WO2011006031A2

WO2011006031A2 - Polymer film for protecting glass sheets

Info

Publication number: WO2011006031A2
Application number: PCT/US2010/041466
Authority: WO
Inventors: Yao-sheng CHEN; Shih-Te Chu; Hongkyu Kim; Szuhsien Mai; Himanshu C Shah
Original assignee: Corning Incorporated
Priority date: 2009-07-10
Filing date: 2010-07-09
Publication date: 2011-01-13
Also published as: CN102596768A; JP5723877B2; JP2012532806A; TWI494227B; KR20120089233A; CN107053796A; KR101819758B1; WO2011006031A3; TW201114604A

Abstract

This disclosure features a method of protecting glass sheets from scratching and staining. A first surface of polymer film is positioned in contact with a major surface of a glass sheet. An additional glass sheet is placed against the polymer film such that a major surface of the additional glass sheet contacts a second surface of the film. Both of the first and second surfaces of the film have a peel value of less than 50g relative to the glass sheets and a surface roughness Ra of greater than 0.5 micron. The method will prevent scratches on a glass surface from the bottom of the draw, during shipment between glass forming plants, as well as during shipment and handling in connection with customers of glass processing plants.

Description

POLYMER FILM FOR PROTECTING GLASS SHEETS

[0001] This application claims the benefit of priority to US Patent Application

No. 61/224584 filed on July 10, 2009.

TECHNICAL FIELD

[0002] The present invention relates to packaging of inorganic sheet materials such as glass sheets. In particular, the present invention relates to processes for packaging and protecting surfaces of glass sheets from scratching and contaminants. The present invention is useful, e.g., for packaging and protecting pristine surfaces of LCD glass substrates.

BACKGROUND

[0003] Glass substrates for the manufacture of LCD displays are required to have pristine surfaces essentially free of scratches and surface particles. The packaging and shipping of such glass sheets have presented significant challenges due to the fact that the materials and process steps involved may cause surface degradation and introduce contaminant thereto. This is especially true for long-distance shipping. In addition, the desire to lower the cost of shipping means that glass sheets should be packaged as densely as possible.

[0004] Various methods were used for the packaging of LCD glass substrates. In one solution illustrated in FIG. 1, three layers of sheets were used between adjacent glass sheets 101 and 103, including two polymer outer sheets 105 and 107 coated onto the surfaces of the glass sheets 101 and 103, respectively and a paper layer 113 sandwiched between the two polymer layers. The approach requires a polymer film coater and polymer films when packing and a film peeler to remove the coated polymer films when de-packing.

[0005] An alternative solution involving the use of a single-layer paper was used to reduce the complexity and number of process steps. A virgin pulp grade paper sheet, processed to have minimal stain and scratch sources contained therein and

supercalendared surfaces with low coefficient of friction and low Sheffield smoothness to minimize scratches and densified to prevent potential contaminants from migrating to the glass surface, was found to be acceptable for a high-density packaging of the glass sheets. However, this approach exhibited significantly more scratch and pit surface defects, when compared with the three-layer approach described above.

[0006] Hence there is a need of a glass sheet packaging method with a desirable combination of cost effectiveness and surface protection capability. The present invention satisfies this and other needs.

SUMMARY

[0007] Several aspects of the present invention are disclosed herein. It is to be understood that the aspects may or may not overlap with one another. Thus, part of one aspect may fall within the scope of another aspect, and vice versa.

[0008] Each aspect is illustrated by a number of embodiments, which, in turn, can include one or more specific embodiments. It is to be understood that the embodiments may or may not overlap with each other. Thus, part of one embodiment, or specific embodiments thereof, may or may not fall within the ambit of another embodiment, or specific embodiments thereof, and vice versa.

[0009] A first aspect of the present invention relates to a method of protecting glass sheets from scratching and stains comprising:

positioning a polymer film on one of said glass sheets, wherein said polymer film has first and second surfaces and said first surface contacts a major surface of said glass sheet;

placing an additional said glass sheet against said polymer film such that a major surface of said additional glass sheet contacts said second surface,

wherein both said first and second surfaces have a peel value of less than 5Og relative to said glass sheets and a surface roughness Ra of greater than 0.5 micron.

[0010] In certain embodiments of the first aspect of the present invention, said polymer film comprises a central core layer of polymer foam and two polymer skin layers sandwiching said core layer, and said core layer and said skin layers are integrally formed with each other. In certain embodiments, said foam core layer includes cells having a size ranging from 0.5 to 20.0 microns. In certain embodiments, said core layer is formed of medium or high density polyethylene and said skin layers are formed of low density polyethylene, linear low density polyethylene or low density polyethylene copolymer.

[0011] In certain embodiments of the first aspect of the present invention, said polymer film comprises a central core layer and two polymer skin layers sandwiching said core layer, said core layer having a stiffness represented by said polymer of said core layer having a tensile modulus of elasticity of greater than 80 kpsi, and said core layer, and said skin layers are integrally formed with each other. In certain embodiments, said core layer is formed of polyethylene terephthalate, polystyrene or polypropylene and said skin layers are formed of low density polyethylene, linear low density polyethylene or low density polyethylene copolymer.

[0012] In certain embodiments of the first aspect of the present invention, said polymer film comprises a central core layer of paper and two polymer skin layers sandwiching said core layer, and said core layer and said skin layers are integrally formed with each other. In certain embodiments, said skin layers are formed of low density polyethylene, linear low density polyethylene or low density polyethylene copolymer.

[0013] In certain embodiments of the first aspect of the present invention, said polymer film comprises a unitary layer of polymer foam formed of polyethylene, polypropylene, polystyrene or biaxially oriented polypropylene. In certain embodiments, said foam core layer includes cells having a size ranging from 0.5 to 20.0 microns.

[0014] In certain embodiments of the first aspect of the present invention, the method comprises repeating said steps of positioning said polymer film and placing an additional said glass sheet against said polymer film until a stack of said glass sheets is arranged with said polymer film between adjacent said glass sheets.

[0015] In certain embodiments of the first aspect of the present invention, said skin layers include a long chain fatty ester or a long chain fatty amide slip agent.

[0016] In certain embodiments of the first aspect of the present invention, said slip agent is present on said first surface and said second surface in an amount ranging from 100 to 5000 ppm. In certain embodiments said slip agent comprises erucamide.

[0017] In certain embodiments of the first aspect of the present invention, said polymer film has a total thickness range from about 60 to 300 microns. In certain embodiments said polymer film comprises a central polymer core layer and two polymer skin layers sandwiching said core layer, and said core layer and said skin layers are integrally formed with each other, said polymer of said skin layers have a tensile modulus of elasticity ranging from 20 to 80 kpsi, said polymer of said core layer has a tensile modulus of elasticity of at least 80 kpsi. In certain embodiments a thickness of each of said skin layers ranges from about 20 to 100 microns. In certain embodiments a thickness of said core layer ranges from about 20 to 100 microns. [0018] In certain embodiments of the first aspect of the present invention, said surface roughness Ra ranges from 1.5 to 15 microns.

[0019] In certain embodiments of the first aspect of the present invention, said first and second surfaces of said polymer film are characterized by a presence of undulations of a size much greater than the surface roughness Ra of said polymer film.

[0020] In certain embodiments of the first aspect of the present invention, said slip agent functions as a barrier layer to protect said glass sheets from staining by components in said skin layer.

[0021] One of more aspects and/or embodiments of the present invention has one or more of the following advantages. First, the use of a separation polymer film having the stated structure does not require the lamination of a polymer film onto a glass sheet surface when packaging or the delamination thereof from the glass surface when de- packaging. Second, where the polymer film comprises a slip agent, the film can function to transfer part of the slip agent to the surface to be protected, which can further enhance the protection to the surface.

[0022] Additional embodiments of the disclosure will be set forth, in part, in the detailed description, and any claims which follow, and in part will be derived from the detailed description, or can be learned by practice of the disclosure. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as disclosed and/or as claimed.

[0023] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate certain examples of the present disclosure and together with the description, serve to explain, without limitation, the principles of the disclosure. Like numbers represent the same elements throughout the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the accompanying drawings:

[0025] FIG. 1 schematically illustrates a prior art surface protection solution using polymer films coated on glass with a sheet of paper sandwiched between the film (a three-layer system);

[0026] FIG. 2 schematically illustrates a first embodiment of the present invention using a film comprising a central foam core layer sandwiched by two polymer skin layers between adjacent glass sheets; [0027] FIG. 3 schematically illustrates a second embodiment of the present invention using a film comprising a central core layer sandwiched by two polymer skin layers wherein the core layer is stiffer than the skin layers, between adjacent glass sheets;

[0028] FIG. 4 schematically illustrates a third embodiment of the present invention using a film comprising a central paper core layer sandwiched by two polymer skin layers, between adjacent glass sheets; and

[0029] FIG. 5 schematically illustrates a fourth embodiment of the present invention using a separation film comprising a unitary foam layer between adjacent glass sheets.

DETAILED DESCRIPTION

[0030] This disclosure features a method of protecting glass sheets from scratching and contamination using a polymer film. Although a polymer film may be interpreted to have a smaller thickness than polymer sheet, the terms polymer film and polymer sheet are used interchangeably in this disclosure. The interleaf polymer film has a low adherence to the glass and has a surface roughness sufficient to provide air pockets that protect the glass from scratches via cushioning and help separate the glass sheets during unpackaging. In addition, a soft polymer (low density polyethylene or linear low density polyethylene) is used to form the outside surfaces of the film, which help to protect the glass surface from particle intrusion and consequently its scratches. This is an improvement over the single sheet of interleaf paper material as well as over the use of Visqueen polymer film on the glass sheets with interleaf paper therebetween. The polymer film of this disclosure will prevent scratches on a glass surface from the bottom of the draw, during shipment between glass forming plants, as well as during shipment to customers and handling at customers of the glass processing plants.

[0031] In the method, a first surface of the polymer film is positioned in contact with a major surface of the glass sheet. An additional glass sheet is placed against the polymer film such that a major surface of the additional glass sheet contacts a second surface of the film. Both of the first and second surfaces of the film have a peel value of less than 5Og (grams) relative to the glass sheets and a surface roughness Ra of greater than 0.5 micron (0.5 xl O ⁶ meters). This peel value is produced according to a procedure by the assignee, Corning, Inc., described below.

[0032] One embodiment of the method features using a polymer film comprising a central core layer of polymer foam and two polymer skin layers sandwiching the core layer. The core layer and skin layers are integrally formed with each other. The foam core layer can include cells having a size ranging from 0.5 to 20.0 microns or greater. The core layer can be formed of low, medium or high density polyethylene, preferably medium density polyethylene. The skin layers are formed of low density polyethylene, linear low density polyethylene or low density polyethylene copolymer, preferably low density polyethylene.

[0033] Another embodiment of the method uses a polymer film comprising a central core layer and two polymer skin layers sandwiching said core layer. The core layer is stiffer than the skin layers and the polymer of the core layer has a stiffness represented by a tensile modulus of elasticity of greater than 80 kpsi, in particular, greater than 100 kpsi (ASTM D638). The core layer and skin layers are integrally formed with each other. The core layer is formed of polyethylene terephthalate (PET), polystyrene or polypropylene. The skin layers are formed of low density polyethylene, linear low density polyethylene or low density polyethylene copolymer.

[0034] Another embodiment of the method uses a polymer film comprising a central core layer of paper and two polymer skin layers sandwiching the core layer. The core layer and skin layers are integrally formed with each other. The skin layers are formed of low density polyethylene, linear low density polyethylene or low density polyethylene copolymer.

[0035] Yet another embodiment of the method uses a polymer film comprising a unitary layer of polymer foam formed of polyethylene, polypropylene, polystyrene or biaxially oriented polypropylene (BOPP). The foam core layer includes cells having a size ranging from 0.5 to 20.0 microns.

[0036] In certain advantageous embodiments of the method there are no other layers of materials besides the polymer film between adjacent glass sheets (i.e., no coatings of polymer film on the glass and no separate paper interleaf material). The skin layers and core layer are considered to be sublayers of the polymer film. However, it is not ruled out that in addition to the presence of the polymer film, additional protective layers may exist over the surface of the glass sheet to be protected in certain embodiments of the present invention.

[0037] Regarding further details of the method, the polymer (before being formed into the skin layer of the film) can have a lower tensile modulus of elasticity ranging from 20 to 80 kpsi (ASTM D638), for example. The higher tensile modulus of elasticity of the core layer can be 80 kpsi, in particular, greater than 100 kpsi (ASTM D638). For the polymer film comprising a unitary layer of foam, it can have either the lower or higher tensile modulus of elasticity, preferably the higher tensile modulus of elasticity. The appropriate tensile modulus of elasticity is that which makes for easy handling of the polymer sheet during loading and unloading processes. The steps of positioning the polymer film and placing an additional glass sheet against the polymer film are repeated until a stack of glass sheets is arranged with the polymer film between adjacent glass sheets. The skin layers can include a long chain fatty ester or a long chain fatty amide slip agent. The slip agent can be present on each of the first and second surfaces of the polymer film in an amount ranging from 100 to 5000 ppm. One specific slip agent is erucamide. In one embodiment, the polymer film can have a total thickness ranging from about 60 to 300 microns, and, in certain embodiments, from 60 to 120 microns. In one embodiment the polymer film can comprise a central core layer and two polymer skin layers sandwiching the core layer; the core layer and skin layers are integrally formed with each other. The skin layer and core layer can have the lower and higher moduli of elasticity, respectively, described above. In this case a thickness of each of the skin layers can range from about 20 to 100 microns, and, in certain embodiments, from 20 to 40 microns. A thickness of the core layer can range from about 20 to 100 microns, and, in certain embodiments, from 20 to 40 microns.

[0038] As used herein, a slip agent provides surface lubrication during and immediately following processing polymer film materials containing the slip agent. If compounded into the polymer film material, the slip agent acts as an internal lubricant that gradually migrates to the surface where it is presented. In this disclosure the lubrication is provided to surfaces of glass sheet that contact the polymer film material containing the slip agent.

[0039] The polymer film of this disclosure offers advantages. First, unlike the

Visqueen film it is not laminated onto the glass sheet and thus, does not require a polymer film coater and a film peeler, which adds extra process steps and increases manufacturing costs. Because the polymer film of this disclosure has a specific surface roughness Ra and low peel value as discussed in the peel procedure below, it does not have the disadvantage of adhering strongly to glass. Also, the polymer film of this disclosure protects glass from scratching better than Glassine paper. The polymer film of this disclosure can be used when packing glass with unground edges for shipment or when packing glass with ground edges for shipment using Coming's DensePak™ process.

[0040] The present invention will be further illustrated by the description of certain specific embodiments, which are not intended to be limiting.

[0041] In one exemplary method, a first surface of the polymer film is positioned in contact with a major surface of the glass sheet. An additional glass sheet is placed against the polymer film such that a major surface of the additional glass sheet contacts a second surface of the film. Both of the first and second surfaces of the film have a peel value of less than 5Og (grams) relative to the glass sheets and a surface roughness Ra of greater than 0.5 micron (0.5 xlO^"6 meters). In certain embodiments, the polymer film can include a central core layer sandwiched by two skin layers. The polymer of the softer skin layer has a tensile modulus of elasticity ranging from 20 to 80 kpsi. If a stiffer material is desired, the polymer (e.g., of the core layer) can have a tensile modulus of elasticity of greater than 80 kpsi. In the case of the unitary layer of foam 501, it can have the lower or higher tensile modulus of elasticity values described herein, preferably the lower value. In addition, the polymer film has a total thickness ranging from about 60 to 300 microns. A thickness of each of the skin layers can range from about 20 to 100 microns. A thickness of the core layer can range from about 20 to 100 microns. The peel value referred to in this disclosure is based on the peel strength test for adhesiveless film by the assignee, Corning Inc., described below. The polymer film can be one of four variations as seen in Figs. 2-5.

[0042] Each of the polymer films 200 in the embodiment illustrated in FIG. 2 comprises a central foam core layer 201 and two outer polymer skin layers 203 sandwiching the core layer. The skin layers have first and second outer surfaces 205, 207 with a low adhering capability with respect to a flat surface of an inorganic glass material. The skin layers can also include a slip agent providing slip functionality. The core layer has outer surfaces 209, 211 that contact inner surfaces 213, 215 of the skin layers, the outer core layer surfaces having a micro or macro cellular structure for cushioning. In all of the figures of this disclosure, for the sake of clarity the polymer films are shown in an exploded view relative to the glass sheets of a stack (glass sheets 101, 103 and 217). In an actual stack under a compressive load the outer skin layer surfaces 205, 207 would contact the inner surfaces 109, 111 of the glass sheets 101, 103 of a first pair of adjacent glass sheets of glass and the outer surfaces 205, 207 would contact inner surfaces 219, 221 of the glass sheets 103, 217 of a second pair of adjacent glass sheets of the stack, and so on, for the remaining polymer films and glass sheets of the stack.

[0043] In the polymer film 200, the polymer for the skin layers is a low density polyethylene (e.g., having a density of less than 0.93g/cm³), linear low density

polyethylene or low density polyethylene copolymer. The polymer for the core layer is a low density polyethylene, a medium density polyethylene (e.g., having a density of about 0.94 g/cm³) or a high density polyethylene (e.g., having a density of 0.94 to 0.97 g/cm³), preferably a medium density polyethylene. The foam of the core layer has cell structures each having a size in a range of about 0.5 to 20.0 microns or larger. Chemical foaming agents that can be used in making the foam of the core layer include, for example, azodicarbonamide; modified azodicarbonamide; 4,4-oxibis(benzenesulfonyl hydrazide); trihydrazinotriazaine. Other chemical foaming agents known in the art could also be used. Physical foaming agents that can be used to make the foam include, for example, nitrogen or halogenated hydrocarbons.

[0044] Each of the polymer films 300 of the embodiment illustrated in FIG. 3 comprises a central polymeric core layer 301 and two outer polymer skin layers 303 sandwiching the core layer. The skin layers have first and second outer surfaces 305, 307 with a low adhering capability with respect to a flat surface of an inorganic glass material. The skin layers can also include a slip agent providing slip functionality. The core layer comprises a layer of polymer different from and having a greater stiffness than the skin layer polymer. Outer surfaces 309, 311 of the core layer contact inner surfaces 313, 315 of the skin layers. In a stack of glass sheets under a compressive load, the first outer surface 305 of the skin layers contacts an inner glass surface 109 of a glass sheet 101 of a first pair of glass sheets of the stack while the second outer surface 307 of the skin layers contacts an inner glass surface 111 of an adjacent glass sheet 103. The first outer surface 305 of the skin layers contacts an inner glass surface 219 of the glass sheet 103 of a second pair of glass sheets of the stack while the second outer surface 307 of the skin layers contacts an inner glass surface 221 of an adjacent glass sheet 217. The alternating of polymer films and glass sheets would continue throughout the stack.

[0045] In the polymer film 300, the polymer for the skin layers is a low density polyethylene (e.g., having a density of less than 0.93 g/cm ), linear low density polyethylene or low density polyethylene copolymer. The polymer for the core layer is stiff polystyrene, polypropylene or PET. A low stiffness of the skin layers is reflected by a skin layer polymer having a tensile modulus of elasticity ranging from 20 to 80 kpsi at room temperature (in particular from about 30 to 60 kpsi). A stiffness of the core layer is reflected by a stiff core layer polymer having a tensile modulus of elasticity of greater than 80 kpsi at room temperature (in particular greater than 150 kpsi). The stiffness of the core layer is such that the mechanical tool that uses suction and mechanical clippers to hold the polymer film can handle the film easily.

[0046] Each of the polymer films 400 of FIG. 4 includes a central paper core layer

401 and two outer polymer skin layers 403 sandwiching the core layer. The skin layers have first and second outer surfaces 405, 407 with a low adhering capability with respect to a flat surface of an inorganic glass material. The skin layers can also include a slip agent providing slip functionality. The polymer for the skin layers is a low density polyethylene (e.g., having a density of less than 0.93 g/cm³), linear low density polyethylene or low density polyethylene copolymer. The paper can be any suitable paper that provides cushioning, for example Kraft paper or paper described in publication WO 2008/002584, which is incorporated herein by reference. The paper provides stiffness and cushioning due to its porosity. The paper has outer surfaces 409, 411 that contact inner surfaces 413, 415 of the skin layers. In a stack of glass sheets under a compressive load, the first outer surface 405 of the skin layers contacts an inner glass surface 109 of a glass sheet 101 of a first pair of glass sheets of the stack while the second outer surface 407 of the skin layers contacts an inner glass surface 111 of an adjacent glass sheet 103. The first outer surface 405 of the skin layers contacts an inner glass surface 219 of the glass sheet 103 of a second pair of glass sheets of the stack while the second outer surface 407 of the skin layers contacts an inner glass surface 221 of an adjacent glass sheet 217. The alternating of polymer films and glass sheets would continue throughout the stack.

[0047] The polymer film of FIG. 5 comprises a unitary layer of polymer foam

501. The foam is made of polyethylene, polystyrene, polypropylene or biaxially oriented polypropylene (BOPP). The foaming agent used to make the foam can be the same as the foaming agents used to produce the foam of the polymer film 200 of FIG. 2. However, when making the foam using BOPP, no foaming agents are used; rather the foam is created by stretching. The foam has outer surfaces 503, 505. In a stack under a compressive load, the first outer surface 503 contacts an inner surface 109 of glass sheet 101 of a first pair of glass sheets of the stack while the second outer surface 505 contacts an inner surface 111 of an adjacent glass sheet 103. The first outer surface 503 contacts an inner surface 219 of glass sheet 103 of a second pair of glass sheets of the stack while the second outer surface 505 contacts an inner surface 221 of an adjacent glass sheet 217. The alternating of polymer films and glass sheets would continue throughout the stack.

[0048] The polymer films have properties making them suitable for providing scratch protection to glass sheets. All of the films described herein can have one or more of the following properties. The polymer film can be used with and without slips agents. The slip functionality or amount of slip agent on each surface of the polymer film, if slip agent is used, ranges from 100 to 5000 ppm and in particular ranges from 500 to 1000 ppm. The slip agent can migrate out of the polymer and presents on both outer surfaces of the film (i.e., from the outer surfaces of the skin layers or the outer surfaces 503, 505 of the foam layer 501). Each side of the polymer film has a low adhering capability with respect to a flat surface of an inorganic glass material of less than 50 g (e.g., from 0 up to 5Og) according to a peel test described below. A surface roughness Ra of the outer surface of each skin layer is greater than 0.5 micron, in particular in a range of 1.5 to 15 microns. The Ra value is an arithmetic average height of peaks of polymeric material on a surface of the polymer films. In contrast, one side of Visqueen film has been known to have a lower surface roughness Ra or a mirror finish in a range of 0.005 to 0.025 microns. Features of the core layers can provide the skin layers with undulations on the surface on a scale much larger than a surface roughness Ra of the skin layers as disclosed herein. For example, these undulations can be provided by the cells of the foam in polymer films 200 and 501 (Figs. 2 and 5), the core layer of polymer film 300 (FIG. 3) and by the paper 400 (FIG. 4). A thickness of the entire film can range from 60 to 300 microns (e.g., 60 to 120 microns). The skin layers can each have a thickness in a range of 20 to 100 microns (e.g., 20 to 40 microns) and the core layers can have a thickness ranging from 20 to 100 microns (e.g., 20 to 40 microns). Selecting the thickness of the film components (e.g., skin layer and core layer) advantageously enables material having a given stiffness to balance different levels of softness and hardness that suit the particular shipping and packaging equipment with potential effects on levels of scratching. Providing stiffness of the polymer film so high they are self standing is not a requirement because a delivery system holds the film with suction and mechanical clippers during loading of the film into the stack of glass sheets. [0049] A function of the polymer film is to prevent scratching by trapping and/or absorbing particles on the surface of the glass, such as glass chips, into the soft outer skin layers of the film. This scratch protection would occur during a compressive loading of the stack of glass sheets on the films. The particles can be submicron and greater in size. The particles can be absorbed by the core layer as well. Slip agents migrate out of the polymer and are presented on outer surfaces of the film into contact with adjacent glass sheets on both sides of the film. The slip agents may permit particles to slide on the slip agent material rather than against the bare glass. No other layers of material besides the polymer film are necessary between adjacent glass sheets (i.e., no paper and no laminated coatings on the glass), though the inclusion of which is not ruled out.

[0050] Compounds that might be suitable as slip agents include at least one long chain fatty acid ester or fatty acid amide. The long chain fatty acid esters and fatty acid amides of this disclosure are derivatives of saturated and unsaturated normal fatty acids ranging from fourteen to thirty-six carbon atoms. Representative fatty acids are, for example, tetradecanoic, pentadecanoic, hexadecanoic, heptadecanoic, octadecanoic, nonadecanoic, eicosanoic, hencosanoic, decosanoic, tetracosanoic, pentacosanoic, tricosanoic, hexacosanoic, triacontanoic, dotriacontanoic, tetratriacontanoic,

hentriacontanoic, pentatriacontanoic, hexatriacontanoic acids, myristic, palmitic, stearic, arachidic, behenic and hexatrieisocontanoic (C₃₆) acids, oleic, palmitoleic, linolenic and cetoleic, and the like.

[0051] Long chain fatty amides are preferred as slip agents, suitable slip agent might include one or more of the following: unsaturated fatty acid monoamide (e.g., oleamide, erucamide, recinoleamide); saturated fatty acid monoamide (preferably, lauramide, palmitamide, arachidamide, behenamide, stearamide, 12 hydroxy stearamide); N-substituted fatty acid amide (e.g., N-stearyl stearamide, N-behenyl behenamide, N- stearyl behenamide, N-behenyl stearamide, N-oleyl oleamide, N-oleyl stearamide, N- stearyl oleamide, N- stearyl erucamide, erucyl erucamide, erucyl stearamide, stearyl erucamide, N-oleyl palmitamide); methylol amide (e.g., methylol stearamide, methylol behenamide); unsaturated fatty acid bis-amide (e.g., ethylene bis-oleamide,

hexamethylene bis-oleamide, N,N'-dioleyl adipamide, ethylene bis oleamide, N,N'-dioleyl sebacamide); saturated or unsaturated fatty acid terra amide; and saturated fatty acid bis- amide (e.g., methylene bis-stearamide, ethylene bis-stearamide, ethylene bis- iso stearamide, ethylene bis-hydroxystearamide, ethylene bis stearamide, ethylene bis- behenamide, hexamethylene bis-stearamide, hexamethylene bis-behenamide,

hexamethylene bis-hydro xystearamide, N,N'-distearyl adipamide, N,N'-distearyl sebacamide).

[0052] Specific long chain fatty amides that may be suitable are erucamide, stearamide, oleamide and behenamide. Fatty amides are commercially available from Humko Chemical Company and include, for example Kemamide S (stearamide), Kemamide U (oleamide), Kemamide E (erucamide). In addition, fatty amides are commercially available from Croda Universal Ltd., and include, for example, Crodamide OR (oleamide), Crodamide ER (erucamide), Crodamide SR (stereamide), Crodamide BR (behenamide).

[0053] The slip agent on the film surface acts as a barrier layer in preventing migratory low molecular organic components in the polymer resin, which would be potential sources of organic stains. Any slip agent present on the glass is washed off at the finishing line using known washing processes including brushes, ultrasound, water jet spraying, and a detergent at a pH of 10-12 (e.g., a potassium hydroxide detergent).

[0054] The slip agent can be provided into the polyethylene resin directly as a powder or mixed as a master batch that is then added to the entire polyethylene batch. The polymer film can be made by casting or blown film processes as known in the art. In the casting process the film is extruded onto a cooling or chilling roll and then passes onto a rubber nip roll which provides the film with suitable surface roughness Ra.

[0055] The following describes procedures for the peel test referred to herein.

Uncoated glass samples are placed on a clean work surface. The glass samples are cleaned. Uncontaminated and untouched 5"x5" samples of film are placed on the glass and rolled with a 10-pound (about 4.5-kg) roller using only two roller strokes. The equipment includes a sample jig on a platform of a TA.XT Texture Analyzer, tensile strength tester. A universal joint is attached to the texture analyzer arm and an S-hook is hung on it. Scotch® double-sided tape is applied to all four edges of a 0.25" thick, 4" square aluminum plate. The plate is suspended by the S-hook via fishing line using four eyehooks at the corners of the plate. The plate is aligned with the brackets and lowered to about 1 mm above the surface. The prepared glass/film sample is centered below the plate. The plate is lowered on top of the glass and film. Four film edges are secured to the double sided tape on the plate, insuring good contact. The sample is slid into the brackets and the glass is tightened to the platform. The device measures force when the plate is pulled up at a rate of 0.2 mm/sec. The plate must be flat at the end of the run. The force needed to lift off the film from the glass is measured and recorded in grams as the peel value.

[0056] This disclosure will now provide a description by way of the following examples, which are for the purpose of illustration and should not be interpreted to limit the invention as defined in the claims.

EXAMPLE 1

[0057] Glass surfaces contacting one polymer film imbibed with erucamide ("SL polymer film") were compared along with glass surfaces contacting clean Glassine paper and glass surfaces with Visqueen film (i.e., the Visqueen/paper/Visqueen system in that film was manually peeled off and removed). The polymer film sample included three sub-layers, one being a central medium density polyethylene core layer. The core layer was made of a foam. Two outer skin layers of low density polyethylene sandwiched the core layer. The total film thickness ranged from about 110 to 120 microns. The skin layers were formulated with erucamide in an amount of 500 ppm. This is the polymer film 200 shown in FIG. 2 and has the tensile modulus of elasticity, surface roughness Ra and peel values described herein. A Generation 8 size glass sheets of 100 for each interleaf type were packed in separate crates then loaded onto the finishing line. Samples 1 and 2 were run during different weeks. Results are listed in Table 1 below.

[0058] Defects are measured by passing a strobing light onto the glass and locating the defects using a scanning camera. Controllable yield is defined as the percentage of glass sheets that passed the quality criteria among the total number of glass sheets tested.

Table 1

[0059] The lowest number of defects and best yields were achieved using manually peeled Visqueen film and the polymer film, whereas the worst yields were from clean Glassine paper.

EXAMPLE 2

[0060] One hundred (100) pieces of glass sheets (Generation 5.5) were packed separately with Glassine paper, Visqueen film, and with the polymer film 200, respectively. The polymer sheet 200 had the peel values, stiffness and surface roughness Ra described herein. The glass was then unpacked and washed. Surface defects were measured as described in Example 1. The results are shown in Table 2 below.

Table 2

[0061] Table 2 shows that the polymer sheet 200 had very good yield and low defect counts compared to the Glassine paper, comparable to that of the Visqueen material.

[0062] Many modifications and variations of the invention will be apparent to those of ordinary skill in the art in light of the foregoing disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than has been specifically shown and described.

Claims

1. A method of protecting glass sheets from scratching and stains comprising: positioning a polymer film on one of said glass sheets, wherein said polymer film has first and second surfaces and said first surface contacts a major surface of said glass sheet; and

2. The method of claim 1 wherein said polymer film comprises a central core layer of polymer foam and two polymer skin layers sandwiching said core layer, and said core layer and said skin layers are integrally formed with each other.

3. The method of claim 2 wherein said foam core layer includes cells having a size ranging from 0.5 to 20.0 microns.

4. The method of claim 3 wherein said core layer is formed of medium or high density polyethylene and said skin layers are formed of low density polyethylene, linear low density polyethylene or low density polyethylene copolymer.

5. The method of any of the preceding claims wherein said polymer film comprises a central core layer and two polymer skin layers sandwiching said core layer, said core layer having a stiffness represented by said polymer of said core layer having a tensile modulus of elasticity of greater than 80 kpsi, and said core layer, and said skin layers are integrally formed with each other.

6. The method of claim 5 wherein said core layer is formed of polyethylene terephthalate, polystyrene or polypropylene and said skin layers are formed of low density polyethylene, linear low density polyethylene or low density polyethylene copolymer.

7. The method of any of the preceding claims 1 to 4 wherein said polymer film comprises a central core layer of paper and two polymer skin layers sandwiching said core layer, and said core layer and said skin layers are integrally formed with each other.

8. The method of claim 7 wherein said skin layers are formed of low density polyethylene, linear low density polyethylene or low density polyethylene copolymer.

9. The method of claim 1 wherein said polymer film comprises a unitary layer of polymer foam formed of polyethylene, polypropylene, polystyrene or biaxially oriented polypropylene.

10. The method of claim 9 wherein said foam core layer includes cells having a size ranging from 0.5 to 20.0 microns.

11. The method of any of the preceding claims comprising repeating said steps of positioning said polymer film and placing an additional said glass sheet against said polymer film until a stack of said glass sheets is arranged with said polymer film between adjacent said glass sheets.

12. The method of any of the preceding claims wherein said first and second surfaces include a long chain fatty ester or a long chain fatty amide slip agent.

13. The method of claim 12 wherein said slip agent is present on said first surface and said second surface in an amount ranging from 100 to 5000 ppm.

14. The method of claim 13 wherein said slip agent is erucamide.

15. The method of any of the preceding claims wherein said polymer film has a total thickness range from about 60 to 300 microns.

16. The method of claim 15 wherein said polymer film comprises a central polymer core layer and two polymer skin layers sandwiching said core layer, and said core layer and said skin layers are integrally formed with each other, said polymer of said skin layers have a tensile modulus of elasticity ranging from 20 to 80 kpsi, said polymer of said core layer has a tensile modulus of elasticity of at least 80 kpsi.

17. The method of claim 16 wherein a thickness of each of said skin layers ranges from about 20 to 100 microns.

18. The method of claim 16 or claim 17 wherein a thickness of said core layer ranges from about 20 to 100 microns.

19. The method of any of the preceding claims wherein said surface roughness Ra ranges from 1.5 to 15 microns.

20. The method of any of the preceding claims wherein said first and second surfaces of said polymer film are characterized by a presence of undulations of a size much greater than the surface roughness Ra of said polymer film.

21. The method of claim 12 wherein said slip agent functions as a barrier layer to protect said glass sheets from staining by components in said skin layer.