Standard Test Method ASTM D 7998-19 for the Cohesive Strength Development of Wood Adhesives

Summary

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.

Abstract

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.

Introduction

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 adhesives^1,2. 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 tes....

Protocol

1. Preparation of substrates

1. Use a substrate surface that is suitable for the application. For wood, use a sliced veneer of about 0.6 to 0.8 mm thick from a reliable producer because these veneers are used for plywood and laminated veneer lumber (LVL) manufacturing. These are obtained from a veneer supplier, as sheets of 0.6 to 0.8 mm thickness and cut into 305 mm on a side. A consistent substrate is a hard maple (Acer saccharum) face veneer because of its surface smoothness and consistent thickness, and it is a diffuse porous and high modulus hardwood. Maple face veneers are commonly used in cabinetry construction and are usually free from defect....

Results

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 conditions^12,13. Further examples can be found in Frihart publicati.......

Discussion

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
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Materials

Name	Company	Catalog Number	Comments
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