Dry l.b. forecasting of commercial tannin adhesives-bonded particleboard by TMA bending

Materials and methods Four tannin-formaldehyde (TF) resins of percentage fo rmaldehyde by weight of 4%, 6%, 8% and 10% on tannin extract solids were te sted for water solutions of mimosa tannin and quebracho tannin commercial a dhesive intermediates prepared according to procedures already reported (Pi zzi 1994) and at respectively 50% and 45% concentration. The glue mixes so obtained were then tested by thermomechanical analysis (TMA) according to p rocedures already reported (Pizzi 1997) by placing 40 mg of resin between t wo plys of beech wood to form a joint of 21 x 6 x 14 mm dimension tested in three points bending for a span of 18 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 10 degrees C/min, from 40 degreesC to 250 degreesC. The minimum value of the deflectio n corresponding to the tighter network formed by the adhesive in the wood j oint is then measured. As the resins had different solids contents, the TMA values were corrected for adhesive quantity in the joint. Duplicate one la yer laboratory particleboard of 350 x 310 x 14 mm dimensions were then prod uced by adding 14% tannin-formaldehyde resin solids content on dry wood par ticles (due to more difficult to bond mixed furnish of beach and softwoods) pressed at a maximum pressure of 0.28 N/mm(2) (Pizzi 1977) followed by a d escending pressing cycle, at 195 degreesC and for a pressing time of 7.5 mi nutes to ensure complete curing. All the panels had densities comprised bet ween 0.695 and 0.704 g/cm(3) (Zhao et al. 1998). The panels after light san ding were tested for dry internal bond (I.B.) strength. The results obtaine d are shown in the Table. Results and discussion The results in the Table indicate clearly that good correlation call exist between the dry internal bond (I.B.) strength of woo d particleboard bonded with tannin-formaldehyde (TF) resins and the minimum defections obtained in three points bending by non-isothermal thermomechan ical analysis (TMA). Thus, the curing of a tannin adhesive can also be foll owed in situ in the wood joint itself, by TMA bending experiments and the r esults obtained can forecast the I.B. of particleboard pressed under standa rd conditions of pressing time and density. The correlation equations obtai ned are of the type I.B. = a(1/f) + b where f is the value of the TMA in mum and I.B. is the value of the interna l bond of the corresponding particleboard in MPa. The numerical values of t he coefficients a and b as well as the value of the coefficient of correlat ion r under the conditions used are shown in the Table. It is of interest t o note that not only excellent correlation of dry I.B. with the TMA f can b e obtained, hut that this often depends on the type of tannin used and is m ore sensitive to conditions of application than in the case of synthetic ad hesives (Zhao et al. 1998). Furthermore the inherently greater variation ob served in the case of the tannin adhesives in relation to similar correlati on equations already obtained for synthetic adhesives indicate that somewha t more complex hardening mechanisms are present in tannin adhesives as alre ady observed by other techniques (Pizzi 1974). As for the synthetic phenolic resins before (Zhao et al. 1998) It must be c learly pointed out that the numerical values of the coefficients of the abo ve equation provide comparison between the I.B.s of panel prepared under id entical conditions only, hence at the same board density, pressing conditio ns, resin loads etc.