Particleboard I.B. forcast by TMA bending in UF adhesives curing

Abstract

Materials and methods Six urea-formaldehyde resins of molar ratios of U:F of respectively 1:1.1, 1:1.2, 1:1.3, 1:1.5, 1:1.8, and 1:2.0 were prepared as follows: To a urea/formaldehyde concentrate (formurea) containing 23% urea and 57% formaldehyde in water was added 22% NaOH to set the pH between 8.3-8.5. The temperature is raised in 50 minutes to 90 ~ maintaining the pH between 7.3-7.6 by small additions of 22% NaOH, and maintained at 90 ~ for approx 20 minutes. The pH decreases by itself and is maintained not lower than 4.7-5.1 and the temperature increased to 98 ~ The reaction is stopped when a water tolerance point of 170% is reached in the case of all resins, by cooling to 40 ~ the second urea added, the pH readjusted to 8.7 with NaOH, and the resin left to mature overnight at 40 ~ The ratio of second urea to initial urea for the preparations above was in the same order as above of 0.35, 0.36, 0.38, 0.41, 0.47 and 0.52 respectively. All the resins were at solids contents in the range 53.3%-54.9%. The resins were then tested by thermomechanical analysis (TMA) by procedures already reported (Pizzi, 1997, Kamoun et al 1998, Garcia and Pizzi 1998, after addition on resin solids of 2% NH4C1 as a 25% solution, by placing 30 mg of glue mix between two plys of beech wood to form a joint of 21 x 6 • 11.5 mm dimension which is tested in three points bending for a span of I8 mm, subjected to an alternating force of 0.1/0.5 N with a 6 s/6 s cycle, at a constant heating rate of !0 s/ minute, from 40 ~ to 250 ~ The minimum value of the deflection corresponding to the tighter network formed by the adhesive on curing is then measured. Duplicate one layer laboratory particleboard of 350 x 310 x 14 mm dimensions were then produced by adding 12% UF resin solids on dry wood particles(to make sure that results good enough to notice a difference would be obtained) pressed at a max pressure of 32 kg/ cm 2 followed by a descending pressure cycle, at 195 ~ and for a pressing time of 4.4 minutes (

Fast advancement and hardening acceleration of low‐condensation alkaline PF resins by esters and copolymerized urea

“…Recent results have shown a clear correlation between the results of nonisothermal TMA curing in bending of aminoplastic and phenolic resinbonded wood joints and the IB strength of wood particleboard prepared using the same resin systems when the boards are prepared under welldefined and reproducible conditions. 30,31 The results shown in Table I for a series of low-condensation and low molecular mass PF resins confirm this finding. Thus, in Table I it is shown that the IB strength of wood particleboard bonded with low-condensation PF resins in which the initial 30,31 All these results confirm the well-known fact that an increase in molar ratio in a PF resin accelerates its curing, increases the IB strength of panels bonded with it, and decreases the TMA deflection (hence, increases the value of the bonded wood joint modulus) 1,30,31 (Fig.…”

“…As the amount of urea addition moves to 30, 36, and 42%, the amount of unreacted urea increases; whereas it seems that for the substituted urea species, a stable maximum is attained, and the relative proportions of hydroxybenzyl urea becomes progressively higher than the proportion of bis(hydroxybenzyl) ureas. All this seems logical if we consider that, because in the resins tested the P : F molar ratio is 1 : 1.7, by increasing the percentage of urea added, the [P ϩ U] : F molar ratio decreases from 1 : 1.52 to, respectively, 1 : 1.44, 1 : 1.37, 1 : 1.1.31, 1 : 1.25, and 1 : 1.20 for the 12,18,24,30,36, and 42% level of urea substitution. Thus, it seems that the greater proportion of urea that coreacts usefully with the phenol in the PUF resin is attained at about 18% molar urea addition, although the proportion of copolymerized urea is still almost as high in the 12 and 24% urea addition resins.…”

Upgrading melamine–urea–formaldehyde polycondensation resins with buffering additives. I. The effect of hexamine sulfate and its limits

“…A constant heating rate of 10°C/min was used. It must be remembered, however, that TMA tests used in this manner correlate mainly with the dry IB results of panels bonded with MUF resins and are no guarantee that correlation will exist with wet strength results in the case of new, experimental resin systems 5, 47–49. In this case, it is necessary to prepare wood particleboard to establish if such correlation exists 47–49.…”

Exterior OSB preparation technology at high moisture content. Part 2: Transfer mechanisms and pressing parameters

“…It has been demonstrated that bond strength development goes through a first stage where mainly entanglement networks can develop , Laigle et al 1998, Zhao et al 1998). The network formed at this stage may be flexible enough to accompany elastic deformations and the duration of this stage may exceed the time taken to close the press.…”