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Home > A to Z of Paper > Properties of Paper

Properties of Paper

Basis Weight (GSM)
Brightness, Whiteness and Colour
Bulk
Dimensional Stability
Folding Endurance (Double Folds)
Formation
Gloss
Machine and Cross Direction
Moisture
Opacity
Porosity
Sizing / Cobb
Smoothness
Stiffness
Stretch (Elongation)
Tearing Resistance
Temperature and Humidity: Conditioning of Paper
Thickness
Wax Pick No. (Surface Strength)
Wire side and Felt side








 

Basis Weight (GSM)
The weight or substance per unit area is obviously fundamental in paper and paper board products. The Basis weight of paper is the weight per unit area. This can be expressed as the weight in grams per square metre (GSM or g/M2), pounds per 1000 sq. ft. or weight in kgs or pounds per ream (500 sheets) of a specific size. REAM WEIGHT is a common term to signify the weight of a lot or batch of paper. Control of basis weight is important as all other properties are affected. Variations in moisture content in paper affects the grammage.



Brightness, Whiteness and Colour
Brightness is defined as the percentage reflectance of blue light only at a wavelength of 457 nm. Whiteness refers to the extent that paper diffusely reflects light of all wave lengths throughout the visible spectrum. Whiteness is an appearance term. Colour is an aesthetic value. Colour may appear different when viewed under a different light source. Brightness is an arbitrarily defined, but carefully standardised, blue reflectance that is used throughout the pulp and paper industry for the control of mill processes and in certain types of research and development programs. Brightness is not whiteness. However, the brightness values of the pulps and pigments going into the paper provide an excellent measure of the maximum whiteness that can be achieved with proper tinting. The colour of paper, like of other materials, depends in a complicated way on the characteristics of the observer and a number of physical factors such as the spectral energy distribution of the illuminant, the geometry of illuminating and viewing, the nature and extent of the surround and the optical characteristics of the paper itself.



Bulk
Bulk is a term used to indicate volume or thickness in relation to weight. It is the reciprocal of density (weight per unit volume). It is calculated from caliper and basis weight. Sheet bulk relates to all other sheet properties. Decrease the bulk or in other words increase the density, and the sheet gets smoother, glossier, less opaque, darker, lower in strength etc.



Dimensional Stability
An important consequence of the absorption and de-absorption of moisture by paper is the change in dimension that usually accompanies changes in moisture content. Such changes in dimension may seriously affect register in printing processes and interfere with the use of such items as tabulating cards. Uneven dimensional changes cause undesirable cockling and curling. Dimensional changes in paper originate in the swelling and contraction of the individual fibres. It has been observed that cellulosic fibres swell in diameter from 15 to 20% in passing from the dry condition to the fibre saturation point. It is impossible to be precise about the degree of this swelling because paper-making fibres differ considerably in this property, and because the irregular cross-section of fibres creates difficulty in defining diameter. Change that occurs in the dimensions of paper with variation in the moisture content is an important consideration in the use of paper. All papers expand with increased moisture content and contract with decreased moisture content, but the rate and extent of changes vary with different papers.



Folding Endurance (Double Folds)
Folding endurance is the paper's capability of withstanding multiple folds before it breaks. It is defined as the number of double folds that a strip of 15 mm wide and 100 mm length can withstand under a specified load before it breaks. It is important for printing grades where the paper is subjected to multiple folds like in books, maps, or pamphlets. Fold test is also important for carton, box boards, ammonia print paper, and cover paper etc. Folding endurance is a requirement in Bond, Ledger, Currency, Map, Blue Print and Record Papers.



Formation
Formation is an indicator of how the fibres and fillers are distributed in the sheet. Formation plays an important role as most of the paper properties depend on it. Paper that is poorly formed will have weak, thin spots and thick spots. These will affect properties like caliper, opacity, strength etc. Paper formation also affects the coating capabilities and printing characteristics of the paper.



Gloss
It is the specularly and diffusely reflected light component measurement against a known standard. Gloss is important for printing such things as magazine advertisements. The level of gloss desired is very dependent on the end use of the paper. Gloss and smoothness are different properties and are not dependent on each other.



Machine and Cross Direction
Paper has a definite grain direction due to greater orientation of fibres in the direction of travel of the paper machine. This grain direction is known as machine direction. The cross direction is the direction of paper at right angles to the machine direction. Some of the properties vary with the MD and CD and hence the values are reported in both the directions. While sheeting the paper, machine and cross direction are to be kept in mind and the sheet cutting to be done to suit the end use requirements. Examples: 1. All printing papers are to be cut in long grain (The biggest dimension in the grain direction). 2. Book papers fold better and the book stays open better if the sheets are out so that the machine direction runs up and down the pages. 3. Wrap around labels for metal cans and bottles are to be cut with the machine direction vertical to obtain greater flexibility about the can. Long grain and Short grain : The sheet is in long grain if the larger dimension is parallel to grain (MD) direction. The sheet is said to be in short grain if the larger dimension is parallel to cross direction (CD).



Moisture
Most physical properties of paper undergo change as a result of variations in moisture content. Water has the effect of plasticising the cellulose fibre and of relaxing and weakening the interfibre 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. Poor moisture control can adversely affect many paper properties.



Opacity
Opacity is the measure of how much light is kept away from passing through a sheet. A perfectly opaque paper is the one that is absolutely impervious to the passage of all visible light. It is the ratio of diffused reflectance and the reflectance of single sheet backed by a black body. Opacity is important in Printing Papers, Book Papers, etc.



Porosity
Because paper is composed of a randomly felted layer of fibre, it follows that the structure has a varying degree of porosity. Thus, the ability of fluids, both liquid and gaseous, to penetrate the structure of paper becomes a property that is both highly significant to the use of paper. Paper is a highly porous material and contains as such as 70% air. Porosity is a highly critical factor in Printing Papers Laminating Paper, Filter Paper, Cigarette Paper. Bag Paper, Antitarnish Paper and Label Paper. Porosity is the measurement of the total connecting air voids, both vertical and horizontal, that exists in a sheet. Porosity of sheet is an indication of absorptivity or the ability of the sheets to accept ink or water. Porosity can also be a factor in a vacuum feeding operation on a printing press.



Sizing / Cobb
Because paper is composed of a randomly felted layer of fibre, it's structure has a varying degree of porosity. Thus, the ability of fluids, both liquid and gaseous, to penetrate the structure of paper becomes a property that is both highly significant to the use of paper. The need to limit the spreading of ink resulted in "sizing" the paper with gelatinous vegetable materials which had the effect of sealing or filling the surface pores. Later, the term "sizing" was applied to the treatment of paper stock prior to the formation of the sheet, with water-repellent materials such as rosin or wax. Resistance towards the penetration of aqueous solution / water is measured by Sizing or Cobb values.



Smoothness
Smoothness is concerned with the surface contour of paper. It is the flatness of the surface under testing conditions which considers roughness, liveliness, and compressibility. In most of the uses of paper, the character of the surface is of great importance. It is common to say that paper has a "smooth" or a "rough" texture. The terms "finish" and "pattern" are frequently used in describing the contour or appearance of paper surfaces. Smoothness in important for writing, where it affects the ease of travel of the pen over the paper surface. Finish is important in bag paper as it is related to the tendency of the bag to slide when stacked. Smoothness of the paper will often determine whether or not it can be successfully printed. Smoothness also gives eye appeal as a rough paper is unattractive.



Stiffness
Stiffness is the measure of force required to bend a paper through a specified angle. Stiffness is an important property for box boards, corrugating medium and to certain extent for printing papers also. A limpy and flimsy paper can cause feeding and delivery problems in larger sheet presses. A sheet that is too stiff will cause problems in copier machines where it must traverse over, under, and around feed rollers. Bond papers also require certain stiffness to be flat in typewriters etc.



Stretch (Elongation)
Stretch is the amount of distortion which paper undergoes under tensile stress. Stretchlelongation is usually expressed, as percent stretch to rupture. Stretch can be related to the paper's ability to conform and maintain conformance to a particular contour, e.g. Copier paper, multicolor offset printing papers, liquids packing cartons base papers etc. It is an important property in sack kraft papers which are used for cement bags etc. Stretch is higher in cross direction than machine direction.



Tearing Resistance
Tearing resistance indicates the behaviour of paper in various end use situations; such as evaluating web runnability, controlling the quality of newsprint and characterising the toughness of packaging papers where the ability to absorb shocks is essential. fibre length and interfibre bonding are both important factors in tearing strength. The fact that longer fibres improve tear strength is well recognised. The explanation is straight forward; longer fibres tend to distribute the stress over a greater area, over more fibres and more bonds, while short fibres allow the stress to be concentrated in a smaller area.



Temperature and Humidity: Conditioning of Paper
Conditioning of paper is also of importance in many printing and converting operations. In addition to the effect of moisture content on physical properties, it also determines the build up of static of the paper sheet subjected to pressure and to friction. The tendency for paper to develop static becomes greater with increasing dryness. Cellulosic fibres are hygroscopic i.e. they are capable of absorbing water from the surrounding atmosphere. The amount of absorbed water depends on the humidity and the temperature of the air in contact with the paper. Hence, changes in temperature and humidity, even slight changes, can often affect the test results. So, it is necessary to maintain standard conditions of humidity and temperature for conditioning.



Thickness
Thickness or Caliper of paper is measured with a micrometre as the perpendicular distance between two circular, plane, parallel surfaces under a pressure of 1 kg./ CM2. Caliper is a critical measurement of uniformity. Variations in caliper, can affect several basic properties including strength, optical and roll quality. Thickness is important in filling cards, printing papers, condenser paper, saturating papers etc.



Wax Pick No. (Surface Strength)
This indicates the surface strength of the paper. This test is important for all uncoated printing papers.



Wire side and Felt side
Also referred as wire side and top side. The side which is in contact with the paper machine wire during paper manufacture is called the wire side. The other side is top side. Certain properties differ between wire and felt side and it is customary to measure these properties on both the sides. In case of paper to be printed on one side only, best results are obtained by printing on felt side. Postage stamps are printed on wire side and then gummed on felt side, where the smoothness is helpful for attaining an even application.


 

 
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