Polyacetal 101: Detailed Information on POM Resin and Its Features

Polyacetal: Detailed Information on POM and Its FeaturesLearn more about Polyacetal or Polyoxymethylene (POM), a semi-crystalline engineering thermoplastic widely used to produce precision parts with high lubricity. Get detailed information on key features which make acetal resins an ideal material of choice in applications ranging from automotive to medical, industrial and many more.

Quick Links to the Detailed Profile:

  »  What is a polyacetal?
  »  Acetal for Your Need: Homopolymer or Copolymer?
  »  Benefits Offered by Polyoxymethylene Resins
  »  What Limits Acetal Resins Use?
  »  Typical Applications in Automotive, E&E, Medical, etc.
  »  Popular Processing Techniques for POM
  »  Properties of Different Types of POM Resins

What is a polyacetal?

Polyacetal, also commonly known as acetal or polyoxymethylene (POM), is a formaldehyde-based, semi-crystalline engineering thermoplastic which contains the functional group of a carbon bonded to two -OR groups. It is 100% recyclable. POM is known as polyformaldehyde, polymethylene glycol and polyoxymethylene glycol.

Molecular Structure of Polyoxymethylene
Molecular Structure of Polyoxymethylene
(Chemical Formula: (CH2O)n)

POM resins are widely used in the production of precision parts for applications demanding good dimensional stability and sliding properties. Some of them include:

The polymer serves as an alternative to metals due to its low friction and wear characteristics as well as its excellent balance of mechanical and chemical properties.

Facts to Know

How polyoxymethylene was developed over the years?

History of Polyoxymethylene Production

Acetal for Your Need: Homopolymer or Copolymer?

Acetal resins are produced by the polymerization of purified formaldehyde [CH2O]. However, different manufacturing processes are used to produce the homopolymer and copolymer versions of POM. In alkaline environments, copolymers are more stable than the homopolymers. On the other hand, homopolymers provide better mechanical properties than copolymers.

POM is commercially available in different form. Homopolymer resins include:

And, popular copolymer resins are available under the following trade names:

» View all POM commercial grades and suppliers in Omnexus Plastics Database

This plastic database is available to all, free of charge. You can filter down your options by property (mechanical, electrical…), applications, conversion mode and many more dimensions.

Comparing Properties of Polyacetal Homopolymer and Copolymer

Acetal homopolymer is produced from anhydrous, monomeric formaldehyde which is polymerized by anionic catalysis in an organic liquid reaction medium. The resulting polymer is stabilized by the reaction to acetic anhydride.

While, the copolymer of POM requires the conversion of formaldehyde into trioxane using acid catalysis and cationic polymerization. The reaction is followed by purification of the trioxane by distillation or extraction to remove water and other active impurities containing hydrogen.

Acetal Copolymer Acetal Homopolymer
  • Easier to process / wider processing window
  • Superior long-term performance (creep resistance, fatigue, endurance, strength retention)
  • Less gassing and odor
  • Heavy metal free colors, i.e. cadmium and lead (safer for workers / environment)
  • Better maintenance of color
  • Under ultraviolet light exposure
  • Faster molding cycles
  • Less mold deposits
  • Stable in alkaline environments
  • Available in several viscosity ranges
  • Greater degree of regularity in their structure
  • Higher tensile strength 
  • Unfilled homopolymer is stiffer and stronger
  • Moderate toughness under repeated impact
  • Allows thinner and lighter part design
  • Shorter molding cycles 
  • Potential for cost reductions
  • Provide better mechanical properties

Benefits Offered by Polyoxymethylene Resins

Polyoxymethyene resins demonstrate well-balanced properties ranging for mechanical to physical and flammability performance. The key benefits of POM resins include:
  • Excellent mechanical properties over a temperature range upto 140°C, down to -40°C
    • High tensile strength, rigidity and toughness (short-term)
    • Low tendency to creep (as compared to nylon) and fatigue (long-term). Not susceptible to environmental stress cracking
  • High degree of crystallinity and excellent dimensional stability
  • Excellent wear resistance
  • Low coefficient of friction
  • Good resistance to organic solvents and chemicals (except phenols) at room temperature
  • Low smoke emission
  • High gloss surfaces
  • Low moisture absorption

POM grades are often produced with various degrees of polymerization resulting in different properties to meet demanding applications. The different forms of POM resins are discussed below:

  1. Standard/Unreinforced Grades

  2. Reinforced Grades: Glass fibers, carbon fibers or glass spheres-reinforced POM grades show high tensile strength or rigidity depending on the type and amount of polymer reinforcement.

  3. High Impact/Toughened Grades: Blending POM resins with rubber, TPU and other polymers results in blends with higher impact strength.

  4. Grades with High Slip/Wear Properties: Modification of POM resins with additives such as graphite, PTFE, mineral fillers, etc. enhances abrasion resistant and slip properties

  5. UV Stabilized Grades: UV stabilizers, such as hindered-amine light stabilizers and UV absorbers are often added to POM resins or its blends to improve the UV stability.

  6. Nanocomposites: Additives, such as CNTs, POSS, ZnO, etc. are used to produce POM nanocomposites

  7. Other Grades: 
    1. Addition of powdered Al or bronze enhances electrical conductivity or heat distortion point of POM resins.
    2. Fluorocarbons lead to good surface lubricity in polyacetal to prevent cracking

Looking for suitable resin for your application? Compare properties of several POM grades (unreinforced, modified, low-friction, mineral-filled) and make the right selection matching to meet your needs.

Acetal Resins Over Metals and Other Thermoplastics

Benefits of Acetal Resins Over Metals and Other Thermoplastics

What Limits Acetal Resins Use?

  • Poor resistance to strong acids, bases and oxidizing agents.
  • Burns easily without flame retardants due to high oxygen content
  • Poor thermal stability without suitable stabilizer system
  • Limited processing temperature range
  • High mold shrinkage
  • Poor resistance to UV radiation. Prolonged exposure lead to color change, enbrittlement, and loss of strength
  • Low surface energy and hence difficult to bond without surface treatment

Acetal Polymers Bonding

One of its limitations, as mentioned above, is bonding issues associated with acetal polymers. However, POM bonding can be improved by applying special treatment processes such as surface etching, flame treatment or mechanical abrasion… Know more about several bonding solutions available for polymeric materials.

Typical Applications of Acetal Resins

Automotive - Modern Fuel Systems

Only a few polymers can withstand permanent contact with the diverse and increasingly aggressive automotive fuels used today and the increasing temperatures encountered in engine compartments. That is why acetal copolymer (POM) is the preferred material for modern fuel systems.

Typical applications in fuel systems are extremely versatile. They include components in fuel caps, fuel filler necks, fuel sender units (e.g. flanges or swirl pot), lifetime filter, valves, fuel pumps, and fuel rails, among many others.

Fuel rails using POM
Fuel Rails
Fuel supply unit using POM
Fuel Supply Unit
Vapor control valves using POM
Vapor Control Valves

Not only do these products have excellent long-term resistance to gasoline, diesel and methanol or ethanol-based fuels, they are also able to withstand temperatures of more than 100°C.(212° F).

Automotive - Interior Appearance

To meet the current trends in automotive interiors for soft, warm finishes, polyoxymethylene offers a molded-in low-gloss effect for automotive interior parts. Also, it provides a system cost benefit when compared to painted components, such as painted PC-ABS. Other benefits include:

  • Durable low-gloss surface
  • Resistant to cleaning solutions
  • Excellent dimensional stability
  • Provides design flexibility

Potential applications within automotive interiors requiring LOW GLOSS include:

Seatbelt adjusters using POM
Seatbelt Adjuster
Fuel Door Release Lever using POM
Seatbelt Adjuster
Speaker Grille using POM
Speaker Grille
Car Lock - Automotive interiors using POM
Car Lock
Automobile HVAC Control Panel Knobs using POM
HVAC Control Panel Knobs
Automotive interior clips using POM
Automotive Clip

POM for Low VOC Performance

The issue of in-vehicle air quality is becoming increasingly important in the automotive industry due to its direct impact on passenger safety and comfort. The reduction of VOCs significantly contributes to an improvement of the cabin air quality. Thus, both POM material suppliers and part manufacturers are keeping it in the mind while developing vehicle components and contributing to a better air quality in vehicles closed environment.

Few examples showcasing the development of POM engineering plastics meeting the growing trend include:

  1. Delrin® 300TE acetal resin by DuPont – DuPont has developed an impact-modified, low-emission grade suitable for use in automotive interiors. According to the company, tests of samples of Delrin® 300TE revealed formaldehyde emissions of 1.0 mg/m² and lower.

  2. DURACON® acetal grades by Polyplastics – Polyplastics has designed low-VOC polyoxymethylene (POM) resin grades utilizing utilizes a technology which reduces the quantity of residual formaldehyde within resin pellets through the use of optimal stabilizers and scavengers. The range includes weather-resistant DURACON® M90-45LV, high-sliding DURACON® NW-02LV resin, a glass-reinforced grade GH-25LV and many more.

  3. Tenac™ -C Z4520 POM Copolymer by Asahi Kasei – This POM grade has been certified as an eco-friendly green material by the China Automotive Technology & Research Center (CATARC) for its low VOC emission performance.

Medical and Healthcare

Materials of construction play a central role in the design of new equipment. As the patient community demands increased safety and accuracy from providers, these dictates are ultimately met through high-performance materials.

Dry powder inhaler - Health care applications using POM
Dry Powder Inhaler
Insulin syringe - Health care applications using POM
Insulin Syringe
Electrical toothbrush - Health care applications using POM
Electrical Tooth Brush

Using engineering plastics in medical technology can help reduce total manufacturing cost, through consolidation of multiple parts into a single unit and by implementation of automated assembly processes. POM addresses the challenges of mission-critical components and offers an array of leading edge materials that are excellent candidates for medical applications.

Acetal copolymers, are easily-processed, highly-crystalline plastics delivering high strength, stiffness, toughness and lubricity over a broad range of temperatures and chemical environment. These polymers offer low extractable and high purity and are FDA Compliant and pharma-friendly – animal and latex free.

Industrial Uses

Pumping, conveying and controlling liquids are important factors in the irrigation, plumbing and process industries. For aqueous fluids, acetal copolymers have an extensive history in parts such as housings, taps and valves, and couplings.

These parts are found in many fluid handling applications including plumbing, irrigation, water softeners, beverage dispensers, water filters, shower heads, sprinklers, water meters, and pumps.

Shower head - Industrial applications using POM
Shower Head
Pipe couplings - Industrial applications using POM
Pipe Couplings
Automatic water valve - Industrial applications using POM
Automatic Water Valve

The acetal copolymer materials offer good flow and moldability, and their very low moisture absorption permits dimensional stability in contact with water.

Consumer Goods

Low fuel permeation to meet new CARB and EPA regulations on evaporative emissions. Small Off-Road Engines (SORE) and other types of gasoline-powered equipment have recently come under new regulations from the US EPA and California (CARB) to limit the amount of evaporative emissions occurring throughout the fuel system, including fuel tanks, caps and hoses.

Fuel tank - Consumer applications using POM
Fuel Tank
Lawn tractor - Consumer applications using POM
Lawn Tractor

Because of its extremely low permeability to gasoline and ethanol, along with excellent long-term chemical resistance and dimensional stability acetal copolymer has been evaluated in small off-road engine fuel tanks found in lawn & garden and other gasoline powered equipment, including recreation vehicles and marine engines, to meet recently adopted CARB and US EPA regulations.

Processing Techniques for POM

Polyacetal resins are supplied in a granulated form and can be molded into a desired shape by applying heat and pressure. It can be processed by all methods suitable for thermoplastics, such as injection molding, extrusion, compression molding, rotational casting or blow molding. Injection molding and extrusion are the most commonly used methods for POM processing.

POM resins must be processed in the temperature range 190 – 230°C and may require drying before forming because it is hygroscopic.

Processing Conditions for Injection Molding

  • Melt temperature
    • Homopolymer resins: 180-230°C
    • Copolymer resins: 190-210°C
  • Mold temperature: 50-150°C. Use higher mold temperatures for precision molding for reduced post-molding shrinkage.
  • Injection pressure: 70-120 MPa
  • Injection speed: Medium to high

Extrusion Processing Conditions

Extrusion is used to produce semi-furnished parts, such as sheets, rods, pipes, filaments, profile sections, etc. which are further machined using traditional methods such as turning, milling, drilling, etc. to form finished parts.

  • Melt temperature: 180-230°C
  • Screw speed: 33-42
  • Die temperature: 175-230°C

Injection Molding / Extrusion: How to Avoid Plastic Quality Crashes

Lightly crosslinked grades are used to produce hollow molding by blow molding.

3D Printing of Acetal Grades

Acetal has found some in-roads into 3D printing in some applications like fan blade, impeller, etc. Its high lubricity surface (with 3-5% on average and as high as 7-10%) makes it interesting for 3D printing especially for difficult to release parts. Also, acetal polymers have high strength which assures dimensional stability up to a maximum continuous service temperature of 80°C (180°F).

Properties of Different Types of POM Resins

Keeping information about the properties of a thermoplastic beforehand is always beneficial. This helps in selecting the right engineering thermoplastic for a particular application. It also assists in evaluating if the end use requirement would be fulfilled or not.

Compare all relevant properties of several POM Grades (Unreinforced, Impact-modified, Low-Friction, Mineral Filled). From physical properties, dimensional stability, electrical performances to fire and thermal properties, find out every possible attribute with its values here.

Property Unreinforced Impact Modified Low Friction Mineral Filled
Dimensional Stability
 Coefficient of Linear Thermal Expansion 10- 15 x 10-5 /°C 12- 13 x 10-5 /°C 10 - 12 x 10-5 /°C 8 - 9 x 10-5 /°C
 Shrinkage 1.8 - 2.5 % 1 - 2.5 % 1.8 - 3 % 1.5 - 2 %
 Water Absorption 24 hours 0.15 - 0.5 % 0.3 - 0.5 % 0.2 - 0.27 % 0.2 - 0.5 %
Electrical Properties
 Arc Resistance 200 - 220 sec 120 sec 126 - 183 sec -
 Dielectric Constant 3.3 - 4.7  4 - 4.3  3 - 4  -
 Dielectric Strength 13.8 - 20 kV/mm 19 kV/mm 16 kV/mm -
 Dissipation Factor 50 - 110 x 10-4 50 - 250 x 10-4 20 - 90 x 10-4 -
 Volume Resistivity 14 - 15 x 1015 Ohm.cm 15 - 16 x 1015 Ohm.cm 15 - 16 x 1015 Ohm.cm -
 Fire Performances
 Fire Resistance (LOI) 18 % 18 % - -
 Flammability UL94 HB HB HB HB
Mechanical Properties
 Elongation at Break 15 - 75 % 60 - 200 % 10 - 70 % 5 - 55 %
 Elongation at Yield 8 - 23 % 10 - 15 % - -
 Flexibility (Flexural Modulus) 2.8 - 3.7 GPa 1.4 - 2.3 GPa 2 - 3 GPa 4 - 5.5 GPa
 Hardness Rockwell M 75 - 94  35 - 79   58 - 94   83 - 90 
 Hardness Shore D 80 - 95  80 - 92  80 - 95  92 - 95 
 Stiffness (Flexural Modulus) 2.8 - 3.7 GPa 1.4 - 2.3 GPa 2 - 3 GPa 4 - 5.5 GPa
 Strength at Break (Tensile) 60 - 70 MPa 45 - 60 MPa 50 - 70 MPa 50 - 75 MPa
 Strength at Yield (Tensile) 54 - 78 MPa 35 - 50 MPa 48 - 69 MPa 54 - 78 MPa
 Toughness (Notched Izod Impact at Room Temperature) 60 - 120 J/m 90 - 250 J/m 25 - 60 J/m 50 - 65 J/m
 Toughness at Low Temperature (Notched Izod Impact at Low Temperature) 53 - 250 J/m - 32 - 53 J/m -
 Young Modulus 2.8 - 3.7 GPa 1.4 - 2.3 GPa 1.8 - 3 GPa 4 - 5.5 GPa
Physical Properties
 Density 1.41 - 1.42 g/cm3 1.3 - 1.35 g/cm3 1.4 - 1.54 g/cm3 1.5 - 1.6 g/cm3
 Glass Transition Temperature -60 - -50 °C -
 Radiation Resistance
 Gamma Radiation Resistance Fair
 UV Light Resistance Poor
 Service Temperature
 Ductile / Brittle Transition Temperature -40 °C -50 to -40 °C -40 °C -
 HDT @0.46 Mpa (67 psi) 158 - 172 °C 145 - 165 °C 168 - 172 °C 158 - 175 °C
 HDT @1.8 Mpa (264 psi) 110 - 136 °C 64 - 90 °C 118 - 136  °C 100 - 140 °C
 Max Continuous Service Temperature 80 - 105 °C 80 - 100 °C 80 - 105 °C 80 - 105 °C
 Min Continuous Service Temperature -40 °C -50 to -40 °C -40 °C -
 Sterilization Resistance (Repeated) Poor
 Thermal Insulation (Thermal Conductivity) 0.31 - 0.37 W/m.K - 0.31 W/m.K -
Chemical Properties
 Acetone @ 100%, 20°C Limited
 Ammonium hydroxide @ 30%, 20°C Satisfying
 Ammonium hydroxide @ diluted, 20°C Satisfying
 Ammonium hydroxide @ diluted, 60°C Satisfying
 Aromatic hydrocarbons @ 20°C Limited
 Benzene @ 100%, 20°C Satisfying
 Butylacetate @ 100%, 20°C
 Chlorinated solvents @ 20°C Limited
 Chloroform @ 20°C Non Satisfactory
 Dioctylphtalate @ 100%, 20°C Limited
 Ethanol @ 96%, 20°C Satisfying
 Ethyleneglycol (Ethane diol) @ 100%, 20°C Limited
 Ethyleneglycol (Ethane diol) @ 100%, 50°C
 Gasoline Satisfying
 Glycerol @ 100%, 20°C Satisfying
 Grease @ 20°C Satisfying
 Hydrogen peroxide @ 30%, 60°C Non Satisfactory
 Kerosene @ 20°C Satisfying
 Methanol @ 100%, 20°C Satisfying
 Methylethyl ketone @ 100%, 20°C Limited
 Mineral oil @ 20°C Satisfying
 Phenol @ 20°C Non Satisfactory
 Silicone oil @ 20°C Limited
 Sodium hydroxide @ 10%, 20°C Satisfying
 Sodium hydroxide @ 10%, 60°C Limited
 Sodium hypochlorite @ 20%, 20°C Non Satisfactory
 Strong acids @ concentrated, 20°C
 Toluene @ 20°C Satisfying
 Toluene @ 60°C Satisfying
 Xylene @ 20°C Satisfying

Commercially Available POM Grades

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