Key Difference – POM-H vs POM-C
POM stands for polyoxymethylene, a high molecular weight thermoplastic polymer which is a widely used for many industrial applications. It is also known as polyacetal, acetal, polyformaldehyde. The POM copolymer of formaldehyde is composed of –CH2O- repeating units. POM polymers, in general, provides excellent mechanical properties like high tensile strength, low friction, high fatigue resistance and, better stiffness and toughness. Furthermore, POM shows high scratch resistance properties and low moisture absorption. Moreover, it is resistant to many strong bases, many organic solvents, and weak acids, However, due to the chemical structure of the POM, it is not stable in acidic conditions (pH <4) and elevated temperatures as the polymer is degraded under these conditions. Hence, the POM is often copolymerized with cyclic ethers such as ethylene oxide or dioxilane to disturb the chemical structure, thus enhancing the stability of the polymer. POM is available in two variants; copolymers (POM-Cs) and homopolymers (POM-Hs). These two types of POM differ in many ways, but the key difference between POM-H and POM-C is their melting point. The melting point of POM-C is between 160-175 °C whereas, that of POM-H is between 172-184 °C. Their applications are determined based on the properties of POM-H and POM-C. This article elaborates the difference between POM-H and POM-C.
Polyoxymethylene
What is POM-H?
POM-H stands for polyoxymethylene homopolymer. When compared to the other variants of POM, the homopolymer has a higher melting point and is 10-15% stronger than the copolymer. However, both variants have same impact properties. POM-H is produced by anionic polymerization of formaldehyde, where the crystallization occurs well, resulting in high stiffness and strength. In general, POM-H has better physical and mechanical properties than POM-C. POM-H are best suited for applications where properties like good abrasion resistance and low coefficient of friction are needed.
What is POM-C?
POM-C stands for polyoxymethylene copolymer. This is produced by cationic polymerization of trioxane. During this process, a small amount of comonomers is added in order to increase the tightness, while lowering the crystallinity. The POM-C however, has low stiffness and strength than POM-H. But its processability is high when compared to POM-H. Because of this reason, POM-C has become the most widely used POM (75% of total POM sales). POM-C is well suited for applications where the property like low coefficient of friction is needed.
What is the difference between POM-H and POM-C?
Full Name
POM-H: Its full name is POM homopolymer.
POM-C: Its full name is POM copolymer.
Produced by
POM-C: It is produced by anionic polymerization of formaldehyde.
POM-H: It is produced by cationic polymerization of trioxane
Properties of POM-H and POM-C
Hardness and Stiffness
POM-H: POM-H is hard and stiff
POM-C: POM-C is not as hard and stiff as POM-H.
Processability
POM-H: Processability is low.
POM-C: Processability is high.
Melting Point
POM-H: Melting point is 172-184 °C.
POM-C: Melting point is 160-175 °C.
Processing temperature
POM-H: Processing temperature of POM-H is 194-244°C.
POM-C: Processing temperature of POM-C is 172-205°C.
Elastic modulus (MPa) (tensile with 0.2% water content)
POM-H: Elastic modulus is 4623.
POM-C: Elastic modulus is 3105.
Glass transition temperature (tg)
POM-H: Glass transition temperature is -85°C.
POM-C: Glass transition temperature is -60°C.
Tensile strength
POM-H: Tensile strength is 70 MPa.
POM-C: Tensile strength is 61 MPa.
Elongation
POM-H: Elongation is 25%.
POM-C: Elongation is 40-75%.
Usage
POM-H: POM-H represents around 25% of total POM sales.
POM-C: POM-C represents around 75% of total POM sales.
Applications
POM-H: Bearings, gears, conveyor belt links, seat belts and grinding accessory of hand mixtures are some examples of POM-H.
POM-C: Electric kettles, water jugs, component with snap fits, chemical pumps, bathroom scales, telephone keypads, housings for domestic applications, etc. are some applications of POM-C.
References:
Cousins, Keith. Plastics and the Market for Small Domestic Appliances: A Report from Rapra’s Industry Analysis Group. iSmithers Rapra Publishing, 1998.
Platt, David K. Engineering and high performance plastics market report: a Rapra market report. iSmithers Rapra Publishing, 2003.
Olabisi, Olagoke, and Kolapo Adewale, eds. Handbook of thermoplastics. Vol. 41. CRC press, 2016.
Image Courtesy:
“Polyoxymethylene” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia