Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a method and apparatus
for heat treatment of a web containing fibres of thermo-
plastic material.
Paper webs have traditionally been formed from
cellulosic fibre derived from various material of vegetable
origin, for example woodpulp. More recently proposals have
been made for the production of so-called synthetic wood
pulp fibres from various polymeræ. Such fibres have a gross
morphology, ie. length:diameter ratio, and diameter of the
same order of magnitude as cellulosic papermaking fibres
derived from woodpulp. They are distinguished from most
synthetic textile fibres by their gross morphology and
diameter.
The use of synthetic fibres in papermaking affords
the possibility of producing webs with novel properties.
One such possibility, provided the synthetic
fibres are of a thermoplastic material, is that of heat
treatment to soften the fibres, and thereby to produce a
partially or wholly consolidated sheet.
It is an object of the invention to provide an
improved method for heat treatment of a web of intermeshed
fibres at least a proportion of which are of a synthetic
thermoplastic material.
According to a first aspect of the invention,
there is provided a method of continuously consolidating and
surface finishing a paper web substantially all the fibres
of which are of a synthetic thermoplastic material, com-
prising the steps of heating the web to a temperature above
the softening point of the synthetic thermoplastic material,
B
the web being supported throughout the time it is above the
softening point of the thermoplastic material, and sub-
sequently cooling the web from a temperature above the
softening point of the synthetic thermoplastic material to a
temperature below the softening point of the thermoplastic
material while the web is in contact with a forming surface,
so that the finish of the forming surface is imparted to the
surface of the web without subjecting the web to substantial
pressure during the time it is so supported.
In a further aspect the invention is a consoli-
dated surface finished web produced by the above method.
It will be appreciated that the present method
consolidates the web by the action of heat rather than
pressure, as is discussed more fully hereafter.
,~
1066470
Preferably, the web is heated by passage round
part of the circumference of a heating roll. The
roll may be heated for example by hot oil or steam,
but the former is preferred as steam presents more
S problems o~ pressure sealing.
Preferably, the web is cooled by passage round
part of the circumference of a cooling roll, the
surface of which constitutes said forming sur$ace.
The cooling roll may be at a temperature below the
soitening point of the thermoplastic material of
which the fibres are made, but still considerably
in excess oi ambient temperature, or it may be at or
below ambient temperature. If the cooling roll is
to be chilled, air or cold water may be used. The
choice of temperature ~or the cooling roll depends
largely on the nature of the web and on process
parameters, as will be discussed more fully hereafter.
The cooling roll finish, which is imparted to
the web, may take various ~orms. For example, it may
be a high gloss ~inish, a matt ~inish, obtained ~or
example by shot-blasting, or an engraved ~inish.
Advantageously, the web is transferred ~rom the
heating roll to the cooling roll by means of a trans~er
roll running in contact with both the heating and cooling
2S rolls. Tbe transier roll has a resilient covering, for
example of rubber, so that in passing between a nip
formed by the transfer and cooling rolls, the roll will
not be subjected to heavy pressure which prevents the
106~i~70
achievement of the desired web properties. Although
the resilient surface of the transfer roll gently
presses the web against the surface of the cooling
roll, the pressure is much less than that used in
conventional hot pressing processes. As a result
of the gentle pressing of the web against the surface
of the cooling roll, imparting of the surface finish
~ oi the cooling roll to the web is facilitated. The
; way in which the surface finish is imparted to the
web is analogous to a casting process, in that the
softened thermoplastic material "flows" and thereby
accurately adopts the finish of the supporting surface.
, Once the web has been heated to a temperature
above the softening point of the thermoplastic material
of which the fibres are made, the web has little
coherence. Consequently, it is necessary for the web
to be supported until it is again at a temperature some
way below the softening point. I~ the web were
unsupported when above or very near the softening
20 ~ point, it would be likely to break, stretch or be
otherwise deformed.
The coherence o$ the softened web depends to some
extent on the extent of consolidation required in the
finished web, and on whether it contains a proportion
of non-thermoplastic fibres, e.g. cellulosic fibres.
If only partial consolidation is being carried out, or
coherence ie imparted by the presence o~ non-thermoplastic
fibres, it may be possible to support the web on air jets.
~ - 5
- Otherwise, a supporting member or members is required.
The type of support which will su~fice wilI be readily
apparent to the skilled worker in the ar~.
A ~urther consequence o~ the lack of coherence
oi the softened web is that if the web is to be
transferred from one support member to another while
in a soit state, the support member from which the
web is being transferred must be treated to facilitate
release of the web. Desirably this is achieved by
means of a release coating, preferably o~ polytetra-
i'luoroethylene. A problem associated with release
coatings for use in the present method and apparatus
i8 that they cannot have a high gloss i'inish or an
engraved ~inish,so i'ar as is known at present. Hence
in order to achieve such a finish, the web must be
transferred while still soi't to a supporting sur~ace
having the desired finish, and on which the web is
cooled to a temperature at which it has the coherence
necessary to be drawn of~ the supporting suriace
without breaking. It might be thought that a iairly
high gloss release coating could be achieved by the
use oi a release sleeve oi the kind sometimes used for
drying cylinders on papermaking or papercoating machines.
It has been iound however that such a sleeve is not
satisiactory on a heating roll which has to run in
contact with another roll, as the sleeve is dama~ed by
the other roll.
Advantageously, the web is pre-heated to a
temperature below the so~tening point oi the synthetic
thermoplastic material before being heated in a
0
separate heating stage to said temperature above the
melting point of the synthetic thermoplastic material.
Pre-heating may ~or example be by means of inra-red
i heaters or preferably by hot air
heating. Pre-heating has the effect of reducing
the temperature gradient through the web when the
web is being heated to a temperature above the
softening point of the thermoplastic material.
Although the present invention preferably employs
heating, transfer and cooling rolls, the web may
alternatively be heated and cooled while on the
surface of either an endless belt or a single roll.
` Heating may for example be by means o$ in~ra-red
heaters or hot air. Cooling may be by cold air, or
may be unforced i~ this affords an adequate rate o~
heat loss. An endless belt or single roll is
satis$actory if it is desired for long periods to make
a web having a particular sur$ace finiæh. However,
i$ a variety o$ sur$ace $inishes are to be applied,
the three roll system is preferred, since only the
cooling roll need be changed.
In order to enable the invention to be more
readily understood, re$erence will now be-made to the
accompanying drawings, which illustrate diagrammatically
and by way o$ example some embodiments thereo$, and
in which -
Fig. 1 is a schematic side view of a $irst
embodiment o$ part o$ a heat consolidation apparatus;
Fig. 2 is a schematic side view o$ an embodiment
o$ a heat consolidation apparatus o$ which part is
- 7
shown in Fig. l;
; Fig. 3 is a partially sectioned partially cut-away
view of a heating roll forming part oi the apparatus '
shown in Figs. 1 and 2;
Fig. 4 is a flow diagram of a heating circuit
ior the heating roll shown in Fig. 3;
Fig. 5 is a schematic side view oi a second
embodiment of part oi a heat consolidation apparatus
corresponding to the part shown in Fig. l; and
Fig. 6 is a schematic side view o~ a third
embodiment o~ part o~ a heat consolidation apparatus
corresponding to the part shown in Figs. 1 and 5.
Re~erring iirst to Fig. 1, part oi a heat
consolidation apparatus comprises a heating roll 1
having a release coating 2. The release coating i8
pre~erably oi polytetrailuoroethylene, but may
alternatively be oi a silicone material or a ceramic
material. A rubber covered ~eed roll 8 is mounted to
run in contact with the roll 1 to ~orm a low pressure
~0 gulding nip 9 into which a web 10 to be consolidated is
fed. The roll 8 need not be positioned so as to de~ine
a nip with the roll 1. It may instead be spaced ~rom
the roll 1, provided it serves satisiactorily to guide
the web on to the suriace o$ the roll 1. The rubber
covering on the roll 8 and on other rolls to be described
subsequently is shown in this and in other Figures to be
described subsequently by hatching around the periphery
oi the roll. A rubber covereq transier roll 5 is also
G
mounted to run in contact with the roll 1 so as to
deiine & nip 6.
- 8 -
106t;~70
The roll 5 is positioned such that the web lO passes
round a major proportion of the circumference of the
roll l before being transferred.
A cooling roll 3 is mounted to run in contact
with the transfer roll 5, so as to define a nip 7 into
which the web lO is fed. Further rolls (not shown) are
provided to guide the web from the roll 3 after it has
passed round slightly more than half the circumference
of the roll 3. The surface 4 of the roll 3 is selected
in accordance with the æurface characteristics it is
desired to impart to the roll. Means (not shown) are
provided to pre-heat the web before it reaches the feed
~, roll 8 and heating roll l. The rolls l and 3 are
arranged to be positively driven by means (not shown) to
lS maintain tension in the web.
In a modification of the apparatus shown in Fig. l,
the roll 5 is omitted and the roll 3 runs in contact with
the roll l. In use, the web lO is then transferred from
the roll l to the roll 3.
Referring now to Fig. 2, there is shown a complete
heat consolidation apparatus including the rolls l, 3, 5
and 8 just described with reference to Fig. l. The web lO
is also shown. There is also shown an unwind reel ll on a
stand ll a, a pre-heating tunnel generally designated 12, a
frame 13 supporting the tunnel l2 and in which the rolls l,
3, S and 8 are journalled (the journals not being shown),
and a conventional type oi reel-up assembly comprising a
rubber-covered roll 14, and a reel }S supported on an arm 16
J -- g
---` 1066~70
arranged to pivot at 17. The equipment for controlled
pivotal movement of the arm 16 and for driving the
reel-up assembly is not shown, for the sake of clarity'.
The pre-heating tunnel 12 is divided by a
horizontal partition 18 into upper and lower ducts 19
and 20 respectively. A vertical partition 21 depending
from the partition 18 defines a vertical duct 22 which
connects the ducts 19 and 20. An inlet port 24 is
provided for introducing hot air into the duct 19. An
outlet port 23 is provided $or exhaust air. Six
guide rolls 25 are disposed in the duct 20, so as to
define a roughly arcuate path for passage of the web 10
through the tunnel. The web 10 enters and leaves the
tunnel through slots (not shown) in the base o~ the
tunnel. A duct floor 34 the shape of which generally
conforms to the web path is disposed above the rolls 25.
Respective elongate nozzles 35 are directed at the
~olls 25.
Rolls 26, 27 and 28 are provided for guiding the
web 10 $rom the reel 11 to the tunnel 12. The rolls
26 and 27 are guide rolls and are journalled in parts
oi the ~rame 13 which are not shown for the sake o$
clarity. The roll 28 is a skew roll positioned in a
stand 29 and its position in the stand 29 is controllable
in conventional manner by a hand wheel 30 to ensure
that the web tension is even across the width of the web.
The degree o$ tension in the web is controlled by
conventional means (not shown), for example a pneumatic
-- 10
10~6~70
,
or electrical controller? associated with the reel 11
and the roll 27.
Guide rolls 31, 32 and 33 are provided for
guiding the web 10 from the roll 3 to the reel-up
assembly already described. During this stage,
the ~eb lS cooled to around ambient temperature.
In use of the apparatus shown in Figs. 1 and 2,
the web 10 is unwound from the reel 11 and the web
tension is suitably adjusted. The web is then
pre-heated in the tunnel 12, as it passes over the
guide rolls 25. Heating is by means of hot air
passing through the inlet port 24, along the duct 20,
through the nozzle 35 so as to impinge on tbe web, up
the duct 22, back along the duct 19 and out through ;
the port 23.
The pre-heated web then passes out of the tunnel 12
and round the feed roll 8 on to the heating roll 1
through the nlp 9. The heating roll is heated
internally by means o~ hot oil circulation. A~ter
~0 passage round the roll 1, the temperature of the web -~
is above the softening point oi the synthetic fibres
in the web. The so~tened web is then trans~erred to
the roll 5, transfer being facilitated by the release
coating on the roll 1. While on the roll 5, the web
cools slightly, before passing into the nip 7 between
the rolls 5 and 3.
The rubber coating on the-roll S prevents the web
being subjected to substantial pressure in the nips 6
and 7, and ensures that such pressure as there is i~
- 1066~70
even across the width of the web. It will be
appreciated that the nip pressure produced by the
rubber coating is much less than would be produced
by a steel transfer roll. Whereas a steel roll
would be in substantially linear contact with the
roll 3, the rubber coating deforms so as to wrap
around the roll slightly, so that the force in the
nip is spread over a large area, i.e. there is a
lower nip pressure. As discussed previously, it
is important to avoid high pressure on the web, since
it prevents the resultant product having the desired
properties.
Although the roll 3 is referred to as a cooling
roll, it must be emphasised that the expression
"cooling" is used in relation to the temperature of
the web arriving at the roll. The cooling roll may
still be very hot compared with ambient temperature,
but still be cooler than the web, Alternatively, the
cooling roll may be maintained at or below ambient
temperatures, e.g. by air or water çooling. The
temperatures chosen ior the rolls 1 and 3 will be
discussed more $ully hereafter.
On the first part oi the cooling roll, the web
. i8 still soft. This is important, since the surface
of the web in contact with the roll 3 can then "flow"
such that the surface finish of the roll is cast on
to the surface oi the ~eb in contact with the roll.
The surface finish may for example be a high gloss
finish, a matt finish, or an engraved iinish.
_ 12
1066~70
.
The gentle pressing action of the rubber covered
roll 5 in the nip 7 assists the softened web material
to flow into close contact with the roll 3, whereby
the finish oi' the roll 3 is cast on to the web.
By the time the web leaves the cooling roll, it
has cooled to an extent such that it has su~ficient
coherence to be unsupported. The coherence is
such that there is no need for a release coating on
the roll 3, which as discussed previously would prevent
the casting o~ a high gloss or engraved i'inish on to
the web.
On leaving the roll 3, the web passes over guide
rolls 31, 32 and 33 to the reel-up assembly 14, 15, 16.
The temperatures chosen ~or the rolls 1 and 3
depend on the web speed and substance, the diameters
o~ the rolls, the proportion o~ the circumi'erence oi
the rolls contacted by the web, the extent of pre-heating,
and the sur~ace and other characteristics required in
- the ~inished product, The heat supplied to the web
by the time it leaves the roll 1 should be such that
the web is at a temperature slightly above the softening
point o~ the thermoplastic material oi which the
synthetic ~ibres are made. This normally requires the
roll to be considerably hotter than the ~inal web
temperature desired. A high roll temperature is needed
~or a fast web speed, ~or a small roll diameter, and
~or a small proportion oi roll suriace in contact with
~; the web. A lower roll temperature will su~ice i~
pre-heating is increased or web substance decreased.
_ 13
1066~70
By way of example, for a 75 g/m2 web made wholly o~
polyethylene ~ibres and travelling at 50 ft/min,
with an 8-inch diameter roll, 270 of the circumference
a~ which is in contact with the web, a web temperature
o~ 180C is suitable.
The web should cool after leaving the roll 1 and
while on the roll 3 to a temperature below the
softening point oi the thermoplastic material. The
rate of heat loss requlred for this depends partly
on the substance of the web and partly on the web speed,
Ii cooling is too fast, the web may not have time
beiore solidiiying t~ adopt closely the finish of the
cooling roll. For a low substance web running at
low speed, the cooling roll must be fairly hot ii
over-cooling is not to occur. For a high substance
web running at high speed, the rate o~ heat loss must
be greater, and hence the cooling roll must be cooler.
In certain cases, the roll 3 may not need to be heated
at all, By way oi example, ior the web described in
the pre¢eding paragraph, and whlch has been heated as
described in the preceding paragraph, the cooling roll
temperature can be in the range 90 to 120C, depending
to some extent on the sur~ace finish which it is
~ desired to impart to the web.
The construction o~ the heating roll is important,
An oil iilled roll arranged to be heated by electric
immersion heaters may be used, However, it is normally
t necessary to strengthen such a roll by internal radial metal
_ 14
.
webbing. This r.1ay lead to two disadvantages. Firstly,
the webbing~being of metal, has a better heat conductivity
than the oil, and hence "hot spotsl' are produced on the ,
sur~ace of the roll opposite the points o~ contact of the
webbing and the roll cylinder. Secondly, the webbing
may expand and lead to the formation of bumps on the roll
surface. It has therefore been found desirable to use a
roll construction as will now be described with reference
to ~ig. 3,
The roll, generally designated 1, comprises a cylinder
40 fitted with end caps 41 and 42. Within the roll 1 is
a closed inner drum 43. The roll 1 and the drum 43 are
mounted on common trunnions 44 arranged to be mounted in
journals (not shown). An axial channel 45, 46 is formed
in each trunnion and communicates with the interior o~
the roll 1 through radially disposed channels 47, 48. Two
helically disposed fins 49, 50 are mounted on the cylindrical
surface o~ the drum 43 to de~ine,two helical races about
the drum.
In use, hot oil is pumped in the direction o~ the
arrow through the channel 45, ~rom whence it passes into
the interior of the roll 1 through the channels 47. The
oil then passes along the races defined by the fins 49, 50,
- heating the cylinder 40 as it does so. On emerging ~rom
the races, the oil passes through the channels 48 and out
oi the roll along the channel 46, as indicated by the arrow.
A pre~erred oil ~low and heating circuit will now
be described with reference to Fig. 4. By way o~ example,
it will be assumed that the heating roll, shown as l in
_ 15
1066~7~
Fig. 4, is to be supplied with oil at a temperature
oi 180C.
The circuit comprises a heater 60 for heating oil
to a temperature considerably greater than is desired
for the roll 1, for example 300C and 180C respectively.
A main pump 61 is provided for pumping the heated oil
around a closed main circuit 62 and back to heater 60.
A take off pipe 63 leads to a subsidiary circuit 64 for
' heating the roll 1. The subsidiary circuit includes a
subsidiary pump 65. A three-way valve 66 is provided
at the junction of the pipe 63 and the circuit 64. A
return pipe 67 in which is a one-way valve 68 connects
the subsidiary circuit 64 with the main circuit 62. The
valve 68 is arranged to permit oil ilow only from the
subsidiary circuit 64 to the main circuit 62. An inira-
red temperature sensor 69 is provided adjacent the roll 1
to sense its temperature. The sensor is connected to a
controller 70 ior the valve 66. The controller may ior
example be pneumatically or electrically controlled. The
sensing and controlling connections are shown by the
dotted lines in Fig. 4.
In use, oil at 300C is pumped continually round the
closed circuit 62, as shown by the thick lines in Fig. 4.
When the temperature of the roll drops below 180C, the
26 sensor 69 sends a signal to the controller 70, which in
turn causes the valve 66 to open to admit hot oil irom
the circuit 62 along the pipe 63, and into the piping 64.
The hot oil is then circulated by the pump 65 through the
roll 1, the one-way valve 68, and the piping 67 back to
the circuit 62.
- 16
- 1066/?t70
When the roll 1 reaches the desired temperature, the
sensor 69 signals the controller 70 accordingly.
The valve 66 is then altered to cut off flow o~
hot oil from the pipe 63, and instead to permit
circulation of oil at 180C round the closed circuit 64,
by means of the pump 65. This is indicated by the thin
dashed lines in Fig.4. When the temperature next drops
below 180C, the procedure just described is repeated.
I~ precise temperature control o~ the roll 1 is
not needed, the subsidiary circuit can be dispersed with,
and the roll simply installed in the main circuit. It
has been found however that such an arrangement only
permits temperature control within about - 3C. The
arrangement just described permits control to within
about - ~C.
If it is desired to heat the cooling roll 3 to a high
temperature, for example 90 to 120C, a iurther ~ubsidiary
¢ircuit can be installed, in parallel with the circuit 64.
The equipment required, and its operation are as described
with reference to the circuit 64.
Re~erring now to Fig.5, there is shown an alternative
embodiment of the part of the apparatus shown in Fig.l.
The remaining part oi the apparatus is as shown in Fig.2.
The apparatus comprises an endless belt 70 arranged to
run around three triangularly disposed rolls 71, 72 and 73.
The roll 71 is a heating roll, and may be as described
previously with re~erence to the heating roll 1. A ~eed
t roll 74 is provided for guiding a web 10 on to the roll 71
and has the function described previously ~or the roll 8.
_ 17
4'70
A rubber-covered take-o~f roll 75 is provided above the
roll 72, and forms a gentle nip with the roll 72. Means
(not shown) are provided for pre-heating the web (arrows.
denoted Hl), for pre-heating the belt 70 (arrows denoted H2)
and ior cooling the belt (arrows denoted C). The heating
means may be infra-red heaters and the cooling means may be
cool air jets. Depending on the extent of cooling
required, it may be possible to dispense with special
cooling means, in that ordinary unforced heat loss from
the web may result in adequate temperature loss.
In use, the web 10 passes round the feed roll 74
after having been pre-heated, and on to the surface o~ the
belt 70. During passage over the roll 71, the web is
heated to a temperature sufficient to ~often the synthetic
fibres in the web. The web then passes on the belt to
the gentle nip between the rolls 75 and 72 and is then
taken oi~ the belt 70. The rolls 74 and 75 together
hold the web ~irmly against the belt. The surface
iinish of the belt i8 there~ore imparted to the surface
o~ the web while the web is in a softened state.
It will be noted that the web remains on the same
support, i.e. the beltj throughout the process. There is
thus po need for the belt to have a release coating. By
~ the time the web is removed from the belt, the web has
cooled sufficiently to have acquir~d sufficient coherence
to be pulled from the belt witho~t breaking.
Referring now to Fig.6, there is shown another
,; embodiment of part of a heat consolidation apparatus
corresponding to that shown in Fig.l. The remainder of
- 18 -
- l066~7b
the apparatus is as shown in Fig.2. The apparatus
comprises a drivably rotatable roll-80 which is
divided internally into compartments 81, 82 by a non-
rotatable barrier 83, which makes sealed contact with
the inside o~ the roll 80. A rubber-covered feed
roll 84, constructed and functioning in the same way
as the roll 8, shown in ~ig.l, is provided to guide
a web 10 on to the external surface of the roll 80.
A rubber-covered take-of~ roll 85 is provided to guide
the web away from the roll 80 after the web has been
treated. The rolls 84 and 85 are disposed such that
the web contacts a major proportion oi the circumference
of the roll 80.
The compartment 82 is arranged to be heated by hot
oil. The compartment 81 is arranged to be heated, or
cooled, by a suitable fluid, for example oil, steam,
water or air. Means (not shown~,e.g. infra-red heaters,may
be provided for heating the web externally, (as shown
by arrows denoted H). Means (also not shown) e.g. air
~ets, may be provided for cooling the web externally,
depending on the extent of cooling required.
In use, the web 10 is pre-heated and passed round
the roll 84 onto the surface of the heated part of the
drum. The synthetic fibres in the web are thereby soitened.
as On passing beyond the barrier 83, the web is cooled.
While the web is still soft, the surface finish of the
roll 80 ls imparted to the web, since the web is held
firmly against the-roll by means oi the rolls 84 and 85.
- 19
` 106~4~70
After cooling, the web is guided away ~rom the roll 80
by the roll 85.
Since the web is on the surface of the roll 80
; throughout the process, there is no need for the
roll 80 to carry a release coating. This is because
by the time the web is taken off the roll 80, it has
cooled to an extent adequate to give the web the
necessary coherence to prevent breaking of the web.
It may not be necessary for the web to be
heated both from within and without the roll. Only
heating from within or only heating from without may
be used if sufficient heat can be supplied to the web
in that way.
If it is desired to impart particular surface
finsihes to both surfaces of the web, two sets of
apparatus as described herein may be used, positioned
in line so as to treat first one surface and then the
other. Alternatively, the one apparatus may contain
me~ns ior heating and surface finishing the web iirst
on one side and then on the other. It is also possible
to suriace iinish both sides simultaneously between
two cooling rolls, provided these are mounted so as not
to apply substantial pressure to the web.
- 20
