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Physical Properties
Basis Weight
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.
Caliper or Thickness
For a given basis weight, thickness determines how bulky or dense paper is. A well beaten/refined pulp, short fiber pulp such as hard wood or straw pulp, highly filled or loaded paper will show lower thickness for given basis weight. Thickness or Caliper of paper is measured with a micrometer as the perpendicular distance between two circular, plane, parallel surfaces under a pressure of 1 kg./ CM2. Uniform caliper is for good roll building and subsequent printing. 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.
Formation
Formation is an indicator of how uniformly the fibers and fillers are distributed in the sheet. Formation plays an important role as most of the paper properties depend on it. A paper is as strong as its weakest point. A poorly formed sheet will have more weak and thin or thick spots. These will affect properties like caliper, opacity, strength etc. Paper formation also affects the coating capabilities and printing characteristics of the paper.
Machine and Cross Direction
In paper machine approach flow system, when stock passes through pressure screen, the fibers are oriented lengthwise. If the stock velocity from headbox slice is equal or less than wire speed, fibers which are already oriented lengthwise, will align in the direction of wire run. Fiber alignment can be altered to some extent if stock velocity is less than wire speed. So all papers have a definite grain direction due to greater orientation of fibers in the direction of paper machine run. 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. The sheet which have all relevant properties same or almost same in both direction are known as 'square sheet'.
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. E.g. 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).
There is no sure way to determine the MD or CD of a sheet but one crude method which work is; cut a strip of about 1" wide and 2" long paper and moist it. Put this moist sheet on a smooth surface or hand. As sheet will dry it will curl. The direction of curl is CD as paper contract in CD more than MD while drying.
Mechanical properties
Tensile Strength
The tensile force required to produce a rupture in a strip of paperboard, measured in MD & CD, expressed in kN/m. Tensile strength is indicative of fiber strength, fiber bonding and fiber length. Tensile strength can be used as a potential indicator of resistance to web breaking during printing or converting.
Tearing Resistance
Tearing resistance indicates the behavior of paper in various end use situations; such as evaluating web runnability, controlling the quality of newsprint and characterizing the toughness of packaging papers where the ability to absorb shocks is essential. fiber length and inter-fiber bonding are both important factors in tearing strength. The fact that longer fibers improve tear strength is well recognized. The explanation is straight forward; longer fibers tend to distribute the stress over a greater area, over more fibers and more bonds, while short fibers allow the stress to be concentrated in a smaller area.
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.
Optical properties
Brightness
Brightness may or may not add much value to the 'useful' properties of the paper but it is the most important selling feature. It is a bragging right every paper manufacturer want to have that he/she produces most bright paper.
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 arbitrarily defined, but carefully standardized, 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.
Whiteness
Whiteness is the extent to which paper diffusely reflects light of all wavelengths throughout the visible spectrum i.e. the magnitude & uniformity of spectral reflectance measured as the percent light reflectance for the whole wavelength range.
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. The opacity of paper is influenced by thickness, amount and kind of filler, degree of bleaching and coating etc.
Opacity is measured as the percentage of light absorbed by a sheet of paper. Important in book printing where both sides of paper are printed.
Gloss
It is the specularly and diffusely reflected light component measurement against a known standard. Gloss is important for magazine advertisements printing . 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.
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