Acrylic Acid, Propenoic Acid, Vinyl Formic Acid, 79-10-7

Acrylic Acid, Propenoic Acid, Vinyl Formic Acid, 79-10-7

ACRYLIC ACID (Propenoic Acid)

CAS No: 79-10-7 | EC No: 201-177-9 | Molecular Formula: C₃H₄O₂ (CH₂=CHCOOH) | Molecular Weight: 72.06 g/mol

1. Physical & Chemical Properties

Key Chemical Features:

• Vinyl group (CH₂=CH–): Contains double bond for polymerization.

• Carboxylic acid group (–COOH): Acidic, salt formation, esterification capability.

• Polymerization tendency: Readily polymerizes in presence of light, heat, and oxygen. Stabilization is mandatory.

• Corrosivity: Corrosive to metals (especially copper, zinc, iron).

• Exothermic reaction: Releases high heat during polymerization (uncontrolled reaction is hazardous).

2. Comparison with Alternative Monomers

Advantages of Acrylic Acid:

• Highest reactivity (double bond + electron-withdrawing carboxyl)

• Full water miscibility – ideal for aqueous polymerization

• Indispensable for Superabsorbent Polymer (SAP) production

• Lowest cost among acrylic monomers (via propylene oxidation)

Disadvantages:

• Corrosive and irritant (requires special equipment)

• Polymerization stabilization mandatory (inhibitor required; otherwise shelf life is hours)

• High melting point (13.5°C) – solidifies in cold climates

3. Production Methods

A. Main Method: Two-Step Propylene Oxidation (Industrial Standard)

text

Step 1:   CH₃-CH=CH₂ (Propylene) + O₂  →  CH₂=CH-CHO (Acrolein) + H₂O
Step 2:   CH₂=CH-CHO (Acrolein) + ½ O₂  →  CH₂=CH-COOH (Acrylic Acid)

B. Alternative Methods

Industrial note: >95% of global acrylic acid production uses the propylene oxidation method.

C. Product Grades

Inhibitor: MEHQ (Methyl hydroquinone) or HQ (Hydroquinone) – prevents polymerization during storage.

4. Formulation Examples (Industry-Specific)

Generic formulation principles. No brand names used.

A. Superabsorbent Polymer (SAP) – for Hygiene Products

Process:

1. Partially neutralize acrylic acid (cooling required – exothermic).

2. Add crosslinker and initiator.

3. Solution polymerization (temperature ramp: 50→90°C).

4. Cut resulting gel, dry, grind, and sieve.

Product property: Absorbs 300-500 times its own weight in water.

B. Water-Based Acrylic Paint (High Gloss, Low VOC)

Process: Emulsion polymerization (semicontinuous, 80-85°C).

Role of acrylic acid: Enhances latex stability, improves substrate adhesion, provides rheological control.

C. Paper Coating (Gloss and Strength)

Application: Add latex to coating mixture (calcium carbonate + kaolin). Coated paper achieves gloss, matte, or satin finish.

D. Pressure-Sensitive Adhesive (PSA – for Transparent Tape)

Application: Solution or emulsion polymerization. Polymer solution coated onto film; solvent evaporated.

E. Polyacrylic Acid (PAA) – Water Treatment Dispersant

Process: Aqueous solution polymerization (70-90°C). Resulting PAA (Mw 2,000-10,000 g/mol) prevents scale formation in cooling towers.

5. Industry Suitability Table

Scale: ★★★★★ = Critical | ★★★☆☆ = Secondary but important

6. Safety & Storage

Hazard Classification (GHS)

First Aid Measures

Storage Conditions

Special Storage Notes:

• Temperature is critical: Freezes below 14°C. Frozen acrylic acid can cause tank cracking and polymerization.

• Inhibitor level must be monitored: If MEHQ drops below 50 ppm, rapid polymerization risk occurs.

• Air/oxygen: Inhibitor requires oxygen to function; tanks must have air or oxygen atmosphere (not nitrogen).

7. Frequently Asked Questions (FAQ)

Q1: What is the difference between acrylic acid and methacrylic acid?

A: Acrylic acid has a hydrogen atom directly attached to the double bond, while methacrylic acid has a methyl group (CH₃).

• Acrylic acid: More reactive, faster polymerization, higher water absorption (ideal for SAP).

• Methacrylic acid: Lower reactivity, polymer is harder and clearer (ideal for PMMA/plexiglass).

Q2: Why is acrylic acid stabilized with an inhibitor? What is the shelf life without inhibitor?

A: Acrylic acid readily undergoes spontaneous free-radical polymerization in the presence of light, heat, or oxygen. Without inhibitor (99% pure), polymerization begins within hours at room temperature and the exothermic reaction can cause uncontrolled heating, tank rupture, or fire. With MEHQ inhibitor, shelf life is ~12 months.

Q3: What tank materials are compatible with acrylic acid?

A:

• Compatible: Stainless steel (316L, 304), titanium, glass-lined steel (enamel), aluminum (pure), HDPE (small containers, <30°C).

• NOT compatible: Carbon steel, copper, brass, bronze, zinc, galvanized steel, natural rubber, neoprene.

Q4: Why is acrylic acid indispensable for superabsorbent polymer (SAP) production?

A: SAP (diaper material) is crosslinked polyacrylic acid salt. Acrylic acid is essential because:

• Fully water-miscible (ideal for aqueous polymerization)

• High ionic capacity per unit mass (many –COONa groups)

• Can be crosslinked (with MBA)

• Other monomers (e.g., acrylamide) have much lower absorption capacity

Q5: Does acrylic acid cause chemical burns on skin? What PPE is required?

A: Yes, causes severe chemical burns. It is classified as corrosive (GHS H314). Required PPE:

• Hands: Butyl rubber, nitrile, or neoprene gloves (≥0.4 mm)

• Eyes: Chemical safety goggles with face shield

• Body: Chemical-resistant apron (PVC, butyl rubber, neoprene)

• Respiratory: Organic vapor cartridge mask (Type A) if ventilation is inadequate

Q6: Is acrylic acid harmful to the environment?

A: Yes, it is toxic to aquatic organisms (LC50 ≤10 mg/L). Due to low pH, it negatively affects aquatic ecosystems. Must be neutralized (pH 6.5-8.5) and/or biologically treated before discharge to wastewater. In soil, it biodegrades (half-life: days to weeks), but high concentrations can suppress soil microbiota.

Q7: What is the advantage of acrylic acid over methacrylates or acrylate esters?

A:

• Acrylic acid: Fully water-miscible, free acid group allows salt formation, high polarity, lowest cost among acrylic monomers.

• Methyl methacrylate (MMA): Water-immiscible, polymer is very hard and clear (plexiglass), more expensive.

• Butyl acrylate (BA): Water-immiscible, polymer is soft and tacky (PSA), more expensive.

Selection guide: Water treatment, SAP, dispersants → Acrylic acid. Rigid, clear plastic → MMA. Adhesives → BA.

Q8: What should I do if acrylic acid freezes?

A: Acrylic acid freezes at 13.5°C. Never apply direct heat to frozen acrylic acid (hot spots can initiate polymerization). Instead:

1. Move container to a controlled warming environment (25-30°C water bath or warm room).

2. Stir continuously during thawing (ensures uniform temperature distribution).

3. Once thawed, immediately check inhibitor level (freezing does not destroy inhibitor but may cause precipitation).

4. Always perform quality control testing before using previously frozen material.

Q9: What byproducts are present in acrylic acid? How is purity determined?

A: Typical byproducts:

• Acetic acid (≤0.5%)

• Propionic acid (≤0.1%)

• Formaldehyde (≤10 ppm)

• Acrolein (≤5 ppm)

• MEHQ inhibitor (0.02%)

Analytical methods:

• Titration (purity, acid number)

• GC (Gas Chromatography – byproducts)

• Karl-Fischer (water content)

• Color (APHA ≤20)

Q10: Can acrylic acid be produced from bio-based (green) sources?

A: Yes, currently in research and development stages. Methods include:

• Lactic acid dehydration (Cargill, NatureWorks)

• 3-Hydroxypropionic acid (3-HP) pathway (Novozymes, DuPont)

• Fermentation from glycerol or sugars (BASF, BioAmber)

Not yet commercially competitive with propylene oxidation, but bio-based acrylic acid market share may increase over the next 10-20 years as sustainability demand grows.

Q11: What is the difference between glacial acrylic acid (GAA) and technical grade?

A:

Q12: What are the occupational exposure limits for acrylic acid?

A:

8. Summary Table – Pros & Cons

This Technical Data Sheet is for informational purposes only and does not constitute medical or regulatory advice. Always consult local regulations and perform safety testing for your specific