木材へのポリビニルアルコールの吸着について (第3報)

Abstract

ポリマーの化学構造を官能基の反応によって変化させると, ポリマーと吸着媒, ポリマーと溶媒の相互作用が変化し吸着量に影響する。木材に対するポリビニルアルコール (PVA) の吸着について, 残存酢酸基量および主鎖にそって存在する残存酢酸基の分布状態に影響について知見をうるために本研究を行なった。供試PVAは, (1) 常法 (塩酸-酢酸-水溶液の均一系) に従って酢化した均一再酢化PVA, (2) PVAoをメタノール-水系およびメタノール-ベンゼン-水系で苛性ソーダを触媒としてけん化した部分けん化PVAおよび (3) 完全けん化PVAの3種である。吸着媒は精製セロハン (#300) およびヒノキ木粉 (アル・ベン抽出, 40 - 60メッシ) である。溶液濃度の定量は酸化還元滴定法およびヨード呈色PVA水溶液についての比色定量法によった。PVA化学構造の解析は中和滴定による残存酢酸基量の定量, D/C (0. 5％PVA水溶液lmlあたりのヨード呈色度) の測定および各PVA薄膜の赤外線吸収スペクトルの測定によった。上記各種PVA-水-セロハンまたは木粉の系について, PVAの吸着等温線を求めて比較検討した。得られた主な知見は, (1) 木材とセロハンそれぞれへのPVAの吸着について, 吸着等温線を比較すると, 前者の方が吸着が大であり, また, 前者は高濃度ほど平衡吸着量が大きく, 後者では小さい。これらはセロハンが木材よりもポリマーの近ずきうる表面の親水性が高いことおよびPVAと溶媒 (水) との競合吸着を考えることにより理解できる。(2) PVA中の残存酢酸基量の影響は著しく, その13wt. %程度に吸着量の極大が認められる。これはPVAの溶解性と残存酢酸基量の関係に一致し, PVAの水溶液中での分子内および分子間結合 (水酸基間の水素結合またはアセチル基間のvan der Waals結合に基づく) の起りやすさと残存酢酸基量の関係より説明できよう。(3) PVA主鎖にそって存在する残存酢酸基の集団化の度合が高いほど多く吸着される。このような不均一部分けん化PVAは同一分子内に連らなったエステル基の疎水部分と水酸基の親水部分をもち, 高分子界面活性剤的性質をもつといわれているが, これがPVAの木材およびセロハンの吸着に正の影響を与えているものであろう。

When the chemical structure of polymer molecule is varied by copolymerization or by reaction of functional group, the adsorptionability of polymer is affected by it through the change of reciprocal actions among the polymer, the adsorbent and the solvent. The present investigation was undertaken to make clear the effect of the amounts and the distribution of residual acetyl groups in partially acetylated PVA and partially saponified PVAc on the adsorption of the polymers from dilute aqueous solution on wood and related substance. PVA used were (1) partially acetylated PVA prepared from acetic acid and PVA in homogeneous system, (2) partially saponified PVAc obtained by alkali method and (3) perfectly saponified PVAc. The adsorbent used were refined cellophane (# 300) and Hinoki (Japanese cypress) powder (alcohol-benzene extracted, 40-60 mesh). The amounts of polymer adsorbed was determined from the difference in the concentration of polymer solution before and after the sorption period. For the determination of the concentration of polymer solution, (1) a redox titration, made by oxidation of polymer with excess standard dichromate/sulfuric acid mixtures and determination of the excess dichromate with standard ferrous iron solutions, and (2) a spectrophotometric method, based on the red color produced by the reaction of partially saponified PVAc with iodine, was used. The results and discussions were as follows: (1) In comparison between wood and cellophane with respect to thier PVA adsorption isotherms, the positive adsorption which is increased with equilibrium concentration, is occured in the case of the former and the negative adsorption which is decreased with equilibrium concentration is appeared by the latter. This is interpreted by the polymer (PVA) - solvent (water) competition for sites on the surface and by that the cellophane has more hydrophilic surface which is approachable for polymer molecule, than alcohol-benzene extracted wood. (2) It is found that the amounts of residual acetyl groups within PVA molecule have a great influence on its adsorptionability, and that the polymer containing about 13 wt. % residual acetyl groups shows the greatest adsorption tendency. These results coincide with the relations between the solubility of PVA and the amounts of residual acetyl groups, and these are interpreted by the relation between the existence of intra- and inter- molecular bonds in aquenous solution of PVA (caused by hydrogen bonds between hydroxyl groups, or by van der waals bonds between acetyl groups of PVA), and the amounts of residual acetyl groups. (3) The adsorption tendency of PVA is more remarkably as the residual acetyl groups are maldistributed in a polymer chain. It is said, that such partially saponified PVAc having hydrophobic ester groups and hydrophilic hydroxyl groups maldistributed in a chain indicates the properties of polymer surface active agent, and this argument seems to support the above result.