The present invention relates to a method of treating a low hydrohead fibrous porous web material and, for example, to a method that increases the retentive wettability of the web. The invention is also directed to low hydrohead fibrous porous web materials having improved retentive wettability. As used herein the term wettability generally refers to the ability of a material to transmit a fluid through the material. Examples of uses where highly nonwettable materials are desired would be tent and raincoat materials. Conversely, highly wettable materials are desired in, for example, feminine care pad liners and diaper liners. In addition to being wettable, items such as feminine care pad liners and diaper liners are desirably retentively wettable. The term retentively wettable is used herein to designate the degree to which a given material which is wettable retains its ability to be rewet after having been wet and allowed to dry. In other words: how well the material retains it ability to transmit fluids after having already transmitted fluid with intermediate drying.
As is well known, conventional diapers, incontinence garments and feminine care pads have a thin liner that contacts the skin of the wearer and covers the material which absorbs the insulting fluids. These liners are usually made of hydrophobic materials and thus need to be treated with a surface active agent to make them wettable. The liners desirably have low hydrohead values (indicating large pores through the material) because their function is to allow the transmission of fluid with minimal pressure to an adjacent absorbent pad which acts to absorb the fluid. As used herein the term "hydrohead" refers to the ability of a material to support a given column of water under defined conditions. The height of the column of water supported is the hydrohead value. Additionally, the liner should be highly wettable because, as has been previously stated, one of the functions of the liner is to transmit as much fluid to the absorbent material as quickly and completely as possible. Thus, fluid should flow through the liner and into the absorbent before it travels more than a short distance on the surface of the liner. Fluid which fails to flow through the liner is termed "run-off". In some circumstances, run-off may cause undesirable leakage and/or skin irritation problems. Additionally, because feminine care pad liners and, especially, diaper liners continue to be worn after the first of numerous fluid insults, liners used in these items desirably should have a high degree of retentive wettability.
In order to evaluate the retentive wettability characteristics of various liner materials, a stringent run-off test has been developed. While the test is described in detail below, it can generally be stated that the test attempts to approximate or exceed the real-world stresses a porous web material is subjected to when utilized as a diaper liner. This approximation is generally achieved by utilizing 100 milliliters of insulting distilled water test fluid having a surface tension greater than urine and maintaining the porous web material to be tested at a 30 degree incline on top of an absorbent during the insult, i.e. during the run-off test.
In the past, feminine pad liners and diaper liners have been subjected to treatment with surfactants to improve wettability. The liners have been treated with surfactant by (1) passing the formed liner through a bath containing the surfactant in either neat or solution form and drying the liner so that a given amount of the surfactant is deposited on the liner, or (2) spraying a surfactant in either neat or solution form on the fibrous porous web and drying the liner so that a given amount of the surfactant is deposited on the liner, or, (3) adding surfactant to a thermoplastic resin prior to extrusion and formation of the resin into a thermoplastic porous web material. In the later situation, under known process conditions, the added surfactant exudes or migrates to the surface of the fibers of the porous web material during or shortly after fiber formation This phenomenon has been referred to as "blooming" the surfactant. It is believed that blooming results from the insolubility of the surfactant in the thermoplastic polymer as the polymer cools. See U.S. Pat. No. 4,535,020 to Thomas et al (hereafter Thomas et al 020) which demonstrates surfactant blooming in a diaper liner formed from a perforated film.
A liner made from a hydrophobic material, such as a thermoplastic polymer, will not be wet by bodily fluids because the surface tension of the bodily fluid is greater than the critical surface energy of the hydrophobic material Surface tension is the contractile surface force of a fluid where the fluid tries to assume a spherical form and to present the least possible surface area. It is usually measured in dynes per centimeter. Accordingly, because it makes the liner wettable, surfactant is desirably applied to the liner. Application of a surfactant onto a liner material may make a nonwettable liner wettable by at least two mechanisms: (1) Surfactants present on the liner can dissolve into an insulting bodily fluid and lower the surface tension of the resulting solution to more equal the critical surface energy of the liner material. Accordingly, when a surfactant coated liner is insulted with a fluid such as urine, the surfactant acts to lower the surface tension of the fluid and allow the fluid to pass through the liner thus reducing or avoiding unacceptable run-off. In this situation, a certain amount of the surfactant on the liner is lost with each insult and unacceptable run-off occurs at some following insult due to the lack of availability of surfactant to lower the surface tension of the insulting fluid. (2) The surfactant can be coated onto the fibers making up the liner, making the fiber surface of the liner more hydrophilic, i.e., increase the apparent critical surface energy of the fibers. In this situation the liner would be permanently wettable if the surfactant did not dissolve into the insulting fluid.
As any parent will testify, it is an unfortunate event when unacceptable run-off, i.e. non-transfer of urine through the liner and into the absorbent portion of the diaper, occurs. In such a situation leakage of urine from the diaper may occur. A sad aspect of this situation is that, but for the fact that the liner did not allow the urine to pass through to the absorbent material, the diaper may well have had the capacity to absorb the urine. In such a situation the diaper has the capacity to absorb the urine but not the ability.
Accordingly, it has been a goal of those in the art to provide a porous web material which has an improved wettability. This was the initial goal because, if the material is not wettable at all, the material cannot function as a liner in a diaper. It has also been a goal of those in the art to provide a low hydrohead web which will transmit the fluid to a juxtaposed absorbant rather than retain the fluid. Additionally, it has been a goal of those in the art to provide a low hydrohead porous web which has an improved retentive wettability. That is, when dried between insults, the diaper liner has a significant increase in the number of fluid insults that the liner allows to pass through to the absorbent before unacceptable run-off occurs. A desirable goal would be to provide a low hydrohead liner which, prior to unacceptable run-off, would allow fluid to pass through it in multiple, typical-quantity insults in a total quantity at least as great as the absorbing capacity of the absorbent material of the diaper. Thus, the entire absorbing capacity of the diaper could be utilized prior to unacceptable run-off.
Perforated films have been used as diaper liners. See, for example, Thomas et al 020. Corona discharge treatment of films is also old in the art and it is known that corona discharge treatment of a polymer film in the presence of air entails substantial morphological and chemical modifications in the polymer film's surface region. See Catoire et al, "Physicochemical modifications of superficial regions of low-density polyethylene (LDPE) film under corona discharge," Polymer, vol. 25, p. 766, et. seq, June, 1984.
Generally speaking, corona treatment has been utilized to either (1) improve the print fastness on the film, or (2) to perforate the film. For example, U.S. Pat. No. 4,283,291 to Lowther describes an apparatus for providing a corona discharge, and U.S. Pat. No. 3,880,966 to Zimmerman et al discloses a method of using a corona discharge to perforate a crystalline elastic polymer film and thus increase its permeability. U.S. Pat. No. 3,471,597 to Schirmer also discloses a method for perforating a film by corona discharge. U.S. Pat. No. 3,754,117 to Walter discloses an apparatus and method for corona discharge treatment for modifying the surface properties of thin layers or fibers which improve the adhesion of subsequently applied inks or paints or of subsequent bonding.
It also is possible to treat a diaper liner material with a corona discharge and then immediately dip the film in a surfactant solution. Because the corona effect on the material generally starts to immediately decay, it is important to get the corona treated material into the bath as quickly as possible. Such a method is discussed in Japanese KOKAI Patent Number SHO63[1988]-211375. This document discloses a method for producing a nonwoven fabric having a long lasting hydrophilicity. The method involves first treating a nonwoven fabric of synthetic fiber by a corona discharge and then coating the treated fabric with about 2-10 grams of surface active agent per square meter of fabric.
Of particular interest is the fact that Thomas et al 020 is directed to the utilization of corona discharge in conjunction with surfactant treated films to effect improved wettability, i.e. higher fluid transmission rates and therefore decreased run-off of fluid. In this regard Thomas et al 020 states that a perforated film which has been treated with surfactant and which is then corona discharge treated results in a film with very low, zero or near zero fluid run-off on the first run-off test. Thomas et al 020 reports that this effect is accomplished because the corona discharge treatment acts on the chemical additive, the surfactant, to provide the perforated film with a zero or near zero percent run off. Thomas et al 020 postulates that this effect is achieved due to the surfactant providing a greater polarizability to the film than the film would have without the surfactant being added. The corona discharge treatment provides additional polarizing effect and, in combination with the surfactant, provides improved wettability. Thomas et al 020 does not appear to address the question of retentive wettability because all of the test results appear to be directed to run-off testing after a single fluid insult.
While of interest to those in the art, the statements made by Thomas et al 020 appear to be of limited value for a number of reasons. Initially, as stated above, Thomas et al 020 appears to be directed toward improved wettability of perforated films as compared to materials having improved retentive wettability materials. Secondly, Thomas et al 020 uses a run-off test procedure which does not, in the opinion of the present inventors, adequately address the environmental stresses a liner is subjected to when it is utilized in a diaper. For example, the Thomas et al 020 run-off test only utilizes 25 milliliter test fluid amounts and not an amount, such as 100 milliliters, which more closely represents the typical amount that is voided by an infant. See, Goeller et al., Urination in the First Three Years of Life, Nephron, 28: 174-178 (1981) where it is stated that the mean voiding size of 12-18 month old infants is 57.3+/-21.6 milliliters. It is also stated that the mean voiding size of 24 -32 month old infants is 79.3+/-14.9 milliliters. Thirdly, Thomas et al 020 uses a run-off test solution which is stated to contain 0.025 percent of alkylaryl polyether alcohol bearing the trade name "Triton X-100" and manufactured by the Rohm & Haas Company. Triton X-100 is, itself, a surface tension reducing agent. Rohm & Haas literature states that 0.01% of X-100 added to water reduces the surface tension of the solution to 31 dynes/cm at 25 degrees C. Clearly the 0.025 percent of X-100 present in the test solution of Thomas et al 020 should have an effect on the wettability of the perforated film, regardless of the type of treatment to which the perforated film is subjected. By comparison, the run-off test procedure used herein is more demanding because it utilizes distilled water as the test solution. Because the distilled water which was used had a surface tension of about 60 dynes per centimeter, it has a higher surface tension that the surface tension of infant urine which, while it varies somewhat with age, is approximately 55 dynes/centimeter. Lastly, the incline at which the liner material to be tested is placed is merely 10 degrees in the Thomas 020 run-off test as compared to the 30 degree incline of our test.
To demonstrate the effect of adding Triton X-100 to the test solution on wettability, run-off testing, as defined below, was conducted on conventional (non-surfactant treated) 0.8 ounce per square yard porous polypropylene web spunbonded material. In these tests, the test fluid was maintained at 37+/-1 degrees Centigrade. Three samples were run using distilled water as the test fluid and three samples were run where the test solution was 99.9751% distilled water and 0.0249% Triton X-100. Additionally, each of these tests was duplicated with the exception that (1) the incline was reduced to 10 degrees; (2) the quantity of test solution was reduced to twenty-five (25) grams and (3) the test fluid was maintained at room temperature. These modifications were made to approximate the test procedure stated in Thomas et al 020. Using our run-off procedure, the average run-off of the 100 grams distilled water only samples was 90.6+/-0.7 grams (90.6% run-off). Clearly an unacceptable amount of run-off. Compared to this, the average run-off of the distilled water with 0.0249% Triton X-100 was 3.7+/-1.8 grams (3.7% run-off). The effect of the presence of the Triton X-100 is clear. Without any surfactant modification of the test fibrous porous web, the fibrous porous web now has apparently acceptable run-off. If the approximation of the Thomas et al 020 test is utilized, the average run-off of the 25 gram distilled water only sample is 21.5+/-0.7 grams (86% run-off). Lastly, if a 99.9751% distilled water and 0.0249% Triton X-100 fluid is used on the 10 degree incline, the average run-off value is 1.6+/-2.2 grams (6.4%) run-off. Once again, the effect of the presence of the Triton-X100 is clear.
Surface tension measurements of both fluids were made after the measuring equipment had been checked for accuracy against a very pure sample of distilled water. The sample of very pure distilled water gave a value of 71.8 dynes/centimeter, reflecting correct calibration and procedure. The average of three surface tension measurements of the laboratory distilled water used was 59.13 dynes/centimeter reflecting the presence of slight impurities in the laboratory distilled water. After addition of the Triton X-100 to the laboratory distilled water, the average of three surface tension measurements for this fluid was 30.7 dynes/centimeter. This correlates well with our run-off testing of the two fluids. From these data it is clear that the presence of the Triton X-100 in the test fluid affects the run-off test results to make the material appear more wettable.
In summary, the Thomas et al 020 test results tend to identify those materials which somewhat enhance the wettability of a liner material. In other words, those materials whose use results in an improvement for a given, single insult with the insult being of limited quantity. However, Thomas et al 020 appears to fail in identifying the materials which result in improved retentive wettability. Said yet another way, Thomas et al 020 fails to identify those surface active agents whose utilization results in an increase in the number of run-off tests a given material can be subjected to without unacceptable run-off occurring. Accordingly, it can be seen that there still exists a need for a fibrous porous web material having improved retentive wettability so that the web can be used as, for example, a diaper liner. Such a web material desirably would have reduced fluid run-off when multiple insults of at least about 100 milliliter are applied. It is such a material to which our invention is directed. Specifically, our invention is directed toward substances which, in combination with corona discharge treatment, increase the retentive wettability of fibrous porous web materials by increasing the number of run-offs tests the material can be subjected to by at least 50%, without the occurrence of unacceptable run-off. In many instances the number of run-offs prior to unacceptable run-off is increased by at least 100%. In some instances the number of run-offs prior to unacceptable run-off is increased by at least 200%.