Adhesive Cure Monitoring with Acoustic Method

Conditions of adhesive cure process play a very important role in the quality of adhesive joints. It is known that the cure stage of adhesives correlates with the glass transition temperature and strength of adhesion. The objective of our present work was to quantitatively characterize cure of the adhesives with acoustic method. Pulse-echo mode was used for the study. Changes in acoustic parameters during cure time were simultaneously monitored. These changes are discussed with respect to the phases of the adhesive cure process. Dependence of the prepolymer/curing agent ratio on acoustic parameters and dynamics of cure are also investigated. Microstructure of the adhesives was investigated with acoustic microscopy at different times of curing. Changes in the adhesive’s microstructure during curing time were observed. Acoustic techniques can be useful in industrial applications for non-destructive testing of the adhesive cure conditions. Introduction: Adhesive joining is widely used in many manufacturing industries. Requirement of both high productivity and quality means the increasing need to improve quality control. Currently, no proper non-destructive in-field technique exists for adhesive bond joint evaluation. The author’s interest in ultrasonic studies of the adhesive cure process results from the necessity to evaluate the cure state of the material in adhesive metal joints where ultrasound is used for non-destructive evaluation of joint quality [1]. Improper setting of cure parameters during production could lead to joint failure or significantly decreased adhesion strength. Network formation of the epoxy resin due to crosslinking is a complex process. During the cure reaction epoxy undergoes changes from low molecular liquid or paste to a highly crosslinked network. Behavior of the adhesive during the cure process can be monitored by different methods: nuclear magnetic resonance, calorimetry, infra-red or Raman spectroscopy, dielectric measurements, dynamic mechanical analysis (DMA), and ultrasonic technique [2-5]. Acoustic technique has been used by many authors for studying adhesive. However, the previous ultrasonic studies [4-7] on epoxy curing process monitored only changes in the wave parameters and do not show changes in the microstructure of the adhesive during the cure process. Because of the nature of the acoustic wave’s propagation, acoustic technique directly measures the relation between acoustic wave behavior and material characteristics on microscopic scale. Ultrasonic images reflect these changes in mechanical properties of the material during the cure process and correspondingly in the physical state of the material. This technique is able to detect curing defects caused by improper cure conditions or inhomogeneous mixing of resin and hardener. In this study we apply ultrasonic technique for non-destructive characterization of the epoxy adhesive cure process and high-resolution acoustic imaging of the adhesive microstructure. Results and discussion: Commercial low-temperature curing adhesive was used for monitoring the adhesive properties’ changes during hardening. The ultrasonic setup consists of spherically focused 15 MHz immersion transducer, coupled with the adhesive sample in a specially designed test cell (Fig.1.). A water-glycerin mixture was used as a coupling medium to reduce reflection from the coupling medium-epoxy interface. The reaction chamber is 20 mm in diameter and 3 mm thick. PET film is thin enough so its presence does not create an additional reflected pulse at the coupling-epoxy interface. Flexibility of the PET film allows us to measure variation of the sample’s thickness during curing and thus to monitor the material’s density changes. The transducer was driven by the Utex 340 pulser-receiver, operating in pulse-echo mode. Output signal was registered by the Textronix TDS 5052 digital oscilloscope for further processing. PET film water-glycerin