What is the quantitative correspondence between the gel fraction index of silicone pressure-sensitive adhesives and the cohesive strength of the adhesive layer?

I. Definition and Measurement Methods of the Indicators

1. Gel fraction:

   The gel fraction refers to the mass percentage of the crosslinked network in the silicone pressure-sensitive adhesive coated on Teflon high-temperature tape that is insoluble in good solvents (such as toluene). The measurement procedure involves wrapping the pure adhesive film in a filter mesh, extracting it in a Soxhlet extractor for 24 hours, and then drying the insoluble residue to constant weight before calculation.

2. Cohesive strength:

   In Teflon tape, cohesive strength is characterized by high-temperature holding power (shear adhesion failure temperature or shear holding time). The test is typically conducted at 180 °C under a 1 kg load, recording the time until the adhesive layer slips or fails. This value directly reflects the adhesive layer's ability to resist flow and splitting.

II. Quantitative Correspondence Between Gel Fraction and Cohesive Strength

1. Below the critical point (gel fraction < 60%):

   The system has not yet formed a percolating network; the adhesive layer remains in a viscous-flow state, with holding power approaching zero and no practical cohesive strength.

2. Practical range (60%–85%):

   Beyond the critical point, the effective elastic chain density increases sharply with the gel fraction, and cohesive strength grows exponentially or following a high-power law. Increasing the gel fraction from 60% to 70% can raise the high-temperature holding time from minutes to hours; from 70% to 80%, the holding power can increase by an additional 3 to 10 times. The failure mode gradually transitions from extensive cohesive failure to mixed failure.

3. High crosslinking range (> 85%):

   The increase in cohesive strength slows down. Raising the gel fraction from 85% to 95% increases holding time by only 20%–50%, but tack (initial adhesion) drops significantly due to restricted molecular chain mobility. When the gel fraction exceeds 95%, the adhesive layer approaches a pure elastomer state, and pressure-sensitive adhesive properties are essentially lost.

III. Semi-Empirical Model and Typical Data

As summarized by Jiangsu Teflon high-temperature fabric manufacturers, the empirical relationship between shear holding time (t_shear) and gel fraction (g) can be expressed as:

lg(t_shear) = A + B · g (with B approximately 0.1–0.2), indicating an exponential dependence. The values of A and B depend on the MQ resin ratio, adhesive layer thickness, and test temperature.

Typical corresponding data (180 °C static shear, normalized to a holding time of 1 at 65% gel fraction):

| Gel Fraction | Relative Holding Time | Failure Mode |

|--------------|------------------------|---------------|

| 55%          | < 0.1                  | 100% cohesive failure |

| 65%          | 1                      | ~80% cohesive failure |

| 75%          | 8–15                   | Mixed failure |

| 85%          | 50–100                 | Slight residue |

| 92%          | 150–200 (near saturation) | Pure interfacial failure |

It is evident that in the 60%–90% range, every 5 percentage-point increase in gel fraction can lead to a multiple-fold increase in high-temperature cohesive strength.

IV. Control Strategies in Teflon Tape Manufacturing

1. Target gel fraction range:

   For general-purpose heat-resistant winding tapes from Jiangsu Teflon high-temperature fabric manufacturers, the recommended gel fraction is 75%–85%, balancing tack and cohesion. For high-temperature anti-stick conveyor belts (subject to long-term loading at 200–260 °C), the gel fraction must reach 85%–93% to prevent adhesive residue caused by cohesive splitting of the adhesive layer.

2. Interface matching requirements:

   The PTFE surface of Teflon high-temperature tape has extremely low surface energy; therefore, the cohesive strength of the adhesive layer must be higher than the interfacial adhesion strength; otherwise, cohesive failure will occur under stress. If the gel fraction exceeds 93%, the adhesive layer becomes stiff, reducing wetting and conformability to the PTFE substrate, which may lead to edge lifting. In production, it is essential to precisely control crosslinking temperature, time, and catalyst dosage to lock the gel fraction within the target range, achieving a reliable transition from cohesive failure to interfacial failure.

The above information is provided by Jiangsu Okai New Material Technology Co., Ltd.

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