Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-12-01
2001-07-17
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S458000, C525S902000, C526S078000, C526S080000, C526S087000
Reexamination Certificate
active
06262144
ABSTRACT:
The present invention relates to a process for preparing aqueous polymer dispersions, where first of all compounds having at least one ethylenically unsaturated group (monomers) are polymerized by the method of free-radical aqueous emulsion polymerization to give a polymer 1, in disperse distribution in the aqueous medium, and then, in one or more successive polymerization stages i, further monomers are subjected to free-radical polymerization in the presence of the polymer 1 that is in disperse distribution in the aqueous medium, with the proviso that
a) the polymer 1 has a glass transition temperature Tg
1
;
b) the monomers polymerized in each of the polymerization stages i are such that isolated random copolymerization of these monomers would give a polymer i whose glass transition temperature tends with increasing molecular weight toward the limit value Tg
i
;
c) for each Tg
i
the difference Tg
i
-Tg
1
is ≧10° C.;
d) the amount of the monomers polymerized to prepare the polymer 1 in disperse distribution, based on the amount of all of the monomers polymerized in the process, is >85 (preferably ≧87 and particularly preferably ≧90) and ≦99.9% by weight;
e) the total amount of the monomers polymerized in all polymerization stages i, based on the total amount of all of the monomers polymerized in the process, is ≧0.1% by weight and <15 (preferably ≦13 and particularly preferably ≦10) % by weight;
f) in each of the polymerization stages i, the addition of the monomers to be polymerized in this polymerization stage i to the polymerization vessel is made such that at no time up to the end of the addition does the degree of polymerization U
i
of monomers to be polymerized in stage i and monomers already added to the polymerization vessel exceed 50 mol-%;
g) the total amount of the monomers polymerized to prepare the polymer 1, apart from monomers having two conjugated ethylenically unsaturated groups, does not embrace more than 5% by weight of monomers having more than one ethylenically unsaturated group.
U
i
here is defined as the percentage of those monomers that have already been added to the polymerization vessel for polymerization in the i-th polymerization stage whose polymerization has already taken place at the time of the addition of further monomers to be polymerized in the polymerization stage i.
A polymerization stage i is defined as the period of time within which the rate of polymerization (the molar quantity of monomers polymerized per unit time), starting from a very low level, first of all increases and then, after having passed through a maximum, decreases again either as a result of consumption of the monomers to be polymerized in this stage and/or of the polymerization initiator, and/or as a result of deliberate outside interference (e.g. reducing the polymerization temperature, adding polymerization inhibitors), before increasing again with the commencement of the next polymerization stage.
The present invention additionally relates to the aqueous polymer dispersions obtainable using the novel process and to their use for coating, bonding, sealing or impregnating, preferance being given to their use as binders for moldings or coating compositions comprising finely divided mineral and/or organic substances (fillers and/or pigments) and as contact adhesives with improved cohesion.
Aqueous polymer dispersions are fluid systems whose disperse phase, in the aqueous dispersion medium, comprises polymer coils (polymer particles) in substantially stable disperse distribution.
Like polymer solutions when the solvent is evaporated, aqueous polymer dispersions have the property, when the aqueous dispersion medium is evaporated, of forming polymer films, which is why aqueous polymer dispersions are much employed as binders, for example for paints or compositions for coating wood, roofs or leather, and as contact adhesives.
In numerous cases, both such polymer films, or the coating compositions comprising them, and the substrate which is coated with them, are subject to great fluctuations in humidity and/or temperature, resulting in extreme expansions and contractions of the films and/or of the substrate on which they adhere; in other words the films are often subject to high stresses which the films can only withstand undamaged (without fracturing or cracking) when a very large amount of energy is required to induce their fracture.
An appropriate measure of the fracture energy to be performed on a polymer film is, in accordance with EP-B 264 903, the product of elongation at break and tear strength of the polymer film. Accordingly, advantageous polymer films are those whose product of elongation at break and tear strength is high; among the polymer films having a high value of this product, preference is given to those which simultaneously feature both a high elongation at break and a high tear strength. The latter objective is particularly difficult to realize, since it is usually the case that measures to improve the tear strength impair the elongation at break, and vice versa (these mechanical properties of a polymer film are usually determined at a temperature above its Tg).
It is an object of the present invention, therefore, to provide a process whose application, on the one hand, permits an increase in the energy required to bring about the fracture of a film of an aqueous polymer dispersion (“of a polymer 1”) and, on the other hand, at the same time ensures both a very high elongation at break and a very high tear strength of the polymer film.
We have found that this object is achieved by the process defined at the outset for preparing an aqueous polymer dispersion.
Pressure-sensitive adhesives (PSAs) form a permanent tacky film which at room temperature, even under slight pressure, sticks to a very wide variety of surfaces. Pressure-sensitive adhesives are used to produce self-adhesive products such as self-adhesive labels, tapes and films. Products of this kind are very simple to use and make it possible to work rapidly when bonding. In contrast to contact adhesive compositions, no ventilation times are necessary. Moreover, there is no “open time” within which the adhesive bond must be implemented. The quality of a self-adhesive article depends essentially on whether the internal strength (cohesion) and the sticking of the adhesive film on the surface that is to be bonded (adhesion) are set properly in relation to one another in accordance with the utility.
In the case of pressure-sensitive adhesive labels, for example, the level of cohesion must be sufficient for no stringing and no emergence of glue at the edges to occur in the course of stamping and cutting, since otherwise the cutting tools become soiled and the cut faces sticky. At the same time, the adhesion should be at a high level in order to obtain good sticking on the substrate that is to be bonded.
In general it is not possible to optimize adhesion and cohesion independently of one another. There is a desire for measures which either lift the level of both properties or, at least, maintain one property unchanged while improving the other.
U.S. Pat. No. 4,939,190 and EP 215 241 disclose pressure-sensitive adhesives comprising polymers of multistage composition. However, these polymers still do not show the desired level of adhesion or cohesion, or the desired adhesion/cohesion relationship.
Surprisingly, it has also been found that certain polymers, obtainable in accordance with the invention, are excellent pressure-sensitive adhesives having high cohesion and high adhesion. These polymers are defined in more detail below.
Polymerization processes similar to that of the invention are known from Polymer, 1985, Vol. 26, pp. 1359 to 1364, from J. of Appl. Polymer Science, Vol. 56, pp. 793 to 802 (1995), from EP-A 359 562, from EP-A 522 789 and from EP-A 187 505. Disadvantages of these processes, however, are that they are unable to establish satisfactorily the mechanical property profile of the resulting polymer films, as desired in accordance with the invention, and/or th
Mathauer Klemens
Roser Joachim
Schuler Bernhard
Wistuba Eckehardt
Zhao Cheng-Le
BASF - Aktiengesellschaft
Egwim Kelechi C.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wu David W.
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