# Role of fluorocarbons in PUR foams: a cryo-SEM study

Martin Hamann, Alain Carvalho, Marc Schmutz, Luca Fiorucci, Daniel Telkemeyer, Markus Schütte, Wiebke Drenckhan-Andreatta

► To cite this version:

Martin Hamann, Alain Carvalho, Marc Schmutz, Luca Fiorucci, Daniel Telkemeyer, et al. Role of fluorocarbons in PUR foams: a cryo-SEM study. Colloid and Polymer Science, 2025, 303 (12), pp.2463-2479. 10.1007/s00396-025-05475-8. hal-05391390

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Submitted on 1 Dec 2025

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# Role of fluorocarbons in PUR foams: a cryo-SEM study

Martin Hamann¹, Alain Carvalho¹, Marc Schmutz¹, Luca Fiorucci¹, Daniel Telkemeyer², Markus Schütte² and Wiebke Drenckhan-Andreatta¹

1 Institut Charles Sadron, CNRS UPR22 – Université de Strasbourg, France

2 BASF Polyurethanes GmbH, Lemförde, Germany

## Graphical abstract

<figure><img src="image_2.png" /></figure>

## Abstract

The formation of polyurethane (PUR) rigid foams is a complex process involving multiple interrelated chemical and physical mechanisms. One critical parameter for thermal insulation applications is the pore size, which has been shown to be reduced by adding fluorocarbons (FCs) to the foam formulation. While recent studies confirm this phenomenon, the underlying physico-chemical mechanisms remain unclear. In this study, we employ cryogenic scanning electron microscopy (Cryo-SEM) to investigate the evolution of nascent PUR foams and the role of FCs in shaping their morphology. Two PUR foam systems—a simplified "scientific system" and an industrially relevant "technical system"—were analysed under both laboratory and pilot-scale conditions. Our results confirm a recently formulated hypothesis that FCs impact foam structure primarily by increasing the number of entrained gas bubbles which act as heterogeneous nucleation sites. We also put in evidence the formation of interfacial FC films around the bubbles, which may affect foam stabilization and growth dynamics. Additionally, we observe an unexpected stagnation in bubble growth in the presence of FCs, highlighting the need for further investigations. This study provides new insights into the role of FCs in pore size control and may contribute to finding alternative of PUR foam formulations for enhanced thermal insulation performance.
# 1. Introduction

The preparation of a polyurethane rigid (PUR) foam is a highly complex process involving many interrelated steps that depend on various chemical- and physical phenomena. Typically, two-component systems are used for PUR-Foam generation with the "A-component" containing polyols as well as surfactants, catalysts, and blowing agents, while the "B-component" consists of (poly-)isocyanates [1], [2]. The formation of a crosslinked polyurethane (PU) network via polyaddition [3] of the polyols and isocyanates is then initiated very soon after mixing of the two components. In parallel to the PU network formation, foam blowing takes place as chemically inert blowing agents with a low boiling point evaporate due to the strongly exothermic nature of the polyaddition reaction. Besides the evaporation of the physical blowing agents, additional blowing is achieved chemically by adding water to the initial formulation which reacts with isocyanates to liberate CO2. Due to this liberation of gas, the reacting matrix is transferred to a liquid foam state. The liquid foam, in turn, is transferred to a solid foam once the network-forming reactions have progressed sufficiently which eventually yields the final PUR foam.

An important question is to understand how the interplay of these different mechanisms controls the final pore size of the foam. In particular, for thermal insulation, small pore sizes are searched for [4]. It is known that the pore size can be reduced upon the addition of fluorocarbons (FCs) to the initial PUR foam formulation [5], [6], [7]. Recently, we conducted a detailed PUR cup foam investigation to quantify the FC-driven pore size reduction [8]. The main focus of this cup foam study was to correlate the FC concentration in the PUR foam formulation to the characteristic pore size of the final PUR foam. In this context, we found that the FC-driven pore size reduction appears to be a systematic effect and that its relative extent seems to depend little on the systems we chose for PUR foam preparation. Thus, we concluded that there is probably one common mechanism at play, whose physico-chemical origins have yet to be elucidated. In the search for this mechanism, we investigated in detail the influence of FCs on the initial mixing step [9]. We could show that the presence of FCs increases significantly the number of entrained gas bubbles which serve as heterogenous nucleation sites for the foam blowing, leading naturally to smaller pore sizes [10]. However, to fully grasp the mechanisms of the FC-driven pore size reduction in PUR foams, we ideally need to follow the full evolution from the initial mixing step to the final solid foam. Unfortunately, studying nascent PUR foams, i.e. reacting PUR foams during their generation, by optical means is a challenging endeavour. Primarily, this is owed to the rapid reaction kinetics of PUR foam systems. Process times measured from the onset of foam blowing to complete foam solidification usually range between several tens of seconds and a few minutes. Furthermore, due to the dispersed nature of foams in general, their optical characterization is very often limited to the observation of just a few layers of foam in contact with some observation window [11], [12], [13]. Reignier et al. [14] recently reported a neat way to bypass most of these difficulties. They succeeded in monitoring nascent PUR foams by drawing samples after defined reaction times which they then froze in liquid nitrogen to quench all ongoing reactions. Therefore, the samples captured the nature of the nascent PUR foam at the moment when they were frozen. The frozen samples were then investigated using scanning electron microscopy under cryogenic conditions (cryo-SEM) such that the morphology of the frozen sample remained intact during the investigation. This approach enabled visualizing the microscopic features of a nascent PUR foam and following the evolution of these features with progressing PUR foam generation. The excellent study
of Reignier et al. [14] focused predominantly on analysing the morphology of the
dispersed liquid reactive mixture containing the A- and the B-component directly after
mixing and its evolution with progressing foam blowing and solidification. However, for
their study they used an FC-free PUR foam system blown only by CO2 and isopentane.

This motivated us to conduct a similar study to focus particularly on potential effects
caused by the presence of fluorocarbons. For this purpose, we exploit two PUR foam
model systems which we shall call (1) the “scientific system” and the (2) “technical
system”. In the industrially relevant “technical system”, the A-component contains three
different macromolecular polyether polyols (Table 1). In contrast, the A-component of
the much simpler “scientific system” contains only one low-molecular polyol
tripropylene glycol (TPG, see Table 1). Other than that, the utilised surfactant, catalysts,
blowing agents etc. are the same for both systems (see Section 2).

As a very first step, we used the scientific system to study unreactive systems consisting
of the A-component only. Here, A-components were prepared systematically including
or excluding the fluorocarbon from the formulation to highlight any potential influence.
Next, the cryo-SEM experiments were conducted with reactive systems, i.e. both the A-
and the B-component of the scientific system were used with and without the
fluorocarbon being present. For both variations of the reacting scientific system, the
evolution of PUR foam generation was followed by drawing samples after different
reaction times and freezing them in liquid nitrogen to halt any ongoing processes. All
investigations concerning the scientific system were performed at the lab-scale, i.e. small,
cup-sized PUR foam batches were prepared using table top motorised stirrers. In contrast,
PUR foams based on the technical system were prepared under industrial conditions on a
pilot-plant at BASF Polyurethanes GmbH. Here, a high pressure mixing- and dispensing
machine was used for the blending of the A- and B-component to initiate PUR foam
generation – with and without FC. Moreover, the catalyst concentration was varied to
study how the reaction kinetics influences the generation of the porous morphology
during PUR foam preparation. To the best of our knowledge, a comparable study
monitoring PUR foam generation under technical conditions via cryo-SEM has not been
reported before. Thus, the results shown in the following are very likely the first their
kind.

**2. Materials, methods and formulations**

**2.1. Materials**

For the “scientific system”, tripropylene glycol (TPG, 97 % supplied by Sigma Aldrich)
was used as a low-molecular polyol. In contrast, for the “technical system” a blend of the
macromolecular polyether polyols “Polyetherol 90178”, “Polyetherol 90094”, and
“Polyetherol 90060” was supplied by BASF. The names of all polyols, the used blend, as
well as some main characteristics of that blend are given in Table 1.
<table><caption>Table 1: Overview of the composition of the polyether polyol blend used for the “technical system” and the “scientific system”. Moreover, information on the hydroxyl value HV and the viscosity η of both systems is provided.</caption><thead><tr><th>Substance</th><th>Polyol blend<br>[wt.%]<br>(tech. sys. only)</th><th>Hydroxyl<br>value HV<br>[mg KOH/g]</th><th>Viscosity η<br>[mPas]<br>(T = 25°C)</th></tr></thead><tbody><tr><td colspan="4" style="text-align: center; font-weight: bold;">Technical System</td></tr><tr><td>Polyetherol 90060<br>(Polyetherolyol produced by<br>propoxylation of a mixture of<br>sucrose and glycerol)</td><td>50</td><td>490</td><td>8450</td></tr><tr><td>Polyetherol 90094<br>(Polyetherolyol produced by<br>propoxylation of 2-Ethyl-2-<br>(hydroxymethyl)-propan-1,3-diol)</td><td>10</td><td>160</td><td>300</td></tr><tr><td>Polyetherol 90178<br>(Polyetherolyol produced by<br>propoxylation of glycerol)</td><td>40</td><td>390</td><td>12800</td></tr><tr><td>Weighted mean</td><td></td><td>420</td><td>9380</td></tr><tr><td colspan="4" style="text-align: center; font-weight: bold;">Scientific System</td></tr><tr><td>Tripropylene glycol (TPG)</td><td>100</td><td>570</td><td>78</td></tr></tbody></table>

The full formulations of the scientific and technical systems are listed in Table 2 and Table 3, respectively. For the scientific system (Table 2), we used the tertiary amine-based catalyst Jeffcat ZR-50 (1-[Bis[3-(dimethylamino)propyl]amino]-2-propanol; supplied by Huntsman), whereas for the technical system (Table 3) a catalyst blend consisting of tertiary amines (provided by BASF) was used. The silicone-based surfactant Tegostab B8491 (supplied by Evonik Industries) was used for both systems. Tap-water was used as a chemical blowing agent, while cyclopentane (≥ 95%, BASF) and the fluorocarbon FA-188 (perfluoro(4-methylpent-2-ene); supplied by 3M) were used as physical blowing agents. For both systems, the B-component consisted of the isocyanate Lupranat M20S supplied by BASF. Lupranat M20S is an oligomeric form of methylene diphenyldiisocyanate (MDI) with an NCO-content of ca. 31.5 wt.% and an average functionality of 3. All chemicals were used as received without prior purification.

For the scientific system, the formulations were prepared by weighing all substances into plastic syringes (60 mL plastic syringes) that are used later for the double-syringe mixing and air entrainment (see Section 1). The A- and B-component are prepared in separate syringes, which allowed combining the reactive compounds on demand via quickly ejecting the content of both syringes into a paper cup, where the actual mixing followed (see Section 1). Note that the A-component was pre-homogenized within the plastic syringe via manual whipping until it looked completely homogeneous and translucent. All formulations were used within 30 – 60 minutes after their preparation.

More information on the preparation of the technical system is given in Section 1.
<table><caption>Table 2: Formulations of the "scientific system". In some experiments, only mixtures of the A-component with the blowing agents where investigated, while in other experiments the fully reactive mixture is used. All concentration are given with respect to 100 wt.% of the base A-component.</caption><thead><tr><th colspan="3">Scientific system</th></tr><tr><th>Component</th><th>Substance</th><th>Weight percent [wt.%]</th></tr></thead><tbody><tr><td rowspan="5">A-component</td><td colspan="2">Base A-component<br>(Σ m = 18.0 g)</td></tr><tr><td>Tripropylene glycol (TPG)</td><td>93.95</td></tr><tr><td>Surfactant (Tegostab B8491)</td><td>2.00</td></tr><tr><td>Water</td><td>2.50</td></tr><tr><td>Catalyst (Jeffcat ZR-50)</td><td>1.55</td></tr><tr><td rowspan="5">B-component</td><td colspan="2">Physical blowing agents</td></tr><tr><td>Cyclopentane (CP)</td><td>0</td><td>0</td><td>13.5</td><td>13.0</td></tr><tr><td>FA-188 (perfluoro(4-methylpent-2-ene) (FC))</td><td>0</td><td>2</td><td>0</td><td>2</td></tr><tr><td colspan="2">(m = 35.64 g)</td></tr><tr><td>Lupranat M20S (I<sub>NCO</sub> = 120)</td><td>Not used for reactive foaming</td><td>198</td></tr></tbody></table>

<table><caption>Table 3: Formulation of the "technical system". The composition of the polyol blend is given in Table 1. All concentration are given with respect to 100 wt.% of the base A-component.</caption><thead><tr><th colspan="3">Technical system</th></tr><tr><th>Component</th><th>Substance</th><th>Weight percent [wt.%]</th></tr></thead><tbody><tr><td rowspan="5">A-component</td><td colspan="2">Base A-component<br>(Σ m = 369 mL)</td></tr><tr><td>Label</td><td>"slow system"</td><td>"fast system"</td></tr><tr><td>Polyether polyol blend</td><td>94.72</td><td>93.95</td></tr><tr><td>Surfactant<br>(Tegostab B8491)</td><td colspan="2">2.00</td></tr><tr><td>Water</td><td colspan="2">2.50</td></tr><tr><td></td><td>Catalyst blend</td><td>0.78</td><td>1.55</td></tr><tr><td rowspan="5">B-component</td><td colspan="2">Physical blowing agents</td></tr><tr><td>Cyclopentane</td><td>13.5</td><td>13.0</td><td>13.5</td><td>13.0</td></tr><tr><td>FA-188</td><td>0</td><td>2</td><td>0</td><td>2</td></tr><tr><td colspan="2">(m = 431 mL)</td></tr><tr><td>Lupranat M20S<br>(I<sub>NCO</sub> = 120)</td><td>137.2</td><td>136.7</td></tr></tbody></table>
## 2.2. Methods

*Cryo-SEM sample acquisition: Non-reacting A-components based on the “scientific system”*

First, only the (non-reactive) A-component of the scientific system was studied via cryo-SEM. Here, four different variations of the A-component were prepared according to the formulations shown in Table 2. After pre-homogenizing the mixture in the syringe (see Section 2.1), they were fully homogenized and aerated using a home-built double-syringe mixing device described in more detail in previous publications [10], [15]. In short, the syringe that contained the A-component (20 mL) was connected to another 60 mL syringe that contained 17 mL of air via a narrow silicone tubing and a three-way valve. A periodic motion of the syringe pistons, powered by a crankshaft of the double-syringe mixing device, pushed both fluids together for 50 cycles through the narrow connection tubing which efficiently mixes the contents of both syringes. Immediately after the air entrainment step, a sample of the bubbly A-component was deposited on a commercial cryo-SEM sample stub and plunged in slush nitrogen to instantly freeze the sample and maintain its morphology in a frozen state. The subsequent cryo-SEM procedure is described in more detail below.

*Cryo-SEM sample acquisition: Reacting PUR foam systems based on the “scientific system”*

To perform cryo-SEM on reactive PUR foams, cup-sized (735 mL) batches of PUR foams (“cup foams”) based on the scientific system were prepared according to the formulations listed in Table 2. A more detailed description of the procedure is given in [8]. After the blending of the reactive components of the PUR foam system, samples of the nascent PUR cup foam were drawn in time intervals of ~10 seconds using a 3 mL plastic pipette and transferred within 2 – 3 seconds into home-built cryo-SEM specimen stubs (with the dimensions given in Figure 1). Immediately after the specimen stub was loaded with the nascent PUR foam sample, it was immersed in liquid nitrogen to halt the PUR foam-forming reactions. Once frozen, the samples were kept under liquid nitrogen continuously until cryo-SEM analysis was conducted.

<figure><img src="image_3.png" /><figcaption>Figure 1: Schematic drawing of the home-built cryo-SEM sample stubs used for the cryo-SEM investigation of nascent PUR foams.</figcaption></figure>

*Cryo-SEM sample acquisition: Reacting PUR foams with the “technical system”*

PUR free-rise foams were prepared using a pilot-scale high pressure mixing- and dispensing machine (mixing head: MT-18 by Hennecke GmbH; dispensing unit: Puromat 30/80 IQ by Elastogran GmbH) at BASF Polyurethanes GmbH, Lemförde (Germany). Initially, the A-component and the B-component were prepared and homogenized
separately in barrels of 120 L according to the formulations given in Table 3. Then, the prepared components were pumped into the respective storage tanks (60 L each) of the *Puromat*. Finally, both components were mixed using a mixing pressure of 140 bar and batches of ~800 mL were ejected with an output rate of 250 g/s for each generated free-rise PUR foam. After the liquid reactive mixture had been ejected from the mixing head, samples of the nascent free-rise PUR foam were drawn in time intervals of 7-8 seconds using a 3 mL plastic pipette and transferred into home-built cryo-SEM specimen stubs (Figure 1). Immediately after the specimen stub was loaded with the nascent PUR foam sample, it was immersed in liquid nitrogen to halt the PUR foam-forming reactions. Once frozen, the samples were kept under liquid nitrogen continuously until cryo-SEM analysis was conducted.

The experimentally accessible time span for drawing samples of the nascent PUR foam was around 30-40 seconds. After that, drawing samples was no longer possible, as the nascent PUR foam became too viscous. Moreover, it shall be clearly emphasized that the samples were not drawn successively from one and the same reacting PUR foam but that only one sample per liberated PUR foam batch was drawn. This is owed to both the complicated handling of drawing and freezing the samples and the fast reaction kinetics of the PUR foam. Therefore, several PUR foam batches of each studied formulation (Table 2 and Table 3) were ejected and samples were drawn after different waiting periods to maintain the sampling interval of 7 – 8 seconds.

### Cryo-SEM analysis

To investigate the acquired samples of the nascent PUR foams quenched mid-reaction, one frozen sample was retrieved from the liquid nitrogen tank and transferred to a *Quorum* cryo-preparation chamber. Within the cryo-preparation chamber, the surface layer of the frozen PUR foam sample was carefully scratched off at some positions with a blade to expose the actual morphology of the sample. Then, the sample was transferred from the preparation chamber to the electron microscope (*Hitachi SU 8010*, ICS Strasbourg) via an air-lock system. All micrographs were recorded using secondary electron detection with an accelerating voltage of 1.0 kV and a beam current of 10 µA. Cryogenic conditions were maintained throughout the entire procedure. The software *ImageJ* by *National Institutes of Health* was used to optimize brightness and contrast settings of the acquired micrographs and to perform image analysis. For all statistical analyses (for example to obtain mean bubble or drop sizes), we analyse 20-30 objects in at least three different images taken at different positions of a sample.

### Characteristic reaction times of the studied PUR foam systems

Table 4 gives an overview over the “cream time” and the “foam rise time” [1], [16] of the different PUR foam systems studied throughout the cryo-SEM study. These characteristic times were determined using preliminary cup foam trials.

<table><caption>Table 4: Overview of the characteristic reaction times of the PUR foam systems studied in the scope of the cryo-SEM study.</caption><thead><tr><th rowspan="2">Characteristic time</th><th rowspan="2">Scientific system<br>(Formulation Table 2)</th><th colspan="2">Technical system<br>(Formulation Table 3)</th></tr><tr><th>Slow</th><th>Fast</th></tr></thead><tbody><tr><td>Cream time</td><td>15 s</td><td>7 – 8 s</td><td>5 s</td></tr><tr><td>Foam rise time</td><td>90 s</td><td>70 s</td><td>50 s</td></tr></tbody></table>
# 3. Results and Discussion

## 3.1. A-component only – scientific system

At the outset we investigate only the A-component of the “scientific system” (Table 2) in the absence of chemical reactions. The A-component contains all important additives such as surfactants, catalyst, chemical and physical blowing agents. Since they are not all miscible, a first analysis of the microstructure of this mixture via cryo-SEM is considered relevant for understanding the reactive PUR foam system in a second step. To investigate if the presence of fluorocarbons (FCs) already influences the nature of the A-component per se, two variations of the A-component were analysed: one without FC and another one that contained 2 wt.% FC. Moreover, this investigation was repeated for A-components with and without cyclopentane (CP). Interestingly, the CP used in this study is fully soluble in the FC-free A-component at the concentration used (13.5 wt.%), while the FC is not soluble. Yet, the FC is partially soluble (ca 30%) in the CP. Hence the presence of CP and/or FC may have different effects on the repartition of the different gasses and liquids in the A-component. Thus, altogether, four different A-components were studied (Table 2). To mimic typical processing conditions, the homogenisation of the A-component and the controlled entrainment of air bubbles into the A-component was promoted via double-syringe mixing [10], [15] of the A-component and some air (Section 2.1).

Cryo-SEM images of the simplest system, i.e. the A-component of the scientific system without FC and without CP, frozen immediately after mixing, are shown in Figure 2. Figure 2a gives a large-scale view of the frozen A-component. One can see a continuous “rocky landscape” with some debris which most likely originate from the sample preparation (Section 1). More importantly, spherical craters with radii of $\langle r \rangle = (20 \pm 5)$ µm can be spotted. These are imprints of air bubbles that were dispersed within the A-component during the double-syringe mixing prior to freezing. Higher magnifications in Figure 2b,c reveal that the A-component appears homogeneous except for a certain roughness of the surface at the submicron-scale Figure 2c. Other than that, no noteworthy particularities can be seen in the sample.

Next, a CP-free A-component of the scientific system containing 2 wt.% FC was investigated. The corresponding cryo-SEM images are shown in Figure 3a-e for increasing magnification. Comparing Figure 3a and Figure 2a seems to suggest at first glance that the addition of fluorocarbons to the A-component did not result in fundamental changes. Again, a continuous matrix is observed with round craters of a mean radius of $\langle r \rangle = (18 \pm 6)$ µm corresponding to imprints of dispersed air bubbles. However, upon closer inspection of Figure 3b and c, one finds that a second population of much smaller craters with $\langle r \rangle = (1.0 \pm 0.5)$ µm coexists with the air bubble imprints. These originate from the small, immiscible FC-droplets dispersed within the A-component. Moreover, one notices the residue of a thin layer deposited at the interface between an air bubble and the A-component (pink arrows in Figure 3d,e). As shall be discussed later, we interpret this layer as an FC deposition on the bubble surface.
<figure><img src="image_4.png" /><figcaption>Figure 2: Cryo-SEM images of the bubbly A-component of the scientific system (Table 2) without fluorocarbon and without cyclopentane at different levels of magnification: a) 100x, b) 4000x, c) 20000x. The yellow circles in a) show the imprints of bubbles.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 3: Cryo-SEM images of the bubbly A-component of the scientific system with fluorocarbon (FC) but without cyclopentane (CP) at different levels of magnification: a) 300x, b) 1500x, c) 1200x, d) 4000x, e) 10,000x. Pink circles show the imprints of FC droplets, while pink arrows indicate a FC film.</figcaption></figure>

Next, we investigated A-components containing CP (with and without FC). Figure 4 shows cryo-SEM images of the CP-containing A-components in the absence (top) and presence (bottom) of FC. In contrast to the A-components without CP (Figure 2 and Figure 3), the frozen A-component appears “smoother” in the presence of CP, as the fractured pieces have rounded edges and corners. However, at this stage we cannot say whether this an experimental artefact. In Figure 4 we see again bubble imprints of about 10 µm radius and the much smaller imprints of the insoluble FC droplets. Interestingly, a comparison of the mean size of the FC-droplets in A-components without and with CP reveals that the FC-droplets are much smaller if the A-component contains CP: $\langle r \rangle = (0.2 \pm 0.02)$ µm with CP (ex. Figure 4b.3) vs. $\langle r \rangle = (1 \pm 0.5)$ µm without CP (ex. Figure 3b,c). This may be due to a change in interfacial tension and/or the viscosity ratio of the polyol- and FC phase, since CP is soluble in both. More importantly, the film-like deposition at the interface between an air bubble and the surrounding A-component appears again only in presence of fluorocarbons (pink arrows in Figure 4b.4-6).
<figure><img src="image_6.png" /><figcaption>Figure 4: a) Cryo-SEM images of A-components based on the scientific system (Table 2) with cyclopentane (CP) a) in absence, and b) in presence of fluorocarbon (FC). Yellow circles show air bubble imprints, pink circles imprints of FC droplets and pink arrows FC films deposited on bubble surfaces.</figcaption></figure>

The observation of “film-type” structures around the bubbles is coherent with academic literature showing the spontaneous adsorption of liquid FC or condensation of gaseous FC on liquid surfaces [9], [17]. If the system is saturated in FC, this can lead to the spontaneous formation of macroscopically thick FC films since they reduce the total interfacial energy of the system. To test this hypothesis in our context, we measured the interfacial tensions $\gamma_{FC,air}$ between FC and air (configuration $\alpha$ in Figure 5) and $\gamma_{FC,TPG}$ between FC and TPG (with and without surfactant, configuration $\beta$ & $\beta^*$ in Figure 5, respectively). The results are shown in Figure 5. We then measured the interfacial tension $\gamma_{TPG,air,FC}$ between air and TPG containing FC. If the FC forms a macroscopic film, the overall interfacial tension measured in the experiment should be the sum of the interfacial tension of both interfaces, i.e.: $\gamma_{TPG,air,FC} = \gamma_{FC,air} + \gamma_{FC,TPG}$. This is indeed the case, as confirmed by comparing the predictions (“$\alpha + \beta^*$”, “$\alpha + \beta^*$”) and the corresponding measured values (“Exp.”) in Figure 5. This was also shown to hold in aqueous systems [9], [17], [18]. A simple demonstration of the desire of the FC to wrap around an air bubble in TPG is shown in Figure 6: simply bringing an FC droplet in contact with a bubble is enough to create a violent motion of the FC wrapping around the bubble in order to reduce the overall energy of the system. While we show this here for TPG, the values of interfacial tensions of polyols are all in the range of TPG which is why we comfortably expect this phenomenon to hold in all PU mixtures upon addition of FC. We therefore believe that this explains the appearance of the films in the Cryo-SEM images in the presence of FC.
<figure><img src="image_7.png" /></figure>

Figure 5: Measurement of individual interfacial tensions of different formulations to probe whether FC-film covered air bubbles in TPG-based A-components are viable. The legend shows the different interfacial configurations that need to be considered, when assuming an FC-film covered air bubble in TPG-based A-components in (a) absence and (b) presence of surfactants. The interfacial tension values given in the bar charts are actual experimentally measured values of the respective interfacial tensions shown in the legend.

<figure><img src="image_8.png" /></figure>

Figure 6: A fluorocarbon droplet rapidly spreads on an air bubble (both held by a needle) in TPG + surfactant when brought in contact. Here the motion is so violent that it detaches the air bubble from the needle.

In summary, the analysis of the pure A-components shows that

* In absence of FC all components are miscible.

* The double syringe mixing integrates gas bubbles of about 20 µm radius.

* The FC is not miscible and creates droplets of about 0.2-1 µm radius, with droplet radii being in the lower range in presence of CP.
* In presence of FC, thin films are deposited on the bubble surfaces in line with interfacial energy arguments.

## 3.2. Reactive PUR foams – scientific system

We now turn to reactive PUR foam systems containing both the A- and B-component of the “scientific system” (Table 2). In analogy to Section 3.1, the FC was systematically included or excluded from the formulationAll formulations now contain CP and were prepared according to the protocol given in Section 2.1. Figure 7 (without FC) and Figure 8 (with FC) show different magnifications of Cryo-SEM images obtained upon quenching the reaction after increasing reaction times (approximately 10, 20 and 30 s).

<figure><img src="image_9.png" /><figcaption>Figure 7: Cryo-SEM images of samples of the reacting scientific system in absence of the fluorocarbon frozen after different reaction times at different levels of magnification.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 8: Cryo-SEM images of samples of the reacting scientific system in presence of the fluorocarbon frozen after different reaction times at different levels of magnification. Pink circles correspond to imprints of FC droplets, while pink arrows indicate and FC film.</figcaption></figure>

For the FC-free system (Figure 7), the earliest reaction time successfully sampled was *t* = 12 s (Figure 7.a1–a3) which is before the “cream time” of this system (Table 4; ~ 15 s). As a reminder, the “cream time” [1], [16] is the period between the end of the blending step and the onset of a first foam rise indicating the liberation of blowing agent vapor. Hence, the PUR foam mixture is still in its “infancy” at this point and foam blowing has yet to happen. At this stage, the reacting system does not appear significantly different from the non-reacting system shown in Figure 3 and Figure 4. Again, a continuous matrix is observed with some embedded craters of $\langle r \rangle = (35 \pm 15)$ µm created by air bubbles entrained during the blending of the A- and B-component. This finding is in line with the work by Reignier et al. [14] who also report in their cryo-SEM study that craters observed at this stage of the reaction are most probably due to air entrainment. Note that the mean bubble radius ($\langle r \rangle = 32$ µm) found by Reignier et al. [14] is similar to what we observe. However, further comparison between our results and those reported by Reignier et al. [14] also reveals differences. Reignier et al. [14] reported that a sample comparably early in the reaction progress exhibited a dispersed morphology in the continuous matrix surrounding the air bubbles. The authors assigned this finding to an initial incompatibility of the A- and B-component such that a B-in-A-emulsion is likely to be formed during the blending step [14]. In contrast, the continuous matrix of the reacting system shown in Figure 7.a1–a3 appears uniform. Thus, the A- and B-component of our scientific system are either miscible or compatibilise very quickly, i.e. before 12 s.

With increasing reaction time (Figure 7b and c), the growth of the bubbles can be observed. This growth can be linked to the liberation of CO₂ and cyclopentane vapor (+FC vapor if present) in the course of PUR foam blowing which inflate *only* the pre-
existing air bubbles. We emphasize at this point that this result is also perfectly in line
with recent findings by us [8], [10] or by Brondi et al. [19] who reported that the
nucleation of new bubbles during PUR foam blowing is highly unlikely since the pre-
existing air bubbles absorb the liberated blowing agent vapor+CO₂ (type IV non-classical
nucleation).

In Figure 8 we show results of the same experimental protocol as in Figure 7, yet this
time in presence of 2 wt% FC. Similarly to the FC-free system, a continuous matrix with
some embedded bubble craters was observed at the earliest sampled reaction time (t = 12
s) (Figure 8a1–a3). Compared to the FC-free system, it appears that the characteristic
bubble size is slightly smaller (⟨r⟩ = 25 ± 6 µm), although reliable statistics are lacking
due to the small number of bubbles observed in the reactive systems (typically several
tens of bubbles). The continuous matrix again appears smooth and uniform even at higher
levels of magnification. As in the FC-containing A-component (Section 3.1), Figure 8a1
and a2 show micron-sized craters which originate from dispersed FC-droplets.

Analogously to the FC-free system, a continuous growth of the bubble imprints can be
seen with increasing reaction time. The most interesting observation is the bright, film-
like layer that can be seen at the interface between a bubble and the continuous reactive
matrix in Figure 8.b3, similar to the observations in the A-component only (Figure 3 and
Figure 4), which we interpret as deposition of FC

3.3. Reactive PUR foams - Technical system prepared under industrial conditions

Last but not least, cryo-SEM analysis was also performed with the industrially relevant “technical system” (Table 3). In contrast to the experiments with the “scientific system” they were prepared using a pilot-scale high pressure mixing- and dispensing machine at BASF Polyurethanes GmbH in Lemförde, Germany. We used two different catalyst concentrations in the A-component to vary the reaction kinetics (“slow system”: 0.78 wt.%; “fast system”: 1.55 wt.%; see Table 3). For both systems, PUR foam generation was performed either in absence or presence of 2 wt.% FC. Thus, four different variations of the technical system were studied in total.

Figure 9 and Figure 10 show different magnifications of cryo-SEM images obtained after quenching the reaction at increasing reaction times for the slow (Figure 9) and the fast (Figure 10) system in absence of FC.

Let us discuss first the “slow system” (Figure 9). For the earliest sample (t = 7-8 s, Figure 9.a1-a3), one finds a continuous reactive matrix with some embedded bubble craters similarly to what was observed in the scientific system in Section 1. Note that a reaction time of 7-8 s corresponds to the cream time of the slow system (Table 4). Hence, Figure 9.a1-a3 displays well what the reactive mixture of the A- and B-component looks like at a very early stage of the PUR foam generation process, as foam blowing is just about to start and urethane network formation has not yet progressed noticeably. The fact that the polyols of the A-component and the isocyanate of the B-component have not yet had the chance to react much with each other is further reflected by Figure 9.a2 which nicely exposes dispersed “patches” (indicated by green arrows) in the continuous matrix surrounding the bubbles. Analogously to the observation reported by Reignier et al. [14], these “patches” are emulsified droplets of the B-component in the A-component confirming that the reactive mixture is in fact a “bubbly emulsion” during the very first moments of PUR foam generation. Note that the size of the emulsified (isocyanate)
droplets is of the same order of magnitude as the size of the bubbles ($\langle r \rangle = (10 \pm 5) \mu m$). Compared to the PUR foams prepared in our laboratory, the bubbles entrained in the machine trial are measurably smaller due to the higher energy input during the turbulent blending of the reactive components with the high-pressure mixing- and dispensing machine. As in the case of the scientific system, the entrained air bubbles grow significantly with progressing reaction times due to the liberation of CO$_2$ and CP vapor which diffuse into the pre-existing bubbles. Moreover, the emulsion-like morphology of the continuous matrix surrounding the air bubbles is no longer observed after 14-15 seconds, which suggests that the reaction has sufficiently advanced to compatibilise the reactive components. Except for a continuous growth of the bubbles due to ongoing gas/vapour liberation, no peculiar observations were made with increasing reaction times. Unfortunately, samples could not be drawn successfully for reaction times longer than ~35 seconds, as the nascent PUR foams eventually became too viscous to be transferred into the cryo-SEM sample holder.

In contrast to the “slow system”, the “fast system” shown in Figure 10 exhibits visibly larger bubbles at comparable reaction times. Already for the very first sample acquired after 7-8 seconds, bubbles of $\langle r \rangle = (25 \pm 10) \mu m$ can be observed. However, it must be emphasized that due to the increased catalyst concentration, the cream time occurs earlier for the faster system (~5 seconds; Table 4). As a consequence, PUR foam blowing had already started when the first sample was acquired 7-8 seconds after the start of the PUR foam preparation (Figure 10.a1–a3). Moreover, no emulsion-like morphology of the reacting matrix is observed. Thus, as expected, a higher catalyst concentration also accelerates the compatibilization of the A- and B-component. Other than that, the bubble imprints also grow with progressing reaction times. Note that the latest possible sampling time was 26 seconds. Due to the faster reaction progress, the reacting system quickly became too viscous to be transferred into the cryo-SEM sample holders.

As can undeniably be seen in Figure 9 and Figure 10, the growth of the initially entrained bubbles with progressing PUR foam generation due to the liberation of CO$_2$ and CP vapor is observed for *both* systems. To distinguish more quantitatively how the bubble growth is affected by the catalyst concentration, we analysed the size of the bubble imprints as a function of reaction time for both systems. To obtain a characteristic bubble size despite their irregular shape, we measured the area of the observed (irregular) bubbles (20 on average) and calculated the radius of the equivalent circle. Note that all characteristic bubble sizes were corrected by a factor of 1.27 which accounts for the fact that the sizes of actually three-dimensional bubbles are evaluated using arbitrary two-dimensional cuts of the bubbles [20]. Figure 13 shows the evolution of the average bubble radius as a function of reaction time for the two catalyst concentrations.

Figure 13 reveals some very interesting facts. Firstly, for both systems, to first approximation, the evolution of the mean bubble radius with time in the observable time window seems well described by a linear dependence. Secondly, the growth rate (the slope of the line) is about $3 \mu m s^{-1}$ and seems to depend little on catalyst concentration! The offset of the red line fitted through the characteristic bubble sizes of the slow system (3.9 s) matches approximately the difference in the cream times (Table 4) between the slow (7-8 s) and the fast system (~5 s). Consequently, PUR foam blowing is just induced earlier with increasing catalyst concentration but the factors governing the bubble growth rate are different ones. This observation is in line with the fact that most of the foam blowing is created by the CP and that the growth rate is essentially fixed by the CP diffusion through the matrix [21], [22], [23], [24]. Finally, we emphasize that the
observed linear trend is surprising, since one commonly assumes a $\langle r \rangle \sim t^{1/2}$-dependence [24] as a consequence of the blowing agents having to diffuse through the matrix into the bubbles. One reason for this deviation may be the fact that our system evolves faster than those reported in the literature, leading rapidly to a state in which interactions between bubbles invalidate the assumptions underlying the $t^{1/2}$-predictions [21]. More detailed investigations are therefore necessary to draw conclusions on the reported linera dependencies.

As the simultaneously progressing PUR foam solidification will limit bubble growth at some point, the characteristic bubble size should rapidly converge to a certain final value. This value, in turn, should then correspond to the PUR foam's characteristic pore size, as preceding studies [14], [25], [26], [27] suggest that the bubbles introduced with the initial blending step are sufficient to serve as precursors for all pores found in the final PUR foam by taking up the liberated gases. For our systems, the final pore radii are of the order of 150 µm.

Let us now consider formulations containing FC. Analogously to the FC-free systems, Figure 11 shows the cryo-SEM images obtained for the slow system in presence of 2wt.% FC with progressing reaction time, while Figure 12 shows the cryo-SEM images of the fast system in presence of FC.

Figure 11.a displays again bubble imprints distributed in the reactive mixture of very similar size ($\langle r \rangle = (10 \pm 5) \mu m$) in comparison to the FC-free formulation. While in most images the continuous matrix appears homogeneous in the FC-containing system (Figure 11.a), Figure 11.b3 indicates that emulsified isocyanate droplets are present. This is surprising, because at a comparable reaction time of 14-15s for the FC-free system (Figure 9.b) the emulsified droplets are no longer observed. Besides that, a continuous growth of the air bubbles similarly to the FC-free slow system is observed. However, one observation clearly distinguishes the FC-containing, slow system from its FC-free counterpart. As can be seen in Figure 11.a2 and a3, several bubble imprints show peculiar "lenses" at the bottom of the bubble imprints while black shadows are observed on the opposite end of the imprint. We expect these to be residues of a thin, interfacial FC-film. but the reason why only lenses concentrated at one side of the bubbles are observed may be found in the high density of FC ($\rho = 1.62 \ g \ cm^{-3}$), leading to gravity-driven drainage towards the bottom of the bubble. While the appearance of the black shadows at the opposite end of the bubble is not completely understood, we speculate that this phenomenon is related to the formation of Newton black films or of dewetted zones at the bubble interface due to the drainage of the FC-film towards the bottom. At later stages of the PUR foam generation, the FC-films can no longer be seen, as the fluorocarbon is very volatile and evaporates upon liberation of reaction heat.

Next, let us compare the fast system in presence of FC (Figure 12) to the FC-free reference (Figure 10). Figure 12 reveals the same finding as the comparison between the FC-free and the FC-containing slow systemAs for the slow system, the occurrence of lenses and films at the interface between air bubbles and the surrounding continuous matrix can be linked to the presence of the FC. One might be confused by the fact that in Figure 12 the lenses appear at the top of the air bubbles and not at the bottom as shown in Figure 11. However, this is due to the fact that the orientation of the images with respect to gravity was rotated by 180°.

As before with the fluorocarbon-free systems, we plot the characteristic size of the bubble imprints as a function of reaction time for the slow- and the fast system in presence of FC
in Figure 13.b. A linear growth of the bubble size is suggested for both the slow and the fast system in the first 25 seconds of the reaction. In contrast to the FC-free systems, however, the growth rate is somewhat smaller, namely ~2 µm/s. While for the FC-free systems the linear trend remained intact throughout the studied range of reactions times (Figure 13.a), the bubble growth stagnates after a reaction time of roughly 25 seconds for the FC-containing systems. The reason for this peculiar behaviour is not at all clear to us, since it is known that the bubbles will reach a final radius of about 80-90 µm, i.e. they still need to grow a little beyond the plateau value. One possible explanation may be that the evaporation in the presence of the FC film is different: we know that this film also contains CP due to the partial FC/CP miscibility. Hence the initial evaporation stage may be entirely driven by evaporation from these mixed films in direct contact with the bubble. When the films are consumed, a plateau may arise, which is then followed by the more traditional diffusion from the matrix. While it seems to be a reproducible phenomenon occurring independently of the catalyst concentration, one also needs to consider the very large error bars in this regime, asking for repetition of these measurements to confirm such that the interpretation of the data should be handled with care and suggested trends might not be very reliable. Despite the fact that we cannot yet explain the diagram in Figure 13 in much detail, we can capture nonetheless that the presence of FCs seems to impact the growth of the pre-existing air bubbles.

Last but not least, to compare attentively the cryo-SEM micrographs of the earliest samples of both the slow- and the fast systems, we plot in Figure 14 the density of bubbles (number of bubbles per imaged area) for the different systems. This graph clearly demonstrates that the density of bubbles is indeed significantly increased in the presence of FC for both the slow- and the fast system. Moreover, it appears as if the bubble density is higher for the slow system in general. However, one should keep in mind that all the data shown in Figure 14 was acquired from the earliest samples, i.e. 7-8 seconds into the PUR foam generation process. At this point, bubble inflation due to foam blowing had already started for the fast system (cream time = 5 seconds) but not for the slow system (cream time = 7-8 s). As a consequence, the air bubbles are larger with respect to the observed area for the fast system which impacts the bubble density.

Unfortunately, the type of statistical analysis presented for the technical system could not be carried out for the scientific system. The limited number of bubbles in the lab-produced samples permits only qualitative observations and does not support a robust quantitative statistical treatment. Performing such an analysis would require a significantly larger number of samples—an experimentally demanding task that was beyond the scope of this study.
<figure><img src="image_11.png" /><figcaption>Figure 9: Cryo-SEM images of PUR foams generated under industrial conditions using the technical system with a lower catalyst concentration (“slow system”; Table 3) in the absence of the fluorocarbon (FC). The images show samples frozen after different reaction times at different levels of magnification. Yellow circles show bubble imprints, while green arrows indicate examples of isocyanate drops.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 10: Cryo-SEM images of PUR foams generated under industrial conditions using the technical system with the higher catalyst concentration (“fast system”, Table 3) in absence of the fluorocarbon (FC). The images show samples frozen after different reaction times at different levels of magnification.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 11: Cryo-SEM images of PUR foams generated under industrial conditions using the "technical system" with the lower catalyst concentration ("slow system"; $c_{cat}$ = 0.77 wt.% in A-component) in **presence** of the fluorocarbon. The images show samples frozen after different reaction times at different levels of magnification. Yellow circles indicate bubbles, pink arrows FC depositions and green arrows isocyanate droplets.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Figure 12: Cryo-SEM images of PUR foams generated under industrial conditions using the "technical system" with the higher catalyst concentration ("fast system"; $c_{cat}$ = 1.55 wt.% in A-component) in **presence** of the fluorocarbon. The images show samples frozen after different reaction times at different levels of magnification.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Figure 13: Mean bubble size as a function of reaction time for the “technical system (Table 3) with two different catalyst concentrations (“slow” and “fast” system in Table 3) in (a) the absence of fluorocarbon (FC) and in the presence of 2wt% FC. In (a) The data sets are fitted by a line giving an approximate growth rate of 3 µm/s (slope) while the lines in (b) are guides to the eye giving an initial approximate growth rate of 3 µm/s.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Figure 14: Observed density of bubble imprints (number of bubbles per imaged area) within the liquid reactive matrix 7-8 seconds after ejection from a high pressure mixing- and dispensing machine for both the slow system and the fast system in absence and presence of the fluorocarbon.</figcaption></figure>
# 4 Conclusion

Inspired by the study of Reignier at al. [14] we used Cryo-SEM imaging to investigate the morphology and evolution of the reacting mixture of nascent PUR foams. For this purpose, samples of evolving PUR foam systems were frozen in liquid nitrogen at different moments of the PUR foam generation process to quench all ongoing reactions. We then visualized the morphology of the frozen samples via scanning electron microscopy under cryogenic conditions (cryo-SEM). Having quenched samples of the evolving PUR foams after different reaction times, we could exploit this cryo-SEM approach to follow and reconstruct the processes at play during PUR foam generation.

In contrast to the investigations by Reignier at al. [14], our specific interest was directed towards understanding the influence of the presence of fluorocarbon (FC) in the blend, known to produce smaller pore sizes in the final PUR foam [21], [28]. In previous work we investigated the morphology of the A-component only, using optical microscopy, and its impact on the morphology of the final solid PUR foam [8], [9], [10]. In those studies, we not only confirmed that the addition of FC to PUR foam formulations systematically reduces the characteristic pore size of the final PUR foam, but we also put in evidence some general tendencies with FC concentration. Unfortunately, those PUR foam studies could not provide information on the morphology of the reactive mixture and the associated evolution of the foam – hence this additional piece of work.

First, we studied only the isolated A-component of our simple scientific model system whose A-component contains tripropylene glycol as polyol. We found that the fluorocarbon forms micron-sized droplets within the A-component and that the size of these FC-droplets is diminished when the A-component contains cyclopentane (CP). We argue that this is due to the fact that the FC perfluoro(4-methylpent-2-ene) is partially soluble in CP. Hence, dissolving cyclopentane in the A-component likely “improves” the compatibility with the FC, probably by impacting the interfacial tension and the viscosity ratio of the phases in a way that they can be mixed more easily. We could also show that bubbles (purposely) entrained within the A-component during mixing can be nicely seen as round craters within the frozen, continuous A-component. Moreover, we found that a film-like deposition at the interface between a bubble imprint and the surrounding A-component can be observed only in presence of fluorocarbons. As we show in Section 3.1, the deposition of an interfacial FC-film can be explained by interfacial energy arguments and has been confirmed in the literature for aqueous systems [9], [17].

Next, we investigated reacting PUR foams based on the scientific system by blending the A-component and the B-component (Table 2) to trigger reactive PUR foam generation. The cryo-SEM analysis revealed that the air bubble craters are inflated due to the liberation of CO₂ and cyclopentane vapor during PUR foam generation. This observation goes well in line with previous studies that also report that if bubbles are already present, it is highly unlikely for the liberated gases to nucleate new bubbles due to the very high energy barriers of nucleation processes. Instead, the pre-existing bubbles are blown up via “heterogeneous nucleation” [10], [14], [19], [25]. As for the isolated A-component, the residue of a thin film-like layer at the interface between gas bubbles and the reactive PUR foam mixture was observed only when the FC was added to the PUR foam formulation (left of Figure 15).

Subsequently, we repeated the investigation of nascent PUR foams via cryo-SEM with an industrially relevant system containing a blend of three polyether polyols which we refer to as the
“technical system” (Table 1 and Table 3). While all experimentation concerning the scientific system was conducted on a lab-scale, foams based on the technical system were prepared on a pilot-plant in cooperation with BASF Polyurethanes GmbH. Again, PUR foam generation was followed by drawing samples of nascent PUR foams successively, freezing them in liquid nitrogen, and finally analysing them via cryo-SEM. To the best of our knowledge, we are the first to employ this approach for the investigation of nascent PUR foams prepared under actual industrial conditions. In the scope of this study, we investigated two different catalyst concentrations (“slow system” vs. “fast system”) combined with the presence or absence of FC.

The Cryo-SEM analysis revealed that the reactive components of the “technical system” are initially incompatible and form an emulsion due to the rigorous blending in the high pressure mixing- and dispensing machine. In parallel, bubbles are entrained into the system while the reactive components are being blended. In this context, the characteristic size of the incorporated bubbles and the emulsion droplets is rather similar with radii in the range of 5 – 15 µm. Eventually, very interesting phenomena were observed with progressing PUR foam generation. For instance, the initially emulsion-like reactive matrix becomes smooth and uniform rather soon after the onset of PUR foam blowing and solidification. This is owed to the urethane-forming polyaddition reaction between the polyols of the A-component and the isocyanates of the B-component which leads to a compatibilization of both components. In parallel, the incorporated bubbles are continuously inflated due to the liberation of CO2 and physical blowing agent vapor as was the case for the scientific system. More interestingly, we observed a linear bubble growth with a rate of ~3 µm/s for both catalyst concentrations in absence of FC. This is in contrast with what is commonly expected. One shall note here however that contrary to other reference systems [21], [23], [24], our results were obtained through freezing industrial samples at different reaction times. This complicated sampling leads to a reduced number of datapoints, making the quantitative interpretation of the results complicated. Future work needs to explore these question more quantitatively.

In contrast, the addition of the fluorocarbon affects the bubble growth significantly. While a linear trend with a rate slightly lower than that of the FC-free reference systems ($Δr/Δt \approx 2$ µm/s) was also observed in the early stages of PUR foam generation, a stagnation of the bubble growth occurred after ca. 25 seconds at an average bubble radius of approximately 50 µm. This is surprising, since the bubbles are known to grow again at a later stage to reach final radii of approximately 80-90 µm. An explanation for this behaviour may be given by including the observation of FC films/lenses on the bubble surfaces (Figure 15). Evaporation from these lenses/films (containing also CP due to the partial miscibility) into the growing bubbles is very different to the growth of bubble controlled by the diffusion of the blowing agent through the matrix. Future work needs to investigate these phenomena, ideally with ultra-fast 3D tomography for better statistics and reduced artefacts.
<figure><img src="image_17.png" /><figcaption>Figure 15: Examples of films observed at the interface between an air bubble and the reactive PUR foam mixture in presence of the fluorocarbon for both the scientific system and the technical system.</figcaption></figure>

We also found that the number of bubbles entrained during the blending of the reactive components is increased when the PUR foam system contains the fluorocarbon. We emphasize at this point that (cryo-)SEM is not a perfect tool for studying the number of bubbles within the reactive mixture quantitatively, as only few bubbles can be observed at the same time. However, the bubble counts differ significantly in absence/presence of the fluorocarbon such that an actual impact of the fluorocarbon on the entrainment of air bubbles during blending of the reactive components should not be ruled out. This is insofar important, as different studies [14], [19], [26], [27] recently pointed out the crucial importance of bubbles entrapped during the initial blending of the reactive components on the formation of the porous PUR foam morphology.

Future studies therefore have to investigate in what way the FC impacts the initial air entrainment and whether the presence of the FC films on the bubble surfaces plays a role in the foaming process. In the case of aqueous foams, for example, it has been shown that the FC improves significantly foam stability [17], [18].

# 5 Acknowledgements

The authors thank the PLAMICS and InCa platforms of the Institut Charles Sadron for their close collaboration on this project.

# 6 Declarations

## 6.1 Funding

The authors gratefully acknowledge the financial support granted by BASF SE for this study. This work was also partly financed by an ERC Consolidator Grant (agreement 819511 – METAFOAM. It was conducted in the framework of the Interdisciplinary Institute HiFunMat, as part of the ITI
2021–2028 program of the University of Strasbourg, CNRS and Inserm, supported by the IdEx Unistra (ANR-10-IDEX-0002) and SFRI (STRATUS project, ANR-20-SFRI-0012) under the framework of the French Investments for the Future Program.

## 6.2 Author contributions

M.H., Marc. S., A.C., W.D and Markus.S. designed the study; D. T. adviced on formulations; M.H. and A.C. performed the experimental work; M.H. performed the analysis of the data; M.H., L.F. and W.D. interpreted the experimental results; M.H., L.F. and W.D. wrote the first draft of the manuscript; all authors proofread and amended the different manuscript versions and validated the final version; W.D. and Markus S. acquired the funding and supervised the overall project.

## 6.3 Other declarations

Ethics, Consent to Participate, and Consent to Publish declarations: not applicable.

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