Thermal properties of boron-bitrogen containing phenol-formaldehyde resin

Abstract The boron-nitrogen containing phenol formaldehyde resin (BNPFR) was synthesized from phenol, formaldehyde, boric acid and ammonia water. The thermal properties of BNPFR were determined by the methods of torsional braid analysis (TBA) and thermogravimetric analysis (TGA). The results show that BNPFR have a T_g about 210ºC and good thermostability. The thermal degradation kinetics of BNPFR was investigated by TGA. It is illustrated that the thermal degradation reaction of the BNPFR follows first-order mechanism, and the half-weight loss temperature is about 150ºC higher than general phenol-formaldehyde resin.
Keywords Phenol-formaldehyde resin, Boric acid, Nitrogen, thermal analysis

Table 1 Material Proportion and Serial Number of BNPFR

    The molecular structure of BNPFR has the following approximate form:

2.3 Torsional braid analysis (TBA)                 
Specimens, prepared by dipping heat-cleaned glass fiber braid in the tetrahydrofuran solution of the BNPFR, were completely evaporated in vacuum.
1. The air oven was first heated up to a desired temperature and kept for a certain period of time until the system reached the equilibrium state. Specimens were quickly set into the thermostatic baths at a curing temperature T_c between 140 and 220ºC. After curing for 1h, respectively, they were taken out and cooled to room temperature, then their T_gs were determined by TBA with a heating rate of 2ºC/min.
2. Uncured resin was inserted into the GDP-4 TBA analyzer to determine the available curing temperature of different boron-nitrogen proportion resins.
2.4 TGA measurements               
After curing at 180ºC for 4h, the thermal analysis was carried out on the Shimadzu DT-40 thermogravimetric analysis (TGA). About 8mg of the sample, which had been completely cured and ground to fine powers, was put into a platinum cell and placed on the detector plate, then the furnace was heated at a heating rate of 10ºC/min to 800ºC.

3. RESULTS AND DISCUSSION
3.1 T_g value and the curing process                    
Generally, the T_g of a crosslinked resin system is related to the conversion, which depends upon the curing conditions, such as temperature and time. With these curing condition varying, the T_g of the system will be changed. So that T_g has been used directly as a parameter for conversion analysis of curing reaction. Because of the T_g of thermosetting resin is easy to be measured by TBA, it is particularly useful for the measurement at high conversion and after vitrification. In this paper, the T_g values are measured for the BNPFR specimen cured isothermally at different temperatures for various periods. Different glass transition behaviors occurred for the 2.2N resin samples with different degrees of curing temperature were shown in Table 2. As seen from Table 2, the BNPFR cured at lower temperature have a low T_g compared with those cured at higher temperature. It is because that during the curing process, cross linking reaction has occurred and the weight-average molecular weight increases, the crossing linking density increases and the mobility of the chains decreases, the chances for the molecules to collide with each other are reduced and the intermolecular reaction at this low temperature is limited. But the samples have a higher curing degree at higher temperature and the T_g value get higher. If the resin was cured at 220ºC, there is no loss modulus peak appeared in TBA curves from 20ºC to 280ºC, this is due to that BNPFRs have good heat-resistance.

    To determine the influence of boron-nitrogen proportion on T_gs, the TBA experiment of uncured resins were carried out, the results are shown in Fig.1, where E is the modulus and F is the mechanical loss tangent. As seen from Fig.1, at low temperature, the resin has not been cured, the modulus is low, and with the temperature rising the resin begins to be cured, the modulus increases. When the temperature approaches a finite value, the resin was completely cured, the modulus does not change during a certain temperature range. From Fig.1 and Table 3, it can be seen that there are the same T_g to BNPFRs of different boron-nitrogen proportion, which illustrate that nitrogen content basically have no influence on T_g of BNPFRs.

Fig.1 The TBA curve of BNPFR: (1) 1.0N, (2) 2.2N, (3) 5.5N, at heating rate of 2ºC/min.

Table 3 the T_g of different resins cured at 200ºC for 1h

3.2 Thermal stability and degradation kinetics
It can be seen from Fig.2, that common formaldehyde resin (PFR) has higher weight loss rate than boron-nitrogen containing phenol-formaldehyde resin. The weight loss for common PFR is over 90% at 600ºC, while the BNPFR is only 32% at the same temperature. The start temperature of weight loss is at about 420ºC, higher than that of common PFR, and the temperature of half-weight loss is about 150ºC higher than that of PFR. It is due to the formation of phenol borate linkage, which reduces the formation of ether linkage, so that the heat- resistance of BNPFR has increased greatly.
    As can be seen from Fig.2, the resins with B-N containing have the same weight loss before 320ºC, which is about 3%. We deem that it is caused by the evaporation of water and small molecules and it has not been considered as the degradation kinetics. The weight loss of BNPFR resins increased obviously from 420ºC, but the common resin decomposed distinctly from 320ºC, which indicates that the heat-resistance of common resin is inferior to BNPFR. According to the TGA curves (Fig.2), the degradation process can be divided into two stages. In the first stage (about 420-600ºC), the weight loss is about 20%, it may be caused by the oxidation and breakage of some ether linkages and carbonyl groups. In the second stage (about 600-800ºC), the weight loss is about 55%, it is due to most ether linkage, carbonyl group, some borate B-O linkage and benzene ring may be oxidized and broken in this stage.

Fig.2 Thermogravimetric analysis of PFR (1) common PFR, (2) 5.5N, (3) 2.2N, (4) 1.0N, at heating rate of 10ºC/min in air.

    To determine the kinetic parameters of the decomposition from the thermo- gravimetric data, the first step is to evaluate the conversion of the reaction. In dynamic TGA experiments, the weight change of the sample is regarded as a function of temperature and the conversion can be expressed as
            Eq. (1)
    where is the sample weight in i stage, is the residual weight of at temperature T. Therefore, with TGA curves and Eq. (1), the conversions are calculated for different degradation stages ^[7,8].
    In this work, considering the multi-stage degradation process of BNPFR, the TGA data were analyzed on the basis of the Madhusdanan-Krishnan-Ninan method ^[8], which can be expressed by the following equation:
                           Eq. (2)
    Where A is the pre-exponential factor in the Arrhenius equation, E is the apparent activation energy, R is the universal gas constant, f is the heating rate, T is absolute temperature, and is the integral form of the conversion dependence function. Where . The correct form of depends on the proper mechanism of the decomposition reaction ^[8]. Different expressions of for some solid-state reaction mechanisms can be described as the following: in first order, is ; in second order, is ; in third order, is .
    According to the above equations, the activation energy can be obtained at different heating rates from fitting versus plots. For different degradation stages, the apparent activation energies and pre-exponential factors were all tested for different mechanism functions. Some results are listed in Table 4.

Table 4 The kinetic parameters of thermal degradation of 2.2N resin for different mechanism function at 10ºC/min heating rate

4. CONCLUSION
1. TBA can be used effectively to study the curing process of BNPFR and determine T_g.
2. The N content has little influence on T_g and the proper curing temperature of BNPFR is 200ºC. The T_g of BNPFR is at about 210ºC.
3. The modulus of the resin increases with the curing temperature increasing, and holding a certain value during a temperature range.
4. The thermal degradation process of BNPFR can be divided into two stages, and the thermal decomposition kinetics all follows the first reaction order.
5. The thermo-stability of BNPFR is better than common PR.