Comparative Study on Polyamide 6 Toughnessusing Multiple Melt-Kneading Techniques

Akira Ishigami, Takuya Konno, Takashi Kurose, Shotaro Nishitsuji, Masaru Ishikawa and Hiroshi Ito*

Graduate School of Organic Materials Science, Yamagata University, Japan

Submission: July 6, 2018; Published: July 20, 2018

*Corresponding author: Hiroshi Ito, Graduate School of Organic Materials Science, Yamagata University, Japan, Tel/Fax: +81-238-26-3081;Email: ihiroshi@yz.yamagata-u.ac.jp

How to cite this article: Akira I, Takuya K, Takashi K, Shotaro N, Masaru I, Hiroshi I. Comparative Study on Polyamide 6 Toughness using Multiple Melt-Kneading Techniques. Academ J Polym Sci. 2018; 1(3): 555563. DOI: 10.19080/AJOP.2018.01.555563

Abstract

In the present work, we tried to toughen polyamide 6 (PA6) by blending a small amount of low molecular-weight polyethylene (LMWPE). We obtained the PA6/LMWPE blends with different morphologies using multiple melt-kneading methods such as the uniaxial melt-kneading, the eight-axial screw melt-kneading, and the high shearing method. As a result, it was elucidated that the LMWPE was homogeneously and finely dispersed by using the eight-axial melt-kneading method. On the other hand, we found that for high shearing method the LMWPE was finely dispersed to a degree close to the compatible system. The result of the three-point bending test showed that the PA6 was brittle fractured, but it changed to ductile fracture when a small amount of LMWPE was blended. Also, the fracture displacement was extensively improved. The results of the comparison by the melt-kneading methods revealed that the toughness of the PA6 was improved even by adding a small amount of LMWPE (about 2%) when they were blended using the eight-axial screw melt-kneading machine. The cross-sectional observation in the bending test showed that voids were produced from the LMWPE. We elucidated that the production of voids was induced with low stress by adding fragile LMWPE, resulting in the improvement of the bending toughness. Moreover, we found that the LMWPE needs to be finely dispersed with an appropriate particle size in order to exhibit ductile properties.

Keywords:Polymer blend; Higher order structure; Kneading extruder; High-shear rotational processing; High speed impact test

Abbrevations: PA6: Polyamide 6; LMWPE: Low Molecular-Weight Polyethylene; TM: Melting Point; TG: Glass Transition Temperature; MFR: Melt Flow Rate; MW: Molecular Weight; HSP: High-Shear Process Kneader; OSE : Octa Screw Kneading Extruder

Introduction

In recent years, due to the rise in environmental consciousness, weight reduction aimed at reducing the fuel consumption of automobiles is proceeding. Under these circumstances, the proportion of using light polymer in automobiles is increasing. However, extremely high mechanical properties are required for materials loaded in automobiles. For that reason, the number of cases has been increasing where a single type of polymer material cannot meet requirements in automobiles. One of the methods to solve this problem is to use polymer blend, a mixture of different kinds of polymer materials. Up to now, the research and development on polymer blend method have been extensively conducted as a method that exhibits various properties and functions. As a result, many polymer alloy materials with various characteristics have been proposed [1,2]. In particular, for polymer materials showing brittle fracture behavior, it is tried to change them to be ductile by blending elastomer and gum. However, with these methods, the rigidity of the polymer materials is generally reduced while increasing ductility [3,4].

A crystalline polymer, polyamide 6 (PA6), is an engineering plastic that has extremely high mechanical property because of the strong molecular-molecular interaction of hydrogen bonds in amide group. Although PA6 has strong rigidity, it causes brittle fracture when imposed with a large displacement. Hence it is very important to change the property of PA6 to be ductile while maintaining its high rigidity [5]

In the present study, we tried to improve the toughness of PA6 by blending a small amount of low molecular-weight polyethylene (LMWPE). The purpose of the work is to clarify the relation between the inner structure and the mechanical property of the polymer blends with different morphologies produced by three different kinds of melt-kneading methods.

Experimental Part

Samples

PA6 (T802, Toyobo Co., Ltd.) was used as the matrix resin. The glass transition temperature (Tg) and the melting point (Tm) of PA6 were 50 °C and 218 °C, respectively. The melt flowrate (MFR) was 26g/10min at 230 °C. As a modifier of PA6,LMWPE (Hi wax 1105A, Mitsubishi Chemical Co.) was used. TheLMWPE was denatured by maleic anhydride. The Tg was 104°C, and the average molecular weight (Mw) of the LMWPE was1,900. The molded film of LMWPE is an extremely brittle andlow-strength material exhibiting a tensile strength of 1.4MPaand a breaking strain of 0.006 in a tensile test. The blend ratioof the samples were PA6/LMWPE=98:2 and 95:5 wt%, and thekneading temperature was 250 °C.

Melt-kneading methods

The blend ratio, the details of the melt-kneading machines,and the kneading conditions for the present samples are shownin Table 1. The melt-kneading machines used were Single screwextruder (SSE) (CER40, Hoshi Plastic Co., Ltd.), Octa screwkneading extruder (OSE) (Technobell Co., Ltd.), and High-shearprocess kneader (HSP) (Niigata Machine Techno CO., Ltd). TheOSE was used for low-shear kneading. Therefore, it is expectedthat we obtain the products with relatively large domain diameter.The OSE is a machine where all eight screws engage and rotate inthe same direction. Since the resin stays among eight screws inthe OSE, the long-time kneading is possible compared with theSSE. The HSP is a batch-type kneading machine and has a specialscrew with a small hole in the center (return hole). Hence, it ispossible to knead a certain amount of resin at arbitrary rotationspeed and time [6-9].

The samples after kneading were pelletized, and then moldedto three-point bending test pieces and flat plate test pieces usingthe injection molding machine.

SEM observation

The samples after kneading were frozen and fracturedin liquid nitrogen, and the fracture surface was observed by ascanning electron microscopy (SEM).

Mechanical properties and observation of plastic deformation region

Three-point bending tests were conducted for specimenswith notches. The Poisson contraction at the tip of the notch issuppressed due to the constraint of the distortion. Hence thepure uniaxial stretching test becomes possible. This test methodis known to be effective for the evaluation of toughness (ductileor brittle) for polymer blend materials [10]. As additionalexperiments, the bending test was stopped at the displacementjust before the fracture or at the maximum displacement (10mm),and the sample was fixed by embedding with epoxy resin.The plane of the sample perpendicular to the tip of the notchwas then scraped to 20μm thick thin film using a microtome.The void-producing region in the plane was observed with apolarization microscope.

Impact resistance evaluation

For evaluating the impact strength at the time of rapid deformation,a high-speed impact test (punching) was conducted. Thesample molded to a plane shape was punched by a test rod of10mm diameter, and the stress was measured. The speed of theimpact test was 10m･s-1.

Results and Discussion

Morphology observation

Figure 1 shows the cross-sectional SEM images for PA6/LMWPE kneaded by SSE (a) and OSE (b). For the SSE-kneadedsample, LMWPE aggregates with a maximum particle size of 3μmwere observed. The size of the aggregates was not homogeneous.For the OSE-kneaded sample, we confirmed the LMWPEaggregates with a particle size from 1μm to 2μm. The size of theaggregates was relatively homogeneous compared with that ofthe SSE-kneaded sample. For the HSP-kneaded sample, LMWPEaggregates were not observed. The reason would be that theLMWPE was extremely finely dispersed in this case.

Three point bending test and observation of plastic deformation region

Figure 2 displays the results of the three-point bending testfor pure PA6 and PA6/LMWPE blends produced by respectivekneading methods. Only OSE kneaded-samples were preparedwith LMWPE addition amount of 2 and 5wt%. For the purePA6, SSE-kneaded sample, and HPS-kneaded sample, they werefinally brittle fractured. In particular, for the HSP-kneadedblend, the fracture behavior was same as that of the pure PA6,thus the effect of the LMWPE blending could not be confirmed.On the other hand, the OSE-kneaded sample showed plasticdeformation, and it was not fractured even at the maximumdisplacement (10mm) resulting in the general yield. The OSEkneaded sample (LMWPE: 5wt%) showed better breakingelongation than the system in which LMWPE(added 2wt%),but the maximum bending strength greatly decreased. Thisresult seems to be due to excessive addition of brittle LMWPE. Figure 3 presents the polarization microscope image of theplastic deformation region produced at the tip of the notch inthe bending test. For the pure PA6 (a), voids were not producedeven at 3.2mm deformation, just before the fracture. Whilefor SSE-kneaded blend (b), the plastic deformation regionoriginating from the voids formation was confirmed for thesample just before the fracture (displacement: 6mm). In thiscase, the blending of LMWPE induced the void formation, andthe Poisson contraction among the voids became possible, whichled to the ductile behavior of the sample compared with the purePA6. However, the stress was locally concentrated because of theinhomogeneous size of the dispersed LMWPE. Therefore, it isconsidered that the sample finally resulted in the brittle fracture.

For the OSE-kneaded sample (c), we confirmed the plasticdeformation regions that are wider than those for the SSEkneadedone at the same displacement (6mm). Comparedwith the SSE-kneaded sample, the OSE-kneaded one exhibitedmorphology where the size of the LMWPE aggregates was smalland homogeneous. Therefore, the distance among the LMWPEaggregates was homogeneous, so the sample was homogeneouslydeformed due to the dispersion of the stress. As a result, it isconsidered that the sample resulted in the general yield withoutthe brittle fracture even at the maximum displacement.

For the HSP-kneaded sample (d), we observed a fine plasticdeformation region at the displacement (3.2mm), just beforethe fracture. By the high shearing kneading, the LMWPE showedthe dispersion condition that is close to the homogeneouscompatibility system. As a result, voids expansion did not proceedbecause the size of the produced voids was extremely small.Thus, the stress constraint was not relaxed because the Poissoncontraction was not sufficient. This would be the reason thatthe HSP-kneaded sample finally resulted in the brittle fracture.Based on the above results, we controlled fragile LMWPE toexhibit fine (about 1μm size) and homogeneous higher-orderstructure. By doing so, the void formation was induced, and thestress was relaxed. As a result, we succeeded in improving thetoughness of PA6. On the other hand, the HSP-knead blendedsample with excessively finely dispersed LMWPE did not have ahigher-order structure. We consider that the blending of LMWPEdid not lead to improve the toughness of PA6 in this case becausethe stress relaxation function did not work.

High-speed impact test

Figure 4 shows the load-displacement curve in the highspeedimpact test at 10m.s-1. At this speed, both the PA6 andthe HSP-kneaded samples were brittle fractured. The reasonis that in both samples the cracks were developed because thestress was not relaxed in the deformation at the high speed.On the other hand, the SSE- and OSE-kneaded samples showedductile fracture. For these samples, the LMWPE that is easy to befractured existed as domains with 1-3μm diameter. The stressrelaxation effect expressed even at high speed of 10m･s-1 becausethe LMWPE became the origin of voids. Based on these results,we succeeded in leading the SSE- and OSE-kneaded blends tothe ductile fracture. On the other hand, the effect of the stressrelaxation effect in the HSP-kneaded sample that has extremelyfine dispersed structure of the LMWPE was not sufficient. Weconsider this is the reason why the HSP-kneaded sample led tothe brittle fracture.

Conclusion

In the present work, we tried to toughen PA6 by blending asmall amount of LMWPE. We obtained the PA6/LMWPE blendswith different morphologies using multiple melt-kneadingmethods such as the uniaxial melt-kneading, the eight-axial screwmelt-kneading, and the high shearing method. As a result, it was possible to lead to ductile fracture while maintaining rigidity byadding a small amount of LMWPE to PA6. Kneading by OSE waseffective for finely dispersing LMWPE and equalizing dispersedparticle size, and the properties of the blended material werealso the most stable. This result seems to be because Poissoncontraction was possible even at high speed deformation, andPA6 could lead to ductile fracture, since LMWPE which breakseasily is the starting point of void formation. However, whenLMWPE was dispersed in a region close to the nano order andhomogeneous system, the effect of adding LMWPE did notappear. It has been found that it was important to form a higherorder structure having an island phase size larger than a certainlevel for toughening by polymer blending.

Acknowledgement

This research was partially funded by Impulsing ParadigmChange Through disruptive Technologies (ImPACT) Program ofCouncil for Science, Technology and Innovation (Cabinet Office,Government of Japan).

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