Dimensional Stabilization of Particleboard by Formaldehyde Treatment

Abstract

尿素 - ホルムアルデヒド (UF) 系, メラミン - ホルムアルデヒド (MF) 系, 及びフェノール - ホルムアルデヒド (PF) 系の3種類の樹脂接着剤を用いて作られた, 市販のパーティクルボードに対して, ホルムアルデヒド処理を適用し, 寸法安定性の改良を試みた。処理試片について, 寸法安定性, 吸湿性, 及び機械的性質を測定し, 他の木質材料との性能の比較を行った。その結果, 二酸化イオウを触媒とした, 120℃における24時間までの処理で, 厚さ膨潤率, 及び線膨潤率は, パーティクルボードの種類に関係なく, 反応時間とともに低下し, 抗膨潤能 (ASE) は厚さ方向で70%, 面方向で60%に達した。耐水性の低いUF系のボードでさえ, 冷水浸せきにおける寸法安定性は, UF系樹脂接着剤で作られた屋内用合板に匹敵する程度にまで向上した。膨潤の抑制効果は, 水浸せき時だけでなく, 繊維飽和点以下においても発現し, 平衡含水率は無処理の約半分にまで低下した。これらの結果は, 水分吸着性の低下がパーティクルボードの寸法安定化に大きく寄与していることを示す。常態の曲げ強さは, 70%のASEレベルで20 - 30%低下したが, 湿潤状態の曲げ強さは無処理とほとんど変わらず, また曲げ弾性係数は20 - 40%増大した。これに対して, 剥離強度の低下は, とくにUF系ボードにおいて深刻であった。

A vapor phase treatment with formaldehyde (formalization) was applied to the dimensional stabilization of three types of commercial particleboard : urea-formaldehyde (UF), melamine-formaldehyde (MF), and phenol-formaldehyde (PF) types. Dimensional stability, hygroscopic property, and mechanical properties of the treated particleboard were compared with those of the other wood-based materials. By a sulfur dioxide catalyzed formalization up to 24hr at 120℃, thickness swelling and linear expansion decreased with reaction time irrespective of the types of particleboard. The antiswelling efficiency (ASE) reached 70 and 60% in thickness and linear directions, respectively. Even a low waterproofing UF type particleboard, the dimensional stability in water soaking at room temperature was enhanced to the same degree as that for UF type plywood. The restraint of swelling was revealed not only in water soaking state but also below fiber saturation point, and the equilibrium moisture content decreased to about a half of the untreated one. These results show that the decrease of moisture adsorptive property contributed effectively and largely to the dimensional stabilization of the particleboard. The modulus of rupture (MOR) in dry state decreased by 20 to 30% when ASE reached at the level of 70%, whereas the water saturated state MOR and the modulus of elasticity (MOE) was unchanged and increased by 20 to 40%, respectively. On the other hand, the loss of internal bond was much more serious than that of bending strength, especially in UF type particleboard.