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Weekly UpdatesExterior-grade melamine-urea-formaldehyde (MUF) resin adhesives are considered too costly to replace urea-formaldehyde (UF) resins for interior applications. Melamine-modified UF resins with reduced melamine content levels have been developed to improve durability and moisture resistance properties. These low-melamine content UF resins have been relatively popular in Europe (Dunky 1995) and in the Asia-Pacific region (Maylor 1995, Parker and Crew 1999) for many years. More recently, melamine-modified UF resins were also shown to yield particleboards with significantly lower formaldehyde emissions than the control UF resins (Graves 1993, Rammon 1997).
It is generally recognized that formulation of low formaldehyde emission UF resins are accomplished primarily by decreasing the formaldehyde/urea (F/U) ratio (i.e., down to 1.2 or even 1.1 ratio). But, it was shown that lower F/U ratio, while yielding substantially lower formaldehyde emission, also resulted in longer resin cure time and lower internal bond (IB) strength (Hse et al. 1994). A method most commonly used to cope with this formaldehyde problem is to react formaldehyde with urea initially at a much higher F/U ratio and strong acidic condition to form the backbone of the resin system; then, after attaining a desired degree of condensation, additional urea is added to adjust the F/U ratio to meet the desired low F/U ratio in final resin products. Similar methods of reacting urea and formaldehyde at strong acidic pH and adjusting F/U ratio to attain the desired low formaldehyde emission were applied in formulating MUF resin adhesives.
Although the low-melamine content MUF resins have not been widely accepted in North America, interest in the development of cost effective resin systems for upgrading particleboard and medium density fiberboard (MDF) remains high because of increasing exports of particleboards to Japanese and Asian markets. This paper is one of a series to describe efforts to develop a low melamine content MUF resin system to improve gluebond durability and formaldehyde emissions of particleboard. The study involved two experiments: 1) formulation of MUF resins based on a UF polymer catalyzed with strong acidic pH and 2) determination of the effects of increased melamine content and melamine reaction pH on the performance of MUF resins.
Experiment I--Formulation of MUF resin based on UF polymer catalyzed with a strong acidic pH
The two guidelines for this experiment were 1) to react formaldehyde and urea at strong acidic pH of 1.0 to form a UF polymer as a backbone structure and then coreact with melamine to form the MUF resin and 2) to set the maximum melamine contents in the system not to exceed 0.035 mol (i.e., 4.39% weight basis) per each mole of urea in a UF polymer at U/F ratio of 1.2. This was for economic considerations because the MUF resin developed was intended to upgrade UF resin for interior applications.
Experimental procedure
Resin preparation.--All of the resins were fabricated in the laboratory by reacting formaldehyde (3 mol) and urea (1 mol) at pH 1.0 for 30 minutes at 70[degrees]C. Then the pH was adjusted to either 4.5 or 6.5 and melamine (either 0.0625 or 0.0875 mol) was added to react for 30 minutes at 80[degrees]C. Thereafter, 1.5 mol of urea was added in three equal parts at 30-minute intervals at 80[degrees]C, and finally reaction was terminated by cooling to room temperature within 10 minutes. The final molar ratios of F/U/M were 1.2F/IU/0.025M and 1.2F/1U/0.035M. Furthermore, with two levels of melamine reaction pH (4.5 and 6.5) and three resin replications for each condition, a total of 12 resins (two F/U/M ratios x two melamine reaction pH x three resin replications) were fabricated.
Particleboard manufacture.--All of the panels were prepared in the laboratory with wood particles obtained from a local particleboard plant, The particles were classified in the plant as core materials with a mean moisture content (MC) of 3 percent. The particles were stored in polyethylene bags directly from the dry-end of the mill dryer and were used in the laboratory without further treatment.
To prepare each panel, the wood furnishes were weighed (target board density was 48 pcf [0.769 g/[cm.sup.3]]) and placed in a rotating drum-type blender. The resin, 4.5 percent based on oven-dry weight of wood, was then weighed and applied by an air-atomizing nozzle with air line pressure maintained at 40 psi. Wax and a catalyst were not used in the study. After blending, the wood furnishes were carefully felted into a 19- by 20-in. (48.3- by 50.8-cm) box to form the mat. The mat was transferred immediately to a 40- by 40-in. (101.6-by 101.6-cm) single-opening hot-press with the platen temperature regulated at 375[degrees]F (190.6[degrees]C). Sufficient pressure (about 550 psi [3,792 kPa]) was applied so that the platen closed to 1/2-in.(1.27-cm)-thickness stops in approximately 45 seconds. Press times were 4 minutes. Board manufacture replication was two boards per condition.
Particleboard testing.--All of the boards were conditioned in a chamber at 50 percent relative humidity (RH) and 80[degrees]F (26.7[degrees]C) before testing, ending with a MC that averaged 5.5 percent. After conditioning, each board was cut to yield ten 2- by 2-inch (5.08- by 5.08-cm) specimens for tensile strength perpendicular to the lace, eight 2.75- by 5-inch (6.99-by 12.7-cm) desicator samples for formaldehyde release testing, and four 6- by 6-inch (15.24- by 15.24-cm) dimensional stability specimens (thickness swell [TS]). The internal bond (IB) strength test was performed in accordance with the ASTM standard for evaluating the properties of wood-based fiber and particle panel materials (D 1037-93). For TS evaluation, a 24-hour water soak was employed. The TS values measured changes in thickness after the specimens were submerged in water at room temperature for 24 hours.
Free formaldehyde and formaldehyde emission measurement.--Free formaldehyde in the MUF resin was determined by a slightly modified sodium sulfite method as described in a previous study (Hse et al. 1994). For the formaldehyde emission measurement, the test was performed in accordance with the National Particleboard Association (NPA) 2-hour-desicator test.
Results of Experiment I
Table 1 summarizes the properties of MUF resins and bond performances (i.e., IB and formaldehyde emission) of the particleboards. Variance analysis indicated that the changes in melamine reaction pH and F/U/M molar ratio significantly effected the formaldehyde emission but not IB strength. As shown in Duncan's multiple range tests, higher melamine content and lower melamine reaction pH resulted in lower formaldehyde emissions (Table 2).
It is important to note, however, that:
I. the MUF resin contains a fairly large amount of un-reacted formaldehyde (free formaldehyde), even though melamine is known to be highly reactive with formaldehyde,
2. average values of gluebond strength were very low considering the well recognized functional fortification of melamine to UF resin, and
3. average values of formaldehyde emission were much higher than the average values of the commercial particleboard products in the market today which are in general controlled at 1 [micro]g/mL or less.
These results strongly suggest that the resin formulation and the reaction of MUF were inadequate and there is some room for further improvement in the MUF resin.
It is generally recognized that current commercial low formaldehyde emission UF resins are primarily formulated at an extremely low F/U ratio of 1.2 or lower. Since urea has four functional groups, at a F/U ratio of 1.2, the formaldehyde per functionality is calculated to be 0.30 (i.e., 1.2/4). On the other hand, in the MUF resin system, the functional group for melamine is six. Therefore, for a molar ratio of 1.2F/1U/0.035M resin system, the formaldehyde per functionality is calculated to be 0.285 {1.2/[(1 x 4) + (0.035 x 6)]}. The computation did not take into consideration that melamine is more reactive toward formaldehyde than urea under the same reaction condition. Therefore, even a slightly lower formaldehyde per functionality of MUF resin than was calculated could be considered as an indication that insufficient formaldehyde in the resin system caused an overall adverse effect to the reaction, which in turn resulted in poor gluebond strength. Thus, it is likely that improvements in resin performance would be expected by increasing the formaldehyde per functionality ratio.
With increased formaldehyde in the system, the next step was to adjust the pH. The molecular structure of UF resin reacted at a strong acidic pH of 1.0, in general, favored the formation of methylene linkage and a cyclic uron compounds. It also tended to build up resin viscosity very rapidly, and the possibility of insoluble gel formation was also greatly increased, implying a very narrow window of safety in working with pH = 1.0. Therefore, in order to induce flexibility for reaction pH adjustment, it was decided to raise the reaction pH slightly in the experiment.
Based on these considerations, a subexperiment was conducted by increasing formaldehyde from 1.2 to 1.38 mol with three acidic pH controls (1.60, 1.40, and 1.20) to fabricate UF resin polymers and coreacted with two melamine content levels (1.38F/1U/0.025M and 1.38F/1U/0.035M). Thus, 18 MUF resins were formulated (3 pH x 2 melamine content x 3 resin replications).
Again, the resins were prepared by reacting the formaldehyde (3 mol) and urea (1 mol) at required acidic pH for 30 minutes at 70[degrees]C. Then, pH was adjusted to 4.5 and coreacted with melamine (either 0.0544 or 0.0761 mol) for 30 minutes at 80[degrees]C. Thereafter, 1.174 mol of urea were added in three equal parts at 30-minute intervals at 80[degrees]C, and finally the reaction was terminated by cooling to room temperature. Resin preparations were replicated three times and two panels were made for each resin. Average IB and formaldehyde emission are summarized in Table 3.
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