What is SC-tek?
SC-tek – is a novel industrial process for applying durable superhydrophobic and oleophobic coatings on fabrics and non-woven materials. What makes SC-tek technology unique is that it produces an ultra-thin uniform protective coating around each single fabric fiber. This is achieved by using supercritical CO2 as a solvent medium for depositing hydrophobic agent. SC-tek coating deposition is a dry process, it doesn’t use or pollute water, which makes it environmentally friendly.
|Individual fibers are coated
Under a microscope one can see, that threads of fabrics consist of individual fibers only several micrometers thick. SC-tek process is unique in that it provides a uniform ultrathin protective coating around each individual fiber.
Find out more abour supercritical fluids and supercritical CO2
When substances are compressed and heated above a certain limit that is called a “critical point”, the boundary between a liquid state of substance and the gas state disappears. This is what is known as “supercritical state”. In such state the substance has a relatively high density, like that of a liquid, but its molecules are very mobile, just like molecules in gases.
||The states of CO2
This picture shows a phase diagram for CO2 state depending on temperature and pressure. The insets show photographs of an ocular to high pressure vessel with CO2 inside. Along the white boundaries different states can co-exist in equilibrium. Moving up along liquid-gas line, the boundary between the liquid and the gas is clearly visible in the ocular. As we get very close to the critical point that boundary is going to start fading out, until it disappears completely beyond the critical point.
Supercritical fluids, like supercritical water or supercritical carbon dioxide, can be used as an environmentally friendly substitute for volatile organic solvents. Specifically, supercritical CO2, with its mild critical point, can be used as an inexpensive green solvent for a wide range of hydrophobic compounds including certain polymers.
Advantages of SC-tek process over traditional methods of textiles hydrophobization
Water- and oil-repellent finishing processes currently used by the textile industry, usually involve depositing protective agents in liquid media by placing the roll of fabric into a water-based dispersion of the protective agent, followed by drying process and thermal annealing for initiating the chemical bounding of the coating to the fabric.
|Liquid-based hydrophobization methods
Both spraying and padding technics include two consecutive stages: a) deposition of liquid solution or dispersion on fabric, and b) the following fabric drying stage.
Such liquid-based methods are relatively simple. However using liquids as a deposition media leads to a number of issues:
- The drying process leads to high energy consumption,
- Waste water polluted by the agent needs to be re-purified, which leads to extra costs,
- High surface tension of liquids doesn’t allow for a uniform coating inside small pores of the material due to capillary effects and coating defects caused by drying process.
Completely dry SC-tek process eliminates these issues and allows for:
- Greatly reduced energy consumption (savings up to ~1 kWh per kg),
- No waste waters and associated costs,
- Uniform coatings even inside small pores of the material, which enables high water- and oil-repellent properties as well as high coating durability. Fabrics are still water- and oil-repellent even after 20 machine wash cycles.
Find out more about the way capillary forces affect thin coatings
During evaporation of liquid solvent, the meniscus, formed by liquid-gas boundary, is sweeping along the coating of deposited hydrophobic agent.
When using supercritical CO2 as a fluid, the liquid-gas boundary is not present, this allows for deposition of thin uniform coatings.
SC-tek process takes place in supercritical CO2, in which there is no gas-liquid boundary present and, hence, no corresponding capillary forces. The protective coating is formed during gradual decrease in pressure, when CO2 is transformed straight to gas state, and the protective agent loses its solubility in CO2 forming a uniform coating on the surface of the treated material. This allows for covering the treated surface with a thin and uniform protective coating without the defects, specific for liquid hydrophobization methods.
How does it work?
The process of depositing coatings on fabrics is taking place in a specifically designed high pressure vessel, which can withstand elevated temperature and pressure necessary for transforming CO2 into supercritical state, in which it is able to dissolve hydrophobic polymers.
The outline of the process is as follows:
|1. A roll of fabric is placed inside a high pressure vessel along with hydrophobic and crosslinking agents.
2. The vessel is sealed and pressurized with carbon dioxide.
|3. As the pressure and temperature rise, the CO2 is transformed to supercritical state, in which it can dissolve the hydrophobic polymer and the crosslinking agent.||4. The crosslinking agent reacts with polymer chains and surface of textile fibers.
5. After the vessel is decompressed a durable water repellent film is formed on textile fibers’ surface chemically bound to it.
The durability and wash resistance of the coatings is provided by a cross-linking agent that chemicaly bounds polymer molecules to fibers’ surface. The procedure is easily automated.