Borofloat glass Technical Information

General description

BOROFLOAT® 33 is a borosilicate float glass 3.3 to DIN ISO 3585. The float process provides BOROFLOAT® 33 with a parallel and fire polished surface on both sides and, as a result, an outstanding optical quality.

The glass’s composition is the result of the use of raw materials which are difficult to melt. For this reason minor inhomogeneities (e.g. gaseous inclusions) may occur.

These inhomogeneities are subject to the quality controls carried out at Schott and comply with EN 1748 T1, Category A.

They have no adverse effect on the function or alternatively the suitability for use of the glass.

Forms available

flat sheets in thicknesses from 0.7 mm to 21 mm
flat sheets in stock sizes or cut-to-size panels
parallel and fire polished surfaces
edge and corner worked
bent (on request)
coated
drilled
Available as individual sheets or boxed

Dimensions

Spectraglass can supply BOROFLOAT® 33 in the following dimentions: Items in bold may need to be ordered in.

Thickness (mm)	Width (mm)	Length (mm)	Tolerance (mm)
0.7	1150	850	± 0.1
1.1			± 0.2
1.75			± 0.2
2.00			± 0.2
2.25			± 0.2
2.75			± 0.2
3.30			± 0.2
3.80			± 0.2
5.00			± 0.2
5.50			± 0.2
6.50			± 0.2
7.50			± 0.3
9.00			± 0.3
11.00			± 0.3
13.00			± 0.3
15.00			± 0.3
16.00			± 0.5
17.00			± 0.5
18.00			± 0.5
19.00			± 0.5
20.00			± 0.7
21.00			± 0.7

Other dimentions are avalible including cutting to customer specifications.

Thermal properties

Coefficient of mean linear thermal expansion

a(2O3OO~C) = 3.3 x (106 K-’) (test method according to ISO 7991)

Thermal conductivity

X = 1 .2 W I (m - K) (at 20 °C)

Specific heat

cP(2O~C,QO~C) = 0.9 - 1 0~ J I (kg - K)

Resistance to thermalgradients (RTG)

The resistance to thermal gradients is a measure of how well a material canresist temperature differences on a restricted area, e.g. the temperaturedifference between the hot area in the centre of a panel and the cold edgearea.

The measurement method developed specifically by Schott Glas for thedetermination of the RTG is ideal for carrying out comparative measurementswith other types of glass. The value determined can, however, vary inpractice.The following values were determined with a surface which had beensubjected beforehand to a defined degree of damage.

For short-term usage (< 1 00 h):For long-term usage (»= 1 00 h):RTG = 100 K RTG = 90 K

Measurement method:
Test panels are heated up in the centre area and the edge area is maintai-ned at a constant temperature. The RTG value is quoted as the tempera-ture difference between the hot centre of the panel and the cold edgearea, where «= 5 % of the test pieces were rejected due to breakage caused by thermal stress.
A contributory factor in the results of this test is the edge finish and state ofthe surface. For this reason the test pieces were sanded before the test in adefined manner with SiC 40 grade sandpaper to simulate an extremedegree of damage in use.

Higher RTG values are achieved with panels having less surface damage.

Resistance to thermal shock(RTS)

The resistance to thermal shock is a measure of a panel’s ability to with-stand a sudden thermal shock (e.g. splashing water on an inner oven doorpanel).

Glass Thickness	RTS
«= 3.8 mm:	RTS = 1 75 K
5 to 5.5 mm:	RTS = 1 60 K
6.5 to 1 5 mm:	RTS = 1 50 K
> 15 mm:	RTS = 1 25 K

Measurement method:
Test panels with defined surface damage (with SiC 220 grade sandpaper)are heated up and then 50 ml of cold water (room temperature) is pouredon them in the centre. The RTS value as quoted is the difference betweenthe temperature of the hot panel and the temperature of the cold water,where «= 5 % of the test pieces are rejected due to breakage. Even with thisvalue deviations may be encountered in practice.

Maximum operating tempe-ratures (taking into account5.2.4 and 5.2.5)

Short-Term Usage	Long-Term Usage
(total < 1 0 h):	(total »= 1 0 h):
Tmax = 500 °C	Tmax = 450 °C

The maximum operating temperatures indicated only apply if the RTG andRTS values quoted above are also observed.

Mechanical properties

Density	p = 2.2 g I cm3 (at 25 °C)
Young’s modulus (modulus of elasticity)	E = 63 1O~ Pa The test is carried out following DIN 1 3316.A tensile or compressive load is applied to a rod-shaped test piece toproduce a change in length, the amount of which depends on the materialand the load. The modulus of elasticity describes the relationship betweenthe tension and the material’s change in length.
Poisson’s ratio	The test is carried out following DIN 1 331 6.A mechanical stress applied longitudinally produces both an increase inlength and lateral contraction. As the length changes, so do thedimensions of the piece at right angles to the load.Poisson’s ratio is defined as the ratio of lateral contraction to longitudinalextension.
Knoop hardness	HK01120 = 480
Bending strength	~bB = 25 MPa
Impact resistance	The impact resistance of BOROFLOAT® 33 is dependent on the way it isfitted, the size and thickness of the panel, the type of impact involved,boreholes and their arrangement and various other parameters. Comments on impact resistance are, therefore, only possible with know-ledge of the particular application (chiefly in conjunction with application-specific standards which have to be complied with as regards strengthrequirements). Nominal values on request
Comments on mechanical strength	Indications of the strength of glass and glass ceramic must take into account the special properties of these materials. In the technical sense glasses and glass ceramics are ideally elastic*), brittle materials, which exhibit no tendency to flow. This causes, when incontact with analogously hard materials, the setting up of surface damagein the form of fine nicks and cracks. Furthermore, when glasses and glassceramics are subjected to a mechanical load, the build-up of critical stressat the points of such nicks and cracks cannot be relieved by plastic flow, asis possible with such materials as metals. The consequence of this behaviour is that the structurally caused highstrength of glasses and glass ceramics (»= 1 04 N/mm2) is irrelevant in practice. It is reduced by the effect of unavoidable surface defects occur-ring in use (in the case of unprotected surfaces) to a practical value rangeof approx. 20 to 200 N/mm2 bending strength — depending on surface state and test conditions. The strength of glass and glass ceramic is therefore not a material constant(as for example its density), but is dependent upon finish condition (md. edge finish) usage condition (type and distribution of surface defects) time-related conditions or alternatively the duration of the effectivebending load surrounding conditions (corrosive substances, e.g. hydrofluoric acid) the area subjected to load (e.g. bow) and is subject in accordance with the type and distribution of the surface defects to a statistical distribution. An appropriate forecasting concept for the determination of technicalstrength has been drawn up by G. Exner.
*) Materials are designated to be "ideally elastic" where there is a linear connectionbetween the load applied to them and their deformation up to the point of breakage, i.e.the material does not hove the ability of, for example, a strong steel to break down the load on it by plastic deformation.Exner, G. Glastechnische Berichte 56(1983), pp. 299-312ctm