Isophthalic Acid (CAS 121-91-5)
Isophthalic acid, this white crystalline powder is the key meta-isomer of phthalic acid and serves as an essential monomer in the production of high-performance unsaturated polyester resins (UPR), powder coatings, and specialty polymers. Its unique meta-position structure delivers superior chemical resistance, thermal stability, and mechanical properties compared to ortho- or para-phthalic acids.
Key Chemical Properties
- Molecular Formula: C₈H₆O₄
- Molecular Weight: 166.13 g/mol
- Appearance: White crystalline powder or granules, odorless
- Melting Point: 345–348 °C (sublimes)
- Density: 1.54 g/cm³ at 25 °C
- Water Solubility: 0.01 g/100 mL at 25 °C (very low)
- Flash Point: 216 °C (closed cup)
- Purity: Available in ≥99.5% (standard industrial grade) and ≥99.8% (premium grade)
Common Synonyms
- 1,3-Benzenedicarboxylic acid
- m-Phthalic acid
- meta-Phthalic acid
- IPA
- Benzene-1,3-dicarboxylic acid
- Isophthalic acid
Primary Applications and Uses
Isophthalic acid is widely used across high-performance resin and coating industries:
- Unsaturated Polyester Resins (UPR): Main application (over 70% of global consumption) – provides excellent corrosion resistance, hydrolytic stability, and mechanical strength for marine, automotive, chemical storage tanks, pipes, and gel coats.
- Powder Coatings: Improves flow, gloss, and weather resistance in architectural, automotive, and industrial powder coatings.
- Alkyd Resins & Coatings: Enhances drying speed, hardness, and chemical resistance in high-quality paints and enamels.
- PET Copolymers & Specialty Polyesters: Used as a co-monomer to improve barrier properties, heat resistance, and clarity in bottles and films.
- Other Applications: Plasticizers, adhesives, printing inks, pharmaceuticals intermediates, and flame-retardant additives.
Its meta-structure gives resins outstanding performance in harsh environments where terephthalic acid (PTA) or phthalic anhydride alone cannot meet requirements.
Product Specifications
| Parameter | Specification |
|---|---|
| CAS Number | 121-91-5 |
| Purity | ≥99.5% (industrial) / ≥99.8% (premium) |
| Appearance | White crystalline powder/granules |
| Acid Value | 675–680 mg KOH/g |
| Moisture Content | ≤0.2% |
| Ash Content | ≤0.005% |
| Iron Content | ≤2.0 ppm |
| 3-Carboxybenzaldehyde | ≤50 ppm |
| Packaging | 25 kg bags, 500 kg, 1000 kg big bags |
| Shelf Life | 24 months (proper storage) |
Safety and Handling
Isophthalic acid is stable under normal conditions and classified as low toxicity. In dust form it may cause mild irritation to eyes, skin, and respiratory tract. It is a combustible dust when finely dispersed. Use appropriate personal protective equipment including dust mask, safety goggles, and gloves. Handle in well-ventilated areas. Store in a cool, dry, well-ventilated warehouse away from strong oxidizing agents and bases. Keep containers tightly closed. In case of contact, rinse with plenty of water. Dispose of waste in accordance with local environmental regulations.
Frequently Asked Questions (FAQs)
Q1: What is the CAS number of isophthalic acid?
A: The CAS registry number is 121-91-5.
Q2: What is isophthalic acid mainly used for?
A: It is primarily used as a monomer in unsaturated polyester resins (UPR) for corrosion-resistant applications, powder coatings, and specialty polyesters.
Q3: What is the difference between isophthalic acid and terephthalic acid?
A: Isophthalic acid (meta-isomer) provides superior chemical and hydrolytic resistance, while terephthalic acid (para-isomer) is preferred for high-molecular-weight PET fibers and bottles.
Q4: How should isophthalic acid be stored?
A: Store in a cool, dry, well-ventilated area below 30 °C. Protect from moisture and direct sunlight. Keep away from strong oxidizers and bases.
Q5: What purity levels are available?
A: Standard industrial grade ≥99.5%, premium polymer/coating grade ≥99.8%, with customized low-impurity profiles available on request.
Q6: Is isophthalic acid hazardous?
A: It has low toxicity but may cause mild irritation and is combustible as dust. Standard industrial hygiene and dust-control measures are recommended.
Q7: What are the main synonyms for isophthalic acid?
A: 1,3-Benzenedicarboxylic acid, m-phthalic acid, meta-phthalic acid, IPA, and benzene-1,3-dicarboxylic acid.
Q8: Can isophthalic acid be used in food-contact applications?
A: High-purity grades meeting relevant food-contact regulations are available for specific resin applications; please confirm regulatory compliance for your end use.
Q9: What packaging options are available for bulk orders?
A: 25 kg multi-layer paper bags, 500–1000 kg FIBC big bags, and full container load shipments.
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