Light transparent body of coextruded synthetic resin

What is disclosed is a light-transparent body comprising a first coextruded layer of a first extrudable light-transparent synthetic resin and a second coextruded layer of a second extrudable light-transparent synthetic resin different from said first resin, wherein said first and second coextruded layers are adjacent and wherein at least said first layer contains a dyestuff capable of fluorescing in the visible region and said second, adjacent, layer is free of the dyestuff present in said first layer.

The present invention relates to a light-transparent body comprising a 
plurality of coextruded layers of synthetic resin and to methods for 
making the same. 
In the manufacture of light-transparent bodies of coextruded synthetic 
resin comprising at least two layers of different, extrudable, transparent 
synthetic resins, the difficulty arises of determining the thickness and 
uniformity of the synthetic resin layers forming the body by observation 
of its surface or of a cut edge. This difficulty is particularly great if 
the layers of the body are of the same color or, more particularly, if 
they are colorless. To be sure, different synthetic resins often have 
different absorption properties at different wavelengths. Nevertheless, 
for thin layers it is hard to determine layer thickness sufficiently 
exactly from the absorption of penetrating radiation of such a wavelength. 
The present invention has as its object the preparation of multi-layered 
coextruded light-transparent bodies of synthetic resin in which the 
thickness and uniformity of at least one of the layers present can be 
determined easily and surely by looking at the body or at a cut edge 
thereof. This object has been achieved by the synthetic resin body 
described and claimed herein and by the method of making it. 
The present invention involves the coloring of at least one layer of such a 
body with a dye fluorescing in the visible region, which dye preferably 
does not absorb in the visible region and the fluorescence of which is 
stimulated by ultraviolet radiation. The light from the fluorescence 
travels in part through the surface of the resin layer to the outside and 
can be detected by observing the layer. The remaining portion of the 
fluorescent light is transmitted within the layer by total reflection and 
emerges on the cut edges of the layer. Whereas the fluorescent light 
emerging through the surface is proportional to the layer thickness, the 
light emerging on a cut edge additionally depends on the size of the area 
which is irradiated. Non-uniformities in the fluorescing layer are for the 
most part detectable by simple observation of the irradiated synthetic 
resin body, without further agencies. For a more exact investigation, a 
light-measuring apparatus is recommended. A visual observation of the cut 
edges permits a visual determination of the outline of the fluorescing 
layer and the form of its boundary surface with the neighboring, 
non-fluorescing layer. For an exact determination of layer thickness from 
the intensity of the fluorescent light emerging from an edge, a 
light-measuring apparatus is usually used. In this case, layer thickness 
is determined as a function of the intensity of the activiting radiation, 
of the intensity of the fluorescent light, of the concentration of the 
fluorescing dye, and of the size and geometry of the test body being 
investigated. Suitably, a standard curve is prepared which takes these 
parameters into consideration. 
If the synthetic resin body comprises more than two layers, more than one 
layer thereof may contain a fluorescent dyestuff. However, those layers 
whose thickness and uniformity are determined should not be adjacent any 
other layer which contains the same fluorescent dyestuff in an effective 
amount. However, various layers can contain different fluorescent 
dyestuffs which emit fluorescent light capable of being differentiated or 
which are excited to fluorescence by different radiation. If the synthetic 
resin body contains several fluorescing layers which are separated by 
non-fluorescing layers, the fluorescent light detectable by observation 
under certain circumstances given only information concerning the sum of 
the thicknesses of the fluorescing layers. The separating layers can have 
an absorbent effect on the irradiation which initiates fluorescence, so 
that if the irradiation is from one side of the body only, only one of the 
layers containing a fluorescent dyestuff may fluoresce. In this case, two 
separated layers can be investigated separately for their uniformity from 
both sides of the body.

Both FIGS. 1 and 2 show a body of coextruded synthetic resin comprising a 
non-fluorescing layer 2 having thereon at least one adjacent fluorescing 
layer 1, as particularly shown in FIG. 1. In the embodiment of FIG. 2, the 
body comprises a second fluorescing layer 1', with layers 1 and 1' forming 
the exterior sides of the synthetic resin body. 
Suitable fluorescent dyestuffs which are preferably colorless in the 
visible region and which can be homogeneously distributed in thermoplastic 
forming masses are known in the art. Dyestuffs which can be stimulated by 
ultraviolet radiation in the region from 230 to 380 nanometers and which 
emit blue fluorescent light are preferred. Examples of such kinds of 
fluorescent dyestuffs are derivatives of 
4,4'-bis-triazinylamino-stilbene-2,2'-disulfonic acid, coumarin, 
bis-benzoxazolyl compounds, bis-benzimidazolyl compounds, benztriazoles, 
pyrazolines, naphthalic acid imides, and bis-styryl-benzene. If the 
fluorescent dyestuff merely serves for monitoring the preparation of the 
body, it need only be stable for a short period of time. The fluorescent 
dyestuffs are suitably added in a concentration from 1 to 10,000 parts per 
million, preferably from 100 to 1,000 parts per million (i.e. 0.01 to 
0.1%), calculated on the weight of the synthetic resin. 
In the preferred case, all layers of the synthetic resin body are 
transparent and colorless. The non-fluorescing layer, which as a rule 
forms the core of the synthetic resin body and is the heaviest layer, can 
be a solid body or can define a hollow body, for example a hollow body 
such as a tube or a double walled sheet the walls of which are supported 
by intermediate supports. The layer thickness can be from 1 to 10 
millimeters, for example, without taking into consideration any cavities: 
in the case of films having many layers, the layer thickness can also be 
less than this. The fluorescing layer is as a rule less than 1 mm thick 
and, preferably, at most 0.1 mm thick. Thicknesses below 0.001 mm are no 
longer suitable for the fluorescing layer, since they are difficult to 
prepare and the fluorescent effect is also then too weak. For the reasons 
discussed above, but also for protection against weathering, it can be 
suitable to provide the non-fluorescing layer with an ultraviolet absorber 
in conventional amounts. Suitable absorbers are, for example, esters of 
2-cyano-3,3-diphenyl-acrylic acid, benzophenone, and benztriazoles such as 
2-hydroxy-5-methyl-phenylbenztriazole. The fluorescing layers can contain 
ultraviolet absorbers of such a nature and in such amounts that the 
fluorescent effect is not annulled. 
The outer layer of a multi-layered coextruded body of synthetic resin often 
must protect the underlying layers from the influence of weathering and 
for this reason as a rule is generally made of a material which is highly 
weather resistant. Polymethylmethacrylate or copolymeric synthetic resins 
predominantly comprising methyl methacrylate are preferred. These resins 
are often also suitable as adhesion promoters for further layers to be 
applied thereto, for example scratch-resistant layers. The preferred 
synthetic resin bodies according to the present invention thus contain one 
or two surface layers from the aforementioned homo- or co-polymers of 
methyl methacrylate which contains a fluorescent dyestuff. The core 
preferably comprises a layer of a different resin, for example a 
polycarbonate--which is preferred--, or of polyvinyl chloride, 
impact-resistant modified polymethylmethacrylate resins, polyethylene, 
polystyrene, or styrenebutadiene copolymers. It is self-explanatory that 
all the layers must consist of thermoplastic extrudable synthetic resins. 
The technique of coextruding different synthetic resins which are melted in 
separated extruders and are brought together in a common co-extrusion 
nozzle to form a multi-layered synthetic resin body is known in the art 
and is performed in its usual manner for the purposes of the present 
invention. However, at least one thermoplastic extrudable, 
light-transparent forming mass is used which contains a dyestuff capable 
of fluorescing in the visible region, whereas at least one further, 
different, forming mass is also employed which does not contain this 
dyestuff. Suitably, the continuously flowing synthetic resin sheet is 
irradiated with radiation providing fluorescence somewhere after leaving 
the extrusion nozzle, but in any event before that site in the emerging 
strand of the body at which the strand is cut up into segments or is 
rolled up. In this way, possible non-uniformities in the fluorescing layer 
can be detected by visual observation. For a more certain determination, a 
cut edge can be continuously or periodically produced and the fluorescent 
light emerging therefrom can be measured. 
A better understanding of the present invention and of its many advantages 
will be had by referring to the following specific Example, given by way 
of illustration. 
EXAMPLE 1 
Preparation of polycarbonate panels having good transparency and good 
resistance to weathering 
Good weather resistance is obtained by coating with a thin film of 
polymethylmethacrylate (PMMA). So that the toughness properties of the 
polycarbonate (PC) panels are maintained as much as possible, the PMMA 
layer shall not be thicker than 30 microns. 
A glass-clear polycarbonate train, 400 mm wide and 3 mm thick, is formed at 
a temperature of 270.degree. C. using a coextrusion nozzle having a nozzle 
imput of 0.4 meter/minute and is simultaneously covered with glass-clear 
polymethylmethacrylate at 220.degree. C. in the nozzle, using a 
three-layer nozzle [cf. multi-layered nozzles in Michaeli, 
"Extrusionswerkzeug fuer Kunststoffe" ("Extrusion Apparatus for Synthetic 
Resins"), Hanser Verlag 1979]. For control of the thickness distribution 
of the PMMA-layer, an adjustable dam is present in the nozzle, as usual. 
In advance, 0.02 percent of 2,5-bis[s'-t-butylbenzoxazolyl(2')]thiophene, 
an optical brightener commercially available under the name "UVITEX OB", 
is added to the PMMA granulate fed to the PMMA extruder. The nozzle exit 
is extensively protected from daylight. The surface of the emerging train 
is irradiated with a mercury high-pressure radiation source having a 
black-glass bulb of the HQV type, 125 watts, manufactured by Osram. It can 
be determined, from variations in the fluorescence, if the polycarbonate 
train is uniformly coated all over with PMMA. If this is not the case, the 
flow of forming material is corrected using the dam. In this way, the PMMA 
layer thickness can be maintained in the region from 20-30 microns over a 
production time of several hours.