One of the more ubiquitous polymeric materials used in households and businesses is polyethylene, which is manufactured in low density (LDPE) or high density (HDPE) forms. Although similar, each form has properties that make it useful for formulation, manufacture, and recycling. Low density PE consists of polymer chains that do not pack into the crystal structure as well as HDPE materials. LDPE is used for rigid containers and plastic film applications such as plastic bags and film wrap. HDPE has high tensile strength, and is used in products and packaging such as milk jugs, detergent bottles, butter tubs, garbage containers, water pipes, and even hard hats.
The density of polyethylene in bulk pellets and single-layer films is easily measured and classified using several standard ISO/ASTM techniques, such as immersion method, density gradient method or ultrasound. However, these techniques can only measure PE in its “pure” form—which can be challenging when used in multilayer films such as packaging structures or film sheets for building materials or crop protection. Here, extensive sample preparations such as microtoming, or separating layers by dissolving in solvents, are often required to isolate the PE layer before analysis. These techniques are labor-intensive and time-consuming. If only there were an easier, more efficient way to look at PE in multilayer materials.
Enter Raman spectroscopy. This analytical technique is sensitive to changes in the molecular structure of materials. In the case of PE, Raman is quite adept at determining crystallinity, which informs a material’s density—in this case, high density or low density. Raman microscopy, which uses spectroscopy as the microscopic scale, is a confocal technique. This means the analyst can probe layers of materials in situ without needing to separate multilayer materials.
Application scientists at Thermo Fisher studied the feasibility of using confocal Raman microscopy for PE film density analysis, both qualitatively and quantitatively. They were able to demonstrate that Raman microscopy, in combination with the discriminant analysis method, can be successfully applied to distinguish HDPE and LDPE in both pellet and film forms.