PO FILE COPY 113 1:31 REMOVE FROM FILE EFFECTS CIF MOISTURE ON EACTERIAIL WEAKENING CIF CASEIN-BONDED PLYWOOD INFORMATION REVIEWED AND REAFFIRMED 1963

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1 EFFECTS CF MOSTURE O EACTERAL WEAKEG CF CASE-BODED LYWOOD 277 April 957 FORMATO REVEWED AD REAFFRMED 963 O FLE COY 3 3 REMOVE FROM FLE i 3 ML!!! a' FOREST RODUCTS LABORATORY MADSO 5 WSCOS UTED STATES DEARTMET OF AGRCULTURE FOREST SERVCE fn Cperatin with the University f Wiscnsin

2 EFFECTS OF MOSTURE O BACTERAL WEAKEG OF CASE-BODED LYWOOD By CATERE G DUCA, athlgist Frest rducts Labratry - Frest Service U S Department f Agriculture Summary Shear tests were made n standard specimens f 3/6-inch, casein-bnded plywd t determine the effects f bacterial infectin f the glue line with the wd at varius misture cntent levels The results indicated that within 3 days the bacteria, all f which were capable f digesting casein, did nt materially weaken the glue jints when the misture cntent f the wd was belw abut 55 percent Abve 55 percent, hwever, the bacteria caused cnsiderably mre weakening f the jints than culd be accunted fr by hydrlysis alne Slight bacterial deteriratin f the glue was indicated within 48 hurs, indicating that bacteria may becme active in a very shrt time if the misture cntent f the wd is high enugh The questin remains whether mlds may als weaken glue bnd ntrductin revius experience and labratry tests have shwn that the bnd f caseinglued plywd may weaken and ultimately fail when expsed cntinuusly t warm, humid cnditins This failure has been variusly attributed t chemical hydrlysis, bilgical deteriratin f the prtein material in the glue, r mechanical stresses induced by shrinking and swelling Christensen and Msesa shwed that naturally ccurring infectins f mlds and bacteria play an imprtant rle in the deteriratin f casein glue, at least when the misture cntent f the wd is abve 5 percent Their Maintained at Madisn, Wis, in cperatin with the University f Wiscnsin -Christensen, C M, and Mses, C S Mlds and bacteria that delaminate plywd bnded with casein and sybean glues Fr ath Spec Release Rept Agriculture-Madisn

3 findings, hwever, did nt fully establish whether there may be weakening f the glue bnd because f hydrlysis alne, r whether bth mlds and bacteria can deterirate the bnd The present study was undertaken t ascertain what effect different amunts f misture in casein-glued plywd may have n the rate f bacterial deteriratin f the bnd and hw much deteriratin may be expected frm the same amunts f misture in the absence f any micr-rganisms Materials and Methds f Test Lngitudinally matched sets f 3-ply casein-glue bnded panels were made frm rtary-cut, yellw birch veneer /6 inch thick After cnditining at 8 F and 65 percent relative humidity fr 3 weeks, the panels were cut int standard - by 3-/4-inch shear-test specimens with the grain f the face plies running lengthwise (The tw standard saw cuts in the face plies were made just befre the specimens were tested fr shear strength) The specimens were then returned t the cnditining rm fr 6 mnths befre being tested Specimens fr each test variable were selected at randm The test variables cnsidered were as fllws Specimens at varius levels f misture cntent incubated singly in clsed bttles fr 3 days at 93 F (34 C) Grup A - Specimens with 8 percent misture cntent, sterilized, nt inculated Grup 32 - Specimens as in Grup A except nt sterilized Grup - Specimens saked in distilled water under partial vacuum, dried t different levels f misture cntent frm 3 t percent, placed singly in the incubatin bttles, and finally sterilized t free them f infectin Grup D - Specimens as in Grup C except that, fllwing sterilizatin, they were aseptically inculated with a mixed suspensin f the five selected bacteria Specimens wetted and incubated simultaneusly by saking in distilled water fr 48 hurs Grup El - Specimens and sak water sterilized Grup F - Specimens as in Grup E except that, fllwing sterilizatin, they were inculated by incrprating a mixed suspensin f the five selected bacteria in the sak water 5-Grups in which the actin f micr-rganisms was prevented by the dryness f the wd r by sterilizatin Rept

4 Grup G - either specimens nr sak water sterilized r inculated Thus, any such infectin ccurred largely thrugh natural infectin f the glue The tests were made in screw-tp glass jars 2 inches in crss sectin and 4-7/8 inches high at the shulder T prevent unequal absrptin f any free misture that might cndense n the walls f the jars during sterilizatin, the specimens were suspended frm a wire sldered t the metal lids Misture lsses during testing were prevented by sealing the lids t the jars with a mixture f warm paraffin and beeswax An attempt was made t bring the misture cntent levels f the specimens t pints ranging between 3 and percent T get a reasnably unifrm distributin f misture within each specimen, the cnditined samples were first placed in distilled water under a reduced pressure f 6 centimeters f mercury fr hur They were then distributed in desiccatrs and dried fr perids varying frm 3 minutes t 4 hurs in rder t bring the misture cntent f the specimens t varying misture cntent levels within the desired range f misture cntent The specimens t be sterilized were autclaved fr 3 minutes at 5 punds f steam pressure n tw successive days reliminary tests had indicated that all micr-rganisms in the glue wuld be killed by this prcedure The inculum cnsisted f a mixed culture f five strains f bacteria selected frm amng thse islated when dilutins f several manufactured casein glues were made and planted n tryptne-sdium-caseinate-glucse agar and a 2 percent trypticase agar adjusted t a p f 7 t 8 Thse selected had previusly been fund independently able t delaminate plywd bnded with a casein glue After the specimens were sterilized and cled, thse t be inculated were dipped fr 3 secnds in a suspensin f the 5 strains f bacteria; the cntrls were similarly dipped in sterile distilled water Care was taken in bth instances t avid any cntaminatin The material, except that used in the initial dry and 48-hur sak tests, was incubated fr 3 days at 93 F (34 C) Shear strength values were btained fr all specimens in each test categry Attempts were als made t islate frm the glue lines bacteria used t inculate the specimens r any cntaminants that might have been intrduced T d this, the specimens were first flamed t eliminate surface cntaminants The plies were separated with a flamed knife, and scrapings f the glue frm the center and edge were placed n plates cntaining trypticase and malt agars and in tryptne-glucse brth They were then incubated fr 4 days at 93 F (34 C) under bth aerbic and anaerbic cnditins Rept

5 Results and Cnclusins The shear strength and final misture cntent f each specimen fllwing incubatin are shwn in tables, 2, and 3, and average values fr the 3-day incubatin test are shwn in figure The findings and cnclusins may be summarized as fllws Cmparisns (table ) f test grups A and B, and f grups G and E, shw that sterilizatin in itself had n sizable effect n the strength f either the dry specimens r thse saked fr 48 hurs 2 Cmparisn (table ) f the strength f the nnsterile specimens (grups G and F) with thse f the sterile specimens (grup E) gives sme indicatin that bacteria may have caused small deteriratin f the glue during the 48-hur sak 3 Cnsideratin f specimen grup C in figure disclses that, in the absence f active micr-rganisms, there was a prgressive reductin in strength during the 3-day strage perid as the misture cntent increased frm 8 t apprximately 55 percent Further, but much smaller, reductins in strength ccurred as the misture f the specimens was increased frm 55 t apprximately percent The maximum reductins left the jint strength f the specimens apprximately ne-third as large as the presumed riginal strength f the dry specimens f grups A and B Attempts t make islatins frm these specimens indicated that all bacteria, yeasts, and mlds had been killed by the sterilizing prcess and that n later cntaminatins by micr-rganisms had ccurred Therefre, the lsses in strength must have been due t hydrlysis rather than t bilgical deteriratin f the glue 4 The sterilized and subsequently inculated specimens incubated 3 days (grup D) shwed jint strength values nly slightly lwer than thse f the crrespnding uninculated specimens (grup C) when misture cntent levels f the wd ranged between 35 and 55 percent Abve 55 percent misture cntent, hwever, the jint strength f the inculated specimens was fr the mst part less than half f that f the uninculated specimens (fig ) A large share f this difference ccurred in the apprximate misture range f 55 t 65 percent Bacteria were islated frm the glue line f all the inculated specimens with a misture cntent f 55 percent r mre Because the nly difference between the inculated (grup D) and uninculated (grup C) specimens was the presence f living bacteria, it may reasnably be cncluded that the smaller shear strength values f the inculated specimens in the misture range abve abut 55 percent were the result f bacterial actin n the glue lines Rept

6 C) 3 n u- \,r u-` \ in u\ u\ us c CV CV r4l CV t -4- r " 4-, CU CV CV n CO /4 4-, -- d p) 3-) 3-) m CO - += n O O-- -' c CO \JO zz C-- t- 4 ' t O pi a c tr t a a a in in n Ln Ln kr, te R rn, c en$ Lt-) O -- CV ( - ni,- CO cv cu C (4 c V a 2) t 4 O 4 4 M $= $arg 3, , 4 CO 5 -m - 9 i - C- r4l C- O \ tr t- ' ci 2 -t 3= 4, Ca, 9 T) ) +, ) m c) 7 LC\ 7 Le ` g -\ alit\ n C r - /4a a f, cd - W "- C7 4-, ed a, cd () $- 3, , 4-, - g td -- al ll _4- / a3 t-- t- CD r4-7 -,- 5 ta M 34t )- 3-) - 3 c), cy, ca )) Ot rce ell 4-, ,-, A rx' p, 3, c) - U) - ' A g +a ta O -VO 4 u) A , 4 3, - CO 4-, t tel t 8 te- t--- ti ul MD 4-4- m +3 g 3,, CO f r% tr 54? `,E`, c er u- O t O , at C) OD CO c t- c CO CO OD c,, pq 4 2 2, bp () (t) 2 4 S a " 4 q(5 q p w t g -p ) 3 g3 g e ;"7 (7 CO t> S m D , C) t- 44 4, al V, 44 4GO rc$ +a34 /4 rc) -p C) 35 w q' Q) fl ' 4 n- 4, 4 m 4 ' 47 " U p rc/ El S i wi Rept 277

7 447 n f Fa , - n n c n c 4 (? CO n t--- - CO n O MD ri - O -- t-- rt t c? 8 Fa an +3 to t-- ti", La- CT -- n V p F r- - F-u- C-' 75 m a gm ii' 7 ' g 8 -S) C) ti g t ls' 443) la r - R,-C, an l E--,4, cl R ck,sa a- \O CO n,- n an MD n VD c \ '3 VD % r 2 -i - V 5,, --3 cl) 4' '3 n c ' `',,4 c t--, w Ss\ k t--- S 9 4 Ap A CO cn ti 4 At n n CO in n in n K ii 2 Lr 43 ig -ig E t - O 4 Fa W ' M [- 4 n v -4- an CA t- n - in n in t CO in t i in c,, "R p A r \ n 4- n O 4- cs / rnia q,i, C E \C 4-3 r, a, Oi CO eri +3 O 3 4-, t -n t, /4 n n an, n n \ n n an - an an an an s- eo as 2 ( 4fO F -2 C- n an n ) Rept 277

8 Table 3--Shear strength f casein-bnded plywd specimens tested after 3 days at varius levels f misture cntentl anel Specimens sterilized, inculated! (Grup D) Misture Strength/ Misture Strength/ Misture Strength/ Misture Strength/ Misture Stre Misture Strength/ cntent cntent cntent cntent cntent cntent ercent ercent ercent ercent ercent, ercent A ! ; c, t ' ! ; 53 g 5-6! 9- i 6-36! a ' ! 56 r 8- g ; 7 r c r ; % ; 28- ; ; 4 ; 4-32 ; g 7 32 ; , D! , r! 2 56 ; ; t! 54 i s 85- i s 5 6- f E i ; r ! 6 75-! r ; ah, t!! g ; ,! ! 2 Average ; ; c Aercent misture (based n vendry weights) at time specimens were brken 2t can be assumed frm representative culturing f the glue lines that bacteria were present in all unsterilized specimens and in sterilized and subsequently inculated specimens There was n evidence f living bacteria in sterilized, uninculated specimens /The first value represents the shear strength in punds per square inch; the secnd value represents the percentage f wd failure Each value is fr ne specimen, except where averages are given Sept 277

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