Paper machine felt with enhanced two-sided structure

A felt provides a uniform paper-supporting surface having good water storage and water release properties. These qualities are effected by a bat including one or more layers of fine fibers being needled onto a base containing perforations and cavities. The free surfaces of the bat fibers needled into the cavities are reduced in a special production step following the needling operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a paper machine felt for the support and 
dewatering of running paper webs in paper machines as well as a method of 
manufacture for paper machine felts. Such felts include a lower carrier 
layer, also known as a base, and an upper layer of fibers, also known as a 
bat. 
2. Description of the Related Art 
The function of the carrier layer is to impart tensile strength to the felt 
in machine direction as well as in cross machine direction. The function 
of the bat layer is to support the paper web. In addition, both layers 
must remain permeable to water while under compression, and the carrier 
layer should be capable of storing water when compressed. Such felts can 
be made of wool, plastic, plastic fiber, or a blend of such materials. 
In felts currently most frequently used, the carrier layer or base includes 
one or several layers of plastic fabric. To the top side of the base, one 
or several layers of random fiber bat layers are needled. During the 
needling operation, a mechanical bond is formed between bat and base by 
sharp-edged needles which have tooth-like notches in their edges that pull 
fibers from the bat and plug them vertically into or through the base or 
carrier layer. After the needles are retracted, the fibers remain stuck in 
the base by friction forces. 
These types of felts are suited for a wide range of applications, but in 
some applications bottlenecks in dewatering become apparent. In 
particular, the extraction of water by air purging by use of suction boxes 
and suction rolls becomes increasingly ineffectual with rising machine 
running speed because the duration of the suction pulse diminishes. 
In order to compensate for this shortcoming, the bat layers of the felts 
are designed more open and coarser. As a consequence, the uniformity of 
paper support is reduced. Also, in wet presses of paper machines, "felt 
marking" of the paper surface by the felt surface occurs, which is 
detrimental to paper quality. Further, the dryness of the paper after wet 
pressing is reduced by increasing backflow of water from the felt to the 
paper on the outgoing end of the pressing zone. 
Furthermore, the bat fibers plugged into the interstices of the base layer 
retain water similarly to paint brushes by use of their large free surface 
area. Between these bat fibers, dirt particles and other matter can get 
stuck, reducing the water storage capacity of the interstices in the base 
layer still further. 
The net water storage capacity of the felt, defined as the maximum water 
content minus the remaining water content after a water removing step, is 
to a great deal influenced by the number and length of the fibers plugged 
into the interstices of the base. Such fibers determine the strength of 
the bond between bat and base. 
The negative effect of bat fibers within the interstices or cavities of the 
base was recognized a long time ago, and as an attempted solution to this 
deficiency, a product is described by Edward Race in APR (Aligemeine 
Papier-Rundschau) No. 37/38, 1970, pages 1378 to 1388. This product is 
intended to overcome the mentioned deficiencies. In particular, see page 
1386, middle column, last paragraph; page 1388, first column; and FIG. 6. 
Despite a special needling technique, the storage capacity of this product 
is small in comparison with the thickness of the base, and the felt is 
required to be sufficiently coarse in order to be dewatered by air purging 
over suction box slots. This felt design, however, as a consequence of 
manufacturing problems, did not prevail. This felt design was succeeded by 
open felt structures with coarse bat layers attached to both sides of a 
stiff base layer fabric. 
SUMMARY OF THE INVENTION 
The present invention improves upon a felt as described above in a way that 
increases its net water storage capacity, facilitates water release in a 
downward direction, or to the inside of the felt loop, and improves the 
uniformity of paper support by the bat. In particular, the downward water 
release is improved to an extent that an air purging device through the 
felt is rendered unnecessary in order to obtain sufficient drainage. 
In addition, the paper-supporting bat is made small-pored and dense to the 
extent that a rewetting of the paper by the felt on the outgoing end of 
the pressing zone is avoided or greatly reduced. The density of the bat is 
so high that drainage by air purging does not occur, even at higher 
pressure differences through the felt. 
The free surface area of the fibers introduced by needling into the 
interstices and cavities of the base is reduced in a following treatment 
step. This reduction not only improves the water release from the 
interstices or cavities of the base but also improves the bonding strength 
between bat and base. In this way, it becomes possible to achieve 
sufficient bonding strength with fewer bat fibers plugged into the base, 
thus increasing the water storage capacity of the base. This effect is 
accomplished by gluing the bat fibers protruding into the interstices of 
the base to each other or to the walls of the base material. 
According to another embodiment of the invention, the bat fibers protruding 
into the interstices or cavities of the base are shrunk in length, 
starting from their lower ends. Along with a thickening of the fiber ends, 
the specific surface, or surface area per unit volume, of the fibers is 
reduced in this way. The thicker ends of the fibers, like rivet heads, 
increase the separation resistance between bat and base. 
The improvement of water storage in, and water release from, the 
interstices of the base as described above renders unnecessary the air 
purging action as a method of felt dewatering, and the fibers of the bat 
can be made finer and packed denser than before. The cross-sectional area 
of the fibers may be reduced to a range from 0.00002 to 0.0003 mm.sup.2. 
With these fine fibers, the support of the paper web is improved to an 
extent that the paper surface is no longer marked by local intrusions into 
the felt during pressing. The bat, of course, can include fiber layers of 
different degrees of coarseness. 
Structure and thickness of the bat depend predominantly upon the uniformity 
of support provided by the base. The thinner the bat is, the lower will be 
its flow resistance. The thicker the bat, the greater is its capability to 
equalize nonuniformities of the base structure. If the topography of the 
base surface is very fine, about 200 g/m.sup.2 of bat fiber will be 
sufficient. If it is coarser, more mass in the bat will be required to 
establish uniform paper support. 
If a loose bat layer is placed on the base, too many stitches are necessary 
to compact the fibers of the bat sufficiently, and in consequence, too 
many bat fibers are unnecessarily plugged into the interstices of the 
base. In order to avoid this undesired overplugging, at least part of the 
bat layers are pre-compacted before they are needled to the base. 
A precompacted intermediary layer can be provided between the base and top 
bat layer. This intermediary layer may be formed of a material of inferior 
strength. The higher strength fibers needled through it provide good 
bonding strength to the base. In the same manner, a foil or membrane can 
be needled into the felt sandwich. 
An originally watertight foil can be rendered pervious by the needling 
stitches. The carrier layer or base can also be formed of a perforated 
foil which, for example, contains cavities with openings to the bottom 
side. 
As reinforcement, the carrier layer may contain reinforcement threads 
extending in machine direction as well as in cross machine direction. 
These threads improve the tear resistance of the foil. 
In order to facilitate handling during installation and removal of felts in 
the machine, the mass per unit area of the base must not be higher than 
what is necessary to satisfy the demands on strength and water storage 
capacity of the specific application. On the other hand, there are 
applications wherein sufficiently large cavities or interstices are 
required in order to accommodate a large quantity of water to be drained 
from the paper in a pressing step. The mass per unit area of the base can 
be from 500 to 2000 g/m.sup.2. 
Although compaction of felts by post-needling treatment is known, e.g., by 
heat application and mechanical compression, the desired effect of 
increasing the net storage capacity of the felt is not obtained by these 
general methods. On the contrary, these methods tend to reduce the cavity 
volume and thus the water storage capacity. Therefore, it is a special 
feature of the invention to reshape the fibers in the base specifically by 
selective treatment from the bottom side of the felt in such a way that 
their specific surface area in the interstices or cavities will be 
decreased. 
One method to this end provides for flowing or spraying of a treatment 
fluid, such as formic acid, into the cavities. As a response to the 
softening effect, the pre-stressed fiber can shrink in length and 
simultaneously develop a tacky surface that may bond or glue to the 
internal surfaces of the base or of other bat fibers in the cavities. 
Another method of shrinking the length of the fibers and thickening the 
fiber ends in the cavities includes thermal softening or even melting of 
the fiber ends. 
One provision to achieve this result includes moving the underside of the 
felt slowly over hot air nozzles that blow hot air into the cavities and 
effect the thermal reshaping of the fiber ends. 
In another embodiment, the heating of the fiber ends is accomplished by 
electromagnetic radiation in a suitable wave length range somewhere 
between ultraviolet and radio waves. 
During thermal reshaping, the fiber ends are intensely heated, whereas the 
base is not excessively affected by the heat application. One method to 
obtain this result includes making the fibers of an opaque material or 
selecting the wavelength of the electromagnetic waves to fit within a 
range of high energy absorption of the bat fiber material. 
On the other hand, it may also be advantageous to render the exposed 
surfaces of the base layer less susceptible to heat absorption by using 
reflective surfaces, a transparent body, or an insulating coating. The 
material of the base layer may also have a higher softening or melting 
point than that of the bat fibers. 
In still another embodiment of the production method, the decrease of the 
free surface area of the bat fibers needled into the cavities of the base 
is effected in three steps. These three steps protect the base layer or 
base fabric as much as possible from undesired thermal effects and reduce 
the bat fiber surface area in the cavities of the base as much as 
possible. In the first step, fibers protruding from the bottom side of the 
base are cut or flamed off. In the second step, the most exposed parts of 
the base, e.g., the bottom knuckles of the base fabric threads, are coated 
with a protective layer. In the third step, heat is conducted or convected 
into the cavities of the base. Regardless of whether hot air or radiation 
heat is applied, the protective coat effects a reduction of temperature at 
the most exposed parts of the base layer. 
The protective coating layer has reflecting and/or insulating qualities. It 
is sufficiently heat-resistant to outlast the short duration of the 
heating pulse. It is also easily removable, like loose chalk, or soluble. 
The thermal effect, particularly in the case of electromagnetic radiation, 
can be increased by adding directional components other than perpendicular 
to the felt plane. These inclined components ensure in a woven base that 
cavities at a larger depth from the bottom surface are reached. These 
cavities would be shielded by nearby threads during purely perpendicular 
irradiation. This three dimensional radiation can be effected, for 
example, by a laser combined with a lens of short focal length that 
concentrates the parallel laser light to a small spot within the base 
layer. Of course, other radiation sources and their combinations with 
reflectors or lenses are also possible. 
With sufficiently high energy density, parts of the fibers in the cavities 
can be burned or evaporated, thereby enlarging the free storage capacity 
in the cavities as desired. 
With hot air application, the range of depth of the thermal effect into the 
base layer can be extended by sucking air through the top surface of the 
felt in the area of hot air application from the bottom side. The air 
blown into the cavities also cools in the cavities via heat transmission 
to the fibers. This cooled air should not escape from the cavities in a 
direction counter to the direction of the hot air stream in order to avoid 
cooling off the hot air stream by intermixing. By sucking the cooled air 
through the bat to the top side, this cooling by intermixing is avoided or 
greatly reduced. 
In order to obtain a good bond between the base layer and the bat layer, 
even with greatly reduced length and number of fibers in the cavities of 
the base, the uppermost weft threads of the base layer are formed of spun 
yarns. The other threads of warp or weft of the three-layer or four-layer 
base fabric (three to four layers of weft threads) are formed of 
monofilaments strands or of strands of two to eight monofilaments twisted 
together. Strands formed of a single monofilament have the advantage of 
stiffness and hence a larger storage capacity for water in the base fabric 
under compression in the pressing zone. Strands formed of several 
monofilaments twisted together are more pliable and therefore better 
suited as a base for needling. 
In many grades of printing paper, different degrees of smoothness of the 
two paper surfaces is undesired. By pressing a paper web in a 
double-felted pressing zone, with one felt being of the new design with a 
fine and smooth bat and the other felt being of conventional coarse and 
open design, e.g., to render it sufficiently porous to function in a 
vacuum transfer position, the two paper surfaces are given different 
degrees of smoothness after pressing. 
In order to avoid this two-sidedness and also to suppress rewetting of the 
paper from the felt, another feature of the invention provides for a very 
thin top layer of bat, including coarse fibers, being placed upon the main 
body of finer bat fibers before needling. The coarse (thicker) fibers of 
this thin top layer imparts an embossing effect to the paper surface that 
is comparable to that created on the other surface by the conventional 
felt. Because of the fine fibers immediately supporting the thin layer of 
coarse fibers, rewetting of the paper by the felt at or following the 
pressing zone exit is avoided. However, the layer of coarse fibers must be 
very thin, in the range of 20 to 100 g/m.sup.2, in order to escape 
rewetting. 
Because during needling the uppermost layers of thicker fibers will be 
gripped by the notches of the needles and will be drawn into the base 
layer, the necessary bonding strength between base layer and bat can be 
achieved with fewer fibers and smaller free fiber surfaces in the cavities 
of the base layer if the fiber diameter (d) is larger. This is because 
bond strength depends on both the cross section of the fibers connecting 
the base and the bat and on the surface area on the circumference (cross 
section increases with d.sup.2 and surface area increases only 
proportional to d). 
A very general feature of the invention provides for actions that will 
counteract or diminish the restriction and blockage of interstices or 
cavities in the base layer that result from the needling process.

Corresponding reference characters indicate corresponding parts throughout 
the several views. The exemplifications set out herein illustrate one 
preferred embodiment of the invention, in one form, and such 
exemplifications are not to be construed as limiting the scope of the 
invention in any manner. 
DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings and particularly to FIG. 1, there is shown a 
bat layer 2 supported by a substantially permeable base layer 1. The base 
layer 1 is a woven fabric belt composed of longitudinal threads 3 and 
cross direction threads 4. In the left-hand half of FIG. 1, some polyamide 
fibers 5 from the bat 2 are needled all the way through the base 1. In the 
right-hand half of the figure, these fibers are deformed or reshaped by 
shrinking in their length direction and thickening of their ends 6. Due to 
the deformation, the specific surface of ends 6 is substantially less than 
that of fibers 5. 
In FIG. 2, a cross section of a felt with a base layer 1 and a bat layer 2 
is again shown. The felt is moved in the direction of arrow 7 over a 
treatment nozzle 8. A treatment fluid 9 is transported through its bottom 
face into the base layer 1. In the left-hand half of FIG. 2, 
still-untreated fiber ends or fiber end loops 5 can be seen. In the 
right-hand half of FIG. 2, the fiber ends and end loops 6 are shrunk and 
thickened in response to softening by the treatment fluid 9 and are bonded 
to the base layer threads 3 and 4. The fiber end loop 10 is just beginning 
to soften and shrink. 
In FIG. 3 there is again a cross-sectional element of felt shown with a 
base layer 1 and a bat layer 2. The element is being moved in the 
direction of arrow 7 past a treatment nozzle 8. The untreated fibers 5 in 
the left-hand half melt in the stream of hot air 11 issuing from the 
nozzle 8 and form thickened ends 6. Base 1 can be formed of a material 
having a heat resistance greater than that of the material of bat 2. 
FIG. 4 shows a different felt construction containing an intermediary layer 
12 between base layer 1 and top bat layer 2. This intermediary layer 12 
can be made of fibers or of foil. Although only three layers are shown in 
FIG. 4, there may be more layers contained in the felt structure for 
special requirements. For instance, the base 1 may be composed of two 
layers and the bat 2 of three or four layers that are made from finest 
fibers in the top layer and become coarser in the downward direction. 
FIG. 5 shows a felt that includes a base layer 1 that contains cavities 13 
machined into its bottom part and into which bat fibers 5 are plugged 
(left-hand half). Bat fibers 5 are shortened and thickened (6) by 
post-needling treatment (right-hand half). 
FIG. 6 shows basic steps of production of the felt structure. Proceeding 
from left to right, bat fibers 5 are moved downward into the base layer 1 
by needles 14 containing notches 15 at their edges that grip the fibers 5 
during their downward movement. In an additional treatment step, the fiber 
ends 5 are transformed into the shape 6 at right. 
FIG. 7 shows still another cross section of a felt 20 composed of base 
layer 1 and bat layer 2. The threads 3 and 4 of the base layer 1 are 
shielded by a protecting layer 16 from the heat supplied by 
electromagnetic radiation 17 and 22, which respectively issue from a lense 
18 and a mirror 21 illuminated by a light source 19. Mirror 21 or lense 18 
focus the electro-magnetic radiation into the inside of the cavities of 
the base layer 1, thus increasing the local temperature sufficiently to 
melt fiber ends 5 into clumps 6 during the movement of the felt 20 in the 
direction of arrow 7 past the radiators 18 and 21. 
Modified as described, the paper machine felt is capable of easily storing 
and later discharging water from the cavities or interstices of the base 
layer, by air displacement or centrifugal forces, through the bottom or 
inside surface of the felt loop. 
In FIG. 8, 31 is a cross-sectional element of a felt with a 3-layered base 
fabric 32 to which a bat 33 is needled. The base fabric 32, serving as 
carrier layer, is composed of an upper row of weft threads 34 made of 
yarns spun from thin fibers, a middle row of weft threads 35, each twisted 
from five monofilaments 36, and a bottom row of weft threads 37 in the 
shape of single-strand stiff monofilaments. 
The weft threads 34, 35 and 37 are kept together by monofilament warp 
threads 38, 38', 38", 38'" and 38"", and form together with them the 
carrier layer 32. The order of the warp threads along different paths in 
the base fabric repeats itself in a recurring pattern across the full area 
of the felt. As the warp threads 38, 38' and 38" connect only the two 
upper rows of weft threads, the remaining warp threads 38'" and 38"" 
interweave the bottom row of weft threads 37 with the remaining structure 
of the carrier layer 32. Thus, base 32 includes a top mesh layer that is 
finer and less open than a bottom mesh layer. 
The bat layer 33 is composed of substantially randomly distributed fibers 
39 oriented in the plane of the felt. Individual strands 40 of fibers 39 
are drawn into and partly through the base layer 32 by the needling 
action. The ends 40' of the fibers 40 are either free ends 41 or end loops 
42. The part of the fibers 40' shown in dotted lines may be burned, 
evaporated or contracted to a thickened section or a bead 43 during the 
following reshaping operation so that only the sections of fiber strands 
40 which are shown in full line remain. 
If the shortening of fibers 40' as shown in the dotted sections is done by 
radiation energy, a substantial portion of this radiation energy can be 
applied in an inclined direction as indicated by arrows 44 and 45, rather 
than in a vertical direction, e.g., between 50.degree. and 70.degree. 
relative to the felt plane. In this way, even more remote cavities in the 
higher parts of the base layer 32 will be reached by radiation. 
In FIG. 9, a cross-sectional element 51 of a felt is shown, composed of a 
woven carrier layer 52 and a bat layer 53 resting on top of it. The 
carrier layer 52 is composed of threads 54 and 55 extending in a direction 
substantially perpendicular to the plane of the figure, and threads 56 
extending in a direction substantially parallel to the plane of the 
figure. The bat 53 is composed of fibers 59 predominantly extending 
substantially parallel to the plane of the felt 51. Parts of these fibers 
59 are deviated into and through the base 52 in strands 60 by the needling 
action. The strands 60 are composed of individual fibers 60' with ends 61. 
During post-treatment, following the needling operation, the lower parts of 
the fibers 60' are shortened to a position close to the bottom surface of 
the base layer 52, as indicated by beads 62. In other words, during this 
first step of post-treatment, the sections of the threads between the 
original ends 61 and the beads 62 will be removed. In a second step, a 
protective coating layer 57 is applied to the bottom knuckles of threads 
55, and protective coating 58 is applied to the bottom knuckles of threads 
56. These coating layers 57 and 58 serve as heat protection during the 
third step of the treatment, during which heat is applied in concentrated 
form into the cavities of the base layer 52 in order to shrink the length 
of fibers 60' further up to the beads 63. 
While this invention has been described as having a preferred design, the 
present invention can be further modified within the spirit and scope of 
this disclosure. This application is therefore intended to cover any 
variations, uses, or adaptations of the invention using its general 
principles. Further, this application is intended to cover such departures 
from the present disclosure as come within known or customary practice in 
the art to which this invention pertains and which fall within the limits 
of the appended claims.