1. Introduction
Silica and carbon black are reinforcing fillers that are
widely used in rubber. Because of their different surface
characteristics, both fillers can form agglomerates but the
cause of such agglomeration is not the same, leading to a
difference in dispersion ability. The filler–filler interaction
of carbon black is mainly generated through relatively
weak Van der Waal forces, which can be readily broken
during mixing. In contrast, silica agglomeration is due to
hydrogen bonding in addition to Van der Waal forces and
other physical interactions, leading to a much stronger
filler–filler interaction [1]. Silica has a high polarity and a
hydrophilic surface due to silanol groups on its surface.
Consequently, silica is incompatible with non-polar
rubbers such as natural rubber (NR), styrene–butadiene
rubber (SBR) and butadiene rubber (BR), but better compatible
with polar rubbers such as polychloroprene rubber
(CR) [2] and acrylonitrile butadiene rubber (NBR) [3]. Use
of silica in non-polar rubbers without any compatibilizers
results in poor dispersion and consequently inferior properties
as a result of these strong filler–filler interactions
[4]. In addition, polar functional groups on the silica
surface may form hydrogen bonds with other polar components
in rubber compounds such as accelerators. So, commonly
basic accelerators are adsorbed on the acidic surface
of silica which negatively affects cure properties [5].
Successful use of silica for rubber reinforcement therefore
requires commonly silane coupling agents to enhance
silica–rubber interaction and silica dispersion as well as
to prevent accelerator adsorption on the silica surface.
Some polar rubbers bearing functional groups which
can interact with the silica surface have been studied as alternatives to silane coupling agents to enhance the interaction
in silica-filled compounds [6–9]. A non polar rubber
such as polybutadiene was chemically modified to have
epoxy groups on the main chain and the interaction of such
epoxidized rubber with silica was consequently improved,
as observed by a decrease of the Payne effect and a change
of damping behavior in the glass transition region. The
improvement of silica–rubber interaction was also demonstrated
by a higher bound rubber content and better silica
dispersion in the matrix [10]. Incorporation of silica into an
epoxidized SBR showed a strong interaction between the
silica and the epoxidized functional groups due to the
highly polar character of the epoxidized rubber [11].
Epoxidized natural rubber (ENR) is also higher polar
than virgin NR due to the epoxide groups in its structure.
The chemical and physical properties of ENR change
according to the epoxide contents, such as heat and swelling
resistances [12]. By its polar functional groups, ENR
interacts with hydroxyl groups on the silica surfaces [13].
It has been reported already that the mechanical properties
of silica-filled ENR without coupling agent are higher than
those of silica-filled virgin NR [14–15]. This was claimed to
be due to improved interaction between ENR and the silica
surface via hydrogen bonds. ENR as the rubber component
in tire compounds has been reported to improve the silica
dispersion leading to improved processability, better storage
stability and tire performance [13,16–18]. ENR was
used as a compatibilizer in silica-filled NR/NBR blends to
enhance the mechanical properties [19]. In case of silicafilled
NBR vulcanizates, the use of ENR as a compatibilizer
improved tensile strength, tear strength and abrasion loss
[20], as well as lowered the loss tangent [21], as the ENR
acts as a coupling agent because it can self-crosslink with
NBR and also link with silica at the vulcanization temperature
[20–21]. Furthermore, ENR was used as a compatibilizer
in organoclay-filled NR composites wherein cure
characteristics, tensile and dynamic mechanical properties
were improved [22–23]. Even though ENR has been used to
compatibilize between silica and various rubber matrices,
the use of ENR as compatbilizer for silica-filled virgin NR
has not yet been extensively studied.
In the present study, ENRs with different mole% of
epoxide groups are used as compatibilizers in a range of
2.5–15 phr for virgin NR/silica compounds. The effects of
ENR contents and mole% of epoxide are investigated. The
epoxide groups are supposed to interact with the silanol
groups of the silica and so to improve silica–rubber compatibilization,
while the unmodified parts of the ENR structure
is compatible with the NR matrix.