Some new and older Polymers

“Technical notes compiled with the assistance of ChatGPT.”

B  Polyacetal Practical comparison Property	POM-H Homopolymer	POM-C Copolymer	Modified / Terpolymer POM
Stiffness	Highest	Slightly lower	Depends on grade
Tensile strength	Highest	Slightly lower	Depends on filler
Thermal stability	Lower	Better	Usually improved
Chemical resistance	Good	Better	Grade-dependent
Hot-water resistance	Moderate	Better	Grade-dependent
Wear resistance	Excellent	Excellent	Can be superior
Risk of degradation	Higher	Lower	Usually lower
Centre-line porosity	More likely	Less likely	Grade-dependent
Processing safety margin	Narrower	Wider	Usually wider

Important processing notes

Acetal must never be mixed with PVC, halogenated materials, strong acids, or contaminated regrind, because decomposition can produce formaldehyde and acidic gases. Also avoid excessive residence time and overheating.

Typical processing temperatures are roughly:

• POM-C: about 180–210°C
• POM-H: about 190–215°C

The exact temperatures depend on the supplier’s grade.

In simple terms

• POM-H: strongest and stiffest, but more sensitive.
• POM-C: more stable and chemically resistant; often the safer general-purpose choice.
• Modified / terpolymer POM: tailored grades for wear, impact, UV, glass reinforcement, low friction, or special applications.

Top of Form

The backbone of acetals, meaning POM / polyoxymethylene, is mainly an alternating carbon–oxygen chain:

–CH₂–O–CH₂–O–CH₂–O–

or written as the repeating unit:

[–CH₂–O–]ₙ

So the backbone is not a normal carbon–carbon chain like polyethylene. It is a polyether-type backbone made from repeated methylene oxide units.

1. Acetal homopolymer backbone — POM-H

The homopolymer is almost entirely:

[–CH₂–O–]ₙ

This gives it a very regular chain structure, which allows high crystallinity.

That is why POM-H usually has:

• high stiffness
• high tensile strength
• good fatigue resistance
• good creep resistance
• relatively high shrinkage
• possible centre-line porosity in thick mouldings

2. Acetal copolymer backbone — POM-C

The copolymer is mostly the same:

–CH₂–O–CH₂–O–CH₂–O–

but with occasional comonomer units inserted, commonly from ethylene oxide or dioxolane chemistry.

Simplified:

–CH₂–O–CH₂–O–CH₂–CH₂–O–CH₂–O–

The occasional –CH₂–CH₂–O– unit interrupts the perfect POM chain.

This slightly reduces crystallinity, but improves:

• thermal stability
• processing safety
• resistance to hot water
• resistance to alkalis
• resistance to unzipping degradation

Why the backbone matters

The –CH₂–O– backbone is strong and regular, but it has one important weakness: under heat, acid contamination, or incorrect processing, the chain can “unzip” and release formaldehyde.

That is why acetals must be processed carefully and kept away from materials such as PVC, acids, and contaminated regrind.

In simple terms:

Acetal has a repeating carbon–oxygen backbone: –CH₂–O–.
The homopolymer is very regular and strong; the copolymer has occasional interruptions that improve stability.

1. Top of FormBottom of Form

B,  Polyketones

Polyketones are a family of engineering thermoplastics where the polymer backbone contains ketone groups:

–CO–

The ketone group is very polar and stiff, which gives polyketones good mechanical strength, chemical resistance, and heat performance.

There are two main groups worth separating:

1. Aliphatic polyketone — PK

This is the newer commercial “polyketone” most often meant in plastics processing.

Basic structure

It is commonly made from carbon monoxide + ethylene, sometimes with a small amount of propylene.

Simplified structure:

–CH₂–CH₂–CO–CH₂–CH₂–CO–

So the chain has regular alternating hydrocarbon and ketone units.

A terpolymer version contains some propylene units:

ethylene + propylene + carbon monoxide

This improves processability and modifies melting point and toughness.

Main properties of aliphatic PK

Polyketone has a very interesting balance of properties:

Property	General behaviour
Strength	High
Stiffness	Good, usually between PA and POM
Toughness	Very good
Wear resistance	Excellent
Chemical resistance	Excellent, especially to fuels and hydrocarbons
Hydrolysis resistance	Better than polyamides
Moisture absorption	Much lower than nylon
Barrier properties	Very good
Fatigue resistance	Good
Creep resistance	Good
Processing	Injection moulding and extrusion possible

Advantages compared with nylon

Compared with PA6 or PA66, aliphatic polyketone usually has:

• much lower moisture absorption
• better dimensional stability in humid conditions
• better fuel and chemical resistance
• very good wear and friction behaviour
• good toughness without needing absorbed moisture

This makes it useful where nylon changes dimensions or properties because of water absorption.

Advantages compared with acetal

Compared with POM/acetal, polyketone can offer:

• better resistance to some chemicals
• better impact toughness
• better barrier properties
• good wear properties
• no formaldehyde-type decomposition issue like POM

However, acetal may still be better for some very precise, low-friction, low-creep applications.

Typical applications

• fuel-system components
• gears
• wear strips
• conveyor parts
• pump components
• valve components
• electrical/electronic parts
• under-bonnet automotive parts
• tubing and pipe applications
• food-contact components, depending on grade approval

Filled grades

Like nylon, PK can be compounded with:

• glass fibre
• carbon fibre
• PTFE
• mineral fillers
• impact modifiers
• flame retardants
• lubricants

Glass-filled PK can become a very strong, stiff, dimensionally stable engineering material.

2. Aromatic polyketones — PEEK, PEK, PEKK, PEKEKK

These are high-performance polymers, not usually called simply “polyketone” in normal trade, but chemically they are polyketones because they contain ketone groups.

Examples:

• PEEK — polyether ether ketone
• PEK — polyether ketone
• PEKK — polyether ketone ketone
• PEKEKK — polyether ketone ether ketone ketone

PEEK simplified structure

PEEK has aromatic rings, ether links and ketone links:

aryl–O–aryl–O–aryl–CO–

The aromatic rings give very high heat resistance and stiffness.

Properties of aromatic polyketones

Property	Behaviour
Heat resistance	Excellent
Continuous service temperature	Very high
Chemical resistance	Excellent
Mechanical strength	Very high
Wear resistance	Excellent
Flame resistance	Naturally good
Cost	Very high
Processing temperature	Very high

PEEK, for example, is processed at much higher temperatures than normal engineering plastics, often around 360–400°C melt temperature, depending on grade.

Typical applications

• aerospace components
• medical implants
• high-temperature electrical parts
• oil and gas components
• compressor parts
• high-performance bearings
• high-temperature gears
• seals and back-up rings

Simple comparison with common engineering plastics

Material	Main character
PA6 / PA66	Strong and tough, but absorbs water
POM / Acetal	Very good dimensional precision and low friction
PK / Aliphatic polyketone	Tough, chemically resistant, low moisture uptake, excellent wear
PBT	Good electrical and dimensional properties
PPS	Higher heat and chemical resistance
PEEK	Very high-performance, very expensive

Practical processing notes for aliphatic PK

Aliphatic polyketone generally processes more like an engineering thermoplastic than like PEEK.

Typical considerations:

• Drying may be required depending on supplier recommendation and storage conditions.
• Avoid excessive residence time.
• Use corrosion-resistant tooling if processing chemically aggressive compounded grades.
• Glass-filled grades will be abrasive to screws, barrels and non-return valves.
• Mould temperature control is important for crystallinity, shrinkage and surface finish.

Important point

When someone says “polyketone” in normal plastics business today, they usually mean aliphatic polyketone PK, not PEEK. PEEK is technically an aromatic polyketone, but commercially it is usually referred to by its own name.

In simple terms:

Polyketone PK is a tough, chemically resistant, low-moisture engineering plastic that sits between nylon, acetal and higher-performance polymers.

Warning:            Ketones should not be processed with contaminated with PVC as this can result in dehydrochlorination with HCl  gas emitted.

Compliments of Composite Plastics of Johannesburg