Bioengineering
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Standard Test Method ASTM D 7998-19 for the Cohesive Strength Development of Wood Adhesives
Chapters
Summary May 17th, 2020
We present a procedure, ASTM D7998-19, for a rapid and more consistent evaluation of both dry and wet strength of adhesive bonds on wood. The method can also be used to provide information on strength development as a function of temperature and time or strength retention up to 250 °C.
Transcript
The Abes Automatic Bond evaluation system is the only method for testing sheer bond strength as the adhesive is being cured because it is much faster and easier to use. It is the best method for developing new adhesives for wood bonding. This method can be applied to other adhesive systems for which it would be helpful to determine the strength of the adhesive, but other substrates may be required.
To prepare the substrate, select a substrate surface that is suitable for the application. For wood, use a slice of veneer that is about 0.6 to 0.8 millimeters thick. Avoid veneers that are excessively wavy, have an uneven surface and or contain defects including discoloration.
At least one day before its use, condition the wood unstacked at 21 degrees Celsius and 50%relative humidity. To prepare the specimen from the selected substrate, after conditioning, check the veneer for any cracks, discoloration or grain irregularities to be avoided when cutting and confirm the functionality of the pneumatically driven specimen cutting device. Next, use a special di cutter to trim the specimen to a 20 by 117 by 0.6 to 0.8 millimeter dimension and place an at least 150 by 300 millimeter piece of veneer under the cutting blades with the veneer grain parallel to the long direction.
Before initiating the bonding process, make sure that the automated bonding evaluation system equipment is operating properly according to the standard operating procedure. And set the low pressure gauge to 0.2 mega pascals. The high pressure gauge to 0.2 megapascals, the pole gauge to 0.55 megapascals and the cool air gauge to 0.2 megapascals.
Clean the platinums of any adhesive that resulted from squeezing out from the prior sample and adjust the platinum temperature to the desired temperature. Then equilibrate the platinums. For adhesive bonding of the specimen, first, tear each specimen on a balance.
Before spreading five milligrams of the experimental adhesive over the terminal 0.5 centimeters of each specimen providing sufficient coverage of the bonding area without excessive squeeze out upon application of the second specimen. Then re weigh the specimen and place a second specimen onto the adhesive, taking care that the two specimens are aligned. To bond the specimens, close the grips on the automated bonding evaluation system tester, making sure that the sample is aligned with the tester and press Start.
To have the 120 degree Celsius platinums, apply pressure to the overlapped section for two minutes. At the end of the heat pressing time, retract the platinums and grips and place the specimens at 21 degrees Celsius and 50%relative humidity overnight. The next morning, insert a conditioned sample into the automated bonding evaluation system tester and press Start.
To have the instrument, pull one end of the specimen through the servo drive while the other end of the sample pulls on a load cell attached to the grips until the bond breaks. Plans are hot and can cause burns if touched. Also, be sure to keep your hands away from the plans when they're closing.
The computer will record the maximum force the sample can withstand as the bond strength. To determine the rate of strength development of an adhesive to estimate the press time required for large scale products, begin the strength testing at a 100 degree Celsius platinum temperature for 10 seconds. At the end of the bonding time, retract the platinums and use the air cooling feature of the automated bonding evaluation system to cool the sample to near room temperature.
When the sample has cooled, test the sample again at the same temperature, but for an increased period of bonding time until increasing the time results in little or no increase in strength. When all of the bond times have been tested, raise the temperature by 10 degrees Celsius and repeat the heated pressure applications for increasing periods of time until there is no longer any linear section of strength versus time at the bonding times. If the product needs to meet a certain temperature resistance, heat the platinums to the temperature above which the adhesive starts to soften and insert the bonded sample into the automated bonding evaluation system unit.
Close the platinums onto the pre bonded sample for two minutes and measure the bond strength as demonstrated to determine any thermal softening of the adhesive compared to the 120 degree Celsius bonding temperature. Then repeat the test after a 30 minute heat pressure application and determine the strength of the adhesive. If the adhesive is thermally degraded as the main objective is to determine the adhesive strength or rate of cohesive strength development.
Be sure to test if the failure is within the adhesive itself due to a failure in the adhesion of the substrate or due to a substrate failure. If a substrate failure occurs then the adhesive had sufficient strength. While cohesive failure in the bulk adhesive indicates an adhesive weakness.
As this set of exothermic strength development plots demonstrates as the bonding temperature increases, higher bonding strengths can be developed in shorter bond pressing times. In this derived plot of regressed bonding rate against temperature illustrates an increased strength development rate can be achieved in less time at higher temperatures. In this image of failed samples and adhesion failure due to the adhesive being stronger than the wood resulting in the bond breaking in the wood and wood failure can be observed.
While in this sample, a cohesive failure due to the adhesive being weaker than the wood resulting in the bond breaking within the adhesive is shown. Remember to make sure that the bonded areas overlap by half a centimeter. As this area is critical to determining the strength of the bond.
The heat sensitivity of the bonds can be measured to determine the temperature at which the adhesive will degrade or fail.
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