Лако-красочные материалы — производство

Table 2 summarizes the general parameters of importance for wood adhesives. Research and development in adhesives and resins are mainly driven by the requirements of the bonding and production processes and by the intended properties of the wood-based panels. These requirements are summarized in Table 3.

The necessity to achieve shorter press times is omnipresent within the woodworking industry, to keep production costs low. An increased production rate gives the chance to reduce production costs. This is only valid when the market is able to absorb such a high level of production. Shorter press times within a given production line and for certain types of wood-based panels can be achieved by, among others:

highly reactive adhesive resins possessing rapid gelling and hardening and steep increase in bonding strength even at a low degree of chemical curing highly reactive adhesive glue mixes obtained by the addition of accelerators, special hardeners, crosslinkers, and others

the optimization of the pressing process, e. g., by increasing the effect of the steam shock by (i) increased press temperatures, (ii) a more marked difference in the moisture content between the surface and the core layer of the panel before hot pressing, or (iii) an additional steam injection step. constancy of as many parameters of the production process as possible.

UF, urea-formaldehyde resin; MUF, melamine fortified UF resin; MF/MUF, melamine and melamine-urea resins (MF resins are only used mixed/coreacted with UF resins; MUPF, melamine-urea-phenol-formaldehyde resin; PF/PUF, phenol and phenol-urea-formaldehyde resin; (P)RF, resorcinol-(phenol-)formaldehyde resin; PMDI, polymeric methylenediisocyanate; PVAc, polyvinylacetate adhesive; old nat. adhesives, old (historic) nat­ural adhesives (e. g., starch, glutin, casein adhesives); nat. adhesives, natural adhesives (e. g., tannins, lignins, carbohydrates); inorg. adhesives, inorganic adhesives (e. g., cement, gypsum); activation: activation constituents of wood to function as adhesives (i. e., lignin).

V20, particleboard according to DIN 68761 (parts 1 and 4, FPY, FPO), DIN 68763 (V20) and EN 312-2 to 4 and 312-6; V100, particleboard according to DIN 68763 and EN 312-5 and 312-7, option 2 (internal bond after boil test according to EN 1087-1); V313, particleboard according to EN 312-5 and 312-7, option 1 (cycle test accord­ing to EN 321); FP, hardboard (wet process) according to EN 622-2; MDF, medium density fiberboard according to EN 622-5; PLW, plywood according to EN 636 with various resistance against influence of moisture and water; HLB, laminated beams; MH, solid wood panels according to OeNORM B 3021 to B 3023 (prEN 12775, prEN 13353 part 1 to 3, prEN 13017-1 and 2, prEN 13354); ven., veneering and covering with foils; furn., production of furniture.

aPartly powder resins.

bBoards with reduced thickness swelling, e. g., for laminate flooring. cSpecial production method.

Table 2 General Requirements for Wood Adhesives

Composition, solids content, viscosity, purity Color and smell

Sufficient storage stability for given transport and storage conditions Easy application

Low transport and application risks Proper gluing quality Climate resistance

Hardening characteristic: reactivity, hardening, crosslinking Compatibility for additives Cold tack behavior

Ecological behavior: Life cycle analysis (LCA), waste water, disposal, etc.

Emission of monomers, Volatile organic compounds (VOC), formaldehyde during production of the wood-based panels and during their use

Shorter press times, shorter cycle times

Better hygroscopic behavior of boards (e. g., lower thickness swelling, higher resistance against the influence of humidity and water, better outdoor performance)

Cheaper raw materials and alternative products Modification of the wood surface

Life cycle assessment, energy and raw material balances, recycling and reuse Reduction of emissions during the production and the use of wood-based panels

Cheaper raw materials are another way to reduce production costs. This includes, for example, the minimization of the melamine content in a MUF resin, to produce boards with reduced thickness swelling or increased resistance against the influence of water and high humidity of the surrounding air. Impeding factors (often temporary) can be the shortage of raw materials for the adhesives, as was the case with methanol and melamine during the 1990s.

Life cycle analysis and recycling of bonded wood boards also concerns the adhesive resins used, since adhesives and resins are one of the major raw materials in the production of wood-based panels. This includes, for example, the impact of the adhesives on various environmental issues such as waste water and effluent management, noxious gas emission during panel production and from the finished boards, or the reuse of panels to burn for energy generation. Furthermore, for certain recycling processes the type of resin has also a crucial influence on their feasibility and efficiency.

Gas emission from wood-based panels during their production can be caused by chemicals inherent to wood itself, such as terpenes or free acids, as well as by volatile compounds and residual monomers coming from the adhesive. The emission of form­aldehyde especially is a matter of concern, but so are possible emissions and discharges of free phenols or other materials. The formaldehyde emission noted only after panel manufacture and adhesive resin hardening is due, on the one hand, to the residual, unreacted formaldehyde present in urea-formaldehyde (UF)-bonded boards, or as gas trapped in the wood or dissolved in the moisture still present in the panel. On the other hand, in aminoplastic resins the hydrolysis of weakly bonded formaldehyde from N- methylol groups, acetals, and hemiacetals as well as in more severe cases of hydrolysis (e. g., at high relative humidity) from methylene ether bridges, increases again the content of emittable formaldehyde after resin hardening. In contrast to phenolic resins, a per­manent reservoir of potentially emittable formaldehyde is the consequence of the pres­ence of these weakly bonded structures. This explains the continuous, yet low, release of formaldehyde from UF-bonded wood-based panels even over long periods. However, the level of emission depends on the environmental conditions, a fact which may be described by the resin hydrolysis rate which indicates if this formaldehyde reservoir will or will not lead to unpleasantly high emission values [1-4]. The higher this hydrolysis rate is, the higher is the potential reservoir of formaldehyde which contributes to sub­sequent formaldehyde emission. The problem of formaldehyde emission after adhesive hardening in panel manufacture can fortunately be regarded today as solved, due to clear and stringent emission regulations in many European and other countries and to successful long term R&D investement by the chemical industry and the wood working industry.

The so-called El-emission class regulations shown in Table 4 for different panel products describe the level of formaldehyde emission which is low enough to prevent

Table 4 Actual Regulations Concerning Formaldehyde Emission from Wood-Based Panels According to the German Regulation of Prohibition of Chemicals (formerly Regulation of Hazardous Substances) for E1 Emission Class (the Lowest Emission Types panels)

(a) Maximum steady state concentration in a climate chamber:

0.1 ppm (prEN 717-1; 1995)

(b) Laboratory test methods (based on experimental correlation experiences):

Particleboard: 6.5mg/100g dry board as perforator value (EN 120; 1992)

MDF: 7.0mg/100g dry board as perforator value (EN 120; 1992)

Plywood: 2.5mg/h-m2 with gas analysis method (EN 717-2)

Particleboard and MDF: correction of the perforator value to 6.5% board moisture content

any danger, irritation, or inflammation of the mucous membranes in the eyes, nose, and mouth. However, it is important that not only the boards themselves, but also veneering and carpenters’ adhesives, lacquers, varnishes, and other sources of formaldehyde be controlled, since they also might contribute to a close environment formaldehyde steady-state concentration [1-4].