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Whiteness

Instrumental determination of the whiteness of materials is a technically complex task. There are too many influencing factors that have technical uncertainty. There is, for example, the problem of maintaining and checking standards, the stability of light sources – they all “age” and it is almost impossible to find two with the same characteristics, there is a problem of the sensitivity of measuring instruments when measuring paper of different colors, etc.

Measurements in the visible part of the spectrum are also affected by invisible, short-wave, ultraviolet radiation. Strictly speaking, to evaluate the optical characteristics of a material, it is necessary to determine its entire reflection spectrum. However, in practice it is more convenient to compare two numbers when comparing samples, which does not allow for comparison of continuous spectra.

• white
  
  ( Brightness
  
  ), as the reflectivity of waves in the range of 457 nm;
• b
  
  CIE quality
  
  ( Whitness
  
  ), calculated from chromaticity coordinates;
• CIE lightness
  
  , defined in chromaticity coordinates L
  
  , a
  
  ^*
  , b
  
  ^*
  .

Standard whiteness (Brightness) of paper is the coefficient of diffuse reflection of the surface of the paper when illuminated by a specific light source, measured at a wavelength of 457 nm.

Whiteness is measured by photometers and spectrophotometers. So whiteness is measured according to the standards of different countries and according to the international standard. When measured according to GOST 30113, which coincides with the ISO 2470 standard, whiteness can exceed 100 percent.

When measuring whiteness (as with other color measurements), it is important to specify the lighting source under which the measurements are taken. As a rule, four such sources are used: “A”, “B”, “C”, “D65”. Source “A” reproduces the conditions of average artificial lighting with incandescent electric lamps, “B” – a direct sunlight lamp, “C” – a fluorescent mercury lamp, “D65” – daylight.

Since measurements using this method are carried out in a narrow range of the spectrum (about 457 nm), and the human eye sees the entire spectrum from 400 to 700 nm, the correlation with visual assessment is not always good.

CIE whiteness (Whitness), calculated from chromaticity coordinates and color coordinates (for this, the CIE-Tint value is determined) the value gives an impression of the degree of whiteness of the sample containing OOB and tint dye elements. This gives a fairly accurate correlation with the human eye and is one of the best methods for measuring whiteness.

Disadvantages of this measurement system: 

• As an official test, it can only be used to compare samples tested with the same spectrophotometer at the same time. This is due to differences in devices and light sources;
• The sample to be measured must be sufficiently white. Newsprint, for example, gives erroneous results. The dark blue tint overestimates the CIE whiteness values.

CIE lightness, determined in conjunction with the a* and b* chromaticity coordinates. And represents the difference between black and white. For perfect white L = 100. For perfect black – 0.

As an illustration of the differences in determining the whiteness of paper depending on the method and instruments used, we present slightly processed data from a report given at the Paper Industry Technical Association (PITA) conference in Manchester in October 1997 by A. Tyndall (Clariant) ) “Production and measurement of whiteness.”

Measurements of the same paper sample were made with three spectrophotometers:

• Elrepho 2000 using a computer program from Clariant;
• Datacolor Spectraflash 500;
• Minolta CM-2002

Smoothness (roughness)

The geometry of the paper surface is characterized by its smoothness or roughness.

The “surface geometry” of paper is determined not only by micro-irregularities, but also by macro-irregularities. The former are determined by microgeometry, the latter by the distribution of mass over the area.

There is a group of most common methods in which smoothness is measured using air flow.

The most common measurement methods are the Bendtsen Sheffield and Parker apparatus (roughness). Becca (smoothness).

The essence of the Beck method is to measure the time required for air of a certain volume to pass into a vacuum chamber between the surfaces of the test sample and a polished glass plate of a certain area, pressed with a certain pressure. Smoothness is measured in seconds. The higher the smoothness, the greater the value of the indicator.

There are no strict relationships between the values ​​of smoothness (roughness) indicators measured by different methods. There is a qualitative relationship between the Beck smoothness and Bendtsen roughness values.

Bendtsen and Sheffield instruments measure the flow of air passing at constant pressure between the surface of the ring and a sheet of paper.

Bendtsen roughness is expressed in ml/min, Sheffield in Sheffield units.

The figures show qualitative relationships between parameters determined by different methods. They make it possible to evaluate the nature of changes in one parameter depending on changes in another and can help in comparing the smoothness and roughness of samples measured by different methods.

The Parker method (PPS) is used to measure the roughness of paper and cardboard under conditions similar to those of a printing press. The Parker roughness measurement result is expressed in microns.

How does paper weight affect the quality of office printing?

The weight of office printing paper depends on the printer specifications. Each printing device manufacturer indicates in its technical specifications the size and weight of the paper used. For example, the marking “Printing material format – A4, density from 64 to 163 g/m ²
” means that this printer is only suitable for paper up to A4 size, and the maximum permissible weight is 163 g/m². When you try to print on paper heavier than 163 gsm ²
, for example using whatman paper 200 g/m ²
, you can easily damage the printer’s feed mechanism.

The standard paper weight for printer and copier is 80 gsm ²
, and the format is A4 and A3. This type of office paper is suitable for printers with any printing technology. There are many brands on the market, but recently the domestic paper brand “Snegurochka” has become more popular. The quality of the paper meets international standards, and the cost, naturally, is significantly lower. The total costs of document management directly depend on the price of consumables. That is why large companies with large monthly volumes of document flow massively use Snegurochka paper, saving enormous amounts of money annually.

Each manufacturer of printing equipment produces its own brand of office paper, guaranteeing high quality prints exclusively on original products of its own production. But the high cost of printed paper from world brands has only led to attempts to use non-original products.

Main indicators of the quality of office paper Snegurochka:

• ideal sheet geometry ensures correct paper grip and eliminates the possibility of “chewing” and skewing;
• the absence of dust separation and burrs on the edges of the sheet is the key to long-term and reliable operation of the printer;
• precise cutting of sheets guarantees protection against paper gluing and jamming;
• a sufficient percentage of opacity allows the paper to be used for duplex printing;
• uniformity of paper pulp and high degree of whiteness improve print quality.

When you need to know paper weight

However, it is unlikely that you keep ultra-sensitive scales and several square meters of paper of different thicknesses at home in order to estimate how thick the booklets or leaflets you need.

Purpose of paper and cardboard

The difference between paper and cardboard also lies in their purpose.

Newspapers are printed exclusively on paper – I haven’t seen them on cardboard yet 😉 Internal sheets of multi-page products: books, brochures and magazines are printed on paper, but the cover can already be made of thick paper or cardboard.

Cardboard is used more for the manufacture of packaging products due to its greater bulk and rigidity. Boxes and boxes, sleeves and shells – everything that requires rigidity and strength.

✎ ADVICE
– for printing covers, boxes, shells, etc. Always agree on a specific sample of material, put your signatures and the printing house’s on it, and store it carefully in a safe. Believe me, sometimes this turns out to be a decisive factor in clarifying a poorly developing relationship with a performer.

Separately, we should consider cardboards used for the production of containers; here corrugated and MGC (micro-corrugated cardboard), container cardboard, boxboard and glued from flat layers (slimed) cardboard appear.

It is also necessary to separately consider special types of papers and the restrictions imposed during printing due to their specific properties.

Frequently used types of paper are delivered to the printing house on pallets:

The choice of paper types is huge. The photo below shows just a couple of catalogs (and there are dozens of such catalogs in our printing house):

FAQ ➤
Is there a difference between 200 and 300 gsm paper ²
?

There is a difference. For one manufacturer (for example, Finnish paper), thickness of glossy coated paper 200 g/m ²
is 0.147 mm, for 300 g/m ²
– 0.23 mm. Matte paper is thicker: two hundred grams – 0.185 mm, three hundred – 0.288 mm.

The thickness of the paper, its weight per square meter affects its practical use.

For example, for printing fiction and textbooks, paper from 60 to 80 g/m is used ²
, offset, printing or coated/light coated. If the publication contains many illustrations of good quality, coated paper from 90 to 150 g/m ² is used
.

For booklets, flyers, leaflets and similar products, paper from 80 to 170-200 g/m is mostly used ²
– the thicker, the more representative) lightweight papers are in demand for handouts and mailing.

Brochure covers start from economical paper options 130-150 g/m ²
and in some cases reach 300.

If the situation with European securities is clear, then Asian producers (China, South Korea) are distinguished not only by lower prices, but also by their wild imagination in terms of technical parameters. Paper similar in appearance to European paper may have a lower density, but is thicker due to its bulk and, often, you can immediately forget about printing large fills of uniform density on such papers. However, they are perfect for comics 😉

A couple more frequently asked questions.

FAQ ➤ 
Will the image and text show through on the reverse side? Should I use thicker paper?

It will definitely happen if you try hard enough. You can illuminate the reverse side with a spotlight 😉 In such cases, only binding or corrugated cardboard can save you, but do you need it? Translucency should be checked under natural conditions of use – if it is a brochure, then in a sewn brochure, and not by observing the open sun through a separate sheet.

FAQ ➤ 
How is paper density related to thickness? What other parameters are there that determine the quality of paper and cardboard?

Generally, paper thickness increases with density. But, as stated above, this is a non-linear relationship and varies greatly depending on the variety and manufacturer.

In addition to thickness and density, there is whiteness and degree of coating. The extremely important parameters are plumpness and hardness, and here manufacturers, especially friendly China, have such diversity that they themselves have difficulty understanding it. But it’s cheap! 🙂

Coloring and paper color

An object is perceived as luminous if its emission spectrum coincides (or overlaps) with the spectrum of visible radiation. Paper as an emitter can be considered when burning, when, depending on its composition, it emits in the red-yellow or even green-blue region, as well as when dyes are introduced into it that increase the emission of visible rays by the paper.

This becomes possible by treating paper with so-called optical brightening agents. These substances, absorbing the energy of the invisible ultraviolet sector of the spectrum, emit additional energy in the visible blue region, giving the paper the appearance of whiteness and brightness.

When light falls on a surface, it is reflected to one degree or another. The reflection of the surface of highly finished paper is partly specular, that is, an incident parallel beam of light remains parallel after reflection.

A perfectly white surface reflects all incident rays without absorbing anything. The gray surface uniformly absorbs light waves of different lengths. The light reflected from it does not change its spectral composition, only the intensity of the radiation changes.

All other surfaces reflect light differently at different wavelengths. Thus, red surfaces absorb waves in the green and blue regions of the spectrum, reflecting red ones. All technologies for producing color in production are based on the principle of selective absorption.

Strictly speaking, it is necessary to distinguish between such concepts as color and color of an object. Coloring is the ability of an object to reflect radiation with certain wavelengths, and color is the result of the implementation of this ability in certain lighting conditions.

Indeed, the white color of the paper can have a different color depending on the lighting. In addition, you need to distinguish between brightness and color. Brightness (lightness) is a quantitative characteristic of color that evaluates the amount of light radiation reflected by the surface.

Sampling

When sampling, it is necessary to follow the sequence of operations:

• from game
  products select units
  products;
• sheets are selected from product units
  ;
• sample sheets (samples) are selected and cut from the selected sheets
  ;
• in accordance with the requirements of standards for specific test methods, samples are cut
  for testing.

Sheets must not have wrinkles or folds and must be flat. They must be cut from undamaged sheets of products. The edges of the selected sheets must be parallel to the machine and cross direction of the paper. The sample sheets should be approximately (300 x 450) mm in size.

When handling sample sheets, care must be taken to protect them from exposure to sunlight, liquids, changes in humidity and other undesirable influences (GOST Sampling for determining average quality).

To bring test conditions into comparable conditions, paper samples are brought to certain standard conditions for humidity and temperature before testing. And the tests themselves are carried out under these conditions. This bringing of samples to standard conditions is called conditioning.

Conditioning conditions are of three types, as indicated in the table. Conditioning conditions at 50% relative humidity are more commonly used. Special conditions are used, for example, when conditioning banknote paper.

The samples are kept until they reach equilibrium moisture content, which is considered achieved if, with two successive weighings of the sample carried out after 1 hour, the last mass differs from the previous one by no more than 0.25%.

When storing and testing samples, the equilibrium humidity should not change (GOST 13523–78. Method of conditioning samples).

US Standard Paper Weight

Determining paper density by the weight of the paper package was previously widely used in the UK, Europe, as well as in North America, where it is still used. The popularity of the method in Europe fell after the international standardization of paper sizes under ISO 216 and its national predecessors in European countries.

Weight is defined as the weight of a ream of uncut paper in pounds (lb) (Note: Often, instead of pounds, reams of paper are written # after the weight number). There are many types of uncut paper that are used in various industries: for office paper and cardboard, Bond, Cover and Index are used.

So if you have a sheet of 20lb Bond Letter, it will be the same thickness as 20lb Bond Legal, although the sheets will weigh differently. But a sheet of 28lb Bond Letter sized paper will not be the same size as a 28lb Cover Letter sized paper since the uncut Bond and Cover sheets are different.

The paper most often used in offices has a weight of 20-24 pounds (lb). Often suppliers will omit the word Bond on the packaging and simply list the weight as 20 lbs or 24 lbs, since other packaging formats always write Index or Cover unless the default Bond type has been used.

Newsprint has its own type, Newsprint, which measures 24″ x 36″ at uncut sheet size. This is significantly thicker than the untrimmed Bond and Cover types, so the most common 30lb Newsprint is actually significantly thinner than the 20lb Bond type. The next page talks about the different types of stock types of uncut sheets of paper.

Tables for converting paper weight from g/m2 to lb and vice versa.

Example: difference between cardboard and paper 300 g/m2

• cardboard contains coarser and larger fibers than paper;
• cardboard with the same mass per square meter is thicker – paper is no thicker than 0.25 mm, cardboard is 0.3-0.5 mm;
• cardboard has a layered structure, the inner layers can be made from a coarse, cheaper fraction of fibers;
• cardboard is stiffer;
• paper contains more sizing and its surface is (almost always) smoother.

FAQ ➤ 
Difference between Kraft Paper and Kraft Cardboard

As mentioned above, the main difference is in the structure. Paper is homogeneous (with the exception of a possible surface coating – coating, for example), cardboard is multi-layered and/or contains thicker and longer fibers of cellulose and wood pulp. Therefore, cardboard of the same weight as paper will be much stiffer and stronger.

Both kraft paper and cardboard can be either ordinary, for mass consumption, or designer. Designer ones are distinguished by a more uniform structure and a “noble” external texture. Kraft paper and kraft cardboard are used in cases where it is necessary to ensure high strength and better moisture and grease protection of the packaging. Designer varieties are used to emphasize the unusualness of the product and the manufacturer’s commitment to protecting the environment – such an environmental fashion.

As you can see in the photo below, kraft cardboard sheets are more rigid than paper, the sheets hold their shape well and do not sag. When kraft cardboard is torn, a layered structure is visible, while the paper structure is homogeneous.

Tensile strength (tear resistance)

Tensile strength is characterized by breaking force, tensile strength, tensile strength index, breaking load, breaking length and is determined according to GOST 13525.

1 “Semi-finished fibrous products, paper, cardboard. Methods for determining tensile strength and tensile elongation”, according to ISO 1924/1 “Paper and cardboard. Determination of tensile strength.” The method consists of stretching a strip of a test sample of a certain size, the ends of which are fixed in clamps, until it breaks at a constant rate of increasing the distance between the clamps.

When determining tensile strength characteristics, strips of paper 15 mm wide, standard lengths, most often 180, 150 mm, are tested on a tensile testing machine.

Breaking force is the force required to break a strip, divided by its width.

Tensile strength represents the breaking force divided by the cross-sectional area of ​​the specimen.

The magnitude of the breaking load is the force at which the test strip breaks.

The tensile strength index is calculated as the ratio of the tensile strength to the weight of paper with an area of ​​1 m2.

Breaking length is the conventional length, expressed in meters, of a strip of paper (cardboard) that breaks under its own weight when suspended vertically at one end.

Sizes of rolls, sheets, braids

The dimensions of paper sheets (paper format) and the width of rolls are determined using a metal ruler and a metal tape measure (GOST 21102).

The width of paper and cardboard in rolls is determined by measuring the width of the sheets taken from the roll.

For sizes up to 1 m, measurements are made with a metal ruler, for sizes over 1 m – with a metal tape measure.

Measurements using a ruler and tape measure are made with an accuracy of 1 mm.

Sheet and roll paper sizes are standardized.

GOST 9327 “Paper and paper products. Consumer Sizes” defines standard sheet paper sizes.

According to GOST 29314 (ISO 478), ISO 593, the following formats are established:

• roll width: 43, 45, 64, 86, 90, 122, 128;
• uncut sheet sizes: 90 x 128;
• uncut sheet sizes: 86 x 122;
• uncut sheet sizes: 64 x 90;
• uncut sheet sizes: 61 x 86;
• uncut sheet sizes: 45 x 64;
• uncut sheet sizes: 43 x 61.

According to GOST ISO 217‑2021, the sheet format is designated by two sizes in millimeters. The format can also be supplemented by indicating the direction of cutting the sheet using the letters: LG and SG.

The first sheet size refers to the side perpendicular to the machine direction, the second size refers to the side parallel to the machine direction. Thus, the format of a sheet of longitudinally cut paper with dimensions of 430×610 mm is designated as 430×610 mm LG, and a cross-cut sheet is designated as 610×430 mm SG.

Slitting paper (LG) is a sheet of paper whose long side is parallel to the machine direction.

Cross-cut paper (SG) – a sheet of paper whose short side is parallel to the machine direction.

The above is illustrated by the figure.

Production tolerances for formats are established by agreement between trading partners.

Cutting sheets of paper, i.e. the degree of mismatch between the sides when bending sheets is determined according to GOST 21102–97.

Surface resistance to plucking

The most important indicator of the quality of offset paper is the resistance of the surface to plucking. The determination of this indicator according to GOST 24356 “Method for determining printing properties” does not have quantitative correspondence with the standards ISO 3783 “Paper and cardboard.

Determination of plucking resistance using IGT (electrical model)” and ISO 3782 “Paper and board. Determination of plucking resistance. Increasing velocity method using IGT (pendulum and spring type) apparatus.”

Among them, perhaps the most standardized instrument for determining this indicator is the instrument of IGT Testing Systems, the successor to the world famous Dutch Institute of Printing Technologies TNO – IGT.

The value of plucking resistance on this device is determined in m/s and characterizes the minimum value of the printing speed at which damage to the surface begins (violation of continuity, massive lifting of fibers or other surface elements) of the paper in the gap of the printing pair.

To have an idea of ​​the comparability of indicators, you need to know the characteristics of the resin used for printing on the proof printing machine, as well as the clamping pressure.

Instruments for assessing the printing properties of paper (they are also called proofing devices) allow for a comprehensive assessment of the printed material and printing inks – here are just a few of them: the tendency of inks to emulsify, assessment of the transparency of inks, paper roughness, the tendency of paper to absorb inks, assessment of the ability to print on a wet basis, the tendency of the paper to dust.

Here, surface durability is assessed using the Dennison number, which corresponds to the minimum number of sticks at which the integrity of the paper surface does not break when they are separated from it.

Types of cardboard packaging.

Packaging from Micro Corrugated Cardboard This material consists of three parts: two outer flat layers and a fluting (accordion) in the middle, the total thickness of the material is 1-1.5 mm. One of the outer layers is sealed. Durable, relatively inexpensive box. These boxes are mainly used for champagne, wine, glassware sets, for appliances, and for large and heavy products.

Cardboard packaging. The most common packaging. These are, for example, boxes for tea, toothpaste, sugar, cereals, cosmetics, etc. Cardboard thicknesses range from 0.3-0.5mm. On one side it is coated, smooth, shiny, on the other it is a little rough and can be white, brown, brown.

Packaging made of mouthguard, tight-fitting or otherwise made of binding cardboard, beer. The thickness of the material is 0.7-2mm solid cardboard. These are expensive boxes, very rigid, not dismountable, and are often supplemented with various supports, for example, made of polyurethane. Covered with paper by hand. Example; gift boxes, boxes for product samples, magnetic boxes, for beauty boxes, etc.

SLIM, When you need a tight box, but are not happy with the fluting inside the sheet. These are two sheets of cardboard glued together. The total thickness is approximately 1mm. Such boxes are made for gifts and very often for wine and cognac.

Small circulations. Boxes made on a flatbed plotter. Can be made from cardboard, MCG, SLIM, with or without printing. The difference from circulation ones is that you can make small quantities, so-called mini circulations of 1-100 pieces. And for reasonable money. Or make trial boxes, design development, to check the installation of products and the durability of the structure.

Characteristics of sorption properties

Being a capillary-porous colloid, paper is in an unstable dynamic interaction with the surrounding moisture, absorbing or releasing water, tending to an equilibrium moisture content under given conditions.

The sorption properties of paper in relation to oil are also important to characterize its interaction with printing inks.

The property of paper is also known as hydrophobicity, which characterizes its tendency to be wetted by water. The higher the hydrophobic properties of paper, the more difficult it is to wet with water. A characteristic of short-term hydrophobicity is the paper sizing indicator.

The degree of sizing characterizes the ability of the ink to blur when writing with a fountain pen, giving so-called “whiskers” when writing a line instead of a clear edge line. The method is not used often and is replaced in paper standards by the Cobb absorption method.

To a certain extent, at a constant mass of 1 m2, hydrophobicity is assessed by the absorption of water during unilateral wetting for 20, 60 seconds or for another time using a Cobb device (GOST 12605).

The same device can also determine the amount of oil absorption, however, it does not correlate well with the conditions of the printing process, therefore, to determine the amount of oil absorption, it is better to use tests on the IGT proof printing machine discussed earlier.

At the same time, it should be borne in mind that for materials that do not absorb oil well, the method must be modified and the time for the disappearance of the shiny trace of a drop of oil on paper to be determined.

The tendency of paper to absorb water can be characterized by its equilibrium humidity at a certain relative humidity and ambient temperature, and which is determined by the amount of weight loss of a paper sample when dried to constant humidity (GOST 13525. 19).

When intaglio printing with toluene-based inks, the indicator of paper absorption by xylene is important, which is currently used to assess the interaction of paper in general with organic solutions (GOST 12603).