Process for in situ glyoxylation of polyacrylamide

Abstract

The invention relates to a process for in-situ glyoxylation of polyacrylamide in paper mills, board mills and the like, wherein a discontinuous batch glyoxylation reaction of an aqueous reaction mixture is performed in a reactor vessel equipped with a drive stirrer to form an aqueous polymer composition comprising glyoxylated polyacrylamide. The process comprises forming or obtaining an aqueous reaction mixture having a starting viscosity and comprising a polyacrylamide base polymer having a weight average molecular weight of 30000 - 300000 g / mol and glyoxal. The concentration of the polyacrylamide base polymer in the reaction mixture is 1.5% - 8% higher than the critical concentration of the polyacrylamide base polymer. A base feed is added to the reaction mixture while measuring the viscosity of the reaction mixture and / or a process variable related to the viscosity of the reaction mixture and allowing the in-situ glyoxylation reaction of the polyacrylamide base polymer in the reaction mixture to proceed. When a predetermined final viscosity value is reached, the base feed is terminated and an acid is added to the reaction mixture. The aqueous polymer composition comprising glyoxylated polyacrylamide is removed from the reaction vessel.

Description

Technical Field

[0001] The present invention relates to a method for in-situ acetaldehyde acidification of polyacrylamide in paper mills, cardboard mills, etc., according to the preamble of the appended independent claims. Background Technology

[0002] Paperboard, especially corrugated container board, is one of the most commonly used packaging materials in the world due to its low price, lightweight structure, and recyclability. However, container board also has limitations. One of the main drawbacks of container board is its poor water and moisture resistance. The main structural unit of container board is based on amphiphilic cellulose fibers, whose hydrophilic hydroxyl groups give the fibers mechanical strength, but also make them susceptible to moisture. This means that in damp conditions, the strength of the paperboard can deteriorate rapidly.

[0003] Today, many goods are produced in countries with humid environments (e.g., Southeast Asia) and shipped worldwide. Corrugated cardboard is also used to package goods that generate moisture or humidity, such as fruits, vegetables, or frozen foods, and / or stored in humid environments, such as in refrigerated storage spaces. As the packaging is exposed to moisture from the environment or from the packaged goods during transport and / or storage, the strength of the packaging decreases. Ultimately, this can even lead to the collapse of the packaging. Therefore, there is a growing need for improved strength properties for cardboard and equivalent fiber products, even under humid conditions.

[0004] Another challenge in packaging made from corrugated cardboard is what's known as scorecracking. Scorecracking refers to fiber dislocation on the outer side of the cardboard along the score line on the edge area of ​​the packaging. When folded along the score line, the outer surface of the cardboard is subjected to tension. Surface adhesive can improve the tensile strength of the cardboard, but the cardboard may become brittle because the strain does not increase. Scorecracking is a quality defect and should be minimized when manufacturing packaging from cardboard and the like.

[0005] Glyoxylated polyacrylamide (GPAM) and compositions containing glyoxylated polyacrylamide are commonly used in the manufacture of paper, paperboard, etc., to improve the properties of the final product. Polyacrylamide polymers can be formed by polymerizing acrylamide and cationic monomers capable of forming ionic bonds with anionic fiber surfaces. The molecular weight (i.e., length) of the polyacrylamide backbone is an important parameter for polymer functionality, as a long backbone provides sufficient size to allow adhesion and bonding between fiber surfaces. Glyoxylation crosslinks the polyacrylamide polymer. Compared to linear polyacrylamide polymers of similar molecular weight, the crosslinked structure improves the drainage and dehydration capabilities of polyacrylamide and has less negative impact on sheet formation.

[0006] Glyoxylated polyacrylamide is a reactive polymer that can covalently bond with cellulose, and thus provides improvements in properties such as strength. For example, the dry and / or wet strength properties of paper and paperboard can be improved by adding glyoxylated polyacrylamide to a stock suspension. Generally, the good forming and bonding capabilities provided by glyoxylated polyacrylamide are beneficial for the strength properties of the final paper or paperboard as measured by the ring crush test (RCT), burst strength, and Concora media test (CMT). These parameters describing strength are particularly important in the manufacture of linerboard, testlinerboard, or fluting board grades.

[0007] High molecular weight glyoxylated polyacrylamide is advantageous for the strength properties of paper or paperboard, especially when it is manufactured from furnishings containing recycled fibers. In these cases, one of the challenges is often the high ash content of the furnishings, which necessitates a sufficiently high molecular weight from the polyacrylamide polymer backbone. Otherwise, inorganic particles between these fibers can hinder bond formation, and the glyoxylated polyacrylamide will not effectively contact the fibers and form bonds between them. Therefore, the physical dimensions (i.e., molecular weight) of the glyoxylated polyacrylamide polymer are crucial for strength improvement efficiency.

[0008] However, glyoxylated polyacrylamide compositions, especially high molecular weight polyacrylamide compositions, present challenges for their commercial use. Glyoxylated polyacrylamide is known to have insufficient storage stability. As mentioned above, glyoxylated polyacrylamide is formed by crosslinking a polyacrylamide-based polymer using glyoxal. The resulting glyoxylated polyacrylamide polymer is not entirely stable because glyoxal in the aqueous polymer solution tends to continue the crosslinking reaction with the polyacrylamide base polymer. This means that the viscosity of the GPAM aqueous solution typically increases during storage as the crosslinking reaction proceeds. Finally, continued crosslinking can even lead to gel formation and render glyoxylated polyacrylamide unsuitable for use in the manufacture of paper, paperboard, etc. Therefore, glyoxylated polyacrylamide and its aqueous solutions have a limited shelf life.

[0009] Increasing the molecular weight of the polyacrylamide base polymer to achieve the desired strength properties can lead to deep gel formation. To reduce the risks of crosslinking and gel formation, glyoxylated polyacrylamide polymers are often manufactured, transported, and stored in aqueous solutions with a relatively low concentration of active glyoxylated polyacrylamide polymer, for example, 5%–7% by weight. This means that the transportation and storage costs of glyoxylated polyacrylamide compositions are relatively high due to the large volumes involved.

[0010] Due to limitations in storage stability, glyoxylated polyacrylamide polymers, especially high molecular weight glyoxylated polyacrylamide polymers, are sometimes added to the process immediately after in-situ manufacturing in paper mills or cardboard plants. Several continuous and discontinuous methods have been proposed for the in-situ manufacture of glyoxylated polyacrylamide.

[0011] For example, US2008 / 0064819 discloses a method for preparing cellulose reactive functionalized polyvinylamide adducts, wherein the concentration of the vinylamide polymer is below, equal to, or above but not exceeding 1% of a critical concentration. The critical concentration is defined as the concentration of the vinylamide polymer above which the viscosity of the reaction mixture obtained from the forward process of adduct formation increases, and below which the viscosity of the reaction mixture decreases. Turbidity measurements are used to monitor the glyoxylation reaction.

[0012] However, in-situ manufacturing of glyoxylated polyacrylamide has proven complex in practice. Maintaining a suitable throughput for industrial production—that is, keeping the reaction time short enough—in a continuous process to produce glyoxylated polyacrylamide of consistently acceptable quality has been challenging. In particular, controlling various process parameters and conditions is complex. For example, precise pH control on an industrial scale requires expensive technologies such as ceramic pH meters, as well as continuous maintenance.

[0013] Conventionally, in-situ glyoxylated polyacrylamide products are processed at a concentration of approximately 2 wt%. The relatively low polymer concentrations used in in-situ manufacturing require large production units to produce the quantities of glyoxylated polyacrylamide required by large paperboard or paper mills. However, the available free space in paper and paperboard mills is limited and can be an obstacle to in-situ glyoxylation. The size of water purification units also increases when low polymer concentrations require large amounts of purified water, highlighting issues related to available free space in the plant. High freshwater intake will also automatically increase the amount of wastewater generated. Permissible wastewater volumes are typically limited by the plant's environmental permits and can become a limiting factor for water consumption.

[0014] Furthermore, the availability of water for the glyoxylation reaction can be limited, and the water quality can be relatively low, with significant seasonal variations. Poor water quality can complicate or even hinder the use of turbidity measurements used to track the glyoxylation reaction. Additionally, poor water quality (such as high hardness, high alkalinity, or high concentrations of colloidal substances) necessitates the introduction of a purification stage into the manufacturing process. In some geographical locations, the raw water used in the plant has high hardness and alkalinity, making it unsuitable for manufacturing glyoxylated polyacrylamide. The only available water for in-situ glyoxylation is typically boiler feedwater from the plant's steam system condensate, but the quantity of this purified water is usually very limited. Summary of the Invention

[0015] The purpose of this invention is to minimize, or possibly even eliminate, the drawbacks present in the prior art.

[0016] Another object of the present invention is to provide a simple, robust and effective method for in-situ production of acetaldehyde-oxidized polyacrylamide.

[0017] Another object of the present invention is to provide a method for in-situ production of acetaldehyde-oxidized polyacrylamide, which is easy to control and wherein water consumption is reduced.

[0018] These objectives are achieved by the invention having the features provided in the characterizing portion of the independent claims. Some preferred embodiments of the invention are provided in the dependent claims.

[0019] Where applicable, the embodiments mentioned herein relate to all aspects of the invention, even if they are not always mentioned separately.

[0020] In a typical method according to the invention for in-situ glyoxylation of polyacrylamide in paper mills, paperboard mills, etc., wherein a discontinuous batch glyoxylation reaction of an aqueous reaction mixture is carried out in a reactor vessel equipped with a driven stirrer to form an aqueous polymer composition containing glyoxylated polyacrylamide for use in the manufacture of paper, paperboard, etc., the method includes:

[0021] - To form or obtain an aqueous reaction mixture having an initial viscosity value and comprising a polyacrylamide base polymer having a weight-average molecular weight of 30,000-300,000 g / mol and glyoxal, wherein the concentration of the polyacrylamide base polymer in the reaction mixture is 1.5%-8% higher than the critical concentration of the polyacrylamide base polymer, preferably 2%-8%.

[0022] - Optionally, the temperature of the reaction mixture can be adjusted to a temperature range of 15-40°C, more preferably 20-30°C;

[0023] - Add alkali feed to the reaction mixture while measuring the viscosity of the reaction mixture and / or process variables related to the viscosity of the reaction mixture;

[0024] - Allows for in-situ glyoxylation of the polyacrylamide-based polymer in the reaction mixture;

[0025] - Stop feeding alkali when the predetermined final viscosity value is obtained;

[0026] - Add the acid to the reaction mixture; and

[0027] - Remove the aqueous polymer composition containing glyoxylated polyacrylamide from the reaction vessel.

[0028] The typical use of the acetaldehyde-oxidized polyacrylamide prepared according to the method of the present invention is in the manufacture of paper and paperboard, with a preferred dosage of 0.5-3 kg / t.

[0029] It has now been unexpectedly discovered that proper control of the in-situ glyoxylation reaction of polyacrylamide does not require complex and / or time-consuming online pH or turbidity measurements. The progress of the glyoxylation process can be effectively controlled by simple measurements of viscosity and / or viscosity-related parameters, without the need for complex measurement schemes involving multiple different parameters and / or online pH sensors (such as pH meters). The method of the present invention can be carried out using equipment already commonly found in paper mills and board mills, making it practically suitable and easily adaptable. Furthermore, it has been observed that the concentration of the polyacrylamide base polymer in the reaction mixture can be increased, which allows for a reduction in the amount of water required to produce glyoxylated polyacrylamide.

[0030] The aqueous polymer compositions containing glyoxylated polyacrylamide, obtainable through this invention, can increase the short-span compression strength (SCT) of paper, paperboard, etc., especially under high humidity conditions. Therefore, this invention can provide final paper or paperboard with properties that make them suitable for packaging intended for harsh environments and / or for harsh items (e.g., fruits or frozen foods). Using the aqueous polymer compositions containing glyoxylated polyacrylamide, obtainable through this invention, can also help reduce packaging weight and thereby reduce CO2 emissions associated with packaging and transportation. High SCT strength under high humidity conditions is particularly desirable for slotting to reduce the risk of packaging collapse during storage and / or transportation.

[0031] The aqueous polymer compositions containing glyoxylated polyacrylamide that can be obtained through this invention also reduce the risk of notch cracking. It has been observed that the aqueous polymer compositions containing glyoxylated polyacrylamide that can be obtained through this invention can significantly increase the tensile energy absorption (TEA) of paper, paperboard, etc. In this way, coated corrugated board and / or waste paper coated corrugated board can be manufactured, which have improved strength properties and can withstand tension when folded along the notch lines during packaging production.

[0032] In the context of this invention, the terms "humid conditions" and "high humidity conditions" are used interchangeably, and they refer to environmental conditions with high humidity, wherein the relative humidity is ≥80%, preferably ≥85%, and more preferably ≥90%. For example, under high humidity conditions, the relative humidity (RH) can be 80%-100%. Tests for evaluating high humidity characteristics, such as SCT strength under high humidity, can be performed, for example, using test strips air-treated at 85% RH and 23°C for at least 4 hours. High humidity conditions typically increase the moisture content of paper or paperboard webs to >7%, which reduces the amount of hydrogen bonds between web components. This can lead to a decrease in web strength if countermeasures such as those of this invention are not taken.

[0033] The weight-average molecular weight (Mw) of the base polymer used for the purposes of this invention was measured using SEC / GPC assays along with PEO (polyethylene oxide) calibration. The weight-average molecular weight (Mw) was determined by size exclusion chromatography (SEC) using an Agilent 1100SE chromatography system with an integrated pump, autosampler, and degasser. The eluent was a buffer solution (0.3125M CH3COOH + 0.3125M CH3COONa) at a flow rate of 0.5 mL / min at 35 °C. Typical sample concentrations were 2–4 mg / mL, with an injection volume of 50 μL. Ethylene glycol (1 mg / mL) was used as the flow marker. The column assembly consisted of three columns (TSKgel PWXL guard column and two TSKgel GMPWXL columns). Detection was performed using an Agilent refractive index detector (T = 35 °C). Molecular weights were determined using standard (column) calibration with poly(ethylene oxide) / poly(ethylene glycol) narrow molecular weight distribution standards (Polymer Standards Service).

[0034] In this invention, polyacrylamide that has undergone in-situ glyoxylation in paper mills, cardboard plants, etc., is subjected to a discontinuous batch glyoxylation reaction in an aqueous reaction mixture within a reactor vessel equipped with a driven stirrer. The term "discontinuous batch glyoxylation reaction" refers to the process where reaction mixture materials (e.g., polyacrylamide base polymer, glyoxal, water, etc.) are introduced into the reaction vessel at the start of the glyoxylation process, and the resulting reaction mixture remains in the reaction vessel until the reaction is complete. During the glyoxylation reaction, an aqueous polymer composition containing glyoxylated polyacrylamide is formed. The aqueous polymer composition containing glyoxylated polyacrylamide polymer is then removed from the reaction vessel. The reaction cycle of the discontinuous batch glyoxylation reaction begins when a batch of reaction mixture material is introduced into the reaction vessel (including possible temperature adjustment time, etc.), and ends when the reaction vessel is ready to receive the next batch of reaction mixture material after the aqueous polymer composition is removed from the reaction vessel (including necessary evacuation and rinsing time). In this invention, the reaction cycle can be less than 130 min, preferably less than 120 min, and more preferably less than 100 min. For example, the reaction cycle time can be 20-130 min or 20-120 min, preferably 30-100 min, more preferably 35-90 min or 45-90 min, and sometimes even 40-75 min, which enables the efficient production of acetaldehyde-treated polyacrylamide for use in paper mills, cardboard mills, etc.

[0035] At the start of the acetaldehyde acidification reaction, an aqueous reaction mixture is obtained or formed in the reaction vessel, the aqueous reaction mixture comprising at least one polyacrylamide base polymer having a weight-average molecular weight of 30,000-300,000 g / mol and glyoxal. According to one embodiment, the reaction mixture is formed by separately feeding or metering appropriate amounts of the reaction mixture materials (including the polyacrylamide base polymer, glyoxal, and dilution water) into the reaction vessel. Alternatively and preferably, a premix comprising the polyacrylamide base polymer and glyoxal can be used. The premix can be produced in situ, for example by arranging a static mixer before the batch reactor and simultaneously feeding the reaction mixture materials (i.e., water, base polymer, glyoxal) into the batch reactor through the static mixer. Using a premix produced in situ can shorten the reaction cycle time and make the process more efficient. Alternatively, a premix comprising the polyacrylamide base polymer and glyoxal can be prepared off-site and the mixed mixture delivered. Using a mixed premix can sometimes be more convenient because it avoids handling concentrated glyoxal solutions.

[0036] The concentration of the polyacrylamide base polymer in the reaction mixture is 1.5% to 8% units above the critical concentration of the polyacrylamide base polymer, preferably 2% to 8% units, more preferably 3% to 7% units. This means that if the critical concentration is, for example, 2% by weight, then 1.5% units above 2% by weight would be 3.5% by weight. Even a relatively small increase in the concentration of the polyacrylamide base polymer can significantly reduce the total water consumption of the glyoxylation reaction in practice. This provides an unexpected advantage, especially when freshwater availability is severely limited, such as during summer months and / or drought periods. The critical concentration is defined in the manner disclosed in US2008 / 0064819. The critical concentration is defined as the concentration of the polyacrylamide base polymer above which the viscosity of the reaction mixture increases due to the forward progression of the glyoxylation of the polyacrylamide, and below which the viscosity of the reaction mixture decreases due to the forward progression of the glyoxylation of the polyacrylamide. The critical concentration for the glyoxylation of a specific polyacrylamide base polymer can be empirically determined through studies involving the glyoxylation of polyacrylamide base polymers, as described in US 2008 / 0064819.

[0037] Surprisingly, when the concentration of the polyacrylamide base polymer in the reaction mixture is above the critical concentration of the polyacrylamide base polymer by 1.5%-8%, preferably 2%-8%, and more preferably 3%-7%, the acetaldehyde acidification reaction can be carried out without a significant risk of gel formation, while operating within a concentration range that provides appropriate viscosity increase for controlling the reaction.

[0038] A base feed is added to the reaction mixture while the viscosity of the reaction mixture and / or process variables related to the viscosity of the reaction mixture are measured. The base feed can be added as a continuous feed or in portions of two or more. The base feed can be constant, i.e., the volume of base in a continuous feed remains the same throughout the feeding period, or the feed can be variable, i.e., the volume of base can decrease or increase during the feeding period. When added in portions, each portion can have an equal volume during the feeding period, or the volume of each portion can increase or decrease. It is also possible that the time interval between portions is the same throughout the feeding period, or that it can increase or decrease during the feeding period.

[0039] The feed duration of the alkali feed can preferably be up to 120 minutes. The feed duration refers to the time from when the alkali feed begins to be added to the reaction mixture to when the alkali feed ends. It is generally desirable that the feed duration be not too long, which allows for efficient online industrial processes. The feed duration of the alkali feed for in-situ glyoxylate acidification reactions can be in the range of 5-120 minutes or 10-100 minutes, preferably 15-80 minutes, more preferably 20-60 minutes, and even more preferably 20-50 minutes.

[0040] NaOH is typically used as a base.

[0041] According to a preferred embodiment of the invention, the alkali feed is added at a rate of 40-500 g NaOH, preferably 50-300 g NaOH or 100-200 g NaOH, per metric tonne of reaction mixture (i.e., the total amount of water, polyacrylamide base polymer, and glyoxal). This has been observed to provide an appropriate reaction rate for the glyoxylation reaction: too slow an alkali feed may reduce the reaction rate, while too fast an alkali feed increases the risk of gelation during the glyoxylation reaction.

[0042] According to a preferred embodiment, the alkali feed can be added at a rate of 40-500 g NaOH, preferably 50-300 g NaOH or 100-200 g NaOH, based on the activity of the reaction mixture (i.e., the total amount of water, polyacrylamide base polymer and glyoxal added) per hour per metric ton of reaction mixture.

[0043] When the alkali feed begins, the glyoxylation reaction (i.e., crosslinking of the polyacrylamide base polymer) commences, allowing in-situ glyoxylation of the polyacrylamide base polymer in the reaction mixture. During the alkali feed period, the viscosity of the reaction mixture and / or viscosity-related process variables are measured and tracked, preferably online. Viscosity and / or viscosity-related process variables provide a more reliable way to control the glyoxylation reaction than, for example, pH measurements. For example, viscometers do not require frequent calibration or are as prone to drift as pH meters. Viscosity-related measurements are also insensitive to water quality. For example, the hardness or colloidal concentration of the water used does not affect viscosity, even though they significantly interfere with routine turbidity measurements. Measurements of viscosity and / or viscosity-related process variables can be continuous, or they can be performed at preselected, preferably short, time intervals. The obtained measurements are used to determine the appropriate endpoint of the alkali feed and the glyoxylation reaction, i.e., when the desired level of crosslinking is reached.

[0044] According to a preferred embodiment of the invention, the viscosity of the reaction mixture is indirectly measured by measuring process variables related to the viscosity of the reaction mixture. For example, the viscosity of the reaction mixture can be estimated, measured, or determined by measuring the torque and / or power consumption of a motor-driven agitator in the reaction vessel, preferably the torque driving the agitator. As the viscosity of the reaction mixture increases, the power consumption and torque of the agitator increase. Measuring the power consumption and / or torque of the agitator provides an easy and reliable way to indirectly monitor and measure changes in the viscosity of the reaction mixture without the need for complex sensor systems, etc.

[0045] Alternatively or additionally, the viscosity of the reaction mixture can be measured from the reaction mixture using a rotational viscometer, an oscillating viscometer, or a vibratory viscometer. The power consumption or torque driving the agitator, as well as the viscosity of the reaction mixture, can also be measured using one of these viscometers.

[0046] In-situ glyoxylation of the polyacrylamide base polymer in the reaction mixture is permitted until a predetermined final viscosity value or viscosity level is reached. The alkali feed is terminated when the predetermined final viscosity value or predetermined process variable value related to the viscosity of the reaction mixture is reached. This means that the endpoint of the alkali feed is determined solely by the measured viscosity of the reaction mixture and / or the process variable related to the viscosity of the reaction mixture. In this way, problems associated with using online pH meters, which can sometimes be insufficiently reliable and / or unreliable, are avoided. Furthermore, since the glyoxylation reaction is not controlled by pH measurement, the reaction cycle time can be significantly reduced, thus providing improved efficiency for in-situ glyoxylation. Moreover, in the method of this invention, the alkali consumption of the reaction mixture is not determined before the alkali feed is added to the reaction mixture. The amount of alkali feed is determined solely based on the viscosity of the reaction mixture and / or the process variable related to the viscosity of the reaction mixture. Because no additional determinations (e.g., by titration or calculation of alkali consumption) are required, this method is significantly faster without degrading the properties of the obtained glyoxylated polyacrylamide polymer.

[0047] According to one embodiment of the invention, the alkali feed is preferably terminated when the viscosity of the reaction mixture increases by at least 20%, preferably at least 40%, more preferably at least 50%, and even more preferably at least 70% from the initial viscosity value. The viscosity increase from the initial viscosity value can be in the range of 20%-250%, preferably 40%-200%, more preferably 50%-170%, and even more preferably 70%-150%. This means that at the moment the alkali feed is terminated, the viscosity value of the reaction mixture is at least 20%, preferably at least 40%, more preferably at least 50%, and even more preferably at least 70% higher than the initial viscosity value of the reaction mixture.

[0048] After the alkali feed is finished, the glyoxylation reaction can still proceed, but at a slower rate. This is beneficial in the later stages of the glyoxylation reaction because it provides more time for the reaction to proceed rapidly to increase the viscosity of the reaction mixture (which occurs at the end of the glyoxylation reaction) and to make the measurement or estimation of the final viscosity more accurate.

[0049] After the alkali feed is finished, acid is added to the reaction mixture. The acid can be added immediately after the alkali feed is finished or some time afterward, during which time the acetaldehyde acidification reaction is still permitted. When the predetermined final viscosity value or level is obtained, acid is added in an amount sufficient to lower the pH of the reaction mixture to pH ≤ 8, preferably ≤ 7, more preferably ≤ 5. Lowering the pH effectively halts the progression of the acetaldehyde acidification reaction, and the crosslinking of the polyacrylamide chains is virtually stopped or at least significantly reduced. The pH of the reaction mixture can be lowered to a pH range of 2.5-5, preferably 3-4. The pH is lowered by adding an acid such as formic acid or sulfuric acid to the reaction mixture.

[0050] If desired, the pH of the reaction mixture can be measured during the alkali feed, but this is not mandatory. Typically, the alkali feed causes the pH of the reaction mixture to be adjusted from the initial pH to a reaction pH in the range of pH 8-10, preferably 8.5-9.5, and sometimes 8.7-9.5, without pH control or measurement. Before the alkali feed, the pH of the reaction mixture, i.e., the initial pH, is <8.

[0051] According to a preferred embodiment of the invention, the method, and particularly the alkali feeding step, does not involve pH measurement. This means that there is no online pH control of the reaction mixture during the alkali feeding period or during the glyoxylation reaction. The process of alkali feeding or glyoxylation is carried out without simultaneous pH control of the reaction mixture.

[0052] During the feeding of the alkali into the reaction mixture, the reaction mixture is effectively mixed by a driven agitator, and mixing continues throughout the glyoxylation reaction. The reaction vessel typically has a capacity of ≤8m. 3 Preferably ≤7m 3 More preferably ≤6m 3 or ≤5m 3 Sometimes even better, ≤4m 3 A relatively small reactor volume. The reactor volume can be, for example, between 0.5 and 8 m³. 3 Preferred size: 0.75-7m 3 More preferably 1-6m 3 or 1-4m 3 Within a certain range. In some embodiments, the reactor volume can be 4-8 m³. 3 5-7m is preferred 3 The relatively small reactor volume allows for efficient mixing of the reaction mixture using conventional driven agitators designed for industrial applications. Furthermore, the relatively small reaction vessel is easier to install on-site at paper mills or cardboard plants. It can even be made portable; for example, it can be mounted on a transport pallet and moved using a forklift.

[0053] Preferably, the reaction vessel does not contain any bypass loop that would allow the reaction mixture or a portion thereof to circulate outside the reaction vessel before the glyoxylation reaction is completed. Therefore, the reaction vessel has no bypass loop or similar. If present, the reaction mixture within the reaction vessel can be measured as needed, meaning there is no need for a bypass loop to remove process samples from it. This is a significant advantage, as maintaining a bypass loop or similar in the industrial production of glyoxylated polyacrylamide polymers is typically complex.

[0054] The temperature of the reaction mixture can be optionally adjusted to a range of 15-45°C or 15-40°C, preferably 20-40°C, more preferably 20-35°C or 20-30°C. According to one embodiment, the temperature of the reaction mixture can be adjusted to 15-35°C, preferably 17-30°C, more preferably 18-25°C or 20-22°C. Adjusting the temperature of the reaction mixture provides increased stability to the glyoxylation reaction. Temperature adjustment can be performed before the start of the alkali feed, i.e., before the start of the glyoxylation reaction and / or during the glyoxylation reaction. Temperature adjustment can be achieved by using a reaction vessel that can be cooled / heated. Another alternative to adjusting the temperature of the reaction mixture is to add hot or cold water to the reaction mixture. For example, the water used to form the reaction mixture can be heated or cooled to a suitable temperature. According to one embodiment of the invention, the temperature of the reaction mixture is measured during the glyoxylation reaction, for example, by using a standard temperature sensor installed in a suitable location in the reaction vessel. Preferably, the temperature of the reaction mixture can be measured and adjusted throughout the glyoxylation reaction.

[0055] After the addition of acid, the viscosity of the reaction mixture can be in the range of 20-60 cP or 20-50 cP, preferably 25-40 cP, more preferably 25-35 cP. Therefore, the viscosity of the reaction mixture is typically in the range of 6-100 cP. This viscosity value provides a suitable level of crosslinking without any risk of gel formation. The viscosity of the reaction mixture after the addition of acid is at least twice and at most nine times the initial viscosity of the reaction mixture. Preferably, the viscosity of the reaction mixture after the addition of acid can be 1.5-10 times, preferably 2-7 times, more preferably 2-5 times, and even more preferably 2.5-5 times, the initial viscosity of the reaction mixture. The initial viscosity of the reaction mixture can be determined by one of the measurement methods known per se and / or described elsewhere in this application, immediately following the addition of the alkali feed to the reaction mixture. According to one embodiment of the invention, the initial viscosity of the reaction mixture can be in the range of 4-15 cP, preferably 6-12 cP, more preferably 7-10 cP.

[0056] After the addition of acid, the resulting aqueous polymer composition containing glyoxylated polyacrylamide is removed from the reaction vessel. The resulting polymer composition containing glyoxylated polyacrylamide can be removed from the reaction vessel immediately or after a suitable storage period, preferably immediately. The glyoxylated polyacrylamide can be used immediately after the glyoxylation reaction is complete in the production of paper, paperboard, etc., or the glyoxylated polyacrylamide can be stored first in the reaction vessel or in a separate storage container. According to one embodiment of the invention, the glyoxylated polyacrylamide can be stored for 0.1-100 hours, preferably 0.5-10 hours, before its use in the production of paper, paperboard, etc.

[0057] According to a preferred embodiment of the invention, an aqueous polymer composition containing glyoxylated polyacrylamide is transferred from a reaction vessel to a manufacturing process for paper, paperboard, etc., via piping through an optional storage container, preferably immediately after the glyoxylation reaction is completed. The obtained aqueous polymer composition containing glyoxylated polyacrylamide is used in the manufacturing process of paper, paperboard, tissue paper, etc., by feeding it into a fiber suspension before manufacturing webs. Preferably, the polymer composition is directly transferred to the fiber suspension, which forms one or more layers of the final fiber product, by pumping through a pipeline. The ability to use the obtained polymer composition directly is advantageous because it minimizes the risk of gelation that may occur during long-term storage.

[0058] By using the present invention, glyoxal is efficiently consumed in the acetaldehyde acidification reaction, and the resulting aqueous polymer composition contains a low amount of residual glyoxal. The aqueous polymer composition containing acetaldehyde-acidified polyacrylamide preferably contains 0.1 wt% to 1.5 wt%, preferably 0.2 wt% to 1 wt%, and more preferably 0.2 wt% to 0.99 wt% of residual glyoxal, calculated from the total weight of the aqueous polymer composition.

[0059] The aqueous polymer composition containing acetaldehyde-acidified polyacrylamide obtained by the method of the present invention can have a viscosity of >20 mPas, preferably >25 mPas and / or <50 mPas, preferably <35 mPas, at 25°C as measured by a Brookfield viscometer.

[0060] The method of the present invention can use polyacrylamide base polymers with relatively high molecular weights. According to one embodiment of the invention, the polyacrylamide base polymer can have a weight-average molecular weight in the range of 30,000-300,000 g / mol, preferably 50,000-300,000 g / mol, more preferably 90,000-250,000 g / mol, even more preferably 100,000-200,000 g / mol or 110,000-200,000 g / mol, and sometimes from 155,000-200,000 g / mol. The weight-average molecular weight of the base polymer can be, for example, 115,000-190,000 g / mol, preferably 120,000-170,000 g / mol, more preferably 130,000-160,000 g / mol. As explained above, the use of high molecular weight polyacrylamide base polymers is typically associated with a significant risk of gel formation during the glyoxylation reaction, but the method of the present invention reduces or eliminates this risk. The higher the molecular weight of the base polymer, the larger the molecular size of the final cross-linked structure, in which the base polymer chains are cross-linked with glyoxal. Larger structures provide improved strength and dehydration properties, especially for formulations containing recycled fibers and / or with high ash content.

[0061] Preferably, the polyacrylamide base polymer is cationic. The polyacrylamide base polymer can be obtained by polymerizing acrylamide with 3-50 mol%, 3-50 mol%, preferably 3-35 mol%, more preferably 7-30 mol%, and even more preferably 11-16 mol% of a hydrolyzed stable cationic monomer. According to one embodiment, the polyacrylamide base polymer can be obtained by polymerizing acrylamide with 6-8 mol% or 11-14 mol% of a hydrolyzed stable cationic monomer. For example, the polyacrylamide base polymer can be obtained by polymerizing acrylamide with 11-17 mol%, preferably 11-15 mol% of a hydrolyzed stable cationic monomer. It has been observed that when the amount of cationic monomer is about 10 mol%, ash retention in the produced paper or paperboard increases, which reduces the obtained strength effect. According to one embodiment, the cationic monomer may be selected from diallyl dimethylammonium chloride (DADMAC), 3-(acryloylaminopropyl)trimethylammonium chloride (APTAC), 3-(methacryloylaminopropyl)trimethylammonium chloride (MAPTAC), or any combination thereof. Preferably, the cationic monomer is diallyl dimethylammonium chloride (DADMAC). These cationic monomers, especially in the stated amounts, are capable of providing hydrolytic stability to the reaction mixture.

[0062] According to one embodiment, the reaction mixture may have a solids content of 3-8 wt%, preferably 3-7 wt%, and more preferably 4-6 wt%, calculated from the total weight of the reaction mixture. The solids content of the reaction mixture may be, for example, 4.1 wt%-6.5 wt%, preferably 4.4 wt%-6.5 wt%, and more preferably 4.4 wt%-6.0 wt%. Preferably, the solids content of the reaction mixture can be adjusted to a suitable level by adding water to the reaction mixture before adding the calculated amount of alkali. A minimum solids content, i.e., the concentration of the polyacrylamide base polymer, is necessary for viscosity increase, which provides a preferred signal for terminating the acetaldehyde acidification reaction.

[0063] Before adding the alkali, the pH of the reaction mixture can be in the range of 2 to <8, preferably 3-7 and more preferably 3-6.

[0064] According to one embodiment of the invention, the polyacrylamide base polymer and glyoxal are provided as a ready-made acidic premix for forming the reaction mixture. This means that it is not necessary to mix the base polymer and glyoxal separately, which reduces the handling of hazardous glyoxal in the plant environment and thus improves occupational safety. For example, the premix may contain a polyacrylamide base polymer (containing at least 5 mol-% cationic monomer and having a weight-average molecular weight (MW) in the range of 50,000-350,000 g / mol) and 0.1-2 wt-% glyoxal calculated by the total weight of the aqueous prepolymer composition. Typically, the pH of the premix is ​​in the range of 2-4, preferably 2.2-3.5, and more preferably 2.5-3.3. When the pH of the reaction mixture is adjusted to an alkaline pH, the crosslinking reaction of the premix is ​​activated.

[0065] According to one aspect, the present invention also relates to an aqueous reaction mixture or reactive composition comprising a polyacrylamide base polymer and glyoxal. The composition comprises a polyacrylamide base polymer obtained by polymerization of (meth)acrylamide with 10-25 mol-% or 10-18 mol-%, preferably 11-17 mol-%, more preferably 11-15 mol-%, a hydrolyzed stable monomer, the base polymer having a weight-average molecular weight in the range of 115,000-200,000 g / mol, preferably 115,000-190,000 g / mol, more preferably 120,000-170,000 g / mol or 130,000-160,000 g / mol; and 6-25 wt%, preferably 10-20 wt%, more preferably 12-18 wt%, of glyoxal calculated from the dry weight of the prepolymer composition. Suitable hydrolyzed stable cationic monomers have been defined elsewhere in this application. This reaction mixture or composition is suitable for use in the method of the present invention and provides glyoxylated polyacrylamide, which provides improved strength and / or dehydration effect. The reaction mixture or composition may have a solids content in the range of 3% to 7% by weight, preferably 4% to 6.5% by weight, and more preferably 4.5% to 6.5% by weight or 4.5% to 6.0% by weight.

[0066] The aqueous polymer compositions containing glyoxylated polyacrylamide prepared according to the present invention are particularly suitable for use as dry strength and / or dehydrating agents in the manufacture of paper or paperboard. The aqueous polymer compositions containing glyoxylated polyacrylamide prepared according to the present invention provide good dry strength and / or dehydration results, especially when used in the manufacture of paper or paperboard containing recycled fibers.

[0067] According to a preferred embodiment, the aqueous polymer composition can be used in the manufacture of paper or paperboard to improve the strength properties of the paper or paperboard under high humidity conditions. This composition is particularly suitable for improving the SCT strength or tensile energy absorption of paper or paperboard.

[0068] According to one embodiment, the aqueous polymer composition containing glyoxylated polyacrylamide prepared by the present invention is particularly suitable for manufacturing fibrous webs, which can have a content of at least 20 g / m². 2 Preferred weight is at least 60g / m 2 More preferably at least 80g / m 2 or even better, at least 100g / m 2 The basis weight (on a dry weight basis). For example, the basis weight of fiber webs can range from 20 to 500 g / m² on a dry weight basis. 2 Preferred size: 50-400g / m 2 Preferred size: 60-350g / m2 Or sometimes even 100-200g / m 2 Within the range.

[0069] According to one embodiment, the present invention is suitable for manufacturing fiber webs formed in a multilayer board selected from testliner, kraftliner, or corrugated medium. The testliner may include a layered structure comprising two layers and up to four layers, and / or having a g / m² content of 80-350 g / m². 2 Basis weight within a range. Corrugated paper media can have a single-layer structure. The basis weight can be between 110-180 g / m³. 2 Within the range. Detailed Implementation

[0070] Some embodiments of the invention are described in more detail in the following non-limiting examples.

[0071] Example

[0072] Example 1: In-situ production of acetaldehyde-acidified polyacrylamide with continuous alkali feeding.

[0073] In-situ production of glyoxylated polyacrylamide is carried out in a unit comprising a reactor vessel, pumps, and feed lines for the polyacrylamide base polymer, glyoxal, sodium hydroxide, and sulfuric acid. A process water feed line is connected to this unit. The reactor vessel has a volume of 1000 liters and is equipped with a Heidolph Hei-Torque Precision 400 mixer for torque monitoring. The reactor vessel is further equipped with an Anton Paar L-Vis 510 online viscometer.

[0074] The polyacrylamide base polymer is in the form of an aqueous solution of a copolymer obtained by polymerizing acrylamide and 14 mol-% diallyldimethylammonium chloride. The weight-average molecular weight (Mw) of the polyacrylamide base polymer is 150,000 g / mol and the critical concentration is approximately 1.5%. The dry content of the aqueous solution of this copolymer is 27.5% by weight. Glyoxal is used as a 40% by weight aqueous solution. Sodium hydroxide is used as a 50% by weight aqueous solution. Sulfuric acid is used as a 37% by weight aqueous solution.

[0075] The original process water contained approximately 150 mg / L of calcium and was softened by ion exchange treatment before being fed into the reactor vessel for in-situ production of acetaldehyde-oxidized polyacrylamide.

[0076] A premixture comprising a polyacrylamide base polymer and glyoxal, produced in situ, was used. The premixture was prepared by mixing 827 kg of softened process water, 144 kg of the polyacrylamide base polymer, and 18 kg of glyoxal in a reactor vessel. The reaction temperature of the premixture was 23°C.

[0077] Sodium hydroxide (50% solution) was diluted to a 2% concentration with water and pumped into a reactor vessel containing the premix. Two tests were performed: OSG Test A and OSG Test B. The flow rate of 50% NaOH in OSG Test A was 141 ml / h. The flow rate of 50% NaOH in OSG Test B was 281 ml / h. The viscosity of the reaction mixture was measured using an Anton Paar viscometer, and the torque level of the Heidolph mixer was monitored as a function of the 50% NaOH dosage time. The measured viscosity and torque values ​​are shown in Table 1.

[0078] Table 1 shows the viscosity and torque values ​​measured as a function of the flow time of the NaOH feed (50%).

[0079]

[0080]

[0081] When the viscosity of the reaction mixture reached 20 cP (62 min in OSG test A and 30 min in OSG test B), the NaOH feed was stopped. After the NaOH feed was stopped, the viscosity continued to increase. When the viscosity reached 30 cP, the reaction mixture was acidified with 860 g of sulfuric acid (37%).

[0082] After adding the acid, the reaction mixture was mixed for 5 minutes. The resulting in-situ produced aqueous polymer composition containing glyoxylated polyacrylamide was transferred to a storage tank and analyzed. The analytical results are given in Table 2. The calculated amounts of NaOH for OSG test A and OSG test B are given in Table 3.

[0083] Table 2 shows the analytical results of the aqueous polymer composition containing glyoxylated polyacrylamide produced in situ in Example 1.

[0084]

[0085]

[0086] Table 3 shows the calculated amount of NaOH in Example 1.

[0087] OSG Test A OSG Test B NaOH 50% flow rate, ml / h 141 281 NaOH 100% Dosage Rate, g / h 106 211 NaOH 50% consumption, ml / ton reaction mixture 155 187 NaOH 100%, g / ton reaction mixture 116 140

[0088] Example 1 shows that the rate of glyoxylation can be affected by changing the alkali (NaOH) feed rate. When the NaOH dosage rate is appropriately selected, pH measurement is not necessary for controlling the glyoxylation reaction.

[0089] Example 2: In-situ production of acetaldehyde-acidified polyacrylamide, with alkali feed carried out in batches.

[0090] The same reactor setup and the same reaction premix were used as in the examples. The temperature of the premix was 23°C.

[0091] NaOH (50%) was pre-diluted to a 2% solution using softened process water. The diluted NaOH solution was fed into the reaction mixture in 1-liter batches. Each 1-liter batch was fed over a 10-minute period. When one batch was completed, there was a 1-minute delay before the next 1-liter batch was started. The feeding of the 2% NaOH solution was stopped when the viscosity reached 20 cP, after which the mixture was acidified with 750 g of sulfuric acid (37%). The measured viscosity values ​​are shown as a function of time in Table 4.

[0092] Table 4 shows the viscosity values ​​measured as a function of time and batch feeding of NaOH (as a 2% solution).

[0093]

[0094]

[0095] The obtained in-situ produced aqueous polymer composition containing glyoxylated acrylamide was analyzed. The analytical results are given in Table 5. The calculated amount of NaOH for OSG test C is given in Table 6.

[0096] Table 5 shows the analytical results of the aqueous polymer composition containing glyoxylated polyacrylamide produced in situ in Example 2.

[0097]

[0098] Table 6 shows the calculated amount of NaOH in Example 2.

[0099] OSG Test C NaOH 2% flow rate, l / h 5.4 NaOH 100% Dosage Rate, g / h 108 NaOH 2% consumption, l / ton reaction mixture 6.8 NaOH 100%, g / ton reaction mixture 136

[0100] The results from Example 2 show that when the total dosage rate is kept slow enough and the NaOH has sufficient time to be properly mixed, the alkali can be fed in batches without a pump.

[0101] It will be apparent to those skilled in the art that the present invention is not limited to the embodiments described above, but rather that the present invention may vary within the scope of the following claims.

Claims

1. A method for in-situ glyoxylation of polyacrylamide in paper mills or cardboard mills, wherein, A discontinuous batch glyoxylation reaction of an aqueous reaction mixture is carried out in a reaction vessel equipped with a driven stirrer to form an aqueous polymer composition comprising glyoxylated polyacrylamide for use in the manufacture of paper or paperboard, the method comprising: - To form or obtain an aqueous reaction mixture having an initial viscosity and comprising a polyacrylamide base polymer having a weight-average molecular weight of 30,000-300,000 g / mol and glyoxal, wherein the concentration of the polyacrylamide base polymer in the reaction mixture is 1.5%-8% higher than the critical concentration of the polyacrylamide base polymer, wherein the critical concentration is defined as the concentration of the polyacrylamide base polymer above which the viscosity of the reaction mixture increases due to the forward progression of the glyoxylation of polyacrylamide, and below which the viscosity of the reaction mixture decreases due to the forward progression of the glyoxylation of polyacrylamide. - Add alkali feed to the reaction mixture. - Measure the viscosity of the reaction mixture and / or process variables related to the viscosity of the reaction mixture; - Allows for in-situ glyoxylation of the polyacrylamide-based polymer in the reaction mixture; - The alkali feed is terminated when the predetermined final viscosity value is reached; - Add the acid to the reaction mixture; and - Remove the aqueous polymer composition containing glyoxylated polyacrylamide from the reaction vessel.

2. The method according to claim 1, characterized in that, Before adding alkali feed to the reaction mixture, the temperature of the reaction mixture is adjusted to a range of 15-40°C.

3. The method according to claim 1, characterized in that, The aqueous polymer composition is transferred from the reaction vessel to the paper or paperboard manufacturing process via a pipeline through a storage container.

4. The method according to any one of claims 1 to 3, characterized in that, The alkali feed is added at a rate of 40-500 g NaOH per ton of the reaction mixture.

5. The method according to claim 1, characterized in that, The polyacrylamide base polymer has a weight-average molecular weight in the range of 50,000-300,000 g / mol.

6. The method according to claim 1, characterized in that, The polyacrylamide base polymer is obtained by polymerizing acrylamide with 3-50 mol% diallyl dimethyl ammonium chloride (DADMAC).

7. The method according to claim 1, characterized in that, The reaction mixture has a solids content of 3-8 wt%.

8. The method according to claim 1, characterized in that, For the alkali feed, the in-situ acetaldehyde acidification reaction has a feed period in the range of 5-120 minutes.

9. The method according to claim 1, characterized in that, The polyacrylamide base polymer and glyoxal are provided as a premix for forming the reaction mixture.

10. The method according to claim 2, characterized in that, The temperature of the reaction mixture is measured and adjusted throughout the glyoxylation reaction.

11. The method according to claim 1, characterized in that, The viscosity of the reaction mixture is measured by measuring the torque of the motor-driven stirrer.

12. The method according to claim 1, characterized in that, The viscosity of the reaction mixture is measured using a rotational viscometer or an oscillating viscometer.

13. The method according to claim 1, characterized in that, The initial viscosity of the reaction mixture is in the range of 4-15 cP.

14. The method according to claim 1, characterized in that, The alkali feed is stopped when the viscosity of the reaction mixture increases by at least 20% from the initial viscosity value.

15. The method according to claim 1, characterized in that, After the acid is added, the viscosity of the reaction mixture is in the range of 20-60 cP.

16. The method according to claim 1, characterized in that, After the acid is added, the viscosity of the reaction mixture is 2 to 7 times the initial viscosity of the reaction mixture.

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