GB1598199A - Adhesive resins containing sulphur - Google Patents

Description

(54) ADHESIVE RESINS CONTAINING SULFUR (71) I, CARL BEAT MEYER, a Citizen of Switzerland, of 6800 42nd Northeast, Seattle, State of Washington, United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention is concerned with certain improvements in thermo-setting adhesive compositions containing a heat-curable urea-formaldehyde resin.

There is an increasing need for new adhesives which are strong and cheap for use in making bonded wood products. Because of dwindling forest resources, solid wood has been replaced in many instances by products, such as particle board, which are made from materials which used to be considered wood wastes. In such products, adhesives account for up to 10% of the weight and up to 50% of the cost of the product, and they substantially determine the nature and quality of the resulting product. Furthermore, new building technology encourages the production of self-supporting, prefabricated laminated wood panels and other bonded material which need to fulfill reliably exacting standards.

Ideally, adhesives for such applications should consist of readily available ingredients and should be mixed and formulated easily, preferably using customary procedures or simpler modifications thereof. For this purpose, the adhesives should have a long storage time, should be quick setting on application and should be non-toxic, nonodorous, and useable with standard equipment. Additionally, the bonded product obtained with the adhesive should have good strength, elasticity, durability, acceptable color and no odor.Furthermore, the adhesive should be useful over a wide range of working conditions because, even though good bonding strength may be possible under certain conditions, the resin may not be acceptable if such results cannot be readily achieved over a wide variation of operations so as to accommodate accepted local production methods or the use of different types of materials. For example, a company for making bonded wood products might use Southern Pine Veneer in one location and Douglas Fir in another, and a good wood adhesive should be suitable for use with either kind of wood. Finally, the adhesive and its application should not be costly.

A variety of adhesive resins are now in use which satisfy at least some of the forest product industry's requirements. However, only few of the presently known adhesives are suitable for large-scale use. Of these, the isocyanates are excellent for making particle board. However, they are expensive and their application requires stringent quality control.

Melamine-formaldehyde resins are also costly. Phenol-formaldehyde resin has many of the desirable physical properties and is now the standard glue for making plywood and for exterior use, but its color and price make it undesirable for assembling particle boards, which contain up to 10% of their weight in glue. Furthermore, the phenols cure slowly, and these adhesives are dark and thus not suitable for gluing thin, decorative veneer.

For large-volume applications, especially wood particle board, urea-formaldehyde resins are used almost exclusively because they have been the cheapest. most reliable adhesive known. Unfortunately, their water resistance is limited, and in order to obtain good bonding characteristics, an excess of formaldehyde is necessary. This gives the board an objectionable odor which persists and can be noticed in the processed product for many months. Other glues are either too expensive or not used because they impart objectionable properties to the product or cannot be applied with present equipment and methods used in the forest product industry.

The principal object of the present invention is to provide an adhesive which is suitable for use in making bonded wood products or the like which meets the requirements set forth above and which obviates difficulties and disadvantages encountered with prior art adhesives.

A more specific object of the invention is to modify prior art wood adhesives so as to provide an adhesive which is capable of use over a wider variety of conditions to give bonded products having optimum properties.

A further object of the invention is to provide bonding compositions which make possible a significant reduction in cost while, at the same time, permitting the use of conventional equipment and process conditions.

Other objects will also be hereinafter apparent from the description of the invention which follows: Broadly stated, the above and other objects are realized by adding elemental sulfur to conventional wood adhesive formulations, based on heat-curable urea-formaldehyde condensation products. The added sulfur may be used to correspondingly reduce the amount of resin normally used or the sulfur may be simply incorporated into a conventional formulation containing the normal amount of resin. Thus, in one illustrative example, 165%, preferably 350%, by weight, of the urea-formaldehyde resin solids in a conventional adhesive for making pressed wood particle board may be replaced by an equal amount of sulfur added in elemental form.

The adhesive compositions of the invention will contain from 0.0599%, preferably 165%, and most preferably 350%, by weight, of added sulfur, based on total solids content. The sulfur-containing adhesive compositions of the present invention possess bonding strengths equal to or greater than the adhesive containing the same total amount of solids but no sulfur. Surprisingly, the substitution of 10% upwards (e.g., 1060io) of adhesives with sulfur provides resulting adhesives exhibiting comparable or increased strengths. The remaining solids will comprise one or more of the conventional urea-formaldehyde adhesive resins, possibly with minor amounts of optional additives, such as fillers and extenders.

The compositions of the invention are easily prepared by adding the desired amount of elemental sulfur to the indicated adhesive resins in liquid medium, advantageously water, with appropriate mixing. The addition of a further amount of water may be desirable in some cases to give the final composition the viscosity preferred for use.

The resulting sulfur-containing suspension can be applied to wood particles or other material to be bonded and then cured using the same conditions and equipment as normally employed with the corresponding sulfur-free adhesives. This is a particularly important advantage of the present compositions since their use does not require modification of existing processes and equipment for making bonded wood products.

The compositions of the invention can be used in a wide variety of ways, for example, as hot setting formulations for making wood particle boards, the manufacture of plywood, the assembly of laminated boards, the impregnation of wood, paper and textiles, for surface coatings or as adherence layers for adhesives for connecting metals or metal alloys to wood, rubber, aminoplasts or phenoplasts. Furthermore, the present compositions are suitable as fillers for adhesives or molding products, as gap-filling cements for plywood and as fillers for knotholes or the like. Additionally, these compositions can serve as extenders for adhesives, as modifiers, as molding materials and, in general, wherever a fast-setting, high-molecular weight adhesive resin is needed.

While the compositions of the invention may be prepared by adding only elemental sulfur to otherwise conventional wood adhesive resins of formulations containing the same, various modifications are also contemplated. For example, the desired sulfur content may be obtained using a mixture of elemental sulfur and one or more polysu Ifides, i.e., metal or organic polysulfides. The polysulfides may be preformed or they may be formed in situ. Additionally, as a further modification, sulfur in elemental form or partly in combined form (e.g. H2S or polysulphide) may be added at any of a variety of production stages for the resins.Thus, for example, some of the elemental sulfur might be dissolved in sodium hydroxide or in an aqueous formaldehyde solution, or melted with urea, before the normal glue preparation procedure begins, or the sulfur might be added in part at any intermediate preparation steps, or the sulfur may be added to the finished conventional adhesive when it is ready to be applied. For this purpose, commercial grade sulfur can be mixed with a commercial resin and the mixture milled or ground by a variety of techniques, or the sulfur can be first ground and then added to the adhesive. The viscosity of the resin solution can be adjusted by adding various additives, often plain water. Furthermore, the composition of the invention can be used in mixed batches which can be processed jointly with conventional resins. Furthermore, high-strength compositions can be blended to reinforce parts of boards which will be used to attach the board to hinges, frames or studs, or a specially low-priced composition can be used to prepare the core or intermediate layers of insulating boards which do not need to have high elasticity or strength. The preparation of the adherent surface is usually similar or identical to that used for conventional adhesives.

It will be appreciated that an inherent advantage of the invention is the fact that elemental sulfur is available in large quantities. Additionally, it is non-toxic and nonodorous and has properties which are intrinsically compatible with the hydroxy groups and double bonds of wood. For the purpose of this invention, elemental sulfur is defined as any allotrope of this element, soluble or not, that contains up to 10% of any natural or artificially added impurity. Claus sulfur, Frasch sulfur and sulfur recovered from SO2 abatement are usually 98% pure or even better and are suitable for use in the compositions of the present invention in any of the commercial forms. The physical state of the sulfur will influence its reactivity and bonding rate.Preferably, the sulfur is ground, if necessary, so that it is compatible with commercial glue spray and spreader systems, but such need not be ground to the extent necessary to achieve a homogenous mixture.

While not wishing to be limited to the reasons therefor, it appears that the success of the invention is due to some kind of synergistic effect resulting from the combined use of sulfur and resin. The reasons for this are not completely understood, but apparently the sulfur reacts with the adhesive resin, the wood and possibly the natural wood resin, and these components and/or their reaction products otherwise coact to give results which are not attainable using either sulfur or the resin components alone.

Thus, while it has been known that elemental sulfur and polysulfides, when properly applied, can yield a strong, quick-setting and water-resistant bond to wood (see, for example, U.S. Patents Nos. 3,855,054 and 3,252,815), these materials have not previously been suitable for large-scale use because they have to be applied at a temperature too high to be compatible with wood, the product must be formed at high pressure, and during the manufacturing, unpleasant and corrosive vapors are released. Furthermore, the products, due to the manufacturing methods, are of undesirably high density and brittle.

It has been discovered that in the bonding reaction with wood above 140"C, sulfur extracts some of the natural wood resins with which it is miscible and slowly reacts, and partly exchanges position in the cellular structure, giving an excellent surface bond.

However, during this process, because of temperature and change in solubility, the wood loses its moisture and its surface water film and thus some of its hydrogen bonding.

This leads slowly to irreversible, destructive changes at and below the glue line. All known economical modifiers and plasticizers which could prevent the brittleness also react destructively with wood and, furthermore, exude a lasting, undesirable odor. However, by using the sulfur and wood resin adhesives together according to the present invention, it has been found possible to take advantage of the desirable wood bonding characteristics of sulfur while avoiding the indicated disadvantages such as brittleness and odor previously encountered.

That sulfur reacts with aldehyde resins and their precursors is a well known fact. Thus, it is known, for example, that sulfur reacts with phenols alone or with phenols in the presence of caustic to yield phenoplasts (see, for example, U.S. Patents Nos. 3,717,682 and 2,035,098) which have useful and attractive properties and can be reinforced with 5-150 weight percent sulfur to yield curable resins (see U.S. Patent No. 3,438,931). The compounds referred to herein as phenols are those potentially reactive aromatic hydroxy compounds, such as phenol, resorcinol, xylenol, cresylic acid or other mono- or dihydric phenols known in the art. Unfortunately, however, such phenols release H2S during the initial step of the reaction with sulfur, when polythioliphenols are formed (see again U.S. Patent No. 3,717,682).Furthermore, the resulting resins are not watersoluble, and the setting of these resins requires too much time or too high a temperature (see Cherubim, Kaut. Gummi 19, 676, 1966) and otherwise defies conventional wood or forest product industry techniques to find use in making bonded wood products.

Urea-sulfur mixtures have also recently been carefully studied for use as fertilizer compositions (see U.S. Patent No. 3,313,613; Ger. Offen. 2,451,723; U.S. Def. Publ. T 912,014; and U.S. Patent No. 3,295,950).

Sulfur is miscible with urea to a limited extent and does not react quickly at the temperature at which wood adhesives must be used. With formaldehyde solutions, sulfur reacts over the entire aqueous temperature range, and in the pH range (see U.S. Patent No. 2,174,000) of 1 to 10 forms a variety of rubbery resins from which molded articles with many outstanding properties can be shaped (see U.S. Patents Nos. 3,303,166; 3,342,620; 2,195,248; 2,174,000; 2,429,859; 2,454,635; 2,255,228; 2,012,347; 2,206,641; 2,255,228; 2,039,206; 1,890,191; 1,991,765; and 1,964,725; and "The Chemistry of Synthetic Resins," Reinhold, 1935, p. 1183).

These compositions suffer from similar drawbacks as the sulfur phenoplasts and cannot be used with conventional equipment and techniques for making bonded wood products.

Notwithstanding the problems indicated by the prior art, when using sulfur alone for wood bonding purposes or for reaction with urea-formaldehyde resins or their precursors, the combined use of elemental sulfur and urea-formaldehyde resins, as proposed herein offers numerous advantages. These apparently result from a combination of possible reactions between, for example, sulfur and wood, sulfur and formaldehyde, and sulfur and urea to yield strong elastic and chemically stable bonds. This is apparently partly due to the fact that whatever reaction products are formed allow the wood to retain much of its natural moisture, and to "work", i.e., expand and contract in response to changes in humidity in the environment.

Retaining such natural properties of wood is vital for establishing a strong, lasting product. Likewise, the present adhesive facilitates the bond between wood products and metal or metal alloys. Additionally, as noted, the compositions of the invention avoid certain disadvantages which are inherent in prior wood adhesive compositions based on the use of aldehyde resins. Thus, for example, the present adhesives are substantially more economical than urea-formaldehyde or phenol-formaldehyde adhesives; they are more moisture resistant than conventional ureaformaldehyde, and they give equal or better mechanical properties as measured by many of the common ASTM 1037 and 1038 tests.

Furthermore, the compositions of the invention can be made in such a manner as to optimize strength, to minimize costs or to optimize elasticity while still meeting other standard requirements for particle board or plywood adhesives. As noted, the present compositions can be applied with equipment and methods identical to those conventionally used in the art. A further advantage of the present compositions is their color. Thus, for example, sulfur urea-formaldehyde products are light yellow. Furthermore, the invention yields odorless materials which lack the objectionable odor of formaldehyde that is common to products prepared by conventional formaldehyde adhesives. Additionally, the bonded products of the invention lack the odor of organic or inorganic sulfides which is common to sulfur-phenoplasts, formaldehyde-sulfur compositions and to modified sulfur.Furthermore, in some of the present compositions, the sulfur appears to decrease the inflammability of the products, probably by a mechanisni similar to that observed when sulfur is added to polyurethane resins and foams (see U.S. Patent No. 3,542,701), and it may also prevent the discoloring observed for polyurethane (see U.S. Patent No. 3,222,301).

It will be appreciated that the basic chemical composition of the invention always includes added elemental sulfur. In any event, the added elemental sulfur may ultimately be present in the adhesive composition or bonded products obtained therewith, at least in part, in the form of a variety of reaction products.

The compositions of the invention are generally prepared in aqueous medium, and if caustic is present when the elemental sulfur is added, the sulfur slowly reacts to form polysulfides of an average rank of 4.5 and sulfite. In the resin reactions of this invention, unreacted elemental sulfur, polysulfides and sulfite all contribute to the formation of products. If desirable, adhesive compositions can be formulated with polysulfide and sulfite as separate ingredients, but this is usually not economical.

As noted, the total sulfur content of the present compositions varies from 0.0599% on the basis of total solids. The formaldehyde-to-urea ratio preferably will range from 1:50 to 50:1, i.e., the formaldehyde content may exceed that normally used in ureaformaldehyde resins, but should not be more than one mole in excess of the sum of the urea and sulfur. The best formulations contain mixtures in which there is a slight surplus (e.g., about 15%) of sulfur in excess of that needed to theoretically react with all other reactable components.

Depending on the physical and chemical form of the ingredients and their ratio, and depending on pH, temperature, reaction time and reaction sequence, the adhesive resin contains different reaction products, different degrees of polymerization and different degrees of cross linkage, which gives the adhesive different tack and other physical and chemical properties, which, in turn, determine the reaction during the setting of the adhesive, and thus the process conditions and final properties of the product. For example, formulations in which formaldehyde is slowly premixed with part of the sulfur tend to yield tough products (see E.

Baumann,- Ber. 23, 60, 1890) which contain polymethylene polysulfides which exhibit excellent high-temperature stability and are usually solvent resistant; high-sulfur resins possess high tensile strength, but tend to be more brittle: If the reaction of formaldehyde and sulfur or its derivatives is conducted at low pH, trithiane is formed as an intermediate (see Walker, "Formaldehyde", ACS Monograph Series, Rheinhold Publ., London 1960).

The role of the various ingredients in the present invention is not in all compositions the same and. furthermore, different intermediates may engage in different synergistic interactions. For example, in low-sulfur compositions, the sulfur partly provides S-S linkage and stabilization of terminal groups, forms some thio-aldehyde products and serves as filler. All these functions can be used to improve the bonding of the ureaformaldehyde or phenol-formaldehyde resin.

In high-sulfur compositions, sulfur and its copolymerization products which are partly miscible in natural wood resins substantially contribute to the adhesive bonding function, and the urea-formaldehyde serves partly as a wetting agent, as well as polymer component.

In intermediate compositions, the urea-formaldehyde component help prolong the water-soluble stage of the reaction, while the sulfur helps to shorten the setting time and substantially reduces the solubility of the fully set adhesive.

The invention is illustrated, but not limited, by the following examples. As used herein, all percentages and ratios are by weight unless otherwise noted.

EXAMPLE I 95 grams commercial grade prilled sulfur from a sour gas Claus plant was mixed with 160 grams commerical urea-formaldehyde particle board resin containing 95 grams, i.e., 60% resin solid, and water was added to adjust total solid to 60 weight percent of mixture. The mixture was blended for 2 minutes in a mechanical blender until the sulfur broke into particles and the mixture could pass through the nozzles of a commercial paint spray gun. At the time of application, the viscosity was about 160 cp (at 25do) and the gel time about 3 minutes at 100"C.

100 grams of this glue was applied to 950 grams of Douglas Fir chips with a dimension of 0.015xapp. l/8-3/8xapp. 1 inch and a moisture content of about 6%. This composition was used to press sixteen 10X10X5/8 inch particle test boards having a density of 0.70--0.75 gr/cm3, i.e., the density of the commercially most common particle board.

The glue application, mat formation and pressing followed common practice known to those skilled in the art. The boards were pressed at 300"F for about 5 minutes and then stored for 1 week at room temperature in air at 70"F and 50% humidity. The finished boards contained urea-formaldehyde and sulfur in a weight ratio of 1:1, and an average 6 weight percent of resin solid. After 24 hours soaking in room-temperature water, the sixteen sulfur-containing boards exhibited only about 13--20% thickness swell, while sixteen similar sulfur-free boards made without including sulfur in the urea-formaldehyde glue swelled about 4250%, i.e., about 3 times more.

The boards made with the sulfur-containing glue exhibited an average tensile strength of 110 lb/sq inch as compared to an average value of 105 Ib/sq inch for boards made from the same chips and the same urea-formaldehyde glue without sulfur, and the same total resin solid content. Both batches of boards had the same color, hardness and odor.

EXAMPLE 2 Several boards were made according to the same procedure as Example 1 but flowers of sulfur were used, the total resin solid content was 8% (average) and the ratio of the ureaformaldehyde to sulfur was 9:1. These boards and several boards made with 8 weight percent of solid resin urea-formaldehyde without sulfur both had average tensile strengths of 150 lb/sq inch and a similar modulus of rupture.

As in Example 1, the sulfur-containing boards could be immediately recognized after soaking for 24 hours in room-temperature water because of their distinctly lesser swelling. The sulfur-containing boards also were distinctly more fire resistant than sulfur-free samples, as measured by ASTM1692-59 tests. This corresponds to the effect sulfur is known to have on the fire resistance of polyurethanes (V. Raamsdonk, U.S. Patent No. 3,542,701 and British Patent No.

1,107,237), and might be connected with the increased fire resistance elemental sulfur is known to gain from admixture of various organic materials (Ludwig U.S. Patent No.

3,440,064; Kobbe U.S. Patent No. 1,853,818).

EXAMPLE 3 A sulfur urea-formaldehyde resin was prepared from 250 grams 47% formaldehyde solution and 130 grams urea by mixing for 60 minutes at 75"C at pH 8.5, and subsequent condensation at 80"C at pH 5.0 until the viscosity reached 85 cp, according to procedures known to those skilled in the art. 190 grams of this resin were blended with 90 grams commercial prilled sulfur and 80 grams water were added. The viscosity of the resulting glue was 180 cp and the gel time was 9 minutes at 100"C. The sulfur-free resin had a viscosity of 200 cp and a gel time of 7 minutes.

The glue containing urea-formaldehyde and sulfur in a weight ratio of 1:1 was applied to 3 kg wood chips. A mat was prepared and an 18 x 18 x 5/16 inch particle board was pressed with techniques known to those skilled in the art. The internal bond strength was 130 Ib/sq inch, the modulus of rupture was 5430 Ib/sq inch, the modulus of elasticity was 600,000 lb/sq inch and the swelling after 96 hours of soaking in roomtemperature water corresponded to a weight gain of 69%. All tests were conducted according to ASTM 1037-72a.

EXAMPLES A 4*11 The urea-formaldehyde resin of Example 3 was mixed with sulfur to give a ratio of urea-formaldehyde resin solid to sulfur of 99:1; 9:1; 4:1; 3:2; 1:1; 2:3; 1:4; and 1:9. In all cases, the total resin solid, sulfur and ureaformaldehyde combined added up to 120 grams and was applied to 1600 grams wood chips containing about 5% moisture to yield a total resin solid content of 8%, by weight.

The viscosity of the resins varied between 160 and 200 cp at the time of application.

The gel times of the resins were between 7 and Il minutes at 100 C.

Boards were made as in Example 3. All of those boards had comparable internal bond strengths of l20j20 Ib/sq inch. Boards made using a urea-formaldehyde:sulfur ratio equal to I were distinctly strongest, and the strength seemed to decrease with shift in sulfur content towards both higher and lower values. After soaking the boards for 24 hours in room-temperature water, it was noted that swelling decreased almost linearly with sulfur content. However, after 90 hours of soaking, all samples had swollen similarly.

All boards had the yellow color characteristic for urea-formaldehyde bonded board. Compositions containing 5 or more percent of sulfur were free of the offensive formaldehyde odor common to finished urea-formaldehyde boards. Boards containing more than 50% of sulfur in the resin gave off a faint smell of elemental sulfur during the last minute of hot pressing. When the board was released, and still hot, the vapor immediately ceased and no odor was noticeable. When samples of the boards were stored at 150"C for 20 days, they initially exuded a faint but noticeable smell, reminiscent of organic sulfones. which subsided after about 5 days. At the end of the test, sulfur-containing boards were not recognizable from sulfur-free boards.

EXAMPLE 12 A resin containing urea-formaldehyde in the ratio 1:1.6 was fortified with 40% furfuryl alcohol according to practice known to those skilled in the art (U.S. Patent No.2,518,388).

90 grams of this resin and 30 grams wettable sulfur were mixed and applied to 1 kg commercial wood woodchips as used for the intermediate layer of a 5-layer particle board.

The face and center layers were formed from the above, fortified urea-formaldehyde glue and commercial wood chips used for face and center, but without any sulfur. This board had comparable properties to a board made with the same procedure, but without sulfur in any of the layers.

EXAMPLE 13 A I X I foot board was made according to Example 11, but 1% of a commercial wax of the type used in particle board manufacturing was added to the glue composition.

Again, the sulfur-containing and the conventional board exhibited similar properties, but in the soak test, the swelling of all boards was reduced.

EXAMPLE 14 Pairs of pine blocks 1x2x2 inch were preheated to 160'C and the glues of Examples 3, 5, 7, 9 and 11 were spread on the faces which were then assembled and clamped to yield glue lines of about 0.1, 1 and 3 mm thickness. The tensile strength of all samples exceeded 300 Ib/sq inch, and 3 samples prepared from pine blocks quickly preheated to 200on and jointed by an adhesive made from 95% liquid sulfur to which 5% ureaformaldehyde was added immediately before a glue line of 1 mm was assembled, had tensile strengths in excess of 325 Ib/sq inch (=24 kg/cm2). In all three samples, failure occurred across solid wood without damage to the glue line.

EXAMPLE 15 A 3-layer plywood was constructed from three sheets of 1 footxl foot X 1/10 inch hard wood veneers according to procedures and practice known to those skilled in the art, and pressed for 11 minutes at 150"C. The glue was prepared from the resin of Example 1, using a commercial extender. 30 grams of the glue were spread on the I foot X I foot samples using a commercial glue spreader.

The sulfur-containing glue and a conventionally-formulated glue using the same resin exhibited similar sheer properties according to ASTM test 1038.

EXAMPLE 16 Commercial urea-formaldehyde adhesive was used to prepare grade Type III plywood, but instead of using 150% wheat flour as an extender, 150% of a mixture of sulfur and wheat flour was used.

The adhesives containing extenders containing ratios of 1:10, 1:3, 1:1, 3:1 and 10:1 sulfur to wheat flour were tested. A ratio of 1:3 to 3:1 was found to be particularly suitable, probably because the wheat flour can absorb water while maintaining a high viscosity. The mixtures are thus suitable for the separate control of water and viscosity.

EXAMPLE 17 The composition of Example 1 was mixed with wood flour and ground walnut meal and used in place of boat, router, circular or dogbone plugs to make patches to fill knotholes and to repair defects. The resulting hot-cured patches had good adhesion, a pale color, could be easily sanded and painted and the product was not brittle.

EXAMPLE 18 A urea-formaldehyde resin was prepared according to Example 3, but part of the sulfur was added to NaOH used to prepare the composition, the remainder of the sulphur being added to the finished resin. The properties of the resin were similar to those in Example 3.

EXAMPLE 19 The resin composition was that of Example 3, but part of the sulfur was melted with the urea and stirred, and the solidified mixture was ground and used as if it were urea, and the rest of the sulfur was added when the resin was finished. Products prepared with this resin had similar properties to those of Example 3.

EXAMPLE 20 The composition was the same as that in Example 3, but part of the sulfur was added to the formaldehyde and part after the resin was prepared. The properties of products made with this resin were similar to those of Example 3.

EXAMPLE 21 The composition was the same as that in Example 3. However, instead of elemental sulfur, a mixture of elemental sulfur with sodium sulfide with a sulfur rank of 4 was used. The procedure was that used in Example 20. The properties of products made with this resin were similar to those of Examples 3 and 20.

EXAMPLE 22 The composition was the same as that in Example 3. However, instead of a part of the elemental sulfur, H2S was forced into the formaldehyde solution which was carefully kept neutral with NaOH to minimize polymer formation. In this reaction, polymethylene polysùlfides are known to be formed. The reaction products were processed as if they were formaldehyde solutions in the preparation for the resin of Example 3.

EXAMPLE 23 The composition of Example 1 was mixed with wood flour and carbon black and molded into a shallow dish which was cured for 30 minutes at 1 30'C. The product serves as an ashtray.

EXAMPLE 24 The sulfur-containing adhesive resins of Examples 1--15 were used to impregnate the surface of particle board and cured as in Example 1. It was found that the surface could be painted without the application of a primer.

The present invention provides a simple modification of the urea-formaldehyde glue used in making pressed boards by either replacing some of the urea-formaldehyde resin with elemental sulfur or by simply adding the sulfur to the otherwise conventional formulation. Advantageously, the ratio of urea resin to sulfur in the glue will be in the range of 10:1 to 1:10. The sulfur-modified adhesives of the present invention are thus in all respects compatible with existing conventional procedures for formulating pressed wood products and the like.

WHAT I CLAIM IS: 1. A thermo-setting urea-aldehyde adhesive composition, suitable for use in making bonded wood products, obtained by mixing and/or reacting a thermo-setting urea-aldehyde adhesive resin, and/or a precursor thereof, with sulphur at least partly in the elemental form, whereby the adhesive composition comprises from 0.05 to 99%, by weight, on a solids basis, of elemental sulphur.

2. A composition according to Claim 1, comprising from 3 to 50% by weight, on a solids basis, of elemental sulphur.

3. A composition according to Claim 1, wherein the ratio of resin to sulphur is about 1:1.

4. A composition according to any one of Claims 1 to 3, wherein the resin and sulphur are suspended in an aqueous medium.

5. A composition according to any one of Claims 1 to 4, obtained by blending together the sulphur and the resin or a precursor of the resin, in an aqueous medium.

6. A bonded wood product wherein the bonding means comprises the composition according to any one of Claims 1 to 5.

7. A pressed board comprising wood particles bonded together under heat and pressure by the composition according to any one of Claims 1 to 5.

8. A pressed board according to Claim 7, wherein the composition comprises at least 10% by weight, on a solids basis, of elemental sulphur.

9. A laminated wood product according

Claims (1)

1. to Claim 6.

   10. A method of preparing a bonded wood product, which comprises applying the composition, according to any one of Claims 1 to 5, to the wood and then bonding under heat and pressure.

   11. A method according to Claim 10, wherein the wood is in the form of woodchips which are impregnated with the composition of Claim 1.

   12. A wood product coated with the composition according to any one of claims 1 to 5.