J. WU et al.: EFFICIENT PRODUCTION OF SMALL-SIZED SiO2 NANOPARTICLES ...
231–237
EFFICIENT PRODUCTION OF SMALL-SIZED SiO
2
NANOPARTICLES AND THEIR APPLICATION IN A
WATERBORNE ACRYLIC-AMINO VARNISH
U^INKOVITA IZDELAVA NANODELCEV SiO
2
IN NJIHOVA
UPORABA ZA AMINO-AKRILNE LAKE NA VODNI OSNOVI
Jinping Wu
1,2
, Jinxiang Mao
1
, Hong Liu
1
, Xichuan Cao
1
, Minmin Chen
2*
1
School of Materials and Physics, China University of Mining and Technology, Xuzhou 221116, P. R. China
2
School of Stomatology, Xuzhou Medical University, Xuzhou 221004, P. R. China
Prejem rokopisa – received: 2023-00-08; sprejem za objavo – accepted for publication: 2024-02-28
doi:10.17222/mit.2023.905
In this study, we optimized the preparation of 100–160 nm monodispersed SiO2 nanoparticles and, through doping, investigated
their effects on the physical properties of a water-based acrylic-amino varnish. First, using a non-fixed point feeding technique
based on the half-batch sol-gel method, we enhanced the yield of small-sized monodispersed SiO2 nanoparticles. To reduce the
cost of production and organic-matter pollution, we assessed certain solution parameters including tetraethyl orthosilicate
(TEOS), ethanol (ETOH) and ammonia in a single-reaction system. We found that the gloss, clarity, hardness, adhesion, and
other physical properties of the acrylic-amino varnish were successfully enhanced through an addition of 1.2 % SiO2
nanoparticles.
Keywords: SiO2 nanoparticles, non-fixed point feeding, acrylic-amino varnish, physical properties
V ~lanku avtorji opisujejo {tudijo optimizirane priprave enakomerno dispergiranih nanodelcev SiO2 velikih od 100 do160 nm, ki
naj bi se uporabljali kot dodatek amino-akrilnim lakom na vodni osnovi. Ugotavljali so u~inek dodatka razli~ne koli~ine
izdelanih nanodelcev SiO2 na fizikalne lastnosti kompozita. Uporabili so tehniko polnjenja z nefiksirano to~ko, temelje~o na
pilotni sol-gel metodi. Na ta na~in so u~inkovito izdelali manj{o koli~ino enakomerno dispergiranih nanodelcev SiO2.Dabi
zmanj{ali stro{ke izdelave in onesna`enje z organskimi snovmi so ocenili dejanske parametre raztopine, vklju~no s tetraetil
ortosilikatom (TEOS), etanolom (ETOH) in amonijakom v enojnem reaktorskem sistemu. Ugotovili so, da so se sijaj, ~istost,
trdota, adhezija in druge fizikalne lastnosti kompozitnega amino-akrilnega laka mo~no izbolj{ale z dodatkom 1,2 % SiO2
nanodelcev.
Klju~ne besede: nanodelci SiO2, nefiksirana to~ka polnjenja, amino-akrilni lak na vodni osnovi, fizikalne lastnosti
1 INTRODUCTION
Small-sized SiO
2
nanoparticles in a range of
100–160 nm have attracted significant attention due to
their small particle size, high specific surface area, large
surface adsorption capacity, good dispersion and facile
surface functionalization. These SiO
2
nanoparticles have
broad applications in various fields, such as catalysis,
1,2
packaging materials,
3
coating,
4
textiles
5
and energy bat-
teries.
6
With an increasing demand for small SiO
2
nanoparticles, an economical production method is in-
creasingly required to ensure a uniform particle size and
good monodispersity. So far, the sol-gel method has been
considered to be a simple and efficient method for pre-
paring SiO
2
nanoparticles.
7–9
Numerous studies have
been conducted to investigate the key factors such as re-
actants, reaction time and temperature, primarily aimed
at controlling the size of SiO
2
nanoparticles.
10–12
How-
ever, these investigations have predominantly overlooked
critical aspects such as resource utilization, overall yield
and environmental considerations within the reaction
system.
13–17
These factors are typically influenced by the
amount of tetraethyl orthosilicate (TEOS) and ethanol
(ETOH) present within the reaction system. Pertinent lit-
erature indicates that for the preparation of reaction sys-
tems yielding particles in the size range of 100–160 nm,
the volume ratio of TEOS to ETOH is often lower than
1/9,
10–17
and in certain cases, it can be as low as 1/43.
6
This observation highlights the inherently extensive na-
ture of the reaction system with a high ethanol volume
and the correspondingly low yield. This system is inher-
ently unsuitable for a large-scale production, given its in-
herent limitations, and has the potential to result in sub-
stantial wastage of valuable resources and significant
organic pollution. Therefore, to facilitate a large-scale
production of SiO
2
nanoparticles, a comprehensive eval-
uation and improvement plan will be devised, focusing
on two primary issues: enhancing the production effi-
ciency and minimizing the reliance on organic solvents.
In recent years, SiO
2
nanoparticles have been increas-
ingly used in the field of coatings, mainly through the
use of covalent bonds, hydrogen bonds or van der Waals
forces to closely combine organic and inorganic compo-
nents, as these complement each other to enhance the
Materiali in tehnologije / Materials and technology 58 (2024) 2, 231–237 231
UDK 678.744.32:546.284-31 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(2)231(2024)
*Corresponding author's e-mail:
minminchen@xzhmu.edu.cn (Minmin Chen)

different properties of waterborne coatings.
18–21
The par-
ticle size of SiO
2
nanoparticles varies according to the
requirements as different particle sizes lead to differ-
ences in the steric effect, optical properties, suspension
state and other properties. As the outermost layer of
color paint, varnish is expected to meet high technical re-
quirements regarding the properties such as appearance,
hardness and adhesion. Currently, a two-component var-
nish composed of acrylic resin and amino resin is not
considered a mature technical direction,
22,23
and achiev-
ing the same performance and appearance of sol-
vent-based coatings under the same construction condi-
tions remains a challenge. Therefore, it is essential to
modify waterborne varnishes. Unlike other color paint
modification techniques, the materials used for a varnish
modification should not affect the glossiness and trans-
parency, as this can cause a color difference between the
primer and varnish. Small-sized SiO
2
nanoparticles can
also act as natural UV absorbers with high hardness, sta-
bility and size effects, potentially enhancing different
properties of a waterborne varnish.
In this study, we successfully doubled the production
and reduced the cost of a single-reaction system for the
preparation of small-sized SiO
2
nanoparticles using a
technique based on the half-batch sol-gel method.
24
This
was achieved by adjusting the reaction parameters (tetra-
ethyl orthosilicate (TEOS), ETOH and ammonia) using
innovative non-fixed feeding methods. Our findings pro-
vide potential solutions for addressing the issues of a
high solvent consumption, low resource utilization rates
and low yield associated with conventional sol-gel meth-
ods. Furthermore, we studied the effects of different SiO
2
nanoparticle concentrations on the physical properties of
a waterborne acrylic-amino varnish through doping,
which promoted the development of water-based coat-
ings and reduced environmental pollution.
2 MATERIALS AND METHODS
2.1 Materials
TEOS, ammonia and ETOH were purchased from
Sinopharm Chemical Reagent (China) while deionized
water was produced using an EPED-E2-20TJ water puri-
fication system. Melamine formaldehyde resin, a wetting
agent, and a leveling agent (Bike Chemical) were ob-
tained from Shanghai Kaiin Chemical Co. (China).
Acrylic resin was obtained from Xingtai Fuyu New Ma-
terials Co. (China), ethylene glycol butyrate was pro-
cured from Shanghai Jiushi Chemical Co. (China), and
dipropylene glycol methyl ether was obtained from
Guangzhou Honghai Chemical Co. (China). The rheo-
logical agent (299) was procured from Shanghai
Guangbai New Materials Co. (China), and dimethyl-
ethanolamine was obtained from BASF Co. (Germany).
The substrate consisted of a 120 × 50 × 0.28 cm tinplate
plate, which was purchased from Guangzhou Desmanyi
Instrument Co. (China).
2.2 Preparation of SiO
2 nanoparticles
Figure 1 illustrates the preparation of SiO
2
nano-
particles with non-fixed point feeding based on the
half-batch sol-gel method. TEOS was first dissolved in
an ETOH solution. Subsequently, deionized water,
ETOH and ammonia were mixed evenly with magnetic
stirring. The TEOS-ETOH solution was carefully
drippled onto the bottom of the reaction solution at four
points, covering the first and second feeding locations,
within 2 min.
The specific preparation schemes employed the
non-fixed point feeding approach (Figure 1), with de-
tailed specifications described as follows. Scheme 1
aimed to adjust the volumes of TEOS to the maximum
capacity of the single-reaction system. First, 90 mL
ETOH, 40 mL deionized water, and 5 mL ammonia were
J. WU et al.: EFFICIENT PRODUCTION OF SMALL-SIZED SiO2 NANOPARTICLES ...
232 Materiali in tehnologije / Materials and technology 58 (2024) 2, 231–237
Figure 1: Schematic of the SiO
2
nanoparticle preparation with non-fixed point feeding

stirred at 30 °C for 15 min. Then different volumes of
TEOS (10, 15, 20 and 25) mL and 30 mL ETOH were
fully mixed and quickly added to the above solution for
2 h. Scheme 2 minimized the production costs of the sin-
gle-reaction system by reducing ETOH and fine-tuning
the volume of ammonia. The ETOH solvent was re-
placed with water while keeping the total volume con-
stant, using a fixed amount of 20 mL of TEOS. The vol-
umes of deionized water, ETOH and ammonia were
varied between 50–80 mL, 110–80 mL and 5–3 mL, re-
spectively. The preparation process was the same as for
Scheme 1. The prepared mixture was centrifugally col-
lected by ETOH and water alternately. The obtained
samples were labeled as S
N
(T/E) where N represents the
volume of ammonia, and T/E is the volume ratio of
TEOS to ETOH.
2.3 Preparation of the coating samples
The prepared and undried SiO
2
nanoparticles were
dispersed in water through high-speed stirring and
ultrasonication to form a SiO
2
sol with a certain concen-
tration. Next, 50 % acrylic resin, 2.5 % dimethylethano-
lamine and 6 % cosolvent were completely stirred and
neutralized in a container equipped with a stirring de-
vice. At 700 min
–1
, we added 10 % melamine formalde-
hyde resin, 5 % dipropylene glycol methyl ether and 2 %
ethylene glycol monobutyl ether, and the mixture was
stirred for 10 min. Then, the SiO
2
nanoparticle sol with
different mass percentages of (0.3, 0.6, 0.9, 1.2, 1.5 and
1.8) % was slowly added while stirring the mixture at
1100 min
–1
for 15 min. Finally, the 0.2 % wetting agent
and 0.2 % leveling agent were added at 700 min
–1
and
stirred for 10 min to obtain a SiO
2
nanoparticle compos-
ite acrylic-amino varnish emulsion. The prepared emul-
sions were sprayed on a carbon steel surface, which was
pre-treated with sandpaper. The thickness of the film was
0.26 ± 0.05 mm. After leveling for 5 min, the samples
were transferred to an oven at 140 °C and baked for
15 min.
2.4 Characterization and measurements
The surface morphologies of SiO
2
nanoparticles were
assessed using a JSM-6700F field-emission scanning
electron microscope (FE-SEM) with an accelerating
voltage of 10.0 kV. The size and monodispersity of SiO
2
nanoparticles were characterized through dynamic light
scattering (DLS, NanoBrook Omni, Brookhaven Instru-
ments). The optical absorbance values were determined
in a wavelength range of 190–1100 nm using a UV-Vis
spectrophotometer (Evolution 300, Thermo). The gloss
values of the coating films were measured using a gloss
meter instrument (3 nh gloss meter, Shenzhen Sanenshi
Technology Co., China), according to the GB 1743-1979
standard under the 60° measurement mode. On each
sample, at least five different points were measured, and
the average values were reported. The static water con-
tact angle of the coating film surface was measured using
a contact angle measurement instrument (Shanghai
Zhongchen Digital Technology Equipment Co., China).
A pencil hardness test was performed according to the
GB/T 6739-2006 standard, and an Elcometer 510 auto-
matic pull-out adhesion tester was used for the adhesion
test according to the GB/T 5210-2006 standard.
3 RASULTS AND DISCUSSION
According to Scheme 1, by fixing the ETOH, water
and ammonia volumes in the reaction solution, the influ-
ence of the TEOS volume on the particle size and mor-
phology of the nanoparticles was studied. For the experi-
ments with (10, 15 and 20) mL of the TEOS solution, the
average size of the prepared particles was (120, 135 and
156) nm, respectively, and the variation range of the par-
ticle size was within 30 nm (Figure 2a to 2c). The poly-
mer dispersity index (PDI) varied between 0.039–0.192,
indicating an excellent monodispersity of the nano-
particles. Using the non-fixed point feeding method
(Figure 1), the high-concentration –Si–O– chains or
silanol groups resulting from the hydrolysis of TEOS un-
J. WU et al.: EFFICIENT PRODUCTION OF SMALL-SIZED SiO2 NANOPARTICLES ...
Materiali in tehnologije / Materials and technology 58 (2024) 2, 231–237 233
Figure 2: SEM and DLS images of the SiO
2
nanoparticles with different TEOS volumes: a) S
5
(10/120), b) S
5
(15/120), c) S
5
(20/120), d)
S
5
(25/120), e) S
5
(20/120) in the fixed feeding point

derwent supersaturation and nucleated within a certain
range of the feeding point, while the other free active
groups became diffused. With an increasing TEOS con-
centration, the difference in the active group concentra-
tion between the feeding points and their surroundings
increased. The accelerated diffusion led to a faster
supersaturation between the feeding points, thus increas-
ing the nucleation. Consequently, the average particle
size of the particles obtained with non-fixed point feed-
ing remained relatively constant and was much smaller
than that obtained with fixed point feeding (Figure 2c
and 2e). Moreover, the increase in TEOS improved the
yield of small-sized SiO
2
. Further increase in the TEOS
volume to 25 mL (Figure 2d) prompted the particle ag-
glomeration, which may be due to the close contacts be-
tween particles, dehydration and condensation of surface
hydroxyl groups.
Scheme 1 optimized the TEOS amount in the sin-
gle-reaction system. However, ETOH, as an organic sol-
vent, accounted for the largest proportion in the reaction
system, which may cause organic matter pollution and
waste of resources. To solve this issue, in Scheme 2,
ETOH was replaced with water while keeping the total
volume constant at a fixed volume of 20 mL of TEOS.
When the amount of ETOH is reduced to 110 mL for
S
5
(20/110) (Figure 3a), the nanoparticle average diame-
ter increases to 320 nm in comparison to S
5
(20/120)
(Figure 2c). The increase in the nanoparticle size is as-
cribed to the increased water that accelerated hydrolysis
and condensation of the active group, crystal nucleation
and growth. ETOH is gradually reduced, resulting in a
successive decrease in the particle size (Figures 3d, 3g
and 3j). The reduction of ethanol, the dissolvent of
TEOS, reduces the hydrolysis rate of TEOS, resulting in
smaller size nanoparticles. Although Figure 3j shows an
average small size of 116 nm, both the yield and produc-
tivity are low (Figure 4b). Therefore, to obtain
small-sized nanoparticles with high yield and productiv-
ity, the ammonia volume is slightly adjusted. Then,
small-sized nanoparticles (100–160 nm) with high
monodispersity (a PDI of 0.005–0.126) are prepared (as
shown in Figures 3c, 3e and 3h).
J. WU et al.: EFFICIENT PRODUCTION OF SMALL-SIZED SiO2 NANOPARTICLES ...
234 Materiali in tehnologije / Materials and technology 58 (2024) 2, 231–237
Figure 3: SEM and DLS images of the SiO
2
nanoparticles prepared with different ETOH volumes: a) S
5
(20/110), b) S
4
(20/110), c) S
3
(20/110),
d) S
5
(20/100), e) S
4
(20/100), f) S
3
(20/100), g) S
5
(20/90), h) S
4
(20/90), i) S
3
(20/90), j) S
5
(20/80), k) S
4
(20/80) and l) S
3
(20/80)

The parameters of small-sized monodisperse SiO
2
were optimized in the single-reaction system. Moreover,
the yield and productivity of nanoparticles based on
Scheme 1 and Scheme 2 were calculated as shown in
Figures 4a and 4b. According to Figure 4a, there was a
linear increase in the yield (>97.0 %) of SiO
2
nanoparticles in relation to the volume of TEOS. Addi-
tionally, the production of the single system was in-
creased by 2 times under the condition that the obtained
SiO
2
nanoparticles were homogeneous and exhibiting a
good morphology. As shown in Figure 4b, a productiv-
ity of more than 97.0 % was obtained for S
N
(20/110) and
S
N
(20/100) while maintaining yield. However, there was
a significant decrease in yield and productivity when the
volume of ETOH was reduced to 80 mL, contributing to
a challenging hydrolysis of TEOS in the limited ETOH
environment.
Considering the experimental results, small nano-
particles with high productivity and monodispersity us-
ing Schemes 1 and 2 were selected for the cost calcula-
tion (Table 1). SiO
2
nanoparticles with a uniform
particle size (100–160 nm) and high productivity were
obtained after reducing the ETOH concentration by
19.6 %. Compared with the existing sol-gel preparation
method,
24
the single-reaction system allowed a lower re-
source consumption and cost for producing small SiO
2
J. WU et al.: EFFICIENT PRODUCTION OF SMALL-SIZED SiO2 NANOPARTICLES ...
Materiali in tehnologije / Materials and technology 58 (2024) 2, 231–237 235
Figure 5: a) UV spectrophotometric test and b) gloss test of the acrylic-amino coatings containing SiO
2
nanoparticles with different concentra-
tions
Figure 4: Relationship between the productivity and yield of SiO
2
nanoparticles, in accordance with the TEOS/ETOH volume ratio (V
T
/V
E
):
a) S
5
(T/120) and b) S
N
(20/E)
Table 1: Required V
T
/V
E
ratio and ETOH volume concentration for preparing small SiO
2
nanoparticles
Samples S
5
(10/120) S
5
(15/120) S
5
(20/120) S
3
(20/110) S
4
(20/100) S
4
(20/90)
d (nm) 125 130 156 120 145 150
Cv
(V
ETOH
/V
overall
%) 68.5% 66.6% 64.8% 60.1% 54.3% 48.9%
VT
/V
E
(V
TEOS
/V
ETOH) 1/12 1/8 1/6 2/11 1/5 2/9

nanoparticles. These findings pave the way for an eco-
nomical and environmentally friendly mass production.
As shown in Figure 5a, the coating paint emulsion
with a 0.3–1.2 % particle addition had higher light trans-
mittance than the original emulsion, and the highest
gloss value was 124 GU higher than that of the original
paint film (Figure 5b). Therefore, the addition of SiO
2
nanoparticles did not reduce the transparency and gloss
of the original varnish film. In addition, when the parti-
cle concentration was 1.2 %, the hardness and adhesion
of the paint film increased to 3H and 5.41 MPa, respec-
tively (Table 2). The added nanoparticles filled in the
gaps created by the cross-linking of the organic molecu-
lar chains at high temperatures. However, a large number
of hydroxyl groups on the surface of the particles could
produce hydrogen or chemical bonds with the organic
molecular chains, thus causing the paint film to be more
stable and less prone to damage. In summary, SiO
2
nanoparticles effectively improved the physical proper-
ties of the original coating without changing its appear-
ance.
4 CONCLUSIONS
In this study, we introduced a new feeding method
via the semi-batch sol-gel preparation of SiO
2
nano-
particles. Following this feeding method, the TEOS reac-
tion amount of the single-reaction system doubled, and
the size of the obtained SiO
2
nanoparticles was main-
tained at 100–160 nm. In addition, the amount of ETOH
in the reaction system was further reduced to explore ef-
ficient, green and economical synthesis solutions. Differ-
ent SiO
2
nanoparticle concentrations were mixed into an
acrylic-amino varnish emulsion and we studied the effect
of the concentration on the physical properties of the var-
nish. The results showed that an addition of 1.2 % SiO
2
nanoparticles improved the UV transmittance, adhesion,
hardness and gloss of the coating.
Author contribution
Jinping Wu: Conceptualization, data curation, writ-
ing the original draft. Jinxiang Mao: Writing, reviewing
and editing. Hong Liu: Rivising. Xichuan Cao: Con-
ceptualization, methodology, investigation, supervision.
Minmin Chen: Supervision, writing, reviewing and edit-
ing.
Acknowledgements
This work was supported by the National Natural
Science Foundation of China [grant number
22120102001] and Scientific Research Foundation for
the Talents of Xuzhou Medical University [grant number
D2021032].
Declaration of no competing interest
The authors declare that they have no known compet-
ing financial interests or personal relationships that could
have influenced the work reported in this paper.
Data availability
The data that support the findings of this study are
available from the corresponding authors upon reason-
able request.
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