Nika Science Pop No.2｜Let’s Talk About "Material Aging"

1. What is MaterialAging?

Material aging refersto the irreversible changes in the physical and chemical properties of amaterial during use, caused by the combined effects of environmental factors(such as heat, oxygen, light, moisture, and chemical media) and intrinsicfactors (such as the material's chemical structure and intermolecular forces).This degradation leads to a decline in performance and ultimately reduces itspractical value.   

For example:   

In polymeric materials, aging typicallymanifests as deterioration in physical properties, such as changes in hardness,elasticity, and strength.   

For holographic materials, aging may appear asyellowing, alterations in polymer grating morphology, decreased diffractionefficiency, increased haze, and other performance degradations.

Sample Photos of Material Degradation

Sample Photos of MaterialDegradation

2. Mechanisms ofMaterial Aging

Material aging can occur in varioustypes of materials, including organic materials (e.g.,plastics, rubber, coatings) and inorganic materials (e.g.,metals, ceramics, glass). Below, we discuss the primary aging mechanisms forthese two categories:

Aging of OrganicMaterials:

Refers to the gradual degradation in performance of organic compoundsover time due to environmental factors, such as plastics, rubbers, fibers,coatings, adhesives, etc. Aging can significantly reduce material properties oreven render them nonfunctional. Common causes include:   

1. Oxidation: Most organic materials undergo slow oxidation whenexposed to air, especially those containing double bonds or polycyclic aromatichydrocarbons. This leads to discoloration (e.g., yellowing) and reducedmechanical strength.   

2.Light Exposure: Ultraviolet (UV) radiation triggersphotochemical reactions, causing material breakdown. Common effects includeyellowing/fading, brittleness, and diminished physical properties.   

3. High Temperature: Heat accelerates molecular chain scission,degrading polymers. It also speeds up oxidation, hastening aging.   

4. Moisture: Water infiltration causes swelling, compromisingdimensional stability. Humidity also catalyzes oxidation and hydrolysis.   

5. Microbial Attack: In damp environments, mold and microbes growon surfaces, secreting bioactive substances that degrade materials.   

Methods to Prevent Aging of Organic Materials

While multiple factors contribute to aging, various strategies canmitigate degradation. Common approaches in materials science include:   

1. Additives: Incorporating antioxidants, UV stabilizers, andantimicrobial agents to slow aging.   

2. Surface Treatments: Applying protective coatings (e.g., paint,varnish) to shield against environmental exposure.   

3. Formulation Optimization: Modifying material composition (e.g.,adjusting chemical structure or glass transition temperature, Tg) to enhanceweather/chemical resistance.   

Mechanism of Antioxidants

Mechanisms of Inorganic Material Aging

The aging of inorganicmaterials primarily involves non-carbon-based compounds such as metals andtheir alloys, glass, ceramics, etc. Unlike organic materials, the agingmechanisms of these materials are mainly influenced by physical and chemicalfactors. Below are several common aging phenomena and their causes in inorganicmaterials:

(I) Metals and Alloys

1. Corrosion: Metals are prone tooxidation or reactions with other chemicals in humid air, leading to theformation of surface rust layers. For example, iron rusts due to its reactionwith oxygen and water in the air, forming iron oxide.   

2. Grain Growth: At hightemperatures, the internal grains of metals may grow larger over time,affecting the material's mechanical properties.

3. Phase Transformation: Some alloysmay undergo phase transformations under specific conditions, altering theirproperties, such as martensitic transformation.

(II) Glass

1. Chemical Erosion: Glass issusceptible to erosion in acidic or alkaline environments, especially underhumid conditions.   

2. Thermal Stress: Temperaturefluctuations can induce thermal stress within glass, leading to cracks orfractures.   

(III) Ceramics   

1. Thermal Shock: Ceramicmaterials are highly sensitive to sudden temperature changes, which may causecracking.   

2. Porosity Changes: Pores inceramics may undergo changes at high temperatures, affecting the material'sdensity and strength.   

3. Chemical Erosion: Althoughceramics exhibit high chemical stability, they may still erode in stronglyacidic or alkaline environments.   

Methods to Prevent Aging in Inorganic Materials:   

- For metals, surface treatments (e.g., electroplating, painting) oradding alloying elements can enhance corrosion resistance.   

- For glass, modifications or protective coatings can improve erosionresistance.   

- For ceramics, optimizing sintering processes to reduce porosity orapplying protective coatings can enhance thermal shock resistance.   

3. Why Are OrganicMaterials More Prone to Aging?

Organic materials are considered more susceptible to aging than inorganicmaterials due to their chemical structure, intermolecular interactions, andenvironmental factors. The aging of organic materials manifests in diverseforms, including thermal degradation, photodegradation, and oxidativedegradation, each with specific mechanisms and influencing factors. Thiscomplexity makes organic material aging more challenging to control.   

In contrast, inorganic materials (e.g., metals, ceramics, glass)typically exhibit higher thermal, chemical, and mechanical stability due totheir stronger chemical bonds (e.g., ionic, covalent). Thus, under identicalenvironmental conditions, they generally age more slowly than organicmaterials.   

However, it is important to note that inorganic materials are notentirely immune to aging. They may still experience performance degradationunder specific conditions, such as metal corrosion or ceramic thermal shockcracking. To enhance the durability of organic materials, strategies such asmodifying chemical structures, adding stabilizers, and optimizing usageenvironments are essential.   

Oxidation Process of OrganicMaterials

4. Aging of Holographic Materials

Holographic material aging refers to the gradual degradation of materialperformance during long-term use or storage due to environmental factors suchas temperature, humidity, and light exposure. Photosensitive polymer materialscan be categorized into photopolymers (PP) and holographic polymer-dispersedliquid crystals (HPDLC) based on their composition.   

The aging of holographic materials leads to performance deterioration. InAR glasses applications:   

- Yellowing alters the material's transmission spectrum, causingdistortion in the viewed scenery.   

- Increased haze reduces clarity, making external scenes appearfoggy.   

- Decreased diffraction efficiency dims virtual images, making themdifficult to recognize in bright outdoor environments.   

These effects significantly impair user experience and may even posesafety risks in certain scenarios. Thus, aging resistance is a critical factorin developing holographic materials.   

Material-Specific Aging Mechanisms

1. PP Materials: Aging primarily stems from the degradation offilm-forming resins and polymerizable components. Selecting monomers with highresistance to water/oxygen and appropriate glass transition temperatures (Tg)is essential.   

2. HPDLC Materials: Beyond polymerizable components, the aging resistanceof liquid crystals (LCs) is equally vital. LC composition must be meticulouslydesigned during material development.   

Liquid Crystals: The Core of HPDLC Reliability

LCs are the most critical component in HPDLCs. Their aging resistanceultimately determines the material's reliability. LCs are physical mixtures ofdozens of compounds with distinct structures:

- Highly conjugated compounds provide high birefringence.   

- Polycyclic long-chain compounds ensure high clearing points.   

- Cycloalkyl short-chain compounds enable good low-temperaturemiscibility.   

These compounds work synergistically to deliver optimal performance. LCformulations must be tailored to application requirements:   

1. Consumer-Grade LCs: Prioritizefast response and low driving voltage, requiring low viscosity and highdielectric anisotropy. These typically incorporate highly polar, short-chaincompounds.   

2. Automotive-Grade LCs: Demand highreliability and broad operating temperatures (−40°C to 105°C). Theyuse polycyclic long-chain compounds and nitrogen-free heterocycles for superiorUV/thermal stability. Automotive LCs are now mature technology, having passedrigorous industry validation.   

Nika Optics' HPDLCsexclusively employ automotive-grade LCs, offering exceptional high-temperatureand UV stability. This makes them suitable not only for AR glasses but also forautomotive applications.

Holographic Waveguide (by Nika Optics)

5. Aging Industry Standards and Aging Tests

Different product applications require varying aging standards.Generally, for fast-moving consumer goods like AR glasses, products must pass a240-hour aging test at 60°C/90%RH.Automotive products, however, face much stricter requirements, needing towithstand a 1000-hour test at 85°C/85%RH.

Aging tests simulate theenvironmental conditions and stresses products may encounter during long-termuse. These tests are crucial for evaluating material durability, stability, andreliability. Beyond the aforementioned high-temperature/high-humidity tests,standard aging tests typically include:

1. Low-Temperature Storage: Evaluates theimpact of cold conditions on equipment safety, integrity, and performanceduring storage, operation, and disassembly.

2. Thermal Shock: Assessesproduct adaptability to rapid ambient temperature changes.

3. Damp Heat Cycling: Simulatestropical rainforest environments to determine product/material adaptabilityduring temperature fluctuations with surface condensation.

4. Salt Spray Testing: Createsartificial salt fog environments to evaluate product/metal material corrosionresistance.

5. UV Testing: Simulates sunlight'sultraviolet effects on materials, using UV light sources to rapidly assessweather resistance and projected lifespan.

6. Xenon Arc Testing: Usesfull-spectrum xenon lamps to replicate destructive light waves (visible/UV/IR)found in various environments, supporting R&D and quality control.

7. Sweat Resistance Testing: Evaluatessample resistance to artificial perspiration, assessing performance againstsweat and sebum.

8. Package Vibration: Simulatestransportation vibrations.

9. Package Drop: Recreatesshipping drop impacts.

10. Steel Wool Testing: Assessessurface/material abrasion resistance.

11. Dust Testing: Simulatesairborne particulate effects.

12. Boiling Water + Cross-Cut Testing: Evaluates heat resistance by submerging products in boiling water,combined with adhesion assessment.

Illustration ofConstant Temperature and Humidity Chamber

Aging Test Procedure(Boyuan Liang et al，Influence of Thermal Aging on Dielectric Properties of High Voltage Cable Insulation Layer, Coatings 2023, 13(3), 527;)

6. Testing Parameters of Holographic Materials After Aging   

For the aging of holographic materials, it is generally necessary to testthe changes in parameters such as diffraction efficiency, refractive indexmodulation, haze, grating period, and chromaticity. After the aging experiment,an abnormality analysis is conducted to identify any abnormal test samples. Thelifespan and reliability of the holographic components are then evaluated basedon the failure data, and improvements are made to the design or manufacturingprocess according to these findings.

Illustration of Haze Testing Equipment

7. How to DesignAging-Resistant Materials?   

As previouslymentioned, the main factors affecting material aging include water, oxygen, UVradiation, high temperature, and high humidity. Therefore, by addressing thesefactors with appropriate countermeasures, aging resistance can be achieved. Theaging resistance of holographic materials can generally be realized throughmolecular design, material composition, formulation optimization, andpost-processing techniques.   

Nika Optics hasimproved aging resistance through structural molecular design, maintaining highrefractive index while effectively reducing the number of heteroatoms. Byscreening various antioxidants and UV stabilizers, additives were identifiedthat effectively resist aging without compromising diffraction efficiency,haze, or other key properties, ultimately yielding durable holographicmaterials. The specific approaches are as follows:   

1. Material DesignStrategy   

Taking photopolymermaterials as an example, selecting the right film-forming resin is crucial. Forinstance, polyurethane-based resins exhibit superior weather resistance due totheir extensive hydrogen bonding and moderate chemical cross-linking, resultingin excellent thermal and moisture resistance.

Additionally, sincephotoactive monomers constitute a significant portion of the formulation, theirresistance to yellowing and post-polymerization durability directly affect thematerial’s appearance. Therefore, selectingmonomers with biphenyl heterocyclic structures, high aliphatic ether content,or diphenyl ether groups enhances grating stability. Furthermore, moderatelyreducing nitrogen-containing molecules and incorporating UV absorbers andantioxidants can significantly improve material reliability.

Material Screening Experiments (by Nika Optics)

2. AuxiliaryMeasures such as Encapsulation:
By encapsulating the HOE material within a laminated optical film structure andbonding it with OCA (Optically Clear Adhesive), direct exposure to oxygen andmoisture in the air can be significantly reduced, thereby minimizing oxidationreactions and enhancing long-term stability. The optical film can be selectedfrom types with low birefringence, such as TAC (Triacetyl Cellulose), whichhelps improve the reliability of large-area grating fabrication.

3. Application of UV Stabilizers:
UV-resistant molecules, such as polymerizable photoactive monomers andfilm-forming resin components (e.g., diols), can be designed by incorporatingazobenzene-based structures into their molecular frameworks. This modificationprovides excellent UV resistance, as the unique photoisomerization effect ofazobenzene enhances absorption in the UV spectrum. Moreover, this process isreversible, significantly improving the grating structure’s durabilityagainst UV exposure. Alternatively, physical doping with a certain amount of UVstabilizers can also effectively extend the lifespan of HOEs.

Conclusion:
Although organic materials are more susceptible to environmental factorscompared to inorganic materials, their diversity grants them irreplaceableproperties. Today, organic materials already meet the demands ofhigh-reliability applications, such as liquid crystal materials in LCDs, widelyadopted OLED materials in automotive displays, and various adhesives. Whileaging issues initially plagued these materials upon their introduction,persistent efforts by material scientists and continuous iterations haveenabled their widespread use. Holographic materials share the same fundamentalnature as these organic materials, meaning aging is not an insurmountablechallenge in their development. With targeted molecular design, optimizedformulations, and protective measures, high-performance, durable holographicmaterials can be successfully engineered.