Table 2: FT-IR absorption bands of the tannin and furanic polymers.

Peaks (cm−1)Tannin-formaldehyde polymer (TF)Polyfurfuryl alcohol (PFA)Classic TBRFFormaldehyde-free TBRFTemplate compounds*Assignments

3545–3405LFurfural, furfuryl alcohol, pyrogallol. O–H stretch.
3280LLLPhenol, resorcinol, benzyl alcohol, furfuryl alcohol, pyrogallol, catechin, flavonol.O–H stretch.
3100Sphenol, benzaldehyde, furfural, furfuryl alcohol, benzofuran, flavonol, diphenylmethane.C–H stretch. Aromatics, also PFA [22]
2920MMMMMethanol, diethyl ether, benzyl alcohol, furfuryl alcohol, catechin, diphenylmethane.C–H stretch. Aliphatics [22].

1710LLLLBenzaldehyde, furan, furfuryl alcohol, benzofuran.C=O stretch (furan ring opening), formaldehyde activated tannin.
1610LMLLPhenol, resorcinol, benzyl alcohol, catechin, benzofuran, flavonol, diphenylmethane.C=C stretch. Aromatics (6C) [22, 25] also flavonoids [25].
1560MFurfuryl alcohol, flavonol, benzofuran, diphenylmethane.C=C–H stretch. symm. aromatic (in-plane) [26] also furanics [22, 27].
1505SMMPhenol, guaiacol, benzofuran, furfuryl alcohol.C=C–H stretch. asym. aromatic (in-plane) [27] also furanics [22, 27].

1450LLLResorcinol, benzyl alcohol, benzaldehyde, furfural, pyrogallol, catechin, flavonol, diphenylmethane.C–H bend. asym. aromatic [22, 28] also flavonoids [29], C–O stretch. Aromatic (6C) [17].
1420SFurfuryl alcohol, flavonol, diphenylmethane.C–H bend. In-plane O-containing aromatics; C=C stretch ring B [25]
1360SBenzyl alcohol, furfural, furfuryl alcohol, benzofuran, pyrogallol, catechin, flavonol.C–H bend. symm. in-plane aromatic conjugated sequences of polymer backbones.
1290MSMMPhenol, resorcinol, benzaldeh., pyrogallol, catechin, flavonol.C–O stretch. Aromatics (6C) [25] (also ring B of flavonoids) [29].
1210MSSBenzyl alcohol, guaiacol, flavonol.O–H bend. in-plane deformation.
1160SLMMPhenol, resorcinol, guaiacol, furan, furfural, furfuryl alcohol, benzofuran, pyrogallol.C–O stretch. Aromatics [25, 27] also ring A of flavonoids and also furanics; H–C–C bend. Ring B [25].

1120MSMethanol, diethyl ether, guaiacol, catechin, flavonol.C–O stretch. Aliphatic, methylene-ether bridges. [27]
1080LSPhenol, resorcinol, benzaldehyde, furfural, furfuryl alcohol, catechin, flavonol. C–H bend. also ring B of flavonoids [28, 29]; =C–O–C= stretch [17, 27].
1035SLLBenzofuran.=C–O–C= stretch [27, 28].Breathing oxoaromatic compounds.
1010SMLFurfural, furfuryl alcohol, benzofuran.=C–O–C= stretch [17]. Breathing oxoaromatic compounds, methylene-ether bridges and methoxy activation.

970MResorcinol, furan, furfuryl alcohol, catechin.C–O stretch. Aromatic (6C) also flavonoids [17, 25, 29] and C–H bend. aromatic Out-of-plane asym.
880MSSPhenol, furfural, furfuryl alcohol, catechin, flavanol.C–H bend. out-plane asym., also furanics.
840SSResorcinol, benzyl alcohol, furfural, pyrogallol.C–H bend aromatic out-plane asym [28].
810SBenzyl alcohol, catechin.C–H bend. aromatic out-plane symm.
780LBenzofuran, naphthalene.C–H bend. aromatic out-of plane conjugated furanics.
730SResorcinol, benzyl alcohol, difurfuryl ether, furfural, pyrogallol.C–C–C ring bend. symm., C–H bend. out-of-plane symm. [28]. Also furanics.
680LPhenol, resorcinol, benzaldehyde, catechin, flavonol.C–C–C ring in-plane stretch. C–H bend. out-of plane symm. (6C) [21].
650SMethylfuran, naphthalenediols.C–C–C ring in-plane stretch. Conjugated aromatics.

According to the Spectral Database for Organic Compounds SDBS (http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi).
L = large; M = medium; S = small.