Polyoxymethylene

  • Polyacetal
  • Polyformaldehyd (with short-chain molecules)
  • Polytrioxane
  • Designation: POM
  • Formaldehyde (homopolymer)
  • Trioxane (oligomer )

Thermoplastic

A white solid; partially crystalline

Fixed

1:39 to 1:42 g/cm3

75 % -85 %

2600-3100 MPa

0.35

0.2 % -0.4 % in normal climate

  • 40 % -50 % (POM -H, elongation at break)
  • 27% -31 % (POM -C, elongation at break)
  • 0.31 W · m -1 · K-1 (POM -C)
  • 0.37 W · m -1 · K-1 (POM -H)

1.1 · 10-4 K-1

(Hereinafter abbreviated POM, also polyacetal, Polyformaldehyd or merely acetal ) polyoxymethylene is a high molecular weight thermoplastic. Discovered in the 1920s by Hermann Staudinger as part of its investigations into macromolecules, POM was synthesized in 1952 by DuPont and the production in 1956 applied for a patent. It is used because of its high stiffness, low friction and excellent dimensional stability and thermal stability as an engineering plastic, especially for precision parts.

  • 3.1 bonding

Properties

POM is characterized by high strength, hardness and stiffness over a wide temperature range. It retains its high toughness down to -40 ° C, has a high abrasion resistance, low coefficient of friction, high heat resistance, good slip properties, good electrical and dielectric properties and low water absorption and can replace metals in many cases. The natural color is due to the high crystallinity opaque white, but the material is covered in every color to color. POM has a suitable density of = 1.410 to 1.420 g/cm3 and is for service temperatures up to 130 ° C.

At high processing temperatures above 220 ° C or when burning POM begins to thermally decompose. It forms by factors such as free formaldehyde, which developed a recognizable, irritating odor. This is not only a negative; compared to other engineering plastics is therefore very easy to see when the material is processed incorrectly. Due to requirements in the automotive industry to reduce emissions within a vehicle, manufacturers develop POM emission (so-called formaldehyde- reduced ) types.

Production

A distinction is made between the homo- and copolymers which are prepared by different methods.

Homopolymer

Chemically, the (homo) polymer has the structure - ( CH 2 O ) n and differs mainly by the degree of polymerization of paraformaldehyde. The homopolymer is referred to as POM -H. For stabilization to prevent acid at impact or thermal stress depolymerization, the end groups are closed by etherification or esterification. The homopolymer is usually obtained by direct polymerization of formaldehyde. A typical homopolymer as Delrin (DuPont). Polyoxymethylene can be obtained by three or cationic transition metal centered cationic polymerization of trioxane ( -CH2- O-).

Copolymer

Another possibility to stabilize against acid impact and thermal stress is the production of copolymers, POM -C having the structure: - [( CH2 -O) n- ( CH2-CH2- O-) m], which, by copolymerization of trioxane with one 4 -dioxane is obtained. Here the unstable end groups are degraded by hydrolysis to formaldehyde to stabilize. Typical copolymers are for example Hostaform ( Ticona / Celanese) and Ultra shape (BASF ).

Both forms can be distinguished by the melting point. Homopolymer which melts at 178 ° C, the copolymer at 166 ° C.

Processing

Further processing can be done via injection molding at 195 ° to 225 ° C (POM -H) or 180 to 230 ° C ( POM-C ) and extrusion. Also, by extrusion, the material can be processed, also a chipping processing is possible.

Stick

POM is one of the low surface energy plastics and is without any special surface treatment of only partly glued. By oxidation or etching the surface of the adhesion of adhesives can be improved.

Oxidation can be achieved by flaming with an oxygen- supersaturated flame ( Bunsen burner ). To the flame is guided at a small distance above the surface quickly. For reinforced the bonding surfaces must be etched with 85% phosphoric acid for 10 seconds at 50 ° C and then rinsed with distilled water. The pickled surface can then be bonded, for example with a 2- component adhesive.

Use

  • Engineering: gears, sliding and guiding elements, housing parts, spring elements, chains, bolts, nuts, fan wheels, pump parts, valve body.
  • Electrical Engineering: insulators, bobbins, connectors, parts for electronic devices, such as televisions, phones, etc.
  • Vehicle: Steering column (including lever for light, turn signals ), power windows, door lock systems, joint sockets.
  • Model: Thin, highly stressed parts for model railways, such as bogies and handle bars. POM breaks under load slightly less than ABS, but is translucent and not be painted in bright colors.
  • Medicine: Insulin
  • Furniture: Fittings, locks, handles, hinges, or even curtains roles.
  • Construction: Structural use of glass - sleeves for point fixings
  • Packing: aerosol cans, vehicle tanks, gas ampoules.
  • Sports: Paintball accessories, especially bolts.
  • Clothing: zippers.
  • Music: Picks for stringed instruments (see Tortex ) as a substitute for tortoiseshell formerly used; Since recently, for wind instruments, in particular " Irish " flutes and tin whistles. Perfect substitute material for raven quills in the harpsichord.
  • Gastronomy: in fully automatic coffee this material is used for the brew group.

The use of POM for the manufacture of materials and articles intended to come into contact with food is regulated by the Food, Commodities and Feed Code. May only be used POM with a melt index of 50 g/10 min maximum ( MFI 190 /2, 16). In addition, the maximum permissible amounts of catalysts and other necessary for the production and processing materials are regulated. For storage and packaging of acidic contents with a pH below 2.5 is not suitable POM.

Trade names and market shares

Market leader in the approximately 800,000 t large POM market currently are:

  • Ticona / Celanese Hostaform, KEMATAL, Celcon (approx. 25%),
  • DuPont Delrin (approx. 20%),
  • Polyplastics DURACON (about 15 %),
  • Korea Engineering Plastics Kepital (approx. 12%),
  • Mitsubishi Iupital ( about 7%) and
  • BASF Ultra form (about 5 %).

Other notable producers ( under 3 %):

  • Zakłady Azotowe w Tarnowie - Mościcach Tarnoform
  • Kolon Plastics ( KTP) Kocetal
  • Formosa Plastics Formocon

Significant compounders (below 1% ):

CAS registry number Structural robustness 1,4-Dioxane Insulin pen Griff Rhodia (company)
227297
de