Semi-Crystalline vs Amorphous Polymers: Which One Is More Durable?
Polymers are an incredibly broad and versatile family of materials. They are integrated into many modern items due to their typically low density and low cost relative to traditional materials such as metals. Some engineering plastics have physical properties that allow them to service demanding applications. Two broad types are semi-crystalline polymers and amorphous polymers, both within the thermoplastic classification. Semi-crystalline polymers tend to be more rigid, whereas amorphous polymers are more tolerant of impacts.
This article will compare semi-crystalline vs. amorphous polymers and discuss the types, where they are used, and much more.
A semi-crystalline polymer is one in which the long polymer chains are arranged in a fairly organized molecular structure and packed closely together. This structure results in semi-crystalline polymers having closely defined melting temperatures. It also causes these polymers to have anisotropic flow, meaning the polymer shrinks less in the direction of flow compared to the transverse direction. This can result in dimensional instability. Further, the semi-crystalline structure gives a rigid material with a good wear resistance, but a poor impact resistance.
A semi-crystalline polymer is made up of long polymer chains that typically have repeating units that are strongly attracted to each other in regular ways. This facilitates the regular ordering of the long polymer chains into a semi-crystalline state. However, several different polymers form in this semi-crystalline state. For instance, an example is PBT (polybutylene terephthalate). PBT is formed by repeating units of terephthalic acid, and the aromatic ring orientates itself in a specific plane. This constrains the rotation of the other elements of the chain and therefore promotes an ordered arrangement.
Different types of semi-crystalline polymers are used throughout everyday life. Each type of polymer has its specific set of properties that fits its popular applications, as explained below:
PBT (polybutylene terephthalate) is a polyester, similar to PET (polyethylene terephthalate)—the common material used for soft-drink bottles. PBT crystallizes more efficiently than PET and is used as an engineering thermoplastic in applications such as the automotive and electrical industries. PBT has good toughness and stiffness, as well as creep resistance. It has useful temperature resistance and is a dielectric to prevent electrical discharge. It can also be combined with fiber reinforcing to improve its mechanical properties.
POM (polyoxymethylene) is an acetal plastic with low surface friction and food-safe properties. POM has very good dimensional stability, which means it does not change significantly with changes in environment such as humidity or temperature. POM has two broad variations: the copolymer type POM-C and the homopolymer type POM-H. There are minor, but measurable differences between the two. For instance, the copolymer has better chemical resistance, but the homopolymer has better mechanical strength. POM is used for sliding components such as bushes, as well as components such as hose fittings that are in constant contact with water.
PP (polypropylene) is a widely used semi-crystalline thermoplastic. It has the properties of low surface friction, toughness, and excellent chemical resistance. As a result, it is used in many varied industries such as: food packaging, piping, and even clothing (as fibers).
PA (polyamides) include the popular material nylon. Nylon has a low density, but still good wear resistance and favorable, low-friction properties. Polyamides can be used in multiple ways, such as: rollers and bushes, textiles for fabrics, and filaments for 3D printing.
POK (polyketone) is a semi-crystalline polymer with a unique blend of properties. It has the largest yield strain of all semi-crystalline polymers, which means it can deform significantly without breaking. It exhibits very low moisture absorption. It also has very good low-friction properties and a high melting point of 220 °C. These properties mean that POK finds application in gears and other mechanical components, primarily in the automotive and furniture industries.
To learn more, see our article on Polymer vs. Plastic.
Semi-crystalline polymers are used in many different industries and applications. As a class of material, they tend to have high-temperature resistance (relative to other plastics), act as dielectrics (prevent the flow of electricity), have high durability and stiffness, and also have a low coefficient of friction (they slide easily). Although each specific polymer has variations, the typical combination of properties generally sees semi-crystalline polymers used for moving and sliding parts like rollers and rails, as well as durable housings for consumer goods. They are used in the automotive industry and electrical components. Some are also food-safe and therefore are used in food containers and utensils.
Semi-crystalline polymers have some clear advantages such as:
- A clearly defined temperature at which the solid polymer melts and becomes a fluid.
- High rigidity and toughness.
- Excellent chemical resistance.
- Hard-wearing materials that are tough and resistant to abrasion.
Listed below are some disadvantages of semi-crystalline polymers:
- Illustrates anisotropic flow. They shrink less in the direction of flow than in the transverse direction. This can lead to non-uniform shrinkage and dimensional instability.
- Due to the rigidity of the structure, impacts can cause breakage of the material.
No, semi-crystalline polymers are not simply better than amorphous polymers—they are only different, with different strengths and weaknesses. The successful use of semi-crystalline polymers will depend on matching the demands of the application with the strength of the semi-crystalline material. For instance, semi-crystalline polymers tend to have high stiffness and durability to wear, but poor impact resistance. For applications that demand high wear without impact, a semi-crystalline polymer would be better than an amorphous polymer.
An amorphous polymer is one in which the long polymer chains are arranged haphazardly within the material’s structure—often compared to cooked spaghetti. This disorganized molecular structure influences the properties of amorphous polymers. In particular, they do not have clearly defined melting temperatures, but rather begin to soften while being heated. This occurs because different parts of the polymer structure melt over a range of temperatures. The amorphous structure allows isotropic flow (meaning uniform flow in all directions) and some flexibility and impact resistance.
An amorphous polymer is made of long chains of repeating units (monomers) but particularly arranged in a haphazard, unstructured way, like cooked spaghetti. Many different polymers form an amorphous solid, such as polycarbonate (PC) or ABS. Polycarbonates contain various repeating units that include carbonate groups, whereas ABS is a polymer of the monomers acrylonitrile, butadiene, and styrene. Therefore each amorphous polymer is made of different repeating units, but as a family of materials, they are made up generally of long polymer chains without a structured arrangement.
There are various types of amorphous polymers available as listed and discussed below:
PMMA (polymethyl methacrylate) is a rigid and transparent thermoplastic that is commonly used as a replacement for glass. It provides an incredibly high transmission of light and has higher impact resistance than glass.
Sulfone polymers are polymers that include SO2 linkages within their polymer chain. The three most common variations are PSU (polysulfone), polyphenylsulfone (PPSU), and polyethersulfone (PESU). As a class of polymers, they have a very good temperature resistance, which makes them flame retardant. As a result, sulfone polymers are used in safety equipment, as well as in medical applications and aircraft.
COC (cyclic olefin copolymer) is another clear, amorphous plastic with excellent light transmission. It is used as a glass substitute whenever optical characteristics are important. It can also be manufactured with incredibly high purity, which makes it well-suited to medical applications in which the risk of contamination by packaging cannot be tolerated.
PC (polycarbonate) is a popular transparent plastic that is used in place of glass. Another key property of polycarbonate is its high impact resistance, which allows it to be used as safety glass or bulletproof glass.
PC/ABS is a blend of the two polymers of polycarbonate and acrylonitrile butadiene styrene. The blend aims to improve the properties of the two polymers when combined. Therefore PC/ABS has a high impact resistance and better temperature resistance than ABS on its own. It is typically used in motor vehicle interiors, as well as electrical fittings like outlet faceplates.
ABS (acrylonitrile butadiene styrene) is an amorphous polymer that is opaque. It has a number of favorable properties, such as: high impact resistance, good machinability, and relatively low cost, which make it a widely applied material. It has a relatively low melting temperature, which makes it easier to use in injection molding but it does not withstand high service temperatures. ABS is commonly used in items such as keyboard keys and Lego pieces.
To learn more, see our guide on Polymer vs. Metal.
Amorphous polymers are used in applications like: motor vehicle interiors, keyboards, and electrical outlet faceplates. There are also a number of amorphous polymers that are transparent, including: PC, PMMA, and COC. Therefore, a major application of amorphous polymers is in glass-like applications, particularly those in which resistance to breakage is a priority such as: safety equipment, medical applications, or aircraft.
Amorphous polymers have some key advantages such as:
- Can absorb impact without fracture.
- There are a number of amorphous polymers, such as polycarbonate, PMMA, and COC, which have very good optical performance. Not only are these plastics transparent, but they allow for a high transmission of light.
- Do not have narrow melting point temperatures but rather soften as the temperature warms up. They generally perform relatively well at elevated temperatures, with some even being fire retardants.
The disadvantages of amorphous polymers are listed below:
- Although amorphous polymers have a high impact resistance, they tend to have poor wear resistance.
- Tend to have limited chemical resistance. That means that although there are some chemicals that can withstand prolonged periods, there is a wide range of many other chemicals that may cause swelling, cracking, or other weakness.
It depends. Amorphous polymers are better than semi-crystalline polymers in specific applications that favor the properties of amorphous polymers. These applications require impact resistance and include: keyboard keys, substitute-glass windows, and Lego pieces. However, there are different applications in which semi-crystalline polymers perform better than amorphous polymers. These include rolling and sliding components with low friction and high rigidity. Successful performance is always about matching the requirements of the application to the material that possesses the best blend of suitable properties.
Amorphous polymers differ from semi-crystalline polymers in a number of ways. Amorphous polymers tend to be less rigid and also absorb impacts better. They do not have a clear melting point, but rather a glass transition temperature at which they begin to soften. They tend to wear poorly, with a high surface friction, as opposed to semi-crystalline polymers that tend to have a low coefficient of friction and good wear resistance.
Yes, amorphous and semi-crystalline polymers can be used together. These two different types of thermoplastics are compatible for use together, so long as, individually, each selected polymer is suitable for the application’s environment. In fact, sometimes two different types of thermoplastics are blended in order to achieve specific properties. The degree of crystallinity is one factor that can be varied when manufacturing engineering plastics, again with the purpose of fine-tuning specific mechanical properties.
There are a number of things to consider when selecting which polymer to use. It is important to understand the application in detail, such as the required durability and life span of the component, together with the environment and temperature to which it will be exposed. These factors will determine which is the best polymer to choose, as explained below:
The full life cycle of a piece of equipment needs to be considered at the design stage. The environmental toll of single-use, non-biodegradable components has become evident and should be mitigated wherever possible. Recyclable components also tend to have lower production costs due to the possibility of incorporating recycled material. Thermoplastics are recyclable, as they can melt and harden multiple times, similarly to metal. This is therefore true of both semi-crystalline and amorphous polymers.
The range of available materials will be larger for applications with limited repeated use compared to applications that require continuous service for more than a decade. For short-life-span articles, such as cutlery and food packaging, the longevity of the polymer is not critical, only its stability over a relatively short period (months rather than years). However, for industrial applications, such as rollers on doors, or windows in aircraft, it is required that the polymer last at least a decade, with longer achievable life spans reducing maintenance costs for replacement. The durability of a polymer is a key feature in these applications. Semi-crystalline polymers have a higher wear resistance and generally would be a better selection in these longer-term applications. However, each application must be considered individually—for instance, if safety against impact is important, an amorphous polymer may be a better choice.
The environment can introduce factors such as moisture, UV exposure, and various chemicals. The right polymer for the application will need to be selected according to the specific exposures present. For transparent amorphous polymers such as PC and PMMA, UV stability is important in situations in which they are used as a glass replacement—the polymer should not degrade, yellow, or become cloudy with extended exposure to sunlight. For other applications, such as equipment housings, resistance to moisture and cleaning chemicals are important. There are further industrial applications that handle specific, harsh chemicals for processing—in these instances, semi-crystalline polymers tend to be a better selection as they have a broader chemical resistance. An example of an excellent polymer for these applications is polypropylene (PP).
A fundamental difference between plastics and the metals they can substitute is their melting temperatures and temperature stability. When comparing semi-crystalline and amorphous polymers, it is important to note that semi-crystalline polymers will have a narrow melting temperature, whereas amorphous polymers gradually soften as temperature increases. Amorphous polymers have a glass transition temperature, which is the temperature at which the material changes from a hard, glassy state, to a softer, rubbery state. Due to this gradual softening, amorphous polymers tend to have a higher temperature resistance than semi-crystalline polymers, and therefore will generally be a better selection for applications with higher working temperatures. However, this must be confirmed for the specific amorphous polymer that is to be selected.
The cost-effectiveness of semi-crystalline polymers is comparable to amorphous polymers. In each classification, there are low-cost, bulk polymers with very broad applications. Examples include polypropylene (PP) as semi-crystalline, and ABS as amorphous. There are also higher-cost engineering polymers within both classifications, which have more specific, higher-intensity applications. Examples of these are POK as semi-crystalline, and COC as amorphous. So for either type, there are comparably priced options, from cheaper, larger-volume materials to more expensive tailored materials with specific properties.
The hardness of plastics can be measured using a number of different hardness testing standards. For semi-crystalline and amorphous polymers which tend to be relatively hard, the most relevant standard is ASTM D785. This standard uses the Rockwell hardness testing method. The test therefore uses an indenter (a steel ball of a specific diameter) which is pressed into the material being tested to make a permanent indentation. The depth of the indentation is measured and used to calculate a hardness value.
Depending on the specific plastic to be tested, as well as the thickness and size of the specimen, a slightly different standard hardness test may need to be applied. Standards bodies such as ASTM International (American Society for Testing Materials) provide a range of standard measures for these purposes.
Yes, both amorphous and semi-crystalline polymers are used in 3D printing. Specifically in the popular fused deposition modeling (FDM), some common filaments used include PLA (semi-crystalline) and ABS (amorphous). Variations on this style of 3D printing, in which temperature is used to fuse thermoplastics, also use both amorphous and semi-crystalline polymers.
It depends. Semi-crystalline polymers can be more durable than amorphous polymers when used in applications with high wear. These can include moving parts with rollers and sliding rails. Semi-crystalline polymers are more rigid and have a higher wear resistance than amorphous polymers, and therefore are more durable in these types of applications. However, semi-crystalline polymers tend to have poor impact resistance, and so if impacts are expected, an amorphous polymer may be more durable for that service.
To learn more, see our guide on Durometer.
The fundamental difference between amorphous and semi-crystalline polymers is the molecular arrangement of their long polymer chains. In semi-crystalline polymers, the chains are arranged in a structured manner, and closely packed. In amorphous polymers, they are arranged randomly, like strands of spaghetti. This fundamental difference in molecular structure then defines the other differences between the two types of polymers. Semi-crystalline polymers have a narrow melting temperature range, whereas amorphous polymers do not but rather soften as their temperature is raised.
This article presented semi-crystalline and amorphous polymers, explained each of them, and discussed each material's durability. To learn more about semi-crystalline and amorphous polymers, contact a Xometry representative.
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