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Reprocessing acrylonitrile-butadiene-styrene plastics: structure-property relationships.

INTRODUCTION

ABS (acrylonitrile-butadiene-styrene) is one of the most frequently used polymers in electrical and electronic equipment, as well as having widespread applications in automobiles, communication instruments, and other commodities. A report by the Association of Plastics Manufacturers in Europe (APME) [1] has suggested that the use of recycled plastics in the electrical and electronic sector could be increased if more polystyrene (PS) and ABS recyclate were to become available. This emphasizes the importance of studying the recycling of ABS as an aid to reducing environmental, economic, and energy issues.

Several studies have been carried out on the mechanical recycling of ABS. For example, Potente and Gao [2] studied the recyclability of injection molded lampshade parts based on ABS. They found that, after regrinding and injection molding, the mechanical properties were not significantly affected, apart from a slight reduction in the notched Izod impact strength. Their gel permeation chromatography (GPC) investigation showed that only slight molecular degradation occurred during the injection moulding and/or regrinding. Thus, they considered that the main reason for the reduction in impact strength could be the volatilization of some of the additives used in the virgin material.

In another study, pieces of ABS from computer monitor casings were granulated followed by thermal processing in a torque rheometer [3]. Then the material was regranulated followed by injection molding into mechanical test specimens. This method was used to simulate a typical commercial recycling procedure. After this reprocessing, tensile strength decreased only by a few MPa, tensile modulus increased slightly, strain to failure decreased from about 11% to 6%, and un-notched impact strength decreased from 44 to 31 kJ/[m.sup.2]. It was thought that polymer degradation might have caused the reduction of impact strength and a slight increased stiffness of the recycled material.

Some basic studies on multiple recycling of ABS have been carried out. For example, the effects of reprocessing conditions on the mechanical properties of ABS have been evaluated by varying temperatures and dwell times [4]. In this case, the material was ground and remolded five times. It was found that the mechanical properties varied with reprocessing temperature. Following the use of higher temperatures, Izod impact strength decreased but there were increases in tensile strength and modulus. Also, the variations of these mechanical properties increased with longer processing dwell times. There was a good correspondence between the changes in mechanical properties and the observed structural variations. Degradation of the rubber phase (evidenced by infrared spectroscopy) was thought to be the main reason for the decrease in toughness. Following the most severe molding conditions, toughness reduction was even greater, perhaps because of degradation of the styrene-acrylonitrile (SAN) phase.

In another study, an ABS material was injection molded and recovered for five cycles [5]. During this multiple reprocessing, the strain to failure showed a very slight tendency to decrease. Failure strength slightly increased after five processing cycles. The toughness measured by notched impact strength reduced continuously. The changes of these important properties were probably due to the degradation of the soft polybutadiene component, which was seen from infrared and dynamic mechanical analysis results.

Kim and Kang reprocessed three ABS resins five times using an extruder [6]. After extrusion, the glass transition temperature of the SAN phase was not changed. The effect of repeated extrusions on mechanical properties such as tensile strength, strain to failure and hardness was small. However, impact resistance of all materials decreased after recycling, especially the impact resistance of ABS with the highest polybutadiene content. The reason for the decrease in the impact strength was again thought to be the degradation of the polybutadiene component in ABS.

In the present study, the effects of reprocessing on both the polymer and any small molecules (including additives) present in ABS plastics from waste computer housings are studied. Through the study, the reasons for the changes in mechanical properties after reprocessing are investigated.

EXPERIMENTAL

Materials

Several waste computer equipment housings were screened using FTIR (Fourier transform infrared spectroscopy) and gel permeation chromatography (GPC) to ensure that (i) they were ABS, (ii) they did not contain high molecular weight additives, and (iii) the proportion of additives and oligomers (containing double bonds) was relatively low. Three individual housings (ABS1, ABS2, and ABS3) were selected for the subsequent series of recycling experiments.

Material Reprocessing and Sample Preparation

The recycling process was simulated using an appropriate combination of granulator, torque rheometer, and injection molder.

ABS1 and ABS2 were used to assess the effects on impact and tensile properties respectively, of reprocessing in the torque rheometer at different temperatures and rotational speeds (shear rates). Granulated particles of each individual housing were introduced into a torque rheometer controlled at a specific temperature (190 or 230 or 270[degrees]C). A fixed time of 3 min was allowed for the charge to reach the cavity temperature with the blades rotating at 10 rpm. Then, the rotational speed was increased to 20, 60, or 100 rpm to process for a further 10 min. Following this dwell time, the blades were stopped, the torque rheometer was allowed to cool, and the plastic was removed when the temperature of the processing cavity had reduced to 140[degrees]C. The solid material was granulated again and made into samples for mechanical testing using a Ray Ran Injection Molder.

ABS3 was used to assess the effects of multiple reprocessing. The housing was cut into pieces with a saw and granulated. To provide a control sample (number of cycles, n = 0), some of these granulated particles were directly injection molded into samples for mechanical testing. To simulate one recycle (n = 1), the granulated particles were processed in the torque rheometer at 230[degrees]C for 3 min at 10 rpm, and 10 min at 60 rpm, cooled, granulated, and injection molded. For multiple recycling (n = 2, 3, and 4), the material underwent additional cycles of torque rheometer processing, cooling, and granulating before the final injection molding.

In all cases, the granulated particles were heated in an oven at 80[degrees]C for about 4 h to remove any moisture immediately before both thermal processing in the torque rheometer and injection molding.

Dynamic Mechanical Thermal Analysis (DMTA)

A rheometric dynamic mechanical thermal analyzer MK III was used to test samples in bending geometry using a single cantilever type clamp configuration at a frequency of 1 Hz, with a heating rate of 2[degrees]C/min.

Infrared Spectroscopic Analysis by FTIR

Infrared spectra of bulk samples were recorded on a Perkin-Elmer FTIR spectrometer in ATR (attenuated total reflectance) mode. The absorbance of trans-2-butene-1,4-diyl moieties at 966 [cm.sup.-1] and the absorbance of nitrile moieties at 2238 [cm.sup.-1] were compared with the absorbance of styrene moieties at 1603 [cm.sup.-1], using the ratios [D.sub.1] and [D.sub.2] as defined below [7]. Values of absorbance were determined using the baseline method.

[D.sub.1] = Absorbance at 966 [cm.sup.-1]/Absorbance at 1603 [cm.sup.-1]

[D.sub.2] = Absorbance at 2238 [cm.sup.-1]/Absorbance at 1603 [cm.sup.-1].

ABS is sensitive to oxidation because of the presence of polybutadiene components, which act as oxidation sensitizers and lead to formation of carbonyl groups, which absorb at 1680-1750 [cm.sup.-1] [8]. In the present study, changes of absorption in this region were also investigated.

GPC

GPC System for Assessing Smaller Molecules. A sample of plastic (1.5 g) was placed in a 32 ml long bottle, 25 ml of tetrahydrofuran (THF) was added and the bottle was sealed. The mixture was stored for 24 h to allow the plastic to dissolve, and then put in an ultrasonic bath for 3 h. Subsequently, the solution was kept in the dark until solid particles precipitated completely. GPC samples were drawn from the top of this solution. The molecular weight distribution was analyzed at room temperature with a GPC system equipped with four PLgel columns (dimensions 300 x 7.5 mm, 10 [micro]m particle size, pore sizes 10, [10.sup.2], [10.sup.3], [10.sup.4] nm) and an ultraviolet detector (at 254 nm). THF was used as the eluent at a flow rate of 1.0 ml/min. The GPC system was calibrated with polystyrene (PS) standards (of molecular weights 1,447,000, 401,340, 24,150, 9100, 2700, and 687) obtained from Aldrich Chemical Company.

GPC System for Assessing Large Polymeric Molecules. GPC analysis of the larger polymeric molecules was carried out at RAPRA Technology Ltd. Sample solutions were prepared by adding chloroform (10 ml) to plastic samples (20 mg), sealing the tube, and leaving the mixture overnight to dissolve. The solutions were thoroughly mixed and filtered through a glass fiber prefilter and a 0.2 [micro]m polyamide membrane prior to the chromatography. The molecular weight distribution was determined at 30[degrees]C using two PLgel columns (mixed bed-B, 300 mm length, 10 [micro]m particle size) with refractive index and differential pressure detectors. Chloroform was used as the eluent at a flow rate of 1.0 ml/min. The GPC system was calibrated with a mixture of two PS standards (of molecular weights 3,053,000 and 30,300).

Gas Chromatography/Mass Spectrometry (GC/MS)

Extraction Procedure [9]. Some broken samples of ABS2 and granulated particles of ABS3, which had undergone thermal reprocessing 0, 1, 2, or 3 times in the torque rheometer, but had not been injection molded, were used for identification. A sample of plastic (6-10 g) was placed in a 100 ml round flask and dichloromethane (40-60 ml) was added as extraction agent. The flask was sealed and the mixture was allowed to stand for 24 h, then ultrasonicated for 2 h, and finally filtered through glass wool. The undissolved solid was recovered and extraction was performed again, this time with dichloromethane (30-40 ml) and for 30 min in the ultrasonic bath. The two extracts were collected together and methanol (80-160 ml) was added to precipitate any polymer. The solution was filtered through a glass fiber filter, and then concentrated to 2-3 ml using a rotary evaporator. After filtering again through a glass fiber filter, the concentrated extract was analyzed by GC/MS.

Identification of Extracts by GC/MS. GC/MS was performed at the National Mass Spectrometry Service Centre at University of Wales Swansea. A Fisons GC 8000 gas chromatograph with Fisons MD 800 mass spectrometer (Thermoquest UK) was used. The gas chromatograph was equipped with a fused silica capillary column with 100% polydimethylsiloxane liquid phase (30 or 15 m x 0.25 mm i.d.; film thickness 1.0 [micro]m; Chrompack DB-1). The total ion current (TIC) chromatograms with long retention times were obtained by using the longer column, while the chromatograms with short retention times were obtained by using the shorter column. The chromatographic conditions were as follows: initial temperature 40[degrees]C held for 1 min, then raised at 8[degrees]C/min up to 300[degrees]C and held for 20 min; carrier gas, helium (head pressure 31 kPa); injection temperature, 220[degrees]C; injection, splitless; injection volume, 1 [micro]l; Mass spectrometer: source temperature, 200[degrees]C; electron energy, 70 eV; electron current, 200 [micro]A. GC/MS interface temp: 300[degrees]C. For identification of mass spectra, the NIST/EPA/NIH Mass Spectra Library (version 2.0) was used.

Impact and Tensile Testing

The notched Izod impact tests were performed (on ABS1 and ABS3) at room temperature using a Ray Ran Universal Pendulum Impact System. All injection molded samples were 61.0 mm long, 12.0 mm wide, and 4.0 mm thick with a notch depth of 2.0 mm. Average values and standard deviations were calculated from at least 5 samples of each material.

Tensile testing was carried out on a Hounsfield H25K-S Benchtop Testing Machine at room temperature. Dogbone-shaped samples were tested to failure at an extension rate of 1 mm/min (ABS2) or 5 mm/min (ABS3). The gauge lengths of the injection molded samples were 36.6 mm (ABS2) and 30.1 mm (ABS3), and the width and thickness were 4.0 mm. Average values and standard deviations were calculated from at least 5 samples of each material.

Analysis of Fracture Surfaces by Scanning Electron Microscopy (SEM)

The fracture surfaces of selected samples were gold coated and observed in a Philips XL 30 CP scanning electron microscope, normally operating at 15-20 kV.

RESULTS AND DISCUSSION

DMTA/FTIR/GPC

Effects of Reprocessing at Different Temperatures and Rotational Speeds. Some DMTA results for ABS2 are shown in Fig. 1. The peak between -110 and -60[degrees]C corresponds to the rubber phase [5]. It can be seen that after reprocessing at 270[degrees]C, the glass-transition temperature ([T.sub.g]) of the rubber phase increased, suggesting that crosslinking reactions may have occurred in the rubber phase. There was negligible variation in [T.sub.g] of the SAN phase, which was 110 [+ or -] 1[degrees]C for all ABS2 samples. The same trends were observed in the DMTA spectra of ABS1.

[FIGURE 1 OMITTED]

Absorbance ratios for different bands in the FTIR spectra of ABS1 and ABS2 are shown in Fig. 2. There is a suggestion that the ratio ([D.sub.1]) of trans-2-butene-1,4-diyl moieties to styrene moieties for ABS2 was slightly higher after reprocessing at 190[degrees]C, compared with before reprocessing (although it was not so clear for ABS1). Such an effect may have been caused by some volatile molecules containing benzene rings being vented out of the material during reprocessing. Figure 2 also indicates that [D.sub.1] was slightly lower for samples reprocessed at 270[degrees]C, compared with those reprocessed at 190[degrees]C at the same rotational speed, for both ABS1 and ABS2. This lower [D.sub.1] may have been mainly caused by crosslinking of the rubber phase when reprocessing at the higher temperature. No increase in absorption due to carbonyl groups could be observed in the spectra after reprocessing, even after reprocessing at 270[degrees]C (see Fig. 3). There was no significant change in the ratio ([D.sub.2]) of the absorption of nitrile moieties to the absorption of styrene moieties after reprocessing at the various temperatures and rotational speeds. This is consistent with the DMTA results, which also showed that reprocessing temperature had little effect on the SAN phase. The SAN phase, therefore, seems to be relatively stable under these processing conditions.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The results using the GPC column designed for assessing the smaller molecular species of ABS1 are shown in Fig. 4. The retention time of the high molecular weight polymer molecules is 18-30 min. The second peak (30-35 min) is attributed to oligomers of ABS, additives, and their derivatives. The third peak (35-40 min) is attributed to more small molecules including for example monomers of ABS. Figure 4 indicates that the proportion of small size components eluting between 25 and 35 min increased after reprocessing at high temperature, suggesting that chain scission reactions of ABS might have occurred, but the degree of such scission reactions was very low. The third peak became lower after reprocessing, presumably because of volatilization of the smallest molecules. Similar trends were observed in the equivalent chromatograms of ABS2.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The results from the GPC column used to assess the higher molecular weight species are shown in Figs. 5 and 6. It can be seen that the rotational speed had no apparent effect on the size distribution of the polymeric components (see Fig. 5). The proportion of large size components increased slightly after reprocessing at the highest temperature of 270[degrees]C, but the increase in the high molecular weight component was greater in ABS2 than ABS1 (Figs. 5 and 6). The major component of soluble polymer molecules is SAN. Some literature has suggested that the viscosity of SAN reduces after multiple reprocessing at high temperature [4, 10]. This would suggest that the molecular weight of SAN tends to reduce during processing. The increased proportion of high molecular weight components in ABS2 observed in Fig. 6 is not easily interpreted. It was not seen to any great extent in ABS1 (see Fig. 5) and so does not appear to be common feature. Its occurrence in Fig. 6 may be related to the crosslinking of polybutadiene as suggested by the DMTA experiments, if small amounts of polybutadiene had originally been grafted onto the SAN molecules, but this is a speculative interpretation and needs further investigation.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Effects of Multiple Reprocessing. DMTA results for ABS3 show that after reprocessing at 230[degrees]C for 4 cycles, [T.sub.g] of the rubber phase increased (see Fig. 7). At the same time, with increasing number of reprocessing cycles, the value of the maximum tan [delta] of the rubber phase decreased, and the peak broadened. One explanation for such observations could be that crosslinking and thermo-oxidative degradation occurred in the rubber phase [11]. Multiple reprocessing had no apparent effect on the tan [delta] peak of the SAN phase of ABS3, [T.sub.g] remaining at about 115[degrees]C.

The FTIR results for ABS3 show that, with an increase in the number of reprocessing cycles, the ratio ([D.sub.1]) of trans-2-butene-1,4-diyl moieties to styrene moieties decreased significantly (see Fig. 8). This shows degradation reactions occurred in the rubber phase while the SAN phase was relatively stable. Figure 8 shows that the ratio ([D.sub.2]) of nitrile moieties to styrene moieties reduced slightly during multiple reprocessing. Additionally, FTIR spectra (see Fig. 9) after 4 reprocessing cycles showed evidence for a small increase in intensity of peaks in the range 1680-1750 [cm.sup.-1], which could be due to formation of carbonyl groups.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

The results of the GPC study of the smaller molecular species of ABS3 are shown in Fig. 10. It may be seen that the proportion of small size components eluting between 27 and 35 min increased during multiple reprocessing as also happened during higher temperature single step reprocessing. This suggests that chain scission reactions occurred. Also, the third peak (35-40 min) decreased with increasing number of reprocessing cycles, probably because of volatilization of the smallest molecules. Changes in the larger polymeric molecular components are shown more clearly in the GPC data of Fig. 11. Again, this shows some evidence of an increase in small size polymer molecules after 4 reprocessing cycles, perhaps because of greater oxidative degradation during multiple reprocessing. However, the degree of chain scission reactions is still low. Additionally, the GPC results in Fig. 11 also show that the proportion of large size polymeric components slightly increased after multiple reprocessing, a similar effect (but to a lesser extent) to that seen for single recycling of ABS2 at 270[degrees]C.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

Gas Chromatography/Mass Spectroscopy

The TIC gas chromatograms of extracts from ABS2, before reprocessing, after reprocessing at 230[degrees]C and 100 rpm, and after reprocessing at 270[degrees]C and 20 rpm are shown in Figs. 12a-c, respectively. There are several large peaks in the chromatogram of the ABS2 'before reprocessing' sample (Fig. 12a). Mass spectra of these peaks show that most of them might be given by oligomers of ABS and their derivatives, except the peaks at about 5.7 min (Scan 134) and about 9.0 min (Scan 330). The mass spectrum of the peak at 5.7 min suggests that the compound might be 2-phenyl-2-propanol. The peak almost disappeared following reprocessing. The peak at 9.0 min has been identified to be 2,4-di-tert-butylphenol, a common antioxidant, by comparison with a standard. However, this peak increased after reprocessing, which implies that during reprocessing, some component(s) in ABS2 decomposed to give 2,4-di-tert-butylphenol. Because of the increase in the amount of material following reprocessing, it is possible that all or part of the 2,4-di-tert-butylphenol present before reprocessing is a result of decomposition of the component(s) over the lifetime of the material. After reprocessing (Fig. 12b and c), two peaks (at 11.4 (Scan 474-475) and 12.67 min (Scan 550)) increased substantially, particularly after reprocessing at 270[degrees]C. The library search results for the mass spectra of these peaks show that the peak at 11.4 min might be hexadecanenitrile, and the peak at 12.67 min might be octadecanenitrile. They are probably decomposition products of the lubricant EBS [12]. The main component of EBS is N,N'-ethylenebis-stearamide, but EBS also always contains N,N'-ethylene-bispalmitamide and ethylene-N-palmitamide-N'-stearamide. When it decomposes, hexadecanenitrile and octadecanenitrile can be formed.

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

The chromatograms of extracts from multiply reprocessed ABS3 are shown in Fig. 13. Figures 13a-d are the extracts of ABS3 after reprocessing at 230[degrees]C and 60 rpm for 0, 1, 2, and 3 cycles, respectively. During this multiple reprocessing, some small molecules were lost continuously. For example, the peak at 9.6 min (Scan 368-370) became smaller as the number of cycles increased, with the largest reduction occurring during the first reprocessing cycle. The other apparent changes are that the peaks at 17.5 min (Scan 840-841), 23.2 min, and 25.9 min (Scan 1337-1341) grow with the increasing number of reprocessing cycles. The peak at 17.5 min, which has been confirmed to be 2,4-di-tert-butylphenol by comparison with a standard, increased during reprocessing. This mirrors the situation found during reprocessing of ABS2. The compounds at 23.2 and 25.9 min could be hexadecanenitrile and octadecanenitrile. This indicates that ABS3 perhaps contains EBS, like ABS2. Increasing amounts of EBS would decompose at 230[degrees]C with increasing numbers of reprocessing cycles. The peak at 24.3 min has been identified to be palmitic acid, which is another lubricant commonly used in plastics, by comparison with a standard. After three reprocessing cycles, the peak became smaller relative to other peaks, which indicates that part of the palmitic acid was lost.

[FIGURE 14 OMITTED]

[FIGURE 15 OMITTED]

From the GC/MS results above, it may be seen that some additives have been identified as decomposing during reprocessing, particularly during reprocessing at 270[degrees]C and multiple reprocessing. It seems likely that other similar additives would also be lost by decomposition under these reprocessing conditions.

[FIGURE 16 OMITTED]

Impact and Tensile Properties

The impact and tensile properties of ABS plastics before reprocessing and after reprocessing at different temperatures and rotational speeds are shown in Fig. 14.

From Fig. 14a, it is particularly evident that the impact strength reduced with increasing reprocessing temperature. Various factors have been involved in reduction of impact strength, including reduction in entanglements stabilized by rubber particles [13], chain scission of the graft between the SAN matrix and the rubber phase [14]. As well as these factors, degradation including crosslinking of the rubber phase (as evidenced from the DMTA data presented above) is likely to be a significant factor in the reduced impact strength after reprocessing at higher temperatures. There is also evidence in Fig. 14a that, at the same reprocessing temperature, the impact strength is slightly reduced with increasing rotational speed. Oxidation of the polymer, particularly the rubber component, might have been a significant factor since higher rotational speeds provide more opportunity for oxygen to diffuse into the polymer (A small amount of degradation of the rubber phase would not significantly change the molecular weight distribution [6]).

The tensile properties of ABS2 after reprocessing are shown in Fig. 14b and c. Compared with the properties after reprocessing at 190 and 230[degrees]C, both the tensile modulus and strength were slightly higher after reprocessing at 270[degrees]C. Potential explanations for these higher stiffness and strength values include the loss of small molecules (including lubricant molecules) [15], and crosslinking of the rubber phase. Figure 14 also indicates that rotational speed had little effect on the tensile properties. The strain to failure in these ABS samples was typically about 10%, and although there was more scatter in these data, there were no significant differences between the average figures for each processing condition. This may have been because the effects of defects (formed during material recycling and sample preparation) were more significant than the effects of other factors, such as processing temperature.

Figure 15 shows how the mechanical properties of ABS3 varied as a function of the number of reprocessing cycles, and it is clear that the impact strength is the most sensitive to reprocessing. From Fig. 15a, it can be seen that the impact strength reduced significantly as ABS3 underwent multiple reprocessing. The greatest reduction was during the first reprocessing cycle, impact strength dropping by about 44%. With subsequent reprocessing cycles, the impact strength diminished more slowly. This could be explained by the major loss in volatile molecules found during the first cycle since a loss in impact strength has been associated with loss of these small molecules [15]. Other potential causes of a reduction in impact strength include crosslinking of the rubber phase and scission of the ABS polymer chains. This present study would seem to suggest that the loss of volatiles is a significant factor since the results indicate that volatiles are most substantially reduced during the first reprocessing cycle, whereas crosslinking and scission occur more gradually at each reprocessing cycle.

Figure 15b and c show the tensile properties of ABS3 as a function of the number of reprocessing cycles. It may be seen that the modulus of ABS3 did not change significantly with increasing number of reprocessing cycles, but tensile strength increased slightly. These small increases in strength may again be related to loss of the small molecules (including lubricant molecules) [15] and degradation (including cross-linking) of the rubber phase [5].

Morphology of Fracture Surfaces

Figure 16 shows the morphology of the impact fracture surfaces of ABS1 after reprocessing at different temperatures and rotational speeds. Compared with the fracture surfaces after reprocessing at 20 rpm (Figs. 16a and c), more holes can be seen in the matrix after reprocessing at 100 rpm (Fig. 16b and d). This suggests that degradation of ABS increased. On the other hand, on increasing the reprocessing temperature from 190 to 270[degrees]C, deformation of the matrix reduced. This is possibly related to crosslinking of the rubber phase. A high degree of crosslinking of the rubber phase can reduce the ability of the rubber particles to cavitate. According to recent research, cavitation of the rubber particles can facilitate deformation of the matrix [16]. Possibly, due to degradation of ABS, particularly crosslinking and scission of the rubber phase, the material had become very brittle after reprocessing at 270[degrees]C and 100 rpm (Fig. 16d).

The fracture surfaces from impact tested samples in the multiple reprocessing studies of ABS3 are shown in Fig. 17. The micrograph (Fig. 17a) of a sample, which had not been reprocessed in the torque rheometer (n = 0), exhibits profuse cavitation and extensive matrix shear yielding. After one reprocessing cycle in the torque rheometer (n = 1), most of the area shows relatively brittle fracture features (Fig. 17b). A number of small holes appear on the fracture surface, and the size of the holes corresponds to the size of rubber particles. Figure 17b indicates that most rubber particles did not heavily cavitate and facilitate shearing of the matrix, and that cracking was more generally at the interface between rubber particles and the matrix. This weakened interface, which may have resulted from the observed degradation of the rubber phase, would lead to a reduction in impact strength. It has been suggested that a small amount of degradation of the rubber phase could promote a decrease in adhesion with the SAN matrix, causing stress concentrations and leading to rapid crack propagation between rubber particles and matrix [17]. The effect of reprocessing was more apparent after four reprocessing cycles (Fig. 17c). The degree of deformation of the matrix is noticeably further reduced and the cracks propagated along the interface between particles and matrix. Many large particles can be seen on the fracture surface, together with a number of relatively large voids in the matrix, from which the large particles had been pulled out. The large particles are possibly due to the agglomeration of the rubber particles, but this could not be confirmed by SEM. Figure 17c suggests that, with increasing reprocessing, the original microstructure of the ABS copolymer is being destroyed. This seems quite likely to be a significant factor in explaining the reduction in impact strength during multiple reprocessing.

[FIGURE 17 OMITTED]

CONCLUSIONS

The results suggest that two of the most important factors affecting the properties of reprocessed ABS are loss of the smaller more volatile molecules and degradation (chain scission and particularly crosslinking) of the rubber (polybutadiene) component. It appears that loss of the smaller molecules occurs more readily, that is, even at the lowest reprocessing temperatures and just a single reprocessing cycle, there is significant reduction in the quantity of small molecules. As the recycling conditions become more and more severe, such as higher temperatures or multiple reprocessing, further small volatile molecule loss occurs but the amount and significance of these further losses is progressively reduced. On the other hand, degradation of the polybutadiene phase seems to become progressively more significant with increasingly severe reprocessing conditions. Thus, with increasing reprocessing temperature or increasing number of reprocessing cycles, the significance of the loss of volatile molecules reduces and the importance of polybutadiene degradation increases. These changes that occur during reprocessing alter the ABS morphology, in particular reducing the interfacial bond strength between the SAN matrix and PB rubber particles. These changes have significant effects on the mechanical properties.

Reprocessing of ABS has a much more severe effect on impact properties than tensile properties. This is consistent with the changes outlined above, namely loss of small molecules, degradation of the polybutadiene phase and relatively little change in the SAN phase. Reduced numbers of small molecules and degradation of the rubber phase will both contribute to loss of impact resistance. At the same time, since the load bearing SAN phase is relatively unaffected by the reprocessing, then the tensile properties also remain relatively unchanged. Even the slight increase in tensile properties with increasing temperature might be expected to result from degradation of the rubber phase and loss of small molecules (including lubricants).

ACKNOWLEDGMENTS

We thank RAPRA for some results of GPC and the EPSRC NMSSC at University of Wales Swansea for mass spectra. X B thanks the Chinese Government for a studentship and the University of Wales Swansea for a bursary to cover her tuition fees.

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Xiaojuan Bai, (1,2) D.H. Isaac, (3) K. Smith (1)

(1) Department of Chemistry, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, United Kingdom

(2) School of Material Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, The People's Republic of China

(3) Materials Research Centre, School of Engineering, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, United Kingdom

Correspondence to: X. Bai; e-mail: baixiao814juan@yahoo.com
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