Plastic Automotive Mirrors with Hardcoating Stress Management
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional glass mirrors for automotive use are heavy, difficult to form into complex shapes, prone to shattering, and soft plastic substrates used as alternatives are susceptible to damage from UV radiation, heat, and water, leading to crazing of reflective surfaces.
Innovation Solution
A plastic mirror design with a reflective layer located at the front and a suitable hardcoating in between, controlling internal stress parameters to minimize differential stress and achieve compressive residual stress, preventing crazing and enhancing abrasion resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a reflective layer is applied directly onto the front surface of a plastic substrate, then abrasion resistance is improved, but the plastic substrate becomes susceptible to crazing from UV radiation, heat and water
Solution Approach 1:
A hardcoating layer is introduced as an intermediary between the plastic substrate and the reflective layer. This hardcoating provides both mechanical protection against abrasion and environmental protection against UV radiation, heat and water, preventing crazing while maintaining abrasion resistance. The hardcoating acts as a mediator that separates the conflicting requirements of direct reflective layer application versus substrate protection.
Solution Approach 2:
The mirror system uses a composite structure combining plastic substrate, hardcoating layer, and reflective layer. Each layer provides specific functions: the plastic substrate provides the base structure, the hardcoating provides abrasion and environmental resistance, and the reflective layer provides optical functionality. This composite approach resolves the contradiction by distributing functions across multiple materials.
2Reliability
If a hardcoating is applied to protect the plastic substrate and reflective coating, then environmental exposure protection is improved, but interference bands appear which are cosmetically unacceptable
Solution Approach 1:
The thickness of the hardcoating layer is precisely controlled within specific ranges (e.g., 50-200 nm) to minimize optical interference effects. By adjusting the coating thickness parameter, the mirror achieves both environmental protection and acceptable cosmetic appearance, eliminating visible interference bands while maintaining protective functionality.
3Reliability
If thick reflective layers (20-50nm) are used to achieve low transparency (<4%), then reflectivity is improved, but stress crazing and cracking increase due to high tensile film stresses and thermal expansion mismatch
Solution Approach 1:
The hardcoating layer serves as a stress buffer between the reflective layer and the plastic substrate. It accommodates the high tensile film stresses and thermal expansion coefficient differences that would otherwise cause crazing and cracking in thick reflective layers, enabling the use of sufficient reflectivity thickness without compromising structural integrity.
Solution Approach 2:
The thickness of the reflective layer is optimized within a specific range (20-50nm) to achieve the required reflectivity while managing stress levels. The hardcoating thickness is also adjusted to compensate for stress, creating a balanced system that achieves low transparency without excessive crazing.
4Strength
If glass mirrors are used for automotive applications, then impact resistance is poor, but weight is high and complex shaping is difficult
Solution Approach 1:
The patent replaces traditional glass mirrors with a plastic-based alternative that, while having different mechanical properties, achieves sufficient durability through the protective hardcoating and reflective layer system. The plastic substrate with protective coatings provides adequate service life for automotive applications while offering weight and shaping advantages.
Solution Approach 2:
The composite structure of plastic substrate combined with hardcoating and reflective layer creates a material system that mimics the protective functions of glass while leveraging the advantages of plastic (lower weight, better impact resistance, easier shaping). The composite approach enables the plastic mirror to achieve glass-like durability with plastic-like flexibility.
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 solution results in a durable, abrasion-resistant plastic mirror that passes automotive tests such as salt spray, thermal cycling, and accelerated weathering without interference fringes, ensuring stability and performance in automotive applications.
Implementation Method 1
the hardcoating ensures good adhesion and provides abrasion resistance
Implementation Method 2
locating a reflective layer at the front of a plastic substrate
Implementation Method 3
the use of the intermediate zone between the hardcoating and the reflective layer permits the adoption of the type of abrasion resistant hardcoating materials that are desirable and preferable for use with a plastic substrate, permits the adoption of materials that would be desirable for use as the reflective layer, and also avoids the crazing problem
Implementation Method 4
The replacement of glass with a plastic substrate would offer the ability to mould complex shapes, to integrate retention/clipping features on the rear, to reduce weight, and to achieve a high level of impact resistance
Data Source
Figure 1~2
AI summary
A plastic mirror that includes a plastic substrate having a front surface, a hardcoating on the front surface, a reflective layer, and an intermediate zone between the hardcoating and the reflective layer, the intermediate zone including at least one layer formed from a material selected from the group consisting of metals and metalloids, oxides and nitrides of metals and metalloids, and carbon.