How Do Fillers Affect the Performance of Modified PTFE Sheets?

2026-10-13

A seal design engineer reviews a warranty claim for a chemical pump. The seal was made from pure PTFE and failed after three months. The failure mode was cold flow: the PTFE had deformed under the compressive load and extruded into the clearance gap. The engineer replaces the seal with a Modified PTFE Sheets containing 25 percent glass fiber. The new seal has been running for two years without measurable deformation. The difference is not the PTFE itself. It is the filler. Fillers change the mechanical properties of PTFE in specific ways: they increase creep resistance, reduce wear, improve thermal conductivity, or enhance chemical resistance. But each filler has trade-offs. This guide explains how to select the right filler for a given application.

1. Why Does Pure PTFE Fail Under Load?

Pure PTFE has excellent chemical resistance and a low coefficient of friction. But it has poor mechanical properties under sustained load. It creeps, or cold flows, even at room temperature. The creep rate increases with temperature and load. At 100°C and 10 MPa compressive stress, pure PTFE can deform by 10 percent in 1,000 hours. This deformation causes the seal to lose contact pressure and leak. Pure PTFE also has low wear resistance. It wears rapidly in dynamic applications, especially against rough or hard surfaces. The table below shows the mechanical properties of pure PTFE compared to filled grades.

Property	Pure PTFE	25% Glass Fiber	25% Carbon	40% Bronze
Compressive strength (MPa)	12	18	20	22
Creep deformation at 10 MPa, 100°C, 1000h (%)	10.5	4.2	3.8	3.2
Wear rate (mm³/Nm x 10⁻⁶)	250	45	20	12
Thermal conductivity (W/mK)	0.25	0.35	0.50	0.65
Coefficient of friction	0.05	0.12	0.15	0.18

The data shows that fillers reduce creep deformation by 60 to 70 percent and wear rate by 80 to 95 percent. The trade-off is an increase in the coefficient of friction and a reduction in chemical resistance. In our factory, we produce Modified PTFE Sheets with all of these filler types. Ningbo Taifno PTFE Plastic Products Co., Ltd. has been manufacturing filled PTFE sheets for over 15 years.

2. How Does Each Filler Type Address Specific Failure Modes?

Different fillers address different failure modes. Glass fiber improves creep resistance and compressive strength. Carbon improves wear resistance and thermal conductivity. Bronze improves thermal conductivity and load-bearing capacity. Graphite improves friction and wear characteristics. The table below shows the recommended filler for each failure mode.

Failure mode	Primary cause	Recommended filler	Filler loading
Cold flow (creep)	Sustained compressive load	Glass fiber	15 – 25%
Adhesive wear	Sliding against metal	Carbon or graphite	15 – 25%
Abrasive wear	Hard particles in fluid	Bronze or carbon	25 – 40%
Thermal deformation	High temperature	Bronze or carbon	25 – 40%
Chemical attack	Aggressive media	Carbon (if compatible)	10 – 20%

In our factory, we select the filler based on the application. For a chemical pump handling sulfuric acid, we recommend carbon-filled PTFE because it has good chemical resistance and wear resistance. For a hydraulic cylinder handling phosphate ester, we recommend bronze-filled PTFE because it has high thermal conductivity and load-bearing capacity. Taifno provides a filler selection guide for all of our customers.

3. How Does Filler Loading and Distribution Affect Performance?

The filler loading is the percentage of filler by weight. Higher loading increases creep resistance and wear resistance but reduces elongation and impact strength. The optimal loading depends on the application. The filler distribution also matters. If the filler is not uniformly dispersed, the sheet will have weak spots that fail prematurely. The table below shows the effect of filler loading on mechanical properties.

Filler loading (% by weight)	Compressive strength (MPa)	Creep deformation (%)	Elongation at break (%)	Impact strength (kJ/m²)
0 (pure PTFE)	12	10.5	300	No break
10	15	6.8	220	No break
20	18	4.5	150	No break
30	21	3.2	90	45
40	23	2.8	50	25

Material selection tip: For a seal that must withstand high compressive loads, choose a filler loading of 25 to 30 percent. For a seal that must maintain flexibility, choose a loading of 10 to 15 percent. For a seal that must resist impact, avoid loadings above 30 percent. In our factory, we can produce Modified PTFE Sheets with any filler loading from 5 to 40 percent. We also use a proprietary mixing process that ensures uniform filler distribution. The coefficient of variation of filler content across the sheet is less than 3 percent.

4. What Are the Chemical Compatibility Limits of Filled PTFE Sheets?

The filler can reduce the chemical resistance of PTFE. Glass fiber is attacked by hydrofluoric acid and hot alkalis. Bronze is attacked by acids and ammonia. Carbon is generally compatible with most chemicals but can catalyze the decomposition of some aggressive media. Graphite is compatible with most chemicals but can be oxidized by strong oxidizing agents. The table below shows the chemical compatibility of different filled PTFE sheets.

Chemical	Pure PTFE	Glass-filled	Carbon-filled	Bronze-filled
Sulfuric acid (98%)	Excellent	Good	Excellent	Not recommended
Hydrofluoric acid (50%)	Excellent	Not recommended	Good	Not recommended
Sodium hydroxide (50%)	Excellent	Fair	Excellent	Not recommended
Ammonia (conc.)	Excellent	Good	Excellent	Not recommended
Steam (150°C)	Good	Good	Good	Fair

In our factory, we recommend that the customer verify the chemical compatibility before selecting a filled PTFE sheet. We provide a compatibility chart for over 500 chemicals. Taifno also offers a sample testing service where we immerse the filled sheet in the customer's chemical for 72 hours and measure the weight change and mechanical property retention.

Frequently Asked Questions About Fillers in Modified PTFE Sheets

Question 1: Can I use a glass-filled PTFE sheet in a hydrofluoric acid application?

Answer: No. Glass fiber is attacked by hydrofluoric acid. The acid dissolves the glass, which creates voids in the sheet and releases silica particles into the process. This causes leakage and contamination. For hydrofluoric acid service, we recommend pure PTFE or carbon-filled PTFE. Carbon is resistant to hydrofluoric acid. In our factory, we have tested carbon-filled Modified PTFE Sheets in 50 percent hydrofluoric acid at 50°C for 1,000 hours. The weight change was less than 1 percent, and the tensile strength retention was over 90 percent. We also recommend carbon-filled PTFE for hot alkalis, where glass-filled PTFE would be attacked. The filler selection chart provided by Ningbo Taifno PTFE Plastic Products Co., Ltd. lists the recommended filler for each chemical family.

Question 2: How does the filler affect the sealing force and bolt load retention?

Answer: The filler increases the compressive strength and the creep resistance of the PTFE. This means that the seal can maintain a higher sealing force over time. A pure PTFE seal loses 30 to 50 percent of its initial bolt load after 1,000 hours at 100°C. A glass-filled seal loses only 10 to 15 percent. The higher bolt load retention means that the seal maintains a tighter joint, which reduces the risk of leakage. In our factory, we measure the bolt load retention of our filled PTFE sheets using a stress relaxation test. The test applies a constant strain to the sample and measures the stress decay over 1,000 hours. We provide the stress relaxation curve for each filler type and loading.

Question 3: What is the maximum operating temperature for filled PTFE sheets?

Answer: The maximum operating temperature depends on the filler. Pure PTFE can operate continuously at 260°C. Glass-filled PTFE is limited to 250°C because the glass fiber can soften at higher temperatures. Carbon-filled PTFE can operate at 260°C. Bronze-filled PTFE is limited to 200°C because the bronze can oxidize and the PTFE can degrade. In our factory, we recommend that the operating temperature be at least 20°C below the maximum for continuous service. For intermittent service, the maximum can be used. We also recommend that the thermal expansion of the filled PTFE be considered. The coefficient of thermal expansion of filled PTFE is 30 to 50 percent lower than pure PTFE, which reduces the dimensional change at high temperatures.

Summary for Seal Design Engineers

Fillers change the mechanical properties of Modified PTFE Sheets in specific ways. Glass fiber improves creep resistance. Carbon improves wear resistance and thermal conductivity. Bronze improves load-bearing capacity. Graphite improves friction characteristics. The filler loading and distribution determine the final performance. The chemical compatibility of the filler must be verified for each application. By selecting the right filler for the failure mode and the chemical environment, engineers can extend the service life of PTFE seals by 5 to 10 times. Ningbo Taifno PTFE Plastic Products Co., Ltd. has been manufacturing filled PTFE sheets for over 15 years and provides full material selection support for our customers.

Ningbo Taifno PTFE Plastic Products Co., Ltd. manufactures Modified PTFE Sheets with glass fiber, carbon, bronze, and graphite fillers at loadings from 5 to 40 percent. We provide stress relaxation curves, chemical compatibility charts, and sample testing services.

Need help selecting the right filler for your PTFE seal application? Contact Ningbo Taifno PTFE Plastic Products Co., Ltd. for a free consultation. We will review your failure mode and chemical environment and recommend the optimal filler type and loading.

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