Measuring the Formaldehyde Content from Different Types of Oriented Strand Board Manufactured with Different Thicknesses and Glued with Different Resins

This study measures the formaldehyde content (FC) of different types of oriented strand board (OSB) panels. Panels with different thicknesses were produced on the OSB production line of a prominent manufacturer of board composite materials in the Czech Republic. The resins used were polyurethane (PU1, PU2) and melamine-urea-formaldehyde (MUF). The corrected FC from 3-layer OSB/3 panels (MUF/surface, PU2/core) ranged from 5.95 ± 0.42 to 7.24 ± 0.67 mg formaldehyde/100 g of dry board. The corrected FC of the 3-layer OSB panels bonded with PU2 resin ranged between 0.48±0.04 and 0.57±0.11 mg formaldehyde/100 g dry board. Additionally, the corrected FC of 3-layer OSB/3 panels bonded with PU1 resin ranged from 0.09±0.22 to 0.46±0.40 mg formaldehyde/100 g dry board. Furthermore, the corrected FC of 3-layer OSB/4 panels bonded with PU1 resin ranged from 0.178±0.309 to 0.473±0.027 mg formaldehyde/100 g dry board. These results showed that the FC was limited to the natural content of formaldehyde in solid wood, where the values are extremely low, and which resulted in the production of green product. Additionally, the FC of OSB panels bonded with PU1 was lower than that of panels glued with PU2.

Ključne riječi: građevinske ploče, sadržaj formaldehida, zeleni proizvodi, OSB ploča Oriented strand board (OSB) is an engineered wood-based panel consisting of strands of wood that are bonded together with a synthetic resin; the strands are oriented, partially randomly overlapped, and pressed together in layers (Hodoušek et al., 2015).In the outer layers, strands are generally oriented longitudinally in line with the panel length, whereas in the middle layers, strands generally lie perpendicular to the outer layers.Recent methods for manufacturing OSB require very thin, long chips with the following optimum dimensions: 0.4 to 0.6 mm in thickness, 5 to 20 mm in width, and 60 to 120 mm in length ) Peña et al., 2006;Ohlmeyer et al., 2008;Böhm et al., 2011).
OSB-like plywood is widely used as a structural material in the manufacture of housing, framing, and other industrial products (Böhm et al., 2012;Salem et al., 2013).Longer and thinner chips that are accurately oriented increase the strength, rigidity, and other physical properties of OSB panels (Han et al., 2006 and2007).
Additionally, for good production of OSB, the density of the surface layers should be higher than the density of the intermediate layer, with a "U"-shaped density profi le along the panel thickness (Xu and Winistorfer, 1995;Painter et al., 2006;Böhm et al., 2011).OSB consists of three layers, and it achieves a high level of dimensional stability with excellent mechanical performance.OSB is manufactured primarily from spruce strands, good quality softwood (Böhm et al., 2011;Hodoušek et al., 2015).
Polyurethane or polyisocyanate resins are widely used in OSB worldwide, MDF mills in Europe, and a few MDF mills in North America (Papadopoulos et al., 2002).It is important to understand how these resins react with wood furnish, as this knowledge will determine whether the urethane linkage will strengthen the board and provide durability in different environmental conditions.Theoretically, isocyanate groups can react with hydroxyl groups in wood furnish and form irreversible urethane linkages (Pizzi and Mittal, 2003;Smith, 2012).Isocyanate-bonded composite panels have benefi ts including water-resistant chemical bonds, strong mechanical bonds and toughness, high strength, low resin dosage, extreme moisture resistance, and low swelling (Papadopoulos, 1999).Polyurethane can be used in the core layer alone or in all layers of OSB depending on the quality and/or formaldehyde emission restrictions (Vangronsveld et al., 2010).
Formaldehyde emissions (FE) from wood, wood products, and fl ooring materials are affected by wood type or species, board thickness, resin type, resin additives, panel manufacturing techniques, moisture content, the drying and hot-pressing techniques, and surface fi nishing materials (Yu and Crump, 1999 (Yu and Crump, 1999).Boards bonded with melamine-ureaformaldehyde (MUF) resin exhibit the lowest formaldehyde content (FC) and FE because of the addition of melamine to UF resin (Aydin et al., 2006).
Previously, differences in the bending stress of the upper and lower faces of OSB/3boards and the air permeation rate of OSB/3 and OSB/4 boards have been studied (Ohlmeyer et al., 2008;Hodoušek et al., 2015).In the present study, OSB/3 and 4 boards, which are extensively produced in Europe for their use as heavy-duty load-bearing boards in humid conditions, were evaluated to determine their formaldehyde content with the perforator or extraction method (EN 120, 1993).

Production of OSB/3 and OSB/4 panels 2.1. Proizvodnja ploča OSB/3 i OSB/4
Industrially manufactured OSB/3 and OSB/4 panels with various thicknesses were investigated to determine their formaldehyde content during the period of August 2014.All panels were manufactured using 80 % Norway spruce (Picea abies L.) and 20 % Scots pine (Pinus sylvestris L.).According to EN 300 (2006), OSB/3 boards were manufactured as loadbearing boards for use in humid conditions, and OSB/4 boards were manufactured as heavy-duty load-bearing boards for use in humid conditions.
Briefl y, mixed wood strands of 80 % Norway spruce and 20 % Scots pine of a predetermined shape with a width of 50 mm, length of 130 mm, and a thick-ness of less than 2 mm, were used to manufacture OSB panels.The wood strands were dried, mixed, and bonded with 4 % polyurethane type 1 (PU1) and 2 (PU2).The mat was randomly formed and hot-pressed with 5 to 8 s/1 mm panel thickness at temperatures from 240 to 190 °C (from entry to output of the press).
Other OSB/3 boards were manufactured using melamine-fortifi ed urea formaldehyde resin (MUF) for the surface layers and PU resin for the core layer with the following categories; OSB/3-A: Surface (MUF) and Core (PU2), OSB/3-B: Surface (PU2) and Core (PU2), and OSB/3-C: Surface (PU1) and Core (PU1).The mat was randomly formed and hot-pressed with 10 s/1 mm panel thickness at approximately 220 °C.The ˝ratio of core to surface layers was 60/40.Properties of the resins used are presented in Table 1.Other properties of the resin used for OSB/4 and PU1 have been described previously (Hodoušek et al., 2015).

Measuring formaldehyde content by the
perforator method (EN 120) 2.2.Mjerenje sadržaja formaldehida perforatorskom metodom (EN 120) In Europe and China, the EN 120 perforator method (1993) is the most frequently used procedure for measuring formaldehyde content (FC) as well as for production control in the wood-based panel industry, and it has good correlation with referenced chamber methods (Risholm-Sundman and Wallin, 1999; Salem et al., 2011 and 2012; Liu and Zhu, 2014).
The collected panels were conditioned for 4 weeks at 20 °C and 65 % RH before measuring the FC with the perforator method (EN 120) (Böhm et al., 2012).At least three replicates from each type of OSB were used.Each replicate of approximately 110 g with dimensions of 25×25 mm were taken from OSB panels with various thicknesses.The samples were extracted in 600 mL of boiling toluene for 2 h in a perforator apparatus.The formaldehyde content was expressed as mg HCHO/100 g of dry board (mg/100 g o.d.).The E1 emission limit is ≤ 8 mg/100 g o.d.The measured FC in boards with different moisture content (MC) was normalized to the FC of boards conditioned to 6.5 % MC according to EN 312 (2003).With MC ranging from 3 2.4 Wood material, kg/1-m 3 of the board, ATRO, coniferous) drvni materijal, kg/1-m 3 ploče, ATRO, četinjače) 535 a: Percentage content of component at 0 % moisture / postotni udjel komponente pri sadržaju vode 0 % b: refers to water addition / odnosi se na dodanu vodu c: refers to catalyst addition (hardener addition) / odnosi se na dodani katalizator (dodani očvršćivač) % to 10 %, the EN 120 test value was multiplied by a factor F, which was calculated from the equation: As formaldehyde content is very sensitive to sample humidity, the moisture content was measured by the following formula (EN 322 1993): (2) where H is the moisture content, m 1 is the mass of the test pieces before drying (g), and m 0 is the mass of the test pieces after drying (g).

Determination of formaldehyde by the acetylacetone method 2.3. Određivanje formaldehida acetilacetonskom metodom
The amount of formaldehyde released from panels and absorbed by water was determined photometrically by the acetylacetone spectrophotometric analysis method.This method is based on the Hantzsch reaction, in which aqueous formaldehyde reacts with ammonium ions and acetylacetone to yield diacetyldihydrolutidine (DDL); DDL has an absorption maximum at 412 nm (Nash, 1953).This technique is widely applied as a standard procedure for the specifi c analysis of formaldehyde.

Statistical analysis Statistička analiza
Measured formaldehyde content was analyzed using SAS version 8.2 (2001).The data were analyzed using analysis of variance in CRD to show the significant differences between the formaldehyde content values with Duncan's multiple-range test at 0.05 level of probability.The values are presented as mean ± SD.

Sadržaj formaldehida u različitim OSB pločama
The formaldehyde content of OSB panels was extremely dependent on the moisture content of the samples (Tables 2, 3, and 4 products be near or in equilibrium with the humidity in the test atmosphere (Meyer 1979).
Table 2 shows the measured and corrected FC of 3-layer OSB/3-A panels (MUF/surface, PU2/core).The corrected values ranged between 5.95±0.42 and 7.24±0.67mg formaldehyde/100 g o.d., and these values are in the range of E1 emission class (E1 ≤ 8 mg formaldehyde/100 g o.d.).The formaldehyde values depended on the oven-dry weight of the board, so that moisture content affected the FC measured by EN 120 (Salem et al., 2012).
Table 3 shows the measured and corrected FC of 3-layer OSB-B bonded with PU2 resin.The lowest value (0.48±0.04 mg formaldehyde/100 g o.d.) was found in panels with a thickness of 15 mm, and the highest value (0.57±0.11 mg formaldehyde/100 g o.d.) occurred in panels of 12-mm thickness.The FC of all panels was much lower than required by the E1 emission classifi cation.The variation in minimum and maximum values could be related to the sample taken from the edge of the board or near the center of the board.Table 4 shows the measured and corrected FC of 3-layer OSB/3-Cpanels bonded with PU1 resin.The corrected FC values ranged from 0.09±0.22mg formaldehyde/100 g o.d.(18-mm panels) to 0.46±0.40mg formaldehyde/100 g o.d.(20-mm panels).Table 5 pre-  These results showed that FC was limited to the natural content of formaldehyde in solid wood, where the values are extremely low and result in the production of green products.Additionally, the FC values of panels bonded with PU1 were lower than those of panels bonded with PU2.In contrast, the perforator method measures the total extractable formaldehyde in the board, which shows that not all formaldehyde is emitted at room temperature (Xiong and Zhang, 2010).

CONCLUSIONS ZAKLJUČAK
1.The measured formaldehyde content of all OSB panels produced was lower than the E1 emission class.2. All OSB panels produced with different glues and thicknesses were recognized as green boards.3. Additional parameters will be studied to measure the physical and mechanical properties of different OSB panels and to provide a better evaluation of the product.