Textiles can be found in many application such as medical engineering, aerospace, automotive, building, filtration, and packaging. Increasingly, a hydrophobic character is required for these products to attain effective liquid repellency, self-cleaning, uni-directional liquid transport, or to create barrier coatings on fibre surfaces. Based on the Cassie–Baxter model, the water repellency of materials is achieved by both enhancing nanoscopic roughness of the surfaces and by adding functional chemical groups to modifying their surface energies. For many years, fluorocarbon chemicals have been used to impart these properties. However, following the awareness of the toxic effect of some of these molecules (PFOA: Perfluorooctanoic Acid; PFOS: Perfluorooctane Sulfonate) on human health and the change in current legislation, the market requires more and more frequently fluorine-free water-repellent fabrics.
In this scenario, ArgoChem offers an innovative high-performance fluoro-free water repellent finish developed with the sol-gel technique. Special types of sol-gel precursors are used and after hydrolysis and condensation processes, highly branched networks with functional groups are formed. The final sol-gel product is called “Ceramicoat DWR“.
This newly designed sol-gel system offers a high potential for many textiles, from cellulose to man-made materials.
The proposed finish has a pH of about 8 and can be easily diluted with water.
The best performance can be achieved using an optimized system realized using at the same time:
- CeramiCoat WR (water repellent sol-gel);
- ArgoWett Plus (specific surfactant for water repellency);
- ArgoEXT (crosslinker).
This makes it ideal as a new environmentally-friendly hybrid treatment for water repellency finishings.
Water repellency principle
The investigation on surface water repellency usually involves the measurement of contact angles as the primary data, which indicates the degree of wetting when a solid and liquid interact.
These values can be carried out using a goniometer or tensiometer. In a goniometer, a sessile droplet of water is placed on the fabric surface, and the contact angle between the water droplet and the fabric on the image can be measured. Contact angle hysteresis and roll-off angle could also be evaluated in this measurement.
Considering a liquid drop resting on a flat, horizontal solid surface. The contact angle is defined as the angle formed by the intersection of the liquid-solid interface and the liquid-air interface (geometrically acquired by applying a tangent line from the contact point along the liquid-vapor interface in the droplet profile). The interface where solid, liquid, and air co-exist is referred to as the “three-phase contact line”.
Illustration of contact angles formed by sessile liquid drops on a smooth homogeneous solid surface
A small contact angle is observed when the liquid spreads on the surface, while a large contact angle is observed when the liquid beads on the surface. In addition, small contact angles (< 90°) correspond to high wettability, while large contact angles (>90°) correspond to low wettability.
Surface tension is a property of liquids governed by intermolecular interactions: it originates from the cohesive forces between molecules in a liquid. Thermodynamics tells us that systems strive to attain a state with a maximum amount of favourable interactions. This implies that liquids will shape in such a way that the amount of bulk molecules is maximum and the amount of surface molecules is minimal.
Ideally, the shape of a liquid droplet is determined by the surface tension of the liquid. In a pure liquid, each molecule in the bulk is pulled equally in every direction by neighbouring liquid molecules, resulting in a net force of zero.
However, the molecules exposed at the surface do not have neighbouring in all directions to provide a balanced net force. Instead, they are pulled inward by the neighbouring molecules, creating an internal pressure. As a result, the liquid voluntary contracts its surface area to maintain the lowest surface free energy. The stronger the interactions between the molecules are, the more energy is required to increase the surface area of a liquid. The surface tension of a liquid, γ, is defined as the energy (in Joule) needed to create 1 m² of a new liquid-gas surface area. The dimension of γ is J/m², even if often the dimension N/m (Newton per metre) is used.
From everyday life, we know that small droplets and bubbles are spherical, which gives the minimum surface area for a fixed volume. This intermolecular force to contract the surface is called “surface tension” and it is responsible for the shape of liquid droplets. In practice, external forces such as gravity deform the droplet; consequently, the contact angle is determined by the combination of surface tension and external forces (usually gravity).
Theoretically, the contact angle is expected to be characteristic for a given solid-liquid system in a specific environment.
As well as with the contact angle value, water repellency performance on textile materials can be measured with drop test and spray test.
Drop test can be carried out by AATCC 193 test method, evaluating the treated fabric resistance to wetting by a series of 12 water/alcohol solutions with different surface tensions (1 being 98:2 water-isopropyl alcohol ratio and 12 being 100% isopropanol). Drops of standard test liquids are placed on the fabric surface and are observed for wetting and wicking.
Spray test is performed following American Association of Textiles Chemists and Colorists (AATCC) test method 22. A 180 mm x 180 mm textile sample is fastened in a hoop and placed on an inclined plane at an angle of 45°. Then, 250 ml of distilled water are sprayed onto the face of the test specimen for 30 s from a height of 150 mm. Upon completion of spraying, the hoop is removed and the sample is compared with the rating chart supplied with the apparatus.
The standard spray ratings
Spray rating State
0 Complete wetting of whole upper and lower surface
50 Complete wetting of whole of the upper surface
70 Partial wetting of whole of the upper surface
80 Wetting of upper surface at spray points
90 Slight random sticking or wetting of upper surface
100 No sticking or wetting of upper surface
In order to determine the resistance to simulated rain for fabrics is used the Bundesmann rain test. The test consists of four specimen holders 100 mm in diameter placed over inclined cups with a rotating member rubbing the underside of specimens. Specimens are subjected to simulated rain from a height of 150 cm using filtered water under pressure. Water penetrating through the fabrics is collected in the cups and is measured on a mass basis.
