Bonding between surfaces is a complex phenomenon that depends on many factors like surface preparation, chemistry, roughness, cleanliness, temperature, humidity etc. In general, adhesion tends to decrease over time due to various reasons outlined below. However, the extent and rate of bond deterioration depends on the specific bonded materials, surface treatment, environmental conditions and loading on the bondline.
Mechanisms of Bond Deterioration Over Time
There are several mechanisms which can cause bonding strength to decrease over time:
Physical Aging
Physical aging refers to changes in viscoelastic properties of polymers over time. As adhesives age, their polymer chains can rearrange and form new interactions, leading to increased stiffness and brittleness. This reduces the adhesive’s ability to withstand stresses and strains, making debonding more likely.
Chemical Degradation
Exposure to oxygen, ozone, sunlight (UV radiation) moisture, or high temperatures can all degrade adhesive polymers over time via chemical reactions like oxidation, hydrolysis, photodegradation etc. The degraded adhesive will have poorer mechanical properties.
Interfacial Changes
The interface between the adhesive and the adherend surface can change over time due to:
- Oxidation of surface
- Reorientation of polymer chains
- Diffusion of moisture to interface
- Contamination
These interfacial changes reduce the intrinsic adhesion between the adhesive and adherend.
Creep Rupture
Constant loading on the bond over time, even below ultimate strength levels, can cause time-dependent creep deformation and eventual failure of the adhesive.
Fatigue Failure
Cyclic loading on the bondline due to vibrations or thermal cycling can cause cumulative damage in the adhesive leading to fatigue failure.
Stress Corrosion
Combined effect of sustained loading + chemical environment can lead to slow crack growth and failure over time.
Factors Affecting Adhesive Bond Durability
The rate and extent of adhesive bond deterioration over time depends on many factors:
Adhesive Material
Some adhesive types like epoxies and polyurethanes provide better resistance to aging than thermoplastics. Adhesive chemistry, crosslink density, polymer structure all affect bond durability.
Adherend Materials
Reactivity between adhesive and substrates affects interfacial bond strength and its stability over time. For example, some adhesives bond better to metals vs plastics.
Surface Preparation
Proper surface cleaning, roughness and use of primers/coupling agents improves initial adhesion strength and its retention over time.
Bondline Thickness
Thinner bondlines are generally more durable as stresses are distributed over smaller area.
Environmental Conditions
Exposure to moisture, UV light, high/low temperatures and pollutants accelerate bond deterioration.
Applied Stresses
Higher sustained or cyclic stresses weaken bonds over time. Stress distribution also important – shear vs peel.
Initial Bond Strength
Bonds with higher initial adhesion strength tend to have better durability as a larger stress is required to start debonding process.
Typical Values for Adhesion Loss Over Time
It is difficult to generalize values for adhesion loss over time since it varies significantly depending on all the factors mentioned above. However, typically epoxy adhesive bonds can retain 50-90% of original strength after 5000 hours of aging at room temperature conditions. Cyclic loading and harsh environments significantly accelerate bond deterioration.
Some typical values are shown below for decrease in adhesion strength over time:
| Adhesive Type | Substrate | Environment | Time | Adhesion Loss |
|---|---|---|---|---|
| Acrylic | Glass | Room temperature | 1000 hours | 15% |
| Epoxy | Aluminum | Elevated temperature (50℃) | 500 hours | 25% |
| Polyurethane | FRP | UV radiation | 100 hours | 20% |
These are just representative values – actual adhesion loss rates need to be evaluated for each specific adhesive/substrate combination under expected service conditions. Accelerated aging tests may be required to extrapolate long term bond durability.
Methods to Improve Adhesive Bond Durability
The durability and long-term performance of adhesive bonds can be enhanced by:
Optimize Surface Preparation
Thorough cleaning, roughening, and use of coupling agents improves initial adhesion and retention over time.
Apply Primer
Primers create a chemical bridge between adhesive and substrate, improving bond durability.
Limit Bondline Thickness
Thinner bondlines distribute stresses more evenly and are less prone to degradation.
Choose Durable Adhesive Chemistry
Epoxies, polyurethanes provide better resistance to environmental degradation than some thermoplastics.
Use Adhesive with Higher Tg
Higher glass transition temperature adhesives retain properties and bond strength for longer times/temperatures.
Optimize Cure Conditions
Ensuring recommended cure time, temperature and pressure minimizes residual stresses and maximizes crosslinking.
Minimize Service Stresses
Design joints to evenly distribute stresses and minimize sustained/cyclic loading.
Protect from Environment
Block moisture, UV radiation and temperature extremes to maximize bonded joint life.
Testing Adhesive Bond Durability
Some common test methods used to evaluate adhesive bond durability include:
Accelerated Aging
Expose bonded samples to elevated temperature, humidity, UV radiation to quickly age bonds. Monitor decrease in strength over time.
Creep Rupture Testing
Apply sustained static loads on bonds at various stress levels until failure. Used to estimate creep-rupture lifetimes.
Fatigue Testing
Apply cyclic stresses on bonds until failure. Generate S-N curves to determine fatigue lifetime at service stress levels.
Fracture Mechanics Testing
Monitor crack growth rate under sustained loads to calculate fracture toughness and determine failure conditions.
Both short and long term tests under realistic conditions are needed to thoroughly evaluate bond durability for given application. Specimen geometries, surface preparation, bonding procedures should match end-use components.
Conclusion
Adhesive bond strength generally decreases over time due to physical aging, chemical degradation, interfacial changes and damage accumulation from sustained/cyclic loading. The specific rate and extent of bonding fall off depends highly on the adhesive-substrate system, bonding process, service stresses and operating environment. Properly chosen adhesives and optimized surface preparation can minimize adhesion loss. Accelerated lab tests help estimate long-term bond durability under working conditions. When designed properly, adhesive joints can retain high strength over many years of service life.