Paper 101

What is Paper?

Through various web searches, we could not arrive at a conclusive definition of Paper. However the definition found on Wikipedia seemed the most appropriate, Paper is a thin sheet material produced by mechanically and/or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distributed on the surface, followed by pressing and drying.

https://en.wikipedia.org/wiki/Paper

A Brief History

Whilst the first uses of paper has always been for communicating and writing. In the ancient civilizations we used to write on barks of wood, Clay blocks, textiles, calf skin, amongst other mediums to communicate and preserve their history.

However Paper bears no relation to the writing material that came before it. Some historians think that the idea of papermaking came from felting, a practice that pre‐dated weaving and entailed beating wool until it mashed into a thick fibrous mat. However whilst the answer to the same is a mystery, paper has had a long evolution!

Paper is accredited to be invented in China by Cai Lun, in AD 105, During the rule of the Han Dynasty. The original form of paper is said to have been manufactured using barks of trees, hemp, rags of cloth and fishing nets. The process launched paper into common use.

In the second century CE, paper began to change. It became stronger, thinner, and more commonly and cheaply produced. Most of the earliest kinds of paper, up to and including Cai Lun’s had been made by beating fabric scrap to pulp with a mortar. The pulp was then poured into molds, which sat above tubs until the sheet set. This technique was slow, but so was the demand.

It wasn’t until the 1100’s, that’s when paper arrived to the west and not until 1800’s when paper could be produced using a continuous process. One person responsible for the invention of the said mill is Louis – Nicolas Robert from France. Louis, was granted the patent for continuous paper making machine in 1799.

At the time, Robert was working for Saint‐Léger Didot, with whom he quarrelled over the ownership of the invention. Didot believed that England was a better place to develop the machine. But during the troubled times of the French Revolution, he could not go there himself, so he sent his brother‐in‐ law, John Gamble, an Englishman living in Paris. Through a chain of acquaintances, Gamble was introduced to the brothers Sealy and Henry Fourdrinier, stationers of London, who agreed to finance the project. Gamble was granted British patent 2487 on October 20, 1801. The Fourdrinier machine used a specially woven fabric mesh conveyor belt (known as a wire, as it was once woven from bronze) in the forming section, where a slurry of fibre (usually wood or other vegetable fibres) is drained to create a continuous paper web. The original Fourdrinier forming section used a horizontal drainage area, referred to as the drainage table.

With the help of Bryan Donkin, a skilled and ingenious mechanic, an improved version of the Robert original was installed at Frogmore Paper Mill, Apsley, Hertfordshire, in 1803, followed by another in 1804. A third machine was installed at the Fourdriniers’ own mill at Two Waters. The Fourdriniers also bought a mill at St Neots intending to install two machines there, and the process and machines continued to develop.

Thomas Gilpin is most often credited for creating the first U.S cylinder type papermaking machine at Brandywine Creek, Delaware in 1817. This machine was also developed in England, but it was a cylinder mould machine. The Fourdrinier machine wasn’t introduced into the USA until 1827.

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How is Modern Day Paper Manufactured?

However almost every paper mill in India and in the world is based on the Fourdrinier forming section. The Fourdrinier machine first gained prominence in 1801, in Britain. Whilst the press section and the drying section varies, the broad process remains the same:

Paper Making Process

A Typical Fourdrinier Machine

Process Flow of Paper Manufacturing Explained

Video to demonstrate the method in which paper is made.
Courtesy - SAPPI

Sources of Fibre

Whilst that’s the basic process there are lot of variables which makes the industry extremely interesting. Some of the most popular fibre sources include:

a. Wood Pulp

i. Modern day paper manufacturers manufacture paper using managed plantation/forestry wooden logs. Species of the trees used to make paper are chosen on various factors. Some of the factors involved are:

  • Fibre Length of the cellulose
  • Growing cycle
  • Maintenance cost
  • Thickness of bark
  • Cellulose: Hemi Cellulose: Lignin percentage

ii. What is wood comprised of Chemically:

  • Cellulose ‐ 50% of the wood by weight is made up of Cellulose. During the growing phase of the tree these molecules are organised into fibrils and is crystalline in nature.
  • Lignin – 22 – 33% of a softwood or 16 – 25 % of a hardwood is comprised of Lignin. Lignin in layman’s term is the adhesive that binds the individual cell together. The presence of Lignin in the final pulp used to make paper can hamper the strength and also change the color of the paper over time.
  • Hemicellulose – is a part of the cellulosic complex and is the area between the lignin and the cellulose. The hemicellulose helps in increasing the fibre to fibre bond.

iii. Whilst there are many other considerations the above sum up the most prominent considerations. Cellulosic fibre is mostly classified by it’s length, broadly classified as Short Fibre and Long Fibre. The length of the fibre helps in establishing the strength of the paper, opacity, softness and other physical parameters.

  • Hardwood
    • Hardwood trees are generally trees with broader leaves and are deciduous in nature. These species include Birch, Aspen, Eucalyptus, Acacia, etc.. The length of this cellulosic fibre is 1mm and there by known as Short Fibre.
    • Short Fibre pulp provide better absorption and opacity
  • Softwood
    • Softwood Trees are generally trees with needle like leaves and cone bearing trees. These are evergreen in nature. Species of softwood trees include Pine, Fir, Spruce, etc.. The length of this cellulosic fibre is 3 times that of a hardwood, and hence the name long fibre.
    • Long Fibre pulp provide better strength

iv. Short Fibre and Long Fibre are generally blended by the paper mills to achieve the desired property in the final product. There is no fixed percentage for the same.

b. Alternative Pulp Sources

i. Modern day paper is manufactured using managed plantation/forestry

  • Bamboo
  • Bagasse (Sugarcane Waste)
  • Wheat Straw
  • Cotton

ii. Similar to Hardwood and Softwood, each of these pulp sources have cellulosic fibres that provide different fibre length and other property affecting inter fibre bonding. Sometimes alternative pulp sources are blended with traditional wood based pulp sources to achieve the desired end product.

iii. Alternative Pulp source are popular owing to some sources having lower fibre cost, due to higher yields, lower growth times, etc..

c. Recycled Pulp Sources

i. With more sophistication in waste collection and efficient sorting facilities, paper waste can be sorted and processed more efficiently, increasing the quality of recycled paper.

ii. Recycled waste is generally classified as Post Consumer and Pre Consumer. Post Consumer waste is the collection of waste after the paper/package has been consumed by the end customer and collected from the waste streams. Pre Consumer is defined as manufacturing or processing waste.

iii. Paper is made from post and pre consumer waste. Recycled Tissue is generally made from Preconsumer waste whilst News Paper and Duplex board is made from Post consumer waste.

iv. There are different subclassifications in the same and we will be happy to share the same with you if you would like to have more information about the same.

Different mechanism of making pulp

i. To add another variable, Hardwood and softwood can be processed into pulp using various Chemical and Mechanical methods. These differ as the end pulp can have vastly different properties

ii. Mechanical Pulp – It is the oldest method of making pulp, with the highest yield. Mechanical Pulp (85-95%) however the process causes a lot of damage to the fibre resulting in low strength. Commonly used Mechanical methods2

  • Refiner Mechanical Pulp (RMP)
    • Wood Chips are ground between two large metal disks in a refiner
    • The yield is generally very high
  • Thermo Mechanical Pulp (TMP)
    • Wood chips are in a pressurized refiner and softened with steam. The same helps in mechanical separation of the Cellulose and reduces the damage to the fibres
    • Results in higher fibre length than RMP but lower yield and higher energy costs
  • Chemi-Thermo Mechanical Pulp (CTMP)
    • Wood chips are first chemically or Biologically treated before heating and mechanical defibration.
    • The same helps in reducing the energy costs and improving the quality of the fibre over TMP

iii. Chemical Pulp - In the mechanical pulping process the goal is quite simply to get the highest fibre content. Whilst in Chemical Pulp the goal is to get only the cellulose and Hemicellulose from the wooden matrix. Yield is generally between 45 – 55% and hence the higher costs. The pulp produced using the said process helps in increasing fibre length, strength and also easier to bleach.

  • Kraft Pulp – The wood chips are first treated in an alkaline solution where the chemicals are attacking the lignin to separate it from the cellulosic complex. The process aims at recovering most of the chemicals used to bring down the cost associated and increase efficiency.
    • Wood Chips are ground between two large metal disks in a refiner
    • The yield is generally very high
  • Sulphite Pulp – Is the oldest form of chemical pulp, before the invention of the recovery boiler (used in the kraft pulping process). The Sulphite pulp has a higher yield and is easier to bleach but lower fibre strength
  • Semi – Chemical Pulp – similar to CTMP, however the wood chips are further treated with chemicals before being ground to pulp. The same helps in extracting part of the lignin, however lowers the yield from a CTMP process

A process illustration:

A process illustration.

A video on Fibers:

A video on Fibers.

Pulp Chest
  1. A pulp chest is where the paper mixes different pulp sources and chemical additives. The papermaker can also manufacture using a single source and use the chest to only decrease the consistency of the pulp by adding water
  2. In certain grades of Tissue, we blend hardwood and softwood in different ratios to achieve higher strength, softness and absorption.
  3. In certain packaging grades we use over 5 kinds of pulp source whilst making the board

Drying Paper
  1. Over the last 200 Years the drying of paper has turned to be a game changer for the industry. In the process of paper making we start at close to 99% consistency water and 1% pulp, and we arrive at 90 – 95% fibre and 10 – 5% moisture.
  2. Drying can take place through pressing, vacuum and heating elements. Each of them affect the end result. Too much pressing and vacuum will reduce the bulk whilst excessive heat will be expensive and can burn the fibrils.
  3. The drying process chosen can also result in changing the end feel of the paper. It is the different drying sections that change the grade of paper.
    1. Machine Finished and Machine glazed paper are caused by altering the drying process
      • Machine Finished
        • The drying of the paper happens using drying sections and rollers resulting in a uniform smoothness and dryness on both sides
      • Machine Glazed
        • The paper has been dried against a highly polished drying cylinder commonly known as the Yankee. This results in having one side of the paper a smooth and glossy appearance and the remains relatively rough.
      • Hand Made Paper
        • Hand made paper is made using traditional paper making process before the invention of the fourdriner machine. The process can be illustrated in the following video:

The paper is still manufactured around the world and is used in fine stationary, gift bags, tags and labels.

Kinds of Paper

Paper is broadly classified in the manner it is used.

Manufacturing Processes

Whilst there are many ways that paper is manufactured two of the most prominent forms is Machine Finished and Machine Glazed. Most papers forms are derivative’s of these processes.

Machine Finished Paper

A Machine Finished (MF) paper machine, is usually manufactured, where the paper is dried using multiple rollers after the press section in the mill. The same results in an even finish on both sides of the paper, with the felt side and the top side having similar smoothness and other properties.

Few examples of a MF Paper Machine:

MF process is also used in manufacturing multilayered kraft paper and packaging boards. The difference here is that the fibers in the wet end are added in layers and different orientation to achieve high strength and stiffness.

Few examples include

Machine Glazed Paper (MG)

A Machine Glazed paper machine uses a Yankee Roll for drying the paper. The Yankee is steam/Gas/ Oil heated cylinder roll that dries the paper post press section. MG is a process where you get a glaze on one side (Side facing the Cylinder), something similar to ironing a dress, where you see a sheen on a freshly ironed shirt. The side facing the felt of the machine is not as smooth as the top side. MG Paper is commonly used to manufacture lower Grammage paper ( 11 – 50 GSM) and in certain cases upto 110 GSM.

MG Kraft, MG Tissue, Glassine, Release Base, Greaseproof, Amongst others.

Few examples include the following:

MG Paper production is also used to manufacture Tissue as well! More on that in our tissue section.

Common Paper Properties

  • GSM

    • The primary and most common measurement of the paper. GSM stands for Grammage per Square Meter. GSM or Basis Weight simply means the weight of 1 square meter of the paper. It is calculated using:

    • Technical Standard followed: TAPPI T 410; ISO 536

      1. These are commonly followed standards and have their own testing formats

    • Commonly used procedures

      1. Equipment needed:

        1. Weighing Scale – units in grams
        2. Scale/ Ruler
        3. Scissor

      2. Recommended Procedure:

        1. If measuring tissue or low substance paper use multiple plies
        2. Cut a sample of the paper : 10 cm * 10 cm
        3. Place it on the scale, preferably the scale should be in a closed container as moisture from the air can hamper your reading
        4. Note the weight on the scale

      3. Calculation method:

        2. K Factor based on the length measurement

          Units of Measurement Conversion Factor
          Mass Area K
          Gram Cm2 10,000
          Gram In2 1,550

    • Significance: Since Paper is bought on weight the GSM indicates and differentiates the grades of paper. GSM is also used as an indicator for the bulk and thickness, though with modern production technologies you can achieve higher Bulk using lower GSM and save on costs.

    • Illustration of a GSM testing scale!

  • Caliper
    1. The Caliper Thickness signifies the thickness of one sheet of paper. Commonly measured in Microns
    2. Technical standard followed:
      1. TAPPI 411 and ISO 534
    3. Commonly used procedure
      1. Equipment Needed:

        1. Vernier Scale

      2. Recommended procedure
        1. Measure the paper using a Vernier scale along the length and breadth of the paper. Take 5 - 6 Readings in total and note the same down
        2. If measuring a low substance paper take 10 - 12 sheets of the same and measure the same across the length or breadth with equal pressure applied
          1. Increasing the pressure will hamper your readings and give you uneven readings
        3. Calculation Method
          1. Take a numerical average of the same and note the units measured in
          2. If you have taken multiple sheets/plies, then divide the average by the number of sheets of plies to arrive at the thickness per sheet/ply. Some test reports determine the same in Thickness per 10 Ply

      3. Significance: Caliper Thickness helps in calculating the bulk and also helps the converter determine the machinability in their machine or the end customers machine. Certain Packaging Boards if they are very thick or thin cannot run on the packing machine, as they require a specific micron to work!

  • Dry Tensile Strength
    1. Dry Tensile strength is measured in two directions of the sheet. Dry Tensile Strength signifies which is the Machine Direction and the Cross Direction of the Paper. Machine Direction/ MD is the direction along which the paper is made, and the direction in which the felt carries the fibers. Cross Direction, CD, signifies the direction perpendicular to the Machine Direction. MD is usually higher than the CD.
    2. Technical Standard followed:
      1. TAPPI 494 and ISO 1924
    3. Tensile Strength is measured in Laboratories or at the Paper mill using strips of paper cut in the same direction in a elongator machine calibrated to the standard followed. The machine elongates the paper in opposite directions and identifies the force at which the paper tears.
      1. You can see a sample test being performed here:
    4. Illustration of a Dry Tensile testing machine
    5. Units followed and conversions:
      1. Tensile Strength in metric form is noted as kN/m or Kilo Newton per meter, the same in English units is measured as gf/25 – m or Gram Force per 25 meters of paper
        1. If the Tensile has been measured in pound and inches, multiply the same by 0.1751 to get kN/m
        2. If this has been measured in kg/mm, you can get kN/m by multiplying by 9.807

      2. Breaking Length: Breaking Length is the length of a paper strip in which it would be self supporting. Weak Paper has a breaking length of 500 meters while Kraft paper can have a vale if 8000 m.

    6. Other values arrived while conducting Tensile Strength is the Stretch.
    7. Significance: We get to know how stiff is the paper and how strong is the board. The same influences the wet strength to a certain extent in tissue, machinability and composite strength in packaging. Commonly Dry tensile is also seen as an MD:CD Ratio. A ration more than 2, signifies significantly higher MD and a very weak CD. The same can result in an extremely weak finish product and can lead to failure when handled harshly by the consumer.

  • Wet Tensile Strength
    1. Wet Tensile strength, is used in papers which come in contact with water, to test the breaking point after the paper is moist. Applications for the same is important in the Label, Packaging and Tissue industry. Wet strength labels is used on Refrigerated Labels, Boards with resistance to moisture is used in Frozen Food Packaging and Towels in the tissue paper need the Wet Strength of the paper measured.
    2. Technical standard followed: ISO 3871 TAPPI T456
    3. Testing Method:
      1. Wet tensile strength is measured in the lab, following one of the Technical Standards quoted
      2. Usually the same apparatus and procedure is used as in the dry strength, where the only difference is that the paper is immersed in water was a specified time
      3. Owing to the inconsistency in the same, it is hard to determine the standard time that the product was immersed
      4. Industry standard usually say 5 – 40 seconds for easily saturated product such as tissue and 2- 24 hours for other grades such as paper board. These suggested time periods is to prevent the variation in strength.
      5. After immersion the same method of dry testing is followed.
      6. Crude way for Toilet Tissue:
        1. Take two sheets of toilet paper, approx. 20 cm, place it in a half filled 500 mL water bottle. After shaking it vigorously for 30 seconds the paper should dissolve. The dissolved paper implies that there is no wet strength in the paper
        2. There is no quantitive test and no control to the above and hence it is crude.
      7. Crude way for Paper Towel
        1. Wet your hands and wipe your hands with the towel to see if the towel disintegrates. You can do this with different samples to determine which has the highest wet strength
        2. Again this is crude and not recommended as there is no control or quantitative method.
    4.  Significance:
      1. Wet strength helps identify the ratio between Wet and Dry Strength, eg. Percentage Wet over Dry. If you are doing the ratio ensure that you are using MD values for both or CD Values
      2. A true wet strength is said to have 10 – 15% of the strength of the dry strength.

  • Stretch or Percentage Elongation
    hardwood
    1. Stretch or Percentage Elongation is also measured on a Tensile strength instrument. The Stretch of the paper implies the elongation before the paper ruptures or breaks. The same plays an importance in Tissue as well as other some packing grades such as Sack Kraft, and paper used to make paper bags, etc.. The stretch on the paper is also important in certain web offset printing machines.
    2. Stretch plays an important roll in the applications you would like to apply the property in . For example in Sac Kraft, a higher Stretch % demonstrates lesser chances of failure when the cement or any product is filled in the bag using pressure. Whilst in Tissue stretch is used for determining the Creping percentage and there by the strength and bulk of the paper. Also in Tissue a paper with high Stretch can give the customer a better yield, as lesser meterage of paper is used.
    3. Testing Standard: TAPPI 494
    4. EquipmentL Tensile Testing Machine
    5. Procedure:
      1. You would calculate the Dry tensile of the paper in a similar manner as mentioned above
      2. Note the initial length of the paper
      3. Note the length of the clamps at failure
      4. Use the below formula to calculate:
        1. Lf - The final length between the Clamps at the time of the failure
        2. Li – The initial length between the clamps before starting the test
    6. So is the paper stronger if it has more stretch? No. The same is not true. A paper can have a high stretch percentage obtained from creping the paper amongst other forms, and yet have a weak tensile. Stretch is only used in certain specific applications.

  • Absorption
    1. Water absorption plays a huge impact in the industry. In typical grades of culture paper/writing printing paper, a high absorption implies high ink usage, whilst no absorption implies surfaces needed added processes to be printable. In the packaging and label industry, absorption is seen as a part of the endurance of the package in cold storage and the ability to withstand temperature change when brought out by the consumer in the room temperature. In the Tissue industry absorption can signify the performance of the paper. A tissue with high absorption can reduce consumption
    2. There are two testing standards for Absorption in the paper industry. One is measured as COBB, whilst the other is measured specifically for Tissue.
    3. Testing Standard (Tissue): ISO 12625-8
    4. Testing procedure in short:
      1. A test piece of the Tissue is placed in a cylindrical basket, which is then immersed in water under its own weight. The time required for complete wetting of the test piece is measured and the mass of the water being absorbed then being determined after a stated immersion time followed by a given draining time.
    5. Apparatus Needed:
      1. Water container
      2. Drainer
      3. Timer
      4. Weigh Balance
      5. Cylindrical Basket made from non-corroding steel gage of a total mass of 3 grams
    6.  Procedure:
      1. Prepare the test specimen
        1. 76mm in width and length sufficient for each test species to weigh 5 grams
        2. Prepare 5 samples
        3. Record the mass of each species as mo in grams
      2. Record the mass of the basket as mb in grams
      3. Place the sample test piece in the basket without folding it
      4. Release the basket in the water in horizontal orientation
      5. Start the timer as you release the basket in the water
      6. Observe the wetting of the test piece to determine the moment that complete wetting happens
      7. Note time on the timer as soon as the same happens and record the time for complete wetting
      8. After the complete wetting happens let the sample be submerged in water for an additional 30 seconds
      9. After competition of 30 seconds remove the cylinder from the water, and let the timer continue to run
      10. Hang the cylinder at the Drainer support at a 30o angle
      11. Allow the test piece to drain for a further 60 seconds
      12. Ater 60 seconds weigh the cylinder basket and note it as mn
      13.  Calculation:
        1. Determine the time needed for complete wetting from step 7
        2. Calculate the water absorption capacity in grams of water per grams of test samples as:
    7.  Units:
      1. Grams of water absorbed/ Grams of paper
    8.  Significance:
      1. A high water absorption implies that the paper is efficient and that the consumption by the end user would be lower. Absorption will generally be higher as you compare the Wet tensile strength. But the same does not need to be true in all cases. Certain toilet grades have a better absorption with no wet strength based on their fiber choices and refining techniques.

  • COBB
    1. Significance: A COBB Test lets us know how much water can all other grades of paper and board absorb in a fixed duration. As mentioned in the section of absorption, what is the importance of knowing the amount of water that can be absorbed by the paper or board.
    2. A YouTube video demonstrating the COBB test being conducted:
    3. Testing Standards:
      1. ISO 535 and TAPPI T441
    4. Apparatus and Material Needed:
      1. Water Absorption apparatus to permit one side of the substrate to be wetted uniformly once the soaking process is started and also quick discharge upon completion. The image shown to the right is commonly yse
      2. Weighing Scale
      3. Metal Roller
      4. Timer/ Stopwatch
      5. Graduated Cylinder
      6. Blotting Paper – 200 – 250 GSM
      7. Distilled or deionized Water
    5. Testing Procedure in short for paper and board:
      1. Sampling:
        1. Cut three to four pieces of paper that you would like to test greater than the outer ring of the apparatus
        2. Specimen should be free from folds and wrinkles
        3. For Hard Sized Paper (Absorbing more than 100 GSM of water) use 10 specimens
        4. For Soft Sized Paper (Absorbing less than 100 GSM of water) use 20 specimens
      2. Procedure:
        1. Weigh each specimen to the nearest 0.01 g
        2. Test half the specimen with the wire side up and the other half with the felt side up
          1. For Corrugated Boards, test half the sample with the outer liner facing up and the other half with the inner liner side facing up
        3. Take 100 ml of de-mineralized water in a graduated cylinder and our it in the bowl of the cobb tester
        4. Place the sample with the test side as specified in 2.b, facing the water. It is the side that comes in contact with the water
        5. Place the lid on top of the sample and tighten the screw nob firmly to hold it firmly
        6. Decide if you would like to conduct a COBB (60) or a COBB (120) seconds test
          1. If you decide to do a 60 seconds test stop the timer at 50 seconds
          2. If you decide to do a 120 seconds test stop the timer at 110 seconds
        7. Turn the bowl of the COBB Tester upside down so that the water can come in contact with the test side of the sample, and as you turn the bowl you can start the timer
        8. Wait for 50 seconds or 110 seconds in this position, and on completion turn the bowl back to the starting position. And stop the stop watch
        9. After releasing the clamp, remove the sample carefully
        10. Place the sample test side down between two sheets of blotting paper
        11. Roll the 10 KG Metal roll on the paper, once forward and once backward, without exerting any external pressure on the roller
        12. Weigh the test sample to the nearest 0.01 g
    6. Calculation:
    7. Significance:
      1. A Zero COBB paper is considered to a paper or board that can resist the penetration of water. Such paper and boards are used in Frozen food packaging
      2. A High COBB value implies high absorbency and a Low COBB value implies low absorbency.
      3. A High COBB value is not desired in printing applications as the paper would soak more ink and minerals, but you would desire a high cobb in certain food packaging grades

  • Brightness
    1. Definition:
      1. Brightness is the measurement of the light reflectance of a specific wavelength of blue light (457 Nano Meter)
      2. Brightness is different from Whiteness and Shade
      3. Brightness is a measurement from 0 – 100
      4. However sometimes manufacturer add OBA in the paper. The OBA chemical reflects UV light as visible light in the blue spectrum. This results in an increased measurement of brightness in excess of 100
    2. Testing Standard:
      1. TAPPI T452, ASTM D985 – 97 and ISO Standard 2470
    3. Apparatus Needed:
      1. Brightness Tester
      2. Backing Weight – 1 kg
      3. Cut 7 sheets of the paper to be tested in the dimensions 37*50 mm
    4.  Procedure:
      1. Place the paper in the brightness tester and secure it firmly using the backing weight
      2. Record the brightness for 5 readings to a 0.1% reflectance and note it down
      3. Measure the florescent component of the brightness:
        1. Set the filter selector to the florescent position
        2. Record the highest readings for the 5 sheets
        3. Subtract the above reading from the reading recorded in d.ii.
      4. Some instruments give the florescence level in the brightness
    5. A link to show you a sample testing method:
    6.  Significance:
      1. Brightness implies how bright the paper is under blue light. Brightness when above 100 implies that there is more light reflected off the surface than shown on the same. This implies the presence of Optical Brightening Agents (OBAs) in the paper. Brightness can also be altered by adding OBA and blue dyes to get a brighter paper.

  • Whiteness
    1. Definition:
      1. Whiteness is a measurement of light reflectance across all wavelengths of light comprising the full visible spectrum
      2. The index is devised so that people understand that a higher whiteness of paper under normal and all conditions of light.
      3. The D65 light replicates all the wavelength of the visible spectrum of light
    2.  Standards
      1. CIE is the most commonly used whiteness index which is made under D65 illumination, which is a standard for outdoor light reflectance
    3.  Measurement:
      1. Whiteness is measured similar to brightness but under the D65 Light
    4. A link to show you how Brightness and Whiteness is different:
    5.  Significance:
      1. Whilst adding high OBA will increase the whiteness coefficient also
      2. A paper with a high amount of OBA might appear whiter outside whilst appear duller under indoor lighting conditions

  • Shade
    1. Definition:
      1. Shade is the measurement of the color of the paper. Shade is reported in the form of CIE model of LAB Values
      2. The typical shades of paper include:
        1. True White – Reflects total color spectrum equally
        2. Cream White – absorbs blues and cooler color. Appears yellowish
          1. Also known as Natural Shade
        3. A blue White shade absorbs the warmer colors and reflects more blues or cooler color
          1. Papers with high blue reflectance are often referred to as Bright White or High White Paper
        4. Standard:
          1. The shade of paper is almost invariably measured on the CIE LAB model (CIE L*, a*, b*) as this model covers the full color space, whereas other models such as RGB or CMYK cover a subset of the LAB space.
        5. Testing Method:
          1. Using a CIE compliant Spectrometer
        6. Significance:
          1. Blue white is used for paper because it appears to be a, “whiter”, white, compared to true white.
          2. Cream white absorbs the blues that blue white reflects and therefore has a yellowish look
          3. True white as the name suggests reflects all the spectrum of white evenly and which is how white usually looks
          4. Whilst printing, the CMYK of the image and the LAB of the paper can effect the final printing quality of the product

  • Gloss
    1. Definition:
      1. Gloss describes the mirror like property of coated paper. Gloss is defined as the percentage reflectance at an angle equal to the angle of incidence compared with a standard surface
    2. Testing Standard:
      1. TAPPI T480 measures gloss at 75o and ASTM D1223-93
    3. Apparatus needed:
      1. Gloss Meter
      2. Area of specimen illuminated
      3. Gloss Standards
      4. High gloss working standard
    4.  Method:
      1. Sampling:
        1. Cut at least 10 samples free of blemishes and free of folds and wrinkles
      2. Start the gloss meter and follow the calibration instructions using the gloss standard or high gloss working standard
      3. Start measuring the gloss by placing the paper in the machine or under the machine. Record the reading
      4. Turn the paper by 180o and record the gloss again at the same point
      5. Average the two results and record it for the specimen. Conduct the test for the 10 specimens and take an average. The same would be the gloss for the said surface.
      6. In MG or 1 side coated paper, Folding Box Board or Duplex Board, the gloss will vary massively between the top and bottom surface
      7. In Coated Paper the paper might have a more even gloss on both side. However the gloss might be higher on the felt side, as the paper is slightly smoother.
    5.  Significance:
      1. A higher gloss, will imply a brighter sheen and more reflectance on the printed surface, there by making the print more eye catching and attractive.
      2. Higher gloss also signifies a smoother surface, hence better surface for lamination

  • Opacity
    1. Definition:
      1. Opacity is the measure of the amount of light transmitted through the paper
      2. 100% opacity means no transmitted light
    2. Testing Standard:
      1. ISO 2471 and TAPPI T425
    3. Significance:
      1. Opacity is used to judge the reverse printing on a substrate, as well as in packaging, especially flexible packaging where paper is laminated, a high opacity will prevent the laminates from being seen
      2. Opacity is increased in the paper mill, by increasing the bulk, adding pigments, using short fibers, reducing the refining or by not calendaring the paper. These are only some methods that are in place.
    4. Opacity is measure similar to brightness. Please find the link of one such instrument being used:

  • Softness
    1. Definition:
      1. Softness is a property that is perceived differently by different people. Traditionally Softness, typically used for Tissue Paper, was comprised of handfeel.
    2. Testing standard:
      1. There is no Testing Standard for the same. However TSA is commonly used as an industry standard. TSA is developed eMtec
        1. TSA is a two step analysis, where sound is used to measure the softness and roughness and second is a deformation measurement where stiffness, plasticity, elasticity and Hysteresis is measured.
    3. Testing Apparatus:
      1. Measured by the propitiatory equipment developed by eMtec
    4. Testing process in brief:
      1. The first process is the measurement by sound/ vibrations of the roughness or softness of the paper. The hard fibers/surfaces will lead to a spike in the measurement, and with the second peak observed in the test would indicate the real softness/roughness.
      2. The second part is to check the deformation. A force of 100mN to 600 mN is applied to the sample to measure the other parameters such as elasticity, plasticity and hysteresis is measured. Th deformation is measured in mm/N
      3. Using these parameters a hand feel value is calculated.

  • Bulk
    1. Definition:
      1. Bulk is used to measure the ratio of paper thickness to its weight in cubic centimeters per gram. Bulk is a term used to indicate volume or thickness in relation to weight. Bulk in the packaging industry is used to determine the substance and caliper thickness the filling lines can use, in Publication, bulk is important to determine the appearance of a book, in Hygiene bulk is especially important for retail products so that the products look bigger!
    2. Calculation:
    3. Significance:
      1. An example of bulk if you are in the packaging industry:
        1. A high bulk board will improve your yield, by matching the customer required thickness using lower basis weight
      2. In the publishing industry:
        1. A high bulk paper will make the novel look thicker and richer to the consumer
        2. A low bulk paper will empower News Paper companies to lower distribution costs
      3. In the hygiene industry:
        1. Retail: High bulk products help converters convince the customer that they are getting the most value for their product, as the product is puffier.
        2. Away from Home (AfH): Low bulk towels and toilet rolls, help in loading more sheets in the dispensers there by reducing the refill times

  • Porosity
    1. Definition:
      1. A measure of the extent to which a paper surface will allow the penetration of a gas or liquid, such as air or ink, through its surface. The nature of paper is such that the bonding of the paper fibers produces many tiny air passages throughout the paper, which can either be completely submerged in the paper, extend from the surface down into the interior of the paper, or penetrate completely through the sheet.
      2. Measured using Gurley method or Bendsten or Bekk methods

  • Moisture
    1. Definition:
      1. Almost all grade of paper has some percentage of moisture. Moisture in paper varies from 2 - 12% depending on relative humidity, type of pulp used, degree of refining and chemical used. Most physical properties of paper undergo change as a result of variations in moisture content. Water has the effect of plasticizing the cellulose fiber and of relaxing and weakening the inter-fiber bonding. The electrical resistance and the dielectric constant of paper both vary with moisture content. The absorption and reflectance of certain bands of infrared and microwave radiation by paper are affected by its moisture content. The amount of water present in a sheet of paper is usually expressed as a percent. The amount of water plays an important role in calendaring, printing and converting process. Moisture control is also significant to the economic aspect of paper making. Water comes free. Poor moisture control can adversely affect many paper properties.
    2. Testing Standrards:
      1. TAPPI T412 abd ISO 287
    3. Apparatus required:
      1. Weighing container
      2. Drying Oven
      3. Weighing balance
    4. Testing procedure:
      1. Sampling:
        1. Cut the samples of 10 cms * 10 cms
        2. Ensure you are cutting the samples from the same lot and under the same conditions
        3. Place the samples in the weighing container as soon as you cut the samples
      2. For larger specimens (50g) and above:
        1. Weigh each specimen container to the nearest 0.02 g (W1)
        2. Place the specimen in an oven for 1 hour at 105o C and if the GSM is greater than 224 GSM heat the sample for 2 hour.
        3. Replace the specimen in the container and close it allowing it to cool to room temperature
        4. When cool open the container and allow the air to flow through again so that there is little air circulation
        5. Weigh the container again to 0.02 g (W2)
      3. For small specimens (2g)
        1. Weigh the specimen in a tarred bottle to the nearest 0.02 g (W1)
        2. Place the specimen in the oven and take off the lid. Heat the sample for 30 minutes. If the sample is above 224 GSM heat it for 1 hour at 105o C
        3. Replace the specimen in the container and close it allowing it to cool to room temperature
        4. When cool open the container and allow the air to flow through again so that there is little air circulation
        5. Weigh the container again to 0.02 g (W2)
    5. Reporting Units:
    6.  Significance:
      1. Moisture is required in paper for multiple reasons!

  • Bursting Strength
    1. Definition:
      1. Bursting strength tells how much pressure paper can tolerate before rupture. Bursting strength is measured as the maximum hydrostatic pressure required to rupture the sample by constantly increasing the pressure applied through a rubber diaphragm on 1.20 - inch diameter (30.5 mm) sample.
      2. A Nice Video to illustrate the same:
    2. Testing Standard:
      1. TAPPI T 403
    3. Typical Testing Method
      1. Apparatus needed:
        1. Bursting tester
      2. Sampling Method:
        1. Take at least 20 samples from the specimen you would like to test from different areas of the sheet or reel. Each sample should be atleast 6.2 Cm * 6.2 Cm, avoid taking samples from areas of wrinkles or blemishes.
      3. Clamp the samples securely in the apparatus and begin the test in line with the manufacturers instruction.
      4. Record 10 tests for each side of the paper
      5. For each side report the Bursting strength as Killo Pascal or pounds per square inch and take an average for each side of the paper.
    4. Burst Index can be calculated as per the below:
    5. Other ratios commonly used:
    6.  Significance:
      1. Bursting strength tells us the puncture resistance of the paper or board. It is a factor which is linked with the strength of the box or bag! A box or bag with high burst strength will be more resilient to punctures and logistical damages
      2. However modern day logistics has evolved much, with bursting factor being only an indicative value especially in the corrugating industry. In the corrugating industry people now use Edge Compression Test for corrugated boards and Ring crush values for the Kraft Liners. There by a paper with high BF values but lower RCT and resultant ECT values, can lead to a weaker box!
      3. Bursting strength is still used widely across all industries as an indicative value. Although as mentioned there are more accurate measurements available to determine what grade of paper would best serve your need!

  • Ring Crush Test (RCT)
    1. Definition:
      1. Ring Crush is a traditional test of linerboard and corrugating medium strength. Ring crush measures compression resistance, and this compression strength is considered to relate to the eventual compression strength of combined board made from the component.
      2. RCT is calculated for the Kraftliner or the Testliner. It is not calculated for the Corrugated Board

  • Wax Pick Test
    1. Definition:
      1. A measure of the surface strength of the sample or surface resistance to picking. Pick occurs due to poor internal bonding strength, making it susceptible to adherence to grade wax sticks (Dennison). This test is valid only for uncoated board or paper. For Coated stock IGT pick test is applicable.

  • IGT
    1. Definition:
      1. IGT is a measurement of the surface strength of the paper. A tacky ink is applied to sample of the paper at an increasing speed. As the speed increases the peeling force applied to the paper also increases and the speed at which the fibers begin to be pulled from the sheet is recorded as the IGT. A high IGT (>300) indicates a strong surface strength suitable for demanding offset applications.