Acetal Plastic: What You Need to Know
Learn about this material, its properties, uses, advantages, and alternatives.
Plastic is one of the most essential materials in today’s world, yet not many people understand the variety and differences between plastics. There are thousands of blends of plastics available, making it difficult for even the informed to decide on what to use for their next project.
This article will highlight acetal plastic materials, or polyoxymethylene (POM), and its characteristics as an engineering material, to show where it can be specified and its beneficial qualities.
POM plastic (polyacetal) is a semi-crystalline thermoplastic that is based on the formaldehyde molecule. For this reason, it can also be found as polyformaldehyde, polyethylene glycol, or polyoxymethylene glycol. Its polymers consist of carbon functional groups bonded to two -OR groups, leading to an exceptional blend of mechanical and chemical properties.
Acetal plastic is commonly used as an alternative to metals, thus it finds applications in mechanical gears, electrical components, automotive products, sports equipment, medical products, food equipment, hardware, construction tools, and much more.
Flexural Modulus of Elasticity
Tensile Modulus of Elasticity
Water absorption (when immersed for 24 hrs)
Heat Deflection Temperature (66 psi & 264 psi)
336 °F & 257 °F
Coefficient of Friction
Data Source: Acetal Material Properties | Curbell Plastics
The formation of Acetal plastics begins by distilling hydrocarbon chains into smaller parts and then polymerizing them using catalysts. The method of formulation will depend on the type of acetal plastic and is generally split into homopolymer acetal and copolymer acetal processes.
The organic chemistry of acetal polymers can give even scientists a headache—but a brief description of the process used for the formulation of this semi-crystalline thermoplastic follows the image below, which shows the chemical structure of polyoxymethylene:
Polyoxymethylene chemical structure.
Image Credit: Wikimedia
In the acetal homopolymer plastic process, aqueous formaldehyde building blocks are mixed with alcohols. They react together to form hemiformal, which is then distilled/dehydrated to release the formaldehyde. These monomers are polymerized in the presence of a catalyst to ultimately form the acetal homopolymer.
The copolymer process is much more involved; it first begins when formaldehyde is acid-catalyzed to form 1,3,5-trioxane. Separately, co-monomers dioxolane or ethylene oxide are formed by reacting ethylene glycol with aqueous formaldehyde in catalysis. The trioxane and dioxolane are polymerized together (again, in catalysis), and the resulting polymer is stabilized via melt or solution hydrolysis to remove unstable byproducts and introduce fillers, lubricants, stabilizers, etc.
Below is a table showing a brief comparison between acetal homopolymer vs. copolymer. In general, one would choose homopolymers over copolymers for their increased mechanical properties, while copolymers are generally more stable.
More structurally regular
Enhanced performance in long-term applications (better creep and fatigue resistance)
Molding cycles are shorter
More stable in alkaline environments and in chemical attacks
Enhanced mechanical characteristics (tensile strength, impact strength)
Minimal odor/gas release
Stiffer and stronger
Wider range of processing temps/times
Allows for thin wall designs and lighter parts
Better protected from UV exposure
Comes in a variety of viscosities
More dimensionally stable
Good color retention and requires no heavy metals for coloring
Acetal is a versatile engineering thermoplastic, demonstrating an outstanding balance of strength, performance, and workability. Below is a brief list of the advantages of acetal plastics:
- Exhibits dimensional stability and creep resistance when machined or worked
- Low friction coefficient (or “slippy”), leading to resistance to wear and abrasion
- Low moisture absorption in both wet and dry environments
- High tensile strength and rigidity
- Chemically resistant to fuels and organic solvents
- Low smoke emission
- Highly aesthetic surface finishes
- FDA approved and is 100% recyclable
- Can be impregnated/ blended with graphite, rubbers, glass-filled, nanocomposites, metals, etc., for additional unique material properties.
Acetal plastics come with tradeoffs for their great qualities. Below is a brief list of the disadvantages of acetal plastics:
- Chemically weak to strong acids, bases, and oxidizers
- Prone to quick-burning (without flame retardants) due to high oxygen concentration
- Shrinks in mold significantly
- Poor resistance to UV radiation without additives; will degrade in color and strength if left in the sun
- Difficult to bond/glue without significant surface treatment
- Must manage temperatures when working/machining due to small working range
- Harder to machine than metals
- Toxic if inhaled/ingested in liquid form
Acetal plastic has a variety of applications, both general-purpose and industry-specific. Below is a list of some of the common applications of acetal plastic:
- Mechanical components such as gears, pumps, valves, nuts, fasteners, etc.
- Food equipment such as pumps, conveyors, tanks, etc.
- Automotive components such as power window components, door locks, knobs, indicators, etc.
- Medical device components
- Fixtures, plumbing parts, bearings
- Clothing zippers
- Lawn equipment
and much more.
Acetal plastic comes in a variety of blends and similar formulations, making it easy to find an alternative if it doesn’t exactly fit the necessary material profile. Below is a brief exploration into other plastics that can function in the place of acetal plastics, given some key considerations.
Delrin: Delrin is a brand-specific POM homopolymer from DuPont and is widely utilized in industry. It is distinct from general acetal plastic, as acetal plastic is typically in a copolymer form, while Delrin is a specific homopolymer blend from DuPont. Delrin is generally specified in industrial applications such as valve components, pumps, gears, insulators, rollers, etc.
Nylon: Nylon is a thermoplastic that sports higher temperature resistance, tensile strength, stiffness, and lower costs than acetal plastic, making it a common alternative. Nylon can generally be used in place of acetal plastics; however, it is less dimensionally stable, less chemically/wear-resistant, and much more affected by humidity and moisture. Nylon is used throughout general-purpose applications and can be found in clothing, consumer goods, electronics, etc.
Polybutylene Terephthalate (PBT): PBT is sometimes exchangeable with acetal plastics, as it is nearly identical in mechanical characteristics. The primary consideration with PBT is that it will require more dehumidification and cannot be used in wet environments and has lower shrinkage than acetals. PBT is commonly used in electronics, sports clothing, bathroom fixtures, computer keyboards, household appliance components, etc.
Delrin is a brand-specific homopolymer blend of acetal plastic, while acetal plastic generally refers to the more common copolymer-type material. Delrin is produced and patented under DuPont, while many other companies produce a variety of generic and brand-specific acetals for use in industry. Delrin is typically specified for industrial applications, while acetal plastics include all the possible applications in both industrial and consumer markets. To learn more, see our guide on Acetal vs. Delrin.
Acetals are composed of stable monomers with two -OR group linkages, while hemiacetals are reaction intermediates with one -OR linkage that are highly unstable and are generally only found in cyclic forms (for example, glucose, the sugar molecule, contains a hemiacetal carbon but is a ring molecule). The most important takeaway is that acetal can be made into a stable plastic, while hemiacetals can not and are usually just a part of reaction mechanisms found in the acetalization chapter of organic chemistry class. To learn more, see our guide on Acetal vs. Hemiacetal.
As previously stated, acetal plastics are FDA-approved for food-handling use in their solid form. However, there are three main points to consider when discussing how acetal plastics affect human health:
- off-gassing fumes
- invisible flames
- unstudied long-term exposure
When overheated, acetal plastics will give off gasses that are unmistakably odorous and have an unhealthy smell. These gasses are especially dangerous since formaldehyde (the building block that makes acetal plastics) is given off as vapor/particulates and is a known carcinogen. Fumes will cause moderate to severe irritation to the eyes, nose, and throat, as well as headaches and other short-term symptoms.
Acetal plastics have a major hazard in that if ignited, they produce flames that are invisible to the naked eye. The surface just looks as if it is bubbling, but the flame will ignite any combustible material nearby and can be extremely dangerous if unnoticed. The additional issue is that formaldehyde is off-gassed when the material is burning, creating a double-danger scenario.
Finally, there is no major study to show the long-term health consequences of chronic acetal plastic exposure, but some individuals report increased cancer, thyroid, and general health issues (especially if regularly around vapors/gasses). Regardless of the lack of studies, it is recommended that proper safety equipment, ventilation, and procedures be put in place to reduce all exposure to acetal particles and fumes to minimize this unknown risk.
Acetal plastics in their solid form regularly encounter food and are, as a result, deemed non-toxic by the USDA and FDA. However, if allowed to burn or overheated, acetal plastics release formaldehyde which becomes a significant toxicity hazard for nearby personnel. While the plastic is non-toxic on its own, any fumes from machining, working, heating, etc. constitute an important source of toxicity and should be avoided at all costs.
This article presented what acetal plastic is and its use throughout industry as a versatile engineering material. We hope this article helped readers understand this widely used polymer, its properties, and alternatives that are available for consideration.
Xometry offers acetal plastic material in sheet and rod form factors in various sizes. We also provide 3D printing services and plastic injection molding services for all of your production needs. Visit our website to explore the full range of our capabilities or to request a free, no-obligation quote.
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- Acetal Material Properties | Curbell Plastics
- Polyoxymethylene (Acetal Plastic): POM Material Properties & Applications (specialchem.com)
- Material Properties of Acetal – General Purpose Plastic (dielectricmfg.com)