西加云杉木材单宁铁变色因素及防治

The influence factors and prevention of iron discoloration induced by tannin in the Picea sitchensis wood

为解决西加云杉木材加工中的铁变色问题,研究了西加云杉木材内单宁引发铁变色的因素,并探寻铁变色化学防治方法,为其加工利用提供理论依据。采用CIE-L a b色度体系对西加云杉木材的铁变色现象进行评价,通过控制变量法分别研究木材含水率(40%,60%,80%和饱水状态)、铁离子质量分数(0.01%,0.05%,0.1%,0.5%和1%)、反应温度(25,45,65,85和95℃)及时间(0 min、1 min、5 min、10 min、30 min、1 h、6 h、12 h和24 h)等因素对西加云杉木材表面颜色的影响,并研究了草酸、次磷酸、磷酸二氢钠、乙二胺四乙酸二钠(EDTA·2Na)试剂对西加云杉木材铁变色防治处理的效果。结果表明:以上因素均对西加云杉木材的铁变色有不同程度的影响,其中,铁离子质量分数、反应温度和反应时间等因素都与变色严重程度呈正相关,即铁离子质量分数为0.01%、反应温度为25℃,反应时间为1 min时木材变色程度均最小。当含水率为40%时,变色严重程度相对较低;当含水率高于40%时,变色程度随含水率增加有增大趋势;当含水率为60%左右时,变色程度较为严重;之后,变色程度在一定范围内波动。2%草酸、5%磷酸二氢钠和2%草酸+5%磷酸二氢钠的组合均能有效防治铁变色;3%草酸对消除铁变色有一定效果,当加入0.5%EDTA·2Na螯合剂时效果更优,为避免返色,应选择3%草酸+0.5%EDTA·2Na药剂进行除色处理。当铁离子质量分数较低时,可通过降低木材含水率、避免高温及长时间接触来减轻铁变色现象;通过对其加工表面喷涂2%草酸、5%磷酸二氢钠或2%草酸+5%磷酸二氢钠试剂组合可以有效预防木材铁变色;对于已经发生铁变色的木材,可采用3%草酸配合0.5%EDTA·2Na进行涂刷处理,消除已产生的变色。

To solve the issue of iron discoloration on the surface of Picea sitchensis wood during the processing,the forming mechanism of iron discoloration induced by tannin in wood was analyzed.The prevention and elimination methods of the discoloration phenomena were investigated,which will result in the increase of the value-added utilization of P.sitchensis wood.The CIE-L a b chromaticity system was used to evaluate the iron discoloration phenomena on the wood surfaces.The influence of wood moisture contents(40%,60%,80%and saturated state),iron ion mass ratios(0.01%,0.05%,0.1%,0.5%and 1%),reaction temperatures(25,45,65,85 and 95℃)and reaction time durations(0 min,1 min,5 min,10 min,30 min,1 h,6 h,12 h and 24 h)on wood discoloration were investigated respectively.Disodium ethylene diamine tetra acetic acid(EDTA·2Na),oxalic acid and sodium dihydrogen phosphate were used to prevent the color changes of the wood.Oxalic acid and hyper phosphoric acid acted as the main reagents to eliminate iron discoloration,while EDTA·2Na was as the chelating agent.The reagents used in various combinations could improve the resistance to the color-changing of the wood effectively.The results showed that,the moisture content,iron ion mass ratio,reaction temperature and time had impact on the iron discoloration of P.sitchensis wood.The degree of discoloration was positively correlated to the iron ion mass ratio,reaction temperature and time.When the moisture content was below 40%,the degree of discoloration was low.Further increase of moisture content resulted in the increase of the degree of discoloration,especially when the moisture content was around 60%.When the moisture content was above 60%,the degree of discoloration fluctuated in a certain range.The combination of 2%oxalic acid,5%sodium dihydrogen phosphate or 2%oxalic acid+5%sodium dihydrogen phosphate could effectively prevent iron discoloration.The 3%oxalic acid was a good reagent to individually eliminate the iron discoloration.However,the combination of 3%oxalic acid+0.5%EDTA·2Na had better effect on the removal of iron discoloration.In order to avoid the stain back,the combination of 3%oxalic acid+0.5%EDTA·2Na was the best choice.The tannic iron discoloration of P.sitchensis wood was affected by the wood moisture content,iron ion mass ratio,reaction temperature and time.When the mass ratio of iron ion was low,the discoloration of iron could be alleviated by reducing the moisture content of wood,reaction temperature and time.By spraying 2%oxalic acid,5%sodium dihydrogen phosphate or 2%oxalic acid+5%sodium dihydrogen phosphate on the machined surface of P.sitchensis wood,the iron discoloration could be effectively prevented.When the wood samples were discolored by iron ions,this phenomenon could be eliminated by brushing 3%oxalic acid and chelating agents such as 0.5%EDTA·2Na.