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5 Conclusions: Key issues in improving the properties of natural rubber (NR)

5 Conclusions: Key issues in improving the properties of natural rubber (NR)

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In situ silica to improve the mechanical properties of NR


particles in NR latex, utilization of a non-rubber component in NR may be

one of the keys to further improve the properties of NR materials.



This work was partially supported by the ENO Science Foundation. The

X-ray experiments were performed on the BL-40B2 beamline at SPring-8

with the approval of the Japan Synchrotron Radiation Research Institute

(JASRI) (Proposal Nos 2005B0661-NL2b-np and 2006A1298-NL2b-np,

2012A1688). The authors thank John Wiley & Sons, Springer, Elsevier and

Nippon Gomu Kyokai for copyright permissions.



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Hydrophobic and hydrophilic silica-filled

cross-linked natural rubber (NR):

structure and properties

A. K a t o, NISSAN ARC Ltd, Japan and Y. K o k u b o,

R. Ts u s h i and Y. I k e d a, Kyoto Institute of

Technology, Japan

DOI: 10.1533/9780857096913.1.193

Abstract: The optical characteristics of hydrophobic silica-filled crosslinked natural rubber (NR) samples were independent of the loading

amount, whereas diffusion transmittance and haze of the hydrophilic

silica-filled cross-linked NR samples showed peak values with silica content

in the vicinity of 30 phr. The two types of silica have markedly different

dispersion and aggregation structures in NR. Conventional TEM and 3DTEM observation reveal that dispersion of the hydrophobic rubber is more

homogeneous than hydrophilic and it is also interesting that the distance

between the two perimeters of the silica aggregates of both silica converged

to ca. 1.3 nm. Hydrophobic silica forms a network structure more readily

than hydrophilic. The results of the study reveal that the optical properties of

hydrophobic silica-filled cross-linked NR do not change markedly even if the

silica loading is varied because isolated silica chains decrease sharply with

increased silica content.

Key words: hydrophobic silica, hydrophilic silica, silica-filled natural

rubber, network structure of particulate silica, transparency, haze.


Introduction: Silica reinforcement of natural

rubber (NR)

The presence of hydrophilic silanol groups on a particulate silica surface

induces strong filler-to-filler interactions due to hydrogen bonding, especially

in a non-polar rubbery matrix.1–3 This dominance of filler-to-filler interactions

over filler-to-rubber interactions (usually due to van der Waals forces)

makes it difficult to disperse particulate silica effectively in the rubbery

matrix compared with carbon black.4 In order to overcome this well-known

difficulty, various silane coupling agents are used,1 which improve filler

dispersion in the rubbery matrix by reacting with the silanol groups to

modify the silica surface.

Another effect of silane coupling agents is to prevent adsorption of crosslinking reagents on the silica surface. This needs to be taken into account in

the design of rubber products when a sulfur/accelerator cross-linking system


© 2014 Woodhead Publishing Limited


Chemistry, Manufacture and Applications of Natural Rubber

is used. Successfully combining these effects during processing results in

improvements in mechanical and other properties of particulate silica/rubber

systems.1, 5 Besides conventional mechanical mixing, it is possible to use

a sol-gel technique involving the reaction of tetraethoxysilane to produce

silica in situ.6–11

Both carbon black and particulate silica for rubber reinforcement are of

a smaller diameter than a micron meter. Therefore, both are aggregated to

form linear and mostly branched aggregates, even during manufacturing.

The increased amount of filler in the rubbery matrix induces more and more

filler-to-filler interactions (actually aggregate-to-aggregate interactions), and

promotes the formation of agglomerates. (The agglomerate is the product

of further association of the aggregates.) Accordingly, the strong filler-tofiller interaction, as is the case in hydrophilic silica (e.g., VN-3), results

in the presence of potentially many types of silica, especially in rubbery

matrix, which is usually non-polar; primary silica particles, a few types of

silica aggregates, and a few types of silica agglomerates. This complicated

situation is believed to be the origin of many practical problems when using

particulate silica in rubber processing, such as inhomogeneous dispersion

of the filler in rubbery matrix, poor processability of rubber/silica mixtures,

poor surface appearance of the final products, inferior mechanical properties,

and so on.5,12–14

Another factor to be considered is the interaction between the polymer

matrix and the filler. The opportunities for agglomeration may be reduced when

the filler-to-rubber interaction is comparable to the filler-to-filler interaction.

It is well known that bound rubber is produced in the mixing process of

reinforcing fillers into rubbers. The filler is surrounded by bound rubber

(often called the immobilized layer) consisting mainly of polymer phases

of different molecular mobility.15–18 Carbon black, in particular, disperses

in the rubbery matrix, not as a separate primary particle, but as aggregates

consisting of 5–10 primary particles. It is believed that the carbon black

aggregates are further associated to form a secondary network structure of the

filler chains in which the immobilized polymers connect the filler particles or

aggregates.19–23 (The primary network structure is formed by the cross-linking

of rubbery matrix, usually by covalent bonding. It is therefore permanent.)

This secondary network structure by the filler is more or less transient, but

is thought to be the reason for the marked enhancement of mechanical and

viscoelastic properties of filler-loaded cross-linked rubbers, even including

NR which is self-reinforcing due to strain-induced crystallization.24–31

Filler networking in elastomer composites can be analyzed by transmission

electron microscopy (TEM) and other techniques. The conventional TEM

observation, however, provides a limited microscopic view of the filler

morphology, mainly due to the fact that only two-dimensional images are

obtained. A flocculation study has reported interesting results on the spatial

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5 Conclusions: Key issues in improving the properties of natural rubber (NR)

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