KR20090041691A - Pour point depressants for biodiesels - Google Patents

Abstract

A cold flow improver for biodiesels is provided to improve a CFPP(Cold Filter Plugging Point) and a pour point of fuel oil containing the biodiesels and to improve breathability of a fuel filter remarkably. A cold flow improver for biodiesels contains terpolymers. The terpolymers are acquired from a mixture including a compound of a chemical formula 1, a compound B and a compound C having comonomer. The chemical formula 1 is CH2=CR^1-COOR^2. The R^1 is a hydrogen atom or methyl radical. The R^2 is an alkyl group of C1~C30.

Description

Low Temperature Fluidity Enhancer for Biodiesel {POUR POINT DEPRESSANTS FOR BIODIESELS}

The present invention relates to a low temperature fluidity improver of fuel oil and a fuel oil composition added thereto. Specifically, the present invention relates to a low temperature fluidity improver for fuel oil containing residue oil, a low temperature fluidity improver for improving low temperature fluidity, and a fuel oil composition excellent in wax dispersibility of fuel oil by adding the same. The low temperature fluidity improver is characterized in that it comprises a terpolymer produced by polymerization of a mixture of monomers of alkyl (meth) acrylates having different lengths of alkyl groups.

Fuel oil is a liquid fuel obtained through the refining process of crude oil, and a part of the n-paraffin component having a high melting point, which is not removed in the refining process, is precipitated at a low temperature to block the fuel filter or a part of the n-paraffin component. There is a possibility of sedimentation in the storage tank and the composition of the diesel oil. In particular, when the fuel filter clogging occurs in winter, the fuel supply is not smooth or the vehicle is stopped while driving. Therefore, the fuel oil used in the low temperature region should be one in which no precipitation or solidification of the wax crystal occurs at low temperature. In fuel oil, n-paraffin precipitation with high melting point is the main cause of filter clogging. In fuel oil containing biodiesel, precipitation of methyl ester of saturated fatty acid with high melting point is the main cause of filter clogging, and melting of methyl ester of saturated fatty acid The higher the carbon number of the fatty acid and the smaller the unsaturated double bond, the higher the point.

As a method for testing the low temperature performance of fuel oil, a common method is to measure the pour point and the cold filter plugging point (hereinafter referred to as 'CFPP'). Pour point test method is a method of measuring the pour point defined as the temperature of 2.5 ℃ higher than the temperature at which the liquidity is completely reduced by cooling the fuel oil every time the temperature is lowered to 2.5 ℃. The CFPP test method is a method of evaluating filter clogging whenever the temperature drops 1 ° C while quenching fuel oil and passing it through a 45 micrometer diameter filter.

As a technique for improving the fluidity at low temperature of fuel oil, a method of adding a low temperature fluidity improver to a fuel is known. The low temperature fluidity improver acts on the saturated fatty acid methyl ester crystals of the biodiesel and the wax that precipitates from the fuel at low temperatures to finely disperse the precipitated wax or crystals in the fuel oil.

The fluidity improvers proposed so far include ethylene-propylene copolymers (JP-A-60-35396 and JP-A-60-137997) and ethylene-vinyl acetate copolymer (EVA) (JP-A 55-137193). When a copolymer made of an ethylenic monomer, an aromatic dicarboxylic acid amine salt and an ethylene-vinyl acetate copolymer or an ethylene-propylene copolymer are mixed (Japanese Patent Laid-Open No. 56-92996, Japanese Patent Laid-Open No. 58-1792) ), Mixtures of aromatic dicarboxylic acid amide amine salts with polyester compounds (Japanese Patent Laid-Open No. 6-49464) and the like are known. However, although the above-mentioned low temperature fluidity improvers have an effect in the CFPP test of fuel oil containing no biodiesel, there is little effect on the dispersibility of the precipitated crystals, but rather a problem of deteriorating the air permeability of the filter.

Further low temperature fluidity improvers include surfactant-type additives such as alkenyl succinic acid amide (Japanese Patent Laid-Open No. 56-43391), polyoxyalkylene ester (Japanese Patent Laid-Open No. 57-177092), or polyalkylene polyamine-bis Although mixtures of amide esters and vinyl acetate copolymers or alkyl (meth) acrylates by reaction of alkylene oxide adducts of fatty acid amides with fatty acids (Japanese Patent Publication No. 7-47742) and the like are known, these are also bio It has little effect on the low temperature fluidity of diesel fuel oil.

As another additive, poly (meth) acrylate (PMMA) based low temperature fluidity improver has been researched and developed for nearly 60 years, and was developed by Rohm & Hass Co. in 1937 (USP 2,001,627; 2,100,993). Air) with the addition of PMMA-styrene copolymer (USP 4,756,843; 6,228,819), PMMA-based copolymer with oxygen (USP 3,249,545; 3,052,648), and antioxidant moiety. Copolymer (USP 4,790,948), a technique using a mixture of PMMA copolymer and olefin copolymer (USP 4,149,984; 4,290,925; Great British Patent 1,559,952) and the like are known. US Patent 6,248,141, Exxon, 2001, discloses a new additive obtained by copolymerizing a third ester such as butyl acetate in a copolymer of ethylene and vinyl acetate to lower the CFPP temperature of diesel. have. BASF, Germany, has also filed a patent on the use of homopolymers of ethylenically unsaturated esters to improve the efficiency of low temperature flow improvers for fuel oil compositions. Applied in Germany, Korea, Japan, the United States, etc., at least one kind having a C ₁ to C ₁₈ alkyl group in order to improve the low temperature fluidity of fatty acid esters of monohydric alcohols derived from rapeseed oil, sunflower oil and / or soybean oil Of an additive containing acrylic acid and a copolymer of 8 to 21 mol% vinyl ester, or one type of C ₈ to C ₁₆ alkyl ester of ethylenically unsaturated dicarboxylic acid and at least one type of C ₁₀ to C ₂₀ olefin. It is known to achieve a CFPP value of -20 ° C by blending an additive containing a comb tooth polymer (Korean Patent Publication No. 10-2007-0075325). In addition, there is Korea Patent Application No. 2005-200637 of the same company as an improved patent application of the above patent application. On the other hand, among the European patents, a Japanese patent issued by Sanyo Co., Ltd. in 2002 and registered in 2004. The copolymer of ethylene / vinylacetate is vinyl neodecanoate, vinyl 2-ethylhexanoate, 4-methyl-1-pentene. , By applying a terpolymer obtained by copolymerizing a third monomer such as 2-ethylhexyl acrylate as a low temperature fluidity improver, adding 300 to 1000 ppm to diesel oil and measuring CFPP to improve the low temperature fluidity. The contents are known.

As described above, many patents have been applied to improve the low temperature fluidity of fuel oils themselves. Patents related to fluidity improvement are mainly dominant, and low temperature fluidity improvers developed by these patented technologies have little effect as the biodiesel content increases.

The present invention improves the air permeability of the fuel filter by improving the pour point and CFPP and dispersing the precipitated wax and saturated fatty acid methyl ester crystals in the fuel oil, for the fuel oil containing the biodiesel, namely It is another object of the present invention to provide a low temperature fluidity improver for biodiesel and a fuel oil composition containing the same.

The present inventors control the length of the alkyl group of the alkyl (meth) acrylate monomer when using a low temperature fluidity improver comprising a terpolymer prepared from a mixture of monomers of alkyl (meth) acrylates having different alkyl groups. Low temperature fluidity of the fuel oil containing biodiesel is prevented by preventing crystal growth of wax crystals and saturated fatty acid methyl esters precipitated by cooling of the fuel oil and finely dispersing the crystals in the fuel oil, thereby reducing filter clogging. The present invention was completed by discovering that it can be improved.

That is, the present invention is a mixture comprising a comonomer of Compound A of Formula 1, Compound B of Formula 2, and Compound C of Formula 3, wherein the alkyl groups of the respective comonomers are different from each other, and each of the comonomers It relates to a low temperature fluidity improver containing a terpolymer obtained from a mixture having a ratio of 0.1 to 0.95: 0.1 to 0.95: 0.01 to 0.2 molar ratio.

[Formula 1]

CH ₂ = CR ¹ -COOR ²

[Wherein, R ¹ is a hydrogen atom or a methyl group,

R ² is a C ₁ to C ₃₀ alkyl group, preferably a C ₁₀ to C ₂₀ alkyl group, particularly preferably a C ₈ to C ₁₈ alkyl group]

Suitable acrylic acid esters in Compound A of Formula 1 include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth ) Acrylate, hexyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, decyl (meth) acrylate, dodecyl (meth) acrylate, tetradecyl (meth) acrylate , Hexadecyl (meth) acrylate, octadecyl (meth) acrylate and mixtures of these comonomers.

[Formula 2]

CH ₂ = CR ^{1 '} -COOR ^2'

[Wherein, R ^{1 ′} Is a hydrogen atom or a methyl group,

R ^{2 '} Is a C ₆ -C ₃₀ alkyl group, preferably a C ₁₀ -C ₂₀ alkyl group, particularly preferably a C ₁₂ -C ₁₈ alkyl group.

Suitable acrylic esters in compound B of formula (2) include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth ) Acrylate, hexyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, decyl (meth) acrylate, dodecyl (meth) acrylate, tetradecyl (meth) acrylate , Hexadecyl (meth) acrylate, octadecyl (meth) acrylate and mixtures of these comonomers.

[Formula 3]

CH ₂ = CR ^{1 ''} -COOR ^{2 ''}

In which R ^{1 ''} Is a hydrogen atom or a methyl group,

R ^{2 ''} Is a hydroxyalkyl (of which the alkyl group is a C ₁ to C ₁₈ alkyl group, preferably a C ₂ to C ₁₀ alkyl group, particularly preferably a C ₂ to C ₆ alkyl group), norbornyl, morpholine or morpholine alkyl ( Among these, the alkyl group is a C ₁ -C ₁₈ alkyl group, preferably a C ₂ -C ₁₀ alkyl group, particularly preferably C ₂ -C ₆ Alkyl group), adamantane alkyl (of which the alkyl group is a C ₁ to C ₁₈ alkyl group, preferably a C ₂ to C ₁₀ alkyl group, particularly preferably a C ₂ to C ₆ alkyl group), and cyclopentadienyl alkyl (in this one, the alkyl group is C ₁ ~C ₁₈ alkyl group, preferably a C ₂ ~C ₁₀ alkyl group, particularly preferably a C ₂ ~C ₆ alkyl group)].

Suitable acrylic esters in compound C of formula (3) include, for example, hydroxyethyl (meth) acrylate, norbornane methyl (meth) acrylate, cyclohexyl (meth) acrylate, morpholine (meth) acrylate, Adamantane ethyl (meth) acrylate, 2- (dimethylamino) ethyl (meth) acrylate, ethylene glycol dicyclo pentyl ether (meth) acrylate and mixtures of these comonomers.

Ternary copolymers are prepared through radical polymerization using monomers of the compounds represented by Formulas 1 to 3, and the weight average molecular weight (Mw) of the copolymer thus prepared is preferably 5,000 to 100,000 g / mol, in particular Preferably it is 10,000-50,000 g / mol. The molecular weight distribution (PD = Mw / Mn) of the copolymer is preferably 1.0 to 5.0, particularly preferably 1.1 to 3.0.

In particular, preferred terpolymers are those containing 1 to 20 mol%, preferably 2 to 15 mol%, of the acrylic acid ester of formula (3).

In addition, the ratio of the acrylic ester of the formula (1) to the acrylic ester of the formula (2) is preferably in the range of 30 to 80 mol%, particularly preferably in the range of 45 to 75 mol%.

The present invention also provides a diesel containing biodiesel comprising the terpolymer as a low temperature fluidity improver. Biodiesel in the present invention is corn oil (cottonseed oil), linseed oil (linseed oil), peanut oil (peanut oil), rapeseed oil (rapeseed oil), safflower oil, soybean oil (soybean) oil, sunflower oil, palm oil, but are not limited to these. The content of biodiesel is in the range of 0.5 to 50% by weight, preferably 0.5 to 20% by weight, based on the weight of light oil. The content of the terpolymer is in the range of 100 to 10,000 ppm by weight of diesel.

When the low temperature fluidity improving agent of the present invention is added to fuel oil containing biodiesel, the fluidity and low temperature filter plugging point of fuel oil are improved, the wax precipitated at low temperature is dispersed in diesel oil, and the air permeability of the filter of fuel oil is not impaired. Excellent effect is not achieved.

Hereinafter, the content of the present invention will be described in detail.

The terpolymer of the present invention may be prepared by adding n-dodecyl mercaptan and n-butyl mercaptan, which are chain transfer agents, to a compound represented by Chemical Formulas 1 to 3 at an activation temperature of a free radical initiator in a solvent in the presence of a solvent. Can be prepared by carrying out a radical polymerization reaction using a free radical initiator. Free radical initiators that can be used in the preparation method preferably include peroxides or azo compounds.

Specific free radical initiators include, for example, benzoyl peroxide, t-butylperoxy acetate, di-t-butylperoxide, t-butylperoxy pivalate, t-butylperoxy-2-ethyl as peroxide compounds. Hexanoate, t-butylperoxy neodecanoate, t-butylperoxy isobutyrate, t-butylperoxy benzoate, t-amylperoxy neodecanoate, t-amylperoxy pivalate, t- Amylperoxy-2-ethyl hexanoate, and 2,2'-azobisisobutyronitrile (AIBN), 1,1'- azobiscyclohexanecarbonitrile, 2,2'-azobis as an azo compound -2-methylpropionamidine and the like.

In this invention, an azo compound is preferable and 2,2'- azobisisobutyronitrile (AIBN) is especially preferable. The free radical initiator may be used by addition in the form dissolved in a solvent or in the form dissolved in a monomer by methods known in the art.

The content of the free radical initiator is from 0.05 to 10% by weight, particularly preferably from 0.5% by weight, based on the weight of the monomer mixture.

The production method of the present invention can be carried out in the presence of a solvent, for example, aliphatic hydrocarbons or aromatic hydrocarbons such as toluene, n-hexane, tetrahydrofuran, xylene, benzene and the like.

The molecular weight of the prepared terpolymer was measured using GPC, number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution (PD = Mw / Mn) was analyzed. The low temperature properties of the terpolymer were analyzed by the ASTMD97 method for the pour point and cloud point by adding a certain amount to diesel (ULSD). Cold filter plugging point (CFPP) was analyzed by KS M2411 method.

Hereinafter, although an Example demonstrates this invention still in detail, this invention is not limited to these Examples.

[ Example  One]

0.5 wt% of initiator AIBN was placed in a 100 ml round container, vacuumed, and filled with nitrogen. 70 wt% of toluene was added as a solvent in a nitrogen atmosphere and stirred until the initiator was completely dissolved. When the initiator was completely dissolved, lauryl methacrylate, stearyl methacrylate, 2-hydroylethyl methacrylate were injected into the vessel in a nitrogen atmosphere at a weight ratio of 3: 6: 1. 0.5 wt% of a chain transfer agent, n-dodecyl mercaptan, was injected to appropriately control the molecular weight. After freeze-thaw was performed three times to remove oxygen in the solution, the vessel was immersed in an oil bath, and the reaction temperature was adjusted to 60 ° C., followed by polymerization. The proportion of monomers in all experiments was calculated by weight ratio and injected.

After the completion of the polymerization, the reaction temperature was lowered to room temperature, and then the polymerization reaction product was dissolved in a small amount of tetrahydrofuran and precipitated in an excess of nonsolvent methanol. The obtained polymer was allowed to settle, the supernatant was removed, dissolved in a small amount of tetrahydrofuran, and the excess was added again to recover by precipitating methanol. The recovered polymer was dried in a vacuum oven at 30 ° C. for 24 hours, and the resulting polymer was analyzed for molecular weight through GPC. The molecular weight of the prepared polymer is as shown in the following [Table 1].

[ Example  2-5]

The polymer was prepared in the same manner as in Example 1 by varying the composition of the monomer as shown in [Table 1].

[ Comparative example  1-3]

The polymer was prepared in the same manner as in Example 1 by varying the composition of the monomer as shown in [Table 1].

[ Comparative example  4 and 5]

Comparative Example 4 is an acryloid 156 of ethyl Co., Ltd., which is a PMMA-based low temperature fluidity improver currently commercially available as a low temperature fluidity improver.

Comparative Example 5 is an EVA-based additive currently being added to diesel.

Weight ratio Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Lauryl methacrylate 6 6 6 3 3 4 3 4 - - Stearyl methacrylate 3 3 3 6 6 6 7 4 - - Hydroxyethyl methacrylate One - - One - - - 2 - - Cyclohexyl methacrylate - One - - One - - - - - Norbornane Methyl Methacrylate - - One - - - - - - - Molecular Weight Mn 9,000 28,000 28,000 28,000 11,000 17,000 19,000 26,000 24,000 3,300 Mw 18,000 58,000 54,000 57,000 23,000 56,000 59,000 61,000 57,000 9,400 PD (= Mw / Mn) 2.0 2.1 1.9 2.0 2.0 3.2 3.0 2.3 2.0 2.8 Solubility * Transparency Transparency Transparency Transparency Transparency Transparency Transparency opacity Transparency Transparency

* Solubility when 1wt% of polymer is added in diesel fuel containing 5% of biodiesel.

Additives used in commercialization with the prepared polymers were added to diesel fuel containing 5% of biodiesel to evaluate low-temperature fluidity such as pour point, cloud point and CFPP. The results are shown in [Table 2].

 Item  Addition ppm Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Pour Point oC 0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 -10.0 500 -17.5 -20.0 -17.5 -22.5 -20.0 -10.0 -12.5 -12.5 -15.5 -12.5 1000 -25.0 -25.0 -22.5 -25.0 -22.5 -12.5 -17.5 -15.0 -17.5 -17.5 2000 -25.0 -25.0 -22.5 -27.5 -25.0 -17.5 -22.5 -17.5 -27.5 -22.5 5000 -27.5 -27.5 -22.5 -22.5 -32.5 -22.5 -25.0 -20.0 -35.0 -25.0 Cloud point ℃ 0 -4.0 -4.0 -4.0 -4.0 -4.0 -4.0 -4.0 -4.0 -4.0 -4.0 500 -7.0 -6.0 -8.0 -7.0 -6.0 -4.0 -5.0 -4.0 -4.0 -4.0 1000 -8.0 -7.0 -8.0 -8.0 -8.0 -5.0 -6.0 -4.0 -4.0 -4.0 2000 -7.0 -8.0 -9.0 -9.0 -10.0 -6.0 -7.0 -5.0 -5.0 -3.0 5000 -8.0 -7.0 -10.0 -8.0 -12.0 -7.0 -8.0 -4.0 -5.0 -3.0 CFPP ℃ 0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 -5.0 500 -7.0 -8.0 -8.0 -9.0 -10.0 -7.0 -7.0 -6.0 -6.0 -10.0 1000 -9.0 -10.0 -11.0 -10.0 -12.0 -7.0 -8.0 -7.0 -6.0 -16.0 2000 -10.0 -11.0 -17.0 -13.0 -15.0 -8.0 -8.0 -7.0 -7.0 -15.0 5000 -10.0 -10.0 -18.0 -17.0 -16.0 -8.0 -9.0 -7.0 -8.0 -16.0

As shown in Table 2, it can be seen that the low-temperature fluidity of the polymer of the present invention is excellent in physical properties such as pour point, cloud point, and CFPP. The low temperature fluidity improver of Comparative Example 4 showed very good physical properties at the pour point, but it was almost ineffective at CFPP. The additive of Comparative Example 5 had relatively good physical properties at the pour point and CFPP, but the cloud point had worse properties. It was.

Claims (14)

 A low temperature fluidity improver containing a terpolymer obtained from a mixture comprising a compound A of Formula 1, a compound B of Formula 2, and a comonomer of Compound C of Formula 3: [Formula 1] CH ₂ = CR ¹ -COOR ² In the formula, R ¹ is a hydrogen atom or a methyl group, R ² is a C ₁ -C ₃₀ alkyl group; [Formula 2] CH ₂ = CR ^{1 '} -COOR ^2' Wherein R ^{1 '} Is a hydrogen atom or a methyl group, R ^{2 '} Is a C ₆ -C ₃₀ alkyl group; [Formula 3] CH ₂ = CR ^{1 ''} -COOR ^{2 ''} Wherein R ^{1 ''} Is a hydrogen atom or a methyl group, R ^{2 ''} is a hydroxy C ₁ -C ₁₈ alkyl group, norbornyl, morpholine or morpholine C ₁ -C ₁₈ alkyl group. The low temperature fluidity improver according to claim 1, wherein the proportion of the comonomers of the compounds A to C of the formulas 1 to 3 in the mixture is in the range of 0.1 to 0.95: 0.1 to 0.95: 0.01 to 0.2 molar ratio. The compound A according to claim 1 or 2, wherein the compound A of formula (1) is methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth ) Acrylate, hexyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, decyl (meth) acrylate, dodecyl (meth) acrylate, tetradecyl (meth) acrylate And hexadecyl (meth) acrylate, octadecyl (meth) acrylate, and mixtures of these comonomers. The compound B according to claim 1 or 2, wherein the compound B of formula (2) is methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth ) Acrylate, hexyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, decyl (meth) acrylate, dodecyl (meth) acrylate, tetradecyl (meth) acrylic Low temperature fluidity improver, characterized in that selected from the group consisting of latex, hexadecyl (meth) acrylate, octadecyl (meth) acrylate and mixtures of these comonomers. The compound C according to claim 1 or 2, wherein the compound C of formula (3) is hydroxyethyl (meth) acrylate, norbornane (meth) acrylate, cyclohexyl (meth) acrylate, morpholine (meth) acrylate, Low temperature fluidity, characterized in that it is selected from the group consisting of tantan ethyl (meth) acrylate, 2- (dimethylamino) ethyl (meth) acrylate, ethylene glycol dicyclopentyl ether (meth) acrylate, and mixtures of these comonomers Enhancer. The low temperature fluidity improver according to claim 1 or 2, wherein the weight average molecular weight (Mw) of the terpolymer is 5,000 to 100,000 g / mol. The low-temperature fluidity improver according to claim 1 or 2, wherein the molecular weight distribution (PD = Mw / Mn) of the terpolymer is 1.0 to 5.0. A fuel oil composition comprising the low temperature fluidity improver of claim 1. 10. The fuel oil composition of claim 8, wherein the fuel oil composition comprises biodiesel. A process for producing the terpolymer of claim 1 comprising the following: To the comonomer mixture of compounds 1 to 3 of formulas 1 to 3 according to claim 1 at the activation temperature of the free radical initiator in the presence of a solvent, n-dodecyl mercaptan and n-butyl mercaptan Add and perform radical polymerization using free radical initiator. The method of claim 10 wherein the free radical initiator is selected from the group consisting of peroxide compounds and azo compounds. 12. The peroxide compound according to claim 11, wherein the peroxide compound that is a free radical initiator is benzoyl peroxide, t-butylperoxy acetate, di-t-butylperoxide, t-butylperoxy pivalate, t-butylperoxy-2-ethyl hexa Noate, t-butylperoxy neodecanoate, t-butylperoxy isobutyrate, t-butylperoxy benzoate, t-amylperoxy neodecanoate, t-amylperoxy pivalate and t-amyl A method characterized in that it is selected from the group consisting of peroxy-2-ethyl hexanoate. The azo compound as a free radical initiator according to claim 11, wherein the azo compound is 2,2'-azobisiso butyronitrile (AIBN), 1,1'-azobiscyclohexanecarbonitrile and 2,2'-azobis-2. -Methylpropionamidine. The process according to claim 10, wherein the content of the free radical initiator is from 0.05 to 10% by weight, based on the comonomer mixture.