Amino resins are a versatile class of materials with a wide range of applications. They are typically formed by the reaction of an amine with a formaldehyde, and can be further modified with a variety of other additives.
Amino resins are used in a variety of industries, including:
- Woodworking: Amino resins are a major component of adhesives used in the woodworking industry. They are used to bond together wood particles to form products such as particleboard, MDF, and plywood.
- Coatings: Amino resins are used in a variety of coatings, including paints, varnishes, and adhesives. They provide good adhesion, hardness, and chemical resistance.
- Plastics: Amino resins can be used to produce a variety of plastics, including molded parts, films, and sheets. They are also used as a binder in composites.
- Textiles: Amino resins are used to improve the strength and durability of textiles. They are also used to produce flame-retardant fabrics.
- Other applications: Amino resins have a variety of other applications, including in the production of foams, grinding wheels, and ion-exchange resins.
The largest consumer of amino resins is the woodworking industry. Amino resins have largely replaced natural glues based on proteins, such as gluten, casein, soybean, and blood albumin glues.
Table of Contents
1. Resin Glues for the Woodworking Industry
Amino resin wood glues are a type of adhesive that is commonly used in the woodworking industry. They are available in two forms: liquid and powder. Liquid amino resin glues have a shelf life of several weeks to six months, while powder amino resin glues have a shelf life of one year or longer.
To use amino resin wood glue, the resin must first be mixed with a curing agent. The curing agent can be a liquid or a powder. Once the resin and curing agent have been mixed, the glue can be applied to the wood surfaces that are to be bonded.
Amino resin wood glues are typically used to bond wood particles together to form products such as particleboard, MDF, and plywood. They can also be used to bond plywood and blockboard together, and to glue wood components for furniture construction.
Amino resin wood glues are typically inexpensive and have short press times. They also have adequate technical properties for most purposes. However, they are not as water-resistant as melamine or phenol resin glues.
The most important amino resin wood glue from an economic point of view is urea resin glue. This is because urea resin glue is inexpensive and has short press times. Urea resin glue is typically used to produce wood particle board.
Melamine resin glues and mixed condensates of formaldehyde, urea, melamine, and/or phenol are also used in the woodworking industry. These glues are more water-resistant than urea resin glue, but they are also more expensive.
MDF resins are a type of amino resin wood glue that has become increasingly important since the 1980s. MDF resins are low-viscosity urea–formaldehyde resins that are used in the production of medium-density fiberboard.
Amino resin wood glues are a versatile and economical type of adhesive that is used in a variety of applications in the woodworking industry.
2. Impregnating Resins
Urea-formaldehyde or melamine-formaldehyde impregnating resins are used in treating papers for decorative applications. The properties of amino resins, including brightness, hardness, abrasion resistance, scratch resistance, and photostability, make them suited for this function.
The papers that undergo impregnation serve a dual purpose: either as constituents in the creation of decorative laminates or as coatings for wood particle boards. Papers impregnated with melamine resin has greater heat stability and water resistance compared to the urea resin.
The process includes drying the amino resin-impregnated paper webs in a furnace operating at temperatures ranging from 70 to 200°C. Then, the dried sheets are stored in climate-controlled environments until their uses.
A diverse array of amino resin-impregnated papers find requisition in the production of decorative laminates. Of particular importance are papers designed to confer ornamental patterns upon boards composed of numerous layers of phenol resin-impregnated kraft paper, known as high pressure laminate (HPL) and continuously pressed laminate (CPL).
However, a substantial portion of contemporary laminate manufacturing hinges upon the direct application of amino resin-impregnated decorative papers onto suitable substrates, such as particle board, medium-density fiberboard (MDF), or high-density fiberboard (HDF). These laminates are commonly abbreviated as DPL (direct pressed laminate) or LPM (low pressure melamine).
Ground films and decorative films, similar to veneers, are also treated with amino resin impregnation. They find analogous application in the furniture industry. Generally derived from urea resins, these films are also produced to some extent from mixtures of melamine and urea resins.
3. Molding materials
Molding materials are composites that consist of a filler, such as cellulose or sawdust, and a binder. The binder can be a urea resin or a melamine resin.
Molding materials made with melamine resin are superior to those made with urea resin because they have better hardening properties, higher mechanical strength, and greater resistance to tracking currents and moisture.
Molding materials are used to make a variety of objects, including:
- Electrical plugs, switches, covers, and telephone accessories
- Buttons
- Camping, hospital, and canteen tableware
- Junction boxes and insulating components
Mixed resins that combine some of the properties of the individual resins are also used.
4. Raw materials for surface coatings
Amino resins are not very useful as surface coatings on their own. They are typically etherified with butanol or isobutanol and dissolved in these solvents. Some brands are also modified with methanol, ethanol, or propanols.
Amino resins are too brittle to be used alone as binders in surface coatings. However, they are very useful in combination with other binders. Urea resins and melamine resins are the most important cross-linking components for alkyd resins.
Physically drying finishes are obtained by adding amino resins to nitrocellulose finishes. The addition of these resins improves the hardness, body, and stability to light. The principal field of use is in furniture finishing.
Acid-curable finishes are obtained by adding alkyd resins or saturated polyester resins. These finishes can be hardened by adding an acid, such as hydrochloric acid, p-toluenesulfonic acid, or a phosphoric acid derivative, at an elevated temperature.
The resulting coatings are very hard and resistant to scratches, solvents, and the effects of light. Urea resins are more important than melamine resins, particularly in furniture finishing and parquet sealing.
Heat-cured finishes are obtained by combining amino resins with alkyd resins, saturated polyester resins, heat-curable acrylate resins, or epoxy resins. Because of their good mechanical properties, gloss, and resistance, these baked finishes are extremely important in industrial metal finishing, for example, for automobile bodywork, household appliances, metal furniture, and containers.
Amino resins used in the surface-coating industry have a shelf life of six months to several years at room temperature. They are dispatched in plain or galvanized sheet metal containers. Only stainless steel or aluminum tanks are used for transport by tanker trucks and railroad tank cars.
5. Paper Auxiliaries
In the paper industry, amino resins find use in enhancing both the strength of paper when it’s dry and, more importantly, when it’s wet. Unlike the textile industry, these resins are mixed with the paper material before the reaction occurs.
They stick to the fibers and then transform due to a chemical reaction that takes place in an acidic environment. It takes around two to four weeks for the paper to reach its maximum strength by this process.
Depending on the type of fiber being used, adding about 1% to 5% of resin by weight is enough to make the paper strong when it’s wet. Amino resins also have a role in giving papers a water-resistant finish, especially when the paper has coatings made of starch.
Among these resins, the ones derived from urea are particularly important. They start as compounds with a certain structure and are changed by adding amines to give them a positive charge, or by adding sulfites to make them negative.
Once modified, these compounds react further in an acidic environment. The end result is a type of resin that can dissolve in water. This property helps them stick to paper fibers, especially the ones that are negatively charged. However, if we want to use negatively charged resins, we need to include aluminum sulfate.
Melamine resins also play a crucial role in making paper strong when it’s wet. These resins react with acids to create strongly acidic materials that carry a positive charge. These materials are kept stable by an electric charge and are made up of partially condensed hydroxymethyl melamines.
Melamine resins that have been altered with aminocaproic acid can replace, either alone or in combination with other substances, some or all of the soap-like compounds that are used in treating paper.
Another way these resins are used is in making starch pastes that can resist moisture. These pastes are used to bond corrugated cardboard.
6. Textile Auxiliaries
Textile auxiliaries based on urea, melamine, and formaldehyde are predominantly monomeric and do not form resins. They react with the primary hydroxyl groups of cellulose to form additional, stronger covalent bonds. Self-condensation takes place on the surface of synthetic fibers.
The pure urea–formaldehyde adducts, mono- and dihydroxymethylurea, are water soluble and crystallize readily. They are used to provide wrinkle-resistant and wash-and-wear finishes for cellulose textiles and mixtures of cellulose fibers with synthetics and wool.
Hydroxymethyl compounds of urea that exhibit a high degree of condensation and possess a high concentration of ether groups are used to produce highly elastic stiffening effects and good wash resistance in cellulose fabrics.
An important group is constituted by hydroxymethyl compounds of cyclic urea derivatives. Acyclic compounds, such as various alkyl carbamates, are also typical finishing agents.
Hydroxymethyl melamine derivatives and their ethers are of some importance for providing high-grade finishes, but they have largely been replaced by the reactant types. The finishes obtained using hydroxymethylmelamines are more wash resistant than those made with hydroxymethylureas.
Hydroxymethylmelamine compounds are preferred for embossed finishes where the fabric can be damaged mechanically.
7. Leather Auxiliaries
Resin retannage is a process of strengthening chrome-tanned leather using hydroxymethyl compounds of urea, thiourea, melamine, dicyanodiamide, and ethers of these.
The aqueous solutions of the resins should penetrate the hide before condensation begins. The acid condensation catalyst (pH 3.9–4.5) should not reduce the period of activity of the tanning liquor or damage the hide.
The resins can be rendered cationic using amines or anionic by means of a bisulfite. The anionic resins based on melamine or dicyanodiamide make leather very strong and supple and leave the color unchanged.
The hydroxymethyl groups react with the active groups of the collagen fiber, which has a high content of glycine, proline, and oxyproline.
8. Other Uses
The number of possible uses for amino resins is constantly increasing. However, the importance of some uses, such as flame-proofing agents, has declined.
Some applications, such as casting resins, have proved unsatisfactory in practice. Other applications are still in the development stage and their industrial suitability cannot yet be assessed.
Reference
- Amino Resins; Ullmann’s Encyclopedia of Industrial Chemistry. – https://onlinelibrary.wiley.com/doi/10.1002/14356007.a02_115.pub2
