What Is Softening Point?
The resins are noncrystalline amorphous materials that soften gradually over a range of temperatures. Therefore, the softening point is controlled by the average molecular weight of the resin.
The softening point is also related to the intrinsic viscosity, hardness, and brittleness of resins. The softening point is defined as the temperature at which the resin flows under a given load on heating.
Several standard methods have been proposed to determine the softening point of resins.
List of Proposed to Determine the Softening Point
- Ring and Ball Method (R&B)
- Kriimer—Sarnow Method
- Mettler Softening Point Method
- Plate—Plate Stress Rheonzeter Test
Ring and Ball Method (R&B)
ASTM D 3461-76, DIN ISO 4625. This method is the most frequently used to determine the softening point of resins. Show above figure illustrates the experimental device used to determine the R&B softening point.
The resin is melted into a metal ring and left to cool. The ring is placed in a special metallic device, which is placed into a water or glycerol bath.
A steel ball of given diameter and mass is placed on the ring and the bath is heated at a given rate. The temperature at which the ball forces the softening resin downward is noted as the softening point.
DIN 53 180. This is the oldest method used to determine the softening point of resins and is relatively similar to the R&B method.
Instead of a ring, a small glass tube that is open at both ends is used and the load is a small mercury drop. The softening point is obtained as the temperature at which the mercury drop breaks through the softening resin and falls.
Mettler Softening Point Method
1. Heating element, 2. Platinum resistance thermometer, 3. Sample, 4. Light source, 5. Furnace, 6. Sample cup, 7. Photocell, 8. Collector sleeve.
ASTM D 3461-76. This is the most recent method. This automatic method measures the temperature at which the resin flows out of a sample cup under its own weight.
The temperature is recorded when the first drop crosses the light path of a photocell (show in the figure). This method is quite accurate and reproducible.
Also, read: Core Cutter Method | What is Compaction of Soil
Plate—Plate Stress Rheonzeter Test.
The resin is placed between the two steel plates of a stress-controlled rheometer, maintaining a gap larger than 0.5 cm. The upper plate is oscillated at a given frequency, whereas the lower plate is heated.
The variation of the storage and loss moduli as a function of the temperature is monitored. The softening temperature can be estimated from the temperature at the cross-over between the two moduli.
Ring-Ball Test for Softening Point of Bitumen
The closely defined method used for bitumen is described in BS 4692. In the ring-and-ball test, samples of bitumen are prepared by pouring the bitumen, which has been heated under specified conditions, into metal rings which serve as molds.
Preparation of the bitumen samples must be carried out in a controlled manner detailed in BS 4692. The determination is usually carried out in duplicate and the apparatus is designed to hold two discs.
Apparatus for Softening Point of Bitumen
- A brass shouldered ring. (Show above figure)
- A steel ball, 9.53 mm (3/8”) in diameter, weighing between 3.45 and 3.55 grams. ( Show above figure)
Ball Centering Guide
- A guide for centering the ball and made of brass. (Show above figure)
- The rings shall be supported on a brass ring holder. (Show above figure)
Brass Pouring Plate
- A flat, smooth brass plate approximately 75 by 50 mm that has been treated to prevent the bituminous material from adhering to it.
- Suitable treatment is to coat the plate just before use with a thin layer of a mixture of glycerin and dextrin, talc, or china clay.
- A glass vessel, capable of being heated, not less than 85 mm in diameter and not less than 120 mm in depth from the bottom of the flare.
Thermometer having a range from -2 to +80°C.
Procedure for Softening Point of Bitumen
The determination is usually carried out in duplicate and the apparatus is designed to hold two discs.
The apparatus is assembled with the rings, the appropriate thermometer, and ball guides in position, and the bath is filled to a height of 50 mm above the upper surface of the rings, as show above figure, with freshly boiled distilled water at a temperature of 5 °C when the softening point is below 80 °C, the water temperature being maintained at 5 °C for 15 min.
Using forceps, a steel ball, previously cooled to 5 °C, having a diameter of 9.53 mm and weighing 3.50 ± 0.05 g, is placed upon each sample disc contained within a tapered brass ring with a larger internal diameter of 17.5 mm.
A ball guide previously placed over the disc assists in placing the ball in position. The bath is then heated and the water stirred so that the temperature rises at a uniform rate of 5 ± 0.5 °C/min until the bitumen softens and allows the ball to pass through the ring.
The rate of temperature rise should not be averaged over the test period and any determination where the rate does not fall within the specified limits after the first 3 min must be rejected.
For each ring and ball, the temperature shown by the thermometer is taken at the instant the sample surrounding the ball touches the bottom plate. If, however, the difference between the values obtained in the duplicate determinations exceeds 1 °C the test must be repeated.
When the softening point is expected to be above 80 °C the water is replaced with glycerol, which is maintained at a temperature of 35 °C for 15 min before placing a steel ball at the same temperature on to the disc.
Observations & Results for Softening Point of Bitumen
|1||45.2 °C||45.1 °C|
Softening Point of Bitumen Lab Report
The experiment has been performed successfully and the softening point came out to be 45.1oC. Bitumen having this softening point should not be used in DDIT Nadiyad as the city’s highest maximum temperature was 40.3 °C recorded on March 30, 1990 And 46 °C was recorded on May 26, 2013.
At the time the meteorological office recorded this official temperature in the shade, it reported a heat index in direct sunlight of 55 °C. So to avoid the problems of Rutting and bleeding this bitumen should not be used in Lahore.