Films with reduced particle formation

Abstract

Disclosed herein is a membrane having a first surface, a second surface opposite the first surface, a skin on the first surface having pores visible when viewed at 10,000 magnification, and a pore size gradient, where the pore size increases from the second surface to the skin.

Description

[Technical Field]

[0001] SUMMARY OF THE DISCLOSURE The present disclosure generally relates to membranes that exhibit reduced particle formation on the surface facing the substrate during formation. [Background technology]

[0002] The semiconductor industry relies on wet etching and cleaning processes to manufacture wafers. The liquids used in wet etching and cleaning processes are filtered to remove micro-contaminants from the liquid. In some embodiments, these wet etching and cleaning applications require filters with membranes capable of transporting filtered media at a minimum flow rate of 10 liters / minute. Such high flow rates require a minimum flow rate of 2,000 LMH / bar (liters / meter). 2 A minimum flux in the range of (bar / hour) / hour) is required. Suitable membranes that meet the flow rate and flux requirements include dip-cast polymer membranes, such as polysulfone-type membranes. However, the dip-casting process can cause particles or beads to form on the open side of the membrane. Particles are not necessarily removed during membrane / filter cleaning, and particles can shed during use of a filter incorporating the membrane, thereby reducing the effectiveness of the filter. A need exists for membranes that have reduced particle formation and, therefore, potentially reduced particle shedding. Summary of the Invention

[0003] In a first embodiment, the membrane comprises a first surface; a second surface opposite the first surface; a skin on the first surface having pores visible when viewed at 10,000x magnification; and a pore size gradient, where the pore size increases from the second surface to the skin.

[0004] Second embodiment according to the first embodiment: The membrane is selected from the group consisting of polysulfone, polyethersulfone, polyphenylsulfone, polyarylsulfone, polyimide, polyamideimide and polyvinylidene fluoride.

[0005] A third embodiment according to any of the previous embodiments: the membrane has a skin on the second surface that is free of visible pores when viewed at 10,000 magnification.

[0006] A fourth aspect according to any of the preceding aspects: the membrane has an average bubble point in the range of about 40 psi to about 75 psi, as measured according to test method B of ASTM F316-03(2011), using ethoxynonafluorobutane (HFE-7200) as the wetting fluid and flowing the wetting fluid from the first surface to the second surface.

[0007]

[0023] A fifth aspect according to any of the preceding aspects: the membrane has an average bubble point in the range of about 75 psi to about 150 psi, as measured according to Test Method B of ASTM F316-03(2011), using ethoxynonafluorobutane (HFE-7200) as the wetting fluid and flowing the wetting fluid from the second surface to the first surface.

[0008] A sixth embodiment according to any of the previous embodiments: the membrane has a thickness in the range of about 40 microns to about 150 microns.

[0009] A seventh embodiment according to any of the previous embodiments: the skin thickness of the first surface ranges from greater than 0 to about 2 microns.

[0010] An eighth embodiment according to any of the previous embodiments: the skin of the first surface has a porosity of about 15% or less.

[0011] A ninth embodiment according to any of the previous embodiments: the second surface has a porosity in the range of about 10% to about 60%.

[0012] A tenth embodiment according to any of the previous embodiments: the second surface has a greater porosity than the skin of the first surface.

[0013] In an eleventh embodiment, the filter comprises the membrane of any of the previous embodiments.

[0014] A twelfth aspect according to the eleventh aspect: the filter, when subjected to a particle shedding test, the membrane sheds less than 300 particles over a 60 minute period.

[0015] A thirteenth aspect according to the eleventh aspect: the filter, when subjected to a particle shedding test, the membrane sheds less than 200 particles over a 60 minute period.

[0016] A fourteenth aspect according to the eleventh aspect: When the filter is subjected to a particle shedding test, the membrane sheds less than 100 particles over a 60 minute period.

[0017] In a fifteenth embodiment, a method of forming a membrane comprises casting a polymer solution on a hydrophilic support to form a membrane, wherein the membrane comprises a first surface; a second surface in contact with the hydrophilic support and opposite the first surface; a skin on the first surface having pores visible when viewed at 10,000 magnification; and a pore size gradient, wherein the pore size increases from the second surface to the skin.

[0018] Sixteenth embodiment according to the fifteenth embodiment: the hydrophilic support is polyester.

[0019] Seventeenth embodiment according to the sixteenth embodiment: the hydrophilic support is biaxially oriented polyethylene terephthalate.

[0020] An eighteenth embodiment according to any of the fifteenth to seventeenth embodiments: further comprising immersing the hydrophilic support in the polymer solution in a water bath.

[0021] A nineteenth embodiment according to the eighteenth embodiment: the water bath has a temperature in the range of about 0°C to about 40°C.

[0022] A twentieth embodiment according to any of the fifteenth to nineteenth embodiments: the membrane has a polymer content in the range of about 10% to about 30% by weight.

[0023] Twenty-first embodiment according to the twentieth embodiment: the membrane has a polymer content in the range of about 10% to about 15% by weight.

[0024] A twenty-second embodiment according to the twentieth embodiment: the membrane has a polymer content ranging from about 15% to about 30% by weight.

[0025] A twenty-third embodiment according to any of the fifteenth to twenty-second embodiments: the polymer solution comprises a polymer, a solvent, and a non-solvent.

[0026] The present disclosure may be more fully understood in consideration of the following description of various illustrative embodiments in connection with the accompanying drawings. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is an exemplary cross-sectional view of a membrane disclosed herein obtained using a SEM (scanning electron microscope) at 2,500 magnification. [Figure 2] 1 is a photograph of the open surface of the membrane taken using an SEM at 5,000 magnification. [Figure 3A] 1 is a photograph of the open-side surface of an exemplary membrane in Example 2, including the skin, taken using an SEM at 10,000 magnification. [Figure 3B] 1 is a cross-sectional photograph of the open side of an exemplary membrane in Example 2 showing the skin taken at 10,000 magnification using an SEM. [Figure 3C] 1 is a photograph of the dense side surface of an exemplary membrane in Example 2 taken using an SEM at 10,000 magnification. [Figure 3D] 1 is a cross-sectional photograph of the dense side of an exemplary membrane in Example 2 taken using an SEM at 10,000 magnification. [Figure 4A] 1 is a photograph of the open-side surface of an exemplary membrane in Example 3, including the skin, taken using an SEM at 10,000 magnification. [Figure 4B] 1 is a cross-sectional photograph of the open side of an exemplary membrane in Example 3 showing the skin taken at 10,000 magnification using an SEM. [Figure 4C] 1 is a photograph of the dense side surface of an exemplary membrane in Example 3 taken using an SEM at 10,000 magnification. [Figure 4D]1 is a cross-sectional photograph of the dense side of an exemplary membrane in Example 3 taken using an SEM at 10,000 magnification. [Figure 5] 1 is a plot showing the number of particles shed on the y-axis and time (minutes) on the x-axis for the membranes tested in Example 4. [Figure 6A] 1 is a photograph of the open-side surface of the skinned membrane of Example 5 taken at 5,000 magnification using an SEM. [Figure 6B] 1 is a photograph of the open-side surface of the skinned membrane of Example 2 taken at 5,000 magnification using an SEM. DETAILED DESCRIPTION OF THE INVENTION

[0028] While the present disclosure is susceptible to various modifications and alternative forms, features thereof have been shown by way of example in the drawings and will be described in detail. It is to be understood, however, that there is no intention to limit aspects of the disclosure to the particular illustrative embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure.

[0029] As used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. As used in this specification and the appended claims, the term "or" is generally used in its sense including "and / or" unless the context clearly dictates otherwise.

[0030] The term "about" generally refers to a range of numbers that are considered equivalent to a stated value (e.g., having the same function or result). In many instances, the term "about" may include numbers that are rounded to the nearest significant figure.

[0031] Numerical ranges expressed using endpoints include all numbers subsumed within that range (eg, 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5).

[0032] The following detailed description should be read with reference to the drawings, in which like elements are numbered the same in different drawings. The detailed description and drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the invention. The depicted illustrative embodiments are intended as examples only. Selected features of any illustrative embodiment may be incorporated into additional embodiments, unless expressly stated to the contrary.

[0033] Disclosed herein is a membrane having a first surface and a second surface opposite the first surface. The membrane also has a pore size gradient across its cross section, with the pore size in the membrane increasing from the second surface to the skin formed at the first surface. The second surface is referred to herein as the "dense side." The first surface, which has the skin covering the larger pores, is referred to herein as the "open side." The skin on the open side, also referred to herein as the "incomplete skin," is a portion of the membrane that has relatively fewer pores than adjacent portions, but still has some pores visible under a scanning electron microscope (SEM) at 10,000 magnification. The presence of the skin on the open side of the membrane is believed to reduce particle formation on the open side of the membrane, potentially resulting in less particle shedding from the membrane during use.

[0034] FIG. 1 shows a cross-sectional view of an exemplary membrane 100 having a first surface 102 and a second surface 104 opposite the first surface 102. A skin 106 is formed on the first surface 102. There are pores throughout the thickness of the membrane 100. In some embodiments, there is a pore size gradient across the membrane cross-section, with the pores increasing in size from the second surface 104 to the skin 106, as indicated by the arrows in FIG. 1. Membranes with a pore size gradient are also referred to as asymmetric. The first surface 102 is the open side, and the second surface 104 is the dense side. The skin 106 is an imperfect skin in that it has pores visible at 10,000 magnification using an SEM. In some embodiments, a skin may be formed on the second surface that is a "perfect" skin, meaning that there are no pores visible at 10,000 magnification using an SEM. In some embodiments, the second surface can have a skin with pores that are less than 1 micron in size when viewed at 10,000 magnification using an SEM.

[0035] In some embodiments, membrane 100 is a polymer. In some embodiments, polymers used in the membrane include, but are not limited to, polysulfone, polyethersulfone, polyphenylsulfone, polyarylsulfone, polyimide, polyamideimide, and polyvinylidene fluoride. In some embodiments, membrane 100 has a fluoride content of about 10% to about 30% by weight, about 10% to about 27% by weight, about 10% to about 25% by weight, about 10% to about 20% by weight, about 10% to about 15% by weight, about 12% to about 30% by weight, about 12% to about 27% by weight, about 12% to about 25% by weight, about 12% to about 20% by weight, about 12% to about 15% by weight, about 15% to about 20 ... The membranes may be prepared from solutions having polymer contents ranging from about 15% to about 30% by weight, about 15% to about 27% by weight, about 15% to about 25% by weight, about 15% to about 20% by weight, about 20% to about 30% by weight, about 20% to about 27% by weight, about 20% to about 25% by weight, about 25% to about 30% by weight, or about 25% to about 27% by weight, including all ranges and subranges therein. As discussed in Example 4 below and with reference to Figure 5, increasing the amount of polymer in the membrane is believed to reduce the amount of particle formation on the open side of the membrane.

[0036] In some embodiments, the open side skin 106 has a thickness in the range of greater than 0 to about 2 microns, greater than 0 to about 1.5 microns, greater than 0 to about 1 micron, about 0.5 microns to about 2 microns, about 0.5 microns to about 1.5 microns, about 1 micron to about 2 microns, about 1 micron to about 1.5 microns, and all ranges and subranges therein. In some embodiments, the membrane 100 has a thickness in the range of about 40 microns to about 150 microns, about 40 microns to about 125 microns, about 40 microns to about 100 microns, about 60 microns to about 150 microns, about 60 microns to about 125 microns, about 60 microns to about 100 microns, about 75 microns to about 150 microns, about 75 microns to about 125 microns, or about 75 microns to about 100 microns.

[0037] In some embodiments, the open-side skin 106 has a porosity of about 15% or less, about 10% or less, or about 5% or less, as assessed by viewing the surface of the open-side skin with an SEM under 10,000x magnification. In some embodiments, the second surface 104 has a porosity of about 10% to about 60%, about 10% to about 50%, about 10% to about 40%, about 15% to about 60%, about 15% to about 50%, about 15% to about 40%, about 20% to about 60%, about 20% to about 50%, or about 20% to about 40%, as assessed by viewing the surface of the second surface 104 with an SEM under 10,000x magnification. In some embodiments, the second surface 104 has a greater porosity than the open-side skin 106, as assessed by viewing the surface with an SEM under 10,000x magnification.

[0038] The mean bubble point (also referred to as mean flow pore pressure) of a membrane may be measured according to ASTM F316-03(2011), entitled "Standard Test Method for Pore Size Characterization of Membrane Filters by Bubble Point and Mean Flow Pore Test," using Test Method B, modified to use ethoxynonafluorobutane (HFE-7200) available from 3M as the wetting fluid. In some embodiments, the membrane has a mean bubble point in the range of about 40 psi to about 75 psi, measured with the wetting fluid flowing from the open side to the dense side. In some embodiments, the membrane has a mean bubble point in the range of about 75 psi to about 150 psi, measured with the wetting fluid flowing from the dense side to the open side.

[0039] In some embodiments, the membranes disclosed herein are manufactured by a dip casting process. The process involves forming a solution containing a polymer, one or more solvents, and one or more non-solvents. As noted above, polymers for membranes include, but are not limited to, polysulfone, polyethersulfone, polyphenylsulfone, polyarylsulfone, polyimide, and polyamideimide.

[0040] In some embodiments, the solution comprises about 10% to about 30% by weight, about 10% to about 27% by weight, about 10% to about 25% by weight, about 10% to about 20% by weight, about 10% to about 15% by weight, about 12% to about 30% by weight, about 12% to about 27% by weight, about 12% to about 25% by weight, about 12% to about 20% by weight, about 12% to about 15% by weight, about 1 The polymer content ranges from 5% to about 30% by weight, from about 15% to about 27% by weight, from about 15% to about 25% by weight, from about 15% to about 20% by weight, from about 20% to about 30% by weight, from about 20% to about 27% by weight, from about 20% to about 25% by weight, from about 25% to about 30% by weight, or from about 25% to about 27% by weight, and all ranges and subranges therein.

[0041] In some embodiments, the solution comprises about 20% to about 90% by weight, about 20% to about 80% by weight, about 20% to about 70% by weight, about 20% to about 60% by weight, about 20% to about 50% by weight, about 20% to about 40% by weight, about 30% to about 90% by weight, about 30% to about 80% by weight, about 30% to about 70% by weight, about 30% to about 60% by weight, about 30% to about 50% by weight, about The solvent content ranges from 40% to about 90% by weight, from about 40% to about 80% by weight, from about 40% to about 70% by weight, from about 40% to about 60% by weight, from about 50% to about 90% by weight, from about 50% to about 80% by weight, from about 50% to about 70% by weight, from about 60% to about 90% by weight, from about 60% to about 80% by weight, from about 70% to about 90% by weight, and all ranges and subranges therein. Suitable solvents include, but are not limited to, dimethylformamide, dimethylacetamide, dioxane, n-methylpyrrolidone, dimethylsulfoxide, chloroform, tetramethylurea, tetrachloroethane, and mixtures thereof.

[0042] In some embodiments, the solution may comprise 0% to about 70% by weight, about 0% to about 60% by weight, 0% to about 50% by weight, 0% to about 40% by weight, 0% to about 30% by weight, 0% to about 20% by weight, about 10% to about 70% by weight, about 10% to about 60% by weight, about 10% to about 50% by weight, about 10% to about 40% by weight, about 10% to about 30% by weight, The non-solvent content ranges from about 20% to about 70% by weight, from about 20% to about 60% by weight, from about 20% to about 50% by weight, from about 20% to about 40% by weight, from about 20% to about 30% by weight, from about 40% to about 70% by weight, from about 40% to about 60% by weight, from about 50% to about 70% by weight, from about 60% to about 70% by weight, and all ranges and subranges therein. Suitable non-solvents include, but are not limited to, alcohols (e.g., methanol, ethanol, isopropanol, amyl alcohol, hexanol, heptanol, octanol, ethylene glycol, or triethylene glycol), alkanes (e.g., propane, hexane, heptane, or octane), ketones (e.g., acetone, methyl ethyl ketone, or methyl isobutyl ketone), nitropropane, ethers (e.g., butyl ether, propylene glycol monomethyl ether (PGME), or tripropylene glycol methyl ether (TPM)), ethyl acetate, amyl acetate, water, acids (e.g., propionic acid), or bases, and mixtures thereof.

[0043] The process also includes casting the solution onto a moving belt or rotating drum covered with a hydrophilic support film and immersing the casting solution in a water bath to form a membrane. In some embodiments, the hydrophilic film is a polyester film, e.g., a biaxially oriented polyethylene terephthalate film such as Mylar®. In some embodiments, the hydrophilic film may be a polyester film, e.g., a biaxially oriented polyethylene terephthalate film having a hydrophilic coating on one or both sides, such as Melinex® 462. In some embodiments, the water bath is at a temperature between about 0°C and about 40°C, between about 0°C and about 35°C, between about 0°C and about 30°C, between about 0°C and about 25°C, between about 0°C and about 20°C, between about 0°C and about 15°C, between about 0°C and about 10°C, between about 5°C and about 40°C, between about 5°C and about 35°C, between about 5°C and about 30°C, between about 5°C and about 25°C, between about 5°C and about 20°C, between about 5°C and about 15°C, between about 5°C and about 10°C, between about 10°C and about 40°C, between about 10°C and about 35°C, between about 10°C and about 30°C, or between about 10°C. The temperature of the water bath is maintained at a temperature ranging from about 25°C to about 25°C, about 10°C to about 20°C, about 10°C to about 15°C, about 15°C to about 40°C, about 15°C to about 35°C, about 15°C to about 30°C, about 15°C to about 25°C, about 15°C to about 20°C, about 20°C to about 40°C, about 20°C to about 35°C, about 20°C to about 30°C, about 20°C to about 25°C, about 25°C to about 40°C, about 25°C to about 35°C, about 25°C to about 30°C, and any range or subrange therein. As discussed in Example 5 below, lowering the temperature of the water bath is believed to reduce the pore size in the skin and decrease the amount of particle formation on the open side of the membrane.

[0044] It is believed that the methods disclosed herein result in fewer particles on the open side as a result of an incomplete open side skin, thereby resulting in fewer particle shedding.

[0045] The membranes disclosed herein can have any convenient geometry, including, but not limited to, flat sheets, corrugated sheets, or hollow fibers. In some embodiments, the membranes disclosed herein are incorporated into filters by placing the membrane inside a filter housing. [Example]

[0046] Example 1 A polyethersulfone membrane was formed by preparing a solution containing 13.9 wt. % polyethersulfone, 45.5 wt. % n-methylpyrrolidone, and 40.6 wt. % propionic acid. The solution was cast onto a moving belt covered with a hydrophobic film, Mylar® A. The solution was passed through an immersion water bath having a temperature of approximately 25°C. The formed membrane was asymmetric, with a dense side facing away from the hydrophobic Mylar® A film and an open side in contact with the hydrophobic Mylar® A film. Figure 2 is a photograph of the open surface of the membrane taken at 5,000x magnification using an SEM (scanning electron microscope).

[0047] Example 2 A polyethersulfone membrane was formed using the same process as in Example 1, except that the solution was cast onto a moving belt covered with a hydrophilic Melinex® 462 film. The formed membrane was asymmetric, with a dense side facing away from the hydrophilic film and an open side in contact with the hydrophilic film. Unexpectedly, the open side of the membrane had an incomplete skin approximately 0.5 microns thick. Figure 3A is a photograph of the open-side surface of the membrane with the skin, taken at 10,000x magnification using an SEM. As can be seen, the skin has pores on the surface. Figure 3B is a cross-sectional photograph of the open side of the membrane, taken at 10,000x magnification using an SEM, showing the skin. The skin is shown at the bottom of the photograph. Figure 3C is a photograph of the dense side surface of the membrane, taken at 10,000x magnification using an SEM. Figure 3D is a cross-sectional photograph of the dense side of the membrane, taken at 10,000x magnification using an SEM.

[0048] Example 3 A polyethersulfone membrane was formed using the same process as in Example 2, except that a solution of 15.5 wt. % polyethersulfone, 44.4 wt. % n-methylpyrrolidone, and 40.1 wt. % propionic acid was cast onto a rotating drum covered with a hydrophilic Melinex® 462 film. The formed membrane was asymmetric, with a dense side facing away from the hydrophilic film and an open side in contact with the hydrophilic film. Unexpectedly, the open side of the membrane had a skin approximately 0.5 microns thick. Figure 4A shows a photograph of the open-side surface of the membrane with the skin, taken at 10,000 magnification using an SEM. As can be seen, the skin has pores on the surface. Figure 4B shows a cross-sectional photograph of the open side of the membrane showing the skin, taken at 10,000 magnification using an SEM. The skin is shown at the bottom of the photograph. Figure 4C shows a photograph of the dense side of the membrane, taken at 10,000 magnification using an SEM. FIG. 4D is a cross-sectional photograph of the dense side of the membrane taken using an SEM at 10,000 magnification.

[0049] Example 4 The membranes of Examples 1-3 were subjected to a particle shedding test in which particle shedding was counted for 60 minutes using a KS-18FX (40 nm) Rion dust meter. Figure 5 shows the results, with time (minutes) on the x-axis and the number of particles shed on the y-axis. The data show that particle shedding is reduced when a hydrophilic film is used instead of a hydrophobic film. The data also show that the membrane of Example 3, which has 15.5 wt% polymer, sheds fewer particles than the membrane of Example 2, which has 13.9 wt% polymer, indicating that increasing the polymer concentration in the membrane appears to result in reduced particle shedding.

[0050] The particle shedding test procedure involves placing a membrane in a filter. Ammonium hydroxide (NH4OH) was passed through the guard filter, then through the test filter, and then through a KS-18FX (40 nm) Rion dust meter. Ammonium hydroxide was then passed through the dust meter until there were no visible bubbles in the sample line, and the flow through the dust meter dropped to a range of 10-20 cc / min. The flow rate through the test filter was approximately 3 liters per minute. The dust meter counted particles shed from the membrane in the test filter for 60 minutes. Thus, in some embodiments, a filter containing a membrane disclosed herein was subjected to a particle shedding test, and the membrane may shed less than 300 particles in 60 minutes, less than 200 particles in 60 minutes, or less than 100 particles.

[0051] Example 5 Polyethersulfone membranes were formed using the same procedure outlined in Example 2, except that the water bath had a temperature of 17° C. Figure 6A is a photograph of the open-side surface of a skinned membrane taken with an SEM (scanning electron microscope) at 5,000 magnification. In comparison, Figure 6B is a photograph of the open-side surface of the skinned membrane of Example 2 taken with an SEM at 5,000 magnification. As can be seen by comparing the photographs, the membrane produced using the cooler water bath had smaller pores in the open-side skin.

[0052] Having thus described several illustrative embodiments of the present disclosure, those skilled in the art will readily recognize that still other embodiments are possible and can be used within the scope of the claims appended hereto. Numerous advantages of the present disclosure covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, illustrative only. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts, without exceeding the scope of the present disclosure. The scope of the present disclosure, of course, is defined in the language in which the appended claims are expressed.

Claims

1. A filter including a membrane, The membrane comprises: a first surface; a second surface opposite the first surface; a skin having holes on a first surface when viewed at a magnification of 10,000; and a pore size gradient, wherein the pore size increases from the second surface to the skin; a second surface having a skin that is free of pores when viewed at 10,000 magnification; having an average bubble point in the range of 40 psi to 75 psi when measured according to ASTM F316-03(2011), Test Method B, using ethoxynonafluorobutane (HFE-7200) as the wetting fluid and flowing the wetting fluid from the first surface to the second surface; When the filter is subjected to a particle shedding test, the membrane sheds less than 300 particles in 60 minutes. filter.

2. The filter described in claim 1, wherein the membrane is selected from the group consisting of polysulfone, polyethersulfone, polyphenylsulfone, polyarylsulfone, polyimide, polyamideimide and polyvinylidene fluoride.

3. 3. The filter of claim 1 or 2, wherein the first surface skin has a porosity of 15% or less.

4. A filter according to any one of claims 1 to 3, wherein the second surface has a porosity in the range of 10% to 60%.

5. A method for forming a membrane included in a filter according to any one of claims 1 to 4, comprising: Casting a polymer solution onto a hydrophilic support to form a membrane A method comprising:

6. The method of claim 5 wherein the hydrophilic support is a polyester.

7. The method according to claim 5 or 6, wherein the polymer solution has a polymer content in the range of 10% to 30% by weight.

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