Silane Coupling Agent In The Metal Pretreatment And Organic Coating Applied Research

As the "molecular bridge" adhering materials with different properties, silane coupling agents has been applied widely in composite materials, coatings and adhesive agents. However it is a new and environment-protecting surface-treatment technology to apply silane coupling agents in metal pretreatments, which has great advantages such as low cost, wide application and excellent adhesive strength to organic coatings, therefore attracts many experts' attention. It should be noted that there are many factors limiting this technology to be applied in industry, which include the selection of silanes to metal substrates and organic coatings, the process of silane hydrolysis and the preparation of silane films. Consequently, this dissertation paid attention to the following work: selecting three optimal silanes(KH-550,KH-560, KBM-7103), studying their processes of hydrolysis and the preparation of silane films, testing and characterizing the properties and structure of the films, and applying this technology to coatings such as polyethylene and fluororesin which have poor adhesion. As a result, many important conclusions were obtained.On the basis of discussing the mechanism of the hydrolysis and condensation of silanes, the hydrolysis process was studied to decide its parameter. The hydrolysis degree of silanes was reflected by using the conductivity testing method on-line not disturbing and destroying the solution system. It was found out that the stability of silane solution was related to hydrolysis solvents, hydrolysis time, concentration, pH value, temperature and additive. In order to obtain more silanol, hydrolyzing in mixing solvent with water and ethanol (or methanol) had great better effect than only in pure water or ethanol. With hydrolysis time and concentration increasing, the stability of silane solution decreased. Due to pH value significantly affecting the rate of hydrolysis, so high pH value contributed to hydrolyze and low pH value helped to condense. The optimum hydrolysis temperature was between 20 to 40℃. Glycerine could reduce the probability of the silanol condensing.The silane films were prepared by steel substrates dipping in silane solution and then curing at high temperature. The influence of the preparation process of films on the properties of films was verified by measuring and testing the adhesive and anti-corrosion capability of films. Firstly, the adhesive strength of films rose and then reduced with the hydrolysis time, concentration and curing time and temperature increasing. Secondly, the capability of anti-corrosion of films outperformedphosphating, and might be different because of the kind of silanes. Finally, the films had more excellent performance of anti-alkali and worse performance of anti-acid.The chemical structure and property of silane films were characterized and analyzed by a series of detection methods which included RA-IR (reflection absorption infrared spectra), XPS(X-ray photoelectron spectroscopy), SEM(scanning electron microscope) and DSC. At the same time, the thermal decomposition temperature of KH-560 and KBM-7103 films was measured by TG and the thermal decomposition kinetics of KH-560 films was studied, too. In the mixing solvent, silanes not only dissolved but hydrolyzed to produce silanol. The silanol highly crosslinked to polysiloxane with low degree of polymerization. The amount of silanol gradually decreased and the amount of polysiloxane increased with the concentration and curing temperature rising. Moreover the content of the atom of Si in films gradually increased and then decreased and the atom of Fe changed conversely with the curing temperature enhancing, which changed all conversely with the solution concentration. According to the results, we can draw the conclusion that the silane film thickness depended primarily on the solution concentration. It was also found from the results of XPS that silanes reacted with the group of hydroxyl of the steel surface. KH-560 films had no good heat-resisting and its thermal decomposition temperature was 50°C lower than that of KBM-7103 films.Effect of various pretreatments on the adhesive strength between PE or FEP coatings and metal substrates was investigated in order to prove the advantage of silanes in metal pretreatments. The results indicated that KH-560 films could remarkably improved PE coatings' adhesion. Furthermore the pretreatment by blasting with KH-560 had better effect on PE coatings' adhesion whose adhesive strength was higher than that of pretreatment only by phosphating or blasting. Moreover its adhesive strength even only dropped 10.9% after 20 times thermal shock, which was much lower than that of other pretreatments. On the other hand, the metal pretreatment by silanes had no effect on FEP coatings because this coatings had high melt temperature at which the silane films began to decompose.