Curing and thermal property of boron-containing o-cresol formaldehyde resin

Xia Liya, Gao Jungang, Yu Zhenxia
(Supervision Institution of Quality&Technology, College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)

Received Dec. 7, 2003.

Abstract The curing and thermal degradation process of boron-containing o-cresol formaldehyde resin (BOCNR) was studied by infrared spectroscopy (IR) and thermogravimetry analysis (TGA). The results show that hexatomic ring containing coordinate linkage of boron-oxygen formed in the curing process of BOCNR, and the coordinated oxygen atom was offered by phenol hydroxyl. TGA results show that BOCNR have good thermostability and its degradation processes can be divided into three stages. In the second and third stages, the decomposition reactions are all following first mechanism function.
Keyword Boron-containing phenol-formaldehyde resin, o-cresol, Boric acid, Thermal analysis

1. INTRODUCTION
The boron-containing phenol-formaldehyde resin is a modified phenolic resin, which is obtained by introducing boron to the main chain of common phenol-formaldehyde resin. This resin can be converted into a three-dimentional cross-linked thermoset network by self-cross-linking reaction during curing process, so it has many excellent performances, such as thermostability, mechanical strength, electric properties and defence of neutron radiation. It is suitable for manufacturing laminated and moldable composite materials, insulated materials, ablation and abrasion resistant materials. With the variation of the raw materials used in the synthesis process, various type of boron-containing phenol-formaldehyde resins have been reported, such as boron-containing phenol-formaldehyde resin (BPFR), boron-containing bisphenol-A formaldehyde resin (BBPFAR) ^[1-4]. While the synthetic structure and thermal properties of boron-containing o-cresol formaldehyde resin (BOCNR) have not been investigated.
    In this work, the structure changes of BOCNR during curing were monitored by Fourier-transform infrared (FTIR) spectrometry; the weight changes and degradation kinetics were studied by thermogravimetry analysis (TGA).

2. EXPERIMENTAL
2.1 Materials
O-cresol (OCN), Boric acid, 37% aqueous formalin, acetone and sodium hydroxide were all analytically pure grade, which were supplied by Tianjin Chemical Reagent Co. of China.
2.2 Synthesis of BOCNR
O-cresol, aqueous formalin and NaOH were introduced into a three-necked flask, equipped with a stirrer, a thermometer and a condenser. The mixture was stirred and heated to 70^oC, then the reaction was maintained at this temperature for 1h. When the water was removed in vacuum, salicylalcohol of o-cresol was obtained. In the second step, boric acid was added to this system, heated to 102-110^oC and held the temperature in the above range for 45min .Then the water formed in the reaction was removed in vacuum. Finally the yellow solid BOCNR was obtained.
2.3 Infrared spectrum analysis
A Fourier-transform infrared (FTIR) spectrometer (Bro-Rad FTS-40 USA) was used to investigate the structure changes of the BOCNR during the curing and thermal degradation. The BOCNR was dissolved in acetone and then coated as a thin film on a potassium bromide plate. When the solvent in the film had completely evaporated in vacuum, the potassium bromide plate was scanned by the FTIR instrument. Then it was scanned after being cured at different temperatures. The principal absorption bands appear as follows ^[4,5]: the benzene ring is at 1600cm^-1, the borate B-O is at 1350cm^-1, phenol hydroxyl C-O is at 1250cm^-1, the -CH₂- group appears at 1450cm^-1, methylol group is at 1020cm^-1, ether linkage C-O is at 1100cm^-1, carbonyl group is at 1650cm^-1. Quantitative analysis was doing according to the literature ^[5]. The benzene ring absorption at 1600cm-1 was used as internal standard. According to the Beer-lambert law A=lgI/I0, the ratios of absorbance A1350/A 1600（borate value）, A1250/A1600 (phenol hydroxyl value), A1100/A1600 (ether value), A1020/A1600 (methylol value), A1650/A1600 (carbonyl value) were obtained.
2.4 Thermal analysis
A Shimadzu TGA-40 JP thermogravimetric apparatus was used to determine the weight loss behaviour of BOCNR during degradation. About 8mg BOCNR powder cured at 180℃ for 4h was introduced into the thermo-balance, then heated to 900^oC at 10^oC/min heating rate in air.

3. RESULT AND DISCUSSION
3.1. Structure of boron-containing o-cresol formaldehyde resin
The process of synthesizing BOCNR by the method of formalin was divided into two steps. Salicylalcohol of OCN was formed in the first step, methylol groups were mainly at ortho and para positions of the phenyl ring [6], and then it reacted with boric acid. According to the literature [3], the reactivity of methylol group with boric acid was higher than that of phenol hydroxyl. So in the second step, the reaction of boric acid with methylol group is prior to that of boric acid with phenol hydroxyl. The reaction can be described as Scheme 1.

    Fig.1 and Table 1 show the IR absorption variation of BOCNR during the curing reaction. As it can be seen from Fig.1 and Table 1, below 160^oC, the absorption of borate B-O linkage was increased with the rising of curing temperature, while, the absorption of methylol groups and phenol hydoxyls were decreased. This is caused by the reaction between methylol group and phenol hydroxyl group with unreacted -OH groups in boric acid. Since most of methylol groups had been reacted in the synthesizing process, the reaction of phenol hydroxyl group with unreacted -OH groups inboric acid is the main reaction. This can also be proved by the disappearance of phenol hydroxyl group at 160^oC. So the main reaction in the curing process is described as Scheme 2.

Table1 Changes of functional group values of BOCNR during curing process

    According to the literature [1,3 ], in the curing resin, when the hexatomic ring containing B←O coordination bond formed, the IR absorption band of B-O borate at 1350cm-1 would disappear. As shown in Fig.1 and Table 1, the borate value decreased after cured at 160^oC. This showed that the hexatomic ring structure containing B←O coordination bond formed at higher temperatures and the coordinated oxygen was offered by phenol hydroxyl because most of methylol groups had been reacted. The reaction and final molecular structure may be described as Scheme 3.

3.2 Thermal stability and degradation kinetics of BOCNR
As shown in Fig.1, the spectra had an ether linkage absorption peak at 1050cm^-1, formed from condensation reaction of benzyl hydroxyl groups. The ether linkage was oxidized at higher temperature to form a carbonyl group. The carbonyl value of uncured resin is only 0.06, however, after it was cured at 160^oC, the carbonyl value is increased to 0.18. The reaction is described as Scheme 4.


    Fig.2 shows the IR absorption variation of BOCNR during thermal degradation. As it can be seen from Fig.2, with the rising temperature, the absorption peak of ether linkage at 1100cm-1 decreases first, then the absorption of carbonyl group decreases gradually. At 300^oC about 1h, the absorption of carbonyl group disappears, while the absorption of -CH₂- (at 1430cm^-1) and the benzene ring are very strong. So ether linkage and carbonyl group in the resin intensely affect the thermal stability of BOCNR.
    A Shimadzu TGA-40 thermogravimrter was used to determine the weight loss behaviour of BOCNR. As shown from Fig.3, the common phenol-formaldehyde resin (PFR) has higher weight loss rates than that of BOCNR. The weight loss for common PFR is over 99% at 580^oC, while the BOCNR is only 34.7 % at 580^oC. The temperature of semi-weight loss is about 227^oC higher than that of common PFR, and the start temperature of weight loss (280^oC) is about 50^oC higher than that of PFR. The thermostability of BOCNR is close to that of boron-containing bisphenal-A formaldehyde resin (BBPFAR)^[3] and BPFR^[7]. The start temperatures of weight loss of BBPFAR and BPFR are 310^oC and 315^oC respectively. And the semi-weight loss of BBPFAR and BPFR are all at 580^oC which are lower than that of BOCNR.
    According to the TGA curves (Fig.3), the degradation process can be divided into three stages. In the first stage (about 260-417^oC), the total weight loss for BOCNR resin at the 10^oC/min heating rate is about 4.56% , which is caused by the evaporation of water and small molecules. In the second stage (417-580^oC) and third stage (580-900^oC), the weight losses are 30% and 32% respectively. Related with structure changes shown in Fig.2, the weight loss in the second stage may be caused by the oxidation and breakage of ether linkages and carbonyl groups. In the third stage, -CH₂- group、borate B-O linkage、benzene ring may be oxidized and broken.
    The following kinetic equation was assumed to hold for the reaction ^[8,9]

where A is the pre-exponential factor in the Arrhenius equation, E is the apparent activation energy, R is the universal gas constant, j is the heating rate, T is absolute temperature, and G(a) is the integral form of the conversion dependence function. The correct form of G(a) depends on the proper mechanism of the decomposition reaction ^[9]. Different expressions of  G(a) for some solid-state reaction mechanisms can be described as follows: first order, G(a) is -ln(1-a); second order, G(a) is 1/(1-a); third order, G(a) is 1/(1-a)² .
    According to the above equation, the activation energy can be obtained at different heating rates from fitting the ln[G(a)/T] versus 1/T plots. For different degradation stages, the apparent activation energies and pre-exponential factors were all tested for different mechanism functions. The results are listed in Table 2.

Table 2 Kinetic parameters of thermal degradation of BOCNR for different mechanism function at 10^oC/min heating rate

    As shown in the Table 2, for the same degradation stage at a given heating rate, the correlation values for different mechanisms are different. According to the principle that the probable mechanism has high correlation coefficient value and low standard deviation value, the mechanism function and other kinetic parameters can be obtained, and the results are listed in Table 3.

Table 3 Kinetic parameters of thermal degradation of BOCNR at 10^oC/min heating rate

4. CONCLUSIONS
During the curing process of BOCNR, borate B-O group and hexatomic ring containing coordinate linkage of boron-oxygen was formed and the coordinated oxygen atom was offered by phenol hydroxyl. Thermal degradation of BOCNR begins with the oxidation and breakage of ether linkage and carbonyl group. The concentration of phenol hydroxyl, methylol group and carbonyl group in the cured resin is the most important factor that affected the thermostabilitis of BOCNR. The thermostabilitis of BOCNR are more excellent than that of common PFR.

Fig.3 Thermogravimetric analysis (1) BOCNR, (2) PFR, at heating rate of 10^oC/min in air

    The TGA results show that the decomposition process of BOCNR can be divided into three stages, and in the second and third stage the decomposition reactions all follows first reaction order.

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