Document ID: EPA-HQ-OPPT-2012-0018-0463
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2013-06-10T04:00Z

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 urea-formaldehyde (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.  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,   2) 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].  

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 MDI
listed 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.

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 low-cost, 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	  HYPERLINK "http://en.wikipedia.org/wiki/CAS_registry_number" \o
"CAS registry number"  CAS Registry  Number	  HYPERLINK
"http://en.wikipedia.org/wiki/Chemical_formula" \o "Chemical formula" 
Molecular Formula 

	  HYPERLINK "http://en.wikipedia.org/wiki/Melting_point" \o "Melting
point"  Melting Point 

(°C)	Boiling Point

(°C)	Vapor Pressure

Pa (°C)	  HYPERLINK "http://en.wikipedia.org/wiki/Solubility" \o
"Solubility"  Solubility  in   HYPERLINK
"http://en.wikipedia.org/wiki/Water" \o "Water"  Water 

g / L  ( °C)

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

Ureaa	[  HYPERLINK
"http://www.commonchemistry.org/ChemicalDetail.aspx?ref=57-13-6" \o
"http://www.commonchemistry.org/ChemicalDetail.aspx?ref=57-13-6" 
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)

	Sublimesc	0.35 (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

   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.

  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  (  HYPERLINK
"http://www.regulations.gov"  http://www.regulations.gov )  )

   HYPERLINK
"http://www.gmzinc.com/index.php?page=lupranate---polymeric-mdi-and-deri
vatives" 
http://www.gmzinc.com/index.php?page=lupranate---polymeric-mdi-and-deriv
atives   is one example of many.