Лако-красочные материалы — производство

The resin used has a crucial influence on the properties of wood-based panels. Depending on the requirements, different resin types are selected for use. Whereas UF resins are mainly used for interior boards (for use in dry conditions, e. g., in furniture manufactur­ing), a higher water resistance can be achieved by incoroporating melamine and also some phenol into the resin (melamine fortified UF resins, MUF, MUPF, PMUF). The level of melamine addition and especially the resin manufacturing sequence used in relation to how melamine is incorporated in the resin can be very different. The different types of these resins which exist today are given in Table 7. The different resistances of these resins against hydrolysis are based on their differences at the molecular level. The methylene bridge linking the nitrogens of amido groups can be split rather easily by water attack in UF resins. The same is not so easy in the case of M(U)F resins, mainly due to the much lower water solubility of melamine itself which is a consequence of the water repellency

Table 7 Molar Ratios F/(NH2)2 of MUF/MUPF Resins Currently in Use in the Wood-Based

Panels Industry

F/(NH2)2 molar ratio Resin type

1.20 to 1.35 Resins for water resistant plywood, in the case of the addition of a

formaldehyde catcher

0.98 to 1.15 E1 particleboard resin and E1 MDF resin for water resistant boards

(PB: EN 312-5 and 312-7; MDF: EN 622-5). For particleboards according to option 1 (V313 cycle test) MUF resins can be used; for boards according to option 2 (V100 2h boiling test, tested wet) MUPF or MUF with a special approval is necessary. In this case, especially for the MDF production, formaldehyde catchers are added

^ 1.00 Special resins for boards with very low formaldehyde emission during board

service [81,82]

characteristic of the triazine ring of melamine. The equivalent methylene bridge is instead very stable to hydrolytic attack in phenolic resins. The melamine fortified products, how­ever, are much more expensive due to the much higher price of melamine compared to urea. Therefore, the content of melamine in these resins is as high as strictly necessary but always as low as possible.

A MUF resin, at parity of all other conditions, yields a lower pH drop after addition of the hardener than a UF resin [46]. This lower drop of the pH due to the buffer capacity of the triazine ring of melamine, however, also causes a decrease of the hardening rate of the resin and, therefore, a lengthening of its gel time [1], hence a lengthening of the hot press time is necessary. This is also seen in the shifts of the exothermic differential scanning calorimetry (DSC) peak of hardening which are observed in thermal experiments [47].

The deterioration of a bondline and hence its durability under conditions of weath­ering is determined essentially by:

The failure of the resin (low hydrolysis resistance, degradation of the hardened resin causing loss of bonding strength).

The failure of the interface between the resin and the wood surface (replacement of physical bondings between resin and reactive wood surface sites by water or other nonresin chemicals). The adhesion of UF resins to cellulose is sensitive to water not only due to the already mentioned lability to hydrolysis of the methylene bridge and of its partial reversibility, but also because theoretical calculations have shown that on most cellulose sites the average adhesion of water to cellulose is stronger than that of UF oligomers [8,48]. Thus, water can displace hardened UF resins from the surface of a wood joint. The inverse effect is valid for PF resins [8,49].

The breaking of bondings due to mechanical forces and stresses: water causes swelling and, therefore, movement of the structural components of the wood-based panels (cyclic stresses due to swelling and shrinking, including stress rupture).

The durability of a glue line can be enhanced by the incorporation of hydrophobic chains into the hardened network. This was done by introducing urea-capped di- and trifunctional amines containing aliphatic chains into the resin structure or by using the hydrochloride salts of some of these amines as a curing agent [50-54]. By this approach some flexibility is introduced into the hardened network, which should decrease internal stresses.

In UF resins the aminomethylene link is susceptible to hydrolysis and, therefore, it is unstable at higher relative humidity, especially at elevated temperatures [55,56]. Water also causes degradation of the UF resin with greater devastating effect the higher is the tem­perature of the water in which the boards are immersed. This different behavior of boards at different temperatures also is the basis for standard tests on which is based the classification of bondlines, resins, and bonded wood products. These classes include the lowest require­ments (interior use) for the normal production of UF-bonded boards up to water and weather resistant boards (V100 boiling test, V313 cycle test, water and boil proof (WBP), and others) according to various national and international standard specifications.

Hardened UF resins can also be hydrolyzed by moisture or water, due to the relative weakness of the bond between the nitrogen of the urea and the carbon of the methylene bridge, and this is especially so at higher temperatures. During this reaction the methylene bridge is eliminated as formaldehyde [57,58]. The amount of liberated formaldehyde can be taken under certain circumstances as a measure of the resistance of the resin against hydrolysis. The main parameters influencing the rate and extent of the hydrolysis are temperature, pH, and degree of hardening of the resin [59]. The acid which has induced the hardening of the resin can also and especially induce such a hydrolysis and hence loss of bonding strength.

Another approach to increase the resistance of UF resins against hydrolysis is there­fore, based on the fact that the resin acid hardening causes acid residues in the glue line. Myers [60] pointed out that in the case of such an acid hardening system the decrease in the durability of adhesive bonds could be initiated both by the hydrolysis of the wood cell wall polymers adjacent to the glue line as well as in the case of UF-bonded products by acid — catalyzed resin degradation. A neutral pH glue line, therefore, should show a distinctly higher hydrolysis resistance. The amount of hardener (acids, acidic substances, latent hard­eners) therefore should always be adjusted to the desired hardening conditions (press temperature, press time, and other parameters) and never follow ‘‘the more the better.’’ Thus, too high an addition of hardener can cause brittleness of the cured resin and a very high acid residue in the glue line. However, glue-line neutralization must not take place as long as the hardening reaction is ongoing, otherwise this would delay or even prevent curing. This aspect is quite a challenge which in practice has not yet really been solved. Higuchi and Sakata [61] found that a complete removal of acidic substances by soaking plywood test specimens in an aqueous sodium bicarbonate solution resulted in considerable increase in water resistance of UF glue lines. Another attempt was made by these authors [62,63] using glass powder as an acid scavenger, which reacts only slowly with the remaining acid of the glue line and, therefore, does not interfere with acid hardening of the resin. Dutkiewicz [64] obtained some good results in the neutralization of the inherent acidity of a hardened UF-bonded glue line by the addition of polymers containing amino or amido groups. All these solutions, however, are not used as yet in broader industrial applications.

Laminate floorings require a very low, long term (24 h) thickness swelling of the MDF/high density fiberboard (HDF) or particleboard cores of which they are composed. Requirements usually are a maximum value of 8 or 10%, sometimes a maximum value of 6% or even lower, all figures based on the original thickness of the board. Such low percentage thickness swelling results cannot usually be obtained by just using straight UF resins, whereas the incorporation of melamine in the resin is a suitable way to achieve the desired results. Other possibilities could be a pretreatment of the particles or the fibers (e. g., acetylation) or a special posttreatment of the board. The necessary melamine content in the resin depends on various parameters, e. g., the type of wood furnish, the pressing parameters (pressure profile, density profile), and on resin consumption which can vary between a few percent up to more than 30%, based on liquid adhesive resin. Due to the considerable cost of melamine itself the content of melamine must always be only as high as necessary but as low as possible. Other important parameters are the resin manufactur­ing procedure, which considerably influences the thickness swelling of the boards even at the same adhesive solids content and at the same content of melamine.

Melamine fortified UF resins and MUF resins can be manufactured in a variety of ways, for example:

(i) By cocondensation of melamine, urea, and formaldehyde in a multistep reac­tion [65-69]. In this regard a comprehensive study of the various reaction types was done by Mercer and Pizzi [70]. They especially compared the sequence of the additions of melamine and urea.

(ii) By mixing of an MF resin with a UF resin according to the desired composition of the resin [71-73].

(iii) By addition of melamine in various forms (pure melamine, MF/MUF powder resin) to a UF resin during the application of the glue mix. In the case of the addition of pure melamine, a UF resin of a higher molar ratio must be used, otherwise there is not enough formaldehyde available to react with the mela­mine in order to incorporate it into the resin.

(iv) Melamine also can be added in the form of melamine salts such as acetates, formates, or oxalates [74-78], which decompose in the aqueous resin mix only at higher temperatures and enable some savings of melamine for the same degree of water resistance compared to original MUF resins. Additionally they act as a hardener. Some of the reasons why melamine salts yield a saving in melamine content have also been identified [74].

The higher the content of melamine, the higher is the stability of the hardened resin towards the influence of humidity and water (hydrolysis resistance) [79,80]. Resins con­taining melamine can be characterized by the molar ratio F/(NH2)2 (Table 7) or by the triple molar ratio F:U:M. The mass portion of melamine in the resin can be described based on (i) the liquid resin, (ii) the resin solids content, or (iii) the sum of urea and melamine in the resin.

One of the most interesting tasks is to clarify if there is a real cocondensation within MUF resins or if two independent networks are formed, which only penetrate each other. The application of MUF resins is very similar to the UF resins, with the difference that the level of hardener addition is usually much higher.

MUPF resins are mainly used for the production of so-called V100 exterior grade boards according to DIN 68763 and EN 312-5 and 312-7, option 2. They contain small amounts of phenol. Production procedures are described in patents and in the literature [83-87] and a coreaction has been demonstrated here, although often not contributing to resin effectiveness [83,84,88,89].

PMF/PMUF resins, in which the amount of phenol is much higher than in MUPF resins, usually contain only little or no urea at all. The analysis of the molecular structure of these resins has shown that either there is no cocondensation between the phenol and the melamine, but that there exist two distinct networks [90-93], or that cocondensation can indeed occur [88]. The reason for this is the different reactivities of the phenol methy — lols and the melamine methylols, depending under which pH conditions the reaction is carried out.