Chemistry Report Analyzing the Substitutes 

for Urea-Formaldehyde (UF) Resins to Reduce 

Emissions from Composite Wood Products

Prepared by:

Greg Fritz, Chemist

USEPA / OCSPP / OPPT / EETD

Industrial Chemistry Branch

Through:

Tracy Williamson

Branch Chief

USEPA / OCSPP / OPPT / EETD

Industrial Chemistry Branch

CONTAINS NO TSCA CBI

06 July 2011
Introduction

The purpose of this document is to provide chemistry information to assist the workgroup in assessing risk to

both workers and consumers from exposures and hazards associated with additives to or substitutes for ureaformaldehyde (UF) resins used to manufacture composite wood products. This document examines the resin 

composition, additives used to enhance resin adhesive function, and additives used to scavenge either unreacted 

or released formaldehyde. The document does not consider the contribution of chemicals that are inherently 

present in the wood, although formaldehyde is one of those chemicals.1 The chemicals reviewed in this 

document were chosen based on the California Air Resources Board’s (CARB) research into the methods 

composite wood product manufacturers would use to comply with the formaldehyde emission standards that

CARB promulgated in 2008 (REF 1-2). The different chemicals that could be used to reduce or eliminate 

formaldehyde emissions were then subject to a hazard prioritization process by EPA’s Risk Assessment 

Division (RAD) to eliminate additives or resin monomers with low toxicity (REF 3). Only those chemicals 

with moderate to high hazard potential were subject to a detailed exposure analysis. A physicochemical 

properties table of the chemicals of interest is provided in the Appendix at the end of this report. 

Resins

The resins used today are manufactured with high oligomeric content that is crosslinked with the wood and 

cured under high temperature/pressure conditions to form an adhesive which binds the different wood 

feedstocks together to form the composite wood products. UF resins are commonly used to manufacture 

hardwood plywood (HWPW), medium density fiberboard (MDF), and particle board (PB). Other wood 

products, like softwood [or structural] plywood (SWPW) and oriented strandboard (OSB), are formed 

predominantly using phenol formaldehyde (PF) resins. 

Urea-Formaldehyde (UF) Resin

Three sources of formaldehyde that can be released from UF resin bonded composite wood products have been 

reported (REF 4): 1) free (unreacted) formaldehyde in the UF resin, 22) formaldehyde generated during resin 

curing and use when processed wood products are being manufactured; and 3) formaldehyde generated from 

the exposure of the processed wood products to heat and humidity .

 The amount of free formaldehyde depends 

on the U:F ratio and the reaction conditions (pH, temperature, time) during the UF resin formation. Most resins 

are formed in a two-step process that consists of adding formaldehyde and urea under basic conditions followed 

by an additional charge of urea under acidic conditions (REF 5). Unlike most thermoset resins that form 

irreversible chemical bonds, the UF resin is believed to degrade and release formaldehyde in the presence of 

moisture. Regardless of source, formaldehyde emissions are why scavengers and barrier techniques are often 

used. Any excess urea will be discussed in the context of additives and scavengers based on the processing 

methodology [urea additions occur after the initial UF resin is formed]. 

 

1 For more information on the other chemicals associated with wood products see Baumann M., et al. Volatile organic chemical 

emissions from composite wood products: a review. USDA Forest Service, Forest Products Laboratory, Technology Summaries; 

1996. p. 5–12 where composite wood product emissions were compared with sawdust emissions.

2

 EPA received a public comment from Arclin, Georgia-Pacific Chemicals, and Hexion indicating that the free formaldehyde content 

of the base resin does not “predict the potential of the resin to emit formaldehyde in either the manufacturing plant or from the final 

product.” (EPA-HQ-OPPT-2008-0627-0072 (http://www.regulations.gov) )
Phenol Formaldehyde (PF) Resins

The phenol formaldehyde (PF) resins used in wood products are another major product used as wood adhesives. 

(REF 2) While most studies indicate that the PF resins have low formaldehyde emissions , this is only the case 

for the phenyl methylene bridge formation that occurs in the latter state of curing and adhesive formation. 

Varying pH, temperature, time, and feedstock P/F ratio all affect the type and properties of the product formed

and the amount of potential formaldehyde that can be released. The initial reaction of formaldehyde and phenol 

is prepared with molar excess of formaldehyde that produces an intermediate that has multiple hydroxymethyl

groups attached to the aromatic rings. In addition, there are methyl ether linkages that can release formaldehyde 

during condensation and curing stages of wood processing. It is highly unlikely that the PF resin have uncured 

oligomeric content which would contribute to release of or consumer exposure to formaldehyde, since the 

termini are active methylol groups that are known to cure even at room temperature. The final high temperature 

and pressure cure will completely and irreversibly bond resin to the wood substrate (REF 5,6,7). The final 

products are stable to high humidity and temperature and can be used for outdoor and bathroom/kitchen 

applications where exposure to moisture will occur without losing product efficacy (REF 8). Since the 

initial reaction occurs with excess formaldehyde the phenol is the limiting reagent and is consumed in the 

reaction. 

Tannins containing Resins

Other phenolic products, namely tannins, have been used to make PF-like resins for wood adhesives (REF 2). 

However, these products are too expensive to be used as one for one substitutes for UF resins, so the tannins [or 

other plant phenolic sources like cashew nut shell liquid (CNSL)] are typically used as additives. In the case of

tannin containing resins, methanol can be added to the formulation to slow the fast curing time which improves

product shelf life (REF 2). Any excess methanol would likely be driven off during wood processing because of 

its high volatility. 

Methylene Diphenyl Diisocyanate (MDI)

PolyMDI (pMDI) is generally a more viscous version of MDI monomer. The production of pMDI, used for 

composite wood products resins, involves multiple steps. First, the acid catalyzed condensation of aniline with 

formaldehyde occurs to give oligomeric di- and poly- amines (MDA). The percentage distribution of the 

homologues and isomers of MDA varies. In the next reaction, the MDA mixtures are phosgenated without 

purification to produce pMDI which contain high concentration of MDI monomer. Most commercial pMDI

resins have at least 20% unreacted MDI3listed on their material safety data sheets (MSDS) (REF 1). The pMDI 

resins are generally low molecular weight components that penetrate into the wood surface and react with 

moisture or cellulosic molecules to form a stronger bond than one expects with the other types of adhesives

(REF 9). The chemistry and reactivity of these resins and the corresponding adhesive formation with wood has 

been demonstrated using 2-D NMR techniques (REF 10, 11). Furthermore, the strong bonds and fast reactivity

of pMDI resins make them useful additives to UF and PF resins to further lower formaldehyde emissions (REF 

11). The MDI is consumed in these applications.

 

3 http://www.gmzinc.com/index.php?page=lupranate---polymeric-mdi-and-derivatives is one example of many.
Protein Based Resins

Historically, protein based resins derived from natural sources were the initial adhesive of choice for wood 

based products (REF 1, 2, 12). Their use declined substantially after World War II with the availability of lowcost, durable synthetic adhesives (REF 5). However, because of concerns about formaldehyde exposure, use of 

protein based resins in composite wood products has increased in recent years. One protein based resin

(PureBond) uses a prepolymer which is based on amine epichlorohydrin coupling products that makes binding 

with the protein based resin more efficacious as a wood adhesive (REF 1, 2, 13). The Pure Bond product 

couples protein with a polyamine –epichlorohydrin pre-polymer resin to make the wood adhesive. The 

polyamine epichlorohydrin resin is manufactured by Hercules Corp. This pre-adhesive prepolymer protein 

resin has been manufactured at a separate facility for decades with documentation that there is no unreacted 

epichlorhydrin present in the downstream products (REF 14). 

Polyvinyl Alcohol/Acetate (PVA/PVAC) Resins 

Polyvinyl alcohol / polyvinyl acetate (PVA/PVAC) based adhesives are thermoplastic resin chemicals. They

are different from the thermoset resins in that the mode of action is not chemical reactivity but a change in 

physical form [from liquid to solid phase] to produce the bonding property. Residual vinyl acetate monomer in 

the polymer is generally limited to much less than 1% (MSDS sheets report below 0.3% typical composition).

Vinyl acetate monomer (VAM) is known to undergo acid, base, or metal catalyzed hydrolysis and alcoholysis at 

fast rates of conversion (REF 15). The moisture content of the wood, alcohol groups in the wood, and the 

adhesive formulation components should provide sufficient reactants to consume any unreacted VAM, making 

it unlikely that the composite wood products would contain the unreacted monomer. Finally, PVA modification 

and partial hydrolysis to form some alcohol species that can react with the wood chemicals has been 

demonstrated with positive adhesion results (REF 16). The added benefit to using PVAC that has been partially 

hydrolyzed to PVA is that the hydrolysis reaction destroys any residual monomer. Some manufacturers list no 

monomer contamination on those MSDS documents (REF 17, 18). 

Additives and Scavengers

Different additives and scavengers are sometimes used for enhanced wood bonding and formaldehyde capture. 

There are three classes of products that will be discussed in this report.

Crosslinking / binding agents 

Triacetin [glycerol triacetate] is used with both phenol urea formaldehyde (PUF) and pMDI resins to enhance 

bonding and reduce formaldehyde emissions (REF 2, 19, 20). The triacetin additives are believed to penetrate 

wood like the other low molecular weight chemicals and react with trapped moisture. The initial byproducts of 

triacetin hydrolysis are acetic acid and glycerol, which crosslink the wood fiber with the resin components. 

While the exact mechanism is unknown, the resulting bonded wood is stronger, curing times and temperatures 

are reduced, and these pressed wood products have fewer unreacted resin monomers remaining compared to 

wood bonded with resin without the added triacetin. Alternatively, binding can be accomplished by reacting the 

alcohol type resins (like PVA) with pMDI (REF 21) and converting the thermoplastic resin into a thermoset 

resin. The small pMDI molecules crosslink PVA alcohol groups or can be used as described previously and 

react with the wood chemistry. Certain chromium metal salts have been added to PVA resin formulations to 

promote adhesion by crosslinking the PVA alcohols and wood structure alcohols to form metal oxide bonds, 
which are water resistant (REF 1, 2, 22). These salts are in the Cr(III) oxidation state, but Cr(III) ions have the 

theoretical ability to leach out of treated substrates. There is no literature evidence that such leaching or 

migration of the Cr(III) salt occurs. It is contrary to the noted stability and water resistance of chromium treated 

products. If the Cr(III) salt were to leach out, it could potentially convert to the Cr(VI) oxidation state, which 

would be a concern.

Nitrogen based formaldehyde scavengers

Nitrogen based additives (urea, melamine, hexamine, ammonia, proteins) are used during resin formation,

producing the corresponding mixed resins like MUF [melamine / urea / formaldehyde] . When incorporated 

into the resin backbone these additives are significantly less likely to be released during the lifetime of the 

composite wood product. (REF 23) The additives that are formulated after the resin has been formed are added 

to scavenge the unreacted formaldehyde or any potential formaldehyde that forms from resin decomposition. 

Since these additives may not be chemically bound in a polymer background, they are potentially available to 

be released from the final composite wood product. These additives may alternatively be impregnated into

barrier components of composite wood products (e.g. melamine placed in paper sheets that are laminated onto 

the surface of wood products) which function to bind formaldehyde as it is released or physically prevent the 

formaldehyde from leaving the coated substrate. The additives with low molecular weight may have the 

potential to diffuse from the wood matrix in the final product, but the P-Chem properties of these neat 

chemicals would generally preclude much volatility. In addition, should the additives react with formaldehyde, 

as intended, the corresponding adducts will be much more likely to irreversibly bind to the wood chemical 

moieties based on the chemistry of Mannich type base adducts (REF 1, 2, 24). 

Phenolic based formaldehyde scavengers

Phenols or other aromatic plant sources of phenolic compounds can be additives in pressed wood products 

much the way the nitrogen based additives are used. They can be incorporated into the polymer backbone 

[phenol urea formaldehyde (PUF) resins] or they can be added separately to capture formaldehyde as it is 

generated before it is released. Formaldehyde reacts with phenolic additives to form methylol groups which can 

further react with the wood or with other resin chemicals (REF 1, 2). While there is little research on the 

process of how additives work, there is much evidence that they do effectively reduce formaldehyde emissions 

from treated composite wood products (REF 25, 26). In some cases, additional methylene sources are added to 

enhance the resin formation, using compounds like tris(hydroxymethyl) nitromethane (TN) to crosslink with 

phenolic active sites present with the other wood or resin chemicals (REF 27). 
References 

REF 1. State of California, California Environmental Protection Agency, Air Resources Board

Supplement To The Final Statement Of Reasons For Rulemaking Adoption Of The Airborne Toxic Control Measure To 

Reduce Formaldehyde Emissions From Composite Wood Products. April 26, 2007 

REF 2. 

 State of California, California Environmental Protection Agency, Air Resources Board

Staff Report: Initial Statement Of Reasons For Proposed Rulemaking Public Hearing To Consider

Adoption Of The Proposed Airborne Toxic Control Measure To Reduce Formaldehyde Emissions From Composite Wood.

Products. March 9, 2007

REF 3. Risk Assessment Divison USEPA. Human Health Hazard Summaries for Chemicals Used in Alternative Resin 

Technologies Replacing Urea-Formaldehyde Resins (Draft). 2011

REF 4. Shin-ichiro Tohmura, Chung-Yun Hse, and Mitsuo Higuchi. Formaldehyde emission and high-temperature 

stability of cured

urea-formaldehyde resins Journal of Wood Science 46, 303-309. 2000

REF 5. A.H. Connor. Wood: Adhesives. In: Buschow KHJ (ed). Encyclopedia of Materials: Science and Technology, p. 

9583-9599. Elsevier Science Ltd., Amsterdam. ISBN: 0-08-0431526. 2001

REF 6. 

 Gilles Finaz, Robert Michon, Michele Rasclard, Michel. United States Patent 4,467,051. Method of producing a 

cellular resin suitable for use as an Insulating material. 1984

REF 7. John A. Emery. “Formaldehyde Release from Wood Products Bonded with Phenol Formaldehyde

Adhesives”, Chapter 3 in Formaldehyde Release from Wood Products, American Chemical Society

Symposium 316, American Chemical Society, Washington DC. 1986

REF 8. Young-Kyu Lee, Dae-Jun Kim, Hyun-Joong Kim, Teak-Sung Hwang, Miriam Rafailovich, and Jonathan Sokolov.

Activation Energy and Curing Behavior of Resol- and Novolac-Type Phenolic Resins by Differential Scanning Calorimetry 

and Thermogravimetric Analysis; Journal of Applied Polymer Science, Vol. 89, 2589–2596. 2003

REF 9. US EPA Sources and Factors Affecting Indoor Emissions from Engineered Wood Products: Summary and 

Evaluation of Current Literature EPA/600/SR-96/067. 1996

REF 10. Charles R. Frihart, Daniel J. Yellea, John Ralph, Robert J. Moon, Donald S. Stone, and Joseph E. Jakes. 

Enhanced understanding of the relationship between chemical modification and mechanical properties of wood. 

presented at the 9th Pacific Rim Biobased Composites Symposium Rotorua, New Zealand 2008

REF 11. Mike Gruver. Penetration and Performance of pMDI Resin on Selected Wood Species presented at the Wood 

Adhesives USDA Forest Service Conference San Diego , CA. 2005

REF 12. Diane Greer. New Crop of Innovative Materials Aims for Green Benefits, Durability: Soy-Based Adhesives for 

Plywood. Construction Resources, p. 3, March 2006

REF 13. USEPA Presidential Green Chemistry Awards Summary : Greener Synthetic Pathway Award Winner. 

Development and Commercial Application of Environmentally Friendly Adhesives for Wood Composites. 2007. 

REF 14. U. Mass, Lowell Chapter 4: Formaldehyde in the Five Chemicals Alternatives Assessment Study June 2006

REF 15. H. Yildirim Erbil Vinyl acetate emulsion polymerization and copolymerization with acrylic monomers ISBN 0-

8493-2303-7 pg. 130 CRC Press LLC Boca Raton, FL. 2000

REF 16. Lijun Qiao, Phil K. Coveny, Allan J. Easteal, Modifications of poly(vinyl alcohol) for use in poly(vinyl acetate) 

emulsion wood adhesives. Pigment & Resin Technology 31(2) 88 – 95. 2002
REF 17. Acros Organics Poly(vinyl alcohol) hydrolyzed CASRN: 9002-89-5 MSDS 2009

REF 18. Alfa Aesar Poly(vinyl acetate) CASRN: 9003-20-7 MSDS 2009

REF 19. Krishnakumar Rangachari and Kays Chinai for Rayoneir Products and Financial Services Co. United States 

Patent 6310268 Non-ionic plasticizer additives for wood pulps and absorbent cores. 2001

REF 20. David W. Park and Frank R. Hunter for Weyerhaeuser Corp. US Patent 5,710,434 Isocyanate Impregnating 

Compositions. 1998

REF 21. Helmut Zecha, Rudolf Weissgerber, and Francis Petrocelli for Air Products and Polymers. United States Patent 

6,794,466. Shear thinning vinyl acetate based polymer latex composition, especially for adhesives. 2004

REF 22. Hartmut Brabetz, Christof Kemenater, Franz Six, and Wilhelm Kaiser for Wacker-Chemie GmbH. US Patent 

4,118,357 Adhesive aqueous dispersions of polyvinyl alcohol graft polymers with acidic hardeners and process of 

production. 1978 

REF 23. Antonio Pizzi - Some considerations on future trends in wood adhesives. Presented at COST E34 Conference 

Innovations in Wood Adhesives. Beil, Switzerland. 2004

REF 24. Syed H. Imam, Sherald H. Gordon, Lijun Mao and Liang Chen. Environmentally friendly wood adhesive from a 

renewable plant polymer: characteristics and optimization. Polymer Degradation and Stability, 73(3), 529-533. 2001

REF 25 . Composite Panel Association Technical Bulletin. The Role of Laminates and Coatings as VOC Emission 

Barriers in Composite Wood Panels. 2003

REF 26. Manfred Dunky, Antonio Pizzi, and Marc Van Leemput. COST ACTION E13 Work Group #1 Report on State of 

the Art. Wood Adhesion and Glued Products. ISBN 92-894-4891-1. 2002

REF 27. Jeffrey E. Martin, Albert F. Vozella, Golden F. Watts, and Edwin R. Luckman for Ashland Oil, Inc. United 

States Patent 4,341,668 Aqueous Composition Containing Aldehyde Condensate and Use Thereof. 1982
Physical and Chemical Properties of Select Chemicals Used in Processed Wood Products

Common Chemical

Name

CAS 

Registry

Number

Molecular 

Formula

Melting Point

(°C)

Boiling Point

(°C)

Vapor Pressure

Pa (°C)

Solubility in Water

g / L ( °C)

Ammoniaa[7664-41-7] NH3 Gas @ 25 -33.4 857 kPa (20 °C) 517 (20 °C)

Ureaa[57-13-6] CH4N2O 133–135(dec) not applicable 80 (20 °C) 1080 (20 °C)

Melaminea[108-78-1] C3H6N6 > 280 

(dec)

not applicable 4.7 x 10-8 (20 °C) 3.1 (20 °C)

Hexaminea[100-97-0] C6H12N4 280 (dec) Sublimesc0.35 (20 °C) 895 (20 °C)

Glycerol triacetatea[102-76-1] C9H14O6 3 258 0.33 (25 °C) 70 (25 °C)

Tris(hydroxymethyl)a

nitromethane

[126-11-4] C4H9NO5 175 (dec) not applicable 20 (25 °C) est 

1950 (94 °C) 

2200 (20 °C) 

500 (sat’d 10 °C) 

_ 

Methanola[67-56-1] CH4O -97 64 – 65 12700 (20 °C) Miscible

Methylene 

Diphenylisocyanateb

[101-68-8] C15H10N2O2 39 - 43 > 300 (dec) 0.13 (40 °C) Reacts with water

Vinyl acetatea

(monomer)

[108-05-4] C5H6O2 -93 72 - 73 11 – 12 kPa (20 

°C)

20 - 23 (20 °C)

Phenola[108-95-2] C6H6O 40 - 41 181 - 182 29 - 48 (20 °C) 82 - 84 (20 °C)

Epichlorohydrina [106-89-8] C3H5ClO -57 115 - 117 1.3 – 1.7 kPa (20

°C)

65 (20 °C)

Chromium (III)

b 

nitrate

[13548-38-4]

[7789-02-8] 

Cr [NO3]3

+ (9 H2O) 66

not applicable not applicable Soluble

810 g/Ld

P-Chem properties are provided from a variety of sources including but not limited to: a) OECD SIDS, USEPA HPV Robust 

summaries or IUCLID reports; b) WHO IARC and IPCS reports; c) manufacturer MSDS, or d) WWW sites ChemSpider, Wikipedia, 

ChemLook