In addition to the established adhesion promoter AR 300-80 (which is based on diphenylsilanediol), also other silicone-containing compounds may be used to render hydrophilic surfaces more hydrophobic. Prerequisite for this to occur is that the silanes used for this purpose possess one or two reactive leaving groups which are able to react with the surface.
Reactive groups are for example chlorides, alcoholates or hydroxy groups, so that HCl, the free alcohols, or water are formed during the condensation reaction. Reactive groups however show marked differences with respect to their reactivity.
It is generally recommended to thoroughly clean all substrates before use. Frequently, just intensive rinsing with acetone and DI water is sufficient for this purpose. For more strongly contaminated surfaces, the use of O2 plasma, hot basic or acidic piranha solution is required. The subsequent thorough rinse with DI water is mandatory. Any direct contact of acidic piranha solution with acetone must be avoided since explosive acetone peroxides may be formed!
Chlorosilanes generally react readily even far below room temperature with the substrate, thereby cleaving off HCl. Alcoholates of silanes react considerably more slowly with surfaces at temperatures not below 30 °C, but higher temperatures accelerate the condensation reaction. In contrast, hydroxy silanes require much higher temperatures, for example about 170 – 180 °C for AR 300-80. These high SB temperatures are however unfavourable for many substrates. The use of Si alcoholates with soft-bake temperatures in a range of only about 70 °C – 90 °C thus provides a very good alternative. And, in contrast to chlorosilanes, no corrosive HCl is released. It also became apparent that the new variant of our adhesion promoter AR 300-80new which is based on these Si alcoholates is equally suitable as adhesion promoter as the previous variant. The comparable good effectiveness of AR 300-80 (new) was evaluated in detail by means of contact angle measurements.
In the following, we will briefly present a contact angle measurement (according to the sessile droplet method), taking a paraffin surface with different solvents as an example.
Contact angle measurements fall into two categories, either static or dynamic measurements. If a static contact angle is used, the liquid droplet to be analysed is deposited on a surface of a solid. The droplet diameter should amount to 2 to 6 mm, since the measured contact angle is in this case independent of the diameter of the drop. Dynamic contact angles in contrast describe processes at a liquid-solid interface during wetting and de-wetting processes. In this case, the advancing (enlargement of the droplet volume) and the receding angle (reduction of the droplet volume) are determined and the contact angle hysteresis is evaluated.
Static contact angles can be determined using the tangent method. The entire profile of the lying drop is here adapted to a general conic section equation. The derivation of this equation at the base line yields the slope at the three-phase boundary and thus the contact angle.
Young equation (Source: Wikipedia)
σL = Surface tension of the liquid
σS = Surface tension of the solid
σLS = Interfacial energy between liquid and solid
ϴ = Contact angle
The following figures show video recordings of drops of different solvents on paraffin for the measurement of the static contact angle q. The individual images illustrate how drop shape and thus also the contact angle changes as a function of the surface properties of the solvent. The static contact angle is 109 ° for water, 88 ° for ethylene glycol, and becomes substantially smaller (37 °) for long-chain alcohols. The difference between the contact angle of water and ethylene glycol becomes clearly apparent in the different drop shape and thus a different wetting behavior (difference of about 20 °).
Water on paraffin, q = 109 °
Ethylene glycol on paraffin, q = 88 °
Octanol on paraffin, q = 37 °