Polyoxymethylene (POM): All You Need to Know

Polyoxymethylene (POM), also known as acetal plastic, is an engineering thermoplastic used in precision parts that require high stiffness, low friction, and excellent dimensional stability. It is commonly found in applications such as automotive, consumer electronics, and medical devices due to its desirable mechanical properties and ease of fabrication.

 

What is Polyoxymethylene?

Polyoxymethylene (POM), also known as acetal plastic, is an engineering thermoplastic used in precision parts that require high stiffness, low friction, and excellent dimensional stability.

Development of Polyoxymethylene (POM)

The development of POM dates back to the early 20th century, but it wasn't until the 1950s that its potential as a commercial plastic was fully realized. The first successful synthesis of POM was achieved by Hermann Staudinger, who later won a Nobel Prize for his pioneering work in polymer chemistry. Industrial production began in the 1960s, led by companies like DuPont and Celanese, marking the start of its widespread use in various industries.

Production of Polyoxymethylene (POM)

Homopolymer

Homopolymer POM is produced by the polymerization of formaldehyde. This type of POM is characterized by a very high crystallinity, leading to superior mechanical properties such as stiffness and strength. However, it can be more prone to thermal degradation and has lower chemical resistance compared to its copolymer counterpart.

Copolymer

Copolymer POM is made by polymerizing formaldehyde with other monomers like ethylene oxide. This results in a material with lower crystallinity, which improves its thermal stability and chemical resistance. The copolymer variant of POM is often preferred for applications that require exposure to harsh environments.

What Is the Difference Between Copolymer and Homopolymer Acetal Plastic?

The main difference between copolymer and homopolymer acetal plastics lies in their chemical structure and resulting properties. Homopolymer acetal has a higher degree of crystallinity, leading to higher stiffness and strength but lower thermal stability and chemical resistance. Copolymer acetal, on the other hand, has improved thermal stability and chemical resistance but slightly lower mechanical properties.

Fabrication of Polyoxymethylene (POM)

POM can be fabricated using various methods such as injection molding, extrusion, and machining. Injection molding is the most common method, allowing for high precision and complex shapes. Extrusion is used for producing continuous shapes like rods and sheets, while machining is employed for creating detailed and intricate parts.

Typical Mechanical Properties

POM is known for its exceptional mechanical properties, which include:

• High tensile strength: POM can withstand significant force without breaking.
• Low friction: It offers smooth movement in applications requiring sliding or rotational motion.
• Excellent dimensional stability: POM maintains its shape and size under various conditions.
• High stiffness and rigidity: It provides structural integrity in demanding applications.
• Good fatigue resistance: POM can endure repeated stress without failure.

These properties make POM an ideal choice for applications that require precision, durability, and reliability.

Availability and Price

POM is widely available in various forms, including sheets, rods, and custom shapes. It is produced by several major manufacturers globally, ensuring a stable supply. The price of POM can vary depending on the type (homopolymer or copolymer), the form, and the supplier. Generally, POM is considered a cost-effective material given its performance characteristics.

Advantages and Disadvantages of Polyoxymethylene (POM)

Advantages

• High mechanical strength and rigidity
• Low coefficient of friction
• Excellent dimensional stability
• Good wear resistance
• Low moisture absorption
• Easy to machine and fabricate

Disadvantages

• Limited thermal stability in homopolymers
• Potential for thermal degradation
• Limited resistance to strong acids and bases
• Can be prone to creep under sustained load

What Are the Alternatives to Polyoxymethylene (POM)?

While POM is a highly versatile material, there are alternatives that might be suitable depending on specific application requirements. Some alternatives include:

• Nylon (Polyamide): Offers excellent toughness and wear resistance.
• Polyethylene (PE): Known for its impact resistance and low cost.
• Polypropylene (PP): Provides good chemical resistance and fatigue resistance.
• Polycarbonate (PC): Offers high impact resistance and optical clarity.
• Acrylic (PMMA): Known for its transparency and weather resistance.

Each of these alternatives has its own set of properties that may make them more suitable for certain applications compared to POM.

What Is the Difference Between Acetal and Hemiacetal?

Acetal and hemiacetal refer to different chemical structures. Acetals are compounds with two ether (R-O-R) groups attached to the same carbon atom, while hemiacetals have one ether group and one hydroxyl (R-OH) group attached to the same carbon. POM, being an acetal polymer, consists of repeated acetal units in its structure, contributing to its stability and mechanical properties.

Machining

Machining POM is relatively straightforward due to its excellent machinability. It can be easily cut, drilled, milled, and turned using standard machining tools. The low friction and high dimensional stability of POM ensure precise and clean cuts, making it a preferred material for manufacturing precision parts.

Bonding

Bonding POM can be challenging due to its low surface energy, which makes it difficult for adhesives to adhere. However, with proper surface preparation and the use of specific adhesives designed for low-energy plastics, strong bonds can be achieved. Common methods for improving adhesion include surface roughening and the use of primer agents.

Usage

POM is used in a wide range of applications due to its versatile properties. Some common uses include:

• Consumer electronics: Housings, connectors, and switches.
• Industrial machinery: Bushings, cams, and conveyor components.
• Household goods: Handles, zippers, and fasteners.

Its combination of strength, durability, and low friction makes POM suitable for many precision applications.

Degradation

POM can degrade when exposed to high temperatures, UV radiation, and certain chemicals. Thermal degradation can result in the release of formaldehyde, a toxic gas, while UV exposure can cause discoloration and brittleness. Chemical degradation occurs when POM is exposed to strong acids or bases, which can break down its molecular structure. Proper stabilization and the use of protective coatings can help mitigate these degradation issues.

In conclusion, Polyoxymethylene (POM) is a highly versatile engineering plastic with a wide range of applications due to its excellent mechanical properties, ease of fabrication, and availability. While it has some limitations, its advantages make it a preferred choice in many industries. Understanding the differences between homopolymer and copolymer variants, as well as proper handling and machining techniques, can maximize the benefits of using POM in various applications.