Acrylate monomers look interchangeable on a price list: colourless liquids, similar CAS families, tonne prices within a band of each other. In a reactor they are not. The ester group decides the glass transition temperature (Tg) of the polymer, whether the film coalesces at the application temperature, and how much coalescent the recipe needs on top. This guide works through the families the way a formulator does: Tg first, then film formation, then crosslinking, then the specification lines that change the economics.

Start From Tg, Not From a Price List

Every acrylate carries an ester side chain, and its size and bulk set the glass transition temperature of the homopolymer. The ladder is well established: poly(methyl acrylate) sits around 10 °C, poly(ethyl acrylate) near −24 °C, poly(butyl acrylate) near −54 °C, poly(2-ethylhexyl acrylate) below −65 °C. At the hard end, poly(acrylic acid) is around 106 °C and the bicyclic isobornyl acrylate homopolymer lands near 90–110 °C. Moving the ester from ethyl to butyl buys thirty degrees of softness; branching the octyl chain buys another ten.

Copolymer Tg follows the Fox equation: 1/Tg = w₁/Tg₁ + w₂/Tg₂, temperatures in kelvin. A 50/50 blend of butyl acrylate (219 K) and acrylic acid (379 K) gives 1/Tg ≈ 0.00360, so Tg ≈ 5 °C. That single calculation says the coating will not form a useful film at room temperature without help — more soft monomer, or a coalescent. Skipping the arithmetic surfaces later as a blocking or cracking complaint.

Soft Monomers: Butyl Acrylate Against 2-Ethylhexyl Acrylate

The two workhorse soft monomers overlap in function but not in behaviour. Butyl acrylate (CAS 141-32-2) is the default for flexible films, adhesives and textile backings; its straight C4 chain gives clean −54 °C soft-segment behaviour, and its boiling point of 145 °C means it stays in the film during drying, so the plasticising effect is permanent.

2-Ethylhexyl acrylate (CAS 103-11-7) is the deeper-soft option: melting point −90 °C, boiling range 215–219 °C, density 0.885 g/mL. The branched C8 chain lowers Tg further and adds hydrophobicity, which matters for exterior paints and sealants exposed to rain. Because it is so involatile, EHA acts as a built-in plasticiser — but over-dosing it produces permanently tacky surfaces, so the usual compromise is BA as the backbone with 10–20% EHA where low-temperature flexibility is specified.

Ethyl acrylate occupies an intermediate slot. Its homopolymer Tg of −24 °C is soft but not extreme, and its boiling point of 99 °C means it partly flashes off during drying, nudging the final film harder than the feed suggests. Practical notes: the flash point is around 15 °C, so it ships as flammable liquid, and suppliers recommend cold storage (2–8 °C) because it polymerises more readily than the heavier esters.

Hard Monomers: Where Acrylic Acid and Isobornyl Acrylate Earn Their Keep

Acrylic acid (CAS 79-10-7, melting point 14 °C, boiling point 140.9 °C) is the cheapest way to raise Tg and the standard route to adhesion. At 2–6% in a BA-rich emulsion it anchors the film to concrete, wood and metal through carboxyl groups, and after neutralisation with ammonia it also thickens the latex. The costs are real: carboxyls are hydrophilic, so water sensitivity rises with every percent, and the monomer is corrosive. On storage, acrylic acid self-condenses by Michael addition into the dimer, an assay drop that shows up if drums sit warm — check the dimer specification, not just GC purity.

When you need hardness without acidity, isobornyl acrylate is the monomer of the moment. The rigid bicyclic bornyl group delivers a homopolymer Tg comparable to styrene but with better weatherability and no aromatic content, which is why it has displaced styrene in many vinyl ester and unsaturated polyester resins. It doubles as a low-viscosity reactive diluent, cutting resin viscosity while reacting fully into the network. Watch the colour specification: optical and resin grades are sold against APHA limits, and yellowed IBOA will tint a clear coat.

Specialty Monomers: DMAAm and Acryloylmorpholine

Two tertiary amides extend the family into water-soluble territory. N,N-Dimethylacrylamide (CAS 2680-03-7) polymerises to a hydrophilic, non-ionic polymer whose tertiary amide resists hydrolysis — unlike acrylamide chemistry it cannot unzip back to acrylic acid at neutral pH. That stability plus salt tolerance makes it standard in hydrogels, contact-lens materials, oilfield fluids and high-solids coatings. Suppliers quote GC assay because the impurities that matter are polymerisation poisons, not minor homologues.

N-Acryloylmorpholine (CAS 5117-12-4) brings the same tertiary-amide stability in a cyclic structure with a very low freezing point, so it ships and handles as a neat liquid in winter. Its polymers hold viscosity in brines, hence its use in oilfield polymers, water-treatment flocculants and specialty papers. For both, ask for water content and inhibitor level explicitly; at these price points they are too often quoted as "typical" rather than guaranteed.

Crosslinking: Four Routes Out of a Thermoplastic Film

A linear acrylate film blocks, swells and re-emulsifies. Four chemistries fix that. First, difunctional ethylene dimethacrylate copolymerised at 0.5–2% builds the network during polymerisation itself — standard in ion-exchange beads, dental resins and absorbent polymers where gel fraction is a sold specification. Second, the ketone–hydrazide system pairs diacetone acrylamide with adipic dihydrazide (CAS 1071-93-8): the adduct stays latent in the drum and only condenses as water leaves the drying film, giving waterborne coatings months of pot life. Third, glyoxal (40% aqueous grade) crosslinks hydroxyl-bearing polymers such as PVA, starch and cellulosics at low temperature — the workhorse of paper sizing and textile finishes. Fourth, silane grafting: copolymerise 1–2% vinyltrimethoxysilane into the latex, and the methoxy groups hydrolyse to silanols that condense after application, turning a thermoplastic emulsion into a moisture-curing, water- and scratch-resistant one-pack film.

One more handle: vinylbenzyl chloride (chloromethyl styrene) carries a benzyl chloride group that quaternises amines after polymerisation — the route to anion-exchange resins and antimicrobial quaternary coatings on an ordinary acrylate backbone.

What the Spec Sheet Hides: Inhibitor, Water, Colour and Storage

Before comparing quotes, walk the specification top to bottom. Acrylates ship with 10–15 ppm MEHQ (acrylic acid typically 200 ppm), a number that matters in both directions: too little risks polymerisation in a warm warehouse, too much slows your own reaction. Note that MEHQ needs dissolved oxygen to function, so a supplier who blankets drums with nitrogen has quietly disabled the inhibitor; ask what headspace gas they use. Water: emulsion polymerisation tolerates it, solution and silane systems do not. GC assay hides dimer and oligomer content; acrylic acid should be quoted with a dimer limit.

Logistics is part of the price. Ethyl and butyl acrylate carry flash points near 15–17 °C and move as Class 3 flammable liquids; EHA and IBOA do not, which changes freight economics. A Tg above ambient typically also carries 3–8% of a slow glycol ether such as dipropylene glycol or a mid-boiling solvent like propylene glycol monomethyl ether, and that adder can swing the real cost of a waterborne formulation by several percent. Compare monomers on delivered, inhibited, polymerisable content per tonne — never on drum price alone — and the "cheaper" quote often reorders itself.