7.2 Physical vs Chemical Changes and Polymerization

Physical change versus chemical reaction on a nail plate

NIC Domain 3 asks you to recognize physical changes and recognize chemical reactions. Those are not trivia categories. They decide whether you think a service "just dried" or whether you understand that you created a new material on someone's keratin.

A physical change alters form—shape, wetness, temperature, or how mixed something looks—without making a new substance. Ice melting is physical. Acetone evaporating off a wipe is physical. Polish solvents leaving a film that was already dissolved in the bottle is the salon version.

A chemical change makes new substances by breaking and forming bonds. Polymerization is the chemical change that pays the bills in a nail salon: small units become a solid network you can file. Heat, light, or a catalyst may start it. Water will not reverse it. If the enhancement will not rinse off in the finger bowl, you probably did chemistry.

Exam sentence: If you mixed powder into liquid and it hardened into a material that was not in either bottle, that is a chemical polymerization. If color polish became dry after the solvents floated away, that is typically taught as physical.

Side-by-side at the table

Catalysts and initiators do not become the enhancement. They start or speed the reaction. Polymer powder carries an initiator that, once wetted with monomer, begins the chain. Dip activator is a catalyst sprayed or brushed to speed resin set. Gel photoinitiators need photons, not a spray. If you borrow an activator from the dip drawer to "hurry" a wet acrylic bead, you are mixing systems (7.3), not being efficient.

Polymerization is the liquid-and-powder story

The professional liquid-and-powder service is a chain-growth polymerization you perform with a brush.

1. Monomer liquid (typically EMA) supplies reactive double-bond molecules.
2. Polymer powder (typically PMMA beads) supplies bulk and an initiator.
3. The wet bead is a slurry. It is still mostly unreacted monomer plus beads.
4. The initiator starts a chain. Monomer units add to each other.
5. The bead goes from glossy-wet to satin to a new solid polymer shaped as a nail.

That last step is not drying. Drying would mean a solvent left and the same solid remained. Here the solid did not exist until the reaction ran. You can file it, you can soak it slowly in acetone, and you cannot rinse it off like lotion. That is the chemical-change test.

Heat during a correct acrylic set is a mild exotherm—the reaction releases energy. A violent hot spot usually means too much product in one lump, a too-wet bead against the plate, or a catalyst mismatch. The client saying "it burned" is chemistry plus technique, not a personality problem.

Bead ratio is a chemistry trap you can see

Students fail this by describing a too-wet nail as "I needed more time to dry." Time does not convert a flood of monomer into a balanced polymer. A too-dry bead does not "need the lamp." Liquid-and-powder is not a UV system unless you are on a hybrid product that the manufacturer designed that way.

Gel curing is photochemistry, not a hair dryer

UV/LED gel sits on the plate as a viscous mixture of oligomers, photoinitiator, and other additives. In the dark it is still a chemical waiting for a trigger. Under a lamp of the correct wavelength and time:

• The photoinitiator absorbs UV or LED energy.
• It generates species that start polymerization / cross-linking of the oligomers.
• The gel becomes a cured polymer network.

That is a photochemical reaction. It is a chemical change. Saying "the lamp dried the gel" is the same error as saying "the acrylic dried." Some surface solvents or inhibitors may still wipe off as a tacky layer; the cure underneath is still polymerization.

Lamp heat is not the cure. Clients feel warmth because (a) LEDs and UV sources produce some heat and (b) a thick layer can polymerize so fast the exotherm spikes. A heat spike is a reaction running too hot in a too-thick film, a wrong lamp, or a gel not matched to that bulb set—not proof you "cooked it dry." An uncured layer stays allergenic (7.4) because leftover reactive molecules can still touch skin.

Manufacturer directions control thickness, coat count, and time. 18VAC41-20-270 F 5 forbids using a product in a manner disapproved by the FDA. Cranking an unlisted lamp or stacking gel until it boils the nail bed is not "artist preference." It is off-label chemistry.

Polish remains the contrast case: base, color, and top typically set because solvents evaporate and the dissolved resins coalesce. You can often redissolve that film with remover without needing to break a densely cross-linked enhancement network. That is why NIC still uses polish as the physical-change example and acrylic/gel as the chemical-reaction example.

MMA versus EMA — precise, not folklore

This is the highest-stakes chemistry item on a Virginia paper. Write it the way the sources write it. Do not invent a Virginia statute that recites "methyl methacrylate."

What MMA and EMA are. Both are methacrylate monomers. Methyl methacrylate (MMA) is a small, aggressive monomer with a sharp odor and a very hard, very adherent polymer. Ethyl methacrylate (EMA) is the professional liquid used in modern liquid-and-powder systems. Cosmetic Ingredient Review (CIR) has treated EMA as safe for nail enhancement when used with directions to avoid skin contact because methacrylates can sensitize.

What FDA actually did. FDA's nail-care page and Cosmetics Safety Q&A (prohibited ingredients) tell the same history:

• In the early 1970s, FDA received complaints of injury from artificial nails containing methyl methacrylate monomer, including fingernail damage and deformity and contact dermatitis.
• FDA concluded that liquid methyl methacrylate was a poisonous and deleterious substance that should not be used in fingernail preparations.
• FDA removed products containing 100 percent liquid MMA monomer through court proceedings: a preliminary injunction, seizure actions, and voluntary recalls.
• FDA currently states: "No regulation specifically prohibits the use of methyl methacrylate monomer in cosmetic products."
• FDA also states it is against the law to market a cosmetic that is harmful when used as directed or in the customary way, even if the ingredient is not specifically prohibited by a regulation.

Those sentences can all be true at once. Students who memorize only "MMA is banned" miss the exam nuance. Students who memorize only "FDA allows MMA" miss the injury history and the adulterated-cosmetic rule.

Why EMA is the professional monomer anyway. MMA liquid is infamous in salons for a harsh odor, adhesion so aggressive that the enhancement can tear or deform the natural plate rather than flex, difficulty of removal (it does not soak like EMA systems), and nail damage when the client or a poorly trained tech rips it. Residual unreacted monomer on skin is a sensitization route. EMA systems still require no skin contact; they are the industry replacement, not a toy version of MMA.

What Virginia actually wrote. 18VAC41-20-270 F 4: no shop, salon, school, or facility providing cosmetology or nail care services will have on the premises cosmetic products containing hazardous substances that have been banned by the FDA for use in cosmetic products. 270 F 5: no product will be used in a manner that is disapproved by the FDA.

Read F 4 and F 5 slowly. Virginia did not drop the letters "MMA" into 18VAC41-20. The Board incorporated FDA's banned-hazardous-substance and disapproved-use lines. A 100% MMA monomer product of the type FDA seized is the historical match to "poisonous and deleterious" fingernail liquid. Using a harmful monomer in an enhancement, off-label, on damaged nails, or in a way FDA would treat as adulterated use, is an F 5 problem even when someone argues "there is no MMA-specific FDA regulation." Do not stock mystery bulk monomer. Do not decant unlabeled liquid. Do not treat "it sticks better than EMA" as a professional argument.

NIC practical overlay. For the sculptured-nail section, verbal instructions require a low-odor/less-odor product. Special-attention rules: monomer must be low-odor/less-odor and factory sealed in its original container with manufacturer's label. Polymer and primer (if used) stay in original manufacturer-labeled containers. That is how the practical keeps you off bulk, off unlabeled, and off harsh-odor liquids without writing an organic-chemistry lecture into the timing sheet.

Polymer versus monomer leftover. FDA notes that once acrylics form, traces of reactive monomer can remain and can cause redness, swelling, and pain in people who have become sensitive to methacrylates. The polymer itself is typically quite inert. The unreacted liquid is the sensitizer. That is why you do not wipe monomer on your sleeve all day (7.3) and why you do not flood the eponychium (7.4).

Chapter 11 will make the bead. This section is the sentence you owe Domain 3: hardening liquid-and-powder is chemical polymerization; polish flash-off is typically physical; gel in the lamp is photochemical; MMA liquid is historical FDA injury chemistry plus Virginia F 4/F 5, not a word that appears in 18VAC41-20.