Natural Rubber Silica And Silane Biology Essay

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Natural rubber modified by sulphur was first discovered in 1839 by Charles Goodyear. Amazing improvements in processing and mechanical properties of raw rubber encouraged rubber engineers to optimise the use of chemical additives including curing agents, accelerators, activators, processing aids, anti-degradants, and fillers [1]. Right amount and type of these additives and uniform dispersion in rubber directly influences the properties of cured product.

The most widely used curing agent for natural rubber is still considered to be sulphur because sulphur-based systems can contribute to the formation of thermally stable covalent bonds between the chains of natural rubber via the C=C bonds, enhancing mechanical properties effectively [ [2] ]. Bifunctional organosilanes pre-treated silanised silicas are excellent fillers to reinforce mechanical properties of natural rubber hence offering significant benefits for example better durability and performance, and longer service life [ [3] ].

However, there are problems using silicas to reinforce sulphur-cure system based natural rubber and mixing time, reaction of the chemical curatives and blooming of the rubber chemicals are affected by silica[1-3]. Mixing time would affect the dispersion of chemical curatives and rubber chains would break after a long time mixing. Slow reaction of the chemical curatives would reduce the crosslink properties and thus damage the rubber properties. Blooming is the excessive use of partly soluble chemical curatives and the lack of equilibrium in concentration, which leads to the transportation of chemical curatives onto the rubber surface, resulting in a thin blooming layer. This would result in low crosslink density and bad mechanical properties of cured rubber.

This literature review is aimed to find and collect information on current silica- reinforced sulphur-cured natural rubber and try to develop new methods for to reduce the number of chemical curatives in rubber compounds and to achieve efficient use of chemical curatives to stop them from migrating to the rubber surfaces.

1. Natural Rubber, silica and silane.

1.1 Natural rubber

Natural rubber (NR), also called polyisoprene, is extracted from field latex tapped from Hevea brasiliensis rubber tree and contains 25-45% solid rubber. Raw natural rubber contains impurities such as proteins (2.5%), acetone-soluble resins (2.5%), sugars (small amount) and ashes (0.3%), which may constitute 5-8% of the whole rubber. It should be noted that parts of these impurities are antioxidants and activators of cure, for instance the tracing copper and manganese element in raw rubber, which helps it to have a longer storage time at ambient temperature [ [4] ]. Different processing conditions always bring about different amounts of the impurities in the rubber [ [5] ].

1.1.1 Structure and composition of NR

NR consists of polymer chains having over 97% cis-1,4 polyisoprene. The formula of cis-1,4 polyisoprene is shown in Fig. 1.1. Since the rubber contains nearly all the same isomer, it can be recognised to be a stereoregular structure with outstanding regularity. Thus when the rubber is stretched, it would crystallise. Also, from the structure of cis-1,4 polyisoprene, it can be found that each isoprene unit has one double bond. The double bond is reactive and reacte with oxygen or ozone, it is also able to react with sulphur for vulcanisation.

imagesCAX3M3A9.jpg

Fig. 1.1 The chemical structure of NR.

The average molecular weight of NR is from 200,000 to 400,000, and the relative broad molecular weight distribution determines its good processibility because of relative good banding on the mill and low compound viscosity.

1.1.2 Properties of natural rubber

Physical Properties

Different constituents in NR may lead to slight differences in its physical properties. For instance, different storage temperature, causing different degree of crystallisation, also referred to as different density. Some basic physical properties of NR are listed in Table 1.1.

Table 1.1 Physical properties of NR [5].

Density (g/cm3)

0.934

Refractive index

1.5215-1.5238

specific heat (J/kg.K)

0.502

Specific resistivity (Ohm.m)

1015 

Heat of combustion (kJ/g)

44.16 

Thermal conductivity (W/m.K)

0.13-0.142

Note: All the properties are measured at 20oC.

Mechanical properties of NR

Regarding the poor mechanical properties of raw NR, it must be reinforced when it is used in industrial applications. Since the strain crystallisation behaviour mentioned before, high tensile strength (20 MPa) and the tear resistance can be obtained even in raw rubber. Of course, adding reinforcing fillers and curing the rubber will give a much better mechanical properties of NR including hardness, stiffness, tensile strength, abrasion resistance, elongation at break, tear resistance, elastic resilience. However, the specific values of mechanical properties are influenced by the amount of fillers added to compounds and the degree of vulcanisation.

Applications of natural rubber

Most of the applications of NR derive from their unique chemical and physical properties. Nowadays, the use of natural rubber is divided into three main parts. A small portion of NR latex is used for producing adhesive tapes, rubber solutions and art gums. A large portion of the solid rubber from latex is vulcanised as hard rubbers, but the largest amount of solid rubber from latex is vulcanised NR, where it shares both the elastic property and good mechanical property. Types of NR applications corresponding to their unique properties are listed in Table 1.2. In addition, there are several kinds of new NR materials including liquid NR, powdered NR, epoxidized NR, thermoplastic NR, Oil-extended NR, since dry rubber processing is energy consuming and cannot be recycled.

Table 1.2 Applications and their corresponding properties of NR [4- [6] ].

Properties

Applications

Strain crystallisation

Thin-walled, soft products: balloons, surgical gloves

Strain crystallisation, outstanding physical properties

Car tires

High elasticity, low hysteresis

Railway/Automotive suspension systems, automotive bumpers

Outstanding physical properties

Engineering componets

Outstanding physical properties

Civil Engineering: Bridge bearings, vibration isolation of buildings, flood control measures, earthquake resistant buildings.

 ?

Medical applications: surgical gloves, catheters, contraceptive condoms, self-adhesive surgical dressings.

Silica and silane

The chemistry of silica surfaces.

The most important feature of silicas is the existence of functional silanol groups on their surfaces (see in Fig. 1.2), which makes a polar silica surface and the silicas in a normal state of hydration [ [7] , [8] ]. As a result, the silica particles are cohesive, and there is a requirement that silica particles must be dispersed well to improve the mechanical properties of the rubber vulcanisates. [1, 5, 6]. At the same time, the silanol groups make the silica fillers acidic, resulting in an absorption of vulcanisating accelerators. For this reason, the vulcanisation process cannot proceed completely with insufficient amount of accelerators, and the cure times and cure rates are influenced by the acidic silica [ [9] ].

silica.jpg

Fig. 1.2 Formula of silica surfaces. [ [10] ]

The reinforcing effects of silica fillers.

The reinforcing effects are attributed to the morphology of silica due to its influence on the dispersion state of the silica fillers. Typically, there are three kinds of morphology of silica, which are the primary particles, aggregates and agglomerates [7]. Since the silica particle shave high surface energy, the aggregates of silica tend to form agglomerates with the tendency of the re-agglomeration after mixing, also namely as secondary agglomeration [7, [11] ].

Table. 1.3 Types of silica morphology and their corresponding dimensions and forming reasons [7].

Types of morphology

Primary particles

Aggregates

Agglomerates

Cross-sectional dimensions

5-100nm

100-500nm

1-40m

Reason of formation

-----

Chemical and physical-chemical interactions of primary particles

Condense the aggregates by Van der Waals force

Modification of silica surfaces.

Since silica fillers cannot meet today's level of technology, the way to solve this problem is the modification of its surfaces, including surface physical modification and chemical modification. In the chemical modification, two kinds of chemicals are in normal use. The first method is to graft chemical groups on the silica surfaces, in this method, there is no chemical reaction, and the other method is to use coupling agents to treat the surfaces with a chemical reaction between the silica and rubber. Chemicals for this method are the "titanate based coupling agents, zirconated based coupling agents and other metal complex coupling agents." [ [12] ]

1.3 Silane coupling agents.

Sliane coupling agents seems to be the most important chemicals for silica modification, especially the bifunctional organosilans [4]. The typical formula of bifunctional organosilanes can be expressed as X3-mRmSi(CH2)nY. X stands for the hydrolysable group, for instance halogen, alkoxyl and acetoxyl groups, Y stands for the groups which are chemical reactive with the rubbers, such as amino, epoxy, acrylate, vinyle and sulphur-containing groups. The number of m and n for the bifunctional silane coupling agents often equals to 0 and 3 respectively [12]. Among the bifunctional silane coupling agents, bis (3-triethoxysilylpropyl-) tetrasulphane (TESPT) also called as Si-69, --mercaptopropyltrimethoxy silane and 3-thiocyanatopropyl triethoxy silane (Si-264) are in common use [10, [13] ]. The formulas of these sillane coupling agents are shown as follow.

Coupling agent.jpg

Fig. 1.3 The formulas of Si-69 and Si-264 [13].

Fillers to reinforce mechanical properties of natural rubber.

The use of fillers in rubber is nearly as old as the use of rubber. Because the technical reasons require a range of mechanical properties for different applications, the use of fillers in rubber becomes necessary [5, 6]. Currently, several mechanisms are available describing mechanisms of reinforcement of the mechanical properties. They are the kinetic theory, the Blanchard-Parkinson theory and the chain extensibility limit theory [ [14] ]. Adding in reinforcing fillers to raw rubber results, in higher hardness, and tensile properties are enhanced, in particular, and better abrasion and tear resistance [14, [15] ]. In addition, with the addition of fillers, an increase in processibility and other changes for instance increases in density, colour or price can be obtained [5, 7].

Properties vs filler content.jpg

Fig. 2.1 Effects of filler content on NR properties. [7]

2.1 Types of fillers to reinforce mechanical properties of natural rubber.

According to their colours, the fillers can be divided into two classes: the carbon blacks and the light coloured fillers. For the light coloured fillers, the colloidal silica, calcium and aluminium silicate, alumina gel, silica, talcum, metal oxide and chalk are the main classes. At the same time, the types of fillers also can be classified as high active, medium active and inactive fillers according to their activities [ [16] ].

2.1.1 Carbon blacks

Surface activity

Carbon blacks for rubber grade commonly include appreciable amounts of chemically combined hydrogen (0.2-1%), oxygen (0.4-1%) and sometimes sulphur (up to 1%). The carbon black surface often contains organic carboxyl, carboxyl, quinine or lactone groups, among these groups, the reactive organic groups which furnish affinity to rubber cannot be found in non-black fillers [4, 5]. These surface groups play an important role due to their influences on the rate of cure. The carbon black filler surfaces will interact more strongly with those polar elastomers, for instance, the nitrile rubber. Therefore, it can be concluded that there is no large influence on reinforcement of NR as a consequence of its non-polar nature. While in a physical sense, the adsorptive activity of reinforcing carbon blacks is not homogeneous, since there are less than 5% more activit sites than the majority of the surface. On the whole, the reinforcing properties depend much more on the physical adsorption activity of the filler surfaces rather than the chemical nature of the filler surface [4].

2.1.2 Non-black fillers.

In the rubber industry, the most important non-black fillers can be recognised as the precipitated fillers. Precipitated silicas are most widely used for commercial applications. The advantage of using this filler is good dispersion and low viscosity in rubber with low rolling resistance in tyre applications [5, 7]. Currently, some types of precipitated silica filler has been widely used in commercial rubber products, for instance, the HiSil® EZ35, Zeosil® 1165 MP38, Ultrasil® VN 338 and KS® 40438 [7]. The most common active non-black fillers are the silicas, which can be obtained by solution process or pyrogenic process, and the inactive non-black fillers involve chalks, kaolins, kieselerdes, zinc oxide, aluminium oxyhydrate, talcum and micro talcum [5].

2.2 Carbon blacks vs. silicas

2.2.1 Colour

Carbon blacks are the traditional reinforcing fillers for NR compounds since their outstanding reinforcement properties, in particular, the better abrasion resistance than others kinds of fillers [17]. However, because of the black colour of carbon blacks, it can only be used in manufacturing black coloured rubber articles. Moreover, the black colour makes the rubber absorb more UV sunlight, which helps to protect the rubber against UV ageing and hence delays its degradation. In order to find a solution for the issues mentioned above, the use of reinforcing silica fillers has been developed for durable coloured rubber articles [7-9].

2.2.2 Surface activity

Regarding the difference between the surface chemistry of carbon black and silica, it seems that silica is more difficult to attain a good dispersion for optimum reinforcing properties. The reason can be attributed to the secondary agglomeration of silica [4, 7]. Also, the strong filler-filler interaction influence the hardness, tear resistance, and dynamic modulus, and at the same time, it brings about a high viscosity which is not good for rubber mixing and rubber processibility and this causes an excessive wear and tear of the processing equipment [3, 4, 7].

2.2.3 Mechanical properties of carbon black- and silica- reinforced rubbers.

Compared to reinforcing carbon blacks, the NR reinforced by pure silicas show nearly equal tensile strength and tear resistance, however, the abrasion resistance is lower by 15-20%, strain values and hardness are lower than the rubber reinforced by carbon blacks. Aside from the mechanical properties, the electrical property is better too [5, [17] ].

For the dynamic mechanical properties, it is remarkable that silica has a lower hysteresis compared with carbon black [18]. Generally, the storage (G'), loss modulus (G'') and the hysteresis (tan) of the rubber compounds increase with combination of reinforcing carbon black. The comparison of hysteresis for carbon black and silica fillers against temperatures are shown in Fig. 2.2. It can be found that silica owns a lower hysteresis value in a certain temperature range approximate from -30oC to 90 oC [7, [18] ].

Fig 2.2 Hysteresis value of carbon blacks (â-) and silicas (â-‹) reinforced rubbers at different temperatures. [7]

According to the comparisons above, some aspects, such as parts of mechanical properties of reinforced rubber compound and the dispersion state in rubber, it seems that carbon black is more suitable for reinforcement, while for the specific area, the light coloured fillers must be used instead of black fillers. In addition, the comparison merely takes place between pure silica fillers and carbon blacks, there is still a large promotion possibility for using modified silicas, such as the precipitated silicas, the silica coated carbon black and the silanized silicas. This project is mainly focused on the silanized silica fillers.

Silanized silica nanofillers.

Due to the hydration and acidic nature of silica fillers, there are several issues with the use of this filler in rubber. They are insufficient dispersion of silica particles in rubber compounds, which leads to more processing problems, the vulcanisation process is more time consuming and the lower crosslink density are obtained, since the filler interferes with the action of accelerators, and the alkaline vulcanisation condition is not satisfied . Moreover, the polarity between silica filler and rubber may be too large [5, 7-9]. Thus, in order to increase the reinforce efficiency of silicas, they should be combined with silane coupling agents to overcome the limitations.

Methods for silica silanisation.

The process of silanisation can take place by two different methods. In the first method, the silica and silane coupling agents are put into a mixer together, after their preliminary mixing, the rubber is added. For the second method, the silanisation process happens in situ, which means the silica has been mixed with rubber before the silanisation by the silane coupling agent [2].

2.3.2 Use of silanized silica nanofillers to reinforce mechanical properties of NR.

Using silanised silica nanofillers is beneficial to improve the capability of reinforcing silicas in non-polar rubber compounds and also increases the network properties by the formation of covalent sulphur crosslinks in rubbers. The bis (3-triethoxysilylpropyl-) tetrasulphane (TESPT) is a bifunctional polysulfidic organosilanes, also called as Si 69, which has both ethoxy and tetrasulphane reactive groups [ [19] ]. The formula of TESPE is shown in Fig. 2.3.

未命名.jpg

Fig. 2.3 Chemical structure of TESPT [3].

The ethoxy groups on TESPT are capable of reacting with the silanol groups on the surfaces of silicas, leading to the formation of stable chemical bonds between the silica and the TESPT coupling agent. On the other hand, the reactivity of the tetrasulphane groups is influenced by the accelerators and reactive temperatures, which means if the rubber system contains accelerators at suitable cure temperature, the unsaturated rubbers can be partially vulcanised by the reaction between the sulphur in TESPT via the double bonds and the formation of stable sulphur bonds [2,3,8, [20] ]. The reaction process can be expressed in Fig. 2.4. Only at high enough temperatures, it is possible that the vulcanisation process can take place since the decomposition of polysulfide begins to generate free sulphur for cure at elevated temperature. If the cure temperature is not high enough, it cannot achieve the optimum reinforcing effect by using the TESPT coupling agent, and the dispersion of the silica fillers in rubber is not as good as expected, also resulting in heat build-up in rubber compounds. Moreover, ethanol can be generated through the hydrolyzation process of silane coupling agent, which leads to a formation of bubbles in the rubber articles [8].

silaised silica.jpg

Fig. 2.4 Schematic of the TESPT treatment for silica surface and the reaction between TESPT and unsaturated rubber [ [21] ].

As a result, precipitated silica pre-treated with bifunctional TESPT coupling agent can be regarded as a "crosslinking filler" for it can both reinforce the rubber and crosslink it at the same time. In addition, after TESPT pre-treatment, the number of silanol groups on the silica surfaces decrease, and it is more difficult for the residual silanol groups to recat with the rubber chains because of steric hindrancet [2]. It can also be found that the viscosity of rubber compounds decreases because of the weaker filler-filler interaction and the cure properties improve, since the problems caused by the acidic nature of the filler are remedied [2]. On the whole, with the breakthrough of using silanised silica fillers gradual instead of carbon blacks, the silanised silicas are found to be the best solutions in some applications which need low heat build-up and high wet traction [9].

Cure systems in natural rubber filled with silica.

Types of vulcanising systems.

Generally, there are two large classes of vulcanising systems. One is the non-sulphur vulcanising system, such as peroxide, urethane, borane, etc. The other one is the sulphur vulcanising system. In practice, the sulphur vulcanising systems are predominantly used for NR [5, 6].

Sulphur vulcanising system.

Typically, a sulphur vulcanising system consists of elemental sulphur, accelerators which are used to increase the rate of cure and activators which are used for activating the accelerators, and there are three broad types of sulphur vulcanising systems: the conventional (high sulphur) vulcanising (CV) system; semi-efficient vulcanising (EV) system; and the EV system [ [22] ].

Table. 3.1 Three types of vulcanising systems and their corresponding characteristics [5, 6].

Types

Sulphur concentration

Accelerator concentration

Approximate range of E values obtained

Crosslink structures

Conventional

2.0-3.5

0.5-1.0

10-25

Polysulfide (C-Sx-C, X)

Semi-EV

1.0-2.0

1.0-2.5

4-8

Disulfide crosslinks (C-S-S-C)

EV

0.3-1.0

2.0-6.0

1.5-4

Monosulfidic (C-S-C)

Note: The E is defined as the number of network-combined sulphur atoms present per physically-effective chemical crosslink.

According to Table. 3.1, it can be found that the crosslink structures and degree of vulcanisation are different in the three types of vulcanising systems. This result leads to a difference in the heat build-up of rubber compounds, which can be described in an ascending order, the CV, the semi-EV and the EV [5, 6]. Because the crosslinks formed in CV system is polysulfide which is more flexible, the dynamic properties and the heat build-up behaviour are better than the vulcanisates formed in the other two systems [13, [23] ].

3.2.1 Vulcanisation process.

"Vulcanisation is the conversion of rubber molecules into a network by the formation of crosslinks. [5]" The degree of vulcanisation can be estimated by the crosslink density, it depends on the amount, activity of vulcanisation agents (sulphur) and the vulcanisation time at elevated vulcanisation temperatures [5]. Fig. 3.1 shows the process of vulcanisation, and the difference in sulphur amount resulting in different rubber properties.

DSC04969.JPG

Fig. 3.1 Un-crosslinked and crosslinked rubber [5].

3.2.2 Cure characteristics.

Cure characteristics can be described by the scorch time (ts2), which is the onset of cure, cure time (t90 or t95), the optimum cure time, and ∆ torque (maximum torque minus minimum torque) of NR, and they can be obtained from the oscillating disc rheometer (ODR) cure traces. The torque can be used as a symbol for the crosslink density of NR [1].

Cure systems in NR filled with silanized silica.

3.3.1 The effect of vulcanising system of reinforcing efficiency of silanized silica nanofiller.

In 2004, Sae-oui [13] pointed out that the different kinds of vulcanising systems including the conventional, semi-EV and EV systems can bring about different reinforcing efficiency (see in Table. 3.2).

Table. 3.2 Tendency of reinforcing efficiency of various curing systems.

Vulcanising systems

Compression set

Heat build up

Swelling ratio

Difference in dynamic shear modulus

CV

 

 

 

 

Semi-EV

EV

As said before, the heat build up in CV vulcanising system is better than in Semi-EV and EV systems, the difference in compression set properties in different vulcanising systems are also related with the different crosslink structures formed in various systems. Since the monosulfidic crosslink formed in EV system is much harder than the polysulfidic and disulfidic crosslinks. For the dynamic shear modulus, the large number of accelerators to react with silanol groups on the surface of silica filler in EV system lead to a less strong filler-filler interaction [7, 13].

3.3.2 TBBS accelerator, ZnO activator and elemental sulphur.

The sulphur vulcanising system for silanised silica filled NR consists of an accelerator, N-tert-butyl-2-benzothiazole sulfenamide (TBBS), an activator, zinc oxide (ZnO) and element sulphur, which is contained in the TESPT coupling agent. The melting temperatures for TBBS is at 109oC, and are over 1000oC for ZnO and silanised silica [1]. By adding in the TBBS accelerators, the tetrasulfane groups of TESPT coupling agents can be activated, which leads to an increase in the crosslink density of NR [1]. Except for the basic constituents in the vulcanising system, stearic acid is always used in a combination with the TBBS and ZnO of the filled NR because it can bring an improvement on the solubility of ZnO [ [24] ].

Saeed et al reported [1] that the optimum contents for each constituent are measured by detecting the optimum cure properties in a process which increases the constituent content progressively. For TBBS accelerators, it can be obviously seen that the increasing rate of ∆torque is much slower after 6phr (see Fig. 3.2), thus, the 6phr TBBS loading can be regarded as the most efficiency content for optimizing the chemical bonding between TESPT and the NR chains.

Torque vs.jpg

Fig. 3.2 Torque against content of TBBS accelerators for the silanized silica NR [1].

torque vs ZnO.jpg

Fig. 3.3 Torque against content of ZnO activators for the silanized silica NR (TBBS content: 6phr) [1].

As shown in Fig. 3.3, there is a steep increase in ∆Torque before 0.3phr was added, when the content of ZnO is beyond 0.3phr, the incremental increases in torque becomes slower. Apparently, 0.3phr is the optimum ZnO loading for increasing the efficiency of TBBS and the formation of chemical bond between tetrasulphane groups and NR.

CBS accelerator, ZnO activator and elemental sulphur.

According to A. Ansarifar et al [2], an efficient cure system for a silanized silica-filled NR consisted of N-cyclohexyl-2-benzothiazole sulphonamide (CBS) accelerator, ZnO activator and sulphur (including in TESPT). The process which was used to measure these optimum amounts of CBS and ZnO was similar to the procedure described above. The certain composition of the two cure systems are listed in Table. 3.2.

Table. 3.2 Compositions of the two cure systems.

Constituents 

1st cure system

2nd cure system

Accelerator

Type

TBBS

CBS

Content (phr)

6

4

Activator

Type

ZnO

ZnO

Content (phr)

0.3

1

Elemental sulphur

Type

Tetrasulphane groups in TESPT

Tetrasulphane groups in TESPT

Content (phr)

----

----

Note: All the loadings of accelerators and activators were for the NR rubber with 60 phr silanized silica fillers.

To achieve the optimum cure properties, the accelerator loading decreased with an increase in silica loading, while the activator loading was independent on the silica loading.

Because of the high accelerator content both in the 1st and 2nd system, these two systems were regarded as EV vulcanising systems. Moreover, some small amounts of elemental sulphur were also added to the vulcanising system in order to obtain a higher crosslink density and better mechanical properties. [3, 13, [25] ]

A novel vulcanisation system.

Nano-ZnO activator used in silanized silica filled NR/BR vulcanising systems.

Kim et al [ [26] ] reported that, in an unfilled NR/BR system, only 20%wt of nano-ZnO can have the same capability as the conventional ZnO due to the much higher specific surface area of nano-ZnO. Also, it was aid that the function of nano-ZnO was more remarkable in the silanized silica filled system than the unfilled system. The reason is that the silanized silica helped to improve the dispersion of nano-ZnO in NR/BR blend. By comparison, the cure characteristics of NR/BR filled with nano-ZnO in the range from 0.1 to 3.0phr, it is selected that the composition of this vulcanising system is consisted of 2,2'-dithiobenzothiazole (MBTS) 1.5phr, and tetramethyl thiuram monosulfide (TMTM) 0.5phr accelerators, nano-ZnO activators (1phr), stearic acid (1phr). Sulphur (1phr), TESPT coupling agent (2phr).

Mechanical properties of silica-filled NR cured with sulphur systems.

Mechanical properties.

In general, hardness, abrasion resistance, compression set, tear properties, and tensile properties can be regarded as the evidence for improved rubber mechanical properties [ [27] ].

4.1.1 Mechanical properties influenced by the silanization method.

The cure systems and the mechanical properties of three different types of filler with NR are listed in Table. 4.1.

Table. 4.1 The formulations of cure systems of NR with various reinforcing fillers and their mechanical properties [12].

Compound (phr)

NR

NR

NR

Types of fillers

Carbon black

Carbon black/silica

Silanized silica

Filler loadings

40

45/15

silica 30

Silane coupling agent 6

Zinc oxide

5

5

5

Stearic acid

2

2

2

Accelerator 1

0.75

1.1

0.6

Accelerator 2

--

-- 

-- 

Elemental sulphur

2.5

2

2.5

Total

10.25

10.1

10.1

Tensile strength (MPa)

28

-- 

24

Elongation at break (%)

590

531

1087

Tear strength (kJ/m2)

150 (N)

30

10

Hardness (Shore A)

62

68

55

Abrasion loss (mm3)

124

-- 

-- 

Modulus at 100% strain

-- 

2.52

-- 

As shown in Table. 4.1, it can be seen that the mechanical properties of carbon black filled rubber and carbon black/silica filled rubber are similar, and the mechanical properties of silanized silica filled NR are as good as the other two compounds. It can be concluded that the silanized silica affects the rubber properties positively [2, 12].

4.1.2 Comparison of mechanical properties of NR before and after the silanisation process.

In recent years, a new method has been developed by Ansarifar and co-workers to overcome the problems associated with the in situ silanisation of silica by liquid silanes [3]. It is remarkable that the mechanical properties of silanized silica are improved obviously, which is shown in Table. 4.2. After comparing the values of tensile strength and hardness, they are both higher than the unfilled NR. This is because the sulphur in TESPT forms chemical bonds or bridges between the silica surfaces and NR chains, resulting in a much higher filler-rubber interaction, and also a higher crosslink density [3, 25]. In a similar study, the improvement of mechanical properties of a silanized silica-filled NR was also confirmed by Idrus [25] and Kim [26].

Table. 4.2 Composition of sulphur curing system and mechanical properties of unfilled and silanized silica filled NR [3].

Compound (phr)

NR

NR

NR

Types of fillers

silica 

Silanized silica

Silanized silica

Filler loadings

10 

silica 10

silica 10

 

TESPT 6

Zinc oxide

1

1

1

Stearic acid

2

2

2

CBS

7

7

7

Elemental sulphur

0.2

0.2

0

Tensile strength

18.7

26.9

17.7

Elongation at break (%)

1591

1305

1486

Tear strength (kJ/m2)

19.5

15.2

14.7

Compression set (%)

10.5

10.3

15.3

Hardness (Shore A)

26

34

28

Moreover, it has been shown by Sae-oui [13] that the mechanical properties improved after using silanized silica nanofiller in NR in the CV, semi-EV and EV systems. Apart from the NR, Ansarifar [10, 20, 21], Idrus [25], Yasin [24] showed that using silanized silica nanofiller also improved the mechanical properties of other types of rubber, for instance, the polybutadiene rubber (BR), the styrene butadiene rubber (SBR), and the acrylonitrile butadiene rubber (NBR).

Blooming.

"Blooming is a process of diffusion of chemical additives dissolved in the rubber to the surface, followed by crystallization."[1]

Formation of blooming.

Excessive use of the curing chemicals, which are partially soluble in rubber, cause them to diffuse to the rubber surface and form a thin layer which is called blooming.

Characterisation of blooming.

Generally, the blooming layer can be characterised by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) techniques. FTIR technique can be used to detect the characteristic chemical group of each ingredient of the rubber surface, confirming the presence of the compounding ingredients in the bloom. SEM technique can go through the surface morphology. Saeed [1] reported that the starting point of blooming and the thickness of the blooming layer can be detected by using SEM (see in Fig. 4.1 & Fig. 4.2).

SEM.jpg

Fig. 4.1 Blooming point on the rubber surface, the rubber is stored 60days at ambient temperature [1].

sem.png

Fig. 4.2 Morphology of the freeze fracture surface detected by SEM [1].

4.2.3 Effects of blooming.

According to Saeed [1], the mechanical properties of the rubber vulcanisate including the hardness, tensile strength, elongation at break of rubber were not influenced by the TBBS accelerator migration to the rubber surface. However, the hardness increased since the TBBS bloomed to the surface, the fatigue life decreased due to the existence of the blooming layer on the rubber surface and cracks initiate and propagated more easily in the rubber as result of the blooming of the chemical curatives on the rubber surface. In addition, the blooming layer was harmful to health, safety and environmental.

Aims and objectives of this project

The overall aim of this project is to eliminate the blooming of chemical curatives from the surface of natural rubber.

The objectives are:

1. To mix raw natural rubber with silanised silica nanofiller (precipitated silica pre-treated with TESPT coupling agent) and fully cure the rubber by adding different amounts of accelerator TBBS and ZnO to the silica filled rubber.

2. Store the cured rubbers at ambient temperature for up to 4 weeks and then perform FTIR and ATR on the rubber surface to chemically analyse the composition of the rubber surfaces to determine whether TBBS and ZnO have bloomed to the surface.

3. Measure a loading for TBBS where blooming stops altogether.

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