DURABILITY OF WATER-RESISTANT 

WOODWORKING GLUES 

No. 1530 

(Revised) September 1956 
DURABILITY OF WATER-RESISTANT WOODWORKING GLUES 

By 

Forest Products Laboratory,1 Forest Service 

U. S. Department of Agriculture 

- - -

The increasing use of glues in the fabrication of items of wood and wood 

products, the increasing variety of the items, and the development of new 

glues and important modifications of old ones, all serve to increase the 

problem of selecting a satisfactory glue for a particular purpose. Selecting the proper adhesive requires, among other things, a knowledge of the 

conditions to which the glued item will be exposed in normal service, and 

a knowledge of the characteristics of the different glues available. Since 

essentially all of the woodworking glues now in common use are capable 

of producing joints that are initially strong when dry, the choice of a glue 

centers largely on the durability under conditions other than those within 

the range of human comfort, and on the practical consideration of cost 

and adaptability to production procedures. 

The range of conditions under which different glued items are expected to 

serve vary widely. Much of the furniture in the north temperate zone, 

for example, is seldom exposed to temperatures much beyond the human 

comfort zone nor to atmospheres damp enough to permit mold growth. 

Toward the other extreme, glued laminated timbers are being used in the 

construction of bridges, where they will be expected to retain a high percentage of their strength even though installed in damp locations, repeatedly wetted and dried, and exposed to wide variations in temperature. Some 

glued wood products used in boat construction may be continuously immersed, while other members, like the ribs, may be continuously damp 

in some locations, warm and dry in others, and hot in still others. Glued 

wood parts in aircraft may be repeatedly wetted and dried, continuously 

damp, or subjected to extremely high temperatures, depending on the location of use. Temperatures of about 215° F. have been recorded immediately beneath the dark-colored plywood wing skins of aircraft during 

1 

Maintained at Madison, Wis., in cooperation with the University of 

Wisconsin. 

Report No. 1530 (revised) -1-
Arizona summers. Some glued wood elements used in house construction 

may be fully exposed to the weather, others to an occasional accidental 

wetting and prolonged dampness, while others may remain dry but reach 

temperatures somewhat above 150° F. during warm summer days. 

Requirements as to permanence also vary widely. In some cases, the 

glue joints need hold only until the item is fully assembled, while other 

uses, as in glued structural members in housing, the glue joints will be 

expected to retain approximately full strength and quality for an indefinite 

number of years. 

In general, three methods of measuring the durability of glued joints have 

been employed: 

1. Laboratory exposure tests of plywood 

2. Weathering tests of plywood panels 

3. Exposure tests of laminated beams 

Each of these methods will be considered in turn in sections of this report. 

For each method of testing, an evaluation of the glues for which data are 

available, will be presented. 

Forest Products Laboratory Exposure Tests of Plywood

2 

Exposure tests of plywood under controlled conditions in the Laboratory 

have supplied a major part of the available data on the durability of glues. 3 

To date these tests have given information on the resistance of various 

glues in plywood joints exposed to severe moisture conditions for periods 

up to 10 years and to extreme temperature conditions for periods up to 3 

years. 

2 

Data presented in this section were originally compiled under the direction of H. D. Bruce. They are the result of the work of many investigators, but particularly Don Brouse, W. Z. Olson, and R. F. Blomquist. 

3 

Blomquist, R. F. Effect of High and Low Temperatures on Resin Glue 

Joints in Birch Plywood. Forest Products Laboratory Report No. 1345, 

Revised. 

Report No. 1530 -2- GPO 826965-2 
Test Procedure 

Most of the Laboratory data are based on tests of standard plywood shear 

specimens that were prepared from 1/16-inch yellow birch veneer and 

glued at 12 percent moisture content. Manufacturers' recommendations 

governed the gluing conditions used. Control samples were tested both 

wet and dry after a 1-week conditioning period, and the remaining specimens were subjected to various conditions of exposure, At regular intervals, samples were removed and shear strength was determined. 

Specimens were generally tested in the condition at which they were removed. Specimens subjected to cyclic exposures were usually tested 

after the dry part of the cycle, 

Exposure Conditions 

No single exposure test can be used for an accurate evaluation of the durability of glued joints under varied service conditions. By means of wellselected sets of conditions to which each glue has been exposed, however, 

the individual factors that lead to the deterioration of glued joints can be 

isolated, and the relative resistance of particular glues to these conditions can be compared. The properties of glued joints determined in this 

way include resistance to water, to molds and other micro-organisms, to 

high and low temperatures, and include ability to withstand mechanical 

stresses caused by restraint of swelling and shrinking due to the crossply construction of the specimens. From a knowledge of these properties, 

gained as a result of extensive laboratory tests, glues can be rated with 

regard to their effectiveness under known conditions of service. 

The exposure conditions used in these laboratory tests, together with a 

statement of the significance of each, follow: 

Continuous soaking in water at room temperature. --This is primarily a 

test of water resistance, although bacterial action may be involved, particularly with protein glues. 

Continuous exposure to 97 percent relative humidity and 80° F. --In this 

test, resistance to a combination of mold and other micro-organisms and 

ability to withstand the effects of moisture content approaching fiber saturation are measured. 

Soaking-drying cycle. --This test consists of repeated cycles of alternate 

soaking in water at room temperature for 2 days and drying at 80° F. and 

Report No. 1530 -3-
30 percent relative humidity for 12 days Resistance to mechanical 

stresses, to water, and possibly to biological deterioration is necessary 

for maintenance of strength under this exposure cycle. 

High and low relative humidity cycle. --In this test, a period of 2 weeks 

at 80° F. and 97 perecent relative humidity is followed by a 2-weekperiod 

at 80° F. and 30 percent relative humidity in a repeating cycle. Mechanical strength, resistance to molds and other micro-organisms, and 

ability to withstand the effects of moisture content approaching fiber 

saturation are measured. 

Continuous exposure at 80° F. and 65 percent relative humidity. --The 

effect of aging of the glued joint under mild conditions is measured. 

Continuous exposure at 158° F. and 20 percent relative humidity.--The 

effect of high temperature at low moisture content is indicated. 

Continuous exposure at 158° F. and 60 percent relative humidity. --In 

this test, the combined effect of high temperature and moderate moisture 

content is indicated. 

Room-temperature--high-temperature cycle. --This cycle, consisting of 

repeated alternate exposures for 16 hours at 80° F. and 65 percent relative humidity and 8 hours at 158° F. and 20 percent relative humidity, 

measures the combined effect of high temperature and moderate to low 

moisture content, together with resistance to mild swelling and shrinking stresses. 

Room-temperature--low-temperature cycle. --This repeated cycle of 

alternate exposures for 16 hours at 80° F. and 65 percent relative humidity and 8 hours at -20° F. measures the resistance of moisturecontaining glued joints to extremely low temperatures. Early results 

obtained in this test were based upon a low temperature of -67° F. over 

dry ice. 

Continuous exposure at 200° F. and 20 percent relative humidity. --This 

is an accelerated test method that measures the comparative resistance 

of glue and wood to high temperature. 

Results of Laboratory Exposure Tests on Eight Types of Glue 

The results of exposure to these conditions for various periods of time 

on the dry shear strength of yellow birch plywood specimens glued with 

Report No. 1530 -4-
eight types of glue are shown in figures 1 to 10. Shear-test results for 

room-temperature-setting urea resin, casein, intermediate-temperature

phenol resin (alkaline type), hot-press urea resin, fortified urea resin, 

resorcinol resin, hot-press phenol resin, and melamine resin glues are 

plotted separately. The curves shown in each graph represent average 

joint strength after exposure as a percentage of original dry shear strength 

The vertical bars indicate average percentages of wood failure. Differences between glues of the same type are not revealed by the curves in 

figures 1 to 10. Some variations in durability within particular types of 

glue are discussed later in this section. 

In addition to the eight types of glue for which exposure data are discussed 

in this section, animal and vegetable (starch) glues have been tested in 

plywood joints under relatively mild exposure conditions. In exposures 

involving continuous 30 percent relative humidity and cycles of 60 and 30 

percent relative humidity, no deterioration was evident throughout more 

than 3 years of exposure. In a cycle consisting of alternating 80 percent 

and 30 percent relative humidity, however, glues of both types decreased 

rapidly in strength, and joints failed completely in about 1-1/2 years. 

Similar failures occurred in 8 months when plywood specimens were exposed to a 90 percent - 30 percent relative humidity cycle, in 3-1/2 

months when exposed to a 97 percent - 30 percent relative humidity cycle, 

and during the first cycle when subjected to alternate soaking and drying. 

After continuous soaking in water at room temperature (fig. 1), joints 

glued with casein glues dropped below 40 percent of their original dry 

strength within 2 months, and failed completely after 1-1/2 years. Woodfailure percentages estimated from casein glue joints were low. Joints 

glued with room-temperature-setting and hot-press urea resins and hotpress phenol resins showed high wood-failure values in shear tests 

throughout 4-1/2 to 6 years of exposure, which indicates that declining 

wood strength rather than declining strength of the glue is shown by the 

shear-strength curve. Test data for fortified hot-press urea resin, 

resorcinol, melamine, and intermediate-temperature phenol glues are 

available for at least 30 months of exposure, during which period high 

strengths and high percentages of wood failure were maintained. 

4 Other Laboratory tests-of urea-resin-glued plywood and laminated joints 

made from a number of different species and immersed in water and in 4 

percent salt solution at room temperature and at 110° F. have shown that, 

4 

Blomquist, R. F. Summary of Available Data on the Stability of Glue 

Joints in Laminated Oak Keels in Fresh and Sea Water. Forest 

Products Laboratory Unnumbered Report. 1943. 

Report No. 1530 -5-
under these conditions, deterioration of the glue bond occurs, particularly 

with certain woods such as white oak and Douglas-fir. In yellow birch, 

similar deterioration occurred when specimens were soaked in salt water. 

Apparently some chemical reaction, possibly the liberation of acids, when 

certain woods are soaked in water or in salt solution is the cause of the 

rapid decrease in strength. Intermediate-temperature phenol, melamine, 

and resorcinol resins maintained the full strength of the wood in joints 

subjected to prolonged soaking in salt water. 

Under continuous exposure to 97 percent relative humidity and 80° F. 

(fig. 2), casein glue dropped below 40 percent of its original dry strength

in 4 months, and failed completely in 2-1/2 years. Hot-press and roomtemperature-setting urea resins fell to approximately the 60 percent level 

in 1 year, but continued to show some measurable strength for several 

years. The hot-press ureas still retained a small percentage of their 

original strength after 7 years, although the cold-setting urea resin (all 

of which met the Army-Navy Aeronautical Specification AN-G-8 of April 

25, 1942), failed completely in 5 years. The wood undoubtedly deteriorated 

under this severe exposure condition and contributed to the reduction in 

joint strength. Decreased percentages of wood failure indicate, however, 

that the glue bonds also broke down. The strength of fortified urea-resin 

glue joints was not much better than that of the other urea resins after 42 

months of exposure, although wood-failure percentages remained high. 

After 7 years of exposure, the hot-press phenol glue joints retained better 

than 40 percent of their original strength, and the high percentages of wood 

failure indicate that joint strength was limited by the declining strength of 

the wood. The intermediate-temperature phenol, resorcinol, and hotpress melamine glues were comparable to the hot-press phenol-resin 

glues. 

After exposure to the soaking-drying cycle (fig. 3), casein glues failed 

rapidly. They dropped below 40 percent of their original dry strength 

within 4 months. The room-temperature-setting urea glues dropped below 60 percent of their original strength within 18 months, and continued 

to lose strength throughout the 4-year exposure period. Decreasing 

strength was accompanied by very low wood-failure values. The hotpress and fortified hot-press urea resins maintained 60 percent of their 

original joint strength for 3 years, but were not quite so durable as the 

hot-press phenol resins. The hot-press phenol resins maintained a high 

level of wood failure throughout the 6-year exposure period, which indicates high durability of the glue. The intermediate-temperature phenol, 

resorcinol, and hot-press melamine glues retained high strength and 

wood-failure values, and appear to be comparable to hot-press phenols 

in durability. 

Report No. 1530 -6- GPO 826965-5 
In the high and low relative humidity cycle (fig. 4), the hot-press and the 

intermediate-temperature phenol, resorcinol, and hot-press melamine 

resins retained approximately 80 percent of their original strength after 

30 months of exposure. The hot-press phenols, for which data are available for a longer period, remained at this level even after 6 years. These 

glues maintained high percentages of wood failure under this exposure 

cycle. Casein glue failed rapidly, and dropped below 60 percent of the 

original joint strength within 1 year. Room-temperature-setting urearesin glue fell to approximately the 60 percent level within 1 year, and 

continued to decline rapidly in strength. Recreasing percentages of 

wood failure indicated deterioration of the glue bond. The hot-press urea 

resins showed somewhat better resistance to alternating high and low relative humidity conditions, but strength values were very low, and tested 

specimens showed no wood failure after 4 years of exposure. The fortified hot-press ureas retained higher strength than the hot-press ureas, 

but some of them showed a gradual decline in wood-failure values over a 

30-month period. 

When continuously maintained at 80° F. and 65 percent relative humidity 

for a period of 3 years (fig. 5), the room-temperature-setting urea resins 

were the only glues tested that showed a trend toward decreasing average 

joint strength. This weakening was confirmed by a regular drop in percentage of wood failure. At the end of 3 years of exposure, the average 

joint strength of glues of this type conforming to Army-Navy Aeronautical 

Specification AN-G-8 (April 25, 1942) bad decreased to 72 percent of the 

original values, but considerable differences existed between individual 

glues. All other types of glue maintained high levels of strength throughout 3 years of exposure to this mild condition. 

At 158° F. and 20 percent relative humidity (fig. 6), room-temperaturesetting and hot-press urea resins showed a rapid decline in joint strength 

and percentage of wood failure, and fell below 60 percent of their original 

strength within 6 months and 12 months, respectively. The remaining 

thermosetting resins and casein retained higher joint strengths over the 

exposure period of 30 months for which data are available, although some 

fortified urea-resin glues showed a tendency to decrease in strength and 

wood failure, and casein glues had dropped to approximately the 55 percent level at the end of 30 months. Joints glued with the phenol, resorcinol, 

and melamine resins declined somewhat in strength under this exposure, 

but retained high percentages of wood failure, which indicates deterioration of the wood rather than of the glue. 

Report No. 1530 -7-
The higher moisture content prevailing under continuous exposure at 

158° F. and 60 percent relative humidity (fig, 7) caused a more rapid 

breakdown of room-temperature-setting urea-resin glue joints.Within 

2 months of exposure, room-temperature-setting urea-resin glue joints 

fell below 50 percent of their original joint strength, and only a few specimens retained measurable joint strength at the end of 9 months. The only 

hot-press urea resin in these tests deteriorated less rapidly, but it had 

lost practically all strength after 18 months. Results obtained in up to 

18 months of exposure show considerable decrease in the strength and 

percentage of wood failure of joints glued with some fortified hot-press 

ureas. Variations in glues of this general type are discussed later in 

this section. Joints glued with alkaline intermediate-temperature phenol, 

hot-press melamine, and resorcinol resins showed high average percentages of wood failure after various periods of exposure up to 18 months. 

Their strength appears to be limited by the decreasing strength of the 

wood under this high-temperature exposure. Although they were not 

generally characterized by high wood-failure values, joints glued with 

casein and hot-press phenol resin both retained high strength under this 

exposure. 

Exposure to the room temperature - high temperature cycle (fig. 8) caused 

room-temperature-setting urea-resin glued joints to decrease considerably in strength. Within 6 months, strength values for the room-temperattire-setting urea-resin glues dropped below 60 percent of original strength 

and were accompanied by a noticeable decline in wood-failure values. Joint 

strengths obtained with some fortified hot-press ureas and with hot-press 

urea resin decreased only moderately, but they were accompanied by a 

dropping off in wood-failure percentages over exposure periods ranging 

from 18 to 36 months. 

the same rate as hot-press urea, but showed no evident trend toward reduced wood-failure values. Hot-press and intermediate-temperature phenol, 

resorcinol, and melamine glues retained high wood-failure values throughout exposure periods of 18 to 36 months, and joint strength appeared to be 

limited by the strength of the wood. Gradual weakening of wood takes place

under these exposure conditions. 

Casein glue joints decreased in strength at about 

The room temperature - low temperature cycle (fig. 9) showed little difference among the glues tested during 1 to 3 years of exposure. All retained moderately high to high joint-strength values, although with the 

room-temperature-setting urea, wood-failure percentages were negligible 

after 3 years and a gradual decline in strength was evident under this condition of exposure. 

Report No. 1530 -8-
Continuous exposure at 200° F. and 20 percent relative humidity (fig. 10) 

accelerated the rate of deterioration of the room-temperature-setting, 

hot-press, and some fortified hot-press urea resins shown by the exposures at 158° F. Room-temperature-setting urea resins failed completely 

in 6 months, and hot-press urea resin in 1 year. The fortified urea resins 

retained some strength after 18 months, but wood-failure values were 

negligible after this period of exposure. With the remaining thermosetting resins and casein included in these tests, the decreasing strength 

curves over a 30-month period are similar and appear to reflect chiefly 

the weakening effect on wood of prolonged periods of exposure to high 

temperature. 

Durability Variations within Particular Types of Glue 

The results discussed in the preceding section are based on average data 

collected for each type of glue. Within any glue type, some glues are 

superior to others of the same type, and there are also modifications of 

certain types of glue that cannot be included readily in a generalized summary. It is particularly important to consider such differences in the 

room-temperature-setting and hot-press urea resins, casein, intermediate-temperature phenol, and fortified urea-resin types. 

Room-temperature-setting urea resin. --The results shown in figures 1 to 

10 are based only on glues of this type that conformed to Army-Navy 

Specification AN-G-8 (April 25, 1942). In figure 11, relative joint-strength 

values after exposure for periods up io 30 months to continuous 80° F. and 

65 percent relative humidity, repeated cycles of 97 percent and 30 percent 

relative humidity at 80° F., continuous 158° F. and 60 percent relative 

humidity, and to the room temperature - high temperature cycle, are 

shown for the most durable and the least durable room-temperature-setting 

urea-resin glues that have been tested at the Forest Products Laboratory, 

including glues that failed to conform to Specification AN-G-8. Average 

results are also shown for glues that met the requirements of Specification 

AN-G-8. Although considerable differences are shown between the best 

and the poorest glues of this type, the trend toward rapid decline in 

strength under high humidity and high temperature conditions is shown by 

all the room-temperature-setting urea resins, and the curves based upon 

average results obtained with AN-G-8 glues appear to represent the better 

glues of this type reasonably well. 

Hot-press urea resin. --For many purposes, urea-resin glues are extended with flour to reduce glue-line costs. The durability of glues 

Report No. 1530 -9-
modified in this way cannot be estimated from the results shown in figures 

1 to 10, which are based upon unextended glues. Birch plywood specimens 

glued with an unextended hot-press urea resin and with the same resin 

containing various amounts of rye-flour extender up to 200 percent (based 

on the dry weight of the resin) were therefore placed in 4 of the regular 

laboratory plywood exposure tests. The results shown in figure 12 cover 

a 7-year period of exposure. Actual shear strengths are plotted in this 

figure. Figure 12A shows the effect of soaking on the wet strength of the 

unextended and flour-extended resins. The original wet strength of the 

glue extended 200 percent was approximately one-half that of the unextended resin, and a nearly constant difference in strength was maintained 

throughout the entire period of exposure as the strength of both glues 

gradually decreased. With 100 percent flour extension, intermediate 

strength values were obtained. 

When specimens were subjected to alternate soaking and drying, shear 

tests were made following the dry portion of the cycle (fig. 12B). The 

effect of extension on original strength was relatively slight. The unextended and the 100-percent-extended urea resins decreased in strength 

at about the same rate as when they were continuously soaked. The specimens glued with resin containing 200 percent extender decreased somewhat more rapidly and failed after 6 years of exposure. 

Rapid deterioration of flour-extended urea resins occurred as a result of 

attack by molds when specimens were exposed continuously to 97 percent 

relative humidity (fig. 12C) and to the high and low relative humidity cycle 

(fig. 12D). Under continuous exposure to high humidity, the joints glued 

with 200-percent-extended resin failed completely in 2 months, and those 

with 100 percent extender failed in I8 months. The 50-percent-extended 

resin showed only slightly higher strength than the resin extended 100 percent, although complete failure did not occur until 4 years of exposure. 

The unextended urea resin declined gradually in strength and wood failure, 

and failed completely after 7 years of continuous exposure to high humidity. 

In the high and low humidity cycle, joints glued with 200-percent-extended 

resin failed completely in 6 months, and those with 100 percent extender 

failed in 2 years. With 50 percent extender, durability was again only 

slightly better. The unextended urea resin failed gradually under these 

conditions, similarly to its performance under continuous high-humidity 

exposure. 

Casein. --Continuous exposure at 80° F. and 97 percent relative humidity 

was shown in figure 2 to result in the rapid breakdown of casein-glued 

joints. Experiments were therefore started to show the effect of adding 

Report No. 1530 -10-
preservatives to casein glue to increase its durability under exposure to 

high humidity conditions favorable to the growth of micro-organisms. 

Two preservatives, beta naphthol and creosote, were added to a commercial casein glue and to Forest Products Laboratory formula 4B casein 

glue in exposure tests of birch plywood. The addition of 10 percent beta 

naphthol or 20 percent of creosote to the glue, based on the weight of the 

dry glue powder, resulted in clearly increased durability when specimens 

were exposed continuously to 95 to 99 percent relative humidity and 80° F. 

Under these conditions of exposure, considerable decay had developed in 

the wood at the end of 20 months. Tests made after longer exposure 

periods measured decreasing wood strength rather than the quality of the 

glue joints. 

More recently developed preservatives, such as chlorinated phenols and 

their sodium salts, show a similar effectiveness in preventing organic 

deterioration of casein glues under high-humidity exposures that are 

especially favorable for the growth of molds and other micro-organisms, 5 

The improvement in mold resistance obtained with such preservatives 

added to two commercial casein glues is shown in figure 13. Retention 

of more than 60 percent of original dry strength and relatively high percentages of wood failure is shown throughout an 18-month period of continuous exposure at 80° F. and 97 percent relative humidity by the glues 

containing 5 percent pentachlorophenal (fig. 13A). The same commercial 

casein glues without preservative, however, characteristically decreased 

rapidly in strength under the same condition of exposure. Evidence that 

the effectiveness of casein glue to which a preservative has been added 

is not appreciably changed at normal or elevated temperatures is given 

in figure 13 (B and C), which is based on exposure of the same commercial glues at 80° F. and hi percent relative humidity and at 158° F. and 

60 percent relative humidity for 18 months. 

Short-term tests of birch plywood glued with casein glue containing various preservatives and subjected to extreme conditions favorable to mold 

development showed improved retention of joint strength compared to plywood glued with casein glue without preservative.5 in these accelerated 

tests, sapwood shear-test specimens were soaked in water containing 

mold spores and then stored in a moisture-saturated atmosphere at 72° 

to 80° F. before they were tested. Results are presented in table 1. 

Under these conditions, joints glued with a commercial casein glue without 

5 

Kaufert, F. H. Increasing the Durability of Casein due Joints with 

Preservatives. Forest Products Laboratory Report No. 1332. 

Report No. 1530 -11-
preservative had failed almost completely in 1 week, whereas the glues 

containing 5 percent of preservative retained considerable strength, in 

some cases with high percentages of wood failure, after 16 weeks of exposure. 

Although evidence obtained from 48-hour soaking tests indicates that the 

resistance of casein glues to short periods of soaking is not consistently 

improved by the addition of preservatives, disinfected casein-glued specimens have been found to resist continuous soaking in water better than 

specimens that were subject to bacterial attack when similarly exposed. 

The possibility is indicated that some improvement may result from using 

preservatives that are effective against bacteria.6 Results of prolonged 

periods of soaking or soaking-drying cycles are not available for casein 

glues containing a preservative. 

Intermediate-temperature phenol resin. --Although intermediate-temperature phenol resins are available in two distinct types, alkaline catalyzed 

and acid catalyzed, the durability values shown in figures 1 to 10 for intermediate-temperature phenols are based upon results obtained only 

from alkaline-catalyzed glues. In some of the earlier tests, the glued 

joints were cured for 16 hours at 140° F. Later work has shown that 

these conditions are not sufficient for complete cure of many glues of the 

intermediate-temperature phenol type, and in later tests curing periods 

as long as 8 hours at 220° F. were used to assure complete cure of the 

resin. 

The average values shown in figures 1 to 10 indicate that the alkalinecatalyzed intermediate-temperature phenol resins, when properly cured, 

are comparable in durability to the hot-press phenols. Results obtained 

with acid-catalyzed intermediate-temperature phenols were considerably 

different. In figure 14, a typical acid phenol resin is compared with a 

typical alkaline phenol under three conditions: (1) continuous exposure 

at 80° F. and 65 percent relative humidity, (2) exposure to repeated cycles 

of soaking and drying, and (3) continuous exposure at 158° F. and 20 percent relative humidity. The acid-type phenol failed to produce joints 

comparable in strength to those obtained with the alkaline-type glue, and, 

although no further deterioration was evident throughout 30 months of 

exposure to moderate humidity at 80° F., to the soaking-drying cycle, 

or to high temperature and low humidity, the original difference in joint 

6 

Christensen, Clyde M., and Moses, C. S. Molds and Bacteria That Delaminate Plywood Bonded with Casein and Soybean Glues. Bureau of 

Plant Industry, Soils, and Agricultural Engineering, Forest Pathology

Release No. 25, 1945. 

Report No. 1530 -12-
strength was retained throughout the test period. Since the veneer used 

was matched for both glues, the high percentages of wood failure retained 

throughout all exposures indicate damage to the wood by the acid-catalyzed 

glue. 

When the resins were partially cured at 140° F. for 16 hours, the differences between alkaline- and acid-catalyzed intermediate-temperature 

phenols were not so great, but damage to the wood by the acid-catalyzed 

phenol was evident.3 

Fortified urea resins. --Fortified urea resins may be classified as melamine-urea or as resorcinol-urea resins on the basis of the type of fortifier added to the urea-formaldehyde resin. The curves shown in figures 

1 to 10 for fortified urea resin are average values obtained for glues of 

this entire class, Differences between the performance of a melamineurea resin glue and a resorcinol-fortified urea resin glue over a period 

of 18 to 30 months of exposure to high temperatures and to high humidity 

are shown in figure 15. These data, which can be compared with similar 

data for other glues in figures 1 through 10, indicate that the melamine 

urea resin glue was generally more durable than the resorcinol-fortified 

urea resin glue. The melamine-urea tended to approach the performance 

of the straight melamine- and phenol-resin glues during the relatively 

short exposure of 18 months. 

More extensive studies of the performance of these two types of glues in 

birch plywood have since been made. 7 A series of laboratory-formulated 

glues containing different proportions of each fortifier with the urea resin 

were exposed under controlled conditions over a 4-year period. Results 

indicated that incorporation of at least 20 percent of melamine resin or 10 

percent of resorcinol with added paraformaldehyde in the urea resin 

distinctly improved the performance of birch plywood under continuous exposure at 158° F. and 20 percent relative humidity, and under repeated 

cycles of high and low humidity at 80° F. Fortification was least effective 

under continuous exposure at 80° F. and 97 percent relative humidity. 

Generally, the improvement with either melamine resin or resorcinol 

fortifier was in direct proportion to the amount of fortifier included, the 

most significant improvement being noted between the straight urea resin 

and the lowest amount of fortifier of each type. The performance of glues 

containing the highest proportions of each fortifier was almost as good as 

that of the straight melamine-resin glue. 

7 Blomquist, R. R., and Olson, W. Z. Durability of Fortified Urea-Resin 

Glues in Plywood Joints, Forest Products Journal, Feb. 1955, p. 50 

Report No. 1530 -13-
Weathering Tests of Plywood Panels 

Tests of unprotected plywood panels subjected to weathering have provided 

data on the durability of five types of glues under the variable conditions 

of outdoor exposure. Plywood test panels, 24 inches square, that were 

made of different species and different thicknesses of veneer, have been 

exposed since 1936. Records of their first 10 years of performance have 

been recorded. An equal number of yellow birch, yellow-poplar, and 

Douglas-fir panels was included in these tests, Three-ply panels were 

made from 1/16-inch veneer, and 5-ply panels from 1/8-inch veneer. 

Each panel has been visually inspected at regular intervals and rated on 

the basis of the integrity of the glue joints. The percentage of open joint 

area and the percentage of wood failure were considered in arriving at 

the rating for each glue shown in figure 16. The glues included in these 

tests, in order of decreasing durability, were as follows: hot-press 

phenol, intermediate-temperature phenol, room-temperature-setting 

and hot-press urea resins, and casein (without preservative). The relatively high rating of the hot-press and intermediate-temperature phenols 

indicates their superiority by a considerable margin over the urea resins. 

The intermediate-temperature phenol rating is based on one glue of the 

acid type that was cold-pressed and later cured in a kiln at 155° F. for 

24 hours. 

Exposure Tests of Laminated Beams 

The durability of several glues has been studied by exposing laminated 

beams to various conditions over periods up to 5 years. Casin and 

room-temperature-setting urea resin were used to glue 9 laminations of 

nominal 1-inch southern yellow pine and Douglas-fir lumber into beams 

7 by 7 inches in cross section and 5 feet 4 inches long. An early acidtype phenol resin was also included in these tests, but results obtained 

with this glue are not presented since they are not typical of modern intermediate-temperature phenols. The beams were glued at approximately 12 percent moisture content, and were subjected to several conditions, including outdoor exposure and exposure in an unheated shed. 

Similar southern yellow pine and Douglas-fir beams, in tests including 

casein containing 10 percent pentachlorophenol as an additional glue, 

were exposed continuously at 80° F. and 97 percent relative humidity. 

In the latter test, redwood heartwood beams were also glued with casein 

containing no preservative to determine whether the greater durability 

of redwood had any effect on the durability of glue joints. At intervals, 

sections were removed from the ends of these beams, and block-shear 

specimens were prepared and tested, after they were reconditioned to 

approximately 12 percent moisture content, to determine the strength

of the glued joints. 

Report No. 1530 -14- GPO 826965-4 
In a more recent and extensive study to determine the suitability of various glues for laminating white oak and other species for boat timbers and 

other purposes involving severe exposure, several hundred beams 4 feet 

long and usually 6 by 6 inches in cross section were laminated with intermediate-temperature phenol, resorcinol, and melamine-resin glues and 

cured at various temperatures. Sections from these beams were tested 

while dry to determine the original shear strength of the glue joints. 

Other sections were immersed in salt water to determine the effect of 

continuous soaking in salt water upon the strength of the glue joints. The 

beams themselves were placed on outdoor exposure racks without protection, except that one end was painted. At intervals, a short section was 

removed from the painted end to provide material for shear testing, and 

the cut end was repainted before further exposure. The unpainted end 

was inspected regularly for evidence of delamination. 

Results of Laminated Beam Exposure Tests 

Exposure to covered, unheated conditions. --The durability of joints made 

with casein (without preservative) and room-temperature-setting urea 

resin and exposed to covered, unheated conditions for periods up to 5 

years is shown in table 2. Under these conditions, no definite indication 

of loss of joint strength with prolonged exposure is evident. 

Continuous exposure at 80° F. and 97 percent relative humidity. --Data 

obtained in these tests are presented in table 3. After 6 months’ of exposure, casein without preservative failed to show appreciable strength 

with any of the 3 species tested. The greater resistance of redwood to 

decay had no apparent effect on the rate of joint strength loss. Casin 

glue containing 10 percent pentachlorophenol showed a remarkably increased durability, particularly in the Douglas-fir heartwood beams, 

which still retained nearly 80 percent of their original joint strength after 

5 years. Joints in the less durable southern yellow pine sapwood beams 

were effectively protected by the preservative for less than 1 year under 

this condition of exposure, which was extremely favorable to mold and 

other fungus development. The difference in effectiveness of the preservative between pine and Douglas-fir may have been due in part to the 

difference in moisture content, which was consistently about 2 percent 

lower in the Douglas-fir. The room-temperature-setting urea resin 

maintained good joint strength for about 1 year, but decreasing amounts 

of wood failure confirm the evidence of progressive weakening of this 

glue at 97 percent relative humidity and 80° F. shown by the Laboratory

plywood exposure tests. 

Report No. 1530 -15-
Exposure to outdoor conditions. --Under outdoor exposure, casein and 

urea-resin glues showed a tendency toward decreasing joint strength 

during the 5-year exposure period, as shown in table 4. Urea-resin 

glued joints lost strength at approximately the same rate in southern 

yellow pine and Douglas-fir. After 5 years of exposure, the joints retained between 30 and 40 percent of their original strength, but the 

beams showed serious amounts of delamination. Southern yellow pine 

beams glued with casein were completely delaminated after 4 years. 

In Douglas-fir, casein-glued joints still retained high strength values 

after 5 years of outdoor exposure, but considerable delamination had 

developed in these beams. 

The difference in durability of casein-glued joints between Douglas-fir 

and southern yellow pine may have been due to differences in moisture 

content observed throughout the period of exposure. Although all beams 

were glued at approximately 12 percent moisture content, the range of 

moisture content throughout the exposure period varied as follows: 

southern yellow pine, 14.4 to 30.0 percent; Douglas-fir, 11.9 to 18.9 

percent. The higher moisture content in the pine beams favors the development of molds and other micro-organisms known to be detrimental 

to casein glue. It appears that the lower moisture content that prevailed 

in the Douglas-fir was sufficient to prevent this form of deterioration in 

casein glue, which is known to cause rapid loss of joint strength at moisture content values above about 20 percent. 

Results of the study involving the determination of joint strength and 

measurement of delamination in beams of white oak, Douglas-fir, and 

southern yellow pine after exposure to the weather and to fresh and salt 

water confirm, in general, the durability of intermediate-temperature 

phenol, melamine, and resorcinol glues as determined from tests of 

plywood. Resistance to delamination while exposed outdoors was found 

to be a more critical test of glue-joint durability than retention of joint 

strength under continuous soaking in either fresh or salt water. These 

tests also showed that resin glues of the intermediate-temperature phenol, 

melamine, arid resorcinol types need a more complete cure to develop 

maximum resistance to delamination in timbers than is indicated by plywood tests. 

Typical measurements of delamination taken at intervals throughout an 

exposure period of 36 months on white oak beams glued with an intermediate-temperature phenol resin cured at temperatures of 110°, 160°, 

180°, and 210° F. are shown in figure 17A. Similar results for a resorcinol-resin glue cured at 80°, 110°, and 140° F. are shown in figure 

Report No. 1530 
17B. Curing at lower temperatures is generally adequate to produce a 

comparable degree of resistance to delamination in lower density species. 

Delamination and joint-strength data have established the fact that certain intermediate-temperature phenol, melamine, and resorcinol resins, 

when properly used and cured, are capable of producing glue joints that 

develop the full strength of the wood, either wet or dry, and maintain 

sufficient strength to avoid appreciable delamination under prolonged 

periods of exposure to weathering without protection. 

Summary 

Various methods of determining the durability of glues have been reviewed, 

and comparisons have been made of hot-press and intermediate-temperature phenol resins, hot-press melamine resins, resorcinol resin, fortified hot-press, hot-press, and room-temperature-setting urea resins, 

casein, animal, and vegetable (starch) glues on the basis of these tests. 

Most of the data available have been secured from Forest Products Laboratory tests of birch plywood specimens involving 10 constant and 10 

cyclic exposures to moisture and to low and high temperatures. Additional information has been supplied by exposure tests of plywood panels 

of different wood species, by exposure tests of laminated beams, and by 

accelerated tests of various sorts. 

The durable nature of the phenol resins is evident under all conditions of 

exposure used in these tests. In general, the strength of the glue joint is 

largely limited by the ability of the wood itself to resist the conditions of 

exposure. In the intermediate-temperature phenols, however, pronounced 

differences exist between certain alkaline and acid glues. When fully 

cured through the application of heat, the more acid glue appears to cause 

deterioration of the wood, with resultant low joint strength, although high 

percentages of wood failure are maintained. The alkaline-catalyzed intermediate-temperature phenol glues do not show this effect. 

Resorcinol and hot-press melamine glues withstand severe conditions of 

exposure to heat and moisture, and appear to be similar to phenol resins 

in durability. 

Most forms of urea-resin glue are moderate to low in resistance to exposure at high humidities and ordinary temperatures, and low in durability under conditions involving high temperature, especially combinations of high temperatures and high humidities. The effect of high 

Report No. 1530 -17-
temperature is most severe in room-temperature-setting urea resin, and 

decreases in severity with the hot-press and fortified urea-resin types. 

Under continuous exposure at 158° F., room-temperature-setting urea 

resin was the least durable glue tested, whereas urea resins containing 

equal or larger proportions of melamine or resorcinol resin tend to approach but not equal the durability of phenol resins. Room-temperaturesetting urea resins are more acid than other glues of the urea-resin type, 

and many of them appear to lose appreciable strength over a period of 3 

years when merely aged at 80° F. and 65 percent relative humidity. All 

urea-resin glue types in birch plywood are good to high in their resistance 

to continuous soaking at room temperature and to soaking-drying cycles, 

but cycles of low and high humidity and exposure to weather cause more 

rapid deterioration of most of these glues. Urea resin highly fortified 

with melamine resin appears to be comparable to phenol resin in resisting cycles of high and low relative humidity. 

Room-temperature-setting urea resins, when used with certain woods 

such as yellow birch, failed when continuously soaked in salt water. With 

other species such as oak, room-temperature-setting urea resins failed 

to withstand prolonged immersion in either fresh or salt water. Flourextended urea resins are moderately reduced in resistance to soaking, 

and the degree of reduction depends upon the amount of flour extension. 

Attack by molds under high humidity conditions is evident even for urea 

resins that contain only small amounts of flour extender, however, and 

deterioration under these conditions is very rapid in the case of highlyextended urea glues 

Casein glue without preservative lacks durability under all conditions of 

exposure to weathering or to relative humidity above 90 percent. Casein 

shows good resistance to high temperatures. The inclusion of preservative chemicals, such as chlorinated phenols, increases the durability of 

casein-glued joints at high relative humidities without apparent detrimental effects on durability in other respects. The effect, however, is 

somewhat temporary. Inclusion of such chemicals does not in itself 

assure permanence when exposed continuously over a period of years to 

warm, damp atmospheres. 

Animal and vegetable (starch) glues maintain good strength in wood for 

long periods of time as long as the wood remains dry. These glues are 

not resistant to moisture, and deteriorate rapidly when exposed to relative humidities above 80 percent. 

Where service conditions are such that durability under extremes of high 

and low humidity and in cycles involving high temperature is important, 

Report No. 1530 -18-
and where methods of applying sufficient heat are available, the hotpress and intermediate-temperature phenol resins, hot-press melamines, 

and resorcinols offer the most satisfactory durability characteristics of 

the glues that have been tested. 

In many operations, the selection of glue is limited to those that set at 

room temperature. Of the room-temperature-setting glues included in 

these tests, casein without preservative is not durable under cycles involving high humidity due to its low mold resistance, although when suitable preservative chemicals are added the resistance of this glue to 

molds is improved. Room-temperature-setting urea resin loses strength 

under exposures involving high relative humidity at room temperature. 

Under continuous exposure at 97 percent relative humidity and 80° F. 

neither joints bonded with urea-resin nor those bonded with casein, with 

or without preservative, can be relied upon to remain permanently strong 

over a period of years. In resistance to exposures involving high temperatures, the superiority of casein over room-temperature-setting urea 

resin is clearly shown. When service conditions involve occasional or 

frequent soaking in addition to high temperature and extreme humidity 

exposures, neither casein nor room-temperature-setting urea glue appears to be satisfactory, The maximum degree of durability available 

at room temperatures is provided by the resorcinal resins. Although 

resorcinol resins are not fully cured at room temperatures and may require beating to somewhat elevated temperatures for certain critical uses, 

such as in white oak boat timbers, glues of this type cured at room temperature are highly durable in plywood specimens under all conditions of 

exposure used in these tests. 

Report No. 1530 -19- 1. -41 
Table 1. -- Comparative resistance of a commercial casein glue with and without preservative as measured 

by tests of standard yellow birch plywood shear specimens after exposure to conditions 

favorable to mold development 

1

The first value represents joint strength in pounds per square inch; the second value represents the percentage of wood failure. Each value represents an average of eight shear-test results. 

2

Mold test as described in Forest Products Laboratory Report No. 1344, revised 

3 

Based on dry weight of glue powder. 
Table 2. -- Durability of joints in unpainted laminated 

beams exposed to unheated conditions 

while covered 

CASEIN WITHOUT PRESERVATIVE 

ROOM-TEMPERATURE-SETTING UREA RESIN 

Report No. 1530 
Table 3. -- Durability of joints in unpainted laminated beams continuously exposed to 97 percent relative humidity 

and 80° F. 

CASEIN WITHOUT PRESERVATIVE 

CASEIN PLUS 10 PERCENT PENTACLOROPHENOL 

ROOM-TEMPERATURE-SETTING UREA RESIN 

Report NO. 1530 
Table 4. -- Durability of joints in unpainted laminated 

beams under outdoor exposure 

CASEIN WITHOUT PRESERVATIVE 

ROOM-TEMPERATURE-SETTING UREA RESIN 

Report No. 1530 
Figure 1.--Resistance of birch plywood joints, glued with eight types of 

ZM 71047 F glues, to continuous soaking in water at room temperatures. 
Figure 2.--Resistance of birch plywood joints, glued with eight types of 

glues, to continuous exposure at 97 percent relative humidity and 80° F. 

ZM 71048 F 
Figure 3.--Resistance of birch plywood joints, glued with eight types of 

glues and exposed to a repeating cycle of 2 days’ soaking in water at room 

temperature followed by 12 days’ drying at 80° F. and 30 percent relative Z M 71049 F humidity. 
Figure 4.--Resistance of birch plywood joints, glued with eight types of 

glues, and exposed to a repeating cycle consisting of 2 weeks in 97 percent

relative humidity at 80° F., followed by 2 weeks in 30 percent relative. 

humidity at 80° F. 

Z M 71050 F 
Figure 5.--Resistance of birch plywood joints, glued with eight types of 

glues and kept under continuous exposure at 80°F. and 65 percent relative Z M 71051 F humidity. 
Figure 6.--Resistance of birch plywood joints, glued with eight types of 

glues and exposed continuously at 158° F. and 20 percent relative humidity. 

Z M 71052 F 
Figure 7.--Resistance of birch plywood joints, glued with eight types of 

ZM 71053 F glued and exposed continuously at 158° F. and 60 percent relative humidity. 
Figure 8.--Resistance of birch plywood joints, glued with eight types of 

glues and exposed to a repeating cycle of 8 hours at 158° F. and 20 percent

relative humidity followed by 16 hours at 80° F. and 65 percent relative Z M 71054 F humidity. 
Figure 9.--Resistance of bircy plywood joints, glued with eight types of 

glues and exposed to a repeating cycle consisting of 8 hours at -20° F. 

Z M 71055 F followed by 16 hours at 80° F. and 65 percent relative humidity. 

Figure 11.--Resistance of birch plywood specimens, glued with 

different cold-setting urea-resin glues, to (A) continuous 

exposure at 80° F. and 65 percent relative humidity, (B) 

repeated cycles of alternatingf high (97 percent) and low 

(30 percent) relative humidity at 80° F., (C) continuous 

exposure at 158° F. adn 60 percent relative humidity, and 

(D) repeated cycles of exposure at room temperature and 

158°F. Z M 71057 F 
Figure 12.--Resistance of birch plywood specimens, glued with unextended and 

rye-flour-extended hot-press urea resin, to (A) continuous soaking, (B)

repeated cycles of soaking and drying, (C) continuous exposure at 80° F. 

and 97 percent relative humidity, and (D) repeated cycles of high (97

percent) and low (30 percent) relative humidity. 

Z M 71058 F 
Figure 13.--Resistance of birch plywood joints, glued with different casein 

glues. with and without the addition of pentachlorophenol under (A) continuous 

exposure at 80° F. and 97 percent relative humidity, (B) continuous exposure 

at 80° F. and 65 percent relative humidity, and (C) continuous exposure at 

158° F. and 60 percent relative humidity. Zm 71059 F 
Figure 14.--Resistance of birch plywood joints, glued with alkaline and acid 

intermediate-temperature phenol resisn,, under (A) continuous exposure at 

80° F. and 65 percent relative humidity, (B) repeated cycles of soaking and 

drying, and (C) continuous exposrue at 158° F. and 20 percent relative 

humidity. 

Z M 71060 F 
Figure 15.--Resistance of birch plywood joints, glued with 

melamine- and resorcinol-fortified urea resins, under (A)

repeated cycles of exposure to high and low humidity, B 

repeated cycles of exposure at room temperatures and 158° F., 

(C) continuous exposure at 158° F. and 60 percent relative 

humidity, and (D) continuous exposure at 200° F. and 20 

percent relative humitdity. Z M 71061 F 

Figure 17.--Resistance to delamination of glue joints in white oak beams cured 

at various temperatures and exposed to the weather without protection, (A)

intermediate-temperature phenol-resin glue, (B) resorcinol-resin glue. Z M 71063 F 

GPO 826968-3