Floor Mats With Microfiber Backing Material

Abstract

An improved floor mat designed to maintain its location in relation to the carpet upon which it is placed. The floor mat comprises a layer of carpet, a layer of support material attached to a layer of carpet, and a layer of loop-pile microfiber material attached to the layer of support material opposite the layer of carpet. The layer of loop-pile microfiber material can include a plurality of split microfibers. The split microfibers can include a generally acute triangular shape wherein a portion of the generally acute triangular shape includes at least one generally arcuate side.

Description

BACKGROUND

[0001] Technical Field

[0002] The present disclosure relates generally to floor mats and more particularly, without limitation, to floor mats having a microfiber backing designed to having a friction interface with the surface one which the floor mat is placed, such as carpet, to retain the floor mat in its desired position.

[0003] Background Art

[0004] Referring generally now to FIGS. 1A-1C, an example of loop-pile construction as known in the textile and carpet art is shown. This type of product manufacturing basically includes a needle (1) partially pushing a length of material (2), such as a strand, yarn, cord, etc., through a primary layer (2), or backing layer. A looper (4) then grabs the material (2) to form the loop as the needle retracts. Another part of the material is then pushed through the backing layer at a second location. This type of carpet manufacturing of can be referred to as tuft manufacturing, e.g. tufted carpet. This type of carpeting generally begins with the loop-pile formation and then the tops of the loops are cut leaving freestanding pieces as shown in the illustration. This is called cut-pile because the loop-pile formation is cut. Another type of textile/carpet manufacturing is referred to as woven. Woven is where interlaced pieces of the material are formed together in an alternating over-under pattern. This creates a generally "interlocked" product. If during the manufacture of the woven product the interlacing pieces of the material are not drawn taunt, "loop type" pieces can form in the woven product. These loose pieces can resemble the loop pile construction just described.

[0005] Floor mats, as known in the art, are typically used to either 1) protect an underlying surface, such as a floor, 2) change the engagement surface of the underlying surface for a user of that surface (e.g. change the coefficient of friction as the user walks), or 3) change the aesthetic look to the surface. Floor mats can be composed of many different materials but are generally composed of a type of plastic or a fabric. The fabric can be generally referred to as carpet. These floor mats have many uses in many locations, including residential, commercial, industrial, and in vehicles.

[0006] One area in particular is the use of floor mats in vehicles to protect and decorate the vehicle interior. Specifically the floor mats are used on the floor of the vehicle, or floor board. The floor board itself is usually covered by carpet used to protect the underlying materials of the floor board and give an aesthetically pleasing look to the passengers and/or driver of the vehicle. This carpet is normally permanently attached to the floor board and is typically comprised of tufted cut-pile carpet or non-woven carpet/fabric. It can be expensive to replace the carpet that is fixed to the floor board. As such, floor mats are often placed in locations of this carpeting that received the most wear and tear, or use, during operation of the vehicle.

[0007] Floor mats generally are known in the art and conventionally comprised of elastomeric materials that have enough flexibility to generally conform to the shape and contours of the vehicle floors. These floor mats can also include carpet attached to the elastomeric material that can provide an aesthetically pleasing look to the users of the vehicle and/or match the carpet attached to the floor board. The elastomeric material and conventional carpet used in floor mats typically do not sufficiently interact with the carpet on the floor boards to maintain the position of the floor mat when subject to slight forces during use.

[0008] Movement of floor mats on carpet can damage the carpet and potentially be unsafe. For example, a person can slip on a floor mat when walking along a carpeted surface if the floor mat shifts. Additionally, a floor mat in a vehicle can move during ingress and egress of a person into the vehicle. This can cause a safety hazard and potentially interfere with operation of the vehicle if the floor mat is in the driver's area. This movement also can increase the wear and tear on the carpet it is trying to protect by additional stress to the underlying carpet as the floor mat moves.

[0009] Numerous prior attempts have been made to address the problem of movement of floor mats relative to underlying floor board carpeting. For example, U.S. Pat. Nos. 4,425,690, 6,381,806, and 7,329,451 are examples of prior art attempts. These patents include various mechanical fasteners designed to secure or maintain the locale of the floor mat relative to the floor. These concepts have various drawbacks based upon their design and configurations.

[0010] What is needed then is an improved floor mat that maintains its position relative to the carpet upon which it is placed. This needed floor mat is lacking in the art.

BRIEF SUMMARY

[0011] The present disclosure provides an improved floor mat designed to maintain its location in relation to the carpet upon which it is placed.

[0012] In some embodiments, the present disclosure provides a floor mat comprising a layer of carpet, a layer of primary support material attached to a layer of carpet with an adhesive, and a layer of loop-pile microfiber material attached to the layer of support material opposite the layer of carpet with adhesive. The layer of loop-pile microfiber material can include a plurality of split microfibers. In an embodiment, the split microfibers can include a generally acute triangular shape wherein a portion of the generally acute triangular shape includes at least one generally arcuate side.

[0013] In an embodiment, the layer of loop-pile microfiber material can include an area, a plurality of piles with each pile formed by at least one strand of microfibers, and each pile can include a height. The loop-pile microfiber material can have a pile height as desired by a user of the floor mat. The pile height can generally fall somewhere in the range from approximately 0.1 millimeters to 15 millimeters as desired. Within this range, the actual variance of the pile height for a given layer of loop-pile microfiber material is preferably smaller. For example, in an embodiment, the height of each pile can range from approximately 1 millimeter to 10 millimeters. In an embodiment, the height of each pile can range from approximately 5.0 millimeters to 10.0 millimeters. In another embodiment, the height of each pile can range from approximately 1.0 millimeter to 3.0 millimeters. In another embodiment, the height of each pile can range from approximately 1.5 millimeters to 4.0 millimeters. In another embodiment, the height of each pile can range from approximately 2.0 millimeters to 3.0 millimeters.

[0014] In an embodiment, the layer of loop-pile microfiber material can include a pile density with the pile density measured by the number of piles per square centimeter of the area of the layer of the loop-pile microfiber material. The loop-pile microfiber material can have a pile density as desired by a designer of the floor mat. The pile density can generally fall somewhere in the range from approximately 5 piles per centimeter squared to 500 piles per centimeter squared as desired. Within this range, the actual variance of the pile density for a given layer of loop-pile microfiber material is relatively small. For example, in an embodiment, the pile density can be any general density as selected and desired from the range and vary approximately +/-10% piles per centimeter squared for that selected density.

[0015] In a preferred embodiment, the layer of loop-pile microfiber material includes a plurality of piles and each of the plurality of piles includes strands of microfiber filaments which are split into smaller segments. Each strand of microfiber can comprise filaments wherein the number of filaments per strand, ranges from approximately 500 to 750. In another embodiment, the number of filaments per strand can range from approximately 10 to 10,000. Each of the filaments can be split into segments ranging from 1 to 64.

[0016] In an embodiment, the primary layer of support material is composed of materials such as polyethylene terephthalate, polypropylene, and polyamide. In another embodiment, the primary layer of support material is coated with latex or other types of adhesive. In another embodiment, a secondary layer of support material such as non-woven, foam, plastic, rubber, etc. is laminated to the primary layer and bonded with adhesive to provide body, support, rigidity, retention and a host of other properties.

[0017] In another embodiment, the present disclosure provides a floor mat comprising a support material having a first side and a second side, carpet attached to the first side of the support material, and loop-pile constructed microfiber material attached to the second side. The loop-pile microfiber material includes a plurality of piles with each pile including a height and microfiber strands having split filaments exposed and extending from the strands. In this embodiment, the split filaments can include edges that extend from the strand. These edges can comprise a generally triangular shape, such as an acute triangular shape, with a portion of the generally triangular shape including at least one generally arcuate side.

[0018] The floor mat can further consist of the loop-pile microfiber material having an area and a pile density measured in the number of piles per square centimeter of the area of the loop-pile microfiber material. In this embodiment, the height of each pile can range from approximately 1 millimeter to 3 millimeters, the pile density can range from approximately 10 piles per centimeter squared to 400 piles per centimeter squared, and a number of filaments per microfiber strand can range from approximately 100 to 10,000.

[0019] In another embodiment, the present disclosure provides a floor mat having a layer of carpet having a front side and a back side with a layer of loop-pile microfiber material operatively attached to the back side of the carpet. The microfiber material can include a plurality of piles and each of the plurality of piles can include microfiber strands. Each strand of microfiber can include filaments with the number of filaments per pile as part of the split ranging from approximately 100 to 10,000. Each microfiber filament can be split into multiple fibers. In this embodiment, a layer of support material can further be included and positioned between the layer of carpet and the layer of loop-pile microfiber material.

[0020] Numerous other objects, features and advantages of the present disclosure will be readily apparent to those skilled in the art upon a reading of the following description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1A is an example of loop-pile carpeting construction.

[0022] FIG. 1B is an example of loop-pile carpeting.

[0023] FIG. 1C is an example of cut-pile carpeting.

[0024] FIG. 2 is an image showing of an embodiment of a floor mat made in accordance with the current disclosure laid against carpeting.

[0025] FIG. 3A is a perspective view of the carpet side of an embodiment of a floor mat made in accordance with the current disclosure.

[0026] FIG. 3B is a perspective view of the microfiber side of an embodiment of a floor mat made in accordance with the current disclosure.

[0027] FIG. 4 is view of the microfiber side of a floor mat made in accordance with the current disclosure.

[0028] FIG. 5 is view of the carpet side of a floor mat made in accordance with the current disclosure.

[0029] FIG. 6 is an end view of the floor mat made of FIGS. 2-5.

[0030] FIG. 7 is a microscopic picture of a floor mat made in accordance with the current disclosure.

[0031] FIG. 8 is a cross sectional view of the image shown in FIG. 7 before dyes are added to the microfiber material.

[0032] FIG. 9 is an image from an electron microscope of a floor mat made in accordance with the current disclosure shown at a magnification level of 200.times..

[0033] FIG. 10 is a cross sectional view of the image shown in FIG. 9 taken at 1000.times. magnification by an electron microscope.

[0034] FIG. 11 is a cross sectional view showing an example of the pile density of a floor mat made in accordance with the current disclosure.

[0035] FIG. 12 is a cross sectional view of 1 microfiber after it has been split showing the filaments extending therefrom.

[0036] FIG. 13 is a cross sectional view of the comparison between split and unsplit filaments taken with an electron microscope.

[0037] FIG. 14 is an example of the testing of the retention characteristics of a floor mat made in accordance with the current disclosure.

[0038] FIG. 15 is a cross sectional view of the comparison of floor mats made in accordance with the current disclosure. One is constructed with the support material and one without.

DETAILED DESCRIPTION

[0039] Referring generally now to FIGS. 1-14, one embodiment of the present disclosure provides a floor mat generally designated by the numeral 10. It is understood that, for the sake of clarity, not all reference numbers are included in each drawing. In addition, positional terms such as a "upper," "lower," "side," "top," "bottom," "vertical," "horizontal," etc. refer to the apparatus when in the orientation shown in the drawing. The skilled artisan will recognize that objects in accordance with the present disclosure can assume different orientations when in use.

[0040] Referring generally now to FIG. 1B, an example of loop-pile carpet as known in the art is shown. This carpeting basically includes a sub-straight to which a length of material is connected to at either end forming a loop. Cut-pile carpeting is shown in FIG. 1C. This type of carpeting generally begins with the loop-pile carpeting formation and then the tops of the loops are then cut leaving freestanding pieces as shown in illustration. This is called cut-pile because the loop-pile is cut.

[0041] The floor mat 10 is used in conjunction with carpet 12 on a floor, or other surface such as a floor board of a vehicle. The floor mat 10 is designed to maintain its positioning on the carpet 12 and have a high resistance to displacement, which can also be described as a retention, of the floor mat 10 in relation to the location it is placed on the carpet 12. The carpet 12 can be any type of carpet known in the art including loop-pile, cut-pile, other styles and variations thereof. The floor mat 10 works in conjunction with the various types of carpet 12 to facilitate retaining its position with respect to that carpet 12.

[0042] In a preferred embodiment, the floor mat 10 comprises a layer of carpet 14, a layer of support material 16 attached to the layer of carpet 14, and a layer of loop-pile microfiber material 18 attached to the layer of support material 16 opposite the layer of carpet 14.

[0043] The layer of carpet 14, which can be described as carpet 14, can include any type of carpet known in the art including loop-pile, cut-pile, other forms of carpet, and variations thereof. This carpet 14 provides a surface for a user of the floor mat 10 and can be colored or decorated to be aesthetically pleasing as desired.

[0044] The layer of support material 16, which can be described as support material 16, can be attached to the carpet 14 to provide enough stability and rigidity in the overall floor mat 10 as desired. The support material 16 can be comprised of those materials known in the art to provide rigidity including various types of plastic, including polyethylene terephthalate (PET), polypropylene (PP), polyamide, and other types of polymers in woven or non-woven fabric styles often coated with various forms of latex adhesive. For example, the support material 16 can be a PET non-woven fabric material used to provide the level of rigidity needed to be able to position the floor mat as desired on a floor, including the floor of a vehicle. The various desired types of plastic, including the PET, can also provide a level of flexibility such that the floor mat 10 can contour to the surface it is placed. For example, the floor mat 10 used with these types of support material 16 can contour to the floor board of a vehicle yet have enough rigidity to be positioned and placed as needed by a user of the floor mats 10.

[0045] The layer of loop-pile microfiber material 18, which can also be described as microfiber material 18, can include a plurality of split microfibers 20 that are designed to engage the carpet 12 to maintain the positioning of the floor mat 10. In a preferred embodiment, the split microfibers 20 include a generally triangular shape, such as an acute triangular shape, as best seen in FIG. 10. In this embodiment, the triangular shape can include at least one generally arcuate side that generally conforms to the overall outer diameter dimension of the split microfiber 20.

[0046] In this embodiment, the split microfiber 20, including the edges of the shapes that the split microfiber 20 takes, such as the triangular shape, can act as a gripping mechanism to the carpet 12. Specifically, they can act as a gripping mechanism to entangle the fibers of the carpet 12, such as tuft cut-pile carpet fibers. Generally, the fibers of the carpet 12 typically protrude and can engage and be intertwined in between the microfiber material 18, and more specifically, the split microfibers 20. This entanglement of the carpet fibers and the split microfibers 20 yields a strong frictional resistance that counteracts a force designed to remove or slide the floor mat 10 across the carpet 12.

[0047] The split microfibers 20 have an advantage over microfiber materials made from extracted microfiber construction. For example, the split microfiber construction includes the various edges that act as the gripping mechanisms. Conversely, extracted type microfibers have smooth, round, and/or oval cross sections similar to natural fibers that lack this gripping feature. As such, the extracted type microfibers do not have the frictional interaction and the resistance to movement that the split microfibers 20 exhibit. As such, the smooth extracted type microfibers will not have the resistance to movement of the floor mat 10 in relation to the carpet 12 when used as part of a floor mat.

[0048] Further, the loop-pile construction provides an additional advantage of the microfiber material 18 in this disclosure. For example, the generally uneven loop-pile construction that includes various porous openings and gaps within adjacent portions of the microfiber material 18 creates a surface condition that promotes entanglement of the fibers of the carpet 12 to the split microfibers 20 of the microfiber material 18. The fiber of the carpet 12 easily protrudes into the surface of the loop-pile construction of the microfiber material 18, such as in between the various split microfibers 20, to promote an engagement and frictional resistance to movement between the split microfibers 20 and the carpet 12. The surface condition created by the loop-pile height of the microfiber material 18 when combined with the surface edges of the split microfibers 20 creates a superior side-to-side frictional resistance in relation to the fibers of the carpet 12 to prevent and/or substantially resist the sliding motion between the carpet 12 and the microfiber material 18 of the floor mat 10.

[0049] The inventor discovered that the loop-pile woven construction of the microfiber material 18 with the split microfibers 20 is superior to other types of microfiber material and normal fiber material. Twill, satin, and plain woven styles of microfiber material and various non-woven microfiber materials, such as microfiber material bonded together by mechanical, thermal, or chemical means, result in a relatively smooth, dense, flat surface when constructed as a sheet of material. That smooth, flat surface can generally prevent the fibers of the carpet 12 from embedding into the surface. As such, these woven or non-woven microfibers limit the entanglement level, and friction, between those microfiber materials and the carpet 12. Alternately stated, a friction grip between the mating surfaces of the woven or non-woven microfibers is not as substantial as the frictional grip of the loop-pile microfiber material 18 with split microfibers 20. Additionally, traditional natural fibers, such as, but not limited to, cotton, does not have the engagement surfaces present in the loop-pile microfiber material 18 with the split microfibers 20. As such, the natural fiber materials as used in a floor mat do not have the same frictional gripping ability in relation to the carpet 12.

[0050] Further, the level of sliding or tension can specifically relate to the edges and number of engagement surfaces present in the split microfibers 20 of the microfiber material 18 as constructed with the porous loop-pile construction of the current floor mat 10. Those combined features can enable a maximum level of entanglement between the microfiber material 18 and the fibers of the carpet 12 to provide an improved and high level of frictional resistance to movement between the floor mat 10 and the carpet 12.

[0051] In construction of a floor mat 10 in accordance with the current disclosure, the microfiber material 18 can include an area and a plurality of piles 22 with each pile 22 formed by at least 1 strand of microfiber. The piles 22 can include a height and a pile density measured in the number of piles per square centimeter of the area of the microfiber 18. In an embodiment, the height of each pile 22 ranges from approximately 1.0 millimeters to approximately 5.0 millimeters, more preferably from approximately 1.5 millimeter to approximately 4.0 millimeters, and most preferably from approximately 2.0 millimeters to approximately 3.0 millimeters. The pile density can be a specific density for a given microfiber material and that particular material can have a density ranging from 5 piles per centimeter squared to approximately 25 piles per centimeter squared. However, the pile density is usually a specific density within that range, as desired, and can vary approximately +/-10% for that specific selected and desired pile density for the given microfiber material.

[0052] Further, a microfiber of the microfiber material 18 can be measure and defined in terms of its linear mass density measured in a denier. A microfiber can have a denier of less than 1 of a given material relative to the density of that material. Examples of microfibers can include: 1 D of PP gives an average diameter of 12.46 micrometers; 1 D of PET gives an average diameter of 10.12 micrometers; 1 D of Nylon gives an average diameter of 11.13 micrometers; and 1 D of high density polyethylene (HDPE) gives an average diameter of 12.19 micrometers. These values can obviously vary in accordance with manufacturing tolerances and design preferences and can have the diameters, depending on the exact material selected, ranging from approximately 1 micrometer to approximately 14 micrometers, from approximately 4 micrometer to approximately 13 micrometers, and from approximately 11 micrometers to 13 micrometers. Further, these diameters that equate to 1 denier size, while the actual fibers could be less than 1 denier by definition. The diameter of the microfiber can be circular in nature or oval in nature and the general length of the diameter measured accordingly.

[0053] The split microfibers 20 of the microfiber material 18 can further comprise filaments 24 as part of the split. This is best seen in FIG. 12. The number of filaments per microfiber as part of the split can range from approximately 100 to approximately 10,000, and more preferably range from approximately 300 to approximately 5,000.

[0054] In another preferred embodiment, a floor mat 10 made in accordance with the current disclosure can be described as having a support material 16 having a first side 15 and a second side 17 with the carpet 14 attached the first side 15 of the support material 16. Microfiber material 18, constructed in a loop and pile manner, can be attached to the second side 17. The microfiber material 18 can include any characteristics as described herein.

[0055] In another preferred embodiment, a floor mat can be constructed without the support material 16. In this embodiment the floor mat 11 includes a layer of carpet 14 having a front side 26 and a back side 28 and a layer of microfiber material 18 is attached the back side 28 of the carpet 14. The microfiber material 18 in this embodiment can include any characteristics of the microfiber material 18 as described herein.

[0056] In testing, the inventors discovered that the retention comparison of the currently disclosed floor mat with the loop-pile microfiber material 18 having a split microfibers 20 had an increased resistance to movement, which can also be described as a retention factor, when compared to other materials. For example, the current microfiber material 18 with the split microfiber 20 was tested in a retention comparison where a maximum load was discovered. This maximum load was the point at the point of which the microfiber material 18 began to skid when placed on carpet 12. This level for the microfiber material 18 having a split microfibers 20 was approximately 1.24 (Kgf). Normal cotton fibers had a maximum load of 0.025 Kgf. Woven microfiber had a maximum load of 0.09 Kgf. Non-woven microfiber had a maximum load of 0.15 Kgf. The sample areas tested were approximately 400 centimeters squared and there was a tensile speed of 100 millimeters per minute. This data shows that the microfiber material 18 in a loop and pile construction with a split microfiber 20 has an increased retention characteristic when compared to other microfibers and normal fibers known in the industry.

[0057] Thus, it is seen that the apparatus and methods disclosed herein achieve the ends and advantages previously mentioned. Numerous changes in the arrangement and construction of the parts and steps will be readily apparent to those skilled in the art, and are encompassed within the scope and spirit of the present disclosure.

Claims

1. A floor mat comprising: a layer of carpet; a layer of support material attached to the layer of carpet; and a layer of loop pile microfiber material attached to the layer of support material opposite the layer of carpet, the layer of loop pile microfiber material including a plurality of split microfibers.

2. The floor mat of claim 1, wherein the split microfibers include generally an acute triangular shape.

3. The floor mat of claim 2, wherein a portion of the generally acute triangular shapes include at least one generally arcuate side.

4. The floor mat of claim 1, wherein the layer of loop pile microfiber material includes: an area; a plurality of piles with each pile formed by at least one microfiber and including a height; and a pile density measured in the number of piles per square centimeter of the area of the layer of loop pile microfiber material,

5. The floor mat of claim 4, wherein the height of each pile ranges from approximately 1.0 millimeters to approximately 12.0 millimeters.

6. The floor mat of claim 4, wherein the height of each pile ranges from approximately 1.5 millimeters to approximately 4.0 millimeters.

7. The floor mat of claim 4, wherein the height of each pile ranges from approximately 2.0 millimeters to approximately 3.0 millimeters.

8. The floor mat of claim 4, wherein the pile density ranges from approximately 5 piles per cm.sup.2 to approximately 500 piles per cm.sup.2.

9. The floor mat of claim 8, wherein the pile density ranges from approximately +/-10% piles per cm.sup.2

10. The floor mat of claim 4, wherein the pile density ranges from approximately +/-10% piles per cm.sup.2.

11. The floor mat of claim 1, wherein the layer of loop pile microfiber material includes: a plurality of piles and each of the plurality of piles includes one of the split microfibers; each split microfiber comprises segments as part of the split; and wherein the number of filaments per pile ranges from approximately 10 to approximately 10,000 and the number of segments as part of the split ranges from 1 to 64.

12. The floor mat of claim 11, wherein the number of filaments per pile ranges from approximately 500 to approximately 750.

13. The floor mat of claim 1, wherein the layer of support material is composed of polyethylene terephthalate, polypropylene, or polyamide.

14. A floor mat comprising: support material having a first side and a second side; carpet attached to the first side of the support material; loop pile constructed microfiber material attached to the second side, the loop pile microfiber material including a plurality of piles with each pile including a height and a split microfiber having segments exposed and extending along the split.

15. The floor mat of claim 14, wherein the segments include a generally triangular shape with a portion of the generally triangular shapes including at least one generally arcuate side.

16. The floor mat of claim 14, wherein: the loop pile microfiber material includes an area and a pile density measured in the number of piles per square centimeter of the area of the loop pile microfiber material; the height of each pile ranges from approximately 1 millimeter to approximately 12 millimeters and the pile density ranges from approximately 5 piles per cm.sup.2 to approximately 500 piles per cm.sup.2; and the number of filaments per pile ranges from approximately 10 to approximately 10,000 and the number of segments as part of the split ranges from 1 to 64.

17. The floor mat of claim 14, wherein the layer of support material is composed of polyethylene terephthalate, polypropylene, or polyamide.

18. A floor mat comprising: a layer of carpet having a front side and a backside; a layer of loop pile microfiber material operatively attached to the backside of the carpet, the microfiber material includes a plurality of piles and each of the plurality of piles includes at least one filament having at least one split microfiber with each split microfiber comprising segments as part of the split, wherein the number of filaments per pile ranges from approximately 10 to approximately 10,000 and the number of segments as part of each split ranges from 1 to 64.

19. The floor mat of claim 18, wherein the split microfibers include a plurality of generally protruding edges extending from the split microfiber surface.

20. The floor mat of claim 18, wherein the layer of loop pile microfiber material includes: an area:, a plurality of piles with each pile including a height, wherein the height of each pile ranges from approximately 1 millimeter to 12 millimeters; and a pile density measured in the number of piles per square centimeter of area of the layer of loop pile microfiber material, wherein the pile density ranges from approximately 5 piles per cm.sup.2 to 500 piles per cm.sup.2.