Thermosetting plastic
Encyclopedia
A thermosetting plastic, also known as a thermoset, is polymer
material that irreversibly cures
. The cure may be done through heat (generally above 200 °C (392 °F)), through a chemical reaction (two-part epoxy
, for example), or irradiation
such as electron beam processing
.
Thermoset materials are usually liquid or malleable prior to curing and designed to be molded
into their final form, or used as adhesive
s. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC).
According to IUPAC recommendation: A thermosetting polymer is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. Curing can be induced by the action of heat or suitable radiation
, or both. A cured thermosetting polymer is called a thermoset.
or rubber
by a cross-link
ing process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D
structure. The cross-link
ing process forms a molecule with a larger molecular weight, resulting in a material with a higher melt
ing point. During the reaction, the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material.
Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melt
ed and re-shaped after it is cured. This implies that thermosets cannot be recycled, except as filler material.
materials due to this three dimensional network of bonds (cross-linking), and are also better suited to high-temperature
applications up to the decomposition temperature. However, they are more brittle. Many thermosetting polymers are difficult to recycle.
Some methods of molding thermosets are:
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
material that irreversibly cures
Curing (chemistry)
Curing is a term in polymer chemistry and process engineering that refers to the toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation, electron beam or heat...
. The cure may be done through heat (generally above 200 °C (392 °F)), through a chemical reaction (two-part epoxy
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....
, for example), or irradiation
Irradiation
Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to...
such as electron beam processing
Electron beam processing
Electron beam processing or electron irradiation is a process which involves using electrons, usually of high energy, to treat an object for a variety of purposes. This may take place under elevated temperatures and nitrogen atmosphere...
.
Thermoset materials are usually liquid or malleable prior to curing and designed to be molded
Molding (process)
Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....
into their final form, or used as adhesive
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...
s. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC).
According to IUPAC recommendation: A thermosetting polymer is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. Curing can be induced by the action of heat or suitable radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
, or both. A cured thermosetting polymer is called a thermoset.
Process
The curing process transforms the resin into a plasticPlastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
or rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
by a cross-link
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...
ing process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...
structure. The cross-link
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...
ing process forms a molecule with a larger molecular weight, resulting in a material with a higher melt
Melt
Melt can refer to:* Melting, in physics, the process of heating a solid substance to a liquid*Melt , the semi-liquid material used in steelmaking and glassblowing*Melt inclusions, a feature of igneous rock...
ing point. During the reaction, the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material.
Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melt
Melt
Melt can refer to:* Melting, in physics, the process of heating a solid substance to a liquid*Melt , the semi-liquid material used in steelmaking and glassblowing*Melt inclusions, a feature of igneous rock...
ed and re-shaped after it is cured. This implies that thermosets cannot be recycled, except as filler material.
Properties
Thermoset materials are generally stronger than thermoplasticThermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...
materials due to this three dimensional network of bonds (cross-linking), and are also better suited to high-temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
applications up to the decomposition temperature. However, they are more brittle. Many thermosetting polymers are difficult to recycle.
Examples
Some examples of thermosets are:- Polyester fibreglass systems: sheet molding compounds and bulk molding compounds)
- Vulcanized rubberVulcanizationVulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...
- Bakelite, a phenolPhenolPhenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
-formaldehydeFormaldehydeFormaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...
resin used in electrical insulators and plasticware - DuroplastDuroplastDuroplast is a composite thermosetting plastic, a close relative of formica and bakelite. It is a resin plastic reinforced with fibers making it a fiber-reinforced plastic similar to glass-reinforced plastic.-Uses:...
, light but strong material, similar to bakelite used for making car parts - Urea-formaldehydeUrea-formaldehydeUrea-formaldehyde, also known as urea-methanal, named so for its common synthesis pathway and overall structure, is a non-transparent thermosetting resin or plastic, made from urea and formaldehyde heated in the presence of a mild base such as ammonia or pyridine...
foam used in plywoodPlywoodPlywood is a type of manufactured timber made from thin sheets of wood veneer. It is one of the most widely used wood products. It is flexible, inexpensive, workable, re-usable, and can usually be locally manufactured...
, particleboard and medium-density fiberboard - Melamine resinMelamine resinMelamine resin or melamine formaldehyde is a hard, thermosetting plastic material made from melamine and formaldehyde by polymerization. In its butylated form, it is dissolved in n-butanol and xylene. It is then used to cross-link with alkyd, epoxy, acrylic and polyester resins, used in surface...
used on worktop surfaces - Epoxy resinEpoxyEpoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....
used as the matrix component in many fiber reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic) - Polyimides used in printed circuit boards and in body parts of modern airplanes
- Cyanate esters or polycyanurates for electronics applications with high demands on dielectric properties and high glass temperature requirements in composites
- Mold or mold runners (the black plastic part in integrated circuits or semiconductors)
Some methods of molding thermosets are:
- Reactive injection moldingInjection moldingInjection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity...
(used for objects such as milk bottle crates) - Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables)
- Compression moldingCompression moldingCompression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are...
(used to shape most thermosetting plastics) - Spin castingSpin castingSpin casting, also known as centrifugal rubber mold casting , is a method of utilizing centrifugal force to produce castings from a rubber mold. Typically, a disc-shaped mold is spun along its central axis at a set speed. The casting material, usually molten metal or liquid thermoset plastic is...
(used for producing fishing lureFishing lureA fishing lure is an object attached to the end of a fishing line which is designed to resemble and move like the prey of a fish. The purpose of the lure is to use movement, vibration, and colour to catch the fish's attention so it bites the hook...
s and jigsJig (fishing)Jigging is the practice of fishing with a jig, a type of fishing lure. A jig consists of a lead sinker with a hook molded into it and usually covered by a soft body to attract fish. Jigs are intended to create a jerky, vertical motion, as opposed to spinnerbaits which move through the water...
, gaming miniatures, figurineFigurineA figurine is a statuette that represents a human, deity or animal. Figurines may be realistic or iconic, depending on the skill and intention of the creator. The earliest were made of stone or clay...
s, emblems as well as production and replacement parts)