We encounter plastics every day, but few of us realize the incredible diversity of materials that make up our modern world. One such material, polyacetal, is a true unsung hero, quietly powering countless products we rely on.
The Power of Polyacetal:
Polyacetal, also known as POM, is a remarkable material with a long history of innovation. It’s tough, durable, and resistant to wear and tear, making it ideal for a wide range of applications. From the gears in your car to the handles on your kitchen appliances, polyacetal is quietly working behind the scenes, making our lives easier and more efficient.
A History of Innovation:
Polyacetal’s story began in the 1920s, but it wasn’t until the mid-1950s that scientists overcame the challenges of thermal stability, paving the way for its widespread use. Since then, polyacetal has become a staple in the plastics industry, known for its reliability and performance.
Homopolymers vs. Copolymers:
Polyacetal comes in two main varieties: homopolymers and copolymers. Homopolymers are made from a single type of monomer, while copolymers are made from two or more different monomers. Copolymers offer a wider processing window, making them more popular among manufacturers.
The Strength of Reinforcement:
To further enhance the properties of polyacetal, manufacturers often add glass fibers to the material. This reinforcement increases the strength and stiffness of the polyacetal, making it even more durable and resistant to deformation.
Flame Retardancy and Identification:
While polyacetal is generally a safe and reliable material, it does have one drawback: it’s not inherently flame-retardant. Special additives are required to make it fire-resistant. And to distinguish between homopolymers and copolymers, a simple test using triethanolamine (TEA) can be used.
The Takeaway: A Material for a Better World
Polyacetal is a versatile and reliable material that’s playing a vital role in our modern world. From the gears in our machines to the handles on our tools, polyacetal is making a difference, making our lives easier, safer, and more efficient. So, the next time you encounter a product made with polyacetal, remember the unsung hero behind it, a material that’s quietly shaping our world.
Polyacetal materials have been a commercial choice for more than 60 years. Due to the lack of standardized nomenclature in our industry, these materials are often referred to as polyacetal (named after the original terminating group on the chain), polyformaldehyde (formaldehyde is the starting material for polymerization), and polyoxymethylene (POM). The last one seems to have gained widespread use as an official abbreviation, with its name derived from the chemical group shown in Figure 1, which represents a repeating unit in a polymer chain.
Polyoxymethylene was discovered at the laboratory level as early as the 1920s, but it was not until the mid-1950s that problems related to the thermal stability of this material were solved. Some who process this material will insist that the problem remains unresolved because overheating the polyoxymethylene polymer produces large amounts of formaldehyde gas. But in fact, the process stability of this material is much better today than it was when the first batch of materials was introduced.
The earliest materials were homopolymers, meaning that each repeating unit in each individual polymer chain looked like the unit in Figure 1. A few years later, polyoxymethylene copolymer was created and commercialized, and since then, molders and end-users have been able to choose between these two families. But the reasons for choosing a homopolymer or a copolymer are not always clear to the person making the decision.
Performance differences between POM copolymers and homopolymers
Processors tend to use copolymers because they offer a wider processing window. Because acetal is highly crystalline, it must be heated above the melting point before processing. Homopolymers have a higher structural regularity; therefore, they are more crystalline than copolymers. Another type of monomer used to make the copolymer has longer hydrocarbon bonds, which increases the spacing between oxygen atoms in the polymer chain. These oxygen atoms are the most susceptible points for thermal and oxidative degradation, so the lower the number of these oxygen atoms, the more resistant the material is to degradation.
But reducing the level of structural regularity also reduces crystallinity. This is a good thing for processors because the copolymer has a melting point about 10°C lower than the homopolymer. Therefore, copolymers can be processed at lower melting temperatures. Additionally, the greater the spacing between vulnerable oxygen atoms, the more resistant the material is to higher melting temperatures. As a result, copolymers have a wider processing window because of advantages at both ends of the thermal spectrum.
However, the copolymer has low crystallinity, resulting in reduced strength and modulus, and the upper limit of short-term use temperature conditions is also reduced. A typical homopolymer has about 15% higher tensile strength and nearly 13% higher modulus than the corresponding copolymer. Higher crystallinity also means improved load-carrying properties and better resistance to fatigue and creep.
While homopolymers have better short-term heat resistance, the presence of additional carbon-hydrogen bonds in the copolymer backbone improves oxidation resistance. This means better performance retention after long-term exposure to high temperatures and better chemical resistance – especially in acidic and alkaline environments.
How to identify POM homopolymer or copolymer
The diagnostic test that determines whether a polyacetal part is molded from a homopolymer or a copolymer is called TEA. TEA stands for triethanolamine and is a very strong base. If a homopolymer sample is placed in high temperature TEA, it will dissolve fairly quickly, whereas the copolymer will not. A very important application where this difference in chemical resistance becomes a factor is exposure to hot water, especially when the water contains chloride. Any prolonged exposure to high temperatures will eventually reduce the difference in mechanical properties, as copolymers will withstand the exposure better than homopolymers.
There is an interesting and often overlooked difference between these two materials that can be observed by examining the strength and modulus of unfilled and glass-filled material grades. We have already mentioned that unfilled homopolymers are harder and stronger than unfilled copolymers.
However, if we look at the datasheets for the 20% fiberglass-filled grades of both materials, we see something very unusual. After adding glass fiber, the strength of the copolymer increased by 60% and the modulus increased by 2.5 times. While the modulus of the homopolymer also increases, the improvement is smaller, resulting in the glass-containing copolymer actually being stiffer than the filled homopolymer. Furthermore, the tensile strength of the homopolymer actually decreases as glass is added.
The difference between POM homopolymer & copolymer in modification
The reason for this different result after glass fiber addition has to do with the way the glass interacts with the polymer. In most polymers, fiberglass plays a reinforcing role: it adds both strength and stiffness. In order to achieve this, a good bond must form between the polymer and the glass fiber. Some polymers, such as nylon, have chemistries that naturally bond with glass, and the resulting performance improvements reflect this. Other materials, such as polypropylene, do not have a natural affinity for glass, so some modification of the polymer is required to achieve this bond.
This modification is called coupling or chemical coupling, and it has been used in polyoxymethylene copolymers to produce the observed improvements in mechanical properties. However, the chemistry of the homopolymer makes coupling nearly impossible. Therefore, fiberglass acts more like a filler than a reinforcement, adding stiffness but not strength. This is an important distinction when evaluating these two material families in the filled state.
Another interesting phenomenon that applies equally to both families is that it is difficult to confer high flame retardant properties on acetals with today’s technology. The traditional route of using halogenated flame retardant systems is not suitable for acetals because these polymers react violently with any chlorinated or brominated species. Polymer chemists are a very smart bunch, and maybe at some point in the future someone will develop an efficient flame-retardant polyoxymethylene.