It has long been a necessity, particularly within the textile industry, to provide substrates, such as apparel fabrics, as one example, that exhibit a number of simultaneous wash-durable properties. Most notably, water repellency, oil repellency, stain resistance, and stain release characteristics are highly desirable to facilitate cleaning of substrates, if not to prevent complete staining thereof. Unfortunately, provision of such simultaneous and wash-durable characteristics has been severely limited due to the general difficulties with meeting certain surface energy requirements throughout the wash-durable life of such a substrate. Generally, coatings or other treatments have not been readily available or widely known that can provide coexistent water and oil repellency and stain release on a wash durable basis to fabrics (or other surfaces) because the surface energy profile required for one of these properties is disparately different from the surface energy profile required to impart the other property at the same time.
Although there have been some instances of initial simultaneous existence of both properties on certain substrates (as noted below), unfortunately, the degree of wash-durability thereof has been unacceptable for long-term utilization of target substrates. As a result, any significant reduction in either oil or water repellency consequently reduces stain repellency as well. With a reduced propensity to repel stains, the ability to effectuate proper stain release may likewise be diminished, particularly upon exposure to greater degrees of staining and wherein the surface energy profile needed for proper stain release function (which is similar to that needed to impart the aforementioned water and oil repellency properties) is compromised (e.g., is not wash-durable).
Hence, truly effective wash-durable, long-term, stain repellent and stain release treatments have not been forthcoming, since simultaneous prevention of both polar (aqueous) and non-polar (olefinic) liquid penetration into such fabric surfaces has been very difficult to achieve that can withstand extended common laundering procedures. This problem with prior oil and water repellent surface treatments is most prominently observed on typical high stain substrates such as cotton-containing fabrics. Such fabrics are generally difficult to modify at their surfaces to the extent necessary to impart both oil and water repellent features thereto and to retain an acceptable hand. These at least three properties (stain release, water repellency, and oil repellency) are simply unavailable to the textile industry on a wash-durable basis due to the aforementioned surface energy issues. A description of such surface energy properties helps to permit a better understanding of such a phenomenon.
A fundamental physical property of any material is its surface energy. This property is usually expressed in mJ/m2. Depending on the magnitude of this property, the material may be classified as having a high surface energy or a low surface energy. This property depends generally on the composition of the substrate. For example, a substrate having a surface that contains a significant portion of polar, hydrophilic groups, such as hydroxyl groups, carboxylic acid groups, amine groups, and the like, generally exhibits a high surface energy. Conversely, a substrate having a surface that contains a significant portion of non-polar, hydrophobic groups, such as silicone, fluorinated groups, and the like, generally exhibits a low surface energy. It is readily known that when a polar liquid, such as water, is placed in contact with the surface of a substrate, the liquid will spontaneously wet the surface only if the surface tension of the liquid is lower than the surface energy of the substrate. Conversely, if the surface tension of the liquid is higher than the surface energy of the substrate, spontaneous wetting will not readily occur, and the liquid will remain pooled on the surface of the substrate.
As one might expect then, substrate surface energy modification has long been a major field of research for a variety of materials and for a multitude of reasons. For instance, it is often desirable to increase the surface energy of a substrate to facilitate its ability to absorb liquid or to increase the adhesion between a coating and a substrate. Practical examples include the chemical treatment of paper or plastic to enhance their wetting with printing inks and corona treatment of plastic to increase the adhesion between the plastic and another material, such as for the aluminum coating of Mylar® films in packaging applications. Textile substrates have also been modified to create substrates with high surface energy which results in a textile substrate that is hydrophilic and that exhibits improved comfort and stain release properties. As one example, the detergent industry has employed this technique for determining effective methods of cleaning various textile substrates.
Surface energy modification has also been utilized in other coating applications, such as to produce non-stick surfaces exhibiting low surface energy through the application of Teflon™ to cookware and cooking utensils. Textile substrates have also been modified with low surface energy treatments in order to produce textile substrates that are hydrophobic and that exhibit repellent properties (such as for water repellent rainwear).
It has commonly been observed that substrates treated with fluorinated polymers generally exhibit a contact angle of greater than 100 degrees with water. The advancing and receding contact angles are very similar. The major component of the surface energy of such treatments is dispersive. Substrates treated with dual functional repellents, such as disclosed in U.S. Pat. No. 3,574,791 to Sherman et al., generally exhibit lower contact angles with water when compared with traditional fluorochemical repellents, and therefore, tend to exhibit lower repellency. The measured surface energy contains significant dispersive and polar components. Differences can usually be measured between the advancing and receding contact angles.
In some instances, a measurable degree of hysteresis exists between the advancing and receding contact angle, indicating that the surface energy has changed in the presence of a liquid. Barring liquid adsorption, hysteresis is indicative that the surface energy has changed (kinetically or thermodynamically) in the presence of a liquid or environmental condition. This measurable degree of hysteresis provides further evidence that the substrate is autoadapting to its environment. One method for achieving ideal performance for textile applications would be obtained from a composition that provides high advancing contact angles (i.e., >90 degrees), exhibiting non-porous behavior, to impart stain resistance and provides low receding contact angles (i.e., <90 degrees), exhibiting porous behavior, to impart stain release to the substrate. Another method to achieve ideal performance for such applications would be obtained from a composition that imparts high advancing and high receding contact angles between a staining substance and the substrate, followed by low advancing and receding contact angles during exposure to a cleaning procedure.
It would be desirable for a porous or stainable surface to exhibit high contact angles versus a variety of liquids to prevent adsorption or staining. It would also be desirable for such surfaces to adapt to a change in their environment, such as in a cleaning medium, to enhance removal of stains and soil. Other environmental conditions that could induce a change in the surface energy of a substrate include changes in temperature, moisture content, and other environmental factors. Highly desirable would be a surface that reversibly adapts to its environment, such that the surface is stain resistant and cleanable and retains this effect through a number of use cycles. In many end-use applications such as apparel, carpet, upholstery, and the like, appearance retention of the product is extremely important. While stain resistant treatments have been developed for each of these exemplary applications, it has been found, that much like stain resistant apparel treatments, such treatments have an adverse effect on subsequent cleaning. Thus, it would be highly desirable to develop soil and stain resistant textile substrates, regardless of the end-use application, that possess enhanced cleanability using appropriate cleaning techniques.
With the development of XPS, SIMS, and other surface analytical techniques, it has become possible to detect certain chemical groups at the surface of materials. For instance, one can measure the concentration and depth profile of functional groups, such as CF3 moieties commonly found in fluoropolymer stain resist chemicals. Through appropriate sample preparation techniques, it is also possible to observe changes that take place on the surface of a substrate and that occur as a result of changes in the environment to which the substrate is exposed. For example, a substrate that is observed to contain predominately low surface energy groups, such as CF3 groups, under a first set of conditions can be shown to contain significant hydrophilic high surface energy groups, such as hydroxyl groups, at its surface under a different, second set of conditions. This polarity change typically allows the surface of the substrate to wet (i.e., absorb liquid), thereby enhancing stain release. As the substrate's environment is returned to the first set of conditions, one can observe, for example, the CF3 groups return to the substrate's surface, thus, returning the substrate to its low surface energy, stain resistant state.
Some treatment compositions, such as polymers, possess other properties, such as glass transition temperature, which may influence the ultimate performance of the treated substrate. For instance a hard polymer that is characterized by a high glass transition temperature may provide increased protection against wetting, especially forcibly wetting. However, this stiff, high glass transition polymer would likely require more work to adapt to changes in its environment due to less intra-polymer flexibility. In addition, the polymer molecular weight and addition of co-monomers may enhance wetting, adhesion, chemical reactivity, and durability for a variety of substrates as well.
As should thus be evident, modification to provide a proper surface energy profile to impart simultaneous wash-durable oil repellency, water repellency, stain resistance, and stain release properties to a target substrate has been sought after for many years without success.
The invention as described herein illustrates that certain combinations of chemicals and processing conditions permit and/or facilitate tailoring of the surface properties of a target substrate to obtain the desired balance of surface energy profiles to impart simultaneous repellency and stain release characteristics thereto. Furthermore, this unique combination of features has surprisingly been shown to be quite durable upon exposure to routine as well as industrial cleaning methods.