Properties of Recycled Polypropylene / Chloroprene Rubber Blends : The Effects of Dynamic Vulcanisation

This work examined the tensile, thermal, morphology and swelling properties of unvulcanised and dynamically vulcanised recycled polypropylene (RPP)/chloroprene rubber (CR) blends at different blend ratios. The results indicated that the tensile strength and Young's modulus of both the unvulcanised and dynamically vulcanised RPP/CR blends were reduced. However, elongation at the breaking point increased with increasing CR content. In contrast, the dynamic vulcanisation enhanced the tensile properties, melting temperature (Tm), enthalpy (ΔHf) and crystallinity (Xb) in comparison with the unvulcanised RPP/CR blends. The swelling percentage of the dynamically vulcanised RPP/CR blends was lower than the unvulcanised blends due to the presence of crosslinks after the dynamic vulcanisation occurred.


Compression Moulding of Blends
Each of the RPP/CR blends were compressed into sheet form using a compression moulding machine, model GT 7014 with a temperature and pressure of 180°C and 170 kg cm -2 , respectively.The blend samples were preheated for 8 minutes and then compressed for 6 min.After that, the samples were subsequently cooled under pressure for 4 min.The samples were then cut into dumbbell shapes using a Wallace dumbbell cutter.

Tensile Properties
Tensile strength, Young's modulus and the elongation at break were measured using an Instron Tensile Machine Model 5569, according to ASTM D638.The gauge length and crosshead speed were 50 mm and 50 mm min -1 , respectively.The tensile test was performed at 25°C ± 3°C.Five samples of each composition were tested, and the average values were recorded.

Swelling Behaviour
The swelling percentage of the specimens were measured according to ASTM D471.The dimensions of the specimens were 30 × 5 × 1.3 mm.First, the specimens were weighed (initial weight) using a Mettler balance.Then, the specimens were immersed in toluene at room temperature for 72 h.After that, the specimens were taken out from the toluene and wiped with tissue paper to remove excess toluene and then weighted again (swollen weight); the swelling percentage of the specimens was calculated using Equation 1:

2.6
The ten electron the spe electrost 2.7 the tensile strength of the dynamically vulcanised RPP/CR blends.Moreover, the improvement of the tensile strength was also due to the crosslinked rubber particles attaching to the RPP matrix, which increased the extent of deformation before failure.Van Dyke et al. 14 claimed that the blends of dynamically vulcanised polyamide 12 (PA12)/chlorobutyl rubber (CIIR) with different curatives improved the tensile strength compared to unvulcanised blends.The same observation was also reported by Narathichat et al. 15 in the dynamically cured natural rubber/PA12 blend system.
Figure 1: Effect of blend ratios on tensile strength of unvulcanised and dynamically vulcanised RPP/CR blends.
Figure 2 illustrates the effect of blend ratios on Young's modulus of the unvulcanised and dynamically vulcanised RPP/CR blends.It is apparent that Young's modulus of both blends decreased with increasing CR content due to a reduced stiffness of the blends.At similar blend ratios, the dynamically vulcanised RPP/CR blends exhibited a higher Young's modulus than the unvulcanised RPP/CR blends.This suggests the presence of crosslinks in the CR phase since there was an improved stiffness of the dynamically vulcanised RPP/CR blends.Accordingly, the Young's modulus is dependent on the crosslink density, and the increase in the crosslink density is reflected by the enhancement in the Young's modulus under dynamic vulcanisation.
Figure 2: Effect of blend ratios on Young's modulus of unvulcanised and dynamically vulcanised RPP/CR blends.
Figure 3 shows the elongation at break of the unvulcanised and dynamically vulcanised RPP/CR blends with different blend ratios.It was observed that the elongation at break increased with the increasing CR content.Nakason et al. 16 claimed that the tendency towards recovery to the original shape of elongated samples is higher for the blends with a higher content of CR.Therefore, the dynamically vulcanised RPP/CR blends exhibited a higher elongation at the break in compared to the unvulcanised RPP/CR blends.The improvement of elongation at the break of the dynamically vulcanised blends was due to an improved dispersion of the CR particles in the RPP matrix.Dearmitt 17 reported that the improved dispersion would help to prevent agglomeration and consequently to maintain a superior elongation at break.][20][21] Figure 3: Effect of blend ratios on elongation at break of unvulcanised and dynamically vulcanised RPP/CR blends.

Swelling Properties
Figure 4(a) shows the effect of blend ratios on the swelling percentage of unvulcanised and dynamically vulcanised RPP/CR blends at compositions of 100/0, 70/30 and 40/60 in toluene for 72 h.The swelling percentage is an indicator of the degree of crosslinking.It can be observed that the swelling percentage of dynamically vulcanised RPP/CR blends was lower than the unvulcanised blends.This indicated that the resistance to chemical penetration of the dynamically vulcanised RPP/CR blends was higher compared to the unvulcanised RPP/CR blends.This is due to the presence of crosslinks in the dynamically vulcanised RPP/CR blends, which hindered the penetration of toluene into the RPP/CR blends.Anandhan et al. 22 claimed that the swelling index of the dynamically vulcanised blends was less than the unvulcanised blends because crosslinks present in the rubber phase of the vulcanised blends limit the transport of solvent molecules into the blends.
The equilibrium swelling percentage of RPP/CR blends is shown in Figure 4(b).
The equilibrium swelling percentage of both unvulcanised and dynamically vulcanised RPP/CR blends increased with increasing CR content for all the compositions of the blends.The blends became less stiff and more permeable to the toluene as the CR content increases within the blends.However, the dynamically vulcanised RPP/CR blends exhibited lower equilibrium swelling percentage compared to the unvulcanised blends.This is due to the presence of the crosslinks, which restricted the penetration of toluene in RPP/CR blends.Similar findings were reported by Ismail et al., 23 where they found that the swelling index of dynamically vulcanised PVCw/NBR blends were lower than unvulcanised PVCv/NBR and PVCw/NBR blends.This is because the blends become stiffer and less penetrable to chemicals as the crosslink density in dynamically vulcanised PVCw/NBR blends increased.

Morphology Study
The SEM micrographs of the RPP/CR blends at compositions of 100/0, 70/30 and 40/60 are shown in Figures 5, 6 and 7, respectively.The SEM micrographs of neat RPP show the homogeneous surfaces with a brittle nature, as shown in Figure 5. Figure 6 shows the tensile fractured surfaces of unvulcanised RPP/CR blends at a blend ratio of 70/30, which exhibited a large number of holes formed by the detachment of the CR particles from the RPP matrix due to poor interfacial interaction.Furthermore, the SEM micrographs of the unvulcanised RPP/CR blends at a blend ratio of 40/60 showed larger CR particles dispersed in the RPP matrix (Figure 7).This result indicates that poor interfacial adhesion between the RPP and CR and the agglomeration of CR particles occurs at higher contents of CR.In contrast, the SEM micrographs of the dynamically vulcanised RPP/CR blends at 70/30 and 40/60 are displayed in Figures 8 and 9, respectively.It can be observed that both of SEM micrographs for the dynamically vulcanised RPP/CR blends exhibited CR particles that are better dispersed in the RPP matrix compared to unvulcanised RPP/CR blends, as well as have fewer holes on the RPP surface.This is because the dynamic vulcanisation of TPEs caused the CR particles to be more uniformly dispersed in the RPP matrix and thus enhanced the tensile strength of the TPEs, as discussed in Section 3.1.Kumar et al. 24 claimed that during dynamic vulcanisation, the crosslinked rubber becomes more finely and uniformly distributed in the plastic matrix.Martin et al. 25 reported that the dynamic crosslink of the elastomer in molten thermoplastics leads to a very fine and homogeneous morphology, resulting in an improvement of the tensile properties.

Differential Scanning Calorimetry (DSC)
The DSC curves of unvulcanised and dynamically vulcanised RPP/CR blends at compositions of 100/0 and 40/60 are presented in Figure 10.Table 2 summarises the DSC data of unvulcanised and dynamically vulcanised RPP/CR blends at different blend ratios.In Table 2, it is shown that the values of ΔH f , and X b for both the unvulcanised and dynamically vulcanised RPP/CR blends decreased with the increasing CR content.The inclusion of CR into the blends limits the crystallising tendency of the RPP and thereby reduced the ΔH f and X b of the blends.Nevertheless, the dynamically vulcanised RPP/CR blends exhibited slightly higher values of T m , ΔH f and X b compared to the unvulcanised RPP/CR blends.This indicated that the formation of crosslinks in the dynamically vulcanised RPP/CR blends enhanced the melting and crystallisation behaviour.
The crosslinked CR phase held the structure of the blends more firmly together and improved the crystallite stability, as well as raised the melting temperature compared to unvulcanised blends.A similar result was reported by Hernández et al., 26 who reported that the dynamic vulcanisation of PP/NR blends exhibited a higher ΔH f compared to unvulcanised and static vulcanised blends.They also claimed that when the blends are dynamically vulcanised, the rubber particle size decreased, and the crystallisation process was less hindered, which caused the enthalpy of fusion of the dynamically vulcanised blends to increase.

Figure 4 :
Figure 4: (a) Swelling percentage; and (b) Equilibrium swelling percentage of unvulcanised and dynamically vulcanised RPP/CR blends at different blend ratios.

Figure 10 :
Figure 10: Comparison of DSC curves between RPP, unvulcanised and dynamically vulcanised RPP/CR blends at 60 php of CR content.

Table 1 :
The formulations of unvulcanised and dynamically vulcanised of RPP/CR blends