Paper machine pressing fabrics

Description

[0001] This invention relates to paper machine pressing fabrics and has particular reference to paper machine pressing fabrics formed of a polyamide material and having improved longevity and resistance to flattening in the surface layer.

[0002] In paper making machines, a slurry of paper making constituents referred to as "furnish" is deposited on a forming fabric or "wire" and the liquid constituent furnish is drawn or extracted through the fabric or wire to produce a self-cohesive sheet. This self-cohesive sheet is then passed to a pressing and then a drying section of the paper making machine.

[0003] In the pressing section, the paper sheet is transported by a pressing fabric to a pair of rollers where the fabric and the paper sheet pass between the nip of the rollers to dewater by mechanical pressure the paper sheet.

[0004] The paper sheet itself may contain other types of chemical finishes and additives and could be, at the same time, subjected to an elevated temperature in order to aid the dewatering. The paper making pressing fabric together with its sheet of paper tends, therefore, to be subjected to immense pressure at elevated temperatures in a rigorous chemical environment. Paper making pressing fabrics are generally produced by needling batt fiber to a woven base fabric. The fabric has multiple functions: first, to support the formed paper sheet through the press section of the machine; secondly, to accept sheet water expressed at each press nip; and last, to impart a finish or surface texture to the paper sheet.

[0005] The batt fiber layer constitutes a "cushion" and support for the paper sheet while the woven base provides the primary channels or voids to accept expressed sheet water. The quality of the batt fiber layer and in particular its surface properties and its consistency of surface properties with time, is of extreme importance. As the surface properties of the batt fiber layer deteriorates, so does the quality of the surface of the paper sheet being pressed, as well as, the uniformity of water removal.

[0006] In the nip of the press rolls, the batt fibers are bent and deformed under great pressure and at great frequency. Hence the mechanical properties of the batt fiber are of considerable importance in such processes.

[0007] Polyamide 6 and polyamide 6,6 have been used extensively in the manufacture of paper machine pressing fabrics. These polymers are readily formable as fibers and their fiber characteristics can be controlled to make acceptable paper making pressing fabrics.

[0008] Treatment of polyamide (PA) fibers in general has been proposed, but none have been specifically concerned with the improvement of the mechanical performance of fibers as used in paper machine clothing. Among the prior proposals for the treatment of polyamide fibers are the following:-

[0009] United States Patent Specification No. 2,434,247 relates to the modification of synthetic linear polyamides with formaldehyde and an alcohol to obtain a more elastic nylon for woven fabrics.

[0010] United States Patent Specification No. 2,430,953 is concerned with producing nylon yarns with a softer hand, better drape, and in addition, a higher melting point, better dye receptivity and increased resistnce to solvents. The polyamide yarns are reacted with formaldehyde at a pH below 3 in alcohol.

[0011] United States Patent Specification No. 2,425,334 relates to modification of polyamides with formaldehyde at a pH less than 3 so that they can be cold drawn by no more than 75% of their original lengths.

[0012] United States Patent Specification No. 3,276,839 describes a process for finishing and dyeing synthetic nylon textile materials in the presence of phenol and an aliphatic anion-active compound in alkaline medium.

[0013] United States Patent Specification No. 3,322,488 refers to sulfomethylated bisphenol-formaldehyde condensation products used as dye resists for synthetic polyamide fibers.

[0014] United States Patent Specification No. 2,533,100 relates to a process for treating synthetic linear polyamides with thio derivatives of phenols or their corresponding reduction products.

[0015] United States Patent Specification No. 2,388,676 is concerned with improving the light stability of polyamide fibers by forming a catecholaldehyde resin within the nylon.

[0016] In each of the above cases the nylon materials exhibit improved properties for use in apparrel or decorative assembly, i.e. use in conditions that will not be subjected to the agressive physical and chemical environment of a paper making machine.

[0017] As paper machine technology improves, speeds, operating temperatures and pressures increase with a result that the tendency of existing pressing fabrics to compact and wear is increased. Furthermore, increased degradation with increasing temperature of operation and increasing speed of the machine results in a significantly shorter service life for these fabrics.

[0018] The present applicant has discovered that a paper making pressing fabric using fibers of various polyamides having an increased amine end group concentration can be further processed and modified to produce significantly improved properties.

[0019] According to the present invention there is provided a paper making pressing fabric comprising a fiber layer formed of various polyamides characterised in that said polyamide has an amine end group concentration such that the amine end group/carboxy end group ratio is greater than 1 which has been subjected to a phenol/aldehyde treatment process to increase the intrinsic viscosity of by at least 4% compared with the untreated material.

[0020] In one embodiment of the invention, the amine end group concentration is at least 60 equivalents/10⁶g. Where the polyamide is polyamide 6,6, the intrinsic viscosity after treatment process is preferably not less than 1.3 dl/g; said fiber layer can just be the surface layer or layers of the batt structure and/or any portion of the batt up to 100% of the batt structure.

[0021] In another aspect of the present invention there is provided a method of making a paper making pressing fabric which method comprises treating a polyamide fiber having an amine end group concentration of at least 60 equivalent/10⁶ gram with an aldehyde/phenol treatment process by contacting the fiber with an aqueous solution of an aldehyde at an elevated temperature and with an aqueous solution of a phenol at an elevated temperature, the treatment process being carried out at a temperature for a time sufficient to impart an increase in intrinsic viscosity of the polyamide by at least 4% and thereafter forming a layer of said fibers and needling onto a woven base fabric. The fiber may be formed into a batt layer prior to attaching to the base fabric either before or after the aldehyde/phenol treatment procedure.

[0022] Since the woven base fabrics contain yarns of monofilament, multifilament, or spun from staple fibers, that are usually polyamide based, the technology of the present invention can be utilized for these yarn forms since they are also exposed to the heat, pressure and chemical environment already described. Benefits of dimensional stability, strength retention and maintenance of water handling capacity are all vital, and will be improved using yarns prepared by this invention.

[0023] Also, while usual manufacturing techniques apply a batt formed of fiber onto a woven base fabric, this should not limit the scope of this invention. Other methods to apply fiber, to anchor the fiber instead of mechanically needling, whether the base is woven or not, or whether there is even a support base should all be considered in producing a pressing fabric.

[0024] The word "fiber" should also not be limited in the sense of size dimensions or shape, but be considered a component. In this sense, therefore, for the purposes of this specification, the use of the term "fiber" should also include "yarns".

[0025] Phenols which can be used in this process include phenol, pyrocatechol, resorcinol, hydroquinone, pyrogallol and phloroglucinol, as well as, substituted products of the aforementioned compounds provided at least two reactive sites are left open on the aromatic ring. In addition, bisphenols, dihydroxy-naphthalenes and the like could also be utilized.

[0026] Aldehydes applicable to this process are formaldehyde, formaldehyde liberating compounds, oxaldehyde, malonaldehyde, succinaldehyde, glutaraldehyde, adipaldehyde, pimelaldehyde, suberaldehyde, azelaldehyde and sebacaldehyde, and the like, as well as their substituted counterparts.

[0027] The treated polyamide batt fibers show an increase in intrinsic viscosity up to 25% depending on the phenolic compound and subsequent heat treatment employed. These fibers also exhibit higher wet elongations relative to standard polyamide fibers both before, but especially after the treatment process. There is also a significant improvement in chemical resistance particularly in terms of resistance to degradation of physical properties caused by oxidation by chlorine containing compounds during use on a paper making machine. Fibers also show increased resistance to abrasion resulting from the pressing of papers containing fillers, such as ground calcium carbonate. Such pressing fabrics have exhibited as much as 100% longer lifetime in use, particularly in hostile chemical and mechanically abrasive environments.

[0028] Following is a description by way of example only of methods of carrying the invention into effect.

EXAMPLE 1

[0029] Polyamide 6,6 15d staple fiber having an amine end group concentration of 66.4 equivalent/10⁶ gram and an intrinsic viscosity of 1.35 dl/g was subjected to a phenolic treatment process as described below.

[0030] The fiber was first immersed in an aqueous bath consisting of 0.5% ethylene diamine and 2.0% formaldehyde at 130°F for 30 minutes. After a rinse, the fiber is then subjected to a second aqueous solution of 1.5% resorcinol and 1% formaldehyde at 130°F for one hour. Followed by a rinse cycle, the sample was subsequently air dried and heated 10 minutes at 300°F.

[0031] The Table 1 below compares intrinsic viscosities and tensile properties of fiber processed with the above formulation and untreated PA 6,6 fibers having high and low amine end groups. Table 2 includes data describing the properties after exposure to sodium hypochlorite solution in a standard degradation test.

[0032] A 6% increase in intrinsic viscosity is observed for the high amine end group fibers after treatment. In addition, these fibers have higher elongations compared to the Control. This higher elongation contributes to the higher Tensile Energy Absorption or "toughness" values. This parameter is important in considering end-use requirements such as resiliency which is an important factor on a paper machine. The treated high amine end group fibers maintain high elongation and tensile energy absorption relative to the control even after exposure to hypochlorite.

EXAMPLE 2

[0033] Additional PA 6,6 fiber samples having low and high amine concentration were treated as described in Example 1 above (designated as "X" treatment) using 1/10 and 1/2 the chemical concentrations. The samples were then subjected to a simulated papermaking test in which they were exposed to repeated compression for a given number cycles and were reviewed and judged on a relative ranking on a scale of 0 to 5.

[0034] Examination of Table 3 below shows that the treated high amine end group fibers give superior properties in terms of improved ranking on the simulated papermaking press. According to the table, the lower the ranking the better the results.

EXAMPLE 3

[0035] One field trial was run on the third press position of a paper machine which produces uncoated printing and writing grades of paper, mainly bond and copy papers.

[0036] The particular machine has been identified to have extremely high chlorine residuals in their stock water and shower water (in excess of 2.0 ppm).

[0037] The standard fabrics have been chemically treated with the best known art to offer a degree of protection to the polyamide fiber in the batt structure. Even so, effective fabric life is less than three weeks, with substantial wear and compaction causing loss of efficient water removal. In extreme cases, fabrics actually tore off the machine due to the degraded state.

[0038] At the time of the trial, the previous three fabrics had run 12 days each, one tearing off. The trial fabric structure utilized was composed of 100% of the modified polyamide fiber, with the addition of the special process steps of the invention. Our trial ran 12 days and was scheduled off. The fabric showed no signs of instability and our analysis of the used fabric revealed low weight loss, no fiber fibrillation and fatigue, and little loss to bulk and water handling capacity.

[0039] Subsequent fabrics exposed to the invention's process have run as long as 39 days on this position with the fabrics still in a usable physical condition at time of removal. Competitive fabrics have run no longer than 14 days during this time period.

Table 1.

Initial Fiber Properties
			Tensile Properties
			Yield		Fail
Fiber	Amine End Group Concentration (Equivalents/10⁶ g)	Intrinsic Viscosity (dl/g)	(g)	Elongation (%)	(g)	Elongation (%)	Tensile Energy Absorption (gm-cm)
PA6,6 untreated	66.4	1.35	39±4	29±2	65±8	92±15	88.0
PA6,6 treated	--	1.43	44±4	30±2	63±7	90±8	93.2
Control PA6,6 untreated	36.9	1.33	52±2	27±2	74±4	63±8	74.7
Control PA6,6 treated	--	1.29	52±3	29±2	70±4	68±13	70.6

Table 2.

Fiber Properties after Exposure to Hypochlorite
				Tensile Properties
				Yield		Fail
Fiber	Amine End* Group Concentration (Equivalents/10⁶ g)	Intrinsic Viscosity (dl/g)	% Retained IV	(g)	Elongation (%)	(g)	Elongation (%)	Tensile Energy Absorption (gm-cm)
PA6,6 untreated	66.4	0.70	52	None		11±2	11±2	1.6
PA6,6 treated	--	1.33	93	42±5	30±2	61±7	91±11	91.5
Control PA6,6 untreated	36.9	0.81	61	None		23±4	18±8	5.3
Control PA6,6 treated	--	1.20	93	None		55±3	43±7	29.9

*Value before hypochloride degradation.

Table 3

Fibers	Treatment	Papermaking Press Ranking (Scale 0-5) 700,000 Cycles
High amine end group Control	--	3.5
High amine end group	1/10 X	2.2
High amine end group	1/2 X	1.9
High amine end group	X	1.9
Low amine end group Control	--	3.7
Low amine end group	1/10 X	3.8
Low amine end group	X	3.3

Claims

1. A polymeric material from the family of polyamides characterised in that said polyamides have an amine end group concentration of at least 60 equivalents/10⁶g, and have been subjected to a phenol/aldehyde treatment process, to increase the intrinsic viscosity by at least 4% compared with standard materials.

2. A polymeric material and process as described in claim 1 which is subsequently exposed to a heat treatment to further increase the intrinsic viscosity by at least 4%.

3. Fibers and yarns formed from the polymeric material as claimed in claim 1 and claim 2.

4. A pressing fabric composed of fibers and/or yarns as described in claim 3.

5. A paper making machine pressing fabric comprising a fiber layer formed of polyamide fibers
characterised in that said polyamide fibers have an end group concentration such that the ratio of the amine end groups/carboxy end groups is greater than 1, and have been subjected to a phenol/aldehyde treatment to increase the intrinsic viscosity by at least 4% compared with the untreated material.

6. A pressing fabric as claimed in claim 5 wherein the excess amine end group concentration is at least 60 equivalents/10⁶g.

7. A pressing fabric as claimed in claim 5 wherein that the polyamide is polyamide 6,6 and the intrinsic viscosity after treatment is not less than 1.3 dl/g.

8. A pressing fabric as claimed in claim 5 wherein that the fiber is contacted with an aqueous solution of aldehyde at an elevated temperature and with an aqueous solution of phenol at an elevated temperature, the treatments being carried out at a temperature and for a time sufficient to impart an increase in intrinsic viscosity of the polyamide being at least 4% and thereafter forming a layer of said fiber on a base fabric.

9. A pressing fabric as claimed in claim 8 wherein the fiber is formed into a batt layer prior to attaching to the base fabric either before or after the aldehyde/phenol treatment.

10. A pressing fabric as claimed in claim 5 wherein the phenols used in the treatment of the fiber are selected from the group consisting of phenol, pyrocatechol, resorcinol, hydroquinone, pyrogallol and phloroglucinol.

11. A pressing fabric as claimed in claim 5 wherein the aldehydes used in the process are selected from the group consisting of formaldehyde, formaldehyde liberating compounds, oxaldehyde, malonaldehyde, succinaldehyde, glutaraldehyde, adipaldehyde, pimelaldehyde, suberaldehyde, azelaldehyde and sebacaldehyde.

12. A pressing fabric as claimed in claim 5 wherein the polyamide is selected from the group consisting of polyamide 6 and polyamide 6,6.