fig1 and 2 show two alternative embodiments of the surface of sheet material using the inventions claimed herein . the sheet 10 of fig1 has surface depressions 14 that are elongated oval - like in shape and having length of up to about 0 . 135 inches . between the oval - shaped depressions are raised areas 16 . the oval - like depressions are non - uniform in size and have the appearance of being be randomly disposed , varying in length from very small ( as small as 0 . 025 inches or even smaller ) up to about 0 . 135 . the larger depressions have a width of about 0 . 25 inches . this yields an aspect ratio ( i . e ., a ratio of length to width ) of about 5 . 4 to 1 . in the fig1 embodiment , the differences in elevation between the bottom of the depressions 14 and the maximum height of the raised areas 16 is about 2 mils in this particular example , wherein the gauge of the film without the foaming agent is 0 . 6 mils . the increase in apparent thickness of the layer is due to the inclusion of a foaming agent . however , the height difference will depend on several factors including the speed of extrusion , the amount of foaming agent used , and the type of material extruded . fig2 shows an embodiment wherein the depressions 24 are substantially more elongated than the depressions 14 in the embodiment of fig1 . as in fig1 , the embodiment of fig2 has raised areas 26 that surround the depressions 24 . the difference in the shape of the depressions of the fig1 embodiment as compared to the fig2 embodiment is the result of a different foaming agent , faster speed of extrusion and / or the amount of the foaming agent used . in both examples , the extrusion die openings for the three layers were the same , and the type of material used to form the middle and bottom layers were the same . the two different embodiments shown in fig1 and 2 were made using two different grades of foaming agent . the material of fig1 was made using foaming agent cfac - 5050 mt supplied by polyfil corporation , while the material of fig2 was made using foaming agent anc - 0540 - 445 also supplied by polyfil corporation . in both cases , the same amount of foaming agent was used , i . e ., up to about 2 % by weight of the resin / agent mixture . the materials of fig1 and 2 were extruded using a blown film process at an extrusion speed of about 90 feet per minute . thus , the only significant difference between process used to make the material of fig1 , as compared with fig2 , was the particular foaming agent used . this difference resulted in a product having a somewhat different color in that the material of fig2 is somewhat darker that the fig2 material . in addition , and significant for purposes of its use as a surgical drape , the fig2 material is much more easily torn in the direction of the extrusion than the material of fig1 . the material of fig2 can be easily torn by hand , without the use of any tools and without the use of excessive force . the tear - ability of the material can be varied by altering the extrusion speed , although the range of speed may be limited to between 80 and 100 feet per minute . if the extrusion is performed too fast , sufficient foaming action will not occur and if it is performed too slow , the bubbles will pop . however the particular speed required will depend upon the results desired and upon the specific foaming agent and other materials used . the depressions and adjacent raised portions are the result of the inclusion of a foaming agent in the feedstock of the plastic that is used in the process of extruding the sheet , and the depressions are the result of the stretching and bursting of bubbles formed by the foaming agent as the sheet leaves the extrusion die . the coefficient of friction ( cof ) provided by the use of the previously mentioned foaming agent gives results in terms of frictional properties that compare well to standard nonwoven drapes , as shown in the following table as measured by astm d - 1894 ( again , the particular results achieved will vary depending upon the particular materials and extrusion speed used to make the material ): the depressions and adjacent raised portions also contribute to another beneficial property of the claimed invention , namely its hydrophilic property . while the particular materials used in the preferred embodiments do not necessarily absorb moisture , the surface texture of the material is believed to disturb the interfacial forces of liquid droplets which contact the surface of the material , thereby breaking the droplets up into smaller droplets which tend to wet out on the material &# 39 ; s surface . moreover , the smaller liquid droplets tend to “ stick ” within the depressions on surface of the material , such that flow of the smaller liquid droplets is inhibited . for example , the two preferred embodiments disclosed herein can retain water droplets with a diameter on the order of 1 / 16 ″- ⅛ ″, even when oriented in a vertical position . such a property is desirable for surgical covering material to prevent moisture from contacting floor surfaces , thereby rendering the floor surfaces slippery , during surgery . the surface depressions and adjacent raised portions also contribute to the non - glare characteristic of the claimed inventions . the surface roughness of the material tends to disperse light to reduce luminance and glare , thereby mitigating eye strain and fatigue during surgery . the textured surface of the claimed invention can be characterized as satin - like . the fig1 embodiment was made using a blown film extrusion line with a foaming agent manufactured by polyfil , i . e ., its cfac - 5050 mt chemical foam concentrate , added to a low linear density polyethylene resin as the outer textured layer . the fig2 embodiment was made the same way , but with a different foaming agent also manufactured by polyfil , i . e ., its anc - 0540 - 445 . fig3 and 4 are perspective views of the fig1 and 2 embodiments , respectively , and they show the texture of the surfaces of the embodiments formed by the raised areas 16 surrounding the depressions 14 in fig3 and the raised areas 26 surrounding the depressions 24 in fig4 . fig5 is an enlarged cross - sectional or end view of the fig1 ( and fig3 ) embodiment showing the two layers 30 and 40 that underlie the textured surface layer 12 , 22 coextruded with that layer . the middle , non - exposed layer 30 and the bottom layer exposed layer 40 are made of a butane linear low density polyethylene . in both the examples of fig1 and 2 , the thickness of the layers is approximately as follows : the outer exposed layer is extruded at about 0 . 6 mils but with the foaming agent increased about three - fold to about 2 mils , the middle layer of butane is about 0 . 5 to 0 . 75 mils , and the inner exposed layer is also about 0 . 5 to 0 . 75 mils . the average combined thickness of the middle and inner layer is about 1 . 5 mils , which when combined with the outer textured layer ( after expansion by foaming ) yields an overall thickness of about 3 . 5 mils , as measured from the height of the raised sections adjacent to the surface depressions . the thicknesses and the exact types of plastics used to form the various layers can vary significantly . for example , ldpe , lldpe , pp , eva and ema are all materials that foam well and could be used to make materials with appropriate coefficients of friction and appropriate tear resistance , depending upon the particular requirements specified by a customer or other designer . the embodiment of fig2 is similar to the embodiment of fig1 with regard to the number and thicknesses of the layers . in the embodiment of fig2 , however , the depressions are more elongated and narrower than in fig1 . for example the surface depression 24 in 2 has a length of about 0 . 3 inches , and a width of about 0 . 025 inches , yielding an aspect ratio of about 12 to 1 . the surface layer of the fig2 embodiment is comprised of raised striations of plastic that are longer and straighter than the raised areas of the fig1 embodiment . as a result , the fig2 embodiment is more easily torn . this tearability , however , may be useful in some instances where the surgical team wants to remove tubing or other components from a patient . in such instances , a cover that can be readily torn by hand may be afford an advantage over tougher covers , such as those containing a nonwoven surface layer . indeed , for certain specific kinds of surgeries , a combination of a tougher cover , such as the fig1 embodiment , and a section of a more easily torn cover , such as that of the fig2 embodiment , may be ideal . for example , because the materials of both fig1 and 2 are all plastic ( without any nonwoven material ), one or more windows may be cut out from a sheet made of the tougher fig1 material , and a section of material made in accordance with fig2 can by attached by heat sealing over ( or under ) the window cut from the tougher material . fig6 shows a patient drape 50 covering most of a patient 52 . windows 32 and 34 have been cut from the drape 50 , and patches 33 and 35 , respectively , of a more easily torn material , such as that of the fig2 embodiment discussed above , are place over and heat sealed to the outside surface of the drape . alternatively , the patches could be heat sealed , or sealingly attached by other methods , to the inside surface of the drape 50 . the present invention has been described by reference to examples , which are presently considered to be the most practical and best embodiments . however , it is important to understand that the invention is not limited to the disclosed description or to the particular examples shown and described , but on the contrary , is intended to cover various modifications , improvements and equivalent arrangements that fall within the spirit and scope of the appended claims .
a surgical covering material formed by extruding a sheet , either by a blown film or a cast film process . the upper exposed layer of the material is made by using a foaming agent to create elongated depressions that are surrounded by raised areas . the alternating raised and depressed areas provide the material with a coefficient of friction that compares well to standard drape materials that include a non - woven layer . the raised and depressed areas also provide the material with hydrophilic and non - glare properties that make is a highly effective drape or covering material for use in making mayo stand covers , back table covers and patient drapes . the size and shape of the depressions may be varied to create materials of different toughness or tearability , and differing materials may be combined to form customized drapes with the ability to gain access or make pathways through the drape .