Pingpong article

Rubber hardness can change due to air exposure, but it depends on the mechanism:

1. Oxidation

When rubber is exposed to oxygen in air (especially at elevated temperatures), the polymer chains can oxidize.

Oxidation may cause crosslinking (chains bond together), which makes the rubber harder and less elastic.

In some cases, oxidation can instead lead to chain scission (breaking of chains), which may make the rubber softer, brittle, or cracked over time.

2. Ozone Cracking

In unsaturated rubbers (like natural rubber, SBR, or nitrile), ozone in air can attack double bonds.

This causes surface cracks and embrittlement, increasing the apparent hardness and reducing flexibility.

3. Moisture & Air Pollutants

Humidity itself usually doesn’t change hardness significantly, but combined with oxygen/ozone it accelerates aging.

Pollutants like NOx or SO₂ in air also accelerate degradation.

4. Practical Observation

Fresh rubber products (tires, seals, gaskets, shoe soles, etc.) are soft and elastic.

Over time in air, especially sunlight + heat, they harden, lose elasticity, and sometimes crack.

This is essentially the increase of hardness value (Shore A or IRHD) with aging.

👉 So, the hardness value of rubber generally increases with air exposure, mainly because of oxidative crosslinking, though the exact effect depends on the rubber type and environment.

Hardness (Shore A)
80 |                                     *
78 |                                  *
76 |                               *
74 |                            *
72 |                         *
70 |                      *
68 |
66 |                   *
64 |                *
62 |
60 | * 
    -----------------------------------------
      0   7   14  21  28  35  42
              Exposure Time (days)

Here’s an example chart showing rubber hardness increasing over time under accelerated air aging.

The initial hardness was about 60 Shore A.
After ~6 weeks of air exposure at elevated temperature, it rose to nearly 78 Shore A.

This reflects the typical trend: rubber hardens as it oxidizes and crosslinks when exposed to air.

Pingpong info

Rubber tranformation