木质素改性脲醛树脂对竹层积材甲醛释放量及胶合性能的影响

Effect of lignin modified urea-formaldehyde resin on the formaldehyde emission and bonding performance of bamboo laminated timber

针对竹层积材中甲醛释放量过高的问题,以碳化竹片为原料,利用木质素作为脲醛树脂胶黏剂的甲醛捕捉改性剂,对脲醛树脂胶黏剂进行共混改性后压制双层竹层积材。采用木质素添加量和组坯方式的双因素分析法,探讨木质素改性脲醛树脂胶黏剂对竹层积材甲醛释放量及胶合性能的影响;采用环境扫描电子显微镜(ESEM)对木质素改性后的竹层积材胶合界面进行微观形貌分析。木质素的加入使竹层积材的甲醛释放量明显降低,各组坯方式下竹层积材的甲醛释放量差异较小,均可达到GB 18580—2001标准规定的E2级;随着木质素添加量的增加,竹层积材的剪切强度逐渐增大,竹黄面与竹黄面无节组坯试件(II)、竹黄面与竹青面无节组坯试件(IO)及竹黄面与竹黄面有节组坯试件(Node)的剪切强度均在木质素添加量为40%时达到最大,分别为7.6,8.0和8.5 MPa,相比空白组分别提高了85%,70%和41%;对于竹黄面与竹黄面组坯试件,带节试件的胶层剪切强度大于无节组;由ESEM可知,碳化竹材胶合界面被压缩甚至压溃,表面细胞不同程度呈扁平碎片状,胶黏剂主要渗透到竹材的表层破坏细胞,多数为薄壁细胞,位于竹材表层的维管束中偶尔也会有胶黏剂存在,极少量胶黏剂可能通过裂隙进入竹材更深部位的细胞。结果表明:在相同木质素添加量条件下,组坯方式对竹层积材甲醛释放量几乎没有影响;黄-黄无节组坯试件、黄-青无节组坯试件及黄-黄有节组坯试件胶层剪切强度随着木质素添加量的增加逐渐增大,黄-青组坯试件的胶层剪切强度普遍优于黄-黄组坯试件,黄-黄有节组坯试件胶层剪切强度均大于无节试件;ESEM分析表明,由于碳化竹片表面易被压缩压溃,木质素含量较大的高黏度胶黏剂缺乏有效渗透,致使胶黏剂集聚在压溃细胞表面,竹片胶合界面有效胶层厚度增大,从而导致胶层剪切强度增加。

In order to reduce formaldehyde emission in bamboo laminated timber, the lignin was used as a formaldehyde catching agent to modify urea-formaldehyde （UF） resin, and two-ply bamboo laminated timber was fabricated by using the modified UF-lignin resin with carbonized bamboo strips. The lignin contents and assemble patterns of bamboo strip were analyzed by two-factor analysis method, aiming to investigate the impact of the modified UF-lignin on formaldehyde emission and bonding properties of the bamboo laminated timber. To examine the penetration of the modified UF-lignin resin in the interface, the microscopy of bonding line of the bamboo laminated timber was characterized by the environmental scanning electron microscope （ESEM）. The formaldehyde emission of the bamboo laminated timber reduced obviously with the addition of lignin, while different assemble patterns had slight influence on formaldehyde emission. However, all of them reached the E2 level according to GB 18580-2001. The shear strength of bamboo laminated timber increased with the increase in lignin content. The shear strength of the inner-inner pattern （IO） specimen, the inner-outer （IO） specimen and the inner-inner pattern with node （Node） specimen reached the maximum of 7.6, 8.0 and 8.5 MPa, respectively, when the lignin content was 40%, which increased by 85%, 70% and 41%, respectively, compared with those of control groups. The shear strength of inner-inner pattern specimens with nodes was greater than those of the specimens without nodes. ESEM results showed that the bonding interface of carbonized bamboo was compressed or even crushed, the surface cells showed flat pieces in some degrees, and the adhesive was penetrated into damaged cells. It was also observed that there were mainly parenchyma cells on the bamboo surfaces, and the adhesive was occasionally found in the vascular bundle located on the bamboo surfaces, while a little fraction of adhesive may penetrate into the cell in deeper part of bamboo through the slit. The results re- vealed that the assembly pattern had slight impact on bamboo laminated timber under the same lignin content. With the increase in lignin content, the shear strength of the inner-inner pattern specimen, the inner-outer specimen and the in- ner-inner pattern with node specimen increased. The shear strength of the inner-outer specimen was generally greater than that of the inner-inner pattern specimen, and the shear strength of inner-inner pattern with node specimen was greater than that of the specimen without node. The ESEM results showed that the carbonized bamboo surface was easy to crush, and the adhesive with high lignin content and high viscosity lacked effective permeability. As a result, the adhesive was concentrated in the crushing cell, the thickening bond line was formed on the bamboo surfaces, and the bonding properties were improved.