Aqueous polymer dispersions

Details Aqueous polymer dispersions Aqueous polymer dispersions

2002-02-19

2003-09-16

Wu, David W. (Department: 1713)

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...

C524S588000, C524S558000

Reexamination Certificate

active

06620881

ABSTRACT:

The present invention relates to aqueous dispersions of acrylic polymers for the finishing treatment of leather.
The invention specifically relates to films obtained from aqueous dispersions of acrylic polymers which confer, on the treated leather, a very good combination of softness, elasticity, abrasion-resistance, both dry and wet, resistance to water and to solvents, very good print acceptance and good adhesion to the leather substrate.
It is known, from the prior art, that the technique of finishing a leather, known as finishing, involves the use of polymer binders which are capable of forming films, which have suitable mechanical properties and which are characterized by good adhesion to the substrate. After the drying operation, the polymer film must achieve a degree of crosslinking capable of conferring, on the treated leather, characteristics of softness, of elasticity and of abrasion-resistance. This feature is particularly important in the case of leathers (splits) subjected to pressing and molding operations. In this case, it is necessary for the polymer film to withstand the print action, namely for it not to exhibit cracks and for it to be, at the same time, sufficiently plastic to retain the desired print profiles.
In the prior art, twin-component systems are used to achieve this very good combination of properties. For example, use may be made of functionalized acrylic dispersions, polyurethane dispersions, and the like, in combination with crosslinking agents, such as, for example, polyaziridines, polycarbodiimides, poly-isocyanates, and the like. The polyurethane dispersions give a very good combination of properties to the end manufactured article but exhibit the disadvantage of being too expensive. In the twin-component systems, the crosslinking agents react, generally at room temperature at the time of the formation of the film, with the functional groups present in the dispersed polymer chains. The use of twin-component systems results in the disadvantage of a very limited pot life, once the components have been mixed. Furthermore, it is necessary to stress the very high toxicity of some crosslinking agents, for example polyaziridines. These twin-component systems are described, for example, in the article in the scientific review “Hide Leathers Tanning Materials”, Year 68, No. 5, Sep.-Oct. 1992, entitled “Prior art and possible medium-term developments of new leather finishing technologies, in particular with water-based solvents”, by A. Cozzolino, pages 503-520.
An alternative to the twin-component systems in the prior art is composed of commercially available self-crosslinkable dispersions, in which the crosslinking takes place via functional groups already present in the polymer chains interacting during the phase of coalescence of the dispersion. Acrylic dispersions functionalized with N-methylolacrylamide (NMA) can be mentioned as example. The disadvantage of these dispersions is that they require a high temperature, of greater than 100° C., in order for the crosslinking to be completed within times which are acceptable industrially. See Chemical Abstracts, 127 192133f, “Preparation of an emulsion of modified polyacrylate comprising two functionalities as leather finishing agent”; Chemical Abstracts, 128 90271c, “Preparation of an acrylic coating resin of the MS series with interpenetrating network technology”.
Other self-crosslinkable dispersions are those comprising divalent metals, for example transition metals, disclosed in EP 789 082 and U.S. Pat. No. 5,723,782. The disadvantage of these dispersions is that they comprise a concentration of heavy metals which is often incompatible with the laws of some countries.
Self-crosslinkable aqueous dispersions based on acrylic polymers functionalized with a (meth)acrolein are also known. These dispersions are those which are the most readily used on the market, given that they make it possible to obtain polymer films characterized by a good compromise between mechanical properties, the resistance to solvents and the absence of stickiness. Even if, from the viewpoint of the final user, they do not exhibit a problem of a toxicological nature, the systems based on (meth)acrolein are difficult to handle for the producer of the polymer dispersion. (Meth)acrolein indeed exhibits the disadvantage of being a highly reactive, volatile and extremely toxic monomer, which results in problems from the viewpoint of transportation and handling.
The need was felt to have available self-crosslinkable aqueous dispersions which do not imply, for the producer of the dispersions, the disadvantages mentioned above of the (meth)acrolein-based systems but which confer, on the treated leather, a very good combination of softness, elasticity, abrasion-resistance, both dry and wet, resistance to water and to solvents, very good print acceptance and good adhesion to the leather substrate.
One object of the present invention is thus aqueous dispersions of polymers comprising the following constituents,
a) from 40 to 95% by weight of one or more monomers comprising an ethylenic unsaturation, the corresponding homopolymer of which has a glass transition temperature (T
g
) of less than 0° C., preferably of less than −20° C.; the constituent a) also optionally comprising the monomers comprising the functional group defined in d), in a proportion of 20-50% by weight with respect to the amount defined in d);
b) from 4 to 58% by weight of one or more monomers comprising an ethylenic unsaturation, the corresponding homopolymer of which has a T
g
of greater than 0° C., preferably of greater than 40° C.; (meth)acrolein being excluded from the constituent b), optionally also comprising the monomers comprising the functional groups defined in d), in a proportion of 20-50% by weight with respect to the amount defined in d);
c) from 0.05 to 4% by weight of one or more monomers comprising at least two ethylenic unsaturations;
d) from 0.2 to 5%, preferably 0.5-3%, by weight of one or more monomers comprising an unsaturation of ethylenic type and at least one functional group capable of interacting at low temperature, lower than 100° C., in the crosslinking process in the coalescence phase, (meth)acrolein and acrylonitrile being excluded from the constituent d).
Preferably, the dispersions of the invention comprise the following constituents:
a) from 40 to 95% by weight of one or more monomers comprising an ethylenic unsaturation, the corresponding homopolymer of which has a glass transition temperature (T
g
) of less than 0° C., preferably of less than −20° C.; the monomers comprising the functional groups defined in d) being excluded from the constituent a);
b) from 4 to 58% by weight of one or more monomers comprising an ethylenic unsaturation, the corresponding homopolymer of which has a T
g
of greater than 0° C., preferably of greater than 40° C.; (meth)acrolein and the monomers comprising the functional groups defined in d) being excluded from the constituent b);
c) from 0.05 to 2% by weight of one or more monomers comprising at least two ethylenic unsaturations;
d) from 0.2 to 5%, preferably 0.5-3%, by weight of one or more monomers comprising an unsaturation of ethylenic type and at least one functional group capable of interacting at low temperature (lower than 100° C.) in the crosslinking process in the coalescence phase, (meth)acrolein and acrylonitrile being excluded from the constituent d).
Preferably, in the dispersions of the invention, the constituent d) is added after approximately two thirds of the polymerization of the constituents a)+b)+c) with respect to the total of a)+b)+c).
Use may be made, as constituent a), of monomers of (meth)acrylic or vinyl type, and the like. Generally, the monomers of the constituent a) have from 4 to 20 carbon atoms, preferably from 5 to 15 carbon atoms. Acrylic or methacrylic esters, for example ethyl acrylate, butyl acrylate or 2-ethylhexyl (meth)acrylate, can be mentioned, for example.
Use may be made, as constituent b), of monomers of (meth)acrylic or vinyl type

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