Лако-красочные материалы — производство

A. Adhesives for Wood-Based Panels (Polyisocyanates)

Adhesives based on isocyanates (especially PMDI) have been used for more than 25 years in the wood-based panel industry [40,41,200-203], but still have a relatively low consump­tion volume compared to systems based on UF, MUF, or PF resins. The main application is the production of waterproof panels, but there is also the production of panels from raw materials that are difficult to glue, such as straw, bagasse, rice shells, or sugar cane bagasse. PMDI can be used as an adhesive for wood-based products such as exterior particleboard, exterior OSB, laminated strand lumber (LSL), MDF, or other specially engineered composites. During hot pressing the viscosity of PMDI is lowered, allowing it to flow across and penetrate below the surface, locking in the wood subsurface as has been shown by Roll [204]. The low wetting angle of PMDI compared to water-based condensation resins allows a rapid penetration into the wood surface; however, this also might result in starved bondlines [205].

PMDI is produced during the manufacturing of monomeric MDI. The PMDI pro­duced industrially by phosgenation of di-, tri-, and higher amines contains a mixture of the three different MDI isomers, triisocyanates, and different polyisocyanates, and thus the structure and the molar mass depend on the number of phenyl groups. This distribution influences to a great extent the reactivity, but also the usual characteristics such as vis­cosity, flow, and wetting behavior as well as the penetration behavior into the wood surface. The structure and the molar mass depend on the number of aromatic rings [206]. For PMDI the distribution of the three monomeric isomers has a great influence on the quality, because the reactivities of the various isomers (4,40-, 2,40-, and 2,20-MDI) differ significantly [207]. The greater the portion of the 2,20- and 2,40-isomers, the lower the reactivity. This can lead to different bonding strengths as well as to residual low reactive isomers in the wood-based panels produced. In the monomeric form (MDI) the function­ality is 2 and the NCO content is 33.5%, while PMDI has an average functionality of 2.7 with an NCO content of approximately 30.5%. The HCl content is usually below 200 ppm. PMDI is cheaper than pure MDI and has a lower melting point (liquid at room tempera­ture) due to the increased asymmetry. It is less prone to dimerization and, as a conse­quence, it is more stable during storage than pure MDI. PMDI is used whenever the color of the finished adhesive is not of concern [208].

The excellent application properties of PMDI and of the wood-based panels pro­duced with it are based on the special properties of PMDI, especially the excellent wetting behavior of a wood surface when compared to waterborne polycondensation resins. Due to this fact surfaces with poor wetting behavior such as straw can also be bonded. According to Larimer [209] the wetting angles for PMDI on various surfaces are much lower than for UF resins. Additionally, these resins show a good penetration behavior into the wood surface, which seems to be determined by the small molar mass of PMDI when compared with polycondensation resins. Marcinko et al. [210] found in their measure­ments, using solid state 13C-NMR, DSC, fluorescence microscopy, and DMA, that PMDI could penetrate 5-10 times further into wood than PF resins. PMDI not only penetrates the macroscopic hollows of the wood substance, but even penetrates the polymer structure of the wood. This enables good mechanical anchoring. The good wetting and penetration behavior of PMDI can sometimes cause starved glue lines. Due to PMDI’s high reactivity and its low molar mass, a special interfacial layer between the wood surface and the adhesive appears to form. If hardening is quicker than the thermodynamically induced desorption during the hardening reaction, then a polyurea/biuret network might form interpenetrating the wood constituents network. Covalent bondings as well as secondary forces can help to avoid desorption reactions during hardening.

Johns [211] showed that isocyanates spread easily on a wood surface; 2 to 3% of isocyanate was enough to form a film completely covering the wood strands, which is not possible even with 6% of a phenolic resin. The good mobility of MDI is based on several parameters [211]:

MDI contains no water; it cannot lose its mobility during adsorption on the wood surface

its low surface tension (ca. 50mN/m) compared to water (76mN/m)

its low viscosity.

The impossibility of diluting PMDI with water was solved by the introduction of emulsified PMDI, often called EMDI, which allows an even distribution of the adhesive during the gluing process. EMDI is a product of the reaction of PMDI with polyglycols. EMDI is manufactured under high pressure and dispersed in water.

The isocyanate group in PMDI is characterized by high reactivity towards all sub­stances which contain active hydrogens. The main hardening reaction proceeds via reac­tion with water to the final amide group, while at the same time CO2 is split off. The water necessary to induce the hardening reaction is applied together with the PMDI (spraying together with the PMDI or spraying of an aqueous dispersion of PMDI in water) or is present in wood in sufficient amount. The amine group then reacts further with another isocyanate group to form a polyurea structure:

R—N=C=O + H2O! R—NH2 + CO2

R—NH2 + O—C—N — R0 ! R—NH—(C—O)—NH—R0

The reaction of an isocyanate group with a hydroxyl group leads to the so-called urethane bonding:

R—N—C—O + HO—R0 ! R—NH—(C—O)—O—R0

Such a reaction can theoretically also occur between an isocyanate group and an — OH group of cellulose or lignin to form a covalent bond. These bonds are usually of greater durability than purely physical bonds. If one could manage to force such a reaction to occur in an industrially useful short curing time, when the reaction of the isocyanate groups of the PMDI with water is suppressed, the probability of the formation of such covalent bonds and with this the quality of the bonding should increase, leading to higher bond strengths and especially a higher resistance against the influence of humidity.

If another isocyanate group reacts with an amide hydrogen within the polyurea structure formed, a branching point is formed (biuret group):

R"—N—C—O + R—NH—(C—O)—NH—R0 ! R—N—(C—O)—NH—R0

I

(C—O)—NH—R00

During the hardening of PMDI Frazier et al. [212] have found the formation of urethanes, polyureas, biurets, and triurets/polyurets. The proportions of the various compounds depend on the working and hardening conditions. The forming of the network is especially influenced by the ratio between the isocyanate and water. The formation of a urethane seems to be possible for low molar mass isocyanates, as e. g., the usual industrial PMDIs, under slightly alkaline conditions. It can also be assumed that the forming of a urethane especially occurs by reaction with lignin. This bond, however, seems not to be stable at higher temperatures (120°C) for longer times.

Hydrophobic polyols should be able to repel and eliminate water from the wood surface and, therefore to fortify the reaction of the isocyanate group with the hydroxyl groups of the wood surface [213]. Umemura et al. [214] and other workers [27] compared the reaction of isocyanate with water and small amounts of polyols using DMA. The bonding strength and the thermal stability increased by adding dipropylene glycol with molar mass in the 400-1000 range.

Usually no hardeners are added during the production of wood-based panels (par­ticle board, MDF, OSB, engineered wood products) using PMDI as adhesive. With special additives a distinct acceleration of the hardening reaction and hence shorter press times or lower press temperatures can be achieved [209]. This fact is especially interesting for cold­setting systems as well as for the production of particleboards. Possible catalysts are tertiary amines (e. g., triethanol amine, triethylamine, N, N-dimethylcyclohexylamine) and metal catalysts, based on organic compounds of tin, lead, cobalt, and mercury [208,215-218].

Compared to other adhesives, PMDI possesses various advantages, but also some disadvantages (Table 13). For the production of plywood the addition of extenders is recommended [221-226] or the mixing with other resins [191,192] as alone PMDI cannot be used for plywood. In the production of OSB (especially for the two types OSB/3 and OSB/4 according to European Norm EN 300) often PMDI is used in the core layer.