Polyurethane is a versatile plastic material used in a wide variety of applications, from furniture and flooring to insulation and adhesives. Polyurethane is prized for its durability, flexibility, and resistance to water, chemicals, and abrasion. However, some people wonder if polyurethane has an expiration date. Can polyurethane degrade over time to the point where it should no longer be used? Let’s take a closer look at how polyurethane ages and when it may be time to replace polyurethane materials.
What is Polyurethane?
Polyurethane is a polymer made by combining an isocyanate compound with a polyol resin blend. The isocyanate acts as a crosslinking agent to join the polymer chains together into a durable, three-dimensional network. Polyurethane polymers come in various forms, including rigid and flexible foams, coatings, adhesives, sealants, and elastomers.
The excellent properties of polyurethane come from its crosslinked structure. The isocyanate links give it strength, while the polyol blend provides flexibility and impact resistance. Polyurethane has high tensile and tear strength, is abrasion and corrosion resistant, and has low temperature flexibility. It also has excellent adhesion to many substrates.
How Does Polyurethane Age?
Like all polymers, polyurethane is susceptible to gradual degradation over time from environmental factors like heat, moisture, and UV radiation. However, polyurethane ages very slowly compared to many other plastics due to its robust crosslinked structure.
Here are some of the main ways polyurethane can degrade over time:
Thermal Oxidation
Exposure to heat speeds up oxidation, which breaks polymer chains. This causes polyurethane to become brittle and discolored. Rigid polyurethane foams are more vulnerable to thermal degradation than elastomers and coatings.
Hydrolysis
Moisture reacts with polyurethane polymers, causing chain scission. This leads to embrittlement, shrinking, and cracking. Flexible polyurethanes absorb more moisture and degrade faster than rigid types.
UV Radiation
Sunlight breaks down urethane bonds in polyurethane, especially at the surface. This leads to chalking, discoloration, and formation of microcracks. Pigments and UV stabilizers help slow UV damage.
Chemical Exposure
Some solvents, acids, and alkalis can react with polyurethane polymers, causing swelling, softening, and chain scission. Abrasion while chemicals are present accelerates degradation.
Fatigue
Repeated flexing stresses polyurethane bonds over time, resulting in microscopic cracks that expand and merge to form fractures. Rigid polyurethanes have better fatigue resistance than elastomeric types.
Typical Polyurethane Lifespan
Most polyurethanes are designed to last for decades with proper formulation and manufacturing. Here are the typical lifespans for different polyurethane product categories:
| Product | Typical Lifespan |
|---|---|
| Furniture foam & padding | 10-30 years |
| Rigid foam insulation | 20-50 years |
| Automotive seats | 5-15 years |
| Wheels & rollers | 5-10 years |
| Coatings | 2-15 years |
| Adhesives | 2-10 years |
| Sealants | 5-20 years |
| Elastomers | 5-10 years |
Polyurethane foams and elastomers tend to have longer lifespans than coatings and adhesives. Factors like formulation, manufacturing quality, and end use conditions significantly affect actual product lifetimes.
Signs of Aging in Polyurethane
Here are some telltale signs that a polyurethane material may be reaching the end of its usable life:
Visible Cracking
Cracks and fractures in the surface indicate the polyurethane network has degraded significantly. Stress can cause small cracks to form and propagate over time.
Powdering Surface
A chalky surface happens when polyurethane degrades through UV exposure, oxidation, and hydrolysis. The chalky layer continues to erode away gradually.
Discoloration
Fading, yellowing, or darkening of polyurethane points to UV damage, heat exposure, or chemical degradation.
Brittleness
Polyurethane that has become excessively stiff, lost flexibility, and is prone to cracking is likely quite aged. This happens due to chain scission from oxidation, hydrolysis, and heat exposure.
Permanent Deformation
Elastomers and flexible foams that no longer spring back after compression have degraded polymer networks due to fatigue, heat, or chemical exposure.
Shrinkage
Notable shrinking or contraction means the polyurethane has likely undergone hydrolysis. Loss of plasticizer oils over time can also cause shrinkage.
Bond Failure
Delamination or detachment of polyurethane from the substrate signals loss of adhesive properties from aging. This can result from oxidation, hydrolysis, or fatigue.
Testing Aged Polyurethane
To evaluate whether an aged polyurethane still retains adequate properties for continued use, manufacturers can conduct standardized tests:
Hardness
Hardness testing with a durometer or Shore scale measures any change in rigidity. Hardness increase points to embrittlement.
Tensile Properties
Tensile strength, elongation, and modulus characterize changes in mechanical performance. Lower values signify polymer degradation.
Compression Set
Testing how well a polyurethane recovers from compression reveals loss of elasticity from aging. Higher compression set correlates with poorer recovery.
Tear Strength
Measuring tear strength helps identify loss of toughness. Lower values mean the polyurethane is more prone to ripping or tearing in use.
Abrasion Resistance
Abrasion testing characterizes wear resistance retention after aging. Higher volume loss indicates the surface has degraded.
Adhesion Testing
Pull-off or peel tests quantify changes in polyurethane bond strength resulting from aging. Lower force to separate means weaker adhesion.
Rejuvenating Aged Polyurethane
In some cases, the properties of aged polyurethane can be partially restored to extend its useful life. Here are a few rejuvenation methods:
Surface Treatments
Applying a fresh topcoat, such as paint or polyurethane, helps restore UV and chemical resistance. This works best on products with sound underlying foam.
Flexible Foam Revitalization
Injecting plasticizer oils into flexible slab foams can improve elasticity and fatigue life. This helps offset plasticizer evaporation over time.
Annealing
Briefly heating rigid polyurethane to a moderate temperature below its heat distortion point mobilizes polymer chains for improved flexibility.
Partial Remanufacture
Worn or damaged sections of a polyurethane product can be replaced while retaining the still functional core. Examples include foam cushion replacement and tread bonding.
However, if the bulk polyurethane is too degraded, attempts at rejuvenation may not restore satisfactory performance.
When to Replace Aged Polyurethane
Here are some general guidelines on when it is time to replace polyurethane materials rather than attempt to refresh them:
– Visible cracks penetrate deeper than just the surface layer
– More than 30% of the polyurethane surface shows signs of UV degradation
– Flexible foams have lost more than 50% of their original support capacity
– Elastomers permanently deform more than 25% after compression
– Adhesives no longer provide adequate bond strength for the application
– Polyurethane insulation value has dropped by R-3 or more
– Automotive seating has visible tears, frame damage, or major fatigue
Performing standardized tests of the aged polyurethane properties provides quantitative data to help inform replacement versus rejuvenation decisions.
Disposing of Aged Polyurethane
Once polyurethane reaches the point where it can no longer serve its intended purpose, it must be disposed of properly. Here are some guidelines for polyurethane disposal:
Recycling
Some polyurethane products like flexible foams can be recycled. Recycled polyurethane is commonly turned into carpet padding or rebounded for use in molded products.
Energy Recovery
Incineration of polyurethane in waste-to-energy facilities helps recover the caloric value from the chemical bonds. This generates usable heat and electricity.
Landfill
Polyurethane waste that cannot be recycled or incinerated for energy ends up in landfills. Polyurethane typically degrades very slowly in landfills.
Auto Shredder Residue
After metals are recovered from auto shredder residue, the remaining polyurethane foam and other plastic shreds are commonly landfilled.
Illegal Dumping
Discarded polyurethane waste is sometimes illegally dumped to avoid landfill fees. This pollutes the environment and should be avoided.
Proper polyurethane disposal depends on waste stream options available in a particular region. Disposal costs may also be a factor in replacement decisions for aged polyurethane.
Conclusion
Polyurethane has excellent inherent resistance to aging and degradation thanks to its robust crosslinked molecular structure. However, after many years of use, environmental exposure takes a toll, causing polyurethane to gradually lose properties. Telltale signs like cracking, powdering, and discoloration indicate the polyurethane has reached the end of its lifespan. While rejuvenation is sometimes possible, testing is needed to determine if the aged polyurethane can still perform adequately. Knowing when to replace rather than refurbish aged polyurethane is critical for both performance and safety. Proper disposal also helps limit environmental impact as polyurethanes reach the inevitable end of their very long service lives.
