Stock material or miscellaneous articles – Composite – Of polyamide
Reexamination Certificate
1999-12-15
2001-11-27
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of polyamide
C428S516000, C525S107000, C525S438000, C525S453000, C525S528000, C525S529000, C525S533000
Reexamination Certificate
active
06322900
ABSTRACT:
The present invention relates to a powder coating which is particularly suitable as a coating for packaging containers.
Liquid coating materials are nowadays preferably used for the coating of packaging containers. These materials give rise to numerous environmental problems as a result of their solvent content. This also applies to cases where aqueous coating materials are employed.
It is therefore increasingly being attempted to replace these coating materials by low-solvent or solvent-free substitutes. For example, thermoplastic powder coatings have already been used frequently for the covering of can weld seams. These products are prepared by expensive cold milling from the corresponding thermoplastics.
Furthermore, EP-B 119 164 discloses thermosetting powder coatings for the weld-seam covering of metal containers which are used to hold foods or beverages. The binder in these thermosetting powder coatings is a mixture of an aromatic epoxy resin having on average not more than 2 epoxide groups per molecule and of an aromatic epoxy resin having on average more than 2 epoxide groups per molecule. The hardener used is the condensation product of bisphenol A diglycidyl ether with bisphenol A, and/or an acidic polyester based on trimellitic anhydride/aliphatic polyol. In EP-B-119 164, however, there are no indications regarding appropriate particle sizes and particle-size distributions of the powder coatings. A further disadvantage is that these powder coatings are suitable only for weld-seam covering.
EP-B-10 805 discloses powder coatings for the interior coating of cans, containing a polyester having carboxyl groups and an OH number of less than 10 mg of KOH/g, and an epoxy resin. As curing catalyst these powder coatings contain choline derivatives. The powder coatings have a mean particle size of between 20 and 150 &mgr;m. However, EP-B-10 805 contains no information on how can interior coatings can be obtained that give coherent films even at coat thicknesses ≦15 &mgr;m. Moreover, as a result of the low OH number of the polyester, these powder coatings have the disadvantage of only poor crosslinking. Correspondingly, this system shows drying times which are unacceptable in practice, ranging from 10 to 40 minutes at from 150 to 220° C., whereas the drying time in modern production plants is at maximum 20 to 30 s at an article temperature of from 260 to 280° C.
U.S. Pat. No. 4,497,837 discloses powder coatings for the interior coating of cans and can lids, containing an epoxy resin and aromatic amines, Lewis acids or acid anhydrides as hardeners. The powder coatings have a mean particle size of between 20 and 150 &mgr;m, preferably from 30 to 70 &mgr;m. A disadvantage with these systems is the high minimum coat thickness of 38 &mgr;m in order to achieve coatings without excessive porosity. Furthermore, these powder coatings have the disadvantage that oven residence times of between 5 and 12 minutes are necessary in order to cure the systems described.
Furthermore, U.S. Pat. No. 3,962,486 discloses powder coatings for the interior coating of cans that likewise include an epoxy resin and aromatic amines, epoxy-amine adducts or acid anhydrides. By using the plasma spray coating technique it is possible to produce coatings which meet the usual requirements placed on interior coatings of food packaging even at low coat thicknesses of less than 13 &mgr;m. To ensure the ability for application by the plasma spray technique, the only powder coatings which it is permitted to use are those having a maximum particle size ≦100 &mgr;m and a sufficiently low melt viscosity.
The use of aminic hardeners, however, leads to inadequate sterilization resistance in the resulting coatings. Further disadvantages are that epoxy resins cured with amines tend toward embrittlement and have very poor elasticities. Acid anhydride hardeners have the disadvantage that they are highly irritant, with the result that particular precautionary measures are necessary when formulating the powder coatings.
Finally, U.S. Pat. No. 4,183,974 discloses powder coatings for the interior coating of cans, containing an epoxy resin and an amine hardener. These powder coatings have mean particle sizes of between 1 and 100 &mgr;m, preferably between 1 and 10 &mgr;m. The resulting coatings do indeed have the required low porosity even at coat thicknesses of ≦13 &mgr;m; however, the resulting coatings are in need of improvement. Further disadvantages are that epoxy resins cured with amines tend toward embrittlement and have very poor elasticities.
Coatings produced from powder coating materials for preserve cans, moreover, are also known from German Patent Applications P 40 38 681.3 and P 42 04 266.6.
The results with powder coatings have overall not been satisfactory to date; in particular, increased coat thicknesses are required in order to obtain a uniform appearance. Furthermore, when so-called double cans which are produced for preserve cans are broken open, the broken edges of the coating material are unsatisfactory.
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Aylward D.
Dawson Robert
Meyers Diane R.
PPG Industries Ohio Inc.
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