Accepted Manuscript

Investigation of dynamic characteristics of nano-size calcium carbonate added

in natural rubber vulcanizate

Qinghong Fang, Bo Song, Tiam-Ting Tee, Lee Tin Sin, David Hui, Soo-Tueen

Bee

PII: S1359-8368(14)00013-4

DOI: http://dx.doi.org/10.1016/j.compositesb.2014.01.010

Reference: JCOMB 2881

To appear in: Composites: Part B

Received Date: 29 October 2013

Revised Date: 17 December 2013

Accepted Date: 3 January 2014

Please cite this article as: Fang, Q., Song, B., Tee, T-T., Sin, L.T., Hui, D., Bee, S-T., Investigation of dynamic

characteristics of nano-size calcium carbonate added in natural rubber vulcanizate, Composites: Part B (2014), doi:

http://dx.doi.org/10.1016/j.compositesb.2014.01.010

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1

Investigation of dynamic characteristics of nano-size calcium carbonate added in

natural rubber vulcanizate

Qinghong Fanga,b*, Bo Songa ,Tiam-Ting Teec, Lee Tin Sinc,*, David Huid, Soo-Tueen Beec

a School of Materials Science and Engineering, Shenyang University of Chemical Technology,

Shenyang Economical and Technological Development Zone, Street No. 11, Shenyang

110142, China

b State Key Laboratory of Organic-Inorganic Composites, Shenyang University of Chemical

Technology, Shenyang Economical and Technological Development Zone, Street No. 11,

Shenyang 110142, China

c Department of Chemical Engineering, Faculty of Engineering and Science, University Tunku

Abdul Rahman, Jalan Genting Kelang, 53300 Setapak, Kuala Lumpur, Malaysia

d Department of Mechanical Engineering, University of New Orleans, New Orleans, LA

70148, USA

Abstract

The nano-calcium carbonates (NCC) with spherical and chain polymorphs and 30 nm, 50 nm,

and 80 nm sizes of cube shape particle have been used to prepare nano-calcium carbonate

(nano-CaCO3)/natural rubber (NR) nano-composite. The influence of NCC on the properties

of rubber vulcanizates such as Mullins effect, Payne-effect, loss factor and the dynamic

compressed heat generation on the structure of nano-composite were investigated. The results

2

showed that the Mullins effect of rubber composite filled chain shape NCC was high and it

was comparable to the large particle size (80 nm) of cubic NCC. For the analysis of Payne

effect, the value of G′Δ of rubber composite filled with spherical shape has the lowest value

due to weaker filler network resulted largest inter-aggregate distance occurred in the rubber

matrix. Meanwhile, the chain and large particle size cubic NCC have more significant ΔG’

with the increasing of strain. The value of damping factor corresponds to energy loss showed

that large particle size NCC has more pronounced values. Both chain and 80 nm cubic NCC

have highest rising of temperature compared to spherical NCC added rubber composites.

Keywords: A. Hybrid B. Mechanical properties; E. Cure: Nano-size calcium carbonate

*Corresponding authors. Tel: +86 2489 3881 53 and Tel: +60 3 4107 9802

Email: fqh80@126.com (Qinghong Fang) and leets@utar.edu.my (Lee Tin Sin)

1. Introduction

Calcium carbonate (CaCO3) has been widely used as filler in plastics and rubber

industry. It is produced from chalk, limestone, or marble found in upper layers of the earth’s

crust. CaCO3 source from natural ground is the most common and cheapest used in the

plastics and rubber industry. There is also exist of chemically produced form of CaCO3 known

as precipitated CaCO3 which is finer and high purity, yet also more costly than the natural

type. The most widely reason of blending CaCO3 with polymer is to reduce cost without

3

scarifying the tensile strength significantly. In addition, CaCO3 can act as processing aids,

toughener, improved productivity from a combination of high thermal conductivity and lower

specific heat in comparison to the polymer materials relatively. According to Khanna and

Xanthos [1] that all these benefits can further be optimized with the selection of appropriate

particle size distribution and surface treatments with hydrophobic agent such as stearic acid,

silane and etc.

While nano-size CaCO3 (NCC) has been produced for 25 years ago [2], the

applications of NCC have gained great attention of the researchers in recent decade because

of NCC particles can produce higher modulus as well as increasing the impact strength in the

acrylonitrile-butadiene-styrene (ABS) system as compared to micro-scale CaCO3 [3].

Manroshan and Baharin [4] observed that acrylic dispersed NCC added in vulcanized latex

showed modulus at 100% elongation and modulus at 300% elongation increased with NCC

loading. At the mean time, tensile strength and elongation at break increased up to 10 phr of

filler loading and then decreased again. Recently study conducted by He et al. [5] on the

compression properties of NCC/epoxy and its fiber composites revealed a remarkable

improvement of 13.5%, 6.1%, 42.5% and 106.3% in compressive strength, elastic modulus,

displacement and the total fracture work of epoxy resin cast filled with 4 wt.% NCC

contrasted to neat epoxy casts. It showed that the modified nano-CaCO3 particles had a

strengthening and toughening effect. Also, Kumar et al. [6] conducted morphological analysis

on nanocomposites fractured surfaces found that that the NCC stearic acid modification

induced homogeneous and fine dispersion of nanoparticles into polymer as well as strong

4

interfacial adhesion between the two phases. An increment in the Tg and storage modulus of

the resulting nanocomposites was observed with the increasing of CaCO3 ratio. Moreover,

thermogravimetric results showed a lower degradation temperature with the increase of

CaCO3 ratio in the polymer matrix.

In the rubber industry, NCC is also commonly used as the filler for

acrylonitrile-butadiene rubber [8], styrene-butadiene rubber (SBR) [9], chloroprene rubber [10]

and etc. Addition of NCC can produce outstanding stiffness, toughness, and dimensional

stability rubber compound. Nevertheless, the outstanding performance of rubber compound

by addition of NCC is still greatly depending on the dispersion of its nano-particles in rubber

matrix [7]. Hence, the depth understanding on the relationship between microstructure and

mechanical properties of NCC are essential to improve the end-use properties of rubber

composite. Most of the elastomeric components in practical applications are deformed

statically and dynamically where specific dynamic properties characterizations are crucially

required. Commonly, the durability of elastomeric compounds was analyzed in accordance to

the effect of strain amplitude on the dynamic modulus. The modulus of filled rubbers

decreases with increasing of applied dynamic strain up to intermediate amplitudes. After

adding the filler, the low strain modulus Go rises more than the high strain modulus G∞,

resulting in a non-linear viscoelastic behavior, which is known as Payne-effect Go-G∞ [11,12].

The Payne effect happens in rubber vulcanizates due to the diminishing of filler-filler

interactions or separation of polymer chains from filler surface when subjected to strain.

Ramier et al. [13] reported the Payne effect of the styrene-butadiene-rubber vulcanizates can

5

be reduced by silane treatment of the nano-size silica. On the other hand, the improvement of

mechanical properties, however, is always limited because NCC with high surface energy

tends to agglomerate. Such condition was observed by Qu et al. [14] who compared the

mechanical properties of bulk NCC and co-precipitated NCC in SBR vulcanizates. They

found that when the amount of co-precipitated NCC and bulk NCC is identical, the

mechanical properties of the former can achieve tensile strength of 13.38 MPa which was

superior over the later. This was due to the NCC in the former had better dispersion and

interface bonding force than that in the later, which led to the better mechanical properties.

Zhang et al. [9] showed that the surface modified NCC also exhibited better processing

capability than that of carbon black. Subsequently, they suggested that the processabilty of

carbon black filled rubber could be improved by the combination of NCC.

This study is aiming to analyze the mechanical properties of rubber nano-composites

filled with the NCC in the context of Mullins effect, Payne effect, the loss factor tan δ and

dynamic heat generation. In particular, the influence of specific surface area, polymorph,

structure and different of particles size of NCC on the Mullins effect and Payne-effect of

natural rubber (NR) composite were investigated. The strength of the filler network and the

filler-polymer interaction in the green compound and vulcanizate were studied using a wide

range of shear amplitudes performance to correlate with the fracture mechanism [16].

6

2 Experimental

2.1 Materials

Natural rubber (NR) grade SCR 20 was supplied by Xi Shuang Ban Na Tian Zheng

Trade Co., Ltd., China Nano-size calcium carbonate (NCC) with cube shape with particle size

30 nm, 50 nm, 80 nm，spherical shape, and chain shape were purchased from Henankeli New

Material Co., Ltd, China. Zinc oxide, stearic acid, sulfur, N-isopropyl-n'

-phenyl-p-phenylenediamine (IPPD 4010), N-oxyoliechylene benzothiazole-2-sulfenamid

(NOBS) were obtained from Rhein Chemie Rheinau GmbH, Germany. IPPD 4010 is used as

antioxidant and NOBS is used as curing accelerator. All were used as received.

2.2. Preparation of rubber nano-composites

The blends of rubber were prepared in accordance with the basic combination of

natural rubber (100 phr), zinc oxide (5 phr), stearic acid (2 phr), IPPD 4010 (2 phr), NOBS

(0.75 phr) and sulfur (2.5 phr). Meanwhile, the amount of NCC was varied accordingly. All

these ingredients were compounded using a two rolls mill machine with the cooling water

heat removal function. The prepared compounds were moulded into sheets using a hydraulic

press at 150°C and 10 MPa. All specimens were then cut into form of testing sheets.

7

2.3 Dynamic mechanical analysis

Dynamic performance and Payne effects of NCC rubber vulcanizates were analyzed

using rubber process analyzer (RPA 2000, Alpha Technologies Akron, Ohio, United States)

under temperature 60 oC, frequency 1 Hz, strain range: 1％-100％. Tensile and Mullins effect

of the nanocomposite were measured using an Instron tensometer with strain range: 1-300%.

In cyclic strain tests, the shear modulus can be simply expressed as a complex modulus,

GiGG ′′+′=* where G′ is the store energy modulus, G ′′ is the loss energy modulus and i

is the imaginary unit. The loss angle tangent is given by GG ′′′= /tan δ [17]. Meanwhile,

heat generation of the sample was tested by heat compression testing machine GT-RH-2000

(Gotech Testing Machines, Inc., Taiwan) at 1.0 MPa prestress, 55 oC and 5.71 mm stroke. The

morphologies of the NCC were inspected on a field emission scanning electron microscope

(Model S-4800, Hitachi).

3. Results and discussion

3.1 Mechanical and morphologies analyses

Figure 1 shows the SEM images of the spherical, chain, and cube shapes of NCC. It

can be noticed that the spherical shape of NCC has low aspect ratio compare to the others. As

shown in Figure 2 regardless type of NCC, when amounts of NCC added in rubber

vulcanizates increased, the tensile strength initially improved up to an optimum level at 60-80

phr and followed by exhibiting inferiority effects. The poor dispersion of NCC caused inferior

8

effect to the polymer matrix. In other words, the inverse relationship is due to the

non-homogeneous distribution of NCC agglomerates that causes stress concentration on the

NR matrix when subjected to extension [8]. The phase separation destroyed the continuity of

rubber matrix while further reducing the interfacial interaction of rubber matrix with NCC

which weakened the mechanical performance of the composite [19]. On the other hand, it was

also found that the tensile strength of the spherical NCC added NR vulcanizates exhibited the

highest tensile strength. This can be explained that the spherical shape of NCC has larger

surface area which can interact well with the polymer matrix. Although the stearic acid is

added to react with zinc oxide to promote the crosslinking of natural rubber, the addition of

excess stearic acid can also act as the surface modifier to improve the hydrophobic especially

the large surface area of spherical nano-size particles of NCC [20-22]. In addition, the mimic

homogenous dispersion of spherical NCC in rubber vulcanizates matrix as observed in Figure

3(A) also attributes to the superior tensile strength of spherical NCC added NR vulcanizates.

By referring to Figure 3(A), the surface morphologies of spherical NCC added NR

vulcanizates was observed to be smooth without the occurrence of NCC particles

agglomeration and voids. This also indicates the excellent interfacial adhesion between

spherical NCC particles and NR matrix which could effectively transfer the stress from rubber

matrix to spherical NCC particles during stretching, thus providing the reinforcing effect to

NR matrix. Unlike the chain type NCC which initially believed to act like a fiber which can

provide reinforcement effect to the rubber vulcanizates. The results showed that chain type

NCC has worse effect and possessed similar outcomes with the 80 nm cube type NCC when

added at high amount 100 phr. This is due to the chains and cube NCC particles tend to

9

agglomerate together into larger aggregates particles in rubber vulcanizates matrix as

observed in Figure 3(B) and 3(C). The agglomeration of NCC could reduce the effective

interfacial adhesion between NCC particles and rubber vulcanizates matrix and cause the

occurrence of voids between the NCC aggregates and rubber matrix. This could further cause

the NCC aggregates to act as stress concentrator point when subjected to stretching and thus

lowering the tensile strength of chain and cube NCC added NR vulcanizates. This indicates

that NCC tends to work as filler in rubber vulcanizates even though it has high aspect ratio.

This can be also evidenced by analyzing different particles sizes of cube shape NCC. The 80

nm cube NCC possessed generally lowest tensile strength among the three particles of cube

shape NCC. The small size cube NCC not only promotes dispersion in the rubber vulcanizates

matrix, while the nano-sized can embedded well into the entangled chains of polymer matrix.

This would produce superior external forces transfer over the entire rubber composite results

outstanding mechanical performance.

Meanwhile, analysis on the elongation shows that low loading level of NCC (< 60 phr)

has better elongation at break compared high loading level. This is because low quantities of

NCC would not cause disruption to the entangled polymer chains to slide freely when

subjected to extension. Besides, the phenomenon can also due to the inter-aggregate distance

becomes smaller with increasing surface area of NCC and thus the probability of forming a

network raises. Meanwhile, it can also be observed that the both spherical and chain shape

NCC possessed high elongation at break when added 60 phr below. This may be due to the

low cross-sectional area of the spherical and chain shape NCC would induce a lubricating like

10

effect which promoted the flowability of the polymer chains [23]. Moreover, this can be

further justified whereby the large 80 nm cube shape NCC has lowest elongation at break.

Such larger particle size of NCC would cause phase discontinuity subsequently ruin the

elongation of rubber vulcanizates. Importantly, this effect becomes pronounced due to the

hydrophilic surface of NCC which reduces the interfacial interaction with rubber vulcanizate

matrix. Nevertheless, such incompatible condition has been minimized with the blending of

strearic acid transform the NCC surface with hydrophobic characteristic. Also, it is expected

that sharp edge of cube NCC would exhibited low elongation at break compared to other

shapes because shape edge tends to cause more inter-molecular abrasion of rubber chains

when extension occurs. This will cause unfavourable stress concentration within the polymer

matrix which leads to earlier failure of sample when subjected to external forces.

3.2 Mullins effect

When a rubber vulcanizate specimen is subjected to static cyclic loading, it will

demonstrate non-linear elastic behaviour as well as damage-induced stress-softening

phenomena which is known as Mullins effect. The Mullins effect is the irreversible softening

of the stress-strain curve that occurs whenever the load of the rubber vulcanizate increases

beyond its all-time maximum value. As such, Figure 4 shows the Mullins effect rubber

vulcanizates composites with 60 phr NCC. It was found that the spherical shape NCC

possessed the smallest Mullins effect of rubber vulcanizate where the different between 1st

time maximum loading (4 MPa) and 2nd time maximum loading (3 MPa) is 1 MPa. This is

11

mainly attributed to that the spherical NCC can easily slip between macromolecular chains in

NR matrix. Therefore, it can decrease the stress softness. Besides, the good dispersion and

low agglomeration of nanoparticle also contributed to the low Mullins effect of spherical

NCC. Low agglomeration of filler is favourable in order to avoid disruption of the polymer

matrix continuity while enabling better interfacial interaction of filler and polymer.

Meanwhile, the rubber vulcanizates filled with 80 nm cubic NCC experienced the most

pronounce Mullins effect with the different reaches 2 MPa. Indeed, such pronounce effect was

also found with the chain shape NCC which has the Mullins effect almost comparable to the

large particle size (80 nm) of cubic NCC. This can be explained where large particle size

NCC can hinder the mobility of macromolecular chains when pulling. This will cause higher

stress concentration at the localized spot to cause chains breakage to occur easily [24]. Further

investigation by comparing the 2nd and 3rd loading found that only cubic NCC with 80 nm

particle size has most pronounce different (0.5 MPa) among the specimens. This is due to the

large particle NCC lack of mobility requires subsequent re-orientation within the entangled

macromolecule chains. Such substantial re-orientation would still lead to formation of internal

stress whereby the breakage of chains expected to occur in minor causing loss of mechanical

strength. For specimens containing spherical and chains NCC still undergoing insignificant

changes of stress after 2nd loading. This is mainly due to viscoelastic behaviour of

disentanglement of macromolecules as well as minor debonding of polymer chains which

caused insignificant loss of mechanical performance.

12

3.3 Payne effect

As seen in Figure 5 shear modulus ( G′ ) of the NR vulcanizate composites

corresponding to different types of NCC gradually increased with the increasing of strain

[12,25]. This phenomenon is caused by the destruction-reformation of filler–filler networks

and adsorption-deposrtion of polymeric chains at the filler interface of the rubber vulcanizates

[26]. The fact is that upon loading of NCC, the inter-aggregate distances become smaller with

rising of filler content, therefore the probability for the formation of a filler network increases.

Previous researchers [26] suggested that such network structure is due to the breakdown of

carbon black network structure from van der Walls-London attractive forces between carbon

black particles. Meanwhile, there is also researcher believe that the Payne effect is due to the

debonding process that take place at the interface between the bulk and bound rubber [27].

Under large deformation, the rubber layers that cover the filler will develop micro-voids

which lead to lowering the modulus of the rubber vulcanizates.

It can be found that the rubber vulcanizates filled NCC exhibited different

Payne-effect corresponds to the polymorph and particles size of NCC. The value of G′Δ of

rubber composite filled with spherical shape has the lowest value due to weaker filler network

resulted largest inter-aggregate distance occurred in the rubber matrix. In addition, the

spherical shape NCC with high uniformity structure also possesses capability to deform

smoothly during straining. Although both the aggregating size and the inter-aggregate

distance decreased with reduced primary particle diameter, the compatibility between

13

nanoparticle and rubber are improved significantly. Nanoparticle is covered by NR matrix

firstly followed by the formation of aggregate together. Thus, the different of the modulus are

insignificant even at high strain. Further analysis found that the chain NCC has higher G′Δ

with the increasing of strain. This may be due to the initial high aspect ratio structure of chain

NCC has experienced breakage under excessive deformation. Hence, this has caused

indirectly irreversible loss of rigidity of the rubber vulcanizate. Besides that, by comparing

the different particle size of cube shape NCC, the 30 nm cubic NCC exhibits the lowest Payne

effect which corresponds to the lower filler network and highest surface area. It is also

interesting to find out that the large particle size of cubic NCC experienced more pronounced

drop of G′Δ . Indeed, the 30 nm cubic NCC’s storage modulus G’ when the strain exceeded

20 % was the highest among the three particles size of cubic NCC. Such phenomenon can be

described whereby the substantial strain can cause large inter-chain separation due to the

motion of large particle size in between the entangled polymer chains. Thus, the regularity of

the macromolecule vulcanizate has been affected leading to inferiority of rigidity. Moreover,

the similar tendency is displayed by the graph tan δ versus strain (Figure 5). The smallest the

inter-aggregate distance corresponds to the higher probability for the formation of a filler

network. Consequently, the extent of filler networking is more obvious when the surface area

rises. It should be noted that the high surface activity of the nanostructure results in high

interaction with the rubber phase able to prevent NCC network formation and reducing the

low strain modulus.

14

The plots of tan δ or damping factor versus strain of both polymorph (spherical shape

and chain) and particle of different size (30 nm, 50 nm and 80 nm) composites are illustrated

in Figure 6. The damping peak of the composites with NCC continuously increases with

increasing strain. The tan δ of cube (30nm) and spherical shape are lower than the others.

With the increasing of strain, the filler network develops and stabilizes a hysteresis behaviour

of composite at small and intermediate strains. At high strain, when part of the filler network

is broken down, the composite is no longer stabilized and the tan δ rises drastically. The filled

sample exhibits the highest tan δ level which is due to the large particle size. In fact, the

hysteresis results from the breakdown of the filler network and straining disruption would

dissipate energy. All these occurred are because of the mobility of rubber segments has been

restricted by the diameter increases in NCC composite. For the different of NCC polymorph,

the tan δ of NCC composite filled with nanoparticles are lowered only when both the filler

structure in the rubber and the surface activity of the filler are high. The structure of chain

shape is complex and has large surface area, so it results in high interaction with the rubber

phase and hindering the flow of the rubber segments. Finally, as can be seen from the tested

results in Figure 7, there is indication of relationship between temperature rising and tan δ. It

also can be seen that their rise trend are consistent. Energy loss transformed as heat generation,

which leads to temperature rising of the sample during dynamic compression. In other words,

a more stabilize NCC filled system would exhibited minimum energy loss with good

durability of its performance

15

4. Conclusions

This work focused on investigating the influence of NCC morphological parameters

(polymorph and different particle sizes) on mechanical and dynamic properties of NR

vulcanizates. It concluded that the addition of NCC is not limited to affect the mechanical

strength; NCC has also exhibited significant influence on the dynamic properties of the rubber

composites corresponding to the polymorph and its particle sizes. Hence, based on the results

of this study, the following information can be concluded:

1. The tensile strength and elongation of rubber composites were dependable on the

polymorph and particle sizes of NCC. Spherical NCC has outstanding mechanical strength

resulted from the good dispersion in the NR composites as found in SEM observation.

This finding is identical to 30 nm cubic NCC which also has better mechanical properties

when compared to 50 nm and 80 nm cubic NCC. The large particle size and high aspect

ratio of chain shape NCC is expected to cause stress concentration spot in the rubber

matrix which lead to inferior of mechanical performance.

2. The Mullins effect of rubber composite filled chain shape NCC was large and it is

comparable to the large particle size (80 nm) of cubic NCC. This is mainly due to the

large particle size NCC would hinder the mobility of macromolecular when pulling,

subsequently leading to stress concentration and chains breakage.

3. The value of G′Δ of rubber composite filled with spherical shape has the lowest value

due to weaker filler network resulted largest inter-aggregate distance occurred in the

16

rubber matrix. Meanwhile, the chain and large particle size cubic NCC have more

significant G′Δ with the increasing of strain. Such large structure of NCC tends to

experienced breakage easily when straining.

4. The value of damping factor corresponds to energy loss showed that large particle size

NCC has more pronounced values. Energy loss from the straining produced heat caused

increasing of temperature. Subsequently, both chain and 80 nm cubic NCC have highest

rising of temperature compared to spherical NCC added rubber composites.

Acknowledgments

This research is under the project of state key laboratory of organic-inorganic

composite (201304). The authors would like to thank for the financial support from National

Natural Science Foundation of China (51173110, 51103086), the Liaoning Province Natural

Science Foundation (201102173).

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Figure 1 SEM images of NCC with (A) spherical shape, (B) chain shape and (C)

cube shape

Figure 1

Figure 1 Tensile strength and elongation at break of NR vulcanizates added with

different polymorph and particle sizes of NCC

0

5

10

15

20

25

30

20 40 60 80 100

Amount of CaCO3 (phr)

Te

ns

ile

str

en

gth

(MP

a)

Spherical shape Chain Cube 30 nm Cube 50 nm Cube 80 nm

400

500

600

700

800

900

20 40 60 80 100

Amount of CaCO3 (phr)

Elo

ng

ati

on

at

bre

ak

(%)

Spherical shape Chain Cube 30 nm Cube 50 nm Cube 80 nm

Figure 2

Figure 3 SEM images of rubber vulcanizate nanocomposite with NCC (A) spherical

shape, (B) chain shape and (C) cube shape

Figure 3

Figure 4 Mullins effects of rubber vulcanizate composites with 60 phr NCC (A)

Spherical , (B) Chain (C) Cube 30 nm (D) Cube 50 nm (E) Cube 80 nm

(C) (D)

(E)

(A) (B)

Figure 4

Figure 5 Payne effect of rubber vulcanizate with 60 phr NCC

Figure 5

Figure 6 Damping factor (tan δ) of rubber vulcanizate with 60 phr NCC

Figure 6

Figure 7 Effect of heat generation of rubber vulcanizate with 60 phr NCC

0

5

10

15

20

25

Spherical

shape

Chain Cube 30 nm Cube 50 nm Cube 80 nm

Nano-sized CaCO3

Tem

pera

ture

(oC

)

Figure 7