Preparation of polymer powders

Details Preparation of polymer powders Preparation of polymer powders

2001-08-21

2002-10-22

Boykin, Terressa M. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Treating polymer containing material or treating a solid...

Reexamination Certificate

active

06469135

ABSTRACT:

DESCRIPTION
The present invention relates to a process for preparing polymer powders by evaporating the volatile constituents from aqueous dispersions of film-forming polymers P in the presence of drying assistants T selected from the salts of oligomeric arylsulfonic acid-formaldehyde condensation products. It also relates to the polymer powders obtainable by this process and to their use.
Aqueous dispersions of film-forming polymers find broad application as, for example, binders, especially for synthetic resin plasters or highly pigmented interior paints, adhesives or coating compositions. Frequently, however, what is desired is to use not the aqueous polymer dispersion itself but rather the polymer present therein in powder form.
To obtain the film-forming polymer in powder form, the dispersion is subjected to a drying operation in which the volatile constituents of the dispersion are evaporated by a suitable method, for example, by means of spray or freeze drying. It is necessary to bear in mind that during the evaporation of the aqueous dispersion medium the polymer particles of the aqueous dispersion aggregate irreversibly and may form secondary particles. Secondary particle formation leads to a coarsening of the polymer powder, resulting in poorer redispersibility, generally associated with poorer performance properties of the powder. Moreover, it leads to the formation of deposits on the dryer walls and so reduces the powder yield.
In order to prevent or at least reduce the irreversible formation of secondary particles during powder preparation, drying assistants are used. They are frequently referred to also as spraying assistants, since spray drying in particular promotes the formation of irreversibly agglomerated secondary particles. This effect becomes more pronounced the lower the glass transition temperature (and thus the softening temperature 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 deposits which remain adhering to the dryer wall, and so bring about an increase in the powder yield.
Drying assistants which are frequently used, not least owing to their favorable price, are the salts, preferably the alkali metal, alkaline earth metal or ammonium salts, of condensation products of aromatic sulfonic acids with formaldehyde (arylsulfonic acid-formaldehyde condensates).
DE-A-24 45 813 describes a pulverulent polymer, redispersible in aqueous systems, which comprises as drying assistant from 1 to 20% by weight of a water-soluble condensate of aromatic hydrocarbons and formaldehyde that contains sulfonic acid or sulfonate groups, examples being phenolsulfonic or naphthalenesulfonic acid-formaldehyde condensates.
WO 98/03577 discloses salts of naphthalenesulfonic acid-formaldehyde condensates having a number average molecular weight M
n
of less than 1500 daltons which possess particularly good spraying assistant properties.
Similarly, EP-A-407 889 describes the use of a water-soluble alkali metal or alkaline earth metal salt of a phenolsulfonic acid-formaldehyde condensate as a spraying assistant for preparing water-redispersible addition polymer powders.
WO 98/03576 likewise discloses salts of phenolsulfonic acid-formaldehyde condensates which have a number average molecular weight M
n
of less than 1500 daltons and possess particularly good spraying assistant properties.
The use of drying agents, especially arylsulfonic acid-formaldehyde condensates, nevertheless has the disadvantage that the film formation of the powder particles in the redispersed state, which is necessary for the application, is adversely affected.
It is an object of the present invention to provide a process for preparing polymer powders which uses drying assistants based on oligomeric arylsulfonic acid-formaldehyde condensates to give polymer powders having better film-forming properties.
We have found that this object is achieved by a process in which the powder is prepared from an aqueous polymer dispersion comprising not only the oligomeric arylsulfonic acid-formaldehyde condensates but also at least 1.5% by weight, based on the polymer in the dispersion, of at least one anionic surface-active compound AO which has at least one C
6
-C
32
alkyl group.
The present invention accordingly provides a process for preparing polymer powders by evaporating the volatile constituents from aqueous dispersions of film-forming polymers P in the presence of drying assistants T selected from the salts of oligomeric arylsulfonic acid-formaldehyde condensates wherein prior to the evaporation of the volatile constituents the aqueous polymer dispersion comprises at least 1.5% by weight, preferably at least 1.7% by weight, based on the polymer in the dispersion, of at least one anionic surface-active compound AO having at least one C
6
-C
32
alkyl group.
Film-forming denotes that when the water is evaporated (during drying) the polymer particles of the aqueous dispersion or the powder particles in an aqueous redispersed preparation coalesce above a temperature specific to them, the minimum film-forming temperature MFT, and form a coherent polymer film. Aqueous redispersed preparation denotes an aqueous dispersion of the polymer powder.
In general, the amount of the anionic surface-active compound AO, based on the polymer in the dispersion, will not exceed 10% by weight and preferably will not exceed 5% by weight. In particular, the amount is in the range from 1.5 to 4% by weight and with particular preference in the range from 1.7 to 3% by weight.
Compounds AO suitable in accordance with the invention are salts of those compounds which have at least one C
6
-C
32
alkyl group, in particular a C
8
-C
22
alkyl group, and a functionality suitable for salt formation with a base, e.g., a carboxylate, sulfonate, phosphonate, phosphate or sulfate group, preferably a sulfonate group. Preferred salts have alkali metal, alkaline earth metal or ammonium counterions and especially sodium, potassium or calcium counterions. Sodium salts are particularly preferred. Such compounds are also known from the prior art as anionic emulsifiers; see, for example, Houben-weyl, Methoden der organischen Chemie, Volume XIV/1, Makromolekulare Stoffe [Macromolecular substances], Georg-Thieme-Verlag, Stuttgart, 1961, pp. 192-208.
Hereinbelow, the expressions C
n
-C
m
relate to the number of carbons of a respective class of compound that is possible in the context of the invention. Alkyl groups may be linear or branched. C
n
-C
m
alkylaryl stands for aryl groups which carry a C
n
-C
m
alkyl radical.
Examples of suitable compounds AO are salts of alkyl sulfates derived from linear or branched alcohols, e.g., fatty alcohols or oxo alcohols (alkyl: C
8
-C
32
), of sulfuric monoesters with ethoxylated alkanols derived from linear or branched alcohols, e.g., fatty alcohols or oxo alcohols (EO units: 2 to 50, alkyl: C
8
to C
32
), of sulfuric monoesters with ethoxylated alkylphenols having a preferably linear alkyl radical (EO units: 2 to 50, alkyl: C
6
-C
22
), of alkylsulfonic acids (alkyl: C
6
-C
32
), of dialkyl esters of sulfosuccinic acid (alkyl: C
6
to C
32
) and of alkylarylsulfonic acids with a preferably linear alkyl radical (alkyl: C
6
to C
32
). Suitable anionic emulsifiers are also the salts of the di(C
6
-C
32
alkyl) derivatives of bis(phenylsulfonyl) ether and the technical mixtures thereof which are in commerce, for example, as DOWFAX® 2A1 from DOW Chemical Co.
Preferred compounds AO are the compounds of the formula I,
in which
R
1
is a C
6
-C
32
alkyl and preferably a C
8
-C
22
alkyl group,
R
2
is hydrogen, C
1
-C
4
alkyl, a fused benzene ring unsubstituted or substituted by C
1
-C
4
alkyl, or is a phenoxy radical with or without a C
6
-C
32
alkyl group and/or a sulfonate group, and
X is a cation equivalent and is preferably an alkali metal cation, especially a sodium or potassium ion, is one equivalent of an alkaline earth metal cation, especially ½

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