Wood Handbook, Wood as an Engineering Material

Chapter 10 Adhesives with Wood Materials: Bond Formation and Performance

Construction Joints

Elastomeric construction adhesives are commonly used

in the light-frame construction industry for field assembly

of panelized floor and wall systems. Structural panels are

bonded to floor joists and wall studs with mastic adhesives

that have the unique capability of bridging gaps up to

6.5 mm (1/4 in.) between rough and poorly fitting surfaces

(Fig. 10–12). Without any premixing, the adhesive is extrud-

ed in a bead along framing members with a hand-held caulk-

ing gun or a pressurized dispenser similar to that shown in

Figure 10–7. Nails or screws provide the only pressure for

bonding, and they hold materials in position while the adhe-

sive sets. Elastomerics are also uniquely tolerant of the tem-

perature and moisture content variations at field construction

sites. Although they do not deliver the strength and durabil-

ity of conventional structural adhesives, elastomerics are

strong and flexible enough to give long-term performance

under most conditions of installation and service.

Construction adhesives enable a nailed or screwed floor

system to act to some degree as a composite assembly with

increased stiffness. Greater stiffness permits joists to be

longer and spaced more widely, with one layer of plywood

subflooring replacing two. Floors are less bouncy with fewer

squeaks and nail pops or screw pulls. However, structural

design of the composite assembly is based only on the in-

creased stiffness of nailed or screwed panel and framing ma-

terials. The strength contributed by the adhesive cannot be

factored into the engineering design but provides increased

value to the homeowner.

Testing and Performance

Testing is necessary to ensure that adhesively bonded ma-

terials hold together within a given service environment for

the life of the structure. Many methods are available to test

bonding performance, particularly for bonded assemblies.

Generally, these testing methods attempt to predict how

bonded joints are likely to perform in a specific loading

mode (shear, tensile, creep) in an assembly at specific

temperature and moisture conditions for a specific time.

Most performance tests are short term. They are based on

chemical, mechanical, and rheological laboratory tests of

adhesive polymers, adhesives, and bonds. Intermediate-term

tests of products that are conducted in pilot operations and

field experiments are integrated with short-term laboratory

tests in an effort to extrapolate these data into long-term per-

formance. Long-term tests of bonded assemblies under ac-

tual environmental exposures are conducted, but this infor-

mation may not be available for 10 to 30 years. Therefore,

short-term tests are extensively used to predict long-term

performance. As we learn the relationships between chemi-

cal structure and mechanical performance, and as companies

are under continued pressure to launch new products, the

reliance on short-term testing is increasing.

Analytical, Chemical, and Mechanical Testing

of Polymers

Although many methods of characterizing adhesives are

available, this section only briefly mentions some of the

most important and common methods.

Figure 10–11. End-to-edge-grain joints: A, plain; B,
miter; C, dowel; D, mortise and tenon; E, dado tongue
and rabbet; F, slip or lock corner; G, dovetail; H,
blocked; I, tongue-and-groove.

Figure 10–12. Gap-filling construction adhesive in

field-assembled plywood floor system.