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Installments: Getting the big picture from adhesive test data

Aug. 3/10; Volume 30/Number 4

By Graham Capobianco

Screen Shot 2015-08-13 at 11.26.39 AMOf all the floor coverings used today, wood is the most demanding when it comes to adhesives. Wood flooring adhesives must be able to sufficiently bond to plywood or concrete, and must also be flexible enough to hold the wood while it goes through dimensional changes over the course of its lifetime and maintain a strong bond with the substrate as the flooring goes through these changes. All the while, it can’t exert too much strain on the substrate because that might cause it to fail. While many adhesives on the market illustrate their ability to withstand these stresses, some leave out one very important factor—elastic modulus.

Elongation at break, or elasticity, is the ratio between the initial length and final length after breakage of a test specimen, often expressed as a percentage. This testing is performed by applying a nominal load to a test specimen and stretching it at a defined speed (stress) until the specimen breaks or cracks. The speed and load used are based on minimum values, though the load increases over time and until break.

With wood flooring adhesives elasticity is important, as wood flooring is susceptible to fluctuations in temperature and humidity. Without an HVAC system constantly monitoring and controlling temperature and humidity levels, it is impossible to prevent wood flooring from experiencing these changes in most environments. If an adhesive is too rigid, these dimensional changes may exert enough force onto the substrate to cause a cohesive substrate failure. If it is too elastic, it may not be strong enough to hold wood.

Some adhesives have a reported elasticity of 150% or more, which make wood flooring prone to gapping, cupping, deformation and bond failure if they are not strong enough to handle the strain the wood can exert on it. These complications are exacerbated when wood experiences quick and dramatic changes in temperature and humidity during de-acclimation, home vacancy, power outages or HVAC equipment malfunction. To ensure adhesives can resist these effects while remaining elastic, it’s important to understand the elastic (e) modulus.

Elastic (e) modulus is the ratio of tensile stress to tensile strain of a test specimen, typically measured in n/mm2 or PSI. It is the amount of pressure an adhesive can withstand while being stretched before breaking. This test is performed using equipment similar to elasticity testing, but the test specimen is usually exposed to maximum speed (stress) and load (strain) values. The applied load and strain are measured at break or at a specific elongation percentage in order to calculate the amount of pressure the test specimen can withstand, usually measured at 25% elongation.

The important criterion of this test is the load; though 25% may seem low compared to elasticity data, 25% elongation can represent a real world scenario. Even the most unstable wood species will only experience a maximum dimensional change of 11% to 12%. Even if we doubled the maximum dimensional change, this would still fall under 25%.

Though adhesives may be advertised with high elasticity results, adhesives with a high elasticity modulus are better suited to handle dimensional changes in wood.