BE1027998B1 - PRODUCTION PROCESS OF THE CATALYST - Google Patents

Abstract

The present invention relates to an improved catalyst for aminoplast resins that typically rely on an acid catalyst to initiate the crosslinking reaction. The improved catalyst, according to the invention, offers better control over the curing, i.e. the crosslinking reaction, thereby improving the quality of the resulting resin after curing. The improved catalyst is based on the encapsulation of said acid catalysts in microcapsules, with a controllable release of said acid catalyst under certain ambient reaction conditions. The present invention further provides methods of obtaining such an improved catalyst as well as its uses.

Description

' BE2019/5766

PRODUCTION PROCESS OF THE CATALYST

FIELD OF THE INVENTION The present invention relates to an improved catalyst for aminoplast resins which typically rely on an acid catalyst to initiate the crosslinking reaction. The improved catalyst, according to the invention, offers better control over the curing, i.e. the crosslinking reaction, thereby improving the quality of the resulting resin after curing. The improved catalyst is based on encapsulating said acid catalysts in microcapsules, with a controllable release of said acid catalyst under certain ambient reaction conditions. The present invention further provides methods of obtaining such an improved catalyst as well as its uses.

BACKGROUND OF THE INVENTION Aminoplast resins, urea-formaldehyde (UF) resins, melamine-formaldehyde (MF) resins find wide industrial application. Because of their characteristic tensile strength and water repellency, their use is mentioned as binders for cellulosic, glass fiber and polymeric materials, as well as composite mixtures thereof. Aminoplast resins are thermosetting resins made by the reaction of an aminoplast resin precursor (e.g., an amine such as melamine, urea, or an amide) with an aldehyde (e.g., formaldehyde). The resins can be used in a variety of applications including molding, protective coatings, ion exchange resins and adhesives, just to name a few. Common thermosetting aminoplast resins are trimethylolmelamine, methylolurea, dimethylolurea, ethylenediamine, benzoguanamine, fully alkylated melamine and partially alkylated melamine. Aminoplast resins are also very useful as cross-linking agents for other polymers, such as acrylic polymers (e.g., amino- or hydroxyl-functional: acrylic polymers, polyesters, epoxides, phenols, and urethanes). A curing reaction catalyst is often used for crosslinking aminoplast resins, especially for urea-formaldehyde (UF) resins. Cure reaction catalysts are typically acid catalysts, including phosphoric acid cure catalyst, sulfonic acid compound cure catalysts such as toluene sulfonic acid and dodecyl benzene sulfonic acid. Such acid catalysts are often used, especially for urea-formaldehyde (UF) resins. When added to the resin composition in the conventional manner, crosslinking would begin already during mixing and would tend to result in uneven curing of the resin, which could result in a defective final product. It is an object of the present invention to provide an improved process in the manufacture of such resins, with improved control over the crosslinking reaction.

© BE2019/5766

SUMMARY OF THE INVENTION In a first embodiment, the present invention provides an aminoplast resin catalyst characterized in that it comprises an encapsulated acid catalyst; in particular comprising a microencapsulated acid catalyst. In an embodiment of the invention, the acid catalyst in said aminoplast resin catalyst is encapsulated in a polymeric spherical shell. In a particular embodiment, the acid catalyst is encapsulated in an aminoplast polymer cell; more particularly in a melamine-formaldehyde (MF) shell. In one embodiment of the invention, the acid catalyst is encapsulated in polymer spherical cells ranging from about 5 µm to about 30 µm; in particular having a volume weight average particle size of about 10 µm to about 20 µm.

In one embodiment of the invention, the acid catalyst used in the aminoplast resin catalyst, according to the invention, is the water-insoluble acid catalysts known to those skilled in the art and used in the manufacture of aminoplast resins. In a particular embodiment, the acid catalysts are water-insoluble carboxylic acids, such as, for example, selected from the group consisting of pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid and linoleic acid; more particularly, the acid catalyst used in the aminoplast resin catalyst of the invention is heptanoic acid. In an embodiment of the invention, the aminoplast resin catalyst consisting of the encapsulated acid catalyst is a liquid composition, especially an aqueous composition containing up to about 50% by weight of the encapsulated acid catalyst, especially up to about 40% by weight, more especially from about 10% to about 30% by weight of the encapsulated acid catalyst. In one embodiment of the invention, the aminoplast resin catalyst is a liquid composition containing about 30% by weight of the encapsulated acid catalyst. The encapsulated acid catalyst used in such an aminoplast resin catalyst composition consists of a microencapsulated acid catalyst as defined before the stable dispersion in said composition. Hence, in a further Embodiment, the present invention provides an emulsion for use as an aminoplast resin catalyst consisting of a microencapsulated acid catalyst as previously defined. In an alternative embodiment, the aminoplast resin catalyst comprising the encapsulated acid catalyst is produced as a semi-solid, e.g., as a wet cake wherein the aqueous composition containing the encapsulated acid catalyst is mixed with a solid, such as, for example, resin binders, which are included in the resin-based products such as moisture-resistant MDF (medium density fibreboard) or composite pallet blocks are used. Accordingly, it is an object of the present invention to provide a semi-solid for use as an aminoplast resin catalyst consisting of the aqueous composition containing the encapsulated acid catalyst of the invention, especially a semi-solid, consisting of up to 75%, especially between and about 60-75% of solids.

In a further aspect, the present invention provides a method of curing an aminoplast resin, which method comprises using an aminoplast resin catalyst as described herein; particularly comprising the step of mixing the aminoplast resin catalyst with the components typically used in the manufacture of an aminoplast resin well known to those skilled in the art. In one embodiment of said method, further comprises the step of applying such mixture to a surface to be coated or laminated with the aminoplast resin and exposing said surface to pressure of at least two bar; or to a pressure of at least two bar and an elevated temperature, i.e. at least above the glass transition temperature (Tg) of the shells encapsulating the acid catalyst. In particular at a pressure of at least 30 bar; more in particular at least 50 bar; even more in particular at least 70 bar. In a preferred embodiment up to a pressure of about 75 bar and a temperature of about 200°C.

It is also an object of the present invention to provide a method of manufacturing the aminoplast catalyst resin as provided herein, which method comprises an interfacial polymerization process to encapsulate the acid catalyst. In one embodiment of said method of encapsulating the acid catalyst, hereinafter referred to as "The first acid catalyst or the encapsulated acid catalyst" comprises; - providing an aqueous solution of a pre-condensate of a casing material; - mixing discrete droplets of the first acid catalyst with said aqueous solution, by adding discrete droplets of the first acid catalyst to said solution; - adding a second acid catalyst to the solution thus obtained to catalyze the polymerization of the dissolved precondensate, and - agitating the solution thus obtained during the polymerization reaction, to give the encapsulation of the first acid catalyst.

In a particular embodiment, the manufacturing process is characterized in that the first acid catalyst is a water-insoluble acid catalyst known to those skilled in the art.

° BE2019/5766 and used in the manufacture of aminoplast resins. In a particular embodiment, said acid catalysts are water-insoluble carboxylic acids, such as, for example, selected from the group consisting of pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid and linoleic acid; more particularly, the first acid catalyst used according to the invention in the aminoplast resin catalyst is heptanoic acid.

In one embodiment, the production process is characterized in that the aqueous solution of a pre-condensate consists of about and between 3 to 30% by weight of the pre-condensate. In the processes of the invention, the precondensate is a urethane prepolymer; in particular with at least two -NCO groups at molecular ends thereof. In a preferred embodiment, the precondensate is methylolurea. In one embodiment, the manufacturing method is characterized in that the solution is maintained between 20 and 90°C during the polymerization reaction.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 - Size distribution diagram of the hepatanoic acid capsules obtained in the example of the present application. In said example, hepatanoic acid is encapsulated in melamine-formaldehyde (MF) shells.

Figure 2 - Scanning electro-microscopy (SEM) images of hepatanoic acid MF capsules obtained in the example of the present application.

DETAILED DESCRIPTION OF THE INVENTION — Microencapsulation: Microencapsulation is a technique in which a core material is incorporated into a (usually) polymeric spherical shell. One of the first applications of microencapsulation was for "self-copying paper" for making copies on paper; ink was encapsulated and the capsules applied to the back surface of paper. The action of pressure (e.g. writing or typing) breaks capsules, releases ink and transfers a copy to the flat underlying paper. Now microencapsulation is widely used to encapsulate fragrances, flavors and similar materials. The material of the shells are typically polymeric and may be synthetic such as aminoplast polymers (urea-formaldehyde, melamine-formaldehyde and the like), polyacrylic, polyurethane, or may be of natural origin, e.g. derived from gelatin or alginate. Capsules can be designed to be robust and fully retain the core material, or they can be designed to release the core slowly (e.g. by diffusion) or more quickly by an external activation such as pH or temperature. A common activation is pressure or friction, where the capsules are ruptured, releasing the encapsulated core.

Capsules containing a water-insoluble core (oily materials} are readily prepared by forming an oil-in-water emulsion of the core with a suitable surfactant; a water-soluble polymer-forming material (eg a urea-formaldehyde resin), is added and a polymer shell is formed on the outer surface of the emulsion droplets, usually by the action of heat.

Aminoplast Resins: Aminoplast resins are produced by the reaction of formaldehyde with amino functional molecules, the most important of which are urea and melamine. Depending on the desired application, a range of properties can be obtained by varying the amino to formaldehyde ratio. The processing of these materials is also very sensitive to the polymerization environment, such as temperature and pH, because these resins, like phenols, cure by a condensation reaction; they are generally processed under pressure, in closed moulds.

Crosslinking aminoplast resins: For crosslinking aminoplast resins, an acid catalyst is often used, especially for urea-formaldehyde (UF) resins. This also requires heat.

As an alternative to a conventional acid catalyst, an encapsulated form has been sought to better control crosslinking and to reduce or prevent certain defects in the appearance of the resin film after curing.

The material of the shell surrounding the liquid acid catalyst core to form the microcapsule can be any suitable polymeric material which is impermeable to the materials in the liquid core and the materials that may contact the outer surface of the shell. The shell wall of the microcapsule can be composed of a wide variety of polymeric materials, including gelatin, polyurethane, polyolefin, polyamide, polyester, polysaccharide, silicone resins, chitosan and epoxy resins. Many of these types of polymeric microcapsule shell materials are further described and illustrated in U.S. Pat. pat. No. 3,870,542.

The highly preferred materials for the shell wall of the microcapsule are aminoplast polymers comprising the reactive products of, for example, urea or melamine and aldehyde, e.g. formaldehyde. Such materials are those capable of polymerizing in an acidic state, from a water-soluble prepolymer or precondensate state. Polymers formed from such pre-condensate materials under acidic conditions are

° BE2019/5766 insoluble in water and can provide the required capsule brittleness properties, to allow for the subsequent breakage of the capsule.

Microcapsules having the liquid cores and polymer shell walls described above can be prepared by any conventional method that produces capsules of the required size, friability and water insolubility. In general, such methods as coacervation and interfacial polymerization can be used in known manner to produce microcapsules of the desired properties. Such methods are described in U.S. Patent No. 3,870,542, U.S. Pat

3,415,758 and U.S. Patent 3,041, 288.

Microcapsules made from aminoplast polymer shell materials can be made by an interfacial polymerization process as detailed in U.S. Patent No. 3,516,941. In such interfacial polymerization reactions, the prepolymer or precondensate is typically selected from urethane prepolymer having at least two -NCO- groups at molecular ends thereof; polyorganosiloxanediol; polysulfide prepolymer; epoxy resin obtained by reaction between bisphenol A and epichlorohydrin; and a reactive polyolefin derivative having a reactive group such as an acid chloride or a thionyl chloride group. In a preferred embodiment, the wall component is formed from urethane prepolymer by interfacial polycondensation of the urethane prepolymer mixed in water or an aqueous solution containing a composition of amine at interfaces between the mixed particles and the mixing medium. In a preferred embodiment, the precondensate is methylolurea.

In a particular embodiment, an aqueous solution of a precondensate (e.g., methylolurea) is formed which consists of about 3% to 30% by weight of the precondensate. Water-insoluble liquid core material, such as the liquid acid catalyst mentioned below, is mixed with this solution in the form of individual droplets of microscopic size. While maintaining the solution temperature between 20 and 90°C, acid is then added to catalyze the polymerization of the dissolved precondensate. If the solution is rapidly agitated during this polymerization step, shells of water-insoluble aminoplast polymer form and envelop the mixed droplets of the liquid core material. Capsules suitable for use in the present invention can be produced by a similar process. Preferably, the polymer of the shell is a melamine-formaldehyde resin or contains a layer of this polymer.

Typically, the microcapsules vary in size and can be from diameters from 1 to 300 µm, preferably from 2 to 100 µm, more preferably from 2 to 50 µm. The weight ratio of

/ BE2019/5766 the shell to the liquid core will usually be 1:500 to 1:5000. If the ratio is less than 1:10,000, the resulting casing may be too thin. If the ratio is greater than 1:100, the resulting wall may be too strong to tear easily. The exact details depend on the casing used.

The microcapsules of the present invention must be brittle in nature. Brittleness refers to the tendency of the microcapsules to rupture or break open when subjected to direct external pressure or shear forces.

A series of organic acids were evaluated in the above production processes; the key performance features were that the acid should be substantially insoluble in water to aid encapsulation by the above technique. A series of simple organic mono-acids were initially evaluated and heptanoic acid was found to have the optimum properties of low solubility in water (0.24g/100ml) and a sufficiently rapid effect on the crosslinking of UF resin.

Mono acids smaller than heptanoic acid were too soluble to encapsulate - hexanoic acid has a solubility of 1.1g/100ml in water and pentanoic acid has a solubility of 5g/100ml; both catalyze the mixing of UF-resin faster than heptanoic acid (in a non-encapsulated form).

Octanoic acid is less soluble in water than heptanoic acid (0.07g/100ml) and is more easily encapsulated, but the rate of crosslinking reaction of UF resin is significantly reduced. Monoacids, higher than linoleic acid, are increasingly insoluble in water and thus have a negligible effect on catalyzing the crosslinking of UF resins.

EXAMPLE Encapsulating Heptanoic Acid: The heptanoic acid is encapsulated in melamine-formaldehyde (MF) shells to produce an emulsion containing about 30% capsules; the heptanoic acid content of the capsules is +/- 70% as measured by thermogravimetric analysis (TGA) and by titration of a diluted solution of the capsules with standard NaOH solution.

The use of a polymeric surfactant improves the encapsulation of the partially soluble heptanoic acid. A number of polymeric surfactants were evaluated before selecting poly(styrene-maleic anhydride) (PSMA). PSMA must first be dissolved by neutralization with sufficient NaOH to ensure complete dissolution.

© BE2019/5766 The first step is to start with the formation of an MF prepolymer; by mixing Cymel 285 (an MF resin) with Floset 150L (a polyacrylamide) with PSMA solution and adjusting it to pH 5.3 (with acetic acid) and heating for a while to 25°C. Then heptanoic acid is added added and the mixture is emulsified under high shear to create emulsion droplets of a suitable size (generally <30 µm, preferably 15 µm); the pH drops to 4.8 - 4.9 due to the slight solubility of heptanoic acid, but is not adjusted. The entire mixture is heated to 80°C over a period of 90 minutes and allowed to remain at 80°C for an additional 60 minutes before being cooled to ambient temperature.

The capsules thus formed are characterized by particle size analysis (Figure 1), TGA/titration for heptanoic acid content (data not shown), and imaged using scanning electromicroscopy (Figure 2}, to ensure satisfactory capsule formation.

Cross-linking of UF resins with heptanoic acid capsules: These resins are used to bond/produce a hard, durable surface for many household goods, such as furniture or wood effect panelling. The entire assembly is subjected to heat and pressure to cure the resin and produce the final article.

The use of a conventional acid catalyst may promote premature curing during mixing or uneven surface mixing under heat/pressure if the mixing is not uniform, leading to a defective finished product. By encapsulating the catalyst, it is protected from the resin because it is inside the polymer shell and cannot initiate crosslinking. More effective and consistent mixing is possible if there is no chance of the cross-linking reaction starting prematurely.

Uniform distribution of the encapsulated catalyst in the resin mixture provides more consistent mixing behavior throughout the article, resulting in improved product quality.

Claims (16)

CONCLUSIONS 1. An aminoplast resin catalyst is characterized as containing an encapsulated acid catalyst. The aminoplast resin catalyst of claim 1, wherein the acid catalyst is encapsulated in a polymer shell; in particular an aminoplast polymer shell. The aminoplast resin catalyst according to claim 1 or 2, wherein the acid catalyst is encapsulated in microcapsules; especially in polymeric spherical cells ranging from about 5 µm to about 30 µm. The aminoplast resin catalyst according to any one of claims 1 to 3, wherein the acid catalyst are water-insoluble carboxylic acids, such as, for example, selected from the group consisting of pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid and linoleic acid; more particularly, the first acid catalyst used according to the invention in the aminoplast resin catalyst is heptanoic acid. The aminoplast resin catalyst according to any one of the preceding claims consists of a liquid composition containing the encapsulated acid catalyst. The aminoplast resin catalyst of claim 5, wherein the liquid composition contains up to about 50% by weight of the encapsulated acid catalyst. An emulsion for use as an aminoplast resin catalyst comprising a microencapsulated acid catalyst as defined in any one of the preceding claims. A method for curing aminoplast resin, according to the method comprising using an aminoplast resin catalyst as defined in any one of the preceding claims. The method according to claim 8, comprising mixing said aminoplast resin catalyst with components commonly used in the manufacture of aminoplast resin. The method of claim 8 or 9, further comprising the steps of applying such mixture to a surface to be coated or laminated with the aminoplast resin, and exposing said surface to a pressure of at least two bar; or to a pressure of at least two bar and an elevated temperature. The method of claim 10, wherein the surface is exposed to a pressure of about 75 bar and a temperature of about 200°C. A method of manufacturing the aminoplast catalyst resin according to any one of claims 1 to 6, wherein said method includes an interfacial polymerization process to encapsulate the acid catalyst. The method of claim 12, comprising the method of encapsulating the acid catalyst; - providing an aqueous solution of a pre-condensate of a casing material; - mixing discrete droplets of the first acid catalyst with said aqueous solution, by adding discrete droplets to said solution - adding a second acid catalyst to the solution thus obtained to catalyze the polymerization of the dissolved pre-condensate and - agitating the solution thus obtained during the polymerization reaction, to give the encapsulation of the first acid catalyst. The method of claim 13, wherein the first acid catalyst are water-insoluble carboxylic acids, such as, for example, selected from the group consisting of pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid and linoleic acid; more particularly, the first acid catalyst used according to the invention in the aminoplast resin catalyst is heptanoic acid. The method of claim 13 or 14, wherein the aqueous solution of a pre-condensate contains about and between 3 and 30% by weight of the pre-condensate; in particular of a urethane prepolymer; more particularly of methylolurea. The method of any one of claims 13 to 15, wherein the solution is maintained between 20 and 90°C during the polymerization reaction.