Elastomers - Thermoplastic polymers, elastomers and additives

The elastomers are unique to polymers and exhibit an extraordinary reversible extension with low hysteresis and minimal permanent assembly. The elastomers are materials with a high elongation, and an excellent elastic recovery, in practice they can be stretched and then return to the initial position. In the market there are two types of elastomers: natural and elastomers or synthetic rubbers. They are macromolecular materials that exhibit a long range of elasticity at room temperature.

They are characterized by a low elastic modulus initially stretching in the range of 106-107 dynes / cm2 and a high degree of stretching almost instantaneously complete and reversible. By reducing the temperature of the elastomer it becomes brittle. The natural gums, according to UNI 7703, are obtained by coagulation of the latex (rubber) obtained from tropical plants and collected by the incision of the trunk.

They provide excellent mechanical properties, but poor resistance to weathering, temperature and many chemical compounds. Synthetic rubbers are produced from simple hydrocarbons, through a polymerization. Currently, different artificial elastomers are available, which have different chemical and physical-mechanical characteristics depending on their nature. Normally some synthetic rubbers are not used as such, but are subject to vulcanization, which consists of a thermal treatment of rubber mixed with appropriate additives such as sulfur, peroxides, epoxy.

During this operation rubber cross-linking occurs, that is, creating bonds between the molecular chains that prevent sliding by applying mutual, rubber loses plasticity and stickiness. During the use process, the vulcanization has been completed and the product takes the final form. Depending on the type of article to be obtained, the vulcanization is carried out in closed molds (casting), or between rotating cylinders (calendering), or in extruders.

TPE Thermoplastic Elastomer, also called thermoplastic rubber.

TPE is the general name of thermoplastic elastomer, also called thermoplastic rubber.

The thermoplastic elastomers are classified, according to ISO 1043, with the abbreviation TPE (thermoplastic elastomers), at the end of the acronym TPE, it is added to a letter that determines the chemical nature:

TPE-Often abbreviated as TPA or COPA, PEPA, PEBA

TPE-E TPEE often abbreviated or TPC-ET, COPE, TEEE

TPE-S often abbreviated as TPS or SBS, SEBS

TPE-U often abbreviated as TPU

TPE-V often abbreviated to TPV

TPE-O often TPO

For reference to the classification of elastomers with the DIN / ISO 1629 standard, which is derived from the ASTM D 1418-79 standard. The last letter identification code defines the basic group in which the polymer belongs, while the initial letters provide specific information and in many cases exclusively define the elastomer list. Below is not complete, but contains many of the most commonly used elastomers.

This rule divides synthetic rubbers into 5 groups:

Group M (from English Methylene), with saturated polyethylene chains or saturated polymethylene chain

Polymers containing saturated polyethylene polymer chains: EPDM, EPM, AEM, ACM, CSM, FEPM, FFPM, FPM, FFKM

There are no polymers like this

Group O, with oxygen in the polymer chain

Polymers containing oxygen atoms: CO, ECO

Group Q, with oxygen and silicon in the polymer chain

Polymers containing oxygen and silicon atoms: FMQ, MQ, PMQ, PVMQ, qualified majority voting

Group R (from English Rubber), rubber similar natural poly-isoprene, with the polymer chain containing unsaturated carbon

Polymers containing unsaturated carbon: BIIR, BR, IIR, CR, IIR, IR, NBR, SBR, CIIR, XNBR, HNBR

Group T, with carbon, oxygen and sulfur in the polymer chain

Group U (of Urethane), with carbon, oxygen and nitrogen in the polymer chain

Polymers containing carbon, oxygen and nitrogen: AU, EU.

Group Z, with phosphorus and nitrogen in the polymer chain

Elastomers can also be broadly divided into service performance in three different groups. Within these groups there are answers to practically all the application needs

Elastomers for general applications, such as NR and SBR, which deteriorate in aggressive environments, such as hot air, mineral oils, fuels, oxidants, ozone. The advantage of these materials is their low price, in addition to the discrete low temperature performance

Elastomers, high performance, such as CR, NBR and EPDM, provide good performance even in aggressive environment at the expense of a slight price increase compared to the products described above.

Special elastomers, How FFKM, FPM, FMQ and qualified majority voting, provide high performance that meet the specific needs of the designer. The increase in cost is high, however.

B indicates halogenated rubbers (for example BIIR)

C indicates halogenated rubbers (for example, CIIIR)

E indicates polymer obtained by an emulsion process (e.g., eSBR)

H indicates hydrogenated rubber (for example, HNBR)

OE (from the English Oil-Extended), indicates a polymer to which oil has been added (for example, OE-SBR)

S indicates polymer obtained by a process in solution (for example, sSBR)

And it usually indicates thermoplastic properties

X indicates presence of carboxyl groups (e.g., XNBR)

diene polymers and copolymers

isoprene polymers (natural rubber)

polybutadiene; styrene, nitrile rubber; copolymers, butadiene-vinylpyridine

chlorobutadiene polymers (neoprene)

polymers and copolymers of monoolefins

isobutene polymers

polyethylene elastomers

fluoro-olefin elastomers

acrylic elastomers

polymers and copolymers of ethyl acrylate and butyl acrylate

fluoroacrylate polymers

thiokol

silicone rubbers

polyester tires of block copolymers

copolyester

Thanks to the stereospecific catalysts, it was possible to synthesize a polyisoprene with properties similar to those of natural rubber. Natural rubber is just polyisoprene. One of the most common methods of crosslinking is the so-called "rubber vulcanization" process, which is used for the crosslinking of polymers ("unsaturated"), which contain double bonds in the molecular chain, such example in polyisoprene ( natural rubber). Curing involves heating the polymer in the presence of sulfur (S): it is not the breakage of the double bonds and the formation of sulfur bridges between two different products cathene.I synthesized with stereo-specific catalysts are superior to natural, but for isomeric purity, and lower cost for various types of production. Among them, cis 1,4-polyisoprene is by far the most important for the elastic properties and the mechanical properties imparted to vulcanizates, properties that derive from the high level of isomeric purity of the material that, in the case of Natural rubber, is established for values ​​higher than 99% from 1,4-cis. The high isomeric purity of the natural polymer determines the remarkable tendency to crystallization, even at room temperature; The maximum degree of crystallinity that the material is capable of having is limited to 25-30%, which is more than enough to increase the Mooney hardness and viscosity over time.

The SBR rubber synthesized by the cold emulsion process is called by the acronym E-SBR, while the SBR rubber synthesized by anionic polymerization in solution, is known by the acronym S-SBR. They have discrete mechanical characteristics, they are used, for their low cost, in applications that do not require the replacement of natural rubber. A typical characteristic of E-SBR rubbers is the presence of highly branched structures, which can lead to the formation of the insoluble fraction (gel). There are at least 90 types of E-SBR, the characteristics of which are identified by the established conventional abbreviations of the IISRP. The anionic polymerization in a hydrocarbon-based solvent, a mixture of styrene and butadiene does not provide a copolymer with co-monomers randomly distributed, but a block copolymer. Good mechanical properties, can have a good value at breaking load, resistance to permanent deformation, elastic recovery and good resistance to fatigue, wear or tear. And 'possible to produce non-toxic products. Good dielectric properties, compatible with silicone oils, water and solutions of acids, bases and diluted salts. Low resistance to oxygen agents, ozone, UV radiation and oxidants, except in appropriately formulated compounds. It is not compatible with mineral, vegetable and animal oils, aliphatic, aromatic and chlorinated. Low resistance to heat, nothing that calls. Good resistance to cold. Working temperature -45 to 100 ° C. Glass transition temperature of -60 ° C. Alternative to natural rubber in many applications and produced a number of quality, synthetic rubber is the most widespread thanks to its use in tires.

Acrylonitrile-butadiene gives rise to synthetic rubber resistant to organic solvents and flexible at low temperature. They show the action of aliphatic hydrocarbons (less aromatic), non-polar solvents, oils and fats, and resistance to temperature up to 150 ° C. They are sensitive to oxidation. Nitrile rubbers according to ISO 1629 are identified by the initials NBR. A serious defect of nitrile rubbers, compared to acrylic and fluorinated rubbers, is represented by the limited resistance to high temperature, due to the presence in the macromolecular structure of double bonds to be removed by hydrogenation. An increase in acrylonitrile leads to an increase in resistance to oil action. It also has good mechanical properties, low permanent deformation and good gas impermeability. Remarkable chemical resistance due to its saturated polymer chain. Excellent resistance in the presence of gas (methane, propane, butane, propane). Good resistance to mineral oils and fats, animal, vegetable, silicone, hydraulic H, HL, H-LP, aliphatic hydrocarbons, non-polar solvents, non-flammable liquids HFA, HFB, HFC, all "hot water and salt solutions such fatty acids, dilute acids and bases, good resistance to heat and aging (premature aging of hot air), resistance to low heat, low resistance to UV radiation, ozone and weathering, low dielectric properties. with aromatic and chlorinated hydrocarbons, ketones, esters, phenols, polar solvents and concentrated acids, can be non-toxic Hardness 30 to 95 Sh. Working temperature -40 / 30 + 120 ° C (+ 100 ° C in air). To ensure a longer duration of the particular NBR, it is recommended to avoid direct contact with air, is used in immersion applications or maintenance of a protective layer of lubricating oil Increase the percentage of ACN (acrylonitrile) in the nitrile rubber compounds are not able to have a greater resistance to gasoline and mineral oils, greater elasticity and lower gas permeability, but also less flexibility at low temperatures and a worse compression set. Depending on the content of the NBR ACN can be divided into five groups:

NBR is the most common material used for its good mechanical properties and resistance to greasing oils and minerals. Its properties are determined by the content of acrylonitrile (ACN between 18 and 50%). A low content of ACN ensures good flexibility at low temperatures, but offers limited resistance to oils and fuels; Increase the content of ACN, the flexibility at low temperatures decreases, while increasing the resistance to oils and the standard fuels. The NBR has an average content of ACN to adapt to a wide range of applications with balanced characteristics. The NBR has good mechanical properties such as high abrasion resistance, low gas permeability and good resistance to mineral oils and fats, hydraulic oils H, HL, H-LP, non-flammable liquids HFA, HFB, HFC, aliphatic hydrocarbons , silicone fats and oils and water at 80 ° C. In general, NBR is not resistant to aromatic and chlorinated hydrocarbons, fuels with a high aromatic content, polar solvents, glycol-based brake fluids and non-flammable HDF hydraulic fluids. In addition, it has a poor resistance to ozone, exposure to atmospheric agents and aging, but, in most applications, it has no adverse effects.

The HNBR is obtained by hydrogenation or partial NBR. This implies a considerable improvement of resistance to heat, ozone and aging, which results in excellent mechanical characteristics. Resistance to liquids is comparable to that of NBR. The HNBR has good resistance to some refrigerant fluids. Exceptional frame of mechanical values, resistance to abrasion, high resistance to permanent deformation (compression set), the good behavior of 'aging and high temperatures. Chemical resistance to acids and mineral oils, animals, vegetables, silicone, aliphatic, aromatic and chlorinated hydrocarbons, polar solvents, oxygen, ozone, water, steam, gas, freon and their substitutes. And 'possible to produce non-toxic products. Hardness 50 to 90 Sh. -40 to 150 ° C.

High tensile strength, high breaking strength (also hot), high resistance to abrasion. High adhesion and cohesive strength. Good chemical resistance in the presence of hydrocarbons, vegetable oils, water, steam, gas, acids and diluted bases. Good resistance to heat and aging, resistance to flame nothing. You can not make non-toxic products. Especially suitable for the coating of wheels and rollers are subject to great wear and high temperatures, impossible conditions of use of polyurethane and too expensive for other elastomers. Hardness 65 to 90 Sh. Working temperature -30 145.

Butadiene rubber, a 1,3-butadiene polymer, is classified as a general purpose rubber. Polybutadiene rubber, also referred to simply as butadiene rubber, is predominantly based on cis-1,4-polybutadiene. The structure of the polybutadiene [-CH2-CH = CH-CH2-] n obtained from 1,3 butadiene (CH2 = CH-CH = CH2) indicates that, preferably, the 1,4-addition is given, being noteworthy that The carbon chain also has a double connection. Polybutadiene (PB) is a synthetic rubber, it is a polymer formed from the polymerization process of 1,3-butadiene monomer. Polybutadiene is a homopolymer of butadiene, C4H6, and is obtained by solution polymerization, the most vulgar. It can also be polymerized by emulsion. The polybutadiene polymerizes by addition, both the 1,2-vinyl form and the trans-1,4 or cis-1,4 form. The five forms according to which the butadiene unit can be attached to the polymer chain are:

The polyisobutene homopolymer is an amorphous material with a glass transition temperature of _70 ° C; its high structural regularity implies crystallization to deformation, but a minimum measure for cooling (more than six months for _33 ° C). The difficulty to crystallize at low temperatures is due to the low flexibility of the molecular chain caused by the presence of the two methyl groups in carbon atoms alternating main molecular axis that determine a distortion of the molecular chain. The most widely used is a copolymer with a small amount (max 5%) of isoprene, which is called "butyl rubber", compounds based on this polymer has a low tensile strength, but good abrasion resistance , shear, temperature, and permeability especially particularly under gas, therefore, are used for the manufacture of air chambers, membranes for autoclaves, etc. These cable insulation due to its essentially saturated tire characteristics, the IIR requires particularly effective vulcanization systems.

EPM represents a copolymer of ethylene and propylene monomers. EPM is completely saturated and therefore requires vulcanization by radiation or products that release free radicals, such as organic peroxides.

EPDM is a terpolymer based on three monomers: ethylene, propylene and an unconjugated diene (ethylidene norbornene ENB) The EPDM grades have a residual unsaturation in the side chains and, therefore, can be cured with sulfur and accelerators. Its resistance to heat is clearly better than natural rubber, SBR and butadiene rubber. EPDM materials generally have a high resistance to hot water, steam, aging and chemicals, and are suitable for a wide range of temperature applications. EPDM has a good resistance to hot water and steam, detergents, potassium hydroxide solutions, sodium hydroxide solutions, silicone oil and fats, many polar solvents, various diluted acids and chemicals, are totally unsuitable for use with all mineral oil products (lubricants, fuels), for glycol-based brake fluids usually special grades are used. They can be cured with sulfur or peroxide. The EPDM cured with peroxide are suitable for the highest temperature ranges and have a much lower compression. They can be used between -45 ° C and +130 ° C (cured with peroxide -50 ° C to +150 ° C).

CR or Polychloroprene Rubber is a homopolymer of Chlorobutadiene, or Chloroprene. Chloroprene rubber is a general name given to a number of unsaturated synthetic rubber based on polychloroprene and manufactured by emulsion curing. It stands out for having a balanced combination of properties that makes it the multifunctional rubber par excellence. The chlorine atom increases the level of resistance to oils, placing it between natural rubber and nitrile rubber, has excellent resistance to ozone, aging and weathering and also good mechanical properties. They have a medium resistance to mineral oils, and are suitable for use with many refrigerants, offering the most balanced set of desirable properties. Acceptable resistance in vulcanizates with hardness from 60 Shore A. The chloroprene rubber shows a marked tendency to crystallization. This happens because small crystals form within the macromolecules; its effect is more or less marked. The application temperature range is -40 ° C to + 100 ° C.

Polymers and copolymers of ethyl acrylate and butyl: They have a good resistance to oxygen and ozone even at high temperatures, and to degradation by UV light, they are used especially for coatings and to impart impact resistance to some plastics. Fluoroacrylate polymers: They have good resistance to fuels, lubricants, hydraulic fluids. They are expensive and are mainly used in aviation applications. Despite the presence of potentially active secondary groups for vulcanization, polyethylacrylate is not curable with peroxide. The ACM exhibit excellent resistance characteristics at high temperature, aging, and solvent oils; cheaper compared to NBR medium and high content of acrylonitrile, have comparable oil resistance, but are better resistant to high temperatures, especially in the presence of oils containing additives based on substances that cause sulfur over-vulcanization of the nitrile rubbers and determine the loss of elastic properties. Being saturated rubbers, resist oxidation, ozone, sunlight and aliphatic hydrocarbons, but the presence of ester groups makes them susceptible to hydrolysis. ACM is mainly used in the automotive industry as it is resistant to engine, transmission and ATF oils even at high temperatures. The application temperature range is -20 ° C to +150 ° C

The larger size of the fluorine atom compared to those of the hydrogen atom, the bond dissociation energy greater than that of the CH bond, are the causes of chemical stability at high temperatures and in contact with chemicals, FKM and FFKM rubbers. . The VDF homopolymer is a highly crystalline material with a Tg temperature of -40 ° C and a melting point of 165 ° C. The copolymerization with HFP and, in the case of the FFKM tires, with TFE determines the interruption of the regular and crystallizable sequences, providing non-crystallinity and a sufficiently low glass transition temperature.

Fluorosilicone has characteristics similar to silicone compounds, but the presence of fluorine in the composition highly increases the compatibility with animal and mineral oils for the entire range of working temperature, the aliphatic and aromatic hydrocarbons, acids. On the contrary, there is a reduction in performance at high temperatures. Quite good mechanical properties, excellent resistance to radiation (gamma and UV rays), excellent dielectric properties. Excellent resistance to oxidation, ozone and weathering. Good resistance to water and dilute saline solutions. Compatible with chlorinated solvents, alcohols and glycols. It is not compatible with silicone and alkali oils (also diluted), water vapor above 120 ° C. Low gas impermeability, self-extinguishing. Hardness 30 to 80 Sh. Working temperature -50 175 ° C (with peaks over 220 ° C).

An important family of synthetic elastomers with silicon and oxygen atoms instead of carbon atoms is silicone. They have a different nature from that of all other elastomers, since their molecules are chains of carbon atoms, but alternate between silicon and oxygen, which are linked to the alkyl side groups. The main characteristic is the resistance to extreme temperature, in fact do not change up to 250 ° C and maintain the elasticity of up to -150 ° C. They also resist well to oxygen and still hot ozone. The mechanical properties are limited, the resistance to chlorinated hydrocarbons and oxygenated solvents is low. The presence of carbon black would result in the development of high temperature gas and favor combustion, therefore, only reinforcing and mineral charges are used (silica, kaolin, calcium carbonate). Vulcanization is often carried out by irradiation with high energy particles. So why silicone elastomers have extreme resistance to high and low temperatures, the absence of components that tend to evaporate or release excellent gas oxidation resistance dielectric characteristics, hydrolysis and action many discrete chemical microorganisms are used for medical equipment, prostheses , toys, masks, electrical insulation, joints.

Perfect behavior at high and low temperatures: the main peculiarity of silicone is to maintain a good flexibility throughout the range of temperature of use (even at extremely low temperatures). The characteristics of purity and non-toxicity make it particularly suitable for food and biomedical applications. Silicon is also the only flexible insulating material that presents, together with excellent heat resistance, high resistance to radiation (gamma and UV rays). Of good resistance to high fire. On the basis of the needs that can be used compounds is highly electro-conductive, it is highly insulating. Excellent resistance to oxidation, ozone and weathering. Saline solutions good resistance to water and diluted, hot water up to 100 ° C and for short periods of steam up to 120 ° C. Compatible with diluted acids and bases, aliphatic oils, motor oils, animal and vegetable oils, transformer oils, brake fluids and hydraulic fluids based on glycol HFD-R and HFD-S, aromatic and chlorinated solvents. It is not compatible with mineral oils and fats and silicones, aromatic hydrocarbons, polar solvents, concentrated acids and alkalis. Low gas impermeability. Hardness 10 to 90 Sh. Working temperature -60 200 ° C (270 ° C, even with peaks of more than 300 ° C for VMQ, up to -110 ° C for PVMQ). Vitreous transition temperature of -120 ° C. The "polysiloxane" is the real name of silicones but when they were discovered it was thought that they had only "silicon" atoms in the main chain, from which they took the name. When the real structure was discovered it was too late, and the name remained: silicones are inorganic polymers that is devoid of carbon atoms in the main chain, its skeleton is a chain of silicon atoms alternating to oxygen atoms, each atom of Silicon has two groups attached, and they can be organic. It is mainly composed of methyl groups, vinyl, and phenolic silicones and allow to obtain good elastomers because its main chain is very flexible, the bonds between a silicon atom and two oxygen atoms attached to it are very flexible: the angle formed by these bonds It can open and close easily like a scissors making the chain flexible.