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Delrin® | White Paper
How To Maximize the
Property Advantages of
Delrin Acetal Homopolymer
Over Acetal Copolymer
®
A guide for design engineers
Delrin® | White Paper
Overview
Delrin® acetal homopolymer resin—also called Delrin® POM-H or Delrin® homopolymer—is one of the most
crystalline engineering thermoplastics available. Delrin is a highly adaptable material that bridges the gap
between plastics and metals and offers unique properties. It is specified for high load mechanical
applications and precision parts, where strength, stiffness, stability and reliability are important. Applications
range from gears, safety restraint components, automotive door mechanisms and conveyor system
components to medical delivery devices, ski bindings, zip fasteners and many other applications across
a wide range of products and industries.
When compared to acetal copolymers, Delrin® homopolymer combines higher fatigue and creep resistance
with overall superior toughness and higher tensile strength and stiffness, allowing for thinner and lighter part
design, and the potential for reduced part production cost.
This white paper describes the differences between Delrin® homopolymer and acetal copolymer in detail and
explains how to make the best use of the higher level of mechanical performance of Delrin® homopolymer over
that of acetal copolymer in order to extract the most value for all stakeholders.
.
Table of contents
Introduction…..................................................................................................... 3
What is an acetal polymer?…… …...................................................................... 3
Key properties of Delrin® acetal homopolymer…............................................ 5
Mechanical properties comparison in detail….................................................6
When to use Delrin® acetal homopolymer….....................................................9
Customer benefits of mechanical properties…..............................................10
How to exploit the superior mechanical properties of Delrin® acetal
homopolymer vs. acetal copolymer…........................................................11
Case Study: Toughened acetal copolymer replaced by Delrin® 300TE........12
How all stakeholders can benefit...................................................................13
Conclusion….....................................................................................................14
Delrin® | White Paper
Introduction
Acetal polymer already plays a major role in the advanced materials revolution. It is being used in a broad
range of applications in the automotive, food processing, life sciences, safety and manufacturing industries,
to name but a few. There are different kinds of acetals with different levels of performance. Suppliers of
acetals often offer equivalent grades of copolymer as a substitute for a certain grade of homopolymer, and it
is not always clear why one would use acetal homopolymer over acetal copolymer, or vice versa. However, it
would be a mistake to think that all acetal resins are the same.
What is an acetal polymer?
Acetal, also called polyoxymethylene (POM), was first discovered in 1920 but was not produced on a large
scale until 1960. At its simplest, POM represents a repeating unit of CH2O. Whereas the Delrin® acetal
homopolymer manufacturing process maintains a straight chain of CH2O monomers with end caps, other
manufacturers of acetals add one of several possible comonomers that appear in the chain on average every
70-100 repeating units (see Fig. 1).
Figure 1. Molecular structure of acetals
In addition, the typical chemistry of acetal copolymer polymerization produces roughly 2-8 weight percent of
cyclic low molecular weight chains (oligomers) that, in general, do not participate in the structure and function
of the material.
The consequences of that molecular difference are exhibited in the crystalline packing of the polymer
(Fig. 2). The purity of the uniform backbone in Delrin® homopolymer allows for a more organized stacking of
the polymer into larger crystalline domains as the polymer solidifies. Meanwhile, the additional comonomer
units in the copolymer disrupt this organization, ending the stacking locally and, ultimately, limiting the size of
the crystalline domains.
Delrin® | White Paper
Figure 2. Delrin® homopolymer vs. acetal copolymer crystalline structure in 2-D
The purity of the uniform backbone in Delrin® homopolymer allows for a
more organized stacking of the polymer into larger crystalline domains as
the polymer solidifies.
Delrin® homopolymer has a uniform backbone with a larger crystalline block structure (lamellae) than acetal
copolymers. These lamellae grow into much larger structures called spherulites which have a mix of
crystalline and amorphous regions.
The mechanical properties that are exhibited by the acetal resins can be envisioned as resulting largely from
the strength of the molecular chain bonds between regions. When a load is applied, these inter-region chains
will first stretch and ultimately peel away from nearby crystalline regions.
Delrin has bigger crystalline structures which favor the formation of a stronger interaction among the chains
in the inter-lamellae regions because of a higher surface of contact among the spherulites.
When smaller spherulites are formed (as with acetal copolymer), their smaller radius limits the surface
contacts among the crystalline domains, decreasing the interactions in the inter-lamellae regions. The empty
space among the smaller spherulites (ultimately filled by smaller molecules, oligomers and potentially
additives) leads to reduced strength and toughness.
Higher bond strength can also absorb more energy during impact and help reset the structure after impact
energy is absorbed, retaining the original shape.
Delrin® | White Paper
Key properties of Delrin® acetal homopolymer
Delrin® is a highly adaptable thermoplastic material that bridges the gap between plastics and metals for a
wide range of technical applications and offers unique properties.
The table below illustrates how the properties of Delrin can translate to the functional performance of parts.
Properties and features of Delrin®
Applicability to parts
Very hard and stiff, combining high mechanical
strength with rigidity
• The stiffest, strongest and most fatigue-resistant
unreinforced (unfilled) engineering polymer available
• Suitable for wide range of high performance
components with long use lifetimes
Low creep and low moisture absorption
Excellent dimensional stability and long operational lifecycles
High fatigue life under cyclical loadings
Suitable for long life, wear-resistant parts
High impact resistance even at low
temperatures
Can withstand repeated shock loads
High yield strain
Good spring-back capabilities, ideal for snap-fit connections
Low coefficient of friction with inherent
lubricity for non-stick properties
Suitable for maintenance-free moving and sliding parts
Retains toughness over wide temperature span
from -40°C to 90°C (intermittent to 120°C)
Doesn’t get brittle at low temperatures; suitable for winter
sports equipment
High electrical resistivity with low static properties
Suitable for electrical applications and in fuel-related and
similar applications
Corrosion-resistant with high tolerance to
moisture, petrol, solvents and organic
chemicals at ambient temperatures
Ideal for many automotive and industrial process
components
Outstanding acoustic properties
Provides low-noise components for doors, etc.
Favorable molding characteristics
Exhibits better flow rates than standard copolymer acetals
for consistent precision molding performance
Smooth, glossy, aesthetic surface finish
Ideal for visible applications and premium consumer products
Easily worked/processed with conventional
machinery and skills
Minimal investment and learning curve required
Very low weight alternative to metals
Half the weight of aluminum and six times lighter than steel
Recyclable
As a thermoplastic material, Delrin can be recycled using
industry standard recovery techniques
Specific application requirements
Delrin offers an unparalleled range of modified resins for
special applications and extended performance including:
• Medical
• Ultra-low friction
• Food contact
• Antistatic
• Very-low VOC emission grades
• UV stabilized
• Low global warming potential
• Fiber reinforced
Delrin® | White Paper
Mechanical properties comparison in detail
Delrin® homopolymer can be used to replace metal to achieve corrosion resistance, lighter weight and
single-stage manufacturing while bypassing multiple machining steps.
Delrin can also be used to replace polyamides (i.e. nylons) when higher stiffness and thinner parts with
better creep and fatigue resistance are needed. More significantly, Delrin is not as sensitive to moisture as
polyamides, resulting in more consistent mechanical properties and part dimensions. Compared to
polypropylene and polyesters (i.e. polybutylene terephthalate), Delrin also exhibits many advantages including
higher stiffness for thinner parts and better performance in wear and friction environments.
Measured against amorphous engineering plastics (i.e. ABS and polycarbonate), Delrin outperforms in
chemical resistance and stiffness, and shows better performance in wear and friction environments.
Acetal polymers come in various molecular weight families, depending on the manufacturer. Delrin®
homopolymer is available in molecular weight families designated 100 through 900, with 100 being the
highest molecular weight. They are shown in the table below (Fig. 3) with the general property trends that
arise as a direct consequence of molecular weight.
Higher molecular
weight and viscosity
delivers increased
toughness, impact
strength and
elongation.
Preferred by
Designers
Flow family Melt-mass Melt-volume
flow rate flow rate
2.4
Higher flow rates
deliver ease of filling
in thinner walled parts,
a more crystalline
structure and increased
Preferred by stiffness and strength
Molders
Figure 3. Property trends with changes in flow family
The mechanical performance versus flow rate trends are shown in more detail in the graph below (Fig. 4).
The high molecular weight of Delrin® 100CPE delivers dramatically better toughness characteristics while
exhibiting only slight decreases in strength/stiffness characteristics. In general, a designer may prefer to
work with the highest molecular weight materials since they typically deliver the best combination of
properties, while the molder may prefer to work with the lowest molecular weight material for its high flow
characteristics to ensure proper part filling and better surface finish. Thus, a balancing act must be achieved
during material selection.
Figure 4. Detailed comparison of
properties of Delrin® homopolymer
grades as a function of flow rate
Delrin® | White Paper
Figure 5 below shows the comparison of the general purpose offering in the Delrin® product line—Delrin®
500CPE against general purpose acetal copolymer offerings. Not only does Delrin® homopolymer
outperform the strength and stiffness characteristics of acetal copolymer, but it surpasses the toughness
characteristics such as impact strength and strain, at the same time delivering a significant increase in flow
rate, resulting in easier molding.
Figure 5. Comparison of Delrin®
500CPE homopolymer vs. general
purpose acetal copolymer
Delrin® can replace metal to achieve corrosion resistance, lighter weight and
single-stage manufacturing while bypassing multiple machining steps. It can also
replace polyamides (i.e. nylons) when higher stiffness and thinner parts
with better creep and fatigue resistance are needed.
The mechanical performance gap is magnified when the higher performance high molecular weight acetal
copolymer is stacked up against a medium-high molecular weight Delrin® homopolymer resin, Delrin®
300CPE (Fig. 6). The much higher mechanical performance to molecular weight ratio of Delrin delivers the
balance between the needs of the designer and the molder.
Figure 6. Comparison of Delrin® 300CPE
homopolymer vs. high molecular weight
standard acetal copolymer
Delrin® | White Paper
The gap widens further when equally high molecular weight resins are matched up. The graph below (Fig. 7)
of high molecular weight copolymer vs. Delrin® 100CPE, the flagship of the Delrin brand, illustrates why Delrin®
homopolymer is clearly the premium acetal. With toughness characteristics generally twice those of acetal
copolymer and higher strength and stiffness, Delrin® homopolymer can provide the safety margins needed for
critical parts.
Figure 7. Comparison of Delrin® 100CPE
homopolymer vs. high molecular weight
standard acetal copolymer
The inherent toughness of unmodified Delrin® homopolymer grades is, in many cases, much higher than that of
impact-modified copolymer grades.
Figure 8 shows how Delrin® 100CPE homopolymer, unmodified for impact toughness, stacks up against acetal
copolymer modified with what is estimated to be approximately 20% reinforcement. The higher inherent
stiffness of Delrin is magnified further, and the added toughener provides only small improvements in the low
temperature impact resistance of acetal copolymer. In other words, many impact-modified acetal copolymer
grades are more brittle than standard Delrin® 100CPE homopolymer at low temperatures.
Figure 8. Comparison of
Delrin® 100CPE homopolymer
vs. acetal copolymer grades
enhanced with up to 20%
toughener
Delrin® | White Paper
The inherent toughness of unmodified Delrin® homopolymer grades is, in many
cases, much higher than that of impact-modified copolymer grades.
In addition to the standard tests shown above, Delrin® 500CPE outperformed general purpose acetal
copolymer by a factor of five in internal flexural fatigue resistance testing, while Delrin® 100CPE outperformed
HMW acetal copolymer by a factor of ten. Even more significantly, the nucleated grades Delrin® 111DP and
311DP outperformed both acetal copolymer resins by almost three orders of magnitude, as shown below.
Figure 9. Comparison of resistance to
flexural fatigue at 33MPa loading of various
Delrin® homopolymer grades vs.