纸张的力学性能以抗张强度为最主要的表征指标, 按照GB/T 12914—2008《纸和纸板抗张强度的测定》的要求, 通常需要使用抗张拉力仪进行测试, 但这种方法所需样品尺寸较大(长度大于180 mm, 宽度约15 mm)、 损耗样品量较多(平均消耗10个样品能获得1个有效数据), 不适合用于测试珍贵的纸质文物。 利用静态热机械分析仪(TMA)微损取样、 灵敏精确的特点, 首次将其用于纸质文物的力学测试, 尝试建立微小取样测试的新方法。 以故宫棚壁糊饰修复用手工竹纸为研究对象, 依据故宫乾隆花园的实际环境监测结果设计老化条件, 对手工竹纸进行紫外、 干热和湿热加速老化实验, 采用TMA测试老化前后竹纸样品的抗张强度, 结合SEM、 红外光谱分析样品的微观形貌与化学结构的变化情况, 分析竹纸的老化特点。 结果表明: TMA能够有效测出老化后强度较低的手工竹纸的力学强度, 其中, 湿热老化后竹纸保留的抗张强度最高(上升1.01%), 干热老化次之(下降15.11%), 紫外老化后强度最低(下降63.85%); 通过SEM观察到湿热、 干热老化样品的纤维结构无明显变化, 紫外老化样品的纤维结构出现明显断裂破损; 红外光谱分析表明紫外组样品在1 715 cm-1出现吸收峰, 纤维素氧化生成羰基, 900~1 200 cm-1处纤维素指纹区的一些吸收峰强度变弱, 纤维素的糖苷键大量断裂, 说明该组样品纤维素降解程度最为严重, 纸张强度下降程度最为显著; 三种测试结果能够相互印证。 除此之外, TMA所需试样尺寸仅在毫米级别(长度10~20 mm、 宽度约1 mm), 远小于常规拉力仪试样尺寸, 测试精度高, 仅需约7个样品就能达到较好的重复性, 所需样品量仅为常规拉力仪方法的0.5%, 是一种微损精确的测试方法。 研究表明TMA对低强度纸张具有良好的适用性, 有望将其开发成脆弱纸质文物力学性能表征的新方法。

The mechanical properties of paper are mainly characterized by tensile strength. According to GB/T 12914—2008, this indicator is usually tested with a tensile machine. However, this method requires a large sample size and many samples, which is unsuitable for precious paper cultural relics. In this work, Thermomechanical Analyzer (TMA) is used for mechanical testing of paper cultural relics for the first time to establish a new evaluation method by taking advantage of its precise and nondestructive feature. The environmental changes in the Qianlong Garden of the Palace Museum were monitored, and the aging conditions were designed based on the monitoring results. UV, dry-heat, and humid-heat accelerated aging experiments were carried out on the handmade bamboo paper, which is usually used to restore Pengbihushi in the Palace Museum. TMA, SEM and FTIR were used to test the changes in tensile strength, micro-morphology, and chemical structure of bamboo paper samples before and after aging to analyze the aging characteristics of bamboo paper. The results show that TMA can effectively measure the mechanical strength of bamboo paper with low strength after aging. The tensile strength of bamboo paper after humid-heat aging was the highest (up 1.01%), followed by dry-heat aging (down 15.11%), and after UV aging the lowest (down 63.85%). Correspondingly, it was observed by SEM that there was no obvious change in the fiber structure of the humid-heat and dry-heat aging samples, but the fiber structure of the UV aging samples showed obvious fracture and damage. Infrared spectroscopy analysis also showed that the UV aging sample had the lowest absorption peak in the cellulose fingerprint region, indicating that the cellulose degradation was the most serious. The three test results can confirm each other. In addition, the size of the sample required by TMA is only at the millimeter level (10~20 mm in length, about 1 mm in width), which is much smaller than that of a conventional tensile machine. Moreover, the TMA test only requires about 7 samples to achieve good repeatability and accuracy. TMA has good applicability to low strength paper, so it is expected to explore further the possibility of applying TMA to precious ancient paper with extremely low strength.