Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.

1177/0095244315618698

Effect of polyethylene glycol intercalated organoclay on vulcanisation

characteristics and reinforcement of natural rubber nanocomposites

Upul N Ratnayake1, 2, Dileepa E Prematunga1, Chaminda Peiris3,Veranja Karunaratne1,

Gehan AJ Amaratunga1, 4

1
Sri Lanka Institute of Nanotechnology (Pvt) Ltd, Homagama, Sri Lanka.
2
Rubber Research Institute, Ratmalana, Sri Lanka.
3
Camoplast Solideal, Sri Lanka.
4
Departnent of Engineering, University of Cambridge, Cambridge, UK.

Corresponding author:

Upul N Ratnayake, Rubber Research Institute, Telewala Road, Ratmalana ,

Sri Lanka.

Email: un_ratnayake@yahoo.co.uk
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

Abstract

Organically modified montmorillonite (OMMT) clay was intercalated with low

molecular weight polyethylene glycol (PEG) oligomer at melt stage. The intercalation

behaviour of PEG into the OMMT clay galleries and its interaction with clay platelets

were characterized with X-ray diffraction (XRD) and differential scanning calorimetric

(DSC) techniques.

A natural rubber (NR)-organoclay nanocomposite (NROCN) was prepared by melt

compounding of NR with polyethylene glycol treated organoclay (P-OMMT) and other

compounding chemicals using a laboratory scale internal mixer. XRD analysis of the

nanocomposites revealed the intercalation of NR molecules into the P-OMMT clay

galleries and subsequent exfoliation during the melt compounding process.

Vulcanization characteristics of the NROCN, especially processing safety and optimum

curing time, has been interpreted with reference to the organic modifier of the

montmorillonite clay, PEG modification and the degree of exfoliation.

Solid-state mechanical properties of P-OMMT clay filled NROCN vulcanisates have

shown a significant enhancement in stiffness and strength characteristics whilst without

scarifying the elasticity of the nanocomposites. Results have been explained in terms of

the degree of exfoliation, dispersability of the organoclay and strain induced

crystallization of the natural rubber.

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

Keywords

Nanocomposites, natural rubber, organoclay, intercalation, exfoliation, polyethylene

glycol, strain induced crystallization

Introduction

Soft elastomeric materials turn into useful engineering materials when they are

reinforced with higher percentage of particulate fillers especially carbon black and

silica, for many industrial applications. However, higher loading levels of these

reinforcing fillers, particularly carbon black, impart a significant loss in elastic

behaviour of the natural rubber (NR) vulcanisates, an inherent property of NR, and also

cause difficulties in processing of such highly filled NR compounds (Arroyo et al.,

2003). In recent past, polymeric nanocomposites, a new class of composite materials,

based on layered silicates (clay) have inspired the scientific and industrial community as

an alternative for conventional polymeric composites because of their unique material

properties such as mechanical, thermal, electrical and barrier properties in comparison

to their conventional counterparts (Vaia and Wagner, 2004; Tjong, 2006; Paul and

Robeson, 2008). Montmorillonite (MMT) clay, which belongs to a general family of 2:1

layered silicates, has attracted as a potential layered silicate material for synthesizing

polymeric nanocomposite materials since MMT clay particles can be separated into

individual layers/platelets with 1 nm thickness while being amenable for surface

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

modification through ion exchange reactions (Alexandre and Dubois, 2000;

Giannelisetal, 1999). Generally, hydrophilic MMT clay needs to be converted into

organophilic to make it more compatible with non-polar elastomers such as NR.

Natural rubber is an important engineering material for numerous applications such as

tyres, automotive and engineering components, etc. In recent years, layered silicates,

especially organically modified MMT clay (organoclay), have been identified as a

potential reinforcing nanomaterials for NR, substituting the conventional reinforcing

filler, because of their ability to enhance the mechanical properties and to impart new

functionality to NR vulcanisates at very low loading levels (i.e. less than 10 phr.)

(Cataldo,2007; Tabsan et al, 2010; Rattanasom et al, 2009). NR/organoclay

nanocomposites have been synthesised with enhanced material properties using

different preparative techniques such as direct melt blending, solution blending and

latex compounding by different research groups (Sengupta et al., 2007; Qureshi and

Qammar,2010; Karger-Kocsis and Varghese,2003). However, irrespective of the

preparation techniques, exfoliation of the organoclay particles into individual clay

platelets of nanometre scale thickness and dispersing them uniformly within the NR

matrix are the key factors to enhance the physical properties of the NR compounds. The

degree of organoclay exfoliation/dispersion and the interaction between organoclay and

the NR determines the extent of property enhancement in rubber compounds (Sengupta

et al., 2007; Arroyo et al., 2007). One of the main obstacles to achieving the highest
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

degree of exfoliation of the clay within the NR matrix is the limited compatibility

between NR and organoclay materials.

Main objective of this research study was to evaluate the possibilities of improving the

degree of exfoliation and subsequent dispersion of clay in the NR matrix in view of

improving the reinforcement by the intercalation of organoclay with low molecular

weight polyethylene glycol (PEG). On the basis of intercalation of PEG into the clay

galleries, a new preparation method for NR/clay nanocomposites is explored. The effect

of PEG modified organoclay on processability in terms of curing characteristics and the

reinforcement of the NR nanocomposite materials is also described.

Experimental method

Materials

NR in the form of ribbed smoked sheet (RSS) was used as the elastomeric material for

preparing NR/organoclay nanocomposites. Montmorillonite clay (Cloisite20A),

modified with dimethyl dihydrogenated tallow quaternary ammonium chloride and with

a density of 1.77 g/cm3was used as the organoclay and was supplied by Southern Clay

Products, USA.

Polyethylene glycol oligomer with an average molecular weight of 4000 g/mol

(PEG4000) was used as a co-intercalant. Compounding ingredients such as sulphur,

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

accelerator, ZnO and antioxidants, etc. are of commercial grade and were supplied by

Camoplast Solideal, Sri Lanka.

Preparation of PEG intercalated OMMT (P-OMMT) clay

Organically modified montmorillonite (OMMT) clay powder was ground with the PEG

powder at different weight ratios. The resultant OMMT/PEG mixture was heated in an

oven at 85 oC for 60 minutes and further grounded after the heat treatment. Table 1

depicts the composition of each OMMT modified with PEG (P-OMMT clay).The

resultant P-OMMT clay was analysed with X-ray diffraction (XRD) and differential

scanning calorimetric (DSC) techniques to investigate the intercalation ability of PEG

molecules into the organoclay galleries and its interaction with the clay platelets.

Glass transition (Tg) temperature of PEG in P-OMMT clay relative to pure PEG

oligomer was measured with a differential scanning calorimeter (TA instrument, model

Q200) of temperature modulation mode (MDSC) with a temperature modulation

amplitude and modulation period of 1 oC and 100 seconds respectively at a heating rate

of 2 oC/min.

Preparation of natural rubber-organoclay nanocomposite (NROCN) materials

NROCN was prepared by melt compounding of NR and organoclay in the presence of

vulcanising chemicals in a laboratory scale internal mixer (Haake Rheomix 600)

operating at 60 0C (set temperature) with an initial rotor speed of 80 rpm for 8 minutes

followed another 4 minutes with a rotor speed of 60 rpm. The mixing conditions were
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

predetermined as the most appropriate mixing cycle. Table 2 presents the exact

formulation for each NROCN material.

Table1. Compositions of P-OMMT clay

P-OMMT OMMT PEG 4000

Clay Parts Parts

OMMT 1.00 0.00

P-OMMT -1 1.00 0.25

P-OMMT -2 1.00 0.50

P-OMMT -3 1.00 0.75

P-OMMT -4 1.00 1.00

Vulcanisation characteristics

The effect of OMMT and its modification with PEG on the vulcanisation characteristics

of the organoclay filled rubber compounds were studied with a Moving Die Rheometer

(MDR-Ektron EKT-2000S model) at 150 oC for 30 minutes.

Vulcanisation parameters such as scorch time (ts2), optimum cure time (t90), cure rate

index, etc. are derived from the rheographs obtained from the Moving Die Rheometer

(MDR).

The effect of P-OMMT clay on cure kinetics was further investigated with a differential

scanning calorimeter (DSC), using TA instrument model Q200, at a heating rate of 10

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

o
C/min. under nitrogen atmosphere. DSC thermograms were obtained across the

temperature range from 40-300 oC.

Table 2. Compound formulations for NR/organoclay nanocomposites (NROCN’s)

Compounding NR/organoclay nanocomposites (NROCN’s)

Ingredients, phr NR-gum NROCN4 NROCN4-P NROCN6 NROCN6-P

NR 100 100 100 100 100

OMMT (OC) - 4 - 6 -

P-OMMT - - 4 - 6

ZnO 5 5 5 5 5

Stearic acid 1 1 1 1 1

IPPD a 1 1 1 1 1

Sulphur 2.5 2.5 2.5 2.5 2.5

MBTS b 1 1 1 1 1

a
n-isopropyl-n-phenyl-n-phenylenediamine

b
Mecarptobenzothiazoloe disulphide

Characterisation of NROCN structure

NROCN sheets of 2 mm thickness were prepared by compression moulding at 150 oC

for the optimum cure time (t90) derived from the vulcanisation data. X-ray diffraction

(XRD) analysis of the NROCN vulcanisate materials was performed with a Bruker D8
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

diffractometer at a wave length of 1.54 Å of Cu Kα radiations to evaluate the

organoclay structure with in the NR nanocomposites and to determine the interlayer

spacing of the clay. X-ray diffractograms of compression moulded sheets of the NR

nanocomposite materials were achieved by scanning over a Bragg angle (2θ) range from

1-10 o at a rate of 0.01o/seconds. The conventional Bragg equation (nλ = 2d sin θ) was

used to calculate the interlayer spacing of the clay in the nanocomposite materials. λ is

the wave length of X-rays, d is the crystal lattice spacing, θ is the angle between

incident radiation and the scattering plane, and n is the order of reflection.

Clay dispersion and distribution within the NR nanocomposite vulcanizate materials

was observed under a Scanning Electron Microscope (SEM), model Hitachi SU 6000, at

an acceleration voltage of 10 kV.

Reinforcing effect of organoclay in NR nanocomposite vulcanisates

Solid state mechanical properties of organoclay filled NR nanocomposite vulcanisates

were analysed to evaluate the reinforcement effect of P-OMMT clay on NR compounds

with respect to the NR-gum compound containing no clay. Tensile properties and tear

strength of the NROCN vulcanisates were analysed using Instron, model 3365

Universal Tensometer according to ISO 37: 2005 (E) and ISO 34 -1: 2004 (E) standard

methods respectively. The extension of the samples was measured using a video

extensometer fixed to the Tensometer. NROCN vulcanisate samples was analysed for

shore A hardness using the Bareiss DigiTest hardness meter as ASTM D 2240 standard
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

method whilst rebound resilience was measured using the rebound resilience elasticity

tester as ISO method of 4662-1986 (E).

Results and discussion

Intercalation of PEG oligomer into clay

PEG treated OMMT (P-OMMT) clay containing different weight ratios of PEG (as

shown in Table 1) was characterised with the X-ray diffraction technique to determine

the optimum PEG to OMMT ratio required to achieve the maximum interlayer spacing

between the silicate layers of P-OMMT clay. Figure 1 illustrates the X-ray diffraction

spectra of OMMT and P-OMMT clays. The characteristic d001 diffraction peak of

OMMT is at 3.70 o with an interlayer spacing of 23.8 Å. As shown in Figure 1, when

OMMT was further surface modified with PEG oligomer, all 001 diffraction peaks of P-

OMMT clays (i.e. P-OMMT-1, P-OMMT-2, P-OMMT-3, and P-OMMT-4) were

shifted towards a lower Bragg angle. This implied that the melted PEG molecules had

diffused into the clay galleries during the heat treatment, resulting in a higher interlayer

spacing. The driving force for the diffusion of PEG molecules into the clay gallery is

likely to be the attractive forces between polar groups of PEG and polar sites of the

silicate layers.
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

Figure 1. X-ray diffraction spectra for OMMT and P-OMMT clays prepared with

different ratio of PEG

However, P-OMMT-1synthesised by melt mixing of 0.25 part of PEG with 1part of

OMMT showed two diffraction peaks overlapping together at 2.95o and 3.70o. The latter

position was similar to the diffraction peak of pure OMMT. This would suggest that P-

OMMT-1 clay is a mixture of OMMT intercalated with PEG and unmodified OMMT

clay. This shows that PEG concentration of 0.25 parts with respect to 1 part of OMMT

is not sufficient to intercalate all organoclay particles. On the other hand, all other P-

OMMT clay (i.e. P-OMMT-2, P-OMMT-3, and P-OMMT-4) containing more than 0.25

parts of PEG showed a single diffraction peak at a lower Bragg angle than the
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

diffraction peak of organoclay (OMMT), indicating that all organoclay was modified

with PEG. However, as shown in Figure 1, if PEG concentration is increased more than

0.5 parts, the interlayer spacing of P-OMMT clay is not further expanded significantly.

This is attributed to the fact that intercalated PEG has reached the saturation and as a

result additional PEG remains outside the clay gallery. Zhu et al. is showed that higher

ratio of PEG/OMMT would not further expand the interlayer spacing due the saturation

of PEG molecules within the clay gallery (Zhua et al., 2013). Based on these X-ray

diffraction results, P-OMMT-2 was chosen as the most suitable clay for the preparation

of NR/organoclay nanocomposites.

P-OMMT clay was further analysed with DSC in the temperature modulation mode to

ascertain the interaction between PEG molecules and OMMT clay platelets. Figure 2

illustrates the DSC traces of PEG and P-OMMT clay samples prepared with different

PEG weight ratios. PEG with an average molecular weight of 4000 mol/g shows the

glass transition temperature (Tg) at -15.90 oC whilst all P-OMMT clay samples show the

Tg values in the range of 14 – 13.3 oC, an increase of about 2 - 2.5 oC. This indicates

that that comparatively a higher energy is required for the onset of molecular motion of

PEG within the P-OMMT clay. Hydroxyl groups of the PEG oligomer are expected to

form hydrogen bonding with the polar sites of the silicate layers and hence the cohesive

energy of intermolecular chains is increased. As a result, PEG, which is confined within

the P-OMMT clay gallery, has a higher Tg compared to that of pure PEG. X-ray
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

diffraction and Tg data suggest that low molecular weight PEG molecules intercalate

into the clay galleries and interact with the clay platelets.

Figure 2. DSC thermograms of PEG and P-OMMT clay samples

Vulcanisation characteristics of NROCN compounds

NR/organoclay compounds prepared according to the formulations shown in Table 2

were analysed to study the effect of organic modification, especially the PEG

modification of clay, on vulcanisation characteristics. Vulcanisation curves of the

NROCN are presented in Figure 3 and vulcanisation parameters such as maximum

torque (MH), scorch time (ts2), curing time (t90), etc., derived from the cure curves are

shown in Table 3.
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

Figure 3. Vulcanisation curves of the NROCN nanocomposites

All NROCN compounds filled with either OMMT (NROCN4 and NROCN6) or P-

OMMT (NROCN4-P and NROCN6-P) showed a higher maximum torque (MH) and a

higher delta torque (ΔM, torque difference between MH and ML) than that of the NR-

gum compound. In general, ΔM of a NR compound is an indication of the crosslink

density of the compound and the reinforcement achieved with filler (Qureshi and

Qammar,2010; Chakraborty et al., 2010). Torque difference (ΔM) of the

nanocomposites was not remarkably altered with the increase of clay concentration

employed in this study. However, as illustrated in Table 3, NROCN compounds

containing P-OMMT clay have a higher ΔM than the nanocomposites containing

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

equivalent quantity of OMMT clay, demonstrating that NROCN compounds prepared

with P-OMMT clay achieved a higher crosslink density and, therefore, are likely to

have a better reinforcement.

Table 3. Vulcanization characteristics of NROCN containing different

concentrations of OMMT and P-OMMT

Maximum Cure

Nanocomposite torque, Δ Torque Scorch Curing rate

materials MH MH-ML time (ts2) time (t90) index

kgf-cm kgf-cm sec. sec. min-1

NR-gum 6.52 5.23 245 492 24.27

NROCN4 9.06 7.71 76 290 28.09

NROCN4-P 9.62 8.44 58 267 28.73

NROCN6 8.29 7.14 73 313 25.00

NROCN6-P 9.50 8.32 61 305 24.88

NR-gum compound cured with a conventional vulcanisation system as shown in Table 2

exhibited delayed onset of cure as shown from the scorch time (ts2) of 245 sec. along

with a longer optimum curing time (t90) of 492 sec. Addition of either OMMT or P-

OMMT into the NR showed a significant reduction of scorch time and the optimum

cure time; scorch time of NROCN4 is reduced from 245 sec. to 76 sec. whilst optimum
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

cure time is compressed from 492 sec. to 290 sec. As a result, although organoclay

accelerates the vulcanisation reaction as evidenced by higher cure rate index of the NR

nanocomposites (Table 3), processing safety of the NROCN compounds is significantly

reduced with the addition of organoclay. This could mean that organic modification

(dimethyl dihydrogenated tallow quaternary ammonium) of the organoclay acts as an

accelerator for the vulcanisation reaction by reducing the activation energy of the curing

process, resulting in early onset of curing. Lopez-Machado et al. (2004) also reported

that MMT clay containing organic intercalant, especially alkyl ammonium types,

accelerates the vulcanization reaction by forming a complex between the intercalant and

Zn ions. More importantly, when P-OMMT clay was used instead of OMMT clay, the

NROCN compounds (i.e. NROCN4-P and NRLOCN6-P) show further reduction in

scorch time and optimum cure time, suggesting that P-OMMT clay further accelerated

the vulcanisation process. This is achieved in both NROCN4-P and NROCN6-P due to

the fact that PEG modification of OMMT clay enhances the intercalation of rubber

molecules into the organoclay gallery and subsequent exfoliation/uniform dispersion of

clay during the compounding process. The enhanced exfoliation/ delaminating process

of P-OMMT within in the NR matrix would further allow the alkyl ammonium

intercalant ions to participate in the vulcanization process effectively.

Vulcanisation kinetics of NROCN compounds were also evaluated with DSC to further

study the effect of organic modification of montmorillonite clay on curing behaviour of

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

NR. Figure 4 shows vulcanisation exothermic peak of NR-gum compound starts at 176
o
C whereas the vulcanisation exothermic peaks of NROCN4 and NROCN4-P start at

142 oC and 136 oC respectively, showing that the NROCN4-P has the lowest on set of

cure. In contrast to the NR gum compound, both the DSC thermograms of NROCN4

and NROCN4-P show two vulcanisation exothermic peaks overlapping together where

the smaller peak is responsible for lower temperature onset of curing of the NR

nanocomposite compounds. The smaller exothermic peak could be due to the

vulcanisation process initiated by the organic modifier of the organoclay. These

vulcanisation kinetics data obtained from DSC thermograms are in agreement with the

scorch data obtained from rheometric analysis (Table 3) and hence further confirmed

the significant effect of organic modification of the clay on the vulcanisation reaction of

the NR compounds.
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

Figure 4. DSC vulcanization exothermic thermograms of NROCN’s

Characterization of NROCN structure

Figure 5 presents the X-ray diffraction spectra of NR nanocomposites prepared with

organoclay. The diffraction peaks of all the NROCN vulcanisate materials prepared

with either OMMT or P-OMMT shifted to lower 2θ compared to that of the diffraction

peak of pure organoclay. For example, when NR nanocomposite was prepared with 4

phr of OMMT clay (i.e. NROCN4), the characteristic diffraction peak of OMMT is

shifted from 3.7o to 2.16 o and, as a result, interlayer spacing of OMMT in the NROCN4

was increased up to 40.8 Å compared to the interlayer spacing of pure OMMT of 23.8

Å . However, NROCN4-P vulcanisate material contained the same clay concentration as

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

NROCN4 but with P-OMMT clay showed a further shifting of the diffraction peak

towards a lower Bragg angle of 2.08 o, resulting in a interlayer spacing of 42.4 Å. This

is clear evidence that when pure organoclay (OMMT) is modified with PEG oligomer,

diffusion of rubber molecules along with other low molecular weight substances used in

compounding process is further effective during the melt compounding process because

of the improved compatibility between the NR matrix and the P-OMMT clay.

Carretero-Gonzalez et al. (2008) has also reported that PEG behaves as a dispersing

agent for clay and facilitates the intercalation of rubber molecules into the clay galleries

resulting in improved clay dispersion in the NR matrix.

Figure 5. X-ray diffraction spectra of NROCN vulcanisate materials.

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

Higher interlayer spacing of clay in the NROCN4-P material facilitates the exfoliation

process of the clay within the NR matrix during the high shear melt compounding in the

internal mixer because of the reduced Vander Wall attractive forces resulted due to the

expanded gallery space. Hence, P-OMMT clay particles exfoliate to either single clay

platelets or to smaller clay stacks with a few clay platelets. A similar X-ray diffraction

pattern was shown in NRLOCN6-P vulcanisate material containing 6 phr of P-OMMT

clay in comparison to that of NROCN6 prepared with 6 phr of OMMT. Both diffraction

peaks (001) of NROCN4-P and NROCN6-P vulcanisate materials occur with a lower

intensity than the corresponding nanocomposite materials containing OMMT clay. This

might have been due to the comparatively higher degree of exfoliation achieved with P-

OMMT clay in NROCN vulcanisate materials. Additionally, another diffraction peak at

4.9o of 2θ appears in the NROCN4-P and the NROCN6-P materials. It is likely that un-

modified montmorillonite clay (diffraction peak at 2θ = 6.8 o) which are within the

OMMT clay is intercalated with PEG oligomer and as a result the corresponding peak

has shifted towards a lower Bragg angle.

Since X-ray diffraction data does not provide accurate information on clay dispersion

and distribution within the nanocomposite materials, SEM analysis was also performed

on both NROCN4 and NROCN4-P vulcanisates to evaluate the PEG effect on clay

dispersability and distribution (Figure 6-a and 6-b).

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Figure 6. SEM micrographs of natural rubber/organoclay nanocomposites; a.

NROCN4 and b. NROCN4-P

As shown in Figure 6, NROCN4 vulcanizate sample (6-a) contains larger clay particles

and less uniform distribution of OMMT clay within the nanocomposite matrix in

comparison to NROCN4-P (6-b). It clearly shows a significant enhancement of clay

dispersability of P-OMMT clay in the NROCN4-P vulcanisate, resulting in smaller clay

particles/stacks. As a result of improved dispersion and compatibility with NR, uniform

distribution of P-OMMT clay is achieved in the NROCN4-P vulcanizate as shown in

Figure 6-b.
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Mechanical properties of NROCN vulcanisate materials

The effect of organoclay (OMMT) and its further modification with PEG on

reinforcement of the NR nanocomposite vulcanisates were evaluated by determining the

tensile properties of the nanocomposite materials.

Figure 7. Stress-strain curves of the NR/organoclay nanocomposites (NROCN)

filled with different loading of OMMT or P-OMMT clay

Figure 7 presents the stress-strain curves of the nanocomposite vulcanisates containing

either OMMT or P-OMMT clay. Tensile properties relevant to stress-strain curves and

other mechanical properties such as hardness tear strength and rebound resilience are
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

shown in Table 4. As clearly illustrated in the stress-strain curves, the highest tensile

properties were shown in P-OMMT clay filled NROCN vulcanisates (NROCN4-P and

NROCN6-P) compared to that of NR-gum vulcanisate and OMMT clay filled NROCN

vulcanisates (NROCN4 and NROCN6). As shown in Table 4, tensile strength and

modulus at 300 % elongation (M300) of NROCN4 vulcanisate were increased by about

26 % and 85 %, respectively whereas the same parameters of NROCN4-P vulcanisate

were increased by about 50 % and 160 %, respectively, with respect to that of NR-gum

vulcanisate, demonstrating the higher reinforcing ability of the P-OMMT clay. This can

be explained by considering the formation ofNROCN4-P material where rubber

molecules are effectively intercalated into the P-OMMT clay galleries and there by clay

particles are exfoliated into clay platelets (which are 1 nm thickness) or smaller stacks

with few clay platelets as confirmed by X-ray diffraction and SEM data (Figures 5 and

6). As a result, the aspect ratio as well as surface area to volume ratio of the clay

platelets/stacks is considerably higher in the NROCN4-P nanocomposite material than

that of the NROCN4 nanocomposite. The increase in specific surface area of clay

platelets/stacks and the improved compatibility between the clay platelets and rubber

molecules through PEG molecules would lead to a stronger interaction between clay

and rubber molecules, resulting in higher tensile properties of NROCN4-P. Kim at al.

(2011) have also shown the effect of PEG on preparation of SBR/organoclay

compounds filled with binary (i.e. organoclay/silica) and tertiary (i.e.

 Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

organoclay/silica/carbon black) filler system and it was observed the enhancement of

modulus at 100 % and 300 % elongation in comparison to the binary and tertiary

particulates filled SBR/organoclay compounds prepared without PEG. These results

were attributed to the improvement in organoclay dispersion within the SBR matrix and

interaction between PEG and organoclay.

Similar to the NROCN4-P vulcanisate material, NROCN6-P vulcanisate containing 6

phr of P-OMMT has better tensile properties (i.e. tensile strength and modulus) than

that of NROCN6 vulcanisate.

Table 4. Mechanical properties of NROCN vulcanisate materials

Tensile Elongation 300% 500 % Tear

Nanocomposite Hardness Rebound
strength at break Modulus Modulus strength
material Shore A resilience
MPa % MPa MPa kN/m

NR-gum 33.0 ±0.6 15.39 ±0.82 769±36 1.23±0.13 2.90±0.42 26.92±1.8 80

NROCN4 43.3 ±1.5 19.39 ±0.96 630±17 2.28±0.21 7.79±1.27 34.70±2.3 79

NRLOCN4-P 45.5 ±0.5 22.93 ±0.74 551±16 3.20±0.16 15.86±1.6 36.67±1.6 82

NROCN6 41.2 ±1.2 16.19 ±1.05 654±18 2.09±0.13 5.92±0.82 28.92±1.4 76

NROCN6-P 43.8 ±1.9 20.72 ±2.62 596±75 2.89±0.06 12.0±2.04 28.02±1.4 79

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

However, when the clay loading is increased from 4 to 6 phr, the tensile properties

showed a slight reduction in the NROCN vulcanisates containing either OMMT or P-

OMMT clay. This could be because of the highest possible degree of exfoliation of clay

is achieved with 4 phr of clay. Any further increase of clay loading would not further

increase the degree of exfoliation and, therefore, clay particles are remained as stacks

with a lower aspect ratio. As a result, higher clay loading would enhance the

intercalated clay structure within the rubber matrix, resulting in slight reduction in

tensile properties. Qu et al.(2009) also observed the similar tensile strength

characteristics with the increase of organoclay loading levels in NR/clay

nanocomposites.

It is well known that natural rubber has a characteristic feature of strain induced

crystallisation. In general, NR molecules start to align towards the stress direction when

it is stretched about 300 % or above and subsequently show comparatively a higher

modules and strength characteristics upon stretching. As evident in stress-strain curves

(Figure 7) and Table 4, NROCN4 vulcanisate material has 7.79 MPa of 500 % modulus

(M500), an increase of about 170 % in comparison to the NR-gum vulcanisate. In

addition, M500 of NROCN4-P was further increased from 7.79 MPa to 15.86 MPa,

indicating a greater effect of P-OMMT clay on strain induced crystallization. This

would suggest that the clay morphology within the rubber nanocomposite materials

plays an important role in achieving strain induced crystallisation. Uniform dispersion

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

of P-OMMT clay at nanoscale would further promote the strain induced crystallisation,

resulting in higher strength characteristics and modulus upon stretching. Recently, Qu at

al. (2009) and Carretero-Gonzalez et al. (2008) have shown a remarkable enhancement

in strain induced crystallisation for organoclay filled NR nanocomposites. However, as

shown in Table 4, NROCN6-P showed a less strain induced crystallisation than

NROCN4-P as measured by M500. This can be explained that degree of exfoliation of

clay in NROCN6-P is comparatively less than that of NROCN4-P and, as a result,

higher percentage of larger clay stacks, which are not nanoscale thicknesses, remained

within the NROCN6-P material. The larger clay stacks, which are similar to

conventional filler, could not promote the strain induced crystallisation.

Tear strength, which indicates the resistance to crack initiation and propagation, of

NROCN vulcanisates showed a similar trend to those of tensile properties; P-OMMT

clay filled NROCN vulcanisates showed a better tear strength than OMMT clay filled

NROCN vulcanisates. Higher degree of exfoliation and enhanced compatibility between

clay and NR achieved in P-OMMT clay filled NROCN’s impart better resistance to

failure under an applied tensile stress. The stress generated at the NR - P-OMMT clay

interface is effectively transferred to the NR bulk and, as a result, resistance to crack

initiation is increased in NROCN materials prepared with the P-OMMT clay.

Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

Hardness values of the nanocomposite vulcanisates (Table 4), further confirmed that P-

OMMT clay has a better reinforcing capability than OMMT for NR compounds because

of the uniform dispersion of P-OMMT clay in the NR matrix.

Reduction of elasticity as measured with rebound resilience is common in highly filled

NR compounds, especially carbon black filled NR compounds, due to the part of the

deformational energy is dissipated as heat energy. As shown in Table 4, when NROCN

prepared with a lower loading of either P-OMMT or OMMT, the rebound resilience

becomes almost similar to that of the NR-gum vulcanisate, indicating that most of the

deformational energy is stored as elastic energy without dissipating as heat. As a result,

the nanocomposite vulcanisates, particularly the nanocomposites containing P-OMMT

clay, have a similar elastic nature as NR gum vulcanisate.

Conclusion

X-ray diffraction and DSC data revealed that PEG oligomer diffused into the OMMT

clay galleries and interacted with clay layers during the melt mixing process and as a

result, P-OMMT clay has a higher interlayer spacing than the pure organoclay

(OMMT).

Based on the PEG intercalation process, a new preparation method has been

demonstrated for NROCN materials by melt compounding of NR with P-OMMT clay,

with an enhanced degree of exfoliation and uniform dispersion of the clay. The
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

processability studies of the NROCN, evaluated in terms of vulcanization kinetics, have

shown that addition of organoclay accelerates the sulphur vulcanization reaction whilst

early initiation of the onset of curing, resulting in reduced processing safety. A marked

improvement in tensile properties of NROCN vulcanisate material relative to NR-gum

vulcanisate confirms the reinforcing effect of the exfoliated clay platelets/stacks

especially when NROCN is formed with P-OMMT clay. Higher strain induced

crystallization achieved in the P-OMMT clay filled NROCN further confirms a higher

degree of exfoliation and uniform dispersion of P-OMMT clay within the NR matrix.

The NROCN vulcanisate, especially the NR nanocomposite containing P-OMMT clay,

was significantly reinforced with a lower clay loading without compromising the

elasticity of the NR nanocomposite vulcanisate. As a result of the higher degree of

exfoliation and compatibility between clay and NR, the P-OMMT clay, which is

synthesized by the intercalation of PEG oligomer, is a better reinforcing material than

the conventional pure OMMT clay for NR compounds.

Acknowledgements

Authors wish to thank Dr. Nanada Fernando of Loadstar manufacturing division of

Camoplast Solideal, for the support rendered through valuable discussion. Authors are

also grateful to Research & Development unit of Loadstar manufacturing division of

CamoplastSolideal for supplying raw materials and for processing of some samples.
Journal of Elastomers and Plastics, 48(8), 711-727. doi:10.1177/0095244315618698

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