Method for producing a filter material

Abstract

A method for producing a filter material comprising a cellulosic material and a positively charged polyelectrolyte comprising polyvinylamine (PVAm) wherein the method comprises at least the steps of: providing a feedstock comprising the cellulosic material, the cellulosic material comprising cellulosic fibers; the polyvinylamine is added to the feedstock such that the concentration of PVAm in the feedstock is in the range of 0.5-2.0 wt% of the dry weight of cellulosic fibers in the polyelectrolyte relative to the feedstock, and the cellulosic material adsorbs the polyvinylamine.

Description

Technical Field

[0001] The invention relates to a method for producing a filter material comprising cellulose fibers.

[0002] The invention also relates to a filter material produced by the process according to the invention. Background Art

[0003] In recent years, people have become more aware of the need for indoor and outdoor air filtration and purification.

[0004] The focus on energy efficiency and indoor air quality has led to the emergence of many air filtration products (such as HEPA filters, etc.) that are designed to remove small particles, allergens and even microorganisms from the air.

[0005] Many of these purification technologies and practices are costly, energy inefficient, and / or require significant technical expertise and complexity. Traditional approaches to reducing these complexities require extensive handling or specially designed equipment. Unfortunately, the development of low-cost technologies does not adequately address the removal of harmful chemical and biological contaminants (e.g., bacteria and viruses). For example, simple point-of-use purification devices (e.g., filters attached to room air supply ducts or portable masks) do not adequately remove oil mist, particles from fires, bacteria, and viruses unless relatively expensive membrane technologies or strong chemical oxidants (e.g., halogens or reactive oxygen species) are used.

[0006] It is well known to use particles, microparticles or fibers of natural or synthetic materials for filtration. These materials are usually used alone or in mixtures. In some cases, a material (called a binder) is used to hold the individual particles or fibers together. The prior art describes techniques for producing porous carbon blocks using polymer binders.

[0007] Therefore, there is still a need in the field of filtration for simple, safe, inexpensive filter materials as starting materials for the production of different types of filters. There is also a need for filter materials made from natural raw materials that are biodegradable and derived from renewable resources. Summary of the invention

[0008] The object of the present invention is to eliminate at least some of the disadvantages of the prior art and to provide a method for producing a filter material according to claim 1 .

[0009] In a first aspect of the present invention, there is provided a method for producing a filter material, the filter material comprising a cellulose material and a positively charged polyelectrolyte, the polyelectrolyte comprising polyvinylamine (PVAm), wherein the method comprises at least the following steps:

[0010] - providing a raw material comprising the cellulosic material, the cellulosic material comprising cellulose fibers;

[0011] - adding the polyvinylamine to the feedstock such that the concentration of the polyvinylamine in the feedstock is in the interval of 0.5-2.0 wt-% of the dry weight of the cellulose fibers in the feedstock, and

[0012] - allowing the cellulosic material to absorb the polyvinylamine.

[0013] The process of the present invention is a wet forming process, wherein the feedstock is an aqueous suspension comprising a cellulosic material.

[0014] In one embodiment, the method preferably further comprises the following steps:

[0015] - forming a wet web of a feedstock comprising a cellulosic material having adsorbed polyelectrolyte;

[0016] - dewatering the wet web by pressing, and

[0017] - Drying the wet web.

[0018] In one embodiment, the method for producing a filter material according to claim 1 further comprises a wet forming process, the wet forming process comprising the following steps:

[0019] - providing a forming tool comprising a three-dimensional (3D) shape of a forming portion;

[0020] - bringing a forming tool of said 3D shape comprising a forming part into contact with said raw material;

[0021] - Applying a vacuum suction device so that the raw material is sucked into the forming part and a 3D filter material of a desired thickness is formed on the forming part;

[0022] - removing the 3D filter material from the shaped portion, and

[0023] - Dehydrating the 3D filter material by pressing and / or drying to form a shaped 3D filter material.

[0024] The method of the present invention can produce a filter material with greatly improved air permeability. Since the filter material has high hydrophobicity, it will not absorb moisture and has a good filtering effect on hydrophobic substances.

[0025] The dry web of filter material can be rolled into filter material rolls and cut at a later stage into filter material of suitable size, ie filter material sheets, for use, for example, in ventilation systems and fan systems or as starting material for the production of, for example, face masks and mouthguards.

[0026] The shape of the formed 3D filter material is preferably complementary to the system or device in which it is to be used, or adapted to better fit a person's face.

[0027] Polyelectrolytes are polymers with electrolyte groups in their repeating units. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers electrically charged. Polyelectrolytes therefore have properties similar to those of electrolytes (salts) and polymers (high molecular weight compounds), and are sometimes called polysalts. Like salts, their solutions conduct electricity.

[0028] The polyelectrolyte added to the raw material is a positively charged polyelectrolyte. The positively charged polyelectrolyte is adsorbed on the surface of the cellulosic material, and may be adsorbed on the surface of the negatively charged fibers of the cellulosic material. The positively charged polyelectrolyte adsorbed on the fibers of the cellulosic material affects the negative charge of the cellulosic fibers by modifying the negative charge of the fibers to be less negative or changing the charge of the fibers to a net zero charge of the cellulosic material. For highly positively charged polyelectrolytes adsorbed on the cellulosic material, the net charge of the cellulosic material may even be positive.

[0029] The positively charged polyelectrolyte is preferably selected from polyvinylamine (PVAm), polyacrylamide, chitosan, cationic gelatin, polydiallyldimethylammonium chloride (polyDADMAC), polyallylamine and / or polyvinylamine.

[0030] In one embodiment, the polyelectrolyte is preferably polyvinylamine (PVAm), including unmodified PVAm or PVAm modified with linear or branched and optionally substituted alkyl chains. PVAm has a very high content of primary amine functional groups and is one of the technical polymers with the highest charge density.

[0031] The PVAm may preferably be unmodified.

[0032] The PVAm is added to the feedstock so that the PVAm concentration in the feedstock is greater than 0.01 wt-% PVAm based on the dry weight of cellulose fibers contained in the feedstock.

[0033] Preferably, the PVAm concentration in the feedstock is equal to or greater than 0.1 wt-% PVAm, more preferably equal to or greater than 0.5 wt-%.

[0034] Preferably, the PVAm concentration in the feedstock is 0.5-2.0 wt-%, based on the dry weight of cellulose fibers contained in the feedstock.

[0035] In one embodiment, the method comprises adjusting the pH of the feedstock to the interval of pH 7 to pH 11; preferably in the interval of pH 8 to pH 10.5, more preferably in the interval of pH 9 to pH 10.

[0036] In one embodiment, the method comprises adding NaCl to the raw material and adjusting the NaCl concentration to 0.1-1.2 wt-% based on the weight of the cellulose fibers contained in the raw material, preferably 0.2-1.0 wt-% per cellulose fiber weight, more preferably 0.3-0.8 wt-% per cellulose fiber weight.

[0037] Adding salt to a polyelectrolyte solution can effectively reduce or screen the electrostatic repulsions within the polyelectrolyte chain and between different parts of the polyelectrolyte chain, which in turn makes the polyelectrolyte chain less rigid and thus allows it to curl up. In the same way, the charges between adsorbed polyelectrolytes are also screened, so in practical cases, the amount adsorbed to the surface can be increased when salt is added.

[0038] In one embodiment of the method of the present invention, the steps of the method are carried out in the following order: NaCl is added to the raw material. The addition of NaCl is followed by a step of adjusting the pH value. After adjusting the pH value of the raw material, a step of adding a polyelectrolyte to the raw material is carried out.

[0039] In one embodiment, the cellulosic material preferably comprises high yield pulp (HYP), in which individual cellulosic fibers are separated from wood raw materials, after softening the wood lignin at high temperature, by mechanical treatment of wood chips in a disc refiner or logs in a wood grinder and / or by chemical pretreatment, such as mechanical pulp, refiner mechanical pulp (RMP), thermomechanical pulp (TMP), chemothermomechanical pulp (CTMP), defiberized fiber material, high temperature chemothermomechanical pulp (HTCTMP), chemothermomechanical pulp (CMP), stone ground wood (SGW) and pressure ground wood (PGW) or mixtures thereof. The wood yield in these types of pulping processes is very high, usually exceeding 90%.

[0040] In one embodiment, the polyelectrolyte is preferably PVAm, and the cellulosic material preferably comprises HYP, preferably CTMP, and possibly mixed with other kinds of HYP. The dry weight of the prepared filter material is preferably in the interval of 10-240 gsm.

[0041] In one embodiment, the polyelectrolyte is preferably PVAm and the cellulosic material is preferably kraft pulp, sulfate pulp, sulfite pulp, recycled paper and paperboard, waste paper, nanopulp, dissolving pulp, deinked pulp (DIP) or recycled fiber or a mixture thereof.

[0042] In one embodiment, the polyelectrolyte is preferably PVAm, and the cellulosic material preferably comprises kraft pulp, and the dry weight of the prepared filter material is preferably in the range of 10-240 gsm.

[0043] In one embodiment, the polyelectrolyte is preferably PVAm, and the cellulosic material is preferably non-wood pulp, such as straw pulp, hemp pulp, bagasse pulp, and the like.

[0044] The present invention also relates to a filter material comprising a cellulose material and a positively charged polyelectrolyte, wherein the polyelectrolyte comprises polyvinylamine (PVAm), wherein the cellulose material comprises cellulose fibers, and wherein the positively charged polyelectrolyte comprises polyvinylamine (PVAm), adsorbed on the cellulose fibers.

[0045] Filter materials comprising cellulose materials with a modified, less negative charge have the advantage of providing higher air permeability, i.e. lower air resistance. This means that less energy is required to press / suck / push air through the filter material compared to conventional filter materials.

[0046] Since the fibers are modified to be less negatively charged or even uncharged, the filter material has improved hydrophobicity. Hydrophobic fiber structures are preferred because hydrophobic filter materials do not absorb water and become wet, while the adsorption capacity of hydrophobic substances (such as hydrocarbons, for example grease, oil, oil mist, fat, non-polar particles and carbon compounds) is greatly improved.

[0047] In addition, since the negative charge of the fiber is modified to be less, the lipophilicity of the filter material is improved. The absorption capacity of the filter material for lipids and lipid particles will be improved.

[0048] It has also surprisingly been found that the fibrous structure is easier to dehydrate if the negative charge of the fibers is modified to be less negative. An easier to dehydrate fibrous structure means that less energy is required to dehydrate the fibrous structure during the production of the filter material.

[0049] For positively charged polyelectrolytes, bacteria and / or viruses are captured and bound to the filter material, probably due to the fact that the positively charged polymer adsorbed onto the cellulose material attracts negatively charged bacteria and / or viruses.

[0050] The filter material is preferably a filter material net, a filter sheet, a shaped three-dimensional filter material, a foam filter material or a mixture thereof. The filter material net can be easily cut into appropriate sizes and shapes to fit any ventilation device or to be converted into a mask or a mouth guard mask.

[0051] In one embodiment, the polyelectrolyte is preferably PVAm and the cellulosic material preferably comprises HYP, preferably CTMP, possibly mixed with other types of HYP. The dry weight of the produced filter material is preferably in the interval of 10-240 gsm.

[0052] The filter material can be used, for example, as a ventilation filter or air filter in a fan system. The filter material can also be used in face masks and mouth guard masks. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The above aspects and many attendant advantages of the present invention will become more readily understood by reference to the following detailed description taken in conjunction with the accompanying drawings, in which:

[0054] The present invention will be described in more detail with reference to the accompanying drawings, in which:

[0055] Figure 1 The drainage results for CTMP and kraft pulps with and without 1 ml / g PVAm fibers are shown;

[0056] Figure 2 The WRV results of CTMP and kraft pulp with and without 1 ml / g PVAm fibers are shown;

[0057] Figure 3 CTMP 20 and 100 g / m 2 The moisture content (g / ) and vacuum dehydration residence time (ms) with and without 1 ml / g PVAm fibers;

[0058] Figure 4 Kraft pulp 20 and 100 g / m 2 The moisture content (g / ) and vacuum dehydration residence time (ms) with and without 1 ml / g PVAm fibers;

[0059] Figure 5 Shows 60g / m 2 Force-stress curves of CTMP with and without the addition of 1 ml / g PVAm fibers;

[0060] Figure 6 Shows 200g / m 2 Force-strain curves of CTMP with and without the addition of 1 ml / g PVAm fibers (washed and unwashed CTMP);

[0061] Figure 7 Shows 200g / m 2 Force-stress curves of kraft pulp with and without the addition of 1 ml / g PVAm fibers;

[0062] Figure 8 Air permeability calculated according to Bendtsen, with 0.7 kPa instead of 1.47 kPa, is shown for single and double layers of 200 g / m with and without 1 ml / g PVAm fibers. 2 CTMP (washed and unwashed CTMP),

[0063] Fig. 9 Air permeability calculated according to Bendtsen, with 0.7 kPa instead of 1.47 kPa, is shown for single and double layers of 200 g / m with and without 1 ml / g PVAm fibers. 2 Kraft pulp. DETAILED DESCRIPTION

[0064] Before the present invention is disclosed and described in detail, it should be understood that all percentages mentioned are calculated by weight unless explicitly stated. The term "about" associated with numerical values ​​throughout the specification and claims represents the precision interval familiar and accepted by those skilled in the art. The interval is ±10%.

[0065] Cellulosic material in this context means a fibrous material comprising cellulose fibers. The cellulose material can be in a dry or moist state. The cellulose material in dry form has a fibrous structure. The cellulose material can be diluted, for example, in an aqueous solution to form an aqueous pulp suspension known as a stock. The cellulose material can also be a wet or dry paper web or sheet. Different types of cellulose materials are illustrated below.

[0066] By feedstock is meant an aqueous slurry solution, suspension comprising cellulosic material; ie comprising cellulosic fibers.

[0067] The following detailed description and the examples included therein are intended only to describe and illustrate certain embodiments of the present invention and are not intended to limit the scope of the present invention in any way.

[0068] experiment

[0069] fiber

[0070] Chemical thermomechanical pulp (CTMP) fibers of Norway spruce (Picea abies) (hereinafter referred to as CTMP fibers) were supplied by Rottneros AB (Sunne, Sweden). According to the supplier, the yield of CTMP fibers was 93-95% and the lignin content was 20-25%. The fine fiber content measured by online Pulpeye was 35% with a standard deviation of 4.5%. The pH value of CTMP was pH 7.

[0071] Bleached chemical softwood pulp fiber (hereafter referred to as kraft fiber) of a mixture of Norway spruce and Scots pine (Pinus sylvestris) was supplied by Stora Enso AB (Skoghall, Sweden). The kraft fiber was refined in a mill. The pH value of kraft pulp was 8.4.

[0072] Chemicals

[0073] Polyvinylamine (PVAm) was supplied by BASF SE (Ludwigshafen, Germany). The average molecular weight of PVAm was 340,000 g / mol.

[0074] Both CTMP fiber and Kraft pulp were prepared at 0.2% w / w with and without PVAm. Before the addition of PVAm, the starting pH of the CTMP feedstock and the Kraft pulp feedstock were pH 7 and pH 8.4, respectively. Solutions containing PVAm were prepared according to the following procedure. 5.7 ml of 2M NaCl was added per gram of dry pulp. The pH was adjusted to >9.5. 1 ml / gram of fiber of PVAm (10 g / l) was then added to the solution.

[0075] Fiber potential analysis

[0076] The fiber charge of cellulose materials with and without PVAm was analyzed based on Zeta potential. Cellulosic materials without PVAm addition were compared with cellulose materials with PVAm addition. The cellulose materials analyzed were CTMP and Kraft pulp.

[0077] 6.5 grams of cellulosic material were dissolved in 1 liter of water. The Zeta potential of the fiber suspension was measured twice in a fiber potential analyzer. After the measurement, 0.1 wt-% NaCl was added per weight of cellulose fiber, the pH was then adjusted to 9.6, and 6.5 ml of PVAm (10 g / l) was added, i.e. 1 wt-% PVAm per dry weight of cellulose fiber. The suspension was mixed, allowed to stand for 10 minutes, and then rinsed thoroughly with water three times to ensure that there was no free PVAm in the suspension. The Zeta potential of the suspension containing PVAm was measured twice in a fiber potential analyzer.

[0078] One of the CTMP solutions containing PVAm was half-washed, where about 40% (6 liters out of 16 liters) of the total volume of the non-slurry phase in the solution was replaced with fresh water without adding any chemicals. The PVAm already attached to the fiber surface is believed to be unaffected by the washing, and it is more likely that only the proportion of chemicals in the water was affected.

[0079] Dehydration, tensile and air permeability testing

[0080] Dewatering of the different slurries was examined by measuring dewatering resistance (°SR), water retention values ​​(g / g) and vacuum dewatering using a commercial forming fabric (SSB type, air permeability 325 cfm) in a custom built laboratory vacuum suction box. Prior to vacuum dewatering, isotropic sheets were formed in a manual sheet former and the stock was thoroughly stirred to ensure consistent forming. The equipment used in this study was the same as described in Granevald, R., Nilsson, L. S. and Stenstrom, S. in the article "Impact of different forming fabric parameters on sheet solids content during vacuum dewatering", Nordic Pulp & Paper Research Journal (2004), 19(4), p. 428-433, except that a plate with a single 5 mm opening was used. The vacuum level was set to -40 kPa and the basis weights on the sheets were 20 and 100 g / m 2 , the residence time is 0, 1, 2.5, 5, 10, 20 ms. After each test, the dryness is measured according to ISO638:2008.

[0081] The sheets were also formed in a standardized sheet former, flat pressed and dried at pressures of 60 and 200 g / m2. The sheet former includes stirring of the raw materials to achieve consistent forming. The sheets were dried in a standard climate according to ISO187:1990, where all tensile and air permeability tests were also performed. 2 The sheets were tested for air permeability. However, the pressure for the air permeability test was adjusted to a lower pressure than the standard because the sheets containing PVAm were too open during the fiber forming process to use the original setting. The air permeability measurements in this study were performed in the machine at 0.7 kPa pressure instead of 1.47 kPa. According to ISO1924-3:2008, the air permeability of 60 and 200 g / m 2 The sheets were subjected to tensile testing.

[0082] Results and Discussion

[0083] Figure 1 and Figure 2The results of the drainage resistance (°SR) and water retention value (WRV) of the tested pulps are shown. Examining the different pulps under study, the drainage simulates early dewatering according to Schopper-Riegler, i.e. water flows between the fibers during the early stages of formation, and the water retention value simulates later dewatering around a high vacuum suction box or wet press, with significantly higher initial dryness. The drainage and water retention results ( Figure 1-2 ) showed that the addition of PVAm mainly affected early dehydration, presumably by preventing the healing mechanism and blockage of the flow channel. Based on the WRV results, under these conditions, the addition of PVAm had little effect on late dehydration, which can be explained by the fact that the addition of PVAm does not affect the swelling of the fibers, but rather affects the flow behavior in low-viscosity solutions. However, no evidence was provided regarding fiber swelling.

[0084] The dehydration results are expressed as the change of water content (g / g) with vacuum residence time (ms). Figure 3-4 Dehydration behavior measured by laboratory-scale vacuum dehydration equipment ( Figure 3-4 ) also showed that the early stages of dewatering were most affected by PVAm addition. This can be explained by the same repair and flow channel blocking mechanisms as drainage (°SR). There was a significant difference in the dewatering effect at vacuum residence times of 0-5 ms, where PVAm addition dewatered more effectively than the corresponding reference slurry. When comparing 20 to 100 g / m 2 When adding PVAm to the sheet, the greater the dehydration resistance, the greater the effect. After adding PVAm, the dehydration rate of CTMP pulp is higher than that of kraft pulp.

[0085] At longer residence times (10-20ms), the addition of PVAm resulted in longer dewatering times, which could explain the interaction between the added chemicals and the formed fabric. At the end of the test series, the permeability of the formed fabric was significantly reduced, which naturally led to slower dewatering. This is not to say that the reasoning behind the dewatering mechanism is self-contradictory, but it is an important observation. When PVAm is added in this way on an industrial scale, the formed fabric cannot be sealed by the chemicals. The sealing behavior must be prevented. This study did not include such an investigation, but this issue could probably be addressed by performing an additional washing step after the addition of the chemicals. In addition, poor forming will lead to less efficient vacuum dewatering, and air will flow easily through patches with fewer fibers and leave more water in areas with more flocculation.

[0086] Figure 5-7 The strength measurements of kraft paper and CTMP slurries with and without PVAm are shown in the form of stress-strain curves, where Figure 5 The results show that the 60 g / m2 fiber with and without the addition of 1 ml / g PVAm 2The force-stress curve of CTMP was repeated 10 times according to the standard (ISO 1924-3:2008). Figure 6 The results show that the addition and non-addition of 1 ml / gram PVAm fiber 200g / m 2 The force-stress curve of CTMP was repeated 10 times according to the standard (ISO 1924-3:2008). The slurry with PVAm added was either washed as described above or not. Figure 7 The results show that the addition and non-addition of 1 ml / gram PVAm fiber 200g / m 2 The force-stress curve of kraft pulp was repeated 10 times according to the standard (ISO 1924-3:2008).

[0087] From the tensile test results of the different sheets, no significant differences were found between the reference and PVAm additions, although PVAm is generally considered a dry strength agent ( Figure 3-4 This is because the formability of PVAm sheets is significantly poorer, 200g / m 2 The tensile results of PVAm CTMP sheets vary widely ( Figure 3 The strength property advantage gained by adding PVAm is offset by a decrease in formability. One of the slurries did have significantly higher tensile strength when washed before forming ( Figure 3 ). The higher tensile strength of the washed PVAm slurry is due to the fact that the washed slurry is more similar to the reference slurry than the unwashed slurry and therefore has better formability. Washing removes some of the chemicals in the water that correspond to poor formability, but the strength-enhancing properties associated with PVAm attached to the fibers remain.

[0088] Figure 5-7 Strength measurements of sheets of Kraft and CTMP slurries with and without PVAm are shown in Figure 2 in the form of force-strain curves. When looking at the tensile test results for the different sheets, there is no significant increase in the tensile stress exponent between the reference and PVAm additions, even though PVAm is generally considered a dry strength agent ( Figure 5 ). The strength property advantage gained by adding PVAm is offset by reduced formability. One of the slurries was washed before forming and had significantly higher tensile strength (Table 2). The higher tensile strength of the washed PVAm slurry compared to the unwashed slurry is due to the fact that the washed slurry is more similar to the reference slurry than the unwashed slurry and therefore has better formability. Washing removes some of the chemicals in the water that correspond to poor formability, but the strength-enhancing properties associated with the PVAm attached to the fibers remain.

[0089] Table 1-3 below shows the yield stress (σ T W), yield strain (ε T ), elastic modulus (E W ) and tensile energy absorption (W T W ), mean, standard deviation, and confidence interval.

[0090] Table 1. 60 g / m with and without PVAm 2 CTMP sheet tensile measurement statistics

[0091]

[0092] Table 2. 200 g / m2 with and without PVAm, washed and unwashed 2 Statistics of tensile measurements of CTMP sheets.

[0093]

[0094] Table 3. 200 g / m with and without PVAm 2 Statistics on tensile measurements of kraft paper sheets

[0095]

[0096] Figure 8-9 The air permeability calculated by Bendtsen for single and double sheets is shown, with 0.7 kPa instead of 1.47 kPa. The CTMP calculated by Bendtsen for 200 g / m 2 Air permeability of single and double sheets with 0.7 kPa instead of 1.47 kPa, with and without 1 ml / g PVAm fibers (washed and unwashed). Note that the CTMP PVAm unwashed single sheet could not be measured because the flow rate in the machine was too high and the value was not 120 μm / Pa*s. The error bars represent 95% confidence intervals ( Figure 8 ). shows the Bendtsen calculated 200 g / m 2 Air permeability of single and double sheets of Kraft pulp with 0.7 kPa instead of 1.47 kPa, with and without 1 ml / g PVAm fibers (washed and unwashed). Note that the flow rate for the Kraft reference double sheet is zero. Error bars represent 95% confidence intervals ( Fig. 9 ).

[0097] Air permeability ( Figure 8-9) give clear results, namely that the addition of PVAm in all cases results in a more open sheet structure, allowing more air to flow through. The higher air permeability can be related to the increased flocculation, which results in poorer formability of the PVAm sheets than the reference sheets, and also to the bonding of fines to the fiber surface. In both cases, the resulting sheets are more open to flowing air. Figure 8 showed that the washed slurry had less effect on air permeability, which is consistent with the observations related to formability in tensile tests, where most of the chemicals in the water were removed.

[0098] Tables 4 and 5 list the fiber potential analysis results of CTMP and kraft pulp, respectively.

[0099] Table 4. Zeta potential measurement statistics of CTMP suspensions / stocks with and without PVAm.

[0100]

[0101] The results in Table 4 clearly show that the addition of PVAm to the CTMP suspension increases the Zeta potential from a mean of -87.4 mV to a mean of -5.7 mV, i.e., an increase of 81.7 mV in the Zeta potential.

[0102] The measured zeta potential values ​​indicated that the electronegativity of the negatively charged CTMP decreased due to the addition of the positively charged PVAm.

[0103] Table 5. Zeta potential measurement statistics of Kraft pulp suspensions / feedstocks with and without PVAm.

[0104]

[0105] The results in Table 5 clearly show that the addition of PVAm to the kraft pulp suspension increases the Zeta potential from a mean of about -113 mV to a mean of about 92 mV, which is an increase of 205 mV in Zeta potential.

[0106] The measured zeta potential values ​​showed that the negatively charged kraft pulp had become positively charged due to the addition of positively charged PVAm.

[0107] Air permeability measurement

[0108] Because of the unexpected discovery that the air permeability of cellulose sheets produced in the presence of PVAm was improved, as described above, additional air permeability measurements were performed. The results are listed in Table 6.

[0109] Cellulosic sheets containing unwashed CTMP, 100 gsm, and six different amounts of added PVAm were tested along with a reference cellulose sheet without added PVAm.

[0110] Table 6. Air permeability measured at 125 Pa pressure, area 20 m 2 , expressed in cubic feet per minute (CFM). All PVAm percentages in the table are weight percentages based on the dry weight of the fiber.

[0111] PVAm 0％ 0,01％ 0,10％ 0,50％ 1,0％ 2,0％ 6,0％ CFM1 2,22 2,28 5,88 19 16,3 18 4,53 CFM2 2,14 2,08 11,4 16,4 7,67 24 4,26 CFM3 1,95 1,85 18,7 13,5 6,39 14,7 3,8 MV1-3 2,10 2,07 11,99 16,30 10,12 18,90 4,20 STDEV 0,14 0,22 6,43 2,75 5,39 4,71 0,37

[0112] Adding very small amounts (0.01 wt-%) of PVAm seemed to have no effect on air permeability. The mean air permeability of the CTMP sheets with 0.01 wt-% PVAm addition was at the same level as the air permeability of the reference CTMP sheets; 2.07 CFM and 2.10 CFM, respectively.

[0113] However, the addition of 0.1 wt-% PVAm increases the air permeability of the CTMP sheets by about six times, i.e., by about six times compared to the reference sheets without the addition of PVAm.

[0114] The highest air permeability value (18.90 CFM) was measured for the CTMP sheet with 2.0 wt-% PVAm addition, i.e., a nine-fold increase compared to the reference sheet.

[0115] Further increasing the amount of PVAm added, to 6.0 wt-% PVAm, resulted in a decrease in air permeability. For 6.0 wt-% PVAm, the measured air permeability was 4.20 CFM, which means that the amount of air flowing through the CTMP sheet containing PVAm is twice as much, i.e., a factor of 2, compared to the CTMP reference without PVAm added.

[0116] The results lead to the conclusion that in order to increase the air permeability of the filter material to be produced, the PVAm concentration of the raw material is appropriately greater than 0.01 wt-%.

[0117] Furthermore, concentrations above 6.0 wt-% PVAm are most likely to increase air permeability compared to the CTMP reference. However, adding PVAm at a higher concentration than required to increase the air permeability of the filter material may be meaningless.

[0118] The conclusions drawn from these experiments are as follows:

[0119] Air permeability is significantly higher for sheets made from unwashed pulp with PVAm added because the presence of PVAm may limit the self-repairing mechanism and flow channel blocking mechanism during dewatering. In the absence of PVAm during dewatering, the self-repairing mechanism occurs. Uncovered patches on the forming fabric increase water outflow at these locations, resulting in more fiber coverage of the holes. However, if PVAm is present in the early dewatering stages, it is recommended to add PVAm to limit self-repairing and maintain the increased dewatering flow due to poor forming fabric formability and uncovered patches.

[0120] The initial dewatering rate is faster for PVAm-added slurries, and this effect is greater for CTMP and higher basis weights. The enhanced dewatering effect also depends on the bonding mechanism of PVAm and its effect on self-healing and flow channel clogging.

[0121] Strength increases with the addition of PVAm, but only if the slurry is washed before sheet forming. Unwashed slurries containing PVAm may produce poor formability, resulting in lower slurry strength, even though local areas may be stronger due to the presence of PVAm.

[0122] Wet-end addition of PVAm was effective for both CTMP and Kraft pulps under the conditions used in this study. Addition of PVAm to the stock suspension at the wet end of the paper machine has great potential for applications requiring high air permeability products. PVAm can also be used as a dewatering enhancer, but attention must be paid to the potential for formability issues.

[0123] An example of the method of the present invention for producing a filter material will now be described.

[0124] The headbox of the paper machine is filled with an aqueous pulp suspension (also referred to as "stock") having a consistency of 0.05-10 wt% fiber, preferably 0.1-3 wt% fiber, more preferably 0.1-0.3 wt% fiber. The pulp is any one of chemothermomechanical pulp (CTMP), defiberized fiber material, high temperature chemothermomechanical pulp (HTCTMP), chemothermomechanical pulp (CMP), stone groundwood (SGW) and pressure groundwood (PGW) or a mixture thereof, kraft pulp, sulfite pulp, unbleached chemical pulp, defiberized fiber material, bagasse, straw, hemp, bamboo, DIP, recycled paper and paperboard, waste paper, RMP, CSP NSSC nano pulp, dissolving pulp, recycled fiber or a mixture thereof.

[0125] A positively charged polyelectrolyte is added to the feedstock.

[0126] Preferably, the polyelectrolyte is PVAm.

[0127] The PVAm is added to the feedstock such that the PVAm concentration in the feedstock is greater than 0.01 wt-% based on the dry weight of cellulose fibers contained in the feedstock.

[0128] The preferred concentration of PVAm in the feedstock may be equal to or higher than 0.1 wt-% based on the dry weight of cellulose fibers contained in the feedstock.

[0129] Preferably, the PVAm concentration in the feedstock may be equal to or higher than 0.5 wt-% based on the dry weight of cellulose fibers contained in the feedstock.

[0130] A more preferred concentration of PVAm in the feedstock may be 0.5-2.0 wt-%.

[0131] Preferably, PVAm is added to the stock inside or before (upstream) the headbox of the paper machine.

[0132] It may be preferred to adjust the pH of the feedstock.

[0133] The pH of the raw material can be adjusted so that the pH of the raw material is in the range of pH 7 to pH 11, preferably in the range of pH 8 to pH 10.5, and more preferably in the range of pH 9 to pH 10. The pH adjustment is not limited to a specific acid or base, but can be accomplished by adding any conventional acid (e.g., H2SO4, HCl) or base (e.g., NaOH), depending on the starting pH of the raw material.

[0134] It will be appreciated that pH adjustment can be performed after or before addition of the PVAm.

[0135] It may be preferred to add a salt in the form of NaCl to the feedstock. Suitably, NaCl is added to the feedstock such that the concentration of NaCl in the feedstock is in the interval of 0.1-1.2 wt-% NaCl per weight of cellulose fibers contained in the feedstock, preferably in a concentration of 0.2-1.0 wt-% NaCl per weight of cellulose fibers, more preferably in a concentration of 0.3-0.8 wt-% NaCl per weight of cellulose fibers.

[0136] NaCl can be added to the feedstock before or after the addition of PVAm.

[0137] NaCl can be added to the feedstock before or after adjusting the pH of the feedstock.

[0138] In certain embodiments, it may be advantageous if the NaCl is added before the pH adjustment step and the PVAm is added after the pH adjustment step.

[0139] After adding PVAm to the stock and possibly adding NaCl and adjusting the pH, the headbox delivers the stock to the forming section, where the forming fabric receives the stock containing fibers from the headbox. In the forming section, dewatering occurs while the fibers form a wet paper web.

[0140] The method preferably comprises a pressing section for further dewatering the wet paper web by pressing. The pressing section preferably comprises at least one pressing roll.

[0141] The method preferably further comprises a drying section for drying the paper web to a final dryness. The drying section is located after / downstream of the pressing section. After the pressing section, the paper web is conveyed to the drying section.

[0142] The drying section may comprise hot air drying, one or several drying cylinders (eg Yankee cylinders), microwave and / or infrared drying, but other drying techniques are also conceivable.

[0143] When the web leaves the dryer section, the web has reached its final dryness.

[0144] Producing filter materials.

[0145] In a preferred embodiment, the feedstock comprises HYP, preferably CTMP, in a concentration of 0.1-0.3 wt-% fiber.

[0146] NaCl is added to the stock at a point in the headbox or before (upstream of) the headbox such that the NaCl concentration in the stock is in the interval of 0.3-0.8 wt-% NaCl based on the dry weight of the cellulose fibers.

[0147] After adding NaCl, the pH is adjusted to pH 9-pH 10, preferably to pH 9.5.

[0148] The PVAm is added after the pH is adjusted. Preferably, the PVAm is added to a concentration of 0.5-2.0 wt-% of the dry weight of the cellulose fibers in the feedstock.

[0149] The headbox transports the raw material to the forming wire in the forming section to form a wet web of cellulose material. The wet web further passes through the pressing section and the drying section for final drying. Thus, the filter material is produced.

[0150] The above embodiment is an example of a web production procedure for making filter material. However, it is conceivable that the filter material can be produced by other papermaking wet forming processes.

[0151] For example, the filter material can be produced by a wet forming process, using a three-dimensional (3D) shaped forming tool, which includes a forming part that contacts the slurry suspension. The slurry suspension is sucked onto the forming part, such as by vacuum suction, until a fiber layer of the desired thickness is formed. The slurry wet layer is dehydrated by pressing and / or drying to form a shaped 3D filter material.

[0152] As will be appreciated by those skilled in the art, numerous changes and modifications may be made to the above-described and other embodiments of the invention without departing from the scope of the invention as defined in the appended claims.

[0153] For example, it is contemplated that the steps of preparing the feedstock may be performed in a different order than described above. In some embodiments, the polyelectrolyte may be added, for example, to the feedstock comprising fibers prior to the pH adjustment and salt addition steps.

[0154] It is envisaged that more than one layer of polyelectrolyte may be adsorbed onto the cellulosic material. A second layer of negatively charged polyelectrolyte may be provided, for example, over a first layer of positively charged polyelectrolyte. Embodiments are envisaged where there are more than two layers of charged polyelectrolytes adsorbed onto the cellulosic material.

[0155] Those skilled in the art will recognize that the present invention contemplates any fiber-based manufacturing method, including conventional wet-laid forming procedures and dry-laid forming procedures, wet-laid forming procedures, dry-laid forming procedures, and three-dimensional (3D) printing techniques.

[0156] It is also conceivable that the filter material is in the form of a solid foam.

[0157] It should be noted that the above aspects may become the subject of their own protection, therefore in separate divisional applications. Thus, it is foreseeable that the aspect of improved dewatering in the forming section due to the addition of PVAm may require separate protection, for example, because it may also apply to other concepts than those defined in the independent claims of this application.

Claims

1. A method for producing a filter material comprising a cellulose material and a positively charged polyelectrolyte, wherein the polyelectrolyte comprises polyvinylamine (PVAm), wherein the method comprises at least the following steps: - providing a raw material comprising the cellulosic material, the cellulosic material comprising cellulose fibers; - adding the polyvinylamine to the feedstock such that the concentration of PVAm in the feedstock is in the interval of 0.5-2.0 wt-% of the dry weight of cellulose fibers in the feedstock, and - allowing the cellulosic material to absorb the polyvinylamine.

2. The method for producing a filter material according to claim 1, wherein the method further comprises the following steps: - forming a wet web of said feedstock comprising said cellulosic material and said polyvinylamine; - dewatering the wet web by pressing; - Drying the wet web.

3. The method for producing a filter material according to claim 1, wherein the method further comprises a wet molding process, the wet molding process comprising the following steps: - providing a forming tool comprising a three-dimensional (3D) shape of a forming portion; - bringing a forming tool of said 3D shape comprising a forming part into contact with said raw material; - Applying a vacuum suction device so that the raw material is sucked into the forming part and a 3D filter material of a desired thickness is formed on the forming part; - removing the 3D filter material from the shaped portion, and - Dehydrating the 3D filter material by pressing and / or drying to form a shaped 3D filter material.

4. The method for producing a filter material according to any one of the preceding claims, wherein the method further comprises: A step of adjusting the pH of the raw material to within the range of pH 7 to pH 11; preferably within the range of pH 8 to pH 10.5, more preferably within the range of pH 9 to pH 10.

5. The method for producing a filter material according to any one of the preceding claims, wherein the method further comprises the step of adding NaCl to the raw material, wherein the concentration of NaCl is 0.1-1.2 wt-% of the dry weight of the cellulose fibers contained in the raw material, preferably 0.2-1.0 wt-% of the dry weight of the cellulose fibers, and more preferably 0.3-0.8 wt-% of the dry weight of the cellulose fibers.

6. The method for producing a filter material according to any one of the preceding claims, wherein NaCl is added to the raw material, wherein the addition of NaCl is followed by a step of adjusting the pH; and after adjusting the pH of the raw material, a step of adding a polyelectrolyte to the raw material is performed.

7. The method for producing a filter material according to any of the preceding claims, wherein the cellulosic material is selected from non-wood pulp, wood pulp, mechanical pulp, refiner mechanical pulp (RMP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), defiberized fibrous material, high temperature chemithermomechanical pulp (HTCTMP), chemimechanical pulp (CMP), stone groundwood (SGW) and pressure groundwood (PGW), kraft pulp, sulfate pulp, sulfite pulp, recycled paper and paperboard, waste paper, nano pulp, dissolving pulp, deinked pulp, recycled fibers or mixtures thereof.

8. A filter material comprising a cellulose material and a positively charged polyelectrolyte, wherein the polyelectrolyte comprises polyvinylamine (PVAm), and wherein the filter material is manufactured by the method of any one of claims 1 to 7, characterized in that: The filter material is a filter material net, a sheet filter material or a shaped three-dimensional filter material.

9. The filter material according to claim 8, characterized in that The filter material is lipophilic and hydrophobic.

10. The filter material according to claim 8 or 9, characterized in that: The cellulosic material includes non-wood pulp, wood pulp, mechanical pulp, refiner mechanical pulp (RMP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), defiberized fibrous material, high temperature chemithermomechanical pulp (HTCTMP), chemithermomechanical pulp (CMP), stone groundwood (SGW) and pressure groundwood (PGW), kraft pulp, sulfate pulp, sulfite pulp, recycled paper and paperboard, waste paper, nano pulp, dissolving pulp, deinked pulp, recycled fiber or a mixture thereof.

11. The filter material according to any one of claims 8 to 10, characterized in that: The polyelectrolyte is polyvinylamine (PVAm), selected from any one of unmodified PVAm, and PVAm modified with a linear or branched and optionally substituted alkyl chain; preferably, the PVAm is unmodified.

12. The filter material according to any one of claims 8 to 11, characterized in that: The polyelectrolyte is PVAm, the cellulosic material is HYP, preferably CTMP, and the dry weight is in the range of 10-240 gsm.

13. A filtration system, characterized in that The filter device comprises a filter material as defined in any one of claims 8-12.

14. A filter for an air ventilator or a mask, characterized in that The filter comprises a filter material as defined in any one of claims 8-13.

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