Vinylidene Chloride Polymer Microgel Powders and Acrylic Fibers Containing Same

The present invention relates to flame-retardant acrylic fibers and, more particularly, to acrylic fibers containing vinylidene chloride polymers.

Acrylic fibers, i.e., those containing at least 85 weight percent acrylonitrile, are well known in the art and widely used in commercial textile manufacture due to their excellent physical properties. However, such fibers have the distinct disadvantage that they ignite rather easily upon exposure to flames or high temperatures. Attempts to remedy this deficiency have been many and varied. For example, copolymerization of acrylonitrile with a halogen-containing monomer such as vinylidene chloride has resulted in fibers having considerably improved flame resistance. Typically, however, such copolymer fibers (generally classed as modacrylic fibers) also exhibit reduced sticking temperature and high shrinkage at elevated temperatures, such as upon exposure to boiling water or steam.

In order to provide a better balance between flame-retardancy and other desirable physical properties, attempts have also been made to blend various copolymers of acrylonitrile. For example, U.S. Pat. No. 3,963,790 is directed to a flame-resistant fiber wherein at least 50 weight percent of the fiber is a copolymer of about 79.5 to about 58 weight percent of acrylonitrile, 20 to 40 weight percent vinylidene chloride and 0.5 to 2 weight percent of a comonomer which enhances dyeability and 1 to 50 weight percent of the fiber is a second copolymer of 20 to 50 weight percent of acrylonitrile and 50 to 80 weight percent of vinylidene chloride. Inasmuch as such fibers consist of no more than a blend of two modacrylic fiber-forming polymers, it is inherent that they cannot be expected to exhibit the excellent properties of acrylic fibers.

Other attempts at providing flame-retardant acrylic fibers have involved the use of a variety of extraneous additives, such as antimony oxides, halogenated paraffins, halogenated hydrocarbons, and low molecular weight phosphate esters. It could be expected that only moderate decrease in beneficial physical properties would result if a minor amount of such an additive were dispersed throughout the continuous phase of an acrylic fiber structure. However, it has been the typical experience that effective utilization of these additives has undesirably required their presence in rather high concentrations.

It is, therefore, an object of the present invention to provide a flame-retardant acrylic fiber by utilizing an additive whose presence would only moderately affect the desirable inherent physical properties, such as high sticking temperature and low shrinkage, of an unmodified acrylic fiber. It is a further object to provide a novel vinylidene chloride polymer which is particularly suited for use as such an additive.

In one embodiment, the present invention provides a crosslinked vinylidene chloride polymer microgel powder which can be incorporated in acrylic fibers as a flame-retardant additive. The microgel powder is recovered from a latex obtained by emulsion polymerizing in sequence

(a) a first monomer mixture comprising about 85 to about 95 parts by weight of vinylidene chloride, about 5 to about 15 parts by weight of a copolymerizable ethylenically unsaturated comonomer, and a minor amount of a copolymerizable crosslinking polyfunctional comonomer, wherein the polymer resulting from the polymerization of the first monomer mixture has a gel content in the range of about 1 to about 50 percent;

(b) a minor amount of a polyfunctional comonomer for providing graft sites on the product of (a); and

(c) about 10 to about 25 weight percent, based on the weight of the first monomer mixture, of a second monomer mixture comprising about 85 to about 95 weight percent acrylonitrile and about 5 to about 15 weight percent of an ethylenically unsaturated comonomer copolymerizable with acrylonitrile.

The microgels in the resulting latex have a diameter less than about 1 micron.

In another embodiment, the present invention provides a spinning solution for the preparation of flame-retardant acrylic fibers comprising (a) a fiber-forming polymer containing at least 85 weight percent acrylonitrile; (b) a solvent for the fiber-forming polymer; and (c) a crosslinked vinylidene chloride polymer microgel powder as hereinbefore described.

In a further embodiment, the present invention provides a process for preparing such a spinning solution by (a) forming a generally uniform dispersion of the solvent and the crosslinked vinylidene chloride polymer microgel powder and (b) admixing the fiber-forming polymer with the so-formed dispersion.

In yet another embodiment, the present invention is directed to an acrylic fiber obtained by spinning such a solution.

The fibers of the present invention are to be generally classed as "acrylic fibers" inasmuch as the fiber-forming polymer contains at least 85 weight percent acrylonitrile. Methods for preparing such fiber-forming polymers are well known in the art, as are the comonomers that can be polymerized with the acrylonitrile. Exemplary comonomers include neutral monomers such as methyl acrylate, methyl methacrylate, and vinyl acetate; and ionic monomers such as sodium styrene sulfonate, sodium methallyl sulfonate and the vinyl pyridines.

The crosslinked vinylidene chloride polymer microgel powders of the present invention are recovered from latices whch have been prepared by polymerization in an aqueous emulsion according to processes well known in the art. Preferably, the polymerization is carried out by an essentially continuous, carefully controlled addition of the requisite polymerization constituents (including polymerization initiator systems if desired) to the aqueous medium.

Generally, it is preferred to start the polymerization by adding a small amount of the first monomer mixture (as defined hereinafter), or a monomer mixture which is similar in composition to the first monomer mixture, to the aqueous medium and then adding the desired polymerization initiator to form a polymeric seed latex to aid in the control of particle size. The aqueous medium in which the seed latex is formed will contain the necessary surfactants to form the emulsion and will generally be adjusted to the desired pH value, as is well known in the art. Following the formation of the polymeric seed latex, the remaining amount of the first monomer mixture (including initiator) is continuously added under carefully controlled conditions to the aqueous medium, thereby forming a "core" for the final polymeric product.

In accordance with the present invention, the first monomer mixture comprises about 85 to about 95, preferably about 90 to 95, parts by weight of vinylidene chloride; about 5 to about 15, preferably about 5 to about 10, parts by weight of a copolymerizable ethylenically unsaturated comonomer; and a minor amount of a copolymerizable crosslinking polyfunctional comonomer.

Exemplary copolymerizable ethylenically unsaturated monomers which can be utilized in the first monomer mixture include the alkyl esters of acrylic and methacrylic acids such as methyl acrylate and methyl methacrylate; nitriles of ethylenically unsaturated carboxylic acids such as acrylonitrile and methacrylonitrile; and other monomers known to polymerize with vinylidene chloride. Of these, acrylonitrile is preferred.

Exemplary copolymerizable crosslinking polyfunctional comonomers which can be employed include 1,3-butylene glycol dimethacrylate, 1,3-butylene glycol diacrylate, 1,4-butane diol dimethacrylate, 1,4-butane diol diacrylate, allyl acrylate, allyl methacrylate, vinyl acrylate, vinyl methacrylate, and the like. The actual amount of crosslinking polyfunctional monomer is not critical and will vary with the crosslinking efficiency of the particular monomer employed. However, it is desirable to utilize sufficient crosslinking monomer to provide a gel content of about 1 to about 50, preferably about 10 to about 40, percent in the polymer obtained by the polymerization of the first monomer mixture. Typically, the amount of crosslinking monomer used in the first monomer mixture to obtain such gel contents will range from about 1 to about 10, preferably about 1 to about 4, parts by weight.

As used herein, "percent gel" is determined by the following technique: Add 36.6 ml tetrahydrofuran (THF) and a predetermined amount (W.sub.S), usually about 0.7-1.2 g, of the desired microgel to a 50 ml centrifuge tube. Cap the tube and then agitate it overnight (usually about 12 hours) on a horizontal agitator. Thereafter, centrifuge the tube at 19,000 rpm for about 1 hour at 5.degree. C. Extract 10 ml of the resulting supernatant liquid and place it into an evaporating dish. Evaporate most of the THF over low heat and then complete the drying by placing the dish in an oven for about 1 hour at 40.degree. C. Finally, determine the weight (W.sub.F) of resin in the dish and calculate gel content by the following formula:

when the polymerization of the first monomer mixture is substantially or completely finished, a minor amount of a polyfunctional monomer is added to the polymerization vessel to provide graft sites for the subsequent polymerization of the second monomer mixture (as hereinafter defined). The amount of graft-site polyfunctional monomer will generally range from about 1 to about 5 percent of the weight of the first monomer mixture. The graft-site polyfunctional monomer may be the same as, or different than, the crosslinking monomer used in the first monomer mixture. Accordingly, those monomers which have heretofore been described as being suitable for use as crosslinking monomers are also suitable for use as graft-site monomers. In addition, conjugated diolefinically unsaturated monomers, such as 1,3-butadiene and the like, are particularly suitable graft-site monomers inasmuch as they typically provide residual unsaturation in the polymers prepared therefrom.

Shortly after the graft-site polyfunctional monomer has been polymerized, e.g., within about 1 hour after adding the graft-site monomer to the polymerization vessel, the second monomer mixture (including initiator) is continuously added under carefully controlled conditions to the aqueous medium. In accordance with the present invention, the second monomer mixture comprises about 85 to about 95 preferably about 85 to about 90, weight percent of acrylonitrile with the remainder comprising an ethylenically unsaturated comonomer copolymerizable therewith, e.g., the alkyl esters of acrylic and methacrylic acids such as methyl acrylate and methyl methacrylate. The amount of the second monomer mixture used will range from about 10 to about 25, preferably from about 15 to about 20, percent of the weight of the first monomer mixture.

The diameter of the microgels in the resulting latex should be less than about 1 micron. Preferably, the mean microgel size is in the range of about 0.1 to about 0.5 micron, with substantially all of the microgels falling within that range. Microgels having a diameter greater than 1 micron will have a tendency to plug a spinnerette used in the fiber-forming process and, accordingly, will generally not provide acceptable fibers.

The crosslinked vinylidene chloride polymer microgel powders of the present invention are recovered from the resulting latices by conventional techniques, preferably by coagulating the latex and then washing and drying the coagulum to produce a fine powder. The optimum temperature for coagulation will vary depending upon the type and amount of comonomer employed in preparing the microgels and particularly upon the second order transition temperature of the so-formed microgels. Generally, the coagulation temperature will be in the range of about 50.degree. to 90.degree. C., preferably from about 50.degree. to 70.degree. C.

The solvent used in the spinning solution does not appear to be critical. It is expected that conventional solvents for acrylic fiber-forming polymers can be employed. Examples of such solvents include dimethyl formamide, dimethyl acetamide, dimethyl sulfoxide, and the like. The concentration of polymers in the solvent may vary, depending upon the viscosity desired, the spinning process which will be used, and other factors known to those in the art. Normally, the viscosity of the spinning solution will range from about 500 to 1000 poises. The method of forming the spinning solution, however, appears to be critical. If the fiber-forming polymer is added to the solvent prior to the addition of the vinylidene chloride polymer microgel powder, a uniform dispersion of the latter cannot be obtained in a reasonable period of time. In contrast, if the vinylidene chloride polymer microgel powder is added to the solvent first, it can be readily dispersed in less than 30 minutes.

In accordance with the present invention, the spinning solution is spun into fibers by processes well known in the art. The resulting acrylic fibers will contain an effective flame-retarding amount of the vinylidene chloride polymer microgel powder. Generally, the amount will range from about 35 to about 45 percent of the weight of the fiber. Stated differently, it is desirable that the resulting fiber have a chlorine content of at least about 20 percent, perferably from about 20 to about 30 percent.

It will be understood that the fibers of the present invention may contain further modifying ingredients such as heat and light stabilizers, placticizers, antistatic agents, lubricants, optical brightners, conventional flame-retardant synergists, and like modifiers without departing from the scope of the invention, whether they are added before, during or after spinning.

The following specific examples are intended to illustrate the invention without limiting the scope thereof.

In accordance with the present invention, the following recipe and technique were used to prepare a crosslinked vinylidene chloride polymer microgel powder which can be incorporated into an acrylic fiber as a flame-retardant additive.

Initial water phase

1800 g distilled water

15 g AREOSOL MA80 emulsifier (80% active)

pH adjusted to 3.5 with acetic acid

Reducing agent

9.75 g HYDROSULFITE AWC in 1000 g aqueous solution

Feed rate=10 g/hr

Initiator

13.5 g of 70% t-butyl hydroperoxide (TBHP) in 1000 g aqueous solution

Freed rate=10 g/hr

Aqueous emulsifier stream

236 g DOWFAX 2Al emulsifier (45% active) in 1600 g aqueous solution

Used 800 g in 20 hours (Feed rate=40 g/hr)

Monomers for seed latex

50 g acrylonitrile (VCN)

450 g vinylidene chloride (VDC)

10 g 1,3-butylene glycol dimethacrylate (BGDM)

Used 150 g in seed latex reaction

First monomer mixture

200 g VCN

80 g BGDM

3800 g VDC

Used 2500 g in 20 hours (Feed rate=125 g/hr)

Polyfunctional monomer

85 g 1,3-butadiene added at end of monomer feeding

Second monomer mixture

315 g methyl acrylate

1785 g VCN

Used 375 g in 3 hours (Feed rate=125 g/hr)

The initial water phase was introduced into a two-gallon reactor and placed under vacuum of about 25 inch Hg for 10 minutes while being heated to 40.degree. C. with agitation at 100 rpm. When the water phase had reached a temperature of about 40.degree. C., the vacuum was shut off and 150 g of seed latex monomer was introduced. Immediately, the reducing agent and initiator streams were introduced at 10 g/hr. When the seed latex reaction had proceeded to approximately 4 psi pressure drop from the maximum pressure achieved, introduction of the first monomer mixture at 125 g/hr and the emulsifier stream at 40 g/hr was begun and continued for 20 hours while concurrently pumping reducing agent and initiator each at 10 g/hr. After 20 hours of feeding the first monomer mixture, this stream was shut off and 85 g of butadiene was shot into the reactor. Within 15 minutes, pumping of the second monomer mixture was begun at 125 g/hr while continuing to feed emulsifier at 40 g/hr and the reducing agent and initiator each at 10 g/hr. The second monomer mixture feed and the emulsifier stream were shut off after 3 hours and the initiator and reducing agent were pumped for 3 additional hours at 10 g/hr each to complete the reaction. The latex was cooled to room temperature to prepare for polymer recovery.

The microgel powder was recovered from the latex by a common coagulation method: 7000 ml water and 35 g alum were mixed and heated to 50.degree. C. Then, with vigorous agitation, the 2700 ml latex was slowly added and the temperature was raised to 70.degree. C. and held at that temperature for five minutes. The mixture of water, alum, and polymer crumb was then rapidly cooled in ice and the wet polymer powder was collected in a centrifuge and washed with water for 10 minutes. Thereafter, the powder was air dried at room temperature.

The performance of a crosslinked vinylidene chloride polymer microgel powder as a flame-retardant additive in an acrylic fiber was evaluated.

A spinning solution was prepared by first adding 148 g of the powder obtained in Example 1 to 650 g of dimethyl formamide and thoroughly mixing the ingredients at room temperature in a Tekmar Dispax high shear agitator. The mixture was then heated to 50.degree. C. to ensure complete dispersion. The so-formed dispersion was slightly turbid, but uniform and relatively low in viscosity.

Next, 202 g of a fiber-forming polymer comprising a polymer used to prepare the commercial acrylic fiber CRESLAN.RTM. T61 was added and the mixture was stirred for 3 hours at a rate sufficient to maintain the temperature below 85.degree. C. (no external heating was required).

The resulting spinning solution was spun into fibers using a spinning unit having a reservoir, pump, filter, and spinnerette. The spinnerette was immersed in a coagulation bath comprising a mixture of 16 parts water and 84 parts dimethyl formamide at about 25.degree. C. The fibers were then passed over a wash roll, oriented under steam, and then relaxed under steam and gathered on a hot chrome roll at 105.degree. C. and then wound on spools.

The resulting fibers, having been subjected to a net draw of 5.0, were tested for physical properties. For comparison, fibers prepared in a similar manner, but without adding the vinylidene chloride polymer microgel powder, were tested. The results of these tests are shown in Table I.

The shrink test above was conducted by immersing the yarn in boiling water for 3 minutes and thereafter drying it without tension. Neither yarn had good shrinkage values, indicating that relaxation during the spinning process was poorly conducted. Nevertheless, the shrinkage values were very comparable for the same spinning conditions. The limiting oxygen index (LOI) was measured by well known procedures.

It can be seen from these tests that the fibers of the present invention have enhanced flame-retardancy (as indicated by LOI measurements) without a substantial loss of other beneficial physical properties, as compared to fibers which do not contain a crosslinked vinylidene chloride polymer microgel powder. Similar good results are obtained by using any of the microgel powders within the scope of the present invention as hereafter claimed.