Effects of BET Surface Area and Silica Hydrophobicity on Natural Rubber Latex Foam Using the Dunlop Process

Abstract

To reinforce natural rubber latex foam, fumed silica and precipitated silica are introduced into latex foam prepared using the Dunlop process as fillers. Four types of silica, including Aerosil 200 (hydrophilic fumed silica), Reolosil DM30, Aerosil R972 (hydrophobic fumed silica), and Sipernat 22S (precipitated silica), are investigated. The latex foam with added silica presents better mechanical and physical properties compared with the non-silica foam. The hydrophobic nature of the fumed silica has better dispersion in natural rubber compared to hydrophilic silica. The specific surface area of silica particles (BET) also significantly influences the properties of the latex foam, with larger specific surface areas resulting in better dispersity in the rubber matrix. It was observed that exceeding 2 phr led to difficulties in the foaming process (bulking). Furthermore, higher loading of silica also affected the rubber foam, resulting in an increased shrinkage percentage, hardness, compression set, and crosslink density. The crosslink density increased from 11.0 ± 0.2 mol/cm³ for non-silica rubber to 11.6 ± 0.6 mol/cm³ for Reolosil DM30. Reolosil DM30 also had the highest hardness, with a hardness value of 52.0 ± 2.1 IRHD, compared to 45.0 ± 1.3 IRHD for non-silica foam rubber and 48 ± 2.4 IRHD for hydrophilic fumed silica Aerosil 200. Hydrophobic fumed silica also had the highest ability to return to its original shape, with a recovery percentage of 88.0% ± 3.5% compared to the other fumed silica. Overall, hydrophobic fumed silica had better results than hydrophilic silica in both fumed and precipitated silica.

Full text

Citation: Assadakorn, D.; Liu, G.;
Hao, K.; Bai, L.; Liu, F.; Xu, Y.; Guo, L.;
Liu, H. Effects of BET Surface Area
and Silica Hydrophobicity on Natural
Rubber Latex Foam Using the Dunlop
Process. Polymers 2024,16, 3076.
https://doi.org/10.3390/
polym16213076
Academic Editor: Changwoon Nah
Received: 24 September 2024
Revised: 18 October 2024
Accepted: 28 October 2024
Published: 31 October 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
Effects of BET Surface Area and Silica Hydrophobicity on
Natural Rubber Latex Foam Using the Dunlop Process
Danvanichkul Assadakorn 1, Gongxu Liu 1, Kuanfa Hao 1, Lichen Bai 1, Fumin Liu 1, Yuan Xu 2, Lei Guo 1,2,*
and Haichao Liu 2, *
1College of Electromechanical Engineering, Qingdao University of Science & Technology,
Qingdao 266061, China; assadakorndan@gmail.com (D.A.); l18765487207@163.com (G.L.);
jayhao0730@163.com (K.H.); 15684195159@163.com (L.B.); liufumin@qust.edu.cn (F.L.)
2National Engineering Research Center of Advanced Tire Equipment and Key Materials,
Qingdao University of Science & Technology, Qingdao 266061, China; 18792469648@163.com
*Correspondence: 06weny@163.com (L.G.); liuhaichao@qust.edu.cn (H.L.)
Abstract: To reinforce natural rubber latex foam, fumed silica and precipitated silica are introduced
into latex foam prepared using the Dunlop process as fillers. Four types of silica, including Aerosil 200
(hydrophilic fumed silica), Reolosil DM30, Aerosil R972 (hydrophobic fumed silica), and Sipernat 22S
(precipitated silica), are investigated. The latex foam with added silica presents better mechanical and
physical properties compared with the non-silica foam. The hydrophobic nature of the fumed silica
has better dispersion in natural rubber compared to hydrophilic silica. The specific surface area of
silica particles (BET) also significantly influences the properties of the latex foam, with larger specific
surface areas resulting in better dispersity in the rubber matrix. It was observed that exceeding 2 phr
led to difficulties in the foaming process (bulking). Furthermore, higher loading of silica also affected
the rubber foam, resulting in an increased shrinkage percentage, hardness, compression set, and
crosslink density. The crosslink density increased from 11.0
±
0.2 mol/cm
3
for non-silica rubber to
11.6
±
0.6 mol/cm
3
for Reolosil DM30. Reolosil DM30 also had the highest hardness, with a hardness
value of 52.0
±
2.1 IRHD, compared to 45.0
±
1.3 IRHD for non-silica foam rubber and 48
±
2.4 IRHD
for hydrophilic fumed silica Aerosil 200. Hydrophobic fumed silica also had the highest ability to
return to its original shape, with a recovery percentage of 88.0%
±
3.5% compared to the other fumed
silica. Overall, hydrophobic fumed silica had better results than hydrophilic silica in both fumed and
precipitated silica.
Keywords: latex foam; silica; Dunlop process; hydrophobicity; BET surface area
1. Introduction
Natural rubber, sourced from rubber trees, is a versatile material known for its elasticity.
It finds wide applications in diverse industries, including bedding, thermal insulation,
and automotive components [
1
]. Various types of foam rubber exist, each tailored to meet
specific needs. The manufacturing process of foam rubber is relatively straightforward,
with fillers playing a pivotal role in shaping the properties of the final product. Common
fillers include calcium carbonate, silica, and carbon black [
2
,
3
]. These fillers serve as primary
reinforcing agents, contributing to the structural integrity and functional characteristics
of rubber compounds. Calcium carbonate, silica, and carbon black each exhibit unique
properties that influence the mechanical and physical attributes of the rubber, as well as
its surface chemistry and coloration [
2
,
4
]. The choice of filler depends on the industry
and the specific type of rubber. In the tire industry, for example, carbon black is the
primary filler of choice due to its exceptional reinforcing properties and compatibility
with rubber compounds [
5
]. The addition of carbon black significantly enhances the
mechanical strength, wear resistance, and overall performance of rubber used in tire
manufacturing [
6
,
7
]. In the production of foam rubber, the utilization of fillers takes a
Polymers 2024,16, 3076. https://doi.org/10.3390/polym16213076 https://www.mdpi.com/journal/polymers

Polymers 2024,16, 3076 2 of 18
different trajectory. Calcium carbonate stands out as a cost-effective and highly compatible
filler. Its widespread use is attributed to its affordability and versatility, making it a
preferred choice in various formulations. Similarly, silica plays a crucial role in shaping the
characteristics of foam rubber. Beyond its reinforcing capabilities, silica introduces specific
surface interactions, altering the material’s properties and surface chemistry [8].
Silica is an oxide of silicon with the chemical formula SiO
2
, commonly presented as
a white powder. Its polar structure results in strong filler–filler interactions and adsorp-
tion [
8
,
9
]. As non-black fillers, silica performs well as a rubber reinforcing filler. Generally,
carbon black reinforcement provides a higher modulus than silica reinforcement, but silica-
reinforced rubber excels in several aspects, such as aging resistance, abrasion resistance,
and adhesion properties [
10
,
11
]. Synthetic amorphous silica can be primarily categorized
into two types: precipitated silica and fumed silica. The differentiation between these
two variants lies in their distinct manufacturing processes and particle dimensions. Fumed
silica, produced through the hydrolysis of silicon tetrachloride in a flame, is inherently
hydrophilic due to its method of synthesis. The high surface area and fine particle size of
fumed silica contribute to its excellent moisture absorption properties [
12
]. Hydrophilic
fumed silica is particularly advantageous in applications requiring moisture absorption,
such as in pharmaceuticals, cosmetics, and certain adhesive formulations [
13
,
14
]. Recogniz-
ing the demand for hydrophobic characteristics, surface modifications are often employed
in the production of hydrophobic fumed silica. The surface treatment involves the intro-
duction of hydrophobic functionalities, enhancing fumed silica’s resistance to moisture
absorption, expanding the range of applications, improving dispersion, impacting rheo-
logical properties, and tailoring surface chemistry [
15
,
16
]. By contrast, precipitated silica
is typically hydrophilic in its natural state. The production process of precipitated silica
involves the acid precipitation of sodium silicate, resulting in a material with a hydrophilic
surface [
17
]. Unlike fumed silica, which can be inherently hydrophilic or hydrophobic
depending on its production process and subsequent surface treatments, the particle dimen-
sions of precipitated silica are influenced by the specific conditions during precipitation.
This type of synthetic silica has found diverse applications owing to its versatility and
cost-effectiveness [17,18].
To the best of our knowledge, fumed and precipitated silica have not been widely used
as fillers in the production of natural foam rubber using the Dunlop method. However,
some researchers have explored the use of fumed silica and precipitated silica as fillers in
the rubber and polymer industry. Bayat and Fasihi (2019) [
19
] studied the effect of coupling
agents in natural rubber silica composite foam. They used different contents of precipitated
silica and found that it was well-dispersed in natural rubber. The results showed that as
the silica concentration increased, the density also increased, while the cell size decreased
with higher silica content. Luo et al. [
20
] investigated the interaction between fumed silica
and epoxidized natural rubber. Their research indicated that fumed silica had a strong
interaction with epoxidized natural rubber and effectively contributed to crosslink density.
Prasertsri et al. [
21
] studied the reinforcement of natural rubber using fumed silica and
precipitated silica mixed by two-roll mills. The results showed that the samples using
fumed silica as a filler had higher hardness, stiffness, and tensile strength compared to
those using precipitated silica with the same content. However, the heat build-up in the
samples containing fumed silica was significantly higher, indicating that the use of fumed
silica as a filler must also consider the application requirements of the final product.
The aim of this present work is to study the reinforcement of natural rubber latex
foam using fumed silica and precipitated silica. Latex foams were produced using different
types of silica, including fumed silica (both hydrophilic and hydrophobic varieties) and
precipitated silica. Zaborski et al. [
12
] highlighted the importance of considering the surface
area of silica particles. Consequently, the effect of specific surface area of silica particles
(BET) was also considered. The crosslink density, mechanical properties, as well as surface
morphologies of latex foam were investigated to gain further insights into the reinforcing
mechanism of silica in latex foam.

Polymers 2024,16, 3076 3 of 18
2. Experiment
2.1. Material
Table 1displays the materials used in this work. Natural rubber latex with a high
ammonia concentration (60% dry rubber content) was provided by Sri Trang Agro-Industry
Public Company Limited (“STA”) (Surat Thani, Thailand). Three types of fumed sil-
ica and one type of precipitated silica were obtained from Evonik Degussa Co., Ltd.
(Essen, Germany). Specifically, these included hydrophilic fumed silica (Aerosil 200,
purity > 99.8%
, BET surface area 200
±
25 m
2
/g) and hydrophobic fumed silica (Aerosil
R972,
purity > 99.8%
, BET surface area 110
±
20 m
2
/g). Hydrophobic fumed silica (Reolosil
DM30, purity > 99.8%, BET surface area 230
±
20 m
2
/g) was purchased from Tokuyama
Chemicals (Zhejiang) Co., Ltd. (Jiaxing, China). The hydrophilic precipitated silica (Siper-
nat 22s, purity > 97%, BET surface area 190 m
2
/g) was also purchased from Evonik Degussa
Co., Ltd. (Essen, Germany). All silica samples were sourced from the same suppliers to
ensure consistency; however, Tokuyama Chemicals did not have the specific silica spec-
ifications required for this study. Consequently, three types of silica were obtained from
Tokuyama Chemicals and one type from Evonik Degussa Co., Ltd. (Essen, Germany),
which met the specifications for pH, tamped density, and other critical parameters. Key
characteristics of silica samples are displayed in Table 2. Sulfur was used as a vulcanizing
agent and various additives were used, such as zinc oxide (ZnO
2
), potassium, zinc-N-
diethyldithiocarbamate, zinc 2-mercaptobenzothiazole, Wingstay L, diphenyl guanidine,
and sodium silicofluoride. These chemicals were prepared by Qingdao Amita Natural
Latex., Co. Ltd. (Qingdao, China).
Table 1. Formulation of latex foam in parts per hundred of rubber (phr).
Materials Weight (phr)
High Ammonia concentrated natural latex 60% DRC 100
Potassium-oleate solution 10% 1
Sulphur dispersion 50% 2.5
Zinc-N-diethyldithiocarbamate dispersion 50% 1
Zinc 2-mercaptobenzothiazole dispersion 50% 1
Wing stay L dispersion 1
Zinc Oxide dispersion of 50% 4
Diphenyl guanidine dispersion 50% 1
Sodium silicofluoride dispersion 20% 2.5
Filler dispersion
fumed or precipitated silica 0.5, 1, 1.5, 2
Table 2. Key characteristics of silica samples used in this study.
Silica Name
Specific Surface
Area
(m2/g)
Carbon
Content (%)
Tamped
Density
(g/cm3)
Ph Value
(4% Suspension)
SiO2Content
(wt.%)
Hydrophilic/
Hydrophobic
Aerosil 972 110 ±20 0.6–1.2 Approx. 50 3.6–5.5 ≥99.8 hydrophobic
Reolosil Dm30 235 ±20 1.7 Approx. 50 4.8 ≥99.8 hydrophobic
Aerosil 200 200 ±25 N/A * Approx. 50 3.7–4.7 ≥99.8 hydrophilic
Sipernat 22s 190 N/A * Approx. 90 6.5 ≥97 hydrophilic
* N/A indicates that the information is not available in the product specifications.
2.2. Rubber Compounds and Foam Preparation
Figure 1shows the schematic diagram of the foaming process using the Dunlop
method. The NR compound was prepared by using a stirring machine to mix the compo-
nents as in Table 1with a mechanical stirring speed of 450 rpm at room temperature of
15
◦
C. This initial low speed was chosen to ensure homogeneous mixing without risking
premature vulcanization.

Polymers 2024,16, 3076 4 of 18
Polymers 2024, 16, x FOR PEER REVIEW 4 of 19
Figure 1 shows the schematic diagram of the foaming process using the Dunlop
method. The NR compound was prepared by using a stirring machine to mix the compo-
nents as in Table 1 with a mechanical stirring speed of 450 rpm at room temperature of
15 °C. This initial low speed was chosen to ensure homogeneous mixing without risking
premature vulcanization.
Figure 1. Schematic diagram of the Dunlop process.
Mixing process: Natural rubber was added into the stirring machine, stirred for 4 min
to remove the ammonia, which is essential for improving the viscosity and stability of the
latex and facilitating beer foam expansion. Afterward, the other ingredients consisting
of K-oleate, sulfur, zinc Diethyldithiocarbamate, zinc 2-mercaptobenzothiazole, Wingstay
L, and diphenyl guanidine were added and stirred for 3 min. These ingredients were in-
cluded to enhance the crosslinking and stability of the rubber matrix during the vulcani-
zation process.
Silica concentration: Silica was gradually added, while continuously stirring for 3
min. The concentration of each type of silica was controlled by adding four levels of con-
tent: 0.5 phr, 1 phr, 1.5 phr, and 2 phr. This range was selected to systematically assess the
effects of varying silica concentrations on the foam’s mechanical properties. Concentra-
tions exceeding 2.5 phr were avoided based on preliminary experiments conducted with
this specific formulation, which indicated that higher silica content resulted in poor mix-
ing and the onset of pre-vulcanization. This behavior compromised the uniformity and
performance of the rubber compound. It is important to note that these observations are
specific to the formulation used in this study, and other rubber formulations may exhibit
different behaviors under similar conditions. Therefore, while the selected silica concen-
trations are appropriate for this particular system, further research may be necessary to
explore the optimal silica levels for different formulations.
Foaming process: After mixing all these ingredients, the stirring speed was increased
to 1200 rpm and the mixture was stirred for 4 min during the foaming process. This higher
speed facilitates the uniform distribution of air bubbles, crucial for achieving the desired
foam structure. The gelling agent, zinc oxide, was added and stirred for 3 min, and then
Sodium silicofluoride dispersion was added and stirred for 2 min, respectively. The total
mixing time for all samples was set to 19 min.
Temperature control: To mitigate the risk of premature vulcanization and gelation
associated with heat generation, the room temperature was maintained at 15 °C
Figure 1. Schematic diagram of the Dunlop process.
Mixing process: Natural rubber was added into the stirring machine, stirred for
4 min
to remove the ammonia, which is essential for improving the viscosity and stability
of the latex and facilitating better foam expansion. Afterward, the other ingredients
consisting of K-oleate, sulfur, zinc Diethyldithiocarbamate, zinc 2-mercaptobenzothiazole,
Wingstay L, and diphenyl guanidine were added and stirred for 3 min. These ingredients
were included to enhance the crosslinking and stability of the rubber matrix during the
vulcanization process.
Silica concentration: Silica was gradually added, while continuously stirring for 3 min.
The concentration of each type of silica was controlled by adding four levels of content:
0.5 phr, 1 phr, 1.5 phr, and 2 phr. This range was selected to systematically assess the
effects of varying silica concentrations on the foam’s mechanical properties. Concentrations
exceeding 2.5 phr were avoided based on preliminary experiments conducted with this
specific formulation, which indicated that higher silica content resulted in poor mixing and
the onset of pre-vulcanization. This behavior compromised the uniformity and performance
of the rubber compound. It is important to note that these observations are specific to
the formulation used in this study, and other rubber formulations may exhibit different
behaviors under similar conditions. Therefore, while the selected silica concentrations are
appropriate for this particular system, further research may be necessary to explore the
optimal silica levels for different formulations.
Foaming process: After mixing all these ingredients, the stirring speed was increased
to 1200 rpm and the mixture was stirred for 4 min during the foaming process. This higher
speed facilitates the uniform distribution of air bubbles, crucial for achieving the desired
foam structure. The gelling agent, zinc oxide, was added and stirred for 3 min, and then
Sodium silicofluoride dispersion was added and stirred for 2 min, respectively. The total
mixing time for all samples was set to 19 min.
Temperature control: To mitigate the risk of premature vulcanization and gelation
associated with heat generation, the room temperature was maintained at 15
◦
C throughout
the mixing process. This temperature control helps to maintain the stability of the latex and
prevent premature curing, which could otherwise adversely affect the foam structure.
Gelling process: After mixing all the materials according to the formulation in Table 1,
foam rubber was put in the rectangular glass mold of 13
×
19
×
9 cm
3
, and the foam sample
was placed in the oven at a temperature of 30
◦
C for 7 min as a gelling process. This gelling
temperature was selected to promote the initial setting of the foam structure without

Polymers 2024,16, 3076 5 of 18
triggering full vulcanization, which is essential for maintaining the desired expansion
characteristics
Vulcanization process: Curing of samples was conducted at 100
◦
C for 17 min in a
steam oven. This temperature and time were selected based on preliminary studies and
experimental trials that demonstrated optimal crosslinking and mechanical performance
under these conditions. These parameters were fine-tuned to ensure effective vulcanization,
as the experiments indicated that this duration allowed for the best balance between curing
and maintaining the desired material properties.
Washing and drying process: All samples after curing were washed with deionized
water and placed in the oven at 60 ◦C for 24 h for drying.
2.3. Characterizations
2.3.1. Foam Density
Foam densities were determined by cutting samples into 30 mm
×
30 mm
×
30 mm,
according to ASTM D3574 [
22
]. The density was calculated using the mass and volume by
the following equation:
ρf=mf
vf
(1)
where, mfis the mass of the foam sample and vfis the volume.
2.3.2. Crosslink Density
The samples were cut to a small size (4
×
2
×
25 mm
3
) for a crosslink density test
based on the equilibrium swelling method. The crosslink density was calculated by using
the Flory–Rehner equation [23,24]:
−ln(1−Vr)−Vr−χv2
r=vsηswell v1/3
r−vr
2(2)
where
Vr
is the volume fraction of rubber in swollen gel,
χ
is the rubber–solvent interaction
parameter,
vs
is the molar volume of toluene (106.8 cm
3
/
mol−1
),
ηswell
is the swelling of
the compounded rubber (mol/cm−3).
Vr
was tested according to ISO 1817 [
25
]. Samples were weighed and then swollen in
toluene for 1 week. Then, the weights of the samples were measured, and the liquid on the
surface of the samples was removed using filter paper. The samples were then dried at a
temperature of 80
◦
C for 48 h. The weight after drying was measured and used to calculate
the volume fraction of rubber in the swollen gel using the following equation [26,27]:
Vr=hw0∅1−α
ρri
hw0∅1−α
ρr+(w1−w2)
ρsi(3)
α=w1−w0
w1
(4)
∅=1−α(5)
where
w0
,
w1
are the weights (g) of samples before and after immersed in toluene, respec-
tively,
∅
is the mass fraction of natural rubber (amount of natural rubber/total quantity),
α
is the mass fraction loss during swelling in toluene, and
ρs
and
ρr
are the densities (g/
cm3
)
of toluene and the rubber composites, respectively.
χ=0.34 +Vsδs−δp2/RT (6)
where
δs
and
δp
are the solubility parameters of toluene (8.26) and natural rubber, which
are 8.26 and 8.91
cal0.5/cm1.5
, respectively.
R
is the gas constant (cal/mol-K) and
T
is the
absolute temperature (K) [28,29].

Polymers 2024,16, 3076 6 of 18
2.3.3. Microstructural Analysis Using Scanning Electron Microscope (SEM)
The foam morphologies of the samples were determined using a scanning electron
microscope (SEM). The samples were first surface-coated with gold using a sputter coater
to ensure good electrical conductivity between the samples and the aluminum stub, thus
preventing any charging effect during the observation. The SEM used was JEOL JSM-7500F
(Tokyo, Japan). The acceleration voltage was set at 5 kV, and magnifications of
×
50 and
×
10,000 were used to observe the pore structure of the latex foam and the distribution of
silica filler in the rubber matrix. The average diameter of cells was measured using ImageJ
software (version 1.54g) from at least 300 different pores.
2.3.4. Fourier Transform Infrared Spectroscopy Analysis (FTIR)
The chemical functional groups of the foam rubber samples were determined using
attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy with a
VERTEX 70 FT-IR spectrometer (Waltham, MA, USA). The foam rubber samples, sized
10 mm
square with a
10 mm
thickness, were placed on a Ge crystal probe and analyzed in
the wave number range of 4000–600 cm−1.
2.3.5. Compression Set
In the compression test according to the ASTM D3574 standard, the specimen size
was 50
±
1 mm (2-inch) square with a thickness of 25
±
1 mm (1 inch). The samples were
compressed by using a force transducer to reduce the thickness of the original sample to
50% and placed in an oven at a temperature of 70
◦
C
±
1
◦
C for 22 h, then the samples were
taken out of the oven and the transducer and stayed in the atmosphere for 30 min. Then, the
thickness of samples was measured. Figure 2shows the schematic diagram for the test of
the compression set. Three samples of each type were tested and the average was reported.
The thickness measured was calculated by using the following two equations [30,31]:
Cd=[( t0−t1)/t0]×100 (7)
recovery percentage =t1
t0
×100 (8)
Polymers 2024, 16, x FOR PEER REVIEW 6 of 19
quantity), α is the mass fraction loss during swelling in toluene, and 𝜌 and 𝜌 are the
densities (g/cm) of toluene and the rubber composites, respectively.
χ=0.34𝑉
𝛿−𝛿
/𝑅𝑇 (6)
where 𝛿 and 𝛿 are the solubility parameters of toluene (8.26) and natural rubber,
which are 8.26 and 8.91 cal./cm., respectively. 𝑅 is the gas constant (cal/mol-K) and
𝑇 is the absolute temperature (K) [28,29].
2.3.3. Microstructural Analysis Using Scanning Electron Microscope (SEM)
The foam morphologies of the samples were determined using a scanning electron
microscope (SEM). The samples were first surface-coated with gold using a spuer coater
to ensure good electrical conductivity between the samples and the aluminum stub, thus
preventing any charging effect during the observation. The SEM used was JEOL JSM-
7500F (Tokyo, Japan). The acceleration voltage was set at 5 kV, and magnifications of ×50
and ×10,000 were used to observe the pore structure of the latex foam and the distribution
of silica filler in the rubber matrix. The average diameter of cells was measured using Im-
ageJ software (version 1.54g) from at least 300 different pores.
2.3.4. Fourier Transform Infrared Spectroscopy Analysis (FTIR)
The chemical functional groups of the foam rubber samples were determined using
aenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy with a
VERTEX 70 FT-IR spectrometer (Waltham, MA, USA). The foam rubber samples, sized 10
mm square with a 10 mm thickness, were placed on a Ge crystal probe and analyzed in
the wave number range of 4000–600 cm⁻
1
.
2.3.5. Compression Set
In the compression test according to the ASTM D3574 standard, the specimen size
was 50 ± 1 mm (2-inch) square with a thickness of 25 ± 1 mm (1 inch). The samples were
compressed by using a force transducer to reduce the thickness of the original sample to
50% and placed in an oven at a temperature of 70 °C ± 1 °C for 22 h, then the samples
were taken out of the oven and the transducer and stayed in the atmosphere for 30 min.
Then, the thickness of samples was measured. Figure 2 shows the schematic diagram for
the test of the compression set. Three samples of each type were tested and the average
was reported. The thickness measured was calculated by using the following two equa-
tions [30,31]:
𝐶=󰇟(𝑡−𝑡
)/𝑡󰇠 100 (7)
𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑦 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 = 



 100 (8)
𝐶 is the calculated percentage expressing the permanent deformation in relation to
the total deformation, accounting for the transducer height (%). 𝑡 is the original height
or thickness of the foam sample before compression, 𝑡 is the height or thickness of the
foam sample after compression.
Figure 2. Schematic diagram for testing the compression set.
Figure 2. Schematic diagram for testing the compression set.
Cd
is the calculated percentage expressing the permanent deformation in relation to
the total deformation, accounting for the transducer height (%).
t0
is the original height or
thickness of the foam sample before compression,
t1
is the height or thickness of the foam
sample after compression.
2.3.6. Hardness Test
A hardness test was conducted using the International Rubber Hardness Degree
(IRHD) tester, specifically the Wallace H14 Macro IRHD tester (Dorking, UK), in accordance
with ASTM D1415 standards [
32
]. Specimens were precision-cut to dimensions of 10 mm in
thickness, forming squares with sides measuring 20 mm at a controlled room temperature
of 23
◦
C. For each specimen, hardness measurements were taken at five distinct points
distributed across the surface. The median value of these measurements, rounded to the
nearest IRHD, was calculated to determine the final hardness value. This approach ensures

Polymers 2024,16, 3076 7 of 18
a representative and accurate characterization of the material’s hardness based on multiple
points on each specimen.
2.3.7. Percentage Shrinkage
The percentage shrinkage was determined in accordance with the standard of ASTM
D1055 [
33
] by measuring the dimensions of the sample. All sides, including width, length,
height, and the height of the center, were measured. The percentage shrinkage was calcu-
lated by comparing the changes in these dimensions between the sample after vulcanization
and the dimensions of the mold. The equation below was used to calculate the percent-
age shrinkage:
Percentage shrinkage =X1−X2
X1×100 (9)
where
X1
= dimension of sides (cm),
X2
= dimensions of sides of rubber after vulcanization.
3. Discussion and Results
The experiment was divided into two major parts for analysis. The first part investi-
gated different types of silica, including fumed and precipitated silica, with a focus on both
hydrophilic and hydrophobic varieties, as well as particle size. These factors were found to
influence foam morphology. The second part studied the impact of silica content on the
vulcanization of the latex foam produced using the Dunlop process.
3.1. Microstructural Analysis
The surface morphologies of the latex foams were examined using SEM, as shown in
Figure 3. The figure illustrates the structures of the foam samples produced by different
types and concentrations of silica. According to Figure 3, it is evident that a higher
concentration of silica results in a smaller cell size and a more uniform distribution of
the cell structure. At a concentration of 1 phr, the SEM results demonstrate that the BET
specific surface area and hydrophobicity/hydrophilicity of the silicas play a decisive role in
determining the foam microstructure. Latex foams containing fumed hydrophobic silicas,
such as Reolosil DM30 and Aerosil R972, exhibited a mixture of larger and smaller pore
sizes; while fumed hydrophilic silica (Aerosil 200) and precipitated silica (Sipernat 22S)
produced predominantly medium and smaller pore sizes (Figure 3). Increasing the silica
concentration to 2 phr made the distinctions between the silica types less pronounced, as all
the silicas were able to effectively promote a finer and more uniform cellular structure in the
latex foam [
34
,
35
]. After mixing with natural rubber, it increased the viscosity of the rubber
mixture, which reduced the bubble size during the foaming process [
36
]. Furthermore, a
higher concentration of silica increased the nucleation sites for bubble formation, leading to
the formation of a larger number of smaller bubbles, rather than fewer larger ones [37,38].
The ImageJ program was used to analyze the cell diameters, as shown in Figure 4.
Fumed silica, Aerosil R972, has the smallest average pore diameters at both 1 phr and
2 phr concentrations. By contrast, fumed silica, Reolosil DM30, has the largest average
pore diameter in both concentrations. These results suggest that particles with a larger
BET specific surface area tend to produce larger average pore diameters. However, the
hydrophilic fumed silica, Aerosil 200, which has nearly the same BET specific surface area
as Reolosil DM30, resulted in smaller average pore diameters. Specifically, the average
pore diameter of Aerosil 200 was 38.1
±
1.3
µ
m at 1 phr and 29.8
±
2.3
µ
m at 2 phr. With
regard to Reolosil DM30, the average pore diameters were 40.8
±
7.9
µ
m at 1 phr and
34.9 ±2.0 µm
at 2 phr. The precipitated silica, Sipernat 22S, which has a similar BET specific
surface area, exhibited slightly different average pore diameters (40.3
±
1.4
µ
m at 1 phr
and 32.8 ±1.7 µm at 2 phr) compared to hydrophobic fumed silica, Reolosil DM30.

Polymers 2024,16, 3076 8 of 18
Polymers 2024, 16, x FOR PEER REVIEW 8 of 19
Figure 3. SEM image of natural latex foam (a) without silica, (b) with DM30 at 1 phr, (c) with DM30
at 2 phr, (d) with Aerosil R972 at 1 phr, (e) with Aerosil R972 at 2 phr, (f) with Aerosil 200 at 1 phr,
(g) with Aerosil 200 at 2 phr, (h) with Sip22s at 1 phr, and (i) with Sip22s at 2 phr. SEM images of
foam rubber at 50× magnification. A 100 µm scale bar is included for reference.
The ImageJ program was used to analyze the cell diameters, as shown in Figure 4.
Fumed silica, Aerosil R972, has the smallest average pore diameters at both 1 phr and 2
phr concentrations. By contrast, fumed silica, Reolosil DM30, has the largest average pore
diameter in both concentrations. These results suggest that particles with a larger BET
specific surface area tend to produce larger average pore diameters. However, the hydro-
philic fumed silica, Aerosil 200, which has nearly the same BET specific surface area as
Reolosil DM30, resulted in smaller average pore diameters. Specifically, the average pore
diameter of Aerosil 200 was 38.1 ± 1.3 µm at 1 phr and 29.8 ± 2.3 µm at 2 phr. With regard
to Reolosil DM30, the average pore diameters were 40.8 ± 7.9 µm at 1 phr and 34.9 ± 2.0
µm at 2 phr. The precipitated silica, Sipernat 22S, which has a similar BET specific surface
area, exhibited slightly different average pore diameters (40.3 ± 1.4 µm at 1 phr and 32.8 ±
1.7 µm at 2 phr) compared to hydrophobic fumed silica, Reolosil DM30.
There were slight differences between hydrophobic and hydrophilic types in the av-
erage pore diameters of silica. However, the pore structures of the foam rubber were dis-
tinct, as can be seen from Figure 3. While increasing the silica concentration resulted in
smaller and more uniform cell sizes overall, there was notable variability between
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
Figure 3. SEM image of natural latex foam (a) without silica, (b) with DM30 at 1 phr, (c) with DM30
at 2 phr, (d) with Aerosil R972 at 1 phr, (e) with Aerosil R972 at 2 phr, (f) with Aerosil 200 at 1 phr,
(g) with Aerosil 200 at 2 phr, (h) with Sip22s at 1 phr, and (i) with Sip22s at 2 phr. SEM images of
foam rubber at 50×magnification. A 100 µm scale bar is included for reference.
There were slight differences between hydrophobic and hydrophilic types in the
average pore diameters of silica. However, the pore structures of the foam rubber were
distinct, as can be seen from Figure 3. While increasing the silica concentration resulted
in smaller and more uniform cell sizes overall, there was notable variability between
different silica types, particularly at 1 phr concentrations. Fumed hydrophobic silicas, such
as Reolosil DM30, exhibited a broader range of pore sizes, reflected in larger standard
deviations. By contrast, the hydrophilic fumed silica, Aerosil 200, and the precipitated
silica, Sipernat 22S, produced more consistently sized, smaller pores.
From the SEM figures at 10,000
×
magnification, as shown in Figure 5, the hydrophobic
fumed silica was observed to disperse more evenly within the natural rubber matrix
compared to the hydrophilic fumed silica. The hydrophilic silica showed more pronounced
aggregation, suggesting that hydrophobic silica had better compatibility with the natural
rubber matrix [
38
]. Additionally, it can be seen that the BET surface area of the silica
particles is a critical factor in their distribution within the rubber matrix. Silica particles
with a larger BET surface area tended to have better dispersions [39].

Polymers 2024,16, 3076 9 of 18
Polymers 2024, 16, x FOR PEER REVIEW 9 of 19
different silica types, particularly at 1 phr concentrations. Fumed hydrophobic silicas,
such as Reolosil DM30, exhibited a broader range of pore sizes, reflected in larger standard
deviations. By contrast, the hydrophilic fumed silica, Aerosil 200, and the precipitated sil-
ica, Sipernat 22S, produced more consistently sized, smaller pores.
Figure 4. Average pores diameter of silicas (a) without silica, (b) with DM30 at 1 phr, (c) with DM30
at 2 phr, (d) with Aerosil R972 at 1 phr, (e) with Aerosil R972 at 2 phr, (f) with Aerosil 200 at 1 phr,
(g) with Aerosil 200 at 2 phr, (h) with Sip22s at 1 phr, and (i) with Sip22s at 2 phr.
From the SEM figures at 10,000× magnification, as shown in Figure 5, the hydropho-
bic fumed silica was observed to disperse more evenly within the natural rubber matrix
compared to the hydrophilic fumed silica. The hydrophilic silica showed more pro-
nounced aggregation, suggesting that hydrophobic silica had beer compatibility with
the natural rubber matrix [38]. Additionally, it can be seen that the BET surface area of the
silica particles is a critical factor in their distribution within the rubber matrix. Silica par-
ticles with a larger BET surface area tended to have beer dispersions [39].
Note: SEM images at 50× magnification were captured with a 100 µm scale bar in
Figure 3 for accurate interpretation of the pore structures, while SEM images at 10,000×
Figure 4. Average pores diameter of silicas (a) without silica, (b) with DM30 at 1 phr, (c) with DM30
at 2 phr, (d) with Aerosil R972 at 1 phr, (e) with Aerosil R972 at 2 phr, (f) with Aerosil 200 at 1 phr,
(g) with Aerosil 200 at 2 phr, (h) with Sip22s at 1 phr, and (i) with Sip22s at 2 phr.
Note: SEM images at 50
×
magnification were captured with a 100
µ
m scale bar in Figure 3
for accurate interpretation of the pore structures, while SEM images at
10,000×magnification
were captured with a 1
µ
m scale bar in Figure 5to provide detailed visualization of
silica dispersion.
3.2. FTIR
ATR-FTIR spectroscopy was used to analyze the chemical interactions between nat-
ural rubber and different types and concentrations of silica (Figure 6). While the FTIR
spectra did not reveal the presence of new functional groups across different silica types,
variations in the intensity of existing peaks were observed, indicating possible physical
interactions between silica and the rubber matrix. A band was observed at 1084 cm
−1
,
corresponding to C–O–Si stretching vibrations. The presence of this band, even though
subtle, suggests possible interactions between the silica particles and the natural rubber

Polymers 2024,16, 3076 10 of 18
matrix [
40
]. These changes in peak intensity, rather than the appearance of new peaks,
imply that the interactions are likely physical rather than chemical in nature at the tested
silica concentrations.
Polymers 2024, 16, x FOR PEER REVIEW 10 of 19
magnification were captured with a 1 µm scale bar in Figure 5 to provide detailed visu-
alization of silica dispersion.
Figure 5. Morphology of the distribution of silicas in natural rubber. (a) DM30 at 2 phr, (b) Aerosil
R972 at 2 phr, (c) Aerosil 200 at 2 phr, and (d) Sip22s at 2 phr. SEM images of foam rubber at 10,000×
magnification. A 1 µm scale bar is included for reference.
3.2. FTIR
ATR-FTIR spectroscopy was used to analyze the chemical interactions between nat-
ural rubber and different types and concentrations of silica (Figure 6). While the FTIR
spectra did not reveal the presence of new functional groups across different silica types,
variations in the intensity of existing peaks were observed, indicating possible physical
interactions between silica and the rubber matrix. A band was observed at 1084 cm⁻
1
, cor-
responding to C–O–Si stretching vibrations. The presence of this band, even though subtle,
suggests possible interactions between the silica particles and the natural rubber matrix
[40]. These changes in peak intensity, rather than the appearance of new peaks, imply that
the interactions are likely physical rather than chemical in nature at the tested silica con-
centrations.
Additionally, the peak intensity at 3469 cm⁻
1
, associated with the vibrations of hy-
droxyl groups (O-H), was notably higher in samples containing hydrophilic silicas, such
as Aerosil 200 and precipitated, Sipernat 22S. This is consistent with their higher surface
hydroxyl group density. Conversely, the hydrophobic silicas (Aerosil R972 and Reolosil
DM30) exhibited a lower intensity in this region, which aligns with their lower hydroxyl
group content. This distinction between hydrophilic and hydrophobic silicas reflects the
expected behavior of these materials, where hydrophobic silicas have fewer surface hy-
droxyl groups due to their treatment to reduce moisture affinity.
(a) (b)
(c) (d)
Figure 5. Morphology of the distribution of silicas in natural rubber. (a) DM30 at 2 phr, (b) Aerosil
R972 at 2 phr, (c) Aerosil 200 at 2 phr, and (d) Sip22s at 2 phr. SEM images of foam rubber at 10,000
×
magnification. A 1 µm scale bar is included for reference.
Polymers 2024, 16, x FOR PEER REVIEW 11 of 19
(a) (b)
Figure 6. FTIR results of latex foam with different types of silica at 1 phr (a) and with different
concentrations of silica (b).
The changes in peak intensities support the idea that silica interacts with the natural
rubber matrix in a physical manner, affecting the overall dispersion of the silica within the
rubber. The beer dispersion of hydrophobic silicas, such as Aerosil R972 and Reolosil
DM30, likely contributes to the improved mechanical properties observed in these sam-
ples. It is also important to note that FTIR’s sensitivity may not be sufficient to detect more
subtle interactions at the low silica concentrations used in this study. Therefore, while no
significant chemical bonding was detected between the silica and natural rubber, The ob-
served variations in peak intensities at specific wavenumbers (1084 cm⁻1 and 3469 cm⁻1)
indicate that physical interactions significantly influence the performance of the silica-
filled rubber foam [41].
3.3. Foam Density
Figure 7 shows the densities of different types of silica with different contents. The
density of the foam samples increased linearly with the increase in silica content for all
types of silica. The highest density was observed for the samples containing 2 phr of Re-
olosil DM30 hydrophobic fumed silica (0.120 ± 0.005 g/cm3), followed by those with 2 phr
of Aerosil 200 hydrophilic fumed silica (0.119 ± 0.006 g/cm3), then 2 phr of Aerosil R972
hydrophobic silica (0.113 ± 0.003 g/cm3), and finally 2 phr of Sipernat 22S precipitated sil-
ica (0.110 ± 0.004 g/cm3). The density of the non-silica foam rubber was 0.100 ± 0.002 g/cm3.
The differences in density between the samples can be aributed to the ability of the silicas
to create a finer and more uniform cellular structure, as observed in the SEM images. The
higher the silica loading, the greater the reinforcement of the rubber matrix, leading to
higher foam densities [42,43].
Figure 6. FTIR results of latex foam with different types of silica at 1 phr (a) and with different
concentrations of silica (b).

Polymers 2024,16, 3076 11 of 18
Additionally, the peak intensity at 3469 cm
−1
, associated with the vibrations of hy-
droxyl groups (O-H), was notably higher in samples containing hydrophilic silicas, such
as Aerosil 200 and precipitated, Sipernat 22S. This is consistent with their higher surface
hydroxyl group density. Conversely, the hydrophobic silicas (Aerosil R972 and Reolosil
DM30) exhibited a lower intensity in this region, which aligns with their lower hydroxyl
group content. This distinction between hydrophilic and hydrophobic silicas reflects the ex-
pected behavior of these materials, where hydrophobic silicas have fewer surface hydroxyl
groups due to their treatment to reduce moisture affinity.
The changes in peak intensities support the idea that silica interacts with the natural
rubber matrix in a physical manner, affecting the overall dispersion of the silica within the
rubber. The better dispersion of hydrophobic silicas, such as Aerosil R972 and Reolosil
DM30, likely contributes to the improved mechanical properties observed in these samples.
It is also important to note that FTIR’s sensitivity may not be sufficient to detect more
subtle interactions at the low silica concentrations used in this study. Therefore, while
no significant chemical bonding was detected between the silica and natural rubber, The
observed variations in peak intensities at specific wavenumbers (1084 cm
−1
and 3469 cm
−1
)
indicate that physical interactions significantly influence the performance of the silica-filled
rubber foam [41].
3.3. Foam Density
Figure 7shows the densities of different types of silica with different contents. The
density of the foam samples increased linearly with the increase in silica content for all types
of silica. The highest density was observed for the samples containing 2 phr of Reolosil
DM30 hydrophobic fumed silica (0.120
±
0.005 g/cm
3
), followed by those with 2 phr of
Aerosil 200 hydrophilic fumed silica (0.119
±
0.006 g/cm
3
), then 2 phr of Aerosil R972
hydrophobic silica (0.113
±
0.003 g/cm
3
), and finally 2 phr of Sipernat 22S precipitated silica
(0.110
±
0.004 g/cm
3
). The density of the non-silica foam rubber was
0.100 ±0.002 g/cm3
.
The differences in density between the samples can be attributed to the ability of the silicas
to create a finer and more uniform cellular structure, as observed in the SEM images. The
higher the silica loading, the greater the reinforcement of the rubber matrix, leading to
higher foam densities [42,43].
Polymers 2024, 16, x FOR PEER REVIEW 12 of 19
Figure 7. Density of different types of silica.
3.4. Crosslink Density
The crosslink density of unfilled and silica-filled samples was estimated using the
swelling method in toluene solvent, as shown in Figure 8. The swelling rate of samples
with different silica loadings was nearly uniform, suggesting that the loading of silica had
minimal influence on the swelling rate [44]. Despite slight differences in crosslink density,
it can be observed that higher silica loadings corresponded to higher crosslink densities.
The addition of silica promoted the dispersion of the vulcanization system in latex. During
the mixing process of silica and latex, the fusion of rubber molecular chains with silica
occurred, enhancing the crosslinking uniformity and efficiency [45].
At a silica concentration of 1 phr, the crosslink density was comparable to that of the
non-silica sample. However, with increasing silica concentrations, crosslink density also
increased. This is because the rubber cells decreased while the silica cells increased, leav-
ing some silica cells unswollen. The crosslink density of non-silica foam rubber was (11.0
± 0.2) × 10
−3
mol/cm
3
. Among the types of silica, the fumed silica, Reolosil DM30, exhibited
the highest crosslink density values (11.6 ± 0.6) × 10
−3
mol/cm
3
due to its high BET surface
area and hydrophobic properties.
Hydrophobic silica particles, such as Reolosil DM30, repel water and have a higher
affinity for non-polar solvents [46,47]. When incorporated into a rubber matrix, hydropho-
bic silica is less likely to interact with water molecules and may instead interact with non-
polar segments of the rubber chains. This interaction promotes the formation of crosslinks
between polymer chains during vulcanization, resulting in a higher crosslink density
compared to systems without silica or with hydrophilic silicas [48].
Although the precipitated silica, Sipernat 22S, has a lower specific surface area, its
higher bulk density compared to all three types of fumed silica can affect crosslink density
[39]. A higher bulk density reduces cell space, resulting in higher silica cell concentrations.
This suggests that the properties of silica, including surface chemistry and bulk density,
play crucial roles in influencing crosslink density within rubber composites.
0.085
0.09
0.095
0.1
0.105
0.11
0.115
0.12
0.125
0.13
00.511.52
Aerosill 972
Aerosil 200
Reolosil dm-
30
Sipernat 22s
Density(g/cm3)
Silica contents(phr)
Figure 7. Density of different types of silica.
3.4. Crosslink Density
The crosslink density of unfilled and silica-filled samples was estimated using the
swelling method in toluene solvent, as shown in Figure 8. The swelling rate of samples

Polymers 2024,16, 3076 12 of 18
with different silica loadings was nearly uniform, suggesting that the loading of silica had
minimal influence on the swelling rate [
44
]. Despite slight differences in crosslink density,
it can be observed that higher silica loadings corresponded to higher crosslink densities.
The addition of silica promoted the dispersion of the vulcanization system in latex. During
the mixing process of silica and latex, the fusion of rubber molecular chains with silica
occurred, enhancing the crosslinking uniformity and efficiency [45].
Polymers 2024, 16, x FOR PEER REVIEW 13 of 19
Figure 8. (a) Crosslink densities of different types of silica, (b) swelling ration of different types of silica.
3.5. Compression Set
The compression set values, as shown in the above figures, varied with silica loading
(0, 0.5, 1, 1.5 and 2 phr) across the four different types of silica. It was observed that the
compression set values increased with higher silica loading. The hydrophobic fumed silica,
Reolosil DM30, exhibited the highest values, followed by the hydrophobic fumed silica,
Aerosil R972, the hydrophilic precipitated silica, Sipernat 22S, and finally the hydrophilic
fumed silica, Aerosil 200.
Compression set values serve as an indicator of the elasticity of the latex foam, with
lower values indicating higher elasticity and ability to return to its original shape [49]. The
compression set value of the non-silica sample was the highest (24.0% ± 0.7%), due to the
unevenly distributed pore characteristics within the rubber matrix [50,51]. By contrast,
silica-reinforced latex foam exhibited a more even distribution of cells. The increased
amount of silica affected the rubber matrix, resulting in foam rubber with smaller and
evenly arranged rubber cells. The hydrophobic fumed silica, Reolosil DM30, with its high
specific surface area, influenced the performance of the reinforced rubber foam. A larger
BET typically results in a higher contact area between rubber cells and silica, meaning
stronger interaction between the silica filler and natural rubber, potentially leading to
lower compression set properties. This means that the foam rubber retains its shape beer
after compression [7,12]. Conversely, a lower specific surface area may reduce the propen-
sity for agglomeration, thus enhancing the compression set properties. At 2 phr, the com-
pression set of Reolosil DM30 was 12.0% ± 1.0%, while Aerosil R972, another hydrophobic
fumed silica with a lower BET, had a compression set value of 14.0% ± 0.6% (Figure 9).
Additionally, hydrophilic silica has a higher affinity for water and may retain water
molecules within the material. This water absorption can lead to swelling of the rubber
matrix, affecting its elastic recovery properties. Consequently, rubber composites contain-
ing hydrophilic silica may exhibit higher compression set values due to increased perma-
nent deformation under compression. This is reflected in the results: at 2 phr, Aerosil 200,
a fumed silica with hydrophilic nature, had the highest compression set value of 17.9% ±
0.6% because of its poorer and more uneven, porous distribution [48], as shown in Figure
3. Similarly, the compression set of precipitated silica, which is also hydrophilic in nature,
had a higher compression set value (14.8% ± 0.8%) than both hydrophobic silicas.
Figure 8. (a) Crosslink densities of different types of silica, (b) swelling ration of different types of silica.
At a silica concentration of 1 phr, the crosslink density was comparable to that of
the non-silica sample. However, with increasing silica concentrations, crosslink density
also increased. This is because the rubber cells decreased while the silica cells increased,
leaving some silica cells unswollen. The crosslink density of non-silica foam rubber was
(11.0 ±0.2) ×10−3mol/cm3
. Among the types of silica, the fumed silica, Reolosil DM30,
exhibited the highest crosslink density values (11.6
±
0.6)
×
10
−3
mol/cm
3
due to its high
BET surface area and hydrophobic properties.
Hydrophobic silica particles, such as Reolosil DM30, repel water and have a higher
affinity for non-polar solvents [
46
,
47
]. When incorporated into a rubber matrix, hydropho-
bic silica is less likely to interact with water molecules and may instead interact with
non-polar segments of the rubber chains. This interaction promotes the formation of
crosslinks between polymer chains during vulcanization, resulting in a higher crosslink
density compared to systems without silica or with hydrophilic silicas [48].
Although the precipitated silica, Sipernat 22S, has a lower specific surface area, its
higher bulk density compared to all three types of fumed silica can affect crosslink den-
sity [
39
]. A higher bulk density reduces cell space, resulting in higher silica cell concen-
trations. This suggests that the properties of silica, including surface chemistry and bulk
density, play crucial roles in influencing crosslink density within rubber composites.
3.5. Compression Set
The compression set values, as shown in the above figures, varied with silica loading
(0, 0.5, 1, 1.5 and 2 phr) across the four different types of silica. It was observed that the
compression set values increased with higher silica loading. The hydrophobic fumed silica,
Reolosil DM30, exhibited the highest values, followed by the hydrophobic fumed silica,
Aerosil R972, the hydrophilic precipitated silica, Sipernat 22S, and finally the hydrophilic
fumed silica, Aerosil 200.
Compression set values serve as an indicator of the elasticity of the latex foam, with
lower values indicating higher elasticity and ability to return to its original shape [
49
]. The
compression set value of the non-silica sample was the highest (24.0%
±
0.7%), due to the
unevenly distributed pore characteristics within the rubber matrix [
50
,
51
]. By contrast,

Polymers 2024,16, 3076 13 of 18
silica-reinforced latex foam exhibited a more even distribution of cells. The increased
amount of silica affected the rubber matrix, resulting in foam rubber with smaller and
evenly arranged rubber cells. The hydrophobic fumed silica, Reolosil DM30, with its high
specific surface area, influenced the performance of the reinforced rubber foam. A larger
BET typically results in a higher contact area between rubber cells and silica, meaning
stronger interaction between the silica filler and natural rubber, potentially leading to lower
compression set properties. This means that the foam rubber retains its shape better after
compression [
7
,
12
]. Conversely, a lower specific surface area may reduce the propensity for
agglomeration, thus enhancing the compression set properties. At 2 phr, the compression
set of Reolosil DM30 was 12.0%
±
1.0%, while Aerosil R972, another hydrophobic fumed
silica with a lower BET, had a compression set value of 14.0% ±0.6% (Figure 9).
Polymers 2024, 16, x FOR PEER REVIEW 14 of 19
Figure 9. (a) Compression set of different types of silica; (b) recovery percentage of different types of silica.
3.6. Hardness
Increasing the loading of silica in rubber composites typically results in increased
hardness, as demonstrated in Figure 10. Non-silica foam exhibited a hardness value of
45.0 ± 1.3 IRHD, and by incorporating silica into the rubber, the hardness of the foams
increased accordingly. As silica loading increases, more filler particles disperse through-
out the rubber matrix. The addition of silica contributes to reducing the size of bubbles or
cells within the foam structure. This reduction in cell size is facilitated by the increased
surface area available for interaction between the silica particles and the rubber chains [52].
Consequently, stronger interfacial adhesion forms between the silica particles and the rub-
ber matrix. With higher silica loading, the foam structure allows less space between the
rubber cells, leading to a denser foam structure. This densification significantly contrib-
utes to increased hardness [50]. By minimizing the space between cells, the foam becomes
less compressible and more resistant to deformation.
A comparison of different silica fillers revealed that the hydrophobic silica, particu-
larly Reolosil DM30 and Aerosil R972, exhibited the highest enhancement in rubber prop-
erties compared to the hydrophilic silica, Sipernat 22S and Aerosil 200. At a concentration
of 2 phr, Aerosil R972 and Reolosil DM30 demonstrated hardness values of 51.0 ± 2.0
IRHD and 52.0 ± 2.1 IRHD, respectively, while Sipernat 22S and Aerosil 200 exhibited
hardness values of 48.5 ± 2.4 IRHD and 48.0 ± 2.4 IRHD, respectively. One significant factor
contributing to this observation is the BET surface area of the silica fillers. Reolosil DM30,
known for its hydrophobic nature, possesses the highest BET surface area among the
tested silica fillers. The higher BET surface area allows for a greater number of active sites
to be available for interaction with the rubber matrix, promoting stronger filler–matrix
adhesion and ultimately resulting in increased hardness [38,53].
Figure 9. (a) Compression set of different types of silica; (b) recovery percentage of different types
of silica.
Additionally, hydrophilic silica has a higher affinity for water and may retain water
molecules within the material. This water absorption can lead to swelling of the rubber
matrix, affecting its elastic recovery properties. Consequently, rubber composites con-
taining hydrophilic silica may exhibit higher compression set values due to increased
permanent deformation under compression. This is reflected in the results: at 2 phr, Aerosil
200, a fumed silica with hydrophilic nature, had the highest compression set value of
17.9% ±0.6%
because of its poorer and more uneven, porous distribution [
48
], as shown in
Figure 3. Similarly, the compression set of precipitated silica, which is also hydrophilic in
nature, had a higher compression set value (14.8% ±0.8%) than both hydrophobic silicas.
3.6. Hardness
Increasing the loading of silica in rubber composites typically results in increased
hardness, as demonstrated in Figure 10. Non-silica foam exhibited a hardness value of
45.0
±
1.3 IRHD, and by incorporating silica into the rubber, the hardness of the foams
increased accordingly. As silica loading increases, more filler particles disperse throughout
the rubber matrix. The addition of silica contributes to reducing the size of bubbles or
cells within the foam structure. This reduction in cell size is facilitated by the increased
surface area available for interaction between the silica particles and the rubber chains [
52
].
Consequently, stronger interfacial adhesion forms between the silica particles and the
rubber matrix. With higher silica loading, the foam structure allows less space between the
rubber cells, leading to a denser foam structure. This densification significantly contributes
to increased hardness [
50
]. By minimizing the space between cells, the foam becomes less
compressible and more resistant to deformation.

Polymers 2024,16, 3076 14 of 18
Polymers 2024, 16, x FOR PEER REVIEW 15 of 19
Figure 10. Hardness of different types of silica with different contents.
3.7. Percentage Shrinkage
Figure 11 depicts the percentage shrinkage of latex foam for four distinct silica con-
centrations. It is evident that the various silica concentrations resulted in different per-
centage shrinkage values. The addition of silica led to a notable reduction in the shrinkage
of the center height in comparison to non-silica rubber. However, the height and length
of the samples exhibited a gradual increase in percentage shrinkage with higher silica
loadings. In the width dimension, there was no clear trend, but adding silica loadings at
0.5 and 1 phr could reduce the percentage shrinkage. An intriguing observation was made
regarding the height of the center, where the percentage shrinkage exhibited a decreasing
trend as the silica loadings increased. The samples’ height and length were smaller and
shorter with higher silica loadings, but the center of the samples showed an opposite trend,
with the height being greater.
This phenomenon of shrinkage may be aributed to the rubber’s relatively low re-
sistance to swelling in solvents and incomplete foaming rates, resulting in varying densi-
ties of the rubber [54]. Furthermore, variations in the quantity of the solids and the types
of chemicals employed in the formula, including the foaming agent, can give rise to un-
certainties in the mechanical properties of the rubber and affect the percentage of shrink-
age [55].
In this experiment, the incorporation of silica as a filler resulted in a reduction in the
percentage shrinkage of rubber in the compound. The incorporation of silica as a powder
is effectively linked with rubber particles, resulting in a thicker and harder rubber matrix.
Consequently, as the silica loadings increased, the shrinkage percentage decreased due to
the densification and reinforcement effects of silica on the rubber matrix [56].
Figure 10. Hardness of different types of silica with different contents.
A comparison of different silica fillers revealed that the hydrophobic silica, particularly
Reolosil DM30 and Aerosil R972, exhibited the highest enhancement in rubber properties
compared to the hydrophilic silica, Sipernat 22S and Aerosil 200. At a concentration of
2 phr
,
Aerosil R972 and Reolosil DM30 demonstrated hardness values of 51.0
±
2.0 IRHD and
52.0
±
2.1 IRHD, respectively, while Sipernat 22S and Aerosil 200 exhibited hardness values
of 48.5
±
2.4 IRHD and 48.0
±
2.4 IRHD, respectively. One significant factor contributing
to this observation is the BET surface area of the silica fillers. Reolosil DM30, known for its
hydrophobic nature, possesses the highest BET surface area among the tested silica fillers.
The higher BET surface area allows for a greater number of active sites to be available
for interaction with the rubber matrix, promoting stronger filler–matrix adhesion and
ultimately resulting in increased hardness [38,53].
3.7. Percentage Shrinkage
Figure 11 depicts the percentage shrinkage of latex foam for four distinct silica concen-
trations. It is evident that the various silica concentrations resulted in different percentage
shrinkage values. The addition of silica led to a notable reduction in the shrinkage of the
center height in comparison to non-silica rubber. However, the height and length of the
samples exhibited a gradual increase in percentage shrinkage with higher silica loadings.
In the width dimension, there was no clear trend, but adding silica loadings at 0.5 and
1 phr
could reduce the percentage shrinkage. An intriguing observation was made regarding the
height of the center, where the percentage shrinkage exhibited a decreasing trend as the
silica loadings increased. The samples’ height and length were smaller and shorter with
higher silica loadings, but the center of the samples showed an opposite trend, with the
height being greater.
This phenomenon of shrinkage may be attributed to the rubber’s relatively low resis-
tance to swelling in solvents and incomplete foaming rates, resulting in varying densities of
the rubber [
54
]. Furthermore, variations in the quantity of the solids and the types of chem-
icals employed in the formula, including the foaming agent, can give rise to uncertainties
in the mechanical properties of the rubber and affect the percentage of shrinkage [55].
In this experiment, the incorporation of silica as a filler resulted in a reduction in the
percentage shrinkage of rubber in the compound. The incorporation of silica as a powder
is effectively linked with rubber particles, resulting in a thicker and harder rubber matrix.
Consequently, as the silica loadings increased, the shrinkage percentage decreased due to
the densification and reinforcement effects of silica on the rubber matrix [56].

Polymers 2024,16, 3076 15 of 18
Polymers 2024, 16, x FOR PEER REVIEW 16 of 19
Figure 11. Percentage shrinkage of different types of silica compared with sides of sample: (a) width;
(b) length; (c) height; and (d) height of center.
4. Conclusions
This study investigated the influence of different types and concentrations of silica
as fillers, including hydrophilic and hydrophobic fumed silicas, as well as precipitated
silica, on the properties of natural rubber foam produced using the Dunlop process. The
physical properties, swelling, hardness, and shrinkage behavior of the natural rubber
foam were determined. Four different types of silica were added to the rubber formulation
with loadings ranging from 0 to 2 phr. The results indicated that the incorporation of silica
fillers—both fumed and precipitated—could be evenly distributed in the rubber matrix.
Hydrophobic fumed silicas exhibited beer dispersion and enhanced the physical and
mechanical properties of the natural rubber foam. In the SEM analysis, it was observed
that the hydrophobic silicas had a more homogeneous dispersion and integration with the
rubber matrix, leading to improved reinforcement and physical/mechanical performance
compared to the hydrophilic silica. Even with a lower specific surface area (BET), the hy-
drophobic fumed silica, Aerosil R972, was still beer dispersed compared to hydrophilic
silica with a higher BET. Increasing loadings of silica had a noticeable effect on the poros-
ity of the foam rubber; higher silica loadings resulted in smaller foam pores due to an
increase in crosslink density as the silica concentration increased, which led to a more
consistent pore structure in the foam rubber matrix. By contrast, a lower specific surface
area made the average pore diameter smaller, as observed with the hydrophilic fumed
silica, Aerosil 200 (29.4 ± 1.7 µm). Higher loadings of silica also increased the foam density
and hardness. The compression set decreased as silica loading increased, meaning the
foam rubber could recover its original shape beer when silica filler was added. Hydro-
phobic fumed silica with a higher specific surface area, Reolosil DM30, provided the best
overall reinforcement and improvement in the properties of natural rubber foam. Even
Figure 11. Percentage shrinkage of different types of silica compared with sides of sample: (a) width;
(b) length; (c) height; and (d) height of center.
4. Conclusions
This study investigated the influence of different types and concentrations of silica
as fillers, including hydrophilic and hydrophobic fumed silicas, as well as precipitated
silica, on the properties of natural rubber foam produced using the Dunlop process. The
physical properties, swelling, hardness, and shrinkage behavior of the natural rubber foam
were determined. Four different types of silica were added to the rubber formulation with
loadings ranging from 0 to 2 phr. The results indicated that the incorporation of silica
fillers—both fumed and precipitated—could be evenly distributed in the rubber matrix.
Hydrophobic fumed silicas exhibited better dispersion and enhanced the physical and
mechanical properties of the natural rubber foam. In the SEM analysis, it was observed
that the hydrophobic silicas had a more homogeneous dispersion and integration with the
rubber matrix, leading to improved reinforcement and physical/mechanical performance
compared to the hydrophilic silica. Even with a lower specific surface area (BET), the
hydrophobic fumed silica, Aerosil R972, was still better dispersed compared to hydrophilic
silica with a higher BET. Increasing loadings of silica had a noticeable effect on the porosity
of the foam rubber; higher silica loadings resulted in smaller foam pores due to an increase
in crosslink density as the silica concentration increased, which led to a more consistent
pore structure in the foam rubber matrix. By contrast, a lower specific surface area made
the average pore diameter smaller, as observed with the hydrophilic fumed silica, Aerosil
200 (29.4
±
1.7
µ
m). Higher loadings of silica also increased the foam density and hardness.
The compression set decreased as silica loading increased, meaning the foam rubber could
recover its original shape better when silica filler was added. Hydrophobic fumed silica
with a higher specific surface area, Reolosil DM30, provided the best overall reinforcement
and improvement in the properties of natural rubber foam. Even with its lower specific

Polymers 2024,16, 3076 16 of 18
surface area, the hydrophobic fumed silica, Aerosil R972, still resulted in greater hardness
and recovery percentages compared to hydrophilic silica. However, excessive silica loading
resulted in an increase in the shrinkage of the foam, except for the height of the center foam,
which showed a decrease in shrinkage. In this experiment, the Dunlop process was used as
the foaming method. Loadings of silica higher than 2 phr were not suitable, as excessive
fillers impaired the foaming process and prevented some silica from mixing well with the
natural rubber. FTIR spectra analysis was used to try to confirm the chemical interaction
between silica and natural rubber. However, the results of the FTIR analysis were not clear
enough to identify differences in the chemical bonds between hydrophobic and hydrophilic
silica and natural rubber, as the silica loading was too low. It only showed a lower peak of
the O-H band in hydrophobic silica compared to hydrophilic silica.
Author Contributions: Conceptualization, L.G. and H.L.; funding acquisition, L.G. and H.L.;
investigation
, D.A., G.L., K.H. and L.B.; methodology, G.L. and F.L.; project administration, Y.X.;
writing—original draft, D.A. and Y.X.; writing—review and editing, F.L. and H.L. All authors have
read and agreed to the published version of the manuscript.
Funding: This research was funded by the National Natural Science Foundation of China (
No. 52373101
),
Natural Science Foundation of Shandong Province, China (No. ZR202102180625 and ZR2020QE207),
and Postdoctoral Research Foundation of China (No. 2023M733754).
Institutional Review Board Statement: Not applicable.
Data Availability Statement: The original contributions presented in the study are included in the
article, further inquiries can be directed to the corresponding authors.
Conflicts of Interest: There are no conflicts of interest to declare.
References
1.
Sobhy, M.; El-Nashar, D.E.; Maziad, N.A. Cure Characteristics and Physicomechanical Properties of Calcium Carbonate Rein-
forcement Rubber Composites. Egypt. J. Solids 2003,26, 241–257.
2.
Cui, C.; Ding, H.; Cao, L.Y.; Chen, D. Preparation of CaCO
3
-SiO
2
Composite with Core-Shell Structure and Its Application in
Silicone Rubber. Przem. Chem. 2015,17, 128–133. [CrossRef]
3. Kraus, G. Interactions of Elastomers and Reinforcing Fillers. Rubber Chem. Technol. 1965,38, 1070–1114. [CrossRef]
4.
Prasertsri, S.; Rattanasom, N. Mechanical and Damping Properties of Silica/Natural Rubber Composites Prepared from Latex
System. Polym. Test. 2011,30, 515–526. [CrossRef]
5.
Joon-Hyung, K. The Foaming Characteristics and Physical Properties of Natural Rubber Foams: Effects of Carbon Black Content
and Foaming Pressure. J. Appl. Polym. Sci. 2016,12, 795–801.
6.
Ghosh, A.K.; Maiti, S.; Adhikari, B.; Ray, G.S.; Mustafi, S. Effect of Modified Carbon Black on the Properties of Natural Rubber
Vulcanizate. J. Appl. Polym. Sci. 1997,66, 683–693. [CrossRef]
7.
Chuayjuljit, S.; Imvittaya, A.; Na-Ranong, N.; Potiyaraj, P. Effects of Particle Size and Amount of Carbon Black and Calcium
Carbonate on Curing Characteristics and Dynamic Mechanical Properties of Natural Rubber. J. Met. Mater. Miner. 2022,12, 51–57.
8. Callister, W.D. Materials Science and Engineering: An Introduction, 7th ed.; John Wiley & Sons, Inc.: New York, NY, USA, 2000; p. 871.
9.
Rattanasom, N.; Saowapark, T.; Deeprasertkul, C. Reinforcement of Natural Rubber with Silica/Carbon Black Hybrid Filler.
Polym. Test. 2007,26, 369–377. [CrossRef]
10. Taikum, O.; Scholz, M. The Last 100 Years of Fumed Silica in Rubber Reinforcement. Rubber World 2020,242, 35–44.
11.
Huang, L.; Li, Q.; Wang, X.; Zhang, J.; Yang, Z.; Zhou, W. Structural Analyses of the Bound Rubber in Silica-Filled Silicone Rubber
Nanocomposites Reveal Mechanisms of Filler-Rubber Interaction. Compos. Sci. Technol. 2023,233, 109905.
12.
Zaborski, M.; Vidal, A.; Ligner, G.; Balard, H.; Papirer, E.; Burneau, A. Comparative Study of the Surface Hydroxyl Groups of
Fumed and Precipitated Silicas. I. Grafting and Chemical Characterization. Langmuir 1989,5, 447–451. [CrossRef]
13.
Protsak, I.; Tertykh, V.A.; Pakhlov, E.; Deryło-Marczewska, A. Modification of Fumed Silica Surface with Mixtures of Poly-
organosiloxanes and Dialkyl Carbonates. Prog. Org. Coat. 2017,106, 163–169. [CrossRef]
14.
Maciá-Agulló, T.G.; Fernández-García, J.C.; Torró-Palau, A.M.; Barceló, A.C.O.; Martín-Martínez, J.M. Hydrophobic or Hy-
drophilic Fumed Silica as Filler of Polyurethane Adhesives. J. Adhes. Sci. Technol. 1995,50, 265–277. [CrossRef]
15.
Hassanajili, S.; Sajedi, M.T. Fumed Silica/Polyurethane Nanocomposites: Effect of Silica Concentration and Its Surface Modifica-
tion on Rheology and Mechanical Properties. J. Appl. Polym. Sci. 2016,25, 697–710. [CrossRef]
16.
Gun’ko, V.M.; Zarko, V.I.; Leboda, R.; Chibowski, E.; Grzegorczyk, W. Features of Fumed Silica Coverage with Silanes Having
Three or Two Groups Reacting with the Surface. Colloids Surf. 2000,166, 187–201. [CrossRef]
17.
Ettlinger, M.; Ladwig, T.; Weise, A. Surface Modified Fumed Silicas for Modern Coatings. Prog. Org. Coat. 2000,40, 31–34.
[CrossRef]

Polymers 2024,16, 3076 17 of 18
18.
Evans, L.R.; Hope, J.C.; Okel, T. Use of Precipitated Silica to Improve Brass-Coated Wire-to-Rubber Adhesion. Rubber World 2020,
214, 21–26.
19.
Bayat, H.; Fasihi, M. Effect of Coupling Agent on the Morphological Characteristics of Natural Rubber/Silica Composite Foams.
ePolymers 2019,19, 430–436. [CrossRef]
20.
Luo, Y.; Wang, Y.Q.; Zhong, J.; He, C.Z.; Li, Y.Z.; Peng, Z. Interaction Between Fumed-Silica and Epoxidized Natural Rubber.
J. Inorg. Organomet. Polym. Mater. 2011,21, 777–783. [CrossRef]
21.
Prasertsri, S.; Rattanasom, N. Fumed and Precipitated Silica Reinforced Natural Rubber Composites Prepared from Latex System:
Mechanical and Dynamic Properties. Polym. Test. 2012,31, 593–605. [CrossRef]
22.
ASTM D3574; Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams. ASTM
International: West Conshohocken, PA, USA, 2020.
23.
Hong, Y.; Weng, G.; Liu, Y.; Fu, K.; Chang, A.; Chen, Z. Effect of Silane Coupling Agent on the Fatigue Crack Propagation of
Silica-Filled Natural Rubber. J. Appl. Polym. Sci. 2015,132, 41980.
24.
Lee, J.; Kim, K.; Kim, J.; Lee, H.; Lee, D.; Jeon, H. Influence of the Silanes on the Crosslink Density and Crosslink Structure of
Silica-Filled Solution Styrene Butadiene Rubber Compounds. Mol. Cryst. Liq. Cryst. 2016,24, 711–727. [CrossRef]
25.
ISO 1817; Rubber, Vulcanized or Thermoplastic—Determination of the Effect of Liquids. International Organization for Standard-
ization: Geneva, Switzerland, 2015.
26.
Liang, T.; Isayev, A.I. Effect of Ultrasonic Extrusion of Star Styrene-Butadiene Rubber on Properties of Carbon Black- and
Silica-Filled Compounds and Vulcanizates. J. Appl. Polym. Sci. 2019,136, 47451. [CrossRef]
27.
Abd-El-Messieh, S.L.; El-Nashar, D.E.; Khafagi, M. Compatibility Investigation of Microwave Irradiated Acrylonitrile Butadi-
ene/Ethylene Propylene Diene Rubber Blends. Polym. Plast. Technol. Eng. 2004,43, 135–158. [CrossRef]
28.
Bernal-Ortega, P.; Anyszka, R.; Morishita, Y.; di Ronza, R.; Blume, A. Determination of the Crosslink Density of Silica-Filled
Styrene Butadiene Rubber Compounds by Different Analytical Methods. Polym. Bull. 2024,81, 995–1018. [CrossRef]
29.
Tangboriboonrat, P.; Suchiva, K.; Kuhakarn, S. Characterization of Non-Crosslinked Natural Rubber Latex by Phase Transfer
Technique. Polymer 1994,35, 5144–5145. [CrossRef]
30.
Timothy, J.J.; Meschke, G. A Cascade Continuum Micromechanics Model for the Effective Elastic Properties of Porous Materials.
Int. J. Solids Struct. 2016,83, 1–12. [CrossRef]
31.
Bashir, A.S.; Manusamy, Y.; Chew, T.L.; Ismail, H.; Ramasamy, S. Mechanical, Thermal, and Morphological Properties of (Eggshell
Powder)-Filled Natural Rubber Latex Foam. J. Vinyl Addit. Technol. 2015,23, 3–12. [CrossRef]
32.
ASTM D1415; Standard Test Method for Rubber Property—International Hardness. ASTM International: West Conshohocken,
PA, USA, 2018.
33. ASTM D1055; Standard Specification for Rubber Latices—Synthetic. ASTM International: West Conshohocken, PA, USA, 2021.
34.
Xiang, B.; Li, J.; Dong, X.; Wang, Y.; Zhou, Y. Microcellular Silicone Rubber Foams: The Influence of Reinforcing Agent on Cellular
Morphology and Nucleation. Polym. Eng. Sci. 2018,59, 5–14. [CrossRef]
35.
Sholeh, M.; Rochmadi, R.; Sulistyo, H.; Budhijanto, B.; Doloksaribu, M. The Effect of Nanostructured Silica Synthesis Temperature
on the Characteristics of Silica Filled Natural Rubber Composite. IOP Conf. Ser. Mater. Sci. Eng. 2021,1143, 012011. [CrossRef]
36.
Kumkrong, N.; Dittanet, P.; Sae-Oui, P.; Loykulnant, S.; Prapainainar, P. Properties of Silica/Natural Rubber Composite Film and
Foam: Effects of Silica Content and Sulfur Vulcanization System. J. Polym. Res. 2022,29, 302. [CrossRef]
37.
Wang, M.; Tu, H.; Murphy, L.J.; Mahmud, K. Carbon—Silica Dual Phase Filler, A New Generation Reinforcing Agent for Rubber:
Part VIII. Surface Characterization by IGC. J. Polym. Sci. Part A Polym. Chem. 2000,73, 666–677. [CrossRef]
38.
Boonkerd, K.; Chuayjuljit, S.; Abdulraman, D.; Jaranrangsup, W. Silica-Rich Filler for the Reinforcement in Natural Rubber.
J. Polym. Sci. Part A Polym. Chem. 2012,85, 1–13. [CrossRef]
39.
Kawaguchi, M. Stability and Rheological Properties of Silica Suspensions in Water-Immiscible Liquids. Colloids Surf. A Physicochem.
Eng. Asp. 2020,278, 102139. [CrossRef] [PubMed]
40.
Kralevich, M.L.; Koenig, L.J. FTIR Analysis of Silica-Filled Natural Rubber. J. Polym. Sci. Part A Polym. Chem. 1998,71, 300–309.
[CrossRef]
41.
Heideman, G.; Datta, R.N.; Noordermeer, J.W.M.; Van Baarle, B. Fundamental aspects of sulfur vulcanization. Rubber Chem.
Technol. 2005,78, 245–268. [CrossRef]
42. Greenwood, N.N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, UK, 1997.
43.
Bayat, H.; Fasihi, M. Curing Characteristics and Cellular Morphology of Natural Rubber/Silica Composite Foams. J. Mater. Sci.
2019,77, 3171–3184. [CrossRef]
44.
Thammathadanukul, V.; O’Haver, J.H.; Harwell, J.H.; Osuwan, S.; Na-Ranong, N.; Waddell, W.H. Comparison of Rubber
Reinforcement Using Various Surface-Modified Precipitated Silicas. Rubber Chem. Technol. 1996,59, 1741–1750. [CrossRef]
45. Sombatsompop, N. Investigation of Swelling Behavior of NR Vulcanisates. Polym. Eng. Sci. 1998,37, 19–39. [CrossRef]
46.
Mishra, S.; Mitra, R. Rational Design in Suspension Rheology of Cellular Silica. Adv. Colloid Interface Sci. 2010,370, 102–113.
[CrossRef]
47.
Jhang, C. Preparation and Characterization of Waterborne Polyurethane with Fumed Silica Composites. Master’s Thesis, National
Taipei University of Technology, Taipei, Taiwan, 2013, pp. 1–80. Available online: https://www.airitilibrary.com/Article/Detail/
U0006-2706201311253000 (accessed on 18 October 2024).
48.
Mathias, J.; Wannemacher, G. Basic Characteristics and Applications of Aerosil. J. Colloid Interface Sci. 1988,125, 61–68. [CrossRef]

Polymers 2024,16, 3076 18 of 18
49.
Suethao, S.; Phongphanphanee, S.; Wong-ekkabut, J.; Smitthipong, W. The Relationship between the Morphology and Elasticity of
Natural Rubber Foam Based on the Concentration of the Chemical Blowing Agent. Materials 2021,13, 1091. [CrossRef] [PubMed]
50.
Wei, C.; Lu, A.; Su-ming, S.; Wei, X.; Zho, X.; Sun, J. Establishment of Constitutive Model of Silicone Rubber Foams Based on
Statistical Theory of Rubber Elasticity. Chin. J. Polym. Sci. 2018,36, 1077–1083. [CrossRef]
51.
Suethao, S.; Ponloa, W.; Phongphanphanee, S.; Wong-ekkabut, J.; Smitthipong, W. Current Challenges in Thermodynamic Aspects
of Rubber Foam. Nature 2021,11, 6097. [CrossRef] [PubMed]
52.
Sattayanurak, S.; Noordermeer, J.W.; Sahakaro, K.; Kaewsakul, W.; Dierkes, W.K.; Blume, A. Silica-Reinforced Natural Rubber:
Synergistic Effects by Addition of Small Amounts of Secondary Fillers to Silica-Reinforced Natural Rubber Tire Tread Compounds.
Adv. Mater. 2019,2019, 5891051. [CrossRef]
53.
Liu, D.; Zhao, Y.; Zhang, Y.; Xu, X.; Zhang, H.; Gao, Y. Correlation between Mechanical Properties and Microscopic Structures of
an Optimized Silica Fraction in Silicone Rubber. Compos. Part B 2018,165, 373–379. [CrossRef]
54.
Ariff, M.Z.; Zakaria, Z.; Tay, L.H.; Lee, S.Y. Effect of Foaming Temperature and Rubber Grades on Properties of Natural Rubber
Foams. J. Appl. Polym. Sci. 2007,107, 2531–2538. [CrossRef]
55.
Pajarito, B.B.; Quizon, M.A.E.; Marquez, R.A.L.; Bermio, P.B.C. Modeling the Hardness and Mechanical Properties of Moulded
Natural Rubber Pad. Mater. Sci. Forum 2017,890, 54–58. [CrossRef]
56.
Muromtsev, N.D.; Gertseva, O.; Pichkhidze, Y.S. The Prediction of the Shrinkage of a Profile Based on Ethylene Propylene Rubber.
Rubber Chem. Technol. 2014,41, 59–62. [CrossRef]
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual
author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to
people or property resulting from any ideas, methods, instructions or products referred to in the content.