FORMALDEHYDE BARRIER EFFICIENCY OF MELAMINE IMPREGNATED PAPER AND MELAMINE EDGE-BANDING TAPE SURFACED PARTICLEBOARD AND MDF UČINKOVITOST PAPIRJA, IMPREGNIRANEGA Z MELAMINSKO SMOLO, IN MELAMINSKEGA ROBNEGA TRAKU PRI OBLAGANJU IVERNIH PLOŠČ IN MDF-

Particleboard and MDF are the most usable materials for furniture production, where emission of formaldehyde should be as low as possible. In order to improve aesthetic properties of particleboard and MDF they are covered with appropriate surfacing material. In most cases, melamine impregnated paper is used for face layers. But covering faces should be accompanied with covered edges, as due to low density of core layer formaldehyde much more easily emits from edges than from faces. The presented research shows differences in formaldehyde emission rate at unsurfaced and surfaced MDF and particleboard. For face layer, melamine impregnated papers with grammages of 130 g/m2 and 200 g/m2 were used, while for covering edges, melamine edge-banding tape was applied. In the research it was determined that higher barrier effect was achieved when impregnated paper with a grammage of 200 g/m2 was used. When surface was completely covered with impregnated paper and melamine edge-band tape, formaldehyde emission was lowered by between 83 % and 94 %.


INTRODUCTION UVOD
Formaldehyde is one of most criticized property of particle and fibre-based wood based panels. Woodbased panel producer and with them related resin producers are constantly looking for the solutions to improve (lower) formaldehyde emission from panels, especially those used for furniture. One of the most widely used wood-based panels for furniture are particleboards (PB). The usability of raw (unsurfaced) particleboards in furniture application is rare due to their unappealing look and less resistant surface especially against water/moisture and mechanical damag-es. To improve aesthetic look, resistance towards water/moisture, scratch and abrasion resistance boards are surfaced/laminated/overlaid. (Groah et al., 1984;Nemli and Ҫolakoğlu, 2005;Nemli and Hiziroglu, 2009;Istek et al., 2010;Bardak et al. 2011). Surfacing/laminating/overlaying describes a process where impregnated paper, foil or laminate type of covering material is applied on the wood-based panel surface using pressure and elevated temperature. In addition to that, also edges are mostly protected by using different edging-banding materials like ABS (Acrylonitrile Butadien Styrene), PVC, melamine, solid wood or veneer (Tankut and Tankut, 2010).
Although surfacing of PB, even just with very thin impregnated paper (thickness < 0.3 mm; paper grammage or base weight between 45 g/m 2 and 200 g/m 2 ), lowers the formaldehyde emission (Liu et al., 2015), the issues related to formaldehyde emissions are still present. Despite the fact that over the last few decade's producers of wood-based panels significantly lowered formaldehyde emission (Salthammer et al. 2010), nowadays we are facing different challenge related to the indoor formaldehyde concentrations. Today, building practice as well as users demand high-energy efficient buildings, which is related to the decreased air exchange rate inside the building. Such low air exchange activity enables accumulation of formaldehyde that emits from materials. The concentration of formaldehyde in indoor air, although emitting from materials with low formaldehyde concentration (like PB or MDF, where concentration of formaldehyde is lower than 0.1 ppm as permitted by EN 312 or EN 622-1) can accumulate to such level/concentration that could cause eye and respiratory irritation.
As mentioned before, surfacing of PB and MDF can lower formaldehyde emission, but closing the surface is not enough. As was shown in research conducted by Grigoriou (1987), it is important to seal edges as well. Although the edge apparent surface area is lower than the face surface area, the formaldehyde emission from edges is higher than that from or through face layer. According to Barry and Corneau (2006), 96 % reduction of formaldehyde emission could be achieved if panels were well surfaced and edges covered. Effect of panel area to volume ratio on release of volatile organic compounds at surfaced particleboards were also investi-gated by Yali et al. (2018). It was determined that increase in panel area to volume ratio led to the increase in volatile organic compounds concentration.
Today's furniture producer usually are sealing only visible edges in order to save money and make cheaper products, hence leaving at least two edges open and with that enabling easy emission of formaldehyde from board to indoor air. Easy emission of formaldehyde from panels is further enhanced due drilling different holes. Some of them are used for shelf holders, but the majority of them are left open what enables, despite covered face, the emission of formaldehyde.
The aim of this work is to show the efficiency of creating a formaldehyde emission barrier by applying melamine impregnated paper and melamine based edging-banding material.

MATERIALS AND METHODS MATERIALI IN METODE DELA
For the purpose of investigation furniture grade, three-layer particleboard (PB), thickness 18 mm and density 650 kg/m 3 , and MDF, thickness 12 mm and density 760 kg/m 3 , available on Slovenian market (contributed by the firm Starman) were used. The impregnated papers used in the research were contributed by Melamin Kočevje, Slovenia. Two types of impregnated paper were used, specifically paper with grammages of 130 g/m 2 (IP130) and 200 g/m 2 (IP200). Melamine based edge-banding tape available on Slovenian market was used (contributed by the firm Blažič, robni trakovi, d.o.o., Slovenia).
After seven-day exposure to normal climate condition (temperature 20±1°C and relative air humid- ity 65±5 %), 500×500 mm 2 panels were surfaced with melamine impregnated paper. The surfacing was done in a laboratory press, where upper heating plate was set at the temperature of 185°C and lower heating plate at 180°C. The pressure was 3 N/mm 2 , while pressing time was 30 seconds. After pressing, the boards were cooled at room temperature for 24 hours, followed by cutting to the sample size of 400×50 mm 2 . On some samples, melamine based edge-banding tape was applied (Table 1). For the application of melamine edge-banding tape, hot-melt adhesive was used. Until measurement, samples were placed in airtight plastic bag and stored at normal climate conditions. The determination of formaldehyde emission was done according to SIST EN 717-2 (gas analysis method). The samples with dimensions 400×50 mm 2 were placed in a chamber that was heated to the temperature of 60 ± 0.5°C. The level of humidity in the chamber was 2 ± 1 % and the air flow into the chamber was 60 ± 3 l/h. Four sets of wash bottles containing from 30 to 40 mL of distilled water were connected to the chamber. Formaldehyde emission was measured for 4 hours at hourly intervals. After 4 hours the water solution from wash bottles was transferred into a 250 mL volumetric flask to which distilled water was added to the indicated mark. The water solution (10 mL) was transferred from the volumetric flask to another flask to which we added 10 mL of acetyl acetone and 10 mL of ammonium acetate solution. The flask was stoppered, shaken and placed in a water bath with temperature 40°C for 15 minutes. The solution was then placed in a dark chamber for one hour. The absorption of the solution was determined using a spectrophotometer at 412 nm. Formaldehyde emission rate (FER) was calculated according to equations 1 and 2.

RESULTS AND DISCUSSION REZULTATI IN RAZPRAVA
Exposed surface plays important role when formaldehyde emission is considered ( Table 2).
The application of impregnated paper and edgebanding material significantly lowers formaldehyde emission (Figures 1 and 2).
The barrier effect of impregnated paper depends on the grammage of the paper. Higher barrier effect (at PB) concerned paper with higher grammage (0.88 compared to 0.69). The impregnated paper with higher grammage is more effective than formaldehyde barrier due to its higher thickness (0.15 mm to 0.2 mm compared to 0.1 mm to 0.12 mm) and more adhesive (around 120 g/m 2 compared to 80 g/m 2 ), hence it seals the faces more efficiently. Although we surfaced MDF board with impregnated paper with a grammage of 200, the barrier efficiency was not as high as observed at PB. The barrier effect at board B2/200 (MDF sur- The reason for formaldehyde barrier effect at thin melamine impregnated paper (thickness less than 0.3 mm) is related firstly due to the creation of additional diffusion resistant layer in addition to already existing diffusion resistance face layer (due higher density compared to core layer). Second reason is due to the pressing conditions. Panels are pressed at a high temperature (usually higher than 180°C), which causes heat and mass flow towards the core layer, and with that also migration of formaldehyde from face into core layer, which results in the accumulation of formaldehyde in core layer. Similar effect was also determined by Grigoriou (1987). During pressing, formaldehyde also emits from panel through edges (with moisture evaporation) and after opening of the press. This was also determined during the experiment. The unsurfaced (raw) particleboard was exposed to same conditions as occurring during surfacing (upper heating plate 185°C, lower heating plate 180°C, pressing pressure 3 N/mm 2 , pressing time 30 seconds). The FER for "tempered" panel was 0.1183 mg/h (compared to raw particleboard FER which was 0.1831).
As could be seen by comparing the results in Table  2 and Figure 1, panel surfacing is an effective way to reduce formaldehyde emission, but in order to prevent easy emission from the panels, edges also needs to be closed. Closing edges lowers formaldehyde emission by at least 40 % (Figure 2).
The formaldehyde emission rate from panels with opened edges was between 0.0665 and 0.0774 mg/h but when edges were closed with melamine edge-band tape, FER was lowered by between 42 % and 59 %. Despite the fact that surface of the faces is much higher than edge surface (0.04 m 2 compared to 0.0162 m 2 at PB respectively 0.0108 m 2 at MDF) the barrier effect of edge-band is high. The reason lies in the diffusion resistance. The density of core layer is lower than surface layer, hence the formaldehyde diffusion resistance of core layer is lower as well. Formaldehyde emits from panel through the path that offers least resistance and since faces present highest diffusion resistance (higher density) formaldehyde emits from edges, but when edges are closed (melamine edge-band) it can be assumed that the majority of formaldehyde emission occurs through semipermeable décor paper.
In Figures 3 and 4, the comparison of FER values of unsurfaced and surfaced panels is shown.
In order to achieve the highest possible barrier effect, all surfaced boards need to be covered. In case some surfaces like edges or even drilled holes are left open, then higher formaldehyde emissions at the customers' end can occur.

CONCLUSIONS ZAKLJUČKI
The results of our experiment pointed at the importance of closing the surfaces of particleboard and MDF in order to lower the formaldehyde emission.
The highest barrier effect was determined when melamine impregnated paper with a grammage of 200 g/m 2 was used.