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Isophthalic Acid, known as Isophthalic Acid in Turkish, is an aromatic dicarboxylic acid with the molecular formula C₈H₆O₄. This raw material, identified by CAS No 121-91-5, is an important intermediate used in the production of polyester, unsaturated polyester, alkyd, coating resin and special polymer because it carries two carboxylic acid groups at positions 1 and 3 of the benzene ring.
It is evaluated in polyester resins to improve water resistance, chemical resistance, mechanical properties and outdoor performance. The molecular geometry formed by the two carboxyl groups in the meta position provides different polymer chain arrangement and resin properties than other benzene dicarboxylic acids such as terephthalic acid.
It is also offered as high purity commercial quality under the name Purified Isophthalic Acid or PIA. PIA; It is an important aromatic monomer used in the production of unsaturated polyester resins, gelcoats, coating resins, PET-based copolymers and different special polyesters.
Two carboxylic acid groups undergo an esterification reaction with glycols to form polyester bonds. When used in combination with neopentyl glycol, ethylene glycol, propylene glycol, and multifunctional alcohols, the resin's hardness, flexibility, hydrolysis resistance, and chemical resistance can be adjusted according to the application.
It is preferred in applications where higher water and chemical resistance is targeted compared to orthophthalic systems. Marine, gelcoat, glass fiber reinforced composite, tank, pipe and corrosion resistant composite systems are important examples of these usage areas.
In PET and other thermoplastic polyesters, it can be used as a comonomer. Adding a limited amount of isophthalate units to the chain can help adjust crystallization behavior, processability, and optical properties.
It is a white crystalline solid with a high melting temperature, and its volatility is very low at room temperature. In contrast, when processed into fine powder, the risk of dust formation in the air and dust explosion under suitable conditions should be considered.
In commercial use, not only chemical purity; color, moisture, acid value, ash, metal impurities and trace components from the production process are also important. Especially in light-colored polyester and coating resins, low-color and high-purity quality can be preferred.
Final product performance depends not only on the amount of isophytalic acid; but also on glycol type, other acid monomers, molecular weight, cross-linking system and production conditions.
WHAT IS ISOPHTHALIC ACID?
Isophthalic Acid is an aromatic dicarboxylic acid with two carboxylic acid groups at positions 1 and 3 of the benzene ring. It is systematically identified by the name Benzene-1,3-dicarboxylic Acid, and the commercial product is usually in the form of a white or colorless crystalline powder.
It is one of the three positional isomers of the phthalic acid family: in Phthalic Acid, the carboxyl groups are located at position 1,2, in Isophthalic Acid at position 1,3, and in Terephthalic Acid at position 1,4.
While this difference in position may seem small, it is significant for polymer chemistry. The connection angle on the aromatic ring affects the geometry, packing, crystallization, and final physical properties of the polyester chain.
Its molecular weight is approximately 166.13 g/mol. Its nominal functionality in polyesterization reactions is 2, as it carries two carboxylic acid functions; both -COOH groups react with hydroxyl groups in glycol, and water is released during esterification.
It is one of the common aromatic acid monomers in unsaturated polyester resins. Using it in combination with unsaturated ingredients such as maleic anhydride or fumaric acid and suitable glycols, "isophthalic polyester" resins can be prepared, which can then be cross-linked with styrene.
Gelcoat production is one of its important applications. Eastman states that unsaturated polyester gelcoats prepared using Purified Isophthalic Acid and neopentyl glycol can provide good weather, water and stain resistance.
In addition to thermosetting polyesters, it is also considered as a comonomer in PET-based copolymers. Adding isophthalate units to the PET chain can change the rate or degree of crystallization by making it difficult to pack in a full order; this property is important in bottle, sheet, film and special polyester applications.
It can also be used in the production of polyester polyols. These polyols are reacted with polyisocyanates to become part of polyurethane coating, elastomer, or foam systems.
LOTTE Chemical defines Purified Isophthalic Acid as a high value-added monomer used in PET, paint, and different polyester applications. Industrial PIA production is usually carried out through the oxidation of meta-xylene followed by purification steps.
It is also involved in the synthesis of special polymers resistant to high temperatures; As a result of polycondensation with m-phenylenediamine, poly(M-phenylene isophthalamide) type aromatic polyamides can be prepared. Its use as a ligand in metal-organic coordination structures is mostly at the laboratory scale.
Isophthalic Acid and Purified Isophthalic Acid refer to the same basic molecule; however, the PIA designation usually describes the high-purity industrial grade suitable for polyester production. Therefore, not only the CAS number should be specified at the time of purchase, but also the desired grade of quality.
It is more soluble in molten glycols and generally more reactive compared to Terephthalic Acid. This difference can shorten the time required to reach a complete reaction in certain polyester synthesis routes.
COMMON PRODUCT AND CHEMICAL NAMES
Isophthalic Acid
Isophytalic Acid
Purified Isophthalic Acid (PIA)
Benzene-1,3-dicarboxylic Acid
m-Phthalic Acid
CAS NO, EC / EINECS NO AND CHEMICAL IDENTIFICATION
Product Name: Isophthalic Acid
English name: Isophytalic Acid
Common trade name: Purified Isophthalic Acid
Abbreviation: PIA
Systematic name: Benzene-1,3-dicarboxylic Acid
Chemical class: Aromatic dicarboxylic acid
CAS No: 121-91-5
EC/EINECS No: 204-506-4
Molecular formula: C₈H₆O₄
Condensed formula: C₆H₄(COOH)₂
Molecular weight: Approximately 166.13 g/mol
Carboxylic acid functionality: 2
Physical form: White or colorless crystalline powder
Melting/sublimation zone: Approx. 341–348 °C, depending on source and method
Density: Approx. 1.54 g/cm³
Water solubility: Very low in cold water, limited in hot water
Volatility: Negligible at room temperature
Its main functions: Polyester monomer, resin raw material, PET comonomer, unsaturated polyester raw material and special polymer building block
Note: Laboratory products have a 99% assay, while industrial PIA grades offer different specifications for color, moisture and impurity values according to the manufacturer; therefore, the quality grade must be specified at the time of ordering.
CHEMICAL STRUCTURE AND MOLECULAR PROPERTIES OF ISOPHTHALIC ACID
Isophthalic Acid consists of two carboxylic acid groups located in the meta position on the benzene ring; i.e., it has a 1,3-disabsorbed aromatic ring structure. Each carboxyl group undergoes a condensation reaction with glycols, resulting in the formation of ester bonds.
During polyesterization, the reaction takes place between the carboxyl and hydroxyl groups, as the water is removed as a by-product, the equilibrium proceeds in the direction of polymer formation. As the molecule is bifunctional, the chain can grow in two directions, and long chains of polyester are formed by the repeated reaction of a large number of acid and glycol molecules.
The benzene ring imparts aromatic rigidity to the polymer chain. This property can contribute to the resin's mechanical strength, thermal behavior, and chemical resistance properties.
Meta-link geometry prevents the chain from running completely linear. Terephthalic Acid tends to form a more linear and symmetrical structure, while Isophthalic Acid creates a geometric irregularity in the chain, which affects crystallization and solubility behavior.
In PET-based copolymers, its function is largely due to this structural irregularity. Reducing crystallization can improve optical clarity in some applications; it can also have an impact on melt processing behavior and thermal transitions.
In unsaturated polyester resins, it forms the aromatic and saturated dicarboxylic acid portion of the chain. Since it does not have double bonds in its structure, it does not participate in radical cross-linking with the reactive diluent; the double bonds required for cross-linking come from maleic anhydride or fumaric acid.
When used in combination with a hydrolysis-resistant glycol such as neopentyl glycol, the durability of gelcoat and outdoor polyesters can be improved. The NPG/PIA combination is therefore one of the frequently used approaches in high-performance unsaturated polyester chemistry.
PHYSICAL AND CHEMICAL PROPERTIES OF ISOPHTHALIC ACID
Isophthalic Acid exists as a white or colorless crystalline solid under normal conditions. The high melting temperature and very low room temperature volatility cause the product to be processed quite differently from low molecular weight liquid organic acids.
General technical specifications:
Physical appearance: White or colorless crystalline powder
Chemical character: Aromatic dicarboxylic acid
Molecular formula: C₈H₆O₄
Molecular weight: Approximately 166.13 g/mol
CAS No: 121-91-5
EC/EINECS No: 204-506-4
Functionality: 2 carboxylic acid groups
Melting point: Approx. 341–348 °C (ICSC: 345–348 °C) depending on the source
Density: Approx. 1.54 g/cm³
Water solubility: Very low in cold water; limited but higher in hot water
Volatility at room temperature: Negligible
Log Pow: Approx. 1.66
Autoignition temperature: Depending on the source, approx. 650–700 °C
Flash point: The classic liquid flash point approach is not applied for the solid product
Boiling behavior: May tend to sublimate at high temperature instead of normal boiling
The ICSC states that Isophthalic Acid in powder or granular form may present a dust explosion hazard when mixed with air. Therefore, dust accumulation and fine particle formation in the air must be controlled.
The high melting temperature makes it difficult to feed the feedstock as a low-temperature melt. This is why the glycol phase, temperature, and mixing system are important in polyester reactions; the reaction proceeds through chemical esterification, not just physical dissolution.
These values are for general technical demonstration purposes. Commercial product acceptance criteria should be determined according to manufacturer TDS and batch COA information.
HOW DOES ISOPHTHALIC ACID WORK AT THE MOLECULAR LEVEL?
Its main function is to form polyester bonds by reacting with hydroxyl function molecules through two carboxylic acid groups. In esterification with glycol, carboxyl and hydroxyl groups are combined, ester bonds are formed and water is released.
As water is removed from the system, the equilibrium shifts in the direction of high conversion; oligomers are formed first, followed by higher molecular weight polyester chains. Branching occurs when polyols carrying three or more hydroxyl groups are used; this property is used in the design of alkyd and thermosetting polyester resins.
The aromatic ring adds a rigid segment to the chain, contributing to mechanical strength, stiffness, and thermal properties. The meta-junction angle, on the other hand, limits regular packing; this effect is particularly important in altering the crystallization behavior in PET copolymers.
Its main functions at the resin level are:
Forming ester bonds with glycols and becoming part of the polyester chain
Providing aromatic rigidity
Building an unsaturated polyester backbone
To contribute to the improvement of water and chemical resistance
Contributing to outdoor resistance
Producing gelcoat resins
Changing crystallization in PET copolymers
Being the acid component in the synthesis of polyester polyol and alkyd resin
Being a monomer in the production of aromatic polyamide
Acting as ligands for coordination polymers
IN WHICH SECTORS IS ISOPHTHALIC ACID USED?
It is an important aromatic monomer, especially in sectors where polyester chemistry is used. Purified Isophthalic Acid grades can be preferred in the production of resins and polymers that require high purity.
Main areas of use:
Unsaturated polyester resins and gelcoats
Glass fiber reinforced plastics and marine composites
Corrosion-resistant composites, chemical storage tanks, and piping
Artificial marble and casting polyester resins
Paint, polyester coating and alkyd resins
Polyester polyols and polyurethane raw materials
PET copolymers, PET bottle resins, polyester sheet and film
Packaging polyesters and special polyester fibers
Production of aromatic polyamides (poly(m-phenylene isophthalamide))
Special engineering polymers and metal-organic coordination polymers
The strongest commercial area is unsaturated polyester resins; resins with higher water and chemical resistance are used as an alternative to orthophthalic systems when required. In PET and other thermoplastic polyesters, it often acts as a property regulator comonomer rather than a main monomer, and the level of use is determined by the final crystallization, optical and processing properties.
ISOPHTHALIC ACID'S UNSATURATED POLYESTER, GELCOAT, COMPOSITE, AND COATING APPLICATIONS
Unsaturated polyester resins are one of the most well-known uses. In these systems, together with the appropriate glycols and unsaturated acid or anhydrides, polyester forms the backbone.
Possible functions in unsaturated polyester applications:
Adding aromatic structure to the polyester spine
Improving water and hydrolysis resistance
Helping to increase chemical resistance
Supporting outdoor performance and mechanical strength
Preparing gelcoat, marine, tank, pipe and artificial marble resins
Isophthalic polyester resins can be used in applications where higher performance is targeted than general-purpose orthophthalic resins. In the marine industry, gelcoat and lamination resins need to withstand water, moisture, sunlight and temperature changes for long periods of time; therefore, they are evaluated in boat and marine composites.
The sterically conserved structure of neopentyl glycol promotes hydrolysis strength, while isophthalic acid contributes to the aromatic and meta-bonded polyester backbone. The NPG/PIA combination is therefore important in high-performance gelcoat chemistry.
In chemical storage tanks and pipes, the right type of polyester should be selected according to the chemical that the resin will come into contact with. Isophthalic polyester can provide better resistance than general-purpose resins; however, in very aggressive environments, different resin systems such as vinyl ester may be required.
The gelcoat surface appearance is also affected by the pigment, filler, thixotropic additive, UV stabilizer, and curing system; the use of isophytalic acid alone does not determine the entire outdoor performance.
In coating resins, it can be incorporated into the polyester or alkyd chain, contributing to the improvement of stiffness and chemical resistance properties. In polyester polyols, on the other hand, hydroxyl-tipped polyesters are prepared by reacting with glycols; these can be cross-linked with polyisocyanates and converted into polyurethane systems used in automotive, industrial maintenance, and specialized protective coatings.
USE OF ISOPHTHALIC ACID IN PET, COPOLYESTER, FIBER AND SPECIAL POLYMER APPLICATIONS
Its role in thermoplastic polyesters is different from its use in unsaturated polyesters. In PET systems, it is mostly used as a comonomer, replacing a portion of the parent dicarboxylic acid.
Standard PET consists largely of terephthalic acid and ethylene glycol units, and the regular money junction allows crystallization to a certain extent. Meta-linked isophthalate units disrupt this order; it becomes difficult to pack the molecules in the same order, and the crystallization rate decreases, increasing the amorphous character.
This effect is particularly useful for controlled copolymerization in some packaging and sheet applications where optical clarity is required. The processing temperature, crystallization behavior, and thermal transitions of copolyesters containing isophytalic acid may differ from those of the parent PET resin; therefore, the amount of comonomer should be determined according to the target properties.
The use of comonomers in polyester fibers can be evaluated in order to develop dyeability, amorphous structure or special thermal properties; the recipe varies according to the production technology used.
Another important area of use is the synthesis of aromatic polyamide. Poly(m-phenylene isophthalamide) can be obtained as a result of polycondensation with m-Phenylenediamine; this polymer class is among the special engineering materials that show high thermal resistance due to its aromatic backbone.
In laboratory and special materials chemistry, it can act as a binder in the synthesis of metal-organic networks and coordination polymers thanks to its two carboxylate groups.
IS ISOPHTHALIC ACID USED IN FOOD, COSMETIC AND PHARMACEUTICAL APPLICATIONS?
It is an industrial monomer mainly used in the production of polyester and specialty polymers. It is not directly used as a food additive, flavoring agent, or nutritional ingredient, and it does not have a FEMA, JECFA, or E number that directly refers to its use in food formulation.
The fact that PET or other polyester packaging can come into contact with food does not mean that the monomer is a direct food additive. Food contact suitability should be evaluated based on the final polymer composition, migration values and the legislation of the relevant country.
It is not a general-purpose formulation raw material in the cosmetics industry; the use of polyester in cosmetic packaging does not mean that monomeric isophthalic acid is used as a cosmetic ingredient.
In the pharmaceutical industry, it is not considered as a standard active substance or general pharmaceutical excipient. In pharmaceutical packaging or medical equipment polyesters, residual monomers and extractable substances should be checked separately on the final product.
INDUSTRIAL APPLICATIONS OF ISOPHTHALIC ACID
Thanks to its two carboxylic acid groups and aromatic structure, it is used as a reactive monomer in many polyester and special polymer systems.
Featured industrial functions:
Production of unsaturated polyester, gelcoat, marine and composite resin
Production of tanks, pipes and corrosion-resistant polyester
Production of artificial marble resin and polyester coating resin
Synthesis of alkyd resin and polyester polyol
PET copolymer production and modification of polyester crystallization
Production of transparent polyester, sheet, film and special fibers
Synthesis of aromatic polyamide, engineering polymer and coordination polymer
The acid/hydroxyl ratio is one of the basic calculation parameters in polyester synthesis; the functionality of monomers and their equivalents directly affect the target molecular weight and end group structure. Different glycols, such as ethylene glycol, propylene glycol, diethylene glycol, and neopentyl glycol, can form polyesters with different properties with the same acid.
In the production of unsaturated polyester, the amount of unsaturated acid or anhydride affects the cross-linking density, while isophytalic acid determines the durability and aromatic character of the resin backbone. The reaction temperature increases the esterification rate; however, excessive temperature increases the risk of color formation and side reactions.
The removal of water generated during the reaction is critical for the progress of the transformation. This is why reactor design, mixing and condensate removal system are important in industrial systems.
The main parameters to be monitored in industrial use:
Purity, acid value, moisture, color, ash and metal impurities
Particle size and melting behavior
Glycol type, purity, and moisture content
Acid/OH equivalent ratio
Reaction temperature, esterification time and water removal efficiency
Resin acid value, hydroxyl value, viscosity and color
Curing behavior, water resistance, chemical resistance and outdoor resistance
WHAT PROBLEMS CAN ISOPHTHALIC ACID HELP SOLVE?
It can contribute to the solution of various technical problems associated with the need for water resistance, chemical resistance, outdoor performance and controlled crystallization in polyester resins. Actual performance depends on the combined effect of all monomers used and process conditions.
Insufficient water resistance in orthophthalic polyester
General-purpose orthophthalic resins may not be sufficient for some high-performance applications in long-term water contact. Isophthalic acid-based backbone can be evaluated in systems aimed at higher hydrolysis and water resistance.
Low outdoor strength in gelcoat
Gelcoats are exposed to UV light, moisture, temperature change, and surface contaminants. Isophytalic acid and the appropriate glycol system can contribute to the development of a more durable polyester backbone.
Low resin resistance in chemical storage equipment
In tanks and pipes, resin can come into contact with various chemicals for a long time. Isophthalic polyester resins can be considered when higher chemical resistance is required than general-purpose systems.
Crystallization in PET is too fast
The regular terephthalate structure can crystallize quickly in some processes. When used as a comonomer, it helps control the crystallization rate by disrupting chain regularity.
Insufficient transparency in polyester sheet
High crystallinity can cause light scattering. Copolymerization with isophytalic acid can support improving optical clarity by increasing amorphous character in certain resin designs.
Need for hardness and strength in coating resin
The aromatic ring provides rigidity to the resin. Together with the appropriate glycol and other monomers, it can help adjust the stiffness and resistance profile of coating polyesters.
It should not be assumed that every grade will provide the same resin color or process behavior. Especially in high-purity PIA applications, color, moisture and impurity profile should be controlled on a batch basis.
THINGS TO CONSIDER WHEN USING ISOPHTHALIC ACID
In addition to chemical purity, particle properties, moisture and dust control are important in use. During the polyester reaction, the equivalent calculation, reaction temperature and water removal conditions must be carefully managed.
Usage considerations:
The CAS number and the desired grade of Purified Isophthalic Acid should be verified.
Purity, moisture, color, acid value and, if necessary, particle size, ash and metal impurities should be checked.
The glycol type should be selected correctly, glycol moisture should be controlled.
Acid/OH equivalent calculation should be made and other acid monomers should be taken into account.
The proportion of unsaturated monomers in unsaturated polyester should be controlled.
The reaction temperature should be kept controlled and the water removal system should be sufficient.
Resin acid value, viscosity and color development should be monitored.
Dust generation and accumulation should be minimized.
TDS, SDS and COA should be examined.
Terephthalic Acid or Phthalic Anhydride is not a direct equivalent of isophytalic acid. The resin formulation must be re-evaluated as the molecular geometry and reaction behavior have changed.
High reaction temperatures can accelerate polyesterization but increase color development; therefore, the temperature must be optimized according to the resin system used. The dispersion of fine dust into the air, on the other hand, must be considered in terms of both worker exposure and dust explosion.
INITIAL FORMULATION APPROACHES FOR ISOPHTHALIC ACID
The following examples are initial approaches designed to illustrate how Isophthalic Acid can be used in resin and process development. These are not verified commercial recipes or direct manufacturing instructions.
The percentages given are initial values by weight (w/w). The actual ratios should be optimized by laboratory trials according to the targeted acid value, glycol structure, molecular weight, unsaturation level and final product properties.
EXAMPLE 1: ISOPHTHALIC ACID BASED UNSATURATED POLYESTER RESIN SCREENING
Objective: To compare the effect of Isophthalic Acid ratio on water resistance, chemical resistance, viscosity and mechanical properties of resin.
Reactor charge (esterification stage, total charge = 100%):
Resin A (low isophytalic acid): Isophthalic Acid 18.1% / Maleic Anhydride 32.0% / Neopentyl Glycol 49.9%
Resin B (medium isophytalic acid): Isophthalic Acid 33.7% / Maleic Anhydride 19.9% / Neopentyl Glycol 46.4%
Resin C (high isophytalic acid): Isophthalic Acid 47.3% / Maleic Anhydride 9.3% / Neopentyl Glycol 43.4%
Additional Components:
Esterification catalyst (e.g., tin-based): Approximately 0.05% relative to total charge
Dilution: Prepared polyester is mixed at 65% and styrene at 35%; an inhibitor such as hydroquinone is added to styrene at about 0.01% relative to the total resin.
Curing system (during application): Approximately 1.5% MEKP and 0.2% cobalt octoate (6% Co) according to resin
Application approach:
Glycols and isophytalic acid are taken into the reactor, unsaturated anhydride is added, and esterification is initiated.
The resulting water is removed in a controlled manner.
During the reaction, the acid value is monitored and the resin is cooled when the target value is reached.
The resin is diluted in the same proportion as styrene, and the viscosity is measured.
Standard samples are cured.
Water absorption, chemical resistance, bending and tensile properties are compared.
The most balanced system is taken to composite tests.
EXAMPLE 2: ISOPHTHALIC ACID / NEOPENTYL GLYCOL GELCOAT FORMULATION SCREENING
Objective: To evaluate the effect of isophytalic acid and neopentyl glycol based polyester backbone on gelcoat water, stain and outdoor resistance.
The starting formula is (total = 100%):
IPA/NPG based unsaturated polyester resin (with styrene): 58.0%
Styrene (viscosity adjustment): 7.0%
Titanium dioxide (white pigment): 12.0%
Filler (calcium carbonate or talc): 18.5%
Fumed silica (thixotropic contribution): 2.0%
Wetting and dispersing additive: 0.8%
UV absorber: 0.8%
HALS (light stabilizer): 0.4%
Antifoam/air-releasing additive: 0.3%
Cobalt octoate (6% Co, promoter): 0.2%
Curing system (during application): Approximately 1.5–2.0% MEKP compared to gelcoat
Application approach:
Resin, styrene and additives are mixed; pigment and filler are distributed at high speed.
Viscosity and thixotropy are adjusted by adding fumed silica last.
For comparison, the resin part (58.0%) is replaced with orthophthalic gelcoat resin in the same recipe and a control sample is prepared.
It is applied in standard film thickness and prepared under the same curing conditions.
Initial brightness and color are measured, followed by water exposure and stain resistance.
In accelerated outdoor aging, color change, gloss retention and surface degradation are compared.
EXAMPLE 3: ISOPHTHALIC ACID COMONOMER SCREENING IN PET COPOLYESTER
Purpose: To evaluate the effect of isophythalic acid addition on polyester crystallization, optical appearance, and thermal behavior.
Esterification stage charge (total charge = 100%; ethylene glycol to total acid mole ratio approximately 1.2):
Control: Terephthalic Acid 69.0% / Isophthalic Acid 0% / Ethylene Glycol 31.0%
Sample 1 (2% of diacid IPA): Terephthalic Acid 67.6% / Isophthalic Acid 1.4% / Ethylene Glycol 31.0%
Sample 2 (5% of diacid IPA): Terephthalic Acid 65.6% / Isophthalic Acid 3.4% / Ethylene Glycol 31.0%
Sample 3 (10% of diacid IPA): Terephthalic Acid 62.1% / Isophthalic Acid 6.9% / Ethylene Glycol 31.0%
Additional Component:
Polycondensation catalyst (e.g. antimony-based): Approx. 200–300 ppm depending on charge
Application approach:
Mixtures of acids and glycols are esterified in the same conditions.
The total acid/glycol equivalent balance is maintained.
It is polymerized under the same polycondensation conditions.
Intrinsic viscosity is compared with molecular weight.
Glass transition temperature, melting and crystallization behavior are examined by DSC.
Transparency or blurriness is assessed.
By comparing the processing behavior, the appropriate comonomer level for the target application is determined.
SIMILAR PRODUCTS AND ALTERNATIVES TO ISOPHTHALIC ACID
When selecting an alternative, carrying only two carboxylic acid groups is not a sufficient comparison. The position of the carboxyl groups, aromatic or aliphatic structure, and functionality significantly alter the properties of the polyester chain. Below, products used for similar purposes with Isophthalic Acid are compared in terms of chain structure, performance effect, typical use, and interchangeability.
ISOPHTHALIC ACID AND TEREPHTHALIC ACID
Terephthalic Acid is an aromatic dicarboxylic acid that carries two carboxylic acid groups at position 1,4 in the benzene ring. Isophthalic Acid, on the other hand, is at position 1,3, i.e., in meta structure.
The more symmetrical money structure of Terephthalic Acid supports the regular packaging of polyester chains and provides high crystallinity. Therefore, PET is the main monomer of polyester fibers and bottle resins. Isophthalic Acid, on the other hand, reduces crystallization by creating geometric irregularities in the chain; It is preferred in unsaturated polyester resins to increase water and chemical resistance.
The two products are often used together rather than as alternatives to each other. In PET copolymers, Terephthalic Acid is the main acid and Isophthalic Acid is the comonomer. The process should be re-evaluated, as replacing each other in the same proportion will change the melting, dissolution and reaction behavior.
ISOPHTHALIC ACID AND PHTHALIC ANHYDRIDE
Phthalic Anhydride is the anhydride form of phthalic acid in the ortho position and is widely used in the production of general-purpose alkyd and unsaturated polyester. It is generally advantageous over Isophthalic Acid in terms of cost.
In terms of performance, orthophthalic polyesters may lag behind isophthalic systems in terms of water and chemical resistance. Isophthalic polyester systems, on the other hand, are for gelcoat, marine and tank applications where water, chemical and outdoor resistance is more important.
Phthalic Anhydride can be considered as a cost-oriented alternative; however, performance degradation may occur in applications with high strength requirements. Since the esterification of the dicarboxylic acid with the opening of the anhydride group creates different process profiles, the recipe cannot be transferred verbatim.
ISOPHTHALIC ACID AND ADIPIC ACID
Adipic Acid is an aliphatic dicarboxylic acid and gives the polyester chain a flexible segment. Isophthalic Acid, on the other hand, is aromatic and more rigid; it increases hardness and chemical resistance.
Adipic Acid; It stands out in coating systems where polyester polyol, alkyd and flexibility are important. Isophthalic Acid is used in systems aimed at hardness, water resistance and outdoor performance.
The two monomers are usually complementary raw materials, not alternatives. By using them together in the same recipe, the hardness-flexibility balance can be adjusted. One-to-one substitute glass can cause significant changes in transition temperature, viscosity and chemical resistance.
ISOPHTHALIC ACID AND MALEIC ANHYDRIDE / FUMARIC ACID
Maleic Anhydride and Fumaric Acid are unsaturated components and impart double bonds to unsaturated polyester resins that will later participate in radical cross-linking with styrene. Isophthalic Acid, on the other hand, is a saturated aromatic acid; it does not participate in cross-linking, determining the durability and aromatic character of the resin backbone.
Therefore, in unsaturated polyester resins, these raw materials are monomers that complement each other rather than being alternatives. The amount of Maleic Anhydride or Fumaric Acid affects cross-link density, while the level of Isophthalic Acid alters the backbone properties.
COMPARISON SUMMARY
In terms of rigidity and water resistance, Isophthalic Acid has a significant advantage over Phthalic Anhydride and Adipic Acid. In terms of crystallization, Terephthalic Acid forms the highest crystallinity, and Isophthalic Acid forms the most irregular chain structure. Adipic Acid stands out in systems where flexibility is required, Phthalic Anhydride stands out in general-purpose resins where cost is at the forefront, and Maleic Anhydride or Fumaric Acid stands out for cross-linking density.
In the selection of alternatives, the targeted performance, resin type, cost and process conditions should be evaluated together; laboratory trials should be performed for each substitute.
QUALITY CONTROL AND PURCHASING CRITERIA FOR ISOPHTHALIC ACID
When purchasing, it is not enough to compare only the CAS number and price. Especially in polyester production, purity, moisture, color and trace impurities can affect the quality of the final resin.
Recommended information to review before purchasing:
Product name, manufacturer, and whether it is Purified Isophthalic Acid quality
CAS and EC/EINECS number
Purity, acidity, moisture and color
Particle size, ash content and metal impurities
Organic trace impurities
Physical appearance and flow characteristics
Storage conditions and shelf life
Packaging type, net quantity, batch number, production date and origin information
TDS, SDS, COA and REACH information
COA shows the actual analysis results of a given batch; TDS is the standard specification, while SDS includes safe handling, dust control, storage, and emergency information. For PIA used in polyester production, it is useful to track color, moisture, and impurity values on a batch-by-batch basis.
Purity is especially important in light-colored resins; trace amounts of color-giving impurities can affect the final resin color in high-temperature polyesterization. Moisture affects the reaction calculation and flow behavior in storage; particle size affects feeding, dust formation and dispersion rate in glycol.
When switching to a new supplier, laboratory or pilot scale polyester synthesis should be performed to compare reaction time, resin color, acid value, viscosity and application performance.
STORAGE AND HANDLING OF ISOPHTHALIC ACID
Storage: It should be stored in dry, tightly closed packages and in the area protected from contamination. The product should be protected from cross-contamination caused by moisture, foreign particles and other raw materials.
Dust control: Dust can be generated in the air when processed into fine particles. Dust spread and dust accumulation on surfaces should be reduced as much as possible in feeding, discharging and conveying systems.
Fire and explosion: Although it is a solid with low volatility, the ICSC states that fine particles can form explosive powder mixtures with air. It is important to prevent dust accumulation and use appropriate electrical equipment.
Transmission: Dry, clean and appropriate equipment should be used, and the metal should be protected from other acids or sources of colorant contamination.
Transport: Packaging formats and transport classification may vary by product and region; up-to-date information should be checked from the supplier's SDS document.
ISOPHTHALIC ACID SAFETY INFORMATION
It is a crystalline solid with low volatility and according to current ICSC information, there is no general hazard classification under the GHS criteria. However, dust exposure and the risk of dust explosion should not be ignored.
The ICSC states that mechanical irritation to the eyes may occur with short-term exposure, and fine dust can quickly reach disturbing concentrations in the air. The Sigma-Aldrich SDS information, on the other hand, recommends storing the product dry and tightly closed, avoiding dust inhalation, and limiting dust generation in the spill.
Dust control and appropriate equipment selection are important in industrial transfer, grinding or feeding processes. In safety applications, the current SDS certificate of the purchased product should be taken as a basis.
WHAT TO CONSIDER WHEN BUYING ISOPHTHALIC ACID?
In the procurement, it should first be determined in which polymer or resin system the product will be used. The quality level required for unsaturated polyester, gelcoat, PET copolymer or laboratory synthesis may not be the same. If required in the demand for industrial polyester production, the quality expectation of "Purified Isophthalic Acid" or "PIA" should be clearly stated.
Things to clarify before purchasing:
Use sector and end application
Purified Isophthalic Acid requirement and minimum purity
Moisture, color, acidity and particle size limits
Ash, metal and organic impurity limits
Manufacturer and origin preference
Packaging need and estimated order quantity
TDS, SDS, COA and REACH certificates
If substituting with an existing product, current TDS and COA information
Price; It varies depending on purity, manufacturer, origin, PIA quality, order volume, packaging and current raw material-logistics conditions. Comparison based solely on ton or kilogram price may not show quality differences such as resin color and process yield.
ISOPHTHALIC ACID SUPPLY FROM ATAMAN KIMYA
Isophthalic Acid; It is an aromatic dicarboxylic acid used in the production of unsaturated polyester, gelcoat, composite, PET, coating, alkyd, polyester polyol and special polymers. It is important for companies planning to purchase the product to determine the purity and quality criteria suitable for the area of use before ordering.
Companies that want to get information about the supply of Isophthalic Acid from Ataman Kimya can request information about product suitability, current supply conditions, packaging options and price offers by sharing their usage areas and target technical specifications.
The current stock status, manufacturer, origin, Purified Isophthalic Acid quality class, purity, packaging and other technical specifications of the product should be confirmed from Ataman Kimya before ordering.
Recommended information to be shared in the request for proposal:
Sector of use, end application, and type of resin or polymer to be produced
Purified Isophthalic Acid requirement and minimum purity
Moisture, color, acidity and particle size requirements
Estimated order quantity and desired packaging type
Requested technical documentation and TDS or COA information of the current product, if applicable
FREQUENTLY ASKED QUESTIONS ABOUT ISOPHTHALIC ACID
What is Isophthalic Acid?
It is an aromatic dicarboxylic acid that carries two carboxylic acid groups in the commodity position in the benzene ring. It is used in the production of polyester, unsaturated polyester, PET copolymer and different special polymers.
What is the Turkish name of Isophthalic Acid?
It is called Isophytalic Acid.
What is the CAS number of Isophthalic Acid?
CAS No is 121-91-5.
What is the EC number of Isophthalic Acid?
EC/EINECS No is 204-506-4.
What is the molecular formula and molecular weight of Isophthalic Acid?
Its formula is C₈H₆O₄, and its molecular weight is approximately 166.13 g/mol.
What does PIA mean?
It is the common industrial abbreviation for Purified Isophthalic Acid.
Is Isophthalic Acid the same chemical as Purified Isophthalic Acid?
They are based on the same basic molecule. The term PIA usually refers to the high-purity commercial grade used in the production of polyester and polymers.
What is the melting point of Isophthalic Acid?
Depending on the source and measurement conditions, values are given in the range of approximately 341–348 °C.
Is Isophthalic Acid soluble in water?
It has very low solubility in cold water. Its solubility increases in hot water but is still limited.
What is the density of Isophthalic Acid?
It is approximately 1.54 g/cm³.
Why is Isophthalic Acid used in the production of polyester?
Two carboxylic acid groups are esterified with glycols, resulting in the formation of polyester chains.
What is Isophthalic polyester?
It is a class of unsaturated polyester resins in which isophystantic acid is one of the acid monomers. It can be used in applications where higher durability is targeted than general-purpose orthophthalic polyesters.
Is Isophthalic Acid used in gelcoat and marine applications?
Yes; it can be used especially in gelcoat and boat and marine composites that require outdoor and water resistance.
Is Isophthalic Acid used in PET how does it affect crystallization?
It is used as a comonomer. Metalinked isophthalate units can alter crystallization behavior by disrupting regular terephthalate chain packing.
Can Isophthalic Acid make polyester more transparent?
In certain copolyester designs, reducing crystallization can contribute to improving optical clarity. The effect depends on the amount of comonomer and the process.
Is Isophthalic Acid and Terephthalic Acid the same?
Isophthalic Acid is 1,3-meta, while Terephthalic Acid is a 1,4-para-linked dicarboxylic acid.
What is the difference between Isophthalic Acid and Phthalic Anhydride?
Phthalic Anhydride is an anhydride of orthophthalic structure and is widely used in general-purpose polyester and alkyds. Isophthalic Acid is a meta dicarboxylic acid and can be evaluated in high-performance polyesters.
Do Isophthalic Acid and Maleic Anhydride do the same job?
Isophthalic Acid affects the strength properties of the polyester backbone, while Maleic Anhydride provides the necessary double bonds for cross-linking.
Is Isophthalic Acid used in the production of polyester polyol and alkyd?
Yes; polyester polyols and certain alkyd resins can be prepared by reacting with glycols, which can be used in polyurethane systems.
Can Isophthalic Acid be used in the production of aromatic polyamide?
It can be used in the preparation of poly(m-phenylene isophthalamide) type aromatic polyamides by polycondensation with m-Phenylenediamine.
Can Isophthalic Acid powder pose a hazard?
Dust inhalation should be limited and it should be taken into account that fine particles can produce a dust explosion with air. The ICSC defines the product as a flammable solid.
What is the commercial purity of Isophthalic Acid?
It varies with quality. Laboratory grade products may have 99% assay; the actual specification for industrial Purified Isophthalic Acid should be checked from the manufacturer's TDS.
What specifications and documents are important when purchasing Isophthalic Acid?
Purity, moisture, color, acid value, particle structure and application-related impurity limits are important. TDS, SDS and COA are basic documents; REACH and other regulatory documents may also be requested depending on the application.
How should Isophthalic Acid be stored?
It should be stored in dry and clean conditions, in tightly closed packaging, protected from contamination and dust spread.
ISOPHTHALIC ACID – TECHNICAL SUMMARY
Isophthalic Acid, known as Isophthalic Acid in Turkish, is an aromatic dicarboxylic acid with the formula C₈H₆O₄. It is defined by CAS No 121-91-5 and EC No 204-506-4, with a molecular weight of approximately 166.13 g/mol.
Two carboxylic acid groups are present at positions 1 and 3 in the benzene ring. This metalinkage forms polymer chain properties that differ from Phthalic Acid's ortho and Terephthalic Acid's para geometry.
It esterifies with glycols to form polyester bonds; thanks to its two carboxylic acid functions, it can be used as a monomer in the production of unsaturated polyester, alkyd, polyester polyol, PET copolymer and other special polyesters.
Unsaturated polyester resins are especially important for products with improved water, chemical and outdoor resistance. Marine gelcoats, glass fiber reinforced composites, tanks, pipes and corrosion-resistant polyester systems are among the main applications.
Unsaturated polyester gelcoats prepared with a combination of Neopentyl glycol and Purified Isophthalic Acid can be evaluated in applications targeting high weather conditions, water and stain resistance.
In PET and other copolyesters, regular terephthalate helps control crystallization behavior by altering the chain structure; this property can be used to adjust transparency, processability, and thermal properties.
Under normal conditions, it is a crystalline solid and shows a high melting temperature of about 341–348 °C. Although its volatility at room temperature is very low, its fine powder can form an explosive mixture with air.
Purity, moisture, color, acid value, particle structure and application-specific impurity limits should be checked when purchasing. If Purified Isophthalic Acid quality is desired, this requirement should be clearly stated in the request.
The product should be kept in dry, clean and closed packaging; contamination and dust formation in the air during storage and transfer should be limited. For safe use, the current SDS document of the purchased product should be taken as basis.
It is recommended that companies planning to supply Isophthalic Acid from Ataman Kimya confirm the current product and offer conditions by sharing the usage area, Purified Isophthalic Acid quality requirement, target purity, moisture and color values, packaging quantity and technical document needs.
