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BY-NC-ND 3.0 license Open Access Published by De Gruyter July 31, 2013

Morphology and properties of a photopolymer/clay nanocomposite prepared by a rapid prototyping system

  • Shih-Hsuan Chiu , Sigit Tri Wicaksono , Kun-Ting Chen and Sheng-Hong Pong EMAIL logo

Abstract

This study represents an in situ polymerization by preparation of tetrafunction polyester acrylate mixed with 1,6-hexanediol diacrylate/clay nanocomposite by digital light processor rapid prototyping. The morphology of nano-clay fillers and the dispersing agent in the photopolymer matrix are investigated by scanning electron microscopy (SEM). Degradation temperature, tensile strength, impact strength, and hardness are characterized by using thermogravimetric analysis, universal tensile machine, Izod impact tester, and hardness shore A tester, respectively. Results show that the effect of clay loading with an appropriate amount of dispersant tends to significantly increase not only the tensile strength and hardness but also the degradation temperature of photopolymer/clay nanocomposite; however, the impact strength is not affected. In the same conditions, as visualized on SEM images, the nanocomposite tends to form the exfoliated structure with agglomeration of clay, which is caused by uneven distribution of nano-clay in the photopolymer matrix.

1 Introduction

In recent decades, nanocompositing of polymers has become a popular issue to achieve dramatic improvements in mechanical, physical, and thermal as well as electrical properties.

Clay has many unique characteristics such as better mechanical strength, flame retardant, and gas barrier properties. It has become one of the promising candidates of inorganic particles for polymer nanocomposite due to relatively low cost [1–7]. In general, there are three different types of polymer/clay nanocomposite depending on the way clay is laid into the polymer matrix viz. intercalated nanocomposites, flocculated nanocomposites, and exfoliated nanocomposites [7].

The polymer/inorganic nanocomposites can be prepared by various methods depending on the type of monomers and nanomaterials, for instance, bulk polymerization, photoinitiated polymerization, emulsion polymerization, in situ thermal polymerization, and copolymerization in solution [8]. Polymer/clay nanocomposites have been conventionally synthesized via melt intercalation or common solvent mixing. However, this process releases a large amount of volatile organic compounds [9]. It produces un-cross-linked polymer. Therefore, many researchers have been using the in situ crosslinking polymerization through photoinitiated polymerization process to produce cross-linked photopolymer/clay nanocomposites. The photoinitiated polymerization inherently is ultrarapid in nature and has temperature-independent reactions and spatial and temporal control of initiation, and it is also environmentally friendly [4, 9].

Recent research has devised various clay-photopolymer nanocomposites based on acrylate (radically initiated photopolymer) and epoxy (cationic initiated photopolymer) systems. Fawn et al. [3] proposed UV-curable epoxy acrylates reinforced with two different organically modified montmorillonites and an unmodified montmorillonite. Decker et al. [10] synthesized different types of nanocomposite materials by photoinitiated crosslinking polymerization of epoxy, vinyl ether, and acrylate-based resins containing small amount of organoclay filler with an UV-cured process. These nanocomposites show some improvements in both thermal and mechanical properties without any significant change in the photopolymerization rate in comparison with the pristine polymer. However, the improvements are far below than those are observed in linear polymer systems such as polyamides, polyolefins, polystyrene, and polyurethanes, which are completely soluble only in the organic solvents.

Chiu and Wu [11] used tetrafunction polyester acrylate (TPA) mixed with 1,6-hexanediol diacrylate (HDDA) as a matrix and small amount of nano-silica as the filler. The liquid composite was cured from liquid to solid by digital light processor rapid prototyping (RP) machine through layer-by-layer process to form the desired prototype. The tensile strength and hardness of HDDA-TPA/silica nanocomposite were increased approximately by 50% compared with the original HDDA-TPA one. To derive better performance on prototypes, in this research, we further investigate the morphology and properties of the nanocomposites proposed in the previous study [11] by replacing the filler silica with clay under a digital light processor RP machine.

2 Experimental

The reagents used, preparation steps, and experimental items are described as follows.

2.1 Reagents

TPA (manufacturer: Henkel, type: photomer 5430, molecular formula: C11H13O). HDDA (manufacturer: Henkel, type: photomer 4017, molecular formula: C12H18O4). Nano-clay (manufacturer: Paikong, Inc., type: PK805, size: 65 nm). Dispersant (manufacturer: Degussa, type: TEGO Dispers 680 UV). Photoinitiator (manufacturers: Ciba, type: IRGACURE 784, molecular formula: C29H17TiF3N2).

2.2 Materials preparation

At first, TPA and HDDA were mixed in 1:1 of ratio for 30 min by a stirrer at 1000 rpm. Then, the nano-clay and dispersant were added into the mixed resin and stirred for approximately 24 hours at 1000 rpm at room temperature. The dispersant was used to avoid agglomeration of nano-clay in the photopolymer matrix. Subsequently, photoinitiator was added into the liquid photopolymer/clay nanocomposite. In last, the liquid photopolymer/clay nanocomposite was exposed in a digital light processor RP machine (as shown in Figure 1A and B) to form a solid part layer-by-layer (each layer of 100 μm thickness) by RP machine.

Figure 1 (A) Schematic and (B) setup of digital light processor RP machine.
Figure 1

(A) Schematic and (B) setup of digital light processor RP machine.

The number of total layers depends on the required thickness in relation to the mechanical testing to meet the standard of measurements. Photoinitiator was added in small amount to catalyze photocuring reaction under the exposure of visible light to form the solid part of photopolymer/clay nanocomposite. The formulations of photopolymer/clay nanocomposite are listed in Table 1.

Table 1

Formulation of photopolymer/clay nanocomposite.

SampleTPA/HDDA (phr)Photoinitiator (phr)Carbon black (phr)Dispersant (phr)
L00D0001001.5
L10D0001001.510
L20D0001001.520
L30D0001001.530
L10D0101001.511
L20D0101001.521
L30D0101001.531
L10D1001001.5110
L20D1001001.5210
L30D1001001.5310

2.3 Tests

The mechanical properties such as tensile strength and hardness were measured and analyzed by using universal tensile machine JIA701 (the sample size is schematically described in Figure 2) at 30 mm/min test speed and shore A type durometer hardness tester, respectively. The impact property was measured by using Izod impact testing machine based on ASTM D256 standard (the specimen size is 64×12.7×3.2 mm without notch). The thermal stability was analyzed by using Thermogravimetric Analyzer Hi-Res TGA2950.

Figure 2 Schematic size of tensile test specimen (in mm).
Figure 2

Schematic size of tensile test specimen (in mm).

3 Results and discussion

3.1 Morphology analysis

The photopolymer/clay nanocomposites were examined for their morphology by scanning electron microscopy (SEM) using JEOL JSM-6390LV at 5 kV. The results show that the sample without any addition of fillers and dispersant (Figure 3A) has the smoothest morphology. The morphology becomes rough with addition of 2 phr clay as shown in Figure 3B. The rough morphology was resulted by agglomeration of clay particles in the photopolymer matrix during the curing process, which was caused by undistributed of clay fillers in the photopolymer matrix resulting by the absence of dispersing agent. Sarkar et al. [7] stated that the clay tends to be exfoliated as individual silicate layers separated in the polymer matrix by average distances that depend only on the clay loading. In this study, the clay at 2 phr loading without dispersant underwent agglomeration in the photopolymer matrix. Therefore, some researches suggested that the modification of the clay with appropriate surfactant should be able to distribute it uniformly in the polymer matrix [3]. However, in the present case, addition of 1 phr of dispersant resulted in smoother morphology as shown in Figure 3C. This was effected by dispersant able to distribute the clay uniformly in the photopolymer matrix. However, excess dispersant (∼10 phr in weight) will destroy the morphology of the sample. The overloading of dispersant causes the inhomogeneous distribution of clay in the photopolymer matrix. The excess of dispersant contributes to agglomeration of the clay, which destroys the clay surfaces. Therefore, the interface bonding between clay and photopolymer matrix become weaker (the agglomeration of dispersant itself is not obvious). The inhomogeneous distribution was indicated by the different size of clay that dispersed in the photopolymer matrix as shown in Figure 3D. This phenomenon was consistent with the result in our previous study that the optimum dispersant to the filler content ratio was approximately 1:1 to distribute silica filler into the photopolymer matrix [11].

Figure 3 SEM images of the sample M 3500X (A) without any addition, (B) 2 phr clay-no dispersant, (C) 2 phr clay-1 phr dispersant, and (D) 2 phr clay-10 phr dispersant.
Figure 3

SEM images of the sample M 3500X (A) without any addition, (B) 2 phr clay-no dispersant, (C) 2 phr clay-1 phr dispersant, and (D) 2 phr clay-10 phr dispersant.

3.2 Thermogravimetric analysis

Figure 4 shows the thermogravimetric analysis plots for various clay loadings of 0, 1, 2, and 3 phr nano-clay. Generally, the addition of nano-clay would significantly enhance the thermal stability of the photopolymer/clay nanocomposite. As indicated in Figure 4, the onset of degradation temperature is shifted to a higher level. It is contrary to the earlier observations, where the clay containing ammonium organomodifiers in polyamide matrix degrading via a Hoffman degradation mechanism could shift onset of decomposition temperature to a lower value [12, 13]. However, our findings are consistent with the work of Zheng and Wilkie [14] where they reported, in experiments with a variety of polymer matrices containing ORG2-modified clay and also modified with polycaprolactone, enhancement in the thermal stability of the nanocomposite. Thus, the overall thermal stability can be affected by the organomodifier in the clay as well as the polymer matrix. This suggests that a better interaction of the ORG1-modified clay with the epoxy acrylate leads to these property enhancements [3].

Figure 4 Differential thermal analysis plots for different clay loading.
Figure 4

Differential thermal analysis plots for different clay loading.

Initially, the degradation temperature of the sample reaches up to 268°C in sample without any addition of clay (Figure 4). The addition of a small amount of nano-clay (1 phr) incorporated with 1 phr of dispersant results in increase of the degradation temperature of photopolymer/clay nanocomposite by 30% up to 366°C. The effect on increase of the degradation temperature after addition of 2 phr clay and the same amount of dispersant (1 phr) in the photopolymer matrix is more obvious. The degradation temperature increases by 60% up to 433°C. This suggests that 2 phr clay incorporated with 1 phr of dispersant contributes the better interaction with the acrylate-based photopolymer, which leads to these property enhancements.

However, further addition of clay into photopolymer matrix can no longer increase the thermal stabilization. As shown in Figure 4, the 3 phr of clay shifts the onset of degradation to lower temperature near 423°C. The clay loading more than 2 phr ceases the further increase of degradation temperature. It is consistent with the results of Fawn et al.; they found, by using differential scanning calorimetry measurements, a significant increase of Tg up to 10°C by adding 1 phr clay into photopolymer matrix [15]. However, Tg tended to decrease after addition of more clay at 3 and 5 phr. Another disadvantage of dispersant addition is shown in the same figure. Addition of excess dispersant for approximately 10 phr results in decrease of the degradation temperature from 433°C to 414°C. It indicates that excess dispersant destroys the bounding of the clay in the photopolymer matrix.

3.3 Mechanical properties

The data resulted with several mechanical properties measurements are shown as follows.

The mechanical properties for different clay loading with different dispersant dosages are shown in Figures 57.

Figure 5 Tensile strength as a function of clay loading.
Figure 5

Tensile strength as a function of clay loading.

Figure 6 Hardness as a function of clay loading.
Figure 6

Hardness as a function of clay loading.

Figure 7 Impact strength as a function of clay loading.
Figure 7

Impact strength as a function of clay loading.

Tensile strength (all data are shown in Table 2) shows an abrupt increase upon addition of clay in acrylate-based photopolymer. A twofold increase in tensile strength is seen for all samples. Interestingly, the tensile strength is drastically increased initially. However, upon increasing the clay loading for more than 2 phr, the tensile strength tends to decrease. However, it is still high when compared with the pristine photopolymer. The sample without dispersant shows the maximum tensile strength is with 1 phr of clay loading. Afterwards, it decreases with increasing clay loading. The tensile strength decreases with clay loading more than 2 phr when 1 or 10 phr of dispersant is present. The highest tensile strength is observed for the sample with 1 phr dispersant and 1 phr of clay loading. These observations suggest that the clay loading contributes to the increase in tensile strength of photopolymer/clay nanocomposite significantly but only when it is in the appropriate content. This occurs due to the optimum contribution in appropriate amount that disperses the nanoclay and further controls network structure in the photopolymer matrix [9]. Higher than the optimum amount of the dispersant into the photopolymer matrix will rather decrease the tensile strength. It is consistent with the SEM results depicting that excess dispersant will destroy the morphology of photopolymer/clay nanocomposite, thus decreasing its tensile strength. The maximum tensile strength in this study is achieved by the sample with 2 phr of the clay loading with 1 phr of dispersant. Comparing with the photopolymer/clay nanocomposite results with some earlier works [1, 3, 7, 9], tensile strength results in this study are almost two times higher, which may probably be due to different sample preparation as explained in the experimental section.

Table 2

Mechanical properties of photopolymer/clay nanocomposite.

SampleTensile strength (MPa)Elongation at break (%)Young’s modulus (GPa)
L00D00073.9410.400.71
L10D000142.4810.891.23
L20D000136.588.931.12
L30D000116.4710.641.09
L10D010173.458.222.07
L20D010182.727.752.11
L30D010131.259.351.52
L10D10076.515.691.35
L20D100171.205.962.54
L30D100170.795.752.05

Figure 6 shows the hardness shore A as a function of clay loading for different dispersant dosages. It indicates that the hardness abruptly increases by 60% only at initial clay loading; further clay loading has no contribution in the enhancement of the hardness. This may probably be due to the depth of indentation, which is not deep enough to penetrate the several layers of sample in the shore A hardness measurement. Therefore, the measured hardness represents the hardness of the first surface of the samples only. However, these overall results show that the hardness values are almost similar compared with other results of different photopolymer systems [1, 7].

Figure 7 shows the stable impact energy of photopolymer/clay nanocomposite as a function of clay loading for different dispersant dosages (from 1 to 3 phr). It indicates that both dispersant and clay loading have no contribution in the increase of the impact energy of the photopolymer/clay nanocomposite. In addition, these results are also close to the results of Sarkar et al. [7], although their results showed occurrence of an initial increase at 5 phr of the clay loading. The reason that the tensile strength increased, but Izod was almost same may be due to the different mechanism on energy absorption rate between the tensile and the impact loading. The energy absorption during impact is greatly affected by the interface bonding between the fillers and the matrices. The impact test is performed by applying a sudden shock (very high speed load/impulse) to the material so that it fractures. The impact load transfers into the sample so fast that the bonding between clay fillers up to 3 phr and the matrix cannot hold the impact loading.

4 Conclusions

The morphology and properties of TPA mixed with HDDA/clay nanocomposite prepared by digital light processor RP are studied. SEM images show the morphologic changes in structure of nano-clay in the photopolymer matrix. It tends to form the exfoliated structure with appropriate amount of dispersant and clay loading with some agglomeration of clay that is caused by uneven distribution of nano-clay in the photopolymer matrix. The effect of clay loading with the appropriate amount of dispersant is limited to not only increase in the mechanical properties such as the tensile strength and hardness but also increase in the degradation temperature of photopolymer/clay nanocomposite is apparent. However, there is no significant effect on the impact strength.


Corresponding author: Sheng-Hong Pong, Department of Creative Product and Technological Application, Lan Yang Institute of Technology, No. 79 Fu Shin Road, Tou Chen, I Lan 261, Taiwan, Phone: +886-3-977-1997 ext. 285, Fax: +886-3-977-1997 ext. 790, e-mail:

References

[1] Wang Y, Zhang H, Wu J, Yang J, Zhang L. J. Appl. Polym. Sci. 2004, 96, 318–323.Search in Google Scholar

[2] Guymon CA, Kwame OA. Macromolecules 2008, 42, 180–188.10.1021/ma801688qSearch in Google Scholar

[3] Fawn MU, Webster DC, Davuluri SP, Wong SC. Eur. Polym. J. 2006, 42, 2596–2605.Search in Google Scholar

[4] Benfari S, Decker C, Keller L, Zahouily K. Eur. Polym. J. 2004, 40, 493–501.Search in Google Scholar

[5] Kassim A, Adzmi F, Mahmud E. Matrl. Sci. 2004, 10, 255–259.Search in Google Scholar

[6] Shichang Lv, Zhong W, Li S, Wenfang S. Eur. Polym. J. 2008, 44, 1613–1620.Search in Google Scholar

[7] Sarkar M, Kausik D, Sankar G, Banerjee A. Bull. Mater. Sci. 2008, 31, 23–28.Search in Google Scholar

[8] Lavinia M, Nicoleta P, Smaranda I, Adriana P, Gheorghe I. J. Appl. Polym. Sci. 2011, 119, 1820–1826.Search in Google Scholar

[9] Kwame O, Schall J, Allan CG. Macromolecules 2009, 42, 3275–3274.10.1021/ma802656xSearch in Google Scholar

[10] Decker C, Keller L, Zahouily K, Benfarhi S. J. Polym. 2005, 46, 3275–3284.Search in Google Scholar

[11] Chiu SH, Wu DC. J. Appl. Polym. Sci. 2007, 107, 3529–3534.Search in Google Scholar

[12] Xie W, Gao Z, Liu K, Pan WP, Richard V, Hunter D, Singh A. Thermochim. Acta 2001, 367–368, 339–351.10.1016/S0040-6031(00)00690-0Search in Google Scholar

[13] Jang BN, Wilkie CA. Polymer 2005, 46, 3264–3274.10.1016/j.polymer.2005.02.078Search in Google Scholar

[14] Zheng X, Wilkie CA. Polym. Degrad. Stabl. 2003, 82, 411–420.Search in Google Scholar

[15] Fawn MU, Davuluri SP, Wong SC, Webster DC. J. Polym. 2004, 45, 6175–6187.Search in Google Scholar

Received: 2012-5-23
Accepted: 2013-6-16
Published Online: 2013-07-31
Published in Print: 2014-03-01

©2014 by Walter de Gruyter Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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