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Pele OyPaper under light
For optical properties it is important that paper is porous (like snow). Paper under light shows the following main phenomena and paper properties:
• Specular reflection gloss and smoothness• Scattered reflection brightness and opacity• Scattered absorption color and opacity• Refraction opacity• Transmission opacity
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Snow - white
Ice – ”glassy”
Reflection
Refraction
Absorption
Transmission
Incident light
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Light reflection from printed image
For optical properties most important is how air is distributed in the paper and for strength how fibers are distributed.
For good multicolor pictures it is important that paper reflects all wavelengths i.e. is white.
Picture: Evans, DuPont
Pele OyKubelka-Munk theory and light scattering
Paper is rough and porous material. This means that it reflects scattered light from the surface but also deeper from the paper.
Light scattering coefficient is a material property, which tells how much from the incoming light the material can reflect as scattered light.
Light scattering coefficient can be calculated from paper reflection measurements.
Refraction index of main paper components are very similar. Every surface between paper and air refracts light and scattering is better when there are more these surfaces.
The higher the paper density is the less there is light scattering.
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Pele OyAbsorption coefficient
Absorption coefficient can be calculated from the measured values of R∞ and R0. It depends on the special chemical groups in molecules. Typically these groups are present in all kind of dirt.
Lignin in a pulp has molecules which reflect only yellow light (=absorbs blue light) and thus reduce brightness very much.
Pulp bleaching is basically additional removal of lignin from the pulp.
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Pele OyReflection and transmission
R∞ is the reflection coefficient of so thick sheet pile that no light goes through. It correlates with brightness measurements.
From the formula one can see that scattering coefficient must be high and absorption coefficient low to get high brightness.
Several specialty papers require some or high transparency. These can be called glassine papers.
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Reflection factor of a sheet against a black backing Reflection factor of a large stack of sheets
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Opacity definition
ISO Opacity =
Reflection factor of a sheet against a black backing Reflection factor of a sheet against a standard backingTappi Opacity =
100 % Opacity
Playing cards with black core
Pele OyOpacity measurement
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In the paper industry, there are two different standards for opacity measurement. ISO 2471 (Printing Opacity) and TAPPI T425 (Contrast Ratio) are the relevant standards.
ISO 2471 utilizes the diffuse illumination and 0° viewing geometry (d/0°) which is the same as the ISO brightness geometry.
TAPPI T425 uses 15° illumination and diffuse viewing(15°/d) originally developed by Bausch & Lomb in the 1930's.
The illuminants of ISO and TAPPI are different where effective wavelengths are 557 and 572 nm. Also the measured ISO and TAPPI opacities are different.
ISO 2471 TAPPI T425
Geometry d/0° 15°/d
White Backing R∞ R0.89
Wavelength, nm 557 572
Pele OyOpacity and brightness
Opacity % is 100*R0/R∞. This simple formula tells that opacity is lower when brightness is higher.
It is very difficult to get high opacity when brightness is high. This is the reason that newsprint grammage can be about 45 gsm but copy paper must be about 80 gsm.
Opacity relates to the show-through of the printed image from the opposite side of the sheet, or the sheet under it.
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Standard illuminants
A = 2856 ºK, “electric lamp”B = 4874 ºK C = 6504 ºK, ”indoor daylight” D65 = 6774 ºK, ”outdoor daylight”D65 = more UV
D65 is outdoor daylight, where UV is fully included. C is about indoor daylight (less UV). Illuminants C and D65 are used for paper measurements. The big difference is that D65
includes more UV light (wavelengths less than 400 nm). Optical brighteners (OBA = FWA) convert this UV light to visible blue light thus increasing
brightness values.
Pele OyBrightness measurements
There are two basic types of brightness measurements: directional and diffuse.
Directional brightness (TAPPI brightness – TAPPI 452) employs the 45º/0º geometry of the original GE-Photovolt instrument. It has been the standard in the U.S. and Japan. Fiber orientation has effect on the TAPPI brightness measurement.
Diffuse brightness employs a D/0º geometry where D indicates diffuse illumination from a sphere, making it insensitive to sample orientation. Diffuse brightness is the standard in much of the rest of the world.
Two types of diffuse brightness are commonly discussed, and a standard exists for both of them.
ISO C Brightness (ISO 2470-1, TAPPI 525) uses Illuminant C level of UV energy. It simulates normal office lighting conditions.
ISO D65 Brightness uses Illuminant D65 (daylight) according to ISO 2470-2.
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Sample
Photodetector
Light source
Sample
Photodetector
Light source
Directional
Diffuse
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D65 illuminant and measuring slot of brightness
For brightness measurement a narrow wavelength ”slot” has been standardised. The dominant wavelength of this slot is 457 nm (blue) and the range is ± 44 nm. If paper is yellow it reflects less blue light. This measure is effective for bleaching (less lignin less yellowish).
D65 illuminant includes lot of UV light, which can be converted to blue light by using optical brighteners. This increases brightness.
When measuring and calculating whiteness all wavelength have effect and blue dyes increase whiteness.
0
20
40
60
80
100
120
350 400 450 500 550 600 650 700
Brightness is measured from the slot which is under the white curve.
Pele OyBrightness weighting function
When pulp is bleached the reflectance of all wavelengths increases. However, the increase is largest at blue end of the spectrum.
Pulp brightness measurement is not only the measurement of total reflection but especially the increase of blue reflection (or decrease of yellow lignin absorption).
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UV-light and copy paper reflection spectrum
Brightness is measured with dominant wavelength of 457 nm. Illuminants C and especially D65 give higher brightness when optical brighteners are used. Actually reflection is lower close to 400 nm, where absorption is higher and the energy is moved to higher wavelengths.
D65C
No UV-light
Picture: Nils Pauler
457
Pele OyWhiteness
Whiteness is the ratio of Red, Green and Blue reflectance. It is an attribute of a diffusing surface which denotes its similarity in color to preferred or standard white.
Measurable properties CIE Whiteness (ISO 11475) and tint equations can be stated as follows. W = 2.41L* – 4.45b*(1–0.009(L*–96))–141.4 T = –1.58a*–0.38b*
Wavelength, nm
Brightness Whiteness
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Brightness and whiteness of copy papers
D65 brightness of European copy papers can be more than 100% with D65 light and optical brightening agents.
Whiteness is about 50 %-unit higher than conventional ISO-brightness and D65 brightness about 10 %-unit higher than ISO brightness with C illuminant.
60708090
100110120130140150160
1 2 3 4 5 6 7 8 9 10 11Paper suppliers
%
Brightness, C/2º, ISO 2470:1999
Brightness, D65/10º, SCAN P-66
Whiteness, D65/10º, ISO 11475:1999
Recycled base
Pele OyFluorescent whitening agent (FWA or OBA)
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Fluorescent Whitening Agent (FWA or OBA) is used to increase the white appearance of papers by absorbing invisible ultraviolet light and re-emitting it in the blue region of the visible spectrum. It is widely used in Europe to make bright surface.
This strategy can compensate for a yellow tint of many types of pulps that have been bleached to moderate levels.
Pele OyFluorescent component of brightness
Fluorescent component is the additional brightness obtained from the use of optical brighteners. It is determined by measuring the sample with and then without the effect of UV energy on the sample.
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Picture: Technidyne
Pele Oy Brightness comparisons
When comparing brightness it is important to know what method and light has been used. The following data is an example of differences.
Data: http://www.crableengineering.com/presentations/
TechnidyneD65 ISO TAPPI CIE D65 CIE C ISO-TAPPI D65 -ISO
A 98,9 91,8 89,9 126,6 106,0 1,9 7,1B 95,3 89,0 87,3 117,5 98,6 1,7 6,4C 105,6 95,1 91,8 139,3 112,3 3,3 10,5D 111,2 99,8 96,5 161,9 134,5 3,3 11,4E 110,6 98,9 95,5 162,0 133,9 3,4 11,7F 112,3 100,4 96,0 149,7 122,2 4,4 11,8G 113,5 100,9 96,4 160,5 132,2 4,5 12,6H 110,1 98,7 94,7 145,3 117,4 4,0 11,4I 112,3 100,2 95,9 161,2 133,6 4,3 12,1J 93,0 88,6 90,0 110,8 98,9 -1,4 4,4K 102,0 92,5 90,6 130,9 106,5 1,9 9,5L 110,1 99,1 95,0 152,5 126,3 4,1 11,1M 110,1 99,5 96,1 150,6 124,5 3,4 10,7N 107,2 96,7 94,1 153,6 127,9 2,6 10,6O 106,9 96,3 93,0 143,0 115,7 3,3 10,6P 110,3 99,2 96,1 146,3 119,0 3,1 11,1Q 109,6 99,0 96,1 143,5 118,1 2,9 10,5R 113,9 101,5 97,4 149,9 121,8 4,1 12,4S 111,6 99,3 95,0 163,0 134,6 4,3 12,3
Brightness Elrepho Whiteness ElrephoSample
Delta Brightness
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Pele OyPaper gloss metering principle
Tappi T 480 defines the specular gloss of paper and paperboard at 75 degrees (15 ° from the plane of paper). This method is suitable for low to moderate-gloss coated and uncoated papers as well as for most ink films on paper or paperboard.
The standard describes the technical requirements for a corresponding gloss meter. Gloss measurements are normally made using standard equipment like Hunter,
Lehman and Zehntner and giving a single mean gloss value. But, a sample can have a high mean gloss value (normally considered as good) but at the same time have a high gloss variation which is disturbing when looking at a printed picture. The micro gloss method can quantify such disturbing gloss textures and ‘glare effects’ on printed and unprinted surfaces.
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Pele OyExample of gloss instrument
It must be remembered that paper gloss is different in different directions depending on fiber orientation. Machine direction gloss is highest and cross machine gloss lowest.
The Technidyne PROFILE/Plus Gloss automatically measures the gloss at 75º in the MD and CD according to the following Industry Standards: TAPPI Method T 480, ISO 8254-1.
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Effect of Raw Materials
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Uncoated paper raw materials
Mechanical printings and woodree uncoated papers.
Material Mech. % WF % Comment
Fibers 60 – 100 70 - 100 Wood or non-wood fibres
Fillers 40 – 0 30 - 0 Mineral or synthetic pigments
Surface size - 0 - 5 Starch, CMC, PVA, synthetic size,optical brighteners etc.
Functional chemicals 0 – 1 0 – 2 Internal sizes, dyes etc.
(effect on paper properties)
Performance chemicals for process
<1 <1 Retention aids, defoamers, biocides etc. (effect on process performance)
Water 5 – 10 4 – 7 To be in balance with air humidity
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Fillers and coatings in papers
To improve optical properties of paper mineral pigments are used in papermaking. They can be added as a filler before headbox or to the surface as a coating with binders.
Paper GradesFiller
Pigment %
Surface sizeper side
g/m2
Coatingper side
g/m2
Woodcontaining
Newsprint, TMP/GWNewsprint, DIP
0 - 55 - 15
00 - 1.5
00
Unctd Mechanical, TD, BulkySC
5 - 1515 - 35
00
0 - 5 0
Ctd Mechanical, LWCMWC, HWC
5 - 158 - 18
00 - 2
5 - 1520 - 40
Woodfree
Uncoated Woodfree, CopyPrinting
15 - 3010 - 25
1 - 21 - 2
00 - 5
Coated Woodfree, standardPremium Art
10 - 1512 - 18
0 - 20 - 2
10 - 1520 - 35
Pele OyLight scattering coefficient of pulps
Finer fibers and less bonding give better light scattering (opacity & brightness)
When mechanical pulps have lower freeness light scattering is better (more refining, more unbonded fines)
When chemical pulps have lower freeness light scattering is lower (more bonding, practically no unbonded fines)
Harwood has better scattering than softwood
The more chemicals in pulping is used the lower light scattering will be
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Pele OyBrightness and chromophores
Brightness is not increasing linearly in bleaching. To make very bright pulp requires too much effort and bleaching chemicals.
It is good to know in papermaking that very small amounts of lower brightness components, such as mechanical pulp, low brightness clay or dirty process water reduces brightness very fast.
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Fillers and Coating Pigments
Pele OyMain pigment requirements
High lightscattering
Carbonate,PCC
Colourprinting
Low basisweight
Highbrightness
Goodopacity
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Effects of filler addition
Positive effects Water removal Better formation Drying shrinkage dimensional stability
Brightness, opacity and color Ink absorption more uniform Smoothness and gloss Costs and printability
Negative effects All general strength properties Surface strength , dusting Internal bond strength Stiffness Carbonate requires > 7 pH Runnability Retention , two-sidedness Wire, felt and machine wear
With PCC bulk and porosity can increase, with other fillers they decrease
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Fillers - agglomerated or dispersed?
Fillers should be first fixed to the fibres when they are dispersed. This would guarantee good retention, strength and optical effect (brightness and opacity) at the same time.
Only fibres,good strength,
Low opacity
Fillers well dispersed low retention,
low strength, good opacity
Fillers agglomerated good retention
and strength, low opacity
Picture: E.Gruber
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Filler distribution
Filler can either fill paper pores (left) or distribute evenly on fibre surfaces (right). Even distribution has good optical effect but reduces strength more.
Good filler distributionBad filler distribution
Pictures: Robert A Gill
Pele OyMain raw material requirements
High light scattering improves opacity and brightness. High absorption coefficient improves opacity but decreases brightness.
Fillers and pigments are good raw materials for optical properties (low k and high s).
Highbrightness
Goodopacity
High lightscattering
High lightabsorption
Low lightabsorption
Property s kHigh brightness + –High opacity + +
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Total mineral content of paper & board
Source: Omya
Pele OyMain fillers and coating pigments
Titanium dioxide is a special filler with high refractive index. It is the only possibility to improve opacity of impregnated and waxed papers. This is the reason that décor paper includes titanium dioxide.
When making opacity and brightness TiO2 is a very expensive filler or coating pigment. It is used in U.S. for this purpose but not in Europe where brightness and opacity are made with less expensive means.
Titanium dioxide price is more than three times pulp price while carbonate filler price can be less than half of pulp price.
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Mineral Avg size Refractive Scattering Brightness Density HardnessPigment Index coefficient
µm cm2/g % kg/m3 Mohs
Clay 0,2-2,0 1,55-1,57 1100-1200 80-92 2580 2-2,5
Calcined clay 0,7-1,5 1,60 2600-3000 90-95 2600 4,0-5,0
GCC 0,7-3,0 1,5-1,7 1400-1700 85-95 2710-2930 3,0-4,0
PCC 0,3-3,0 1,5-1,7 2200-6000 96-100 2710-3830 3,0TiO2 Anatase 0,2-0,4 2,5-2,55 4500-6000 98-100 3820-3970 5,5-6,0TiO2 Rutile 0,2-0,4 2,6-2,9 4500-6000 98-100 4230-5500 6,0-7,0
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Effect of filler content on tensile strength
Dry tensile strength is reduced about 50% when a normal 20% loading is used. Initial wet strength reduces even more.
Higher particle size gives better strength but optical effect will be lower due to lower light scattering.
Täyteainepitoisuus, %
Picture: Robert A Gill Filler content, %
Tens
ile, k
m
Particle size increases
Pele OyCritical properties of titanium dioxide
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Anatase, RI = 2.5Rutile, RI = 2.7(more effective)
Several properties of TiO2 are different compared to other fillers. However, if the effects are compared at the same level of opacity increase, the detrimental effects with TiO2 are lower than with several other fillers and pigments.
Even if the price of TiO2 can be up to ten times compared to lowest price fillers, the cost can be lower because the usage can be only 10% of the use of main filler (1-2% of paper for printing papers).
Because the share of TiO2 as a filler is low a good retention is very important. TiO2 absorbs UV light and the effect of OBA is reduced with TiO2. OBA is more used in
Europe and TiO2 in North America.
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Minerals in papermaking
Minerals are a fast growing raw materials of papermaking. Total amount of minerals in paper and board is globally over 10%. We are back in stone age.
Especially consumption of carbonates has been growing fast because they are white, easy to find everywhere and less expensive than fibers or clay.
Stone Forest in Kunming, China
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Paper Color and Printing
Illuminant Eye and brain
Pele OyColor mixing
When lights are mixed it is additive mixing (we add energy). When inks and paints are mixed it is subtractive mixing (we add absorption and reduce energy from reflection).
Pure red and green light produce yellow, red and blue make magenta, blue and green combine to make cyan, and all three together, when mixed at full intensity, create white.
For mixing of dye pigments, it is better to use the secondary colors, since they mix subtractively instead of additively. Using Cyan, Yellow and Magenta toners we can create colors on paper.
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Red
BlueGreen
Yellow
MagentaCyan
Pele OyColor gamut
In digital printing, when we talk about “gamut” we mean the color space of a device.
Devices can be divided into two categories:• RGB devices like scanners, monitors, digital
cameras etc.• CMYK devices like laser printers, offset etc.
Normally, a CMYK printer device color space will be smaller (less saturated and fewer colors) than a RGB capture device color space.
A typical RGB color space A typical CMYK color space
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Pele OyRGB color system
Light with a wavelength between 600 and 700 nm is known as red light. Light with a wavelength between 500 and 600 nm is known as green light. Light with a wavelength between 400 and 500 nm is known as blue light. By combining Red, Green and Blue light we can create all the colors of the
visible light-spectrum.
400 500 600 700 nmBlue Green Red
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Blue Green Red
Cyan 1 1 0Magenta 1 0 1Yellow 0 1 1
1 = Reflection0 = Absorption
Pele OyWhite paper reflection
In theory, white paper reflects all colors. This is a theoretical statement, because different brands of paper have a different color. This is why in color management it is very important to know what paper we are using. This is not only true for the output, but also for the original.
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Pele OyYellow toner absorbs blue light
Yellow is the complement of blue. Yellow toner absorbs blue light and reflects green and red light. The reflected “G” and “R” light are seen as yellow.
R
G B
Y
C M
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Pele OyMagenta toner absorbs green light
Magenta is the complement of green. Magenta toner absorbs green light and reflects blue and red light. The reflected “B” and “R” light are seen as magenta.
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R
G B
Y
C M
Pele OyCyan toner absorbs red light
Cyan is the complement of red. Cyan toner absorbs red light and reflects green and blue light. The reflected “B” and “G” light are seen as cyan.
R
G B
Y
C M45
Pele OyMixing subtractive colors
Equal amounts of magenta and yellow toner produces red. Equal amounts of cyan and yellow toner produces green. Equal amounts of magenta and cyan toner produces blue.
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Y
C M
Pele OyProcess black
In theory, equal amounts of C, M and Y produce black. This black is called “Process Black”. In reality, it is virtually impossible to produce true black using cyan, magenta and
yellow toner. Depending on the used toners or inks, the result can vary form deep blue to be brown or gray.
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Pure black Process black
Pele OyYellow school bus
Yellow surface absorbs mainly blue rays.
48Picture: HunterLab
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The CIE Lab model
CIE Lab is the second of two systems adopted by the CIE. It is an attempt to reduce the distortion in color distances.
Lab is based on XYZ, but is non-linear, to try to mimic the human senses.
• L is a luminance scale.• a and b are color axes.
Although not perfect, it is the most useful system today.
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Lab colour system
Vertical axis L is black and white axis.
b is yellow-blue axis. Bright paper has negative b values i.e. paper is bluish.
a is red-green axis. Bright paper has positive a values i.e. paper is reddish.
Blue-red paper looks brighter than yellow-green.
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Multicolor process CMYK
+
+
+
=
=
=
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Some Practical Issues
Pele OyPapermaking variables and paper properties
53Source: Michael Evans
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Colour of white papers
Kuva: Jouni Marttila
L*a*b*
L
Close to the neutral point all papers look white. When looking more closely, higher quality papers are bluish (-b values) and lower quality papers are yellowish (+b values).
Pele OyExample of commercial papers
Brightness ISO (%) in relation to Opacity
90,0
91,0
92,0
93,0
94,0
95,0
96,0
97,0
98,0
65,0
65,5
66,0
66,5
67,0
67,5
68,0
68,5
69,0
69,5
70,0
70,5
71,0
71,5
72,0
SC-ASC-B
NorCal 2011
Development of NorCal in relation to the competition > Measurements resulted from print trials.
UPM ECOStora Enso Publipress matt
Stora Enso Envi Press
SCA Grapho verde
MY Joy
Stora Enso maxauSCA Grapho verde
UPM ECO
Brightness C2 (%)
MY Joy Blue
Opacity (%)
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AF&PA brightness comparison
73-78.9
79-82.9
83-84.9
85-87.9
88+
81.9 and below
82-86.9
87-90.9
91 +NO.1
NO.2
NO.3
NO.4
GE BRIGHTNESS METER
Premium
Old New
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Pele OyHow to improve brightness and opacity?
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Action s k brightness opacityBleaching –– ++ –Mechanical pulp refining + +Chemical pulp refining –– – –Adding carbonate filler ++ – + ++Adding OBA/FWA – +Adding blue dye + – +Adding other dyes ++ –– ++Wet pressing – – –Drying paper more + + +Surface sizing – – –Calendering – – –
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.
Greetings from Finland.Thank You for reading this much.
Finally a test of color blindness for you
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Color blindness test 1
5
5
8
9
What are the numbers inside circles?
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Color blindness test 2
3
10
5
9
What are the numbers inside circles?
