Лако-красочные материалы — производство

A very important application for UF adhesives is in the manufacture of particleboard. The glue mix is generally composed of a liquid resin to which water has been added to decrease viscosity and to facilitate spraying, plus small amounts of ammonium chloride or sulfate and small amounts of ammonia solution. Small quantities of insecticides, wax emulsion, and fire-retarding agents (such as ammonium phosphates) are added before spraying the adhesive onto the wood chips. Pressing temperatures and maximum pressures used in the cycle are in the range of 150 to 200°C and 2 to 35kg/cm2, respectively.

The moisture content of glued furnish chips is 7 to 8% for the board core and 10 to 12% for the surface. The resin contents used (i. e., solids) are 6 to 8% for board core and 10 to 11% for board surfaces, but such proportions might be higher for the weaker low emission adhesives used today and depending for the application envisaged [i. e., particle­board or medium density fiberboard (MDF)].

It must be realized that on curing, the viscosity of UF resins changes, not only at a different rate but also in a different manner according to the temperature. The viscosity gradually increases with the temperature up to ± 50° C. Above 60° C the viscosity quite rapidly reaches a maximum and then decreases. This indicates that the resin tends to degrade under prolonged heating at high temperatures (Fig. 3). To avoid this problem, the UF-bonded particleboard must never be pressed for too long, and must never have a ‘‘hotstack’’ or “postcure’’ period after pressing. They must preferably be cooled after manufacture to avoid deterioration in strength and quality. The cured UF resins degrade rapidly at any temperature at a pH below 2. The viscosity for a good particleboard resin is on the order of 100 to 450 cP (at 20°C) [17]. While this rule is true, the development that UF resins have undergone in the past 15-20 years in order to decrease drastically the levels of formaldehyde emission has led to new formulations which have very different charac­teristics and behavior. In some respects, and at least partially some of the old rules are no longer completely valid. This is the case with the rule of trying to avoid hotstacking of UF bonded boards [2,18].

Thus, when a panel is taken out of the press it gives off a considerable amount of moisture and its temperature is quite high. If a board in such a condition is immediately placed in an oven the temperature of which is higher than 75°C some degradation with consequent loss of performance will occur, this being shown to be due mainly by some progressive degradation of the UF adhesive hardened network [2,8]. Conversely, if the board is just cooled down there will not be any further curing of the resin. The predomi­nance of the effect derived from the first of these two considerations has led to the need to

Figure 3 Viscosity of a UF resin as a function of time at different temperatures. Traditional resin of high F/U molar ratio.

limit the heat conservation of UF particleboard after pressing, hence to today’s wide­spread practice of cooling the board after pressing [2,8]. As a consequence, decrease of board performance by resin degradation is in the main avoided but an unexploited reser­voir of further potential strength of the resin achievable by further curing is wasted. It has many times been reported that the mechanical performance of aminoplastic resin-bonded particleboard cannot be improved by hot posttreatment with the exception of physical properties such as the homogenizing of the moisture content throughout the board and stress reduction improving the board dimensional stability [19].

Results obtained by a series of techniques for the curing of several resin systems [18,20-24] have indicated, however, that posttreatment and hotstacking (postcuring) con­ditions capable of improving the mechanical performance of aminoplastic resin-bonded particleboard without any degradation should instead exist. This is of some importance, firstly because the performance of UF — and melamine-urea-formaldehyde (MUF)-bonded particleboard could be improved with very little process change from the present industrial conditions to yield better board performance (or the same performance at lower adhesive content levels), and secondly because at parity of board performance such an approach may well lead to the use of even shorter industrial press cycles than today, even for aminoplastic resins.

From the experimental results obtained [18] it is evident that: (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-bonded joints and panels without any further joint and hardened adhesive degradation, as the value of strength reached during postcuring is always con­sistently higher than the value at which the strength stabilizes after complete curing during the ‘‘pressing’’ cycle [18]. (2) Postcuring could also be used in principle and for the same reasons to further shorten the pressing time of UF-bonded joint and panels when well — defined postcuring conditions are used [18]. (3) There is clear indication that 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 [18].

The molecular level reasons for this behavior can be deduced also by bonded wood panel internal bond (IB) behavior. The IB performance improvements for instance 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 thermomechanical analysis (TMA) of aminoplastic and phenolic resins [20-24]. These are able to counterbalance well the degradative trend to which the aminoplastic resin should be subjected. Furthermore, in modern resins of lower formal — dehyde/urea 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 ratios when postcured under the same conditions [2,8]. 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 value will then improve, within certain limits, with postcuring in boards bonded with modern, lower formaldehyde aminoplastic adhesives [18].

A model to describe the decrease in temperature under different conditions of a particleboard after hot pressing has been developed and this model is shown to correlate well with experimental results of board temperature variation after pressing, both on cooling and during postcuring under different conditions [18]. From this, conditions of temperature and time favorable to improve panel performance by postcuring treatments were also determined [18]. The validity of the improvements forecasted under such con­ditions was then confirmed at molecular level for UF adhesive/wood joints by TMA testing, and finally confirmed by testing the mechanical performance of laboratory boards prepared under the postcuring treatment conditions identified [18]. The improve­ments in panel performance observed were explained on the basis of already described [24] and well-known molecular level rearrangements of the cured adhesive network and of the shifts in their relative importance in modern, lower formaldehyde content UF adhesives. The conclusion was that modern, lower formaldehyde content UF adhesives can consid­erably benefit as regards board performance from short period hot postcuring at tempera­tures in the 60 and 100° C range, a trend in clear contrast with the degradation and loss of performance this practice was known to induce [2,16] in the older, very much higher formaldehyde content aminoplastic resins of the past. Consequences of economical and technical interest derive from this, as the findings also imply lower adhesive consumptions and possibly even faster press cycles at parity with present resin performance, if simple postcuring procedures such as after pressing hotstacking (rather than board cooling as at present) are implemented for UF-bonded particle and other types of boards [18].

Figure 4 [18,20,23] shows that the slower is the heating rate the more evident is the entanglement plateau and the higher is its value of the modulus due initially to entangle­ment. This confirms that linear growth of the polycondensate can be maximized by decreasing the temperature at which polycondensation is carried out (this is likely to be valid both in the reactor during preparation of the UF resin as well as in the resin curing stages on the substrate). Figure 4 indicates that this effect becomes more marked the slower is the rate of heating applied. It implies that polycondensates grow mostly linearly to a higher degree of polymerization, before tridimensional cross-linking starts, the slower

is the rate of heating. This might depend on the reactivities of urea sites with formaldehyde which are in the approximate ratio 9:3:1 respectively for the first-reacted, second-reacted, and third-reacted urea sites [2]. The slower heating rates used decrease molecular move­ment and hence further decrease the chance of the third urea site reacting, hence favoring more linear growth of the polycondensate. Tridimensional covalent networking will still occur, and a tridimensional cross-linked network will still be the final product of the reaction, but will occur later when the polymer has grown to greater lengths. The most important observation from Fig. 4, however, is the considerably higher value of the modulus at slower heating rates, which must also be viewed in the same context as above: it relates to the polymer having time to adjust by better utilization of empty volume spaces, the same reason that gives a lower value of the glass transition temperature Tg the slower is the rate of heating. The extent of the effect observed is considerable: the maximum value of the modulus once the resin is tridimensionally cross-linked for the 40°C/min case is lower, due to early tridimensional immobilization of the resin in a less tight tridimensional covalent network, than the value of the modulus of just the entangle­ment network observed for the 15°C/min and slower heating rate curves.

It is important here to point out that the concept widespread in wood panel manufacture that a resin capable of a faster pressing and curing time (a faster curing resin for example) is giving better panel strength is only subjected to the exact definition of the concept of time of curing (and of pressing) in Fig. 4 above. Thus, in Fig. 4 a fast resin, as fast as being able to reproduce the 40 s/mm curve (which is in line with today’s rates of curing for wood particleboard panels) will only be able to give to the joint the strength equivalent to 1.5 GPa modulus, while a slower resin which is in principle cap­able of yielding a modulus three times stronger at 4.5 GPa (reproducing the 5 s/mm curve) has no strength (less than 0.2 GPa) at the same curing time used to maximize the strength result of the faster resin. Thus it is preposterous to define one resin as better than another unless the concept of time is also defined and the two resins are seen in this ‘‘time’’ context. This insight leads also to two consequences. (i) It contributes to explaining why in modern UF resins one can improve strength by after pressing hotstacking: this is equivalent to passing in Fig. 4 from the faster curve to one of the slower curves after hot pressing, one curve that allows the system to reach a higher strength value as shown in the figure. (ii) A faster curing resin needs to be engineered to give not only a faster curing time as this will only yield an ultimate lower strength due to the looser and hence weaker network produced, but also to be able to concomitantly obtain a higher degree of cross-linking of the network to counterbalance the weakening caused by the faster curing rate of the resin: this needs to be introduced by varying resin parameters and by other techniques. To obtain a good yet faster adhesive the two effects must both be taken into account.

As important as viscosity is resin flow, which reflects viscosity under hot-pressing conditions. Resin flow is a determining factor in manufacturing good particleboard. Excessive flow causes the resin to soak into the wood particles and causes glue-line starva­tion; insufficient flow causes insufficient contact surface. The gel time generally used at 100°C for glue mixes of UF-bonded particleboard is 3 to 12min, with 30 s to 3min for board faces and cores, respectively. The actual gel time in the press depends on the press temperature and is considerably shorter.