Abstract:
Method of producing a particle or group of particles having a coating of at least two, preferably at least three, outside each other located thin layers of interacting polymers, at which the particle or group of particles is treated in consecutive steps with solutions of the interacting polymers. Excess of the previous polymer is removed between each treatment step alternatively the respective polymer is added only in such an amount in each step that substantially all polymer is absorbed to the particle surface. It is also referred to a paper- or nonwoven product containing fibers and/or fillers containing particles or groups of particles of the mentioned kind.

Description:
TECHNICAL FIELD  
         [0001]    The present invention refers to a method of producing a particle or group of particles having a coating of at least two, preferably at least three, outside each other located thin layers of interacting polymers, at which the particle or group of particles is treated in consecutive steps with solutions of the interacting polymers. The particles consists at first hand of fibers or filler particles. Such a coating modifies the properties of the particles as well as the properties of the products, e g paper and nonwoven, in which the treated fibers and/or filler particles are contained.  
         BACKGROUND OF THE INVENTION  
         [0002]    The increased use of recovered fibers in paper production and the use of components with poorer bonding properties, such as mineral fillers, have increased the need for more effective dry strength agents in the paper. Traditionally two different methods have been used for adding strength improving chemicals to the paper, viz. by adding chemicals at the wet end of the paper process or by surface application by means of a size press. Wet end addition is usually more effective than surface application counted per kg utilized product. In order to maintain the addition made in the wet end in the paper sheet, wet end chemicals are mainly exclusively cationic, and for making them less sensitive to dissolved and colloidal materials and the increased concentration of electrolytes caused by the increased closing of the systems, their cationic charge is usually increased. This leads in turn to a decreased saturation adsorption of the additive chemicals to the fibers, which leads to a reduced maximum effect of the additive chemicals. This involves that there is a need both for new methods of applying strength-improving chemicals to the paper, and new chemical systems.  
           [0003]    Besides there is an increased need for improving the opacity of the finished paper. Since the today most frequently used strength agents contribute negatively to the opacity the need for new methods of developing strength in the paper is further reinforced.  
           [0004]    Such a way would be to utilize size presses to a higher extent, but this would however lead to large reductions of the manufacturing capacity and the production economy since the paper has to be dried once farther depending on the rewet it is exerted to in the size press.  
           [0005]    This involves that there is a great need for new ways of treating fibers and other particles contained in the paper, such as filler particles, in the wet end of the paper machine.  
           [0006]    Also treatment with a similar process such as size presses may be of interest if the quality of the produced paper is herewith increased in a satisfactory way, so that the above mentioned drawbacks will be of less importance.  
           [0007]    It is known to build up thin multilayers of electro active polymers on an electrostatically charged substrate for use in optics, such as sensors, friction reduction etc. This is described in for example in  Thin Solid Films,  210/211 (1992) 831-835 and in  Thin Solid Films,  244 (1994) 806-809. The substrate is herewith immersed alternatingly in diluted solutions of a polycation with an intermediate rinsing in order to remove rests of the previous polyion which is not bonded to the substrate. The thickness of each deposited layer is described to be between 5-20 Å. There is no indication that the treated substrates could be particles, such as fibers.  
           [0008]    In U.S. Pat. No. 5,338,407 there is disclosed a method for improving the dry strength properties of paper, at which an anionic carboxy methyl guar or carboxy methyl hydroxy ethyl guar and a cationic guar is added to the furnish. These two components are either added in mixture or separately. There is no indication that the treatment is made under such conditions that a double layer is built up on the fibers with one component in one layer and the other component in the other layer.  
           [0009]    In the U.S. Pat. Nos. 5,507,914 and 5,185,062 there are disclosed methods for improving the dewatering properties and the retention of paper by adding anionic and cationic polymers to the pulp. There is no indication that the treatment takes place under such conditions that a double- or multilayer is created on the pulp fibers with the anionic component in one layer and the cationic component in the other layer.  
           [0010]    Dual surface treatment of filler particles with anionic and cationic polymers is disclosed in EP-A-0 850 879, WO 95/32335, U.S. Pat. Nos. 4,495,245 and 4,925,530. There is no indication that the treatment takes place under such controlled conditions that a double- or multilayer is created on the pulp fibers with the anionic component in one layer and the cationic component in the other layer.  
         The Object and Most Important Features of the Invention  
         [0011]    The object of the present invention is to provide a method for producing particles or groups of particles, especially fibers and/or filler particles, having a coating of at least two, preferably at least three, outside each other located thin layers of interacting polymers, at which the particle or group of particles is treated in consecutive steps with solutions of the interacting polymers. This has been provided by the fact that excess of the previous polymer is removed between each treatment step alternatively that the respective polymer is added only in such an amount in each step that substantially all polymer is adsorbed to the particle surface.  
           [0012]    The particles or groups of particles may be of optional type, however fibers, e g cellulosic fibers, regenerated fibers and different types of synthetic fibers, and filler particles are mainly concerned.  
           [0013]    The interacting polymers are preferably alternating cationic and anionic polyelectrolytes, but they may also be so called zwitter ions.  
           [0014]    The thickness of each of said thin layers is preferably between 3 and 100 Å, more preferably between 7 and 20 Å.  
           [0015]    The invention further refers to a paper- or nonwoven product, which contains fibers and/or filler particles produced by the method described above. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0016]    [0016]FIG. 1 shows in the form of bar charts the tensile strength index of sheets made of cellulosic fibers with different numbers of applied polymer layers.  
         [0017]    [0017]FIG. 2 shows in the form of bar charts the tensile strength index of paper containing different amounts of filler particles treated according to the invention.  
         [0018]    [0018]FIG. 3 shows a tensile strength index-opacity diagram for paper containing different amounts of filler particles treated according to the invention.  
         [0019]    [0019]FIG. 4 shows in the form of a bar chart the increase of tensile strength of paper containing pulp fibers coated with an anionic and a cationic polymer added at the same time and added separately in six consecutive steps rinsing away the excess of polymer between each step. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0020]    According to the present invention particles or groups of particles, e g fibers or filler particles, are treated with interacting polymers in order to build up thin multilayers of the interacting polymers on the particle surface. In principle the technique described in for example above mentioned articles from  thin Solid Films  is used in case alternating cationic and anionic polyelectrolytes are used as interacting polymers, with the difference that according to the invention the substrate is fibers or other particles or groups of particles.  
         [0021]    The particles are treated in consecutive steps with solutions of the interacting polymers, at which the treatment time for each step is sufficient for forming a layer of the desired molecular thickness, preferably of the magnitude 5-100 Å. The interaction between the particles can be in the form of electrostatic forces, at which the polymers consist of alternating cationic and anionic polyelectrolytes, or by interaction between nonionic polymers by means of e g dispersion forces or hydrogen bonds. Examples of this type of interaction between nonionic polymers are adsorption of polyethylene oxide on unbleached cellulosic fibers and complex formation between polyethylene oxide and polyacrylic acid.  
         [0022]    In case the interacting polymers are alternating cationic and anionic polyelectrolytes the first layer should be a cationic polymer for particles or groups of particles having an anionic surface, which for example is the case for cellulosic fibers, and vice versa. Possible excess of the previous polyelectrolyte can be removed between every treatment step, e g by rinsing with water. Alternatively the addition is controlled in such a way that no excess amount of the respective polymer is added in each step, so that substantially all polymer is adsorbed to the particle surface.  
         [0023]    The method is based on electrostatic attraction between oppositely charged polyelectrolytes for building the desired multilayers. By treating the fibers in consecutive steps with a solution containing polyions of opposite charge and permit these spontaneously to adsorb to the particle surface, multilayers of the stated kind are built up. In principle all types of polyelectrolytes may be used.  
         [0024]    Through such a treatment of particles or groups of particles, such as fibers, it is possible to make new types of surface modifications to them. By for example treating fibers with consecutive layers of hydrophobic, charged polyelectrolytes it would be possible for example to develop new types of hydrophobizing chemicals for the hydrophobization of paper. It would also be possible to build up “intelligent” surface layers on fibers, which alter the properties with temperature, pH, salt content etc. These changes could for example be based on fundamental knowledge about modern theories on interaction between polymers and surfactants.  
         [0025]    Further applications are ion-exchanging fibers where “membranes” with ion-exchanging properties are provided on the fiber surface, wet strength agents where the added polymers are reactive with the fibers and with each other, in order to provide permanent bonds between the fibers and for the production of highly swelling surface layers, where the added chemicals form swollen gel structures on the fiber surface for use in absorbent hygiene products. Another possible application are new types of fibers for printing paper, where the adsorbed polymers change colour when they are exerted to an electric, magnetic or electromagnetic field. Such polymers are available today.  
         [0026]    The fibers that are treated with the method according to the invention can be of optional kind, natural as well as synthetic fibers. Mainly cellulosic fiber are concerned. However it would be possible to treat synthetic fibers, for example for giving them a more hydrophilic surface.  
         [0027]    Also groups of fibers or particles can be treated according to the method.  
         [0028]    Examples of suitable anionic and cationic polyelectrolytes which may be used in the method according to the invention are given below.  
         [0029]    Anionic polyelectrolytes: Anionic starch with different degrees of substitution, polystyrene sulphonate, carboxy methyl cellulose with different degrees of substitution, anionic galactoglucomannan, polyphosphoric acid, polymethacrylic acid, polyvinyl sulphate, alginate, copolymers of acryl amide and acrylic acid or 2-acrylic amide-2-alkylpropane sulphonic acid.  
         [0030]    Cationic polyelectrolyte: Cationic galactoglucomannan, polyvinyl amine, polyvinyl pyridine and its N-alkyl derivatives, polyvinyl pyrrolidone, chitosan, alginate, modified polyacryl amides, polydiallyl dialkyl, cationic amide amines, condensation products between dicyane diamides, formaldehyde and an ammonium salt, reaction products between epichlorhydrine, polyepichlorhydrine and ammonia, primary and secondary amines, polymers formed by reaction between ditertiary amines or secondary amines or dihaloalkanes, polyethylene imines and polymers formed by polymerisation of N-(dialkylaminoalkyl)acrylic amide monomers.  
       EXAMPLE 1  
       [0031]    The example below shows the increase of tensile strength of sheets made in a dynamic sheet former. The pulp that is used was bleached SWK (softwood sulphate pulp) beaten in accordance with SCAN-C 18:65, diluted to 3 g/l and pH adjusted to 8. PVAM (polyvinyl amine), a cationic polymer, was added in excess and was given time to react after which the excess of polymer was washed away from the fiber suspension by means of water. After that CMC (carboxy methyl cellulose), an anionic polymer, was added in excess, and after 10 minutes non-adsorbed polymer was removed through washing. Admixture of PVAm and CMC was repeated in several steps. After each addition of CMC the so called dynamic sheet former was used for making sheets having a basis weight of 80 g/m 2 . The sheets were tested with respect to tensile strength according to SCAN-P 67:93. The results are shown in Table 1 below and proves clearly an improvement of tensile strength index with the number of applied polymer layers.  
                   TABLE 1                       Polymer layer (P = PV Am, C = CMC)   Tensile strength index (kNm/kg)                   No polymer   39.0       P   53.6       PC   56.6       PCPC   69.4       PCPCPC   75.8                  
 
         [0032]    The results are also shown in FIG. 1 in the drawings.  
       EXAMPLE 2  
       [0033]    In this example fillers for papermaking have been used which have been treated through multilayer adsorption with the same polymers as in Example 1 above, i e polyvinyl amine and carboxy methyl cellulose. Paper sheets of 80 g/m 2  were made in a dynamic sheet former. The sheets were tested with respect to ash content, tensile strength index and opacity. The results are shown in Table 2 below. Ash content here means the content of filler treated as above and which has been added to the paper.  
                                             TABLE 2                               Tensile           Polymer layer       strength index   Opacity       (P = PV Am, C = CMC)   Ash content (%)   (kNm/kg)   (%)                                No polymer   0.0   47.2   78.8       No polymer   20.6   19.7   87.1       P   7.5   38.3   82.9       P   14.3   30.5   86.7       P   21.7   23.8   89.5       PCP   7.5   37.9   83.5       PCP   13.9   31.7   87.8       PCP   24.3   26.6   90.7       PCPCP   9.0   40.0   84.7       PCPCP   18.0   35.9   87.0       PCPCP   28.0   30.0   191.0                   
 
         [0034]    The results are also shown in FIG. 2 and  3 . FIG. 2 shows the effect on strength in the paper containing different amounts of filler particles treated according to the invention. FIG. 3 shows the effect on the opacity of paper when using different amounts of fillers treated according to Example 2 above.  
       EXAMPLE 3  
       [0035]    Two types of polymers were used: an anionic polyacryl amide (A-PAM), Percol 155 from Ciba, and a cationic polydimethyl diallyl ammonium chloride (DMDAAC) also from Ciba. The pulp that was used was unrefined, fully bleached longfiber pulp, pH 8 in all tests. The dosing of the polymers were done in two different ways:  
         [0036]    A) all at the same time and in this case 3.9 kg/ton A-PAM was added first and then 6.6 kg/ton polyDMDAAC was added.  
         [0037]    B) Six layers of 1.1 kg/ton polyDMDAAC and 0.65 kg/ton A-PAM in the respective layers were added. Excess of polymer was removed between the dosings.  
         [0038]    Handsheets were then made and the strength (tensile strength index) was measured. In FIG. 4 the strength increase in % is shown for the two different cases. As can clearly be seen it is much more effective to add the polymers in layers in a controlled manner.