阅读说明：本技术 使亲水性聚合物生物材料增塑和致密化的方法以及亲水性聚合物生物材料 (Method for plasticizing and densifying hydrophilic polymeric biomaterials and hydrophilic polymeric biomaterials ) 是由 迪克·桑德贝里 贝内迪克特·奈泽斯 于 2020-05-18 设计创作，主要内容包括：本发明涉及使亲水性聚合物生物材料增塑和致密化的方法,所述亲水性聚合物生物材料具有至少一个表面,所述方法包括以下步骤：使待压缩的亲水性聚合物生物材料的表面软化；通过在升高的温度下向所述亲水性聚合物生物材料的经软化的表面上施加升高的压力持续预定时间段来压缩亲水性聚合物生物材料；降低施加至亲水性聚合物生物材料的温度,以及之后降低施加至亲水性聚合物生物材料的压力；其中向待致密化的亲水性聚合物生物材料的所述表面添加增塑液体,增塑液体为基于非咪唑的离子液体(IL)、有机超强碱或深共融溶剂(DES)。(The present invention relates to a method of plasticizing and densifying a hydrophilic polymeric biomaterial, the hydrophilic polymeric biomaterial having at least one surface, the method comprising the steps of: softening the surface of the hydrophilic polymeric biomaterial to be compressed; by subjecting the substrate to an elevated temperatureApplying elevated pressure on the softened surface of the aqueous polymeric biomaterial for a predetermined period of time to compress the hydrophilic polymeric biomaterial; reducing the temperature applied to the hydrophilic polymeric biomaterial and then reducing the pressure applied to the hydrophilic polymeric biomaterial; wherein a plasticizing liquid is added to the surface of the hydrophilic polymeric biomaterial to be densified, the plasticizing liquid being non-imidazole based An Ionic Liquid (IL), an organic superbase or a Deep Eutectic Solvent (DES).)

1. A continuous, semi-continuous or static process for plasticizing and densifying a hydrophilic polymeric biomaterial, the hydrophilic polymeric biomaterial having at least one surface, the process comprising the steps of:

a) providing the hydrophilic polymeric biomaterial;

b) softening the surface of the hydrophilic polymeric biomaterial to be compressed by applying heat to increase the temperature of the hydrophilic polymeric biomaterial;

c) compressing the hydrophilic polymeric biomaterial by applying an elevated pressure at an elevated temperature onto the softened surface of the hydrophilic polymeric biomaterial obtained in step b) for a predetermined period of time;

d) reducing the temperature applied to the hydrophilic polymeric biomaterial;

e) reducing the pressure applied to the hydrophilic polymeric biomaterial;

characterised in that step b is preceded by a step of densification of the preformSaid surface of said hydrophilic polymeric biomaterial is supplemented with a plasticizing liquid such that said liquid penetrates said surface during step b, and characterized in that said plasticizing liquid is based on non-imidazoleAn Ionic Liquid (IL), an organic superbase or a Deep Eutectic Solvent (DES).

2. The method of claim 1, wherein the non-imidazole-based isIs made by combining an amidine-based cation or a guanidine-based cation with a carboxylic acid.

3. The method of claim 1 or 2, wherein the plasticizing liquid is recovered from the hydrophilic polymeric biomaterial before, during, or after compressing the hydrophilic polymeric biomaterial.

4. The method of any one of claims 1 to 3, wherein step b) is performed by applying water vapor and/or heat to the surface of the hydrophilic polymeric biomaterial for a time sufficient to soften the surface to a predetermined depth.

5. The method according to claim 4, wherein step b) is performed by direct heating, hot plate heating, steam heating, boiling in water or dielectric heating, preferably by direct heating, hot plate heating or dielectric heating.

6. The method according to any one of claims 1 to 5, wherein step b) further comprises piercing the surface of the hydrophilic polymeric biomaterial at a plurality of locations to provide a plurality of piercing points on the surface to facilitate penetration of the plasticizing liquid into the material to a predetermined depth.

7. The method according to claim 6, wherein the puncture site comprises a hole having a depth of 1mm to 5mm, preferably 2mm to 4mm, even more preferably 3mm to 4mm, the hole having a diameter of about 0.1mm to 1.0mm, preferably 0.1mm to 0.5mm, and more preferably 0.2 mm.

8. The process according to any of the preceding claims, wherein the elevated temperature in step c) is in the range of about 80 ℃ to 270 ℃, preferably below 250 ℃, even more preferably below 200 ℃.

9. The method according to any one of the preceding claims, wherein the hydrophilic polymeric biomaterial is wood, wood-based material, and/or other lignocellulosic material.

10. The method of any one of the preceding claims, wherein the hydrophilic polymeric biomaterial is a low density wood species, and wherein the low density wood species may be a low density softwood or a low density hardwood.

11. The method according to claim 10, wherein the low density wood species is selected from wood species having a dry density of less than 600kg/m prior to treatment³The kind of (2).

12. The method of any one of the preceding claims, wherein the hydrophilic polymeric biomaterial may be in the form of sawn timber, raw timber, planks, sheets, barks, shavings, and/or chips.

13. The method of any one of the preceding claims, wherein the hydrophilic polymeric biomaterial has: at least one surface, possibly two or more surfaces; and a core, and the hydrophilic polymeric biomaterial is plasticized and densified on any or all of its surfaces, and optionally, the hydrophilic polymeric biomaterial is additionally plasticized and densified at the core of the material such that the entire material is plasticized and densified.

14. A densified hydrophilic polymeric biomaterial, characterized in that it has been plasticized and densified by the method according to any one of claims 1 to 13.

15. The densified hydrophilic polymeric biomaterial of claim 13, wherein a plasticizing liquid is substantially absent from the densified hydrophilic polymeric biomaterial obtained by the method.

Technical Field

The present invention relates to the field of densifying hydrophilic polymeric biomaterials, particularly low density wood species.

Background

In recent years, surface densification of wood boards has become an increasingly interesting subject. Forming a layer of densified wood tissue a few millimeters thick just below the surface can result in a two-fold increase in hardness (Gong et al, 2010; Laine et al, 2013a) and opens up new opportunities for using low density wood species in high value products such as wood flooring or window frames.

Generally, the densification process consists of three stages: the wood tissue is softened, then actually compressed, and finally the compressed wood is cured to prevent elastic recoil and moisture-induced set-recovery (Sandberg et al, 2013). During the last decades, many different densification methods have been investigated, as well as the influence of densification process parameters on wood properties. The first published studies of surface densification of wood are probably performed by Tarkow et al (1968). In 1990, Inoue et al reported a rather complex technique involving cutting grooves in the surface of wood to facilitate softening with water. Pizzi et al (2005) used friction welding equipment for surface densification and Rautkari et al (2009), which employed similar principles, found a positive correlation between the level of densification and hardness brought about by the friction welding method. Most recent studies have used batch type hot presses with a cooling system where the combination of moisture and heat plasticizes or at least softens the wood sufficiently to allow the wood tissue to deform easily and densify the wood surface. Laine et al (2013a) achieved nearly a two-fold increase in brinell hardness using this method. An even greater hardness increase was achieved by softening the wood in boiling water prior to densification (Lamason and Gong, 2007).

The surface densification process in hot presses takes only a few minutes (laie, 2014) and is therefore suitable in principle for conversion to an industrial process (Neyses et al, 2016). However, a major obstacle hindering the widespread commercialization of surface densified wood products is perhaps elastic springback, particularly the fixing-recovery of compressed wood tissue after exposure to moisture. By introducing a cooling stage to bring the temperature of the densified wood below 80 ℃ before the pressure is released, the elastic recoil (Neyses, 2016) that occurs immediately upon release of the compression force is greatly reduced. Moisture-induced fixing-recovery can be eliminated by chemical modification, by impregnation with a resin, or by hot-water-mechanical post-treatment (Kutnar et al, 2015). As early as the 90 s of the 20 th century, Inoue et al (1993) showed: set-back can be practically eliminated by post-steaming the compressed wood at 200 ℃ for 1 minute or at 180 ℃ for 8 minutes. Similar results were reported by Navi and Heger (2004). Kutnar et al (2012) almost eliminated the fixing-recovery by performing the densification process in a closed system under saturated steam conditions at 170 ℃. Similar effects were observed in other studies (Fang et al, 2006; Inoue et al, 2008; Gong et al, 2010). Laine et al (2012) show: the thermal post-treatment almost eliminates the fixing-recovery, although with a treatment time of several hours. Other studies have successfully employed impregnation treatments to reduce set-back, primarily by using conventional binders or resins as additives (Stamm, 1964; Gabrielli and Kamke, 2010; Pfriem et al, 2012; Khalil et al, 2014). Unfortunately, these established methods are either very time consuming or difficult to convert to continuous processes, and they often cause environmental problems.

One of the main theories describing the underlying mechanisms of elastic recoil and fixation-recovery is proposed by Navi and Sandberg (2012) and further described by Navi and Pizzi (2014), which propose that the fixation-recovery is due to elastic recovery of the deformation of crystalline cellulose, which is "frozen" inside the plastically deformed matrix of lignin and hemicellulose. The re-softening of the matrix after it has been densified (e.g. by moisture) results in the recovery of elastic deformation in the crystalline cellulose. Our hypothesis is that if plastic deformation of crystalline cellulose can be achieved in the initial plasticizing/softening stage prior to densification, the set-back will be greatly reduced. However, this is difficult to achieve with existing methods of plasticizing/softening wood, such as a combination of moisture and heat, or treatment with gaseous ammonia (Schuerch et al, 1966).

Disclosure of Invention

It is an object of the present invention to provide a method for densifying hydrophilic polymeric biomaterials, wherein the method is industrially advantageous. Furthermore, it is an object of the present invention to provide a method for densifying hydrophilic polymeric biomaterials, which reduces the set-back and spring-back while increasing the hardness of the densified hydrophilic polymeric biomaterials. Furthermore, it is an object to provide such a method: the method is used to plasticize a precisely defined volume of a piece of hydrophilic polymeric biomaterial (e.g., a surface area of sawn timber to a depth) so that subsequent densification can be confined to that defined volume.

According to a first aspect, this object is achieved by providing a continuous, semi-continuous or static process for plasticizing and densifying a hydrophilic polymeric biomaterial, said hydrophilic polymeric biomaterial having at least one surface, said process comprising the steps of:

a) providing a hydrophilic polymeric biomaterial;

b) softening the surface of the hydrophilic polymeric biomaterial to be compressed by applying heat to increase the temperature of the hydrophilic polymeric biomaterial;

c) compressing the hydrophilic polymeric biomaterial by applying an elevated pressure at an elevated temperature onto the softened surface of the hydrophilic polymeric biomaterial obtained in step b) for a predetermined period of time;

d) reducing the temperature applied to the hydrophilic polymeric biomaterial;

e) reducing the pressure applied to the hydrophilic polymeric biomaterial;

wherein a plasticizing liquid is added to the surface of the hydrophilic polymeric biomaterial to be densified prior to step b), such that the liquid penetrates the surface during step b), and wherein the plasticizing liquid is non-imidazole-basedIonic Liquids (IL), organic superbases or Deep co-melting solvents (Deep Eutectic Solvent, DES).

Based on non-imidazolesThe Ionic Liquid (IL) of (a) may be made from an amidine-based cation or a guanidine-based cation in combination with a carboxylic acid.

The plasticizing liquid may be recovered from the hydrophilic polymeric biomaterial before, during, or after compression of the hydrophilic polymeric biomaterial.

Step b) may be performed by applying water vapor and/or heat to the surface of the hydrophilic polymeric biomaterial for a time sufficient to soften the surface to a predetermined depth.

Step b) can be carried out by direct heating, hotplate heating, steam heating, boiling in water or dielectric heating, preferably by direct heating, hotplate heating or dielectric heating.

Step b) may further comprise puncturing the surface of the hydrophilic polymeric biomaterial at a plurality of locations to provide a plurality of puncture points on the surface to facilitate penetration of the plasticizing liquid into the material to a predetermined depth.

The puncture site may comprise a hole having a depth of 1mm to 5mm, preferably 2mm to 4mm, even more preferably 3mm to 4mm, said hole having a diameter of about 0.1mm to 1.0mm, preferably 0.1mm to 0.5mm, more preferably 0.2 mm.

The elevated temperature in step c) may be in the range of about 80 ℃ to 270 ℃, preferably below 250 ℃, even more preferably below 200 ℃.

The hydrophilic polymeric biomaterial may be wood, wood-based materials, and/or other lignocellulosic materials. Further, the hydrophilic polymeric biomaterial may be a low density wood species, wherein the low density wood species may be a low density softwood or a low density hardwood. The low density wood species may be selected from wood species having a dry density of less than 600kg/m prior to treatment³The kind of (2). The hydrophilic polymeric biomaterial may be in the form of sawn timber, logs, planks (planks), boards (boards), veneers (veneers) and/or shavings (wafers).

In one embodiment, the hydrophilic polymeric biomaterial has at least one surface, possibly two or more surfaces, and a core, and is plasticized and densified on either or all of its surfaces, and optionally, the hydrophilic polymeric biomaterial is additionally plasticized and densified at the core of the hydrophilic polymeric biomaterial, such that the bulk of the treated hydrophilic polymeric biomaterial is compressed and plasticized.

According to a second aspect, there is provided a densified hydrophilic polymeric biomaterial that has been compressed and plasticized by a method according to the first aspect and any embodiment thereof.

According to one embodiment of the densified hydrophilic polymeric biomaterial, the plasticizing liquid is substantially absent from the densified hydrophilic polymeric biomaterial obtained by the method according to the first aspect.

Drawings

FIG. 1: proposed is a mechanism for breaking hydrogen bonds between cellulose chains with ionic liquids.

FIG. 2: overview of fix-restore. According to table 1, the dark grey bars are untreated sample sets and the light grey bars are chemically treated sample sets. Error bars show ± 1 standard deviation.

FIG. 3: average fixation-recovery as a function of densification temperature. The dark curves correspond to group C150 and groups R200 through R270, and the light curves correspond to groups 200DBU through 270 DBU.

FIG. 4: average brinell hardness before densification (left bar) and after densification (right bar). Error bars show ± 1 standard deviation.

FIG. 5: near the subsurface densification of (a) the chemically treated sample and (b) the untreated sample. The horizontal bars show the distance between the annual rings.

FIG. 6: densification process flow from stages (a) to (e). It shows the point of addition of the plasticizing liquid to the surface of the hydrophilic polymeric biomaterial to be densified, and the point at which the plasticizing liquid can be recovered in the process.

Definition of

As used herein, the term "ionic liquid" refers to a salt comprising at least one cation and at least one anion, which salt is in liquid form at a temperature below 100 ℃. The Ionic Liquid (IL) may be in liquid form at less than 50 ℃, 40 ℃, 30 ℃, 25 ℃ or 20 ℃. Some ILs have melting points below room temperature; wherein some of them have melting points even below 0 ℃. IL has a low degree of symmetry. In addition, the charge of the cation as well as the charge of the anion is distributed over a larger volume of molecules by resonance. Therefore, solidification of the IL will occur at lower temperatures.

As used herein, the term "first generation IL" refers to an IL that: wherein the cation is generally organic and may be, for example, imidazolePyridine compoundPyrrolidine as a therapeutic agentAmmonium and sulfonium, and wherein the anion can be organic or inorganic. The organic anion may be, for example, an alkyl sulfate, a tosylate orA mesylate salt. The inorganic anion may be, for example, bis (trifluoromethylsulfonyl) imide, hexafluorophosphate, tetrafluoroborate or a halide such as fluoride (F)^-) Chloride (Cl)^-) Bromide (Br)^-) Iodide (I)^-) And astatide (At)^-)。

As used herein, the term "organic superbase" refers to an organic very basic compound or corrosive substance with high affinity for protons. An organic superbase is defined as an organic compound with a basicity greater than that of a proton sponge with a conjugated pKa of 12.1. Organic superbases are almost always charge neutral nitrogen-containing species. Despite the great proton affinity, organic superbases exhibit low nucleophilicity. Organic superbases include phosphazenes, amidines and guanidines. Other organic compounds may also conform to the physicochemical or structural definition of "superbases". Proton chelators such as aromatic proton sponges and 3, 7-diazabicyclo [3.3.1] nonane (bispidine) are also superbases.

As used herein, the term acid-base conjugated IL refers to a relatively new class of IL, which is typically composed of a mixture of an organic superbase and a carboxylic acid. This type of IL is distillable, they can rapidly dissolve cellulose to high concentrations, and can recover it from cellulosic material with a recovery rate of greater than 99% (King et al, 2011). The term acid-base conjugated IL also includes so-called "switchable" IL, wherein gaseous CO is used at ambient pressure and room temperature₂An organic superbase and an alcohol are bubbled to produce IL. The IL may be returned to its original composition by bubbling nitrogen or argon into the solution (parviaien, 2016). Acid-base conjugates are cheaper, more environmentally friendly and easier to recycle than first generation IL.

As used herein, the term "deep eutectic solvent" (DES) refers to a relatively new class of chemicals that can be used as solvents, reactants, and catalysts. DES are generally classified as ionic liquids, but they can also be obtained from non-ionic substances. Typically, DES is by quaternary ammonium orThe halide salt of the cation (acting as a hydrogen bond acceptor,HBA) with a Hydrogen Bond Donor (HBD) such as urea, glycerol or ethylene glycol. The DES is characterized in that: it exhibits a lower melting point compared to HBA or HBD because strong hydrogen bonding prevents crystallization of the parent component. DES, in particular, exhibits certain advantages over ionic liquids and first generation IL, such as ease of preparation from two or more components without the need for separate purification steps. DES can be obtained from readily available, bio-based, inexpensive, low toxicity materials that can be regenerated in some cases. Thus, the price level of DES is an order of magnitude lower than that of the first generation IL. While DES exhibits higher toxicity than some of its individual components, DES has relatively low toxicity, is biodegradable, and has low vapor emission. These properties make DES one of the most promising solvents and chemicals for sustainable material production.

As used herein, the term "hydrophilic polymeric biomaterial" includes wood, wood-based materials, and other lignocellulosic materials. As used herein, wood is a complex polymeric structure consisting of lignin and carbohydrates, i.e., cellulose and hemicellulose, which in any form and/or shape, i.e., fiber, wood chips, shavings, wood skin, or sawn timber, form the visible lignocellulosic structure of wood. Examples of wood-based materials are MDF (Medium Density fiberboard), chip board (particle board), OSB (Oriented strand board), plywood (plywood), and cross-laminated wood (cross-laminated wood). Examples of other lignocellulosic materials are dry matter or biomass comprising mainly cellulose and lignin, such as bamboo and reed canary grass. There is a strong correlation between the properties of the hydrophilic polymeric biomaterial (e.g., wood) and the properties of the particular tree or plant from which it is produced. The density of wood varies from species to species, within a species and even within a tree. The density of a hydrophilic polymeric biomaterial is related to its hardness and other mechanical properties. The hydrophilic polymeric biomaterial, and in particular wood, may be in the form of sawn timber, logs, planks, veneers, shavings and/or chips. In the present disclosure, the term "wood" is often used because it is the material that is most often subjected to the densification process, but this should not be considered limiting because the scope of the present invention encompasses hydrophilic polymeric biomaterials as defined above.

As used herein, the term "densifying" or "densifying … …" refers to the process of softening, compressing or otherwise shaping a material, followed by solidifying the material to prevent the material from returning to the original shape, i.e., the material has been formed under plastic deformation. For simplicity, the term "compression" is used hereinafter for the forming step, but this may therefore require another form of forming of the material in question. Typically, plastic deformation is irreversible. However, an object that is within the plastic deformation range will first undergo an elastic deformation (which is reversible), and thus will partially return to its original shape when the force causing the deformation is removed. This phenomenon is called rebound.

As used herein, the term "springback" refers to the geometric change made to a material at the end of a forming process when the material is released from the force of the forming tool, as well as the ability or tendency of the bent or formed elastic material to at least partially recover its elastic deformation. Alternatively, the term "elastically resilient" may be used. This is a common problem in the art of shaped wood, well known to the skilled person.

A "plasticizer" is an additive that increases the ductility or reduces the viscosity of a material, i.e., a plasticizer softens the material but does not necessarily make it plastically formable.

The plasticizer may be "weak" or "strong". Weak plasticizers soften materials to allow large deformations without destroying the macrostructure, but the deformations are not purely plastic.

In practice, this process means treating a material, such as wood, with a weak plasticizer, and then compressing the material, thereby densifying. Some solidification is then required so that the densified elastomeric component is "frozen" in a quasi-stable state and the material remains densified and does not return to its original shape before the material was softened and compressed. Densification processes are well known in the art and are commonly used to increase the density of wood or wood-based materials such as plywood, chipboard and MDF. When re-plasticized, the material deformed by the weak plasticizer will return to its original shape.

Strong plasticizers allow the material to deform in a purely plastic manner, with the elastic component being absent or negligible. The material deformed by the strong plasticizer will remain stable over time even when re-plasticized.

Unlike metals, for example, hydrophilic polymeric biomaterials such as wood may exhibit "virtual plastic deformation," meaning that after being formed by a process similar to that defined herein as "densification," the wood may behave "plastically deformed" until it is exposed to elevated moisture or heat conditions or other treatments that alter the internal molecular balance, thereby reforming toward its original shape. This phenomenon is called fix-recovery.

As used herein, the term "set-back" refers to a process in which a shaped wood will return toward its original shape when exposed to, for example, water or moisture. This is a common problem in the field of softened or shaped wood, well known to the skilled person.

As used herein, the term "shaped wood" refers to wood whose shape has been altered by compression, densification, drawing, bending or molding.

As used herein, a "plasticizing liquid" is a liquid used to facilitate softening and plasticizing of a material and/or to help solidify or densify the material during a densification process that will reduce or eliminate spring back and/or set-back effects.

Detailed Description

At the beginning of the 21 st century, the possibility of dissolving and plasticizing cellulose by treatment with so-called Ionic Liquids (IL) was reported (Swatloski et al, 2002;etc., 2007). As mentioned above, IL is a salt that is liquid at temperatures typically below 100 ℃ (Hanabusa et al, 2018), meaning that it is composed of ions rather than electrically neutral moleculesAnd (4) forming. In addition to being considered as a "green solvent" for cellulose, IL has also been reported to have a positive effect on antifungal and antimicrobial activity as well as on fire resistance and UV stability of wood (Pernak et al, 2004; Foksowicz-Flaczyk and Walentowska, 2012; Patachia et al, 2012; Miyafuji and Fujiwara, 2013). The first generation of IL's for dissolving cellulose are generally based on imidazolesIs usually combined with a halogen anion such as chloride or bromide. Zhang et al (2017) reported a mechanism that relies on the synergistic action of cations and anions to break hydrogen bonds between cellulose chains (FIG. 1). For example 1-ethyl-3-methylimidazoleThe small anion of the chloride interacts with the hydrogen atoms of the cellulose hydroxyl groups, while the larger imidazoleThe cation is bonded to a less sterically hindered oxygen atom. Ou et al (2014) reported that IL treatment converted wood into a thermoplastic material with no or very little elastic deformation under load. This indicates that elastic rebound and fixation-recovery can be greatly reduced, i.e., the IL acts as a strong plasticizer.

Based on imidazolesThe IL's of (1) are highly efficient cellulose solvents, but they are also expensive (Hanabusa et al, 2018). For this reason, many researchers have been focusing on finding cheaper alternatives, and some studies explored the possibility of IL made from amidine-based cations or guanidine-based cations in combination with carboxylic acids as anions (e.g. propionic acid or acetic acid) (King et al, 2011). Due to the overbased nature of these amidines and guanidines, they are also known as organic superbases (Ishikawa, 2009). Such an IL is referred to as an acid-base conjugated IL. Parviainen et al (2013) and Hanabusa et al (2018) report that this new generation IL ratio is based on imidazoleIs relatively cheaper and easier to recover, while maintaining the ability to dissolve cellulose: (Etc., 2010; domiguez de Maria, 2014). Since wood itself is somewhat acidic, pretreatment with only amidine or guanidine components as cations was tested in this study to see if in situ IL formation with the acid groups of wood as anions was occurring. The in situ IL will then plasticize the cellulose.

The inventors hypothesized that IL pretreatment would increase the hardness of the densified wood surface compared to wood softened with heat and moisture and then densified (fig. 4). Two reasons are expected: first, the level of "strong" plasticization is generally higher, resulting in more intense densification, as compared to softening the wood with heat and moisture; secondly, for untreated wood, the heated press platens will rapidly dry the outermost wood surface on contact and reduce the softening level in this area. Thus, the density peak after densification will be slightly lower than the surface. However, for IL treated wood, moisture is not required to achieve the maximum softening level, and this will shift the density peak to the outermost regions of the wood surface. This proposed effect stems from the analysis of the strain versus time curve of the brinell hardness test reported in previous studies (Neyses et al, 2017). As is evident from the experimental section below, this hypothesis was confirmed in the experiments performed.

Therefore, the purpose of the studies disclosed in the experimental section below was to evaluate the effect of IL and organic super base pretreatment on the elastic resilience, set-recovery and brinell hardness of surface densified wood. Various combinations of chemical compounds, solution concentrations, and densification temperatures were tested. Since Deep Eutectic Solvents (DES) may also be classified as ionic liquids, as disclosed in the definitions section above, DES is also included within the scope of the present disclosure. The method according to the present disclosure is a continuous, semi-continuous or static method of plasticizing and densifying a hydrophilic polymeric biomaterial having at least one surface. The method comprises the following steps:

a) providing a hydrophilic polymeric biomaterial;

b) softening the surface of the hydrophilic polymeric biomaterial to be compressed by applying heat to increase the temperature of the hydrophilic polymeric biomaterial;

c) compressing a hydrophilic polymeric biomaterial by applying elevated pressure at elevated temperature onto the softened surface of the wood obtained in step b) for a predetermined period of time;

d) reducing the temperature applied to the hydrophilic polymeric biomaterial;

e) reducing the pressure applied to the hydrophilic polymeric biomaterial;

wherein a plasticizing liquid is added to the surface of the hydrophilic polymeric biomaterial to be densified prior to step b, such that the liquid penetrates the surface during step b, and wherein the plasticizing liquid is non-imidazole-basedAn Ionic Liquid (IL), an organic superbase or a Deep Eutectic Solvent (DES).

The softening of the hydrophilic polymeric biomaterial may be carried out on at least one surface for a predetermined amount of time such that the first outer portion intended to be compressed and thus densified will be softened to a predetermined depth below the surface while the core below the first outer portion may not be softened and thus not densified during the subsequent compression step. When the material has more than one surface, it is also possible to soften only one surface of the material, i.e. the first outer portion, without softening the subsequent outer portions, e.g. the second, third, fourth, etc. Thus, only one of the several surfaces is softened and thereby densified by the methods herein. It is also possible to soften the material throughout the entire material so that all portions of the material (both the first outer portion and the optional subsequent outer portion) as well as the core are softened, whereby the compression results in the entire material being compressed and densified throughout its entire depth. The latter method will result in a more dense material than the first mentioned method. The skilled person is able to determine the amount of time a particular material needs to be heated in step b) to soften the material, as well as the pressure and temperature required in order to compress and densify the softened material in step c).

By using IL, organic superbase or DES as plasticizing liquid, no plasticizing liquid is consumed. The plasticizing liquid can be recovered, since it can be collected during or after compression of the material and reused in subsequent batches. Once the hydrophilic polymeric material is softened by the combination of plasticizing liquid and heat, the hydrogen bonds between the cellulose chains will be broken and remain so until the material is densified and new bonds can be formed with the aid of the extrusion energy. For this reason, it is not necessary to keep the plasticizing liquid in the hydrophilic polymer biomaterial to achieve the desired reduction in fixation-recovery. This theory is supported by (Khakalo et al, 2019). The plasticizing liquid can thus be discharged from the material during or after compression and can be reused. It may be possible to recover at least 80%, or at least 85%, or at least 90% of the plasticizing liquid during the process of the present disclosure. Thus, the plasticizing liquid may be reused thereafter when processing a subsequent batch of hydrophilic polymeric biomaterial. Thus, the method of the present disclosure is cost-effective. The possibility of reusing the plasticizing liquid is also an environmental advantage, since less resources are required for providing a new plasticizing liquid, compared to using glue or resin or the like to reduce the springback and the fixing-recovery of the material, which must be provided in a new batch each time the previous method is performed. By performing the method of the present disclosure, no or low levels of IL, DES, or organic superbase are consumed, whereas in conventional methods resins or gums or other hardeners are consumed.

Furthermore, the method according to the prior art uses for example a glue, a resin, maltodextrin or another hardening agent, which hardens within the hydrophilic polymeric biomaterial and thus remains therein. Thus, in the prior art, it is the gum, resin, maltodextrin or other hardening agent that keeps the material compressed and densified. In contrast, by using the methods of the present disclosure, the resulting compressed hydrophilic polymeric biomaterial will be substantially absent of the plasticizing liquid, and the stiffening of the treated hydrophilic polymeric biomaterial is due only to the compression process and not to the stiffening substance therein. As mentioned above, once the hydrogen bonds between the cellulose chains are broken, the plasticizing liquid is no longer used and can therefore be discharged from the material. The compression during the elevated pressure and elevated temperature is sufficient to compress and densify the material with broken hydrogen bonds. Without being bound by it, one theory is: hydrogen bonds will be reformed in a new arrangement within the material in the compressed state during the compression step so that the compressed form will remain compressed even when the elevated pressure and temperature are removed.

A further advantage of using the method according to the present disclosure is that there is no risk of leaching during the service life of the densified product, since the plasticizing liquid will in principle not remain within the material after the method is completed. Thus, there is no hardener within the material that risks leaching out of the material.

Thus, the present disclosure also provides a densified hydrophilic polymeric biomaterial that has been densified by the methods of the present disclosure. The densified hydrophilic polymeric biomaterial does not contain any resins, gums, or other hardening agents such as maltodextrin. The densified hydrophilic polymeric biomaterial contains substantially no hardening agent. Thus, a densified hydrophilic polymeric biomaterial that contains substantially no plasticizing liquid is obtained after completion of the method of the present disclosure.

By using the method of the present disclosure, the fiber orientation of the hydrophilic polymeric biomaterial does not have to be considered when entering the hydrophilic polymeric biomaterial into the process. Thus, when feeding the hydrophilic polymeric biomaterial into, for example, an automated system for performing the methods of the present disclosure, it is not necessary to control the orientation of the fibers in the hydrophilic polymeric biomaterial. Therefore, it is easy to expand the process of the method, and to perform a continuous or semi-continuous process of the method and a static process of the method. Softening of hydrophilic polymeric biomaterials has traditionally been performed by applying water vapor or heat to the surface of the hydrophilic polymeric biomaterials for a time sufficient to soften the surface to a predetermined depth. Softening can also be carried out by direct heating, hot plate heating, steam heating, boiling in water or dielectric heating. Conventional procedures for softening hydrophilic polymeric biomaterials are well known to the skilled artisan. According to the present disclosure, the softening is preferably performed by direct heating, hot plate heating or dielectric heating. This is because the softening based on water vapour may reduce the efficiency of the plasticizing liquid in the present process.

By plasticizing the hydrophilic polymeric biomaterial with any of the plasticizing liquids mentioned herein, a method is obtained that will allow a user to define the volume of the hydrophilic polymeric biomaterial to be densified. For example, the hydrophilic polymeric biomaterial may be soaked in a plasticizing liquid. By controlling the soaking time of the material in the plasticizing liquid, depending on the amount of time, the penetration of the liquid into the material, and thus the depth of the plasticized material, can be controlled and defined. Furthermore, the heat may be controlled according to the elevated temperature applied to the material and/or the amount of time the elevated temperature is applied in order to control and define the volume to be densified by the present method. This is particularly advantageous when surface densification is required, for example densification of a portion of the sawn timber. Furthermore, it appears that the use of the plasticizing liquid mentioned herein to plasticize hydrophilic polymeric biomaterials is stronger than the prior art. In plasticizing and densifying hydrophilic polymeric biomaterials, unless the entire material is plasticized, there is always a transition zone between the already plasticized hydrophilic polymeric biomaterial and the non-plasticized hydrophilic polymeric biomaterial. This transition zone can be minimized by using the method according to the present disclosure compared to using the method according to the prior art. Since the transition zone itself is not a fully densified zone, a smaller or narrower transition zone is advantageous and thus tends to increase the level of set-back.

The surface of the hydrophilic polymeric biomaterial may be pierced at a plurality of locations to provide a plurality of puncture points on the surface to facilitate softening of the material and penetration of the plasticizing liquid into the surface to a predetermined depth of the material. The puncture site comprises a hole having a depth of 1mm to 5mm, preferably 2mm to 4mm, even more preferably 3mm to 4mm, the diameter of the hole being about 0.51mm to 1.0mm, preferably 0.8mm to 0.5mm, and more preferably 0.1mm to 0.2 mm. Thus, as described above, the puncture site may enable the plasticizing liquid to penetrate only the first outer portion of the hydrophilic polymeric biomaterial or throughout the bulk of the hydrophilic polymeric biomaterial (including both the first outer portion of the hydrophilic polymeric biomaterial and the core).

By using the method according to the present disclosure, the inventors have seen a significant reduction in the set-back of compressed wood. As shown in the experimental section below, wood samples treated according to the methods of the present disclosure exhibit significantly reduced set-back compared to wood samples treated by the same procedure but without the plasticizing liquid in the form of an ionic liquid or an organic superbase. Interestingly, the inventors could demonstrate that more cost-effective acid-base conjugated IL's enable at least the first generation of imidazole-based ILAs much fixation-recovery as IL. Indeed, all the IL and organic superbases proposed in the experimental section below have much stronger effects than the IL tested in earlier studies not included in the present invention (Neyses et al, 2017).

Hydrophilic polymeric biomaterials treated with polymeric resins and then densified according to the prior art may exhibit a significant reduction in fixation-recovery (Gabrielli et al, 2010) and an increase in hardness over densified wood without such treatment (Pfriem et al, 2011). The increase in hardness can be almost as high as that achieved by the method according to the present disclosure (furfuryl alcohol impregnation with a 33.9% weight percent increase). However, methods such as the methods reported by Gabrielli et al (2010) and Pfriem et al (2011) consume significant time and material resources. Resin processing requires a closed system, i.e. autoclave/pressure vessel, and takes several hours. Furthermore, the process cannot be performed in a continuous manner due to the need for a closed system. As previously mentioned above, the resin remains in the densified material and cannot be recycled.

The inventors further observed a reduction in springback by using the method according to the present disclosure. Wood treated with the method according to the prior art may have an elastic rebound of about 10% to 20%. Hydrophilic polymeric biomaterials that have been treated according to the present invention exhibit an elastic recoil of about 0% to 5% or 0 ± 3%, which means that the recoil is less than the uncertainty of the measurement step.

The hardness of the hydrophilic polymeric biomaterials treated with the methods of the present disclosure is significantly increased. As shown in the experimental section below, wood samples treated according to the method of the present invention showed a significant increase in brinell hardness compared to wood samples treated by the same procedure but without the plasticizing liquid in the form of an ionic liquid or an organic superbase.

Another advantage of the process of the present disclosure is that the temperature used in the compression step of the process, step c), can be reduced. By using the present method, the temperature applied during compression may be lower than in the prior art, e.g. 80 ℃ to 270 ℃ or 100 ℃ to 270 ℃, preferably lower than 250 ℃, more preferably lower than 200 ℃, and the temperature may even be lower than 150 ℃ or lower than 100 ℃.

As described in the "definitions" above, the hydrophilic polymeric biomaterials can be wood, wood-based materials, and other lignocellulosic materials. The hydrophilic polymeric biomaterial may be in the form of sawn timber, logs, planks, sheets, veneers, shavings and/or chips. Preferably, the hydrophilic polymeric biomaterial is a low density wood species. However, it is also possible to subject the high density species to the present method to even further increase its hardness and/or density.

The low density wood species that can be densified by the present process may be selected from any so-called softwood or hardwood. The low density wood species are preferably selected from wood species having a dry density of less than 600kg/m prior to treatment³The kind of (2). The low density wood species may also be selected from: cork wood, for example, Pinus (Pinophyta) such as Pinaceae (Pinaceae), Araucaceae (Araucaceae), Podocarpaceae (Podocarpaceae), Sciaonaceae (Sciadopityaceae), Cupressaceae (Cupressaceae), Cephalotaxaceae (Cephalotaxaceae), and Taxaceae (Taxaceae); or hardwoods such as linden (Tilia), alder (Alnus) and Populus (Populus). However, any low density wood speciesMay be treated according to the present method in order to densify and harden the low density wood species. The hydrophilic polymeric biomaterial may be in the form of sawn timber, logs, planks, sheets, veneers, shavings and/or chips.

Accordingly, the present inventors provide a method of: this method will increase the hardness of the hydrophilic polymeric biomaterial and reduce elastic recoil and fixation-recovery as compared to hydrophilic polymeric biomaterials treated with prior art methods or without any pretreatment.

Experimental part

Materials and methods

Sample preparation

The study was conducted on a knotless edgewood sample of scotch pine (Pinus sylvestris L.) of dimensions 124mm (L) by 25mm (T) by 18.5mm (R). The annual rings are oriented within ± 15 ° parallel to the tangential wood surface to be densified. The samples were kept in a climate chamber at a temperature of 20 ℃ and a Relative Humidity (RH) of 65% until they reached Equilibrium Moisture Content (EMC). The samples are grouped into seven classes according to their density and samples belonging to the highest or lowest density class are discarded. The samples were then selected so that each sample set (table 1) had the same number of samples of each density category.

Preparation of chemicals

Samples were diluted with ethanol to different concentrations of first generation IL, non-imidazole basedAn acid-base conjugated ionic liquid prepared by combining an amidine-based cation or a guanidine-based cation (organic superbase) with a carboxylic acid, or an organic superbase treatment (table 1). IL 1-butyl-3-methylimidazoleChloride (Bmim [ Cl ]]) And the organic superbases 1, 5-diazabicyclo (4.3.0) non-5-ene (DBN), 1, 8-diazabicyclo (5.4.0) undec-7-ene (CDBU) and 1,1,3, 3-tetramethyl-guanidine (TMG) were purchased from Acros Organics. Another IL was prepared by mixing DBN with propionic acid in equimolar ratio to give DBN propionate (DBN [ EtCO 2H)]) And then the product is obtained.

Chemical pretreatment

All sample groups to be chemically treated were oven dried at 103 ℃ to MC of 0% before pretreatment. A chemical solution is first applied with a pipette onto the tangential bark-side oriented wood surface to be densified. To keep the solution volume consistent, the contents of one pipette fill were distributed over the surface of both samples. Within one minute, the solution was completely absorbed by the wood. This procedure was repeated three more times for each sample. To facilitate the penetration of the chemicals into the wood, the surface was pierced with a 3mm to 4mm deep hole having a diameter of about 0.8 mm. Only group 250NDBU (table 1) had no holes. After pretreatment, the sample was placed in an oven at 60 ℃ for 14 hours to evaporate the ethanol. Omitting this step can lead to destruction of the wood during the densification stage due to too high steam pressure.

Densification process

The samples were densified in the radial direction in a press with one platen, heated and equipped with a water cooling system. Two or three samples were densified simultaneously. During the first 10 seconds of the process, a low pressure is applied sufficient to maintain proper contact between the wood surface and the heated press platens. The pressure is then increased to a level that will only lead to densification if the wood tissue is sufficiently softened. Depending on the parameters and processing, a target thickness of 14.98mm (set by the metal stops in the press) was achieved after a time of 90 to 240 seconds (table 1). Regardless of the time taken to reach the target compression, the sample was held at the set temperature for a total of 240 seconds, after which the cooling system was started. The press was opened when the press platen temperature reached 60 ℃, then the springback was measured (see "elastic springback" below), after which the test specimens were held in an oven at 60 ℃ until the fix-recovery measurement was performed (see "fix-recovery" below).

Grouping of samples

Table 1 lists the sample sets with their respective treatment and densification parameters. Group C150 is a reference group densified with parameters similar to those commonly used in surface densification steps. The purpose of groups R270 to R200 was to study the effect of thermal modification on rebound and set-recovery and to separate it from the effect of chemical pretreatment. After densification with the same parameters as set C150, the four sets were placed back in the press at a temperature of 270 ℃ to 200 ℃ (corresponding to the densification temperature of the chemically treated samples) for 240 seconds.

TABLE 1Test group: concentration in CE-ethanol; t-press platen temperature; p-densification pressure; t 1-time to reach target thickness; t 2-Total time in the Press before Cooling

Elastic resilience

The elastic rebound (SB) was determined from measurements with a digital caliper measuring with a resolution of 0.01mm and an accuracy of ± 0.03mm (depending on the manufacturer). SB is defined as:

wherein B is₀The original dimension of the sample in the densification direction before densification, B_cTarget dimensions of the sample after densification (14.98 mm in this case), and B'_cThe actual dimensions of the samples after densification. Thus, the rebound may vary between 0% and 100%, where 0% means no rebound and 100% means full rebound.

Fixation-restoration

Fixation-recovery measurements were performed on specimens cut from the densified specimens at dimensions of 20mm (L) by 25mm (T) by about 15mm (R). Prior to the fixing-recovery step, the samples were kept in an oven at 60 ℃ to prevent rewetting. The samples were measured with digital calipers after one or two cycles of soaking in water at room temperature for at least 14 hours and then oven drying at 60 ℃ for 24 hours. The fix-restore (SR) is defined as:

wherein R is₀Oven dried dimension for the sample in the densification Direction before densification, R_cIs the size of the sample after densification, and R'_cThe dimensions of the densified samples after the wet-dry cycle. The fixation-recovery may vary between 0% and 100%, wherein 0% means no fixation-recovery, i.e. the compression set is completely fixed, and 100% means complete recovery. After the first wet-dry cycle, the samples showed cupping of the densified surface and for this reason, for R'_cThe average of the maximum and minimum dimensions on the surface of each specimen was used.

By this method of calculating the set-back, the initial uncompressed oven dried dimensions are unknown for samples densified at 14.8% or 5.0% MC. In this case, it is assumed that the oven dried size of the samples is equal to the average size of all samples densified at 0% MC. Since the standard deviation of the dimensions of these samples is only 0.07mm, this assumption seems to be sufficiently accurate.

Brinell hardness

Brinell Hardness (HB) measurements were measured according to standard EN 1534 (but with some modifications (CEN 2010)). A Zwick Roell Zwick iLine 2.5TS universal tester equipped with a 2.5kN load cell and a steel ball having a diameter (D) of 10mm was used. The force was increased at a rate of 4 kN/min to a nominal force (F) of 1kN, which was held for 25 seconds before release.

According to the standard, the brinell hardness is calculated from the diameter of the indentation in the test specimen, but the brinell hardness is calculated from the depth of the indentation. The reason for this modification to the test procedure is that it is difficult to measure the diameter of the indentation in the wood. In contrast to steel or other metals, wood does not leave impressions with sharp circular boundaries due to its anisotropic nature. However, the depth (h) of indentation can be measured with a hardness testing apparatus having high accuracy and precision. The HB values in this study were used only for comparison between the test groups. Similar methods were performed before Niemz and Stubi (2000) and Lane et al (2013 a). Accordingly, the brinell Hardness (HB) is defined herein as:

a total of 30 hardness measurements were made for each sample set, 15 for the densified surfaces and 15 for the opposite undensified surfaces as control measurements. Each set of 30 measurements was distributed over five specimens, with six measurements per specimen.

Effect of treatment on Wood color

The color change due to the treatment and the color change due to the densification process were evaluated in a qualitative manner by visual inspection (data not shown). There is some discoloration, but in general, wood treated with the present method only discolors to a depth of less than 0.1mm at the surface. Such surface discoloration is easily planed or ground off prior to use of the wood and does not have a negative impact on the usability of the densified wood obtained by the present method.

Data analysis

All collected data were processed in Microsoft Excel, after which Principal Component Analysis (PCA) screening was performed using the software package SIMCA supplied by umetics, sweden. With the help of PCA (a multivariate data analysis method), correlations can be found in large datasets that are difficult to detect with univariate data analysis methods.

Results and discussion

This study tested the effect of pretreatment with Ionic Liquids (IL) or organic superbases on the elastic resilience, set-recovery and brinell hardness of surface densified sugren.

Elastic resilience

The rebound of untreated surface densified scotland pine is reported to be about 50% without a cooling stage after the densification stage (Neyses et al, 2017). With the cooling phase, the springback can be reduced to 10% to 15% (Laine et al, 2013 b; Neyses et al, 2017), and this is confirmed in this study by group C150 (which has an average springback of 11%). For all other groups, the dimensions of the sample in the densification direction were the same as the target compression within the accuracy of the measurement, so the spring back was negligible.

Laine et al (2013c) suggested that the reduction in spring back when using a cooling stage in surface densification of scotland pine is a result of high temperatures during densification. However, we propose that a low degree of resilience is the result of a very effective plasticization of wood modified by IL or superbase combined with a very low deformation of the unmodified wood volume under the modified wood during the densification process.

Fixation-restoration

Figure 2 shows the average set-back of the untreated sample set after two wet-dry cycles. For all groups treated with the plasticizing liquid, there was no significant difference in set-back between the first wet-dry cycle and the second wet-dry cycle, i.e. set-back did not increase after the first wet-dry cycle, which means that the plasticizing liquid acts as a strong plasticizer and the densified wood remains stable over time. For the untreated control group, there was a slight increase between the first wet-dry cycle and the second wet-dry cycle. The set-back for the control (C150) was 90%, and the set-back for group R270 was likely to decrease slightly to 68% as the densification temperature increased. All groups treated with plasticizing liquid showed a strong reduction of set-recovery and at the highest densification temperature it was only 10%, almost completely eliminated.

The reduction in the setting-recovery can be attributed to both the press platen temperature and the chemical treatment, but the setting-recovery of the R200 to R270 groups is related to the setting of the wood treated with the plasticizing liquid at the corresponding temperature-The recovery ratio shows that: the thermal effect, i.e. thermal degradation, is much less than the effect of the chemical treatment/plasticizing liquid. In the case of all the plasticizing liquids tested, the fixing-recovery can be reduced to less than 20%. Surprisingly, it is advantageous in many respects to base them on non-imidazolesWith a first generation of IL 1-butyl-3-methylimidazole and an organic superbaseThe chloride is as effective.

For DBU treatment, an increase in press platen temperature or an increase in chemical concentration reduces the set-back. The MC increase of the sample before densification (250WDBU) or the absence of pierced holes in the surface to enhance penetration of the chemicals (250NDBU) increased fixation-recovery. It is not possible to see any effect on the fixing-recovery of the annual ring orientation change in the cross section of the specimen.

Fig. 3 shows the relationship between average fixing-recovery and densification temperature. The lighter curves (squares) correspond to the samples without chemical treatment (C150 and R200 to R270 groups) and the darker curves correspond to the 270 to 200DBU groups. The decrease in fixation-recovery increases with increasing temperature, and in the case of samples treated with DBU, this relationship appears to be almost linear.

Comparison of the 250DBU group, 250HDBU group and 250LDBU group (where the concentration of DBU in ethanol is 20%, 30% and 10%, respectively) shows that: an increase in concentration results in a decrease in fixation-recovery, but further studies are required to determine the exact nature of this relationship.

Fixation-recovery of group 250WDBU (MC ═ 5%) indicates that moisture is detrimental to the efficacy of pretreatment with DBU, as expected based on published studies on dissolving cellulose in IL: (b ═ b: (b ═ b:)Etc., 2010). Zavrel et al (2009) report: the water molecules form a hydrodynamic shell around the IL molecule, and this inhibits the interaction of IL with celluloseDirect interaction between them. Nevertheless, the reduction in fixation-recovery was also significant, from 81% to only 37%.

The surface of nearly all of the groups of samples were pierced with small holes to facilitate penetration of the chemical solution into the wood. Unexpectedly, the set-recovery of the set 250NDBU without holes was only slightly higher than the set 250DBU otherwise treated and densified in the same manner. Depending on the possible applications of the surface densified wood and its requirements, it may therefore not be necessary to pierce the surface.

Brinell hardness

Figure 4 shows the brinell hardness of the undensified test piece surface and the densified test piece surface. The average brinell hardness of the undensified surfaces did not differ significantly. The densification process results in an increase in the hardness of the densified surface. Densification without chemical treatment (group C150 and group R270 to group R200) resulted in a hardness of from 13.4N/mm²To 26.2N/mm²Two-fold increase and no difference between groups. This increase in hardness is consistent with the results of previous studies (Gong et al, 2010; Laine et al, 2013 a; Neyses et al, 2017). The chemically treated samples showed an increase in HB to 2.7 times after densification, i.e., from 12.9N/mm²To 34.4N/mm²This is a significant improvement over the HB of the untreated and densified sample. The lack of significant differences in HB between the different groups treated with plasticizing liquids again indicates a non-imidazole based that is more economical, easier to recover and more environmentally friendlyThe Ionic Liquid (IL) and superbase of (a) function at least as well as the first generation IL.

It appears that the reason for the higher HB of the chemically treated samples is a more thorough and targeted plasticization than the untreated samples. Fig. 5a shows: for the chemically treated samples, the region of highest densification level was located at the outermost surface of the sample, and in this regard, the samples shown are representative of all of the chemically treated samples. There appears to be a layer having a thickness of about 1.2mm and a three-fold increase in density just below the surface. The density then gradually decreases towards the core of the sample. In fig. 5b, which shows untreated and densified samples, it appears that the density peaks of these samples are not located at the very surface, but slightly below the surface. This is probably because the heated press platens dried the sample surface, thus inhibiting plasticization in this area. For chemically treated samples, this phenomenon is insignificant, since their plasticization is independent of moisture. Another observation that supports our hypothesis is the bulging of the side surfaces in the densified regions (fig. 5): the chemically treated sample was convex at the outermost surface, while the untreated sample was slightly convex below the surface.

Since the brinell hardness method is a volume-based measurement, it does not appear to be representative of the perceived surface hardness of the test specimen. Theoretically, a three-fold increase in observed density should result in a three-fold increase in hardness, but this is not confirmed by the calculated HB value. Perhaps another type of hardness or scratch resistance measurement would provide more meaningful results, at least for the intended use of the densified wood. For this reason, we believe that it may be worthwhile to study the relationship between different hardness and scratch resistance measurement methods and surface densified wood treated in different ways. This is particularly important for possible applications of the surface densified wood product.

Conclusion

Ionic Liquid (IL) 1-butyl-3-methylimidazoleChloride and 1, 5-diazabicyclo (4.3.0) non-5-enePropionates and the organic superbases 1, 5-diazabicyclo (4.3.0) non-5-ene, 1, 8-diazabicyclo (5.4.0) undec-7-ene and 1,1,3, 3-tetramethylguanidine have proven to be strong plasticizers which can reduce or even eliminate the elastic and fixing-recovery components of the deformation of surface-densified wood. Elastic recoil was eliminated and the fixation-recovery after two wet/dry cycles in water was reduced to a value as low as 10% (100)% means complete recovery of deformation). Superbases and based on non-imidazolesAcid-base conjugated IL's, e.g. made from amidine-based cations or guanidine-based cations in combination with carboxylic acids, have more or less the same effect as the first generation IL, which is an encouraging result, as these are arguably more attractive in terms of industrial implementation.

The results discussed above for using non-imidazole based ILs made from amidine based cations or guanidine based cations in combination with carboxylic acids would most likely also apply when using other non-imidazole based ILs. Have proven to be based on imidazolesSuch non-imidazole based on the ability of IL to similarly solubilize celluloseExamples of IL's of (a) are carboxylate-based IL (Kasprzak et al, 2019); quaternary ammonium ILs (Diez et al, 2019), which are advantageous from an environmental point of view because they are derived from grass, softwood or hardwood lignin; biodegradable dimethylpyridine-based compositionsIL (Samikannu et al, 2018); or ammonium-based IL and pyridine-basedIL (Rieland and Love, 2020). Typically, these are based on non-imidazolesThe cellulose-dissolving capacity of IL (a) is related to its ability to form hydrogen bonds with the hydroxyl groups of the cellulose chain. It is recommended that the anions start to react, swelling the biomaterial and also hydrogen bonding the generally bulky cations, thereby forming "solvation cages" that produce rows between cellulose chainsRepellency (Rieland and Love, 2020).

The chemical treatment resulted in a brinell Hardness (HB) 1.3 times higher than that of the untreated and densified samples, and 2.7 times higher relative to the undensified surface. This means that HB of chemically treated scotch pine is close to a level similar to that of oak, a species widely used for high quality wood flooring.

The process described herein can be viewed as a precursor to the development of a continuous open system surface densification process. A closed system is not necessary and both chemical treatment and densification are possible in a continuous manner. However, further research is required to optimize the process.

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