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| Grammage (Basis weight) |
This parameter relates available surface area of paper or
its weight. It is important due to several reasons: |
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| Thickness (Caliper) | Since thickness of paper is affected by its moisture content, due care is to be taken regard to conditioning of paper. | ||||||||||||||||
| Apparent density / bulk | Typical values of apparent density range from 0.75(in loosely formed or less dense papers) to 1.20 for highly bonded sheets. Since density of cellulose (major paper making ingredient) is 1.5,lower value of apparent density means larger fraction of paper volume having air and vice-versa .In most papers,roughly 50% of volume is air. Apparent density affects nearly all properties of paper. The inverse of apparent densityis called as 'Bulk'. A bulky paper will be more opaque and soft. Bulky paper may cause more ink absorption during printing. | ||||||||||||||||
| Smoothness | In comparision to a perfectly flat surface, paper surface may have hills
and valleys .Presence of hills and valleys to lesser extent will
characterise a smoother surface.Thus we have papers of varying degree of
smoothness being referred as having either 'smooth' or 'rough' texture. Smoother paper will give better print reproduction but will be stiffer .Generally smoothness is achieved by calendering / supercalendering .Higher calendering might give reduced brightness and poor opacity. |
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| Porosity | Paper is porous i.e. paper has air-space in between its constituents namely fibres and fines. Consequently, density of paper is less and these spaces give way for permeability of air and liquid. | ||||||||||||||||
| Strength Properties | Strength of a material refers to the ability of material to withstand applied load. Printing / writing paper or base paper for coating experience different types and magnitude of load during converting operations and their use. It is useful to know the extent of load that these papers will be able to sustain to avoid breaks during printing or coating operations. | ||||||||||||||||
| Tensile Strength | Tensile strength is dependent upon gammage (hence thickness) of specimen. In order that tensile strength of specimens with varying grammage may be compared , the term Breaking length is used. This is defined as the length of specimen which breaks under its own weight. | ||||||||||||||||
| Tear Strength | This parameter indicates the resistance of paper to tearing action. Tear strength is defined as amount of energy consumed in continuing the tearing of a given paper. Tear factor is tear strength(gmf) per unit grammage (g/m2). | ||||||||||||||||
| Double fold | This parameter represents the resistance of paper to repeated folding /
unfolding as is experienced in case of ledger bank notes etc.It also
Indicates deterioration of paper with time. Consequently , folding
endurance is of importance for currency, ledger , map, blue print, record
papers where resistance against repeated folding / unfolding is
required alongwith longevity. When paper undergoes repeated folding , the fibres strain initially. Gradually fiber-fiber bonding starts loosening resulting into decrease of tensile strength. Consequently , folding endurance is determination of modified tensile strength. For example the paper failing after experiencing 1000 double folds , in comparision to the other failing after 100 double folds, will not have 10 times more life than weaker paper against folding / unfolding stress while in use but roughly only 1.5 times life. Thus number of folds represented log to the base 10(Log 10) gives better comparision of folding strength. |
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| Bending Stiffness |
Paper reqiure certain stiffness no matter what is its use .Copying
nachines could not function properly without proper bending stiffness.
Even when we read our daily news paper we hold the paper for which some
bending stiffness is required. Bending stiffness represents the capability of paper / board to resist the forces responsible for bending action. It also indicates the capability of paper to support its own weight while clamped in cantilever form. |
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| Optical Properties | Appearance of paper and ease in reading a paper printed / written on both sides is defined by optical properties namely brightness , opacity and gloss. Interaction of light with paper is basic for understanding as to how these properties are defined and measured in case of paper. First let us discuss paper light interaction. | ||||||||||||||||
| Light-Sheet Interaction | When light falls on any material , a part of it is reflected , another
part is absorbed and the rest is transmitted through the body. In case of
paper , for most of the applications, only the reflected and transmitted
fractions are of greater importance.
Appearance of paper depends on nature and amount of reflectance/transmittance, in following manner-
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| Brightness | A paper sheet appears brighter if it reflects (diffused) preferentially
blue colour (wave length=457nm).Pulp is bleached before making cultural
paper, so that the resulting sheet formed is bright. This is because a
sheet formed of unbleached pulp looks pale and hence is not pleasing in
appearance.The degree of bleaching is measured by the extent of blue
colour reflectance . Due to these reasons , brightness of a sheet is
measured as the diffused reflectance of blue colour from an opaque pad of
such sheets. Brightness differs from whiteness in the sense that whiteness refers to the uniformity of reflectance of different colours (wavelength).Most white papers have total reflectance ranging from 50-90%. Brightness is affected by the addition of dyestuff. A small amount of blue dyestuff is often added in furnish for making white paper. However,it does not increase brightness. |
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| Opacity | This property refers to see through characteristic of a paper sheet. Consider a
paper printed on both the sides , when you read one side of this paper,
you dread the impression of the text printed on the other side. The
quality of printing might be good but look through characteristic makes
overall bad printing. This is due to less opacity of paper and is
attributed to larger degree of diffused transmission. Thus a perfectly
opaque paper is one which is absolutely impervious to all visible light
(zero diffused transmission). Black paper used to wrap photographic films
is nearest to perfect opaque paper. However , normal book paper have
opacity of approximately 90%. Opacity increases with increase in grammage of sheet. Addition of fillers , dyestuff or pigment also increases opacity of paper. However , effect of beating ,pressing or calendering decreases the opacity. Highly bleached pulp also have less opacity. |
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| Gloss | Gloss is the attribute of surfaces which is responsible for their shiny appearance.
Thus a glossy surface will be shiny. It is associated with specular
reflection from the given surface. Thus in some surfaces , the amount of
light reflected in different directions will be same but in some ,the
reflecttion along 90o ( i = 0o) will be more than
those along other directions. The latter surfaces are termed as 'Glossy'
and the former , at the other extreme , are termed 'matt'. So in glossy
surface, there is selective reflection along a particular direction (angle
0o) over others. Gloss is related to 'luster' , the sudden selective reflection a light and to 'glare' , the desirable reflection (again in a particular direction) of excessive bright light. It is a property which is characteristic of surfaces (unlike diffuse reflection which depends upon nature of material bulk). ( i ) if we don't change the furnish used in paper making , a smooth paper will be glossier than a rough paper, ( ii ) if a paper surface has chemical whose specular reflection factor is more , the surface will have higher gloss. |
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| Sheen | For surfaces with low gloss e.g. low finished , uncoated paper 'matt' , gloss measurement is done at 85o angle and field angles . The result is usually reported as 'Sheen'. |