酚醛树脂中亚甲基对热降解的影响

EFFECT OF METHYLENE SUBSTITUENTS ON THE THERMAL DEGRADATION OF PHENOLIC RESIN

将固化后的酚醛树脂在不同温度下进行热处理，对固化后的样品进行热重分析，对热处理后的试样进行傅立叶红外光谱分析．实验结果表明，酚醛树脂的热降解与亚甲基的取代位有关．酚醛树脂中的亚甲基分两个阶段进行热解降，350～450％的温度区间主要是部分邻-邻（0-0’）位亚甲基和邻．对（o-p）位亚甲基的分解，400～620％温度区间为对-对（p-p’）位亚甲基的分解，p-p’位亚甲基比o-o’位亚甲基的起始热分解温度高50％左右．

The phenolic resin was synthesized by condensation of formaldehyde and phenol, with a molar ratio of 1.2,in aqueous solutions containing ammonia at 90℃ for 1.5 h under vacuum dehydration. The synthesized resin was cured at 170℃ for 3 h and then characterized by using TG under nitrogen. The cured resin was heat-treated at 300,350,400,450,500,550 and 600℃ ,respectively and then characterized with FTIR. In the FTIR spectra of the cured samples,the absorption bands of the methylene groups for orth-orth （o-o＇）, orth-para （o-p） and para-para （p-p＇） substituents were at 1437 - 1440 cm^-1, 1452 - 1456 cm^-1 and 1473- 1477 cm^-1, respectively. The variations of the individual absorbance with heat-treatment temperature were compared in terms of the relative absorbance, i. e. Ix/I1610, where I1610 is the absorbance of aromatic ring backbone stretching at 1610 cm^-1 The experimental results show that the thermal degradation of the phenolic resin is significantly related to the substitution position of the methylene groups. The para-substitution of methylene groups was suddenly transferred to the orth-one in some content during the condensation. Otherwise, in the cured process, para-substitution was prior formed because its activity is higher than that of the ortho-one. According to the TG and FTIR spectra, the decomposition process of methylene groups could be divided into two stages. The bond cleavages of o-o＇ and p-p＇ substituents occurred at 350 - 450℃ and 400 - 620℃, respectively. It means the o-o＇ substituent is thermally unstable and then partially cyclized to form xanthene by intramolecular etherification. Also, the benzylic-bonded keto- intermediates could be formed through keto-enol tantomerism. The benzylic bond energy in the keto-form is at least 168 kJ/mol less than that of the original aromatic structure. Therefore, at higher temperatures,the o-o＇ substituent of the methylene groups was prior decomposed. For the p-p＇ substituents, the hydroxyl groups are distant form each other, and the benzylic-bonded keto-intermediate formation is hindered. Finally, the decomposition temperature is higher than that of the o-o＇ substituents.