PEEK, Nylon 6/6: Semi-Crystalline Polymers Explained

Structure of Semi-Crystalline Polymers

Semi-crystalline polymers consist of both crystalline regions and amorphous regions. They have areas that form well-ordered polymer chains, as shown below, along with regions where the polymer chains are randomly ordered. Lamellae refer to the orderly structured crystalline regions. While the loose polymer chains, often called tie molecules, make-up the amorphous regions. This assembly is called a spherulite. There are billions of them in only a few grams of a polymer material.

There are really no crystalline polymers. They would be too brittle. Rather materials such as diamonds, salt and most metals are crystalline. The addition of amorphous regions give plastics toughness or in other words the ability to bend without breaking and to absorb impact.

Types of Semi-crystalline Polymers

Amorphous polymers include PEI Ultem® and polycarbonate. Just knowing whether a plastic is semi-crystalline or amorphous will not determine which polymer to use in your application. For example, Ultem® PEI exhibits excellent thermal properties, good chemical resistance, inherent flame retardancy and exceptional dimensional stability. Thus it is important to examine a polymer for all its characteristics when making a choice in a given application.

Properties

The amount of crystallinity present in the plastic influences properties such as hardness, strength, chemical resistance, stiffness, optical clarity, and melting point.

The properties of semi-crystalline polymers include: 

Shrinkage:

The shrinkage rate of semi-crystalline plastics is typically higher than amorphous plastics. And even different semi-crystalline polymers have different shrinkage rates depending on their chemical structure and degree of crystallinity. The range of shrinkage is generally between 1% and 4% depending on the specific material. Unfilled PEEK has a shrinkage rate of 1–1.8% while nylon 6/6 has a rate of 1.5%. However, polycarbonate, an amorphous polymer, has a rate of 0.6%.

Crystallization effect:

When a semi-crystalline plastic cools, the polymer chains form crystalline regions, which are denser than the amorphous regions in the plastic. This causes significant shrinkage. 

Molecular structure:

These plastics are made up tightly packed, well organized molecular chains. Thus they are stronger than amorphous polymers 

Processing factors:

The temperature of the mold, cooling rate, and melt temperature of a plastic significantly affect the degree of crystallization and, therefore, the shrinkage rate.

Durability:

Semi-crystalline plastics are tough and durable. They can also handle heavy loads. 

Melting point:

They have a sharp melting point. This means they remain solid until they reach a specific temperature and then rapidly melt into a liquid. 

Chemical resistance:

They are more resistant to prolonged chemical exposure than amorphous plastics. For example PEEK is resistant to 100% hydrochloric acid as well as Jet fuel.

Thermal stability:

They perform more consistently and reliably in high-heat applications than amorphous polymers. 

Wear properties:

Semi-crystalline polymers have a low coefficient of friction. This makes them ideal for machinery bearings and automotive components designed to reduce wear and improve fuel efficiency. Also, they are used for wear resistance in medical implants, prosthetics, sports equipment, and in propellers to minimize drag.

Stress resistance:

Semi-crystalline plastics endure stress-cracking and fatigue better than amorphous plastics. Low stress cracking and fatigue are primarily relevant in engineering applications where components experience repeated loading cycles including situations such as: welded structures, engine arts, turbine blades, high-pressure pipelines, automotive components and machinery components under variable temperature conditions.  

Clarity:

As the percentage of crystallinity increases, the polymer becomes less clear. Thus as the density increases, the clarity decreases as light passing through slows down.