CN106661490B

CN106661490B - Hydraulic fluid in plastic injection molding process

Info

Publication number: CN106661490B
Application number: CN201580043890.7A
Authority: CN
Inventors: F·劳特瓦瑟; F－O·玛灵; R·克尔博; T·巴特尔斯; T·施梅尔; S·麦尔; M·阿利伯特
Original assignee: Evonik Operations GmbH
Current assignee: Evonik Operations GmbH
Priority date: 2014-08-18
Filing date: 2015-08-07
Publication date: 2020-02-28
Anticipated expiration: 2035-08-07
Also published as: EP3183324B1; US20170218295A1; CA2957330A1; WO2016026719A1; PL3183324T3; EP3183324A1; CA2957330C; CN106661490A

Abstract

The present invention relates to the use of hydraulic fluid in a plastic injection molding process. It has thus been unexpectedly found that the use of hydraulic fluids with the right combination of physical parameters, such as viscosity grade, viscosity index, density and dispersancy, allows significant energy savings in plastic injection molding Processes (PIM). The PIM process is an industrial process for manufacturing plastic parts under well controlled temperature, pressure and cycle time. The energy consumption of the process has become more important in the last years, but other parameters, such as process stability and accuracy of the plastic part parameters as well as mechanical protection and long oil change periods, must be satisfactory.

Description

Hydraulic fluid in plastic injection molding process

Technical Field

Background

Injection molding is a manufacturing method for producing parts by injecting material into a mold at well controlled temperature, pressure and cycle time. Injection molding can be performed using a number of materials including metals, glass, elastomers, conditioners (formulations) and most polymers, which are typically thermoplastic and thermoset. The material for the part is fed into a heated barrel, mixed, and forced into the mold cavity where it cools and hardens into the mold cavity configuration.

The power required for this injection molding process depends on the various movements in the molding machine, but also varies between the materials used. The Robert Todd book Manufacturing process Reference Guide states that power requirements depend on the specific gravity, melting point, thermal conductivity, part size, and molding rate of the material. The injection molding machine is driven by a hydraulic system, wherein electrical energy is converted into mechanical energy by hydraulic energy. The energy reaches the driver in the form of pressure and volume flow. When power is transmitted by hydraulic pressure, energy losses due to flow losses and friction are observed. Furthermore, the compression of the hydraulic fluid leads to the formation of frictional heat, which must be controlled, for example, by cooling. The type of pump and the control of such a pump also have a great influence on the degree of effectiveness of the moulding machine in the treatment of plastics.

In the prior art, some efforts have been made to save energy by improving injection molding machines. For example, in EP 0403041, a special ac servomotor is used for the pump connected to the hydraulic user. In US 4,020,633, a completely new concept for the entire hydraulic drive system of an injection molding machine is disclosed. However, none of these concepts touch the hydraulic fluid used herein. Thus, additional energy savings must be possible by optimizing these fluids.

EP 2337832 discloses a method of reducing noise generation in a hydraulic system, the method comprising contacting a hydraulic fluid comprising a polyalkyl (meth) acrylate polymer with the hydraulic system. The hydraulic fluid has a viscosity index VI of at least 130. The polyalkyl (meth) acrylate has a molecular weight in the range from 10000 to 200000 g/mol and is obtained by polymerizing a mixture of ethylenically unsaturated monomers, said mixture preferably containing 50 to 95 wt.% of C₉To C₁₆And 1 to 30% by weight of C₁To C₈。

The object of the invention described in EP 2337832 is to achieve a reduction in noise by increasing the oil viscosity at higher temperatures. For this effect, high viscosity and high density are beneficial, and the high VI of the fluid is used to increase the viscosity at the operating temperature.

In the present invention, a completely different approach is used to improve energy efficiency. High VI is used to enable a reduction in base fluid viscosity. This reduced viscosity combined with the low density of the hydraulic base fluid increases the efficiency of the injection molding process. The reduction of the noise level of the hydraulic fluid according to the invention compared to EP 2337832 is not expected.

EP 2157159 discloses hydraulic fluids containing esters as base oil, said esters containing at least two ring structures. It is described therein that the hydraulic fluid has low energy loss due to compression and exhibits excellent responsiveness when used in a hydraulic circuit. Thus, the hydraulic fluid achieves energy-saving high-speed operation and high control accuracy in the hydraulic circuit.

EP 1987118 discloses the use of a fluid having a viscosity improvement index of at least 130, comprising C in a mixture of an API group II or III mineral oil and a poly α olefin having a molecular weight of less than 10,000g/mol, in a hydraulic system such as an engine or an electric motor₁To C₆(meth) acrylic acid ester, C₇To C₄₀(meth) acrylic acid esters and optionally with (meth)) Copolymers of other monomers with which the acrylate is copolymerizable. There is no indication that such fluids can be used in injection molding machines, nor that a particular composition of the fluid can be used in such machines.

However, there is still a need for further research into possible alternative hydraulic fluid compositions to be used in hydraulic systems that experience high working pressures, as in plastic injection molding processes.

Disclosure of Invention

Purpose(s) to

Improving energy efficiency is a general objective in the technical field of injection molding. Generally, such an object is achieved by constructional improvements to the units of the hydraulic system that provide mechanical energy. However, further improvements with respect to this objective are still needed. It is therefore an object of the present invention to provide a method for saving energy, increasing productivity, avoiding heating, improving air release and avoiding cavitation within a wide temperature operating window in a hydraulic system used in a plastic injection molding process.

In particular, the object of the present invention is to improve the performance of the hydraulic system in a plastic injection molding machine, wherein energy is saved by at least 5% and up to 10% compared to the performance of the machine when run with a standard fluid having a VI of about 100 as recommended by the manufacturer of the injection molding machine. It is also an object to achieve an energy saving of more than 10% for a single, very energy consuming method step.

In particular, it is an object of the present invention to achieve such energy savings by providing a new hydraulic fluid for use in a plastic injection molding machine.

Other objects not explicitly discussed herein may become apparent from the prior art, the description, the claims or the exemplary embodiments hereinafter.

Detailed Description

The prior art documents given above relating to injection moulding processes attempt to reduce the energy consumption, but without changing the oil parameters. After exhaustive research, the inventors have unexpectedly found that hydraulic fluids play a critical role for energy saving in plastic injection molding processes, in particular that some hydraulic fluid compositions adjusted to the right physical parameters allow energy savings of up to 5% or more in the overall plastic injection molding Process (PIM) or more than 10% for certain steps of the PIM process, in most cases up to 15%. Indeed, by adjusting the viscosity grade, viscosity index, density and dispersancy of the hydraulic fluid as defined in claim 1, the inventors found that a significant amount of energy can be advantageously saved, even by operating under high pressure conditions common in PIM processes.

In detail, the above discussed objects have been achieved by a new method of reducing the energy consumption of a hydraulic system in industrial hydraulic applications, preferably in a plastic injection molding process or in a process comprising a hydraulic press. In this method, hydraulic fluid is used in the plastic injection molding process. Wherein the hydraulic fluid composition comprises (i) a polyalkyl (meth) acrylate viscosity index improver and (ii) a base oil.

Wherein the polyalkyl (meth) acrylate viscosity index improver (i) comprises at least monomer units a) and b) and optionally monomer units c) and/or d). Preferably, component (i) has a weight average molecular weight (M) of 20,000 to 100,000g/mol_w). More preferably, the molecular weight M_wBetween 30,000 and 85,000g/mol, particularly preferably between 40,000 and 70,000 g/mol. The polyalkyl (meth) acrylate viscosity index improver has a polydispersity index of between 1 and 4, preferably between 1.2 and 3.0, most preferably between 1.5 and 2.5.

The polyalkyl (meth) acrylate viscosity index improver (i) contains from 5 to 40% by weight, preferably from 7 to 30% by weight, more particularly preferably from 10 to 25% by weight, of recurring units obtained by copolymerization of the monomer a) and from 50 to 95% by weight, preferably from 60 to 93% by weight, more particularly preferably from 70 to 90% by weight, of recurring units obtained by copolymerization of the monomer b). In a particular embodiment of the invention, the amount of compound of the formula (II) is between 75 and 90% by weight, particularly preferably between 70 and 80% by weight.

Wherein the monomers a) are one or more ethylenically unsaturated ester compounds of the formula (I)

Wherein R is H or CH₃，R¹Represents a linear or branched alkyl radical having 1 to 6 carbon atoms, R²And R³Independently represents H or a group of formula-COOR ', wherein R' is H or an alkyl group having 1 to 5 carbon atoms.

Examples of component a) are, inter alia, (meth) acrylates, fumarates and maleates derived from saturated alcohols such as methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, t-butyl (meth) acrylate and/or pentyl (meth) acrylate; cycloalkyl (meth) acrylates, such as cyclopentyl (meth) acrylate. Methacrylates are preferred over acrylates.

The monomers b) are one or more ethylenically unsaturated ester compounds of the formula (II)

Wherein R is H or CH₃，R⁴Represents a linear or branched alkyl radical having from 7 to 15 carbon atoms, R⁵And R⁶Independently represents H or a group of formula-COOR ", wherein R" is H or an alkyl group having 6 to 15 carbon atoms.

Among these are (meth) acrylates, fumarates and maleates derived from saturated alcohols, such as n-hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, heptyl (meth) acrylate, 2-tert-butyl heptyl (meth) acrylate, octyl (meth) acrylate, 3-isopropyl heptyl (meth) acrylate, nonyl (meth) acrylate, decyl (meth) acrylate, undecyl (meth) acrylate, 5-methylundecyl (meth) acrylate, dodecyl (meth) acrylate, 2-methyldodecyl (meth) acrylate, tridecyl (meth) acrylate, 5-methyltrodecyl (meth) acrylate, tetradecyl (meth) acrylate, and/or pentadecyl (meth) acrylate.

The polyalkyl (meth) acrylate viscosity index improver (i) may also contain other components in the form of monomers copolymerizable with at least one of components a) and b). These further monomers are in particular components c) and d), where the maximum concentration of c) is 30% by weight and the maximum concentration of d) is 10% by weight.

Wherein the monomers c) are one or more ethylenically unsaturated ester compounds of the formula (III)

Wherein R is H or CH₃，R⁷Denotes a linear or branched alkyl radical having from 16 to 30 carbon atoms, R⁸And R⁹Independently represents H or a group of formula-COOR '", wherein R'" is H or an alkyl group having from 16 to 30 carbon atoms.

Examples of component c) are, inter alia, (meth) acrylates, fumarates and maleates derived from saturated alcohols such as 2-methylhexadecyl (meth) acrylate, heptadecyl (meth) acrylate, 5-isopropylheptadecyl (meth) acrylate, 4-tert-butyloctadecyl (meth) acrylate, 5-ethyloctadecyl (meth) acrylate, 3-isopropyloctadecyl (meth) acrylate, octadecyl (meth) acrylate, nonadecyl (meth) acrylate, eicosyl (meth) acrylate, hexadecyleicosyl (meth) acrylate, stearyleicosyl (meth) acrylate and/or docosyl (meth) acrylate.

Optionally, the polyalkyl (meth) acrylate viscosity index improver (i) contains in polymerized form 5 to 20% by weight of monomer a), 70 to 90% by weight of monomer b) and 2 to 25% by weight of monomer c).

Monomer d) is at least one N-dispersant monomer. Such N-dispersant monomers are preferably compounds of the formula (IV)

Wherein R is¹⁰、R¹¹And R¹²Independently is H or alkyl having 1 to 5 carbon atoms, R¹³Is a group C (Y) X-R¹⁴Wherein X ═ O or NH and Y is (═ O) or (═ NR)¹⁵) Wherein R is¹⁵Is an alkyl or aryl group. R¹⁴Represents a quilt group NR¹⁶R¹⁷Substituted, linear or branched alkyl having 1 to 20 carbon atoms, wherein R¹⁶And R¹⁷Independently represents H or a linear or branched alkyl group having 1 to 8 carbon atoms, or wherein R¹⁶And R¹⁷Is part of a 4 to 8 membered saturated or unsaturated ring optionally containing one or more heteroatoms selected from nitrogen, oxygen or sulfur, wherein said ring may be further substituted by alkyl or aryl.

Or, R¹³Is a group NR¹⁸R¹⁹Wherein R is¹⁸And R¹⁹Is part of a 4 to 8 membered saturated or unsaturated ring containing at least one carbon atom as part of the ring which forms a double bond with a heteroatom selected from nitrogen, oxygen or sulfur, wherein the ring may be further substituted with an alkyl or aryl group.

Preferably, the dispersant monomer d) of the polymer (i) is at least one monomer selected from: n-vinyl type monomers, (meth) acrylates, (meth) acrylamides, (meth) acrylimides, each of which has a N-containing dispersing moiety in the side chain. It is particularly preferred that the N-dispersant monomer is at least one monomer selected from the group consisting of N-vinylpyrrolidone, N-dimethylaminoethyl methacrylate and N, N-dimethylaminopropyl methacrylamide.

Optionally, the polyalkyl (meth) acrylate viscosity index improver (i) contains 5 to 25 wt.% of monomer c) and 1 to 7 wt.% of monomer d), both in polymerized form. In particular, the viscosity index improver (i) contains, in polymerized form, from 10 to 20% by weight of monomer c) and from 2 to 5% by weight of at least one N-dispersant monomer d).

For the present invention, the base oil (II) is selected from API group I, II, III or IV base oils or mixtures thereof. By mixing with the above viscosityIndex Improver (VII) one of these base oils or a mixture of at least two of these base oils is used together, the formulated hydraulic fluid of the invention having a fresh oil viscosity index of at least 160, a viscosity at 40 ℃ of from 15cSt to 51cSt and 800kg/m³To 890kg/m³Is particularly preferred is an API group IV base oil in the form of a poly α olefin (PAO) or a mixture of API group I to IV base oils containing at least 50 wt% of a poly α olefin.

Synthetic hydrocarbons, particularly polyolefins, are known in the art as API group IV base oils. These compounds are obtainable by polymerization of alkenes, in particular alkenes having 3 to 12 carbon atoms, such as propene, 1-hexene, 1-octene, 1-decene and 1-dodecene, or mixtures of these alkenes. Preferred PAOs have a number average molecular weight in the range of from 200 to 10000g/mol, more preferably from 500 to 5000 g/mol.

In particular, the hydraulic fluid composition comprises from 70 to 95 wt%, more preferably from 80 to 95 wt%, even more preferably from 80 to 90 wt% of a base oil (II) selected from API group I, II, III or IV base oils or mixtures thereof, and from 5 to 30 wt%, more preferably from 5 to 20 wt%, even more preferably from 10 to 20 wt% of a polyalkyl (meth) acrylate viscosity index improver (I). Particularly suitable are hydraulic fluids according to the invention having a viscosity index of at least 180, preferably at least 200, particularly preferably at least 250, and a viscosity of from 15cSt to 36cSt, preferably between 15cSt and 28cSt, particularly preferably between 19cST and 28cST, at 40 ℃. It is furthermore advantageous if the hydraulic fluid has a volume of 800kg/m³To 860kg/m³Preferably 800kg/m³To 840kg/m³15 ℃ density of (g).

In calculating the composition of the hydraulic fluid, it must be taken into account that the Viscosity Index Improver (VII) may be added in the solvent. In a preferred embodiment of the invention, such solvent is also an API group I, II, III or IV oil. It is particularly preferred that such solvent is the same as the base oil of the composition. Independently of the solvent used herein, it must be counted as part of the base oil in the composition. Typically, the VII solution added contains 20 to 40 wt% solvent.

The viscosity index can be determined according to ASTM D2270.

The hydraulic fluid composition according to the invention may also contain dispersant inhibitor packages (DI packages) to improve parameters such as foam, corrosion, oxidation, wear and the like. Such DI packages may contain antioxidants, defoamers, corrosion inhibitors, and/or at least one phosphorus or sulfur containing antiwear agent.

Technical advantages of the invention

High VI hydraulic fluids are commonly used in mobile applications, such as excavators. In these applications, the hydraulic fluid must handle a wide range of temperatures-very low starting temperatures in winter and very high temperatures under heavy load conditions. A high VI for the fluid is required to keep the viscosity as close as possible to the optimum. The optimum value is defined by a balance between the mechanical efficiency required for thin oil and the volumetric efficiency required for thick oil to minimize losses in the pump due to internal leakage. Under normal operating conditions, particularly under heavy duty conditions, volumetric efficiency becomes the dominant factor, and viscosity index improvers can greatly improve efficiency by increasing the viscosity of the fluid.

Injection molding applications are quite different compared to excavators. The external temperature is constant, the duty cycle is well defined and heavy load conditions are avoided as much as possible. For this reason, the oil temperature is fairly constant and high VI base fluids are not generally used. Typically, the manufacturer of an injection molding machine recommends ISO46 single stage flow.

For these reasons, the advantages of high VI fluids in applications such as injection molding are not expected to be seen, but we have unexpectedly found that significant energy savings are achieved when using low viscosity hydraulic fluids with high VI. In stark contrast to the well described energy savings with high VI fluids in excavators, the improvement in efficiency in injection molding is greatest at low load conditions.

Unexpectedly, the method as defined above or as defined in claim 1 not only achieves the above-mentioned objects, but also advantageously provides an improved oil life and thus a longer oil change period of the hydraulic system.

Furthermore, the system performance of the hydraulic system may be improved. The expression "system performance" refers to the work productivity achieved by the hydraulic system over a defined period of time. In particular, the system performance may be improved by at least 5%, more preferably by at least 10%. In a preferred system, the hourly duty cycle can be improved.

Synthesis of viscosity index improver

For the synthesis of the polyalkyl (meth) acrylate viscosity index improver (i), the above monomer mixture may be polymerized by any known method. Conventional free radical polymerization can be carried out using conventional free radical initiators. These initiators are well known in the art. Examples of such radical initiators are azo initiators such as 2,2 '-Azobisisobutyronitrile (AIBN), 2' -azobis (2-methylbutyronitrile) and 1, 1-azo-bicyclohexanecarbonitrile; peroxy compounds, for example methyl ethyl ketone peroxide, acetylacetone peroxide, dilauroyl peroxide, tert-butyl per-2-ethylhexanoate, ketone peroxide, methyl isobutyl ketone peroxide, cyclohexanone peroxide, dibenzoyl peroxide, tert-butyl perbenzoate, tert-butyl peroxyisopropylcarbonate, 2, 5-bis (2-ethylhexanoylperoxy) -2, 5-dimethylhexane, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3, 5, 5-trimethylhexanoate, dicumyl peroxide, 1-bis (tert-butylperoxy) cyclohexane, 1-bis (tert-butylperoxy) -3,3, 5-trimethylcyclohexane, cumene hydroperoxide and tert-butyl hydroperoxide.

Poly (meth) acrylates having a lower molecular weight can be obtained by using chain transfer agents. Such techniques are widely known and practiced in the polymer industry and are described in Odian, Principles of polymerization, 1991.

In addition, new polymerization techniques, such as ATRP (atom transfer radical polymerization) and or RAFT (reversible addition fragmentation chain transfer) can be used to obtain useful polymers derived from alkyl esters. These methods are well known. ATRP reaction methods are described, for example, in J.Am.chem.Soc. by J-S.Wang et al, Vol.117, p.5614-5615 (1995), and Macromolecules by Matyjaszewski, Vol.28, p.7901-7910 (1995). Furthermore, patent applications WO 96/30421, WO 97/47661, WO 97/18247, WO 98/40415 and WO 99/10387 disclose variants of the ATRP explained above, to which reference is explicitly made for disclosure purposes. The RAFT process is fully described, for example, in WO 98/01478, to which explicit reference is made for disclosure purposes.

The polymerization can be carried out under normal pressure, reduced pressure or elevated pressure. The polymerization temperature is also not critical. However, it is usually in the range of-20 to 200 ℃, preferably 60 to 120 ℃, and is not subject to any limitation thereby. The polymerization can be carried out with or without the use of a solvent. The term "solvent" is to be understood broadly herein. According to a preferred embodiment, the polymer is obtainable by polymerization in API group I, II or III mineral oil or in API group IV synthetic oil.

Detailed Description

Examples

The invention is further illustrated in the following non-limiting examples and comparative examples (reference oils). The following examples serve to further illustrate preferred embodiments according to the invention, but are not intended to limit the invention. All results are shown in tables 1 and 2.

Test and oil

To determine energy consumption, different test oils were compared to a reference (ISO VG 46 single stage Castrol HyspinDF Top 46, VI ═ 100).

The following hydraulic fluids were used:

TABLE 1 Hydraulic fluid formulations

The polyalkylmethacrylate viscosity index improver PAMA-1 consists of 13 wt% of methyl methacrylate and 87 wt% of methacrylic acid C_12-14Alkyl esters (M)_w52,000g/mol, PDI 2.1), which is dissolved in highly refined mineral oil.

The polyalkylmethacrylate viscosity index improver PAMA-2 consists of methyl methacrylate in 10 wt% and methyl propylene in 90 wt%Acid C_12-15Alkyl esters (M)_w58,000g/mol, PDI 2.0), which is dissolved in highly refined mineral oil.

Performance of	Method of producing a composite material
		Kinematic viscosity at 40 ℃ in mm²/s	ASTM D445
Kinematic viscosity at 100 ℃ in mm²/s	ASTM D445
		VI	ASTM D2270
Density at 15 ℃ in kg/L	ASTM D1298

The injection molding machine used to generate the data was Krauss Maffei KM 80/380 CX. The energy consumption of the hydraulic pump was calculated by measuring the voltage and current of the pump motor with external test equipment (measuring amplifier MX 840 PAKAP; element MX 403B for voltage recording, 1000V; both from Hottinger Baldwin Messtechnik GmbH). Prior to testing, the system was flushed with hydraulic fluid to be used, and the oil parameters were checked to ensure that the previous oil was properly cleaned and that no mixing with the previous oil occurred. Table 1 shows viscosity measurement data for fresh oil, oil injected for testing, and oil collected after testing.

During the test, use

-running a forming cycle with a forming compound, said compound being carried out in cycle a

Reactive-Liquid cf30OA monomer mixture coverage.

The evaluation of the data was focused on the process step without polymer to avoid any influence of polymer properties on the results.

Drawings

FIG. 1 description of a typical injection molding cycle

The cycle starts when the mold is closed (step 1) and then pressure is built up (step 2a), which is required in order to keep the mold closed during the injection process. After moving the extruder to the die (step 2b), the material is injected (step 3) and the working pressure is maintained to compensate for material shrinkage during molding (step 4). Optionally, can employ

Method steps coat the workpiece (step 4.1, applied in cycle a). After the cooling phase has started, the extruder is moved back (steps 5 and 6). At the end of the cooling phase, the mold is opened (step 7) and the workpiece can be removed (step 8).

Table 2 shows the differences in energy consumption (negative savings) found for cycle a, cycle B and the evaluations of step 1 and step 2 derived from the cycle a data.

Period A, step 1+ step 2(2a +2b) + step 4.1+ step 7+ step 8

Period B step 1+ step 2(2a +2B) + step 7+ step 8

During this period, steps 1, 2, 4.1, 7 and 8 are independent of the injected material. Thus, the energy saving is not dependent on the plastic material properties.

Coating step 4.1 is optional, and is

Part of a method. Period A (with coating) and period B (without coating)The impact of this step on energy savings was evaluated.

TABLE 2 differences in energy consumption with the investigated hydraulic fluids

Period A Material independent method step, Using

Method step

Period B Material independent Process step, noneMethod step

Step 1+ step 2-step completely independent of the material before injection

Based on the above results, it is clearly demonstrated that the physical parameters of the base oil in combination with the viscosity index improver defined in claim 1 are crucial in order to observe energy savings in hydraulic systems used under high pressure conditions of plastic injection molding processes.

Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims.

Claims

1. Method for reducing the energy consumption of a hydraulic system in a plastic injection molding process, said hydraulic system comprising a hydraulic fluid, characterized in that the hydraulic fluid composition comprises

(i) A polyalkyl (meth) acrylate viscosity index improver comprising the following monomer units

a)10 to 25% by weight of one or more ethylenically unsaturated ester compounds of formula (I)

Wherein R is H or CH₃,

R¹Represents a linear or branched alkyl group having 1 to 6 carbon atoms,

R²and R³Independently represents H or a group of formula-COOR ', wherein R' is H or an alkyl group having 1 to 5 carbon atoms, and

b) from 75 to 90% by weight of one or more ethylenically unsaturated ester compounds of the formula (II)

Wherein R is H or CH₃,

R⁴Represents a linear or branched alkyl group having 7 to 15 carbon atoms,

R⁵and R⁶Independently represents H or a group of formula-COOR 'wherein R' is H or an alkyl group having 6 to 15 carbon atoms,

wherein the polyalkyl (meth) acrylate viscosity index improver (i) has a weight average molecular weight (M)_w) Is from 40,000 to 70,000g/mol, and

(ii) a base oil selected from the group consisting of API group I, II, III or IV base oils or mixtures thereof,

wherein the hydraulic fluid is formulated to have

A fresh oil viscosity index of at least 160,

15mm²s to 51mm²(ii) a viscosity at 40 ℃ in terms of/s,

800kg/m³to 860kg/m³15 ℃ density of (g).

2. Method according to claim 1, characterized in that the hydraulic fluid has 800kg/m³To 840kg/m³15 ℃ density of (g).

3. The method of claim 1, wherein the hydraulic fluid has a viscosity index of at least 180, 15-36mm²Viscosity at 40 ℃ of/s and 800-³15 ℃ density of (g).

4. The method of claim 1, wherein the hydraulic fluid has a viscosity index of at least 200, 15-28mm²Viscosity at 40 ℃ of/s and 800-³15 ℃ density of (g).

5. The method of claim 1, wherein the hydraulic fluid has a viscosity index of at least 250, 19mm²S to 28mm²Viscosity at 40 ℃ of/s and 800-³15 ℃ density of (g).

6. The method of any of claims 1-5, wherein the polyalkyl (meth) acrylate viscosity index improver has a polydispersity index of 1.5 to 2.5.

7. The method of any of claims 1-5, wherein the hydraulic fluid composition comprises

70 to 95 wt% of a base oil selected from the group consisting of API group I, II, III or IV base oils or mixtures thereof, and

5 to 30 wt% of a polyalkyl (meth) acrylate viscosity index improver.

8. The method of claim 7, wherein the hydraulic fluid composition comprises

80 to 95 weight percent of a base oil, and

5 to 20 wt% of a polyalkyl (meth) acrylate viscosity index improver.

9. The process of any of claims 1-5 wherein the base oil contains at least 50 wt.% poly α olefins.

10. The process according to claim 9, wherein the poly α olefin has a number average molecular weight in the range of 200 to 10000 g/mol.

11. The method of any of claims 1-5, wherein the hydraulic fluid composition comprises a dispersant-inhibitor package comprising an antioxidant, an anti-foam agent, an anti-corrosion agent, and/or at least one phosphorus or sulfur containing antiwear agent.