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1 Introduction: Silica reinforcement of natural rubber (NR)

1 Introduction: Silica reinforcement of natural rubber (NR)

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194



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



Hydrophobic and hydrophilic silica-filled cross-linked NR



195



interpenetration of neighboring flocculated filler clusters,16 and focuses on

the mechanical response of uncross-linked composites under the small strain

during heat treatment (annealing). It has been shown that some movement of

particles takes place, depending on the particle size, molar mass of the polymer,

and relative magnitude of polymer-to-filler and filler-to-filler interactions. The

results provide experimental evidence for a kinetic cluster–cluster aggregation

(CCA) mechanism of filler particles in the rubbery matrix, which may result

in the formation of a filler network structure.32 Dielectric investigations have

revealed that charge transport above the percolation threshold is limited by

a hopping or tunnelling mechanism of charge carriers across a small gap (of

the order of nanometers) between carbon black particles.33

On the other hand, mechanical flexibility can be one of the unique properties

of optically transparent materials.3 However, there have been few studies

on transparent elastomers to date, perhaps because few raw rubbers have

good transparency. Polysiloxane is an exception, and one of the authors

has studied a method of controlling the refractive index of polysiloxane by

chemically modifying the polymer having a hydrosilyl group (Si-H). 34 The

polymer was reacted with various compounds in order to introduce various

side groups by hydrosilylation (the reaction of Si-H with CH2=C– group).

Rubber technology can provide us with a more convenient approach than

chemically modifying the transparent elastomers. The process of mixing

rubber and filler particles, which commonly occurs when dispersing silica

filler into rubber, may be of use in preparing transparent elastomers. With

this in mind, the effects of hydrophobic and hydrophilic silica particles on

the optical properties are to be studied in this chapter.

Additionally, the three-dimensional morphology of silica particles in the

rubbery matrix was reported among the soft nanocomposites.3,8,10,35–37 The

morphology (i.e., agglomeration state) of hydrophilic silica particles (VN-3) in

NR was investigated three-dimensionally using three-dimensional transmission

electron microscopy (3D-TEM or electron tomography38) and the results

were examined in relation to the optical transparency of the materials. 3DTEM, which is a combination of TEM and computerized tomography, is a

powerful tool for demonstrating the three-dimensional structural parameters

of nanocomposites.36,37 Moreover, since the optical transparency or opacity

of silica-filled rubber has not been fully deduced, this study investigated

the relationship between the transparency and morphology of hydrophilic

silica-filled cross-linked NR.3

This chapter focuses on experiments where peroxide cross-linked NR was

filled with commercial hydrophilic silica particles of nanometer size (VN-3).

By combining the experimental results with some theoretical considerations,

the optical anomaly is inferred to originate from multiple light scattering

due to isolated chains of hydrophilic silica. Such chains are present in the

network-like structure of hydrophilic silica, but one end of the chains is not



196



Chemistry, Manufacture and Applications of Natural Rubber



connected to the network even after the formation of a percolated hydrophilic

silica network. To evaluate the optical transparency of hydrophobic silica-filled

cross-linked NR, diffusion transmittance and haze are measured by using

the standard light C representing average daylight.3 After the percolation

behavior is measured, morphology of the hydrophobic silica network is

observed through 3D-TEM and transparency of hydrophobic silica-filled

cured NR is compared with that of hydrophilic silica-filled NR.



7.2



Testing hydrophobic and hydrophilic silica

fillers: sample preparation



In this study, two types of particulate silicas were used: Nipsil VN-3 from

Toso Silica Co., as hydrophilic silica, and Aerosil RX200 (surface modified

by trimethyl silyl groups) from Evonik Deguss, Japan as hydrophobic. The

grade of NR used was RSS No. 1. The components for preparing hydrophobic

and hydrophilic silica-filled cross-linked NRs are shown in Table 7.1.

The only component that was altered in these series of cross-linked NR

samples was the silica loading, which varied from 0 to 80 phr (grams of

additive per 100 g of rubber). The quantity of dicumyl peroxide (DCP), a

cross-linking agent, was 1 phr, and identical in all samples. Raw rubber and

the compounding agents were first kneaded on an open two-roll mill, and

subjected to compression molding at a pressure of 100–150 kg/cm2 at 155°C

for 30 min to obtain cross-linked NR sheets of 1 mm in thickness. In the

following sections, hydrophobic and hydrophilic silica-filled cross-linked NR

samples will be referred to as NR-P-#-RX and NR-P-#-VN, respectively,

where # indicates the amount of silica in phr.



7.3



Methods for analyzing silica filler behavior in

cross-linked NR matrix



7.3.1 Measuring volume resistivity

The volume resistivity of the samples was measured by the three-electrode

method, using an ultra-high resistance meter (Advantest Corp., R8340A), with

a main electrode (stainless steel; 25 mm dia.), a guard electrode (stainless

steel; internal/external dia. of 38/50 mm) and a counter electrode (stainless

steel; 50 mm dia.). A sheet-shaped sample (50 ¥ 50 ¥ 1 mm) was tightly

sandwiched between the main and guard electrodes and the counter electrode

and the guard electrode were grounded to prevent current from flowing along

the sample surface. The applied voltage was 100 V/min. or 1 V/min. Volume

resistivity was calculated from the current that flowed through the sample when

a certain specified voltage was applied. This measurement method conforms

to the procedure specified in the relevant Japanese Industrial Standard (ISO



Table 7.1 Preparing hydrophobic and hydrophilic silica-filled peroxide cross-linked NR a

Sample



NR-P-OPX



NR-P-10RX



NR-P-20RX



NR-P-30RX



NR-P-40RX



NR-P-60RX



NR-P-80RX



NR

Dicumyl peroxide (phrb)

Silica RXc (phr)



100

1

0



100

1

10



100

1

20



100

1

30



100

1

40



100

1

60



100

1

80



Sample



NR-P-OVN



NR-P-10VN



NR-P-20VN



NR-P-30VN



NR-P-40VN



NR-P-60VN



NR-P-80VN



NR

Dicumyl peroxide (phra)

Silica VN3d (phr)



100

1

0



100

1

10



100

1

20



100

1

30



100

1

40



100

1

60



100

1

80



a



Cross-linking conditions: 30 min. at 155°C under 100–150 kg/cm2.

Grams per one hundred grams of rubber.

c

Aerosil RX200 (trimethyl silyl group treated silica, average radius = ca. 12 nm) from Evonik Degussa Japan Corpporation.

d

Nipsil VN-3 (average primary diameter = ca. 16 nm) from Tosoh Silica Corporation.

b



198



Chemistry, Manufacture and Applications of Natural Rubber



D-257-90, ‘Standard Test Method for DC Resistance on Insulating Materials’).

Sample cells were also loaded in a constant-temperature bath (the TR-43C

provided by Advantest Corp.) which kept the temperature at 296 K (23°C)

for approximately 20 min. Subsequently, the current that flowed through

the sample upon the application of voltage was measured to determine the

volume resistivity of a heat treated sample.



7.3.2 Optical microscopic observation

Using a Nikon optical microscope fitted with a close-up lens, photographs

were taken of cross-linked NR samples placed on sheets of graph paper,

illuminated from behind. This method enabled us to observe the samples in

transmission mode.



7.3.3 Diffusion transmittance and haze measurement

The diffusion transmittance and haze of the cross-linked NR samples were

measured using a fully automatic direct-reading haze meter from Suga Test

Instruments Co., Ltd (HGM-2). This measuring procedure conforms to Japan

Industrial Standard JIS K 7136:2000 (‘Plastics: determination of haze for

transparent materials’). The halogen lamp used as the light source produced

a single beam of standard illuminant C, representing average daylight with a

correlated color temperature of approximately 6774 K. The sample size was

70 ¥ 70 ¥ 1 mm. Diffusion transmittance and haze are widely used indices

of transparency in the optics industry.



7.3.4 Conventional transmission electron microscopy

(TEM) observation

Small and thin pieces of the cross-linked NR film were first embedded in a

plastic resin and then frozen at the temperature of liquid nitrogen (–198°C).

Ultra-thin samples of several tens of nm in thickness were then cut out with

an ultra-microtome. Unstained and uncoated ultra-thin samples were observed

using a Hitachi H-800 TEM at an accelerating voltage of 200 kV.



7.3.5 Three-dimensional TEM (3D-TEM) measurements

Ultra-thin samples for 3D-TEM observation were cut with an ultra-microtome

from the cross-linked NR films frozen at –198°C. They were then subjected

to a pre-treatment process using the Nissan ACR-SG method to make their

thickness uniform and their surface smooth. The samples obtained in this

way for 3D-TEM observation measured approximately 500 nm in length,

500 nm in width and 200 nm in thickness.



H 3C

C



CH



H 3C



H 2C



CH2

C



CH

H 2C



H 3C



trans



cis



CH2

C

H 3C



H 3C



H 3C

C



CH

H 2C



C



CH



CH2



H 2C



n



CH2



CH

H 2C



OH



w



a



Plate I (Chapter 2) Chemical structure of NR. Head (a) and end group

(w) structure is not proven.



OPP



OPP



Farnesyl pyrophosphate



Isopentenyl pyrophosphate



cis-prenyltransferase

metal2– cofactor

OPP



2



HOPP

cis-prenyltransferase

metal2– cofactor

2



>5000



OPP



HOPP



Plate II (Chapter 2) Synthesis of NR in H. brasiliensis. OPP stands for

the pyrophosphate end-group and HOPP represents pyrophosphoric

acid.



F



H 3C

G C

H 3C



F



H

CH



A≤

CH2



H 2C

I



C≤

CH



B≤ C

H 3C

E≤



trans



CH2



H 2C

D≤



B¢ C

H 3C







CH

H 2C





cis



E

H 3C



E≤¢

H 3C



C

CH



B

C



C



H 2C

CH2

D n A≤¢



CH2

A



B



200



H 2C

D≤¢



OH



w



a



O

||

–OCR



C≤¢

CH





trans-trans

w-trans

A≤



C



B≤

w-trans

C

B≤¢

a

C



160



Plate III (Chapter 2)

2001).



C≤¢

a

CH



120

13



a

C O

H2



O



A D E



trans

–CH2



E≤







trans

F

–CH3

w

–CH3



CH3



80

40

0

Chemical shift (ppm from TMS)



C of NR from L. volemus (Tanaka et al., 1994,



Enzyme





Initiation

n







O

P

P Mt2+

O

O

O



O



 2+Mt



PP



O



Allylic pyrophosphate initiator



Enzyme





Propagation



2+







O OO O

Mt P

P Mt2+

O

O

O





PP







PP



+











PP



H H



HPP



PP



Plate IV (Chapter 2) Proposed natural living carbocationic polymerization (NLCP) mechanism of NR biosynthesis (Puskas

et al., 2006).



Unreactive



Reactive



X

SiMe3

1







El



El



- SiMe3



1

El

X

- SiMe3X



El



X



X



n



O

- SiMe3X





O





Cl O

O



El = elecrophilic initiator



Plate V (Chapter 2) Attempted ‘bio-inspired’ synthesis of cis-1,4polyisoprene (Yokozawa and Yokoyama, 2005).



H 3C

CH3

H

H3C C OH



H 2C



H 3C



CH2



B(C6F5)3



H

H 3C C



CH2Cl2, -30 °C

or

H2O, +20 °C

OCH3



H 2C



C C

H



CH2



CH



C CH

CH2



n



OCH3



Plate VI (Chapter 2) Chemical scheme of cationic polymerization of IP

with 1-(4-methoxyphenyl) ethanol as the initiator and B(C6F5)3 as the

co-initiator (Kostjuk et al., 2011).



B phase



A phase

¥4



B phase



¥6

: ZnO cluster

: Absorbed sulfur and CBS



IR-1-Z1-S1.5

A phase



: Network domain containing ZnS

: Solubilized zinc stearate in rubbery matrix

(a)



(b)



(c)



Plate VII (Chapter 4) Proposed models to explain the inhomogeneity of network structure in isoprene rubber vulcanizate.

(a) Two-phase model of network structure: A phase, matrix with low network-chain density; B phase, network domain

with high network-chain density. (b) Inhomogeneity around ZnO clusters in the matrix, where mesh size (x) and size

of network domain (X) are also displayed. x and X should be referred to the measured x and X shown in Table 4.1. (c)

One example of two-phase network structure with a high reality, although the distance between the domains is not

yet revealed. (Reprinted from Ikeda, Y., Higashitani, N., Hijikata, K., Kokubo, Y., Morita, Y., Shibayama, M., Osaka, N.,

Suzuki, T., Endo, H., Kohjiya, S., Vulcanization: new focus on a traditional technology by small-angle neutron scattering,

Macromolecules, 42, 2741–2748, copyright © 2009, with permission from the American Chemical Society.)



Increase of sulfur and

CBS



Increase of

ZnO

: ZnO cluster

: Absorbed sulfur and CBS

: Network domain containing ZnS

: Solubilized zinc stearate in rubbery matrix



Plate VIII (Chapter 4) Schematic presentation of the formation of

two-phase inhomogeneity in rubber vulcanizates on the basis of

the amounts of sulfur, CBS (accelerator) and ZnO in the presence

of stearic acid of 2 phr. (Reprinted from Ikeda, Y., Higashitani, N.,

Hijikata, K., Kokubo, Y., Morita, Y., Shibayama, M., Osaka, N., Suzuki,

T., Endo, H., Kohjiya, S., Vulcanization: new focus on a traditional

technology by small-angle neutron scattering, Macromolecules, 42,

2741–2748, copyright © 2009, with permission from the American

Chemical Society.)



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