Ambient Temperature Self-Blowing Tannin-Humins Biofoams

Ambient temperature self-blowing tannin–furanic foams have been prepared by substituting a great part—even a majority—of furfuryl alcohol with humins, a polyfuranic material derived from the acid treatment at high temperature of fructose. Closed-cell foams were prepared at room temperature and curing, while interconnected-cell foams were prepared at 80 °C and curing, this being due to the more vigorous evaporation of the solvent. These foams appear to present similar characteristics as other tannin–furanic foams based only on furfuryl alcohol. A series of tannin–humins–furfuryl alcohol oligomer structures have been defined indicating that all three reagents co-react. Humins appeared to react well with condensed tannins, even higher molecular weight humins species, and even at ambient temperature, but they react slower than furfuryl alcohol. This is due to their high average molecular weight and high viscosity, causing their reaction with other species to be diffusion controlled. Thus, small increases in solvent led to foams with less cracks and open structures. It showed that furfuryl alcohol appears to also have a role as a humins solvent, and not just as a co-reagent and self-polymerization heat generator for foam expansion and hardening. Stress-strain for the different foams showed a higher compressive strength for both the foam with the lowest and the highest proportion of humins, thus in the dominant proportions of either furfuryl alcohol or the humins. Thus, due to their slower reactivity as their proportion increases to a certain critical level, more of them do proportionally participate within the expansion/curing time of the foam to the reaction.

Formulations 1 (control) 2 and 3 were self-blown and cured at ambient temperature (24 • C), while formulation 4 was cured at 80 • C to qualitatively observe the difference in morphology between foam 3 and foam 4. Their differences in composition were used to ensure a stronger cell walls for foam 4, as broken cell walls were expected by curing at 80 • C.
After these initial formulations the systematic study that followed used the formulations shown in Table 1. All the formulations were self-blown at ambient temperature (23 • C). Table 1. The formulation of tannin-humins foams.

MALDI-ToF Analysis
Samples for matrix-assisted laser desorption ionization time-of-flight (MALDI-ToF) analysis were prepared first by dissolving 5 mg of sample powder in 10 mL of a 50:50 v/v acetone/water solution. Then 10 mg of this solution was added to 10 µL of a 2,5-dihydroxy benzoic acid (DHB) matrix to obtain a homogeneous solution. The locations dedicated to the samples on the analysis plaque were first covered with 2 µL of a NaCl solution 0.1 M in 2:1 v/v methanol/water, and pre-dried. Then, 1 µL of the sample solution was placed on its dedicated location and the plaque was dried again. The reference substance used for the equipment calibration was red phosphorus. MALDI-ToF spectra were obtained using an Axima-Performance mass spectrometer from Shimadzu Biotech (Kratos Analytical Shimadzu Europe Ltd., Manchester, UK) using a linear polarity positive tuning mode. The measurements were carried out making 1000 profiles per sample with two shots accumulated per profile. The spectra precision is of ±1 Da.
For the MALDI ToF analysis the foam sample was ground to a very fine powder homogeneously covered on the target point of dedicated analysis plaque through series of steps before testing. The target testing spots were first covered with the 1.5 µL of a 0.1 M NaCl (in a methanol:water mixture (1:1)) and then completely dried. Secondly, 7.5 mg of samples were dissolved into a 1 mL 50:50 v/v water/acetone mixture solution. Then, 1.5 µL of this solution was mixed with 1.5 µL of DHB (2,5-dihydroxy benzoic acid) matrix to obtain a homogenous solution. Finally, 1.5 µL of sample-DHB solution were placed on the corresponding testing spots, and dried completely. Before each test, the device was calibrated by using as reference red phosphorous. The measurements were carried out making 1000 profiles per sample with two shots accumulated per profile. The spectra precision is of ±1 Da.

FTIR
Fourier transform infra-red (FTIR) analysis was carried out using a Shimadzu IR Affinity-1 (Shimadzu Europe Ltd., Manchester, UK) spectrophotometer. A blank sample tablet of potassium bromide, ACS reagent from ACROS Organics (Geel, Belgium), was prepared for the reference spectra. Similar tablets were prepared by mixing potassium bromide with 5% by weight of the sample powders. The spectra were plotted in percentage transmittance by combining 32 scans with a resolution of 2.0 cm −1 in the 400-4000 cm −1 range.

Apparent Density
According to the standard method of ASTM D1622-08, all testing foam samples were prepared to a size of 30 × 30 × 30 mm 3 . The ratio of weight to cubic volume of the specimen volume was defined as density. Five sample repeats were tested for each foam.

Scanning Electron Microscopy (SEM) Analysis
Scanning electron microscopy (SEM, Hitachi TM-3000) (Milexia, Paris, France) was used to analyze the microstructure and morphology of the foams obtained. All samples were made into 0.5 cm 2 (cross section). Then, a thin layer of gold-palladium was sputtered on the surface of the foams so that a better definition could be obtained.

Compression Strength
The samples were cut into a uniform size of 25 × 25 × 25 mm. A universal testing machine (Instron 3300, Elancourt, France) was used to test the compression strength of the foams. The direction of load was parallel to that of the foam rise under ambient conditions. The crosshead rate was fixed at 2.0 mm·min −1 . At least three sample repeats were tested for each foam.

Results and Discussion
Humins are known to have a polyfuranic structure [21][22][23]. However, they are a mixture of a variety of polyfuranic oligomers rather than possessing a unitary polyfuranic structure. To further clarify the types of oligomers present in the humins used for foam formulation, and to ascertain their suitability for this task, MALDI ToF analysis was carried out. Figures 1-6 report the more significant MALDI ToF spectra (more spectra are presented in the Supplementary Material) and Table 2 reports the assigned structures that could be interpreted from the spectra. From Table 2 it can be noticed that in the mix derived from the hydrolysis of fructose either at 180 or 150 • C there are a considerable number of small furan-derived monomers, dimers and trimers, of which in particular structures of type I (                            Two initial tannin-humins-FA formulations were prepared including two furanic foams controls. The two tannin-humins-FA were expanded and hardened one at ambient temperature (23 °C) and the other at 80 °C to see how preparation temperature influenced the morphology of the finished foams. The differences observed by scanning electron microscopy (SEM) were indeed quite major as can be seen in Figure 7. The foam prepared at ambient temperature presented a closed cells structure while the one prepared at 80 °C clearly presented an interconnected cells structure, with a number of open pores and cell walls breaks. The reason for this difference is due to the more vigorous evaporation of water at 80 °C breaking weaker cell wall sites in the structure. This does not occur at ambient temperature. This means that a foam of this type prepared at ambient temperature, once stabilized, is more suitable for thermal isolation applications, while when prepared at 80 °C it I more

Da With Na+
Polymers 2020, 12, x FOR PEER REVIEW 11 of 29 Two initial tannin-humins-FA formulations were prepared including two furanic foams controls. The two tannin-humins-FA were expanded and hardened one at ambient temperature (23 °C) and the other at 80 °C to see how preparation temperature influenced the morphology of the finished foams. The differences observed by scanning electron microscopy (SEM) were indeed quite major as can be seen in Figure 7. The foam prepared at ambient temperature presented a closed cells structure while the one prepared at 80 °C clearly presented an interconnected cells structure, with a number of open pores and cell walls breaks. The reason for this difference is due to the more vigorous evaporation of water at 80 °C breaking weaker cell wall sites in the structure. This does not occur at ambient temperature. This means that a foam of this type prepared at ambient temperature, once suitability for this task, MALDI ToF analysis was carried out. Figures 1-6 report the more significant MALDI ToF spectra (more spectra are presented in the Supplementary Material) and Table 2 reports the assigned structures that could be interpreted from the spectra. From Table 2 it can be noticed that in the mix derived from the hydrolysis of fructose either at 180 or 150 °C there are a considerable number of small furan-derived monomers, dimers and trimers, of which in particular structures of type I (139 Da Peak), II (203 Da peak) and III (284 Da peak) and their derivates are examples (Scheme 1) Small furanic species present in the humins used.
More complex structures such as IV and V, both corresponding to the 322 Da peak, already previously identified [22] have also been confirmed (Scheme 2). Scheme 1. Small furanic species present in the humins used.
More complex structures such as IV and V, both corresponding to the 322 Da peak, already previously identified [22] have also been confirmed (Scheme 2). Furanic structures still linked to condensed fructose moieties also appear to be present such as structure VI, assigned to the peak at 392 Da, also already previously identified [22] (Scheme 3). It is the progression from structures I, II and III that however brings to the more classical representation of the polyfuranic structure of acid-derived fructose humins, this being represented among others by the peaks at 1227, 1250 and 1351 Da (VII), such as in Scheme 4 and higher more complex oligomers. This served to control that the humins prepared presented the polyfuranic structure expected and needed to formulate the tannin-humins foams.  Furanic structures still linked to condensed fructose moieties also appear to be present such as structure VI, assigned to the peak at 392 Da, also already previously identified [22] (Scheme 3). Furanic structures still linked to condensed fructose moieties also appear to be present such as structure VI, assigned to the peak at 392 Da, also already previously identified [22] (Scheme 3). It is the progression from structures I, II and III that however brings to the more classical representation of the polyfuranic structure of acid-derived fructose humins, this being represented among others by the peaks at 1227, 1250 and 1351 Da (VII), such as in Scheme 4 and higher more complex oligomers. This served to control that the humins prepared presented the polyfuranic structure expected and needed to formulate the tannin-humins foams.  It is the progression from structures I, II and III that however brings to the more classical representation of the polyfuranic structure of acid-derived fructose humins, this being represented among others by the peaks at 1227, 1250 and 1351 Da (VII), such as in Scheme 4 and higher more complex oligomers. This served to control that the humins prepared presented the polyfuranic structure expected and needed to formulate the tannin-humins foams. Furanic structures still linked to condensed fructose moieties also appear to be present such as structure VI, assigned to the peak at 392 Da, also already previously identified [22] (Scheme 3). It is the progression from structures I, II and III that however brings to the more classical representation of the polyfuranic structure of acid-derived fructose humins, this being represented among others by the peaks at 1227, 1250 and 1351 Da (VII), such as in Scheme 4 and higher more complex oligomers. This served to control that the humins prepared presented the polyfuranic structure expected and needed to formulate the tannin-humins foams.  Two initial tannin-humins-FA formulations were prepared including two furanic foams controls. The two tannin-humins-FA were expanded and hardened one at ambient temperature (23 • C) and the other at 80 • C to see how preparation temperature influenced the morphology of the finished foams. The differences observed by scanning electron microscopy (SEM) were indeed quite major as can be seen in Figure 7. The foam prepared at ambient temperature presented a closed cells structure while the one prepared at 80 • C clearly presented an interconnected cells structure, with a number of open pores and cell walls breaks. The reason for this difference is due to the more vigorous evaporation of water at 80 • C breaking weaker cell wall sites in the structure. This does not occur at ambient temperature. This means that a foam of this type prepared at ambient temperature, once stabilized, is more suitable for thermal isolation applications, while when prepared at 80 • C it I more suitable for acoustic insulation [12]. The foams in Table 1 were prepared to observe the effect of the differences in formulation on foam characteristics. The influence of small differences in the proportion of the water present and of the relative differences in humins to furfuryl alcohol proportions were studied. The results obtained, shown in Table 3 and Figures 8-11, indicated remarkable differences in behavior or structure and characteristics. SEM observation (Figures 8-10) showed that: (i) a small variation in the proportion of water (Figure 8a,b) does not appear to change much the structure of the foam when cured at ambient temperature; (ii) a decrease in the proportion of furfuryl alcohol, even if not major, appears to cause some cell wall rupture (Figure 9a,b) and a more open cells structure, even if the apparent uniformity of the cell wall still appears rather uniform and strong; (iii) a variation in the relative proportions of humins to furfuryl alcohol shows instead much more apparent structural differences (Figure 10 a-d): when the proportion of humins is low and FA is dominant, a closed cell structure predominates (Figure 10a). This at first transforms itself in a slightly more porous structure at the following two higher levels of humins (Figure 10b,c) proportion to return to a more closed cell structure at the highest relative proportions of humins used (Figure 10d). The foams in Table 1 were prepared to observe the effect of the differences in formulation on foam characteristics. The influence of small differences in the proportion of the water present and of the relative differences in humins to furfuryl alcohol proportions were studied. The results obtained, shown in Table 3 and Figures 8-11, indicated remarkable differences in behavior or structure and characteristics. SEM observation (Figures 8-10) showed that: (i) a small variation in the proportion of water (Figure 8a,b) does not appear to change much the structure of the foam when cured at ambient temperature; (ii) a decrease in the proportion of furfuryl alcohol, even if not major, appears to cause some cell wall rupture (Figure 9a,b) and a more open cells structure, even if the apparent uniformity of the cell wall still appears rather uniform and strong; (iii) a variation in the relative proportions of humins to furfuryl alcohol shows instead much more apparent structural differences (Figure 10a-d): when the proportion of humins is low and FA is dominant, a closed cell structure predominates ( Figure 10a). This at first transforms itself in a slightly more porous structure at the following two higher levels of humins (Figure 10b,c) proportion to return to a more closed cell structure at the highest relative proportions of humins used (Figure 10d).         These SEM observations explain the results of compression strength observed in Figure 11, and derives from the results shown in Table 3. Thus, in Figure 11 the curves of stress-strain of the different foams indicate higher compression strength as a function of strain for both the foam with the lowest and the highest proportion of humins in relation to furfuryl alcohol, and lower compression strength for the two intermediate humins proportion cases. This indicates that humins appear to be, in general, either less or slower reacting than furfuryl alcohol at ambient temperature by the time the foam is set, but that as their proportion increases to a certain critical level more of them do proportionally participate within the expansion/curing time of the foam to the reaction to contribute to its strength characteristics. This observation infers that, if moderately higher foaming/curing temperatures are used, humins would most likely participate to a greater measure to the strength of the cell walls, but These SEM observations explain the results of compression strength observed in Figure 11, and derives from the results shown in Table 3. Thus, in Figure 11 the curves of stress-strain of the different foams indicate higher compression strength as a function of strain for both the foam with the lowest and the highest proportion of humins in relation to furfuryl alcohol, and lower compression strength for the two intermediate humins proportion cases. This indicates that humins appear to be, in general, either less or slower reacting than furfuryl alcohol at ambient temperature by the time the foam is set, but that as their proportion increases to a certain critical level more of them do proportionally participate within the expansion/curing time of the foam to the reaction to contribute to its strength characteristics. This observation infers that, if moderately higher foaming/curing temperatures are used, humins would most likely participate to a greater measure to the strength of the cell walls, but would also most likely present a predominant interconnected cells structure. These conclusions are confirmed and supported by the results for overall foam density, compression strength and particularly specific compression strength in Table 2, where in effect the overall density at first progressively decreases from the H2.0 to the H3.7 and H4.5 (humins percentage on total furanic material of respectively 25%, 38% and 43%), and then progressively increases passing from the H4.5 to the H5.5 and the H5.9 (humins percentage on total furanic material of respectively 43%, 48% and 54%) all these parameters peaking for the H5.9. The compression strength and specific compression strength do follow the same trend as the density and of the strength/strain curves in Figure 11.
Polymers 2020, 12, x FOR PEER REVIEW 15 of 29 confirmed and supported by the results for overall foam density, compression strength and particularly specific compression strength in Table 2, where in effect the overall density at first progressively decreases from the H2.0 to the H3.7 and H4.5 (humins percentage on total furanic material of respectively 25%, 38% and 43%), and then progressively increases passing from the H4.5 to the H5.5 and the H5.9 (humins percentage on total furanic material of respectively 43%, 48% and 54%) all these parameters peaking for the H5.9. The compression strength and specific compression strength do follow the same trend as the density and of the strength/strain curves in Figure 11. However, it is interesting to observe the effect of a small increase of water on the H3.7 formulation, for which the density increases slightly but leads to an impressive increase in compression strength and more noticeably of specific compression strength. This result, supported by the SEM observations, indicate that the apparent lower reactivity of humins is due, partly or even mainly, to the hindrance to its reaction consequence of its high viscosity. The reaction is then clearly diffusion-controlled, a drawback that can be partially be eliminated or decreased by adding small proportions of a solvent (in this case water). The picture that is then obtained about the use of humins for tannin-humins biofoams is that a higher proportion of humins can be used to replace in the formulation furfuryl alcohol-if small to moderate proportions of a suitable solvent, for example water, are added. An increase in temperature can also help in decreasing the humins viscosity, but while curing the reactivity/mobility problem of the material will also contribute to increase the proportion of open cells in the foam structure. It is interesting too that the compression stress at >1 MPa is rather good, and it is higher than the majority of the expanded polystyrene foams used for isolation, and higher than for the foams of humins alone [28].
An example of the appearance of the foams prepared is shown in Figure 12. The appearance of the initial exploratory foams is also shown in Figure SM3 in the Supplementary Material. However, it is interesting to observe the effect of a small increase of water on the H3.7 formulation, for which the density increases slightly but leads to an impressive increase in compression strength and more noticeably of specific compression strength. This result, supported by the SEM observations, indicate that the apparent lower reactivity of humins is due, partly or even mainly, to the hindrance to its reaction consequence of its high viscosity. The reaction is then clearly diffusion-controlled, a drawback that can be partially be eliminated or decreased by adding small proportions of a solvent (in this case water). The picture that is then obtained about the use of humins for tannin-humins biofoams is that a higher proportion of humins can be used to replace in the formulation furfuryl alcohol-if small to moderate proportions of a suitable solvent, for example water, are added. An increase in temperature can also help in decreasing the humins viscosity, but while curing the reactivity/mobility problem of the material will also contribute to increase the proportion of open cells in the foam structure. It is interesting too that the compression stress at >1 MPa is rather good, and it is higher than the majority of the expanded polystyrene foams used for isolation, and higher than for the foams of humins alone [28].
An example of the appearance of the foams prepared is shown in Figure 12. The appearance of the initial exploratory foams is also shown in Figure SM3  The FT-IR of the raw humins and of the different humins-FA mixes show some interesting features as well ( Figure 13). The raw humins show the peaks at 1038 cm −1 and the very broad peak between 3000 and 3600 cm −1 of hydrogen bonded alcohol groups. The three peaks at around 1700 cm −1 correspond to aldehyde groups under different surrounding conditions, while the 1500 cm −1 peak is assigned to alkane groups asymmetric stretching, and the peak at 800 cm −1 to alkene groups present in structures as that of the 1351 Da peak in the MALDI ToF spectra. In the experimental humins-FA mixes the peak that appear superimposed to the humins groups are the 1310 cm −1 and 1029 cm −1 assigned to the stretching of the hydrogen-bonded alcohol function of furfuryl alcohol, which appears just very near to the 1038 cm −1 peak of humins, The furanic structures C=C groups of both humins and furfuryl alcohol both appear at around 1200 cm −1 with two vicinal sharp peaks, the smaller assigned to the humins and the taller and sharper to the furfuryl alcohol.
It is of interest to observe the results of the MALDI ToF spectra of the finished foams, to see if the three main constituents-mainly tannin extract, furfuryl alcohol and humins oligomers-have jointly reacted. The list of the compounds identified for the F-4.5H+6FA+1.5W in Figure 14a-f is shown in Table 4. Several compounds and oligomers present in the MALDI spectra of the humins alone are also present in the MALDI spectra of the tannin-humins-FA foam, such as the ones at 621, 635, 1009, 1035, 1136 Da, and others. Additionally, present are the characteristic peaks of unreacted flavonoid monomers and dimers such as 304 and 327Da, both for a gallocatechin monomer without and with Na+, 604 Da for a robinetinidin or catechin dimer with Na+, 607 Da for a gallocatechin dimer without Na+, 897 Da for a (gallocatechin)2-robinetinidin trimer without Na+, 910 Da for a gallocatechin trimer without Na+, etc. However, there are a considerable number of peaks assigned to reaction products of flavonoid monomers and dimers with humins smaller molecular weight species as well as with both humins species and furfuryl alcohol. Some of the more significative ones will be discussed here (Scheme 5). The FT-IR of the raw humins and of the different humins-FA mixes show some interesting features as well ( Figure 13). The raw humins show the peaks at 1038 cm −1 and the very broad peak between 3000 and 3600 cm −1 of hydrogen bonded alcohol groups. The three peaks at around 1700 cm −1 correspond to aldehyde groups under different surrounding conditions, while the 1500 cm −1 peak is assigned to alkane groups asymmetric stretching, and the peak at 800 cm −1 to alkene groups present in structures as that of the 1351 Da peak in the MALDI ToF spectra. In the experimental humins-FA mixes the peak that appear superimposed to the humins groups are the 1310 cm −1 and 1029 cm −1 assigned to the stretching of the hydrogen-bonded alcohol function of furfuryl alcohol, which appears just very near to the 1038 cm −1 peak of humins, The furanic structures C=C groups of both humins and furfuryl alcohol both appear at around 1200 cm −1 with two vicinal sharp peaks, the smaller assigned to the humins and the taller and sharper to the furfuryl alcohol.
It is of interest to observe the results of the MALDI ToF spectra of the finished foams, to see if the three main constituents-mainly tannin extract, furfuryl alcohol and humins oligomers-have jointly reacted. The list of the compounds identified for the F-4.5 H + 6 FA+1.5 W in Figure 14a-f is shown in Table 4. Several compounds and oligomers present in the MALDI spectra of the humins alone are also present in the MALDI spectra of the tannin-humins-FA foam, such as the ones at 621, 635, 1009, 1035, 1136 Da, and others. Additionally, present are the characteristic peaks of unreacted flavonoid monomers and dimers such as 304 and 327 Da, both for a gallocatechin monomer without and with Na+, 604 Da for a robinetinidin or catechin dimer with Na+, 607 Da for a gallocatechin dimer without Na+, 897 Da for a (gallocatechin) 2 -robinetinidin trimer without Na+, 910 Da for a gallocatechin trimer without Na+, etc. However, there are a considerable number of peaks assigned to reaction products of flavonoid monomers and dimers with humins smaller molecular weight species as well as with both humins species and furfuryl alcohol. Some of the more significative ones will be discussed here                       The peak at 411 Da is assigned to structure VIII above. Examples of the structures of some of the ones that follows in the series are 605 Da (XIV) and 798 Da (XV) without Na+. The species at 1008 Da (XVI) with Na+ is part of the same series. It is clear that this series of oligomers is formed by fragments of increasingly higher molecular weight humins having reacted with flavonoids (Scheme 8).
Equally the series 393, 587, 781, 975, 1169, 1363, 1558 Da are also separated by the same repeating unit above, this being a series of lesser peaks. This series comes exclusively from the reaction of smaller humins molecules to form the complex and sizable polyfuranic structure of the humin polymer. The first term of this series (393 Da, XVII) is shown in Scheme 9. The peak at 411 Da is assigned to structure VIII above. Examples of the structures of some of the ones that follows in the series are 605 Da (XIV) and 798 Da (XV) without Na+. The species at 1008 Da (XVI) with Na+ is part of the same series. It is clear that this series of oligomers is formed by fragments of increasingly higher molecular weight humins having reacted with flavonoids (Scheme 8 To arrive finally to some of the higher molecular weight structures such as structure XVIII (1945 Da) where a rather sizeable humins oligomer has linked to a flavonoid dimer onto which are already linked some smaller molecular weights humins compounds (Scheme 10). The peak at 411 Da is assigned to structure VIII above. Examples of the structures of some of the ones that follows in the series are 605 Da (XIV) and 798 Da (XV) without Na+. The species at 1008 Da (XVI) with Na+ is part of the same series. It is clear that this series of oligomers is formed by fragments of increasingly higher molecular weight humins having reacted with flavonoids (Scheme 8 To arrive finally to some of the higher molecular weight structures such as structure XVIII (1945 Da) where a rather sizeable humins oligomer has linked to a flavonoid dimer onto which are already linked some smaller molecular weights humins compounds (Scheme 10).

Scheme 9.
Example of reaction of simpler humins structure capable of reacting with itself to form more complex humins networks.
To arrive finally to some of the higher molecular weight structures such as structure XVIII (1945 Da) where a rather sizeable humins oligomer has linked to a flavonoid dimer onto which are already linked some smaller molecular weights humins compounds (Scheme 10).
All the above indicates clearly that there is reaction between the humins oligomers and the tannin flavonoids, and principally that any humins furanic hydroxymethyl and aldehyde group, even on very high molecular weight humins oligomers, can react with the flavonoid tannin sites. The relatively high molecular weight species in the humins mix does then explain why its reaction with the tannin is slower than that of furfuryl alcohol. In the case of humins, as already remarked above, it is their bulky molecular size that determines that their reaction is heavily diffusion controlled, hence slower [33,34].
The reaction of tannin with furfuryl alcohol is particularly well studied and defined [35] and the mixed flavonoid-furfuryl alcohol-humins oligomers attest that all the three species co-react. All the above indicates clearly that there is reaction between the humins oligomers and the tannin flavonoids, and principally that any humins furanic hydroxymethyl and aldehyde group, even on very high molecular weight humins oligomers, can react with the flavonoid tannin sites. The relatively high molecular weight species in the humins mix does then explain why its reaction with the tannin is slower than that of furfuryl alcohol. In the case of humins, as already remarked above, it is their bulky molecular size that determines that their reaction is heavily diffusion controlled, hence slower [33,34]. The reaction of tannin with furfuryl alcohol is particularly well studied and defined [35] and the mixed flavonoid-furfuryl alcohol-humins oligomers attest that all the three species co-react.
The above considerations explain the not ruptured morphology of the foam caused by even a very small increase in solvent. It indicates that humins foams will react better if their viscosity is lowered, either by moderately increasing their temperature, or by adding relatively low proportions of suitable solvents.
It explains why the use of slightly higher proportions of water have this effect, and why furfuryl alcohol works also as an adequate solvent to mobilize humins. When furfuryl alcohol is used not only as a reagent but also as a humins solvent, what must be considered is the relative balance of the two materials: too low a proportion of furfuryl alcohol will increase the effect of diffusion hindrance to the reaction of the humins decreasing their reaction capability, while too much of the more reactive furfuryl alcohol may limit the role of the humins in the co-reaction.

Conclusions
Ambient temperature self-blowing tannin-furanic foams, with both closed and interconnected cells, in which a great part of furfuryl alcohol has been substituted by humins, a polyfuranic material derived from the acid treatment at high temperature of fructose, have been prepared. Ambient temperature foam expansion yielded closed cells foams, while the use of a higher curing temperature (80 °C) yielded open cells foams. It must be pointed out the difference in calling some foams "opencell" rather than "interconnected cells" foams. This difference is important, as foams having an interconnected cellular structure may combine some of the advantages of closed-cell foams (mechanical performance) and open-cell foams (absorption, acoustic applications).
They appear to present the equivalent characteristics of tannin-furanic foams prepared previously, such as self-extinguishing fire resistance, compression strength but with the advantage of a greater ease of close cells formation. The reaction of condensed tannin, furfuryl alcohol and humins has been shown to occur by the oligomers mix formed, namely tannin-humins, tanninfurfuryl alcohol and in particular tannin-humins-furfuryl alcohol, oligomer types and their structures having been determined. Humins have been shown to react well with condensed tannins, even at ambient temperature, but the reaction is slowed down by their high viscosity in relation to that of tannin with furfuryl alcohol. While even higher molecular weight humins species do appear Scheme 10. Example of higher molecular weight structure in the foams derived from the reaction of humins higher and lower molecular weight oligomers with flavonoid oligomers of the tannin.
The above considerations explain the not ruptured morphology of the foam caused by even a very small increase in solvent. It indicates that humins foams will react better if their viscosity is lowered, either by moderately increasing their temperature, or by adding relatively low proportions of suitable solvents.
It explains why the use of slightly higher proportions of water have this effect, and why furfuryl alcohol works also as an adequate solvent to mobilize humins. When furfuryl alcohol is used not only as a reagent but also as a humins solvent, what must be considered is the relative balance of the two materials: too low a proportion of furfuryl alcohol will increase the effect of diffusion hindrance to the reaction of the humins decreasing their reaction capability, while too much of the more reactive furfuryl alcohol may limit the role of the humins in the co-reaction.

Conclusions
Ambient temperature self-blowing tannin-furanic foams, with both closed and interconnected cells, in which a great part of furfuryl alcohol has been substituted by humins, a polyfuranic material derived from the acid treatment at high temperature of fructose, have been prepared. Ambient temperature foam expansion yielded closed cells foams, while the use of a higher curing temperature (80 • C) yielded open cells foams. It must be pointed out the difference in calling some foams "open-cell" rather than "interconnected cells" foams. This difference is important, as foams having an interconnected cellular structure may combine some of the advantages of closed-cell foams (mechanical performance) and open-cell foams (absorption, acoustic applications).
They appear to present the equivalent characteristics of tannin-furanic foams prepared previously, such as self-extinguishing fire resistance, compression strength but with the advantage of a greater ease of close cells formation. The reaction of condensed tannin, furfuryl alcohol and humins has been shown to occur by the oligomers mix formed, namely tannin-humins, tannin-furfuryl alcohol and in particular tannin-humins-furfuryl alcohol, oligomer types and their structures having been determined. Humins have been shown to react well with condensed tannins, even at ambient temperature, but the reaction is slowed down by their high viscosity in relation to that of tannin with furfuryl alcohol. While even higher molecular weight humins species do appear to react with tannin, their lower rate of reaction is due to their rather high molecular weight, hence their high viscosity, causing the reaction to mainly be diffusion controlled. This feature appears to explain why even small increases in solvent proportions improve the results, and indicates the additional role of furfuryl alcohol as humins solvent and not just as a co-reagent contributing to both cross-linking and to the heat increase leading to foam expansion and its hardening.
Funding: This research received no external funding.