Patent Description:
Viscosity index (VI) is a commonly used method of measuring a fluid's change of viscosity in relation to temperature. The higher the VI, the smaller the relative change in viscosity with temperature. VI improvers (also known as viscosity modifiers) are additives that increase the viscosity of the fluid throughout its useful temperature range.

Viscosity modifiers are polymeric molecules that are sensitive to temperature. At elevated temperatures, the polymeric chain is better solvated by the solvent that leads to an increase in hydrodynamic radius of the polymer in solution. Higher hydrodynamic radius equals an increase in thickening power and therefore an increase in VI.

When linear polymers are dissolved in a solvent such as oil, each individual polymer strand is separated from its neighbours and in solution exists in isolation. The polymer is fully solvated by the oil, but still retains a coiled structured, with oil solvent molecules filling the empty spaces within the loosened coil. The polymer then adopts an ellipsoid or spherical form and occupies a discrete volume known as the hydrodynamic volume of the polymer coil (see Scheme <NUM> below).

Dissolving polymer in a solvent such as oil is accompanied by a large increase viscosity, and this is due to the presence of these larger scale hydrodynamic spheres. The size of the hydrodynamic sphere volume determines the magnitude of the viscosity increase. Polymers yielding a high hydrodynamic volume, either due to a high molecular weight or strong associating with the oil solvent, give a relatively higher increases in the viscosity of the solution.

Increasing temperature increases the solvency of the oil, which, in turn, promotes the uncoiling of the polymer and results in a larger hydrodynamic volume.

The hydrodynamic volume of a polymer in solution depends on many parameters, such as for example the polymer chain length and composition. At low temperatures, the long-chained fiber-like polymers are more tightly coiled and contribute relatively little to viscosity. The polymers uncoil at higher temperatures, occupying a larger hydrodynamic volume and increasingly boosting viscosity as shown in the following Scheme <NUM>. <CHM>
<IMG>.

Main disadvantage of the coiling/uncoiling mechanism is that the VI effect is strongly dependent on the molecular weight of the polymer. In addition, shear stability and thickening effect are linked to the molecular weight. Shear stability limits have always been a challenge for high VI formulations, but requirements are getting constantly more demanding. A more recent challenge are low viscosity formulations that do not leave much space for thickening polymer species. Therefore, a decoupling of VI effect, shear stability and thickening would be highly desirable.

The use of a viscosity index improver in an oil of lubricating viscosity to impart desired low and high temperature viscometrics and other viscosity properties is well known. Some of the most commonly used polymers in lubricating oils include olefin copolymers (OCP), polyalkyl(meth)acrylates (PAMAs) and hydrogenated poly-(styrene-co-conjugated dienes).

The widely reported mechanism of how polymers improve VI is that polymers raise the viscosity of the fluid proportionately more at higher temperatures than at lower temperatures due to expansion of the polymer coil with increasing temperature. Not much is known in the art about other mechanisms that could be used to provide a VI effect.

<NPL>) describes a transition of specific diblock PAMA polymer aggregates from vesicles to worms. This transition occurs around <NUM> and drastically increases the viscosity of the solutions which are conducted in dodecane as a lubricant model system. Armes does not disclose the use of triblock copolymers according to the present invention with a weight-average molecular weight of <NUM>,<NUM> to <NUM>,<NUM>/mol for improving the viscosity index of lubricating oil compositions.

<CIT> describes the use of (meth)acrylate block copolymers as VI improvers. The block copolymers consist of oil soluble and insoluble blocks and mainly diblock and triblock structures are reported. In case of triblock copolymers, the oil-insoluble part is in the middle and the outer parts are oil-soluble. The working examples comprise at least <NUM> mol% of acrylic monomers and show number-average molecular weight between <NUM>,<NUM> and <NUM>,<NUM>/mol. The use of triblock copolymers according to the present invention with a weight-average molecular weight of <NUM>,<NUM> to <NUM>,<NUM>/mol for improving the viscosity index of lubricating oil compositions is not disclosed.

Diblock polymers consisting of a hydrogenated diene and a styrene block (also described as HSDs) are commonly used in the lubricant field (<NPL>). These polymers form micelles in solution which provide a huge thickening effect even at very low treat rates. These aggregates are known to lose their thickening power under shear which is used to reach low HTHS values with rather thick oils. Their permanent shear loss is quite low relative to their thickening power (see <CIT>); if temperature is raised too much the aggregates break down and the thickening effect is lost. No exceptional VI effect is provided by these polymers (see <CIT>). The oil soluble part can also be polyisobutylene (see <CIT>). The use of triblock copolymers according to the present invention with a weight-average molecular weight of <NUM>,<NUM> to <NUM>,<NUM>/mol for improving the viscosity index of lubricating oil compositions is not disclosed.

<CIT> discloses a selectively hydrogenated butadiene-isoprene-butadiene triblock copolymer with a weight-average molecular weight of <NUM>/mol (polymer A) and a process for improving viscosity index of lubricating oil compositions.

<CIT> and <CIT> describe methacrylate blocks which are used to replace the insoluble styrene block or the oil soluble hydrogenated diene block.

<CIT> describes diblock polymers including PAMA blocks which are prepared in combination with polypropylene and micelle formation in PAO4 of these polymers was investigated. The use of triblock copolymers according to the present invention with a weight-average molecular weight of <NUM>,<NUM> to <NUM>,<NUM>/mol for improving the viscosity index of lubricating oil compositions is not disclosed.

Pure PAMA diblocks with a dispersant block are described in <CIT> and the use of these polymers as emulsifiers in engine oils was investigated. The use of triblock copolymers according to the present invention with a weight-average molecular weight of <NUM>,<NUM> to <NUM>,<NUM>/mol for improving the viscosity index of lubricating oil compositions is not disclosed.

It was now surprisingly found that triblock copolymers comprising at least one oil-soluble part and at least one oil-insoluble part and having a relatively low molecular weight show an associative mechanism at higher temperatures and can be used as viscosity index improvers in lubricating oil compositions.

Associative mechanism in this connection means that relatively small molecules self-organize at elevated temperature in order to form bigger structures with an increased hydrodynamic radius. In this way, thickening contribution at low temperatures can be minimized. As the small molecules are too small to be destroyed by mechanical shear, the bigger assemblies could reform after destruction by shear forces.

One embodiment of the present invention is therefore directed to a process for improving the viscosity index of lubricating oil compositions by adding a triblock copolymer of general formula (Ib).

The weight-average molecular weight is determined by gel permeation chromatography against polymeric standards. Depending on the nature and composition of the copolymer, polyalkylmethacrylates (PMMA) are used as corresponding standard.

Suitable polyalkyl(meth)acrylates to be used as segments A can be prepared from monomer mixtures, comprising:.

The content of each component (A1) and (A2) is based on the total composition of segments A. In a particular embodiment, the proportions of components (A1) and (A2) add up to <NUM>% by weight.

The term "(meth)acrylate" refers to both, esters of acrylic acid and esters of methacrylic acid. Methacrylates are preferably used in accordance with the present invention.

The C<NUM>-<NUM>-alkyl (meth)acrylates for use in accordance with the invention are esters of (meth)acrylic acid and straight chain or branched alcohols having <NUM> to <NUM> carbon atoms. The term "C<NUM>-<NUM>-alkyl (meth)acrylates" encompasses individual (meth)acrylic esters with an alcohol of a particular length, and likewise mixtures of (meth)acrylic esters with alcohols of different lengths.

Suitable C<NUM>-<NUM>-alkyl (meth)acrylates include, for example, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate), iso-propyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate and tert-butyl (meth)acrylate. Particularly preferred C<NUM>-<NUM>-alkyl (meth)acrylates are methyl (meth)acrylate and n-butyl (meth)acrylate; methyl methacrylate and n-butyl methacrylate are especially preferred.

The C<NUM>-<NUM>-alkyl (meth)acrylates for use in accordance with the invention are esters of (meth)acrylic acid and straight chain or branched alcohols having <NUM> to <NUM> carbon atoms. The term "C<NUM>-<NUM>-alkyl (meth)acrylates" encompasses individual (meth)acrylic esters with an alcohol of a particular length, and likewise mixtures of methacrylic esters with alcohols of different lengths.

Suitable C<NUM>-<NUM>-alkyl (meth)acrylates include, for example, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, <NUM>-ethylhexyl (meth)acrylate, <NUM> tert-butylheptyl (meth)acrylate, octyl (meth)acrylate, <NUM>-isopropylheptyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, <NUM>-methylundecyl (meth)acrylate, dodecyl (meth)acrylate, <NUM>-methyldodecyl (meth)acrylate, tridecyl (meth)acrylate, <NUM>-methyltridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, <NUM>-methylhexadecyl (meth)acrylate, <NUM>-dodecylhexadecyl (meth)acrylate, heptadecyl (meth)acrylate, <NUM>-isopropylheptadecyl (meth)acrylate, <NUM>-tert-butyloctadecyl (meth)acrylate, <NUM>-ethyloctadecyl (meth)acrylate, <NUM>-isopropyloctadecyl (meth)acrylate, octadecyl (meth)acrylate, <NUM>-decyloctadecyl (meth)acrylate, <NUM>-tetradecyloctadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, cetyleicosyl (meth)acrylate, stearyleicosyl (meth)acrylate, docosyl (meth)acrylate and/or eicosyltetratriacontyl (meth)acrylate. <NUM>-decyl-tetradecyl (meth)acrylate, <NUM>-decyloctadecyl (meth)acrylate, <NUM>-dodecyl-<NUM>-hexadecyl (meth)acrylate, <NUM>,<NUM>-octyl-<NUM>-dodecyl (meth)acrylate, <NUM>-tetradecyl-octadecyl (meth)acrylate, <NUM>,<NUM>-tetradecyl-octadecyl (meth)acrylate and <NUM>-hexadecyl-eicosyl (meth)acrylate.

Suitable C<NUM>-<NUM>-alkyl (meth)acrylates include, for example, decyl (meth)acrylate, undecyl (meth)acrylate, <NUM>-methylundecyl (meth)acrylate, dodecyl (meth)acrylate, <NUM>-methyldodecyl (meth)acrylate, tridecyl (meth)acrylate, <NUM>-methyltridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate and hexadecyl (meth)acrylate.

Amongst these, particularly preferred are methacrylic esters of a linear C<NUM>-<NUM>-alcohol mixture (C<NUM>-<NUM>-alkyl-methacrylate).

Suitable polyalkyl(meth)acrylates to be used as segment B can be prepared from monomer mixtures, comprising:.

The content of each component (B1) and (B2) is based on the total composition of segments B. In a particular embodiment, the proportions of components (B1) and (B2) add up to <NUM>% by weight.

The C<NUM>-<NUM>-alkyl (meth)acrylates, C<NUM>-<NUM>-alkyl (meth)acrylates, C<NUM>-<NUM>-alkyl (meth)acrylates and styrenes for use in accordance with the invention are as defined further above.

The resulting polymers are characterized by a number-average molecular weight Mn in the range of <NUM>,<NUM> to <NUM>,<NUM>/mol, determined by gel permeation chromatography to DIN <NUM>-<NUM> in tetrahydrofuran as eluent and PMMA for calibration.

They are further characterized by a weight-average molecular weight Mw, determined by gel permeation chromatography to DIN <NUM>-<NUM> in tetrahydrofuran as eluent and PMMA for calibration. The polydispersity index PDI of the block polymers according to the present invention is in the range of <NUM> to <NUM>.

Triblock polymers of general formula (Ib) can be generally prepared by any living or controlled polymerization technique. To synthesize copolymers with segments of different polarity, the block copolymers of the present invention are preferably prepared by controlled radical polymerization.

These processes generally combine a typical free-radical initiator with a free radical stabilizing compound to control the polymerization process and produce polymers of a specific composition, and having a controlled molecular weight and narrow molecular weight range. The free-radical initiators used may be those known in the art, including, but not limited to peroxy compounds, peroxides, hydroperoxides and azo compounds which decompose thermally to provide free radicals.

Examples of controlled radical polymerization techniques are generally known the person skilled in the art and include, but are not limited to, atom transfer radical polymerization (ATRP), reversible addition fragmentation chain transfer polymerization (RAFT), nitroxide-mediated polymerization (NMP), boron-mediated polymerization, and catalytic chain transfer polymerization (CCT). Descriptions and comparisons of these types of polymerizations are described in the <NPL>.

Another embodiment of the present invention is directed to a lubricating oil composition, comprising:.

characterized in that the lubricating oil compositions has a VI of at least <NUM>, determined to ASTM D2270.

The content of each component (A) and (B) is based on the total weight of the lubricating oil composition.

In a particular embodiment, the proportions of components (A) and (B) add up to <NUM>% by weight.

The lubricating oil composition is characterized by a VI of at least <NUM>, preferably at least <NUM>, determined to ASTM D2270, and a kinematic viscosity at <NUM> of <NUM> to <NUM><NUM>/s, preferably <NUM> to <NUM><NUM>/s, determined to ASTM D445.

The base oil to be used in the additive composition comprises an oil of lubricating viscosity. Such base oils are defined as specified by the American Petroleum Institute (API) (see <NPL>").

The API currently defines five groups of lubricant base stocks (<NPL>). Groups I, II and III are mineral oils which are classified by the amount of saturates and sulphur they contain and by their viscosity indices; Group IV are polyalphaolefins; and Group V are all others, including e.g. ester oils. The table below illustrates these API classifications.

The kinematic viscosity at <NUM> (KV<NUM>) of appropriate apolar base oils used to prepare an additive composition or lubricating composition in accordance with the present invention is preferably in the range of <NUM><NUM>/s to <NUM><NUM>/s, more preferably in the range of <NUM><NUM>/s to <NUM><NUM>/s, according to ASTM D445.

Further base oils which can be used in accordance with the present invention are Group II-III Fischer-Tropsch derived base oils.

Fischer-Tropsch derived base oils are known in the art. By the term "Fischer-Tropsch derived" is meant that a base oil is, or is derived from, a synthesis product of a Fischer-Tropsch process. A Fischer-Tropsch derived base oil may also be referred to as a GTL (Gas-To-Liquids) base oil. Suitable Fischer-Tropsch derived base oils that may be conveniently used as the base oil in the lubricating composition of the present invention are those as for example disclosed in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

The lubricating oil composition is further characterized in that the base oil (B) comprises:.

The content of each component (B1) and (B2) is based on the total weight of the base oil (B).

In a particular embodiment, the proportions of components (B1) and (B2) add up to <NUM>% by weight.

The lubricating oil composition according to the invention may also contain, as component (C), further additives selected from the group consisting of pour point depressants, dispersants, defoamers, detergents, demulsifiers, antioxidants, antiwear additives, extreme pressure additives, friction modifiers, anticorrosion additives, dyes and mixtures thereof.

Triblock polymers with insoluble outer parts are known in the art for their ability to aggregate in special micelles. These micelles are described as flower-like as both ends of the polymer are shielded from the solvent in the inner part of the micelle. This is illustrated by the following Scheme <NUM>.

A special feature of these micelles is that the polymer ends can also be part of two different micelles, which is called bridging. The thermodynamic balance in between bridging and non- bridging polymer chains is very complex. <CHM>
<IMG>.

When dissolved in apolar fluids such as mineral oils, the triblock polymers according to the present invention show a very pronounced thickening behavior which is not typical for polymers of such low molecular weight indicating an associative thickening mechanism. Another indicator for this hypothesis is that thickening power increases strongly with higher polymer concentration. Surprisingly, this thickening effect becomes even stronger at elevated temperatures, completely opposite to what was expected. Using this effect oils with very exceptional temperature/viscosity behavior can be formulated. Formulations with extremely high VIs can be formulated in this way. Even behavior that cannot be described by the VI anymore due to lack of definition can be generated in this way. For example, it is possible to generate oil formulations with a higher viscosity at <NUM> than at <NUM>.

As the effect is based on self-organization of low-molecular weight polymers the system is less susceptible to destruction by mechanical shear. In fact no permanent shear loss was observed in standard industry tests.

The described polymer solutions are usually clear and flowable over the whole relevant temperature range. Neither precipitation at low temperatures nor at high temperatures is observed. For a few polymer compositions precipitation at elevated temperatures can be observed dependent on the solvent. This may be an indicator that the general mechanism is based on reduced miscibility at elevated temperatures which can result in a miscibility gap for some solvent/polymer combinations.

By application of the triblock copolymers of the present invention in lubricating oil compositions, formulations with exceptional VIs can be generated.

Another embodiment of the present invention is directed to the use of block copolymers as outlined further above to provide a positive VI effect in lubricating oil compositions, characterized in that the lubricating oil compositions have a VI of <NUM> or greater, preferably of <NUM> or greater, determined to ASTM D2270.

Another embodiment of the present invention is directed to a method for improving the viscosity index (VI) of a lubricating oil composition, the method comprising the step of adding a block copolymer of general formula (I) to a lubricating oil composition.

A further object of the present invention is directed to the use of the above-described lubricating oil composition as hydraulic fluid, transmission fluid, gear oil or motor oil.

The invention has been further illustrated by the following non-limiting examples.

The triblock copolymers according to the present invention and the comparative examples were characterized with respect to their molecular weight, PDI and thermal properties (differential scanning calorimetry - DSC).

The number-average molecular weights Mn and the weight-average molecular weights Mw were determined by gel permeation chromatography (GPC) to DIN <NUM>-<NUM> in tetrahydrofuran as eluent and polymeric standards like polystyrene or polymethylmethacrylate (PMMA) for calibration. Determining the thermal properties (Tg and Tm) of the block copolymers employed in the present invention was carried out by differential scanning calorimetry (DSC) according to DSC method DIN <NUM>-<NUM>.

The block copolymers and the statistical polyester-copolymer prepared have hydroxyl groups as end groups. The concentration of OH groups is determined titrimetrically in mg KOH/g polymer according to DIN <NUM>-<NUM>.

The additive compositions including the block copolymers according to the present invention and comparative examples were characterized with respect to their viscosity index (VI) to ASTM D2270 and their kinematic viscosity at <NUM> (KV<NUM>) and <NUM> (KV<NUM>) to ASTM D445.

The lubricating oil compositions including the block copolymers according to the present invention and comparative examples were characterized with respect to kinematic viscosity at -<NUM> (KV-<NUM>), <NUM> (KV<NUM>) and <NUM> (KV<NUM>) to ASTM D445, their viscosity index (VI) to ASTM D2270, their cloud point to ASTM D5771 and their pour point to ASTM D5950.

KRL measurements to determine the shear loss were also done to CEC L-<NUM>-A-<NUM> (<NUM> hours, <NUM> and 5kN).

In connection with the evaluation of hydraulic formulations, foam tests to ASTM D892 were run, air release was determined to DIN ISO <NUM>.

The Brookfield viscosity was determined to DIN <NUM>.

<NUM> of pentamethyldiethylenetriamine (PMDETA), <NUM> CuBr and <NUM> of lauryl methacrylate (LMA) were dissolved in <NUM> of Chevron 100R oil. The solution was purged with nitrogen for <NUM> minutes and heated to <NUM>. After addition of <NUM> of ethylene bis(<NUM>-bromoisobutyrate) the reaction mixture was heated to <NUM>. <NUM> of methyl methacrylate were added after <NUM> and another <NUM> later <NUM> DDM were added. The mixture was stirred for <NUM>, cooled to room temperature and purified by pressure filtration. After filtration, a clear and slightly yellow colored highly viscous liquid was obtained which was applied without further purification.

<NUM> methyl methacrylate and <NUM> <NUM>-cyanobutanyl-<NUM>-yl <NUM>,<NUM>-dimethyl-<NUM>-pyrazole-<NUM>-carbodithioate were dissolved in <NUM> of toluene and purged with nitrogen for <NUM> minutes. The mixture was heated to <NUM> and <NUM> <NUM>,<NUM>'-azobis(<NUM>-methylbutyronitrile) was added. After <NUM>, <NUM> <NUM>,<NUM>'-azobis(<NUM>-methylbutyronitrile) dissolved in <NUM> lauryl methacrylate was added and after stirring for another <NUM> <NUM> <NUM>,<NUM>'-azobis(<NUM>-methylbutyronitrile) dissolved in <NUM> methyl methacrylate was added. The mixture was stirred overnight. The toluene was removed by vacuum distillation and replaced by <NUM> of Nexbase <NUM>. The highly viscous yellow liquid was used without further purification.

A round-bottom flask equipped with a glass stir rod, nitrogen inlet, reflux condenser and thermometer was charged with <NUM> of Chevron 100R oil, <NUM> lauryl methacrylate, <NUM> methyl methacrylate and <NUM> DDM. The mixture was heated up to <NUM> while stirring and nitrogen bubbling for inertion. Then <NUM>-stage feed for <NUM> hours feed of a mixture consisting of <NUM> tert-butyl-perhexanoate and <NUM> Chevron 100R oil was started. After the feed end the mixture was stirred for an additional <NUM> minutes. The colorless viscous liquid was used without further purification.

Table <NUM> shows the reaction mixtures used to prepare examples <NUM>-<NUM> which are PAMA-based triblock copolymers A-B-A according to general formula (Ib) and comparative examples. The reaction was carried out by using Chevron 100R as base oil.

Example <NUM> is a comparative example because the content of segments A (MMA) is lower than the ranges described in the present invention.

Examples <NUM> and <NUM> are in accordance with the present invention and were prepared by following the protocol as described in Synthesis <NUM>.

Example <NUM> is a comparative example as it is an A-B diblock copolymer. As monofunctional initiator was used ethyl-alpha-bromoisobutyrate.

Example <NUM> is a comparative example as it is an inverse triblock copolymer B-A-B; i.e. the oil-soluble segment B forms the outer parts and the oil-insoluble segment A forms the inner part. As the inner block A was synthesized first, the synthesis was performed in toluene because the A-block alone was insoluble in oil. After polymerization, the whole amount of toluene was exchanged by the same amount of Chevron 100R oil.

The net compositions of the resulting block copolymers <NUM>-<NUM> are shown in the following Table <NUM> together with their characteristic weight-average molecular weights Mw, number-average molecular weights Mn as well as polydispersity indices.

It can be seen that their weight-average molecular weights are within the claimed range of <NUM>,<NUM> to <NUM>,<NUM>/mol.

Table <NUM> shows the reaction mixtures used to prepare examples <NUM>-<NUM> which are PAMA-based block copolymers A-B-A according to general formula (Ib). They were prepared by following the protocol of Synthesis <NUM>.

Examples <NUM>-<NUM> are all in accordance with the present invention. The synthesis was performed in toluene as solvent. After polymerization, the whole amount of toluene was exchanged by the same amount of NB3020.

As Examples <NUM>-<NUM> are all prepared by RAFT polymerization, the different blocks A and B are built up consecutively. That means that for each block A half of the corresponding monomer mixture is added to the reaction mixture. Regarding the initiator, for each block A and B is added one third of the whole amount.

To demonstrate the thickening effect of triblock copolymers according to the present invention on the viscosity of base oils, solutions with different polymer content were prepared and corresponding KV<NUM> and KV<NUM> values as well as viscosity indices were measured. The results are shown in the following Table <NUM>.

Table <NUM> shows that the triblock polymers according to the present invention provide a very strong VI lift in lubricating oil compositions. This origin of the effect is the high thickening power at elevated temperatures. Contrary to standard VI improver technology, this effect can be achieved at comparably low viscosities at <NUM> which is highly desirable for modern low viscosity lubricants. Another important factor compared to the existing VI improver technology is that the effect is achieved with low molecular weight polymers which are known to be more resistant to mechanical shear forces. Adjustment of the polymer parameters allows fine-tuning of the desired thickening power at <NUM>.

Such an effect is not known for previously reported triblock systems in the patent literature as they do not match the requirements for geometry and polarity of the claimed polymers, which is required for aggregates that show this unusual effect.

Comparative example <NUM> demonstrates how the effect vanishes when the apolar A segments become too short.

Claim 1:
Process for improving the viscosity index of lubricating oil compositions by adding a triblock copolymer of general formula (Ib)

        A-B-A     (Ib),

wherein each segment A is prepared from a monomer mixture comprising:
(A1) <NUM> to <NUM>% by weight of monomers being selected from the group consisting of C<NUM>-<NUM>-alkyl (meth)acrylates, styrene, benzyl (meth)acrylate and mixtures thereof, preferably C<NUM>-<NUM>-alkyl (meth)acrylates; and
(A2) <NUM> to <NUM>% by weight of C<NUM>-<NUM>-alkyl (meth)acrylates, preferably C<NUM>-<NUM>-alkyl (meth)acrylates,
based on the total weight of segments A, and
segment B is prepared from a monomer mixture comprising:
(B1) <NUM> to <NUM>% by weight of monomers being selected from the group consisting of C<NUM>-<NUM>-alkyl (meth)acrylates, preferably C<NUM>-<NUM>-alkyl (meth)acrylates; and
(B2) <NUM> to <NUM>% by weight of monomers being selected from the group consisting of C<NUM>-<NUM>-alkyl (meth)acrylates, styrene, benzyl (meth)acrylate and mixtures thereof, preferably C<NUM>-<NUM>-alkyl (meth)acrylates,
based on the total weight of segments B,
characterized in that the weight-average molecular weight of the triblock copolymer is in the range of <NUM>,<NUM> to <NUM>,<NUM>/mol, preferably <NUM>,<NUM> to <NUM>,<NUM>/mol, more preferably <NUM>,<NUM> to <NUM>,<NUM>/mol, determined by gel permeation chromatography against PMMA standards,
characterized in that the triblock copolymer of general formula (Ib) consists of:
(a) <NUM> to <NUM>% by weight of segments A and
(b) <NUM> to <NUM>% by weight of segment B,
based on the total composition of the triblock copolymer.