Thermoplastic Natural Rubber Based on Polyester Blends: Influence of Different Types of Natural Rubber and Polyesters on Cure, Mechanical, Rheological, Morphological, Thermal, Relaxation Properties and Biodegradation

doi:10.21203/rs.3.rs-1613369/v1

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Thermoplastic Natural Rubber Based on Polyester Blends: Influence of Different Types of Natural Rubber and Polyesters on Cure, Mechanical, Rheological, Morphological, Thermal, Relaxation Properties and Biodegradation

https://doi.org/10.21203/rs.3.rs-1613369/v1

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Novel green biodegradable thermoplastic natural rubber (GB-TPNR) based on natural rubber and polyester blends was prepared by dynamic vulcanization. The main objective was to improve mechanical and rheological properties, thermal stability and biodegradability of the natural rubber/polyester thermoplastic vulcanizates (TPVs). Three types of natural rubber: unmodified NR, and epoxidized natural rubbers with 25 and 50 mole % epoxide (i.e., ENR-25 and ENR-50); and two polyester types: polybutylene succinate (PBS) and poly(butylene succinate-co-butylene adipate) (PBSA) were used as blend components. It was found that the dynamically cured ENR-50/PBS blends exhibited superior mechanical, rheological, thermal and stress relaxation properties relative to ENR-25/PBS and unmodified NR/PBS TPVs. This is attributed to stronger chemical interactions between the polar functional groups of ENR-50 and PBS, which caused strong interfacial adhesion and hence the smallest vulcanized rubber domains dispersed in the PBS matrix. Furthermore, stress relaxation, thermal resistance, mechanical strength and degree of crystallinity of PBS phase increased with epoxide content in the modified rubber. Moreover, the dynamically blended ENR-50/PBSA showed larger vulcanized rubber domains along with poorer mechanical, relaxation and thermal properties but easier biodegradation than the ENR-50/PBS blend.

Thermoplastic vulcanizate (TPV)

Epoxidized natural rubber (ENR)

Poly(butylene succinate)

Poly(butylene succinate-co-adipate)

Biodegradability

Thermoplastic elastomers (TPEs) are a family of rubber-like materials that combine the thermoset characteristics of cross-linked rubber together with the melt processability, recyclability, and processing advantages of thermoplastics [1].¹ Thus, the TPEs can be processed by conventional thermoplastic processing techniques such as extrusion, blow molding, injection molding, vacuum forming, and calendaring [2]. Typically, TPEs can be divided into six basic types: styrenic thermoplastic elastomers, multiblock copolymers, hard polymer-elastomer combinations, graft copolymers, ionomers, and core–shell morphologies [3]. The hard polymer-elastomer combinations can be subcategorized to two types: simple blend (SB) and thermoplastic vulcanzate (TPV) or dynamic vulcanizate (DV). The simple blend is prepared by blending thermoplastic and elastomer components without added curatives. On the other hand, the TPV is produced via dynamic vulcanization of an immiscible blend of a thermoplastic and an elastomer component. Therefore, the final morphology of TPV is a two-phase system that consists of the cross-linked elastomer domains finely dispersed throughout the thermoplastic matrix phase during melt mixing, and during the use of the material [4].

Natural rubber (NR) has been known as a green and renewable material sourced from rubber trees. It is a suitable candidate elastomer component for bio-based thermoplastic elastomer materials. The thermoplastic elastomers based on natural rubber and a thermoplastic blend have been known as thermoplastic natural rubbers (TPNR). Due to the poor level of polarity in NR, chemical modifications of NR that incorporate polar functional groups have been devised to gain NR derivatives such as epoxidized natural rubber (ENR) and maleated natural rubber (MNR). The epoxidized natural rubber (ENR) is one of the most frequently prepared and used modified NR. That is, ENR has been used in TPNR by blending it with various types of thermoplastics, including polypropylene (PP) [5, 6], high-density polyethylene (HDPE) [7, 8], low-density polyethylene (LDPE) [9], and polystyrene (PS) [10].

Poly(butylene succinate) (PBS) and it copolymers, including poly(butylene succinate-co-butylene adipate) (PBSA), have been recognized as thermoplastic materials with excellent biodegradability due to their stereochemistry, the flexibility of molecular chains, the crystallinity of the polymers, and the surface wettability [11]. In addition, PBS has desirable melt processability, thermal stability, and mechanical properties, closely comparable to those of the widely-used polyolefins (i.e., PE and PP) [12]. Among the commercially available biodegradable plastics, poly(butylene succinate-co-adipate) (PBSA) is a promising thermoplastic aliphatic polyester with many interesting properties, including excellent processability in extrusion, thermoforming, film blowing, and injection molding; biodegradability in different environments; and good mechanical performance similar to that of polyolefins [13]. Blending of PBS and PBSA with other biopolymers can promote some unique characteristics that cannot be obtained from a single polymer. PBS has been used as a blend component in various polymer blends, such as PBS/polylactic acid (PLA) [14, 15], PBS/polybutylene terephthalate (PBT) [16], PBS/polypropylene carbonate (PPC) [17], and PBS/polyhydroxy butyrate (PHB) blends [18]. On the other hand, the PBSA has been used as a blend component with various biopolymers including PBSA/poly(lactic acid) (PLA) [19], PBSA/PBS/PLA [20], and PLA/PBSA/PBAT (poly(butylene adipate-co-terephthalate)) blends [21].

In this work, novel green biodegradable thermoplastic natural rubbers (GB-TPNR) based on natural rubber and polyester blends were prepared by dynamic vulcanization. The main aim was to improve mechanical, morphological and rheological properties, thermal stability, and biodegradability of the dynamically cured natural rubber/polyester blends. Three types of NR (i.e., unmodified NR, ENR-25, and ENR-50)) together with two types of polyester (i.e., PBS and PBSA) were exploited to prepare the novel green biodegradable thermoplastic natural rubbers (GB-TPNR) or green TPVs in a systematic exploration.

2.1 Materials

Polybutylene succinate (PBS), Bionolle™ grade 1001 MD and poly(butylene succinate-co-butylene adipate) (PBSA), Bionolle™ grade 3001 MD used as thermoplastic blend components were manufactured by Showa Denko Co. Ltd. (Tokyo, Japan). Block natural rubber, grade STR 5L (ρ = 0.92 g/cm³) used as unmodified NR, was manufactured by Rubber Authority of Thailand (RAOT) (Nakhon Si Thammarat, Thailand). Epoxidized natural rubber with epoxide contents of 25 and 50 mol % (i.e., ENR-25 (ρ = 0.97 g/cm³) and ENR-50 (ρ = 1.03 g/cm³)), were manufactured by Muang Mai Guthrie Co. Ltd. (Surat Thani, Thailand). Stearic acid (activator) was manufactured by Unichema International B.V., (Gouda, the Netherlands). Zinc oxide (activator) was manufactured by Lanxess, (Leverkusen, Germany). Wingstay®L antioxidant was manufactured by Eliokem lnc. (Ohio, USA). The N-tert-butyl-2 benzothiazolesulfenamide (TBBS) cure accelerator was manufactured by Rhein Chemie Rheinau GmbH, (Mannheim, Germany). The sulfur curing agent was manufactured by Pergan GmbH, (Bocholt, Germany).

2.2 Preparation of dynamically cured natural rubber/polyester blends

Three alternative types of natural rubber (i.e., unmodified NR (STR 5L), ENR-25, and ENR-50) and two types of polyester (i.e., PBS and PBSA) were used to prepare dynamically cured blends or thermoplastic vulcanizates (TPVs) with a fixed blend ratio of rubber/polyester = 40/60 wt%. All materials were first dried in a hot air oven at 60°C for about 24 h. The natural rubber compound was then prepared by using the formulation and mixing steps shown in Table 1 (Step I). After conditioning the prepared rubber compound for at least 24 h, the dynamically cured NR/polyester blends or TPVs were then prepared via dynamic vulcanization, using the mixing schedule shown in Table 1 (Step II). In Step I, the rubber compounding used the conventional sulfur vulcanization (CV) system. This was done by mixing rubber and chemical ingredients in a Haake Rheocord 600 laboratory internal mixer, Thermo Electron Corporation, (Karlsruhe, Germany) with the mixing capability of 78 cm³ and a fill factor of 0.85. The mixing was performed at 60ºC, at a rotor speed 60 rpm and for a total mixing time of 10 min. Natural rubber was first masticated for about 1 min before subsequently adding the other chemical ingredients: ZnO, steric acid, wingstay®L, TBBS, and sulfur. The rubber compound was eventually dumped and conditioned at room temperature at least 24 h. In Step II, the dynamic vulcanization of rubber/polyester blends was performed in the same machine, a Haake Rheocord 600 laboratory internal mixer, at 140ºC and rotor speed of 100 rpm for a total mixing time of 7 min. The dried polyester (PBS or PBSA) was first incorporated into the mixing chamber and mixed for about 1 min. The rubber compound was then added into the mixing chamber and the mixing was continued for another 6 min. Then, the mix was dumped from the mixing chamber. After conditioning for about 24 h, the blend materials were ground to small granules and then pelletized. The dumbbell-shaped specimens were then fabricated by injection molding using a Babyplast 6/10P-T, Cronoplast, S.L., (Barcelona, Spain) at 170ºC with a fast mold cooling to 40ºC. The TPV materials were eventually conditioned for about 24 h before testing and characterization.

Table 1

Compounding formulation and mixing schedule during preparation of rubber compound (Step I) and during dynamic vulcanization of rubber/polyester blends (Step II).
Step	Ingredient	Quantity (phr)	Time (min)
I	Rubber (NR, ENR-25 and ENR-50)	100	0.00
	ZnO	5	1.00
	Stearic acid	1	1.00
	Wingstay®L	1	1.00
	TBBS	0.9	5.00
	Sulfur	3	7.00
	Dump		10:00
II	Polyester (PBS and PBSA)	60	0:00
	Rubber compound (NR, ENR-25 and ENR-50)	40	1:00
	Dump		7:00

2.3 Cure characteristics

Cure characteristics of NR, ENR-25 and ENR-50 compounds were determined according to ISO 6502 using a moving die rheometer, D-MDR 3000 from MonTech Werkstoffprüfmaschinen GmbH (Buchen, Germany). The rheometer test was performed at 1.67 Hz, angular amplitude of 0.5°, and at 160°C for 20 min. Then, the minimum torque (M_L), maximum torque (M_H), torque difference or delta torque (M_H -M_L), scorch time (T_s2), and optimum cure time (T_c90) were determined from the cure curves.

2.4 Rheological properties

Rheological or flow properties of dynamically cured rubber/polyester blends were determined by using a high pressure capillary rheometer, GöttfertRheo-Tester 2000 from Göttfert Werkstoff-Prüfmaschinen GmbH, (Buchen, Germany). The test was carried out in a wide shear rate range from 10 to 2000 s^− 1 at 150°C. The capillary die with 1.5 mm diameter, 30 mm length, 180º entry angle, and an aspect ratio (L/D) of 20/1 was used by fitting it to the rheometer barrel. Then, the tested sample (about 25 g) was loaded into the rheometer barrel and preheated for about 5 min under an automatically pressurized condition to get a compact mass. The piston was programmed to automatically travel downward to purge the excess molten material and eliminate bubbles present in the melt. The apparent shear viscosity (η_s) and apparent shear stress (${\dot{\gamma }}_{app}$) were determined using a derivative of the Poiseuille law, as described elsewhere [22].

2.5 Morphological properties

Morphological properties of the dynamically cured rubber/polyester blends were investigated by two different techniques: scanning electron microscopy (SEM), and atomic force microscopy (AFM). For SEM imaging, cryogenically fractured surfaces were firstly prepared by cracking the samples in liquid nitrogen. Then, then the polyester continuous phase (PBS or PBSA) was selectively removed by extracting with two types of solvents: dimethyl sulfoxide (DMSO) at 190°C for 15 min, and chloroform at room temperature for 30 min. The sample was then dried at room temperature for 24 h before transferring to dry in a vacuum oven for about 24 h. The sample was then imaged using a scanning electron microscope, ZeissSupra-40 VP, Carl Zeiss Microscopy GmbH, (Oberkochen, Germany). From the SEM micrographs, the size of rubber domains dispersed in polyester matrix was determined in terms of the number-average (D_n), weight-average (D_w), and volume-average (D_v) domain diameters using the following equations [23]:

$${D}_{n}= \frac{\sum {N}_{i}{D}_{i}}{\sum {N}_{i}}$$

$${D}_{w}= \frac{\sum {N}_{i}{D}_{i}^{2}}{\sum {N}_{i}{D}_{i}}$$

$${D}_{v}= \frac{\sum {N}_{i}{D}_{i}^{4}}{\sum {N}_{i}{D}_{i}^{3}}$$

where N_i is the number of particles with the diameter D_i

For AFM imaging, the sample was first cut with a glass knife at -70°C to obtain an ultrasmooth surface before characterizing by a Nanosurf easyScan 2 Atomic force microscope (AFM), (Liestal, Switzerland). AFM images were captured at room temperature in a tapping mode using a Nano-sensor cantilever (i.e., the NCL-R type) with 250 scans per sample.

2.6 Mechanical properties

Tensile properties were determined using dumbbell shaped specimens (type 5 A) in a Zwick Z 1545 tensile testing machine, Zwick GmbH & Co., Ltd., (Ulm, Germany), which was operated at a crosshead speed of 200 mm/min and at 23 ± 2°C according to ISO 527. The Young’s modulus, tensile strength and elongation at break were determined from the stress-strain curves. Furthermore, hardness (Shore A) of the samples was also determined using a durometer (Shore A) according to ISO 868.

2.7 Thermal properties

Differential scanning calorimetry (DSC) of dynamically cured natural rubber/polyester blends was characterized by using a Mettler Toledo DSC823e from Schwerzenbach, (Zurich, Switzerland). The samples (∼15 mg) ware placed into alumina crucibles and the first heating scan was performed in the temperature range from − 80°C to 200°C with a heating rate of 10°C/min. This was to remove the thermal history of the material. The second scan was then carried out by cooling the sample from 200°C to -80°C with the cooling rate of 10°C/min. Finally, the third scan was performed by heating the sample in the same temperature range from − 80°C to 200°C with the same heating rate of 10°C/min. From the DSC thermograms, glass transition temperatures (T_g), crystallization temperature (T_c), melting temperature (T_m), and degree of crystallinity (X_c) for the blend components were determined. It is noted that the degree of crystallinity of PBS and PBSA were determined by the following relation:

$$\% \text{D}\text{e}\text{g}\text{r}\text{e}\text{e} \text{o}\text{f} \text{c}\text{r}\text{y}\text{s}\text{t}\text{a}\text{l}\text{l}\text{i}\text{n}\text{e} \left({X}_{C}\right)= \frac{\varDelta {H}_{m}}{\varDelta {H}_{m}^{0}} \times \frac{100}{W}$$

where $\varDelta {H}_{m}$, $\varDelta {H}_{m}^{o}$ and W are the melting enthalpy, melting enthalpy for 100% crystalline PBS or PBSA (110.3 [24] or 142 J/g [25]), and the weight fraction (wt%) of either PBS or PBSA in the rubber/polyester blends, respectively.

Furthermore, thermogravimetric analysis (TGA) of the natural rubber/polyester TPVs was also performed in nitrogen and oxygen atmospheres. A sample (∼15 mg) was placed in a ceramic pan, and then heated from 25°C to 650°C in nitrogen atmosphere and then the oxygen atmosphere was switched on and heating continued from 650°C to 900°C with the same heating rate of 10°C/min.

2.8 Temperature scanning stress relaxation (TSSR)

The stress relaxation behaviors of dynamically cured natural rubber/polyester blends were characterized by using a TSSR meter from Brabender GmbH, (Duisburg, Germany). The dumbbell-shaped samples (type 5A, ISO 527) were firstly placed in the TSSR chamber at an initial temperature of 23°C, and a tensile strain of 50% was applied. The sample was conditioned at this isothermal state for about 2 h to eliminate the thermal history and also storage hardening of natural rubber. After that, the non-isothermal conditions were applied to the sample by gradually raising temperature from 23°C to 120°C at a slow heating rate of 2 K/min. After the thermal relaxation (or rupture) of the sample, the system was automatically cooled down to room temperature. The force-temperature curve and relaxation spectrum for each sample were recorded and displayed. Various thermal characteristics namely T₁₀, T₅₀, and T₉₀ were determined from the force-temperature relationships. It is noted that T_x stands for the temperature at which the stress has decreased by x% from the initial stress.

2.9 Biodegradability

The dumbbell-shaped specimens (type 5A, ISO 527) was firstly fabricated by injection molding, and they were then dried at 50°C for about 24 h in a hot air over before biodegradation test. The dried samples were buried in soil at a depth of about 0.07 m for various durations from 1 to 4 weeks. Some burials of samples were terminated for testing every week. The samples after removal from soil were cleaned with distillated water and dried at 50°C for 24 h, and the sample weight was recorded. A metallographic microscope, Carl Zeiss Microscopy Gmbh, (Oberkochen, Germany), was then used to visualize the surface topology of the samples. The biodegradation in terms of weight loss was then calculated by the following equation:

$$Weight loss \left(\%\right)= \frac{{W}_{0}-{W}_{t}}{{W}_{0}}\times 100$$

where W₀ and W_t are the masses of the sample before and after soil burial, respectively.

3.1 Curing characteristics

Figure 1 shows the cure curves of NR, ENR-25, and ENR-50 compounds. Also, cure properties in terms of minimum, maximum torques and torque difference, scorch time and cure time for NR, ENR-25, and ENR-50 compounds are given in Table 2. In Fig. 1, it is seen that the cure curve of unmodified NR is of a plateau type, while slight reversion is seen for ENR-25 and ENR-50 compounds. This might be due to breakdown of disulfidic and polysulfidic crosslinks and other weak linkages during thermal treatment under high shear, which would cause reversion for the ENR compounds [26]. In Table 2, it is seen that the ENR-25 and ENR-50 compounds have lower minimum torque (M_L) but higher maximum torque (M_H) and torque difference (M_H-M_L) than the NR compound. The lower M_L of ENR indicates that the ENR has lower initial plasticity, Mooney viscosity, and also lower molecular weight due to the epoxidation process [24]. Also, extensive mastication of rubber resulted in more chain scission of ENR molecules than of NR molecules [27]. The higher M_H and M_H-M_L for both types of ENRs than NR compound suggests that the epoxide groups in ENR molecules cause higher stiffness due to restricting ENR molecular movements by their polarity and also by chemical interactions between the ENR molecules. Also, ENRs are capable of self-cross-linking at epoxide groups [24] and hence a higher crosslink density is expected in ENR than in the unmodified NR. Moreover, the ENR-50 compound exhibits shorter scorch (T_s2) and cure times (T_C90) than those of the ENR-25 and NR compounds. This might be due to having more double bonds in ENR-50 replaced by epoxide groups than in ENR-25.

Table 2

Cure properties in terms of minimum and maximum torque and torque difference, scorch time, and cure time, for the NR, ENR-25, and ENR-50 compounds.
Sample	Min. torque (M_L;dNm)	Max. torque (M_H ;dNm)	Torque difference (M_H-M_L; dNm)	Scorch time (T_s2; min)	Cure time (T_c95; min)
NR	1.2	8.4	7.2	5.0	8.7
ENR-25	0.6	9.2	8.7	2.3	7.3
ENR-50	0.8	9.5	8.7	1.9	6.0

3.2 Mixing torque during preparation of TPVs

Figure 2 shows the time profiles of mixing torque and mixing temperature during preparation of dynamically cured rubber/polyester blends with different types of natural rubber and polyester. In the mixing torque-time curves, three peaks are clearly seen, with the first and second peaks related to addition of polyester (i.e., PBS or PBSA) and natural rubber compound into the mixing chamber. The mixing torque clearly decreases after the addition of polyester and rubber compound due to softening effect before and after melting of polymers at elevated temperature and mechanical shear. After the second peak (at the mixing time of about 3 min) the mixing torque starts to increase due to the dynamic vulcanization of rubber phase and this creates the third peak of the mixing torque-time relation. It is clear that the dynamically cured ENR-50/PBS shows a higher peak with larger area underneath the curve than the ENR-25/PBS and NR/PBS blends. This matches well the larger maximum torque and torque difference of ENR-50 compound (Table 2). Also, the ENR-50 compound has shorter scorch and cure times (Table 2), with earlier and faster increase in the mixing torque of ENR-50/polyester blend, compared to the ENR-25/PBS and unmodified NR/PBS blends. After the maximum torque of the third peak at about 5.5 to 6.0 min, the mixing torque decreases until the end of mixing at about 7 min. A strong reversion is seen for the dynamically cured ENR-50/PBS, ENR-25/PBS and ENR-50/PBSA blends while almost a plateau type mixing curve is seen for the dynamically cured and NR/PBS blend. This matches to the static cure characteristics of each rubber compound (Fig. 1), i.e., a strong reversion was observed for ENR-50 and ENR-25 compounds but the NR compound showed a plateau cure curve. Furthermore, it is observed that at the final mixing time of 7 min, the dynamically cured ENR-50/PBS shows a larger torque than the ENR-25/PBS and NR/PBS blends. This is attributed to higher crosslink density of the ENR-50 compound that corresponds to a higher torque difference (Table 1). In addition, the dynamically cured ENR-50/PBS blend shows higher final mixing torque than the ENR-50/PBSA blend. This can be attributed to the different chemical structures of PBSA and PBS that causes different chemical interactions between ENR and polyester phase. Also, lower melt viscosity, degree of crystallinity and melting temperature of PBSA may contribute to its lower final mixing torque than for the ENR-50/PBSA TPV.

In Fig. 2, the mixing temperature-time curves show patterns related to the mixing torque-time curves. That is, the first and second drops in mixing temperature correspond to the first and second peaks of the mixing torque-time, after the polyester and rubber compound were added. However, after the loading of rubber compounds, the mixing temperature gradually increased due to the energy input via mechanical shear and the exothermic sulfur vulcanization reactions with rubber. At the final mixing time of 7 min, it is clearly seen that the dynamically cured ENR-50/PBS shows higher temperature than the ENR-25/PBS and NR/PBS blends. Furthermore, the dynamically cured ENR-50/PBS has a higher final mixing temperature than the ENR-50/PBSA blend. This is due to the higher shear heating and the heat from exothermic crosslink reactions in dynamically cured ENR-50/PBS blend with a higher crosslink density (Table 2).

3.3 Morphological properties

Figures 3 and 4 show SEM micrographs of dynamically cured rubber/polyester blends with different types of natural rubber and polyester (i.e., NR/PBS, ENR-25/PBS, ENR-50/PBS and ENR-50/PBSA). The dual phase morphology with the vulcanized rubber domains dispersed in polyester matrix is clearly seen. Figure 3 shows the SEM results for TPVs from which the polyester phase (PBS or PBSA) was preferentially extracted by DMSO at an elevated temperature. It is clear that remaining not dissolved vulcanized rubber domains adhered to the TPV surface. Table 3 summarizes the average particle diameters of the dynamically cured rubber/polyester blends with different types of natural rubber and polyesters. Clearly that size of the vulcanized rubber domains decreased with epoxide content of the rubber. It is anticipated that the size of vulcanized rubber domains may relate to crosslink density and chemical interactions between rubber and polyester phases. That is, a higher degree of crosslinking and higher chemical interactions between the rubber with high epoxide content and polyester phases promote higher shear and elongational viscosities during dynamic vulcanization. This promotes more break-up of the vulcanizing rubber phase to smaller sized rubber particles during mixing with simultaneous dynamic vulcanization. Therefore, the dynamically cured ENR-50/PBS blend (Fig. 3(c)) with the highest maximum torque and torque difference of ENR-50 compound (Table 2) and the highest third peak and final mixing torque (Fig. 2) shows the smallest spherical vulcanized ENR-50 domains dispersed in PBS matrix.

Table 3

Average particle diameters in the dynamically cured rubber/polyester blends with different types of natural rubber and polyester.
Sample	Number average particle diameter, D_n (µm)	Weight average particle diameter, D_w (µm)	Volume average particle diameter, D_v (µm)
NR/PBS	1.3	1.4	1.6
ENR-25/PBS	1.0	1.0	1.1
ENR-50/PBS	0.9	0.9	1.0
ENR-50/PBSA	1.9	2.1	2.4

Figure 4 shows SEM micrographs of dynamically cured rubber/polyester blends with different types of natural rubber and polyester after surface extraction with chloroform at room temperature. It is seen that the crosslinked rubber phase has a honeycomb-like network structure in the dynamically vulcanized rubber/polyester blends, where the dark phase is the rubber domains residing in the lighter appearing PBS or PBSA phase. It is again clear that the average particle size of the vulcanized rubber domain decreased with epoxide content of the rubber. This can be clearly described by the proposed model of interfacial adhesion between different phases of the dynamically cured unmodified NR/polyester and modified NR (ENR)/polyester blends, as illustrated in Fig. 5. It is seen that the modified natural rubber (i.e., ENR-25 and ENR-50) has high chemical interactions and also self-crosslinks can occur, these both increase with the epoxide content in rubber. Therefore, the dynamically cured ENR-50/PBS blend (Fig. 3(c), and Fig. 4(c)) exhibited the highest interfacial interactions via the polar functional groups of PBS and the epoxide groups in ENR-50, having the lowest interfacial tension but the highest interfacial adhesion. This led to the formation of the finest dispersion of vulcanized rubber domains in PBS matrix. On the other hand, the unmodified NR has very poor interactions with PBS but higher interfacial tension, and hence larger vulcanized rubber domains are seen in the dynamically cured NR/PBS blend (Fig. 3(a), and Fig. 4(a)). In addition, the polyester type also affects the morphological properties of the ENR-50/polyester TPVs. In Figs. 3 and 4, smaller vulcanized rubber domains are seen in the dynamically cured ENR-50/PBS as compared to the ENR-50/PBSA blend. This might be due to higher polarity of PBS than PBSA, which caused higher interfacial adhesion between ENR and PBS phases. This is seen in the higher mixing torque of the dynamically cured ENR-50/PBS blend than the ENR-50/PBSA blend (Fig. 2). Hence, higher shear and elongational viscosities were encountered in the dynamically cured ENR-50/PBS blend, resulting in more break-up in the vulcanizing ENR phase during dynamic vulcanization, eventually giving smaller vulcanized ENR domains dispersed in the PBS phase.

Figure 6 shows AFM images of dynamically cured rubber/polyester blends with different rubber and polyester types. It is clear that the AFM results corroborate the SEM observations. That is, the two-phase systems are clearly seen with darker regions for the vulcanized rubber domains while the lighter ones represent the polyester phase. It is clear that the darker regions increase with increasing of epoxide content in rubber. The results confirm that the dynamically cured ENR-50/PBS blend had the finest grained vulcanized rubber domains. Also, the dynamically cured ENR-50/PBS blend showed smaller vulcanized rubber domains than the ENR-50/PBSA blend.

3.4 Rheological properties

Figure 7 shows logarithmic plots of apparent shear viscosity and apparent shear stress as functions of apparent shear rate for the dynamically cured rubber/polyester blends with different rubber and polyester types, measured at 150°C. It is seen that the dynamically cured ENR-50/PBS blend shows the highest apparent shear viscosity and apparent shear stress at a given apparent shear rate, while the dynamically cured NR/PBS blends have the lowest values. On the other hand, the dynamically cured ENR-25/PBS blend shows intermediate values. This corresponds to the sizes of vulcanized rubber domains (Table 3 and Figs. 3 and 4). That is, the ENR-50/PBS TPV with smallest vulcanized rubber domains exhibited the highest shear stress and shear viscosity due to higher interfacial adhesion increasing flow resistance. In a comparison between polyester types, it is seen that the dynamically cured ENR-50/PBS blend exhibited much higher apparent shear viscosity and shear stress than the ENR-50/PBSA blend. This again corresponds to larger vulcanized rubber domains in the dynamically cured of ENR-50/PBSA blend due to lower polarity, melting temperature and degree of crystallinity of the PBSA phase. Furthermore, it is clear that the shear viscosity and shear stress at a given shear rate have the same trend as the mixing torque (Fig. 2). That is, the dynamically cured ENR-50/PBS blend showed a higher final mixing torque than the dynamically cured ENR-25/PBS, NR-50/PBS and ENR-50/PBSA blends. Therefore, it is concluded that the morphological properties of the TPVs have a consistent relationship to the flow properties of the TPV melts.

3.5 Mechanical properties

Figure 8 shows stress-strain curves of dynamically cured rubber/polyester blends with different rubber and polyester types. Also, Table 4 shows mechanical properties in terms of Young’s modulus, tensile strength, elongation at break, hardness and toughness of the dynamically cured rubber/polyester blends. It is seen that the initial slope of the stress-strain curve or Young’s modulus (Table 4) and area underneath the curve (which indicates toughness) increased with epoxide content of rubber, reflecting an increased stiffness of the materials. Therefore, the Young’s modulus, toughness and stiffness properties of dynamically cured ENR-50/PBS blend were higher than those of the ENR-25/PBS, and NR/PBS blends. In Table 4, it is also seen that tensile strength and hardness properties show the same rank order as ENR-50/PBS > ENR-25/PBS > NR/PBS. This corresponds to the apparent shear viscosities and shear stresses (Fig. 7), the sizes of vulcanized rubber domains (Figs. 3 to 5), and the final mixing torques (Fig. 2). In Table 2, it is also seen that the elongation at break shows the opposite trend to the stiffness with an increase in epoxide content. This is due to higher chemical interfacial interactions between the PBS and rubber molecules with higher epoxied content, as described in the proposed model of Fig. 5 (b). With an increase in interfacial adhesion there is decreased interfacial tension, causing the formation of smaller vulcanized rubber domains with greater interfacial area in the dynamically cured ENR-50/PBS blend. Also, SEM micrographs of tensile fracture surfaces of dynamically cured rubber/polyester blends were used to compare with the trend in mechanical properties, as shown in Fig. 9. It is seen that, the unmodified NR/PBS TPV (Fig. 9 (a)) has a smoother surface with large vulcanized rubber domains imbedded in continuous PBS matrix (also in Figs. 3 and 4), with presumably very low interactions between the two different phases. On the other hand, the TPVs of the modified NR/PBS blends (i.e., ENR-25/PBS and ENR-50/PBS blends) show rougher surfaces with smaller vulcanized rubber domains embedded in the continuous PBS phase. This is attributed to higher chemical interactions between the phases, as seen in connecting domains among them due to high polarity of both polymer phases. This matches the improvement of mechanical and other related properties of the rubber/polyester blends. In Fig. 8 and Table 4, showing a comparison between PBS and PBSA blends, it is seen that the ENR-50/PBSA TPV shows much lower Young’s modulus, toughness, tensile strength, and hardness that the ENR-50/PBS TPV. This reflects lower interfacial interactions between ENR-50 and PBSA than ENR-50 and PBS also the PBSA has a lower degree of crystallinity.

Table 4

Mechanical properties in terms of Young’s modulus, tensile strength, elongation at break, hardness and toughness for the dynamically cured rubber/polyester blends with different types of natural rubber and polyester
Sample	Young’s modulus (MPa)	Tensile strength (MPa)	Elongation at break (%)	Hardness (shore A)	Toughness
NR/PBS	9.5 ± 0.5	16.3 ± 0.5	245.3 ± 3.5	89.8 ± 1.4	3450.6
ENR-25/PBS	10.5 ± 0.2	18.0 ± 0.4	236.3 ± 9.2	90.4 ± 1.5	3632.4
ENR-50/PBS	10.7 ± 0.2	19.7 ± 0.2	198.2 ± 5.9	90.6 ± 1.6	3082.0
ENR-50/PBSA	6.0 ± 0.1	13.8 ± 0.5	300.7 ± 4.9	83.6 ± 4.0	3213.9

3.6 Thermogravimetric analysis (TGA)

Figure 10 shows thermogravimetry (TGA) and derivative thermogravimetry (DTG) thermograms of dynamically cured rubber/polyester blends with different rubber and polyester types. Also, Table 5 demonstrates the decomposition temperature (T_d), temperature at 5% (T₅) and 90% (T₉₀) weight losses, and percent weight loss at 425°C and 900°C for the dynamically cured rubber/polyester blends with different types of natural rubber and polyester. A single decomposition step is seen. This corresponds to the DTG peak in the temperature range from 250°C to 500°C under nitrogen atmosphere. Therefore, the DTG peaks represent the decomposition temperature (T_d) of the TPV materials. It is clear that the dynamically cured ENR-50/PBS has a higher T_d than the ENR-25/PBS and NR/PBS blends. This indicates better thermal stability of the dynamically cured ENR-50/PBS blend. The main reason for this is higher chemical interactions between two phases, together with larger interfacial area due to the smallest vulcanized rubber domains (Figs. 3 and 4) in ENR-50/PBS TPV. In Fig. 10 and Table 5, it is surprisingly seen that the decomposition temperature (T_d) of the dynamically cured ENR-50/PBSA is higher than that of the ENR-50/PBS blend. This is attributed to lower thermal stability of pure PBS than pure PBSA (as seen the TGA thermograms in the right top corner of Fig. 10). Even though the PBSA has lower content of ester functional groups and hence lower polarity than PBS, it has intrinsically a higher thermal resistance than the PBS. This may be because the ester functional groups contain weak chemical bonds and are prone to degrade at an elevated temperature. Therefore, the molecular structures of the polyesters play more important roles than their chemical interactions with rubber. In Table 5, it is seen that the highest weight losses at 425 and 900°C were in the dynamically cured NR/PBS when compared with the dynamically cured ENR-25/PBS and ENR-50/PBS blends. Furthermore, the ENR-50/PBSA blend shows a lower weight losses at 425 and 900°C than the ENR-50/PBS blend.

Table 5

Decomposition temperature (T_d), temperatures at 5% (T₅) and 90% (T₉₀) weight losses, and percent weight loss at 425°C and 900°C for the dynamically cured rubber/polyester blends with different types of natural rubber and polyester.
Sample	T₅ (°C)	T₉₀ (°C)	T_d (°C)	Weight loss (%)
Sample	T₅ (°C)	T₉₀ (°C)	T_d (°C)	at 425°C	at 900°C
NR/PBS	358.3	443.0	409.6	83.6	98.8
ENR-25/PBS	352.7	460.3	409.6	75.6	97.0
ENR-50/PBS	359.3	455.7	412.9	75.6	97.9
ENR-50/PBSA	361.7	453.3	414.8	73.0	97.3

3.7 Differential scanning calorimetry (DSC)

Figure 11 shows differential scanning calorimetry (DSC) thermograms of dynamically cured rubber/polyester blends with different rubber and polyester types. Also, Table 6 summarizes glass transition temperature (T_g), crystallization temperature (T_c), crystalline melting temperature (T_m), heat of fusion (∆H_f) and degree of crystallinity (X_c) for the dynamically cured rubber/polyester blends. It is seen that the first cooling scan from 200°C to -80°C gives glass transition temperature (T_g), and crystalline melting temperature (T_m), while the second heating scan in the same temperature range gives the crystallization temperature (T_c). In Table 6, it is clear that the glass transition temperature (T_g) of rubber phase in the dynamically cured rubber/PBS blends is shifted to a higher temperature with increased epoxide content in natural rubber, while the T_g of PBS phase remains the same at -30.3°C. The two T_gs confirm the dual phase morphology of the immiscible natural rubber/polyester blends. Furthermore, it is seen that the T_g of ENR-50 in dynamically cured ENR-50/PBSA blend (-15.2°C) is lower than the T_g of ENR-50 phase in ENR-50/PBS blend (-14.2°C), while the T_g of PBSA phase in ENR-50/PBSA blend (-42.8°C) is lower than the T_g of PBS phase in ENR-50/PBS blend (-30.3°C). In Fig. 11 and Table 6, it is also seen that the crystallization temperature (T_c) and crystalline melting temperature (T_m) of pure PBS are higher than T_c and T_m of PBS phase in rubber/PBS blends. Furthermore, the T_c and T_m of PBS phase in the ENR-25/PBS, and NR/PBS TPVs are not different but they are marginally higher in the ENR-50/PBS blend. This indicates that blending PBS with natural rubber reduced the regularity of PBS molecules and hence decreased T_c and T_m. Also, the polarity of ENR molecules had a small effect on T_c and T_m of PBS in the blends. Furthermore, the lower T_c and T_m of the PBSA phase in the ENR-50/PBSA blend than that of the PBS phase in the ENR-50/PBS blend were due to lower T_c and T_m of the pure PBSA. In addition, blending of polyester with rubber also lowered the degree of crystallinity in the PBS phase of NR/PBS and ENR-25/PBS blends, as compared with the pure PBS (Table 6). However, in ENR-50/PBS and ENR-50/PBSA blends, the degrees of crystallinity (X_c) of PBS (58.7%) and PBSA (33.9°%) phases in natural rubber/polyester TPVs were higher than of the pure PBS (56.0%) and PBSA (27.2%), respectively. The polarity may promote regularity of polyester molecules and hence a higher degree of crystallinity.

Table 6

Glass transition temperature (T_g), crystallization temperature (T_c), crystalline melting temperature (T_m), heat of fusion *(∆H*_f) and degree of crystallinity (X_c) for the dynamically cured rubber/polyester blends with different types of natural rubber and polyester.
Sample	T_g(rubber) (˚C)	T_g(polyester) (˚C)	T_c (˚C)	T_m (˚C)	∆H_f (J/g)	X_c (%)
Pure PBS	-	-29.6	81.3	116.0	61.8	56.0
Pure PBSA	-	-38.7	51.6	94.8	38.6	27.2
NR/PBS	-60.1	-30.3	72.2	114.5	33.8	51.0
ENR-25/PBS	-40.8	-30.3	72.2	114.5	35.5	53.6
ENR-50/PBS	-14.2	-30.3	74.7	114.7	38.9	58.7
ENR-50/PBSA	-15.2	-42.8	51.4	97.1	28.9	33.9

3.8 Temperature scanning stress relaxation (TSSR)

Figure 12 shows normalized force-temperature and relaxation spectra-temperature relations of dynamically cured rubber/polyester blends with different rubber and polyester types. The stress relaxation during increasing temperature during non-isothermal test conditions relates to physical and/or chemical interactions and chain scission of polymer main chains. In the normalized force-temperature curves, plateau regions are clearly seen in the temperature range from 23°C to 35°C. Then strong decrease in the normalized forces is observed with increasing temperature. Table 7 illustrates various properties based on the normalized force-temperature curves including initial stress (σ_o), T_x (T₁₀, T₅₀, and T₉₀) and rubber index (RI) of the dynamically cured rubber/polyester blends. It is clearly seen that the dynamically cured ENR-50/PBS blend shows higher normalized force‐temperature curve together with initial stress, T₁₀, T₅₀, and T₉₀ than the ENR‐25/PBS and NR/PBS blends. This is due to higher chemical interactions of the polar functional groups in ENR‐50 and PBS which increased stiffness and other related mechanical and chemical properties. This observation is in agreement with the trends of apparent shear viscosity, apparent shear stress (Fig. 7), Young’s modulus and tensile properties (Fig. 8 and Table 4). Comparison between the two types of polyester (PBS and PBSA) shows that the dynamically cured ENR‐50/PBSA had a stronger decrease of normalized forces or more relaxation during temperature increase than the ENR‐50/PBS TPV. Also, the dynamically cured ENR‐50/PBS blend shows higher initial stress, T₁₀, T₅₀, and T₉₀ than the ENR‐50/PBSA blend (Table 7). This may be related to higher interfacial forces in the ENR-50/PBS blend as described previously, and also to lower T_c and T_m of PBSA (Table 6). In addition, all types of TPVs show similar rubber indexes in Table 6.

Table 7

Initial stress (σ_o), T_x (T₁₀, T₅₀, *and T*₉₀) and rubber index (RI) for the dynamically cured rubber/polyester blends with different types of natural rubber and polyester.
Sample	σ_o (MPa)	Temperature (°C)			Rubber Index (RI)
Sample	σ_o (MPa)	T₁₀	T₅₀	T₉₀	Rubber Index (RI)
NR/PBS	8.45	40.1	68.2	103.2	0.58
ENR-25/PBS	8.48	40.2	69.0	104.1	0.58
ENR-50/PBS	9.69	41.0	71.5	107.7	0.58
ENR-50/PBSA	5.37	38.0	60.6	86.5	0.59

In Fig. 12, it is clear that the dynamically cured rubber/polyester blends exhibited two relaxation peaks in relaxation spectra-temperature curves. The first peaks are clear in the temperature ranges from 88°C to 110°C (for rubber/PBS TPVs) and from 68°C to 80°C (for rubber/PBSA TPV). This relates to crystalline melting temperature of the polyester phase (i.e., PBSA or PBS) in the rubber/polyester TPVs (Fig. 11). Furthermore, the first peak of relaxation spectra-temperature curves of dynamically cured ENR-50/PBS has higher temperature and area underneath the peak than the ENR-25/PBS and NR/PBS blends. This is attributed to chemical interactions between the different phases and degree of crystallinity (X_c) in the polyester phase of rubber/polyester TPVs. That is, the ENR-50/PBS TPV has higher chemical interactions, interfacial adhesion and X_c than ENR-25/PBS and NR/PBS blends. This correlates to the smallest vulcanized rubber domains of the ENR-50/PBS TPV (Figs. 3, 4 and 6), the higher shear stress and viscosity (Fig. 7), and the highest mechanical properties (Fig. 8 and Table 4). In Fig. 12, the second peaks of the relaxation spectra are seen in the temperature ranges from 105°C to 114°C (in rubber/PBS blends) and from 83°C to 89°C (in rubber/PBSA blend). This relates to melting temperature of polyester component in the blends. This is more or less correlated to the T_m of pure PBS and pure PBSA at 94°C and 114°C and T_m of PBS and PBSA in the blends at about 114°C and 97°C, respectively.

3.9 Biodegradability

Figure 13 shows % weight losses of dynamically cured rubber/polyester blends after soil burial test for 4 weeks. It is seen that the weight loss increased with duration of burial, which indicates increasing of biodegradation of the dynamically cured rubber/polyester blends. In the natural rubber/PBS TPVs, the maximum weight loss and the hence level of biodegradation is seen in the dynamically cured NR/PBS blend (i.e., about 0.55% weight loss) at 4 weeks of burial. This is because the natural rubber/PBS blend has a low level of interfacial interactions with larger vulcanized rubber domains (Figs. 3 and 4) and a lower degree of crystallinity of PBS phase (Table 6). This may create more and larger voids in the TPV sample that facilitate exposure to moisture and microorganisms [22, 24]. Also, the unmodified NR may contain higher level of proteins, lipids and other ingredients that can be digested by bacteria and other microorganisms [28]. Therefore, the biodegradation of dynamically cured NR/PBS was higher than of the ENR-25/PBS, ENR-50/PBS blends, respectively. In Fig. 13, it is also seen that the biodegradation of dynamically cured ENR-50/PBSA is higher than of the ENR-50/PBS, ENR-25/PBS and NR/PBS blends. This may be due to the adipic acid units in PBSA reducing chain regularity and hence lowering the degree of crystallinity with more amorphous regions, which are more susceptible to attack by bacteria and fungi in soil [29]. Also, very large vulcanized rubber domains of ENR-50/PBSA TPVs (Figs. 3 and 4) create more large voids that are prone to have a microorganism attack.

Figure 14 shows metallographic micrographs for the various types of dynamically cured rubber/polyester blends after soil burial for 4 weeks. It can be seen that there are a number of large holes or cavities at the sample surface, which indicate locations where the microorganism activities took place. It is clear that the dynamically cured NR/PBS blend shows more large holes on the surface than the other types of rubber/PBS TPVs. Also, the dynamically rographs confirm the trend of weight loss and hence the level of biodegradation for the different types of TPV materials (Fig. 13). cured ENR-50/PBSA blend displays a rougher surface and some more fungal colonization than the dynamically cured ENR-50/PBS blend. Therefore, the metallographic mic

Green biodegradable thermoplastic natural rubber (GB-TPNR) was prepared by dynamical curing of natural rubber/polyester blends. The morphological, rheological, mechanical, stress relaxation, and thermal properties as well as biodegradability were analyzed. It was found that apparent shear viscosity, shear stress, Young’s modulus, tensile strength, glass transition temperature (T_g) of the rubber phase, and degree of crystallinity of the PBS phase in rubber/PBS TPVs increased with epoxide content of the NR. However, the ENR-50/PBSA TPV exhibited poorer properties than the ENR-50/PBS TPV. This may be due to adipic acid as a third unit in PBSA molecules, which reduced chemical interactions of PBSA with rubber, and hence increased irregularity of the polyester molecules. Furthermore, SEM and AFM micrographs confirmed the dual phase morphology of the TPVs with micron sized vulcanized rubber domains dispersed in the polyester matrix. In addition, the dynamically cured ENR-50/PBS with high content of polar functional groups in ENR and PBS molecules exhibited the smallest vulcanized rubber domains, with the highest interfacial area and interfacial interactions, but the lowest interfacial tension. It was also found that the NR/PBS with poor interactions between the phases, and with higher level of non-rubber components, possessed higher biodegradability. We also found that the ENR-50/PBSA TPV showed higher biodegradability than the ENR-50/PBS, ENR-25/PBS and NR/PBS, due to lower chemical interactions and lower degree of crystallinity, which provided more voids for exposure to the microorganisms.

Acknowledgments

This research was financially supported by the Thailand Research Fund (TRF) through the Royal Golden Jubilee Ph.D. Program (Grant no. PHD/0208/2557) to Dr. Charoen Nakason as principal researcher, and to Miss Parisa Faibunchan as the research assistant. Also, the authors would like to express their gratitude to Faculty of Engineering and Computer Science, University of Applied Sciences Osnabrück, Germany, which is gratefully acknowledged for providing access to its facilities. In addition, we would like to thank Dr. Seppo Karrila for his assistance with the manuscript preparation.

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Thermoplastic Natural Rubber Based on Polyester Blends: Influence of Different Types of Natural Rubber and Polyesters on Cure, Mechanical, Rheological, Morphological, Thermal, Relaxation Properties and Biodegradation

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Abstract

Figures

1. Introduction

2. Experimental

2.1 Materials

2.2 Preparation of dynamically cured natural rubber/polyester blends

2.3 Cure characteristics

2.4 Rheological properties

2.5 Morphological properties

2.6 Mechanical properties

2.7 Thermal properties

2.8 Temperature scanning stress relaxation (TSSR)

2.9 Biodegradability

3. Results And Discussion

3.1 Curing characteristics

3.2 Mixing torque during preparation of TPVs

3.3 Morphological properties

3.4 Rheological properties

3.5 Mechanical properties

3.6 Thermogravimetric analysis (TGA)

3.7 Differential scanning calorimetry (DSC)

3.8 Temperature scanning stress relaxation (TSSR)

3.9 Biodegradability

4. Conclusions

Declarations

Acknowledgments

References

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