9
0 . 115110
Lib 62-D
U . S. Department of Agriculture, Forest Servic e
FOREST PRODUCTS LABORATOR Y
In cooperation with the University of Wisconsi n
MADISON, WISCONSIN
R o 77
ADHESION IN THE PAINTIN Q
AND IN THE GLUING OF WOOD
By F . L . BROWNE
Senior Chemis t
Published i n
INDUSTRIAL & ENGINEERING CHEMISTR Y
March, 1931
A.DE1SION IN T H4- PAINTING AND IN 'H3 GLUING OF WOO D1
_fr.
By
F, L . Browne ,
Senior Chemis t
Study of the behavior of coatings of house paint on exterio r
softwood surfaces (1 to 3) reveals a fundamental]] un atisfactory form o f
failure . Instead of eroding superficially until it bec+nines too thin t o
conceal the underlying wood, the coating shatters thoroughly and some o f
the fragments fall off bodily, laying wood bare in patches . Such appear s
to be the normal mode of disintegration for paint of good quality, althoug h
with, sraW
.dlfii$ of poor formula or with paints applied under ver y
untaty
t ,
orms of very
ovnafiy
rapid failure in '!_
ched away fairly
f~s sets in first ove r
uniformly . The i9,
k
the s mmerWt ,
`be left
e the neighboring
springwood
,complete
Flaking of this kind i s
disastrous
. 1. .
gives ri t :a marked difference in the
dtirabI11'V .0f co
. e r Ong as they cover boards containing much o r
litter, i
rwookf tO ; , $ trates improvements in both appearance an d
d abi t'y 4
be effected through certai n
4VAIVeletad bt
well-kn6m
2e
lt form
' to)
leaves . roughpa,n d
' certa
t o lidnI '
iita ;,Rs . ~~.
te.r. geI. ,
e,d
ie im•
ice
DI,f' .&A04
A
a
Ftngs.
f:roa-tia
rts ve1y'.•~argely a
Permanen t
Agrogt,nd
'
woo
a
if i cant ly
in g
ila
t ot5 s3 .
mayhemi,
t eithe r
Simon of !Nit
-r been the chie f
c procedure o f
coatings and wood, d controlled. The
hods
ti
During recent years material progress in knowledge of the natur e
of adhesion to wood has been gained from research having for its practical
objective improvement in the gluing of wood, Studies by the Forest Product s
Laboratory (21), by the British Royal Adreraft Astablishment (8), and b y
McBain and his co-workers (10, 12 to 12) have proved especially fruitful .
It is therefore desirable to review some of the successful methods o f
attack and the generalizations that have been established and to conside r
their possible bearing upon the problem of adhesion between coatings an d
wood .
Methods of .;xperimenting ThatHaveProved Fruitful
Two kinds of experiments have proved esreciaily fruitful i n
studying the adhesion of glue to wood -- namely, mechanical tests of th e
strength of glued joints and microscopical observation of transvers e
sections through glued joints, the mechanical strength of which had bee n
determined .
Since two sizable pieces of wood may be joined with glue, it i s
comparatively easy to grasp each piece separately and to tear them apar t
in a testing machine (22) . The entire procedure must of course b e
standardized very carefully if the results are to be reproducible . I t
must also be remembered that such tests measure the strength of the joint ,
not the strength of the glue . Strong joints made with wood of the sam e
species and similar density and grain have approximately the same strength ,
since the strength of the joint is roughly that of a solid piece of woo d
of the same kind when tested similarly, and the fracture is found wholl y
or largely in the wood itself . Joints that fail without tearing the wood ,
and prove materially weaker than the wood itself, are weak joints . Mos t
glues commonly used in woodworking are-capable of making strong joint s
provided that suitable conditions are maintained while making them, but th e
gluing technic necessary to obtain good joints may be materially differen t
for glues of different kind or grade .
It is much more difficult to devise a mechanical measure of th e
bond between coating and wood achieved in painting, because only one membe r
is large enough to grasp, the coating being too thin . The difficulty i s
not necessarily insurmountable, however, and the benefits to be derive d
from such a direct measure of an elementary requirement of pain t
serviceableness are so certain that the invention of a satisfactory techni c
of testing is clearly essential to progress by the scientific a s
distinguished from the empirical method of attack .
Microscopical study of thin, transverse sections of stron g
and weak glued joints between wood surfaces was necessary for interpretin g
the results of mechanical strength tests and obtaining an insight into the
R977
-2-
nature of adhesion (5) . Suitable methods were developed for softenin g
the wood and slue sufficiently to permit cutting sections with a microtom e
and then staining them differentially so as to make it easy to see jus t
where the glue had gone (21) . Similar methods are applicable to coated
wood, although in some cases it is necessary to embed the coating in a
protective material to keep it intact while cutting the sections .
Definite payer of Glue Necessary for Strong Wood Joint s
Strong glued joints always have a definite layer of glu e
sandwiched between the two wood surfaces that are joined . (2) . The laye r
is thick enough to be seen easily with the aid of a microscope and ofte n
with the unaided eye, but within wide limits the thialaw, s of the la as ha s
no bearing on the strength of the joint . Wood jo' .nts lacking such a Jaye r
of glue are weak joints of the kind known as "starved . Joints may .c" twi n
a layer of glue and, still be weak, however, if the layer of glue is :.i c
continuous or was not brought intimately in contact with all parts of th e
wood surfaces during the gluing operation . Dried joints and chilled joint s
belong to this category .
It may be that a layer of glue of perceptible thickness i s
requisite for strong joints only when the surface to which the glu e
adheres is sensibly rough, as it is with wood . Joints between non-porou s
solids having very smooth surfaces, such as polished metal, fused glass ,
or surfaces of single crystals, are much stronger when the layer o f
adhesive, which must be complete and continuous, is exceedingly thin ,
perhaps but a few millimicrons in thickness (1$, 12) . Moreover, smooth ,
non-porous surfaces may be joined strongly with adhesives of much mor e
diverse physical nature than those effective in wood joints . Pure
crystalline substances and even pure liquids have been found affective ,
in joining smooth, non-porous surfaces (11 to 12), but all glues known t o
be suitable for joining wood are properly classifiable as colloid jellies .
The thicker layer of glue necessary when the surfaces joine d
are rough and porous instead of smooth and impervious may result from th e
fact that the glue must act as a wood filler to occupy completely th e
cavities near the surface before a suitable layer of glue can be built up .
If it did not fill the cavities in this way, contact between glue laye r
and rough surface would be discontinuous and therefore imperfect . During
the drying of aqueous glues there is a marked contraction in volume . To
keep the cavities full, glue from the surface layer must be drawn int o
them to compensate for the loss by shrinkage . Sufficient glue for the
purpose can hardly be supplied practically without having a laye r
comparable in volume with the amount needed to fill the cavitias . The
necessary flow of glue from layer to cavity can take place satisfactoril y
in colloids that remain viscous or plastic during transformation from so l
to gel, but it is not compatible with the sharper transitions• from liqui d
or solution to crystalline solid .
R977
-3-
Coatings of paint are continuous layers comparable in thicknes s
with the layers of glue required in good glued joints . Coatings diffe r
from layers of glue, of course, in the facts that they adhere to wood o n
only one instead of both sides, that they are applied with relativel y
little pressure, and that as a rule they do not cause wood to swell . Like
glues, coatings must first fill the wood cavities near the surface if the y
are to establish continuous contact with the wood. Roughly speaking, the
first or priming coat of paint applied to wood serves as a filler and a
really continuous coating is seldom obtained until the second 'coat o f
paint is applied . Change in volume takes place when coatings dry i n
consequence both of evaporation of thinners and oxidation of drying oils .
In subsequent service volume changes continue, not only because o f
continued oxidation (6, 7) but also because of fluctuating temperatur e
and moisture content, for paints are definitely hygroscopic (20) . During
such changes in volume the wood cavities can remain filled and the coatin g
can remain in continuous contact with the wood only as long as the coatin g
retains the property of plasticity especially characteristic of elasti c
jellies . In paints the capacity for undergoing plastic deformation i s
provided by the gelatinous matrix, the linbxyn, and not by the pigments .
It therefore seems clear that adhesion between wood and coatings of pain t
is pri arily a problem of vehicle rather than of pigment composition .
aI
' 100pPenetration Not Necessary for Strong Adhesio n
Strong glued joints require very little penetration of glue
into the wood (2), but to establish firm contact with all parts of th e
mood surface the glue must, of course, follow all its irregularities .
wood are likely
ary that the glue
the wood is a kind
hardwoods or lik e
r than the lumin a
ways into the
siderably deepe r
ed with large vessels o r
asier to glue strongly
woods .
_
:•n .
n the othe r
,
or
1;
„ ten a
land ~,
L cte zed by
Water from the glue solution penetrates the wood far more deepl y
than the glue jelly . When the glue contains truly soluble ingredients ,
such as sodium hydroxide, their presence can be demonstrated in-parts o f
the wood many fiber diameters beyond the deepest point attained by the glu e
jelly .
In the lumina of tracheids paint cannot penetrate beyond thos e
cavities that open directly on the surface, because the particles o f
paint pigments are much larger than the openings in the pit membrane s
through which communication must take place . Figure 1Cshows how a paint
coating lies on the surface of a softwood . However, the liquids of pain t
pass through readily enough, and therefore penetrate much more deeply .
When painters speak of the absorption of paint by wood they mea n
absorption of paint liquids ; the pigments are left behind at the surface .
:enough liquid is thus drained away from the priming coat materially t o
alter the composition of the paint . The liquids penetrate farther i n
the hard sammerwood of the softwoods than they do in the springwood ,
which has much larger lumina . In this respect penetration of paint liquid s
differs strikingly from that of glue jelly . Since it is the summerwoo d
from which coatings first begin to flake, permanent adhesion of coating s
to wood obviously is not a matter of obtaining deep penetration of th e
paint liquids .
Behavior of Tendrils of Glue Penetrating Wood Cavitie s
probably Significan t
Conclusions about the nature of the adhesion between glue an d
wood nave been drawn from the shapes assumed by tendrils of glue tha t
have penetrated into wood cavities opening into the surface (5) . It is
clear that glue solution entering the open end of a cavity pushes forwar d
in the form of a column of solution filling the cross section of the
tubular cavity, usually completely . As the joint dries the solution give s
up water and shrinks greatly in volume . The tendril of glue evidentl y
adheres firmly to the surfaces wherever contact has once been made . It is
rarely dislodged or its contact with the layer of glue severed by shrinkag e
during drying . At the open end of the cavity the tendril draws in mor e
glue from the layer, while at its remote end the concave meniscus i s
sucked back deeply, leaving a long, tubular shell of glue clinging t o
the walls of the cavity . Small bubbles of air enclosed in the column o f
solution fail to disrupt the tendril as it shrinks, and remain a s
ellipsoids enclosed within the tendril .
As a rule cross sections through cavities containing tendril s
exhibit either a plug of glue filling the space completely or a thic k
ring of glue fitting snugly in the cavity . Occasionally, however, ther e
is a mass of glue„ contained loosely within a cavity', shriveled into t
.n1€' Mlle column of gl ,
*ape .tom t isltt :d, Ittv,i
in the Oavity had :
fled the cross sectio n
the wa1lis, perha•',>
e of an oily or resino u
-these fq '",i' '
11146'; Town in Figur e
glue,
both
in aqueous solu
c-bae, ,u
7
.c
vusl7•
to
the
wood
itself once sat i
t
.
mad- .
solutt'; . at that point
pletely or had not we t
incrustation there .
B . The facts prov e
ion and as dried jelly ,
,tact has been
'i 1=
' II
- Whether pa,1.nt penet'fates into the 1
is of coarselike
glue
has
no
t
beI
textured hardwoods aid forms' tendrils that
s
ver,
that
paint
oil
yet been determined . There is reason to think,
o not readily form
penetrating deeply into the Ii.ra,o3st:Pile of softwgo
view
of the large amount
tendrils. Few, if any, appear Lat Figure lti~
ne
number
of lumina foun d
of liquid
drl ?j. ~6 i1 t
dieting pa
dried
coatings
plugged with ltaso rn t~1e4i
bb p&o are .1stur'
.
work
will
be
on softaa4s' is : swIlmei;
Wen:. kf
more
rimental
,lfinown
rw~o
the
absorbed
oil
goe
s
hecessai, ha m! i
.'it ' 3a b4
wood
and
linoxyn
can
and what deduct ~
the. , a •siM
ieigkt
tanebaa7i
be : d 'adn .
No Relation &stablished Between Mechanical Properties of Glue
Films and Strength of Wood Joint s
Siloodwoxing g3ti's 4' different kinds have been prepared in th e
, 16) .
form of thin films for st
'df t■hiaii,.- kiechanical properties (12,
feer
making
iu,
Relation between film progig' t9,a s of glues and th ` w
e
nts
between
hard
mapl
strong wood_ joints is ne; e•6ly• lacking ,(4) . Good;
r
square inch in shea
blocks that'cull withs to id Aloie than 3000 pounds
can be made 'venial e,a;e'ki &. f 't} e- Eallowin4 ; (a) an 4 luur: , films of whic h
withstand'•t2000 '.
ca per sQ'ra ineiz : h tensiBr3i and are moderatel y
flexible ; (b) ' anti: .ue co~t~ ir~g 1.5 times its weight of dextrose ,
square inch and are exceedfilms of which break apt' about
10' 15(141.414
ingly fie ble ; (c) com ercial sdciixIm silic4te, films of which break at
1900 pound, par square inch and are exceed j' krittle .
Paint coati:4 s have likewise been ., epared in the form of films
for mechanical tests, .and it he's sometimes 'Oeen assumed that paint s
satisfactory for use del wood m si,yield_fis that are reasonably stron g
:_5riilh woodworking glue s
and moderately flexil e (C),'. The 'eager
suggests that wheil .bt .j ail-lesion betwee7r4 e"e>i„eting and wood can b e
established the- aasi-on may not be valid.
.f~
Figure l .--Photomicrographs of cross section s
through (A) a strong joint, (B) a starve d
joint between surfaces of a hardwood, an d
(0) a paint coating on a softwood .
.1`y r~x
Adhesion of Glue to
Summerwood
is Strong
It has sometimes been argued that glue does not adhere strongl y
to the summerwood of softwoods, because glued joints after breaking ofte n
reveal badly torn patches of springwood while the areas of summerwood remai n
smoothly intact . Consideration of the loads required to break good joint s
between softwood surfaces, however, proves that the appearance is deceptiv e
and that the adhesion to summerwood is in fact strong .
The strength of good joints approximates the strength of th e
wood joined . The wood strength and hence the joint strength are directl y
proportional to the density of the wood, which in turn increases accordin g
to the proportion of summerwood it contains . Thus strong joints in heavy
softwoods having much summerwood take higher testing loads than joints i n
light softwoods, although the surface presented for gluing by the heavie r
wood consists more largely of hard summerwood . If adhesion to summerwoo d
were weak -- that is, weaker than the strength of springwood -- join t
strength would decrease rather than increase with summerwood content .
This fact that joint strength not only increases, but even increases i n
about the same proportion as the wood density, shows clearly that th e
summerwood bears its share of the load applied to good joints and tha t
adhesion . of glue to summerwood is indeed firm .
Aged coatings, unlike glue, do not adhere firmly to summerwoo d
(1 to 3) . The heavy softwoods slough coatings much sooner than the ligh t
softwoods . Among woods of equal density, coatings flake sooner the wide r
the annual growth rings and in consequence the wider te bands of summersts a
wood .. This cQntx'ast-bs,r
lpait c
fundamental difference in tt• natetre of tk "
in spite of
other similarities in their behavior .
a .
There are in general at least , two kinds- of: gtthesion : ■ specifi c
adi ice„ each is duo tic) tmatamele-culax forces of t skind repponsible fo r
solution, we
Ong, ald. a x4r1a4ion, and (b) mechanical adhesion .; which comes
from embedment of the =ad .bes*i:ve in %avities in the surface
(Specifadhsonmybe
opep ativ~e at either smooth or rough
suraces, bu t
mechanical adhesion exists at'ro-ugli surfaces only . Mott people seem to
place more confidence in mechanical than in specific adhesion, and yet th e
more powarftl forces are undoubtedly those of specific adhesion . There
seems also' ` to be a t•amdea4ey to take it for granted that at rough surface s
adhesion is predomi
1 if not 'exclusi'Voly, mechanical .
Among craftsmen the vibw has long prevailed that adhesion o f
glue to wood is mechanical . This conception led to gluing technic giving
maximum penetration of glue into the wood and has resulted in starve d
joints, which are the most common type of weak joints, at least wit h
animal glue . Some scientists likewise consider the adhesion of glue t o
wood essentially mechanical (12, 3 , 14, 15, 16) . Yet it has been show n
definitely that wood-working glues adhere firmly to the walls of the woo d
cavities and are not merely embedded in the visible or the microscopi c
interstices (4, 5) . as far as is now known, these walls may be smooth ,
in which case adhesion is necessarily specific . If they are rough, it is i n
an ultra-microscopic sense . Difficulties both theoretical and practica l
make it unprofitable to extend the concept of mechanical adhesion t o
interstices of ultra-microscopic size .
The springwood of softwoods offers much better anchorage fo r
mechanical adhesion than the siraerwood, because the tracheids of th e
springwood have thin walls and large lumina, while sum ::zerwood tracheids
have thick walls and small, flattened lumina . Where adhesion is specific ,
springwood still has an advantage over mnpa erwood in that, being rougher ,
it presents more interface per unit of gross area . The advantage fo r
springwood, however, decreases as the strength of the specific bon d
increases . Finally, when specific adhesion is strong enough for the join t
to withstand stress as great as the strength of the wood itself, the
adhesion is for all practical purposes equally strong on summerwood an d
springwood, although the breaking of the joint is still likely to sho w
more tearing of the weaker springwood than of the stronger summerwood .
The nature of the adhesion between linoxyn and wood remain s
to be determined . The ease with which paint oils penetrate wood indicate s
that, while liquid, they wet or adhere to wood readily . The change from
liquid oil to linoxyn is a gradual one and the linoxyn is known to contai n
liquid oil for a long time after it has hardened (11) . At least during
this time linoxyn probably adheres to wood specifically . The flaking of
aged coatings from the sumuerwood of softwoods indicates, however, that a
fundamental change in the nature of the adhesion may have taken place ,
corresponding perhaps to the disappearance of liquid oil from the linoxy n
jel1,y,F its marked contraction in vole, and a'
liri ite't-=ratty
patteim '(6 , 1) . adhesion of ;aged coatin gg 'may bi st r:Ictly mechanical
and tie lumina of summerwood tracheids mylt-h6n'' ble too . small to affor d
adegiata anchorage . If fit re 'study of the ac
ion of coatings confirm s
t•h%g view, the problem -of imp-roving paints for od is one of obtainin g
pent specific adhe's-ion between linoxyn . ;, er nes•pithe4r)matrix fo r
pigments, and wood.
1. The problem of adhesion between paint coatings and wood i s
essentially similar to that of adhesion between glue and wood, which ha s
been much more successfully studied . With glue, insight into the natur e
of adhesion was gained by measuring the strength of glued wood joints an d
by observing, under the microscope, sections through wood joints . Simila r
methods should be fruitful in studying the adhesion of coatings, though a
technic for measuring the force required to tear coatings from wood mus t
first be devised .
2. If the conditions for adhesion are similar for coating s
and for glue, then the widely held opinion that the paint oils must penetrat e
deeply into the wood is unsound . The coating must follow the surfac e
Irregularities of the wood exactly, but the filtering off of paint liquid s
by the absorption into the wood that now takes place is both unnecessar y
and undesirable . Where paint oils do penetrate into wood cavities, the y
should form tendrils projecting from the coating as glue does .
3. Where adhesion between coatings and wood is strong, th e
mechanical properties of films of paint may be less significant than has
sometimes been supposed .
4. Glues adhere to wood specifically and find no difficult y
in clinging to the summerwood of softwoods . Possibly paint coatings adher e
specifically while young, but after they age the adhesion may becom e
mechanical only, whereupon the lumina in mummarwood prove too small fo r
adequate anchorage . If Such is fact, paint as made at present must be s o
modified that* specific adhesion will be maintained indefinitely .
•.r-
~ l~~l~
y
-
'
Fr i
•
'Ai '
'7T
Egrature Cited,
~IJ
Y
1•
, .-''i'}'Ii i
r
.i3
rnishiroduction Clubs, Of
1. , 22
pril 7, 1 70) ; Pain t
7 . Clark, G . L ., , and Tschentke, H . L ., Ind .
8 . Do ]~ s r W
Ae rx t. .
D:_,
.•
. P .t-tmfor , C . B ., I'lW-5r= O.W)naut . So( (Gt . Britain) ,
0: 929) .
9. Ge nt, 444,, as
10. Lee, W . B .. , 'Ind .
11 , Lem, '
8 6X19
21, 621 (1929) .
Aurnit e' $
I, .
rtZ'kilgeraziur, .
. Z1.
776 <Il
.
K ; ;. and '
12 . McBain, J .. W :, Iz~L. Ant
:. Chem,.- 12', 10015 I
)' ,
13 . M 3at,n., ",F . W ., and H.opk .s s , D . G ., Dept .
Rs• igiar ch (Gt . Br i t a i n ) , , : ,dine p i:,ve s Re s
McBain, J . W.,'
. , and Herkins, D. G ., J .
15 . McBain, J . W. ,
16 . . McBain, J . lfii-. , and Lee, W. B ., J . So.c . C,4h
;. . Yi d.l
17 . McBain, J . IL ,
lg . McBain, J . W . ,
19. McBain, J . W ., and Lee, W . B . , J . Phys .
]xem:, 4148 (1928) .
20. Nelson, H . A ., Proc . Am. Soc . Testing Materials, 21 II ,- H"3'-- (1921) ;
3 II, 356 ( 1 923) .
R.977
-10 -
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270
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