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Katowice 22. - 24. 9. 2010

Drying methodsand their influence on

paper properties

Jiří Neuvirt

National Library of the Czech republic

Paper

• Papermaking fiberstextile fibers: (linen, hemp, cotton, …)

wood fibers

The main component of all is

cellulose polymer

forming long chains with hydroxyl groups capable to form hydrogen

bonds – the secret of the fiber strength and their ability to stick

together to form the paper sheet.

O-HO-H O-HO-HO-H

Paper

Dry fibers do not “stick” together –

hydrogen bonds have to be activated

For the activation we need a water environment –

the way how to do it is job for papermakers

Paper produced on paper machine

is anisotropic in the plane

MD - machine direction

CD - cross-machine direction

Mechanical properties

(tensile strength, dimension stability, rigidity, etc.)

are direction dependent

- more fibers are oriented in MD

- paper dry under tension along MD

Paper origins on sieve by dewatering the water suspension of activated

papermaking fibers.

Hand-made paper

is isotropic in the plane

Mechanical properties are

independent of direction in the

plane

CDMD

By inverse process (irrigation) paper can be destroyed

Water penetrating a paper loose inter-fiber bonds

O-HO-H O-HO-HO-H

O-H O-H O-H

O-HO-H

H-O-HH-O-H

H-O-H

H-O-H

H-O-H

Hydrogen bonds between fibers are broken and progressively substituted by hydrogen

bonds with water molecules. A similar process exist in internal structure of fibers.

A layer of water molecules serve as a lubricant.

Fibers slip instead of bonding

Dry paper

Wet paper

Water type Where Water content

depends on

Effect

Bounded

Free

In internal

structure of

fibers

In the space

between fibers

In the space

between

sheets

Relative humidity of

surrounding air and

fiber type and history

Up to 30%

Porosity, capillarity

Size degree

Time of water contact

Up to 120%

External pressure

Fiber swelling,

change of the sheet

size

MD : CD : z = 1 : 2 : 10

Loosen inter-fiber

bonds

Difficult manipulation

Water in paper

Expanded books on library shelf in the room with

100% relative humidityPhotograph of Randy Silverman

When books are submerged in vessel with water for 24 hours

they pick up different quantity of water according quality of paper

Selected books published in period

1767 - 1979

0 20 40 60 80 100 120 140 160

1767

1785

1796

1804

1827

1845

1892

1898

1899

1901

1902

1904

1906

1906

1910

1911

1922

1923

1927

1950

1966

1979

Mass increment by wetting (%)

DA

TE

(Y

ear)

Water content in booksafter 24 hour dipping

After 24 hour dipping the

books are closed, gently

squeezed, and drained

over night at 100% r.h.

Principal drying methods

Thermal methodsAccelerate water evaporation by temperature rising

Evaporated water is removed by ventilation (air

exchange)

Vacuum methodsAccelerate water evaporation by decreasing of

atmospheric pressure

Evaporated water is removed through vacuum

pump or freezing unit

Free air drying Intensive air circulation and ventilation

at room temperature

Principal drying methods

Thermal methods

Hot air 105°C

Wet air (up to 60°C, r.h. from 70 to 20%)

Microwaves

Vacuum methodsVacuum drying at t > 0°C and p > 6,1 mbar

Vacuum freeze drying at t < 0°C, p < 6,0 mbar

Vacuum packing

Free air drying

Thermal methods

Thermal methods

Hot air

In principle the row of documents is interleaved every

approx. 6 to 10 cm by aluminum plate and pressed

together. The plates facilitate heat transfer.

The row is placed in the chamber with intensive circulation

of hot (105°C) and dry air.

Documents are over dried (bone dry).

This method was used only for quick drying of documents

containing important information which is possible to

transform to digital form and paper form then thrown out

as it is without any cultural or historical value.

Thermal methods

Wet air (up to 60°C, r.h. from 70 to 20%)

Books are dried under controlled conditions (temp., r.h.)

Wood drying kiln represent this method in industrial size. We

used it after flood in 2002 for drying more than 20.000

books.

I think we were the first who used this method for drying

books. We had to solve book placement, manipulation, and

fixation

The only thing we had at beginning were 2 empty chambers

with intensive air circulation and with selectable

temperature and humidity. See the following picture.

Thermal methodsWet air (continuation)

Wood-drying kiln in Kralupy/Vlt

Thermal methodsWet air (continuation)

Wood-drying kiln interior

before we started

One of two powerful

centrifugal fan

Thermal methods

Wet air (up to 60°C, r.h. from 70 to 20%)

We had to solve:

book placement,

manipulation,

fixation

Manipulation and placement: Books are loaded into the

chamber on trolleys having two shelves

Thermal methodsWet air (continuation)

Advantage of trolleys:

- More trolleys can be

loaded at the time outside

the chamber (under the

shelter)

- The loading of the

chamber is very compact

(15 trolleys, often more

than 1000 books)

The book fixation:

Is needed to prevent deformation

and save air circulation around each book.

Books on the book trolley were stacked into columns

interleaved by doublet of unglazed ceramic tiles and loaded by

concrete block on the top.

One tile of the doublet contains glued

distance cubes creating a ventilation gap

between them.

Thermal methodsWet air (continuation)

Thermal methodsWet air (continuation)

Book columns consist of unit sandwiches.

Composition of sandwich is illustrated here

Tile doublet

Absorptive layer

Nonwoven PP

BOOK

Nonwoven PP – prevents sticking of absorptive layer on the book board

Absorptive layer (old newspapers) – prevents tiles against contamination from the book

Atmosphere

of the first batch loading

December 2002

Deformation of frozen bookthe result of neighborhood of very different book

sizes during freezing

Frozen books arriveUnpackingPreselecting:

exclusion of leather and

parchment bondsThawing frozen book

books in crates are placed in the

chamber overnight at 30 deg C and

70% r.h. -- the way to flatten

deformation of frozen books

Thermal methodsWet air (continuation)

Thermal methodsWet air (continuation)

The next day wet books are loaded into

the chamber and drying begins.

Temperature is set to 60 deg. C and

relative humidity to 70%

After 7 to 14 days drying is finished

After 2 - 3 days r.h. is set to 30%

Big formats:maps

plans on tracing paper

Instead of trolleys we use waterproof wood-board (250 x 125 cm)

The board is covered with tile-doublets

then with layer of old newspapers

layer of wet plans

layer of old newspapers

covered with tiles serving as a weight to suppress deformation

See the next 4 photos

Thermal methodsWet air (continuation)

Combined temperature and humidity sensor is placed in wet

signal book to follow the drying process in different parts of

the book.

Thermal methodsWet air (continuation)

0

20

40

60

80

100

0 1 2 3 4 5 6 7

Rel. h

um

idit

y (

%)

Time (days)

Record of r.h. in book and chamber

in book

space

in spine

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6 7

Tem

pera

ture

(oC

)

Time (days)

Record of teperature in book and chamber

in book

space

in spine

Thermal methodsWet air (continuation)

The record of sensor`s data during 7 days of drying

The principle of this method is the same as we use in kitchen

MW oven:

Liquid water has strong absorption of microwave

radiation

MW absorption cause heating of matter

The higher is water concentration in a space the more

heat is developed in that space and rises temperature

Elevated temperature promote water vaporization

(drying)

Thermal methodsMicrowaves

Advantages:

Energy is absorbed in the wet space of dried matter

Heat generate inside the wet book

Thermal methodsMicrowaves (continuation)

Disadvantages:

It is hard to get homogeneous MW field in closed

space so it is a danger of overheating (carbonization)

of some area of dried matter

My personal opinion: it is a perspective method

It is the only method able to heat material from inside

and selectively heats the wet parts.

After overcoming radiation inhomogeneity problems and

in connection with vacuum, it is to be

a useful and economic method

Thermal methodsMicrowaves (continuation)

Practical experience with this method will be presented by Dr. Ďurovič

Vacuum methods

Vacuum drying at t > 0°C and p > 6,1 mbar

Vacuum freeze drying at t < 0°C, p < 6,0 mbar

Vacuum packing

Principle:

Reduction of pressure depress the boiling point of water and facilitate

intensive water evaporation at given temperature.

Intensive evaporation cause decreasing of dried material temperature and

lowering intensity of evaporation.

To save the intensity of evaporation we are to transfer heat to dried books.

Equipment:

Drying proceeds in chamber furnished with heated shelves and connected

with vacuum pump capable to draw off pure water vapors.

Vacuum methodsVacuum drying (pressure > 6.1 mbar)

Books are stacked on the heated shelves and loaded on the top of each column to

restrain book deformation. The “success” depends on the book size variation.

Vacuum methodsVacuum drying (continuation)

load

books

heated shelve

Vacuum pump

outlet

Heat transfer from heated shelves to the book column is suppressed when the

bottom books become dry (heat conductivity fall down).

To save the drying intensity we are to increase the shelve temperature.

Bottom books are always over dried

Principle: is similar to vacuum drying but the water form at pressure

value bellow 6 mbar is ice or vapour.

Reduction of pressure depress the boiling point temperature of water under

0°C (3 mbar: -8°C). At this temperature water exist in the form of ice and

“boiling” of ice we call sublimation.

Intensive sublimation cause decreasing of dried material temperature and

we are to transfer heat to dried books to save the sublimation intensity.

Equipment:

Drying proceeds in chamber furnished with heated shelves and connected

via freezer with vacuum pump. The pump is capable to draw off water

vapors which condense in the freezer as ice.

If opening between chamber and freezer is large enough traveling of water

vapors to freezer is spontaneous and pressure in the chamber stay bellow

6.0 mbar even if vacuum pump is switched off.

Vacuum methodsVacuum freeze drying (pressure < 6.0 mbar)

Books are stacked on the heated shelves and loaded on the top of each column to

restrain book deformation. The “success” depends on the book size variation.

Vacuum methodsVacuum freeze drying (continuation)

load

books

heated shelve

Vacuum pump

outlet

Heat transfer from heated shelves to the book column is suppressed when the

bottom book become dry (heat conductivity fall down).

To save the drying intensity we are to increase the shelve temperature.

Bottom books are always over dried. Periodical change of book position helps to

overcome this effect.

Freezer

Book stacking alternative:

Vacuum methodsVacuum freeze drying (continuation)

load

books

heated shelve

Vacuum pump

outlet

Freezer

The books are firmly pressed together (bungee cord), spine down, on the shelve to

help maintain their shape during drying.

Pressure is to be controlled as the book thickness decrease

Advantage: all books have the same opportunity get heat from the shelve

Vacuum methodsVacuum freeze drying (continuation)

Exclusiveness of the vacuum freeze drying (lyophilisation) is in fact that there is

not liquid water in material so this implicate:

There is not the wick effect

The wick effect means transport of liquid water from wet

interior to the dry surface.

The wick effect cause accumulation of water soluble matter on

the book surface during drying which results in coloring and

fragility of edges.

Inter fiber bonding is not fully recovered

Ice escape and there is not environment facilitating fiber

approach after separation by ice matter

Principle:

Wet books are overlaid with nonwoven sheet and with old

newspapers and then vacuum packed in PE sack.

Water from the book move to dry newspapers by wick effect

and evaporation until equilibrium is reached.

Packing is repeated until the book is dry.

Vacuum methodsVacuum pack

PP nonwoven sheet

old newspapers

BOOK

atmospheric

pressure

Equipment:

Vacuum packing machine, PE pouches, and “manpower”

Vacuum methodsVacuum pack (continuation)

By simple calculation we can estimate theoretical number of packing operations

needed to dry the book from water content 100% to the content 6 – 7%.

Relative mass of dry newspapers (dry book = 100)

Theoretical number of packing operations

25% 12

50% 7

100% 4

150% 3

300% 2

The assumption is:WWe open the pack after equilibration of

the water content between newspapers

and book.

hThe newspapers are oven dry.

Real number is about

1 – 2 packing operations more

Interleaving book block by filter

paper accelerate equilibration

Principle:

In a low relative humidity environment (30% RH), wet books

are stood on end on tables and fanned open to stimulate

evaporation. Supports to prevent books from falling over is to

be used for very wet books or books without boards.

Equipment:

Electric fans to ensure effective air circulation

Free air drying

Good appetite

Illustration of fixed book drying

• PLATE

PLATE

Middle of the book Book margin

Thickness reduction of dried book margin release its compression

by plates – the space formed enable distortion of pages and board

Standard print wet and frozen

Water content: approx. 30%

Dried standard prints.

Left: vacuum freeze dried - OK. Right: vacuum dried - blurred

Ink stability

Paper expansion phenomenawater

•Recovery and regeneration of

interfiber bonds

•Thickness increase <15%

•Interrupted interfiber bonds

are not recovered

•Thickness increase >15%

•Loss of transparency (tracing

paper)

Dry paperWet expanded paper

Frozen more expanded paper

Freezing

Freeze drying

“wet” drying

Comparison with orig. paper

Wetting and drying size changes

Original book = 100%

100

110

120

130

140

150

Book hight Book width Book thickness Paper block

thickness

Ch

an

ge (

%)

wet dried freeze dried

Expansion of tracing paper after freeze drying

Freeze dried

rewetted and

air dried part

Freeze dried

only part

Changes of the book thicknesseffect of drying methods

ORIGINAL WET FROZEN

DRIED UP

Under

controlled

atmosphereLyophilisation

BOOK

WITH

COVER

100% 121% 133% 114% 125%

BOOK

BLOCK*)100% 118% --- 111% 123%

*) Calendered paper

0

2

4

6

8

10

12

0,0 0,5 1,0 1,5

kW

h/k

g

Average heating power input (kW)

Specific energy consumption (kWh/kg water)

Influence of heating power input

on specific energy consumption (SEC)

of vacuum freeze drying

Theoretical heat of sublimation0,787 kWh/kg

The higher the heating power input is

the lower is the specific energy

needed for evaporation of 1kg water

from the books.

„Theoretical“ value means the heat

of sublimation of ice at 0°C and

atmospheric pressure

Chamber was loaded to

maximum capacity

Mass of evaporated water

was 200 – 250 kg per batch

Results of 7 experiments with identical material – office paper in plastic pockets