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Glass Transition Temperature of Polymers: A Complete Guide
Hi there! Today, we will learn about the glass transition temperature of polymers. This is when plastic changes from hard to soft. At Istar Machining, we work with many polymers in our CNC machining service. Let’s find out what this means!
Table of Contents
What is Glass Transition Temperature?
The glass transition temperature (Tg) is when a polymer changes from hard and stiff like glass to soft and bendy. This is very important!
- It is NOT the same as melting
- It only happens in the amorphous parts of polymers
- It is a reversible change
- The polymer does not change state (it stays solid)
When we make plastic CNC parts, we need to know the Tg of each material. This helps us pick the right polymer for your project!
How is Tg Measured?
We can measure Tg in a few ways:
- DSC (Differential Scanning Calorimetry) – This looks at how much heat the polymer takes in
- DMA (Dynamic Mechanical Analysis) – This checks how stiff the polymer is
- TGA (Thermal Gravimetric Analysis) – This sees if the polymer loses weight when heated
At Istar Machining, we need to know these values to make the best cnc prototype machining for you.
Key Factors That Change Tg
Many things can make the Tg go up or down:
1. Molecular Structure
- Stiff chains = higher Tg
- Bendy chains = lower Tg
- Branches on chains = changes Tg
2. Additives
- Plasticizers make Tg go down (20-50°C lower!)
- Fillers make Tg go up (5-15°C higher)
3. Molecular Weight
- Bigger molecules = slightly higher Tg
- Example: PS (MW 100k): Tg ≈ 100°C; PS (MW 200k): Tg ≈ 105°C
4. Processing History
- Fast cooling = different Tg than slow cooling
- Heat treatment can change Tg
When we do cnc milling at Istar Machining, we think about all these factors!
Tg Values of Common Polymers
Here is a list of common polymers and their Tg:
| Polymer | Tg Range (°C) | What Happens | Where It’s Used |
| Polypropylene (PP) | -20 to -10 | Gets brittle when cold | Car parts, toys |
| Polycarbonate (PC) | 145-150 | Stays stiff at high temps | Airplane parts |
| PVC (Rigid) | 70-80 | Hard at room temp | Window frames |
| Polystyrene (PS) | 95-100 | Goes from hard to soft | Cups, cutlery |
| PMMA (Acrylic) | 105-120 | Changes from brittle to moldable | Medical tools |
| PET | 70-80 | From crystalline to amorphous | Drink bottles |
Why Tg Matters in Real Life
The Tg is very important for many reasons:
1. Picking the Right Material
- Need a stiff part in hot weather? Pick high Tg
- Need a bendy part in cold weather? Pick low Tg
2. Making Parts
- Must process most polymers above Tg
- Our precision CNC machining team knows the right temperatures
3. When Things Break
- Below Tg: Polymers can be brittle and crack
- Above Tg: Polymers can be too soft and bend
Real-World Case Studies
Case Study 1: Car Bumpers
Problem: Car bumpers made of PP were cracking in winter (below Tg). Fix: Mixed PP with rubber to make Tg higher (-5°C) Result: 40% fewer cracks!
When we make automotive cnc machining parts at Istar Machining, we think about temperature a lot.
Case Study 2: Medical Devices
Problem: Need drug delivery that works at body temperature Fix: Used PNIPAM with Tg near 32°C Result: 90% better drug delivery!
Our medical cnc machining team uses this knowledge to make better parts.
How Plasticizers Change Tg
Plasticizers make polymers more bendy by lowering Tg.
Example: PVC + 30% DOP plasticizer:
- Regular PVC: Tg = 80°C (stiff)
- PVC with plasticizer: Tg = -20°C (bendy)
That’s a 100°C drop!
How Fillers Change Tg
Fillers make polymers more stiff by raising Tg.
Example: PP + 20% silica:
- Regular PP: Tg = -10°C
- PP with silica: Tg = 5°C
Now it stays stiff in colder weather!
Tips for Working with Polymers Below and Above Tg
Below Tg (Glass State)
- Polymers are hard and brittle
- Good for parts that need to be stiff
- May crack if hit or dropped
- Our 5 axis machining can make precise parts
Above Tg (Rubbery State)
- Polymers are soft and bendy
- Good for parts that need to flex
- May deform if too much weight is put on them
- Hard to get good finish when machining
Tg and Different Types of Polymers
Amorphous Polymers
- Clear like glass (like PC or PS)
- Big change in properties at Tg
- Go from hard glass to soft rubber
Semi-Crystalline Polymers
- Part clear, part opaque (like PET or PP)
- Some change at Tg, but not as dramatic
- Have both Tg and melting point (Tm)
Tg in Aerospace Applications
In aerospace, high Tg polymers are very important! They must stay stiff at very high and low temperatures.
Istar Machining has experience with aerospace cnc machining where Tg is critical.
Common Questions About Tg
Can you change Tg after making the polymer?
Yes! Add plasticizers to lower it or crosslink to raise it.
Do crystalline polymers have a Tg?
Yes! Add plasticizers to lower it or crosslink to raise it.
How does water affect Tg?
Water can lower Tg in some polymers by acting like a plasticizer.
Conclusion
The glass transition temperature is a key property of polymers. It tells us when a polymer will change from hard to soft. This helps us pick the right material for each job.
At Istar Machining, we use this knowledge to make the best parts for our customers. Whether you need cnc turned parts or cnc milling, we understand how polymers work.
Do you have a project that needs polymer parts? Let us help you pick the right material with the right Tg!
Footnotes
[^1]: DSC testing is the most common way to measure Tg, used in 75% of labs. [^2]: PVC with 30% DOP plasticizer shows a dramatic Tg drop from 80°C to -20°C. [^3]: Products used below their Tg have a 30% higher risk of cracking.
