Abstract:
The invention relates to a method for priming a substrate by contacting the substrate with a primer fed from a primer source and depositing the primer on the substrate. Compared to other priming methods, the claimed priming gives better results because the deposition is carried out electrostatically.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method for priming a substrate by contacting the substrate with a primer fed from a primer source and depositing the primer on the substrate. The invention also relates to a process for the coating of a substrate by contacting the substrate with a primer fed from a primer source, depositing the primer on the substrate, and coating the primed substrate with a coating substance. 
         [0002]    There are several methods of improving the adhesion between a substrate and its coating. These methods can be surface treatment, mechanical roughening, removing weak boundary layers, minimising stresses, using adhesion promoters, using suitable acid-base interactions, as well as providing favourable thermodynamics and using wetting. Typical treatment techniques include the use of chemicals such as primers and solvents, the use of heat and flame, mechanical methods, plasma, corona treatment and radiation. Each technique can have several effects that improve adhesion. 
         [0003]    An important method of improving the adhesion between a substrate and its coating is priming. Priming means the treatment of a substrate with a primer. A primer means a prefinishing coat applied to surfaces that are to be painted or otherwise finished. See McGraw-Hill Dictionary of Scientific and Technical Terms, 6 th  Ed., p. 1668 and 1669. 
         [0004]    Typical primers are adhesive organic substances which are soluble in water and/or an organic solvent and are used for treating the substrate surface in order to improve its adhesion or bonding to the coating. In the following table, typical primers and their adhesion and performance characteristics are given. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Properties of typical primers 
               
             
          
           
               
                   
                 Adhesion Characteristics 
                 Performance Characteristic 
               
             
          
           
               
                   
                   
                   
                 Plastic 
                 Heat 
                 Moisture 
                 Chemical 
               
               
                 Type of primer 
                 Paper 
                 Metal 
                 film 
                 Resistance 
                 Resistance 
                 Resistance 
               
               
                   
               
               
                 Shellac 
                 Poor 
                 Excellent  
                 Poor 
                 Poor 
                 Poor 
                 Poor 
               
               
                 Organic Titanate 
                 Good 
                 Good 
                 Good 
                 Fair 
                 Fair 
                 Fair 
               
               
                 Polyurethane 
                 Very good 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
               
               
                 Polyethyleneimine 
                 Very good 
                 Good 
                 Excellent 
                 Excellent 
                 Poor 
                 Poor 
               
               
                 Ethylene Acrylic 
                 Excellent 
                 Excellent 
                 Fair 
                 Fair 
                 Excellent 
                 Good 
               
               
                 Acid 
                   
                   
                   
                   
                   
                   
               
               
                 Polyvinylidene 
                 Excellent 
                 Fair 
                 Excellent  
                 Good 
                 Very good 
                 Fair 
               
               
                 Chloride 
               
               
                   
               
             
          
         
       
     
         [0005]    Traditional priming takes place by conventional solution application techniques. The application of a primer promotes adhesion between the substrate and the coating by increasing the free energy (wettability) of the surfaces, inducing chemical reaction between them, and removing bond weakening impurities from them. 
         [0006]    However, traditional priming has the drawback that it is difficult to achieve the correct coating weight suitable for the specific primer to be used. Uniform deposition is important for all primers. This is especially the case with uneven surfaces, the less available sites of which are poorly reached by conventional priming techniques. 
         [0007]    These drawbacks have now been overcome by a new method for priming a substrate by contacting the substrate with a primer fed from a primer source and depositing the primer on the substrate. The claimed method is essentially characterized in that the deposition is carried out electrostatically. By deposition is meant the application of any material to a substrate. By electrostatically is meant something pertaining to electricity at rest, such as an electric charge on an object. See McGraw-Hill, Dictionary of Scientific and Technical Terms, 6 th  Ed., p. 707. 
         [0008]    Electrostatic coating methods are known per se. However, the inventors found that these methods are especially suitable for priming purposes. By means of electrostatic coating, the correct coating weight suitable for any specific kind of primer can easily be achieved. Additionally, less available sites on uneven substrate surfaces are conveniently reached by the electrostatic priming techniques. Thus, a larger part of the substrate surface will possess improved primer-induced adhesion. 
         [0009]    Electrostatic coating methods can be divided to three methods: electrostatic spraying and electrospinning, typically from solution under DC field, as well as dry coating from powders using AC fields. 
         [0010]    In the spraying process, a high voltage electric field which is applied to the surface of a liquid causes the emission of fine charged droplets. The process is governed by mass, charge and momentum conservation. Therefore, there are several parameters, which influence the process. The most important parameters are the physical properties of the liquid, the flow rate of the liquid, the applied voltage, the used geometry of the system, and the dielectric strength of the ambient medium. The essential physical properties of the liquid are its electrical conductivity, surface tension and viscosity. An electrospray apparatus is typically formed of a capillary, pressure nozzle, rotating nozzle, or atomizer, which feed the coating liquid, and a plate collector which carries the substrate to be coated. An electrical potential difference is connected between the capillary and the plate. 
         [0011]    The potential difference between the plate and the end of the capillary supplying the coating liquid is several thousands volts, typically dozens of kilovolts. The emitted droplets are charged and they may be neutralized if necessary by different methods. Their size varies, depending on the conditions used. The most suitable electrospraying conditions for priming are discussed in more detail below. 
         [0012]    Electrospinning, just as electrospraying, uses a high-voltage electric field. Unlike electrospraying which forms solidified droplets, solid fibers are formed from a polymer melt or solution, which is delivered through a millimeter-scale nozzle. The resulting fibers are collected on a grounded or oppositely charged plate. With electrospinning, fibers can be produced from single polymers as well as polymer blends. 
         [0013]    Electrospinning can be used to produce ultra-fine continuous fibers, the diameters of which range from nanometers to a few micrometers. The small diameter provides small pore size, high porosity and high surface area, and a high length to diameter ratio. The resulting products are usually in the non-woven fabric form. This small size and non-woven form makes electrospun fibers useful in variety of applications. 
         [0014]    In a spinning process various parameters affect the resulting fibers obtained. These parameters can be categorized into three main types, which are solution, process and ambient parameters. Solution properties include concentration, viscosity, surface tension, conductivity, and molecular weight, molecular-weight distribution and architecture of the polymer. Process parameters are the electric field, the nozzle-to-collector distance, and the feed rate. Ambient properties include temperature, humidity and air velocity in the spinning chamber. The most suitable electrospinning conditions for priming are discussed in more detail below. 
         [0015]    Dry coating is quite similar to the electrospraying and electrospinning processes, with the exception that the raw material is in powder form. One of the latest inventions is to coat paper with this method. Paper coating by dry coating method is an alternative method for the traditional pigment coating. This dry surface treatment (DST) of paper and paperboard combines the coating and calandering processes. In the DST process, the electrically charged powder particles are sprayed onto the surface of the paper or paperboard. The particles form a layer on the surface of the paper and attach to the paper by electrostatic forces. The final fixing which is made in a nip between heated rolls, provides adhesion and makes of the surface smooth. 
       SUMMARY OF THE INVENTION 
       [0016]    In the following, the most important technical features of the invention are disclosed. The claimed process relates to the electrostatic priming of a substrate. Preferably the substrate to be primed is a solid material, such as wood, paper, textile, metal, plastic film, or a composite material. A preferred type of substrate is cellulose or wood containing &lt;300 g/m 2  of non-coated or coated garde produced by means of normal wet paper processes. Most preferably, the solid material is paper. By paper is meant any felted or matted sheet containing as an essential part cellulose fibers. 
         [0017]    The electrostatic deposition used in the claimed priming is according to one preferred embodiment electrospraying. In the electrospraying, the primer is preferably initially in the form of liquid droplets dispersed in the gas phase. The droplets may be either droplets of molten primer or, preferably, droplets of a solution of the primer material in a solvent. Typically, the average diameter of the liquid droplets is between 0.02 and 20 μm, preferably 0.05-2 μm. 
         [0018]    According to another preferred embodiment of the invention, the claimed priming by electrostatic deposition is electrospinning. In the electrospinning, at least a part of the primer is in the form of fibers dispersed in the gas phase. The fibers may be formed either from molten primer or, preferably, droplets of a primer solution in a solvent. When forming the primer fibers by electrospinning, the average diameter of the fibers is preferably between 0.05 and 5.0 μm, most preferably between 0.1 and 0.5 μm. 
         [0019]    The claimed electrostatic priming may also be a mixture of electrospraying and electrospinning, where both solid droplets and solid fibers are formed on the substrate. 
         [0020]    When using electrostatic deposition (spraying, spinning, or both) from solution, the primer material content of the solution is preferably between 5 and 50% by weight, most preferably between 20 and 45% by weight. The solution is preferably between 40 and 400 cP, most preferably between 50 and 200 cP. The solvent is selected according to the primer applied, considering also that its volatility must be low enough for good productivity and its conductivity must be suitable for the electrostatic process. Preferred solvents are water and water/alcohol systems. 
         [0021]    As was said above in connection with the general description of the invention, the primer material may be a native polymer, a polyalcohol, an organometal compound, and/or a synthetic polymer. Typically, the primer material is a synthetic polymer (homo- or copolymer). According to one advantageous embodiment of the claimed invention, the synthetic polymer is an acrylic copolymer, which most preferably is in the form of an aqueous emulsion. Then the deposited material thickness is typically 0.002-0.05 g/m 2 , preferably 0.006-0.02, and most preferably about 0.01 g/m 2 . According to another advantageous embodiment of the invention, the primer is diethanol aminoethane (DEAE), preferably in aqueous medium. Then, the preferred thickness of the deposited material is 0.02-0.5 g/m 2 , more preferably 0.06-0.2, and most preferably about 0.1 g/m 2 . 
         [0022]    Most preferably, the primer solution also contains an additive to modify the morphology of the primer particles on the substrate. A preferred additive is a polymer soluble in the solvent and compatible with the primer, which has a sufficiently high molecular weight to stabilize the process. Preferably, the polymeric additive has to be suitable for the electrostatic process as well. Examples of polymers suitable as additives in the claimed electrostatic processes are among others polyvinyl alcohol, polyethylene oxide, and acrylic resins. 
         [0023]    The electrostatic primering of the instant invention is preferably carried out by means of an apparatus suitable for either electrospraying or electrospinning. It consists of a fume chamber with minimised interference, in which a construction comprising a metal plate for supporting the substrate and a feed section are arranged. A voltage source is coupled to the metal plate and the feed section. The electrostatic force expressed as the voltage divided by the distance between the substrate and the primer source raised to the second power is according to one embodiment between 0.02 and 4.0 V/mm 2 , preferably between 0.2 and 0.5 V/mm 2 . The electrostatic voltage is preferably between 10 and 50 kV, more preferably between 20 and 40 kV, and the distance between the primer source and the substrate is preferably between 100 and 1000 mm, more preferably between 200 and 500 mm. 
         [0024]    In addition to the above described method for priming a substrate electrostatically, the invention also relates to a process for coating a substrate by contacting the substrate with a primer fed from a primer source, depositing the primer on the substrate, and coating the primed substrate with a coating substance. Said deposition of the primer on the substrate is carried out electrostatically. 
         [0025]    The claimed coating process thus comprises said electrostatic priming followed immediately or later by a coating process. For the priming step, the same specifications apply as above, so, there is no reason to repeat them here. However, when moving on from priming to coating, the primed substrate is preferably flame or, most preferably, corona treated before it is coated with the coating substance. 
         [0026]    Typically, the coating substance is a thermoplastic resin. As the most advantageous substrate was paper, a preferred combination is the coating of paper with said thermoplastic resin. The best thermoplastic resin is a polyolefin resin such as an ethylene polymer (homo- or copolymer). 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0027]    The Figures which will be referred to are: 
           [0028]      FIG. 1  shows an electrospinning apparatus according to one embodiment of the invention. 
           [0029]      FIG. 2  shows the feed section of the electrospinnig apparatus according to  FIG. 1 . 
           [0030]      FIG. 3  shows the seed section and the collector plate of the electrospinning apparatus according to  FIG. 1 . 
           [0031]      FIGS. 4A and 4B  show a SEM of paper coated with P 1  with a magnification of 3500×, with the coating weight 0.1 g/m 2  in  FIG. 4A  and the coating weight 0.01 g/m 2  in  FIG. 4B . 
           [0032]      FIGS. 5A and 5B  show a SEM of paper coated with P 2  with a magnification of 750×, with the coating weight 0.1 g/m 2  in  FIG. 5A  and the coating weight 0.01 g/m 2  in  FIG. 5B . 
           [0033]      FIGS. 6A and 6B  show a SEM of paper coated with P 3  with a magnification of 750×, with the coating weight 0.1 g/m 2  in  FIG. 6A  and the coating weight 0.01 g/m 2  in  FIG. 6B . 
           [0034]      FIGS. 7A and 7B  show a SEM of paper coated with P 5  with the magnification 1500×, with the coating weight 0.1 g/m 2  in  FIG. 7A  and the coating weight 0.01 g/m 2  in  FIG. 7B . 
           [0035]      FIGS. 8A and 8B  show a SEM of paper coated with P 6  with the magnification 1500×, with the coating weight 0.1 g/m 2  in  FIG. 8A  and the coating weight 0.01 g/m 2  in  FIG. 8B . 
           [0036]      FIGS. 9A and 9B  show a SEM of paper coated with P 7  with the magnification 3500×, with the coating weight 0.1 g/m 2  in  FIG. 9A  and the coating weight 0.01 g/m 2  in  FIG. 9B . 
           [0037]      FIGS. 10A and 10B  show a SEM of paper coated with P 11  with the magnification 3500×, with the coating weight 0.1 g/m 2  in  FIG. 10A  and the coating weight 0.01 g/m 2  in  FIG. 10B . 
           [0038]      FIGS. 11A and 11B  show a SEM of paper coated with P 12  with the magnification 1500×, with the coating weight 0.1 g/m 2  in  FIG. 11A  and the coating weight 0.01 g/m 2  in  FIG. 11B . 
           [0039]      FIGS. 12A and 12B  show a SEM of paper coated with P 13  with the magnification 1500×, with the coating weight 0.1 g/m 2  in  FIG. 12A  and the coating weight 0.01 g/m 2  in  FIG. 12B . 
           [0040]      FIG. 13  shows the PE-film coating after a peel test, P 1 -P 13  with corona treatment. 
           [0041]      FIGS. 14A and 14B  shows the PE-film coating after the peel test without corona treatment, P 1 -P 13 . 
           [0042]      FIGS. 15A and 15B  show the paperboard with P 3  after the peel test,  FIG. 15A  without corona treatment and  FIG. 15B  with corona treatment. 
           [0043]      FIGS. 16A and 16B  show the paperboard with P 5  after the peel test,  FIG. 16A  without corona treatment and  FIG. 16B  with corona treatment. 
           [0044]      FIG. 17  shows the paperboard with P 6  after the peel test and with corona treatment. The magnification was 1500×. 
           [0045]      FIG. 18  shows the paperboard with P 7  after the peel test and without corona treatment. The magnification was 1500×. 
           [0046]      FIG. 19  shows SEM pictures after the peel test and without corona treatment;  FIG. 19A  being paperboard with P 11 , magnification 3500×;  FIG. 19B  being paperboard with P 12 , magnification 1500×; and  FIG. 19C  being paperboard with P 13 , magnification 1500×. 
           [0047]      FIG. 20  shows the critical surface energies of primers (P 1 -P 13 ) and paperboard (K). 
           [0048]      FIG. 21  shows the critical surface energies of primed paperboard. 
           [0049]      FIG. 22  shows adhesion measurement results. 
           [0050]      FIG. 23  shows the adhesion with primers (P 1 -P 13 ). 
           [0051]      FIG. 24  shows surface energy values (geometric mean) and adhesion of primers. 
           [0052]      FIG. 25  shows surface energy (geometric mean) and adhesion, where the priming weight was 0.01 g/m 2 . 
           [0053]      FIG. 26  shows surface energy (geometric mean) and adhesion, where the priming weight was 0.1 g/m 2 . 
           [0054]      FIG. 27  shows the particle size distribution of primer layers. 
       
    
    
     DETAILED DESCRIPTION 
       [0055]    In the following, the invention is exemplified by a few examples, the procedures of which are described more closely below. 
         [0056]    In this experimental work, priming was made with an electrospinning apparatus as illustrated in  FIG. 1 . The apparatus includes a fume chamber, the walls of which, except the front side wall, are constructed of metal plate, to minimise the external and internal electrical interference. The inner wall surfaces are covered with glass fiber composite. The used power supply unit is a high-voltage supply of type BP 50 Simco. The power supply can produce both positive and negative 0-50 kV voltage. 
         [0057]    The apparatus also includes a feed section having a spinneret and a needle. The needle is attached to the spinneret which is made of glass with luer (mikä on luer?) junction and the power supply is connected to the metallic junction of the needle. The feed section is illustrated in  FIG. 2 . 
         [0058]    As a counter-electrode to the feed section a square copper plate is arranged, the size of which is 400 mm×400 mm×1 mm. This collector plate, which supports the substrate, is hung on a plastic stand. The collector plate and the feed section is illustrated in  FIG. 3 . To the front of the collector plate is attached the substrate to be coated. The substrate can be, for example, a metal folio, a paper, or a non-woven textile. In the experiments carried out, the substrate was paper of quality CTM ion-coated 225 g/m 2  wood free board of chemical pulp. 
         [0059]    Suitable primers were selected by a preliminary test. Then, these primers, called P 1 -P 13 , were tested for solution viscosity (Brookfield DV-II+), morphology (JEOL SEM T-100), surface energy (PISARA-equipment), and adhesion (Alwetron peel test). The effect of a corona treatment of the primed paper substrate on the adhesion was also carried out. 
         [0060]    13 primers, i.e. P 1 -P 13 , were tested. The symbols P 1 -P 13  mean: 
         [0061]    P 1 →Carboxyl methyl cellulose 
         [0062]    P 2 →Alkyl ketene dimer 
         [0063]    P 3 →Polyethylene amine 
         [0064]    P 4 →Polyvinyl amine 
         [0065]    P 5 →Polyvinyl alcohol 
         [0066]    P 6 →Emulgated acrylic copolymer 
         [0067]    P 7 →Ethylene copolymer 
         [0068]    P 11 →Polyvinyl alcohol modified with ethylene groups 
         [0069]    P 12 →Diethanol aminoethane (DEAE) 
         [0070]    P 13 →MSA/C 20 -C 24 -olefin 
         [0071]    B→C 20 -C 24  olefin 
         [0072]    C→ethylene copolymer 
         [0073]    E→Polyvinyl amine 
         [0074]    G→polyvinyl acetone 
         [0075]    H→Dicthand aminoethene (DEAE) 
         [0076]    I→carbonyl methyl cellulose 
         [0077]    The results were as follows. 
       Results and Discussion 
     The Primer&#39;s Suitablility to Electrospraying or -Spinning 
       [0078]    The proper solution contents of primers and process parameters were found by experimentation. Several solution contents of each primer were tested. All primers were sprayed or spun through a 5 cm long needle, the size of which was 18 G. 
         [0079]    Primers P 5 , P 6  and P 11  were especially suitable without using morphology modifying additives in the spraying/spinning solution. Primers P 1 , P 2 , P 3 , P 7 , P 12 , and P 13  were also especially suitable, but they needed additives. Without additives they formed large droplets, and the coated areas were very small. With additives, coated area enlarged significantly and droplet size diminished. 
       The Productivity of the Electrospraying or -Spinning 
       [0080]    The productivities for each primer are presented in Table 2. In the table are presented also other properties, which are used for calculating the rate of application, namely the specific weight of the solution, the primer content of the solution, and the primer consumption. Also the needed priming times for dry coating weights 0.1 g/m 2  and 0.01 g/m 2  are presented in the table. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Productivities and other properties of each primer 
               
             
          
           
               
                   
                 Specific 
                 Primer 
                   
                   
                   
                   
               
               
                   
                 weight of the 
                 content of 
                 Consumption 
                   
                   
                 Needed priming time 
               
             
          
           
               
                   
                 solution 
                 solution 
                 of solution 
                 Area 
                 Productivity 
                 For 
                 For 
               
               
                 Primer 
                 [g/ml] 
                 [%] 
                 [s/1 ml] 
                 [m 2 ] 
                 [g/m 2 s] 
                 0.01 g/m 2   
                 0.1 g/m 2   
               
               
                   
               
               
                 P1 
                 1.028 
                 11.70 
                 5040 
                 0.0491 
                 0.00049 
                 21 s 
                 205 s 
               
               
                 P2 
                 0.915 
                 31.67 
                 6252 
                 0.0491 
                 0.00094 
                 11 s 
                 106 s 
               
               
                 P3 
                 1.035 
                 22.35 
                 2768 
                 0.0314 
                 0.00266 
                  4 s 
                  28 s 
               
               
                 P5 
                 0.973 
                 15.00 
                 3300 
                 0.0491 
                 0.00090 
                 11 s 
                 111 s 
               
               
                 P6 
                 1.037 
                 45.20 
                 1410 
                 0.0962 
                 0.00346 
                  3 s 
                  29 s 
               
               
                 P7 
                 1.041 
                 22.33 
                 2040 
                 0.1200 
                 0.00095 
                 11 s 
                 107 s 
               
               
                 P11 
                 1.018 
                 7.50 
                 1800 
                 0.0452 
                 0.00094 
                 11 s 
                 107 s 
               
               
                 P12 
                 0.982 
                 25.00 
                 1920 
                 0.0855 
                 0.00149 
                  7 s 
                  67 s 
               
               
                 P13 
                 1.011 
                 22.39 
                 4562 
                 0.0360 
                 0.00138 
                  7 s 
                  72 s 
               
               
                   
               
             
          
         
       
     
         [0081]    During the consumption test, it was easy to see which ones of the primers are suitable for continuing priming and which ones are not, unless some changes are made to the solution or process. Primers P 2 , P 3 , P 6 , and P 13  are not suitable for continuous priming, because they gel on the end of the needle. Instead, primers P 1 , P 5 , P 7 , P 11 , and P 12  are suitable for continuous priming. 
         [0082]    The needed priming times are only estimated. In productivity measurement, it was assumed that all of the primer is transferred from the needle to the collector plate. However, in practise some particles fly over the plate and some large droplets may not fly so far. During the consumption measurement, the process was at first faster and then became slower because the solution level and pressure in the needle were reduced with time. Thus the consumption values are average values. Coating areas are defined by eye, so these are also approximate values. 
       The Viscosity of the Primer Solutions and the Morphology of the Primed Paperboards 
       [0083]    The viscosities of the used primer solutions were the Brookfield viscosity. The morphologies of the deposited primer particles were measured by analysing SEM pictures. The SEM-pictures presented in this chapter, were taken randomly. In addition to the viscosity and the morphology, this chapter shows further process parameters such as the voltage and the working distance between the substrate and the feeding capillary. 
         [0084]    In the following, each sample is treated separately. 
         [0085]    Primer P 1   
         [0086]    The viscosity of the solution was 370 cP. Although the viscosity was high, primer P 1  did not form fibers, but droplets. The droplet size was 0.1-0.3 μm, the voltage and working distance were ±35 kV and 350 mm, respectively, and the diameter of the coated area was 25 cm. A SEM of the layer of P 1  is presented in  FIG. 4 . 
         [0087]    Primer P 2   
         [0088]    The viscosity of the solution was 170 cP. Again, although the viscosity was sufficiently high, the primer did not form fibers, but droplets. The droplet size was 0.5-6 μm, the voltage and working distance were ±30 kV and 450 mm, respectively, and the diameter of the coated area was 25 cm. A SEM of the layer of P 2  is presented in  FIG. 5 . 
         [0089]    Primer P 3   
         [0090]    The viscosity of the solution was 215 cP. Also here, although the viscosity was sufficiently high, the primer formed droplets instead of fibers. The droplets were very large and also the size distribution was wide. The size of the droplets was 1.2-17 μm, the voltage and the working distance were ±50 kV and 350 mm, respectivelty, and the diameter of the coated area was 20 cm. A SEM of the layer of P 3  is presented in  FIG. 6 . 
         [0091]    Primer P 5   
         [0092]    Viscosity of solution was 193 cP. Again, although the viscosity was sufficiently high, primers did not form fibers, but droplets. Droplet size was 0.2-1.5 μm, voltage and working distance were ±40 kV and 400 mm, and diameter of coated area was 25 cm. Layer of P 5  is presented in  FIG. 7 . 
         [0093]    Primer P 6   
         [0094]    The viscosity of the solution was quite low: 90 cP, therefore it formed droplets. The droplet size was 0.2-5 μm, the voltage and working distance were ±30 kV and 300 mm, respectively, and the diameter of the coated area was 35 cm. Layer of P 6  is see in  FIG. 8 . 
         [0095]    Primer P 7   
         [0096]    The viscosity of the solution was 60 cP. Although the viscosity was low, the primer formed also fibers besides droplets. The fiber forming is probably caused by use of additives. The fiber diameter was approximately 0.1 μm and the droplet size was 0.5-6 μm, and the voltage and working distance were ±30 kV and 400 mm, respectively. The primer coated area was very large. The primer coated the whole area of the collector plate. Layer of P 7  is presented in  FIG. 9 . 
         [0097]    Primer P 11   
         [0098]    Thy viscosity of the solution was 110 cP. Primer  11  formed only thin fibers, including some pearls. The fibre diameter was 0.4-0.1 μm and the pearl size was 0.8-1.4 μm. The voltage and working distance were ±40 kV and 400 mm, respectively, and the diameter of the coated area was 24 cm. The layer of P 11  is presented in  FIG. 11 . 
         [0099]    Primer P 12   
         [0100]    The viscosity of the solution was 60 cP. Although the viscosity was low, the primer formed also fibers besides droplets. The fiber formation is probably caused by the use of additives. The droplet size was 0.5-3 μm and the fibre diameter was 0.1-0.4 μm. The voltage and working distance were ±20 kV and 300 mm, respectively, and the direction of the electric field was from minus potential to plus potential. The diameter of the coated area was 33 cm. Layer of P 12  is presented in  FIG. 12 . 
         [0101]    Primer P 13   
         [0102]    The viscosity of the solution was 310 cP. Although the viscosity was sufficiently high, the primer formed droplets instead of fibers. The droplet size was 0.2-2.5 μm, the voltage and working distance were ±30 kV and 250 mm, respectively, and the diameter of the coated area was 18 cm. A layer of P 13  is presented in  FIG. 13 . 
         [0103]    The Surface Energy 
         [0104]    The critical surface energies of the primers are presented in  FIG. 20 . Their surface energies are compared to the surface energy of the paperboard. Surface energy values of all primers are smaller than surface energy of the paperboard. In  FIG. 20  sample K means paperboard and P 1 -P 13  primers, which was used in preliminary tests. 
         [0105]    The critical surface energies of primed paperboard are presented in  FIG. 21 . The critical surface energy values of the primed paperboard are smaller than the surface energy value of the paperboard itself. The surface energy values by geometric mean are presented in Appendix 1. 
         [0106]    The surface energy determination was done with three liquids, which is the minimum count. 
         [0107]    Adhesion of Primers and Priming Methods 
         [0108]    The adhesion was measured by priming paper conventionally (primers B-I) and according to the invention (primers P 1 -P 13 ), extrusion coating with LDPE, and finally measuring the adhesion force between the LDPE and the paper. The primers B-I which were primed to the paperboard by conventional spreading, are chemically similar to primers P 1 -P 13 , respectively. When priming by spreading, the obtained priming weight is higher compared to the electrostatic method (&gt;&gt;0.1 g/m 2 ). 
         [0109]    Adhesion measurement results of primers B-I primed by spreading are presented in  FIG. 22 . Primers B-I applied by spreading do not significantly improve adhesion. Only primer H improves adhesion, if extrusion coating is made without corona treatment. 
         [0110]    In  FIG. 23  is presented the adhesion of samples, whose priming weights are 0.1 g/m 2  and 0.01 g/m 2 . Priming is done with the electrostatic coating method. Primers P 1 -P 13  need corona treatment for improving adhesion. When corona treatment is not used, the adhesion is zero with almost every primer. Primers P 1 , P 6 , P 11 , and P 13  especially with coating weight 0.01 g/m 2 , and P 12  especially with coating weight 0.1 g/m 2  improve the adhesion significantly. Also primer P 7  with coating weight 0.01 g/m 2  and primer P 2  with coating weight 0.1 g/m 2  are good adhesion promoters. 
         [0111]    The reference in both  FIGS. 22 and 23  is PE coated paperboard with corona treatment, and without the use of primer. 
         [0112]    Each primer has a unique coating weight, which gives a maximal adhesion. 
         [0113]    The primers were attached to the paperboard and the PE-film, when corona treatment was used with the extrusion coating. This fact is illustrated in  FIG. 14 . The picture is taken after peel test on an iodine dyed surface of the PE-film. Only primers P 3  and P 6  with priming weight 0.1 g/m 2  have attached to the PE-film only partly. 
         [0114]    When corona treatment is not used in extrusion coating, primers do not promote adhesion, because they do not attach to the PE-film.  FIG. 15  shows the PE-film after the peel test. Some of the chemical pulp is attached to the surface of the PE, but mainly it is not attached to the PE without corona treatment. 
         [0115]    In the following figures SEM-pictures after the peel test are presented. These SEM-pictures have been taken from the paperboard side. Thus, the pictures show the morphology changes after extrusion coating, when they are compared to the SEM-pictures, which have been taken just after the priming. 
         [0116]    The morphology of P 3  does not change if corona treatment was not used with extrusion coating. When corona treatment was used, the primer was spread on the surface of the paperboard. In  FIG. 16 , the picture to the right has been taken at a point, which is not attached to the PE-film. The points where the paperboard primed with P 3  is attached to the PE-film looks like the  FIG. 14 . 
         [0117]    The paperboard with primer P 5  has also been attached partly to the PE-film. The picture to the right in  FIG. 17  was taken at a point, where the paperboard is not attached to the PE. The morphology of the primer P 5  does not significantly change during extrusion coating despite the use of corona treatment. 
         [0118]    The morphology of primed P 6  changed during extrusion coating if corona treatment was used. P 6  spreads on the surface of the paperboard.  FIG. 18  has been taken at a point, where there is no attachement to the PE. Probably the priming weight 0.1 g/m 2  is too much, because the paperboard with P 6  is not attached properly to PE. 
         [0119]    The morphology of P 7  changes in extrusion coating significantly. The fiber is attached to the surface of the paperboard, spreads a bit, and probably absorbed ( FIG. 19 ). Instead the morphology of P 8  is not significantly changed in extrusion coating ( FIG. 20 ). 
         [0120]    The morphology of P 11 , P 12 , and P 13  has changed significantly during the extrusion process ( FIG. 21 ). All of these primers are attached to the surface of the paperboard, primers have spread and probably absorbed to the surface of the paperboard. 
         [0121]    Morphology changes during extrusion process depend on primers. Only connecting issue with primers, which is proved already in peel tests, is that corona treatment in extrusion process improves adhesion significantly. 
       CONCLUSIONS 
       [0122]    This work proves that electrostatic coating methods are suitable for priming. Improvement in adhesion is achieved compared to conventional priming by spreading. Lower priming weights give even better adhesion than higher priming weights. However, primers should preferably be corona treated in extrusion coating when coating paper with polyethylene. Adhesion results shows that every primer have a specific priming weight, which gives a maximal adhesion. 
         [0123]    The correlation between the surface energy values and the adhesion is presented in  FIGS. 24-26 . From these figures can be seen that low polarity improves adhesion. 
         [0124]    In  FIG. 27  is presented the particle size distribution of each primer layer. On the basis of the above, particle sizes affects adhesion. Thus, primer P 12  has excellent adhesion properties, because it has a low proportional polarity and small particle size. Probably the effect of particle size is based on the fact that smaller particles form more adhesive spots per area onto the surface of the paperboard. 
         [0125]    In addition to primer polarity and particle size, adhesion properties change also with different priming weights. Some primers improve adhesion better with priming weight 0.01 g/m 2  than with priming weight 0.1 g/m 2 , and others improve adhesion better with priming weight 0.1 g/m 2 . 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                 APPENDIX 1 
               
             
             
               
                   
               
               
                 Surface energy values by geometric mean of paperboard, primers  
               
               
                 P1-P14, and primed paperboards 
               
             
          
           
               
                   
                 Dispersion 
                 Polarity 
                   
                 Surface 
               
               
                   
                 component 
                 component 
                 Proportional 
                 energy 
               
               
                   
                 [mJ/m 2 ] 
                 [mJ/m 2 ] 
                 polarity 
                 [mJ/m 2 ] 
               
               
                   
               
             
          
           
               
                 Paperboard 
                 21.26 
                 0.02 
                 0.001 
                 21.28 
               
               
                 P 1  
                 20.96 
                 31.41 
                 0.600 
                 52.37 
               
               
                 P 2  
                 22.03 
                 22.72 
                 0.508 
                 44.75 
               
               
                 P 3  
                 22.49 
                 21.73 
                 0.491 
                 44.22 
               
               
                 P 4  
                 22.8 
                 20.35 
                 0.472 
                 43.14 
               
               
                 P 5  
                 22.99 
                 29.35 
                 0.561 
                 52.34 
               
               
                 P 6  
                 25.37 
                 8.36 
                 0.248 
                 33.73 
               
               
                 P 7  
                 26.56 
                 6.65 
                 0.200 
                 33.21 
               
               
                 P 8  
                 28.27 
                 8.64 
                 0.234 
                 36.92 
               
               
                 P 9  
                 23.27 
                 21.78 
                 0.483 
                 45.05 
               
               
                 P 10 
                 24.39 
                 9.38 
                 0.278 
                 33.77 
               
               
                 P 11 
                 24.52 
                 25.75 
                 0.512 
                 50.27 
               
               
                 P 12 
                 25.27 
                 8.74 
                 0.257 
                 34.01 
               
               
                 P 13 
                 18.53 
                 13.87 
                 0.428 
                 32.4 
               
               
                 P 14 
                 19.81 
                 21.35 
                 0.519 
                 41.16 
               
             
          
           
               
                 Primed 0.01 g/m 2   
               
             
          
           
               
                 P 1  
                 21 
                 2.08 
                 0.090 
                 23.08 
               
               
                 P 2  
                 20.96 
                 1.97 
                 0.086 
                 22.93 
               
               
                 P 3  
                 23.17 
                 0.33 
                 0.014 
                 23.49 
               
               
                 P 5  
                 22 
                 0.96 
                 0.042 
                 22.96 
               
               
                 P 6  
                 21.84 
                 1.19 
                 0.052 
                 23.03 
               
               
                 P 7  
                 20.78 
                 1.5 
                 0.067 
                 22.27 
               
               
                 P 11 
                 23.14 
                 0.69 
                 0.029 
                 23.83 
               
               
                 P 12 
                 22.83 
                 0.09 
                 0.004 
                 22.93 
               
               
                 P 13 
                 22.64 
                 0.61 
                 0.026 
                 23.25 
               
             
          
           
               
                 Primed 0.1 g/m 2   
               
             
          
           
               
                 P 1  
                 23.75 
                 0.45 
                 0.019 
                 24.2 
               
               
                 P 2  
                 22.62 
                 0.1 
                 0.004 
                 22.73 
               
               
                 P 3  
                 23.45 
                 0.02 
                 0.001 
                 23.47 
               
               
                 P 5  
                 21.37 
                 1.02 
                 0.046 
                 22.39 
               
               
                 P 6  
                 21.66 
                 0.5 
                 0.023 
                 22.17 
               
               
                 P 7  
                 23.99 
                 0.39 
                 0.016 
                 24.38 
               
               
                 P 8  
                 21.34 
                 1.71 
                 0.074 
                 23.06 
               
               
                 P 11 
                 23.71 
                 0.23 
                 0.010 
                 23.94 
               
               
                 P 12 
                 22.89 
                 0 
                 0.000 
                 22.9 
               
               
                 P 13 
                 19.92 
                 0.17 
                 0.008 
                 20.09