**1. Introduction**

The use of silica has become of growing importance in tires because of reduced fuel consumption in automotive transport and consequently preservation of the environment. The high-dispersion silica reinforcement was introduced in the early 1990, by Michelin in passenger tire tread rubbers, the so-called green tire [1], which offers approximately 30% lower rolling resistance, resulting in 5% fuel savings and lower carbon dioxide emission to the environment [2, 3].

The core of the high-dispersion silica technology is the nanoscale reaction of the 4–6 silanol groups per nm2 on the surface of the 20–30 nm diameter primary silica particles with a coupling agent. This reaction reduces the hydrophilic character of the filler and increases its compatibility with the rubber polymer [2, 4, 5]. This reaction is due to take place in the same processing step as mixing of the tire compound and is very difficult to lead to completion [6]. The coupling agent eventually creates a chemical link between the primary silica particles and the rubber molecules during the later processing steps of vulcanization [7, 8].

Silica technology as it is used today employs solution-polymerized synthetic elastomers like solution styrene-butadiene rubber (sSBR) and solution butadiene rubber (BR). On the other hand, the great majority of rubber polymers used for carbon-black reinforced tire applications are emulsion polymers such as natural rubber and emulsion styrene-butadiene rubber (E-SBR). Research on reinforcement of the silica-silane system has been extensively investigated, and most of the early silica compound development involved natural rubber as the base polymer [9]. Up until now, natural rubber is not fully utilized in silica technology due to

postulation of its ineffectiveness with silane coupling agent. An in-rubber study of the interaction of silica with proteins present in natural rubber shows that proteins compete with the coupling agents for reaction with the silica during mixing, making the silane less efficient for improving dispersion and filler-polymer coupling and consequently affecting the final properties of the compound [10].

Natural rubber is a linear, long-chain polymer with repeating isoprene units (C5H8) [11]. It has a density of 0.913 at 20°C and glass transition temperature of −72°C [12]. The average molecular weight (Mw) of commercial NR is 1–1.5 × 106 , and number average molecular weights are 3–5 × 105 [13]. The commercial NR is produced from coagulation of latex which is tapped from *Hevea brasiliensis* or rubber tree. The composition of Hevea latex is mainly rubber hydrocarbon of 30–45% weight; nonrubber components for 3–5% and the balance is water. The nonrubber components are proteins, lipids, amino acids, amine, carbohydrates, inorganic materials, and minerals [14].

The fundamental structure of NR has been revealed by NMR studies as a linear rubber chain consisting of initiating terminal (ω-terminal), two trans-1,4 isoprene units, long sequence of 1000–3000 cis-1,4 isoprene units, and chain-end terminal (α-terminal) as shown in **Figure 1** [15–17]. The ω-terminal consists of mono- and diphosphate groups linked with phospholipids by hydrogen bond or ionic bond [18]. The α-terminal is a modified dimethylallyl unit linked with functional groups, which is associated with proteins to form cross-linking via hydrogen bonding. These functional groups at both terminals are presumed to play a role in the branching and gel formation in NR [19, 20]. These secondary structures play a significant role in the strain-induced crystallization of unvulcanized and vulcanized natural rubber [21].

Modification of natural rubber is carried out to improve the behavior of NR during rubber product processing and to improve the in-service performance of the products which can be outside its traditional applications. NR is modified through physical and chemical processes. The major modifications of NR are outlined in **Figure 2** [12]. The physical modification of NR incorporates additives and other compounding ingredients which are not chemically reacting with rubber. Examples of physical-modified form of NR are oil extended NR, thermoplastic NR, powdered NR, and deproteinized NR. The chemical modification of NR involves reacting the NR chains chemically by attachment of pendant functional groups, grafting of different polymers along the rubber chains or through intramolecular changes and bond rearrangement. The chemical-modified form of NR includes epoxidized NR (ENR) [22], hydrogenated NR, and poly-methyl methacrylate (PMMA)-grafted NR.

The first use of silica in truck tire treads based on NR had shown an improvement in tear properties in terms of cut and chip behavior, but the amount used was limited to 25 phr in order to avoid negative effects on tread wear [2]. Higher amounts of silica require coupling agents, which at that time were not used for

**47**

applications [22, 31, 32].

**Figure 2.**

*Modification of natural rubber.*

*Silica-Reinforced Deproteinized Natural Rubber DOI: http://dx.doi.org/10.5772/intechopen.85678*

silica. From a study on optimization of the mixing conditions of silica-filled NR compounds with silane, an increase of mixing temperature and time for the silica modification with silane enhances the compatibility between silica and NR through a chemical bond between bis-triethoxysilylpropyl-tetrasulfide (TESPT) and the rubber [23]. The overall properties are dependent on the extent of this interaction [23]. The field tests with truck treads demonstrated that the rolling resistance can be improved by 30% when TESPT-modified silica is used in comparison to N220 carbon black. The tread wear index (abrasion resistance) decreases by no more than

High reinforcement of properties for silica in NR and other rubbers needs to employ silane coupling agent [2, 7, 9, 25]. High reinforcing effect of silica is obtained with ENR without the use of coupling agents [26, 27] or reduced amount of coupling agent [28]. The vulcanizate properties of silica-filled ENR are comparable to carbon black compound at similar loading and are superior to a silica-filled NR without silane coupling agent (**Table 1**) [26, 29]. A comparison of physical properties of ENR-25—silica tread compound without coupling agent to sSBR/ BR—silica compound is shown in **Table 2** [30]. In contrast with sSBR/BR, the ENR compound shows better reinforcement with silica without coupling agent. ENR exhibits both excellent wet grip and very low rolling resistance properties which is an attractive choice for tire tread compound for both passenger car and bus/truck

Alternative approaches in incorporating silica into NR as nanocomposites are via in situ sol-gel technique, admicellar polymerization, and polymer-encapsulated silica [33]. Using sol-gel process, solid rubber is swollen in a silica precursor, e.g., tetraethyl orthosilicate (TEOS), followed by the sol-gel reaction, and silica content in the range of 15–22 wt % is achievable with stiffer and stronger tensile strength of 24 MPa. The in situ sol-gel technique on ENR with 3-aminopropyltriethoxysilane (APS) has been reported [34]. The ENR-APS sol-gel system with 28% sol-gel silica has higher tensile properties compared to a conventional ENR-sulfur-cured silica vulcanizate at 27% silica content with no silane coupling agent (**Table 3**) [35]. In

5%, and wet traction shows little changes [24].

*Silica-Reinforced Deproteinized Natural Rubber DOI: http://dx.doi.org/10.5772/intechopen.85678*

*Silicon Materials*

postulation of its ineffectiveness with silane coupling agent. An in-rubber study of the interaction of silica with proteins present in natural rubber shows that proteins compete with the coupling agents for reaction with the silica during mixing, making the silane less efficient for improving dispersion and filler-polymer coupling and

Natural rubber is a linear, long-chain polymer with repeating isoprene units (C5H8) [11]. It has a density of 0.913 at 20°C and glass transition temperature of −72°C [12]. The average molecular weight (Mw) of commercial NR is 1–1.5 × 106

produced from coagulation of latex which is tapped from *Hevea brasiliensis* or rubber tree. The composition of Hevea latex is mainly rubber hydrocarbon of 30–45% weight; nonrubber components for 3–5% and the balance is water. The nonrubber components are proteins, lipids, amino acids, amine, carbohydrates, inorganic

The fundamental structure of NR has been revealed by NMR studies as a linear rubber chain consisting of initiating terminal (ω-terminal), two trans-1,4 isoprene units, long sequence of 1000–3000 cis-1,4 isoprene units, and chain-end terminal (α-terminal) as shown in **Figure 1** [15–17]. The ω-terminal consists of mono- and diphosphate groups linked with phospholipids by hydrogen bond or ionic bond [18]. The α-terminal is a modified dimethylallyl unit linked with functional groups, which is associated with proteins to form cross-linking via hydrogen bonding. These functional groups at both terminals are presumed to play a role in the branching and gel formation in NR [19, 20]. These secondary structures play a significant role in the strain-induced crystallization of unvulcanized and vulcanized natural rubber [21]. Modification of natural rubber is carried out to improve the behavior of NR during rubber product processing and to improve the in-service performance of the products which can be outside its traditional applications. NR is modified through physical and chemical processes. The major modifications of NR are outlined in **Figure 2** [12]. The physical modification of NR incorporates additives and other compounding ingredients which are not chemically reacting with rubber. Examples of physical-modified form of NR are oil extended NR, thermoplastic NR, powdered NR, and deproteinized NR. The chemical modification of NR involves reacting the NR chains chemically by attachment of pendant functional groups, grafting of different polymers along the rubber chains or through intramolecular changes and bond rearrangement. The chemical-modified form of NR includes epoxidized NR (ENR) [22], hydrogenated NR, and poly-methyl methacrylate (PMMA)-grafted NR. The first use of silica in truck tire treads based on NR had shown an improvement in tear properties in terms of cut and chip behavior, but the amount used was limited to 25 phr in order to avoid negative effects on tread wear [2]. Higher amounts of silica require coupling agents, which at that time were not used for

*A linear rubber chain structure with naturally occurring network associated with proteins and phospholipids.*

,

[13]. The commercial NR is

consequently affecting the final properties of the compound [10].

and number average molecular weights are 3–5 × 105

materials, and minerals [14].

**46**

**Figure 1.**

**Figure 2.** *Modification of natural rubber.*

silica. From a study on optimization of the mixing conditions of silica-filled NR compounds with silane, an increase of mixing temperature and time for the silica modification with silane enhances the compatibility between silica and NR through a chemical bond between bis-triethoxysilylpropyl-tetrasulfide (TESPT) and the rubber [23]. The overall properties are dependent on the extent of this interaction [23]. The field tests with truck treads demonstrated that the rolling resistance can be improved by 30% when TESPT-modified silica is used in comparison to N220 carbon black. The tread wear index (abrasion resistance) decreases by no more than 5%, and wet traction shows little changes [24].

High reinforcement of properties for silica in NR and other rubbers needs to employ silane coupling agent [2, 7, 9, 25]. High reinforcing effect of silica is obtained with ENR without the use of coupling agents [26, 27] or reduced amount of coupling agent [28]. The vulcanizate properties of silica-filled ENR are comparable to carbon black compound at similar loading and are superior to a silica-filled NR without silane coupling agent (**Table 1**) [26, 29]. A comparison of physical properties of ENR-25—silica tread compound without coupling agent to sSBR/ BR—silica compound is shown in **Table 2** [30]. In contrast with sSBR/BR, the ENR compound shows better reinforcement with silica without coupling agent. ENR exhibits both excellent wet grip and very low rolling resistance properties which is an attractive choice for tire tread compound for both passenger car and bus/truck applications [22, 31, 32].

Alternative approaches in incorporating silica into NR as nanocomposites are via in situ sol-gel technique, admicellar polymerization, and polymer-encapsulated silica [33]. Using sol-gel process, solid rubber is swollen in a silica precursor, e.g., tetraethyl orthosilicate (TEOS), followed by the sol-gel reaction, and silica content in the range of 15–22 wt % is achievable with stiffer and stronger tensile strength of 24 MPa. The in situ sol-gel technique on ENR with 3-aminopropyltriethoxysilane (APS) has been reported [34]. The ENR-APS sol-gel system with 28% sol-gel silica has higher tensile properties compared to a conventional ENR-sulfur-cured silica vulcanizate at 27% silica content with no silane coupling agent (**Table 3**) [35]. In


*a Rubber formulation in phr: rubber 100, filler, oil 5, ZnO 5, StA 2, antioxidant 2, S 2, MBS 1.5. b Rubber formulation in phr: rubber 100, filler, oil 5, ZnO 5, StA 2, antioxidant 2, S 2, MBS 1.5, DPG 0.5.*

## **Table 1.**

*Comparison of physical properties of black and silica-filled ENR vulcanizates.*


## **Table 2.**

*Comparison of physical properties of ENR-silica and sSBR/BR tread compounds.*


#### **Table 3.**

*Comparison of tensile properties of ENR-silica sulfur cured with ENR-APS sol-gel system.*

admicellar polymerization, a bilayer of surfactant (admicelle) is formed on the surface of silica and polymerization of the monomer in the admicelle resulting in an ultrathin layer of polymer covering the silica [33, 36, 37]. The silica modified with admicellar polymerization has superior performance compared to those reinforced with unmodified or silica modified with typical coupling agents [37].

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**Table 5.**

**Table 4.**

*Silica-Reinforced Deproteinized Natural Rubber DOI: http://dx.doi.org/10.5772/intechopen.85678*

Purification of natural rubbers from nonrubber components is possible especially the protein removal. The deproteinization process of natural rubber yields a rubber with reduced protein content significantly as well as low ash and volatile matter which is known as deproteinized natural rubber (DPNR). The deproteinization process can be carried out with natural rubber latex via enzymatic treatment [38–40] or urea treatment [41, 42]. The principle of deproteinization with enzymatic treatment is to hydrolyze the proteins in the natural rubber latex into water soluble which will be removed during washing process of manufacturing the

Structural changes of NR branch points are proposed to occur with the deproteinization process [15]. This is based on the findings that a linear rubber chain contains two types of functional groups at both terminals. After deproteinization, the branch points formed by the functional groups associated with proteins at the α-terminal through hydrogen bonding decompose and leave the branch points from phospholipid at the α-terminal. The long-chain branching in the purified NR originated from the interaction of phospholipids which link the rubber chain together [43–45]. The phospholipids are associated together by the formation of a

The commercially available DPNR is Pureprena, which is produced by the Malaysian Rubber Board and licensed to Felda Rubber Industries Sdn Bhd. Pureprena is a purified form of natural rubber and has a very-low-nitrogen, ash, and volatile matter contents as well as being lighter in color (**Table 4**). DPNR is less prone to storage hardening than a normal NR grade. When compounded using an efficient vulcanization (EV) system, DPNR has low creep and stress relaxation, low water absorption, low compression set, and a more consistent modulus when subjected to variable humidity conditions [46]. DPNR gives superior rubber compounds with excellent dynamic properties which are suitable for engineering

Dirt retained on 44 μm aperture (% wt) 0.20 max 0.01 max Ash content, (% wt) 1.00 max 0.15 max Nitrogen content, (% wt) 0.60 max 0.12 max Volatile matter content, (% wt) 0.80 max 0.30 max

*Comparison of raw rubber properties specification of DPNR (Pureprena) and natural rubber (SMR20).*

Low water absorption Underwater applications, large shock absorbers Good dynamic properties Anti-vibration mountings, surge fenders Low protein and low ash Medical, pharmaceutical, and food applications

bushes, helicopter rotor bearings

**SMR 20**

Hydromounts, seals, joint rings, large shock absorbers, suspension

**Specification of Pureprena**

**Properties Specification of** 

**Features Applications**

**2. Deproteinized natural rubber**

rubber [41, 42].

micelle structure.

applications (**Table 5**) [46].

Low stress relaxation and

*Areas of application of DPNR.*

low creep
