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Effect of crystallinity level on the double yielding behavior of polyamide 6
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POLYMER TESTING Polymer Testing 25 (2006) 452–459 www.elsevier.com/locate/polytest
Data Interpretation
Effect of crystallinity level on the double yielding behavior of polyamide 6 Gui-Fang Shan, Wei Yang, Ming-bo Yang, Bang-hu Xie, Zhong-ming Li, Jian-min Feng State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065, Sichuan, People’s Republic of China Received 14 December 2005; accepted 27 January 2006
Abstract A complex double yielding behavior is observed in the engineering stress–strain curves of injection molded specimens of polyamide 6 (PA6) under tensile loading, and a simple method is put forward to judge the apparentness of the double yielding process. By thermal treatment, the effect of the crystallinity level on the double yielding behavior is studied in some detail. The results show that the second yield stress becomes larger than the first after the thermal treatment, which is contrary to the case without thermal treatment. With the annealing time decreasing and the annealing temperature increasing, the percentage crystallinity becomes higher, and the second yield point is much more apparent with a decrease of crystallinity of the specimens. A possible crystallinity window may exist in double yielding behavior of PA6 material. These results indirectly show that the second yield point is not only associated with the deformation of the crystalline region. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polyamide 6; Crystallinity; Double yielding; Stress-Strain behaviour
1. Introduction It has long been known that the deformation behavior and yielding properties are very important characteristics of semi-crystalline polymers. Conventionally, the yielding behavior of thermoplastics under tensile loading is often characterized by only one yield maximum [1]. Since the recognition of the double yielding process in branched and heated pressed polyethylene specimens by Popli and Mandelkern in 1987 [2], many related investigations Corresponding author. Fax: +86 28 8540 5324.
E-mail address: [email protected] (M.-b. Yang). 0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.01.013
have concentrated in understanding this complex phenomenon and some researchers have deemed that the double yielding behavior is a general phenomenon to be expected in semi-crystalline polymers but not in amorphous ones [3–7]. Recently, Adhikari et al. [8] have made an important discovery and detected the existence of double yielding in nanostructured amorphous polymer. Subsequently, Li et al. [9] have also reported the morphology-dependent double yielding behavior in injection molded polycarbonate/polyethylene blend, which is a typically incompatible blending system. Some possible deformation models and mechanisms have been postulated to
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explain the origin of this special phenomenon [10–12]. For example, Se´gue´la et al. [11,12] have pointed out that the two yield points are due to the slip of the crystal blocks past each other in the mosaic crystalline structure (heterogenous slip) and the homogeneous shear of the crystal blocks (homogeneous slip). While attempting to understand the double yielding behavior of polyethylenes and other polymer systems, several factors, including the structure changes, deformation temperature, strain rate, crystallinity degree, lamellae thickness, perfection of crystallites, composition distribution, branching degree and branching content, etc., must be evaluated [2–7,10–12]. A series of investigations concerning the influence of crystallinity level on double yielding in crystalline polymers has been carried out in the recent years. Lucas et al. [5] have studied a set of linear polyethylene with different crystallinity levels and observed that samples with crystallinity contents higher than 50% show only one yield point, samples with crystallinity levels in the range 20–50% show two yield points and samples with crystallinity levels less than 20% only display a very broad yield process, similar to a rubber-like deformation. Se´gue´la and Darras [13] have made a comprehensive study on the double yield of polyethylenes and drawn the conclusion that the double yielding behavior is a common feature to polyethylene and ethylene copolymers, regardless of the crystallinity level. Furthermore, the experimental results from Muramatsu and Lando [4] have proved that the first yield point of poly(tetramethylene terephthalate) and its copolymers becomes less apparent with an increase of crystallinity level of the specimens, and the second yield point becomes much more apparent with the increase of crystallinity level. In our previous papers [14,15], the special double yielding phenomenon of injection molded polyamide 6 (PA6) and glass bead-filled PA6 composites under tensile loading has been studied by means of various measurements such as differential scanning calorimetry (DSC) and X-ray diffraction (XRD). In this study, to further elucidate the relation between the crystallinity level and the double yielding behavior, PA6 injection specimens with a wide range of crystallinity level were prepared and a simple method to judge the first and second yielding grade was considered.
453
2. Experimental procedure 2.1. Material The Polyamide 6 (PA6) resin used in this study was a commercial product of Xinhui Meida-DSM Nylon Slice Company Ltd, supplied in pellets, with the trade mark M52800, as described previously [14,15]. The melting temperature of PA6 measured by DSC was 225 1C. The resin was dried for 12 h under vacuum at 100 1C before processing to avoid its hydrolytic degradation. 2.2. Sample preparation After drying in vacuum at 100 1C over 12 h to remove the moisture, PA6 were injection molded into dog-bone tensile samples and impact samples on an injection-molding machine made by Nissan, Japan, with a temperature profile of 230, 240, 250, and 245 1C from the feeding zone to the nozzle, an injection velocity of 14% (the maximal injection velocity is 45 g/s), and an injection pressure and holding pressure of 30% (the maximal injection pressure is about 187 MPa), respectively. The mold temperature was 40 1C. The molded samples were then annealed at 100 1C and 190 1C for different time and annealed for 2 h at 130, 160, 190, and 205 1C, respectively. 2.3. Tensile deformation The tensile test was performed at room temperature according to ISO 527-1-1993 on an Instron Series IX universal test machine by using the dogbone specimens. The distance between the grips was 110 mm as recommended in the standard. The crosshead speeds of the apparatus were 5, 10, 20, 50, 100, and 200 mm/min respectively, with the strain being determined on a 50 mm length zone in the middle part of the specimens by an extensometer. At least five specimens were used for each measurement and the average results were reported here. The nominal stress and nominal strain are defined as the ratio of the draw force to the initial cross-section of the sample and the ratio of the extensometer displacement to the initial gauge length of the sample, respectively. On the other hand, the nominal strain rate is the ratio of the crosshead speed to the initial gauge length of the sample. Thus the strain rates were 1.67 103, 0.33 102, 0.67 102,
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1.67 102, respectively.
0.33 101
and
0.67 101 s1,
2.4. Differential scanning calorimetry (DSC) procedures The thermal analyses were carried out using a Netzsch DSC-204 apparatus from 120 to 250 1C at a scanning rate of 10 1C/min with 50 mL/min N2 protection, with the sample weight in the range of 5–10 mg. The melting of indium and zinc samples was used to calibrate the temperature and the heatflow scales at the same heating rate. The weight fraction crystallinity (XC-DSC) of neat PA6 can be determined from the enthalpy obtained using the following equation: X c ð%Þ ¼
DH c 100, DH 0m
(1)
where DH c is the apparent enthalpy of fusion of sample, DH 0m is the extrapolated value of the enthalpy corresponding to the melting of 100% crystalline sample. Two main crystals of a- and g-form of neat PA6 are usually observed in most cases and for this reason, an average value of 190 J/g has been chosen for DH 0m [16,17]. 2.5. X-ray diffraction (XRD) XRD patterns of the samples were recorded using a Philips X’Pert Graphics and Identify instrument (PRO MPX) at room temperature. The Cu Ka (wave length is 1.54056 A˚) irradiation source was operated at 50 KV and 30 mA. Patterns were recorded by monitoring diffractions from 101 to 501, and the scanning speed was 21/min.
of the necked region [8,18]. Fig. 1 shows that the first yield zone occurs at about 5% strain and the second one at about 20% strain. It is well-known that the Eyring formalization [19,20] for thermally activated rate processed has been the most widely used model for studying the yield mechanism of glassy as well as semi-crystalline polymers. However, generally little information has been obtained concerning how to judge the apparentness of the double yielding. In the present paper, the authors will introduce a simple method to indirectly define the apparentness of the first and second yield process under tensile loading. As shown in Fig. 1, (a1 is defined as the expandable angle during the first stress drop and a2 that between the first stress plateau region and the second stress drop; s1 denotes the first yield stress drop value and s2 the second stress drop value. We deem that a for a smaller s value, as well as for a bigger s value, the yield process is more apparent. Relatively speaking, the expandable angle a is the more important parameter. The a1 and a2 values can be determined as shown in Fig. 2, and the s1 and s2 values can be calculated easily. In this paper, such a definition will be adopted chiefly to describe the double yielding of injection molded PA6 samples with different crystallinity levels. Fig. 3 illustrates the engineering stress–strain curves as a function of strain rate of the neat PA6. The results clearly indicated that the PA6 polymer exhibits a single yield point at high strain rate, but as the strain rate decreases, a second yield point gradually develops. Another definite trend is that 70 I σ1
60
3. Results and discussion
As reported in previous papers [14,15], two distinct stress drops and plateau regions were observed in the injection-molded PA6 samples under tensile loading and the larger stress drop occurred at the second yield point, where necking took place simultaneously. The primary reason for the sudden decrease of stress in the stress–strain curves is that the stress is calculated using the initial width and thickness of the test sample and the strain hardening of the necked material is not sufficient to counteract the reduction in the cross-sectional area
50 stress (MPa)
3.1. A simple method to define the yield process
α1
σ2
II
α2
40 30 20 v=10mm/min
10 0 0
10
20
30 strain (%)
40
50
60
Fig. 1. Typical stress–strain curve in the yield zone obtained for injection-molded sample of PA6: (I) first yield point and (II) second yield point.
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455
Table 1 The a and s values of neat PA6 under a wide range of strain rate
60 α1
stress (MPa)
50
α2
40 30 slope= 6.36109 slope=-1.15353 slope=-0.384661 slope=-1.16283
20 10
Strain rate (s1)
a1 (deg)
a2 (deg)
s1
s2
1.67 103 0.33 102 0.67 102 1.67 102 0.33 101 0.67 101
53.1 49.0 43.1 39.5 40.4 37.4
130.9 120.7 129.6 152.8 147.8 153.3
4.50 4.47 5.21 5.96 7.48 8.69
9.71 12.54 14.15 16.91 17.66 18.12
0 0
10
20
30 strain (%)
40
50
60
Fig. 2. The sketch map of how to calculate the a1 and a2 value.
80 5
70
4
Stress (MPa)
60
6
3
2 1
50 40
1: 1.67 × 10-3 s-1 2: 0.33 × 10-2 s-1 3: 0.67 × 10-2 s-1 4: 1.67 × 10-2 s-1 5: 0.33 × 10-1 s-1 6: 0.67 × 10-1 s-1
30 20 10 0 -5
0
5
10
15
20 25 30 Strain (%)
35
40
45
50
Fig. 3. Double yielding behavior of PA6 in a wide range of strain rate.
with the strain rate increasing, the first yield stress increases owing to the shorter relaxation time of the stretched samples. According to the method described, the a and s values of PA6 samples tested under different strain rates are listed in Table 1. As shown in Table 1, it is evident that a1 is much lower and a2 is much larger with an increase of the strain rate, which means that the first yield point becomes more apparent, while the second yield point becomes less apparent with increasing strain rate. In addition, when the strain rate reaches 1.67 102 s1, the first yield stress plateau region becomes much narrower and sharper and the yielding behavior at high strain rate is much more close to the single yielding process (see Fig. 3).
3.2. Influence of the crystallinity level of polymer material Mandelkern and Popli [2] have postulated that the Flory and Yoon [21] mechanism of partial melting and recrystallization for the plastic deformation of semi-crystalline polymers can explain the origin of the double yielding. In order to further probe the effect of crystallinity level on the double yielding behavior of PA6, the present work has taken into account neat PA6 spanning the range of crystal weight fraction 0.30–0.50. At the same time, we also apply the method introduced above to judge the apparentness of the double yielding behavior. The nominal stress–strain curves of samples thermally treated following the procedure described above are depicted in Figs. 4 and 5, respectively. A strain rate of 0.67 102 s1 was chosen for the tensile test. The stress–strain curves drawn here do not include the segment over 70% in strain for the sake of more clearly demonstrated curves before necking. Two yield points can also be observed before necking in these test pieces. In comparison with the case of unannealed PA6, some remarkable features appear in annealed samples. The main difference is that for annealed samples, the level of the second yield stress is higher than that of the first yield stress. In contrast, the double yielding behavior of unannealed samples exhibits different behavior and the second yield stress value is lower than the first one. Moreover, it is notable that the double yielding behavior becomes more apparent after the heat treatment. The calculated a2 and s2 values are summarized in Table 2. Table 2 gives the information on how the second yield point gradually changes after the different thermal treatments. It is clear that the s2 value increases and the a2 value decreases a little with the annealing time increasing, while the s2 value decreases and the a2 value increases a little with
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70
II
I
3
60
4 5
stress (MPa)
50 2
1
40
1-0h 2-1h 3-3h 4-7h 5 -13 h
30 20 10 0 0
20
10
30 40 strain (%)
50
60
70
Fig. 4. The engineering stress–strain curves of PA6 with different annealing times (T ¼ 100 1C).
80 70
4
stress (MPa)
60
3 2
50 40
1
5
30 20 10
1 - unannealed 2 -130°C 3 -160°C 4 -190°C 5 -205°C
0 0
10
20
30 40 strain (%)
50
60
70
Fig. 5. The engineering stress–strain curves of PA6 with different annealing temperatures (t ¼ 2 h). Table 2 The a and s values of different thermal treated PA6 tanneal (h)
a2 (deg)
s2
Tanneal (1C)
a2 (deg)
s2
1 3 7 13
137.9 125.0 131.1 129.2
13.4 15.1 16.4 17.0
130 160 190 205
124.3 125.9 128.4 —
19.96 19.38 15.69 —
the annealing temperature increasing. That is to say, reducing the annealing time and increasing the annealing temperature is likely to depress the double yielding phenomenon of the neat PA6. These changes on the yield pattern might be related to the crystallinity levels of the polymer material.
The corresponding DSC melting curves and the crystallinity levels calculated by Eq. (1) are shown in Fig. 6 and Tables 3 and 4. The crystallinity data from Tables 3 and 4 make it clear that the crystallinity level decreases with the annealing time increasing, while increases with the annealing temperature increasing. It is normal that annealing treatment causes lamellar thickening as well as an increase in percentage crystallinity since the molecule chains have enough time and energy to arrange in the crystal lattice [18]. As to the reason why the crystallinity decreases with the annealing time increasing in our experiment, a much deeper investigation will be undertaken later. We repeated the experiment and got the same results. It is possible that the annealing temperature set at 100 1C in our study is too low to give enough energy to drive the molecule chain to move quickly and arrange in the crystal lattice, for the glass transition temperature of PA6 in our study is about 70 1C. Combining Figs. 4–6 and Tables 2 and 4 with the method suggested in this paper, it seems that the second yield point of neat PA6 will be depressed when the crystallinity levels of the sample increase and the second yielding process is not only associated with the deformation of the crystalline region. When the degree of crystallinity reaches a certain level, there is only one yield point in existence on the engineering stress-strain curves, as is illustrated in Fig. 5. Lucas et al. [5] has considered that the double yielding of the polyethylenes is only present in samples with the crystallinity level of 20–50%. It is similarly believable that a crystallinity window is also present in the double yielding of PA6. Some authors [8] have regarded that the existence of two distinct yield points in semicrystalline polymer is attributed to the deformation of the amorphous phase for the first yield and the deformation of the crystalline phase for the second phase. Thus, the second yield point should become larger with the crystallinity level increasing. However, the results of our experiments demonstrates that the second yield is not only associated with the deformation of the crystalline region. The DSC melting curves of PA6 in Fig. 6 show that a series of stepwise annealing treatments at steadily increasing temperature will result in a double endothermic peak. Moreover, the additional annealing peak at lower temperature shifts to higher temperature accompanied by an increase in its peak area magnitude. The results of the XRD show that this peak does not belong to the g-crystal form of
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Endotherm >
Endotherm >
G.-F. Shan et al. / Polymer Testing 25 (2006) 452–459
13h 7h 3h
160
180 200 220 240 Temperature (°C)
205 °C
I
190 °C I
1h
(a)
457
160 °C 130 °C
I 260
120 (b)
140
160 180 200 220 Temperature (°C)
240
260
Fig. 6. DSC heating curves of specimens of annealed PA6: (a) different annealing times and (b) different annealing temperature.
Table 3 DSC data and crystallinity of PA6 with different anneal time during heating scans (the annealing temperature is 100 1C) Anneal time Tm (1C) (h)
DHm (J/g)
Crystallinity (%)
Tonset (1C)
Tend (1C)
1 3 7 13
77.52 76.35 63.52 60.25
40.80 40.18 33.43 31.70
218.88 217.25 208.62 221.48
225.13 226.23 229.61 225.80
223.10 223.71 225.85 224.08
20.25
20.63
23.67
23.23
38.17
Table 4 DSC data and crystallinity of PA6 with different anneal temperatures during heating scans (the annealing time is 2 h) Anneal Tm (1C) temperature (1C)
DHm (J/g) Crystallinity (%)
Tonset (1C)
Tend (1C)
130
67.16 1.50 69.34 3.85 53.91 25.6 79.86
218.35 141.89 219.10 162.87 213.42 199.93 216.98
228.48 155.09 225.04 177.45 225.97 210.24 226.85
160 190 205
224.37 147.58 222.91 169.69 222.67 206.32 224.54
35.6 0.79 36.50 2.02 28.37 13.47 42.03
PA6. It has been reported that PA6 crystals can exist in two major forms including the monoclinic a-crystal and monoclinic (or pseudo-hexagonal) g-crystal form [17]. Two main reflections can be observed at about 2y ¼ 201 and 23.71, which are attributed to the a100 and a002/202 crystal planes, respectively. The reflection peaks at 2y ¼ 10:71 and 21.41 are associated with g020 and g001 crystal planes of PA6, respectively [22–25]. Thus, it is easy to observe in Fig. 7 that only the a-crystal form is present in the annealed PA6 samples. According to Yan et al. [26], the additional endothermic peak in
190°C
160°C
0
10
20
30
40
50
2θ Fig. 7. The XRD patterns of neat PA6 at different annealing temperatures.
the DSC melting curve is attributed to the melting peak of the imperfect crystallites under the low annealing temperature, while Wang et al. [27] considered that endotherm I is due to the melting of secondary lamellae. The crystal parameters, including the interplanar distance and the crystallite size, can be determined using the Bragg formula and Scherrer equation that have been indicated in a previous publication [15], and the corresponding results are listed in Table 5. These data suggest that both the interplanar distance and the crystallite size increase with the annealing temperature increasing, indicating that a process of crystal perfection takes place and the regularity of the crystal structure is much better at higher annealing temperature. In order to gain a better insight into the effect of crystallinity level on the double yielding behavior of PA6 and the multiple melting behavior of annealed
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Table 5 The crystal parameters of neat PA6 with different annealing temperatures (hkl)
a200 a002/202
PA6-160 1C
PA6-190 1C
PA6-160 1C
PA6-190 1C
PA6-160 1C
PA6-190 1C
20.5 23.7
20.3 23.5
0.43 0.38
0.44 0.38
2.50 2.51
3.57 3.58
are marked on the DSC curves. These results are also consistent with the conclusion that the second yield point is much more apparent with a decrease of the crystallinity of the specimens. Perhaps the second yield point will disappear as a result of the crystallinity percentage above about 42%. There are also two melting endothermic peaks on the DSC heating scan curves and the temperature of the lowmelting peaks increases with the annealing time increasing.
80 70
Stress (MPa)
2
1
60
3
50 4
40 30
1: unannealed 2: 0.5h 3: 1.5h 4: 3 h 5: 7 h 6: 12 h
5
20
Hhkl (nm)
dhkl (nm)
2y (deg)
6
10 0
4. Conclusion
-10 0
5
10
15
20 25 30 Strain (%)
35
40
45
50
Fig. 8. The engineering stress–strain curves of PA6 with different annealing times (T ¼ 190 1C).
Endotherm >
48.87% (12.0h) 45.12% (7.0h) 42.94% (3.0h) 42.84% (1.5h) 39.06% (0.5h)
140
160
180 200 Temperature (°C)
220
240
Fig. 9. DSC heating curves of annealed PA6 specimens with different annealing times (T ¼ 190 1C).
PA6 sample under high annealing temperature, some samples were annealed at different annealing time and the annealing temperature was set at 190 1C. The corresponding stress–strain curves and the DSC melting curves are shown in Figs. 8 and 9. The calculated crystallinity levels based on Eq. (1)
Double yield points before necking were observed in injection-molded specimens of polyamide 6 (PA6) under tensile loading. Generally speaking, the second yield point becomes much more apparent with the strain rate decreasing. A simple method is put forward to judge the apparentness of the first and second yield point. We have pointed out that the expandable angle a and the stress drops s are the important indexes in estimating the yielding process. If the a value is much smaller or the s value is much bigger, the yield point is much more apparent. Furthermore, using the method mentioned above, the relation between the degree of crystallinity and the double yielding has also been discussed. The level of the second yield stress becomes larger than that of the first yield stress after thermal treatment, which is contrary to the case without thermal treatment. The first yield point becomes more apparent with an increase of the crystallinity of the specimens, while the second yield point becomes less apparent with an increase of the crystallinity. These results show that the second yield is not only associated with the deformation of the crystalline region. Acknowledgements The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (Grant no. 50503014) and the financial support on the Fundamental of Application
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Research Project from Sinopec Corp (Grant no. X504004). References [1] I.M. Ward, Mechanical Properties of Solid Polymers, 2nd Ed, Wiley, New York, 1983 (Chapter 11). [2] R. Popli, L. Madelkern, Influence of structural and morphological factors on the mechanical-properties of the polyethylenes, J. Polym. Sci. Polym. Phys. Ed. 25 (1987) 441–483. [3] N.W. Brooks, R.A. Duckett, I.M. Ward, Investigation into double yield points in polyethylene, Polymer 33 (1992) 1872–1880. [4] S. Muramatsu, J.B. Lando, Double yield points in poly (tetramethylene terephthalate) and its copolymers under tensile loading, Polym. Eng. Sci. 35 (1995) 1077–1085. [5] J.C. Lucas, M.D. Failla, F.L. Smith, L. Mandelkern, A.J. Peacock, Study of the phase-structure of linear polyethylene by means of small-angle X-ray-scattering and raman-spectroscopy, Polym. Eng. Sci. 35 (1995) 1117–1123. [6] J.L. Feijoo, J.J. Sa´nchez, A.J. Mu¨ller, The phenomenon of double yielding in oriented high density polyethylene films, J. Mater. Sci. Lett. 16 (1997) 1721–1724. [7] G. Spathis, E. Kontou, Nonlinear viscoelastic model for the prediction of double yielding in a linear low density polyethylene film, J. Appl. Polym. Sci. 91 (2004) 3519–3527. [8] R. Adhikari, M. Buschnakowski, S. Henning, Double yielding in a styrene/butadiene star block copolymer, Macromol. Rapid Commun. 25 (2004) 653–658. [9] Z.M. Li, C.G. Huang, W. Yang, Macromol. Morphology dependent double yielding in injection molded polycarbonate/polyethylene blend, Mater. Eng. 289 (2004) 1004–1011. [10] P.J. Flory, D.Y. Yoon, Molecular morphology in semicrystalline polymer, Nature 272 (1978) 226–229. [11] R. Se´gue´la, F. Rietsch, Double yield-point in polyethylene under tensile loading, J. Mater. Sci. Lett. 9 (1990) 46–47. [12] R. Se´gue´la, V. Gaucher-Miri, Tensile yield of polyethylene and related copolymers: mechanical and structural evidences of two thermally activated processes, Macromolecules 30 (1997) 1158–1167.
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POLYMER TESTING Polymer Testing 25 (2006) 452–459 www.elsevier.com/locate/polytest
Data Interpretation
Effect of crystallinity level on the double yielding behavior of polyamide 6 Gui-Fang Shan, Wei Yang, Ming-bo Yang, Bang-hu Xie, Zhong-ming Li, Jian-min Feng State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065, Sichuan, People’s Republic of China Received 14 December 2005; accepted 27 January 2006
Abstract A complex double yielding behavior is observed in the engineering stress–strain curves of injection molded specimens of polyamide 6 (PA6) under tensile loading, and a simple method is put forward to judge the apparentness of the double yielding process. By thermal treatment, the effect of the crystallinity level on the double yielding behavior is studied in some detail. The results show that the second yield stress becomes larger than the first after the thermal treatment, which is contrary to the case without thermal treatment. With the annealing time decreasing and the annealing temperature increasing, the percentage crystallinity becomes higher, and the second yield point is much more apparent with a decrease of crystallinity of the specimens. A possible crystallinity window may exist in double yielding behavior of PA6 material. These results indirectly show that the second yield point is not only associated with the deformation of the crystalline region. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polyamide 6; Crystallinity; Double yielding; Stress-Strain behaviour
1. Introduction It has long been known that the deformation behavior and yielding properties are very important characteristics of semi-crystalline polymers. Conventionally, the yielding behavior of thermoplastics under tensile loading is often characterized by only one yield maximum [1]. Since the recognition of the double yielding process in branched and heated pressed polyethylene specimens by Popli and Mandelkern in 1987 [2], many related investigations Corresponding author. Fax: +86 28 8540 5324.
E-mail address: [email protected] (M.-b. Yang). 0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.01.013
have concentrated in understanding this complex phenomenon and some researchers have deemed that the double yielding behavior is a general phenomenon to be expected in semi-crystalline polymers but not in amorphous ones [3–7]. Recently, Adhikari et al. [8] have made an important discovery and detected the existence of double yielding in nanostructured amorphous polymer. Subsequently, Li et al. [9] have also reported the morphology-dependent double yielding behavior in injection molded polycarbonate/polyethylene blend, which is a typically incompatible blending system. Some possible deformation models and mechanisms have been postulated to
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explain the origin of this special phenomenon [10–12]. For example, Se´gue´la et al. [11,12] have pointed out that the two yield points are due to the slip of the crystal blocks past each other in the mosaic crystalline structure (heterogenous slip) and the homogeneous shear of the crystal blocks (homogeneous slip). While attempting to understand the double yielding behavior of polyethylenes and other polymer systems, several factors, including the structure changes, deformation temperature, strain rate, crystallinity degree, lamellae thickness, perfection of crystallites, composition distribution, branching degree and branching content, etc., must be evaluated [2–7,10–12]. A series of investigations concerning the influence of crystallinity level on double yielding in crystalline polymers has been carried out in the recent years. Lucas et al. [5] have studied a set of linear polyethylene with different crystallinity levels and observed that samples with crystallinity contents higher than 50% show only one yield point, samples with crystallinity levels in the range 20–50% show two yield points and samples with crystallinity levels less than 20% only display a very broad yield process, similar to a rubber-like deformation. Se´gue´la and Darras [13] have made a comprehensive study on the double yield of polyethylenes and drawn the conclusion that the double yielding behavior is a common feature to polyethylene and ethylene copolymers, regardless of the crystallinity level. Furthermore, the experimental results from Muramatsu and Lando [4] have proved that the first yield point of poly(tetramethylene terephthalate) and its copolymers becomes less apparent with an increase of crystallinity level of the specimens, and the second yield point becomes much more apparent with the increase of crystallinity level. In our previous papers [14,15], the special double yielding phenomenon of injection molded polyamide 6 (PA6) and glass bead-filled PA6 composites under tensile loading has been studied by means of various measurements such as differential scanning calorimetry (DSC) and X-ray diffraction (XRD). In this study, to further elucidate the relation between the crystallinity level and the double yielding behavior, PA6 injection specimens with a wide range of crystallinity level were prepared and a simple method to judge the first and second yielding grade was considered.
453
2. Experimental procedure 2.1. Material The Polyamide 6 (PA6) resin used in this study was a commercial product of Xinhui Meida-DSM Nylon Slice Company Ltd, supplied in pellets, with the trade mark M52800, as described previously [14,15]. The melting temperature of PA6 measured by DSC was 225 1C. The resin was dried for 12 h under vacuum at 100 1C before processing to avoid its hydrolytic degradation. 2.2. Sample preparation After drying in vacuum at 100 1C over 12 h to remove the moisture, PA6 were injection molded into dog-bone tensile samples and impact samples on an injection-molding machine made by Nissan, Japan, with a temperature profile of 230, 240, 250, and 245 1C from the feeding zone to the nozzle, an injection velocity of 14% (the maximal injection velocity is 45 g/s), and an injection pressure and holding pressure of 30% (the maximal injection pressure is about 187 MPa), respectively. The mold temperature was 40 1C. The molded samples were then annealed at 100 1C and 190 1C for different time and annealed for 2 h at 130, 160, 190, and 205 1C, respectively. 2.3. Tensile deformation The tensile test was performed at room temperature according to ISO 527-1-1993 on an Instron Series IX universal test machine by using the dogbone specimens. The distance between the grips was 110 mm as recommended in the standard. The crosshead speeds of the apparatus were 5, 10, 20, 50, 100, and 200 mm/min respectively, with the strain being determined on a 50 mm length zone in the middle part of the specimens by an extensometer. At least five specimens were used for each measurement and the average results were reported here. The nominal stress and nominal strain are defined as the ratio of the draw force to the initial cross-section of the sample and the ratio of the extensometer displacement to the initial gauge length of the sample, respectively. On the other hand, the nominal strain rate is the ratio of the crosshead speed to the initial gauge length of the sample. Thus the strain rates were 1.67 103, 0.33 102, 0.67 102,
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1.67 102, respectively.
0.33 101
and
0.67 101 s1,
2.4. Differential scanning calorimetry (DSC) procedures The thermal analyses were carried out using a Netzsch DSC-204 apparatus from 120 to 250 1C at a scanning rate of 10 1C/min with 50 mL/min N2 protection, with the sample weight in the range of 5–10 mg. The melting of indium and zinc samples was used to calibrate the temperature and the heatflow scales at the same heating rate. The weight fraction crystallinity (XC-DSC) of neat PA6 can be determined from the enthalpy obtained using the following equation: X c ð%Þ ¼
DH c 100, DH 0m
(1)
where DH c is the apparent enthalpy of fusion of sample, DH 0m is the extrapolated value of the enthalpy corresponding to the melting of 100% crystalline sample. Two main crystals of a- and g-form of neat PA6 are usually observed in most cases and for this reason, an average value of 190 J/g has been chosen for DH 0m [16,17]. 2.5. X-ray diffraction (XRD) XRD patterns of the samples were recorded using a Philips X’Pert Graphics and Identify instrument (PRO MPX) at room temperature. The Cu Ka (wave length is 1.54056 A˚) irradiation source was operated at 50 KV and 30 mA. Patterns were recorded by monitoring diffractions from 101 to 501, and the scanning speed was 21/min.
of the necked region [8,18]. Fig. 1 shows that the first yield zone occurs at about 5% strain and the second one at about 20% strain. It is well-known that the Eyring formalization [19,20] for thermally activated rate processed has been the most widely used model for studying the yield mechanism of glassy as well as semi-crystalline polymers. However, generally little information has been obtained concerning how to judge the apparentness of the double yielding. In the present paper, the authors will introduce a simple method to indirectly define the apparentness of the first and second yield process under tensile loading. As shown in Fig. 1, (a1 is defined as the expandable angle during the first stress drop and a2 that between the first stress plateau region and the second stress drop; s1 denotes the first yield stress drop value and s2 the second stress drop value. We deem that a for a smaller s value, as well as for a bigger s value, the yield process is more apparent. Relatively speaking, the expandable angle a is the more important parameter. The a1 and a2 values can be determined as shown in Fig. 2, and the s1 and s2 values can be calculated easily. In this paper, such a definition will be adopted chiefly to describe the double yielding of injection molded PA6 samples with different crystallinity levels. Fig. 3 illustrates the engineering stress–strain curves as a function of strain rate of the neat PA6. The results clearly indicated that the PA6 polymer exhibits a single yield point at high strain rate, but as the strain rate decreases, a second yield point gradually develops. Another definite trend is that 70 I σ1
60
3. Results and discussion
As reported in previous papers [14,15], two distinct stress drops and plateau regions were observed in the injection-molded PA6 samples under tensile loading and the larger stress drop occurred at the second yield point, where necking took place simultaneously. The primary reason for the sudden decrease of stress in the stress–strain curves is that the stress is calculated using the initial width and thickness of the test sample and the strain hardening of the necked material is not sufficient to counteract the reduction in the cross-sectional area
50 stress (MPa)
3.1. A simple method to define the yield process
α1
σ2
II
α2
40 30 20 v=10mm/min
10 0 0
10
20
30 strain (%)
40
50
60
Fig. 1. Typical stress–strain curve in the yield zone obtained for injection-molded sample of PA6: (I) first yield point and (II) second yield point.
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70
455
Table 1 The a and s values of neat PA6 under a wide range of strain rate
60 α1
stress (MPa)
50
α2
40 30 slope= 6.36109 slope=-1.15353 slope=-0.384661 slope=-1.16283
20 10
Strain rate (s1)
a1 (deg)
a2 (deg)
s1
s2
1.67 103 0.33 102 0.67 102 1.67 102 0.33 101 0.67 101
53.1 49.0 43.1 39.5 40.4 37.4
130.9 120.7 129.6 152.8 147.8 153.3
4.50 4.47 5.21 5.96 7.48 8.69
9.71 12.54 14.15 16.91 17.66 18.12
0 0
10
20
30 strain (%)
40
50
60
Fig. 2. The sketch map of how to calculate the a1 and a2 value.
80 5
70
4
Stress (MPa)
60
6
3
2 1
50 40
1: 1.67 × 10-3 s-1 2: 0.33 × 10-2 s-1 3: 0.67 × 10-2 s-1 4: 1.67 × 10-2 s-1 5: 0.33 × 10-1 s-1 6: 0.67 × 10-1 s-1
30 20 10 0 -5
0
5
10
15
20 25 30 Strain (%)
35
40
45
50
Fig. 3. Double yielding behavior of PA6 in a wide range of strain rate.
with the strain rate increasing, the first yield stress increases owing to the shorter relaxation time of the stretched samples. According to the method described, the a and s values of PA6 samples tested under different strain rates are listed in Table 1. As shown in Table 1, it is evident that a1 is much lower and a2 is much larger with an increase of the strain rate, which means that the first yield point becomes more apparent, while the second yield point becomes less apparent with increasing strain rate. In addition, when the strain rate reaches 1.67 102 s1, the first yield stress plateau region becomes much narrower and sharper and the yielding behavior at high strain rate is much more close to the single yielding process (see Fig. 3).
3.2. Influence of the crystallinity level of polymer material Mandelkern and Popli [2] have postulated that the Flory and Yoon [21] mechanism of partial melting and recrystallization for the plastic deformation of semi-crystalline polymers can explain the origin of the double yielding. In order to further probe the effect of crystallinity level on the double yielding behavior of PA6, the present work has taken into account neat PA6 spanning the range of crystal weight fraction 0.30–0.50. At the same time, we also apply the method introduced above to judge the apparentness of the double yielding behavior. The nominal stress–strain curves of samples thermally treated following the procedure described above are depicted in Figs. 4 and 5, respectively. A strain rate of 0.67 102 s1 was chosen for the tensile test. The stress–strain curves drawn here do not include the segment over 70% in strain for the sake of more clearly demonstrated curves before necking. Two yield points can also be observed before necking in these test pieces. In comparison with the case of unannealed PA6, some remarkable features appear in annealed samples. The main difference is that for annealed samples, the level of the second yield stress is higher than that of the first yield stress. In contrast, the double yielding behavior of unannealed samples exhibits different behavior and the second yield stress value is lower than the first one. Moreover, it is notable that the double yielding behavior becomes more apparent after the heat treatment. The calculated a2 and s2 values are summarized in Table 2. Table 2 gives the information on how the second yield point gradually changes after the different thermal treatments. It is clear that the s2 value increases and the a2 value decreases a little with the annealing time increasing, while the s2 value decreases and the a2 value increases a little with
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70
II
I
3
60
4 5
stress (MPa)
50 2
1
40
1-0h 2-1h 3-3h 4-7h 5 -13 h
30 20 10 0 0
20
10
30 40 strain (%)
50
60
70
Fig. 4. The engineering stress–strain curves of PA6 with different annealing times (T ¼ 100 1C).
80 70
4
stress (MPa)
60
3 2
50 40
1
5
30 20 10
1 - unannealed 2 -130°C 3 -160°C 4 -190°C 5 -205°C
0 0
10
20
30 40 strain (%)
50
60
70
Fig. 5. The engineering stress–strain curves of PA6 with different annealing temperatures (t ¼ 2 h). Table 2 The a and s values of different thermal treated PA6 tanneal (h)
a2 (deg)
s2
Tanneal (1C)
a2 (deg)
s2
1 3 7 13
137.9 125.0 131.1 129.2
13.4 15.1 16.4 17.0
130 160 190 205
124.3 125.9 128.4 —
19.96 19.38 15.69 —
the annealing temperature increasing. That is to say, reducing the annealing time and increasing the annealing temperature is likely to depress the double yielding phenomenon of the neat PA6. These changes on the yield pattern might be related to the crystallinity levels of the polymer material.
The corresponding DSC melting curves and the crystallinity levels calculated by Eq. (1) are shown in Fig. 6 and Tables 3 and 4. The crystallinity data from Tables 3 and 4 make it clear that the crystallinity level decreases with the annealing time increasing, while increases with the annealing temperature increasing. It is normal that annealing treatment causes lamellar thickening as well as an increase in percentage crystallinity since the molecule chains have enough time and energy to arrange in the crystal lattice [18]. As to the reason why the crystallinity decreases with the annealing time increasing in our experiment, a much deeper investigation will be undertaken later. We repeated the experiment and got the same results. It is possible that the annealing temperature set at 100 1C in our study is too low to give enough energy to drive the molecule chain to move quickly and arrange in the crystal lattice, for the glass transition temperature of PA6 in our study is about 70 1C. Combining Figs. 4–6 and Tables 2 and 4 with the method suggested in this paper, it seems that the second yield point of neat PA6 will be depressed when the crystallinity levels of the sample increase and the second yielding process is not only associated with the deformation of the crystalline region. When the degree of crystallinity reaches a certain level, there is only one yield point in existence on the engineering stress-strain curves, as is illustrated in Fig. 5. Lucas et al. [5] has considered that the double yielding of the polyethylenes is only present in samples with the crystallinity level of 20–50%. It is similarly believable that a crystallinity window is also present in the double yielding of PA6. Some authors [8] have regarded that the existence of two distinct yield points in semicrystalline polymer is attributed to the deformation of the amorphous phase for the first yield and the deformation of the crystalline phase for the second phase. Thus, the second yield point should become larger with the crystallinity level increasing. However, the results of our experiments demonstrates that the second yield is not only associated with the deformation of the crystalline region. The DSC melting curves of PA6 in Fig. 6 show that a series of stepwise annealing treatments at steadily increasing temperature will result in a double endothermic peak. Moreover, the additional annealing peak at lower temperature shifts to higher temperature accompanied by an increase in its peak area magnitude. The results of the XRD show that this peak does not belong to the g-crystal form of
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Endotherm >
Endotherm >
G.-F. Shan et al. / Polymer Testing 25 (2006) 452–459
13h 7h 3h
160
180 200 220 240 Temperature (°C)
205 °C
I
190 °C I
1h
(a)
457
160 °C 130 °C
I 260
120 (b)
140
160 180 200 220 Temperature (°C)
240
260
Fig. 6. DSC heating curves of specimens of annealed PA6: (a) different annealing times and (b) different annealing temperature.
Table 3 DSC data and crystallinity of PA6 with different anneal time during heating scans (the annealing temperature is 100 1C) Anneal time Tm (1C) (h)
DHm (J/g)
Crystallinity (%)
Tonset (1C)
Tend (1C)
1 3 7 13
77.52 76.35 63.52 60.25
40.80 40.18 33.43 31.70
218.88 217.25 208.62 221.48
225.13 226.23 229.61 225.80
223.10 223.71 225.85 224.08
20.25
20.63
23.67
23.23
38.17
Table 4 DSC data and crystallinity of PA6 with different anneal temperatures during heating scans (the annealing time is 2 h) Anneal Tm (1C) temperature (1C)
DHm (J/g) Crystallinity (%)
Tonset (1C)
Tend (1C)
130
67.16 1.50 69.34 3.85 53.91 25.6 79.86
218.35 141.89 219.10 162.87 213.42 199.93 216.98
228.48 155.09 225.04 177.45 225.97 210.24 226.85
160 190 205
224.37 147.58 222.91 169.69 222.67 206.32 224.54
35.6 0.79 36.50 2.02 28.37 13.47 42.03
PA6. It has been reported that PA6 crystals can exist in two major forms including the monoclinic a-crystal and monoclinic (or pseudo-hexagonal) g-crystal form [17]. Two main reflections can be observed at about 2y ¼ 201 and 23.71, which are attributed to the a100 and a002/202 crystal planes, respectively. The reflection peaks at 2y ¼ 10:71 and 21.41 are associated with g020 and g001 crystal planes of PA6, respectively [22–25]. Thus, it is easy to observe in Fig. 7 that only the a-crystal form is present in the annealed PA6 samples. According to Yan et al. [26], the additional endothermic peak in
190°C
160°C
0
10
20
30
40
50
2θ Fig. 7. The XRD patterns of neat PA6 at different annealing temperatures.
the DSC melting curve is attributed to the melting peak of the imperfect crystallites under the low annealing temperature, while Wang et al. [27] considered that endotherm I is due to the melting of secondary lamellae. The crystal parameters, including the interplanar distance and the crystallite size, can be determined using the Bragg formula and Scherrer equation that have been indicated in a previous publication [15], and the corresponding results are listed in Table 5. These data suggest that both the interplanar distance and the crystallite size increase with the annealing temperature increasing, indicating that a process of crystal perfection takes place and the regularity of the crystal structure is much better at higher annealing temperature. In order to gain a better insight into the effect of crystallinity level on the double yielding behavior of PA6 and the multiple melting behavior of annealed
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Table 5 The crystal parameters of neat PA6 with different annealing temperatures (hkl)
a200 a002/202
PA6-160 1C
PA6-190 1C
PA6-160 1C
PA6-190 1C
PA6-160 1C
PA6-190 1C
20.5 23.7
20.3 23.5
0.43 0.38
0.44 0.38
2.50 2.51
3.57 3.58
are marked on the DSC curves. These results are also consistent with the conclusion that the second yield point is much more apparent with a decrease of the crystallinity of the specimens. Perhaps the second yield point will disappear as a result of the crystallinity percentage above about 42%. There are also two melting endothermic peaks on the DSC heating scan curves and the temperature of the lowmelting peaks increases with the annealing time increasing.
80 70
Stress (MPa)
2
1
60
3
50 4
40 30
1: unannealed 2: 0.5h 3: 1.5h 4: 3 h 5: 7 h 6: 12 h
5
20
Hhkl (nm)
dhkl (nm)
2y (deg)
6
10 0
4. Conclusion
-10 0
5
10
15
20 25 30 Strain (%)
35
40
45
50
Fig. 8. The engineering stress–strain curves of PA6 with different annealing times (T ¼ 190 1C).
Endotherm >
48.87% (12.0h) 45.12% (7.0h) 42.94% (3.0h) 42.84% (1.5h) 39.06% (0.5h)
140
160
180 200 Temperature (°C)
220
240
Fig. 9. DSC heating curves of annealed PA6 specimens with different annealing times (T ¼ 190 1C).
PA6 sample under high annealing temperature, some samples were annealed at different annealing time and the annealing temperature was set at 190 1C. The corresponding stress–strain curves and the DSC melting curves are shown in Figs. 8 and 9. The calculated crystallinity levels based on Eq. (1)
Double yield points before necking were observed in injection-molded specimens of polyamide 6 (PA6) under tensile loading. Generally speaking, the second yield point becomes much more apparent with the strain rate decreasing. A simple method is put forward to judge the apparentness of the first and second yield point. We have pointed out that the expandable angle a and the stress drops s are the important indexes in estimating the yielding process. If the a value is much smaller or the s value is much bigger, the yield point is much more apparent. Furthermore, using the method mentioned above, the relation between the degree of crystallinity and the double yielding has also been discussed. The level of the second yield stress becomes larger than that of the first yield stress after thermal treatment, which is contrary to the case without thermal treatment. The first yield point becomes more apparent with an increase of the crystallinity of the specimens, while the second yield point becomes less apparent with an increase of the crystallinity. These results show that the second yield is not only associated with the deformation of the crystalline region. Acknowledgements The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (Grant no. 50503014) and the financial support on the Fundamental of Application
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