Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Patent
1999-01-20
2000-02-22
Szekely, Peter A.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
524 3, 524 5, 524 6, 524 8, 528487, 528491, 528493, 528502E, C08K 300, C04B 1604, C04B 2422, C04B 2430, C08G 818
Patent
active
060281677
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the use of phenolsulfonic 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 naphtalenesulfonic 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.degree. 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 naphtalenesulfonic acid-formaldehyde condensate. Here again, there is no information on the molecular weight of the naphtal
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Angel Maximilian
Dragon Andree
Pakusch Joachim
Roser Joachim
Sack Heinrich
BASF - Aktiengesellschaft
Szekely Peter A.
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