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
2000-11-01
2004-03-23
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...
C524S457000, C524S504000
Reexamination Certificate
active
06710112
ABSTRACT:
The present invention relates to aqueous polymer dispersions in which the polymer particles have two different polymer phases P1 and P2 having different theoretical glass transition temperatures T
g
(1)
and T
g
(2)
.
The invention also relates to a process for preparing such polymer dispersions. The invention further relates to the use of the polymer dispersions as binders in coating compositions. The invention relates additionally to latex paints comprising such polymer dispersions as binders.
Paints are commonly divided into three categories in accordance with their ability to reflect light:
1. flat paints having a specular gloss of less than 15% reflectance,
2. semi-gloss paints having a specular gloss of about 35% to 50% reflectance, and
3. high-gloss paints having a specular gloss of >70% reflectance,
based in each case on light having a 60° angle of incidence.
Solventborne paints can easily be formulated into these three categories. In the case of latex paints, i.e., paints comprising not only a pigment as coloring constituent but also an aqueous, film-forming polymer dispersion as binder, it is difficult to achieve a high specular gloss. The lower gloss of latex paints in comparison to oil-based paints has its origin in the process of film formation. In comparison to the polymer of the oil paints, which is dissolved at the molecular level, latex polymers are usually of higher molecular weight and are present in the form of individual particles. A retarded or greatly restricted flow of the macromolecules during the process of film formation is the result. This, and remanent textures, are the essential reasons why only a low gloss can usually be obtained with latex paints. In pigmented coating compositions based on aqueous polymer dispersions the quality of the coating depends essentially on the ability of the polymer particles, as the coating composition dries, to bind the pigment particles and any filler present and to form a coherent polymeric film. Of course, the higher the proportion of pigments and fillers in the coating composition, the more difficult this process is.
EP-A 429 207 describes aqueous polymer dispersions whose polymer particles have a core-shell structure, the core-forming polymers having a higher glass transition temperature than the polymers which form the shell. The polymer particles have a size in the range from 20 to 70 nm. The polymer dispersions described therein are used to prepare coating compositions with low levels of pigmentation having improved gloss and improved blocking resistance. A disadvantage is the low particle size, which leads to viscosity problems and stability problems during the preparation of the polymer dispersions.
U.S. Pat. No. 5,182,327 describes aqueous polymer dispersions and high-gloss latex paints prepared from them. The average molecular weight of the polymers present in the dispersions is below 150,000. Furthermore, the polymers are functionalized with from 3 to 20% by weight of an olefinic carboxylic acid. The paints exhibit poor blocking resistance, probably on account of the low molecular weight. Furthermore, in the wet state the coatings are sensitive to mechanical influences. Their scrub resistance (abrasion resistance), in particular, leaves something to be desired.
U.S. Pat. No. 5,506,282 describes aqueous coating compositions based on polymer dispersions which contain two different types of polymer particle having different particle diameters. EP-A 466 409 likewise describes a blend of two different aqueous polymer dispersions, in which one of the polymer particle types has a glass transition temperature above room temperature and the other polymer particle type has a glass transition temperature of below 20° C. EP-A 761 778 discloses similar coating compositions, the polymer particles in this case having not only a different glass transition temperature but also different particle sizes.
Coating compositions containing different types of polymer particle are, of course, more complex to prepare, since the different types of polymer particle must be prepared in separate polymerization reactions.
It is an object of the present invention to provide aqueous polymer dispersions which are easy to prepare and which in particular in coating compositions ensure high gloss, good mechanical strength and a high blocking resistance of the coating.
We have found that this object is achieved by means of aqueous polymer dispersions in which the polymer particles have a minimum film-forming temperature of below 65° C. and contain the two polymer phases P1 and P2 each with different glass transition temperatures T
g
(1)
and T
g
(2)
, a chain transfer reagent having been used in the preparation of one of the polymer phases.
The present invention accordingly provides aqueous polymer dispersions having a minimum film-forming temperature of below +65° C. comprising at least one film-forming polymer in the form of dispersed polymer particles comprising a polymer phase P1 and a different polymer phase P2, the polymer dispersion being obtainable by free-radical aqueous emulsion polymerization comprising the following steps:
i) polymerization of a first monomer charge M1 to give a polymer P1 having a theoretical glass transition temperature T
g
(1)
(acccording to Fox) and
ii) polymerization of a second monomer charge M2 to give a polymer P2 having a theoretical glass transition temperature T
g
(2)
(according to Fox) which is different from T
g
(1)
in the aqueous dispersion of the polymer P1,
at least one chain transfer reagent being used either in the polymerization of the monomer charge M1 or in the polymerization of the monomer charge M2.
In accordance with the invention, the polymer phases P1 and P2 have different glass transition temperatures T
g
(1)
and T
g
(2)
. The difference between the glass transition temperatures is generally at least 10 K, preferably at least 20 K, in particular at least 40 K. With very particular preference, the difference between the theoretical glass transition temperatures is from 40 to 150 kelvins.
The term theoretical glass transition temperature as used here and below is the glass transition temperature T
g
(1)
or T
g
(2)
, respectively, as calculated by the method of Fox on the basis of the monomer composition of the monomer charge M1 and of the monomer charge M2. According to Fox (T. G. Fox, Bull. Am. Phys. Soc. (Ser. II) 1, 123 [1956] and Ullmann's Enzyklopädie der technischen Chemie, Weinheim (1980), pp. 17, 18) the glass transition temperature of copolymers at high molecular masses is given in good approximation by
1
T
g
=
X
1
T
g
1
+
X
2
T
g
2
+
…
X
n
T
g
n
where x
1
, X
2
, . . . , X
n
are the mass fractions of the monomers 1, 2, . . . , n and T
g
1
, T
g
2
, . . . , T
g
n
are the glass transition temperatures of the polymers composed in each case of only one of the monomers 1, 2, . . . , n in degrees Kelvin. These are known, for example, from Ullmann's Encyclopedia of Industrial Chemistry, VCH, Weinheim, Vol. A 21 (1992) p., 169 or from J. Brandrup, E. H. Immergut, Polymer Handbook 3
rd
ed, J. Wiley, New York 1989.
In accordance with the invention, the monomer charge M2 is preferably chosen such that the theoretical glass transition temperature (according to Fox) of the resulting polymer phase P2 lies above the theoretical glass transition temperature of the polymer P1 prepared first of all. The monomer charge M2 then preferably has a composition leading to a theoretical glass transition temperature T
g
(2)
of the polymer phase P2 which lies above 30° C., preferably above 40° C. and, in particular, in the range from 50° C. to 120° C.
For T
g
(2)
>T
g
(1)
, the monomer charge MI preferably has a monomer composition leading to a theoretical glass transition temperature T
g
(1)
of the resulting polymer phase P1 which lies in the range from −40° to +40° C., preferably in the range from −30° C. to +30° C. and, with very particular preference, in the range from −10° C. to +25° C.
Where T
g
(1)
&g
Dittrich Uwe
Röckel Harald
Sandor Mario
Zhao Cheng-Le
BASF - Aktiengesellschaft
Yoon Tae H.
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