Лако-красочные материалы — производство

Table 9 summarizes some resin glue mixes for different applications in the production of particleboard and MDF. Table 10 summarizes various resin glue mixes for different applications in the production of plywood, parquet flooring, and furniture.

II. PHENOLIC RESINS

Phenolic resins [phenol-formaldehyde (PF) resins] show complete resistance to hydrolysis of the C-C bond between the aromatic nucleus and the methylene bridge and, therefore, are used for water and weather resistant glue lines and boards such as water and weather proof particleboards, OSB, MDF, or plywood for use under exterior weather conditions. Another advantage of phenolic resins is the very low formaldehyde emission in service, after hardening, also due to the stability of the methylene bridges between aromatic nuclei. The disadvantages of phenolic resins are the distinctly longer press times necessary for hardening when compared to UF resins, the dark color of the glue line and of the board surface as well as a higher equilibrium moisture content of the boards due to the hygro — scopicity of the high alkali content of the board.

The preparation procedure of a phenolic resin is a multistage process, characterized by the time, sequence, and amount (in the case of several steps) of the additions of phenol, formaldehyde, and alkali as the most important raw materials. Similarly to all other formaldehyde condensation resins two main reactions predominate:

Methylolation: there is no special preference for ortho or para substitution, preference which, however, could be achieved using special catalysts [104-106].

Methylolation is strongly exothermic and includes the risk of an uncontrolled reaction [107].

Condensation methylene and methylene ether linkages are formed; the latter do not exist at high alkaline conditions. During this stage chains are formed, still carrying free methylol groups. The reaction is stopped just by cooling down the preparation reactor thus preventing resin gelling.

Phenolic resins contain oligomeric and polymeric chains as well as monomeric methylol — phenols, free formaldehyde, and unreacted phenol. The contents of both monomers have to be minimized by the proper preparation procedure. Various preparation procedures are described in the literature and in patents [108-117].

Special PF resins consisting of a two-phase system of a highly condensed and inso­luble PF resin and a lower condensation standard PF resin have also been prepared [118] and used industrially. Another two-phase resin consists of a highly condensed PF resin still in an aqueous solution and a PF dispersion [119]. The purpose of such special resins is the gluing of panel products of higher moisture content wood, where the danger of over­penetration of the resin into the wood surface would cause a starved glue line and other serious problems.

The properties of the resins are determined mainly by the F/P molar ratio, the concentration of phenol and formaldehyde in the resin, the type and amount of the preparation catalyst (in most cases alkaline), and the reaction conditions. The reaction

itself is performed in an aqueous system without addition of organic solvents. The higher the F/P molar ratio, the higher is the reactivity of the resin hence the higher its hardening rate [120], the degree of branching, and the three-dimensional crosslinking. At lower F/P molar ratios linear molecules are formed preferably. Chow et al. [121] found an increase in the bonding strength of plywood with increasing F/P molar ratio; however, the bonding strength remained constant for molar ratios higher than 1.4. This value is still distinctly lower than the common industrial molar ratios of PF resins for wood adhesives.

Usually sodium hydroxide is used as a catalyst, in an amount up to one mole per mole phenol (molar ratio NaOH/P), which corresponds to a portion of alkali in the liquid resin of approximately 10% by weight. The pH of a phenolic resin is in the range of 10 to 13. The preponderant part of the alkali is free NaOH, and a smaller part is present as sodium phenate. The alkali is necessary to keep the resin water soluble via the phenate ion forma­tion in order to achieve a degree of condensation as high as possible at a viscosity that still can be used in practice. Additionally the alkali content significantly lowers the viscosity of the reaction mixture. Thus, the higher the alkali content of the resin, the higher is its possible degree of condensation, hence the greater is the reactivity of the resin and the higher its hardening rate and, therefore, the shorter is the necessary press time.

High alkali contents have also some disadvantages. The equilibrium moisture content in humid climates increases with the alkaline content as do some hygroscopic-dependent properties (longitudinal stability, thickness swelling, water absorption), and some mechan­ical properties (creep behavior) become worse. The alkali content also causes a cleavage of the acetyl groups of the hemicelluloses. This leads to an enhanced emission of acetic acid compared to UF-bonded boards. The higher the alkali content, the higher is the emission of acetic acid. In European Norms EN 312-5 and 312-7 the content of alkali is limited to 2.0% for the whole board and 1.7% for the face layer, both figures being based on oven-dried mass of the board.

Besides NaOH, other basic catalysts can be and are used, such as Ba(OH)2, LiOH, Na2CO3, ammonia, or hexamine. However, with some notable exceptions, these are not used in practice. The type of catalyst significantly determines the properties of the resins [122-124]. Replacing alkali in PF-bonded boards could give some advantages. Ammonia being a gas evaporates during the hot press process and does not therefore contribute to the alkali content and the hygroscopicity of the boards. It is important to hold a fairly high pH as long as possible during hot pressing in order to guarantee a high reactivity and hence a short press time [125,126].

The condensation process of PF resins can be followed by monitoring the increase in viscosity and by gel permeation chromatography (GPC) to measure the molar mass distribution. Chromatograms have been obtained by Duval et al. [122], Ellis and Steiner [127], Gobec et al. [128], Kim et al. [129], and Nieh and Sellers [130].

The penetration behavior strongly depends on the molar masses present in the resin: the higher the molar masses (approximately equivalent to the viscosity of the resin at the same solid content), the worse is the wettability and the lower is the penetration into the wood surface [131,132]. The lower molar masses are responsible for the good wettability, however, too low a molar mass can cause overpenetration and hence starved glue lines. Contact angles of phenolic resins on wood increase strongly with the viscosity of the resin, which increases with the molar masses [133]. The higher molar masses remain at the wood surface and form the glue line, but they will not anchor as well in the wood surface. Depending on the porosity of the wood surface, a certain portion with higher molar masses must be present to avoid an overpenetration into the wood, causing a starved glue line; this means a certain ratio between low and high molar masses is necessary

[127,130,134-141]. Gollob and co-workers [109,142] found a decrease in the wood failure with increased molar mass averages of PF resins.

The penetration behavior of resins into the wood surface also is influenced by various other parameters, such as wood species, amount of glue spread, press temperature, and pressure and hardening time. The temperature of the wood surface and of the glue line and hence the viscosity of the resin (which itself also depends on the degree of advance­ment of the resin at the time of measuring) influences the penetration behavior of the resin [143]. Table 11 summarizes the properties of various PF resins.

The contents of free monomers (formaldehyde, phenol) depend on the type of the resin and the preparation procedure. Usual values are <0.3 mass% for the free formal­dehyde and <0.1 mass% for free phenol.

The storage stability of liquid PF resins ranges from a few weeks up to several months, depending on the degree of condensation, the content of alkali, and the viscosity. An important parameter for the length of the possible storage time is the viscosity of the resin, with regards to both the proper application onto the wood surface during blending as well as the danger that the resin might gel in its storage tank. The lower the alkali content, the lower the storage stability. The aging behavior can also be monitored using GPC [144].

The molecular characterization of PF adhesive resins is done in similar way to that of all the other condensation resins by determining:

the molar ratios of the main components: F/P/NaOH; F/P; NaOH/P the composition of the resins, based on liquid form of delivery the degree of condensation and molar mass distribution, molar mass averages the content of reactive sites and functional groups and their distribution in the resin, type of bridges between the aromatic rings of the phenol molecule, branching sites and others.

Due to the hydrolysis-resistant C-C bonding between the aromatic ring and the methylene group it is not possible to determine the molar ratio in the usual chemical way. This is only possible by 13C-NMR. The F/P molar ratio of PF resins is usually between 1.8 and 2.5, depending on the type of resin. The higher the molar ratio, the higher its reactivity as well as its storage stability. However, the hardened resin is more brittle due to its higher level of crosslinking.