PROCESS FOR CONTINUOUSLY OR SEMI-CONTINUOUSLY DYEING A POLY (M-PHENYLENEISOPHTHALAMIDE) FIBRE

Abstract: A process for continuously or semi-continuously dyeing a poly(m-phenyleneisophthalamide) fibre, comprises the steps of (1) contacting a dyeable poly(m­ phenyleneisophthalamide) fibre with a dyeing solution of an organic swelling agent selected from the group consisting of N-methylpyrrolidone, dimethylsulphoxide, and dimethylacetamide adapted to swell said fibre, and a solvent-compatible dye dissolved in said solution; and (2) heating the poly(m-phenyleneisophthalamide) fibre treated in step (1) to fix said dye to said fibre. An aqueous dimethylsulphoxide solution is used as the preferred swelling agent.

Description: This invention relates to a process for continuously or semi-continuously dyeing poly(m-phenyleneisophthalamide) - aramid - fibres in which the dye is introduced into the fibre while the fibre is in a solvent-swollen state. A flame retardant is introduced into the fibre simultaneously with the dye. Aramid fibres are highly resistant to heat decomposition, have inherent flame retardant properties and are frequently used in working wear for special environments where flame retardant properties are required. Fabrics made of these fibres are extremely strong and durable, and have been widely adopted for use in the protective clothing field, particularly for military applications where personnel have the potential to be exposed to fire and flame, such as aircraft pilots, tank crews and the like. Meta-linked aromatic polyamide fibres (aramid fibres) are made from high molecular weight polymers that are highly crystalline and have either a high or no glass transition temperature. These inherent desirable properties of aramid fibres also create difficulties for fibre processing in other areas; specifically, aramids are difficult to dye. Fibre suppliers currently recommend a complicated exhaust dyeing procedure with a high carrier (acetophenone) content; the process is conducted at high temperatures over long periods of time and often results in a product having an unpleasant odor. Such dyeing conditions require substantial amounts of energy both to maintain dyeing temperature and for the treatment of waste dye baths. Polar organic solvents have also been used to swell the fibre or create voids in the fibre structure to enhance dyeability. These procedures involve solvent exhaust treatments at elevated temperatures with subsequent dyeing. Another source of dyed aramid fibre is solution dyed aramid yarn, available from the fibre producer, prepared by solution dyeing in which a quantity of dye or pigment is mixed with the molten polymer prior to extrusion of the polymer into fine fibres; the dye or pigment becomes part of the fibre structure. Solution dyed fibres are more costly than the undyed fibres due, in part, to the additional costs of manufacture, and must be used in the colour provided by the supplier leaving the weaver with only a limited choice of colours. Solution dyed fibres offer relatively good lightfastness whereas some undyed aramid fibres, particularly NOMEX (Trade Mark of E.I. duPont, Wilmington, Delaware, USA), yellow following exposure to UV light. Because of this potential for yellowing, although deep, rich colourations, particularly dark blue and navy blue, are achievable, they still lack acceptable lightfastness. More recently, a process has been described in US-A-4,525,168 in which acid or anionic dyes are introduced into aramid fibres by coupling the dye to a dye site receptor which, in turn, is attached to the fibre. The process includes first swelling the fibre in a strong polar solvent and, while in the swollen condition, introducing a substance capable of forming a strong chemical bond with an anionic dye into the swollen fibre. This dye site receptor substance is an amine, typically hexamethylenediamine. The procedure described requires at least three steps, first pre-treating the fibre in a solution of solvent/swelling agent, the diamine and a wetting agent, then drying to shrink the fibre and incorporate the diamine dye site receptor into the fibre. The thus pre-treated fabric is then dyed with an anionic dye. Aramid fibres described and purported to be successfully dyed in US-A-4,198,494 are sold under the trademarks NOMEX and KEVLAR by duPont and under the trade mark CONEX by Teijin Limited of Tokyo, Japan. The present invention seeks to provide a process for (a) continuously or semi-continuously dyeing a dyeable, compatible aramid fibre that will yield acceptable colour fastness without detracting from the inherent flame resistance and strength properties of the aramid fibres, and (b) continuously dyeing large quantities of compatible aramid fabric on a commercial scale at less cost than prior procedures. Accordingly, the present invention provides a process for continuously or semi-continuously dyeing a poly(m-phenyleneisophthalamide) fibre characterised by comprising the steps of: (a) contacting a dyeable poly(m-phenyleneisophthalamide) fibre with a dyebath solution containing (1) an organic polar solvent swelling agent selected from the group consisting of dimethylsulphoxide, N-methylpyrrolidone and dimethylacetamide, (2) a compatible inert diluent to dilute the swelling agent and protect the fibre from degradation, (3) a dye which is for dyeing the fibre and which is dissolved in the solution, and (4) a flame retardant, and (b) heating the fibre to fix the dye to the fibre. The swelling agent may be adapted to swell the fibre and allow the dye to enter into and become fixed in the fibre. The swelling agent and inert diluent may be present in proportions such that the mechanical strength of the dyed fibre is at least 80% of the strength of untreated fibre. The solution may be maintained at a temperature in the range of room temperature up to about 93°C (200°F). If desired, the temperature may be from about 60°C (140°F) to about 93°C (200°F). The fibre treated in step (a) may be held at ambient temperature for a time sufficient to fix said dye to said fibre. The fibre may be washed to remove any residual dye and organic swelling agent. Preferably, the diluent is selected from the group consisting of water, xylene, ethylene glycol, lower alcohols and 4-butyrolactone. Preferably, the dye is selected from the group consisting of acid dyes, mordant dyes, basic dyes, direct dyes, disperse dyes and reactive dyes. Advantageously, the strength of the dyed fibre is at least 90% of the strength of an untreated fibre. The weight ratio of swelling agent to inert diluent may be from about 70:30 to about 90:10. Preferably the organic swelling agent is dimethylsulphoxide. In accordance with one aspect of the present invention when the poly (m-phenyleneisophthalamide) fibres are produced using a flame retardant, then there is provided a woven or knit fabric of dyed poly(m-phenyleneisophthalamide) fibres characterised by having a limiting oxygen index (ASTM D-2863-77) in the range of 28 to 45. The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- Figure 1 is a schematic illustration of a process according to the present invention of applying a dye, a swelling agent and a flame retardant from a hot pad bath to a poly(m-phenyleneisophthalamide)-containing fabric, fixing the dye and drying the fabric over a stack of steam cans, washing to remove any residual swelling agent, drying the fabric on a second set of steam cans, and taking the dyed fabric up on a roll; Figure 2 is a schematic illustration of a process according to the present invention of applying a dye, a swelling agent and a flame retardant from a pad bath onto a poly(m-phenyleneisophthalamide)-containing fabric, drying and fixing a poly(m-phenyleneisophthalamide-containing fabric in a tenter oven, followed by washing and drying on a stack of steam cans; FIGURE 3 is a schematic illustration of a process according to the present invention of applying a dye pad bath at elevated temperature to a poly(m-phenyleneisophthalamide)-containing fabric, holding the fabric at ambient conditions for a period of time to fix the dye, followed by washing and drying; FIGURE 4 is a schematic illustration of a process according to the present invention of dyeing a fabric on a semi-continuous basis at an elevated temperature by padding a dye, a swelling agent and a flame retardant onto a poly(m-phenyleneisophthalamide)-containing fabric, batching the wet fabric on a roll for an extended period of time to fix the dye, then unwinding, washing and drying the dyed fabric; and FIGURE 5 is a graph showing reflectance value (KSSUM), a measure of colour, as a function of treatment of dwell time or poly(m-phenyleneisophthalamide) fibres in the fibre swelling agent/dye, at several temperatures. The flame retardant/performance properties of fabrics dyed by a process according to the present invention are significantly improved, far better than if after treated with a fire-retardant finish applied from an aqueous solution following the dyeing and fixing operation. LOI values, as described in more detail below, may be as high as 44% for the simultaneously dyed and flame retarded T-455 NOMEX fabric produced by a process according to the present invention. As a means of comparison, undyed T-455 NOMEX fabric has an LOI of 26.6%. Fibre swelling is accomplished in an aqueous solution of one or more fibre swelling agents. The following polar organic solvents have been found to be preferred swelling agents for poly(m-phenyleneisophthalamide) fibre: N-methylpyrrolidone dimethylsulphoxide (DMSO) dimethylacetamide (DMAc) Conveniently, these swelling agents are mixed with a compatible diluent, usually water, in various amounts; the swelling agent is present in a major amount, that is, more than half of the total weight of the solution. As an illustration, good dye fixation was obtained in a continuous pad-oven-dry process using dimethylsulphoxide (DMSO) and water in ratios of DMSO:water of 70:30 to 90:10 with best results at the 90:10 level. Fibres amenable to a process according to the present invention are generally known as aromatic polyamides or aramids and are made from a polymer known chemically as poly(m-phenyleneisophthalamide), i.e., the meta isomer which is the polycondensation product of metaphenylenediamine and isophthalic acid. Below is a listing of fibres now commercially available identified by fibre name (usually a trademark) and producer: Selection of a suitable aromatic polyamide amenable to a process according to the present invention can be conveniently made by subjecting a fibre sample to an abbreviated test to determine fibre dyeability. Experience indicates that fibres of the para isomer, poly(p-phenyleneterephthalamide), represented commercially by duPont's KEVLAR and Enka-Glanzstoff's ARENKA, as well as Rhone-Poulenc's KERMEL and polybenzimidazole (PBI), are merely stained or changed in colour but are not dyed by a process according to the present invention. Accordingly, as used in the text of this application and in the claims that follow, the expressions "aramid" and "aromatic polyamide fibre", when pertaining to a process according to the present invention, will signify the meta isomer. Blends of poly(m-phenyleneisophthalamide) fibres with other fibres, including fibres of the para isomer, may be subjected to the dyeing process in which case only the meta isomer fibres will be dyed. The diluted polar organic solvent used in a process according to the present invention has the ability to swell the aromatic polyamide fibre to be dyed with minimum or no damage to the fibre itself. Many polar organic solvents will successfully swell aromatic polyamide fibres to introduce a dye into the fibre but damage the fibre itself and are thus unsuited for use in undiluted form. Fibre damage can be mitigated or avoided by including an otherwise inert and compatible diluent such as water in the swelling agent system. An important application of fabrics made of aramid fibres is the protection of military personnel. To be fully acceptable for military applications, dyed aromatic polyamide fabrics must meet minimum strength requirements as defined in U.S.A. MIL-C-83429A for solution dyed fabrics. For convenience, comparison of the undyed (greige) T-455 fabric with the solution-dyed T-456 fabric and the dyed fabric resulting from the process according to the present invention will be made. Highly polar organic solvents are notorious for degrading mechanical properties of aramid-type fibres, possibly by dissolving or solvating the polymer. To accommodate for this potential concern, the swelling agent system selected, when used at the appropriate temperatures and under the usual processing conditions, will result in a dyed aromatic polyamide fibre or fabric exhibiting at least 80%, preferably at least 90% if not identical to the strength of either the greige T-455 fibre or fabric as the case may be. Expressed conversely, the successfully dyed fibre or fabric exhibits no more than a 20% loss in strength, and preferably far less strength loss, and still will be acceptable for most applications. The swelling agent system is composed of at least two components: (1) an organic polar solvent, and (2) a compatible, miscible "inert" diluent (inert in the sense that it does not itself enter into the dyeing process or interfere with the dyeing process) to minimize any damage that the polar organic solvent may cause to the fibre. It will be appreciated that the proportion of organic solvent to diluent, as well as the identity of each of the components, will vary depending upon several factors including the colour to be achieved and the nature of the specific poly(m-phenyleneisophthalamide) fibre to be dyed, among others. Suitable swelling agents are selected from dimethylsulphoxide (DMSO), dimethylacetamide (DMAc), and N-methylpyrrolidone; DMSO is preferred. Suitable inert diluents include water, xylene (ortho, meta or para-dimethylbenzene), lower alkene glycols such as ethylene glycol and propylene glycol, alcohols such as n-propanol, methanol, benzyl alcohol, 4-butyrolactone, all of which are compatible with DMSO as the swelling agent, or other relatively high boiling organic liquids otherwise suited to the dyeing process. The selection of swelling agent and diluent is guided by optimum colour yield balanced with minimum fibre damage. While not wishing to be bound to any particular theory or mode of operation, experience leads the inventor to believe that the swelling agent modifies the aromatic polyamide fibre by allowing the dye and flame retardant to enter the fibre. Examination by mass spectroscopy fails to reveal any swelling agent (DMSO) in a fibre dyed by the process of this invention. The mechanism of dye attachment to the fibre is less clear but is believed to be a physical entrapment rather than a chemical covalent bonding. The absence of swelling agent in the fibre following treatment provides an odor-free product, allowing the swelling agent to be more efficiently recovered and permits practice of the invention without untoward environmental concerns. The particular type of dyestuff used in the process is not critical and may be selected from acid, mordant, basic, direct, disperse and reactive,and probably pigment or vat dyes. Especially good results with high colour yields are obtained with the following classes of dyes, particular examples given parenthetically: acid dyes (Acid Green 25), mordant dyes (Mordant Orange 6), basic dyes (Basic Blue 77), direct dyes (Direct Red 79), disperse dyes (Disperse Blue 56) and reactive dyes (Reactive Violet 1). Mixtures of two or more dyes from the same class or two or more dyes of different classes are contemplated. The dye selected will be compatible with and function effectively in the swelling agent system. One or more flame-retardant agents in amounts sufficient to increase the already inherent flame resistant properties of the fabrics are included in the dyebath to achieve simultaneous dyeing and flame-retardant treatment of a fabric. Conventional flame retardants may be used provided that they are compatible with other components of the system, notably the swelling agent, and impart the required degree of flame resistance to the treated aramid fibres. Flame retardant agent concentrations from 0.1% to about 20% are contemplated; however, the upper limit as a practical matter will be determined by the degree of performance required balanced against the cost of the FR chemical or system used. Concentrations in the range of about 1% to about 15% have been shown to be effective in increasing LOI values from 26.6% for greige NOMEX T-455 to 44% for NOMEX T-455 that has been simultaneously dyed and flame-retardant treated by a process according to the present invention. Amounts as little as 1% add-on flame-retardant agents results in an LOI value of 30+% for the dyed flame-retardanttreated fabric made by a process according to the present invention. Fixation of the flame retardant and the dye is by heating such as using a tenter frame, drying on steam cans or the like. Preferred flame-retardant agents used in a process according to the present invention are thermally stable cyclic phosphonate esters prepared by reacting alkyl-halogen-free esters with a bicyclic phosphite. As a class these cyclic phosphonate esters are represented by one of the following formulae: where a is 0 or 1; b is 0, 1 or 2, c is 1, 2 or 3 and a+b+c is 3 ; R and R′ are the same or different and are alkyl (C₁-C₈), phenyl, halophenyl, hydroxyphenyl, tolyl, xylyl, benzyl, phenethyl, hydroxyethyl, phenoxyethyl, or dibromophenoxymethyl; R² is alkyl (C₁-C₄); and R³ is lower alkyl (C₁-C₄) or hydroxyalkyl (C₁-C₄) or where d is 0, 1 or 2; e is 1, 2 or 3; R² is alkyl (C₁-C₄); R³ is lower alkyl (C₁-C₄) or hydroxyalkyl (C₁-C₄); R⁴ is alkyl (C₁-C₄) phenyl, halophenyl, hydroxyphenyl, hydroxyethyl, phenoxyethyl, dibromophenoxyethyl, tolyl, xylyl, benzyl, or phenethyl; and R⁵ is monovalent alkyl (C₁-C₆), chlorophenyl, bromophenyl, dibromophenyl, tribromophenyl, hydroxyphenyl, naphthyl, tolyl, xylyl, benzyl, or phenethyl; divalent alkylene (C₁-C₆), vinylene, o-phenylene, m-phenylene, p-phenylene, tetrachlorophenylene (o, m, or p), or tetrabromophenylene (o, m, or p); or trivalent phenyl. The preferred compounds are represented by the formula: in which X is 0 or 1, and usually a 50:50 mixture of the mono- and di-esters. The preparation of these cyclic phosphonate esters and their use as flame retardants are described in US-A-3,789,091 and US-A-3,849,368. In addition to the swelling agent, the inert diluent(s) and the dye, the customary dye pad bath additives and auxiliaries may be included, such as softeners (to improve hand), UV absorbing agents, IR absorbing agents, antistatic agents, water repellants, anti-foaming agents, and the like. Alternatively, these and other treatments may be applied to the fabric as a post-treatment finish after dyeing, heating, washing and drying are completed. Preferably the dyed fabric is water washed to remove any residual swelling agent remaining on the fabric. Typically, the wash water remains clear (uncoloured) indicating good dye fixation. Greige fibres that are dyed by a process according to the present invention (as distinguished from solution-dyed fibres in which a colouring agent is included in the molten resin prior to fibre formation) are virtually free of acetophenone and chlorinated solvents such as perchloroethylene. Residual DMSO amounts in fibres dyed by a process according to the present invention have been measured at less than 0.012 ppm. The dyed fibres have a strength retention of at least 80% of the undyed fibres. These properties distinguish products produced by the process from aramids dyed by the conventional process, using acetophenone as a dye carrier, which retain that solvent tenaciously, and NOMEX dyed by the STX process (Rhone-Poulenc Chemie, France, a 90:10 v/v mixture of perchloroethylene:methanol as the dyeing medium) in which the fibres retain small amounts of perchloroethylene. The physical form of the fibre to be dyed is also open to wide variation at the convenience of the user. Most dyeing operations and equipment are suited to treatment of woven or knit fabrics in the open width as illustrated in Figures 1 - 4. It is also possible to slasher dye the fibres in yarn form and thereafter weave or knit the yarns into the item desired. Testing procedures that were used in the examples are described in detail as follows: Flame Retardant Federal Test Method 5903 (USA) is intended for use in determining the resistance of cloth to flame and glow propagation and tendency to char. A rectangular cloth test specimen (70mm x 120mm) with the long dimension parallel to the warp or fill direction is placed in a holder and suspended vertically in a cabinet with the lower end 1.9cm (0.75 inch) above the top of a Fisher gas burner. A synthetic gas mixture consisting primarily of hydrogen and methane is supplied to the burner. After the specimen is mounted in the cabinet and the door closed, the burner flame is applied vertically at the middle of the lower edge of the specimen for 12 seconds. The specimen continues to flame after the burner is extinguished. The time in seconds the specimen continues to glow after the specimen has ceased to flame is reported as afterglow time; if the specimen glows for more than 30 seconds, it is removed from the test cabinet, taking care not to fan the glow, and suspended in a draft-free area in the same vertical position as in the test cabinet. Char length, the distance (in mm) from the end of the specimen, which was exposed to the flame, to the end of a lengthwise tear through the center of the charred area to the highest peak in the charred area, is also measured. Five specimens from each sample are usually measured and the results averaged. Flame Retardant Federal Test Method 5905 (USA), flame contact test -- a measurement of the resistance of textiles and other materials to flame propagation that exposes the specimen to the flame source for a longer period of time than test method 5903. A test specimen the same size as in the above method is exposed to a high temperature butane gas flame 7.6cm (3 inch) in height by vertical suspension in the flame for 12 seconds, the lowest part of the specimen always 3.8cm (1.5 inch) above the centre of the burner. At the end of 12 seconds, the specimen is withdrawn from the flame slowly, and afterflaming is timed. Then the specimen is re-introduced into the flame and again slowly withdrawn after 12 seconds and any afterflame timed. For each 12-second exposure the results are reported as: ignites, propagates flame; ignites but is self-extinguishing; is ignition resistant; melts; shrinks away from the flame; or drops flaming pieces. In the examples that follow, all parts and percentages are by weight. Limiting Oxygen Index (LOI) is a method of measuring the minimum oxygen concentration needed to support candle-like combustion of a sample according to ASTM D-2863-77. A test specimen is placed vertically in a glass cylinder, ignited, and a mixture of oxygen and nitrogen is flowed upwardly through the column. An initial oxygen concentration is selected, the specimen ignited from the top and the length of burning and the time are noted. The oxygen concentration is adjusted, the specimen is re-ignited (or a new specimen inserted), and the test is repeated until the lowest concentration of oxygen needed to support burning is reached. The continuous dyeing process of this invention is time and temperature dependent -- higher temperatures and longer treatment times favor higher reflectance values, expressed in the graph of Figure 5 as KSSUM, a measure of colour. Highest KSSUM values are obtained where the treatment time is at least 30 minutes and the dyebath is at least 60°C (140°F); this value improves slightly as the temperature increases (see the line connecting the + data points). By contrast, very short treatment times (box line) achieve only about half the KSSUM values even at treatment temperatures of 93°C (200°F). This information together with related data and comparisons will provide the operator with ample guidance to carry out the process of the invention. The invention will now be explained with reference to the following examples: Example Continuous dyeing of Type 455 woven Nomex in open width was accomplished as follows: three pad baths were prepared each containing 90 parts by weight DMSO and 10 parts by weight water to which was added a mixture of 1.20% Irgalan Olive 3 BL 13 (Acid Green 70), 0.09% Intralan Orange P2, and 0.09% Nylanthrene Yellow SL 20 (Acid Yellow 198) to make sage green. The first pad bath contained no fire retardant, the second 2.5% of Antiblaze 19 and the third bath contained 15.0% Antiblaze 19. The dyebath was padded onto T-455 Nomex at 93°C (200°F) from a heated bath at a speed of 6 m (20 yards) per minute and a pad pressure of 14060kg/m² (20 psi) resulting in a wet pick-up of approximately 90%. The padded fabric was then dried on steam cans maintained at 121°C (250°F) for about 24 seconds resulting in a fabric temperature of about 82-101°C (180-215°F). The fabric was then washed and dried in an oven. Samples of the fabric so treated were then subjected to testing for flame-resistant properties including Limiting Oxygen Index (LOI) and Federal Test Methods (FTM) 5903 and 5905. LOI values are reported for the treated fabric, after scouring and after 25 launderings; W is width, F is fill. Results of the tests are given in the following Table:

Claims: A process for continuously or semi-continuously dyeing a poly(m-phenyleneisophthalamide) fibre characterised by comprising the steps of: (a) contacting a dyeable poly(m-phenyleneisophthalamide) fibre with a dyebath solution containing (1) an organic polar solvent swelling agent selected from the group consisting of dimethylsulphoxide, N-methylpyrrolidone and dimethylacetamide, (2) a compatible inert diluent to dilute the swelling agent and protect the fibre from degradation, (3) a dye which is for dyeing the fibre and which is dissolved in the solution, and (4) a flame retardant, and (b) heating the fibre to fix the dye to the fibre. A process as claimed in claim 1, characterised in that the solution is maintained at a temperature in the range of room temperature up to about 93°C (200°F). A process as claimed in claim 1 or 2 characterised in that the fibre is washed to remove any residual dye and organic swelling agent. A process as claimed in any preceding claim characterised in that the diluent is selected from the group consisting of water, xylene, ethylene glycol, lower alcohols and 4-butyrolactone. A process as claimed in any preceding claim characterised in that the dye is selected from the group consisting of acid dyes, mordant dyes, basic dyes, direct dyes, disperse dyes and reactive dyes. A process as claimed in any preceding claim characterised in that the weight ratio of swelling agent to inert diluent is from about 70 : 30 to about 90 : 10. A process as claimed in any preceding claim characterised in that the organic swelling agent is dimethylsulphoxide. A woven or knit fabric of dyed poly(m-phenyleneisophthalamide) fibres produced by a process as claimed in any of claims 1 to 7 characterised by having a limiting oxygen index (ASTM D-2863-77) in the range of 28 to 45.

Inventors: CATES, BARBARA J., DAVIS, JAMES K., FITZGERALD, TANYA E., RUSSELL, ERNEST K.
Applicants: BURLINGTON INDUSTRIES, INC.
Application year: 1987
Main classification: D06P3
IPC codes: D06P3, D06P1