Stock material or miscellaneous articles – Surface property or characteristic of web – sheet or block – Surface modified glass
Reexamination Certificate
2002-03-01
2004-03-30
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Surface property or characteristic of web, sheet or block
Surface modified glass
C428S409000, C501S053000, C501S055000, C501S068000, C501S069000, C501S072000
Reexamination Certificate
active
06713180
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to glazings of tempered glass, especially, but not exclusively,
1. Field of the Invention
Glazings of thin tempered glass (normally tempered float glass) for automotive use, a method of tempering a glazing, and to a novel glass composition suitable for use in the tempered glazings of the invention and in the method of the invention.
2. Discussion of Related Art
Prior to the 1970's, automotive glazings were generally 4 mm or 5 mm thick or even thicker. The first oil crisis of the 1970's encouraged a move towards thinner glazings for automotive use, especially in Europe and Japan, and towards solving the problems encountered in producing thin tempered glazings having the fracture characteristics required to meet official standards. In order to meet European standards, it was found necessary (because of the fracture characteristics of the glass) to provide a higher tempering stress together with an appropriate stress distribution (see, for example, UK patents GB 1 512 163 and GB 2 000 117) in order to achieve the required fracture patterns on breakage. Moreover, because of the reduced thickness of the glass, it was more difficult to achieve the temperature differential between the surface and core of the glass required to produce a given tempering stress. While satisfactory tempering was achieved in thicknesses of about 3 mm, the difficulties of tempering thinner glasses by conventional processes have inhibited progress in reducing glass thickness further so that, about 25 years after the introduction of such thin tempered automotive glazings, the commercial production of tempered automotive glazings having a thickness of less than 3.1 mm remains difficult.
We have now found that glazings, especially but not exclusively thinner glazings, can be more readily tempered including tempered to meet glazing standards (e.g. such as European automotive glazing standards) if the glass composition is appropriately modified, especially if the glass composition is modified to significantly increase its coefficient of thermal expansion and/or reduce its Fracture Toughness.
Certain selected glass compositions have previously been proposed for use in thin automotive glazing. International Patent Application WO 96/28394 relates to glass sheets of thickness in the range 2 to 3 mm having a total iron content (measured as Fe
2
O
3
) of 0.85 to 2% by weight, and specified optical properties, including a visible light transmission of greater than 70% and a total energy transmission of less than 50%. The glasses specifically described have a high alkali metal oxide content (ranging from 14.4% to 15.8% by weight) a magnesium oxide content ranging from 0.25% to 3.8% by weight and a calcium oxide content ranging from 8.4% to 8.6% by weight. The specification refers to the possibility of tempering single sheets of such thin glass for use in automotive side glazings, but make no reference to the difficulty of achieving commercially satisfactory tempering in practice.
International Patent Application WO 99/44952 relates to a sheet of soda lime silica glass designed to be heat tempered and characterised by a very high coefficient &agr; of thermal expansion greater that 100×10
−7
K
−1
(although it does not specify the range of temperatures over which &agr; is to be measured), a Young's Modulus E higher than 60 Gpa and a thermal conductivity K less than 0.9 Wm
−1
K
−1
. The invention is said to make possible glass sheets of thickness lower than 2.5 mm which can be tempered to the requirements of ECE Regulation R43 using apparatus previously envisaged for the tempering of 3.15 mm glass. The particular glasses described all have a very high alkali metal oxide content (in the range 19.9 to 22.3% by weight) resulting in low durability and making the glasses expensive to produce.
SUMMARY OF THE INVENTION
According to the present invention there is provided a thermally tempered glazing of soda lime silica glass produced by tempering a pane of glass having a coefficient of thermal expansion, &agr;, greater than 93×10
−7
° C.
−1
and/or a Fracture Toughness, FT, of less than 0.72 MPam
½
. The invention is especially, but not exclusively, applicable to tempered glass panes less than 3 mm thick and to the tempering of such panes.
DESCRIPITION OF THE PREFERRED EMBODIMENTS
For the purpose of the present specification and claims, &agr; is the value of the coefficient of thermal expansion per degree Centigrade of the glass measured over the range 100° C. to 300° C.; it is measured in accordance with ASTM E228 at constant heating rate. Preferably the coefficient of thermal expansion is at least 95×10
−7
per degree Centigrade, although modification of the composition to achieve a coefficient of thermal expansion greater than or equal to 100×10
−7
, while beneficial to assist tempering, will generally be avoided on cost and durability grounds.
Toughness is the energy per unit area (Joules per square metre) required to make a crack grow. Fracture Toughness, FT, is related to Young's modulus and surface energy by
FT=(2×Surface Energy×Young's Modulus/1
−v
2
)
½
where v is Poisson's ratio. For the purpose of the present specification and claims, it is determined by indenting a bar of glass using a Vickers indenter at a load sufficient to produce cracks at the corners of the indentation, and then breaking the bar in a 3- or 4-point bend test and the determining fracture stress, &sgr;
f
, in Pascals required for breakage. The Fracture Toughness of the glass, assuming it is in the fully annealed state*, is then given by
FT
=&eegr;(
E/H
)
⅛
&sgr;
f
¾
P
¼
where &eegr; is a constant, E is Young's modulus, H is the hardness of the glass, and P is the load used to create the indentation.
Flat glass will be the fully annealed state if it has been heated at the annealing temperature for one hour and cooled at 2° C. per minute to room temperature.
The constant &eegr; is determined with reference to FIG. 8.20 in Fracture of Brittle Solids (Brian Lawn, Cambridge University Press 1993). Applying values of E=70 GPa, H=5.5 GPa and FT=0.75 MPam
{fraction (1/2 )}
for soda lime silica glass the value of &eegr; is &eegr;=0.44.
If the glass is not in the fully annealed state*, it is necessary to apply a correction for residual stress to the Fracture Toughness calculated using the above equation. In practice, it is convenient to measure the Fracture Toughness of glass in the fully annealed state.
Preferably, the glass has a Fracture Toughness of less than or equal to 0.70 MPam
½
, especially less than or equal to 0.68 MPam
½
.
In a preferred embodiment of the invention, the glass has a coefficient of thermal expansion &agr; (° C.
−1
in the range 100° to 300° C.) and a Fracture Toughness, FT (in MPam
½
) such that
α
×
10
7
FT
≥
135
preferably ≧140, and especially ≧145.
It has been found that an increase in the alkali metal oxide content of the glass tends to increase the coefficient of thermal expansion, and while it is well known that glass can be produced with high alkali metal contents (patents relating to glass compositions for production by the float process typically propose an alkali metal oxide content in a range up to about 20%), an increase in alkali metal oxide content generally increases the cost of the glass and reduces its durability. In consequence, commercially available float glass generally has an alkali metal oxide content in the range of 13 to 14% by weight, and glasses with higher alkali metal oxide contents are not used in the production of thermally tempered glazings, especially automotive glazings. We have found that increasing the alkali metal oxide content by a relatively small amount results in a surprising increase in the ease of tempering (as measured, for example, by the particle count on fracture) of the glass (e
Butcher Louise Sara
Jones Kevin
Torr Ashley Carl
Woodward Alan Charles
Blackwell-Rudasill G. A.
Jones Deborah
Marshall & Melhorn LLC
Pilkington plc
LandOfFree
Improvements in or relating to tempered glazings and glass... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Improvements in or relating to tempered glazings and glass..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Improvements in or relating to tempered glazings and glass... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3189947