Laminated glass and its production process

1. A laminated glass comprising a plurality of glass plates bent into a predetermined shape, and an interlayer interposed between the plurality of glass plates, at least two glass plates among the plurality of glass plates being a thick glass plate and a thin glass plate differing in the plate thickness;

wherein at any temperature between the annealing point and the softening point of the thick glass plate, the thick glass plate between the two glass plates differing in the plate thickness, has a lower viscosity than the thin glass plate, and

wherein the two glass plates differing in the plate thickness satisfy the formula 1<y<(1.22−0.206×x) and the formula 1<z<(1.15−0.131×x),

wherein x (x=t 2 /t 1 ) is the ratio of the plate thickness (t 2 ) of the thin glass plate to the plate thickness (t 1 ) of the thick glass plate at room temperature;

y (y=log 10 η2/log 10 η1) is the ratio of the logarithm (log 10 η2) of the viscosity of the thin mass plate to the logarithm (log 10 η1) of the viscosity of the thick glass plate, at the annealing point of the thick glass plate; and

z (z=log 10 η4/log 10 η3) is the ratio of the logarithm (log 10 η4) of the viscosity of the thin glass plate to the logarithm (log 10 η3) of the viscosity of the thick glass plate, at the softening point of the thick glass plate.

2. The laminated glass according to claim 1 , wherein the two glass plates differing in the plate thickness have different glass compositions.

3. The laminated glass according to claim 1 , wherein the two glass plates differing in the plate thickness satisfy the formula 1<y<(1.20−0.206×x) and the formula 1<z<(1.13−0.131×x).

4. The laminated glass according to claim 1 , wherein the two glass plates differing in the plate thickness satisfy the formula 0.3≦x≦0.9.

5. The laminated glass according to claim 1 , wherein the two glass plates differing in the plate thickness satisfy the formula 1.017≦y.

6. The laminated glass according to claim 1 , wherein the laminated glass is window glass for a vehicle, the number of the glass plates constituting the laminated glass is two, and a convex curved surface of the laminated glass is constituted by a convex curved surface of the thick glass plate.

7. The laminated glass according to claim 1 ,

wherein the thin glass plate is a soda lime glass plate having a composition comprising, as calculated as oxides:

Al 2 O 3 : 0 to 3.5 mass %, and

Na 2 O and K 2 O in total: 12.0 to 14.5 mass %; and

the thick glass plate is a soda lime glass plate having a composition comprising, as calculated as oxides:

Al 2 O 3 : 0 to 2.0 mass %, and

Na 2 O and K 2 O in total: 13.0 to 15.5 mass %.

8. The laminated glass according to claim 1 ,

wherein the thin glass plate is a glass plate having a composition comprising, as calculated as oxides:

SiO 2 : 68.0 to 75.0 mass %,

Al 2 O 3 : 0 to 3.5 mass %,

CaO: 7.0 to 13.0 mass %,

MgO: 0 to 7.0 mass %,

Na 2 O: 12.0 to 15.0 mass %,

K 2 O: 0 to 3.0 mass %, and

Na 2 O and K 2 O in total: 12.0 to 14.5 mass %; and

the thick glass plate is a glass plate having a composition comprising, as calculated as oxides:

SiO 2 : 68.0 to 75.0 mass %,

Al 2 O 3 : 0 to 2.0 mass %,

CaO: 7.0 to 13.0 mass %,

MgO: 0 to 7.0 mass %,

Na 2 O: 12.0 to 15.0 mass %,

K 2 O: 0 to 3.0 mass %, and

Na 2 O and K 2 O in total 13.0 to 15.5 mass %.

9. The laminated glass according to claim 1 , wherein the β-OH value (mm-1) is from 0.1 to 0.4.

10. A process for producing the laminated glass of claim 1 , which comprises:

heating said plurality of glass plates to the vicinity of the softening point and bending them into a predetermined shape to provide a plurality of bent glass plates;

laminating the plurality of bent glass plates with an interlayer interposed there between to provide laminated glass plates and an interlayer; and

pressure bonding the laminated glass plates and the interlayer to form laminated glass.

11. The process according to claim 10 , wherein the two glass plates differing in the plate thickness have different glass compositions.

12. The process according to claim 10 , wherein the two glass plates differing in the plate thickness satisfy the formula 0.3≦x≦0.9.

13. The process according to claim 10 , wherein the thin glass plate is a glass plate having a composition comprising, as calculated as oxides:

SiO 2 : 68.0 to 75.0 mass %,

Al 2 O 3 : 0 to 3.5 mass %,

CaO: 7.0 to 13.0 mass %,

MgO: 0 to 7.0 mass %,

Na 2 O: 12.0 to 15.0 mass %,

K 2 O: 0 to 3.0 mass %, and

Na 2 O and K 2 O in total: 12.0 to 14.5 mass %; and

the thick glass plate is a glass plate having a composition comprising, as calculated as oxides:

SiO 2 : 68.0 to 75.0 mass %,

Al 2 O 3 : 0 to 2.0 mass %,

CaO: 7.0 to 13.0 mass %,

MgO: 0 to 7.0 mass %,

Na 2 O: 12.0 to 15.0 mass %,

K 2 O: 0 to 3.0 mass %, and

Na 2 O and K 2 O in total: 13.0 to 15.5 mass %.

14. The process according to claim 10 , wherein the β-OH value (mm-1) is from 0.1 to 0.4.

15. The laminated glass according to claim 1 , wherein the two glass plates differing in the plate thickness satisfy the formula 0.3≦x≦0.9 and the formula 1.017≦y.

16. The laminated glass according to claim 3 , wherein the laminated glass is window glass for a vehicle, the number of the glass plates constituting the laminated glass is two, and a convex curved surface of the laminated glass is constituted by a convex curved surface of the thick glass plate.