Fluoropolymer Applications in Khaladkar Author Applied Plastics Engineering Thermoforming of Single and Film Properties of Plastics McKeen Author The Effect of Creep and Other Alfredo Campo Author Fatigue and Tribological Compounding Precipitated Kurtz Editor Landrock Author Sina Ebnesajjad Author TPEs are generally low modulus, flexible materials that can be stretched repeatedly to at least twice their original length at room temperature with an ability to return to their approximate original length when stress is released.
They can be processed with the efficiency and economy of thermoplastics and can be molded with other olefin based materials, such as polypropylene, polyethylene without the use of adhesives. Various mechanical designs were implanted within the parts to ensure tight fit in case of other substrates like polyamides, acrylonitrile butadiene styrene and so on. Rubbery materials have high degree of flexibility in their elongated polymeric chains which binds them into an ordered network structure [ 1 ].
Due to this mobility and flexibility the long chains when subjected to external pressure or stress change their configuration. Thus chain allied to a structure gives a solid feature preventing them to flow under pressure. Due this fact, this type of materials may be stretched or pulled up to several times of its original length. Withdrawing external forces, it rapidly restore its nearly original dimensions, with essentially no residual or nonrecoverable strain. TPEs serve a wide range of markets:. One of the major usages of TPE is in encapsulations.
By encapsulation we mean covering of an object with some material to protect and to insulate it. Polymeric materials are used extensively as encapsulants for coils, motor windings and micro electronics packaging industry. Use of TPEs speed manufacturing with low cost compared with any other material and technology.
TPE gives both the characteristics like thermosets along with the process and design flexibility same as plastics thereby enhancing wide design alternatives and cost reduction facility. In this chapter we will concern our focus on Polyolefins. The basic characteristics of thermoplastic elastomer greatly depend on polymers used in their manufacturing process based on their application.
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These properties can be modified, however, through the appropriate addition of compounding ingredients. Some are added to accelerate cross-linking in order to impart the exact cure time required some improve processability, while others improve the properties of the finished product.
Some ingredients are added to deliver the highest levels of performance in the end product. In some applications, compounding is required to reduce cost, diluents and extenders are sometimes added to minimize the ratio of high valued components within the system. By this, certain compromises in the mechanical and other properties are obtained, but for some specific high end applications it is worthwhile. Processing of the compounded material can also influence the end product properties.
The major ingredient that affect the properties of the finished product and which have its own importance while making an end product are classified as:. The neat or raw polymer, is the fundamental or main ingredient in determining the properties of the compound or the end product. It is selected in such a way so as to optimize service life and processing requirements with cost effectiveness as one of the main parameter taken into account. Polymers with high molecular weight can give very tough material as the final product.
Again this criterion sometimes becomes a disadvantage for some specific product. So depending on the end use application selection of polymer is yet challenging. The most important part of filler is to provide better reinforcement and secondly to reduce the cost of the end product. Generally we have two basic types of fillers. One is the reinforcing type which can reinforce the system according to need and the other is the diluent type which are generally used for calibrating the physical properties of the system. They become more reinforcing as the particle size decreases [ 4 ].
As mentioned above fillers with high reinforcing character can make a compound very hard and rigid which in turn can result in poor flow behavior. Carbon blacks are generally alkaline in nature and tend to accelerate cure time. Other non-black fillers may be acidic and can retard cure as well as absorb moisture, which can result in blistering problems during the processing stage. Several nano-fillers like super-fine clays have a high surface area as compared to their volume and can induce better mechanical behavior.
They are more expensive than general fillers, the same weight of material goes further because the particles are so much finer.
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These speed up the cure mechanism of the system. Accelerators increase the rate of cross-linking and therefore decrease both production time and cost. Accelerators can help to control the cure speed and cure time and also other material properties. They can be incorporated within the system at any amount and sometimes may be more than one accelerators are used in a single formulation.
Some important accelerators include peroxides for curing which plays dual character in the network. They can act like an accelerator as well as a modifier for the physical properties of the system. These are compounds added to materials to suppress or delay the flames and prevent fire spreading. Most of the TPEs promote combustion and thus the end by-products can be extremely hazardous and dangerous.
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Due to this fact manufacturers induce flame retardants to improve their flame resistance. There are several flame retardants that can be added to the compound, either inorganic or organic. These include antimony trioxide, zinc borate, aluminum hydroxide and chlorinated paraffins and so on. Curatives are very crucial for compounding of elastomers. They are the crosslinking agents generally used to connect separate neat polymers. The type of curative used varies upon the elastomer selected.
Like in sulfur-cured, sulfur donors give better heat stability as they tend to give single sulfur crosslinks. Peroxide cures promotes good thermal stability due to the short length of the cross-links between the polymer chains. Fluorocarbons, along with some other polymer types, can have their own specialized cure systems.
Plasticizers help to reduce the hardness with given filler loading and also gives better filler addition and dispersion. They should be compatible with the matrix polymer. Special types of plasticizers can improve the low temperature flexibility of some rubber. Process aids can also assist with filler dispersion, although they are normally added to improve processability.
The cure characteristic is one of the major properties of an elastomer to study before its application to different areas. As the compound cures within the hot platens it gradually becomes stiffer. This is measured via a strain gauge connected to an oscillating rotor in contact with the elastomer. The resistance to torque or the stiffness of the material is plotted on a graph against time, known as a rheograph.
This information shows the behavior of the moulding characteristics, since the rheograph shows the time available to load the press, the time of cure and the final state of cure. The ultimate state of cure is not always a straight line but can also be a slope [ 5 ].
For some elastomers the cure continues in a different way which could be explained as the heat is actually breaking the polymer chain rather than the crosslinks formed during the curing stage. Abrasion resistance is important where friction, rough surface, industrial use is a key factor. Hard TPOs have good abrasion resistance. Abrasion damage can occur when there is dynamic motion against an abrasive surface.
Standard abrasion tests depend on producing relative motion between a rubber sample and an abrasive surface, pressed together by a fixed force. These tests do not correlate particularly well with application experience. It is sometimes believed that tensile strength is related to abrasion resistance, higher the tensile strength higher is the abrasion resistant property while a high tensile strength compound can have good abrasion resistance. Abrasion resistance is mainly related more to the polymer taken and the nature of compounding ingredient used [ 6 ]. Abrasion resistant elastomers must therefore be specifically developed.
Polyolefin based TPEs show good abrasion resistant character. Tear strength is a measure of the resistance of an elastomer to tearing. It is measured using a tensile test machine operating at a constant rate of traverse until the test piece breaks. Various types of test pieces can be used, and depending on the method employed the maximum or median force achieved is used to calculate the tear strength.
Parameters affecting the adhesion and bond ability: The application of elastomers at high temperature is generally determined by their chemical structure and stability and varies depending on the elastomers being used. In course of developing high temperature product, these elastomers can be attacked by several chemical species like hydrogen, oxygen, and other groups which results in some chemical reaction and as a result the effectiveness or power increases at high temperature.
Another parameter responsible for affecting the adhesion and bond ability is the degradative chemical reactions. It may hamper the product in two specific ways. Breaking the molecular chains or cross-links, thus softening the rubber because they weaken the network. Additional cross-linking, hardening the rubber, often characterized by a hard, cracked and degraded skin occuring on the elastomer component [ 7 ]. One more important criterion responsible for adhesion and bond ability is the perfect selection of the materials based on the end product needed. For example elastomers significantly weaken at high temperatures so in the case of seals, can result in a significant reduction in extrusion resistance.
For applications involving elevated temperatures, especially at high pressures, anti-extrusion elements may also need to be used, either incorporated into the seal design or added as an additional component when manufacturing the seal. Elastomers when cooled to sufficiently low temperatures show the characteristics of glass, including hardness, stiffness and brittleness, and do not behave in the readily deformable manner usually associated with elastomers.
As the temperatures rises, the segments of the polymer chain gain sufficient energy to rotate and vibrate. At high temperatures full segmental rotation is possible and the material behaves in the rubbery way. The usefulness of an elastomer at low temperatures is dependent on whether the material is above its glass transition temperature Tg , where it will still behave elastically, or below its Tg, where the material will be hard and relatively brittle.
Polyolefins also named as poly alkenes are simple long chain hydrocarbon. They are prepared by the reaction of an alkene as a monmer with general formula C n H 2n. The most commonly used Polyolefins are low and high density polyethylene copolymer, polypropylene copolymer and polymethyl pentene. Polyethylene and polypropylene are produced by long chain polymerization of olefins ethylene and propylene respectively.
The metallocene catalyst helps to instigate polymerization of the ethylene and co-monomer sequences and while further increasing the co-monomer content will produce polymers with higher elasticity due to the fact that the co-monomer incorporation disrupts the polyethylene crystallinity.
The molecular weight of the copolymer will help determine its processing characteristics and end-use performance properties. Higher in molecular weights higher is the polymer toughness. Hence depending on the end use application one can choose the appropriate copolymer with molecular weight according to the requirement. POEs are produced using refined metallocene catalyst. These catalysts have a constrained transition metal such as Ti, Zr, sandwiched between one or more cyclopentadienyl ring structures to form a sterically hindered polymerization site.
The catalyst is usually first mixed with an activator or co-catalyst, which can significantly enhance the polymerization efficiency rate to beyond a million units of polymer per unit of catalyst [ 8 - 9 ]. Hardcover ISBN: Imprint: William Andrew. Published Date: 6th June Page Count: View all volumes in this series: Plastics Design Library.
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Institutional Subscription. Free Shipping Free global shipping No minimum order. Introduction 1. Elasticity and Elastomers 1. Thermoplastic Elastomers 2. Brief History of Thermoplastic Elastomers 3. Additives 3. Antioxidants 3. Light Stabilizers 3. Nucleating Agents 3. Flame Retardants 3. Colorants 3. Antistatic Agents 3. Slip Agents 3. Antiblocking Agents 3. Processing Aids 3. Fillers and Reinforcements 3. Plasticizers 3. Other Additives 3. Selection of Additives 3. Health, Hygiene, and Safety 4. Processing Methods Applicable to Thermoplastic Elastomers 4. Introduction 4.
Mixing and Blending 4. Extrusion 4. Injection Molding 4. Compression Molding 4.
Transfer Molding 4. Blow Molding 4. Rotational Molding 4. Foaming of Thermoplastics 4. Thermoforming 4. Calendering 4. Secondary Manufacturing Processes 4. General Processing Technology of Thermoplastic Elastomers 4. Process Simulation 4. Product Development and Testing 5. Styrenic Block Copolymers 5.
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Introduction 5. Polystyrene—Polydiene Block Copolymers 5.