DURABILITY OF RESIN-TREATED PAPER HONEYCOMB CORE

7/28/2019 DURABILITY OF RESIN-TREATED PAPER HONEYCOMB CORE

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DURABILITY OF RESIN-TREATED PAPER HONEYCOMB CORE

By

K. H. BOLLER, Engineer

Forest Products Laboratory,1 Forest ServiceU. S. Department of Agriculture

- - -

Abstract

Thi s report presents the results of tests to determine the durabil i ty oresin-treated paper honeycomb core. Durability was determined by flatwcompression and tension tests of core alone and core in sandwich with tplywood and with aluminum facings that had been exposed to outdoor weating and to accelerated aging. The investigationenough protection to core so that weathering for

terious effect on the core. Bare core, however,of its tensile and 55 percent of its compressiveweathering. Accelerated aging had little effect

I ntroduction

showed that facings pr5 years had only sligh

retained only about 35strength after 5 yearson core strength.

One of the factors that determine the usefulness of a product is its du

bi l i t y. Even though the product of manufacture and resecarch is strong to withstand applied loads, the product must also be durable enough to stand changes in climatic conditions over a period of time. The durabof building materials is especially important because their usefulness musually be extended over long periods.

Information on the durabil ity of a new product such as sandwich panels paper honeycomb cores is especially meager. Exposures and subsequent mcal tests can be used to determine the degree of deterioration when the

terial is exposed in nonstressed condition. Aging tests of book and docpapers stored for extremely long periods of time show that the quality papers made with purified rag and wood pulp fibers would be good after

1Maintained at Madison, Wis., in cooperation with the University of Wis


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decades or more if they were not exposed to acidic atmospheres.2, 3,4

However, information on the aging of resin-treated papers, especiallysandwich construction, is needed. This research on aging and durabof a sandwich--type building material was undertaken to fi ll that gacooperation with the Douglas Aircraft Company of Santa Monica, Calif

evaluation was made of the effects of accelerated aging cycles and ofyears of natural weathering cycles on cores and sandwich panels.

This investigation is intended to offer a compari son of the detrimeneffects of a standard accelerated aging environment and of natural aoutdoors. The cycles of accelerated aging are, of course, conducted controlled laboratory conditions, while the cycles of natural weathewere experienced outdoors under varying conditions at two sites. On

door weathering location was at Madison, Wis., and the other near L oAngeles, Calif.

This report presents the results of observations and mechanical testssandwich panels and paper honeycomb core material after exposure to various environments. Mechanical tests were made in tension and comsion in a direction parallel to the flutes of the core, that is, flatto the sandwich panel. The various environments include a control c

tion (at 75 F. and 64 percent relative humidity), outdoor weatherinWisconsin and California, and accelerated aging that consisted of cycof extreme temperature and relative humidity conditions.

Material

The sandwich panels and core panels were furnished by the cooperatocore panels consisted of a resin-treated paper honeycomb core made opound kraft paper, expanded to a honeycomb having 7/16-inch hexagonand then treated with 35 percent of an alcohol-soluble phenolic restions of this core material were evaluated (1) alone or without facin(2) with plywood facings, and (3) with aluminum facings.

The plywood facings, 0.070 inch thick, consisted of three-ply exterio

yellow-poplar. The aluminum facings consi sted of 0.064-inch-thick a7075 aluminum. These facings were bonded to the paper cores with a

polyvinyl butyra1 adhesive according to the standard procedure of th

2Kimberly, A. E . , and Emley, A. L. A Study of the Deterioration of BPapers in L ibraries National Bureau of Standards Misc. Pub. NO.


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each period three panels were removed, dimensional and weight owere made, and then specimens were cut from the panels for streThese mechanical test specimens were conditioned to temperature equil ibrium at 75 F. and 64 percent relative humidity before mtests were conducted.

The other natural weathering site was also in a temperate zone, located on the seashore, within 100 yards of the ocean, at E l SeCal i f . This location is at 33 latitude in a temperate zone whsummer and winter air temperatures range from 100 F. to 35 F.there are contrasting daily changes in temperature, humidity, anvelocity. The site is protected by a 4-foot bank of broken chunconcrete, but the wind carries highly humid air and salt spray tpanels. Representative panels were exposed here as at Madison, After their respective periods of exposure they were shipped to Forest P roducts L aboratory for observations, cutting, conditiontesting. The periods of exposure were 0, 1, 9, 18, and 60 monning February 23, 1954,

(3) Accelerated Weathering.--The accelerated weathering consiste

series of controlled conditions of extreme temperature and relahumidity. Each cycle consisted of the following exposures:

(a) Immersed in water at 122 F. for 1 hour.

(b) Sprayed with wet steam at 194 to 200 for 3 hours.

(c) Stored at 10 F . for 20 hours.

(d) Heated in dry air at 212 F . for 3 hours.

(e) Sprayed with wet stream at 194 to 200 F. for 3 hours.

(f) Heated in dry air at 212 F. for 18 hours.

Representative panels were exposed for from zero to six cycles,

each cycle the panels were conditioned to equilibrium temperatuweight conditions at 75 F . and 64 percent relative humidity. vations of weight and dimensions were made before cutting the pspecimens. The specimens were again conditioned and tested at 50 percent relative humidity. Mechanical strength tests were tecompression


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All the specimens were tested between the heads of a universal-typmachine. The upper head held a spherical seat through which the loapplied, and it traveled at a head speed of 0.006 inch per minute.the test, at regular intervals of load, deformation data were obtameans of a Marten's mirror compressometer of 1-inch gage length mo

two opposite sides of the specimen.

The tensile specimens were 2-5/8 by 2 by 2 inches. These specimenalso tested in a direction parallel to the flutes. The load on thwood facings, and hence on the flutes, was applied through 2- by 22-inch maple blocks. The load on the aluminum-faced specimens andthe core-only specimens was applied through aluminum blocks of the size. The 2-inch cubes were glued to their specimens with an epoxset at 200 F. under pressure of 15 pounds per square inch. The l

applied to the 2-inch cubes by bolts that were perpendicular to thand at right angles to each other. The load was applied through aspeed of 0.025 inch per minute in a universal-type test machine.

Presentation of Data

Table 1 presents observations after the various periods of exposurtive to appearance, dimensions, and weight of each panel. The obsare on sandwich and core panels subjected to only the control con75 F. and 64 percent relative humidity. The effect of exposure tconstant condition is negligible. I t can be seen that there is vdimensional or weight change and no change in appearance.

Table 2 presents observations on sandwich and core panels subjecteWisconsin outdoor weathering. According to these observations and

(1) the aluminum-faced panels became corroded and discolored by so(2) the core-only panels became distorted and discolored, and (3) plywood-faced panel after 5 years had its top ply completely erodethe glue film.

Table 3 presents observations on sandwich and core panels subjecteCalifornia outdoor weathering. According to these observations anout benefit of original data before exposure, California outdoor e

causes the core panel to be distorted. The discoloration of aluminfaced panels is toward a whitish gray due to the salt deposit, and top ply of plywood eroded away.

Figures 1 through 7 show the appearance of weathered panels.

Table 4 presents the observations on sandwich and core panels sub


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into specimens for machanical tests. E ach cycle represents 6 expso that the first panel was evaluated for strength after 6 exposuthe last panel was evaluated after 36 exposures. The data show mmensional and weight changes but in general all panels appeared stand this accelerated aging exposure.

Tables 5, 6, and 7 present the tensile and compressive propertiesaluminum-faced, plywood-faced, and core-only panels after they hadsubjected to their respective exposure end reconditioned to equiconditions.

Discussion of Results

The results presented in this investigation bring out four degreeseverity of weathering or exposure on three constructions. Theseconstructions were basically paper honeycomb cores with three degprotecti on. One construction with aluminum facings had the protof a metal on two sides which shielded the core at the center of greatly retarded the movement of moisture, and reflected the sun

The construction with plywood facings also protected the core mato a lesser degree as shown in figure 8, The core alone, howeverentirely unprotected so that it received the full amount of tempechange, moisture, and sun's rays.

The factors that influence the effect of aging and weathering in exposure conditions may be recognized as follows: (1) The controwere exposed to constant temperature and relative humidity condithe absence of sunlight; (2) the accelerated weathering panels werto variable temperature and moisture conditions for short periodsand also with the absence of sunlight; (3) the panels weathered oCalifornia were exposed to variable temperatures, humidity, sunlisalt spray; and (4) the Wisconsin panels were also exposed to vatemperature, humidity; and sunlight, but the local conditions caustemperature range that was greater than that in California. No swas experienced but snow, hail, and soot was added.

The mechanical property that best measures the effects of these vdegrees of aging on the core is the maximum compressive strengthto the flutes. I n this compression test only the paper core, afrespective aging, resisted the compressive forces until failure oI n the tension test however the weakest link such as paper to


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controlled atmosphere, the panels at each period of exposure have a pressive strength about 45 pounds per square inch stronger than the

two types of panels. The core alone is the weakest, and the plywoopanels appear to have intermediate strength.

The sandwich panel facings protected the cores to the extent that nappreciable damage was done to the cores during either natural oraccelerated aging and weathering. Each of the points on figure 8 the average of five specimens so they show a trend in spite of theter of individual r esults shown on the tables. I t can be observed there was somewhat of an increase in strength for the earlier perioof exposure, This trend was probably due to additional curing of resin. The core panels had no protection and hence the three exporeduced the core strength. The accelerated exposure caused about 1

percent reduction in strength and the outdoor exposures caused abou5 percent r educti on. The sunlight may have had a detrimental effethe unprotected core material.

Tensile strengths of the paper honeycomb sandwich constructions afaging are summarized in figure 9. The tensile strength shown heresents the flatwise strength of the entire sandwich construction.trends in strength for earlier periods of exposure probably reflec

tional cure in resin or adhesives.The plywood-faced sandwich

had the lowest tensile values because its failure was in the plywooperpendicular to the grain. Thi s type of fai lure only indi cates tthe strength of the bond between facing and core and the strength core exceeded the observed values. For the aluminum-faced sandwicfailures usually occurred in the bond between the facing and the cThi s type of fai lure again only indi cates that the core is strongerthe bond. For the core-only material, failures usually occurred inpaper core and hence the values shown are indicative of the effect

aging. Tensile strength of the core-only material shows that the controlled exposure and the accelerated aging had very little effecthe strength. However, there is a substantial reduction in strengtto natural outdoor exposure. That reduction is about 60 percent, wagain may be due to the effect of the sunlight on the unprotected phoneycomb.

These mechanical tests have shown four degrees of exposure in whicdeterioration due to accelerated aging was compared to that due to

natural aging and weathering. The deterioration due to acceleratedunfortunately, was not severe enough to develop a definite trend. scatter of individual results obscures the slight trend that mightSince there is not a measurable deterioration due toerated aging, there is not a correlation between theaccelerated aging and the number of years of natural

six cycles ofnumber of cycweathering.


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(1) Aging in constant temperature and relative humidity conditions effect on the strength of sandwich and core panels after 5 years,

(2) Accelerated aging by six cycles of variable temperatures and humidity had only a slight tendency to reduce strength.

(3) Natural weathering at Wisconsin and California outdoor sites conly a slight reduction in strength of sandwich construction.

(4) Aluminum or plywood facings provided protection to the core mso that the latter was only slightly affected by natural weatherin

(5) Unprotected core material, however, retained only about 35 pe

of its tensile strength and 55 percent of its compressive strength5 years of natural exposure.


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Table 1. --Sandwich panels subjected to aging and controlledexpossure at 75 F. and 64 percent relativehumidity--Continued

ALUMINUM FACING AND PAPER HONEYCOMB CORE

PAPER HONEYCOMB CORE ONLY

(Page 1 of 2)


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Table 1. - - Sandwich panels subjected to aging and controlledexposure at 75 F. and 64 percent relativehumidity - - Continued

PLYWOOD FACING AND PAPER HONEYCOMB CORE

(Page 2 of 2)


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Table 2.--Sandwich panels subjected to Wisconsin outdoorexposure




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Table 2.--Sandwich panels subjected to Wisconsin outdoorexposure--Continued

(Page 2 of 2)


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Table 3.--Sandwich panels subjected to California outdoorseaside) exposure



PLYWOOD FACING AND PAPER HONEYCOMB CORE


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Table 4. - -Sandwich panels subjected to accelerated weathering




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Figure 1. --Sandwich and core panels on exposure rack at Madison W

as they appeared after one day of exposure.

ZM 95274 F


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Figure 2. --Sandwich and core panels on exposure rack at Wis., after 1 month of exposure.

ZM 95499 F


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Figure 3. --Sandwich and core panels on exposure rack

at Madison, Wis., after exposure for 18 months.


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Figure5.--Corepa

nelsafter5years'exposuretoWisconsinou

tdoors(left),Californiaout-

doors(center),a

ndcontrol(right).

ZM

115292


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DURABILITY OF RESIN-TREATED PAPER HONEYCOMB CORE

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