Analysis of the technical difficulties of surface bonding of mechanical seals made of polytetrafluoroethylene

PTFE (polytetrafluoroethylene PTFE) has excellent chemical stability, electrical insulation, self-lubrication, non-flammability, aging resistance, high and low temperature adaptability and chemical corrosion resistance, and has high mechanical properties, is a comprehensive performance of dual-use engineering plastics. However, because of the poor wettability of PTFE surface, it cannot be well bonded, thus greatly limiting its scope of use. In order to make PTFE more widely used, it is necessary to treat its surface to improve the bonding performance and enhance the bonding strength.

Analysis of the reasons why PTFE is difficult to bond

PTFE is difficult to bond, from its physical properties analysis, mainly for the following reasons:

First, the surface energy is low, the critical surface tension is generally only 1.85 × 10-2N/m. PTFE forward contact angle (θd) for 118 °, backward contact angle (θr) for 91 °, contact angle (θ) for 104 °, is the largest of all materials, and the larger the contact angle, the smaller the degree of wetting, that is, the poorer the wettability, the adhesive can not fully wet, PTFE, thus can not be well adhered to the PTFE;

　Second, the crystallinity of large, good chemical stability. PTFE PTFE swelling and dissolution are more difficult than non-crystalline polymers, adhesive coated in PTFE surface is difficult to polymer molecular chain diffusion and entanglement, can not form a strong adhesion;

Third, PTFE PTFE structure is highly symmetrical, and is a non-polar polymer. Adhesive adsorption on PTFE surface is caused by intermolecular forces, such forces include orientation, induction and dispersion forces. And PTFE PTFE non-polar surface does not have the formation of orientation and induction force conditions, can only form a weak dispersion force, and therefore its adhesion performance is poor;　　

Fourth, the solubility parameter of PTFE is very small, so the adhesion with other substances is also very small.　Therefore, to solve the problem of PTFE difficult to bond, generally should be modified from the surface to improve its adhesive properties and the development of new adhesive two aspects to start.