Polyoxymethylene
Encyclopedia
Thermoplastic
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...
used in precision parts that require high stiffness, low friction and excellent dimensional stability.
It was discovered by Hermann Staudinger
Hermann Staudinger
- External links :* Staudinger's * Staudinger's Nobel Lecture *....
, a German chemist who received the 1953 Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
in Chemistry. He studied the polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
and structure of POM in the 1920s to research the theory of macromolecules, which he characterized as polymers. Due to initial problems with thermal stability, POM was not commercialized.
First synthesized by DuPont research chemists around 1952, the company filed for patent protection of the homopolymer in 1956 and completed construction of a plant to produce Delrin at Parkersburg, West Virginia
Parkersburg, West Virginia
As of the census of 2000, there were 33,099 people, 14,467 households, and 8,767 families residing in the city. In 2006 the U.S. Census Bureau estimated that Parkersburg's population had decreased 4.4% to 31,755. The population density was 2,800.5 people per square mile . There were 16,100 housing...
, in 1960. Celanese
Celanese
Celanese Corporation is a Fortune 500 global technology and specialty materials company with its headquarters in Dallas, Texas. The company is a leading producer of acetyl products, which are intermediate chemicals for nearly all major industries, and is the world's largest producer of vinyl...
completed its study on a copolymer in 1960, producing Celcon in 1962 and Hostaform in 1963 in Kelsterbach, Germany, under a limited partnership with Ticona
Ticona
Ticona is an international manufacturer of engineering polymers. The company's head office is located at the Ticona/Celanese-Plant in Florence, Kentucky, USA, since 2005.-Company history:...
.
Properties
POM is characterized by its high strength, hardness and rigidity to -40°C. POM is intrinsically opaque white, due to its high crystalline composition, but it is available in all colors. POM has a density of ρ = 1.410 - 1.420 g / cm3.POM homopolymer is a semi-crystalline polymer (75-85% crystalline) with a melting point of 175° Celsius. The POM copolymer has a slightly lower melting point of 162 – 173° Celsius.
POM is a tough material with a very low coefficient of friction. However, it is susceptible to polymer degradation
Polymer degradation
Polymer degradation is a change in the properties—tensile strength, colour, shape, etc.—of a polymer or polymer-based product under the influence of one or more environmental factors such as heat, light or chemicals such as acids, alkalis and some salts...
catalyzed by acids, which is why both polymer types are stabilized. Both homopolymer and copolymer have chain end groups (introduced via end capping) which resist depolymerization
Depolymerization
Depolymerization is the process of converting a polymer into a monomer or a mixture of monomers.Thioglycolysis, thiolysis and phloroglucinolysis are reactions used to study condensed tannins by means of their depolymerisation. Thioglycolysis is also used to study lignin....
. With the copolymer, the second unit normally is a C2 (ethylene glycol) or C4 (1,4-butanediol) unit, which is introduced via its cyclic acetal (which can be made from the diol and formaldehyde) or cyclic ether (e.g. ethylene oxide). These units resist chain cleavage, because the O-linkage is now no longer an acetal group, but an ether linkage, which is stable to hydrolysis. POM is sensitive to oxidation, and an anti-oxidant is normally added to molding grades of the material.
POM advantages:
- High abrasionAbrasion (mechanical)Abrasion is the process of scuffing, scratching, wearing down, marring, or rubbing away. It can be intentionally imposed in a controlled process using an abrasive...
resistance - Low coefficient of friction
- High heat resistance
- Good electrical and dielectric properties
- Low water absorption
Production
Different manufacturing processes are used to produce the homopolymer and copolymer versions of POM.Homopolymer
To make polyoxymethylene homopolymer, anhydrous
Anhydrous
As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...
formaldehyde must be generated. The principal method is by reaction of the aqueous formaldehyde with an alcohol to create a hemiformal, dehydration of the hemiformal/water mixture (either by extraction
Extraction (chemistry)
Extraction in chemistry is a separation process consisting in the separation of a substance from a matrix. It may refer to Liquid-liquid extraction, and Solid phase extraction....
or vacuum distillation
Vacuum distillation
Vacuum distillation is a method of distillation whereby the pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure causing evaporation of the most volatile liquid...
) and release of the formaldehyde by heating the hemiformal. The formaldehyde is then polymerized by anionic catalysis and the resulting polymer stabilized by reaction with acetic anhydride
Acetic anhydride
Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula 2O. Commonly abbreviated Ac2O, it is the simplest isolatable acid anhydride and is a widely used reagent in organic synthesis...
. A typical example is DuPont’s Delrin.
Copolymer
To make polyoxymethylene copolymer, formaldehyde is generally converted to trioxane
Trioxane
Trioxane refers to a pair of isomeric organic compounds having the molecular formula C3H6O3. Each contains a six membered ring with three carbon atoms and three oxygen atoms.The two isomers are...
. This is done by acid catalysis
Acid catalysis
In acid catalysis and base catalysis a chemical reaction is catalyzed by an acid or a base. The acid is often the proton and the base is often a hydroxyl ion. Typical reactions catalyzed by proton transfer are esterfications and aldol reactions. In these reactions the conjugate acid of the carbonyl...
(either sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
or acidic ion exchange resins) followed by purificaiton of the trioxane by distillation and/or extraction to remove water and other active hydrogen containing impurities. Typical copolymers are Hostaform from Ticona
Ticona
Ticona is an international manufacturer of engineering polymers. The company's head office is located at the Ticona/Celanese-Plant in Florence, Kentucky, USA, since 2005.-Company history:...
and Ultraform from BASF
BASF
BASF SE is the largest chemical company in the world and is headquartered in Germany. BASF originally stood for Badische Anilin- und Soda-Fabrik . Today, the four letters are a registered trademark and the company is listed on the Frankfurt Stock Exchange, London Stock Exchange, and Zurich Stock...
.
The co-monomer is typically dioxolane
Dioxolane
Dioxolane is a heterocyclic acetal with the chemical formula 2O2CH2. It is related to tetrahydrofuran by interchange of one oxygen for a CH2 group. The corresponding saturated 6-membered C4O2 rings are called dioxanes. The isomeric 1,2-dioxolane is an peroxide...
but ethylene oxide
Ethylene oxide
Ethylene oxide, also called oxirane, is the organic compound with the formula . It is a cyclic ether. This means that it is composed of two alkyl groups attached to an oxygen atom in a cyclic shape . This colorless flammable gas with a faintly sweet odor is the simplest epoxide, a three-membered...
can also be used. Dioxolane is formed by reaction of ethylene glycol
Ethylene glycol
Ethylene glycol is an organic compound widely used as an automotive antifreeze and a precursor to polymers. In its pure form, it is an odorless, colorless, syrupy, sweet-tasting liquid...
with aqueous formaldehyde over an acid catalyst. Other diols can also be used.
Trioxane and Dioxolane are polymerized using an acid catalyst, often boron trifluoride etherate, BF3 OEt2. The polymerization can take place in a non-polar solvent (in which case the polymer forms as a slurry) or in neat trioxane (e.g. in an extruder). After polymerization, the acidic catalyst must be deactivated and the polymer stabilized by melt or solution hydrolysis in order to remove the unstable end groups.
Stable polymer is melt compounded, adding thermal and oxidative stabilizers and optionally lubricants and miscellaneous fillers.
Fabrication
POM is supplied in a granulated form and can be formed into the desired shape by applying heat and pressure. The two most common forming methods employed are injection moldingInjection molding
Injection 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...
and extrusion
Extrusion
Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section...
. Rotational molding
Rotational molding
Rotational molding, also known as rotomolding, rotocasting or spin casting, is a molding process for creating many kinds of mostly hollow items, typically of plastic....
and blow molding
Blow molding
Blow molding is a manufacturing process by which hollow plastic parts are formed. In general, there are three main types of blow molding: extrusion blow molding, injection blow molding, and stretch blow molding. The blow molding process begins with melting down the plastic and forming it into a...
are also possible.
Typical applications for injection-molded POM include high performance engineering components (e.g. gear wheels, ski bindings, fasteners, lock systems) and the material is widely used in the automotive and consumer electronics industry. There are special grades that offer higher mechanical toughness, stiffness or low friction/ wear properties.
POM is commonly extruded as continuous lengths of round or rectangular section. These sections can be cut to length and sold as bar or sheet stock for machining.
Machining
When supplied as extruded bar or sheet, POM may be machined using traditional methods such as turning, milling, drilling etc. These techniques are best employed where production economics do not merit the expense of melt processing. The material is free-cutting, but does require sharp tools with a high clearance angle. The use of soluble cutting lubricant is not necessary, but is recommended.Because the material lacks the rigidity of most metals, care should be taken to use light clamping forces and sufficient support for the work piece.
Machined POM can be dimensionally unstable, especially with parts that have large variations in wall thicknesses. It is recommended that such features are ‘designed-out’ e.g. by adding fillets or strengthening ribs. Annealing of pre-machined parts before final finishing is an alternative. A rule-of-thumb is that in general, small components machined in POM suffer from less warping.
Bonding
POM is typically very difficult to bond. Special processes and treatments have been developed to improve bonding. Typically these processes involve surface etching, flame treatment or mechanical abrasion.Typical etching processes involve chromic acid
Chromic acid
The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the...
at elevated temperatures. DuPont has a patented process for treating acetal homopolymer called satinizing which creates anchor points on the surface, giving an adhesive something to grab. There are also processes involving oxygen plasma and corona discharge.
Once the surface is prepared, a number of adhesives can be used for bonding. These include epoxies, polyurethanes, and cyanoacrylates. Epoxies have shown 150-500 psi shear strength on mechanically abraded surfaces and 500-1000 psi on chemically treated surfaces. Cyanoacrylates are useful for bonding to metal, leather, rubber and other plastics.
Solvent welding is typically unsuccessful on acetal polymers, due to the excellent solvent resistance of acetal. Thermal welding through various methods has been used successfully on both homopolymer and copolymer.
Usage
- Mechanical gears, sliding and guiding elements, housing parts, springsSpring (device)A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
, chains, screws, nuts, fan wheels, pump parts, valve bodies. - Electrical Engineering: insulators, bobbinBobbinA bobbin is a spindle or cylinder, with or without flanges, on which wire, yarn, thread or film is wound. Bobbins are typically found in sewing machines, cameras, and within electronic equipment....
s, connectorsElectrical connectorAn electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...
, parts for electronic devices such as televisions, telephones, etc. - Vehicle: Fuel sender unit, Light stock (including shifter for light, turn signal), power windows, door lock systems, articulated shells.
- Model: Thin, later claimed Modellbahnen parts, such as bogies and handle bars. POM breaks under load slightly less than ABSAcrylonitrile butadiene styreneAcrylonitrile butadiene styrene is a common thermoplastic. Its melting point is approximately 105 °C ....
, but in bright translucent colors, and not paintable. - Medical: insulin pen, Metered dose inhalers (MDI)
- Furniture: hardwareHardwareHardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....
, locksLock (device)A lock is a mechanical or electronic fastening device that is released by a physical object or secret information , or combination of more than one of these....
, handlesHandle (grip)A handle is a part of, or attachment to, an object that can be moved or used by hand. The design of each type of handle involves substantial ergonomic issues, even where these are dealt with intuitively or by following tradition...
, hingesHingesHinges may refer to:* Hinges, Pas-de-Calais, France* Hinges , an Andy Partridge album...
, or even curtains roles. - Construction: Structural Glass - pod holder for point
- Packaging: aerosol cans, vehicle tanks.
- Sports: Paintball accessories. It is often used for machined parts of paintball markers that do not require the strength of aluminum, such as handles and reciprocating bolts. POM is also used in airsoft guns, in order to reduce the noise of the piston.
- Clothing: zippers.
- Music: picksPlectrumA plectrum is a small flat tool used to pluck or strum a stringed instrument. For hand-held instruments such as guitars and mandolins, the plectrum is often called a pick, and is a separate tool held in the player's hand...
, Irish flutes, BagpipesBagpipesBagpipes are a class of musical instrument, aerophones, using enclosed reeds fed from a constant reservoir of air in the form of a bag. Though the Scottish Great Highland Bagpipe and Irish uilleann pipes have the greatest international visibility, bagpipes of many different types come from...
, Practice chanterPractice chanterThe bagpipe practice chanter is a double reed woodwind instrument whose main function is as an adjunct to the Great Highland Bagpipe. In this respect, its purpose is twofold. First, it is the instrument used to learn how to play the Great Highland Bagpipes before taking up the bagpipes themselves...
s and harpsichord plectra. - Dining: Fully automatic coffee brewers; knife handles (particularly folding knives)
- The Food and Drug AdministrationFood and Drug AdministrationThe Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...
has approved Delrin for use in the food industry.
Degradation
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
and oxidation by agents such as mineral acids and chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
. POM homopolymer is also susceptible to alkaline attack and is more susceptible to degradation in hot water. Both POM homopolymer and copolymer are stabilized to mitigate these types of degradation. Thus low levels of chlorine in potable water supplies (1–3 ppm) can be sufficient enough to cause stress corrosion cracking
Stress corrosion cracking
Stress corrosion cracking is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when...
to develop, a problem which has been experienced in both the USA and Europe in domestic and commercial water supply systems. Defective mouldings are most sensitive to cracking, but normal mouldings will succumb if the water is hot.
Widespread failure of acetal mouldings in potable and hot water supplies resulted in one of the largest class action
Class action
In law, a class action, a class suit, or a representative action is a form of lawsuit in which a large group of people collectively bring a claim to court and/or in which a class of defendants is being sued...
s in the USA when acetal plumbing
Plumbing
Plumbing is the system of pipes and drains installed in a building for the distribution of potable drinking water and the removal of waterborne wastes, and the skilled trade of working with pipes, tubing and plumbing fixtures in such systems. A plumber is someone who installs or repairs piping...
fittings cracked and caused flooding of homes, a problem exacerbated by similar problems with polybutylene
Polybutylene
Polybutylene is a polyolefin or saturated polymer with the chemical formula n. It should not be confused with polybutene, a low molecular weight oligomer with a different repeat unit....
pipework. The acetal fittings tended to fail first, followed by the pipework.
In chemistry applications, whilst the polymer is often suitable for the majority of glassware work, it can succumb to catastrophic failure. An example of this would be using the polymer clips on hot areas of the glassware (such as a flask to column, column to head or head to condenser joint during distillation). As the polymer is sensitive to both chlorine and acid hydrolysis, it may perform very poorly when exposed to the reactive gases, particularly hydrogen chloride. Failures in this latter instance can occur with seemingly unimportant exposures from well sealed joints, and do so without warning and rapidly (the component will split or fall apart). This can be a significant health hazard as the glass may open or smash. Here, PTFE or a high grade stainless steel may be a more appropriate choice.
Trade names & shares
Leading POM producers (approximately 800 kt market):- TiconaTiconaTicona is an international manufacturer of engineering polymers. The company's head office is located at the Ticona/Celanese-Plant in Florence, Kentucky, USA, since 2005.-Company history:...
Hostaform, Celcon (approx. 25%) - DuPontDuPontE. I. du Pont de Nemours and Company , commonly referred to as DuPont, is an American chemical company that was founded in July 1802 as a gunpowder mill by Eleuthère Irénée du Pont. DuPont was the world's third largest chemical company based on market capitalization and ninth based on revenue in 2009...
Delrin (approx. 20%) - Polyplastic Duracon (approx. 15%)
- Korea Engineering Plastics Kepital (approx. 12%)
- MitsubishiMitsubishiThe Mitsubishi Group , Mitsubishi Group of Companies, or Mitsubishi Companies is a Japanese multinational conglomerate company that consists of a range of autonomous businesses which share the Mitsubishi brand, trademark and legacy...
Lupital (ca. 7%) - BASFBASFBASF SE is the largest chemical company in the world and is headquartered in Germany. BASF originally stood for Badische Anilin- und Soda-Fabrik . Today, the four letters are a registered trademark and the company is listed on the Frankfurt Stock Exchange, London Stock Exchange, and Zurich Stock...
Ultraform (ca. 5%)
Other significant producers (less than 3%):
- Zaklady Tarnoform
- KTP Kocetal
Significant coupounders (less than 1%):
- RhodiaRhodia (company)Rhodia is a group specialized in fine chemistry, synthetic fibers and polymers. Rhodia is listed on the Paris Stock Exchange on NYSE Euronext. The company services the consumer goods, automotive, energy, manufacturing and processes and electronics markets...
Sniatal - SchulmanSchulmanSchulman, also spelled Schulmann, is a surname, usually that of a Jewish person. The name is derived from the Yiddish word shul...
Schulaform
See also
- Forensic engineeringForensic engineeringForensic engineering is the investigation of materials, products, structures or components that fail or do not operate or function as intended, causing personal injury or damage to property. The consequences of failure are dealt with by the law of product liability. The field also deals with...
- ParaformaldehydeParaformaldehydeParaformaldehyde is the smallest polyoxymethylene, it is the condensation reaction product of formaldehyde with a typical degree of polymerization of 8–100 units. Paraformaldehyde commonly has a slight odor of formaldehyde due to decomposition...
- PolymerPolymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
- Polymer degradationPolymer degradationPolymer degradation is a change in the properties—tensile strength, colour, shape, etc.—of a polymer or polymer-based product under the influence of one or more environmental factors such as heat, light or chemicals such as acids, alkalis and some salts...
- ResinResinResin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
- Dalziel HammickDalziel HammickDalziel Llewellyn Hammick FRS , was an English research chemist. His major work was in synthetic organic chemistry. He promulgated Hammick's rule, which predicts the order of substitution in benzene derivatives, while the Hammick reaction is used in the synthesis of larger molecules.-Early life:The...