Weidi Hea,
Nian Liua,
Xiaolang Chen*ac,
Jianbing Guo*b and
Tao Weib
aKey Laboratory of Advanced Materials Technology Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China. E-mail: chenxl612@sina.com
bNational Engineering Research Center for Compounding and Modification of Polymer Materials, Guiyang 550014, China. E-mail: guojianbing1015@126.com
cThe State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China
First published on 5th April 2016
In this article, the influence of relatively long hygrothermal ageing on long glass fibre reinforced polyamide 6 (LGF/PA6) composites filled with sepiolite (Sep) are analyzed. The composites with different weight fractions of Sep are exposed in a hygrothermal environment of 80 °C and RH 95% for 0–20 days. The mechanical properties with different ageing time show a decrease in tensile strength and an increase in impact strength. The scanning electronic microscopy (SEM) photos of the impact fracture indicate better adhesion of the interface between the glass fibre and polymer matrix in the composites filled with Sep after the ageing process. Then the thermal stability and degradation kinetics of the composites with different filler fractions and ageing times are studied by thermal gravimetric analysis (TGA) with the methods of Kissinger, Friedman and Flynn–Wall–Ozawa in dynamic measurements. Calculated apparent activation energy (E) by the three methods shows that the composites without Sep have an obvious decrease in the value of E after ageing, however, the E values of the composites filled with Sep have a slight increase.
Sepiolite (Sep) is a family of fibrous hydrated magnesium silicate with the theoretical half unit-cell formula Si12O30Mg8(OH)4(OH2)4·8H2O, similar to the 2:
1 layered structure of montmorillonite, consisting of two tetrahedral silica sheets enclosing a central sheet of octahedral magnesia.6–8 These layers create a unique paralleled tunnels structure which has a high specific surface area and pore volume to accommodate exchangeable ions, radicals and water molecules and provide an ion exchange ability and water absorption capacity.9 Because of the hollow structure and the excellent absorption capacities, Sep has some advantages of low weight, high intensity and low price to be widely used in polymer matrix composites.10 In recently, some progress has been achieved in the organic modification of Sep by surfactant and coupling agent grafting.11–13
It has been well established that the properties of engineer plastics are affected in different environment factors. However, there is lack of information on the LGF reinforced PA6 composites for serving in hygrothermal environment, especially with regard to the thermal degradation kinetics after ageing. In this work, mechanical properties of LGF reinforced PA6 composites filled with Sep are evaluated both by differing weight content of Sep and different hygrothermal ageing time. The thermal stability and degradation kinetics of the unaged and aged composites with different contents of Sep, as well as the apparent activation energy of the process, are analyzed by three typical methods. The aim of this work is to evaluate the protection effect of Sep filled in PA6/LGF composites and find out the feasibility of using Sep to fill LGF reinforced PA6 composites in hygrothermal environment by thermal kinetic analysis methods.
PA6/Sep masterbatches were prepared by mixing different fraction of Sep and PA6 of using a two-screw extruder (TSE-40A, L/D = 40, D = 40 mm, Coperion Keya machinery, Co., Ltd., China) at the temperatures of 200–250 °C. The PA6 was dried for 4 hours at 90 °C to remove moisture water before use. The PA6/LGF composites were blended in the same two-screw extruder and fibres were kept at the fraction of 50 wt%. The continuous strand was then cut into pellets with the length of 11 mm for injection molding.
PA6/LGF extrudates were dried at 90 °C for 4 hours, and then were blended with the PA6/Sep masterbatches (at the mass ratio to make the content of glass fibre staying at 30 wt% in the composites). Blended composites were injection molded at 220–265 °C into standard specimens for testing and characterization. PA6/LGF/Sep composites were put into a hygrothermal testing chamber which keeps the temperature at 80 °C and relative humidity of 95%. The hygrothermal ageing lasts 20 days, and each 5 days several specimens were taken out for testing.
The variations of impact strength of PA6/LGF/Sep with different contents of sepiolite are displayed in Fig. 2. In the first 5 days of ageing, it is clearly observed that the impact strength of all the samples increases significantly, and then have a slight decrease with increasing ageing time, however, the values of impact strength still stay at a much higher level than unaged ones. Because of the water absorption of PA6, it can be attributed to the effect of physical water plasticization caused by the diffused water in PA6 matrix. The decrease of tensile strength and increase of impact strength show that the material has become ductile.
More clearly, to analysis the effect of sepiolite on the mechanical properties of PA6/LGF composites in hygrothermal ageing procedure, the retention rate of tensile and impact strength of PA6/LGF composites with various sepiolites contents in different ageing time is calculated and exhibited in Fig. 3 and 4, respectively. The addition of Sep has a very large effect on the retention rate of tensile and impact strength. It can be clearly found from Fig. 3 that the retention rate of tensile strength of PA6/LGF sample after 10 days ageing can be recognized to a boundary. The sample with 6% wt Sep has a higher retention rate of more than 80% in tensile strength while that of the sample with 10% wt is lower than 70% because of the excess filler. It is obviously observed that the physical water plasticization makes the retention rate of impact strength over 150% in all samples. For those samples filled with various contents of sepiolite, the rates are higher than that of PA6/LGF sample, especially for the sample with 8% wt Sep, a nearly 200% retention rate is observed clearly. These results from the retention rate of mechanical properties above indicates that certain content (6–8%) of sepiolite in PA6/LGF/Sep composites can keep tensile strength at a stable level and increase the retention rate of impact strength in long time ageing, which is positive for the composites to serve in hygrothermal environment. By the way, the samples filled with 10% wt sepiolites show a worse retention rate of mechanical properties because an excessive addition of filler (sepiolites) can cause a severe deterioration in mechanical properties.
In value of number, PA6/LGF/Sep shows a global decreasing trend both in tensile and impact strength with the addition of sepiolite. We infer that the mechanical properties of the composites are mainly relying on the LGF. It can also be indicated that the sepiolite has some rejected effects with glass fibre on the reinforcing polymer composites. Fortunately, because of the existence of enough (30% wt) LGF, the mechanical properties of the composites are still staying at a relatively high level for application in many areas. What's more, for all the different content of sepiolite, the samples with 8% wt sepiolite have the most stable tensile and impact strength and the highest retention rate of impact strength in the hygrothermal ageing period from 5 to 20 d. For those samples, the variations of the tensile and impact strength were less than 3.1% and 5.5%, respectively. By the way, when the content of sepiolite reaches 10%, it will be difficult to extrude the masterbatches. So the content of 8% wt of sepiolite will be typical to be the stabilizer of PA6/LGF composites analyzed below.
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Fig. 5 The SEM images of the impact section morphology with 500× magnification: (A) PA6/LGF, unaged; (B) PA6/LGF, 20 d aged; (C) PA6/LGF/Sep 8%, unaged; (D) PA6/LGF/Sep 8%, 20 d aged. |
Temperature characteristic point | T5% (°C) | Tmax (°C) | ||||||
---|---|---|---|---|---|---|---|---|
Sep 0% 0 d | Sep 0% 20 d | Sep 8% 0 d | Sep 8% 20 d | Sep 0% 0 d | Sep 0% 20 d | Sep 8% 0 d | Sep 8% 20 d | |
10 °C min−1 | 353.4 | 354.1 | 361.8 | 358.8 | 458.9 | 445.50 | 445.8 | 450.4 |
20 °C min−1 | 377.5 | 354.0 | 378.0 | 381.5 | 479.1 | 474.10 | 459.5 | 464.6 |
30 °C min−1 | 375.1 | 396.1 | 392.7 | 390.6 | 485.1 | 477.17 | 469.1 | 470.5 |
40 °C min−1 | 394.6 | 394.4 | 399.9 | 399.3 | 488.5 | 474.06 | 474.4 | 478.6 |
The TGA of PA6/LGF and PA6/LGF/Sep composites with different hygrothermal ageing time at the same heating rate of 10 °C min−1 are exhibited in Fig. 7. From 0 day to 15 days ageing, there are very slight variations in TGA curves for PA6/LGF composites, however, the ageing time reaches 20 d, an obvious decrease in Tmax, which means a serious decrease in thermal stability. It is due to the chain breakage of PA6 molecule after the hygrothermal ageing which include not only interface debonding but also matrix degradation caused by water corrosion. With enough ageing time, degraded PA6 with shorter molecular chain has a worse thermal stability. Furthermore, it can be observed that the degradation residues have a slight decrease with enough long time ageing (15 and 20 d).
For the PA6/LGF/Sep composites, it can be found from Fig. 7(B) that TGA curves move to higher temperature with longer ageing time, which means an increase in thermal stability. It is very interesting that the residues of each sample has a different degree of variation, it is also attributed to the migration of sepiolite from inner structure to sample surface even out of samples in different period of hygrothermal ageing. A small amount of sepiolite which absorbed water flow out from samples as narrated above which can cause a slight change of sample weight. Because of this course, it is difficult to explain the changes only with TGA data at one constant heating rate. Then the variation of thermal stability can be analyzed by kinetic methods thereinafter in details.
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Fig. 8 The TGA curves with different heating rates of 10, 20, 30, and 40 °C min−1 for (A) PA6/LGF, unaged; (B) PA6/LGF, 20 d aged; (C) PA6/LGF/8% Sep, unaged; (D) PA6/LGF/8% Sep, 20 d aged. |
Temperature characteristic point | dα/dT (%/°C) | |||
---|---|---|---|---|
Sep 0%, 0 d | Sep 0%, 20 d | Sep 8%, 0 d | Sep 8%, 20 d | |
10 °C min−1 | 1.445 | 1.865 | 1.464 | 1.503 |
20 °C min−1 | 1.350 | 1.278 | 1.496 | 1.463 |
30 °C min−1 | 1.288 | 1.244 | 1.407 | 1.359 |
40 °C min−1 | 1.309 | 1.562 | 1.460 | 1.264 |
In the kinetic studies of this work, we first assume that the isothermal rate of conversion, dα/dT, is a linear function of the reactant concentration loss and of the temperature-independent rate constant, k, and a temperature independent function of conversion, it can be represented by the equation of:
dα/dt = β(dα/dT) = k(T)f(α) = k(1 − α)n | (1) |
k = A![]() | (2) |
In which E is the activation energy of the reaction, A is the frequency factor, T is the reaction temperature, and R is the gas constant. Combination of eqn (1) and (2) gives the following equation:
dα/dT = A(1 − α)n![]() | (3) |
Kissinger method17 thinks that apparent order of reaction (n) can be ignored when temperature reaches the maximum weight losing temperature (Tmax). It shows that Tmax can be used to calculate the apparent Arrhenius activation energy by this equation:
ln(β/Tmax) = ln(AR/E) − E/RTmax | (4) |
On the other hand, Friedman18 gives the following equation:
ln(β![]() | (5) |
The analysis of Kissinger and Friedman method are exhibited in Fig. 9, which shows the plots of ln(β/Tmax) and ln(βdα/dT) versus 1/T at varying conversion, respectively. Application of the analytical technique to the data presented in Tables 1 and 2 gives the mean values of activation energy calculated from the 4 different heating rates which are summarized in Table 3. For PA6/LGF 20 d aged samples, Tmax of the heating rate of 40 °C min−1 is much lower than that of 30 °C min. This may be attributed to the diffusion of water molecule in PA6 molecular chain which leads to a partial hydrolyzation after long time hygrothermal ageing. So only the heating rates of 10–30 °C min−1 were used to calculate the value of E in this sample.
Samples | PA6/LGF 0 d | PA6/LGF 20 d | PA6/LGF/Sep 0 d | PA6/LGF/Sep 20 d |
---|---|---|---|---|
Kissinger E (kJ mol−1) correlation coefficient | 193.41 | 124.25 | 201.77 | 215.68 |
0.9537 | 0.9037 | 0.9963 | 0.9922 | |
Friedman E (kJ mol−1) correlation coefficient | 188.59 | 78.45 | 210.53 | 200.43 |
0.9500 | 0.7771 | 0.9943 | 0.9890 |
From the E results, for PA6/LGF samples without sepiolite, it is obviously seen that the value of E has a serious decrease of 35.8% and 58.4% by Kissinger and Friedman methods with longer ageing time, respectively. This indicates that the hygrothermal ageing results in a sharp decline in the thermal stability of the composites, which can also be attributed to the hydrolyzation of PA6 molecular chain. Those samples filled with sepiolites show a relatively steady value of E with longer ageing time of which variations are 7.5% increase and 4.8% decrease by Kissinger and Friedman methods, respectively.
The above technique of Kissinger and Friedman has some merit in providing kinetic data, but it is defective that only use one data point (at Tmax) for the whole degradation stage. Another kinetic method used in this work is Flynn–Wall–Ozawa method,19,20 which is probably the most general derivative method.21 This method uses an integral solution and it is also independent of the degradation mechanism. Then from Doyle approximation, we can get the following equation:
log![]() | (6) |
In this work, we use conversion values in the range of 20–60% with this method. The plot of logβ against 1/T at a fixed degree of conversion α should give a straight whose slope is proportional to E, including the temperature with different partial mass loss rate of 0.2, 0.3, 0.4, 0.5, and 0.6. The analysis of Flynn–Wall–Ozawa is shown in Fig. 10, and the E values corresponding to different conversions are given in Fig. 11.
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Fig. 10 The log![]() |
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Fig. 11 The activation energy (E) of unaged and 20 d aged samples with partial mass loss by Flynn–Wall–Ozawa method. |
Fig. 11 reveals that unaged PA6/LGF shows a steady level of E at α value of 0.2–0.6. Then after 20 d hygrothermal ageing, an obvious decrease of 27.8% of E can be observed as same as the results calculated by the other two methods. What's more, a decrease of E with the partial mass loss increasing occurs, which means the aged samples appear to have a lower E in the later stages of the degradation. It is indicated that aged PA6/LGF is in a lower and lower energy barrier state of thermal kinetics with the progress of thermal degradation. On the contrary, the PA6/LGF/Sep filled with 8% wt sepiolites display only a slightly increase of E after ageing as the same trend of the results calculated by Kissinger method. With the addition of sepiolites, a much more steady thermal kinetics state of the samples has been achieved.
Then the E values calculated by all the three methods are listed in Table 4. It is clearly observed that the E values of the unaged and aged composites filled with 8% wt sepiolite are higher than that without sepiolite calculated by the three methods. It means that the sepiolites can improve the thermal stability of PA6/LGF, which can be used to enhance the flame retardancy of the composites. Then for 20 d aged ones, E values of samples without sepiolite have a varying degrees of decline calculated by the three methods while the composites filled with sepiolite show a slightly increase of 6.9% and 7.3% by Kissinger and Flynn–Wall–Ozawa method, respectively.
Samples | Kissinger E (kJ mol−1) | Friedman E (kJ mol−1) | F–W–O E (kJ mol−1) |
---|---|---|---|
PA6/LGF 0 d | 178.130 | 188.586 | 177.845 |
PA6/LGF 20 d | 124.252 | 78.451 | 128.481 |
PA6/LGF/Sep 0 d | 201.772 | 210.527 | 182.608 |
PA6/LGF/Sep 20 d | 215.681 | 200.433 | 196.012 |
Through analyzing the E obtained from the Kissinger, Friedman and Flynn–Wall–Ozawa methods, it is found that the effect of long time hygrothermal ageing on the E of the composites filled with sepiolites is much lesser than that of the composites without it. It can be attributed to the water absorption of sepiolite which reduces the water diffusion into PA6 matrix and protects PA6 and glass fibre from water corrosion including the PA6 molecular chain hydrolyzation and interface debonding. The presence of sepiolite makes PA6/LGF composites have a better thermal stability to stay at a relatively steady state in both thermodynamics and kinetics against hygrothermal environment.
This journal is © The Royal Society of Chemistry 2016 |