Laundry and Heat

Why is heat damaging to clothing? When heat is applied to clothing fibers, it provides more energy than the fiber can handle, damaging the material. This damage can include shrinkage, permanent deformation, fiber weakening, and loss of elasticity.


Chemically, most clothing fibers are polymers — long chains of many small, repeating molecules called monomers. Depending on the monomer, different fabrics behave differently when they are exposed to heat.

Cotton

Cotton is made up of cellulose. Cellulose belongs to the class of polymers called polysaccharides, which are long chains of monosaccharides. Monosaccharides are chemically classified as sugars, while polysaccharides are carbohydrates.


Cellulose is a long, linear repeating chain of glucose molecules, specifically β-D-glucose.

Glucose contains alcohol groups (-OH) that form hydrogen bonds with both oxygen molecules in both the same chain and different chains.


These hydrogen bonds zip the polymer chains together and stiffen them, resulting in a resilient, dense fiber. Cotton fibers are woven under tension, producing long, narrow strands. Because these hydrogen bonds form while the fibers are under tension, internal strain is stored in the fibers.

Heat in the Washer

Heat causes cotton’s hydrogen bonds to partially break, allowing them to temporarily form hydrogen bonds with water. The hotter the water, the more hydrogen bonding with water can occur. This is essentially absorbency, which is why cotton swatches in hot water swell more and absorb more water than those in cold water. Motion in the washer helps fibers slide past one another, allowing more water to penetrate and more relaxation within fibers to occur.


This disruption of existing hydrogen bonds between cellulose chains allows the fibers and yarns —- previously held under mechanical tension from spinning, weaving or knitting, and finishing —- to relax into a shorter, lower-energy configuration.

Heat in the Dryer

As the water evaporates, new hydrogen bonds form between nearby glucose units, locking the fibers into this relaxed state. Most dimensional shrinkage therefore occurs in the washer, where heat, moisture, and agitation allow molecular and structural rearrangement.


By the time the garment reaches the dryer, the fiber structure has largely reorganized. The dryer primarily determines how permanently this new configuration is set. Higher dryer temperatures remove residual moisture more quickly and promote stronger hydrogen-bond fixation, often increasing permanent shrinkage rather than relaxing the fabric further. Importantly, hot water does not chemically damage cotton under normal laundering conditions. The covalent bonds of the cellulose backbone remain intact. Shrinkage and wear arise from physical reorganization (changes in hydrogen bonding and fiber alignment) rather than chemical degradation.

Shrinking vs Wrinkling

Shrinkage occurs when polymer chains reform hydrogen bonds along the fiber’s length, releasing the internal tension stored during spinning and weaving.

Wrinkles form when polymer chains hydrogen bond in bent or folded positions perpendicular to the fiber’s length, producing visible creases.

Common advice is to remove clothing while slightly damp from the dryer and fold immediately. At a molecular level, this makes sense — damp fibers are still hydrogen bonding with water and have not yet re-bonded with themselves. Folding the garment prevents the fibers from forming perpendicular hydrogen bonds, helping to avoid wrinkles.

Animal Fibers (wool, silk, cashmere)

Animal fibers like wool, silk, alpaca, and cashmere are polymers of proteins, primarily keratin (wool, alpaca, cashmere) or fibroin (silk). The general term for this structure is polypeptide. These proteins are held together by hydrogen bonds, disulfide bonds (in keratin), and other secondary interactions.


These fibers behave similarly to cotton in some ways, but they have a key additional feature — scales. Except for silk, animal fibers have tiny, overlapping flaps on their surface (especially pronounced in wool) that run lengthwise in a single direction. These scales are microscopic, but they provide extra resistance to breakage and trap air, improving insulation.


When wool and other similar animal fibers are agitated (e.g., in washing or drying machines), these scales rub against each other and interlock, a process known as felting, which can dramatically shrink the fabric.


Water plays a major role: it breaks hydrogen bonds within the protein, making fibers more flexible and allowing the scales to interlock more easily. Hot water intensifies this effect — it penetrates the fiber, competes for hydrogen bonds, swells the protein, and lifts the outer cuticle scales. Combined with motion, this leads to fiber migration and felting, causing major, often irreversible shrinkage.


Hot air, by contrast, supplies thermal energy without directly breaking hydrogen bonds or lifting the scales. Felting requires mechanical agitation and the interlocking of scales. Moisture greatly accelerates this process by softening fibers and allowing the scales to move more freely, but heat plus motion alone can still cause some felting, just less intensely. In short, wet heat felts wool faster and more aggressively, while dry heat can still cause shrinkage if there is enough agitation.


Why doesn’t wool wrinkle?

Wool resists wrinkling because its fibers are elastic and structurally constrained in a way that cotton fibers are not. Wool is made of keratin, a protein polymer whose chains are coiled and covalently crosslinked by disulfide bonds, giving the fiber a built-in springiness. When a wool garment is bent or compressed, the fibers store elastic energy and naturally try to return to their original shape. Even if moisture and heat temporarily disrupt hydrogen bonds, the underlying covalent framework prevents the chains from sliding into new, bent configurations. As the fiber cools or dries, elastic recoil straightens it first, and hydrogen bonds reform in this low-strain geometry rather than locking in a crease. Cotton, by contrast, relies almost entirely on hydrogen bonding between straight cellulose chains, so when it dries while folded, those bonds easily set wrinkles in place.

Synthetics

Synthetics like polyester and nylon are long-chain polymers. They are smooth, mostly non-polar, and held together along each chain by strong covalent bonds, while neighboring chains interact via weaker London dispersion forces (LDFs). This non-polar nature is why many activewear fabrics are sweat-wicking: sweat (or body oils, also mostly non-polar) adheres to the synthetic chains through LDFs.

Heat

Because synthetics are largely non-polar, they do not absorb much water. Their fibers do not swell, and there are few internal hydrogen bonds to disrupt. This is why synthetics shrink less than natural fabrics.


However, heat can still cause shrinkage or damage. LDFs are weak and become even weaker at higher temperatures. LDFs arise from temporary fluctuations in electron density that induce dipoles in neighboring molecules. As temperature rises, molecules move faster and spread farther apart, increasing the distance between them. Because intermolecular attractions decrease rapidly with distance, LDFs weaken, allowing polymer chains to slide past one another.


Synthetics are thermoplastic, meaning that as heat is applied, polymer chains can shift slightly relative to their neighbors. When the fibers cool, LDFs reform, locking the chains in their new positions. This creates small deformations in the fabric, which appear as wrinkles, slight shrinkage, or other forms of damage.

How does Water and Heat impact dye?

Clothing colors are held by different types of dyes, which adhere to fibers through various bonding mechanisms. Regardless of the dye type, water can cause color to fade by penetrating fibers, swelling them, and dislodging dye molecules. Several factors influence how much color is lost:

Because of this, it’s important not to wash colored clothing with light or white items in hot water, as dye can leach and stain lighter fabrics.