木材へのポリ酢酸ピニル吸着におけるけん化度依存性

Abstract

木材へのポリ酢酸ビニル (PVAc) の吸着について. PVAcをけん化し側鎖の酢酸基の一部を水酸基に換えるととによる吸着性の変化を検討した。また, 未けん化のPVAcについては, 分子量, 吸着媒(木材)の形状, 前膨潤剤の種類の影響についても検討した。供試PVAcは重合度25 (PVAc 25) 及び300 (PVAc 300) のもので, けん化度依存性の検討にはPVAc25を種々の程度にけん化して用いた。吸着媒はヒノキの60 - 80メッシュ木粉及び4mm厚木口試験片で, これらを前膨潤剤 (ジメチルスルホキシドあるいはメタノール) で膨潤後, ベンゼンに溶媒置換して用いた。得られた主な知見は, (1) 木粉への吸着において. PVAc 25はPVAc 300の倍以上の吸着を示す。これは分子量の増大に伴なって吸着に有効な表面 (一時空隙表面) が減少するためと考えられる。(2) PVAc 25, PVAc 300ともに, 木口試験片には吸着されない。木口試験片では木粉に比して溶媒置換が困難であるため, 木材との親和性の高い前膨潤剤が多く残留し, これがPVAcの吸着を阻害するためと考えられる。(3) 部分けん化PVAcの木粉への吸着量は, けん化度約15％まではけん化度の増大とともに増加傾向を示すが, それ以上では減少する。これは15％以下のけん化度では, PVAcの酢酸基がより寸法の小さい水酸基に変わることにより分子屈曲性が高まり, 一時空隙への侵入が容易になるためと考えられる。一方, 15％以上のけん化度では, 電子供与性, 受容性を合せ持つ水酸基の増加により, 分子内水素結合の機会が増え, 分子屈曲性が低下する効果とともに, 水酸基というより吸着力の強い官能基を持つことで吸着セグメント数が増加し, 一分子あたりの吸着占有面積が増大するという効果が優勢となるため, 吸着量が減少すると考えられる。(4) (3) の結果から. PVAcに電子供与性, 受容性をかね持つ水酸基を導入しても, 少なくとも吸着量を増加させることには大きな意味を持たないといえる。

The adsorption of polyvinyl acetate (PVAc) from benzene solution onto swollen wood has been investigated in connection with the effect of saponification of PVAc. The effects of the molecular weight, the shape of adsorbents (wood meal and wood cross-section) and the sort of preswelling agents were also studied for non-saponified PVAc. Two kinds of non-saponified PVAc having the degrees of polymerization of 25 (PVAc 25) and 300 (PVAc 300), and five kinds of partially saponified PVAc with various degrees of hydrolysis obtained by saponification of PVAc 25 were used as adsorbates. The adsorbents (60-80 mesh wood meal and wood cross-section with 4 mm thickness of Hinoki) were swollen in a preswelling agent (dimethyl sulfoxide: DMSO, or methyl alcohol: MeOH in an exceptional case for the cross-section), and successively exchanged to benzene before adsorption. The results obtained are as follows: (1) PVAc 25 is not adsorbed on the wood meal over twice as much as PVAc 300. This attributes to the decrease of the effective surface (the surface of fine capillaries in the swollen cell wall of wood) for the adsorption with increasing the molecular weight of PVAc. (2) Both PVAc 25 and PVAc 300 are not adsorbed on the wood crosssections swollen both in DMSO and MeOH, though they are adsorptive on the wood meal swollen in DMSO. This should be dependent on the inhibiting effect of the preswelling agents remained after the solvent exchange for the adsorption of PVAc. The residual amount of a preswelling agent should be larger in the wood cross-section than in the wood meal because of its larger dimension. (3) The adsorbed amount of PVAc on the wood meal tends to increase slightly with incresing the degree of hydrolysis until about 15%, and then decreases above 15% hydrolysis. In relatively low hydrolysis, the flexibility of PVAc chain will increase with increasing hydrolysis, because the steric hindrance in PVAc molecules decreases owing to the exchange of acetyl groups to smaller hydroxyl groups. This leads to the incease of the adsorbed amount on porous adsorbents. However, in relatively high hydrolysis, the chance of intra-molecular hydrogen bonding should increase with increasing the hydroxyl groups having an ability to both donate and accept electrons. Such intra-molecular hydrogen bonding reduces the flexibility of PVAc molecules. The increase of hydroxyl groups should result in flat conformation of PVAc on surface because of thier stronger affinity to the the surface sites than acetyl groups, and so the neccessary area for adsorption of one molecule will increase. These can explain the decrease of adsorbed amount above 15% hydrolysis. (4) From the result (3), it can be said that the introduction of hydroxyl groups in PVAc molecules is not so effective for the purpose of increasing the amount of PVAc adsorbed on swollen wood.