Dear Dr. Marc Yuri Madayag ,

I suggest you to have a look at the interesting, following papers:

-A prototype of a portable optical sensor for the detection of transparent and translucent microplastics in freshwater

Benjamin O.Asamoah, Boniphace Kanyathare, Matthieu Roussey, Kai-ErikPeiponen

Chemosphere, Vol. 231, Pages 161-167 (2019)

Available at: https://www.sciencedirect.com/science/article/pii/S0045653519310203

-Design and Development of the Trash Spliter with Three Different Sensors

Despaleri Perangin Angin, Hendrik Siagian, Eka Dodi Suryantu, Rahayu Sashanti and Marcopolo

MECnIT-IOP Conf. Series: Journal of Physics: Conf. Series 1007 012057 (2018)

Available at: https://iopscience.iop.org/article/10.1088/1742-6596/1007/1/012057/pdf

Best regards, Pierluigi Traverso

In addition to papers suggested by Dr.Pierluigi Traverso, I will suggest to have a look at the following article.

https://www.researchgate.net/publication/315480619_Application_of_Capacitance_Proximity_Sensor_for_the_Identification_of_Paper_and_Plastic_from_Recycling_Materials

The most suitable contactless detector is the capacitive detector. where the material to be sensed is brought in the electrostatic field of a metallic electrode. One needs only to measure the displacement current on approaching the plastic material.

Best wishes

This article will be also helpful;

Article Application of Capacitance Proximity Sensor for the Identifi...

Bests,