EP3807373A1

EP3807373A1 - Waterborne adhesive comprising crosslinkable microfibrillated cellulose

Info

Publication number: EP3807373A1
Application number: EP19819018.3A
Authority: EP
Inventors: Gisela CUNHA; Lena LÖNNEMARK
Original assignee: Stora Enso Oyj
Current assignee: Stora Enso Oyj
Priority date: 2018-06-13
Filing date: 2019-06-11
Publication date: 2021-04-21
Also published as: SE1850727A1; EP3807373A4; WO2019239299A1; SE543251C2; JP2021527153A

Abstract

The present invention relates to a waterborne adhesive comprising crosslinkable microfibrillated cellulose (MFC), wherein the crosslinkable MFC is phosphorylated MFC, as well as a method for adhering surfaces together using said adhesive.

Description

WATERBORNE ADHESIVE COMPRISING CROSSLINKABLE MICROFIBRILLATED CELLULOSE

The present invention relates to a waterborne adhesive comprising crosslinkable

microfibrillated cellulose (MFC), as well as a method for adhering surfaces together using said adhesive.

BACKGROUND

Waterborne adhesives are becoming increasingly popular, since they represent an environmentally friendly and economically viable alternative to solvent-based counterparts. However, the performance of current paper and/or wood waterborne adhesives based on natural sources, such as starch or proteins, is reduced under moist conditions, due to the intrinsic water sensitivity of these natural sources, especially if no external crosslinker is added to the formulation.

On the other hand, waterborne adhesives with high wet performance usually consist of latexes comprising fossil-fuel based monomers; this is the case for synthetic rubbers, polyacrylates and polyurethanes. It is therefore desirable to develop waterborne adhesives with improved performance in moist conditions and which are based on natural sources.

Microfibrillated cellulose (MFC) comprises partly or totally fibrillated cellulose or lignocellulose fibers. The liberated fibrils have a diameter less than 100 nm, whereas the actual fibril diameter or particle size distribution and/or aspect ratio (length/width) depends on the source and the manufacturing methods. The smallest fibril is called elementary fibril and has a diameter of approximately 2-4 nm (see e.g. Chinga-Carrasco, G., Nanoscale research letters 2011, 6 :417), while it is common that the aggregated form of the elementary fibrils, also defined as microfibril, is the main product that is obtained when making MFC e.g . by using an extended refining process or pressure-drop disintegration process (see Fengel, D., Tappi J ., March 1970, Vol 53, No. 3.) . Depending on the source and the manufacturing process, the length of the fibrils can vary from around 1 to more than 10 micrometers. A coarse MFC grade might contain a substantial fraction of fibrillated fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), with a certain amount of fibrils liberated from the tracheid (cellulose fiber) .

There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as large surface area or its ability to form a gel like material at low solids (1-5 wt%) when dispersed in water.

MFC exhibits useful chemical and mechanical properties. Chemical surface modification of MFC has the potential to improve the properties of MFC itself, e.g. mechanical strength and water absorbance and - in certain circumstances - elasticity/flexibility.

Patent publications in this field include US20090298976, US2017183820, JPH063398 and JP2017189164.

SUMMARY

It has been verified that when an aqueous dispersion of crosslinkable phosphorylated MFC is applied between two substrates and subsequently dried, it can deliver strong adhesive properties (strong bonding), providing that the drying occurs at a high enough temperature to trigger crosslinking, i.e. above 60 °C. Moreover, since crosslinkable MFC can crosslink with paper-based substrates and the like, it is expected that these types of adhesive exhibit high performance even under moist conditions. Such properties are not achieved with

conventional waterborne adhesives based on natural sources.

Thus, a good adhesion with good wet performances is achieved by adding a dispersion comprising phosphorylated MFC as an adhesive and triggering the adhesion by heating. The dispersion may also comprise other components to further improve the adhesion, e.g. starch.

So, in a first aspect the present invention relates to a waterborne adhesive comprising crosslinkable microfibrillated cellulose (MFC) dispersed in an aqueous solvent. Preferably, the waterborne adhesive does not comprise additional crosslinking agents.

In a second aspect, the present invention relates to method for adhering a first and a second surface together, said method comprising the steps of: a. applying a waterborne adhesive as defined herein, to at least one of said first or second surfaces; b. placing said first and second surfaces in contact with each other, such that the waterborne adhesive is located between said surfaces; c. treating said waterborne adhesive so as to provide crosslinking of the crosslinkable MFC, thereby adhering said first and second surfaces together. In a third aspect, the use of an aqueous dispersion of crosslinkable microfibrillated cellulose (MFC) as a waterborne adhesive is provided . Further aspects of the invention are provided in the following text and in the dependent claims.

DETAILED DISCLOSURE

In a first aspect, a waterborne adhesive is provided, which comprises crosslinkable microfibrillated cellulose (MFC) dispersed in an aqueous solvent.

Microfibrillated cellulose (MFC) or so called cellulose microfibrils (CMF) shall in the context of the present application mean a nano-scale cellulose particle fiber or fibril with at least one dimension less than 100 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. The cellulose fiber is preferably fibrillated to such an extent that the final specific surface area of the formed MFC is from about 1 to about 300 m²/g, such as from 1 to 200 m²/g or more preferably 50-200 m²/g when determined for a freeze-dried material with the BET method .

Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre treatment steps are usually required in order to make MFC manufacturing both energy- efficient and sustainable. The cellulose fibers of the pulp to be supplied may thus be pre treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl, aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example "TEMPO"), or quaternary ammonium (cationic cellulose) . After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC or NFC.

The nanofibrillar cellulose may contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of the pre-treated fibers, e.g . hydrolysed, pre swelled, or oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, single - or twin-screw extruder, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on the MFC manufacturing method, the product might also contain fines, or nanocrystalline cellulose or e.g. other chemicals present in wood fibers or in papermaking process. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated. MFC can be produced from wood cellulose fibers, both from hardwood or softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from pulp including pulp from virgin fiber, e.g. mechanical, chemical and/or thermomechanical pulps. It can also be made from broke or recycled paper.

The above described definition of MFC includes, but is not limited to, the proposed TAPPI standard W13021 on cellulose nano or microfibril (CMF) defining a cellulose nanofiber material containing multiple elementary fibrils with both crystalline and amorphous regions, having a high aspect ratio with width of 5-30 nm and aspect ratio usually greater than 50.

Chemically-modified MFC comprising crosslinkable groups is "crosslinkable MFC".

Crosslinkable MFC forms bonds between the MFC fibrils upon crosslinking, e.g. by heat- treatment or pH treatment. The crosslinkable MFCs are phosphorylated microfibrillated cellulose (P-MFC).

Phosphorylated microfibrillated cellulose (P-MFC) is typically obtained by reacting cellulose pulp fibers with a phosphorylating agent such as phosphoric acid, and subsequently fibrillating the fibers to P-MFC. One particular method involves providing a suspension of cellulose pulp fibers in water, and phosphorylating the cellulose pulp fibers in said water suspension with a phosphorylating agent, followed by fibrillation with methods common in the art. Suitable phosphorylating agents include phosphoric acid, phosphorus pentaoxide, phosphorus oxychloride, diammonium hydrogen phosphate and sodium dihydrogen phosphate.

In the reaction to form P-MFC, alcohol functionalities (-OH) in the cellulose are converted to phosphate groups (-OPO3² ) . In this manner, crosslinkable functional groups (phosphate groups) are introduced to the pulp fibers or microfibrillated cellulose.

Typically, the P-MFC is in the form of its sodium salt.

The aqueous solvent used in the waterborne adhesive may consist solely of water, without additives. However, the aqueous solvent may include additives common in the field of adhesives, such as starch, which can improve the adhesion. To improve miscibility/solubility, the aqueous solvent may include non-aqueous, water-miscible solvents such as alcohols, but this is less desirable from an environmental point of view.

The waterborne adhesive suitably comprises more than 25%, preferably more than 50%, such as e.g. more than 75% by dry weight crosslinkable MFC on a dry basis. The waterborne adhesive may comprise additional components, such as other MFC grades and/or inorganic fillers. Of these, non-modified MFC is preferred, as good crosslinking with crosslinkable MFC can be obtained.

Crosslinkable MFC can form crosslinks (and thereby good adhesion) itself, without additional crosslinking agents. Therefore, in one preferred embodiment, the waterborne adhesive does not comprise additional crosslinking agents.

In a second aspect, a method is provided for adhering a first and a second surface together. The method comprises the general steps of: a. applying a waterborne adhesive as defined herein, to at least one of said first or second surfaces; b. placing said first and second surfaces in contact with each other, such that the waterborne adhesive is located between said surfaces; c. treating said waterborne adhesive so as to provide crosslinking of the MFC, thereby adhering said first and second surfaces together wherein said crosslinkable MFC is phosphorylated microfibrillated cellulose.

The treatment in step c is heat treatment, suitably at a temperature of between 60 and 200 °C, preferably between 70 and 120 °C.

The treatment in step c. may take place for a time of between 2 and 180 minutes, preferably between 10 and 180 minutes.

Due to the cellulosic nature of the crosslinkable MFC, and the nature of the crosslinking, good adhesion of cellulose based materials such as e.g. paper, cardboard or wood can take place.

It is therefore preferred that said first and/or said second surfaces comprise or consist of cellulose-based materials such as e.g. paper, cardboard or wood.

All details of the waterborne adhesive relating to the first aspect of the invention are also relevant to the second aspect of the invention. In particular, it is preferred that the waterborne adhesive used in step a. does not comprise additional crosslinking agents.

In a third aspect, the use of an aqueous dispersion of crosslinkable microfibrillated cellulose (MFC) as a waterborne adhesive is also provided. All details of the crosslinkable microfibrillated cellulose and the aqueous dispersion are as provided above are also relevant for this aspect of the invention.

Although the invention has been described with reference to a number of aspects and embodiments, these aspects and embodiments may be combined by the person skilled in the art, while remaining within the scope of the present invention.

Schematic description of the figures:

Figure 1 : Shows variation of ABES dry strength with pressing time of veneer samples glued with different starch based adhesives as described in Example 1.

Figure 2: Shows variation of ABES wet strength with soaking time of veneer samples glued with different starch-based adhesives as described in Example 2.

Example 1:

Dry strength of P-MFC-containing starch-based adhesives Samples:

• Enzymatically pre-treated native MFC (N-MFC; 4.65% solids content)

• Phosphorylated MFC (P-MFC; degree of functionalization=0.49 mmol/g; 95.5% solids content)

• Starch (Modified starch Mylbond 210; ~95% solids content)

Method:

All adhesives were prepared at a 10% solid content as following :

25% N-MFC + 75% starch : To 134.31 g of native MFC, 96.10 g of deionized water was added and mixed for 1.5 minutes with IKA ULTRA-TURRAX T50 disperser equipped with a 40 mm dissolver disc blade at a speed of 3000 rpm. Subsequently, 18.23 g of starch and then 1.04 g of 50% NaOH-solution were added under mixing (mixing for a total of 2.5 minutes).

25% P-MFC + 75% starch : To 6.55 g of phosphorylated MFC powder, 224.18 g of deionized water was added and mixed for 1.5 minutes with IKA ULTRA-TURRAX T50 disperser equipped with a 40 mm dissolver disc blade at a speed of 3000 rpm. Subsequently, 18.23 g of starch and then 1.04 g of 50% NaOH-solution were added under mixing (mixing for a total of 2.5 minutes).

100% starch : To 66.67 g of cooked starch (see below for preparation), 165.10 g of deionized water was added and mixed for 1.5 minutes with IKA ULTRA-TURRAX T50 disperser equipped with a 40 mm dissolver disc blade at a speed of 3000 rpm. Subsequently, 18.23 g of starch was added under mixing (mixing for a total of 2.5 minutes). The cooked starch was prepared by mixing 306.25 g water, 37.5 g of starch, 6.25 g of 50% NaOH-solution with IKA ULTRA- TURRAX T50 disperser equipped with a 40 mm dissolver disc blade at the same time as heating to 45 °C, and finally adding a further 50 g deionized water.

Testing:

Samples were tested using Automated Bonding Evaluation System (ABES). Adhesive was applied using a pipette on a 100 mm² area (20 x 5 mm) of a beech veneer and using a 100 pm applicator rod to remove excess glue. Pressing temperature was set to 105 °C and samples were pressed up to 120 s. Cooling time was set to 3 s and pulling time to 4 s. Three replicates were tested for each sample at each pressing time.

Results:

The dry bond strength of P-MFC-containing starch-based adhesive is higher than that of pure starch adhesive, given than the pressing time (which induces drying) is at least 60 s. At 120 s pressing time the dry bond strength of P-MFC-containing starch-based adhesive is also slightly higher than that of the benchmark N-MFC-containing starch-based adhesive (Table 1 and Figure 1). This indicates that the crosslinking ability of P-MFC can improve the bond strength of water-based starch-based adhesives.

Table 1. ABES dry strength at different pressing times for veneer samples glued with different starch-based adhesives.

Example 2: Wet strength of P-MFC-containing starch-based adhesives Samples:

Same as in Example 1.

Method:

Same as in Example 1. Testing:

Samples were tested using Automated Bonding Evaluation System (ABES). Adhesive was applied using a pipette on a 100 mm² area (20 x 5 mm) of a beech veneer and using a 100 pm applicator rod to remove excess glue. Pressing temperature was set to 105 °C and pressing time was set to 120 s (for drying). Cooling time was set to 3 s and pulling disengaged. Veneer samples were put to soak in deionized water up to 60 s. Thereafter the samples were briefly wiped with a paper towel and put back into ABES for pulling test, 4 s. Three replicates were tested for each sample at each pressing time.

Results:

The wet bond strength of P-MFC-containing starch-based adhesive (pressing time 120 s) is higher than that of pure starch adhesive, irrespectively of the soaking time. After 60 s soaking time, the wet bond strength of P-MFC-containing starch-based adhesive is almost 2 times higher than that of the benchmark N-MFC-containing starch-based adhesive (Table 2 and Figure 2). This indicates that the crosslinking ability of P-MFC can highly improve the wet strength of waterborne starch-based adhesives. Table 2. ABES wet strength after different soaking times for veneer samples glued with different starch-based adhesives for 120 s.

Claims

1. A waterborne adhesive comprising crosslinkable microfibrillated cellulose (MFC) dispersed in an aqueous solvent, wherein the crosslinkable MFC is phosphorylated

microfibrillated cellulose (P-MFC).

2. The waterborne adhesive according to claim 1, wherein the crosslinkable MFC is phosphorylated microfibrillated cellulose (P-MFC), in the form of its sodium salt.

3. The waterborne adhesive according to any one of the preceding claims, comprising more than 25%, preferably more than 50%, such as e.g. more than 75% by dry weight crosslinkable MFC.

4. The waterborne adhesive according to any one of the preceding claims, comprising additional components, such as other MFC grades and/or inorganic fillers.

5. The waterborne adhesive according to any one of the preceding claims, further comprising starch.

6. The waterborne adhesive according to any one of the preceding claims, wherein the waterborne adhesive does not comprise additional crosslinking agents.

7. A method for adhering a first and a second surface together, said method comprising the steps of: a. applying a waterborne adhesive as defined in any one of the preceding claims, to at least one of said first or second surfaces; b. placing said first and second surfaces in contact with each other, such that the waterborne adhesive is located between said surfaces; c. treating said waterborne adhesive so as to provide crosslinking of the crosslinkable MFC, thereby adhering said first and second surfaces together, wherein said crosslinkable MFC is phosphorylated microfibrillated cellulose.

8. The method according to claim 7, wherein said treatment in step c is heat treatment, suitably at a temperature of between 60 and 200 °C, preferably between 70 and 120 °C.

9. The method according to any one of claims 7-8, wherein said treatment takes place for a time of between 2 and 180 minutes.

10. The method according to any one of claims 7-9, wherein said first and/or said second surfaces comprise or consist of cellulose-based materials such as e.g. paper, cardboard or wood.

11. The method according to any one of claims 7-10, wherein the waterborne adhesive used in step a. does not comprise additional crosslinking agents

12. Use of an aqueous dispersion of crosslinkable microfibrillated cellulose (MFC) according to any one of claims 1-6 as a waterborne adhesive.