To develop a method to economically produce MDF and particleboard products with ultra low formaldehyde emissions (<0.05 ppm), different accelerators for PF resins and a formaldehyde scavenger / hardener for UF resins were evaluated by means of gel time tests, lap shear tests and panel tests. The following conclusions and recommendations are made:
1. Particleboard and MDF products with formaldehyde emissions well below 0.05 ppm can be produced with a phenol-formaldehyde adhesive. These products can easily meet the most stringent formaldehyde emission standards for wood composite panel products such as the Japanese F**** standard (or E0). The products also have excellent durability in water, being suitable for both interior and exterior applications. These advantages effectively solve the two difficult problems commonly associated with today’s UF-bonded PB and MDF products: high formaldehyde emission and low water durability. The disadvantages of using PF adhesives are darker color on panel surface and higher water absorption.
2. Phenol-formaldehyde resin is a more efficient wood adhesive than urea-formaldehyde resin. Using PF at slightly more than one half of the UF loading rate, PF-bonded particleboard and MDF showed better mechanical properties than UF-bonded particleboard and MDF, and about the same thickness swell after 24-hour water soak.
3. The cure speed of phenol-formaldehyde resin can be dramatically improved with the use of resorcinol as an accelerator. At 205°C (400°F, normal OSB press temperature) and a resorcinol loading rate of 2% of liquid PF resin weight, a commercial OSB face phenolic resin used for MDF manufacturing showed comparable cure speed to that of an uncatalyzed commercial UF resin of E2 type at 182°C (360°F, normal MDF press temperature in a multi-opening press). At 2% resorcinol loading rate and 205°C, the same PF resin showed slightly slower cure speed to that of the same UF resin catalyzed by 0.5% ammonium chloride at 182°C during particleboard production.
4. The experiments revealed that there is potential to reduce PF resin consumption to one half of the normal UF resin consumption in particleboard and MDF manufacture and still maintain the desirable physical and mechanical properties. The cost of using PF adhesive to produce particleboard and MDF should be substantially lower than previously thought. Given the fact that the current oil price is high and PF resin cost is sensitive to oil price, the economic viability of using PF adhesive for particleboard and MDF production should be evaluated carefully. Nonetheless, the findings from this project have provided the basis for a more optimistic view on the economic viability of PF-bonded MDF and particleboard.
5. Ethylene carbonate, propylene carbonate and triacetin are well known PF accelerators in the literature but it was revealed in this research project that these esters cause substantial loss of bonding strength, particularly in the case of PF resin with higher alkalinity. Therefore, they are not recommended for the manufacture of PF-bonded MDF and particleboard. On the other hand, resorcinol is not only an effective PF accelerator but also preserves most of the bonding strength.
6. Ethylene carbonate, propylene carbonate and triacetin are very effective in reducing PF resin gel times. The gel time reduction is pH-dependent. Higher pH leads to shorter gel time.
7. Combining gel time test with lap shear test is a far more reliable approach to evaluate and predict PF adhesive (and conceivably UF adhesive) cure speed in wood composite panel manufacturing than using gel time test alone.
8. SUH-511M is an effective formaldehyde scavenger and hardener for UF resins but it seemed to produce lower board bending strength and increase thickness swell and water absorption.
9. A mill trial of producing MDF using resorcinol-accelerated phenolic resin as an adhesive is recommended.
10. A mill trial of producing particleboard using resorcinol-accelerated phenolic resin as an adhesive is recommended.