The separation softening point difference test is the principal method for evaluating the hot storage stability of polymer modified asphalt binders, especially SBS-modified bitumen. Conducted by accredited third-party laboratories in accordance with ASTM D7173, ASTM D36, EN 13399, and the Chinese highway standard JTG E20 (T 0661), the test measures the difference between the ring-and-ball softening points of the top and bottom portions of a binder specimen after prolonged high-temperature conditioning. This article explains the principle, apparatus, procedure, calculation, acceptance criteria, and practical applications of this determination.
Technical Overview
Polymer modified asphalt (PMA), particularly styrene-butadiene-styrene (SBS) modified bitumen, is produced by dispersing a polymer phase into base asphalt through high-shear milling. Because the polymer and the base binder differ in density and chemical compatibility, the polymer-rich phase tends to migrate upward when the material is held in heated storage tanks. This phenomenon, known as separation or segregation, causes the upper layer of the stored binder to become polymer-rich while the lower layer becomes polymer-lean, resulting in non-uniform performance across the tank.
The separation softening point difference quantifies this instability. A specimen of modified asphalt is conditioned vertically in a closed aluminum tube (commonly called a cigar tube) at 163 °C for 48 hours to simulate hot storage. The solidified specimen is then divided into three equal sections, and the ring-and-ball softening points of the top and bottom sections are determined separately. The difference between the two values serves as the index of storage stability: a small difference indicates a homogeneous, stable binder, while a large difference indicates a strong tendency to separate.
The test is governed by several recognized standards, including ASTM D7173 (Standard Practice for Determining the Separation Tendency of Polymer from Polymer Modified Asphalt), ASTM D36 (softening point by ring and ball), EN 13399 (storage stability of polymer modified bitumen), and the Chinese highway standard JTG E20-2011 (T 0661) together with the specification JTG F40-2004. In most specifications for SBS-modified asphalt, the separation softening point difference must not exceed 2.5 °C.
Testing Samples
SBS-Modified Asphalt Binders
Thermoplastic elastomer (SBS) modified asphalt is the primary application of this test. SBS-modified binders are widely used in surface courses, intersections, bridge deck pavements, and heavy-traffic roadways, where their improved elasticity, high-temperature rutting resistance, and low-temperature cracking resistance depend on a uniform polymer dispersion.
Other Polymer-Modified Binders
The method can also be applied to binders modified with polyethylene (PE), ethylene-vinyl acetate (EVA), or styrene-butadiene rubber (SBR), subject to the requirements of the governing specification. For crumb rubber modified asphalt and chemically curing systems such as epoxy asphalt, the separation mechanism differs, and alternative criteria or modified procedures are typically specified instead.
Sample Requirements and Submission
A minimum of approximately 1 kg of binder is recommended so that replicate tube tests and retests can be performed. Before testing, the sample is heated gently to a fluid state, typically between 150 °C and 175 °C, and stirred to ensure homogeneity. Direct flame heating, local overheating, and prolonged exposure at high temperature must be avoided, as they can alter the polymer network and distort the test result.
Testing Items
The determination comprises the following reported items, each expressed in degrees Celsius:
- Softening point of the top section of the conditioned specimen (ring and ball method)
- Softening point of the bottom section of the conditioned specimen (ring and ball method)
- Separation softening point difference, calculated as the difference between the top and bottom values
- Comparison of the difference against the specification limit, typically a maximum of 2.5 °C for SBS-modified asphalt
- Supplementary observations, such as visible polymer clots, gel formation, or abnormal flow behavior during specimen preparation
Testing Methods
Apparatus and Materials
The test requires a vertical aluminum tube with an inner diameter of approximately 25 mm and a height of approximately 140 mm, closed at one end; a forced-ventilation oven capable of maintaining 163 ± 5 °C; a refrigerator or freezer for solidifying the specimen; a cutting device for sectioning the tube; and a complete ring-and-ball softening point apparatus conforming to ASTM D36, EN 1427, or JTG E20 (T 0606), including standard rings, balls, a bath, and a calibrated thermometer.
Sample Conditioning Procedure
- Heat the modified asphalt sample with continuous stirring until fully fluid and homogeneous, taking care to avoid overheating or entraining air bubbles.
- Pour approximately 50 g of the sample into the upright aluminum tube, taking care that the sample does not contact the tube walls above the fill line.
- Close and crimp the open end of the tube to seal it, then wipe the exterior clean.
- Place the tube vertically in the oven at 163 °C and maintain it for 48 hours without disturbance.
- Remove the tube, allow it to cool briefly, then chill it until the specimen is completely solidified, typically for at least 4 hours.
- Cut the tube into three sections of equal height and separate them; retain the top and bottom sections and discard the middle section.
Softening Point Determination
The top and bottom sections are melted separately in an oven at an appropriate temperature, such as 163 °C, with gentle stirring until sufficiently fluid. Each portion is then tested for softening point using the ring-and-ball method: a steel ball is placed on a bitumen disk cast in a brass ring, and the assembly is heated in a water or glycerol bath at a controlled rate. The temperature at which the specimen softens and the ball sinks a defined distance is recorded as the softening point. All determination steps follow the requirements of ASTM D36, EN 1427, or JTG E20 (T 0606).
Calculation and Expression of Results
The separation softening point difference is calculated by subtracting the softening point of the bottom section from that of the top section. Because the polymer phase normally migrates upward, the top value is frequently higher; the evaluation is generally based on the absolute magnitude of the difference. Results are reported to the precision required by the governing standard, commonly the nearest 0.5 °C, together with the individual top and bottom values, the test conditions, and the applicable specification limit.
Acceptance Criteria and Precision
Under JTG F40-2004 and most international project specifications, SBS-modified asphalt must exhibit a separation softening point difference of no more than 2.5 °C after 48 hours at 163 °C; some stricter project specifications impose 2.2 °C. Duplicate softening point determinations should agree within the repeatability limits of the ring-and-ball method. Key factors influencing the result include polymer type and dosage, shearing intensity and duration during production, the use of compatibilizers or stabilizers, sulfur cross-linking, and the actual storage time and temperature.
Applications
The separation softening point difference determination is an essential quality control and acceptance tool throughout the modified asphalt supply chain. Typical applications include:
- Factory quality control at modified asphalt production plants, verifying that shearing parameters and stabilizer dosages produce a storage-stable binder
- Acceptance testing for expressway and arterial pavement projects where SBS-modified asphalt is mandated by specification
- Hot storage management at asphalt mixing plants, confirming binder uniformity in heated tanks before and during production
- Research and development of polymer modifiers, cross-linking agents, and stabilizer systems
- Compliance demonstration against specifications such as JTG F40-2004 and the classification framework of EN 14023
- Assessment of binders intended for bridge deck waterproofing layers, airport pavements, and other premium applications
FAQ
What does a large separation softening point difference indicate?
A large difference indicates that the polymer phase has migrated during hot storage, making the binder non-uniform. The polymer-rich top layer will show an abnormally high softening point while the bottom layer will be depleted of polymer, compromising the rutting resistance, elasticity, and durability expected from the modified binder.
Why is the specimen conditioned at 163 °C for 48 hours?
The conditioning regime simulates the severe but realistic conditions of hot storage in production and mixing plant tanks. The temperature of 163 °C is aligned with other binder tests such as rolling thin film oven conditioning, and the 48-hour hold is long enough to reveal any latent tendency of the polymer network to separate.
What is the typical acceptance limit?
For SBS-modified asphalt, the commonly specified limit is a difference of no more than 2.5 °C, as required by JTG F40-2004. Some project specifications and high-grade binder requirements tighten this limit to 2.2 °C.
Why is the middle third of the specimen discarded?
The top and bottom sections represent the extremes of the separation gradient and therefore provide the most sensitive measure of instability. The middle section is a transition zone and is excluded to avoid diluting the measured difference.
Is the test applicable to crumb rubber modified asphalt?
Not directly. Crumb rubber particles interact with the base binder through swelling rather than full dissolution, so separation is assessed using different visual and performance criteria. The cigar-tube softening point difference method is primarily intended for polymer modified binders such as SBS-modified asphalt.
How can a manufacturer reduce the separation difference?
Effective measures include optimizing shearing time, temperature, and colloid mill gap; increasing polymer compatibility through proper stabilizer or cross-linking agent selection; controlling polymer dosage; and ensuring that storage tanks are agitated and that storage time at high temperature is minimized.