!MAIMED' OF PAPIREG-TO-PAPREG APAPREG-TC-131ral GLUM JOINTS

November 1945

NO. 1538

UNITED STATES DEPARTMENT OF AGRICULTUREFOREST SERVICE

FOREST PRODUCTS LABORATORYMadison, Wisconsin

In Cooperation with the University d Wisconsin

DURABILITY OF PAPREG-TO-PAPREG AND

PAPREG-TO-BIRCH GLUED JOINTS1-

By

HERBERT W. EIOKNER, Engineer

Introduction

A series of gluing studies on papreg was made and reported in ForestProducts Laboratory Report No. 1348 (Revised) "The Gluing of Laminated PaperPlastic (Papreg)" in which such factors as the preparation of gluing sur-faces, evaluation of glues for the bonding, the range of good gluing condi-tions, the gluing of papreg to wood, and the nail gluing of thin papreg wereinvestigated. In general, these studies show that, with the proper glues,surface preparation; and gluing conditions, joints that have high initialstrength can be obtained between papreg and papreg, and papreg and wood.

It is the purpose of this study to determine the durability of suchwell-glued joints when aged under controlled conditions of moisture andtemperature. Joints between papreg surfaces and between papreg and woodwere prepared with five types of glue and were subjected to six exposureconditions. To detect changes in joint quality with age, specimens are tobe tested at the ends of various periods up to 5 years. This report pre-sents the results of the tests after periods of exposure up to 1 year.

Experimental Procedure

Papreg

The 1/16-inch-thick papreg used for this study was pupared by press-

ing together approximately 30 sheets of Hitscherlich paper,- impregnated withan alcohol-soluble, phenol-formaldehyde resin. Stainless steel cauls wereused. All sheets were laminated with the fiber direction of the papers par-allel, under a pressure of 250 pounds per square inch for 20 minutes at320° F.

'This is one of a series of progress reports prepared by the Forest ProductsLaboratory relating to the use of wood in aircraft. Results here reportedare preliminary and may be revised as additional data become available.

2 • •-This impregnated paper was manufactured by a commercial paper company fol-lowing recommendations of the Forest Products Laboratory.

Report No. 1538 -1-

The resulting papreg had a density of 1.3 to 1.4 grams per cubiccentimeter and a resin content of 30 to 35 percent.

Wood.

Yellow birch veneer (1/16 inch thick), selected for straightness ofgrain, was used in preparation of the papreg-to-wood joints.

Preparation of Material for Gluing

The birch veneer and papreg were conditioned for at least 1 week at80° F. and 30 percent relative humidity before being cut into 5- by 12-inchpieces in preparation for gluing. The fiber direction was parallel to the5-inch dimension of the pieces.

Number 1/2 garnet sandpaper was used on a portable belt sander toremove 0.002 to 0.003 inch from the face of the papreg panels before theywere bonded together or bonded to the wood,

Type of Specimen

Panels 5 by 12 inches were prepared by gluing together two of the1/16-inch papreg pieces or one of the papreg pieces and a piece of theyellow birch veneer,

From the completed panel, 10 lap hear specimens of the type shownin figure 1 were cut.

Glues

The following glues were used in preparinL; the joints:

a cold-setting, urea resin,an alkaline, low-temperature phenolic resin,an 'acid, low-temperature phenolic resin,a casein clue plus a preservative,

3a low-temperature resorcinol resin,-

Gluing Conditions

Glue spread.--A double spread was used to provide a total of 15 to 20grams of wet glue per square foot of glue joint.

The resorcinol glue employed in t'ais study was not typical of currentresorcinol-formaldehyde glues in that it was not adequately water-resistantwhen cured at 75° Y, and was shortly replaced thereafter by an improvedformulation.

6pOrt NO, 1538 -2-

Assembly time,--A closed assembly time of 10 to 20 minutes was used.

Pressure.--A pressure of 150 pounds per square inch was employed forall gluing.

Curing time and temperature.--The joints glued with the urea-resin,casein, and low-temperature resorcinol glues were pressed for 18 hours at75° F. The joints prepared with the two low-temperature phenolic resinglues were pressed for 18 hours at 160° F.

Conditioning.--All joints were conditioned at least 1 week at 30 -per-cent relative humidity and 80° F. before being cut into specimens and placedin the exposure racks.

Distribution of Specimens

From the 48 panels or 480 specimens prepared with each glue, 60 speci-mens were selected as controls and the remaining 420 specimens were dividedequally among six conditions of exposure, Ten specimens were tested fromeach condition after aging for 1, 3, and 6 months and 1 year, Ten specimenswill be tested after 2, 4, and 5 years.

Distribution was made so that not more than two specimens per panelwere included in any of the six exposure conditions and not more than twospecimens per panel were tested as controls nor will be at any testing inter-val.

Exposure

The six conditions of exposure are:

1. Continuous exposure to 80° and 30 percent relative humidity;test dry.

2. Continuous soaking at room temperature; test wet.

3. Continuous exposure to 80° F. and 97 percent relative humidity;test on removal from the 97 percent relative humidity room.

4. A repeating cycle of 2 weeks at 80° F. and 97 percent relativehumidity and 2 weeks at 80° F. and 30 percent relative humidity; test onremoval from the 30 percent relative humidity room.

5. Continuous exposure to 200° F. and 20 percent relative humid-ity; test after cooling to room temperature.

6. A repeating cycle of 1 day at 200° F. and 20 percent relativehumidity and 1 day at -25° F.; test at room temperature. When the scheduleof changes falls on Sunday, the specimens are allowed to remain 2 days atthe previous condition.

Report No, 1538

Testing

The specimens were tested in shear in a plywood-joint testing machinewhich loaded the specimen at a rate of 700 to 800 pounds per minute.

Summary of Results of First Year

The joint shear strength and the estimated percentage of failures inwood and.pacreg specimens tested after exposure to the various conditionsduring the first year are given in tables 1 and 2.

The control specimens all had high joint strengths and the averagepapreg and birch failures were over 9(1 ) percent. The joints glued with theresorcinol resin had the lowest average control strengths of 531 and 575pounds per square inch for papreg-to-wood and yiapreg-to-papreg joints, re-spectively, as compared to minimum control values of 636 and 674 pounds persquare inch for the joints bonded with the other glues, but showed the mostsignificant change (an increase) in strength of any of the glued joints whenaged at 800 P. and 30 percent relative humidity. The resorcinol glue usedwas one of the early glues of this type to be developed and it is possiblethat the curing conditions used did not produce a fully cured glue joint.There was some decrease in papreg-and-birch failures in urea-resin gluedpapreg--to-.birch joints after aging for 3 months to 1 year at 80° F. and 30percent relative humidity.

There was a pronounced loss of strength in the resorcinol- andcasein-glued joints after 1 month of soaking. The joints glued with theacid, low-temperature phenol had somewhat lower strength than the jointsglued with the alkaline, low-temperature phenol or the cold-setting urea-resin glues.

After soaking for 1 year; the joints glued with the two low-tempera-ture phenolic resins and the cold-setting urea-resin glues had averagepapreg and birch failures of over 90 percent. The casein-glued joints hadless than 15 percent papreg or birch failure after this exposure, and theresorcinol-glued joints had average papreg failures of 31 percent in thepapreg--to-papreg joints exposed for 1 year.

Continuous exposure to 97 percent relative humidity at 80 0 P. causedgeneral loss of joint strength but much of this loss can be attributed tothe higher moisture content at test. Tne joints glued with the casein andresorcinol glues may have lost more than can thus be accounted for, but theresults of further tests are required to make this certain.

The alternating cycle of 97 percent and 30 percent relative humidityat 80° F. has not, in 1 year, caused any significant change in either thestrength or type of failure of any of the glued joints with the possibleexception of the urea glue between papreg and birch.

Report No. 1538 -4-

The joints glued with the cold-setting urea-resin failed very rapidlywhen exposed continuously to 200° F. In 1 month, the papreg-to-papreg andpapreg-to-birch joints failed over 95 percent in the glue, ana, at the end of1 year the joint strength was one-tnird or less that of the original.

Papreg-to-papreg joints glued with the four other glues continued tomaintain over 75 percent ?apreg failure at the end of 1 year exposure to2000 F. but the resorcinol and acid-low-temperature-phenolic-resin gluedjoints lost strength. The loss of strength in the acid-phenolic-resin gluedspecimens was probably due to the effect of heat and acid on the papreg, butthis explanation would not appear to hold for resorcinol-glued joints sincethe pH of the resorcinol glue is not low.

The papreg-to-birch joints glued with the phenolic-resin and resorcinolglues developed over 90 percent wood or papreg failure after 1 year of expo-sure to 200° F„ but the joint strengths were greatly reduced. Many of thejoint specimens failed in the birch ply in tension. The casein-glued jointsdecreased somewhat in percentage of wood or papreg failure but the jointstrengths were higher than those obtained with any of the other glues. Thejoints made with urea glue failed conspicuously at 200° P.

The cycle of 2000and -25° F. repeated for 1 year reduced thestrength of the papreg-to-birch joints, that were urea-resin glued, to thesame level as the continuous 2.000 F. exposure for the same period, but therate of deterioration of the glue joint in the papreg-to-papreg specimenswas slower than when exposed continuously.

After 1 year of exposure to the high- and low-temperature cycle, thepapreg-to-papreg and papreg-to-birch joints glued with the three other resinglues failed over 90 percent in the wood or papreg, -with low strength onlyin the joints glued with the acid, low-temperature phenolic resin.

The papreg-to-papreg siecimens glued with casein withstood the high-low temperature cycle fairly well with papreg failure 90 percent or more, andwith relatively high joint strength, but the papreg-to-birch joints had only35 percent papreg or wood failure after 1 year of exposure and loss ofstrength was more pronounced.

Summary

All of the glues (casein, acid and alkaline low-temperature phenolicresin, cold-setting urea resin, and resorcinol resin) produced well-gluedoriginal joints of papreg-to-papreg, and apreg-to-wooa, but there was someevidence that the resorcinol,glued joints were somewhat undercured.

The strength of joints glued with the acid low-temperature phenolicresi.r ware much lower after certain exposures, but consistently showed highpercenGages of wood or papreg failure.

Report No. 1538 .5-

The joints glued with the alkaline low temperature phenolic resinretained a high percentage of joint strength and had Wood or papreg failuresof 96 percent or better under all conditions.

The preserved..casein glued joints showed only a small loss of strengthat continuous exposure to 97 percent relative humidity, and practically nonefrom the alternating 97 and 30 percent relative-humidity cycle. Continuoussoaking, however, greatly weakened the strength of these joints., At the hightemperature exposures, the Casein-glued joints maintained relatively highstrengths, but the percentage , of wood or papreg failure in the papreg-to-woodjoints was considerably less than the percentage of papreg failure in thepapreg-to-papreg joints.

tThe cold-setting, ureawresin glued joints had good durability at allconditions except where high temperatures were involved. Exposures involvingtemperatures of 200° F, quickly reduced the strength of these joints.

The resorcinol-,glued joints, as pre)ared for this study, were probablyundercured as they did not have the durability under- continuous soakin thatmight be exoected for well-cured joints. Under the other conditions of expo-sure, these glued joints had slightly higher strengths than those clued withthe acid low-temperature phenolic resin,

Report No. 1538

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