Introduction

Phenolic foam (PF) is a crispy foam made of phenolic resins (phenol formaldehyde resins or phenoplasts), hardeners and some other chemical additives1. The PFs are made in two general structures; closed and open-cell foams2. The closed-cell PFs are versatile materials utilized in a wide variety of uses, e.g., thermal insulation3, fire performance4, moisture resistance5 and structural strength6. These abilities made the PF foams perfect for 3rd-generation insulation7 in various industries, e.g., aerospace, petrochemical and construction8. However, the open-cell PFs are used to manufacture catalysts, separators and even drug carriers9.

Unlike most foams, which are flammable and produce smoke and change their dimensions when burned, the perfect thermal stability without any drip and toxic smoke during burning has made the PFs a good candidate for construction applications10. Although there are other polymeric materials to use as insulators in construction, i.e., polyurethane, polystyrene and polyethylene, the PFs due to their fantastic heat resistance, better insulation, good chemical resistance and flame retardant properties attracted the attention of many scientists and investors11. However, weak mechanical properties, crispy structure tendency to crumble without desirable toughness and higher final costs have limited the usage of PFs in the insulation application12. In recent years, many scientists and polymer researchers have tried to resolve the mentioned disadvantages and improve the production technology of PF foams. In conjunction with the improvement of the PFs’ mechanical, thermal and microstructural properties, two approaches, i.e., the chemical modification and filler addition, are recently provided13. The foam density is a very important factor to evaluate when considering the mechanical properties of PFs which are affected by the cell wall and the cavity sizes14. Xiao et al.15 studied the effects of kaolin powder and glass fiber as fillers on the mechanical and thermal properties of the reinforced PFs. Their results showed that kaolin facilitated the foaming process of PF leading to small cells with thicker walls and good mechanical characteristics as well as thermal stability because of embedding the filler particles in the PF backbones. On the other hand, the glass fibers hindered the gas release followed by increasing the PF thermal stability. Li et al.16 evaluated the impact of different nanoparticles on the PF cell size. They found that the addition of 1 and 2 weight% (wt%) of titanium nitride caused a 54 and 43% decrease in the foam cell size to 102 and 126 μm, respectively. Moreover, he and his colleagues in another research work17 observed a 29, 54 and 26% reduction in the mean cell size by incorporating 0.5, 2 and 5 wt% of multi-walled carbon nanotubes (MWCNT), respectively. Yang et al.18 decreased the foam cell size by 30, 32 and 27% with the addition of MWCNT-functionalized by pristine, COOH and NH2, respectively. The efforts to improve the mechanical and microstructural characteristics of PF foams were also carried out by clay fillers, like Cloisite19 and Attapulgite20. Yuan et al.21 illustrated a reduction in the cell dimensions by 26% by adding 0.03 phr (parts per hundred rubber) of poly(n-butyl acrylate)-silica core-shell nanoparticles. In contrast, Li et al.22 exhibited a 17% enhancement in the compressive strength of the PF foam tanks to the 3 wt% powder of nitrile butadiene rubber (NBRP), while the cell size simultaneously raised by 50%. In addition, the chemical modification route is also accomplished to promote the mechanical, thermal and microstructural features of PFs. There are a few research works related to the influence of chemical modification on the microstructure followed by the mechanical and thermal properties of PFs. The chemical modifiers act as a surfactant to control the resin viscosity23,24. Bo et al.23 increased the PF cell size by 59 and 18% with the addition of 3 and 7 wt% oil-based polyurethane, respectively. This behavior reduced the compressive strength of the prepared foam. In another work, Liu et al.24 considered the effect of the modified polyethylene glycol (PEG) on the cell size and the improvement of the mechanical properties, i.e., flexural and compressive strength of the PF foams. Their resultant data indicated that the mean cavity size was increased by 66, 17 and 17% with 4.5 wt% of modified PEG (grades 200, 400 and 600) by boron, respectively.

Despite many efforts to improve the mechanical, microstructural and thermal stability of PF foams, the manufactured PFs are still far from the desired properties for insulation application. On the other hand, using additional materials except the required chemicals to produce PF foams, i.e., fillers and modified particles, imposes extra costs. Changing and modifying the used chemicals in the foaming process can affect the aforementioned properties of the PF foams. Surface active agents are well-known materials that are used in the PF manufacturing process to stabilize the emulsion droplets and gas formed during the mixing step and curing process. Surfactants reduce the interfacial tension by adsorbing at the interface of the droplet-phenolic resin and hinder the coalescence phenomenon before the foaming stability25. Changing the surface tension of the solution can lead to mechanisms influencing the physical characteristics of the foam, e.g., bubble size, expansion ratio and foamability26. In this regard, Qui et al.27 unmasked the impact of surfactants on the foaming kinetics of polymer solutions and the thermophysical features of foams. Yu et al.28 also proved the strong relationship between the interfacial tension of polymer solution and the polymer foam properties. According to the literature, the investigation of the foaming potential of surfactants is still at the beginning of the road29. So far, no research has been conducted comparing the effects of various surfactant types on the structure and properties of phenolic foams. In this research work, the impact of different surfactants in the producing process on the microstructural, mechanical and thermal properties, and water uptake of the prepared PF foams was comprehensively investigated.

Materials and methods

Materials

Phenolic resin (type IL800, density 1.2 g.cm− 3, viscosity 700 mPa.s, solid content 75 ± 3%) was purchased from Resitan Co. (Tehran, Iran). The acid of p-Toluenesulfonic (PTSA) (purity ≥ 98%) as a curing agent, n-pentane (purity ≥ 99.0%) as a foaming agent, urea (purity ≥ 99%) as an anti-odor, Tween80 (polysorbate) as a nonionic surfactant, SLS30 (sodium lauryl sulfate), ALS70 (ammonium lauryl sulfate) and SLES270 (sodium laureth sulfate) as anionic surfactants were bought from Merck & Co., Inc. (Germany).

Foaming process

To manufacture a PF foam, first, 100 phr of phenolic resin IL800 was mixed with 4 phr of n-pentane and 1.2 phr of an anti-odor (urea) in a stirrer under the temperature and rotor speed of 50 °C and 1200 rpm, respectively, for 2 min. Then the surfactant with the amount of 14 phr was added to the mixture and mixed for another 2 min at the same condition. Afterward, a 1:1 mixture of PTSA and deionized water with the amount of 20 phr was added to the compound and then stirred for 1.5 min under the same circumstances. After the mixing process, the mixture was poured into a mold and put in an oven at a temperature of 85 °C and allowed to foam for 15 min. In the end, the produced foam was removed from the mold and cooled at the ambient temperature for 24 h. In the present research work, different surfactants were used to prepare the PF foams. In the formulations presented in Table 1, the PF samples prepared with the Tween80, SLS30, ALS70 and SLES270 surfactants were symbolized with FT, FL, FA and FS, respectively.

Table 1 The details of the prepared phenolic foam formulations.
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Tests performed on the PF foams

Scanning electron microscopy (SEM)

To consider the morphology, porosity and cell structure of the prepared foams, the field emission SEM (FSEM) images were taken from the samples using FEI Inspect F (FEI Company, Hillsboro, Oregon, USA) with an accelerating voltage of 20 kV. To prepare the samples for the FSEM test, first, liquid nitrogen was used to break the samples, and then they were coated with a thin gold layer.

Density measurement

The density of the prepared PF foam was achieved using the buoyancy method utilizing the densimeter of DM3000 made by MonTech Company (Germany) according to the standard of ASTM D297. The density was calculated based of an average on ten samples for each formulation.

Thermogravimetric analysis

Thermogravimetric analysis (TGA) for the PF samples was performed by PerkinElmer-TGA8000 (USA) with a heating rate of 10 °C/min from 25 to 600 °C. The TGA test was implemented for each PF sample to evaluate the thermal stability of the foams. The mass of the analyzed foam sample was 5 ± 0.5 mg. Moreover, the differential thermogravimetric analysis (DTG) was also performed on the PFs using the aforementioned apparatus to measure the degradation rate.

Fire resistance test

The fire resistance test was carried out according to the standard of ASTM D3806. The overall plan and descriptions of the device components were reported in ASTM D3806 in detail. Figure 1 shows the applied apparatus at different side views. The test was implemented under ambient circumstances. As seen in the figure the bottom of the chamber (the plate where the sample is put) was made of stainless steel. A very sensitive thermometer was connected to the beneath of the plate which detects the plate temperature. At the beginning of the test, the plate was directly exposed to constant fire for 3 min and the plate temperature was detected 115 °C. After that, the foam sample was put on the plate and exposed to the fire for the same time duration and the temperature variation was registered. There is a point if the detected temperature raised more than the reference temperature of 115 °C means extra heat was created by the sample due to burning.

Fig. 1
figure 1

The custom-made apparatus to test the fire resistance of the PF foam samples.

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Compression test

Compression analysis of the prepared PF samples was performed based on the standard of ASTM D1621 using the Zwick/ Roell Z030 universal testing machine (USA). Ten different samples for each formulation were cut at the dimensions of 25.43 mm3. Next, each sample was placed between the plates made of stainless steel and then a uniform load was applied on the upper plate with a constant rate of 2.5 mm/min. The maximum strength determined for the ten samples for each PF formulation at 10% core deformation (according to ASTM D1621) was averaged as the compressive strength.

Water absorption analysis

To determine the hydrophilicity/hydrophobicity level of the prepared PF samples, a water absorption test was implemented. First, five different samples per formulation with the same dimensions were immersed in water for 3 days. The weight of the samples was detected after each 24 h. The difference between the weights of the samples at time 0 and after each 24 h was used to calculate the amount of absorbed water in the samples. Finally, the average water absorption amount (WA) was calculated based on an average of five samples for each formulation.

Results and discussions

As shown in Fig. 2, there is a complex procedure consisting of nucleation, growth and stability followed by solidification in the foaming process. As seen in the figure, the created bubbles have also gone through four stages of nucleation, growth, collision and combination, during the foaming process. In this work, the effects of Tween80 as a nonionic and SLS30, ALS70 and SLES270 as anionic surfactants on the aforementioned features of the PF foams were comprehensively studied. For more clarity, the chemical structure of the used surfactants in this research work is schematically presented in Fig. 2.

Fig. 2
figure 2

Schematics of the foaming process and molecular structure of the surfactants used to manufacture the PF foams; in the molecular structures: gray, blue, red, yellow, purple and white balls are the carbon, nitrogen, oxygen, sulfur, sodium and hydrogen atoms, respectively. The letters of a, b … e represent the repeating units in the molecular structure.

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Foam density

Figure 3 shows the obtained average density for all the prepared PF foams. As seen in the figure, the lowest and highest densities belong to the FT and FA foam samples, respectively. The sample prepared with the SLS30 surfactant almost has the same as the FA density. However, the SLES270 surfactant formed a foam with a modest foam density. The cell structure, i.e., cell size, cell wall thickness and cell density, has a direct impact on the foam apparent density.

Fig. 3
figure 3

Radar chart for average density (with a \(\:\sim\)1% errorbar) for the FT, FS, FL and FA foam samples

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The densities obtained in this research work are much lower than the PF foams provided by other researchers; Saz-Orozco et al.30 (they produced the PF-incorporated Eucalyptus cellulose fibers by Tween 40 as a surfactant, phenol-4-sulfonic acid as a catalyst and n-pentane as a foaming agent), Song et al.31 (they prepared the PF-filled multi-wall carbon nanotube/graphene by using PTSA catalyst), Zhou et al.32 (they provided a PF foam by graphene oxide, general-purpose silicone surfactant and n-pentane and hexane as blowing agent) prepared phenolic-based foams with a foam density of 160, 66 and 80 kg/m3.

Microstructural properties

To consider the effect of the surfactant type on the microstructural characteristics of the prepared PF foam, the FSEM images were taken from the sample surface. The FSEMs are exhibited in Fig. 4a–d for all the FT, FS, FL and FA samples, respectively.

Fig. 4
figure 4

The FSEM images taken from the surface of the PF foams prepared by the (a) Tween80, (b) SLES270, (c) SLS30 and (d) ALS70 surfactants.

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As seen in the figure, the morphology of the created cells in the produced foams are all open-cell, except FT which has a semi-open-cell structure. The FT foam consists of very large cells with a thinner cell wall thickness compared to the other samples (see Fig. 4a). The FL and FA samples have the same morphology and cell size. The cell characteristics and morphology were perfect in the FS sample with the smallest cell size, good distribution and a desirable cell wall thickness. To create a better understanding of the influence of the applied surfactants on the microstructural properties of the PF foams, the cell characteristics were quantified. It should be mentioned that, in the calculation procedure of the microstructural quantities, almost 200 cells were used to compute the mean cell size (based on an average of 200 cells) and probability distribution function (PDF) of the cell sizes using several SEM images with different magnifications. The cell density (Nc), volume expansion ratio (VER) and cell wall thickness (δ) were obtained using the following equations33:

$$\:\text{V}\text{E}\text{R}=\raisebox{1ex}{${{\uprho\:}}_{\text{p}}$}\!\left/\:\!\raisebox{-1ex}{${{\uprho\:}}_{\text{r}}$}\right.$$
(1)
$$\:{\text{N}}_{\text{c}}\cong\:\frac{{10}^{4}}{{\text{d}}_{\text{a}}^{3}}[1-\frac{1}{\text{V}\text{E}\text{R}}]$$
(2)
$$\:{\updelta\:}={\text{d}}_{\text{a}}\left[\frac{1}{\sqrt{1-\frac{1}{\text{V}\text{E}\text{R}}}}-1\right]$$
(3)

in which ρf and ρr are the densities of the PF foam and the resin compound for each formulation, respectively. The da indicates the cell diameter. Figure 5 illustrates the calculated cell size, density and cell wall thickness for all the samples.

Fig. 5
figure 5

Radar chart for the averaged cell characteristics, (a) cell size, (b) cell density and (c) cell wall thickness, for the FT, FS, FL and FA foam samples.

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Figure 5 proves the claims presented above. As exhibited in Fig. 5, the sample with the lowest cell size, the highest cell density and a suitable cell wall thickness belongs to the FS formulation. The prepared foam samples have lower cell sizes than the phenolic-based foam with cell sizes in the range of 390–450 μm manufactured by Liu et al.24 (they produced a PF foam with acetic acid/phosphorous acid/distilled water, petroleum-based ether as a blowing agent and Tween 80 surfactant), while they are higher than the cell size of the foams presented by Bo et al.23 (they prepared a PF foam by Tween 80 surfactant, commercial curing reagent and silicon oil), 100–130 μm. The FL and FA foams have the same cell morphology and characteristics, and the FT foam has the worst microstructural features. What is clear is that the SLES, SLS and ALS surfactants with their ionic heads acted much better than the Tween80 emulsifier without any ions in its molecular structure. It could be concluded that the anionic surfactant can act better than nonionic ones to create a foam with more desirable microstructural properties. In the PF foam preparation process, the surfactants played a key role in promoting the post-foaming procedure, the collision and merging of the created microbubbles (see Fig. 2). This mechanism changed the foamability of the phenolic resin. As shown in Fig. 2, the surfactants diffuse into the solution to reach the interface of the formed nuclei-resin, then, with their hydrophilic-lipophilic nature, create bubbles by reducing the interfacial tension. This mechanism also helps to prevent the coalescence phenomenon before the formed bubbles become stable. To reveal the hidden insight into the effect of the surfactant chemistry on the microstructural properties of the PF foams, the hydrophilic-lipophilic balance (HLB) was calculated for all the surfactants based on Ontiveros et al.34. The values of 40, 11.62, 10.03 and 14.9 were obtained for the SLES, SLS, ALS and Tween80 surfactants, respectively. According to the achieved results, the SLES has an HLB number much further than the other surfactants because of its higher polarity. The high polarity is due to the existence of both ion and ether functional groups at the head of the SLES surfactant. The high hydrophilicity of SLES resulted in a better micelle formation followed by better bubble creation and coalition in the FS recipe. This issue led to the most favorable impact on the foam microstructural characteristics because the SLES molecules acted as a better solubilizer in comparison with other ones35,36,37. According to Fig. 2, the same molecular structure of the SLS and ALS (except for the surfactant head) led to almost the same HLB number in the range of 10–12, which resulted in nearly the same microstructural characteristics for the prepared FL and FA foam samples. Although the HLB of SLS and ALS is lower than Tween80, more molecules per same molecular weight as well as the polarity of the surfactant head (due to the existence of ions) led to a better performance for anionic surfactants compared to the nonionic ones38.

For more consideration, the PDF graph of the foam cells for all the samples was provided using the FSEM images, and the graphs were reported in Fig. 6. As exhibited in the figure, the PDF-cell size graph for the FS sample is much thinner than the other foams which means a perfect homogeneity in the foam cell sizes. The FT and FL and particularly FA samples have a wide distribution. Therefore, the SLES270 surfactant led to a foam made of cells of almost the same sizes.

Fig. 6
figure 6

The probability distribution function (PDF) of the foam cell sizes for (a) FT, (b) FS, (c) FL and (d) FA samples.

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Fire resistance

One of the best criteria to evaluate the thermal insulation, thermal stability and heat resistance of the PF foams is the fire resistance test. In this research work, the fire resistance test was done by putting a rectangular cube-shaped sample for each formulation on the plate in the chamber (see Fig. 1). Then the sample was exposed to constant fire for 3 min. Figure 7a1–d1,a2–d2 illustrate the sample pictures before and after the fire resistance test, respectively.

Fig. 7
figure 7

The pictures of (a1 and a2) FT, (b1 and b2) FS, (c1 and c2) FL and (d1 and d2) FA samples before and after the fire resistance test, respectively.

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As shown in Fig. 7a1,a2, the FT sample was completely destroyed by the fire, while the decomposition behavior was partial in the other foam samples. According to the figure, there was almost no dimensional reduction in the FA foam. After the fire resistance test, the foam samples were qualitatively tested as well. The FS sample kept its partial flexibility even after the fire exposition while the fragility of the FL and FA samples increased drastically. A slight stress and loading led to crushing the FL and FA foam structure, while this phenomenon was not observed in the FS sample due to its slight flexibility. Based on the observations, the thermal stability of the FA foam was more than FS > FL > FT. Despite the mentioned physical observations, the amount of smoke created during the fire exposition was also investigated. From the remnant of the fire resistance test presented in Fig. 7, the FT sample produced a huge amount of smoke when exposed to the fire, while the trend of FS < FL < FA was found in the smoke amount in the other samples.

To create a better understanding of the thermal insulation ability of the foam samples, the temperature under the plate (see Fig. 1) was detected by a sensitive thermometer. The results of the temperature as a function of time during the test for all the samples were reported in Fig. 8.

Fig. 8
figure 8

The time dependency of the detected temperature by a sensitive thermometer located under the plate embedded in the test chamber for the FT, FS, FL and FA foam samples (see Fig. 1).

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According to Fig. 8, the temperature variation with the time was very intense in the FT sample because the fire caused a total decomposition of the sample followed by direct contact with the metal plate. The intensity of the temperature-time graph for the other samples was FA > FL > FS. Therefore, the SLES270 surfactant produced a sample with the best thermal insulation compared to the other samples because of the lowest slope in the curve (see Fig. 8). Factually, the good thermal insulation feature of the FS foam led to a resistance against the heat diffusion through the sample to increase the temperature of the plate. According to Hasanzadeh et al.39 research work on the thermal insulation behavior of polymer foams, the cell characteristics have a strong effect on the foam’s thermal properties, particularly thermal conductivity which has a direct impact on the foam’s thermal insulation feature. Their results showed that a decrease in the cell size and an increase in the cell wall thickness reduces the overall thermal conductivity of the polymer foam significantly40. In fact, the trapped air in the created bubbles in the foaming process acts as a heat insulator because of its very poor thermal conductivity, 2.623 − 6.763 × 10− 2 W/mK41. For this reason, the FS sample with the lowest cell size, highest cell density and perfect cell distribution illustrates the highest thermal insulation.

To create a deep insight into the thermal behavior of the FS sample as a foam with the most desirable thermal behavior, the TGA combined with the DTG test was applied to the foam sample by 600 °C. The resulting thermal graphs are presented in Fig. 9a together.

Fig. 9
figure 9

The TGA and DTG results as a function of the temperature for (a) FS and (b) FT foam samples.

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As seen in Fig. 9a, the weight loss in the FS sample was almost 5% by 100 °C. After the temperature, the main decomposition of the FS sample began with a more intense slope in the TGA curve. Moreover, the DTG graph also showed peaks at the temperature range of 100–350 °C meaning rapid decompositions at the given temperatures. With raising the temperature more than 350 °C, the oxidation of the remaining structure occurred with the most variations slopes of the thermal graph, and the highest peaks in the DTG curve were observed at the range of 350–600 °C. The sample showed good thermal stability because 55% of the sample remained after the heat exposure at a temperature of 600 °C. To compare the FS thermal behavior as the best fire resistance sample with the FT foam as the worst sample, the TGA combined DTG graph for the foam made of Tween80 was also presented in Fig. 9b. As exhibited in Fig. 9b, both FS and FT foams almost have the same TGA and DTG behavior. The 5% weight loss, main decomposition and oxidation phenomena occurred in the temperature ranges of 30–200, 200–350 and 350–600 °C, and the maximum weight loss was detected at 57% for the FT sample. The fire resistance and TGA + DTG tests together indicate that the FS sample may have the same thermal degradation behavior as the FT foam but it resists much more than the FT sample against the fire exposition. Therefore, the mentioned thermal tests complement each other and they are not individually a good criterion to evaluate the PF foam thermal behavior.

Compressive strength

The compressive strength (CS) of a polymeric foam has great importance in many industrial applications, particularly insulation42. It should be mentioned that the detected maximum load applied on the sample foam at 10% of the sample deformation (according to ASTM D1621) - extracted from the stress-displacement graphs of the compressive test (see Fig. 10) - was registered as a compressive strength. The obtained specific CS calculated by the CS/foam density based on an average of ten samples for each formulation is shown in Fig. 11. Comparing the specific CS instead of the pure CS removes the effect of the foam density on the mechanical properties.

Fig. 10
figure 10

The stress-displacement graphs of the compressive test for all the FT, FS, FL and FA foam samples.

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Fig. 11
figure 11

Radar chart for the results of the compressive test for all the FT, FS, FL and FA foam samples; the specific compressive strength is obtained by compressive strength/foam density, which is based on an average of the resultant data with a \(\:\sim\)2% errorbar.

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According to Fig. 11, the FS sample with the highest cell density and the most perfect cell size and distribution has the furthest specific CS compared to the other ones. The regular cell morphology in the foam structure leads to a great distribution of the applied load on the sample which causes an increase in the foam to withstand the pressure. In contrast, irregular cell distribution in the FT, FL and FA foam structures and even their bigger cell sizes led to a nonuniform distribution of the applied stress resulting in a lower specific CS. The phenomenon enhanced the specific CS of the FS sample two times compared to the three other foams.

In comparison with the literature, the specific compressive strength obtained in this work is lower than the values of 9.5 and 3.3 kPa.m3/kg reported by Shen et al.43 (they produced a PF foam-reinforced by aramid and glass fibers) and Yuan et al.44 (they prepared a PF foam filled with glass fibers modified with phosphorus consisting of polyurethane), respectively. The obvious difference between the results is due to using fillers and fibers in the PF composites manufactured by the mentioned scientists.

Water absorption

The hydrophilicity of phenolic resins is at a very low level due to its aromatic structure45. In some applications, it is needed to increase the hydrophilicity of the PF foams such as femoral foams, while in others, like thermal insulation applications, it is vice versa46. The water tendency in the PF foams can lead to serious damage like corrosion when it is used as a thermal insulator around metallic equipment47. Hence, controlling the hydrophilicity/hydrophobicity level in the prepared PF foam is important. The used surfactants in the research work due to their tendency to both hydrophilic and hydrophobic materials can affect the water sorption amount in the manufactured foams. In the present research, the effect of the chemical structure of the Tween80, SLES, SLS and ALS surfactants on the hydrophobicity of the FT, FS, FL and FA foams was evaluated by measuring the water sorption amount in them.

The water absorption (WA) amount for all the foam samples was calculated using (weight of dried foam-weight of wet foam)/weight of dried foam, and the resultant figure as the function of time is shown in Fig. 12.

Fig. 12
figure 12

Water absorption amount for all the immersed PF samples in water as a function of time.

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As exhibited in the figure, the WA value increased with time and after a given time the variations changed to an almost constant trend. The Tween80 surfactant, due to its lower polarity compared to the other emulsifiers, resulted in a foam with the lowest WA amount. The obtained WA value for the FT foam has good conformity with the results published by DSouza et al.2 (they prepared a PF foam with cyclopentane, pentane and hexane as blowing agents, acetic acid for neutralization and PTSA). Moreover, the most water compatibility belongs to the FL and FA foams, respectively. However, these two samples have almost the same water sorption behavior. According to the results, the FS sample has the second lowest WA value among the presented formulations. The intermolecular interactions between the ions of the surfactant molecules and water led to the attraction between them and the capturing of the water molecules in the foam structure. The more the non-bonded interactions caused more water sorption capacity for the foam sample. The SLS and ALS have more surfactant molecules than the SLES at the same weight% due to their lower molecular weights. This issue increased the ion numbers in the system, followed by more interaction between the foam and water, which resulted in a higher amount of WA.

According to the all achieved results in the present research work, the FS sample has the most favorable foam properties from the microstructural, thermal stability and insulation, physical and mechanical properties as well as lower water sorption amount compared to others. Therefore, the PF foam prepared by SLES270 surfactant is a good candidate to manufacture a PF-based foam for thermal insulation application.

Conclusions

In this work, the influence of surfactant chemistry on the microstructural, hydrophilicity, compressive and thermal properties of the PF foams were studied. Accordingly, two surfactant families, nonionic and anionic, were used to manufacture PF foam. The FT (20.2 ± 0.2 kg/m3) and (FA and FL with almost the same value, 45.4 ± 0.4 and 42.72 ± 0.4 kg/m3) showed the lowest and highest densities, respectively. However, the SLES produced a PF foam with a moderate density (38.01 ± 0.4 kg/m3). An open-cell morphology was observed for all the foams, except FT with a semi-open-cell structure consisting of very large cells (cell diameter = 432.6 ± 22 μm). The foams made by SLS and ALS had the same morphology, cell size of 338.5 ± 18.5 and 390.9 ± 19 μm, the δ = 7.6 and 10 μm and cell density = 2.5 and 1.6cell/mm3 × 105, respectively. The FS foam had the most perfect microstructural properties with the smallest cell size, 195.3 ± 9 μm, the highest Nc, 12.9cell/mm3 × 105, and a desirable cell wall thickness, δ = 3.9 μm. According to the test measuring the thermal insulation features, the FS sample had a ~ 50, 40 and 25% more thermal insulation capability than the FT, FL and FA foams, respectively. Among the used emulsifiers, the anionic SLES has the most desirable impact on the PF foam microstructural features because of its higher polarity of the hydrophilic head. The higher polarity of the SLES emulsifier caused better micelle creation followed by better bubble formation and coalition in the FS recipe. The existence of ions in the FL and FA foams enhanced the water absorption amounts of the samples even more than the FS sample. Whereas the FT sample, due to its nonionic, components had the lowest WA value. According to the resultant data, the good thermal properties of FS and FA samples (FS > FA > FL > FT) have turned the foams into good candidates for thermal insulation properties. Among them, the PF foam made of SLES270 surfactant has the most favorable characteristics required for thermal insulation application because of its good resistance against heat diffusion and perfect compressive strength. Whereas, the FL and FA samples, due to their good hydrophilicity behavior compared to the other foams, are great for floral foam application. Because their open-cell morphology and higher hydrophilicity increase the water absorption amount.