1. OBA or FWA Training for the Color & Appearance of Paper
Crable Engineering LLC,
The Color Solutions Company
OBA or FWA Training for the Color & Appearance of Paper
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2. What is an OBA?
OBA stands for Optical Brightening Agent, also known as FWA (Fluorescent Whitening Agent), optical brightener, fluorescent dye, or simply whitener.
Basically it is a special type of dye.
They are used in most grades of white paper and board (even in newsprint) to brighten and whiten the paper.
Color of the OBA in its concentrated form is generally amber (dark yellow).
Mostly they are anionic (or negative) in charge.
pH is typically between 8.5 and 11.0.
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3. How do OBAs work?
OBAs absorb a portion of the invisible Ultra Violet (UV) rays from the sun or another light source.
They reemit that energy as visible, blue light.
This phenomenon, called fluorescence, causes the total visible reflected light from a sample to increase, particularly in the blue range.
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4. Overuse of OBAs
OBAs are effective as long as they find new places to bind to the fiber or to surface starch (size press).
When there are no more bonding sites available, the OBA molecules will start attaching to each other.
Once the OBA molecules start attaching to themselves, a combination of their “blue” fluorescence and the yellow of their own natural color produces a phenomenon called “greening over.”
When “greening over” occurs, the paper actually starts to turn green in shade and the brightness and whiteness drop.
Fluorescence measurement will continue to increase even though brightness does not when green over occurs.
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5. OBA Greenover (Overuse)
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6. How many types of OBA are there?
There are many different types of OBAs used across multiple industries, such as paper, laundry detergent, plastics, and textiles.
But, there are three (3) main types used in the paper industry: disulphonated, tetrasulphonated, and hexasulphonated.
There is even at least one cationic OBA used in the paper industry – Clariant’s Leucophor FTS. It is used in some inkjet grades where the size press starch has a positive charge due to other additives.
3V, Greenville, Clariant, and now TFM are the major suppliers of OBA to the paper industry in the U.S., Europe, and Brazil.
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7. Which OBAs are Mostly Used?
In North America, we use mainly three types of OBA: disulphonated, tetrasulphonated, and hexasulphonated.
Disulphonated OBAs (2 sulphonate groups) have the lowest solubility in water and the greatest affinity for cellulose fiber of any whitener type. They are recommended exclusively for the wet-end of the paper machine. Disulphonated OBAs have the highest potential to green over when overdosed.
Tetrasulphonated (4 sulphonate groups) is the most universal of the OBAs. It can be used in the pulp and size press and can be used in the wet-end of the paper machine. If used in the wet-end of the paper machine, though, tetras have a greater impact on charge (negative effect on wet end chemistry) than disulphos. They can be used at the size press but will not build to very high brightnesses when applied there. Tetras will also green over when used excessively.
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8. What are the OBAs that we use?
Hexasulphonated OBAs (6 sulphonate groups) have the highest solubility in water and are used exclusively in the size press and in coatings. Hexas are used to build to the highest brightness and whiteness levels, and they have the lowest potential to cause green over.
*Modified tetrasulphonated OBAs are specialty products where the tetrasulphonated chemistry has been slightly altered to allow them to build to higher brightness levels than normal tetras. They are used exclusively in the size press. Hexas have mostly replaced them now since hexa prices have dropped.
NOTE: tetrasulphonateds are the only type of OBA approved for use in food contact grades.
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9. How do they compare for cost?
Prices for tetrasulphonated OBA are the lowest and averaged perhaps $0.55/lb and $0.65/lb in 2008 (Eastern USA).
The price for hexasulphonated OBAs were about $0.65/lb to $0.70/lb in 2008 (Eastern USA). Hexa used to cost much more, but it is now much closer in cost to tetra.
Disulphonated OBAs are the most expensive with pricing of approximately $1.00/lb in 2008 depending on whether they are received in bulk (tank trucks) or in totes (Eastern USA).
NOTE: Prices can rise sharply with raw material shortages or gross price increases.
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10. How much OBA should be added?
The right OBAs should be added at each location to gradually build to the final brightness/whiteness.
At the wet end of the paper machine, if OBA is needed, it is preferable to add disulphonated to minimize the impact on wet end chemistry. Also this type of OBA has the best fiber affinity. Efficiency is good at dosages up to about 10 #/ton.
In the size press, tetrasulphonated should be used at lower dosages because it is stronger than hexasulphonated. But at dosages above about 15 to 20 #/ton the tetrasulphonated no longer builds. Hexasulphonated should be used at loadings above 20 #/ton in the size press.
Too much TOTAL use of OBA will lead to green over, but adding it in portions allows optimal build internally and on the surface.
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11. How much can OBA help brightness & whiteness?
A maximum of 10 points (theoretical) of TAPPI (GE) brightness can be obtained from the use of OBAs. In practice, the brightness boost from OBA is usually closer to 7 points.
If that 10 points is used up by adding an excess of OBA at the wet end of the paper machine, then additional use of OBA on the paper machine will have little to no positive impact.
WHITENESS
OBA can have a significant impact in raising CIE whiteness.
Use of OBA can raise whiteness by more than 10 points.
Making the sheet slightly bluer, though, is a lot more cost effective.
Using the right amount of OBA at the best addition points is still key to keeping cost down.
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12. Can too much OBA be added at a single addition point?
Yes, it is possible to add too much OBA at one addition point.
Too much added at any one point will either cause green over or simply waste OBA and drive up cost.
Think of building brightness and whiteness like a drying profile; it should be done thoughtfully to be efficient.
A suggested way to build brightness/whiteness:
Treat new bonding sites (e.g. at refiners or fan pump): up to 10 #/ton
Then treat the surface of the sheet (e.g. size press or coating): up to about 15 #/ton (tetrasulphonated) or 40 #/ton (hexasulphonated)
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13. What can hurt OBA performance?
Cationic materials such as retention aids, alum, and coagulants can interfere with OBA. Their use should be kept to a minimum, and they should be diluted and added as far from the OBA as possible.
Acid conditions tend to have a quenching effect on OBA.
Lignin containing fibers such as those in BCTMP will reduce OBA effectiveness by absorbing UV energy.
Fillers can compete with OBA for UV energy. Titanium dioxide is the worst in this regard, particularly rutile titanium dioxide.
Yellow and black dyes are very bad for OBA performance.
Heavy metals such as iron, manganese, and copper – often found in process water – interfere with OBA.
Poor starch quality (yellow) reduces OBA efficiency, too.
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14. How can OBA affect runnability?
OBAs have a negative charge and they increase the cationic demand of the paper machine wet end.
They can, therefore, strongly impact retention and can lead to sheet breaks if not monitored and carefully controlled.
In order of their negative impact on wet end chemistry, OBAs are listed:
Hexasulphonated – worst (added at the size press; comes back to the wet end through broke)
Tetrasulphonated – middle
Disulphonated – least impact (best)
Tetrasulphonated OBAs added in the wet end of the paper machine can be problematic at moderate to high dosages.
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15. What test measurements will OBA affect?
OBA addition will impact:
Fluorescence
Brightness
Whiteness
b* value (slightly on the Elrepho)
L* value (slightly)
a* value (lower – green – if overused)
Increase OBA to raise fluorescence.
Increase OBA (to its optimum point) to achieve brightness and whiteness.
Decrease OBA if greening (lower a* value) is observed.
Decrease OBA to the minimum flow while maintaining specifications for cost savings.
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16. Why is OBA reduction important?
OBA, like many chemicals, has oil as one of its elementary ingredients.
With oil prices rising and fluctuating, OBA prices could increase dramatically.
Significant savings can be realized by systematically reducing OBA use which is not improving optical properties. One North American mill saved over $2,000,000 per year while making the same grades.
With global demand for OBA going up & possible supply disruptions, it could become difficult to get enough material . . at least in a timely manner.
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17. How Can We Save OBA?
Simply put, it is very easy to overuse OBA without realizing it.
Regular efforts should be made to cut OBA flow while observing brightness and whiteness. This is particularly true after grade changes and sheet breaks.
If a cut in OBA flow does not result in a drop in brightness and whiteness, then it is being overused.
Use more high brightness filler (PCC).
Avoid OBA leaks and spills.
Compare OBA usage to that of previous runs and continually try to reduce it. Also create a “dashboard.”
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18. How Do We Use the Dashboard?
Create it using a straightforward template or screen form.
The Dashboard is designed to give a quick overview of most of the factors that influence OBA consumption.
It should be referred to regularly to see if there are any process changes (e.g. increase in dye flow or drop in pulp brightness) than could increase OBA usage.
Corrective action should be taken immediately to correct the process and allow OBA flow to be reduced.
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19. What does ColorLok® do to OBAs?
ColorLok® can be a size press additive that interferes with the OBA’s ability to fluoresce and thus build brightness and whiteness. It can also cause them to precipitate out of the starch. Hexas are most resistant to precipitation.
ColorLok® chemicals usually impart a yellow-green color to the paper.
This shift in shade has to be overcome by a combination of increasing the flow of the tinting dye(s) and usually using more OBA.
ColorLok® makes the size press addition of OBA less efficient than it would otherwise be.
Therefore, it is appropriate to try using more disulphonated OBA in the wet end of the paper machine to achieve brightness and whiteness targets, if necessary.
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20. Are there safety or handling issues with OBAs?
OBAs are not harmful when handled with appropriate care.
They do not contain any toxic chemical components.
They will exhibit fluorescence (blue glow) under black light.
In some OBAs, with urea or glycol used as solvents/stabilizers, sensitivity can result if it comes into contact with the skin. NOTE: This is not generally an issue.
When spilled, OBAs can form a slippery amber puddle.
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21. Control scenarios with questions . . .
APPENDIX
Control scenarios with questions
and answers.
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22. Scenario 1 TARGETS: L* = 95.70 TAPPI Brightness = 92.0
a* = CIE Whiteness = 145.0
b* =
ACTUAL VALUES:
L* = TAPPI Brightness = 93.1
a* = CIE Whiteness = 146.5
b* =
SETPOINTS:
Tetra OBA is being added to the size 30 cc/min top and bottom.
Blue dye is being added at 29 cc/min.
Red dye is being added at 20 cc/min.
QUESTION:
What can be done to save OBA and thus cut costs?
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23. Scenario 2 TARGETS: L* = 95.70 TAPPI Brightness = 92.0
a* = CIE Whiteness = 145.0
b* =
ACTUAL VALUES:
L* = TAPPI Brightness = 91.6
a* = CIE Whiteness = 142.2
b* =
SETPOINTS:
Hexa OBA is being added to the size 50 cc/min top and bottom.
Blue dye is being added at 25 cc/min.
Red dye is being added at 17 cc/min.
QUESTION:
What can be done to save OBA and thus cut costs?
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24. Scenario 3 TARGETS: L* = 96.38 TAPPI Brightness = 96.0
a* = CIE Whiteness = 153.0
b* =
ACTUAL VALUES:
L* = TAPPI Brightness = 97.1
a* = CIE Whiteness = 149.7
b* =
SETPOINTS:
Hexa OBA is being added to the size 65 cc/min top and bottom.
Di OBA is being added to the wet end at 50 cc/min.
Blue dye is being added at 37 cc/min.
Red dye is being added at 39 cc/min.
QUESTION:
What can be done to save OBA and thus cut costs?
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25. Scenario 4 TARGETS: L* = 96.25 TAPPI Brightness = 96.0
a* = CIE Whiteness = 150.0
b* =
ACTUAL VALUES:
L* = TAPPI Brightness = 95.6
a* = CIE Whiteness = 153.2
b* =
SETPOINTS:
Hexa OBA is being added to the size cc/min top and bottom.
Di OBA is being added in the wet end at 35 cc/min.
Blue dye is being added at 29 cc/min.
Red dye is being added at 20 cc/min.
QUESTION:
What can be done to save OBA and thus cut costs?
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26. Scenario 5 TARGETS: L* = 96.05 TAPPI Brightness = 96.0
a* = CIE Whiteness = 153.0
b* =
ACTUAL VALUES:
L* = TAPPI Brightness = 94.2
a* = CIE Whiteness = 149.9
b* =
SETPOINTS:
Hexa OBA is being added to the size 70 cc/min top and bottom.
Di OBA is being added to the wet end at 23 cc/min.
Blue dye is being added at 32 cc/min.
Red dye is being added at 26 cc/min.
QUESTION:
What can be done to save OBA and thus cut costs?
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27. Scenario 1 - Answers
Delta L* = , Delta a* = +0.30, Delta b* =
Delta TAPPI Brightness = +1.1, Delta Whiteness = + 1.5
The sheet is light, red, blue, and high in both brightness and whiteness.
Blue dye should be cut (perhaps 29 cc/min to 26 cc/min);
Red dye possibly could be cut too (20 cc/min to 19 cc/min);
OBA flow to the size press should be reduced on both sides – perhaps 30 cc/min to 22 cc/min.
Continue to adjust dye flows to achieve shade, and reduce OBA until TAPPI brightness is close to 92. CIE whiteness will drop slightly.
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28. Scenario 2 - Answers
Delta L* = , Delta a* = , Delta b* =
Delta TAPPI Brightness = - 0.4, Delta Whiteness = - 2.8
The sheet is dark, green, yellow, and low in both brightness and whiteness.
OBA flow to the size press should be reduced slightly on both sides – perhaps 50 cc/min to 45 cc/min to evaluate possible green over.
Blue dye should be increased slightly (perhaps 25 cc/min to cc/min) to make the sheet bluer;
Adjust OBA flows up or down to improve brightness and eliminate green over (if there is any). Adjust dyes to match shade. CIE whiteness should rise slightly with the adjustment to be bluer.
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29. Scenario 3 - Answers
Delta L* = , Delta a* = , Delta b* =
Delta TAPPI Brightness = + 1.1, Delta Whiteness = - 3.3
The sheet is slightly dark, green, and yellow. It is high in brightness but low in whiteness.
There is strong evidence of greening over occurring.
The flow of di OBA is high; it could be reduced from 50 cc/min to 35 cc/min or 30 cc/min.
OBA flow to the size press should be reduced on both sides – perhaps 65 cc/min to 55 cc/min to evaluate green over.
Dyes should not be adjusted until the green over is eliminated.
Continue to reduce OBA flows to improve brightness and eliminate green over. Then adjust dyes to match shade. CIE whiteness should rise slightly.
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30. Scenario 4 - Answers
Delta L* = , Delta a* = , Delta b* =
Delta TAPPI Brightness = - 0.4, Delta Whiteness = + 3.2
The sheet is slightly dark, red, and blue. It is low in brightness but high in whiteness.
The flow of Blue dye is high making the sheet blue and slightly red. (NOTE: It’s a reddish-blue dye.)
Cut Blue dye from 29 cc/min to perhaps 26 cc/min.
Reduce Red dye by 1 to 2 cc/min if the sheet remains red.
This may offer the opportunity then to cut some OBA because the extra dyes are dulling the sheet.
Continue to try reducing size press OBA flows as the brightness allows.
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31. Scenario 5 - Answers
Delta L* = , Delta a* = , Delta b* =
Delta TAPPI Brightness = - 1.8, Delta Whiteness = - 3.1
The sheet is slightly dark, red, and yellow. It is low in brightness and whiteness.
Reduce Red dye by 1 to 2 cc/min to eliminate the red.
Add Blue dye only if necessary to maintain b* value.
The di OBA is at a low dosage and the hexa OBA is at a high dosage.
Reduce the hexa OBA to the size press from 70 cc/min each side to 60 cc/min each side. Increase the di OBA in the wet end to 35 cc/min.
Continue to try reducing size press OBA flows as the brightness allows.
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32. END
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