Definition and determination of the rheological properties critical for the microapplication of adhesives

The microapplication of adhesives is extremely dependent on their rheological properties. In most cases, the viscosity values indicated by the manufacturers are not sufficient for the precise predetermination of the behaviour of the adhesives in the application process. Therefore, relevant parameters were established for the various applications by means of screen and stencil printing, ink jet as well as stamping. The adhesive quantity, the transferred volume and the thread drawing were investigated in greater detail in preliminary tests. The photographs taken with a high-speed camera have proven to be suitable, especially for the thread drawing. The deposition of the adhesive during the stamping lasts less than one second and therefore cannot be seen by the human eye. The deposition and detachment of the threads were recorded in the photographic mode of 1,000 images per second and could then be investigated and compared precisely by decreasing the image sequence. Together with the project-accompanying committee, the transferred adhesive quantity was stipulated as the decisive variable for the assessment of the stamping operation in the case of the adhesive application by means of stamping. In order to determine which rheological variable exerts a decisive influence on the adhesive transfer, the Q1-9226 adhesive, a two-component silicon adhesive, was investigated depending on the processing time, i.e. on the age of the adhesive after the mixing. The rheological properties of the adhesive were expected to change depending on the processing time. This procedure has the advantage that variables such as the filler content and the density of the adhesive may be regarded as constant and no longer have to be taken into account. The adhesive was characterised with both shearing and elongating deformation depending on the processing time. It was possible to establish that not only the storage modulus and the loss factor but also the elongational viscosity with high elongations alter as the processing time of the adhesive increases and that these correlate with the transferred adhesive quantity. In order to selector optimise the adhesive with regard to the highest possible adhesive transfer, it is recommended to consider the loss factor tan delta in the frequency range from 1 Hz to 10 Hz since this takes account of the viscous and elastic properties of the adhesive which obviously both have an influence on the transferred adhesive quantity. Entnommen aus <a_href="http://www.fiz-technik.de/db/b_tema.htm" target="_blank">TEMA</a>