New approach on Phenol-Formaldehyde resins for Wood Composites
Type of DegreeMaster's Thesis
Forestry and Wildlife Science
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Almost two-thirds of the state of Alabama is covered by forest. This generates a substantial market for wood products. Engineered wood products named particleboards (PB) are useful for low load bearing applications. Even though the market for PB has been growing for the past few years, there are concerns about the release of hazardous gases (e.g., formaldehyde emissions) from PB. These gases are generated when the adhesives do not cure entirely creating the tridimensional structures that allow for the bonding between the wood substrates. Instead, there are small unreacted quantities of chemicals that can generate these gases. The reaction of Phenol-formaldehyde (PF) resins, more precisely Novolac-type resins, represents a solution for this problem because they use small quantities of formaldehyde during their synthesis. Hence, the proposed systems have the advantage that most of the formaldehyde is consumed during the synthesis reaction, leaving few or no excess to be released after the pressing step. In this study, I will pursue a different approach to the conventional synthesis method. The monomers will be added separately during the blending step instead of using the commercially available pre-polymerized resins. The hypothesis was that the PF Novolac resins synthesized at room temperature and within the composite during pressing can develop the dimensional stability and mechanical properties that meet the industrial requirements to perform according to their applications. However, careful attention to formaldehyde emission was performed under a hood and monitored closely to see if complete polymerization in the board was possible and human safety could be preserved. The experimental analysis was divided into two parts. In the first part, different ratios of phenol to formaldehyde chemical were used to synthesize different resins that had to be tested in order to determine which is the combination of reactants and also which is the optimum quantity of catalyst that provide the best adhesion. For that purpose, we tested the lap shear properties for different combinations according to the standard DIN EN 205 with the dimension 80mm long, 20mm wide, and 5mm thick . For this procedure, lap shear samples were pressed in 15.24 by 15.24 cm platens heated to 150ºC and 1.38 MPa for 10 minutes. The same press was used to make small particleboards of 12.7cm long, 11.43cm wide and an average of 12mm thick. Similar panels were manufactured using urea-formaldehyde (UF) adhesive and used as the control. Due to the differences in densities that we encountered while making the PB, the variation in density values affected the mechanical properties. An Analysis of Covariance was run to eliminate the effect of this covariant in the final properties. The results showed that only for the Modulus of rupture, the control panels performed better than the suggested treatments. For all the other properties: thickness swelling, edge swelling, modulus of elasticity and internal boding, there was not significant difference between the control and the treatments. The final analysis also demonstrated that for water absorption, the suggested treatments outperformed the results for the UF resins.