1. Introduction to Acrylic Acid
Acrylic acid (IUPAC name: propenoic acid) is an unsaturated monocarboxylic acid with the molecular formula C₃H₄O₂. It is a clear, colorless liquid with a pungent odor and is primarily used as a precursor for acrylate esters and polyacrylic acid polymers. Its structural features—a vinyl group conjugated with a carboxylic acid—make it a highly reactive molecule suitable for a wide variety of chemical transformations. As a key monomer in the production of superabsorbent polymers, coatings, adhesives, and textiles, acrylic acid holds a prominent place in industrial chemistry.
This article will explore acrylic acid’s chemical behavior, industrial-scale synthesis, and diverse applications, providing a thorough understanding from the perspective of a chemical engineer.
2. Chemical Structure and Properties
2.1 Molecular Structure
- Molecular formula: C₃H₄O₂
- Molecular weight: 72.06 g/mol
- CAS Number: 79-10-7
- Structure: CH₂=CH–COOH
- Functional groups:
- Vinyl group (–CH=CH₂)
- Carboxylic acid group (–COOH)
Acrylic acid features a planar molecule due to the sp² hybridization of its carbon atoms. The conjugation between the carbon–carbon double bond and the carboxylic acid group increases its chemical reactivity compared to saturated carboxylic acids.
2.2 Physical Properties
| Property | Value |
| Appearance | Clear, colorless liquid |
| Odor | Acrid, pungent |
| Melting point | 13 °C |
| Boiling point | 141 °C |
| Density | 1.051 g/cm³ at 20 °C |
| Solubility | Miscible with water, ethanol, and ether |
| Vapor pressure | ~3.8 mmHg at 20 °C |
| Flash point | ~50 °C (closed cup) |
Acrylic acid is moderately volatile and flammable. It polymerizes easily in the presence of heat or light, often requiring inhibitors (e.g., hydroquinone) during storage.
3. Chemical Properties and Reactivity
Acrylic acid’s reactivity arises from its dual functional groups:
3.1 Addition Reactions at the Vinyl Group
- Free-radical polymerization:
Acrylic acid readily undergoes radical polymerization to form poly(acrylic acid) (PAA), a water-absorbent polymer. - Michael addition:
The conjugated double bond undergoes nucleophilic conjugate addition with amines, thiols, or enolates. - Hydrohalogenation and hydration:
Similar to other alkenes, acrylic acid reacts with HX, water (in acid catalysis), and other electrophiles.
3.2 Carboxylic Acid Reactions
- Esterification:
Reacts with alcohols in the presence of acid catalysts to produce acrylate esters. - Salt formation:
Reacts with bases (e.g., NaOH) to form acrylate salts. - Amide formation:
Forms acrylamides upon reaction with amines or ammonia.
3.3 Polymerization Behavior
Acrylic acid can be polymerized via:
- Free-radical polymerization
- Solution polymerization
- Emulsion polymerization
- Inverse suspension polymerization
Its reactivity ratio with other monomers (e.g., acrylamide, methacrylic acid) allows copolymer production tailored for specific applications.
4. Production Processes
4.1 Historical Context
Initially, acrylic acid was synthesized via hydrolysis of acrylonitrile or from acetylene and carbon monoxide. These methods were eventually replaced by more efficient and cost-effective propene oxidation processes.
4.2 Modern Industrial Synthesis: Two-Step Oxidation of Propene
The dominant method today is a two-step catalytic oxidation of propene (propylene) using air (oxygen) over metal oxide catalysts.
4.2.1 Reaction Steps
Step 1: Oxidation of propene to acrolein
CH₃–CH=CH₂+O2→CH2=CH–CHO+H2O
Step 2: Oxidation of acrolein to acrylic acid
CH₂=CH–CHO+O2→CH2=CH–COOH
Overall Reaction:
CH₃–CH=CH₂+1.5O2→CH2=CH–COOH+H2O
4.2.2 Catalysts Used
Step 1: Bismuth molybdate (Bi–Mo–O) based catalysts
Step 2: Vanadium-molybdenum mixed oxide catalysts (V–Mo–O)
4.2.3 Process Conditions Temperatures: 250–350 °CPressure: Atmospheric to slight overpressure (1–3 atm)Yields: >85% acrylic acid yield per pass
4.2.4 Process Flow Air and propene are mixed and passed over the first catalyst bed for acrolein formation.Acrolein-rich gas stream flows into a second reactor for further oxidation to acrylic acid.Quenching and absorption: The gas mixture is rapidly cooled and acrylic acid is absorbed in water or solvents.Purification: Distillation removes water, acetic acid, and other byproducts.
4.3 Alternative Routes (Under Development or Niche Use) 4.3.1 From Glycerol (Bio-Based Route) With increased interest in renewable feedstocks, glycerol (a byproduct of biodiesel) is being investigated as a precursor: C₃H₈O₃→CH2=CH–CHO→CH2=CH–COOH Advantages: Bio-renewableCarbon-neutral potential Challenges: Lower yieldsCatalyst deactivationHigher cost 4.3.2 From Lactic Acid or 3-Hydroxypropionic Acid These biomass-derived intermediates can also be converted into acrylic acid, but commercial viability remains limited.
5. Industrial Applications
Acrylic acid’s applications are mainly through its polymer and ester derivatives.
5.1 Superabsorbent Polymers (SAPs)
- Main use: Over 50% of global acrylic acid is used to produce polyacrylic acid and its salts (e.g., sodium polyacrylate).
- Application: Diapers, adult incontinence products, sanitary napkins
- Function: SAPs can absorb and retain hundreds of times their weight in water
5.2 Acrylate Esters
Produced via esterification with alcohols:
- Methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate
Applications:
- Paints and coatings (binder resins)
- Pressure-sensitive adhesives (PSAs)
- Sealants and caulks
- Textile finishes
These esters offer flexibility, UV resistance, and adhesion properties.
5.3 Coatings and Paints
- Acrylic resins based on acrylic acid are used in automotive finishes, architectural paints, and industrial coatings.
- Their advantages include:
- Fast drying
- Weather and UV resistance
- Gloss retention
5.4 Adhesives and Sealants
- Acrylic copolymers with acrylamide, styrene, or vinyl acetate enhance adhesion, durability, and water resistance.
- Used in:
- Construction
- Packaging
- Electronics
5.5 Detergents and Water Treatment
- Polyacrylic acid acts as a dispersant and scale inhibitor in:
- Cooling water systems
- Boilers
- Detergents (prevents re-deposition of soil)
5.6 Textiles and Leather
- Acrylic acid-based copolymers are used in:
- Textile sizing
- Finishes
- Dye thickening agents
They provide flexibility, water absorbency, and binding capability.
5.7 Oil and Gas Industry
- Drilling fluids and enhanced oil recovery (EOR) formulations use acrylic-based polymers to modify viscosity and fluid behavior.
6. Health, Safety, and Environmental Considerations (Continued)
6.1 Health Hazards (Continued)
- Inhalation: Prolonged exposure to acrylic acid vapors can lead to respiratory tract irritation, coughing, and difficulty breathing. High concentrations may cause pulmonary edema.
- Skin contact: Causes severe skin burns. Even diluted solutions can result in dermatitis upon prolonged exposure.
- Eye contact: Direct exposure can lead to severe eye damage or blindness.
- Ingestion: Harmful if swallowed, causing burns to the digestive tract.
- Carcinogenicity: Currently, acrylic acid is not classified as a confirmed human carcinogen, but long-term studies are ongoing.
- Occupational exposure limits (depending on local regulations):
- ACGIH TLV: 2 ppm (TWA)
- OSHA PEL: 10 ppm (TWA)
6.2 Safety Considerations in Storage and Handling
- Polymerization hazard: Acrylic acid can polymerize spontaneously, especially when exposed to heat, light, or peroxides. This reaction is exothermic and can cause container rupture.
- Inhibitors: Commercial acrylic acid typically contains stabilizers such as hydroquinone or MEHQ (monomethyl ether of hydroquinone) to prevent runaway polymerization.
- Storage:
- Store under nitrogen atmosphere in stainless steel tanks with temperature control (< 25°C).
- Avoid contact with oxidizing agents, bases, and amines.
- Keep away from heat and sunlight.
6.3 Environmental Impact
- Aquatic toxicity: Acrylic acid is moderately toxic to aquatic life. It is biodegradable but can cause localized environmental damage if released in large quantities.
- Air emissions: Volatile emissions contribute to VOC (volatile organic compound) load, which can impact air quality and contribute to smog formation.
- Spill management:
- Small spills: Neutralize with sodium bicarbonate and absorb with inert material.
- Large spills: Evacuate area, contain with dikes, and collect for proper disposal.
6.4 Regulatory and Compliance
- Subject to chemical control laws such as:
- REACH (EU)
- TSCA (USA)
- GHS classification:
- Flammable liquid, Category 4
- Acute toxicity (oral, dermal, inhalation): Category 4
- Skin corrosion: Category 1A
- Aquatic toxicity: Category 3
Industries using acrylic acid must implement strict exposure controls, employee training, and emergency response protocols.
7. Market Overview and Industry Trends
7.1 Global Production and Consumption
As of recent market data, global acrylic acid production exceeds 8 million metric tons per year, with Asia-Pacific (particularly China) being the dominant producer and consumer, followed by North America and Europe.
7.2 Market Segmentation by Application
| Application Area | % Share of Global Acrylic Acid Use |
| Superabsorbent polymers | ~50% |
| Surface coatings | ~20% |
| Adhesives and sealants | ~10% |
| Textiles, leather, paper | ~10% |
| Water treatment and others | ~10% |
7.3 Trends and Drivers
- Growing hygiene product demand: The global rise in disposable diapers, particularly in developing countries, is the primary driver for superabsorbent polymer demand.
- Bio-based production: Sustainability pressures are pushing R&D toward renewable routes (e.g., glycerol, lactic acid).
- Water treatment growth: Urbanization and industrialization are increasing the demand for acrylic acid-based dispersants and scale inhibitors.
- Paint and coatings innovation: The shift toward waterborne and low-VOC paints enhances the demand for acrylic binders.
7.4 Challenges
- Feedstock volatility: The price of propene, the primary raw material, is subject to petrochemical market fluctuations.
- Environmental regulations: VOC restrictions and carbon footprint goals are tightening around petrochemical-derived acrylic acid.
- Competition from methacrylic acid and biodegradable polymers in certain niche applications.
8. Emerging Technologies and Research Directions
8.1 Bio-Based Acrylic Acid
Several routes for renewable production are under study or in pilot stages:
- Glycerol-based: Catalytic dehydration to acrolein followed by oxidation.
- Lactic acid-based: Through dehydration to acrylic acid.
- 3-Hydroxypropionic acid (3-HP) pathway: From glucose fermentation to 3-HP, then dehydration.
These technologies aim to reduce greenhouse gas emissions, lessen dependency on fossil fuels, and create sustainable value chains.
Challenges include:
- Catalyst development and lifetime
- Separation and purification complexity
- Cost competitiveness with petrochemical routes
8.2 Green Polymerization Techniques
Efforts are underway to:
- Use controlled radical polymerization (e.g., RAFT, ATRP) to create well-defined acrylic polymers
- Develop solvent-free or aqueous polymerization systems
- Introduce bio-degradable or stimuli-responsive polymers for medical or agricultural applications
9. Conclusion
Acrylic acid is a highly versatile, industrially significant monomer with a unique combination of a vinyl group and a carboxylic acid, enabling it to participate in a wide range of chemical transformations. Its largest impact lies in the production of superabsorbent polymers, which are integral to the hygiene industry. Beyond this, its use in coatings, adhesives, textiles, and water treatment demonstrates its breadth of application.
From a chemical engineering perspective, the two-step catalytic oxidation of propene remains the most efficient and economically viable production route, though bio-based alternatives are receiving increasing attention due to sustainability concerns.
In terms of handling, acrylic acid requires careful storage and inhibition to avoid hazardous polymerization, along with measures to control exposure and environmental emissions.
With growing demand across multiple industries and rising environmental consciousness, the future of acrylic acid production and use will likely focus on greener, safer, and more sustainable technologies. For chemical engineers, this presents both a challenge and an opportunity to innovate within a mature yet evolving sector.