| ACETAL - COMPARISON GUIDE |
|Acetal or Polyoxymethylene (POM), its chemical name, is a widely used crystalline engineering thermoplastic produced through the polymerization of formaldehyde. It is characterized as having an excellent combination of physical properties that makes it suitable for numerous industrial applications. |
There are two types of Acetal resins available: Acetal Co-polymer and Acetal Homo-polymer, sold under the tradename Delrin®. Each type of Acetal has its own set of advantages and disadvantages.
| HOMOPOLYMER ACETALS: || |
| COPOLYMER ACETALS: |
| Acetal Homopolymer resins are currently made by E.I. du Pont de Nemours & Co. Inc. under the trade name DELRIN®. Homopolymer Acetals offer the following benefits: |
- Stiffer than Copolymer Acetals.
- 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 Acetals (195ºF vs. 185ºF).
| || Acetal Copolymer resins are currently produced under the trade names ULTRAFORM® (BASF Corp.) and CELCON® (Ticona). |
Copolymer Acetals offer the following advantages over Homopolymer Acetals:
- Improved dimensional stability over comparable Homopolymer formulations due to its lower level of crystallinity.
- Better chemical resistance with high pH (basic) solutions.
- Available in both extruded and compression molded grades.
- Lower centerline porosity than Homopolymer Acetals in extruded shapes.
- Porosity-Free grades available in both extruded and compression molded grades.
|In most cases, Homopolymer and Copolymer Acetals can be interchanged because many material properties are approximately 10% of each other. Typical physical properites are shown on the following pages. |
Perhaps the most significant difference between Homopolymer and Copolymer Acetal relates to the phenomenon known as “Centerline Porosity”. It is most prominent in all grades of DELRIN® Homopolymer Acetal shapes, 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. 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.
- Provides areas where the bacteria can grow in food processing applications. The cause of the porosity is shrinkage. During the extrusion process, the outside skin of the shape cools before the interior region. As the interior 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.