The influence of PMMA grafted Halloysite Nanotubes on the by malj


									The influence of PMMA grafted Halloysite Nanotubes on the
            structure and property of epoxy acrylate resin

    Junheng Zhang1, Qinhua Qiu2, Wei Yeat Yek2, Feidi Wang3, Zhixin Jia1*, Baochun Guo1,
                                       Demin Jia1

  College of Materials Science and Engineering, South China University of Technology,
  Guangzhou 510640, China
  Sika Guangzhou Ltd., Guangzhou 510530, China
  Faculty of Chemical Engineering and Light Industry, Guangdong University of Technology,
  Guangzhou 510090, China

Abstract: Addition of nanoparticels to epoxy acrylate (EA) resin can significantly improve their
mechanical properties. Halloysite nanotubes (HNTs) are one kind of novel reinforcing materials
for polymers. In present work, HNTs were compounded to epoxy acylate resin to improve the
abrasistent resistant and toughness of the resin. To improve the embedding of HNTs within the
epoxy acrylate matrix, the HNTs surface was grafted by poly (methyl methacrylate) (PMMA). The
morphology of the grafted HNTs particles demonstrates a core-shell structure with HNTs as the
core and the polymer as the shell. PMMA was chemically attached to the HNTs particles as
indicated by the FTIR and XPS results. The grafted HNTs then compounded with EA resin and
nanocomposites coatings are formed via UV-curing method. The EA/HNTs nanocomposites
exhibited markedly improved properties as compared to neat EA, e.g., improved hardness,
flexibility, impact energy and abrasion resistance. The well dispersal of HNTs in the matrix is
attributed to the modification on the surface of HNTs.
Key words: halloysite nanotubes/epoxy acrylate/grafting/naocomposites/UV curing

1. Introduction
      UV-curable coatings offer considerable attention for the rapid and solvent-free curing of
coating films[1]. Nanoparticles, such as silica[2], clay nanolayer[3-5], zinc oxide[6,7], zirconia[8,9] and
titanium dioxide[10], have been recently used in UV curable nanocomposites to improve the
physical mechanical properties of coatings, especially promote hardness and result in a strong
increase in scratch and abrasion resistance [11-14]. However, the key issue of preparing
nanocomposites is how to disperse the inorganic nanoparticles in the polymer matrix uniformly
and to avoid aggregation and phase separation. Therefore, the modification of the nanoparticles
surface can offer superior performance, improved dispersibility and increased compatibility of
nanoparticles in the matrix. Due to the surface modification of nano-sized SiO2, UV curable
acrylate formulations show markedly improved properties as compared to neat polymers, e.g.,
increased microhardness and modulus, improved scratch and abrasion resistance, higher gas
barriers and temperature resistance[15-17].
      Halloysite is defined as a two-layered aluminosilicate, chemically similar to kaolin, which
has a predominantly hollow tubular structure in the submicrometer range. HNTs are cheap,
abundantly available, and durable, with high mechanical strength and biocompatible. HNTs have
been demonstrated to be an ideal component for fabricating high performance polymer
nanocomposites, such as polypropylene [18], polyamide [19], and styrene rubber [20], particularly for
strengthening and toughening epoxies [21-23].
      It is well known that the dispersion state of the filler and the interfacial properties are two
critical factors in determining the ultimate performance of the polymer composites [24]. For large
surface area (specific area = ~60 m2/g), however, HNTs can hardly be effective reinforcement of
polymers because of the unsatisfied interfacial bonding and agglomeration in the polymer
matrix[25]. Furthermore, surface modifications to HNTs may decrease the possibility of particle
agglomeration, and provide good wetting and bonding of HNTs with polymers [26]. With the
improvement in particle dispersion in matrix, enhancements in the mechanical properties of the
halloysite–nanocomposites have been achieved [27,28].
      In this study, surface grafting with poly (methyl methacrylate) (PMMA) was conducted to
obtain HNTs/EA nanocomposites with improved homogeneity of HNTs in the matrix and with
enhanced mechanical performance.

2. Experimental
2.1. Materials
     HNTs were mined from Hubei province, China and purified according to the reported
procedure [20]. Methyl methacrylate (MMA) was purched from Sinopharm Chemical Reagent Co.
Ltd. and distilled under reduced pressure. 2, 2-Azobis(isobutyronitrile) (AIBN) was purchased
from Sinopharm Chemical Reagent Co. Ltd., and recrystallized from ethanol prior to use.
2-Propanol, chloroform and all other chemicals were purchased from Sinopharm Chemical
Reagent Co. Ltd. and dried over 4 Å sieves for 24 h before use. 3-(trimethoxysilyl) propyl
methacrylate (MPS) was purched from Dow Corning Co., Ltd. and used as received. Bisphenol A
epoxy di(meth)acrylate (EB605) was purched from UCB Co., and used without further
purification. The photoinitiators 1-hydroxycyclohexyl benzophenone (Irgacure-184) was purched
from Ciba Co.

2.2. Synthesis of m-HNTs
      The synthetic procedure of HNTs-PMMA is described in Scheme 1. First, the surface of
HNTs was functionalized with MPS. 10.0 g HNTs was added in solution (5.0 g MPS dissolved in
100.0 ml toluene), the mixture was stirred and refluxed for 12 h. Then the suspension was filtered
using a PTFE membrane with a pore size of 0.22 μm by using a large amount of toluene to wash
off the unreacted reagents. Finally, the solids HNTs-MPS were obtained after vacuum drying at
60 ℃ for 24h.
      5.0 g MPS-HNTs, 5.0 g MMA and 50 ml 2-propanol were added into a 150 cm3 three-necked
flask and then ultrasonicated for 30 min under N2 atmosphere. AIBN was dissolved and added into
the aforementioned suspension with stirring and at refluxed for 12 h. When the reaction was
complete, the product was cooled by ice water and poured into large methanol to induce
precipitation. The grafted products were washed several times with methanol, and then the
products were dried at 60 ℃ for 24h under vacuum. Exactly weight of the above product was
extracted with the chloroform in a soxhlets extractor for 72h to remove PMMA homopolymer. The
pure copolymers were dried at 60 ℃ for 24h vacuum. The percentage of grafting and grafting
efficiency was determined by TGA [29,30].
2.3. Photopolymerisation of nanocomposites
      The EB605 was mixed with different amounts of HNTs-PMMA and ultrasonicated for 30 min.
Then the photo-initiation (Irgacure-184) was added and treated with ultrasonication for 5 min. At
all times samples were not exposed to light. After degassing under reduced pressure, the above
liquid resin was dropped onto glass plates or tinplates. The thickness of resultant coatings were
controlled about 20 μm. Samples were cured with the UV lamp (1 KW) for 30 sec.

                     Scheme 1 Procedure for preparation of HNTs-PMMA

2.4. Characterizations
Characterization of PMMA Grafted HNTs
     Fourier Transform Infrared Spectrometer (FTIR) spectra were obtained from a Bruker
VERTEX 70. The spectrum was scanned 16 times at a resolution of 4 cm-1. X-ray photoelectron
spectroscopy (XPS) spectra of the HNTs and HNTs-PMMA were recorded using a Kratos Axis
Ultra DLD with an aluminum (mono) Kα source (1486.6 eV). Thermogravimetric analysis (TGA)
was performed on a TA Q5000 instrument in nitrogen atmosphere, at a heating rate of 10 ℃/min
from 30 ℃ to 700 ℃. The morphologies of HNTs and HNTs-PMMA were examined on a JEOL

Characterization of nanocomposites
     The pencil hardness of the nanocomposites coatings was tested with a pencil sclerometer
according to ISO 15184-98. The flexibility of the coatings was determined with an YZQ-II
cylindrical mandrel bend tester in accordance with ASTM D4338-97. Impact resistance was
performed on a CJQ-II paint film impact tester, according to the ASTM D1709-09. All above
mentioned testing instruments are manufactured by Shanghai Pushen Chemical Industry
Machinery Co., China. Glossness (60°) was determined according to ASTM D2457-03 on an
ETB-0833 glossnessmeter (Factory Yisitong Electronic Instruments Factory, China). The
transmittance test was visualized on a WGT-S Hazemeter (Shanghai Precision & Scientific
Instrument Co,.Ltd) according to ASTM D1003-61. The abrasion resistance of neat EA resin and
nanocomposites coatings was measured with an Abrasion Tester JM-IV (Tianjin Jinke Chemical
Industry Machinery Co., China) according to ISO7784.2-97. The abrasion values are given by the
mass losses after 1000 cycles under a force of 250N. The morphology of the fracture surfaces of
the specimens after impact testing and the worn surfaces of nanocomposites coatings after wear
experiments were observed on a FEI Nova NanoSEM 430.

3. Results and discussion
3.1. Characterization of HNTs and m-HNTs
     FTIR and XPS measurements have been performed on HNTs and HNTs-PMMA. Figure 1
displays the FT-IR spectra of HNTs, HNTs-MPS and HNTs-PMMA. Spectra of HNTs-MPS
exhibits new absorption peaks at 1720 cm-1 (C=O) and 2953 cm-1 (C-H). Furthermore, the peaks
of OH stretching band at 3703cm-1 and 3628 cm-1 become weaker, since hydroxyl groups on the
surface of HNTs has been partially reacted with methoxy groups of MPS. Spectra of
HNTs-PMMA shows that the C-H stretching vibration at 2953cm-1 and C=O vibration at 1732cm-1,
the -CH3 bending bonds are beheld 1393cm-1, and the -C(O)-O-C stretching are confirmed in the
region between 1244cm-1 and 1153cm-1. Figure 2 shows the survey XPS spectra of HNTs and
HNTs-PMMA, the surface element contents of C and O increase and that of Si and Al decrease
after the PMMA had been grafted (Table 1). This can be attributed to exposure of HNTs
underneath. The FTIR and XPS results show that PMMA has been covalently grafted onto the
surfaces of HNTs.

                 Figure 1 FT-IR spectra of HNTs, HNTs-MPS and HNTs-PMMA

                             a                                       b
                        Figure 2 XPS spectra of HNTs and HNTs-PMMA

                       Table 1 Surface composition data from XPS analysis

     The percentage of grafting and grafting efficiency of MMA to HNTs can be calculated by
TGA result. Figure 3 shows the thermogravimetric curves of HNTs, HNTs-MPS and
HNTs-PMMA after chloroform extraction 72h. It is observed that the grafted polymer begins to
decompose at about 250 ℃ and ends at 500 ℃, which ascertains that polymer is bonding onto
the HNTs surfaces. The percentage of grafting and grafting efficiency is 11.4% and 17.8%,

             Figure 3 TGA thermograms of HNTs, HNTs-MPS and HNTs-g-PMMA

     The morphology of HNTs and HNTs-PMMA is compared using TEM observation. It can be
seen that the surface of the HNTs is rather smooth with a tube diameter of about 50 nm (Figure 4
a). As shown in Figure 4 b, it is clearly indicated that the surface of HNTs-PMMA is mostly
covered with an outer layer of about 2-5 nm thick. The morphology of the HNTs-PMMA particles
demonstrate a core-shell structure with HNTs as the core and polymeric as the shell. Therefore, we
can conclude that the PMMA are grafted onto the HNTs.

                   a                                                        b

                    c                                                  d
                  Figure 4 TEM images of (a, b) HNTs and (c, d) HNTs-PMMA

3.2. Mechanical and Optic properties of Nanocomposites Coatings
     Table 2 shows the mechanical and optic properties of nanocomposites coatings. As seen in
table 2, the pencil hardness shows continuous improvement with increasing HNTs-PMMA content.
This hardness increase is due to the hard HNTs nanoparticles that can probably in part migrate
toward the surface of the coating [31,32], and the increase of hardness is always related to an
increase on scratch and abrasion resistance[33]. It noticeably found that the impact energy of
nanocomposites coatings increased 50% compare to neat EA resin. It accords with the previously
studies which demonstrated that blending epoxy with an appropriate amount of HNTs could
significantly increase impact strength of hybrid resin [22,23]. Furthermore, the flexibility of coatings
has slight increases. However, the transmittance of coatings decreases as the particles loading
increasing, but it retained transmittance above 80%.

   Table 2 Mechanical and optic properties of nanocomposite coatings with different content of

3.3. Morphology of Nanocomposites Coatings
      To investigate the toughening mechanisms, the fracture surfaces of the broken specimens are
examined by using SEM. The fracture surface of neat EA resin (Figure 5 a) is rather smooth, and
shows typical characteristics of brittle fracture. The fracture surface of nanocomposites coating
filled with 0.5wt_% HNTs-PMMA or 1 wt_% HNTs-PMMA is relatively rough. The
improvements of toughness and flexibility can be attributed to mechanisms such as crack front
pinning or bridging, due to the large aspect ratios of HNTs, and crack deflection and twisting, as
well as plastic deformation [26,34]. It can be seen that particles dispersion in the nanocomposite is
basically uniform in figure 5 c. Figure 5 c shows some broken HNTs on the fracture surface,
though they are most left in the matrix because of the well adhesion between the HNTs and epoxy.
Therefore, it is critical that modification of HNTs can effectively reduce cluster size and enhance
the adhesion between the particles and matrix.

                     a                                                           b

                    d                                                             c
Figure 5 SEM micrographs of the fracture surface of (a): EA; (b): EA/HNTs nanocomposite with
       0.5_wt% HNTs-PMMA ;(c, d): EA/HNTs nanocomposite with 1_wt% HNTs-PMMA

3.4. Friction and wear behaviors of UV Cured Films
     Figure 6 indicates the effect of HNTs-PMMA content on abrasion resistance of
nanocomposites coatings. It can be clearly seen that the incorporation of HNTs-PMMA
significantly decreases the wear volume ratio, though; there was a pronounced minimum around
1_wt% HNTs-PMMA content. It is found that when the content of HNTs-PMMA exceeds 1_wt%,
the wear volume ratio of HNTs-PMMA/EA nanocomposites coating increases slightly with
increasing HNTs-PMMA content. This wear minimum in the range of a few weight per cent
nanoparticles content is typical for nanocomposites[35]. Only small amounts of nanoparticles can
be uniformly mixed in the polymer matrix when every nanoparticle can be properly wetted for its
large specific surface. As the content of HNTs-PMMA exceeding this limit, it usually causes
embrittlement of the composites and promotes susceptibility to surface fatigue [36]. SEM
inspections of the worn surfaces give the informations about the wear mechanisms. The SEM
images of the wear marks of neat resin coating and nanocomposites coating with 1_wt%
HNTs-PMMA are shown in figure 7. The corresponding surface of neat EA resin coating is rather
rough, displaying plucked and ploughed marks. The worn surface of neat EA resin coating shows
signs of adhesion and abrasive wear. It can be seen that obvious ploughed furrows appear on the
worn surface of neat resin coating. By contrast, the worn surface of nanocomposite coating with
1wt% HNTs-PMMA is smoother. Some looser wear debris but few and smaller scratches can be
found on the surface. This phenomenon corresponds to the increased wear resistance. As similar as
the prominent friction and wear mechanisms of MWNTs/EP nanocomposites[36-39], the
incorporation of HNTs into EA reisn increases the mechanical properties (both hardness and
toughening) of resin. During the course of friction and wear, HNTs serve as spacers preventing the
close touch between the grinding wheel face and the nanocomposites coating block, otherwise, the
self-lubricate properties of HNTs result in enhancing the abrasion resistance. Therefore, the
HNTs-PMMA/EA nanocomposites show much better abrasion resistance than neat EA resin.

          Figure 6 The dependence of abrasion resistance of EA resin coatings on the content of
                                           HNTs-PMMA particles

                        a                                                              b

                        c                                                              d

                         e                                                             f
    Figure 7 SEM micrographs of the worn surface of (a,b): EA; (c, d, e, f): HNTs-PMMA/EA
                                     nanocomposite with 1_wt% HNTs

4. Conclusions
     Chemically grafting of PMMA onto the surface of HNTs is accomplished through in situ
polymerization. The HNTs-PMMA is found to improve the basic mechanical performance of EA
coating effectively, particularly toughness and abrasion resistant. The pencil hardness improves
three higher grades with 5wt_% HNTs-PMMA content. The wear volume of coating containing
1_wt% HNTs is reduced to quarter, as compared to that of neat EA resin coating. The wear
reductions are likely due to the enhanced hardness and toughness. Especially, HNTs serves as
spacers slowing the wear rate and reduces the friction coefficient, the self-lubricate properties of
HNTs also increase the abrasion resistant.

Acknowledgements: This work was financially supported by National Nature Science
Foundation of China with grant number of 51003031.

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Captions for the tables
Table 1 Surface composition data from XPS analysis
Table 2 Mechanical and optic properties of nanocomposite coatings with different content of HNTs-PMMA

                             Table 1 Surface composition data from XPS analysis
                                                     Surface concentration (%)
                                                   O 1s       C 1s   Si 2p    Al 2p
                                      HNTs         54.3       15.3   16.7         13.7
                                HNTs-PMMA          51.2       32.6    9.7         6.5

   Table 2 Mechanical and optic properties of nanocomposite coatings with different content of HNTs-PMMA

  HNTs-PMMA content,          Hardness,        Flexibility,          Impact energy,      Transmittance,   Gloss,

          wt_%                   H                mm                    Kg·cm                  %           60°

            0                     3                 6                        20              92.0         80.6

           0.5                    4                 6                        22              87.0         89.2

            1                     5                 5                        30              84.3         127.3

           2.5                    6                 5                        35              83.3         119.0

           5.0                    6                 5                        25              80.5         103.5

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