What is a Polyacetal?
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
(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:
- Automotive
- Electrical & electronic
- Industrial
- Drug Delivery
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.
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How polyoxymethylene was developed over the years?
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Acetal for Your Need
Acetal for Your Need
Acetal resins are produced by the polymerization of purified formaldehyde [CH2O]. However, different manufacturing processes are used to produce the homopolymer andcopolymer 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
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- 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
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Benefits and Limitations of POM Resins
Benefits and Limitations of POM Resins
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:
- Standard/Unreinforced Grades
- 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.
- High Impact/Toughened Grades: Blending POM resins with rubber, TPU and other polymers results in blends with higher impact strength.
- 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
- 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.
- Nanocomposites: Additives, such as CNTs, POSS, ZnO, etc. are used to produce POM nanocomposites
- Other Grades:
- Addition of powdered Al or bronze enhances electrical conductivity or heat distortion point of POM resins.
- 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.
Check out the benefits of acetal resins over metals and other thermoplastics below.
| Benefits over Metals |
Benefits over Thermoplastics
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- Design flexibility
- High strength-to-weight ratio
- Color matching possibility
- Inherent lubricity
- Lower finished part cost
- Opportunities for parts consolidation
- Chemical/corrosion resistance
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- Low coefficient of friction
- Good creep resistance
- Good toughness/impact resistance
- Hard surface with good appearance
- High strength and stiffness
- Excellent dimensional stability
- Excellent chemical resistance
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POM Resins – What Limits Its 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.
Processing Techniques for POM
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
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).