Photopolymer Film Adhesive Diffusion Barrier for Hologram Colour Stability
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Solution Overview
Problem
Commercially available adhesives cause a significant colour shift in holograms when bonded to exposed photopolymer films, particularly in RGB and true-colour holograms, due to wavelength shifts exceeding 20 nm, distorting the overall perceived colour.
Innovation Solution
A laminate construction where the adhesive layer is designed as a diffusion barrier for the fluorourethane additive, ensuring no time-dependent change in absorption spectrum, using specific adhesive compounds like synthetic rubbers, polyester-based hotmelt adhesives, and urethane-based adhesives with high network density, preventing peak intensity differences greater than 3% in the absorption spectrum.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If commercially available adhesive sheets or liquid adhesive formulations are used to bond the photopolymer film to a substrate, then bonding is achieved, but a colour shift in the hologram occurs due to wavelength shifts exceeding 20 nm
Solution Approach 1:
A release liner is introduced as an intermediary layer between the adhesive and the photopolymer film. The release liner prevents direct contact between the adhesive and the photopolymer film, thereby preventing diffusion of the fluorourethane additive into the adhesive while still allowing bonding to occur. This mediator layer solves the contradiction by enabling reliable bonding without compromising hologram colour accuracy.
Solution Approach 2:
A thin release liner film is used to physically separate the adhesive from the photopolymer film. This thin film barrier is sufficient to prevent molecular diffusion of the fluorourethane additive while maintaining the structural integrity needed for bonding. The flexible nature of the thin film allows it to conform during the bonding process.
2Manufacturing precision
If the adhesive layer is designed as a diffusion barrier for the fluorourethane additive, then wavelength shift is prevented, but the adhesive selection becomes more restricted
Solution Approach 1:
The release liner serves as a universal intermediary that works with various adhesive types (pressure-sensitive adhesives, hotmelt adhesives, UV-curing adhesives). Instead of restricting adhesive selection, the release liner provides a standardized solution that maintains hologram colour accuracy while allowing versatility in adhesive choice.
Solution Approach 2:
The bonding system is segmented into distinct functional layers: the photopolymer film layer, the release liner barrier layer, and the adhesive layer. This segmentation allows each layer to be optimized independently - the release liner specifically for preventing diffusion, while adhesives can be selected based on bonding requirements without compromising colour accuracy.
3Strength
If the fluorourethane additive diffuses into the adhesive layer, then bonding is enhanced, but the absorption spectrum changes causing colour shift in the hologram
Solution Approach 1:
The release liner acts as a diffusion barrier that prevents the fluorourethane additive from migrating into the adhesive layer. This intermediary layer maintains the chemical stability of both the photopolymer film and the adhesive, preventing changes in the absorption spectrum while still allowing mechanical bonding to occur.
Solution Approach 2:
The potential harmful effect of fluorourethane additive diffusion is converted into a benefit by using the release liner to control the diffusion process. The release liner itself becomes beneficial by providing a controlled interface that prevents unwanted chemical interaction while enabling mechanical bonding, thus transforming the potential problem into a solution.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The laminate prevents any wavelength shift of more than 20 nm during bonding, maintaining the original colour integrity of the hologram, as demonstrated by ATR-IR measurements showing no significant absorbance changes over time.
Implementation Method 1
the layer of adhesive is designed as a diffusion barrier for the fluorourethane additive C)
Implementation Method 2
on ATR-IR measurement there is no time-dependent change in the absorption spectrum
Data Source
AI summary
The present invention relates to a layer composite comprising an exposed photopolymer film and an adhesive layer which is connected to the photopolymer film at least in certain regions, wherein the photopolymer film comprises crosslinked polyurethane matrix polymers A), crosslinked writing monomers B) and a monomeric fluoroethane additive C), wherein the adhesive layer is in the form of a diffusion barrier for the fluoroethane additive C). The invention also relates to the use of the layer composite for producing chip cards, ID documents or 3D images, as a product protective label, as a label, in banknotes in the form of a strip or window or as holographically optical elements in displays.