Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2001-04-26
2003-09-23
Yoon, Tae H. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S487000, C524S490000, C524S904000, C525S934000, C106S270000, C428S484100
Reexamination Certificate
active
06624229
ABSTRACT:
The invention relates to a novel powder coating material which delivers low-noise powder coatings. The present invention further relates to the use of pulverulent, preferably hard linear polyethylene waxes to suppress noises associated with the sliding stressing of powder coatings.
For a number of years, coating materials known as powder coatings have been continually increasing their field of use. Powder coating materials are in the form of a dry powder which is applied in finely divided form to the surface of the article that is to be coated, generally with the aid of electrostatic charging, and is then “dried”, i.e., baked. During baking, which customarily takes place at temperatures between 130 and 220° C., crosslinking reactions and film forming result in the solid coat of paint, or the powder coatings, on the workpiece. Powder coating materials of this kind offer a range of advantages. For instance, owing to the pulverulent consistency of the coating material, there is no need to work with solvents. With powder coating materials, accordingly, all of the environmental problems normally associated with the evaporation of the solvents disappear.
Moreover, the heating energy otherwise needed to evaporate the solvents is spared. A further advantage of powder coating materials is that material which has not remained adhering to the workpiece can be collected from the floor of the powder booth and used again without difficulty. In this way it is possible to achieve a degree of powder utilization of up to 98%. Besides cost advantages, this increased degree of utilization is accompanied by a far lesser environmental burden than in the case of customary coating materials.
Powder coating materials are prepared from binders, curing agents, fillers, pigments, and additives. With regard to the binders and curing agents used, modern-day powder coating materials are based in general on one of the following polymer classes: epoxides; polyesters; polyester/isocyanates (poly-urethanes); or acrylates.
A problem associated with the use of powder coating materials, however, is to obtain a surface having sufficiently good properties. Commonly, there are considerable defects in the baked powder coating film, which reduce the surface smoothness. These surface defects result in part from the fact that the powder of the coating material, applied irregularly with its own structure, does not have sufficient time during baking to flow out during its liquid phase (with a minimum viscosity value) and, in so doing, to provide uniform coverage of the surface of the workpiece. The reason for this is that the process of melting and the achievement of the minimum viscosity is accompanied by the beginning of crosslinking of the binders and hence of polymerization of the coating material. A further important cause of irregular film surfaces is the phenomenon known as cratering. In general, it is impurities in the powder coating material that cause such defects.
In order to improve the surface smoothness of powder coatings, leveling additives have been used for a fairly long time. These additives are intended at the same time to suppress craters. For these purposes it is preferred to use acrylate heteropolymers and homopolymers in the molecular weight range of Mw=5 000 to 200 000. Typical examples thereof are Acronal 4 F® and Byk-360 P® (Acronal 4 F® is a brand of BASF AG; Byk-360 P® is a brand of Byk-Mallinckrodt).
Acronal 4 F® is a poly(n-butyl acrylate). According to its specification, this additive is solvent-free and has nonvolatile fractions of less than 1.5% on 30 min at 140° C. As a resin, it is soft, tough, and tacky, difficult to saponify, and has excellent light stability and aging stability. Used alone, Acronal 4 F® results in a tacky film of low strength. Byk-360 P® is an acrylate polymer which has been applied to a specific silica. The active substance is compounded at 60% to form a white, free-flowing powder. This leveling additive is equally effective in powder coating materials based on epoxy, acrylate, polyester, and polyurethane. It is mixed with resin, curing agent, and pigments and then extruded with all of the components.
Occasionally, low molecular mass polyesters are used as well.
The results obtained do not, however, meet the requirements imposed on the surface.
It is true that a considerable advance has been provided here by polyvinyl ethers, especially polyvinyl ethyl ethers having a number-average molecular weight Mn of from 1000 to 2000 and a mass-average molecular weight Mw of from 3000 to 4000.
However, these additives are likewise unable to suppress or prevent completely the noises, especially the squeaking and scratching noises, which are manifested, in many cases unpleasantly, during the sliding stressing of powder coatings. These noises occur, for example, with slide rails that are coated with powder coating materials when slide carriages are moved in them. Slide rails of this kind are used, for example, in the case of automobile bodies in order to mount and guide the slide carriages of the windows and of the seats.
EP 0 750 024 discloses, for example, additive formulations which are incorporated into waxes. However, it does not describe the production of low-squeak powder coatings. The prior art also discloses the use of wax as a grinding aid. The wax here is added only following extrusion. In other words, the wax is used at temperatures which lie below the extrusion temperature of the powder coating material. As a result, however, the wax is not incorporated homogeneously, i.e., is not an integral component of the powder coating material. The present invention set itself the object of providing powder coating materials and powder coatings which, with a view to the abovementioned low-noise requirements are improved in relation to the prior art.
This object is achieved in accordance with the invention by the powder coating material which comprises homogeneously distributed therein at least one pulverulent polyethylene wax. It is preferable in this case for the powder coating material to be preparable by adding the polyethylene wax to the powder coating mixture, prior to or during the extrusion of the powder coating material, in such a way that it is homogeneously incorporated. With preference, the powder coating material contains from 0.05 to 5% by weight, preferably from 0.5 to 2% by weight, of a polyethylene.
In accordance with the invention, the wax is not used as a grinding aid following extrusion but instead is incorporated in such a way that it becomes an integral part of the powder coating formulation. Accordingly, incorporation takes place preferably by addition of the wax prior to or during extrusion of the powder coating material, i.e., conjoint extrusion of the wax and of the powder coating material. The extrusion per se may be carried out in accordance with the commonly known methods.
The wax in the resulting powder coating brings about a degree of gloss of the resulting—i.e., cured —powder coating that corresponds to a slight flatting effect. This flatting corresponds advantageously to a degree of gloss of from 30 to 70%, in particular from 40 to 60%, measured at an angle of 60°.
The present invention further provides a process for preparing a powder coating material, in which at least one polyethylene wax is homogeneously distributed in the powder coating material. It is preferred to incorporate from 0.05 to 5% by weight, preferably from 0.5 to 2% by weight, into the powder coating material. The polyethylene wax is preferably incorporated at temperatures which correspond approximately to the extruder temperature during the preparation of the powder coating materials. It is preferred, accordingly, to incorporate the wax during the extrusion of the powder coating material. Nevertheless, it is possible in accordance with the invention to use all methods which ensure homogeneous distribution.
It is preferable for the dropping point of the waxes used to lie within the range of the extruder temperature during the preparation of the powder coating material. In
Boysen Rolf
Perdun Birgit
Riestenpatt Helmut
BASF Coatings AG
Yoon Tae H.
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