METHOD FOR PRODUCING PAPER, BOARD OR THE LIKE
The present invention relates to a method for producing paper, board or the like, according to preambles of the enclosed independent claim.
Fillers, such as clay, calcium carbonate, calcium sulphate or talc are used in paper and board making to reduce costs and to improve optical properties of paper or board, such as opacity and light reflectance. Fillers are added to the fibre stock before paper machine. For coated paper grades coating pigments, which comprise the same minerals, may partly enter to the paper via the broke, which is recycled back to paper making process. The content of fillers and coating pigments is typically measured through ash content measurement by burning the stock or paper sample in 525 °C. The base paper for uncoated fine paper and for coated fine paper is made from softwood and hardwood and its ash content is typically 18 - 24 %. The base paper for 100 % softwood based uncoated fine paper and for coated fine paper has an ash content typically 10 — 17 %.
Increase in paper’s filler content reduces the strength properties of the formed paper. It is assumed that the filler particles interfere with the fibre-fibre bonds, and thus impair the paper strength. It has been observed that the filler particle size is an important variable for paper strength, as large filler particles do not interfere with the fibre-fibre bonding as much as small filler particles. Thus, for paper strength it would be advisable to increase the size of the individual filler particles. Large individual filler particles may, however, cause problems in other parts of the papermaking process, for example they may be more difficult to retain under high shear forces and result in retention problems.
An object of this invention is to minimise or even eliminate the disadvantages existing in the prior art.
An object of the present invention is to provide an effective and simple method for increasing the filler content in paper, board or the like, while maintaining its strength properties and/or optical properties.
An object of the present invention is to increase filler content of paper or board in order to reduce papermaking costs while maintaining the strength properties of the paper or board.
These objects are attained with a methodhaving the characteristics presented below in the characterising part of the independent claim.
Typical method according to the present invention for producing paper, board or the like, comprises - obtaining an aqueous filler dispersion, comprising particles of at least one first filler and at least one different second filler, the filler dispersion having a floe size distribution with original mean chord length value, - bringing the filler dispersion into a contact with a pre-treatment agent and forming floes, which comprise first and second filler particles, - combining the pre-treated filler dispersion with a stock of fibres, and - changing the floe size distribution of the filler dispersion with the pre-treating agent so that mean chord length value increases at least with 2 % from the original mean chord length value.
Typical agglomerate comprises - at least one first filler particle, - a plurality of second different filler particles attached to the at least one first filler particle by a flocculating agent, whereby the average particle size by weight of the first filler particles is 1.5 - 4.0 nm, the size of the second filler particles is 0.5 - 2.4 nm, provided that the second filler particles are smaller than the first filler particles, and the size of the agglomerate is < 40 pm.
Typical use of a method according to the invention is for producing following paper grades: SC, LWC, newsprint, fine paper, folding boxboard, white top linerboard or white lined chipboard.
Now it has been surprisingly found out that filler content of produced paper may be significantly increased, while maintaining the strength and optical properties of the final paper by controllably pre-treating a filler dispersion, which comprises at least two different fillers, so that the formed floes in the filler dispersion are sufficiently large, i.e. that mean chord length value increases at least with 2 % from the original mean chord length value. It was suddenly realised that the optimisation of the size of the formed filler floes provides unexpected advantages. For example, it has been observed that the filler content may be increased by 1 - 5 % while still maintaining acceptable paper strength. When using the present invention, the light scattering coefficient for the final produced paper is kept constant or almost constant in constant filler level. Light scattering values are typically in the range of ± 5 %.
In this application flocculation is understood as a process of contact and adhesion whereby the particles of a dispersion form larger-size clusters. “Flocculation” is used synonymously with the terms agglomeration and aggregation, and “floe” is used synonymously with the term agglomerate and aggregate. Floe is here understood as a cluster, comprising at least two, preferably a plurality of primary particles attached to each other, the size of the cluster being larger than the size of the individual primary particles. In this application the first and second filler particles are the primary particles forming the floes.
In this application the value “average molecular weight” is used to describe the magnitude of the polymer chain length. Average molecular weight values are calculated from intrinsic viscosity results measured in a known manner in 1N NaCI at 25 °C. The capillary selected is appropriate, and in the measurements of this application an Ubbelohde capillary viscometer with constant K=0.005228 was used. The average molecular weight is then calculated from intrinsic viscosity result in a known manner using Mark-Houwink equation [Γ|]=Κ·Μ3, where [Π] is intrinsic viscosity, M molecular weight (g/mol), and K and are parameters given in Polymer Handbook, Fourth Edition, Volume 2, Editors: J. Brandrup, E.H. Immergut and E.A. Grulke, John Wiley & Sons, Inc., USA, 1999, p. VI1/11 for poly(acrylamide-co-N,N,N-trimethyl aminoethyl chloride acrylate), 70 % acrylamide. Accordingly, value of parameter K is 0.0105 ml/g and value of parameter a is 0.3. The average molecular weight range given for the parameters in used conditions is 450 000 - 2 700 000 g/mol, but the same parameters are used to describe the magnitude of molecular weight also outside this range.
According to one embodiment of the invention the average particle size by weight of the first filler particles is 1.8 - 3.5 nm, preferably 2.0 - 3.0 nm, and the size of the second filler particles is 0.6 - 2.0 nm, preferably 0.7 - 1.8 nm, provided that the second filler particles are smaller than the first filler particles.
According to one preferred embodiment of the invention the floe size distribution of the filler dispersion is changed with the pre-treating agent so that mean chord length value increases 2-100 %, typically 3-60 %, preferably 5-40 % from the original mean chord length value. All the floe size values in this application have been measured by using Focused Beam Reflectance Measurement (FBRM). Used FBRM apparatus is Lasentec FBRM Model D600L by Laser Sensor Technology, Redmond, WA, USA, Serial No. 1106, and its detector is D600L-HC22-K, Serial No. 961. A more detailed description of the floe size measurements is given in the experimental section.
According to one embodiment of the invention the pre-treatment agent is a polymer having an average molecular weight (MW) in the range 200 000 - 5 000 000 g/mol, preferably 350 000 - 4 000 000 g/mol, more preferably < 2 000 000 g/mol, still more preferably 500 000 - 1 900 000 g/mol. It has been observed that the size of the formed floes or aggregates can be optimised by using a polymer with a relatively low average molecular weight.
Pre-treatment agent is typically used as an aqueous dispersion. The pre-treatment agent may be added to a filler dispersion comprising two different fillers, or the filler dispersion comprising two different fillers may be added to the aqueous solution of the pre-treatment agent, or the pre-treatment agent may be mixed simultaneously with a first filler dispersion comprising a first filler and a second filler dispersion comprising a second filler. Typically the pre-treatment agent brought simultaneously in contact with a first filler dispersion flow comprising a first filler and a second filler dispersion flow comprising a second filler. Thus the pretreatment agent solution and the filler dispersion are mixed together and the filler flocks are formed before the resulting dispersion mixture is introduced to the fibre stock. This means that normally the pre-treatment of the fillers is performed on-line by mixing two filler dispersion flows with the pre-treatment agent solution flow just before the pre-treated filler dispersion is added to the fibre stock. Typically the introduction of the pre-treatment agent to the filler dispersion is performed < 10 minutes, preferably < 30 seconds, more preferably < 20 seconds, before the addition of the pre-treated filler dispersion to the stock.
According to one embodiment of the invention the pre-treatment agent is added to the white water which is circulated back to the fibre stock preparation. The white water comprises filler minerals and fibre fines, which have length < 0.2 mm. According to one preferred embodiment the pre-treatment agent is brought in contact with filler dispersion in an environment which is substantially fibre free, i.e. does not comprise fibres having length > 0.2 mm. Thus the adsorption of the pretreatment agent to the fibre surface may be avoided and the formation filler floes or agglomerated guaranteed.
Typically the pre-treatment agent is added to the aqueous filler dispersion in an amount of at minimum 0.01 weight-%. The dosage amount is of 0.01 - 0.5 weight-%, preferably 0.02 - 0.1 weight-%, calculated to the dry solids weight of the filler dispersion.
The pre-treatment agent may be selected from a group consisting polyacrylamide (PAM), glyoxalated polyacrylamide (GPAM), polyethyleneimine (PEI), polyamine, polyvinylamine (PVAM), poly-DADMAC, DADMAC-acrylamide copolymer, polyamidoamine epihalohydrin (PAE), chitosan, polysaccharide derivatives and any of their mixtures. Polyacrylamide may be cationic, anionic or amphoteric. Typically the charge density of the pre-treatment agent is 0.1 - 7 meq/g, preferably 0.2 - 5 meq/g, more preferably 0.5 - 2 meq/g, determined at pH 7.
According to one embodiment of the invention the pre-treatment agent is high cationic starch, which has a degree of substitution, DS, > 0.06, preferably >0.1. The high cationic starch is preferably only slightly degragded or non-degraded, and modified solely by cationisation. Most preferably the used starch is non-degraded and non-cross-linked. Suitable high cationic starches are of natural origin, for example, potato, rice, corn, waxy corn, wheat, barley, sweet potato or tapioca starch, potato starch being preferred. Suitable starches preferably have an amylopectin content > 70 %, preferably >75%.
In case the pre-treatment agent comprises more than one polymer, any second or following polymer is added simultaneously with the first polymer to the aqueous filler dispersion. Preferably the pre-treatment agent is one single liquid solution comprising a first polymer selected from the group specified above and possible other second or following polymers.
According to one preferred embodiment of the invention the pre-treatment agent is cationic polyacrylamide having a molecular weight (MW) in the range of 200 000 -2 000 000 g/mol. According to one preferred embodiment of the present invention the pre-treatment agent is cationic polyacrylamide having an average molecular weight (MW) in the range of 400 000 - 2 000 000 g/mol, typically 400 000 -1 900 000, more typically 500 000 - 1 900 000, preferably 1 000 000 - 1 900 000 g/mol, more preferably 1 200 000 - 1 900 000 g/mol. Cationic polyacrylamide may be produced by copolymerizing acrylamide with a cationic monomer or methacrylamide with a cationic monomer. The cationic monomer may be selected from the group consisting methacryloyloxyethyltrimethyl ammonium chloride, acryloyloxyethyltrimethyl ammonium chloride, 3- (methacrylamido) propyltrimethyl ammonium chloride, 3-(acryloylamido) propyltrimethyl ammonium chloride, diallyldimethyl ammonium chloride, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, dimethylaminopropylacrylamide, dimethylamino-propylmethacrylamide, or a similar monomer. According to one preferred embodiment of the invention cationic polyacrylamide is copolymer of acrylamide or methacrylamide with (meth)acryloyloxyethyltrimethyl ammonium chloride. Cationic polyacrylamide may also contain other monomers, as long as its net charge is cationic and it has an acrylamide/methacrylamide backbone. An acrylamide or methacrylamide based polymer may also be treated after the polymerisation to render it cationic, for example, by using Hofmann or Mannich reactions.
Cationic polyacrylamide may be prepared by conventional radical-initiation polymerization methods. The polymerisation of the cationic polyacrylamide may be performed by using solution polymerisation in water, gel-like solution polymerisation in water, aqueous dispersion polymerisation, dispersion polymerisation in an organic medium or emulsion polymerisation in an organic medium. The cationic polyacrylamide final product may be obtained either as an emulsion in an organic medium, aqueous dispersion, or as solution in water, or as a dry powder or dry granules after optional filtration and drying steps following the polymerisation. Typically cationic polyacrylamide is used as a solution, the dosing concentration of the cationic polyacrylamide being 0.01 - 0.5 weight-%, preferably 0.1 - 0.3 weight-% in the solution.
The charge density of the cationic polyacrylamide is typically 2-30 mol-%, preferably 3-20 mol-%, more preferably 5-15 mol-%. The relatively low charge density of the cationic polyacrylamide assists the formation of filler floes with suitable, preferred size.
First filler particles are different from the second filler particles in the sense that the first filler particles and the second filler particles have (a) different average particle size by weight, D(50), measured with Sedigraph-method referred in this application(b) different chemical composition and/or (c) different crystal structure. Typically the size ratio between the first filler particles and the second filler particles is 1.05-2.0, preferably 1.1 - 1.6.
Sedigraph-method for particle size measurement is performed as follows:
Used apparatus is Sedigraph III 5120, Micromeritics Instrument Corporation, Norcross, GA, USA. Sedigraph III 5120 is used for measuring the particle size distribution of pigment slurries and powders. Typical measuring range is 0.2 - 100 pm. Measurement is based on measuring the sedimentation rate of particles in a suspension by X-ray beam. Measured sedimentation rates are converted to particle sizes using the so-called Stokes’ law. Total sample volume is 50 ml and concentration 4 %. Used solvent is Daxad 23 by Micromeritics (sodium lignosulfonate, 0.02 %).
According to one embodiment of the invention the first and/or second filler are selected from a group consisting of ground calcium carbonate, precipitated calcium carbonate, clay, talc, gypsum, titanium dioxide, synthetic silicate, aluminium trihydrate, barium sulphate, magnesium oxide or their mixtures. Preferably the fillers, which are used in making or paper or board, suitable for use in the present invention, and content of which is increased, are clay, ground or precipitated calcium carbonate, calcium sulphate, titanium dioxide, synthetic silicate or talc, or any of their mixtures. The typical particle size of the filler used in the invention depends on the filler quality. Thus clay has a typical average particle diameter in the range of 500 - 1000 nm, calcium carbonate in the range of 200 -400 nm, talc in the range of 1000 - 10 000 nm, titanium dioxide in the range of 150 - 350 nm and synthetic silicate in the range of 100 - 400 nm.
According to one especially advantageous embodiment of the present invention the first and second filler is selected from the following combinations: GCC/clay and TiCVcIay.
Pre-treatment agent may be added to the filler dispersion in amount < 1000 g/ ton total amount filler, typically 10 - 1000 g/ton, preferably < 300 g/ton, more preferably 20 - 300 g/ton, still more preferably 30 -- 150 g/ton total amount filler. Total amount filler comprises here both the first filler and the second filler.
According to one embodiment of the invention in addition to the pre-treatment agent also inorganic colloidal particles, which have an average particle size in water < 100 nm, are added to the filler dispersion in order to improve the filler retention to the formed paper web. Inorganic colloidal particles are at least partly negatively charged particles, whose average diameter length in water is < 100 nm, preferably in the range of 1 to 100 nm. The average particle diameter of inorganic colloid is in the range of 1 - 80 nm, preferably 1 - 50 nm, more preferably in the range of 1 - 25 nm. The specific area, (BET), which naturally depends on the particle size, is preferably in the range of 30 - 1000 m2/g, more advantageously in the range of 100 - 1000 m2/g. The inorganic colloidal particles have anionic groups on their surface, which groups may be e. g. counter-ions of dissolved metal cations. Typical inorganic colloidal particles comprise colloidal silicate particles, such as synthetic silicates, silicates of Mg- and Al-type, colloidal silica, and polysilicate microgel, polysilicic acid microgel and aluminium-modified derivatives of these. Synthetic silicates include e. g. fumed or alloyed silica, silica gel and synthetic metal silicates. Silicates of Mg- and Al-type comprise expanded clay types, i.e. smectite, such as montmorillonite, hectorite, vermiculite, baidelite, saponite and sauconite, and their silicate alloys and derivatives.
Pre-treatment agent and inorganic colloidal particles may be added to the filler dispersion either simultaneously or sequentially in any order to the filler dispersion.
The method according to invention may be used for producing super calendered (SC) paper, ultralight weight coated (ULWC) paper, light weight coated (LWC) paper, medium weight coated (MWC) paper, heavy weight coated (HWC) paper, machine finished coated (MFC) paper, uncoated woodfree (UWF) paper, woodfree coated (WFC) paper, lightweight coated (LWCO) printing paper, SC offset (SCO) printing paper, machine finished specialties (MFS), multilayer coated paper, inkjet paper, copy paper, newsprint paper, folding boxboard, white top linerboard or white lined chipboard. The method is preferably used for producing following paper or board grades: super calendered (SC) paper, lightweight coated (LWC) paper, newsprint paper, fine paper, folding boxboard, white top linerboard or white lined chipboard. Typical coated paper, such as LWC comprises mechanical pulp around 40 - 60 weight-%, bleached softwood pulp around 25 - 40 weight-% and fillers and/or coating agents around 20 - 35 weight-%. SC paper comprises mechanical pulp around 70 - 90 weight-% and long fibered cellulose pulp around 10-30 %. Typical grammage values for different paper grades may be: 40 - 80 g/m2 for SC, 40 - 70 g/m2 for LWC, 70-130 g/m2 for MWC, 50 - 70 g/m2 for MFC, 40 - 200 g/m2 for UWF, 70 - 90 g/m2 for MWC, 100- 135 g/m2 for HWC, 40 - 200 g/m2 for WFC. The paper or board may comprise also recycled fibres. According to one embodiment of the invention the filler content of the paper or board is increased, whereby the ash content in is >34 % for super calendered (SC) paper, >25 % for uncoated woodfree (UWF) and coated woodfree base paper and >15 % for newsprint paper, LWC base paper and board grades, the ash content being measured by burning the stock sample completely in 525 °C.
The invention is described in more detail below with reference to the enclosed schematic drawing, in which
Figure 1 shows a first embodiment of the present invention
Figure 1 shows an first embodiment of the present invention, where thick stock is led through thick stock pipeline 1 from preceding stock preparation stages (not shown) to off-machine silo 9. From the off-machine silo 9 thick stock is led by a primary fan pump 2 to a centrifugal cleaner 3 and from centrifugal cleaner 3 further to a deaeration unit 4. From deaeration unit 4 thick stock is fed by headbox fan pump 5 to machine screen 6 and further to headbox 7.
Filler is added to the fibre stock after deaeration unit 4. A first filler is fed from a first filler chest 12 and a second different filler is feed from a second filler chest 12’ by using first and second filler pumps 13, 13’. Filler pipe 14 is connected to the pipeline transferring stock from deaeration unit 4 to machine screen 6.The flows of the first and second fillers are combined before addition of the pre-treatment agent at feeding point 15, which is located between filler chest 12 and connection point of filler pipe 14 and main stock pipeline.
From the headbox 7 the fibre stock comprising pre-treated filler is fed to wire section 8 of the paper machine.
It is also possible to add pre-treatment agent at one or several addition points 19. Suitable addition points may be selected on basis of the process, used filler and pre-treatment agent.
EXPRIMENTAL
General procedure for conducting Focused Beam Reflectance Measurement (FBRM) tests
Used FBRM apparatus is Lasentec FBRM Model D600L by Laser Sensor Technology, Redmond, WA, USA, Serial No. 1106, and its detector is D600L-FIC22-K, Serial No. 961. The FBRM instrument is a flocculation analyzer using a highly focused laser beam and back-scattered geometry as a principle of operation. From the collected data the FBRM instrument yields chord length distribution, mean of the chord length values and the number of particles detected. The measurement range of the device is 1 - 1000 pm.
The following step-wise procedure is used for determining the floe size: (1) at moment 0 s and at stirring rate of 1000 rpm a 500 ml filler sample which is diluted to 1 % consistency is poured into a vessel, (2) at moment 30 s pretreatment agent is dosed into the filler sample, and (3) at moment 45 s floe size of the sample is measured .
The pretreatment polymer doses are based on solids per dry matter weight of the filler, in unit’s g/t. The overall consistency of filler was produced by drying it in a heating chamber at a temperature of 100-105 °C.
Example 1
Example 1 illustrates pretreatment of two fillers with a pre-treatment agent.
Filler sample is used in the form of slurry with a desired solids content. Filler sample comprises 50 % of ground calcium carbonate, GCC, under the trade name Flydrocarb 65, having average particle size D(50) 1.78 pm measured with Sedigraph-method, and 50 % of clay under the trade name Intramax 60, having average particle size D(50) 1.98 pm measured with Sedigraph-method. Filler portions in the sample are based on dry solids content. Filler sample is obtained by diluting with tap water.
Two pre-treatment agents are tested. First pre-treatment agent is denoted as pretreatment agent A, which is cationic polyacrylamide with charge 30 mol-%, and with molecular weight 1 400 000 g/mol. Second pre-treatment agent is denoted pre-treatment agent B, a cationic dispersion polyacrylamide product with total active content of 30 %. The cationic polyacrylamide content is 15 % and poly-DADMAC content is 15 %. The cationic polyacrylamide is a copolymer of acrylamide and acryloyloxyethyltrimethyl ammonium chloride, the share of the cationic monomer being 25 mol-%. The molecular weight of the cationic polyacrylamide is 5 000 000 g/mol. The pretreatment polymer doses are based on solids per dry matter weight of the filler, unit g/t. The overall consistency of filler is produced by drying it in a heating chamber at a temperature of 100-105 °C.
Mean chord length before and after pre-treatment are measured by using FBRM apparatus and procedure described above. The obtained results for mean chord length before and after pre-treatment are presented in Table 1.
Table 1 Mean chord length and increase of mean chord length.
Example 1 shows that mean chord length increases as a function of pre-treatment agent A or B.
Example 2
Example 2 illustrates how the pre-treated filler acts as a filler in papermaking.
The fibre stock comprises thermo mechanical pulp from a paper mill, and it is diluted with tap water to 0.5 % consistency.
Ground calcium carbonate, GCC, under the trade name Hydrocarb 65, having average particle size D(50) 1.78 pm measured with Sedigraph-method, and clay under the trade name Intramax 60 having average particle size D(50) 1.98 pm measured with Sedigraph-method, are used in the experiments. GCC and clay are mixed together as 50/50 proportion based on dry solids contents. The filler is treated in the form of slurry.
Filler dispersion is pre-treated with pre-treatment agent A, which is cationic polyacrylamide with charge 30 mol-%, and with molecular weight 1 400 000 g/mol. Two doses are tested, 35 g/t or 70 g/t, given as solids, and calculated based on dry solids content of the filler dispersion. Pre-treatment agent is added to the filler dispersion in the form of diluted aqueous slurry and mixed with magnetic stirrer.
Pre-treated filler dispersion is added to the fibre stock sample 20 s before sheet forming to achieve desired filler content of sheets. Retention aid, cationic polyacrylamide , with charge 1 meq/g and molecular weight 6 400 000 g/mol, is added to sample 10 s before hand sheet forming.
Hand sheets are prepared with Rapid-Köthen hand sheet former according to the standard ISO 5269-2:2004. Target grammage for hand sheets is 80 g/m2
Tensile strength and light scattering values are determined for hand sheet according to the standards ISO 1924-2:2008 and ISO 9416:2009. The measured values are presented in Table 2.
Table 2 Filler content, tensile index and light scattering values for formed hand sheets.
Example 3
Example 3 illustrates how the pre-treated filler acts as a filler in papermaking.
Example 3 is conducted at different time as Example 2 and different fibre stock is used. The fibre stock comprisese thermo mechanical pulp from a paper mill, and it is diluted with tap water to 0.5 % consistency.
Ground calcium carbonate, GCC, under the trade name Hydrocarb 65 having average particle size D(50) 1.78 pm measured with Sedigraph-method, and clay under the trade name Intramax 60 having average particle size D(50) 1.98 pm measured with Sedigraph-method, are used in the experiments. GCC and clay are mixed together as 50/50 proportion based on dry solids contents. The filler is treated in the form of slurry.
Filler dispersion is pre-treated with pre-treatment agent B, which is a cationic dispersion polyacrylamide product with total active content of 30 %. The cationic polyacrylamide content is 15 % and poly-DADMAC content is 15 %. The cationic polyacrylamide is a copolymer of acrylamide and acryloyloxyethyltrimethyl ammonium chloride, the share of the cationic monomer being 25 mol-%. The molecular weight of the cationic polyacrylamide is 5 000 000 g/mol. Two doses are tested, 90 g/t or 240 g/t, given as solids, and calculated based on dry solids content of the filler dispersion. Pre-treatment agent is added to the filler dispersion in the form of diluted aqueous slurry and mixed with magnetic stirrer.
Pre-treated filler dispersion is added to the fibre stock sample 20 s before sheet forming to achieve desired filler content of sheets. Retention aid, cationic polyacrylamide, with charge 1 meq/g and molecular weight 6 400 000 g/mol, is added to sample 10 s before hand sheet forming.
Hand sheets are prepared with Rapid-Köthen hand sheet former according to the standard ISO 5269-2:2004. Target grammage for hand sheets is 80 g/m2
Tensile strength and light scattering values are determined for hand sheet according to the standards ISO 1924-2:2008 and ISO 9416:2009. The measured values are presented in Table 3.
Table 3 Filler content, tensile index and light scattering values for formed hand sheets.
These examples 2 and 3 show that with pretreated fillers it is possible to have higher tensile strength and light scattering properties for paper.
Mean chord length of treated filler composition should not be increased too much, because then light scattering properties will be decreased. The excess amount of polymer can also decrease tensile strength.
Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.