Polyurethanes of Improved Characteristics Biocompatible Urea-Modified Polyurethane From Castor Oil

The present invention is related to polyurethanes, and particularly to urea-modified polyurethanes.

The chemistry of urethane polymers extends over a very broad scope of chemical structures, producing products varying widely in properties and uses. A common variation from the basic urethane backbone structure (i.e.. the carbamate linkage) is the incorporation of urea linkages, which is generally accomplished by the use of diamines as chain extenders.

Among the many uses of polyurethanes are those involving medical devices, where polyurethanes are used as structural materials for separatory devices and prosthetic devices, and also as adhesives in such devices. The polyurethanes assume a variety of forms, in these materials, including sheets, tubing, structural components, and two-component adhesive composition. In many cases, the polymer is in intimate contact with internal human tissue. In such applications, compatibility with the tissue is essential, and certain types of polyurethanes, notably those in which the polyol component is castor oil or various polyols similar in structure and properties, have been found to provide biocompatible products.

The various polyurethanes and their components have certain disadvantages which affect their utility, particularly in medical use. Biocompatibility, as mentioned above, is a problem in many cases. In addition, the product color is important, particularly when the product must be observed during use. This may occur for example when one is monitoring a clinical treatment process or a patient's progress, where a high degree of clarity and transparency is desired. In two-component polyurethane systems, the stability of each of the components and possible dangers to those handling them (such as toxicity considerations, particularly carcogenicity) are a concern. Still further, certain combinations of diamines and polyols have a strong tendency to react with each other, thereby interfering with the formation of the carbamate groups in the basic polyurethane curing reaction, which not only interferes with the polyurethane formation but results in a loss of the chain-extending function of the diamine.

It has now been discovered that a narrowly defined class of diamines has unique qualities when used as chain extenders or modifiers in the formation of urea-modified polyurethanes. These diamines are nontoxic, highly stable, and avoid interfering reactions with polyols, particularly those such as castor oil and its analogs, which have been found to be particularly biocompatible. These diamines thus produce an exceptional polyurethane product.

The critical diamines in accordance with the present invention are those having the formula ##STR2##

In this formula, R.sup.1 and R.sup.2 are either the same or different, and each is a straight-chain alkyl group of two to six carbon atoms. In preferred embodiments, R.sup.1 and R.sup.2 each contain two to four carbon atoms, with ethyl and n-propyl particularly preferred. The symbols m and n are either the same or different and denote integers which may be either 1 or 2. In preferred embodiments, m and n are both 1. In further preferred embodiments, the diamine is symmetrical, the amino groups are located at the 4-positions on the phenyl rings, and the alkyl substituents are located at the 3-positions, or the 3- and 5-positions (in the case of either or both of m and n being 2). Examples of diamines within the scope of the invention are:

Di(2-ethyl-4-aminophenyl)methane

Di(2-n-propyl-4-aminophenyl)methane

Di(3-ethyl-4-aminophenyl)methane

Di(3-n-propyl-4-aminophenyl)methane

Di(3-n-butyl-4-aminophenyl)methane

Di(3-n-hexyl-4-aminophenyl)methane

Di(3-amino-4-ethylphenyl)methane

Di(3-amino-4-n-propylphenyl)methane

Di(3,5-diethyl-4-aminophenyl)methane

Di(3,5-di-n-propyl-4-aminophenyl)methane

Di(2,6-diethyl-4-aminophenyl)methane

Further examples are asymmetrical analogs of the above, involving various combinations of the substituted-phenyl groups.

Using these diamines, polyurethanes may be prepared in accordance with the present invention by any conventional technique, including both one-shot and two-shot methods. One-shot methods may be performed using polyurethane catalysts known to those skilled in the art. Examples include triethylene diamine and various organotin compounds such as di-n-butyl tin diacetate. Two-shot methods are likewise performed using conventional techniques. In general, the two-shot or two-package polyurethanes are prepared by first reacting a portion of the polyol with an excess of the polyisocyanate to form an isocyanate-terminated prepolymer (component A of the system). The prepolymer is then reacted with the remainder of the polyol, which is in the form of a solution containing a diamine within the scope of the above formula (component B).

The parameters of a two-package system may vary widely in value. In most cases, however, prepolymers with an isocyanate content of about 5 to about 30%. preferably about 10% to about 20%, will provide the best results. Likewise, a diamine content of component B ranging from about 0.5% to about 20%, preferably about 1% to about 10% (by weight) will provide the best results.

Relative amounts of components A and B used in forming the finished product may also vary. Preferred proportions will generally fall within the range of about 40% to about 60% by weight for each component. Particularly preferred proportions will be about 40% to about 50% by weight of component A. with the remainder being component B. The optimum levels and proportions in each case will vary with the desired properties and intended use of the finished product.

The reaction to form the polyurethane product may be conducted under conventional operating conditions. In the two-shot method, the components may be mixed at room temperature in the preselected proportions, particularly when the desired product is an adhesive. The product is then permitted to stand at room temperature, and will generally cure within about 5-10 hours. The two components may be prepared in advance, and sold shipped and stored as such, leaving the final polyurethane reaction to the user.

Polyisocyanates useful in the present invention are preferably diisocyanates. A wide range of diisocyanates may be used, including aromatic diisocyanates, aliphatic diisocyanates, cycloaliphatic diisocyanates, and blends of aromatic and aliphatic diisocyanates. Aromatic diisocyanates are preferred. Examples of diisocyanates are as follows:

ethylene diisocyanate

propylene diisocyanate

trimethylene diisocyanate

hexamethylene diisocyanate

cyclopentylene-1.3-diisocyanate

cyclohexylene-1.3-diisocyanate

cyclohexylene-1,4-diisocyanate

1,4-di(isocyanatomethyl)cyclohexane

1,3-di(isocyanatomethyl)cyclohexane

m-phenylene diisocyanate

p-phenylene diisocyanate

2,4-toluene diisocyanate

2,6-toluene diisocyanate

4,4'-methylenebis-(phenyl isocyanate)

3,3'-methylenebis-(phenyl isocyanate)

dibenzyl-4,4'-diisocyanate

1,4-naphthalene diisocyanate

1,5 naphthalene diisocyanate polymeric methylenebis(phenyl isocyanates), such as those obtained by the phosgenation of aniline-formaldehyde-derived polyamines

3,3'-bitolylene 6,6'-diisocyanate isophorone diisocyanate 4,4'-methylenebis(cyclohexyl diisocyanate) 1,6-diisocyanato-2,2,4,4-tetramethylhexane 1,6-diisocyanato-2,2,4-trimethylhexane

Preferred polyols for use in the present invention are mono-, di- and triglycerides of aliphatic carboxylic acids of 10 carbon atoms or more, particularly triglycerides of hydroxy-substituted aliphatic carboxylic acids. Notable examples are natural oils, particularly castor oil. A typical composition of castor oil is a combination of triglycerides glycerides of the following fatty acids:

The following example is offered for purposes of illustration, and is intended neither to limit nor define the invention in any manner.

This example compares various diamines within the scope of the present invention to those outside the scope of the present invention, in terms of stability, toxicity, and reactivity in forming the polyurethane. Those within the scope of the invention are the first two entries, MBOEA and MDPA. As the data in the table demonstrates, these two species are superior in overall properties to all others tested.

The foregoing is offered primarily for purposes of illustration. It will be readily apparent to those skilled in the art that variations, modifications and further substitutions of the elements and features of molecular structure, system parameters and operating conditions described above may be made without departing from the spirit and scope of the invention.