Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND FILM FOR PRODUCING LAMINATED SAFETY GLASS PANES
The invention relates to a process and a film for
producing laminated safety glass, composed of at least
two panes of glass and of an intermediate layer made
from plasticized, partially acetalized polyvinyl
alcohol, in particular of plasticized polyvinyl butyral
(PVB), in a single-stage process not using an
autoclave.
Laminated safety glass, composed of two panes of glass
and of an adhesive film which bonds the panes of glass
and is made of polyvinyl butyral (PVB) for example, are
used in particular as windshields in motor vehicles,
and one pane of glass here may if desired be replaced
by a sheet of plastic, especially one made from an
amorphous polyamide, or from a transparent PMMA,
polycarbonate or polyester. The construction sector,
too, uses this type of silicate glass/silicate glass
laminate or silicate glass/plastic laminate as window
glass or as partitioning, for example. Here, if desired
and depending on the application, e.g. bulletproof
glass, use may also be made of multiple laminates, i.e.
laminates which are composed of more than two load-
bearing layers.
Whereas laminated safety glass for the construction
sector is intended to have, inter alia, the highest
possible adhesion between glass and adhesive film, in
laminated safety glass for motor vehicles it is
desirable to have a specified adhesion level which is
not too high.
Wherever the term polyvinyl butyral (PVB) is used in
this application, it is intended to include other
polymers whose properties affecting their usability are
similar to those of (plasticized) polyvinyl butyral.
These include partially acetalized polyvinyl alcohols
in general, and, for example, unplasticized ethylene-
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vinyl acetate copolymers. Particular properties
regarded as relevant here are surface tack, softness
and elastomeric properties at room temperature.
Description of the prior art
Plasticized films based on polyvinyl butyral (PVB) used
for producing laminated safety glass are very soft and
tacky, even at room temperature. Although the high tack
is indispensable for holding the glass-film-glass
laminate together in laminated glass, it nevertheless
has to be eliminated temporarily during transport and
during processing to produce this glass. It is well
known that for this purpose the films can be provided
with a roughened surface on one or both sides. The
roughnesses, measured as RZ to DIN 4768, are usually
from 15 to 35 pm. A typical process for producing films
of this type with a roughened surface is disclosed in
EP 0 185 863 Bl.
In producing laminate safety glass a two-stage
manufacturing process is generally used. A first step
prepares what is known as a prelaminate, composed of
two panes of glass and of the adhesive film arranged
therebetween and usually made from PVB. A second step
compresses this prelaminate in an autoclave at
temperatures of from about 130 to 145 C and at
pressures of at least 11 bar to give the actual
laminated safety glass. An example of a description of
this process is given in the introduction to
EP 0 331 648 A2. The water content of the PVB film used
has a decisive effect on the bonding power of the
laminate. It is usual to maintain a water content of
from 0.40 to 0.55o by weight of the film - with very
tight tolerances. The use of an autoclave means that
this known two-stage process is relatively complicated
and requires high capital expenditure, and is therefore
uneconomic for small production quantities.
EP 0 331 648 A2 discloses a process for producing
laminated safety glass without using an autoclave. In
...
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this known process the panes of glass, with the PVB
film lying therebetween, are bonded together in an
airtight evacuated sack at an elevated temperature,
using the pressure of the surrounding air. That
publication also discloses that, to prevent bubble
formation within the laminate, the PVB film used should
have the lowest possible water content. However,the
drying of a film and the handling of a film dried in
this way is relatively complicated.
DE 196 43 404 Al discloses another process for
producing laminated safety glass without using an
autoclave, by using a plasticized, partially acetalized
polyvinyl alcohol film with water content below 0.350
by weight, based on the weight of the film, and with a
useful content of an organosilicon-functional silane
which increases adhesion. However, precise adjustment
to such a low water content is very difficult.
In US 5,536,347 Al, a process for producing laminated
safety glass without using an autoclave uses a PVB film
which has a specific geometric structure on both sides.
Structures of this type can only be produced by using
specific embossing processes and are unsuitable for
many PVB-film-production plants. Other processes for
producing films with regularly shaped, geometrically
arranged surface structures are disclosed in
DE 32 08 820 C2 and US 5,478,412 Al, but there is no
disclosure here of use in a process which does not use
an autoclave.
US 5,425,977 Al discloses a process for producing PVB
films with a regular surface structure and with
roughness Rz below 75 pm, preferably below 50 pm. There
is no disclosure of use in a process which does not use
an autoclave. The same applies to the process of
US 4,925,725. The film described there can again only
be produced by a complicated embossing process.
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Obj ec t
It is an object of the present invention to eliminate
the disadvantages described while providing a process
for producing laminated safety glass composed of at
least two panes of glass and of an intermediate layer
made from plasticized, partially acetalized polyvinyl
alcohol in a single-stage process not using an
autoclave which gives bubble-free laminated safety
glass at a sufficient level of process reliability. A
further object of the invention is to provide an
intermediate layer which is made from plasticized,
partially acetalized polyvinyl alcohol, can be produced
without a complicated embossing process, and can be
used in a process not using an autoclave.
Summary of the invention
The invention achieves this object by a process for producing laminated
safety glass, composed of at least two panes of glass and of an
intermediate layer made from plasticized, partially acetalized polyvinyl
alcohol, in a single-stage process not using an autoclave, which
comprises using a plasticized, partially acetalized polyvinyl alcohol film
with roughness Rz of from 70 to 150 pm.
As such, the object of the invention is achieved by using a PVB film with,
applied to one side or preferably to both sides, a surface structure with
roughness RZ > 70 pm and Rz < 150 pm, preferably in conjunction with
one or more features from the subclaims. If the surface roughness
Rz is below 70 pm, the edges of the laminated safety
glass seal during the heating phase before all of the
air can be evacuated from the PVB/glass interface. This
results in air inclusions (bubbles) in the center of
the laminated safety glass. If the surface roughness is
too high (Rz > 150 pm) more energy is needed to melt
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the roughness peaks and the process becomes
unacceptably protracted.
It is preferable for the surface structure to be
applied in the extrusion process, immediately prior to
discharge of the plastic melt from the extrusion die,
via what may be called the flow- or melt-fracture
process of EP 0 185 863 Bl. Various levels of roughness
can be produced as required by varying the discharge
gap width and the temperature of the die lips directly
at the discharge from the die. This process gives
random roughness, which is' approximately isotropic.
This means that the roughness value is approximately
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the same when measured in any direction, while the
individual protrusions and depressions have a random
arrangement of height and distribution.
Wherever this text uses the term surface roughness RZ
or roughness Rz, the surface roughness measurement is
made in accordance with DIN 4768 or DIN EN ISO 4287 and
DIN ISO 4288. The measurement devices used to measure
surface roughness must comply with EN ISO 3274. The
profile filters used must correspond to
DIN EN ISO 11562.
The laminated safety glass production process
preferably employed for the invention, using the film
employed according to the invention, is a single-stage
vacuum process based on the vacuum-ring or vacuum-sack
process. In the 'vacuum-sack process the unbonded
glass/film/glass sandwich is placed in a plastic or
rubber sack. This is then sealed so as to be airtight
and provided with a suction valve, preferably in the
center of the pane. In the case of large-format panes
it is also possible to provide two suction valves.
Using a vacuum pump, the pressure is then reduced to
below 200 mbar, and evacuation proceeds for 5 min at
room temperature. While retaining the reduced pressure,
the sandwich is then heated to 140 C in a circulating-
air cabinet. Depending on the heating power used, the
heating rate here is from 4 to 6 C/min, and therefore
the holding temperature of 140 C is reached after from
20 to 30 min. The temperature is then held at 140 C for
30 min. Cooling must then take place to at least 60 C
before the pressure is restored to atmospheric
pressure. Fig. 1 shows the temperature profile for this
preferred one-stage process.
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Examples of the working of the invention, and
comparative examples
Example 1
Preliminary experiments are first carried out using PVB
films having various roughnesses. Table 1 shows the
average surface roughnesses Rz of the individual films:
Film Average surface roughness Rz (pm)
side 1/side 2
PVB 1 35/35
PVB 2 55/55
PVB 3 85/85
PVB 4 115/115
Table 1
Flat laminated safety glass specimens in 500 x 1100 mm
format were produced by the single-stage vacuum process
described above. Films PVB 1 and PVB 2 here exhibited
air inclusions in the central region of the panes. This
indicates that edge sealing occurred prior to complete
evacuation of the panes. In contrast, films PVB 3 and
PVB 4 were completely transparent. Table 2 shows a
comparison of bonding properties with a laminated
safety glass (PVB 4) produced by the two-stage process:
Film PVB 4
Laminated safety glass Autoclave Vacuum
test (two-stage process
process, (single-stage
comparative process,
example) inventive)
Moisture [~] 0.43 0.43
Pummel adhesion F/F 5 5
Sn/Sn 4 4
Shear-test adhesion F/Sn [N/mm2] 13.9 13.6
Bake test in heating 140 150
cabinet at temp. C
internal bubbles 4 none
Table 2 (F = fire side of glass, Sn = tin side of
glass)
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The values for adhesion between film and glass,
expressed in Table 2 via the pummel value and the shear
test, are at the same level for both types of laminated
safety glass. After the test known as the bake test,
which gives information on the quality of bond produced
and in particular on any tendency to form bubbles, the
laminated safety glass produced by the single-stage
process gives no bubbles even at 150 C.
Example 2 (inventive)
On the basis of these preliminary experiments with flat
panes, two different types of windshield glass were now
produced by the single-stage process. Film PVB 4 was
used. Table 3 shows the dimensions of the windshields
(WSS) used:
Type Width Height Longitudinal Transverse
flex flex
min/max (mm) min/max (mm) min/max (mm) max (mm)
WSS 1 1100/1410 620/660 90/30 0
WSS 2 1170/1510 660/780 110/50 17
Table 3
After the process, both windshields had satisfactory
optical properties. The adhesion values and the result
of the bake test correspond to the values from
Example 1, Table 2.
All of the films in Examples 1 and 2 were produced by
what may be called the flow- or melt-fracture process
of EP 0 185 863 Bl. This process gives rough surfaces
with random distribution of protrusions and
depressions, without any regular patterns or
structures.
