A nonwoven fabric is a textile structure consisting of a mat of fibers held together with a bonding material. The fibers can be partially oriented or they can be completely randomly distributed. Latex is often used as the binder for the fibers in nonwoven fabrics.
Nonwoven fabrics are popular owing to the simplicity and economy of their production since the traditional weaving operations are not used; hence, less equipment, less space, and fewer personnel are required. Nonwoven fabrics can also be produced from what would normally be considered as waste fibers, and useful characteristics are obtained which may not be provided by woven or knitted fabrics.
Enormous quantity of fibers are consumed annually in applications of nonwoven fabrics such as clothing, interliners, filters, automotive door panels, heat and electrical insulation, packaging, sanitary napkins, fillers for quilted structures, wiping cloths, towels, masks, wall coverings, shoe uppers and liners, curtains and draperies, tea bags, simulated leather, gaskets, luggage, ribbons, and diapers.
In the medical/surgical field, nonwoven fabrics are used in surgical drapes, surgical caps and gowns, as wraps for surgical instruments and the like. The surgical use requires that the fabric used be sterilized. In the case of latex-impregnated fabric used for drapes, gowns and caps, such items are wrapped and sealed in polyolefin bags and then are sterilized with gamma ray radiation. At time of use, the bags are opened and the doctors and nurses put on the caps and gowns and use the drapes to cover the patient during surgery. When the bags are opened, they often emit odors. In the case of the wraps, after being washed, surgical instruments are stacked in a tray and then wrapped in the latex-impregnated nonwoven fabric and sterilized in steam. After steam sterilization, the wrapped instrument trays are removed from the sterilization unit. When the door to the sterilization unit is opened to remove the wrapped instrument tray, an odor is emitted from the fabric that can be quite potent.
Therefore, in order to sell nonwoven fabric bonded with a latex, the post-sterilization odor should be minimized.
In the past, antioxidants have been added to unsaturated latexes to prevent degradation of the unsaturated polymer backbone in the presence of air, ozone, heat, and light. Examples of unsaturated latexes include polybutadiene, poly(butadiene-styrene), poly(butadiene-acrylonitrile), polyisoprene, and polychloroprene. Saturated latexes are prepared by homopolymerizing esters of acrylic or methacrylic acids or copolymerizing esters of acrylic or methacrylic acids with other vinyl monomers such as acrylonitrile, styrene, vinyl chloride, and vinyl acetate. The saturated latexes do not need protection of antioxidants and whenever antioxidants were added to saturated latexes, they were added for colloidal stabilization, which is unrelated to the antioxidant function.
Hydrophilic acrylic latexes based on poly(ethyl acrylate) have been available in the past for bonding nonwoven fibers. Such latexes were prepared by polymerizing in excess of 90 weight parts ethyl acrylate; less than 5 parts of each N-methylol acrylamide, acrylamide, or acrylonitrile; a low level of an emulsifier; and less than 1 weight part of an antioxidant selected from hindered and partially hindered phenols, such as a mixture of 2,2'-di-t-butyl Bisphenol A, 2-t-butyl-2'-.alpha.-methylbenzyl Bisphenol A, 2,6-di-t-butylphenol, 2-t-butyl-isopropylphenol, etc. The antioxidant was added as an emulsion to provide additional colloidal stability.
The Spaulding U.S. Pat. No. 3,539,434 describes nonwoven, bonded articles which retain a high level of physical properties even after prolonged exposure to aging and treatment with typical dry cleaning solvents. The physical properties of concern to Spaulding are tensile strength and elongation. The noted advantages are realized by admixing with an acrylate ester latex binder an antioxidant and a chelating agent. Amount of the antioxidant is 0.1 to 3 weight parts whereas amount of the chelating agent is 0.2 to 3 weight parts, based on 100 weight parts of the latex polymer.
The latex of the Spaulding patent is prepared by polymerizing an acrylate ester, a carboxylic acid, an N-alkylol amide, and other conventional additives. Suitable antioxidant is selected from phenolics, especially hindered phenolic antioxidants, and phosphite esters. The chelating agent is selected from polycarboxylic acid/amine type of chelating or complexing agents which conform to the structural formula ##STR1## where X and Y are alkali metal carboxylate groups of the formula ##STR2## wherein n is a number from 1 to 6, or a carboxylic acid groups of the formula ##STR3## wherein n is a number from 1 to 6, or one but not both may be a hydroxyalkyl group of the formula EQU (--CH.sub.2 --)CH.sub.2 CH
wherein n is a number from 1 to 6; and Z is an alkali metal carboxylate or carboxylic acid groups as defined for X and Y or a ##STR4## group wherein R' is a methylene grouping of the formula ##STR5## wherein n is a number from 1 to 12, a cyclohexane or benzene ring, and X' and Y' are alkali metal carboxylate, carboxylic acid or hydroxyalkyl groups as defined for X and Y, and one but not both may be an alkyl group containing from 1 to 20 carbon atoms or a grouping which conforms to the structure ##STR6## wherein R", X" and Y" are defined as for R', X and Y above; said phenolic antioxidant present in an amount from about 0.2 to 3 parts by weight based on 100 parts by weight of the acrylic polymer and said chelating agent present in an amount from about 0.2 to 3 parts by weight based on 100 parts by weight of the acrylic polymer.
Examples of the chelating agents fitting the above description are: ethylenediamine tetraacetic acid, the sodium salt of ethylenediamine tetraacetic acid, nitrilotriacetic acid, sodium salt of cyclohexanediamine tetraacetic acid, pentasodium diethylenetriamine pentaacetate, trisodium hydroxyethyl ethylenediamine triacetate. Other chelating agents such as sodium hexahydroxy heptannoate, sodium gluconate, citric acid, dihydroxy ethyl glycine may also be employed if desired.
At bottom of column 2 of the Spaulding patent, it is stated that it is known to add antioxidants to acrylate polymers. When this is done and such acrylate polymers are used alone, as in a cast film or a molded article, the antioxidants appear to effectively retard degradation. However, when such acrylate polymers are used as binders in nonwovens, only partial protection is achieved. Also, the addition of chelating agents to acrylate polymers without antioxidants, does not result in the polymer protection against degradation. It is only when the antioxidants are used in combination with the chelating agents in acrylate polymer latexes, that improved resistance to aging and to typical dry cleaning solvents is realized.