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 I1610is 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 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.