Treating glass sheets to heal vents that result in breakage during thermal treatment

This invention relates to applying a protective composition to the vicinity of a scored region of a glass sheet. The scored region is weakened and develops vents as a result of the scoring. Such vents are likely to develop breakage when the glass sheet is subjected to a thermal treatment associated with tempering. The application of the protective composition is made before the glass sheet is subjected to thermal processing by heating to above its strain point followed by rapid cooling below its strain point. The protective composition flows at a lower temperature than the glass sheet to which it is applied and is capable of healing the scored region during the thermal treatment to which the glass sheet is subjected after the application of said protective composition, thereby reducing the tendency of a vent to develop into glass breakage during said thermal treatment.

BACKGROUND OF THE INVENTION 
The present invention relates to treating glass sheets, and particularly 
refers to a method of reducing vents that result in breakage during 
thermal treatment commonly associated with tempering of glass sheets. 
Glass sheets are scored when they are cut and/or drilled to provide holes 
extending through the thickness of the glass. The scored regions usually 
have vents associated therewith. 
When glass sheets are tempered, they are heated to above the strain point 
of the glass, and even as high as a temperature approaching the glass 
softening point. After the glass attains a temperature sufficient for 
tempering, it is chilled rapidly. Stresses are established temporarily in 
the glass during this rapid cooling. These stresses may become so severe 
as to cause the glass sheet to fracture during the tempering operation. 
Glass fracturing during tempering is frequently observed in the vicinity 
of vents near those regions that are scored when the glass is cut, such as 
for drilling holes through the glass. 
Many fabricated products of glass require thermal treatment involving 
heating the sheet to between its strain point and its melting point as 
part of their fabrication. Among the many articles of commerce that 
require such thermal treatment are tempered and partly tempered windows, 
lenses, cover plates and automotive and other vehicle glazing closures, 
such as windshields, backlights, vent panes, And sidelights. 
Many movable glazing closures for present day vehicles must be drilled to 
produce holes that receive actuating rods associated with motors or cranks 
or other actuating mechanisms for opening and closing the glazing 
closures. Also, many of the articles of commerce fabricated from glass 
sheets require pre-cutting to irregular outlines. Unless considerable care 
is taken to seam the edges of the pre-cut sheets or to smooth the drilled 
portion formed into a hole, vents form along the edge of the glass 
periphery around the drilled portion. Such vents sometimes result in 
breakage during the thermal treatment. This breakage increases in 
frequency when the glass sheets so fabricated have a nominal thickness of 
5/32 inch (4 millimeters) and less instead of thicker glass sheets that 
were used for similar parts in the prior art. 
The prior art recognizes the fact that vents are associated with glass 
breakage during the tempering operation involving the thermal treatment 
described previously. It is well known that a glass sheet to be tempered 
adequately must be heated to a temperature sufficient to lower the 
viscosity of the glass sheet to enable it to flow sufficiently when 
chilled rapidly to establish a surface region stressed in compression 
surrounding an interior region stressed in tension. If the glass sheet is 
heated to a temperature above the optimum temperature for tempering, the 
glass sheet tends to lose its shape before it is subjected to the rapid 
cooling step. The lower the maximum temperature to which the glass sheet 
is heated, the greater is the likelihood that the glass sheet will 
fracture, particularly when temporary tension stresses arise during 
thermal treatment associated with tempering that cause a vent to develop 
into glass breakage. The need to heat each glass sheet to a maximum 
temperature within a narrow temperature range suitable for tempering is 
difficult to obtain in a commercial mass production operation where 
successive glass sheets precut to an identical outline of a production 
pattern differ in thickness and in distributions of stress around the 
scored regions so that a uniform time-temperature cycle applied to each 
glass sheet in a series of identical production pattern outlines does not 
necessarily avoid breakage of some of the treated glass sheets, 
particularly when the sheets are scored to produce holes through their 
thickness. 
In the past, several different techniques have been suggested for reducing 
the losses due to breakage originating in the vicinity of vents in 
thermally treated glass articles at scored regions. These include 
providing a different thermal treatment for the region likely to be the 
source of breakage than the remainder of the glass sheet undergoing 
thermal treatment to develop a temper. Some of these techniques include 
blocking or insulating heat locally to provide a reduction in damage in 
the blocked or insulated region; applying a heat absorbing and 
re-radiating material in the vicinity of the scored region to cause the 
glass to selectively heal any vents by increasing the local temperature in 
the vicinity of the scored region; using an insulating material to prevent 
heat loss and reduce temporary stress levels locally during a thermal 
treatment involved in tempering, and selectively applying a heat source 
locally to the vicinity of an aperture during thermal treatment involved 
in tempering. In addition, it has been recognized that edges, as well as 
drilled holes, become weakened as a result of scoring and cutting to 
divide larger sheets into smaller sheets, and these have been strengthened 
by selective seaming, by localized heating, by application of 
fluoride-containing compounds and by applying materials having a low 
coefficient of thermal expansion compared to the glass such as a glassy 
frit having a melting point less than the rest of the glass sheet along 
seamed edges that require strengthening. 
DESCRIPTION OF THE PRIOR ART 
The following patents were discovered in a novelty search for the present 
invention. 
U.S. Pat. No. 3,149,945 to Bertrand and Acloque and U.S. Pat. No. 3,251,670 
to Acloque disclose methods of differentially treating glass objects 
thermally by giving different areas of the same objects different thermal 
treatments. A selected zone is coated with a coating material such as a 
carbon black, mica flakes or aluminum paint prior to the thermal treatment 
so as to provide an area of less stress than other areas and thereby 
provide a limit to the portion of the glass sheet that breaks if the glass 
object is fractured. If one of a pair of adjacent areas that are thermally 
treated differently breaks, the break stops at the boundary with the other 
adjacent area and does not penetrate into the other adjacent area. 
U.S. Pat. No. 2,871,623 to Marini refers to a method of bending glass sheet 
differentially by applying a material that selectively absorbs and locally 
re-radiates heat (such as carbon black dispensed in a liquid vehicle) to 
enhance localized bending of a glass sheet while the sheet is mounted on a 
bending mold and subjected to thermal treatment to cause the glass to sag 
and be shaped. 
U.S. Pat. No. 3,551,234 to Britton discloses a method of repairing breaks 
in the glass linings of glass lined vessels by grinding an enlargement of 
a break, filling the enlarged hole and seat so formed with an epoxy or 
silicone cement and positioning a glass disc in the enlarged seat. 
U.S. Pat. No. 3,765,859 to Seymour strengthens a scored edge of a glass 
sheet by applying an edge protecting composition taken from the class of 
compositions having a low coefficient of expansion compared to glass 
and/or a heat transfer coefficient not greater than glass while 
maintaining the major surfaces inwardly of each of said surfaces on or 
adjacent the peripheral edge substantially free of the edge protecting 
composition. The glass with its edge so treated is then subjected to 
thermal treatment which results in a tempered glass sheet. 
U.S. Pat. No. 3,827,872 to Augustin and Long discloses a glass sheet 
tempering method in which the trailing edge of the glass sheet supported 
on a glass handling fixture is preheated prior to the glass sheet entering 
into a glass heating chamber to reduce local stress. 
U.S. Pat. No. 3,843,472 to Toussaint, Laroche, Schotty and Lambert 
strengthens an edge of a glass sheet by initially smoothening the surface 
and subjecting the smoothened edge to a chemical tempering treatment and 
to a rectifying treatment involving the application of fluoride ions to 
the edge for reducing or removing stress concentrations when the edge is 
subjected to tensile loading. 
U.S. Pat. Nos. 3,894,858; 3,988,136; and 4,000,997 to Rogers disclose 
various techniques of applying an insulating material such as a thin tape 
of asbestos or the like to provide heat insulation to locally reduce the 
rate of glass surface cooling in certain regions of a cathode ray tube 
during the process of salvaging such tubes. 
The following patents relate to the treatment of glass sheets having 
apertures through the thickness thereof and methods taken to reduce the 
incidence of fracture in the vicinity of the score mark that is involved 
with producing a hole through the thickness of the glass sheet. 
U.S. Pat. No. 3,352,655 to Barch and Posney discloses a method of 
controlling heat application to repair scored regions of a glass sheet 
that involves applying to the vicinity of the periphery of a hole drilled 
through the thickness of the glass sheet a composition suitable to 
selectively absorb and locally re-radiate heat to help heal the scores or 
vents formed in the vicinity of the periphery of each hole. Finely divided 
carbon and/or finely divided copper are ingredients of especially suitable 
compositions used for this purpose. 
U.S. Pat. No. 3,960,535 to Hamilton and Oelke discloses a method of bending 
glass sheets having holes extending through their thickness that involves 
directing localized heat to selective portions of the glass sheet that are 
apertured or provided with holes during the bending of the sheets to 
condition the sheet for subsequent tempering. 
U.S. Pat. No. 4,023,945 to Boaz discloses cutting a relief opening in a 
glass sheet from a hole drilled through the glass sheet thickness to an 
edge of the glass sheet and thereafter carrying out a conventional 
tempering operation. The relief opening is alleged to reduce the breakage 
which occurs in the glass sheet because of the stresses set up therein 
during the quenching portion of the glass tempering operation. 
SUMMARY OF THE PRESENT INVENTION 
The present invention relates to treating glass sheets to heal vents that 
result in breakage during thermal treatment by a method which comprises 
applying to a scored region of the glass sheet a protective composition 
that adheres to the scored region at the temperature of application 
(usually approximately room temperature), and that has a lower viscosity 
than the composition of the glass sheet in the temperature range of the 
thermal treatment required for tempering prior to subjecting the glass 
sheet to tempering. This protective composition is applied in a form that 
adheres to the scored region of the glass sheet at room temperature and 
which is capable of flowing into and healing the scored region while the 
glass sheet is subjected to the thermal treatment. Thus, when the glass 
sheet with the protective composition applied to the scored region is 
subjected to conventional thermal treatment involved in tempering, the 
vents are healed to virtually eliminate breakage entirely during the 
thermal treatment involved in tempering, even through glass sheets 
untreated in the scored region are likely to break during identical 
thermal treatment. According to a specific embodiment of the present 
invention for use with a glass sheet composed of a soda-lime-silica glass 
composition, the protective composition is applied in the form of a liquid 
carrier containing as an essential ingredient a composition having a high 
concentration ratio of sodium ions that does not decompose prior to 
forming a glassy composition having a higher soda to lime ratio than that 
of the glass sheet. A most preferred composition contains a sodium 
silicate composition as the glass forming ingredient. The most preferred 
method of application is in the form of a viscous aqueous dispersion. 
Other suitable protective compositions contain non-alkali metals such as 
lead, whose oxides provide additives for soda-lime silica glasses that 
tend to develop glass compositions having a lower viscosity than that of 
the basic glass composition that they modify. 
The present invention will be understood better in the light of a 
description of a preferred embodiment and other variations thereof which 
follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND ALTERNATIVES 
Referred to the drawings, FIG. 1 shows an applicator means in the form of a 
pencil-like member 10 having a felt tip 12 at one end thereof. The 
pencil-like member 10 is immersed into a container 14 containing a 
protective composition 16. 
A preferred protective composition for use in performing the present 
invention is sodium silicate. A preferred composition is an aqueous, 
viscous solution of sodium silicate. A very effective water solution of 
sodium silicate has a specific gravity of 41 degrees Baume'. 
The position occupied by the felt tip relative to a hole 18 to which the 
sodium silicate composition is applied is shown in FIG. 2. In FIG. 3, the 
hole 18 is shown with a thin film 16 of sodium silicate composition which 
adheres to the peripheral wall of the hole extending through the thickness 
of the glass sheet G. A suitable film thickness is approximately 1 mil 
(0.0254 mm). A typical position of the hole 18 relative to the extent of 
the glass sheet is shown in FIG. 4. The tendency for the hole to cause 
breakage in the glass is associated with vents that form in the vicinity 
of the hole during the drilling of the hole through the thickness of the 
glass sheet G. 
The apertured glass sheet with the viscous, adherent sodium silicate 
composition applied to the periphery of the hole throughout its extent is 
then subjected to a typical thermal treatment involved in tempering which 
involves heating the glass sheet to the vicinity of 1200.degree. F. 
(650.degree. C.) and then chilling the heated glass sheet sufficiently 
rapidly to establish a stress pattern of desired magnitude and 
configuration throughout the thickness of the glass. The application of 
the sodium silicate composition prior to the thermal treatment is believed 
to result in a chemical fusion of the sodium silicate, which has a lower 
viscosity than the float glass substrate throughout the temperature region 
to which the glass and the applied coating is heated. Sodium silicate is 
believed to chemically combine with the float glass to form a glass which 
is of an intermediate glass composition. The intermediate glass flows into 
the vents and in effect becomes fire or heat polished as it approaches its 
melting point. 
The sodium silicate composition has a sufficiently low enough viscosity for 
a high melt state at approximately 1200.degree. F. (650.degree. C.) to 
flow. Normally, commercial float glass requires a temperature in excess of 
1600.degree. F. (870.degree. C.) to obtain the same viscosity state. It is 
believed that the ready flow of the sodium silicate glass composition 
compared to that of float glass sufficiently heals vents that are formed 
during cutting so as to reduce the tendency of the glass to develop high 
tension stresses during the thermal treatment involved in tempering that 
would cause the glass sheet to break during thermal treatment. 
In order to prove the theory that an intermediate glass product is formed 
by applying the sodium silicate to the periphery of a hole followed by 
thermal treatment associated with tempering, various float glass samples 
were provided with half-inch diameter holes, some were treated with sodium 
silicate and control samples were untreated. All the samples were analyzed 
for sodium and calcium concentration in the vicinity of the hole. The 
surface composition of the treated areas was found to be relatively poor 
in calcium and rich in sodium compared to the surface composition of the 
untreated areas. Furthermore, in depth analysis of the treated samples 
after the samples (including the treated and controlled samples) were 
subjected to a thermal treatment similar to that involved in a commercial 
glass sheet tempering operation, showed that the treated samples had a 
sodium to calcium concentration ratio that decreased inwardly from the 
exposed treated surface. The control specimens which were not treated had 
virtually uniform sodium to calcium concentration ratios at different scan 
depths. 
ALTERNATE COMPOSITIONS 
While the sodium silicate compositions are most suitable because of the 
fact that the sodium silicate is soluble in water and can be easily 
applied at room temperature using a felt tip pencil-like member, other 
materials also seem to improve the tempering operation by reducing the 
breakage in the vicinity of the scored regions. For example, a dispersion 
of sodium silicate in oil applied directly to the peripheral wall of a 
hole prior to thermal treatment simulating tempering reduces glass 
breakage frequency when the applied composition is sufficiently viscous to 
avoid running over the major surface of the glass sheet. Other 
compositions tested for less breakage include aqueous solutions or 
dispersions of sodium chloride, sodium sulfate, potassium sulfate, sodium 
borate, lithium carbonate, barium sulfate, and HCl solutions of lead 
oxide. The other materials enumerated were tested after three days of a 
production operation in which sodium silicate was applied to the inner 
periphery of each aperture of each apertured glass sheet without any 
breakage in the vicinity of the holes. The first change involved treating 
10 consecutive sheets of glass identical to those produced without 
breakage for three days in the same manner as regular production except 
that the application of sodium silicate was omitted. Subsequently, 10 
consecutive glass sheets were treated with various materials and the 
number of plates lost in production noted for each treatment. The results 
are tabulated in Table I that follows: 
TABLE I 
______________________________________ 
RESULTS OF DIFFERENT TREATMENTS OF 
APERTURES UNDER CONDITIONS PRODUCING 100% 
YIELD ON VENT HOLE HEALING WITH SODIUM 
SILICATE 
Holes Sheets Lost 
Hole Treatment Treated (Comments) 
______________________________________ 
None 10 5 (Control 
samples) 
Concentrated NaCl in water 
10 0 
Concentrated LiCO.sub.3 in water 
10 3 
Sodium silicate 
(30% water added to 41.degree. Baume') 
10 0 
Sodium sulfate 
(concentrated aqueous solution) 
10 0 
Concentrated NaNO.sub.3 in water 
10 10 (NaNO.sub.3 
decomposed) 
Concentrated sodium borate 
in water 10 3 
Concentrated potassium sulfate 
in water 10 2 
Concentrated LiNO.sub.3 in water 
10 5 
Concentrated BaSO.sub.4 in water 
10 5 
Concentrated PbO in HCl 
10 2 
______________________________________ 
On the following day, the furnace conditions were changed by lowering the 
top temperature the glass sheets attained while conveyed through the 
furnace so that only 9 of 10 apertured glass sheets treated at their holes 
with sodium silicate were produced successfully. Under these more nearly 
marginal production conditions, only 5 of 10 apertured glass sheets 
treated with a concentrated NaCl solution in water and only 5 of 10 
apertured glass sheets treated with a concentrated Na.sub.2 SO.sub.4 
solution in water were successfully produced. Improving the furnace 
temperature conditions caused a resumption of good yields for apertured 
glass sheets treated with sodium silicate, sodium chloride and sodium 
sulfate, the only ones retested. 
Aqueous solutions of NaCl and Na.sub.2 SO.sub.4 are neater to apply than 
sodium silicate. The resulting residues from NaCl and Na.sub.2 SO.sub.4 
aqueous compositions are thin white deposits. The remains from a sodium 
silicate application is slightly messier, but also results in a white 
solidified film on the circumferential wall of the hole to which it is 
applied. 
From the results of the experiments, it was concluded that vents can be 
inhibited from developing into glass breakage during a thermal treatment 
characteristic of glass sheet tempering by applying a composition that 
flows more readily than the glass sheet in the temperature range of the 
thermal treatment so that it flows into the vents during said thermal 
treatment. Preferably, the composition does not decompose into a 
composition that lacks utility for this intended purpose (such as sodium 
nitrite). It is believed that the composition contains a metal, probably 
in ionic form, that forms a glassy composition on the scored glass surface 
to which it is applied when subjected to the same thermal treatment cycle 
as the treated apertured glass sheet. The glassy composition of the thin 
coating thus formed has a higher concentration of a metal that tends to 
reduce the viscosity of the resulting glass composition to one that is 
less than that of the composition of the glass sheet. These metals include 
the sodium and lead. 
The composition applied to the perimeter of the hole must adhere to the 
hole sufficiently at room temperature and be sufficiently viscous to avoid 
flowing over a major glass sheet surface. Such flowing may introduce 
surface area defects of an optical nature that would impair the desired 
uniform optical properties of the tempered glass article that results. 
The article that results from the performance of the improved method based 
on the present invention comprises a glass sheet having a hole extending 
entirely through its thickness, the composition of the glass sheet having 
a given viscosity range in the temperature range of thermal treatment 
required for tempering. The resulting article is characterized by a 
coating consisting essentially of a glassy composition having a lower 
viscosity than that of the glass sheet throughout the temperature range 
required for tempering bonded to the wall of said hole. When the glass 
sheet is composed of a soda-lime-silica glass composition, the coating 
that forms on the interior wall of the hole during the thermal treatment 
incidental to tempering is composed of a glassy composition having a 
higher concentration ratio of soda or lead to lime than the glass sheet. 
The coating on the periphery of the hole has a maximum thickness of 
approximately 1 mil (0.0254 mm). 
The treated glass sheet may also be shaped and/or exposed to a film forming 
composition between the heating step and the rapid cooling step of the 
tempering operation. Also, if desired, a glass sheet (having a scored 
region) that is to be annealed rather than tempered, with or without the 
other processing steps such as shaping and/or coating, may have its scored 
region treated prior to the thermal treatment of heating followed by more 
gradual, controlled cooling characteristic of annealing. However, in 
annealing, the temporary stresses induced in the glass are not as large as 
those induced during the rapid cooling step of tempering. Therefore, it is 
less likely that such treatment of the scored region prior to thermal 
treatment will be needed if the glass sheet is to be annealed rather than 
tempered. 
The form of the invention shown and described in this disclosure represents 
an illustrative preferred embodiment and certain modifications thereof. It 
is understood that various changes, such as applying alternative viscosity 
reducing compositions to a scored region, may be made without departing 
from the gist of the invention as defined in the claimed subject matter 
which follows.