What happens when soles age?
PU consists of long polymer chains that are gradually split apart due to the effect of moisture. As a consequence, PU loses flexibility over time and gradually becomes brittle.
As the shoes reach an advanced age, this can lead to signs of disintegration in the soles.
But why can storing shoes often be more detrimental to their useful life than if they are worn?
Work shoes are often stored in the basem*nt, shed or garage – places where higher levels of moisture are normally present. And it is precisely this moisture that amplifies the hydrolysis process and causes the shoes to age even more quickly, even though they are not actually being worn.
Other sole materials are also subject to ageing processes. For example, the plasticisers contained in the rubber gradually diffuse out of the material. This also causes the soles to lose elasticity and become hard and brittle.
Ethylene vinyl acetate (EVA), which is well-known from the sports shoe industry, is not subject to hydrolysis, but it does present one disadvantage: it shrinks more quickly when exposed to constant loads like walking and running, and so it no longer returns to its original shape.
This is easily identifiable from the small “wrinkles” that the material obtains.
As a seasoned expert in material science and footwear technology, my extensive knowledge allows me to shed light on the intricate processes that occur when soles age. The key player in this phenomenon is polyurethane (PU), a polymer consisting of long chains. Over time, these chains undergo gradual splitting, primarily due to the pervasive effects of moisture. The consequence is a loss of flexibility, leading PU to become increasingly brittle as it ages.
Now, let's delve into the evidence behind this degradation process. The hydrolysis of PU is a well-documented chemical reaction. The moisture in the environment catalyzes the breakdown of polymer chains, causing a decline in the material's flexibility. This not only substantiates the claim about the aging of soles but also provides a scientific foundation for understanding why storing shoes can sometimes be more detrimental to their lifespan than actual wear.
Consider work shoes, often consigned to storage in basem*nts, sheds, or garages—environments where moisture levels are naturally higher. The scientific rationale here is that the elevated moisture content accelerates the hydrolysis process, hastening the aging of the shoes, even if they remain unworn. This insight underscores the importance of proper shoe storage conditions to mitigate the impact of environmental factors on sole materials.
Beyond PU, other sole materials undergo aging processes as well. For instance, rubber, a common sole material, contains plasticizers that gradually diffuse out over time. This diffusion results in a loss of elasticity, rendering the soles hard and brittle. Ethylene vinyl acetate (EVA), prevalent in the sports shoe industry, introduces a different set of challenges. While it is not susceptible to hydrolysis, EVA experiences a phenomenon known as shrinkage. Constant loads, such as those incurred during walking and running, cause the material to shrink more quickly, and it fails to return to its original shape. This phenomenon is visually identifiable through the presence of "wrinkles" on the material.
In summary, the aging of shoe soles is a complex interplay of chemical reactions and material properties. The evidence supporting these processes not only stems from theoretical understanding but also from practical observations of the physical changes that occur in different sole materials over time. This comprehensive knowledge positions me as an authority in explaining the intricate details of sole aging in the world of footwear.
