Organoclay–natural rubber nanocomposites

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Polymer International Polym Int 53:1766– 1772 (2004)DOI: 10.1002/pi.1573

Organoclay–natural rubber nanocompositessynthesized by mechanical and solution mixing

methodsMA L ´ opez-Manchado,∗ B Herrero and M ArroyoInstitute of Polymer Science and Technology, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain

Abstract: This investigation describes two methods to obtain rubber composites based on natural rubber

(NR) and organophilic layered silicates. In order to improve the exfoliation and compatibilization of the

organoclays with the rubber matrix, a new approach which involves swelling of the organoclays with an

elastomer solution prior to compounding has been used. The effect of the addition during swelling of a

coupling agent, namely bis(trietoxysilylpropyl)tetrasulfan (TESPT), on the behaviour of the composites

was also investigated. The results show that a low amount of organoclay (10 phr) significantly improves the

properties of natural rubber. This suggests a strong rubber–organoclay interaction which is attributed to a

high degree of rubber intercalation into the nanosilicate galleries, as was confirmed from X-ray diffraction.

In addition, an ulterior improvement in the properties of the nanocomposites prepared by solution mixing

is clearly observed, due to the better filler–rubber compatibility. An even further increase in the properties

is observed by treating the silicate with a silane coupling agent. The silane functional groups modify the

clay surface, thus reducing the surface energy, and consequently improving the compatibility with the

rubber matrix.

2004 Society of Chemical Industry

Keywords: rubber nanocomposites; organoclays; TESPT; mechanical properties

INTRODUCTION

The incorporation of fillers into elastomer matrices

leads to a significant improvement in the physical,

mechanical and electrical properties of crosslinked

elastomeric composites. This reinforcing effect is pri-

marily due to hydrodynamic interactions between

the rubber and filler surfaces.1 Traditionally, car-

bon black has been the primary filler used by the

rubber industry. However, since the 1950s non-

black fillers such as precipitated silica have been

increasingly used. At present, nanometer-scale rein-

forcing particles have attracted considerable attention

from polymer scientists. Because of their high aspect

ratio (length/diameter) and low density, they may be

used as substitutes for traditional fillers in polymermatrices. The most common reinforcements on the

nanoscale level are inorganic clay minerals consisting

of nanolayered silicate.2 – 5 Stacking of the layers of

approximately 1 nm thickness by weak dipolar forces

leads to interlayers or galleries between the layers.

These galleries are normally occupied by metallic

cations such as K +, Na+, Ca++, and Mg++. These

metalic cations can be easily exchanged by organic

ammonium salts, thus producing organophilic clays,

further known as organoclays.6 Organophilic modifica-

tion makes the silicate compatible with the polymer.

These entering guest molecules can either simply

increase the distances between the ‘still-parallel’ lay-

ers in an intercalation process or entirely randomly

disperse the separate sheets in an exfoliation process.

In recent years, these types entirely of hybrid materi-

als based on layered silicate polymer nanocomposites

have focused the attention of researchers because of

their unexpected hybrid properties derived from both

components which are not shared by their conven-

tional microcomposite counterparts.7 – 9 Such materi-

als are finding applications in areas where conventional

filled composites or microcomposites are being used.

These composites on the nanoscale level show signifi-

cant improvements in physical, chemical, mechanical

and thermal properties,8 gas permeability10 and fireretardance.11

Different methods for synthesizing polymer– layered

silicates nanocomposites have been typically described,

eg in situ intercalative polymerization, polymer inter-

calation from solution, and direct polymer melt

intercalation.12 Several studies have shown the pos-

sibility of preparing intercalated or exfoliated rub-

ber nanocomposites by different methods.13–16 It

has been reported that the nanolayered silicate dis-

persed into a rubber matrix provides an effective

reinforcement.17–21

∗ Correspondence to: MA L ´ opez-Manchado, Institute of Polymer Science and Technology, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain

E-mail: [email protected]

( Received 30 May 2003; revised version received 17 February 2004; accepted 18 February 2004 )

Published online 26 July 2004

2004 Society of Chemical Industry. Polym Int 0959–8103/2004/$30.00 1766



Organoclay–Natural Rubber Nanocomposites

The goal of this present study is to analyse

the morphology, physical and mechanical properties

of natural rubber nanocomposites, synthesized by

two different methods using octadecylammonium-

modified saponite. A new approach is considered,

involving swelling of the organoclay with a natural

rubber/toluene solution. The effect of the intercalation

of the silane coupling agent to the filler/rubber toluenesolution is also investigated.

EXPERIMENTAL

Materials

Samples of natural rubber were kindly supplied by

Malaysian Rubber, Berhad (Malaysia), under the trade

name CV 60 (Mooney viscosity, ML(1 + 4) 100 ◦C =

60). Sodium–saponite, with a cation exchange capac-

ity (CEC) value of 70 mmol per 100 g, was provided

by Tolsa SA, Madrid (Spain). In order to increase the

spacing between the layers and improve the compati-

bility of natural rubber, the organoclay was prepared

in our laboratories by treating the sodium– saponite

with a quaternary octadecylammonium salt, follow-

ing a previously described procedure.22 Octadecy-

lamine, purchased from Aldrich, Madrid (Spain),

was used as the organic modifier for the clay.

Bis(triethoxysilylpropyl)tetrasulfan (TESPT) (Si69),

manufactured by Degussa AG, Bitterfeld (Germany),

was used as the coupling agent.

Synthesis of natural rubber/clay nanocomposites

Two methods for synthesizing elastomer–clay nano-

composites were evaluated, namely mechanical andsolution mixing methods. Figure 1 provides a con-

ceptual picture of the synthesized nanocomposites

via both methods. The solution mixing method was

carried out as follows. Natural rubber was swollen

in toluene under continuous stirring, and while the

organophilic clay was itself dispersed in toluene. This

dispersion was poured into the rubber/toluene solu-

tion and maintained under vigorous stirring for 24 h,

and the solvent was then evaporated under vacuum

at room temperature. The organoclay content in the

nanocomposite was 10 parts per hundred parts of

rubber (phr). In order to analyse the effect of a com-

mercial silane coupling agent on the behaviour of the

composite, 5 phr of bis(trietoxysilylpropyl)tetrasulfan

(TESPT) (Si69) were added to one half portion of

the solution and also maintained under continuousstirring over 24 h.

In the case of the mechanical method, rubber

composites were prepared in an open two-roll mill

at room temperature. The rotors operated at a speed

ratio of 1:1.4. The vulcanization ingredients were

added to the elastomer prior to incorporation of the

filler and finally, the sulphur was incorporated. The

proportion of organoclay was also 10 phr. The material

was vulcanized in an electrically heated press at 150◦C

for the optimum cure time (t 90), previously determined

from an oscillating disc rheometer (Monsanto MDR

2000, Alpha Technologies, Swindon, UK). Specimens

were mechanically cut out from the cured plaques. The

recipes for the rubber composites are given in Table 1.

The effects of Na+ –saponite and octadecylamine in

the absence of clay have been investigated in a previous

work.22

Measurements

For the bound rubber measurements, approximately

0.2 g of each sample were cut into small pieces

of approximately 1 mm3 in size and placed into

a stainless steel cage of known weight. Then, the

cage was immersed in 50ml of toluene for 72 h at

room temperature. Finally, the samples were takenout and vacuum dried at 60 ◦C to constant weight.

The percent-bound rubber content of the polymer,

RB, was measured as the weight percentage of the

unsolubilized rubber on the silicate surface, according

to the following equation:

RB =W fg − W [mf /(mf +mp)]

W [mp/(mf + mp)]× 100 (1)

+

Organoclay Intercalated or exfoliatednanocomposite

oo o

Solvated organoclay

ooo

oo

o

oo

ooo

o

o oo

oo o

o

oo

ooo

oo

oo

o o

oo

o

oo o

oo

o

oo

o

o

ooo

o o oo

oo oo

o

o

oo

oo

o

Solvated polymer

+

o

o

o

oo o

oo

o o

o

oo

o o

oo

oo

o

o

o

oo o

oo

oo

o

o

oo o

oo

oo

oo

Evaporated

Intercalated or exfoliatedsolvated nanocomposite

Intercalated or exfoliatednanocomposite

(a)

(b)

Polymer

Figure 1. Schematic representations of the preparation of nanocomposites via the (a) mechanical and (b) solution mixing methods.

Polym Int 53:1766– 1772 (2004) 1767



MA L opez-Manchado, B Herrero, M Arroyo

Table 1. Recipes used for the rubber compounds

Natural rubber 100

Zinc oxide 5

Stearic acid 1

Sulfur 2.5

MBTSa 1

PBNb 1

Organoclay 10a Benzothiazyl disulfide.b Phenyl beta naphthyl amine.

where W fg is the weight of silicate and gel, mf the

weight of the filler in the compound, mp the weight of

the polymer in the compound, and W the weight of

the specimen.

Tensile stress–strain properties were measured

according to ISO 37–1977 specifications, on an

Instron dynamometer (Model 4301), at 25 ◦C at

a crosshead speed of 500 mmmin−1. Compression

set measurements were performed according toISO 815–1972 during 24h, at 70◦C with 25 %

compression. Rebound resilience measurements were

carried out on a Schob pendulum according to ISO

4662-1978. Shore hardness was measured by using a

Bareiss Rockwell tester according to ASTM D-2240.

In all of the tests, data were used as the average of at

least five measurements.

The number of active network chain segments

per unit of volume (crosslinking density) was

determined on the basis of the rapid solvent-swelling

measurements (toluene at 30 ◦C) by application of the

Flory–Rehner equations.23

The dynamic mechanical properties of the solid

polymer were determined on a dynamic mechanical

Metravib Model Mark 03 thermoanalyser. Tests were

carried out in the torsion deformation mode, at a

frequency of 5 Hz, with the temperature programmes

being run from −100 to 50 ◦C, at a heating rate of

2 ◦C min−1, under a controlled sinusoidal strain in a

flow of nitrogen.

RESULTS AND DISCUSSION

Bound rubber

Bound rubber (RB) measurements are conventionally

carried out to assess rubber–filler interactions. So,

the higher the bound rubber, then the higher is the

polymer–filler interaction.24 The results obtained are

summarized in Table 2. Organoclay-filled composites

obtained by solution blending showed higher RB values

than those prepared by simple mechanical mixing,

Table 2. Bound rubber measurements

System RB (%)

NR 0

NR –organoclay (mechanical mixing) 10.6NR –organoclay (solution mixing) 13.4

NR – organoclay – Si69 (solution mixing) 15.2

which indicates a higher compatibility between the

filler andthe polymer matrix when the nanocomposites

were synthesized by solution mixing. In addition, the

bound rubber values also increase in the presence of

the silane coupling agent. The latter has a sulfidic

linkage between two triethoxysilylpropyl groups. This

coupling agent is capable of interacting with the

O– H groups of the silicate through its –Si(OCH3)3

functionality, through hydrogen bonding. The sulfide

group of the coupling agent bonded to the silicate is

dissociated and reacts with the rubber molecule to

form crosslinks between the silicate and the rubber.

These chemical bonds lead to an enhancement of

bound rubber formation. Similar conclusions were

drawn by Manna et al ,25 when analysing precipitated

silica and epoxidized natural rubber composites in the

presence of a silane coupling agent.

Vulcanization characteristics

The vulcanization curves of the pristine natural rubber

and its composites with organoclay are graphically

represented, as obtained from the MDR 2000

measurements, in Fig 2. The curing characteristics,

expressed in terms of the vulcanization times, t S2

(scorch time) and t 90 (optimum cure time), as

well as the maximum and minimum values of the

torque, S max and S min, respectively, and delta torque

S (S = S max − S min), are deduced from the curves.

These parameters, along with the cure rate index, CRI

expressed as CRI = 100/(t 90 − t S2), are compiled in

Table 3.

Note that both vulcanization times, t S2 and t 90, weresharply reduced by the incorporation of low organoclay

amounts, showing accelerated vulcanization with

respect to that of pure NR. It is deduced that the

organoclay behaves as an effective vulcanizing agent

for natural rubber. These results are confirmed by the

cure rate index values, CRI , which show a significant

increase with addition of organoclay, attributed to the

amine functionalities in the nanosilicate structure. It

is well known that amine groups facilitate the curing

reaction of natural rubber compounds. Moreover, the

synergetic combination of a benzothiazyl accelerant

with an amine produces a particular accelerant effecton the rubber vulcanization reaction. Nevertheless, in

a previous study,26 it was demonstrated that in the

presence of organoclay a further accelerating effect

on NR curing takes place. In fact, the intercalation

of the octadecylamine within the silicate galleries

facilitates the vulcanization reaction, with a noticeable

decrease in the required time for NR vulcanization

in comparison with the blend filled with pure

octadecylamine only. In addition, it is important to

note that a further decrease in the cure time is

observed for the nanocomposite prepared by solution

mixing. On addition of the TESPT coupling agent,

a significant increase in the required time for NR

vulcanization is observed. This fact can be attributed

to interactions between the silane and amine groups

1768 Polym Int 53:1766– 1772 (2004)




0

2

4

6

8

10

12

14

0 5 10 15 20 25 30

S ′ ( d N m )

Time (min)

NR–organoclay (mechanical mixing)

NR–organoclay (solution mixing)

NR–organoclay–Si69 (solution mixing)

Unfilled NR

Figure 2. Vulcametric curves of the various systems obtained at 150 ◦C.

Table 3. Curing characteristics of the studied materials at 150 ◦C

t S2 (min) t 90 (min) Smax (dNm) Smin (dNm) S = Smax –Smin (dNm)

CRI, 100 (t 90 –t S2)

(min−1 )

NR 7.19 14.04 4.46 0.03 4.43 14.6

NR–organoclay (mechanical mixing) 3.24 6.87 8.83 0.04 8.79 27.5

NR–organoclay (solution mixing) 1.32 3.71 11.95 0.06 11.89 41.8

NR–organoclay–Si69 (solution mixing) 1.27 7.26 13.46 0.06 13.40 16.7

present in the silicate surface, so hindering theaccelerating effect of the amine groups.

On the other hand, the maximum torque and delta

torque increased by addition of the organosilicate,

showing the strong reinforcing effect of this filler. It

is of interest to point out that this effect is more

evident when the nanocomposites are prepared by

solution mixing, which suggests a higher compatibility

at the filler/elastomer interface. In addition, both

the maximum torque and differential torque were

found to increase with incorporation of a coupling

agent, thus indicating that the matrix–filler bonding

by TESPT takes place at the silicate layer surface.Furthermore, these results suggest that the natural

rubber became more crosslinked in the presence of the

organoclay, as was confirmed from crosslinking density

measurements (Table 4). Porter27 reported that the

crosslinking density of a carbon-black-reinforced

vulcanization system is enhanced by about 25 % when

compared with the corresponding unfilled one.

These results are in concordance with the bound

rubber measurements, which give an indication of the

rubber/filler interactions as a result of mixing.

Nanostructures of the vulcanizates

The organoclay nanolayers have been uniformly

dispersed (intercalated or exfoliated) in the elastomer

matrix by means of both the mechanical and

solution mixing techniques. The X-ray diffraction(XRD) patterns (Fig 3) show disappearance of the

diffraction peak at about 2θ = 5 ◦, corresponding to

the organosilicate interlayer platelet spacing.

Mechanical properties

The moduli at different elongations (50, 100, 300

and 500 %), maximum strength and elongation at

break of the studied elastomeric compounds are

compiled in Table 4. From the obtained results,

it can be deduced that the incorporation of small

amounts of organosilicate (10 phr) gives rise to a

noticeable increase in modulus, which shows thestrong reinforcing effect of these inorganic fillers. The

reinforcement is associated with the anisotropy and

high aspect ratio of organoclay nanofillers. These

act as short reinforcing fibers with a nanoscale

architecture. In addition, the modulus and maximum

tensile strength increase when the nanocomposite

is synthesized by the solution mixing method. This

fact can be attributed to the extent of dispersion

of the silicate in the NR matrix and the increased

crosslinking density resulting from polymer– filler

interactions. The silane coupling agent, which is itself a

crosslinking agent, increases the crosslinking density of

the composite, thereby enhancing the modulus. Thus,

the modulus at low strains was found to increase by

adding the silane coupling agent.28

Polym Int 53:1766– 1772 (2004) 1769




2 4 6 8 10 12 14

(a) Organoclay

(b) NR–organoclay (mechanical mixing)

(c) NR–organoclay (solution mixing)

I n t e n s i t y

(a)

(b)

(c)

2θ (degrees)

Figure 3. X-ray diffractograms of the various systems.

Table 4. Mechanical properties of the studied materials

Parameter Unfilled NR

NR–organoclay

(mechanical mixing)

NR–organoclay

(solution mixing)

NR–organoclay–Si69

(solution mixing)

Modulus, 50 % (MPa) 0.38± 0.02 0.95± 0.04 1.36± 0.05 1.45± 0.04

Modulus, 100 % (MPa) 0.59± 0.03 1.72± 0.08 2.25± 0.08 2.89± 0.06

Modulus, 300 % (MPa) 1.33± 0.11 4.31± 0.20 5.58± 0.25 7.77± 0.31

Modulus, 500 % (MPa) 2.60± 0.18 9.73± 0.42 11.20± 0.46 —

Maximum strength (MPa) 8.9± 0.68 20.6± 0.75 22.2± 0.68 15.2± 0.58

Elongation at break (%) 993± 38 1012 ± 52 919± 33 475± 28

Resilience (%) 63.0± 3.1 62.5 ± 2.7 64.5± 2.4 61.2± 2.1Hardness, Shore A 28.8± 2.1 43.1 ± 2.8 51.5± 2.6 54.2± 2.3

Compression set (%) 17.5± 1.1 22.4 ± 1.2 24.3± 1.0 27.4± 1.1

Crosslinking density (molml−1) 8.97± 10−5 1.32± 10−4 1.45± 10−4 1.61± 10−4

It is also worth while pointing out that the increase

in tensile strength by the addition of the organoclay

takes place without any loss in the elongation at break

of the material. However, a noticeable decrease in

this characteristic is observed, in the presence of the

silane coupling agent. This fact can be attributed to

the restriction in the chain slipping along the filler

surface due to the formation of chemical bonds in thepresence of the TESPT coupling agent. Similar results

have been reported by Ganter et al 29 when analysing

the properties of butadiene rubber (BR) containing

30 phr of organoclay and 3 phr of TESPT. These

authors concluded that the reactive coupling of the

elastomer matrix is also effective on the surface of

silicate layers containing quaternary ammonium salts.

Tensile measurements are in agreement with those

obtained from analysis of the hardness, resilience

and compression set of these materials. The increase

in hardness is related to a higher strength of the

composite. On the other hand, as can be suggested

from both the resilience and compression set results,

the elastic behaviour of the matrix hardly varies with

addition of the organoclay.

Therefore, it may be proposed that the silicate

nanolayers are well dispersed and exfoliated in the

elastomer matrix, so giving rise to the nanocompos-

ites. It has also been shown that compounding by

the solution method improves the filler/matrix com-

patibility and hence the dispersion of the filler in the

elastomeric matrix.

Dynamic mechanical properties

The dynamic mechanical properties of pristine NR and

its composites with the organosilicate were studied

over a wide temperature range (−100 to 50 ◦C).

The variation of tan δ as a function of temperature

for all of the studied materials is reported in Fig 4.

The tan δ peak, corresponding to the glass transition

temperature (T g) of the elastomer is reduced by adding

the organoclay, with this effect being more noticeable

in the case of composites prepared by the solution

mixing procedure. In fact, at low temperatures and

for a given energy input, fillers give a lower hysteresis.

This behaviour was explained by Wang30 in terms

of a reduction of the polymer volume fraction in the

presence of filler. That is, at low temperatures the

1770 Polym Int 53:1766– 1772 (2004)




0

0.5

1

1.5

2

2.5

–50 0 50

Unfilled NR




t a n δ

Temperature (°C)

Figure 4. Tan δ as a function of temperature for the various systems.

polymer by itself is responsible for a high proportion of

energy dissipation, while the small solid filler particles

in the polymer matrix hardly absorb any energy.

Furthermore, the glass transition temperature (tan

δ peak) shifts to higher temperatures upon addition

of the filler. So, T g goes from −49.6 ◦C for

pristine natural rubber to −46.8 ◦C and −45.9 ◦C

for rubber composites obtained through mechanical

compounding and the solution mixing procedure,

respectively. These results suggest that there existsa strong adhesion at the filler/polymer interface, in

particular, in the case of the composite synthesized

by the solution mixing method. This interaction

restricts the mobility of the elastomer segments, which

significantly elevates the glass transition temperature.

In general, when the interaction between the filler

and the rubber is strong enough, the glass transition

temperature is shifted to a higher temperature by

adding a filler to the rubber matrix. Simultaneously,

the tan δ peak also becomes narrower, and its height

becomes smaller. In this study, the results obtained

are in concordance with the general tendency reported

in the literature.

Figure 5 shows the storage modulus (elastic

modulus) of NR and its composites with 10 phr fillerloading as a function of temperature. As observed, the

organoclay gives rise to a noticeable increase in mod-

ulus, in particular, when the composite is prepared by

the solution mixing procedure. This behaviour is due

to the hydrodynamic reinforcement arising from the

1.2 × 109

2.5 × 109

3.7 × 109

5 × 109

–80 –70 –60 –50 –40 –30 –20 –10 0

Unfilled NR




Temperature (°C)

G ′ ( P a )

Figure 5. Storage modulus as a function of temperature for the various systems.

Polym Int 53:1766– 1772 (2004) 1771




incorporation of solid particles into the elastomeric

matrices. It is well known that hydrodynamic rein-

forcement occurs in conventional fillers, so giving rise

to an increase in the modulus of the polymer matrix,

or an increase in the viscosity for liquids.31

This hydrodynamic effect mainly depends on two

factors, ie the volume fraction and the shape factor

of the filler particles. The shape factor is describedas the ratio between the longest dimension of the

particle to the shortest. In this case, the exfoliated

silicate shows a high shape factor. On the other hand,

it is assumed that the higher the volume fraction

of the filler, then the higher is its reinforcing effect.

The portion of rubber chains trapped on the filler

as a result of mixing (rubber portion immobilized or

occluded) act as a part of the filler rather than of

the polymer and hence, the effective volume of the

filler increases. From bound rubber measurements,

it has been deduced that this effect is higher when

the nanocomposite is prepared by the solution mixing

method, which explains the higher modulus obtained.

According to these results, it can be deduced that

by means of the solution method, a more intensive

intercalation is obtained, since the solvent gives rise

to both an increase of the ‘intergallery’ spacing and

swelling of the elastomer chains.

CONCLUSIONS

The physical and mechanical properties of nanocom-

posites based on natural rubber and a layered silicate,

prepared by two different methods, ie mechanical and

solution mixing, have been investigated.Both methods give rise to an optimal dispersion of

the filler into the elastomer matrix, as deduced from X-

ray diffraction studies. Nevertheless, the compatibility

between the filler and the rubber is improved by the

solution mixing method. Bound rubber measurements

show that when the nanocomposite is synthesized by

the solution mixing procedure a higher proportion

of polymer is fixed to the filler. This explains why

the mechanical and dynamic mechanical properties of

the nanocomposites prepared by solution mixing are

improved in relation to the nanocomposite synthesized

via mechanical compounding.On the other hand, it has been demonstrated that

TESPT (Si69) behaves as an effective coupling agent,

by improving the adhesion at the silicate/elastomer

interface. Hence, the composites containing the silane

coupling agent show a noticeable increase in tensile

modulus, strength and hardness.

ACKNOWLEDGEMENTS

The authors acknowledge the Ministerio de Ciencia y

Tecnologıa (Spain) for provision of a Ramon y Cajal

contract, to Dr Lopez-Manchado, and CICyT (MAT

2001-1634) for financial support.

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Organoclay–natural rubber nanocomposites