Heat Setting Drawn PTFE Fiber to Reduce Shrinkage
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Solution Overview
Problem
Dispersion spun PTFE yarns experience decreased thermal stability and elongation when drawn, leading to significant shrinkage during heat treatment, which is problematic for hot gas filtration applications where temperatures exceed 260 degrees Celsius.
Innovation Solution
The process involves sintering and drawing PTFE fibers, followed by heat setting above the Tg of PTFE molecules to relax internal stresses, thereby maintaining the fiber's alignment and reducing shrinkage at elevated temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the PTFE yarn is drawn to increase productivity and tenacity, then the yarn tenacity is improved, but the yarn thermal stability and elongation prior to break decrease
Solution Approach 1:
The patent applies preliminary action by heat setting the drawn yarn at or above the PTFE glass transition temperature (Tg) before the yarn is put into service. This preliminary heat treatment relaxes internal stresses and stabilizes the molecular alignment achieved during drawing, preventing subsequent shrinkage and maintaining thermal stability. The heat setting step is performed in advance to prepare the yarn for high-temperature service conditions.
Solution Approach 2:
The patent utilizes parameter changes by controlling the temperature parameter during heat setting at or above the PTFE Tg (320-350°C). By changing the thermal parameter to this specific range, the molecular mobility is increased enough to allow stress relaxation and reorganization, yet controlled to maintain the drawn configuration. This parameter change transforms the yarn from a stressed, unstable state to a relaxed, stable state.
2Productivity
If the PTFE yarn is drawn to increase productivity, then the production efficiency is improved, but the yarn shrinkage at elevated temperatures increases
Solution Approach 1:
The heat setting step is applied as a preliminary action after drawing and before the yarn is used in filtration applications. This preliminary treatment at or above PTFE Tg stabilizes the yarn dimensions by relaxing internal stresses, ensuring that the yarn will not undergo significant shrinkage during subsequent high-temperature service, thus maintaining filter surface area and productivity.
Solution Approach 2:
The patent converts the harmful effect of internal stresses (which cause shrinkage) into a benefit by deliberately applying heat at or above Tg to relax these stresses. The harmful shrinkage tendency is transformed into a controlled stabilization process, where the same molecular mobility that causes shrinkage is harnessed to achieve a stable, low-shrinkage configuration.
3Shape
If the drawn PTFE fiber is quickly cooled below the Tg to freeze molecular alignment, then the aligned molecules are maintained in place, but the internal stresses are not relaxed causing subsequent shrinkage
Solution Approach 1:
The patent inverts the conventional cooling approach by not quickly cooling the drawn yarn below Tg to freeze alignment. Instead, it holds the yarn at or above Tg to allow stress relaxation, then controls the cooling process. This inversion recognizes that freezing alignment quickly creates internal stresses, whereas controlled holding at elevated temperature relaxes stresses while maintaining acceptable alignment through the subsequent controlled cooling.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method results in PTFE fibers with improved thermal stability and elongation, exhibiting less than 9%, 15%, and 5% shrinkage at 300 degrees Celsius for 400 denier, 400 denier, and 1200 denier fibers respectively, enhancing their performance in high-temperature applications.
Implementation Method 1
Sintering the dispersion spun, intermediate PTFE fiber structure causes the PTFE particles in the structure to coalesce and entangle thus forming a continuous PTFE filament fiber
Implementation Method 2
the continuous PTFE filament fiber is later heated near or above the PTFE molecule's Tg, for example during hot gas filtration applications
Implementation Method 3
by maintaining the drawn fiber at or above the Tg while the fiber is held at length relaxes the internal stresses within the fiber created by drawing
Data Source
AI summary
A dispersion spun polytetrafluoroethylene fiber exhibiting improved elongation prior to fiber break and increased thermal stability, the fiber prepared by forming a spin mix containing a dispersion of poly(tetrafluoroethylene) particles, forming an intermediate fluoropolymer fiber structure from the spin mix, sintering the intermediate fluoropolymer fiber structure and forming a continuous fluoropolymer filament yarn, drawing the continuous fluoropolymer filament yarn, and thereafter heat setting the continuous fluoropolymer filament yarn.