UV-protected plastic sheet or foil with UV-absorbing outer layer.

The invention relates to an ultraviolet-protected plastic plate or sheet, comprising a core layer of a to UV-radiation-sensitive plastic and at least one side of the core layer a covering layer of lesser thickness than the core layer of a plastics material with a content of UV-absorbing constituents, which is sufficient for the substantial or complete absorption of the incident UV-radiation, one of said layers contains an activatable by UV radiation, the fluorescent dye.

State of the art

A generic smooth plastic plate is known from German Utility Model G 81 13 747th In a typical embodiment, the plastic plate includes a thicker core layer without fluorescent dye and at least one thinner covering layer containing a fluorescent dye. By UV irradiation, the top layer is excited to fluoresce. The presence of the cover layer and its uniformity can be recognized by the occurrence and the luminance distribution of the fluorescent light, which is utilized for monitoring the production process.

In a particular embodiment of the known plastic sheet a thicker core layer containing a UV absorber between two fluorescent dye layers containing trapped. By UV irradiation from both sides, both outer layers may be observed separately because the UV radiation is able to penetrate the core layer.

Frequently a thicker core layer is provided from a sensitive to UV radiation plastic protective thinner layers of a UV-absorbing plastic. Especially in this case, the monitoring of the layer thickness by a fluorescent dye contained in the coating layer is difficult, because the exciting UV radiation is absorbed and will have no effect.

Problem and Solution

The invention has the object, in this case to monitor the thickness and effectiveness of the layers during production - for example, by co-extrusion of the core and outer layers - to enable.

To meet this object, the invention makes use again of the fluorescent light, which is emitted from a contained in the plastic plate fluorescent dye under UV radiation. However, while in the known plastic sheet, the fluorescent dye is included in the thin top layer in order to have them light up itself, according to the invention the fluorescent dye in the core layer.

Fluorescence therefore only occurs when the incident UV-radiation in the covering layer is not absorbed.

The invention is accordingly a UV-protected plastic plate or sheet containing absorbent UV a core layer of a to UV-radiation-sensitive plastic and at least one side of the core layer a covering layer of lesser thickness than the core layer of a plastics material with a content of ingredients, which is sufficient for substantial or complete absorption of the incident UV radiation. According to the invention, the core layer contains an activatable by UV radiation, the fluorescent dye.

If this irradiated alone with an energetic ultraviolet radiation, it would emit fluorescent light. However, when the top layer, sufficiently fulfilled their task of protecting the core layer against UV radiation, as in the core layer no more fluorescence light is generated. If the surface as a result of a manufacturing defect in places missing or as thin or as little effect, that a considerable portion of the incident UV radiation penetrates into the core layer, so there fluorescence radiation is triggered. The appearance of fluorescence light is therefore an indication of defects in the surface layer. In this way, the manufacturing process can be monitored.

A particular advantage of the invention is that the target through the cover layer protective effect is based on the same effect as the prevention of the fluorescence radiation, namely the absorption of UV radiation in the covering layer. Regardless of whether this protection is affected by the absence or a too small thickness of the cover layer or by the absence or degradation of the UV absorber or of a combination of such defects that the appearance of the fluorescent light always shows a lack of the top layer to which remedy is. To detect variations in thickness, although a small decrease in penetrating into the core layer of UV radiation would be sufficient, however, the intended protection of the core layer is achieved only in the substantial or complete absorption of the harmful radiation. A UV absorber which is only partially absorbs the harmful UV radiation to a large extent, but the exciting ultraviolet radiation, would suffice for the purposes of the invention.

Embodiments of the invention

Under UV-sensitive plastics are understood to be the age on prolonged exposure to UV radiation, in particular of the UV component of daylight or sunlight, as exposed by yellowing, fading, embrittlement, dulling the surface or similar symptoms in the radiation surfaces. Most plastics have a certain UV sensitivity, at least when they are exposed in the southern latitudes of the solar radiation. Even polymethyl methacrylate, which is regarded as the plastic with the highest aging, and weather is often equipped for outdoor use with UV absorbers.

A higher UV sensitivity have plastics based on olefins, dienes and vinyl aromatics. Pronounced UV sensitive are polyvinyl chloride plastics and having aromatic groups in the main chain, such as polycarbonate plastics, its most famous representative is the bisphenol A polycarbonate. Plastic plates made of polycarbonate are mainly used with one or both sides protective layers of UV-absorbing polycarbonate in trade.

It is understood that plastic materials are used for the core and outer layers, which are adapted to the requirements of the manufacturing process in a conventional manner. So plastic molding compounds must be used with appropriate melt or solution viscosity for extrusion or coextrusion. Transparent, especially colorless and clear plastics are preferred.

The terms "sheet or film" must be seen comprehensively. These include films less than 1 mm down to 10 microns, solid slabs of 1 to 100 mm, in particular 1 to 10 mm, as well as hollow-chamber plates, which are composed of parallel layers and integrally interposed webs and usually has an overall thickness of 4 to 50 mm have. All these sheets or films are predominantly optionally produced by continuous extrusion, followed by smoothing in a roll stack. Preferably, all the layers involved in the construction of the plates or films are smooth and flat and of uniform thickness.

The UV-absorbing coatings are typically applied during the manufacturing process. A common method is to coat the web surface with a solvent-based paint containing a UV absorber and a plastic material as a binder and is then dried. Preferably, the outer layers are produced in a conventional manner together with the core layer by coextrusion. For this purpose, the method of the adapter co-extrusion can be applied, is generated at the cover layer from the Kernschicht- and the material per a thermoplastic melt strand. In coextrusion, the strands are brought together and adhesively bonded and molded in the distribution channel of a connected slit to the desired width and thickness. Alternatively, the method of the multilayer extrusion can be applied, in which the two melt strands are formed into separate distribution channels to the desired width and thickness, and combined only shortly before the nozzle tip to form a multilayer.

In the co-extrusion, it has been difficult to monitor the thickness and uniformity of transparent and UV-absorbing layers during production. When the core layer itself contains small quantities of UV absorber, which is usual, as the thickness of the covering layer can not follow with reference to the total absorption of the plate in the UV range. The invention is therefore particularly advantageous for the production of sheets and films by coextrusion of the core and outer layers.

Outer layers may be arranged on one or both sides of the sheets and films. They extend generally over the entire surface of the coated core layer side. They are significantly thinner than the core layer and can for example have a thickness of 2 to 100 micrometers. In general, the covering layer accounts for not more than 5% of the core layer thickness, preferably 0.5 to 4%.

Preferably, the outer layers are directly adhered to the core layer and form at the same time at least one surface of the plate or foil. However, this is not required for the operation of the invention, so that, if desired, various modifications are possible. The core and outer layers may themselves be composed of several layers or include other types of layers in between. The UV-absorbing cover layer may be covered by other layers, eg anti-scratch or wasserspreitendem material. All such embodiments belong to the invention, provided a UV-absorbing layer is present that absorbs the light incident on the plate or foil surface UV radiation to such an extent that the fluorescent dye in the core layer does not, or only excited in recognizable weakened contribution to fluorescence light formation becomes.

Preferably the plastic of the core layer contains small amounts of UV-absorbing constituents, but significantly less than the outer layers. Typical is a content of 0.05 to 1 wt .-% of UV absorbers. He can come for example from recycled sheet or film material when doing the UV-absorbing layers are processed as well. The content of UV absorbing components in the core layer, however, must not be so high that it suppressed the exciting radiation and prevents the fluorescence.

The plastic of the outer layer is preferably the same as the core layer. In the top layer, the UV sensitivity of the plastic is sufficiently suppressed by the content of UV-absorbing constituents. Problems that are based on differences between the core and the outer layer plastic, such as insufficient adhesion or in layers different physical, chemical or corrosion behavior are excluded in this case from the outset.

The invention may, however, it also, to use a different plastic material than for the core layer for the top layer. Especially important is its UV absorbing property. It must be all the greater the thinner the top layer. The UV absorption can be a property of the plastic itself or be achieved by interference of UV absorbers. UV-absorbing polymers may be obtained by use of monomers or comonomers with UV-absorbing groups, for example 3- (2-benzotriazolyl) -2-hydroxy-5-tert-octylbenzylmethacrylamid. Among the non-polymer-bound UV absorbers are preferably those of low volatility. Suitable UV absorbers are, for example benzophenones such as 2-hydroxy-4n-octoxy-benzophenone, benzotriazoles such as 2- (2-hydroxy-5-methyl-phenyl) benzotriazole.

The covering layer material is at a layer thickness of at most 1 mm absorb the major part of the UV radiation in the range of 280 to 380 nm, preferably more than 80%, in particular 90 to 100%. For this purpose, generally from 2 to 20 wt .-%, preferably 5 to 15 wt .-% of said UV-absorbing constituents, based on the weight of the facing material needed.

According to the invention, the core layer contains a fluorescent dye which emits in the action of UV radiation in the range of 400 to 500 nm fluorescent light in the visible range. The fluorescent dye is chosen such that the resulting fluorescent light is not absorbed by any coloring in the core or the cover layer. The fluorescent light can be observed in the plan view of the UV-irradiated plate or foil and if necessary quantitatively measured in individual surface sections. With the proviso that the fluorescent dye is evenly distributed in the core layer, the measured value of the fluorescent light gives information about the existing per unit area amount of the UV absorber. In the continuous production of a plate or a foil webs transversely to the production direction traversing measuring device may be used to detect the distribution of the UV absorber over the web width. The measurement with a line camera across the entire web width is possible.

Suitable fluorescent dyes that are preferably colorless in the visible range and can be distributed homogeneously in thermoplastics are known. Preferably, fluorescent dyes that are excited by UV radiation in the range of 230 to 380 nm and emit blue fluorescent light is. Examples of such fluorescent dyes are derivatives of 4,4'-bis-triazinylamino-stilbene-2,2'-disulfonic acid, coumarin, bis-benzoxazolyl compounds, bis-benzimidazolyl compounds, benzotriazoles, pyrazolines, Naphthalsäureimiden and bis-styryl-benzene. If the fluorescent dye is only used for monitoring the production, it needs only a short time to be stable. The fluorescent dye is used advantageously in a concentration of 1 to 10,000 ppm, preferably 10 to 1000 ppm (0.001 to 0.1 wt .-%), based on the weight of the plastic.

If the core layer itself contains UV-absorbing constituents, the penetration depth of the UV radiation is limited. The concentration of the fluorescent dye must therefore be dimensioned so high that in the invaded zone of the UV radiation sufficient fluorescence is triggered.

EXAMPLE

Through adapter coextrusion at a melt temperature of 290^OC is a 4 mm thick plate produced web of bis-phenol A polycarbonate. To form the core layer, a molding compound is in a content of 0.3 wt .-% of 2- (hydroxy-3-sec-butyl-5-tert-butyl-phenyl) benzotriazole as UV absorber and 0,001 % of 2,2 '- (2,5-thiophenediyl) bis (1,1-dimethylethyl) -benzoxazole (Uvitex® OB, Ciba-Geigy) used as a fluorescent dye. For the top layer is the same molding material without fluorescent dye, but used with a content of 10 wt .-% of the above-mentioned UV absorber.

At a thickness of the cover layer of 70 micrometers occurs on irradiation with a UV lamp no fluorescence. Topcoat zones of lesser thickness to fall by fluorescent light. Improved setting the adapter the formation of fluorescent zones with insufficient UV absorption can be avoided.