1. Field of the Invention
The invention relates to a transparent composite system comprising polymethyl methacrylate (PMMA) plates and an intermediate layer of an elastomeric resin.
2. Discussion of the Background
Polymethyl methacrylate, and copolymers of methyl methacrylate, exhibit characteristics which are desirable for glazing materials: particularly high transparency and good weather resistance, and--in comparison with silicate glass--a lower weight. But these advantageous, optical characteristics of PMMA and its copolymers are diminished by the brittleness of these materials. This brittleness exhibits itself in the splintering off of shards or parts of the PMMA when it undergoes mechanical stress.
This intrinsic brittleness of PMMA (in addition to its poor scratch resistance and its flammability) naturally reduces its application, especially as a construction material or in areas of high traffic or public use. Several attempts to eliminate these disadvantages have been reported. (see H. Rauch-Puntigam, Th. Volker, Acryl- und Methacrylverbindungen, Springer-Verlag 1967, p. 291).
For example, German Patent Application 29 14 223 recommends, e.g. a clear, transparent synthetic resin composite plate consisting of a clear, transparent plastic base plate with a thickness of at least 1 mm and with parallel surfaces, at least one clear, transparent plastic foil with a scratch resistance not less than that of the base plate, and at least one clear, transparent adhesive resin layer positioned between the base plate and plastic foil, which adheres to the foil in fixed manner and which can be pulled off the base plate.
Japanese Application 61 132 343 (Chem. Abstr. 105, 154387e) discloses an acrylic glass laminate consisting of two exterior hard plastic plates and an intermediate plate of soft plastic with an electrically conductive layer on or near the surface. The intermediate layer is, e.g., an MMA-methacrylate-elastomer equipped on both sides with an antistatic agent and an acrylate adhesive.
German Application 31 28 985 discloses laminated, thermoplastic objects. The laminates are obtained by joining at least one PMMA film with at least one impact-resistant methacrylate polymer by calendaring.
European Patent 419 166 describes co-extruded multi-layer webs consisting of a complex configuration of individual layers with a thickness of 0.3 to 6 mm each. The first exterior layer consists of a blend of polyvinylidene fluoride with M.sub.n &lt;250,000, 30-60% by weight C1-C8 alkyl methacrylate with M.sub.n .ltoreq.150,000, and possibly 10-30% by weight of a graft elastomer of C1-C8 alkyl acrylate, grafted and cross-linked with allylmethacrylate, as well as at least one second layer of an alkyl methacrylate homopolymer or copolymer. The co-extrusion is performed through a multiple extrusion nozzle.
There are also papers on laminates from different plastics. Illinger et al. in Polym. Sci. Technol. 1975, 9A (Adhes. Sci. Technol) 217-232, e.g., proposes laminates of acrylate-polyurethane-polycarbonate for the protection of windshields, whereby it is emphasized that the impact resistance and the optical clarity of the laminates depend on the chemical composition of the intermediate adhesion layer. European Patent Application 138 083 proposes polymer mixtures as an intermediate adhesion layer for laminates, said intermediate adhesion layers consisting of copolymers of firstly 81-99 weight parts olefin and unsaturated carboxylic acids and secondly of 1-19 weight parts thermoplastic elastomers, whereby the former are present dispersed as particles. The use of fiber inserts, particularly glass and carbon fiber inserts, has also been considered in attempting to improve the splintering safety of thermoplasts, such as acrylic glasses (cf. European Patent 0 510 927).