GB2223234A - Glass interleaving, anti-staining, and black spot eliminating materials - Google Patents

Abstract

An interleaving material for minimizing scratching, staining, and black spotting of glass sheets is provided, comprising fine particles of polypropylene, and a solid, weakly acidic water soluble material. The acidic material may be present as fine particles having a particle size less than largest particles of polypropylene or a coating on the surface of the glass sheets. The coating may also include a water-soluble surfactant and a water-soluble polymeric resin.

Description

GLASS INTERLEAVING, ANTI-STAINING, AND BLACK SPOT ELIMINATING MATERIALS The present invention is directed to agents and compositions useful as interleaving materials. More particularly, the present invention is directed to materials useful for the separation of glass sheets and for the reduction and elimination of stains and black spots on glass products encountered in the glass and mirror industries and also to methods employing these materials.

In the storage and transport of glass sheets, the usual practice is to interpose a solid material between the sheets to minimize scratching or for blocking of the glass surfaces. Sheets of paper have been used for this purpose but this separation technique has proven time consuming and costly.

A less expensive and equally effective method of separating glass sheets has been employed in recent years. Particulate matter, both natural and syn thetic, is interposed between the glass sheets.

Thus, natural materials such as wood flour and beads formed from synthetic materials such as polyethylene, polystyrene, polyacrylates, and polymethacrylates have been employed as interleaving materials. While providing a less expensive and facile method for effectively separating and reducing scratching of glass plates, such materials and methods have been ineffective in solving some problems and have caused others.

One of these additional problems is the formation of stains on the surfaces of the glass due to reactions of cations present in the glass with carbonic acid. This is particularly noticeable under hot, humid conditions in which carbon dioxide and moisture in the air combine to form carbonic acid which reacts with the sodium present in the glass and attacks the glass surfaces.

To eliminate the surface staining of glass sheets, the use of weak acids either to coat the glass sheets, ccat the particulate matter employed as the interleaving material, or as particles in combination with particles of interleaving material has proven fairly effective as an anti-staining method.

Many of the synthetic materials employed as particles of intc tav-r!g raterial pArrtcurarly polymethyl metbacrylate, have resulted in "black spots".

This term s descriptsve of a surface blemish on the glass contiguous to a mirror surface. It results from a mechanical or chemical decomposition or partial solution of the material employed as the interleaving agent. Thus, under some time/temperature conditions the interleaving material tends to depo lymerize or otherwise decompose leaving traces of undesirable material on the glass surface. It is believed that the residual material on the surface of the glass interferes with the oxidation-reduction reaction that produces the mirrored surface.

Accordingly, a nonuniform mirror results. In some instances, the lubricant employed for the cutting surfaces of the glass cutting machines tends to soften, swell, and partially dissolve the interleaving material, also leaving an undesirable residue on the surfaces of the glass which results in black spotting.

In addition to the problems experienced in the plating industry from using polymethyl methacrylate, this interleaving material suffers from an additional shortcoming. The particles of polymethyl methacrylate are hard and relatively incompressible. In the particle sizes employed in the glass industry, they create a "slippery" sensation, such as the type of sensation observed with small ball bearings, when interposed between two hard surfaces. Accordingly, they create a hazard in the work place when spilled on the floor.

Technological developments in many fields which employ glass have created new demands for high purity glass. The purity refers not only to the composition of the glass but also to the surfaces thereof. Many industries are seeking uniform and flawless surfaces.

The present invention provides a material which is effective to provide suitable interleaving and to significantly reduce or eliminate staining due to surface etching and black spotting during the mirroring process. The present invention is also directed to methods of employing such interleaving/anti-staining material.

The interleaving material of the present invention includes fine particles of polypropylene and a solid, water-soluble, weakly acidic material. The latter component may be present in the form of fine particles itself, or may be present as a coating on the glass sheets. The acidic material is preferably an organic acid, particularly a carboxylic acid, and most preferably one having more than one carboxylic acid functional group present in the molecule. An example of a preferred weakly acidic substance is the dicarboxylic acid adipic acid.

When the weakly acidic material is present as particles, it is employed at a concentration in the range of 75% to 25%, by weight, based on the total weight cf the snte,leav.r.g coluposition while the polypropylene particles are present in a concentration of 25% to 75%, by weight.

The polypropylene particles which form part of the present invention are quite effective as an interleaving agent in preventing scratching. Because of the preferred shape of the polypropylene particles (preferably spherical or ellipsoidal) and the small tendency of polypropylene to adhere to other substances, such particles exhibit a natural lubricating or release quality desirable in an interleaving material. That is, the glass sheets can move freely with respect to one another or can be easily separated.

In being more compressible than the currently employed polymethyl methacrylate, the polypropylene particles also reduce hazards in the work area. This polymeric material is also very stable with respect to time and the temperatures which may be encountered under glass storage and transport conditions. It is also resistant to all but the most severe temperature and humidity conditions. This material demonstrates little tendency to degrade or depolymerize and, as a result, black spotting during mirroring, under actual process conditions, has been reduced by 50% to 100% (since neither polypropylene nor any possible decomposition products show any tendency to adhere to glass sraccs). The polypropylene also shows little tendency to soften or dissolve in solvents or lubricants typically employed in the glass or plating industries.

The polypropylene particles or mixture of polypropylene and weakly acidic particles are relatively inexpensive and are as simple to use as polymethyl methacrylate compositions currently employed. The means of application and the apparatus used therefor do not need to be varied and no more than minor adjustments need to be made. When overspray onto the plant floor occurs, the slippery sensation experienced with polymethyl methacrylate materials is not demonstrated. The interleaving/anti-staining materials of the present invention are also easily removed from glass surfaces with simple washing procedures.

The present invention provides an interleaving material for minimizing scratching, staining and black spotting of glass sheets, comprising fine particles of polypropylene and a solid, weakly acidic water soluble material. The acidic material may comprise fine particles having a particle size less than the largest particle size of the polypropylene particles. The acidic material may comprise a coating on the surface of the glass sheets. The interleaving material is, based on the total weight of all particles, present in a coKce,il+ration of 25% to 755, by weight, and the particles of the acidic material are present in a cQncentr -ion of 75f- t 25%, by weight. The interleaving materials may include particles of polypropylene and particles of acidic material each present in concentrations of 50%, by weight. The interleaving material may have polypropylene particles with particle sizes corresponding to the range of 30 to 200 U.S. standard mesh. The acidic material which may be used may comprise a carboxylic acid which may be a dibasic carboxylic acid, e.g., adipic acid. The polypropylene particles may be substantially spherical or ellipsoidal and the interleaving material may also include a water-soluble surfactant.The surfactant may comprise a non-ionic surfactant from the group consisting of alkylphenoxypoly(ethyleneoxy)ethanols, alkylphenoxypoly(propyleneoxy)ethanols, alkylpoly (ethyleneoxy) ethanols, alkylpoly (propyleneoxy) ethanols and ethoxylated acetylenic diols. The interleaving material may further include a water-soluble polymeric resin which may comprise a polyolefin glycol, an alkoxypolyolefin glycol, or a polyvinyl pyrrolidone.

The present invention also provides an interleaving material comprising fine particles of polypropylene and a solid, weakly acidic water soluble material with an acidic material which may comprise fine particles having a particle size less than the iargest particle size of the polypropylene particles.

The polypropylene particles of this interleavinq riâvêri l may be substantialiy spherical or eilipsoidal and have particle sizes in the range of 30 to 200 U.S.

standard mesh, with the particles of acidic material comprising adipic acid particles. The particles of polypropylene and adipic acid may be present in about equal amounts by weight.

The present invention also provides an interleaving material for minimizing scratching, staining, and black spotting of glass sheets comprising fine particles of polypropylene and a solid, weakly acidic water soluble material, wherein the acidic material may comprise a coating on the surface of the glass sheets. The polypropylene particles are substantially spherical or ellipsoidal and have particle sizes in the range of 30 to 200 U.S.

standard mesh. The coating of adipic acid may further include a nonionic water-soluble resin selected from the groups consisting of alkylphenoxypoly (ethyleneoxy) ethanols, alkylphenoxypoly(propyleneoxy)ethanols, alkylpoly(ethyleneoxy)ethanols, alkylpoly(propyleneoxy)ethanols, and ethoxylated acetylenic diols, along with a water-soluble polymeric resin selected from the group consisting of a polyolefin glyco', an alkoxypolyolefin glycol, and a polyvinyl pyrrolidone.

The present invention also provides an article of manufacture comprising a plurality of spaced apart glass sheets, the surfaces of the sheets having a coating thereon comprising a weakly acidic materal; and particles of polypropylene interspersed between and separating the glass sheets. The article of manufacture may include polypropylene particles which have particle sizes corresponding to the range of 30 to 200 U.S. standard mesh screen. The weakly acidic material may comprise a dibasic carboxylic acid, e.g., adipic acid. The coatings of the article of manufacture may include a water-soluble surfactant and may include a water-soluble polymeric resin.

The present invention is directed to an effective interleaving material which substantially reduces or eliminates blemishes on the surface of sheet glass from typical sources of such surface marring.

Thus, the present invention serves to eliminate surface stains caused by chemical etching or black spots related to polymer abrasion and deposit on the glass surface as well as mechanically caused scratches.

The present invention employs compositions which include fine particles of polypropylene and a weakly acidic material. As used herein, the term "'weakly acidic material" refers to those materials which dissociate only slightly in an aqueous liquid to form an acidic solution. Such weakly acidic materials may be prevent also as fine particles or may be used as a coating on the glass s'irfaces.

Polypropylene appears to be the ideal material for use as an interleaving agent. As a fine particulate powder, polypropylene is at least as effective, if not more effective, as any particulate interleaving material currently used to eliminate surface scratching. In addition, having a greater compres sibility, the particles of polypropylene show significantly less tendency to either abrade or crush compared to polymethyl methacrylate, the primary material used today as an interleaving agent. In addition, polypropylene is not readily dissolved or swollen in solvents currently used in glass cutting apparatuses. Polypropylene also shows a significant resistance to decompose or depolymerize under adverse time/temprature conditions. Accordingly, another source of adverse residue deposition is eliminated.

In addition, the low modulus of compressibility pro-.

vides the interleaving material with a spongy characteristic. Thus, powdered polypropylene does not have the slippery sensation (as in the sense of being like miniature ball bearings) of harder polymethyl methacrylate which creates a hazard in the work place when overspill occurs. As a result, there is little tendency for residue to be deposited or black spots to appear on the surface of the glass during the mirroring process. Polypropylene particles are also easily .e..ovcd by washing with only water or with aqueous detergent solutions because of the lack of adhesion of polypropylene and its low density, particularly when compared with the most widely used interleaving agent, polymethyl methacrylate.

Although any type of polypropylene may be employed, preferred are polypropylenes having softening points above 93.3 C (200"F), preferably above 148.9"C (300 F). Stereo-regular, for example isotactic polypropylene, is preferred. Selection of particle size is of some importance in the present invention, particularly the particle sizes of the polypropylene.

If the particle sizes are too large, mechanical handling suffers. Material which is suitable is that which has particle sizes passed by a 30 mesh (U.S.

standard) sieve and retained by a 200 mesh sieve (500 to 74 microns). Preferably, the particle sizes should be between 40 to 120 U.S. standard mesh (354 to 125 microns).

Particularly preferred are those polypropylene particles which have particle size ranges in which greater than 90% of the particles are retained on a 100 U.S. standard mesh sieve. Polypropylene particles having the following composition, based on particle sizes as measured by U.S. standard mesh sizes, have proven effective.

Sieve Size (mesh) % Particles Retained ~ Ut - A 40 0.4 - 1.8 iO it .3 - 97. 2 200 0 - 2.3 The narrowness of the particle size distribution range provides a uniform separation of the glass plates and also reduces focal points which are potential fulcrums for points of glass fracture. The pre ferred form of the polypropylene particles employed in the present invention is spherical or elliptical.

Flakes of polypropylene are not as effective as the preferred particle form. A material particularly suited to the present invention is spherical or elliptical polypropylene particles available from Himont Corporation, designated as 6323PM or 6523PM.

Suitable for use in the present invention is any weakly acidic material which exists as a solid in the temperature range in which it is expected that glass plates will be stacked, stored, or shipped. Sheet glass is frequently stacked shortly after manufacturing. Typically, the temperature of the glass surface is about 71.1"C (160"F) or less.

The weakly acidic material should be moderately soluble in water and capable of forming an aqueous acidic solution in which the acid is slightly dissociated. It is preferred that the weakly acidic material not be so soluble in water that under high humidity conditions it dissolves readily and can then 5e lost from the glass surface by runnfng Of.

Although inorganic acid salts, such as ammonium chloride, may be used, organic acids, particularly carboxylic acids and preferably dibasic carboxylic acids, especially dibasic aliphatic carboxylic acids, are employed in the present invention. The carboxylic and dicarboxylic acids which are preferred have 3 to 10 carbon atoms. This generally provides a suitable solubility in water, particularly of the preferred dibasic aliphatic carboxylic acids. Examples of the latter are succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, citric acid, ascorbic acid, fumaric, oxalic acid, and maleic acid. Adipic acid is most preferred because of its solubility, dissociation constant, and cost. When employed in particle form, food grade adipic acid is preferred, such as that available from E. I. Dupont de Nemours, Inc.

In addition to the carboxylic acids, preferably dicarboxylic acids of the sort described above, hydroxy acids, particularly hydroxy dicarboxylic acids, may also be used. In addition, mixtures of suitable acids may be employed. sThe weakly acidic material may be employed either as (1) fine particles in a mixture with the polypropylene particles or (2) a coating applied to the surface of a glass sheet. The Present invention, practiced in accordance with either (1) or (2), provides equally effective reduction in scratching and black spotting and both (1) and (2) are superior to the polymethyl methacrylate systems.

However, although both (1) and (2) provide excellent protection against alkali metal carbonate staining, the use of a coating containing a weakly acidic material proves particularly effective in eliminating this problem.

When used in particle form with the polypropyl ene particles, the particle size of the weakly acidic material should be no larger than the largest parti cl-es of polypropylene. Preferably, the weakly acidic material should be in crystalline form and a majority (90%) should pass a 120 U.S. standard mesh screen.

Whether used as a coating or in particle form, the weakly acidic material should be used at a coverage rate of 75.4 to 161.5 mg/m2 (7 to 15 mg/ft2) of glass surface area, preferably 86.1 to 107.6 mg/m2 (8 to 10 mg/ft2) of glass surface area. When in particulate form, the weakly acidic material may be combined with particulate polypropylene to form a mixture of particles in which the weakly acidic material is present, based on the total weight of the composition, in a ccncentration of 25% to 75%, and the polypropylene material is present in an amount, by weight, of 75% to 25% by weight. Most preferred, particularly when adipic acid is employed, is a mixture of 50%, by weight, of weakly acidic material particles and 50%, by weight, of polypropylene particues.Such a mixture is applied to the glass at a concentration of 161.5 to 215.3 mgs of mixture/m2 (15 to 20 mgs of mixture/ft2) of glass surface. Although higher concentrations of polypropylene powder may be employed, results achieved with the specified concentrations are satisfactory and no additional benefits accrue from the use of higher concentrations of polypropylene powder. On the contrary, higher concentrations are merely wasteful of the material.

When the weakly acidic material is employed as a coating on the surface of the glass, it is typically applied to the surface in the form of an aqueous solution. Preferably, the solution contains a watersoluble surfactant. The surfactant assists in dissolving and dispersing the weakly acidic material and any other components employed in the coating solution, such as a water-soluble polymer (discussed below). Suitable for use as the surfactant in the coating solution are water-soluble nonionic surfactants having above room temperature cloud points.

They are preferably of the type which are derived from the reaction of fatty alcchol hydrophobes with ethylene oxide, propylene oxide, or mixtures of such oxides, preferably ethylene oxide. Examples of preferred nonionic surfactants are water-soluble alkylphenoxypoly(ethyleneoxy)ethanols (ethoxylated alkylphenols), alkylphenoxypoly(propyleneoxy)ethanols, alkylpoly (ethyleneoxy) ethanols (ethoxylated ^7^èlAtic alcohols), aikylpoly(propylcneoxy) ethanols, and ethoxylated acetylenic diols.Most preferred are the alkylphenoxypoly(ethyleneoxy)ethanols and the al kylphenoxypoly (propylenboxy) ethanols formed from C10 to C15 fatty alcohol hydrophobes and having mole ratios of combined ethylene (propylene) oxide to hydrophobe varying between 7 to 40, preferably 9 or 10.

Ethoxylated acetylenic diols also constitute a preferred group of nonionic, water-soluble surfactants. Examples of preferred surfactants include nonylphenoxypoly(ethyleneoxy)ethanol, available commercially under the trademarks TritonTM N-101 (from Rohm & Haas Chemical Company) and GAFe C0630 (from GAF Corporation), and a decyne diol hydrophobe ethoxylated with 65%, by weight, ethylene oxide (available under the trademark Surfy-nol 465 from Airco Chemicals & Plastics). Surfactants which are preferred in the present invention are those which are capable of significantly reducing the surface tension of water. Thus, suitable surfactants should be capable of reducing the surface tension of pure water, 72 dynes/cm2, to no higher than about 40 dynes/cm2.

When polypropylene particles are employed with a glass coating containing a weakly acidic material, which coating is applied to the surface of a glass as a solution primarily containing the weakly acidic material and a surfactant, aqueous solutions are employed whiCh contain, on a weight basis, trom 0.05% to 0.5 surfactant, based on the total weight of solution. Pèferably, the solution contains, by weight, 0.1% to 0.3% surfactant, most preferably about 0.15% surfactant.The concentration of weakly acidic material is determined by the coverage rate discussed above and the maximum solubility of the weakly acidic material in the surfactant-containing solution For the preferred adipic acid, and for many of the other preferred acidic materials, a concentration of no more than about 5%, by weight, based on the total weight of solution, is suitable.

Although a coating or layer of adipic acid applied to a glass surface with an acid/surfactant solution, as described above, is quite effective in providing intimate contact between the acid and the surface of the glass, a more effective means of creating intimate contact between the weakly acidic material and the glass surface is achieved by providing a weakly acidic material-containing layer on the surface of the glass which comprises a continuous film on the glass surface which serves as a matrix securing dispersed acidic material. This film is formed not only from the weakly acidic material and surfactant, discussed above, but also includes a water-soluble or partially water-soluble polymer, such as a polymeric resin.The continuous film provided. by inclusion of a water-soluble pol"r'.er assures a more uniform coating with the absence of any dis~or.tinuity or holes in the coating where moisture could penetrate to the surface of the glass. The choice of water-soluble polymer is determined by a number of factors. Any water-soluble polymer which does not adversely react with the weakly acidic material or surfactant employed is suitable for use in the present invention providing that it is soluble in the aqueous solution employed at the temperatures and within the concentration range used with the application apparatus employed.Thus, the water-soluble polymer should provide a continuous film tq the coating, such film continuity being obtained with concentrations of components in the aqueous solution which provide concomitant viscosities that are suitable to the apparatus used to apply the coating solution to the surface of the glass.

Preferred as the water-soluble polymers suitable for use are polyolefin glycols and alkoxypolyolefin glycols, such as polyethylene glycols and methoxypolyethylene glycols (collectively referred to under the trademark CarbowaxTMs, and polyvinyl pyrrolidone. When polyethylene glycol is employed as the water-soluble polymer resin in the present invention, molecular weights between 3,500 to 20,000 are suitable, with molecular weights of 4,000 to 10,000 being preferred and a molecular weight of about 8,000 being most preferred. iMen polyvinyl pyrrolidone is the water-soluble polymeric resin chosen, polymers having K factors (a property related to viscosity) of 15 to 90 are suitable.This corresponds to number average molecular weights of 10,000 to 36,000. The polymers with lower K factors tend to be tackier than those with higher K factors.

It is generally preferred not to use materials which are particularly tacky. However, polyvinyl pyrrolidones which have lower K factors are generally more soluble, a desirable characteristic.

Accordingly, the choice of a particular water-soluble polymer will depend on the apparatus, other components, and the conditions under which it is being used. However, those polyvinyl pyrrolidone polymers having lower K factors, i.e., close to 15, are generally preferred.

The weakly acidic material/surfactant/watersoluble polymer compositions are generally formulated so as to provide the coverage rate indicated above for weakly acidi material of 75.4 to 161.5 mg/m2 (7 to 15 mg/ft2) of glass surface area, preferably 86.1 to 107.6 mg/m2 (8 to 10 mg/ft2) of glass surface area. On a dry basis, the weakly acidic material constitutes, by weight of the entire dry composition, 20% to 60%, preferably 40% to 45%. The water-soluble polymer is present in an amount, by weight, of 10% to 50%, preferably 40% te 45%. The surfactant is generally present in an amount, by weight, of 6t to 20%, preferably about 13%.The components of the weakly acidic materiai/surfactant/water-soluble polymer are combined with sufficient water to prepare an aqueous solution comprising 1% to 5%, by weight, of the mixture.

Although both the surfactant and polymeric resin have been described above as water-soluble materials, it is possible to use surfactants and polymeric resins which are not soluble in water in the present invention, if an appropriate solvent were employed.

Suitable solvents include alcohols, ethers, esters, ketones, and aqueous solutions containing one or more of these solvents. However, it is preferable not to use such solvents because of the hazards of flammability and toxicity which they present and the excellent results achieved with water-soluble materials and water as a solvent.

Treatment of the glass surface to deposit thereon a coating of weakly acidic material, mixture of weakly acidic material and surfactant, or mixture of weakly acidic material, surfactant, and watersoluble polymer resin may be accomplished by any conventional technique. Typically this is done by spraying one of these acidic solutions on the surface of tb: glass and preferably when the glass is at a temperature which is above room temperature, particularly a temperature which results in rapid evapora tion of water fo the aqueous solution, for example; a glass tmeare of 5 to 850C.

The mixture is applied to the glass surface by means of a pneumatic, sonic, or pressure sprayer, the particular device used depending on the particular acid solution employed and in particular its viscosity. The spraying devices employed preferably pro duce a droplet size of 20 to 40 microns. It is important that the nozzle of the device not be easily clogged but if this occurs that the condition be easily resolved. Because of somewhat higher viscosities encountered when polyvinyl pyrrolidone is employed as the water-soluble polymeric resin, a pressure sprayer is preferred because of the higher pressures which may be achieved.

Although the polypropylene particles may be sprayed onto the glass surface any time after a solution of weakly acidic material has been applied, it is preferred to allow sufficient time for the solvent to evaporate.

The solution of weakly acidic material optionally, but preferably, containing a nonionic surfactant and a water-soluble polymeric resin may be prepared by any suitable conventional technique. Thus, the individual components may be added sequentially, with stirring, to the aqueous solvent. Alternatively, individual solutions of the separate components may be prepared and then combined. Preferably. when a sl- ton is to be prepared containing either surfactant or, most preferably, a mixture cf surfactant and water-soluble polymeric resin, in addition to the weakly acidic material, the dry components may be combined and mixed to obtain a uniform mixture.When an acid/surfactant/water-soluble polymer is to be prepared, it is preferable to initially blend the first two mentioned components together and thereafter add the water-soluble polymeric resin. The dry mixture is then blended using, preferably, a sigmatype mixer which provides sufficient shear to break up lumps which may form. The dry mixture may then be stirred into the aqueous solvent or may be placed in moisture-type containers.

The polypropylene particles or mixture of particles of both polypropylene and weakly acidic material may be applied to the surfaces of the glass sheets by conventional techniques using known apparatuses.

Typically, powder applicators are employed. In particular, any apparatus may be used which results in a uniform weight of material being applied over the surface of the glass. The preferred apparatus is the type of device used in the printing industry, known as an electrostatic sprayer.

Example 1: Interleaving With Particulate Mixture of Polypropylene Powder and Adipic Acid: In a double helix ribbon mixer was placed 340.2 kg. (750 lbs.) of food grade adipic acid (obtained from E. I. Dupont de Nemours, Inc.). An equal weight of poly- propylene particles (obtained from Himont as product no. 6323PM) was placed in the mixer and the mixture blended for 30 minutes. While still mixing, the adipic acid/polypropylene mixture was slowly delivered to a rotating screening mechanism (Sweko) having a two deck screen area. The first deck screen was provided with a 30 U.S. standard mesh screen having openings not less than about 5n0 microns. The material passing through the 30 U.S. standard mesh screen was transferred to the upper surface of a 200 U.S.

standard mesh screen. All material not retained by the screen was discarded.

In applying the mixture of particles of polypro pylene and adipic acid to the surfaces of glass sheets, a portion of the mixture having particle sizes between 30 and 200 U.S. standard mesh screen was transferred to an OxyDry electrostatic sprayer.

The sprayer was actuated to deposit a uniform layer of particles of polypropylene and adipic acid on the surfaces of glass sheets.

An "accelerated test" was employed to determine the resistance of the glass to alkali metal carbonate staining The conditions employed in the test approximate a six month to one year exposure of glass stored under typical warehouse conditions. Several sheets of glass were stacked vertically in a chamber having a relative humidity of 90% and a temperature of 32.2eC (90"F). The glass was examined at 24 hour intervals for signs of staining. The glass showed no indication of staining after a period of 3 days in the chamber, a time considered equivalent to a period of six months to one year under normal conditions.

A second test was performed to determine the effectiveness of the mixture of particles of polypropylene and adipic acid in preventing scratches or abrasions. The test was conducted by stacking several coated pieces of glass on a bed which rotates on a 360 off-center cam. Such rotation simulates handling of glass sheets under normal conditions in which glass is rubbed against an interleaving agent. The sheets were examined every 24 hours to detect surface scratches as well as abrasion of the interleaving agent on the surface of the glass. After a period of 7 days no scratches or abrasion marks were discernible.

Example 2: Treatment of Glass Sheets With Adipic Acid Solution and Interleaving of Adipic Acid Coated Sheets With Polypropylene Particles: To 196.6 kg. (433.4 lbs.) of food grade adipic acid (obtained from E. I. Dupont de Nemours, Inc.) was added 60.4 kg. (133.2 lbs.) of Surfy-nole 465 (available from Airco Chemicals and Plastics) and 196.6 kg. (433.4 lbs.) of polyvinyl pyrrolidone resin having a K factor of 90 and a number average molecular weight of 35,000 (available from GAF Corporation).

The acid, surfactant, and water-soluble polymer resin were mixed in the dry state using a sigma mixer. The material was then packaged into 11.3 kg. (25 lbs.) containers havina a moisture barrier. A portion was set aside to prepare a solution for application to glass surfaces.

A 1.68%, by weight, aqueous solution of the above acid/surfactant/water-soluble polymer resin was prepared by combining 0.064 kg (5s1-l lbs.) of mixture and 3.7 kg. (8.25 isis.) of water. The solution was then transferred to a pressure type sprayer. The sprayer was adjusted to produce a 20 micron droplet and the solution was applied to the glass at a glass temperature of 68.30C (155'F). After the solution had dried (about 20 seconds) to a film on the glass surface, polypropylene particles were applied to the surface of the glass sheet. The type of polypropylene particles and sizes thereof described in Example 1 were employed and the manner of application was also the same as that described in Example 1.

The accelerated humidity and abrasion tests were performed as in Example 1. The same results were observed.

Claims (24)

1. An interleaving material for minimizing scratching, staining, and black spotting of glass sheets, comprising: fine particles of polypropylene, and a solid, weakly acidic water soluble mate- rial.

2. An interleaving material according to claim 1 wherein said acidic material comprises fine particles having a particle size less than the largest particle size of said polypropylene particles.

3. Ar: interleaving material according to claim 1 wherein said acidic material comprises a coating on the surface of said glass sheets.

4. An interleaving material according to claim 2 wherein1 based on the total weight of all particles, said particles of polypropylene are present in 3 conscr.tration of 25% to 75%, by weight, and said particles of said acidic material are present in a concentration of 75% to 25%, by weight.

5. An interleaving material according to claim 4 wherein said particles of polypropylene and said particles of acidic material are each present in concentrations of about 50%, by weight.

6. An interleaving material according to claim 1 wherein said polypropylene particles have particle sizes corresponding to the range of 30 to 200 U.S.

standard mesh.

7. An interleaving material according to claim 1 wherein said acidic material comprises a carboxylic acid.

e. An interleaving material according to claim 7 wherein said carboxylic acid is a dibasic carboxylic acid.

9. An interleaving material according to claim 8 wherein said dibasic carboxylic acid is adipic acid.

10. An interleaving material according to claim 1 wherein said polypropylene particles are substantially spherical cr ellipsoidal.

11. An interleaving material according to claim 3, further including a water-soluble surfactant.

12. An interleaving material according to claim 11 wherein said surfactant comprises a nonionic surfactant from the group consisting of alkylphenoxy poly (ethyleneoxy) ethanols, alkylphenoxypoly(propyleneoxy) ethanols, alkylpoly (ethyleneoxy)ethanols, alkylpoly (propyleneoxy) ethanols and ethoxylated acetylenic diols.

13. An interleaving material according to claim 11, further including a water-soluble polymeric resin.

14. An interleaving material according to claim 13 wherein said water-soluble polymeric resin comprises a polyolefin glycol, an alkoxypolyolefin glycol, or a polyvinyl pyrrolidone.

15. An interleaving material according to claim 2 wherein said polypropylene particles are substantially spherical or ellipsoidal and have particle sizes in the range of 30 to 200 U.S. standard mesh, said particles of acidic material comprise adipic acid particles, and said polypropylene particles and said aspic acid particles are present in about equal amounts us by weight.

16. An interleaving material according to claim 3 wherein said polypropylene particles are substantially spherical or ellipsoidal and have particle sizes in the range of 30 to 200 U.S. standard mesh, and said coating of adipic acid further includes a nonionic water-soluble resin selected from the group consisting of alkylphenoxypoly(ethyleneoxy)ethanols, alkylphenoxypoly(propyleneoxy)ethanols, alkylpoly (ethyleneoxy) ethanols, alkylpoly (propyleneoxy) ethanols, and ethoxylated acetylenic diols and a water-soluble polymeric resin selected from the group consisting of a polyolefin glycol, a alkoxypolyolefin glycol, and a polyvinyl pyrrolidone.

17. An article of manufacture comprising: (a) a plurality of spaced apart glass sheets, the surfaces of said sheets having a coating thereon comprising a weakly acidic material; and (b) particles of polypropylene interspersed between and separating said glass sheets.

18. The article ol manufacture according to claim 17 wherein said polypropylene particles have particle sizes corresponding to the range of 30 to 200 U.S. standard mesh screen.

1. The article of manufacture according to claim 17 wherein said weakly acidic material comprises a dibasic carboxylic acid.

20. The article of manufacture according to claim 19 wherein said dibasic carboxylic acid is adipic acid.

21. The article of manufacture according to claim 17 wherein said coating further includes a water-soluble surfactant.

22. The article of manufacture according to claim 17 wherein said coating further includes a water-soluble polymeric resin.

23. The interleaving material substantially as described in the specification.

24. An article of manufacture substantially as described in the specification.