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ADHESIVES AST TULIP APPLICATION
TO FABRICATION OF
FARM STRUCTURES
February 19€0
No. 2178
FOREST PRODUCTS LABORATORY
n Cooperation with the University of Wisconsin
ADHESIVES AND THEIR APPLICATION
TO FABRICATION OF FARM STRUCTURES!
By
DON BROUSE, Engineer
Forest Products Laboratory,— Forest Service
U. S. Department of Agriculture
Summary
Application and service characteristics of presently available wood
working glues are reviewed, with emphasis on durability of adhesive
bonds under conditions of service characteristic of farm structures.
Casein glues are convenient to use and are adequate for items that will
not become repeatedly or continuously damp or wet in service. Phenol-,
resorcinol-, and melamine-resin adhesives have adequately proven
durability under severe service conditions. Phenol- and melaminetype adhesives require rather high temperatures (250 0 to 310° F.) for
adequate curing. Resorcinol and phenol resorcinol adhesives may be
cured adequately for many purposes at 70° F. and above and, consequently, are favored for farm structures.
Development of the Art of Gluing
The gluing of wood is a very old procedure, apparently going back as
far as reliable history does (3).-3-- The earliest recorded use of glues
!Presented at the winter meeting of the American Society of Agricultural
Engineers at Chicago, M., December 1959, on a program arranged by
the Farm Structures Division.
2
—Maintained at Madison, Wis. , in cooperation with the University of
Wisconsin.
!Underlined numbers in parentheses refer to Literature Cited at the end
of this report.
Report No. 2178
-1
with wood seems to be for the purpose of enhancing the beauty of the
product. The earliest use of glues to produce items that could not
be fabricated from wood and mechanical fastenings seems to have
passed without record. By the 17th century, however, the production
of glued-wood items, which could not have been carved from solid wood,
was well advanced, as the very fine violins of that period illustrate.
One of the most important developments in the field of glued- wood products
occurred in the latter part of the 19th century when the commercial
production of a crossbanded assembly of glued veneers, now known as
plywood, got under way. This outstanding development made available a product that was better than a solid wood panel in dimensional
stability in width and in resistance to splitting. In this century, the
use of glued-wood products has expanded to an almost unbelievable
extent, as illustrated by the increase in the production of Douglas-fir
plywood from 153 million s q uare feet (3/8-inch base) in 1925 to 6, 240
billion square feet in 1958, and by the increased use of structural elements of glued-laminated wood.
The use of glue to fabricate items of wood is not the answer to all problems of the wood-using industry. Some of the advantages and disadvantages to consider are listed as follows:
Advantages
(1) Gluing side grain to parallel side grain can develop the full strength
of the wood, a goal that cannot be achieved by mechanical fasteners.
(2) The use of glue permits the fabrication of wood items having desirable
properties that can be realized in no other way, as for example plywood
with its high degree of stabilization in width.
(3) Gluing permits production of items for which large timbers are no
longer available, such as large arches and ship timbers.
(4) The use of glue permits fabrication of large structural elements from
thin lumber that, before laminating, can be dried easily to uniform and
suitable moisture contents and thoroughly treated with preservatives,
if so desired.
(5) Gluing is often the most satisfactory way of assembling such items as
hollow- and solid-core flush doors.
Report No. 2178
-2
(6) Glues permit the fabrication of strong but light structural elements,
such as box beams, stressed-skin panels, and sandwich-type constructions.
Disadvantages
(1) Solid wood items, providing they will serve satisfactorily, are less
costly for many uses than glued-up items, as for example, the heavy
beams and columns in mill-type construction.
(2) No , one glue has all the desirable characteristics. Hence, the glue
must be selected that will meet the probable use requirements of the
item.
(3) No glue is foolproof. To realize the advantages of glued construction,
all joints must be well made. To produce joints of consistently high
quality requires skill and a rather exacting control of gluing conditions.
(4) Improper use of glues and glued products may result in costly repairs
or replacements.
(5) End-to-end or end-to-side butt joints of adequate strength cannot be
made with any glue and gluing techniques now available.
Development of Glues
The two essential elements in making strong and durable glue joints are
(1) the selection of the proper glue for the probable service conditions
to which the glued item will be exposed in normal service, and (2)
the maintenance of good gluing conditions to insure the best possible
results with the glue selected.
At the present time, the woodworker has available some 10 different
types of glues and several mixtures or combinations of these basic types.
Some are of natural origin and some are based on synthetic resins. Those
now being widely used have been on the market long enough so that their
characteristics and limitations are reasonably well known. Other resins
and combinations have exhibited some promise as woodworking adhesives,
but they have not been sufficiently evaluated to permit completely reliable
judgments of their characteristics.
Report No. 2178
-3
The more important developments in adhesives all have taken place
in this century. Until about 1903, when starch glues for wood gluing
came on the market, the only type of glue known to be adequate for
production work was that produced from the skins, tendons, and bones
of animals, usually cattle.
In this century, the glues that were developed and accepted by industry
include starch (in 1903), casein and blood (during World War I), soy
bean (in 1924), phenol resin and urea resin (in 1932), and melamine resin,
resorcinol resin, and polyvinyl resin (all during World War II).
In a similar field, the period since about 1942 has seen significant developments in adhesives for bonding metal to metal. These developments are
of importance to the wood industry, because they provide a means of
gluing metal to wood with strong and durable bonds. Some of these
developments also have characteristics, such as self-reactivity, which
may prove useful in special cases for the bonding of wood to wood.
Still other developments in the field of "contact adhesives" (those
requiring only a low bonding pressure for a brief period) are likewise
of interest to the wood user. While at the present, these contact
adhesives are not considered suitable for purposes requiring very strong
and durable bonds, they are useful for such purposes as bonding plastic
tops to built-in cabinets and bonding the finish floor to subfloors. This
field likewise appears promising and improvements seem probable.
Characteristics of Woodworking Glues
All of the adhesives, now widely used in the woodworking industry are
capable of forming strong bonds that will remain so if the glued product
is kept dry and at temperatures no higher than the human-comfort range.
With a few exceptions, a glue for domestic or office furniture could be
selected largely on the basis of price, or how its operating characteristics
fit the available equipment and production schedules (4).
The difference between these glues lies in their resistance to the more
severe conditions of service and in the technique of application (1).
The following tabulation, taken from the Wood Handbook (5), summarizes
information of importance to the users of woodworking glues or glued-wood
products:
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From the information in the column headed "Properties, " it is not
difficult to slect the type of glue suitable for the probable service conditions under which the glued item is expected to perform. Possibly
the greatest chance for error in selecting an adhesive occurs when the
common understanding of "normal service" does not include periods of
"abnormal conditions" that frequently develop. As an example, several
military structures were erected in World War II under a demand
schedule that required using any material available that would serve
with reasonable satisfaction for an estimated period of about 5 years.
On these buildings, interior-type plywood was frequently used as the
exterior covering of the walls. On the assumption that the "normal
service" was for 5 years, the choice was reasonable. In this case,
however, "abnormal conditions" prevailed;- the "temporary use" extended for 10 years or more, and the interior-type plywood on the
exterior surfaces could not be expected to serve satisfactorily over
such a prolonged period.
Importance of Gluing Techniques
Besides selection of a glue suitable for the expected use, the gluing
operation must be well controlled to produce consistently strong bonds (2).
In addition to the limited information in the preceeding tabulation, the
quality of glue bonds is dependent on (1) conditioning the wood to a uniform moisture content at a level suitable for the conditions of service;
(2) preparing mating surfaces that are smooth and well fitted mechanically;
(3) spreading an adequate amount of well-mixed glue uniformly over the
faying surfaces; (4) adjusting the interval between spreading and pressing
to suit the characteristics of the glue; (5) applying an adequate and uniform gluing pressure; (6) maintaining the gluing pressure long enough to
permit the glue to set; and (7) in some cases, conditioning before
further work is done on the item.
The lasting qualities of well-made glue joints are reasonably well known,
but the long-term serviceability of poorly made joints is open to question
even under service conditions that may be quite normal.
Report No. 2178
-8
Application to Farm Structures
With this information on glues as a background, their application to farm
structures and the fabrication of products may be considered.
Barns
Among the most striking examples of glued-wood products in farm structures are glued-laminated rafters that form the gracefully curved roofs
of barns (fig. 1). This application raises two questions that are typical
of all uses of glued-wood products in structures: What type of glue
should be selected, and what control of gluing conditions must be maintained to insure adequate strength and durability of bond? Some laminated rafters bonded with casein glue have failed because the condi- tions of service permitted the development of molds and other microorganisms to which casein glues are not permanently resistant. Such
unfavorable conditions are very likely to develop in northern, dairy barns
during the colder seasons, when the moisture in the air from the stables
condenses on the roof members. Highly resistant resorcinol or phenol
resorcinol types of glue should be used in barns that are reasonably
tight and are used to shelter animals. It would probably be advisable, to
purchase these laminated rafters from established laminators rather
than to fabricate them with improvised equipment and local help. While
not impossible, it is unlikely that the farmer could control all the factors
necessary to insure bonds of uniform and adequate strength.
Moisture barriers installed at or near the interior of the walls and
ceiling of the stable area will prevent the high moisture-laden air from
coming in contact with the wood structural members and the insulation.
Good construction practices of this type prevent decay of the wood, paint
failures, and delamination of laminated members that are bonded with
casein or interior types of glues. Due to openings to the barn loft and
possible punctures and deterioration of the moisture barrier, however,
a still safer procedure would be to use rafters constructed of the more
resistant glues.
While dairy barns have been used to illustrate the effect of condensation
on the choice of glues, the principles apply also to any farm building in
which condensation of moisture on the structural elements is a possibility,
such as laying and broiler houses for poultry and farrowing and nursery
quarters for hogs.
Report No. 2178
-9
Many older barns are of the heavy timber type of construction. The
timbers were often sawed at local yards from logs furnished by the
farmer from his woodlot. Under such conditions, the cost of raw
material for this type of construction may well be lower than that of
the laminated rafter type, but the cost of labor required to erect the
laminated-rafter structure is lower (fig. 2). The overall costs,
therefore, are not necessarily in favor of the heavy timber construction.
Plywood is another glued-wood product that is used in barn construction.
The buyer has a choice of both hardwood and softwood plywood in
various species and types. The term "type" refers to the resistance
of the glue to severe service conditions. Douglas-fir and other West
Coast softwood plywood are available in two types (6), Exterior and Interior,
which are indicative of the intended use. The commercial standards for
hardwood plywood (7) list four types, Technical and Types 1, 2, and 3.
Technical and Type 1 require a fully waterproof and weatherproof bond,
Type 2 utilizes a moderately water-resistant bond, and Type 3 has
somewhat lower water resistance than Type 2.
Any plywood used on the exterior surfaces should, of course, be Exterior or
Type 1. Likewise, plywood in exterior locations where it may be
repeatedly wetted and dried also should be of Exterior or Type 1. Plywood that will be in contact with the ground or in locations that remain
warm, and damp for long periods should be of Type 1 or Exterior type, and
treated with wood preservatives if long service is expected.
Silos
Glued-wood products can likewise be used in the construction of silos.
Exterior or Type 1 plywood should be used for both the outer and the
inner surface material, and laminated members, bonded with a highly
resistant glue, should be used for framing. In considering the probable
service conditions, only the phenol- or melamine-bonded plywood and the
phenol resorcinol-bonded laminates are likely to be suitable. Framing
members that are not of a decay-resistant species also should have
preservative treatment. With some unloading systems, an interior
surface of metal or enamelized metal would probably serve more satisfactorily than the surfaces of bare plywood. If so, adhesives and bonding
procedures are available by which reliable bonds of metal to wood may
be achieved.
Report No. 2178
-10-
Implement Sheds
Another kind of farm building in which glued-wood products could be
used to advantage is the implement shed. The term "implement shed"
here includes those structures that are not heated, or regularly occupied
by livestock, and in which the temperature and relative humidity are
essentially equal to outside conditions (fig. 3). Depending on design and
overall costs, glued-laminated rafters, roof trusses with glued gussets,
and plywood may be used. Such glued items require that a choice of
glues be made. Since these sheds are unheated and not so tight as to
prevent a rather free circulation of air between the inside and the outside,
no serious condensation problems should occur.
Roof trusses that are protected by a tight, well-maintained roof with
adequate overhang, should give satisfactory service, with casein-glue
bonds between the gussets and the frame, although, of course, the more
resistant glues would provide a greater margin of safety. While a good
casein bond may be made at temperatures below 70 0 F., the advisability
of fabricating the trusses on the job is open to question. If lumber at
a desirable moisture content between 10 and 15 percent can be smoothly
surfaced shortly before gluing, if a protective shelter is available, and
if the workmen are reasonably skilled in the use of glue, the chances of
producing serviceable trusses are good. If the lumber is rough and
poorly seasoned, and if the trusses must be fabricated more or less in
the open by unskilled workers with hurriedly improvised equipment,
then nailing the gussets in place would be the safer procedure.
Plywood, of course, can be used to advantage for such sheds. Generally,
the siding will be of single thickness and, consequently, only Exterior or
Type 1 plywood should be considered. In most cases, it can be assumed
that the shed will be painted for the sake of appearance. If so, it may
prove desirable to purchase plywood overlaid with a resin-impregnated
paper, because the bulk of commercial plywood consists of species that
do not hold paint particularly well. Interior-type plywood may give
satisfactory service for roof sheathing, provided it is covered with a tight
well-maintained roofing and the exposed edges are treated with water
repellent and then protected either mechanically or by well-maintained
paint coatings.
The use of Type 2 hardwood plywood that has been glued with a straight
urea-resin glue is questionable, because the temperature may become
high enough to weaken urea-resin bonds. Roofs of implement sheds
and similar structures, moreover, are not always well maintained,
Report No. 2178
-11-
because an occasional leak is unlikely to cause any serious deterioration
of the implements. For this reason, the use of Exterior or Type 1 plywood would be a safer procedure.
Exterior-type plywood has been suggested as the only roof covering
that is laid directly on the rafters with no cover on the weather side.
While the glue bonds in Exterior or Type 1 plywood are probably
adequate for this severe service, there are some disadvantages. The
surfaces will check markedly in a short time, darken, and become rough.
Some separation of summerwood from springwood also may develop.
The general appearance then may become rather unsightly. If the outside surfaces are to be painted, satisfactory paint service can scarcely
be expected from conventional house paints, because the wood species
commonly used for commercial plywood will not hold paint well, and the
low-angle exposure of roofs is much more severe on paints than are
vertical exposures. If painting is considered essential, three coats of
aluminum paint, or a primer of aluminum paint followed by two coats of
iron oxide paint, would probably be the best choice.
More important, probably, is the decay hazard. Few of the species
commonly used for plywood are highly resistant to decay, and water
seeping into the exposed edges may create conditions favorable to decay.
Some of these disadvantages can be overcome by the use of preservatively
treated plywood that is overlaid with a resin-impregnated paper or sheet
plastics, but the final cost may then exceed that of conventional roofs.
While not often seen on farms, shelters are occasionally designed that
violate some of the principles of the use of glued-wood items. Some
shelters, whose cross section resembles that of the quonset huts, have
curved glued laminated beams supported at the ends by concrete abutments
as their main structural elements. The laminates bonded with casein
glue that are protected by a roof extending down over the abutment may
give satisfactory service. If the roof ends above the abutments, however,
unsatisfactory service of casein-bonded laminates result because of
delamination and, ultimately, decay in the exposed ends.
Shelters
• The remarks about the use of glued-wood items in implement sheds apply
equally well to fixed or movable shelters consisting essentially of a roof
under which livestock may find shelter. In this type of open structure,
Report No. 2178
-12-
condensation is not likely to cause serious trouble because free circulation
of air tends to keep temperatures and relative humidities, inside and out,
at nearly the same level. On roofs that are tight and well maintained,
rafters may be laminated with casein glue, provided that the ends are
sloped back and both the ends and the part of the rafters extending beyond
the walls are kept well painted.
With wood trusses, casein glue should serve satisfactorily for bonding
the gusset plates to the framing members, again assuming a tight and
well-maintained roof. On roofs that will be well maintained, Interiortype plywood can be used for roof sheathing, provided that the exposed
edges are brushed with water-repellent preservatives and kept well
painted.
The use of Type 2 plywood bonded with straight urea-resin glues may be
questionable because of possible temperature effects. The safer procedure, of course, is to use a phenol resorcinol type of glue for rafters
or gussets and Exterior or Type 1 plywood for roof sheathing. Exterior
or Type 1 plywood can be used as the only roof covering, if appearance
and absolute freedom from leaks are not important. The serviceable
life of bare plywood for roofing has not been established, however, and,
unless a tight roof is maintained, phenol resorcinol glues should be used
for bonding the gussets on the trusses.
The shelter type of construction serves to illustrate the cases in which
the expected length of service is a factor to be considered. In permanent
structures, the few added dollars that result from the use of more
resistant glues and Exterior or Type 1 plywoods are well spent. If the
service life of the item is expected to be short, 5 years or less, taking
somewhat greater chances on material using the less resistant, but
cheaper glues, may be justified.
Dwellings
In farm dwellings, just as in urban dwellings,. glued-wood items and
structures find a wide and probably increasing use. The structural
elements (walls, floors, and sometimes roofs) of many of .the prefabricated
houses are essentially sandwich-type constructions,formed by gluing plywood to frames of solid wood. The outside, plywood surfaces should,
of course, be Exterior or Type 1 and should be bonded to the framing with
one of the highly resistant glues. So far as glue bonds are concerned,
Interior-type plywood for inside facing of the exterior walls would
Report No. 2178
-13-
probably give satisfactory service, but using wall units with Interior
plywood on one side and Exterior plywood on the other side , offers
chances of error during fabrication. As a general rule, Interior or
Type 2 plywood would be suitable for interior partitions, and the plywood could be bonded to the frames with casein glues.
In conventionally built houses, plywood is often used for subfloors,.
roof sheathing, and wall sheathing. The choice of the type, as in all
other cases, depends on the expected service conditions. For example,
Interior-type plywood for subfloors should be satisfactory over a full
basement and on floors above the first, assuming, of course, that the
builder provided for adequate protection from rain and moisture during
the construction of the house. Type 2 hardwood plywood bonded with a
moderately fortified urea-resin glue or better would likewise be suitable
under these conditions. If the house has no basement, or only a partial
one, and the crawl space is inadequately ventilated, Exterior or Type 1
plywood would be the safer choice.
Somewhat similar considerations arise with the use of plywood for wall
and roof sheathing. If it is assumed that moisture barriers are properly
installed, that water will not gain access through open joints or cracks
in the siding or around windows or doors, and that the temperature does
not rise too much above the human-comfort range, then Interior or Type
2 plywood should serve satisfactorily. Since these ideal conditions may
not always be maintained during the life of the house, the use of Type 1
or Exterior plywood for wall and roof sheathing seems the safer choice.
In the northern States, snow or ice dams that frequently form near the
eaves and permit water from rain or melting snow above the eaves to
seep back under the shingles, thus wetting the sheathing, are added
hazards. Occasionally, builders use Exterior-type roof sheathing near
the eaves and Interior types above.
Other Applications
Implements
Outside of the field of farm structures, glued-wood products may find
use in farm implements. It is desirable for the user of such implements
to know the limitations of the glued-wood product, if any, and be prepared to make necessary repairs by gluing with the types of glues required for this service. While the farm implement field has been taken
Report No. 2178
-14-
over to some extent by metals, some parts made of plywood and glued
laminates certainly give good service in many items. Since many farm
implements must withstand long-continued exposure to the' elements, any
glued products used should be bonded with the most resistant glues,
such as, phenol resin for the plywood and resorcinol or phenol resorcinol
for the laminates. In addition, any parts that are usually in contact
with or very near the ground and those located where water will accumulate
and remain indefinitely should be treated with preservatives.
Recreational Equipment
In a somewhat lighter vein, glued-wood products are found in much of
the recreational equipment that is common to both farm and urban
families, and a knowledge of the characteristics of glued-wood products
is helpful in the use and repair of such items. If the horse lover has
a good trotter or pacer, he is likely to find that the better thills for the
sulky are glued-wood laminates, bent to form during the gluing operation.
His sons and daughters likewise will probably be using glued-wood
laminates in such recreational equipment as baseball bats, bowling
pins, tennis rackets, bows, and skis. If any of these must be repaired
by gluing, the glue should be selected on the basis of probable conditions
of service and storage, recognizing that it is never a mistake to use a
glue with greater resistance than the minimum required.
Probable Future
As the country's supply of large sawtimber decreases, more glued
products may be expected on the market, because gluing offers a
method of fabricating large or specially formed products from the
supply of small timber that is available. For example, serious consideration is being given to producing 2 by 4's by gluing together two
1 by 4's. When well glued with resorcinol or phenol resorcinol glues,
such laminates should serve as well as solid 2 by 4's in any use. At
least one company is producing edge- and end-glued nominal 1-inch
lumber as a substitute for solid wood in order to utilize effectively
their short and narrow stock.
Report No. 2178
-15-
A considerable amount of experimenting has been done on lumber overlaid
with resin-impregnated paper or vulcanized fiber in order to permit
use of lower grades of lumber. This provides a surface that will retain
paint well and be suitable for a product with miscellaneous uses. One
of the most promising uses for such a composite of paper and lumber
appears to be for the wide siding that is in demand for architectural
and style reasons.
Literature Cited
(1)
Blomquist, R. H.
1954. Evaluation of Glues and Glued Products. Jour. of
Forest Products Research Society 4 (5): 0-299.
Madison, Wis.
(2)
Freas, A. D. and Selbo, M. L.
1954. Fabrication and Design of Glued Laminated Wood
Structural Members. U. S. Department of Agriculture
Tech. Bull. No. 1069.
(3)
Perry, Thomas D.
1942. Modern Plywood. Pittman Publishing Corp. , New York.
(4)
Truax, T. R.
1929. The Gluing of Wood. U. S. Department of Agriculture
Dept. Bull. No. 1500.
(5)) U. S. Forest Products Laboratory.
1955. Wood Handbook. U. S. Department of Agriculture
Handbook No. 72.
(6)) U. S. Office of Technical Services.
1959. Commercial Standard CS 45-55, Douglas-fir Plywood.
U. S. Department of Commerce.
1956. Commercial Standard CS 35-56, Hardwood Plywood.
U. S. Department of Commerce.