Note: Descriptions are shown in the official language in which they were submitted.
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Backing Fabrics for Papermakin~ Machine Covering Materials
The present invention relates to backing fabrics for papermaking machine felt
with
improved properties, preferably improved wear resistance and in particular
improved dimensional stability during paper manufacture.
Processes for the production of monofilaments from thermoplastic polymers are
in
principle known (c.f. Handbuch der Kunststofftechnik II, C. Hanser Verlag,
Munich 1986, pp. 295-319).
Paper production on modern papermaking machines involving sheet forming
(forming part), mechanical dewatering (pressing part) and thermal dewatering
(drying part), smoothing and rolling is known from Lehrbuch der Papier- and
Kartonerzeugung (VEB Fachbuchverlag 1987, p. 190 ff).
Fabrics employed in the forming part consist predominantly of polyester
monofilaments. In order to improve the abrasion resistance monofilaments of
polyamides together with polyester monofilaments in an alternating pick-and-
shot
arrangement on the machine side are also used.
In the pressing part the basic fabrics for the pressing felt or wet pressing
felt are
' produced almost exclusively from polyamide fibres and polyamide
monofilaments,
preferably from pure polyamide-6 but also from polyamide-66. A nonwoven layer
of polyamide fibres is needled onto the base fabrics consisting of polyamide
monofilaments in a second processing stage and this layer is thereby
mechanically
anchored in the said base fabric.
Dry screens on the other hand normally consist of polyester monofilaments that
are
largely stabilised by means of suitable products, for example Stabaxol (a
commercial product available from Rheinchemie, Mannheim), against hydrolytic
decomposition.
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The commercially available press felts made from polyamide-6 monofilaments
have
on account of their high abrasion resistance, compressibility and very good
recovery
of the felts after passing through the press nip major advantages compared to
press
felts of other materials, e.g. polypropylene, polyester, wool or other types
of
polyamide (e.g. PA 6.10, PA 6.12).
A significant disadvantage of these press felts is however the lack of
dimensional
stability in the event of machine downtimes. The materials polyamide-6 and
polyamide-66 absorb up to 10 wt.% of water in a wet environment. The length
and
thickness of the monofilaments changes with the absorption of water. In
particular
the change in length means that in the event of malfunctions or downtimes of
the
papermaking machine due to other causes the felts have a different weight and
fabric
density in the wet zones than in the dry zones. After dealing with the
malfunctions
and starting up the papermaking machine again no high-quality paper can be
IS produced with these felts until the felts have re-established the same
water content
and the same density and width over the whole area.
Furthermore the change in width often means that the full working width of the
papermaking machine cannot be utilised since the felts extend beyond the
maximum
width of the machine and are damaged at their edges.
' There has therefore been no lack of attempts to improve the dimensional
stability of
press felts in wet/dry cycles.
One possibility is to 'use outer fabric constructions.
The use of other materials in the warp of the fabrics is widespread, for
example the
replacement of polyamide-6 or polyamide-66 monofilaments by filaments which
absorb substantially less moisture under high ambient moisture conditions and
in
which the dimensions of the fabrics consequently change only slightly.
Monofilaments of polyamide 6.10 and polyamide 6.12 have proved suitable.
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A disadvantage of these fabrics and of the felt produced therefrom is however
the
significantly reduced wear resistance when used in papermaking machines
compared
to fabrics of polyamide-6 monofilaments and felts produced therefrom.
It has now surprisingly been found that the disadvantages of the lack of wear
resistance can be avoided and can be replaced by the advantages of a good
dimensional stability if the warp of the basic fabric consists of combination
twisted
yarns that contain monofilaments of polyamide-6 as well as also monofilaments
of
polyamide 6.10 or polyamide 6.12.
The object of the invention is achieved if in the production of the backing
fabric
there are used combination twisted yarns with 1 to 20 monofilaments of
polyamide-6 and 20 to 1 monofilaments of polyamide 6.10, polyamide 6.12,
polyamide 11 or polyamide 12 in the warp instead of twisted yarns of polyamide-
6
monofilaments.
Moreover, the fabrics produced in this way also have a significantly improved
economic utility since the raw materials polyamide-6 and polyamide-66 are
industrially more readily available and can be re-used in many recycling
systems
after economic utilisation.
' A particular advantage of the process according to the invention is that
twisted yarns
of an even number of the materials used as well as also an odd number of these
materials can be twisted with one another. In this way specific, calculable
dimensional changes of the twisted yarns or fabrics produced therefrom can be
established and the economic utility can optionally also be improved.
The following examples demonstrate the advantages according to the invention
of
the combination twisted yarns, without restricting the possibilities of these
combinations.
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Examples
Monofilament
Commercial Product Diameter
Polyamide 6 X 201 0.20 mm
Polyamide 6.10 ATF 2311 0.20 mm
Polyamide 6.12 ATF 23 0.20 mm
Manufacturer: Bayer Faser GmbH
Pre-twisted yarns
Pre-twisted yarns of construction 0.20 mm x 2 were produced on an Allma Saurer
AZB-T type yarn twisting machine at 304 revolutions/metre
Experimental part V 1: X 201/X 201, 0.20 mm x 2, S 304 revolutions/metre
Experimental part V 2: X 201/ATF 2311, 0.20 mm x 2, S 304 revolutions/metre
Experimental part V 3: ATF 2311, 0.20 mm x 2, S 304 revolutions/metre
Experimental part V 4: ATF 2300 0.20 mm x 2, S 304 revolutions/metre
' Comparison example 1
Pre-twisted yarns of polyamide 6, experimental part V 1, were processed on an
Allma Saurer AZB-T type yarn twisting machine to form a balanced annular
twisted
yarn of construction 0.2 mm x 2 x 2 with S 304/Z 260 revolutions.
The twisted yarn was then fixed tension-free in a heating cabinet for 5
minutes
at 160°C and cut into pieces of length 1.00 m. The exact length and the
weight of
the sample pieces was determined. Following this the samples were then stored
tension-free for 24 hours in a water bath at 20°C, removed from the
water, dried, and
the change in length as well as the weight were determined.
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The twisted yarn was then dried for 24 hours at 80°C in a circulating
air drying
cabinet and the change in length and weight loss were again determined. This
cycle
was repeated three times. The changes in length between the wet/dry cycles are
summarised in Table 1.
The abrasion resistance of the twisted yarns was determined by an abrasion
test
developed in-house. For this, the monofilaments and twisted yarns are drawn
cyclically under a defined load over a grinding roller until they break. The
number
of grinding cycles is a measure of the abrasion resistance.
Comparison example 2
Pre-twisted yarns of polyamide 6.10 (ATF 2311), experimental part V 3, 0.20 mm
were processed into an annular twisted yarn as described in comparison example
1.
The change in length after wet/dry alternating cycles as well as the abrasion
resistance were also determined as described in comparison example 1. The
results
are summarised in Table l .
Example 1
Pre-twisted yarn V 1 and pre-twisted yarn V 2 were processed into an annular
twisted yarn as described in comparison example 1. The annular twisted yarn
had a
proportion of PA 6.10 of 25%. The change in length after wet/dry alternating
cycles
as well as the abrasion resistance was also determined as described in
comparison
example I . The results are summarised in Table 1.
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Example 2
Pre-twisted yarn V 1 and pre-twisted yarn V 3 were processed into an annular
twisted yarn as described in comparison example 1. The annular twisted yarn
had a
proportion of PA 6.10 of 50%. The change in length after wet/dry alternating
cycles
as well as the abrasion resistance was also determined as described in
comparison
example 1. The results are summarised in Table 1.
Example 3
Pre-twisted yarn V 2 and pre-twisted yarn V 2 were processed into an annular
twisted yarn as described in comparison example 1. The annular twisted yarn
had a
proportion of PA 6.10 of 50%. The change in length after wetldry alternating
cycles
as well as the abrasion resistance was also determined as described in
comparison
example 1. The results are summarised in Table 1.
Example 4
Pre-twisted yarn V 3 and pre-twisted yarn V 2 were processed into an annular
twisted yarn as described in comparison example 1. The annular twisted yarn
had a
proportion of PA 6.10 of 75%. The change in length after wet/dry alternating
cycles
' as well as the abrasion resistance was also determined as described in
comparison
example 1. The results are summarised in Table 1.
Example 5
Pre-twisted yarn V 4 and pre-twisted yarn V 1 were processed into an annular
twisted yarn as described in comparison example 1. The annular twisted yarn
had a
proportion of PA 6.12 of 50%. The change in length after wet/dry alternating
cycles
as well as the abrasion resistance was also determined as described in
comparison
example 1. The results are summarised in Table 1.
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Table 1
Propn.Propn. Water Water Abrasion Behaviour
PA PA 6.10AbsorptionElongationmin - max
6 1~ 1'
Cycles
Comp. Ex. 100 0 6.8 3.0 260 - 350
1
Comp. Ex. 0 100 2.8 1.2 220 - 290
2
Example 75 25 6.0 2.7 260 - 320
1
Example 50 50 5.2 2.0 250 - 295
2
Example 50 50 4.9 1.9 255 - 305
3
Example 25 75 3.8 1.6 225 - 290
4
Propn.
PA 6.12
Example 50 50 5.1 2.1 245 - 300
5
1' Mean value from three measurement cycles
