γ-巯丙基三乙氧基硅烷对饱水木质文物的脱水定型机理研究

Study on the mechanism of dimensional stabilization of waterlogged archaeological wood treated byγ-mercaptopropyltriethoxysilane

饱水木质文物的脱水定型是该类文物最核心的保护需求之一,围绕这一保护需求已研究了天然树脂、聚乙二醇、糖类、乙二醛等多种保护加固材料,近期研究表明有机硅氧烷具有优异的脱水定型效果,其中γ-巯丙基三乙氧基硅烷(MPTES)最佳,但含巯基材料与木质文物的相互作用尚未得到较好的解释,对其进一步地研究或可为饱水木质文物保护材料的研发提供新的思路和借鉴。在本研究中以MPTES处理后的南海一号出水船板马尾松(Pinus massoniana)为研究材料,通过增重率、收缩率、扫描电子显微镜观察(SEM)和傅里叶变换红外光谱分析(FTIR),研究其脱水定型效果与巯基的关系以探讨其加固机理。结果表明:1)MPTES处理后的古代饱水木材样品具有良好的尺寸稳定性(最低3.5%体积收缩),脱水定型效果印证了以往研究中的结果,并进一步得到在低增重率下(约干重的50%)较为理想的效果;2)SEM观察表明硅氧烷均匀地分布于细胞壁内部而非沉积于细胞腔内;3)处理条件(时间、浓度或水分含量)、宏观指标(增重率和收缩率)、显微形貌变化与FTIR图谱中芳醚伸缩振动吸收峰、聚硅氧烷相关吸收峰均呈现明显的相关性。根据上述结果,着重讨论了巯基与木质素反应的机理,认为两者的反应可能与海洋出水饱水木质文物在还原性埋藏过程中有机硫化物的形成过程、造纸工业中硫酸盐制浆法的初期反应原理具有相似性,涉及在酸或碱催化下木质素醌甲基化合物中间体的形成和巯基进一步与该中间体的加成反应,最终得到硫醚的结构。由于MPTES水解后同时具备能与木质素反应的巯基和能与纤维素羟基发生缩合或氢键作用的Si-OH,使该材料具有多样的化学结合能力,能够加强纤维素和木质素自身和互相之间多种界面结合力,提高饱水木质文物中残余组分的整体性和强度,从而获得更好的脱水定型效果。本研究在一定程度上阐明了MPTES加固饱水木质文物后获得良好脱水定型效果的原理,在此基础上进一步认为,在饱水木质文物保护材料引入适度反应性的巯基等基团可进一步提高材料的脱水定型效果,这一思路在未来饱水木质文物保护材料的研发中具备良好的研究前景和应用价值。

Dimensional stability is one of the principal protection goals for waterlogged archaeological wood.Various materials,including natural resins,polyethylene glycol,sugars,glyoxal,etc.,have been developed for dimensional stabilization of waterlogged archaeological wood upon drying.Recent studies demonstrated the excellent performance of organosilicon compounds,among whichγ-mercaptopropyltriethoxysilane(MPTES)gave the best results.However,the interaction between MPTES and waterlogged archaeological wood has not been well explained.A further study on this topic might provide new insights into designing better consolidants for waterlogged archaeological wood.In this paper,archaeological pine wood(Pinus massoniana)from the NanhaiⅠshipwreck was used as the research material,and the effectiveness of maintaining dimensional stability upon drying by MPTES and its mechanism are further explored and discussed through weight percentage gain,shrinkage,observation using SEM and FTIR techniques.The results show that:1)the samples treated by MPTES show desirable dimensional stability(3.5%volumetric shrinkage)and that the effectiveness of MPTES is confirmed at low weight percentage gains(around 50%);2)the resultant silicone is distributed uniformly in cell walls rather than in cell lumina;3)with an increase in treatment duration,the water content,or concentration,the absorption assigned to aryl ether decreased,which is negatively correlated to the weight percentage gains and related to the FTIR absorptions of silicone.On the basis of the above results,the reaction between mercapto group and lignin,as well as the consolidation mechanism of MPTES are discussed.It is proposed that the reaction between the mercapto group and lignin may be similar to the formation of organic sulfur in waterlogged archaeological wood buried in redox condition and the reaction mechanism of the initial step of the Kraft pulping process,which involves the formation of lignin quinone methides,catalyzed by acid or base,followed by the addition of mercapto groups,to form thioether structures.Due to the Si-OH and-SH groups in hydrolyzed MPTES,MPTES has multiple interactions with both lignin and cellulose,which help to build stronger interfacial bonding forces between wood components,and therefore increase the integrity and mechanical strength of waterlogged archaeological wood.In this study,the mechanism of dimensional stabilization of waterlogged archaeological wood upon drying after treatment withγ-mercaptopropyltriethoxysilane is proposed.It is further believed that introducing moderately reactive groups such as mercapto group can help increase the dimensional stabilization ability of WAW consolidating materials and may have promising research and application values in the future design and development of better materials for waterlogged archaeological wood protection.