酚醛树脂中亚甲基对热降解的影响

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(o-o′)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.

Effect of Methylene Substituents on the Thermal Degradation of Phenolic Resin

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.I_x/I_(1610),where I_(1610) 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,parasubstitution 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 tautomerism.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.