Toughening modification of Poly(vinyl chloride)/ [alpha]-methylstyrene-acrylonitrile-butadiene-styrene copolymer blends via adding chlorinated polyethylene.
Nowadays poly (vinyl chloride) (PVC) has already been widely used in the form of pipes, sheets, cables, wood composites, etc (1-7). However, for rigid PVC, its low heat distortion temperature (HDT) has restricted its wider use especially in some harsh environment and conditions. A very effective way to solve this deficiency is via adding a polymer that both exhibits a higher glass transition temperature ([T.sub.g]) and is compatible with PVC (8). Based on this discipline, [alpha]-methylstyrene/styrene/ acrylonitrile copolymer (9), [alpha]-methylstyrene/acrylonitrile copolymer ([alpha]-MSAN) (10-13), chlorinated PVC (14), (15), imide polymers (16), (17) and styrene/maleic anhydride copolymer (18), (19) have already been reported. Unfortunately, incorporation of these polymers inevitably contributes to the loss in fracture toughness, albeit they can enhance the HDT of PVC.
Despite the aforementioned drawback, rigid PVC is very sensitive to the notch and its notched impact strength is very low especially at low temperature. To overcome this disadvantage, several toughening modifier such as chlorinated polyethylene (CPE), acrylic resin (ACR), methyl methacrylate-butadiene-styrene copolymer (MBS), nitrile butadiene rubber (NBR) and ethylene-vinyl acetate copolymer (EVA) have been introduced and widely reported (2). But none of these polymers play a positive role in the HDT of PVC, and high dosage of these modifiers even result in a reduction in HDT (2), (20). Thus, it is very meaningful and necessary to overcome these two deficiencies of rigid PVC simultaneously.
To solve these problems at the same time, both a-methylstyrene/acrylonitrile copolymer and toughening modifiers (CPE and ACR) are introduced in our previous work to produce a ternary blends that is combined with high toughness and high HDT (21), (22). [alpha]-MSAN has a higher glass transition temperature compared with that of pure PVC (10-13), and its miscibility window between PVC and a-MSAN is very narrow (23), indicating that addition of a-MSAN could improve the HDT of PVC but embrittle rigid PVC. Either incorporation of CPE or ACR into PVC/a-MSAN binary blends could increase the toughness (21), (22), and ACR exhibits a higher toughening efficiency (22).
In this article, PVC blends with largely improved toughness have been successfully prepared via blending PVC, [alpha]-methylstyrene-acrylonitrile-butadiene-styrene copolymer (AMS-ABS) and CPE together. AMS-ABS is the product of modified [alpha]-MSAN. Like [alpha]-MSAN, it is compatible with PVC and has a higher HDT than PVC (24). Moreover, the polybutadiene rubber that is integrated with a-methylstyrene-acrylonitrile-styrene matrix could improve the toughness of PVC to some extent, which is justified by our previous paper (25). On the other hand, CPE has a long reputation to act as an impact modifier for PVC (20). CPE with 36 wt% chlorine is immiscible with PVC (26) and could provide PVC with excellent impact strength and processability (27-29). Although no literatures about AMS-ABS/CPE blends has been reported, Hwang and Kim (30) reported that the impact strength of styrene-acrylonitrile copolymer (SAN) was dramatically increased with the introduction of CPE and the brittle-ductile transition (BDT) was observed at the composition of 30-40 wt% CPE. Thus, it is logical to incorporate CPE in PVC/AMS-ABS binary blends for further toughening.
To characterize the structure and properties of the blends, the effect of CPE on the [T.sub.g], HDT and thermal stability of PVC/AMS-ABS (70/30) blends was investigated. When it comes to the mechanical properties, both toughness and strength were determined. The toughening mechanism was also proposed based on the results of morphology.
Materials and Sample Preparation
PVC used in this study was a suspension grade resin (S-1000) provided by Sinopec Qilu, China with a K value of 66. The AMS-ABS copolymer, with the commercial name of BLENDEX 703 and 14 wt% of butadiene, was supplied by Chemtura, America. CPE (135A), with 36 wt% of chlorine, was produced by Weifang Yaxing Chemical. Other additives, such as organotin, calcium stearate and polyethylene wax were all industrial grade and utilized as stabilizer, metal and PVC surface lubricant and slip lubricant, respectively.
PVC 70 phr, AMS-ABS 30 phr, organotin 1.5 phr, calcium stearate 1 phr and polyethylene wax 0.8 phr were premixed in a high-speed mixer at 85[degrees]C for 10 min to obtain the PVC compound. The obtained PVC/AMS-ABS (70/30) compound and CPE were melt mixed with a two-roll mill at 180[degrees]C for 10 min, followed by molding into the sheets with the thickness of 2 and 4 mm by compression-molding at 180[degrees]C. The content of CPE was varied, and the blend ratios of PVC/AMS-ABS/CPE ternary blends were based on the mass fraction of polymers (70/ 30/0, 70/30/3, 70/30/5, 70/30/10, 70/30/12, 70/30/15, and 70/30/20).
Glass Transition Temperature
The glass transition temperature values were determined by using a differential scanning calorimeter (Q200, TA). Each sample that is about 10 mg was first scanned from room temperature to 180[degrees]C at a heating rate of 40[degrees]C [min.sup.-1], then quickly followed by being quenched to 0[degrees]C at a heating rate of 40[degrees]C [min.sup.-1] and scanned for the second time to 180[degrees]C at a heating rate of 10[degrees]C [min.sup.-1]. The glass transition temperature was defined as the midpoint of the transition.
Heat Distortion Temperature
To determine the heat distortion temperature, Vicat/ HDT equipment (ZWK1302-2, Shenzhen SANS Testing Machine, China) was used. The development of the tests were conducted at a heating rate of 120[degrees]C [h.sup.-1] under the maximum bending stress of 1.80 and 0.45 MPa, respectively, following ISO 75-1.
Thermogravimetric analysis (TGA) was carried out by using a TGA (Pyris 1, Perkin-Elmer, USA) in a nitrogen flow (20 ml [min.sup.-1]]). Each scan was conducted from 50 to 600[degrees]C with a heating rate of 20[degrees]C [min.sup.-1].
Tensile tests and flexural tests were carried out by a universal testing machine (CMT 5254, Shenzhen SANS Testing Machine, China) according to ISO 527 and ISO 178 with a speed of 5 and 2 mm [min.sup.-1], respectively. To investigate the influence of CPE on the notch sensitivity of the blends, different notch depth including 2.0, 3.0, 4.0, 5.0, 5.5, and 6.0 mm were introduced in each blends. The notched Izod impact strength of specimens with different notch depth was determined by an impact tester (UJ-4, Chengde Machine Factory, China), following ISO 180.
Scanning electron microscopy (JSM-5900, JEOL, Japan) was used to observe the impact-fractured surfaces of the blends with an accelerating voltage of 15 kV. The chosen specimens were with a notch depth of 2.0 mm. The impact-fractured surfaces were coated with gold before viewing and the observed location was laid in the central regions of the surfaces. Neither staining nor any other chemical treatments were used in this study.
RESULT AND DISCUSSION
Glass Transition Temperature Differential scanning calorimetry (DSC) curves of pure PVC and different blends are presented in Fig. I. Pure PVC/AMS-ABS (70/30) blends exhibit a single [T.sub.g] of 88.8[degrees]C, the value of which is higher than that of pure PVC (80.5[degrees]C) but lower than that of pure AMS-ABS (120.0[degrees]C). This feature indicates that PVC is miscible with the [alpha]-methylstyrene/acrylonitrile-styrene component in AMS-ABS. With regard to the chlorinated polyethylene, the glass transition behaviors are not exhibited due to the lack of low-temperature condition in DSC tests. However, it is expected to be--21.2[degrees]C for CPE with 36% of chlorine following the below equation (31):
[T.sub.g]([degrees]C) = 0.09[x.sup.2]--4.64x + 29.19 (1)
where x is the content of chlorine. With the incorporation of CPE into PVC/AMS-ABS (70/30) binary blends, one can see that no obvious changes occur in the glass transition behaviors, and the ternary blends still exhibit a single [T.sub.g]. This feature at least suggests that incorporation of CPE does not play a negative role in the compatibility between PVC and AMS-ABS.
In addition, as shown in Table 1, the [T.sub.g] almost remains constant with different blends and no significant shifts to lower temperature are observed even with the introduction of CPE. For AMS-ABS, the [T.sub.g] of polybuta-diene is --80 [degrees]C (32). On the basis of this fact, we can conclude that both CPE and the PB component in AMS-ABS can be dispersed in the PVC/AMS-ABS matrix. In other words, a dual-phase structure with PVC/AMS-ABS as the continuous phase while CPE and polybutadiene component as the dispersed phase would be expected in the ternary blends system.
TABLE 1. Glass transition temperature of pure PVC and different blends. Tg([degrees]C) CPE content (phr) PVC PVC/AMS-ABS (70/30) matrix 0 80.5 88.8 3 3 -- 88.7 5 -- 91.8 10 -- 92.1 12 -- 91.6 15 -- 92.7 20 -- 92.7
Heat Distortion Temperature
HDT is used to evaluate the heat resistance of different blends, and the curves are listed in Fig. 2. As shown in Fig. 2, HDT of PVC/AMS-ABS (70/30) binary blends in this work is 85.6C, which is higher than that of pure PVC (22), indicating that incorporation of AMS-ABS could improve the heat resistance of PVC. With regard to the ternary blends, HDT almost remains constant and only a slight decrease in HDT is observed at high level of CPE. For amorphous polymers, the higher the [T.sub.g] is, the higher HDT can be obtained (8). Because the [T.sub.g] of different blends almost do not vary with different compositions, the results of HDT are consistent with the results of DSC.
Dynamic thermogravimetric and the derivative thermog-ravimetric (DIG) curves are shown in Fig. 3. For pure PVC/AMS-ABS (70/30) binary blends, the thermal degradation consists of three basic degradation steps. Like the behavior of the binary blends, the ternary blends also exhibit three degradation steps. The first degradation step, which is at the temperature up to about 369T, presents the process of dehydrochlorination and the formation of polyenes caused by PVC and CPE (33). The second and the third step occur in the temperature range of 369-444[degrees]C and 444-500[degrees]C, respectively. The former is corresponded to the degradation of AMS-ABS and the latter is attributed to the elimination of low molecular weight hydrocarbons from the polyenes residue (34). In addition, as seen in the DTG curves, the temperature at the first peak is shifted to higher values combined with lower maximal rate of degradation (height of the peak) as the content of CPE increases. This indicates that the degradation state of binary blends can slightly decrease with the introduction of CPE in the first degradation step. Interestingly, no obvious changes in the second peak of the DIG curve are observed. Because the degradation of AMS-ABS mainly occurs in the second degradation step, it can be concluded that the interaction between CPE and [alpha]-MSAN is weak (22). With regard to the third step of degradation, the maximal rate increases as the content of CPE increases.
The detailed results obtained from the curves of TG and DTG of different blends are summarized in Table 2. The [T.sub.onset], increase from 285 to 288[degrees]C with the introduction of 15 phr CPE, though the effect of CPE in [T.sub.onset] can be ignored when the content of CPE is 5 phr. Similar results can be gained when it comes to the T10%. However, the introduction of CPE increases T50% effectively. All these results indicate that CPE does not play a negative role in the stability of PVC/AMS-ABS binary blends. These results are expected, since CPE exhibits a higher thermal stability than PVC. Similar to PVC, the dehydro-chlorination is also the dominant reaction in the CPE degradation (34), (35). However, PVC exhibits a fast zipper dehydrochlorination, while the dehydrochlorination of CPE is a slow elimination of random chlorine and occurs at a relatively higher temperature (33), (34). The chlorine radical that is formed from the scission of PVC could diffuse into the CPE phase for reinitiation of the dehydro-chlorination of CPE, and thus the rate of dehydrochlorina-tion decreases. With regards to the weight loss, increasing the content of CPE decreases the weight loss of the first degradation step but increases that of the third degradation step. This change could be explained by the dilution effect caused by the introduction of CPE. In sum, incorporation of CPE does not play a negative role in the thermal stability of PVC/ AMS-ABS (70/30) blends.
TABLE 2. TG analysis results of PVC/AMS-ABS/CPE (70/30/varible) blends. CPE [T.sub.onset] [T.sub.10%] [T.sub.50%] Weight content (a) (a) loss (phr) (a) (%) ([degrees] C) ([degrees] ([degrees] Step 1 Step Step C) C) 2 3 0 285 298 358 52.2 22.7 14.0 5 284 299 368 50.2 22.4 17.0 15 288 303 376 48.3 22.0 19.4 (a) [T.sub.onset%], [T.sub.10%]. and [T.sub.50%] are the temperature corresponding to 5, 10, 50 wt % of weight loss respectively.
As shown in Fig. 4, incorporation of CPE can drastically improve the toughness of PVC/AMS-ABS (70/30) blends. We first take the impact strength of the specimens with 2-mm depth as examples. Pure PVC/AMS-ABS (70/ 30) blends exhibits a relatively lower toughness with an impact strength of 12.9 Id [m.sup.-2]. A BDT is found to occur in the range of 5-15 phr CPE, in which the impact strength increases from 23.3 to 110.0 kJ [m.sup.-2], indicating that the introduction of CPE could give the blends system with appreciable increase in toughness. With further addition of CPE, the influence of CPE on the increase in impact strength becomes less. The impact strength of specimens with 2-mm depth is enhanced by about 21.0 times and 8.5 times in comparison with pure PVC (5.0 Id [m.sup.-2]) (12) and PVC/AMS-ABS (70/30) blends, respectively, when the dosage of CPE is 15 phr. On the other hand, the maximum value of the impact strength in the ternary blends is at the same level of super-tough nylon (36-38). However, for PVC/CPE binary blends, it is reported that more than 20 phr CPE are required to enhance impact strength to about 80.0 kJ [m.sup.-2] (39), (40), while addition of 15 phr CPE (21), (41) only contributes to a 4.0 times increase in impact strength of PVC/[alpha]-MSAN (70/30) blends. Both of the above two mentioned binary blends systems exhibit lower impact strength compared with the ternary blends system in this article. And one of the obvious differences between AMS-ABS and [alpha]-MSAN is the existence of polybutadiene component. Thus, we can conclude that the polybutadiene component might be responsible for such a drastic increase in toughness, and a synergistic toughening effect is achieved by the combination of CPE and polybutadiene component. The toughening mechanism will be discussed in the next section.
It is well-known that rigid PVC is sensitive to the notch depth (2), (20), and via adding elastomers this deficiency can be overcome. Thus, we investigate the influence of CPE on the notch sensitivity of PVC/AMS-ABS blends. As can be seen in Fig. 4, the impact strength of different blends exhibits the same BDT, even though the notch depth is varied. To better understand the influence of notch depth on the impact strength, the retention rate of impact strength of different notch depth is shown in Table 3. As the content of CPE increases, the retention rate of impact strength exhibits an increasing trend, when the notch depth is relatively low. For example, with the addition of 15 phr CPE, the retention rate of impact strength of specimens with 3-mm notch depth increases from 67.4% for pure PVC/AMS-ABS (70/30) blends to 87.4%. This indicates that incorporation of CPE could be helpful to increase the resistance to notch sensitivity. However, as the notch depth continues to increase, the improvement in the retention rate becomes less. Anyway, the impact strength of blends containing CPE is still considerably higher than that of pure PVC/AMS-ABS (70/30) blends with the same notch depth.
TABLE 3. Retention rate of impact strength with different notch depth (%). Retention rate (a) (%) CPE content (phr) 3.0 mm 4.0 mm 5.0 mm 5.5 mm 6.0 mm 0 67.4 55.8 50.4 36.7 34.2 3 69.8 59.5 53.8 49.8 49.7 5 75.1 56.3 52.0 43.8 43.7 10 81.6 53.7 38.5 39.0 34.0 12 83.8 59.0 42.7 40.6 32.0 15 87.4 63.3 45.0 32.0 28.0 20 86.9 60.9 43.4 34.8 33.5 (a.) The retention rate is defined as the percentage of impact strength for the blends with a certain notch depth compared with the impact strength of that blends with a 2-mm notch depth.
With regard to the elongation at break, pure PVC/ AMS-ABS (70/30) blends exhibits an elongation at break of 114%. With the incorporation of 15 phr CPE, the elongation at break increases to 187 %. Thus, addition of CPE could improve the tensile ductility of PVC/AMS-ABS (70/30) blends significantly.
Although drastic improvement in toughness and ductility are observed, the loss in the strength and modulus is accompanied as shown in Figs. 5 and 6. With the addition of 15 phr CPE, the tensile strength, flexural strength and flexural modulus are 38.7, 50.5, and 1670 MPa compared with 50.7, 73.8, and 2413 MPa for PVC/AMS-ABS (70/ 30) binary blends. And further addition of CPE contributes to a continuous decrease. This decrease is owing to the lower strength and modulus of CPE (42). It is worthy noting that both tensile and flexural properties are almost the same and still remain at a high level, when the content of CPE is less than 5 phr. This is in agreement with the BDT of impact strength.
SEM is used to observe the impact-fracture surface of different blends and the SEM micrographs are shown in Fig. 7. Some voids are observed for PVC/AMS-ABS binary blends and PVC/AMS-ABS/CPE ternary blends, indicating a dual-phase structure that is actually a necessary condition for rubber toughened plastic (43), (44). These voids are resulted due to the debonding of rubbers particles, which is consistent with the results of DSC that both the CPE component and polybutadiene component are not miscible with the matrix.
The surfaces of the fractured pure PVC/AMS-ABS blends is relatively flat (see Fig. 7A), and only slight domain distortions are observed, thus indicating that the impact strength of PVC/AMS-ABS blends is not high. This is an expected result, since in this situation the matrix mainly bear the stress and the toughening effect contributed by the polyethylene component is very limited, although the debonding of rubber particles could absorb a considerable amount of energy (44). As the CPE content increase to 5 phr (see Fig. 7B), the surface becomes relatively rough, and some pseudo-fibril structures are observed. This reveals that the impact strength is enhanced. The images of pure PVC/AMS-ABS blends and ternary blends (see Fig. 8) with 5 phr CPE clearly support the toughening effect, since some stress whitening area can be observed for ternary blends containing 5 phr CPE.
With further addition of CPE, it can be seen in Fig. 8 that the ternary blends experienced partial breakage under the impact loading. The stress whitening area exhibit a semi-crescent shape, and its area increases until the dosage of CPE is 15 phr, which is consistent with the results of impact strength (2). The SEM micrograph of the ternary blends containing 10 phr CPE (see Fig. 7C) exhibits a very interesting morphology. Besides the voiding at the interface, the pseudo-fibril has successfully transformed into the fibril with a very short length. In addition, these fibrils with relatively short length act as the path arresters during the impact loading, and thus enhanced the impact toughness (45). As the CPE content increases to 15 phr, the observed surface (see Fig. 7D) is completely deformed and shows extensive drawing of the matrix. The length of fibrils becomes longer, hence its capacity to bridge and retard the propagation of the crack also increase (39).
Based on the above facts, the toughening mechanism can be summarized as below. The existence of polybuta-diene particles in AMS-ABS could not be sufficient to promote the shear yielding of the whole matrix, which is mainly due to the insufficient dosage. In other words, the polybutadiene particles are isolated, and the stress field around one certain particle is seldom affected by its neighboring one. The observed fibrils in the ternary blends indicate shear yielding is the main mechanism. According to Wu's theory (46-50), for rubber toughened plastics, the BDT usually occurs at a critical surface-to-surface interparticle distance or the critical matrix--liga-ment thickness (47).
[[tea].sub.c]= [d.sub.c]([[pi]/6[[PHI].sub.r]).sup.1/3]-1] (2)
where [[tea].sub.c] is the critical matrix--ligament thickness, dc is the critical rubber particle diameter, Or is the rubber volume fraction, r is the average surface-to-surface interparticle distance, and is strongly depended upon the rubber volume fraction. For a given polymer/rubber blends system, if [tea] > [[tea].sub.c], the matrix yielding could not be pervade over the entire matrix. In contrast, if [tea] < [[tea].sub.c], the shear yielding can be drawn in the whole matrix and the blends will be tough. In this system, incorporation of CPE could be inserted into the places between different polybuta-diene particles. Because CPE particles themselves can be dispersed in the matrix, this is very beneficial and actually reduces surface-to-surface interparticle distance. The stress field between different rubber particles (both poly-butadiene particles and CPE particles) begins to overlap, and once the continuum percolation of the stress volume around the rubber particles are accomplished, the BDT in impact strength will be achieved simultaneously. The observed increase in the length of fibrils also corroborates this assumption. As the CPE content further increases (higher than 15 phr CPE), the stress field has already been saturated, so its positive role in the toughness becomes less. However, the proposed toughening mechanism is still required to be substantiated by the transmission electron microscope (TEM) study. On the basis of the SEM micrographs, we are not able to distinguish either polyethylene phase or CPE phase, and there is a lack of information about whether these two components interact with each other.
PVC/AMS-ABS/CPE with largely improved toughness ternary blends has been prepared via melt blending. Addition of CPE exerts little influence on the HDT and thermal stability of the blends. The glass transition temperature almost remains unchanged over all composition, indicating CPE is immiscible with PVC/AMS-ABS matrix. With regard to mechanical properties, super-tough behavior has been observed for blends containing 15 phr CPE. Addition of 15 phr CPE results in an increase in impact strength by about 21.0 times and 8.5 times in comparison with that of pure PVC (5.0 kJ [m.sup.-2]) and that of PVC/AMS-ABS (70/30) blends, respectively. The value of the impact strength is even close to the supertough nylon. In addition, incorporation of CPE decreases tensile strength and flexural properties. Both fibrils and voids are observed in the fractured surface of the ternary blends, and the length fibrils increase with the increasing amount of CPE. Based on the morphology, it is suggested that shear yielding is the main toughening mechanisms, and the improvement in toughness can be well-explained by Wu's model. So far, there exists a limitation of this work, that is, the existence of polybutadiene is prone to cause ultraviolet irradiation degradation and thermal oxygen degradation resulting in the destruction of polymer chains. Ultraviolet absorbent or titanium pigment should be added so that the weatherability of the blends can be enhanced. That work is being carried out and will be performed in our future work.
Correspondence to: Jun Zhang; e-mail: email@example.com
Contract grant sponsor: Scientific Achievement Transformation Foundation of Jiangsu Province; contract grant number: BA2010017; contract grant sponsor: Priority Academic Program Development of Jiangsu Higher Education Institutions.
Published online in Wiley Online Library (wileyonlinelibrary.com). [C] 2013 Society of Plastics Engineers
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|Author:||Zhang, Zhen; Zhang, Jun; Liu, Hongyong|
|Publication:||Polymer Engineering and Science|
|Date:||Feb 1, 2014|
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