Effect of Glass Flake

Effect of glass flake on performance of tyre inner-liners


The objective of this project is to investigate the effect of glass flake on the mechanical properties and the tensile crack growth behaviour of the rubber and in particular the effect of surface coating and investigate the breakdown of flake during mixing and the effect of processing on orientation of the flake. Bromobutyl rubber compound filled with glass flake both silane coated and uncoated and a control compound containing no glass flake will be cured and tested on a testing machine, a tensile fatigue machine and SEM.

1. Development and Construction of a Tyre

The tyre is a product which is widely used today. It is ring-shaped and inflated with air to support the car body and protect the wheel by providing a flexible cushion which absorbs shock between the wheel and the ground [1].

1.1. Development of Tyres

The earliest tyres were made of wood which can be told by the wagon of ancient China. Today, nearly all tyres are pneumatic tyres. After the invention of vulcanized rubber in 1844 by Charles Goodyear, the first air-filled tyre was invented in 1888 which was used for bicycles. Michelin introduced pneumatic automobile tyres in 1895 and radial tyres in 1946. In 1947 B. F. Goodrich invented the tubeless tyre.

1.2. Construction of a Tyre

The main construction of a radial tyre for automobiles is illustrated in 1. The tyre is a high performance polymeric composite of many rubbery components including of a tread area which includes the tread cup, base and cushion compounds, and the casing which includes the bead, body ply, steel belt and coat, sidewall, apex, chafer and inner liner [3]. The inner liner is a thin rubber sheet which has low gas permeability to keep high-pressure air inside. The necessary structural strength and dimensional stability is provided by body plies. The sidewall gives the sides of tyre good abrasion resistance and environment resistance [4]. The beads are used to fit the tyre to the wheel.

2. Rubbers for the Tyre Inner Liner

The inner liner is the inner-most layer of the tyre structure for maintaining the internal air pressure. For a tubeless tyre, the inner liner it is designed to prevent the permeation of air out of the tyre and so maintain internal pressure. It will prevent moisture from entering the tyre cavity. It is also formulated to provide flex fatigue resistance, aging resistance and adhesion to the body ply coat compound. The tyre can only achieve optimum handling, traction, rolling and wear resistance when it has the optimum inflation pressure. So the inner liner should be made with a very impermeable rubber compound to minimize the pressure loss for car tyres [5].

The tyre inner liner is composed of a rubber having lower air permeability than the rubber employed in other layers of the tyre. At present, this layer is usually based on butyl rubber or a halobutyl rubber due to their excellent impermeability to air, good flex fatigue properties, weathering resistance, and oxidative stability. The halobutyl (brominated and chlorinated butyl) rubber provides much higher vulcanization rates and improves the compatibility with highly unsaturated elastomers, so that the inner liner can chemically bond to the body of the tyre. Chloro and bromobutyl rubbers have similar values of permeability constant, but bromobutyl rubber is more reactive than chlorobuty rubber. A comparision of advantages of using chlorobuty rubber and bromobutyl rubber is given in Table 1:

Table 1 Comparison of Chlorobutyl and Bromobutyl Rubber Compounds [6]

Chlorobutyl advantages

Bromobutyl advantages

Sorch resistance

Shorter cure induction times

Good impermeability

Good impermeability

Reversion resistance

Faster cure rate

Thermal stability

Adhesion to other substrates

Compounds show rouble-free operations

Broad range of cure system types

2.1. Chemical Composition

Butyl rubber (IIR) is the oldest speciality rubber together with NBR and CR, it is made of 97 to 99.5 mole% isobutylene and 0.5 to 3 mole% isoprene which provides the double bond required for sulphur vulcanization. It is synthesised by cationic copolymerization in methylene chloride solvent with AlCl3 as a catalyst [7]. The chemical structure of IIR and BIIR is shown in 4 below:

2.2. Properties of Butyl Rubber

Properties of bromobutyl rubber and chlorobutyl rubber are similar to the butyl rubber but BIIR has even lower permeability, better weather, heat, chemical and ozone resistance and higher hysteresis than BIIR [7]. CIIR is between BIIR and IIR.

2.2.1. Permeability

According to Massey [8], the most important property of butyl rubbers is the very low gas and liquid permeability, without a high Tg. She explains that at 65℃, the air permeability of SBR is about 80% that of natural rubber while butyl shows only 10% on the same scale. During controlled road tests on cars driven 60 mph for 100 miles per day, it was found that butyl is at least 8 times better than natural rubber in air retention. [9] It also can be got from her book that although the solubility of gases in butyl rubber is similar to that in other hydrocarbon polymers, the diffusion rate through butyl is far lower than in other hydrocarbon polymers. According to Massey, the low permeability is caused by the apparent good packing of the polyisobutylene portion of the butyl molecule and its low fractional free volume.

2.2.2. Physical and Chemical Properties

Butyl rubber is white to light gray, odorless and tasteless, density 0.917 g/cm3, glass transition temperature -67 to -69 ° C, heat capacity 1.95 kJ/kgK. [7]

Butyl rubber has good chemical stability and thermal stability, it is not soluble in ethanol and acetone, has good resistance to animal and vegetable oils, oxygen and ozone, acid and alkali. It also has good cold-hardiness, airtightness, watertight and electrical insulation.

2.2.3. Mechanical Properties and Crack Growth Behaviour

The tire inner liner must provide adequate impermeability, resistance to moisture penetration, adhesion to the casing, and good crack and fatigue resistance [3]. The elastomer failure theory is based on Griffith's energy balance concept, which is a criterion for the advance of a crack through a brittle material perpendicular to the applied tensile stress [10]. The fatigue failure mechanism consists of three phases; in the first phase cracks nucleate; in the second phase cracks grow; in the last phase, the material fails. The cracks nucleate when the molecular structure of the material has defects, local stress concentration or other impurities. The cracks need energy, which is called the tearing energy, to grow. The tearing energy, which means the energy released per unit area of crack surface growth, is provided by either the strain energy in the deformed rubber or work done by the applied forces or both [11].

The tearing energy is given by the equation 1 below:

Where T = tearing energy; W = elastic energy stored in specimen; A = area of one fracture surface of the crack. [12] According to Rivlin and Thomas [13], crack growth happens when the energy release rate is above a critical value, no matter what type of the specimen they used. The critical energy release rate, Tcr, is a fundamental property of the material.

Usually, two types of specimens are used in the study of crack growth experiments which are single edge notched sample and the trouser tear test sample. Rivlin and Thomas [13] gave the following equation (2) for tearing energy of a single edge notched sample ( 5):

Where W = recoverable strain energy density perunit volume; a = crack length; k = strain-dependent parameter.

The equation of trouser specimen ( 6) is given by equation (3):

Where T= the energy release rate, F = the applied force, λ= the extension ratio, t= the specimen thickness, W= strain energy density in the “legs” of the specimen, b= the “leg” width.

3. Filler Reinforcement

Although rubber has numerous outstanding properties, reinforcing fillers are necessarily added into rubber in most cases to gain the appropriate properties for specific applications. Fillers are particulate solids, essentially rigid and non-deformable, they are mainly used to reinforce the rubber, reduce the material costs and improve the processing [9]. Reinforcement is primarily the enhancement of strength and strength-related properties, abrasion resistance, hardness, modulus, as well as resistance to fatigue and cracking [15, 16]. Generally, the mechanical properties of rubber composites can be improved by incorporating fillers [11].

3.1. Mechanism

Reinforcement is a phenomenon that has many forms and applications and generally requires composite of at least two elements [17]. Although fillers have been used for many years, the mechanisms of reinforcement are not completely understood. According to Bokobza [18], a filled network can be regarded as a two-phase-system of rigid particles surrounded by an elastomeric network formed by flexible chains linked together by chemical junction ( 4).

The interaction between filler and rubber is the key factor in the reinforcement. The interactions can be classified into two types, physical interaction and chemical interaction. The physical interaction comes from long-range Van der Waals forces between the filler surface and rubber, it is the most widespread and important interaction. The chemical interaction also very important when physical interaction is weak, coupling agents are required in this case.

Filler morphology, such as particle size, structure, total surface area, can also affect the reinforcement of elastomer. The finer the filler particle size, the larger total area surface area of rubber filler interaction, then the higher strength of rubber compound [11].

3.2. Filler Interaction

Carbon black and silica, which are used as main reinforcing fillers, are discussed in the following two sections.

3.2.1. Carbon Black

Carbon black is composed of almost entirely of carbon with disordered graphite like layers. There are some oxygen functional groups on the surface of carbon black, like alcohol, carbonyl and carboxylic acid group, so it can create strong polar interaction with elastomers. The types of carbon black depend mainly on their specific surface, the ratio of the total outer surface of the filler over its mass, their structure and their surface activity, and these parameters have very significant effect on the reinforcement [19].

In the 1940s, due to the development of the investigation tools and increasing needs of synthetic rubbers in automotive industry, carbon black was proved to be a useful reinforcement [20]. Nowadays, carbon black has been extensively employed to be fillers for numerous rubber products. When carbon black is added into rubbers, the mechanical and physical properties of rubber are enhanced, such as tensile strength, tear strength, modulus, abrasion resistance and durability, owing to the filler to filler and filler to elastomer physical and chemical interactions [21]. The reinforce ability of carbon black in rubbers is mainly decided by the following three parameters: the loading, the surface area and the structure, for example, the tear resistance increases with the increase surface area, decreases with the increase structure and first increase to an optimum then decreases with the increase load.

3.2.2. Silica

Silica is also an important filler material now, which was started to be taken into consideration in the 1980s when the benefit of silane treated surface was found out [20]. Generally speaking, the modulus of a carbon black-reinforced rubber is higher than a silica-reinforced one, whereas, a silica-reinforced rubber has excellent tear strength, abrasion resistance, aging resistance, adhesion properties combined with low rolling resistance [21].

The structure of silica is shown in 7, each silicon atom is connected to 4 oxygen atoms with covalent bond and there are many polar hydroxyl groups on the surface. That means silica has weak non-specific interactions, in other words, silica has very weak interaction with non-polar rubbers.

3.2.3. Silane Coupling Agents

As can be seen from part 3.2.2. that the interaction between silica and non-polar rubbers is weak. The use of silane coupling agents, such as bis(triethoxysilylpropyl)-tetrasulphide (TESPT) is a good way to chemically bond the filler to the rubber. Silane coupling agents, which has functional groups that can react with the silica and rubber, has been used for over 30 years to enhance the performane of the fillers in rubber compounds. TESPT donates sulphur to the rubber compound or plays the role of accelerator during vulcanization to make a crosslink. After the reaction, the surface of silica is covered by a layer of non-polar molecluar chains, hence it become less polar and can interact with non-polar rubbers. By this way, the interaction of rubber and silica is improved and the reinforcement of silica is enhanced. [23] The reaction of the TESPT—attached to silica to hydrocarbon rubber is shown in .

3.3. Glass Flake

Stephen and Mason [24] point out that glass flakes are small platelets of glass with high surface areas due to high aspect ratios and are typically 5μ or 2μ thick, and 100μ to 1000μ in diameter. Different formulations of glass flake, such as E glass and C glass, are used for different environments. For example, E glass is a general purpose glass, whereas C glass is used for additional chemical resistance that is required against acids.

The main purpose of adding glass flake into the rubber is to create a longer, more winding path for gas when it transports through a rubber sheet, as illustrated in 4.

In Stephen and Mason's work, the air permeability results for the inner liner compound are shown in 3 which illustrates that the permeability decreased with increased glass flake concentrations and there was a limiting effect of concentration (~20phr), above which the permeability did not reduce any more. At 20phr, the permeability of the rubber sheet is reduced by approximately 45%, which is significant.It also can be seen from their work that addition of glass flake caused a rapid increase in modulus with a subsequent decrease in tensile strength and elongation to break. Angle tear properties showed that the addition of glass flake tends to increase the tear resistance of the compound, by a small amount (maximum 12%).

3.4. Other Fillers

3.5.1 Clay

Recently, as reinforcing fillers, plate-like particles such as clay minerals have attracted a lot of attention because of their outstanding mechanical properties. These sheet-like platelets possess very high aspect ratio and large surface areas, because they are about 1 nm in thickness and 100–1000 nm in width and length. Moreover, these platelets are easy to align in an ordered manner. Wu et al. [25] reported that rubber–clay nanocomposites would be expected to possess different modulus values depending on different aspect ratios, the moldulus increasing with increasing aspect ratio ( 9, Vf is the volume fraction of filler and φm is the maximum volume fraction of filler). They also said that due to the high aspect ratio and good planar orientation of dispersed thin platelets of the layered silicate, rubber–clay nanocomposites exhibited high hardness, high modulus, high tensile and tear strengths and low gas permeability.

Lately, Stephen et al. [8] R. Stephen, C. Ranganathaiah, S. Varghese, K. Joseph and S. Thomas, Gas transport through nano and micro composites of natural previous termrubbernext term (NR) and their blends with carboxylated styrene butadiene previous termrubbernext term (XSBR) latex membranes, Polymer 47 (2006), pp. 858–870. Article | http://www.sciencedirect.com/scidirimg/icon_pdf.gifPDF (265 K) | View Record in Scopus | Cited By in Scopus (21)[10] reported that the high aspect ratio of layered silicates not only provided reinforcement of nanocomposites but also lowered the gas permeability through the layered silicate reinforced latex membranes, which is meaningful to tyre inner-liner industry.

3.5.2 Mica

Mica is a kind of low-cost mineral available in Brazil. It can easily be cleaved into thin, resilient, highly flexible sheets and possesses excellent heat, electrical, and chemical resistance. The chemical composition of mica is about 86% potassium aluminium silicates with different proportions of magnesium, iron, lithium or fluorine. [26]

Recently, use mica as a filler in rubbers started to be taken into consideration, and it has been proved that in styrene butadiene rubber partial replacement of carbon black by mica does not reduce much of the mechanical properties. Debnath et al. [27] reported that mica can reduce the permeability to gases and liquids in rubbers. Furtado et al. reported from their work that a system made up of 5 phr mica and 40 phr carbon black has better modulus and similar tensile and tear strength and elongation at break than the system with 40 phr of carbon black only. They pointed out that 5 phr mica was sufficient to reduce the fatigue life of these carbon black filled rubber but the reduction disappeared when added larger quantities of mica.


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