Polycarbonate plastic panel.

p0001The invention relates to a plastic sheet consisting of a core layer of polycarbonate plastics and, at least on one side of the core layer, from a co-extruded outer layer of plastic which contains at least 3 wt .-% of a UV absorber.

p0002Polycarbonate plastics have only limited weather resistance. Especially by UV radiation they are damaged. In the plastic introduced UV absorbers inside a plastic sheet of polycarbonate are not able to protect, but the surface. For their protection is applied to DE-OS 1694273 a paint layer, which may consist of 3 parts of a UV absorber and 5 parts of polymethyl methacrylate or polycarbonate.

p0003According to DE-OS 28 32 676 a polymethyl methacrylate is applied containing a UV absorber in the production of polycarbonate plastic sheet by coextrusion.

p0004In both cases Polymethacrylatkunststoffe are preferably used for the cover layer because they are highly weather-resistant, and its surface is less than the carbonates of poly damaged by UV radiation. A drawback of the top layer of polymethacrylate is their - compared to polycarbonates - lower toughness. If the polycarbonate plastic sheet under tension, for example in the bent state, postponed, so come into the convex surface at tensile stresses that can cause cracking in the top layer. To the naked eye appears the increasing cracking as gradual dulling of the initially shiny surface. Furthermore the cracks have a tendency to continue running in the polycarbonate core layer so that the coated panel does not retain the high breaking strength of the polycarbonate resin. Especially the fracture susceptibility Hagelachlag increases. These disadvantages make itself hollow profile plates even more noticeable because there occur in curved laying especially high strain on the convex side.

Problem and Solution

p0005The aim of the invention was to avoid caused by the polymethyl methacrylate reduction of breaking strength, without sacrificing the protection of the UV absorber contained therein.

p0006It has surprisingly been found that the disadvantages can be avoided, but the protective effect of the coating layer is fully retained, if this consists of coextruded polycarbonate plastic. Through the use of similar material for the core layer and the outer layer both layers have the same extensibility so that no increased cracks occur even under stress in the cover layer. is surprising that at the surface of the covering layer no rapid weathering occurs when the top layer is at least 3, preferably at least 5 wt._-% contains a UV absorber, although at the surface of the polycarbonate cover layer of the UV radiation is fully exposed. The absorption of UV radiation can only occur inside the top layer. The observed benefits only occur when a produced by coextrusion with the core layer topcoat.

Embodiments of the invention

p0007The simplest embodiment is shown in Figure 1 in a partial cross section of a single-side coated full board. Figure 2 shows in a diagram, as a preferred embodiment of an extruded hollow panel with double-sided coating.

The core layer (1)

p0008a full panel may have a thickness of, for example 2 to 10 mm, preferably 3 to 8 mm. In general, the outer sides of the core layer are mutually parallel and flat, but they may also be corrugated or - in particular in the installed state - be uniaxially bent. The core layer may also be curved dome-shaped or three-dimensionally deformed in another way.

p0009The core layer (1) can also be one or more hollow chambers (2), which preferably mutually extend in parallel in a straight line through the core layer. Preferably, it is composed by double-skin or triple plates in which the core layer (1) consists of two flat, parallel walls (3,4), which are connected by webs (5) integral with each other. The hollow chambers (2) can optionally be subdivided by one or more further partitions. The walls (3,4) and webs (5) typically have a wall thickness of 0.3 to 2 mm. The core layer including the hollow chambers is for example 5 to 50, preferably 6 to 10 mm thick.

p0010The surface area and shape of the core layer is subject to the requirements of the intended use and is under the technical production conditions largely arbitrary. Lie flat panels have suitably a rectangular surface and edge lengths of 0.5 to 6 m. Longer full plates or sheets can be rolled.

The outer layer (6)

p0011The core layer (1) on at least one, preferably on both sides a top layer (6), which differs from the core layer by a higher content of UV-absorbers. Its thickness should be sufficient to a perpendicular incident UV radiation from 280 to 380 nm to more than 90% being preferred, to absorb more than 99% (based on the non-reflected part). For sufficient thickness of 5 to 50 in order from, but layer thicknesses can be applied to 100 microns.

p0012The cover layer contains at least 3 wt .-%, and preferably 5 to 15 wt .-% of a conventional UV absorber for plastics. The preferred content of UV absorbers is between 0.4 and 2.5 g / m²But may optionally up to 5 g / m² be increased. The concentration is therefore selected to be higher, the thinner the top layer, so that the required ADSORPTION occurs within the outer layer. Below 0.4 g / m² is no longer guaranteed to weathering of protecting the surface of the cover layer. The protection is all the more effective, the higher the absorber amount used per square meter of total lies. Suitable UV absorbers are 2-hydroxy-4-n-octoxybenzo- phenone, 2-hydroxy-4-methoxybenzophenone, 2- (2'-hydroxy-5'-methylphenyl) benzotriazole, and other derivatives des'2-hy- droxy -benzophenons or benzotriazole, further 2,4-Dihydroxybenzoylfuran, phenyl salicylate, Resorcindisalicylat, Resorcinmono- and di-benzoate, benzyl benzoate, stilbene, ß-methylumbelliferone and its benzoate. Many other UV absorbers are known and commercially available. Particularly preferred are UV-absorber of low volatility at the processing temperature, that is, particularly those with the highest possible molecular weight. The UV absorber should be at the selected concentration with the plastic material homogeneously miscible. The core layer can be completely free of UV absorbers or including in significantly lower concentration than the top layer. A concentration of 0.5 wt .-% is not exceeded.

p0013The polycarbonate plastic is derived usually from an aromatic bisphenol or mixture of such bisphenols, especially bisphenol A. All polycarbonate plastics, which can be processed into sheets of high strength and light transmittance, are suitable as a material for both the core layer and the outer layer ,

p0014These layers are preferably, but not necessarily of the same extrudable plastic material, except for the concentration of the UV absorber. You can be clear and colorless or colored, clouded or pigmented in the usual way, but the invention has for highly light permeable masses are the most important.

p0015The cover layer is formed simultaneously with the core layer by coextrusion.

p0016Man used for a known multi-fuel slot nozzles in the two extruders, the plasticized compositions for the core and outer layer are pressed. Extrusion takes place at temperatures between 240 and 300 ° C.

application

p0017The panels according to the invention are suitable as glazing and structural elements, especially outdoors, where exposed to sunlight page carries the top layer. Advantageously they are used where expansion forces can act on the panels, eg for curved laying of full panels or polycarbonate sheets. This installation method is frequently used in greenhouse construction and has in the past led to severe hail damage. The invention such damage.

example

p0018By co-extrusion by means of a multi-fuel slit nozzle which is fed by two extruders, a two-layered polycarbonate sheet is generated. In an extruder an extrudable thermoplastic polycarbonate molding composition without UV absorber is processed. The second extruder conveys a mixture of 90 wt .-% of the same polycarbonate molding composition and 10 wt .-% 2-hydroxy-4-n-octoxybenzophenone as UV absorbers in the multi-component nozzle. The flow of this mixture forms a cover layer of 30 microns thickness on a surface of the core layer. This is formed from the pure polycarbonate molding compound and is 3mm thick. The plastic melt is smoothed after leaving the nozzle in a three-roll polishing stack.

p0019In continuous sunlight, the coated panel casts a uniform shade, which reveals no surface defects. A similar picture emerges from the surface of the panel to a certain screen reflected sunlight.

p0020The panel thus produced is elastically bent with a curvature radius of 0.5 mm and subjected to weathering in outdoor. After two years of weathering, the panel is free of cracks. For comparison, an extruded, smoothed by smoothing plate is then coated on the extrusion on a surface. The paint is applied evenly by opposing the direction of extrusion coating application rollers. The varnish solution consisting of 18% polycarbonate having a molecular weight of about 20,000, 2% of the above-mentioned UV absorber and 80% dichloroethane. The solvent is then driven off in a drying oven at gradually increasing temperatures up to 100 ° C. The final coating thickness is 30 pm.

p0021The silhouette of the panel shows some pimples, orange peel-like surface structures and optical disturbances due to incomplete history and inclusions of dust particles.

p0022In the weathering of the elastically curved panel in the manner described above are numerous cracks already after 1 year, some, after 2 years recognizable. The cracking is to be regarded as a result of remaining in the polycarbonate solvent residues and the low molecular weight. The lower molecular weight is required to achieve a high concentration coating, whereby additional surface defects are avoided as a result of skin formation upon evaporation of the solvent and flow problems.