The invention discloses an emulsion polymerization process for the preparation of acrylonitrile-styrene-butadiene (ABS) graft copolymer latexes having consid erably reduced residual monomer content, ABS graft copolymers obtained by said process, thermoplastic molding compositions comprising it, and the use, in particular in the automotive industry.

In the automotive market, the application of ABS plastic materials is increasing year by year, especially in the four-wheeler auto sector. Owing to the excep tional properties of ABS plastic materials, automotive manufacturers mostly pre fer it for both interior as well as exterior applications. ABS molding compositions are used for variety of automotive application because it provides balanced characteristics of impact strength, dimensional stability, flowability, chemical resistance and heat resistance, which normally other general-purpose thermo plastics cannot deliver. ABS molding compositions are used for many interior auto-components and thus a low content of volatile organic compounds (VOC) is desirable and required from the automobile manufacturers, in particular in the four-wheeler auto sector. There are legal and environmental regulations and stringent laws prevailing in most of the European countries related to VOC of interior parts used for automobiles.

In general, most manufacturers of ABS graft copolymers follow the emulsion polymerization method nowadays. It is known that ABS graft copolymers ob tained by emulsion polymerization retain a higher amount of residual, unreacted monomers in comparison to ABS graft copolymers obtained by mass polymeri zation, but emulsion polymerized ABS graft copolymers have the advantage of better mechanical properties.

In emulsion technique, in order to reduce residual monomers, the reaction has to achieve a higher conversion. However, beyond a certain level due to higher crosslinking, mechanical properties especially impact properties are highly af fected. An alternative route mostly followed is to add monomer scavenging chemicals or additives in the polymerization stage or later. However, these chemicals can also have impact on the final properties of the product and lead to additional cost of production. US 4,301,264 discloses a process for reducing the residual styrene monomer content of ABS polymer dispersions. In said process, at first an ABS latex is prepared by grafting styrene and acrylonitrile (SAN) on to a polybutadiene latex by emulsion polymerization using a redox initiator based on cumene hydroper oxide. Then, after a polymerization conversion of at least 90% in a separate vessel, an activated cumene hydroperoxide is added to the obtained ABS latex which is followed by a heat treatment. US 7,897,686 describes a method for preparing an acrylonitrile-butadiene- styrene (ABS) graft rubber latex having a low residual monomer content of up to 5000 ppm. The graft rubber latex is obtained by grafting SAN on to a polybuta diene rubber latex (particle size of 250 to 500 nm) in an emulsion polymeriza tion. In the graft copolymerization, in a first step, 10 to 30 wt.-% of the co- monomer mixture together with a polystyrene or SAN-copolymer latex (particle size 20 to 100 nm) are added with an initiator (1-1); then, in a second step, 70 to 90 wt.-% of the comonomer mixture are added with further initiator (I-2). When the conversion rate of the polymerization is at least 94 %, in a third step, a re dox polymerization initiator (I-3) is additionally added. Initiator (1-1) is preferably a redox initiator based on t-butylhydroperoxide; initiator (I-2) is preferably cu- menehydro peroxide, and redox initiator (I-3) is preferably based on cumenehy- dro peroxide.

US 2002/0111435 describes a process for the preparation of ABS graft rubber polymers having reduced residual monomer content. The graft rubber latex is obtained by emulsion polymerization of SAN co-monomers in presence of a polybutadiene rubber latex according to a fed batch process, wherein the initia tor or the initiator (redox) system is added to the reaction mixture in specific por tions within certain time intervals. Preferred initiators are peroxodisulfates, as well as organic hydroperoxides. In the examples either a redox initiator based on tert-butyl hydroperoxide, or potassium peroxodisulfate has been used as initiator.

Even though the residual monomer content of the ABS graft rubber copolymers obtained by said prior art processes is reduced, their mechanical properties are often still in need of improvement. It is one objective of the invention to provide an improved process for the prepa ration of an acrylonitrile-styrene-butadiene (ABS) graft copolymer by emulsion polymerization which does not alter any conditions or parameters of the butadi- ene rubber latex preparation and wherein no additional residual monomer scav engers (e.g. amines such as aryl(methylene amine)i-3, dialkyl amines, bisul phate or sulphite salts, thiols etc.) or such additives are added. Moreover, such a process shall be provided in which the entire grafting of the butadiene rubber latex with the graft monomers can be carried out in one reactor. ABS graft co- polymers obtained by said process shall have a reduced residual monomer con tent (range preferably 3000-4000 ppm) and further ABS molding compositions comprising said ABS graft copolymers have good mechanical properties.

The object of the invention is achieved by providing a process and a graft rub- ber copolymer obtained by said process in accordance with the claims.

Thus, one subject of the invention is a process for the preparation of a graft rubber copolymer (A), which process comprises the following steps: a) emulsion polymerization of butadiene or a mixture of butadiene and at least one monomer copolymerizable with butadiene to obtain at least one starting butadiene rubber latex (S-A1) having a median weight particle diameter Dso of equal to or less than 120 nm; b) subjecting of said at least one starting butadiene rubber latex (S-A1) to agglomeration, preferably by use of an organic acid, more preferably or ganic acid anhydride, to obtain at least one agglomerated butadiene rub ber latex (A1) with a median weight particle diameter Dso of 150-2000 nm (= graft substrate); c) grafting of said at least one agglomerated butadiene rubber latex (A1) by emulsion polymerization of styrene and acrylonitrile, preferably in a weight ratio of 95:5 to 65:35, in presence of at least one agglomerated butadiene rubber latex (A1) to obtain a graft rubber copolymer (A) at a temperature of 40 to 90°C, it being possible for styrene and/or acrylonitrile to be replaced partially (less than 50 wt.-%) by alpha-methylstyrene, methyl methacrylate, maleic anhydride or N-phenylmaleimide or mixtures thereof; wherein step c) comprises sub-steps d), c2) and c3): c1) sludge feeding:

• feeding of 10 to 45 wt.-%, preferably 20 to 40 wt.-%, more preferably 25 to 30 wt.-% of styrene and acrylonitrile - based on the total amount sty rene and acrylonitrile - in one portion to the at least one agglomerated butadiene rubber latex (A1), and

• addition of 0.01 to 0.06 parts by weight of at least one redox system initi ator (1-1) selected from hydrogen peroxide or at least one organic perox ide - based on 100 parts by weight of styrene and acrylonitrile and ag glomerated butadiene rubber latex (A1) -, then - after addition of initiator (1-1 )) -

• polymerization for 30 to 90 minutes to obtain a reaction mixture (RM-c1 ); c2) incremental feeding: then, preferably within 5 hours, more preferably within 3 hours, to the re action mixture obtained in step d),

• feeding the remaining amount of styrene and acrylonitrile - based on the total amount of styrene and acrylonitrile - in portions (preferably equally divided portions fed in regular intervals) or continuously, and

• further addition of 0.05 to 0.12 parts by weight of said at least one redox system initiator (1-1) - based on 100 parts by weight of styrene and acry lonitrile and agglomerated butadiene rubber latex (A1) - to obtain a reac tion mixture (RM-c2); c3) boost addition: then, to the reaction mixture obtained in step c2),

• addition of 0.05 to 0.40 parts by weight - based on 100 parts by weight of styrene and acrylonitrile and agglomerated butadiene rubber latex (A1) - of at least one inorganic free radical initiator (I-2), in particular inorganic per-salt, preferably alkali persulfate, more preferred potassium persul fate, and then

• continuation of the polymerization for 30 to 90 minutes to obtain graft rubber copolymer (A).

Wt.-% means percent by weight; pbw means parts by weight.

Butadiene means 1,3-butadiene. In the context of steps d), c2) and c3) the term “styrene and acrylonitrile” means styrene and acrylonitrile which independently can be partially (less than 50 wt.-%) replaced by alpha-methylstyrene, methyl methacrylate, maleic anhy dride or N-phenylmaleimide or mixtures thereof as before defined in step c).

The median weight particle diameter D50, also known as the D50 value of the integral mass distribution is defined as the value at which 50 wt.-% of the parti cles have a diameter smaller than the D50 value and 50 wt.-% of the particles have a diameter larger than the D50 value. In the present application the weight- average particle diameter D _w , in particular the median weight particle diameter D50, is determined with a disc centrifuge (e.g.: CPS Instruments Inc. DC 24000 with a disc rotational speed of 24000 rpm).

The weight-average particle diameter D _w is defined by the following formula (see G. Lagaly, O. Schulz and R. Ziemehl, Dispersionen und Emulsionen: Eine EinfOhrung in die Kolloidik feinverteilter Stoffe einschlieBlich der Tonminerale, Darmstadt: Steinkopf-Verlag 1997, ISBN 3-7985 -1087-3, page 282, formula 8.3b):

Dw = sum ( n, ^* d, ⁴ ) / sum( n, ^* d, ³ ) n,: number of particles of diameter d,.

The summation is performed from the smallest to largest diameter of the parti cles size distribution. It should be mentioned that for a particles size distribution of particles with the same density which is the case for the starting rubber lati- ces and agglomerated rubber latices the volume average particle size diameter Dv is equal to the weight average particle size diameter Dw.

Steps a) and b) of the process according to the invention are described e.g. in WO 2012/022710.

Preparation of starting butadiene rubber (S-A1)

In step a) of the process according to the invention butadiene, or a mixture of butadiene and at least one monomer co-polymerizable with butadiene, is poly- merized by emulsion polymerization to obtain at least one starting butadiene rubber latex (S-A1) having a median weight particle diameter D50 of equal to or less than 120 nm.

Step a) of the process of the invention is described e.g. in WO 2012/022710.

The term “butadiene rubber latex” means polybutadiene latices produced by emulsion polymerization of butadiene and less than 50 wt.-% (based on the to tal amount of monomers used for the production of polybutadiene polymers) of one or more monomers that are copolymerizable with butadiene as comono mers.

Examples for such monomers include isoprene, chloroprene, acrylonitrile, sty rene, alpha-methylstyrene, Ci-C4-alkylstyrenes, C-i-Cs-alkylacrylates, C-i-Cs- alkylmethacrylates, alkyleneglycol diacrylates, alkylenglycol dimethacrylates, divinylbenzol; preferred monomers are styrene and/or acrylonitrile, more prefer ably styrene.

Preferably, butadiene is used alone or mixed with up to 30 wt.-%, preferably up to 20 wt.-%, more preferably up to 15 wt.-% styrene and/or acrylonitrile, prefer ably styrene.

Preferably a mixture of butadiene with 1 to 30 wt.-%, preferably 3 to 20 wt.-%, more preferably 5 to 15 wt.-% styrene is used for the preparation of the starting butadiene rubber latex (S-A1). Preferably for the emulsion polymerization a plant based, in particular a resin acid-based, emulsifier is used. More preferably as emulsifier only resin-acid based emulsifiers are used in step a).

As resin acid-based emulsifiers, those are being used in particular for the pro duction of the starting rubber latices by emulsion polymerization that contain alkaline salts of the resin acids. Salts of the resin acids are also known as resin soaps. Examples include alkaline soaps as sodium or potassium salts from dis- proportionated and/or dehydrated and/or hydrated and/or partially hydrated gum resin with a content of dehydroabietic acid of at least 30 wt.-% and preferably a content of abietic acid of maximally 1 wt.-%. Furthermore, alkaline soaps as sodium or potassium salts of tall resins or tall oils can be used with a content of dehydroabietic acid of preferably at least 30 wt.-%, a content of abietic acid of preferably maximally 1 wt.-% and a fatty acid content of preferably less than 1 wt.-%.

Mixtures of the aforementioned emulsifiers can also be used for the production of the starting rubber latices. The use of alkaline soaps as sodium or potassium salts from disproportionated and/or dehydrated and/or hydrated and/or partially hydrated gum resin with a content of dehydroabietic acid of at least 30 wt.-% and a content of abietic acid of maximally 1 wt.-% is advantageous.

Preferably the emulsifier is added in such a concentration that the final particle size of the starting butadiene rubber latex (S-A1) achieved is from 60 to 110 nm (median weight particle diameter D50).

Suitable molecular-weight regulators for the production of the starting butadiene rubber latices(S-AI), include, for example, alkylmercaptans, such as n- do- decylmercaptan, tert-dodecylmercaptan, dimeric alpha-methylstyrene and ter- pinolene.

In step a) of the process according to the invention inorganic and organic perox ides, e.g. hydrogen peroxide, di-tert-butyl peroxide, cumene hydroperoxide, di- cyclohexylpercarbonate, tert-butylhydroperoxide, p-menthanehydroperoxide, azo initiators such as azobisisobutyronitrile, inorganic per-salts such as ammo- nium, sodium or potassium persulfate, potassium perphosphate, sodium perbo rate as well as redox systems can be taken into consideration as initiators of the emulsion polymerization of butadiene or a styrene-butadiene mixture.

Redox systems generally consist of an organic oxidizing agent and a reducing agent, additional heavy-metal ions can be present in the reaction medium (see Houben-Weyl, Methoden d. Organischen Chemie, Volume 14/1 , pp. 263 - 297).

Preferably at least one alkalipersulfate initiator, in particular potassium persul fate, is used in step a) of the process according to the invention.

More preferably butadiene, or a mixture of butadiene and at least one monomer copolymerizable with butadiene, is polymerized by emulsion polymerization us ing, in particular persulfates, in particular potassium persulfate, as an initiator and a resin-acid based emulsifier to obtain the starting butadiene rubber latices (S-A1 ).

Moreover, salts, acids and bases can be used in the emulsion polymerization for producing the starting rubber latices. With acids and bases the pH value, with salts the viscosity of the latices is adjusted during the emulsion polymerisa tion. Examples for acids include sulfuric acid, hydrochloric acid, phosphoric ac- id; examples for bases include sodium hydroxide solution, potassium hydroxide solution; examples for salts include chlorides, sulfates, phosphates as sodium or potassium salts. The preferred base is sodium hydroxide solution and the preferred salt is tetrasodium pyrophosphate. The pH value of the fine-particle rubber latices is between pH 7 and pH 13, preferably between 8 and pH 12, particularly preferably between pH 9 and pH 12. With respect to the optionally used salts it can be referred to in addition to those mentioned in US 2003/0139514, which include for example alkali salts such as alkali halides, ni trates, sulfates, phosphates, pyrophosphates, preferably tetrasodium pyrophos phate, sodium sulfate, sodium chloride or potassium chloride. The amount of the optional salt may include 0 to 2, preferred 0 to 1 weight- percent relative to the latex solids.

Polymerization temperature in the preparation of the starting butadiene rubber latices (S-A1) is generally 25°C to 160°C, preferably 40°C to 90°C. Work can be carried out under the usual temperature control, e.g. isothermally. It is also pos sible to carry out polymerization in such a way that the temperature difference between the beginning and the end of the reaction is at least 2 °C, or at least 5°C, or at least 10°C starting with a lower temperature. It is possible to first provide all substances used, i.e. water, monomers, emulsi fiers, molecular-weight regulators, initiators, bases, acids and salts at the be ginning of polymerization. Furthermore, it is also possible to first provide only a part of the substances used at the beginning of the polymerization, and to first provide other substances used only partially, and to feed the remaining part dur- ing the polymerization. It has proved to be advantageous to first provide only a part of the monomers and molecular-weight regulators up to 35% and to feed the largest part. Fur thermore, it has proved advantageous to first provide the emulsifiers for the most part at least 65% or completely and to feed the remainder. Furthermore, it has proved advantageous to first provide the initiators, bases, acids and salts for the most part at least 65% or completely and to feed the remainder. Fur thermore, it has proved advantageous to first provide water for the most part at least 50% or completely and to feed the remainder. After the polymerization is finished the starting rubber latex can be cooled down to 50°C or lower and as far as the monomer conversion is not completed the not reacted monomers, e.g. butadiene can be removed by devolatilization at re duced pressure if necessary. The at least one, preferably one, starting butadiene rubber latex (S-A1) prefera bly has a median weight particle diameter D50 of equal to or less than 110 nm, particularly equal to or less than 90 nm.

The gel content of the starting rubber latices is preferably 30 to 98% by wt., preferably 50 to 95% by wt. based on the water unsoluble solids of said latices. The values indicated for the gel content are based on the determination accord ing to the wire cage method in toluene (see Flouben-Weyl, Methoden der Or- ganischen Chemie, Makromolekulare Stoffe, part 1 , page 307 (1961 ) Thieme Verlag Stuttgart). The gel contents of the starting rubber latices can be adjusted in a manner known in principle by applying suitable reaction conditions (e.g., high reaction temperature and/or polymerization up to high conversion as well as, optionally, addition of substances with a cross-linking effect for achieving a high gel content, or, e.g., low reaction temperature and/or termination of the polymerization reaction prior to the occurrence of a cross-linkage that is too comprehensive as well as, optionally, addition of molecular-weight regulators such as, for example n-dodecylmercaptan or tert- dodecylmercaptan for achiev ing a low gel content).

The solid content of the starting rubber latices is preferably 25 to 55% by wt. (evaporation sample at 180°C for 25 min. in drying cabinet), more preferably 30 to 50% by wt., particularly preferably 35 to 45% by wt. The degree of conversion (calculated from the solid content of a sample and the mass of the substances used) of the monomers used in the emulsion polymeri zation preferably is larger than 50%, more preferably larger than 60%, particu larly preferably larger than 70%, very particularly preferably larger than 80%, in each case based on the sum of monomers. Moreover, the degree of conversion of the monomers used is preferably lower than 99%, more preferably lower than 97%, particularly preferably lower than 96%, very particularly preferably lower than 95%, in each case based on the sum of monomers.

Preparation of the agglomerated butadiene rubber latex (A1)

In step b) of the process according to the invention said at least one starting butadiene rubber latex (S-A1) is subjected to agglomeration, preferably by addi tion of an organic acid, to obtain at least one agglomerated butadiene rubber latex (A1) with a median weight particle diameter Dsoof 150 to 2000 nm (= graft substrate).

The median weight average particle diameter D50 of the agglomerated rubber latices is preferably 160 to 1000 nm, more preferably 170 to 800 nm, more pre ferred 200 to 600 nm, preferred 250 to 500 nm, very preferred 300 to 400 nm.

Preferably agglomeration of the starting butadiene rubber latex (S-A1) is carried out by the addition of at least one organic acid, in particular carboxylic acid, preferably organic acid anhydride, more preferably carboxylic acid anhydride, and still more preferably acetic anhydride.

Production of the agglomerated rubber latices (A1) is preferably carried out by mixing the starting butadiene rubber latices with the afore-mentioned acids and/or acid anhydrides.

After agglomeration is complete, preferably restabilization with a base prefera bly potassium hydroxide solution is carried out.

Preferably, acetic anhydride is used for agglomeration. However, other organic anhydrides can also be used. It is also possible to use mixtures of acetic anhy dride with acetic acid or mixtures of organic anhydrides with acetic acid or other carboxylic acids. Once the agglomeration is complete, the agglomerated rubber latex (A1) is preferably stabilized by addition of further emulsifier while adjusting the pH val ue of the latex (A1 ) to a pH value (at 20°C) between pH 7.5 and pH 11 , prefera- bly of at least 8, particular preferably of at least 8.5, in order to minimize the formation of coagulum and to increase the formation of a stable agglomerated rubber latex (A1) with a uniform particle size. As further emulsifier preferably rosin-acid based emulsifiers as described above in step a) are used. The pH value is adjusted by use of bases such as sodium hydroxide solution or prefer- ably potassium hydroxide solution.

In a preferred embodiment of the process of the invention first, the starting rub ber latex is provided, wherein, in a preferred form, the solid content of this latex is adjusted to at most 50% by wt., more preferably at most 45% by wt, and par- ticularly preferably at most 40% by wt. by the addition of water. The temperature of the starting rubber latex, optionally mixed with water, can be adjusted in a broad range of from 0°C to 70°C, preferably of from 0°C to 60°C, and particular ly preferably of from 15°C to 50°C. Preferably at this temperature, a mixture of preferably acetic anhydride and water, which was prepared by mixing, is added to the starting rubber latex under good mixing. The addition of the acetic anhy dride-water mixture and the mixing with the starting rubber latex should take place within a time span of two minutes at most in order to keep the coagulate formation as small as possible. In the process of the invention the coagulate formation cannot be avoided completely, but the amount of coagulate can be limited advantageously by this measure to significantly less than 1 % by wt, generally to significantly less than 0.5% by wt based on the solids of the starting rubber latex used.

Preferably the mixing ratio of the organic acid anhydride-water mixture, in par- ticular acetic anhydride-water mixture, used in the agglomeration step is 1 :5 to 1 : 50 parts by mass, preferably 1 :7.5 to 1 :40, particularly preferably 1 : 10 to 1 :30. When the organic acid anhydride-water mixture, preferably acetic anhy dride-water mixture, is added, agglomeration of the fine-particle rubber particles within the starting butadiene rubber latex (S-A1) to form larger rubber particles starts and is finished after 5 to 60 minutes according to the adjusted tempera ture. The rubber latex is not stirred or mixed in this phase. The agglomeration, the increase in size of the rubber particles, comes to a standstill when the entire amount of acetic anhydride is hydrolyzed and the pH value of the rubber latex does not drop any further.

Coagulate which has possibly formed is removed from the agglomerated rubber latex in particular by filtering (e.g. a filter with a mesh width of 50 pm).

Preparation of graft rubber copolymer (A)

In step c) of the process according to the invention styrene and acrylonitrile, preferably in a weight ratio of 95:5 to 65:35, more preferably 80:20 to 70:30, are polymerized by emulsion polymerization in presence of at least one agglomer ated butadiene rubber latex (A1) to obtain a graft rubber copolymer (A) at a temperature of 40 to 90°C, it being possible for styrene and/or acrylonitrile to be replaced partially (less than 50 wt.-% based on the total amount of monomers used in step c))) by alpha-methylstyrene, methyl methacrylate, maleic anhydride or N-phenylmaleimide or mixtures thereof.

The preparation of the graft rubber polymers (A) may be carried out, as desired, by grafting of only one agglomerated butadiene rubber latex (A1) or by the common grafting of more than one agglomerated butadiene rubber lattices (A1) during one reaction.

Alternatively mixtures of graft rubber copolymers (A) can be obtained by first individual grafting of an agglomerated butadiene rubber latex (A1) (e.g. having different particle size) and then mixing the individually obtained graft rubber co polymers.

Step c) of the process according to the invention comprises sub-steps d), c2) and c3).

Sub-steps d), c2) and c3) comprised in step c) are carried out in the order first d), then c2) and then c3).

Step c) is preferably carried out in one reactor.

Step c) is carried out at a temperature of 40 to 90°C, preferably 50 to 80°C, more preferably 60 to 75°C. In step c) acrylonitrile and styrene - optionally independently replaced partially (less than 50 wt.-% based on the total amount of acrylonitrile and styrene) by alpha-methylstyrene, methyl methacrylate, maleic anhydride or N- phenylmaleimide or mixtures thereof - are used in a total amount of 15 to 60 wt.-%, preferably from 20 to 50 wt.-%, and the at least one agglomerated diene butadiene rubber latex (A1) is used in a total amount of 40 to 85 wt.-% , prefer ably from 50 to 80 wt.-% (in each case based on the solid).

Preferably styrene and acrylonitrile are not partially replaced by one of the afore-mentioned co-monomers; preferably styrene and acrylonitrile are polymer ized alone in a weight ratio of 95:5 to 65:35, preferably 80:20 to 65:35.

In step c) as emulsifier there may be used conventional anionic emulsifiers such as alkyl sulfates, alkyl sulfonates, ar-alkyl sulfonates and soaps of saturated or unsaturated fatty acids as well as above mentioned resin acid-based emulsifiers or tall resin emulsifiers. Resin acid-based emulsifiers or tall resin emulsifiers are used preferably, resin acid-based emulsifiers (resin soaps) are in particular pre ferred. Molecular-weight regulators may additionally be used in the graft polymerization step c) preferably in amounts of from 0.01 to 2 wt. %, particularly preferably in amounts of from 0.05 to 1 wt. % (in each case based on the total amount of monomers in the graft polymerization step). Suitable molecular-weight regula tors are, for example, alkylmercaptans, such as n-dodecyimercaptan, tert- do- decylmercaptan (TDDM), dimeric alpha-methylstyrene, terpinols.

In the context of steps d), c2) and c3) the term “styrene and acrylonitrile” means styrene and acrylonitrile which independently can be partially (less than 50 wt.-%) replaced by alpha-methylstyrene, methyl methacrylate, maleic anhy- dride or N-phenylmaleimide or mixtures thereof as hereinbefore defined in step c).

In step d) - the so-called sludge feeding - in presence of the at least one ag glomerated butadiene rubber latex (A1 ) 10 to 45 wt.-%, preferably 20 to 40 wt.- %, more preferably 25 to 30 wt.-%, of styrene and acrylonitrile - based on the total amount of styrene and acrylonitrile - are fed in one portion, and 0.01 to 0.06 parts by weight of at least one redox system initiator (1-1) selected from hydrogen peroxide or at least one organic peroxide - based on 100 parts by weight of styrene and acrylonitrile and rubber latex (A) - is added.

Preferably styrene and acrylonitrile are fed in one portion within 10 and 20 minutes.

In step c1) the addition of the redox system initiator (1-1) may be done simulta neously with or preferably after the feeding of styrene and acrylonitrile. The ad dition of the redox system initiator (1-1) may be done in at least one, preferably one, portion.

The redox system initiator (1-1) used in step d) is hydrogen peroxide or at least one organic peroxide selected from the group consisting of di-tert-butyl perox ide, cumene hydroperoxide (CHP), dicyclohexyl percarbonate, tert-butyl hy droperoxide, p-menthane hydroperoxide, diisopropylbenzene hydroperoxide and dibenzoylperoxide. Organic peroxides are preferred, cumene hydroperox ide is in particular preferred.

Redox systems generally consist of an oxidizing agent and a reducing agent, it being possible for heavy metal ions (e.g. Fe ⁺⁺ ) additionally to be present in the reaction medium (see Houben-Weyl, Methoden der Organischen Chemie, Vol ume 14/1, p. 263 to 297). In the process according to the invention the redox system initiator (1-1) is used as the oxidizing agent.

For the other components of the redox system comprising the redox system initiator (1-1), any reducing agent and metal component known from literature can be used. Preferably the redox system is an aqueous redox system compris ing redox system initiator (1-1).

A particular preferred redox system comprises cumene hydroperoxide, dextrose and FeSC .

Preferably 0.03 to 0.05 parts by weight, more preferred about 0.04 parts by weight of the at least one redox system initiator (1-1) - based on 100 parts by weight of styrene and acrylonitrile and rubber latex (A) - are used in step d). Then, after the addition of initiator (1-1) in step d), the polymerization is started and carried out for 30 to 90 minutes, preferably 45 to 75 minutes, to obtain a reaction mixture (RM-c1).

Then, in step c2), the so-called incremental feeding, preferably within 5 hours, more preferably within 3 hours, to the reaction mixture (RM-c1) obtained in step d), the remaining amount of styrene and acrylonitrile - based on the total amount of styrene and acrylonitrile - is fed in portions or continuously, and 0.05 to 0.12 parts by weight of at least one redox system initiator (1-1) as afore mentioned - based on 100 parts by weight of styrene and acrylonitrile and rub ber latex (A) - is further added to obtain a reaction mixture (RM-c2).

Preferably, the feeding of the remaining amount of styrene and acrylonitrile is carried out in equally divided portions in regular intervals.

Generally in step c2) the addition of the redox system initiator (1-1) is done sim ultaneously with the feeding of styrene and acrylonitrile. In step c2) the addition of the redox system initiator (1-1) may be done in at least one portion or contin uously (within preferably 5 hours, more preferably 3 hours).

Preferably 0.06 to 0.10 parts by weight, more preferred about 0.08 parts by weight, of the at least one redox system initiator (1-1) - based on 100 parts by weight of styrene and acrylonitrile and rubber latex (A) - are used in step c2).

Step c2) is preferably completed within 5 hours, more preferably within 3 hours.

Then, in step c3) - the so-called boost addition - to the reaction mixture (RM-c2) obtained in step c2) 0.05 to 0.40 parts by weight - based on 100 parts by weight of styrene and acrylonitrile and agglomerated butadiene rubber latex (A1) - of at least one inorganic free radical initiator (I-2) is added, and then, af ter the addition of initiator (I-2), polymerization is generally continued for 45 to 90 minutes, preferably 45 to 75 minutes, to obtain graft rubber copolymer (A).

Generally the inorganic free radical initiator (I-2) is added in one portion. The at least one inorganic free radical initiator (I-2) is in particular an inorganic per-salt, preferably an alkali persulfate, more preferred potassium persulfate (KPS). Generally the inorganic free radical initiator (I-2) is water soluble.

Preferably 0.07 to 0.35 parts by weight, more preferred about 0.10 to 0.20 parts by weight, in particular 0.13 to 0.17 parts by weight, based on 100 parts by weight of styrene and acrylonitrile and agglomerated butadiene rubber latex (A1 ), of the at least one inorganic free radical initiator (I-2) are used in step c3).

The process according to the invention ensures that all available monomers take part in the polymerization reaction and help to achieve a higher conversion rate. The reactivity and decomposition rate of the inorganic free radical initiator (I-2) is also comparatively higher compared to organic redox system initiators used in prior art processes which also improves the conversion.

The work-up of the graft rubber copolymers (A) is carried out by common pro cedures, e.g. by coagulation with salts, e.g. Epsom salt and/or acids, washing, drying or by spray drying.

A further subject of the invention is a graft rubber copolymer obtained by the process according to the invention.

Thermoplastic Molding Compositions

A further subject of the invention is a thermoplastic molding composition com prising at least one graft rubber copolymer (A) obtained by the process accord ing to the invention and at least one rubber-free vinylaromatic polymer (B).

Suitable rubber-free vinyl aromatic polymers (B) are in particular copolymers of styrene and acrylonitrile (SAN) in a weight ratio of from 95:5 to 50:50, preferably 78:22 to 55:45, more preferably 75:25 to 65:35, most preferred 72:28 to 70:30, it being possible for styrene and/or acrylonitrile to be replaced wholly or partially by alpha-methylstyrene, methyl methacrylate, maleic anhydride or N- phenylmaleimide. It is preferred that styrene and acrylonitrile are not replaced by one of the afore-mentioned comonomers. Preferred are SAN-copolymers of styrene and acrylonitrile alone. Said SAN-copolymers preferably have weight average molecular weights Mw of from 85,000 to 250,000 g/mol, more preferably 100,000 to 225,000 g/mol.

The weight average molar mass M _w is determined by GPC (solvent: tetrahydro- furan, polystyrene as polymer standard) with UV detection according to DIN 55672-1:2016-03.

Said SAN-copolymers often have a melt flow index (MFI) of 20 to 75 g/10 min (measured according to ASTM D 1238 (ISO 1133:1-2011) at 220°C and 10 kg load).

Details relating to the preparation of such resins are described, for example, in DE-A 2 420 358 and DE-A 2 724 360 and in Kunststoff-Handbuch ([Plastics Handbook], Vieweg-Daumiller, volume V, (Polystyrol [Polystyrene]), Carl- Hanser-Verlag, Munich, 1969, pp. 122 ff. , lines 12 ff.). Such copolymers pre pared by mass (bulk) or solution polymerization in, for example, toluene or ethylbenzene, have proved to be particularly suitable.

In general, the thermoplastic molding composition according to the invention may comprise the graft rubber copolymer (A) in amounts of from 10 to 50 wt.-%, preferably from 15 to 45 wt.-%, more preferably from 20 to 40 wt.-%, and the rubber-free vinylaromatic polymer (B), preferably a copolymer of styrene and acrylonitrile, in amounts of from 50 to 90 wt.-%, preferably from 55 to 85 wt.-%, more preferably from 60 to 80 wt.-%. The sum of components (A) and (B) totals 100 wt.-%.

Optionally the thermoplastic molding composition according to the invention may comprise at least one additive and/or processing aid (C). If present, com ponent (C) is generally used in amounts of 0.01 to 10 parts by weight, prefera- bly 0.10 to 5.0 parts by weight, based on 100 parts by weight of the total of (A) + (B).

Suitable additives and/or processing aids are e.g. antioxidants, UV stabilizers, peroxide destroyers, antistatics, lubricants, release agents, flame retarding agents, fillers or reinforcing agents (glass fibers, carbon fibers, etc.) as well as colorants. In addition to the mentioned polymer components (A) and (B), these polymer compositions according to the invention may contain further rubber-free ther moplastic resins (TP) not composed of vinyl monomers, such thermoplastic res- ins being used in amounts of up to 1000 parts by weight, preferably up to 700 parts by weight and particularly preferably up to 500 parts by weight (in each case based on 100 parts by weight of the total of (A) + (B).

The thermoplastic resins (TP) as the rubber-free copolymer in the thermoplastic molding compositions according to the invention which are used in addition to the mentioned polymer components (A) and (B), include for example polycon densation products, for example aromatic polycarbonates, aromatic polyester carbonates, polyesters, polyamides. Suitable thermoplastic polycarbonates and polyester carbonates are known (see, for example, DE-A 1 495626, DE-A 2232877, DE-A 2703376, DE-A 2 714544, DE-A 3000610, DE-A 3832396, DE-A 3077934) and are further described in detail as well as suitable polyamides in WO 2012/022710 (p. 14 - 18) to which reference is in particular made.

The molding compositions according to the invention are produced by mixing graft rubber copolymer (A) according to the invention and the rubber-free vinyl- aromatic polymer (B) and, optionally, further polymers (TP) and conventional additives and/or processing aids (C) in conventional mixing apparatuses (pref- erably in multi-cylinder mills, mixing extruders or internal kneaders).

Accordingly, the invention also provides a process for the production of the thermoplastic molding compositions according to the invention, wherein compo nents (A) and (B) and, optionally, further polymers (TP) and conventional addi- tives and/or processing aids (C) are mixed and compounded and extruded at elevated temperature, generally at temperatures of from 150°C to 300°C. Dur ing the production, working up, further processing and final shaping, the re quired or useful additives and/or processing aids (C) can be added to the ther moplastic molding materials. The final shaping can be carried out on commercially available processing ma chines, and comprises, for example, injection-molding processing, plate extru sion with optionally subsequent hot forming, cold forming, extrusion of tubes and profiles and calender processing.

The process according to the invention leads to a higher conversion of the graft polymerization reaction and a significant reduction in residual monomers pre sent in the graft rubber copolymer (A). Thermoplastic molding compositions according to the invention have a signifi cantly reduced content of unreacted monomers (= residual monomers). The products obtained according to the process of the invention are environment- friendly without compromising any mechanical properties and application per formance. On the other hand, the effluent treatment - which is known in the art and can be carried out by common procedures - is eased by lowered residual monomer in the effluent water.

A further subject of the invention is the use of graft rubber copolymers (A) ob tained according to the process of the invention in the automotive sector, in par- ticular for automotive interior applications, and the use of thermoplastic molding compositions according the invention in the automotive sector, in particular for automotive interior applications.

The invention is further illustrated by the examples and the claims.

Examples

Test Methods Particle Size Dw / Dso For measuring the weight average particle size Dw (in particular the median weight particle diameter D50) with the disc centrifuge DC 24000 by CPS In struments Inc. equipped with a low density disc, an aqueous sugar solution of 17.1 ml_ with a density gradient of 8 to 20% by wt. of saccharose in the centri fuge disc was used, in order to achieve a stable flotation behavior of the parti- cles. A polybutadiene latex with a narrow distribution and a mean particle size of 405 nm was used for calibration. The measurements were carried out at a rotational speed of the disc of 24,000 r.p.m. by injecting 0.1 ml_ of a diluted rubber dispersion into an aqueous 24% by wt. saccharose solution. The calculation of the weight average particle size Dw was performed by means of the formula Dw = sum ( n, ^* di ⁴ ) / sum( n, ^* d, ³ ) n,: number of particles of diameter d,.

Molar Mass M _w

The weight average molar mass M _w is determined by GPC (solvent: tetrahydro- furan, polystyrene as polymer standard) with UV detection according to DIN 55672-1:2016-03.

Melt Flow Index (MFI) or Melt Flow Rate (MFR)

MFI/MFR test was performed on ABS pellets (ISO 1133 standard, ASTM 1238, 220°C/10 kg load) using a MFI-machine of CEAST, Italy.

Notched Izod Impact Strength (N I IS) Test

Izod impact tests were performed on molded and notched specimens (ASTM D 256 standard) using instrument of CEAST (part of Instron’s product line), Italy.

Tensile Strength (TS) and Tensile Modulus (TM) Test

Tensile tests (ASTM D 638) were carried out at 23°C using a Universal testing Machine (UTM) of Instron, UK. Flexural Strength (FS) and Flexural Modulus (FM) Test

Flexural tests (ASTM D 790 standard) were carried out at 23°C using a UTM of Lloyd Instruments, UK.

Fleat deflection temperature (HDT) Heat deflection temperature test was performed on injection molded specimen (ASTM D 648) using an instrument of Zwick Roell GmbH, Germany.

VICAT Softening Temperature (VST)

Vicat softening temperature test was performed on injection molded test speci- men (ASTM D 1525-09 standard) using a machine of Zwick Roell GmbH, DE. Test was carried out at a heating rate of 120°C/hr (Method B) at 50 N loads. Rockwell Hardness

Hardness of the injection molded test specimen (ISO - 2039/2-11) was carried out on Fuel Instruments and Engineers Pvt Ltd, India.

The norms and standards are the actual versions.

Analysis of Residuals

Residual analysis is carried out using a Gas Chromatograph with FID of Perkin Elmer, USA.

The following materials were used:

Component (A)

Fine-particle butadiene rubber latex (S-A1)

The fine-particle butadiene rubber latex (S-A1) which is used for the agglomera tion step was produced by emulsion polymerization using tert- dodecylmercaptan as chain transfer agent and potassium persulfate as initiator at temperatures from 60° to 80°C. The addition of potassium persulfate marked the beginning of the polymerization. The fine-particle butadiene rubber latex (S- A1) was cooled below 50°C and the non-reacted monomers were removed par tially under vacuum (200 to 500 mbar) at temperatures below 50°C which de- fines the end of the polymerization. Then the latex solids (in % per weight) were determined by evaporation of a sample at 180°C for 25 min. in a drying cabinet. The monomer conversion is calculated from the measured latex solids. The bu tadiene rubber latex (S-A1) is characterized by the following parameters, see table 1.

Latex S-A1 -1

No seed latex is used. As emulsifier the potassium salt of a disproportionated rosin (amount of potassium dehydroabietate: 52 wt.-%, potassium abietate: 0 wt.-%) and as salt tetrasodium pyrophosphate is used.

Table 1 : Composition of the butadiene rubber latex S-A1

K = W ^* (1 -1 4 ^* S) ^* Dw

W = decomposed potassium persulfate [parts per 100 parts rubber]

S = salt amount in percent relative to the weight of solids of the rubber latex Dw = weight average particle size (= median particle diameter D50) of the fine- particle butadiene rubber latex (S-A1 )

Production of the coarse-particle, agglomerated butadiene rubber latices (A1 ) The production of the coarse-particle, agglomerated butadiene rubber latices (A1 ) was performed with the specified amounts mentioned in table 2. The fine- particle butadiene rubber latex (S-A1 ) was provided first at 25°C and was ad justed if necessary with deionized water to a certain concentration and stirred. To this dispersion an amount of acetic anhydride based on 100 parts of the sol ids from the fine-particle butadiene rubber latex (S-A1 ) as fresh produced aque- ous mixture with a concentration of 4.58 wt.-% was added and the total mixture was stirred for 60 seconds. After this the agglomeration was carried out for 30 minutes without stirring. Subsequently KOH was added as a 3 to 5 wt.-% aque ous solution to the agglomerated latex and mixed by stirring. After filtration through a 50 pm filter the amount of coagulate as solid mass based on 100 parts solids of the fine-particle butadiene rubber latex (S-A1 ) was determined. The solid content of the agglomerated butadiene rubber latex (A), the pH value und the median weight particle diameter D50 was determined. Table 2: Production of coarse-particle, agglomerated butadiene rubber latex(A1 )

Production of the graft rubber copolymers (A) Table 3 shows the recipe for the grafting. The amount of the additional compo nents - other than the main components including the butadiene rubber latex (A1 ) and acrylonitrile and styrene - are given in parts by weight (pbw) based on 100 parts by weight (of the sum of the total amount) of butadiene rubber latex (A1 ) and acrylonitrile and styrene monomer. The weight ratio of acrylonitrile and styrene is 26:74.

Table 3: Recipe for grafting **TSPP = tetrasodiumpyrophosphate

Examples 1 and 2 (invention)

Mixtures (= latex A1) of the coarse-particle, agglomerated butadiene rubber lat- ices A1-1 and A1-2 (ratio 50:50, calculated as solids of the rubber latices (A1)) were diluted with water to a solid content of 27.5 wt.-% and heated to 68°C in a reactor.

In the initial sludge feeding step d), said reactor was flooded with styrene and acrylonitrile in the amounts given in Table 3 within 15 minutes. TDDM as mo lecular weight regulator was additionally added along with the monomer charge. At the same time when the monomer slug feed was completed, the polymeriza- tion was started by feeding cumene hydroperoxide (CHP) together with a po tassium salt of disproportionated resin (amount of potassium dehydroabietate: 52 wt.-%, potassium abietate: 0 wt.-%) as aqueous solution and separately an aqueous solution of dextrose, tetrasodium pyrophosphate (TSPP) and iron-(ll)- sulfate were fed to the reactor within 5 minutes. The temperature was kept at 68°C. The polymerization was carried out for one further hour.

Then, in the incremental feeding step c2) to the reaction mixture obtained in step d), the remaining amount of styrene and acrylonitrile (see Table 3) was fed in equally divided portions in regular intervals within 3 hours and simultane- ously further CHP as redox system in amounts according to Table 3 wa added within 180 minutes. Further emulsifier and TDDM was also added. This incre mental feeding of monomers increases the graft ratio and improves the conver sion. The temperature was kept at 68°C. The incremental feeding was complet ed within 3 hours.

In the final boost addition grafting step c3) to the reaction mixture obtained in step c2), potassium persulfate (KPS) was added within 10 minutes in the amounts given in Table 3. Then, after addition of KPS the polymerization was continued for one further hour. The temperature was kept at 68°C. This boost addition of KPS as initiator ensures maximum conversion. Then, the obtained graft rubber latex (= graft rubber copolymer A) was cooled to ambient temperature. The graft rubber latex was stabilized with ca. 0.6 wt.-parts of a phenolic antioxidant and precipitated with sulfuric acid, washed with water and the wet graft powder was dried at 70°C (residual humidity less than 0.5 wt.- %).

Samples A-2 and A-3 of graft rubber copolymer A were obtained according to said process. Comparative Examples 1 and 2 (samples A1 and A5) are regular ABS graft rubber copolymer powders. The samples A1 and A5 were prepared by general grafting methods using a particular initiator (cp. Table 3).

Comparative Example 1 Mixtures (= latex A1 ) of the coarse-particle, agglomerated butadiene rubber lat- ices A1-1 and A1-2 (ratio 50:50, calculated as solids of the rubber latices (A1)) were diluted with water to a solid content of 27.5 wt.-% and heated to 68°C in a reactor. Grafting steps d ) and c2) were carried out in accordance with inventive Examples 1 and 2, but in step C3 (boost charging) CHP and a redox system were added in a single portion (amounts see Table 3). Then, the polymerization was continued for one further hour. The reaction conditions of step c3) are as described for inventive Examples 1 and 2 above.

Comparative Example 2

Mixtures (= latex A1) of the coarse-particle, agglomerated butadiene rubber lat ices A1-1 and A1-2 (ratio 50:50, calculated as solids of the rubber latices (A1)) were diluted with water to a solid content of 27.5 wt.-% and heated to 68°C in a reactor. Then for grafting in the sludge feeding step d ) KPS was used as initia- tor (reaction medium and amounts see Table 3). The amounts of the rubber latex (A1 ) and of the monomers are the same as in inventive Examples 1 and 2. Post sludge feeding, no hold up was given and in the following incremental feeding step c2), the entire portion of remaining monomer (see Table 3) was fed in regular intervals and in equally divided proportions, but no further initiator was added. The entire step is completed in 4 hours. In the boost charging step c3) potassium persulphate was added to the reaction vessel (see Table 3). Then, the polymerization was continued for one further hour. All other reaction conditions of the grafting steps d) to c3) are as described for inventive Exam ples 1 and 2 above. Comparative Example 3

Mixtures (= latex A1) of the coarse-particle, agglomerated butadiene rubber lat- ices A1-1 and A1-2 (ratio 50:50, calculated as solids of the rubber latices (A1)) were diluted with water to a solid content of 27.5 wt.-% and heated to 68°C in a reactor.

Then for grafting in the sludge feeding step d) KPS was used as initiator (reac tion medium and amounts see Table 3). The amounts of the rubber latex (A1) and of the monomers are the same as in inventive Examples 1 and 2.

Post sludge feeding, no hold up was given and in the following incremental feeding step c2), the entire portion of remaining monomer (see Table 3) was fed in regular intervals and in equally divided proportions, but no further initiator was added. The entire step was completed in 4 hours. In the boost charging step c3) CHP and a redox system were added to the reaction vessel in one portion (amounts see Table 3). Then, the polymerization was continued for one further hour. All other reaction conditions of the grafting steps d) to c3) are as de scribed for inventive Examples 1 and 2 above. Table 4: Analysis of the residuals

*VCH =vinyl cyclohexane Table 4 shows that the total content of the residuals, in particular the residual monomers, of the graft rubber copolymers A-2 and A-3 of inventive examples 1 and 2 is significantly lower than in comparative example 1. The inventive pro cess ensures that all available monomers take part in the polymerization reac- tion and help to achieve a higher conversion rate. The reactivity and decompo sition rate of KPS - used in the final grafting step c3) - is also comparatively higher compared to CHP which also improves the conversion.

The graft rubber copolymers A-5 of comparative example 2 formed with only a KPS initiator show also a substantial reduction in residuals but the core rubber gets highly cross-linked. This undesirably affects all the mechanical properties of the final ABS molding compositions (cp. Tables 7 and 8). In contrast the graft rubber copolymers A-2 and A-3 of inventive examples 1 and 2 have no adverse effect on the mechanical properties of the final ABS molding compositions (cp. Tables 7 and 8) due to optimized formation of crosslinks. Thus, it was surpris ingly found that adding KPS only in the boost addition step does not have any impact on crosslinking related mechanical property reduction of the final ABS molding compositions. The lowest total content of residuals is obtained for graft rubber copolymer A-4 of comparative example 3. However, the obtained rubber morphology typically with higher cross linking in the core is disadvantageous and the mechanical properties of the final ABS molding compositions are also undesirably affected (cp. Tables 7 and 8).

Component (B)

B-1 : Statistical copolymer from styrene and acrylonitrile with a ratio of polymer ized styrene to acrylonitrile of 73:27 with a weight average molecular weight Mw of 107,000 g/mol, a polydispersity of Mw/Mn of 2.4 and a melt flow rate (MFR) (220°C/10 kg load) of 65 g/10 minutes, produced by free radical solution polymerization.

B-2: Statistical copolymer from styrene and acrylonitrile with a ratio of polymer ized styrene to acrylonitrile of 71 :29 with a weight average molecular weight Mw of 140,000 g/mol, a polydispersity of Mw/Mn of 2.5 and a melt flow rate (MFR) (220°C/10 kg load) of 30 g/10 minutes, produced by free radical solution polymerization. Additives/Processing aids (C)

C-1: Ethylene bis-stearamide with trade name ‘Palmowax’ obtained from Pal- mamide Sdn Bhd, Malaysia

C-2: distearylpentaerythrityldiphosphite (SPEP) from Addivant, Switzerland C 3: Silicon oil having a kinematic viscosity of 1000 centi Stokes obtained from KK Chempro India Pvt Ltd C 4: Magnesium Stearate from Sunshine organics C 5: Metal oxide received from Kyowa Chemicals. Thermoplastic compositions

Each sample A-1 to A-5 of graft rubber polymers (A), SAN-copolymer (B-l) or (B-2), and the afore-mentioned further components were mixed (composition of polymer blend see Tables 5 and 6, batch size 5 kg) for 2 minutes in a high speed mixer to obtain good dispersion and a uniform premix and then said pre- mix was melt blended in a twin-screw extruder at a speed of 80 rpm and using an incremental temperature profile from 190 to 220° C for the different barrel zones. The extruded strands were cooled in a water bath, air-dried and pelletized. For the mechanical tests standard test specimens (ASTM test bars) of the obtained blend were injection moulded at a temperature of 190 to 230°C. Table 5: Composition of ABS molding compound set 1 Table 6: Composition of ABS molding compound set 2

The mechanical test results, MFR, Vicat Softening Temperature (VST), Heat deflection temperature (HDT) and Rockwell Hardness of the compositions are presented in Tables 7 and 8.

Table 7: Properties of ABS molding compound set 1

Table 8: Properties of ABS molding compound set 2

The results shown in Table 4 and Tables 7 and 8 clearly prove that the ABS molding compositions comprising graft copolymers A-2 or A-3 of inventive ex amples 1 and 2 have a significant reduction of residuals while maintaining good mechanical properties.

This effect can be achieved with the combination of two different specific initia tors in the grafting step. It has been shown that individual initiators in the graft ing step (cp. comparative example 1) results in inferior residuals or adverse property change (cp. comparative example 2). Also the sequence of initiators is of importance otherwise there will be different rubber morphology typically with higher cross link in the core which leads to inferior mechanical properties (cp. comparative example 3). The products obtained according to the inventive process clearly demonstrate the process robustness since no coagulum or other structural disproportionation or any property deterioration has been found.