Coatings for minimizing changes in the moisture content of wood

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Material Information

Title:
Coatings for minimizing changes in the moisture content of wood
Series Title:
Technical note ;
Physical Description:
5 p. : ill. ; 20 cm.
Language:
English
Creator:
Forest Products Laboratory (U.S.)
Publisher:
U.S. Dept. of Agriculture, U.S. Forest Service, Forest Products Laboratory
Place of Publication:
Madison, Wis
Publication Date:
Edition:
Rev. Dec. 1952.

Subjects

Subjects / Keywords:
Wood preservatives   ( lcsh )
Wood -- Moisture   ( lcsh )
Genre:
federal government publication   ( marcgt )
non-fiction   ( marcgt )

Notes

General Note:
Caption title.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 029721877
oclc - 761382032
System ID:
AA00025025:00001

Full Text
U 3 j t / ,-


TECHNICAL NOTE NUMBER 181
UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE
FOREST PRODUCTS LABORATORY
MADISON B. WISCONSIN /-VISED December 1952



COATINGS FOR MINIMIZE G [HANOE& INN
THE MOISTURE CONTENT F WOO) / .. ,' /


Shrinking and swelling and the accompanying stresses 1 'ftl& cause
warping, checking, and weathering are brought about by chang' t-le
moisture content. Such changes occur whenever wood is exposed to
varying atmospheric conditions. Effective protection against fluctuating
atmospheric conditions is furnished by coatings of various moisture re-
tardant finishes, provided that the coating is applied to all surfaces of
wood through which moisture might gain access. No coating is entirely
moisture-proof, however, and there is as yet no way of keeping mois-
ture out of wood that's exposed to dampness constantly or for prolonged
periods.

Tests of the moisture-excluding ability of coatings of many kinds have
been made at the Forest Products Laboratory. (The method of test is
described and more complete data are given in U. S. Department of
Agriculture Circular No. 128 obtainable from the Superintendent of Docu-
ments, Goyernment Printing Office, Washington, D. C., for 10. ) Ta-
bles 1 and 2 list coatings suitable for interior use only. The coatings
in table 3 may be used for exterior as well as interior work; some are
more durable than others. Ordinarily, moisture-excluding ability is
only one of several properties that must be considered in selecting a
coating for a given use.

The data ate for coatings only a few weeks old and not yet exposed to
the weather. The effectiveness of many coatings improves slightly with
age. Good exterior coatings either retain their maximum effectiveness
for a considerable time or lose effectiveness slowly. As long as the
original appearance and integrity of the coatings are retained, most of
the effectiveness remains. Paint that is faded or chalking remains effec-
tive if vigorous rubbing removes the chalk and discloses a glossy film
underneath. Deep chalking, checking, or cracking indicates serious
impairment of the effectiveness.
U. S. Dept. of Agricilture Library
C/O Agricultural Experiment Stations
Ath Mrs. I. K. Cresap, Librarian
Library 209 Horticulture Bldg.
Gainesville, E lcrida







The numericalvalues for percentage effectiveness are to be considered
in a relative rather than an absolute sense because percentage effective-
ness varies materially with the conditions under which exposure to mois-
ture takes place. Coatings are much more effective, for example, against
a brief exposure to water inliquid form than they are against orie or two
weeks of exposure to moisture-saturated air. The values for effective
coatings (60 percent oit over) are reliable in the sense that they can be
reproduced closely on repeating the test; values for ineffective coatings
(less than 20 percent) must be regarded as rough approximations only.
The percentages shown are based on average amounts of moisture ab-
sorbed per unit surface area by newly coated and by uncoated wood pan-
els subjected to a relative humidity of 95 to 100 percent for 14 days. In-
terior coatings were tested on birch panels only, exterior coatings on
white pine, redwood, Douglas-fir, and southern yellow pine.

The data obtained lead to the following generalizations:

1. To retard the exchange of moisture between wood and air, a substan-
tial coating that is relatively impervious to moisture is necessary.
Merely "plugging the wood pores" is not sufficient.

2. The first coat (primer) applied to bare wood rarely forms a substan-
tial, impervious coating.

3. Linseed oil alone is low in effectiveness even when a substantial
coating has been achieved. The effectiveness of drying oils is greatly
increased by incorporating resins with them (making varnishes), or by
adding pigments (making paints). The more resin or pigment incorpor-
ated, within practical limits, the greater the effectiveness. As a rule,
paints are more effective than varnishes, and enamels (pigments added
to varnish) more so than either.

4. Aluminum powder as apigmentmakes especially effective and dura-
ble coatings. It can be used with many liquids, but for exteriors the
liquid itself should be suitable for exterior use. Aluminum paints may
be used as complete coatings or as primers for coatings of other paints.

5. Asphalt and pitch paints are highly effective, inexpensive, and rea-
sonably durable if well made, but they are dark in color and can rarely
be painted over satisfactorily with light-colored paints.

6. Nitrocellulose wood lacquers are considered suitable for interior
use only, and do not prove so effective as interior varnishes, though
more effective than some exterior varnishes.


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Table 1. --Moisture-excluding effectiveness of coatings suitable for in-
terior use only

Percentage
effectivenessI


Coating:
Three
Heavy
Three
Three
Three
Three
Three
Three


coats of aluminum powder in gloss oil ....
coating of paraffin... ...................
coats of rubbing varnish ................
coats of shellac .........................
coats of enamel (cellulose lacquer vehicle)
coats of cellulose lacquer ...............
coats of gloss oil bronzing liquid ........
coats of furniture wax ..................


Table 2. --Moisture-excluding effectiveness of some typical interior
wall primers, flat paints, and semigloss wall paints!-


Percentage
effectivenessI


1 coat


2 coats


Coating:
Wall
Wall
Wall

Two
Two
Two

Two
Two 4
Two


primer A
primer B
primer C


:oats
coats
:oats

:oats
:oats
zoats


flat wall paint A
flat wall paint B
flat wall paint C

semigloss wall paint A
semigloss wall paint B
semigloss wall paint C


One coat wall primer A + one coat flat
wall paint A
One coat wall primer B + one coat flat
wall paint B
One coat wall primer C + one coat flat
wall paint C


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13
4
11

83
52
41


67







Table 2. --Moisture-excluding effectiveness of some typical interior
wall primers, flat paints, and semigloss wall paints.
(continued)
Percentage
effectiveness!
1 coat Z coats
Coating (continued):
One coat wall primer and semigloss wall
paint A -- 82
One coat wall primer and semigloss wall
paint B -- 69
One coat wall primer and semigloss wall
paint C -- 27


Table 3. --Moisture-excluding effectiveness of coatings suitable for ex-
terior or interior use

Coating Description Percentage effectivenessi
number 1 coat 2 coats 3 coats

1 Aluminum powder in asphalt
or pitch paint vehicle .......... -- 98
2 Aluminum powder in No. 16
vehicle ....................... 39 88 95
3 Extra fine aluminum powder in
No. 16 vehicle ................. 78 92 94
4 Aluminum powder in "alkyd"
type synthetic vehicle .......... 15 81 93
5 White lead in a vehicle similar
to No. 16...................... 62 86 91
6 One coat of No. 2 plus two
coats of No. 17 ................ 39 86 91
7 White lead in No. 19 vehicle ...... 24 85 91
8 Aluminum powder in No. 23
vehicle ............... .... .... 9 61 90
9 Asphalt or pitch paint ........... -- -- 90
10 Aluminum powder in bodied
linseed oil vehicle ............. 26 84 89
11 One coat of No. 8 plus two coats
of No. 17..................... 9 62 86
12 White lead in No. 23 vehicle .... 7 62 83
13 Aluminum in linseed oil ......... 14 57 77
14 Aluminum and red lead in
linseed oil .................... 7 65 75


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Table 3. --Moisture-excluding effectiveness of coatings suitable for ex-
terior or interior use (continued)

Coating Description Percentage effectiveness-
number 1 coat 2 coats 3 coats

15 Linseed oil house paint con-
taining zinc oxide and other
white pigments with or with-
out tinting colors ............ 30 69 73
16 Phenol-aldehyde synthetic
resin, 50-gal. varnish ....... 5 49 73
17 Linseed oil house paint con-
taining no zinc oxide, such as
common "lead and oil". ..... . 20 57 70
18 Red lead in linseed oil ......... 15 56 67
19 Ester gum resin, 33-gal.
spar varnish ................ 6 37 65
20 Graphite in linseed oil ......... 4 58 64
21 Red linseed oil barn paint, pig-
ment 98 percent pure iron
oxide....................... .25 53 56
22 Red linseed oil barn paint, pig-
ment Venetian red containing
40 percent iron oxide ......... 1 25 45
23 Ester gum resin, 75-gal. long-
oil spar varnish ............. 3 14 35
24 Linseed oil containing paint
drier....................... .3 5 21

-Perfect protection would be represented by 100 percent effectiveness;
the complete lack of protection in the case of uncoated wood, by zero.
These data are to be considered only as approximations for relative
comparison. Results vary significantly according to the kind of wood
on which tests are made, and according to the thickness of the coat-
ing.
AThe finishes designated A are made by one manufacturer, B by another,
and C by a third.


A F r i c u I t ur*-M-ad i -on


ZU45264F


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