Thin Circular Polarizer Film Stack for Bendable Displays

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Solution Overview

Problem

Traditional thick film stack circular polarizers are not mechanically robust enough to survive the tight bend radii required for bendable and rollable emissive displays, and attempts to make thin circular polarizers have resulted in films that crack or break during bending and folding.

Innovation Solution

A thin film stack comprising a quarter-wave retarder, a transparent aliphatic cross-linked polyurethane layer, and a polyvinyl alcohol (PVOH) layer, where the polyurethane layer acts as a primer to enhance mechanical integrity and resist cracking.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Strength

If traditional thick film stack circular polarizers are used, then mechanical strength is improved, but thickness increases making them unsuitable for bendable and rollable displays

Engineering Contradiction:

Improvemechanical strengthVSAvoidthickness

Core Design Contradiction:

StrengthVSLength of stationary object

Solution Approach 1:

The patent changes the material parameters by selecting specific polymers with appropriate glass transition temperatures and mechanical properties. The use of cross-linked polyurethane with Tg between -50°C and 0°C and polyester with Tg between 40°C and 80°C creates a thin film that maintains mechanical strength while achieving thickness below 50 μm, resolving the contradiction between strength and thickness.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent employs a composite structure consisting of multiple layers including cross-linked polyurethane, polyester, and polyvinyl alcohol layers. This composite material approach allows each layer to contribute specific properties - the polyurethane provides flexibility and crack resistance, the polyester provides structural support, and the PVOH provides polarizing function - achieving both thinness and mechanical robustness simultaneously.

Inventive Principle:

Principle #40Composite materials

2Adaptability or versatility

If thin circular polarizer films are made to reduce thickness, then adaptability for bendable displays is improved, but mechanical integrity deteriorates causing cracks and breaks during bending and folding

Engineering Contradiction:

Improveadaptability for bendable displaysVSAvoidmechanical integrity

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The patent uses flexible thin films with carefully selected material properties, specifically cross-linked polyurethane with low glass transition temperature (-50°C to 0°C) that remains flexible at operating temperatures. This flexibility allows the film to bend and fold without cracking, while the cross-linked structure provides mechanical integrity and prevents film failure during repeated bending cycles.

Inventive Principle:

Principle #30Flexible shells and thin films

Solution Approach 2:

The patent incorporates a cross-linked polyurethane layer that acts as a cushioning layer before bending occurs. This layer absorbs and distributes stress during bending and folding, preventing stress concentration that would lead to cracks. The low Tg of this layer ensures it remains compliant and cushioning throughout the bending process, protecting the overall film structure from mechanical failure.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

Data Source

PatentUS12321000B2Film stacks for thin circular polarizers

Publication Date: 2025.06.03 3M INNOVATIVE PROPERTIES CO

AI summary

Film stacks comprise a quarter-wave retarder and a transparent aliphatic cross-linked polyurethane layer disposed on a major surface of the quarter-wave retarder. The transparent aliphatic cross-linked polyurethane layer has a glass transition temperature in a range from 11 to 27° C. and a Tan Delta peak value in a range from 0.5 to 2.5.