Open-cell shaped article made of acetalized polyvinyl alcohol and process for its manufacture

The acetalization of polyvinyl alcohol in an acidic aqueous medium in the presence of a wetting agent yields an open-cell, solid foam when the reaction medium contains gas bubbles which are preferably produced by stirring air into the reaction mixture. By carrying out the acetalization in the presence of a nucleic acid a porous shaped article is obtained in which the cells are uniformly distributed. The dried shaped article absorbs a multiple of its weight of water. It can be used for household and industrial purposes.

This invention relates to a shaped article made of acetalized polyvinyl 
alcohols and to a process for its manufacture. 
The manufacture of foam material with open cells by reaction of polyvinyl 
alcohol with formaldehyde in an aqueous medium has been known for a long 
time (cf. U.S. Pat. No. 2,609,347). The reaction is carried out at a 
temperature of from 20.degree. to 60.degree. C. in the presence of an acid 
catalyst, for example, sulfuric acid, while gas bubbles being dispersed in 
the reaction mixture. The gas bubbles are produced by beating air into the 
reaction mixture. Wetting agents are additionally used in order to ensure 
the formation of bubbles of uniform size and a better distribution of the 
bubbles in the mixture. In addition, the wetting agent serves to stabilize 
the sponge formed. The apparent specific gravity of the sponge obtained in 
this manner is in the range of from 0.03 to 0.15, preferably 0.05 to 0.06. 
The said process has the disadvantage that--above all when it is carried 
out on an industrial scale--pores of varying size are formed which are non 
uniformly distributed in the mixture as in the course of the acetalization 
reaction the bubbles cannot be prevented completely from rising in the 
reaction mixture, the homogeneity of the pore distribution becoming 
impaired. Consequently, the mass obtained tends to shrink whereby the 
absorptivity and water retentivity of the sponge is reduced. In an 
industrial manufacturing process this volume contraction is combined with 
an increase in waste and cuttings. Moreover, the known process requires a 
long reaction time, i.e. 54 hours at a temperature of 30.degree. C., so 
that it is uneconomical. 
It is the object of the present invention to provide a sponge-like, porous 
shaped article on the basis of acetalized polyvinyl alcohol in which 
intercommunicating cells having a uniform a size as possible are 
distributed as uniformly as possible and which can be manufactured on an 
industrial scale in economic manner. 
The invention therefore provides an open-cell shaped article made of 
acetalized polyvinyl alcohol which is characterized in that the shaped 
article (a) consists of acetalized polyvinyl alcohol which, prior to 
acetalization, has an ester number of 50 to 270 mg of KOH/g and the 4% by 
weight aqueous solution of which has a viscosity of 4 to 50 mPa.s at 
20.degree. C., (b) has cells with diameters in the range of from 0.5 to 5 
mm in uniform distribution and (c) contains a nucleic acid. 
The invention also provides a process for the manufacture of an open-cell 
shaped article made of acetalized polyvinyl alcohol by acetalization of 
polyvinyl alcohol in an acidic aqueous medium containing gas bubbles in 
the presence of a conventional wetting agent, which comprises acetalizing 
a polyvinyl alcohol having an ester number of 50 to 270 mg KOH/g the 4% by 
weight aqueous solution of which has a viscosity of 4 to 50 mPa.s at 
20.degree. C., in the presence of a nucleic acid in an amount sufficient 
to act as foam stabilizer. 
As starting material for the manufacture of the shaped article according to 
the invention a partially saponified polyvinyl alcohol having an ester 
number of 50 to 270 mg KOH/g, preferably 100 to 200 mg KOH/g, is used. The 
4% by weight aqueous solution of the polyvinyl alcohol has a viscosity of 
4 to 50 mPa.s, preferably 10 to 26 mPa.s, measured at 20.degree. C. 
according to DIN 53,015. During the acetalization reaction the polyvinyl 
alcohol has the form of a froth. For acetalization of the polyvinyl 
alcohol an aliphatic aldehyde or dialdehyde having from 1 to 4 carbon 
atoms is used. The use of formaldehyde or paraformaldehyde is preferred. 
The aldehyde is preferably used in the form of an aqueous solution. The 
acetalization is carried out in an acidic aqueous medium. A strong mineral 
acid, preferably sulfuric acid, is used as catalyst which is 
advantageously employed in the form of an aqueous solution having an acid 
content in the range of from 30 to 80% by weight, preferably 35 to 65% by 
weight. 
The polyvinyl alcohol is acetalized in the presence of a conventional 
wetting agent, preferably an anionically effective wetting agent. Suitable 
wetting agents are, in particular, alkali metal salts, preferably sodium 
salts, of alkyl sulfonates, aryl sulfonates, alkylaryl sulfonates and of 
the corresponding sulfates, phosphates or phosphonates, for example, 
sodium lauryl sulfate, sodium dodecylbenzenesulfonate and sodium oleyl 
methyl tauride. 
An essential feature of the shaped article according to the invention is 
the fact that it has cells having a diameter of 0.5 to 5 mm, preferably 
0.5 to 3 mm and contains a nucleic acid. Preferably the nucleic acid 
content is in the range of from 0.2 to 2% by weight. 
It is the characteristic feature of the process of the invention that the 
acetalization of the polyvinyl alcohol is carried out in the presence of 
nucleic acid in an amount sufficient to stabilize the foam. The nucleic 
acid is preferably used in an amount of from 0.5 to 5% by weight, more 
preferably 1 to 3% by weight, calculated on the total amount of polyvinyl 
alcohol and nucleic acid. Suitable nucleic acids are ribonucleic acids 
(RNS) as well as desoxyribonucleic acids (DNS) or mixtures thereof. It is 
recommended for economical reasons to use, instead of isolated nucleic 
acids, extracts containing nucleic acids, for example as obtained in the 
production of microbial bio-proteins. Extracts of this type have a content 
of nucleic acid of 40 to 80% by weight, preferably 50 to 60% by weight. 
Suitable starting materials are micro-organisms, for example bacteria such 
as Methylomonas clara ATCC 31226 and yeasts such as Candida lipolytica 
ATCC 20383 which may be obtained by cultivation on paraffins in the 
presence of an aqueous nutrient medium. The extracts can be obtained in 
known manner by extraction of a purified microbial cell mass with water at 
a temperature of from 30.degree. to 95.degree. C., preferably 40.degree. 
to 70.degree. C. (cf. German Auslegeschrift No. 2,633,666). According to 
the invention, this extract is preferably used in the form of an aqueous 
solution or dispersion containing 1 to 5% by weight, preferably 2 to 4% by 
weight, of extract. 
In the manufacturing process of the invention the nucleic acid serves as 
foam stabilizer. Part of the nucleic acid, preferably 25 to 35% by weight 
of the amount used, remains in chemically bound form in the final shaped 
article and acts therein as internal plasticizer. The amount of chemically 
bound nucleic acid in the shaped article is determined by analysis of the 
phosphorus content. 
To carry out the process of the invention it proved advantageous first to 
prepare a froth from an aqueous solution of the polyvinyl alcohol, the 
wetting agent and the nucleic acid by homogeneous distribution of an inert 
gas in the reaction mixture, preferably by beating in air at a stirring 
speed of 500 to 2,500 rpm, preferably 1,000 to 2,000 rpm. The volume of 
froth should be at most 50% of the final volume of the finished shaped 
article. After the "prefoaming", the aldehyde and the catalyst are added 
to the froth, the temperature of which should not exceed 40.degree. C. 
Thereupon the mixture is allowed to stand in a closed mold at a 
temperature of from 65.degree. to 100.degree. C., preferably 70.degree. to 
90.degree. C. Depending on the size of the mold, the reaction time is from 
2 to 4 hours. The porous shaped article obtained without shrinking is then 
washed with water and dried until its weight remains constant, the drying 
temperature preferably being in the range of from 70.degree. to 
100.degree. C. 
The dried shaped article has open cells and a soft, velvety surface. It has 
an absorption capacity for water of 1,500 to 1,800% by weight (for 
comparison a natural sponge absorbs less than 1,000% by weight of water). 
The open-cell shaped article according to the invention is suitable as 
sponge for industrial and household purposes, for example for washing cars 
and for bathing.

The following examples illustrate the invention, the percentages being by 
weight unless otherwise stated. 
EXAMPLE 1 
250 g of a 20% aqueous solution of polyvinyl alcohol, having an ester 
number of 140 mg of KOH/g, the 4% aqueous solution of which has a 
viscosity of 18 mPa.s, are poured into a beaker and, while stirring at 500 
rpm with a rapid stirrer, a mixture of (a) 30 g of a 4% aqueous solution 
of sodium oleyl methyl tauride and (b) a suspension of 1.2 g of nucleic 
acid extract obtained from Methylomonas clara and containing 55% of 
nucleic acid, 5% of phosphorus and 15% of proteins in 58 g of water is 
added in portions. While beating in air at a stirring speed of 2,000 rpm, 
a froth having a volume of 0.5 l is prepared to which 60 g of a 30% 
aqueous formaldehyde solution and thereafter 65 g of a 60% aqueous 
sulfuric acid are added. During this operation the temperature of the 
mixture is maintained at about 35.degree. C. 
As soon as the froth has acquired a creamy consistency, it is poured in an 
acid-resistant 1 l mold which is closed by a cover. The mold is allowed to 
stand without movement for 4 hours at 75.degree. C. Thereafter, the sponge 
obtained, the volume of which has not diminished with respect to the 
volume of the froth, is washed with water. After drying at 80.degree. C., 
the sponge has intercommunicating cells having diameters of from 1 to 2 mm 
which are uniformly distributed. The dried sponge has a content of nucleic 
acid of 0.8%. It is elastic and absorbs 15 times it dry weight of water. 
EXAMPLE 2 
Example 1 is repeated with the following components: 250 g of polyvinyl 
alcohol solution (as defined in Example 1), 50 g of sodium oleyl methyl 
tauride solution (as defined in Example 1), 90 g of 2% suspension of the 
nucleic acid extract as defined in Example 1, 20 g of paraformaldehyde and 
100 g of 60% sulfuric acid. 
The dried sponge has intercommunicating cells having diameters of from 0.5 
to 1 mm in uniform distribution. The nucleic acid content of the dried 
sponge is found to be 1%. The sponge is elastic and absorbs 16 times its 
dry weight of water. 
EXAMPLE 3 
Example 1 is repeated with the following components: 250 g of polyvinyl 
alcohol solution (as defined in Example 1), 50 g of sodium oleyl methyl 
tauride solution (as defined in Example 1), 100 g of 2% suspension of the 
nucleic acid extract as defined in Example 1, 20 g of paraformaldehyde, 
100 g of 60% sulfuric acid and 1 g of pulverized cellulose (fiber length 
0.5 mm). 
The pulverized cellulose is added to the froth simultaneously with the 
paraformaldehyde. 
The dried sponge obtained has intercommunicating cells having diameters of 
from 0.5 to 1 mm in uniform distribution and a content of nucleic acid of 
1.4%. The sponge is elastic and absorbs 18 times its dry weight of water. 
COMATIVE EXAMPLES 
250 g of the polyvinyl alcohol solution as defined in Example 1 is given in 
a beaker and, while stirring at 500 rpm with a rapid stirrer, 30 g of the 
sodium oleyl methyl tauride solution as defined in Example 1 are added in 
portions. A preliminary froth is prepared from the mixture as described in 
Example 1. 
(a) After transfer into a 1 liter mold as specified in Example 1 and 
heating to 75.degree. C., a uniform sponge is not obtained, rather a 
gel-like liquid forms in which the cells become larger towards the 
surface. 
(b) When the mixture which has been preliminary frothed is allowed to react 
for 24 hours at 35.degree. C., an intensive shrinkage occurs and the cells 
in the sponge obtained increase in diameter from the bottom upward. 
Moreover, the sponge is not resilient and phosphorus cannot be detected 
therein.