Creating Ceramic Pores with Cellulose Fibers
Organic cellulose and lignocellulose particles modified for use in the ceramic industry can provide high-class purity combined with consistent product performance.
Organic fibers touch our lives each day—from the food we eat and the products used in our homes to medicines, vitamins, and even innovative ceramics. High-quality organic fibers and particles derived from vegetable raw materials (e.g., cellulose, wood fibers, or cereal and fruit fibers) for industrial purposes also help manufacturers keep the ecological cycle as a top priority.
Organic cellulose and lignocellulose particles modified for use in the ceramic industry can provide high-class purity combined with consistent product performance. Quality is guaranteed by certified quality control, and variety enables optimal usage. Different structures lead to various effects in the ceramic production process, as well as the performance of the final ceramic products. The tailor-made selection of natural particles enables the controlled improvement of specific applications. Products are available in various sizes and structures, including long fibers, cubic particles, granules, spheres, and cellulose gels.
During sintering, the natural particles are completely burned out so that pores remain. Pore volume, pore structure and pore size distribution are controlled. Advantages of higher porosity include:
- Lightweight materials
- Controlled permeability
- Higher specific inner surface
- Acoustic and thermal insulation
- Improved thermal shock resistance
- Increased capillary activity
Flexural strength decreases with an increase of porosity. With cellulose particles, stability is on a high level even for porosities up to 60%. The lower the particle size of the pore creator, the lower its impact on flexural strength. As a result, microporosity becomes attractive for applications with high stability requirements.
Pore size distribution can be determined according to mercury intrusion porosimetry and micro-tomography (µCT) measurements). The pore channel is defined by the contact area of the single pores. For fibrous particles, the contact area corresponds to the pore diameter. This diameter is uniform and therefore results in a unimodal pore size distribution. The contact area varies for granules and spheres, which leads to a wide pore size distribution (bimodal or even multimodal).
Testing was recently undertaken using cellulose fibers as pore creators for ceramics.* The effect on pore characterization was evaluated as the cellulose particle structure and size were adjusted. Three different types of cellulose were used:
- Type 1—cellulose particles (1.49 g/cm³ density, 18 µm average particle length, 15 µm average particle thickness)
- Type 2—microcrystalline cellulose granules (1.53 g/cm³, 150 µm average granule diameter)
- Type 3—microcrystalline cellulose spheres (1.50 g/cm³, 300 µm average sphere diameter)
Investigations were made according to processing mediated by ionotropic gelation (see Figure 1).** After the first step of alginate dissolution at slightly alkaline conditions, the aluminum oxide ceramic particles are added and then stabilized by trichloroacetic acid. The cellulose particles (15 vol%) are then added, and the mixture must be cooled (< 10°C) before the calcium iodate can be added as a crosslinker. Calcium cations initiate the gelation. The green body is dried at 20°C for four days, and shrinkage is around 10%. Sample discs (27 mm diameter x 3 mm thickness) are cut and grinded.
Finally, the gelled parts are sintered (1,350°C for 2 hrs), which leads to the chemical consolidation of ceramic particles. The cellulose particles are simultaneously burned out so that pores remain. Shrinkage after sintering is around 17%.
*Testing performed by J. Rettenmaier & Söhne GmbH (JRS) in cooperation with the Advanced Ceramics group at the University of Bremen using JRS ARBOCEL® particles. **Developed by the Advanced Ceramics group at the University of Bremen.
Cellulose is the raw material base of all three products. Microcrystalline cellulose (MCC) is chemically treated to increase the amount of the crystalline content. The structure of MCC is therefore different than standard cellulose, which creates different pore structures after sintering:
- Cellulose particles → oblong pore channels
- MCC particlesnearly → cubic pores
- MCC spheres → ball-shaped pores
The pore structure is specifically controlled by choosing the equivalent particle structure (see Figure 2).
Pores can be differentiated in open pores, closed pores and a combination of both. The preferred kind of pore strongly depends on the requested functionality (see Figure 3).
Total porosity is defined as the sum of open and closed pores (see Figure 4). An increase of total porosity leads to an increase of open pores. Starting at a total porosity level of nearly 10%, closed pore structures become less prevalent. The reason is that with a higher amount of pores, the single pores necessarily come into direct contact. The connection of several closed pores will result in the creation of new pore channels (open pores).
As pore creators, cellulose particles:
- create twice as much total porosity as used fiber content (e.g., 30% total porosity realized by 15% cellulose)
- follow the typical positive correlation of total porosity and open porosity (e.g., 30% total porosity = 10% closed pores + 20% open pores; and 40% total porosity = 10% closed pores + 30% open pores)
Pore size distribution is detected according to mercury intrusion porosimetry. The pore channel is defined by the contact area of the single pores (see Figure 5).
For fibrous particles, the contact area corresponds to the pore diameter. Here the particle diameter is uniform and therefore results in a unimodal pore size distribution. Due to the nature of cellulose particles, the diameter is always in the range of 20-30 µm, independent of the particle length. Consequently, the pore size distribution is similar for all fine cellulose particles. In contrast to this study, rougher cellulose grades (> 1,000 µm) tend to agglomerate, which causes coarser and varying pores.
For granules and spheres, the contact area varies. Variation of contact area leads to a wide pore size distribution (bimodal or multimodal), as illustrated in Figure 6. Even though the diameter of the cellulose granules is very uniform, granules tend to break at high shear forces during the ceramic production process. This leads to uncontrolled final granule sizes and contact areas. Pore size distribution is unpredictable and strongly depends on the production process parameters.
These results should be evaluated and discussed in the context of the final ceramic application. Regarding pore structure, it could be possible to induce a controlled breaking behavior of the ceramic part or to improve vessel growth in medical applications (e.g., bone replacements).
Increased pore quantity will lead to lightweight materials for the automotive industry. In addition, it can improve the thermal shock resistance for refractory ceramics, and it results in a higher inner specific surface area for catalyst carriers.
In filtration applications, specific pore sizes enable a controlled permeability and an accurate separation of the single components. Microporosity will be an advantage for applications with high stability requirements.
The next step will be to investigate the influence of pore structure, quantity and size on strength parameters. Alternative natural fibers will also be examined, including recycled paper fibers, wood fibers, fibers from annual plants (e.g., corn, oats), and specialty fibers/particles from nut husks or olive pits.
1. Brandes, C.; Treccani, L.; Kroll, S.; and Rezwan, K., “Gel Casting of Free Shapeable Ceramic Membranes with Adjustable Pore Size for Ultra and Microfiltration,” Journal of the American Ceramic Society, 2014, 97(5), pp.1393-1401.
2. Brandes, C.; Hoog Antink, M.; Kroll, S.; Treccani, L.; and Rezwan, K., “Aluminium Acetate as Alternative Cross-linker for Temperature Controlled Gel-casting and Joining of Ceramics,” Journal of the European Ceramic Society, 2016, 36(5), pp. 1241-1251, http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.011.
3. Brandes, C.; Megne Tague, C.; Kroll, S.; Treccani, L.; and Rezwan, K., “Gel Casting of Large Area Micro- and Sub-Micropatterned Thin Ceramic Tapes,” Ceramics International, 2016, 42(4), p.5036-5044.
4. Brandes C., “Complex Structured Ceramic Components by Combination of Temperature Controlled Ionotropic Gelation and Plastic Processing,” Dissertation, University of Bremen, Advanced Ceramics, 2016, http://publica.fraunhofer.de/documents/N-393788.html.