Лако-красочные материалы — производство

Urea-formaldehyde resins [1-9] are based on a series of consecutive reactions of urea and formaldehyde. Using different conditions of reaction and preparation a practically endless variety of condensed UF chemical structures is possible. UF resins are thermosetting resins and consist of linear or branched oligomers and polymers always admixed with some amounts of monomers. The presence of some unreacted urea is often helpful to achieve specific effects, e. g., a better storage stability of the resin. The presence of free formaldehyde has, however, both positive and negative effects. On the one hand, it is necessary to induce the subsequent hardening reaction while, on the other hand, it causes a certain level of formaldehyde emission during the hot press, resin hardening cycle. Even in the hardened state, low levels of residual formaldehyde can lead to the displeasing odor of formaldehyde emission from the boards while in service. This fact has changed significantly the composition and formulation of UF resins during the past 20 years.

After hardening, UF resins consist of insoluble, three-dimensional networks which cannot be melted or thermoformed again. In their application stage UF resins are used as water solutions or dispersions or even in the form of still soluble spray dried powders. These, however, in most cases have to be redissolved and redispersed in water for application.

Despite the fact that UF resins consist of only the two main components, namely urea and formaldehyde, a broad variety of possible reactions and resin structures can be achieved. The basic characteristics of UF resins can be ascribed at a molecular level to:

their high reactivity

their waterborne state, which renders these resins ideal for use in the woodworking industry

the reversibility of their aminomethylene bridge, which also explains the low resis­tance of UF resins to water and moisture attack, especially at higher tempera­tures; this is also one of the reasons for the hydrolysis leading to subsequent formaldehyde emission.

The reaction of urea and formaldehyde is basically a two-step process, usually con­sisting of an alkaline methylolation (hydroxymethylation) step and an acid condensation step. The methylolation reaction, which usually is performed at a high molar ratio (F/U = 1.8 to 2.5), is the addition of up to three (four in theory) molecules of bifunctional formaldehyde to one molecule of urea to give methylolureas; the types and the proportions of the formed methylol groups depend on the molar ratio F/U. Each methylolation step has its own rate constant kh with different values for the forward and the backward reactions. The formation of these methylol groups mostly depends on the molar ratio F/U. The higher the molar ratio used, the higher the molecular weight the methylolated species formed tends to be. The UF resin itself is formed in the acid condensation step, where still the same high molar ratios as in the alkaline methylolation step is used (F/ U = 1.8 to 2.5): the methylol groups, urea and the free formaldehyde react with linear and partly branched molecules with medium and even higher molar masses, forming the polydisperse molar mass distribution pattern characteristic of UF resins. Molar ratios lower than approximately 1.8 during this acid condensation step tend to cause resin precipitation.

The final UF resin has a low F/U molar ratio obtained by the addition of the so-called second urea, which might also be added in several steps [8,9]. The second urea process step needs particular care. It is important for the production of resins with good performance, especially at the very low molar ratios usually in use now in the production of particle­boards and MDFs. This last step also includes the distillation of the resin solution to usually 66% resin solids content, which is performed by vacuum distillation in the reactor itself or in a thin layer evaporator. Industrial manufacturing procedures usually are pro­prietary and are described in depth in the literature only in rare cases [7-11].

The type of bonding between the urea molecules depends on the conditions used: low temperatures and slightly acid pHs favor the formation of methylene ether bridges (-CH2- O-CH2-) and higher temperatures and lower pHs lead preferentially to the formation of more stable methylene bridges (-CH2-). Ether bridges can be rearranged to methylene bridges by splitting off formaldehyde. One ether bridge needs two formaldehyde molecules and additionally it is not as stable as a methylene bridge, hence it is highly recommended to follow procedures that minimize the formation of such ether groups in UF resins. In the literature other types of resin preparation procedures are also described. Some of these yield uron structures in high proportion [12-15] or triazinone rings in the resins [15-17]. The latter are formed by the reaction of ammonia or an amine, respectively, with urea and an excess of formaldehyde under alkaline conditions. These resins are used, e. g., to enhance the wet strength of paper.

The following chemical species are present in UF resins:

free formaldehyde, which is in steady state with the remaining methylol groups and the post-added urea

monomeric methylol groups, which have been formed mainly by the reaction of the post-added urea with the high content of free formaldehyde at the still high molar ratio of the acid condensation step

oligomeric methylol groups, which have not reacted further in the acid condensation reaction or which have been formed by the above-mentioned reaction of post — added urea

molecules with higher molar masses, which constitute the real polymer portion of the resin.

The condensation reaction as well as the increase in the molar mass can also be monitored by gel permeation chromatography (GPC) [18,19]. At longer acid condensation steps, molecules with higher molar mass form and the GPC peaks shift to lower elution volumes.

Because of the necessity to limit the subsequent formaldehyde emission, the molar ratio F/U has been decreased constantly over the years [20]. The main differences between the UF resins with high and low formaldehyde content are the reactivity of the resin due to the different contents of free formaldehyde and the degree of crosslinking in the cured network. The main challenge has been to reduce the content of formaldehyde in the UF resins and to achieve this without any major changes in the performance of the resins. In theory this is not possible, because formaldehyde is the reactive partner in the reaction of urea and formaldehyde during the condensation reaction as well as curing. Decreasing the molar ratio F/U means lowering the degree of branching and crosslinking in the hardened network, which unavoidably leads to a lower cohesive bonding strength. The degree of crosslinking is directly related to the molar ratio of the two components.

The UF resin formulators have revolutionized UF resin chemistry in the past 30 years. For example, in a straight UF resin for wood particleboard the above mentioned molar ratio F/U was approximately 1.6 at the end of the 1970s. It is now 1.02-1.08, but the requirements for the boards (e. g., internal bond strength or percent thickness swelling in water) as given in the quality standards are still unaltered. Also the reactivity of the resin during hardening, besides the degree of crosslinking of the cured resins, depends on the availability of free formaldehyde in the system.

It has, however, to be considered that it is neither the content of free formaldehyde itself nor the molar ratio which should be taken as the decisive and only criterion for the classification of a resin concerning its subsequent level of formaldehyde emission. In reality the composition of the glue mix as well as the various process parameters during board production also determine the level of formaldehyde emission. Depending on the type of board and the process of application, it is sometimes recommended to use a UF resin with a low molar ration F/U (e. g., F/U = 1.03), hence presenting a low content of free formaldehyde; while sometimes the use of a resin with higher molar ratio (e. g., F/U = 1.10) to which a formaldehyde catcher has been added in the glue mix will give better results. Which of these two possible ways is the better one in practice can only be decided by trial and error in each case.

The higher the molar ratio F/U, the higher is the content of free formaldehyde in the resin. Assuming stable conditions in the resins, which means that, e. g., post-added urea has had enough time to react with the resin, the content of free formaldehyde is very similar even for different manufacturing procedures. The content of formaldehyde in a straight UF resin is approximately 0.1% at F/U = 1.1 and 1% at F/U = 1.8 [19-21]. It also decreases with time due to aging reactions where this formaldehyde reacts further. Table 5 summarizes the various influences of the molar ratio F/U on various properties of wood — based panels. Table 6 summerizes the influence of the molar rations F/U and F/(NH2)2,

Table 5 Influence of the Molar Ratio on Various Properties of UF-Bonded Wood-Based Panels

Decreasing the molar ratio leads to

a decrease of the formaldehyde emission during the production of the wood-based panels

the subsequent formaldehyde emission the mechanical properties the degree of hardening

an increase of the thickness swelling and the water absorption
the susceptibility of hydrolysis

respectively, of pure and melamine fortified UF resins currently in use in the wood-based panels industry.

The molar mass distribution of UF resins is determined

by the degree of condensation and

by the addition of urea (and sometimes also other components) after the condensa­tion step; this again shifts the resin mass distribution towards lower average molar masses.

For this reason the molar mass distribution is much broader than for other poly­mers: it starts at the low molar mass monomers (the molecular weight of formaldehyde is 30, for urea it is 60) and goes up to more polymerized structures. It is not clearly known, however, what are really the highest molar masses in a UF resin. Molar masses of up to 500,000, determined by light scattering, have been reported [18,22]. The conditions of molecular level shear within the chromatographic columns [23] should guarantee that all physically bonded clusters, caused by the interaction of the polar groups present in the resins and which might simulate too high a molar mass, are separated and that these high numbers between 100,000 and 500,000, measured using low angle laser light scatter­ing (LALLS) coupled to GPC, really do describe the macromolecular structure of a UF resin in the right manner. A second important argument for this statement is the fact that up to such a high molar mass the on-line calibration curve determined in the GPC-LALLS run is stable and more or less linear. It does not show any sudden transition as would be the case of a too sharp increase in apparent molar mass if molecular clustering occurred again after the material has passed through the column.

The molar mass distribution (and the degree of condensation) is one of the most important characteristics of the resin and it determines several properties of the resin. Consequence of highly condensed resin structures (high molar masses) are:

the viscosity at a given solids content increases [19,24]

the flowing ability is reduced

the wetting behavior of a wood surface becomes worse [24]

the penetration into the wood surface is reduced [25,26]

the distribution of the resin on the furnish (particles, fibers) worsens

the water dilutability of the resin becomes lower

the portion of the resin that remains soluble in water decreases [22]

Diluting the resin with a surplus of water causes precipitation of parts of the resin. These parts preferably contain the higher molar mass molecules of the resin and their relative proportion increases at higher degrees of condensation [22]. Information on cor­relations between the molar mass distribution (degree of condensation) and mechanical and hygroscopic properties of the boards produced, however, is rather rare and often equivocal [7,19,27-29].

The influence of the degree of condensation is mostly felt during the application and the hardening reaction (wetting behavior and penetration into the wood surface which depend on the degree of condensation). At higher temperatures, during the curing hot press cycle, the viscosity of the resin drops, before the onset of hardening again leads to an increase of viscosity. With this temporary lowering of the viscosity the adhesive wetting behavior improves significantly, but its substrate penetration behavior also changes. The reactivity of an aminoplastic resin seems to be independent of its viscosity (degrees of condensation), at parity of molar ratio. Ferg [30] mentioned that the bonding strength increased with the degree of condensation of the applied UF resin. The higher molar masses (higher viscosity resin fractions) give a more stable glue line and determine the cohesive properties of the hardened resin [7]. Also Rice [29] and Narkarai and Wantanabe [28] reported that the resistance of a bondline against water attack and redrying increased with the viscosity of the resin. The reason again might be that resins with an advanced degree of condensation remain to a greater extent in the glue line, avoiding resin over­absorption by the substrate and hence avoiding starving of the bondline. Rice [29] found an increase of the thickness of the glue line with an increased viscosity of the resin, obviously due to its lower penetration into the wood substrate. However, it must be taken into consideration that the strength and stability of a glue line decrease with increased glue-line thickness [31]. According to the findings of Sodhi [32] the bonding strength decreases the longer is the waiting time before application of the glue mix. Once the hardening reaction has started and, therefore, the average molar mass has started to increase, the worse the resin wetting behavior and its penetration in the wood surface appears to be.