Thermally expandable cellulose microspheres having a low expansion temperature

Abstract

The present invention relates to thermally expandable microspheres comprising a polymeric shell surrounding a hollow core, the hollow core comprising a blowing agent, and the polymeric shell comprising a carboxylate-functionalized cellulose, the thermally expandable microspheres having an expansion initiation temperature (T) of 80°C to less than 135°C. Start The present invention further relates to a process for preparing expandable microspheres, as well as to thermally expandable microspheres obtained by such a process, comprising mixing a carboxylate-functionalized cellulose, an organic solvent, a blowing agent, and optionally a polymeric shell enhancer, and thereafter spraying the mixture thus obtained into a drying apparatus to produce thermally expandable microspheres having a polymeric shell surrounding a hollow core, wherein the polymeric shell comprises the carboxylate-functionalized cellulose and the hollow core comprises the blowing agent.

Description

Technical Field

[0001] The present invention relates to thermally expandable microspheres made from a cellulose-based biopolymer having a low expansion temperature, and also to a process for its manufacture.

Background Art

[0002] Thermally expandable microspheres are known in the art and are described, for example, in US Patent No. 3,615,972, International Publication No. 00 / 37547, and International Publication No. 2007 / 091960. Some examples are sold under the trade name Expancel®. These can be expanded to form very low weight and low density fillers and can find use in applications such as foamed resins or low density resins, paints and coatings, cement, inks, and crack fillers. In many cases, consumer products containing expandable microspheres include lightweight shoe soles (e.g., for running shoes), textured coatings such as wallpaper, sunlight reflective and insulating coatings, food package sealants, wine corks, artificial leather, foams for protective helmet liners, and automotive weatherstripping.

[0003] Thermally expandable polymer microspheres typically include a thermoplastic polymer shell and a hollow core containing a blowing agent that expands upon heating. Examples of blowing agents include low boiling point hydrocarbons or halogenated hydrocarbons, which are liquids at room temperature but vaporize upon heating. To produce expanded microspheres, the thermally expandable microspheres are heated so that the thermoplastic polymer shell softens and the blowing agent vaporizes and expands, thus expanding the microspheres. Typically, the diameter of the microspheres can increase 1.5 to 8 times during expansion. Thermally expandable microspheres are commercially available in various forms, for example, as dry free-flowing particles, as aqueous slurries, or as partially dehydrated wet cakes.

[0004] Expandable microspheres can be produced, for example, by polymerizing ethylenically unsaturated monomers in the presence of a blowing agent using a suspension polymerization process. Typical monomers include those based on acrylates, acrylonitrile, acrylamide, vinylidene dichloride, and styrene. A problem associated with such thermoplastic polymers is that they typically derive from petrochemical products and not from sustainable sources. In addition, many polymers are non-biodegradable or at least biodegrade very slowly, posing a risk of cumulative accumulation in the environment. However, simply replacing the monomers with alternatives from more sustainable sources is not always straightforward, as it is necessary to ensure that acceptable expansion performance is maintained. For example, the polymer needs to have an appropriate surface energy to obtain core-shell particles in a suspension polymerization reaction such that the blowing agent is encapsulated. In addition, the resulting polymer must have good gas barrier properties to be able to retain the blowing agent. Furthermore, the polymer needs to have suitable viscoelastic properties such that the glass transition temperature T g is exceeded to allow the shell to stretch during expansion. Thus, replacing conventional monomers with bio-based monomers is not simple.

[0005] Expandable microspheres are described where at least a portion of the monomers making up the thermoplastic shell are bio-based and derived from renewable resources.

[0006] WO 2019 / 043235 describes polymers containing lactone monomers having the following general formula. [Chemical formula] In the formula, R 1 ~R 4 are each independently selected from H and C 1~4 alkyl.

[0007] International Publication No. WO 2019 / 101749 describes a copolymer comprising a dialkyl itaconate monomer having the following general formula. [Chemical formula] In the formula, each of R 1 and R 2 is separately selected from alkyl groups.

[0008] International Publication No. WO 2020 / 099440 (PCT / EP2019 / 081076), which is a published patent application, discloses thermally expandable microspheres made from cellulose-based biopolymers. The polymer shell of these microspheres contains carboxylate-functionalized cellulose having a glass transition temperature (T g ) of at least 125 °C. All of the microspheres exemplified in this application have a temperature (T Start ) at which expansion begins that is at least 135 °C or higher. However, in some applications, there is a need for microspheres having a lower T Start .

[0009] Therefore, there remains a need for alternative thermoplastic expandable microspheres, at least in part, derived from sustainable sources for the thermoplastic polymer shell. Further, there remains a need to provide expandable microspheres where the thermoplastic polymer shell is at least in part derived from sustainable sources and where the expandable microspheres have a low expansion temperature and other desirable expansion characteristics, such as, for example, expanded microspheres of preferably low density. Further, when such expandable microspheres are derived from sustainable sources, it is desirable for them to have sufficient or still improved storage stability. Accordingly, the present invention is directed to finding thermally expandable polymer microspheres that have desirable expansion characteristics, such as a preferably low density of the expanded microspheres at a low expansion temperature (i.e., the temperature at which expansion begins), for example, by using a biopolymer, and that at the same time preferably have sufficient storage stability or even improved storage stability. SUMMARY OF THE INVENTION

[0010] The present invention is directed to thermally expandable microspheres comprising a polymer shell surrounding a hollow core, the hollow core containing a blowing agent and the polymer shell containing carboxylate-functionalized cellulose, the thermally expandable microspheres having a temperature (T Start ) at which expansion begins of from 80° C. to less than 135° C.

[0011] The present invention is directed not only to a process for preparing expandable microspheres but also to the thermally expandable microspheres obtained by such a process, the process comprising mixing carboxylate-functionalized cellulose, an organic solvent, a blowing agent, and optionally a polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor, and then spraying the mixture thus obtained into a drying apparatus to produce thermally expandable microspheres having a polymer shell surrounding a hollow core, the polymer shell containing carboxylate-functionalized cellulose within the hollow core and the hollow core containing a blowing agent.

Brief Description of the Drawings

[0012]

Fig. 1A

Fig. 1B

Fig. 2

Modes for Carrying Out the Invention

[0013] One aspect of the present invention is a thermally expandable microsphere comprising a polymer shell surrounding a hollow core, the hollow core containing a blowing agent, and the polymer shell containing carboxylate-functionalized cellulose, the thermally expandable microsphere having a temperature (T Start ) at which expansion begins from 80°C to less than 135°C.

[0014] The expandable microsphere is based on a polymer shell containing carboxylate-functionalized cellulose. The functional group is a carboxylate group, or two or more carboxylate groups, which are typically selected from C 1 ~C 12 carboxylates. Thus, the term "carboxylate-functionalized cellulose" means that the cellulose contains at least one carboxylate group. The carboxylate moiety forms part of the link between the carboxylate functional group and the cellulose, i.e., the cellulose is linked to the carboxylate functional group via an ester bond.

[0015] The polymer shell can comprise one or more polymer components or can consist of one or more polymer components, where at least one component, two or more components, or all of the polymer components are selected from such carboxylate-functionalized cellulose. If the shell contains polymers other than those described herein (i.e., carboxylate-functionalized cellulose), their content is typically less than 50% by weight, for example, less than 30% by weight, or less than 10% by weight, for example, 9% by weight or less, 5% by weight or less, or even 2% by weight or less. These percentages are based on the total polymer content of the shell.

[0016] In an embodiment, the carboxylate functional group on the carboxylate-functionalized cellulose can be represented by formula (1).

Chemical formula

[0017] In formula (1), A is selected from -H, -OH, -OR b , -C(O)OH, and -C(O)OR b . In an embodiment, A is selected from -H and -C(O)OH.

[0018] R a may not be present, i.e., it is possible to attach A directly to the C=O group. However, if present, R a can be selected from saturated or unsaturated aliphatic groups having 1 to 11 carbon atoms and can be linear, branched, or cyclic.

[0019] R a can also be selected from 5- and 6-membered aromatic rings.

[0020] R a can optionally contain one or more substituents selected from -OH, halides, C 1~4 alkyl, and C 1~4 alkoxy, where C 1~4Alkyl and C 1~4 The alkoxy group is optionally substituted with one or more groups selected from halides and -OH.

[0021] In an embodiment, R a contains 1 to 7 carbon atoms, for example, 1 to 5, or 1 to 3 carbon atoms.

[0022] In each occurrence, R b is independently selected from a C 1~4 alkyl group (for example, a C 1~2 alkyl group) and optionally has one or more substituents selected from halides and -OH groups. In an embodiment, the C 1~4 alkyl group or the C 1~2 alkyl group is unsubstituted.

[0023] In an embodiment, R a is a saturated straight-chain or branched-chain

Chemical formula

Chemical formula

[0024] In each occurrence, R c is independently selected from H, -OH, halide, C 1~4 alkyl, and C 1~4 alkoxy, and the C 1~4 alkyl and C 1~4 alkoxy groups are optionally substituted with one or more groups selected from halides and -OH.

[0025] In other embodiments, R a is an unsaturated straight-chain or branched-chain containing a "y" double bond

Chemical formula

[0026] In a further embodiment, R a includes a "y" double bond

Chemical Structure

[0027] In yet a further embodiment, R a is

Chemical Structure

[0028] In yet a further embodiment, R a can be a linear or branched aliphatic group containing a cycloaliphatic ring or an aromatic ring. Thus, R a has 11 or fewer carbon atoms

Chemical Structure

Chemical Structure

[0029] The halide is typically selected from F and Cl. However, in an embodiment, the functional group does not include a halide, whereby groups A, R a R b and Rc There is no halide present therein.

[0030] In an embodiment, at least one R c group is H. In other embodiments, no more than two R c groups are other than H, and in a further embodiment, no more than one R c group is other than H. In yet a further embodiment, all R c groups are H.

[0031] R a , R b , and R c In the above definitions, when there are two or more -OH substituents, typically there is no more than one -OH substituent per carbon atom.

[0032] In certain embodiments, R a is an optionally substituted C 1 ~C 8 aliphatic (alkylene) group. In other embodiments, R a is an optionally substituted C 6 aromatic ring. In a further embodiment, R a is unsubstituted.

[0033] In embodiments, the functional groups on the cellulose substituents are selected from acetate, propionate, butyrate, pentanoate, hexanoate, heptanoate, octanoate, and phthalate. In a further embodiment, it is selected from acetate, propionate, and butyrate, preferably selected from propionate and butyrate.

[0034] The degree of substitution (DS) of the hydroxyl groups of cellulose by one or more carboxylate groups can range from 0.9 to 3.5, and in embodiments, ranges from 1.5 to 3.5, for example, ranges from 2.5 to 3.3.

[0035] Optionally, other functional groups may be present in the functionalized cellulose. For example, the -OH groups on cellulose molecules that have not yet been substituted with carboxylate functionality can be substituted with alkoxy groups, or for example C 1 ~C 6 two or more alkoxy groups selected from alkoxy groups. In other embodiments, although less preferred, the -OH groups can be replaced with halide groups, such as F or Cl. When such other functional groups are present, their molar amounts are lower than those of one or more carboxylate groups. In embodiments, the degree of substitution of cellulose by other functional groups is 1 or less, for example, 0.5 or less or 0.2 or less. In further embodiments, the degree of substitution by groups other than carboxylate groups is 0.1 or less.

[0036] In embodiments, the carboxylate-functionalized cellulose contains propionate groups or butyrate groups, preferably propionate groups.

[0037] In embodiments, cellulose can be functionalized with two or more different carboxylate groups, preferably functionalized with two different carboxylate groups. In a preferred embodiment, the carboxylate-functionalized cellulose is functionalized with at least two, preferably two different carboxylate functionalities as described above. In a more preferred embodiment, the carboxylate-functionalized cellulose is functionalized with at least an acetate group and at least one additional group selected from propionate groups, butyrate groups, pentanoate groups, hexanoate groups, heptanoate groups, octanoate groups, and phthalate groups. In an even more preferred embodiment, the carboxylate-functionalized cellulose is functionalized with at least an acetate group and at least one additional group (i.e., for additional carboxylate groups, R a is aliphatic and unsubstituted C 2 or C 3is a base and A is H), and is functionalized. For example, carboxylate-functionalized cellulose is functionalized with acetate groups and butyrate groups, or carboxylate-functionalized cellulose is functionalized with acetate groups and propionate groups.

[0038] In embodiments, the glass transition temperature (T g ) of the carboxylate-functionalized cellulose that forms the shell of the microsphere or at least a part of the shell of the microsphere is at least 80 °C. T g can be measured using differential scanning calorimetry (DSC), such as by using the method described by Nishio et al; Cellulose, 2006(13), 245-259. Here, a 5 mg sample is first heated from ambient temperature (25 °C) to 240 °C at a rate of 20 °C / min under a nitrogen atmosphere, and then immediately quenched to -50 °C. After that, for the second time, it is heated from -50 °C to 240 °C at a rate of 20 °C / min under a nitrogen atmosphere, and the calculation of T g is based on the second heating cycle.

[0039] In further embodiments, the T g of the carboxylate-functionalized cellulose is at least 90 °C, for example, at least 100 °C, at least 110 °C, or at least 120 °C. In embodiments, the T g of the carboxylate-functionalized cellulose is 150 °C or lower, for example, 135 °C or lower, or 125 °C or lower (such as 124 °C or lower). In embodiments, T g is in the range of 80 to 150 °C, for example, in the range of 80 to 135 °C, 80 to 125 °C, or 80 to 124 °C. In further embodiments, T g is in the range of 90 to 124 °C, for example, in the range of 100 to 124 °C or 110 to 124 °C.

[0040] The melting point of the carboxylate-functionalized cellulose is typically T gExceeding the value, in embodiments, it exceeds 125 °C. In embodiments, the melting point exceeds 150 °C. The melting point is typically 270 °C or lower, for example, 250 °C or lower.

[0041] The T of carboxylate-functionalized cellulose g and the melting point can be modified or controlled by changing the functional groups on the functionalized cellulose or by changing the molecular weight.

[0042] The thermally expandable microspheres are hollow, the shell contains carboxylate-functionalized cellulose therein, and the hollow center or core contains one or more blowing agents. The carboxylate-functionalized cellulose used to prepare the microspheres typically has a density of 1.1 - 1.35 g / cm 3 In the expanded microspheres, the density is typically less than 1 g / cm 3 and ranges from 0.005 - 0.8 g / cm 3 or from 0.01 - 0.6 g / cm 3 is preferred. In a further embodiment, the density of the expanded microspheres ranges from 0.01 - 0.4 g / cm 3 for example, in the range of 0.01 - 0.2 g / cm 3 preferably in the range of 0.01 - 0.15 g / cm 3 and so on. Higher densities, especially densities of 1 g / cm 3 and above generally mean that the microsphere sample is not suitable for use.

[0043] In embodiments, the number average molecular weight (M n ) of the functionalized cellulose used to form the microspheres ranges from 1,000 - 700,000, for example, in the range of 2,000 - 500,000. In embodiments, it is, for example, in the range of 10,000 - 100,000, for example, in the range of 10,000 - 80,000.

[0044] Examples of suitable carboxylate-functionalized celluloses include cellulose acetate propionate (CAP) and cellulose acetate butyrate (CAB), particularly those having a number average molecular weight (M n ) in the range of 2,000 to 100,000 Da, such as in the range of 2,000 to 80,000 Da, preferably in the range of 2,000 to 30,000 Da, and more preferably in the range of 10,000 to 30,000 Da.

[0045] The most preferred carboxylate-functionalized cellulose is cellulose acetate propionate (CAP) having a number average molecular weight (M n ) in the range of 2,000 to 100,000 Da, such as in the range of 2,000 to 80,000 Da, preferably in the range of 2,000 to 30,000 Da, more preferably in the range of 10,000 to 30,000 Da, and most preferably in the range of 10,000 to 25,000 Da.

[0046] The thermally expandable microspheres have a temperature (T Start ) at which expansion starts that is from 80 °C to less than 135 °C. The temperature at which expansion starts is referred to as T Start , while the temperature at which maximum expansion is reached is referred to as T max . T Start and T Max may be determined using standard measurement techniques generally known to those skilled in the art. For example, T Start and T Maxcan be determined in a temperature rise experiment, for example, by using a Mettler-Toledo thermomechanical analyzer (e.g., Mettler-Toledo TMA / SDTA 841e) with a heating rate of 20 °C / min and a load (net) of 0.06 N. In such a temperature rise experiment, a sample of a known weight of the thermally expandable microspheres is heated at a constant heating rate of 20 °C / min under a load (net) of 0.06 N. When the expansion of the thermally expandable microspheres begins, the volume of the sample increases and the load moves upward. From such measurements, an expansion thermogram (an exemplary thermogram is shown in FIG. 2) is obtained, where the vertical axis indicates the height that moves the load upward and the horizontal axis indicates the temperature. T Start and T Max can be determined from this expansion thermogram, for example, using STARe software from Mettler-Toledo.

[0047] In an embodiment, the thermally expandable microspheres have a T Start in the range from 90 °C to less than 135 °C. The T Start less than 135 °C can be, for example, a T Start of 134 °C or less, 132 °C or less, 130 °C or less, 125 °C or less, or even 120 °C or less. Therefore, the thermally expandable microspheres may have a T Start in the range of 80 °C to 134 °C, preferably 80 °C to 132 °C, more preferably 80 °C to 130 °C. Even more preferably, the thermally expandable microspheres have a T Start in the range of 90 °C to 130 °C, most preferably 95 °C to 128 °C.

[0048] In an embodiment, to further enhance the properties of the polymer shell, the polymer shell of the thermally expandable microspheres can include a polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor. The crosslinking agent may react with groups on the carboxylate-functionalized cellulose. The hydrogen bond donor may interact with groups on the carboxylate-functionalized cellulose via hydrogen bonds. Whether a particular compound reacts or interacts with the groups of the carboxylate-functionalized cellulose of the polymer shell depends not only on the chemical properties of the compound but also on the process conditions used when preparing the microspheres. By adding a polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor, the barrier properties of the polymer shell can be further improved, and the mechanical properties of the polymer shell, and thus the expansion properties of the microspheres, can be enhanced.

[0049] In particular, when the polymer shell enhancer is in the form of a crosslinking agent or a hydrogen bond donor, the polymer shell enhancer is typically a low molecular weight compound having a molecular weight of less than, for example, 2000 g / mol, preferably less than 1500 g / mol, more preferably less than 1000 g / mol, and still more preferably less than 500 g / mol. For example, the polymer shell enhancer may have a molecular weight in the range of 20 to 500 g / mol, preferably 30 to 400 g / mol, and still more preferably 40 to 300 g / mol.

[0050] In principle, any crosslinking agent known in the art for crosslinking polymers, and preferably any crosslinking agent that has been described for use in preparing expandable microspheres, may be used as the crosslinking agent for preparing the expandable microspheres of the present invention. An overview of such potential crosslinking agents is provided, for example, in European Patent No. 1288272 A1 and U.S. Patent No. 6617364 B2.

[0051] The crosslinking agent is a polyfunctional compound having two or more polymerizable functional groups suitable for crosslinking a polymer by reacting with the functional groups of the polymer to be crosslinked. The polymerizable functional groups of the crosslinking agent may be, for example, carbon-carbon double bonds ((meth)acrylate, etc.), isocyanates, aldehydes, carbonyl halides (carbonyl chloride, etc.), or anhydrides, epoxides, hydroxyl groups, aldehydes, melamines. These groups may react with suitable functional groups of the polymer to be crosslinked, such as carbon-carbon double bonds, isocyanates, amine groups, or hydroxyl groups of the polymer to be crosslinked.

[0052] To prepare the expandable microspheres of the present invention, preferably a bifunctional crosslinking agent, i.e., such a crosslinking agent having two functional groups suitable for crosslinking a polymer by reacting with the functional groups of the polymer to be crosslinked, is used. For example, the bifunctional crosslinking agent has polymerizable functional groups of the crosslinking agent as described above. In the context of the present invention, the polymer to be crosslinked is carboxylate-functionalized cellulose.

[0053] A hydrogen bond donor is a compound having a hydrogen atom covalently bonded to a more electronegative atom or group, particularly including nitrogen, oxygen, and fluorine, and these hydrogen atoms form intermolecular hydrogen bonds with the functional groups (hydrogen bond acceptors) of the carboxylate-functionalized cellulose (carboxylate groups, hydroxyl groups, and ether groups, etc., particularly carboxylate groups).

[0054] In an embodiment, the crosslinking agent is a crosslinking agent containing an anhydride group, preferably a dianhydride crosslinking agent, i.e., a crosslinking agent having two anhydride groups.

[0055] In an embodiment, the dianhydride compound is preferably selected from the group consisting of 1,2,4,5-benzenetetracarboxylic dianhydride, benzophenone tetracarboxylic dianhydride, ethylenediaminetetraacetic dianhydride, butanetetracarboxylic dianhydride, ethylenediaminetetraacetic anhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, cyclobutane-1,2,3,4-tetracarboxylic dianhydride, 4,4′-oxydiphthalic anhydride, and tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride. More preferably, the dianhydride compound is selected from the group consisting of 1,2,4,5-benzenetetracarboxylic dianhydride, benzophenone tetracarboxylic dianhydride, and ethylenediaminetetraacetic dianhydride, even more preferably selected from the group consisting of 1,2,4,5-benzenetetracarboxylic dianhydride or benzophenone tetracarboxylic dianhydride. Particularly preferred is that the dianhydride compound is 1,2,4,5-benzenetetracarboxylic dianhydride. 1,2,4,5-benzenetetracarboxylic dianhydride is also known as pyromellitic dianhydride and is so called herein.

[0056] The polymer shell enhancer is preferably a hydrogen bond donor.

[0057] Examples of suitable hydrogen bond donors are hydrolyzed anhydrides (e.g., dianhydrides such as those exemplified above, which are hydrolyzed, i.e., hydrolyzed), alcohols (e.g., 1,3-butanediol, glycerol, polycaprolactone-triol, pentaerythritol, ascorbic acid, sorbitol, polyethylene glycol (20) sorbitan monolaurate (also called Tween 20)), amides (e.g., urea, biuret, succinamide), and carboxylic acids (e.g., citric acid, maleic acid, succinic acid, pyromellitic acid, lactic acid, tartaric acid, ethylenediaminetetraacetic acid (EDTA), butanetetracarboxylic acid, e.g., 1,2,3,4-butanetetracarboxylic acid (BTCA)).

[0058] In an embodiment, the hydrogen bond donor is a carboxylic acid, that is, a compound containing at least one carboxylic acid group of a monocarboxylic acid, a dicarboxylic acid, a tricarboxylic acid, a tetracarboxylic acid, or a polycarboxylic acid (such as a polycarboxylic acid polymer, etc.). Preferably, the hydrogen bond donor is a carboxylic acid containing at least two carboxylic acid groups (-COOH), particularly a carboxylic acid containing a dicarboxylic acid, a tricarboxylic acid, or a tetracarboxylic acid.

[0059] Examples of monocarboxylic acids are formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, and lactic acid.

[0060] Examples of dicarboxylic acids are adipic acid, maleic acid, succinic acid, tartaric acid, and aldaric acid.

[0061] Examples of tricarboxylic acids are citric acid and isocitric acid.

[0062] Examples of tetracarboxylic acids are pyromellitic acid, ethylenediaminetetraacetic acid (EDTA), and 1,2,3,4 - butanetetracarboxylic acid (BTCA).

[0063] When the hydrogen bond donor is a carboxylic acid, the hydrogen bond donor may have a molecular weight in the range of 20 to 2000 g / mol, preferably in the range of 20 to 500 g / mol, more preferably in the range of 30 to 400 g / mol, and still more preferably in the range of 40 to 300 g / mol.

[0064] A compound containing an anhydride group (such as a dianhydride compound, etc.) may be hydrolyzed (for example, partially hydrolyzed or completely hydrolyzed), and then the hydrolyzed anhydride group may form an intermolecular hydrogen bond with the functional group of the carboxylate-functionalized cellulose and then act as a hydrogen bond donor. In an embodiment, the hydrogen bond donor is a compound containing a hydrolyzed anhydride group, preferably a hydrolyzed dianhydride compound, that is, a compound having two hydrolyzed anhydride groups.

[0065] Thus, in an embodiment, the hydrogen bond donor may be a compound containing a hydrolyzed anhydride group, preferably selected from the group consisting of hydrolyzed 1,2,4,5-benzenetetracarboxylic dianhydride, hydrolyzed benzophenonetetracarboxylic dianhydride, hydrolyzed ethylenediaminetetraacetic dianhydride, hydrolyzed butanetetracarboxylic dianhydride, hydrolyzed ethylenediaminetetraacetic anhydride, hydrolyzed 3,3,4,4-biphenyltetracarboxylic dianhydride, hydrolyzed bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, hydrolyzed cyclobutane-1,2,3,4-tetracarboxylic dianhydride, hydrolyzed 4,4'-oxydiphthalic anhydride, and hydrolyzed tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride. More preferably, the hydrogen bond donor may be selected from the group consisting of hydrolyzed 1,2,4,5-benzenetetracarboxylic dianhydride, hydrolyzed benzophenonetetracarboxylic dianhydride, and hydrolyzed ethylenediaminetetraacetic dianhydride, and most preferably, may be selected from the group consisting of hydrolyzed 1,2,4,5-benzenetetracarboxylic dianhydride or hydrolyzed benzophenonetetracarboxylic dianhydride.

[0066] The hydrogen bond donor may also be selected from the group consisting of 1,2,4,5-benzenetetracarboxylic acid, citric acid, tartaric acid, 1,2,3,4-butanetetracarboxylic acid (BTCA), and maleic acid.

[0067] In particular, the hydrogen bond donor is selected from the group consisting of 1,2,4,5-benzenetetracarboxylic acid, citric acid, tartaric acid, 1,2,3,4-butanetetracarboxylic acid (BTCA), and maleic acid.

[0068] More specifically, the hydrogen bond donor is selected from the group consisting of 1,2,4,5-benzenetetracarboxylic acid, citric acid, tartaric acid, and 1,2,3,4-butanetetracarboxylic acid (BTCA).

[0069] Preferably, the hydrogen bond donor is a tricarboxylic acid, or a tetracarboxylic acid such as citric acid, 1,2,4,5-benzenetetracarboxylic acid or 1,2,3,4-butanetetracarboxylic acid (BTCA).

[0070] In certain embodiments, the hydrogen bond donor is a tetracarboxylic acid such as 1,2,4,5-benzenetetracarboxylic acid or 1,2,3,4-butanetetracarboxylic acid (BTCA). Most preferably, the hydrogen bond donor is 1,2,3,4-butanetetracarboxylic acid (BTCA).

[0071] The amount of the polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor used to prepare the expandable microspheres of the present invention is not particularly limited.

[0072] However, in one embodiment, the polymer shell enhancer is a crosslinking agent used in an amount of 0.1 to 10 moles of the polymerizable functional group of the crosslinking agent per mole of the functional group suitable for crosslinking the carboxylate-functionalized cellulose. This means that the molar ratio of the polymerizable functional group of the crosslinking agent to the functional group suitable for crosslinking the carboxylate-functionalized cellulose is in the range of 0.1 / 1 (mol / mol) to 10 / 1 (mol / mol). Preferably, the crosslinking agent is used in an amount of 0.5 to 5 moles of the polymerizable functional group of the crosslinking agent per mole of the functional group suitable for crosslinking the carboxylate-functionalized cellulose. Even more preferably, the crosslinking agent is used in an amount of 0.5 to 3 moles of the polymerizable functional group of the crosslinking agent per mole of the functional group suitable for crosslinking the carboxylate-functionalized cellulose, and most preferably, the crosslinking agent is used in an amount of 1.4 to 2.2 moles of the polymerizable functional group of the crosslinking agent per mole of the functional group suitable for crosslinking the carboxylate-functionalized cellulose. The crosslinking agent may also be used in an amount of 1.5 moles of the polymerizable functional group of the crosslinking agent per mole of the functional group suitable for crosslinking the carboxylate-functionalized cellulose.

[0073] For example, when the polymer shell enhancer is a dianhydride crosslinking agent, the dianhydride crosslinking agent (i.e., a crosslinking agent having two polymerizable functional groups of the crosslinking agent) is used in an amount of 0.05 to 5 moles of the functional group per mole of the functional group suitable for crosslinking the carboxylate-functionalized cellulose. This means that the molar ratio of the dianhydride crosslinking agent to the functional group suitable for crosslinking the carboxylate-functionalized cellulose is in the range of 0.05 / 1 (mol / mol) to 5 / 1 (mol / mol). Preferably, the dianhydride crosslinking agent is used in an amount of 0.25 to 2.5 moles of the crosslinking agent per mole of the functional group suitable for crosslinking the carboxylate-functionalized cellulose. Even more preferably, the dianhydride crosslinking agent is used in an amount of 0.25 to 1.5 moles of the crosslinking agent per mole of the functional group suitable for crosslinking the carboxylate-functionalized cellulose, and most preferably, the dianhydride crosslinking agent is used in an amount of 0.7 to 1.1 moles of the crosslinking agent per mole of the functional group suitable for crosslinking the carboxylate-functionalized cellulose. The dianhydride crosslinking agent may also be used in an amount of 0.75 moles of the crosslinking agent per mole of the functional group suitable for crosslinking the carboxylate-functionalized cellulose.

[0074] Functional groups suitable for crosslinking carboxylate-functionalized cellulose are preferably hydroxyl groups. Thus, in a preferred embodiment, the dianhydride crosslinking agent is used in an amount of 0.05 to 5 moles of crosslinking agent per mole of hydroxyl groups of the carboxylate-functionalized cellulose. This means that the molar ratio of the dianhydride crosslinking agent to the hydroxyl groups of the carboxylate-functionalized cellulose ranges from 0.05 / 1 (mol / mol) to 5 / 1 (mol / mol). Preferably, the dianhydride crosslinking agent is used in an amount of 0.25 to 2.5 moles of crosslinking agent per mole of hydroxyl groups of the carboxylate-functionalized cellulose. Even more preferably, the dianhydride crosslinking agent is used in an amount of 0.25 to 1.5 moles of crosslinking agent per mole of hydroxyl groups of the carboxylate-functionalized cellulose, and most preferably, the dianhydride crosslinking agent is used in an amount of 0.7 to 1.1 moles of crosslinking agent per mole of hydroxyl groups of the carboxylate-functionalized cellulose. The dianhydride crosslinking agent may also be used in an amount of 0.75 moles of crosslinking agent per mole of hydroxyl groups of the carboxylate-functionalized cellulose.

[0075] The amount of the polymer shell enhancer used may be, in particular when the polymer shell enhancer is a hydrogen bond donor, 0 to 50% by weight based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose. In embodiments, it can be in the range of 0.01 to 40% by weight, such as in the range of 0.1 to 30% by weight, in the range of 1 to 30% by weight, or even in the range of 5 to 25% by weight (such as in the range of 10 to 20% by weight), and the % by weight is based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose.

[0076] For example, when the polymer shell comprises cellulose acetate propionate (CAP) or cellulose acetate butyrate (CAB), the amount of the polymer shell enhancer used may be, in particular when the polymer shell enhancer is a hydrogen bond donor, from 0 to 50% by weight, based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose. In embodiments, it can be in the range of 0.01 to 30% by weight, for example, in the range of 0.1 to 30% by weight, in the range of 1 to 25% by weight, or even in the range of 2 to 20% by weight (such as in the range of 5 to 20% by weight or in the range of 10 to 20% by weight), and the % by weight is based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose.

[0077] In further embodiments, the polymer shell can include particles for improving the mechanical properties and gas barrier of the polymer shell, and thus also acts as a polymer shell enhancer. Examples of such particles are talc, montmorillonite, nanocrystalline cellulose, and various types of clays (such as bentonite).

[0078] Multiple factors can result in high density. For example, high density can result from a low microsphere yield, i.e., the proportion of microspheres in the polymeric material is too low to reduce the overall density to an acceptable level. Another problem is insufficient expansion characteristics, which can occur when there are too many microspheres and there is not enough blowing agent to allow for proper expansion. This can be due to the polymeric shell being too permeable to the blowing agent or the formation of so-called "multi-core" microspheres, i.e., there are multiple blowing agent-containing cores within the shell (e.g., like a microsphere-shaped foam or sponge) instead of a single blowing agent-containing core. In such multi-core microspheres, the concentration of the blowing agent is typically too low to properly reduce the density. Another cause is the aggregation or agglomeration of the polymer, resulting in poor production of microspheres and a higher density material. A high proportion of the aggregated material or microspheres that do not expand sufficiently can also lead to significant inhomogeneities in the expansion characteristics of the resulting microsphere product. This is particularly undesirable for applications sensitive to the surface, such as coatings where a smooth finish is desired.

[0079] Exemplary cross-sections of single-core and multi-core microspheres are provided in FIGS. 1A and 1B, respectively, where the polymeric region 1 is represented by the hatched area and the blowing agent-containing region 2 is represented by the blank area.

[0080] One or more blowing agents generally have a boiling point above 25 °C at a pressure of 5.0 bara, or above 25 °C at a pressure of 3.0 bara, where "bara" is an abbreviation for "bar-absolute". In embodiments, they have a boiling point above 25 °C at atmospheric pressure (1.013 bara). Typically, they have a boiling point of 250 °C or less (e.g., 220 °C or less, or 200 °C or less) at atmospheric pressure. They are preferably inert and do not react with the functionalized cellulose shell. The boiling point at high pressure can be calculated using the Clausius-Clapeyron equation.

[0081] Examples of blowing agents include dialkyl ethers, alkanes, and halocarbons (e.g., chlorocarbons, fluorocarbons, or chlorofluorocarbons). In embodiments, the dialkyl ether contains two alkyl groups each selected from C 2 ~C 5 alkyl groups. In embodiments, the alkane is a C 4 ~C 12 alkane. In embodiments, the haloalkane is selected from C 2 ~C 10 haloalkanes. The haloalkane may contain one or more halogen atoms selected from chlorine and fluorine. The alkyl or haloalkyl groups in the dialkyl ether, alkane, and haloalkane can be linear, branched, or cyclic. One or a mixture of one or more blowing agents can be used.

[0082] In embodiments, for environmental reasons, one or more blowing agents are selected from alkyl ethers and alkanes, and in further embodiments, one or more blowing agents are selected from alkanes. Haloalkanes are preferably avoided due to their potential ozone-depleting properties and their generally higher global warming potential.

[0083] Examples of suitable blowing agents that can be used include n - pentane, isopentane, neopentane, cyclopentane, cyclohexane, n - butane, isobutane, isohexane, neohexane, heptane, isoheptane, octane, isooctane, isodecane, and isododecane. In a preferred embodiment, the blowing agent is a C 4 ~C 12 selected from isoalkanes.

[0084] In the expandable microspheres, one or more blowing agents are typically present in an amount of 5 to 50% by weight, based on the total weight of the functionalized cellulose and the blowing agent(s), for example, in an amount in the range of 5 to 45% by weight, or 10 to 40% by weight.

[0085] Carboxylate - functionalized cellulose materials can be purchased commercially or can be made by known means, such as by mixing cellulose with a suitable carboxylic acid in the presence of a strong acid such as sulfuric acid, or, for example, by a base - catalyzed reaction of cellulose with an acyl chloride as described in, for example, Nishio et al; Cellulose, 2006(13), 245 - 259.

[0086] In certain embodiments, the polymeric shell comprises cellulose acetate propionate (CAP) having a number - average molecular weight (M n ) in the range of 10,000 to 100,000 Da (in the range of 10,000 to 80,000 Da, preferably in the range of 10,000 to 30,000 Da, more preferably in the range of 10,000 to 25,000 Da, etc.), and the thermally expandable microspheres exhibit an expansion start temperature (T Start ) from 80°C to less than 135°C.

[0087] The amount of the hydrogen bond donor in the form of the carboxylic acid used may be from 0 to 50% by weight based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose. In embodiments, it may be in the range of 0.01 to 40% by weight, for example, in the range of 0.1 to 30% by weight, in the range of 1 to 30% by weight, or even in the range of 5 to 25% by weight (such as in the range of 10 to 20% by weight), and the % by weight is based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose.

[0088] For example, when the polymer shell contains cellulose acetate propionate (CAP) or cellulose acetate butyrate (CAB), the amount of the hydrogen bond donor in the form of the carboxylic acid used may be from 0 to 50% by weight based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose. In embodiments, it may be in the range of 0.01 to 30% by weight, for example, in the range of 0.1 to 30% by weight, in the range of 1 to 25% by weight, or even in the range of 2 to 20% by weight (such as in the range of 5 to 20% by weight or in the range of 10 to 20% by weight), and the % by weight is based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose.

[0089] The expandable microspheres of the present invention can be obtained by a spray drying process, which includes mixing a carboxylate-functionalized cellulose, an organic solvent, a foaming agent, and optionally a polymer shell enhancer in the form of a cross-linking agent or a hydrogen bond donor, and then spraying the thus obtained mixture into a drying device to produce thermally expandable microspheres having a polymer shell surrounding a hollow core, wherein the polymer shell contains the carboxylate-functionalized cellulose within the hollow core and the hollow core contains the foaming agent.

[0090] In principle, the spray drying apparatus for carrying out the spray drying process is not limited, and any conventional and commercially available spray drying apparatus can be used for the spray drying process. A typical spray drying apparatus suitable for the process described herein comprises a drying chamber equipped with a nozzle, an inlet for the drying gas, and an outlet connecting the drying chamber to a cyclone. The liquid to be atomized is usually sprayed into the drying chamber through a nozzle (although it may be located in any other part of the spray dryer), usually located at the top of the spray chamber, usually in combination with a spray gas. In the drying chamber, the atomized liquid is dried by the drying gas supplied through an inlet for the drying gas in the spray chamber. The inlet for the drying gas may be located, for example, immediately adjacent to the nozzle. The atomized liquid dries and forms particles. The particles thus obtained are then fed into the cyclone together with the drying gas through an outlet of the drying chamber, usually located in the bottom area of the drying chamber. In the cyclone, the particles are separated from the drying air. The drying air may be further filtered to remove any residual particles from the drying air.

[0091] A suitable spray drying apparatus for carrying out the spray drying process is the Buchi Mini Spray Dryer B-290, commercially available from Buchi / Switzerland.

[0092] The order of addition of the carboxylate-functionalized cellulose, the organic solvent, the foaming agent, and the polymeric shell enhancer in the form of any crosslinking agent or hydrogen bond donor for mixing is not restricted and any order can be chosen.

[0093] However, in a preferred embodiment, in the process of making the expandable microspheres, the carboxylate-functionalized cellulose is first mixed with the organic solvent, and then in a further step, the foaming agent and optionally the polymeric shell enhancer in the form of a crosslinking agent or hydrogen bond donor are added to the mixture.

[0094] The mixing of carboxylate-functionalized cellulose may be carried out at ambient temperature, but temperatures in the range of 5 to 75 °C can be used. The mixing is usually carried out until the carboxylate-functionalized cellulose is completely dissolved in the organic solvent.

[0095] In an embodiment, the mixture of carboxylate-functionalized cellulose and the organic solvent can be left standing or stirred for a period, for example, between 1 and 100 hours, or between 2 and 50 hours. This can be carried out at a temperature in the range of 10 to 95 °C, for example, at a temperature of 20 to 90 °C.

[0096] In a further step, a blowing agent and optionally a polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor are added to the mixture of carboxylate-functionalized cellulose and the organic solvent. When the polymer shell enhancer is added, the order of addition of the blowing agent and the polymer shell enhancer is not critical, and thus, the blowing agent may be added first followed by the polymer shell enhancer, or the polymer shell enhancer may be added first followed by the blowing agent. Also, this mixing step can be carried out at ambient temperature, but temperatures in the range of 5 to 75 °C can be used. Also, this mixing step is usually carried out until the blowing agent and optionally the polymer shell enhancer are completely dissolved in the organic solvent.

[0097] After adding the blowing agent and optionally the polymer shell enhancer to the mixture of carboxylate-functionalized cellulose and the organic solvent, the mixture thus obtained may be further stirred for a period, for example, between 1 and 100 hours, or between 2 and 50 hours. This can also be at a temperature in the range of 10 to 95 °C, for example, at a temperature of 20 to 90 °C.

[0098] A mixture comprising a carboxylate-functionalized cellulose, an organic solvent, a blowing agent, and optionally a polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor is then sprayed into a drying device to produce thermally expandable microspheres as described herein. The drying device may be a spray drying device as described above.

[0099] Any spray gas sprayed through the nozzle together with the atomized liquid is not particularly limited and may be any suitable spray gas known to those skilled in the art. For example, the spray gas may be selected from nitrogen, carbon dioxide, (pressurized) air, noble gases (such as argon). Preferably, in the method for producing expandable microspheres as described herein, a spray gas is used, and more preferably, this spray gas is nitrogen.

[0100] Also, the drying gas is not particularly limited and may be any suitable drying gas known to those skilled in the art. For example, the spray gas may also be selected from nitrogen, carbon dioxide, (pressurized) air, noble gases (such as argon). It is preferred that the drying gas is nitrogen.

[0101] Further process parameters for operating the spray drying device, such as the spray gas flow rate, the inlet temperature of the drying gas when entering the drying chamber, the supply rate of the atomized liquid, and the inhaler speed and atomizer speed for circulating the drying gas within the spray drying device, can be easily selected by those skilled in the art.

[0102] It has been found that expandable microspheres comprising a polymer shell surrounding a hollow core can be obtained by the above method, the hollow core contains a blowing agent, the polymer shell contains a carboxylate-functionalized cellulose, and the thermally expandable microspheres have a starting temperature (T Start ) for expansion from 80 °C to less than 135 °C. This method has been further found to be particularly suitable for obtaining such expandable microspheres, and the polymer shell further contains a polymer shell enhancer (such as a carboxylic acid) in the form of a crosslinking agent or a hydrogen bond donor, particularly in the form of a hydrogen bond donor.

[0103] The organic solvent can be selected from organic solvents having one or more functional groups selected from, for example, esters, amides, aldehydes, ketones, alcohols (including glycols), and ethers having 3 to 12 carbon atoms. In an embodiment, the ester, ketone, and ether may be part of a cyclic structure. Further examples include haloalkanes having 1 to 6 carbon atoms and halo-carboxylic acids having 1 to 6 carbon atoms, where the halogen is selected from fluorine, chlorine, bromine, and iodine.

[0104] Examples of organic solvents that can be used include ethyl acetate, ethyl formate, methyl acetate, n-propyl formate, isopropyl formate, n-propyl acetate, isopropyl acetate, isobutyl acetate, n-butyl acetate, n-pentyl formate, isopentyl formate, n-pentyl acetate, isopentyl acetate, ethyl propionate, isobutyl isobutyrate, n-butyl propionate, ethyl 3-ethoxypropionate, 2-ethylhexyl acetate, acetone, methyl ethyl ketone, diethyl ketone, methyl isobutyl ketone, methyl isoamyl ketone, methyl n-amyl ketone, mesityl oxide, acetophenone, cyclohexanone, diethyl phthalate, ethyl lactate, benzyl acetate, butyrolactone, acetylacetone, methylcyclohexanone, benzaldehyde, diisobutyl ketone, diacetone alcohol, ethylene glycol, glyceryl-α-monochlorohydrin, propylene glycol, glycol ethers (e.g., propylene glycol monomethyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, propylene glycol monotert-butyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether), glycol ether esters (e.g., ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, ethylene glycol monobutyl ether acetate, diethylene glycol diacetate), n-propyl alcohol, isopropyl alcohol, n-butanol, sec-butanol, isobutanol, benzyl alcohol, diisopropyl ether, dimethoxymethane, dimethoxyethane, 1,4-dioxane, 1,3-dioxolane, tetrahydrofuran, anisole, phenetole, and dimethylformamide. Other examples of solvents include dimethyl sulfoxide, toluene, xylene, n-methyl-2-pyrrolidone, methyl chloride, chloroform, carbon tetrachloride, trichloroacetic acid, methyl bromide, methyl iodide, trichloroethylene, and tetrachloroethylene. The organic solvent can be a mixture of two or more solvents.The organic solvent(s) can contain water, but typically, the water content of the organic solvent(s) is less than 5% by weight, i.e., 0 - 5% by weight of water, for example, 0 - 1% by weight of water.

[0105] In an embodiment, the solvent is selected from one or more of ethyl acetate, methyl acetate, ethyl formate, and acetone. The solvent is particularly preferably acetone.

[0106] Typically, the carboxylate-functionalized cellulose content in the mixture for spray drying is typically in the range of 0.1 - 50% by weight. In an embodiment, the amount can be in the range of 1 - 40% by weight, for example, in the range of 5 - 35% by weight, or even in the range of 10 - 30% by weight. % by weight is based on the total weight of the mixture for spray drying.

[0107] The amount of the foaming agent(s) in the mixture for spray drying is typically in the range of 0.5 - 50% by weight. In an embodiment, the amount can be in the range of 0.5 - 40% by weight, for example, in the range of 1 - 30% by weight, or even in the range of 5 - 25% by weight. In an embodiment, the weight of the foaming agent in the mixture for spray drying is not more than the weight of the carboxylate-functionalized cellulose. For example, the weight ratio of the foaming agent to the carboxylate-functionalized cellulose can be 1.5 or less, for example, 1.3 or less, or even 1.1 or less. In an embodiment, the minimum weight ratio is 0.1, or in a further embodiment 0.2. In an embodiment, the weight ratio of the foaming agent to the carboxylate-functionalized cellulose in the organic phase is in the range of 0.1 - 1.5, such as in the range of 0.2 - 1.3, or even in the range of 0.3 - 1.1.

[0108] The amount of the polymer shell enhancer in the form of a crosslinking agent or hydrogen bond donor in the mixture for spray drying is typically in the range of 0 to 15% by weight. In embodiments, the amount is in the range of 0.01 to 15% by weight, for example, it can be in the range of 0.05 to 10% by weight, in the range of 0.1 to 8% by weight, or even in the range of 1 to 6% by weight. The % by weight is based on the total weight of the carboxylate-functionalized cellulose, foaming agent, polymer shell enhancer, and solvent in the mixture for spray drying.

[0109] For example, when cellulose acetate propionate (CAP) or cellulose acetate butyrate (CAB) is used as the polymer, the amount of the polymer shell enhancer used may be 0 to 15% by weight. In embodiments, the amount is in the range of 0.01 to 15% by weight, for example, it can be in the range of 0.05 to 10% by weight, in the range of 0.1 to 8% by weight, or even in the range of 0.5 to 5.4% by weight. The % by weight is based on the total weight of the carboxylate-functionalized cellulose, foaming agent, polymer shell enhancer and solvent in the mixture for spray drying.

[0110] The amount of the organic solvent totals 100% by weight. The amount of the organic solvent is preferably at least 30% by weight, more preferably at least 40% by weight, and even more preferably at least 50% by weight. The % by weight is based on the total weight of the mixture for spray drying.

[0111] When the polymer shell enhancer is a crosslinking agent or a hydrogen bond donor, especially a hydrogen bond donor, the amount of the polymer shell enhancer in the mixture for spray drying may also be 0 to 50% by weight based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose in the mixture for spray drying. In embodiments, the amount is in the range of 0.1 to 40% by weight, for example, it can be in the range of 0.5 to 35% by weight, in the range of 1 to 30% by weight, or even in the range of 2 to 25% by weight. The % by weight is based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose in the mixture for spray drying.

[0112] The unexpanded microspheres typically have a volume average particle size (diameter) in the range of 1 to 500 μm (such as 5 to 200 μm), or in embodiments, in the range of 10 to 100 μm, or even in the range of 15 to 80 μm, which is the D(0.5) value.

[0113] The diameter of the expanded microspheres is typically in the range of 1.5 to 8 times larger in diameter than the unexpanded microspheres, for example, 2 to 7 times or 3 to 6 times its original diameter.

[0114] The particle size is preferably measured using light scattering techniques, such as laser diffraction like low angle laser light scattering (LALLS). They can also be measured by image analysis from photographs or electron micrograph images of the microspheres before or after expansion.

[0115] To expand the expandable microspheres, they can be heated to a temperature above the boiling point of the blowing agent and the T g of the functionalized cellulose, and can also be heated to a temperature below the melting point of the microspheres. To interrupt the expansion, the microspheres can be cooled back down to below the T g of the functionalized cellulose and / or the boiling point of the blowing agent.

[0116] As a method of heating the expandable microspheres, for example, as described in International Publication No. WO 2004 / 056549, International Publication No. WO 2014 / 198532, and International Publication No. WO 2016 / 091847, it may be mentioned to bring them into direct or indirect contact with a heat transfer medium such as steam or pressurized steam. In a further embodiment, direct or indirect contact with other heated gases (for example, air or nitrogen) optionally mixed with steam can be used. In still further embodiments where indirect heating is used, a liquid heat transfer medium (for example, heated oil) may be used. In another embodiment, IR radiation can be used to heat the microspheres.

[0117] The expansion properties of the thermally expandable thermoplastic microspheres can be evaluated using a thermomechanical analyzer (for example, Mettler TMA 841), and quantitative data can be obtained from images using suitable software such as STARe software.

[0118] The expandable or expanded thermoplastic microspheres may be provided in an unexpanded form, for example, for local expansion at their place of use, or they can be pre-expanded before shipping to the final place of use.

[0119] The microspheres can find use in numerous applications in the manufacture of, for example, paper (for example, embossed paper, paper fillers, adhesives), inks, cork, cementitious compositions, adhesives, foams, insulation materials, coatings, rubber-based products, thermoplastics, thermosetting resins, ceramics, non-woven composites, fillers, etc., for example, to provide lightweight fillers in such applications.

[0120] The thermally expandable microspheres described in this specification can be thermally expandable when dry, wet, or in a slurry. They can also retain the blowing agent for a long period of time, for example, for at least one week, at least one month, or at least four months, etc. Furthermore, their expansion is typically irreversible, that is, cooling the microspheres after thermal expansion does not result in their shrinkage such that they return to their size before expansion.

[0121] Another aspect of the present invention is a process for preparing thermally expandable microspheres, which comprises mixing carboxylate-functionalized cellulose, an organic solvent, a blowing agent, and optionally a polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor, and then spraying the thus-obtained mixture into a drying device to produce thermally expandable microspheres having a polymer shell surrounding a hollow core, the polymer shell containing carboxylate-functionalized cellulose within the hollow core, and the hollow core containing the blowing agent.

[0122] Not only the process parameters, spray drying device, carboxylate-functionalized cellulose, organic solvent, blowing agent, and any polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor, but also their amounts are the same as those already described above and are equally applicable to the process according to the second aspect of the present invention.

[0123] In an embodiment, in the process for preparing thermally expandable microspheres, when the polymer shell enhancer is a crosslinking agent, the crosslinking agent is added in an amount corresponding to a molar ratio of the polymerizable functional group of the crosslinking agent to the functional group suitable for crosslinking the carboxylate-functionalized cellulose in the range of 0.1 / 1 (mol / mol) to 10 / 1 (mol / mol), preferably in the range of 0.5 / 1 (mol / mol) to 5 / 1 (mol / mol), more preferably in the range of 0.5 / 1 (mol / mol) to 3 / 1 (mol / mol), and most preferably in the range of 1.4 / 1 (mol / mol) to 2.2 / 1 (mol / mol).

[0124] For example, when the polymer shell enhancer is a dianhydride crosslinking agent, the dianhydride crosslinking agent (i.e., a crosslinking agent having two polymerizable functional groups of the crosslinking agent) is in the range of 0.05 / 1 (mol / mol) to 5 / 1 (mol / mol), preferably in the range of 0.25 / 1 (mol / mol) to 2.5 / 1 (mol / mol), more preferably in the range of 0.25 / 1 (mol / mol) to 1.5 / 1 (mol / mol), and most preferably in the range of 0.7 / 1 (mol / mol) to 1.1 / 1 (mol / mol), and is added in an amount corresponding to the molar ratio of the crosslinking agent to the functional group suitable for crosslinking the carboxylate-functionalized cellulose.

[0125] In a preferred embodiment, the dianhydride crosslinking agent is in the range of 0.05 / 1 (mol / mol) to 5 / 1 (mol / mol), preferably in the range of 0.25 / 1 (mol / mol) to 2.5 / 1 (mol / mol), more preferably in the range from 0.25 / 1 (mol / mol) to 1.5 / 1 (mol / mol), and most preferably in the range of 0.7 / 1 (mol / mol) to 1.1 / 1 (mol / mol), and is added in an amount corresponding to the molar ratio of the dianhydride crosslinking agent to the hydroxyl group of the carboxylate-functionalized cellulose.

[0126] In a further embodiment, in the process of preparing the thermally expandable microspheres, no catalyst (amines for crosslinking, such as triethylamine, etc.) is added to the mixture sprayed in the drying device. This may improve the storage stability of the expandable microspheres obtained by the process. Further, when no catalyst is used in this process, the obtained microspheres may improve their expansion performance with storage.

[0127] In a further embodiment, particularly when the polymer shell enhancer is an anhydride, the process of preparing the thermally expandable microspheres further includes a step of storing the prepared thermally expandable microspheres for at least two weeks after preparation and preferably for at least four weeks before expanding the thermally expandable microspheres. Surprisingly, it has been found that the storage of the prepared thermally expandable microspheres may improve the expansion density of the thermally expandable microspheres. The expansion density represents the density of the microspheres at the maximum expansion of the thermally expandable microspheres. The expansion density can be determined using standard measurement techniques generally known to those skilled in the art. For example, the expansion density can also be determined by analyzing the expansion thermogram obtained by the above-described temperature increase experiment for the determination of T Start i.e., by using a Mettler-Toledo thermomechanical analyzer such as a Mettler-Toledo TMA / SDTA 841e and, for example, by using STARe software manufactured by Mettler-Toledo. Using such an apparatus, the typical value determined for the density is called the TMA density. The TMA density is calculated using the formula of dividing the weight of the sample [g] by the volume increase of the sample [dm 3 at maximum expansion. A lower TMA density usually exhibits more desirable expansion characteristics. A TMA density of 0.2 g / cm 3 or less is considered desirable, and a TMA density of at least 0.15 g / cm 3 or less is particularly considered desirable.

[0128] In a further aspect, the present invention also relates to thermally expandable microspheres obtained by the process for preparing thermally expandable microspheres as described above. The present invention includes the following aspects. Item 1. A thermally expandable microsphere comprising a polymer shell surrounding a hollow core, wherein the hollow core contains a blowing agent and the polymer shell contains carboxylate-functionalized cellulose, and the thermally expandable microsphere has an expansion start temperature (T StartThermally expandable microspheres having Item 2. T Start is 90°C to 130°C, preferably 95°C to 120°C, the thermally expandable microspheres according to Item 1. Item 3. The thermally expandable microspheres according to Item 1 or 2, wherein the polymer shell further contains a polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor. Item 4. The crosslinking agent is a dianhydride crosslinking agent, preferably selected from the group consisting of 1,2,4,5-benzenetetracarboxylic dianhydride, benzophenonetetracarboxylic dianhydride, ethylenediaminetetraacetic dianhydride, butanetetracarboxylic dianhydride, ethylenediaminetetraacetic anhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, cyclobutane-1,2,3,4-tetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride and tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride, more preferably selected from the group consisting of 1,2,4,5-benzenetetracarboxylic dianhydride, benzophenonetetracarboxylic dianhydride and ethylenediaminetetraacetic dianhydride, and most preferably 1,2,4,5-benzenetetracarboxylic dianhydride or benzophenonetetracarboxylic dianhydride, the thermally expandable microspheres according to Item 3. Item 5. The molar ratio of the polymerizable functional group of the crosslinking agent to the functional group suitable for crosslinking the carboxylate-functionalized cellulose is in the range of 0.1 / 1 (mol / mol) to 10 / 1 (mol / mol), preferably in the range of 0.5 / 1 (mol / mol) to 5 / 1 (mol / mol), and most preferably in the range of 1.4 / 1 (mol / mol) to 2.2 / 1 (mol / mol), the thermally expandable microspheres according to Item 3 or 4. Item 6. The thermally expandable microspheres according to Item 3, wherein the polymer shell enhancer is a hydrogen bond donor selected from the group consisting of alcohols, amides, carboxylic acids, and hydrolyzed anhydrides. Item 7. The hydrogen bond donor is a hydrolyzed dianhydride compound, preferably selected from the group consisting of hydrolyzed 1,2,4,5-benzenetetracarboxylic dianhydride, hydrolyzed benzophenonetetracarboxylic dianhydride, hydrolyzed ethylenediaminetetraacetic dianhydride, hydrolyzed butanetetracarboxylic dianhydride, hydrolyzed ethylenediaminetetraacetic anhydride, hydrolyzed 3,3′,4,4′-biphenyltetracarboxylic dianhydride, hydrolyzed bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, hydrolyzed cyclobutane-1,2,3,4-tetracarboxylic dianhydride, hydrolyzed 4,4′-oxydiphthalic anhydride, and hydrolyzed tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride; more preferably selected from the group consisting of hydrolyzed 1,2,4,5-benzenetetracarboxylic dianhydride, hydrolyzed benzophenonetetracarboxylic dianhydride, and hydrolyzed ethylenediaminetetraacetic dianhydride; and most preferably, the thermally expandable microsphere according to Item 6, which is hydrolyzed 1,2,4,5-benzenetetracarboxylic dianhydride or hydrolyzed benzophenonetetracarboxylic dianhydride. Item 8. The hydrogen bond donor is a carboxylic acid, preferably selected from the group consisting of 1,2,4,5-benzenetetracarboxylic acid, citric acid, tartaric acid, 1,2,3,4-butanetetracarboxylic acid (BTCA), maleic acid, and any combination thereof; more preferably selected from the group consisting of citric acid, 1,2,4,5-benzenetetracarboxylic acid, and 1,2,3,4-butanetetracarboxylic acid (BTCA); most preferably, the thermally expandable microsphere according to Item 6, which is 1,2,3,4-butanetetracarboxylic acid (BTCA). Item 9. The amount of the hydrogen bond donor is in the range of 0.01 to 50% by weight, preferably in the range of 0.1 to 30% by weight, more preferably in the range of 1 to 25% by weight, or still more preferably in the range of 2 to 20% by weight, and most preferably in the range of 10 to 20% by weight, and the % by weight is based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose, the thermally expandable microspheres according to item 7 or 8. Item 10. The carboxylate-functionalized cellulose contains a carboxylate group of formula (1),

Chemical formula

Chem.

Chem.

Chem.

Chem.

Chem.

Chem.

Chem.

Example

[0129] The following examples are intended to illustrate the present invention. - The swelling properties were evaluated using a Mettler TMA / SDTA 841e thermomechanical analyzer interfaced to a PC running STARe software. The sample to be analyzed was prepared from 0.5 mg (+ / - 0.02 mg) of thermally expandable microspheres contained in an aluminum oxide crucible with a diameter of 6.8 mm and a depth of 4.0 mm. The crucible was sealed using a lid of aluminum oxide with a diameter of 6.1 mm. Using the TMA swelling probe type, the temperature of the sample was raised from about 30 °C to 240 °C at a heating rate of 20 °C / min while applying a load of 0.06 N (net) with the probe. The vertical displacement of the probe was measured to analyze the swelling properties. Initial temperature of swelling (T start ): The temperature (°C) at which the displacement of the probe began. - Maximum swelling temperature (T max ): The temperature (°C) at which the displacement of the probe reached its maximum. - Maximum displacement (L max ): The displacement of the probe (μm) when the displacement of the probe reached its maximum. - TMA density: When the displacement of the probe reached its maximum value, the sample weight (d) divided by the volume increase of the sample (dm 3 ).

[0130] The parameters were determined as illustrated by the exemplary embodiment shown in FIG. 2.

[0131] The volatile content in the microspheres was determined using a Mettler Toledo TGA / DSC1 TGA instrument.

[0132] Gas chromatography - flame ionization detection (GC - FID) analysis was performed using an Agilent 7697A Headspace in combination with an Agilent 7890A GC.

[0133] Differential scanning calorimetry (DSC) was obtained using a Mettler Toledo DSC 822e device.

[0134] General synthesis method: In the experiment of Example 1, a solution of a carboxylate-functionalized cellulose polymer in a suitable organic solvent was prepared by dissolving the polymer overnight using a magnetic stirrer.

[0135] A blowing agent was added to the solution and the mixture was stirred for 5 minutes to redissolve any precipitated polymer.

[0136] If applicable, a polymer shell enhancer was further added to the solution and the mixture was stirred for an additional 10 minutes to dissolve the polymer shell enhancer.

[0137] In the experiments of Examples 2 - 5, all components (solvent, polymer, hydrogen bond donor, and blowing agent) were mixed and left stirring overnight.

[0138] The mixture thus obtained (for Example 1) was then spray dried using a Buchi Mini Spray Dryer B-290. Nitrogen was used as the spray gas at a feed rate of 238 l / h. The feed rate of the mixture to be spray dried was 9 ml / min. The temperature of the drying gas at the inlet was 70 °C and the aspirator speed was 38 m 3 / h. The temperature at the outlet was approximately 52 - 54 °C and the spray time was approximately 4 minutes.

[0139] The mixture thus obtained (for Examples 2 - 5) was then spray dried using a Buchi Mini Spray Dryer B-290. Nitrogen was used as the spray gas at a feed rate of 238 l / h. The feed rate of the mixture to be spray dried was measured to be approximately 12 - 13 ml / min. The temperature of the drying gas at the inlet was 70 °C and the aspirator speed was 38 m 3 / h. The temperature at the outlet was approximately 52 - 54 °C and the spray time was approximately 4 minutes.

[0140] The dried solid was collected from the bottom of the cyclone and analyzed within a few days or after storage, as shown in Tables 3, 6, and 7.

[0141] Table 1 lists the carboxylate-functionalized cellulose polymers used to prepare the microspheres. They were either cellulose acetate propionate (CAP) or cellulose acetate butyrate (CAB).

[0142] **Table 1** (1) DS = degree of substitution. Total DS = sum of the DS of the individual substituents (2) Number average molecular weight (unit: Da) provided by the supplier (3) Glass transition temperature provided by the supplier (Eastman). The other values were measured using the DSC method of Nishio et al. described above. (4) Melting point provided by the supplier - Not specified / Not measured

[0143] Example 1 The details of the prepared samples are shown in Table 2, and the data regarding the obtained microspheres are shown in Table 3.

[0144] **Table 2** (1) Actn = acetone (2) IO = isooctane (3) PMDA = pyromellitic dianhydride BPTD = benzophenone tetracarboxylic dianhydride (4) Eq = equivalent eq (polymer shell enhancer) = molar ratio of the polymer shell enhancer to the hydroxyl groups of the carboxylate-functionalized cellulose. For example, an eq (polymer shell enhancer) of 0.5 for the dianhydride compound means that the molar ratio of the anhydride groups of the dianhydride compound to the hydroxyl groups of the polymer is 1:1.

[0145] **Table 3** (1) Volatiles of microspheres measured by TGA (2) Temperature at which the microspheres began to expand (3) Temperature at which maximum microsphere expansion was observed * Measurement of properties after storage at room temperature for 4 weeks ** Measurement of properties after storage at room temperature for 2 weeks *** Measurement of properties after storage at room temperature for 5 weeks ‡ Measured by GC-FID (gas chromatography - flame ionization detection)

[0146] These data demonstrate that bio-based microspheres having an expansion start temperature (T Start ) from 80 °C to less than 135 °C and favorable expansion properties can be produced using carboxylate-functionalized cellulose compositions. Generally, higher quality microspheres are obtained when a polymer shell enhancer such as PMDA or BPDT is additionally used in the preparation of the microspheres. Moreover, the microspheres of Experiments 3, 4, 7, and 26 - 28 show a significant improvement in the density properties of the microspheres over storage periods of 2 weeks or even 4 weeks, with no substantial loss of expansion performance.

[0147] Example 2 The effects of various dosages of different polymer shell enhancers on T start , T max , and TMA density were investigated using CAP1 as the polymer for the polymer shell.

[0148] The mixtures for all the experiments of Example 2 contained 5.5 g of CAP1, 32 g of acetone, and 3.65 g of isooctane (i.e., 40% by weight based on the total weight of isooctane and CAP1). As shown in Table 4, the polymer shell enhancer was added at various dosages of 5 - 40% by weight (based on the total weight of the polymer shell enhancer and CAP1). (PMA = pyromellitic acid, BTCA = 1,2,3,4 - butanetetracarboxylic acid).

[0149] [Table 4] (1) Based on the total weight of the polymer shell enhancer and CAP1 (2) Based on the total weight of the microspheres, the volatile content of the microspheres measured by TGA ‡Measured by GC - FID (gas chromatography - flame ionization detection)

[0150] Example 3 The effects of various dosages of different polymer shell enhancers on T start T max and the TMA density were investigated using CAB2 as the polymer for the polymer shell.

[0151] The mixtures for all the experiments of Example 3 contained 5 g of CAB2 and 3.3 g of isooctane. For the experiments using citric acid, tartaric acid, and BTCA, CAB2 was dissolved in 19.8 g of acetone. For the experiments using PMA, CAB2 was dissolved in 20.8 g of acetone (dosage 10%), 31.8 g of acetone (dosage 15%), and 39.8 g of acetone (dosage 20%). As shown in Table 5, the polymer shell enhancer was added at various dosages of 10 - 20% by weight (based on the total weight of the polymer shell enhancer and CAB2). (PMA = pyromellitic acid, BTCA = 1,2,3,4 - butanetetracarboxylic acid).

[0152] [Table 5] (1) Based on the total weight of the polymer shell enhancer and CAB2 (2) Volatiles of the microspheres measured by TGA

[0153] The data obtained in Examples 2 and 3 show that bio-based microspheres with an expansion start temperature (T Start ) from 80 °C to less than 135 °C and a preferably low TMA density can be produced using a carboxylate-functionalized cellulose composition. The data further show that microspheres with a particularly low TMA density can be obtained by adjusting the amount of the polymer shell enhancer added.

[0154] Example 4: A storage test was carried out to evaluate the storage stability of the microspheres of the present invention after storage for 4 weeks. Table 6 shows the T start , T max , and TMA density characteristics of the microspheres from the experiment of Example 2 above after 4 weeks of storage. For example, the data provided in Table 4 above for Experiment 28 from Example 2 are for newly prepared microspheres, and the data for the corresponding Experiment 28 (4w) provided in Table 6 below are for the exact same microspheres, and the only difference is that they were measured after 4 weeks of storage of the microspheres.

[0155]

Table 6

[0156] The data show that during 4 weeks of storage, although the microspheres slightly release the blowing agent, unexpectedly, the expansion characteristics T start , T maxand there is no significant change in the TMA density, and in some cases, even an improvement has been demonstrated, indicating the desirable storage stability of the microspheres of the present invention.

[0157] Example 5: Further storage tests were carried out using the microspheres of Example 3 above in the same manner as in Example 4 above. The results are summarized in Table 7.

[0158]

Table 7

Claims

1. A thermally expandable single-core microsphere comprising a polymer shell surrounding a single hollow core, wherein the single hollow core contains a blowing agent and the polymer shell contains carboxylate-functionalized cellulose, the carboxylate-functionalized cellulose contains at least two different carboxylate functionalities, and the thermally expandable single-core microsphere has an expansion start temperature (T Ｓｔａｒｔ ), which is from 80 °C to less than 135 °C, and the polymer shell contains a polymer other than carboxylate-functionalized cellulose in an amount of less than 50% by weight based on the total polymer content of the polymer shell. A thermally expandable single-core microsphere.

2. T Ｓｔａｒｔ The thermally expandable single-core microsphere according to claim 1, wherein T is from 90°C to 130°C.

3. The thermally expandable single-core microsphere according to claim 1 or 2, wherein the polymer shell further comprises a polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor.

4. The thermally expandable single-core microsphere according to claim 3, wherein the crosslinking agent is a dianhydride crosslinking agent.

5. The thermally expandable single-core microsphere according to claim 3 or 4, wherein the molar ratio of the polymerizable functional group of the crosslinking agent to the functional group suitable for crosslinking the carboxylate-functionalized cellulose is in the range of 0.1 / 1 (mol / mol) to 10 / 1 (mol / mol).

6. The thermally expandable single-core microsphere according to claim 3, wherein the polymer shell enhancer is a hydrogen bond donor selected from the group consisting of alcohols, amides, carboxylic acids, and hydrolyzed anhydrides.

7. The thermally expandable single-core microsphere according to claim 6, wherein the hydrogen bond donor is a hydrolyzed dianhydride compound.

8. The thermally expandable single-core microsphere according to claim 6, wherein the hydrogen bond donor is a carboxylic acid.

9. The thermally expandable single-core microsphere according to claim 7 or 8, wherein the amount of the hydrogen bond donor is 0.01 to 50% by weight, and the % by weight is based on the total weight of the polymer shell enhancer and the carboxylate-functionalized cellulose.

10. The carboxylate-functionalized cellulose contains a carboxylate group of formula (1), 【Chemical 1】 wherein, A is selected from -H, -OH, -OR ｂ , -C(O)OH, and -C(O)OR ｂ and R ａ is absent, whereby A is directly bonded to the C=O group; or R ａ is a saturated or unsaturated aliphatic group having 1 to 11 carbon atoms, which may be linear, branched, or cyclic, and is selected from 5- and 6-membered aromatic rings; wherein R ａ is optionally substituted with one or more substituents selected from -OH, halides, C １～４ alkyl, and C １～４ alkoxy, and the C １～４ alkyl and C １～４ alkoxy groups are optionally substituted with one or more groups selected from halides and -OH, and In each occurrence, R ｂ is independently selected from C １～４ alkyl groups and optionally has one or more substituents selected from halide groups and -OH groups The thermally expandable single-core microsphere according to any one of claims 1 to 9.

11. The following conditions, namely, (a) A is selected from H and C(O)OH, (b) R ａ contains from 1 to 7 carbon atoms, (c) R ａ is (i) a saturated straight-chain or branched-chain 【Chemical 2】 , (ii) [Chemical Formula 3] , (iii) an unsaturated straight-chain or branched-chain aliphatic containing "y" double bonds 【Chemical Formula 4】 , (iv) containing "y" double bonds 【Chemical Formula 5】 , and (v) 【Chemical Formula 6】 is selected from, wherein, v is an integer in the range of 1 to 11, w is an integer in the range of 3 to 11, x is an integer in the range of 2 to 11, y is 1 or 2, z is 5 or 6, in each occurrence R ｃ is independently selected from H, -OH, halide, C １～４ alkyl, and C １～４ alkoxy, and the C １～４ alkyl and C １～４ alkoxy groups are optionally substituted with one or more groups selected from halide and -OH (d) R ａ has 11 or fewer carbon atoms 【Chemical Formula 7】 is selected from, wherein, E is as defined in (c) above 【Chemical 8】 is, p and r are each independently an integer from 0 to 8, and p + r is at least 1, q and s are each the number of double bonds in each acyclic aliphatic component, and one or more of each independently selected from 0, 1, and 2 apply. The thermally expandable single-core microsphere according to any one of claims 1 to 10.

12. The carboxylate-functionalized cellulose is optionally substituted C １ -C ８ aliphatic carboxylate groups, and optionally substituted C ６ The thermally expandable single-core microsphere according to any one of claims 1 to 11, which contains carboxylate functionality and is selected from carboxylate groups containing an aromatic ring.

13. The carboxylate-functionalized cellulose is cellulose acetate propionate (CAP) having a number average molecular weight (M ｎ ) in the range of 2,000 to 30,000 Da, or cellulose acetate butyrate (CAB) having a number average molecular weight (M ｎ ) in the range of 2,000 to 30,000 Da, the thermally expandable single-core microsphere according to any one of claims 1 to 12.

14. A process for preparing a thermally expandable single-core microsphere, the process comprising mixing a carboxylate-functionalized cellulose, an organic solvent, a foaming agent, and optionally a polymer shell enhancer, and then spraying the thus-obtained mixture into a drying device to produce a thermally expandable single-core microsphere having a polymer shell surrounding a single hollow core, the polymer shell containing carboxylate-functionalized cellulose and the single hollow core containing a foaming agent, the polymer shell having a polymer other than carboxylate-functionalized cellulose in an amount of less than 50% by weight based on the total polymer content of the polymer shell.

15. The process according to claim 14, wherein a polymer shell enhancer in the form of a crosslinking agent or a hydrogen bond donor is further added.

16. The polymer shell enhancer is a hydrogen bond donor, and the hydrogen bond donor is added in an amount in the range of 0 to 15% by weight, the % by weight being based on the total weight of the carboxylate-functionalized cellulose, foaming agent, polymer shell enhancer, and solvent in the mixture for spray drying.

17. The process according to any one of claims 14 to 16, further comprising a step of storing the prepared thermally expandable single-core microsphere for at least two weeks after preparation before expanding the thermally expandable single-core microsphere.

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