CA1074953A - Parting material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Packaging glass sheets employing a parting material between contiguous sheets which is a mixture of good flour and as ammonium salt, an aqueous solution of which salt will not form as insoluble compound when in contact with gases normally found in the atmosphere.

Description

10~7~953 The present invention relates generally to the packing, shipping and storing of glass sheets. More particularly it has to do with protecting the surfaces of such sheets while they are in stacks or packages, and with an improved form of specially treated and si~ed particulate separating means for that purpose.
Perhap~ one of the greatest problems encountered in the shipping and storing of glass sheets, while they are packed or stacked in fsce-to-fsce relation, is that of retaining the origi-nal color, clarity, transparency and surface quality of the glass in the package or stack. To do 80 successfully requires that the contiguou~ glass surfaces be protected at all times and for ex-tended periods from both physical injury and chemical change.
With thi~ in mind, it has been common practice for many years to use interleaved sheets of paper as separators between stacked glass sheets and, more recently, alternative type~ of separator means, including various particulate packing materials, haYe been suggeated. Similarly, the application of various kinds of acida and acidic substances have been proposed as a means of reducing or preventing staining, etching and other chemically in-1 20 duced in~ury to or discoloration of glass surfaces.
However, none of the separator or packing means avail-able to date, provides the kind of glass and surface quality in-surance that must be had in a day and age that employs commercial glass in such critical environments as sight openings, mirrors, lenses and the like for everything from automobiLes to scientific instruments.
The present invention, on the other hand, makea it pos-sible to protectively pack and store, and/or ship, glass sheets in stack~ and under extreme and adverse handling, atmospheric and environmental conditions, for extended periods of time, with a minimal risk of physical in~ury or deterioration in surface .

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i(~'74'3S3 quality and it does this by a method o~ stacking that utilizes a mixture of relatively inert and of chemically active particulates to adequately protect and separate the glass sheets in a special stacked assembly.
- Broadly speaking, the particulate separator means, or parting material, employed in the method and in the stacked unit of the invention is a mixture of wood flour with a particular ammonium compound and it is a primary ob~ect of the invention to utilize a mixture of this character in providing a different, efficient and economical way of protecting the surfaces of glass sheets.
Another ob~ect is to accomplish this general aim by providing the essentially binary separator mixture in selected proportions and particle sizes for use in the method and in particular densities between the glass sheets of the stacked unit ` of the invention.
, Another object is to provide an improved procedure for ' applying the particular binary mixture of the invention to, and for maintaining it on, the glass surfaces to be protected for an adequate length of time.
According to the present invention, there is provided a ~- method of packaging glass sheets which includes the step of electrostatically spraying a surface of the sheets to be packed with a layer of parting material having a moisture content of ~-less than 3 per cent, and comprising a mixture of from 25 to 60 parts of wood flour, with from 40 to 75 parts of an ammonium ' salt, a solution of which in water will not form an insoluble compound when in contact with gases found in the atmosphere, in particle sizes at least 75 per cent of which are between 100 and 270 mesh, and in a density in excess of 3 milligrams per square foot.

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A~so, according to this invention, there is provided a parting material for use between contiguous glass sheets in packing the same, comprising a mixture of from 25 to 60 parts of hard wood flour with from 75 to 40 parts of an ammonium salt that in a water solution will not form insoluble compounds when in contact with atmospheric gases, in particle sizes at least 75 per - cent of which are between 100 and 270 mesh.
In the accompanying drawings:
Fig. 1 is a plan view of a part of a packing line along which glass sheets are passed and treated preparatory to packing or stacking them for shipment or storage;
Fig. 2 is a vertical, sectional view taken substantially along the line 2--2 in Fig. l, and illustrating the ~` operation of a special electrostatic spray applicator for the separator mixture;
Yy Fig. 3 is a fragmentary side elevation of a packing bench showing a shipping box in the process of having glass sheets stacked therein; and Fig. 4 is a fragmentary perspective view of a railway ¦ 20 car with a rack or buck load of large glass sheets stacked and held in position thereon.
Although not always the case, packing or storing of glass sheets ordinarily takes place very shortly after the glass Y has been produced and cut into sheets and at or near the end of a conveyor, such as indicated at 10 in Fig. 1, upon which sheets 11 v~ are being moved into the packing or storage area.
~; In accordance with the present invention, there is applied to at least one surface of each of a series of glass sheets that are to be stacked into shipping boxes or containers ~ 30 or onto .''~, ..... .

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~074953 transporting or storing equipment, a pflrticulate mixture of a protective separAting material and, as illustrated in Fig. 1, this may be done very effectively by passing the sheets 11 through an electrostatic spraying device 12 as they move from left to right along the conveyor 10.
Selection of a completely adequate protective separator mixture to be sprflyed in accordance with the invention is,of course, of primary importance from a number of standpoints.
First, such a mixture must be capable of adequately separating and protecting the glass sheet~, not only from physical in~ury S but also from chemical attack even under adverse climatic condi-tions; second, it must be of a character that is capable of being applied to the glass surfaces in a properly dispersed pattern of the required coating density; and, finally, it must be of a char-acter to remain on the glass and withstand and/or react accept-ably to any and all conditions that can be encountered, either during or after the coating of the glass and still be readily and completely removable whenever desired.
Against this demanding background, it was discovered ~ ~ 20 that by mixing wood flour with a particular kind of ammonium ,~ salt, there was provided a surprisingly more effective protective separator material that when properly compounded, si~ed and dis-tributed over the glass, gave remarkably good results over un-usually extended periods and under abnormally adverse physical circumstances and climatic conditions and yet, could be easily removed from the glass by a simple water wash.
The particular ammonium salt is and must be one of a :. :
select group of ammonium compounds, water solutions of which will ~ not form insoluble compounds when they come in contact with carbon S 30 or sulphur dioxide (the gase~ normally present in the atmosphere), or when they come in contact with sulphur triox$de or hydrogen sulphide (gases that are sometimes present in the atmosphere) - 4 - ~

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107~9S3 and which ammonium compounds include ammonium chloride, ammonium nitrate, smmonium sulphate and dihydrogen ammonium phosphate.
The flour employed should be of a hard wood, such as maple and ammonium chloride has given especially good results in actual practice as the member of the group of selected ammonium compounds to be used in the essentially binary mixture. Also, best results are obtained when the two essentisl ingredients of the mixture are employed in substantially equal parts, although within a plus or minus range of 3 per cent, there is no notice-able difference in performance.
Moreover, a greater reduction in the amount of wood - flour wieh a corresponding increase in the amount of the ammonium compound or salt can be resorted to if desired, or to meet spec-ial circumstances, without seriously affecting the efficacy of .
the mixture. For example, a ratio of 25 parts by weight of wood flour to 75 parts of the ammonium compound will adequately pro-tect the glass sheets against physical in~ury while effectively preventing staining even under extremely humid conditions. How-ever, a reduction of the amount of the ammonium salt in the mix-ture to a point below 40 per cent by weight, is not recommended.
Along with the proportions of wood flour to ammonium compound, the sizes of particles in the mixture must also be con-sidered; first, for their effect on the efficacy of the mixture for its intended purpose, but also in connection with the neces-sity of avoiding clogging and insuring efficient operation of the electrostatic spraying or applicating equipment and of reducing atmospheric dusting and equipment contamination therefrom.
Generally speaking, good results from all of these standpoints will be obtained when at least 75 percent of the particle sizes in the mixture are between S0 and 270 mesh. Thus, atl~factory ttn~es of Che detlred p~rtlcle ~lzet, with tcceptable .,.

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107~953 ranges of sizes above and below these, are set forth in the table below:
~ No. 40 mesh Less thao .5 per cent ; No. 50 mesh 2 - 2.5 per cent No. 70 mesh 11 - 14 per cent No. 100 mesh 29 - 31 per cent No. 140 mesh 19 - 23 per cent No. 200 mesh 11 - 15 per cent No. 270 mesh 5 - 6 per cent No. 325 mesh 3.5 - 5 per cent Smaller than 325 mesh 10 - 12 per cent And an example of desired and acceptable particle sizes in the mixture is:
No. 40 mesh .1 per cent No. 50 mesh 2.25 per cent No. 70 mesh 12.5 per cent No. 100 mesh 30 per cent No. 140 mesh 21 per cent No. 200 mesh 13 per cent .: 20 No. 270 mesh 5.5 per cent ~:
No. 325 mesh . 4 per cent Less than 325 mesh 11 per cent The mesh or particle sizes discussed throughout this , application are based on United States Standard Sieve numbers ~` and both the wood flour and the ammonium sale may be reduced to ; particles of the desired size either before or after mixing or :` blending them together or they may be purchased in the particle sizes desired. However, the preferred procedure is to first mix approximately equal parts by weight of wood flour and ammonium compound, as received in relatively larger commercially available particle sizes, in a rotary blender, and to then re-I duce the components of the resulting mixture to particle sizes :!

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:~ , 107~9S3 that are within the ranges given in the first table above, ina high speed hammer mill having stirrup-type hammers. This not only gives the particle sizes desired but, because of the high hest generated and the lsrge volumes of air used during milling, i the milled mixture i9 found tO be a highly dispersed product with a very low moisture content.
This low moisture content i8 an important factor be-cause wood flour, as obtained commercially, may contain around ~ ~ 8 per cent moisture, whereas the maximum acceptable moisture con-r. 10 tent for the protective separator mixture of the invention is ap-preciably less than 3 per cent and preferably is below 2 per cent.
The hammer milling operation makes it possible for the moisture content of the si~ed mixture to be brought down to around 1 per cent or less.
This magnitude of moisture content is entirely satis-factory, particularly when the mixture is to be used immediately .. . .
or stored ~n an airtight container but, as an optional further treatment, the particles of the dry mixture may be coated with a suitable moisture barrier such as hydrophobic silica. When this is done, it is important that the hot, dry protective mix-., ture be coated in increments rather than on a one-shot basis to insure maximum particle coating. For example, in coating 22 pounds of the mixture with 100 grams of hydrophobic silica, there may first be placed in a rotary blender, silica in an amount equal to 1 per cent by weight of the total working volume i., of the blender. To this may be added no more than one-quarter A ~ of the working capscity of the blender or one-quarter of the i mixture to be coated, and the whole blended or mixed for from 3 to 5 minutes. A second one-quarter increment is then added ~ 30 and blended for another 3 to 5 minutea, after which a third in-'r crement is added and mixed. Finally, the fourth increment is added and the whole mixed for 10 minutes.

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lV7~953 Once the binary protectlve mixture has been formulated, whether or not it6 particles have also been coated with a mols-ture barrier, it i9 fed to a~ appllcator, such as the device 12 of Fig. 1, for flpplication to glass sheets 11 moving therebeneath.
To insure adequate protection ln stacking the glass, it i8 neces-sary that the protective mlxture be applied with a high degree of particle dispersion and close particle proximity and, for best results, it is desirable to provide a uniform coating having a density of 20 milligrams per square foot, although coverages as low as 3.4 milligrams per square foot have performed acceptably and coverages of anywhere between 10 and 30 milligrams per square foot are commercially practicable.
Also, to prevent possible separation of the components of the mixture by reason of the density difference existing be_ tween them, it is recommended that the binary mixture of the in-vention be applied in the absence of a forced air carrier. Con-sequently, the u~e of an electrostatic spray applicator as here ~ contemplated, is a highly desirable, if not an essential, part of ,' :' , the coating procedure.
A proven device of this character is that shown at 12 in Figs. 1 and 2, which utilizes gravity to propel charged par-i ticles of the mixture to the moving glass sheets 11 passing therebelow. More specifically, with this device, particles 13 of the separator mixture are fed from a hopper 14 to an etched metering roller 15 and through ionized air in an electrostatic field induced by electronic tubes 16 onto the glass 11 below. A -very high degree of particle dispersion comes from the like charged particle repulsion which sets up a whirling cloud of ~ charged particles in the field and, under influence of gravity, ; 30 particles from the whirling cloud settle to the glass sheets and there form a close~ but highly dispersed, particle deposition pattern.

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, 10'~4~53 Wiper blades 17, in combination with the vertically adjustable metering roller 15, give precise metering control and the chamberized side frames 18 of the hopper 14 provide a curtain to confine airborne particles of the mixture. Further, the hopper 14 may be heflted to further, or to initially, or to again, reduce the moisture content of the mixture being fed when-ever the procedure being followed makes it necessary or desirable.
Finally, the electrostatic adhesion of the protective mixture settling onto, and 80 being deposited on, the moving glass sheets i9 significantly increased by precharging the sheets as they move into coating position. This may be done by the use of a charge bar 19 arranged as closely as possible to the glass and approximately six inches up stream from the device 12. The bar must be capable of providing a high static field on the glass passing through its field and the glass may be precharged either positively or negatively. Preferably, a negative field is em-ployed, but this is primarily because an equally high positive ~ field may cause sizzling or arcing.
`, Immediately after coating the glass sheets with the ~; 20 protective parting mixture in the manner above described, they may be removed from the conveyor 10 and stacked directly into a box 20 or the like on a bench 21 and without the packer having had to do anything about separating or protecting them. Never-theless, the packed glass will be adequately protected from phys-ical damage during packing and will also be effectively protected from both physical and chemical attack for a considerable period thereafter. The same thing is true with sheets that are to be packed, as on open bucks, for storage and, of course, glass sheets of any size and shape can be treated and handled in the same way.
For example, sheet~ 80 large that they must be packed on racks and shipped on open flat cars 22 (Fig. 4~, have been successfully coated, handled, packed and shipped in the manner described.

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10~4953 The following example describes more speclficfllly an early test procedure that hfls since been repeatedly employed in commercifll operation to successfully practice the invention:
Exsmple On the premise that the protective mixture of the in-vention must be made up of particle sizes that are large enough to resi~t being blown away by interfering drafts but small enough to have good electrostatic adhesion, equsl part~ by weight of ammonium chloride (Allied Chemical Company's standard crystal growth with 78 per cent of the particles +70 to +140 mesh) and maple wood flour (Conner Forest Industries Grade #95FG), as they were received from the suppliers, were taken as the starting materials. An additional 4 per cent by weight of the wood flour il was added to offset the fact that the wood flour contained more moisture than the ammonium chloride and these ingredients were mixed in a Patterson-Kelly blender by rotating the same, using the inten~ifying bsr, for 10 minutes.
The resulting mixture was then hammer milled in a mill ; having a rotor speed of 8,000 to 10,000 rpm, equipped with stirrup hammers and 1/16 inch separator screen (round hole), with the air intake adjusted for stable maximum feed rate and using full to one-half feed rate speed. The following mesh distribution was obtained: -I MeshRetained on Screen ! ~ No. 501.5 per cent , No. 7010.0 per cent No. 10025.1 per cent No. 14024.0 per cent No. 20018.5 per cent No. 2708.1 per cent ' No. 3255.2 per cent Pan 7.6 per cent "
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, Three sets of five 12 inch by 12 inch by 1/8 inch thick pieces of float glao-s were coated on ~heir top sides with the above mixture using the electrostatic spray applicator 1~
to provide uniform coatings containing 3.4 mg, 10.4 mg and 22.6 mg respectively. The fif~een coated glass s~mples were then placed in a high humidity chamber.
After twenty-nine day~, the two sets of 5 sheets that were coated with the 3.4 mg per square foot and 10.4 mg per square foot, were removed and inspection showed that no physical damage or staining had occurred.
Specifically, the initial in~pection showed the test sheets to be unbroken, loose and moderately weS but without standing water. The outside, uncoated, exposed glass faces were severely weathered, showing the typical alkaline leach that occurs with severely water attacked glass. The coatings were then removed from the individual sheets by plain water washing and inspection of the washed sheets showed no staining of, i5 scratches on or abrasion damage to what had been the coated sur-faces of any of them. After sixty days, the set of five sheets coated with the 22.6 mg per square foot were removed and in-spected in the same manner and with the same results.
From this, it was evident that a well dispersed coating of as little as 3.4 mg per square foot of the protective separator material of the invention will protect the coated glass from the . type of physical injury incidental to packing and storing and . .
will also provide at least thirty days protection against stain-ing, even under high cyclical humidity conditions. Nevertheless, ~ because of the possibility of some coating material being lost J' during handling, coatings of at least 10 mg per square foot and up to 20 mg per square foot, are being recommended although heavier coatings may also be used, depending on the particular ~ time, humidity and handling conditions expected to be encountered.
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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of packaging glass sheets which includes the step of electrostatically spraying a surface of the sheets to be packed with a layer of parting material having a moisture content of less than 3 per cent and comprising a mixture of from 25 to 60 parts of wood flour with from 40 to 75 parts of an ammonium salt, a solution of which in water will not form an insoluble compound when in contact with gases found in the atmosphere, in particle sizes at least 75 percent of which are between 100 and 270 mesh, and in a density in excess of 3 milligrams per square foot.

2. A method as claimed in claim 1, characterized in that said ammonium compound and said wood flour are mixed in a rotating blender using an intensifier bar, and said mixture is milled to the specified particle sizes in a high speed hammer mill equipped with stirrup hammers.

3. A method as claimed in claim 1, characterized in that said sheets are precharged, and said parting material mixture is fed to said electrostatic spraying through a heated hopper to reduce the moisture content thereof.

4. A parting material for use between contiguous glass sheets in packing the same, comprising a mixture of from 25 to 60 parts of hard wood flour with from 75 to 40 parts of an ammonium salt that in a water solution will not form insoluble compounds when in contact with atmospheric gases, in particle sizes at least 75 percent of which are between 100 and 270 mesh, and having a moisture content of less than 3 percent.

5. A parting material as claimed in claim 4, charac-terized in that said parting material has a moisture content below 2 per cent and the particles are coated with a moisture barrier.

6. A parting material as claimed in claim 5, characterized in that the moisture barrier is hydrophobic silica.

7. A method of producing a parting material for use between contiguous glass sheets, comprising mixing from 25 to 60 parts by weight of hard wood flour with from 75 to 40 parts of an ammonium salt that in a water solution will not yield insoluble precipitates when in contact with the atmosphere in a rotary blender, hammer milling said mixture to reduce the particles thereof to sizes at least 75 per cent of which are between 100 and 270 mesh, and then coating said particles with a moisture barrier.

8. A method as claimed in claim 7, characterized in that said particles are coated in increments with hydrophobic silica.