Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2001-07-11
2003-07-08
Reddick, Judy M. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S047000, C524S052000, C524S053000, C524S202000, C524S203000, C524S307000, C524S316000, C524S405000, C524S416000, C524S423000, C524S434000, C524S437000, C524S444000, C524S445000, C524S446000, C524S451000, C524S512000
Reexamination Certificate
active
06590013
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hardener for use in urea-formaldehyde and urea-melamine-formaldehyde based adhesives. The invention furthermore relates to an adhesive composition comprising a urea-formaldehyde or urea-melamine-formaldehyde resin and a hardener therefor, which adhesive composition provides faster curing rate, better adhesion and a wider application range than prior art adhesives. Further, the invention allows wide variations in the resin:hardener ratio without compromising the quality of the glue joint obtained.
The invention furthermore relates to the use of said hardener for curing urea-formaldehyde or urea-melamine-formaldehyde resins.
BACKGROUND ART
Urea-formaldehyde (UF) resins and urea-melamine-formaldehyde (UMF) resins are widely used in industry for gluing wood and as binders for the production of wood-based materials, including chipboard, plywood and various fibreboards, in the fortification of starch adhesives for the manufacture of paper bags and corrugated box boards, the production of insulating foams, high quality sandpaper and parquet flooring.
Urea and melamine adhesives are simple reaction products of urea and/or melamine with formaldehyde. They are set or cured by the use of a hardener, either by heat or at room temperature. Hardeners are either acidic substances themselves, or they are capable of liberating acid when mixed with the resin. Ammonium salts of strong acids are widely used hardeners.
U.S. Pat. No. 2,873,260 discloses a craze-resistant, acid-hardenable, gap-filling adhesive comprising a water-soluble urea-formaldehyde condensate, wherein the powdery hardener is selected among ammonium chloride and ammonium sulphate.
GB 761,777 discloses a dry composition comprising a dry, powdered urea- or thioureaaldehyde condensation product in the intermediate water-soluble, hardenable stage in admixture with dry, finely divided aluminum sulphate as a catalyst. Such powdery products are inconvenient to handle and create an unhealthy environment compared to liquid UF or UMF resins and liquid hardeners.
Further, it is known from GB 2,274,084 A to use, as the hardener an aqueous solution of ammonium chloride, alkaline urea, and hexamine. The resulting salt solution provides, however, a glue having a very poor filling capacity.
Further, GB 2,245,578 A discloses an aqueous batch for curing a formaldehyde-containing resin, containing ammonium, aluminium and sulphate ions, and urea. Such a curing system may provide an acceptable curing rate, but a very low viscosity adhesive is unavoidably obtained, resulting in a poor filling capacity.
EP 0249747 B1 discloses a curing agent composition for curing urea-formaldehyde or urea-melamine-formaldehyde adhesive resins comprising an ammonium salt of an inorganic or organic acid, wherein said composition further comprises an acid, urea, a glycol or a functional derivative thereof and, optionally, a metal salt. However, said curing agent composition comprises no polyvinyl acetate, resulting in a poor flexibility in the glue line.
WO 86/00915 discloses a method of adhering or cementing together two or more substrates by the use of a liquid composition comprising an organic polyhydroxy compound, an ammonium phosphate, water-dispersible polyvinyl acetate, water-dispersible formaldehyde resin, polyamido compound and water-hardenable setting agent. The composition, which comprises up to 5% of polyvinyl acetate, more preferably 2 to 5% thereof, does not include any metal chloride, resulting in an unsatisfactorily long curing time. Also the pH of said liquid composition is relatively high, which further reduces the curing rate. Finally, the liquid composition is produced by blending together all ingredients in the form of a dry powder, whereupon water is mixed with said powder to form a slurry (see the example). The use of a powder has, however, a number of drawbacks, as already pointed out above, such as dust problems, resulting in an unhealthy working environment, and necessitates an extra process step compared to the use of a liquid system.
As it appears from the above, a need still exists for an adhesive composition, which fulfils the requirements of a safe and rapid gluing of wood parts of different origin, such as softwood, e.g. coniferous tree, as well as hardwood, such as e.g. teak wood. Further, it would be desirable to have an adhesive composition which can be used for gluing wood of different qualities and of different moisture or oil content.
BRIEF DESCRIPTION OF THE INVENTION
It has now surprisingly been found that the use of a hardener comprising a polyvinyl acetate (PVA) emulsion together with a metal chloride and an ammonium salt of an organic or inorganic acid for curing a urea-formaldehyde or a urea-melamine-formaldehyde resin, results in an adhesive composition having a considerably increased curing rate, an acceptable viscosity, better adhesion and greater tolerance towards substrate type and presents fewer problems to the working environments than prior art adhesives. Further, the glue-line has a better water—resistance and better mechanical properties than prior art adhesives.
In a first aspect the present invention thus relates to a hardener for use in urea-formaldehyde or urea-melamine-formaldehyde based adhesives, comprising a metal chloride and an ammonium salt of an organic or inorganic acid, wherein the metal chloride is selected among aluminum, zinc and magnesium chloride, the ammonium salt is selected among mono- and di-ammonium phosphate, ammonium borate and ammonium citrate, wherein the weight ratio of metal chloride to ammonium salt is in the range of 3:1-0.5:1, and the hardener further comprises a polyvinyl acetate emulsion.
In a second aspect the present invention relates to an adhesive composition comprising a urea-formaldehyde or urea-melamine-formaldehyde resin and the above disclosed hardener.
Furthermore, the present invention relates to the use of the above disclosed hardener for curing urea-formaldehyde or urea-melamine-formaldehyde resins.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.
BEST MODE FOR CARRYING OUT THE INVENTION
The metal chloride used as one component of the hardener is selected among aluminum chloride, zinc chloride and magnesium chloride, preferably being aluminum chloride.
As a further component of the hardener an ammonium salt of an organic or inorganic acid is used, said salt being selected among mono- and di-ammonium phosphate, ammonium borate and ammonium citrate, preferably being mono-ammonium phosphate.
Thus, it has been found that the metal chlorides disclosed above, when used as an acid-liberating curing agent, will decrease the pH of the hardener more efficiently than the conventionally used ammonium salts, provided an ammonium salt is used concomitantly. Further, a higher stability of the hardener is obtained compared to ammonium chloride containing hardeners, the latter tending to be destabilised when mixed with a polyvinyl acetate emulsion.
The polyvinyl acetate (PVA) emulsion used in the hardener comprises polyvinyl acetates having a molecular weight in the range of 2,000 to 20,000, a viscosity in the range of 5,000 to 40,000 cps (mPa·s), preferably about 10,000 to 25,000 at 23° C., and a solids content of 45 to 65% by weight. The PVA is preferably an acid-modified PVA, such as a PVA modified by e.g. acetyl or propionyl groups.
By the use of polyvinyl acetate it is possible to decrease the pH of the hardener to 1 or even lower without breaking the uniform phase of polymers. This results in a considerably increased curing rate. Furthermore, the PVA component provides an increased tack and an increased impact strength com
Andersen Bjørn
Brehmer Bo Gao
A/S F. Heimann & Co.
Birch & Stewart Kolasch & Birch, LLP
Reddick Judy M.
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