Lignin structure, properties, function and uses

Posted on May 28, 2015 by admin

Lignin is an aromatic complex polymer of monolignols (aromatic alcohols) which is found in plant cell walls. Later than cellulose, lignin is the most abundant natural polymer in the planet. It is not composed of carbohydrate monomers whereas the rest of polymers found in plant cell walls are composed of carbohydrate monomers for example cellulose. In wood, it contains about 17-32% of the weight of moisture free wood depends on the species, age of wood etc; such as in soft wood average 28%, whereas 24% in hardwood. Moreover, it is not equally distributed throughout the same wood; different parts may content different percentages.

Due to highly complex chemical nature, the lignin structure is still unknown and there are few physical and chemical properties of lignin are known. Still the isolation of it’s unachievable. All the properties of lignin are determined depends on its derivatives. Nearly all of the lignin in plants is insoluble in inert solvents. The lignin structure is amorphous and the color is dark brown.

Lignin structure is not unique; depending on the species, lignin structure is composed of up to three different phenyl propane monomers such as coniferyl alcohol, syringyl alcohol and coumaryl alcohol units; coniferyl alcohol dominant in softwoods, syringyl in hardwoods and coumaryl alcohol in grasses and agricultural crops. These individual units can be composed in various probable bonding patterns. The shape is not linear whereas cellulose is linear; it is highly branched or cross-linked polymer. The molecule of it contains many reactive functional groups as like phenol propane unit, hydroxyl groups, methoxyl groups, carbonyl groups etc. These groups exhibit their characteristic reactions in suitable conditions.
monomers for lignin structure
The lignin plays many significant biological roles in plants; the various function of lignin is:

  • It is responsible for providing construction of plants as like grasses, bushes and rigid trees. In contrast, some mosses don’t contain any lignin at all.
  • The lignin in plants is less hydrophilic compare to cellulose and hemicelluloses; hence it prevents the absorption of water.
  • The lignin in plants also works as a barrier against attack by insects and fungi.
  • It transports liquids all over the plant tissues.

Commercial uses of Lignin: It can be utilized in many industries such as pulp & paper, agriculture and construction. Some uses of Lignin are described in below:

  • In sulfite pulping process lignosulfonates is an important byproduct; it contains sulfonate (-SO3-) groups, therefore it is soluble in water. lignosulfonates can be used as dispersants, binders, complexing agents and emulsifying agents.
  • It can be used as alternative raw materials for plastics.
  • Lignin in plants also burns very effectively, It can be used as bio-based alternative to petroleum.
  • The other uses of Lignin are animal feed, coatings, agricultural chemicals, micronutrients, natural binders, adhesives, resins, and in the manufacturing of vanillin and textile dyes.
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Pulp Bleaching Sequences

Posted on April 29, 2015 by admin

Globally, there are several bleaching sequences are shown in different pulp mills to achieve the bleaching action. Different chemicals are used in each stage usually with washing between two stages to removes products. The preference of bleaching sequences depends on many things such as pulp categories, environment issue, operating cost, product quality of final bleached pulp. The sulfite pulps and hardwood Kraft pulps contains lower lignin, therefore it is easier bleaching than softwood Kraft pulps. On the other hands, non wood pulps are easily bleached compare to wood pulps; as a result shorter bleaching sequences are used in here such as CEH, CEHH. The bleaching of pulp with elemental chlorine and chlorine based chemicals has become a most important universal environmental problem. They produce many hazard organic compounds (such as organic halides) during bleaching process. Therefore, many leading pulp mills modify their bleaching sequences so that it can be decreased the impact on environment. The oxygen, ozone, hydrogen peroxide and Chlorine dioxide are more environmentally bleaching agent compatible to chlorine and they reduces significantly the consumption of chlorine. Oxygen is cheaper than Chlorine dioxide and hydrogen peroxide; hence it reduces the production cost.

Bleaching sequences can be divided into two segments depends on their function such as delignification segment and brightening segment. The function of delignification agent is to remove the lignin. Normally the initial stages of a sequence are delignification segment; the oxygen and chlorination stages are included in this segment. The main function of brightening agent is to raise the brightness of the pulp. The hypochlorite, Chlorine dioxide, hydrogen peroxide etc stages are included into this segment. Usually, the most of the brightening segment chemicals are highly selective chemicals (such as Chlorine dioxide and hydrogen peroxide) and that are used in later stages when the lignin content are low and the cellulose is vulnerable to degradation.

The commonly applied chemical treatments of pulp and their symbols are bellow.
Chlorination (C): Elemental chlorine reacts with pulp in acidic medium.
Alkaline extraction (E): Dissolution of reaction product with NaOH.
Chlorine Dioxide (D): Chlorine Dioxide reacts with pulp in acidic medium.
Oxygen (O): Molecular oxygen reacts with pulp in alkaline medium at high pressure.
Hypochlorite (H): Hypochlorite reacts with pulp in alkaline medium.
Peroxide (P): Peroxide react with pulp in alkaline medium.
Ozone (Z): In this stage Ozone reacts with pulp in acidic medium.
Chelats (Q): Chelating stage
Eo – extraction stage reinforced with oxygen
Eop – extraction stage reinforced with oxygen and hydrogen peroxide
X = enzymes stage
Y = hydrosulfite stage
Paa = peracetic acid stage
Pp – pressurized hydrogen peroxide stage;
q – Semi-chelating stage (conducted in not ideal conditions);

The most common bleaching sequences (using the above symbols) that are wildly used by the pulp and paper mills are CEH (for semi bleached pulp), CEHH, CHEH, CEHEH, CCHEHH, CED, CEHD, CEHED, CEHDD, CEHDH, CEDED,CEHDED, CCHEDH, CDEODED, OCDEHD, CEHEDP, CEDPD, CEDEHD. (DC)(EPO)DED, D(EPO)DED, (D/Z)(EPO)DED, OD(EPO)D, OD(EPO)DD, OD(EPO)DP, OD(EPO)DZ, O(Z/D)(EPO)DP, (C+D)EODED, DEODED, OZEOD, YP (two stage for mechanical pulp). Among them CEH, CEHH, CHEH are used for lower brightness pulps; CEHEH, CCHEHH, CED, CEHD, CEHED are used for medium brightness pulp (up to 85% GE brightness) and CEDED, CEHDED, CCHEDH, CDEODED, OCDEHD are used for high brightness (around 90 or 90 plus). Hydrogen peroxide containing bleach stage produce high brightness pulps for example CEHDP and CEDPD produce above 90 plus percent G.E brightness pulp.

A bleaching sequence and their flow diagram are shown below:
Bleaching sequence and flow diagram

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What is bleaching of pulp

Posted on March 26, 2015 by admin

The objective of pulp bleaching is to produce white pulp. Unbleached pulps are not suitable for manufacturing of high grade of white paper. Since all the processing pulp brightness is very low, especially Kraft pulp. The sulfite pulping process produces comparatively bright chemical pulps, while the others pulping process such as kraft, soda and semichemical are produce quite dark pulp. The approximate brightness of unbleached kraft pulp range is 15-30, whereas sulfite pulp range is 50-65. In modern world most of the pulp is manufactured by Kraft pulping process. The pulp is not only made of cellulose and hemicelluloses, but it also contains some impurities which are never completely removed in the pulping process. Inherently, cellulose and hemicelluloses are white in color and don’t contribute to pulp colorization. Impurities as like lignin, colored organic matter are predominantly impart color to the pulp. Generally the chromophoric groups of lignin are responsible for this.

Two principal reactions are occurred during in the chemical bleaching of pulps. One type of reactions is to solubilization the coloring matter and almost totally removes them. The other type of reactions is to changing the coloring matters to a colorless form by selectively destroy some of the chromophoric groups which is as stable as possible to color and heat.

pulp bleaching
Oxidative mechanisms are believed to convert part of the lignin’s phenolic groups to quinone-like substance that are known to absorb light. Heavy metal ions (e.g., iron and copper) are also known to form colored complexes with the phenolic groups. Extractive materials can contribute to the color of mechanical pulps made from resinous woods.

The pulp brightness is measured by its ability to reflect monochromatic light in comparison to a known standard. To measure the pulp brightness two most widely common processes are Elrepho and ISO method. In Elrepho method the used instrument is Zeiss Elrepho reflectance meter which provides a diffuse light source and standard reflecting surface is magnesium oxide block or snow. If a surface reflect all the applied monochromatic light colors in the same proportions is said to be pure white such as pure cellulose will reflect normal light in its true proportions. Fully bleached sulfite pulps can test as high as 94% Elrepho.

The principal pulp bleaching agents are chlorine, chlorine dioxide, hypochlorite, peroxide, chlorite, oxygen and ozone.

Definition of pulp bleaching: Removal of colored substance or alter the coloring material from chemical pulp to increase its brightness and cleanliness, without deleterious effect on the physical and chemical properties of the pulp.

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Pulp screening

Posted on February 16, 2015 by admin

Screening equipment is the important operational system for pulp processing. After separation of the black liquor from the fiber in the washing section the pulp contains coarser fibers, foreign matter and dirt as like pieces of bark, digester brick, cement etc. Hence this unwanted and troublesome matter should be removed to produce first quality pulp. Pulp screening equipment is applied in this regards.

There are two categories screens are used in pulp mills; coarse screens and fine screens. These screening equipment works basis on vibrating, shaking, gravity centrifugal and centripetal forces.

The major types of coarse screen are two types sliver screens and knotters. The examples of sliver screens such as bull screen, rotary scraper, rotary oscillating and the knotters such as worm knotter, Johnson knotter screen, diaphragm screen etc.

On the other hands, the major types of fine screen are Cowan, IMPCO, diaphragm or flat screen, Quiller, bird pulp screen, vertical type of centrifugal screens, inward-flow rotary type screen, bird inward-flow screen, Jones inward flow screen, Walpole outward flow screen, APMEW outward flow, APMEW inward flow, Ahlfors Swedish screens etc.

A simplified “cascade arrangement” of screens is shown in Figure below:
Pulp Screening
Screen selection
Screen selection is a significant matter. All type of screen is not suitable for all kinds of fiber. To choice a screen it should be consider the kind of fiber, cost and repair, power consumption, efficiency, capacity, space required etc.

Various factor affects on screening performance
There are various factor that effects on screening performance such as stock consistency, type of fiber, type and size of holes, type of plate cleaning mechanism, level of coarse fiber and foreign matter, rejection rate, flow configuration, flow rate, stock temperature etc.

Water (consistency) has an important function in whole the steps in the pulp screening. In screening section, water acts as a conveyer for the fiber. The consistency of the pulp of the storage tanks may be 3 to 4 percent. When the pulp leaving the storage tank, the consistency should be reduced by adding of white water (previously used water or rewater). Depending on the nature of pulp the inlet consistency of the screening equipment should be 0.35 to 0.85 for optimum result. When the consistency comes over the 0.85 percent, then the rejections will increases. The proper consistency of the stock ought to be 0.40 to 0.50 percent depending, on the type of pulp screening.

The diameter of the pulp screening equipment holes is another important variable for screening performance. In case of coarse screen it should not be more than 3/16 ins in diameter; but it is better to keep 1/8 ins in diameter. Whereas it ought to be 0.045 to 0.0625 ins in case of fine screen. It decides the minimum size of particle that will be rejected.

After fine screening the rejected stock may around 10%; it rejects such fibers that are not too coarse. Hence it may advice to rescreen the stock with the proper dilution to better recover. It may be coarse screen. The accepted stock of fine screening is excellent quality for supply to next stage.

The coarse screening removes the heavier particles such as very coarse fibers, knots, shives, dirt and sand and it may be 4 to 5 percent of the whole stock. It may differ basis on kinds of fiber and type of screening and the holes diameter. The accepted stock of coarse screening is send to the fine screening inlet side to rescreen for greater efficiency in screening section.

Besides pulp screening equipment, the screening section may contain centi-cleaner and filter for Different paper grades required different qualities of pulps. Centi-cleaners are used to separate the fine dirt from the fiber and to deliver a clean pulp suitable for bleaching.

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Pulp Washing

Posted on January 18, 2015 by admin

There are different methods are available for pulp washing such as rotary vacuum washer, rotary pressure washer, pressure and atmospheric diffusion washers, horizontal belt washer and dilution/extraction equipment. Among them the rotary vacuum washer method is well-known to paper makers for long times. This method of pulp washing is employing a series of rotary vacuum washers working in a countercurrent flow system. Here, rotary vacuum washer is described.

Used chemical recover is the most important part for Kraft pulping process. The chemicals are separated from fiber as black liquor. Concentration of the black liquor is very significant for recovery; on the other hand pulp should be well washed. In one word, the liquor should be minimum dilution to get better evaporation benefits and better wash pulp.
pulp washing by rotary vacuum drum washer
Rotary Vacuum Washer may contain three or four single stages. These arrangements are necessary to achieve an overall satisfactory elimination of 99% of the “washable” liquor solids. Every single stage contains a rotating vacuum drum washer. The drum washer is a cylinder that covered with clothes. The clothes of washer may be made by synthetic fabric or metal wire. Washer drum rotated in a vat, which contains the mixer of pulp and liquor. When the rotating drum washer enters the mixer, then the vacuum is started and a thick layer of pulp developed on the clothes of washer. The pulp layer is disconnect by a plate (known as doctor blade) and ready for next stage. The black liquor is collected in a tank. To keep minimum dilution of the black liquor a typical countercurrent mechanism arranges.

There are many factors that affect brown stock washing displacement efficiency such as fiber characteristics, shower characteristics, sheet formation/thickness, and operating factors. Fiber characteristics involve Pulping process, Species, Stock hardness and freeness; whereas Shower Characteristics engages the method of showering application, nozzles, temperature and shower distribution. If the shower does not place at right position and does not choice right nozzles, then it may creates foam and affects on brown stock washing.

Dilution factor, air in stock, stock temperature, fabric mesh and fabric unclean are most significant operating factors. Normally, a higher dilution improves pulp washing performance. But at this time it dilute the black liquor that effects on evaporation. Hence, it is batter to keep balanced to get washing benefit achieved. In this system a specific amount of air is continuously pulled throughout the pulp sheet, it may create foaming problems and affects on pulp washing. It could be overcome by installing large seal tanks; it helps air to escape and foam bubbles to break. Sheet formation that depends on loading, Vat consistency and rotational speed also affects on pulp washing.

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Kraft pulping process

Posted on December 31, 2014 by admin

Sulfate or Kraft pulp is one of the chemical pulping processes. It is manufactured by alkaline method. Kraft pulping process is the dominating pulp manufacturing process in the world. Today this process is most widely used with a production of 70-80% of the total Pulp.

Raw material:
Although soft wood are mostly used but any type of soft or hard pulp wood can be used for Kraft pulp manufacturing. The benefits of soft wood are that cellulose fiber is thin and long.

Cooking chemicals, this is known as white liquor. The white liquor contains the active cooking chemicals NaoH and Na2S. NaoH and Na2S are used for cooking the wood chips in the digester. There is no fixed percentage of the chemicals NaoH and Na2S in the white liquor, but it is better to keeping 25-35% sulfidity (based on Total Titratable Alkali). Low percentage of sulfidity (especially below 15%) may affect on the reaction rate and pulp quality.

Black liquor is used as makeup liquor to balance the liquid requirement. The ratio of the liquor to wood might be between 3 and 5 approximately. The concentration of the liquor should not be more dilute, it may be affect on the reaction rate. Furthermore steam is used for receiving temperature and pressure. Pressure and temperature are very important for cooking process. During Kraft pulping process above 360 ⁰F temperature the strength of the fiber and yield percentage are greatly affected.

Below there is a Kraft pulping process diagram
Kraft pulping process
Description
Digester cooking process is two type; batch type and continuous type process. The capacity of the digester may be 10-20 ton pulp.

In batch type process, chips are taken a certain volume of the digester. Then a calculated amount of white liquor (contains NaOH and Na2S) or caustic soda and black liquor (if needed) are taken into the digester, so that the cooking liquor soak and cover the chips. After the digester filled with chips and liquor, then the heat is applied on contents by cooking liquor circulation method though a heat exchanger. The circulating liquor is collecting from the middle point of the vessel through a pump and deliver to the top and bottom point of the vessel. Temperature is control from 320 to 350 ⁰F. And pressure is control from 110 PSI to 150 PSI. To reach maximum temperature and pressure, it is needed from 1.5 hours to 3.0 hours. After reaching the maximum temperature and pressure then, the content is allowed to stay one to three hours to complete the cooking reaction. This time is called cooking time. The cooking time varies basis on uses purpose of the pulp. At this time the chips becomes softened. After completing the cooking time the content are ready to discharge into blow tank.

Pressure release control valve: At the top of the vessel, there is a pressure release control valve to release air and other non-condensable gases.

The main benefit of the Kraft pulping process is that used pulping chemicals can be recovered economically. About 90% of the chemicals can be recovered.

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Tensile strength of paper

Posted on November 19, 2014 by admin

What is the tensile strength of paper?
The tensile strength is the maximum stress to break a strip of paper sheet. It is one of the most important basic physical properties of paper and paperboard. The tensile strength is different basis on fiber direction. Since the fiber orientation is dissimilar between machine direction (MD) and cross direction CD), hence the tensile strength is measured in both directions. Machine direction is the direction of the paper web which is running on the machine whereas cross direction means the direction, which is perpendicular to the paper sheet that is running on the machine during paper making. It is greater in machine direction than in cross direction. It is calculated with the force per unit width and express as N/m.

The tensile strength test of paper sheet is like to the other materials test, but the method of expressing is different. For most cases the tensile strength is generally expressed in terms of load per unit cross-sectional area, whereas in paper industry it is stated in terms of load per unit of the test specimen. If the tensile strength of paper is lower, then the quality of the paper is lowered and it in need to be increased by improving different factor.

Relation of the tensile strength of paper

Tensile strength is used to find out how resistant paper is to a web break. The strength, length and bonding of fiber, degree of fiber refining and the direction of the fiber are the main sources of the tensile strength of paper. It is also depends on the quality and quantity of fillers used. It is a significant factor for many applications as like printing, converter and packaging papers.

Tensile index and its calculation
Tensile index is defined with tensile strength divided by basis weight and express as Nm/g.
Tensile Strength = N/m
Basis Weight = g/m2
Hence, Tensile index (TI) = (N/m)/(g/m2 ) = Nm/g

Tensile strength test machine

Several types of tensile strength testing apparatus are available, working on horizontally or vertically oriented specimen. There are five types of tensile strength tester apparatus used in paper industry such as rigid crosshead type, inclined plane type, hydraulic type and spring type. Among them pendulum type tensile strength test is most commonly used.

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Tear strength of paper

Posted on November 4, 2014 by admin

What is the tear strength of paper?
The tear strength of paper means the resistance of a paper sheet to tearing force that it is subjected to. It is another important basic physical property of paper and paperboard. It is measured in both machine direction (MD) & cross direction (CD) and expressed as mN (mili Newton). Machine direction means the direction of the paper web which is running on the machine during paper making. On the other hand cross direction means the direction, which is perpendicular to the paper sheet that is running on the machine during paper making.

Factor of the tear strength of paper
Tearing resistance depends on the degree of fiber refining, related to inter fiber bonding, the fiber strength, the fiber length, the quality and quantity of fillers used. Among of them fiber length and fiber bonding are most important factor. Longer fibers increase the tear strength because it is able to distribute the stress over more fibers and more bonds, whereas short fibers concentrated the stress in a smaller region. The direction of the fiber is another important parameter for tear strength. It is larger in lateral direction of the fiber than in longitudinal direction. Appropriate refining is increased the tearing strength, whereas insufficient and extra refining decreased. It is also decreased by the uses of more filler.

Applications
The tearing strength property is significant factor for many applications involving cover papers, wrapping, toughness of packaging papers, bond papers, envelope papers, printer and converter.

Tear factor and its calculation
Tear factor is calculated as tear strength per unit basis weight and expressed as mN/g/m2 or dm2 . Tearing factor = Tearing strength/grammage
If the tearing strength = x mN
basis weight = y g/m2
Then the tear factor = x/y mN* m2/g or 100*x/y dm2

Types of instruments
There are two types of instruments are used for tear strength measurement, for example Elmendorf & Trouser tear device. Elmendorf tear test are used most commonly.

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How to increase the caliper of the paper

Posted on October 22, 2014 by admin

The caliper or thickness is one of the important basic physical properties of paper and paperboard. It is depends on refined stock, category of fiber, dryer temperature, load on press section and calendars, moister content, filler etc for a given basis weight. Thickness is related to bulk or dense of paper.

Uniform caliper or thickness is very important for mother roll building up, quality of paper, printing papers, condenser paper, saturating papers etc. If the caliper variant then it could be affect some fundamental properties including the quality of the paper as like bad formation, uneven drying, strength, stiffness, bad mother roll development, optical quality etc. Furthermore cutter machine have to face problem for bad mother roll building up. Caliper is most important for printing press; print quality and press runnability is affected by caliper variation. Hence controlling the caliper is very significant for a papermaker; he is always trying to keep optimum and uniform caliper.

How to increase the caliper of the paper
Caliper is affected by refining of the fiber; well refined or beaten of stock reduce the thickness. In addition consistency in the headbox also effect on it. So a papermaker always should check out the degree SR and consistency of the stock and should be keep it at optimum level.

Press and calendars nip pressure, number of press section, number of calendars and stacks are significant for caliper. If press and calendars load is reduce then increase it. On the other hand, if it is increase than decrease the caliper. Hence it should be used optimum press and calendars load.

Caliper is also affected by ash content or filler. If ash content is reduced then increase the thickness. Using of PCC as filler increased the caliper.

Short fiber pulp such as hard wood or straw pulp can reduce the caliper. Mechanical pulp such as BCTMP increases the caliper.
If first group dryer temperature differs then the caliper also differ. It should be low to increase the caliper.

Moisture content is another parameter for caliper. It should be kept standard level.

The measuring instrument is micrometer for thickness or Caliper of paper and expressed as mili-microns. The distance is taken in perpendicular under a pressure of 1 kg/ cm2.

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Basis Weight of paper

Posted on October 15, 2014 by admin

The basis weight or grammage is basic physical property of paper and paperboard. The Basis weight of paper is calculating by the weight per unit area. It is mainly expressed as GSM (grams per square meter) and pounds per ream (500 sheets) of a specific size. It is also expressed in kg/m2, pounds per 1000 sq. ft etc. Grammage are used in non-metric countries. There are many grade of paper are found in market such as printing 80 GSM, 60 GSM, 70 GSM etc. 80 GSM means, the weight of one square meter is 80 grams.

Basis weight is very important for paper production, productivity and quality of paper. If the basis weight fluctuates then it will be hamper the quality of the paper such as bad formation, uneven drying and calendaring, calendar cutting, bad mother roll development, blackish etc. Moreover cutter machine have to face problem for bad mother roll development. So it is very important for a papermaker to controlling basis weight.

Every paper machine is designed depends on specific range of basis weight. Some paper machine has auto controlling system. On the other hand some paper machine has manual control system. You can control basis weight by speed adjustment, valve control or by slice adjustment. Papermakers always make every effort to obtain all desired properties of paper with minimum probable basis weight.

Generally papermakers check the basis weight after every half an hour during paper production. It may be weight with moisture or without moisture. Without moisture is a standard system. For this purpose the paper sheet is drying into an oven at 105ºC temperature.

Production of a paper machine is depends on speed of the machine, deckle and basis weight. If the basis weight is 80 GSM, deckle 3.00 m and the paper machine speed is 200 m/min then the production would be 80*3.00*200g per minute.

Normally ream weight is calculated for commercial purpose. It is given a specific size and it’s weight in kg or pounds for a ream. For example 23’×36′ = 36 pounds. Here 23’×36′ is a paper size and the ream weight is 36 pounds. Off course it is related to basis weight.

Explanation of the size 23’×36′ = 36 pounds:
Area of a sheet = 23’×36′ = 828 ins2 ≈ 5341.9 cm2
Area of a ream or 500 sheet = (5341.9*500) cm2 = 2670950 cm2 = 267.095 m2
Total ream weight = 36 pounds ≈ 16290 g
Hence, Basis weight of the paper = 16290/267.095 ≈ 61 g/m2

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