Coated paper – types, application in printing, characteristics, differences from offset paper

Technological process for producing coated paper. abstract. other. 2022-10-05

CONTENTS:

Introduction

Technological

coated paper making process

List

sources used

Applications

INTRODUCTION

It’s hard to imagine our life without printing

products, it is also difficult to imagine a newspaper, magazine or book without paper. Paper

meets many consumer, technological and economic requirements.

A thin, fairly strong sheet of paper has a smooth surface, whiteness

which ensures image quality. Porous structure of paper

helps paint adhere to it. With all its advantages, the paper has

disadvantages, the main of which is sensitivity to moisture, and

also heterogeneity of structure and properties. Paper is a thin sheet

material consisting of vegetable plants intertwined and fastened together

fibers [7]

Paper is a very ancient invention. Father of Paper

The Chinese Pai Lun is believed to have invented paper in 105 AD. Did

then it was like this: scraps of silk wool, rags, old fishing nets were crushed and

thrown into a vat of water, shaken until it turned out homogeneous, watery

a mushy mass that was scooped out with a bamboo net. Sediment remaining

lie in an even layer on the mesh and dry. This principle still works today,

only the means of production, scale, speed and raw materials have changed.

By the way, environmentalists found out that officials, lawyers

and financiers are breaking all records in terms of writing paper consumption. One such worker for

a year writes up (in terms of trees) – as many as 17 coniferous trunks. Also by

According to statistics in Russia, only 0.1% of paper is produced from waste paper. In Europe this

the figure reaches 50%, and in Japan even 65% of new paper is made from old paper

waste paper.

And now some information about coated paper.

Coated paper is one of the most popular grades of paper on

modern printing market. Coating is a way to improve the quality of paper

and cardboard. For the production of coated paper, a coating based on
is used

on mineral materials. The coating process significantly levels out

rough surface of the paper web, which improves its appearance

view

and is important for ensuring high quality printing.

TECHNOLOGICAL PROCESS FOR MANUFACTURING COATED COAT

PAPER

Raw materials.

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 1. A) Cellulose fibers B) Fibers

cellulose under a microscope

The basis of any paper is cellulose fibers

(Fig. 1). These fibers can be obtained from different sources – wood,

straw, hemp, rice or paper itself. Vast majority

The paper used today requires wood as a raw material (Fig. 2).

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 2. Wood

As a rule, all grades of paper contain a mixture of

hard (birch) and soft (spruce, pine) wood species. Soft varieties give

long fibers, giving the paper good strength properties, but hard

varieties produce short fibers, which improve quality

indicators.

There are also alternative methods

obtaining raw materials. For example, for making high quality paper

They use a mixture of cotton and wood fibers, or only cotton fibers.

Cotton produces very long and strong fibers that make paper

of the highest quality (Fig. 3).

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 3. A) Cotton fiber B) Plant

cotton

Semi-finished products for paper production can

serve: waste paper; semi-rag rag; semicellulose (Fig. 4). Technologies with

using paper waste allows us to obtain high quality paper, not

while affecting natural sources. Papers of this type

Widely used for newspaper production, packaging, etc. [3]

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 4. Waste paper

Production of cellulose.

At the machine factory first

rip off

from wood bark (Fig. 5, 6, 7). Cambio machines remove bark more efficiently by

compared to other machines on the market. In addition, Cambio machines

causes minimal damage to logs and removes much less fiber

wood This can be easily verified by the presence of wood in the bark after debarking.

The only manufacturer of original Cambio debarkers is Söderhamn

Eriksson AB (Sweden).

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 5. Cambio debarker

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 6. Feed rollers and cutting tool

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 7. Demonstration of cutting operation

tools

Next is a special chipping machine

grinds wood into small chips

(Fig. 8, 9).

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 8 .
Bruks drum chipper

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 9. Wood chips

A further stage of transformation of wood into

paper is the separation of cellulose fibers from the mass of wood chips. To achieve this

For this purpose, there are two methods – chemical and mechanical. These methods allow

obtain cellulose with a wide range of properties, from which it can then

paper is produced for various purposes.

The most economical way to obtain wood

pulp – mechanical

: at a woodworking enterprise

The wood chips are crushed in a refiner to crumbs, which are mixed with water. Paper,

made on the basis of such cellulose, is fragile and most often goes to

production, for example, of newspapers. [6]

This method of producing cellulose consists of a simple

supplying wood to a rotating crushing millstone (refiner) that splinters

fibers from raw materials. Percentage of raw materials used for a given

method is very high – about 98% of the raw materials are processed into pulp, but

the percentage spent on energy is also high. Among other things, this

the mass will contain not only cellulose fibers, but also non-cellulosic lignin.

Thanks to this component of plants, the pulp will have a brown color

shade [1].

Therefore, higher quality paper is made from

pulp obtained chemical

way. From such wood

pulps make paper for books, brochures and fashion magazines, as well as durable

wrapping materials.

In this case, the chips are sorted by size at

special sieves and sent to cooking. Wood is boiled in special machines,

where the acid is added (Figure 10). [6]

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 10. Digester

The peeled and boiled wood is filtered and

washed to remove impurities.

Freshly produced pulp is still

has a natural brown color. But already in this form it is suitable for

production of a number of products (for example, packaging), but for high-quality paper,

in need of high whiteness, this is not enough.

For higher quality paper

this pulp is usually subjected to multi-stage bleaching

(fig.

eleven).

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 11. Pulp bleaching steps

First stage – oxygen bleaching,

giving the pulp a light brown tint. Bright white cellulose

the mass is obtained at the last stage of bleaching (Fig. 12).

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 12. Whitening

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 13. Bleaching tower

Whitening occurs in specialized

high structures (Fig. 13).

Historically, this stage took place using

chlorine, but since chlorine is harmful to the environment, it is now widely

Non-chlorine bleaches are used. For example, whitening with hydrogen peroxide

is becoming an increasingly interesting process due to the fact that the degree

Contamination of wash water is significantly lower. This cellulose is called

chlorine-free and is used to produce “ecological” paper.

During bleaching, lignin dissolves or

is destroyed. It should be said in advance that the degree of whiteness of the paper increases

and white pigments that are added during the process, as well as colorless ones

fluorescent substances (converting ultraviolet radiation into

blue), which are called optical brighteners. They add paper

slightly bluish tint, balancing the existing slight yellowness and behind

due to this increasing the effect of whiteness. [10]

After bleaching, the pulp goes through another

one stage. At this stage, the cellulose mass is finally ground. At

When ground, the fibers are given the required thickness and physical properties. Grinding


produced in batch and continuous devices

(rolls, conical and disc mills, refiners and others). [3] Refiners

Refines fiber well without shortening, increasing the strength properties of paper,

especially resistance to tearing and fracture (Fig. 14). This mass is good

dewatered on the paper machine screen. Disc refiners have

greater power and performance, require less space for them

placement and capital costs for installation, easier and cheaper to maintain, consume

Less energy required for grinding and more economical to operate than conical mills.

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 14. Disc refiner

So, when the mass is ready, the next stage is

paper making.

Paper production

Initial stage of coated paper production

not much different from the standard papermaking process. Pulp

the mass contains approximately 5-7% cellulose and 93-95% water.[1] She’s served

pump from the mass preparation department first to the machine pool, from where

enters the paper machine screen through an inlet device. [ 10]

This is how the paper web is gradually formed.

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 15. Flat grid diagram

paper machine: 1 – machine pool; 2 – pump; 3 – permanent tank

pressure; 4 – conical mill; 5 – mixing pump; 6 – valve; 7 –

cleaning equipment; 8 – head box; 9 – mesh part; 10 – chest shaft;

11 – cable shaft; 12 – register rolls; 13 – suction boxes; 14 – decisive

roller (eguter); /5 – correct roller; 16 – press part; 17 – press

shafts; 18 – woolen cloth; 19 – drying part; 20 and 21 – drying cylinders;

22 – calender; 23 – refrigeration cylinders; 24 – roll up; 25 – longitudinally split

machine *

*Source: [8]

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 16. Simplified papermaking diagram

machines: 1 – mesh section, 2 – pressing section, 3 – drying section, 4 –

finishing section *

*Source: [7]

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 17.
Mesh section

Grid Section

is a fine mesh running between two roles (Fig. 17). Herself

mesh is woven from threads of various copper alloys or synthetic materials and

stretched on two shafts and rotates all the time, carrying the paper pulp forward.

To prevent the paper pulp from flowing down, restrictors are installed along the edges of the mesh

rulers. The formation of the paper web begins at the mesh section,

called sheet forming. As the paper pulp moves along the conveyor belt

part of the water contained in it flows out through the mesh holes, and paper

the fibers begin to intertwine with each other, forming the so-called roll

tape. Dewatering of the paper pulp and forming of the paper web occurs in

due to the free flow and suction action of the register rollers. For

obtaining a more uniform paper web longitudinally and transversely

directions, at a machine speed of no more than 300 m/min, the register part is sometimes

is subjected to shaking in the transverse direction. Further dehydration

occurs above the suction boxes under the influence of the vacuum created by

special vacuum pumps. When producing high-grade papers above them

A light leveling roller (eguter) is often installed. It also serves for

applying watermarks to paper. After this, the mesh along with the paper sheet

passes over the couch shaft, which has from one to three suction chambers.

After the mesh section, the paper contains another 80% water.

Next, this mixture enters the spin section (press section

), where from

it extracts another 10 – 15% of the liquid (Fig. 18).

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 18. Spin section

The spin section also affects the thickness of the final

sheet and the smoothness of its surface.[3] Raw already formed sheet under

under the action of pressure and vacuum passes through a number of rollers. Some rollers

squeeze out the water, others, heated from the inside by steam, dry it, others

polished.

At the end of the grid section there is still damp paper

the web is moved to the pressing section, which is also called “wet

pressing.” [6]

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 19. Drying section

After the spinning section, the paper sheet is dry to

45% goes to drying section

(longest), consisting of

a series of heated rollers (cast iron rotating drying cylinders, which

heated from the inside by steam). The paper web moves from the upper cylinder to

the bottom one, then to the adjacent top one, etc. In this case, the paper is dried to

residual moisture content 5-7%. On modern paper machines in

the second half of the drying part is usually placed with a twin-shaft size press for

surface sizing of paper and application of a surface layer. Drying part

Some paper machines are equipped with automatic feed regulators

pair into cylinders, devices for automatic threading of paper web on

drying cylinders, etc. Steam is collected under a hood located above the whole

drying part of the paper machine, and then exhausted

fans outwards (Fig. 19). At this stage the canvas is final

thickens and dehydrates.

During the manufacturing process, paper is a must

undergoes sizing

between drying sections. This is necessary in order to

to reduce the degree of absorption of the printing ink binder. For gluing

use a special substance that is applied to the surface of the paper and improves it

its printing properties. To do this, it is given hydrophobic properties: into paper

the mass is introduced with rosin glue, paraffin emulsion, alumina and others

substances that promote adhesion (so-called sizing) (Fig. 20); for

increasing the connection between fibers and increasing mechanical strength and rigidity

add starch and animal glue; to increase paper strength when wet

condition – urea and melamine-formaldehyde (urea) resins (Fig. 21).

[3] Nowadays, synthetic chemically neutral additives are increasingly used

– paper containing such additives is called neutral paper

sizing.

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 20. A) Paraffin                                     

B) Rosin

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 21. Urea resins

A distinction is made between bulk paper sizing and

superficial. In the first case, sizing agents are introduced directly into

paper pulp before casting paper on the paper machine, due to

why these substances are distributed approximately evenly throughout the entire thickness of the sheet. Vo

in the second case, ready-made paper is subjected to sizing by applying

surface (on one or both sides) of the glue. In this case, sizing

substances are distributed mainly on the surface, but the sheet thickness remains

unglued. When gluing using the second method, significant savings are achieved

sizing agents and eliminate their loss with wastewater. However for

such gluing requires special equipment, and for

There is no need for sizing in the mass. Therefore, more than 50% of paper produced and

the cardboard is glued together. Sometimes both methods are used simultaneously. [11]

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 22. Sizing presses

Procedure for adding sizing agents to

the fibrous mass is called machine sizing, and in the case when sizing

carried out at the final stage of paper production, it is called

superficial. This sizing is carried out using special sizing presses

(Fig. 22). This procedure reduces the likelihood of fibers being plucked during printing.

[1]

During hot drying of paper, resin

melts and adheres to cellulose fibers, making them water-repellent.

Therefore, sizing in the mass does not significantly reduce the porosity of the paper, but greatly

reduces fiber wettability; adhesives reduce paper porosity and

reduce its absorption capacity. [10]

Some types of paper, such as absorbent and

electrically insulating, produced without sizing and filling. Paper from

hemp pulp and rice paper are whiter than wood pulp paper, therefore

often does not require additional chemical bleaching of fibers.[3]

The next important stage of production is calendering

(finishing stage)

. This is a surface treatment of paper that allows

increase its strength, smoothness and gloss. Calendering is a passage

web of paper between rollers with an elastic or hard surface, which

produced under pressure and at high temperature. Calander presents

is a vertical battery of 5-8 metal shafts. Rotating in pairs

heated metal rollers press the paper web, dry it and shape it

smoothness (Fig. 23). The more the paper is pressed by the calender rolls, the more it

less plump and more transparent. In the production of high-quality coated

paper, a balance between the smoothness of the base paper and opacity should be maintained;

This is especially important for thin papers, which contain substances necessary

to obtain a coated coating, comparable to the cellulose fiber content.

[5]

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 23. Calender

Calendering is carried out by machine

calenders installed at the end of the drying part or on free-standing ones

supercalenders.

Supercalendered paper

called calendered or polished; paper put through machine

calender and having less gloss – with “machine smooth” paper. Smoothness

paper obtained by calendering depends on the type of roll (it is higher

when combining cast iron and paper rolls), their temperature (when heated

smoothness increases), paper composition and moisture content (paper containing more

kaolin, better calendered), as well as pressure between the calender shafts.

Calender (French calandre), press with horizontal

arranged shafts (from 2 to 20), between which material (fabric,
) is passed

paper, rubber) in the form of rolls or sheets to increase its density,

increasing smoothness, embossing a design or pattern. Calender rollers have

smooth polished surface; are usually made of metal. Required

the pressure between the shafts is achieved by the own weight of the shafts, and in some

cases and additional pressure from pneumatic or hydraulic devices.

Supercalender (from super. and calender), machine for

finishing paper, that is, increasing the smoothness of its surface, adding gloss,

seals, embossing, etc.; consists of 6-12 shafts (metal and paper),

between which a paper web is passed. Metal supercalender rollers

are made of cast iron, their surface is ground (Fig. 24).

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 24 .
A) Supercalanr                               B)

Supercalender circuit

The paper is inserted into the calender from above, between the

the upper and subsequent cylinders; therefore every surface touches

alternately cast iron and paper surface. Approximately in the middle of the calendar

there are two adjacent paper cylinders that turn the surfaces

paper: the one that initially touched the cast iron cylinders will touch the paper ones

cylinders, and vice versa.

Glossiness is imparted to hard mirror

surface of cast iron cylinders; paper cylinders only provide

counterpressure and, due to their relatively rough surface, provide

greater adhesion of the paper, because it thus slides on a smooth surface

metal cylinder. [10]

To obtain paper rolls, paper circles

pressed onto a steel core under pressure up to 45 MN/m2 (450 kgf/cm2),

then the shaft is turned and ground. Hardness of paper rolls for calendering

printing paper – 36-40 units according to the Shore method; line pressure when finishing

within 100-350 kgf/cm.

Speed ​​of the paper web in

supercalender reaches 900 m/min. To prevent paper from breaking

Bypass paper guide shafts are used, which reduces paper stress in the area

calendering. [3]

Paper machine is

the “heart” of the paper mill, since all the preparatory

the installation is designed taking into account its performance (Fig. 25, 26). On

in continuous cycle enterprises the machine operates non-stop 24 hours a day

during the whole year. It is stopped only on major holidays and on

repair; these stops should be planned in order to optimize

number of car stops. [10]

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 25. Paper machines

Length of paper making machine for production

newsprint reaches 115 m, weight about 3500 tons, height of individual parts up to

15 m, power of all electric motors (including paper preparation equipment

mass) about 30,000 kW. The drive of individual sections of the machine is carried out

DC motors. Within 1 hour, such a machine consumes up to 45 tons

pair. Automatic devices regulate the process of ebbing and drying paper at

high speeds. Highly equipped with automatic instruments, accuracy

adjustments and execution of the paper machine make it possible to reduce the number of

workers directly servicing it, up to 3-8 people. [3]

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 26. Paper machine

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 27. APP Paper Mill,

China

Now about what paper actually does

coated – about its coating, or, in other words, about layers of coating

.

In order to obtain coated paper, you need binders that create

coated coating. One of the best in quality, but also the most expensive

the filler is titanium dioxide. Manufacturers use others, more

economical fillers, for example kaolin. According to some forecasts, expected

increase in the use of calcium carbonate (chalk) as a filler as in

production of coated paper and for filling. But for now the championship

belongs to kaolin. [1]

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 28. Kaolin deposit. England,

Cornwall

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 29. Kaolin

Coated paper coating can be applied

both once and repeatedly.

Double coating, usually

provides for coating on the paper machine and on a separate

coating installation (Fig. 25) (Appendix B). However, there are also such

paper machines in which double coating is carried out by

sequential coating application. In this case, a second coating can be applied

either on the already dried first one (wet-on-dry method), or without drying

the first (wet-on-wet method). Three or more multiple coverage is obtained

for combined coating of paper on built-in papermaking machines

or on stand-alone devices. The first layer is applied to a damp base

to increase layer adhesion. It provides a kind of primer upon which

subsequent layers lay down much more evenly. Thanks to this, it increases

mechanical strength of the coating. The paper after coating has a matte surface,

however, when calendered it takes on a much lighter shine than uncoated

paper.

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 25. Coating of paper

Coating is divided into light, full and

cast. With different types of processing, not only the quantity varies

applied composition, but the nature of its penetration into the structure also changes

paper web (Fig. 27). The depth of penetration of the composition can be as follows

significant, sometimes even bordering on impregnation, and small.

First layer applied in papermaking

machine (machine coating), small in weight – about 4 g/m2. Second

the layer is larger – 20-25 g/m2 (up to 40 g/m2) – and is applied in a coating machine. At

using scraper coating, excess mixture is removed with a flexible knife (scraper) or air

jet (air scraper) (Fig. 26), and with cast coating the layer is sprayed with

using a series of nozzles, which ensures a more uniform application. Cast

Coating is an expensive process and is commonly used in the production of some

designer grades, as well as some label papers.

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 26. Scraper coating *

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 27. Degrees of coating *

*Source: [12]

Coating can also be one-sided

(label papers) and double-sided (most printing papers).

The density of the coating layer varies from 5-6g/m2 to 18g/m2 and higher.

As a rule, the larger the coating layer, the higher the quality of the printed material

surfaces.

In recent years, coating devices and

Calendering is included in the drying part of the paper machine. Main advantage

such design solutions result in significant savings

production areas. The inclusion of coating installations in the paper machine became

possible after the development of a film-type roller device, using

in which the coating paste is applied simultaneously to both sides of the paper web

while maintaining the contour profile of the paper. New coating designs

the devices have two shafts, each of which is adjacent to a dosing shaft,

rotating at a speed different from the speed of the application roller, thanks to

why shear forces arise in the film of the applied coating composition,

ensuring film stability at high speed.

Coated paper - types, application in printing, characteristics, differences from offset paper

Figure 26. Coating of paper on a Voith installation

Paper

The method of curtain
is very promising

coating

. The device for its implementation does not contain mechanical

elements for leveling the paste over the surface of the canvas. Paste is forced out

through a narrow slit in the inlet chamber and, under the influence of gravity, falls onto

fabric in the form of a curtain. The curtain changes at the point of contact with the fabric

direction of movement and stretches, forming on the surface of the paper web

a thin film that is dried with air (Fig. 26).

The disadvantage of the curtain coating method is

the fact that the coating is applied only to one side of the canvas. Cover weight

layer is regulated by the speed of movement of the paper web and curtains, as well as

dry matter content in the paste.

Paste dispensing is completed until it is

contact with the canvas, due to which the uniformity of the layer thickness does not depend on

unevenness of the surface of the base fabric. The result is high

the degree of uniformity of the web coating in the machine and transverse directions, and

absence of mechanical parts in contact with the web reduces requirements

to the strength of the paper web and reduces the number of breaks. [5]

After completion of the coating process, paper

the web is processed in a supercalender.

And finally, a smooth white ribbon comes out of the machine

and is wound into a huge roll. Most coated paper comes in

sheets, and packaged in bundles. This packaging protects the paper as much as possible

from external influences during storage and transportation. Typically paper

factories are equipped with the most modern and precise cutting equipment. However

Less, some coated paper is sold in rolls, but, as a rule, this paper

designed for gravure printing or hot-rotary offset printing

drying (HSWO). [12]

Coated paper - types, application in printing, characteristics, differences from offset paper

Coated paper - types, application in printing, characteristics, differences from offset paper

Coated paper - types, application in printing, characteristics, differences from offset paper

Appendix A

General scheme of paper production

Coated paper - types, application in printing, characteristics, differences from offset paper

Appendix B

production paper coating machine

19th century paper machine

Coated paper - types, application in printing, characteristics, differences from offset paper

Appendix B

Scheme of a separate coating installation

Coated paper - types, application in printing, characteristics, differences from offset paper

Appendix D

Leaders in the production of coated paper:

UPM, Germany:

Stora

Enso, Finland

Kotka

Mills, Finland

APP, China:

Stora

Enso, Finland *

*Source: [4].

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