Polyacetal, also known as acetal or polyoxymethylene (POM), is a formaldehyde-based, semi-crystalline engineering thermoplastic containing the functional group of a carbon bonded to two -OR groups. Other names of POM are Polyformaldehyde, Polymethylene glycol, Polyoxymethylene glycol. It is 100% recyclable.
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 thanks to its low friction & wear characteristics and its excellent balance of mechanical and chemical properties.
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Acetal for Your Need: Homopolymer or Copolymer?
Acetal resins are produced by the polymerization of purified formaldehyde [CH
2O]. However,
different manufacturing processes are used to produce the homopolymer and the
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. Here's how polyoxymethylene developed over the years:
POM is commercially available as
homopolymer under trade names, some of them include:
And,
Copolymer form under trade names:
Comparing Homopolymer Vs 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 reaction to acetic anhydride.
The
copolymer of POM requires that the formaldehyde is converted to trioxane, which can be done by acid catalysis and cationic polymerization. The reaction 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 deposit
- 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
- 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 at room temperature.(except phenols)
- 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:
- Standard/Unreinforced Grades
- Reinforced Grades: Glass fibers, carbon fibers or glass spheres-filled grades show high tensile strength or rigidity depending on the type and amount of polymer reinforcement used
- High-impact/Toughened Grades: POM blends with rubber, TPU etc. lead to higher impact strength
- Grades with High Slip/Wear Properties: Modification with additives such as graphite, PTFE, mineral fillers, etc. enhances abrasion resistant and slip properties
- UV Stabilized Grades: To improve the UV stability, hindered-amine light stabilizers and UV adsorbers are often added to the blend.
- Nanocomposites with CNTs, POSS, ZnO etc.
- Other Grades:
- Addition of powdered Al or bronze enhances electrical conductivity or heat distortion point.
- Fluorocarbons lead to good surface lubricity 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 your needs!
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
Typical Applications of Acetal Resins
Automotive – Fuel Systems and Interior Applications
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 |
Fuel Supply Unit |
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 appearance 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 Adjuster |
Seatbelt Adjuster |
Speaker Grille |
| |
Car Lock |
HVAC Control Panel Knobs |
Automotive Clip |
|
|
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 |
Insulin Syringe |
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 controling 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 |
Pipe Couplings |
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 |
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.
Popular Processing Techniques for POM
Polyacetal resins are supplied in a granulated form and, with heat and pressure, can be molded into a desired shape.
It can be
processed by all methods suitable for thermoplastics: injection molding, extrusion, compression molding, rotational casting or blow molding. However,
injection molding and
extrusion are the most important methods used.
It must be processed in the temperature range 190 – 230°C and may require drying before forming because it is hygroscopic.
Injection Molding:
- Melt Temperature-
- Homopolymer: 180-230°C
- Copolymer: 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:
Extrusion is used to produce
semi-furnished parts like sheets, rods, pipes, fimalents, 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.
Acetal has found some in-roads into
3D printing in some applications like fan blade, impeller… 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).
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.