain tertiary amines are somewhat suitable in this catalytic area they nevertheless do not have a sufficiently high tertiary amine content in terms of the number of tertiary amines compared to overall molecular weight. It is believed that the higher the tertiary amine content the more rapid the catalytic activity in the polyurethane art.

It would therefore be a substantial advance in the art if a new class of amine catalysts were discovered which overcome some of the just enumerated disadvantages of the prior art.

A new class of compounds has been discovered which have been found useful as polyurethane catalyst. These compounds have the following structural formula: ##STR2## where R is lower alkyl, R.sub.1 is selected from the group consisting of hydrogen, alkyl, aryl, substituted aryl, alkaryl, substituted alkaryl, aralkyl, and substituted aralkyl.

The compounds here may be prepared by resort to a wide variety of synthetic techniques. However, preferably these compositions are prepared by first making bis(dialkylaminopropyl)-amine or the alkyl (R.sub.1) derivative thereof. The bis-amine again may also be prepared by a variety of known techniques. However, one excellent mode of preparation involves reaction of a dialkylamine such as dimethylamine with acrylonitrile followed by hydrogenation of the resultant condensate to produce a dialkylaminopropylamine. In producing the dialkylaminopropylamine, one also produces bis-(dialkylaminopropyl)-amine which may be removed from dialkylaminopropylamine by conventional means such as distillation and the like. Preferably then R contains 1-4 carbon atoms. Also, R.sub.1 when alkyl is preferably lower alkyl containing 1-4 carbon atoms.

The above bis-amines then readily react with a variety of olefin oxides characterized by the formula: ##STR3## where R.sub.1 has a significance as above stated. Thus, the bis-amine is essentially derivatized by addition of an olefin oxide thereto such as an alkylene oxide. Mixtures of olefin oxides may also be employed to form the catalyst compounds of the invention.

The olefin oxide which may be employed as reactants alone or as mixtures include ethylene oxide, propylene oxide, the isomeric normal butylene oxides, hexylene oxide, octylene oxide, dodecene oxide, styrene oxide and cyclohexene oxide. Alkyl, cyano, nitro, halo, and other additional substituents may also be present in the above olefin oxides or others.

Greatly preferred olefin oxide reactants with the bis-amine are alkylene oxides which will contain from 2 up to 18 carbon atoms. Most preferably, these include ethylene oxide and propylene oxide.

The reaction of an olefin oxide such as an alkylene oxide with a hydroxy-containing or amine-containing reactant as here is well known to those skilled in the art. Such reaction is normally carried out under basic conditions established through the use of alkali metals, their hydroxides, oxides and hydrides, and in some cases basic amines. The reaction of the bis-amine with olefin oxides is essentially on a mol per mol basis.

The compounds here possess a number of useful characteristics making them exceptionally attractive as polyurethane catalysts. For example, the just defined compounds have a high tertiary amine content and resultant rapid catalytic activity in the polyurethane foam area. Tertiary amine content is calculated as the number of tertiary amines divided by the molecular weight times 1,000. In addition, the compounds here are also relatively non-volatile and possess little if any odor. With respect to the products, there are no solids handling problems such as are present with well known polyurethane catalysts as triethylenediamine. The catalysts of the invention are particularly desirable in foaming urethane formations in that they provide a sufficient delay in the foaming operation to aid in processing. Yet the catalysts also give good foams with desirable tack-free times. As noted above this delay time is particularly desirable in molding applications to allow sufficient time to situate the prefoam mix in the mold. Lastly, the compounds are easily prepared as typically described above, and are relatively inexpensive.

To prepare polyurethanes using the catalysts here any aromatic polyisocyanate may be used. Typical aromatic polyisocyanates include m-phenylene diisocyanate, p-phenylene diisocyanate, polymethylene polyphenylisocyanate, 2,4-toluene diisocyanate, 2,6-tolylene diisocyanate, dianisidine diisocyanate, bitolylene diisocyanate, naphthalene-1,4-diisocyanate, diphenylene-4,4'-diisocyanate, aliphatic-aromatic diisocyanates, such as xylylene-1,4-diisocyanate, xylylene-1,4-diisocyanate, xylylene-1,3-diisocyanate, bis(4-isocyanatophenyl) methane, bis(3-methyl-4-isocyanatophenyl) methane, and 4,4'-diphenylpropane diisocyanate.

Greatly preferred aromatic polyisocyanates used in the practice of the invention are 2,4- and 2,6-toluene diisocyanates and methylene-bridged polyphenyl polyisocyanate mixtures which have a functionality of from about 2 to about 4. These latter isocyanate compounds are generally produced by the phosgenation of corresponding methylene bridged polyphenyl polyamines, which are conventionally produced by the reaction of formaldehyde and primary aromatic amines, such as aniline, in the presence of hydrochloric acid and/or other acidic catalysts. Known processes for preparing the methylene-bridged polyphenyl polyamines and corresponding methylene-bridged polyphenyl polyisocyanates therefrom are described in the literature and in many patents, for example, U.S. Pat. Nos. 2,683,730; 2,950,263; 3,012,008; 3,344,162; and 3,362,979.

Most preferred methylene-bridged polyphenyl polyisocyanate mixtures used here contain from about 20 to about 100 weight percent methylene diphenyldiisocyanate isomers with the remainder being polymethylene polyphenyl diisocyanates having higher functionalities and higher molecular weights. Typical of these are polyphenyl polyisocyanate mixtures containing about 20 to 100 weight percent methylene diphenyldiisocyanate isomers, of which 20 to about 95 weight percent thereof if the 4,4'-isomer with the remainder being polymethylene polyphenyl polyisocyanates of higher molecular weight and functionality that have an average functionality of from about 2.1 to about 3.5. The isocyanate mixtures are known commercially available materials and can be prepared by the process described in U.S. Pat. No. 3,362,979, issued Jan. 9, 1968 to Floyd E. Bentley.

The hydroxyl-containing polyol component which reacts with the isocyanate may suitably be a polyester polyol or a polyether polyol having a hydroxyl number ranging from about 700 to about 25, or lower. When it is desired to provide a flexible foam, the hydroxyl number is preferably in the range from about 25 to 60. For rigid foams, the hydroxyl number is preferably in the range from 350 to 700. Semi-rigid foams of a desired flexibility are provided when the hydroxyl number is intermediate to the ranges just given.

When the polyol is a polyester, it is preferably to use, as the polyester, a resin having a relatively high hydroxyl value and a relatively low acid value made from the reaction of a polycaroxylic acid with a polyhydric alcohol. The acid component of the polyester is preferably of the divasic or polybasic type and is usually free of reactive unsaturation, such as ethylenic groups or acetylenic groups. The unsaturation such as occurs in the rings of such aromatic acids as phthalic acid, terephthalic acid, isophthalic acid, or the like, is non-ethylenic and non-reactive. Thus, aromatic acids may be employed for the acid component. Aliphatic acids, such as succinic acid, adipic acid, sebacic acid, azelaic acid, etc., may also be employed. The alcohol component for the polyester should preferably contain a plurality of hydroxyl groups and is preferably an aliphatic alcohol, such as ethylene glycol, propylene glycol, dipropylene glycol, diethylene glycol, glycerol, pentaerthyritol, trimethylolethane, trimethylolpropane, mannitol, sorbitol, or methyl glucoside. Mixtures of two or more of the above identified alcohols may be employed also if desired. When a flexible urethane foam is desired, the polyol should preferably have an average functionality of from about 2 to about 4 and a molecular weight of from about 2,000 to about 4,000. For rigid foams, the finctionality of the polyol component is preferably from about 4 to about 7.

When the hydroxyl-containing component is a polyether polyol for use in flexible polyurethane foam, the polyol may be an alkylene oxide adduct of a polyhydric alcohol with a functionality of from about 2 to about 4. The alkylene oxide may be suitable by ethylene oxide, propylene oxide, or 1,2-butylene oxide, or a mixture of some or all of these. The polyol will suitable have a molecular weight within the range of from about 2000 to about 7000. For flexible polyether polyurethane foams, the alkylene oxide is preferably propylene oxide or a mixture of propylene oxide and ethylene oxide.

For rigid polyether polyurethane foams, the polyol should have a functionality of from about 4 to about 7 and a molecular weight of from about 300 to about 1200. Polyols for rigid polyether polyurethane foams may be made in various ways including the addition of an alkylene oxide as above to a polyhydric alcohol with a functionality of from 4 to 7. These polyols may also be, for example, Mannich condensation products of a phenol, an alkanolamine, and formaldehyde, which Mannich condensation product is then reacted with an alkylene oxide. See U.S. Pat. No. 3,297,597.

The amount of hydroxyl-containing polyol compound to be used relative to the isocyanate compound in both polyester and polyether foams normally should be such that the isocyanato groups are present in at least an equivalent amount, and preferably, in slight excess, compared with the free hydroxyl groups. Preferably, the ingredients will be proportioned so as to provide from about 1.05 to about 1.5 mol equivalents of isocyanato groups per mol equivalent of hydroxyl groups. However, for certain shock absorbing foams we have found that by using the catalysts of our invention the mol equivalents of isocyanate to hydroxyl groups can be as low as 0.4.

When water is used, the amount of water, based on the hydroxyl compound, is suitable within the range of about 0.05 to about 5.0 mol per mol equivalent of hydroxy compound.

It is within the scope of the present invention to utilize an extraneously added inert blowing agent such as a gas or gas-producing material. For example, halogenated low-boiling hydrocarbons, such as trichloromonofluoromethane and methylene chloride, carbon dioxide, nitrogen, etc., may be used. The inert blowing agent reduces the amount of excess isocyanate and water that is required in preparing flexible urethane foam. For a rigid foam, it is preferable to avoid the use of water and to use exclusively the extraneous blowing agent. Selection of the proper blowing agent is well within the knowledge of those skilled in the art.

The catalysts discovered here are useful in the preparation of rigid or flexible polyester or polyether polyurethane foams based on the combined weight of the hydroxyl-containing compound and polyisocyanate, are employed in an amount of from about 0.05 to about 4.0 weight percent. More often that the amount of catalyst used is 0.1-1.0 weight percent.

The catalysts of this invention may be used either alone or in a mixture with one or more other catalysts such as other tertiary amines or with an organic tin compound or other polyurethane catalysts. The organic tin compound, particularly useful in making flexible foams may suitably be a stannous or stannic compound, such as a stannous salt of a carboxylic acid, a trialkyltin oxide, a dialkyltin dihalide, a dialkyltin oxide, etc., wherein the organic groups of the organic portion of the tin compound are hydrocarbon groups containing from 1 to 8 carbon atoms. For example, dibutylin dilaurate, dibutylin diacetate, diethyltin diacetate, dihexyltin diacetate, di-2-ethylhexyltin oxide, dioctyltin dioxide, stannous octoate, stannour oleate, etc., or a mixture thereof, may be used.

Such other tertiary amines include trialkylamines (e.g. trimethylamine, triethylamine), heterocyclic amines, such as N-alkylmorpholines (e.g. N-methylmorpholine, N-ethylmorpholine, etc.), 1,4-dimethylpiperazine, triethylenediamine, etc., aliphatic polyamines, such as N,N,N'N'-tetramethyl-1,3-butanediamine.

Conventional formulation ingredients are also employed, such as, for example, foam stabilizers also known as silicone oils or emulsifiers. The foam stabilizer may be an organic silane or siloxane. For example, compounds may be used having the formula:

wherein R is an alkyl group containing from 1 to 4 carbon atoms; n is an integer of from 4 to 8; m is an integer of 20 to 40; and the oxyalkylene groups are derived from propylene oxide and ethylene oxide. See, for example, U.S. Pat. No. 3,194,773.

In preparing a flexible foam, the ingredients may be simultaneously, intimately mixed with each other by the so-called "one-shot" method to provide a foam by a one-step process. In this instance, water should comprise at least a part (e.g., 10 to 100%) of the blowing agent. The foregoing methods are known to those skilled in the art, as evidenced by the following publication: duPont Foam Bulletin, "Evaluation of Some Polyols in One-Shot Resilient Foams", Mar. 22, 1960.

When it is desired to prepare rigid foams, the "one-shot" method or the so-called "quasi-prepolymer method" is employed, wherein the hydroxyl-containing component preferably contains from about 4 to 7 reactive hydroxyl groups, on the average, per molecule.

In accordance with the "quasi-prepolymer method", a portion of the hydroxyl-containing component is reacted in the absence of a catalyst with the polyisocyanate component in proportions so as to provide from about 20 percent to about 40 percent of free isocyanato groups in the reaction product, based on the polyol. To prepare a foam, the remaining portion of the polyol is added and the two components are allowed to react in the presence of catalytic systems such as those discussed above and other appropriate additives, such as blowing agents, foam stabilizing agents, fire retardants, etc. The blowing agent (e.g., a halogenated lower aliphatic hydrocarbon), the foam-stabilizing agent, the fire retardant, etc., may be added to either the prepolymer or remaining polyol, or both, prior to the mixing of the component, whereby at the end of the reaction a rigid polyurethane foam is provided.

Urethane elastomers and coatings may be prepared also by known techniques in accordance with the present invention wherein a tertiary amine of this invention is used as a catalyst. See, for example, duPont Bulletin PB-2 by Remington and Lorenz, entitled "The Chemistry of Urethane Coatings".

The invention will be illustrated further with respect to the following specific examples, which are given by way of illustration and not as limitations on the scope of this invention.

A mixture of 187 g bis(3-dimethylaminopropyl) amine and 68 g propylene oxide was sealed (in nitrogen atmosphere) in a stainless steel 1 liter autoclave and heated at 100.degree. C for 2 hours, then at 130.degree. C for 3 hours. NMR and IR spectroscopy revealed the major product was bis-(3-dimethylaminopropyl) (2-hydroxypropyl)amine.

This example illustrates use of the urethane catalysts here in a flexible urethane formulation. A high speed stirrer was used to mix 48.4 parts of toluene diisocyanate with the following blend of components:

The blended components were poured into a standard mold and allowed to rise. The observed properties are recorded below:

This example illustrates the use of the urethane catalysts here in a rigid urethane formulation. The components below were blended with a high speed stirrer, then poured into a standard mold and allowed to rise.

The rise characteristics observed were recorded below: