A new measurement technique to determine the frictional force between a plug and the pipe wall in dense phase pneumatic conveying was developed. Four strain gauges were installed on the outer surface of the pipe wall and strains were measured at different locations (top, back, bottom, and front) of the pipe wall, where the strain signals were obtained. The frictional forces were determined for different experimental conditions such as the superficial air velocity and the amount of material being transported. Polyester and polystyrenes with different size and shape, and polyolefin materials were tested in this study. It was found that the frictional force is related to the plug length as well as material properties such as size, shape of the polymer particles and contact area with the wall. In addition, it was found that the frictional force was the highest at the top or sides of the pipe and was smallest at the bottom of the pipe depending on the particle characteristics. The ratio of shear stress to the overall driving pressure for the plug decreased when the plug length is increased. (C) 2012 Elsevier B.V. All rights reserved.