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Polyurethane reactive adhesives, also known as curing hot melts, have been on the market for several years. While their use has grown rapidly in the woodworking and furniture industries, there are still misconceptions about their chemistry and applications.
Professional furniture manufacturers are highly skilled in woodworking, but many who spoke with 3M at the AWFS Fair in Las Vegas admitted that they aren’t experts when it comes to adhesive selection and use. To help furniture makers improve productivity, 3M technicians answered these common assembly questions about PURs.
Q:What is the difference between a PUR and other bonding systems?
By Debra Bettes, Product Marketing Supervisor, 3M Industrial Adhesives and Tapes Division
PURs create durable bonds that resist temperature differentials, weathering, moisture and chemicals. Once a polyurethane crosslinks and forms a permanent bond, it can never be broken.
A:Bonding systems such as hot melts, liquid PVA-type adhesives and one-part room temperature applied urethanes can be used effectively for various tasks, but they all have some limitations. PURs, however, offer the combined benefits of various adhesives and can overcome many obstacles that other options cannot.
● Hot-melt adhesives will typically join parts very quickly, but are not recommended for permanent, high-strength, load-bearing bonds. They also will not yield the thin glue line that can be obtained with PURs and usually only allow seconds for assembly.
● PVA adhesives, commonly known as white and yellow glue, typically do a nice job of producing a high-strength bond with thin glue lines. However, they require long clamping times, and the resulting bonds do not allow for movement in case of wood shrinkage or expansion. The advantage of PURs is that you can usually obtain handling strength in seconds, which allows the parts to be removed from the clamp and eliminates the need for nails or brads to secure the parts until a glue system dries.
● One-part room temperature urethane adhesives also produce high-strength, flexible bond lines, but still require clamping time. Many of these urethanes also tend to foam, which forces adhesive out of the glue line. This foaming makes cleanup time-consuming and can also create a foam or honeycomb glue line that could potentially decrease the strength of the bond.
With PURs, the adhesive does not foam and any squeezed-out adhesive can be removed by hand or with a plastic putty knife once it has turned solid, usually within two to four minutes. Cleanup is generally very easy, as long as excess adhesive is removed in the first 15 minutes. There are also PUR formulas that will allow for fixturing in up to two to three minutes. PURs can also bond dissimilar substrates such as wood to plastic.
Q:Do PURs form structural-strength bonds?
A:Yes, within minutes, PURs
have the holding strength of
more than 1,000 pounds. PUR
adhesives form a bond in two stages.
First they cool and form an initial
bond and then crosslink molecu-
larly. They absorb moisture from
the wood and from the air to form
a permanent bond. Once a polyure-
continued
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