3.1 Mechanical and rheological properties
3.1.1 Effects of changing the polyolefin raw material
Both type and origin of the PO raw material were considered to be an influencing factor in the properties of PO/ABS blends even in the range of immiscibility. Therefore, three different POs (a commercial PP and HDPE, as well as the w-HDPE) were used in the experiments.
As shown in Fig. 4, the effects of changing the PO raw material on the Charpy impact strength, the tensile strength and elongation at break were examined at two different compositions of PO/ABS.
Approximately the same Charpy impact strength were measured at 40% PO content (40/60 PO/ABS) (Fig. 4a) and only that of the HDPE/ABS blend was a slightly higher value. This suggests that the solubility and compatibility values of POs were similar in this composition. Increasing the PO concentration up to 60%, the impact strength of PP/ABS blend decreased by 38%, but the HDPE/ABS and w-HDPE/ABS blend almost kept their previous impact strength values. No effects of shifting the polyolefin ratio was experienced in the case of w-HDPE in the studied polyolefin concentration range.
Performance of every blend was higher in 60/40 PO/ABS than in 40/60 manifested in tensile strength (Fig. 4b). The weakest tensile strength was provided by w-HDPE containing blends as expected but almost the same value was achieved by addition of 60% w-HDPE as by 40% PP addition into the ABS.
Shifting the ratio of PO/ABS in w-HDPE containing samples did not have any remarkable effect on the elongation at break (Fig. 4c) being really low. However, it can be clearly seen that almost the same elongation at break values were achieved at 40% PO content for all PO types. A general improvement was observed in the elongation at break of commercial PO-rich blends, but the standard deviation was not negligible especially in PP-rich blends taking tensile properties and compositions into account.
Considering the aspects and goals of waste management w-HDPE has been chosen for the further experiments, such as compatibilizing.
3.1.2 Compatibilization of GTR containing w-HDPE/ABS blends
At the stage of GTR introduction and compatibilization during the experiments, ratio of w-HDPE and ABS was kept at constant values i.e. at 40/60 or 60/40 and GTR filler was incorporated in the blends in concentration of 10 wt%, 20 wt% and 30 wt%.
Addition of GTR to 40/60 w-HDPE/ABS can be established to slightly improve impact resistance of the blends (Fig. 5), however, a positive trend with increasing GTR content could be observed only in the strength values of the experimentally compatibilized (AD-EXP) samples. There was a progressive decreasing of impact strength of the uncompatibilized (AD-FREE) blends above 10% GTR content, which reflected the weakening of interfacial adhesion in these blends. Using experimental compatibilizer, almost the same impact strength and standard deviation was obtained for the samples with different GTR contents, indicating a good homogeneity in the presence of this additive even at the highest GTR content. This compatibilizer showed such a balanced impact resistance with increasing GTR content, which allowed to introduce waste rubber in a higher concentration into the blend. At the same time, the highest impact strength was achieved at 20% GTR content of AD-EXP sample. In the case of commercial additive (AD-COM), standard deviations of impact strength were very high, which can be attributed to the significant structural inhomogeneity of specimens.
Taking into account the Charpy impact strength values of 60/40 w-HDPE/ABS blend (Fig. 5b), the incorporation of GTR was proven to be beneficial, moreover, a linear trend in the impact strength was observed for the uncompatibilized and compatibilized blends as well. Significant differences in the effect of additives could not be revealed until 30% GTR content. For example at 30% GTR content, the impact strength was improved by 28% and 21% with the current dosage of the experimental and commercial additives, respectively, but standard deviation of the AD-COM strength values was twice as high as that of the AD-EXP ones.
It can be concluded, that higher GTR content (20% and 30%) was more favorable to obtain an enhanced compatibilization with the AD-EXP in both studied blends (40/60 and 60/40 w-HDPE/ABS), and in w-HDPE-rich experimentally compatibilized blend, the impact strength could be enhanced by 61% compared to the ABS-rich one compatibilized by AD-COM. Mostly, in the case of the 60/40 w-HDPE/ABS blends, better strength results were achieved because the w-HDPE could behave as a matrix material in the system.
In addition, the tensile properties such as the tensile strength (Fig. 6a) and elongation at break (Fig. 6b) of the blends were also measured.
First, the tensile strength values of the 40/60 w-HDPE/ABS blends were shown in Fig. 6a. Although the addition of GTR into the 40/60 w-HDPE/ABS blends resulted in decreasing tensile strength with increasing GTR content, the decrease was somewhat compensated by the addition of AD-EXP. In the case of 10% GTR content, the tensile strength was almost the same as that of the uncompatibilized blend without any GTR content. Increasing the GTR content up to 20%, a similar phenomenon occurred, the AD-EXP could compensate the effect of increased GTR content on the tensile strength. It should be mentioned that in the field of waste recycling, the reuse of a higher GTR content in blends with additives without the property deterioration is really advantageous. Nevertheless, the addition of 30% GTR with or without any additive significantly decreased the tensile strength. At the same time, the AD-COM could not improve the tensile strength of GTR containing blends. Considering the strength values of the GTR containing samples, the effectiveness of the AD-COM was far below that of the AD-EXP. Nevertheless, the most outstanding phenomenon was the balanced performance of the AD-EXP in the whole studied range of GTR concentrations.
Next, the changes of the tensile strength of the 60/40 w-HDPE/ABS blends were presented in Fig. 6b. Besides compatibilization concern, the effect of GTR loading on the tensile strength of the blends can be well established in Fig. 6b. In the case of AD-FREE blends, a drastic decrease of tensile strength was observed at 20% and 30% GTR content. At the same time, the addition of 10% GTR practically did not reduce the tensile strength of the 60/40 w-HDPE/ABS blend compared to that of the 40/60 w-HDPE/ABS one. Application of AD-EXP caused a smaller reduction of the tensile strength value at higher (20% and 30%) GTR content compared to the value of the uncompatibilized composite.
In the case of the AD-COM, the addition of 10% GTR slightly reduced the tensile strength, but a higher GTR content caused significant decrease. Based on that trend of the tensile strength, the commercial additive had a higher compatibilizing effectiveness at lower GTR content, probably because of the higher miscibility of the two raw polymers in that ratio. Although the blends with the experimental additive did not show outstanding strength values, the decrease of tensile strength was lower at 20% and 30% GTR content even up to 20% meanwhile drastic decrease was realized in case of the other two compositions. At the same time, the AD-COM produced a slightly higher reduction of the tensile strength at higher (20% and 30%) GTR content than the AD-EXP.
Finally, it should be noted that the tensile strength results of both studied w-HDPE/ABS blends were similar in magnitude.
The elongation at break values of the 40/60 and 60/40 w-HDPE/ABS blends are presented in Fig. 7. The additive-free 40/60 w-HDPE/ABS blends and their composites with GTR (Fig. 7a, AD-FREE) had low elongation at break values with high standard deviation and the compatibilization did not improve those values significantly (AD-EXP, AD-COM). Therefore, it can be concluded that those polymer blends had almost the same elongation at break values in the presence or absence of compatibilizer. In the case of the 60/40 w-HDPE/ABS blends (Fig. 7b), the elastic behavior of GTR is evidenced in increased elongation at break values. The additive-free blends with different GTR content had similar elongation at break values with high standard deviation, and the addition of AD-COM practically did not change those values. Application of AD-EXP showed excellent performance since a significant increase was noticed in the elongation at break values, which was independent from GTR content.
Flow behavior of the blends was studied by MFI measurements. MFI values of the uncompatibilized and compatibilized w-HDPE/ABS blends with different GTR content were represented in Fig. 8. It is well-known that solid particles are able to influence the melt flow indices of the polymers.
MFI showed linear trend with increasing GTR content and MFI values of GTR-free blends are halved at 30% GTR loading in both w-HDPE/ABS compositions, MFI values of blends with AD-COM and closer to MFI of AD-FREE blends with increasing GTR concentration.
3.2 Structure characterization
3.2.1 XRD analysis of the blends
It is well known that the crystal structure of the crystalline HDPE belongs to the orthorhombic crystal system and to the Pnam space group. Corresponding to the 00–060-0986 PDF, its lattice parameters are the followings: a = 7.465 Å, b = 4.951 Å and c = 2.560 Å. The lattice parameter c (along the PE chains) is equivalent to the length of one monomer unit (C2H4) . XRD analysis is suitable to characterize the crystal structure and crystallinity of polymer blends [39, 40]. In this case, the crystallinity can describe the ratio of crystalline part in a mixture of crystalline and amorphous materials .
As a part of the present study, XRD patterns of the pure w-HDPE and ABS, as well as the blends (with 0% and 20% GTR content) were measured in three different areas. Some representative XRD patterns of samples are given in Fig. 9. In the XRD pattern of the pure w-HDPE (100% w-HDPE), the crystalline HDPE produced sharp reflections (e.g. +) due to diffraction, whereas the amorphous HDPE produced a broad reflection (in the 8–28° two-theta range) indicating only some short-range order in the atomic arrangement. The XRD pattern of the raw ABS (100% ABS) showed several pronounced reflections (o), which was characteristic for its predominant crystalline state. In the XRD patterns of the blends, the characteristic reflections of crystalline HDPE were present, while the reflections of crystalline ABS were absent. Simultaneously, the broad reflection in the 8–28° two-theta range strengthened representing the transformation of ABS from crystalline to glassy state. To characterize the changes of crystalline and amorphous parts in the samples, the crystallinity values were determined using the following equation :
where A110 and A200 were the area of the 110 and 200 reflections, respectively, Aam was the area of the amorphous reflection in the 8–28° two-theta range, and the K correction parameter was 1.4 .
As indicated in Fig. 9, the calculated crystallinity values of the pure w-HDPE (100% w-HDPE) and the uncompatibilized 40/60 w-HDPE/ABS blend at 0% GTR content (AD-FREE) were 0.58 and 0.23, respectively; these values confirmed that the mass fraction of crystalline HDPE phase was related to the mass fraction of w-HDPE (40%). This also proved the complete transformation of ABS from crystalline to glassy phase. It was also interesting that the crystallinity values at 20% GTR content was almost the same (approximately 0.23), which suggested a slight (~10%) increase in the mass fraction of the crystalline HDPE during the preparation of these ternary blends.
Using three parallel XRD patterns of the 40/60 and 60/40 w-HDPE/ABS blends at 0% and 20% GTR content, the crystallinity values were determined (Fig. 10). In the case of the 40/60 w-HDPE/ABS blends at 0%, the mean crystallinity values were close to each other with a relatively small standard deviation. This presented that the additives practically had no effect on the mass fraction of the crystalline HDPE. At 20% GTR content, the mean crystallinity values were very similar, but with a higher standard deviation, which indicated the inhomogeneity of these samples. It should be mentioned that the lowest standard deviation was obtained using AD-EXP. This also strengthened the good compatibilization effect of this additive. In the case of the 60/40 w-HDPE/ABS blends at 0% GTR content, the mean crystallinity values were at around 0.35, which value also proved that the mass fraction of crystalline HDPE mainly depended on the mass fraction of w-HDPE (60%). There were also relatively small standard deviations, which characterized the fairly good homogeneity of the uncompatibilized and compatibilized blends without addition of GTR. In the case of 20% GTR content, all samples showed slightly decreased mean crystallinity values, but only the AD-FREE sample had an increased standard deviation. The experimental and commercial additive decreased the standard deviation and enhanced the compatibility of the w-HDPE/ABS blend with the GTR. The AD-EXP sample showed the highest mean crystallinity value (0.32), which indicated a slightly (~8%) increased mass fraction of the crystalline HDPE.
Characterization of GTR containing blends was supplemented with FT-IR evaluation. FT-IR spectra of blends with 20% GTR were compared in Fig. 11. The other, GTR containing samples gave the same characteristic peaks, therefore, only the results of the methylene and methyl absorption peaks are involved (Table 3.). Possible changes in the structure of the blend were followed by the ratio of integrated areas of methyl and methylene characteristic bands at 2955 cm-1 and 2912 cm-1.
Additive containing w-HDPE/ABS blends without GTR had a higher methylene/methyl ratio than the additive free counterparts, but no difference was measured between the two additive containing blends.
Ratio of the integrated area of the methylene groups was independent from the GTR content in blends of w-HDPE/ABS in 40/60 without compatibilizer and with the commercially available one. In w-HDPE rich blends the aforementioned area of the same compositions changed to a higher extent with the GTR content in the whole range investigated.
Blends containing the experimental additive showed higher variability with GTR content in both compositions of w-HDPE/ABS.
3.2.3 Blend morphology
Tensile fracture surfaces of the w-HDPE/ABS 60/40 blends with GTR concentrations of 0% and 20% can be seen in Fig. 12a-c.
Fig. 12a shows the morphology of uncompatibilized 60/40 w-HDPE/ABS blend where a relatively crowded structure was observed without any voids and filament formation reflected in the low value of elongation at break as well. The two polymers located in layers on each other but they can be distinguished obviously: w-HDPE created the crystalline and continuous phase of the blend while ABS domains gave the amorphous dispersed part with flat tensile surface.
Incorporation of 20% GTR into the blend of w-HDPE/ABS (Fig. 12b) broke the continuity and uniformity of the structure which revealed in the void formation around GTR particles among others. Furthermore the aforementioned lamellar and concise structure of the blend eliminated and polymer burrs around the particle were not observed leading to the conclusion that relatively lower strain was needed for the separation of the interface.
Effects of the compatibilization can be clearly seen in the Figures of 12 c and d. Blends compatibilized by experimental or commercial additive showed more structured polymer phase and more homogenous dispersion of ABS particles in the w-HDPE matrix, as expected. A remarkable reduction in the particle size of ABS was noticed as an effect of compatibilizing.
A completely embedded particle was observed in the Fig. 12c which illustrated the tensile fracture of the blend compatibilized by the experimental additive. Besides, burr formation around the particle could be seen indicating the higher strength of the embedding resulted by improved adhesion between the polymer-rubber phases. SEM micrograph and the results of the tensile test were totally in sync with each other since the improvement in tensile strength and elongation at break was approximately 60%. Moreover a better dispersion of ABS domains was detected in comparison with the uncompatibilized blend supported by the lower standard deviation values of tensile properties. ABS particles appeared in spherical shape in uncompatibilized blend and blend compatibilized by AD-EXP which led to the increment of interfaces.
The rough surface of the GTR promotes and boost the adhesion between the polymer and rubber particle especially in compatibilized blends. Separation located at the point where the surface of GTR particle was flat so the interlocking was hindered by the geometry of the GTR particles, while in uncompatibilized blend the rubber particle possessed rough surface at the point of separation.