Fracture and impact strength of poly(vinyl chloride)/methyl methacrylate/butadiene/styrene polymer blends.
It is known that plastic failure changes between ductile and brittle behavior, depending on polymer type, shape/size of the test specimen, loading (test method) and test conditions (e.g. test temperature and test speed). A comprehensive explanation for this complicated fracture behavior has been made, using the relationship between [[Sigma].sub.c], (the craze initiation stress) and [[Sigma].sub.y] (the shear yield initiation stress) (1). It was suggested that when [[Sigma].sub.c] [less than] [[Sigma].sub.y], brittle fracture occurs with crazing, and when [[Sigma].sub.c] [greater than] [[Sigma].sub.y], ductile fracture occurs with shear yielding.
There are several methods for introducing rubbers for greater impact strength, such as simple blending or grafting by polymerization; the latter is often used commercially. A typical example is acrylonitrile-butadiene-styrene (ABS), which involves graft polymerization onto rubber particles.
Poly(vinyl chloride) (PVC) offers the advantages of excellent clarity and low cost, but it has a disadvantage of low impact resistance. Therefore, methyl methacrylate-butadiene-styrene (MBS) resin has been developed for improving the impact resistance of PVC. The MBS resin is a copolymer in which styrene and methyl methacrylate are graft polymerized onto butadiene-styrene rubber particles. The addition of MBS can provide high impact resistance and yet clear PVC for bottles or sheets.
Table 1. Methyl Methacrylate-Butadiene-Styrene Resin Characteristics. SBR MMA Styrene Particle Diameter MBS (%) (%) (%) ([Angstrom]) MBS-1 70 15 15 840 MBS-2 70 15 15 1690 MBS-3 70 15 15 2050 MBS-4 70 15 15 2350
Complicated fracture behavior is observed in the experiments of PVC/MBS blends, depending on test methods and/or conditions. There have been several explanations (2-4). In our previous paper, fracture behavior was classified into three types, defined by fracture mode (ductile/brittle), and the degree of stress whitening on deformation. The mechanisms of energy absorption in each fracture type were discussed (5). It was suggested that a variety of fracture behavior could be classified into three types of ductile fracture without whitening (Fracture I), ductile fracture with whitening (Fracture II), and brittle fracture with little whitening (Fracture III), and that shear yielding occurs mainly in Fracture 1, both shear yielding and crazing occur in Fracture II, and rubber deformation and partial crazing occur in Fracture III. A general relationship between the types of fracture and the mechanism of energy absorption was developed. It was presumed that the mechanism of energy absorption was within the three-type-classification.
Table 2. Conditions of Tensile Test. Tensile Condition Notch Speed (m/s) Condition A V 3.7 Condition B R = 2mm 3.7 Condition C R = 5mm 3.7 Condition D R = 2mm 1.6 x [10.sup.-4]
In this paper, the fracture and impact resistance of PVC/MBS blends in Izod impact tests, carried out with different MBS levels and particle size are explained by classifying them into three types. This paper also describes the characteristics of the factors controlling crazing and shear yielding by tensile experiments using various types of MBS, and an explanation is offered for the dependence of Izod impact strength of PVC/MBS on MBS particle size and with a maximum around a particle size of 2000 [Angstrom].
Fifteen weight-percent of methyl methacrylate and 15 wt% of styrene were graft polymerized onto 70 wt% of styrene-butadiene rubber particles. Four types of MBS resin of having different particle size were prepared (Table 1).
A commercial PVC resin (Kanevinyl S-1007, P = 700), MBS resin and octyltin mercaptide stabilizer (1%) were mixed for 5 min on a heated roll at 160 [degrees] C, and made into a sheet of 1.3 mm thickness. The sheet was then compression molded for 15 min at 180 [degrees] C to form 5.0 mm thickness, and test specimens, with a V notch cut by using a circular saw for Izod impact test, were made from this sheet. For tensile test, the sheet of 1.3 mm thickness was compression molded for 15 min at 180 [degrees] C to form 1 mm thickness and tensile specimens with three types of V notches, shown in Fig. 1, were made from the sheet.
Izod impact strength measurements were carried out by using an Izod impact tester (Toyo Seiki). Tensile measurements were made using an Autograph IS-500 (Shimazu) and CEAST MK-400 (CEAST). SEM and TEM analysis of the fractured surfaces were done with a Hitachi S-520 scanning electron microscope and Nihon Denshi JEM-1200EX transmission electron microscope. For TEM observations, ultrathin specimens were cut in a vertical direction to the fracture surface and stained with osmium tetraoxide.
Calculation of Inter-Particle Distance
MBS particles were dispersed uniformly in the PVC as detailed later. The inter-particle distance of the MBS was calculated by the following procedures: When 100g of PVC (specific gravity 1.389) and Xg of MBS (specific gravity 0.985) are mixed and molded, the total weight of MBS in 1 [cm.sup.3] of the specimen, designated by m, can be calculated using Eq 1. A relationship between the number of MBS particles, N, and the radius of one particle, r, is given in Eq 2. When small balls are closed-packed in 1 [cm.sup.3], then the relation between the radius of one ball R and the number of the packed balls is expressed by Eq 3. Then the inter-particle distance of the MBS is calculated by Eq 4. [ILLUSTRATION FOR FIGURE 2 OMITTED].
m = X / (100/1.389) + (X/0.985)
m = (4[Pi][r.sup.3]/3) x 0.985 x N(2)
(4[Pi][R.sup.3]/3) x N = 0.74(3)
L = 2R - 2r = 2(R - r) (4)
RESULTS AND DISCUSSION
Fracture Behavior and Izod Impact Strength
The relationships between the Izod impact strength and MBS particle size and concentration are shown in Fig. 3. Figure 4 shows the dependences of Izod impact strength, fracture mode, and the degree of whitening on MBS content for a MBS particle size on 2350 [Angstrom]. As shown in Figs. 3 and 4, brittle fracture, with little whitening (Fracture III) is observed on fracture and the Izod impact strength is low at an MBS level of 1 to 6 phr. According to our previous paper, rubber deformation and crazing occur mainly in Fracture III (5). Beyond the MBS level of 10 phr, then ductile fracture with whitening (Fracture II) can be seen and the Izod impact strength becomes high. It is suggested that shear yielding and crazing occur in Fracture II. These ideas are reasonable from the microscopic observations of the fractured surface shown in Figs. 5 and 6. Figure 5 shows that the beard-like pattern, characteristic of shear yielding, is not observed in the region of MBS level of 1 to 6 phr. This means that shear yielding did not occur, Figure 6 shows crazes in the fractured surface, therefore, rubber deformation and crazing should occur mainly in this region. Beyond the MBS level of 10 phr, the beard-like pattern, some traces of crazing, voiding in rubbers which is thought to be evidence that yielding occurs after crazing are observed, and these results are in accord with the ideas discussed in our previous paper (5).
Figure 7 indicates that the relation between the Izod impact strength of PVC/MBS blends and MBS particle size shows a maximum around a particle size of 2000 [Angstrom] (6, 7). This phenomenon suggests that the mechanism of energy absorption changes. Fracture below and above 2000 [Angstrom] are classified into different types and will be analyzed further.
The dependences of the Izod impact strength, the fracture mode, and the degree of whitening on MBS particle size are indicated in Fig. 7 for an MBS level of 10 phr. When the particle size is 840 to 1690 [Angstrom], brittle fracture with little whitening (Fracture III) is observed, and the Izod impact strength is low. In Fracture III, rubber deformation and crazing mainly occur. When the MBS particle size is 1690 to 2350 [Angstrom], ductile fracture with whitening (Fracture II) is observed and the Izod impact strength is high. Shear yielding and crazing occur in Fracture II. In Fig. 8, the beard-like pattern is not observed for particle sizes of 840 to 1690 [Angstrom]. This means that shear yielding does not occur in this region. Crazes can be seen at the fractured surface shown in Fig. 9. This shows that rubber deformation and crazing occur mainly in this range of particle size. When the particle size is 1690 to 2350 [Angstrom], then the beard-like pattern, characteristic of shear yielding, is seen [ILLUSTRATION FOR FIGURE 8 OMITTED], and voiding in rubber is thought to be evidence of shear yielding after crazing [ILLUSTRATION FOR FIGURE 9 OMITTED]. These phenomena are in accord with our previous paper (5).
As above, the fracture behavior of PVC/MBS blends was classified and analyzed, with the following results: Up to a MBS particle size of 2000 [Angstrom], the Izod impact strength increases with particle size, and crazing is the main and energy-absorbing mode. Above 2000 [Angstrom], the Izod impact strength decreases with increasing particle size, and shear yielding also occurs. The impact energy is thus absorbed by both crazing and shear yielding. It is supposed that these differences in Izod impact dependence on particle size imply a different mechanisms of energy absorption between these two regions (the influence of MBS particle size upon shear yielding is assumed to be opposite to that upon crazing).
The fracture behavior of PVC/MBS blend changes significantly, dependent on which occurs mainly, crazing or shear yielding. Inducing shear yielding is important for improving the Izod impact strength.
Factors Controlling Crazing and Shear Yielding
Tensile experiments were carried out at diverse test conditions using four types of MBS of different particle size for studying the factors controlling crazing and shear yielding which are known to influence fracture behavior and the impact resistance. The MBS level was changed from 0.5 to 16 phr, three different notch shapes were used (notch tip; R = 2 mm, R = 5 mm, and V-notch). The tensile speed was 3.7 m/s for the fast deformation and 1.6 x [10.sup.-4] m/s for the slow, see Table 2. Pictures of the fractured specimens are shown in Figs. 10 to 13. Fracture can be classified into three types defined by the combination of the fracture mode and the degree of whitening. The same specimen can show a variety of fracture patterns. Under Condition A (V-notch, 3.7 m/s) and Condition B (notch tip; R = 2 mm, 3.7 m/s), most specimens show brittle fracture with little whitening (Fracture III), except for samples of highest MBS level (10 to 16 phr), which show ductile fracture with whitening (Fracture II) [ILLUSTRATION FOR FIGURE 14 OMITTED]. Under Condition C (notch tip; R = 5 mm, 3.7 m/s), ductile fracture with whitening (Fracture II) is mainly observed. Under Condition D (notch tip; R = 2 mm, 1.6 x [10.sup.-4] m/s), ductile fracture without whitening (Fracture I) is mainly observed.
These results show that crazing mainly occurs under Condition A and B, as shown in Figs. 15 and 16. The beard-like pattern, characteristic of shear yielding is not observed, see Fig. 15 (MBS level of 0.5 to 6 phr, Fracture III), and crazing can be seen in Fig. 16. The same specimen fractured under Condition D shows ductile fracture without whitening, and has the beard-like pattern shown in Fig. 17. Figure 18 shows no crazing, and MBS rubber particles are elongated in one direction indicating shear yielding.
As explained above, the fracture of PVC/MBS blends change among brittle fracture with little whitening (Fracture III), ductile fracture with whitening (Fracture II), and ductile fracture without whitening (Fracture I), by changing the notch type and the tensile speed.
The energy absorption important for shear yielding is calculated from the tensile test results of Condition A[similar to]D, as shown in Fig. 19. The energy absorption increases with MBS level for all conditions. For Condition A under which crazing mainly occurs, the energy absorption increases with MBS particle size, and for Condition D under which shear yielding occurs mainly, the amount of energy absorption decreases with MBS particle size. The dependence of energy absorption on the inter-particle distance of MBS is shown in Fig. 20. For Condition A and Condition B, under which energy is mainly absorbed by crazing, the energy absorption is strongly affected by the inter-particle distance and MBS particle size. For Condition D, under which shear yielding mainly occurs, the energy absorption is independent of MBS particle size, depending only on the inter-particle distance. It is suggested that the impact energy absorption of nylon/rubber blend depends only on the inter-particle distance of the rubber particles (8). Hence, the result of Condition D seems to be the same as for nylon/rubber blends. The dependence of yield stress on the inter-particle distance shown in Fig. 21 indicates that yield stress of PVC/MBS blend is affected, not only by the inter-particle distance, but is also strongly dependent upon MBS particle size. Figure 22 shows the dependence of breaking strain on the inter-particle distance. It is obvious that breaking strain of PVC/MBS blends depends only on the inter-particle distance. Therefore, the energy absorption by shear yielding strongly depends on MBS inter-particle distance. This may be ascribed to the dependence of breaking strain on the inter-particle distance, and not to yield stress.
From the above tensile experiment results under Condition A[similar to]D, we conclude that the energy absorption by crazing decreases with increasing MBS particle size and with decreasing inter-particle distance. We conclude that the energy absorption by shear yielding is independent of MBS particle size and is controlled by inter-particle distance, that is, when MBS levels are the same. MBS of smaller size is effective for enhancing the energy absorption by shear yielding.
The phenomenon of the Izod impact strength of PVC/MBS blend increasing with MBS particle size up to 2000 [Angstrom] and decreasing above 2000 [Angstrom] can be explained: below 2000 [Angstrom], the energy absorption by crazing dominates the total energy absorption. The energy absorption by crazing increases with MBS particle size. Above 2000 [Angstrom], the energy absorption by shear yielding is dominant, and the energy absorption by shear yielding increases with decreasing inter-particle distance: That is to say, with decreasing MBS particle size.
The following conclusions were derived from the above experiment:
1) The fracture of PVC/MBS in Izod impact tests may be classified into two types, brittle fracture with little whitening and ductile fracture with whitening. In the former, crazing and partial rubber deformation occur, and in the latter, crazing and shear yielding occur.
2) The dependence of the Izod impact strength of PVC/MBS blend on MBS particle size confirms the maximum around the MBS particle size of 2000 [Angstrom]. When the particle size is smaller, the Izod impact strength increases with MBS particle size, and crazing mainly occurs. When MBS particle size is larger than 2000 [Angstrom], then the Izod impact strength, in contrast, decreases with increasing MBS particle size, with crazing and shear yielding dominating.
3) The fracture of PVC/MBS blends changes significantly, controlled by the energy absorbing modes of shear yielding and crazing. Induced shear yielding is important for increasing the Izod impact strength.
4) Tensile experiments of PVC/MBS blends carried out under various conditions show that the amount of energy absorption increases with decreasing MBS inter-particle distance and with increasing MBS particle size, when crazing is the main energy absorbing mode. The inter-particle distance of MBS dominates the amount of energy absorption when shear yielding is the main energy absorbing mode.
5) Therefore, the phenomenon of Izod impact strength of PVC/MBS blends showing the maximum around the MBS particle size of 2000 [Angstrom] can be explained as follows: Below 2000 [Angstrom], the energy absorption by crazing is dominant. The energy absorption by crazing increases with MBS particle size. Above 2000 [Angstrom], the energy absorption by shear yielding is dominant, and the energy absorption by shear yielding increases on decreasing the inter-particle distance, that is to say, decreasing the MBS particle size.
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|Author:||Takaki, Akira; Yasui, Hideo; Narisawa, Ikuo|
|Publication:||Polymer Engineering and Science|
|Date:||Jan 1, 1997|
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