Amorphous vs Semi-Crystalline plastic from a designer perspective

The difference between amorphous and semi-crystalline plastic is something that every product designer should address. Here is what I find essential from a designer perspective

The difference in the thermal behavior

Amorphous and semi-crystalline plastics react to temperature in a different way. If we observe this closer, it will help us to understand the essential difference in their structure.  This can be done in practice by heating some plastic items, for example, Plasticprop samples.

Let’s have polystyrene PS as an example of amorphous plastic. Place the sample into an oven and start heating it slowly. If you take it out every 20°C (gloves recommended), you can notice that the sample gradually turns softer and softer. At 70°C the sample can already be bent quite easily. When the temperature of app. 95°C  is reached, the sample totally (and more rapidly) loses its stiffness and collapses under its own weight. Polystyrene has reached its material-specific Glass Transition temperature (Tg). Amorphous plastic can only be used in temperatures below their Tg.

Semi-crystalline polymers have a partly different structure. A portion, of their polymer chains, 20-80% depending on the material, have arranged to tight and strictly orientated crystals. The remaining chains are in an amorphous state surrounding the crystals. Because of the amorphous part, semi-crystalline plastics do have a Tg as well.

Let’s place a Plasticprop sample made of PA6 in the oven. At 50°C, it feels slightly more flexible than it did at room temperature. But if you take it out of the oven again at 60°C, the difference is bigger. PA6 has exceeded its Tg temperature, which is app. 55°C. The amorphous part of the polymer is now free to move but the crystals still hold the polymer structure together.  The sample has turned from glassy to rubbery (or leathery) state. Semi-crystalline plastic can be used on both sides of its Tg, but the mechanical behavior of the material does not stay the same. Below Tg, in their glassy state, semi-crystalline plastics are stiffer and stronger but at the same time more brittle. Many semi-crystalline polymers are above their Tg in the room temperature (POM, Tg app. -60 °C; HD-PE, Tg  -110°C, both approximately).

Elevating the temperature further will lead to a gradual decrease in modulus and strength. At 200°C the sample feels interestingly similar to the Plasticprops sample of TPE-S (SEBS). When the temperature reaches 220°C, the crystals dissolute and the solid piece turns to low viscosity liquid. The PA6 sample has then reached its Melting Point, Tm.

In both cases, amorphous and semi-crystalline, the process is reversible. In the case of semi-crystalline polymers, the temperature at which the polymer chains have reordered as crystals during the cooling process is known as Crystallization Temperature, Tc. This is far above the service temperature of the material.

 

How to address this in my design?

As said, amorphous plastics can be used only below their Tg. But it is vital to understand that their mechanical properties do not remain the same through the whole service temperature area. The higher the temperature, the lower the strength and modulus of plastic. PMMA, for example, at 60°C has completely different mechanical characteristics as PMMA at 23°C. Many plastic manufacturers are quite optimistic about the service-temperature they provide in their material data-sheets. The given temperatures may be on the very edge of reaching the Tg. You need to be skeptical when reading the data-sheets and bear in mind that temperature has a great influence on the mechanical properties of any polymer.

When using semi-crystalline plastic you need to check if its Tg is within the service temperature of the product. If so, make sure (by testing and exploring reference samples) that your requirements are filled on both sides of the Tg. For example, the Tg of PP homopolymer is -10°C. The ductility of a water bucket in Scandinavian winter might be surprisingly low.

Transparency

Amorphous plastics are generally transparent. Crystals block the light, which makes semi-crystalline plastics opaque. PP-random-copolymer is the only semi-crystalline transparent material, although even at its best it is slightly milky. Some modified grades of PA6 are available as well, but they should be described as translucent rather than transparent.

How to address this in my design?

Amorphous plastics have their weaknesses that are explained later in this article. I might spoil your excitement, but those are related to chemical resistance, continuous stress and friction/wear. Amorphous plastics do not serve well in machine design purposes. Combining mechanical functions to your transparent component must not be done with too much optimism. Problems are likely to occur. PP-RC is often the safest choice.

Straightness and dimensional accuracy

During the crystallization, the polymer chains are packed very firmly into a small volume. A large number of amorphous polymer chains are compressed into one crystal block. Due to crystallization, the shrinkage of semicrystalline plastics is higher. In the case of amorphous plastics the shrinkage is closer to the influence of negative thermal expansion.

Furthermore, amorphous plastics shrink evenly in every direction. In the injection molding process, the crystals of semi-crystalline plastics are oriented in the direction of the flow. As a result, the shrinkage in the direction against the flow is higher than the shrinkage in the flow direction.

Due to higher and uneven shrinkage products that are made of semi-crystalline plastics tend to warp more. It is more challenging to produce them straight.

 

How to address this in my design?

It is easier to produce a straight and dimensionally accurate product using amorphous plastic. Due to the mechanical functions or service environment of the product it is, however, often safer to use semi-crystalline plastic.

Regardless of what plastic you use, the following basic rules should be applied:

  1. Component geometry: Uniform wall thickness, no deep wells (difficult to cool), large radius rather than sharp corners.
  2. Tooling: Efficient cooling, proper gate location. These are important topics you need to discuss with your tooling supplier.

Surface quality

Higher shrinkage of semi-crystalline plastics easily leads to more visible sink marks. The surface of semi-crystalline plastics also tends to be slightly oily. Amorphous plastics typically provide a dry and shiny surface appearance.

How to address this in my design?

Amorphous plastics are the right choice for visually demanding components such as Bluetooth loudspeakers or WiFi routers. If the product is challenged by harsh service environment or mechanical use, semi-crystalline plastics are a safer choice. PBT, PA6, and PA12, for example, do provide a nice shiny surface. If ribs etc. are properly designed thin enough (app 0,6x wall thickness) the sink marks are not evident.

Compounds of semi-crystalline and amorphous plastics, such as PBT/PC and ABS/PA, are also available.  The idea of these is to combine the advantages of both amorphous and semi-crystalline plastics. Sometimes it works.

Paintability, printability, gluability

Amorphous plastics are usually easy to paint, print and glue. With semi-crystalline, it’s the opposite. The crystals for some reason repel chemical substances.

How to address this in my design?

Don’t take it as given that all plastic components would be easy to print, paint or glue.

The higher the level of crystallinity, the more challenging it is to get something to stick on the surface. If you try scratching the printed text on Plasticprop sample made of POM, you’ll notice that the text peels off quite easily. PBT is quite easy to print or even paint, although it is semi-crystalline.

Gluing is hardly a process for modern high volume manufacturing, snap joints should be preferred instead. The same goes for painting. Glossy colored surface can be achieved without secondary operations, although exterior car parts are today commonly painted PP+EPDM+talk. Outdoor components made of PC are lacquered with a UV protective silicate layer.

Chemical resistance

Chemicals challenge every plastic product, especially together with elevated temperature and continuous load. In general, semi-crystalline plastics endure different chemicals considerably better than amorphous plastics.

How to address this in my design?

Amorphous plastics suit well as covers for electronic appliances, toys or picture frames. But in chemically demanding service conditions that include oil, petrol or strong cleaning detergents, etc, the use of semi-crystalline plastic should be required rather than recommended. If you are uncertain, semi-crystalline plastic is always a safer choice.

Resistance against continuous stress

Stress-cracking due to continuous load is the most common reason for plastic products to fail. The long-term load can be also cyclic, fatigue is another common reason for failures. Semi-crystalline plastics endure these both much better than amorphous plastics.

How to address this in my design?

If your component subjected to continuous or cyclic load, semi-crystalline plastic is a better choice. But this is not enough. You should also keep the long term stress level considerably lower (1/5 is a good starting point) that the short term data would suggest. If you must use amorphous plastic, the difference should be even higher. This is not “over-engineering”, it is the most efficient way to prevent failures in the long term use.

Friction and wear

As said earlier in this article, the surface of semi-crystalline plastics is slightly oily. This gives a good indication of their bearing properties. The friction coefficient of semi-crystalline plastics is smaller than that of amorphous. They also resist wear better.

How to address this in my design?

Lower friction and better wear resistance are another two good reasons to favor semi-crystalline plastics in mechanisms and machine design applications.

SLS 3D printing is a common method for prototyping. The material used in SLS sintering is PA12. If you end up using amorphous plastic, the friction between the moving parts of the final product may be totally different than what you experienced with your 3D-printed models.