The control of crystallinity in fluoroplastics is a crucial factor affecting their transparency, mechanical properties, thermal stability, and processability.
1. The influence of crystallinity on the properties of fluoroplastics
High crystallinity (such as PTFE reaching 90%–98%):
·Improve strength, hardness, heat resistance, and chemical stability
"But it leads to opacity (light scattering at the crystal region interface) 18."
Low crystallinity (e.g., FEP can be less than 5%):
Enhance transparency and improve processing fluidity
"But it may reduce high-temperature dimensional stability."
2. Main control methods
00001.
Cooling rate regulation
00002.
Rapid cooling (such as FEP quenching through a water bath): inhibits the orderly arrangement of molecular chains, reduces crystallinity, and enhances transparency
Slow cooling (such as high mold temperature molding): promotes crystallization, increases crystallinity, and is suitable for applications requiring high strength.46
00003.
Mold temperature control
00004.
·The higher the mold temperature, the stronger the activity of the molecular chain, and the easier it is to crystallize
For example, the crystallinity of PFA is about 45% when the mold temperature is 140–160℃, and it can be increased to 58% when the temperature is raised to 190℃
·Semiconductor-grade PFA tubing achieves a crystallinity level of less than 30% by controlling mold temperature, thereby enhancing transparency
00005.
Molecular structure design
00006.
·The introduction of comonomers (such as adding 15%–20% HFP to FEP) disrupts the chain regularity and inhibits crystallization1
Although the perfluoroalkoxy side chain of PFA is partially crystalline, the crystallinity can be controlled at 40%–50% through specific processes
00007.
Nucleating agent and heat treatment
00008.
Nucleating agents can refine grains and increase the crystallization rate, but most of them tend to reduce transparency (use with caution for transparent products)
·Heat treatment can refine the crystal structure, enhance crystallinity and heat resistance, but excessive crystallization leading to embrittlement should be avoided
00009.
Optimization of processing technology
00010.
· Preheat in stages (e.g. PFA: 80℃→120℃→180℃) to prevent precrystallization2
Melt temperature control: When PCTFE is heated above 250℃, its crystallinity decreases to 30%–40%, resulting in more uniform crystal grains
In-mold vibration (such as PCTFE) can reduce grain size by 30%–40% and enhance mechanical properties
3. Typical crystallinity range of fluoroplastics
material
Crystallinity range
Typical application characteristics
PTFE
90%–98%
Opaque, high wear resistance 18
FEP
≤5%
High transparency, easy to process
PFA
30%–70% (for high-end products, it can be less than 30%)
Translucent to transparent, heat-resistant to 28 degrees Celsius
PVDF
Controllable (semi-crystalline)
Adjustable mechanical properties and chemical resistance
PCTFE
30%–60%
It depends on the processing temperature
