Лако-красочные материалы — производство

Several effective techniques to consistently and markedly decrease the melamine content in MUF wood adhesives without any loss of performance have also been recently developed. Some of these formulation systems and techniques are already in the early stages of industrialization. Among these melamine/acid salts, such as melamine acetate (Formula 5), function both as efficient hidden hardeners of UF resins for plywood as well as upgrad­ing the performance of simple UF resins for plywood by approximately 10% by mass melamine grafting to yield comparable strength durability of premanufactured MUF resins of 30 to 40% melamine mass content, hence of resins of much higher mass content of melamine. In short a MUF resin of melamine:urea weight ratio 10:90 will perform in certain applications such as exterior plywood as a premanufactured MUF resin of mela — mine:urea between 30:70 and 40:60 [21-24]. The system works both (i) by simple addition of the melamine salt in the UF glue mix eliminating the need to premanufacture a MUF resin. The effectiveness of melamine grafting in the glue mix and during hot pressing has been found to depend on the relative solubility of the melamine salt which depends on both the acid strength of the acid as well as the number of acid functions in the salt. (ii) By use of salts in which the excess acid has been eliminated from the salt, hence melamine monoacetate with no loose acid residue. The salt can be added in the resin factory to a UF resin and the mix sold as a MUF resin as pot life is indefinite and the resin needs the addition of a classical hardener for aminoplastic resins such as sodium sulfate or sodium chloride for hardening. The solubility of the salts used increases with increase in tempera­ture. The reasons why traditional, premanufactured MUF resins waste 2/3 or more of the melamine used in them, and why such a melamine salt addition system is so much more effective by not wasting melamine were presented in the same study [21].

How is it possible that addition of a melamine salt to a UF glue mix in a melami — ne:urea mass ratio of 10:90 yields plywood of comparable water resistance to a prereacted MUF resin of melamine:urea mass ratio in the range 30:70 to 40:60? As a consequence of what is presented above it is now possible to answer such a question. In the preparation of precopolymerized MUF resins, hence of today’s normal, commercial MUF resins, during the high temperature preparation reaction the melamine also reacts with formaldehyde to form short MF chains which are then bound to the more abundant UF chains. Hardening of MUF resins has been proven to occur almost exclusively by cross-linking through — CH2- bridges connecting two melamines [20,25] as, due to its much lower reactivity, urea is not greatly involved. The use of melamine salts at ambient temperature in the glue mix instead ensures that only single melamine molecules are singly and separately grafted on the UF resin chain.

—U-CH2-M-CH2-M-CH2-M against —U-CH2-M

to yield rather different cross-linked networks than those of a standard MUF reactor — made resin [21-26].

As to cross-link the system only a very small amount of melamine molecules for each UF chain is needed to achieve the same effect, to have several chains of MF as in standard MUF resins does not improve the bond strength because (i) only one of the melamines in the chain will react, the other not participating at all in final cross-linking, and (ii) the bonding strength will also not be improved by having even all the melamines of the MF chain react all in the same space zone of the network as shown in the first network formula above: on the contrary, the highly localized position on vicinal sites in the network of a high density of cross-links might well render the resin far too rigid and far too brittle (which indeed is the case for most melamine-based resins). It is then clear that at least 2/3 of the melamine presently used in MUF resins is actually wasted and does not contribute much to the final results other than in a damaging manner, this being unavoidable as a consequence of the system of preparation used. The new system presented greatly improves on the present situation, not only on ease of handling (only a UF resin and a melamine salt as a hardener are needed rather than a more sophisticated MUF resin), but also on the amount of melamine needed (just approximately 1/3 of present consumption for equal exterior-grade bonding performance) with potentially considerable economic advantages as melamine is generally expensive.

The results of a 2 year field weathering test in Europe have confirmed that a UF resin to which has been added 15% melamine acetate salt at the glue mix stage, to obtain a melamine:urea mass ratio of 10:90 solids on solids, imparts a better durability and better exterior performance to plywood glue lines than traditionally reactor-coreacted MUF resins of melamine:urea mass ratio of 33:66 and even of commercial, prereacted PMUF resin where the relative mass proportions of the materials in the resin are 10:30:60 [23].

Postcuring of aminoplastic-bonded wood joints has always been avoided due to the evident degradation induced by heat and humidity on the aminoplastic resin hardened network. This is a known fact and it is for this reason that boards bonded with UF, MUF, and MF resins are traditionally cooled as rapidly as possible after manufacture. However, tightening of formaldehyde emission regulations has caused considerable progress in ami — noplastic formulations, especially much smaller molar ratios, and hence today’s amino — plastic adhesives are indeed very different materials than those of 10-20 years ago. A recent study [27,28] has shown that the postulate on the avoidance of postcuring of aminoplastic resin bonded joints is under many conditions no longer valid. Thus, (1) postcuring (for example by hotstacking in the simpler cases, by an oven or other heat treatment in more sophisticated cases) can be used in principle and under well-defined conditions to improve the performance of UF and MUF-bonded joints and panels without any further joint and hardened adhesive degradation, as the value of the modulus reached during postcuring is always consistently higher than the value at which the modulus stabilizes after complete curing during the ‘‘pressing’’ cycle. (2) Postcuring could also be used in principle and for the same reasons to further shorten the pressing time of MUF-bonded joints and panels when well-defined postcuring conditions are used or to decrease the proportion of adhesive used at parity of performance [27,28]. (3) There is clear indication that under certain conditions, even when adhesive degradation starts, the application of the posttreatment reestablishes the value of the joint’s strength to a value higher than its maximum value obtained during curing. Some of the best posttreat­ment schedules have also been presented [27] (Fig. 5).

The performance improvements in the internal bond (IB) strength of bonded wood panels are introduced by the series of reactions pertaining to internal methylene ether bridge rearrangements to a tighter methylene bridge network which have already been observed and extensively discussed in the analysis of aminoplastic and phenolic resins [27,29-31]. These are able to counterbalance well the degradative trend to which the aminoplastic resin should be subjected. Furthermore, in modern resins of lower F:(U + M) molar ratio the amount of methylene ether bridges formed in curing is much lower. Thus, disruption by postcuring of the already formed resin network by internal resin rearrangements will be milder, if at all present, and will definitely not yield the marked degradation and even collapse of the structure of the network which characterizes older resins of much higher molar ratio when postcured under the same conditions [32,33]. In short, notwithstanding the internal rearrangement the network will stand and stand

quite strongly: no, or hardly any decrease of IB strength will be noticeable. For modern, lower molar ratio aminoplastic adhesives, since the resin network does not noticeably degrade or collapse with postcuring, only the tightening of the network derived by further bridge formation by reaction within the network of the few formaldehyde molecules released by the now mild internal rearrangement will be noticeable: the IB strength value will then improve with postcuring in boards bonded with modern, lower formalde­hyde aminoplastic adhesives.

There are important differences in the behavior of MUF resins prepared in different ways, and hence at the level of their performance as binders of wood panels, due both to their differences at the level of the resin structure and to the type and distribution of the molecular species formed before hardening, as well as to the differences in the structure of the final hardened networks. An example of three types of MUF resins examined can illustrate this point. (i) A sequential MUF in which the UF was prepared first and then melamine coreacted afterwards once the UF polymer had been formed [8], a last small urea addition also being carried out for a final (M + U):F molar ratio of 1:1.5 and M:U weight ratio of 47:53, (ii) a MUF resin in which the great majority of the urea and of the melamine were premixed and then reacted simultaneously to form the resin, followed by addition of small amounts of both last melamine and last urea, for a (M + U):F molar ratio of 1:1.5 and M:U weight ratio of 47:53, and (iii) a UF resin of molar ration 1:1.5 to which has been added 15% by weight on resin solids of monoacetate of melamine in the glue mix for a final (M + U):F molar ratio of 1:1.39 and M:U weight ratio of 14:86. The proportion and type of chemical species formed which can be calculated by the molar proportions of the reagent, the manner in which these are combined during the reaction under different conditions as well as the rate reaction constants of urea and melamine with formaldehyde lead to the conclusion, confirmed by 13C nuclear magnetic resonance (NMR), that the distribution of species for resins (i), (ii), and (iii) are as follows (their relative proportions are indicated in Formulas 6, 7, and 8).

Case (i) above presents the following predominant chemical species (Formula 6), where M attached to the UF polymer is in the form of both a single melamine as well as in the form of a melamine formaldehyde short oligomer.

+ 0.05HOCH2—fu-CH2-)hf U-CH2’)j—OH + 0.50 HOCH2—(-и-СН2)и(и-СН2^г-

сн2{м] снгон

ш

+ UNREACTED MELAMINE + UNREACTED UREA

Formula 8

Thus an MF resin drowned in mostly unreacted urea and where M attached to the UF polymer is in the form of both a single melamine (M and M framed) as well as in the form of a MF short oligomer (M framed).

The structure of the three resins when still in liquid form explains the appearance of their structure after hardening. Thus, hardened MUF resins of formulation type (ii) will present structures as presented in Formula 8 and thus will waste the benefit of a consider­able proportion of the melamine used. Hardened MUF resins of type (i) will present structures intermediate between those shown in Formulas 7 and 8 (but tending more to the type of Formula 8) and thus while also wasting a considerable proportion of the
melamine used, this will be less than for formulations of type (ii): the strength and water resistance results of MUFs of type (iii) will then be noticeably better at parity of all other conditions than what is obtainable with resins of type (ii), as indeed has been shown to be the case. MUF resin formulations of type (iii), those of melamine acetate type, will give hardened structures according to Formula 7 without wasting much melamine and giving hence the best performance, with the limitation of proportion already mentioned and explained above. This can be seen by comparing the strength results obtained by constant heating rate thermomechanical analysis (TMA) [26]. A MUF formulation of type (iii) containing 20% melamine acetate performs almost as well as a good formulation of type (i) which contains two and a half times more melamine. They both perform much better than a formulation of type (ii) [26] with some notable exceptions [38].

Another recent approach which has shown considerable promise in markedly decreasing the percentage of adhesive solids on a board, and hence in markedly decreasing melamine content, has been found almost by chance. It is based on the addition of certain additives to the MUF resin. Additives have been found that are both able to decrease melamine content in MUF resins at parity of performance, as well as able to decrease the percentage of any MUF resin needed for bonding while still conserving the same adhesive and joint performance. This second class of additives works for UF adhesives too, but less well, while it gives acceptable results for PF resins, but it is at its best in the case of MUF resins. This second class of additives is the acetals [34-36], methylal and ethylal being the two most appropriate due to their cost to performance ratio, which do not release for­maldehyde at pHs higher than 1 [35]. Methylal has according to results reported by the Environmental Protection Agency (EPA) an LD50 value of 10,000 against that of 100 in the case of formaldehyde, and is thus classed as nontoxic. The addition of these materials to the glue mix of formaldehyde-based resin improves considerably its mechanical resis­tance and the performance of the bonded joint.

This is in general valid for MUFs, UFs, and PFs, but the effect is particularly evident for the MUF resins [35]. Decreases in MUF resin solids content of as much as 33% while conserving the same performance are reported in the case of wood particleboard. In Fig. 6 are shown the continuous heating rate TMA curves of modulus as a function of tempera­ture for an MUF resin of 1:1.2 (M + U):F molar ratio. Similar but much less extreme

trends are obtained also for UF and PF resins. In the case of MUF resins the addition of 10% additive on resin solids yields laboratory particleboard in which one can decrease the percentage of resin solids on the board by between 20% and 25% without any loss of performance. Similarly, at equal resin solids the strength of a particleboard is 33% higher when 10% additive on resin solids is added to the glue mix. Addition of 20% methylal on the board yields, in the case of the same resin, the same strength with 30% less adhesive (and hence less melamine) [35].

What is the mechanism of action of methylal, ethylal, and some other acetals to achieve such a feat? Their excellent solvent action on melamine and higher molecular weight ligomers. The cases shown earlier in this chapter referring to melamine salts and the loss of effectiveness due to wastage of melamine are applicable in this case too. Melamine when added to a reacting mixture during resin manufacture is not really soluble. It reacts then in heterogeneous phase with the other components of the resin, some of it being in a transient state in equilibrium between being in solution and being out of solution, and thus its efficacity is partially, but noticeably reduced. The intro­duction of an excellent solvent, none better than these acetals was known before, brings the totality of the reaction into homogeneous phase with a consequent, notice­able improvement in both the effectiveness of reaction and the effectiveness of mela­mine utilization.

A different class of additives from those above but also able to decrease melamine content in MUF resins at parity of performance also exist. They are based on the addition in the glue mix of 1 to 5% additive and allow preparation of MUF copolymers, prema­nufactured in a traditional manner, in which either the proportion of melamine is lower, for example a 20:80 by weight M:U resin to which the additive has been added performing as well as a M:U 50:50 resin, or alternatively to upgrade a top of the range M:U 50:50 MUF adhesive to an exterior performance comparable and even superior to that of PF resins [37-39]. Several different types of additives can achieve this but they are all based on the preparation and acid stabilization of imines, or better of iminomethylene bases [38,39], and of their addition to the MUF resin. Thus, the effect is still the same whether the imines/iminomethylene bases, acid-anion stabilized, are prepared by coreaction of ammo­nia and formaldehyde [38,39], or for instance as described for acid-anion stabilized decom­position of hexamethylenetetramine [38,39] (Fig. 7). The structure of the imines and the iminomethylene bases yielding this effect are very similar indeed to the structure of the acetal additives presented above, the -NH — bridge of the imines having the same function of the — O — bridge of the acetals. The imines/iminomethylene bases have the added dimen­sion, however, that the nitrogen can function as a knot of tridimensional cross-linking itself, which the oxygen bridge obviously cannot do. The amount of nitrogen-based addi­tive that can be used is limited by its higher sensitivity to water in the hardened network. This is not the case of the possibly less effective oxygen-based additives, which can be used in greater amount: one property balances the other. The oxygen bridge conversely presents perhaps a better longer-term thermal stability than the nitrogen-based bridges. These are only very relative, rather subjective advantages. What is instead important is that the similarity of structure indicates that in the main (but not completely) the mode of action of all these additives may appear to be the same, but often different effects are at work, namely first a considerable improvement of the viscoelastic dissipation of the energy of the glue line and bonded joint without a drop in cross-linking density. The differences between the different additives is then due to additional, although rather important effects such as the solvent effect of the acetals in the MUF resins, and the increase in reaction rate [25] and buffer effect [38] of the iminomethylene basis, as well as others. It is on the basis of

this similarity of structure and effect that a scale of additives providing similar effects to different levels has been established (see Formula 9) [40].

It must be pointed out that a TMA strength improvement of 100% on the MUF resin without methylal (this is achieved by addition of 20% methylal on resin solids) corresponds in the actual wood particleboard to an increase of IB strength of 33%. This means that of all the compounds shown above only the acetals, such as methylal and ethylal, as well as the similarly structured imine/iminomethylene bases discussed above (for which the effect on strength is more marked) are capable of marked improve­ments in IB strength at the actual wood panel level.

These developments are of use for MUF resins not just in the field of wood adhe­sives, or of other binders in general, but also to improve and upgrade the performance of

I3 CHO

H3

resins in other applications such as that of melamine-based impregnated paper laminates, where they have been shown to improve considerably the storage stability of paper impregnating resins [36].

As regards the more application-bound physical aspects of MUF resins these can be applied in different ways, this too sometimes having a bearing on other types of additives used. Thus to the normal case of a MUF plus its hardener one can add cases in which a formaldehyde depressant such as a low condensation MF, UF, or MUF precondensate or one of their mixes is added; sometimes this is in combination with an accelerator based on the same principle. Such an approach is more used in other resins, but it has been shown and reported as being feasible also for MUF resins [41].