Method for manufacture of smooth and glossy papers and apparatus

A method and a machine are described for drying of fibrous web, especially suitable for high speed machines producing printing papers. High drying rates are obtained by subsequently pressing the two surfaces of wet web onto two large diameter dryer cylinders heated to between 100.degree. and 150.degree. C. In the first nip, the web is pressed on the dryer cylinder by a felted press roll, while and unfelted smooth roll is used to press the web on the second dryer. Drying rates obtained when practicing the invention are substantially greater than those found in conventional dryer sections. The product obtained according to this method is 30% stronger than the conventionally dried uncalendered paper, and without calendering has a smoothness and gloss similar to those of calendered conventional papers.

BACKGROUND OF THE INVENTION 
1. Field of invention 
The present invention relates to a continuous process, and apparatus for 
drying wet fibrous webs, and more particularly, it applies to the drying 
of wet printing papers such as newsprint. A new drying method is disclosed 
which, compared with the conventional method, employs a smaller number of 
dryers, leads to better machine runnability, and yields products with 
greater tensile strength, better surface properties and printing 
characteristics. This method is particularly suitable for drying wet 
cellulosic webs intended for printing products such as newsprint and bond 
paper. 
2. Description of Prior Art 
In the production of paper, a suspension of cellulosic fibres is ejected on 
an advancing forming fabric which retains a large portion of fibres and 
fine cellulosic material, and transmits a large portion of water. 
Additional water is removed from the wet cellulosic web by mechanical 
compression between two rotating press rolls. Water remaining in the 
pressed web is removed by evaporation in a dryer section of the paper 
machine. 
Two elementary processes involved in paper drying are the heat transfer 
from the heating medium into the wet web (i.e. heat transfer) and 
transportation of water vapours away from the substance (i.e. mass 
transfer). The heat transfer is proportional to the temperature difference 
between the heat source and the wet web, and inversely proportional to the 
heat transfer resistance by the boundary layers between the web and the 
source of heat. An intimate contact between the heating surface and the 
wet web is, therefore, desirable for good heat transfer. A high 
temperature of the heat source is also desirable, however, the temperature 
of the dryer is limited by some practical considerations. For example, if 
the wet web is in contact with a very hot body, an insulating layer of 
steam is formed between the heat source and the web, and the rate of heat 
transfer is reduced. The high temperature of dryers could cause a 
reduction in paper quality. High temperature operation of steam heated 
dryers, also requires elevated steam pressures. 
The evaporation of water from wet webs at temperatures below the boiling 
point of water is possible only if water vapours are carried away from the 
web by the drying air. The rate of this mass transfer is reduced if the 
web is insulated from the surrounding air by, for example, a drying 
fabric. On the other hand, mass transfer is enhanced by impingement of hot 
air on the web. 
In the most common method of paper drying, the web is passed around a 
series of internally-heated rotating cylinders known as "dryers", which 
are usually arranged in an upper and a lower row. The advancing web is 
heated by direct contact with a portion of the cylinder surface. This 
drying method is well known, and is described for example in U.S. Pat. No. 
2,299,460. To improve the web-dryer contact, the wet web is often 
sandwiched to the dryer surface by dryer fabrics. One such fabric might 
wrap a part of the surface of the upper row of dryers, while another dryer 
fabric might wrap the bottom part of the lower row of dryers. 
One disadvantage associated with this method of drying is the large number 
of cylinders required to dry the paper. For example, 1986 survey of 
Canadian newsprint dryers revealed that the majority of machines operating 
at, or above 800 m/min had between 35 and 50 dryers, with diameters of 1.5 
m or 1.8 m (N. N. Sayegh, I. I. Pikulik, and H. I. Simonsen, Pulp Paper 
Can, Dec. 1987). 
Such a large number of dryers represents extensive capital, operating and 
maintenance costs, and also contributes to the great length of the 
machine, and to a large demand for building space. Another disadvantage of 
conventional paper drying methods is the transfer of unsupported, weak, 
wet web between two consecutive cylinders. At high machine speeds, the wet 
web passing unsupported through the air is unstable and, reacting to small 
variations in the process, has a tendency to oscillate or "flutter". An 
excessive sheet flutter can cause deformations and wrinkling of the sheet, 
reducing the product quality, or completely breaking the sheet and 
interrupting production. To reduce the frequency of sheet breaks, the 
machine speed is sometimes kept low, even though this leads to a decrease 
in production. 
To reduce the problems associated with the movement of unsupported sheets 
between cylinders, on some rapidly operating paper machines, the wet paper 
proceeds around the initial drying cylinders adjacent to a single drying 
fabric. With this single felted arrangement, the paper is suported by the 
fabric as it advances between the dryers, which reduces the tendency of 
the web to flutter, and the frequecy of the breaks. 
The single felted arrangement is used primarily, but not exclusively, on 
the initial section of cylinders where the sheet is very moist and weak, 
while the open draw is utilized between the remaining drying cylinders. 
Application of a single drying fabric in a serpentine configuration is 
described for example in U.S. Pat. No. 4,172,007. 
A disadvantage of the single felted run is that only one half of dryers 
(usually those in the upper row) come into direct contact with the paper, 
while the other half of dryers are separated from the paper by the dryer 
fabric which reduces the amount of heat transferred from these dryers to 
wet web. Consequently, more dryers, or higher dryer temperatures are 
required with the serpentine arrangement of dryer felt. 
Recently, a dryer section called "total Bel Run" was described in which the 
serpentine arrangement was extended over the whole length of the dryer 
section. While in the regular serpentine run the bottom row of cylinders 
is separated from wet web by drying fabric, in the Total Bel Run, the 
bottom cylinders are replaced by small diameter vacuum rolls (Beloit 
Canada Technical Seminar, Montreal Jan. 26, 1988). Such a dryer section is 
capable of operating at high speeds, but has the disadvantage of being 
even longer than the conventional dryer section. 
Another method sometimes used for drying paper employs the "Minton Dryer" 
(U.S. Pat. No. 1,147,809) in which the drying cylinder is located within a 
large evacuated chamber. At the decreased air pressure, the boiling point 
of water is reduced, which could potentially increase the rate of heat 
transfer. The disadvantage of Minton dryers is that, in the absence of 
drying fabrics, the intimate contact between the dryer and the web is not 
established, and the rate of heat transfer is low. Another problem 
associated with the Minton Dryer is the necessity to disrupt the vacuum 
whenever a sheet break occurs. In the absence of any support for the wet 
web during the transfer between dryers, this method cannot be used on fast 
machines. 
In yet another drying method, the wet web is supported by jets of heated 
gas which provide the heat required for the evaporation of water, and 
carry away water vapours. This operation is described for example in U.S. 
Pat. No. 3,739,491. The heat transfer rates achieved with this method are 
high, and the mechanical stress on the wet web is low. However, the 
cellulosic web dried without contact with a supporting medium shrinks 
unevenly and subsequently develops undesirable deviation from polarity 
called cockle, which lowers the product quality and might lead to wrinkles 
or cuts during calendering. Difficulties with threading the dryer after a 
web break are another disadvantage of this drying method, presently used 
mainly for heavy basis weight grades or for initial drying of light basis 
weight products. 
In a different method, the web is dried entirely on a single, 
large-diameter, rotating, steam-heated cylinder known as "MG cylinder" or 
"Yankee dryer". A notable feature of web drying on a single cylinder is 
that the initial contact between the dryer surface and the web is 
established in a press nip. A metal, rubber covered press roll wrapped by 
a press felt or a fabric, presses the web onto the dryer surface by a 
force of about 30 to 80 kN/m. Upon pressing the soft wet-web fibres 
establish an intimate contact with the surface of the dryer which leads to 
a high heat transfer and drying rates. On the majority of fast modern 
Yankee machines, the drying rate is further enhanced by impingement of hot 
air on the paper adhering to the dryer surface. The jets of preheated air 
come from the so called "high velocity" hood, which surrounds a large 
portion of the Yankee dryer. Typically, about half of the drying energy is 
derived from the steam inside the Yankee dryer, while the other half is 
supplied by the hot air. 
When paper is completely dried on a single, large diameter dryer, it 
adheres strongly to the dryer surface and cannot be safely peeled off 
without breaking the sheet, especially if the basis weight of the paper is 
low. In production of creped tissue paper, the web is dried entirely on a 
single Yankee dryer, and the dry product is separated from the dryer 
surface by a creping blade. The separated paper is densely wrinkled by the 
action of the blade, and usually has from 25 to 120 crepe ridges per inch. 
Paper creped in this manner has low tensile strength, high bulk, softness 
and water absorbency, and a rough surface. These properties make creped 
paper a good material for hygienic products, but unsuitable for 
application as a printing paper. 
Heavier basis weight, often partially dried, cellulosic webs are sometimes 
also pressed to large diameter driers, called MG cylinders. These stronger 
and less adhesive webs might be peeled from the dryer surface, giving a 
product which has one side smooth and glossy, or "machine glazed" (hence 
MG cylinder). However, two sides of a product treated in this way are very 
different, namely the web side which was in contact with the dryer surface 
becomes smoother and glossier than the reverse side. Such a product is 
suitable for products such as folding boxes in which only one side is 
visible, while lower demands are placed on the board side inside the box. 
Thus Yankee drying is presently used especially for light basis weight 
hygienic or wrapping papers which are removed from the dryer by a creping 
blade, and MG cylinders are used for some paperboard products in which the 
difference in the two paper sides is desirable. 
Recently, another method of water removal was described (U.S. Pat. No. 
4,324,613) in which the cellulosic web is pressed to a cylinder heated to 
temperatures much higher than the boiling point of water, for example 
150.degree.-250.degree. C. This process, sometimes called "Impulse Drying" 
is based on the generation of high pressure steam on contact of the wet 
web with the hot dryer surface in the press nip. The front of high 
pressure steam formed at the wall of the hot dryer advances rapidly 
through the paper thickness and expels a large proportion of liquid water 
contained in the cellulosic web into the adjacent felt. Since the 
prevalent portion of water is removed in liquid form, this process is a 
special case of paper pressing, rather than paper drying. Large steam 
pressure on the boundary of the roll and the paper causes the paper to 
separate from the roll immediately upon its exit from the nip. 
Disadvantages of paper drying include product two-sidedness and, under 
certain conditions, splitting of paper to two plies by high steam pressure 
within the sheet. Generation of steam in the press nip requires a certain 
nip residence time, which might limit the usefulness of this drying method 
for high-speed machines. No commercial high speed Impulse Drying 
installation exists at the present time. 
The essential requirements of printing paper include good surface 
smoothness, identical properties of two paper sides, and resistance of the 
superficial fibres and fines to their removal by tacky ink during the 
printing process (low linting propensity). Regardless of the printing 
technique applied, the printing quality of paper improves with improving 
surface smoothness. Therefore, the smoothness of all printing papers is 
enhanced by calendering the dry paper in one or several nips formed by 
polished calender rolls. The results of calendering of paper include 
decreased roughness and increased gloss, which are desirable, and reduced 
paper thickness which is desirable only for some grades. The undesirable 
results include a decrease in the tensile, tear and burst strength of 
paper, and a reduction of the cohesion of the superficial fibres and fines 
with the rest of the web. Superficial material which was partially 
detached by the action of calender rolls, or by other means, might be 
removed during printing of paper by tacky ink and accumulates on the 
printing plates. The accumulation of this "lint" on the printing cylinder 
or on the printing blanket causes the appearance of undesirable print 
"mottle". Therefore, linting propensity is a serious defect of printing 
papers. 
Desirable properties of printing papers include low roughness, high gloss, 
large tensile ant tear strength, low linting propensity and no difference 
in the characteristics of the two sides of paper. While smoothness and 
gloss of paper can be improved by calendering, this treatment has a 
negative effect on the strength and linting propensity of paper. 
Therefore, other and more expensive methods, such as the application of 
more expensive pulps to furnish, are sometimes used to reduce the amount 
of calendering required to optimize the properties of printing papers made 
of mechanical pulps. Clearly it is desirable to develop a process which 
would produce a smoother and glossier paper, especially newsprint, without 
negatively affecting the strength and linting properties of paper. 
The equality of the surface characteristics on the two sides of paper is 
another important requirement of printing grades of paper. The effect on 
the print quality of small, but consistent deviations from the optimum 
values of the surface roughness, gloss, or fines content might be to some 
extent compensated for by modification of process parameters on the 
printing machine. However, a difference in the printing characteristics of 
the two paper sides, so called two-sidedness, results in a very noticeable 
and therefore undesirable difference in the print quality of two facing 
pages. 
The importance placed by the industry on two-sidedness has been 
demonstrated by conversion, during the last 20 years, of the majority of 
newsprint formers from fourdriniers to twin-formers. The lower 
two-sidedness of the sheet dewatered in a more symmetrical manner on a 
twin-former was the main driving force for these modifications. Paper 
proceeding through a conventional, cylinder dryer section contacts with 
its alternative sides the consecutive dryers, or series of dryers in the 
Total Bel Run arrangement. Drying through both sides has been considered 
essential to prevent the development of two-sidedness. Yankee or HG dryers 
have not been considered suitable for printing grades of paper because 
they produce creped or grossly two-sided products. 
SUMMARY OF THE INVENTION 
A method has been discovered for the continuous drying of endless 
cellulosic webs at a drying rate greater than that normally achieved on 
cylinder dryers. The paper dried according to this method is stronger, 
smoother, glossier, and has a greater surface strength than paper dried by 
conventional methods. The method is particularly suitable for drying wet 
cellulosic webs intended for printing products such as newsprint and bond 
paper. The method consists of drying a water-containing cellulosic web for 
paper on at least two heated cylinders in such a manner that one paper 
side is adjacent to the surface of the first cylinder and the other side 
is adjacent to the surface of the second cylinder. 
In accordance with the invention the water-containing cellulosic web is fed 
onto a first heated cylinder and a first side of the web is pressed 
against a smooth surface of the first heated cylinder with the first side 
contacting the smooth surface. The resulting partially dried web is 
removed from the first cylinder and fed onto a second heated cylinder and 
is pressed against a smooth surface of the second heated cylinder with a 
second side of the web contacting the smooth surface of the second 
cylinder; the second side being opposed to the first side, whereafter the 
resulting dried cellulosic web is removed from said second cylinder. The 
pressing of the partially dried web against the smooth surface of the 
second cylinder is carried out by pressing with a smooth, impermeable 
press roll applied against the web in contact with the first side. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
Suitably the first and second cylinders are large diameter cylinders, 
having a diameter of 3 to 8, usually about 6 meters, and the cylinders 
have a surface temperature of about 105.degree. to 130.degree. C., 
preferably about 118.degree. C. 
The nip load at the pressing of the web against the first and second 
cylinders is suitably 65 to 150 kN/m, preferably about 100 kN/m. 
The web suitably travels :n contact with the cylinders at the regular 
velocity of the paper machine for example 600 to 1500 m/min., and the 
method is particularly suitable for machines operating at speeds above 
1000 m/min. The drying rate achieved is between 70 and 100, typically 
about 85 kg/m.sup.2 h on the first cylinder and 15 to 30, typically about 
25 kg/m.sup.2 h on the second cylinder. 
In the first pressing stage the web is pressed against the first cylinder 
by pressing a porous, compressible substrate, for example, a felt, in 
contacting engagement with the second side of the web. 
Thus in a particular embodiment the web, previously dewatered in a 
conventional press section, is pressed onto the first large diameter dryer 
by means of a felt, backed by a press roll equipped with some superficial 
cavities to accept water escaping from the press nip. The partially dried 
web is pressed to the second large diameter cylinder by an unfelted smooth 
press roll. The large diameter dryers operate without dryer fabrics and in 
the preferred embodiment, they are equipped with high velocity hoods, 
infrared heaters or other external means of drying. Drying of webs pressed 
on the surface of a smooth dryer leads to paper with higher strength, 
gloss, and smoothness than that of the conventionally dried paper. The 
smooth unfelted press roll on the second large diameter dryer preserves 
the paper smoothness developed on the first dryer. The drying rates 
obtained by this method are 6 to 10 times greater than those achieved on 
conventional dryer sections.

DESCRIPTION OF PREFERRED EMBODIMENTS WITH REFERENCE TO THE DRAWINGS 
The drying apparatus consists of two large diameter cylinders 1, 2 and two 
press rolls 3, 4 equipped with web transfer devices (FIG. 1). Web 5, is 
carried by the press felt 6 to the nip formed by the press roll 3 and 
dryer 1. The transfer device consists of an air doctor 7, and a vacuum 
roll 8. The combined effect of the air doctor 7 and the vacuum roll 8 
transfers the web from dryer 1 to the conveyor fabric 9. The web is picked 
up by the solid smooth press roll 4 and pressed onto the second dryer 2. 
The partially or completely dried web is removed from the dryer 2 by the 
air doctor 10 and, with the assistance of the vacuum roll 11 is 
transferred on the conveyor fabric 20 which carried it to the subsequent 
machine part such as an after-dryer or a calender. 
The web 5 arrives to dryer 1 from a conventional press with a solids 
content between about 30% and 46% as it enters the nip formed by the press 
roll 3 and the dryer 1. In the press nip, the web is compressed by the 
press felt 6 to the dryer surface and an intimate contact is established 
between the web fibres and the surface of the dryers. The nip loads 
required to establish such a contact resemble those commonly used in the 
press sections of paper machines (I. I. Pikulik and I. T. Pye, Survey of 
Press Sections of Canadian Pulp, Paper and Board Machines, TS CPPA, 
Montreal, 1986). Higher nip loads, for example those above 100 kN/m, might 
lead to a better water removal in the press nip and a slightly greater 
solids content of web leaving the nip. However, we have found that high 
nip loads are not required for the achievement of high heat transfer 
rates, and therefore high drying rates. Relatively low nip loads similar 
to those often used in conventional first presses, for example 25 to 40 
kN/m are sufficient for development of an adequate drying rate on the 
first dryer. On the other hand, high nip loads enhance the development of 
high gloss and the smoothness of paper, low roughness and higher paper 
gloss are obtained when press nip loads of 40 to 150 kN/m are used. 
The surface of the dryers 1 and 2 is smoothly polished because 
characteristics of roll surface are duplicated on the paper surface and a 
low roughness is desirable for good printing properties of paper. As is 
common with MG cylinders or dryers of tissue machines, the backing press 
rolls 3 might be equipped with some cavities such as suction holes, blind 
drilled holes, or grooves, which can receive water expressed from the web 
and press felt. On the other hand, the press roll 4 must be smooth without 
any large superficial features. Roll 4 is in contact with the smooth and 
glossy side of paper which was previously glazed by dryer 1. In a 
conventional felted press nip, such as that formed between dryer 1 and 
roll 3 or between conventional press rolls, one paper side is compressed 
against the surface of the press felt. The surface of the felt is rougher 
and stiffer than that of moist paper. Compression of the smooth bottom 
side of web 5 in a conventional manner, that is by a press felt 12 as 
shown in FIG. 2 increases the roughness and decreases the gloss of this 
paper side, and largely eliminates improvements of the paper surface 
achieved on the first dryer. Therefore, a unique feature of this invention 
is the compression of the web by a smooth, impermeable roll 4 on the 
surface of heated cylinder 2. Conventional press roll covers made of hard 
rubber or other material are suitable for this roll. Loads in the nip 
formed by press roll 4 and dryer 2 will depend on the desired properties 
of the product. Higher nip loads increase the drying rate on the second 
dryer and smoothness of the top side of paper. The smoothness of the top 
paper side is also greater if the humidity of paper arriving at the second 
dryer is increased. Equal smoothness and gloss of the two paper sides can 
be achieved by adjusting dryer surface temperature, parameters of high 
velocity hoods, and nip loads of the two dryers. 
Water removal capacity of a dryer is doubled when it is equipped with a so 
called high velocity hood as indicated in FIG. 3. The jets of preheated 
air which impinge from the high velocity hood 13 on web 5 as it proceeds 
around the dryer provide the heat required for evaporation of water and 
carry away water vapour away from the vicinity of the paper. The high 
velocity hood can be used when this invention is applied to high speed 
machines, heavy basis weight printing papers, or whenever an increase in 
the drying capacity is required. The construction and operating parameters 
of the high velocity hood are not the subject of this invention. A hood 
such as that described by T. Gardner [Tappi, 47(4) 210 (1964)] or other 
efficient hoods could be employed. Alternatively, other external heat 
sources, for example infra red heat could be used to increase the drying 
rate. 
When the wet web is dried according to this invention almost all water is 
evaporated from the web which is in close contact with the smooth surface 
of dryers. Under these conditions the web cannot preferentially shrink in 
certain areas and develop deviation from planarity. Therefore paper dried 
to a solids content above about 80% on two large diameter driers does not 
develop the undesirable small scale deviation from planarity or "cockle". 
Dryers 1 and 2 can be heated by various means, such as internally by 
compressed steam or direct flame, electrical induction, infrared radiation 
or externally e.g. by electrical induction or other means. The majority of 
Yankee dryers and MG cylinders currently used, are heated internally by 
medium pressure steam, and similar techniques are also applicable to the 
present invention. The optimum temperature of the dryer surface will 
depend on the properties of the web. If the dryer temperature is too high 
and the web has a low permeability to water vapours, a layer of 
pressurized steam could develop between the dryer surface and the paper in 
a manner similar to that used in impulse drying. At high machine speeds 
and short nip residence time this steam would not displace liquid water as 
it occurs during impulse drying, but it could partially separate the web 
from the dryer wall, reduce the area of intimate contact between the dryer 
and the fibres. This would result in a lower heat transfer rate and a 
slower rate of drying. Too low a temperature at the drier surface would 
result in a low temperature difference between paper and dryer, and 
therefore a low heat transfer rate, and a low rate of drying. The dryer 
surface temperature might range from about 100.degree. C. to about 
170.degree. C., and temperatures in the range of 108.degree. C. to 
140.degree. C. were found to be especially convenient for the drying of 
newsprint. 
The amount of water removal on the dryers described in this invention 
depends on the web-dryer contact time which, in turn, depends on the 
diameter of the dryers and on machine speed. The diameter of the dryers is 
determined by the basis weight of products, moisture content in the 
pressed web, and machine speed. Dryers with large diameters can provide 
the residence time required for complete drying of low basis weight 
printing grades. However, various practical reasons restrict the size of 
dryers. For example, modern machines producing creped papers usually 
employ a single dryer with a diameter of about 6 to 7 m. 
The best fibre-cylinder contact, and consequently the highest drying rate 
is developed when the web is pressed to the dryer surface at a low solids 
content, because cellulosic fibres swollen with water are maleable and 
conform to the surface of the dryer. Therefore the highest drying rates 
are obtained when a web is pressed to, and completely dried on a single 
cylinder. However, the drying capacity of a single cylinder is too low for 
a fast paper machine. Also, paper dried on a single dryer adheres too 
strongly to its surface, is difficult to peel from the dryer, and is very 
two-sided. To avoid these problems, paper has to be dried on at least two 
cylinders. In the preferred embodiment of the invention, the web is 
pressed on the first dryer cylinder with the solids content similar to 
that obtained at the end of a conventional press section, namely 35% to 
50%. The web is removed from the first cylinder at a solids content of 55% 
to 75% and pressed to the second dryer. At this higher solids content, the 
cellulosic fibres are less swollen, more rigid and a less perfect contact 
with the surface of the second dryer will be established upon pressing. 
For this and other reasons, the rate of drying is lower on the second 
dryer than on the first dryer. 
In the preferred embodiment, paper is dried to its final solids content, 
namely about 90%, on just two large diameter dryers, as this method of 
drying provides a high drying rate, good paper surface smoothness and 
gloss and improved paper strength. A smaller, but still substantial 
improvement in the paper smoothness and gloss can be obtained when paper 
is pressed to a dryer with a smaller diameter. Paper partially dried in 
this manner can then be dried further by other methods. The two dryers 
equipped with presser rolls could be installed within conventional 
cylinder dryer section to improve the surface properties of the product. A 
scheme of a conventional dryer section is shown in FIG. 4. A conventional 
dryer section modified by installation of two press roll equipped dryers 
is shown in FIG. 5. The dryer section modified as shown in FIG. 5 provides 
an increased rate of drying only on two dryers, 14 and 15, to which paper 
is pressed by press rolls 16. Additional dryer cylinders can be equipped 
with press rolls if further increase in the drying capacity and improvemnt 
in the paper surface properties is desirable. However, press rolls acting 
on the wet webs on the initial dryers where the web solids content is too 
low, must be felted to provide a route for escape of water removed from 
paper in the press nip, and press rolls acting on dryer webs (for example 
with solids content of 57% or greater), should be unfelted and smooth to 
prevent marking of the sheet. Paper could be pressed to any dryer 
cylinder, however, as the web solids content increases, beneficial effects 
of web pressing on dryers declines. 
The combined effect of high heat transfer rate from the dryer and 
impingement of hot air from a high velocity hood produce a high drying 
rate. For example, drying rates of 160 kg water per m.sup.2 per hour are 
achieved at dryers of tissue machines. In our trials, described below, 
drying rates of about 90 kg water per m.sup.2 per hour were obtained when 
drying newsprint, even without the assistance of air impingement. In 
comparison, the average drying rate on conventional newsprint drying 
cylinders is only about 15 kg water per m.sup.2 per hour (TAPPI Technical 
Information Sheet 0404-15, revised in 1986). 
In the preferred embodiment, the web is dried on two dryers with diameters 
of about 5-7 m. Two large diameter dryers, even if equipped with high 
velocity hoods, might not be able to entirely dry the web on very rapid 
machines, especially if they produce products with a higher basis weight. 
If more drying capacity is required, additional dryers can be used to 
complete drying of the web after the second dryer. In this configuration, 
the web pressed onto the dryer surface has a relatively low solids 
content, as required for the development of good paper surface properties. 
Once high gloss and smoothness are achieved in the paper, after drying on 
the two consecutive large dryers to a solids content of 70% or more, these 
desirable properties are retained in paper which was subsequently dried by 
other means. This additional drying can be accomplished on conventional 
drying cylinders, or by other technique. For example, the excessive 
moisture which remains in the web leaving the second dryer could be 
evaporated on a dryer which combines the air impingement with an air 
passage through the sheet (U.S. Pat. No. 3,248,798), and which is known as 
Papridryer. 
The following are some examples of experiments made using the process of 
this invention: 
EXAMPLE 1 
Showing that paper pressed to just one dryer is stronger, and has one 
smooth and one glossy side. 
Newsprint sheet with a basis weight of 50 g/m.sup.2 was prepared on a pilot 
paper machine equipped with a twin former and operating at 800 m/min, from 
a furnish composed of 18% softwood kraft pulp and 82% stone groundwood 
pulp. The sheet was pressed on a paper machine in two press nips loaded to 
45 and 90 kN/m respectively, and reeled at a solids content close to 40%. 
Further treatment of the sheet was carried out on the pilot drying machine 
shown in FIG. 6. The wet paper was unwound from the reel 18 and carried at 
a speed of 100 m/min by the press felt 6 into a nip formed by the press 
roll 3 and dryer roll 1. The dryer roll was heated externally by 
electrical induction. The press nip load was 100 kN/m and the roll 
temperature was about 125.degree. C. The diameter of the press roll 3 was 
0.76 m and that of the dryer 1 was 0.88 m. The residence time of paper on 
the dryer roll was about 1.60 s which corresponds to a residence time on a 
dryer with a diameter of 6 m, operating at 700 m/min. The solids content 
of the sheet at reel 18 was 39.2% and of that at reel 19 was 61.2%. This 
corresponds to a drying rate of 104 kg water per m.sup.2 of dryer per 
hour. 
In a control experiment, a similar sheet was pressed under similar 
conditions, however, the dryer 1 was not heated. Samples from both 
experiments were completely dried on a rotary photographic dryer while 
sandwiched between two blotters, conditioned overnight at 25.degree. C. 
and 50% relative humidity and tested. Some physical and surface properties 
of both samples are presented in Table I. 
TABLE I 
__________________________________________________________________________ 
Selected Properties of a Newsprint Sample Dried According to This 
Invention and of a Control Sample 
Smoothness PPS-S10 (.mu.m) 
Hunter Gloss 75% (%) 
MD Breaking 
Tensile Energy 
Tear Index 
Sample Top Side 
Bottom Side 
Top Side 
Bottom Side 
Length (km) 
Absorption (mJ/g) 
(mNm.sup.2 /g) 
__________________________________________________________________________ 
Control 
6.4 6.8 5.6 5.0 3.5 245 6.3 
Paper 3.5 7.6 19.2 4.6 4.8 397 6.2 
treated on 
one side only 
__________________________________________________________________________ 
Surface properties measured on both sides of the control sample which was 
pressed on a cold dryer roll, namely PPS-S10 of about 7 m and Hunter gloss 
close to 6% are typical for conventionally-made uncalendered newsprint. 
The roughness and gloss of the bottom side of paper which was pressed 
against the felt and then proceeded around the heated press roll, were 
similar to those measured for the control sample. On the other hand, the 
top side of the sample which was directly adjacent to the dryer surface 
was much smoother and glossier than that of the control sample, or of 
conventionally dried newsprint paper. 
Although the properties listed in Table I were measured on uncalendered 
paper, the PPS-S10 roughness of 3.5 and Hunter gloss of 19% found on the 
top side of newsprint dried according to this invention are similar to 
those usually obtained only on fully-calendered newsprint. This indicates 
that paper made according to this invention requires either substantially 
less calendering than the conventional paper or no calendering at all. 
Since about 30% of paper tensile strength is normally lost during 
calendering, paper made according to this invention can retain more of its 
original strength. A commonly used criterion of paper strength is the 
breaking length, which is the length of the paper strip at which it would 
break by its own weight. Using this criterion, the sample dried according 
to this invention was 37% stronger than the control sample. The tensile 
energy absorption (TEA), is a reflection of both the tensile strength and 
stretch of paper. Paper prepared according to the invention had TEA 38% 
greater than the control sample. While an increase of the tensile strength 
by conventional methods, such as by refining or by addition of strength 
chemicals, is often accompanied by a decline of the tear strength, this 
negative effect did not occur when the invention was applied. 
EXAMPLE 2 
Showing that smoothness developed on the first dryer is destroyed by 
pressing paper with a felt to the second dryer. 
Wet newsprint web was prepared as described in Example 1, and treated on 
the pilot dryer machine shown in FIG. 6. Paper was unwound from reel 18, 
passed through the press nip and over the dryer 1 and collected on reel 
19. A sample of paper treated in this manner was removed, and reel 19 was 
relocated to position 18. The sheet was then passed again through the nip 
and over the dryer in such a manner that the paper side that had faced the 
felt during the first pass faced the dryer on the second pass. Solids 
content of the initial paper was 43.8%, after the first pass 76.3%, and 
after the second pass 80.3%. Some parameters of operation and paper test 
results from this experiment are shown in Table II. 
Data in Table II indicate that on the first pass through the drying 
machine, a substantial improvement of roughness and gloss occurred on the 
top sheet surface, which was in contact with the dryer roll. The bottom 
side of the sheet which, during the first pass was pressed against the 
felt, had its roughness unchanged, and its gloss improved only marginally 
when compared with the control sheet. 
TABLE II 
__________________________________________________________________________ 
Some Operation Parameters and Paper Properties. 
Dryer Temp. 
Nip Load 
Drying Rate 
Roughness PPS-S10 (.mu.m) 
Hunter gloss 75.degree. (%) 
(.degree.C.) 
(kN/m) 
(kg/m.sup.2 h) 
Top Bottom Top Bottom 
__________________________________________________________________________ 
Starting sheet 6.9 7.2 6.2 6.8 
First pass 
121 150 85 4.1 7.2 10.8 9.8 
Second Pass 
100 150 25 5.8 3.6 6.9 16.5 
pressed by a felt 
__________________________________________________________________________ 
During the second pass, the bottom side of the paper was pressed against 
the dryer surface. As shown in Table II, this resulted in a dramatic 
decrease in roughness, which dropped to 3.6 m, and improvement in gloss, 
which increased to 16.5%. Similar values of roughness and gloss are 
usually found on fully-calendered newsprint paper. In contrast with this, 
surface characteristics of the bottom side of the paper deteriorated 
during the second pass. Subsequent to a compression by the press felt in 
the second nip formed by the press roll and the dryer, the gloss and 
roughness of the paper top side became similar to that found on 
uncalendered conventional newsprint. 
This example demonstrates that a substantial improvement of the bottom 
paper surface can be achieved, even on the second dryer onto which the web 
is pressed by means of a felted roll as it is practiced in conventional MG 
or Yankee drying. However, such an operation destroys the smoothness of 
the paper's top side. Furthermore, the experiment indicates that a 
substantially higher drying rate, namely 80.5 kg/m.sup.2 h is achieved on 
web which was pressed onto the dryer at a low solids content of 43.8% than 
the drying rate (25.3 kg/m.sup.2 h) achieved when the same web is 
introduced at a higher starting web solids content of 67.3%. It is 
therefore desirable to dry paper on just two, large diameter dryers as 
this allows operation with the highest incoming web solids contents. In 
this experiment, the dryer residence time of paper was similar to that 
achieved on a 6 m diameter dryer operating at 700 m/min. Therefore, this 
result indicates that two dryers on a commercial machine could increase 
the solids content of newsprint from about 44% to about 80 %, even when 
they are not equipped with air impingement hoods. 
EXAMPLE 3 
Showing improvements in smoothness, strength and printing quality of paper 
dried according to this invention. 
The newsprint web was prepared as described in Example 1, and treated on 
the pilot dryer machine shown in FIG. 6, equipped with a smooth, hard 
press roll 3. Paper was unwound from reel 18 in the press nip, was 
compressed between the felt 6 and the dryer 1, proceeded around the dryer 
1, and was collected on the reel 19. The reel 19 with the partially dried 
and was collected on the reel 19. The reel 19 with the partially dried 
paper was then relocated to the position 18 and the felt 6 was removed. 
The sheet was passed again through the nip formed by the smooth roll 3 and 
dryer 1, proceeded over the dryer in such a manner that the paper side 
that contacted the felt during the first pass faced the dryer during the 
second pass, while the reverse side faced the hard and smooth press roll. 
This procedure simulated the apparatus shown in FIG. 1. The solids content 
of the original paper was 37.9%, after the first pass 66.3%, and after the 
second pass 74.8%. Samples were removed from the original paper and paper 
treated according to this invention, and both were dried sandwiched 
between blotters on a photographic dryer. 
Some physical properties of the paper prepared according to this invention 
are in Table III. Compared with those of the control sample, the sample 
prepared according to the invention had almost 50% greater breaking length 
and internal bond, and had more than twice the tensile energy absorption. 
The control sample had somewhat greater tear index and scattering 
coeffient and both samples have similar opacity. This example demonstrates 
the surprising improvement in the strength and smoothness of newsprint 
dried according to this invention, and also indicates that the difference 
in the smoothness of two paper sides created on the first dryer can be 
eliminated on the second dryer. 
TABLE III 
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Properties of Newsprint Dried on Two Consecutive Dryers According to this 
Invention 
Parker Print Smoothness .mu.m 
Breaking Length, MD 
Stretch MD 
TEA* 
Tear Index 
Internal Bond, 
TAPPI 
Sample 
Top Bottom km % mJ/g 
mNm.sup.2 /g 
kJ/m.sup.2 
Opacity 
__________________________________________________________________________ 
% 
Control 
8.0 7.6 4.1 1.0 11.0 
4.4 157 91.4 
Invention 
4.5 4.8 6.1 1.4 25.7 
3.6 307 91.0 
__________________________________________________________________________ 
*Tensile Energy Absorption 
Paper characterized in Table III was pressed at 100 kN/m and dried on a 
cylinder heated to about 118.degree. C. Other paper samples which were 
pressed at lower nip loads such as 20 or 60 kN/m or dried at a higher 
temperature, such as 145.degree. C., were weaker and rougher, indicating 
that the optimum operating conditions are close to 118.degree. C. and 100 
kN/m. Compared with conventionally pressed paper, the paper described in 
Table III develops the required bulk and smoothness with less calendering. 
An important quality criterion of printing paper grades is the print 
density index, which is a measure of the surface darkness achieved by a 
certain quantity of ink. Table IV indicates that, although the paper 
prepared according to this invention was only lightly calendered, it had a 
print density index similar to that of heavily calendered conventional 
paper. 
TABLE IV 
__________________________________________________________________________ 
Printing Properties of Paper Dried According to this Invention 
Drying Calender nip 
Smoothness (PPS, .mu.m) 
Print density index (m.sup.2 /g) 
Bulk 
Conditions 
loads (kN/m) 
Top Bottom 
Top Bottom (cm.sup.3 /g) 
__________________________________________________________________________ 
Conventional 
20 + 40 + 60 
2.98 2.88 0.42 0.29 1.48 
Invention 
60 kN/m, 120.degree. C. 
20 + 40 
2.95 2.86 0.39 0.39 1.46 
100 kN/m, 120.degree. C. 
20 3.24 3.28 0.36 0.41 1.39 
__________________________________________________________________________ 
EXAMPLE 4 
Showing web solids contents which can be obtained on two large diameter 
dryers equipped with press rolls. 
At a specified drying rate, the amount of water removed from a web pressed 
onto a heated cylinder depends on the web residence time. The average 
drying rates obtained in several experiments similar to those described in 
Examples 2 and 3 were 88 kg/m.sup.2 h for the first pass and 23 kg/m.sup.2 
h for the second pass. If it is assumed that similar drying rates could be 
obtained on a large diameter industrial dryer, the solids content 
obtainable at various machine speeds could be calculated. Table V contains 
solids contents calculated for a newsprint sheet with a basis weight of 50 
g/m.sup.2 previously pressed to a solids content of 45% and dried on two 6 
m diameter driers, the perimeter of each of which is wrapped by the web, 
for a distance of 17 m. Drying rates of 88 kg/m.sup.2 h and 23 kg/m.sup.2 
h are assumed for the two passes. 
TABLE V 
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Calculated Solids Content of Newsprint After the First and Second Dryer, 
at Three Machine Speeds for Two Dryers Without Hoods. 
Speed Dryer Residence 
Drying Rate kg/m.sup.2 h 
Web Solids Content (%) 
(m/min) Time (s) 1st Dryer 
2nd Dryer After 1st Dryer 
After 2nd 
__________________________________________________________________________ 
Dryer 
700 1.46 88 23 66 76 
1000 1.02 88 23 58 62 
1300 1.78 88 23 54 57 
__________________________________________________________________________ 
Experience with Yankee drying of tissue indicates the drying rate can be 
doubled by impingement of the hot air from high velocity hood on the web 
proceeding on the surface of a heated drying cylinder. Assuming that the 
drying rates obtained on our pilot drying machine could be doubled by 
impingement of hot air as indicated in FIG. 3, then drying rates obtained 
on the first and second dryers would be 176 and 46 kg/m.sup.2 h. Table VI 
presents solids contents calculated for the same conditions as those 
described in Table V, but assuming that both dryers employ a high velocity 
hood. 
TABLE VI 
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Solids Content Calculated for Dryers Equipped with High Velocity Hoods. 
All Other Conditions Are Similar to Those Assumed for Data in Table IV 
Speed 
Dryer Residence 
Drying Rate kg/m.sup.2 h 
Web Solids Content (%) 
(m/min) 
Time (s) 1st Dryer 
2nd Dryer 
After 1st Dryer 
After 2nd Dryer 
__________________________________________________________________________ 
700 1.46 176 46 100 100 
1000 1.02 176 46 81 100 
1300 0.78 176 46 68 79 
__________________________________________________________________________ 
The average speed of Canadian newsprint machines in 1986 was about 700 
m/min [N. N. Sayegh and I. I. Pikulik, Pulp Paper Can., 88 912) T470 
(1987)], and, at the present time, only a few of the fastest machines 
operate at speeds in the vicinity of 1300 m/min. Data in Table V indicate 
that two dryers with a diameter of 6 m, equipped with high velocity hoods 
would be capable of completley drying newsprint on all but a few of the 
fastest machines.