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montmorillonite nanocomposites — Prepared by the twin-screw extruder mixed technique
International Communications in Heat and Mass Transfer 38 (2011) 37–43
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International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Mechanical properties of polyamide-6/montmorillonite nanocomposites — Prepared by the twin-screw extruder mixed technique☆ Sung-Po Liu a,⁎, Shyh-Shin Hwang a, Jui-Ming Yeh b, Chi-Chang Hung a a b
Department of Mechanical Engineering, Ching Yun University, Jung-Li 32097, Taiwan, ROC Department of Chemistry and Center for Nanotechnology at CYCU, Chung-Yuan Christian University, Chung-Li, Taiwan 32023, Taiwan, ROC
a r t i c l e
i n f o
Available online 18 November 2010 Keywords: Polyamide-6 Montmorillonite Nanocomposites Twin-screw extruder Mechanical property
a b s t r a c t In this paper, we present the effects of incorporated montmorillonite (MMT) on a surface and the bulk mechanical properties of as-synthesized PA6/MMT composites that are prepared using the twin-screw extruder mixed technique. The as-prepared polymer–clay nanocomposite (PCN) materials in the form of a pellet subsequently characterized using the powder X-ray diffraction (XRD) and the transmission electron microscopy (TEM). In this experiment, the surface mechanical property studies (i.e., wear resistance and hardness) show that the integration of MMTs exhibited a distinct increase on shore hardness was up to 5 wt.% of MMTs loaded in composites. Moreover, the enhancement of wear resistance of as-prepared composites, compared to pure PA6, can be further identified by the Scanning Electron Microscopy (SEM) observation of the surface morphology after testing. On the other hand, for the bulk mechanical property studies (i.e., tensile strength and flexural strength), we found that the composites containing 3 wt.% of MMTs in the PA6 matrix exhibited the best performance in tensile strength and flexural strength. It means that this composition of MMTs exhibits good compatibility with the PA6 matrix. Generally, PCN materials show an obvious enhancement of mechanical properties of neat polymer by an incorporated low loading of organophilic-clay platelets into a PA6 matrix used for the evaluation of the mechanical properties of the as-prepared samples. Furthermore, it was found that at higher MMT loading (e.g. 5 wt.%), MMTs were to be aggregated in the polymer matrix, as observed in TEM. Also, the result leads to an obvious decrease in tensile and flexural strength tests. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Recently, layered materials such as smectite clay (e.g., montmorillonite, MMT) have attracted intense research interests for the preparation of polymer–clay nanocomposite (PCN) materials. PCN materials usually demonstrate unique properties superior to traditional composites and conventional materials. In general, they combine both the characteristics of inorganic nanofillers and organic polymers at the molecular level. Currently, the PCN material is found to be a promising system due to the fact that the clay possesses a high aspect ratio and a platy morphology. It can be employed to boost the physical properties (e.g., thermal stability [1], fire retardant [2], gas barrier [3], and corrosion protection [4–15]) of bulk polymers, and mechanical properties are a particularly significant issue to study application and development for PCN materials. Kim and White [16] reported a variety of organic modified MMTs to understand the contribution of the organophilicity of organoclay on the formation of the polymer/clay nanocomposite. ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (S.-P. Liu). 0735-1933/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2010.10.003
Nylon 6 is a polymer developed by Paul Schlack at IG Farben to reproduce the properties of Nylon 6,6 without violating the patent of its production. Unlike most other nylons, Nylon 6 is not a condensation polymer and instead is formed by a ring-opening polymerization. Nylon 6 begins as pure caprolactam. As caprolactam has 6 carbon atoms, it got the name Nylon 6. When caprolactam is heated at about 533 K in an inert atmosphere of nitrogen for about 4– 5 hours, the ring breaks and undergoes polymerization. Then the molten mass is passed through spinnerets to form fibres of Nylon 6. Nylon 6 fibres are tough, possessing high tensile strength, as well as elasticity and lustre. They are wrinkle-proof and highly resistant to abrasion and chemicals such as acids and alkalis. The fibres can absorb up to 2.4% of water, although this lowers tensile strength. Nylon 6 is used as thread in bristles for toothbrushes, surgical sutures, and strings for acoustic and classical musical instruments, including guitars, violins, violas, and cellos. It is also used in the manufacture of a large variety of threads, ropes, filaments, nets, and tire cords, as well as hosiery and knitted garments. It can also be used in gun frames, such as those used by Glock, which are made with a composite of Nylon 6 and other polymers. It has the potential to be used as a technical nutrient [17]. Several attempts to prepare PA6–clay nanocomposites have been reported. For example, Garcia et al. [18] reported that the composites of
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nanometer-sized silica (SiO2) filler incorporated in Nylon 6 polymer were prepared by compression molding. Their friction and wear properties were investigated on a pin on disk tribometer by running a flat pin of steel against a composite disc. Yu et al. [19] reported that the tribological behaviour of Nylon 66, Nylon 66/organoclay nanocomposites and Nylon 66/ (SEBS-g-MA+ organoclay) nanocomposites was studied by means of a pin-on-disk apparatus. Recently, several attempts have been reported to study the mechanical properties of PA6–clay nanocomposites. For example, several research groups have reported the tensile and flexural strength of PA6–clay nanocomposite materials [20,21]. On the other hand, thermal, impact properties and morphologies of PA6–clay nanocomposites have also been reported [22–24]. However, wear resistance of PA6–clay nanocomposites has seldom been mentioned. Therefore, in this paper we present the first evaluation of wear resistance of PA6–clay nanocomposites and the first to make a comparative study on the mechanical properties of the effect of organoclay (MMTs) on shore hardness, tensile strength, and flexural strength of PA6–clay nanocomposites. 2. Experimental sections 2.1. Chemicals and instrumentations Commercial PA6 (Model No. A2700, melting point = 221 °C, specific gravity = 1.13 g/cm3) were purchased as pellets from the Nanopolymer Composites Corporation in Taiwan. The montmorillonite clay (Model No. I.34TCN) was purchased from Nanocor Company in USA. The surfactant of clay I.34TCN is methyl dihyroxyethyl hydrogenated tallow ammonium. A wide-angle X-ray diffraction study of the samples was performed on a Rigaku D/MAX-3C OD-2988N X-ray diffractometer with a copper target and Ni filter at a scanning rate of 4°/ min. The samples for the transmission electron microscopy (TEM) study were first prepared by putting the membrane of PCN (PA6–clay nanocomposite) materials into low-viscosity embedding media epoxy resin capsules with four ingredients (ERL4206 5.0 g, DER736 3.0 g, NSA 13.0 g, and DMAE 0.15 g) and by curing the epoxy resin at 100 ° C for 24 h in a vacuum oven. Then the cured epoxy resin containing PCN materials was microtomed with a Reichert-Jumg Ultracut-E into 60–90 nm slices. Subsequently, one layer of carbon about 10 nm was deposited on these silices on mesh 100 copper nets for TEM observations on a JEOL-200FX, with an acceleration voltage of 120 kV. Centrifugal Ball Mill (Retsch S100) was used to mill the size of organophilic clay. A Scanning Electron Microscopy (SEM) with the model Hitachi S-4100 FE-SEM evaluates the surface morphology of as-prepared composite materials. The twin-screw extruder machine and hot press machine manufactured by Sun Sing Scientific Company (Hong Kong, Model no. SHJ20, co-rotating type, non-intermeshing, max screw speed is 120 rpm, screw diameter is 20 mm, L/D ratio is 40, test temperature is 20 °C to 400 °C, Chamber maximum volume is 200 g) and Long-Chang Company (Taiwan, Model no. FC-60 TON) were used to make the asprepared sample in the form of its standard shape. The tensile and flexural test of PA6–clay specimens were carried out through Universal Testing Instruments manufactured by Hung-Ta Company (Taiwan, Model no. HT-9102). The shore hardness tests were run on a Shore Hardness Test Machine by EXCELLENCE Company (Japan, Model D) for high and low hardness samples, respectively, under an indentation time of 10 seconds at 25 °C. The wearing resistance tests were run on a Wearing Test Machine by TABER Company (Model 5130 ABRASER). 2.2. Preparation of PA6–clay composite materials through twin-screw mixed extruder technique Before blending, PA6 and clay were dried at 85 °C under vacuum oven for 2 h. PA6–clay composites were prepared in a twin-screw
extrude machine at a screw speed of 85 rpm, melt temperature at 210 °C. The melt-mixing procedure of PA6–clay composites was performed by blending samples repeatedly, at least twice, to form products in the shape of pellets, with better combinations. The asprepared PA6–clay composite pellets were subsequently mixed by a twin-screw extrude machine using the twin-screw extrude mixing method to obtain nanocomposites with standard shaped specimens for the following investigations. Melt mixing is a procedure in which the mixture's consistency is an important step in the polymer material processing. Using mechanical force to blend is the most convenient and practical method. It can also change the quality of high polymers through melting technology effectively. It is one of the most frequently used methods in the industry. 2.3. Hot press of PA6–clay composite materials According to the test standard of the experiment project, we designed the mold for the hot-pressing requirement, and then used the hot-press machine to undergo our tests. Place the blending PA6 and clay in the mold, put the mold into a hot-pressing machine after preheating it for 4 min under 240 °C. After the upper and lower templates of the hot-pressing machine are closed for 1 min continuing at 1000 kg/cm2of pressure then pressurize it at 2000 kg/cm2 for 1 min. Take out the mold after the hot press is finished. Last, wait for the mold to cool down, using water cooling to 80 °C then open the mold to take out the specimen. 2.4. Test of surface mechanical properties 2.4.1. Wear-resistance test Wear-resistance test is one of the most convenient and basic properties of the material in mechanical property testing. When one object is contacting with other objects running for a long time, it will cause wearing. So, it is necessary to perform a long period wearing– tearing test. This experiment utilizes wearing test machines to determine the roll wearing and slip wearing characteristic of material, and then show this material surface decreasing ability on wear resisting. The experiment is accomplished according to the ASTM D4060 standard. 2.4.2. Shore hardness test We used the Japan Shore Hardness Testing Machine as the depthdetect instrument that our nano hardness test was to be carried on, the shore hardness experimental method of the hardness being invented by Americans Albert F Shore in 1906, has already been used extensively as the dynamic loading hardness. The Shore Hardness Testing Machine type D calibrated a scale instruction type that improves, as to large-scale specimens or large round type of diameter specimens for fittings that can be fixed and increase its scope of application and characteristic. In addition, it is a quick operation and can reach to 1000 or more times per hour. The machine is easy and simple to operate; even non-technical staff can operate it smoothly. It is also hard to damage the specimen, on the other hand, using other machines after the indenter-hardness test; the specimen needs to be repaired before the indenter can be removed. Furthermore, its assembly cost is cheaper and it can also apply to thin specimens [25]. Through the above-mentioned assessment, this experiment will adopt the type D Shore Hardness Testing Machine. The experiment is completed according to the ASTM D790 standard. 2.5. Test of bulk mechanical properties 2.5.1. Tensile strength test In mechanical properties, tensile strength is one of the most frequently used comparative standards. The definition of tensile
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strength is one that exerts tensile stress the most and tries to extract the test piece before cracking (or plastic deformation). This experiment, according to the ASTM D638 standard, shapes the material into a dumbbell-shaped specimen first, and then carries out the tensile test. Examine at least five specimens in each group, calculate the average, and record the greatest strength of tensile tests. 2.5.2. Flexural strength test Flexural test can measure the deformation energy or fracture strength that bears the bending moment for determining materials. In general, there are three kinds of measurement methods of flexural tests: cantilever, three points, and four points. It is common to use the three-point method to measure in laboratories. Its method is to put the test specimen on two fixed displacements, apply force at the central point with a certain pressing speed, until the test specimen fractures. It can utilize the flexural test to determine Young's modulus. Also, the experiment is done according to the ASTM D790 standard. 3. Results and discussion 3.1. Characterization Fig. 1 showed the wide-angle powder XRD patterns of raw MMT, and a series of PCN materials. For raw MMT, there is any diffraction peak in 2θ = 4.7° (d-spacing = 1.87 nm), as opposed to the diffraction peak of 2θ 2° (d-spacing 4.4 nm) for organoclay, indicating the possibility of having exfoliated silicate nanolayers of organoclay dispersed in the PA6 matrix. With the amount of organoclay increased to 5 wt.%, this implied that there was a small amount of organoclay that couldn't be exfoliated in the PA6 and existed in the form of a partial intercalated or unintercalated layer structure. In addition to the XRD investigation, the internal structure of polymer/clay nanocomposites was also examined using Transmission Electron Microscopy (TEM), which can directly visualize the expanded layering structure in the nanocomposites and the dispersion quality of the silicate layers in the polymer matrix. As shown in Fig. 2(a), we found that the 3 wt.% of MMT shows a good dispersion ability in the PA6 matrix, where the dark line represents clay platelets and the gray/white areas represented the PA6 matrix. Furthermore, 5 wt.% of MMT also shows a good dispersion capability in the polymer matrix. However, an overdose of MMT loading embedded in as-prepared composites may cause the observable aggregation of MMT clusters in the polymer matrix, as shown in Fig. 2(b). It is clear that lamellar nanocomposites has a mixed
Fig. 2. TEM images of the PA6–MMT composites of (a) 3 wt.% MMT (× 50 k) (b) 5 wt.% MMT (× 50 k).
Fig. 1. X-ray diffraction patterns of raw MMT, neat PA6 and PA6–MMT clay.
morphology with major intercalation and minor unintercalated dispersion in the PA6 matrix. The different dispersion morphology of MMT in the polymer matrix could lead to as-prepared composites revealing different mechanical properties. For the property studies, the mechanical property discussed can be categorized into two parts: surface mechanical property (e.g., wear resistance and shore hardness)
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Fig. 3. Effects of the clay content on wear loss of PA6–MMT composites.
and bulk mechanical property (e.g., tensile strength and flexural strength). 3.2. Surface mechanical property studies 3.2.1. Wear-resistance test Test specimens with the dimensions of 100 × 100 × 2.8 (mm) were made. After that, utilize wearing testing machine with a load of 1000 g, measuring wear mass-loss (the influence factor of the wear mass loss amount) by operating the wearing speed at 500 rpm and 1000 rpm, respectively, to probe the wear-resistance properties of asprepared composites. Fig. 3 showed the wearing mass-loss of as-prepared pure PA6 and PA6–MMT composites. For example, the amount of wear mass-loss operated at 500 rpm for as-prepared composites was found to slightly increase from the original of PA6 to PA6MMT3. It indicated that the wear mass-loss of as-prepared composites reduced slightly as the content of MMTs that existed in the composites increased at a low wearing speed of 500 rpm. Moreover, when operated at a higher wearing speed of 1000 rpm, the wear mass-loss amount of asprepared composites was found to reduce significantly. For example, the amount of wear mass-loss for composites changed from the original value of 75 mg (for PA6) to 46 mg (for PA6MMT3), as summarized in Table 1. This implied that the higher content of MMTs (e.g., 3 wt.%) in composites led to an obviously reduction in wear mass-loss amount at a higher wearing speed rate, reflecting that the incorporation of MMTs into PA6 might show better wear-resisting property of as-prepared samples. However, the increase in the wear mass-loss amount for PA6MMT5 may probably be resulted from the aggregation of MMTs in composites. Moreover, the wear resistance of as-prepared composites can be further evaluated by the visual observation of the as-prepared
composites after the wearing test. Fig. 4 showed the Scanning Electron Microscope (SEM) of pure PA6 (a), PA6MMT0.5 (b), PA6MMT1 (c), PA6MMT3 (d) and PA6MMT5 (e), respectively. It showed that the SEM observation of pure PA6 morphology revealed an uneven and rough surface, indicating that the pure PA6 (wear mass-loss amount of 75 mg) had hard surface structures as compared to the pure PS (wear mass-loss amount of 128.74 mg) [26,27]. After the wearing test, the material structures of pure PA6 can't be easily degraded and removed from the surface of as-prepared sample, as shown in Fig. 4(a). However, the surface morphology of PA6–MMTs composites was found to display a relatively smooth pattern as compared to the pure PA6, as illustrated in Fig. 4(b), (c) and (d). This implied that the incorporation of MMTs (except 5 wt.% in Fig. 4(e)) into PA6 might effectively enhance the wear resistance of PA6. The wear mass-loss amount increases at loading higher that 3 wt.%, with the 5 wt.% filled nanocomposite displaying the poorest properties. The phenomenon of optimum clay content has been observed for several systems. However, an explanation for why the wear of the nanocomposite increase beyond optimum clay content is lacking. It is hypothesized that the phenomenon of the optimum clay content is a result of the competitive effects of transfer film development and the formation of abrasive aggregates. Such aggregates are not seen on the wear surfaces indicating that the aggregates are formed in the transfer film due to continuous shear. With increasing filler loading, the ease of formation of these aggregates in the transfer film is enhanced due to the reduced interparticle distance in the parent sample [28]. This conclusion obtained from the SEM observations is consistent with the previous studies related to the wear mass-loss measurement of pure PA6 and corresponding composites. The increase of surface wear resistance of pure PA6 by the incorporation of MMTs may be attributed to the increase of surface hardness of pure polymer by introducing MMTs. This finding can be further identified by the shore hardness measurements of composites as discussed in the following sections.
3.2.2. Shore hardness test The shore hardness is an important parameter for evaluating or designating nanocomposite materials. The hardness data of the PA6– MMTs nanocomposites in the form of a standard shape at various material compositions are given in Fig. 5 and Table 1. These showed that as the MMT content increased, the corresponding shore hardness value (Hs) of composites increased up to 5 wt.% of MMT loading in composites. For example, the pure PA6 showed a low surface hardness of 60.5 Hs. Once you add the MMTs into PA6, the surface hardness is increased to 64.10 Hs for PA6MMT0.5, 67.8 Hs for PA6MMT1, 74.1 Hs for PA6MMT3, and 80.8 Hs for PA6MMT5. This indicated that the surface hardness of the composite materials increased with an increasing MMT content. Thus, the integration of raw MMTs into the PA6 matrix does indeed change the surface characteristics of the composite materials significantly based on the surface mechanical property studies.
Table 1 Relations of composition of PA6–MMT nanocomposites with mechanical properties. Compound
Feed Code
Tensile
Flexural
Shore
Composition
(wt.%)
Strength
Strength
Hardness
(After 1000 rpm) (mg)
PA6 PA6MMT0.5 PA6MMT1 PA6MMT3 PA6MMT5
PA6
MMT
(MPa)
(MPa)
(Hs)
100 99.5 99 97 95
0 0.5 1 3 5
64.12 69.51 71.16 75.92 60.36
64.97 70.95 80.85 84.08 67.60
60.5 64.1 67.8 74.1 80.8
[+ 8.4%] [+ 10.97%] [+ 18.40%] [−5.87%]
[+ 9.2%] [+ 24.44%] [+ 29.41%] [−21.53%]
[+ 5.95%] [+ 12.06%] [+ 22.48%] [+ 33.55%]
Wear Loss
75 65 [+ 13%] 57 [+ 24%] 46 [+ 38.67%] 131 [−74.6%]
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Fig. 4. SEM images of the typical worn surfaces of (a) pure PA6, (b) PA6MMT0.5, (c) PA6MMT1, (d) PA6MMT3 and (e) PA6MMT5 nanocomposites.
3.3. Bulk mechanical property studies 3.3.1. Tensile strength test Relationships between tensile properties and MMT loading as obtained from tensile test on the standard dumbbell-shaped of PA6– MMT composite materials were studied, as shown in Fig. 6. For example, upon addition of MMTs into PA6, the tensile strength of composites increased to 69.51 MPa for PA6MMT0.5, 71.16 MPa for PA6MMT1, and 75.92 MPa for PA6MMT3. Furthermore, an overdose of MMT loading (e.g., 5 wt.%) in composites was found to decrease the tensile strength of as-prepared composite materials, as summarized in Table 1. This indicated that the introduction of MMTs into the polymer matrix does effectively affect the tensile properties of pristine PA6. However, the decrease in tensile strength for PA6MMT5 may probably have resulted
from the aggregation of MMTs in composites. Similar trends were also found in the flexural strength of as-prepared composites. Fig. 7 shows the load–displacement measurements of pure PA6 and their corresponding PA6–MMT composites. Originally, the pure PA6 material was a hard and tough material, adding MMTs makes PA6–MMT composites allowed it to absorb more elastic strain energy. When adding 3 wt.% of MMT, we can obtain the best tensile strength and its elastic modulus is larger showing their strong and tough property. When increasing to 5 wt.% the phenomenon of aggregation makes this PA6–MMT composites turn into a hard and brittle property, though its hardness value increases, yielding point is low, elongation is short, and can absorb less elastic strain energy easily cracking. Similar trends were also found in the flexural strength of asprepared composites.
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Fig. 5. The shore hardness of PA6–MMT composites as a function of the clay content.
Fig. 7. Variation of the load of PA6–MMT composites with displacement.
3.3.2. Flexural strength test Relationships between flexural strength and MMT loading as obtained from the tensile test on the standard dumbbell-shaped of PA6–MMT composite materials was studied, as shown in Fig. 8. For example, upon addition of MMTs into PA6, the flexural strength of composites increased to 70.95 MPa for PA6MMT0.5, 80.85 MPa for PA6MMT1, and 84.08 MPa for PA6MMT3. Furthermore, an overdose of MMT loading (e.g., 5 wt.%) in composites was found to decrease the tensile strength of as-prepared composite materials, as summarized in Table 1. This indicated that the introduction of MMTs into the polymer matrix does effectively affect the flexural properties of pristine PA6. However, the decrease in flexural strength for PA6MMT5 may probably have resulted from the aggregation of MMTs in composites. Because the neat PA6 matrix is comparatively hard and tough. Therefore, although the addition of MMT into the matrix has already improved it's bond strength, the optimum MMT content is a result of the formation of abrasive aggregates. If the MMT content demonstrates better scattering in the PA6 matrix, the flexural strength of the polymers can be improved gradually (elevating at a steady rate).
Morphology for the dispersion capability of MMTs in the PA6 matrix was subsequently characterized by XRD and TEM. We found that the 3 wt.% of MMT loading showed good dispersion capability in the PA6 matrix based on the TEM observation. Moreover, 5 wt.% of MMT loading also showed good dispersion capability in the PA6 matrix. However, some domains of the TEM micrograph also found that it combined MMT cluster. It should be noted that the mechanical studies could be divided into surface and bulk types. In the studies of surface mechanical property (e.g., wear resistance and shore hardness), we found that the integration of MMTs shows a distinctly increasing trend on shore hardness up to 5 wt.% of MMT loading in composites. Moreover, enhancement of wear resistance of as-prepared composites as compared to pure PA6 can be further identified by the Scanning Electron Microscopy (SEM) observation of the surface morphology on as-prepared samples after testing. On the other hand, for the bulk mechanical property studies (i.e., tensile strength and flexural strength), we found that the composites containing 3 wt.% of MMTs in the PA6 matrix exhibited the best performance of tensile strength and flexural strength. It implied that for this type of composition, MMTs exhibited good compatibility with the PA6 matrix. Furthermore, at higher MMTs loading (e.g. 5 wt.%), the raw MMTs were found to be aggregated in the polymer matrix, as observed in TEM, leading to an obvious decrease in tensile and flexural strength tests.
4. Conclusions In this paper, we presented the effect of integrated MMTs on the surface and bulk mechanical properties of as-synthesized PA6–MMT composites prepared from the twin-screw extruder mixed technique.
Fig. 6. The tensile strength of PA6–MMT composites as a function of the clay content.
Fig. 8. Variation of the flexural strength of PA6–MMT composites with the clay content.
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Acknowledgement The financial support of this research by the National Science Council, Taiwan, ROC (subsidy no. NSC 97-2221-E-231-021) is gratefully acknowledged. References [1] H.L. Tyan, Y.C. Liu, K.H. Wei, Effect of reactivity of organics-modified montmorillonite on the thermal and mechanical properties of montmorillonite/polyimide nanocomposites, Chem. Mater. 13 (1) (2001) 222–226. [2] J.W. Gilman, C.L. Jackson, A.B. Morgan, R.J. Hayis, E. Manias, E.P. Giannelis, M. Hilton, D. Wuthenow, S.H. Phillips, Flammability properties of polymer-layeredsilicate nanocomposites. Polypropylene and polystyrene nanocomposites, Chem. Mater. 12 (7) (2000) 1866–1873. [3] T. Lan, P.D. Kaviratna, T.J. Pinnavaia, On the nature of polyimide–clay hybrid composites, Chem. Mater. 6 (1994) 573–575. [4] J.M. Yeh, S.J. Liou, C.Y. Lai, P.C. Wu, T.Y. Tsai, Enhancement of corrosion protection effect in polyaniline via the formation of polyaniline–clay nanocomposite materials, Chem. Mater. 13 (3) (2001) 1131–1136. [5] J.M. Yeh, S.J. Liou, C.Y. Lin, C.Y. Cheng, Y.W. Chang, K.R. Lee, Anticorrosively enhanced pmma–clay nanocomposite materials with quaternary alkylphosphonium salt as an intercalating agent, Chem. Mater. 14 (1) (2002) 154–161. [6] J.M. Yeh, C.P. Chin, Y.C. Chen, C.Y. Ma, K.R. Lee, Y. Wei, S. Li, Enhancement of corrosion protection effect of poly(o-ethoxyaniline) via the formation of poly(oethoxyaniline)–clay nanocomposite materials, Polymer 43 (9) (2002) 2729–2736. [7] J.M. Yeh, C.P. Chin, S.J. Chang, Enhanced corrosion protection coatings prepared from soluble electronically conductive polypyrrole–clay nanocomposite materials, J. Appl. Polym. Sci. 88 (14) (2003) 3264–3272. [8] J.M. Yeh, C.P. Chin, Structure and properties of poly(o-methoxyaniline)–clay nanocomposite materials, J. Appl. Polym. Sci. 88 (4) (2003) 1072–1080. [9] Y.H. Yu, J.M. Yeh, S.J. Liou, Y.P. Chang, Organo-soluble polyimide (TBAPP–OPDA)/ clay nanocomposite materials with advanced anticorrosive properties prepared from solution dispersion technique, Acta Mater. 52 (2) (2004) 475–486. [10] Y.H. Yu, J.M. Yeh, C.C. Jen, H.Y. Huang, P.C. Wu, C.C. Huang, ''Preparation and properties of heterocyclically conjugated poly (3-hexylthiophene)-clay nanocomposite, J. Appl. Polym. Sci. 91 (6) (2004) 3438–3446. [11] J.M. Yeh, S.J. Liou, C.G. Lin, Y.P. Chang, Y.H. Yu, Effective enhancement of anticorrosive properties of polystyrene by polystyrene–clay nanocomposite materials, J. Appl. Polym. Sci. 92 (3) (2004) 1970–1976. [12] J.M. Yeh, S.J. Liou, H.J. Lu, H.Y. Huang, Enhancement of corrosion protection effect of poly(styrene-co-acrylonitrile) by the incorporation of nanolayers of montmorillonite clay into copolymer matrix, J. Appl. Polym. Sci. 92 (4) (2004) 2269–2277.
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[13] J.M. Yeh, C.L. Chen, Y.C. Chen, C.Y. Ma, Y.H. Yu, H.Y. Huang, Enhanced corrosion prevention effect of polysulfone–clay nanocomposite materials prepared by solution dispersion, J. Appl. Polym. Sci. 92 (1) (2004) 631–637. [14] J.M. Yeh, C.L. Chen, T.H. Kuo, W.F. Su, H.Y. Huang, D.J. Liaw, H.Y. Lu, C.F. Liu, Y.H. Yu, Preparation and properties of (BATB–ODPA) polyimide–clay nanocomposite materials, J. Appl. Polym. Sci. 92 (2) (2004) 1072–1079. [15] Y.H. Yu, J.M. Yeh, S.J. Liou, C.L. Chen, D.J. Liaw, H.Y. Lu, Preparation and properties of polyimide–clay nanocomposite materials for anticorrosion application, J. Appl. Polym. Sci. 92 (6) (2004) 3573–3582. [16] Y. Kim, J.L. White, Formation of polymer nanocomposites with various organoclays, J. Appl. Polym. Sci. 96 (2005) 1888–1896. [17] US Patent 2,130,523 ‘Linear polyamides suitable for spinning into strong pliable fibers’, U.S. Patent 2,130,947 ‘Diamine dicarboxylic acid salt’ and U.S. Patent 2,130,948 ‘Synthetic fibers’, all issued September 20, 1938. [18] M. Garcia, M.D. Rooij, L. Winnubst, W.E.V. Zyl, H. Verweij, Friction and wear studies on nylon-6/SiO2 nanocomposites, J. Appl. Polym. Sci. 92 (2004) 1855–1862. [19] S. Yu, Z. Yu, Y.W. Mai, Effects of SEBS-g-MA on tribological behaviour of nylon 66/ organoclay nanocomposites, Tribol. Int. 40 (2007) 855–862. [20] J.W. Cho, D.R. Paul, Nylon-6 nanocomposites by melt compounding polymer, 20018 1083-1094. [21] A.N. Wilkinson, Z. Man, J.L. Stanford, P. Matikainen, M.L. Clemens, G.C. Lees, C.M. Liauw, Tensile properties of melt intercalated polyamide 6–montmorillonite nanocomposites, Compos. Sci. Technol. 67 (15) (2007) 3360–3368. [22] Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kuraauchi, O. Kamigaito, Mechanical properties of nylon 6–clay hybrid, J. Mater. Res. 8 (5) (1993) 1185–1189. [23] H. Wang, C. Zeng, M. Elkovitch, L.J. Lee, K.W. Koelling, Processing and properties of polymeric nano-composites, Polym. Engng. Sci. 41 (11) (2001) 2036–2046. [24] H. Kharbas, P. Nelson, M. Yuan, S. Gong, L.S. Trung, R. Spindler, Effects of nano-fillers and process conditions on the microstructure and mechanical properties of microcellular injection molded polyamide nanocomposites, Polym. Compos. 24 (6) (December 2003) 655–671. [25] T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Electrical conductivity of individual carbon nanotubes, Nature 382 (1996) 54–55. [26] S.P. Liu, W.L. Hsu, K.C. Chang, J.M. Yeh, Enhancement of the surface and bulk mechanical properties of polystyrene through the incorporation of raw multiwalled nanotubes with the twin-screw mixing technique, J. Appl. Polym. Sci. 113 (2009) 992–999. [27] S.P. Liu, I.J. Huang, K.C. Chang, J.M. Yeh, Mechanical properties of polystyrenemontmorillonite nanocomposites — prepared by melt intercalation, J. Appl. Polym. Sci. 115 (2010) 288–296. [28] P. Bhimaraj, D.L. Burris, J. Action, W.G. Sawyer, C.G. Toney, R.W. Siegel, L.S. Schadler, Effect of matrix morphology on the wear and friction behavior of alumina nanoparticle/poly(ethylene) terephthalate composites, Wear 258 (2005) 1437–1443.
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International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Mechanical properties of polyamide-6/montmorillonite nanocomposites — Prepared by the twin-screw extruder mixed technique☆ Sung-Po Liu a,⁎, Shyh-Shin Hwang a, Jui-Ming Yeh b, Chi-Chang Hung a a b
Department of Mechanical Engineering, Ching Yun University, Jung-Li 32097, Taiwan, ROC Department of Chemistry and Center for Nanotechnology at CYCU, Chung-Yuan Christian University, Chung-Li, Taiwan 32023, Taiwan, ROC
a r t i c l e
i n f o
Available online 18 November 2010 Keywords: Polyamide-6 Montmorillonite Nanocomposites Twin-screw extruder Mechanical property
a b s t r a c t In this paper, we present the effects of incorporated montmorillonite (MMT) on a surface and the bulk mechanical properties of as-synthesized PA6/MMT composites that are prepared using the twin-screw extruder mixed technique. The as-prepared polymer–clay nanocomposite (PCN) materials in the form of a pellet subsequently characterized using the powder X-ray diffraction (XRD) and the transmission electron microscopy (TEM). In this experiment, the surface mechanical property studies (i.e., wear resistance and hardness) show that the integration of MMTs exhibited a distinct increase on shore hardness was up to 5 wt.% of MMTs loaded in composites. Moreover, the enhancement of wear resistance of as-prepared composites, compared to pure PA6, can be further identified by the Scanning Electron Microscopy (SEM) observation of the surface morphology after testing. On the other hand, for the bulk mechanical property studies (i.e., tensile strength and flexural strength), we found that the composites containing 3 wt.% of MMTs in the PA6 matrix exhibited the best performance in tensile strength and flexural strength. It means that this composition of MMTs exhibits good compatibility with the PA6 matrix. Generally, PCN materials show an obvious enhancement of mechanical properties of neat polymer by an incorporated low loading of organophilic-clay platelets into a PA6 matrix used for the evaluation of the mechanical properties of the as-prepared samples. Furthermore, it was found that at higher MMT loading (e.g. 5 wt.%), MMTs were to be aggregated in the polymer matrix, as observed in TEM. Also, the result leads to an obvious decrease in tensile and flexural strength tests. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Recently, layered materials such as smectite clay (e.g., montmorillonite, MMT) have attracted intense research interests for the preparation of polymer–clay nanocomposite (PCN) materials. PCN materials usually demonstrate unique properties superior to traditional composites and conventional materials. In general, they combine both the characteristics of inorganic nanofillers and organic polymers at the molecular level. Currently, the PCN material is found to be a promising system due to the fact that the clay possesses a high aspect ratio and a platy morphology. It can be employed to boost the physical properties (e.g., thermal stability [1], fire retardant [2], gas barrier [3], and corrosion protection [4–15]) of bulk polymers, and mechanical properties are a particularly significant issue to study application and development for PCN materials. Kim and White [16] reported a variety of organic modified MMTs to understand the contribution of the organophilicity of organoclay on the formation of the polymer/clay nanocomposite. ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (S.-P. Liu). 0735-1933/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2010.10.003
Nylon 6 is a polymer developed by Paul Schlack at IG Farben to reproduce the properties of Nylon 6,6 without violating the patent of its production. Unlike most other nylons, Nylon 6 is not a condensation polymer and instead is formed by a ring-opening polymerization. Nylon 6 begins as pure caprolactam. As caprolactam has 6 carbon atoms, it got the name Nylon 6. When caprolactam is heated at about 533 K in an inert atmosphere of nitrogen for about 4– 5 hours, the ring breaks and undergoes polymerization. Then the molten mass is passed through spinnerets to form fibres of Nylon 6. Nylon 6 fibres are tough, possessing high tensile strength, as well as elasticity and lustre. They are wrinkle-proof and highly resistant to abrasion and chemicals such as acids and alkalis. The fibres can absorb up to 2.4% of water, although this lowers tensile strength. Nylon 6 is used as thread in bristles for toothbrushes, surgical sutures, and strings for acoustic and classical musical instruments, including guitars, violins, violas, and cellos. It is also used in the manufacture of a large variety of threads, ropes, filaments, nets, and tire cords, as well as hosiery and knitted garments. It can also be used in gun frames, such as those used by Glock, which are made with a composite of Nylon 6 and other polymers. It has the potential to be used as a technical nutrient [17]. Several attempts to prepare PA6–clay nanocomposites have been reported. For example, Garcia et al. [18] reported that the composites of
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nanometer-sized silica (SiO2) filler incorporated in Nylon 6 polymer were prepared by compression molding. Their friction and wear properties were investigated on a pin on disk tribometer by running a flat pin of steel against a composite disc. Yu et al. [19] reported that the tribological behaviour of Nylon 66, Nylon 66/organoclay nanocomposites and Nylon 66/ (SEBS-g-MA+ organoclay) nanocomposites was studied by means of a pin-on-disk apparatus. Recently, several attempts have been reported to study the mechanical properties of PA6–clay nanocomposites. For example, several research groups have reported the tensile and flexural strength of PA6–clay nanocomposite materials [20,21]. On the other hand, thermal, impact properties and morphologies of PA6–clay nanocomposites have also been reported [22–24]. However, wear resistance of PA6–clay nanocomposites has seldom been mentioned. Therefore, in this paper we present the first evaluation of wear resistance of PA6–clay nanocomposites and the first to make a comparative study on the mechanical properties of the effect of organoclay (MMTs) on shore hardness, tensile strength, and flexural strength of PA6–clay nanocomposites. 2. Experimental sections 2.1. Chemicals and instrumentations Commercial PA6 (Model No. A2700, melting point = 221 °C, specific gravity = 1.13 g/cm3) were purchased as pellets from the Nanopolymer Composites Corporation in Taiwan. The montmorillonite clay (Model No. I.34TCN) was purchased from Nanocor Company in USA. The surfactant of clay I.34TCN is methyl dihyroxyethyl hydrogenated tallow ammonium. A wide-angle X-ray diffraction study of the samples was performed on a Rigaku D/MAX-3C OD-2988N X-ray diffractometer with a copper target and Ni filter at a scanning rate of 4°/ min. The samples for the transmission electron microscopy (TEM) study were first prepared by putting the membrane of PCN (PA6–clay nanocomposite) materials into low-viscosity embedding media epoxy resin capsules with four ingredients (ERL4206 5.0 g, DER736 3.0 g, NSA 13.0 g, and DMAE 0.15 g) and by curing the epoxy resin at 100 ° C for 24 h in a vacuum oven. Then the cured epoxy resin containing PCN materials was microtomed with a Reichert-Jumg Ultracut-E into 60–90 nm slices. Subsequently, one layer of carbon about 10 nm was deposited on these silices on mesh 100 copper nets for TEM observations on a JEOL-200FX, with an acceleration voltage of 120 kV. Centrifugal Ball Mill (Retsch S100) was used to mill the size of organophilic clay. A Scanning Electron Microscopy (SEM) with the model Hitachi S-4100 FE-SEM evaluates the surface morphology of as-prepared composite materials. The twin-screw extruder machine and hot press machine manufactured by Sun Sing Scientific Company (Hong Kong, Model no. SHJ20, co-rotating type, non-intermeshing, max screw speed is 120 rpm, screw diameter is 20 mm, L/D ratio is 40, test temperature is 20 °C to 400 °C, Chamber maximum volume is 200 g) and Long-Chang Company (Taiwan, Model no. FC-60 TON) were used to make the asprepared sample in the form of its standard shape. The tensile and flexural test of PA6–clay specimens were carried out through Universal Testing Instruments manufactured by Hung-Ta Company (Taiwan, Model no. HT-9102). The shore hardness tests were run on a Shore Hardness Test Machine by EXCELLENCE Company (Japan, Model D) for high and low hardness samples, respectively, under an indentation time of 10 seconds at 25 °C. The wearing resistance tests were run on a Wearing Test Machine by TABER Company (Model 5130 ABRASER). 2.2. Preparation of PA6–clay composite materials through twin-screw mixed extruder technique Before blending, PA6 and clay were dried at 85 °C under vacuum oven for 2 h. PA6–clay composites were prepared in a twin-screw
extrude machine at a screw speed of 85 rpm, melt temperature at 210 °C. The melt-mixing procedure of PA6–clay composites was performed by blending samples repeatedly, at least twice, to form products in the shape of pellets, with better combinations. The asprepared PA6–clay composite pellets were subsequently mixed by a twin-screw extrude machine using the twin-screw extrude mixing method to obtain nanocomposites with standard shaped specimens for the following investigations. Melt mixing is a procedure in which the mixture's consistency is an important step in the polymer material processing. Using mechanical force to blend is the most convenient and practical method. It can also change the quality of high polymers through melting technology effectively. It is one of the most frequently used methods in the industry. 2.3. Hot press of PA6–clay composite materials According to the test standard of the experiment project, we designed the mold for the hot-pressing requirement, and then used the hot-press machine to undergo our tests. Place the blending PA6 and clay in the mold, put the mold into a hot-pressing machine after preheating it for 4 min under 240 °C. After the upper and lower templates of the hot-pressing machine are closed for 1 min continuing at 1000 kg/cm2of pressure then pressurize it at 2000 kg/cm2 for 1 min. Take out the mold after the hot press is finished. Last, wait for the mold to cool down, using water cooling to 80 °C then open the mold to take out the specimen. 2.4. Test of surface mechanical properties 2.4.1. Wear-resistance test Wear-resistance test is one of the most convenient and basic properties of the material in mechanical property testing. When one object is contacting with other objects running for a long time, it will cause wearing. So, it is necessary to perform a long period wearing– tearing test. This experiment utilizes wearing test machines to determine the roll wearing and slip wearing characteristic of material, and then show this material surface decreasing ability on wear resisting. The experiment is accomplished according to the ASTM D4060 standard. 2.4.2. Shore hardness test We used the Japan Shore Hardness Testing Machine as the depthdetect instrument that our nano hardness test was to be carried on, the shore hardness experimental method of the hardness being invented by Americans Albert F Shore in 1906, has already been used extensively as the dynamic loading hardness. The Shore Hardness Testing Machine type D calibrated a scale instruction type that improves, as to large-scale specimens or large round type of diameter specimens for fittings that can be fixed and increase its scope of application and characteristic. In addition, it is a quick operation and can reach to 1000 or more times per hour. The machine is easy and simple to operate; even non-technical staff can operate it smoothly. It is also hard to damage the specimen, on the other hand, using other machines after the indenter-hardness test; the specimen needs to be repaired before the indenter can be removed. Furthermore, its assembly cost is cheaper and it can also apply to thin specimens [25]. Through the above-mentioned assessment, this experiment will adopt the type D Shore Hardness Testing Machine. The experiment is completed according to the ASTM D790 standard. 2.5. Test of bulk mechanical properties 2.5.1. Tensile strength test In mechanical properties, tensile strength is one of the most frequently used comparative standards. The definition of tensile
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strength is one that exerts tensile stress the most and tries to extract the test piece before cracking (or plastic deformation). This experiment, according to the ASTM D638 standard, shapes the material into a dumbbell-shaped specimen first, and then carries out the tensile test. Examine at least five specimens in each group, calculate the average, and record the greatest strength of tensile tests. 2.5.2. Flexural strength test Flexural test can measure the deformation energy or fracture strength that bears the bending moment for determining materials. In general, there are three kinds of measurement methods of flexural tests: cantilever, three points, and four points. It is common to use the three-point method to measure in laboratories. Its method is to put the test specimen on two fixed displacements, apply force at the central point with a certain pressing speed, until the test specimen fractures. It can utilize the flexural test to determine Young's modulus. Also, the experiment is done according to the ASTM D790 standard. 3. Results and discussion 3.1. Characterization Fig. 1 showed the wide-angle powder XRD patterns of raw MMT, and a series of PCN materials. For raw MMT, there is any diffraction peak in 2θ = 4.7° (d-spacing = 1.87 nm), as opposed to the diffraction peak of 2θ 2° (d-spacing 4.4 nm) for organoclay, indicating the possibility of having exfoliated silicate nanolayers of organoclay dispersed in the PA6 matrix. With the amount of organoclay increased to 5 wt.%, this implied that there was a small amount of organoclay that couldn't be exfoliated in the PA6 and existed in the form of a partial intercalated or unintercalated layer structure. In addition to the XRD investigation, the internal structure of polymer/clay nanocomposites was also examined using Transmission Electron Microscopy (TEM), which can directly visualize the expanded layering structure in the nanocomposites and the dispersion quality of the silicate layers in the polymer matrix. As shown in Fig. 2(a), we found that the 3 wt.% of MMT shows a good dispersion ability in the PA6 matrix, where the dark line represents clay platelets and the gray/white areas represented the PA6 matrix. Furthermore, 5 wt.% of MMT also shows a good dispersion capability in the polymer matrix. However, an overdose of MMT loading embedded in as-prepared composites may cause the observable aggregation of MMT clusters in the polymer matrix, as shown in Fig. 2(b). It is clear that lamellar nanocomposites has a mixed
Fig. 2. TEM images of the PA6–MMT composites of (a) 3 wt.% MMT (× 50 k) (b) 5 wt.% MMT (× 50 k).
Fig. 1. X-ray diffraction patterns of raw MMT, neat PA6 and PA6–MMT clay.
morphology with major intercalation and minor unintercalated dispersion in the PA6 matrix. The different dispersion morphology of MMT in the polymer matrix could lead to as-prepared composites revealing different mechanical properties. For the property studies, the mechanical property discussed can be categorized into two parts: surface mechanical property (e.g., wear resistance and shore hardness)
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Fig. 3. Effects of the clay content on wear loss of PA6–MMT composites.
and bulk mechanical property (e.g., tensile strength and flexural strength). 3.2. Surface mechanical property studies 3.2.1. Wear-resistance test Test specimens with the dimensions of 100 × 100 × 2.8 (mm) were made. After that, utilize wearing testing machine with a load of 1000 g, measuring wear mass-loss (the influence factor of the wear mass loss amount) by operating the wearing speed at 500 rpm and 1000 rpm, respectively, to probe the wear-resistance properties of asprepared composites. Fig. 3 showed the wearing mass-loss of as-prepared pure PA6 and PA6–MMT composites. For example, the amount of wear mass-loss operated at 500 rpm for as-prepared composites was found to slightly increase from the original of PA6 to PA6MMT3. It indicated that the wear mass-loss of as-prepared composites reduced slightly as the content of MMTs that existed in the composites increased at a low wearing speed of 500 rpm. Moreover, when operated at a higher wearing speed of 1000 rpm, the wear mass-loss amount of asprepared composites was found to reduce significantly. For example, the amount of wear mass-loss for composites changed from the original value of 75 mg (for PA6) to 46 mg (for PA6MMT3), as summarized in Table 1. This implied that the higher content of MMTs (e.g., 3 wt.%) in composites led to an obviously reduction in wear mass-loss amount at a higher wearing speed rate, reflecting that the incorporation of MMTs into PA6 might show better wear-resisting property of as-prepared samples. However, the increase in the wear mass-loss amount for PA6MMT5 may probably be resulted from the aggregation of MMTs in composites. Moreover, the wear resistance of as-prepared composites can be further evaluated by the visual observation of the as-prepared
composites after the wearing test. Fig. 4 showed the Scanning Electron Microscope (SEM) of pure PA6 (a), PA6MMT0.5 (b), PA6MMT1 (c), PA6MMT3 (d) and PA6MMT5 (e), respectively. It showed that the SEM observation of pure PA6 morphology revealed an uneven and rough surface, indicating that the pure PA6 (wear mass-loss amount of 75 mg) had hard surface structures as compared to the pure PS (wear mass-loss amount of 128.74 mg) [26,27]. After the wearing test, the material structures of pure PA6 can't be easily degraded and removed from the surface of as-prepared sample, as shown in Fig. 4(a). However, the surface morphology of PA6–MMTs composites was found to display a relatively smooth pattern as compared to the pure PA6, as illustrated in Fig. 4(b), (c) and (d). This implied that the incorporation of MMTs (except 5 wt.% in Fig. 4(e)) into PA6 might effectively enhance the wear resistance of PA6. The wear mass-loss amount increases at loading higher that 3 wt.%, with the 5 wt.% filled nanocomposite displaying the poorest properties. The phenomenon of optimum clay content has been observed for several systems. However, an explanation for why the wear of the nanocomposite increase beyond optimum clay content is lacking. It is hypothesized that the phenomenon of the optimum clay content is a result of the competitive effects of transfer film development and the formation of abrasive aggregates. Such aggregates are not seen on the wear surfaces indicating that the aggregates are formed in the transfer film due to continuous shear. With increasing filler loading, the ease of formation of these aggregates in the transfer film is enhanced due to the reduced interparticle distance in the parent sample [28]. This conclusion obtained from the SEM observations is consistent with the previous studies related to the wear mass-loss measurement of pure PA6 and corresponding composites. The increase of surface wear resistance of pure PA6 by the incorporation of MMTs may be attributed to the increase of surface hardness of pure polymer by introducing MMTs. This finding can be further identified by the shore hardness measurements of composites as discussed in the following sections.
3.2.2. Shore hardness test The shore hardness is an important parameter for evaluating or designating nanocomposite materials. The hardness data of the PA6– MMTs nanocomposites in the form of a standard shape at various material compositions are given in Fig. 5 and Table 1. These showed that as the MMT content increased, the corresponding shore hardness value (Hs) of composites increased up to 5 wt.% of MMT loading in composites. For example, the pure PA6 showed a low surface hardness of 60.5 Hs. Once you add the MMTs into PA6, the surface hardness is increased to 64.10 Hs for PA6MMT0.5, 67.8 Hs for PA6MMT1, 74.1 Hs for PA6MMT3, and 80.8 Hs for PA6MMT5. This indicated that the surface hardness of the composite materials increased with an increasing MMT content. Thus, the integration of raw MMTs into the PA6 matrix does indeed change the surface characteristics of the composite materials significantly based on the surface mechanical property studies.
Table 1 Relations of composition of PA6–MMT nanocomposites with mechanical properties. Compound
Feed Code
Tensile
Flexural
Shore
Composition
(wt.%)
Strength
Strength
Hardness
(After 1000 rpm) (mg)
PA6 PA6MMT0.5 PA6MMT1 PA6MMT3 PA6MMT5
PA6
MMT
(MPa)
(MPa)
(Hs)
100 99.5 99 97 95
0 0.5 1 3 5
64.12 69.51 71.16 75.92 60.36
64.97 70.95 80.85 84.08 67.60
60.5 64.1 67.8 74.1 80.8
[+ 8.4%] [+ 10.97%] [+ 18.40%] [−5.87%]
[+ 9.2%] [+ 24.44%] [+ 29.41%] [−21.53%]
[+ 5.95%] [+ 12.06%] [+ 22.48%] [+ 33.55%]
Wear Loss
75 65 [+ 13%] 57 [+ 24%] 46 [+ 38.67%] 131 [−74.6%]
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Fig. 4. SEM images of the typical worn surfaces of (a) pure PA6, (b) PA6MMT0.5, (c) PA6MMT1, (d) PA6MMT3 and (e) PA6MMT5 nanocomposites.
3.3. Bulk mechanical property studies 3.3.1. Tensile strength test Relationships between tensile properties and MMT loading as obtained from tensile test on the standard dumbbell-shaped of PA6– MMT composite materials were studied, as shown in Fig. 6. For example, upon addition of MMTs into PA6, the tensile strength of composites increased to 69.51 MPa for PA6MMT0.5, 71.16 MPa for PA6MMT1, and 75.92 MPa for PA6MMT3. Furthermore, an overdose of MMT loading (e.g., 5 wt.%) in composites was found to decrease the tensile strength of as-prepared composite materials, as summarized in Table 1. This indicated that the introduction of MMTs into the polymer matrix does effectively affect the tensile properties of pristine PA6. However, the decrease in tensile strength for PA6MMT5 may probably have resulted
from the aggregation of MMTs in composites. Similar trends were also found in the flexural strength of as-prepared composites. Fig. 7 shows the load–displacement measurements of pure PA6 and their corresponding PA6–MMT composites. Originally, the pure PA6 material was a hard and tough material, adding MMTs makes PA6–MMT composites allowed it to absorb more elastic strain energy. When adding 3 wt.% of MMT, we can obtain the best tensile strength and its elastic modulus is larger showing their strong and tough property. When increasing to 5 wt.% the phenomenon of aggregation makes this PA6–MMT composites turn into a hard and brittle property, though its hardness value increases, yielding point is low, elongation is short, and can absorb less elastic strain energy easily cracking. Similar trends were also found in the flexural strength of asprepared composites.
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Fig. 5. The shore hardness of PA6–MMT composites as a function of the clay content.
Fig. 7. Variation of the load of PA6–MMT composites with displacement.
3.3.2. Flexural strength test Relationships between flexural strength and MMT loading as obtained from the tensile test on the standard dumbbell-shaped of PA6–MMT composite materials was studied, as shown in Fig. 8. For example, upon addition of MMTs into PA6, the flexural strength of composites increased to 70.95 MPa for PA6MMT0.5, 80.85 MPa for PA6MMT1, and 84.08 MPa for PA6MMT3. Furthermore, an overdose of MMT loading (e.g., 5 wt.%) in composites was found to decrease the tensile strength of as-prepared composite materials, as summarized in Table 1. This indicated that the introduction of MMTs into the polymer matrix does effectively affect the flexural properties of pristine PA6. However, the decrease in flexural strength for PA6MMT5 may probably have resulted from the aggregation of MMTs in composites. Because the neat PA6 matrix is comparatively hard and tough. Therefore, although the addition of MMT into the matrix has already improved it's bond strength, the optimum MMT content is a result of the formation of abrasive aggregates. If the MMT content demonstrates better scattering in the PA6 matrix, the flexural strength of the polymers can be improved gradually (elevating at a steady rate).
Morphology for the dispersion capability of MMTs in the PA6 matrix was subsequently characterized by XRD and TEM. We found that the 3 wt.% of MMT loading showed good dispersion capability in the PA6 matrix based on the TEM observation. Moreover, 5 wt.% of MMT loading also showed good dispersion capability in the PA6 matrix. However, some domains of the TEM micrograph also found that it combined MMT cluster. It should be noted that the mechanical studies could be divided into surface and bulk types. In the studies of surface mechanical property (e.g., wear resistance and shore hardness), we found that the integration of MMTs shows a distinctly increasing trend on shore hardness up to 5 wt.% of MMT loading in composites. Moreover, enhancement of wear resistance of as-prepared composites as compared to pure PA6 can be further identified by the Scanning Electron Microscopy (SEM) observation of the surface morphology on as-prepared samples after testing. On the other hand, for the bulk mechanical property studies (i.e., tensile strength and flexural strength), we found that the composites containing 3 wt.% of MMTs in the PA6 matrix exhibited the best performance of tensile strength and flexural strength. It implied that for this type of composition, MMTs exhibited good compatibility with the PA6 matrix. Furthermore, at higher MMTs loading (e.g. 5 wt.%), the raw MMTs were found to be aggregated in the polymer matrix, as observed in TEM, leading to an obvious decrease in tensile and flexural strength tests.
4. Conclusions In this paper, we presented the effect of integrated MMTs on the surface and bulk mechanical properties of as-synthesized PA6–MMT composites prepared from the twin-screw extruder mixed technique.
Fig. 6. The tensile strength of PA6–MMT composites as a function of the clay content.
Fig. 8. Variation of the flexural strength of PA6–MMT composites with the clay content.
S.-P. Liu et al. / International Communications in Heat and Mass Transfer 38 (2011) 37–43
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