Methods of production of micro porous coating
1. Wet coagulation process.
2. Thermo coagulation process.
3. Foam coating.
1. Wet coagulation process:
Wet coagulation process : Polyurethane polymer is dissolved in the
organic solvent dimethyl formamide to produce a solution insoluble in
water. This is then coated on to the fabric. The coated fabric is pass
through a conditioning chamber containing water vapour. As the organic
solvent is miscible with water, it is diluted and solid polyurethane
precipitates. The fabric is then washed to remove the solvent, which
leaves behind pores in the coating. Finally the coated fabric is mangled
and dried. This method is not very popular as it requires high capital
cost machine and solvent recovery expensive.
Ø Produces a very fine interconnecting structure of micro pores.
Ø Pores are small enough to keep droplets of water out but, large enough to let the small water vapour molecule.
Ø Micro-porosity can also be created by leaching the salts on treatment of the film with water.
Ø Shows ultrafine pores of <1µm, in addition to a honeycomb skin core structure of 1-20µm pores.
Ø Micro pores was produced due to the subtle difference in the rates of coagulation as the resin particle interface.
Ø Precise
control over the coating operation is required to generate a
consistent, uniform pore structure, preferably below 3µm for optimum
balance of breathability and water proofness.
2. Thermo coagulation process:
Thermo coagulation: Polyurethane is dissolved in an organic solvent and
the resulting solution mixed with water to produce an emulsion. The
emulsion 'paste' is coated on to one side of the fabric. The coated
fabric is then goes through a two stage drying process. The first stage
employs a low temperature to remove the organic solvent, precipitating
the polyurethane. The coating is now mixture of solid polyurethane and
water. The second stage employs a higher temperature to evaporate the
water leaving behind pores in the coating.
Ø PU- based coating technique operates on thermo-coagulation technique.
Ø PU is dissolved in a solvent mixture of methyl ethyl ketone, tolune, and water, having 15-20% solids and coated on the fabric.
Ø The low boiling solvents evaporates and leads to precipitations of PU in the non solvent.
3. Foam Coating:
A mixture of polyurethane and polyurethane/polyacrylic acid esters are
dispersed in water and then foamed. The foam is stabilised with the aid
of additives The foam is then coated on to the one side of the fabric.
The coated fabric is dried to form a microporous coating. It is
important that the foam is open cell to allow penetration of water
vapour but with small enough cells to prevent liquid water penetration.
The fabric is finally calendered under low pressure to compress the
coating. As the foam cells are relatively large, a flurocarbon polymer
water- repellent finish is applied to improve the water resistant
properties. This type of coating production is environmentally friendly as no organic solvents are used.
Ø Water based polyurethane/ polyacrylic acid esters are used.
Ø The foam is stabilized with the aid of additives.
Ø Then the foam is coated on one side of the fabric and dried to form a micro porous coating.
Ø The fabric is finally calendered under low pressure to compress the coating.
Ø The foam cells being relatively large, a flurocarbon polymer based water-repellent finish is applied to improve the water resistant properties.
Ø The
water vapour permeability of foam-coated fabric is higher and their
water resistence was found to be lower than those of non foam coated
fabrics with the same coating thickness.
Ø The water vapor permeability and the water resistance become higher when the foaming speed is increased.
Hydrophilic coating:
Ø Same as hydrophilic membrane.
Ø Transmit vapour by a molecular mechanism involving adsorption diffusion and desorption.
Ø Coating are based on polyurethane, which has been modified by incorporating polyvinyl alcohols and polyethylene oxides.
Ø Chemical affinity for water vapour allowing the diffusion of water vapour through the amorphous regions of the polymer.
Combination of micro porous coating and membrane:
Poly coating and membranes have excellent integrity. This can be conferred in two ways:
1. By a high degree of hydrogen bonding, principally between polar groups in the hydrophilic segments of adjacent polymer chain.
2. By forming covalent cross links between adjacent polymer chain. The
effective length and density of the cross links are variables affecting
polymer swelling and thus vapour permeability.
Biomimetic material:
Biomimetics is the mimicking of biological mechanism, with modification to produce useful artificial items.
Research workers (UK) modelled an opening pore comprising flaps of two
laminated materials with different moisture uptakes life leaf stomata
which opens when the plants needs to increase moisture vapour
transpiration and closes when it need to reduce it.
Biomimetics has now become a commercial reality.
Stomatex (Biomimetic material):
Ø Stomatex is claimed to be 'the most comfortable clothing and footwear systems in the worlds today
Ø Akzo nobel company is marketing stomatex.
Ø This
is closed foam insulating material made from neoprene incorporating a
series of convex domes vented by a tiny aperture at the apex.
Ø Provide a controlled release of water vapour for comfortable wear characteristics.
Ø Respond
to the level of activity by pumping faster as more heat is produced,
returning to a more passive state when the wearer is at rest.
Ø Stomatex
is used in conjunction with Sympatex to produce a breathable water
proof insulating barrier for use in clothing and footwear.
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