Multilayer polymer films for covert marking of mirrored products

The polarization of light reflected from the mirror-like surfaces of glass, plastic, or metal containers, or from foil or metallized film labels, produces pleochroism effects in polymer multilayer coatings. This optical phenomenon can be utilized for both open and hidden product markings. Two types of anisotropic optically active polymer films used for marking transparent coatings and packaging are produced by linear and biaxial stretching. Linear stretching during the production of polymer films creates a shape memory effect and is used to manufacture the outer layer of the coating. This layer compresses and fixes the multilayer structure during heat shrinkage onto cylindrical, conical, or other curved mirror-like surfaces. It has been established that the optical transmission coefficients of regular and polarized light through a shape-memory polyvinyl chloride (PVC) shrink film differ significantly. Depending on the light wavelength and the stress in the film, the transmission decreases several times during the film shrinkage process. In the study of the optical properties of composite polymer films undergoing thermally induced shrinkage, striking color phenomena and a paradoxical effect of the scale factor on the transparency of PVC film for light reflected from mirror surfaces were observed. The polarization of reflected light enables the use of multicolor pleochroism as both eye-catching visual features for consumer engagement and hidden security elements capable of encoding product information in a barcode-like format for discreet package authentication. The feasibility of reading information embedded within a multilayer film has been demonstrated by incorporating a barcode into the middle layer of a biaxially oriented polypropylene film. This embedded data can be accurately captured and decoded using a smartphone camera in combination with a dedicated software application. The pleochroism observed in the multilayer film formed through thermally induced shrinkage of sleeves or cylindrical labels depends on the level of internal stress present in the outer layer of the PVC shrink film. The optimal internal stress is 1-4 MPa for the film shrinkage and fixation on products, corresponding to the 5-65% of the maximum shrinkage strain.