The developmental study of PET-PBT blend revealed that 60/40 PET/PBT blend possess the lowest impact strength as compared to rest of the weight ratio combinations of PET-PBT. Thus, this blend is modified by adding the impact modifier in different proportions (2 and 10%) with the aim to gain more impact strength. The addition of the impact modifier has significantly improved the impact strength, but the wear resistance and the tensile strength both are found to decrease considerably. Thus, to overcome this situation an attempt was made to treat these blends cryogenically, as improvement in the mechanical properties is observed in case of the polymers and the composites reported till now [8–14].
Here the blend B (unmodified 60/40 PET-PBT blend), B1-B2 (B blend modified with 2 and 10% Elvaloy AC), B3-B4 (B blend modified with 2 and 10% PP-cp) are cryogenically treated at -80°C, -140°C and − 185°C for 4, 8, 12, 16, 20 and 24h for each mentioned temperatures. The cryo-treated blends are then characterized for the mechanical, structural and thermal properties to study the effect of the treatment on the considered blends. The cryo-treated blends both unmodified and impact modified, are primarily characterized for the tensile strength and the impact strength, as these mechanical properties are of main concern. The increase in the tensile strength of the samples treated at -185°C is observed. Whereas, for the samples treated at -80°C and − 140°C the decrease in tensile strength is observed. The detailed analysis of this difference in the tensile behavior is attributed to the simultaneous and differential thermal contraction taking place as a result of cryo-treatment. The temperatures − 80°C and − 140°C are not sufficient enough to bring out the simultaneous thermal contraction, which results into de-bonding between the modifier and matrix, ultimately leads to the premature failure of the material. In addition to the temperature parameter, the time for which the treatment is given, is also important to achieve the simultaneous contraction. In the present study, blends with different concentration of modifiers are considered, as the amount of modifier have definite response towards the cryo-treatment. It was observed that the samples with no or less addition of modifier requires 8h time duration, whereas, the samples with higher % of modifier (10%) requires more time duration (12h) to achieve better tensile and impact properties. Thus, in general it can be said that higher modifier content requires more time duration of the cryo-treatment.
In case of untreated impact modified blends, it was observed that tensile strength decreases with increase in % addition of the modifier. The impact strength improvement is in linear relationship with the wt. % of the impact modifier added. Here, the reason for the increase in impact strength is attributed to crazing, shear yielding and cavitation phenomenon mainly observed in the impact modified systems, which are also depicted in Fig. 14. Crazing is a network of fine cracks homogeneously formed throughout the material. Crazing results in fibrils drawn from both sides of a growing crack and bridging the crack walls, resulting in the void formation, thereby volume expansion. In actual scenario, shear band formation should precede crazing by forcing the crazes to propagate through the shear bands as demonstrated by R. P. Kambour et al. [21]. The formation of shear bands involves a limited slip or yielding of one portion of the polymer relative to another, leaving a highly oriented band of material connecting the two portions. Crazing and shear toughening may occur simultaneously in impact modified polymers blends.
Crazing and shear yielding can be enhanced by addition of the impact modifier as they can act as stress concentrators. The impact modified blends involves one more process for the improvement in the impact strength called as cavitation. In the modified blends cavitation of modifier particle has been observed to be an important factor as precursor to shear deformation. The tri-axial stresses state existing in front of the crack tip produces dilation that can result in void formation in the modifier particle or at the modifier particle-matrix interface. Cavitation of modifier particles is a prerequisite for enhanced toughness when shear yielding is the principal mechanism. Practically, the craze, shear band and the cavity formed, consumes extra energy, providing an additional toughening mechanism [22]. This is the main reason for the increase in impact strength. But, simultaneously the tensile strength is gradually decreasing with increase in the impact strength. The reason for such behavior is the poor modifier- matrix interface interaction.
Actually to obtain better tensile strength the interface should be strong, so as to transfer the stresses formed during the tensile loading, from the matrix to the modifier. The weak interfacial bonding will act as a barrier in the stress transfer mechanism which results in early failure of the material. Thus attempts were made to improve the tensile properties of such blends by addition of a compatibilizer [23]. Compatibilization is the process of modification of the interfacial properties of an immiscible modifier and polymer blend to improve the adhesion and blend properties. This process involves the incorporation of block or graft copolymers which are identical to those in the respective phases. But there are many factors such as miscibility, size, stability, chemistry etc., in order to choose the proper compatibilizer for the intended blend system. Thus, the cryogenic treatment can be an economic and compatible alternative in order to improve the interfacial bonding. The proper cryo-treatment results in the alignment of the randomly arranged polymer chains. The aligned structure enables the polymer fibrils or chains to carry very high stresses because of much stronger and stiffer bonding as a result of cryo-treatment.
Apart, on cryogenic treatment the improvement in the interface between modifier and the matrix is taking place which can be supported from the SEM and FTIR analysis. The improved interface enables to transfer the stresses from the matrix to the modifier and improves the tensile strength of the impact modified blends without sacrificing the inherent impact property. Apart from the tensile and impact strength the wear performance of both unmodified and the modified blends is found to be improved on the treatment, irrespective of the wt.% of the modifier added. It is observed that, those samples exhibiting good tensile and impact strength performed excellent for the wear property too. As the samples treated at -185°C for 8h and 12h for the blends B, B1, B3, and B2, B4 respectively have shown the better wear performance. The recorded improvement is nearly 50% as compared to the un-treated samples. Thus to understand the core reason behind this improvement in the wear performance the structural analysis of these samples is performed.
The comparative XRD analysis of un-treated and cryo-treated blends revealed a small negative shift in the peak position (2θ) which indicates the decrease in the material stresses. In addition to this the evaluation of XRD data for the crystallinity and crystallite size measurements, exhibited the increase in crystallinity and decrease in crystallite size on cryo-treatment. XRD results indicate that structural modification of blend occurring as a result of conformational changes which may be due to interfacial interaction. Therefore, to visualize the scenario at the interface the SEM micrographs are taken.
The SEM images of un-treated and cryo-treated unmodified blend have shown the definite modification as far as the surface morphology is concerned. The blend B morphology appears relatively denser to the un-treated blend. Besides, in the Elvaloy AC and PP-cp modified blends also the dense microstructure is observed along with the good interfacial bonding. Figure 6 and Fig. 7 shows the SEM micrographs of all the Elvaloy AC and PP-cp modified blends respectively, treated at -185°C for all the studied time duration (4, 8, 12, 16, 20 & 24) compared with the un-treated material. The trend observed here emphasizes the fact that, the time given for the treatment is crucial to decide the level of bonding between the matrix and the modifier. Thus it was observed that the samples loaded with 2 and 10% of modifier exhibited better interfacial interaction for the treatment time 8 and 12h, respectively at -185°C. Actually the specific time and temperature for the cryo-treatment is required to bring out the synchronized or simultaneous thermal contraction resulting to the improved interface. Whereas, the differential contraction resulted in the de-bonding, this can be depicted from the Fig. 8.
In addition to the surface morphology the worn out surface is also analyzed. Here the untreated and cryo-treated worn surfaces are monitored. The untreated sample of the modified blend revealed the smooth removal of the modifier balls during sliding wear, indicating the weak bonding between the matrix and the modifier. Whereas, in case of cryogenically treated samples, the modifier balls are found to be embedded within the matrix itself imparting good wear performance (Fig. 9). Including improved interface, the improvement in the transfer film produced by cryo -treated samples helped to decrease the wear rate.
The FTIR analysis confirmed the presence of interfacial bonding between the modifier and the matrix. In these two blend systems completely new IR band is detected between the wavenumber 3550–3590 cm− 1 which corresponds to O-H stretching vibrations of hydrogen bonded hydroxyl group indicating the presence of hydrogen bonding between modifier particles and PET-PBT 60/40 matrix. Thus, the probable reactions occurring in the Elvaloy AC and PP-cp modified blends at the interface are predicted in Fig. 15 and Fig. 16, respectively. Including this main modification, the slight positive shift in the peaks in the range of 2900–3000 cm− 1 which represents C-H stretch, is also observed which may have resulted due to the effect of O-H bonding.
The melting point and the melting enthalpy of the untreated and cryo-treated samples in a comparative manner are studied by DSC technique. The aim to carry out the thermal investigation was to note the thermal stability of the sample prior and post the cryogenic treatment. It is observed that on cryo-treatment, the melting temperature in all the studied blends is increased (approximately by 6–10°C). This implies that the material is becoming thermally stable as a result of the cryogenic treatment.