Overview / Chemistry

Basic chemical structures

Building blocks behind each route, drawn from SMILES with RDKit at build time. Stereochemistry is checked against InChIKeys. Polymers such as lignin, tannin, proteins and starch are shown by a representative monomer or model unit, never as an invented full structure.

How a wood adhesive bonds

1 · Wet and penetrate

The liquid adhesive must wet the wood surface and penetrate the cell structure. Viscosity, solids content and surface energy control this step.

2 · Build cohesion

In the press, the binder forms a network through condensation (aldehydes with phenolics or amines), esterification (acids with polyols), addition (isocyanates, epoxides, carbonates with amines) or physical consolidation.

3 · Resist water

Water resistance depends on how dense the crosslinking is and how few hydrophilic groups remain free. This is the step most bio-binders struggle with.

4 · Stay inert

The cured bond should not release formaldehyde or VOCs, support mould growth, or hydrolyse over the product's life.

Incumbent resin chemistry

The formaldehyde-based and fossil systems that every alternative is measured against.

Formaldehyde

Incumbent crosslinker in UF, MUF and PF resins; the substance the new rules limit.

C=O

Urea

Amino co-monomer of UF resins; also paired with glyoxal in UG resins.

NC(N)=O

Phenol

Fossil phenolic monomer of PF resins; the component lignin and tannin aim to replace.

Oc1ccccc1

Melamine

Raises moisture resistance of UF (MUF) resins.

Nc1nc(N)nc(N)n1

Vinyl acetate

Monomer of PVAc, the incumbent for many layer-glued and assembly bonds.

C=COC(C)=O

Lignin

The most abundant aromatic biopolymer, used mainly to replace phenol in PF-type resins.

Phenol

Fossil phenolic monomer of PF resins; the component lignin and tannin aim to replace.

Oc1ccccc1

p-Coumaryl alcohol (H unit)

Monolignol giving H units; abundant in grass lignins.

OC/C=C/c1ccc(O)cc1

Coniferyl alcohol (G unit)

Main monolignol of softwood lignin; one free ring position ortho to the phenol for crosslinking.

OC/C=C/c1ccc(O)c(OC)c1

Sinapyl alcohol (S unit)

Hardwood monolignol; both ortho positions methoxylated, so less reactive toward aldehydes.

OC/C=C/c1cc(OC)c(O)c(OC)c1

Proteins

Soy, casein, zein, gelatine and side-stream proteins; soy with PAE is the commercial reference.

L-Lysine

Amine-rich amino acid; lysine residues in proteins and polylysine react with crosslinkers.

NCCCC[C@H](N)C(O)=O

Epichlorohydrin

Precursor of the azetidinium groups in PAE resins, the common crosslinker for soy adhesives.

ClCC1CO1

Tannins

Condensed tannins from bark and wood, the longest-running industrial bio-phenolic wood adhesive.

(+)-Catechin (flavan-3-ol unit)

Representative monomer unit of condensed tannins; the A-ring is the nucleophilic site toward aldehydes.

O[C@H]1Cc2c(O)cc(O)cc2O[C@@H]1c1ccc(O)c(O)c1

Furfuryl alcohol

Furanic co-reactant in tannin-furanic systems; self-condenses under acid.

OCc1ccco1

Hexamine (hexamethylenetetramine)

Hardener for tannin and lignin resins; it is itself a formaldehyde-releasing compound.

C1N2CN3CN1CN(C2)C3

Carbohydrates

Starch, cellulose derivatives, sugars and chitosan, used as binders, co-reactants or extenders.

alpha-D-Glucopyranose

Repeat unit of starch (amylose and amylopectin); also a sugar feedstock.

OC[C@H]1O[C@H](O)[C@H](O)[C@@H](O)[C@@H]1O

Sucrose

Polyol in citric acid-sucrose adhesives and sugar-based NIPU.

OC[C@H]1O[C@@](CO)(O[C@H]2O[C@H](CO)[C@@H](O)[C@H](O)[C@H]2O)[C@@H](O)[C@@H]1O

D-Glucosamine (stereochemistry omitted)

Repeat unit of chitosan; the amine gives cationic, film-forming behaviour.

NC1C(O)OC(CO)C(O)C1O

Polycarboxylic acid systems

Citric acid with sucrose or other polyols forms ester crosslinks in the hot press without aldehydes.

Sucrose

Polyol in citric acid-sucrose adhesives and sugar-based NIPU.

OC[C@H]1O[C@@](CO)(O[C@H]2O[C@H](CO)[C@@H](O)[C@H](O)[C@H]2O)[C@@H](O)[C@@H]1O

Citric acid

Tricarboxylic acid that forms ester crosslinks with polyols and wood hydroxyls on hot pressing.

OC(=O)CC(O)(CC(O)=O)C(O)=O

Renewable crosslinkers and aldehyde substitutes

Glyoxal, 5-HMF, furanics and bio-based epoxides that replace formaldehyde or fossil crosslinkers.

Urea

Amino co-monomer of UF resins; also paired with glyoxal in UG resins.

NC(N)=O

Epichlorohydrin

Precursor of the azetidinium groups in PAE resins, the common crosslinker for soy adhesives.

ClCC1CO1

Glyoxal

Non-volatile dialdehyde used as a formaldehyde substitute (UG, lignin-glyoxal, tannin-glyoxal).

O=CC=O

5-Hydroxymethylfurfural (5-HMF)

Sugar-derived furanic aldehyde; candidate aldehyde for formaldehyde-free phenolic and amino resins.

O=Cc1ccc(CO)o1

Furfuryl alcohol

Furanic co-reactant in tannin-furanic systems; self-condenses under acid.

OCc1ccco1

Hexamine (hexamethylenetetramine)

Hardener for tannin and lignin resins; it is itself a formaldehyde-releasing compound.

C1N2CN3CN1CN(C2)C3

Non-isocyanate polyurethanes (NIPU)

Cyclic carbonates reacting with amines build polyurethanes without isocyanates or phosgene.

Sucrose

Polyol in citric acid-sucrose adhesives and sugar-based NIPU.

OC[C@H]1O[C@@](CO)(O[C@H]2O[C@H](CO)[C@@H](O)[C@H](O)[C@H]2O)[C@@H](O)[C@@H]1O

Glycerol carbonate

Bio-derivable five-membered cyclic carbonate; reacts with amines to hydroxyurethanes (NIPU).

O=C1OCC(CO)O1

Bio-based acrylics and dispersions (watch)

Watch list for renewable acrylic acid and esters feeding dispersion and assembly adhesives.

Acrylic acid

Monomer for acrylic dispersions and polyacrylic acid; renewable routes are on the watch list.

C=CC(=O)O

Vinyl acetate

Monomer of PVAc, the incumbent for many layer-glued and assembly bonds.

C=COC(C)=O

Structure data: structures.json. Suggest a molecule via a GitHub issue; please include a SMILES and a citable source.