Binder – Polymeric binding agent & its fixation by UV therapy- a brief
Pigment Printing industry can adopt new technology of fixation by UV (Binder) to achieve energy saving by replacing high-temperature curing and thus become environmentally sustainable.
The area of fabric processing particularly dyeing and printing, are the most value-added processes as they make products most pleasant & attractive to consumer choices.
Generally, the dyeing is covering full substrate by immersion into the colour bath while the printing operation is limited to the portion on one side of the fabric surface and restricted to the defined area/ designs one wants to create over it.
So, we can say that printing is a technique equal to localized dyeing. Printing can be done by many dyestuffs categories & with various types of dyes, but comparatively, pigment printing is unique in this respect.
We can say that its ability due to practically waterless operation fulfills the compliance needs of all discharges into sewerage line as a zero discharge process. The cooling water of the blanket is recycled and only replenished over a long period.
The uniqueness of pigment printing is the requirement of no washing off. Postprint operation without water consumption is a salient & attractive feature.
Across the globe, maximum printing activities over fabric are performed by acrylic colours and pigments are favorable choices for almost all fibers and fiber-blends with appropriate wearable qualities of fastness.
Pigments are organic molecules (not soluble) and do not have any attraction towards fibres of any kind either natural or synthetic or man-made substrates.
The technology of pigment colours requires efficient mix or paste which is built up by multi-components individual products involved with various functions to fulfill qualitative aspects of the consumer’s end use of the article.
This pigment paste matrix is one of the most successful aspects of printing via any system, which involves flow ability of paste, viscosity, after print tackiness, and character of transparent & durable film formation.
Let us look into the first important components of the printing paste matrix, i.e., binder. Binder is a chemical that has a bigger size molecule than dye (azo, anthraquinone & phthalocyanine chromophores) engulfs dye particle and when it fixes with the fiber substrate, possess the pigment colour particle along with it firmly to give robust fastness.
A binder by virtue of chemical structure is a monomer that gets converted into polymer or copolymer by forming a web net across the print surface & having the most important aspects and area of study. (Cross linking ability induced).
Of course, with the aid of cross-linking agent/fixer, this does happen to the best level of fixation with the fiber substrate.
The majority of binders are monomers of the type of polyvinyl acetate with acrylate resins as crosslinkers, normally this type of monomer triggers reaction due to elevation in temperature & gets converted into a long-chain polymer (linear and branched) by strongly adhering with the fibers.
But the efficiency is judged by two important functions of the binder system, one part of the macromolecule must have a hydrophobic group that does not react with the fiber but helps in the dispersions process whereby a homogeneous paste is maintained other is crosslink able functions which keep binder adhered to fibers mechanically and not chemically.
Modern all aqueous system comprises of pigment colorant, synthetic thickener, crosslinking agent, binder, softener, and catalyst.
Acrylic acids such as methacrylic acid and methyl methacrylic acid, are two examples of simple acrylic monomers that can be polymerized to resins with a high molecular weight, which will, in turn, form films around the pigments and will adhere to the substrate thus binding the pigment.
While this does accomplish the basic function of the binder, this type of polymer may be more susceptible to removal by rubbing or washing than a binder that would also incorporate more association with the fibers.
For instance, if the binder contained additional sections of polymer known as “branched chains” or “arms”, it could latch onto other segments of the polymer at right angles to the long linear chains.
This three-dimensional nature would be more stable to outside forces such as rubbing and washing. But by nature, once the system becomes three-dimensional it becomes more rigid and thus would make the fabric stiffen. These polymer-forming additions are referred to as “copolymers”.
Cross-linking agent, which is a resin, is usually a melamine compound that links by chemical reaction the binder polymer to itself to increase its durability and enhance fastness. These products, while improving wet fastness, can also increase hand.
Selection of Binder
The selection of binders depends on,
- Simplicity of handling.
- Low cost & toxicity.
- Easily polymerised.
- Does not build up on equipment and stainless property.
- Excellent fastness.
- Non-yellowing tend.
Binder Properties
Some of the binder properties matrix.
| Sr. No. | Binder chemistry | Hand feel Comments | Fastness Dry wet | Use |
| 1 | Acrylic | Soft | Fair Poor | Common |
| 2 | Acrylic + Crosslinker + Softener + Emulsifier | Soft | Fair Poor | Common |
| 2 | Acrylic+ Melamine | Medium | Good Fair | Common |
| 3 | Acrylic + Styrene Acrylate | Hard | Fair Poor | Special |
| 4 | Butadiene Acrylonitrile + Acrylic latex | Hard | Excellent Good | Special |
| 5 | Acrylic butadiene + Acrylonitrile + Melamine | Hard | Good Fair | Common |
| 6 | Vinyl Acrylate | Hard | Fair Fair | Special |
Troubleshooting guide.
| Poor dry crocking | binder quantity insufficient |
| Poor wet crocking | check curing time & temp./add fixer if needed. |
| Poor abrasion | use softener of suitability |
| Poor wash fastness | add crosslinker/fixer to increase adhesion |
| Fabric cracking | use softener |
| Clogged print screen | add emulsifier & ammonia liquor |
| Yellowing | check ph at neutral to feeble alkali or replace the binder |
| Poor light fastness | pigment selection criteria to revisit |
| Poor dry clean | pigment selection criteria to revisit |
Problems associated with pigment printing
One of the problems associated with pigment printing is its development through high-temperature curing.
The temperature required at this stage triggers polymerization of the binder system with possible cross-linking & which finally protects the colorants for ever encapsulation which ultimately reflects into the desired level of fastness. The temperature range is 1500 c and above for more than 5 minutes.
The energy consumption via coal-fired boiler-fossil fuel or furnace oil as combustion oil in oil heating system brings about lots of consumption of such fuels which have the capacity to generate carbon monoxide as air pollution load.
A method to lower down the temperature and protect substrate by additional heat load (by scorching) has brought the attention of many researchers. One of the obvious options is a selection of low-temperature polymerization binders to lower energy consumption.
Yet curing with cross-linking and condensation process does not allow the temperature range below 110 degrees Celsius and to complete swollen binder gel coagulation (clotting) & coalesce (formation in one mass) the entire process requires higher heat load & many minutes and so still not fruitful results have achieved to use into the industrial practice of such binders.
Though extra moisture required to dry the print is minimum 1000 c., or around this cannot be avoided post immediate printing.
Popular chemistry used in binders is a combination of various monomers, when heated up to 1500 degrees Celsius temperature in curing turns into strong binding properties with the help of fixers or cross-linking agents. The film produced in such a way is fast to dry cleaning and laundry washings.
The monomers are based on styrene-butadiene, styrene acrylate, or vinyl –acetate copolymer.
Some important binders’ chemical structures are,
- N-Butyl acrylate
- Acrylonitrile
- Styrene -1, 3 butadiene.
U.V. curing is one of the possibilities of binder fixation via inducement in polymerization against thermal curing. Here the requirement to lower the curing temperature is an energy saver technique. Another way to describe curing by U.V. radiation is named cold-curing through ultraviolet radiation curing.
Obviously, the use of solvents or solvent-based ingredients is eliminated in the printing paste system.
The binder system must have a monomer/oligomer along with photoinitiators. U.V. exposure that triggers of photoinitiator brings about the free radicals’ mechanism which, instantaneously starts polymerization & this is much faster than thermal curing/backing. The time requires is less than a few minutes compared to traditional curing.
Today various types of UV curable monomers/oligomers are available, e.g., polyether, polyester, epoxy, polyacrylate, and urethane acrylates. One suitable photoinitiator the polymer film properties of hardness & flexibility, resistance, adhesion can be controlled by experiments under variable conditions.
Generally, three-stage is considered as initiation, propagation, and termination in the mechanism of radical polymerization. Vinyl monomer types of linkages are well suited with carbon-carbon double bond, where radical initiation is possible.
What is happening during this process?
Photoinitiators absorb light energy to work. Curing lamps must emit UV energy in the spectral area that the photoinitiator absorbs.
1) Photoinitiator absorbs UV light energy creating radicals.
2) Radicals react with acrylate monomers and oligomers inducing cross-linking.
3) Liquid resin system converts to dry/cured product on cross-linking.
U.V. spectrum values/range nm (nanometer). Lower energy higher wavelength.
| UV-C | UV -B | UV-A | VISIBLE LIGHT | I.R. |
| 100-280 nm | 280-315 nm | 315-40 nm | 400-700 nm | 700-1800 nm |
| Short wave UV | Medium wave UV High speed | Long wave UV Depth cure | Rainbow spectrum |
For more details contact ctd@atitra.in
