Tack is an adhesive characteristic that must be considered when layering prepregs. Contradictory requirements are imposed on the tack of prepregs: on the one hand, it is necessary to ensure adhesion of the prepreg to the laid layers or to the shaping equipment, on the other hand, it is necessary to ensure that the binder does not stick to the release film when it is removed.
The tackiness of prepregs largely determines the efficiency of the production of high quality composites using automated stacking processes such as automatic fiber placement or automatic tape stacking . The c of prepregs is regulated by adhesion and cohesive processes, which depend on the parameters of the equipment used in the production process, as well as on the properties of the binder and reinforcing filler, on the type and content of the resin, the amount of inert volatiles, the degree of curing, temperature and humidity, and other factors [2, 3].
Considering the enormous costs of equipment and materials, control of prepeg tack ness is necessary, as lack of prepeg tack control can lead to defects in the laminate or equipment breakdown. The lack of a standardized measurement technique has led to the use of various methods for quantifying the tackiness of a pre-impregnated fibrous material with a thermosetting matrix [4–7].
Using the probe tack method on an Instron 5543 tensile testing machine, the micromechanics of the detachment of a metal cylindrical rod from the adhesive surface was studied. The technique used makes it possible to evaluate the load with uniform tearing of prepreg samples from the separating film at temperatures from 20 to 60 °C, the tear-off rate is 5 mm/min, the clamping force is from 30 N to 50 N, the contact time is 60 — 90 sec. To form the matrix, an epoxidation resin of the ED-20 brand was used with a mass fraction of epoxy groups of 21.3 % produced by the plant. Ya.M. Sverdlov, Dzerzhinsk. The reinforcing filler is ELUR carbon fiber. As a hardener — an industrial amine hardener of the Aramine brand based on aromatic amines, produced by «Stekloplastik». To reduce the viscosity, active diluents were introduced into the polymer matrix: diethylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, oligoester cyclocarbonates.
It has been found that the prepeg tack depends on the compaction load and on the compaction time. With a clamping force of 50 N for 60 sec, the prepeg tack is 150 kN/m2, with a clamping force of 30 N for 30 sec. — 115 kN/m2. If the effect on the prepreg of a load of 30 N is increased to 90 s, the prepeg tack increases to 145 kN /m2. A sharp decrease in prepeg tack is observed with decreasing load, which can be compensated for by increasing the holding time under load. Due to the fact that this method of regulating prepeg tack reduces the process productivity, optimization of the temperature regime during the laying of thermosetting material seems to be the most effective method for regulating prepeg tack.
The tackiness of the prepreg was found to be very sensitive to temperature fluctuations: in all studies, tackiness increases to a maximum and drops to almost zero when the temperature rises to 60 °C. It was found that at low temperatures (20...25 °C) the prepeg tack values are due to insufficient wetting of the interface, which leads to disruption of adhesion between the prepreg and the substrate. At higher temperatures, wetting is improved and the epoxy matrix cannot provide high shear resistance during delamination due to reduced viscosity.
The viscosity of the binder has a decisive influence on the prepeg tack. Requirements for the viscosity of the binder are contradictory: to ensure the wetting of the substrate (as well as the impregnation of the reinforcing filler), a low-viscosity binder is preferred, and at the same time, a high viscosity of the binder is required for shear resistance. Therefore, the effect of active diluents on prepreg tack is of particular interest.
Studies of the dependence of the prepeg tack on the content of active diluents showed that the prepeg tack monotonically increases with an increase in the content of diethylene glycol diglycidyl ether and neopentyl glycol diglycidyl ether. With an increase in temperature to 60 °C, an increase in the prepeg tack based on modified binders was also observed. Apparently, at a temperature of 20 °C, the active diluent contributes to the separation of the metal cylindrical rod from the prepreg surface, and at temperatures above 40 °C, less mobile components of the polymer binder. The results of studying the effect of temperature (short-term exposure), compaction load, compaction time, binder composition on prepeg tack can be explained by the balance of adhesion and cohesion, as well as by the processes that determine adhesion, viscoelastic deformation and the formation of an adhesive bond. It has been found that the optimal prepeg tack values are achieved in the transition region from adhesive to cohesive detachment.