When a product is made from a polymer usually the polymer is first heated until it is molten (the fluid phase), then brought into the correct shape and then cooled down until it is solid again (the glass phase). In the glass phase however, the mobility of the large polymer molecules is too slow to fully conform to the rapidly changing temperature
There is not enough time for the molecules to move into the correct position or shape that corresponds to the current temperature. As a result of this, the polymer molecules “freeze” into far from optimal positions. The volume of the polymer is too high directly after cooling. Excess volume exists between the polymer molecules. The molecules will move closer together later onwards.
In due time the molecules will move themselves into more favourable positions. This causes the volume of the polymer to decrease. The reduction of volume will further reduce the mobility of the polymer molecules. As a result, the process of moving into more favourable positions slows down even more. It is a self-retarding process.
In due time the polymer structure becomes more compacted, which will influence important properties like tensile strength and flexibility. This process of properties changing with time is called “physical aging”.
Physical aging is not limited to polymers only. Many materials in day-to-day practice show aging effects. A famous example is sand. When sand is first poured into a hole then it can be stirred easily. Its surface is too soft to build anything on top of it. The sand grains need to be compacted first. This will happen in time due to small vibrations, but it can be accelerated by using a compactor.
Aging of polymers is a slow and self-retarding process. It may take many years before a final equilibrium is reached. That causes several properties of the polymer to change slowly:
- The polymer becomes denser. The volume of a product made from the polymer will reduce somewhat.
- The tensile strength of the polymer increases. That means that a product made from the polymer becomes stronger with time. Slow deformation of the product under continuous load will become less.
- The stiffness of the polymer increases. It becomes more difficult to bend or deform the plastic product.
- The polymer becomes more brittle. That means that a product made from the polymer may break more easily when it falls.
- The fatigue strength reduces. A product made from the polymer under fluctuating load will have a shorter lifetime.
Aging of polymers can have rather dramatic effects on our life. For example, a water supply pipe in town that suddenly fails after a couple of years of service time. Due to the increasing brittleness, rapid crack growth may occur suddenly releasing many cubic meters of water into the street.
Another, less dramatic result of aging is the plastic hanging basket with an expansive plant that suddenly breaks and falls to the floor.
Summary
- Physical aging is the process of the polymer molecules slowly adapting their conformation to a new temperature below the glass transition temperature.
- During aging the volume of the polymer reduces, which slows-down the mobility of the polymer molecules. Therefore, aging is a self-retarding process.
- Aging influences important product properties. The tensile strength, the stiffness and the brittleness increase with time, the impact strength reduces with time.
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