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1 Introduction: The importance of natural rubber (NR)/cellulose composites

1 Introduction: The importance of natural rubber (NR)/cellulose composites

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NR composites using cellulosic fiber reinforcements


resistance (Jacob et al., 2008; Coran et al., 1974; Ibarra and Chamorro, 1991;

Varghese et al., 1994).

Nowadays cellulose is also produced at a nanoscale in different ways.

Such nanofibrillated cellulose is typically ultra-strong and has, in addition,

several other interesting properties like super-hydrophilicity and interesting

rheological properties. The uses are expected to be in new lightweight

composite materials to be used in transport and electronic applications, but

nanocelluloses can also be used in food, cosmetics, medical, packaging and

many other applications (Seppälä, 2012). According to Oksman, the main

challenge when producing cellulose-based nanocomposites, is to disperse

the nanoreinforcements in the polymer matrix without degradation of the

polymer or the reinforcing phase and also to develop composites based on

nanocellulose using suitable large-scale processing technologies (Oksman,

2012). Klemm and collaborators pointed out that the process for obtaining

nanocellulose is laborious because it involves several steps, namely purification,

bleaching, fibrillation and hydrolysis (Klemm et al., 2006). Another difficulty

is associated with the lack of compatibility of hydrophobic polymers using

nanocellulose in composites, and various chemical modification methods have

been explored in order to address this hurdle (Siro and Plackett, 2010).

To overcome those problems, our research group developed different

procedures to disperse cellulose in the nano-scale and introduce it in

elastomeric composites (Peres et al., 2001; Mano et al., 1975). Although

these patented processes allow nanocellulose in all marketed elastomeric

lattices to be obtained (Zine et al., 2011; Lapa et al., 2007, 2008; Affonso

and Nunes, 1995; Nunes and Affonso, 1999), the present chapter will focus

exclusively on the results obtained for nanocellulose in NR, or nanocellulose

in admixture of NR with other elastomers. The results were promising and the

nanocomposites exhibit clear color, have high mechanical performance and

characteristics of impermeability to gases. The preparation process is simple,

effective and widely applicable to industrial rubber. This chapter provides

an overview of reinforcements based on cellulosic fibers and nanofibers and

their effects on NR composites.


NR/cellulose composites

Short fiber reinforcement of rubber is a subject of interest to a large number

of researchers because of the importance both of end-use applications and

research and development areas (Das et al., 2005; Geethamma et al., 2005;

Goettler and Shen, 1983; Ismail et al., 1997, 2002; Jacob et al., 2008; Murty

and De, 1982; Nassar et al., 1996; Nunes and Visconte, 2000; Varghese et al.,

1994). These composites exhibit the combined behavior of the soft, elastic

rubber matrix and the stiff, strong fibrous reinforcement. The use of short

fibers makes it almost impossible to obtain the high level of reinforcement


Chemistry, Manufacture and Applications of Natural Rubber

that can be attained with long fibers. Still, short-fiber composites are preferred

in products such as V-belts and hoses because of their easy processability

and high green strength, and the possibility of producing complex-shaped

articles (Goettler et al., 1981). When fibers are aligned parallel to the stress

direction, tensile strength develops a characteristic drop with increasing fiber

volume content until a critical fiber level is reached (Jacob et al., 2008). In

addition, natural fibers have advantages due to their renewable nature, low

cost, availability, and ease of chemical and mechanical modification. Many

researchers have reported the processing advantages and improvements in

the mechanical properties of short fiber reinforced rubber composites (Coran

et al., 1974; Ibarra and Chamorro, 1991; Varghese et al., 1994).

The quality of a composite can be increased through the preservation of

high aspect ratio of the fiber, control of the fiber alignment, generation of a

strong interface through physical-mechanical bonding and establishment of

a high degree of dispersion. An aspect ratio of 100–200 is generally required

for effective reinforcement in short elastomer composites (Ibarra et al., 1988;

Varghese et al., 1994). Although the average tensile strength of wood pulp

fibers of about 300 MPa is only a quarter that of glass fiber or 60% that of

nylon fiber, it is still effective in rubber composite because in short fiber

composites, failure commonly occurs in the matrix around fibers lying at an

angle to the applied stress and the high strength (inorganic) reinforcements

tend to be brittle and break during processing (Britt, 1964).

The most widely used natural fibers as filler for NR composites include

jute, bagasse, bamboo, coir fiber, oil palm fibre and sisal.

Jute fiber is a lignocellulosic fiber considered as a hard cellulosic fiber

because of its high tensile modulus and low elongation at break. The effect

of carbon black on the processing characteristics and physical properties of

jute fiber-reinforced composites and the role of silica and carbon black in

promoting adhesion between jute fiber and NR have been studied by Murty

and De (1982).

Bagasse can be regarded as a renewable material as it is the byproduct of

sugarcane stalks after crushing in order to extract juice. Bagasse in natural

and synthetic rubber composites showed both improved tensile strength and

barrier properties and increased rate of rubber degradation in soil (Abdelwahab

et al., 2008; Nassar et al., 1996; Bras et al., 2010).

Bamboo fiber has been selected as reinforcement because bamboo is an

abundant natural resource in tropical countries like Brazil, and its overall

mechanical properties are comparable to those of wood (Lakkad and Patel,

1981). The curing characteristics and mechanical properties of bamboo fiberreinforced NR composites were examined as a function of fiber loading and

bonding agent. Tensile modulus and hardness of composites increase with

increasing filler loading and the presence of bonding agents (Ismail et al.,


NR composites using cellulosic fiber reinforcements


Coir is a versatile lignocellulosic fiber obtained from coconut trees

(Cocos nucifera), which grows extensively in tropical countries. It is an

inexpensive fiber among the various natural fibers available worldwide; it

is not as brittle as glass fiber, being amenable to chemical modification, is

non-toxic and poses no waste disposal problems. Also coir fiber has certain

advantages over other natural fibers. It possesses high weather resistance

due to a higher amount of lignin and consequently absorbs water to a lesser

extent compared to all the other natural fibers. Also the fiber can be stretched

beyond its elastic limit without rupture due to the helical arrangement of

micro-fibrils at 45°. Hence research has been undertaken to identify new

fields of applications for coir such as reinforcement of polymers (Geethamma

et al., 2005; Rout et al., 2001; Arumugam et al., 1989). The efficiency of

coir as reinforcement in rubber composites can be improved by enhancing

the interfacial adhesion between coir and rubber. This can be achieved either

by modifying the surface topology of coir by a suitable pretreatment or by

selecting the proper components of the bonding system (Geethamma et al.,


Sisal fiber is a lignocellulosic fiber and one of the strongest fibers, which

can be used for several applications because of its excellent aging resistance,

unlike NR. The effects of acetylation and bonding agent on thermal aging,

gamma-radiation and ozone resistance of short sisal fiber reinforced NR

natural rubber composites have been evaluated. The ozone resistance of

the samples is better at higher fiber loading, especially in the presence of

bonding agent. In all cases, the performance of composites incorporating

acetylated fiber was better than that of composites incorporating untreated

fiber (Varghese et al., 1994; Iannace et al., 2001; Haseena and Unnikrishnan,

2005; Jacob et al., 2008). Sisal and oil palm fibers appear to be promising

materials because of the high tensile strength of sisal fiber and toughness

of oil palm fiber. Therefore any composite comprising these two fibers will

exhibit the above desirable properties of the individual constituents.

Fibers with chemical and physical surface treatments can also be obtained in

order to improve the mechanical properties of final products (Jacob et al., 2007,

2008). Some examples of fiber treatments are dipping, surface roughening,

and chemical modification. The rubber can also be treated, for example,

by mixing ingredients in the rubber that enhance adhesion or by grafting

functionalities to the rubber polymer (Nunes and Visconte, 2000; Wennekes

and Datta, 2009). The most commonly used commercial dip is resorcinol

formaldehyde latex. The adhesion between many types of commercial fibers

and most elastomers has been overcome by the discovery, for instance, of the

tricomponent systems consisting of hexamethylenetetramine, resorcinol and

high surface area hydrated silica (Creasey and Wager, 1968; Creasey et al.,

1968; Solomon, 1985). The mechanical properties of the composites, such as

modulus, tensile strength and ultimate elongation, depend on fiber orientation,


Chemistry, Manufacture and Applications of Natural Rubber

fiber aspect ratio and adhesion between fiber and matrix (O’Connor, 1977;

Mathew and Joseph, 2007).

Concerning natural fiber reinforced polymer composites, Luyt recently

concluded that in many of the investigated systems, the treatment/modification/

compatibilization is not effective enough in the creation of an interphase with

the right lamellar architecture. In these cases, the fiber does not reinforce

the polymer, acting as filler only (Luyt, 2009).


NR/natural cellulose nanocomposites

Nanocomposites in general are two-phase materials in which one of the phases

has at least one dimension in the nanometer range (1–100 nm). Siro and

Plackett summarize progress in nanocellulose preparation with a particular

focus on microfibrillated cellulose and also discuss recent developments in

bio-nanocomposite fabrication based on nanocellulose (Siro and Plackett,

2010). In a recent publication, Klemm et al. (2011) gave a review detailing

the way to incorporate nanocellulose into composites.

Microfibrils are important fiber wall components, i.e. biological nanostructures. However, due to the classical suffix ‘micro’, microfibrils may

be wrongly associated with micrometer-sized fibrils, which may be 1,000

times larger (>1 mm). According to evidence given in the literature, it

appears that microfibrillated cellulose (MFC) materials may be composed

of nanofibrils, fibrillar fines, fiber fragments, and fibers. This implies that

MFC is not necessarily synonymous with microfibrils, nanofibrils or any

other cellulose nano-structure. However, properly produced MFC materials

contain nano-structures as a main component, i.e. nanofibrils (Chinga-Carrasco,


Cellulose nanofibers and crystals have gained a large interest, not only

in the academic research society but also in industries, during the last few

years. The number of published papers on this topic has increased from some

few publications per year in 2005 to more than 500 in 2011. The research

topics have been extraction methods of nanocelluloses, their properties,

chemical modifications, self-assembling and their use in composites (Oksman,


There are two completely different ways to produce nanocelluloses:

the bio-formation of cellulose by bacteria (Klemm et al., 2005) and the

disintegration of plant celluloses using shear forces in refiner techniques

(Nakagaito and Yano, 2004).

The main advantages resulting from the development of nanoreinforced

composite materials are the attractive properties imparted by the nanometric

size of the reinforcement. There are two reasons for changes in material

properties when the size of the reinforcing phase is reduced down to the

nanometer range. Firstly, the large surface area associated with nanoparticles

NR composites using cellulosic fiber reinforcements


results in numerous interfaces among the constituent intermixed phases that

play an important role in the macroscopic properties. In addition, the mean

distance between particles is much lower as their size is reduced, favoring

particle/particle interactions. Secondly, possible quantum effects may occur,

namely changes in magnetic, optical or electrical properties.

There are different techniques for obtaining nanocellulose reinforcement

in NR composites. Cellulose whisker (CW) was prepared by hydrolysis of

natural microcrystalline cellulose with sulfate acid. The NR/CW composite

was prepared by natural rubber NR latex coagulated with CW suspension. The

results show that CW has a fair reinforcement effect on NR. The modulus,

elongation at break and tear strength of NR composites are further improved

by using CW modified by resorcinol and hexamethylenetetraamine (Gu et al.,


CW can also be isolated from bleached sugar cane bagasse kraft pulp

and is used as a reinforcing element in a NR matrix. The effect of CW

loading on tensile properties, thermal properties, moisture sorption, water

vapor permeation and soil biodegradation was studied. The presence of CW

increased the rate of degradation of rubber in soil, and resulted in increase

in moisture sorption of rubber films up to 5% loading and barrier properties

to water vapor (Bras et al., 2010).

In another paper, cellulose was extracted from the rachis of date palm

tree, characterized and used as reinforcing phase to prepare nanocomposite

films using latex of NR as a matrix. These films were obtained by the

casting/evaporation method. The reinforcing effect was shown to be higher

for nanocomposites with MFC compared to whiskers. This was ascribed

to the higher aspect ratio and possibility of entanglements of the former.

The presence of residual lignin, extractive substances and fatty acids at the

surface of MFC was also suggested to promote higher adhesion levels with

the polymeric matrix (Bendahou et al., 2010).

Some researchers have also prepared CW by acid hydrolysis of bamboo

pulp fibers, having a diameter of 4–14 nm, which were used as the reinforcing

phase in vulcanized NR. Theoretical modeling of the mechanical properties

showed lower performance than predicted and therefore further process

optimization and/or compatibilization are required to reach the maximum

potential of these nanocomposites (Visakh et al., 2012).

The effects of partial replacement of silica with surface modified MCF

on properties of NR nanocomposites were investigated in the paper by Xu

et al. (2012). When MFC was modified, NR/MFC/silica compounds showed

accelerated curing rate and better processing performance. Tear strength,

300% modulus, hardness, heat build-up, compression set and dynamic

mechanical performance were greatly improved and NR/silica compatibility

was reinforced. In addition, a number of advantages such as renewability,

biodegradability, availability, low density, low cost and environmentally


Chemistry, Manufacture and Applications of Natural Rubber

friendly nature make MFC a potential new energy-saving filler to partly

replace silica.


NR/regenerated cellulose nanocomposites

Naturally occurring cellulose exists in parallel strands without intersheet

hydrogen bonding. Regenerated cellulose is thermodynamically more stable

and exists in antiparallel strains with intersheet hydrogen bonding. The

difference in properties between natural and regenerated cellulose arises

due to changes in crystal structure. Cellulose III is amorphous and obtained

by treatment of natural or regenerated cellulose with amines, and cellulose

IV is obtained after treatment of cellulose III with glycerol at very high

temperatures (Sasaki et al., 2003; Klemm et al., 1998).

Regenerated cellulose fibers are a class of materials manufactured from

wood pulp or other natural sources of cellulose. During production, the

constituent cellulose polymer is dispersed into solution at the molecular

level, either by temporary derivatization, by complex formation or by direct

dissolution. The viscous polymer solution is then extruded through spinnerets

and regenerated or precipitated into filaments, which are then washed, dried

and further processed for different applications. The end uses of cellulosic

fibers are many and varied, including applications in apparel or technical

textiles, or non-woven textiles such as wipes and filters, or non-textile

technical materials, including healthcare and medical products (Ibbett et al.,

2008; Moncrief, 1970). The conversion of native cellulose into regenerated

cellulose takes place by breaking intramolecular hydrogen bonds along the

chains to form intermolecular ones. This makes the chains in regenerated

cellulose more flexible. Thus, native cellulose and the commercially available

regenerated cellulose may develop different interactions towards the various

major polymer architectures because of differences in interchain distances

and flexibility, crystallinity and hydrogen bonding between the cellulose

chains (Vigo, 1998).

Nanocomposites of NR/regenerated cellulose were prepared by cocoagulation of natural latex and cellulose xanthate mixtures using two

processes differing in the order of addition of the polymeric mixture and

coagulating solution (Peres et al., 2001; Mano et al., 1975). Kalb and Manley

(1980) demonstrated that cellulose fibers can be formed by precipitation from

solutions by two methods, similar to those described in the patents above,

involving different mechanisms of fiber formation. The fibers resulting from

both processes show properties depending on the stirring speed and coagulant


Nunes et al. (2004) investigated the effect of two procedures for obtaining

nanocomposites of NR/regenerated cellulose. Both processes led to powdered

rubber/cellulose masterbatches. In the first developed system (s1), the co-

NR composites using cellulosic fiber reinforcements


coagulation of the NR/regenerated cellulose mixes was carried out by adding,

under stirring, an equimolar acidic solution of sulfuric acid and zinc sulfate

to a natural latex/cellulose xanthate mixture. In the second system (s2),

the co-coagulation of the NR/regenerated cellulose mixes was carried out

by adding, under stirring, a natural latex/cellulose xanthate mixture to an

equimolar acidic solution of sulfuric acid and zinc sulfate. The precipitated

particles, having the aspect of yellowish crumbs, were rather uniform; their

dimensions were dependent on many variables, such as dilution, rate of

coagulation, speed and shape of the stirrer and temperature. The crumbs do

not aggregate easily and can be utilized as particulate rubber masterbatch

for the usual compounding purposes in a roll mill. Solid rubber or crumbs

were compounded according to a standard NR formulation. The influence

of increasing amounts of regenerated cellulose, varying from 0 to 30 phr

in both cases, was investigated as for rheometric and mechanical properties

and also by WAXD during uniaxial stretching at room temperature.

Tensile strength at break as a function of regenerated cellulose content is

shown in Fig. 10.1 (Nunes et al., 2004) for both systems. In both processes

the best performance was achieved by the addition of 15 phr regenerated

cellulose, although distinct properties were found in the composites because

of the structural arrangement and higher properties refinement caused by

the different order of addition of the polymeric mixture and coagulating


The preparation processes based on cellulose xanthate with elastomeric

lattices are simple, effective and widely applicable to industrial rubber



Stress at break (MPa)

















Regenerated cellulose content (phr)


10.1 Stress at break of NR/regenerated cellulose nanocomposites.


Chemistry, Manufacture and Applications of Natural Rubber

nanocomposites. The coagulation employed to incorporate nanocellulose

into NR succeeded in promoting excellent dispersion, allowing the material

obtained to be classified as a nanocomposite. The nanocomposite character

of these NR/regenerated cellulose systems was ascertained by transmission

electron microscopy (TEM) and is presented in Fig. 10.2 for the 15 phr

regenerated cellulose composite (Nunes et al., 2004; Martins et al., 2004a).

Recently, a similar system, starting from cellulose xanthate with elastomeric

latex was developed (Brandt et al., 2006; German Patent, 2006).

Gas transport and gas solubility in vulcanized NR/regenerated cellulose

were studied, respectively by Nunes et al. (2000, 2005) and Andrio et al.

(2000). The analysis of the results suggests that gas transport is severely

hindered in both the cellulose phase and the cellulose–rubber interphase of

the composites. The adsorption process undergoes a slight decrease when

the cellulose content reduces from 30 phr to 15 phr, being nil in NR. The

rather high adsorption processes detected with carbon dioxide in comparison

with the other gases are attributed to interactions between the quadrupoles

of carbon dioxide and the cellulose filler.

Recently, dielectric spectroscopy of nanocomposites of NR/regenerated

cellulose was determined by Ortiz-Serna et al. (2011). The analysis of the

200 nm

10.2 Transmission electron microscopy of NR/regenerated cellulose

nanocomposites containing 15 phr of regenerated cellulose.

NR composites using cellulosic fiber reinforcements


dynamics of the nanocomposites identifies three relaxation processes: one

b-relaxation associated with the local chain dynamics of cellulose and two

intimately related a-relaxations. The spectra exhibit conductivity phenomena

at low frequencies and high temperatures. The samples were also studied in

the dry state. An explanation is given concerning the cellulose effect on the

dielectric properties of the dry and wet nanocomposites.

Martins et al. have explored the properties of NR/regenerated cellulose

nanocomposites concerning the curing and mechanical properties (Martins

et  al., 2002a, 2004a), aging effect on dynamic and mechanical properties

(Martins et al., 2004b); characterization of uncured nanocomposites (Martins

et al., 2002b, 2005); mechanical and fractographic behavior (Martins et al.,

2003); and interaction between NR and regenerated cellulose (Nunes

et al., 2003).

In conclusion, it can be pointed out that:

Nanocomposites of NR/regenerated cellulose were prepared and the

one containing 15 phr of cellulosic filler was found to present the best


∑Solid state 13C NMR showed the occurrence of physical rubber–filler

interactions which correlated very well with the mechanical results.

∑ Lower T1rH values and a more homogeneous phase were observed for

the composite containing 15 phr of cellulosic filler. This decrease in

molecular mobility corroborates the highest tensile strength for this


∑ X-rays reveal that regenerated cellulose increases the strain-induced

crystallization of NR. From WAXD patterns the best structural arrangement

was found to occur for 15 phr of regenerated cellulose, and the highest

tensile strength presented by this composite seems to be associated with

this feature. The results of WAXD are presented in Fig. 10.3 (Nunes

et al., 2003).

Table 10.1 presents T1r relaxations detected by the protonated main chain

carbons of NR/regenerated cellulose nanocomposites (Nunes et al., 2003).

The sharper lines in the spectra were monitored to measure T1rH in these

systems. From Table 10.1, three different features can be observed:

1. NR/10 and NR/20 composites show a heterogeneous phase, characterized

by distinct T1rH values between the unsaturated and the aliphatic carbon

peaks. The T1rH values of the aliphatic carbons, lower than the ones

for the unfilled NR, indicate a decrease in the molecular mobility of the

rubber, which can suggest that the reaction of vulcanization has occurred

mainly through the allylic carbons.

2. The NR/30 composite shows a homogeneous phase since the T1rH values

for both domains are similar. This composite presents higher molecular

mobility than the other filled ones, much closer to the unfilled NR, which


Chemistry, Manufacture and Applications of Natural Rubber


Intensity (a.u.)





a = 4.5







10 15

20 25

2q (°)




30 35 40



Intensity (a.u.)



a = 4.5



a = 3.5











2q (°)






10.3 WAXD patterns of NR/regenerated cellulose nanocomposites in

the 2q interval from 2° to 40° at room temperature during uniaxial

stretching: (a) NR; (b) NR/15 phr of regenerated cellulose.

suggests that regenerated cellulose no longer affects the NR molecular

motion when present in the amount of 30 phr.

3. The NR/15 composite presents a unique behavior. Among the filled

composites, it shows a more homogeneous phase, and the lower T1rH

values indicate a decrease in the NR molecular mobility. These results

corroborate the mechanical properties.

NR composites using cellulosic fiber reinforcements


Table 10.1 T1rH values of NR/regenerated cellulose nanocomposites



cellulose (phr)

T1rH (ms)










136 ppm 127 ppm 106 ppm 76 ppm 34 ppm 28 ppm 25 ppm


























Blending of two or more rubbers is carried out for three main goals:

improvement in technical properties, better processing, and lower compounding

cost. Many products in the rubber industry, such as tires, are based on blends.

There is an ever increasing technological interest in the use of blends of

dissimilar rubbers to improve specific properties of vulcanizates.

Nunes and Costa (1994) studied the influence of a NR/regenerated cellulose

masterbatch on a composition with ethylene-propylene dieneter polymer

(EPDM). Two types of EPDM were used and the results were analyzed

for the influence of both types of EPDM as well as the relative amounts

of EPDM and NR/regenerated cellulose system in the blends. Properties of

the EPDM/NR/regenerated cellulose nanocompositions can be inferred from

the ones containing 75 phr of the NR/regenerated cellulose system which

showed the best mechanical performance.

Vieira et al. (1996, 1997) performed a series of experiments in blending

NR and butadiene rubber (BR) with cellulose filler. The results of tensile

strength, tear strength and abrasion resistance show that regenerated cellulose

can be considered as reinforcing filler for NR/BR blends. Swelling studies

show that the degree of interaction between the elastomeric phase (NR and

BR) and nanocellulose is improved by increasing the BR content in the

elastomeric matrix. Tensile strength at break data display auto-reinforcement

of NR in NR/BR/regenerated cellulose composites, since the predominance

of NR in the mixtures gives rise to better mechanical performance. Figure

10.4 shows the tensile strength of NR/BR as a function of the cellulose


From research by Fernandes et al. (2011), new nanocomposites of NR,

epoxidized natural rubber (ENR) and regenerated cellulose were obtained. The

influence of the ENR content on cure properties, mechanical and dynamicmechanical properties of vulcanized composites was studied. The amounts

of ENR varied from 0 to 75% and regenerated cellulose was kept at 20 phr

in all composites. NR/regenerated cellulose nanocomposites were prepared

by the co-coagulation method and admixture with ENR on a two-roll mill.

Mano and Nunes (1983) reported some data on the effect of NR/regenerated

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1 Introduction: The importance of natural rubber (NR)/cellulose composites

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