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- und 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.
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 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 other 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.
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.