Amos, Homer C. (Palm Springs, CA)
Strickland, Edward T. (Palm Springs, CA)

05/047453

02/27/1973

06/18/1970

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15/215

15/105, 134/6, 134/10

A47L23/26; B08B7/00; A47L23/00; A47L23/22

15/215,105 134/6-10

Machlin, Leon G.

We claim

1. A cleaning pad adapted to be adhesively and removably secured to the floor outside the doorway and disposed before it to remove dirt and other foreign matter from shoes and wheels in contact therewith, said pad being paper-thin and having an upper coating of water-washable solid adhesive.

2. The cleaning pad of claim 1 wherein said adhesive has a Young's modulus of 20 to 50 p.s.i., a tack of from about 3.5° to about 8°, and a viscosity of from 12 min. to 50 min., as measured by the tests described in the specification.

3. The cleaning pad of claim 1 in which the pad has a lower coating of adhesive with a Young's modulus of 150 to 250 p.s.i., and an internal viscosity and free surface energy to stop at between 30° and 70° and to come within 5° of its final position in 80 to 800 minutes.

4. The cleaning pad of claim 1 in which the pad has a lower coating of adhesive with a Young's modulus of about 210 p.s.i., and an internal viscosity and free surface energy to stop at about 45° and to come within 5° of its final position in about 500 minutes.

5. A pad as in claim 1 for cleaning shoes and wheels substantially as wide as said doorway and longer than a large step taken by a person walking, said pad being adhered to said floor just before said doorway,

6. The pad of claim 5 in which the pad has a lower coating of adhesive with a Young's modulus of about 150 to 250 p.s.i., and an internal viscosity and free surface energy to stop at about 30° to 70° and to come within 5° of its final position in 80 to 800 minutes.

7. The pad of claim 5 wherein said pad upper surface has two marginal non-adhesive covered areas exposing said base material, one on the edge facing the doorway and one on the opposite edge.

8. The pad of claim 5 wherein said adhesive on said upper surface has a modulus of about 30 p.s.i., a tack angle of about 5°, and a viscosity of about 12 minutes.

9. A method for keeping foreign matter from being tracked into a clean room, comprising

10. The method of claim 9 in which the water-washable adhesive is a true-solid elastomer having a Young's modulus of elasticity between about 20 and 50 p.s.i., an internal viscosity and a free surface energy sufficiently high to stop at a position between 3.5° and 8° when a 1-mil film is cast in a 1-mil polyester terephthalate strip, placed over a polished steel cylinder to obtain 100 percent contact area and a 100 gram weight hung on a trailing edge and allowed to peel under static conditions, coming to a point within 5° of its final position in from 12 min. to 50 min.

This invention relates to a hospital or scientific clean room and to a disposable, washable, tacky pad for cleaning shoes and wheels at or just outside the entrance to a clean room.

With the rise of modern technology, "clean rooms" have become increasingly important. In the surgery, scrupulous cleanliness is vitally important to prevent infection, but the continued use of antibiotics has caused the emergence of strains of resistant bacteria, and the unpleasant result has been that hospitals, instead of being sanctuaries of sterility, have become a source of new infections that are extremely difficult to combat.

Similarly, in areas where microcircuitry and delicate space-age instruments are manufactured, one speck of dust can cause failure. Hence, techniques for insuring the highest possible degree of cleanliness have received intensive study.

Two fruitful sources of dirt are shoe soles and cart wheels. Many efforts have been made to find an easy and convenient but yet effective method for removing dust and dirt from shoes, and in the case of surgeries, from the wheels of the cart on which the patient is moved about. Heretofore, such removal has been of varying degrees of efficacy -- or lack of it -- and of inconvenience. For example, air blast systems have not been very effective, and disinfectant wash troughs have been messy and have been impractical for cart wheels.

Sticky pads of two general types have been tried, -- reusable ones and throw-aways. The reusable pads heretofore in use have had to be washed frequently with kerosene or similar solvent, and they have been so expensive that they usually ended up being purchased in too small a size to be adequately effective, and were impractical for cleaning cart wheels. The throw-aways have quickly lost their effectiveness but could not be washed, so that they had to be replaced very often, and again because of their cost have tended to be used in too small a size to be sufficiently effective.

The present invention accepts the fact that both low cost and washability are essential in a practical clean-room shoe-wheel pad, and the required low cost can be achieved only if the washable adhesive film can perform its function properly even when extremely thin. We have discovered that effective operability depends upon a proper combination of physical properties. Having determined those properties, we have found that a very effective pressure-sensitive pad can be produced from several different kinds of materials.

Our new pad is washable in place and can be washed and re-used several times, yet it is inexpensive enough to be very practical when made in a generously sized throw-away, such as four feet by five feet. With the physical properties disclosed below, its two adhesives -- one to stick it to the floor and one to remove the dirt from shoes and wheels -- are very effective at a thickness of only one mil (0.001 inch).

These adhesives may be coated approximately one mil thick on both sides of a strong but inexpensive backing such as 70 lb. bleached kraft paper or 6 mil (0.006 inch) polyethylene which has been treated on both sides to enable adhesion of the adhesive. The resulting inexpensive product is a very effective shoe and wheel cleaner. A third preferred substrate is a vinyl film of 41/2 mils thickness and of average softness, e.g., a Shore hardness of 85; this gives excellent service. When coated with the adhesive having the proper physical properties, it is easily washable many times with soap and water or even plain water. It has a proper balance of tackiness: ample to perform its function, and yet not unpleasant to walk upon. Our pad adheres well to almost any smooth surface, and yet it strips easily from the floor without delamination when ready for replacement.

While both the top-side coating and the underside coating come under the broad classification of pressure-sensitive adhesives, actually they have two entirely different functions. The top-side coating must have a fairly high temporary tack which grabs dirt aggressively and firmly, yet releases it easily when washed. In contrast, the bottom side must have a medium-strength but permanent tack that holds the pad firmly in place in spite of the sustained tension of the stretched vinyl substrate (or other substrate) and in spite of the violent temporary loads induced by shoes and wheels, yet at the same time the whole sheet must not be too difficult to remove from the floor at the time of replacement. The top side is definitely water insoluble and the bottom side is preferably so.

As stated already, it is not the chemical nature of the adhesive which is controlling. It is the physical properties that are critical, and we have found that these can be defined in clear terms. Several materials that are quite different chemically have these and some of these are readily available.

One physical requirement of both adhesives is that they be resilient solids, not viscous liquids. Another one is they must have a low but definite Young's modulus of elasticity. Two further physical properties that are essential are the surface free energy and the internal viscosity, which must have values appropriate to the desired function. The two adhesives -- i.e., the upper, dirt-removing adhesive and the lower, floor-gripping adhesive -- differ from each other in modulus, surface free properties and internal viscosity as will be seen below, but speaking in generalities for the present, the preferred pressure-sensitive adhesives are both water-insoluble, true-solid elastomers; both of these have a relatively low modulus of elasticity; however, one has a relatively low and one a high internal viscosity, and one has a low and one has a medium high free surface energy.

Internal viscosity in a solid may be a new concept to some people, and an example may help: a vinyl garden hose has a high internal viscosity, while gum rubber has a lower internal viscosity. Even though it may be "harder," a gum rubber tube suffers no harm from being run over by a car, while a vinyl garden hose (especially if old and cold) can be cracked to pieces by the same treatment, because it cannot deform quickly enough, due to its higher internal viscosity. If the rubber tube be heavy, it may require more weight to flatten it than to flatten the vinyl hose; moreover, the rubber flattens partway immediately and stays there, while the vinyl slowly flattens out completely. Hence, the vinyl is considered "softer" because it flattens out further, but it is slow to do so, because it has a higher internal viscosity.

Being a true solid, the adhesives used in this invention avoid many disadvantages of the conventional pressure-sensitive adhesives, which are highly viscous liquids having the tendency to creep freely, to deposit on (and even penetrate into) attached articles, and to embed lint and other small particles. In contrast, our true solids derive their adhesiveness from their low modulus of elasticity and their high elongation, which permits them to rapidly deform, without rupture, to a near-perfect molecule-to-molecule fit with articles brought into contact with it, thus allowing the Van der Waals forces of their free surface energy to act.

Another advantage of our preferred true-solid adhesives over conventional pressure-sensitive adhesives is their washability. Any such pad will soon have its surface contaminated with acquired lint, fuzz, dust, and other particles. Our true solids have such high resilience that these particles do not become embedded in the adhesive, but remain on the surface and may easily be washed off completely clean by sponging the surface with warm soapy water. This remarkable effect is contrary to all usual experience with a material which feels like a sticky, viscous, pitch-like liquid. The adhesive, being insoluble in water, is not affected by the washing.

When a foot or wheel engages the surface of the upper adhesive coating, a downward pressure is exerted. The tacky elastomer initially making only point contact with each dirt particle flows to enlarge the surface contact. The yielding of the upper adhesive coating results primarily from the softness of the material, since the modulus is less than 50. The softness and pliability of the upper adhesive coating material enable Van der Waals forces to pull the particle and distort the coating into a perfect fit with the particle's surface. There is a powerful attraction between the contacting surfaces of the particles and the upper adhesive coating, due to these Van der Waals forces. These forces are usually undetectable, since normally the actual contact area between two surfaces apparently in contact is vanishingly small, for instance 1/1000 of 1 percent. When, however, the percentage of actual contact area becomes appreciable, then the Van der Waals forces are strongly in evidence, approaching the strengths of materials (which are, in fact, due to these same forces.) The internal viscosity of the upper adhesive coating is, however, great enough that the coating only slowly and reluctantly changes from this perfectly fitting shape. The particle does not sink into the upper adhesive coating down to the interface between the coating and the substrate, since the coating, though soft, is an elastic solid, not a liquid. As the shoe or roller is rather suddenly removed, the dirt particle though still having a tendency to cling to it does not follow it but remains adherent to the adhesive coating.

Subsequently, when it becomes desirable to clean the coating by washing off the dirt particles so that the full surface again becomes available for re-use, preferably, soapy water having a sufficiently reduced surface tension is used. The extremely high dielectric characteristics of water (dielectric constant of about 80) immediately tend to neutralize the Van der Waals forces in an action which may be described as shorting them out. First, the water attacks the adhesive bond at the edge of the contacting surfaces of the adhesive coating and dirt particles. Since the washing operation takes several seconds, there is plenty of time for the required change of shape to occur, and the internal viscosity of the adhesive coating, which was formerly helpful in picking up the dirt particle, because the material was almost undeformable in the very short time required to yank the particle off the shoe or roller, now has several seconds to reform to its original, generally smooth surface, and so the internal viscosity does not prevent the reforming. Hence, the water at first neutralizes the Van der Waals forces at the edges of the dirt particle, thereby loosening the bond between the particles and the adhesive coating.

The resiliency of the upper adhesive coating is of vital importance. Many tacky materials on the market, though they may show elasticity to a short-term stress, are not true solids, but are very viscous liquids. If particles are not washed off immediately from such materials, they slowly flow around the particles and imbed them, a slow continuation of the initial "wetting" action. Soon, it becomes virtually impossible to wash the particles off, and the utility of such a coating is destroyed if left dirty overnight.

However, the coating of this invention being a true solid, the particles do not sink into it. The upper adhesive coating is deformed during the bonding state into a contour perfectly fitting a portion of each dirt particle, due to operation of the Van der Waals forces, but the partial neutralizing of these forces by the soapy water enables resilient retraction of the adhesive coating. Hence, the adhesive coating attempts to pull back into its original shape, to return to its original generally smooth glossy surface contour. This loosening of the bond and retraction of the adhesive coating is progressive from the bond edge, in that it tends progressively to reduce the bonding surface area until the dirt particles in effect are loosely seated on the surface of the adhesive coating and can be rinsed away. This loosening of the bond is enhanced if the dirt particle is hydrophilic. As practically all lint and dust are either cotton or silicate mineral (both of which are hydrophilic), these particles are easily wetted, so that the bond is additionally loosened by water reaching and wetting the bond surface, facilitating and hastening the loosening of the bond. Hence, the time needed for washing the dirt off the adhesive coating is quite short. If the adhesive coating is (as is preferable) hydrophobic, water and dirt run off together to render the adhesive coating readily dry after washing, and the pad is reusable immediately thereafter.

The invention may be more fully understood by reference to the drawings, wherein:

FIG. 1 is a simplified plan view of a clean room embodying the principles of the invention.

FIG. 2 is a plan view of a pad that may be used at the entrance of the room of FIG. 1.

FIG. 3 is a fragmentary sectional view taken along the line 3--3 of FIG. 2 illustrating one form of construction.

FIG. 4 is a view in perspective of a test-apparatus comprising a polished steel cylinder with scribed lines, with adhesive-coated strip on test and shown in alternative positions.

FIG. 5 is a view in side elevation of a tack-measuring test apparatus usable with this invention.

FIG. 6 is a view in front elevation of the apparatus of FIG. 5.

FIG. 1 illustrates a clean room 10 having a doorway 11 and doors 12 and 12a opening onto a hall 13. According to the present invention a pad 15 embodying the invention is adhesively and removably secured to the floor 14 of the hall 13 just in front of the doorway 11, so that everyone entering the room 10 and all wheels rolled into the room 10 pass across the pad 15. Preferably, the pad 15 is large enough to make it difficult to miss it and difficult to enter the room 10 without the shoes resting briefly in the pad 15 and without the full perimeter of the wheels rolling across the pad.

FIGS. 2 and 3 show the pad 15 in more detail. A base 16, may be of vinyl plastic, e.g., polyvinyl chloride (homopolymer), of polyethylene, or of kraft paper, among other things, but in any event is "paper-thin" and suitably strong and is made of material that accepts the materials to be used as adhesives. A lower adhesive 17 is secured to the lower surface of the base 16 and is used for attaching the pad 15 to the floor 14, and an upper adhesive 18 is secured to the upper surface of the base 16 and is used for removing dirt and other foreign matter from shoes and wheels.

Materials of many chemical types may be used as adhesives. Polyvinyl chlorides can be combined with a suitable plasticizer to give a suitable product. Highly plasticized neoprene is suitable. Some polysulfides and polyurethanes can be used. Many acrylics, such as butyl acrylate and many of its homologues will also be found useful.

It is the physical properties which are important. As mentioned before, each of the adhesives 17 and 18 must be a true resilient solid, not a viscous liquid. Each must be soft enough to conform easily to the shape of the adherend, and it must be insoluble in water. These three physical properties are an absolute necessity in any washable tacky elastomer.

The type of tack -- temporary or permanent -- depends on the requirements of the end use, and the two coatings 17 and 18 differ in this.

A high temporary tack is imparted to the upper coating 18 by using a relatively low Young's modulus (typically 20 to 50 p.s.i. and very good results being obtained at 30 p.s.i.) a relatively high internal viscosity, and a low surface free energy. A low permanent tack is imparted to the lower coating 17 by a medium surface free energy, a low internal viscosity, and a higher Young's modulus, typically 150 to 250 p.s.i. with very good results at about 210 p.s.i.

Internal viscosity is inconvenient to measure in terms of poises, and surface free energy is so difficult to measure in terms of ergs per square centimeter as to be impractical. Therefore, we have devised a simple empirical test to evaluate adhesives for possible use in the present invention.

A FIG. 4 shows a polished steel cylinder 20, two inches in diameter, scribed with fine lines 21 parallel to the axis 22 and 5° apart over 90° of its surface 23. The cylinder 20 is mounted firmly in cantilever with the axis horizontal, in the position shown in FIG. 4. A one mil film of adhesive 24 (which may be either coating 17 or 18) is cast on a polyester terephthalate (mylar) strip 25 of one mil thickness, which is then trimmed to a width of one inch. The cylinder 20 is washed carefully with methyl ethyl ketone, and the strip 25 is placed over the scribed lines 21 and one end 26 draped over the top. The strip 25 is pressed firmly against the cylinder 20 to attain as near as possible 100 percent contact area. A 100-gram weight 27 is hung gently on the trailing end 28 and the time noted at the 90° position. The progress of the peel line is noted from time to time and it should come to a point within 5° of its final position in the times noted below. The total time allowed must be ample to ascertain that the peel line has in fact stopped moving, to insure that internal viscosity is not affecting the result. Twenty-four hours is suggested as a convenient total period; if the line is still moving, the formulation may be discarded as having too high an internal viscosity for a shoe-cleaning pad.

From this test, three things can be learned;

1. The stopping position of the peel line is a measure of the permanent tack of the adhesive in question. Much experience has shown that the optimum for best all-around performance of the lower coating 17 is given by an adhesive that stops at approximately 45°. Values in the range of about 30° to about 70° will give reasonable success, and very good results are obtained in a range of about 35° to about 60°. For the upper coating 17, satisfactory performance requires values of from about 3.5° to about 8°, preferably from about 4° to about 6°, and excellent results are obtained at about 5°.

2. The length of time from start to within 5° of finish (the finish is surprisingly definite) is a measure of the internal viscosity. For the lower coating 17, the time should be in the range of 80 to 800 minutes, preferably from 300 to 700 minutes, and optimum results are generally obtained at about 500 minutes. For the upper coating 18, the adhesive should have a value of from 12 minutes to 50 minutes, preferably of from 15 minutes to 30 minutes.

3. If the creep is too high in an otherwise satisfactory adhesive, the adhesion will be greater than the cohesion, and a residue will be left on the cylinder 60. This adhesive cannot be considered washable; lint and dust and paper fibers will embed in it and soon make it unusable. If used as a coating for the underside, it will have a deposit on the floor.

In the factory a quick test is useful for quality control. Two plywood boards 30 and 31, coated with a low-stick material, such as polyethylene polypropylene, or polytetrafluoroethylene, covered with an adhesive 32 (which may be either coating 17 or 18), are hinged together by a hinge 33 and mounted at two different angles from he horizontal 34 as shown in FIG. 5. A 1-inch diameter stainless steel ball 35 is cleaned carefully and, with freshly washed hands, is rolled up hill on the surface of samples adhered to the boards 30 and 31. The ball 35 comes to a stop and either stays where it stops or rolls back down. Thus we have a go-no go test; the ball 35 should roll back on the steeper slop 31 but stay put on the lower slope 30. This test is objective, since the timing of the stop and possible restart back down are unaffected by the speed and accuracy with which the ball 35 is thrown, and operator error is eliminated.

It has been found by much experience that the adhesive for the lower coating 17 will be of the proper (rather aggressive) tack if the ball 35 rolls back down on a 21/2° slope and does not roll back down on a 2° slope. For the upper adhesive coating 18 the values lie between 6° and 5°. This range may be of course adjusted slightly downward or upward, but only slightly and after much field testing. While this test measures no particular absolute value, it is useful as a quick test of production uniformity. The preferred thickness of each of the coatings 17 and 18 is 1 mil but they can be in the range of 0.3 to 3.0 mils.

Formulations of several types may be used. We make no formula claims, since following the teachings of this disclosure, anyone skilled in the art of pressure-sensitive adhesives can make up suitable compositions from the materials with which he is familiar and prefers to use. Almost any elastomer can be used as a base. For example, any of the following may be used as starting points and modified as desired and will be satisfactory washable pressure-sensitive adhesives if the requisite physical characteristics as taught by this and our companion application are observed and fulfilled. There are also many acrylics, available under various trade names which, again provided that they are selected and formulated to obtain the proper physical properties, will afford very satisfactory results.

Example 1 Water-Washable Tacky Adhesive Elastomer

Ingredient Parts by Weight High-molecular-weight polyvinyl chloride e.g., Geon 121 made by B. F. Goodrich 80 Geon 22 " 222 " 20 Plasticizer, a polyester condensation product of sebacic acid and ethylene glycol of approximately 8,000 molecular weight, e.g., Paraplex G25 made by Rohm and Haas 400 Barium zinc phenate, e.g., Argus Chemical Co., Mark KCB 4

suitable pigment or dye, if desired, may be added. The suggested cure cycle is 10 min. at 380° F.

Similar results may be obtained by substituting a copolymer of polyvinyl chloride and polyvinyl acetate for the polyvinyl chloride, the copolymer preferably having approximately the same physical qualities as that of this example and usually requiring slightly less plasticizer.

Example 2 Polyurethane-type Water-Washable Tacky Composition

Two components are used:

Component A:

ingredient Parts by weight Polyether triol, e.g., Wyandotte TP 4542 100 Toluene di-isocyanate, e.g., Allied Chemical, Nacconate 12

The mixture is held at 160° F for 4 hours.

Component B:

ingredient Polyether triol as in Component A 100 Tin octoate catalyst, e.g., Witco Chemical Co., Tin catalyst C-4 0.5

the two prepared components are mixed in the proportions, by weight, of

Component A 100 Component B 83

Cure 4 hours at 180° F. The resultant composition had an internal viscosity of about 1,800 poises and a Young's modulus of about 60.

Example 3 Polysulfide Type Water-Washable Tacky Material

Ingredient Parts by Weight A. Resin Component ____________________________________________________________ ______________ Polyalkylene polysulfide, e.g., Thiokol LP 31 100 Chlorinated biphenyl, e.g., Monsanto Aroclor 1254 90 Calcium carbonate, e.g., Diamond Alkali Co. Super Multifex 30 Liquid coumarin-indene alkylated phenol, e.g., Neville Chemical Co. 10° Nevillac 20 Bis-Phenol-A and epichlorhydrin epoxy resin, e.g., Shell Chemical Epon 836 3 Stearic acid 0.5 Sulfur 0.1 B. Catalyst Component ____________________________________________________________ ______________ Lead oxide 100 Chlorinated biphenyl, e.g., Monsanto Aroclor 1254 30 Xylene 10 Zinc stearate 2.5 Stearic acid 1.5

Then two components are mixed in the preferred ratio of 100 parts by weight of the resin mix to three and one-half parts of catalyst mix and cured at 160° F. for 2 hours.

Example 4 Neoprene Type Water-Washable Tacky Material

Rubber Solution Parts by Weight Polychloroprene, e.g., Du Pont Neoprene W 100 Toluene 240 Methyl ethyl ketone 160

The mixture is stirred or tumbled until the neoprene is dissolved.

Premix of catalyst, plasticizers,etc

Chlorinated biphenyl, e.g., Monsanto Aroclor 1254 20 Zinc Oxide 10 Magnesium Oxide 5 Phenyl-beta-naphthylamine, e.g., Du Pont Neozone D. 2 Ethyl thiourea, e.g., Du Pont Accelerator NA 22 1

the materials are dispersed in the Aroclor, and then the following materials are added and mixed:

Parts by Weight Monsanto Aroclor 1254 55 Neville Chemical Co. 10° Nevillac 25

The premix is added to the rubber solution and mixed. After it is thoroughly dry, cure for 30 minutes at 250° F.

Example 5 Acrylic Type Water-Washable Tacky Material

Another very successful and effective top coat 18 employs Rohn 8 Haas rhoplex K3, An acrylic latex. This has a modulus of 30 p.s.i., and the following physical properties by the steel cylinder test.

tack angle of 6°

viscosity of 12 minutes

This material gives a high and effective "quick-stick," yet washes so well that after over 100 dirtyings and moppings, and though very scratched and scarred after that much usage, it still was coming clean easily and regaining almost all of its initial tack.

An underside coating 17 which effectively fulfilled its requirements as described above is B. F. Goodrich acrylic latex 2600 × 83 and had the following properties (the comparison is interesting and instructive):

modulus 210 p.s.i. tack angle 43° viscosity 610 minutes

This viscosity may be somewhat higher than necessary. With the relatively high modulus and angle desired, a low viscosity would be difficult to attain, and since the high viscosity does help resist short violent loads, no effort was made to reduce it.

This combination of properties is not very tacky to the touch, whereas the combination listed for the top coat feels quite tacky. The uninitiate feels sure the pad is being applied to the floor upside down, and while he feels the top coat pulling strongly at his shoes, is amazed to find the bottom coat stubbornly holding the pad firmly to the floor. If he experimentally applies a pad upside down, he finds his best efforts cannot stick the pad to the floor -- it quickly comes loose and lies there in a welter of wrinkles.

It has also been found that, whereas the under surface (to be applied to the floor) must be completely covered with adhesive, it is very important to leave a strip perhaps an inch or two wide bare of adhesive at the front and rear upper surface edges of the mat that is, one non-adhesive covered area on the edge facing a doorway and one on the opposite edge. This prevents accidental progressive peeling from the floor, if the pad is placed in this manner.

To those skilled in the art to which this invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting.