Abstract:
A method is described for treating fabrics, yarns and individual fibers to improve the mechanical properties thereof, for example their wrinkle-resistance, by treating the fabric, yarn, and fibers in a solution containing nanoparticles. The nanoparticles include two sizes of particles and b appropriate selection of the nanoparticles the degree and mode of cross-linking in the fabric can be controlled with corresponding control of the mechanical properties.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to methods for the treatment of fibers, yarns, fabrics and textiles by the generation of a crosslinking architecture on a nanometer or micrometer scale. Such architecture can be applied for treatment of fabrics, yarns and fibers, but not limited to the above, for achieving desired and controlled physical and chemical properties. The invention also extends to fibers, fabrics and textiles so treated  
       DESCRIPTION OF THE PRIOR ART  
       [0002]     Fiber or fabric treatments for achieving valued added properties are valuable in textiles, home furnishing, and composite materials industries. Particularly in textile industries, various processes have been developed to achieve wrinkle-free/durable-press (DP) properties or antibacterial properties. For examples: U.S. Pat. No. 4,562,097 discloses a continuous process for creating a uniform foamable functional composition that can be used in the treatment of a textile fabric to improve its properties. U.S. Pat. No. 5,614,591 discloses an aqueous durable press treatment composition comprising a reactive modified ethylene urea resin, such as dimethylol dihydroxy ethylene urea (DMDHEU), a crosslinking acrylic copolymer derived from butyl acrylate and acrylonitrile, and a catalyst. This well-known process can be applied either to fabrics prior to fabrication into garments, or as a garment durable press process imparting durable press properties to fabricated garments. U.S. Pat. Nos. 5,856,245 and 5,869,172 disclosed a curable thixotropic polymer to form barrier webs that are either impermeable to all microorganisms or are impermeable to microorganisms of certain sizes or imparts specific properties to the end product material. U.S. Pat. No. 5,874,164 discloses novel barrier webs that have certain desirable physical qualities such as water resistance, increased durability, improved barrier qualities. This process is also based on a curable shear thinned thixotropic polymer composition, including fabrics that are capable of either selective binding certain microorganisms, particles or molecules depending upon what binding partners are incorporated into the polymer before application to the fabric. U.S. Pat. No. 5,885,303 provides a durable press wrinkle-free process which comprises treating a cellulosic fiber-containing fabric with formaldehyde, a catalyst capable of catalyzing the crosslinking reaction between the formaldehyde and cellulose and a silicone elastomer, heat-curing the treated cellulose fiber-containing fabric, under conditions at which formaldehyde reacts with cellulose in the presence of the catalyst without a substantial loss of formaldehyde before the reaction of the formaldehyde with cellulose to improve the wrinkle resistance of the fabric in the presence of a silicone elastomeric softener to provide higher wrinkle resistance, and better tear strength after washing, with less treatment. U.S. Pat. No. 5,912,116 presents a process based a curable shear thinned thixotropic polymer composition to offer water resistance, increased durability, improved barrier qualities of fabrics. U.S. Pat. No. 6,372,674 discloses a textile treatment process imparts water repellant, stain resistant, and wrinkle-free properties as well as aesthetically pleasing hand properties to a fabric made in whole or in part of fibers having a hydroxyl group, such as cellulosic fibers, though immersion in an aqueous bath and subsequent heating for curing.  
         [0003]     Although the above processes, to some extent, achieved the claimed properties, all suffer drawbacks such as the loss of tensile strength, abrasion resistance, and tear strength. Therefore, others have sought improvements using nanotechnology. WO 01/06054 discloses textile-reactive beads, whose inner sphere contains “payload”—for example, anti-biologic reagents, dyes, and UV-protecting agents, that can bind or attach to the fibers of the textiles or other webs to be treated, to provide permanent attachment of the payload to the textiles. In this process, the procedure for getting payload insides the nanobeads, however, is hard to control; the sizes of the nanobeads have wide distribution, which is ineffective to control the post-curing properties; and the resulting structure after treatment is unknown, which makes it difficult achieve desired properties; moreover, the persistent problem of the loss of mechanical strength of the treated textiles remains unsolved.  
       SUMMARY OF THE INVENTION  
       [0004]     According to the present invention there is provided a method of treating a fabric, yarn or individual fiber comprising the steps of (a) subjecting the fabric, yarn or individual fiber to an aqueous solution containing nanoparticles and a cross-linking agent, (b) drying the fabric, yarn or individual fiber, and (c) curing the fabric, yarn or individual fiber.  
         [0005]     Preferably the nanoparticles comprise first nanoparticles of a first size and second nanoparticles of a second size, the second size being larger than the first. The first nanoparticles may have a diameter in the range of from 18 to 50 nm, and the second nanoparticles may have a diameter in the range of 35 nm to 100 nm.  
         [0006]     The diameter of the first nanoparticles preferably forms a narrow distribution within the preferred range of diameters of the first nanoparticles, and the diameter of the second nanoparticles preferably forms a narrow distribution within the preferred range of diameters of the second nanoparticles.  
         [0007]     In preferred embodiments the number of second nanoparticles in the solution is greater than the number first nanoparticles by a ratio in the range of 1:1 to 4.2:1.  
         [0008]     The nanoparticles may be formed of surface modified polystyrene.  
         [0009]     Preferably the crosslinking agent comprises dimethyl dihydroxy ethylene urea (DMDHEU).  
         [0010]     Preferably the concentration of nanoparticles and cross-linking agent is selected to provide a wet pick-up of 60-70%.  
         [0011]     In preferred embodiments the curing is performed as a single step at a temperature of between 105-170° C. for between 1 to 20 minutes.  
         [0012]     The curing may preferably be performed as a two-step process.  
         [0013]     In one preferred embodiment of the invention prior to step (a) the fabric, yarn or individual fiber is subject to an aqueous solution comprising a cross-linking agent and is then cured under an applied pressure, and wherein the curing of step (c) is carried out under an applied pressure.  
         [0014]     Viewed from another broad aspect the present invention also extends to a fabric material wherein the fibers forming the material are cross-linked by a structure formed of nanoparticles. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]     Some examples of the invention will now be described by way of example and with reference to the accompanying drawings, in which:— 
         [0016]      FIG. 1  is an illustration of bimodal (two different sizes) nanoparticles on the surface of a fabric for generation hierarchical structures,  
         [0017]      FIG. 2  schematically illustrates nanoparticles on the surface of a yarn for generating hierarchical structures,  
         [0018]      FIG. 3  schematically illustrates nanoparticles on the surface of a fiber for generating hierarchical structures,  
         [0019]      FIG. 4  is a scanning electron micrograph of the narrow-dispersed nanoparticles,  
         [0020]      FIG. 5  is a plot illustrating the size distribution of the nanoparticles,  
         [0021]      FIG. 6  is a scanning electron micrograph of a hierarchical structures of a fiber,  
         [0022]      FIG. 7  is a plot showing the increase of the efficiency for the treatment of fabric by using nanoparticles,  
         [0023]      FIG. 8  is a plot showing the relation between recovery angles and the amounts of nanoparticles used, and  
         [0024]      FIG. 9  is a plot showing a comparision of recovery angle and mechanical properties of untreated samples, samples treated by conventional methods, and samples treated by an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0025]     The present invention provides a method for creating controlled, hierarchical crosslinking structure on fibers and fabrics using nanoparticles, thus enhancing the mechanical properties of cotton fabrics and other materials made of fibers.  
         [0026]     Several key features and benefits distinguish embodiments of the present invention from the prior art. As shown in  FIG. 1  nanoparticles with different sizes (bimodal) were applied on the fabric (details of preferred application methods will be described below) to generate domains with distinct mechanical and chemical properties in a controlled fashion, and thus offer desired enhancements.  FIG. 1  is an illustration of the nanoparticles of the two different sizes applied to the surface of a fabric prior to curing. As shown in  FIG. 2 , at the level of individual yarns  1 , crosslinking was accomplished by nanoparticles  2  plus conventional crosslinking agents (e.g. DMDHEU). At the level of a fiber, where the fiber  3  is formed from multiple fibrils  4  which may be natural or synthetic, as shown in  FIGS. 3 and 6 , the density of crosslinking exhibits three different degrees, thus inducing three regions with different morphologies, which contributes to the control of the mechanical properties. Again the crosslinking between individual fibrils in the fiber is achieved by nanoparticles  5  in the presence of conventional crosslinking agents and a heating step. With regard to the cross-linking agent, DMDHEU is one particularly suitable choice. However, other cross-linking agents can be used as well, for example, small molecules with multiple —COOH groups or —CHO groups. The concentration of DMDHEU is 15˜20% (v:v), catalyst (eg MgCl 2  or ZnCl 2 , with the latter being less preferred for biocompatibility reasons) 6˜20 g/L, and the nanoparticles content 3˜10 g/L. Generally, more DMDHEU and more nanoparticles lead to a higher density of cross-linking. However, it is both the amount of the cross-linking and the mode of the cross-linking (the crosslinking density difference on the surface of a cotton fiber and the core of a fiber as shown in  FIG. 6 ) that determine the final performance of the fabric. The amount of nanoparticles used and the sizes of the nanoparticles will affect the mode of cross-linking which is reflected in the recovery angles as can be seen from  FIG. 8  to be discussed further below. The essence of this invention is that the use of nanoparticles controls both the modes and the amount of the cross-linking, thus offers improved mechanical properties. An important aspect of the present invention, at least in its preferred forms, is that through the appropriate use of nanoparticles the amount of cross-linking on the surface of the fibers is increased, and the amount of cross-linking in the core of the fiber is minimized.  
         [0027]     The nanoparticles used comprise a mixture of smaller and larger particles. The smaller particles preferably range from 18 to 50 nm in diameter, while the larger particles will range from 35 to 100 nm in diameter. In any mixture used in an embodiment of the present invention the smaller and larger particles are narrowly distributed within their respective size bands (ie although the smaller particles may range between 18 and 50 nm in any exemplary mixture the range of sizes of the smaller particles will be narrower than that), and the number of larger particles will exceed the number of smaller particles by a ratio in the range of 1:1 to 4.2:1.  FIG. 4  shows a scanning electromicrograph of suitable nanoparticles and  FIG. 5  shows a plot of the size distributions of a preferred example. In this preferred example the smaller nanoparticles have diameters in a narrow band around 33 nm, and the larger particles have a diameter in a narrow band around 40 nm. The size of the nanoparticles can be controlled to provide the foundation for bimodal distribution of the particles on the surface of the fabrics. The nanoparticles and cross-linking agents are provided to the fabric in an aqueous solution, and their amounts are controlled to achieve wet pick-up 60-70%. The curing temperature is preferably from 105˜170° C., and the curing time 1˜20 minutes.  
         [0028]     The function of the nanoparticles is to act as a seed to form a hierarchical nanostructure and any polymer material can be used for the nanoparticles. A suitable material for example is surface modified polystyrene with the surface modification providing the covalent link between the nanoparticles and the fabric. Surface modification may be achieved, for example, by covalently linking —OH or —COOH groups on the surface of the polystyrene during the synthesis of nanoparticles. Other forms of surface modification are possible, however, for example by oxidation of the surface of the polystyrene to form —COOH groups or by reduction to form —OH groups  
         [0029]     For use in the examples below, the polystyrene nanoparticles were synthesized through water emulsion using styrene (ST) and acrylic acid (AA) in a certain weight ratio with or without surfactants at a particular polymerization condition. All emulsion polymerization reactions were carried in a three-neck flask. The flask was equipped with a condenser and inlets for nitrogen. Prior to polymerization, the reaction mixture was degassed by nitrogen flow, and nitrogen was maintained during the synthesis. A typical procedure was follows: 1) Addition of 0.6 g SDS and 0.24 g sodium hydrogen carbonate into 75 ml water to form a solution; 2) addition of 15.6 ml styrene, 2.2 ml acrylic acid and 2.2 ml hydroxyl ethyl methyl acrylate (HEMA) into solution; 3) after 20 minutes stirring, add 0.2 g potassium persulfate (KPS) dissolved in 5 ml water into above solution when temperature increase at 50° C.; and 4) increase the temperature and keep it at 75° C. for another 5 hours until the reaction finished. The morphology of the nanoparticles was examined using a Phillips CM 20 transmission electron microscope (TEM) with an acceleration voltage of 200 kV. The nanoparticles were fished onto a carbon-coated copper grid before examination. Figure shows the TEM results,  FIG. 5  shows the size-distribution of the nanoparticles measured by light scattering. This solution of nanoparticles was then used with crosslinking agents (e.g. DMDHEU) for the treatment of the fabrics.  
         [0030]     Table 1 shows the three compositions of the nanoparticles used in the Examples below.  
                                             TABLE 1                       Sample #   1   2   3                                Water (ml)   80   80   90       Styrene (ml)   15.6   5.5   10       Acrylic acid (ml)   2.2   0.5   1       Hydroxyl ethyl   2.2   0   0       methyl acrylate (ml)       Potassium persulfate (g)   0.2   0.06   0.11       SDS (g)   0.6   10   0       NaHCO 3  (g)   0.24   0.24   0.12       Temperature (° C.)   75   75   75       Time (h)   5   5   5       Size of the nanoparticles (nm)   21 ± 1.5,   33 ± 1.5,   94˜420           35 ± 1.5    39 ± 1.5                   
 
         [0031]     Using these nanoparticles the following examples were prepared. In each of these Examples the nanoparticles were from Sample 2 above with the sizes and relative numbers as shown in  FIG. 4  &amp;  5 :  
       EXAMPLE 1  
       [0032]     100% cotton fabric (160 mm×72 mm, 80/2//×80/2 pinpoint oxford) was immersed in an aqueous solution (27 wt % DMDHEU, 1.4 wt % MgCl 2 , 1.5 wt % nanoparticles, and 4.4 wt % commercial softener) and subject to ultrasonic vibration for 1 minute. The fabric was pressed to give a wet uptake of about 70% and then dried at 80-90° C. for 4 hours, and cured at 140˜150° C. for 15 minutes. Then, the properties of the fabric were tested: the recovery angle of treated fabrics was measured according to the AATCC 66 test of option 2; the tensile test was carried out using Instron 4466 following the ASTM D5034 standard. The results of measurements are: Recovery angle 256°.  
       EXAMPLE 2  
       [0033]     100% cotton fabric (160 mm×72 mm, 80/2//×80/2 pinpoint oxford) was immersed in an aqueous solution of 1.5 wt % nanoparticles, and subject to ultrasonic vibration for 1 minute. Then an aqueous solution containing 27 wt % DMDHEU, 1.4 wt % MgCl 2 , and 4.4 wt % commercial softener was added in the same solution. The fabric was immersed for 5˜10 minutes and pressed to give a wet uptake of about 70%, then dried at 80-90° C. for 4 hours, and cured at 140-150° C. for 15 minutes. Then, the properties of the fabric were tested as in Example 1. The results of measurements were: Recovery angle 262°, tensile retention (72% wft, 85% wrp), and abrasion 27000 revolution.  
       EXAMPLE 3  
       [0034]     100% cotton fabric (160 mm×72 mm, 80/2//×80/2 pinpoint oxford) was immersed in the aqueous solution containing 27 wt % DMDHEU, 1.4 wt % MgCl 2 , and 4.4 wt % commercial softener for 5˜10 minutes. Then 1.5 wt % nanoparticles, were added to the solution and ultrasonic vibration was provided for 1 minute. The fabric as pressed to give a wet uptake of about 70% and then dried at 80-90° C. for 4 hours, and cured at 140-150° C. for 15 minutes. Then, the properties of the fabric were tested as in Example 1. The results of measurements are: Recovery angle 212°.  
       EXAMPLE 4  
       [0035]     This example is of a two-step constrained curing. The fabric was treated with a solution consisting of 15% DMDHEU, MgCl 2  (6 g/L) for 5˜10 minutes. After the excess solution was removed by padding, the wet take-up of samples is ˜65%. After the fabric was air dried, it was cured at 110° C. for 30 minutes between two flat glass plates with applied pressure. After that, the fabric was treated with a solution consisting of 5% DMDHEU, MgCl 2  (3 g/L), and the nanoparticles (0.5-1.5 wt %) for 5˜10 minutes. After the excess solution was removed by padding, the wet take-up of samples is ˜80%. After the fabric was air dried, it was cured at 160° C. for 3 minutes between two flat glass plates with applied pressure. The measured recovery angle was 270˜284°, the tensile strength 68%˜79%, and the tearing strength 47%˜59%.  
       EXAMPLE 5  
       [0036]     100% cotton fabric (160 mm×72 mm, 80/2//×80/2 pinpoint oxford) was immersed in an aqueous solution (30% DMDHEU, 7 wt % MgCl 2 , 0 or 1.5 wt % nanoparticles, and 4.4 wt % commercial softener) for periods of 1, 5, and 10 minutes. The fabric was pressed to give a wet uptake of ˜70% and then dried at 80-90° C. for 4 hours, and cured at 150° C. for 15 minutes. Then, the properties of the fabric were tested: the recovery angle of treated fabrics was measured according to the AATCC 66 test of option 2. Recovery angles are given in  FIG. 7 .  
       EXAMPLE 6  
       [0037]     100% cotton fabric (160 mm×72 mm, 80/2//×80/2 pinpoint oxford) was immersed in an aqueous solution (30% DMDHEU, 7 wt % MgCl 2 , 0 to 1.8 wt % nanoparticles, and 4.4 wt % commercial softener) and ultrasonic for 1 minute. The fabric was pressed to give a wet pickup of ˜70% and dried at 80-90° C. for 4 hours, and cured at 150° C. for 15 minutes. Then, the properties of the fabric were tested: the recovery angle of treated fabrics was measured according to the AATCC 66 test of option 2. Recovery angles are given in  FIG. 8 .  
         [0038]     In these examples the mechanical properties were measured according to existing industrial standards. The recovery angle of the treated fabrics was measured according to the AATCC 66 test of option 2. The grab test was also performed to assess the change of tensile properties of the fibrils after the treatment. The tensile test was carried out using Instron 4466 following the ASTM D5034-1995 standard. The abrasion tests were also carried out under the guideline of ASTM D-4966-1989 standard.  
         [0039]     As shown in  FIG. 7 , the time of immersion fabrics in the bath of nanoparticles is reduced, compared to the treatment without nanoparticles, and thus leads to higher efficiency of the process of the treatment. In this Figure the data for the fabric with nanoparticles is as in Example 5. The plot showing a fabric without the use of nanoparticles is obtained from a similar process as in Example 5 but without the application of the nanoparticles and with the application of DMDHEU.  
         [0040]      FIG. 8  illustrates the recovery angle as a function of the pick-up of the nanoparticles, which can be controlled easily by the concentration of nanoparticles. Other than the varying wt % of the nanoparticles, the data of  FIG. 8  is obtained using the process of Example 6. A recovery angle of greater than 260° is considered to be indicative of excellent wrinkle-resistance and it can be seen from  FIG. 8  that this recovery angle can be equaled or bettered with a wt % of nanoparticles from about 0.02 to at least 1.8 wt %.  
         [0041]      FIG. 9  shows the comparison between non-treated native cotton, conventional treated, and NHCA treated fabric in terms of mechanical properties. And in all physical performance indicators, the NHCA process produces samples with the best result. The data for the “commercial process” fabric is obtained from a fabric treated with a known industrial formulation and sold under the “Brooks Brothers” brand.  
         [0042]     It will thus be seen that the present invention, at least in its preferred forms, provides a method for control the physical and chemical properties of fibers or fabrics via hierarchical crosslinking architecture at nanometer or micrometer scale. The architecture, proved by scanning microscopy study, of the present invention has improved the mechanical properties of a fabric as evidenced by measurement of tensile strength, tear strength, and recovery angle of a cotton fabric. The architecture, which consists a thin film with nanometer or micrometer domains, is generated using nanoparticles and crosslinking reagents. The method of present invention can be applied to various fibers, fabrics, or textiles for enhancing their properties. In comparison with conventional technology, this method gives a well-defined structure, thus offering the potential for property design and control.