Acetal Plastic: Properties, Production, And Uses

What is Acetal Plastic?

Acetal PLastic OR 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.

Properties of Acetal Plastic

Acetal (polyoxymethylene or POM) is a high strength, low friction engineering plastic that has excellent wear properties in both wet and dry environments. Acetal is chemically resistant to hydrocarbons, solvents, and neutral chemicals.

Acetal plastic is coveted for its excellent dimensional stability and machining profile, rivaling even metals in its properties. Below is a table including some useful properties of acetal plastic (note this table is not exhaustive).

Property	ASTM Test	Value
Density	–	1.39-1.43 g/cm3
Tensile Strength	D638	10,000 psi
Flexural Modulus of Elasticity	D790	420 ksi
Tensile Modulus of Elasticity	D638	450 ksi
Water absorption (when immersed for 24 hrs)	D570	0.25%
Heat Deflection Temperature (66 psi & 264 psi)	D648	336 °F & 257 °F
Coefficient of Friction	Dynamic	0.20

Properties Of Acetal That Make It Ideal For Manufacturing

Several of the properties of acetal make it a popular choice for the manufacturing industry. These include:

• Low friction: Conveyor systems, gears, valves, and pump parts all benefit from the low friction of acetal, allowing smooth operation.
• Strength: The overall strength of acetal is another reason it is ideal for moving parts in manufacturing and construction applications. It can glide smoothly over other acetal pieces, resists damage from repeated use, and does not break or dent easily.
• Easy machinability: Even though it is quite strong, with the right conditions, acetal is easily machinable. This means manufacturers can mold it into the exact shape they need
• Colorization: Acetal appears white in its original form, so manufacturers can color it to any shade they want, making it ideal for applications where color coding or visual appeal is important.
• Low water absorption: Acetal resists water absorption quite well, making it a good choice for wet applications in the manufacturing sector.

How Acetal is Formed

Acetals are geminal-diether derivatives of aldehydes or ketones, formed by reaction with two equivalents (or an excess amount) of an alcohol and elimination of water.

Ketone derivatives of this kind were once called ketals, but modern usage has dropped that term. It is important to note that a hemiacetal is formed as an intermediate during the formation of an acetal.

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.

Production of Acetal Plastic

Different manufacturing processes are used to produce the homopolymer and copolymer versions of POM.

Homopolymer

To make polyoxymethylene homopolymer, anhydrous formaldehyde must be generated.

The principal method is by reaction of the aqueous formaldehyde with an alcohol to create a hemiformal, dehydration of the hemiformal/water mixture and release of the formaldehyde by heating the hemiformal.

The formaldehyde is then polymerized by anionic catalysis, and the resulting polymer stabilized by reaction with acetic anhydride.

Due to the manufacturing process, large-diameter cross-sections may have pronounced centerline porosity. A typical example is DuPont’s Delrin.

Copolymer

To make polyoxymethylene copolymer, formaldehyde is generally converted to trioxane.

This is done by acid catalysis followed by purification of the trioxane by distillation and/or extraction to remove water and other active hydrogen-containing impurities.

Typical copolymers are Hostaform from Celanese and Ultraform from BASF.

The co-monomer is typically dioxolane, but ethylene oxide can also be used. Dioxolane is formed by reaction of ethylene glycol with aqueous formaldehyde over an acid catalyst. Other diols can also be used.

Trioxane and dioxolane are polymerized using an acid catalyst, often boron trifluoride etherate, BF₃OEt₂.

The polymerization can take place in a non-polar solvent or in neat trioxane. After polymerization, the acidic catalyst must be deactivated and the polymer stabilized by melt or solution hydrolysis to remove unstable end groups.

Stable polymer is melt-compounded, adding thermal and oxidative stabilizers and optionally lubricants and miscellaneous fillers.

Fabrication

POM is supplied in a granulated form and can be formed into the desired shape by applying heat and pressure.

The two most common forming methods employed are injection molding and extrusion. Rotational molding and blow molding are also possible.

Typical applications for injection-molded POM include high-performance engineering components. The material is widely used in the automotive and consumer electronics industry.

There are special grades that offer higher mechanical toughness, stiffness or low-friction/wear properties.

POM is commonly extruded as continuous lengths of round or rectangular section. These sections can be cut to length and sold as bar or sheet stock for machining.

The Differences Between Copolymer and Homopolymer Acetal Plastics

So, those are the similarities, but what makes Acetal C and Acetal H different? Well, as mentioned previously, Acetal C is a copolymer whilst Acetal H is a homopolymer.

Polymers are formed by linking a large number of units called monomers through chemical reactions otherwise known as polymerisation.

The main difference between copolymers and homopolymers is that homopolymers are created by using one monomer whilst copolymers are produced by using two types of monomer.

This means that Acetal C and Acetal H have slightly different properties.

	ACETAL H	ACETAL C
Acetal C has better chemical properties	Significant centreline porosity Good chemical resistance in PH range 4-9 Hydrolysis resistance up to 60 degrees Celsius Continuous allowable service temperature in air: 90 degrees Celsius Higher outgassing	Reduced centreline porosity Good chemical resistance in PH range 4-13 Hydrolysis resistance up to 85 degrees Celsius Continuous allowable service temperature in air: 100 degrees Celsius Lower outgassing
Acetal H has better mechanical properties	15% higher strength and stiffness 10% better creep resistance Higher Rockwell hardness rating	Less stiff, more ductile

One of the biggest differences between Acetal C and Acetal H is centreline porosity, which is a characteristic of H but not C.

Centreline porosity is caused by gasses trying to escape during the cooling process after the manufacturing process of extrusion or compression.

It can appear as small bubbles in thicker rods or a white line down the middle of each cut edge of a sheet.

There are other slight differences between Acetal C and Acetal H:

• Acetal H has better mechanical properties than Acetal, including higher strength and stiffness, better creep resistance and higher Rockwell hardness rating.
• However, Acetal C has better chemical properties than Acetal H, including reduced centreline porosity, better chemical resistance, better Hydrolosis resistance, higher continuous allowable service temperature in air and less outgassing.

Advantages of Acetal Plastic

• 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.

Disadvantages of Acetal Plastic

• 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.

Common Uses of Acetal Plastic in Manufacturing

Acetal is used for various industry components and products such as machining and parts creation, plastic and metal fabrication, semiconductors, medical equipment and instruments, and food processing and handling.

Some individual examples of applications include:

Common Alternatives to Acetal Plastic

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.

1. 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.

2. 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.

What Is the Difference Between Acetal Plastic and Delrin?

Delrin is a type of acetal homopolymer and therefore generally interchangeable with others of the same class. This means that Delrin still offers the same core benefits of general acetal plastics, such as:

• Excellent dimensional stability
• Friction, fatigue, and abrasion resistance
• Low moisture absorbance, leading to strong performance in wet conditions
• Desirable electrical properties
• Machinability to tight tolerances
• FDA & USDA compliance

The main distinctions between Delrin and other acetal plastics stem from its composition as a homopolymer it has a uniform crystalline structure rather than one incorporating multiple subunits.

This leads to better properties in select areas such as higher stiffness, flex fatigue resistance, and creep resistance when compared to copolymers. Overall, Delrin offers slight but measurable benefits in terms of mechanical strength.

One negative characteristic specific to Delrin over other acetal plastics is its increased centerline porosity an inconsistency in the center of the plastic caused by gas escape in the cooling process.

While not a concern when the center of a rod is to be drilled out, this property can be problematic in certain food and medical related applications as centerline porosity increases the risk of bacterial growth.

Here’s the main takeaway: While Delrin does have some distinguishing properties, acetal plastics share enough in common that they are often interchangeable based on budget and other project-specific characteristics.