Лако-красочные материалы — производство

In their cured state UF resins are nontoxic. Urea itself is also harmless. However, free formaldehyde and formaldehyde generated by slow hydrolysis of the aminoplastic bond are highly reactive and combine easily with proteins in the human body. This may cause a painful inflammation of the mucous membranes of the eyes, nose, and mouth [25]. Even a low concentration of formaldehyde vapor in the air can cause disagreeable irritations of the nose and eyes. However, such irritations usually disappear in a short time without permanent damage. Occasionally, allergic or anaphylactic reactions develop and complete removal from exposure is necessary.

High temperatures and high relative humidity can result in odor problems in a room containing particleboard manufactured with UF resins [25]. The release of formaldehyde from UF particleboard is caused by two factors. It can be due to free formaldehyde present in the board that has not reacted, and it can be due to formaldehyde formed by hydrolysis of the aminoplastic bond as a result of temperature and relative humidity [2,25]. While the first type of release lasts only a short time after manufacture of the particle­board, the second type of release can continue throughout the entire working life of the board. A considerable number of variables influence the emission of formaldehyde from a UF-bonded particleboard. The main ones are the molar ratio of urea to formaldehyde (which influences both types of release), the press temperature, and in service, the ambient temperature and relative humidity.

UF resins for particleboard with urea/formaldehyde molar ratios of 1:1.45, 1:1.32, and 1:1.25 have free formaldehyde contents of 0.8%, 0.3%, and less than 0.2%, respectively [15]. While the current tendency internationally is to use UF resins that have a urea/formaldehyde molar ratio lower than or much lower than 1:1.2, which release much less formaldehyde, these resins perform less well in the production of UF — bonded particleboard [15,17]. In particular, they do not allow as much flexibility in particleboard production as do resins with higher formaldehyde/urea molar ratios. This fact stresses the need for greater control and supervision of the production at particle­board plants where UF resins of low molar ratio are used. An example of the variation in properties between particleboard manufactured with different molar ratio resins is given in Table 1.

It is also necessary to use more hardener when working with UF resins of a lower formaldehyde/urea molar ratio, as the gel time of the resin is slower. Up to 5% urea can sometimes be added to the glue mix to decrease the amount of formaldehyde released during pressing and to decrease the initial amount of free formaldehyde present in the finished board immediately after manufacture. Strict norms have been established in many countries with regard to the limits of formaldehyde emission from particleboard bonded with UF resins [25,29,30]. Recent work indicated that good E1-type UF resins of urea/ formaldehyde molar ratio lower than 1:1.1 can be prepared in a variety of ways [16,26-28]. Although the theoretical basis of this finding has been discussed in part elsewhere [16], to be able to advance a tentative theory for low-formaldehyde-emission UF resins, it is of interest to apply these findings to the formulation and preparation of UF resins of low formaldehyde emission, initially in the laboratory and then at the industrial level. First, these resins can be divided into two broad classes: (1) those resins based on addition of melamine or melamine-formaldehyde (MF) resins to the UF resin, and (2) those UF resins in which very low formaldehyde emission capability is obtained exclusively by the manipulation of their manufacturing parameters. The former class is simply a subset of the second.

The underlying principle of a low-formaldehyde-emission UF resin is that a certain amount of free urea needs to be present to (1) mop up a large amount of the free for­maldehyde that may be present at the end of the preparation, and (2) to mop up the greater part of the free formaldehyde that may be generated during hot curing of the resin. A third possible requirement would be that some free monomeric urea species should still be left to mop up, over a long period of time, some of the formaldehyde that may be liberated during the service life of the board.

Such requirements of a UF resin are fundamentally quite divergent and extreme. They mean that addition of great amounts of urea is needed, possibly at the end of the reaction; such urea will react with the free HCHO present or generated during hot curing, but will also react with the active methylol groups present on the urea resin itself, severely limiting the possibility of cross-linking of the resin and ultimately affecting adversely and diminishing its cured strength. These two sets of divergent requirements indicate that in general, a low-formaldehyde-emission UF formulation must be a compromise between strength and emission requirements. Once this basic conflict of requirements is under­stood, it can be overcome to attain formulations that give both good strength and low formaldehyde emission. A UF resin is a mixture of molecular species: namely, methylo — lurea, UF polymers, and methylolated UF polymers. It has already been proven, both theoretically [8,19] and by applied means [14,27,29] that while monomeric and polymeric methylolated species contribute more to the adhesion of the resin to the wood substrate, it is the polymeric fraction (methylolated and nonmethylolated) that contributes most to the

cohesion of the resin. Thus, a resin to which great amounts of final urea are added will have a proportionally high amount of urea and monomeric methylolated species, giving both good adhesion and low formaldehyde emission, and proportionally a lower amount of prebuilt polymeric species, giving poor cohesion, hence lower strength. Conversely, a resin of final higher formaldehyde/urea molar ratio such as the classic UF resins used for the last few decades, will have a large number of polymeric species, will still be heavily methylolated—most of the methylolated species will be polymeric, however—and will still have a considerable amount of free and potentially free formaldehyde. These resins will have good cohesion and good adhesion, hence good strength, but very high HCHO emission.

The logical manner to avoid the conflicting requirements of the two properties wanted is then to prebuild in some easy and convenient manner the particular mixture of species that will give the correct balance of strength and emission for the applications required. Thus, although UF resins of very low formaldehyde/urea ratios [16] can be prepared by adding great amounts of second and third ureas, the high predominance of urea and other monomeric species in relation to polymer proportions will give boards of poor strength, albeit of very low HCHO emission. The required balance of chemical species and of properties can then be achieved more easily by preparing two or more UF resins, and/or preresins, which are mixed in various amounts to yield the desired balance of acceptable strength and low emission [16,26].