Semi-tempered glass

Semi-tempered glass consists of a monolithic float glass which is thermally treated to increase flexural strength. Semi-tempered glass is produced and labelled in accordance with the European Construction Products Regulation. The product properties and specification requirements are defined in DIN EN ISO 1863-1. The factory production control and appropriate product labelling must be carried out in accordance with DIN EN ISO 1863-2.

Production:

As with thermally tempered safety glass, the cut glass pane is heated above the transformation temperature as with semi-tempered glass. To avoid possible marks and deformations of the softening glass, the glass pane is continuously moved back and forth over rollers in the oven. The homogeneously heated glass is then cooled in a controlled manner. The cooling rate is lower compared to the thermally toughened safety glass process, resulting in a lower pre-stressing.

Material properties:

The compressive stress created on the surfaces increases the flexural strength of semi-tempered glass, which can withstand up to 3 times higher loads.

The fracture pattern is similar to that of untreated float glass. Simplified, it can be said that it breaks radiantly from the starting point into large fragments. Due to the frozen pre-stressing, semi-tempered glass can not be subsequently processed.

If the pre-stressing introduced is uneven, a point shaped or stripe pattern can be detected under certain lighting conditions. To detect this, a high proportion of polarised radiation must be present in the light. Polarised light can be of natural origin or is created by reflection from adjacent surfaces. However, the risk of undesired anisotropies is significantly reduced by the lower pre-stressing level. With modern production know-how these phenomena can be additionally reduced.

As the glass panes are heated above the transformation temperature (softening temperature) during production, they lose their planarity. The “wavy glass surface” creates visual distortions in the viewer and reflection. According to the current standard, the wave depth of 0.30 mm must not be exceeded over a distance of 300 mm. These geometric limits are not sufficient to describe the actual optical quality of the glass. Only when the actual wavelength is directly related to the wave depth can a statement about the optical quality be made.

Millidiopters allow a meaningful description of the optical quality. The wave length and wave depth are thereby evaluated together. The following table shows the resulting millidiopters, depending on the wavelength and wave depth.

Wave depth / wavelength [mm]

/

80.00

100.00

120.00

140.00

160.00

180.00

200.00

220.00

240.00

260.00

280.00

300.00

320.00

340.00

360.00

380.00

0.01

61.69

39.48

27.42

20.14

15.42

12.18

9.87

8.16

6.85

5.84

5.04

4.39

3.86

3.42

3.05

2.73

0.02

123.37

78.96

54.83

40.28

30.84

24.37

19.74

16.31

13.71

11.68

10.07

8.77

7.71

6.83

6.09

5.47

0.03

185.06

118.44

82.25

60.43

46.26

36.55

29.61

24.47

20.56

17.52

15.11

13.16

11.57

10.25

9.14

8.20

0.04

246.74

157.91

109.66

80.57

61.69

48.74

39.48

32.63

27.42

23.36

20.14

17.55

15.42

13.66

12.18

10.94

0.05

308.43

197.39

137.08

100.71

77.11

60.92

49.35

40.78

34.27

29.20

25.18

21.93

19.28

17.08

15.23

13.67

0.06

370.11

236.87

164.49

120.85

92.53

73.11

59.22

48.94

41.12

35.04

30.21

26.32

23.13

20.49

18.28

16.40

0.07

431.80

276.35

191.91

140.99

107.95

85.29

69.09

57.10

47.98

40.88

35.25

30.71

26.99

23.91

21.32

19.14

0.08

493.48

315.83

219.32

161.14

123.37

97.48

78.96

65.25

54.83

46.72

40.28

35.09

30.84

27.32

24.37

21.87

0.09

555.17

355.31

246.74

181.28

138.79

109.66

88.83

73.41

61.69

52.56

45.32

39.48

34.70

30.74

27.42

24.61

0.10

616.85

394.78

274.16

201.42

154.21

121.85

98.70

81.57

68.54

58.40

50.36

43.86

38.55

34.15

30.46

27.34

0.11

678.54

434.26

301.57

221.56

169.63

134.03

108.57

89.72

75.39

64.24

55.39

48.25

42.41

37.57

33.51

30.07

0.12

740.22

473.74

328.99

241.70

185.06

146.22

118.44

97.88

82.25

70.08

60.43

52.64

46.26

40.98

36.55

32.81

0.13

801.91

513.22

356.40

261.85

200.48

158.40

128.30

106.04

89.10

75.92

65.46

57.02

50.12

44.40

39.60

35.54

0.14

863.59

552.70

383.82

281.99

215.90

170.59

138.17

114.19

95.95

81.76

70.50

61.41

53.97

47.81

42.65

38.28

0.15

925.28

592.18

411.23

302.13

231.32

182.77

148.04

122.35

102.81

87.60

75.53

65.80

57.83

51.23

45.69

41.01

0.16

986.96

631.65

438.65

322.27

246.74

194.96

157.91

130.51

109.66

93.44

80.57

70.18

61.69

54.64

48.74

43.74

0.17

1048.65

671.13

466.06

342.41

262.16

207.14

167.78

138.66

116.52

99.28

85.60

74.57

65.54

58.06

51.78

46.48

0.18

1110.33

710.61

493.48

362.56

277.58

219.32

177.65

146.82

123.37

105.12

90.64

78.96

69.40

61.47

54.83

49.21

0.19

1172.02

750.09

520.90

382.70

293.00

231.51

187.52

154.98

130.22

110.96

95.67

83.34

73.25

64.89

57.88

51.95

0.20

1233.70

789.57

548.31

402.84

308.43

243.69

197.39

163.13

137.08

116.80

100.71

87.73

77.11

68.30

60.92

54.68

0.21

1295.39

829.05

575.73

422.98

323.85

255.88

207.26

171.29

143.93

122.64

105.75

92.12

80.96

71.72

63.97

57.41

0.22

1357.07

868.53

603.14

443.13

339.27

268.06

217.13

179.45

150.79

128.48

110.78

96.50

84.82

75.13

67.02

60.15

0.23

1418.76

908.00

630.56

463.27

354.69

280.25

227.00

187.60

157.64

134.32

115.82

100.89

88.67

78.55

70.06

62.88

0.24

1480.44

947.48

657.97

483.41

370.11

292.43

236.87

195.76

164.49

140.16

120.85

105.28

92.53

81.96

73.11

65.62

0.25

1542.13

986.96

685.39

503.55

385.53

304.62

246.74

203.92

171.35

146.00

125.89

109.66

96.38

85.38

76.15

68.35

0.26

1603.81

1026.44

712.80

523.69

400.95

316.80

256.61

212.07

178.20

151.84

130.92

114.05

100.24

88.79

79.20

71.08

0.27

1665.50

1065.92

740.22

543.84

416.37

328.99

266.48

220.23

185.06

157.68

135.96

118.44

104.09

92.21

82.25

73.82

0.28

1727.18

1105.40

767.64

563.98

431.80

341.17

276.35

228.39

191.91

163.52

140.99

122.82

107.95

95.62

85.29

76.55

0.29

1788.87

1144.87

795.05

584.12

447.22

353.36

286.22

236.54

198.76

169.36

146.03

127.21

111.80

99.04

88.34

79.28

0.30

1850.55

1184.35

822.47

604.26

462.64

365.54

296.09

244.70

205.62

175.20

151.07

131.59

115.66

102.45

91.39

82.02

Application:

Semi-tempered glass is always used when there are increased structural requirements and the flexural strength of float glass is insufficient.

The field of application of semi-tempered glass is superimposed with that of thermally toughened safety glass. It is necessary for large area glass elements, which are exposed to high wind loads or structural load bearing elements, such as in glass walls, glass beams and glass columns. Semi-tempered glass is also used for point-fixed glazing where stress concentrations are to be expected in the hole area. Although semi-tempered glass has lower flexural strength compared to safety glass, it is an important component in modern glass construction. The advantage lies in the increased residual capacity of the product. If it is installed in laminated glass, the result is a high load capacity despite any breakage of glass. This leaves enough time to undertake the necessary exchange measures.

Often, semi-tempered glass is also used when the expected temperature differences in a glass element lead to an increased risk of breakage.

Further details on the optical evaluation of the glass surface in Millidiopters can be found in the American standard ASTM C 1651.

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