Introduction

Aminoplastic resins, especially UF-resins, are the main binders used in the industry of wood-based panels. UF-resins are fast curing resins and of uncontested good performance. However, boards bonded with UF-resins are, in general, of limited moisture resistance and emit also detectable amounts of formaldehyde. The amount of formaldehyde liberated from UF-bonded boards depends on a number of endogenic factors related to the resin and the production conditions of the boards. Moreover, exogenic factors, such as the relative humidity and temperature, under which the boards are tested (Peterson et al. 1972, 1973, 1974; Sundin et al. 1992) impact on the formaldehyde release. Additionally, ageing of the boards decreases their formaldehyde release significantly (Sundin and Roffael 1989). Therefore, the assessment of formaldehyde emanation is an intricate issue, to which over the last decades an untold number of publications pertains.

The literature describes many methods for measuring the liberation of formaldehyde from wood-based panels (Roffael 1993) in the laboratory. Recently, continuous methods have been also proposed for assessing the formaldehyde release during production in the factories (Engström 2007, 2008).

In Europe, mainly three laboratory methods for the determination of formaldehyde release have been standardised which are as follows:

  1. 1.

    Extraction method called the perforator method (EN 120)

  2. 2.

    Formaldehyde emission by the Gas analysis method (EN 717-2)

  3. 3.

    Formaldehyde emission by the flask method (EN 717-3)

Besides these methods, the formaldehyde emission of the boards can be measured using the chamber technique (EN 717-1), which is considered to be the reference method.

Among the above-mentioned laboratory methods, the perforator technique gained wide acceptance for assessing the emanation of formaldehyde from wood-based panels; the method can be used for both quality control of the boards in the factory as well as classification of the boards according to the guideline (DIBt-100 1994).

The perforator method (EN 120)

The perforator test is used primarily on specimens of raw or unfinished particleboards (PB), medium density fibreboards (MDF) and oriented strand boards (OSB). It involves boiling test specimens in toluene at nearly 110°C. The extracted formaldehyde is collected in water and analysed photometrically. Within the range above 3 to 9% (3 ≤ u ≤ 9%) moisture content (MC), the measured perforator values are corrected to a common MC (6.5%) using an established equation proposed by Jann and Deppe (1990):

$$ F = - 0. 1 3 3\times u + 1. 8 6 $$

(F perforator correction factor, u moisture content [%])

The correction factor of Jann and Deppe (1990) is based on the assumption that particle and fibreboards change their perforator values to the same extent regardless whether increasing or decreasing the moisture content in the range of 3 ≤ u ≤ 9%. The limit perforator values in Germany for particleboards, MDF and OSB are given in Table 1 according to the German regulations (DIBt-Richtlinie 100).

Table 1 Limit perforator values for boards of the E1-class category
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State of knowledge

In the literature, untold number of publications pertains to the perforator method and its field of application. Mayer (1978) established a curvilinear relationship between the formaldehyde content of the resins as expressed by the molar ratio F:U and the measured perforator value (Fig. 1). As can be seen from Fig. 1, a more or less linear relation exists between the perforator value and the molar ratio of the resin in the range between U:F 1:1.4 and U:F 1:1.8. In the range below U:F 1:1.1, decreasing the formaldehyde content in the resins leads to disproportionally little change in the perforator value of the boards. This indicates that the perforator value in this low region of emission is by far less sensitive to changes in the formaldehyde content of the boards than in the high molar ratio region. This is a serious drawback, since nowadays a substantial amount of particle- and fibreboards are bonded with low-fuming UF-resins with molar ratio U:F of 1:1 or even higher.

Fig. 1
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Change in the formaldehyde release (FESYP-perforator method) with the molar ratio of UF-resins (Mayer 1978)

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Results and discussion

Measurement of the change in the perforator value of aged particle- and fibreboards after climatisation at different relative humidities (30, 65 and 85%) leads to the following results: Increasing relative humidity during storage drives the moisture content of the boards to higher values and increases the measured perforator values as well. However, the corrected perforator values decline in the same direction. This phenomenon was observed in case of particleboards of different thickness (Fig. 2) as well as in case of medium density fibreboards (Fig. 3). In both cases, the correction factor seems to go beyond the mere compensation of the moisture influence on the perforator value. This becomes also evident from Fig. 4 showing that the emission level in the 1 m³ chamber of the boards after preclimatisation to different moisture contents. As can be concluded from the results, boards climatisated to higher moisture content emit also under equilibrium conditions higher amounts of formaldehyde in the chamber, this irrespective of the thickness of the boards. The difference in the emission level may reach about 0.03 ppm.

Fig. 2
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Change in the measured and corrected perforator value of UF-bonded particleboards with increasing moisture content (Witkavel 2004)

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Fig. 3
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Change in the measured and corrected perforator value of UF-bonded fibreboards with increasing moisture content (Witkavel 2004)

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Fig. 4
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Formaldehyde emission (EN 717-1) of particleboards in the 1 m³ chamber (Witkavel 2004)

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Further systematic investigations into the influence of moisture content of three fibreboards on the perforator value showed that the relation between moisture content and perforator values can be represented by a straight line (Fig. 5). However, the slope of the straight line representing the linear relation between moisture content and the perforator value is different for the various fibreboards investigated. The 12 mm fibreboard showed a slope of 0.27 and the 19 mm board a slope of 0.53. This is a strong indicator that the change of the measured perforator value with the moisture content is not the same for all aminoplastic-bonded boards, as originally assumed by the empirical formula published by Jann and Deppe (1990). This conclusion is in line with previous results published by Roffael and Mehlhorn (1980), who found that the molar ratio is an important factor as far as the relation between the perforator value and the moisture content is concerned, as shown in Figs. 6 and 7. All the above cited results unambiguously point out that the correction factor suffers from the deficiency of not including all factors affecting the change of the perforator value with the moisture content. The relation between the perforator content and the moisture content is multifactorial and not as simple as the correction formula implies.

Fig. 5
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Correlation between moisture content and measured perforator value of UF-bonded fibreboards (Witkavel 2004)

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Fig. 6
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Influence of moisture content of approximately 1-year-old UF-bonded particleboards on their measured perforator value; the boards were bonded with an UF-resin with a molar ratio U:F of 1:1.6 (Roffael and Mehlhorn 1980)

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Fig. 7
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Influence of moisture content of approximately 1-year-old UF-bonded particleboards on their measured perforator value; the boards were bonded with an UF-resin with a molar ratio U:F of 1:1.3 (Roffael and Mehlhorn 1980)

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Modification of UF-resins with melamine or melamine resins increases their moisture resistance. Consequently, the change in the measured perforator value of the boards bonded with modified resins as a function of moisture content is less concise than in the case of unmodified resins, as it becomes evident from Fig. 8.

Fig. 8
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Correlation between the perforator value and the moisture content of UF- and MUF-bonded particleboards (Roffael et al. 2001)

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Figure 9 shows a hypothetical ideal case, where the moisture content has no influence on the measured perforator value. In this case, the measured and corrected perforator values would be the same, in other words, the correction factor is 1. Therefore, if any measured perforator value is to be ideally corrected to eliminate the influence of moisture content, the corrected value will be on the ideal line or on a parallel line, where no influence of the moisture content on the perforator value exists. However, as can be seen from Fig. 9, the corrected perforator values using the formula of Jann and Deppe (1990) deviate strongly from the ideal line. The deviation from the ideal case seems to be more pronounced at low perforator value levels.

Fig. 9
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Correction factor according to Jann and Deppe (1990) against the ideal line (Roffael and Witkavel 2002)

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Undisputed hard evidence of the shortcoming of the correction factor can be derived from Figs. 10 and 11, where the measured and corrected perforator values of low-emission boards are plotted against the emission of the boards in the 1 m³ chamber. The measured uncorrected perforator values show a loose correlation with the emission in the chamber. Nevertheless, the emission rises with increasing perforator value (Fig. 10). The corrected perforator values, however, correlate inversely with the emission showing a decrease in the emission with increase in the perforator value (Fig. 11).

Fig. 10
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Correlation between the measured perforator value and formaldehyde emission in the 1 m³ chamber (Roffael 2004)

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Fig. 11
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Correlation between the corrected perforator value and the formaldehyde emission in the 1 m³ chamber (Roffael 2004)

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Conclusion

Summing up the whole results, it becomes clear that the corrected perforator value of the same board decreases with increasing moisture content over 6.5%. Moreover, the change in the measured perforator value with the moisture distinctly is different for boards bonded with different binders. In the very low region of emission, the corrected perforator value could lead to an inverse relation between the perforator value and the emission in the chamber as the correction factor goes beyond counter balancing the moisture effect. As this result is paradoxical, there is enough reason to revise the use of the perforator method for detecting small differences in the emission of boards in the low-emission range.