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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