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.
The properties of cured wood adhesives are difficult to study because of the loss of water and other components to the wood, the influence of wood on the adhesive cure, and the effect of adhesive penetration on the wood interphase; thus, normal testing of a neat adhesive film is generally not useful. Most tests of wood adhesive bond strength are slow, laborious, can be strongly influenced by the wood and do not provide information on the kinetics of cure. Test method ASTM D 7998-19, however, can be used for fast evaluation of the strength of wood bonds. The use of a smooth, uniform, and strong wood surface, like maple face-veneer, and sufficient bonding pressure reduces the adhesion and wood strength effects on bond strength. This method has three main applications. The first is to provide consistent data on bond strength development. The second is to measure the dry and wet strengths of bonded lap shear samples. The third is to better understand the adhesive heat resistance by quickly evaluating thermal sensitivity and distinguishing between thermal softening and thermal degradation.
Wood bonding is the largest single adhesive market and has led to efficient use of forest resources. For many centuries, solid wood was used for most applications, except for furniture construction, with no test criteria except product in-use durability. However, bonded wood products became more common, starting with plywood and glulam beams, using bio-based adhesives1,2. Although these products were satisfactory at the time, the replacement of soy, casein, and blood glues by synthetic adhesives containing formaldehyde led to improved properties. The higher performance of these new adhesives led to defined testing standards with higher performance expectations than achievable with most bio-based adhesives. The synthetic adhesives also made possible the bonding of particles including sawdust to form particleboards, fibers to form fiberboards with varying densities, chips to provide oriented strandboard and parallel strand lumber, veneers to yield plywood and laminated veneer lumber, as well as finger jointed lumber, glulam, cross laminated lumber, and wood I-joists3. Each of these products have their own testing criteria4. Thus, the development of a new adhesive can require a lot of formulation work and extensive testing to determine if there is any potential for developing sufficient strength. This time-consuming testing and the complexity of wood properties and wood bonding5 has limited the development of new adhesives. In addition, the mechanical properties of wood adhesives can be different when cured between wood surfaces as opposed to neat6. Curing in contact with wood allows water and low molecular weight components from the adhesive to escape, in addition to complex interphase and chemical interactions of the adhesive with the wood3,7.
The development of the Automated Bonding Evaluation System (ABES) has been very helpful for understanding the strength development of wood adhesives because it is rapid and easy to use8,9,10. The system is an integral unit that bonds lap-shear samples and then measures the force under tension needed to break the bond. Its utility has led to development of ASTM method D7998-19 that uses this system11. Although this system was originally designed to measure adhesive strength development as a function of temperature and time, it can also measure the heat resistance of cured adhesives, as well as routine bond strength evaluation. Although the ABES test is a very useful preliminary screening tool, like any test, it has its limitations and does not replace all specific product strength and durability testing.
While there are many means of gauging the curing characteristics of adhesives, ranging from gel-time rheometry to differential scanning calorimetry, dynamic mechanical analysis, and spectroscopy of many types, only the ABES method measures the development of mechanical strength. This requires an instrument that is tightly controlled for heating, cooling, and in-place tensile testing11.
1. Preparation of substrates
2. Preparation of specimens
3. Operability of the equipment
4. Bonding of specimens with the adhesive
NOTE: The application of the adhesive is a critical issue for wood adhesives because of the wide variation in viscosity and percent solids going from a lamination adhesive as in plywood to a spray able adhesive for binder applications. Wood adhesives are generally water-borne so evaporation is only a minor problem. However, water soaking into the porous wood is important.
5. Image analysis of failed bonding surface
The procedure has been used extensively for the study of protein adhesives at the Forest Products Laboratory. It has been found that less than 2 MPa wet bond strength was insufficient to warrant further wood adhesive testing, while greater than 3 MPa was a promising result for further testing19. It has been shown to be useful in demonstrating sensitivity of wood processing conditions12,13. Further examples can be found in Frihart publications7. The precision and bias of the method has been determined (Research Report RR:D14-1018) as summarized in ASTM D7998-1911.
Figure 1: Photograph of Specimen Cutter. Please click here to view a larger version of this figure.
Figure 2: Photograph of ABES system (top) and drawing of the apparatus with bonded sample (bottom). Please click here to view a larger version of this figure.
Figure 3: A set of isothermal strength development plots (left) with a derived plot of regressed bonding rate against temperature9. Please click here to view a larger version of this figure.
Figure 4: Analysis of failed sample. Adhesion failure on the left and cohesive failure on the right. Please click here to view a larger version of this figure.
Critical steps in the procedure are as follows: selection of substrates, preparation of specimens, operability of the equipment, and bonding of samples.
The substrate must be strong, have minimal defects (smooth, flat, no cracks and no discoloration. Unsanded, rotary cut cabinetry face veneer of a diffuse porous hardwood with sugar maple (Acer saccharum) preferred. Sanding creates a less even and more fragmented surface7. After conditioning the veneer at 21 °C and 50% RH for at least a day, cut a strip of 20 mm by 117 mm. Apply usually 5 mg of adhesive evenly to 5 mm of the end of one wood strip. With the platens heated to 120 °C, bond the coated strip with another strip with a 5 mm overlap for 2 min in the ABES with the platens closed to form a lap shear sample. After removing the lap shear samples from the ABES unit, they are conditioned overnight before using the ABES unit to test the strength (half at ambient conditions and half after submerging the samples in water). For a measurement of bond strength, the failure must occur in the bonded area. Complete details on equipment specification are given in the ASTM standard11.
The procedure is most useful for evaluation of the strength development of wood adhesives as a function of temperature and time. It is less useful for wood adhesives that cure at room temperature, such as EPI and PUR, because they do not require heat for bonding to wood. Primers for wood adhesives, such as HMR, can be tested, but they are mostly used with room temperature adhesives. Samples with primers could be bonded with veneer pieces that will fit in the ABES with a separate press at room temperature and tested in the ABES.
The significance of small-scale bonding as described in ASTM D-7998-19 is that it is a preliminary evaluation of wood adhesives that can be done quickly and with little labor. The existing methods of testing wood adhesives require larger amounts of adhesive and wood and time to bond large panels of plywood or particleboard which need to be conditioned at a specific temperature and humidity before they are cut up by a professional carpenter into precise samples for testing. Many panels have to be made to test different variables, which can be done more easily and quickly with the ASTM D-7998-19 procedure, ABES. There is no other test method that can determine kinetic cure data of an adhesive.
The authors have nothing to disclose.
This work was supported by the United Soybean Board grant 1940-352-0701-C and the U.S. Department of AgricultureForest Service. We appreciate the support and detailed information from Phil Humphrey of AES.
Adhesive | Supplied by user | ||
Balance | Normal supply house | ||
Mark II Automated Bonding Evaluation System (ABES-II) | Adhesive Evaluation Systems Inc | ||
Pneumatically driven sample cutting device | Adhesive Evaluation Systems Inc | ||
Regular spatula | Normal supply house | ||
Wood supply – Hard maple | Besse Forest Products Group |