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The effects of dryer temperature, sapwood and heartwood, and time elapsing between drying and gluing… Shen, Kuo-Cheng 1958

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THE EFFECTS OF DRYER TEMPERATURE, SAPWOOD AND HEARTWOOD, AND TIME ELAPSING BETWEEN DRYING AND GLUING ON THE GLUING PROPERTIES OF ENGELMANN SPRUCE VENEER by KUO-CHENG SHEN B.S.F.Taiwan prov. C o l l . ©f Agr., China, 1954 A Thesis Submitted in P a r t i a l Fulfilment of The Requirements for the Degree of Master of Forestry in the . Faculty, of Forestry We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1958 ABSTRACT Although Engelmann spruce i s being u t i l i z e d on a commercial scale f o r plywood manufacture i n t h i s province, d i f f i c u l t i e s are s t i l l encountered i n the manufacturing process. The most serious problems are the long drying time required at low temperatures f o r veneers and the frequently u n s a t i s f a c t o r y nature of the glue bond. Com-p l a i n t s have also been voiced about the unique c h a r a c t e r i s t i c s of Engelmann spruce veneer that render i t susceptible to degrade i n storage. Experience has shown that d r i e d veneer stored under manufacturing conditions f o r only a few days w i l l no longer form a good glue bond. The present study was i n i t i a t e d to inv e s t i g a t e the causes behind these d i f f i c u l t i e s . Three f a c t o r s were included: three maximum veneer-drying temperatures (358°F., 378°F. and 393°F.), sapwood and heartwood, and two periods of storage ( 3 and 30 days) i n a f u l l y d r i e d c o n d i t i o n . Two measures of glue-bond strength, breaking-load and per-centage wood-failure, were used as i n d i c a t o r s of glue-bond q u a l i t y . These were determined by three methods of t e s t : plywood- shear, g l u e - l i n e cleavage, untreated and g l u e - l i n e cleavage, t r e a t e d . I t was found that drying temperature and storage time, as studied, had no important e f f e c t upon the glue-bond q u a l i t y . Sapwood veneer gave a lower breaking-load but a higher percentage wood-failure than d i d heartwood veneer. The breaking - load seemed to be a more r e l i a b l e i n d i c a t o r f o r estimating the glue-bond q u a l i t y than d i d the use of per-centage wood-failure. i I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by t h e Head o f my Department o r by h i s r e p r e s e n t a t i v e . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allov»Ted w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f FORESTRY The U n i v e r s i t y o f B r i t i s h Columbia, Vancouver 5 , Canada. Date APRIL 16. 1958 TABLE OP CONTENTS PAGE I . INTRODUCTION 1 I I . FACTORS INFLUENCING PLYWOOD GLUE BONDS 4 A. Veneer D r y i n g Temperature 4 B. Sapwood and H e a r t w o o d Veneer . . 6 C. S t o r a g e Time o f D r i e d Veneer B e f o r e G l u i n g 7 I I I . METHODS OF ESTIMATING GLUE BONDS 9 A. P e r c e n t a g e W o o d - F a i l u r e . 9 B. De l a m i n a t i o n 10 C. M e c h a n i c a l Tests 10 1. V a r i a b l e s A f f e c t i n g t h e M e c h a n i c a l T e s t a o f Glue Bonds ^ 11 2. Plywood-Shear T e s t 12 3. G l u e - l i n e Cleavage T e s t 12 D. C o r r e l a t i o n between B r e a k i n g Load and Pe r c e n t a g e W o o d - F a i l u r e 13 IV. PROCEDURE 14 A. E x p e r i m e n t a l D e s i g n 14 B. M a t e r i a l s 15 1. Veneer C o l l e c t i o n 15 (a) P r e - t r e a t m e n t o f B o l t s b e f o r e P e e l i n g 15 (b) P e e l i n g o f Veneer B o l t s 16 (c ) S e l e c t i o n o f t h e Veneers . . . . 16 (d) G r o u p i n g t h e S e l e c t e d Veneers . . 16 2. Veneer D r y i n g 16 3. Veneer S t o r a g e 18 C. Plywood P a n e l C o n s t r u c t i o n 18 D. P r e p a r a t i o n o f T e s t Specimens f o r Plywood Giue-Bond T e s t s . 19 1. Normal Plywood-Shear T e s t Specimens . 20 2. G l u e - l i n e Cleavage T e s t , U n t r e a t e d Spec-imens 20 3. G l u e - l i n e Cleavage T e s t , T r e a t e d Spec-imens 21 V. RESULTS AND DISCUSSION 21 A. E f f e c t o f D r y i n g Temperature 21 B. E f f e c t o f Sapwood and H e a r t w o o d Veneer. • . 25 C. E f f e c t o f S t o r a g e Time . . . 26 D. , V a r i a t i o n Among the T e s t Methods 26 E. P a n e l s F a i l i n g the I n d u s t r y S t a n d a r d s . . 28 1. E f f e c t o f D r y i n g Temperature 29 2. E f f e c t o f Sapwood v s . Heartwood Veneer 29 3. E f f e c t o f S t o r a g e Time 29 4. C o n c l u s i o n 30 V I . CONCLUSIONS 32 V I I . REFERENCES 34 i i LIST OP TABLES PAGE 1. Veneer D r y i n g Schedu lea . . . . . 37 2 . Average B r e a k i n g Load and P e r c e n t a g e Wood-F a i l u r e Obta ined from the Three D i f f e r e n t T e s t i n g Methods. E a c h Va lue i s an Average o f 150 T e s t Specimens Taken from 15 P a n e l s . . 38 3 . A n a l y s l a o f V a r i a n c e o f B r e a k i n g Load and Percentage W o o d - F a i l u r e Obta ined from Three D i f f e r e n t T e s t i n g Methods 39 4 . A n a l y s i s o f V a r i a n c e Based on the Data i n T a b l e 2 40 5 . Number o f P a n e l s which F a i l to Paas the CSA 0121-54 Standard-, f o r E x t e r i o r Grade Douglas F i r Plywood 40 i i i ACKNOWLEDGEMENT The au thor wishes to express h i s g r a t i t u d e to M r . K. G . Fensom, S u p e r i n t e n d e n t o f the Vancouver L a b o r a t o r y , F o r e s t Products L a b o r a t o r i e s o f C a n a d a , Department o f N o r t h e r n A f f a i r s and N a t i o n a l R e s o u r c e s , f o r the use o f the L a b o r a t o r y ' s f a c i l i t i e s . A p p r e c i a t i o n i s a l s o due to M r . P . L . N o r t h c o t t , Head o f the Plywood and Wood S t r u c t u r e S e c t i o n o f the Vancouver L a b o r a t o r y , f o r h i s a d v i c e and c o n s t r u c t i v e c r i t i c i s m and to D r . R. W. Wellwood of the F a c u l t y of F o r e s t r y at the U n i v e r s i t y of B r i t i s h C o l u m b i a , under whose d i r e c t i o n t h i s t h e s i s p r o j e c t was e a r r i e d o u t . Thanks are a l s o due t o D r . J . H . G . Smi th f o r h i s h e l p i n s t a t i s t i c a l a n a l y s i s and to Mr . R. W. Kennedy and Mr . W. V . Hancock f o r a s s i s t a n c e i n p r e p a r a t i o n of the m a n u s c r i p t . The Simpson and K e l l o g Plywood Company L t d . , Ke lowna , B . C . p r e p a r e d and donated the veneers used and the Monsanto C h e m i c a l Company L t d . p r o v i d e d the g l u e used i n the e x p e r i m e n t . T h e i r g e n e r o s i t y is g r a t e f u l l y acknowledged. i v 1 INTRODUCTION The manufacture o f plywood i s one o f t h e most i m p o r t a n t f o r e s t p r o d u c t s I n d u s t r i e s i n the p r o v i n c e o f B r i t i s h C olumbia. I n 1956, B r i t i s h Columbia produced 85 p e r c e n t o f t h e t o t a l plywood produced i n Canada and, o f t h i s , o v e r 90 p e r c e n t was made from c o n i f e r o u s s p e c i e s ( 6 ) . Douglas f i r has been f o r many y e a r s the l e a d i n g s p e c i e s i n plywood manufacture i n the P a c i f i c N o r t h w e s t . V i r g i n s t a n d s o f Douglas f i r i n the c o a s t a l r e g i o n o f B r i t i s h Columbia a r e b e i n g g r a d u a l l y d e p l e t e d , r e s u l t i n g i n keen c o m p e t i t i o n f o r tre purchase o f Douglas f i r p e e l e r l o g s . T h i s has caused m a n u f a c t u r e r s o f plywood t o seek a l t e r n a t i v e s o u r c e s o f p e e l e r l o g s t o s u p p l y the c o n s t a n t l y expanding plywood i n d u s t r y . D u r i n g t h e p a s t few y e a r s , new t i m b e r s p e c i e s have been u t i l i z e d on a r e s t r i c t e d s c a l e f o r plywood manufacture. Among t h e s e , Engelmann s p r u c e ( P i c e a engelmann! P a r r y ) i s c o n s i d e r e d b y many m a n u f a c t u r e r s to be the most d e s i r a b l e . A huge volume o f mature t i m b e r i s a v a i l a b l e i n the i n t e r i o r o f the p r o v i n c e , an area w i t h g r e a t i n d u s t r i a l p o t e n t i a l . Engelmann s p r u c e grows a t h i g h e l e v a t i o n i n t h e i n t e r i o r mountain r a n g e s . The wood i s s o f t and l i g h t i n w e i g h t , w i t h an average s p e c i f i c g r a v i t y ( a i r - d r y volume) o f 0.59 ( 1 0 ) . I t c l o s e l y r e s embles the wood o f t h e e a s t e r n Canadian s p r u c e s i n appearance and p r o p e r t i e s . The heartwood i s n e a r l y w h i t e , whereas t h e narrow sapwood i s o n l y s l i g h t l y l i g h t e r i n c o l o u r t h a n the heartwood. The wood i s f i n e -2 t e x t u r e d w i t h f a i r l y d i s t i n c t g r o w t h r i n g s . I t i s s t r a i g h t -g r a i n e d and e x h i b i t s m o d e r a t e l y low s h r i n k a g e . Many s m a l l , sound k n o t s a r e u s u a l l y p r e s e n t because o f t h e p e r s i s t e n t n a t u r e o f the b r a n c h e s . The good p h y s i c a l and m e c h a n i c a l p r o p e r t i e s o f Engelmann s p r u c e r e n d e r i t v e r y s u i t a b l e f o r t h e manufacture o f plywood. A p r e l i m i n a r y s t u d y ( 5 5 ) on Engelmann s p r u c e veneer c u t t i n g and d r y i n g p r o p e r t i e s was conducted a t the U n i t e d S t a t e s F o r e s t P r o d u c t s L a b o r a t o r y at M a d i s o n . The r e s u l t s showed t h a t most o f the wood p e e l e d s a t i s f a c t o r i l y on t h e r o t a r y l a t h e and the veneer was e a s i l y d r i e d w i t h l i t t l e d e grade. No g l u i n g d i f f i c u l t i e s were e n c o u n t e r e d i n making a s m a l l number o f sample plywood p a n e l s . At p r e s e n t two companies i n the p r o v i n c e produce Engelmann spruce plywood on a commercial s c a l e . The e n t i r e p r o d u c t i o n i s bonded w i t h w a t e r - p r o o f p h e n o l i c r e s i n g l u e s and i s s p e c i f i e d as s h e a t h i n g g r a d e . Up t o the p r e s e n t t i m e , the major f i e l d o f u t i l i z a t i o n has been as s h e a t h i n g p a n e l s i n c o n s t r u c t i o n work. However, the l i g h t w e i g h t , b r i g h t c o l o u r , and freedom from odour and t a s t e would seem to suggest i t s s u i t a b i l i t y f o r use i n some f o r m o f c o n t a i n e r . A l t h o u g h Engelmann s p r u c e i s c u r r e n t l y b e i n g used f o r the commercial p r o d u c t i o n o f plywood, d i f f i c u l t i e s are s t i l l e n c o u n tered i n the m a n u f a c t u r i n g p r o c e s s . These d i f f i c u l t i e s have more t h a n once f o r c e d t h e companies m a n u f a c t u r i n g i t to c u r t a i l o r h a l t p r o d u c t i o n e n t i r e l y . The l o n g time needed to d r y the veneer a t low t e m p e r a t u r e s i s one o f the most s e r i o u s 3 p r o b l e m s . F u r t h e r m o r e , an u n s a t i s f a c t o r y g l u e bond i s o f t e n o b t a i n e d . Because o f the u n c e r t a i n t y o f the q u a l i t y o f t h e bond, consumer r e s i s t a n c e has been h i g h , and t h i s has g r e a t l y hampered e x p a n s i o n o f the market f o r the p r o d u c t . C o m p l a i n t s have a l s o been v o i c e d about the unique c h a r a c t e r i s t i c o f Engelmann s p r u c e veneer t h a t r e n d e r s i t s u s c e p t i b l e t o degrade i n s t o r a g e . E x p e r i e n c e has shown t h a t v e n e e r s s t o r e d f o r o n l y a few days w i l l no l o n g e r form a good g l u e bond. The l o n g e r t h e time e l a p s i n g between t h e d r y i n g and g l u i n g o f the v e n e e r , the p o o r e r t h e g l u e bond produced. To d a t e t h e r e has been no t h o r o u g h i n v e s t i g a t i o n o f t h e s e problems i n Engelmann s p r u c e plywood m a n u f a c t u r e . The Monsanto Ch e m i c a l Company L t d . o f Vancouver has done some r e s e a r c h work on t h e s e p r o b l e m s , b u t t h i s has been p r i m a r i l y concerned w i t h f o r m u l a t i o n o f t h e p r o p e r a d h e s i v e s f o r Engelmann s p r u c e v e n e e r . T h i s t h e s i s r e p r e s e n t s t h e r e s u l t s o f an i n v e s t i g a t i o n u n d e r t a k e n b y t h e w r i t e r i n c o o p e r a t i o n w i t h the Plywood and Wood S t r u c t u r e S e c t i o n o f the Vancouver L a b o r a t o r y , F o r e s t P r o d u c t s L a b o r a t o r i e s o f Canada, i n t o the major d i f f i c u l t i e s e n c o u n t e r e d i n the manufacture o f Engelmann s p r u c e p l y w o o d . The experiment was d e s i g n e d t o e v a l u a t e t h e g l u i n g p r o p e r t i e s o f b o t h sapwood and heartwood o f Engelmann s p r u c e , exposed t o d i f f e r e n t d r y i n g s c h e d u l e s and s t o r e d f o r d i f f e r e n t t i m e s between veneer d r y i n g and g l u i n g . I f t h e s e f a c t o r s were b e t t e r u n d e r s t o o d , i t might be p o s s i b l e t o d e v e l o p a s u i t a b l e s c h e d u l e f o r m a n u f a c t u r i n g Engelmann s p r u c e plywood w i t h a c o n s i s t e n t l y s a t i s f a c t o r y g l u e bond. 4 FACTORS INFLUENCING PLYWOOD GLUE BONDS There a r e a g r e a t number o f f a c t o r s e n t e r i n g i n t o the p r o d u c t i o n o f good plywood g l u e bonds. M a r i a n ( 2 4 ) has l i s t e d twenty-one o f t h e s e , w h i c h , i n g e n e r a l , can be d i v i d e d I n t o t h r e e major c l a s s i f i c a t i o n s : wood p r o p e r t i e s , g l u e p r o p e r t i e s and g l u i n g o p e r a t i n g . The i n f l u e n c e o f some o f thes e f a c t o r s i s more i m p o r t a n t t h a n t h a t o f o t h e r s . Some o f them can be v a r i e d a t w i l l , s u ch as g l u e s p r e a d , p r e s s u r e and p r e s s i n g t e m p e r a t u r e . Some o f them a r e more o r l e s s f i x e d , such as t h e n a t u r e o f t h e g l u e and m o i s t u r e c o n t e n t o f the ven e e r . Others a r e g i v e n c o n s t a n t s , such as d e n s i t y o f wood s p e c i e s . Many o f t h e s e f a c t o r s a r e f u n c t i o n a l l y I n t e r r e l a t e d to o t h e r s , as f o r i n s t a n c e p r e s s i n g time and p r e s s i n g t e m p e r a t u r e ; o t h e r s , as s u r f a c e c o n d i t i o n o f veneer and p r e s s i n g t e m p e r a t u r e a r e e v i d e n t l y independent o f each o t h e r . Veneer D r y i n g Temperature Veneer d r y i n g i s r e g a r d e d as one o f t h e most i m p o r t a n t phases o f plywood m a n u f a c t u r i n g . I t h o t o n l y d i r e c t l y a f f e c t s plywood q u a l i t y , but a l s o i s s t r o n g l y r e l a t e d to m a n u f a c t u r i n g c o s t s . Plywood m a n u f a c t u r e r s p r e f e r veneer t o be d r i e d a t the h i g h e s t p o s s i b l e t e m p e r a t u r e f o r a s h o r t t i m e , w i t h o u t h a v i n g the veneer degraded by e x c e s s i v e h e a t . The work done by N o r t h c o t t ( 2 9 ) has i n d i c a t e d t h a t , i n the case o f Douglas f i r bonded w i t h a h o t - p r e s s p h e n o l i c r e s i n , t h e i n f e r i o r g l u e bonds produced b y d r y i n g veneer a t 5 h i g h t e m p e r a t u r e s a r e a t t r i b u t a b l e , n o t t o i n t e r f e r e n c e w i t h the g l u e bond, b u t t o the d e g r a d a t i o n i n s t r e n g t h o f t h e wood. The r e s e a r c h o f B r y a n t and S t e n s r u d ( 8 ) and C u r r i e r ( 1 4 ) can be i n t e r p r e t e d s i m i l a r l y . The h e a t i n g o f wood, p a r t i c u l a r l y when h i g h t e m p e r a t u r e I s combined w i t h a l o n g t i m e , g r e a t l y r e d u c e s t h e m e c h a n i c a l s t r e n g t h . McLean(27) f o u n d t h a t t h e h e a t i n g o f wood causes a r e d u c t i o n i n i t s w e i g h t . S i t k a s p r u c e samples h e a t e d i n an oven a t 350°F f o r 120 hours and a t 300°F f o r 400 hours l o s t r e s p e c t i v e l y 1818 .and 10.3 p e r c e n t o f t h e i r o ven-dry w e i g h t . F u r t h e r he found t h a t the l o n g e r the h e a t i n g d u r a t i o n , t h e g r e a t e r the w e i g h t l o s s . By h e a t i n g S i t k a s p r u c e a t 250°F f o r 470 days he succeeded i n r e d u c i n g i t s weight b y 24.5 p e r c e n t . F r a s h o u r and C u r r i e r ( 1 8 ) found t h a t the w a t e r - a b s o r b i n g c a p a c i t y o f veneer i s r e d u c e d b y i n c r e a s i n g t h e d r y i n g temperature w i t h a c o n s t a n t t i m e , or by i n c r e a s i n g t h e d r y i n g time a t a c o n s t a n t t e m p e r a t u r e . The reduced w a t e r -a b s o r b i n g c a p a c i t y o f the veneer c o u l d be a c o n t r i b u t i n g f a c t o r i n p r o d u c i n g i n f e r i o r g l u e bonds because of r e d u c t i o n i n w e t t a b i l i t y o f the veneer by t h e g l u e . Marra(25) s t a t e d t h a t an unanchored g l u e l i n e i s caused by l a c k o f w e t t i n g . Veneer o f re d u c e d w e t t a b i l i t y a l s o may cause a t h i c k e r g l u e l i n e , w h i c h C o c k r e l l and B r u c e ( 1 3 ) have found t o be weaker i n s t r e n g t h t h a n a t h i n g l u e l i n e . The s u r f a c e c o n d i t i o n o f veneer may be p h y s i c a l l y changed d u r i n g d r y i n g , e s p e c i a l l y a t e x c e s s i v e l y h i g h t e m p e r a t u r e s of l o n g d u r a t i o n . I t i s a well-known f a c t 6 t h a t s u r f a c e c h e c k i n g i s one o f the most s e r i o u s d e f e c t s i n d r y i n g lumber. The degrade from t h i s cause i s found t o be much g r e a t e r i n f l a t - g r a i n e d lumber t h a n I n e d g e - g r a i n e d lumber. D u r i n g veneer d r y i n g , the l a t h e - c h e c k s w i l l become deeper and w i d e r as w i l l t h e s u r f a c e c h e c k s , which a r e caused by t h e d i f f e r e n t i a l s h r i n k a g e between the s u r f a c e and I n t e r i o r o f t h e v e n e e r . Bef o r e g l u i n g , the veneer i s s u b j e c t e d l t o s e v e r a l m a n u f a c t u r i n g p r o c e s s e s . L a t h e and s u r f a c e checks may cause s m a l l s p l i n t e r s and s l i v e r s d u r i n g h a n d l i n g . T h e i r p r e s e n c e causes an uneven and r o u g h veneer s u r f a c e . I t was found by K a u f e r t ( 2 0 ) t h a t a r e l a t i v e l y smooth s u r f a c e r e s u l t s i n the s t r o n g e s t g l u e bond. M a x w e l l ( 2 6 ) a l s o i n d i c a t e d t h a t s p l i n t e r e d s u r f a c e s p r o v i d e d poor anchorage f o r g l u e , r e s u l t i n g i n a weak g l u e bond. Sapwood and Heartwood A g r e a t d e a l o f r e s e a r c h work has been done on t h e d i f f e r e n c e between sapwood and heartwood o f the same s p e c i e s . I n o v e r 500,000 t e s t s made by the U n i t e d S t a t e s F o r e s t P r o d u c t s L a b o r a t o r y on the v a r i o u s s p e c i e s o f wood grown i n t h e U n i t e d S t a t e s , no e f f e c t upon the m e c h a n i c a l p r o p e r t i e s of t h e wood has been found due to I t s change from sapwood i n t o h e a r t w o o d ( 3 6 ) . Sapwood, as a r u l e , t a k e s p r e s e r v a t i v e t r e a t m e n t b e t t e r t h a h heartwood because o f i t s h i g h p e r m e a b i l i t y . I t i s o f t e n c o n s i d e r e d t h a t the p e n e t r a t i o n o f g l u e i n t o t h e c e l l c a v i t i e s i s an i m p o r t a n t p r o c e s s i n g l u i n g wood. A p a r t from the f l u i d i t y o f t h e g l u e and the p r e s s u r e a p p l i e d d u r i n g p r e s s i n g ; the p e n e t r a b i l i t y o f wood i t s e l f p r o b a b l y c o n t r o l s , t o a g r e a t 7 e x t e n t , the p e n e t r a t i o n o f g l u e . G e n e r a l l y , good p e n e t r a t i o n o f g l u e i s n e c e s s a r y f o r a c h i e v i n g the deep anchorage w h i c h r e s u l t s I n a s t r o n g g l u e b o n d ( 2 2 ) . I n c o n t r a s t , the p e n e t r a t i o n o f g l u e must be c r i t i c a l l y c o n t r o l l e d i n o r d e r t h a t t h e g l u e f i l m be not e x c e s s i v e l y d e p l e t e d . The r e s u l t i n g s t a r v e d g l u e l i n e , produced by e x c e s s i v e p e n e t r a t i o n o f g l u e , w i l l a g a i n r e s u l t i n a poor b o n d ( 2 5 ) . The phenomenon o f sapwood-heartwood d i f f e r e n c e i s e x p l a i n e d , i n p a r t a t l e a s t , by t h e lower e x t r a c t i v e c o n t e n t o f sapwood. F o r r e s i n o u s s p e c i e s , sapwood u s u a l l y c o n t a i n s l e s s r e s i n t h a n t h e heartwood. I t i s c o n s i d e r e d t h a t t h e heartwood i s more d i f f i c u l t t o g l u e t h a n sapwood(37). The major d i f f e r e n c e between sapwood and heartwood w h i c h a f f e c t s i t s use as veneers i s t h e i n i t i a l m o i s t u r e c o n t e n t . I n c o n i f e r s , sapwood m o i s t u r e c o n t e n t i s u s u a l l y much h i g h e r t h a n t h a t o f heartwood o f t h e same s p e c i e s ( 3 7 ) . Sapwood veneer of h i g h e r i n i t i a l m o i s t u r e c o n t e n t r e q u i r e s a much l o n g e r t ime to d r y a t comparable t e m p e r a t u r e s t h a n does the heartwood v e n e e r . T h i s i s t h e main r e a s o n f o r s e p a r a t i o n o f t h e g r e e n sapwood veneer b e f o r e d r y i n g . S t o r a g e Time o f D r i e d Veneer b e f o r e G l u i n g . R e l i a b l e i n f o r m a t i o n about the e f f e c t o f the time e l a p s i n g between veneer d r y i n g and g l u i n g upon the g l u i n g p r o p e r t i e s i s not a v a i l a b l e a t p r e s e n t . C o n v e n t i o n a l plywood p l a n t s seldom keep d r i e d veneer f o r a l o n g p e r i o d b e f o r e u s i n g i t e x c e p t i n the few c a s e s o f plywood p l a n t s w h i c h do not themselves produce v e n e e r . These p l a n t s must 8 p u r c h a s e d r i e d veneer and i n t h i s i n s t a n c e , i t might p o s s i b l y be s t o r e d o r be i n t r a n s i t f o r a l o n g p e r i o d o f time b e f o r e use. D r i e d veneer may p i c k up m o i s t u r e d u r i n g s t o r a g e . T h i s e f f e c t Is a c c e n t u a t e d when t h e veneers have b e e n d r i e d to an e x t r e m e l y low m o i s t u r e c o n t e n t . The m o i s t u r e c o n t e n t o f wood e q u a l i z e s t o t h a t o f t h e s u r r o u n d i n g atmosphere w i t h i n a r e l a t i v e l y s h o r t t i m e . Moreover, t h e r a t e o f a b s o r p t i o n o f m o i s t u r e i n k i l n - d r i e d lumber i s e s p e c i a l l y r a p i d when t h e lumber i s f i r s t removed f r o m the k i l n . I n a t e s t conducted a t the Vancouver F o r e s t P r o d u c t s L a b o r a t o r y ( 9 ) , t h e m o i s t u r e c o n t e n t o f e n d - c o a t e d , o n e - i n c h Douglas f i r p i l e d i n an open, unheated s t o r a g e shed, i n c r e a s e d from 7.6 p e r c e n t to 8.4 p e r c e n t a f t e r 24 h o u r s , and a f t e r s t o r a g e f o r seven days', i t was 10.4 p e r c e n t . The m o i s t u r e c o n t e n t o f veneer a t t h e t i m e o f g l u i n g i s c o n s i d e r e d a c r i t i c a l f a c t o r i n the p l y w o o d - m a n u f a c t u r i n g p r o c e s s . The r e s u l t s o f work a t P r i n c e s R I s b o r o u g h by N e w a l l and C a r r u t h e r s ( 2 8 ) on b i r c h wood w i t h m o i s t u r e c o n t e n t s o f 4 - l / 2 , 7, 12, 13, 20, 27, and 35 p e r c e n t , g l u e d w i t h c a s e i n , p h e n o l and u r e a - f o r m a l d e h y d e r e s i n s , have shown t h a t the j o i n t s t r e n g t h i s u n a f f e c t e d by m o i s t u r e c o n t e n t s between 4 - l / 2 and 20 p e r c e n t , I f u r e a o r c a s s e i n g l u e s a r e used. W i t h the p h e n o l i c r e s i n , a d e c r e a s e i n bond s t r e n g t h o c c u r s above 12 p e r c e n t m o i s t u r e c o n t e n t . Above 20 p e r c e n t a l l t h r e e g l u e s a r e s e v e r e l y a f f e c t e d by m o i s t u r e c o n t e n t and a t 35 p e r c e n t the p h e n o l i c r e s i n f a i l s t o bond c o m p l e t e l y . 9 Some test work i n t h i s f i e l d was conducted at Madison on maple shear-test blocks and 3/l6-inch birch plywood glued with phenolic, r e s o r c i n o l , and melamine resins at moisture contents of 2, 6, 11, 17, 21 and 25 percent. 01sen(32) concluded from t h i s work that j o i n t strength was not affected by moisture contents between 6 and 20 percent. Another possible source of deterioration of the veneer during storage i s surface contamination by dust. The contaminated surfaces have been considered -to be one cause of ah i n f e r i o r glue bond( 21). •-• • METHODS OF ESTIMATING GLUE BOND QUALITY There are two broad methods of appraising the glue bond qualit y , by non-destructive and by destructive t e s t i n g . The non-destructive methods(16,19) have been developed quite recently and have not been widely employed as yet i n research work on plywood glue bonds. The destructive methods have long been used i n the plywood industry for glue-bond t e s t i n g . Three d i f f e r e n t methods can be distinguished under the general heading of destructive t e s t i n g . These are discussed below. Percentage Wood-Failure In B r i t a i n , percentage wood-failure obtained from the knife-test method(23), i s generally accepted by the plywood industry as a standard estimation of glue-bond quality. In the s p e c i f i c a t i o n set up by the Canadian Standards A s s o c i a t i o n ( l l ) for t e s t i n g exterior-type Douglas f i r plywood bonded with water-io p r o o f p h e n o l i c r e s i n > the p e r c e n t a g e o f wood f a i l u r e i s the c r i t e r i o n f o r e s t i m a t i n g the q u a l i t y o f t h e g l u e bond. I f the average w o o d - f a i l u r e o f t e n p l y w o o d - s h e a r - t e s t specimens i s below 60 p e r c e n t , o r i f more t h a n one o f the specimens i s below 30 p e r c e n t , the t e s t p i e c e f r o m w h i c h t h e specimens o r i g i n a t e d , f a i l s . D e l a m l n a t l o n The p r o c e d u r e used i s to expose t h e t e s t specimens to n a t u r a l w e a t h e r i n g , or t o some f o r m of a r t i f i c a l l y a c c e l e r a t e d w e a t h e r i n g t r e a t m e n t c a l c u l a t e d to s e t up s t r e s s e s i n the g l u e bond. The amount o f d e l a m l n a t l o n w h i c h develops under w e a t h e r i n g i s used as the e s t i m a t i o n o f glue-bond q u a l i t y . The s t r e s s e s s e t up d u r i n g t h e w e a t h e r i n g t r e a t m e n t by the d i f f e r e n t i a l s w e l l i n g o f wood and g l u e and the v a r i a b l e g r a i n d i r e c t i o n s o f the p l i e s g i v e r i s e to a f orm o f m e c h a n i c a l t e s t , but one f o r w h i c h no measure o f the f o r c e i n v o l v e d i s a v a i l a b l e . M e c h a n i c a l T e s t s The p r o c e d u r e i s t o impose i n c r e a s i n g e x t e r n a l f o r c e upon the t e s t specimen u n t i l t h e g l u e bond i s r u p t u r e d . The amount o f l o a d r e c o r d e d i s the c r i t e r i o n f o r a p p r a i s i n g t h e g l u e bond q u a l i t y . Because of t h e h i g h p r e c i s i o n p o s s i b l e i n measuring the r e s u l t s , and the ease o f t e s t i n g , the m e c h a n i c a l method has l o n g been used i n the plywood i n d u s t r y . D i f f e r e n t t ypes o f m e c h a n i c a l t e s t methods have been d e v e l o p e d i n o r d e r to f i t s p e c i f i c r e q u i r e m e n t s . Among them, the plywood s h e a r -t e s t I s the c o n v e n t i o n a l one. Others a r e the g l u e - l i n e c l e a v a g e ( 3 0 ) , t e n s i l e ( 3 3 , 3 4 , 3 8 ) and s h e a r ( l , 1 5 ) t e s t s . 11 1. V a r i a b l e s a f f e c t i n g m e c h a n i c a l t e s t s o f g l u e bonds When one o f t h e m e c h a n i c a l methods i s used t o break down t h e g l u e bond, t h e f a i l u r e may d e v e l o p I n the wood, i n the g l u e l i n e o r a t the wood-glue i n t e r f a c e . Where f a i l u r e w i l l d e v e l o p i s dependent upon t h e magnitude and d i r e c t i o n o f the a p p l i e d f o r c e w i t h r e s p e c t t o t h e weakest p o r t i o n o f the whole p i e c e under t e s t . The amount o f v a r i a b i l i t y i n t h e t e s t r e s u l t s i s due t o the n a t u r a l v a r i a b i l i t y i n the m e c h a n i c a l p r o p e r t i e s o f the wood, g l u e and t h e wood-glue i n t e r f a c e . D i f f e r e n t s p e c i e s o f wood v a r y c o n s i d e r a b l y i n t h e n a t u r e o f t h e i r p h y s i c a l p r o p e r t i e s and r e s u l t a n t b e h a v i o r . I n a m e c h a n i c a l t e s t o f wood-glue j o i n t s , v a r i a t i o n i n wood d e n s i t y , f i b e r a l i g n m e n t , e x t r a c t i v e m a t e r i a l and m o i s t u r e c o n t e n t a r e f a c t o r s o f major i m p o r t a n c e . Glue i s c o n s i d e r e d t o be a more homogeneous m a t e r i a l than wood. The n a t u r e o f the g l u e and the r i g i d i t y a t t a i n e d by the g l u e a t the time o f t e s t w i l l d etermine whether the g l u e l i n e i s s t r o n g o r weak, thus a f f e c t i n g the d i s t r i b u t i o n o f s t r e s s e s imposed d u r i n g l o a d i n g . Glue tends to s h r i n k w h i l e d r y i n g and the r e s t r a i n t o f f e r e d by the a d j a c e n t wood l e a d s t o the development of r e s i d u a l s t r e s s i n t h e g l u e l i n e to w h i c h d e l a m i n a t i o n o f t h e j o i n t may be t r a c e d . The i n t e r f a c e between g l u e and wood i s I n f l u e n c e d by the p h y s i c a l and c h e m i c a l n a t u r e o f b o t h g l u e and wood. These n a t u r a l c h a r a c t e r i s t i c s i n c l u d e t h e p o l a r n a t u r e o f g l u e and wood, s u r f a c e t e n s i o n s a t the i n t e r f a c e , the s u r f a c e c o n d i t i o n o f wood, wood s t r u c t u r e , f i b e r a l i g n m e n t , and v i s c o s i t y o f t h e g l u e . 12 2. Plywood 3hear t e 3 t Many i n v e s t i g a t o r s u s i n g the plywood-shear t e s t t e c h n i q u e have found t h a t t h e r e i s a g r e a t degree o f v a r i a t i o n w i t h i n the t e s t , i n f l u e n c e d by a number o f f a c t o r s . Bensend and P r e s t o n ( 2 ) have s t u d i e d t e n p o s s i b l e causes o f v a r i a b i l i t y by w h i c h the shear t e s t has been i n f l u e n c e d t o d i f f e r e n t d e g r e e s . B e t h e l and Huffman(5) found t h a t the l a t h e - c h e c k o r i e n t a t i o n o f veneer i n f l u e n c e d t h e plywood-shear t e s t . Bensend and P r e s t o n ( 2 ) and C h e l v a r a j a n ( 1 2 ) c o n c l u d e d t h a t the d e p t h o f the saw groove o f the s h e a r t e s t specimen s h o u l d be c o n t r o l l e d ; v a r i a t i o n i n i t s d e p t h r e s u l t e d i n i m p o r t a n t v a r i a t i o n s i n s h e a r - s t r e n g t h v a l u e s . 3. G l u e - l i n e c l e a v a g e t e s t The g l u e - l i n e c l e a v a g e t e s t ( 3 0 ) was developed a t the Vancouver L a b o r a t o r y o f the F o r e s t P r o d u c t s L a b o r a t o r i e s o f Canada. I t has been c l a i m e d to p o s s e s s a c c u r a c y e q u a l to the t e n s i o n s h e a r method(31). T h i s t e s t i s made b y p l a c i n g a k n i f e edge o f 90° a l o n g t h e g l u e ' - l i n e o f a specimen w i t h the g r a i n o f the f a c e p l y a t an a n g l e o f 45° t o the k n i f e edge, and measuring the f o r c e r e q u i r e d to r u p t u r e the glue'^-line. B e s i d e s the a c c u r a t e r e s u l t s o b t a i n e d by t h i s t e s t , o t h e r advantages have been c l a i m e d by N o r t h c o t t ( 3 1 ) as f o l l o w s : 1. The g l u e l i n e o f every specimen i s exposed f o r i n s p e c t i o n (when used w i t h e d g e - g r a i n e d veneer i n w h i c h the c e l l o f the wood i n t e r s e c t s t h e s u r f a c e a t an angle o f 10°). 2. Tfest specimens are easy to manufacture and 13 c o n s e q u e n t l y r e l a t i v e l y i n e x p e n s i v e . 3. Less t e s t i n g time i s r e q u i r e d t h a n w i t h the o t h e r methods compared. 4. A maximum number o f specimens may be p r e p a r e d from a g i v e n sheet o f plywood. So f a r as t h i s t e s t method i s co n c e r n e d , the main d e f e c t , when used w i t h normal plywood, I s t h a t the l o a d i n g f o r c e i s not a p p l i e d p a r a l l e l t o t h e g r a i n o r i e n t a t i o n o f b o t h the c e n t r a l and f a c e p l y . When the g l u e l i n e i s b r o k e n , the veneer p l i e s a r e seldom found to have c o m p l e t e l y s e p a r a t e d a l o n g the g l u e l i n e . C o r r e l a t i o n between B r e a k i n g Load and P e r c e n t a g e W o o d - F a i l u r e <-The major drawback o f u s i n g wood f a i l u r e a s a s a t i s f a c t o r y means f o r e s t i m a t i n g glue-bond s t r e n g t h i s t h a t the c o r r e l a t i o n between the p e r c e n t a g e w o o d - f a i l u r e and b r e a k i n g - l o a d i n shear or c l e a v a g e t e s t s has not been a d e q u a t e l y d e f i n e d . Many i n v e s t i g a t o r s ( 4 , 30, 31) have c o n c l u d e d t h a t p e r c e n t a g e w o o d - f a i l u r e a l o n e i s not a r e l i a b l e means o f a s s e s s i n g t h e q u a l i t y o f a g l u e and i t s bond w h i l e o t h e r s b e l i e v e i t s u p e r i o r t o o t h e r methods. I n t e s t i n g g l u e bonds, c o n s i d e r a b l e v a r i a t i o n i n pe r c e n t a g e wood f a i l u r e f r e q u e n t l y o c c u r s and i s sometimes d i f f i c u l t t o e x p l a i n . C e r t a i n v a r i a t i o n s i n wood, g l u e , g l u i n g o p e r a t i o n , o r t e s t and me a s u r i n g method may have caused the amount o f w o o d - f a i l u r e o f a g l u e j o i n t to v a r y c o n s i d e r a b l y . Wood, by n a t u r e , v a r i e s g r e a t l y i n i t s p h y s i c a l make-up and 14- - 7 behavior. The s p e c i f i c gravity v a r i e s between and within species, and even i n d i f f e r e n t positions within a single tree(7). Bergin(J) found that the wood species with high density usually gave a lower percentage wood-failure than those with a low density. Wood-failure also varied greatly with d i f f e r e n t glues used. Other f a c t o r s may also influence the percentage wood-f a i l u r e . Bergin(4) found that, i n general, where the angle of grain i n r e l a t i o n to the glue l i n e exceeded 20 degrees, an increase i n percentage wood-failure accompanied a high breaking-load. C o c k r e l l and Bruce(13) found that the percentage wood-f a i l u r e was generally lower f o r t h i c k glue l i n e s than f o r t h i n glue l i n e s , with rather pronounced differences f o r the d i f f e r e n t glues. Eickner(17) compared the gluing properties of 15 native woods of the United States with a co l d - s e t t i n g urea r e s i n glue. He found that the average wood-failure was usually higher on jo i n t s glued with the urea r e s i n than on those obtained f o r the same species by Traux(34> using casein glue. PROCEDURE Experimental Design Factor A- Drying temperature. Three drying temperatures were used and designated as low(358°F), medium(378°F) and high(393°F). These rep-resent the maximum temperatures recorded during drying (Table 1). Factor B- Sapwood and heartwood. Two types of veneers were used, one obtained from the wet outer, or sapwood, p o r t i o n o f b o l t s and. t h e o t h e r from the d r i e r i n n e r p o r t i o n , or heartwood. F a c t o r C - S t o r a g e t i m e . Two d i f f e r e n t e l a p s e d times were used between d r y i n g and g l u i n g o f v e n e e r s , namely 3 and 30 days. A t o t a l o f 12 c o m b i n a t i o n s o f c o n d i t i o n s were thus s t u d i e d . F o r each s e t o f c o n d i t i o n s , f i f t e e n 3 - p l y plywood p a n e l s , 15 i n c h e s s q u a r e , were t e s t e d , making a g r a n d t o t a l o f 180 p a n e l s . Each p a n e l p r o v i d e d 30 glue-bond t e s t specimens, o f w h i c h 10 were normal plywood-shear t e s t specimens, 10 u n t r e a t e d g l u e - l i n e c l e a v a g e t e s t specimens, and 10 t r e a t e d g l u e - l i n e c l e a v a g e t e s t specimens. I n a l l , the use o f 12 c o m b i n a t i o n s meant t h a t 5400 glue-bond t e s t specimens were used i n t h i s e x p e r i m e n t . M a t e r i a l s 1. Veneer c o l l e c t i o n The veneers used i n the experiment were c o l l e c t e d and d r i e d a t t h e plywood p l a n t o f the Simpson & K e l l o g Company L t d . at Kelowna, B r i t i s h C olumbia. a. P r e - t r e a t m e n t o f b o l t s b e f o r e p e e l i n g B e f o r e p e e l i n g , b o l t s were debarked and l o a d e d i n t o a c o n c r e t e k i l n f o r 20 hours o f heat c o n d i t i o n i n g . The temperature i n s i d e the k i l n was m a i n t a i n e d a t about 210°F. by t h e h e a t o f exhaust steam from the p l a n t ' s steam veneer d r y e r . T h i s c o n d i t i o n i n g s o f t e n s t h e wood, p r o v i d e s f o r e a s i e r p e e l i n g , and g i v e s t h e veneer a smoother s u r f a c e . 16 b. P e e l i n g o f veneer b l o c k s . The d i a m e t e r o f t h e b l o c k s r a n g e d from 16 to 24 i n c h e s . B o t h the f a c e and cor e veneers were p e e l e d a t the same u n i f o r m t h i c k n e s s o f l / 8 - i n c h . A l l t e s t v e n e e r s were c l i p p e d t o a u n i f o r m s h e e t , 15 i n c h e s i n w i d t h and 100 i n c h e s i n l e n g t h . c. S e l e c t i o n o f t h e veneers A l t o g e t h e r , 120 s h e e t s o f veneer were c o l l e c t e d , o f w h i c h 60 s h e e t s were sapwood and 60 s h e e t s heartwood v e n e e r . S e l e c t i o n was a t random d u r i n g the p e e l i n g o f 135 b o l t s . I n g e n e r a l , each b o l t p r o v i d e d e i t h e r one sheet of sapwood veneer or one s h e e t o f heartwood veneer. A l l s h e e t s were c a r e f u l l y examined t o a s s u r e t h a t no major d e f e c t s , such as c o m p r e s s i o n wood, decay or r e s i n s t r e a k s , were p r e s e n t . M o i s t u r e c o n t e n t o f the green m a t e r i a l was f o u n d t o range from 110 t o 253 p e r c e n t f o r sapwood veneer and from 19 to 90 p e r c e n t f o r heartwood v e n e e r . d. G r o u p i n g t h e s e l e c t e d veneers Veneers were grouped a t random i n t o s i x gr o u p s . Each . group c o n s i s t e d o f twenty s h e e t s o f sapwood veneer o r twenty s h e e t s o f heartwood v e n e e r . Each s h e e t o f the group was marked w i t h a d i f f e r e n t number. 2. Veneer d r y i n g The grouped veneers were d r i e d i n an 1 8 - s e c t i o n , Moore c r o s s - c i r c u l a t i o n , steam h e a t e d , r o l l e r veneer d r y e r . Three d i f f e r e n t d r y i n g s c h e d u l e s ( T a b l e l ) were employed as i n d i c a t e d o i n t h e e x p e r i m e n t a l d e s i g n , namely low (maximum o f 358 F with 25 minutes d r y i n g time f o r heartwood and 40 minutes f o r sapwood 17 veneers), medium (maximum of 378°F with 16 minutes drying time for heartwood and 28-1/2 minutes for sapwood veneers) and high maximum of 393°F with 13 minutes drying time for heartwood and 25 minutes for sapwood veneers). One group of sapwood veneers and one group of heartwood veneers were dried at each temp-erature, but sapwood veneers were kept in the dryer for a longer time than the heartwood veneers because of sapwood»s higher i n i t i a l moisture content. The experimental veneers were run while the dryer was f i l l e d with Engelmann spruce veneers with corresponding moisture content. After drying, the moisture content of the veneer was tested by a commercial moisture meter of the capacitance type and found to range from 1 to 2 percent for both sapwood and heartwood veneers dried at different schedules. One sheet of sapwood veneer, dried at medium temperature, showed a relatively high moisture content and was discarded. The moisture-meter values were checked by the oven-dry method. The dried veneer sheets were then cross-cut by a oiroular saw into six pieces, each 15 inches square, and the extra end pieces of every sheet were collected for moisture content determination. Each group of six pieces obtained i from the veneer cut from one sheet was divided at random into two sets and marked. One set was intended for gluing after 3 days storage and the other for gluing after 30 days storage. The entire 120 pieces of 15-Inch square veneer produced from one group were wrapped in a polythene bag to avoid pickup of moisture en route to Vancouver and during the storage period. 18 3. Veneer s t o r a g e The veneers were t r a n s p o r t e d t o Vancouver, s t i l l i n p o l y t h e n e bags and were s t o r e d i n a c o n s t a n t room t e m p e r a t u r e o f 70°P. Plywood P a n e l C o n s t r u c t i o n The f i r s t groups o f veneers were made i n t o plywood p a n e l s f o l l o w i n g 3 days s t o r a g e a f t e r d r y i n g . G l u i n g and 'pressing o f t h e plywood p a n e l s were performed u s i n g t h e L a b o r a t o r y ' s g l u e s p r e a d e r and h y d r a u l i c p r e s s w i t h e l e c t r i c a l l y h e a t e d p l a t e n s . The 3 - p l y plywood p a n e l s were assembled to s i m u l a t e commercial m a n u f a c t u r e . B o t h t h e f a c e p l i e s had t h e i r t i g h t s i d e s o u t w a r d s , and t h e c o r e p l y was l a i d c r o s s w i s e w i t h r e s p e c t t o b o t h t h e f a c e p l i e s . Due t o the l i m i t e d c a p a c i t y o f the g l u e s p r e a d e r , the core veneers were p r e v i o u s l y c u t I n t o h a l v e s o f 15 x 7 l / 2 i n c h i n s i z e . The g l u e was Monsanto Chemical,Company PP512 l i q u i d p h e n o l - f o r m a l d e h y d e r e s i n , i n a form r e a d y f o r immediate use. T h i s r e s i n g l u e Is b e i n g used a t p r e s e n t by t h e plywood companies i n B r i t i s h Columbia who are making s p r u c e plywood. P a n e l s were made under the f o l l o w i n g s c h e d u l e s : Glue s p r e a d : 50 pounds p e r thousand square f e e t based on l i q u i d w e i g h t o f the g l u e . Assembly t i m e : 5 minutes P r e s s i n g t i m e : 10 minutes P l a t e n t e m p e r a t u r e : 285°P P l a t e o p r e s s u r e : 200 pounds per square i n c h 19 P a n e l s p e r p l a t e n o p e n i n g : 2 M o i s t u r e c o n t e n t o f t h e veneer a t g l u i n g : 1 t o 2 p e r c e n t I m m e d i a t e l y upon l e a v i n g t h e h o t p r e s s , t h e p a n e l s were p l a c e d i n a t h e r m o s t a t i c a l l y - c o n t r o l l e d oven a t 175°Ffor 7 hours: of> h o t s t a c k i n g . The hot s t a c k i n g , o r c o n d i t i o n i n g , was t o ensure c o m p l e t i o n o f cure o f t h e g l u e l i n e . The second group o f veneers was made i n t o plywood p a n e l s f o l l o w i n g 30 days s t o r a g e a f t e r d r y i n g , under c o n d i t i o n s s i m i l a r t o t h e f i r s t group. P r e p a r a t i o n o f T e s t Specimens f o r Plywood Glue-Bond T e s t s A l l t e s t s were performed w i t h a T i n i u s O l s e n u n i v e r s a l t e s t i n g machine h a v i n g a c a p a c i t y o f 20,000 pounds. A f t e r t e s t i n g , t h e b r e a k i n g • l o a d and p e r c e n t a g e w o o d - f a i l u r e o f specimens were r e c o r d e d . P e r c e n t a g e w o o d - f a l l u r e o f t e s t specimens was evaluated i n comparison w i t h a s e t o f specimens p r e p a r e d as a s t a n d a r d by the Douglas f i r Plywood A s s o c i a t i o n i n c o o p e r a t i o n w i t h the Vancouver F o r e s t P r o d u c t s L a b o r a t o r y . B e f o r e d o i n g t h e e x p e r i m e n t a l e s t i m a t i o n s , t h e e s t i m a t o r a t t a i n e d a degree o f p r o f i c i e n c y p e r m i t t i n g him t o e s t i m a t e the s t a n d a r d specimens w i t h a maximum average e r r o r o f 5 p e r c e n t . A c c o r d i n g t o t h e e x p e r i m e n t a l d e s i g n , t h e r e were 5,400 glue-bond t e s t specimens r e q u i r e d i n the experiment f o r t h e t h r e e t e s t methods, i . e . normal p l y w o o d - s h e a r , u n t r e a t e d g l u e - l i n e c l e a v a g e , and t r e a t e d g l u e - l i n e c l e a v a g e . More tha n 30 specimens were c u t from each t e s t p a n e l . From t h e s e , 20 10 specimens were s e l e c t e d randomly f o r each t e s t method. 1. Normal Plywood-shear t e s t specimens S t a n d a r d t e s t specimens were cut i n accordance w i t h the s p e c i f i c a t i o n s e t o u t by t h e Canadian Standards A s s o c i a t i o n ( 1 1 ) . Specimens were thus 3 - l / 4 i n c h i n l e n g t h and 1 - i n c h i n w i d t h w i t h t h e g r a i n o f t h e f a c e p l i e s p a r a l l e l t o the l o n g a x i s . A l / 8 - i n c h - w i d e saw c u t was made t h r o u g h e a c h f a c e p l y , and t w o - t h i r d s t h r o u g h the c o r e p l y , the c u t s b e i n g spaced one i n c h a p a r t i n t h e c e n t e r o f t h e p i e c e , a c c o r d i n g t o the above s p e c i f i c a t i o n s . Twelve t e s t specimens, two o f which r e p r e s e n t e d s p a r e s , were p l a c e d i n a s m a l l l i n e n bag and marked. A l l t h e specimens were s u b j e c t e d t o a 3 - c y c l e , a c c e l e r a t e d - w e a t h e r i n g t r e a t m e n t a f t e r s o a k i n g i n c o l d water f o r 48 h o u r s . Each a c c e l e r a t e d -w e a t h e r i n g c y c l e c o n s i s t e d o f 24 hours o v e n - d r y i n g a t 145°P and 24 hours s o a k i n g I n c o l d water a t room t e m p e r a t u r e . F o l l o w i n g the c o m p l e t i o n o f t h e l a s t c y c l e , which ended a f t e r t h e specimens had been soaked i n c o l d water f o r 24 h o u r s , the specimens were t e s t e d wet. The maximum b r e a k i n g - l o a d o f t e s t specimens was r e c o r d e d i m m e d i a t e l y f o l l o w i n g the o c c u r r e n c e o f r u p t u r e , and the p e r c e n t a g e w o o d - f a i l u r e was e v a l u a t e d when the t e s t specimens had r e t u r n e d t o a d r y c o n d i t i o n . 2. G l u e - l i n e c l e a v a g e t e s t , u n t r e a t e d specimens The g l u e - l i n e c l e a v a g e t e s t specimens were c u t 1-Inch s q u a r e , w i t h the g r a i n o f the f a c e p l i e s r u n n i n g d i a g o n a l l y to the edges o f the specimens; 0 . 0 1 - i n c h o f v a r i a t i o n i n the 21 t e s t specimen dimensions was allowed. Ten specimens were chosen at random from each panel, and tested without further treatment. Both the breaking-load: and percentage wood-failure were recorded immediately after the specimens were ruptured. 3. Glue-line cleavage t e s t , treated specimens Ten specimens were cut from each panel as for the untreated glue-line cleavage test specimens. The specimens were subjected to 14-cycle, accelerated-weathering treatments after soaking for 24 hours i n cold water. Each cycle consisted of 4 hours b o i l i n g i n water and 20 hours oven-drying at 225°P. After completion of the f i n a l c y c l e , the specimens were dry and,ready for the t e s t . RESULTS AND DISCUSSION E f f e c t of Drying Temperature The data of Table 2 indicate that the medium temperature drying schedule resulted i n glue bonds i n f e r i o r to those produced with a high or low drying temperature schedule. P a r t i c u l a r l y , the heartwood veneers dried at the medium temperature schedule gave both the lowest breaking-load and percentage wood-failure when compared with the veneers d r i e d at the other two drying schedules. However, the analyses of variance summarized i n Table 3 shows that there are no s i g n i f i c a n t differences for the drying temperatures (Factor A) i n either breaking load or i n percentage wood-failure regardless of the method of t e s t . I t would appear from t h i s that drying temperatures did not have a s i g n i f i c a n t e f f e c t upon the glue bond quality. I t must be pointed out, however, that the analyses of variance are extremely conservative i n choice of the error term used i n t e s t i n g . A further analysis of variance (Table 4), based on the data outlined i n Table 2, ihdi6ates that when breaking-load and percentage wood-failures are combined there are s i g n i f i c a n t differences due to drying temperatures. This c o n f l i c t between two conclusions drawn separately from Tables 3 and 4 needs further c l a r i f i c a t i o n . One l o g i c a l method of comparison of the severity of the schedules would involve the product of temperature and time. Using the maximum drying temperature i n the dryer section, the comparison i s : For sapwood veneer: High temperature 393 x 25 = 9,725 Medium temperature 31% x 28-1/2 = 10,775 Low temperature 558 x 40 = 14,320 For heartwood veneer: High temperature 393 x 13 = 5,109 Medium temperature 378 x 1 6 s 6,048 Low temperature 358 x 25 = 8,950 This crude mathematical comparison suggests that the medium temperature schedule i s not the most severe of the three. Another b i t of supporting evidence i s that the one known "wet" veneer sheet occurred among the veneers dried at the medium temperatures. I t i s reasonable to i n f e r that, i f drying has a deleterious e f f e c t upon glue bond quality, ~ -:. - ~ 23 quality, the low drying schedule should produce the lowest results in both breaking load and percentage wood-failure as compared with the other two drying schedules, since the product of temperature and time of the low drying schedule is the largest. However, the low drying sohedule produced the most satisfactory glue bonds obtained. It is also suggested that the inferior glue bond quality obtained following the medium drying temperature schedule was due to an unfortunate combination of drying time and temperature which proved to be more damaging to the veneer than a higher temperature applied for a shorter time or a lower temperature for a longer time. A comparison of the sapwood and heartwood veneers dried at the medium temperature sohedule does not bear this out. The fact is that only the heartwood veneers dried at this schedule produced an inferior glue bond using a l l three testing methods, while the sapwood veneers, which were dried at the same temperature for a much longer time yielded good glue bonds quite comparable with those obtained using sapwood veneers dried at both high and low temperature schedules. The unexpectedly poor results from the medium drying temperature schedule are not readily explainable on the basis of the information available. In order to investigate these results, further tests were made. A determination of specific gravity of the test specimens from each of the three different temperature groups failed to show any correlation with lower 24 breaking load and percentage wood f a i l u r e of the specimens tested. Since the breaking load and percentage wood-failure are greatly influenced by the lathe-check orientation of the core p l y (5), the te s t specimens were also examined to determine i f those made from the veneers dried at the medium temperature contained an abnormal number of opening-lathe checks i n the core ply of the plywood-shear test specimens. This also f a i l e d to y i e l d an explanation f o r the r e s u l t s obtained. In summation t h i s research has f a i l e d to e s t a b l i s h a clear-cut r e l a t i o n s h i p between bond qu a l i t y f o r Engelmann spruce veneers and drying temperature i n the range 358°]p to 393°F. In the i n i t i a l design of t h i s experiment, drying temperatures of 300°F, 350°F and 400°F were to be used. Because of r e s t r i c t i o n s imposed by the m i l l supplying the veneers, i t was not possible to obtain the designed temp-eratures. Temperatures of 358°F, 378°E and 393°F were substituted. Since these were maximum readings among nine gauges on an 18-section dryer, the actual temperatures to which the veneer was subjected undoubtedly fluctuated con-siderably about these maxima. The i n f e r i o r glue bonds, p a r t i c u l a r l y lower per-centage wood-failure obtained from the heartwood veneers dried at the medium temperature schedule, may be a t t r i b u t a b l e to inherent c h a r a c t e r i s t i c s of the i n d i v i d u a l tree from 25 which the veneers were out. A further p o s s i b i l i t y i s the occurrence of heartwood veneers of low i n i t i a l moisture content at the time of s e l e c t i o n of veneers f o r drying at the medium temperatures schedule. Neither explanation has a basis i n f a c t , and a more complete study should be made of t h i s anomalous behaviour. E f f e c t of Sapwood and Heartwood Veneer The analyses of variance summarized i n Table J indicate that Factor B (sapwood and heartwood veneer) had a s i g n i f i c a n t e f f e c t on bond qualit y , as recorded by four of the six t e s t methods. Within two of the t e s t methods, percentage wood f a i l u r e showed no s i g n i f i c a n t difference. The analysis of variance summarized i n Table 4 shows that there were no s i g n i f i c a n t differences f o r the sapwood and heartwood veneers. This contradiction can be explained on the basis that the heartwood gave higher breaking loads while the sapwood exhibited higher percentage wood-failure. These two factors operated i n such a way as to o f f s e t each other i n the ca l c u l a t i o n s f o r the analysis. The heartwood veneers gave higher breaking loads and lower wood-failure than sapwood veneers dried at the same temperature. However, because of i t s higher i n i t i a l moisture content, sapwood veneer was subjected to a longer drying period. The reduced mechanical strength of sapwood 26 was anticipated because of i t s longer exposure to heat. The combination of higher wood-failure with lower mechanical strength i s quite l o g i c a l , the external force applied on the glue l i n e of the t e s t specimens breaking the weaker wood more r e a d i l y than the stronger wood. -E f f e c t of Storage Time None of the six t e s t methods of estimating bond quality yielded s i g n i f i c a n t differences between the plywood glued a f t e r 3-days and that a f t e r 30-days storage time as indicated on Tables 3 and 4. Certain estimates (Table 2) yielded contradictory trends; f o r instance the plywood-shear breaking loads indicated higher average strength i n the 30-days panels while the cleavage t e s t on untreated specimens indicated the reverse. Two of the wood-failure estimates indicated lower average wood-failure f o r the 30-days panels while one indicated no difference. Based on the r e s u l t s from t h i s experiment, i t i s reasonable to suppose that, i f the dried veneers were properly stored to maintain the moisture content obtained through drying and contamination of the surfaces was avoided, a short period of storage would not cause any trouble i n gluing. V a r i a t i o n among the Test Methods A l l the t e s t methods followed more or l e s s the same uniform trends. However, i n some cases, the r e s u l t s obtained from one method are opposite to those of another. In some 27 specimens the breaking load varied by more than 100 percent. This difference i n r e s u l t s i s very d i f f i c u l t to explain since each mechanical t e s t method contains a great number of sources causing v a r i a t i o n i n the r e s u l t s obtained. 3Tor example, the lathe-check orientation of the core ply of the plywood-shear test specimen has been considered to be a very important f a c t o r influencing both the breaking load and percentage wood-failure ( 3 ) . A tes t specimen stressed so that lathe-checks open w i l l give a lower breaking load and percentage wood-failure than a specimen stressed so that lathe-checks close. The plywood-shear test specimens made of veneers dried at the medium temperature schedule showed the lowest breaking load a l l through the experiment with one exception. Sapwood veneer from the medium temperature run stored f o r 30 days, produced higher breaking loads than did the matching specimens made of veneers dried at the high temperature schedule. This led to a further investigation of the number of opening lathe-checks and c l o s i n g lathe-checks i n t h i s combination of treatments. I t was found that the number of closing lathe-checks i n the plywood shear specimens were 6 3 , 7 2 , and 80 respectively i n the sets of 130 specimens made of sapwood veneers dried at low, medium and high temperature and glued a f t e r 30-days storage. The breaking loads and percentage wood-failure 28 of the specimens made from sapwood veneers dried at the high temperature schedule was lowest since this group of test specimens contained the highest number of opening lathe-checks. So far as the glue-line cleavage test i s concerned, no s i g n i f i c a n t v a r i a t i o n influencing the test r e s u l t s was revealed i n thi s experiment. One f a c t that should be noted was that the area of the tes t specimens that broke during test did not often cover the whole area subjected to stress. There was a tendency f o r the lower breaking loads to coincide with smaller areas of f a i l u r e . This condition was emphasized In the case of accelerated-weathering-treated specimens. The reduction i n area may be responsible for the high percentage wood-failure i n the treated glue-l i n e cleavage test specimens since the breaking load of those specimens was reduced more than 50 percent and wood-f a i l u r e only dropped about 5 percent a f t e r the specimens were weathered. Some of the specimens tested by the other two methods showed p a r t i c u l a r l y low wood-failures and displayed even increasing percentages of wood-failure after weathering treatment. Considering these two f a c t s i t may be com-'•"olue&ed; that .the., weathering treatment has caused more damage to the veneer than to the glue bond i t s e l f . Therefore, the resu l t s obtained from the weather; --treated cleavage test specimens seem more r e l i a b l e than those from the other test me thods• Panels F a i l i n g the Industry Standard Of most in t e r e s t to industry i s the percentage of 29 panels which f a i l to pass the industry t e s t . The industry standard is„the average wood-failure of ten plywood-sheet specimens from the panel s h a l l exceed 60 percent. The number of panels (out of 15) which f a i l e d to pass th i s test» for each treatment combination, i s recorded i n Table 5 . 1. E f f e c t of temperature Eleven of the low-temperature panels, nineteen of the medium and fourteen of the high temperature f a i l e d the industry t e s t . When subjected to a M t M test, the difference between the means proved not s i g n i f i c a n t , i n other words, on the basis of the industry standard, the medium temperature would not produce s i g n i f i c a n t l y poorer bonds than the low temperature or the high temperature. 2. E f f e c t of sapwood vs. heartwood veneer Thirty-three of the heartwood panels f a i l e d but only eleven of the sapwood. The difference between the means proved s i g n i f i c a n t when the data were subjected to a " t w t e s t . According to the industry standard more f a u l t y bonds occurred i n the heartwood than i n the sapwood panels. 3. E f f e c t of s torage time Twenty-seven of the 30-days panels .and: seventeen for 3-days f a i l e d - the industry test". The differences of the means when subjected to a '>t"'test proved not s i g n i f i c a n t . 30 4. Conclusion The comparisons based on the industry standard, i . e . that panels with average wood-failure less than 60 percent f a i l the t e s t , substantiate the conclusions drawn from Table 2 and 3, regarding the sapwood vs. heartwood, and 3-days vs. 30-days storage f a c t o r s , and strengthen the opinion that the veneers dried at the medium temperature did not produce s i g n i f i c a n t l y poorer bonds. It i s i n t e r e s t i n g to note that a l l treatment combinations produced at least one out of f i f t e e n panels with less than 60 percent wood-failure. One produced as high as ten out of f i f t e e n panels or 67 percent f a i l u r e s by the industry standard. Another point of i n t e r e s t i s the evidence that these low wood f a i l u r e s are probably not i n d i c a t i v e of poor bonds, but are, rather, i n d i c a t i v e of a breakdown, of ;- , the industry standard. This evidence was obtained by a r b i t r a r i l y d i v i d i n g the panels into two groups, one with zero to 69 percent average wood-fallure and the other with average wood-failure of 70 to 100 percent and analysing each group separately. The reason for Including cleavage tests on both untreated and treated specimens was to check t h e - p o s s i b i l i t y that the anticipated low wood-failures were not due to f a u l t y bonds. The accelerated-weathering treatment of 24"hours soaking followed by 14 cycles of b o i l i n g for 4 hours and drying for 20 hours at 225°P was estimated to be severe 31 enough to delaminate faulty bonds. In the case of the untreated specimens the average breaking load of the high-wood-failure panels was 13 percent lower than the average breaking load for the low-wood-failure panels. The same trend was maintained aft e r the accelerated-weathering treatment but the difference was increased to 19 percent. The f a c t that the low-wood-failure specimens maintained t h e i r higher strength after the rather severe accelerated-weathering treatment suggests that the bonds are at l e a s t as good as those with higher wood-failure. In the case of the panels with high wood-failure i n the untreated specimens, the average wood-failure was reduced by 10 percent due to the accelerated-weathering treatment whereas the wood-failure of the i n i t i a l l y low-wood-failure panels was increased by 8 percent. This also supports the thought that low wood-failure is not an i n d i c a t i o n of an i n f e r i o r bond. In other words there i s a reasonable doubt that percent wood-failure i s an accurate estimate of plywood d u r a b i l i t y i n t h i s case. Much more d e t a i l e d research is needed to resolve this problem. This conclusion that there may be c e r t a i n circumstances under which the wood-failure method of estimating bond qua l i t y (service l i f e ) f a i l s to give the correct estimate Is contrary to the evidence of the Douglas F i r Plywood Association (33). Two l i n e s of work seem to be required to clear up the confusion. One is to est a b l i s h for c e r t a i n whether the wood-failure method r e a l l y i s an unreliable estimate of 32 plywood d u r a b i l i t y i n c e r t a i n oases. The other i s to i n t e n s i f y study of the factors which produce low wood-f a i l u r e s but high-strength bonds. CONCLUSIONS 1. In this experiment, with temperature and time confounded, drying temperatures ranging from 358°IP to 393°F showed no s i g n i f i c a n t e f f e c t on glue-bond quality of Engelmann spruce veneers. 2. Plywood made of sapwood veneer showed s i g n i f i c a n t l y lower breaking -loads than that made from heartwood veneer dried at the same temperature with a shorter drying time. However, the percentage of wood-failure of the sapwood veneer was higher than that of the heartwood veneer. 3. Duration of storage of the dried veneer had no e f f e c t upon i t s gluing properties. There was no difference i n glue-bond qua l i t y between the plywood made of veneer stored i n sealed polythene bags f o r 3 days and 30 days a f t e r drying. 4. The breaking load of the te s t specimens seemed to be a more accurate indicator f o r estimating the glue bond quality than did the percentage wood-failure. However, more research i s needed to provide more accurate estimates of bond q u a l i t y than are a v a i l a b l e . 3. Generally speaking, the three d i f f e r e n t t e s t i n g methods used gave r e s u l t s that followed the same pattern but at d i f f e r e n t l e v e l s of magnitude. 33 The glue-line cleavage test of treated specimens seemed to give the most r e l i a b l e r e s u l t s . More research i s needed to esta b l i s h whether the type of low-wood-failure with high-strength-bond studied i n t h i s work i s r e a l l y an i n f e r i o r bond and to es t a b l i s h what conditions create I t . 34 LITERATURE CITED 1. Am. Soc. for Test. Mat. Standards. Philadelphia. 1953. 2. Bensend, J.W. and R.S. Preston. Some causes of v a r i a b i l i t y i n the r e s u l t s of plywood shear t e s t s . U.S. For. Prod. Lab. Rept. No. R 1615. 1946. 3. Bergin, E.G. The gluing c h a r a c t e r i s t i c s of various Eastern Canadian wood species. Repr. from Can. Woodworker. March, 1953. 4. . The sig n i f i c a n c e of wood f a i l u r e i n glued j o i n t s . Repr. from Can. Woodworker. March, 1953. 5. Bethel, J.S. and J.S. Huffman. Influence of lathe check orien t a t i o n on plywood shear t e s t r e s u l t s . School of For., North Carolina State C o l l . Tech. Rept. No. 1. 1951. 6. B.C. For. Serv. Report of the Commissioner r e l a t i n g to the forest resources of B r i t i s h Columbia. V o l . 1. V i c t o r i a . 1956. 7. Brown, H.P., A.J. Panshin and C C . Fo r s a i t h . Textbook of Wood technology. V o l . 1. McGraw-Hill Book Co. Inc., New York. 19491 8. Bryant, B.S. and R.K. Stensrud. Some factors a f f e c t i n g the glue bond qua l i t y of hard-grained Douglas f i r plywood. J . For. Prod. Res. Soc. 4 (4):158-161. 1954. 9. Can. For. Prod. Lab. Moisture content changes i n seasoned lumber i n storage and i n t r a n s i t . Vancouver Lab. B u l l . No. 102. 1952. 10. Can. For. Prod. Lab. Canadian woods: t h e i r properties and uses. King's Printer and Controller of Stationery, Ottawa. 1951. 11. Can. Stand. Assoc. 0121 - 1954 S p e c i f i c a t i o n for Douglas f i r plywood and western softwood construction plywood. Ottawa. 1954. 12. Chelvarajan, B.K. Comparative study of Indian and American plywood shear te s t standard. Indian For. 80 (l):47-53. 1953. 13. C o c k r e l l , R.A. and H.D. Bruce. E f f e c t of thickness of glue l i n e on strength and d u r a b i l i t y of glued rwadd j o i n t s . U.S. For. Prod. Lab. No. R 1616. 1946. 35 14. Cu r r i e r , R.A. High drying temperatures - do they harm Douglas-fir veneer? Oregon For. Prod. Lab. (Uhpubl. Rept.) 1956. 15. Dept. S c i . and Ind. Res. Second report of the adhesives research committee. H.M.S.O., London. 1926. 16. Dietz, A.G.H., H.N. Brockstruck and G. Epstein. Non-destructive determination of mechanical properties and dete r i o r a t i o n of adhesive. Am. Soc. Test. Mat. Rept. No. 134. 1952. 17. Eickner, H.W. The gluing c h a r a c t e r i s t i c s of 15 species of wood with cold-setting urea-resin glue. U.S. For. Prod. Lab. Rept. No. 1342 1 1955. 18. Frashour, R.G. and R.A. Currier. Water absorption of Douglas-fir veneer dried at high temperatures. Oregon For. Prod. Lab. (Unpub. Rept.). 1956. 19. Jayne, B.A. A non-destructive test of glue bond q u a l i t y . For. Prod. J . 5(5):294-301. 1955. , 20. Kaufert, F.H. Preliminary experiments to improve the gluing c h a r a c t e r i s t i c s of re f r a c t o r y plywood surfaces by sanding. U.S. For. Prod. Lab. Mimeo No. 1351. 1945. 21. and W.F. Hutchins. Experiments i n the gluing of wood treated with o i l solutions. U.S. For. Prod. Lab. Rept. No. 1484. 1945. 22. Kitazawa, G. A study of adhesion i n the glue l i n e s of 22 woods of the United States. New York State C o l l . For. Tech. Pub. No. 66. 1946. 23. Knight., R.A.G. Adhesives for wood. Chemical Publishing Co. Inc., New York. 1952. 24. Marian, J.E. The co r r e l a t i o n of 21 variables of the gluing process for wood and s i m i l a r materials. Svenska Traforskning-institutet Trateknik, Meddelande 68B, Stockholm. 1955. 25. Marra, A.A. Glue l i n e doctor. For. Prod. Res. Soc. prep r i n t . 148. 1951. 26. Maxwell, J.W. Shear strength of glue j o i n t s as affected by wood surfaces and pressures. New York State C o l l . For. Tech. Pub. No. 64. 1944. 27. McLean, J.D. Rate of d i s i n t e g r a t i o n of wood under d i f f e r e n t heating conditions. Proc. Am. Wood Preservers' Assoc. 47:153-168. 1951. 36 28. Newwall, R.J. and J.P.S. Carruthers. Influence of moisture content on the gluing of wood. Gt. B r i t . Por. Prod. Res. Lab. Gluing and Glue Progress Rept. No. 41, Part 1. 1946. 29. Northcott, P.L. The e f f e c t of dryer temperatures upon the gluing properties of Douglas-fir veneer. Por. Prod. J . 7(1):10-16. 1957. 30. The development of the glue l i n e cleavage t e s t . J . For. Prod. Res. Soc. 2(5);216-224. 1952. 31. Improved method for evaluating q u a l i t y of phenolic r e s i n bonds of Douglas f i r . Thesis for Master'.s degree i n Por. Eng., University of'B. C. (Unpublished). 1954. 32. 01sen, W.Z. Rate of development of joint strength by four r e s i n glues on 8 species of wood. U.S. Por. Prod. Lab. Rept. No. 1547. 1946. 33. Perkins, N.S. Predicting exterior plywood performances. Por. Prod. Res. Soc. Proc. 4:352-363. 1950. 34. Traux, T.R. The gluing of wood. U.S. Dept. Agr. B u l l . No. 1500. 1929. 35. U.S. Dept. Agr. Wood handbook. Agr. Handbook No. 72. 1955. 36. U.S. For. Prod. Lab. Differences between heartwood and sapwood. Tech. Note No. 189. 1923. 37. U.S. For. Prod. Lab. Engelmann spruce veneer cutting and drying properties. Rept. No. 1766-10. 1953. 38. Wakefield, W.E. The tension normal to the glue^'lihe plywood t e s t . Can. For. Prod. Lab. Mimeo No. 121. 1947. 37 Table 1. Veneer Drying Schedules 1. High temperature (393°P) Time at Steam Temperature readings from gauges reading pressure No. 1 No.2 No.3 No.4 No.5 No.6 No.7 No.8 No.9 p s i 7:00 250 358 372 379 392 388 394 396 392 383 7:30 249 356 365 368 382 380 387 392 390 382 8:00 250 362 374 382 393 386 388 392 391 381 8:30 255 362 374 378 394 392 396 392 397 382 Average 251.3 360 372 377 390 387 392 393 393 382 Drying time: 25 minutes for sapwood veneers 13 minutes for heartwood:veneer 2. Medium Temperature (378° F) 9:30 190 336 350 356 374 376 378 380 373 362 10:00 190 346 356 364 376 372 374 375 372 360 10:30 190 338 352 376 376 378 380 372 363 363 Average 190 340 353 359 375 375 377 378 372 362 Drying time: 28-1/2 minutes for sapwood veneers 16 minutes for heartwood veneer 3. Low Temperature (358°F) 11:30 140 317 332 336 352 354 357 359 352 340 12:00 143 320 331 344 355 352 354 357 350 340 12:30 141 318 334 340 358 357 356 358 350 340 Average 141 318 332 340 355 354 356 358 351 340 Drying time: 40 minutes f o r sapwood veneers 25 minutes for heartwood veneer 38 Table 2. Average Breaking^Load and Percentage Wood-Failure Obtained from the Three Different Testing Methods. Each Value i s an average of 150 Test Specimens Taken from 15 Panels 3 - days storage 30 - days storage Veneer Testing Sapwood Heartwood Sapwood Heartwood dryer tern- methods veneer veneer veneer veneer peratures * B.L.lb. W.F.g B.L.lb. W.F.g B.L.lb. W.F.g B.L.lb. W.F.$ Plywood-shear 136.7 76.0 143.0 67.7 132.1 73.0 147.6 63.5 test Cleavage 358°F testtun- 89.2 77.3 90.4 77.9 78.6 79.6 87.1 76.6 treated Cleavage t e s t , 43.4 75.4 43.2 69.9 37.6 72.9 48.9 64.3 treated Total 269.3 228.7 276.6 215.5 249.3 225.5 283.6 204.4 Plywood-shear 107.6 85.0 121.7 59.6 116.3 72.8 125.6 42.0 test Cleavage 378°F test,un- 81.5 76.5 88.3 63.0 74.0 79.4 86.8 58.0 treated Cleavage t e s t , 36.9 72.4 42.1 56.6 39.7 73.1 40.9 49.9 treated Total 236.0 234.9 252.1 179.2 230.0 225.3 253.2 149.9 Plywood-shear 111.3 79.0 129.4 68.2 110.7 70.0 136.8 59.2 test Cleavage 393 F t e s t * * - 71.5 75.2 91.7 75.2 77.1 78.4 94.8 74.6 treated Cleavage t e s t , 39.0 72.6 46.5 68.3 36.9 69.7 42.2 64.8 t r e a t e d Total 221.8 226.8 267.6 211.7 224.7 218.1 273.8 198.6 # B.L. l b . indicates breaking-load i n pounds W.F.$ indicates percentage wood^failure Table 3. Analysis of Variance of Breaking Load and Percentage Wood-Failure Obtained from three Different Test Methods Breaking-Load Percentage Wood-Failure Methods of estimating Bond Quality Source of v a r i a t i o n D.F. Temperature=Factor A Plywood*Shear 2 Cleavage,untreated 2 Cleavage,treated 2 Sap Vs.heartwood veneer  = Factor B Plywood-* Shear 1 Cleavage, untreated 1 Cleavage, treated 1 Storage time = Factor C Plywood-Shear 1 Cleavage,untreated 1 Cleavage,treated 1 Factor AxFactor B Plywood-Shear 2 Cleavage,untreated 2 Cleavage,treated 2 Factor AxFactor C Plywoodr Shear 2 Cleavage,untreated 2 Cleavage,treated 2 Factor BxFactorC Plywood-Shear 1 Cleavage,untreated 1 Cleavage,treated 1 Fa c torAxFa c tor BxFa c torC Plywood-Shear 2 Cleavage, untreated 2 Cleavage, treated 2 Error Plywood-Shear 1788 i Cleavage, Untreated 1788 Cleavage, treated 1788 i The error terms used i n c a l c u l a t i n g variance r a t i o s for factors A, B and C were obtained by using those Interaction sums of squares that were both s i g n i f i c a n t (5% l e v e l ) and i n c l u s i v e of the appropriate main f a c t o r . *,**, and *** indicate 5%, 1% andO.l^ l e v e l c f signiificant-..respec.tlyely -and .n.s. indica-tes not s i g n i f icant• i . s.s. M.S. F 3. S.S. M.S. F 160,620 3,399 3,605 80,310 1,699 1,802 14.26 0.46 0.89 n.s • n.s. n.s. 12,286 24,089 14,675 6,143 12,044 7,337 0.96 0.53 0.98 102,046 48,123 16,122 19.91 19.87 9.67 94,612 21,149 32,778 8.88 1.69 7.72 5,185 4,656 69 3.19 2.41 0.44 n.s. n.s. n.s. 26.305 50 8,342 8.54 0.02 2.66 11,253 9,407 3,541 5,626 4,703 1,770 3.47 16.86 8.37 •SHS--K-29,173 25,229 15,002 14,586 12,614 7,501 41.03 81.60 23.51 2.471 4,429 97 1,235 2,214 " 48 0.76 7.94 0.23 n.s. n.s. 6,433 488 342 3,216 244 171 9.05 0.88 0.54 1,619 3,581 1,563 1.00 12.84 7.38 n.s. 2,804 2,965 3,134 7.89 1.07 9.82 i'M 4,725 2,362 i:U 11.58 n.s. n.s. +>*" -1,640 503 3,093 11? 1,547 2.31 0.91 4.85 900,857 499,050 378,598 1,622 279 211 635,693 492;553 570,507 "" 355 277 ' .319 v ... .3 . .S CO 40 Table 4. Analysis of Variance Based on the Data i n Table 2. Source of v a r i a t i o n D.F. S.S. M.S. F 1 = B.L. and #W.F. 1 3,626.680 3,626.680 129.858 M = Me thods<*Te sting 2 22,284.333 11,142.167 408.167 C ^ Storage time 1 66.125 66.125 2.422 n.s. B = Sapwood and heartwood 1 1.680 1.680 - n.s • veneer A = Dryer temperatures 2 852.333 426.167 15.612 A X B 2 406.779 203.390 7.451 A X C 2 10.333 5.166 - n.s • A X M 4 211.834 52.959 1.940 n.s. A X I 2 103.445 51.722 1.895 n.s. B X C 1 0.126 0.126 - n.s . B X M 2 75.112 37.556 1.376 n.s . B X I 1 2,016.126 2,016,126 73.856 C X M 2 12.000 6.000 - n.s. C X I 1 78.126 78.126 2.862 n.s. M X I 2 21,657.445 10,828.728 396.686 Residual 45 1,228.398 27.298 Correction 1 427,350.125 Total 72 52,730.875 **and *** indicate s i g n i f i c a n t at 1 % and 0.1^ l e v e l r s p e c t i v e l y and n.s. indicates not s i g n i f i c a n t . Table 5. Number of Panels which F a i l to Pass the CSA 0121-54 Standard* for Exterior Grade Douglas f i r Plywood. Storage Time (Days) 3 30 Sapwood Veneer Dryer Temperatures 358°F 378°F Sapwood Heatwood Sapwood Heartwood Veneer Veneer 4 1 2 4 1 2 6 10 393°F :;• Sapwood Heaifcwood T o t a l 2 1 2 9 17 27 l i Heartwood Veneer Total 11 16 19 14 11 33 44 * (11) 

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