CS267333B1 - A method for producing homopolymers and / or copolymers of vinyl chloride - Google Patents

Abstract

In the homopolymerization and/or copolymerization of vinyl chloride, the dispersant or component of the dispersion system is modified polyvinyl alcohol with a degree of hydrolysis of 60 to 100 H prepared by the action of oxygen or oxygen-containing gas (air, 0, 0,) on polyvinyl alcohol, or partially saponified polyvinyl acetate, usually in the form of an aqueous solution (salt) at a temperature of 0 to 260 °C (molecular 0? at 180 to 220 C) until an acidity number of 2 to 45 mg KOH/g of aqueous solution calculated on 15 H wt. of modified polyvinyl alcohol is not less than twice the value of the acidity number of the starting polyvinyl alcohol. The mixture makes it possible to produce primarily compact, but also porous suspension polymers and copolymers with a controlled and especially high bulk density.

Description

CS 267333 B1 1

The present invention relates to a process for the preparation of suspension homopolymers and / or copolymers by suspension homopolymerization and / or copolymerization of vinyl chloride with the aid of novel protective colloids and / or copolymers. a disperser, allowing the homopolymer or copolymer to be produced by a narrower grain size distribution and higher bulk density. In suspension polymerizations and copolymerization of vinyl monomers, long-term protective colloids, or dispersants, use inorganic and especially organic protective colloids. Inorganic include magnesium hydroxide and phosphorous. Organic include gelatin, maleic anhydride copolymers and other tonal protective colloids, polyvinyl alcohol alkyl cellulose esters such as methyl cellulose, hydroxyethyl cellulose / Zilberman E. N. (ed.):

The hallucinations of polivinilchloride, etr. 66-77. Izdatelstvo "Chimija", Moscow (1968) /. In the case of polyvinyl alcohol, the polymerization stage is to be 1500-1700, the content of acetate groups 20-25% and the pH of its solution 5-7. However, these protective colloids are no longer sufficient to prepare the slurry types of PVC and vinyl chloride copolymers by narrow grain distribution, high bulk density and desired morphology. Therefore, other types of dispersing agents are also contemplated. Thus, a mixture of partially saponified polyvinyl acetate, sorbitol ester with fatty acid and hydroxypropylcellulose is used in an amount of 0.03 to 1% by weight. to vinyl chloride or modified polyvinyl alcohol, prepared by saponification of a vinyl acetate copolymer with 0.5 to 5 mol%. acrylic acid, crotonic acid or vinyl thioesters, optionally with polyvinyl alcohol oxyethylation or its ether-epichlorohydrin. 5-2139 186 (Nov 19, 1977j C 08 f 2/20} Jap.pat. 5-4033 590 (March 12, 1979) ·, C 08 f 2/20; Jap.pat 5-3144 989 (16. 12, 1978), Japanese Patent No. 5,5029,879 (Mar. 1, 1980), Japanese Patent No. 5-7014608 (Jan. 25, 1982), incorporated herein by reference. ) / PVC suspension stabilizer comprises a modified polyvinyl alcohol containing 0.1 to 15 mol% lacquer rings, with a saponification degree of 80 to 95 mol%. Similar (European Pat. 26812) suspension polymerization of vinyl chloride is carried out in the presence of a free radical initiator 0.01 to 0.3% by weight dissolved in the monomer and 0.05 to 1% of the water-soluble polyvinyl alcohol with a viscosity of 15 to 50 mPa.s (in 4% aqueous solution at 20 ° C) C) as well as using a combination of hydroxypropyl methylcellulose with polyvinyl alcohol with 30 to 50 mol% hydrolysis degree for the suapenzene copolymer of vinyl chloride with propylene (European Patent 96363) Similarly prepared by suspension copolymerization of vinyl chloride sj by a single or multiple compactable monomer copolymer using a susceptible stabilizer of a partially saponified polyvinyl alcohol having a polymerization degree of 740, a wash rate of 71.4%, and a solubilization index in metonale at 55 ° C at 10-60% (NSR Pat. 2610021). Despite these and other known methods in terms of the properties of the suspension of PVC or vinyl chloride copolymers, the problem remains the reproducibility of the properties of the modified polyvinyl alcohol types, and then their effect as protective colloids, respectively. suspension polymerization and copolymerization suspenders. Furthermore, the preparation of the products "tailor-made", not least from the point of view of the morphology and distribution of powder grains and their bulk density, not least the technical availability of polyvinyl alcohol-based dispersers. modified polyvinyl alcohol.

The sketched and other problems are solved by the processes for the production of homopolymers and / or copolymers by suspension homopolymerization and / or copolymerization of vinyl chloride at a temperature of 5 to 80 ° C, with the initiation effect of the free radical initiators, protective colloids, the main dispersant or the mixture of dlspergents and auxiliaries, as molecular weight regulators, reducing agents, higher aliphatic alcohols and pH environment regulators, by dispersing the dispersant or component of the dispersing system in an amount of 0.001 to 1% by weight; based on vinyl chloride or a mixture of monomers, the modified polyvinyl alcohol with a degree of hydrolysis of 60 to 100%, prepared by bringing oxygen or oxygen-containing gas to polyvinyl alcohol and / or partially saponified polyvinyl acetate, typically in the form of an aqueous solution and / or sol, at a temperature of from 0 to 260 [deg.] C. until a content of from 2 to 45 mg of KOH / g of aqueous solution and / or a net content of 15% by weight is reached. modified polyvinyl alcohol or at least twice the acidic polyvinyl alcohol and / or partially saponified polyvinyl acetate. An advantage of the process of the invention is the technical availability of modified polyvinyl alcohol, respectively. modified partial saponified polyvinyl acetate as effective protective colloids, good reproducibility of the homopolymerization course as well as vinyl chloride copolymerization, the ability to control the bulk density of the powder, its compactness or porosity relatively well. However, they can use simple dispersants to achieve high bulk density of produced suspensions! and to produce highly compact polychloride polymers and copolymers. In particular, it makes it possible to produce suspension products with a relatively narrow distribution.

Suitable vinyl chloride comonomers include wool acetate, vinylidene chloride, alkyl acrylate, methyl methacrylate, and alkyl methacrylates, maleic anhydride, and alkyl esters of maleic acid, itaconic acid, and alkenes. Protective colloids include cellulose derivatives, especially methylhydroxypropylcellulose, methylcellulose and other alkylcelluloses, hydroxyethylcellulose, hydroxypropylmethylcellulose, further carboxymethylcellulose, polyvinylpyrrolidone, polyvinyl alcohol, partially saponified polyvinyl acetate, styrone / malelic anhydride copolymers and vinyl acetate / malelic anhydride copolymers; salts, esters of polyethylene glycols and the like. However, in particular, modified polyvinyl alcohol is mentioned in the following text.

The other ingredients are secondary dispersants or secondary suspension stabilizers such as partially saponified fats, partially esterified polyols, partial or full esterified sorbitol saturated and unsaturated fatty acids, polyethoxylated sorbitol esters, polyethylene glycol esters, urea-formaldehyde condensates and polycondensates, tert-butylated pentaerythritol and dipentaerythritol and block copolymers ethylene oxide and propylene oxide. Other excipients may be defoamers, molecular weight regulators such as trichlorethylene, chloroform, carbon tetrachloride, 2-mercaptoethanol, aliphatic aldehydes are less suitable. Then reducing agents and antioxidants such as phenols, alkylphenols, especially 2,6-di-tert. butyl 4-methylphenol, organic phosphites, sulfur dioxide, hydrosulfites, nitrites, formaldehyde sulfoxylates, ascorbic acid and polyethylene polyamines. Among the higher aliphatic alcohols, saturated and unsaturated alcohols C0 to C20 are suitable, alcohols to C18, cyclohexanol and alkylcyclohexanol are less preferred. Mixtures of alcohols C8 to C14 formed as a by-product in the production of 2-ethylhexanol by the oxo process are also suitable. Environmental pH regulators include alkali metal hydrophosphates, salts of organic acids, and the like. An effective dispersant of the present invention is a modified polyvinyl alcohol, wherein the term "polyvinyl alcohol" also includes partially saponified polyvinyl acetate in which at least 60 µl of acetyl groups or ester groups have been hydrolyzed or saponified. While modifying polyvinyl alcohol, this may be oxidized with oxygen or oxygen-containing gas in the form of a powder, but most preferably its oxidation is in the form of an aqueous solution, respectively. sol, since they are essentially colloidal solutions, most preferably 4 to 20% polyvinyl alcohol. At lower concentrations, a larger reaction space is needed and, at higher levels, due to the high viscosity of the solution, respectively, the higher the reaction space. solo is more difficult to manipulate. Oxidation leads to an increase in acidity, to the formation of double bonds and to carbonyl groups in the polyvinyl alcohol macromolecules, and generally to an increase in the degree of hydrolysis of the partially saponified polyvinyl alcohol.

In addition to the oxygen itself, oxygen-containing gas such as air or oxygen-enriched air may be used. The effect increases with oxygen pressure, respectively. oxygen partial pressure, such as ozone admixtures and other oxygen oligomers.

On the other hand, atomic oxygen (or oxygen ions) produced, for example, by electrolysis of CS 267333 01 3 water and aqueous solutions of polyvinyl alcohols, can be utilized for the production of polyvinyl alcohol according to the invention, and immediate use can be made of oxldation "in situ" secreted oxygen at the anode. In the oxidation, temperatures from 0 to 260 ° C may be used, but the range in the case of molecular oxygen is preferably 100 to 220 ° C. At lower temperatures, the rate of oxidation decreases and higher temperatures can lead to undesired degradation of polyvinyl alcohol. If gas-containing ozone is also used, temperatures in the range of 20 DEG to 120 DEG C. are also suitable and, when using atomic oxygen or oxygen produced by the " state of the art ", a temperature range of 0 DEG C., most preferably 20 DEG to 90 DEG C. is suitable. C.

Modified polyvinyl alcohol according to the invention can be applied alone or as a mixture of several modified types of polyvinyl alcohol or as a component of a dispersing system consisting of known dispersants such as cellulose derivatives and the like.

The suspension homopolymer or copolymer can still be stabilized in the " raw " suspension in the " mother liquors " or by centrifugation by additives such as radical scavengers, reducing agents, antifoam additives and intistatic agents.

The method of production is carried out primarily discontinuously, but can be exceptionally carried out semi-continuous and continuous. Further details of the process of the invention as well as other advantages are apparent from the examples. Example 1

23 kg of demineralized water, 30 g of mixed glycerol ester with stearic, palmitic and oily acids, and 30 g of mixed glycerol ester with stearic acid, palmitic acid and oil, respectively, are metered into a 50 dm 'stainless steel autoclave fitted with a duplicator, bottom outlet opening, anchor stirrer, thermometer well, piston valve, pressure gauge, and accessories. . partially saponified fat (Emulsifier C, abbreviation EC) with a melting point of 36 ° C, with a monoglyceride content of up to 30¾ wt. Next, 42g of a 50µm mixed initiator xylene solution. containing bis (2-ethylhexyl) peroxydicarbonate (75 mol%), benzoylperoxy-2-ethylhexyl carbonate (22 mol%) and dibenzoyl peroxide (3 mol%) and finally modified polyvinyl alcohol at 17g dry weight in 500 g of aqueous solution, respectively. solo. The modified polyvinyl alcohol is prepared from partially hydrolyzed (alkali-hydrolyzed) polyvinyl acetate with a hydrolysis degree of 88.8 µl of the dynamic viscosity of its aqueous solution, i.e., a concentration of 4% by weight. at 25 ° C = 13.5 mPa.s, in the form of an aqueous solution (3000 g in a 5 dm 3 autoclave) with a concentration of 16 wt. by the action of oxygen at a pressure of 0.1MPa and at a temperature of 200-2 ° C for 1h. The obtained modified polyvinyl alcohol has the following properties: dynamic viscosity of an aqueous solution of 4 wt. at 25 ° C = 8.96 mPa · s; analysis of an aqueous solution (sol) of 16.15 wt. starting acid number = 12.64 mg KOH / g solution; saponification number = 24.55 mg KOH / g solution; degree of hydrolysis = 93.9 mole%. Analysis of dried modified polyvinyl alcohol at 110 ° C for 3h under reduced pressure: acid number = 22.0 mg KOH / g; saponification number = 102.84 mgKOH / g; degree of hydrolysis = 93.3 rel. mol. %, Polymerization degree F = 912.

After dispensing the modified polyvinyl alcohol sol, the autoclave is closed, the carbon black is removed by thoroughly flushing the steam space with nitrogen. Then, 17 kg of vinyl chloride is added to the autoclave and the stirrer (200 rpm) is lowered while the autoclave contents are heated. After 21 min, the desired temperature of 55 ° C is reached and at this temperature a suspension polymerization of vinyl chloride takes place for 5.5h. During this time, the autoclave pressure drops by 0.4 MPa. Subsequently, the unreacted vinyl chloride "degassed", the polyvinyl chloride suspension is filtered off and a "mother liquor" sample is collected. The polyvinyl chloride suspension is dried in a thin layer in a forced air circulation oven at 60 ° C for 24 h.

The vinyl chloride conversion is 85%. Bulk density of the produced polyvinyl chloride suspension = 592 g / dm 3; absorber absorption time = 31 min. 4 CS 267333 B1

The powder distribution according to the grain is such a mesh retention of 1 mm = 0.0 ¾ sieve residue 0.250 mm »0.1 ¾ 0.200 mm« 1.0 ¾ 0.160 mm = 4.4 0,1 0.100 mm ° 35.8 ¾ 0.063 mm · overflow * 33.6 ¾ 25.1 ¾ Example 2

The procedure is similar to that described in Example 1, except that 17 g of unmodified polyvinyl alcohol and 17 g of unmodified polyvinyl alcohol are used instead of modified polyvinyl alcohol. of the polyvinyl acetate polymers specified in Example 1. The polymerization proceeded unevenly, resulting in a " blocking " of the product, thus avoiding the suspension of polyvinyl chloride. Example 3

The procedure is similar to Example 1, except that the partially modified polyvinyl alcohol is prepared under a nitrogen atmosphere at a pressure of 1 bar and a temperature of 200-2 ° C for 4h. The polyvinyl alcohol thus treated is in the form of an aqueous solution, respectively. solo of 4¾ mass. dynamic viscosity = 7,71 mPa.s at 25 ° C. Analyzing okonoentraoii solution 3.76 hmot wt. the dry matter yields an acid value of 5.07 mg KOH / g solution; deletion number = 5.61mgKH / g solution; degree of hydrolysis »98.9 rel. mol. %. Analysis of the dried nitrogen atmosphere of the modified polyvinyl alcohol yields an acid value of 9.50 mgKOH / g; saponification number * 122.41 mg KOH / g; degree of hydrolysis 90.3 rel. mol. %; polymerization stage F 601.

The polymerization is carried out in a manner similar to that of Example 1, but the batch into the autoclave of the polymorphose shows irregularities. After opening the autoclave and washcloth tnkmnr knmpnctný block. Example 4

The procedure is similar to Example 1, but with the difference that the polyvinyl alcohol is modified in the form of an aqueous solution, respectively. sol of partially saponified polyvinyl acetate with a concentration of 16 wt. (3000 g of solution in a rotary autoclave of 5 dm 3) at an oxygen pressure of 0.35 MPa and a temperature of 200-2 ° C during 1h.

The polyvinyl alcohol thus modified has these properties! the viscosity of the aqueous solution was about 4µ. at 25 ° C, 0.55 mPa · s; 15.2% aqueous solution. has an acid value of 8.83 mg KOH / g; saponification number: 24.65 mg KOH / g; degree of hydrolysis «91.1 rolls. mol. The dried modified polyvinyl alcohol has an acid value of 19.1 µm K (µl / µl; the zinc number was 111.06 mg KOH / g); degree of hydrolysis 92.3 rel. mol. H. Polymorphic Rate F = 1122.

Vinyl chloride conversion is 88 88. wherein the polyvinyl chloride powder obtained has a bulk density of 561 g / dm 3 and a absorber absorbency of 31 min. Sewing analysis gives a thirty-second appearance of the mesh on the mesh size of 1 mm »0.7 mass; sieve residue 0,250 mm = 2,2 sieve 0,200 mm «4,8¾} 0,160 mm = 16,0¾} 0,100 mm = 45,6¾ and on sieve 0,063 mm = 17, Bpad with this seam * 13,6¾. Example 5

Similar to Example 1, different types of starting polyvinyl alcohol PVA1) are used, but otherwise similar modification conditions are used. Properties of starting polyvinyl alcohol si) listed in Table 1 and properties of modified polyvinyl alcohol in Table 2. EN 267333 81 5

Table l

Indicator 'Experiment number and qualitative parameters of parvinyl alcohol 5a 5b characteristics PVA1: type (Goshenol) CH-20 • 4% w / w viscosity of the aqueous solution. [mPa.s] 45.2 12.3 surface tension of an aqueous solution at a concentration of 4% by weight. [mNm-] 46 41.5 polymerization degree, P 2300 1122 hydrolysis degree determined from solution [%] 87.5 77.3 and from dried PVA1 [%] 86.1 73.9

Modification of the two starting materials of polyvinyl alcohol is carried out using a 5 dm stainless steel rotary autoclave for each experiment to form 3000 g of a 4% (w / w) aqueous solution of polyvinyl alcohol. The modification is carried out with 0.1 Mpa oxygen pressure at 200-2 ° C for 1 h.

The suspension homopolymerization of the vinyl chloride of both experiments (5a, 5b) is carried out in other similar conditions to that of Example 1. The "mother liquors" of Experiment 5a have a pH of 2.7 and from 5b to 2.7; conductivity /s.10^/ 440, resp. 490; the chloride ion content (C1-4) of 43 and 39mg / dm3. Indeed, the results of both experiments are shown in Table 3.

Table 2

Properties of Modified PVA1 Experiment Number 5 ^ 5b Viscosity of an aqueous solution at a concentration of 4¾. wt. 8.7 7.7 Surface Water Strength 4k Wt. [mNm *] 50.5 46.5 4% w / w aqueous solution. modified PVA1- acid number [mgKOH / g] 2.4 2.8 - saponification number [mg KOH / g] 7.4 13.1 - degree of hydrolysis [rel. mol. 89.6 74.8 Analysis of dried modified PVA1: - acid number [mg KOH / g] 10.5 4.5 - saponification number [mg KOH / g] 118.2 216.7 - degree of hydrolysis [rel. mol. %] 90.8 80.2 6 CS 267333 B1

Table 3

Polymerization and Suspension PVC Indicators Experiment Number 5a 5b Conversion of Vinyl Chloride [%] 86 88 Bulk, Suspension PVC Weight [g / dmJJ 613 464 □ both PVC-absorbed absorbers, [min] 31 9 PVCi Powder Sieve Analysis Sieve Balance 0.250 mm [%] 1.2 12.8 0.200 mm [%] 5.0 22.4 0.160mm [%] 6.4 26.8 O.100mm [%] 36.8 27.4 0.063 mm [%] 25.8 2.4 Overflow over sieve 0.063 mm [%] 24.8 S, 2 Example 6

The procedure is similar to that of Example 1, except that the modified polyvinyl alcohol was prepared by modifying the aqueous solution at a concentration of 16% by weight. polyvinyl alcohol for a rotary outflow of 5 dm 3 at a temperature of 200 ° C and an oxygen pressure of 0.2 MPa for 1 hour, the effect of the amount of dispersant is also appreciated. An aqueous solution of the modified alcohol at a concentration of 4% by weight. has a dynamic viscosity at 25 ° C - 9.39 mPa 3 and an analysis of the aqueous solution at a concentration of 12.4% by weight. and obtain these results, the kyaloethyl * 5.73 mgKOH / g solution number saponified with 16.23 mg KO11 / gi degree of hydrolysis of 92.9 mole%. Analysis by evaporation (at 110 ° C for 3h under reduced pressure) of the modified polyvinyl alcohol yields the following results. acid number 8,17mgK0H / g; saponification number 133.06 mg KOH / gj degree of hydrolysis «89.2% rel. mol.

Three attempts are made to suspend the polymerization of vinyl chloride using modified polyvinyl alcohol in an amount of 0.05%, 0.07% and 0.10% by weight, calculated on vinyl chloride.

Most importantly, the homogeneous suspension homopolymerization of vinyl chloride and the properties of the prepared washing machine as a function of the dispersant content are summarized in Table 4.

Table 4

Indicator Experiment Number 6a 6b 6c Amount of Modified Polyvinyl Alcohol Used as Disperser [% w / v Chloride] 0.05 0.07 0.100 Vinyl Chloride Conversion [%] 89 87 87 Powder Density [g / dm3] 528 615 658 Adla both absorber absorbers [min] 12 25 31 Sewing analysis of powder PVC.Sheet residue 0.25 mm [%] 32.4 10.8 1.2 0.200 mm [%] 21.2 13.8 4.0 0.160 mm [%] 16.0 7.8 10.4 0.100 mm [%] 10.4 22.2 31.2 0.063 mm [%] 7.0 17.6, 40.0 overflow through sieve 0.063 mm [%] 5 , 0 27.0 13.2

Claims (2)

EXAMPLE 7 30 kg of demineralized water and 1.05 kg of vinyl acetate (VAC) are pumped into a 50 dm 3 autoclave specified in Example 1. Further, 75 g (50 cm 3) of trichlorethylene, 20 gdilauroyl peroxide (0.07 mol), 6 g of a xylene peroxide solution of 50% by weight (0.006 mol), specified in Example 1, and various amounts of modified polyvinyl alcohol dissolved in 500 g of water are added. The preparation and properties of the modified polyvinyl alcohol are described in Example 6. Thus, 4 experiments are carried out with varying amounts of modified polyvinyl alcohol as a dispersant in an amount of 0.14%, 0.18% by weight, 0.20% by weight. and 0.30% by weight, calculated on vinyl chloride (VC), used for suspension copolymerization with vinyl acetate (VAC). In comparison to one experiment, methylhydroxypropylcellulose (Methocel F50) is used instead of the modified polyvinyl alcohol as a dispersant in an amount of 0.20% w / v and in another mixture of modified polyvinyl alcohol with methylhydroxypropylcellulose in a weight ratio of 1: 1 in the total amount of 0, 20 wt. / VC. The autoclave is closed in each experiment, purged with nitrogen, and 50 kg of vinyl chloride is introduced. Subsequently, the contents of the autoclave are heated to 66 [deg.] C. for 20-30 min with constant agitation and maintained at this temperature for 6-7 h until a pressure drop of 0.4 MPa. Then unreacted vinyl chloride is discharged from the autoclave and the autoclave contents are filtered off. The vinyl chloride / vinyl acetate suspension obtained is then dried under reduced pressure at 50 ° C for 24 h, weighed and analyzed. Depending on the amount and type of dispersant, the results of six pieces are shown in Table 5. Table 5 Indicator Experiment number 7a 7b 7c 7d 7e 7f Amount of modified PVA1 used [% by weight / VC] 0.14 0.18 0 , 20 0,30 0,00 0,10 Amount of methylhydroxypropylcellulose used [% w / w] 0,00 0,00 0,00 0,00 0,20 0,10 Conversion of monomers [%] 86 84 82 76 82 83 Bulk density of suspension copolymer [g / dnr] 540 652 676 713 484 532 Screen analysis of VC / VAC copolymer powder: mesh retention 1 mm [%] 2.0 0.6 0.2 0.1 0.0 0 , 1 net balance 0.250 mm,% | 22.8 1.4 0.9 0.6 0.4 4.0 on site 0.200 mm [%] 16.6 1.2 1.2 0.1 3.6 8.7 on site 0.160 mm [%] 15.2 4.0 3.5 2.8 18.0 12.3 on site 0.100 mm [%] 22.8 29.2 38.1 36.0 48.4 44.1 on sieve 0.063 mm [%] 9.0 40.0 37.1 37.6 20.4 20.1 and overflow through said sieve C% 3 13.6 23.8 19.2 22.9 9.2 10.7 OBJECT OF THE INVENTION A process for the preparation of suspension homopolymers and / or copolymers of vinyl chloride by suspending homopolymerization and / or copolymerization of vinyl chloride at a temperature of 5 to 80 ° C, the initiating effect of noradical initiators, protective colloids, in particular a dispersant or a mixture of dispersants and excipients such as molecular regulators. B1 weight, reducing agents, higher aliphatic alcohols and pH medium regulators, characterized in that the dispersant or dispersant component is present in an amount of 0.001 to 1% by weight. based on vinyl chloride or a mixture of monomers, modified polyvinyl alcohol with a degree of hydrolysis of 60 to 100% is obtainable by contacting oxygen or oxygen-containing gas to polyvinyl alcohol and / or partially hydrolysed polyvinyl acetate, generally in the form of an aqueous solution and / or sol, priteplote 0 to 260 ° C to achieving an acid value of 2 to 45 mg KOH / g aqueous solution and / or sol, calculated to a concentration of 15% by weight. the modified polyvinyl alcohol or at least twice the acid value of the starting polyvinyl alcohol and / or the partially saponified polyvinyl acetate. 2. Process according to claim 1, characterized in that the oxygen or oxygen-containing gas is also a gas containing atomic oxygen and / or oligomeric oxygen, preferably ozone.