Mixed Ethylene and Propylene Carbonate-Containing Organic Polyisocyanate Adhesive Binder Composition

The present invention relates generally to the compression molding of lignocellulosic materials into composite bodies, sheets and the like and more particularly to an organic di- or polyisocyanate based adhesive binder, which contains a mixture of ethylene carbonate and propylene carbonate, for the lignocellulosic material which imparts equivalent or improved properties to the molded pressed composite product at substantially reduced isocyanate binder levels.

The molding of lignocellulosic and lignocellulosic containing fibers, particles or layers to form composite articles is known. The binders which normally are used are the synthetic resin glues such as a solution of urea-formaldehyde or phenol-formaldehyde resin in water. Composite products containing lignocellulose produced in this way lack durability and are susceptible to moisture conditions and deterioration in certain building purposes to which they may be subjected.

Di- and polyisocyanates as binders for lignocellulose materials have been proposed and are known to give products of increased stability and mechanical strength. Process technical advantages of polyisocyanates have been disclosed in German Offlegungsschrift No. 2,109,686. Two problems associated with the use of polyisocyanate binders are (1) even at reduced binder use levels, the cost of polyisocyanates as compared to the urea-formaldehyde or phenol-formaldehyde resin binders is unfavorable and (2) the viscosity of the normally employed polyisocyanate binders are much higher than the synthetic glue resins and are therefore difficult to apply to the lignocellulose with current application equipment or methods. The advent of technology to emulsify the polyisocyanates in water provided a binder product with reduced viscosity but with a service life of only 2 to 4 hours.

In a co-pending application of James M. Gaul and Tinh Nguyen, Ser. No. 293,564, filed Aug. 17, 1981 an organic polyisocyanate-liquid alkylene carbonate adhesive binder composition is described. In addition to other improved properties, the added liquid cyclic alkylene carbonate, such as propylene carbonate, imparted a reduced viscosity to the binder system allowing the use of present application technology. Because of its crystalline properties and melting point of 36.4.degree. C., ethylene carbonate per se, cannot be sprayed or employed with the polyisocyanates especially the solid or semi-solid di- or polyisocyanates without the application of heat and other costly techniques. It has been discovered however that ethylene carbonate can be completely dissolved in propylene carbonate at an ethylene carbonate weight ratio of from 5 to 80 weight percent at ambient (25.degree.-30.degree. C.) or room temperature to form a mixture which remains liquid and which may be used with an organic di- or polyisocyanate to give an improved adhesive binder composition for the preparation of compression moldable lignocellulosic materials.

The present invention comprises an improved polyisocyanate based adhesive binder composition that has been extended through the addition of a liquid ethylene-carbonate-propylene carbonate mixture and optionally lignin and at the same time avoids the above mentioned disadvantages.

This invention relates to the preparation of a polyisocyanate-ethylene carbonate-propylene carbonate binder composition and its use for the preparation of lignocellulosic composite molded products.

It is the principal object of this invention to provide a lignocellulosic binder and composition, containing a normally crystalline ethylene carbonate in liquid form, which gives properties equivalent to or better than prior molded pressed composite products at dramatically reduced di- or poly-isocyanate (referred to generally as polyisocyanates) levels and cost.

The advantages of the present invention is that levels of the di- or poly-isocyanate necessary to produce a cured pressed composite lignocellulosic product can be substantially reduced while maintaining equivalent or superior composite board physical properties. By employing the liquid ethylene carbonate-propylene carbonate-di- or polyisocyanate combination, with or without the use of lignin as a diluent, the binder resin economics are equivalent or better than the synthetic glue resin economics. The composite products made with the adhesive binders of the present invention have superior physical properties to those prepared with the urea-formaldehyde or phenol-formaldehyde binder systems. The addition of the liquid ethylene-propylene carbonate mixture to the di- or polyisocyanates substantially reduces the overall viscosity of the binder system while exhibiting a marked increase in service life of up to thirty (30) days. The binder system of the present invention have viscosities equivalent to or lower than the formaldehyde type resin systems in water, which allows the instant binder system to be applied to the lignocellulosic chips, particles, etc., using present application technology. As further advantages, the binder system of the present invention can (1) use the normally solid ethylene carbonate and (2) be employed with high molecular weight solid or semi-solid di- or polyisocyanates which by themselves would have properties which would preclude their use as adhesives for lignocellulosic products with current application techniques.

In accordance with the present invention an adhesive binder composition comprising a liquid ethylene carbonate and propylene carbonate mixture containing as high as 80 weight percent ethylene carbonate by weight and an organic di- or polyisocyanate, with or without the addition of lignin, is provided for the preparation of lignocellulosic composite bodies or sheets which comprises shaping in a mold or between mold surfaces in the form of a caul plate or platen a mixture of the lignocellulosic material and the isocyanate based binder of the present invention generally at temperatures of from about 140.degree. C. to 220.degree. C., preferably 160.degree. C. to 190.degree. C., at pressures of from about 100 to 600 psi for a period of from 1 to 10 preferably 3 to 5 minutes, there being provided at the interface of the mixture and mold surface or surfaces a release agent such as a metallic soap.

The organic polyisocyanates are generally applicable as binders in the present invention. Organic polyisocyanates which may be used include aliphatic, alicyclic and aromatic polyisocyanates characterized by containing two or more isocyanate groups. Such polyisocyanates include the diisocyanates and higher functionality isocyanates, particularly the aromatic polyisocyanates. Mixtures of polyisocyanates may be used which for example are the crude mixtures of di- and higher functionality polyisocyanates produced by phosgenation of aniline-formaldehyde condensate or as prepared by the thermal decomposition of the corresponding carbamates dissolved in a suitable solvent as described in U.S. Pat. Nos. 3,962,302 and 3,919,279 both known as crude MDI or PMDI. The organic polyisocyanate may be isocyanate-ended prepolymers made by reacting under standard known conditions, an excess of a polyisocyanate with a polyol which on a polyisocyanate to polyol basis may range from 20:1 to 2:1 and include for example polyethylene glycol, polypropylene glycol, diethylene glycol monobutyl ether, ethylene glycol monoethyl ether, triethylene glycol, etc. as well as glycols or polyglycols partially esterified with carboxylic acids including polyester polyols and polyether polyols. The organic polyisocyanates or isocyanate terminated prepolymer may also be used in the form of an aqueous emulsion by mixing such materials with water in the presence of an emulsifying agent. The isocyanates may also contain impurities or additives such as carbodiimides, isocyanaurate groups, urea, hydrolyzable chlorides and biurets as well as certain release agents. Illustrative of such di- or polyisocyanates which may be employed include, for example, toluene-2,4- and 2,6-diisocyanates or mixtures thereof, diphenylmethane-4,4'-diisocyanate (a solid) and diphenylmethane-2,4'-diisocyanate (a solid) or mixtures of same, i.e., containing about 10 parts by weight 2,4'- or higher, which are liquid at room temperature, polymethylene polyphenyl isocyanates, naphthalene-1,5-diisocyanate, 3,3'-dimethyl diphenylmethane-4,4'-diisocyanate, triphenylmethane triisocyanate, hexamethylene diisocyanate, 3,3'-ditolylene-4,4-diisocyanate, butylene 1,4-diisocyanate, octylene-1,8-diisocyanate, 4-chloro-1,3-phenylene diisocyanate, 1,4-, 1,3-, and 1,2-cyclohexylene diisocyanates and in general the polyisocyanates disclosed in U.S. Pat. No. 3,577,358. The preferred polyisocyanates are the diphenylmethane diisocyanate 2,4' and 4,4' isomers including the 2,2' isomer and the higher functional polyisocyanate and polymethylene polyphenyl isocyanate mixtures, which may contain from about 20 to 85 weight percent of the diphenylmethane diisocyanate isomers. Typical of the preferred polyisocyanates are those sold commercially as "Rubinate-M" (Rubicon Chemicals Inc.) and "Papi" (The Upjohn Co.). In general the organic polyisocyanates will have a molecular weight in the range between about 100 and 10,000 and will be employed in amounts of from about 20 to 95 parts by weight, preferably 50 to 75 parts by weight based on the polyisocyanate-carbonate mixture.

The liquid ethylene carbonate-propylene carbonate mixture is employed with the organic isocyanates in the present invention in amounts of from about 5 to 80 preferably 25 to 50 parts by weight based on the isocyanate-carbonate binder mixture. The ethylene carbonate-propylene carbonate mixture may contain 5 to 80 parts by weight ethylene carbonate but preferably the mixture will contain between 40 to 80 weight percent propylene carbonate and 60 to 20 weight percent ethylene carbonate.

As indicated hereinabove the addition of the liquid ethylene-propylene carbonate mixture substantially reduces the overall viscosity of the binder system providing ease of application. Typical of the viscosities for a PMDI-ethylene-propylene carbonate mixture (60 parts ethylene carbonate and 40 parts propylene carbonate by weight) attained in centipoise, (c.p.) at 25.degree. C. are as follows using a diphenylmethane diisocyanate-polymethylene polyphenyl isocyanate mixture, sold commercially as "Rubinate-M" by Rubicon Chemicals Inc. and having a viscosity of 230 c.p. at 25.degree. C. The viscosities in (c.p.) was determined after aging 1, 7, 12 and 28 days at various ratios by weight of polyisocyanate to the mixed carbonate.

In addition a solid diphenylmethane-4,4'-diisocyanate mixed with the ethylene-propylene carbonate mixture above at a parts by weight ratio of the 4,4' isomer to the carbonate mixture of 2.5:1.0, 2.0:1.5 and 3.0:0.5 and gave a liquid mixture with less than 65 c.p., a clear liquid mixture of 12 c.p. and a liquid mixture of less than 110 c.p. respectively.

The lignin which may be employed as a diluent in the binder system of the present invention are used in amounts of from 0 to 60 weight percent, preferably from 20 to 35 weight percent based on the total polyisocyanate-mixed carbonate-lignin mixture. The exact composition of lignin varies. Lignin derived from a variety of sources may be used in the binder system of the present invention to form the adhesive mixture. One source is a mixture of waste liquors resulting from the chemical pulping of lignocellulose. Suitable lignin which may be employed include, for example, purified pine wood lignin, kraft waste liquor, soda waste liquor, calcium, magnesium, sodium and ammonium base sulfite liquors, chlorinated waste liquors, etc. Lignins from hardwood and softwood sources may be used as well as lignins from acid-precipitated and alkali-reconstituted kraft liquors. The abundant and available chlorinated waste liquors derived from paper mill bleach may be used. The waste liquors may be used in their original condition in the adhesive binder composition of the instant invention. The lignin may also have a wide range of pH and a solids content of from about 10 to 90 weight percent. When employed, the lignin is simply physically mixed with the isocyanate-ethylene-propylene carbonate adhesive mixture when prepared or applied directly to the lignocellulosic material.

Lignocellulose, according to the present invention used to prepare the lignocellulosic-containing composite articles include wood chips, wood fibers, shavings, sawdust, wood wool, cork, bark and the like products from the wood-working industry. Fibers, particles, etc. from other natural products which are lignocellulosic such as straw, flax residues, dried weeds and grasses, nut shells, hulls from cereal crops such as rice and oats and the like may be used. In addition, the lignocellulosic materials may be mixed with inorganic flakes or fibrous material such as glass fibers or wool, mica and asbestos as well as with rubber and plastic materials in particulate form. The lignocellulose may contain a moisture (water) content of up to about 25 percent by weight but preferably contains between 4 and 12 percent by weight.

In manufacturing lignocellulosic composite products for the purpose of this invention, such as flakeboard for example, a small amount of the binder (with or without lignin) along with the lignocellulosic material is simply milled or mixed uniformly together. Generally the isocyanate-mixed carbonate binding agent (with or without lignin) is sprayed onto the material while it is being mixed or agitated in suitable and conventional equipment. Various type mixing equipment such as an intensive-shear mixer or tumbler may be employed. The binder use levels according to the present invention, based on the weight of oven dried (0% moisture content) lignocellulosic material is from about 1.5 to 12 preferably 2.5 to 6.5 weight percent. The lignocellulose-adhesive binder mixture is generally sprinkled or formed in the desired proportions onto a caul plate of aluminum or steel which serves to carry the "cake" into the press to give the desired thickness or product, the plates having been sprayed or coated with a release agent such as the metal soaps like the iron, calcium or zinc stearate compounds. Other conventional release agents such as sodium phenolate and certain tertiary amines may also be employed.

The following examples are provided to illustrate the invention in accordance with the principles of this invention but are not to be construed as limiting the invention in any way except as indicated by the appended claims.

In the Examples which follow, the test results set forth are expressed as ASTM D 1037 values and wherein

580 g. of pine wood chips dried to a moisture content of 6 percent are placed in an open tumbler-mixer. During tumbling, 3.5 parts based on 100 parts by weight of the wood of an isocyanate based binding agent prepared by mixing 50 parts crude liquid diphenylmethane diisocyanate-polymethylene polyphenyl isocyanate (PMDI) mixture having 46.5 percent diphenylmethane diisocyanate content and 50 parts of ethylene carbonate (E.C. 50%)-propylene carbonate (P.C. 50%) mixture are sprayed over the chips by an air pressurized system. Two caul plates 12 inches square are sprayed evenly with a zinc stearate external release agent. Using a former box, a 10.5 inches square "cake" of the chip-binder mixture is formed on the caul plate. The second caul plate is placed over the cake which is pressed to stops at a thickness of 13 mm at 185.degree. C. for 5 minutes after which the pressed cake is released from between the press plates and cut into specimens for physical testing. Test results of the composite board is 158 psi I.B. and a T.S. of 19.7 percent.

The procedure of Example 1 is repeated except that different levels of the ethylene carbonate-propylene carbonate mixture at various E.C. to P.C. ratios are substituted with the crude liquid diphenylmethane diphenylmethane diisocyanate-polymethylene polyphenyl isocyanate (PMDI) mixture having 46.5 percent diphenylmethane diisocyanate content while maintaining the total binder level constant at 3.5 percent based on the weight of the wood. Results are given in Table 1 below.

The procedure of Example 1 is repeated except with the following changes. The total binder level is held constant at 4.5 percent based on the weight of the wood. A third component, lignin, a product resulting from the chemical pulping of lignocellulose, is added to the binder mixture. The liquid diphenylmethane diisocyanate-polymethylene polyphenyl isocyanate (PMDI) mixture having a 46.5 percent diphenylmethane diisocyanate content and carbonate content varies with the 50--50 mixture of E.C.-P.C. mixture over the range specified in Table 2 below.

A number of runs are made in accordance with the procedure of Examples 1 to 10, utilizing 40 parts by weight ethylene carbonate-propylene carbonate mixture at various ratios and 60 parts by weight of various polyisocyanate compositions. The binder level is maintained at 4.5 weight percent based on 100 parts by weight of the wood. The E.C. to P.C. ratios and isocyanates with test results are set forth in Table 3.