What Acrylic Acid (CAS 79-10-7) Is
Acrylic acid, C3H4O2, molecular weight 72.06, is the simplest unsaturated carboxylic acid: a vinyl group bonded directly to a carboxyl group. It is a colourless, corrosive liquid with a tart, acrid odour, miscible with water, alcohols, ethers and chloroform. Polymer grade material is normally 99.0% assay or higher, with water and acetic acid as the main residual components, because those two impurities interfere with radical polymerisation.
The vinyl group is the reason the molecule behaves differently from most acids in a warehouse. It polymerises readily on exposure to heat, light or certain metal ions, so every commercial parcel contains a polymerisation inhibitor, most commonly the monomethyl ether of hydroquinone at roughly 180 to 220 ppm, plus a small amount of dissolved oxygen that the inhibitor needs in order to function.
Key Physical and Chemical Data
| Property | Typical value | Reference method |
|---|---|---|
| Assay as C3H4O2 | 99.0 % minimum, polymer grade | Titration |
| Water | 0.30 % maximum | ASTM D1364 (Karl Fischer) |
| Colour | 20 Pt-Co maximum | ASTM D1209 |
| Density at 20 degrees C | 1.051 g/cm3 | ASTM D4052 |
| Boiling point at 101.3 kPa | 141 degrees C | ASTM D1078 |
| Melting point | 13 degrees C | Freezing curve |
| Flash point, closed cup | 50 degrees C | ASTM D93 |
| Inhibitor (MEHQ) | 180 - 220 ppm | HPLC |
| Transport classification | UN 2218, Class 8, Packing Group II | UN Model Regulations |
Why Industry Buys It
Superabsorbent polymers are the largest outlet: acrylic acid is neutralised with sodium hydroxide and polymerised into a cross-linked gel used in disposable hygiene products and in agricultural water retention. Water treatment is the second major outlet, where acrylic acid copolymers act as scale inhibitors, dispersants and flocculant builders. Thickeners and rheology modifiers for paints and detergents, including alkali-swellable and hydrophobically modified emulsions, depend on the same chemistry. Other users include pressure-sensitive adhesives, textile sizing and printing binders, leather finishing, paper coating binders and dispersants for pigment concentrates.
Grades and How They Are Specified
Two commercial grades dominate. Technical or crude grade is an intermediate stream, normally 94% to 99% assay, sold into processes that can tolerate water and acetic acid. Polymer or glacial grade is 99.0% plus with tightly controlled aldehydes, maleic anhydride and inhibitor, and is the grade on which most purchase specifications are written. Buyers should state assay, water, colour, inhibitor level and aldehyde limits explicitly, together with the test methods, since the two grades can share the same headline assay figure while differing in the trace impurities that decide polymer quality.
Storage and Logistics Points That Decide Quality
Acrylic acid freezes at 13 degrees C, so tanks and drums are held between about 15 and 25 degrees C with trace heating or controlled insulation; a frozen parcel must be thawed evenly and recirculated, because the inhibitor does not redistribute itself in a partially frozen mass. Equipment is 316L stainless steel, polyethylene or PTFE-lined; mild steel and copper alloys are avoided because dissolved iron and copper ions accelerate polymerisation. Air is excluded from the vapour space of storage tanks to limit oxygen uptake and moisture ingress, and parcels are despatched in 200 litre drums, 1000 litre IBCs or stainless road tankers classified UN 2218.
Frequently Asked Questions
Q: What is acrylic acid used for in industry?
A: Its main use is as a monomer. It is neutralised and polymerised into superabsorbent polymers, copolymerised into scale inhibitors and dispersants for water treatment, and converted into thickeners for coatings, adhesives and detergents, plus textile, leather and paper auxiliaries.
Q: How should acrylic acid be bought and specified?
A: Specify polymer grade at 99.0% minimum assay with water at 0.30% maximum, colour at 20 Pt-Co maximum, MEHQ inhibitor at 180 to 220 ppm, and aldehyde limits agreed with the supplier. Request the batch certificate, the lot number and the date of manufacture on every delivery.
Q: Why does acrylic acid polymerise in storage?
A: Heat, light, dissolved metal ions or inhibitor depletion can start radical polymerisation. Keep the parcel below 25 degrees C, out of direct sunlight, in stainless or lined equipment, monitor the inhibitor level during long storage, and never top up an old parcel with fresh material without checking the inhibitor content.
Q: Is acrylic acid dangerous to handle?
A: Yes. It is corrosive and classified UN 2218, Class 8, Packing Group II, with a flash point of 50 degrees C. It attacks skin and eyes and its vapour is irritant, so closed transfer, local extraction, face and hand protection and grounding of transfer lines are standard practice.
Q: Does acrylic acid need a temperature-controlled container in winter?
A: Yes for polymer grade, which freezes at 13 degrees C. Unheated transport risks a frozen or partly frozen parcel; heated or insulated tankers and short transit times are normal practice in cold climates, with recirculation on arrival.