**4. Cellular foam from recycled waste: synthesis, microstructure and material properties**

Generally speaking, cellular foams being porous materials find a large variety of applications, irrespectively of their nature, wherever a lightweight porous material is needed [3], applications as thermal and acoustic insulators being perhaps those most important [54]. Ceramic or glass foam synthesis is traditionally carried out by three routes: (i) replica technique, (ii) use of sacrificial template, and (iii) use of direct foaming agents [2, 55]. There is a common strategy for the first two routes of preparing a precursor of the porous structures at a low temperature. This can be achieved either by impregnation of a "spongelike" material or by using sacrificial particles incorporated in the precursor network. In a subsequent heating step, the sacrificial material is removed, leaving the porous cellular microstructure. In principle this allows to design a specific porous network in the low-temperature synthesis step, which then creates a specific skeleton leading to the porous network during the calcination step. Accordingly, porous structures, ranging from microporous and/or mesoporous to macroporous, could be synthesized [56]. Concerning the third route, the foaming agent is added to the starting mixture. Upon calcination, this agent decomposes generating gas bubbles in the melted material, thus creating the porous structure upon cooling [2, 55]. Typical industrially employed foaming agents are carbonates, particularly in the production of ceramic- and glass-based foams [57].

As stated in the introduction section, there is an increasing attention to the sustainability of material production, and effectively, acoustical sustainable materials, either natural or made from recycled materials, are quite often a valid alternative to traditional synthetic materials [58, 59]. However, the reutilization of glass and ceramic waste generally employs high-energy-demanding production process [9–11], which clearly impacts the sustainability of this route. Furthermore, the use of sacrificial reagents for the synthesis as stated above, clearly contradicts the principles of sustainable chemistry, whereas there is an increasing need for sustainability of both processes and products [60].

We have recently reported synthesis of open-cell foams based on room temperature co-gelling of alginates with glass waste as a viable and sustainable process for production of glass-based cellular. Alginates biopolymers have been used mostly

in biomedical applications [61, 62], but recently different applications have been reported, e.g., fire retardants and insulation materials [63], membranes [64], and fuel cell applications [65]. The synthesis of these materials generally implies formation of a gel structure as the "foaming" principle. The gen consists of a continuous solid porous network with pores filled with a fluid, water in our case. Removal of the liquid from the gel to achieve the porous solid causes a significant collapse of pores due to liquid surface tension, particularly high in the case of water, leading to the so-called xerogels where more than 80–90% of pores initially present collapse. Accordingly, to conserve the gel porous structure, water removal must be carried avoiding the liquid–gas interphase. Under supercritical conditions there is no interface between the liquid and gaseous phase; thus, during removal of the fluid, pore collapse is prevented, obtaining high surface products called aerogels [66]. Similarly, sublimation does not imply liquid surface tension, and using freeze-drying technique, the so-called cryogels are obtained [67]. Notably directional freezing was used for the synthesis of cryogels [68, 69], and even alginate gels could be prepared with either isotropic or anisotropic pore structure according to the freeze-drying conditions [70].

Since our interest was focused on eco-efficient glass/fiberglass recycling methodologies [14, 71], the use of alginates, i.e., a natural product, represents a route to improve the greenness of the process. We showed that alginates effectively incorporate recycled glass powders in the gelation step. When these materials are subjected to freeze-drying, open-cell foam structures are formed. Since we were interested in the influence of the coarse foam structure on the acoustic properties, we do not add other bonding agents such as aggregators and plasticizers [72–75], which could confer either flexibility of rigidity to our composite materials. However, we can anticipate that even if the specimens are flexible [76], we do not find significant variation of sound absorption properties in the range of frequencies here investigated. Last but not least, even if freeze drying is considered an energydemanding unit operation, the comparison of the gelation process with the hightemperature industrial foaming processes showed this process being competitive, less energy-demanding, and cost-effective.

Three samples A, B, and C were prepared containing, respectively, 10 and 20% w/v of glass powders and 20% w/v of fiberglass (see Refs. [14, 71] for details of the syntheses). Recycling fiberglass is difficult and costly being a thermoset composite [13, 77]. In contrast when added in the gel synthesis, they contribute in the creation of the pore structure, leading to a possible, environmentally friendly, recycle route. **Figure 5** shows the microstructure of the samples: macroporous open-cell morphology is observed for all the samples, confirming the efficiency of the proposed methodology for preparing cellular foams. As shown in **Figure 5**(**a1**, **b1**, and **c1**), taken at higher magnifications, both glass and fiberglass are well dispersed and engulfed within the cell walls made of the alginate polymer.

A perusal of **Figure 5**(**a**–**c**) reveals a net change of the shape and alignment of the pores upon varying the nature of the glass-containing materials and its amount (compare also **Table 2**). Samples A and B feature mostly cells of a quadratic/ rectangular form. When the amount of glass powder is increased from 10 to 20% w/v, the orientation of the cells is favored and their dimensions increase. Using fiberglass (sample C), unoriented cells are formed with larger pores compared to sample B. **Table 1** summarizes both the microstructure and mechanical properties of the samples, including the reference rock wool.

Consistently, submillimeter fiber particles, indicated by arrows, are clearly detected in the SEM micrographs reported in **Figure 5**(**c**). This change of powder morphology is even more important by considering the particle number (PN) distribution: about 90% of the glass particles are smaller than 4 μm, whereas for an equal percentage, the dimension increases to ca. 60 μm in the case of the fiberglass

*SEM micrographs: (a) sample A 50, (a1) details of sample A 50, evidencing some of the glass powder inclusions; (b) sample B 50, (b1) details of sample B 2500 evidencing some of the glass powder inclusions; (c) sample C 50, (c1) details of sample C 500 evidencing some of the fiberglass inclusions. Figure adapted*

*Thermal and Acoustic Numerical Simulation of Foams for Constructions*

*DOI: http://dx.doi.org/10.5772/intechopen.91727*

powder.

**33**

**Figure 5.**

*from [15].*

Particle size distribution (PSD) curves were measured on the starting glass and fiberglass powders reported in Ref. [15] which provide some insight into this change of microstructure. The glass powder consists of smaller particles compared to fiberglass: the PSD peaks at ca. 8 μm which increases to ca. 128 μm for the fiberglass. *Thermal and Acoustic Numerical Simulation of Foams for Constructions DOI: http://dx.doi.org/10.5772/intechopen.91727*

in biomedical applications [61, 62], but recently different applications have been reported, e.g., fire retardants and insulation materials [63], membranes [64], and fuel cell applications [65]. The synthesis of these materials generally implies formation of a gel structure as the "foaming" principle. The gen consists of a continuous solid porous network with pores filled with a fluid, water in our case. Removal of the liquid from the gel to achieve the porous solid causes a significant collapse of pores due to liquid surface tension, particularly high in the case of water, leading to the so-called xerogels where more than 80–90% of pores initially present collapse. Accordingly, to conserve the gel porous structure, water removal must be carried avoiding the liquid–gas interphase. Under supercritical conditions there is no interface between the liquid and gaseous phase; thus, during removal of the fluid, pore collapse is prevented, obtaining high surface products called aerogels [66]. Similarly, sublimation does not imply liquid surface tension, and using freeze-drying technique, the so-called cryogels are obtained [67]. Notably directional freezing was used for the synthesis of cryogels [68, 69], and even alginate gels could be prepared with either isotropic or anisotropic pore structure according to the freeze-drying

Since our interest was focused on eco-efficient glass/fiberglass recycling methodologies [14, 71], the use of alginates, i.e., a natural product, represents a route to improve the greenness of the process. We showed that alginates effectively incorporate recycled glass powders in the gelation step. When these materials are subjected to freeze-drying, open-cell foam structures are formed. Since we were interested in the influence of the coarse foam structure on the acoustic properties, we do not add other bonding agents such as aggregators and plasticizers [72–75], which could confer either flexibility of rigidity to our composite materials. However, we can anticipate that even if the specimens are flexible [76], we do not find significant variation of sound absorption properties in the range of frequencies here investigated. Last but not least, even if freeze drying is considered an energydemanding unit operation, the comparison of the gelation process with the hightemperature industrial foaming processes showed this process being competitive,

Three samples A, B, and C were prepared containing, respectively, 10 and 20% w/v of glass powders and 20% w/v of fiberglass (see Refs. [14, 71] for details of the syntheses). Recycling fiberglass is difficult and costly being a thermoset composite [13, 77]. In contrast when added in the gel synthesis, they contribute in the creation of the pore structure, leading to a possible, environmentally friendly, recycle route. **Figure 5** shows the microstructure of the samples: macroporous open-cell morphology is observed for all the samples, confirming the efficiency of the proposed methodology for preparing cellular foams. As shown in **Figure 5**(**a1**, **b1**, and **c1**), taken at higher magnifications, both glass and fiberglass are well dispersed and

A perusal of **Figure 5**(**a**–**c**) reveals a net change of the shape and alignment of the pores upon varying the nature of the glass-containing materials and its amount (compare also **Table 2**). Samples A and B feature mostly cells of a quadratic/ rectangular form. When the amount of glass powder is increased from 10 to 20% w/v, the orientation of the cells is favored and their dimensions increase. Using fiberglass (sample C), unoriented cells are formed with larger pores compared to sample B. **Table 1** summarizes both the microstructure and mechanical properties

Particle size distribution (PSD) curves were measured on the starting glass and fiberglass powders reported in Ref. [15] which provide some insight into this change of microstructure. The glass powder consists of smaller particles compared to fiberglass: the PSD peaks at ca. 8 μm which increases to ca. 128 μm for the fiberglass.

conditions [70].

*Foams - Emerging Technologies*

less energy-demanding, and cost-effective.

engulfed within the cell walls made of the alginate polymer.

of the samples, including the reference rock wool.

**32**

*SEM micrographs: (a) sample A 50, (a1) details of sample A 50, evidencing some of the glass powder inclusions; (b) sample B 50, (b1) details of sample B 2500 evidencing some of the glass powder inclusions; (c) sample C 50, (c1) details of sample C 500 evidencing some of the fiberglass inclusions. Figure adapted from [15].*

Consistently, submillimeter fiber particles, indicated by arrows, are clearly detected in the SEM micrographs reported in **Figure 5**(**c**). This change of powder morphology is even more important by considering the particle number (PN) distribution: about 90% of the glass particles are smaller than 4 μm, whereas for an equal percentage, the dimension increases to ca. 60 μm in the case of the fiberglass powder.


**5.1 Analytical model and acoustic performance**

*DOI: http://dx.doi.org/10.5772/intechopen.91727*

*Thermal and Acoustic Numerical Simulation of Foams for Constructions*

applicable to a novel type of material?

methodology to fibrous materials [48, 50].

B and C [15].

microstructure.

**Material Flow**

Rock wool

*lengths (*Λ*,* Λ'*).*

**Table 2.**

**35**

**resistivity (Eq. (14)) (***σ***) ((N s) m**�**<sup>4</sup>**

**)**

Experimentally measured acoustic absorption coefficient for the samples A, B, and C are reported in **Figure 6**. Samples A and B show comparable shape of the curves where sample B features better global sound-absorbing properties compared to A: the highest absorption coefficient observed for sample B is 0.998 at 2190 Hz. Sample C features a maximum of absorption at about 2100 Hz followed by a slow decline, at variance with samples A and B where a rapid decline is observed. Clearly, different morphologies of sample C compared to A and B lead to different acoustic properties. For comparison, a reference rock wool sample features a nearly linear increase of the

As highlighted above, the application of the analytical model by calculating the JCA parameters using Eqs. (10)–(14) was one of the important aspects of this study. The question is, are the widely employed state-of-the-art parameter formulations

To answer this question, we report, for the sake of conciseness, only the result

In the first instance, in order to model the sound absorption coefficient, the five parameters were calculated according to Eqs. (10)–(14) using the measured densities and the dimensions of the cells evaluated from the SEM micrographs (**Table 2**). As perusal of the data reported in **Table 2** reveals a close similarity of the calculated parameters notwithstanding the dissimilarity in their nature and morphology. This demonstrates that the analytical model is not suitable for this kind of cellular foam

The frequency trends of the sound absorption coefficient, which were calculated using these parameters as input for the TMM procedure, are shown in **Figure 6**. The results (**Figure 6**) show that the analytical model procedure as implemented using Eqs. (10)–(14) cannot be reliably applied to the complex foam structures, at variance with the rock wool sample which is properly modeled. The observation is in line with the above reported comments on the limits of the applicability of this

The above presented microstructural data show that the morphology and dimensions of the foam cells depend on the addition of the glass-containing powders. Since the powder is incorporated into the walls of the cells, increasing its amount will result in an extension of the free path for the wave propagating within the material itself. As a consequence, a modification of the tortuosity parameter is expected. For this reason, we consider the tortuosity factor as calculated by Eq. (11), developed for fibrous like materials, to inadequately describe this type of novel material. Notice that a modification of the tortuosity parameter changes the

sound absorption leaving the thickness of the material unchanged.

**Tortuosity (Eq. (11)) (***α***∞) (–)**

A 24,428 0.92 1.07 2.89 31 57

*Analytical model results: flow resistivity, porosity (*ϕÞ*, tortuosity (*α∞*), tortuosity* α*mod*,∞*, and characteristic*

27,289 0.93 1.06 31 57

**Tortuosity (Eq. (19)) (***α***mod,∞) (–)**

**Viscous characteristic length (Eq. (12)) (***Λ***) (μm)**

**Thermal characteristic length (Eq. (13)) (***Λ***') (μm)**

**Porosity (Eq. (10)) (***ϕ***) (–)**

obtained for sample A, but equivalent results have been obtained for samples

sound absorption coefficient with a maximum value of ca. 0.85 at 2900 Hz.

### **Table 1.**

*Microstructure and properties of the alginate foams: average area and radius of the foam pores, density, and compression modulus.*

The process conditions strongly affect freeze-drying synthesis since directional freezing of the ice particles can be easily achieved leading to novel morphologies such as monoliths [69, 78]. This technique can be widely applied, and also alginatebased gels were produced in an anisotropic form [70]. Ordinary freezing conditions were employed for the synthesis, which suggest that this effect should not be operative in our case. It is well-known that during the crystallization of ice, both solute and suspended particles/gels are segregated from the ice crystals. This may generate an ice-templating effect where the morphology of the material is dictated by the crystallized solvent [79]. A large number of small particles favors heterogeneous nucleation providing a large number of nucleation centers [80, 81]. The large amount of small particles in the glass-containing samples A and B increase the ice front velocity promoting formation of a columnar morphology [82], accounting for the morphology detected by SEM. Sample C contains much less small particles, and the rate of nucleation decreases compared to that of particle growth (ice crystallization). This generates an isotropic pattern of the open cells in sample C. The large pore dimension is in line with the higher particle size of the fiberglass compared to glass materials [79, 80].

Thus, the crystallization conditions and the particle distribution in the starting waste material appear to represent factors capable of directing the microstructure leading to distinct cell morphology and dimension. This is an important aspect as the aim of the study is to find correlation between the microstructure and acoustic properties of these materials.

The data reported in **Table 1** show clear trends for the density and the compression modulus which can be correlated with the dimension of the open cells. For a fixed volume, the higher the pore area, the lower the number of cells, which means that the density increases in the sequence samples A, B and C and the opposite occurs for the compression modulus. Data for a rock wool sample are also included in **Table 1**, as a standard sample for the acoustic studies.
