US3089218A - Textile fabrics and felts for technical - Google Patents

Description

3,8@,Zl8 Patented May 14, 1963 3,089,218 TEXTTLE FABRICS AND FELTS FQR TECHNICAL PURPQSES Glof Snnden, Ljungaverl; Sweden, assignor to Stochholms Superfcsfat Fabrilrs Alrtiebolag, Stockholm, Sweden, a corporation of Weden No Drawing. Filed June 2, i958, Ser. No. 755 ,6923

16 Claims. (ill. 28--78) This invention relates to textile fabrics and felts for technical purposes consisting of polyacrylonitrile fibers with improved tensile strength, modulus and resistance against acidic hydrolysis, particularly at higher temperatures.

More particularly the invention relates to papermakers felt for use in the press section and the dryer section of paper machines, such as Fourdrinier machines.

In manufacturing of paper, paperboard and other board products paperrnakers felts are used for carrying and for dewatering and drying the sheet and moreover to secure a desired sheet surface. These felts are usually prepared as endless belts or belts joined together to endless belts.

Such felts have hitherto for the most part been prepared from twisted wool yarn and woven in a special manner and milled to obtain the desired dimensions.

Owing to friction, abrasion, chemical and bacterial deterioration, such felts have a limited period of use, owing to the fact that they often have to be washed and substituted. Efforts have therefore been made to increase the period of use by adding synthetic resins to the felt, for example phenolformaldehyde resins or alkylated melamine resins, but then it is dilficult to obtain a satisfactory curing of the resin without damaging the temperature sensitive woolen fiber-s. Cotton has also been used as material for papermakers felt. Since the synthetic fibers were introduced on the market, many efforts have been made to use such fibers in these felts. The reason for these attempts has been the defective wearing strength of the natural fibers and their property to swell in moisture and heat, thereby given a thicker and non-permeable felt. 'I hus polyamide fibers have been used as reinforcing materials in woolen wet-felts, while polyesters commonly have been used in dry-end felts.

The most significant disadvantage of the natural fibers as well as of the synthetic fibers hitherto used is their bad chemical resistance against deterioration of the polymer structure and the fiber in the hot and damp conditions of a paper making machine. The deterioration of wool, cotton, polyamide and polyester fibers moreover seems to become remarkably aggravated in the slightly acid conditions caused by the presence of aluminum sulfate in the paper manufacture.

When the acrylonitrile fibers appeared, their outstanding properties were recognized in respect to resistance against degradation by sunlight, mildew, bacteria and acid hydrolysis, and these properties seemed to predestinate this fiber-group for a wide use in the technical field. The practical results were, however, not very successful and today, acrylic fibers have found some limited use in the technical textiles field. The main reason for this lack of success has probably been their poor strength and modulus of elasticity at high temperatures, thus causing creep-effects at high temperatures. Their abrasion resistance and flex life, which are not too good compared with polyamide and polyester fibers but mostly better corn pared with wool and cotton, also limit their applicability in the technical field.

The main drawback of the earlier acrylonitrile fiber from a pure textile point of view was the dyeing difliculties and the fibrillation tendency (fiber-splitting into smaller fibers). To solve these problems, the acrylic fibers were developed in such a direction as to make dyeable copolymers of different compositions and to attain a rather low orientation of the fibers. Consequently the mechanical properties and particularly the creep at high temperatures became still more undesirable. Also the resistance against acidic degradation was impaired.

THE INVENTION BROADLY In accordance with this invention textile fabrics and felts for technical purposes are prepared from fibers or yarns of slightly inter-linked acrylonitrile copolymers. More specifically the acrylonitrile copolymer used should contain at least molar percent of acrylonitrile units and from 0 to about 10 molar percent of monoethylenically unsaturated monomer units copolymerizable with acrylonitrile, inter-linked to a degree of one inter-link per from 2 to 12 polymeric chains by means of an interlinking polyfunctional compound. Said degree of interlinking or cross linking may also be defined as 1 interlink per 1000 to 20,000 monomer units in the polymer, which corresponds with the formation of centrally branched polymer molecules with up to six polymeric chains radiating from the inter-linking central point. This type of polymeric structure has been named multiehain molecules by Flory et al. in respect of polycaprolactams. These radiating polymeric chains can orient independently and build up fiber forming bonds between different multi-chain molecules.

These inter-linked copolymers result in fibers with improved mechanical and elastical properties and an improved creep resistance at higher temperatures as to C. and therefore the range of uses of fibers of this kind may be considerably broader than fibers made from linear copolymers.

The monoethylenically unsaturated monomer preferred in the acrylonitrile copolymer for this purpose is selected from the group consisting of vinyl acetate, acrylic acid, acrylamide, methacrylonitrile, methacrylamide and an ester of acrylic acid and methacrylic acid.

The inter-linking polyfunctional compound can be a compound of different characters. The inter-linking compound can be a diethylenieally or triethylenically unsaturated monomer or a salt bridge formed by a polyvalent base or acid, such as a poly'valent metal. As examples 1' diethylenically unsaturated monomers divinyl benzene, methylene-bis-acrylamide, diallylphthalate, diallylmaleate and ethylene diacrylate (dimers) are mentioned. As an example of a triethylenically unsaturated monomer triacrylylperhydrotriazine (trimer) is stated. The interlinking by a polyethylenically unsaturated compound is performed during the polymerization. The inter-linking by polyvalent metals, however, is preferably performed by an after-treatment of a copolymer containing acid groups or a fiber containing said copolymer.

The content of inter-linking compound should be determined for each inter-linking compound, paying due attention to the requirement of the polymerization process used and the relative reaction rate of acrylonitrile with the agent. We have preferably employed solution or emulsion polymerization in water with water-soluble catalysts. The upper concentration limit has been determined by the necessity of the polymer to be completely soluble in the spinning solvent and of keeping the viscosity of a spinning solution of normal concentration (about 1820%) low enough to permit its technical filtration and de-aeration. In view or" what is said above, the suitable concentration ranges of some inter-linkers have been determined for some suitable inter-linkers. The figures of Table I are stated in moles of inter-linker per 1,000 moles of acrylonitrile. They furthermore relate to a viscosimetric molecular weight of the polymer of about 70,000 calculated from the Staudinger formula.

3 Table I [01 inter-linker per 1000 units acrylonitrile Divinyl benzene About 1. Methylene-bis-acrylamide 0.2-0.6, preferably 0.35. Triacrylylperhydrotriazine 0.05-02, preferably 0.12. Diallylphthalate 0.10.2. Diallylmaleate 0.1-0.8.

As specific examples of the inter-linked copolymer the following compositions are mentioned:

(i) A copolyrner prepared from 97 kg. acrylonitrile and 3 kg. acrylic acid inter-linked by 120 g. methylenebisacrylamide and (ii) A copolymer prepared from 95 kg. acrylonitrile and 5 kg. methylacrylate inter-linked by 60 g. triacrylylperhydrotriazine.

The molecular weight range for fiber forming purposes is from about 30,000 to 100,000, preferably between 40,000 and 90,000, the latter figure corresponding to an intrinsic viscosity of 140 respectively 300 ml./ g. measured in dimethyl formamide solution. In practice the interlinkers rnethylene-bis-acrylamide and triacrylylperhydrotriazine are to be preferred as they involve no hydrolyzable inter-links.

For preparing fibers from copolymers of the kind described the copolyrner is dissolved in an appropriate polymer solvent such as dimethyl formamide, dimethyl acet amide, dimethyl sulfoxide, ethylene carbonate and propylene carbonate. The spinning solution is extruded into a bath that is miscible with the polymer solvent but precipitates the polymer in its filament form. As coagulants water, aqueous solutions, alcohols, such as glycerine, aromatic hydrocarbons, such as benzene and cymene, may be used. The solutions of the inter-linked polymers according to this invention may advantageously be extruded in very slow acting coagulating baths, such as liquid hydrocarbons predominantly consisting of parafiinic hydrocarbons, such as commercial paraifinic kerosenes with a boiling range of ISO-250 C. Solutions of the said interlinked polymers or copolymers may also be extruded into a heated spinning cell in accordance with the common dry-spinning technique. Obviously, mixtures of the said inter-linked copolymers, and other polymers or copolymers may be used for forming filaments as stated. The various spinning techniques are more completely described in various US. patents, e.g. in Patents Nos. 2,404,714 and 2,404,715, and the spinning into liquid hydrocarbon mixtures in the copending patent application Serial Number 327,429, filed Dec. 22, 1952, now abandoned, Serial Number 662,316, filed Aug. 29, 1957, now Patent No. 2,967,085, and Serial Number 662,352, filed May 29, 1957, now Patent No. 2,967,086.

The specific structure of the multi-chain acrylonitrile polymers gives the fiber not only a high resistance to creep but also a higher modulus and a higher strength compared with linear acrylonitrile fibers with the same degree of orientation, while the strain properties are un 4 aifected. The inulti-chain acrylonitrile fibers have a much better breaking tenacity than linear acrylonitrile fibers. The dimensional stability of the multi-chain acrylonitrile fibers is also higher than that of linear acrylonitrile fibers when the material is relaxed satisfactorily after yarn manufacturing and weaving.

The multi-chain acrylonitrile fibers give fabrics with a very high dimensional stability and extremely high resistance to yarn-slippage in contrast to most synthetic fibers. This stability gives the felts a quiet running even at very high speeds. These properties combined with the extremely high bulk of the fiber makes it possible to make dryend felts in the lightest weights with retained dimensional and dynamic stability and also high pliability.

Like linear acrylonitrile fibers, the multi-chain acrylonitrile fibers undergo a very slow hydrolysis under hot acidic aqueous conditions. This hydrolysis, however, is so slow that it does not afiect the properties during years of service, but on the contrary, the hydrolysis involves a further molecular inter-linking of the multi-chain acrylonitrile copolymers containing acid or ester groups. polyacrylonitrile does not show as great a subsequent molecular inter-linking. Fibers containing free acid groups seem to be hydrolysed too fast and can be destroyed. The best results therefore have been achieved by copolymers containing esters of acrylic acid and methacrylic acid, for example methylacrylate. The rate of further interlinking is much higher for the multi-chain molecular structures than for the linear molecular structures. This further inter-linking of the molecules gives an insignificant or no change in the room temperature properties, such as strength and elongation. Table 'IiI shows some results from treatment of difierent fibers in boiling Water and aluminum sulfate solutions. By the acidic treatment the increase in high temperature strength is much more rapid in the multi-chain fibers containing methylacrylate than in the linear copolymers. The 100% acrylonitrile polymer does not show any increase above its original high temperature strength after the acid treatment.

Table III [Resistance of different acrylom'trile fibers to boiling water and boiling solutions of aluminum sulfate. (Samples boiled in glass with reflux and tested in an Instron apparatus)] Initial Resulting strength, Initial strength, Resulting Flber Treatment Time, g./d., elong. strength, g./d., elong. strength,

hours percent, g./d. percent, g./d.,

6. 150 0. 65% RH 65% RH Linear aerylonitrile fiber with {Water 1, 000 2. 3/25 0.25 1. 9/23 0.25 M5l6%r11]1ethacrylate(diiygipjun). 5% lMSOOs 1, 3/2.; 0.25 1. 9/24 0.60 u ti-e ain acry onitri e ers: a er .9 2 0.7 4.2/28 0. 7 gstywet splim) lmiZfscyloniglilel. 112(SO4)3.. 11., 5/24 0.2 4. 2/29 0. 7

0 acry oni ri e me y a a er .6 30 0. 5 3.2/29 0.75 g ayerylatei t 1 +3; 1 {5% A1z(SO4)a- 1, 0(2)? 3.1/37 2.50 7 acry oni ri e acry ie a 1 3.3 343 0.7

azid. }5% A1z(s0i)z i 168 3. 4 31 0. 60 3. 2/ 1. 3

The multi-chain acrylonitrile fibers of this table are inter-linked by 0.050.06% of a trimeric inter-linker (triacrylylperhydrotriazine).

The further inter-linking substantially influences the strength and elongation at higher temperatures, such as 150 C., and accordingly the plastic deformation (the plastic flow) of the fibers decreases with increasing molecular inter-linking. An X-ray investigation has shown that no significant change of the crystallinity of the fibers occurs as a result of the further inter-linking. An infrared analysis, however, indicates an increase in ionic carboxylic groups capable of inter-linking by polyvalent metals, such as aluminum, magnesium, nickel and bivalent copper. The polyvalent metal atom is thereby chemically bonded between two molecules.

Such salt bridges also can be used as the original interlinking units in the fiber, for instance by treating fibers containing acrylic acid groups with aluminum sulfate. The inter-linking by polymerization, however, is to be preferred.

An inter-linked acrylonitrile copolymer containing 97 parts of aerylonitrile, 3 parts of methylacrylate and 0.05 part of triacrylylperhydrotriazine prepared by co-polymerization have valuable properties for papermakers felt, but the valuable properties can be further improved by further inter-linking by means of aluminum sulfate or other acidic substances in the pH-range of 26 in the aqueous medium, where the fiber is treated and boiled.

The valuable influence of the acidic treatment on the high temperature properties has its maximum at about pH 2. Below pH 1 and above pH 8, the multi-chain acrylonitrile fibers begin to lose their high temperature strength, as well as their room temperature strength, but

these specific pH-conditions have no influence in the ordinary paper making.

The fabrics and felts of the inter-linked copolymers of this invention also have valuable properties in respect of water adsorption caused by the strong capillary forces. 0

Of all known fibers the acrylonitrile fibers have the highest rate of wetting-n property of great importance both for dryand wet-end felts in paper machines.

The fabrics and felts have also valuable drying properties in that the water evaporates exceptionally fast. The drying conditions in a paper machine also involve transfer of heat from the drying cylinders to the felt and the paper sheet. As there is little difference in the heat conductivity between different dry fibers, the fiber which has the fastest distribution for water practically will have the best heat conductivity.

Evidently the drying characteristic of the slightly interlinked fibers depends on its capillary properties, which causes both a rapid wetting in contact with the damp sheet and a rapid constant rate of drying in contact with the heated drying cylinders.

There is nothing particular about the construction of the papermakers felt itself. The felt has the form of an endless belt comprising a woven felt base consisting of warp yarns and weft yarns of fibers of the slightly interlinked polyacrylonitrile fibers or filaments. Obviously the specific acrylonitrile fibers can be blended with other fibers, such as wool and cotton or other synthetic fibers when particular effects are desired. This is not recommended, when the best over-all properties are important.

A number of dry-end felts of 100% inter-linked acrylonitrile fibers of 3 denier have been operated in paper machines for experimental purposes. They have run very satisfactorily and after two years of experimental operation the fibers exhibited the same strength and the same abrasional resistance as the original fibers. An increase in high temperature strength has been evident.

The improved textile fabrics and felts of this invention may advantageously be used also as filter medium in filter presses and on continuous filters, such as rotary filters, operating at higher temperatures, particularly when acidic media are treated. Furthermore the increased high tem perature strength is important for such uses as cordage and tire cords.

EXAMPLE 1 Two different copolymers were prepared from 97 kg. acrylonitrile, 3 kg. acrylic acid and g. methylene-bis- -acrylamide respectively, 95 kg. acrylonitrile, 5 kg. methylacrylate, 60 g. triacrylyperhydrotriazine in the following manner. The monomer mixture was gently poured during 3 hours into 400 1. water at 50 to 55 C. containing dissolved l g. ammoniumpersulfate, 1.5 sodiumpyrosulfite and l g. sodiumlaurylalcoholsulfate per liter. The polymerization was continued for 4 hours and then a yield of 95 kg. precipitated and dried polymer was obtained. The polymers had a molecular weight of 60,000-65,000 as calculated from viscosity measurement by the Staudinger equation (see page 967, volume 10 of The Encyclopedia of Chemical Technology, by Kirk- Othmer).

EXAMPLE 2 An 18 percent solution in dirnethylformamide was prepared from a copolymer of Example 1 and extruded without any preheating through a 1000 hole-spinneret, hole-diameter 0.15 mm. with a velocity of 250 ml. per minute. The spinneret was arranged in the bottom of a vertical stem-mended tube of 3 m. length, through which an aromatic free kerosene (boiling range 160-200 C.) with a temperature of C. was running from above to the bottom (counter-flow). The peripheric velocity of the collecting godet in the upper part of the tube was 25 m. per minute. After said godet the fiber was stretched 6 times its original length at 130 C. to another godet with the peripheric speed of m. per minute. After relaxation at 130 C. in air and crimping, the fiber was cut to staple fibers. The fiber was washed with boiling water containing nonionic soap (a polyalkylene oxide) during 30 minutes at pH 4 to remove the content of dimethylformamide and kerosene, rinsed, treated with a catonic agent, Querton 18 AST (an octadecyl dimethyl ethyl ammonium ethyl sulfate), and dried in air at 120 C. The fiber had a tenacity of 3.0 denier (measured microscopically 3.1 to 3.2), a tensile strength of 3.5 g. per denier and an elongation at rupture of about 35 percent. The d-ye receptivity to basic dyes, for example Du Pont Basic Blue, was excellent. The amount of dye saturation was about 10 percent dye in the fiber for the acrylic acid containing fiber and 8 percent for the methylacrylate containing fiber. The fiber containing acrylic acid was more sensitive to heat and boiling in alkaline solution than the fiber containing methylacrylate.

EXAMPLE 3 Papermakers dry end acrylic felts have been woven from fibers spun according to Example 2, and installed in the drying section of different paper-machines.

In a machine, where woolen felts of twice the weight lasted for 46 months, the acrylic felt runs satisfactorily even after 2.4 months use. Fibers taken from the felt at that time had the same strength as they had in the new felt.

In a fast moving machine, where woolen felts lasted for about 2 months only, there was no change in appearance or functionality of the acrylic felt after 8 months use.

I claim:

1. A papermakers felt comprising a woven cloth in which the warp and the filling are composed essentially of a yarn prepared from the fibers of an acrylonitrile copolymer containing at least 90 molar percent of acrylonitrile units and up to about 10 molar percent of monoethylenically unsaturated monomer units, inter-linked to a degree of one inter-link per 1000 to 20,000 monomer units in the polymer by means of an inter-linking polyfunctional compound.

2. A papermakers felt according to claim 1, wherein the monoethylenically unsaturated monomer is selected from the group consisting of vinyl acetate, acrylic acid,

acrylamide, methacrylonitrile, methacrylamide and an ester of acrylic acid and methacrylic acid.

3. A papermakers felt according to claim 1, wherein the inter-linking polyfunctional compound is a polyethylenically unsaturated monomer.

4. A papermaker s felt according to claim 3, wherein the inter-linking compound is a diethylenically unsaturated monomer selected from the group consisting of divinyl benzene, methylene-bis-acrylamide, diallylphthalate, diallylmaleate, ethylene acrylate and ethylene diacrylate.

5. A papermakers felt according to ciaim 1 wherein the inter-linking compound is a triethylenically unsaturated monomer copolymerizable with acrylonitrile.

6. A papermakers felt according to claim 5 wherein the inter-linking compound is triacrylylperhydrotriazine.

7. A papermakers felt according to claim 1 wherein the inter-linking polyfunctional compound is a polyvalent metal forming salt bridges between acidic groups of the polymer.

References Cited in the file of this patent UNITED STATES PATENTS 2,426,728 DAlelio Sept. 2, 1947 2,581,790 Gates Ian. 8, 1952 2,821,458 Evans Jan. 28, 1958 2,821,771 Skeer Feb. 4, 1958 2,861,051 Caldwell Nov. 18, 1958 2,883,371 Thomas et al. Apr. 21, 1959

Claims (1)

1. A PAPERMAKERS'' FELT COMPRISING A WOVEN CLOTH IN WHICH THE WARP AND THE FILLING ARE COMPOSED ESSENTIALLY OF A YARN PREPARED FROM THE FIBERS OF AN ACRULONITRILE COPOLYMER CONTAINING AT LEAST 90 MOLAR PERCENT OF ACRYLONGITRILE UNITS AND UP TO ABOUT 10 MOLAR PERCENT OF MONOETHYLENICALLY UNSATURATED MONOMER UNITS, INTER-LINKED TO A DEGREE OF ONE INTER-LINK PER 1000 TO 20,000 MONOMER UNITS IN THE POLYMER BY MEANS OF AN INTER-LINKING POLYFUNCTIONAL COMPOUND.