In North America, resin binder systems used for OSB production are normally either in powder form such as powder phenol-formaldehyde (PPF) or in liquid form such as liquid phenol-formaldehyde (LPF) resin and diphenylmethane diisocyanate (MDI). To improve resin efficiency and bond quality, recent OSB production has shown a trend toward the utilisation of liquid and powder resin combination system such as LPF/PPF and MDI/PPF. This study was conducted to investigate effects of MDI/PPF combination binder system on strand board performance. The main variables included (1) MDI/PPF combination ratio (100/0, 75/25, 50/50, 25/75, 0/100), (2) mat moisture variation (uniform 7% MC, average 7% MC obtained by mixing 2%MC and 12%MC strands at a ratio of 50/50), (3) storage time of resinated strands prior to pressing (<0.5 h, 1.0 h, 2 h), and (4) resin content/type based on a similar basis of resin cost (2.4% MDI, 3.5% LPF, 3.5% PPF, 0.875%LPF/2.625%PPF).
There was an optimal MDI/PPF combination ratio for improving resin efficiency regarding resin bond quality and cost savings. Homogeneous strand boards bonded with 75/25 and/or 50/50 MDI/PPF combination were statistically comparable to MDI bonded panel regarding internal bond (IB), thickness swelling (TS), water absorption (WA), dry/wet modulus of rupture (MOR) and modulus of elasticity (MOE) properties. A further decrease in the amount of MDI in the combination significantly reduced the board performance.
MDI was susceptible to react with water in the strands after blending, which in turn resulted in a detrimental effect on the panel properties. An increase in storage time significantly increased TS and WA, and decreased wet MOR properties of boards. Increasing storage time also reduced IB and wet MOE properties. However, the reduction was not significant at the 95 percent confidence level.
MDI and MDI/PPF (50/50) binder systems were more tolerant to furnish MC variation compared to PPF resin. A change in mat MC condition from uniform 7% to average 7% significantly increased TS value and reduced dry MOR and wet MOE properties of PPF bonded panels. However, this change in the moisture condition did not affect MDI/PPF panel, but significantly enhanced MDI panel performance in terms of IB, TS, and WA properties. Spraying water to the top of mat prior to pressing did not influence panel performance regarding all panel properties.
The study on different resin systems for strand boards on a similar basis of resin cost showed that MDI produced lower TS and WA panels compared to PPF, LPF and LPF/PPF (25/75) combination. However, no significant difference between these four resin bonded panels was observed in consideration of other panel properties such as IB, dry/wet MOR, and dry/wet MOE.