Nano Finishings in Textiles

IntroductionThe first R&D work of Nanofinshings was made by Dr. Soane.He started the first Nanotechnology based company Nano-Tex in 1998.Prof W Barthlott of Germany led to understand the mechnaism of lotus leaves and owns the patent Lotus-Effect. Dr Xin and Dr Daoud found an efficient way to coat cotton cloth with tiny particles of TiO2 which act as catalysts.

Easy Care-Hydrophobic Nano FinishHydrophobic surfaces are produced by two ways: 1) by creating a rough structure on a hydrophobic surface 2) by modifying a rough surface using materials with low surface free energy.

Using hydrophobic polymer films and attaching hydrophobic monomers, hydrophobic character is imparted to cotton.

Some monomeric hydrocarbon hydrophobes are aluminium and zirconium soaps, metal complexes, waxes and wax like substances, pridinium compounds and other fibre reactive finishes.

Fluorocarbon finishes are a class of hydrophobic finishes, which impart water & oil repellency and have consumer driven properties to fabrics.

Fluorocarbons contain a perfluroalkyl residue having high thermal stability & low reactivity.

Varying the proportion of hydrophobic & hydrophilic groups in the side chains can vary the structure of fluorinated acrylates.

Durable fluorocarbon forming cross-linked networks, forms low energy films that protect the treated fabrics.

The fluorinated side chains of a polyacrylate fluorocarbon finishes form low energy repellent surfaces.

To develop a more durable hydrophobic & oleophobic finish to the fabric, Soane patented a large number of multifunctional (nano) molecules.

These multifunctional molecules used may be block copolymers or graft copolymers having plural functional groups.

In these molecules carboxyl groups are reactive groups, and these may present in the form of poly carboxylic acid or as poly anhydrides such as poly (maleic anhydride).

Perfluorinated group

Oil-repellency test(ATCC 118)

Spray test(ISO 4920)

-CF3 0 50-CF2-CF3 3-4 70

-(CF2) 2-CF3 6-7 70

-(CF2) 4-CF3 7-8 70

-(CF2) 6-CF3 7-8 70

-(CF2) 8-CF3 8 80

Oil and Water repellency of fabrics treated with acrylic polymers

Film of fluorocarbon acrylate polymer based finish

Multifunctional Reactive Molecule disclosed by Dr. Soane

Photocatalytic Self-CleaningA powerful oxidizing agent (catalytic initiators) with UV light is applied for the removal of organic pollutant.

TiO2 is an excellent catalyst in the photodegradation.

Photocatalytic propensity of TiO2 has been attributed to the promotion of an electron from the valence band to the conduction band is brought about by the absorption of a photon of ultra- bandgap.

The electron-hole pair, e-h+ created due to the electron transfer from VB to CB.

In the presence of oxygen and/or H2O, superoxide (O2) and/or hydroxyl (OH) radicals are formed.

These radicals attack adsorbed organic species on the surface of TiO2 and decompose them.

If an electron donor (ED), such as ethanol, methanol and EDTA, is present at the surface then the photogenerated hole can react with it to generate an oxidised product (ED+).

If there is an electron acceptor (EA) present at the surface, such as oxygen or hydrogen peroxide, then the photogenerated conductance band electrons can react with it to generate a reduced product (EA-).

EA + ED EA- + ED

+ TiO2

h 3.2 eV

Many commercial systems, employ the semiconductor photocatalyst TiO2 to oxidize organic pollutants by oxygen, i.e.

Organic pollutant + O2 CO2 + H2O + mineral acid

The TiO2 particle can act as either an electron donor or acceptor for molecules in the surrounding media.

The photo-induced charge separation in bare TiO2 particles has a very short lifetime because of charge recombination.

TiO2

h 3.2 eV

Major processes associated with TiO2 semiconductor particle

Nano Antimicrobial FinishesSilver or silver ions have strong inhibitory and bacterial effects as well as a broad spectrum of antimicrobial activities.

This inhibitory effect of silver ion/silver metal on bacteria has been attributed to the interaction of silver ion with thiol groups in bacteria.

Silver salts such as Silver nitrate with anionic copolymer of styrene, ethyl acrylate, acrylic acid and divinyl benzene are widely used for this interaction.

Silver antimicrobial agent can be produced by treating cross-linked carboxy methyl cellulose (CMC) having > 0.4 carboxy methyl groups with silver nitrate.

Yang has patented the process for preparing a silver nanoparticles containing functional microcapsule.

Microcapsule can be prepared by treating an emulsified solution of a perfume, which is encapsulated with melanin precondensate.

The microcapsule so produced is treated with silver nanoparticle dispersed in water-soluble styrene maleic anhydride polymer solution before it fully dries.

In these microcapsules, the silver nanoparticles are on the surface of the capsule.

For producing highly concentrated stable dispersions of nanosized silver particles, silver nitrate is reduced with ascorbic acid to precipitate metallic silver in acidic solutions according to following reaction:

2Ag+ + C6H8O6 2Ag0 + C6H6O6 + 2H+

Structural view of a Silver nano particle containing functional microcapsule (a). Microcapsule

(b). Inner core contains functional substance

such as perfume, a thermosensitive

pigment, thermal storage material or

pharmaceutical preparation

(c). Outer shell

Super Hydrophobicity – Self Cleaning – Lotus Effect Hydrophobic finishes, lower the surface area and can give a maximum water contact angle of 120 degrees.

Self-cleaning ability and super hydrophobic finish can be obtained with the contact angles above 150 degrees.

By increasing the surface roughness we can get higher contact angles, also provides large geometric area for a relatively small projected area.

Cassie & Baxter first observed the water-repellency of rough surfaces due to the air enclosed between the gaps in the surface.

Barthlott and Neinhuis (1997) investigated the self-cleaning propensity of plant leaves’ rough surface.

The report shows that the majority of wettable leaves were smooth without any prominent surface sculpturing.

Contact angle < 110 degrees. Absence of Epicuticular wax crystals.

In contrast, water repellent leaves exhibited various surface sculptures where water contracted forming spherical droplets.

Contact angles above 150 degrees. Presence of Epicuticular wax crystals with papillose epidermal cells.

Particles adhering to the leaf surfaces were removed entirely from the water repellent leaves.

When a water droplet rolls over a particle, the water droplet captures the particle.

This is because the adhesion between the particle and surface is greater than the adhesion between the particle and water droplet.

This result shows the complete self-cleaning ability by water repellent plant surfaces, which can by demonstrated with sacred lotus (Nelumbo nucifera).

This phenomenon is called Lotus Effect.

ConclusionThe basic mechanisms and the logic of some of these finishes have been explained.

Some nano finishes such as 'Nano-Care', 'Lotus Effect' finishes, Nanosphere based finish and Ag Fresh has been commercialized.

The commercial viability of these finishes will be customer driven.

The new concepts exploited for the development of nano finishes have opened up exciting opportunities for further R&D.