Optical Brightening Agent in paper making

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  • Optical Brightening Agent in paper making
Ngày đăng: 03/07/2013, 11:21 am
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WET END: Addition Points FWA & Colorants
•The number of sulphonic groups on the FWA determines the solubility of the molecules. Highest solubility has hexasulpho, lowest disulpho.
•The lower the solubility, the higher the fiber affinity.
•Only di- and tetrasulpho should be used in Wet End (normally).
•Higher affinity allows later addition point.
•Late addition of FWA means faster response times and less interference with other chemical additives. Always consider the dosage points for other chemicals (especially cationic additives).
•Higher affinity gives higher first pass retention of FWA and consequently faster grade changes.
•The addition level of shading colorants is relatively low: 25-200 g/ton of dry paper.
•Direct Dyes (mainly anionic) have enough affinity to be added without fixation etc.
•Basic Dyes (cationic) should only be used for wood containing (or recycled) pulp. Due to high affinity for lignin, mottling can occur with fiber mixtures (e.g. 20% CTMP/80% Kraft).
•Pigment Colorants have low natural affinity and retention increases with total retention.
•Late addition point of colorant makes the shade control faster.
•Dilution requires purified water and hygienic conditions. Failures in dilution- and dosage systems are common reasons for broke.


WET END: Factors influencing the FWA performance

•Pulp brightness
• Furnish
• Water hardness
• pH
•Cationic additives
• Filler
• Consistency & Contact time
•Shading Colorant

SIZE/FILM PRESS:Factors influencing the FWA performance
•Base Paper: Sizing, pH, Fillers, Liquor pick-up
• Size Press Liquor: Anionic- & Cationic starches
• Carriers: CMC, PVA
Base Paper:
•Sizing: Naturally, the level of Sizing influences the pick-up of the base paper. Furthermore, highly cationic-, neutral sizes and rosin size (with alum) has a negative effect on the size press FWA (as on the wet end FWA). Tetrasulpho’s are more sensitive than hexasulpho.
•pH: A base paper pH below 5.5 has a significantly negative effect on tetrasulpho FWA’s and greening occurs early. Hexasulpho is much less affected. There is also a buffered tetrasulpho formulations available; TINOPAL ESP Liquid, that are tailored for acidic conditions and works well also with highly cationic sizing systems. .
•Liquor pick-up: The higher the amount of FWA carried onto the paper by the starch solution, the higher the whitening effect. The build-up goes to a certain point after which greening starts.
•Fillers: The brightness of the filler has a large impact on the whiteness also in size press. Base paper filled with CaCO3 (esp. PCC) has an increasing effect while e.g. Clay has a decreasing effect on whiteness


COATING: Factors influencing the FWA performance
•Pigments
•Binders
•Co-binders
•Solid content
•Base paper
•FWA distribution in single coat/double coat
•Shading Colorants
Pigments:

Mainly, combinations of china clay (increases gloss) and CaCO3 (increases whiteness) are used as pigments in coating colors. For high-quality coated papers and boards, special products such as satin white, titanium dioxide, aluminium hydroxide, modified clays, and synthetic pigments are also used.
The pigments have very little effect on whitening performance. They primarily affect the base white. The higher the base white, the lower the amount of FWA required to achieve a given effect and the higher the maximum whitening effect.

Binders:
The binders added to coating colors are essentially dispersions based on
– styrene-butadiene lattices
– styrene-acrylate
– vinyl acetate
All binders are anionic and have no affinity for FWA’s and, like pigments, have only a negligible influence on the whitening. The natural color of the latex will however have an influence on the whiteness.
Together with SB-latex, it has been observed that tetrasulpho FWA sometimes gives a higher b*-value (yellower => lower whiteness) than disulpho FWA at the same brightness level.

Co-binders:

FWA’s in coatings require a so called booster, also known as a carrier.
Typical boosters are:
Co-binders: PVA, CMC, starch, etc. which are added to the coating color to adjust the rheology. They also influence the water retention of the coating color.
Polyethylene glycols (PEG): Normally added (up to 40%) in some FWA formulations (mainly tetra- and hexasulpho type), but can also be added directly to the coating color. PEG-boosted FWA’s are especially useful in coating colors with low (<0,5%) or no co-binders or when the highest whiteness levels are required.
However, independently of the amount of co-binder, the PEG-booster always increases the maximal achievable whiteness and shifts the saturation point (the point where greening occurs) to much higher     FWA-concentrations.


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