Two-functional phase-change pressure-sensitive adhesives based on polyisobutylene matrix filled with paraffin wax

Abstract

Paraffin wax was used to impart the function of heat accumulation to pressure sensitive adhesives based on polyisobutylene (PIB) and simultaneously to suppress their cold flow. Laser microinterferometry showed 6% solubility of paraffin wax in PIB at 25 °C with achieving complete miscibility of PIB and wax at temperatures above 46 °C. The thermophysical and rheological properties of mixtures containing from 10% to 60% paraffin wax were investigated. According to calorimetry data, wax reduces the glass transition temperature of PIB, which in turn affects the apparent degree of wax crystallinity, shifting it in different directions. Paraffin wax significantly reduces the effective viscosity of the PIB melt and lowers the values of its storage and loss moduli. This simplifies the processing of adhesives and allows them to be used as easily flowable liquids. Nevertheless, the wax crystallizes during cooling, which leads to a multiple increase in the viscosity of adhesives to a level comparable or even greater than the viscosity of filler-free PIB. In this process, the heat is released in relatively larger quantities than during crystallization of pure wax, as additional heat is produced owing to the loss of solubility of wax in the PIB matrix. With an increase in the wax content, the durability of adhesive joints with steel firstly passes through a minimum, which is associated with the plasticizing effect of paraffin wax. However, the addition of 60% paraffin wax makes it possible to increase the durability of adhesive joints by 20 times, simultaneously reducing the melt viscosity of the adhesive by 4 decimal orders of magnitude. A key fact is that the resulting easy-to-process adhesive allows storing heat, i.e. to play two roles at the same time – a reinforced pressure-sensitive adhesive and a phase-change material.