Treatment of Wood-Based Panel on Formaldehyde Emission
Original source https://www.hindawi.com/journals/ijps/2018/9349721/
3.3.1. Surface Sealing Method
For most wood-based panel products, such as PB or MDF, surface overlays are usually used for decoration purpose, which can effectively reduce the formaldehyde emission.
Barghoor compared the formaldehyde emission of the wood-based panels with different methods for surface overlays using test chamber, and the results are summarized in Table 2.
Barry and Corneau evaluated the effectiveness of 10 different surface treatment methods as emission barriers for formaldehyde and total volatile organic compounds (TVOC). These include paint, UV topcoat, acrylic topcoat, vinyl resin system (ethyl-vinyl acetate), phenolic saturated film, melamine saturated paper, multiple (3) topcoat wet process, foil resin system (polyvinyl acetate), and powder coating. Three MDF and four unfinished PB products from different manufacturers were used in the experiments. The results showed that the epoxy powder coatings used to finish the MDF samples performed the best, achieving 99+% emission reduction in formaldehyde and up to 94% reduction of TVOC emissions (noted as 99+%/94%). This was compared to reductions of 89%/85% for the UV paint and 11% (27% increase) for the acrylic-painted topcoat on MDF. A multiple (3) topcoat wet process treatment showed a 28% reduction in formaldehyde emissions but an increase in TVOC emissions, suggesting that the coating may have a high solvent content. The respective formaldehyde/TVOC emission reductions for the laminates were 99%/88% for phenolic paper laminates, 99%/66% for vinyl, 93%/85% for the 80 g melamine paper, and 73%/75% for the 60 g foil on PB. These very limited data suggest that several of the treatments are very efficient formaldehyde and TVOC emission barriers when applied to PB and MDF, although epoxy powders are typically applied only to MDF.
Chen et al. studied the effect of different surface finishes of PB on reducing TVOC and formaldehyde emission. These finishes and coatings include wood veneer, polypropylene film paper, low-pressure melamine-impregnated paper (80 g, 120 g), water-based polyurethane coatings, and water-based polypropylene coatings. The experiment results showed that formaldehyde and TVOC concentrations were different for surface finishing treatments. Among these finishes and coatings, polypropylene film paper was the best barrier to TVOC and formaldehyde, which reduced 84.18% of TVOC and 71.43% of formaldehyde, while the TVOC concentrations of water-based coatings containing high VOC were higher than those of the unfinished PB. Twenty-one kinds of VOCs were identified from the unfinished PB and 15 kinds of VOCs from the veneer-overlaid PB by the method of GC-MS, and the content of VOCs from the veneer-overlaid PB was over 50% lower than that of VOCs from the unfinished boards.
Park et al. conducted a study to understand the effect of surface laminating materials on the formaldehyde emission from PB and MDF with or without edge sealing, using 24-hour desiccator method. For the PB, the edge sealing could reduce the formaldehyde emission by 37.4% for low-pressure laminate (LPL) and 80.7% for polypropylene (PP) film lamination. The surface-laminated MDF with the sealed edges also showed a decrease in the emission up to 57.8% and 54.3%, with the poly(vinyl chloride) (PVC) film bonded by a solvent-based adhesive or with coating cured by ultraviolet radiation, respectively. However, the coated MDF samples showed 5.3% increase in the emission when their edges were sealed, indicating the formaldehyde was formed the solvent used for coating. Therefore, the type of surface lamination materials on wood-based panels has a great impact on their formaldehyde emission.
3.3.2. Edge Sealing Method
A great number of experiments show that the main channel of formaldehyde emission is the side face, which is generally at least two times more than the surface. So edge sealing can effectively reduce formaldehyde emission when using the wood-based panels in making the decorative components and furniture parts. Kim et al. tested the formaldehyde emissions from PB and MDF by the desiccator according to Japanese Industrial Standard (JIS A 1460), and the edges of each sample were sealed by paraffin film, polyethylene wax, or aluminum foil; the results showed that the difference between the edge seal methods was much smaller than that from the unsealed specimen.
The edge sealing can only slow down the formaldehyde emission rate, while the overall formaldehyde content in the board remains the same. However, the advantage of the edge sealing is to reduce the formaldehyde emission during a certain period so that the formaldehyde effect on the human health is much reduced.
3.4. The Structure of Wood-Based Panel Products on Formaldehyde Emission
The furniture and decoration components made from wood-based panel are usually assembled with hardware, from which many predrilled holes and notches are on the panels. The amount of holes on the side panels of the furniture is usually dozens to hundreds. Many holes are deep to the center of the panel. A greater amount of formaldehyde escapes from these structural holes and notches because the intermediate layer of wood-based panel generally has a higher formaldehyde emission capacity than that from the surface layer. Therefore, it will not be able to truly reflect the actual level of formaldehyde emission if the test of formaldehyde emission from wood-based panel products does not consider the impact of these factors. One of the effective ways to reduce the formaldehyde release is to use the reasonable connection structure to minimize the number of holes on the panels in the practical application of wood-based panels.