Effect of tri(methoxysilyl)propyl methacrylate silane modified nanosilica on some properties of acrylic coating

This paper presents the characteristics of nanosilica modified with different contents of trimethoxysilylpropyl methacrylate silane MPTS as a coupling agent and the effect of modified na

EFFECT OF TRI(METHOXYSILYL)PROPYL METHACRYLATE

SILANE MODIFIED NANOSILICA ON SOME PROPERTIES OF

ACRYLIC COATING

Nguyen Thuy Chinh1, 2, Dao Phi Hung1, 2, Nguyen Anh Hiep2,

Nguyen Xuan Thai2, Thai Hoang1, 2, *

1

Graduate University of Science and Technology, Vietnam Academy of Science and Technology,

18 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam

2

Institute for Tropical Technology, Vietnam Academy of Science and Technology,

18 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam

*

Email: hoangth@itt.vast.vn

Received: 15 June 2021; Accepted for publication: 19 October 2021

Abstract Acrylic emulsion resin is one of popular matrixes for both outside and inside coatings

It has low cost, high UV and weather resistance, good aesthetics and environmentally friendly

The introduction of inorganic additives in nanosize such as SiO2, TiO2, etc into acrylic emulsion

resin could improve the mechanical and thermal properties of the polymer matrix The surface

modification of nanoparticles can enhance the dispersibility of the nanoparticles in the polymer

matrix This paper presents the characteristics of nanosilica modified with different contents of

tri(methoxysilyl)propyl methacrylate silane (MPTS) as a coupling agent and the effect of

modified nanosilica on the abrasion resistance and thermal properties of acrylic emulsion resin

The infrared (IR) spectroscopy and thermo-gravimetric analysis (TGA) were used to evaluate

the presence of MPTS on the surface of modified nanosilica Grafting yield of MPTS on the

surface of nanosilica was calculated from TGA diagrams The obtained results showed that

nanosilica was surface modified successfully by MPTS with a grafting yield of 47 % when using

3 wt.% of MPTS After modification, the modified nanosilica became more hydrophobic and

can disperse well in acrylic emulsion resin The modified nanosilica improved significantly the

abrasion resistance of acrylic coating as compared to neat acrylic resin and acrylic/unmodified

silica nanocoating The MPTS modified nanosilica has the potential to be applied as a

reinforcing additive to acrylic emulsion resins to improve its abrasion resistance

Keywords: organic modified nanosilica, acrylic coating, silane coupling agent, thermal properties

Classification numbers: 2.5.3, 2.9.4

1 INTRODUCTION

In recent years, organic coatings containing nano-additives are widely used not only to

decorate plastics, metals, concrete, glass, etc but also as a protective coating for these materials

under environmental impact Nano-additives with their small size can improve properties

(including mechanical, physical, thermal, anti-corrosion properties, etc.) of organic coatings

[1-2], especially, long-term stability without affecting other coating properties [3] Among the popular organic coatings, acrylic resin coatings are used the most, followed by epoxy resin and alkyd because acrylic resins have valuable properties such as high UV resistance, high aesthetics, and weather resistance Moreover, acrylic emulsion resin is environmentally friendly, low cost and quick drying [4, 5]

Nanosilica has been known as an inorganic reinforcing additive for acrylic resin-based coatings because it is readily dispersed in the resin, therefore, the mechanical properties, weather stability and thermal stability of acrylic resin can be improved [4 - 6] Nanocomposite coatings containing nanosilica have high abrasion resistance, good transparency and weather resistance, therefore, they are used for clear coats [7 - 10] In particular, organic modified nanosilica improves scratch resistance, wear resistance, hardness of the coating In addition, organic treated nanosilica enhances the coating’s anti-corrosion, gloss and color

In our previous publications, silica nanoparticles were modified with isopropyl tri(dioctyl phosphate) titanate coupling agent and incorporated into epoxy resin The dispersion ability of nanosilica in epoxy resin was significantly improved after organic modification, leading to the improvement in the tensile strength, flexural strength, thermal stability, dynamic mechanical properties and flame resistance of neat epoxy in the presence of the above modified nanosilica [11, 12] In another report, the colloidal nanosilica particles (CSPs) were modified with 3-(methacryloyloxy) propyl trimethoxy silane (MPTS) (0.3, 0.45, 0.6, 1.0, 1.5 g/g of MPTS/SiO2),

the application of modified nanosilica was also mentioned in this report [13] X Guo et al

modified nanosilica with MPTS and prepared the modified nanosilica/poly(methylmethacrylate) (PMMA) core-shell composite latex for toughening PVC matrix [14] The organic-inorganic hybrid gels containing nanosilica and nanosilica modified with 5 wt.% of MPTS were prepared [15] Based on thermo–gravimetric and elemental analysis, Francis Pardal et al calculated the grafting yield of MPTS on the surface of silica nanoparticles of 0.45 - 3.93 % [16] The MPTS modified silica nanoparticles were used in rubber, PMMA, poly(terephthalate) (PET) [17-19] In

these literatures, the silica nanoparticles could be modified by in-situ method from

tetraethoxysilane precursor or solution method in toluene or ethanol solvent Although the applications of MPTS modified silica nanoparticles in PVC, PMMA, PET, rubber have been studied, the effect of MPTS content and MPTS modified nanosilica content on abrasion resistance, morphology and thermal stability of acrylic emulsion-based nanocoatings is still limited in research Meanwhile, the abrasion resistance is one of most important properties for decorating paint Therefore, the purpose of this study is to evaluate the effect of nanosilica modified with MPTS on some properties of nanocoatings based on acrylic emulsion resin The characteristics of modified nanosilica as well as the mechanical, thermal and morphological properties of acrylic/nanosilica coatings will be evaluated and discussed

2.1 Materials

Nanosilica (nano-SiO2, Sigma-Aldrich, USA) has an average particle size of 20-30 nm and

a specific surface area of 200 m2 g-1 Silane coupling agent as 3-(trimethoxysilyl)propyl methacrylate (MPTS, Sigma-Aldrich, USA) has a purity of 98 % Acrylic emulsion resin (Primal AC261, Dow Company, USA) has a solid content of 49 ± 1 % Texanol Ester Alcohol (2,2,4-Trimethyl-1,3-pentanediol, monoisobutyrate, Dow Company, USA) has a purity of 99 %, a

density of 0.95 g/mL at 25 oC Others (ethanol 99.7 %, ammonia solution 25 %, acetic acid 99

%, paint additives, etc.) are analytical chemicals from Vietnam and China

2.2 Modification of nanosilica and preparation of acrylic/nanosilica coatings

SiO2 nanoparticles were modified according to the procedure for modification of TiO2 nanoparticles described in our previous report [20] The amount of MPTS varied by 0, 1, 3, 5, 7, and 20 % by weight compared with the nanosilica weight The modified silica nanoparticles were abbreviated as u-SiO2, m-SiO2-1, m-SiO2-3, m-SiO2-5, m-SiO2-7, m-SiO2-20 corresponding to 0, 1, 3, 5, 7, and 20 wt.% of MPTS amount, respectively

To prepare acrylic/nanosilica coatings, unmodified or modified silica nanoparticles were first dispersed in distilled water at a ratio of 1/10 (w/w) by ultrasonicating for 30 minutes (mixture A) At the same time, texanol was added to the AC261 acrylic solution at a ratio of 1.5/100 (w/w) The mixture was stirred on a magnetic stirrer at a speed of 300 rpm for 15 minutes (mixture B) Next, A and B were mixed together and stirred on a magnetic stirrer at 300 rpm for 15 minutes before ultrasonicating for 1 hour to obtain a homogenous mixture The content of components in nanocoatings and designature of samples were presented in Table 1 The acrylic nanocoating containing 1 wt.% of u-SiO2 was also prepared (abbreviated as AuS1.0) Nanocoatings with a thickness of 120 µm (made by Erichsen Film Applicator 360 thickness wiper) were naturally dried for 7 days and stored at room temperature (~ 25 oC) and a relative humidity of 60 % for further tests

Table 1 Content of components (wt.%) in nanocoatings and designature of samples

2.3 Characterization

Characterization of modified nanosilica: Attenuated total reflection infrared (ATR-IR) spectra of nanosilica samples were recorded using a Nicolet iS10 spectrometer (Thermo Scientific, USA) in wavenumbers of 400 cm-1 4000 cm-1 Field emission scanning electron microscopy (FESEM) images of samples were taken on a FESEM S4800 analyzer (Hitachi, Japan) Transmission electron microscopy (TEM) images of samples were obtained using a JEM1010 device (JEOL, Japan) The samples were coated with platinum to increase the conductivity before taking FESEM and TEM images Thermo-gravimetric analysis (TGA) was carried out using a TGA 60H analyzer (Shimadzu, Japan) The samples were heated from room temperature to 900 oC in air with a heating rate of 10 oC/min The samples were dispersed in distilled water for recording UV-Vis spectra The contact angle (CA) of samples was obtained using a SEO Phoenix-150 analyzer (South Korea)

Characterization of acrylic/nanosilica coatings: Their abrasion resistance was determined according to Falling Sand Abrasion Tester method (ASTM D968-15) Their morphology was evaluated by FESEM method using a FESEM S4800 analyzer (Hitachi, Japan)

3 RESULTS AND DISCUSSION 3.1 Chacteristics and morphology of unmodified and modified nanosilica

3.1.1 IR spectra

Wavenumbers (cm -1 )

1000 3000

4000

u-SiO2 (1) m-SiO2-1 (2) m-SiO2-3 (3) m-SiO2-5 (4) m-SiO2-7 (5) m-SiO2-20 (6)

(6)

(5)

(4)

(3)

(2)

(1)

1073

939 1331 1633 1720 2916

Figure 1 IR spectra of unmodified and modified nanosilica

Table 2 Vibrations of functional groups on IR spectra of unmodified and modified nanosilica

m-SiO 2-3 2920 1702 1641 1327 1307 941 1057, 804 595

m-SiO 2-5 2922 1700 1635 1329 1299 937 1058, 804 595

m-SiO 2-7 2918 1701 1633 1326 1303 939 1056, 804 597

m-SiO 2 -20 2916 1720 1633 1331 1299 939 1057, 807 597

The IR spectra of unmodified nanosilica (u-SiO2) and nanosilica modified with silane coupling agent as MPTS (m-SiO2) are presented in Fig 1 The spectrum of u-SiO2 showed peaks

at 1073 cm-1, 809 cm-1 and 572 cm-1 which were attributed to assymetric stretching, symmetric stretching and bending vibrations of the Si-O bond in nanosilica [14, 15, 19] Compared with the

IR spectrum of u-SiO2, the IR spectra of m-SiO2 had some differences The first point was that the intensity of the peak characteristic for Si-O stretching vibration was two times higher as well

as the postition of this peak was shifted slightly (5 - 17 cm-1) due to the resonance of Si-O groups in MPTS coupling agent with Si-O groups in nanosilica The second point was the

appearance of new peaks assigned to vibration of C-H, C=O, C=C, C-O/C-Si groups in MPTS molecules (Table 2) [14, 15] The third point was the new shoulder at 939-941 cm-1 attributed to the vibration of the O-Si-C bond [21] The intensities of this shoulder and the C-H, C=O, C=C, C-O/C-Si peaks were grown up with increasing the MPTS amount for modification These points were evidence for the successful grafting of MPTS on the nanosilica surface During the modification, MPTS was first hydrolyzed to form an organosilanetriol, which then reacted with

OH groups on the surface of nanosilica to form a C-Si-O-Si bond

3.1.2 Thermal behavior

TGA is a popular method used for calculating the amount of silane coupling agent grafted

on the surface of silica nanoparticles [13 - 14, 16 - 17] Therefore, in this study, the thermo-gravimetric (TG) and derivative of the TG (DTG) diagrams of unmodified and modified nanosilica samples (Figure 2) were used for determining the MPTS amount grafted on the surface of nanosilica

Figure 2 TG and DTG curves of unmodified (a) and modified nanosilica with 1 % (b), 3 % (c), 5 %

(d), 7 % (e) and 20% (f) MPTS samples

Table 3 TG, DTG parameters and grafting yield of MPTS of modified nanosilica samples

Sample Weight loss at

600 o C (%) T max ( o C) The initial MPTS

content (%)

Grafting yield of MPTS (%)

From TG and DTG diagrams of unmodified nanosilica, it can be seen that the loss weight

of nanosilica (2.69 %) below 200 oC was due to the loss of water absorbed on the surface of nanosilica (the maximum degradation temperature – Tmax of 118.84oC) After modification, the dehydration peak below 200 oC was not present in the DTG diagrams of the modified nanosilica samples Instead, the maximum degradation peak of MPTS grafted on the surface of nanosilica was presented in the temperature range of 250-550 oC [14, 16] The reason may be that the

reaction of MPTS with OH groups on the surface of nanosilica caused the absence of a water loss peak in the DTG diagram of m-SiO2 When heated to high temperature, the MPTS part grafted on nanosilica was degraded, leading to the appearance of the maximum degradation peak

at 250 - 550 oC With increasing MPTS content, the weight loss of m-SiO2 increased For example, the weight losses at 600 oC of m-SiO2-1, m-SiO2-3, m-SiO2-5, m-SiO2-7 and m-SiO2

-20 were 3.04, 4.10, 4.12, 4.27 and 4.79 %, respectively

The grafting yield of MPTS can be calculated according to this formula:

( ) (1)

As observed from Table 3, MPTS grafting yield of reached its maximum value at the initial MPTS amount of 3 wt.%, and decreased gradually with increasing initial MPTS amount The reason could be that MPTS was grafted on the nanosilica surface, so a layer of MPTS was formed and it covered the nanosilica surface This MPTS layer prevented other MPTS molecules from continuously grafting on the nanosilica surface This could be a sufficient amount of MPTS

to obtain a saturated reaction on the nanosilica surface [22] Moreover, when the MPTS amount used was high, vinyl groups in the excess MPTS could be polymerized, causing spatial obstruction As a result, the grafting yield of MPTS was reduced

3.1.3 Morphology

The FESEM and TEM images of u-SiO2 and m-SiO2-3 (nanosilica modified with 3 wt.% of MPTS) samples are shown in Figure 3 It can be seen that the nanosilica presented a spherical shape with a basic size in the range of 10 - 20 nm (Fig 3a, c) However, they were agglomerated into clusters over 50nm in size due to the hydrogen bonding between OH groups on the surface

of the nanosilica [23]

Figure 3 FESEM and TEM images of u-SiO 2 (a, c) and m-SiO 2-3 (b, d)

After modification, the agglomeration of MPTS modified nanosilica was reduced (Fig 3 b,d) This could be explained by the fact that the hydrophobization of m-SiO2 gradually increased in the presence of MPTS and its hydrophilicity was weakened [14, 24]

3.1.4 Contact angle

The water droplet images on the surface of u-SiO2 and m-SiO2-3 are presented in Fig 4 After dripping, the water droplet was quickly and completely penetrated into the sample surface Therefore, the contact angle of the samples was relative and was measured immediately after water was dropped onto the sample surface and the dropper was withdrawn The relative contact angle of u-SiO2 and m-SiO2-3 was about 18o and 39o, respectively This result indicated that the modification helped slightly improve the hydrophobicity of nanosilica

Figure 4 Water droplet images on the surface of u-SiO 2 (a) and m-SiO 2 -3 (b)

3.2 Chacteristics, properties and morphology of acrylic/silica nanocoatings

3.2.1 Abrasion resistance

The abrasion resistance is one of the important properties of paint coating Acrylic coating had a low abrasion resistance, at 69 L/mil In the case of using 1.0 wt.% u-SiO2 nanoparticles, the abrasion resistance of acrylic coating was significantly improved, up to 107 L/mil This was

a general tendency for nanocoatings containing a low weight fraction of nanosilica [8, 25] It can

be explained that nanosilica could establish suitable bonds and approciate interfacial interaction with polymer matrix Nanosilica particles could improve mechanical properties such the hardness, toughness and static modulus of the nano-composite coats [25] Hence, the abrasion resistance of the nanosilca-filled coating filled was increased

The effect of MPTS content on the abrasion resistance of acrylic/nanosilica coatings containing 1.0 wt.% m-nanosilica is presented in Figure 5 It can be seen that the abrasion resistance of acrylic coatings was enhanced when using nanosilica modified with 1 wt.% and 3 wt.% MPTS as compared to acrylic coatings containing 1.0 wt.% u-SiO2, and then this property

of nanocoatings was reduced with increasing MPTS content for nanosilica modification This was caused by a less dispersal of m-SiO2 nanoparticles in acrylic resin at high content of silane coupling agent since the polymerization of silane led to the agglomeration of m-SiO2 nanoparticles as above discussed Moreover, a low grafting yield of MPTS on the surface of nanosilica at high initial silane content also reduced the efficiency enhancement of the m-SiO2 nanoparticles The nanosilica modified with 3 wt.% MPTS was most effective in improving the abrasion resistance of the acrylic coating, 1.83 times and 1.125 times as compared to neat acrylic and acrylic/nano u-SiO2 coatings, respectively This result suggested that the MPTS content and

the grafting yield of silane on nanosilica affected the abrasion resistance of the acrylic coatings The most suitable content of MPTS for modifying nanosilica, used in acrylic coatings was 3 wt.%

Figure 6 shows the graph of the dependence of abrasion resistance of acrylic coatings on the m-SiO2-3 nanoparticles content The contents of nanosilica modified with 3 wt.% MPTS used in acrylic coatings, were 0.25, 0.5, 1.0, 2.0 wt.% The neat acrylic resin coating had low abrasion resistance, at 69 L/mil The abrasion resistance of acrylic resin filled with u-SiO2 nanoparticles was improved (increased by 1.55 times in comparison with neat coating) The m-SiO2 nanoparticles improved the abrasion resistance of acrylic resin better than u-SiO2 nanoparticles did The abrasion resistance of acrylic coatings had a gradually increasing tendency with raising the m-SiO2 nanoparticles from 0.25 wt.% to 1 wt.% However, the abrasion resistance of the coating decreased when the m-SiO2 nanoparticles content was higher than 1 wt.% The abrasion resistance enhancement of coatings was more significant in the case

of lower weight fraction as confirmed by Malaki et al [25] In this work, 1 wt.% m-SiO2-3 nanoparticles was the most suitable content to prepare acrylic coatings with good abrasion resistance

Figure 5 Abrasion resistance of acrylic

coatings filled with 1.0 wt.% m-SiO 2 nanoparticles

modified with different MPTS contents

Figure 6 Abrasion resistance of acrylic/m-SiO 2 -3 nanoparticles coatings prepared with different m-SiO 2 -3 contents

3.2.2 Morphology

Figure 7 FESEM images of cross surface of acrylic/u-SiO 2 nanoparticles (a) and acrylic/m-SiO 2 -3

nanoparticles coatings

The FESEM images of the cross surface of acrylic/nano u-SiO2 (a) and acrylic/nano m-SiO2-3 coatings are shown in Fig 7 It can be seen that the nano u-SiO2 was agglomerated in

acrylic resin (Fig 7a) The size of u-SiO2 nanoparticles cluster was up to 1 µm The m-SiO2 nanoparticles were uniformly dispersed in acrylic resin (Fig 7b) This may be due to the binding

of silane functional groups on the surface of m-SiO2 nanoparticles with carbonyl groups in acrylic resin, leading to more regular dispersion of m-SiO2 nanoparticles in acrylic resin matrix

4 CONCLUSIONS

In this study, the nanosilica was modified succesfully with silane coupling agent - MPTS and used as an enhancement additive for acrylic emulsion resin coating From the obtained results, the most suitable content of MPTS for surface modification of nanosilica was found at 3 wt.% The organic modification helps to reduce the hydrophilicity and agglomeration of nanosilica The grafting yield of MPTS on the suface of nanosilica reached 47 % at 3 wt.% MPTS The modified silica nanoparticles enhanced the abrasion resistance of the acrylic coating better than unmodified nanosilica The content of 1 wt.% modified nanosilica was the most suitable for improving the abrasion resistance of the acrylic coating (1.83 times higher) The modified nanosilica was well dispersed in acrylic resin coating These results suggest that MPTS modified silica nanoparticles are appropriate enhancement additives for acrylic resin coating

Acknowledgement This research is funded by Vietnam Academy of Science and Technology under grant

number KHCBHH.01/20-22, period of 2020–2022

CRediT authorship contribution statement Nguyen Thuy Chinh: Investigation, Writing - original paper

Nguyen Anh Hiep: Investigation, Formal analysis Dao Phi Hung: Methodology, Validation Nguyen Xuan Thai: Investigation Thai Hoang: Funding acquisition, Methodology, Writing - review & editing All the authors have read and approved the final version of the manuscript

Declaration of competing interest The authors declare that they have no known competing financial

interests or personal relationships that could have appeared to influence the work reported in this paper.

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