Our TECH Talk series episode 16 provides an in-depth review covering the differences between Delrin, Homopolymer Acetal, and Copolymer acetal.

Often, we are asked, "What is the difference between Acetal, Copolymer acetal, homopolymer acetal, and Delrin?"


Acetal is the general term for the family of plastics with the chemical polyoxymethylene (POM). Delrin® is a POM Acetal, but not all POM acetals are Delrin, nor are acetals all the same.

Delrin® is a registered trademarked brand name for the most famous homopolymer acetal resin which DuPont invented.

To understand the differences between Delrin and other acetals, we need to review the polymeric structure of the polyoxymethylene chemical.

POM Acetals are available in two different structures…..a homopolymer and a copolymer structure. Both homopolymer and copolymer acetals are considered semi-crystalline polymers, which are made up of repeating chains of molecules or monomers.

The homopolymer structure consists of a single species of repeating polymer chains or monomers…..while the copolymer consists of two or more types of monomer structures.

Delrin is a homopolymer Acetal offered by many manufactures of stock shapes. And those same manufacturers also offer Copolymer Acetals. So what are the differences?

Both homopolymer and copolymer acetals are very similar in properties and performance, and both offer FDA & USD compliance, excellent machinability, dimensional stability, high strength and stiffness, and wear resistance. However, the difference in structures does result in some minimal performance differences.

The Delrin homopolymer grade offers higher levels of crystallinity due to its tightly uniform molecular structure, which does affect some properties.


Some of the differences are as follows:

Mechanical Properties:

Unfilled homopolymer acetal, or Delrin, offers approximately 10% to 15% higher strength and stiffness than copolymer acetals. Meanwhile, copolymer acetals provide more toughness and ductility.

Chemical Resistance:

The chemical resistance is generally similar but may differ depending on the chemical, exposure temperature, and concentration. Copolymer acetals tend to offer better resistance to chlorine-containing solutions.

Hot Water/Steam Resistance:

Copolymer acetals exhibit substantially more resistance to hydrolysis or hot water and better dimensional stability, especially after prolonged exposure.

Continuous Service Temperature:

The continuous service temperatures for both grades are considered the same….180F. However, copolymer acetal has been reported to resist thermal degradation at higher temperatures better than homopolymer acetal. Delrin homopolymer has a higher heat deflection temperature, indicating it will exhibit higher mechanical properties during short-term exposure.

Wear Resistance:

Wear resistance, Limiting PV, and k-factor are the same for unfilled homopolymer and copolymer acetals.

Machinability:

The machinability of both acetal grades is excellent. The greater stiffness of Delrin® homopolymer does make it superior for small-diameter parts and thin-walled parts, particularly if being screw-machined or manufactured on high-speed CNC lathes.

Appearance:

The cross-section of acetal stock shapes has long been recognized to contain microporosity along the centerline of rod and plate products. The centerline porosity occurs during the extrusion of stock shapes.

Characteristically, copolymer acetals contain significantly less centerline porosity but can still exhibit interconnected micro-porosity ranging from 100 to 150 microns in the center of the stock shape.

This micro-porosity can result in mechanical weakness and leakage of liquids and gases under pressure. Some stock-shape manufacturers of copolymer acetal claim zero centerline porosity. Regardless, it is far superior to the porosity inherent in Delrin homopolymer acetal.

In summary, customers select Delrin Homopolymer acetal when higher mechanical properties are desirable or when machining smaller parts using screw machines or high-speed lathes.

Copolymer Acetal is chosen for applications that require toughness and less centerline porosity…or for larger cross-sections of rod or sheet. In many cases, the larger sizes are not commonly available in homopolymer Acetal due to the porosity in larger cross sections.

In many applications, homopolymer or copolymer will provide similar performance, except when the slight differences between the two are relevant to the application.

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