Лако-красочные материалы — производство

Because of their characteristic rigidity and brittleness in their cured state, when MF resins are used for impregnated paper overlays, small amounts (typically 3 to 5%) of modifying compounds are often copolymerized with the MF resin during its preparation to give better flexibility to the finished product and better viscoelastic dissipation of stress in the joint. Most commonly used are acetoguanamine, e-caprolactam, and p-toluene — sulfonamide (Formula 2).

The effect of these is to decrease cross-linking density in the cured resin due to the lower number of amidic or aminic groups in their molecules. Thus in resin segments where

Formula 2

they are included, only linear segments are possible, decreasing the rigidity and brittleness of the resin. Acetoguanamine is most used for modification of resins for high-pressure paper laminates, while caprolactame, which in water is subject to the following equilibrium (Formula 3),

is used primarily for low-pressure overlays for particleboard. Small amounts of noncopo — lymerized plasticizers such as diethylene glycol can also be used for the same purpose. Due to the peculiar structure of the wood product itself, MF adhesives for particleboard generally do not need the addition of these modifiers. Often, a small amount of dimethyl — formamide, a good solvent for melamine, is added at the beginning of the reaction to ensure that all the melamine is dissolved and is available for reaction. Sugar is often added to lessen cost of the resin. The aldehyde group of sugars have been proven to be able to condense with the amine groups of melamine and hence to copolymerize in the resin. Their quantity in MF resins must be limited to very low percentages, and if possible, sugars should not be used at all, as with aging they tend to cause yellowing, crazing, and cracking of cured MF paper laminates and to have a bad effect on adhesive long-term water resistance in both plywood and particleboard.

MF adhesive resins for plywood and particleboard must be prepared to quite dif­ferent characteristics than those for paper impregnation. The latter must have lower viscosity but still high resin solids content because they need to penetrate the paper sub­strate to a high resin load, to be dried without losing adhesive capability, and only later to be able to bond strongly to a substrate. Instead, MF adhesive resins for plywood and particleboard are generally more condensed, to obtain lower penetrability of the wood substrate (otherwise, some of the adhesive is lost by overpenetration into the substrate). The reverse applies for paper substrates, where the contrasting characteristics desired— good paper penetration and fast curing—can be obtained in several ways during resin preparation. These characteristics can be achieved by producing, for example, a resin with a lower degree of condensation and high methylol group content. Typically, a MF resin of a lower level of condensation with melamine/formaldehyde molar ratio of 1:1.8 to 1:2 will give the desired characteristics. Its high methylol content and somewhat lower degree of polymerization will give low viscosity at a high resin solids content, favoring rapid wetting
and impregnation of the paper substrate, while the high proportion of methylol groups will give it fast cross-linking and curing capabilities.

A second, equally successful approach in to produce a MF resin of lower methylol group content and higher degree of condensation to which a small second addition of melamine (typically, 3 to 5% total melamine) is effected toward the end of resin prepara­tion. The shift to lower viscosity and higher solids content given by a second addition of melamine, shifting to lower values the average of the resin molecular mass distribution, yields a resin of rapid impregnation characteristics. Conversely, the higher degree of polymerization of the major part of the resin gives fast cross-linking, and curing, due to the lower number of reaction steps needed to reach gel point. Typical total melamine/ formaldehyde molar ratios used in this system are 1:1.5 to 1:1.7.

Figures 3 and 4 show typical temperature and pH diagrams for the industrial man­ufacture of MF and MUF resins for adhesives and other applications. The important control parameters to take care of during manufacture are the turbidity point (the point during resin preparation at which addition of a drop of MF reaction mixture to a test tube of cold water gives slight turbidity) and the water tolerance or hydrophobicity point, which marks the end of the reaction. The latter is a direct measure of the extent of condensation of the resin and indicates the percentage of water or mass of liquid on the reaction mixture that the MF resin can tolerate before precipitating out. It is typically set for resins of higher formaldehyde/melamine ratios and lower condensation levels at around 170 to 190%, but for resins of lower formaldehyde/melamine molar ratios and higher condensation levels it is set at around 120%. As can be seen from the diagrams in Fig. 3, once maximum reaction temperature is reached, pH is lowered to 9 to 9.5 to accelerate formation of the polymer. Once the turbidity point is reached, pH is again increased to 9.7 to 10.0, to slow down and more finely control the end point, determined by reaching of the wanted value of the water tolerance point. Industrial MF resins are generally manufactured to a 53 to 55% resin solids content with a final pH of 9.9 to 10.4 (but lower pH values are also used for low-condensation resins). To have acceptable rates of curing if higher pH values are used, higher quantities of hardener need to be used, which is clearly uneconomical. For typical MF resins for low pressure (particleboard),

self-adhesive overlay pressing times of between 30 and 60 s at 170 to 190° C press tempera­ture are required according to the type of resin used. Pressing conditions for particleboard and plywood adhesives are identical to those used for UF resins.

Glue mixing presents different requirements according to the final use of the MF resin. Hardeners are either acids or materials that will liberate acids on addition to the resin or on heating. In MF and MUF adhesives for bonding particleboard and plywood, the use of small percentages of ammonium salts, such as ammonium chloride or ammonium sulfate, is well established and is indeed identical to standard practice in UF resins. In MF adhesives for low — and high-pressure self-adhesive overlays and laminates the situation is quite different. Ammonium salts cannot be used for the latter application for three main reasons. First, evolution of ammonia gas during drying and subsequent hot curing of the MF impregnated paper would cause high porosity of the cured MF overlay. Second, the stability of ammonium salts, in particular of ammonium chloride, might cause MF liquid resin whitening and the MF-impregnated paper to cure and deactivate at ambient tempera­ture after a short time in storage, causing the resin to have lost its adhesive capability by the time it is needed in hot curing. Third, the elimination of ammonia during drying and curing would leave the cured, finished paper laminate essentially very acid due to the residual acid of the hardener left in the system. This badly affects the resistance to water attack of the cured MF surface defeating the primarily advantage for which such surfaces have justly become so popular. Thus a stable, self-neutralizing, non-gas-releasing hardener is needed for such an application. Several have been prepared and one of the most commonly used is the readily formed complex between morpholine and p-toluenesulfonic acid. Morpholine and p-toluenesulfonic acid readily react exothermically to form a complex of essentially neutral pH that is stable up to well above 65°C (Formula 4).

During heat curing of the MF paper overlay in the press, the complex decomposes, the MF resin is hardened by the acid that is liberated, morpholine is not vaporized and lost to the system, and on cooling the complex is reformed, leaving the cured glue line essen­tially neutral.

In MF glue mixing for overlays and laminates, small amounts of release agents to facilitate release from the hot press of the cured bonded overlay are added. Small amounts of defoamers and wetting agents to further facilitate wetting and penetration of the resin in the paper are always added. A typical glue mix is shown in Table 1.

Two strong trends have appeared reasonably recently in the preparation of mela­mine-impregnated paper laminates. First, impregnating machines capable of giving papers in which much cheaper UF resin is substituting as much as 50% of the more expensive MF resin have now been in operation for several years. This equipment is based on a double impregnating bath application: the paper passes through a first bath where it absorbs the UF resin first, the excess on the surfaces being scraped off in-line, and then passes through a second bath where it absorbs the MF resin. The concept is to limit the UF resin to the inside of the paper with the MF resin coating the outside of the paper: the hardened surface after final curing will then have all the waterproof characteristics of a MF paper laminate but at a lower price. Good results are obtained and many machines using this type of process are today in industrial operation. A more recent trend has been to develop MUF copolymers to use with the less costly single impregnating bath machines. A few cases of this route to coping with the high cost of melamine are on record.