Manufacture of Polyurethanes From Bark

Polyurethane compositions and their preparation have been investigated intensively throughout the last few decades. These polymer products have proven extremely versatile and are utilized throughout industry. In particular, urethanes which have been applied as soft or hard elastomer coatings or as adhesives and which are either rigid or flexible and in compact or foamed form have found exceptional commercial utility.

Polyurethanes are polymers characterized by repeating carbamic ester groups. They are produced by reaction of polyisocyanate with polyol. This reaction proceeds readily under a variety of conditions, now well-known in the art.

These conditions, as well as many modifiers of the reaction (such as catalysts, surfactants, blowing agents and the like) are described in detail in Polyurethanes: Chemistry and Technology, Vol. XVI, parts I and II of The High Polymers series of Saunders and Frisch, Interscience Publishers (1962 and 1964). These books are incorporated herein by reference as if they were set forth at length.

This invention relates to the discovery that useful polyurethane compositions may be produced through utilization of bark and/or its components as the polyol in the reaction with polyisocyanate. More particularly, it has been discovered that bark and/or its components possess sufficient active hydrogen in available form to react with polyisocyanate to produce rigid polyurethane compositions.

One of the few uses for the millions of tons of bark which are produced each year in the lumber and paper industries, is as a combustible. Even for this purpose, however, there are many drawbacks. Bark is not a particularly efficient fuel and often gives rise to severe pollution problems. This invention therefore provides a new use for the substantial and extremely inexpensive amounts of this material which may otherwise require disposal.

Bark is ordinarily provided in comminuted form. This form provides an increased surface area which facilitates higher degrees of reaction. These comminuted particles of bark should preferably have a mesh size of from 50 to 325, most preferably from 100 to 300.

In place of the bark, polyol derivatives thereof, for instance tannin or other compositions containing polyols can be used.

The tannins which can be employed in this invention are the astringent, aromatic glucosides common to plants and trees. As is known in the art, tannins commonly include a broad mixture of individual compounds including the gallotannins; galloyl sugars; ellagitannins or diphenylmethyloids; and catecholtannins or phlobatannins. In the context of this invention, however, the particular compound or mixture of compounds present or predominant is of no regard. It is necessary only that one or more members of the class generally recognized as being tannins is present.

The physical form of the tannins at the time of reaction with polyisocyanate is also important in this invention. Tannins may be present either in their normal acidic state or as basic salts and in solid form. Because of its readily availability, however, it is convenient to utilize a material such as commercial wattel tannin.

In the preferred embodiment of this invention, the polyol component for reaction with the isocyanate is bark, most preferably a soft wood bark, and more preferably, pine bark.

Polyisocyanates useful for formation of polyurethanes with the bark include both the polyisocyanate and other diisocyanates known in the art. These polyurethane precursors encompass a large number of individual compounds, most preferred of which are toluene diisocyanate and p-p' diphenylmethane diisocyanate and polymeric diisocyanate (crude MDI).

The stoichiometry of reaction of these polyisocyanates with bark materials varies, dependent largely upon the hydroxyl number of the bark. Optimum stoichiometry may readily be determined as is known in the art however, through correlation of the reactivities of the specific bark or bark components and specific diisocyanate precursor employed. Thus, by analysis of the percent hydroxyl values of the bark and bark components (tannin), the hydroxyl number can readily be calculated and therefrom, optimum proportions of reagents are readily determined.

The results and advantages of this invention do not require exclusive use of bark and or bark components i.e. tannin or tannin materials as the source of polyol for production of polyurethanes. Mixtures of bark material with conventional polyols, such as the glycols and glycerols, can be utilized in combination. In this manner, the known advantages in product properties resultant from these other polyols may also be incorporated into the polyurethanes of this invention. In these circumstances, optimum stoichiometry is again readily determined, as indicated above, based on the hydroxyl number of these polyols. Preferably, however, bark and/or bark components should comprise at least 30%, most preferably from 50 to 100% by weight of the total polyol reagent employed.

In addition to the formation of dense polyurethanes, lighter foam products may also be produced. Foaming, which permits production of polyurethane compositions having densities ranging from about 1 to 40 pounds per cubic foot, may be induced by incorporating conventional blowing agents such as Freon or water within the initial reaction mixture.

Other conventional additives, such as catalysts, surfactants and the like, are also useful for modification of the present polyurethane foam compositions. These additives function in the same manner and by the same means for which they are well-known in the prior art.

In addition to substantially reducing product cost by using an inexpensive or waste bark and/or bark component material in production of polyurethane and polyurethane foams, the present invention has solved one of the major physical drawbacks of the prior art. Known polyurethane compositions are highly flammable and thus have engendered substantial research into methods for overcoming this property. This research has generally involved either the incorporation of flame-retardant materials into the polyurethane or direct alteration of the polymer itself.

Evaluation of these (bark-polyisocyanate) polyurethane foams, it was found that they possess a high degree of thermal stability as well as being fire-retardant (self extinguishing). This inherent flame retardancy of polyurethane foams in general is almost entirely a function of the chemical composition of the solid polymer and the flammability of polyurethane foams can be reduced by using:

(a) components with a high degree of aromaticity

(b) High molecular weight polyols

(c) polyols with a high functionality-- at least 4

(d) aromatic isocyanates with a functionality of 2.3 to 3.2

(e) cyclic rather than open chain polyols

Bark-- which contain the following principal components lignin, cellulose, tannins and related water insoluble phlobaphenes, other organic components-- saponins, mannitol, dulcitol, etc., contain within these components many of the above listed inherent fire-retardant characteristics, i.e., high degree of aromaticity (lignin, tannin, cellulose, etc.), high molecular weight component with high functionality, cyclic rather than open chain polyols. Furthermore, the polyisocyanates (crude MDI) used possess high degree of aromaticity as well as high functionality.

Thus, the fire-retardant properties obtained are mainly derived from the design of the components used.

The following examples are further illustrative of this invention.

Pine bark (100 mesh) 40 grams was blended with polyisocyanate MDI (crude) 100 grams and allowed to remain overnight. The material thusly obtained became very hard and was possessed of large cell size.

An exotherm developed on blending of the aforesaid components and the resultant material was foamed. Water was employed as the blowing agent.

A rigid polyurethane foam was produced from the following ingredients:

The bark was mixed with the polyisocyanate while the other components (catalyst, surfactant and blowing agent) were added while mixing continued. The resultant rigid polyurethane foam had the following properties:

The burning rate (BR) in inches per minutes was calculated by the following equation: ##EQU1##

A low density rigid polyurethane foam was produced from the following ingredients:

Utilizing the technique of Example III, a product was obtained having the following physical properties: