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Thermoplastic vulcanizates based on maleated natural rubber/polypropylene blends: effect of blend ratios on rheological, mechanical, and morphological properties.

INTRODUCTION

Thermoplastic elastomers (TPEs) based on polyolefin rubber-thermoplastic compositions have developed along two distinctly different classes. One class consists of a simple blend and classically meets the definition of a thermoplastic elastomeric olefin (TPO). In the other class, the rubber phase is dynamically vulcanized, giving rise to thermoplastic vulcanizates (TPVs) or dynamic vulcanizates (DVs). The TPVs are characterized by finely dispersed, micron-sized, crosslinked rubber particles distributed in a thermoplastic matrix [1]. Most polyolefin TPEs are based on synthetic rubbers such as ethylene-propylene-diene monomer (EPDM), ethylene propylene rubber (EPR) and butadiene acrylonitrile rubber (NBR), or a modification of them. Interest in natural rubber (NR) and thermoplastic blends has increased recently because of abundance of NR and unique properties of the blends. These materials are known as thermoplastic natural rubbers (TPNRs). Not only raw NR has been used to prepare TPNRs, but the modified form of it has also been widely studied. Eoxidized natural rubber (ENR) is one of the most recognized modified forms currently used to prepare TPNRs by blending with thermoplastics such as poly(vinyl chloride) [2, 3] and poly(ethylene-co-acrylic acid) [4]. Graft copolymers of NR with PMMA [5, 6] have also been used to prepare TPNRs.

In this work, an attempt was made to prepare TPNRs based on maleated natural rubber and PP blends via the dynamic vulcanization process. The effect of the blend ratios on the rheological, mechanical, and morphological properties of the TPVs was investigated.

EXPERIMENTAL

Materials

The polypropylene used as a thermoplastic blend component was an injection-molding grade MD with an MFI of 12 g/10 min at 230[degrees]C and a specific gravity of 0.91 g/[cm.sup.3] and was manufactured by the Thai Polypropylene, Rayong, Thailand. The natural rubber used was air-dried sheet (ADS) manufactured by a local factory operated by Khun Pan Tae Farmer Co-operation (Phattalung, Thailand). The maleic anhydride used to prepare the graft copolymer of maleic anhydride and natural rubber or maleated natural rubber (MNR) was manufactured by Riedel-de Haen, Seelze, Germany. The toluene used as a solvent was manufactured by Lab Scan, Ireland. The zinc oxide used as an activator was manufactured by Global Chemical, Samutprakarn, Thailand. The stearic acid used as an activator was manufactured by Imperial Chemical, Pathumthani, Thailand. The sulfur used as a vulcanizing agent was manufactured by Ajax Chemical, Samutprakarn, Thailand. The N-tert-butyl-2-benzothiazolesulphenamide (Santocure TBBS), used as an accelerator, was manufactured by Flexsys. The polyphenolic additive, Wingstay[R] L, used as an antioxidant was manufactured by Eliokem (OH).

Preparation of Maleated Natural Rubbers

A graft copolymerization of maleic anhydride (MA) onto natural rubber (NR) molecules was prepared in a molten state using a Brabender Plasticorder PLE 331 (Duisberg, Germany) at 180[degrees]C. In our previous work, more grafted MA on the NR molecules and lower gel contents was observed for reaction systems without peroxide [7]. Therefore, only shearing action was used to generate active sites for the grafting reaction. The graft copolymers of MA onto NR molecules or MNRs were prepared by blending ADS and MA in an internal mixer, Brabender Plasticorder PLE 331. The ADS and MA were first dried in a vacuum at 40[degrees]C for 24 h. The internal mixer was then used to masticate ADS at 145[degrees]C with a rotor speed of 60 rpm for 2 min. A predetermined quantity of MA was then incorporated into the internal mixer. The mixing continued for 8 min. The blend product was then sheeted out and cut into small pieces. The crude products were purified by extracting with toluene. FT-IR spectra were recorded on an Omnic ESP Magna-IR 560 Spectrometer at a resolution of 2 [cm.sup.-1], with the spectral range of 4000-400 [cm.sup.-1]. In this work, a level of MA at 8 part per hundred (phr) was selected to prepare the MNR due to higher grafting efficiency and low gel content [8].

Influence of Mixing Technique on Properties of the TPVs Based on MNR/PP Blends

MNR prepared using a level of MA at 8 phr in the grafting reaction was selected to prepare the TPVs based on MNR/PP blends. Two types of mixing techniques were studied. One was to mix all chemical ingredients (i.e., as shown in Table 1) using an internal mixer Brabender Plasticorder PLE 331. Mixing was performed at 180[degrees]C and a rotor speed of 60 rpm with a fill factor of 0.85 using a mixing schedule as shown in Table 2. The other technique was to compound MNR using a two-roll mill at room temperature before blending with PP in the internal mixer. The chemical ingredients used were the same as shown in Table 1 with exclusion of the PP. The mixing was performed on a two-roll mill, at a mixing schedule as shown in Table 3. The MNR compounds were later blended with PP in a Brabender Plasticorder at 180[degrees]C, a rotor speed of 60 rpm and a fill factor of 0.85. The blend products were later cut into small pieces with a Bosco plastic grinder (Samutprakarn, Thailand). Mechanical properties in terms of tensile strength and ultimate elongation were later characterized to determine the most suitable mixing method.

Preparation of TPVs Based on MNR/PP Blends

The mixing method using a two-roll mill gave TPVs with better mechanical properties. Therefore, this mixing method was used throughout the work. Thermoplastic vulcanizates of 60/40 MNR/PP blends were later prepared via dynamic vulcanization during melt mixing, using a Brabender Plasticorder at 180[degrees]C. PP was first preheated for 6 min in the mixing chamber without rotation. It was then melted for 2 min at a rotor speed of 60 rpm. The MNR compound was added and mixing was continued until we obtained a plateau mixing torque. The blend products were finally cut into small pieces with a Bosco plastic grinder. Various blend ratios of MNR/PP (i.e., 20/80, 40/60, 50/50, 60/40, and 80/20) were studied.

Rheological Characterization

A Rosand single bore capillary rheometer (model RH7, Rosand Precision, Gloucestershire, UK) was used to characterize shear flow properties in terms of the relationship between apparent shear stress against apparent shear rate (i.e., flow curves) and apparent shear viscosity against apparent shear rate. The test was carried out at a wide range of shear rates (50-1600 [s.sup.-1]) at a test temperature of 200[degrees]C. The dimensions of the capillary die used were 2-mm diameter, 32-mm length, and 90[degrees] entry angle with an aspect ratio (L/D) of 16/1. The material was first preheated in a rheometer's barrel for 5 min under a pressure of ~3-5 MPa to get a compact mass. The excess molten material was then automatically purged until no bubbles were observed. The test was then carried out at a set of shear rates as programmed in a microprocessor. During the test, the pressure drop across the capillary channel and melt temperature was captured via a data acquisition system. The apparent values of shear stress, shear rate, and shear viscosity were calculated using the derivation of the Poiseuille law for capillary flow [9]. The power law equation was also applied to the relationship between apparent shear stress and shear rate (i.e., flow curves) for MNR/PP blends, using various types and levels of compatibilizers as follows [10]:

[tau] = K([dot.[gamma]])[.sup.n] (1)

where n is the power law index or the flow behavior index, and K is the consistency or viscosity coefficient index.

Mechanical Testing

Tensile testing of the samples was performed at (25 [+ or -] 2)[degrees]C at a crosshead speed of 500 mm/min according to ASTM D412. The instrument used was a Hounsfield Tensometer, model H 10 KS manufactured by the Hounsld Test Equipment, UK. Dumbbell-shaped specimens, 2 mm thick, were prepared by thermoplastic injection molding machine with a capacity of 90 tons (clamping force), Welltec Machinery, Hong Kong. Hardness of the samples was also measured using indentation shore A, according to ASTM D 1415.

Morphological Studies

Morphological studies were carried out using a Leo scanning electron microscope, model VP 1450, manufactured by Leo, Cambridge, UK. Molded samples of the thermoplastic vulcanizates were cryogenically cracked in liquid nitrogen to avoid any possibility of phase deformation. The PP phase was extracted by immersing the fractured surface into hot xylene for 10 min. The samples were later dried in vacuum oven at 40[degrees]C for 3 h. The dried surfaces were then gold-coated and examined by scanning electron microscope.

RESULTS AND DISCUSSION

Mixing Method

There were two mixing methods used to prepare TPVs based on 60/40 MNR/PP blends. One was to blend all chemical ingredients in an internal mixer, Brabender Plasticorder (method I). The other was to compound MNR using a two-roll mill before blending with PP in the internal mixer (method II). Table 4 shows tensile strength and elongation at break of the TPVs prepared from mixing methods I and II. It was found that mixing method II gave higher tensile strength (9.3 vs. 7.9) and elongation at break (250 vs. 230). This may be attributed to better distribution of the chemicals in the MNR. This would cause faster dynamic vulcanization and hence phase separation of small dispersed vulcanized rubber domains in the PP matrix. This proves by a higher mixing torque of the method II, as shown in Fig. 1. It is seen that the mixing torque increased steadily after 4 min of rotation and reached a maximum value at a mixing time of ~6 min. However, the mixing torque of method I marginally increased at a mixing time of 10 min after incorporation of the last component of curative, sulphur (Table 2). The maximum torque of the method I was observed at 12.5 min with a lower magnitude than that of the method II. Also, stronger vulcanized rubber networks were formed. Therefore, mixing method II was selected to use for the preparation of TPVs based on MNR/PP blends throughout this work.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Mixing Torque

Figure 2 shows the mixing torque-time curves of the TPVs prepared from various blend ratios of MNR/PP. It can be seen that the mixing torque rose sharply when the rotors were started because of the resistance exerted on the rotors by the unmelted PP. The torque then decreased because the PP melted on account of mechanical shearing and heat transfer from the heater. This created the first group of peaks of the mixing torque-time curves. The height of the peak increased with an increase in polypropylene contents in the blends. This is because of higher volume of the material at the early stage of mixing. We observed a second group of peaks after incorporation of MNR compounds with the vulcanizing agents. The mixing torque sharply rose and created a second peak because of resistance of the unmelted material. The torque was thereafter reduced because of melting of the MNR compound. The height of the second peak increased with an increasing content of MNR compound in the blends. The mixing torque finally rose again due to dynamic vulcanization of the MNR phase. Plateau mixing torque-time curves were observed after a mixing time of ~400 s. The mixing was continued to ensure the complete vulcanization of the rubber phase. This was done until a total mixing time of ~600 s. It can be seen that the level of the final mixing torque increased with increasing levels of MNR compound. Therefore, the final mixing torque can be ordered as follows: MNR/PP = 80/20 > ENR/PP = 60/40 > ENR/PP = 50/50 > ENR/PP = 40/60 > ENR/PP = 20/80. This may be attributed to an influence of the higher viscosity of the pure MNR than that of the pure PP, as shown in Fig. 3. Also, in morphological studies, TPV products with two-phase morphologies were observed, as shown in Fig. 4. The PP phase was dissolved and this left vulcanized rubber particles adhering at the surfaces. Also, various sizes of holes, depending on the levels of PP, were observed. Therefore, higher PP content caused a more molten continuous phase. As a consequence, a decrease of the mixing torque was observed with an increase in PP content in the blends.

[FIGURE 3 OMITTED]

Rheological Properties

Plots of apparent shear stress versus shear rate (i.e., flow curves) of TPVs based on MNR/PP blends with various blend ratios are shown in Fig. 5. It can be seen that at a given shear rate, the shear stress of the pure PP exhibited the lowest values. The apparent shear stress of the TPVs increased with an increase in MNR contents. Trends of apparent shear stress corresponded to the trends of final mixing torques (Fig. 2) and apparent shear viscosity, as shown in Fig. 6. Therefore, trends of a final mixing torque, apparent shear stress, and apparent shear viscosity can be ranked as follows: MNR/PP = 80/20 > MNR/PP = 60/40 > MNR/PP = 50/50 > MNR/PP = 40/60 > MNR/PP = 20/80 > pure PP. This is attributed to lower flow resistance for the TPVs with higher content of PP. As SEM micrographs shown in Fig. 4, PP was forced into a continuous phase or matrix during the mixing operation even with a very low content of PP such as 20 wt%. Only the PP phase was melted during rheological test. Therefore, higher levels of PP facilitate the melt flow through a capillary channel.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

A power law model was applied to the rheological data of TPVs based on MNR/PP blends with various blend ratios. In Fig. 5, straight lines of the flow curves (on plots of shear rate versus shear stress) in logarithmic plots for all sets of TPVs were observed. The results correspond to the power law equation proposed by Ostwald in Eq. 1 [10]. That is, in the linear relation in the log-log scale of shear stress and shear rate, we can get the slope (n) and intercept (K). Plots of the n and K values against the levels of MNR in the MNR/PP blends are shown in Fig. 7. We found that, the power law indices (n) were less than 1, which indicated pseudoplastic (shear-thinning) behavior of the melts. Furthermore, the n value decreased with an increase in concentration of MNR. This indicates a greater pseudoplasticity in the flow for the melts of MNR/PP blends. The value of n < 0.5 also reflects a deviation of the flow profiles from uniform parabolic patterns (i.e., n = 1 for Newtonian flow) to plug-like flow profiles [11]. The pure MNR exhibited very low n value (i.e., n = 0.17). Therefore, the high shear-thinning fluid flowed through the capillary almost as a plug, moving at a uniform speed as the melt was sliding down against the channel wall. The pure PP exhibited the highest power law index at 0.54. Therefore, the flow patterns display more parabolic-like profiles. In the TPVs, n values lined up between the indices of pure PP and pure MNR. That is, they become lower with an increase in the amount of MNR in the blends. The consistency index, K, is a Newtonian viscosity if n = 1. By definition, the K value is related to the zero-shear viscosity (i.e., shear viscosity at shear rate of zero) of the flowing TPVs. In Fig. 7, it can be seen that the consistency index increased with increasing content of MNR in the blends. This corresponded to the trend of apparent shear viscosity at a given shear rate (Fig. 6).

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Mechanical Properties

Effects of various blend ratios on the mechanical properties (i.e. tensile strength, elongation at break and hardness) of thermoplastic vulcanizates prepared from MNR/PP blends are shown in Figs. 8, 9, and 10, respectively. It can be seen that the tensile strength and hardness decreased with an increasing level of the MNR while the elongation at break showed the contrary. The increasing trend of tensile strength and hardness is attributed to an increasing content of PP, which provides strength and hardness properties to the TPVs. Increasing levels of the MNR caused an increasing trend of the elongation at break. This is attributed to an increasing in the elastomeric component in the TPVs. Also, SEM micrographs as shown in Fig. 4, the sizes of vulcanized rubber domains dispersed in the PP phase decreased with decreasing level of the PP component. This caused an increase in the elastomeric nature of the TPVs. As a consequence, higher extensibility and recovery after prolonged extension was expected. This was proved by evaluation of a permanent set in terms of tension set, as shown in Fig. 11. The tension set was determined by extending the TPV specimen at 100% elongation for 10 min. Dimension of the specimen was measured and compared between before and after elongation. It can be seen that the tension set decreased with an increasing level of MNR in the blends. This indicates an increasing trend of elastomeric properties of the TPVs. Furthermore, tension set for all set of TPVs was lower than 36%. This indicates that the TPVs prepared in this work can be used for industrial applications.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

CONCLUSIONS

MNR was prepared and used to formulate TPVs based on various MNR/PP blends. Influence of the mixing method on mechanical properties was first studied. It was found that tensile strength and elongation at break of the TPVs based on 60/40 MNR/PP blend with mixing method II (i.e. compound the MNR using a two-roll mill before blending with PP in the internal mixer) exhibited better mechanical properties. We therefore used this mixing method to prepare the TPVs throughout the work. We also found that the final mixing torque, apparent shear stress, and apparent shear viscosity of the TPVs increased with increasing levels of MNR in the blends. This may be attributed to higher shear viscosity of the pure MNR than that of the pure PP. Also, in SEM micrographs, the TPVs display two-phase morphologies. Therefore, higher PP caused a more molten continuous phase of the flow during mixing and rheological characterization. Power law index (n) of the MNR was the lowest (i.e., 0.17) while the PP exhibited the highest n value (i.e., 0.54). The power law indices of TPVs lined up between the n values of MNR and PP. They decreased with increasing levels of MNR in the blends. This indicates an increasing pseudoplasticity of the TPVs' melt flow with increasing MNR content. The consistency index of the TPVs showed the same trend as the apparent shear viscosity. The tensile strength and hardness of the TPVs increased with increasing level of PP in the blend while the elongation at break decreased. Furthermore, the elastomeric properties in terms of tension set decreased with increasing levels of MNR in the blends. This indicates higher elastomeric properties of the TPVs. This may be attributed to decreasing trends for sizes of vulcanized rubber particles dispersed in the PP matrix with increasing concentration of the MNR.

ACKNOWLEDGMENTS

A special scholarship granted by Prince of Songkla University, Thailand to one of us (Ms Sitisaiyidah Saiwari) is gratefully acknowledged.

REFERENCES

1. A.Y. Coran, B. Das, and R. Patel, U.S. Patent 4,130,535 (1978).

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8. C. Saivaree, "Thermoplastic Elastomer based on MNR and PP," M.S. Thesis, Prince of Songkla University, Pattani, Thailand (2005).

9. F.N. Cogswell, Polymer Melt Rheology: A Guide for Industrial Practice, Woodhead, Cambridge, 24 (1981).

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Charoen Nakason, Sitisaiyidah Saiwari, Azizon Kaesaman

Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani 94000, Thailand

Correspondence to: C. Nakason; e-mail: ncharoen@bunga.pn.psu.ac.th

Contract grant sponsor: National Metal and Materials Technology Center, NSTDA; contract grant number: MT-B-46-POL-18-199-G.
TABLE 1. Compounding formulation.

Ingredient Quantity (phr)

MNR 60
PP 40
ZnO 3.6
Stearic acid 0.3
Wingstay L 0.6
TBBS 0.6
Sulphur 2.1

TABLE 2. Mixing schedule for preparation of TPVs in an internal mixer.

Activity Mixing time (min)

Warming of PP 6
Melting of PP 2
Incorporation of MNR 2
Stearic acid 1
ZnO 1
Wingstay L 1
TBBS (a) 2
Sulphur Until plateau mixing torque

(a) N-tert-butyl-2-benzothiazolesulphenamide.

TABLE 3. Mixing schedule of rubber compounding on a two-roll mill.

Activity Mixing time (min)

Rubber mastication 5
Incorporation of stearic acid 1
ZnO 1
Wingstay L 1
TBBS (a) 2
Sulphur 2
Compound finishing 3

(a) N-tert-butyl-2-benzothiazolesulphenamide.

TABLE 4. Tensile strength and elongation at break of TPVs based on 60/40
MNR/PP using two different mixing methods.

Mixing Tensile strength Elongation
methods (MPa) at break (%)

Method I 7.9 230
Method II 9.3 250
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Author:Nakason, Charoen; Saiwari, Sitisaiyidah; Kaesaman, Azizon
Publication:Polymer Engineering and Science
Geographic Code:9THAI
Date:May 1, 2006
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