Dioctyl terephthalate (DOTP) is a widely used non-orthophthalate plasticizer whose quality is controlled in China by the national standard GB/T 26509-2011. This article explains the GB testing framework for DOTP, covering sample types, specification items such as purity, color, acidity, moisture, flash point and volume resistivity, and the corresponding standardized test methods. It is intended as a practical reference for manufacturers, formulators, and accredited third-party laboratories performing DOTP quality control and compliance testing.
Technical Overview
Dioctyl terephthalate (DOTP, CAS No. 6422-86-2), chemically known as bis(2-ethylhexyl) terephthalate with the molecular formula C24H38O4 and a molecular weight of approximately 390.6 g/mol, is a primary plasticizer for polyvinyl chloride (PVC) and other polymers. Because the terephthalate ester groups occupy the para positions of the benzene ring, DOTP is structurally distinct from ortho-phthalate plasticizers such as DOP (dioctyl phthalate), and it is increasingly favored in applications requiring lower regulatory risk, better low-temperature flexibility, and higher electrical resistivity.
In China, the quality of industrial DOTP is governed by the national standard GB/T 26509-2011, "Dioctyl terephthalate (DOTP)," issued by the Standardization Administration of China (SAC). The standard defines product classification, technical requirements, test methods, inspection rules, and requirements for marking, packaging, transport, and storage. The test methods referenced in the standard are drawn from the GB/T series of general chemical product test method standards, and results can generally be correlated with equivalent ISO and ASTM methods for international trade purposes.
- Product standard: GB/T 26509-2011 (industrial DOTP)
- Key physicochemical identity parameters: density of approximately 0.981–0.985 g/cm3 at 20 °C, boiling point above 390 °C, and flash point typically above 210 °C
- Complementary method standards: GB/T 3143, GB/T 4472, GB/T 1668, GB/T 1669, GB/T 1671, GB/T 1672, GB/T 3536, and GB/T 6283
Testing Samples
Neat DOTP Product Samples
The most common sample type is the plasticizer itself, taken from production batches, storage tanks, drums, or bulk shipments. Sampling should follow the inspection rules of GB/T 26509-2011, which specify representative sampling from containers, mixing to ensure homogeneity, and collection in clean, dry, sealed glass bottles protected from light and moisture. A typical submission of 500 mL to 1 L is sufficient for a full specification test suite.
PVC Compounds and Finished Polymer Articles
Finished products such as cable insulation, artificial leather, films, and molded parts may be analyzed to determine plasticizer content, composition, or migration behavior. In these cases, the plasticizer is typically extracted with organic solvent and quantified by gas chromatography, following the analytical principles of related GB method standards for phthalate-type esters in plastics and consumer products.
Environmental and Food-Contact Matrices
For compliance screening in food-contact materials, toys, or environmental samples, trace-level determination of DOTP is performed by gas chromatography-mass spectrometry (GC-MS) after solvent extraction, consistent with the GC-MS based approaches used in the corresponding GB standards for ester plasticizer determination.
Testing Items
GB/T 26509-2011 establishes technical requirements for DOTP that are grouped into appearance and physicochemical indicators. Products are commonly classified into grades (for example, superior and first grade), each with corresponding limit values. Typical specification items include:
- Appearance: transparent oily liquid free of visible suspended matter and impurities
- Color (platinum-cobalt, Hazen units): typically limited to 50 or below for high grades
- Density at 20 °C: reference range of approximately 0.981–0.985 g/cm3
- DOTP purity (ester content): typically required at 99.0% or above for superior grades
- Acidity, calculated as terephthalic acid: typically limited to very low levels on the order of hundredths of a percent
- Moisture (water content): typically 0.05% or below
- Flash point: typically 210 °C or above
- Volume resistivity: an electrical-quality indicator commonly on the order of 10^10 Ω·m at 20 °C
- Heat loss on heating: an indicator of volatile impurities such as residual 2-ethylhexanol
Exact limit values for each grade should always be verified against the current official text of GB/T 26509-2011, as only the published standard is normative.
Testing Methods
Appearance and Color Determination
Appearance is assessed visually by inspecting the sample in a color-comparison tube against a white background under transmitted light. Color is quantified as platinum-cobalt (Pt-Co, Hazen) color numbers according to GB/T 3143, which prescribes visual comparison of the sample against standard color reference solutions in matched tubes.
Density Determination
Density at 20 °C is measured in accordance with GB/T 4472, using either a calibrated pycnometer, a density hydrometer, or a digital oscillating-U-tube density meter. Temperature control at 20.0 ± 0.1 °C is critical because ester plasticizers exhibit a measurable density temperature coefficient.
Acidity Determination
Acidity or acid value is determined by GB/T 1668, an alkalimetric titration method for plasticizers. A weighed sample is dissolved in a neutralized ethanol-ether solvent mixture and titrated with standardized sodium hydroxide solution to the phenolphthalein endpoint. The result may be expressed as an acid value in mg KOH/g or recalculated as percent terephthalic acid, reflecting residual unreacted acid from the esterification process.
Moisture Determination
Water content is determined by the Karl Fischer method according to GB/T 6283, using either volumetric or coulometric titration depending on the expected water level. Accurate moisture data are important because excessive water degrades electrical properties and can promote hydrolysis during processing.
Purity and Ester Content by Gas Chromatography
The DOTP content (purity) is determined by gas chromatography as described in the method annex of GB/T 26509-2011. A capillary column with a non-polar or medium-polarity stationary phase and a flame ionization detector (FID) is used, typically with an internal standard for quantification. This method simultaneously resolves residual 2-ethylhexanol, monoesters, and by-product esters such as DOP, providing a full impurity profile of the product.
Flash Point Determination
The flash point is determined by the Cleveland open cup method according to GB/T 3536 or the plasticizer-specific flash point method GB/T 1671, both of which are harmonized in principle with ISO 2592. The sample is heated at a controlled rate and a test flame is passed over the surface until a flash is observed; the result is relevant to safety classification and processing stability.
Volume Resistivity Determination
Volume resistivity is measured according to GB/T 1672, the standard method for liquid plasticizers, using a guarded concentric-electrode cell and a high-resistance measurement system, typically at 20 °C and often supplemented at elevated temperature. This parameter is a decisive quality criterion for DOTP used in wire and cable insulation compounds.
Heat Loss Determination
Applications
DOTP verified through GB-standard testing is used across a broad range of industries where plasticizer purity, electrical performance, and regulatory acceptability are required:
- Wire and cable insulation and sheathing compounds requiring high volume resistivity and heat resistance
- Automotive interior components, sealants, and films requiring low volatility and fogging performance
- Synthetic leather, coated fabrics, flooring, and wallcovering materials
- Medical devices and food-contact articles where non-orthophthalate plasticizers are preferred
- Toys and childcare articles, supported by chemical compliance testing of the finished polymer matrix
For manufacturers exporting products, GB/T test results can be presented alongside ISO- or ASTM-based data from accredited third-party laboratories to demonstrate equivalence and support global market access.
FAQ
Which Chinese national standard applies to DOTP testing?
GB/T 26509-2011 is the current national standard for dioctyl terephthalate. It defines the technical indicators, the referenced test method standards, and the rules for factory inspection and acceptance testing.
Is DOTP the same as DOP?
No. Although both are di(2-ethylhexyl) esters, DOP is the ortho-phthalate (di-2-ethylhexyl phthalate) while DOTP is the para isomer (terephthalate). Gas chromatographic purity testing under GB methods clearly separates the two, which is essential because many international regulations restrict ortho-phthalates but not DOTP.
What instruments are required for a full GB specification test?
A typical laboratory setup includes a gas chromatograph with FID for purity, a Karl Fischer titrator for moisture, a Cleveland open cup flash point apparatus, a density meter or pycnometer, Pt-Co color comparison equipment, an acid-base titration station, and a high-resistance meter with a liquid resistivity cell.
How much sample is needed and what is a typical turnaround time?
A full specification suite usually requires 500 mL to 1 L of sample. Routine testing at accredited third-party laboratories is generally completed within five to ten working days, depending on the scope and whether GC-MS impurity identification is added.
Can GB/T results be compared with ASTM or ISO results?
Yes. Most GB/T methods are technically aligned with international counterparts, such as ASTM D1045 for plasticizer testing, ASTM D92/ISO 2592 for flash point, and ASTM D4052/ISO 12185 for density. Method-specific deviations should be confirmed with the testing laboratory before reporting dual-standard results.
