## Chemistry Report Analyzing the Substitutes f or 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**  **Bra nch Chief**  USEPA / OCSPP / OPPT / EETD Industrial Chemistry Branch **CONTA INS NO TSCA CBI**  06 July 201 1

### 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 formaldeh yde (UF ) resins used to manufacture composite wood products . This document examine s 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 inherentl y 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 C ARB promulgate d in 2008 (REF 1 - 2) . The different ch emicals that could be used to reduce or eliminate formaldeh yde 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). Onl y those chemicals with moderate to high hazard potential were subject to a detailed exposure anal ys is. A phy sicochemical 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 w ith 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 product s . UF resins are commonly used to m anufacture hardwood pl ywood (HWPW), medium density fiberboard (MDF), and particle board (PB) . Other wood products, like softwood [or structural] ply wood (SWPW) and oriented strand board (OSB), are formed predominantly

using phenol formaldehyde (PF) resins . **Urea -**  **Fo rmaldehyde (UF) Resin**  Three sources of formaldehyde that can be release d from UF resin bonded composite wood products have been reported ( REF 4 ) : 1) free (unreacted) formaldehyde in the UF resi n, 2 2) formaldeh yde generat ed during resin curin g and use when proce ssed wood products are being manufactured; and 3) formaldeh yd e generated from the exposure of the processed wood products to heat and humidity . The amount of free formaldeh yd e depends on the U : F ratio and the reaction conditions (pH, t emperature, 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 the rmoset 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 emissio n s are wh y scavengers and barrier techniques are often used. An y exce ss 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, Techn ology Summaries; 1996. p. 5

– 12 where composite wood product emissions were compared with sawdust emissions. 2 EPA received a public comm ent 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 - O PPT - 2008 - 0627 - 0072 ( http://www.regulations.gov) )**Phenol Formaldehyde (PF) Resins** The phenol formaldeh yd e (PF) resins used in wood produ cts 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 phenylmethylene bridge formation that occurs in the latter state of curing and a dhesive 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 formaldeh yde and phenol is prepared with molar excess of formaldeh yde that produces an intermediate that has multiple hydroxy methyl groups attached to the aromatic rings. In addition, there are methylether linkages that can release formaldehyde during condensation and curing stages of wood processing. It is highl y unlikely that the PF resin have uncured oligomeric content which would contribute to release of or consumer exposure to formaldeh yd e, since the termini are active meth yl ol 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 f o r outdoor and bathroom/kitchen applications where exposure to moisture will occur wi thout losing product efficacy ( REF 8 ). Since the initial reaction occurs with excess formaldehyde the phenol is the limiting re a gent and is consumed in the reaction. **Tannins containing Resins**  Other phenolic products, namely tannins, have been used to make PF - like resins for wood adhes i ves ( REF 2) . However, these products are too expensive to be used as one for one substitute s for UF resins , so the tannins [or other plant phenolic sources like cashew nut shell liquid (CNSL) ] are ty pically used as additives. In the case o f tannin containing resins, methanol can be add ed to the formulation to slow the fast curing time which improve s product shelf life ( REF 2 ) . An y excess methanol would likely be driven off during woo d processing because of its high volatility . **Methylene Diphenyl Diisocyanate (MDI)** Poly MDI(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 a cid cataly zed condensation of aniline with formaldeh yde 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 purific ation to produce pMDIwhich contain high concentration of MDI monomer. Most commercial pMDI resin s have at least 2 0% unreacted MDI 3 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 ty pes 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 the m useful additives to UF and PF resins to further low er formaldehyde emissions ( REF 1 1 ). 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 , p rotein based resins derived from natural sources were the initial adhesive of choice for wood based products ( REF 1, 2, 1 2). Their use declined substantially after World War II with the availabi lity of low cost, durable s ynthetic adhesives ( REF 5) . However, because of concerns about formaldehyde exposure, use of protein based resins in composite wood products has increased in recent ye ars. One protein based resin (PureBond) uses a prepoly mer 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, 1 3 ). The Pure Bond product couples protein with a polyamine

– epichloroh ydrin pre - pol ym er resin to mak e the wood adhesive. The polyamine epichlorohydrin resin is manufactured by Hercules Corp . T his pre - adhesive prepoly mer protein resin has been manufactured at a separate facility for decades with documentation that there is no unreacted epichlorh ydrin present in the downstream products ( REF 1 4 ). **Polyvinyl Alcohol/Acetate (PVA/PVAC )**  **Resins** P oly vinyl alcohol / poly

vinyl 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 phy sical form [from liquid to solid phase] to produce the bonding property . Residual v inyl acetate monomer in the pol ym er 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 cataly zed hydrol ys is and alcoholy sis at fast rates of conversion ( REF 1 5 ). The moisture content of the wood , alcohol group s in the w ood, and the adhesive formulation components should provide sufficient reactants to consume an y unreacted VAM , making it unlikely

that the composite wood products would contain the unreacted monomer . Finall y, PVA modification and partial h ydrol ys is to for m some alcohol species that can react with the wood chemi c als has been d emonstrated with positive adhesion results ( REF 16 ). The added benefit to using PVAC that has been partiall y hydrol yz ed to PVA is that the hydrol ys is reaction destroy s any residual m o nomer . Some manufacturers list no monomer contamination on those MSDS documents (REF 1 7 , 1 8 ). 

### Additives and Scavengers D ifferent additives and scavengers are sometimes used for enhanced wood bonding and formaldeh yde capture. There are three classes of products that will be discussed in this report . **C rosslinking**  **/ binding agents** Triacetin [gl yc erol triacetate] is used with both phenol urea formaldeh yd e (PUF) and pMDI resins to enhance bonding and reduce formaldeh yde emissions ( REF 2, 1 9 , 20 ) . T he triacetin additi v es are believed to penetrate wood like the other low molecular weight chemical s and react with trapped moisture . The initial by products of triacetin h ydrol ys is are acetic acid and glycerol , which crosslink the wood fiber with the resi n components. While the exact mechanism is unknown, the resulting bonded wood is stronger, curing times and temperatures are reduced, and the se pressed wood product s have fewer unreacted resin monomers remaining compared to wood bonded with resin without the added triacetin. Alternativel y, binding can be accomplished by reacting the alcohol ty pe resins (like PVA) with pMDI ( REF 2 1 ) and converting the thermoplastic resin into a thermoset re sin. 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 b y crosslinking the PVA alcohols and wood structure alcohols to form metal oxide bo nds,which are water resistant ( REF 1, 2, 2 2 ) . 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** N itrogen based additives (urea, melamine, hexamine, ammonia , proteins ) are us ed during resin formation , producing the corresponding mixed resins like MUF [melamine / urea / formaldehyde] . When incorporated into the resin backbone these additives are si gnificantly less likely to be released during the lifetime of the composite wood product . ( REF 2 3 ) The additives that are formulated after the resin has been formed are added to scavenge the unreacted formaldehyde or an y potential formaldeh yde that forms from resin decomposition . S ince the se additives may not be c h emicall y bound in a polymer background , they are potentiall y available to be released from the final composite wood product . These additives may alternativel y be impregnated into barrier comp onents 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 ph ys icall y prevent the formaldeh yde from leaving the coated substrate. The addi ti ves 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 f ormaldehyde, as inten d ed, the corresponding adducts will be much more likel y to irre versibly bind to the wood chemical moieties based on the chemistry of Mannich type base adducts ( REF 1, 2 , 2 4 ). **Phenolic based formaldehyde scavengers**  Phenols or other aromatic plant sources of phenolic compounds can be additives in pressed wood products much the way

the n itrogen based additives are used. They can be incorporated into the poly mer backbone [phenol urea formaldeh yde (PUF) resins] or they can be added sep arately

to capture formaldehyde as it is generated before it is released. Formaldeh yde 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 2 5 , 2 6 ) . In some cases , addition al methylene sources are added to enhance the resin formation , us ing compounds like tri s(hydroxy methyl ) nitromethane (TN) to crosslink with phenolic active sites present with the other wood or resin chemicals ( REF 2 7 ).

### References

REF 1. State of California, California Environmental Protection Agency , Air Re sources Board Supplement To The Final Statement Of Reasons For Rulemaking Adoption Of The Airborne Toxic Control Measure To Reduce Formaldehyde Emissions From Composite Wo od 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 Emiss ions From Composite W ood . Products . March 9, 2007 REF 3. Risk Assessment Divison US EPA. 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 Wo od Science 46, 303 - 309. 2000 REF 5 . A.H. Connor. Wo od: Adhesives. In: Buschow KHJ (ed). Encyclopedia of Materials: Science and Technology, p. 9583 - 9599. Elsevier Science Ltd., Am sterdam. 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. Em ery . “Formaldehyde Release from Wo od Products Bonded with Phenol Formaldehyde Adhesives”, Chapter 3 in Formaldehyde Release from Wo od Products, American Chemical Society Sy mp osium 316, American Chemical Socie ty , W ashington DC . 1986 REF 8 . Young - Ky u Lee, Dae - Jun Kim, Hyun - Joong Kim, Teak - Sung Hwang, Miriam Rafailovich, and Jonathan Sokolov . Activation Energy and Curing Behavior of Resol - and Novolac - Ty pe Phenolic Resins by Differential Scanning Calorimetry a nd Thermogravimetric Analysis; Journal of Applied Poly m er Science, Vol. 89, 2589 –2596. 2003 REF 9 . US EPA Sources and Factors Affecting Indoor Emissions from Engineered W ood Products: Summary and Evaluation of Current Literature EPA/600/SR - 96/067. 199 6 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 R im Biobased Composites Sy mposium Rotorua, New Zealand 2008 REF 1 1 . Mike Gruver. Penetration and Performance of pMDI Resin on Selected Wo od Species presented at the Wo od Adhesives USDA Forest Service Conference San Diego , CA . 2005 REF 1 2 . Diane Gree r. New Crop of Innovative Materials Aims for Green Benefits, Durability : So y- Based Adhesives for Ply wood . Construction Resources, p. 3, March 2006 REF 1 3 . USEPA Presidential Green Chemistry Awards Summary : Greener Synthetic Pathway Award W inner. Develo pm ent and Commercial Application of Environmentally Friendly Adhesives for Wo od Com posites . 2007 . REF 1 4 . U. Mass, Lowell Chapter 4: Formaldehyde in the Five Chemicals Alternatives Assessment Study June 2006 REF 15. H. Yildirim Erbil Vinyl ace tate emulsion polymerization and copoly me rization 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 ace tate) em ulsion wood adhesives. Pigment & Resin Technology 31(2) 88 – 95. 2002REF 1 7 . Acros Organics Poly(vinyl alcohol) hydrolyzed CASRN: 9002 - 89 - 5 MSDS 2009

REF 1 8 . Alfa Aesar Poly (viny l acetate) CASRN: 9003 - 20 - 7 MSDS 2009 REF 1 9 . 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 We yerhaeuser Corp. US Pate nt 5,710,434 Isoc yanate Im pregnating Compositions . 1998 REF 2 1 . H elm ut Zecha , Rudolf We issgerber, and Francis Petrocelli for Air Products and Poly me rs. United States Patent 6,794,466. Shear thinning vinyl acetate based poly me r latex composition, espec ially for adhesives. 2004 REF 2 2 . Hartmut Brabetz, Christof Kemenater, Franz Six, and W ilhelm Kaiser for Wa cker- Chemie GmbH . US Patent 4,118,357 Adhesive aqueous dispersions of polyvinyl alcohol graft poly me rs with acidic hardeners and process of produ ction. 1978 REF 2 3 . A ntonio Pizzi - Some considerations on future trends in wood adhesives. Presented at COST E34 Conference Innovations in W ood Adhesives . Beil, Switzerland . 2004 REF 2 4 . Syed H. Imam, Sherald H. Gordon, Lijun Mao and Liang Chen . Environmentally friendly wood adhesive from a renewable plant poly me r: characteristics and optimization . Poly me r Degradation and Stability, 73(3), 529 - 533 . 2001 REF 2 5 . Composite Panel Association Technical Bulletin . The Role of Laminates and Coat ings as VOC Emission Barriers in Composite W ood Panels . 2003 REF 2 6 . Manfred Dunk y, Antonio Pizzi, and Marc Van Leemput . COST ACTION E 13 W ork Group #1 Report on State of the Art . Wo od Adhesion and Glued Products . ISBN 92 - 894 - 4891 - 1 . 2002 REF 2 7 . Je ffrey

E. Martin, Albert F. Vozella, Golden F. Wa tts, and Edwin R. Luckman for Ashland Oil, Inc. United States Patent 4,341,668 Aqueous Composition Containing Aldehyde Condensate a nd Use Thereof . 1982**Physical and Chemical Properties of Select Chemical**  **s Used in Processed Wood Products**

**Comm on Chemical**  **Nam e**  **CAS**

**R egistry**  **Number Molecul**  **ar**

**F orm**  **ula Melting**  **P oint** 

**( °C)**  **Boiling Point**

**( °C)**  **Vapor Pressure**

**Pa (**  **°C) Solubility**  **in W**  **ater**

**g / L ( °C) Ammonia**  **a [7664**  **- 41**  **- 7]**  **NH3 Gas @ 25**  **- 33.4**  **857 kPa (**  **20 °C) 517**  **( 20 °C)**  **Urea a**  **[ 57**  **- 13**  **- 6**  **] CH**  **4 N**  **2 O**  **13 3** 
**– 135**  **(dec) not applicable**  **80 (** **20 °C) 1080 (20 °C)**  **Melam ine**  **a [**  **108 -**  **78 -**  **1 ]**  **C 3**  **H 6**  **N 6**  **> 280 (dec)**  **not applicable 4.7**  **x 10**  **- 8**  **(2 0**  **°C) 3.**  **1 (20 °C)**  **Hexamine a**  **[100 -**  **97 -**  **0] C**  **6 H**  **12 N**  **4 280**  **(dec) Subli**  **mes c**  **0.35 (2**  **0 °**  **C) 8**  **95 (2**  **0 °C)**  **Glycerol triacetate a**  **[102 -**  **76 -**  **1] C**  **9 H**  **14 O**  **6 3**  **258 0.33**  **(25 °C) 70**  **(25 °C) Tri**  **s(hydroxymethyl) a**  **nitrom ethane**  **[126 -**  **11 -**  **4] C**  **4 H**  **9 NO**  **5 175 (dec)**  **not applicable 20 (25 °C) est**  **1950 (94 °C) 2200**  **(20 °C)**

**500 (sat’ d 1**  **0 °C)**

**_ Me**  **thanol a**  **[67 -**  **56 -**  **1] CH**  **4 O**  **- 97**  **64**
**– 65**  **12700 (2**  **0 °C)**  **Miscible Me**  **thylene Diphenylisocyanate**  **b [101**  **- 68**  **- 8]**  **C 15**  **H 10**  **N 2**  **O 2**  **39 -**  **43 >**  **300 (dec) 0.13**  **( 40**  **°C) Reacts**  **with water Vinyl acetate**  **a (m**  **onomer) [108**  **- 05**  **- 4]**  **C 5**  **H 6**  **O 2**  **- 9**  **3 72**  **- 73**  **11**
**– 12 kPa**  **(20 °C)**  **20 -**  **23 (20 °C)**  **Phenol a**  **[108 -**  **95 -**  **2] C**  **6 H**  **6 O**  **40 -**  **41 181**  **- 182**  **29 -**  **48 (2**  **0 °C)**  **82 -**  **84 (2**  **0 °C)**  **Epichlorohydrin a**  **[106 -**  **89 -**  **8] C**  **3 H**  **5 ClO**  **- 57**  **115 -**  **117 1.3** 
**– 1.7 kPa**  **(2 0**  **°C) 6**  **5 (2**  **0 °C)**  **Chromium (**  **III )**  **b nitrate**  **[13548 -**  **38 -**  **4]**

**[7789 -**  **02 -**  **8] Cr [**  **NO 3**  **] 3** 

**+ (9 H2O) 66**  **not applicable not applicable**  **Soluble 810 g/L**  **d** P - Chem properties are provided from a variety of sources including but not limited to: a) OECD SIDS, US EPA HPV Robust summaries or IUCLID reports ; b ) WHO IARC and IPCS reports ; c ) manufacturer MSDS, or d) WWW sites ChemSpider, Wikipedia, ChemLook