The Effect of Polymeric Template Density and Solid Loading on the Properties of Ceramic Foam

This paper studies the effect of various polymeric foam template densities and solid loadings on the properties of ceramic foam. The study was based on six different polymeric foam templates with densities ranging from 13.4 to 37.8 kg/m. The templates were impregnated in ceramic slurry with solid loading ranging from 15 to 60 wt. %. Effects of polymeric foam template density and solid loading quantity were evaluated based on porosity, density and mechanical properties of resulted ceramic foam. It was found that the density, porosity and flexural strength of ceramic foam seem to be independent from the template densities when the solid loading is less than 35 wt. %. For the given solid loading, i.e < 35 wt. %, the density and flexural strength are less than 1100 kg/m and 6 MPa, respectively, and porosities are higher than 40 %. The polymer replication method is a versatile method for the production of ceramic foams. It will allow the production of any desired properties of ceramic foam through a simple modification route.


Introduction
Reticulated macroporous ceramic foams are highly porous (75-90 %) ceramics which exhibit an open three-dimensional network structure.They are used in a wide range of applications such as in membranes, absorbents, kiln furniture, catalytic converters, insulation, biomedical devices and as a core material in sandwich construction [1][2][3] or lightweight structural laminates [4] due to their high surface area, high permeability, good thermal properties, high strength, resistance to chemical attacks, and low mass depending on the materials used and fabrication process.
Ceramic foam can be produced using various methods including the replication method [5][6][7][8], burning out of additive [9], starch consolidation [10], the foaming method [11][12][13][14] and gel-casting of foam [15][16][17][18].The most popular method is by polymeric foam replication method that was patented in 1963 by Schwartzwalder and Somers [19].The process involves coating of open-cell polymeric foam with a ceramic slurry followed by burning-out of polymeric foam through a sintering process which yields ceramic foam as the replica of the original polymeric foam used.This method produces ceramic foam with a vast majority of open cell foam microstructure [1].The properties of ceramic foam produced by this method could be adjusted by varying the viscosity of the slurry and the polymeric foam characteristics such as density, and pore size, shape and its distribution.The quality of ceramic coating on the polymeric sponge is strongly dependent on the viscosity of the slurry and the density of the sponge [2].In this paper, an attempt has been made to study the ability of the commercially available polymeric sponge which is widely used in packaging and household industries to be used as a template material for the production of ceramic foam.This study provides useful information regarding the applicability of relatively cheap foam materials compared to the use of custom made templates as reported in the literature [20][21][22].

Experimental Procedure
Six different densities of polymeric foam ranging from 13.4 to 37.8 kg/m 3 obtained from Pexafoam Sdn.Bhd, Malaysia were used as template materials.Each was cut into a dimension of 25 mm × 50 mm × 100 mm for flexural testing.Porcelain powder was produced in-house by mixing ball clay, quartz, kaolin and feldspar in the ratios of 10, 30, 40 and 20 wt.%, respectively.The ceramic slurry was prepared in the range of 15 to 60 wt.%, with distilled water as the medium.The details of the template density used and ceramic slurry composition are shown in Tab.I.The mixture was ball-milled for two hours in a porcelain jar to produce a ceramic slurry, using porcelain balls as the grinding media.Prior to the dipping process, the viscosity of ceramic slurries was measured using the Brookfield viscometer (model RVT) at a spindle speed of 50 rpm.The foam was squeezed manually to remove trapped air then dipped and kept in the slurry for 5 minutes to ensure an adequate filling time of the template.Any excess slurry was removed by wiping the outer surface of the templates.The loaded templates were then dried in an oven at 60 ºC for 72 hours and subsequently at 100 ºC for 1 hour.The sample was further sintered at the temperature and heating rate of 1250 ºC and 5 ºC/min, respectively with two hours of holding time.The density of the ceramic foam was characterized according to ASTM C 271-94 [23] and its porosity was determined using the Archimedes method.Mechanical properties of the ceramic foams at various densities were determined by conducting flexural test according to ASTM C1161-94 [24] using the Instron machine model 8501 at a crosshead speed of 0.5 mm/min.The ceramic foams morphological study was performed using a scanning electron microscope (VPFESEM LEO SUPRA 35 VP).

Morphology of polymeric foam template
The morphology of ceramic foam is directly related to the microstructure of the polymeric foam template.Fig. 1 shows the SEM micrograph of polymeric foam with densities ranging from 13.4 to 37.8 kg/m 3 which have been used as template material in this study.It is observed that each template consisted of spherical-and polyhedral-shaped cells.As the density of the template increases from 13.4 to 37.8 kg/m 3 , the cell sizes decreased, the number of closed cell decreased and the thickness of cell wall increases.Change in density has also brought changes to cell geometry which transform from polyhedral to spherical in shape as shown in Fig. 1(e).It is also observed that the formation of a thin membrane, which acts as the window covering the cell structure, forms a more closed cell structures.However, the cell size, shape, and distribution are fairly uniform for all template densities.The properties of these templates will affect the properties of the ceramic foam produced via polymer replication method.

Processing aspect of ceramic foam
The main parameters affecting the properties of ceramic foam using the replication method are quantity of solid loading and the various template densities.The impregnation process is a very important process in producing ceramic foam via polymeric foam replication method.During this process, viscosity should be sufficient for slurry to enter, fill and uniformly coat the foam structure and retain in the foam under static condition.The best quality of ceramic foam can be produced from a template which will return to its original shape without any distortion after impregnation with ceramic slurry.
As shown in Fig. 2, the viscosity of slurry increased linearly with the increase in solid loading.However, there is a minimum solid loading where the slurry could sufficiently coat the polymeric foam which is 35 wt.%.Below 35 wt.% of solid loading, the slurry is too thin and unable to coat the polymeric foam effectively.On the other hand, a slurry that is too thick caused the non-uniform coating of polymeric foam.Non-uniform coatings result in unhomogeneous coating especially inside the template structure.Unhomogeneous coating of slurry will then contribute to the formation of voids or holes inside the structure after sintering and thus reduce the strength of the ceramic foam as discussed in our previous paper [25].

Effect of template density on the various solid loading
Similar observations can be made for the effects of template density on the quality of impregnated samples at various solid loading.For lower template densities (13.4 and 16.9 kg/m 3 ), good samples are produced from 30 to 45 wt.% solid loadings.Below 25 wt.% solid loading, samples are cracked and at solid loadings higher than 45 wt.%, distorted samples were produced as shown in Fig. 3. On the other hand, higher template density (19.1 -37.8 kg/m 3 ) produced quality samples from 20 to 50 wt.% solid loadings.

Physical properties of the ceramic foam
Illustration for the transformation of template materials to the derived ceramic foam product is shown in Fig. 4. The template density is 37.8 kg/m 3 and 45 wt.% solid loading.The only difference is that the ceramic foam has a smaller dimension due to the firing shrinkage.

Density and porosity
It is clearly seen that the density of all ceramic foams increase with the increase of solid loading for template density as shown in Fig. 5.However, with solid loading of lower than 35 wt.%, the ceramic foam is considered not much dependent on the template density, which is less than 1100 kg/m 3 .This is due to thin coating and complete replication of the template derived from low viscosity ceramic slurry.Low solid loading permits the slurry to enter the foam structure easily, while higher solid loadings will affect the impregnation process especially in the lower template density which has low ability to retain their shapes and structures.This is due to lower stiffness of the template.Therefore, the density of the ceramic foam steeply increased for solid loadings of more than 35 wt.% up to 45 wt.%, as shown in Fig. 5.At these solid loadings, the amount of ceramic materials needed/used to form the structure is more, therefore contributing to higher density ceramic foams.However, at the same solid loadings, low template densities (13.4 -19.1 kg/m 3 ) produced higher ceramic foam density due to their ability to absorb more ceramic mass from high solid loadings slurry.

Fig. 5 Bulk density of ceramic foam as a function of polymeric foam densities and amount of ceramic solid loading
As the density increases, the amount of porosity was expected to decrease as shown in Fig. 6.It is clearly seen that this is a similar trend with density.At solid loadings of lower than 35 wt.%, the porosities of ceramic foam is considered not much dependent on the template density, which is higher than 40 %.

Flexural strength
A similar trend with density was observed for flexural strength of ceramic foam.The flexural strength was increased as the solid loading increased as shown in Fig. 7.It is clearly seen that at solid loadings lower than 35 wt.%, flexural strength is considered not strongly dependent on the template density.The strength is consistent at less than 6 MPa.It is due to the ability of the templates to return to their original shape to produce porous microstructures.Fig. 7 Flexural strength of ceramic foam as a function of polymeric foam densities and amount of ceramic solid loading However, at solid loadings higher than 35 wt.%, the flexural strength was drastically increased for the lower template density (13.4 -19.1 kg/m 3 ) compared to the higher values (25.2 -37.8 kg/m 3 ).This is because the thicker slip will strongly stick to the templates, hence the templates are unable to return to their original shape thus producing rather distorted shape.At the same time, the ceramic foam produced contains lesser amount of porosity.This is expected since the flexural strength of the ceramic foam structure is dependent upon the area within the structure that acts as load-bearing struts.A denser packing of ceramic particles around these areas help to increase the effective load bearing area hence increases the strength of ceramic foam.

Conclusion
It is proven that commercially available polymeric sponge can be used as a template to produce ceramic foam.The properties of ceramic foam are greatly influenced by solid loading, as well as the template density.As the amount of solid loading increased, the density and strength steeply increases while porosity decreases.However, the effect of template density to the properties of ceramic foam is different.The properties of ceramic foam do not depend much on the template density at solid loadings less than 35 wt.%, and vice versa for solid loadings higher than 35 wt.%.The polymer replication is a versatile method for the production of ceramic foams.It will allow for the production of any desired properties of ceramic foam through a simple modification route.
Terengganu and Construction Industrial Development Board Malaysia (CIDB) for their facilities and financial support.

Fig. 2
Fig. 2 Effect of solid loading on the viscosity of porcelain slurry

Fig. 4 (
a) is the microstructure of the template which contains various open and closed cells.It is also consists of different pore sizes, shapes and struts.Some of the struts are covered by thin membrane layers.These template structures will transform into ceramic foam after sintering, as shown in Fig. 4(b).

Fig. 4
Fig. 4 SEM micrograph of (a) polymeric foam template with density of 37.8 kg/m 3 and (b) ceramic foam derived from polymeric foam template at solid loading of 45 wt.%.

Fig. 6
Fig. 6 Porosity of ceramic foam as a function of polymeric foam densities and amount of ceramic solid loading