On preparation,structure and performance of high porosity bulk cellulose aerogel

Abstract

A novel approach has been applied in preparing cellulose aerogel foams by combining NaOH/ thiourea/ H₂O solvent system and the freeze drying technology. The improvement involved adding 60°C ethanol during the formation process of cellulose hydrogel. The effects of the solvent system on the structure and heat insulation performance of aerogel were studied. The gelation time reduced 63% and the mechanical strength of hydrogel reinforced 2·3% with 60°C ethanol. The optimal ratio of the solvent system for cellulose was NaOH 9·5 wt-%, thiourea 4·5 wt-%. Under this condition, the obtained cellulose aerogel have a density range between 0·2 to 0·4 g cm⁻³, and the porosity up to 84·88%. Cellulose aerogel had good heat insulation performance with the lowest thermal conductivity only 0·029 W m K⁻¹. This work provided an effective preparation method for high porosity bulk cellulose aerogel, and showed the application potential of cellulose aerogel as insulation materials in the medium low thermal insulation field.

Keywords

Solvent of cellulose Cellulose aerogel Porosity Heat insulation performance

Introduction

Aerogel is a kind of material prepared by replacing the liquid solvent in a gel by air without substantially altering the network structure or the volume of the gel body.^{1, 2} Owing to the unique pore structure, it has many special properties, such as low density,³ high porosity,^{4, 5} specific surface area,⁶ nanosize pore space,⁷ low acoustic impedance,⁸ low thermal conductivity.⁹ For these important properties, aerogel materials have many great potential applications.^{10, 11, 12} Compared with inorganic aerogels, organic aerogels overcome the fragility in a certain degree, have the unique feature of plasticity and toughness. As the new third-generation material, cellulose aerogel has received intensifying concern.^{13, 14} Cellulose aerogel materials possess features of traditional aerogels,^{15, 16} at the same time they have their own excellent specificity. The preparation of aerogels made from cellulose derivatives needs complicated and non-controllable chemical cross-linking process, however aerogels prepared by unmodified cellulose avoids these processes,¹⁷ which is essential to aerogels form cellulose derivatives.

Owing to hydrogen bonding, complicated aggregation and a high degree of crystallinity, cellulose dose not dissolve in ordinary solvents. At low temperatures, the NaOH/ thiourea/ H₂O system rapidly dissolves cellulose (M_w≤1·2×10⁵) leaving a transparent solution. The solution state can be maintained for quite a long time.¹⁸ Many cellulose materials were successfully prepared with this solution, such as novel cellulose silk,¹⁹ membrane,²⁰ chromatographic column fillers,^{21, 22} composite materials.^{23, 24, 25} And using this kind of solution, cellulose fibril was successfully prepared through trial produce.²⁶ An effective cellulose solvent is an important ingredient in the formulation. However, there is a lack of materials with a rigid frame and high porosity in the literature. The application of cellulose aerogel has been restricted in many fields due to their mechanical properties. The effect of the solvent on the structure and performance of the cellulose aerogels has not been investigated systematically.

In this study NaOH, thiourea and H₂O were selected as solvents. Bulk cellulose aerogels were prepared by the injection mould method. The process improvement reduced the gelation time and reinforced the mechanical strength of hydrogel. The effect of the structure on the performance was analysed. It was investigated about the relationship between the preparation method of cellulose aerogels and the performance.

Experimental

Materials

Cellulose (cotton linter) provided by Hubei Chemical Fiber Co., Ltd, and the α-cellulose content was more than 95%. The cellulose was used without being further purified. M_η was measured to be 1·01×10⁵ with cadoxen as the solvent at 25°C by using viscometer according to [η] = 3·85×10⁻²M_w^0·76(mL g⁻¹).²⁷ Sodium hydroxide (NaOH), thiourea (CH₄N₂S) and ethanol were purchased from Tianjin Beilian Fine Chemicals Co., Ltd., Guangzhou Xilong Chemical Co., Ltd., and Shanghai Green Food Additive Co., Ltd. respectively. All chemical reagents were of analytical grade and directly used unless otherwise mentioned.

Methods

Preparation of cellulose hydrogel and aerogel

In the first step, cellulose was rinsed with distilled water and ethanol repeatedly. Then it was dried and added to the NaOH/ thiourea/ H₂O solution (the percentage of cellulose weight: 2–5 wt-%, the percentage of NaOH weight: 5·5–11·5%, the percentage of thiourea weight: 3–6%) and the swollen cellulose dispersing solution was obtained. The cellulose dispersing solution was injected in a mould and stirred by 750 rev min⁻¹ to disperse cellulose homogeneously. Then the solution was frozen for 24 h. The following solution was thawed at room temperature and hydrogel was obtained. The hydrogel was rinsed with distilled water repeatedly. Finally, bulk cellulose aerogels were obtained through a FreeZone Plus 2·5 L Cascade Console Freeze Dry System (LABCONCO Co.).

Characterisation

Estimation of gelation rate

Gelation rate is characterised with gelation time. As an important parameter to investigate hydrogel forming process, gelation time can be used to measure the reaction rate. Gelation time was recorded to start from the beginning of gel reaction until the hydrogel compound no longer flowed when the mould was tilted at 70°.

Measure of volume shrinkage

Deformation of the gel can be evaluated from the results of freeze drying. Equation 1 was used to calculate the volume shrinkage (1) Where V₀ is the initial volume of hydrogel and V is the final volume of aerogel after freeze drying, each sample was measured three times to obtain the standard deviation.

Estimation of mechanical property

Mechanical property is characterised by compressive strength. Compressive strength was measured by WDW-100E microcomputer-controlled electronic universal testing machine of Jinan Shijin Group Ltd. After drying the regular cylindrical sample at 60°C for 24 h, put it on testing platform. The pressure gauge was dropped at a speed of 0·2 mm s⁻¹ and the pressure was recorded when the aerogel was broken.

It was difficult to measure the tensile strength of the cellulose hydrogel because of its brittle frame; instead compressive method was used to determine the mechanical properties. A series of weights were put and the gross weights were recorded when the first fracture appeared. Compression strength was calculated by the following formula (2) where m is the bearing weight added when the structure of sample collapsed, g is the acceleration of gravity, and S is an effective contact area of the sample.

Estimation of water adsorption

Water adsorption is represented by the adsorbency. The dried aerogels were immersed in water, after aerogels were fully swelled to weight. The adsorbency was expressed as (3) where m₀ is the weight of dried aerogels and m₁ is the weight of aerogels after adsorbing water.

Estimation of porosity

Porosity is an important parameter in the characterisation of aerogels. The samples were dried in the oven for 6 h at 60°C. The dried samples were then completely immersed in ethanol. The samples were then placed in a vacuum until no air bubbles were visually present. The weight of the container was then recorded m₃. Porosity of the sample was calculated according to the following formula (4)

Measurement of thermal conductivity

Thermal conductivity is usually used to estimate the heat insulation performance of insulating materials. In order to investigate the heat insulating efficiency of cellulose aerogels, their thermal conductivity was performed using Conductometer (model: DZDR-PL) of Nanjing Dazhan Institute of Electrome Chemical Technology. Thermal conductivity was measured by the guarded hot plate apparatus. Cellulose materials are stable in normal and low temperature environment (−20–300°C). In the application of thermal insulation field, cellulose aerogel can be used as insulating materials in normal and low temperature environment. In the experiment, the temperature of hot plate was set at 50°C and the temperature of cold plate was set at −15°C. All of the samples were dried at 60°C before the measurement was performed.

Estimation of structure and morphology

The chemical structure of aerogels was investigated and compared with that of natural cellulose by Bruker FTIR spectrometer (model: TENSOR 27) with a resolution of 1 cm⁻¹.

Scanning electron microscopy (SEM) images of aerogel were obtained using a Hitachi S-3000 N scanning electron microscope of Japan's Hitachi with an acceleration voltage of 5 kV after 60 s metallisation with Au.

X-ray diffraction was measured with an X-ray diffractometer (D8ADVANCE, Bruker Axs, Germany). X-ray diffraction patterns with Cukα radiation at 40 kV and 30 mA were recorded in the range of 2–70° and the speed is 2·5° min⁻¹.

Results and discussion

Influences of absolute ethanol to forming of hydrogel

In the formative stage of cellulose hydrogel, the samples were thawed from −18°C to room temperature. In order to improve the performance of hydrogel 60°C ethanol was added. As shown in Fig. 1, the gelation time changed with cellulose concentration without and with 60°C ethanol. The gelation time reduced with the increase of cellulose concentration. Cellulose macromolecules which participated in the network structure increased with the increase of cellulose concentration resulting in a faster rate of reaction. The formation with 60°C alcohol leads to a significantly reduced gelation time. A possible explanation could that during the forming process the rising temperature speeds up the movement of cellulose molecules. It made the cellulose molecules connected with each other. At the same time, ethanol had strong hydrophilicity and could form intense hydrogen bonds with water molecules, which speeded up the separation of water molecules from the hydrophilic coating of NaOH and thiourea molecules. Thus the gelation time was reduced.

The relationship between compressive strength and cellulose concentration was shown in Fig. 2. The compressive strength of hydrogel increased with the increase of cellulose concentration. With the increase in cellulose concentration, the structure of hydrogel became more compact and cellulose chains throughout the hydrogel structure were increased. Cellulose is a kind of reinforcing agent in general, and can improve the compressive properties of materials.^{28, 29} Here a certain amount of celluloses were conducive to reinforce hydrogel. Thus compressive strength was improved with 60°C ethanol. It showed that when temperature increased, the movement of cellulose, thiourea and NaOH molecules were speeded up. More cellulose molecules formed the intramolecular and intermolecular hydrogen bond. The smaller surface tension of ethanol had little influence on the structure of the hydrogel when the solvent volatilised, the network structure of hydrogel was intact, moreover improving the strength of the hydrogel.

At low temperatures, NaOH was activated and had strong capacity to combine with –OH group in NaOH/thiourea aqueous solution. NaOH formed hydrate with a lot of H₂O molecules, cellulose molecules were dispersed in the aqueous solution as molecular level. Thiourea and NaOH molecules attached to the surface of cellulose molecules as hydrophilic coats.³⁰ When adding 60°C ethanol, hydrophilic coats were damaged with the rising temperature. At the same time, the movement of cellulose molecules speeded up. The –OH group had stronger self-association action, it caused the physical cross-linking structure between intramolecular and intermolecular chain to form rapidly. Then water molecules entered into the network structure of cellulose molecules, and finally formed the hydrogel network. Ethanol had a smaller surface tension compared with H₂O molecules, it prevented the destruction to the structure of hydrogel. In addition, ethanol had certain elution capacity to the remaining solvent.

Optimum dissolution conditions

Cellulose is a kind of polyhydroxy compound, and NaOH is a good swollen agent for cellulose. The results from Roy³¹ and Kuo³² showed that, at low temperatures, alkali had a good dissolution effect when the mass fraction was 6–9 wt-%. On average one NaOH molecule could connect with nine water molecules. NaOH hydrate was formed in NaOH aqueous solution. Cellulose reacted with NaOH, the production was alkali cellulose with negative charge. Because of strong hydration, when Na⁺ combined with cellulose, a lot of water molecules could enter into the inside of cellulose macromolecules. Cellulose molecules cause the cellulose molecules to swell in aqueous solution, furthermore causing breakup of the binging force in the amorphous region of the cellulose, while the binding force in the crystalline region was still intact. Therefore, cellulose was not dissolved in the NaOH solution; this was the first step of dissolution. Thiourea molecules contained strong polarity C = S and –NH₂ groups,³³ they have the tendency to form intermolecular hydrogen bonding. The thiourea molecules enter the crystalline region of cellulose braking up the remaining binding forces among the molecules dissolving the cellulose. This was the second step of dissolution process. Therefore, NaOH/thiourea/H₂O solvent system was applied to dissolve cellulose in this work.

In order to determine the influence of the solvent on the performance of cellulose aerogel, mass fraction of NaOH and thiourea were taken in system respectively as influence factors (mass fraction of cellulose was all 5 wt-%). The changes of gelation time and thermal conductivity of cellulose aerogels were investigated, the results were shown in Table 1.

Table 1

Effect of NaOH/Thiourea/H₂O concentration on gelation time and thermal conductivity of aerogels

No.	Solvent		Gelation time	Thermal conductivity/W m⁻¹ K⁻¹
No.	NaOH/wt-%	Thiourea/wt-%	Gelation time	Thermal conductivity/W m⁻¹ K⁻¹
1	5·5	3	154 min	0·039
2	7·5	3	≥24 h	0·038
3	9·5	3	≥24 h	0·034
4	11·5	3	≥24 h	0·036
5	5·5	4·5	≥24 h	0·036
6	7·5	4·5	≥24 h	0·040
7	9·5	4·5	47 min	0·032
8	11·5	4·5	56 min	0·045
9	5·5	6	≥24 h	0·038
10	7·5	6	61 min	0·043
11	9·5	6	94 min	0·046
12	11·5	6	67 min	0·042

When solvent system contained 9·5 wt-%NaOH and 4·5 wt-% thiourea, the shortest gelation time and the lowest thermal conductivity were obtained in Table 1. With this solvent ratio, cellulose aerogel had the fastest reaction rate and had the best heat insulation performance. Therefore, this solvent system was the best dissolution conditions in this work.

Structure and morphology of cellulose aerogels

The infrared spectra of linter cellulose and cellulose aerogels were shown in Fig. 3. The absorption peaks of the cellulose and the aerogel were almost the same. Both of them had obvious characteristic absorption peaks of cellulose macromolecular, it indicated that cellulose didnot derivate during the course of dissolution. The dissolution course was dissolved directly.

In the spectra, strong absorption peak at 3414 cm⁻¹ was the stretching vibration peak of O–H. Absorption peak at 2900 cm⁻¹ was the stretching vibration peak of C–H bond in cellulose molecule. Absorption peak at 1638 cm⁻¹ was caused by hemiacetal group on cellulose 4′ end. Relative strong absorption peak at 1150 cm⁻¹ was the stretching vibration adsorption peak of C–O bond. Absorption peak at 1429 cm⁻¹ represented the stretching vibration peak of CH₂–OH. After cellulose dissolved and formed hydrogels, the absorption peak at 1429 cm⁻¹ decreased, and a weak peak representing CH₂ shear vibration appeared at 1429 cm⁻¹. This showed that during the course of dissolving and forming hydrogel, intramolecular hydrogen bonds were disrupted, while CH₂OH had a conformational transition process and formed new intramolecular hydrogen bonds.³⁴ 896 cm⁻¹ was the characteristic adsorption peak of β-D-glucosidic bond. The changes of infrared adsorption before and after forming of cellulose aerogels showed that there was a phase transformation from cellulose I to cellulose II.

The SEM images of cellulose aerogel were shown in Fig. 4. Cellulose aerogel had a support frame, which formed the three-dimensional network structure. Cellulose aerogel had a fine mechanical performance with the support frame. In the inner aeras, the aerogel displayed a heterogeneous porous structure (Fig. 4a). While compact heterogeneous pore structure was formed on the surface of the aerogel (Fig. 4b). The low density support frame and a high pore size distribution gives aerogels the high porosity. In Fig. 4c the cellulose is present as circular fibres and the areas of non-dissolved cellulose increased with the increase of cellulose concentration. It was conjected, during the process of cellulose forming aerogel, the cross-linking between cellulose macromolecules made up the support space structure of aerogel, and the evaporation of water during the drying process led to the pore structure on the surface of the aerogel.

X-ray diffraction patterns of cellulose aerogels and cellulose were shown in Fig. 5. The original cellulose exhibited a typical cellulose I crystal with three characteristic diffraction peaks at 2θ = 14·7°, 16·2°, 22·2°, while cellulose aerogels exhibited a typical cellulose II crystal with three characteristic diffraction peaks at 2θ = 11·6°, 20·0°, 21·8°. It was inferred from the position of diffraction peak that cellulose had a crystal form transformation during the process of dissolution, it had random intramolecular and intermolecular cross-linking instead of ordered packaging chain structure.

Physical appearance of cellulose aerogels

Volume shrinkage of aerogels reflects the efficiency of drying method. Usually aerogels are dried by supercritical fluid drying, freeze drying or the method of solvent replacement, the volume shrinkage was between 4·0 and 70·0%.^35–37 The result of supercritical fluid drying for cellulose aerogel was the best one, and volume shrinkage could reach to 6·5%,^{38, 39} but the drying was difficult. Volume shrinkage of freeze drying for organic aerogel was 15–30%,⁴⁰ this drying method had mass application. In this paper, the volume shrinkage was between 20·41 and 28·36%, which was in an acceptable range. It showed that freeze drying was a practicable way to prepare cellulose aerogels.

Under the condition of freeze drying, cellulose aerogels had a relatively low density. The density of cellulose aerogels was in the range of 0·2–0·4 g cm⁻³ in Fig. 6. When the cellulose concentration was lower than 3 wt-%, density reduced with the increase of cellulose concentration. Before the dissolving capacity of solvent reached the saturation point, cellulose was dissolved completely, and the obtained aerogels were well distributed and loose. The density of regenerated cellulose aerogel was 0·06–0·3 g cm⁻³, which prepared by NaOH solvent system and CO₂ supercritical fluid drying.⁴¹ In this work when the cellulose concentration was 3 wt-%, the density reached the lowest point, 0·233 g cm⁻³. Limited by the cellulose concentration, 2 wt-% cellulose aerogels had less cellulose content involved in forming the structure of aerogel, which caused the structure of aerogels incomplete. As the cellulose concentration continued to increase, insoluble cellulose increased, the density of the aerogels increased instead. The high cellulose concentration made the aerogels denser.

The pore structure is predominately controlled by the polymer concentration.⁴² The higher the cellulose concentration was, the lower the mean porosity was.⁴³ For freeze dried cellulose solutions of different cellulose concentrations had the same phenomenon.⁴⁴ The porosity of aerogels decreased with the increase of cellulose concentration as shown in Fig. 6. The increase of the number of polymer molecule per unit volume caused the increase of density of polymer network. That caused the probability of polymer nannocytes wound with each other increased accordingly. The polymer nannocytes composed of multiple polymer molecules. Therefore, relatively high polymer concentration was good for the association of polymer molecules. It also made hydrogel easier to form small and uniform condensed phase and caused the porosity decreased. Whereas cellulose concentration was 3 wt-%, porosity reached the highest point of 84·88%. In consideration of incompleted structure of 2 wt-% cellulose aerogel, the porosity is lower. The decrease in the porosity is directly related to the increase in the density of the aerogel.

Compressive strength of cellulose aerogels changed with cellulose concentration as shown in Fig. 7. The compressive strength increased with increase of cellulose concentration reaching a maximum of 8·2MPa at a concentration of 5 wt-%. Moreover up to a concentration of 5 wt-% the non-dissolved cellulose chain reinforced the aerogel frame. The decrease in the compressive strength after 5 wt-% is due to a large amount of non-dissolved cellulose causing phase separation in the area of cellulose agglomeration, which destroys the aerogel structure. In comparison the traditional silica aerogel and other organic aerogel have poor mechanical properties, they are fragile with an average compressive strength of 0·1 MPa.⁴⁵ Whereas cellulose aerogels have good mechanical properties with compressive strength ranging from 5·7 to 8·2 MPa.

From Fig. 8 the effect of cellulose concentration on the thermal conductivity of aerogels is fairly linear. When cellulose concentration reached a certain amount, the amount of non-dissolved cellulose increased. The absolute content of cellulose also increased. Thus heat conduction in solid and thermal conductivity increased accordingly. When the concentration of cellulose was 3%, thermal conductivity reached the lowest point. Compared with 2% cellulose aerogel, 3% cellulose aerogel had relatively completed structure, and had the lower density and the higher porosity.

Conclusions

In this paper, cellulose hydrogel and its bulk aerogels were successfully prepared by using NaOH/thiourea solvent system and freeze drying method. The result showed that in the process of forming hydrogel, the gelation time was reduced and the mechanical strength was reinforced with 60°C ethanol. NaOH/thiourea solvent had fine performance to dissolve cellulose, and the optimum ratio was 9·5 wt-%NaOH, 4·5 wt-% thiourea. Under this condition, the obtained aerogel had the shortest gelation time and lowest thermal conductivity. Cellulose aerogels have a relatively low density range from 0·2 to 0·4 g cm⁻³ and relatively high porosity up to 84·88%. The best performance in terms of thermal conductivity was as low as 0·029 W m⁻¹ K⁻¹. In this work, cellulose aerogel was directly prepared from cellulose. The preparation method was simple, and the obtained aerogel had fine mechanical property and heat insulation performance. Hence, cellulose aerogel provided a vaste potential in medium and low temperature heat insulation material field. But currently the studies about the heat insulation performance and heat insulation mechanism of cellulose aerogel were rather limited. Further work will involve a more thorough investigation into the heat insulation performance of cellulose aerogels.

Footnotes

Acknowledgement

This work was supported by the Important Science & Technology Specific Projects of Hainan Province (no. ZDZX20100009), the Projects of Science Foundation of Hainan Province (no. 512113) and the Young Scientist Project of Hainan University(no. Qnjj1235).

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