Factors and Mechanisms Influencing the Anti-sticking Durability of FEP Coatings on Rubber Molds

Rubber molds are widely used in the manufacturing of tires, shoe soles, seals, and stern bearings. The mold surface quality directly affects the quality and appearance of products. However, the surfaces of rubber molds are contaminated after multiple vulcanizations, causing difficulty in demolding and affecting product quality. Coatings are usually used to improve the anti-sticking performance of rubber molds in industry; however, anti-sticking failure is still one of the main failure modes of rubber mold surface coatings. The factors and mechanisms that affect the anti-sticking durability of coatings are unclear. To improve the service life of commonly used fluorinated ethylene propylene (FEP) coatings on rubber molds, coatings were prepared on flat samples using the air-spraying method, and their anti-sticking durability was evaluated using rubber vulcanization tests. The surface morphology and elemental composition of the samples were examined using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to study the changes with the vulcanization time. A bilayer waterborne FEP coating sample and a single-layer solvent-based FEP coating sample were prepared, and their anti-sticking durabilities were compared with that of a type 45 steel sample. The results showed that Zn and S were added to the surface of the samples after the vulcanization tests. After 300 tests, the surface Zn weight concentrations of the type 45, bilayer waterborne FEP coating, and single-layer solvent-based FEP coating samples were 40.6%, 0.45%, and 1.13%, respectively. X-ray photoelectron spectra (XPS) analysis was conducted for the type 45 steel sample, and it was found that the contaminants were mainly composed of Zn, S, C, and O. ZnS was the main component of the contaminants in the beginning, and as the number of vulcanizations increased, organic deposition occurred in the ZnS layer. The contaminants on the FEP coating samples appeared at the surface defects of the coating. The surface FEP resin film thickness and quality are easier to control for the bilayer waterborne FEP coating than for the single-layer solvent-based FEP coating; thus, the bilayer waterborne FEP coating has better anti-sticking durability. Three bilayer waterborne FEP coating samples with different coating thicknesses (T1: 13.7 mu m, T2: 20.4 mu m, T3: 29 mu m) were prepared by changing the thickness of the topcoat, and their anti-sticking durability was studied and compared. The results showed that the increase in the Zn concentration on the surface of the T1 sample was more significant than those of the T2 and T3 samples, indicating that the topcoat thickness and film quality are the key factors affecting the anti-sticking durability of the bilayer waterborne FEP coating. Bilayer waterborne FEP coating samples with different sintering processes were prepared by changing the sintering temperature (340 degrees C and 380 degrees C) and using a cooling method (furnace cooling, air cooling, and water quenching). The Fourier-transform infrared spectra (FTIR) of the coating samples were obtained, and it was deduced that the sintering process of the coating had a slight effect on the molecular structure of the FEP resin but had an effect on the crystallinity. The water-quenching and air-cooling methods resulted in lower crystallinity of the FEP resin than the furnace-cooling method. The results of the anti-sticking durability test showed that, within 300 vulcanization tests, the sintering process had a slight effect on the anti-sticking durability of the bilayer waterborne FEP coating. The research results provide a theoretical basis for the anti-sticking surface treatment of rubber molds