Homopolymers are plastics with molecular chains containing identical repeating units (mers), while co-polymers chemically contain two different repeating units. Homopolymer acetal is most commonly recognized as Delrin, (a registered trademark of E.I. DuPont) since DuPont is the only manufacturer of homopolymer acetal resin. Copolymer acetal resin is domestically supplied under the tradenames of Celcon (Ticona) and Ultraform (BASF). The chemistry is similar enough that both products are still considered acetals; however, there are some important property differences.

Mechanical Properties:
The strength and stiffness of unfilled homopolymer acetal (Delrin) is approximately 15% greater than the co-polymer acetals.

Hot Water/Steam Resistance:
Copolymer acetals exhibit substantially greater resistance to hydrolysis (degradation by hot water) and better dimensional stability, especially after prolonged exposure.

Chemical Resistance:
In general, the chemical resistance is similar; however, it may be different depending on the chemical, the temperature and concentration encountered. Copolymer acetal offers better resistance to chlorine-containing solutions, including bleach and strong alkalies, both common-sanitizing chemicals.

Continuous Service Temperature:
Generally, the continuous service temperature for both grades is considered the same (180 F). Copolymer acetal has been reported to resist thermal degradation at higher temperatures better than the homopolymer. However, the higher heat deflection temperature of homopolymer acetal indicates it will exhibit greater mechanical properties during short-term exposure.

Machinability:
Generally, the machinability of acetal products is extremely good. The greater stiffness of Delrin makes it superior for screw machined parts and small diameter parts (less than 1” diameter) machined on high speed NC lathes. It is important to recognize the importance of an extruder’s processing sequence, including the annealing cycle on the stability of a stock shape during machining.

Note: Acetron GP stock shapes are supplied with the lowest internal stress to assure the machinist dimensional consistency during and after machining.

Wear Resistance:
The general wear resistance, including limiting PV and K-factor are equivalent for both unfilled acetals. The enhanced bearing and wear grades each have their own wear resistance properties. Inherently, all acetals have lower abrasion resistance than nylons and polyethylenes.

Appearance:
The cross section of acetal stock shapes has long been recognized to contain micro-porosity along the centerline of rod and plate products. The centerline porosity is created during the extrusion of stock shapes. Characteristically, co-polymer acetals yield less centerline porosity, but still exhibit interconnected microporosity ranging from 100 to 150 microns (.1 to .15mm) in stock shapes. This microporosity results in mechanical weakness and leakage of liquids and gases under high pressure.

Note: Acetron GP stock shapes contain no centerline porosity. Any isolated pores are less than 25 microns in diameter.

ACETAL RESINS AND EXTRUDED STOCK SHAPES

Homo-Polymers
DuPont classifies acetal resins according to the following types:

General Purpose Grades
These low to medium viscosity resins include Delrin 100, 111P, 500, 511P, 900, 911 and 1700. These resins are designed for injection-molding processing. The “P” designation used after certain resin numbers denotes that the resin contains additives to enhance processability during injection molding.

Toughened Grades
These super tough high viscosity grades are developed for injection molding processing and include Delrin 100 ST, 500ST, and 500T.

Low friction and Wear Grades
These grades contain lubricants that reduce surface friction. The grades are generally designed for injection molding processing and include Delrin 500 AL, 500CL, 100AF, 500AF, 520MP, 510MP,500TL, 900SP, 100KM. Only Delrin 100AF is designed for extrusion. A blend of Delrin AF 100 with Delrin 100, which results in lower PTFE and therefore lower cost, is also produced for extrusion.

High Stiffness Grades
These grades are glass reinforced for added mechanical strength. The grades which include Delrin 570 NC, 577BK, 510GR, 525GR are designed for injection molding primarily.

UV Stabilized Grades
These grades are designed for added resistance to UV degradation for outdoor use. They contain a chemical UV stabilizer. The grades are Delrin 127UV, 527UV, 927UV 1727UV. Delrin 127UV is most suitable for extrusion. The other grades are primarily for injection molding processing.

Extrusion grades
The grades designated as extrusion grades for the stock shapes market are Delrin II 150E, Delrin II 150SA and Delrin II 550SA. The “II” designation is used for these extrusion-grade resins only. It is appropriate to use the designation or to drop it, as there is only one product for these grades. Of the three grades, 150E is the most common used for extruded shapes.

Co-Polymers
The co-polymer resins are not asked for by grade designation as much as the Delrin grades, although most of the stock shapes supplied are based on co-polymer acetals. One of the earliest resins that was asked for by name, and still is Celcon. The M25 grade is a high viscosity grade designed for extrusion. It is also supplied in glass reinforced and UV stabilized grades. Two other co-polymer resins produced and supplied by BASF and Kapital. Companies that supply stock shapes typically market products under their own tradenames.

Acetron Characteristics
GP General-purpose acetal machining stock produced by Quadrant EPP. This product is fully dense throughout the cross section containing no centerline porosity and is stress relieved to assure the machinist the lowest possible residual stress. It is currently produced from co-polymer acetal resin.