The effect of different mixing methods on morphology and mechanical properties of PP/EPDM/talc blend

Abstract

To overcome the low modulus and low maximum service temperature of polypropylene (PP) and ethylene-propylene-diene monomer (EPDM) blends, incorporation of talc into blend have been considered. In this study, the effect of various mixing methods of morphological behavior and mechanical properties of PP/EPDM/talc compound were investigated. Samples were prepared by different methods in internal mixer and co-rotating twin-screw extruder. It was found, the tensile properties were not affected by applying different mixing methods.The highest impact resistance was related to sample which had pre-blending on two roll mills and finally mixed in the internal mixer. Scanning electron microscopy (SEM) micrographs showed that the rubber dispersed morphology for all samples and the finest morphology was related to sample with the highest impact resistance. Because of pre-blending of EPDM/talc on two roll mills, this sample showed the best dispersion and distribution of talc.

Keywords

Polypropylene EPDM mixing SEM morphology

Introduction

The growing interest in using of thermoplastic elastomers based on polypropylene (PP) and ethylene-propylene-diene monomer (EPDM) in automotive, structure and cosmetic industries are mainly due to their advantages such as low density, high impact resistance, good chemical resistance and low cost. PP is one of the most favorable thermoplastic polymers that has many prominent properties, but the fairly poor hardness at low temperature limits its application fields.^1-4 Thermoplastic elastomers are normally used to harden PP.^5,6 EPDM is an effective material because of its high resistance to oxidation.^7,8 there is a large body publication which describes the mechanical properties, morphology and rheology of these blends.^9-17

However, some limitations in using of this blend are low modulus and low service temperature. To compensate these defects, incorporation of lignocellulosic filler, mineral filler and high-density polyethylene have been considered by researchers.^18-23 Liang et al.²⁴ investigated the effect of pretreatment and concentration of glass bead on the tensile properties of PP/EPDM/glass bead compounds and reported that the elastic modulus increased while yield stress and tensile fracture strength of the composite decreased. Siriwardena et al.²⁵ used the rice husk as a natural filler in PP/EPDM blend and studied the effect of mixing sequence (five methods) on the properties of final compound.The mechanical properties of such ternary composites strongly depend on their composition, characterization of the components, the phase morphology, and in particular relative dispersion of additive components. In the ternary composites containing elastomer and rigid fillers, two different types in phase microstructure may occur, where elastomer and filler particles are dispersed separately on the PP matrix or the rubber encapsulates fillers particles, resulting in a low modulus interlayer between matrix and filler.^26-32 Among the natural and mineral fillers, due to low cost, appropriate reinforcing efficiency and service temperature, talc is a good option for using in PP/EPDM compound. Oksuz et al.³³ studied the effect of concentration of talc on the mechanical, microstructural and thermal properties of PP/EPDM/talc. It was found that elastic modulus increased as the talc content increased up to 9%. Michel et al.³⁴ replaced the ethylene-octane copolymer with EPDM in PP/EPDM/talc and studied the performance of this compound. Using ethylene-octane copolymer in ternary composites provided a significant improvement in impact resistance. They also examine the effect of compounding conditions and composition on the properties of talc/ethylene-octane copolymer/PP compounds. They evaluated the effect of aforesaid parameters in terms of the mechanical properties, filler and copolymer dispersion and polymer matrix degradation during twin-screw processing. Pinto and Bonse³⁵ investigated the effect of paraffinic and plant-made plastic on the mechanical and thermal properties of sawdust. Inácio et al.³⁶ investigated the mechanical and thermal properties of bamboo fiber amplified talc filled PP/EPDM composites.

In this research, the manner and order of mixing the components of the materials for making alloys have been investigated. In the previous published articles, the study of mixing process and the order of mixing, which determines the proper method of mixing, has been neglected and less attention has been paid to it. Here, we emphasized the mixing process and the order of mixing. So, the results of this research can be used in the industrial scale works with the lowest cost and highest efficiency. Also, the various mixing sequences were applied on PP/EPDM/talc in co-rotating twin-screw extruder and internal mixer, then their effect were evaluated by studying their mechanical properties and microstructure of samples. Due to the fact that the purpose was to determine the appropriate sequence and manner to add materials to each other for that the results could be used by craftsmen, in this work, the expected results were obtained.

Experimental

Materials

In alloying processes, the parameters of temperature and revolutions per minute (rpm) are the main parameters that must be optimumed to obtain the optimal conditions for the production of materials. Optimal temperature and rpm based on previous work and experience has been applied to determine the optimal sequence and manner of adding materials.^35,37-39

PP purchased from Hysung Co. Korea with J 740 trade names (MFI = 25 g/10 min, 2.16 kg, 230°C) and EPDM from DSM Co. Netherland with keltan 2340A trade name (ML (1 + 4) at 125°C: 25, ethylene content: 53 weight%). The average particle size of talc was 3.96 microns (ET1).

Sample preparation

All samples were produced with 77/18/5: PP/EPDM/talc weight ratio. In general, in this research, each result value reported was an average of 5–10 measurements, where the maximum deviations from the average value were always less than 0.5%. A Haake Internal mixer (model HBISYS90, USA), single-screw extruder (L/D = 24, D = 19 mm), a laboratory twin-screw extruder (co-rotating-diameter of screw D = 20 mm, L/D = 40) Brabender Co. Model TSE 20 (Germany) and two roll mill (model polymix 200 L, England) were used for producing the sample with different mixing sequence addition listed below:

HPTE: This compound was prepared in Haake internal mixer. As the mixing chamber attained the set temperature, in zero time; PP was charged and allowed to soften for 4 min. The talc powder was then added and allowed to mix for 2.5 min followed by the adding of EPDM. Mixing was continued until a constant torque was recorded. The mixing temperature and rotational speed were 180°C and 80 rpm, respectively.

HETP: talc was added to the EPDM on a two roll mill (for 15 min). Then EPDM/talc poured in mixing chamber of Haake internal mixer and allowed to soften for 3 min and then PP added; mixing continued until a constant torque was obtained. The compound was prepared at 180°C and 80 rpm.

BETP: This compound was prepared in an extruder. Firstly, PP and talc powder were compounded in single extruder. Then PP/talc and EPDM premixed in turbo mixer at room temperature and then fed into the twin-screw extruder. The processing conditions of single and twin-screw extruder are reported in Table 1.

BPTE: Similar to sample HETP, initially EPDM and talc were mixed on a two roll mill and EPDM/talc and PP premixed in turbo mixer and then fed into twin-screw extruder with processing condition similar to before part.

Table 1.

Processing conditions in mixing machines.

Mixing machine	Zone	1	2	3	4	5	Die head	Screw speed (rpm)
Haake single-screw extruder	Temperature (C°)	170	180	190	—	—	195	200
Brabender Plastic order: twin-screw extruder	Temperature (C°)	170	180	190	195	200	200	150

The screw arrangement of twin-screw extruder is shown in Figure 1. The best combination of kneader and reversed blocks conducted the compound to proper dispersion and distribution of talc and EPDM particles in PP matrix. Arrangement of screw in a single-screw extruder was fully flighted without any mixing element.

Figure 1.

The screw profile of twin-screw extruder.

To prepare the sample specimens for various tests all compounds were pressed using Toyoseki, Japan into plaques at 200°C and 25 MPa.

Measurement and characterization

Tensile testing of the specimens was performed according to ASTM D-638 on an MTS (USA) material test machine, model 10/M with cross head speed 50 mm/min. ASTM method D-256 was followed in determining the Izod impact (Zwick: model 54-10) values. A Cambridge stereos can 360 scanning electron microscope (SEM) was used to investigate the cryogenically fractured surface morphology of the specimens. The elastomer particles of EPDM on the fracture surface were etched with n-heptane for 8 h.

Results and discussion

The time dependency of torque for two samples prepared by internal mixer is shown in Figure 2. For HPTE procedure, three peaks were observed, which reflected to the loading of PP, talc and EPDM respectively, while for HETP system there were only two peaks. As seen the torque becomes stable after 8 and 10 min for HEPT, HPTE respectively which indicates completion of mixing. The lower value for HETP may be attributed to pre-blending of EPDM and talc on two roll mill. The same value of the stabilization torque for HETP and HPTE (around 20 NM) indicates that this parameter has not been affected by various mixing sequences.

Figure 2.

Torque–time variation for the samples with different mixing sequense addition in Haake internal mixer.

The HETP system exhibited the highest peak torque when the PP was added to molten EPDM/talc. Since the PP is a hard material, this highest peak was unexpected. Another reason may be due to the charging of higher volume of material into mixing chamber within a very short period.

Figure 3 shows the torque-time variation for samples which obtained by twin-screw extruder. It is clear, there is no significant difference between torques which leads us to conclude that the pre-blending of PP/talc and EPDM/talc did not change the energy consumption of twin-screw extruder at the same output.

Figure 3.

Torque–time variation for the samples with different mixing sequence addition in a laboratory twin-screw extruder.

Since the purpose of the study in this paper was to investigate the order of composition of materials with each other, a fixed percentage of polypropylene, EPDM and talc, based on previous experimental work, was selected. By studying the articles, the optimal percentage of 77% polypropylene, 18% EPDM and 5% talc mineral powder, which were mostly used in industrial applications, were used. The percentage of materials was considered fixed so, the results of the order of composition of the materials to each other can be clearly examined. Rheological properties are usually performed by rheometers and accurate and reliable results are obtained. But sometimes, especially for industrial applications, it is necessary to measure these properties online so that the formulas can be finalized more quickly. Online measurements of torque changes versus time during mixing make it possible to obtain the mixing behavior of the material, which somehow shows the trend of rheological changes. In polymer alloy systems, the morphology of the melt, and consequently, the rheological properties of these systems, is highly dependent on the process conditions, so the results obtained from different rheometers may not be generalizable to the actual process conditions. In systems of polymer alloys with fillers, the flow behavior depends on the amount of additives dispersed in the polymer phase, so measuring the flow behavior during mixing is important and provides reliable rheological properties. Therefore, in order to obtain a diagram of molten material flow in torque rheometers, the amount of torque and rotational speed must be calibrated with stress and shear speed, and this does not seem easy due to the complex geometry of the mixing blades. On the other hand, a rheometer cannot be tested at very high shear rates. Therefore, a dark spot remains in the flow chart. Due to the high limitations in the devices for measuring rheological properties, as well as having a percentage composition of materials in the prepared samples, only the order of the composition of the material has changed and the results of these changes are important. Taken by drawing torque versus time curves in Figures 2 and 3, the mixing process of the samples was examined.

Tensile strength, elastic modulus, strains at break and Izod impact strength of samples are illustrated in Figures 4 and 5 and Table 2. At a glance at these figures it is obvious that with the increase of EPDM to PP matrix, tensile strength and elastic modulus properties decreased while strain at break and impact resistance and increased. This is because of the toughening effect of rubber particles in the compound. The highest impact resistance was related to HETP sample. It seems the incorporation of talc into EPDM by two roll mill leads to the compound with higher modulus than that of virgin EPDM which causes grinding and dispersing become easier. For this sample, the finer rubber dispersed phase compared to other samples is confirmed by SEM micrograph (Figure 6).

Figure 4.

Young’s modulus and tensile strength of various samples.

Figure 5.

Strain at break and Izod impact strength of various samples.

Figure 6.

SEM micrographs of samples.

Table 2.

Mechanical and impact properties of the samples.

Sample	Strain at break (%)	Tensile strength (MPa)	Young’s modulus (MPa)	Izod impact strength (kJ/m²)
PP	9.5	23.8	827.9	5.7
HPTE	38	13.2	694.0	6.9
HETP	56	14.1	674.7	8.2
BPTE	37	13.7	691.0	7.1
BETP	42	14.0	701.3	7.9

Results showed that the tensile properties were not affected by the mixing method significantly. As all the samples obtained with various mixing method showed rubber dispersed phase morphology, therefore the differences between tensile properties were not considerable. Another interesting point is that the two blend prepared by the higher residence time of the PP in the mixing apparatuses (HETP, BPTE) showed the lowest elongation at break. It may be related to possible thermal degradation of PP. However the obtained dispersed morphologies were different and the finest was related to HETP.

This is because of pre-milling of EPDM/talc on the two roll mill. This morphology caused the highest impact resistance and lower elastic modulus among the samples. On the other hand, the better distribution of talc was also observed in this sample and BETP system which means the filler particles coated with EPDM facilitates the better dispersion and distribution of talc. In the other words molten PP restricts the talc dispersion. According to various articles,^40,41 talc particles act as nucleating agents in the compound and if the talc particles are dispersed and distributed properly, it will increase nucleation and thus increase crystallinity. In the BETP sample, homogeneous crystallinity appears with more nuclei and smaller size, resulting in higher impact resistance.

As can be seen in Figure 6 the holes represent the EPDM particle size that was between 0.1 and 4 µm with an average particle size 0.9, 1.3, 2 and 2.6 for HETP, HPTE, BETP and BPTE respectively. By considering the mechanical properties and morphological behavior of samples, it seems that there is no significant difference between efficiency of an internal mixer and twin-screw extruder with appropriate mixing elements. In other words, the different mixing condition which applied to the samples by internal mixer and twin-screw extruder (various shear rates) approximately leads to the same morphology, while the different sequence time and pre-blending of ingredients of compound could change the morphology.

Conclusions

In this study, the effect of the various mixing apparatus and time sequence on PP/EPDM/talc was evaluated by mechanical properties and morphology behavior. It was found that the tensile properties were not affected by different mixing methods. Some results are listed below:

The final stabilization torque for two mixing systems in internal mixer was the same which means this parameter was not affected by different mixing system. In twin-screw extruder mixing system, there was no significant difference between torques at the same output. The variation of tensile properties such as tensile strength and elongation at break and elastic modulus were not considerable which indicates these properties were not affected by applied different mixing method. The high impact resistance was related to HETP due to the finest dispersed morphology of this sample. The systems containing pre-blending of EPDM/talc step on two roll mill showed the better dispersion and distribution of talc in compounds which means covering of filler by EPDM facilitates of filler mixing.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Hamid Yazdani

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