Measuring Relative Viscosity of Polyamide: Methods, Standards & Molecular Weight
What Is Relative Viscosity in Polyamides?
Relative viscosity (RV) is a measure of the molecular weight and chain length of polyamide (nylon) polymers, expressed as the ratio of the viscosity of a polyamide solution to the viscosity of the pure solvent under identical conditions. It is one of the most widely used molecular weight indicators for polyamides in industrial quality control and is directly correlated with the mechanical properties (tensile strength, impact resistance, elongation) and melt processing behavior of the polymer.
Relative viscosity measurement is a fundamental quality control test for polyamide manufacturers and processors across the textile fiber, engineering plastics, automotive, and packaging industries.
Why Relative Viscosity Matters for Polyamides
The molecular weight (and hence relative viscosity) of a polyamide determines:
- Melt viscosity and processability: Higher RV → higher melt viscosity → greater injection pressure and processing temperature required
- Tensile and impact strength: Mechanical properties increase with molecular weight up to a plateau, governed by entanglement density
- Fiber tenacity: Textile PA6 and PA6.6 fibers require specific RV ranges for desired fiber strength and drawing behavior
- Film and packaging properties: Barrier and mechanical properties of PA films depend on controlled molecular weight
A polyamide with RV outside its specified range will either process poorly or deliver inferior mechanical performance—making RV one of the first tests performed on incoming polyamide raw material.
Measuring Relative Viscosity: Standard Methods
ISO 307: Polyamides—Determination of Viscosity Number
The most widely used international standard for polyamide solution viscosity. A solution of known concentration (0.5 g/100 mL or 1.0 g/100 mL) is prepared in a specified solvent. The flow time of the solution and pure solvent through a calibrated Ubbelohde or Cannon-Fenske capillary viscometer at 25°C is measured. Relative viscosity is calculated as:
η_rel = t_solution / t_solvent
Where t_solution and t_solvent are the flow times of the solution and pure solvent.
Viscosity number (VN) is then calculated from η_rel and concentration using the Huggins or Schulz-Blaschke equation.
ASTM D789: Relative Viscosity of Polyamide (Nylon) in Formic Acid
The standard method used in North America for PA6.6 and PA6 quality control. A solution of 11 g polyamide in 100 mL 90% formic acid is prepared and its viscosity measured at 25°C using a calibrated Cannon-Fenske or Ubbelohde viscometer. Relative viscosity is reported as the ratio of solution to solvent flow time.
Typical RV ranges by application:
| PA6.6 Application | Typical RV (ASTM D789) |
| Textile yarn | 38–50 |
| Technical yarn | 50–65 |
| Injection molding | 45–60 |
| Film and packaging | 45–55 |
| High-impact molding | >60 |
Sulfuric Acid Solvent Method (ISO 307 for PA6)
For polyamide 6, 96% sulfuric acid is used as the solvent (ISO 307). This method is more hazardous than formic acid but provides accurate RV for PA6 grades.
Relationship Between RV, Molecular Weight, and Properties
Relative viscosity is related to intrinsic viscosity [η] and to weight-average molecular weight (Mw) through the Mark-Houwink-Sakurada equation:
[η] = K × Mw^a
Where K and a are polymer/solvent/temperature-specific constants. For routine quality control, RV is used directly rather than converting to absolute Mw, since RV correlates empirically with the properties of interest.
Why Choose Infinita Lab for Polyamide Viscosity Testing?
Infinita Lab offers relative viscosity testing per ASTM D789 and ISO 307 for PA6, PA6.6, PA12, and other polyamide grades. Our accredited laboratory network provides precise solution viscosity measurements with fast turnaround for raw material qualification and production quality control programs.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
What is the difference between relative viscosity (RV), inherent viscosity (IV), and intrinsic viscosity ([η])? Relative viscosity (RV or η_rel) is the ratio of solution to solvent flow time—dimensionless. Inherent viscosity (IV or η_inh) is ln(η_rel)/c, where c is concentration. Intrinsic viscosity ([η]) is the limiting value of reduced viscosity as concentration approaches zero—the most rigorous molecular weight indicator. For routine QC, RV is used; for absolute molecular weight determination, [η] extrapolated to zero concentration is required.
Why does moisture affect polyamide relative viscosity measurements? Polyamides are hygroscopic and undergo hydrolytic degradation (chain scission) in the presence of moisture, reducing molecular weight and RV. Samples must be dried (typically at 80°C under vacuum for 16–24 hours) before RV measurement to remove absorbed moisture. Undried samples give artificially low RV readings.
Can relative viscosity predict the mechanical properties of molded PA parts? RV correlates positively with tensile strength, notched impact resistance, and elongation at break in molded polyamide parts within a given grade family. However, additives (glass fiber, impact modifiers, stabilizers) also significantly affect properties, so RV alone is not sufficient to predict finished part performance—it is one of several material qualification criteria.
What capillary viscometer is recommended for ASTM D789 testing? ASTM D789 requires a Cannon-Fenske or equivalent calibrated capillary viscometer with a flow time for the solvent (90% formic acid) of 90–130 seconds. The viscometer must be calibrated and the flow time ratio (solution/solvent) calculated from triplicate measurements for each.
Is relative viscosity measurement applicable to other engineering polymers besides polyamide? Yes. Solution viscometry is also used for PET (intrinsic viscosity per ASTM D4603 in phenol/1,1,2,2-tetrachloroethane), polycarbonate, and cellulose derivatives. Each polymer has specific solvents, concentrations, and temperature conditions optimized for that polymer type. ASTM D789 is specifically for polyamide; different standards apply to other polymers.
