Characteristics of the Surface Tension of Blygold PoluAl

The following study was performed by the Technical University (Hechtingsinstituut) in Delft, Netherlands to determine the wetting of polar liquids on surfaces coated with Blygold products. The data indicates that Blygold PoluAl does not have a high polar content, which means that surfaces coated with Blygold PoluAl (condenser/evaporator coils) repulse liquids; and therefore, are resistant to wetting. This characteristic also makes the surface of the coating resistant to dust/dirt adherence.

Report: HI 2065 Title: Contact Angle Measurements Blygold Coatings Ir. D.P. Borger April 2000

Summary: Contact Angle Measurements have been done to estimate the polarity of coated surfaces.

Substrate:	5 different coatings
Pre-treatment:	-
Adhesive:	-
Keywords:	Coating, Contact Angle, Polar contribution, Surface energy
Opdrachtgever:	Blygold International B.V.
Projectnummer:	2000.22
Filelocatie:	D:\Rapport\2000
Auteur:	ir. D.P. Borger
Gecontroleerd:	ir. A. Kwakernaak

Introduction

The surface energy (g) of a solid surface can be divided into a dispersion (gd) - and a polar (gp) contribution. The dispersion energy contribution is built up from a single interaction, which is generated by the movement of electrons around an atom or molecule. The polar contribution is built up from different forces/interactions like hydrogen bonds, covalent bonds and dipole-dipole interactions. The surface energy, which is defined as the sum of the dispersion and polar contribution of a solid surface, can be determined by contact angle measurements. The larger the polar contribution to the surface energy, the more the surface attracts polar liquids and the higher the wettability of the surface by water.

Problem Definition

At the firm Blygold Int., coatings are used to protect metal surfaces from different kinds of external influences. Before a coating is applied it is important to determine the wetting of polar liquids, such as water, on the coated surface.

Experimental

Contact Angle Measurements are based on the equilibrium between three phases at the solid interface:

· Solid phase

· Liquid phase

· Vapour phase

A drop of fluid on a solid surface will transform itself until it reaches an equilibrium. At the edge of the drop there are three different forces/tensions present: gSV, gSL and gLV (S=Solid, L=Liquid and V=Vapour). This three-phase-equilibrium can be expressed as:

gSV - gSL = gLV · cos q	[1]
q is the contact angle [°]. The surface energy g [N/m] can be written as:
g = gS + gLV - gSL = gLV · (1+cos q)	[2]

To determine the dispersion- and polar contribution to the surface energy g, Owens and Wendt have formulated an empirical equation based on a geometrical average:

g = 2 (( gSd gLd )1/2 + ( gSp gLp )1/2)

[3]

By combining equations 2 and 3, and by measuring the contact angles of 5 different fluids on the solid surface, the dispersion- and polar contribution to the surface energy can be determined. The test fluids selected for the contact angle measurements are: Water, Glycerol, Formamide, 1-bromonaphtalene and tricresylphosphate. The dispersion and polar contribution to the surface tension of these fluids are known at the liquid-vapour interface. This means that only gSd and gSp are unknowns in the combination of equations 2 and 3.

The difference in polar contribution to the total surface energy is, for all coatings, clearly demonstrated by these results. The results of the measurements of the Blygold Polual coatings show that these coatings do not have a highly polar component Especially Blygold Polual has, with a polar component of 3.37 mN/m, a very low polar contribution to the surface energy, which means that the wettability of this surface by polar liquids is poor. This non-polar surface will be repulsive to liquids like water. This can be seen in the result of the contact angle of water, which is very high. Due to this property it is also dirt repellent.

The difference in the dispersion contribution of the coatings can be explained by the difference in density of the coatings. The higher the density the larger the dispersion contribution to the surface energy.