Thermo-mechanical degradation-induced grafting of poly(styrene–acrylonitrile) to chlorinated polyethylene

Polymer Degradation and Stability 97 (2012) 766e770

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Thermo-mechanical degradation-induced grafting of poly(styreneeacrylonitrile) to chlorinated polyethylene Peng Luo, Guozhang Wu* Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science & Engineering, East China University of Science & Technology, Shanghai 200237, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 December 2011 Received in revised form 1 February 2012 Accepted 6 February 2012 Available online 10 February 2012

The thermo-mechanical degradation of acrylonitrile-chlorinated polyethylene-styrene (ACS) terpolymer was investigated by means of high temperature shearing in a Haake Rheomixer. The results showed that the chain scission takes place in the poly(styreneeacrylonitrile) (SAN) component while the residual rate of the ACS resin after extraction of SAN increases gradually with increasing the shearing time. FTIR and DSC analysis confirmed that the increase in the residual rate is due to the thermo-mechanical degradationinduced grafting of SAN to the CPE chain. SEM and TEM observation revealed a remarkable reduction in the CPE domain size and formation of a salami-like structure in which many CPE-g-SAN micelles are embedded in the fine CPE domain. These changes result in a significant improvement in the impact strength of the ACS resin. The mechanism of the thermo-mechanical degradation-induced grafting and the morphology change were discussed. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Acrylonitrile-chlorinated polyethylene-styrene (ACS) terpolymer Thermal degradation Grafting Reactive blending

1. Introduction ACS is a polymer alloy consisting of a dispersed chlorinated polyethylene (CPE) phase and a poly(styreneeacrylonitrile) (SAN) matrix. Although it is similar to ABS both in the microstructure and in general mechanical properties, the ACS resin has attracted vast interest in many fields due to its excellent properties, such as superior weathering resistance, high flame retardant and good anti-electrostatic effect [1e3]. These characteristics are generally ascribed to the replacement of the unsaturated polybutadiene (PB) in ABS by the saturated and chlorine contained CPE elastomer. However, similar to other polymers containing chlorine, ACS resin is liable to dehydrochlorination during the high temperature processing, leading to the undesired color change and processing machine/mold corrosion. It is therefore of great importance to study the thermal degradation behavior of ACS for optimizing the resin’s processing conditions and better understanding the related changes in physical properties. Despite of little work on ACS, much has been reported about thermal degradation of the segmented homopolymers or copolymers of ACS terpolymer, such as PVC, CPE, SAN and ABS. Compared to the ‘‘head to tail’’ pattern of the monomer segments in PVC, CPE has non-regular location of chlorine atoms along the polymer chain and behaves better thermal stability than PVC. Many experimental

* Corresponding author. Fax: þ86 21 64251661. E-mail addresses: [email protected], [email protected] (G. Wu). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.02.006

results have shown [4] that the thermal degradation of CPE depends on the chlorine content and the production process. Many factors, such as the initial morphology of the polyethylene and kinetics of the chlorination reaction [5], have deep impact on the overall properties of the CPE. Anyway, the degradation of CPE is supposed to involve ionic or radical mechanism [6], which is also seen as the degradation mechanism [7e9] of PVC along with sixcenter concerted mechanism [10]. The thermal degradation of ABS is thought to be a radical process where end-chain and random scissions occur [11,12], causing not only the discoloration, but also the deterioration in overall mechanical properties. Some researchers stated that the ABS degradation is limited to the rubbery PB phase as hydrogen abstraction by oxygen is thermodynamically favorable due to the presence of tertiary substituted carbon atoms in the PB phase [13], while others proposed that the property degradation is due to a combination of physical aging in the SAN phase and oxidation of the PB phase [14,15] though the latter might play the main role [13,16,17]. Our initial interest has been on the shear-induced thermal degradation of ACS to clarify the contribution of shear energy toward discoloration. In this work, we revealed that a large amount of SAN grafts to CPE during the high temperature shearing process. This phenomenon is attractive not only because it provides a new clue to investigate the degradation of ACS under the complicated thermal, oxidative and mechanical conditions, but also it paves a potential way for in situ fabrication of CPE-g-SAN. The purpose of the present work is to elucidate the grafting mechanism and to

P. Luo, G. Wu / Polymer Degradation and Stability 97 (2012) 766e770

check whether or not the grafting influences the compatibility between SAN and CPE and thus physical properties of the ACS.

A bulk polymerized ACS resin was kindly provided by Ningbo OCC. The melting flow rate (MFR) of the resin is 2.7 g/10 min as measured at 200  C under a pressure of 5 kg. The resin contains 15 wt% CPE and the chlorine content of the CPE is about 35 wt%. The ACS resin was dried for 4 h in an oven at 80  C, and then 55 g of ACS was introduced to Haake Rheomixer (RC300P) and shearing at 220  C for a given time. The rotating speed of the mixer was set at 100 rpm. Specimens after experiencing the shearing process were finally compression-molded into sheets with a thickness of about 3.2 mm at 200  C for 10 min under a pressure of 10 MPa for impact tests. Soxhlet extraction was performed for 72 h using acetone as solvent to separate the SAN component from ACS. The residual was subjected to distilling-off of the acetone by means of a rotary evaporator. The residual rate then was calculated according to the following formula in which the residual weight means the total weight after extraction and the original weight means the total weight before extraction:

Residual rate % ¼ Residual weight=Original weight  100%

(1)

2.2. Characterization Size exclusion chromatography (SEC, Waters 1515) was applied to measure the molecular weight of SAN extracted from the ACS using THF as a solvent. FTIR spectrum instrument (Nicolet 5700, Thermo Electron Scientific Instruments Corp) was used to check the change in chemical groups during the high temperature shearing. The specimen for FTIR characterization was obtained by casting the residues solution with a doctor blade on a dry and clean glass plate followed by thermal treatment at 60  C for 10 h under vacuum. The thickness of the specimen was about 20e40 mm. The glass transition temperature of ACS was measured using a differential scanning calorimeter (DSC, TA 2910). To erase thermal history; the samples were first heated to 200  C at a rate of 10  C/min and were kept there for 2 min. The samples were then cooled to 40  C. A subsequent scan was performed at the same rate. Morphology observation was performed by scanning electron microscopy (SEM, JEOL JSM-6360-LV) at an accelerating voltage of 15 kV. Samples were taken from the freeze fractured surfaces without etching and sputtered after gold coating. To further clarify the microstructure of the CPE domain, transmission electron microscope (TEM, JEOL JEM-2100) observation was carried out at an accelerating voltage of 100 kV. Samples were cut into ultrathin sections approximately 80 nm in thickness using an ultramicrotome (Ultracut N, Reicherr). Sections were collected on holey copper grids and were carefully stained with osmium tetroxide (OsO4) vapor to enhance the phase contrast between CPE and SAN. The notched Izod impact test was measured at room temperature according to ASTM-D256-2006 using impact tester (MTS, ZBC1400-2). 3. Results and discussion 3.1. SAN chain scission Much has been reported that thermal degradation of CPE initiates at 215  C [6,18]. The shearing temperature in this work was set

Torque / Nm

2.1. Raw materials and sample preparation

40

8

30

7

20

6 Molecular weight

10

0

5

Torque

0

10

20

30

Number-average molecular weight 10 4

2. Experimental

767

4

Shear time / min Fig. 1. Influences of the shearing time on the torque of the ACS melt and the molecular weight of SAN extracted from the melting processed ACS resin.

at 220  C, which is slightly higher than the degradation temperature. Fig. 1 records the variation in torque with the shearing time. It can be observed that, after an abrupt change at the stage of ACS injection, the torque, a parameter usually correlated to the viscosity of the polymer melt, becomes stable at a shearing time of about 10 min, suggesting that the ACS melts completely. We noticed a slight increase of the torque value with further increasing the shearing time. The influence of the high temperature shearing on SAN molecular weight is also shown in Fig. 1. Here, SAN was extracted from the melt-mixed ACS using acetone as a solvent. It is clear that the number-average molecular weight of SAN decreases from 6.8  104 to 5.2  104 after shearing for 30 min. The reduction should be ascribed to a break-up of SAN chains, which is in agreement to the degradation behavior of SAN as reported previously [19,20]. However, this result is opposite to the torque variation in Fig. 1. As the ACS resin used in this work contains 85 wt% SAN, the torque should have been decreased with the chain scission of SAN. There are two possible reasons accounting for the torque increase: one is cross-linking of SAN which increases the viscosity of the ACS melt but cannot be extracted by acetone; the other is the change in the CPE microstructure during the high temperature shearing procedure. 3.2. Residual rate and FTIR analysis of ACS after extraction of SAN The residual rate of ACS after selective extraction of SAN was investigated as shown in Fig. 2. It was found that the residual rate increases with increasing the shearing time. FTIR spectra from the residues showed (see Fig. 3) that peaks at 701, 760, 1500 cm1 corresponding to the vibration of the benzene group and 2237 cm1 to the nitrile group enhance remarkably with increasing time. For better comparison, all of the FTIR spectra are plotted in a normalized scale such that the height of 2927 cm1 keeps equal. This is based on an assumption that total amount of the second carbon remains unchangeable during the thermomechanical processing. Fig. 2 provides the comparison of the residual rate with the FTIR absorbance at 2237 (nitrile) and 760 cm1 (benzene) peaks. Clearly, the increase of the residual rate should be derived from the increase of SAN content in the residue. According to the FTIR spectra in Fig. 3, one can also distinguish the thermo-oxidative degradation of the CPE as the emergence of the new peak at 1740 cm1 and the dehydrochlorination-induced double bond at 970 and 911 cm1, though the height variation of these peaks is relatively weak.

768

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40

0.2

0.15 760cm -1

20

0.1

10

30min

EXO→

30

Absorbance

Residual rate / %

113 ºC

20min 10min

0.05

Original

2237cm -1

0

0

10

20

0

30

-20

Shear time / min Fig. 2. Shearing time dependency of the residual rate and the FTIR absorbance at 2237 (nitrile) and 760 cm1 (benzene) peaks for ACS after extraction of SAN by acetone.

1500cm -1

2237cm -1

760, 701cm -1

30min 20min

10

40 70 100 Temperature / ºC

130

160

Fig. 4. DSC curves for ACS samples sheared at 220  C for different times.

CPE phase should be ascribed to grafting of the high Tg SAN to the CPE chain. Since a relatively low weight fraction of SAN is grafted to CPE, the change in SAN Tg is not remarkable in Fig. 4. However, due to the grafting, one can still distinguish that the glass transition region of SAN becomes broad with increasing the shearing time as shown. 3.4. Morphology observation

10min Original 1740cm -1

4000

2800

1600

Wavenumbers /

cm -1

970, 911cm -1

400

Fig. 3. Influence of shearing time on transmission FTIR spectra of ACS after extraction of SAN.

It should be pointed out that all of the extraction residues were testified to be dissolved in chloroform. This provided a direct evidence demonstrating the degree of cross-linking of CPE phase is very low during the thermo-mechanical degradation. Therefore, it is believed that the slight increase in torque in Fig. 1 and the large increase in the residual rate in Fig. 2 should be derived from grafting of SAN to CPE during the high temperature shearing process. 3.3. Changes in glass transition temperature Fig. 4 shows DSC curves for ACS samples after the high temperature shearing for different times. Two transition regions are observed. The upper one at about 113  C reflects the glass transition of the SAN phase while the lower one around 15  C belongs to the glass transition of the CPE phase in the ACS specimen. A distinguishable increase in glass transition temperature (Tg) of the CPE phase is observed from 14.5  C to 23.8  C after the high temperature shearing for 30 min. It is well known that the cross-linking of a polymer may result in a higher Tg; however, the extraction residues were well dissolved in chloroform and the variation in torque was not obvious during the whole process, demonstrating that the degree of CPE cross-linking is very low during the thermo-mechanical degradation. Therefore, the increase in Tg of

Fig. 5 shows SEM micrographs before and after the thermomechanical degradation. Samples were obtained from freeze fractured surfaces of ACS without etching. The picture in Fig. 5a shows a two-phase morphology, that is, rubber particles (CPE) embedded in a styreneeacrylonitrile copolymer (SAN) matrix. It is observed that the fracture surface becomes very rough and the CPE domain size appearing as white particles reduces greatly for ACS after the shearing at 220  C for 30 min (see Fig. 5b), indicating that the thermo-mechanically degraded specimen promotes the compatibility between the CPE and the SAN phase. Fig. 6 shows the TEM micrographs of ACS where the specimens have been stained by OsO4. For the specimen without experiencing the thermo-mechanical degradation, the CPE particle appears white in color and cannot be stained by OsO4 (see Fig. 6a). However, the CPE domain in Fig. 6b becomes black in color after the thermomechanical degradation. The change in color suggests that there exists dehydrochlorination-induced double bond in the CPE domain which is sensitive to be stained by OsO4. This result is in agreement with FTIR analysis. Furthermore, the domain size Fig. 6b reduces greatly from about 400 nm to even lower than 100 nm. Two coordinating factors might play the key role. One is shear-induced break-up of the CPE droplet, and the other is formation of compatilizer essential for lowering the interfacial free energy between CPE and SAN. During the high temperature shearing process, the CPE domain is supposed to be deformed into microfibril, and then break-up into smaller droplets. Without effective compatilizer, these small droplets coagulate together rapidly. The remarkable reduction in the CPE domain size reminds us again that there exists in situ formed CPE-g-SAN compatilizer. 3.5. Mechanism of thermo-mechanical-induced grafting of SAN to CPE Based on the above results, we propose that grafting of SAN to CPE should derive from the reaction of radical sites between the

P. Luo, G. Wu / Polymer Degradation and Stability 97 (2012) 766e770

769

Fig. 5. SEM Photographs of ACS (a) before and (b) after shearing at 220  C for 30 min.

SAN chain and the CPE chain. On the SAN side, the thermomechanical degradation causes chain scission, leading to the end free radicals. The situation becomes complex on the CPE side. During the high temperature shearing process, the CPE chain is exposed to thermal, oxidative and mechanical degradation. Although it is rather difficult to specify, the degradation is effective to leave a radical site on the CPE chain. Fig. 7 presents a cartoon for grafting and morphology evolution when ACS undergoes the thermo-mechanical degradation. It should be noticed that the residual rate of ACS after the 30 min high temperature shearing is about 36 wt% (see Fig. 2). By subtraction of 15 wt% CPE, one can easily deduce that 21 parts of SAN are grafted to 15 parts of CPE. It seems impossible to localize such a high volume of CPE-g-SAN at the interface between the CPE domain and the SAN matrix. A careful examination of TEM picture

in Fig. 6b reveals that many white particles with a size less than 20 nm are dispersed in the black CPE domain, and thus forms a salamilike structure. This indicates that the excess volume of CPE-g-SAN is embedded in the CPE domain in a form of micelle [21]. 3.6. Impact resistance Thermo-mechanical degradation usually deteriorates the polymer’s impact strength. ABS has been reported as one of the typical cases that the interfacial bond strength between the SAN matrix and rubbery PB particles decreases significantly during the melting process [13,16,22], resulting in a reduction in the impact strength. However, our experiment revealed an opposite result. Fig. 8 shows the variation of the impact strength with shearing time for ACS at 220  C. Clearly, the impact strength increases with the shearing

Fig. 6. Transmission electron micrographs of ACS with different magnifications: (a) before and (b) after shearing at 220  C for 30 min. Samples have been stained by OsO4.

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from the reaction of radical sites between the SAN chain and the CPE chain.

Shearing

Acknowledgments Grafting

Thermo-mechanical degradation

References

Molecule chain of SAN

Shearing

CPE domain CPE-g-SAN micelle Fig. 7. Schematic diagram of grafting and morphology evolution when ACS undergoes thermo-mechanical degradation.

Impact strength / KJ/m2

15

12

9

6

3

0

Original

We thank Ningbo OCC and the National Natural Science Foundation of China (20974033) for financial support.

10min

20min

30min

Shear time Fig. 8. Variation of impact strength with shearing time for ACS at 220  C.

time. Since the degree of CPE cross-linking was testified to be very low during the thermo-mechanical degradation, the positive effect on the impact strength should be ascribed to in situ grafting of SAN to CPE and forming a fine salami-like structure during the thermomechanical degradation of ACS. 4. Conclusions The effect of thermo-mechanical degradation on morphology and impact strength of ACS was investigated. It was found that the impact strength of ACS is enhanced due to in situ grafting of SAN to CPE and forming a fine salami-like structure during the thermomechanical degradation of ACS. FTIR, DSC analysis and extraction tests clarified that a large amount of SAN is grafted onto the CPE chain. It was proposed that grafting of the SAN to CPE is derived

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