Email Us

The Glass Transition Temperature of Polymers

When designing parts for plastic injection molding, there is an important but often overlooked material consideration. It involves the glass transition temperature, or Tg. We'll get into the underlying science later, but for now, keep in mind that there is a temperature at which amorphous materials transition from a glassy/rigid state to a leathery/rubbery state.

What is meant by the glass transition temperature (Tg)?

The glass transition temperature is the temperature at which an amorphous polymer transitions from a hard/glassy state to a soft/leathery state, or vice versa. The Tg is directly related to the strength and potential uses of a material in a particular end-use application. The glass transition temperature is linked to the mechanical properties of a polymer. These include tensile strength, impact strength, modulus of elasticity, and operating temperature range, as shown in Figure 1.

glass-transition-temperature-chart.png

Figure 1: Flexural modulus vs. temperature.

Amorphous vs. semi-crystalline polymers.

There are two classes of polymers: Thermoplastics and Thermosets. Thermoplastic polymers are further divided into two groups: amorphous, such as polycarbonate (PC) and polystyrene (PS), and semi-crystalline, such as polypropylene or acetal.

To take a closer look here, let's take a digression back to high school chemistry class. Don't worry, it won't take long. We'll start on the first day when the teacher asks the question, "What are polymers?" The smart guy sitting up front studying ancient languages for fun knows that "poly" and "mer" are the Greek words for "many parts," so he concludes, "Polymers are long chains of smaller molecules joined together by a process called polymerization, with molecular weights in the hundreds to hundreds of thousands."

Polymer Structure

 Thermoplastic Thermosetting
  Amorph Kristallin
Chain structureRandom/UnorderedOrdered/StableNetworked
Melting pointNot defined/slow softeningClear/crystalline decompositionNo Schelz point
Shrinkage rateLowHighLow
Appearance Transparent OpaqueVaries
 Chemical resistanceLowHighHigh
Examples ABS, PC, PS PP, PET, POMEpoxid, LSR

Long molecular chains

What is meant by molecular weight? More importantly, who cares? It should interest anyone who designs plastic parts. The molecular weight of a polymer determines the length of the "long chains" just mentioned, and thus its physical properties. For example, while a hydrogen molecule weighs only 1.01 g/mol (molar mass) and a carbon molecule weighs 12.01 g/mol, a single molecule of high-density polyethylene (HDPE) - which is nothing more than a chain of these two molecules - can weigh 250,000 g/mol or more.

Whether it's polyethylene terephthalate (PET) molecules weighing 8000 to 31,000 g/mol or polystyrene (PS) molecules weighing 400,000 g/mol, these centipede-like chains of monomers, which are the building blocks of polymers, arrange themselves into either amorphous or semi-crystalline structures.

Polymer morphology

Amorphous polymers have a randomly arranged/disordered chain structure. Below the glass transition temperature, they are hard and brittle. When heated, they slowly soften and become leathery/rubbery. This transition is the glass transition. When further heat is applied, they gradually melt (become moldable) after exceeding Tg to a temperature where the polymer begins to flow viscously. Common examples of amorphous polymers are hard, rigid materials such as polystyrene (PS) and polymethyl methacrylate (PMMA), which are used in their glassy state and well below their glass transition temperature.

Semi-crystalline polymers have highly ordered crystalline regions in addition to amorphous regions. The amorphous regions exhibit the properties just described. In semi-crystalline materials, on the other hand, the crystalline regions remain highly ordered after the glass transition temperature (Tg) is exceeded and form the structure of the material. For this reason, many semi-crystalline materials can be used above their Tg. Semi-crystalline materials such as polypropylene (PP), which has a Tg of about -20 °C, are used above their Tg in applications such as garden furniture, which is robust and flexible in the warm summer months but can become brittle in the cold winters.

Thermoplastic polymers have cross-links that bind their chains together. These bonds form between the chains, forming one large molecule. Think about this the next time you pick up a bowling ball. Cross-linking creates a robust chain structure, allowing elastomeric materials, such as silicone, to be used well above their Tg. Other thermoplastic materials are typically used below their Tg and are quite rigid, such as phenols. The cross-links form such strong bonds between molecular chains that the melting point of thermoset materials is above their decomposition temperature.


Tg for Common Molded Plastics

MaterialTg in degrees Celsius
GPPS - Universal polystyrene100
HDPE - High Density Polyethylene-120
LCP - Liquid Crystal Polymer120
LSR - liquid silicone-125
PC - polycarbonate145
PEEK - Polyetheretherketone140
PEI - Polyetherimide210
PMMA - polymethyl methacrylate90
PP - Polypropylene (atactic)-20
PPS - Polyphenylene sulfone90
PSU - Polysulfone190
SPS - Syndiotactic Polystyrene100

Advantages and disadvantages of polymers

Amorphous polymers are often transparent (polycarbonate and acrylic are two examples) rather than opaque, as are most semi-crystalline materials. They generally have better dimensional stability and are less likely to warp during the molding process. They are generally resistant to hot water and steam (think plumbing materials) and have good stiffness and impact resistance. As mentioned earlier, they tend to soften gradually when exposed to heat.

Partially crystalline thermoplastics have very strong molecular bonds due to their internal structure. This property makes them resistant to aggressive chemicals. Like Teflon, many of them offer a low coefficient of friction, making them a good choice for bearing and wear surfaces or where there is a heavy structural load. They are also much more resistant to fatigue than amorphous polymers. They soften when exposed to heat, but can be used above their Tg because the crystalline regions retain their structure up to the melting temperature of the polymer.

With their cross-linked internal structure, thermoset materials have very good chemical resistance, dimensional stability and heat resistance. Thermoplastic materials vary from transparent to opaque. They can be elastomeric or rigid. They can also be used above and below their Tg and have no melting point.

There is no doubt that we are dealing with a complex subject here. We hope that these technical remarks have helped to bring a little more clarity to this subject area.

Are you working on an precise machining project? Contact us at +86-0755-28025755 or sales@richconn.com.cn. We are always happy to talk about polymers.

Related CNC Machining Services
Related News of CNC Machining
  • What Is Reaming & Reamer And Their DifferencesWhat Is Reaming & Reamer And Their DifferencesSeptember 26, 2023Reaming is an important process in CNC machining services, and its main purpose is to join parts or components so that they can rotate or oscillate relative to each other and move at a specific angle or direction.view
  • Comparison of 7075 Aluminum Alloy vs 6061 Aluminum AlloyComparison of 7075 Aluminum Alloy vs 6061 Aluminum AlloyNovember 27, 2023​Aluminum alloy is a widely used metal material with the advantages of light weight, high strength, corrosion resistance and recyclability. There are many types of aluminum alloys, and different alloying elements and heat treatment processes affect the properties and uses of aluminum alloys.view
  • The Complete Guide to Aerospace Fasteners 2023The Complete Guide to Aerospace Fasteners 2023August 24, 2023The aerospace fastener is an essential part of aerospace equipment which is the core product of modern aviation industry. The main function of air fasteners is to ensure the safety and stability of ai...view
  • What Are Machined Rotors?What Are Machined Rotors?November 8, 2023Welcome to the world of precision engineering, where the subtle details can make a world of difference. In the realm of CNC machining and automotive components, 'Machined Rotors' emerge as a pivotal player, quietly ensuring your safety and vehicle performance.view
  • CNC Machine vs 3D Printer: A Comprehensive ComparisonCNC Machine vs 3D Printer: A Comprehensive ComparisonNovember 6, 2023​As the proud owner of Richconn, a CNC machining service provider, I understand the importance of choosing the right manufacturing technology for your production needs. The decision between CNC machines and 3D printers can be a daunting one, and that's why I'm here to provide you with a comprehensive comparison to help you make an informed choice.view
  • Brass vs Bronze vs Pure Copper vs Copper Alloys: An in-depth look at their differencesBrass vs Bronze vs Pure Copper vs Copper Alloys: An in-depth look at their differencesSeptember 26, 2023The comparison of brass to bronze to copper and its copper alloys is crucial as these metals can be easily confused. In addition, the distinction between these materials is also very subtle. As a result, development engineers or designers can be seriously confused when it comes to material selection. In particular, the similarity of these metal constituent elements makes it more difficult to classify and select them.view
Tel
+86-0755-28025755
Address
1212, Zehua Building, Intersection of Longhua Meilong Road and Donghuanyi Road, Songhe Community, Longhua Street, Longhua District, Shenzhen, GuangDong, China