木材細胞膜中におけるラクチッドの開環重合

Abstract

疎水性オリゴマーまたはポリマーを木材の細胞膜内に充愼し, 木材の寸法安定化を計る方法の1つとして, 木材中での開環重合によるポリエステル合成を試みた。乳酸の環状ダイマーであるlactideは, SnCl_4を触媒としてトルエン中で反応し, かさ効果が十分に期待される分子量のポリ乳酸が生成することが知られたので, ブナ木粉中でのlactideの開環重合条件を検討した。木粉中のポリ乳酸生成量として, 未処理木粉重量を基準にした重量増加率, および, lactideのみを欠いたブランク処理木粉重量を基準にした増加率の2種を算出した。ブランク処理木粉を基準にした重量増加率は, 最高35％に達し, さらに, 種々の重量増加率の処理木粉について (1) 式より算出したカルポニル基の赤外線吸光度インデックスとの間に, ほぼ比例関係が認められた。しかし, 水酸基の赤外線吸光度インデックスに対しては反比例の結果となり, これから, 木材構成成分とlactideまたはポリ乳酸との間に化学反応がおきたことが考えられ, 化学結合の存在が推定された。同様に, マカンバ木口試験片を用いたlactideの開環重合により, 未処理材に対する寸法増加が得られ, 寸法安定化が期待された。

The impregnation of hydrophobic polyester oligomer with good swelling media of wood has been reported for dimensional stabilization of wood.1) Similar effects may be obtained by impregnation and polymerization of monomer within wood cell wall. Polyester synthesis in the wall was, therefore, attempted by ring-opening-polymerization which was thought to be more advantageous for wood than condensation polymerization, because of reacting on the milder condition and forming no by-products. Water-soaked beech wood meal and birch wood specimens were successively replaced in five steps with ethanol, and toluene containing definite amount of stannic chloride. Lactide, a cyclic dimer of lactic acid, was prepared by Kleine's procedure.3) Molecular weight of polymer was determined by titrating end carboxyl groups. Weight increase percent of wood after polymerization was calculated on basis of untreated or blanktreated wood, and IR absorption index by the formula 1. Dimensional change was also measured in the tangential direction. On polymerizing lactide, molecular weight of product increased according to the temperature and concentration of lactide and catalyst in toluene, as shown in Tables 1 and 2. The polymer appeared to have enough size for attaining the improved dimensional stability of wood, compared with the polyethylene glycol treatment.6) The weight increase percent of beech wood meal after polymerization reached 35 percent based on blank-treated wood, depending upon temperatures and concentrations of lactide and stannic chloride. They were explained by mechanism of monomer penetration (osmotic pressure, diffusion or a quantity of monomer in the wall), size of resulting polymer in the wall, and affinity of stannic chloride with cellulose. The weight increase of wood was considered as the deposit of polylactic acid produced within the wall rather than the impregnation of polymer synthesized outside the wall. Figure 1 shows that IR absorbancy of blank-treated wood is broader and weaker than that of untreated wood, and especially, absorbancy of CO group at 1730 cm-1 has heavily diminished by the action of stannic chloride or heat against wood components. Sites and intensities of IR absorption of wood after polymerization has changed with the weight increase percent, depending upon the amount of polymer in the wall. IR absorption indexes of CO and OH group based on absorbancy of CH group at 2880 cm-1 are given in Figures 2 and 3. While the index of CO group is proportional to the weight increase percent, that of OH group is inversely proportional. The results suggest that the hydroxyl group of wood components reacts with lactide or polylactic acid during the polymerization reaction. IR spectra of blank-treated samples show that lower temperature and less stannic chloride will have less effect on wood. Higher dimensional increase percent of birch specimen after polymerization compared with blank-treated wood also shows that polylactic acid is deposited in the wall (Table 4). The adsorption of stannic chloride in the wall would seem to give the blank-treated wood larger dimensional increase than the solvent-replaced sample.