CHEMICAL MODIFICATION OF TANNIN/FURANIC RIGID FOAMS BY ISOCYANATES AND POLYURETHANES

Tannin/furanic rigid foams were modifi ed by either polymeric 4,4’ diphenylmethane diisocyanate (pMDI) or a one-component polyurethane still presenting reactive isocyanate groups (PUR). Several diff erent sequences of mixing the isocyanate and polyurethane with the chemicals used to prepare tannin-based foams were tested. Scanning electron microscopy (SEM) showed that addition of either pMDI or PUR yielded a more ordered microstructure of the foam and a larger cell size. Series of two diff erent cell sizes were noticed: those obtained by the evaporation of the blowing agent used for classical tannin-based foams and those caused by the formation of CO 2 issued by the reaction of the isocyanate groups with water. Th e cell size, density, thermal conductivity and mechanical property of tannin/furanic foams were controlled by changing the amount of blowing agent and weight fraction of pMDI. Th e cell size of pMDI modifi ed foams was largest when the weight fraction of pMDI was 5% and kept decreasing as the pMDI percentage became higher. Th e density of the foams increased with the increase of the weight fraction of pMDI. Th e addition of pMDI increased slightly the thermal conductivity of tannin/furanic foam with the foams still presenting good thermal insulation. Moreover, the addition of pMDI in tannin/furanic foams improved markedly the foam’s mechanical properties.


INTRODUCTION
Tannin/furanic rigid foams are polymeric materials used in the eld of packaging, crash protection, catalyst supports and metal ion adsorption (Meikleham and Pizzi 1994, Pizzi et al. 2008, Tondi and Pizzi 2009, Tondi et al. 2009a,b,c,d).Because of their valuable characteristics of good re resistance and low thermal conductivity, Tannin/furanic rigid foams are particularly useful as insulating materials (Tondi et al. 2008(Tondi et al. , 2009a)).Tannin/furanic rigid foams are networked structures obtained from a highly viscous liquid phase composed of a tannin-formaldehyde resin mixed with furfuryl alcohol (FA).FA is used both for the heat generation due to the exothermic self-condensation and for its polycondensation with the tannin and formaldehyde under acid conditions (Meikleham and Pizzi 1994, Choura et al 1996).A er adding the acid reaction catalyst the foaming process starts and is completed within 2-5 minutes.e expansion to a low density foam of the viscous liquid is caused by the evaporation of a low temperature boiling solvent during the exothermic reaction.Controlled pyrolysis of such materials yields a reticulated vitreous carbon (RVC) having new properties, such as electrical conductivity, higher mechanical strength and even improved resistance to very high temperatures and chemicals (Tondi et al. 2009c,d).us, the applications may be extended to porous electrodes, catalytic lters or in carbon composites.
Polymeric 4,4' diphenylmethane disocyanate (pMDI) and one-component polyurethane still presenting reactive isocyanate groups (PUR) could react during foaming process of tannin/furanic system (Frisch et al. 1983), thus could modify the skeleton and microstructure of tannin/furanic rigid foams.For the foamed materials, not only the nature of the skeleton material properties, but also the microstructure of the foam plays a very important role on the properties of the foams, such as insulation, compressive strength and Young's modulus.So in this paper, pMDI and PUR were added in order to improve these properties of our foams.However, the addition of pMDI and PUR could highly improve the viscosity of the system.Especially when pMDI is added, the mixture acquires an almost gel-like state, which makes rather di cult to mix the foam chemical components.In order to get foams more homogeneous, of lower thermal conductivity and higher compressive strength, ve di erent mixing sequences were tried.

Materials
Commercial tannin, obtained by spray drying of aqueous Mimosa (Acacia mearnsii formerly mollissima, De Wildt) bark extracts and containing 84% phenolic material (Silva Chimica, St.Michele Mondovi, Italy), was used to synthesize the foams.e single component polyurethane (PUR) used was Mirapur 9521 (Geitslich, Schlieren, Switzerland) and the polymeric pMDI of viscosity 199 mPa .s at 25 o C was from DOW Chemicals.e formulation used is shown in table 1. para-Toluene sulphonic acid (pTSA) was added as a 65% water solution and the formaldehyde we used is 37% water solution.Diethyl ether (DE) was used as the blowing agent

Preparation of the foams
Di erent mixing sequences of the materials were tried to optimize the results obtained.e ones that gave best results were as follows: Mixing sequence 1.Tannin and pMDI were rst mixed together.A er stirring by a general blender, the mixture of FA, formaldehyde, water and DE were added.e whole mixture was stirred strongly by the blender and a er 20 s pTSA was added followed by 20 s strong stirring by the blender.
Mixing sequence 2. FA, formaldehyde, water DE, and pMDI were rst mixed together.A er stirring by the blender, the mixture was added to tannin.e whole mixture was stirred strongly by the blender and a er 20 s pTSA was added followed by 20 s strong stirring by the blender.
Mixing sequence 3. Tannin and pMDI were rst mixed together, and then FA was added.A er strong stirring by the blender, the mixture of formaldehyde, water and DE were added.e whole mixture was stirred strongly by the blender and a er 20 s pTSA was added followed by 20 s strong stirring by the blender.
Mixing sequence 4. e mixture of FA and water was added into the mixture of tannin and pMDI.A er strong stirring by the blender the mixture of formaldehyde and DE was added.e whole mixture was stirred strongly by the blender and a er 20 s pTSA was added followed by 20 s strong stirring by the blender.
Mixing sequence 5. e mixture of FA, formaldehyde, water, and DE was added into a 250 mL beaker already containing the tannin, and the whole stirred strongly by the blender for 10 s. e pMDI was then added to the viscous mixture.e whole mixture was stirred strongly by the blender and a er 20 s pTSA was added followed by 20 s strong stirring by the blender.
A black foam was obtained within 2 minutes a er the adding of pTSA.e ve mixing sequences were prepared holding the PUR constant at 10 wt%.For pMDI, preparation followed the procedures of mixing sequences 4 and 5 and the corresponding samples were called p6 and p7.All the foams were prepared according to Table 1, and le to harden and age for 3 days.For comparison, the standard tannin/furanic foam, p0, was prepared according to table 1 only without pMDI.
Based on mixing sequence 5 and formulation table 1, pMDI was maintained in a xed proportion of 10% and the blowing agent DE was varied between 1g and 5g, and the samples obtained were named DE-1, DE-2, DE-4 and DE-5, correspondingly.A erwards the weight fraction of pMDI were changed to 5%, 15% and 20% while maintaining the same proportions of all the other components in table 1, and the samples obtained were designated p-5%, p-15% and p-20%.

Measurements Bulk and skeletal density
Blocks of foam of dimension of 3×3×1.5 cm 3 were weighted to obtain bulk density.Skeletal density, sometimes called true density, was evaluated by helium pycnometry in a lab-made apparatus.e cellular morphologies of the foams were obtained by scanning electron microscope (SEM Hitachi S 4800).ermal conductivity of foam samples of dimension of 3 × 3 × 1.5 cm was measured by the transient plane source method (Hot Disk TPS 2500) at room temperature.e mechanical resistance to compression was investigated with an Instron 4206 universal testing machine at a load rate of 2.0 mm min -1 .

Density and Porosity
A er preparation, in p1 and p2 some small white plastic-like particles were observed.is is consequence of the agglomeration and poor dispersion of the pMDI.us p1 and p2 gave heterogeneous materials, badly mixed and of unmanageable viscosity.As a consequence these two procedures are not discussed further.e bulk density d and skeletal density d s for p3, p4, p5, p6 and p7 are shown in table 2. e porosity P of the samples were calculated by (Tondi et al. 2009a) where d is the bulk density and d s is the skeletal density.e porosity of all the samples is in the range of 91~94%.For the foams modi ed with the PUR there was not much di erence of apparent density and porosity.However, for the pMDI modi ed foams p6 showed a lower density and a higher porosity.
e relationship between the percentage of pMDI and bulk density and porosity is shown in table 3. e bulk density increased and the porosity decreased when the proportion of pMDI increased.
e data t the following linear regression relationships where f is the weight fraction of pMDI and d is the bulk density.With 10% of pMDI, when varying the amount of DE, the bulk density and porosity are shown in table 5. e bulk density decreased rapidly and porosity increased with the increase in the amount of DE. us, in pMDI modi ed tannin foams the density can be modi ed by changing the amount of DE.

Microstructure of the foams
Examples of the cellular structures of the samples are shown in gure 1.For the SEM image of p0, the cells were small, broken and disordered.However, for the foams with pMDI, each cell is connected with its neighbours through more or less circular windows.Moreover, the cell size of the foams with pMDI was larger than p0.It is very interesting to notice that there are many small cells Maderas.among the big cells, thus that two di erent cell sizes are obtained when pMDI is present.e smaller cells are produced by a di erent mechanism with the blowing of DE, namely the production of CO 2 resulting by the reaction of the -N=C=O groups of pMDI with water (Frisch et al. 1983). Maderas.

ermal conductivity
ermal conductivities of the samples are presented in table 3. e ermal conductivities of pMDI modi ed tannin/furanic foams are about 0.054 W/m .K thus slightly higher than the foams without pMDI (at 0.44 W/m .K). is may be caused by the decrease of porosity a er the addition of the pMDI.e foam p6, which had the highest porosity, had the lowest thermal conductivity at 0.47 W/m .K. us, the results do not show too much di erence in thermal conductivity for the foams with and without pMDI, at least no statistically signi cant di erence (P value = 0.325 at the 0.05 level).e thermal conductivity of the foams shown in Table 3 indicates that when the weight fraction of pMDI was under 20%, the thermal conductivity increased gradually.Fitting the results in Table 3 according  where k (W/m .K) is thermal conductivity, f is the weight fraction of pMDI.Eq. 4 shows that k increases very slowly as f increases.e results do not show much di erence in thermal conductivity for the foams with and without pMDI

Compressive strength
For tannin/furanic foams, the stress-strain curves always show three distinct regions: linear elastic, collapse and densi cation.ese three regions are also shown in PUR and pMDI modi ed carbon/furanic foams (Figure 2).PUR modi cation p3 had a better compression resistance than p4 and p5, and better than the pMDI modi ed foams p6 and p7, where p6 had a better compression resistance than p7.Both p3 and p6 were mixed with mixing sequence 3 (cf.Experimental).e stress-strain curves of the pMDI modi ed tannin/furanic foams (Figure 3).e foams behaved as typical elastic fragile cellular solids.Moreover, as the percentage of pMDI increased, the compressive strength improved.As the Diethyl ether decreased, hence the density increased, the compressive strength increased too.Especially when the amount of diethyl ether was lower than 3 g, the compressive strength increased rapidly.e compressive strength and the Young`s modulus are shown in table 2 and gure 3. e results in table 3 show that the compressive strength and the modulus of the foams can become as high as 5.22 MPa and 148.5 MPa respectively.Additionally, the compressive strength depends on the bulk density according to a power law (Sanders and Gibson 2003), where σ s is the yield strength of the (non-porous) solid and the exponent n is associated with the structure and deformation mechanics of the cellular material.eoretical values of n are 1.5, 2 or higher and 1.36 if the foams are open-cells, closed-cells and hollow spheres.Fitting the results of table 2 to Eq.5, the following Eq.6 was obtained, It must be made clear that under the experimental conditions used Eq.6 is only valid when the percentage of pMDI is 0%-20% range.e tting result is shown in gure 4. n = 2.69, and do not follow the theoretical values.is is considered as two di erent cell sizes caused by two di erent forming processes, as disused previously.Table 2 shows the compression strength and Young`s modulus of all the pMDI modi ed tannin/furanic foams were much improved compared with p0.  e results of the PUR modi ed foams are very similar to those of the pMDI modi ed foams, so we only report here the results for pMDI modi ed foams.e similarity of results was due to the PUR used containing a number of active isocyanate groups, thus there was no di erence (other than for viscosity) in adding the PUR or the pMDI.

CONCLUSIONS
Two di erent kinds of forti ers of tannin/furanic foams were used, namely a single component polyurethane (PUR) and polymeric 4,4' diphenylmethane diisocyanate.(pMDI).In the case of the PUR, better dispersion was achieved by mixing the furfuryl alcohol or the mixture of furfuryl alcohol and water into the mixture of tannin and PUR.For pMDI, it was better to add rst the furfuryl alcohol into the mixture of tannin and pMDI.e density and cell size of the foams are larger for pMDI forti ed foams and presents two sizes of cells, the smaller one being caused by the CO 2 generated by the reaction of pMDI with water.e cell size of pMDI modi ed foams had a peak when the weight fraction of pMDI is 5%. is opens a new window to control the sell size, which may play a very important role when our foam is used as catalyst carrier.e density of the foams increased with the increase of the weight fraction of pMDI.However, the density could be adjusted by varying the amount of blowing agent.e addition of pMDI enhanced the thermal conductivity of the foam but not drastically, the pMDI modi ed tannin/furanic foams being still good thermal insulators.Moreover, the addition of pMDI in tannin/furanic foams improved the mechanical property sharply, the compression strength was improved considerably for both PUR modi ed foams and pMDI modi ed foams, the greater the improvement the greater is the proportion of pMDI added.So the pMDI/PUR modi ed foams, with higher mechanical property and low thermal insulation, are supposed to be more suitable to use as thermal insulation layer of the house wall.

Figure 2 .
Figure 2. Stress-strain curves of pMDI modifi ed tannin/furanic foams.a) With different percentage of pMDI; b) With different amount of DE.

Figure 3 .
Figure 3. Stress-strain curves of pMDI modifi ed tannin/furanic foams.a) With different percentage of pMDI; b) With different amount of DE.

Figure 4 .
Figure 4. Relationship between bulk density of pMDI-modifi ed tannin/furanic foams and their compression strength.