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
1999-01-21
2001-02-27
Henderson, Christopher (Department: 1713)
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
active
06194522
ABSTRACT:
The present invention relates to crosslinkable compositions. In particular, the present invention relates to compositions which include a polymer having sulfur and/or phosphorus-containing acid groups and an aliphatic polycarbodiimide cross-linker. The crosslinkable compositions include film-forming compositions such as paints, lacquers, varnishes, sealants, nonwoven fabric binders, leather coatings, and adhesives.
It is well-known that the addition of a cross-linker to a film forming coating formulation can improve certain important properties of the formed coating, such as its hardness, solvent resistance and mar resistance. However, it is desirable to have a crosslinking mechanism which is operative during or after film formation. If the cross-linking mechanism has proceeded materially before film formation has been substantially completed, the overall film formation process will be compromised and the resultant film will be weak and porous with the result that the protective functions of the film will be severely impaired. In some applications heat may be used to assist film formation and crosslinking. However, in many cases, heat is not available so the crosslinking mechanism is desirably triggered under ambient conditions before film formation is complete.
Carbodiimides having the general structural formula of r-(N═C═N-r′)x, wherein r and r′ are aliphatic and/or aromatic groups, have been used as cross-linkers. In this regard, U.S. Pat. No. 4,977,219, EP-A-0277361, et al. disclose aliphatic polycarbodiimide cross-linkers, wherein both r and r′ are aliphatic groups. EP-A-0277361 discloses a mixed aliphatic and aromatic polycarbodiimide cross-linker, wherein each crosslinker molecule contains some aliphatic and some aromatic carbodiimides. EP A-0628582 discloses the use of aromatic polycarbodiimide crosslinkers, wherein both r and r′ are aromatic groups. In all the references cited above, the use of polycarbodiimide crosslinkers is taught in conjunction with carboxylic acid- or carboxylate-containing polymers, and the crosslinking reaction takes place between the carbodiimide groups and the carboxylic acid or carboxylate groups on the polymer. U.S. Pat. No. 5,352,400 discloses that polymers containing carbodiimide groups derived from alpha-methylstyryl-containing isocyanates can be used to crosslink any polymers with active hydrogen atoms which may be present in various groups, including phosphoric acid and partial esters thereof. However, compositions incorporating carboxylic acid containing-polymers, over which the present invention is an improvement, are preferred for crosslinking with polycarbodiimides.
Even though aliphatic polycarbodiimide cross-linkers are used commercially with carboxylate-containing polymers, they do suffer from certain draw backs. In particular, with the aliphatic polycarbodiimide crosslinkers, crosslinking with carboxylate groups is too rapid with the result that the crosslinking mechanism is substantially completed before film formation is completed. Therefore, despite the fact that a rapid cross-linking step will generally lead to an increase in cross-link density and possibly to improvements in solvent resistance, it detracts from the process of film formation. For example, there is a decrease in the important property of mar resistance. The rapid cross-linking therefore leads to the formation of generally poor and brittle coatings as they are not well-knitted. While aromatic polycarbodiimide crosslinkers react more slowly with carboxylate groups than do aliphatic polycarbodiimides, and consequently give coatings with improved mar resistance, the resulting films tend to discolor in the presence of UV light, which is undesirable in many applications.
The present invention seeks to overcome the problems associated with the prior art coatings. In particular, the present invention seeks to provide relatively non-toxic crosslinking coating compositions, which yield good performance, non-yellowing films.
According to a first aspect of the present invention there is provided a crosslinkable composition including a polymer bearing at least two sulfur- or phosphorous-containing acid groups and an aliphatic polycarbodiimide. According to a second aspect of the present invention there is provided a method of crosslinking a polymer composition by forming a polymer bearing at least two sulfur- or phosphorous-containing acid groups, admixing an aliphatic polycarbodiimide, and curing the coating.
An advantage of the present invention is that aliphatic polycarbodiimides do not discolor in the presence of UV light, as do aromatic polycarbodiimides.
A further advantage is that the cross-linking process is much slower than that of the prior art systems incorporating carboxylic acid- and/or carboxylate-containing polymers and an aliphatic polycarbodiimide. It is believed that the cross-linking process may be approximately a hundred times slower. The cross-linking process is slow enough that it substantially occurs during or after film formation with the result that a number of important requirements for a good coating, such as mar resistance, are greatly improved. However, crosslinking is fast enough to be useful in an ambiently cured film in a typical industrial or architectural use.
The present invention therefore rests in the discovery and recognition that a polymer bearing at least two sulfur- and/or phosphorus-containing acid groups reacts more slowly with aliphatic polycarbodiimide crosslinkers than a similar polymer containing carboxylic acid groups, yet still reacts rapidly enough to be useful in commercial applications.
The aliphatic polycarbodiimides of the present invention may be any compound containing two or more aliphatic carbodiimide groups. An aliphatic carbodiimide is defined herein as the grouping X—N═C═N—Y, wherein each of the groups X and Y are attached to the N-atom through an aliphatic C-atom. Examples of useful aliphatic polycarbodiimides include the commercial product Ucarlnk® XL-29SE and formerly commercial products Ucarlnk® XL-20, Ucarlnk®XL-25SE, and Ucarlnk® XL-27HS (all from Union Carbide), the commercial product EX-5558 (from Stahl Holland bv), as well as the compounds described in U.S. Pat. No. 5,081,173, U.S. Pat. No. 5,047,588, U.S. Pat. No.5,136,006, U.S. Pat. No.5,373,080, EP-0241805, U.S. Pat. No. 4,487,964, EP-0259511, EP-0274402, EP-0277361, and U.S. Pat. No. 5,258,481. The aliphatic polycarbodiimides may contain functional groups other than aliphatic carbodiimides, provided that these additional functional groups do not interfere with the ability of the polycarbodiimide to crosslink polymers which contain sulfur and/or phosphorus-containing acid groups. In particular, the polycarbodiimide may contain a minority of aromatic carbodiimide groups. The preferred polycarbodiimides contain only aliphatic carbodiimide groups.
The polymer according to the present invention may be any polymer capable of forming a film under the drying/curing conditions to which it is subjected and which contains two or more sulfur and/or phosphorus-containing acid groups. The polymer may be solventborne or waterborne. Examples of suitable polymers are polyacrylates, polyurethanes, polyureas, polyesters, polyethers, and polyepoxides, as well as blends and hybrids such as graft or block copolymers of the listed polymers. The acid groups on the polymer may be totally or partially neutralized. Preferred is an addition polymer formed by the aqueous emulsion polymerixation of ethylenically-unsaturated monomers.
The polymer composition may be selected and the polymer prepared by conventional techniques known to those generally skilled in the art. Preferred are addition polymers. The polymer may contain one or more of the following copolymerized ethylenically unsaturated monomers such as, for example, C
1
-C
22
linear or branched chain alkyl (meth)acrylates, bornyl (meth)acrylate, isobornyl (meth)acrylate, and the like; hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate; (meth)acrylamide or substituted (m
Bakule Ronald D.
Henderson Christopher
Rohm and Haas Company
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