Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-10-16
2004-06-15
Woodward, Ana L. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S107000, C525S124000, C525S185000, C525S528000, C525S533000
Reexamination Certificate
active
06750300
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a thermosetting hot melt paint composition and to an application method for such hot melt paint compositions. Hot melt paint compositions are typically first heated until the required application viscosity is reached. Subsequently, the compositions are applied and cured, for instance by further raising the temperature to the curing temperature. In general, hot melt coatings are substantially free of volatile organic compounds (VOC).
BACKGROUND OF THE INVENTION
EP-A 0 604 815 discloses a solvent-free thermosetting coating composition for silk screen printing. The coating composition comprises a blend of liquid and solid epoxy resins and has a high viscosity, making this coating composition unsuitable for use in application techniques requiring a low viscosity, such as curtain coating.
Application techniques such as curtain coating are specifically useful when thin layers have to be applied. A particular example of an application requiring thin film layers is mirror backing, when a coating has to be applied to the back of a reflective metallic layer of mirrors to protect them against corrosion and mechanical and chemical impact. These mirror backing coatings are typically applied in one or more layers having a thickness of the order of about 20-100 microns.
Hitherto, paint compositions used in application techniques requiring a low viscosity, such as the mirror backing coating disclosed in EP-A 0 562 660, have contained solvents. However, the use of solvent borne paint compositions is encountering ever more serious environmental difficulties. For environmental as well as economic reasons, the solvent content should be minimized. To acquire anticorrosive properties, mirror backing coatings typically include corrosion inhibitors, most of which are detrimental to people's health or the environment.
SUMMARY OF THE INVENTION
The object of the invention is to provide a substantially VOC-free coating composition which can be used at application temperatures between 100-160° C., or at even lower temperatures using application techniques requiring a low viscosity, such as curtain coating or spray coating.
The object of the invention is achieved with a hot melt paint composition having a Tg below 5° C., preferably below −20° C. and a viscosity below 2,000 mPa.s, preferably below 600 mPa.s, measured at 80° C. at a shear rate of 10 s
−1
. It has been found that such hot melt compositions, unlike the prior art hot melt paints, are suitable for application techniques requiring a low viscosity without the need to add solvents. As a result of the low viscosity of the hot melt paint composition according to the invention, relatively low application temperatures, e.g., below 120° C., can be used while still maintaining good paint properties like flow and appearance. The composition according to the invention can, e.g., be solid or liquid at room temperature.
DETAILED DESCRIPTION OF THE INVENTION
In a preferred embodiment of the hot melt paint composition according to the present invention, the composition includes at least one amorphous binder having a Tg below 5° C., preferably below −20° C., and/or at least one crystalline or semi-crystalline binder having a melting point or melting range below 150° C. Semi-crystalline and/or crystalline binders are generally characterized by a sharp phase change resulting in very low melt viscosities after melting. Also combinations of amorphous and (semi)-crystalline binders can be used. By combining amorphous binders with (semi)-crystalline binders, the melt viscosity of the hot melt paint composition can easily be adjusted to the desired level. Optionally, the binder may have a hyperbranched or dendritic structure.
In order to examine if a binder is crystalline, semi-crystalline, or amorphous, these types of phase changes can be detected by Differential Scanning Calorimetry (DSC), as described in
Encyclopedia of Polymer Science and Engineering,
Volume 4, pages 482-519, 1986 (Wiley Interscience). A binder is considered to be amorphous if it shows a discernible glass transition temperature (Tg) and has neither crystallization nor melting peaks. A resin is considered to be semi-crystalline if it shows a discernible Tg and at least one melting peak. In general, when different melting peaks in a DSC curve are observed, these multiple peaks are specified by a melting range. If a binder does not show any Tg on heating from −60° C., but only a sharp melting peak, the binder is considered to be crystalline.
The binder of the hot melt coating composition may for example be an alkyd resin, an acrylic resin, a polyester resin, a urethane oil or resin, a vinyl polymer, a vinyl halide polymer, a phenol-formaldehyde resin, an amino resin, an epoxy resin, or modifications and/or combinations thereof.
The binder is either self-crosslinking or is used in combination with a corresponding crosslinker, for example blocked or unblocked isocyanates or amino-functional crosslinkers. Suitable examples of amino-functional crosslinkers are for instance melamine-formaldehyde resins like hexamethoxy methylol melamine (HMMM) or trisalkoxy carbonyl amino triazine (TACT) or derivatives thereof. Commercially available HMMM resins are for example Beetle® 370, Beetle® 3745, and Beetle® 3747, all available from BIP. Suitable isocyanates are for example Desmodur® N3300, available from Bayer, and Vestanat® T1890, available from Creanova.
Binders reactive with these isocyanate or amino-functional crosslinkers are for instance hydroxy-functional binders. Preferred hydroxy-functional binders are acrylic polyols, polyether polyols, and polyester polyols. Examples of suitable amorphous polyester polyols are Desmophen® 670, 800, 1155, and Desmophen® VPLS 2249/1, all available from Bayer. A suitable example of a dendritic polyol is Boltorn® H10, available from Perstorp Specialty Chemicals.
Polyol/isocyanate crosslinking systems as such are highly reactive. A suitable way to prevent premature curing is to use internally or externally blocked isocyanates. An example of an internally blocked polyisocyanate crosslinker is a cycloaliphatic polyuretdione based on isophorondiisocyanate, Vestagon® EP-BF 1320, available from Creanova.
Another way to prevent the premature crosslinking of such highly reactive crosslinking systems is the use of two-component systems, requiring separate storage of the reactive components and special application equipment. The reactive components are mixed with each other just before or during application, for instance in a two-component spray-gun. Such a two-component arrangement is not only suitable for isocyanate/polyol crosslinking systems using unblocked isocyanates, but also for other highly reactive crosslinking systems, for instance for epoxy/amine and acryloyl/amine crosslinking systems.
Hot melt coating compositions based on polyol/isocyanate chemistry have the advantage of combining a low VOC level or even zero VOC with a high level of film performance. Such hot melt coating compositions are particularly suitable for use in for example the automotive and the vehicle refinish industry.
Another class of suitable binders consists of epoxy resins, optionally in combination with compounds containing epoxy-reactive groups, e.g., amines, polycarboxylic acids and/or anhydrides.
Particularly suitable epoxy resins are for example Epikote® 828, Epikote® 1001, and Epikote® 1002. Viscous epoxy resins, such as Epikote® 1004, should preferably be used in smaller amounts in order to obtain the required viscosity. All the aforementioned Epikote® resins are available from Shell Chemicals.
If epoxy resins are included in the hot melt coating composition according to the present invention, it is preferred to use a compound containing cyanamide groups and/or derivatives, such as dicyandiamide, accelerated dicyandiamide, substituted dicyandiamide, or cyclic amidines. Mirror backing coatings formulated with cyanamide-functional compounds have the advantage that they can form a complex with the silver ions present i
Koenraadt Martinus Adrianus Anthonius Maria
Mahieu Philip
Qazi Faridoon
Sörensen Kent Raabjerg
Vanderveken Didier
Akzo Nobel N.V.
Parker Lainie E.
Woodward Ana L.
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