Laser diode sensors for measuring mechanical properties of recycled plastics

Uncertainty of mechanical properties is one of obstacles in the use of recycled plastic as raw materials. This paper presents a method for measuring mechanical properties of plastic products that are partially made of recycled plastic. Specifically, the plastic type of interest in this project is polypropylene. The proposed method employs an analysis through a relationship between optical transmittance and tensile strength, which varies with recycled compositions. Adopted from spectroscopy technique, laser diode sensor was used to measure optical transmittance. The sensor is composed of a 1550 nm laser diode as a light source and a photodiode as a detector. Experimental results from laser sensing method are equivalent to results from spectrometry. Optical transmittance, as well as tensile strength, linearly increases with the ratio of recycled plastic. Statistically, every linear function has the coefficient of determination (R²) over 0.95. Thus, the mathematical relationships between optical transmittance and tensile strength were generated. In conclusions, the tensile strength of plastic with different percentages of recycled mixture can be measured by the use of the demonstrated laser diode sensor.