Stock material or miscellaneous articles – Composite
Reexamination Certificate
2000-12-18
2003-06-03
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
C428S412000, C427S428010, C264S129000, C264S130000, C264S211130, C264S211180, C264S211200
Reexamination Certificate
active
06572970
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to planar mouldings consisting of thermoplastics and provided with a water-spreading coating, to two processes for their production and also to their use.
BACKGROUND OF THE INVENTION
The term “water-spreading” is to be understood to mean the property of a surface for forming a contact angle of below 20 degrees with a drop of water applied on it. A water-spreading coating is accordingly a coating that brings about this property of a surface.
Mouldings with a water-spreading surface have the property that water which gets onto their surface is not concentrated there into drops which are separate from one another but rather that the drops spread out and, when they come into contact, run together to form a closed layer. As a result, an improved reflection of light on the surface that is moistened with water and a better transmission of light in the case of transparent mouldings are achieved. In addition, the dripping of water from the underside of the moulding is rendered difficult. These so-called anti-drop properties which impede the formation of drops are demanded, in particular, for various glazing materials consisting of thermoplastics. In such cases it is desired that condensed water or condensate water deposited thereon does not fall off in the form of drops but rather that it runs off, following the downward gradient of the material, in a closed layer or at least in coherent paths on the lower edge.
Behaviour contrary to that of a water-spreading surface is displayed by the water-repellent surface. On water-repellent surfaces, water which gets onto such a surface is concentrated into drops that are separate from one another.
Numerous attempts to provide water-repellent surfaces of plastics with water-spreading layers are known from the literature. According to DE-A 21 61 645, coatings of this type are produced from a mixed polymer consisting of alkyl esters; hydroxyalkyl esters and quaternary aminoalkyl esters of acrylic or methacrylic acid and methylol ethers of methacrylamide by way of crosslinking agent. They firstly take up water, subject to swelling, and gradually pass over into a water-spreading state. However, as a consequence of the swelling the coating becomes soft and sensitive to mechanical damage.
With a view to improving the mechanical strength of water-spreading coatings, inorganic constituents such as colloidal metal oxides, in particular aluminium oxide, or colloidal silicon dioxide have been worked into the coating compositions (EP-A 7 681 877 or EP-A 7 606 193).
In order to achieve higher mechanical stability, coatings with hydrophilic inorganic constituents in a hydrophilic binding agent have been developed. According to JP-A 76 81 877, polyvinyl-chloride films or polymethyl-methacrylate films are covered with a coating consisting of colloidal aluminium oxide by way of hydrophilising, hard constituent and polyvinyl alcohol and ammonium polyacrylate by way of binding agent. However, in the state swollen with water this coating is also sensitive to mechanical loads.
There have also already been attempts to work wetting-friendly agents into the plastic material itself from which the moulding is produced. For instance, water-spreading coverings for greenhouses and similar damp rooms are produced, according to DE-A 2 017 002, from a plastic that contains surface-active agents such as polyalkylene glycol. The water-spreading effect of this additive is not adequate. The resistance of the plastic to weathering is also impaired.
In JP-A 76 06 193 polymethacrylate sheets with a coating consisting of 95 parts of colloidal silicon dioxide and 5 parts of a dispersion of a hydrophobic acrylic resin are proposed as glazing means. However, the adhesion of this coating is totally unsatisfactory. This is true, above all, with respect to the moist state.
A better adhesion of a water-spreading coating on plastic mouldings is achieved, according to EP-A 51 405, with a covering synthesised from two layers, both layers containing colloidal silicon dioxide, a partially hydrolysed polysiloxane and polyvinyl alcohol by way of binding agent. The ratio of silicon to carbon is greater in the lower layer than in the outer layer.
Generalising, it is possible to note that although a coating having a good water-spreading property can usually be achieved with strongly hydrophilic covering materials, as a rule the coating is too soft in the swollen state. If it is desired to counteract this disadvantage by means of a stronger crosslinking or less hydrophilicity, then the water-spreading effect declines simultaneously with the mechanical sensitivity. Although silicon dioxide and various other oxides of metals or semimetals combine the advantages of great hardness and good wettability by means of water without swelling, they have the disadvantage that they do not adhere at all.
To the extent that use is made of binding agents with a view to anchoring the oxides on the surface of the plastic, the wettability of the oxides and hence the water-spreading effect of the coating declines, and the disadvantages of the binding agents appear: mechanical sensitivity in the case of hydrophilic binding agents, and insufficient spreading of water in the case of hydrophobic binding agents.
In DE-A 34 00 079 it was proposed to bind a water-spreading layer consisting substantially completely of silicon dioxide or other metal oxides of colloidal particle size, which itself has an insufficient adhesive strength on the layer of plastic, onto the water-repellent surface of a plastic moulding in firmly adhering manner by means of an adhesion-promoting layer of an organic polymer having polar groups that is not soluble in water and that is substantially not swellable.
All the named coating agents have the disadvantage that they have to be applied from organic solvents.
The processes for applying the coating agents mentioned in the state of the art onto mouldings consisting of thermoplastics are elaborate.
SUMMARY OF THE INVENTION
With respect to the state of the art that has been mentioned it was therefore a matter of making available a rational coating process which follows the production of the mouldings immediately—i.e., on-line—and with which it is preferably possible to apply an aqueous coating agent even without providing a layer of adhesion promoter. In addition, a uniform and smooth coating of the surface of the moulding is to be guaranteed. The object further underlying the present invention is to make available planar mouldings consisting of thermoplastics and provided with a water-spreading coating and also to make possibilities available for their use.
In accordance with the invention, this object is achieved by means of a process comprising:
(i) preparing a planar thermoplastic molding by means of extrusion;
(ii) cooling said planar thermoplastic molding;
(iii) applying an aqueous based coating composition onto at least one side of the cooled planar thermoplastic molding, said coating being applied by means of an application roll immediately after production of said planar thermoplastic molding; and
(iv) drying the applied coating on said planar thermoplastic molding, wherein said aqueous coating composition comprises,
A) 0.005 to 2 parts by weight of a compound represented by general formula (1),
for which,
R
1
is a C
1
-C
30
hydrocarbon radical,
R
2
is hydrogen or a C
1
-C
8
hydrocarbon radical,
A is a single bond or a divalent C
1
-C
30
hydrocarbon radical,
B is a single bond or a divalent C
1
-C
30
hydrocarbon radical,
n is 1, 2, 3 or 4, and
M
n+
is a cation having an n-fold positive charge,
B) 1 to 20 parts by weight of at least one water-insoluble oxide of a metal or of a semimetal,
C) 80 to 100 parts by weight of a mixture of an acid and water, more than 90% of which consists of water,
provided that the pH of said aqueous coating composition is less than 6.
In an embodiment of the present invention, the application roll rotates contrary to the direction of transport of the planar thermoplastic molding that is to be coated (refe
Anders Siegfried
Bier Peter
Capellen Peter
Röhner Jürgen
Bayer Aktiengesellschaft
Dawson Robert
Feely Michael J
Franks James R.
Gil Joseph C.
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