Non-isothermal crystallization, yellowing resistance and mechanical properties of heat-resistant nylon 10T/66/titania dioxide/glass fibre composites

Herein, we report novel heat-resistant nylon 10T/66/titania dioxide/glass fibre (nylon 10T/66/TiO2/GF) composites based on as-synthesised nylon 10T/66, which is a copolymer of poly(decamethylene terephthalamide) (nylon 10T). The non-isothermal crystallization behaviors of nylon 10T/66 and nylon 10T/66/TiO2/GF composites were investigated by differential scanning calorimetry (DSC). Jeziorny and Mo equations were used to analyse the crystallization kinetics, whereas the Kissinger method was applied to calculate the activation energy. It turned out that the introduction of TiO2 and GF could accelerate the crystallization of nylon 10T/66 and exhibited an effective heterogeneous nucleation effect. In addition, we conducted yellowing resistance and mechanical property analysis of the nylon 10T/66/TiO2/GF composites. The above results successfully demonstrated that the heat-resistant nylon 10T/66/TiO2/GF composites possess higher crystallization temperature and crystallization rate, whiter color, and better yellowing resistance and mechanical properties than previously as-synthesised nylon 10T/66. Consequently, nylon 10T/66/TiO2/GF composites have great potential to be used as a heat-resistant engineering plastic.


Introduction
Semi-aromatic nylon, combining the superior thermal stability of aromatic nylon and the excellent melt processability of aliphatic nylon, has been widely applied in the eld of electronics (e.g., connectors, surface mount devices and reectors) and automobile parts (e.g., powertrain components). 1,2 Typical commercial semi-aromatic nylon includes poly(hexamethylene terephthalamide) (nylon 6T) copolymers (Dupont, Solvary et al.), poly(nonamethylene terephthalamide) (nylon 9T) (Kuraray) and nylon 10T (Kingfa and Zenong). Among them, nylon 10T is the only bio-based semi-aromatic heat-resistant nylon. 3 However, the melting point of nylon 10T (316 C) is relatively close to its initial decomposition temperature (350 C) which makes conventional melt processing impractical. 4 Consequently, we introduced the aliphatic nylon 66 chains into the backbones of nylon 10T, and obtained nylon 10T/66 copolymer, 5,6 which possesses better melt processability.
However, the yellower color, poor yellowing resistance and mechanical properties limit the application of the neat nylon 10T/66. As we all know, blending is the most common and effective way to improve polymer performance. 7 Glass ber (GF), due to its high strength and low price, has become one of the most extensive reinforcement materials. 8,9 Li et al. investigated the effect of GF addition on mechanical properties of poly(arylene ether nitriles), and found the tensile strength, exural strength and izod impact strength of poly(arylene ether nitriles) were sharply increased in the presence of GF. 10 Titania dioxide (TiO 2 ) is applied as whiteners in a variety of polymeric composiions. 11 Wang et al. added TiO 2 into knitted fabric, which successfully improved its color and yellowing resistance. 12 At present, many papers have studied the mechanical properties of GF reinforced nylon, and lots of researches have reported the improvement of the yellowing resistance of TiO 2 to nylon, however, to the best of our knowledge, there has been no report that GF, TiO 2 and nylon resin are ternary blended to simultaneously improve their mechanical properties and yellowing resistance properties. In this paper, we prepared nylon 10T/66/TiO 2 /GF by blending as-synthesised nylon 10T/66 with GF and TiO 2 . The morphological structures of nylon 10T/66/ TiO 2 /GF were observed by scanning electron microscopy (SEM). In order to investigate the effect of GF and TiO 2 on the crystallization of polymers and better understand the relationship between the structure and properties of materials, it is essential to study the crystallization kinetics, especially the non-isothermal crystallization kinetics, which is closer to the actual processing condition. 13,14 The non-isothermal kinetics parameters of nylon 10T/66/TiO 2 /GF were carried out using the Jeziorny 15 and the Mo equations. 16 The non-isothermal crystallization activation energy was calculated by Kissinger method. 17 In addition, the yellowing resistance and the mechanical properties of nylon 10T/66/TiO 2 /GF composites were also assessed and reported here.

Synthesis of nylon 10T/66
DA10 (172.3 g, 1 mol), PTA (166.1 g, 1 mol), BA (4.6 g, 0.038 mol) and nylon 66 salt (13.1 g, 0.05 mol) were added into an autoclave. To reduce volatilization of diamine during polymerization, boiled distilled water (100 mL) was added. BA is used to control the molecular weight of copolymers. Then, the autoclave was purged with N 2 for 5 min. At a pressure of about 0.4 MPa pressure and a 300 C set point, the mixture temperature remained at 125 C until all water evaporated. This stage promotes the homogenization of the reaction mixture, with almost no polymerization. When the reaction temperature reached 273 C, meanwhile the pressure was up to 2.0 MPa, the heater set point was changed to 330 C and the temperature was allowed to rise and hold at 280 C. Aer allowing to react for 2 h, the pressure of the autoclave was gradually released to atmospheric pressure in 1 h and the reaction temperature of the sample was increased to 320 C. Then the pressure of the autoclave was evacuated to À0.09 MPa. The reaction was continued for another 0.6 h and the copolymers were obtained. In order to improve conductivity, all samples were coated with gold before testing. The surface morphology was performed by cross-section scanning electron microscopy (SEM, MIRA3 FE-SEM, Czech) with an Oxford energy dispersive spectrometer (EDS).

Preparation of nylon
DSC measurements were carried out on a Mettler 822e equipped with a STAR system, and calibrated with an indium standard. All measurements were under a nitrogen atmosphere (50 mL min À1 ). Sample with mass of 3 mg was heated from 25 C to 320 C at a rate of 50 C min À1 and held this temperature for 5 min in order to eliminate the thermal history. Aer that, the sample was cooled to 30 C at different cooling rates of 5, 10, 20 and 30 C min À1 , respectively. Record the nonisothermal crystallization curves as a function of time.
We injected the standard color plates using the injection moulding machine (HF-036). Then plates were put into the blast oven at the set temperature of 180 C for 0 and 2 hours respectively. The CIELAB color parameters were performed under the Color i5 spectrophotometer. In the CIELAB system, the L, b denote the white-black value and yellow-blue of the materials, respectively.
The standard mechanical test samples of nylon 10T/66/TiO 2 , nylon 10T/66/GF and nylon 10T/66/TiO 2 /GF were prepared by an injection molding machine (HF-036). The tensile and bending properties were measured by Universal Testing Machine CMT6104 according to the ISO527 and ISO178. The impact property was tested on the basis of ISO179 by an impact testing machine XJU-22.

Synthesis of nylon 10T/66
The chemical structure of nylon 10T/66 is determined by 1 H NMR spectra (Fig. 1). The chemical shis, in the range of 3.71-3.62 ppm, originate from the protons of methylene adjacent to the NH group (position 1). The peaks at 1.82-1.75 ppm and 1.48-1.34 can be attributed to the position 2 and 3, respectively. The chemical shis at 7.96-7.93 ppm correspond to the aromatic protons (position T). It is worth noting that the chemical shis of nylon 10T/66 at 2.76 ppm and 1.89 ppm correspond to the positions 4 and 5, respectively, which indicates the formation of nylon 10T/66. Aer testing, the melting point of nylon 10T/66 (305 C) is lower than that of neat nylon 10T (316 C), suggesting that nylon 10T/66 possesses better processability.

SEM and EDS analysis of nylon 10T/66/TiO 2 /GF
SEM and EDS images shown in Fig. 2 are applied to evaluate the dispersion of the GF and TiO 2 in the nylon 10T/66 matrix. In Fig. 2A, the GF phases, which are cylindrical, are well wrapped by the nylon matrix. The white substances in Fig. 2B are TiO 2 , and it can be clearly seen that TiO 2 is well dispersed in the matrix. SEM results indicated the excellent compatibility between the TiO 2 , GF and matrix resin in nylon 10T/66/TiO 2 /GF. Fig. 2C and D represent the EDS dot map distribution images of the titanium and silicon in nylon 10T/66/TiO 2 /GF, respectively. As shown in these two gures, both titanium and silicon elements were uniformly distributed in the nylon matrix (silicon element is derived from GF), suggesting that the outstanding dispersion of the TiO 2 and GF in nylon matrix resin, which is similar to the SEM results.

FT-IR analysis
The FT-IR spectrums of different samples are presented in Fig. 3. The absorbance peaks appeared at 3297 cm À1 are compounding to stretching vibration of N-H, and the peaks at 2926 cm À1 and 2857 cm À1 represent asymmetric and symmetrical stretching vibration of -CH 2 respectively. Moreover, the absorption bands of amide can be reected in these spectrums. In detail, the stretching vibration of C]O, C-N and C-CO are observed at 1626, 1380 and 1018 cm À1 respectively, and the in-plane exural vibration of N-H is revealed at 1542 cm À1 . The absorbance peaks appeared at 864 cm À1 and 624 cm À1 are compounding to out-of-plane exural vibration of C-H. In addition, the absorbance peak at 540 cm À1 is caused by TiO 2 . These results of absorbance peaks prove that both TiO 2 and GF are without inuence upon the chemical bonding structure of nylon 10T/66.

Non-isothermal crystallization behaviors of samples.
The non-isothermal crystallization curves of nylon 10T/66, nylon 10T/66/TiO 2 , nylon 10T/66/GF and nylon 10T/66/TiO 2 / GF at various cooling rate are shown in Fig. 4, and the crystallization peak temperatures (T p ) are summarized in Table 2. It could be clearly found from Fig. 4 that for all samples, the crystallization temperature decreases signicantly and the temperature range becomes wider with increasing cooling rate, which is a common phenomenon of semi-crystalline polymers. 7,18 This suggests that the molecular chain does not   have enough time to overcome the barrier to form crystal nuclei and perfect crystals at higher cooling rate. 19,20 According to the Table 2, for a xed cooling rate, the crystallization temperature of neat nylon 10T/66 increases with the addition of TiO 2 and GF, which indicates that TiO 2 and GF play nucleating agent roles in accelerating the crystallization process of neat nylon 10T/66. 13 Moreover, nylon 10T/66/TiO 2 /GF has the highest crystallization temperature, since TiO 2 and GF have a synergistic.
3.4.2 Non-isothermal crystallization kinetics. Considering the effect of TiO 2 and GF on the crystallization behavior of nylon 10T/66, it is necessary to study the non-isothermal crystallization kinetics of nylon 10T/66/TiO 2 /GF. 14 The relative crystallinity of nylon 10T/66/TiO 2 /GF at a certain temperature can be calculated by the ratio of the area of the crystallization curve from the initial crystallization temperature to the crystallization temperature T to the area of the whole crystallization peak.
Generally, the relationship between relative crystallinity and crystallization temperature can be expressed as follows: where T 0 and T N correspond to the initial and end temperatures of the crystallization curve, respectively. And T represents the crystallization temperature at time t. Fig. 5 presents the plots of X t versus T according to eqn (1) for nylon 10T/66, nylon 10T/66/ TiO 2 , nylon 10T/66/GF and nylon 10T/66/TiO 2 /GF. It can be clearly seen that all the curves show reverse S shape, implying that the cooling rate has a retardation effect on the crystallization. In addition, the lower crystallization onset temperature of all samples was obtained along with the increase of cooling rate, which could be attributed to the higher cooling rate causes the molecular chain to have insufficient time to start crystallization at a higher temperature.
The crystallization time and crystallization temperature can be transformed by the following formula: where F is the cooling rate. Combining eqn (1) and (2), we got the X t ¼ f(t) curves (Fig. 6). The time corresponding to the 50% relative crystallinity (t 1/2 ) can be obtained from the Fig. 6. And the crystallization rate G ¼ 1/t 1/2 , which is listed in Table 2. 21 According to the theory of crystallization kinetics, the bigger value of G means the faster crystallization rate. 22 As presented in Table 2, for each sample, t 1/2 decreases and G increases gradually as the cooling rate increases, indicating faster crystallization rate at the high cooling rate. Interestingly, at the same cooling rate, with the addition of GF and TiO 2 , the crystallization rate increases, which further conrmed they play important roles in the hetero-geneous nucleation. Moreover, the fastest crystallization rate of nylon 10T/66/TiO 2 /GF could be ascribed to the synergistic of the TiO 2 and GF. As for the crystallization rate of the nylon 10T/66/TiO 2 is faster than nylon 10T/66/GF, it is because that the smaller particle size of TiO 2 promotes heterogeneous nucleation to a greater extent.

Jeziorny equation.
In order to study the crystallization mechanism of the materials, some equations were choosen to analysis the crystallization kinetics. Generally, Avrami equation (eqn (3)) is one of the most common methods for investigating the isothermal crystallization processes. 19 Taking the logarithms of both sides, we can obtain: where n is Avrami index and Z t is the crystallization rate constant. Regrettably, Avrami equation is unsuited for describing non-isothermal crystallization process. 23 In order to analysis the non-isothermal crystallization process, Jeziorny modied Z t with the cooling rate, and the modied equation is as follow: 24 where Z c denotes non-isothermal crystallization rate constant. On the basis of eqn (4), the plots of lg[Àln(1 À X t )] against lg t at different cooling rates are shown in Fig. 7. The n and Z c can be determined from the slope and intercept, and they are presented in Table 3. As listed in Table 3 where a ¼ n/m (n, Avrami exponent and m, Ozawa exponent) and F(T) represents the cooling rate required to reach a relative crystallinity at a unit time. 29 Many studies have identied that Mo equation could well describe the non-isothermal crystallization behaviors of polymers, such as the poly(vinyl alcohol)/ starch composite, 30 poly(ethylene terephthalate) composites 31 and poly(butylene succinate) (PBS). 20 Fig. 8 shows the curves of lg F versus lg t of nylon 10T/66, nylon 10T/66/TiO 2 , nylon 10T/ 66/GF and nylon 10T/66/TiO 2 /GF with a series of straight lines (the correlation coefficient R 2 is shown in Table 4, they are all less than 0.99), which indicates the Mo equation is also suitable for analyzing the non-isothermal crystallization behaviors of these nylons. The a and F(T) could be obtained from the slopes and the intercepts, and the values are listed in Table 4. Usually, the bigger F(T) value means the slower crystallization rate. 22 It can be clearly seen from Table 4 that the values of F(T) increases with the relative crystallinity, which indicates that if we want to reach a large relative crystallinity within a certain period of time, we must achieve this by increasing the cooling  rate. For the same relative crystallinity, the F(T) values of nylon 10T/66/GF and nylon 10T/66/TiO 2 are both lower than that of nylon 10T/66, and the value of nylon 10T/66/TiO 2 /GF is the lowest. These indicate that nylon 10T/66/GF and nylon 10T/66/ TiO 2 have faster crystallization rate than nylon 10T/66, and nylon 10T/66/TiO 2 /GF has the fastest crystallization rate, which are in perfect accordance with the results of the Jeziorny analysis. 3.4.5 Crystallization activation energy. Activation energy is an important parameter in the phase transition process, which is related to the energy and barrier of phase transition. 32 Activation energy could effectively reect the crystallization ability of polymers. 33 Kissinger 34 is the most common method for calculating crystallization activation energy. 35 where E denotes the activation energy. The plots of ln(F/T p 2 ) versus 1/T p is shown in Fig. 9. The activation energy could be calculated from the slope of the plots (see Table 5). As listed in Table 5, the R 2 of these samples are less than 0.97, which mean that the lines have a good linear relationship. It also can be clearly found that adding GF and TiO 2 reduces the E value and the E value of nylon 10T/66/TiO 2 /GF is the lowest. In other word, adding GF and TiO 2 improves the crystallization rate of nylon 10T/66 and nylon 10T/66/TiO 2 /GF has the most fast crystallization rate.

XRD analysis
The XRD patterns of different samples are showed in Fig. 10. It can be clearly seen that broad diffraction peaks which belong to     Fig. 11 is the color plates the nylon 10T/66, nylon 10T/66/TiO 2 , nylon 10T/66/GF and nylon 10T/66/TiO 2 /GF aer 0 or 2 hours of thermal oxidation aging at 180 C. It can be clearly seen from Fig. 11 that the addition of TiO 2 can signicantly improve the whiteness of nylon 10T/66 whether 0 or 2 hours of thermal oxygen aging. Although the whiteness of nylon 10T/66/TiO 2 /GF has a small decrease compared with nylon 10T/66/TiO 2 , it still has a signicant improvement over nylon 10T/66. Table 6 shows the comparison of L and b values for different samples. The larger the L and the b values, the whiter and yellower color of the material, respectively. 36,37 As shown in Table 6, nylon 10T/66/TiO 2 /GF has lager L values and smaller b values than nylon 10T/66 whether it is 0 or 2 hours of the hot oxygen aging, which means that the color of the nylon 10T/66/TiO 2 /GF is whiter. This result is same as the visual observation of Fig. 11.

Color comparison and yellowing resistance
In addition, the degree of yellowing, an important indicator of the heat-resistant plastics, could be well reected by the magnitude of the change in Db value, which is equal to b (2 h) minus b (0 h). As shown in Table 6, the Db value of nylon 10T/66/ TiO 2 /GF is signicantly lower than that of neat nylon 10T/66, indicating that nylon 10T/66/TiO 2 /GF has better yellowing resistance neat nylon 10T/66.

Mechanical properties
The mechanical performance parameters of the nylon 10T/66, nylon 10T/66/TiO 2 , nylon 10T/66/GF and nylon 10T/66/TiO 2 / GF are summarized in Table 7. It can be observed that the tensile strength, elongation at break and impact strength of   nylon 10T/66 decreased with the addition of TiO 2 . However, the tensile strength, exural strength and exural modulus are obviously improved by introducing GF, while the elongation at break and impact strength are comparable to nylon 10T/66. These phenomena are attributed that the addition of GF promotes the crystallization and increases surface fracture energy of nylon. Also, it is due to the rigidization effect of GF within the nylon and higher modulus of GF as compared with nylon. 8,9 Clearly, although the mechanical properties of nylon 10T/66/TiO 2 /GF are lower than those of nylon 10T/66/GF, there is still a signicant increase compared with neat nylon 10T/66.
The crystallization curves showed that nylon 10T/66/TiO 2 /GF has a higher crystallization temperature than nylon 10T/66. The crystallization rate G, Jeziorny and Mo analysis revealed that nylon 10T/66/TiO 2 /GF has higher crystallization rate. The Kissinger method was used to calculate the non-isothermal crystallization activation energies, indicating that the E value of nylon 10T/66/TiO 2 /GF is lower than nylon 10T/66. All these results can be attributed to that GF and TiO 2 play strong crucial roles in the heterogeneous nucleation. The color comparison and mechanical properties showed that the yellowing resistance and mechanical properties of nylon 10T/66/TiO 2 /GF were better than nylon 10T/66. The nylon 10T/66/TiO 2 /GF composites possesses higher crystallization temperature, crystallization rate, whiter color, better yellowing resistance and mechanical properties, and has promising applicability in the eld of LED lights.

Conflicts of interest
There are no conicts to declare.