10.3.3 State of R&D
Following are examples of finishing processes using nanotechnology of interest for textiles. Some are already in use, other are at different stages of development.
Dyeing and Printing.
Dyeing is ubiquitous within the textile industry and the process formulation includes: the dye, a dispersant, and, optionally, a thickener. The interest for its improvement is high and, as mentioned, nanotechnology offers various advantaged. By using the dye as nanoparticles, in fact, the amount of the dyeing agent can be greatly reduced, products wastewater is largely diminished, the cost of post-treatment of wastewater minimised and, by replacing the complicated operations, process time shortened.
Coating and Lamination
Coating and Lamination are important processes for the application of nanomaterial on textiles, mainly for fabric finishing, but they can also been used in fiber manufacturing. In particular, they are the most versatile and widely used processes for most of the applications of nanoparticles.
In general, nanotechnology provides the tools for controlling 3 key parameters crucial for thin films performance: chemical composition (and crystalline structure at nano-sized domains), thickness, topography (including nano-scale patterning of thin films' surface).
There are different approaches for coatings and lamination processes. They are briefly described below.
- Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD)
These methods consist of heating the material (converting it into the gas phase) and then depositing it onto the surface. The use of chemical reactants triggers the deposition process. Several techniques are available for depositing the thin film material. Thermal evaporation, magnetron sputtering and pulsed laser deposition are probably the most widely used
Electrodeposition is a coating process based on the action of electric current and is normally used to produce metallic coatings. The deposition is achieved by negatively charging the substrate to be coated and by immersing it into a solution containing a salt of the metal to be deposited.
- Spray coating
There are two main methods: plasma spray coating and thermal spray coating:
Plasma spray coating (also known as plasma arc plating, plasma arc spraying and plasma coating) in which powders are introduced in a cavity that contains the gas stream of a plasma gun. After being melted, the powders are projected onto the surface being coated;
Thermal spraying coating, consisting on heating a feed stock material (powder or wire) and accelerating it to high velocity by a gas stream. Then the particles strike the substrate surface and the particles deform and freeze onto the substrate. The collision speed is an essential element, which directly influences the coating properties.
- Self-assembling monolayers (SAM)
SAM are one-molecule thick thin layers spontaneously formed by a substance, which can be used for many purposes, from scratch resistant coatings for glass to self-cleaning surfaces. The crucial dimension in SAMs is the thickness perpendicular to the plane of the monolayer: this dimension, and the composition along this axis, can be controlled very simply at the scale of 0.1 nm, by controlling the structures of the molecules making up the monolayer. SAMs also provide tailor-made functions: for example, by changing the structures of the organic molecules in straightforward ways, interfacial free energies can be controlled.
- Atomic Layer Deposition (ALD)
ALD is used to obtain ultrathin and conformal thin film structures for many semiconductor and thin film device applications. ALD uses sequential self-limiting surface reactions to achieve control of film growth in the monolayer or sub-monolayer thickness regime. ALD is receiving attention for its potential applications in advanced electronic devices, but it is also of interest in any advanced application that benefits from control of film structure in the nanometer or sub-nanometer scale. ALD is especially suitable for coating of substrates/parts with complex surface topography.
- Sol-Gel technology
In the sol-gel process the precursor is dissolved in a solvent (forming a sol or gel, depending on the reactor conditions) and precipitates due to chemical reactions. The sol-gel process consists on 4 basic steps: hydrolysis, condensation and polymerisation of particles, growth of particles and agglomeration, and formation of networks.
Sol-gel technology is not new and inorganic is being applied in diverse sectors for decades. The inorganic nature of sol-gel layers makes them very strong and wear-resistant, so that one may obtain important effects with nanometric layers. Thanks to its vast applicability, an extensive catalogue of molecules has been developed throughout the years, applicable with the sol-gel process, and allowing to compose layers with a variety of properties.
Recently, several research groups have acknowledged the potentialities of sol-gel for textile treatment. However, receipts and methods which are being currently used in other sectors, are not adapted to textile raw materials. The major problems are related to the use of highly acidic solutions and predominantly organic solvents, high process temperatures and long process sequences. Inorganic sol can be produced from various nanopowders of metal oxides as, for example, TiO2, ZnO, Fe3O4, Al2O3 and SiO2. Using a sol of this type have been obtained durable nanometric coatings to have textiles with hydrophilic or hydrophobic surfaces.
- Nanoscale emulsification
It is an important tool for chemical finishing, through which finishes can be applied to textile materials more evenly and precisely. Finishes can be emulsified into nano-micelles, made into nano-sols or wrapped in nanocapsules that will adhere to textile substrates more evenly. These advanced finishes allow unprecedented hydrophilicity and stain resistance, which can be used in household and domestic textiles.
Grafting is a superficial process used to functionalize materials that do not have functional chemical groups, that is, changing the material surface by adding or modifying chemical groups trough chemical reactions.
A plasma is a partially ionised gas containing ions, electrons, atoms and neutral species.
These particles are highly reactive and plasma treatments can induce chemical and physical transformation of the fabric surface with negligible addition or subtraction of mass, without changing the bulk properties or the use of chemical products.
This treatments deliver materials with new possibilities, which open perspectives to resolve production or design problems or even develop complete new applications. In fact, with plasma treatments can be obtained new stable characteristics/properties that can span from hydrophobic to hydrophilic surfaces (and vice-versa) are possible, to enhanced printing capability, increased dyeing ability, improved adhesion, metallization, and so fort.
Plasma treatments are techniques environmentally friendly and though rather mature technology in many respects, has not been fully exploited yet in textile applications.
Plasma Enhanced Chemical Vapour Deposition (PE-CVD) is probably the widest class of plasma processes. PE-CVD processes can apply many different classes of coatings to tailor the surface of materials, with composition ad properties that span from teflon-like to silica-like to nanocomposite, from super hydrophobic to hydrophilic to hydrogel-like.
PE-CVD is working under vacuum conditions, and therefore in batch, but there are emerging also plasma treatments at atmospheric pressure which look very interesting since they can be used in continuous processes.
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