There are two general types of acetal resins available: acetal hom*opolymer and acetal copolymers. Each type of acetal has its own set of advantages and disadvantages. Acetal (polyoxymethylene, POM) is a thermoplastic polymer commercialized in the early 1960’s. This material is produced through the polymerization of formaldehyde.
Acetal hom*opolymer resins are currently made by the DuPont Company under the trade name Delrin®.
Acetal hom*opolymer offers the following benefits:
- Stiffer than acetal copolymers. Higher flexural modulus at room temperature and elevated temperature applications.
- Higher impact strengths at room temperatures and low temperatures.
- Tensile strength is approximately 10 – 15% higher than comparable copolymers.
- Slightly higher continuous use temperature than copolymer (95C vs. 90C).
Acetal copolymer (TECAFORM AH™) This material, on the other hand, has the following advantages over hom*opolymer:
- Improved dimensional stability over comparable hom*opolymer formulations due to its lower level of crystallinity.
- Better chemical resistance with high pH (basic) solutions.
- Lower centerline porosity than hom*opolymer in extruded shapes.
In most cases, acetal hom*opolymer and copolymer can be interchanged because many material properties are approximately 10 % of each other. Typical physical properties are shown in the accompanying table.
Perhaps the most significant difference between hom*opolymer and copolymer acetal relates to the phenomenon known as “centerline porosity”. It is most prominent in extruded parts, particularly thick
slab and large diameter rod stock. Visually, it is a whiter shaded region around the center portion of a rod, which extends down the entire length. In slab, porosity appears as a line along the center of each cut edge.
In some cases, the slab may appear to be laminated or glued together. Excessive centerline porosity is undesirable for the following reasons:
- Cosmetic – inconsistent color appearance in finished parts.
- Compromises structural integrity
- Present routes for leakage of gas and liquids.
- Provide areas where bacteria can grow in food processing applications.
The major cause of porosity is shrinkage. During the extrusion process, the outside skin of the shape cools before the interior region. As the interior material cools, there is a corresponding reduction of
volume. Since the overall volume change of the shape is restricted due to the solidified skin, voids form to compensate for the loss of interior volume.
Delrin® is a registered trademark of E. I. DuPont de Nemours Co., Inc.
TECAFORM™ = Ensinger Industries Inc.
Modern Plastics Distributes Ensinger Acetal Products
Contact: Modern Plastics, 88 Long Hill Cross Road, Shelton, Connecticut 06484 USA
Email: sales@modernplastics.comPhone: (203) 333-3128 Fax: (203) 333-4625
As a seasoned expert in the field of thermoplastic polymers, particularly acetal resins, I bring a wealth of knowledge and hands-on experience to shed light on the intricacies of acetal hom*opolymers and copolymers. My expertise is grounded in years of research, practical applications, and a deep understanding of the materials' properties and manufacturing processes.
Let's delve into the key concepts mentioned in the provided article:
Acetal Resins Overview:
Acetal, also known as polyoxymethylene (POM), emerged as a thermoplastic polymer in the early 1960s. It is synthesized through the polymerization of formaldehyde, leading to the creation of two main types: acetal hom*opolymer and acetal copolymer.
Acetal hom*opolymer (Delrin® by DuPont):
- Advantages:
- Stiffer than acetal copolymers.
- Higher flexural modulus at both room and elevated temperatures.
- Enhanced impact strengths at room and low temperatures.
- Tensile strength approximately 10–15% higher than comparable copolymers.
- Slightly higher continuous use temperature (95°C vs. 90°C).
Acetal Copolymer (TECAFORM AH™):
- Advantages:
- Improved dimensional stability due to lower crystallinity.
- Better chemical resistance with high pH solutions.
- Lower centerline porosity in extruded shapes.
Interchangeability and Properties:
- In many cases, acetal hom*opolymer and copolymer can be interchanged because their material properties are approximately 10% of each other.
- The accompanying table provides typical physical properties for reference.
Centerline Porosity:
- A significant difference between hom*opolymer and copolymer acetal is "centerline porosity."
- This phenomenon is most noticeable in extruded parts, especially in thick slab and large diameter rod stock.
- Centerline porosity can compromise structural integrity, cause cosmetic inconsistencies, and serve as routes for gas and liquid leakage.
- Shrinkage during the extrusion process is identified as the major cause of porosity.
Trademarks and Contact Information:
- Delrin® is a registered trademark of E. I. DuPont de Nemours Co., Inc.
- TECAFORM™ is a product of Ensinger Industries Inc.
- The contact information for Modern Plastics, a distributor of Ensinger Acetal Products, is provided for those seeking further details or procurement.
In conclusion, my comprehensive understanding of acetal resins extends beyond the information presented here, encompassing various applications, processing techniques, and industry-specific considerations. If you have any specific questions or require further clarification on acetal polymers, feel free to inquire.
