ethylene-vinyl acetate copolymer blends

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POLYMER TESTING Polymer Testing 26 (2007) 388–395 www.elsevier.com/locate/polytest

Material Behaviour

Morphology and fracture behaviour of poly(vinyl chloride)/ ethylene-vinyl acetate copolymer blends Ying Liu, Bang-Hu Xie, Wei Yang, Wei-qin Zhang, Jian-min Feng, Ming-Bo Yang College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, Sichuan, People’s Republic of China Received 13 October 2006; accepted 15 December 2006

Abstract Poly (vinyl chloride) (PVC)/ethylene-vinyl acetate copolymer (EVA) blends were prepared to investigate the effect of EVA content on the phase morphology and fracture behaviour, evaluated by means of the essential work of fracture method (EWF). The results clearly showed the prominent influence of phase transformation on fracture toughness of the blends. At low EVA content (o7 phr), EVA was dispersed in the PVC matrix as separated small particles, and with increase in EVA content increasing the number of EVA particles increased, resulting in diminished distance between the dispersed particles and an increase in specific essential work of fracture (we). The enhanced we values were attributed to the increase of specific essential work of fracture in the necking and tearing stage (we,n), although the specific EWF before yielding (we,y) decreased. When EVA content was up to 9 phr, the EVA particles were observed to adhere to each other and formed a coherent phase, which caused sharp decrease of we as the result of simultaneous decrease of we,y and we,n. On the other hand, the specific plastic work, bwp, was barely affected by the increase of EVA content. r 2007 Elsevier Ltd. All rights reserved. Keywords: Polyvinyl chloride; Ethylene-vinyl acetate copolymer; Essential work of fracture; Morphology

1. Introduction Although polyvinyl chloride (PVC) is one of the most widely used commercial plastics, it has two shortcomings in commercial applications, poor processability and low impact strength. To overcome such shortcomings, the incorporation of elastomers in the PVC matrix is an efficient way that can avoid the escape of low molecular weight plasticizer from the matrix and, concomitantly, prolong the service life of the PVC. Among these Corresponding author. Tel./fax: +86 28 8540 5324.

E-mail address: [email protected] (B.-H. Xie). 0142-9418/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.12.008

elastomers, ethylene-vinyl acetate copolymer (EVA) is popularly employed. Investigation of miscibility, rheological behaviour, processability, surface properties, dynamic viscoelasticity and mechanical properties of PVC/EVA blends [1–5] were extensively reported. It can be found in the literature that, even at low EVA content (o10 wt%), a phase transition occurs with increasing EVA content [6–9]. However, little attention have been paid to the fracture behaviour of PVC/EVA blends, especially the relationship between the morphology and the fracture performance of the blends at various EVA contents, which is of great importance for the better application of the materials.

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The essential work of fracture (EWF) method was originally proposed by Broberg [10] and he characterized the fracture behaviour of metals and this work was further developed by Mai and Cotterell [11–13] to enable evaluation of the fracture behaviour of ductile polymer. Due to the simplicity of experiment and data manipulation, especially its applicability to evaluate the fracture behaviour of ductile polymer sheets and films, the EWF method has been gaining more attention and acceptance [14–16]. Levita [17] investigated the thickness dependence of fracture work parameters of rigid PVC sheets using this method. Maspoch and Arkhireyeva investigated the influences of thickness and test conditions, such as load speed and temperature, on the EWF parameters of unplasticized PVC sheets [18,19]. In this paper, the EWF method was primarily used to study the interrelation of the phase morphology and fracture performance for PVC/EVA blends at different EVA contents (below 10 wt% in the blends).

389

OPDZ

IFPZ Z

l

t W

2. Experiment

Fig. 1. DENT specimen used for EWF test.

2.1. Materials PVC-SG5 (Z ¼ 114, K value between 60 and 66 according to the manufacturer) serving as the matrix was a powder product offered by Yibin Tianyuan Company. EVA (Model Evaflex) with 28 wt% of vinyl acetate was purchased from Du Pont-Mistsui Poly Chemicals Co. Ltd., Japan. Organic stannous stabilizer, di(2-ethylhexyl)phthalate (DOP) and stearic acid were commercial products.

specimens were made perpendicular to the tensile direction with a fresh razor blade. In order to keep the specimens tested under plane-stress state and to avoid edge effects [20], the ligament length was limited in the following range: 3toloW =3.

(1)

The ligament lengths and the thickness of DENT specimens were measured using a microscope and a vernier caliper, respectively.

2.2. Sample preparation

2.3. Fractography

The dried PVC, organic stannous stabilizer, DOP and stearic acid were mixed in a high-speed mixer at pre-selected mass ratios. The resulting mixtures were plasticized on a two roll mill (SK-160B) at 160 1C for 5 min. Then EVA pellets were incorporated into the pre-plasticized mixture at PVC/EVA mass ratios of 100/0,100/3,100/5,100/7,100/9, and mixed for 3 min. The PVC/EVA blends were then compression molded into sheets of 0.75 mm thickness at 16575 1C and 10 MPa. The double edge notched tensile specimens (DENT: length  width ¼ 100  35 mm, illustrated in Fig. 1) were cut from the compression-molded sheets. The sharp pre-cracks of at least 1.2 mm length on both sides of the

The compression-molded sheets of the blends were immersed in liquid nitrogen for about 20 min and then fractured immediately. After sputtering gold on the fracture surface to make the samples conductive, the microstructures of the composites were observed using a scanning electron microscope (SEM: JSM-5900LV, JEOL), with an accelerating voltage of 20 kV. 2.4. Fracture tests The fracture tests were conducted on an Instron (4302) universal test machine equipped with a 500 N load cell at a cross-head speed of 5 mm/min and at a

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test temperature of 2372 1C. The energy consumed from the onset of loading to failure of the samples was calculated from the area of the load–displacement curves. 3. Results and discussion 3.1. Morphology Fig. 2 shows the SEM photos of the fracture surface of PVC/EVA blends with different EVA content. It can be seen in Fig. 2(a) that the fracture surface of plain PVC is quite smooth and no particles could be observed. However, particles and holes which are homogeneously distributed on the fracture surface can be observed for the blends with EVA incorporated, which corresponds to EVA particles and holes left by pulled-out particles, respectively. Shur found that the compatibility of EVA and PVC was considerably influenced by the VA content, and the increase of VA content promoted the interaction between PVC and EVA,

resulting in enhancement of the compatibility between EVA and PVC molecules [7]. Baek et al. further confirmed that when the VA content was relatively low (o45 wt%), PVC/EVA blends were incompatible or only partially compatible [2,9]. The VA content of EVA used in our work being about 28 wt%, incompatibility would be expected according to above theories. However, the experimental results (Figs. 2(b) and 2(c)) show a certain degree of plastic deformation around particles and holes (formed by pulling out particles), indicating that the PVC matrix phase and EVA separated phase have partial compatibility, which is mainly due to their similarity in molecule polarity. As seen in Fig. 2(b), when the EVA content is 3 phr, only a small quantity of spherical particles and holes of 0.05–0.2 mm diameter are spread on the fracture surface, with little coalition of particles and holes. As the EVA content reaches 7 phr, the quantity of particles and holes increases markedly. However, the size of the individual particles remains almost unchanged, although some particles may

Fig. 2. Scanning electron micrographs of the fracture surface at a magnification of  15 000 for PVC/EVA blends at different mass ratios: (a) 100/0; (b) 100/3; (c) 100/7; (d) 100/9.

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connect with each other (Fig. 2(c)). With further increase of EVA content (up to 9 phr), numerous large particles and holes join together (Fig. 2(d)). Baek et al. have showed that at low EVA content (o7.5 wt%, with a VA content of 45 wt%) in PVC/ EVA blends, PVC constitutes the continuous phase, while EVA forms the dispersed phase in the form of individual particles, and when EVA content is up to 7–8 wt%, it begins to form a coherent phase, and finally forms the continuous phase at higher EVA content [9]. Similar phenomena, as well as the formation of co-continuous phase of PVC/EVA blends and even phase transformation, have also been reported in the literature [6–8]. Therefore, it can be deduced that the morphology of blends as shown in Fig. 2(d) is related to the formation of a coherent or continuous phase of EVA in PVC. 3.2. Load– displacement curves Fig. 3 exemplifies the load–displacement curves of DENT specimens with the same ligament length for pure PVC and the blends with different EVA content. It is clear that the shape of the curve was significantly influenced by the EVA content. The neat PVC shows a fracture mode of steady propagation of the crack after the ligament fully yielded, which is consistent with the characteristic of post-yielding behaviour [21]. At first, the load increases quickly to the maximum with slight increase of the displacement. With displacement increasing, a sudden load drop occurs and, subsequently, the load decreases slowly and smoothly until the fracture of the DENT specimen (Fig. 3(a)). Such a fracture process was analogous to that of 200 a

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uPVC sheets reported by Arkhieyeva [19], showing extensive yielding and ductile tearing of the ligament region preceding the failure of DENT specimens [18,19]. Compared to neat PVC, the addition of 3 phr EVA in PVC leads to the reduction of the maximum load as well as the rate and extent of load drop after yielding, as shown in Fig. 3(b). Simultaneously, the displacement in the stage of steady load decline increases, although the displacement before yielding decreases. Furthermore, these variations are more pronounced at higher EVA content (Figs. 3(c) and (d)). At an EVA content of 9 phr, it is hard to distinguish the rapid load drop after the load maximum and the subsequently steady load decline after yielding. The load–displacement curves of DENT specimens with different ligament lengths during EWF tests are shown in Fig. 4. It is obvious that within each group the load–displacement curves are very similar. The maximum load, the displacement to failure and the area under the curves increase regularly with increasing ligament length, indicating that the mode of fracture is independent of ligament length, which further ensures the validity of the EWF method [14–16,18,19]. 3.3. Fracture parameters According to the EWF concept, the total energy required to fracture a pre-cracked specimen can be partitioned into the EWF, We, and the non-essential or plastic work of fracture, Wp [11,22] ( Fig. 1) . We is essentially a surface energy dissipated in the inner fracture process zone (IFPZ) to generate new crack surface during the fracture of the specimen, while Wp is a volume energy dissipated in the outer plastic deformation zone (OPDZ). The relationship can be written as follows:

Load (N)

150

W f ¼ W e þ W p ¼ we lt þ bwp l 2 t,

b 100 c 50 d 0 0.0

0.5

1.0

1.5

2.0

Displacement (mm)

Fig. 3. Load–displacement curves of DENT specimen with ligament length of 4 mm: for PVC/EVA blends at different mass ratios: (a) 100/0; (b) 100/3; (c) 100/7; (d) 100/9.

(2)

where we and wp are the specific EWF and specific non-essential work of fracture (specific plastic work), respectively. b is the shape factor related to the plastic zone and bwp is the plastic work term which represents the total plastic energy consumption of the specimen during the process of deformation. The specific total work of fracture (wf) dissipated per unit ligament area can thus be obtained through the following equation: wf ¼ W f =tl ¼ we þ bwp l.

(3)

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a

450

b

400

400

350

350 Ligament length increasing

Ligament length increasing

300 Load (N)

300 Load (N)

450

250 200

250 200

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150

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50

0

0 0.0

0.5

1.0

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0.5

1.0

Displacement (mm)

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d

300

2.0

2.5

3.0

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350 300

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250

Ligament length increasing Load (N)

Load (N)

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Displacement (mm)

200 150

Ligament length increasing 200 150

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100

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50

0

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

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4.0

Displacement (mm)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Displacement (mm)

Fig. 4. Load–displacement curves for:PVC/EVA blends at different mass ratios: (a) 100/0; (b) 100/3; (c) 100/7; (d) 100/9.

Furthermore, taking the peak of the load–displacement curve as the cut-off point, Wf can be partitioned into the energy dissipated in the stage before yielding, i.e., Wf,y, and that dissipated in the necking–tearing stage after yielding, i.e., Wf,n [20,23]. Accordingly, the following equations can be written: wf ¼ wf ;y þ wf ;n , wf ;y ¼ we;y þ b0 wp;y l;

(4) wf ;n ¼ we;n þ b00 wp;n l,

(5)

where we,y and we,n are the specific EWF before and after yielding, respectively, and b0 wp,y and b00 wp,n are the corresponding plastic work terms. The plots of wf and corresponding wf,y, wf,n versus l of PVC/EVA blends are shown in Fig. 5. It is worthwhile noting that the diagrams present good linear relationships for all the samples, proved by the linear regression coefficient (R2) being in most cases higher than 0.90. The values of we and bwp obtained from the intercept and slope of the wf –l plots extrapolated to zero ligament length and the

fracture parameters at different stages in the fracture process are listed in Table 1. Theoretically, we is a material constant, only dependent on the thickness (t) and independent of the geometry of the specimen, which characterizes the fracture toughness in EWF theory. Mai and Cotterell [11] have proved that we is equivalent to the critical value, JIC, in the J integral method, which has also been validated by other reseachers [24–26]. When the blends contain a low EVA content (3–5 phr), the fracture toughness (we) of PVC/EVA blends is higher than that of neat PVC, as shown in Table 1, which coincided with the increase of the number of tiny EVA particles. In PVC/EVA blends, EVA particles act as stress concentrators and can induce the yielding of the PVC matrix [12]. As EVA content increases, the amount of EVA particles multiplies and the distance among particles decrease, so that the interactions among the stress field are enhanced and, consequently, the fracture toughness, ie. the crack resistance, is improved. When EVA content rises

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b 80

80

70

70

60

60

wf , wy , wn (kJ/m2)

wf , wy , wn (kJ/m2)

a

50 40 30

50 40 30

20

20

10

10 0

0 0

2

4

6 l (mm)

8

10

12

0

2

4

6

8

10

8

10

12

l (mm)

d

c 80

80

70

70

60

60

wf , wy , wn (kJ/m2)

wf , wy , wn (kJ/m2)

393

50 40 30

50 40 30

20

20

10

10

0

0 0

2

4

6

8

10

12

12 0

2

6

4

l (mm)

l (mm)

Fig. 5. wf, wy, wn–l curves of PVC/EVA blends at different mass ratios (a) 100/0; (b) 100/3; (c) 100/7; (d) 100/9 (’, wf ; K, wy;m, wn).

Table 1 Constituting terms of essential work of fracture for the PVC/EVA blends with varied content of EVA PVC/EVA (mass ratio)

we (KJ/m2)

bwp (MJ/m3)

we,y (KJ/m2)

we,n (KJ/m2)

b0 wp,y (MJ/m3)

b00 wp,n (MJ/m3)

100/0 100/3 100/5 100/7 100/9

12.60 14.69 15.78 19.71 13.07

5.85 5.95 4.67 4.54 4.55

7.81 4.34 4.01 5.04 2.91

4.79 10.35 11.22 14.88 10.17

2.92 2.98 1.99 1.53 1.43

2.93 2.97 2.74 2.99 3.11

from 5 to 7 phr, the we value of the blends further increases up to a maximum. This may be related to the morphology of the maximum number of independent tiny EVA particles without the obvious

formation of coherent phase, as shown in SEM photos. In this case, the average distance between particles (including the holes which represent the pulled-out EVA particles) is minimal. Wu has

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proposed that, in polymer/rubber blends, the toughness will increase with decrease is average distance between rubber particles, and even a sharp brittle–tough transition may occur when the average distance is at the critical value [27]. However, when the EVA content is up to 9 phr, the we value of the blend decreases sharply instead of a continual increase. This could be attributed to the formation of an EVA coherent phase, or even a continuous phase. The combination of EVA particles results in the decreasing number and increasing size of the dispersed particles, hence the average distance between the EVA particles increases. Accordingly, it could be concluded that for a certain content, the toughening effect of EVA as the dispersed phase with appropriately fine dimensions would be better than that with larger particle size. On the other hand, it could be speculated that the fracture toughness of the blends is hardly improved, or even decreases to a certain extent, when the EVA content increases to form the co-continuous phase or to start the complete phase inversion. Lach has reported that in poly (styrene-block-butadiene) star block copolymer/polystyrene blends [28] the fracture toughness of the material is significantly affected by the phase transition. It can be seen from Table 2 that when the EVA content is 3–7 phr, the specific EWF before yielding (we,y) decreases, while the specific essential work of fracture in the necking–tearing stage (we,n) increases markedly with increasing EVA content. It is therefore concluded that in such a range of EVA content, the increase of we values is due to the improvement of the crack resistance after yielding. Both the values of we,y and we,n decrease when EVA content increases from 7 to 9 phr. So, the value of we,n was more strongly affected by the number of separated tiny EVA particles and the average distance between them. we and bwp are two energy consumption parameters with intrinsic differences. we represents the energy dissipated in the formation of new surfaces of cracks, which is in fact the work required to fracture a specimen completely, while bwp is usually used to characterize the total plastic work dissipated around the fracture route. It can be seen from Table 1 that when EVA content is lower than 7 phr the bwp values decrease with increasing EVA content, just opposite to the variation of we values. This is in accordance with the results of Karger–Kocsis [29,30] in rubber-toughened, fiber-reinforced and particle-filled polymer systems that we and bwp

cannot be improved simultaneously. Nevertheless, when the majority of EVA is dispersed in the PVC matrix in the form of individual tiny particles, the increment of we is greater than the reduction of bwp with increasing EVA content. When the amount of EVA increases from 7 to 9 phr, bwp also stays unchanged while we decreases. Hence, it is concluded that the aggregation of EVA particles or the formation of a continuous EVA phase shows insignificant effect on plastic energy absorption. Furthermore, according to Table 1, b0 wp,y decreases with increasing EVA content while a variation of b00 wp,n is not evident. 4. Conclusions The fracture behaviour, especially the crack resistance is greatly influenced by the phase morphology of PVC/EVA blends. When EVA is dispersed individually in the PVC matrix as a discontinuous phase, we, i. e. the crack resistance of the blends, increases remarkably with increasing EVA content which leads to small average distances between the particles, while the variation of bwp is very slight, indicating that the influence of EVA content on the total plastic energy consumption is not so great. However, when the EVA content increases to form a coherent or continuous phase, we shows a sharp decrease while bwp is almost unchanged. It is concluded that the effect of increasing EVA content on the fracture performance of PVC/EVA blends was mainly achieved through its pronounced influence on the fracture properties in the stage of necking and tearing. Acknowledgements The authors gratefully acknowledge the financial support of this work by National Natural Science Foundation of China (Grant Nos. 50673066, 10590351) and Fundamental of Application Research Project from Sinopec Corp. (No. X504004). References [1] D.C. Mcconnell, G.M. Mcnally, W.R. Murphy, J. Vinvl. Addit. Technol. 8 (2002) 194. [2] R. Hernandez, J.J. Pena, L. Irusta, A. Santamaria, Eur. Polym. J. 36 (2000) 1011. [3] C. Thaumaturgo, E.E.C. Monteiro, Compos. Sci. Technol. 57 (1997) 1159.

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