Method for reducing roll marking of glass sheets

A method for reducing roll marking of glass sheets is disclosed whereby metal rolls are coated in situ with a boron nitride film.

FIELD OF THE INVENTION 
The present invention relates generally to the art of reducing roll marking 
of glass sheets and more particularly to the art of reducing roll marking 
by forming a protective coating on the rolls. 
THE PRIOR ART 
In the glass industry, rolls for supporting and conveying glass sheets are 
subjected to very high temperatures; e.g., take off rolls in a float glass 
line or conveyor rolls in a tempering or annealing lehr. Since 
temperatures of 900.degree. to 1600.degree. F. (482.degree. to 871.degree. 
C.) are not uncommon in the glass making operation, rolls are generally 
made from a heat resistant material such as asbestos or stainless steel. 
Although asbestos rolls are used extensively in the glass industry, they 
have several disadvantages such as the tendency to deform as a result of 
continuous contact with hot glass. When this occurs, the out-of-round 
rolls mar the bottom surface of the glass, a defect known as roll 
scuffing. Although metal rolls are not as prone to deformation, the metal 
may oxidize and oxide deposits on the roll surface, including tin oxide 
which may be transported from the bath by the glass, can make imprints on 
the glass surface. 
In addition to the above-described defects relating to the marking of glass 
sheets, another problem encountered in the glass producing environment is 
short roll life. The continuous passing of hot glass over rolls causes 
such wear that the rolls, particularly asbestos rolls, have relatively 
short lives. Depending on the temperature, determined by the location of 
the roll, typical roll life may vary from about a week for take off rolls 
which are in a 1200.degree. to 1600.degree. F. (649.degree. to 871.degree. 
C.) environment to several months for lehr rolls in the 200.degree. to 
600.degree. F. (93.degree. to 316.degree. C.) range. 
An asbestos roll with a boron nitride coating found to prolong roll life 
and to minimize deformation of the roll and adhesion to the glass surface 
is disclosed in U.S. Pat. No. 3,853,525 to Gorman. The roll is prepared by 
adhering asbestos discs together on a mandrel, spraying on a solution of 
boron nitride, air drying for 1 to 10 hours at room temperature to 
200.degree. F. (93.degree. C.), then baking at 200.degree. to 1500.degree. 
F. (93.degree. to 815.degree. C.) for 2 to 6 hours before installing the 
roll in a glass producing apparatus. 
In U.S. Pat. No. 3,492,109 to Robinson et al, boron nitride is disclosed 
for use in the glass engaging rolls of a float glass tank. 
In U.S. Pat. No. 3,783,013, Seeman discloses coating metal glassware molds 
with a dispersion of 12 to 53 percent by weight monoaluminum phosphate and 
47 to 88 percent by weight boron nitride. 
SUMMARY OF THE INVENTION 
The present invention involves a method for reducing roll marking of glass 
sheets in a continuous glass producing or treating operation by coating 
metal rolls in situ to form a boron nitride coating which prevents 
oxidation of the metal rolls and also prevents build up of any deposits 
which can cause imprinting defects. 
During the course of operation of a glass production or treatment line, a 
roll coating may wear at high load points. However, since the boron 
nitride is applied in situ according to the present invention, spot 
recoating can be accomplished with minimum down time of the production 
line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Rolls for supporting glass sheets may be formed from conventional stainless 
steel; for example, compositions of 20 to 25 percent chromium, 12 to 20 
percent nickel and the balance iron. For certain applications other alloys 
may be preferred such as those compositions of about 2 percent iron, at 
least 10 percent cobalt and at least 3 percent tungsten as described in 
U.S. Pat. No. 3,443,922. 
The rolls are contacted in place at any temperature which the coating 
apparatus can withstand with an aqueous suspension of boron nitride. The 
coating apparatus may be a brush, roller, spray gun, etc. Spraying is 
preferred where feasible because a very uniform adherent coating is 
produced. However, good results can be achieved with a brush or roller 
where temperature or space constraints prohibit spraying. 
Aqueous boron nitride compositions useful according to the present 
invention are commercially available from Carborundum Corporation under 
the trademark COMBAT.RTM.. These compositions are typically gels of boron 
nitride which contain a high temperature inorganic binder such as aluminum 
oxide, magnesium silicate or zirconium oxide in aqueous medium. Preferred 
compositions include the Type S, a boron nitride gel of the Carborundum 
COMBAT.RTM. series as well as the Type V. 
In a method for treating flat glass sheeting supported by metal rolls, 
whatever the treatment, e.g. annealing, coating, etc., particularly at 
elevated temperatures, whenever roll marking defects begin to appear on 
the glass surface, the metal rolls are contacted with an aqueous solution 
comprising about 4 to 5 volumes of water per volume of concentrated boron 
nitride gel. As soon as the coating has set, the time required varying 
inversely with the temperature, the glass treatment process is resumed. 
The roll treatment according to the present invention may be repeated as 
often as necessary to reduce roll marking of the glass and allows the 
stainless steel rolls to remain in place and in operation for extended 
periods of time, thus increasing the efficiency of the glass treatment 
process. An additional advantage of the present invention is that the 
steam generated by he application of the aqueous composition in situ 
removes adherent particulate contamination deposited on the rolls. 
The present invention will be further understood from the description of 
specific examples which follow. 
EXAMPLE I 
On a glass production line, whereupon flat glass sheeting being treated is 
exhibiting unacceptable roll marking, existing untreated stainless steel 
rolls from the take-off position through the annealing lehr, where 
temperatures range up to 1530.degree. F. (832.degree. C.), are contacted 
with an aqueous composition comprising 1 part of a boron nitride gel 
comprising 60 percent water and 4 parts additional water. The boron 
nitride gel is commercially available from Carborundum Corporation as 
COMBAT.RTM. Type S coating composition and comprises 85 parts boron 
nitride to 15 parts aluminum oxide. The aqueous composition is applied 
with long handled brushes. When flat glass sheeting is again brought into 
contact with the boron nitride coated stainless steel rolls, the roll 
marking defects are substantially reduced. 
EXAMPLE II 
Stainless steel rolls are contacted as in Example I with an aqueous 
composition comprising 5 volumes of water per volume of a boron nitride 
gel available from Carborundum Corporation as COMBAT.RTM. Type V which 
differs from Type S in that magnesium silicate, rather than aluminum 
oxide, is used in ratio of 92 parts boron nitride to 8 parts magnesium 
silicate. Again, a boron nitride coating is formed which substantially 
reduces roll marking of the glass sheeting. 
The above examples are offered to illustrate the present invention, the 
scope of which is defined by the following claims.