Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
2001-04-10
2004-03-23
Padgett, Marianne (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S539000, C427S491000, C427S536000, C427S358000
Reexamination Certificate
active
06709718
ABSTRACT:
BACKGROUND
A method is provided for surface treating a porous sheet material. The surface treatment involves contacting at least one porous surface of the film with plasma at atmospheric pressure and a plasma generating electrode frequency of greater than 1 MHz. This method provides treatment which penetrates into the pores of the sheet material.
Flame, plasma or corona discharge treatments have been used to improve the printability or metal adhesion properties of film surfaces of polymers, such as polypropylene. These treatments are believed to generate oxygen containing functional groups, such as —OH or —COOH, on exposed surfaces of the film. Plasma treatment in the presence of a hydroxyl-donating material, such as methanol, is described in U.S. Pat. No. 5,981,079.
Flame or corona discharge treatments tend to involve subjecting the treated film surfaces to high temperatures. These high temperatures may tend to melt or distort polymers.
Exposing a porous polymeric film to high temperatures can be particularly problematic. For example, U.S. Pat. No. 5,650,451 states that treatment of a porous biaxially oriented high molecular weight film at a temperature of 132 to 145° C. for one second to ten minutes can result in a loss of specific surface area of 20 m
2
/g or more.
SUMMARY
There is provided a method for treating a porous sheet material to render the pore space thereof more hydrophilic, said method comprising contacting said sheet material with plasma under sufficient treating conditions, wherein said treating conditions comprise atmospheric pressure and plasma generating electrode frequency of greater than 1 MHz.
DETAILED DESCRIPTION
The porous sheet material may have an open cell foam structure, a non-woven fiber structure or a woven fiber structure. The open cell foam structure may be described as having a continuous matrix material with a series of interconnected cavities or voids extending from at least one surface of the sheet into a substantial depth, e.g., at least 50 percent, of the sheet. The continuous matrix material may be a thermoplastic material, such as a polyolefin.
The porous sheet material may be a fibrous web substrate, either woven of nonwoven. The fibers may be made of a thermoplastic material, such as a polyolefin, e.g., polyethylene or polypropylene. A particular type of fibrous web material is spunbond polypropylene. Treatment of examples of fibrous web material with various types of plasma is discussed in U.S. Pat. No. 6,106,659.
The porous sheet material may be an extruded film comprising a surface layer of polyolefin and particles of an incompatible material, wherein said layer has a meshed network of polyolefin fibers and striations of layers coplanar with the plane of the film, wherein said layer is porous in a direction perpendicular to the plane of the film and wherein said layer has a void content of at least 20%. Particular examples of such porous sheet materials are described in U.S. Pat. No. 4,861,644.
In U.S. Pat. No. 4,861,644, the microporous material substrate comprises (1) a matrix consisting essentially of linear ultrahigh molecular weight polyolefin, (2) a large proportion of finely divided water-insoluble siliceous filler, and (3) interconnecting pores.
As pointed out in U.S. Pat. No. 4,861,644, inasmuch as ultrahigh molecular weight (UHMW) polyolefin is not a thermoset polymer having an infinite molecular weight, it is technically classified as a thermoplastic. However, because the molecules are essentially very long chains, UHMW polyolefin, and especially UHMW polyethylene, softens when heated but does not flow as a molten liquid in a normal thermoplastic manner. In U.S. Pat. No. 4,861,644, it is stated that the very long chains and the peculiar properties they provide to UHMW polyolefin are believed to contribute in large measure to the desirable properties of the microporous material substrate.
In view of the flow characteristics of UHMW polyethylene, it is difficult to process into the form of a film. As described in U.S. Pat. No. 4,861,644, a processing plasticizer is blended with UHMW polyethylene and precipitated silica to improve film forming characteristics. Examples of such processing plasticizers include processing oil such as paraffinic oil, naphthenic oil, or aromatic oil. After the film is formed the processing plasticizer is removed by an extraction step.
A particular process for forming the film of U.S. Pat. No. 4,861,644 involves mixing filler, thermoplastic organic polymer powder, processing plasticizer and minor amounts of lubricant and antioxidant until a substantially uniform mixture is obtained. The weight ratio of filler to polymer powder employed in forming the mixture is essentially the same as that of the microporous material substrate to be produced. The mixture, together with additional processing plasticizer, is introduced to the heated barrel of a screw extruder. Attached to the extruder is a sheeting die. A continuous sheet formed by the die is forwarded without drawing to a pair of heated calender rolls acting cooperatively to form continuous sheet of lesser thickness than the continuous sheet exiting from the die. The continuous sheet from the calender then passes to a first extraction zone where the processing plasticizer is substantially removed by extraction with an organic liquid which is a good solvent for the processing plasticizer, a poor solvent for the organic polymer, and more volatile than the processing plasticizer. Usually, but not necessarily, both the processing plasticizer and the organic extraction liquid are substantially immiscible with water. The continuous sheet then passes to a second extraction zone where the residual organic extraction liquid is substantially removed by steam and/or water. The continuous sheet is then passed through a forced air dryer for substantial removal of residual water and remaining residual organic extraction liquid. From the dryer the continuous sheet, which is microporous material substrate, is passed to a take-up roll.
After the processing plasticizer is extracted from the film, biaxial stretching may, optionally, take place.
A microporous substrate described in U.S. Pat. No. 4,861,644, or a substrate similar thereto, is believed to be commercially available from PPG Industries, Inc., under the tradename Teslin.
Methods for making films with a surface layer with an open cell pore structure are described in U.S. application Ser. No. 09/079,807, filed May 15, 1998. According to this method a cavitating agent is used with a particular polymeric matrix material, which is high density polyethylene (HDPE) or medium density polyethylene (MDPE). When this material is stretched, separations which form voids are formed not only horizontally, i.e. within or parallel to the plane of the film, but also in the vertical dimension or perpendicular to the plane of the film.
As the term high density polyethylene (HDPE) is used herein, it is defined to mean an ethylene-containing polymer having a density of 0.940 or higher. (Density (d) is expressed as g/cm
3
.) It is noted that the tensile strength of HDPE increases when the density of HDPE increases. One particularly suitable HDPE is the resin sold as M6211 by Equistar. Another particularly suitable HDPE is the resin sold as HDZ128 by Exxon. Other HDPE resins include, for example, BDM 94-25 available from Fina Oil and Chemical Co. Dallas, Tex., and 19C and 19F available from Nova Corporation, Sarnia, Ontario, Canada.
The term “medium density polyethylene” (MDPE) as used herein is defined to mean an ethylene-containing polymer having a density of from about 0.926 to about 0.940. MDPE is readily available, e.g., Dowlex™2038 or Dowlex™2027A from The Dow Chemical Company.
An incompatible material, also referred to herein as a cavitating agent, is blended with HDPE or MDPE to provide a voided layer. Such agents may be added to the HDPE or MDPE prior to extrusion and are capable of generating voids (cavities) in the structure of the film during the film-making process. It is believed that small inhomogeneities introduc
ExxonMobil Oil Corporation
James Rick F.
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