Why do we need to iron our clothes? Why does a paper get crumpled? Why do our fingers get wrinkled when kept in water for long? What is a crease? How is it different from a fold? Why do some things get creases, and others don’t? Let’s see!
A crease/wrinkle is formed when pressure on a solid breaks bonds or forms new ones inside its structure.
Amorphous and Crystalline Regions
Polymers occur in two kinds of orientations in a fabric. While the Crystalline arrangement is parallel, the Amorphous arrangement is more cross-linked and irregular. Since polymers in the Crystalline orientation are parallel, they are much closer to each other and form strong inter-molecular bonds which make them less vulnerable to external disturbances such as water and heat. Thus, they can continue to maintain their bonds and shapes. Most fibers have a combination of both types of orientations – crystalline and amorphous. The more is the crystalline orientation, more is the stability of the fiber and vice versa.
A change in perspective
Why some crystalline structures are brittle while others are not?
They’re both. There is something called Ductile To Brittle Transition Temperature. It is the temperature after which the material enters a zero ductility state, and shall break with the application of an external force. Materials usually enter this state at lower temperatures. Why? Because low temperatures do not allow the atoms or molecules to slide past each other and the lattice is closely packed. So, the structure cannot change its shape and eventually fractures on prolonged exposure to the applied force due to the lack of an alternative.
The Ductile to Brittle Transition Temperature is not the same for all types of lattices. This is because most metals are ductile and malleable at room temperature whereas ceramics are not. They are simply brittle. Drop your coffee mug on the floor for a proof. This does not mean that ceramics do not have a transition temperature. But it is that their Ductile to Brittle Transition Temperatures are higher than room temperature. Simply said, brittle materials are brittle, just because the temperature is below their DBTT. (Titanic didn’t sink just because of the iceberg.)
However, the crystalline structures such as metals do not show a really high compressibility at room temperatures because of their still compact and ordered structures. This is why the temperature of the metal is increased before beating it into sheets or drawing it into wires. Applying heat allows the structure to be more mouldable.
Crease with Ease – Cotton & Paper
Cotton clothes are majorly prone to creasing and wrinkling. Whether we dry them after washing or wear them on a moist sunny day, we cannot stop the crumples from showing up. That is why they require constant ironing.
Cotton has cellulose as its basic composition. The glucose molecules in cellulose are polar in nature due to the inter molecular attraction between oxygen and hydrogen. Which is why they are capable of forming hydrogen bonds because the hydrogen in the molecules bonds with the hydrogen in water. Water acts as a lubricant by forming these bonds and allows the fabric to slide into new orientations. Also, due to its small size water can easily enter between cellulose molecules. When these hydrogen bonds form, the material (here, cotton) enters its plastic phase – it becomes soft and mouldable. Whatever changes in its shape take place in the plastic phase, stay up after the water evaporates unless an external force is applied to reverse them.
When we iron the clothes, these new hydrogen bonds break due to the heat and pressure, and the original shape is reformed. (Reference)
At the macroscopic level, paper crushing is still a mysterious phenomenon. (Read here) Let’s see what happens inside!
A crease should not be confused with a fold. Creasing is when the paper retains or remembers its fold. We may bring the paper back to the smooth state if we fold it softly and not press too hard. But when we apply a stronger press, the sheet develops a crease because bonds at the crease break, sometimes unable to disappear even after ironing. This is also the reason why tearing a paper is easier at the creases. The only thing that holds the paper together at a crease are the surface bonds (which are also broken when we tear it). Same is the case with an Aluminum foil. (Reference)
Dive in here for details about skin wrinkling. If you don’t move your fingers, they lose their creases.
Which Solids Don’t Wrinkle?
This list usually includes synthetic polymers. They cannot form hydrogen bonds because they are interlocked with other chemicals and polymers so as to make their structure strengthened and less prone to creasing. Some examples are polyester, nylon etc. The plus point of interlocking polymers with other chemicals is that they prevent water from penetrating into the fiber thereby further preventing it from interacting with the hydrogen in the structure of the fiber. They are able to keep wrinkle-free only until their glass transition temperature is reached.