Rod-Like Attapulgite Modified by Bifunctional Acrylic Resin As

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Rod-like attapulgite modified by bifunctional acrylic resin as reinforcement for epoxy composites Xiling Niu, Lixia Huo, Chenting Cai, Jinshan Guo, and Hui Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie502869c • Publication Date (Web): 20 Sep 2014 Downloaded from http://pubs.acs.org on October 7, 2014

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Industrial & Engineering Chemistry Research

Rod-like attapulgite modified by bifunctional acrylic resin as reinforcement for epoxy composites Xiling Niua, Lixia Huob, Chenting Caia, Jinshan Guo*a, Hui Zhou*b a：Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu, 730000, China b：Science and Technology on Surface Engineering Laboratory, Lanzhou Institute of Physics, Lanzhou Gansu, 730000, China *Corresponding author: Address: Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, 222# Tianshui Nanlu, Lanzhou 730000, Gansu, China, Jinshan Guo, Tel: +86 931 8912516; fax: +86 931 8912582; E-mail address: [email protected]

ABSTRACT To improve the dispersion stability of attapulgite (ATT) in epoxy (EP), acrylic resin (AC) containing trimethoxysilyl and carboxyl was employed to form a covalent link between attapulgite and epoxy. The rod-like attapulgite was first efficiently activated by means of removing the adsorbed water to release the structural channels under reflux and subsequently functionalized by the bifunctional acrylic resin obtaining grafting efficiency as high as 11wt%. The improvement of the hydrophobic nature of attapulgite was demonstrated by the fact that the contact angle of organically modified attapulgite (108.6o) was much higher than that of the pristine attapulgite (29.9o). Modified attapulgite exhibited homogeneous dispersion in epoxy matrix in forms of 1

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monodispersion or agglomeration by several nanorods. Compared to pristine epoxy, AC-ATT/EP nanocomposites containing only 3 wt% AC-ATT exhibited a significantly increase of 128.9% in storage modulus. In addition, the nanocomposites exhibited remarkable enhancement in the impact strength, Young’s modulus and the tensile strength relative to the pristine epoxy.

1. INTRODUCTION In recent years, nanoparticles have been used extensively to improve properties of polymers, such as mechanical, thermal and optical properties.1 Epoxy (EP) has been widely used in adhesives and coating owing to its good adhesiveness, high modulus, attractive strength, creep resistance and high heat distortion temperature. However, highly crosslinked microstructure of epoxy leads to undesirably brittleness on account of plastic deformation and poor resistance to crack initiation. Therefore toughening of epoxy without sacrificing mechanical strength is necessary to get more effective applications.2, 3 Fibrous or rod-like nanoparticles can efficiently reinforce materials due to their high aspect ratios.4 Carbon nanotubes based nanocomposites exhibit significant improvement in properties, while their high cost is an inevitable limitation on effective applications.5-7 Consequently, the rod-like attapulgite (ATT) is regarded as an attractive candidate for preparing high performance nanocomposites owing to its low cost and immense reserve all over the word.8, 9 The rod-like attapulgite (ATT) is a hydrated magnesium aluminum silicate which contains ribbons of a 2:1 phyllosilicate struction. The theoretical half unit cell formula 2


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of ATT is Mg5Si8O20(OH)2(OH2)4.4H2O, which was first proposed by Bradley.10 The smallest structure unit with the diameter of less than 100 nm and the length from several hundred nanometers to several thousand nanometers brings ATT with large surface and very high aspect ratios. Because of its distinct properties among clay minerals, ATT has attracted much attention as epoxy reinforcement.11 There are three types of water in ATT, namely magnesium coordinated water, adsorbed water in its structure and zeolite water in zeolite-like channels. With abundant hydroxyl groups on the surface, ATT is readily functionalized by silane coupling agent or surfactant.12, 13 Nano-ATT particles modified by silane coupling agent had effectively improved the macroscopic properties of nanocomposites, such as storage modulus (G′), the glass transition temperature (Tg), impact strength and thermal stability.14 In addition, ATT after silylation exhibited stable dispersion in epoxy matrix and high improvement in mechanical properties.15 However, this method needed excess silane coupling agent which would cause high cost of the modification. Compared with silane coupling agent, bifunctional acrylic resin (AC) containing trimethoxysilyl and carboxyl can be covalently grafted onto ATT effectively. It is reported that the dispersion of nanocomposites can be improved when the radii of gyration for polymers is higher than the particle size.16 Moreover, effective interface bonding and the entanglements between the epoxy molecules and the acrylic resin grafted onto ATT can reinforce the compatibility and mechanical properties of nanocomposites.17 Currently, researchers mainly use acid to activate ATT to release hydroxyl, but this process is too complex.18, 19 In addition, that approach often in aqueous solution induces 3



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more adsorbed water which may cause crosslinking of acrylic resins containing trimethoxysilyl. Therefore, a simple and effective method to activate ATT is needed. In our work, the rod-like attapulgite was first efficiently activated under reflux at 110 oC. Thus the structural channels of ATT were released by means of removing the adsorbed water and partial zeolite water. Exposed abundant hydroxyl groups on the surface of ATT are favorable to react with the trimethoxysilyl of acrylic resin, which was synthesized from functional monomers: γ-Methacryloxypropyl trimethoxy silane (KH-570) and acrylic acid (AA). The carboxyl from acrylic resin can interact with epoxy groups from epoxy to form a cross-linked network structure, thus the dispersion of ATT in epoxy matrix and performance of epoxy can be markedly improved. Not only microstructures and macroscopic properties of AC-ATT/EP nanocomposites, but also relationships between them were investigated in detail.

2. EXPERIMENTAL 2.1. Materials ATT was provided by Jiuchuan Nano-material Technology with a concentration of over 98%. The epoxy resin was a standard bis-phenol A (marked as NPEL127E) supplied by South Asia Epoxy Resin Company. The anhydride hardener cis-4-methyl- 1, 2, 3, 6-tetrahydrophthalic anhydride (MeH-HPA) was purchased from the Ziegler Bio-technology. 2-ethyl-4-methyl-1H-imidazole-1-propiononitrile (2E4MZ-CN) serves as imidazole accelerator was provided by the Shikoku Chemicals Corporation. Benzoyl peroxide (BPO) was dried before polymerization, while Methyl methacrylate (MMA), 4


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Butyl Methacrylate (BMA), AA and KH-570 were used as received. 2.2. Preparation of AC-ATT and AC-ATT-EP The acrylic resin (AC) was synthesized from monomers containing MMA, BMA, AA and KH-570 as 35 wt%, 40 wt%, 5 wt% and 20 wt%. The copolymer was synthesized by means of solution radical polymerization in isopropanol with 3.5wt% BPO.20 ATT was first dispersed in toluene under mechanical stirring. Then the dispersion was under reflux at 110 oC to remove the adsorbed water, zeolite water and residual impurities. The acrylic resin (20 wt% of ATT) was dispersed in ATT solution using a stirrer. After mixing AC and ATT for 20 min, the concentrated hydrochloric acid was added to adjust the pH to 4, and then the mixture was under reflux for 6 h. The AC-ATT was centrifuged and washed then repeatedly reflux extracted by Soxhlet extractor for two days. To explore the reaction between AC-ATT and epoxy (EP), we conduct the experiment as below. AC-ATT was dispersed in excess epoxy and stirred for 5 min, then sonicated for 20 min to obtain a homogeneous suspension. The epoxy suspension was placed in an oven and cured by heating the suspension at 140 oC for 2 h. The suspension was washed in acetone and reflux extracted using Soxhlet extractor for two days. 2.3. Preparation of AC-ATT/ EP nanocomposites AC-ATT was first mixed with epoxy at given amount under mechanical stirring then sonicated for 30 min. Subsequently, the solvent in the mixture was removed after vacuum-rotary evaporation. At last, a stoichiometric amount of MeH-HPA and 2E4MZ-CN was added (at weight ratios of 89.1:0.9:100 to epoxy) to the mixture 5



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respectively. After continuous stirring, the mixture was subjected to degas under vacuum. The final mixture was casted into aluminous mold to cure at 120 oC for 2 h. The weight ratios of the AC-ATT in epoxy varied from 0wt% to 7wt% (0 wt%, 1 wt%, 3 wt%, 5 wt% and 7 wt%). 2.4. Characterization The FTIR spectra of the specimens were taken using a Bruker 550 FTIR in the 4000-400cm-1 and the specimens mixed with KBr were pressed into pellets. The TGA was characterized by a Perkin-Elmer TGA-7 system at a heating rate of 10 oC /min from room temperature to 800 oC under the nitrogen atmosphere. The morphology of AC-ATT was characterized by SEM (JSM-6701F), with an accelerating voltage of 5.0 KV. The TEM micrographs of the samples were performed on an instrument of JEM-1200 EX/S transmission electron microscope and the suspension were dropped onto the copper grids after sonicating in acetone. Contact angles (CA) measurements were measured with a contact angle goniometer by means of the sessile drop using a microsyringe by averaging three fresh points at room temperature. The DMA of the cured nanocomposites were measured with the DMTAQ800 at Multi-Freqency-Strain module. The sample was heated from ambient temperature to 250 oC at the heating rate of 5oC/min and the frequency was 10Hz. Impact tests were measured with the mechanical cantilever beam impact testing machine (JJ-50) at ambient temperature. At least five samples were repeatedly tested for each nanocomposites. Tensile tests were performed on CMT4204 at the speed of 5mm/min.

6


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3. RESULT AND DISCUSSION 3.1. Characterization of ATT and AC-ATT Fig. 1 shows the FTIR spectra of AC, ATT, AC-ATT and AC-ATT-EP. The peaks at 2958 cm-1, 1734 cm-1 and 1145 cm-1 correspond to the stretching absorptions of the C-H, C=O and Si-O in the acrylic resin. In comparison with ATT, the characteristic absorption bands at 1652 cm-1 and 3614 cm-1 respectively assigned to the stretching and bending resonances of –OH are weakened in the spectrum of AC-ATT illustrating that partial hydroxyl has reacted with trimethoxysilyl. In the spectrum of AC-ATT, the new peaks at 1730 cm-1 and 2960 cm-1 respectively represent the stretching resonance of C=O and C-H of the acrylic resin.15 The samples were repeatedly extracted by Soxhlet extractor for two days to completely remove adsorptions from the surface of ATT. These characteristic peaks indicate that trimethoxysilyl has reacted with the hydroxyl on the surface of ATT under the catalysis of acid. In the spectrum of AC-ATT-EP, new absorption peak at 1510 cm-1 due to C=C stretching resonance from phenyl ring in epoxy were found compared to those of ATT and AC-ATT. These characteristic peaks indicate that the carboxyl of AC-ATT can react with the epoxy group of epoxy at 140 o

C. (Insert Figure 1)

TGA curves were used to evaluate the grafting efficiency of ATT. The TGA curves for ATT, AC-ATT and AC-ATT-EP are presented in Fig. 2. High grafting efficiency (about 11 wt%) was obtained after condensation reaction between acrylic resin and ATT. In general, a three-stage weight loss was observed in the TGA curves for AC-ATT. The 7



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first stage from 50 to 300 oC is due to the vaporization of residual moisture and loss of oligomer existing in AC-ATT.

In the temperature range of 300-575 oC, a larger weight

loss was found in AC-ATT because of the decomposition of acrylic resin. The third stage from 575 to 680 oC, which only observed in the TGA curves for AC-ATT and AC-ATT-EP attributed to the decomposition of residual carbon chains from organic nanorods. The ATT modified by acrylic resin exhibits a higher degradation temperature (about 300 oC) than those of ATT modified by micromolecule (about 200 oC).15 Enhanced thermal stability indicates that AC resin has been grafted onto ATT by chemical bond and the method employed to active ATT is feasible. Removing the adsorbed water and partial zeolite water can loosen the channel of ATT, thus releasing hydroxyl groups on the surface of ATT. What’s more, the adsorbed water may cause the crosslinking of the AC resin. The second stage decomposition temperature of AC-ATT-EP (about 310 oC) is higher than that of AC-ATT. The maximum loss weight of AC-ATT-EP is about 14 wt%, which is slightly increased comparing to that of AC-ATT. The TGA results imply the thermal stability of AC-ATT-EP has been enhanced compared with AC-ATT. It reveals that the carboxyl group can react with the epoxy group, so the AC-ATT can be used to enhance the performance of epoxy resin. (Insert Figure 2) To explore the dispersion and stability of ATT and AC-ATT in organic solvents, the status of the pristine ATT and AC-ATT dispersed in acetone at the concentration of 8 wt% is shown in figure 3. The pristine ATT began to precipitate within 10 min and then precipitated quickly. Nevertheless, the suspension of AC-ATT was quite stable and no 8


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obvious precipitation was found at the bottom of the bottle even after 30 min. The molecule chains of acrylic resin grafted to the ATT can stretch and entangle with each other, thus retard the precipitation and stabilize the suspension. Therefore, the dispersion stability of ATT in organic solvents was effectively enhanced after modification of ATT by acrylic resin. (Insert Figure 3) Fig. 4(a) and (b) display TEM images of ATT before and after modification. From Fig. 4(a), it can be seen that the pristine ATT agglomerates together in large scale and monodispersed rod-like ATT can hardly be found. However, after modification, the rod-like ATT appears monodispersion or agglomeration by several nanorods and there is seldom agglomeration by lots of nanorods. This indicates that the polarity of nanorods decreases after wrapping by acrylic resin, thus the interaction between nanorods is weakened. In Fig. 4(b), some dark spots on the ATT marked with black arrows should be the entangled acrylic resin. Hence, it is concluded that the acrylic resin has been grafted to the ATT by chemical bond. (Insert Figure 4) Fig. 5(a) and (b) are the contact angle (CA) pictures of ATT and AC-ATT. The contact angle of AC-ATT is around 108.6o, which is much larger than that of pristine ATT (29.9o). It can be inferred that the significant improvement on hydrophobic property of ATT is attributed to the long chain of acrylic resin. It can be speculated that the macromolecule with long molecule chain could wrap ATT more effectively in contrast with the micromolecule. The enhanced hydrophobic property of AC-ATT can be 9



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additional evidence that the acrylic resin has grafted on the ATT by covalent linkage. (Insert Figure 5) 3.2. Morphology of the nanocomposites Fig. 6 displays the surface morphology of EP and AC-ATT/EP. These SEM pictures were taken from the fractured surfaces of EP and AC-ATT/EP molded samples at room temperature. The surface morphology of epoxy is quite smooth and only few crazings are found, suggesting its fragile nature. With adding the AC-ATT to epoxy, the fracture surface is much rougher than epoxy indicating the toughness enhancement of the nanocomposites. Under stress, the nanorods deflect the crack path forming the thin crazings. In Fig. 6 the acrylic modified ATT orients randomly in the epoxy matrix and could be found in the form of a single nanorod, implying an improved compatibility between ATT and epoxy. Small aggregates including two or several nanorods appear when the concentration of the AC-ATT reaches to 7 wt%. Increasing the interface reaction or the length of the grafted molecules will enhance the solvation interaction between additives and the polymer matrix.19,

21

The phenomenon that the size of

exposed AC-ATT nanorods is enlarged compared with that of pristine ATT nanorods is due to the presence of the thin epoxy layer. The modified ATT has linked to the highly cross-linked epoxy by the reaction between carboxyl group and the epoxy group, thus homogeneous dispersion and strong interfacial interaction between AC-ATT and epoxy matrix are obtained. (Insert Figure 6) 3.3. Mechanical properties of AC-ATT/EP nanocomposites 10


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The tanδ of the pristine EP and nanocomposites with different contents of AC-ATT is shown in Fig. 7(a). The glass transition temperatures (Tg) of different contents of the AC-ATT modified epoxy are present in Table 1. It shows that the Tg of AC-ATT/EP (1wt%) increased slightly compared with the pristine epoxy. With the increase of the AC-ATT concentration, the value of Tg decreased. The variation of the Tg of polymer mainly depends on the strength of the interfacial interaction and the dispersion of particles in the polymer matrix.22 Based on the above analysis, we know that AC-ATT has linked to epoxy by chemical bond and is uniformly dispersed in the epoxy matrix as shown in Fig.6. Thus the decreased value of Tg attributes to the reduced cross-linking density. The acrylic resin used to modify ATT plays a crucial role in this work, which can introduce not only carboxyl to react with the epoxy group but also the mobilizable polymer chains of the acrylic resin around the nanoparticles. As a result, the cross-linking density of the zone in which epoxy reacted with acrylic resin is lower than that of the neat epoxy resin. Besides, the conformation changes of the acrylic resin chains result in additional free volume surrounding the rod-like ATT in the epoxy, which in return will decrease the confinement extent of the epoxy grafted to the AC-ATT, thus the Tg of the nanocomposites decreases when the content of AC-ATT is above 1wt%.21 (Insert Figure 7) (Insert Table 1) The storage modulus of the pristine EP and nanocomposites with different contents of AC-ATT is shown in Fig. 7(b). The storage modulus of AC-ATT/EP is apparently higher than that of EP. The increase in storage modulus of AC-ATT/EP reached maximum 11



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(128.9%) when only 3 wt% AC-ATT was added and the additive amount is much lower than reported.23 The homogeneous suspension reinforces the interfacial interaction, thereby reduces the concentration of additives and highly enhances G′ with few AC-ATT.24 The covalent linking between AC-ATT and EP is also supported to improve the storage modulus. However, the storage modulus decreases when the content of AC-ATT is above 3 wt%. Adding more AC-ATT to epoxy will increase the free volume and decrease the cross-link density, thus inducing the decrease of the G′ values. The impact strength of AC-ATT/EP is shown in Fig. 8(a). The impact strength of nanocomposites is much higher than that of pristine epoxy. The impact strength was remarkably enhanced with the increase of AC-ATT concentration when the concentration is below 3 wt%.The impact strength reaches the maximum with 3 wt% AC-ATT and then reduces slightly with further increasing the AC-ATT concentration. The toughness is affected by many factors, such as the ability of energy dissipation, the mobility of nanoparticles and the resistance to crack propagation.25 The acrylic resin served as flexible interlayer can reduce the stress concentration and relax the residual stress on the interface between epoxy and ATT, thus enhancing the impact toughness.26 In addition, the homogenous dispersion of AC-ATT in epoxy matrix could resist the crack propagation resulting in improvement of the toughness. The decrease of the impact strength may be caused by the inhomogeneous distribution and the decreased mobility of the nanoparticals when the concentration of AC-ATT increases. From the SEM images of fractured surfaces of nanocomposites, it can be seen that the nanorods modified by acrylic resin can cause the deflection of the cracking, thus leading to the 12


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enhanced toughness. Fig. 8(b) and (c) show the curves of the Young’s modulus and tensile strength of the nanocomposites with different loadings of AC-ATT. With increasing the concentrations of AC-ATT, both Young’s modulus and the tensile strength improve. By adding 5 wt% AC-ATT into the epoxy matrix, the Young’s modulus and tensile strength of the nanocomposites reaches 18.6 GPa and 75.96 MPa, which are 75.5% and 188.3% increase compared with that of epoxy. The improvement of tensile properties for epoxy is higher than the results reported before.27 These significant enhancements should be related to the modification of ATT and the homogeneous dispersion of AC-ATT in epoxy. The improvement can explain that the carboxyl groups on the surface of AC-ATT have participated in the curing reactions of epoxy. The enhanced interfacial interaction can transfer the stress from epoxy to the rigid ATT and thus the Young’s modulus and tensile strength are improved. The Young’s modulus and tensile strength of the sample (7 wt%) are higher than those of epoxy while they are lower compared with the value of 5 wt% sample. This trend is similar to the impact strength and that may also be caused by the inhomogeneous distribution when the concentration of AC-ATT increases. (Insert Figure 8)

4. CONCLUSIONS In this study, ATT modified by bifunctional acrylic resin containing trimethoxysilyl and carboxyl was employed to enhance the properties of epoxy. The rod-like ATT was 13



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activated by reflux in toluene to remove the adsorbed water and some zeolite water. TGA curves show that 11wt% of acrylic resin has been grafted onto the surface of ATT. The results of contact angle imply that the bifunctional acrylic resin makes a significantly impact on the surface hydrophobicity of nanorods. The high grafting efficiency of acrylic resin molecular chains on the nanorods has resulted in homogeneous dispersion of AC-ATT in epoxy. The greatest enhancements on storage modulus, impact strength, Young’s modulus and the tensile strength appear when the concentration is within the range of 3 wt% to 5 wt%, since uniform dispersion and strong interfacial interaction are achieved. Consequently, the bifunctional acrylic resin modified ATT shows a great potential as reinforcement for epoxy.

AUTHOR INFORMATION Corresponding Author *Tel: +86 931 8912516. Fax: +86 931 8912582. E-mail: [email protected] Notes The authors declare no competing financial interest.

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nanocomposites. Nat Mater. 2007, 6, 278-282. (23) An, L.; Pan, Y.; Shen, X.; Lu, H.; Yang, Y., Rod-like attapulgite/polyimide nanocomposites with simultaneously improved strength, toughness, thermal stability and related mechanisms. J Mater Chem. 2008, 18, 4928. (24) Schilling, T.; Jungblut, S.; Miller, M., Depletion-Induced Percolation in Networks of Nanorods. Phys Rev Lett. 2007, 98, 108303 (25) Hsieh, T. H.; Kinloch, A. J.; Masania, K.; Taylor, A. C.; Sprenger, S., The mechanisms and mechanics of the toughening of epoxy polymers modified with silica nanoparticles. Polymer. 2010, 51, 6284-6294. (26) Li, Y.; Lin, Q.; Chen, L.; Zhou, X., Assembly of triblock copolymer brush at glass fiber/polystyrene interface and its effect on interfacial shear strength. Compos Sci Technol. 2009, 69,1919-1924. (27) Wang, R.; Li, Z.; Wang, Y.; Liu, W.; Deng, L.; Jiao, W.; Yang, F., Effects of 17



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modified attapulgite on the properties of attapulgite/epoxy nanocomposites. Polym Composite. 2013, 34, 22-31.

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Figures:

Fig 1. FTIR spectra of AC, ATT, AC-ATT and AC-ATT-EP.

Fig 2. TGA curves of ATT, AC-ATT and AC-ATT-EP.

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Fig 3. Direct observation of the stability of ATT and AC-ATT in acetone: (a) 0 min, (b) 10 min and (c) 30 min.

Fig 4. TEM Images of (a) pristine ATT and (b) AC-ATT.

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Fig 5. The Contact Angle pictures of (a) ATT and (b) AC-ATT.

Fig 6. SEM images of fractured surfaces of (a) EP and different contents of AC-ATT/EP nanocomposites (b) 1wt%, (c) 3wt%, (d) 5wt% and (e) 7wt%.

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Fig 7. (a) Tan δ and (b) storage modulus curves for different loadings of AC-ATT/EP nanocomposites.

Fig 8. Mechanical properties of AC-ATT/EP with different concentrations: (a) Impact strength, (b) Young’s modulus and (c) Tensile strength.

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Table(s) Table 1. Tg for different loadings of AC-ATT/EP nanocomposites. AC-ATT content

0

1

3

5

7

162.4

164.7

155.2

157.6

160.4

G′ at 50oC (MPa) 1900

2320

4350

3569

2193

Increase (%)

22.1

128.9

87.8

15.4

(wt%) Tg(oC)

-

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Rod-Like Attapulgite Modified by Bifunctional Acrylic Resin As

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