JP4172195B2 - Photosensitive film for multilayer optical recording media - Google Patents

Description

【００５０】
【表２】

表２に示したように、実施例１、２の感光性フィルムにおいて、感光層の光透過率は９８％と高い値を示した。また、膜厚が１００μｍのシートを用いて同様な工程で測定した場合も感光層の光透過率は９６％から９８％の間で安定していた。これらの感光性フィルムを用いて作製した光ディスクにおいては十分な高密度化を達成することができた。さらに、実施例１、２のいずれの場合も空気巻き込み性の評価において良好な結果を示したが、感光層の厚み精度がＲａ値で±０．３０μｍの範囲内にある実施例２において空気の巻き込みがより高水準で防止されていた。
【００５１】
これに対して、比較例１の感光性フィルムにおける感光層の透過率は８６％であり、この感光性フィルムを用いて得られた光ディスクの場合は十分な高密度化を達成することができなかった。
【００５２】
【発明の効果】
以上説明した通り、本発明の多層構造光記録媒体用感光性フィルムは、優れた光学的特性を有するものであり、これにより高密度化に対応可能な多層構造光記録媒体用感光性フィルムを実現することが可能となる。
【図面の簡単な説明】
【図１】（ａ）〜（ｅ）はそれぞれ本発明の感光性フィルムを用いて多層構造光記録媒体を製造するときの各工程における基板又は積層体を示す模式断面図である。
【符号の説明】
１０１…透光性基板、１０２…第１中間反射層、１０５…スタンパ、１０６…スペーサ層、１０７…第２中間反射層。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a multilayer structure optical recording medium photosensitive film.
[0002]
[Prior art]
In recent years, with the development of information technology including computer device technology, computer software technology, communication technology, etc., it has become possible to transmit more information at high speed. In the field of video technology, digitalization has progressed rapidly, and BS digital broadcasting started in December 2000. At the HDTV level, the transfer rate is 23 Mbps, and, for example, a procedure of 20.7 GB or more is required to record one movie of 120 minutes or more. Therefore, there is an increasing demand for a technology for increasing the recording density. In response to this demand, development of a digital versatile disk (DVD) as a high-density recording medium has been further advanced.
[0003]
The read-only optical disc has features such as easy mass duplication and easy random access, and is used for recording media such as video, audio, and computer data. The recording capacity is reduced from a light source wavelength (780 nm to 650 nm), an objective lens having a high aperture (0.45 to 0.6), and various signal processing, and from a CD 688 MB (CD-ROM) to a DVD 4 However, the recording density has been required to be greatly improved before the practical use of blue semiconductor lasers in recent years.
[0004]
Corresponding to such high density, the optical disk multilayer technology in which the information recording layer or layers in which pits or grooves carrying information are formed and the spacer layer or layers overlap each other to form a multilayer Development is underway.
[0005]
[Problems to be solved by the invention]
In an optical recording medium having a multilayer structure, information on the information recording layer is read out by adjusting the focus of the laser to each information recording layer. Therefore, the spacer layer needs to be transparent at the wavelength of the laser beam. In addition, it is necessary to accurately transfer pits or grooves carrying information.
[0006]
On the other hand, when the spacer layer is formed, conventionally, a liquid ultraviolet curable resin is generally applied onto the information recording layer by spin coating and photocured. However, when applying using such a spin coating method, it is difficult to apply a uniform and uniform film thickness at the center and outer periphery of the disk, and the obtained light. The optical characteristics of the recording medium become insufficient, and problems such as a decrease in yield occur.
[0007]
The present invention has been made in order to solve the above-described problems, and provides a photosensitive film for an optical recording medium having a multilayer structure that has excellent optical characteristics and can cope with higher density. With the goal.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, the photosensitive film for spacer of the multilayer optical recording medium of the present invention has a photosensitive layer having a light transmittance of 90% or more at 400 nm after ultraviolet irradiation, and the photosensitive layer comprises: (A) a polymer binder, (b) a urethane acrylate compound or urethane methacrylate compound having at least two polymerizable unsaturated double bonds in the molecule, and (c) 1-hydroxy-cyclohexyl-phenyl-ketone . It is characterized by that.
[0009]
In the present invention, the thickness of the photosensitive layer is preferably 5 to 20 μm.
[0010]
In the present invention, the thickness accuracy of the photosensitive layer is preferably 0.30 μm or less in terms of Ra value.
[0012]
The photosensitive film of the present invention comprises (a) a polymer binder, (b) a urethane acrylate compound or urethane methacrylate compound having at least two polymerizable unsaturated double bonds in the molecule, and (c) 1- A photosensitive resin composition containing hydroxy-cyclohexyl-phenyl-ketone and having a viscosity of 1 to 3000 cps, wherein the components (a) to (c) are contained in the components (a) and (b). The component (a) is 40 to 80 parts by weight, the component (b) is 20 to 60 parts by weight, and the component (c) is 0.1 to 10 parts by weight based on 100 parts by weight of the total amount with the components. It is preferable to laminate a photosensitive resin composition on a support.
[0013]
In the present invention, the refractive index at 400 nm of the photosensitive layer is preferably 1.32 to 1.72 .
[0014]
In the present invention, the birefringence of the photosensitive layer at 400 nm is preferably -20 nm to +20 nm .
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The photosensitive film for a multilayer structure optical recording medium of the present invention comprises a photosensitive layer having a light transmittance of 90% or more at 400 nm after ultraviolet irradiation.
[0016]
In addition, the light transmittance as used in the field of this invention means the value measured after irradiating a 20-micrometer-thick photosensitive layer with the exposure amount of 1 J / cm < ² > of ultraviolet rays at 0.5 degreeC.
[0017]
In the photosensitive film for a multilayer structure optical recording medium of the present invention, the material is not limited as long as the light transmittance of the photosensitive layer satisfies the above conditions, but (a) a polymer binder, (b) at least in the molecule. It preferably contains a polymerizable compound having two polymerizable unsaturated double bonds, and (c) a photoinitiator.
[0018]
Specific examples of the component (a) include polymethyl methacrylate, polyethyl methacrylate, polyvinyl acetate, polyvinyl acetate / acrylate copolymer, vinyl acetate / methacrylate copolymer, ethylene / vinyl acetate copolymer, polystyrene, vinylidene chloride / acrylonitrile copolymer, Vinylidene chloride / methacrylate and vinylidene chloride / vinylidene acetate copolymer, polyvinyl chloride, butadiene / acrylonitrile copolymer, acrylonitrile / butadiene / styrene copolymer, methacrylate / acrylonitrile / butadiene / styrene copolymer, 2-chlorobutadiene-1,3-polymer, chlorine Rubber, styrene / butadiene / styrene copolymer, steel On / isoprene / styrene block copolymers, epoxides containing acrylate or methacrylate groups, copolyesters, polyamides, cellulose esters, cellulose ethers, polycarbonates, polyvinyl acetal, poly formaldehyde. In addition, for example, by using a vinyl monomer having an acidic polar group such as acrylic acid, methacrylic acid, p-vinylbenzoic acid or the like and a polymer binder obtained by copolymerizing with other various vinyl monomers. Also good. Preferred examples of various vinyl monomers include methyl methacrylate, butyl methacrylate, ethyl acrylate, styrene, α-methyl styrene, vinyl toluene, benzyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2- Examples thereof include hydroxyethyl acrylate, glycidyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 2,3-dibromopropyl methacrylate, 3-chloro-2-hydroxypropyl methacrylate, acrylamide, acrylonitrile, methacrylonitrile and the like. These polymer binders may be used individually by 1 type, and may be used in combination of 2 or more type.
[0019]
The weight average molecular weight of component (a) is preferably in the range of 20,000 to 200,000. If the weight average molecular weight is less than 20,000, the flexibility of the resist tends to be low and the resist is liable to be chipped. On the other hand, if it exceeds 200,000, it is difficult to obtain a uniform film and the film thickness accuracy decreases. Tend. In addition, the weight average molecular weight as used in the field of this invention means the value which converted the measurement value obtained by gel permeation chromatography into standard polystyrene.
[0020]
Examples of compounds having at least two polymerizable unsaturated double bonds in the molecule of component (b) include (i) diethylene glycol dimethacrylate, tetraethylene glycol diacrylate, hexapropylene glycol diacrylate, trimethylolpropane tri (Meth) acrylates of polyhydric alcohols such as acrylate, pentaerythritol triacrylate, 1,6-hexanediol diacrylate, dipentaerythritol pentaacrylate, trimethylolpropane trimethacrylate, (ii) 2,2-bis (4-methacrylate) Roxyethoxyphenyl) propane, 2,2-bis (4-acryloxyethoxyphenyl) propane, bisphenol A, acrylic acid or methacrylic acid adducts of epichlorohydrin type epoxy resin, etc. (Meth) acrylate having a benzene ring in a molecule such as low molecular weight unsaturated polyester such as epoxy acrylate, phthalic anhydride-neopentyl glycol-acrylic acid 1: 2: 2 molar ratio condensate, (iii) trimethylol Adduct of propane triglycidyl ether with acrylic acid or methacrylic acid, (iv) urethane acrylate compound or urethane methacrylate obtained by reaction of trimethylhexamethylene diisocyanate with acrylic acid monoester or methacrylic acid monoester of dihydric alcohol A compound etc. can be mentioned.
[0021]
As the photoinitiator of component (c), those conventionally known can be used, and examples thereof include benzophenone, p, p-dimethylaminobenzophenone, p, p-diethylaminobenzophenone, p, p-dichlorobenzophenone, and the like. Benzophenones, mixtures thereof, anthraquinones such as 2-ethylanthraquinone, t-butylanthraquinone, 2-chlorothioxanthone, benzoin isopropyl ether, benzoin ethyl ether, benzyl, 2,4,5-triarylimidazole dimer Etc.
[0022]
The combination of the components (a) to (c) is appropriately selected so that the light transmittance of the photosensitive layer satisfies the above conditions. In the photosensitive layer of the present invention, the total amount of (a) and (b) is 100 parts by weight within the range of 40 to 80 parts by weight of component (a) and 20 to 60 parts by weight of component (b). It is preferable to use 0.1 to 10 parts by weight of (c) photoinitiator with respect to 100 parts by weight. When the amount of the component (a) used is too large, the photocurability is lowered. When the amount of component (c) used is less than 0.1 part by weight, the photocurability tends to be lowered.
[0023]
Furthermore, a dye, a plasticizer, a pigment, a flame retardant, a stabilizer, and the like can be added to the photosensitive layer according to the present invention as necessary. It is also possible to use an adhesion-imparting agent.
[0024]
A photosensitive resin composition comprising (a) a polymer binder, (b) a compound having at least two polymerizable unsaturated double bonds in the molecule, and (c) a photoinitiator on a support. The viscosity at the time of lamination is preferably 1 to 3000 cps, more preferably 1 to 1500 cps, and particularly preferably 200 to 1500 cps for improving the coating appearance. If it is less than 1 cps, movement of the coating liquid tends to occur during coating, and if it exceeds 3000 cps, bubbles tend to be easily involved.
[0025]
The thickness of the photosensitive layer of the photosensitive film for multilayer optical recording media is preferably 100 μm or less, more preferably 70 μm or less, and particularly preferably 20 μm or less. When the thickness exceeds 100 μm, it is difficult to form the photosensitive layer in a uniform and uniform process, and it is difficult to maintain optical characteristics.
[0026]
The thickness accuracy of the photosensitive layer of the photosensitive film for multilayer optical recording media is preferably ± 0.30 μm in Ra value, more preferably ± 0.25 μm, and more preferably ± 0.20 μm. Particularly preferred. When the thickness accuracy exceeds ± 0.30 μm, when the photosensitive layer of the photosensitive film for a multilayer optical recording medium is laminated as the spacer layer or the protective layer between the information recording layers, air bubbles are involved and information is collected. There is a tendency not to read well.
[0027]
The refractive index at 400 nm of the photosensitive layer of the photosensitive film for multilayer optical recording media is preferably 1.59 ± 0.20, and 1.59 ± 0.10 from the viewpoint of recording / reproducing characteristics. It is particularly preferred. If this refractive index exceeds 1.59 ± 0.20, unlike the refractive index value of the substrate material, the aberration tends to shift and information reading tends to be difficult.
[0028]
The birefringence at 400 nm of the photosensitive layer of the photosensitive film for multilayer optical recording media is preferably 0 ± 20 nm, more preferably 0 ± 10 nm from the viewpoint of recording / reproduction characteristics. In general, an optical disc has a mechanism for irradiating a laser beam onto an information recording surface and reading a signal by capturing the reflection. In this case, whether the signal is 1 or 0 is determined by comparing the intensity of the reflected light in the detection unit. Therefore, if the refractive index exceeds ± 20 nm, the polarization direction of the reflected light is deviated from the polarization plane set by the optical system, and the light reaching the detection unit does not weaken, so there is a tendency that the signal cannot be accurately captured. .
[0029]
The photosensitive film for multilayer optical recording media of the present invention has excellent optical characteristics as described above. Accordingly, a multilayer structure optical recording medium comprising the photosensitive layer of the photosensitive film of the present invention as a spacer layer, and the spacer layer and an information recording layer on which bits or grooves for carrying information are laminated, is sufficiently obtained. High density can be achieved.
[0030]
1A to 1E are schematic cross-sectional views showing a substrate or a laminate in each step when a multilayer structure optical recording medium is produced using the photosensitive film of the present invention.
[0031]
In FIG. 1A, a translucent substrate 101 is made of a thermoplastic resin having a predetermined light transmittance, and an information recording surface having an uneven shape corresponding to a pit or groove is formed on one surface thereof. Has been.
[0032]
A first intermediate reflective layer (normal reflective layer) 102 as an information recording layer is formed on the information recording surface of the translucent substrate 101 (FIG. 1B). The first intermediate reflective layer 102 is obtained, for example, by coating a metal thin film or the like with a predetermined thickness, and has a predetermined light reflectance and transmittance.
[0033]
Next, the photosensitive film for multilayer optical recording medium of the present invention is laminated on the pits or grooves carried by the first intermediate reflective layer 102, and further, pits different from the pits or grooves carried by the first intermediate layer 102 Alternatively, the first spacer layer 106 is formed by transferring the stamper 105 to which the groove has been transferred and irradiating ultraviolet rays from the surface opposite to the information recording surface of the translucent substrate 101 to cure the photosensitive film. (FIG. 1 (c)). At the time of laminating the photosensitive film and transferring the short wave 105, it is preferable to use a normal pressure or a vacuum laminator from the viewpoint of followability to the concave and convex pits.
[0034]
In the lamination, the photosensitive layer of the photosensitive film is preferably heated, and the heating temperature is preferably 30 to 150 ° C, more preferably 40 to 120 ° C, and more preferably 50 to 110 ° C. It is particularly preferred that If this temperature is less than 30 ° C., the fluidity of the resin is low and the followability to uneven pits tends to be insufficient, and if it exceeds 150 ° C., low boiling point substances or low boiling point solvents in the resin composition are vaporized and , Tends to cause deformation of the substrate.
[0035]
The speed of the laminate is preferably 0.01 to 2.00 m / min, more preferably 0.01 to 0.50 m / min, and preferably 0.01 to 0.20 m / min. Particularly preferred. When this speed is less than 0.01 m / min, there is a tendency that the resin oozes out, and when it exceeds 2.00 m / min, there is a tendency that followability to uneven pits is insufficient.
[0036]
Further, the pressure of the laminate is preferably 0.3 to 1.0 MPa, more preferably 0.4 to 0.8 MPa, and particularly preferably 0.5 to 0.6 MPa. If the speed is less than 0.3 MPa, the followability to uneven pits tends to be insufficient, and if it exceeds 1.0 MPa, the substrate tends to be deformed.
[0037]
After laminating, for example, the photosensitive layer is photocured by irradiating with actinic rays such as ultraviolet rays, thereby transferring uneven pits present on the information recording layer. In this case, in order to simplify the drive device and increase the reproduction access speed, it is desirable that the information can be reproduced by, for example, irradiation from the same side of the recording medium with a set of optical heads. It is.
[0038]
After the stamper 105 is peeled from the laminated body (disk) (FIG. 1D), a predetermined reflectance formed by vacuum deposition or sputtering on the first spacer layer 106 (the lower surface side in the drawing) is obtained. A second intermediate reflection layer (normal reflection layer) 107 made of a metal thin film or the like is formed (FIG. 1 (e)). Next, a protective layer (not shown) is usually formed on the reflective layer, and a label (not shown) is further formed on the protective layer to produce an optical disc having two pits or grooves. Multi-layering of the optical recording medium is realized by repeating the steps shown in FIGS.
[0039]
In the multilayer optical recording medium thus obtained, each information recording layer is reproduced by focusing light irradiation from the optical head on each information recording layer. Therefore, the transmittance of the spacer layer, that is, the photosensitive layer after photocuring, is required to transmit 90% or more with respect to the wavelength of light of the optical head, and the coating film accuracy is also maintained in order to maintain optical characteristics. It is preferable to keep it within ± 2.0 μm.
[0040]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples. In the examples and comparative examples, “part” means part by weight, and “%” means% by weight.
[0041]
[Example 1]
(Production of photosensitive film for multilayer optical recording media)
A solution of a photosensitive resin composition was obtained using components (a) to (c) shown in Table 1. Next, this photosensitive resin composition solution was uniformly coated on a 25 μm-thick polyethylene terephthalate film, dried for 10 minutes with a 100 ° C. hot air convection dryer, and further laminated with a polyethylene film to obtain a multilayer structure light. A photosensitive film for a recording medium was obtained. The film thickness after drying of the photosensitive layer was 20 μm.
[0042]
(Measurement of light transmittance)
The light transmittance of the photosensitive layer was measured according to the following procedure.
[0043]
The substrate was polycarbonate (thickness: 0.5 mm), and the sheet was 20 μm thick. Using a normal pressure laminator HLM-3000 (Hitachi AIC Co., Ltd.), a photosensitive film for a multilayer structure optical recording medium is laminated under conditions of a roll temperature of 110 ° C., a laminating speed of 1.0 m / min, and a laminating pressure of 0.5 MPa. did. After leaving the sample for 15 minutes or longer, the polyethylene terephthalate sheet was peeled off, and 0.5 hours after the 1 J / cm ² UV irradiation was performed using a large UV irradiator (QRM-2317-F-00 Oak). The light transmittance at 400 nm was measured with a 228A Spectrophotometer (manufactured by Hitachi, Ltd.). The light transmittance was determined by measuring the difference in light transmittance between the polycarbonate substrate and the polycarbonate substrate with the photosensitive film for the multilayer structure optical recording medium. The obtained results are shown in Table 2.
[0044]
(Measurement of thickness accuracy)
The thickness accuracy of the photosensitive layer of the photosensitive film was measured according to the following procedure.
[0045]
The substrate was a Si wafer, and a substrate primer treatment was performed by applying a coupling agent (KBM503 3% methanol solution) onto the substrate. The substrate was naturally dried for about 30 minutes, then washed with acetone, and dust and dust were removed by air blowing. Next, using a normal pressure laminator HLM-3000 (Hitachi AIC Co., Ltd.), the photosensitive film for multilayer structure optical recording medium was rolled at a temperature of 50 ° C., laminating speed 0.2 m / min, laminating pressure 0.5 MPa. Laminated with. At this time, the substrate was not preheated. After peeling the polyethylene terephthalate sheet from the laminated sample and irradiating it with ² J / cm ² ultraviolet rays using a large UV irradiator (QRM-2317-F-00 Oak), 200MM wafer surface shape measuring device (Veeco) (Contact type) DEKTAK V200-Si) was used to measure the thickness unevenness of the photosensitive layer with a load of 3 mg and a measurement range of 5 mm, and the difference between the thickness unevenness peaks and valleys was determined. This measurement was performed three times at different positions, and the average value of the differences between the peaks and valleys was defined as the thickness accuracy Ra. The obtained results are shown in Table 2.
[0046]
(Evaluation of air entrainment)
Using an Ni stamper, pits were transferred to the photosensitive layer of the polycarbonate substrate with the photosensitive film described above to produce an optical disc. At this time, whether air is involved between the photosensitive layer and the Ni stamper is as follows:
A: No entrainment of air was observed B: Almost no entrainment of air was observed C: Evaluation was made based on the recognition of entrainment of air. The obtained results are shown in Table 2.
[0047]
[Example 2]
A photosensitive film was prepared in the same manner as in Example 1 except that the thickness accuracy of the photosensitive layer was 0.13 μm in Ra value, and the light transmittance was measured. Moreover, an optical disk was produced using this polycarbonate with a photosensitive film, and the air entrainment property was evaluated. The obtained results are shown in Table 2.
[0048]
[Comparative Example 1]
A photosensitive film was prepared in the same manner as in Example 1 except that the photosensitive resin composition shown in Table 1 was used, and the thickness accuracy of the photosensitive layer was 0.16 μm in terms of Ra value. It was measured. Moreover, an optical disk was produced using this polycarbonate with a photosensitive film, and the air entrainment property was evaluated. The obtained results are shown in Table 2.
[0049]
[Table 1]

[0050]
[Table 2]

As shown in Table 2, in the photosensitive films of Examples 1 and 2, the light transmittance of the photosensitive layer was as high as 98%. Further, when the sheet having a thickness of 100 μm was measured in the same process, the light transmittance of the photosensitive layer was stable between 96% and 98%. In an optical disk produced using these photosensitive films, a sufficiently high density could be achieved. Further, in both cases of Examples 1 and 2, good results were shown in the evaluation of air entrainment. However, in Example 2 in which the thickness accuracy of the photosensitive layer is within a range of ± 0.30 μm in Ra value, Entrainment was prevented at a higher level.
[0051]
On the other hand, the transmittance of the photosensitive layer in the photosensitive film of Comparative Example 1 is 86%, and in the case of an optical disk obtained using this photosensitive film, sufficient density cannot be achieved. It was.
[0052]
【The invention's effect】
As described above, the photosensitive film for multilayer optical recording media according to the present invention has excellent optical characteristics, thereby realizing a photosensitive film for multilayer optical recording media that can cope with higher density. It becomes possible to do.
[Brief description of the drawings]
FIGS. 1A to 1E are schematic cross-sectional views showing a substrate or a laminate in each step when a multilayer structure optical recording medium is produced using the photosensitive film of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 101 ... Translucent substrate, 102 ... 1st intermediate | middle reflective layer, 105 ... Stamper, 106 ... Spacer layer, 107 ... 2nd intermediate | middle reflective layer.

Claims (7)

A photosensitive layer having a light transmittance of 90% or more at 400 nm after ultraviolet irradiation, the photosensitive layer comprising: (a) a polymer binder; and (b) at least two polymerizable unsaturated double bonds in the molecule. A photosensitive film for spacers of a multilayer structure optical recording medium, comprising a urethane acrylate compound or urethane methacrylate compound having a bond and (c) 1-hydroxy-cyclohexyl-phenyl-ketone .   The photosensitive film for a spacer of a multilayer structure optical recording medium according to claim 1, wherein the photosensitive layer has a thickness of 5 to 20 μm.   3. The photosensitive film for a spacer of a multilayer optical recording medium according to claim 1, wherein the thickness accuracy of the photosensitive layer is 0.30 μm or less in terms of Ra value. (A) a polymer binder, (b) a urethane acrylate compound or urethane methacrylate compound having at least two polymerizable unsaturated double bonds in the molecule, and (c) 1-hydroxy-cyclohexyl-phenyl-ketone. A photosensitive resin composition having a viscosity of 1 to 3000 cps, wherein the content of the components (a) to (c) is 100 parts by weight based on the total amount of the components (a) and (b). The photosensitive resin composition, wherein the component (a) is 40 to 80 parts by weight, the component (b) is 20 to 60 parts by weight, and the component (c) is 0.1 to 10 parts by weight on a support. The photosensitive film for spacers of a multilayer structure optical recording medium according to any one of claims 1 to 3, wherein the photosensitive film is laminated.   The photosensitive film for a spacer of a multilayer structure optical recording medium according to any one of claims 1 to 4, wherein the refractive index at 400 nm of the photosensitive layer is 1.32 to 1.72.   6. The photosensitive film for a spacer of a multilayer structure optical recording medium according to claim 1, wherein the birefringence of the photosensitive layer at 400 nm is −20 nm to +20 nm.   The photosensitive property for the spacer of the multilayer optical recording medium according to any one of claims 1 to 6, wherein the polymer binder has a weight average molecular weight of 20,000 to 200,000. the film.

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