The global pandemic, spike in demand, and shortage of traditional personal protective equipment (PPE) materials capable of viral transmission protection has led to a search for low-cost alternative materials suitable for producing medical gowns and other PPE. This paper discusses the testing and characterization of a new class of potential PPE material made from polyethylene-coated nonwoven paper. A mixture of low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE), which has higher crystallinity than LDPE, is extruded through a slit die and laminated onto a nonwoven paper substrate at high temperature through a nip roll assembly. The paper is fed from a rubber-covered pressure roll into the nip where lamination is achieved by pressing the polyethylene and paper layer together. This formed laminate is rapidly cooled down by a water-cooled chill roll and collected by a wind-up mechanism.
The quality of the PE-coated nonwoven paper finished product depends on a number factors, including the coating thickness and uniformity. The researchers used a Bruker nanoIR3 AFM-IR instrument to chemically characterize various regions of the coated nonwoven paper. Visible color filtered reflectance images of propidium-iodine stained samples with thinner PE coatings (6 and 10 g/m2) showed light and dark regions. AFM-IR spectra of the CH2-scissoring region around 1464 cm-1 confirmed that the lighter regions contained more crystalline PE than the dark regions. AFM-IR single wavenumber absorbance image ratio maps (PE(1464 cm-1)/Cellulose(1062 cm-1)) of a 5-mm x 5-mm rough domain in one of the dark regions clearly indicate that the rough domain has a higher PE concentration than the surrounding continuous matrix. Two types of ~1-mm-diameter pinhole defect morphologies were also examined by AFM-IR. Protruding defects showed a stronger cellulose absorbance at 1062 cm-1 and a reduced PE absorption around 1466 cm-1. Indentation defects exhibited an increase in absorbance around 1060 cm-1 nearer the bottom of the crater. A simultaneous measurement of the relative stiffness of the material was collected by tracking the resonant frequency of the AFM tip. The protruding defect generated higher contact resonance frequencies than the surrounding area, indicating that a PE poor region can introduce mechanical heterogeneity, such as domains with high relative stiffness contrast compared to that of surrounding areas due to exposed cellulose material from base sheet.
It is expected that the nanoscale chemical insights provided by AFM-IR measurements may suggest processing condition changes that ultimately lead to improved material properties.