Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-01-22
2004-07-13
Szekely, Peter (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C528S488000, C528S493000, C528S50200C
Reexamination Certificate
active
06762221
ABSTRACT:
The present invention relates to the use of naphthalenesulfonic acid-formaldehyde condensation products (condensates) as drying assistants, especially in connection with the spray drying of aqueous polymer dispersions.
The present invention also relates to a process for preparing polymer powders which are redispersible in an aqueous medium, and to the redispersible polymer powders and their use.
Aqueous polymer dispersions are widely employed, for example as binders, especially for synthetic-resin renders or highly pigmented interior paints, adhesives or other coating compositions. In many cases, however, it is desired to use not the aqueous polymer dispersion but the polymer in powder form.
To obtain the polymer in powder form, the dispersion must undergo a drying operation, for example spray drying or else freeze drying. In the case of spray drying, the polymer dispersion is sprayed in a stream of hot air and the water is removed; the air used for drying, and the sprayed dispersion, are preferably passed cocurrently through the dryer.
However, the resulting polymer powder has the disadvantage that its redispersibility in an aqueous medium is generally not entirely satisfactory, since the polymer particle diameter distribution which results from redispersion generally differs from that in the initial aqueous dispersion. The reason for this is that aqueous polymer dispersions, unlike polymer solutions, do not form thermodynamically stable systems. Instead, the system tends to reduce the polymer/dispersion interface by combining small primary particles to form larger secondary particles (gel specks, coagulum). In the state of disperse distribution in the aqueous medium, this can be prevented even for a relatively long time by the addition of dispersants, such as emulsifiers and protective colloids. During the drying of aqueous polymer dispersions, however, the action of the dispersants is in many cases inadequate, and irreversible formation of secondary particles takes place to a certain extent, ie. the secondary particles are retained on redispersion and impair the performance characteristics of the aqueous polymer dispersion that are obtainable on redispersion.
To prevent or at least reduce the formation of secondary particles in the course of drying, it has long been known to employ drying assistants. These substances are widely referred to as spray assistants, since spray drying in particular promotes the formation of irreversibly agglomerated secondary particles. This effect is all the more marked the lower glass transition temperature (and thus the softening point or minimum film-forming temperature) of the polymer particles, especially when it is below the drying temperature. At the same time, drying assistants generally reduce the formation of polymer coating which remains adhering to the dryer wall, and thus result in an increase in powder yield.
The use of drying assistants is known from numerous publications. For instance, DE-A-24 45 813 describes a pulverulent polymer which is redispersible in aqueous systems and whose drying assistant comprises from 1 to 20% by weight of a water-soluble sulfo- or sulfonate-functional condensation product formed from aromatic hydrocarbons and formaldehyde. The condensation products involved are, in particular, condensates of phenolsulfonic or naphthalenesulfonic acid with formaldehyde. There is no information on the molecular weight of the condensates used. It is mentioned that the drying of the polymer powders should be undertaken at below the softening point.
EP-A-78 449 describes a process for preparing water-redispersible, blocking-resistant polymer powders by spray drying aqueous dispersions of polymers having glass transition temperatures below 50° C. As spray assistant the dispersions include a water-soluble copolymer of vinylpyrrolidone and vinyl acetate and/or a water-soluble alkali metal salt and/or alkaline earth metal salt of a naphthalenesulfonic acid-formaldehyde condensate. Here again, there is no information on the molecular weight of the naphthalenesulfonic acid-formaldehyde condensates employed. What is evident is the comparatively large amount of spray assistant in the case where the naphthalenesulfonic acid-formaldehyde condensates are used alone (30% by weight in Ex. 4, 50% by weight in Ex. 5, 30% by weight in Ex. 6, based in each case on the polymers). This adversely affects the binder properties of the polymer powders for example by increasing by an undesirable extent the flowability of compositions in which they are used as binder (cf. EP 407 889), or by retarding the setting of cementitious compositions.
Similarly, EP-A-407 889 describes the use of a water-soluble alkali metal salt or alkaline earth metal salt of a phenolsulfonic acid-formaldehyde condensate as a spraying assistant for preparing water-redispersible polymer powders from aqueous polymer dispersions. Yet again, there is no information on the molecular weight of the condensates used.
It is an object of the present invention to provide drying assistants which permit the preparation, from polymer dispersions, of polymer powders which are readily redispersible in water and which do not have the disadvantages of the prior art.
We have found that this object is achieved, surprisingly, by using naphthalenesulfonic acid-formaldehyde condensates having a number-average molecular weight M
n
<1500 as drying assistants.
The present invention therefore relates to the use of naphthalenesulfonic acid-formaldehyde condensates having a number-average molecular weight M
n
<1500 daltons or salts thereof as drying assistants in the drying of aqueous polymer dispersions.
The condensates preferably have mean molecular weights M
n
in the range from 500 to 1500, preferably from 700 to 1250 daltons, determined by means of gel permeation chromatography, as described in the Examples for the preparation of the spraying assistants. The molecular weight distribution or polydispersity (defined as M
w
/M
n
) is in the range from 5 to 15, preferably from 5 to 10. The proportion of condensates with molar masses above 10,000 daltons is preferably less than 25% by weight, in particular less than 20% by weight, of the overall condensate.
The condensates used should generally comprise no more than 25% by weight, preferably no more than 15% by weight, of unreacted naphthalenesulfonic acid (&agr; and/or &bgr; product) and not less than 75% by weight of condensates. Where the condensate is employed in the form of its salts, the salts normally used are alkali metal salts or alkaline earth metal salts or ammonium salts, ie. salts with ammonia or with organic amines such as triethanolamine, diethanolamine and triethylamine. Preference is given to the alkaline earth metal salts, and especially to the calcium salts.
The drying assistants employed in accordance with the invention are generally prepared by condensation of naphthalenesulfonic acid with formaldehyde under acidic—especially sulfuric—reaction conditions. In this reaction the naphthalenesulfonic acid can be introduced as initial charge or can be prepared in situ by sulfonation in accordance with known methods (cf. J. March, Advanced Organic Chemistry, 3
rd
ed., John Wiley, New York 1985, p 473 ff. and literature cited therein). Naphthalenesulfonic acid is preferably prepared in situ by sulfonation with sulfuric acid, preferably concentrated sulfuric acid. Condensation takes place by reacting naphthalenesulfonic acid with formaldehyde under acidic reaction conditions, preferably under sulfuric reaction conditions, and especially in concentrated sulfuric acid. Where the naphthalene acid is prepared in situ, the condensation reaction is initiated by adding formaldehyde to the sulfuric reaction mixture. The molar ratio of formaldehyde to naphthalenesulfonic acid is in the range from 1:1 to 1:2, preferably 1:1.3 to 1:1.7. Formaldehyde is preferably added in the form of an aqueous solution. In order to establish the desired molecular weight, the condensation reaction is generally conducted at from 90 to 110° C., preferably at
Angel Maximilian
Dragon Andree
Pakusch Joachim
Roser Joachim
Sack Heinrich
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Szekely Peter
LandOfFree
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