Proceedings of the International Convention of Society of Wood Science and Technology and

United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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Formaldehyde Emissions from ULEF- and NAF-Bonded 

Commercial Hardwood Plywood as Influenced by 

Temperature and Relative Humidity

Charles R. Frihart

USDA Forest Products Laboratory

Madison, Wisconsin, USA

James M. Wescott

Heartland Resource Technologies

Waunakee, Wisconsin, USA

Michael J. Birkeland

Heartland Resource Technologies

Edgerton, Wisconsin, USA

Kyle M. Gonner

Heartland Resource Technologies

Madison, Wisconsin, USA

Abstract

It is well documented in the literature that temperature and humidity can influence formaldehyde 

emissions from composite panels that are produced using urea-formaldehyde (UF) adhesives. 

This work investigates the effect of temperature and humidity on newer, ultra-low emitting 

formaldehyde urea formaldehyde (ULEF-UF) and no-added formaldehyde (NAF) adhesives. A 

modified version of the EN 717-3 method to collect formaldehyde coupled with the acetylacetone method to quantify formaldehyde emissions was used. Formaldehyde emissions from a 

commercial CARB phase II compliant hardwood-plywood panel bonded with a ULEF-UF resin 

increased greatly when panels were exposed to higher heat and humidity. Furthermore, the rate of 

emission for ULEF-UF panels increased with longer exposure at 100% humidity. In contrast, 

formaldehyde emissions from CARB phase II compliant hardwood-plywood bonded with a NAF 

resin were relatively stable at different temperature and relative humidity conditions and 

decreased over time. This work highlights the potential for long-term formaldehyde emissions 

from the new ULEF-UF, CARB phase II compliant resin systems. 

Keywords soy, urea formaldehyde, formaldehyde emissions, heat, humidity, ULEF
Proceedings of the International Convention of Society of Wood Science and Technology and

United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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Introduction

Increasing concerns over the quality of indoor air has led to recent changes in legislation and a 

general preference for more stringent limits on the quantity of formaldehyde that can be emitted 

from consumer products intended for indoor use. Wood composites bonded with ureaformaldehyde (UF) adhesive have been identified as an important source of indoor formaldehyde 

emissions (Battelle 1996). Formaldehyde release from interior wood composites has been a 

longstanding issue, leading in the 1980s to the adoption of voluntary standards in the United 

States and Europe that placed limits on formaldehyde emissions (ANSI 2009a,b, 2004; European 

Standard 2002). These voluntary standards led to lower formaldehyde-emitting wood composites, 

but in subsequent years, U.S. emissions from standard products stayed constant while products in 

Europe and Japan moved to lower emission levels. 

Recent standards adopted by the California Air Resources Board (CARB) are intended to 

significantly reduce and regulate formaldehyde emissions in composite wood products (ATCM 

2009). The CARB standard is also the basis for national legislation recently passed concerning 

the emission of formaldehyde in interior wood composites (TSCA 2010). The new standards 

have led to new UF adhesives with ultra-low formaldehyde emissions (ULEF-UF) (Dunkey 

2005), and opened the door for no-added-formaldehyde (NAF) adhesives, such as soy-based 

adhesives (Allen et al. 2010, Wescott et al. 2010), polymeric diphenylmethane diisocyanate, and 

certain types of poly(vinyl acetate). Although both classes of adhesives, ULEF-UF and NAF, are 

capable of passing the CARB phase II formaldehyde emissions limits, there is a concern about 

the long-term emission potential of ULEF-UF adhesives when exposed to potentially higher 

temperature or humidity levels than specified in the current testing methodology for measuring 

formaldehyde emissions. 

Although substantial progress has been made on ULEF-UF adhesives, the fundamental chemistry 

of urea formaldehyde is relatively unchanged and may remain susceptible to hydrolysis. The 

reaction of urea with formaldehyde first produces hydroxylmethyolated urea that then condenses 

to yield methylene and dimethylene ether bridged urea polymers (Pizzi 2003, Meyer 1979). 

Although these reactions are not unlike the steps to produce the other formaldehyde-containing 

wood adhesives, the UF polymers are distinct in that they are susceptible to hydrolysis under 

some normal use conditions (Myers 1986a). The reaction shown in Figure 1 for urea and 

formaldehyde illustrates the problem with depolymerization in that it can yield additional free 

formaldehyde, especially when free water is present.

Figure 1. The reaction of urea with formaldehyde to form the urea-formaldehyde polymer.

O

H H

O

H2N NH2

+

O

N

H

N n + H2O

Formaldehyde Urea Urea-Formaldehyde Water

∆ H+ +
Proceedings of the International Convention of Society of Wood Science and Technology and

United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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The presence of free water in the composite panel, as in the case of higher humidity, tends to 

drive the reverse reaction, yielding more free formaldehyde. Myers showed that formaldehyde 

adsorbed onto wood reaches an emission plateau in about 7 days at 80% relative humidity (RH) 

and 27°C, as does phenol-formaldehyde-bonded particleboard. In contrast, UF-bonded 

particleboard emitted formaldehyde for over 30 days without reaching a plateau (Myers 1986b). 

In addition, Myers’s own data and his analysis of the literature data showed that formaldehyde 

emissions increase from UF-bonded wood composites at higher humidity and temperature 

conditions (Myers 1985, Myers and Nagaoka 1981). This work was done with composites 

bonded with more traditional UF-based adhesives; it is unclear if the new ULEF-UF systems 

suffer from the same level of hydrolytic instability and subsequent high formaldehyde emissions.

Wood itself generates significant formaldehyde when exposed to certain conditions common to 

the composite panel manufacturing process (Schäfer and Roffael 2000, Roffael 2006). This socalled “native” formaldehyde has been shown to be transient and rapidly decreases to levels 

below those set by the standards (Birkeland et al. 2010). Production of formaldehyde from wood 

has been shown to occur at conditions of very high heat and would not be expected to be a 

significant source of formaldehyde in composite wood products during service. 

Currently, the primary standard test method in the United States for measuring and regulating

formaldehyde emissions in composite wood panels is the ASTM E 1333 (ASTM 2002) large 

chamber test. Secondary methods can be also used; however, all methods must prove equivalence 

to the primary method. In E 1333, samples are conditioned at 25°C and 50% RH for 7 days and 

then tested at the same temperature and RH conditions. Based upon the available literature, some 

questions arise regarding formaldehyde emissions from composite wood panels:

• Given that the standard test method, ASTM E 1333, uses 25°C (77°F) and 50% relative

humidity, do these conditions represent all the exposure that interior composite wood products 

will experience in service?

• How do the formaldehyde emissions in CARB phase II certified composite wood products 

bonded with ULEF-UF adhesives compare to those bonded with NAF adhesives over a range 

of temperatures and relative humidity that they may reasonably experience in service?

This study focused on answering these questions, using a modified version of EN 717-3 to test 

the effects of temperature and relative humidity on commercial CARB phase II compliant 

plywood bonded with either ULEF-UF or NAF. 

Methods

Samples

Plywood samples used for testing were 3/4-in.- (19-mm-) thick, seven-ply, hardwood plywood 

with maple face and back and mixed softwood cores obtained from a commercial collaborator. 

One plywood specimen was bonded with a ULEF-UF and one was bonded with a soy-based NAF 

adhesive. Samples from both plywood specimens were tested in a small chamber by the 

manufacturer and shown to be CARB phase II compliant (<0.05 ppm). Samples were wrapped 
Proceedings of the International Convention of Society of Wood Science and Technology and

United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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and sealed in plastic within 24 h after hot-pressing and remained in plastic until testing by the 

modified EN 717-3 method. 

Modified EN 717-3 (WKI Bottle Method)

A modified version of the EN-717-3 was conducted in this study; Table 1 defines the specific 

modifications used in this study. From other studies, the repeatability of this method gave a 4% 

coefficient of variation.

Relative humidity was controlled as follows: 30% RH (with saturated MgCl2), 75% RH (with 

saturated NaCl), and 100% RH (RO H2O) (Wexler 1961).

Table 1: Summary of modifications to EN-717-3

EN-717-3 Our method

Temperature 40°C 25°C and 35°C

Test duration 3 h 1–4 days

Relative humidity 

(%) 100 30–100

Detailed Test Procedure

Using a Nalgene® 500-mL wide-mouth polypropylene bottle (Sigma Aldrich, Milwaukee, 

Wiscsonsin), with cap modified with epoxy and a paper clip, 50 mL (via burette) of either H2O

(100% RH), saturated MgCl2 (30% RH), or saturated NaCl (75% RH) solution is placed in the 

bottom. Three 1- by -in. samples are weighed to 0.1 g, stacked on top of each other (faces 

together), and bound with a rubber band. The bound samples are then suspended above the 

solution in the bottle by attaching the rubber band to the paper clip. The bottle is then kept in a 

temperature/humidity controlled room (25°C) or in a water bath (35°C) for the allotted time (24, 

48, or 96 h). The test bottles are then cooled in an ice water bath for 30 min, the samples removed 

carefully, and the solution collected for analysis. The samples are analyzed for formaldehyde on 

the same day using the acetyl-acetone method.

Acetyl-Acetone Method (ONORM 1992, Nash 1953, Belman 1963)

A calibration curve was generated using a standard formaldehyde solution prepared from 37% 

formaldehyde (Sigma Aldrich, Milwaukee, Wisconsin) titrated to determine formaldehyde level. 

A six-point calibration curve was generated using concentrations of 0 to 5.6 μg/mL formaldehyde 

and yielding an R2 = 0.9998. The standards were analyzed as described below to obtain the 

formaldehyde response. Equal parts (2 mL) of 0.4% acetyl-acetone (Fisher Scientific, Fair Lawn, 

New Jersey), 20% ammonium acetate (Daigger Chemical, Vernon Hills, Illinois), and sample 

solution were combined in a test tube. The mixture was heated to 40°C for 15 min in a water 

bath, then cooled to 25°C. The cooled solution was placed in the dark for 1 h. The mixture was 

then analyzed for absorbance with a spectrophotometer at a wavelength of 412 nm. The 

formaldehyde concentration of the sample was determined based on the calibration curve as 

described above. The results were converted to micrograms of formaldehyde emitted per gram of 

wood. Standard solutions were run at 1, 10, and 30 μg/mL to cover the range of formaldehyde 
Proceedings of the International Convention of Society of Wood Science and Technology and

United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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concentrations measured. Controls were run using the acetyl-acetone method with standard 

formaldehyde levels in the presence of MgCl2 and NaCl to ensure no interference from the salts.

Figure 2: Sample set-up for 

modified EN-717-3 method

Results and Discussion

A static formaldehyde emissions technique was employed to assess changes in emissions for 

composite wood products as a function of temperature and humidity. The method was a modified 

version of the EN 717-3 method. The modifications to this method are outlined in Table 1. The 

purpose of the modifications was to allow the test to be run at various temperatures, relative 

humidities, and durations to better understand the formaldehyde emission potential of composite 

panels under a variety of possible exposures. The 100% humidity was higher than typical, but 

accelerated tests are generally run under more severe conditions than normal exposures due to 

shorter times under those conditions. 

Two commercially produced decorative hardwood plywood specimens were evaluated in this 

study. Both specimens were of the same construction, and the only variable was the adhesive 

used to bond the veneers together to produce the final product. One specimen was bonded with a 

ULEF-UF adhesive; the other specimen was bonded with a NAF soy-based system. Samples 

from both specimens were tested by the manufacturer in a small chamber correlated to ASTM E 

1333 prior to our testing, and both were shown to be CARB-phase II compliant. 

The ASTM E 1333-96 “Large Chamber Method” and any correlated “Small Chamber Method” 

per ASTM D 6007-02 must be run at 25°C and 50% relative humidity. It is likely that these 

conditions may be typical for many homes in the United States, in particular, those that contain 
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United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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and operate an air conditioning system. However, there are many instances when these panels are 

used within the interior of a home that they may be subjected to extended periods of time at 

temperatures higher than 25°C and/or relative humidity levels higher than 50%. The data in 

Figures 3a (www.weather.com) and 3b (www.cityrating.com/relativehumidity.asp) show that the 

majority of the United States, in fact, is actually much higher than 50% RH. Most notably, during 

the summer months, the southeastern region has substantially higher RH and temperature than the 

test conditions (Table 2). It is this finding that led us to evaluate composite panel emissions as a 

function of temperature and humidity. 

0 1 1

4

15

35

79

2

0

10

20

30

40

50

60

70

80

90

21 - 30 31 - 40 41 - 50 51 - 60 61 - 70 71 - 80 81 - 90 91 - 100

Number of US Cities in RH Range*

Relative Humidity Ranges (%)

Relative Humidity AM

Figure 3a: AM relative humidity distribution of 

137 U.S. Cities (Average = 79%)

6 9 11

65

45

1 0 0

0

10

20

30

40

50

60

70

21 - 30 31 - 40 41 - 50 51 - 60 61 - 70 71 - 80 81 - 90 91 - 100

Number of US Cities in RH Range*

Relative Humidity Ranges (%)

Relative Humidity PM

Figure 3b: PM relative humidity distribution of 

137 U.S. cities (Average = 55%)
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United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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Table 2: Summer temperature and relative humidity averages for 

select U.S. cities 

Ave. low temp. 

Jun–Aug 

(°F (°C))

AM 

RH

(%)

Ave high temp 

Jun–Aug

(°F (°C))

PM 

RH 

(%)

Chicago, IL 64 (18) 80 82 (28) 62

Fargo, ND 57 (14) 81 80 (27) 64

Houston, TX 75 (24) 90 93 (34) 63

Kansas City, MO 70 (18) 81 88 (31) 63

Las Vegas, NV 76 (24) 39 102 (39) 21

Los Angeles, CA 64 (18) 79 83 (28) 65

Miami, FL 76 (24) 83 91 (33) 61

New York, NY 66 (19) 72 80 (27) 56

Raleigh-Durham, 

NC 67 (19) 85 86 (30) 54

Seattle, WA 55 (13) 83 73 (23) 62

Impact of Relative Humidity

In this section of the study, we used water (100% RH) and saturated solutions of MgCl2 (30% 

RH) and NaCl (75% RH) to control the relative humidity inside the sample bottles. (See the 

experimental section for details.) These experiments were run at both 25°C (77°F) and 35°C 

(95°F). Table 3 shows the results.

Table 3: Formaldehyde emissions in (μg CH2O/g wood) via modified 

EN-717-3 method

Temp.

Days (°C) 30 75 100 30 75 100

1 25 1.8 3.6 5.8 0.7 0.7 1.6

2 25 4.2 6.0 16.7 1.2 1.6 2.9

4 25 5.6 11.4 50.8 1.4 2.4 4.0

1 35 3.1 8.9 24.0 0.5 2.4 4.5

2 35 5.2 15.5 54.2 0.7 2.7 5.0

4 35 9.1 31.3 178.1 2.1 4.4 5.3

% Relative Humidity % Relative Humidity

ULEF-UF NAF

To better analyze the data, several charts were constructed from the data shown in Table 3. 

Figures 4a and 4b show emissions as a function of relative humidity for both the 25°C and 35°C 

data sets. These results clearly show that the ULEF-UF panel emitted significantly higher 

formaldehyde levels when subjected to higher relative humidity levels and that this was further 

exacerbated by a concomitant increase in temperature.
Proceedings of the International Convention of Society of Wood Science and Technology and

United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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0

10

20

30

40

50

60

0 20 40 60 80 100 120

CH2O Emissions (

μg/g)

% Relative Humidity @ 25 C

NAF ULEF-UF

Figure 4a: Formaldehyde emissions at various relative humidity 

levels at 25°C (4 day samples) 

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100 120

CH2O Emissions (

μg/g)

% Relative Humidity @ 35 C

NAF ULEF-UF

Figure 4b: Formaldehyde emissions at various relative humidity 

levels at 35°C (4 day samples) 

Although it was expected that emission levels of panels produced with ULEF-UF adhesive would 

increase more than those produced with NAF adhesive at increasing temperature and humidity, 

the fact that the ULEF-UF panel emitted >33 times the amount of formaldehyde than the NAF 
Proceedings of the International Convention of Society of Wood Science and Technology and

United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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panel at 100% RH/35°C and the ULEF-UF panel’s emissions within its own set would increase 

by >31 times (100% RH/35°C vs 30% RH/25°C) was quite surprising to us.

We were also interested in rate of emissions in this study. To assess rate of emissions, individual 

samples were tested at 1-, 2-, and 4-day increments. Results of this study are shown in Figures 5 

and 6 (note differences in scale for each set of figures). These results show that for all the NAF 

samples, regardless of temperature or relative humidity, rate of formaldehyde emissions 

decreased over time. This is a desired feature as this would suggest that emission levels will only 

improve (reduce) over the life of the product. Interestingly, the ULEF-UF produced panel did not 

show this behavior when subjected to the 100% relative humidity level, and even at the lower 

humidity levels emission rates were either relatively flat or only slightly decreasing. The trends 

were the same regardless of temperature, that is, emissions rate was roughly double at 4 days 

what if was after only 1 day. This could be the result of scavenger consumption or possibly 

sample equilibration and/or diffusion rates. 

Another interesting aspect of these data is the apparent limited or fixed amount of “native” 

formaldehyde present in these panels. With the NAF panels, this native formaldehyde appeared to 

be released faster when the temperature was higher and the relative humidity was higher, hence 

the emissions only increased from 4.5 to 5.3 from day 1 to day 4 within the 100% RH/35°C 

sample set. It appears that once the native formaldehyde was extracted from the moist panel, 

there was virtually no formaldehyde left in the panel, thus the total formaldehyde emitted did not 

change substantially when the test duration was extended. The ULEF-UF panel was produced 

from UF resin, and these resins are well known to hydrolyze and produce additional 

formaldehyde; the NAF panel did not show this behavior.

The effect of temperature and humidity on formaldehyde emission reported in Table 3 is similar 

to the effects seen by Myers and Nagaoka (1981). Using a dynamic chamber method and UFbonded particleboard, they showed that moving from 25°C and 30% RH to 25°C and 75% RH 

resulted in an approximate two-fold increase in formaldehyde emissions. An examination of the 

data in Table 3 shows that at 25°C, the increase from 30% to 75% RH yields a 2.0-, 1.4-, and 2.0-

fold increase in emissions for the 1-, 2-, and 4-day data, respectively. 

Furthermore, when Myers and Nagaoka changed both temperature (from 25°C to 40°C) and RH 

(from 30% to 75%), the resultant increase in formaldehyde emission was approximately six-fold. 

An analysis of data from Table 3 shows that moving from 25°C and 30% RH to 35°C and 75% 

RH yields a 4.9-, 3.7-, and 5.6-fold increase in emissions for the 1-, 2-, and 4-day data, 

respectively. 

Although Myers and Nagaoka (1981) did not conduct analyses at relative humidity levels above 

75%, in a comprehensive literature survey, Myers was able to derive quantitative temperature and 

relative humidity factors at a wide range of temperature and relative humidity conditions (Myers 

1985). Based on these equations, the move from 30% to 90% RH at 25°C is predicted to yield a 

three-fold increase in formaldehyde emissions. The data in Table 3 show that moving from 30% 

to 100% RH at 25°C yields increases in formaldehyde emissions of 3.2, 4.0, and 9.1 fold for the 

1-, 2-, and 4-day data, respectively. The 1-day data agree well with Myers prediction; however, 
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United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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the longer test periods present increasing rates of formaldehyde emission. The mechanism for this 

increase is currently unknown. 

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0 1 2 3 4 5

Emissions Rate (ug/mL per day)

Days @ 25 C (ULEF-UF)

30% 75% 100%

Figure 5a: Emissions rate as a function of % relative humidity for 

ULEF-UF panel at 25°C.

Figure 5b: Emissions rate as a function of % relative humidity for 

NAF panel at 25°C.
Proceedings of the International Convention of Society of Wood Science and Technology and

United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

0 1 2 3 4 5

Emissions Rate (ug/mL per day)

Days @ 35 C (ULEF-UF)

30% 75% 100%

Figure 6a: Emissions rate as a function of % relative humidity for 

ULEF-UF panel at 35°C.

Figure 6b: Emissions rate as a function of % relative humidity for 

NAF panels at 35°C. 

Conclusions

Testing of formaldehyde emissions from CARB II compliant plywood panels using an ultra-low 

emitting formaldehyde (ULEF) UF resin and a no-added-formaldehyde (NAF) resin carried out at 

six different conditions (25°C at 30%, 75%, and 100% relative humidity, and 35°C at 30%, 75%, 

and 100% relative humidity) using a modification of EN 717-3 has shown the following: (1) The 
Proceedings of the International Convention of Society of Wood Science and Technology and

United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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ULEF-UF product emitted more formaldehyde as the temperature and relative humidity 

increased; at 100% humidity, there was an initial delay in this rise probably due to the 

consumption of the scavenger. (2) The NAF plywood product showed that the total formaldehyde 

emissions reached a plateau and decreased rapidly after only a few days under all test conditions. 

Thus, neither the wood nor the NAF are providing any significant potential source of long-term 

formaldehyde as a result of these increased heat and moisture conditions. 

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