Polymers: A Comprehensive Guide to Types, Properties, and Applications
Elastomers
Polymers are able to develop a large elastic deformation if they are subject to relatively small efforts. Some elastomers have extensions of 500% or more and return to their original shape. Elastomers consist of long chain molecules cross-linked. They have elastic properties due to two characteristics: 1) the long molecules are folded tightly when not stretched, and 2) the degree of crosslinking is well below the ternofijos.
Production Technology
Synthetic elastomers are obtained by polymerization processes similar to processes for thermoplastic and thermoset polymers.
Polymerization
There are two methods of polymer synthesis:
- Addition Polymerization: It leads to the double bonds between carbon atoms of the monomers to be opened so as to join with other molecules of monomers. The connections occur at both ends of the macromolecule that is expanding, which were used long chains of simple repetitive.
- Condensation Polymerization: The two monomers react to form a new molecule of the compound that is desired. Also called condensation polymerization since it is obtained as a byproduct of the reaction water that condenses.
Examples of Elastomers
- Polybutadiene: very good elasticity, it is consumed primarily in the manufacture of tires (92%).
- Butadiene-styrene (SBR) in 6:1 ratio: median elasticity, high resistance to aging (resistant to solvents and abrasion) which makes it very useful in the manufacture of tires.
- Polyisoprene all cis-1, 4 (natural rubber)
- Isoprene copolymer and 2-methylpropene (IIR): very resistant to oxidation, has little elasticity, low gas permeability and resistance to tearing. Used in balloons, tire inner chambers for vibration dampers and cable sheathing.
- Neoprene (poly 2-chloro-1 ,3-butadiene): very resistant to oxidation, organic solvents and high temperatures due to the presence of C-Cl bonds. Its elasticity is average. Used in the manufacture of hoses and pipes for conveyance.
Calendar
It is a continuous process of transformation of thermoplastics and elastomers for the manufacture of flexible and semirigid plates of reduced thickness. Consists of passing the pre-laminated material for the air gap of two or more cylinders arranged with their axes parallel, providing a raw sheet whose characteristics are consistent with a set of calibration cylinders, cooling, finishing and collection.
Foams
Polymer foam is a mixture of polymer and gas, which gives the material a porous or cellular structure. Also called cellular polymer, polymer and polymer blowing expanded. The most common polymer foams are polystyrene and polyurethane. Its most important properties are: low density, high strength per unit weight, good thermal insulation.
Rating:
- Elastomer: in which a polymer matrix is capable of a rubber elastic deformation.
- Flexible: in which a polymer matrix is very plastic and soft, such as PVC.
Foaming Processes
The gases used in the polymer foams are air, nitrogen and carbon dioxide. The gas is introduced into the polymer with several methods called foaming processes. These include:
- Liquid resin mixed with air by mechanical agitation, after the polymer is hardened by heat or chemical reaction.
- Mixing a physical blowing agent in the polymer, a gas such as nitrogen or pentane to dissolved in the polymer melt subject to pressure, so that the gas comes out of solution and expands when pressure is then reduced.
- The polymer is mixed with chemicals, called chemical blowing agents that decompose at high temperatures and releases gases in the mixture.
Polystyrene foams: they receive their form for extrusion and molding.
Polyurethane foams: they are made in a one-stage process in which two liquids are mixed and members immediately introduced into a mold or other form, so that the polymer is synthesized at the same time create the shape of the piece.
Crosslinked polyethylene foams: they are a plastic polyethylene foam base. For foaming, use, or a chemical reaction with the help of catalysts, or is exposed to radiation product base of high energy sources such as gamma rays. Generally, we obtain a family of plastic foam or polyethylene foam, closed cell and small with good resistance to temperature and ultraviolet rays, but hardly recyclable.
Additives
Additives alter the molecular structure of the polymer, or add a second phase to plastic, and process in a composite material.
Rating:
The additives are classified according to their function as fillers, plasticizers, dyes, lubricants, flame retardants, crosslinking agents, ultraviolet light absorbers and antioxidants.
Filled materials are being added in the form of particles or fibers into a polymer, to alter the mechanical properties of this or just to reduce material cost. He is also used to improve the dimensional stability and heat. The fillers that improve the mechanical properties agents are called reinforcers, and what s compounds formed in this way are called reinforced plastics; have more stiffness, strength, hardness the original polymer. Example: fillers that are used polymers are cellulose fibers and powders.
Plasticizers: are chemicals that are added to a polymer to make it more soft and flexible, and to improve flow characteristics during training. The plasticizer works by reducing the glass transition temperature below ambient. Below this temperature the polymer is hard and brittle and above this temperature the polymer is soft and rigid.
Dyes: for polymers are of two types: pigments and dyes. The pigments are finely powdered materials, insoluble and must be distributed uniformly throughout polymer was very low concentrations. The dyes are chemicals that are usually applied in liquid form and are soluble in the polymer.
Lubricants: apply the polymer to reduce friction and facilitate the flow into the mold interface.
Flame Retardants: are chemicals that are added to polymers to reduce the flammability of some of the following mechanisms or combination thereof:
Crosslinking agent: they are a variety of ingredients that cause cross-linking reaction or act as catalysts that facilitate it.
Ultraviolet light absorbers and antioxidants: these are additives which reduce susceptibility of the polymer to degrade due to ultraviolet light and oxidation.
Fiber obtained by Polyaddition
The main fibers are polyurethane, elastomeric fibers that are resulting from the polyaddition of diisocyanates with diols. The first fibers were poliaducto of polyoxymethylene and polyoxyethylene obtained from formaldehyde and ethylene oxide, respectively. The formation of these products are considered not polyadducts polymerizations, since each reaction step takes place migration of hydrogen atoms of a monomer to another, then linking the two molecules that are free valences in the proton transfer.
The polyurethane fibers are often used in combination with other fibers to give the article has elasticity.
Fiber obtained by polycondensation
Polyester Fibers
The base material, polyesters, linear policondensadostermoplásticos are chemically formed from a dicarboxylic acid and a di-alcohol. In these products, the ester groups are incorporated as a bridge in the macromolecular chains, whereas the cellulose esters and polyesters are not considered, because in them the ester groups are in side chains.
The mecanismodel process of forming a linear polyester consists of repetitive condensation of bifunctional monomers.
The polyester fibers are elastic and very resistant to traction and the touch, approaching the mechanical properties of polyamide fibers. They are very stable to light, acids, oxidants and solvents, but not too much to the bases, which, concentrated and hot saponified act polyester. Absorb less
Polyamide fibers
Can be obtained by two different procedures, leading to two different types of polyamides. One of them is the polycondensation of diamines with dicarboxylic acids containing both, at least four methylene groups in their molecules, the other method of production, is based on autopolicondensación of aminoicidos (or their lactams) of at least five methylenes. If the number of methylene groups is less, not enough to cause condensation of important textile products.
Polytetrafluoroethylene fibers
Tetrafluoroethylene monomer in the industry is obtained by pyrolysis difluorclorometano formed to treat antimony fluoride chloroform:
The polymerized tetrafluoroethylene peroxidic catalysts in water giving a white powder of polytetrafluoroethylene.
The fibers of polytetrafluoroethylene (Teflon, Fluon, Hydefion) have unusual properties in terms of thermal stability and chemical agents.
Olefinic fibers
Polyolefins are a group of synthetic technique whose main advantage lies in its economy, because they come from by-products of petroleum distillation and cracking. This group includes the polietilenosapropiados for technical textiles, rope, filter cloths, etc.., The polybutadiene and polypropylene, not all are suitable for making fibers, but only the isotactic.
The fibers of polyethylene and polypropylene are obtained by spinning of polyolefins for fusion, ie, by extruding the molten material in through holes.
Polystyrene fibers
The styrene monomer, or vinylbenzene, is a liquid that is obtained in the industry by adding benzene to ethylene in the presence of Cl 3 Al, according to a Friedel-Crafts reaction; ethylbenzene is formed, which dehydrogenated at 600 º C in the presence of ZnO styrene catalyst becomes:
From the properties of polystyrene fibers include its impermeability to water and their high electrical insulator. Trade names are Polyfibre, Algila, Styroflex, etc..
Fibers vinylidene
Are those resulting from the polymerization and copolymerization of vinylidene chloride. This is obtained by chlorination of vinyl chloride and hydrochloric acid separation whitewash of l ,2,2-trichloroethane.
The polymerization is carried by the emulsion method, according to the radical mechanism in the presence of dibenzoyl peroxide. The fibers are obtained by extrusion of the molten filaments.
These fibers copolymer of vinyl chloride / vinylidene chloride show high chemical and abrasion resistance and impermeability to water, but are not very stable to heat, beginning to shrink to 65 ° C and softens at 115 º C approximately.
Fibers obtained by polymerization
Polymerization takes place in the chain of unsaturated monomer units by opening their double bonds, yielding a polymer with the same percentage composition. In any polymerization process features three stages: the initiation reaction, the growth or propagation and chain breaking or termination.
FIBERS
Synthetic fibers those obtained by chemical processes polirreacción from low molecular weight substances through purely synthetic.
Synthetic fibers belong to the group of thermoplastics. To obtain synthetic fibers are suitable only slightly higher linear or branched polymers, since the macromolecules are hard and rigid three-dimensional, must possess certain technological features such as elasticity, elongation, easy pigmentation or dyeing, the chemical stability, atmospheric and biological resistance to the traction and the touch to the fold, not all products have linear macromolecular
Polyester Resins
These resins are made primarily from maleic and phthalic anhydrides with propylene glycol and cross-linking with styrene. It should be noted that the use of these resins with glass fiber reinforcement has been replaced by different materials such as: high strength thermoplastic, wood, carbon steel, glass and acrylic sheet, cement, gypsum, etc..
Epoxy resins
Almost all commercial epoxy resins made from bisphenol A (obtained from phenol and acetone) and epichlorohydrin (produced from allyl alcohol). Its most important properties are: high resistance to temperatures up to 500 ° C, high adhesion to metal surfaces and excellent resistance to chemicals. Epoxy resins are mainly used in coatings of cans, drums, surface finishing equipment and as an adhesive.
Thermosetting resins
These materials are characterized by intersecting polymer chains, forming a resin with a three-dimensional structure that does not melt. Irreversibly polymerize under heat and pressure to form a stiff dough and hard.
TABLE 18. Main thermosetting resins
Phenolic Resins
The reaction between phenol and formaldehyde results in the phenolic or Phenoplasts. There are two types of phenolic resins, the Resols and novolac. The Resols are obtained when using a base catalyst in the polymerization. The product is cross-linking between chains that allow three-dimensional thermoset networks. The novolac is done using acid catalysts. Here the chains are not cross-linking so that the product is soluble and fusible permanently. The most important properties of phenolic thermosets are their hardness, rigidity and resistance to acids. They have excellent insulating properties and can be used continuously at temperatures up to 150 ° C. Phenolic resins are used to make adhesives, insulation, laminates for buildings, furniture, plywood and automotive parts. These resins are cheaper and easier to mold.