The Tu-4 cup viscosity test (also written as T-4 flow cup or coating cup No. 4) is a rapid efflux viscometry method standardized in GB/T 1723 and widely used to measure the viscosity of paints, coatings, inks, adhesives and related fluids. By timing how long a fixed volume of liquid takes to discharge through a calibrated 4 mm orifice, manufacturers can control batch consistency, verify application properties and confirm compliance with product specifications. This article explains the principle, instrumentation, operating procedure, reporting requirements and practical applications of the Tu-4 cup method.
Technical Overview
The Tu-4 cup is an efflux-type (flow cup) viscometer specified in the Chinese national standard GB/T 1723, "Determination of viscosity of paints". The instrument consists of a conical or cylindrical cup, typically made of brass or stainless steel, with a nominal capacity of 100 mL and a precision-machined orifice of 4 mm diameter at the bottom. The test result is expressed as a conditional viscosity: the efflux time, in seconds, required for 100 mL of liquid at a controlled temperature to flow completely out of the cup. The method is best suited to Newtonian or near-Newtonian liquids with efflux times typically between 10 s and 150 s; outside this range, a different orifice size or another viscometer principle should be selected.
Functionally, the Tu-4 cup belongs to the same family of instruments as the Ford cup described in ASTM D1200 and the ISO flow cups described in ISO 2431 (adopted in China as GB/T 6753.4). However, cup geometry, orifice length and calibration differ between these systems, so efflux times obtained with one cup cannot be directly compared with another without using recognized correlation tables. For most routine quality control purposes, the Tu-4 efflux time in seconds is used as the acceptance criterion itself.
Testing Samples
Paints and Coatings
Solvent-borne coatings such as alkyd paints, nitrocellulose lacquers, primers, sealers, varnishes, enamels and general industrial finishes are the primary application of the Tu-4 cup. The method is most reliable for clear, bubble-free liquids whose efflux time falls within the working range of the cup.
Printing Inks and Ink Vehicles
Low-to-medium viscosity liquid inks, including gravure and flexographic ink systems, ink vehicles and solvent dispersions, can be evaluated with the Tu-4 cup for press-readiness and batch-to-batch consistency.
Adhesives, Resins and Binders
Free-flowing adhesives, unsaturated polyester resins, alkyd and acrylic resins, and other binder solutions may be tested, provided they are homogeneous, stable and do not gel or string during outflow.
Other Liquids
The cup can also be applied to chemical solutions, cleaners, oils and similar near-Newtonian fluids when a quick comparative viscosity check is required in a production or field environment.
Testing Items
Typical items evaluated in a Tu-4 cup viscosity test program include:
- Conditional viscosity, reported as efflux time in seconds, determined in accordance with GB/T 1723.
- Comparison of measured efflux time against product specification limits or customer requirements.
- Batch-to-batch consistency checks for incoming materials, in-process intermediates and finished products.
- Approximate conversion of efflux time to kinematic viscosity (mm²/s) using recognized correlation tables, when required.
- Verification of temperature influence on viscosity for products with strict application windows.
Testing Methods
The standard operating procedure per GB/T 1723 is summarized below. The test is normally performed at 25 °C ± 1 °C, and temperature control is critical because viscosity changes measurably with even small temperature deviations.
- Condition the sample to the specified test temperature and, if necessary, strain it to remove skins, gels or coarse particles.
- Clean the cup and orifice with a suitable solvent and dry them completely; verify that no residue blocks the orifice.
- Place the cup level on its stand and seal the orifice with a finger.
- Pour the sample into the cup until it overflows the rim, ensuring the cup is completely filled without entrapped air.
- Draw a clean glass plate across the rim to remove the excess liquid and any surface bubbles.
- Release the orifice and simultaneously start the stopwatch; observe the emerging stream.
- Stop the timer at the moment the stream first breaks, indicating that the liquid level has reached the orifice.
- Record the efflux time; repeat the determination and report the average, together with the test temperature and cup identification.
Good practice includes periodic calibration of the cup against certified standard viscosity oils, avoiding thixotropic or heavily loaded suspensions that shear-thin during outflow, and cleaning the cup immediately after each measurement to prevent orifice buildup. Duplicate determinations should agree within the repeatability limits of the standard; results that exceed these limits indicate poor temperature control, bubbles or partial blockage and should be retested.
Applications
The Tu-4 cup viscosity test is used throughout the coatings and related industries because it is fast, inexpensive and directly interpretable on the shop floor. Typical applications include:
- Production quality control of paints, varnishes and lacquers, where efflux time is a routine release parameter.
- Incoming inspection of raw materials such as resins, solvents and semi-finished intermediates.
- Application tuning, for example adjusting spray, dip, brush or roller application consistency by solvent addition.
- Research and development, where formulators track the viscosity impact of resin content, solvent balance and additive level.
- Field and on-site checks by applicators and contractors before coating application.
- Support of compliance testing in accredited third-party laboratories for product certification and customer dispute resolution.
FAQ
What is the difference between the Tu-4 cup and the ISO flow cup?
Both are efflux cups, but they differ in geometry and calibration. The Tu-4 cup is defined by GB/T 1723 with a 100 mL capacity and a 4 mm orifice, whereas ISO 2431 (GB/T 6753.4) specifies flow cups with 3 mm, 4 mm, 5 mm and 6 mm orifices of different dimensional design. Efflux times from different cup systems must be compared only through recognized conversion tables.
At what temperature should the test be performed?
GB/T 1723 specifies a test temperature of 25 °C ± 1 °C for the Tu-4 cup. ISO 2431 flow cup tests are commonly performed at 23 °C. The actual test temperature must always be reported together with the result, since viscosity is strongly temperature dependent.
Can the Tu-4 efflux time be converted to millipascal-seconds?
Only approximately. The efflux time is a conditional viscosity, and published correlation tables allow an estimation of kinematic viscosity in mm²/s. For accurate dynamic viscosity in mPa·s, a rotational viscometer method such as ISO 2884 (GB/T 9751) should be used.
Is the method suitable for all liquids?
No. The Tu-4 cup is appropriate for Newtonian or near-Newtonian liquids within its 10–150 s working range. Highly thixotropic, shear-thinning or gel-like materials, such as many emulsion paints, are better characterized with a Stormer viscometer per GB/T 9269 (ASTM D562) or a rotational viscometer.
What are common sources of error?
Frequent errors include poor temperature control, air bubbles in the filled cup, partial blockage or residue in the orifice, unlevel cup positioning, delayed or anticipated timing, and inaccurate endpoint judgment. Consistent technique and regular calibration with standard viscosity oils minimize these errors.