Preparation of skim natural rubber and polypropylene blends via melt blending: A study on processability

Abstract

Processability of thermoplastic natural rubber (TPNR) blends is a vital aspect in the preparation of blends for industrial applications and is assessed using different techniques. In this study, skim natural rubber/Polypropylene (SNR/PP) blends were prepared using melt mixing and their processability was examined in terms of mixing torque development, long-term processability, moving die rheometer (MDR) torque, thermo-gravimetric analysis (TGA), melt flow index (MFI) and morphological studies. A series of unvulcanized (UV) and dynamically vulcanized (DV) blends having 70/30, 60/40, 50/50, 40/60, and 30/70 SNR/PP compositions were prepared. Standard Lanka Rubber (SLR) and PP blends having corresponding ratios were also studied for the purpose of comparison. The study reveals that a high percentage of non-rubbers present in the SNR has positively influenced the processability of both UV and DV SNR/PP blends compared to SLR/PP blends as assessed by the mixing torque values. As suggested by the mixing torque development and long-term processability, natural rubber dominant both DV blends cannot be processed. However, they can be processed under low shear rates as shown in the MDR studies irrespective of the rubber type. Therefore, it can be inferred that these rubber dominant blends can be processed into molded products via compression molding technique. TGA studies revealed that a similar degree of protection has been offered to the blends against thermal degradation by SNR and SLR. MFI studies showed that there is no significant difference in flowability between UV SNR/PP blends and SLR/PP blends. However, for DV blends, SNR/PP blends showed a higher flowability than SLR/PP blends. In addition, increase in PP percentage in the blend enhances the flowability while dynamic vulcanization reduces the flowability irrespective of the type of rubber used in the blends. Morphological studies suggest continuous or co-continuous phase structures for UV blends and two-phase structures for DV blends where the plastic phase acts as the continuous phase.

Keywords

Skim natural rubber polypropylene processability dynamic vulcanization thermoplastic natural rubber blends

Introduction

Skim natural rubber (SNR) is manufactured from SNR latex, which is generated during the centrifugation process of field latex as a by-product. It has a variable composition containing 70–85% of natural rubber (cis 1,4-polyisoprene) predominantly consisting of smaller size agglomerates compared to that in conventional rubbers.^1–4 SNR also has comparatively high contents of non-rubber substances such as proteins (10–20%), acetone soluble fatty acids (5–10%), and minor quantities of metal ions.^5,6 The presence of higher amounts of non-rubbers imparts some undesirable properties such as off-color, cure variability, vulnerability to thermal, and oxidative degradations limiting its applications in rubber compounds and vulcanizates.⁷ Consequently, SNR is considered a low-cost raw material suitable for natural rubber (NR) based industrial applications where high product performance is not a concern.^1,8,9

If new applications for SNR are developed or SNR is modified to qualify for advanced industrial applications, it will gain competitive economic advantages through increased demand in expanded areas while assuring value addition. Therefore, the technologies to improve quality and/or to negate the inferior properties of SNR have gained considerable interest.^1,2,4,10,11 Removal of undesirable non-rubber fractions has been identified as a route to improve the quality of SNR latex as far back as the 1950s.^12,13 Membrane separation of skim rubber, deproteinization using various enzymes and other chemicals are some of the techniques that have been used to remove the protein present in the SNR latex.^{1,2,12,14–17} Even though some of the above studies have reported that deproteinization improves mechanical properties and aging characteristics of skim rubber,^1,2,17 this property enhancement is achieved with a loss of mainly proteins, approximately 10–15% weight of the entire material. Instead of removal of proteins, various modifications such as grafting and pre-vulcanization of rubber molecules available in SNR latex have also been reported.^14,18,19 Alex and Nah¹³ have prepared creamed SNR latex treating with an alkali and a surfactant followed by incorporation of organoclay. This treated latex has been processed into dry SNR following the conventional rubber manufacturing processes. The SNR composite with organoclay has shown a faster onset of cure and a higher level of cross-linking.¹³

In spite of the considerable work that has been carried out on the manufacture of value-added or property improved SNR,^1,2,5,17,19 the use of these materials at a commercial scale is rarely reported. This may probably be due to the process complexity and/or un-favorable cost associated with these modification processes. Therefore, it is of great importance to investigate an industrially viable, simple, and well-adaptable conversion process of SNR to be used in potential industrial applications in order to increase the demand and consequently, the value of the SNR.

The blending of elastomers and plastics has become a widely adapted, simple, and cost-effective method to generate industrially important raw materials with combined characteristics of component polymers.^20–27 The blending of NR with various thermoplastic materials forms an industrially important class of continuous or multi-phase materials known as Thermoplastic Natural Rubber (TPNR). A large number of studies is found in the literature on TPNR and its composites produced using different NR types such as ribbed smoked sheets (RSS), technically specified rubber (TSR), etc., and different thermoplastics (TPs) such as polyethylene (PE), polypropylene (PP), etc.^28–30 Dynamic vulcanization of the rubber phase in TPNR has been widely studied as a potential route to improve the performance of these types of blends.^20,31,32 These studies have shown that NR can be blended with thermoplastic materials above its melting temperature to generate useful raw materials even though NR is vulnerable to thermal oxidation due to unsaturation.^7,24 Also, it has been reported that various inferior quality rubber materials such as reclaimed rubbers, ground rubber tyres, and rubber glove waste have been used to blend with thermoplastics to generate TPNR blends and composites.^33–35 These studies have shown that compatibility of constituent plastics and rubber phases and distribution of non-rubber materials in the polymer phases govern the morphology of resultant materials and hence the properties of the blends. SNR being an NR-rich elastomer, has the potential to be used in the preparation of similar types of TPNR blends.

However, only a single study on the processability of SNR and PE blends has been carried out by Balasooriya and co-workers (Anon 2016). As PP is a widely used commodity thermoplastic material having a high softening point than PE and is extensively used in the industry in the preparation of NR/PP blends, it is worthwhile to study the possibility of preparation of SNR and PP-based TPNR blends through melt blending. During melt blending of rubber and plastics, processability is of great importance to assure its adaptability in the industry. Processability depends on many factors including the nature of the component polymers, their ratios, and blending conditions. As mentioned above, SNR is a different rubber material containing a significant percentage of non-rubbers including proteins, lipids, fatty acids, metal ions, etc., consequently possessing a low percentage of rubber. The lower content of dirt in the SNR than SLR is another difference between these two rubbers.^5,36 However, no single study was found in the literature related to SNR/PP blends covering the possible effects of these compositional differences on the processing and performance of SNR/PP blends or comparing these blends with their conventional NR counterparts. Therefore, this study was carried out to examine the melt processability of blends of SNR and PP at different compositions in comparison to that of the blends of SLR and PP. Processability, morphology, and thermal properties of SNR/PP blends were focused in this study.

Materials and methods

Materials

Polypropylene resin copolymer (grade P640 J) with a melt flow rate at 230 °C/2.16 kg value of 10 g/10 min (ASTM D 1238), melting point of 163°C, tensile strength at yield of 27.8 MPa (ASTM D 638 at crosshead speed 50 mm/min) and a specific gravity of 0.91, was supplied by Elastomeric Engineering Company Ltd, Sri Lanka. Standard Lanka Rubber 20 (SLR20) and Skim Natural Rubber (SNR) were supplied by C. W. Mackie PLC., and Glenross Rubber Factory (Pvt.) Ltd. Sri Lanka, respectively. All the other compounding ingredients such as antioxidant (IPPD) and curing ingredients (ZnO, stearic acid, TMTD, TBBS, and sulfur) were of industrial grade and obtained from Glorchem Enterprises, Sri Lanka, and used as received.

Preparation of SLR/PP and SNR/PP blends

Preparation of NR (SLR/SNR) and PP blends was carried out using melt mixing in an internal mixer (MX300- TQ; CAM type rotor) with a fill factor of 0.7. The melt mixing was carried out at the melt temperature of 180°C achieved by adjusting the set temperature of the internal mixer at 200°C and rotor speed of 70 rpm. Both UV and their DV counterparts were prepared. The formulation used for the preparation of DV blends is shown in Table 1. For the preparation of UV blends, the same formulation was used, however, without curing ingredients. The total weight of rubber and PP in a batch was kept at 200 g.

Table 1.

Formulation used in preparation of DV blends.

Ingredient	Quantity/parts per hundred rubber (phr)
N-Isopropyl-N’-phenyl-1,4-phenylenediamine (IPPD)^a	0.75
Zinc oxide (ZnO)^b	2.5
Stearic acid^b	1
Tetramethylthiuram disulphide (TMTD)^b	1.25
N-Tert-butyl-2-benzothiazole sulfenamide (TBBS)^b	1
Sulfur^b	0.5

antioxidant;

curing ingredients.

SLR and SNR were first masticated for a period of 2 min on a two-roll mill with a nip clearance of 3 mm and a friction ratio of 1.25:1. This was carried out to improve the mixing efficiency by lowering the viscosity of high viscous NR to reach approximately similar viscosity of PP at the mixing temperature. PP was first charged and preheated for 5 min in the internal mixer while running the rotor at a speed of 10 rpm. Thereafter, rotor speed was increased to 70 rpm and PP was allowed to melt for 3 min. At the 8^th minute, SLR20 or SNR and antioxidants were added and the mixing was continued. In preparation of UV blends, mixing was continued without the addition of curing ingredients for 20 min in order to maintain the same mixing time used in the preparation of corresponding DV blends. For the preparation of DV blends, accelerators, activators, and vulcanizing agents (sulfur) were added according to the mixing schedule shown in Table 2. For these blends, mixing was carried out for 2 min after the stabilization torque is achieved to assure a homogenized blend. The total mixing time was 20 min. After the mixing, the compound was removed and immediately passed through a two-roll mill once before the sample got hard. The sheeted materials obtained were finally cut into small pieces for the preparation of test specimens.

Table 2.

Mixing schedule.

Activity	Time of the action/min
PP was added and rotor was started to run at 10 rpm	0
Rotor speed was adjusted to intended rpm for mixing	5
NR was added with IPPD	8
Stearic acid and ZnO were added	11
TBBS and TMTD were added	12
Sulfur was added	14
Mixing was stopped and dumped (2 min after the stabilization torque was achieved)	20

Compression molding

The sheeted samples prepared through the internal mixer and two-roll mill were then compressed at a pressure of 13.5 bar and molded into sheets with 2 mm thickness using a hydraulic hot press at 180°C for 15 min including 5 min pre-heating time, 1.5 min for compression and 8.5 min for cooling under pressure.

Processability studies

Mixing torque development:

Mixing torque and melt temperature development during the melt mixing and compounding were recorded to study the material flow, thermal degradation behavior and the viscosity of the material generated in the internal mixer of which technical details were given earlier.

Long-term internal mixer torque development:

In order to study the long-term processability and thermal stability of the TPNRs generated, the blending was continued for additional 10 min mixing under the same blending conditions. Internal mixer torque development with time was recorded. For this study, 30/70 (plastic dominant), 50/50 (intermediate) and 70/30 (rubber dominant) of NR/PP blends were selected.

MDR torque development of DV blends:

This study was carried out using a Moving Die Rheometer (MDR 3000) at 180°C for 30 min. Both types of DV NR-rich NR/PP (70/30) blends (SNR/PP and SLR/PP) were selected. MDR torque developments were recorded.

Thermogravimetric analysis of the blends:

Thermogravimetric analysis was carried out using a Perkin-Elmer Thermo Gravimetric Analyzer (model TGA4000). The samples (about 10 mg) were heated at a rate of 4 °C/min to 72°C from room temperature (28°C) at ambient atmosphere. Heating was carried out from 72–600°C at the same rate under oxygen atmosphere. TGA curves for selected samples of UV and DV blend systems (SLR/PP and SNR/PP) were obtained to study the thermal stability.

Melt flow index:

Melt flow rate analysis was performed using a CEAST melt flow meter according to ASTM D 1238 at 230°C temperature and using a load of 7.575 kg with pre-heating time of 20 min and testing time of 10 s.

Morphological studies:

Blends were characterized using a ZEISS EVO LS 15 Scanning Electron Microscope (SEM). Selected UV blends were solvent-extracted by hot n-hexane for 48 h using soxhlet apparatus whereby the rubbery phase was removed from the blend. DV blends were etched by treating with conc. nitric acid for 48 h at room temperature.

All the samples for SEM studies were sputter-coated with gold and examined within 24 h of testing.

Results and discussion

Mixing torque development

Mixing torque curves of the blends are presented in two different stages; before (0–12 min) and after (12–20 min) addition of curing ingredients. Figure 1 shows the typical mixing torque development curves until the 12^th minute of melt mixing at which curing ingredients are introduced for dynamic vulcanization of rubber phases of SLR/PP and SNR/PP blends containing 70% rubber. For all the blends (UV and DV), during the pre-heating period of 5 min, PP absorbs the heat and gradually softens. As the rotor speed was increased to 70 rpm, the mixing torque rises sharply due to the resistance exerted on the rotors against the rotation at set speed by the unmelted (softened) PP.^37–39 As the PP gets melted due to the mechanical shearing and heat transfer from the heated rotors, the viscosity of the material gradually decreases until the melting process is completed.³⁷ It is seen that at the 8^th minute, PP gets completely melted as evidenced by nearly a constant torque value of around 2.2 Nm achieved for pure PP.⁴⁰ At this point, NR was added along with the IPPD into the mixing chamber and the mixing torque rises suddenly again because of the nervy nature of cold NR (SLR or SNR).^37,38,41 The mixing torque of the blend gradually decreases as the heat softens the NR, and the torque starts to stabilize at around 7 Nm at the 12^th minute indicating the formation of a homogenized material. Both SNR/PP and SLR/PP blend systems show almost a similar torque development pattern until the 12^th minute irrespective of the rubber type. This indicates that no extra energy is required to prepare UV forms of SNR/PP blends compared to SLR counterparts. Therefore, it could be inferred that there is no impact from either high content of non-rubbers or low percentage of rubber present in SNR on the preparation of UV blends through melt mixing at the initial stage (8–12 min). However, it is noted that the on-set stabilization torque of NR/PP blends is approximately 3 times higher than that of pure PP.³² This is attributed to the higher viscosity of rubber (SNR and SLR) compared to PP at the same temperature as reported in the literature.^32,42,43

Figure 1.

Variations of mixing torque of NR dominant 70/30 NR/PP blends and pure PP during 0–12 min mixing period.

Mixing torque development in the second stage (12–20 min) is shown in Figure 2 for both UV and DV blend systems having the same blend ratio (NR/PP: 70/30) discussed above. In the UV blends, the torque continues to be stable until the 20^th minute at which the blend is dumped showing that no degradation has taken place while indicating a good level of mixing.^38,42 As the mixing proceeds beyond the 12^th minute, it is observed that SNR counterparts exhibit a drop of 2–4 units in torque values than their SLR counterparts. With prolonged mixing at a higher temperature, various non-rubber substances such as fatty acids and proteins available in the SNR may act as plasticizers and contribute to lowering the mixing torque than the SLR counterparts offering processing advantages.⁴ For vulcanized blends, after 2 min from the addition of the curing agent (sulfur), the mixing torque rises and maintains at a higher level than the stabilization torque of the UV blends. It confirms that vulcanization has occurred in the rubber phase.³⁰ The lower torque values of DV SNR counterparts may be due to the lower cross-linked density in the rubber phase of the blend as SNR contains a lower percentage of cross-linkable NR molecules.

Figure 2.

Variation of mixing torque of NR dominant 70/30 NR/PP blends during 12–20 min mixing period.

However, both DV blends (SLR and SNR-based blends) have shown a gradual decrement in the torque with prolonged mixing time. As the vulcanization takes place, there is a heat development in the rubber phase due to the exothermic nature of the vulcanization reaction.⁴⁴ This may have resulted in degradation of the material as evidenced by the declining trends observed in torque values as well as the crumb nature of the material. Once the rubber phase is vulcanized, it transforms into the hard vulcanized rubber particles and exists as the dispersed phase in PP continuous phase.²¹ If the amount of the continuous phase is not adequate to hold the vulcanized rubber phase, the material shows characteristics of crumb demonstrating poor processability. In addition, this torque development suggests that SNR blend systems degrade almost at the same rate as SLR blend systems. However, as seen in Figure 3, other blends with higher content of PP yield melt processable materials.

Figure 3.

Variation of mixing torque of (a) 50/50 and (b) 30/70 NR/PP blends during 12–20 min mixing time.

Figure 3(a) and (b) depict the torque development for the second stage (12–20 min) mixing of both UV and DV 50/50 NR/PP and 30/70 NR/PP blends, respectively. As the PP percentage of the blend increases, early stabilization of the mixing torque is observed. This may be due to the decrease of rubber amount and consequent facilitation of melt mixing. As the PP phase becomes dominant, the blend system does not experience a higher heat generation and prevents degradation of the rubber phase due to the reduction of high viscous rubber content in the blend.⁴⁵ It is also seen that the difference in the processability of SNR/PP and SLR/PP blends diminishes as the PP phase increases.

Mixing torque value at the completion of melt mixing of NR/PP blends against the PP percentage is shown in Figure 4. It is seen that for all the blends, stabilization torque decreases with the PP percentage. The stabilization torque of UV and DV blends decreases from 7.7 Nm to 3.5 Nm and 10 Nm to 4.6 Nm, respectively for SLR/PP blends. For SNR/PP blends, the stabilization torque decreases from 5.8 Nm to 3.2 Nm and 8.6 Nm to 5 Nm for UV and DV blends, respectively. However, the significance of these decrements diminishes with the increase of the PP phase as mentioned above. Higher the volume of the continuous PP phase of the blends, the effect of the dispersed phase is lower. Therefore, from the above observations, it could be inferred that rubber percentage and rubber type have no significant impact on the processability of both UV and DV NR/PP blends when the PP percentage exceeds 50%.

Figure 4.

Final mixing torque values of NR/PP blends against blend ratio.

However, it is noted that PP-rich DV SNR/PP blends exhibit slightly higher torque value than their SLR counterparts which is a different observation from other blends. With the decrease of the SNR content, the amount of non-rubber materials responsible for the plasticizer effect may be negligible and interactions form between the interface of the rubber and plastics would be stronger due to the smaller size (higher surface area) SNR agglomerates.

Long-term mixing torque development

As it was seen that rubber dominant vulcanized systems (NR/PP: 70/30) commence degradation even during their melt mixing, long-term processability studies concentrate only on the other blends. In order to examine the long-term processability, mixing was continued for an additional 10 min after the preparation of the blends. The torque registered during the last 10 min is presented in Figure 5. The mixing torque values of the UV blend series except SLR dominant systems reach almost constant but different stabilization torque values during the last 4–5 min indicating their long-term processability (Figure 5(a)).

Figure 5.

Variation of mixing torque of NR/PP (a) UV and (b) DV blends with prolong mixing during 20–30 min.

For both SNR/PP and SLR/PP blends, vulcanized counterparts containing 50% or more plastic phase (Figure 5(b)) register a stable mixing torque during the prolonged mixing inferring that both SNR/PP and SLR/PP blends exhibit similar processability when the PP percentage is more than 50%. Therefore, it can be inferred that stable melt-processable DV blends are possible only for blends having a PP percentage of 50% or higher irrespective of the rubber type.

MDR torque development of DV blends

The long-term processability of DV blends was investigated using a Moving Die Rheometer (MDR) in order to examine the processability at a low shear rate to assess the suitability of material to manufacture molded products. Both 70/30 SNR/PP and SLR/PP DV blends were used for the study. The blends were subjected to a low shear rate in the MDR at 180°C temperature for an extended period and the torque was measured. At the commencement, both blends record a very high torque as they were in a cold state (Figure 6). Once the PP phase gets melted, this viscosity of the blend becomes very low. It is seen that both blends of SLR and SNR show no reduction in MDR torque at the extended time and maintains approximately 1 dNm above zero without a sudden drop. It ensures the long-term processability of these blends as the PP phase is adequate to hold the cross-linked rubber phase⁴⁶ when the blends were processed under a lower shear rate than the shear rates applied in the internal mixer. Therefore, these rubber dominant blends could be processed into molded products via compression molding technique despite the poor processability shown during melt mixing in the internal mixer.

Figure 6.

MDR rheographs of 70/30 NR/PP DV blends.

Thermogravimetric analysis of the samples

TGA is a useful technique to determine the thermal stability of polymers and polymer blends. They slowly lose their useful properties because of polymer chain degradation under various environmental conditions such as heat and light. The thermal stability of the blends depends on the thermal behavior of the component polymers and the interactions between them.²⁶ Thermal stability of a blend can be discussed using the parameters derived from thermograms such as onset temperature of the degradation, the mid-point of the degradation, and the fraction which is not volatile at 600°C, denoted as char residue.⁴⁷ TGA thermograms for raw materials and blends are shown in Figure 7 and their onset degradation temperatures are summarized in Table 3.

Figure 7.

TGA thermograms of (a) pure SNR, SLR and PP, (b) DV SNR/PP and SLR/PP blends and (c) UV and DV 50/50 SNR/PP and SLR/PP blends.

Table 3.

Onset degradation temperatures of NR/PP blends derived from TGA thermograms.

NR/PP blend	Onset degradation temperatures/°C
NR/PP blend	UV	DV
SNR/PP; 70/30	346.20	341.85
SNR/PP; 50/50	337.22	336.23
SNR/PP; 30/70	329.13	319.46
SLR/PP; 70/30	340.23	347.65
SLR/PP; 50/50	332.37	345.29
SLR/PP; 30/70	317.94	317.88

Decomposition of PP starts at the temperature of 300°C and ends around 360°C with a steep decline in the TGA curve, which is lower than that of SNR and SLR for which onset degradation starts at 332°C and 342°C, respectively (Figure 7(a)). Slight weight loss before onset temperature (100–300°C), is observed in SNR due to its higher volatile matter content than SLR.⁴ A lower decomposition temperature of the main polymer is shown by SNR than SLR. SNR contains higher content of metal ions which could catalyze the thermal oxidation of rubber hydrocarbon chains.^5,6 However, no degradation of both rubbers occurs below the processing temperature (180°C) which is above the melting temperature of the PP phase. The decomposition temperature of the blends has increased with the addition of both SNR and SLR compared with the PP as shown in Figure 7(b) and (c) respectively. Similar behavior has been reported in the literature for NR/PP blends.^48,49 Further, it is noticeable from Table 3 that the onset decomposition temperature of both the blends increases with the increase of the NR (SNR and SLR) percentage. This may be elucidated by the decrease in the percentage of thermally unstable PP.

It is shown in the above thermograms that onset degradation of PP commences at a lower temperature than both rubbers due to the presence of a tertiary carbon atom which favors the thermal degradation.⁵⁰ As evident from Figure 7(b), the thermal stability of the blends has improved with the incorporation of either SNR or SLR. Further, with the increase of rubber percentage in the blends, the thermal stability of the blends also increases. This behavior of conventional NR (SLR/RSS)/PP blends has been reported by Varghese,⁴⁹ Azlina,⁵¹ and their co-workers. From this study, it is shown that SNR also offers a similar trend in the variation of thermal stability despite their high content of non-rubbers.

When the onset degradation temperatures of corresponding SLR/PP and SNR/PP blends are considered, SNR has offered higher thermal stability than the SLR for UV blends (Table 3). SNR contains more non-rubbers including chemical substituents having anti-oxidant characteristics and these can absorb the thermal energy providing protection to the PP phase. However, this trend is vice versa for the DV blends. In the vulcanized blends, the degree of cross-linking of SLR/PP blends is higher than that of the SNR/PP counterpart due to the higher content of rubber available in SLR. As the degree of cross-linking increases, it could improve the thermal stability due to the presence of more mono and disulphide cross-links as semi efficient vulcanized system was used in the vulcanization process.⁵²

Dynamic vulcanization of NR/PP blends yields thermally more stable blends than its UV counterparts which is a widely reported observation for rubber-plastic blends.^32,52 Contrary to the SLR/PP blends, dynamic vulcanization of SNR/PP blends results in generating thermally less stable blends due to the metal ions (present in SNR) acting as catalysts for oxidation of rubber phase supported by the extra-heat generated during vulcanization (Figure 7(c)).

Melt flow index

Melt Flow Index (MFI) at low strain rates indicates the processing behavior of polymers even though the actual processing conditions (such as temperature and shear rate) employed in product development differ from those of the conditions used in MFI tests.⁴⁴ However, MFI is widely calculated as an empirical parameter influenced by the physical properties and structure of the materials. In all cases, MFI increases with the increase of PP content showing better flow properties of the blends due to low viscosity of PP (Figure 8(a) and (b)). MFI values of UV blends are very high compared to those of the DV blends. This is because of the less hindrance to the flow of the PP phase in the absence of rigid cross-linked rubber particles as reported in the literature.^49,53,54 From Figure 8(a), it is seen that the flowability of UV blends shows no significant difference with respect to the rubber type used. However, DV SNR/PP blends show a significant increment in MFI compared to SLR/PP counterparts (Figure 8(b)) probably due to the low density of rigid vulcanized rubber particles and the presence of non-rubbers in the plastic phase of the SNR/PP blends.

Figure 8.

MFI results of NR/PP (a) UV blends and (b) DV blends.

Morphological studies

Figure 9(a)–(f) represent the SEM micrographs of the cut surfaces of the UV blends. The rubber phase of the blends is extracted using n-hexane prior to obtaining the SEM micrographs and the extracted rubber phase in the blend is seen as dark voids. Figure 9(a)–(d) clearly show almost similar oriented forms of PP molecules along with uniformly distributed dark voids for both SNR/PP and SLR/PP blends having a PP percentage of 50% or above. Therefore, it can be inferred that these blends possess co-continuous phase morphology and almost similar processability characteristics supporting the observations made in the above-mentioned studies. At high PP percentages, PP being a thermoplastic material, the molecules are more oriented during melt mixing owing to lower viscosity compared to the rubber. Consequently, PP becomes the continuous phase.^13,28 Different morphologies can be clearly seen for NR-rich blends (Figure 9(e)–(f)). In these blends, a lesser number of oriented PP phases and a higher number of dark voids of the rubber phase are seen. This suggests a possible conversion of the morphology of the blends from continuous PP phase to co-continuous or continuous rubber phase as the SNR percentage in the blend exceeds 50%.⁴⁶

Figure 9.

Scanning electron micrographs of both UV NR/PP blends (5K magnification). (a) -30/70 SLR/PP UV (b) -30/70 SNR/PP UV (c) -50/50 SLR/PP UV. (d) -50/50 SNR/PP UV (e) -70/30 SLR/PP UV (f) -70/30 SNR/PP UV.

Figure 10(a)–(f) show the SEM micrographs of DV blends. In DV blends, the vulcanized rubber phase is preferentially extracted by treating with nitric acid while the PP phase remained unaffected. In the micrographs, light portions represent the PP phase and the dark voids represent the vulcanized rubber phase. Irrespective of the rubber/plastic ratio, all the DV blends do not show the elongated nature of the morphology, which is shown in corresponding plastic-rich UV blends. This observation clearly shows the restriction of the orientation of the PP molecules due to the presence of rigid vulcanized rubber particles. Comparison of the corresponding SLR/PP and SNR/PP blends shows comparatively smaller vulcanized rubber domains in the latter. SNR has a higher content of non-rubbers (more than 15%) and hence possesses a high viscosity. Stresses applied during mixing may be transferred to the rubber phase more effectively forming smaller vulcanized rubber domains due to the high viscosity caused by the presence of these non-rubbers and the formation of cross-links in the rubber phase. The high viscosity of SNR may also result in the uneven distribution of these vulcanized rubber domains in the continuous PP phase. In rubber dominant DV SLR/PP blends, domains of similar shape with a wide distribution in size dispersed in a continuous thermoplastic phase are evident⁵⁴ However, the SNR counterparts show more irregular-shaped, smaller, and unevenly distributed rubber particles dispersed in the continuous PP phase.⁵⁵ As there is no significant difference in the morphologies of DV blend systems, it can be inferred that all the blends are processable as thermoplastics irrespective of the rubber type.

Figure 10.

Scanning electron micrographs of both DV NR/PP blends (5K magnification). (a) -30/70 SLR/PP DV (b) -30/70 SNR/PP DV (c) -50/50 SLR/PP DV. (d) -50/50 SNR/PP DV (e)-70/30 SLR/PP DV (f) -70/30 SNR/PP DV.

Conclusions

The processability of SNR/PP and SLR/PP was compared. It is found that the high percentage of non-rubbers present in the SNR has a positive influence on the processability of both UV and DV blends compared to that of SLR/PP blends. However, the processability of the blends becomes inferior to rubber dominant NR/PP blends (70:30) irrespective of rubber type. It is further shown that lesser energy is required for processing SNR/PP blends compared to the SLR counterparts as assessed by the mixing torque development studies. Further studies carried on long-term processability exhibit the possibility of utilization of SNR/PP blends similar to the SLR/PP blends to prepare thermoplastic elastomers without inferior processing characteristics. It is revealed from the TGA studies that a similar degree of protection has been offered to NR/PP blends against the thermal degradation by SNR compared to SLR. According to the MFI studies, dynamic vulcanization results in a reduction of flowability under a low strain rate while the increase of the PP percentage shows an improvement in the flowability of the blend. Also, it is found that the flowability of both blends shows no significant difference at the UV stage. However, DV SNR/PP blends show a considerable increase in flowability compared to SLR/PP counterparts. Further, it is noted that these materials can be processed into molded products via compression molding technique and processability will be easier with increasing PP percentage. Morphological studies suggest continuous or co-continuous morphologies irrespective of either composition or rubber type in the UV blends. For DV blends, discrete vulcanized phases are seen in the continuous PP phase confirming their processability as thermoplastics irrespective of the rubber type.

Footnotes

Acknowledgements

The authors gratefully acknowledge the Faculty of Applied Sciences, Uva Wellassa University, Sri Lanka for the M.Phil. Studentship (UWU/GSMP/2019/003). For preparation and characterization of samples, the authors wish to thank the Rubber Research Institute of Sri Lanka and University of Moratuwa, Sri Lanka.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research: This work was supported by National Research Council of Sri Lanka (Grant no: 17-072).

ORCID iD

Susantha Siriwardena

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