Abstract
A comprehensive literature survey and the results of investigations clearly show that the influence of resin level in particleboards on the perforator value (EN 120) depends on the molar ratio of the resin used as a binder. In case of high molar ratio resins (F:U 1.6:1) increasing the binder content in the boards leads to an increase in the perforator value of the boards. In contrast, boards bonded with very low molar ratio aminoplastic resins with molar ratio F:U of 1:1 (F:NH2 0.5:1) or even lower do not show any noticeable increase in the perforator value on raising the binder level. On the contrary, the measured perforator values may sometimes decline due to increase in the resin content in the boards. The perforator test seems to include two main processes taking place alongside each other: the physical extraction of free formaldehyde and the chemical hydrolysis process of the resin itself, induced by the moisture content of the boards. In case of low molar ratio resins, particularly at very low molar ratios (F:U 1:1 or even lower), free urea may also be present in the resin or added to the resin as a scavenger. Urea seems to react with extracted formaldehyde at high temperature during the extraction process. This lowers the perforator value without necessarily decreasing the emission at ambient conditions. Insofar, inextricably intertwined chemical processes seem to unfold during the extraction process. Additionally, the results show that subtle differences in the perforator value in the very low formaldehyde emission region may remain without noticeable impact on the emission measured by the chamber method (EN 717-1).
Highlights
Decreasing the molar ratio F:U in UF-resins to reach very low formaldehyde emission, as needed by the stringent standards like F**** in Japan (JIS 1460 A 2001), Carb II (2007) (ASTM E 1333) in California (USA) has a negative impact on the reactivity of the resins and leads to extended gel time and to a decrease in the hardening speed of the resin during pressing. Wittmann (1982) and Sundin et al (1987) conducted detailed studies on the influence of decreasing the formaldehyde content in UFresins on the water solubility in particleboards bonded therewith
The influence of resin content on the formaldehyde release measured by the perforator method has been studied in the past by several authors on particleboards bonded with UF-resins of high formaldehyde content (Petersen et al 1972, 1973, 1974; Mayer 1978)
Whereas quantitative data are available addressing the influence of resin content on the perforator value in boards bonded with urea formaldehyde resins of high molar ratio, no systematic studies seem to have been reported related to the influence of molar ratio in UF-resins of molar ratio F:U lower than 1:1
Summary
Decreasing the molar ratio F:U in UF-resins to reach very low formaldehyde emission, as needed by the stringent standards like F**** in Japan (JIS 1460 A 2001), Carb II (2007) (ASTM E 1333) in California (USA) has a negative impact on the reactivity of the resins and leads to extended gel time and to a decrease in the hardening speed of the resin during pressing. Wittmann (1982) and Sundin et al (1987) conducted detailed studies on the influence of decreasing the formaldehyde content in UFresins on the water solubility in particleboards bonded therewith. Wittmann (1982) and Sundin et al (1987) conducted detailed studies on the influence of decreasing the formaldehyde content in UFresins on the water solubility in particleboards bonded therewith. The results reveal that lowering the molar ratio F:U increases significantly the water solubility of the hardened resins in the boards. This clearly indicates that the cross-linking density of UF-resins decreases on reducing the formaldehyde content in the resins to produce emaciated products. The results of Sundin et al (1987) conform to work published earlier by Mayer (1978) revealing that the water solubility of resins in particleboards increases on decreasing the molar ratio F:U in UF-resins
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