Influences of blend proportions and curing systems on dynamic, mechanical, and morphological properties of dynamically cured epoxidized natural rubber/high-density polyethylene blends.
A thermoplastic elastomer (TPE) is defined as a polymeric material with properties and functional performance similar to those of a conventional vulcanized rubber. Also, it can be processed using thermoplastic processing machines. TPEs have attracted increasing interest from both scientific and industrial perspectives. They provide very useful and attractive applications in a variety of markets, such as automotive parts, building materials and construction equipment, wire and cable insulation, and so on (1). The TPEs based on rubber-thermoplastic blends are classified into two distinct classes. One class consists of a simple blend and is commonly called a thermoplastic elastomer polyolfin (TEO) or thermoplastic polyolefin (TPO). The rubber phase of the TPO is an unvulcanized material. In the other class, the rubber phase is dynamically vulcanized, during melt mixing, with a thermoplastic polymer, giving rise to thermoplastic vulcanizates (TPVs), dynamic vulcanizates (DVs), or elastomeric alloys (EAs) (2). It has been well established that future developments will most likely be based on dynamic vulcanization (3), TPEs of natural rubber (NR) and thermoplastic blends are known as thermoplastic natural rubbers (TPNRs). NR has an inherent affinity for some olefin thermoplastics that permits the formation of TPVs characterized by submicron scale morphologies and excellent physical properties (4). Many researchers used various types of thermoplastics to prepare TPNRs. These include polypropylene (PP) (5-10), low-density polyethylene (11), (12), ultra low-density polyethylene (13), linear low-density polyethylene (14), polystyrene (15), polyamide 6 (16), ethylene-vinyl acetate copolymer (EVA), (17) and poly(methyl methacrylate) (PMMA) (18). High-density polyethylene (HDPE) has also been used to prepare TPNR with improved tensile properties and hardness by the addition of liquid natural rubber (LNR) as a compatibilizer (19). The influence of gamma radiation on the tensile properties of TPNR based on NR and HDPE blend was investigated (11). Moreover, structures of natural rubber-based TPNR and their composites with carbon black were investigated by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS) (20). very recently, Nakason et al. pursued extensive research on the preparation of TPNR based on various thermoplastics, such as PP (21-30), PMMA (31-33), and HDPE (4), (34). The majority of these works have used modified forms of NR i.e., epoxidized natural rubber (ENR) and maleated natural rubber (MNR) due to the polarity and solvent resistance of the final products. Various types of blend compatibilizers, therefore, must have the ability to promote the interfacial adhesion between the rubber and plastic phases. These compatibilizers include graft copolymer of PP (23), (30) or HDPE (4) with maleic anhydride, i.e., polypropylene-g-(maleic anhydride) (PP-g-MA) and high-density polyethylene-g-(maleic anhydride) (HDPE-g-MA), respectively, and phenolic modified PP (23), (24), (27), (29), (30) or HDPE (4). In these works, blend compatibilizers (4), (23), (27), (30), curing systems (4), (21), (24), (25), (27), (28), and blend proportions (4), (26), (31), (32) were the main interest because they played very important roles in controlling desirable properties of the TPVs.
In this work, an attempt was made to prepare dynamically cured TPNRs based on blends of ENR and HDPE via a dynamic vulcanization process. Influences of process oil, proportion of ENR and HDPE, as well as curing systems, on mechanical, morphological, and dynamic properties were investigated.
ENR with a level of epoxide groups at 30% mole (ENR-30, having a [T.sub.g] of--28[degrees]C and a number average molecular weight of 5.23 X [10.sup.5] g [mol.sup.-1]) was prepared in-house using high ammonia NR latex with a dry rubber content of ~ 60%. Details of the preparation and characterization processes of the ENR were described in our previous work (24), (35). HDPE as a blend component was manufactured by the Thai Polyethylene, Co., Ltd, (Rayong, Thailand). It was an injection molding grade, H600J with a melt flow index (MFI) of 7.5 g [10.sup.-1] [min.sup.-1] (2.16 kg loads at 190[degrees]C) and a density of 970 kg [m.sup.-3]] Phenolic modified HDPE compatibilizer (here referred to as PhHRJ-HDPE) was also prepared in-house using HDPE, phenolic resin with active hydroxymethyl (methylol) groups of about 6--9% (HRJ-10518 with a softening point of 80--95[degrees]C and a specific gravity of 1.05, manufactured by Schenectady International, Newport, NY) and stannous chloride or tin (II) chloride hydrate catalyst (Sn[Cl.sub.2] 2[H.sub.2]O, a density of 271 kg [m.sup.-3] and a softening point of 38 C from Carlo Erba Reagents, France). The preparation and characterization procedures were described elsewhere (30), (35). The zinc oxide (a density of 557 kg [m.sup.-3] and a decomposition temperature of 1975[degrees]C) used as an activator was manufactured by Global Chemical Co., Ltd, Samutprakarn, Thailand. The stearic acid (a density of 850 kg [m.sup.-3] and a melting point of 69-70[degrees]C) used as an activator was manufactured by Imperial Chemical Co., Ltd. U.K. The sulfur (a melting point at 115[degrees]C) used as a vulcanizing agent was manufactured by Siam Chemical Co., Ltd., Bangkok, Thailand. The N-tert-butyl-2-benzothiazolesulphenamide (Santocure TBBS, having a melting point of 238.4[degrees]C), used as an accelerator, was manufactured by Flexsys (Flexsys America L.P., U.S.A.). Dicumyl peroxide (DCP, having a melting point of 39-41[degrees]C), used as a vulcanizing agent, was manufactured by Akzo Nobel Polymer Chemicals BV, Amersfoort, The Netherlands. Triallyl cyanurate (TAC, melting point of 26-28[degrees]C), a co-agent for peroxide curing system, was manufactured by Fluka Chemie (Buchs, Switzerland). The polyphenolic additive, (Wingstay [R]L, MW 650 g [mol.sup.-1], a density of 1100 kg [m.sup.-3]and a melting point of 115[degrees]C), an antioxidant for NR, was manufactured by Eliokem Inc. (Ohio, USA) to oppose oxidation or inhibit reactions promoted by oxygen or peroxide. White oil composing of paraffinic oil and naphthanic oil (having a specific gravity of 0.84 and a solubility parameter of 16.4 MP[a.sup.1/2]) obtained from Akrochem Co., Ltd. (USA) was used as a plasticizer.
Influence of the Process Oil on Properties of Dynamically Cured ENR/HDPE Blends
In this work, test specimens were prepared using a plastic injection molding process. We first experienced difficulty in the flow ability of the polymer melt to fill the mold cavity. Therefore, oil extended TPVs were exploited to facilitate flow and enhance other properties, in particular, the rubber elasticity. According to our previous work, ENR with 30% mol of epoxide groups (i.e., ENR-30) gave dynamically cured ENR/PP with superior mechanical properties and fine grain of spherical dispersed vulcanized rubber domains (29). In this work, the ENR-30 was therefore dynamically cured during blending with HDPE. The oil extended ENR-30 (i.e., OE-ENR) was first prepared by mixing the ENR-30 with 20 phr of paraffinic oil in a 3-L dispersion kneader at 80[degrees]C(Young Fong Machinery Co., Ltd, Samutsakon, Thailand). The OE-ENR was then compounded using the mixed sulfur and peroxide vulcanization system, which compounding formulation is shown in Table 1. Various compounding ingredients were mixed with OE-ENR on a two-roll mill at room temperature. This mixing schedule is described in Table 2. ENR without the process oil was also compounded for comparison purposes. The rubber compounds were later dynamically cured with HDPE by blending in an internal mixer, Brabender Plastcorder, model PLE331 (Brabender OHG Duisburg, Germany) at 180[degrees]C and a rotor speed of 60 rpm The mixing steps are described in Table 3. A fixed blend ratio of ENR-30/HDPE = 60/40 was used to study influences of the process oil on properties and processability of the TPVs. The process started by warming HDPE in the mixing chamber for 6 min without rotation and followed by mixing the HDPE at a rotor speed of 60 rpm at 180[degrees]C for 2 min. The ENR compound was then added into the mixing chamber and the mixing was continued for 2 min. Finally, the blend compatibilizer (i.e. PhHRJ-HDPE) was added. Blending was continued until a plateau mixing torque was obtained. The blending products were cooled down to room temperature and cut into small pieces using a Bosco plastic grinder (Bosco engineering, Samutparkarn, Thailand). The test specimens of the TPVs were prepared by a plastic injection molding machine (TII-90F, Weltec Machinery Ltd., Hong Kong). The temperatures for the three zones of the barrel were set at 160, 170, and 180[degrees]C, and the injection nozzle was at 180[degrees]. The pressure, velocity, and stoke were set according to guidelines provided by the manufacturer. The injection pressure was set at 100-115 Pa and the injection velocity was 55--65 m [s.sup.-1]. The mechanical and thermal properties were later investigated.
TABLE 1. Compounding formulation of ENR-30 for preparation of dynamically cured ENR/HDPE blends with and without the processing oil. Quantities (phr) Ingredients Without oil (phr) With oil (phr) ENR-30 100.0 100.0 ZnO 5.0 5.0 White oil - 20.0 Stearic acid 1.0 1.0 Wingstay L 1.0 1.0 TBBS 1.0 1.0 Sulfur 0.5 0.5 DCP 1.0 1.0 TAC 0.5 0.5 TABLE 2. Mixing schedule. Descriptions Mixing time (min) Rubber mastication 5 Stearic acid 1 ZnO 1 Wingstay L 1 TBBS 1 Sulfur 1 DCP 1 TAC 1 Compound finishing 2 TABLE 3. Mixing step for the dynamically cured ENR-30/HDPE blends. Descriptions Time (min) Warm HDPE 6 Mix HDPE 2 Oil extended ENR compounds 2 Blend compatibilizer Until reaching the plateau of a mixing (PhHRJ-HDPE) torque
Influence of Blend Proportions on Properties of Dynamically Cured ENR/HDPE Blends
The oil extended ENR-30/HDPE TPVs were prepared using the same compounding formulation as shown in Table 1 and mixing schedule shown in Table 2. In this study, three blend ratios of the oil extended ENR-30/HDPE at 50/50, 60/40 and 75/25 were studied. Blends with ENR-30 content lower than 50% wt were not investigated in this study because they displayed inferior elastomeric properties (30). Dynamic vulcanization of the ENR-30/HDPE blends was performed at 180[degrees]C using an internal mixer at a rotor speed of 60 rpm. The mixing steps are shown in Table 3. Mechanical, dynamic, and morphological properties of the blending products were then characterized.
Influence of Curing Systems on Properties of Dynamically Cured ENR/HDPE Blends
Three different types of curing systems were used to prepare the oil extended ENR/HDPE TPVs via a dynamic vulcanization process: peroxide with co-agent system, mixed sulfur and peroxide curing system and phenolic resin curing system. The oil extended ENR-30 was first prepared and then compounded using a two-roll mill at room temperature with the compounding formulations as shown in Table 4. Dynamically cured ENR-30/HDPE blends at a fixed blend ratio of 60/40 were prepared in an internal mixer at 180[degrees]C and a rotor speed of 60 rpm using the mixing steps as shown in Table 3. Mechanical and morphological properties of the blending products were then characterized.
TABLE 4. Compounding formulation of ENR-30 for the preparation of dynamically cured ENR/HDPE blends. Quantities (phr) Ingredients Peroxide with Mixed Phenolic co-agent peroxide and resin sulfur system system ENR-30 100.0 100.0 100.0 ZnO 5.0 5.0 5.0 Stearic acid 1.0 1.0 1.0 Wingstay L 1.0 1.0 1.0 TBBS - 1.0 - Sulfur - 0.5 - DCP 2.0 1.0 - TAC 1.0 0.5 - SP-1045 - - 4.5 HRJ-10518 - - 4.5 [SnCl.sub.2] * [H.sub.2]O - - 0.6 White oil 20 20 20
The specimens were conditioned at room temperature for 24 h before tensile test. Tensile testing of the samples was performed at (25 [+ or -] 2) [degrees]C at a crosshead speed of 500 mm [min.sup.-1] according to ISO 37. The instrument used was Hounsfield Tensometer, model H 10 KS manufactured by the Hounsfield Test Equipment Co., Ltd, UK. The dumbbell-shaped specimens, 2 mm thick, were prepared by a thermoplastic injection molding machine with a capacity of 90 tons of clamping force (Welltec Machinery Ltd., Hong Kong). Tension set at 100% elongation was also determined at room temperature (25 [+ or -] 2[degrees]C) according to ISO 2285 (ASTM 412). The samples were kept under tension for a fixed elongation and time interval. Then they were released from the clamp, kept aside for another fixed time interval and the changes in the sample dimensions were determined. Hardness of the samples was also measured using indentation Shore A, according to ISO 7619.
Dynamic properties of the oil extended ENR-30/HDPE TPVs were characterized using a rotorless oscillating shear rheometer (RheoTech MDPT, Cuyahoya Falls, Ohio, USA) at 180[degrees]C. The oscillation frequency was set in the range of 1-25 Hz at a constant strain of 3%. This was to assure that samples were tested in the range of linear viscoelasticity of the TPVs. The storage (G') and loss shear (G") modulus, loss factor, tan [delta] = G"/G' as well as the complex viscosity (i.e., [eta]* = 3G*/o) = [eta]" + i[eta]') of the TPVs were characterized.
Morphological studies were carried out using a Leo scanning electron microscope, model VP 1450, manufactured by Leo Co., Ltd., Cambridge, UK. Molded samples of the TPE were cryogenically cracked in liquid nitrogen to avoid any possibility of phase deformation. The HDPE phase was preferentially extracted by dissolving the fractured surface in hot xylene at 100[degrees]C which is below its boiling point (140[degrees]C). The samples were then dried in a vacuum oven at 40[degrees]C for 3 h. The dried surfaces were later gold-coated and examined by scanning electron microscopy.
Thermal Property Characterization
Two techniques were used to follow the thermal behavior of the oil extended ENR-30 and HDPE; differential scanning calorimetry (DSC, TA Instruments, TX), and dynamic mechanical analysis (DMA, PerkinElmer DMA7,Boston, USA). The DSC was performed with the instrument under nitrogen atmosphere. The samples were first heated to 120[degrees]C, maintained at that temperature for 5 min and then quenched to--100[degrees]C to remove the thermal history. Then, a second scan was carried out from--120 to +90[degrees]C at a heating rate of 10[degrees]C [min.sup.-1]. The glass transition temperatures, [T.sub.g], of the samples were determined from the midpoints of the transitions. The DMA experiment was conducted in the tension mode. A temperature-time scan was carried out from--120 to +130[degrees]C at a frequency of 1 Hz. The heating rate was 5[degress]C [min.sup.-1].
RESULTS AND DISCUSSION
Influence of Process Oil on Properties of Dynamically Cured ENR/HDPE Blends
The relationship between stress and strain from the tensile testing of the TPVs with and without the paraffinic oil (process oil) is shown in Fig. 1. It can be seen that the curve of TPV without the oil shows a higher slope at the beginning of the test. This reflects higher elastic modulus. It is a measure of stiffness of the material. Higher stress or strength at fracture of the TPV without the oil was also observed. However, the strain at fracture (i.e., elongation at break) of the oil extended TPV is higher. These details are shown in Table 5. It can be seen that incorporation of the oil caused decreases in hardness and tensile strength but increasing elongation and a lower tension set, i.e., a tendency to recover from prolong extension. This is attributed to the oil distribution and partition in the ENR and HDPE phases causing increasing chain mobility as indicated by the decreasing glass transition temperature ([T.sub.g]) of the ENR from approximately--51.8[degrees]C for the ENR without oil to--58.1[degrees]C for the oil extended ENR, as shown in Fig. 2. It was very difficult to observe [T.sub.g] of the HDPE phase by the DSC technique because a very low amount of the amorphous portion or the high crystalline portion was present in HDPE. Therefore, the [T.sub.g] of HDPE was evaluated from a peak of tan [delta] obtained from a DMA technique, as shown in Fig. 3. It can be seen that the [T.sub.g] of HDPE without the oil is approximately--31[degrees]C. while the oil extended HDPE has [T.sub.g] of--73[degrees]C. The materials having a lower [T.sub.g] exhibit higher elastomeric property in terms of elongation at break and tension set properties with lower strength and hardness properties. This observation was in good agreement with the oil distribution in plastic and elastomer phases in PP/EPDM and PP/SEBS TPVs (36). During the preparation of the test specimens, it was found that the oil extended ENR-30/HDPE TPVs were easier to process as the injection process required a lower injection pressure and the mold filled easily. Therefore, the oil extended ENR-30 was used throughout this study.
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TABLE 5. Mechanical properties of 60/40 ENR-30/HDPE TPVs with and without oil. Mechanical properties Without oil With oil Tensile strength (MPa) 8.32 7.85 Elongation at break (%) 248 269 Hardness (Shore A) 90.0 85.0 Tension set (%) 40 35
Influence of Blend Proportions on Properties of Dynamically Cured ENR/HDPE Blends
Figure 4 shows the stress--strain curves of ENR-30/HDPE TPVs with three blend proportions. Increasing ENR content caused a decrease in stiffness of the materials, i.e., lower elastic moduli which were calculated from the slopes of the stress--strain curves. However, increasing elongation at break of the TPVs was observed upon increasing the level of the rubber component. The mechanical properties are shown in Table 6. Increasing the level of ENR-30 gave higher elongation at break and superior tension set properties of the TPVs but decreased tensile strength and hardness properties. This is attributed to the nature of the blend components, i.e., increasing rubber phase produced increased elastomeric properties in terms of higher elongation at break and lower tension set. On the other hand, increasing plastic phase caused increasing tensile strength and hardness properties. It was also noted that the blends with all proportions understudy, exhibit the TPE properties, i.e., they are elastomeric material with tension set values lower than 50% and they can be processed with plastic injection molding machines. The blend compatibilizer (PhHRJ-HDPE) also played an important role in enhancing various properties. Jarnthong (37) studied the influences of this type of blend compatibilizer on dynamically cured ENR/HDPE blends. Jarnthong found that the blends with the compatibilizer showed superior mechanical, morphological, and elastomeric properties.
TABLE 6. Mechanical properties of ENR-30/HDPE TPVs with various proportions of rubber and HDPE. ENR-30/HDPE Mechanical properties 50/50 60/40 75/25 Tensile strength (MPa) 8.44 7.85 7.32 Elongation at break (%) 211 269 331 Hardness (Shore A) 88 85 77.5 Tension set (%) 45 25 15
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Figure 5 shows the elastic modulus (G') as a function of frequency. It is seen that these blends showed an increase in G' with increasing frequency. This is due to the decrease in time available for molecular relaxation. Also, at a given frequency, the G' values increased with increasing levels of the ENR-30 contents. In this system, the dynamic test was performed at 180[degrees]C where the HDPE matrix was completely molten but the vulcanized rubber domains still remained unmolten. Therefore, the elastic properties of the system greatly depend on the amount of vulcanized rubber domains. As a consequence, higher elastic response (i.e., storage modulus) was observed for the system with the amount of the small particles of vulcanized rubber domains. According to the SEM micrographs in Fig. 6, the extracted surface of the HDPE phase shows cavitations of the removed HDPE phase by xylene extraction. The vulcanized rubber domains are also observed as small particles but they are not in a spherical shape as are the ENR/PP-TPVs (26), (31), (32). This may be attributed to melt viscosity and crystallinity of the plastic phase. The PP at the melting temperature exhibited lower melt viscosity than that of the HDPE in this work. Therefore, the stable spherical vulcanized rubber domains were able to form during dynamic vulcanization. Also, HDPE typically exhibits a higher degree and rate of crystallization than that of PP. During a cooling stage of the crystallization process, the shape of vulcanized rubber dispersed domains in the HDPE matrix is exposed to higher internal force and can be deformed under the influence of the force exerted. Figure 6 shows that the higher ENR-30/HDPE content produces the smaller vulcanized rubber domains and HDPE cavitations. That is, the ENR-30/HDPE = 75/25 shows smaller vulcanized rubber domains than those of ENR-30/HDPE = 60/40 and 50/50, respectively. Smaller rubber domains provide higher surface areas and interfacial force to promote interaction with the blend compatibilizer (i.e., PhHRJ-HDPE) and both phases. This can be a reason for the increases in elastic modulus with increases in rubber content.
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The complex viscosity as a function of frequency is shown in Fig. 7. It is clear that the viscosity decreases with increasing frequency or shear rate (i.e., shear-thinning behavior). Furthermore, higher content of ENR-30 or vulcanized rubber domains produced the higher complex viscosity. That is higher flow resistance was the result. This can be attributed to the TPVs with smaller vulcanized rubber particles exhibiting higher interaction between the two phases because the higher surface area and thus causing better interfacial adhesion. Higher rubber content caused increasing interfacial areas due to decreases in rubber domain size (see Fig. 6). However, increases in rubber proportion may not increase crosslink density in small-sized rubber domains. When crosslink density can be favorably optimized by utilizing the higher amount of rubber content and a suitable amount of curing agent, the interfacial interaction attributed by the blend compatibilizer may improve the material properties.
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Figure 8 shows the effect of tan [delta] on frequency which is a ratio between of loss modulus and storage modulus or the ratio of viscous to elastic properties. It indicates that the TPVs with higher rubber content exhibit a lower tan [delta] or damping factor. That is, the higher the content of vulcanized rubber, the greater the elastic response becomes. This leads to a lower value of the tan [delta]. Therefore, trend of tan [delta] at a given frequency can be ranked as: 75/25 ENR-30/HDPE TPV < 60/40 < ENR-30/HDPE TPV < 50/50 ENR-30/HDPE TPV. This trend also correlates with the decreasing trend of tension set in Table 6, i.e., increasing elastomeric properties of the solid TPVs with increasing ENR content.
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Influence of Curing Systems on Properties of Dynamically Cured ENR/HDPE Blends
Relationship between the stress and strain of the TPVs using different curing reactions is shown in Fig. 9. It can be seen that the curves of TPV with peroxide co-agent and mixed peroxide and sulfur curing systems show similar values of slope (i.e., modulus or stiffness), while the phenolic resin cured TPV exhibited the highest modulus and hence the most stiffness. The area under the stress--strain curve represents the toughness of the material. It is clear that the TPV in the phenolic curing system shows the highest toughness, while the mixed peroxide and sulfur system gave the lowest value. On the other hand, the TPV in the peroxide co-agent system showed the intermediate value. Tensile strength and elongation at break from the stress--strain curves in Fig. 9 are concluded in Table 7. The tension set and hardness properties of the TPVs using the three curing systems are also shown in Table 7. That is, the peroxide co-agent curing system gave the highest tensile strength and hardness but the lowest elongation at break. This reveals high strength properties of the material. This is attributed to the stable C--C linkages that were formed during the dynamic curing of the ENR-30/HDPE blends. In addition, the C--C links are extremely short that they cannot contribute any influence to the entropy of the chains unlike the polysulfidic linkages. This reason can explain the cause for high strength of this material. It is noted that in the presence of peroxide co-agent curing system, HDPE is capable of performing crosslinking reactions between the molecules (38). This is a synergistic effect on the strength and hardness of the TPV. For the mixed sulfur and peroxide curing system, the lowest tensile strength and hardness properties with moderate elongation at break were observed. This is attributed to the occurrence of a rubber network with C--C, C--S and S--S linkages between the ENR molecules. The later two types are the chemical bonds with low bond dissociation energy. Also, the lower amount of peroxide curing agent used in this system caused lower crosslinking of the HDPE. In the phenolic curing system, dynamically cured ENR-30/HDPE occurred via methylol and hydroxyl groups of the phenolic resin and double bond of the rubber molecules. The Chroman ring structures (39), (40) were the intermediate reaction between the phenolic and ENR molecules, probable reaction mechanisms are shown in Schemes 1 and 2 for the reaction of SP-1045 and HRJ-10518 with ENR-30, respectively. The ENR molecules are eventually crosslinked by the phenolic molecules. The phenolic resin consists of an aromatic ring and two ortho-and para-directed reactions in each phenol molecule. Also, abundance of the free polar functional groups in the phenolic resin molecules (i.e., --OH groups) is capable of interacting with the oxirane ring in the ENR molecules. This reaction produces another type of crosslinking network. Apart from these, the reactive functional groups may react with PhHRJ-HDPE compatibilizer at the interface of the vulcanized ENR and HDPE. The higher interfacial force between the ENR and HDPE is a consequence of the reaction. Therefore, the ENR-30/HDPE TPVs treated with the phenolic resin curing system exhibit high strength properties. Also, this type of TPV exhibits the highest elongation at break and the lowest tension set properties, as shown in Table 7. This indicates the highest elastomeric properties among the curing systems studied; that is, the vulcanized rubber consists of long crosslinking segments of the phenolic moiety between the ENR molecules. Also, more chemical interactions occur between the rubber and plastic phases by the reaction and/or interaction of the phenolic modified HDPE compatibilizer. This induces the TPV to be more extended and it has a high tendency to recover to its original shape after a prolong extension.
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TABLE 7. Mechanical properties of ENR-30/HDPE TPVs using three types of vulcanization systems. Curing systems Mechanical Peroxide with Mixed sulfur Phenolic properties co-agent and peroxide resin system Tensile 8.36 7.85 8.15 strength (MPa) Elongation (%) 233 269 347 at break Hardness 87 85 86 (Shore A) Tension set (%) 30 25 15
Figure 10 shows the elastic modulus (G') as a function of frequency of the ENR-30/HDPE TPVs using the three curing systems. It is clear that all TPVs show an increase in G' with increasing frequency because of a shorter time available for molecular relaxation. At a given frequency, the TPV with the phenolic resin curing system shows the highest G', while the peroxide co-agent system exhibits the lowest value. The TPV with the peroxide and sulfur curing systems shows the intermediate value. As mentioned earlier, the phenolic resin cured TPV consists of longer crosslinks of phenolic molecules with capability of interacting with the oxirane rings of ENR molecules, and with the PhHRJ-HDPE compatibilizer at the interface of HDPE and vulcanized ENR domains. These are the reasons for the high elastic response of this material. On the other hand, the dynamically cured TPV with the peroxide co-agent system produced only the C--C linkages which are short and poor in elasticity with less strength. In contrast, the interaction between the vulcanized ENR and the HDPE interface occurred without the inclusion of other polar functional groups. Moreover, the DCP curing agent caused a crosslinking reaction of the HDPE phase. The behavior lowered elastic response of the TPV with the peroxide co-agent curing system. For the TPV with the mixed sulfur and peroxide curing system, the additional elastic linkages of S--S and C--S linkages were formed. In addition, a lower content of DCP was used to possibly lower the crosslinking reaction in the HDPE phase. Therefore, the mixed curing system shows the higher elastic response than that with only peroxide curing system but lower than that of the phenolic resin curing system.
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Figure 11 shows tan [delta] curves as a function of frequency of the dynamically cured ENR-30/HDPE TPV using three curing systems. The TPVs with the phenolic curing system exhibited the lowest tan [delta] or damping factor. The TPVs cured by the peroxide co-agent showed the highest value, while the TPVs cured by the mixed sulfur and peroxide system exhibited the intermediate value of tan [delta]. This corresponds to the tension set properties in Table 7. That is, the TPV cured by the phenolic resin system showed the lowest tension set (i.e., the highest elasticity), while The TPVs with the peroxide co-agent system showed the highest tension set value. Therefore, it can be concluded that the TPVs with the phenolic resin curing system show the highest elastic response. The trend of tan [delta] or elasticity of the materials at a given frequency can be ranked as: TPV with the peroxide co-agent system > TPV with the mixed sulfur and peroxide system > TPV with the phenolic resin system. The influence of different curing systems on phase separation may result in differential spatial heterogeneities and thus affect the mechanical properties of the elastomers, which shall be further explored in Fig. 12.
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Figure 12 shows the SEM micrographs of the TPVs using three types of curing systems. It demonstrates that the surface of the TPV with the phenolic curing system consists of the smallest vulcanized rubber domains and cavitations because of the higher interaction between the HDPE and vulcanized ENR phase caused by the reaction or the interaction of the blend compatibilizers acting as a bridge which joins the two phases. The SEM micrographs of the TPV with the peroxide co-agent system are difficult to see and analyze because the HDPE phase did not dissolve in the hot xylene due to the crosslinked structure of the HDPE phase caused by the peroxide curing system. Therefore, the HDPE still remains on the surface. This prevents us from observing the details of the phase morphology at the surface for this type of TPV. In the mixed peroxide and sulfur system, the amount of crosslink structures is low. Therefore, a large amount of the vulcanized rubber domains and more cavitations of the extracted HDPE were observed. This is why this type of TPVs exhibits inferior mechanical and dynamic properties. On the other hand, the TPVs cured by the phenolic resin system exhibit superior mechanical and dynamic properties because of the small vulcanized rubber domains with high interfacial adhesion between the HDPE and vulcanized ENR phases. The SEM micrographs in Fig. 12 further illustrates that the phenolic resin and peroxide curing systems have caused greater heterogeneities of the system based on their phase separation and gave smaller rubber particles. In another word, higher number of particles is expressed in a unit volume. The mixed peroxide and sulfur system gave lower spatial heterogeneities of the blends based on evidence of larger rubber domains.
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The complex viscosity as a function of frequency in 60/40 ENR-30/HDPE TPVS with three curing systems is shown in Fig. 13. It shows the shear-thinning behavior that was observed. The TPVs with the phenolic resin curing system show the highest complex viscosity because of the newly occurring strong, flexible networks as well as the higher interfacial adhesion at the interface, as discussed earlier. The viscosity of the peroxide co-agent is lower than that of the phenolic resin curing system but marginally higher than that of the mixed sulfur and peroxide cured TPV. This may be attributed to the network structure and higher amount of crosslinks in the HDPE phase for the peroxide co-agent system.
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Dynamically cured ENR/HDPE TPVs were prepared. Influences of the process oil, blend proportion, and curing systems were investigated. We found that the oil extended ENR-30/TPVs exhibited ease in the injection molding process. Final products with better elastomeric properties were observed. Three blend proportions of ENR-30/HDPE at 50/50, 60/40, and 75/25 were studied. Increasing the proportion of ENR increased elastic response in dynamic properties, i.e., storage modulus, complex viscosity, and elastomeric properties in terms of elongation at break, tension set properties and tan [delta]. Three types of curing systems namely peroxide co-agent, mixed sulfur and peroxide, and phenolic resin were used for investigating with a fixed ENR-30/HDPE ratio of 60/40. The TPVs treated with the phenolic curing system exhibit superior mechanical and dynamic properties, and the smallest vulcanized rubber domains. The TPVs with the conventional peroxide co-agent curing system show good mechanical strength properties and poor elastomeric properties. The morphological property of this type of TPV could not be clearly viewed because the crosslinked HDPE matrix was not extracted and obstructed the rubber phase. On the other hand, the TPVs with the mixed sulfur peroxide curing system exhibited intermediate mechanical dynamic properties with a smaller amount of large vulcanized rubber domains than that of the phenolic resin cured TPVs.
The authors thank the Prince of Songkla University at Pattani Campus and Chulalongkorn University for research facilities and other support.
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Charoen Nakason, (1) Methakarn Jarnthong, (1) Azizon Kaesaman, (1) Suda Kiatkamjornwong (2)
(1) Center of Excellence in Natural Rubber Technology, Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani 94000, Thailand
(2) Multidisciplinary Program of Petrochemistry and Polymer Science, Department of Imaging and Printing Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
Correspondence to: Suda Kiatkamjornwong; e-mail: email@example.com
Contract grant sponsor: Thailand Research Fund; contract grant number:
Published online in Wiley InterScience (www.interscience.wiley.com).
[c] 2008 Society of Plastics Engineers
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|Author:||Nakason, Charoen; Jarnthong, Methakarn; Kaesaman, Azizon; Kiatkamjornwong, Suda|
|Publication:||Polymer Engineering and Science|
|Article Type:||Technical report|
|Date:||Feb 1, 2009|
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