Single-component Polyurethane Adhesive For Wet Wood Glue Creep Characteristics

Bonding of timber of high moisture content, namely "gluing wet wood", or gluing of greenwood as it is sometime called, to prepare glulam and fingerjointing became possible with theindustrial introduction of honeymoon, fast-set, separate application phenol-resorcinol-formaldehyde (PRF) adhesives in the early 1980's. While wet wood gluing up to 90% moisturecontent of the timber was already achieved commercially in the early 1980's only more recentlysingle application, one component polyurethane systems for "gluing wet wood" have beenformally approved and widely promoted for use in structural glulam in some Europeancountries. Polyurethanes often impart lower percentage wood failure than what were the olderrequirement, but imparted also excellent joint strength.The main characteristic of reactive polyurethanes, hence still containing reactiveisocyanate groups to yield cross-linking in application, such as those used for glulam, is that thepresence of water in the timber should set off and accelerate the cross-linking, hardeningreaction. In this and in their ease of handling, polyurethane systems are a very interestingproposition for bonding glulam. The main problem all polyurethanes, even cross-linked onespresent, however, is that of creep. This means that in the past even good polyurethane systems,even those well resistant to water and weather deterioration could not be used for structuralapplication. Considering that the majority of the applications for glulam and fingerjointing isfor structural building applications this can be a grave defect indeed. It is the existence of thisproblem which has effectively stopped polyurethanes, good wood adhesives, to be used inglulam applications in the far past, and perhaps their slighter higher cost. Temperature-dependent creep is a somewhat different problem, but critical to the safety of under-roofingstructural timber in hot countries (under-roof temperatures in Mediterranean and other countriescan reach 70℃in summer) and in the behaviour of structures in fires.The concern for creep under load, and for temperature-dependant creep, is only veryrecently starting to be addressed as a serious threat to the structural application for which theseresins have already been approved in a few European countries. The clear existence of thisthreat has elicited a number of responses from the specifying authorities involved, namelyeither the problem has been ignored and dismissed as if it did not exist, or the problem has beenaddressed by reporting only results of polyurethane adhesives which present a low level ofcreep. Recent work however, has clearly shown that of the few types of commercialpolyurethanes approved and used in Europe for this application, some present no creep, somepresent medium-level creep and some a potentially disastrous level of creep. The principle thatpolyurethanes can be excellent structural wood adhesives is not in doubt here. What is important is that knowledge of their structure and network is essential and is determinant to theextent of creep and temperature dependent creep they present in this application.Creep and temperature-dependent creep, characteristic of one component polyurethaneadhesives for wet and green structural wood gluing are two different parameters. Testing ofstandards-approved commercial polyurethane adhesives for this purpose shows that whileambient temperature creep can be avoided or at least greatly limited according to theformulation used this is not the case for temperature-dependent creep. The commercial adhesiveformulation characteristics that minimize or enhance ambient temperature and temperature-dependent creep are identified. Basic principles on the structure of simple polyurethaneadhesives influencing temperature-dependent creep are also identified and discussed.The mechanical performance of seven one component polyurethane adhesive systemsunder increased temperature was analyzed and compared with the chemical composition of theadhesives. The influence of a reduced maximum glue line thickness and the effect of post-curing of the bond line were also evaluated.The study also deals with testing for ambient-temperature creep as well as for temperature-dependent creep, the best, approved, polyurethane adhesive tested by other authors and thenwith trying to correlate temperature-dependent creep with the structure obtained from theanalysis of this best, low-creep commercial one-component polyurethanes approved for thispurpose and commercially available in the European market. The commercial adhesiveformulation characteristics that minimize or enhance ambient temperature and temperature-dependent creep are identified.Traditional fast-set honeymoon adhesive is used to bond with green wood and presentclear capabilities for bonding wet and green wood of very high percentage MC, in this study; ashigh as 55% MC.