Guide Recent Advances in Elastomeric Nanocomposites: 9 (Advanced Structured Materials)

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Samyn, P. Plastics, Rubber and Composites: Macromolecular Engineering, 40 Samyn P. Hassan, Aji P Mathew, E. Hassan, E-W Nahala K. Effect of peroxide treatment on energy consumption of refining and quality ,: International Paperworld. Comparison of two refining methods in production of microfibrillar cellulose: kinetics of refining and energy consumption , International Paperworld.

Mathew, Kristiina Oksman, Art J. Ragauskas, Poly methyl vinyl ether-co-maleic acid —polyethylene glycol nanocomposites crosslinked in-situ with cellulose nanowhiskers , Biomacromolecules ,11, 10, Hassan, K. Oksman Effect of pre-treatment of bagasse pulp on the properties of isolated nanofibers and nanopaper sheets , Wood and fiber science, 42 30 , Kirsi Mikkonen, Aji P.

Mathew , Pirkkalainen K. Jackson-Etang, Ayuk , Aji P. Journal of Applied Polymer Science 5 , , n. Lee Goetz, Aji P. Kristiina Oksman, Aji P. Composites Science and Technology. Aji P. Biomacromolecules, 10, 6, , Benoit Duchemin, Aji P. Mathew, Kristiina Oksman. All-cellulose composites by partial dissolution in the ionic liquid 1-butylmethylimidazolium chloride. Part A, Applied science and manufacturing40, 12, , Dispersion and properties of cellulose nanowhiskers and layered silicates in cellulose acetate butyrate nanocomposites. Mathew, Mohini Sain.

Aji P Mathew, W. Thielsman and Alain Dufresne, Mechanical properties of nanocomposites from sorbitol plasticized starch and tunicin whiskers , J. Cellulose, 13, , Linnea Petersson, Kristiina Oksman, Aji. Mathew, Ayan Chakraborty, Kristiina Oksman and Mohini Sain, The structure and mechanical properties of cellulose nanocomposites prepared by twin screw extrusion. Journal of Applied Polymer Science, 1 , , Mathew, Hima Varghese, S. Polym Sci. Mathew, S. Packirisamy, Ranimol Stephen, Sabu Thomas. Packirisamy, Sabu Thomas.

Packirisamy, and Sabu Thomas, Effect of initiating system, blend ratio and crosslink density on the mechanical properties and failure topography of nano-structured full-interpenetrating polymer networks from natural rubber and polystyrene , Eur. Mathew, G. Groeninckx, H. Radhusch, G. Morphology, mechanical properties and failure topography of semi-interpenetrating polymer networks based on natural rubber and polystyrene , J. Abi S. Aprem, Aji P. Mathew, Kuruvilla Joseph, Sabu Thomas. Sorption and diffusion of acrylonitrile monomer through crosslinked nitrile rubber , J.

Packirisamy, A. Padmanabhan, Sabu Thomas, Kinetics of diffusion if styrene monomer containing divinyl benzene through vulcanized natural rubber , J. Polym Eng. Packirisamy, M. Kumaran, Sabu Thomas. Jonoobi, M. Singapore: World Scientific, Vol. Mathew, A. Postek, M. Herrera M.

Layered Double Hydroxide Polymer Nanocomposites

Postek , M. Kumar ed. Visakh PM, Aji P. Habibi and L. Lucia ed. Oksman Eds, orld Scientific Publishers invited chapter. Self and direct-assembling of bionanomaterails Vol 3. Biobased composite materails, their processing, properties and industrial applications Vol 4. Oral presentation. Aji P Mathew et al, Bio-based membranes and adsorbents for water purification : NanoSelect activities, 4th Dissemenation workshop for the nano4water cluster, April , Stockholm, Sweden Oral presentation.

Hakalahti, M. Surface modification of nanocellulose membranes using thermoresponsive poly N-isopropylacrylamide. Zoheb Karim,Aji P. Zoheb Karim, Aji. Poster presentation. Oral resentation. Naseri N. Zoheb Karim, Water cleaning membranes based on cellulose nanowhiskers: Removal of dyes from contaminated water , 17 April , Nano4water Workshop Dresden. Aji P Mathew, Bio- nanocomposite membranes for water purification: Tailoring the morphology and selectivity for pollutants , Nano4water Workshop, 17 April , Dresden, Germany.

Bibliographic Information

Aji P Mathew, Nanoenabled membranes based on biomaterials for water purification , 2nd Dissemination Workshop of the Nano4water Cluster www. A comparison of two industrial bio-residues , IOP Conf. Jonoobi M, Mathew AP and Oksman K, Study on the energy consumption and cost of cellulose nanofibers produced by mechanical process , Nanoparticles and composites workshop, February , Oulu, Finland. Oksman, Kristiina ; Aji Mathew Composites based on cellulose nanofibers and natural rubber , Aji Mathew, Oksman, Kristiina. Mechanical isolation of cellulose nanofibers and their utilization in novel nanocomposites for medical applications.

Kirsi S. Mikkonen, Madhav P. Mikkonen, Mari I. Yadav, Aji P. The reaction is intentionally stopped by spraying water quenching. The carbon black obtained is filtered and then compressed into pellets of millimetre dimension. During the first split second of the above combustion reaction carbon nodules are formed with dimensions from 20 to nm, depending on the grade of carbon black. On the basis of the proposed definitions these nodules may be considered nano-objects.

However the lifespan of these nodules is very short as they immediately and irreversibly cluster together to form aggregates of sizes between to nm. These aggregates then subsequently combine together to form very solid entities: agglomerates Fig. These agglomerates measure between 1 and 50 microns. The aggregates and agglomerates however, may meet the proposed definition of nanostructured materials.

In aggregates, covalent bonds exist between particles; in agglomerates the bonds between aggregates are electrostatic. Aggregates cannot deaggregate at all. Agglomerates cannot de-agglomerate during standard handling conditions. After the agglomeration reaction, the agglomerates pass through the pelletiser to compact the carbon black into a pellet form of millimetre dimension [24]. Amorphous precipitated silica apart from being used to produce rubber articles is used in many other applications, e. The world production of amorphous precipitated silica is 1.

Silica has been used in the treads of tires for more than twenty years in order to reduce the fuel consumption of vehicles, thus contributing to a reduction in vehicle emissions of greenhouse gases. Amorphous precipitated silica is produced by a two step process: obtaining silicate, and then the production of precipitated silica.

To make the silicate, very pure sand is mixed thoroughly with sodium carbonate. The homogeneous mixture is transferred to a glass oven, heated above C, so that Fig. Then rapid cooling causes solidification and fractionation of the silicate into small granules, a few cubic centimeters in volume. The vitreous silicate is dissolved in water and transferred to a reactor in which, through acidification and under agitation, amorphous silica is precipitated out. During this precipitation, as is the case with carbon black, there is an instantaneous initial formation of elementary particles, from 5 to 40 nm, of a very short lifespan.

These also may be considered nano-objects according to the above definition Fig. These particles, however, immediately and irreversibly cluster to form nondissociable aggregates, from 50 to nm in size, which subsequently bind together to form agglomerates from 1 to 50 microns. The aggregates and agglomerates meet the above definition of nanostructured materials. Aggregates cannot de-aggregate at all. Agglomerates cannot deagglomerate during standard handling conditions [25, 26].

The natural rubber NR forms as a latex, a suspension in water of rubber particles coated with a natural surfactant [27]. The synthetic rubbers SR have been developed for specific purposes, and are made by polymerization of the monomers under pressure and heat in the presence of a catalyst either in suspension, to form latex, or in solution.

The elastomers may be defined as long-chain organics, often polyolefin molecules, consisting of repeating units of one or more monomers, which exhibit very high extensibility combined with an ability to return to the original Fig. Analysis showed that their high extensibility was due to uncoiling of the linear polymer chains from an initially random configuration to one of general alignment. There are several natural limits to both the elastic extension and the permanent set in raw elastomers: i Chain entanglements, which result in stiffening at very high elongations, an effect that increases with molecular weight [28 31].

This level is unrealizable in cross-linked material. Spherulitic crystallinity develops in unstrained polymers e. Crystallinity can also develop at high extension levels as fibers align along the stretching direction, where it increases both stiffness and strength [32 34]. Since elastomers owe their elastic properties to thermal agitation, they are naturally sensitive to temperature variations. In general, their stiffness increases as the temperature is reduced, until a temperature is reached where the material assumes a glass-like consistency, with a corresponding increase in hysteresis, internal viscosity.

The temperature at which this occurs, at the glass transition temperature, Tg, increases with the number and size of the side groups on the polymer chain, for example vinyl groups, benzene rings, chlorine, and the tendency is thus for high glass transition temperature to be associated with high hysteresis and incidentally low air permeability and high friction. Elastomers are usually mixed with several different ingredients to form a compound, the ingredients being chosen to optimize the physical properties of the compound in order to meet the specification for a particular application.

Studies have shown that the process causes sulfur cross-links between two and eight atoms long to form at irregular intervals between chains, by breaking the covalent bonds which are a feature of the polyolefin structure. Efficient vulcanization systems, having short cross-links, tend to have higher modulus and lower extension at break.

The cross-link density can be deduced from swelling tests in solvents, and is usually defined in terms of the molecular weight between crosslinks, M c, typical values being around ,, compared with 78 for the polyisoprene monomer unit, or 26, for a natural rubber molecule. Where the main elastomer chain contains no covalent bonds, it is necessary either to provide such sites by copolymerization with an olefin, or to form cross-links directly between carbon atoms on the chains without using sulfur by radiation or organic peroxides. It was found that some materials, typically sulfenamides or thiazoles, can catalyze cross-linking reactions and are added as accelerators to reduce the reaction time.

In some applications, the reaction rate needs to be reduced and a retarder e. All these additives comprise the curative system. However, it was soon found that very fine powders, particularly carbon black, manufactured by burning oil or natural gas in controlled conditions, could result in a large increase in both strength and modulus, albeit with an increase in hysteresis [37 40].

Reinforcing silicas having similar particle size, but with special surface treatments, have appeared, which appear advantageous in terms of hysteresis. Another means of reducing cost is to add oil to the compound, which may also improve processability, but usually with a small reduction in physical properties. By using highly aromatic oil, it is possible to raise the glass transition temperature, Tg, of the compound, which is found to improve the wet grip of tires, while aliphatic paraffinic oils tend to reduce it.

Soon after the development of cross-linking systems, it was observed that rubber compounds deteriorate with time, an effect that is typically manifest as cracking or crazing of the surface. This is usually due to oxygen or ozone attack on the covalent bonds in the elastomer chains, a process perpetuated by the formation of free radicals. There are a variety of means of reducing the deterioration, e.

In addition to reducing the cracking, these materials may also affect the cross-linking reaction, hence necessitating an increase in the amount of accelerator used. Where rubber is required to adhere to a polymeric or metal substrate e. In some cases, one or more bonding layers such as latex resin mixtures are interposed between the substrate and the rubber, which bond well to both surfaces, while in other cases special compounding ingredients e.

The cord reinforcement acts as the main load bearing component of the composite and enables it to withstand the variety of loads that it is subjected to in its service life while maintaining an acceptable degree of dimensional stability. Cords are made by twisting together a parallel array of fibers to form a singles yarn and then twisting together two or more of these yarns together to form the cord.

Advances in Nanofibre Research

Many different materials can be used as the cord reinforcement, the main requirement being that it is strong in a longitudinal direction and sufficiently flexible to withstand the forces imposed upon it in service. Initially cords were made from. The development of regenerated natural fibers such as rayon and manmade fibers such as nylon, which have a much greater tensile strength, enabled the performance of cord reinforced elastomers to be greatly improved.

In addition to organic polymer materials, some inorganic materials such as glass and steel are also used for cord reinforcement. Rayon is produced from regenerated cellulose. Cellulose is the main structural material of many plants such as trees, grasses, and cotton. The main source of cellulose for rayon production is wood pulp derived from spruce wood.

Nylon was the first commercially successful synthetic fiber, the raw materials being derived from coal or oil. Nylon is a generic name for polyamide fibers composed of linear polymers, the monomer units being linked by amide groups. Polyester is the generic name for polymers with the monomer units joined by ester linkages formed by reacting an acid with an alcohol, the raw materials being derived from petroleum. Aramid fibers are aromatic polyamides formed from an aromatic acid such as terephthalic acid or an aromatic dichloride such as terephthaloyl dichloride and para-phenylene diamine.

The performance of cord reinforced elastomers has improved dramatically, particularly when used in tires, mainly due to the service requirements of car manufacturers. In order to enable these demands to be met there has been continual research and development in polymer chemistry to find new fibers with improved properties.

Two fibers which are new to the marketplace are polyethylene naphthanate and polyolefin ketone. Steel wires of higher tensile strength have been developed; these wires have a slightly higher carbon content and are known as high tensile and superhigh tensile. Steel cords, which are made from a number of steel wires, were first used in radial car tires as the tread stabilizing layer.

In a similar way that steel would not normally be considered as a textile material, glass when extruded as a filament and drawn down fine enough can be processed similarly to a normal textile material. Glass is spun into air from the molten state, it is then rapidly quenched and attenuated to prevent crystallization and coated with size to aid future processing.

Its main properties of interest as tire reinforcement are similar to those of glass but it is lighter and has even higher modulus and strength. Rubber is a widely used material having properties of flexibility, strength and elasticity. The basic raw material, either natural rubber or synthetic rubber, is in latex or solid form, and processed into many different products. The raw rubbers, composed essentially of long polymer chains, are joined together with crosslinks in a process called vulcanization to give the final material its characteristic properties.

Natural rubber hardens below 0 C and softens and weakens above 80 C, losing its strength and becoming tacky. In between these temperatures it can flow under stress and permanent deformation occurs under prolonged strain. These undesirable properties are reduced by vulcanization, in which the reactivity the double bonds impart to the molecule is utilized to make it react with added material to form crosslinks between the chains. The crosslinking increases the useful temperature range of the rubber and hardens the rubber so that it becomes much stronger and does not creep but returns to its original shape on release of.

Its surface properties are improved and its solubility decreased. Sulphur is still the most important vulcanizing compound for natural rubber, but not the only one. The raw rubbers include butadiene rubber, butyl rubber isobuteneisoprene copolymer , synthetic isoprene rubber, ethylene-propylene rubbers, chloroprene rubber the methyl group of isoprene is replaced by chlorine , nitrile rubbers acrylonitrile butadiene copolymer , styrene-butadiene rubber, and silicone rubbers which are polysiloxanes.

Compounds added to the raw latex must be in the form of emulsions or dispersions. They are prepared by milling the substances with distilled or softened water in ball or gravel mills which revolve for anything from a few hours up to several days. Gelatin, casein, glues, soaps etc. Substances added are softeners, fillers, pigments for color, the vulcanizing agents and antioxidants. The latex itself has to be stabilized with surface-active agents to prevent coagulation which can be irreversible.

These agents act by imparting a charge to the surface of the minute rubber particles or by holding an envelope of water around the particle, thereby preventing any mixing. The compounding materials and the latex are mixed and are then ready for dipping, molding, foaming or spreading.

The production sequence is mixing, forming and vulcanizing. The solid rubber and the other materials have to be mixed. This is done with two basic machines, a two-roll mill in which the material is passed between two heavy metal rollers mounted horizontally, and a Banbury mixer, an internal mixer in which the materials are sheared between the internal rollers and the inside of the casing. There are four basic methods of forming the material to the required shape; spreading onto fabric from solution, extruding, calendering and molding.

Sulphur vulcanization, the most common form, involves the formation of polysulphide crosslinks between the chains. If sulphur only is used, curing times of 8 h are necessary. Modern methods using activators and accelerators reduce the curing time, eliminate cyclic structures and can shorten the sulphur links down to one or two sulphur atoms. Shorter sulphur links give greater stability to heat and improve the ageing properties. Fillers range from inert dilutants such as whiting, talc, clays, CaCO 3, etc.

Carbon black is a most important reinforcing agent in tires and tubes and is usually produced by burning oil or natural gas in a limited supply of oxygen. Protective agents prevent ageing or deterioration, which comes about mainly by oxidation. Softeners and lubricants, in the form of Stearic acid, waxes, mineral oils, tars, etc. Certain resins increase the tackiness of the rubber for use on adhesion tapes.

Elastomers are polymers that will elongate when subjected to a tensile force. They will return to the original shape when the force is removed. Rubber is an elastomer. Natural rubber is composed of isoprene units. Isoprene is polymerized into polyisoprene. Natural rubber is soft and tacky when hot. Reacting it with sulfur cross-links the polyisoprene and makes the rubber harder. This process is known as vulcanization.

Synthetic rubber is similar to. One example is polybutadiene. Neoprene is very similar to polybutadiene, but contains chlorine instead of one CH-group. It is more resistant to solvents like oil and gasoline. Another synthetic rubber is styrene-butadiene rubber SBR. It is tougher and more resistant to oxidation than natural rubber, but its mechanical properties are less satisfactory [41 46].

Addition of filler increases hardness of the cured product. All fillers are not created equal, so that there is a range of reinforcement from very high to very low, corresponding to the primary size of the filler particle, from around 10 nm for very fine particle carbon blacks giving high reinforcement, to greater than nm for some calcium carbonate which give low reinforcement. Use of the latter reduces compound cost. The shape and surface chemistry of the filler particle also play an important part in reinforcement.

Some popular fillers are, in order of decreasing reinforcement, carbon blacks and silicas, clays and then whitings. Carbon black is a material of major significance to the rubber industry, so it is no surprise that most rubber products we see in the market place are black in color. There are two common methods of producing carbon black today. Heating natural gas in a silica brick furnace to form hydrogen and carbon produces a moderately reinforcing material called thermal black. Alternatively, if we incompletely burn heavy petroleum fractions, then furnace blacks are produced.

These are the most important blacks in terms of quantity used and available types. Carbon black consists of extremely small particles from around 10 to nm. This gives two primary properties allowing a whole range of grades designated by both a particular particle size surface area and a specific level of structure [47 50].

Results indicate that the scorch and cure times decrease with increasing rubberwood loading. Tensile modulus and hardness of the composites increase with increasing rubberwood loading whereas tensile strength and tear strength show a decrease. SEM studies and rubber-rubberwood adhesion measurements indicate that the increasing rubber wood loading has weakened the rubber-rubber wood interactions [51]. Properties of natural rubber NR filled with various loadings of ultra-fine vulcanized acrylate rubber powder ACMP have been investigated.

ACMP loading was varied from 0 to 20 phr and, after compounding, the compound properties have been determined. Results reveal that increasing ACMP loading leads to improved processability, as evidenced by the reduction of both mixing energy and Mooney viscosity. ACMP, however, has negative effect on cure, that is, both scorch time and optimum cure time are prolonged while the state of cure is reduced with increasing ACMP loading. Due to the reinforcing effect of the fine ACMP particles, both modulus and hardness are found to increase.

The tensile strength is also found to improve with increasing ACMP loading up to 10 phr. However, due to the cure retardation effect and the high thermoplastic nature, the presence of ACMP causes deterioration of elasticity. Significant improvement of thermal aging resistance is found when 10 phr or more of ACMP is added [52]. The puncture and burst properties of short-fiber reinforced polychloroprene rubber under various conditions has been investigated to yield the best mechanical properties.

Certain interphase conditions and higher fiber aspect rations have shown to provide higher puncture and burst stresses at a given fiber content. Since both testing methods measure biaxial properties of reinforced rubber, the relation between the two properties have studied. The discrepancy between regressed puncture and burst force is explained based upon the rubber stiffness due to reinforcing parameters and the stress concentration upon sharp edge.

Overall, it was found that the interphase condition, fiber aspect ratio, and fiber content have an important effect upon puncture and burst properties [53]. Mica or carbon black was used as filler in composites of acrylic rubber. The fillers differ not only in nature, mica being a mineral material and carbon black being organic, but also in form and particle size. The content of filler varied from 0 to 50 phr and its influence on acrylic rubber was evaluated based on cure parameters, mechanical, and swelling properties. The cure parameters allow the conclusion that the presence of mica does not have a negative effect on the cure or processability; the swelling results indicated a weak interaction between acrylic rubber and mica even though the mechanical properties of acrylic rubber composition with 40 phr of mica were found to be similar to those of 20 phr carbon black.

As a result of mica being less expensive than carbon black, is light colored and easily processible, these properties are of industrial importance. All properties analyzed have been compared with gum type composition without filler [54]. A networked silica prepared by interconnecting silica particles with polymeric methylene diphenyl diisocyanate has developed for use as a highly effective reinforcing material for rubber compounds without the need to add silane coupling agents.

Methylene diphenyl diisocyanate incorporated onto the silica surface formed networks among neighboring silica particles with urethane linkages and produced networked silica at low cost. The TEM photographs illustrated the improved dispersion and formation of openings among the silica particles, which could allow easy intrusion of rubber molecules.

The networked silica has showed a high reinforcing performance for styrene-butadiene rubber SBR compounds, suggesting the possibility of replacing the silica reinforcing systems with coupling agents. Due to the absence of any silane-containing coupling agents, the networked silica does not suffer the disadvantages associated with coupling agents. Since the networked silica reinforces rubber compounds by the physical entanglement. Different elastomer-based composites for microwave absorbers are developed.

The influence of chemical character and structure of the polymer matrix and chemical nature and concentration of fillers with high values of the imaginary part of the complex dielectric permittivity and magnetic permeability on the microwave properties of the absorbers is investigated. Natural rubber, styrene butadiene rubber, butadiene rubber, chloroprene rubber, nitrile butadiene rubber have tested as polymer matrix. Graphite, furnace carbon black, acetylene carbon black and active carbon are used as fillers with high dielectric losses; natural magnetite is used as filler with high magnetic losses in the experiments.

The production of hydrosilylated impact polypropylene PP copolymer has been demonstrated in a co-rotating twin-screw extruder. This has been accomplished through a two-step reactive extrusion process that involves: i the formation of terminal double bonds on a commodity polypropylene copolymer through peroxide initiated degradation reactions, and ii the melt-phase functionalization of these double bonds with hydride-terminated polydimethylsiloxane PDMS.

Spectroscopic analysis of the peroxide degraded polypropylene and the purified hydrosilylated polypropylene has been performed to confirm the attachment of PDMS onto the polypropylene chains. In addition, the hydrosilylated polypropylene has been characterized in terms of molecular, rheological, surface, and mechanical properties. Finally, oscillatory shear measurements exhibit an unusual up-turn in the viscosity curve at low frequencies [57]. A theory for incompressible, rubber-like shells of arbitrary geometry undergoing finite rotations and finite strains, including transverse normal strains but neglecting transverse shear strains, is presented [58].

The effect of fillers on the mechanical properties such as stress strain behavior, tensile strength, percentage strain at-break, Young s modulus and tear strength has been investigated. The reinforcement ability of the filler was increased in the order of silica [ HAF-black [ clay [ TiO 2. Filled blends showed improved mechanical properties such as enhanced of strain at-break, when fillers are incorporated. The initial trend of properties for all filled system is the enhancement of properties. When HAF-black is used as the filler, at higher loading strain at- break is found to decrease due to the stiffness of the matrix.

In the case of clay, there is a deterioration of properties occurs on higher loading, which is attributed to dilution effect and all TiO 2 filled system have lower elongation at break than the base polymer. Theoretical models namely Guith s and Kerner s model have been compared with the experimental values of Young s modulus of filled system. The experimental values of modulus are found to be higher than the theoretical values indicating strong interaction between the filler and the matrix.

SEM studies of the tensile and tear fractured surfaces of the filled blends have been carried out. The variation in properties was correlated with the morphology of the system [59]. The use of the sol gel process on general-purpose grade rubbers has reviewed in the absence or presence of silane coupling agents. The sol gel reactions of tetraethoxysilane in epoxidized natural rubber ENR , styrene-butadiene rubber SBR or butadiene rubber BR vulcanizates produced silica generated in situ.

This silica was found to be a good reinforcing agent by investigating tensile and dynamic mechanical properties and morphology observation by transmission electron microscopy TEM. The amount of silica formed was limited by the degree of swelling of the rubber vulcanizate by of tetraethoxysilane which was the precursor of the silica. However, the dispersion of silica generated in situ was better than conventionally added silica due to its formation in place.

Also, it was noted that the diameter distribution of in situ silica was monodispersed. Silane coupling agents, such as mercaptosilane, aminosilane, and bis 3-triethoxysilylpropyl tetrasulfide, were compounded in the vulcanizates and their effects on silica generated in situ were evaluated.

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Their effects were significant. The dispersion of the silica in the rubbery matrix became better and the particle size became smaller and monodispersed, as observed by transmission electron microscopy, which improved mechanical properties. The superior properties of silica generated in situ have studied further to elucidate the mechanism of reinforcement [60].

Fatigue life prediction and evaluation are the key technologies to assure the safety and reliability of automotive rubber components. Fatigue life prediction methodology of vulcanized natural rubber was proposed by incorporating the finite element analysis and fatigue damage parameter determined from fatigue test [61]. Heataging effects on the fatigue life prediction of natural rubber were experimentally investigated. Fatigue test were performed using the 3-dimensional dumbbell specimen, which were aged different amounts.

The Green Lagrange strain at the critical location determined from the finite element method used for evaluating the fatigue damage parameter. Fatigue life prediction equation effectively represented by a single function using the Green Lagrange strain. According to fatigue life prediction equation, fatigue life ambient temperature was longer than at 70 C. Predicted fatigue lives of the rubber component were in fairly good agreements with the experimental fatigue lives within factor of two [61].

The composites with low-ao content have showed the incomplete miscibility, and a part of AO- 80 dissolved into HNBR, while the rest was dispersed into HNBR in the form of deep-submicron-sized microspheres based on the micromorphological and thermal analyses. However, with the increasing AO content, the system became completely miscible.

When blending with AO, the resulting composite exhibited the remarkably improved dynamic mechanical property. The loss tangent peak of the composites gradually shifted to room temperature with the increasing AO content [63]. The successive operations for natural rubber being [64 71]: i Tapping the rubber tree to collect the latex in cups, to be followed by screening to remove contaminants.

A few minutes mixing are typically required to achieve good dispersion of the filler, which may be accomplished in two or three stages to avoid overheating and the risk of scorch premature cure. The cross-linking ingredients are naturally always added last. In normal rubber manufacture a slug of compound is usually placed in a mold of the required shape, and loaded into a press, which applies external load to prevent the mold opening or internal porosity developing in the rubber.

Narrow vents spew holes are provided to allow the slight excess of rubber to escape. Textile and glass cords are usually processed using the traditional spinning and weaving route to form a fabric prior to calendering but steel cords are usually fed from a creel situated in front of the calendar. After spinning the bundle of continuous filaments, these are brought together to form the yarn. This is then given a spin finish and a small amount of twist to aid processing and wound onto a cylindrical shaped center to form a cheese.

The starting material of a steel cord is a hot-rolled wire rod, usually 5. The main production process consists of a series of drawing and heattreatment stages during which the steel rod is reduced in diameter to give a flexible wire suitable for use as a textile material, and also having the desired crystalline structure to give a high-modulus, high-tensile wire. After the final wire drawing stage, several wires are brought together and twisted to form a strand, and for heavier cords, such as those used in truck tires, several of these strands are twisted together to form a cabled cord or further layers of wires are wrapped around a core strand to form a layered cord.

There are several ways of making this fabric, the most common by far being calendering, but spreading and extruding methods can also be used. The process for doing this with steel cords is called the Steelastic process, although a similar process can be used to produce textile cord fabric. The benefit of this technique is that it requires much less space and resources to set up, and produces much less waste material. In the rubber industry, for the design and production of a commercial material that satisfies precise technical requirements, it is necessary to: a Select the right raw materials; b Blend them in the appropriate proportions in suitable equipment; c Form the resultant blend into the desired shape; and d Render the finished product dimensionally stable.

The first stage, known as compounding, refers to the formulation of a blend of rubber and various additives. These are first thoroughly mixed and then formed. All the operations inherent to the blending of the various ingredients and their forming constitute the processing. The physical properties of an unvulcanized elastomer do not allow a manufactured item to retain its dimensional and mechanical characteristics over time; therefore, it is necessary to generate a stable.

Finally, it should be recalled that the definition of the physical and chemical properties of elastomers has been the subject of standardization carried out by the American Society for Testing and Materials ASTM. The standard methods of measurement are continually updated and adapted to new instrumentation and needs, verifying their reliability through cross-tests between different laboratories. Once the characteristics of the finished product have been established, the basic elastomer and the package of ingredients necessary to obtain the properties required are selected.

The recipe of the blend consists of a list of the various ingredients, and instructions on how to mix them to prepare the blend. The unit of measurement used for the quantities by weight of the various constituents of the recipe is phr per hundred rubber , which indicates the quantity of additive required per parts of rubber.

This is the main ingredient and can consist of natural or synthetic rubber, or thermoplastic elastomers. These are the substances needed to generate the three-dimensional network that gives the rubber its typical characteristics except in the case of thermoplastic elastomers that do not need to be vulcanized ; a typical vulcanizer is sulphur, used in quantities in the range of phr.

They are the substances that interact with the vulcanizer to reduce the time of vulcanization; they are used in quantities in the range of phr. Activators are made up of metal oxides such as zinc, lead and magnesium , of carbonates and of alkaline hydroxides; they are added in quantities of 2 3 phr, form chemical compounds with the accelerators and modify the speed of vulcanization and the number of links between the different macromolecules density of cross-linking unlike diamonds and graphite, is not found in nature.

It is the generic term used to indicate a family of materials made up of elementary carbon in the form of aggregated spheroidal particles, obtained by the thermal decomposition of hydrocarbons in a shortage of air. Vulcanization retardants. These are substances that interact with the vulcanizeraccelerator-activator system, creating a period of time during which vulcanization does not take place. They thus ensure the completion of the various transformation operations thereby avoiding pre-vulcanization; they are used in quantities in the range of phr.

Organic acids. Their reaction with the activators provides the cations necessary for the formation of chemical complexes with the accelerators. Monobasic acids with high molecular weights, such as stearic, oleic, lauric, palmitic, and myristic acids, and hydrogenated palm, castor and linseed oils are used in quantities of 1 3 phr. Added in quantities of 1 2 phr, they protect the rubber from oxidation, accelerated by light and ozone, which generally causes a structural modification to a lesser or greater degree in the polymer chain with consequent variations to its mechanical properties.

This category includes secondary amines, which exhibit a high tendency to coloring, phenols hindered by t-butyl groups in the ortho-position primary antioxidants, they reduce the peroxide radicals and organic phosphites secondary antioxidants, they reduce the hydroperoxides. The primary and secondary antioxidants are used in synergic combinations. Initially they were added to rubber in the form of small particles for economic reasons.

The addition of carbon black to natural rubber had a reinforcing effect, improving some characteristics such as resistance to abrasion and teat, and increasing the values of the elastic modulus and of the tensile strength. Fillers are subdivided into two groups: the first includes reinforcing fillers and the second inert fillers.

The inert fillers kaolin, barytes, carbonates of calcium and of magnesium, of iron and of lead , powdered to dimensions of 0. The reinforcing fillers carbon black and silicas , instead, have a major effect on the mechanical and dynamic characteristics of the vulcanizate in that, by interacting with the macromolecules, they take part in the elastic network. Carbon black is an allotropic form of carbon that, unlike diamonds and graphite, is not found in nature.

These are materials capable of improving processability, of reducing the hardness of the vulcanizates and increasing their elasticity and cold. They elong to two main classes: extender oils, derived from the petroleum refining industry, which are suitable for diene-based rubbers SBR, NR and BR , and esters, which are recommended for polar rubbers such as NBR rubbers. Processing auxiliaries. These are additives different sorts introduced to facilitate the incorporation of other ingredients peptizers, adhesion promoters, dispersants , to regulate the process lubricants , or to facilitate the separation of the vulcanized article from the mould release agents.

Among the peptizers of natural rubber is thio-bnaphthol, which is used in quantities of 0. Various additives. They are substances of various types that are added in varying quantities and proportions flame retardants aluminium hydroxide, antimony oxide, zinc borate , antistatics metal powders or fibres, carbon black , colorants metal oxides and substances that increase adhesion to metals during the molding or extrusion phase resorcinol formaldehyde resins and derivatives of isocyanates.

The mixing of solid particles with highly viscous material is a very complex operation, which can be split into three successive stages: incorporation, dispersion and distribution. During the incorporation stage, starting with the separate ingredients of the compound, a homogeneous mass capable of flowing is obtained. Within this stage there can be three further distinct phases: encapsulation, during which the free surface of the elastomer wraps around the fillers, subdivision, during which the reciprocal distances and dimensions of the encapsulated fillers are reduced, deformed in shear or elongation; and, finally, the immobilization of a considerable fraction of the rubber inside the voids contained in the filler aggregates, with the important consequence that shielded polymer plays no part in the flow behavior.

If the compound is subjected to moderate strain, the whole filler agglomerate and the associated entrapped rubber behave as if they were a single unit of filler. Since, when a polymer is mixed with a rigid additive, the viscosity of the compound increases with the volumetric fraction of the additive and its elastic memory diminishes, a blend with a less than optimal dispersion possesses an everincreasing viscosity and an ever-decreasing post-extrusion swelling as compared to a material in which the filler has been effectively dispersed.

The use of plasticizers and oils, which produce greater molecular mobility, allows the polymer to quickly exit the voids of the reinforcing filler aggregates, thereby diminishing the proportion of entrapped rubber, reducing the viscosity of the mass and favoring the subsequent phase of dispersion. A suitable parameter for describing the effectiveness of mixing is the amount of power applied, usually correlated linearly with the rate of dispersion. During mixing, the temperature rises notably and the viscosity decreases along with the amount of applied power; the mixing time should therefore be increased, but not beyond certain limits in order to avoid early vulcanization.

The time interval, measured from the moment that the compound containing vulcanizing agents is heated to the moment at which the reaction of cross-linking starts, is called the scorch time. This time can be modified by using retardants as well as by the choice of vulcanizer and accelerator. From the technological point of view, while for thermoplastic materials broad use is made of continuous mixing, for the processing of rubber, batch mixers are mainly utilized, consisting typically of open mixers two roll mills and of internal mixers.

The reasons for this derive from the lack of availability of rubber in free-flowing granules at a cost comparable to that of rubber in bales, from the difficulty of accurately feeding a large number of ingredients and from the impossibility of adapting continuous mixers to the processing of different types of rubber and formulations without heavy changes. In an open mixer, there are three zones: one is located between the high powered internally cooled rollers a ; another, the bench b , acts as a reservoir to feed the region between the rollers where the process of encapsulation takes place; the third, the belt c , carries the rubber from zone a to zone b.

The rollers rotate in opposite directions at different speeds with a ratio that varies between 1 and 1. Due to its versatility the ability to mix a great variety of elastomers with the same setting , and due to its ability to accept rubber in bales, the internal mixer is the machine most used in the rubber industry. Figure 4 contains a diagram of the two counter-rotating rotors, of the piston, which allows for the introduction of the various ingredients of the blend into the mixing chamber when lifted, but which keeps the rubber in the mixing area when lowered, and of the discharge door.

Cooling water passes through the rollers, the walls of the chamber and the discharge door. Observing the mixing of rubber in a roll mill makes it possible to identify two limiting situations: a dry behavior, which shows a critical factor relating to the shearing of the elastomers the rubber breaks up ; and a cheesy behavior, typical of materials characterized by poor elastic behavior and poor tensile properties, mainly related to the absence of high molecular weight fractions.

Aside from these two limiting cases, the behavior considered to be good is that characterized by the formation of a continuous belt between the rolls. The scale of visual evaluation is associated to a behavior that can be related to the. The principle stems from the theory of the elasticity of rubber at high deformation, and is based on the calculation of the elastic recoverable and viscous non-recoverable contributions of the polymer used in the blend.

A mill consists of two horizontally placed hollow metal cylinders rotating towards each other Fig. The distance between the cylinders mill rolls can be varied, typically between 0. This gap between the rolls is called a nip. Raw gum elastomer is placed into the gap between the two mill rolls, the mill nip. It then bands, as a continuous sheet, onto one of the rolls. The speeds of the two rolls are often different, the back roll rotating faster than the front.

The difference in speed between the two rolls is called the friction ratio and allows a shearing action friction at the nip to disperse the ingredients and to force the compound to stay on one roll, preferably the front one. Mill processing The following description relates primarily to compounds which use sulfur as the crosslinking agent. The key to mixing in a Banbury mixer or a mill Fig. Both mechanisms have been hypothesized and one typical mixing sequence might be as follows: The raw gum elastomer is placed into the nip and allowed to band onto the front roll Internal mixing machines If the rolls of a mill are twisted to produce a corkscrew effect and then a block of steel is placed over the mill nip with the block connected to a steel rod above it, this would be called a ram.

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The ram would move up, to allow addition of ingredients to the nip, and it would move down to force the compound ingredients into the nip. If the whole thing is surrounded in a heavy metal jacket with a chute at the Fig. Raw gum elastomer is dropped through the hopper into the mixing chamber where it is mixed by the rotors.

The ram, pressing on to the rubber mixture, is forced down by a pneumatically or hydraulically controlled cylinder, whose pressure is adjusted to give the best control of the mixing process. Oil may be poured in from the hopper, or injected through a valve in the hopper wall just above the mixing chamber. Mixing can occur between the rotors intermeshing rotors or between the mixing chamber walls and the rotors.

Processing The mixing process will be discussed primarily with reference to unsaturated elastomers which are sulfur cured, unless otherwise stated. The mixing principles are similar to those for the mill. The raw gum elastomer is dropped into the hopper and the ram allowed to move down under pressure; the ram is raised for each addition of material and then lowered, to compact the mixture in the mixing chamber.

Fillers are then added; large total amounts can be added incrementally and after most of the filler has been mixed in, any oil in the formulation may be then be introduced. If oil addition is delayed too long, the filler becomes totally encapsulated by the elastomer and, the oil addition can cause a loss of shearing action, resulting in a slippery mess in the mixing chamber.

During the mixing operation, feedback is received from the electrical power usage indicator, the temperature gauge, the time clock, and, for experienced operators, the sucking sound of the batch and the sound of the electrical motor driving the mixer. Extruders Extruders Fig. The back end has a hopper, sometimes with feed rollers, to put rubber into the screw, and the front end has a head to hold a die, through which the rubber extrudes.

An alternative to the screw extruder is the ram extruder, a well known trade name being Barwell. The ram extruder predates the screw extruder, but it is still used in certain specialized applications. Here, the screw is replaced by a ram, which forces the material through the die. Since the process is discontinuous, a slug of rubber is placed in the barrel, extruded, then another slug introduced, it is suited to making preforms for further use, such as placing into the cavities of molds.

Die swell The die is designed to avoid sudden discontinuities, as the compound moves through it and thus often has a contoured lead section. As the extrusion exits the die, the extrusion can shorten in length and increase in cross section. This is known as die swell, which is dependent on die design, screw speed, i. In practice, die swell can be quite complex and it might be necessary to modify the die a number of times, before the required extrusion shape is achieved. This recognizes that even uncured rubber has complex elastic and plastic behavior.

Like an elastic band it can undergo elastic recovery on exiting the die. Recent extruder design A problem with traditional extruders is the potential for reduced interblending of material as it moves along the screw. This causes uneven temperature distribution in the extrudate, which translates to a variable viscosity and therefore a continuously changing die swell. Layers of compound move along without intermingling, i. Calendars A calendar can be crudely thought of as a very high precision mill, with the rolls stacked on top of one another, with anything from two to four rolls in various configurations.

The distance between the rolls can be varied to control calendared thickness. As with the extruder, the calendar Fig. Calendering is a useful technique, if the final product is to be a roof or tank lining, fabricated hose, expansion joint or indeed any further product which needs accurately dimensioned continuous sheet. Calendering is also used for applying rubber compound to textiles. Sheet from a mill will have a thickness which is much too imprecise, can have a rough surface and may contain some trapped bubbles of air.

A three roll calendar is very popular, where the middle roll is fixed, while the ones above and below it can be moved vertically to adjust the gap between the rolls. A four roll S configuration might be considered more state of the art. Calendars are extremely robust and solidly built machines, and may provide service for many decades. Some of the rolls can be a substantial size, i.

Material thickness control The compound in the nip can generate very large reaction forces pushing against the rolls. Since the middle of. Even this apparently small variation can result in significant material wastage and complications in further processing. To counteract this, an opposing curve called a crown is put into the roll by grinding it. This only goes some way to solving the problem, since deflection forces vary with the compound used, and the thickness of the rubber to be calendared. Feeding the calendar If cold, at room temperature, compound were to be fed to the nip, it would heat up erratically and produce a variable viscosity.

This would cause uncontrolled deflection forces on the rolls and hence an unacceptable thickness variation and surface quality of the rubber. The simplest way to feed the calendar, is to roll small pieces of compound off a mill and immediately put them in the nip. The pieces then spread along the length of the nip and form a rolling bank. Curing equipment The rolls of sheeting have been calendared, the extrusions have been made, the Barwell has produced its preformed pieces, and shapes have been cut from milled sheet.

The final step is to provide sufficient heat to change the uncured compound from a somewhat plastic state, to a dimensionally stable elastic substance, and additionally, in the case of molding, to achieve a final shape. This would be a very basic mold. Molding is by far the most important. Mold design A basic compression mold design is illustrated in Fig. It is very important that the two halves of the mold register fit accurately together. In this case, pins built into the top section fit snugly into holes drilled into the bottom half.

Any looseness between the pin and the hole may cause the top half of the product to be out of alignment with the bottom half. If the fit is too tight, attempts to manually open the mold may prove difficult. There are different ways of introducing compound into the mold, some of which involve modifications to the basic design in Fig. They each confer certain advantages not found in the others. Thus the mold can stay closed while rubber compound is introduced through these holes into the cavity by using the force exerted by the press platen.

Compression molding This is the simplest, cheapest, and probably the most widespread of the three basic molding techniques. It is ideally suited to small quantity production, say, from around fifty to a few thousand of each product annually. Figures 10, 11, One of the keys to successful molding is adequate removal of air while the mold cavity is filling up with rubber. The uncured pieces of compound placed in the mold are known-variously as preforms billets or load weights. For a ball, one might use an elliptically shaped extrusion, cut to an appropriate length from a Barwell.

This shape is important and deliberately chosen so that air in the mold cavity will have a free path of escape when the mold begins to close. Often, residual air remains and various methods have been devised to remove it. One method is to bring the mold pressure back down to zero and then return to full pressure by quickly lowering and raising. This shock treatment is called bumping.

An additional line of attack is to find where air is being trapped in the final cured product and drill a small diameter hole through the mold cavity in the equivalent area; these are called bleeder holes. They permit an alternative escape route for the trapped air together with some rubber. The shape of the preform and also its placement in the mold is important. The uncured rubber, placed in the cavity, might be a single piece or a number of pieces. Backrind Once the compression mold has closed, the compound continues to heat up and attempts to thermally expand.

Its coefficient of expansion can be a least fifteen times greater. Heat transfer How long should it take to cure a compound in a mold? The rubber laboratory uses a rheometer to help determine these using small samples of compound. The rheometer might indicate a cure time to be 25 min at C. If this is then applied to a shop floor molding, it must be remembered that the 25 min is based on the entire compound in the rheometer being at C at approximately the beginning of the 25 min period. Rubber can be very poor at transferring heat, so that for a large bulky part in a shop floor compression mold, it may take hours for heat to be transferred from the mold to the center of the part.

The rheometer estimate of 25 min must now be revised, to take into account a constantly changing temperature throughout the part as the cure progresses. Carbon black is significantly better at transferring heat than a raw gum elastomer, thus for the same bulky part, a carbon black filled compound will vulcanize, through to its center, much sooner than a non-filled gum compound. Transfer molding If we take the top half of a compression mold, then drill transfer holes through it and place a metal collar on the closed mold so as to surround all of the holes, we have in effect converted it into a transfer mold.

All that is needed now is to put rubber compound into the pot and force it through the holes by placing a piston plunger into the pot and using the press platens to force the piston to push the compound down through the pot into the closed mold cavity. This conversion is used in the rubber industry. Alternatively, the transfer pot can be designed to be an integral part of the mold and the piston can be fixed to the upper press platen see Figs.

Injection molding Injection mold consists of a cylinder injection barrel with a ram or screw inside it, so that the rubber compound can be moved towards a nozzle at its end. The nozzle is then pressed against a hole made in the top half of a closed mold. This hole is then connected to smaller holes gates and runners which enter the cavities of the mold. The compound can be presented to the barrel as a continuous.

A ram has a tighter fit in the barrel than a screw and therefore there is less leakage backwards through the barrel; it is also cheaper than a screw. The screw mixes the compound as it moves towards the nozzle, creating more frictional heat and therefore higher temperatures which translate to easier flow and shorter cure times.

A combination of ram and screw is popular. Autoclave curing An autoclave is a cylindrical steel pressure vessel, used to cure extrusions, sheeting and all manner of hand fabricated parts. They can be very small or huge, for example, 30 m long and 3 m or more in diameter. The heat needed to cure is commonly provided by wet steam, often at 0. Since rubber can be considered a thixotropic non-newtonian fluid, the shear between it and the walls of the transfer holes reduces its viscosity, thus allowing the compound to enter the mold cavity more easily. Shear also heats the compound which reduces viscosity and speeds up cure processes might need pressures of 0.

In special cases carbon dioxide or nitrogen might be used, either separately or in combination with wet steam, to provide higher pressures than the wet steam alone could produce at a given temperature. Choosing C as the desired cure temperature would only generate 0. In cases where porosity in the product is a problem it might be advantageous to independently increase the pressure in the autoclave.

If enough nitrogen is introduced to give an extra 0. The preparation of the blend is usually followed by a phase of molding and, subsequently, of vulcanization. The two phases can be simultaneous as in the case of compression molding, which consists in putting a part of the appropriately shaped blend into a mould. The closing of the mould and application of pressure bring about the adaptation of the rubber to the mould, while the elimination of excess material takes place through appropriate holes.

Pneumatic tires are shaped and vulcanized by a similar technique. A technology for the shaping of thermoplastic elastomers, but also for vulcanizable elastomers, is that of injection molding, typical of the plastic industry, but adapted also for elastomers; in this case, the molding and vulcanization phases are also usually simultaneous. Instead, in the case of shaping by extrusion, which is used for the preparation of pipes or tapes, the vulcanization phase is separate and is effected continuously in high temperature baths C , in hot air tunnels or by means of continuous hot pressing systems heated cylinders.

The introduction of bonds between the macromolecules leads to important physical variations of the elastomer, which changes from a fluid soluble in solvents to an insoluble elastic solid characterized by mechanical properties that are technologically useful. The properties of the vulcanized elastomer depend on the number and type of the links that connect the molecular chains. The number and type of the links are, in turn, a function of the degree of advancement of the vulcanization and of the type of accelerator.

As the density of the network increases, there is an increase in the static and the dynamic elastic moduli, at high frequency, and in hardness, while the permanent deformation values after compression decrease.

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The tensile strength, the tear strength and the fatigue resistance reach their maximum. The nature and the characteristics of the intermolecular bonds, as well as their number, have a great influence on the final properties of the vulcanized elastomer: short bonds, the thermal stability and the dynamic properties; improve tensile strength, fatigue resistance and tear strength.

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As vulcanization proceeds, the viscosity of the material increases and this increase is measured by the rise in the torque needed to keep the amplitude of the oscillation constant. Since the measurement is carried out at high temperature, it is assumed that the viscous effect of the material is negligible and that the increase of the torque is proportional to the number of links per unit volume of rubber.

Figure 16 contains a typical vulcanization curve, which shows the trend of the torque as a function of time. The first period induction , in which the viscosity does not change the retardant systems that inhibit the formation of the activator-accelerator complex are active is followed by a period of torque rise and, subsequently, of stabilization of the torque value.

A decrease in this value is symptomatic of breakage of the intermolecular bonds caused by the temperature reversion , while an increase is linked to a further cross-linking of the material Accelerated Sulphur Systems For many years, the adjustment of the scorch time was carried out by means of salicylic or benzoic acid or N-nitrosodiphenylamine until the first vulcanization inhibitors were introduced in they represent an important class of substances that make it possible to reduce the risk of prevulcanization without substantially altering the rate of the process.

The principal and most common one is N- cyclohexylthio phthalimide CTP , which is used in quantities of the order of phr. Molding is by far the most important curing process, where uncross-linked rubber is placed into a heated mold, which gives it the final product shape, and then vulcanizes the material. It can vary in size from a clenched fist to that of an automobile, and can have a single cavity to make one product at a time, or enough cavities to make a hundred or more.

Most rubber molding is based on introducing a solid compound into a mold, although urethanes and silicones can be introduced as solids or liquids.