Acrylic Resins in Textile J
Processing A. C. NUESSLE AND B. B. KINE
Research Laboratories, Rohm & Haas Co., Philadelphia, Pa. e
NTIL a few decades ago, the only polymeric materials em- ployed in the finishing of textiles were naturally occurring
substances such as rosin, gelatin, and various starches and gums. Today, however, the industry utilizes a large variety of synthetic polymers to achieve effects not possible with the natural products.
One group of synthetics which enjoys such use is the acrylics family, composed of resins derived from acrylic acid and related compounds. A simplified picture of the chemical relationship between the acrylics and other synthetic polymers commonly utilized in textile finishing is shown:
ties responsible for their utility in certain applications. Other polymer types are mentioned where it is desirable to show simi- larities or differences, because the use of acrylics in a particular application will be unique only where other polymers do not have the required properties.
GENERAL PHYSICOCHEMICAL CONSIDERATIONS
A linear polymer may exist in any one of three states, depend- ing on its temperature, its chemical nature, and its molecular weight. Below a certain temperature the polymer is frozen in a glassy state, in which it possesses the mechanical properties of a rigid solid. With increasing temperature there occurs a second-
order transition, beyond which the polymer enters a plastic state wherein under small periods of applied stress it has the revers- ible deformation of a rubber. At a still higher temperature the polymer may enter a state of viscous flow, in which nonrevers- ible deformations take place; this occurs most readily with polymers of low molecular weight, but may also occur with high polymers if the temperature is sufficiently elevated. Under
Melamine F Ketone F (generally Vinylidene normal conditions of use, how- ever, most linear polymers em- ployed in the textile industry
polymers I' sym-Ethylenes unsym-Ethylenes
I Formaldehyde Polyesters
halides Styrene Acrvlics
7- condensates I Maleic derivatives Vinyl esters
I Phenol F
The acrylics are a sdbdivision of the unsymmetrically sub- stituted ethylene class, which comprises a large number of resins obtained by the polymerization of monomers containing a CHz=C< group. It is not unexpected, therefore, that in proper- ties and uses they are more closely related to vinyl polymers such as polystyrene or polyvinyl acetate, than to the condensation resins such as the urea-formaldehydes.
The acrylic monomers include both acrylic and methacrylic acids, and their salts, esters, amides, and nitriles:
CH-CH CH-CH CHFCH I
CONHz 2- I
bOOH COONa COOR
Acrylic Acrylate Acrylic Acrylamide acid salt ester
8"" etc. COOH
Acrylonitrile Methacrylic acid
The present paper is concerned with textile uses of acrylics, and especially with some of the chemical and physical proper-
are in either the glassy or the rubbery state.
The transition from glassy to rubbery is a fairly sharp one, but the observed value of the transition temperature may vary widely, depending on the method of test; probably the most critical variable is the rate of testing. A number of mechanical tests have been proposed, involving softening temperature (1 ), brittle point (9), Young's modulus (9), and the like. A typical curve, showing the effect of temperature on torsional stiffness, is given in Figure 1.
A comparison of test methods is outside the scope of this paper. Of greater importance to the present discussion is the fact that, under specific conditions of test, the location of Tg varies markedly with the chemical composition of the polymer. For example, an ethyl acrylate polymer may have a transition temperature of -20' C., while a polymer of ethyl methacrylate may have a Tg of f55" C., as indicated in Figure 2. At room temperature the ethyl polyacrylate would be soft and rubbery, while the ethyl polymethacrylate would be stiff. The stiffness of a polymer in the rubbery state also tends to increase with molecular weight, but most polymers used in textile applications are polymerized to a degree where the effect of molecular size is no longer critical. Accordingly, the transition temperatures of the various polymers, and their relative stiffness a t a given temperature, are determined primarily by their chemical constitution.
The following generalizations may be made concerning the relationship between chemical structure and physical properties of the acrylic polymers.
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t 60 GLASSY
Tg TEMPERATURE - Figure 1. Effect of Temperature on Torsional
Because the alkyl group tends to separate the polymer mole- cules and act as a plasticizer, the lower members of the n-alkyl acrylate series become even softer with increasing length of the alkyl chain. Higher in the series, there is a reversal as a waxy stiffness sets in owing to the tendency of the longer alkyl chains to crystallize. Brittle point measurements, which determine the temperature a t which an arbitrary brittle stiffness is attained, set the minimum at n-octyl(9); however, a test based on relative hardness a t a fixed, more elevated temperature-e.g., room tem- perature-would put the minimum somewhat higher.
The n-alkyl methacrylates similarly become softer, to a mini- mum, by brittle point, a t dodecyl ( 8 ) .
Branched-chain alkyl esters have greater rigidity than the corresponding straight-chain esters. The tert-butyl esters are especially stiff ( 7 ) .
Acrylonitrile and methacrylonitrile polymers are very stiff and have a very high Tg, presumably because of strong hydro- gen-bonding tendencies of the compact, polar nitrile group.
The polyacids, their amides, and their alkali salts, which are so strongly polar as to be generally water-soluble, give stiff films having relatively little thermoplasticity.
The effect of copolymerization tends to be additive, with the copolymer exhibiting physical properties intermediate between those of its components. Thus, the physical as well as chemical properties can be varied over a wide range by suitable choice of monomer combinations.
One other important factor which may influence the physical properties of a polymer film is its mode of preparation. A film laid down from emulsion-Le., latex-may be somewhat less continuous than one laid down from solvent, particularly if the transition temperature is high. As is illustrated in a later sec- tion, this can influence the type of finish obtained on the textile fabric.
METHOD OF APPLICATION
There are four general methods of applying acrylics to textile materials:
FROM WATER SOLUTION. The water-soluble polymers include polyacrylic and polymethacrylic acids, their alkali and am- monium salts, and their amides. Acrylonitrile and the acrylic esters cannot be so applied, unless copolymerized with a sub- stantial proportion of one of the water-soluble acrylics. Applied to textile yarns or fabrics, such materials usually remain mater- soluble, unless insolubilized by some special method, and are therefore most useful where wash-fastness is not required.
The acrylic esters and their co- polymers with acrylonitrile may generally be applied from solu-
FROM SOLVENT SOLUTION.
tion in ethylene dichloride or other suitable solvent. Because of greater fire hazard, toxicity, and expense, this is usually less de- sirable than an application from water medium, but finds use in applications where a very glossy surface coating is desired.
FROM EMULSION POLYMER. The water-insoluble monomers may be polymerized in emulsion form, to produce an aqueouq dispersion or latex. Properly made, such dispersions are stable in storage for a year or more, and may be readily diluted with water to any degree at time of use. On drying, most dispersions are irreversibly broken, so that the finish is wash-fast. Because of the ease of application, this is the most popular method of applying the water-insoluble acrylics to textiles.
APPLIED AS MONOMER. There are many ways of applying an acrylic monomer (or combination of monomers) to a textile material, and subsequently bringing about polymerization in place. By such means, the polymer can in many instances be built up within the fiber, rather than deposited on the surface. Some attention has been given to this method by various in- vestigators, although thus far little commercial utilization has resulted.
It is evident that the acrylics are most often applied either from water solution or from aqueous emulsion.
PHYSICAL PROPERTIES AXD ESD USE. One of the most com- mon objectives of textile finishing is the modification of the hand or feel of a fabric. By this means it is possible to weave a standard fabri: and modify it later to meet the requirements of the current fashion trend. It is also possible to take a light, inexpensive fabric and build it up to simulate a heavier, more expensive fabric. This is generally done by treating the fabric with a polymeric material which will penetrate the yarns and bind the fibers together, and also bond the yarns a t the crossover points. Thus the fabric becomes less multifilamentous, and
t I 11 EA - 20 ROOM TEMP +55
PEkrbiYk 6. - Figure 2. Effect of Chemical Composition on
Transi t ion Tempera ture
therefore less flexible. In order to do this, the polymer must adhere t o the fibers; but assuming good adhesion (as is almost invariably the case with cellulosic fabrics) the effect on fabric Lfhand is then dependent to a large degree on the physical proper- ties of the polymer.
For example, consider the effect of two conlmercially available acrylic dispersions applied to an 80 x 80 cotton sheeting. One of these, designated as emulsion A, when dried doivn on gla% in the absence of fabric, yields a soft, rubbery film, whereas the other, emulsion R, dries to a hard film. Each of these products was padded onto the fabric a t two concentrations and dried at two temperatures. Stiffness tests were then made using the Gurley tester (9) which measures the force required to bend the fabric sample unilaterally. Although mrh a test cannot take into account all of the many factors Tvhich go to make up the complex property known as hand, it does give an objective meawrement of one important fabric property-flexural stiffness. The data are listed in Table I .
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It is apparent that stiffness increases with concentration (as would be expected), and also increases slightly with tempera- ture of drying (because of more com lete fusion of the emulsion particles). Far greater, however, is t%e difference between prod- ucts: Emulsion B, which dries to a hard film, imparts consider- able stiffness to the-fabric, while emulsion A, which dries to a soft, rubbery film, Gves little stiffness but imparts full, heavy feel, which unfortunately cannot easily be reduced to a single numerical quantity. As these materials are both acrylics, it is obvious that a range of effects is available, depending on the type of acrylic selected.
TABLE I. FILM FORM OF EMULSION POLYMER vs. EFFECT ON FABRIC
Drying Gurley Solids Tfmp., Stiffnessa, Hand of
Polymer Film Form on Fabric, C. Mg../In. Fabric %
... 150 6 . 6 Water control . . . . . . . . . 120 6 . 6 Soft A
Soft, rubbery 3 . 9 120 1 0 . 2 3 . 9 150 1 0 . 4 Full, heavy 7 . 7 120 1 2 . 0 7 . 7 150 1 3 . 2
Hard, stiff 4 .1 120 38 Stiff 4 1 150 44 7 . 7 120 59 7 . 7 150 62
a Test made in warp direction.
To illustrate further the variety of effects, and to show how monomer composition controls the physical properties of the film and therefore the type of finish obtained, the following series of polymers was studied:
E 100% ethyl acrylate EM M 100% methyl methacrylate
50% ethyl acrylate, 50% methyl methacrylate
Each was polymerized in a similar system, using an anionic dispersing agent, and to an average chain length in a range where variations would not be critical; therefore, any marked differences between the materials may be attributed to their monomer composition.
Cotton sheeting was dipped into the polymer dispersions, squeezed, and dried either a t room temperature (ea. 25' C.) or at 150' C. Gurley stiffness values were obtained, and the hand was also evaluated subjectively. In addition, portions of each polymer were dried down in the absence of fabric to determine the film properties, The data, presented in Table 11, indicate the direct relation between the physical properties of the dried-down polymer and the effect on fabric. The soft, rubbery polymer (E) gives a full, heavy hand with little added stiffness; the stiff, brittle copolymer (EM) produces a stiff, rather crisp finish; but the even stiffer polymer (M), which has such a high fusing point that the tiny dispersed particles will not coalesce into a continuous film, has very little effect on the hand. A polymer of the latter type, lacking cohesion, may nevertheless have satis- factory adhesion to certain fibers, and in such a case may be used to deluster a very shiny fabric.
The necessity of fusing the dispersed particles to obtain proper cohesion is shown in the effect of drying temperature on copoly- mer EM. Drying at 150' C. almost doubled the stiffness over that obtained a t room temperature.
Each of these polymers, if precipitated from the dispersion and dissolved in a suitable solvent, will then dry down to a con- tinuous film, as there are no longer any discrete particles which need to be fused. The molecular flow required for film formation is maintained during drying by residual solvent, which temporar- ily lowers the transition temperature. Accordingly, polymer M would be expected to give a stiff finish to fabric if applied from solvent. It is also ap- parent that the drying temperature is much less critical when applying from solvent than when applying from aqueous dis- persion.
Data in Table I11 show that this is so.
TABLE 111. EFFECT OF SOLVENT APPLICATIONS Drying GurIey T:mp., Stiffness, Hand of
Solvent Solutions Film Form C. Mg./Inch Fabric Solvent only . . . . . . . 25 7 . 2 Soft E (EA)
150 7 . 1
150 11 E M (EA/MMA) Stiff, flexible 25 65 Stiff
150 73 M (MMA) Very stiff, brittle 25 112
Soft, rubbery 25 12 Full, heavy
a About 4% solids in ethylene dichloride.
Another way in which a dispersion of a hard polymer such as M may be made to give a continuous film, and therefore stiffness to fabric, is by plasticizing it with a softer polymer. In Table IV is shown the effect on fabric stiffness of a physical mixture of poly- mer dispersions E and M, dried on a t 15OoC., as compared with each polymer separately. As th...