1. Field of the Invention
This invention concerns a sheet of plastic material comprising at least two films intended, in particular, for the manufacture of laminated panes known as asymmetric panes, in which the plastic sheet is associated with a monolithic or laminated substrate of glass.
2. Discussion of Background
Asymmetric panes having a sheet of plastic material and a glass substrate can be used, notably, as vehicle panes, such as windshields, lateral panes, rear window panes, roof or roof head of land transportation vehicles, as aircraft panes, and as panes for buildings and ships. Asymmetric panes of this type are described, for example, in FR-A-2 398 606 and EP-A-132 198.
The sheet of plastic material used in these asymmetric panes is generally formed of several layers or films having different properties. In effect, this sheet of plastic material must have surface properties, such as resistance to scratching, resistance to abrasion, resistance to the action and attack from external agents, such as cleaning agents and atmospheric agents. It must, in addition, have properties of adhesion to the glass substrate. It must also, if applicable, have energy-absorber, shock resistance and penetration resistance properties.
Not all of these properties can be provided satisfactorily with a single film of plastic material. This is why, as indicated above, the sheet is generally formed of at least two layers or films.
One of the problems encountered in the manufacture of asymmetric laminated panes is poor bonding of the sheet of plastic material to the glass substrate. For a laminated pane to be considered as a safety pane, it is necessary for the bonding forces between the glass substrate and the sheet of plastic material to lie between two limits. In effect, and notably when the plastic material sheet is to fulfill the function of an energy-absorber and impact resistor, the bond must not be too strong in order to prevent the sheet of plastic material, if the glass should break, from being damaged by the splinters and, on the other hand, in order that the plastic material sheet shall be able to become detached partially from the glass substrate in order to fulfill completely its energy-absorber function. Further, the bond must not be too weak in order to assure the hold between the two assembled components of the laminated pane in all conditions of use of the asymmetric pane, that is to say, in very variable conditions of temperature and humidity and of solar radiation. Moreover, the bond must be maintained over time.
On the other hand, in the case of prolonged storage of the sheet of plastic material before being assembled to the substrate, the adhesive power of the sheet must be maintained.
In order to resolve this problem of adhesion, it has already been proposed to treat the surface of the glass support intended to be in contact with the plastic material sheet with a primer composition containing a coupling agent for glass/plastic material, generally an organosilane.
The treatment of the surface of the glass may be carried out by sputtering, wiping, etc. This solution, although it is satisfactory in terms of the resultant adhesion, does have a major disadvantage; it always necessitates at least one supplementary operation for the assembling process. It, therefore, necessitates an installation that is expensive in time and labor.
To solve the problem of adhesion, it has also been proposed to incorporate a coupling agent into the composition of the film of adhesive plastic material which will contact the glass substrate. A quantity of organosilanes of between 0.05% and 0.2% by weight of the total weight of the adhesive plastic material film generally produces a sufficient adhesion during the period which follows assembly. A higher quantity of organosilanes can lead to adhesion values that are too strong and which may be prejudicial to the energy-absorber properties and resistance to penetration, as indicated above. But, although the appropriate quantities listed above result in a satisfactory adhesion after assembly, these properties do not last with time, notably when the pane is placed in conditions of high temperatures and high humidity, and when the adhesive film has a thickness less than approximately 50 .mu.m.
A need continues to exist for plastic film materials which provide optimum adhesion properties to glass panes.