Process for manufacturing a natural adhesive using tannins

Abstract

The invention relates to a process for manufacturing a natural formaldehyde-free adhesive for MDF wood boards, wherein the adhesive comprises a formulation containing at least three components, which come from renewable sources. These components are selected from tannins, sucrose, starch and citric acid. The process comprises oxidising the sucrose (partially or completely), providing condensed tannins, hydrolysing and oxidising the starch, if present, and mixing the components.

Description

[0001] MANUFACTURING PROCESS OF A NATURAL ADHESIVE USING TANNINS

[0002] Field of Invention:

[0003] The present invention relates to a manufacturing process for a formaldehyde-free, natural adhesive for medium-density fiberboard (MDF) wood panels, wherein the adhesive comprises a formulation of at least three components derived from renewable sources. These components are selected from tannins, sucrose, starch, and citric acid.

[0004] The boards resulting from manufacturing with this adhesive have physical-mechanical properties similar to or better than those that use commercial UF (urea-formaldehyde) resin, as well as significantly lower formaldehyde emission values.

[0005] State of the Art:

[0006] Traditionally, wood-based panels are produced using thermosetting resins, which are primarily formaldehyde-based. The most common types are urea-formaldehyde (UF), phenol-formaldehyde (FF), and melamine-urea-formaldehyde (MUF). Applying these adhesives to materials for interior use can be potentially harmful to health due to the emission of volatile organic compounds (VOCs). Exposure to a mixture of these compounds can result in sensory irritation if concentration levels exceed the maximum recommended threshold.

[0007] On the other hand, highly reactive chemicals such as formaldehyde should not be included in the VOC mixture, and their effects should be evaluated independently. Formaldehyde is a well-known air pollutant with adverse health effects, of particular concern due to its classification as a human carcinogen (IARC, 2004, International Agency for Research on Cancer).

[0008] Tannin-based resins have been used in the wood industry since the 1970s, as a component obtained from biomass waste, primarily tree bark. These resins allow for the manufacture of particleboard through heat treatment, even without catalysts (Pizzi, A., Meikleham, N., Dombo, B., & Roll, W. 1995; “Autocondensation-based, zeroemission, tannin adhesives for particleboard”; European Journal of Wood and Wood Products; 53-3, 201-204). However, these resins exhibit low reactivity. According to this publication, tannin-based resin requires 37.5 s / mm at the laboratory level to achieve a board with physical and mechanical properties comparable to UF resin. This means that the reactivity of this resin with tannins is 4.7 times lower compared to UF at the laboratory level and up to 10 times less reactive compared to the industrial standard, which hinders its use at an industrial level due to loss of production capacity.

[0009] By using crosslinking agents, tannin-based resins improve their reactivity, with much of the publication and patenting focused on the use of formaldehyde. Here are some examples:

[0010] Patent DE19704525A1: "Method of manufacture of low-formaldehyde wood fibre or chip board bonded with tannin formaldehyde resin" explains the method for producing particleboard or fiberboard from powdered and liquid tannins crosslinked with formaldehyde, using urea as a sequestering agent to reduce emissions.

[0011] In DE19528492A1: “Medium density fiber board (MDF) without precursor or formaldehyde emission during drying - comprises treating fibers with a tannin binder component before and formaldehyde or precursor after drying and also saves binder,” a method for producing a tannin-based adhesive for MDF is presented, where the tannins are added to the fibers prior to the drying stage. After drying, formaldehyde is added to the fibers, and then the board is pressed.

[0012] EP1874513B1: “Method for producing reduced-emission, low-thickness swell medium density fiber boards and molded fiber parts” explains a method for the production of MDF where tannin-formaldehyde resins are used in combination with starch and methacrylate reaction resins for boards with densities above 1,000 kg / m³ 3 .

[0013] Tannins can also crosslink with aldehydes other than formaldehyde, such as acetaldehyde and propionaldehyde (Pizzi, A., Rossouw, DDT, & Daling, GME 1980; “The role of aldehydes other than formaldehyde in tannin-based wood adhesives”; Holzforschung und Holzverwertung, 32-4, 101-103), furfural (Jorda, J., Cesprini, E., Barbu, MC, Tondi, G., Zanetti, M., & Král, P. 2022; “Quebracho Tannin Bio-Based Adhesives for Plywood”; Polymers, 14-11, 2257), as well as with glyoxal (Ballerini, A., Despres, A., & Pizzi, A. 2005; “Non-toxic, zero emission tannin-glyoxal adhesives for wood panels”; Holz als Roh-und Werkstoff, 63, 477-478).

[0014] Components that are formaldehyde donors also exhibit crosslinking with tannins, such as hexamine (hexamethylenetetramine), oxazolidines and tris(hydroxymethyl)nitromethane (Trosa, A., & Pizzi, A. 2001; “A no- aldehyde emission hardener for tannin-based wood adhesives for exterior panels”; Holz als Roh-und Werkstoff, 59-4, 266-271).

[0015] Tannins can also crosslink with other naturally occurring components. Among these natural crosslinking agents for tannins is sucrose, whose interaction has been studied in recent years. The reaction mechanism between them involves the decomposition of sucrose to 5-HMF (5-hydroxymethylfurfural) during the hot pressing stage, which allows the formation of dimethylene ether bridges between the two molecules. In a study of the interaction of these two components for the manufacture of particleboard (Zhao, Z., & Umemura, K. (2014); “Investigation of a new natural particleboard adhesive composed of tannin and sucrose”; Journal of Wood Science, 60, 269-277), physical and mechanical properties were achieved with values ​​exceeding the requirements of the JIS A5908 standard. However, pressing times were long (10 minutes for a thickness of 9 mm) and moisture resistance was low (20 to 23% swelling at 24 hrs).

[0016] Acids, on the other hand, can act as catalysts to accelerate the conversion of sucrose to 5-HMF by lowering its activation temperature. For example, the use of sulfuric acid (Zhao, Z., Miao, Y., Yang, Z., Wang, H., Sang, R., Fu, Y., & Yong, Q. (2018); “Effects of sulfuric acid on the curing behavior and bonding performance of tannin-sucrose adhesive”; Polymers, 10-6, 651.) allows for a decrease in this temperature from 205°C–215°C to 136°C–128°C. The physical-mechanical property values ​​in this study improved with the use of this acid and met the requirements of the JIS A5908 standard. However, it is concluded that the adhesive consumption and pressing times obtained are far from the parameters of industrial and even laboratory production.

[0017] Despite the use of various crosslinking agents, tannin-based resins can exhibit low adhesive strength and water resistance. For example, in the case of tannin-formaldehyde resins, phenol-resorcinol-formaldehyde, phenol-formaldehyde, and urea-formaldehyde polymers are used to increase the degree of polymerization, thereby improving water resistance and reducing brittleness (Pizzi, A., & Scharfetter, HO 1978; “The chemistry and development of tannin-based adhesives for exterior plywood”; Journal of applied polymer science, 22-6, 1745-1761). These disadvantages play a significant role in determining the feasibility of a tannin-based adhesive that can compete industrially with current solutions.

[0018] On the other hand, because tannins obtained from bark extractions generally do not undergo a refining stage, they contain alcohols and sugars (mono- and polysaccharides found in wood) that can impair reactivity by reducing the adhesive strength of the resins and increasing water resistance. This impact on the performance of the adhesive formula is significant, as the tannin extract may only be composed of 70-80% active phenols.

[0019] In the case of the present invention, despite the lack of purification of the extracted tannins, the sugars present in the bark do not cause the negative effects of the state of the art, due to the process carried out with the present invention.

[0020] Furthermore, when tannins react, they exhibit a significant increase in molecular weight, which translates into practical difficulties such as a low pot life (Pizzi, A., 1983 “Wood Adhesives”, Ed. A. Pizzi, Marcel Dekker, New York, 1983).

[0021] Therefore, it is necessary to develop an adhesive that uses a natural binding agent, such as tannins, while also incorporating a renewable and natural crosslinking agent. This will result in an adhesive formula free of synthetic chemical compounds with potential risks to human health, promoting sustainable development and enabling certification as emission-free products. Furthermore, this adhesive formulation must address issues such as low reactivity, poor mechanical properties, and low moisture resistance.

[0022] Patent application CL2021000272A1, “A method for producing a natural, formaldehyde-free adhesive for wood panels,” addresses these problems by using a natural binder and crosslinking agent, such as soy flour and oxidized sucrose. The properties of panels manufactured with this adhesive meet the physical and mechanical requirements, with densities and pressing times similar to those observed with UF resin. However, panels manufactured with this natural adhesive have two problems: first, low moisture resistance (swelling in water), with values ​​up to 2.5 times higher than UF resin; and second, poor surface uniformity (high roughness and loose fibers on the surface in the case of MDF).

[0023] In the present invention, it has been discovered that a combination of aldehyde groups from oxidized sucrose and / or oxidized starch, along with condensed tannins of different species, starch, and citric acid, allows for the consolidation of a sustainable, 100% bio-based adhesive that is competitive with amine adhesives currently used in industry in terms of reactivity, physical-mechanical properties, and swelling of the resulting boards when exposed to water. Furthermore, the surface of these boards exhibits good uniformity, providing a smooth and homogeneous finish.

[0024] The formation of these aldehyde groups in sucrose is possible through the use of oxidizing agents such as periodic acid, a periodate salt, or hydrogen peroxide, which generate a glycol cleavage reaction known as the Malaprade oxidative cleavage. Depending on the conformation of the glycols, this reaction can lead to the formation of these aldehyde groups.

[0025] The present invention establishes an adhesive formulation method based on sustainable raw materials that, through oxidation chemistry, allows a high level of crosslinking.

[0026] Figure description:

[0027] Figure 1 represents a graph showing IB values ​​in N / mm 2(Internal bond) measured on MDF boards for the natural adhesive based on condensed radiata pine tannins, oxidized sucrose, and citric acid. The different columns correspond to different formulations according to whether they contain natural starch, oxidized starch, or no starch. Figure 2 represents the thickness swelling values ​​in water after 24 hours of immersion (in percentage). The different columns correspond to different formulations according to whether they contain natural starch, oxidized starch, or no starch.

[0028] Figure 3 shows the formaldehyde emission values ​​according to gas analysis using the DMC (Dynamic Microchamber) measurement method.

[0029] Detailed description of the invention:

[0030] The present invention relates to a manufacturing process for a new formaldehyde-free natural adhesive for MDF wood boards, which comprises a formulation of at least three components, which are derived from renewable sources.

[0031] The first component is tannins, which are abundant as flavonoids in plant tissues and are composed primarily of high molecular weight polyphenols. They are mainly extracted using hot water in combination with organic solvents and alkalis to increase the extraction yield. These types of tannins, called condensed tannins, are the main type of commercially available tannin and exhibit a high level of polymerization when reacting with certain crosslinking agents such as formaldehyde.

[0032] The second component is sucrose, a disaccharide composed of glucose and fructose that occurs naturally in plants. In the adhesive formulation of the present invention, it is used in its oxidized (partially or fully) form, its unoxidized (i.e., pure) form, or a combination of both. Partial oxidation refers to the process in which some of the natural polysaccharide, such as sucrose, remains unreacted. This unreacted amount ranges from 1% to 99% of the total polysaccharide used.

[0033] The third component is citric acid. Optionally, the formulation may also include a fourth component: starch, a natural polysaccharide produced by most plants to store energy. Industrially, it is extracted from the seeds, roots, and tubers of plants such as corn, tapioca, wheat, rice, and potatoes through milling, washing, sieving, and drying processes. In the adhesive formulation, it is used in its natural (unoxidized) form or in combination with its oxidized form.

[0034] With the present invention it was observed that, by achieving an optimal degree of reaction between condensed tannins, a crosslinking agent such as oxidized sucrose (partially or totally), citric acid, and optionally with starch, a natural adhesive can be obtained whose physical-mechanical properties equal and even surpass commercial synthetic resins.

[0035] In the present invention, the reaction between condensed tannins and pure sucrose is further enhanced when the tannins are in the presence of sucrose oxidized (partially or totally) with aldehyde groups. This interaction allows the formation of hemiacetal bonds between the hydroxyl groups of the tannins (-OH) and the aldehydes from the oxidized sucrose (sucrose-aldehyde), thus forming an adhesive with greater reactivity, a higher degree of polymerization, and therefore better mechanical properties and water resistance.

[0036] On the other hand, the aldehyde groups of oxidized sucrose (partially or totally) also react with starch molecules thanks to the hydroxyl groups present in it.

[0037] In the publication by Xu, H., Canisag, H., Mu, B., & Yang, Y. 2015; “Robust and flexible films from 100% starch cross-linked by biobased disaccharide derivative”; ACS Sustainable Chemistry & Engineering, 3-11, 2631-2639, flexible films are manufactured by crosslinking both molecules. However, this publication mentions the lack of studies that evaluate the interaction between sucrose and starch molecules and that also evaluate the low activation energy required to initiate crosslinking (33.22 kJ / mol, the lowest among the most commonly used crosslinking agents with starch), which would imply greater reactivity.

[0038] In the present invention it was obtained that at certain degrees of oxidation of sucrose and in the appropriate proportion, starch cross-links with sucrose-aldehyde to give rise to a reactive adhesive for wood boards in the presence of condensed tannins.

[0039] In turn, when starch oxidizes, functional groups such as aldehyde or carboxyl groups are introduced into its structure. These groups can react with the phenolic groups of tannins in a condensation reaction, forming cross-links between the two molecules. This can result in modifications to the properties of the starch and its ability to form stronger gels or films. This starch oxidation, in combination with condensed tannins and partially or fully oxidized sucrose, increases the reactivity of the adhesive formulation, allowing for shorter pressing times.

[0040] Finally, citric acid fulfills several functions in the formulation of the adhesive of the invention, where on the one hand it has been reported as an efficient agent in the conversion of saccharides to 5-HMF by lowering the temperature required for said conversion (Haworth, WN, & Jones, WGM 1944, 183, “The conversion of sucrose into furan compounds.” Part I. 5-Hydroxymethylfurfuraldehyde and some derivatives. Journal of the Chemical Society, 667-670) which would allow reducing the temperature needed during hot pressing for the polymerization of the adhesive. Furthermore, being a polycarboxylic organic acid, it has three carboxyl groups which interact with the hydroxyl groups forming ester bonds, thus allowing a greater degree of polymerization of the final adhesive (Zhao, Z., Umemura, K., & Kanayama, K. 2016, “Effects of the addition of citric acid on tannin-sucrose adhesive and physical properties of the particleboard.” BioResources, 11-1, 1319-1333).The process of the invention will be exemplified with condensed tannins obtained from radiata pine (Pinus radiata); however, condensed tannins from other species such as Pinus elliottii, Pinus taeda and Pinus pinaster and even those obtained from grapes are equally valid.

[0041] The process of obtaining the adhesive comprises the following stages:

[0042] 1) Sucrose Oxidation: Provide sucrose from a source containing this component, such as refined or unrefined sucrose, molasses, or any other form containing this component in solid or liquid form, preferably at least 50% w / w if in liquid form. React the sucrose with an oxidizing agent selected from periodic acid, sodium metaperiodate, and hydrogen peroxide, preferably hydrogen peroxide, in a molar ratio of 1:1 to 5:1 of oxidizing agent to sucrose, until the sucrose is oxidized between 1% and 100% (partial or total oxidation). Adjust the temperature of the oxidation reaction to a range of 35 to 90°C, preferably 80°C, adjust the pH to a range of 1.0 to 7.0, and adjust the reaction time to a range of 6 to 12 hours.

[0043] pH adjustment can be done using phosphoric acid or sulfuric acid to decrease it, or sodium hydroxide to increase it.

[0044] In the case where the oxidizing agent is sodium metaperiodate, due to the exothermic and kinetic nature of the reaction, the temperature must be controlled so that it does not exceed 40°C in order to control the reaction rate, and the reaction must be additionally protected from light to prevent reaction of the oxidizing agent with it.

[0045] Optionally, the reacted sucrose is cooled in an ice-water bath (0°C) to stop the oxidation reaction. Furthermore, combinations of oxidized (partially or fully) and unoxidized sucrose can be used in the adhesive formulation in ratios from 100 / 0 to 20 / 80 (oxidized sucrose / unoxidized sucrose).

[0046] 2) Preparation of starch when used in adhesive:

[0047] The starch used in the adhesive formulation can come from various sources, such as corn (cornflour), cassava (tapioca), potatoes, wheat, rice, etc. It can be used in solid form (powder) or in aqueous solution, preferably in powder form.

[0048] The type of starch to be used can be food grade or technical grade, of the natural, chemically modified or waxy type, preferring those natural starches without modifications.

[0049] The selected starch undergoes a hydrolysis and oxidation process comprising the following steps: Providing starch powder from a source such as corn, tapioca, wheat, rice, etc. Reacting the starch with an acid that allows its hydrolysis, such as hydrochloric or sulfuric acid, in a ratio of 9:1 to 15:1 starch to acid (w / w) at a concentration between 1 and 3 M (molar) for a stirring time of 3 to 5 hours. Using copper(II) sulfate pentahydrate as a catalyst at a ratio of 0.1% to 0.6% w / w relative to the starch. Reacting the previously hydrolyzed starch with an oxidant selected from: periodic acid, sodium metaperiodate, and hydrogen peroxide, preferably hydrogen peroxide, in a molar ratio of 0.5:1 to 2:1 oxidizing agent relative to starch.Adjust the temperature in the oxidation reaction to a range of 50 to 70°C, preferably 50°C, and adjust the reaction time to a range of 1 to 2 hours. 3) Provide condensed tannins:

[0050] Condensed tannins are provided for use in the formulation of the adhesive in powder form or in a solution, preferably with a solids percentage range of 15% to 40% w / w if in solution.

[0051] The extraction method for obtaining tannins from bark can vary. Some processes use aqueous or alcoholic solvents, followed by heating the solution to specific temperatures depending on the solvent used. Extraction time and bark particle size vary depending on the manufacturer, and some additives, such as NaOH, NaHSO₄, and urea, can be used to increase extraction efficiency.

[0052] 4) Mixing: Add the oxidized sucrose obtained in the oxidation stage along with the condensed tannins, citric acid, and a fraction of water. The amount of water ranges from 20% to 350% relative to the tannins, thus maintaining a homogeneous distribution of the components with an acceptable operating viscosity (below 1000 cP). Optionally, add a quantity of starch powder or solution, either oxidized or unoxidized, and maintain mechanical stirring until a homogeneous solution is obtained. Optionally, add a quantity of unoxidized sucrose powder or solution and maintain mechanical stirring until a homogeneous solution is obtained. Mixing should be carried out for at least 20 minutes.

[0053] Application examples:

[0054] The examples are carried out in the manufacture of MDF boards, where the natural, formaldehyde-free adhesive of the invention is applied. The MDF board (medium-density fiberboard) has the following characteristics: Board thickness: 10 mm. Pressing time: 100 seconds, equivalent to a pressing factor of 10 sec / mm. Press temperature: 200°C. Board density: 720–780 kg / m³ 3 Fiber used: radiata pine from industrial refining.

[0055] Example 1:

[0056] Sufficient adhesive was prepared to glue 2 kg of wood fiber from mechanical refining, according to the methodology of the invention.

[0057] Sucrose with a purity >99% is provided and dissolved in water at room temperature at a concentration of 60% w / w. Hydrogen peroxide is added at a molar ratio of 4:1 to sucrose, with 50% purity. The reaction temperature is controlled to maintain a temperature of 80°C. The oxidation reaction is carried out for 8 hours under constant stirring, yielding a dark amber-colored oxidized product. Once the reaction time is complete, the sample is cooled to stop the reaction.

[0058] Condensed radiata pine tannins are provided as a dry powder along with a volume of water such that a 30% w / w solution is formed. This tannin solution is added directly to the previously oxidized sucrose solution, kept under stirring, at a ratio of 1.5:1 tannins / oxidized sucrose.

[0059] Dry corn starch powder is provided at a ratio of 1.2:1 to dry tannins, along with a volume of water sufficient to form a 43% w / w starch solution. Citric acid is added at a ratio of 0.4:1 to dry tannins. The final mixture is stirred for 20 minutes at room temperature, yielding an adhesive with a concentration of 40.3% w / w.

[0060] The wood fibers are bonded by spray gluing in a rotating drum for a sufficient time until the adhesive is completely dispersed. A quantity of the adhesive preparation is used such that the bonding ratio is 20% for 2 kg of wood fibers (approximately 3% moisture content), and the fibers are then dried to a moisture content of 10% to 12% before being placed in the hot press.

[0061] Example 2:

[0062] Sufficient adhesive was prepared to glue 2 kg of wood fiber from mechanical refining, according to the methodology of the invention.

[0063] Sucrose with a purity >99% is provided and dissolved in water by stirring at room temperature to a concentration of 42% w / w. Sodium metaperiodate (NaICU) is added gradually at a molar ratio of 3:1 moles of NaICU to sucrose. The reaction is protected from light. The pH is adjusted to acidic values ​​within the range of 1.0 to 3.0 by adding phosphoric acid. The oxidation reaction is carried out for 6 hours under constant stirring, yielding an oxidized product.

[0064] Condensed radiata pine tannins are provided as a dry powder along with a volume of water such that a 30% w / w solution is formed. This tannin solution is added directly to the previously oxidized sucrose solution, kept under stirring, at a ratio of 1:1 tannins / oxidized sucrose.

[0065] Citric acid is added at a ratio of 0.2:1 to dry tannins. The final mixture is stirred for 20 minutes at room temperature, resulting in an adhesive with a concentration of 40% w / w. The wood fibers are bonded by spray gluing in a rotating drum for a sufficient time until the adhesive is completely dispersed. A quantity of the adhesive preparation is used such that the bonding ratio is 20% for 2 kg of wood fibers (approximately 3% moisture content), and the fibers are then dried to a moisture content of 10% to 12% before being placed in the hot press.

[0066] Example 3:

[0067] Sufficient adhesive was prepared to glue 2 kg of wood fiber from mechanical refining, according to the methodology of the invention.

[0068] Sucrose with a purity >99% is provided and dissolved in water by stirring at room temperature at a concentration of 53% w / w. 70% hydrogen peroxide is added at a molar ratio of 3.5:1 with respect to the sucrose. The reaction temperature is controlled to maintain it at 80°C. The oxidation reaction is carried out for 8 hours under constant stirring, yielding a dark amber-colored oxidized product. Once the reaction time is complete, the sample is cooled to stop the reaction.

[0069] Condensed radiata pine tannins are provided as a dry powder along with a volume of water such that a 30% w / w solution is formed. This tannin solution is added directly to the previously oxidized sucrose solution, kept under stirring, at a ratio of 2.5:1 tannins / oxidized sucrose.

[0070] Unoxidized sucrose powder is added at a ratio of 0.9:1 to dry tannins.

[0071] Citric acid is added at a ratio of 0.26:1 to dry tannins. The final mixture is stirred for 20 minutes at room temperature, yielding an adhesive with a concentration of 47.4% w / w. The wood fibers are bonded by spray gluing in a rotating drum for a sufficient time until the adhesive is completely dispersed. A quantity of the adhesive preparation is used such that the bonding ratio is 20% for 2 kg of wood fibers (approximately 3% moisture content), and the fibers are then dried to a moisture content of 10% to 12% before being placed in the hot press.

[0072] Example 4:

[0073] Sufficient adhesive was prepared to glue 2 kg of wood fiber from mechanical refining, according to the methodology of the invention.

[0074] Sucrose with a purity >99% is provided and dissolved in water at room temperature at a concentration of 60% w / w. Hydrogen peroxide is added at a molar ratio of 4:1 to sucrose, with 50% purity. The reaction temperature is controlled to maintain a temperature of 80°C. The oxidation reaction is carried out for 8 hours under constant stirring, yielding a dark amber-colored oxidized product. Once the reaction time is complete, the sample is cooled to stop the reaction.

[0075] A solution of oxidized starch is prepared separately by initially adding a mass of corn starch powder, which is hydrolyzed with 2 M hydrochloric acid in a 13:1 ratio of starch to hydrochloric acid (w / w) for 4 hours at 50°C. Then, copper(II) sulfate pentahydrate is added at a ratio of 0.4% w / w relative to starch, and finally, a 70% hydrogen peroxide solution is slowly added until a ratio of 0.8:1 moles of peroxide to moles of starch is reached. The reaction is allowed to continue for 1 hour at 60°C.

[0076] Condensed radiata pine tannins are provided as a dry powder along with a volume of water sufficient to form a 65% w / w solution. This tannin solution is added directly to the previously oxidized sucrose solution, under stirring, at a ratio of 1.4:1 tannins to oxidized sucrose. Dried corn starch powder is provided at a ratio of 0.5:1 to tannins (w / w), along with the oxidized starch solution at a ratio of 0.5:1 starch to tannins (w / w).

[0077] Citric acid is added at a ratio of 0.1:1 to dry tannins. The final mixture is left to stir for 20 minutes at room temperature, yielding an adhesive with a concentration of 50.5% w / w.

[0078] The wood fibers are bonded by spray gluing in a rotating drum for a sufficient time until the adhesive is completely dispersed. A quantity of the adhesive preparation is used such that the bonding ratio is 20% for 2 kg of wood fibers (approximately 3% moisture content), and the fibers are then dried to a moisture content of 10% to 12% before being placed in the hot press.

[0079] Figures 1 and 2 show 5 columns, where:

[0080] The first column on the left, colored light blue, corresponds to the control group, which consists of commercial urea-formaldehyde resin for MDF.

[0081] The second column below, in yellow, represents values ​​obtained for a formulation of tannins, citric acid, sucrose oxidized by hydrogen peroxide and corn starch, according to example 1.

[0082] The third brown column represents values ​​obtained for a formulation of tannins, citric acid, sucrose oxidized by sodium metaperiodate and corn starch, according to example 2.

[0083] The fourth orange column represents values ​​obtained for a formulation of tannins, citric acid and sucrose oxidized by hydrogen peroxide, according to example 3.

[0084] The fifth column in green represents values ​​obtained for a formulation of tannins, citric acid, sucrose oxidized by hydrogen peroxide, corn starch and oxidized starch, according to example 4. In Figure 3, the columns indicate the formaldehyde emission values ​​to the environment, obtained in ppm at 2 hours and after 7 days of cabinet ventilation for the boards manufactured by UF (first 2 columns on the left), for a formulation of tannins, citric acid, sucrose oxidized by hydrogen peroxide and corn starch according to example 1 (columns in the middle of the figure) and for a formulation of tannins, citric acid and sucrose oxidized by hydrogen peroxide according to example 3 (columns on the right side).

[0085] Advantages of the invention:

[0086] The present invention achieves the production of a natural and formaldehyde-free adhesive, whose IB values ​​reach or exceed those manufactured with UF resin under the same forming and pressing conditions.

[0087] Figure 1 shows significant differences between the IB values ​​obtained with the different formulations according to the examples described above. In the first column on the left, the IB value obtained for an MDF using commercial UF resin was 0.55, while all the columns to the right, which correspond to formulations according to the present invention, obtained higher values ​​on average.

[0088] These formulations offer an advantage because all the MDF boards compared were manufactured under the same conditions and with the same amount of resin (20% bonding ratio). The average values ​​were 0.58 (example 1), 0.65 (example 2), 0.69 (example 3), and 0.75 N / mm². 2 for the formulation that includes the use of oxidized starch (example 4).

[0089] Figure 2 shows the thickness variation values ​​after 24 hours of immersion in water. A significant advantage can be observed when using the natural formulation of the present invention, as the swelling values ​​remain close to the control group with UF, and sometimes the variation is even less in formulations that do not include starch (brown and yellow columns for examples 2 and 3, respectively).

[0090] In the first column on the left, the swelling value obtained for an MDF using commercial UF resin was 11%, and the values ​​for the natural formulations averaged 12% (example 1), 7% (example 2), 8% (example 3), and 16% (example 4).

[0091] Figure 3 shows the formaldehyde emission values ​​measured using the DMC method, revealing a significant difference compared to the values ​​obtained with a UF resin, which even includes a formaldehyde sequestrant (urea). The advantage of this formulation lies in the fact that after 7 days of ventilation, the formaldehyde emission values ​​are between 60% lower (example 1) and 70% lower (example 3). These emission values ​​fall within the range attributable to the emissions of radiata pine wood, suggesting that this adhesive formulation does not produce formaldehyde emissions in the resulting board.

Claims

CLAIMS 1. A manufacturing process for a natural, formaldehyde-free adhesive for MDF (medium-density fiberboard) wood panels, which allows for greater moisture resistance and greater uniformity on the surface of the panels, CHARACTERIZED in that it comprises the steps of: a.) sucrose oxidation: providing sucrose from a renewable source containing it; reacting the sucrose with an oxidizing agent, in a molar ratio of 1:1 to 5:1 of oxidizing agent to sucrose, until a sucrose oxidation of between 1% and 100% is obtained; adjusting the temperature in the oxidation reaction in a range of 35°C to 90°C; adjusting the pH in the oxidation reaction in a range of 1.0 to 7.0; adjusting the time of the oxidation reaction in a range of 6 to 12 hours. b.) proportion of condensed tannins: providing condensed tannins in powder form or in the form of a solution; c.) Mixing of components: add the oxidized sucrose obtained in the oxidation stage along with the condensed tannins, plus citric acid and a fraction of water; mix for at least 20 minutes.

2. The process of claim 1, CHARACTERIZED in that the sucrose source comprises: refined sucrose, unrefined sucrose, molasses or some other form containing this component in a solid or liquid state, wherein in the case of being in a liquid state the sucrose is over 50% w / w.

3. The process of claim 1, CHARACTERIZED in that the oxidizing agent is selected from: periodic acid, sodium metaperiodate, and hydrogen peroxide.

4. The process of claim 3, CHARACTERIZED in that the oxidizing agent is hydrogen peroxide.

5. The process of claim 1, CHARACTERIZED in that the temperature in the oxidation reaction is 80°C.

6. The process of claim 1, CHARACTERIZED in that the pH adjustment can be carried out by phosphoric acid or sulfuric acid to decrease it or sodium hydroxide to increase it.

7. The process of claim 3, CHARACTERIZED in that the oxidizing agent is sodium metaperiodate, the temperature is controlled so that it does not exceed 40°C and additionally the oxidation reaction is protected from light.

8. The process of claim 1, CHARACTERIZED in that the sucrose reacted in the oxidation reaction is cooled in an ice-water bath (0°C) to stop the oxidation reaction.

9. The process of claim 1, CHARACTERIZED in that the condensed tannins are provided in solution form in a range of solids percentages between 15% and 40% w / w.

10. The process of claim 1, CHARACTERIZED in that in the mixing stage, the amount of water ranges from 20% to 350% with respect to the tannins, maintaining a homogeneous distribution of the components with a viscosity less than 1,000 cP.

11. The process of claim 1, CHARACTERIZED in that the adhesive optionally further comprises starch and in the mixing stage is added Starch powder or solution, whether oxidized or not, is kept under mechanical agitation until a homogeneous solution is obtained.

12. The process of claim 11, CHARACTERIZED in that the starch comes from selected renewable sources of corn (cornflour), cassava (tapioca), potatoes, wheat, rice, among other sources.

13. The process of claim 12, CHARACTERIZED in that the starch is in a powder state.

14. The process of claim 12, CHARACTERIZED in that the starch is of food or technical grade, of the natural, chemically modified or waxy type.

15. The process of claim 11, CHARACTERIZED in that prior to the mixing stage, the starch is subjected to a hydrolysis and oxidation stage comprising: reacting the starch with an acid that allows its hydrolysis in a ratio of 9:1 to 15:1 of starch to the acid w / w used at a concentration between 1 and 3 M (molar) for a stirring time of between 3 and 5 hours; using copper(II) sulfate pentahydrate as a catalyst at a ratio of 0.1% to 0.6% w / w with respect to the starch; reacting the previously hydrolyzed starch with an oxidant in a molar ratio of 0.5:1 to 2:1 of oxidizing agent to the starch; adjusting the temperature in the oxidation reaction to a range of 50 to 70°C; and adjusting the reaction time to a range of 1 to 2 hours.

16. The process of claim 15, CHARACTERIZED in that the acid for the hydrolysis of starch is selected from hydrochloric or sulfuric acid.

17. The process of claim 15, CHARACTERIZED in that the oxidant that reacts with the hydrolyzed starch is selected from periodic acid, sodium metaperiodate, and hydrogen peroxide.

18. The process of claim 17, CHARACTERIZED in that the oxidant that reacts with the hydrolyzed starch is hydrogen peroxide.

19. The process of claim 15, CHARACTERIZED in that the temperature in the starch oxidation reaction is 50°C.

20. The process of claim 1, CHARACTERIZED in that the adhesive optionally further comprises unoxidized sucrose and in the mixing stage unoxidized sucrose in powder or solution is added and kept under mechanical agitation until a homogeneous solution is obtained.

21. The process of claim 11, CHARACTERIZED in that the adhesive optionally further comprises unoxidized sucrose and in the mixing stage unoxidized sucrose in powder or solution is added and kept under mechanical agitation until a homogeneous solution is obtained.

22. The process of claim 20 or 21, CHARACTERIZED in that the ratio of oxidized sucrose to unoxidized sucrose in the adhesive is between 100 / 0 and 20 / 80.

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