CONTROL CF CONDITIONS IN GLUING
WITIi PROTEIN AND STAPCI-1 GLUES
Revised April 1950
(No. 134O
UNITED STATES 'DEPARTMENT OF AGRICULTURE
SOREST SERVICE
I/FOREST— PRODUCTS LABORATORY
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin
When protein glues are used in the manufacture of plywood, glue
spreads are normally much lower than in assembly gluing. For example, in
the manufacture of moisture resistant Douglas-fir plywood, wet spreads of
55 to 60 pounds per 1,000 square feet of single glue line for cold-press
plywood and 35 to 40 pounds per 1,000 square feet of single glue line for
hot-press plywood are commonly employed. •
Assembly Time
Pressure should be applied and contact made in the joint before the
glue becomes too thick to flow. Glue that is spread on dry wood becomes
thicker on standing. Where pieces of wood are coated with glue and exposed
freely to the air ("open assembly"), a much more rapid thickening of the
glue occurs than where the pieces are laid together as soon as the glue is
spread ("closed assembly").
In some gluing operations, manufacturing conditions permit only short
assembly times, and under such conditions relatively thick glue must be
used. In other operations the . assembly time may vary from less than a
minute to as much as 25 minutes, and the glue must 'remain at a satisfactory
consistency during the entire period. Such a wide variation in assembly
time is not desirable.. Changes in consistency of casein glues are relatively
slow and permit considerable variations in assembly periods, but assembly
periods in excess of 20 minutes should be .avoided. Where this is unavoidable, the glue should be of medium consistency and should be spread somewhat heavier than normal and on both surfaces, A heavy pressure, such as
should be used.
200 pounds per square Effect of
Ifasinpzarp.
With casein, starch, and soybean glues, satisfactory joints are
obtained when the glue is applied at ordinary room temperatures, and
normal variations within this range do not require any important change
in other conditions, such as assembly time or amount of pressure. When
the room and wood temperature are high, as on very hot days, some reduction in assetbly time may be necessary and desirable.
In gluing with animal glue, the wood is sometimes heated before
gluing. The glue cools and thickens more slowly on heated wood than on
wood at ordinary temperatures. If long assembly periods are necessary,
heating the wood may be an advantage, but with short assembly periods it
may 1)e a distinct disadvantage. When gluing is done at ordinary room
temperature it is seldom necessary to heat the wood unless conditions
require slow thickening that cannot be better accomplished, in some other
way.
Reip t. Uo.
1340
-3-
Pressing and Clamping
Pressure shauld be applied to the glue joint by means of clamps,
presses, or other mechanical devices wherever it is possible to do so.
The object of such pressure is to bring the wood surfaces into intimate
contact with the glue, to force air from the joint, and to spread the glue
out into . a continuous film between -the wood layers. The amount of pressure
applied should be goVerned by the consistency of the glue at the time of
pressing, as illustrated in figure 2, and to some extent by the kind of
wood being glued. For softwoods, such as spruce, a pressure of 100 to 150
pounds per square inch, and for hardwoods such as ash, 150 to 20t pounds
per square inch should be used with a relatively thick glue represented by
average casein glue.
Measurinp Pressure
As close a check as practicable should be kept on the amount of
Pressure applied to joints by means of clamps, pressure screws, and similar
equipment. Figure 3 illustrates some of the common devices used for
applying pressure by hand, and in table 1 are pressure data relating to
their use. The approximate loads applied by screws with square threads may
be calculated from the formula given in table 1.
Uniform andSustained Pressure
Where relatively thin pieces are being glued, blocks and cauls should
be used between the clamp or press and the layers being glued, to distribute
the load from the point of contact to other parts not directly under the
load and to prevent clamps from crushing the wood, It is especially important that the cauls be true and uniform in dimension. Besides the
neglect to use proper blocking and cauls, some other principal causes of
unequal pressure on joints (which must be carefully checked and, if possible,
prevented) are: (1) irregular surfaces of the wood pieces being glued; (2)
unequal thickness or width of stock; (3) improper spacing of the pressurebearing members; and (4) deflection and other imperfections in press, clamps,
or other pressing equipment.
Clamps or other devices , should be kept closely adjusted for a short
time after the first application of the load in order to insure that the
correct amount of pressure is maintained. The pressure first applied to
glue joints tends to decrease as the glue squeezes out from between the
wood layers or distributes itself in the joint,
Rept. No. 1340
-4-
Duration of Pressure
For gluing at room temperature, pressure should be maintained for
hours or more. Where it is convenient to do so, it is better to maintain pressure from one day to the next. Where thick layers of absorptive
wood are being glued, the pressing period, may be reduced to 2 hours but
an additional conditioning period must then be provided before the machining and finishing operations begin. Longer pressing periods are desirable,
as this enables the joints to reach a higher proportion of their final
strength before being disturbed. Since the movement of water from the glue
line is one of the main factors in the increase in joint strength for
animal and starch glues, short pressure periods are favored by a concentrated
glue, a thin spread, warm, dry, thick layers of wood, and a warm room. The
setting reaction in case of moisture resistant casein, soybean, and blood
glues involves a chemical reaction in addition to the removal of water.
Consequently, the rate of setting of these glues can be greatly increased
by the application of higher temperatures. This is illustrated by the
relatively .short pressing cycles used in the manufacture of moisture-resistant plywood with these glues by means of hot-pressing. The rate at
which casein glue joints gain.strength when cured at room temperature (7080° F.), is illustrated in figure 4.
Drying and Conditioninglued Stock
The moisture added in gluing should be allowed to dry out or to
distribute itself thoroughly in the wood before any machining or finishing
is done, in order to insure the full strength of the joint, lower the
tendency of the glued member to warp, and reduce difficulties with sunken
joints and other surface irregularities.
Time Required for ConditioninF4
In laminated and similar constructions, where the percentage of
moisture added is relative]y small, the moisture from glue need not
actually be eliminated, but should be allowed sufficient time to distribute
itself throughout the wood. In laminated items and core stock, sunken
joints and other surface irregularities may be largely eliminated by conditioning for 1 week at room temperatures or for 2 days at 120° F. It is
advisable to dry out the surplus moisture from cold-pressed plywood by
conditioning it in a panel kiln or chamber that can be heated to about
120° F. The plywood should be placed on stickers not less than 3/4 inch
square, which should be arranged in vertical rows, one above the other and
parallel to the direction in which the air is circulating.
If the products are to be used in heated buildings or dwellings,
drying and conditioning to an average moisture content of 6 to 10 percent
is desirable and, with a panel kiln, can be accomplished easily in about
Rept. No. 1340
-5-
•
24 hours. Drying to excessively low moisture content materially increases
warping, opening of joints, and other defects, and is, therefore, to be
avoided.
•
Rept. No. 1340
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Fig. 1. Defects developed in cutting and
drying veneer: (1) badly honeycombed from
severe drying conditions (regularly spaced
alecks on upper surface produced in cutting);
(2) veneer sheets wrinkled by improper drying; (3) plywood joint failure due to veneer
badly caecked in cutting.
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Fig. 1,A.--Satisfactory and unsatisfaotory glue joints
resulting from different gluing conditions with casein
glue.
ZMS731F