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What is the reason for the poor adhesion of PTFE bellows?

2019-12-06

Most of the bellows damage is due to mechanical kneading rather than use in production engineering. In order to prevent production from being forced to stop due to damage to the diaphragm valve, in practice, the PTFE bellows is usually replaced before the specified service life. Because the PTFE bellows can be used in almost all kinds of production processes, it is more economical to replace the number of PTFE bellows or repair costs.

The reasons for the poor adhesion of PTFE bellows are summarized as follows:

1. Adhesive adsorption on the surface of PTFE tube is caused by intermolecular effect force, which includes orientation force, induction force and dispersion force. The non-polar appearance of PTFE does not have the conditions for forming the orientation force and the induction force, but can only form a weak dispersion force, so its adhesion function is poor;

2. The external surface energy is low, and the critical external surface tension is generally only 1.85 × 10-2N / m. The forward contact angle (θd) of the PTFE tube is 118o, the backward contact angle (θr) is 91o, and the contact angle (θ) is 104o, which is the highest among all materials. The larger the contact angle, the lower the humidity. That is, the worse the wetness is, the adhesive cannot be sufficiently wet with PTFE, and then cannot adhere well to PTFE.

3. The solubility parameter SP value of the tetrafluoro tube is very small, so the adhesion with other substances is also very small. From the above analysis, we can see that to solve the problem of difficult bonding of PTFE tubes, we should generally start from the two aspects of surface modification to improve its bonding function and develop new adhesives.

4. Large crystallinity, good chemical stability, swelling and dissolving of PTFE tube is more difficult than non-crystalline polymer. When the adhesive is applied on the surface of PTFE tube, it is difficult to cause mutual dispersion and entanglement of polymer molecular chains. , Can not constitute a strong adhesion.

Because the effects of PTFE bellows are different, the requirements for their service life are also different. When the bellows works in a tight state, the agreed shrinkage displacement is larger than the displacement in the working tension state, so when designing the PTFE bellows, it should work as tightly as possible.


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