What is Dropping Point? What is Softening Point?

Dropping and softening point determination is one of the few methods available to thermally characterize ointments, synthetic and natural resins, edible fats, greases, waxes, fatty polymers, bitumen, tars, etc. Analysis of dropping and softening points are mainly conducted during the quality control process. It can also be of value during the research and development stages of determining the operating temperatures and process parameters of varying materials.

On this page, you will gain essential knowledge about dropping and softening point techniques. Furthermore, practical tips and hints for daily work are provided.

In automated dropping point determination, the entire drop point test is video recorded and image analysis is used to automatically detect the first drop that escapes the sample cup when it passes through a virtual white rectangle located underneath the cup orifice. While detecting this, the furnace temperature is measured and recorded at a resolution of 0.1 °C.

• Video image analysis from METTLER TOLEDO's DP70 and DP90
  
• Evaluation area: pictured within the marked rectangle area
  
• Detection occurs when a drop has passed through the rectangle

In automated softening point determination, the leading edge of the softened sample is marked with a stepping line in the video. Once the stepping line has passed, a virtual line appears that is located 19 mm below the cup orifice. The corresponding furnace temperature is measured and recorded at a resolution of 0.1 °C. The automated Dropping Point device from METTLER TOLEDO also generates a novel length diagram, which indicates the rheological behavior of the sample.

• Video image analysis from METTLER TOLEDO's DP70 and DP90
  
• Evaluation area: pictured within the marked rectangular area including the stepping line at 19 mm
  
• Detection occurs when the stepping line reaches a defined distance

Watch this video to learn more about the benefits of automated dropping point instruments:

A manual method for the dropping point determination of edible oils and fats, which is described in the DIN 51801 standard is the Ubbelohde method. A dedicated one tenth of a degree resolution thermometer is directly immersed in contact with the sample residing in a cup. The setup is positioned in a test tube and the heating is performed with a water bath. The dropping point temperature should be noted as soon as the event is detected visually. The results of this test are strongly dependent on the operator's experience.

The DIN standard requires a sample cup whose dimensions are different from that used in the AOCS, ASTM and other dropping point methods. Comparative dropping point tests with both cups performed on an automatic METTLER TOLEDO instrument revealed no significant difference in the results obtained.

In conclusion, with regards to result quality and comparability, the automatic dropping point test of edible oils and fats is a perfect alternative to the manual Ubbelohde method.

A manual method for the dropping point test of waxes is described in the ASTM D127 standard. The setup requires a chilled water bath, two ASTM thermometers with one tenth of a degree resolution and two test tubes. The water bath should be maintained at 16 °C. Both ASTM thermometers should be cooled to 16 °C. The pre-cooled ASTM thermometers are immersed into the molten sample. Both thermometers with the adherent sample are then cooled for 5 minutes at 16 °C in the water bath before they are placed into test tubes that are immersed in the water bath at the same temperature. A dual heating ramp procedure is then applied: first the temperature is raised by 2 °C/min to 38 °C and then by 1 °C/min until the first drop of the molten substance leaves the thermometer. The temperature is noted at this point.  

Compared to the automatic method, visual event detection requires experienced operators in order to achieve sufficient results repeatability. Result variation will be higher due to the operator's bias. In addition to the comparatively tedious sample preparation and dual heating ramp procedure, the required ASTM thermometer includes poisonous mercury. The use of such devices is controversial and in the future, laboratories will seek out other alternatives.

The automated methods provide unambiguous advantages and can be a considered viable alternatives to the manual dropping point test of waxes.

METTLER TOLEDO's Dropping Point systems for softening point determination operate according the cup-and-ball method. This setup differs in various respects. The temperature control is ensured by a metal block heating principle and the cup-and-ball temperature is recorded by a digital thermometer. The sample is placed in a cup and can flow freely downwards through an aperture in the cup. As with the ring-and-ball setup, a ball also promotes the flow of the sample, however here it is blocked by the smaller diameter of the cup and does not flow through with the sample. The analysis takes place in a glass container that is disposed of after the experiment, thus avoiding furnace contamination.

The question that often arises is whether the two techniques deliver the same results. ASTM methods explicitly state that they have been designed to reproduce the results of the ring-and-ball methods. This is proven by ASTM Interlaboratory studies that were conducted. An example of the results of 11 different laboratories on 7 resin substances (six-fold measurements) is detailed below:

The international standards ASTM D3104, ASTM D3461, ASTM D3954, ASTM D6090 and AOCS Cc 18-80 are specifically based on automatic dropping point and softening point measurement with METTLER TOLEDO's Dropping Point instruments.

For an overview of international norms and standards for dropping and softening points including manual methods, visit www.mt.com/MPDP-norms.

The ASTM D3104 and D3461 standards are called softening point according to METTLER TOLEDO or METTLER TOLEDO's cup-and-ball. They are based on the automatic softening point detection principle with the METTLER TOLEDO Dropping Point instruments. In the D3104 standard, no extra weighting of the sample in the cup is required, whereas in the D3461 standard, a lead ball of specified dimensions is required to enforce the flow of the sample from the cup. In both cases, the test is performed in a furnace and the softening point temperature is detected and recorded automatically. The ASTM D36 or ISO EN 1427 methods specify the softening point test according to the ring-and-ball method. In contrast to METTLER TOLEDO's standards, a different test setup including liquid-medium bath heating is used. The ASTM D566 or ISO EN 2176 are the basis for lubricating grease dropping point testing, using either a liquid medium bath or an aluminum furnace block heating. In both cases, the sample needs to be prepared in the dropping point cup according to an exactly specified procedure. In both standards, the dropping point event is detected manually. The IP 396 standard adheres to these standards in respect to sample preparation, but is based on a dual heating ramp procedure (See diagram) and automatic dropping point event detection.

METTLER TOLEDO's Dropping Point instruments allow fully automated dropping point detection of lubricating grease exactly according to the requirements of the IP 396 standard. In all standards, tight limits for repeatability and reproducibility are given, which serve as a guideline to assess the result quality. In the bitumen/pitch/asphalt standards, these specifications are tight, whereas in the lubricant grease standards the corresponding values are comparatively high. This is due to the fact that the sample integrity of the grease is destroyed during the heating procedure, especially if temperatures above 200 °C need to be applied.

IP 396 workflow requirements

• T_initial = T_ambient + 10 °C
• 10 °C/min T-ramp to T_start
• T_start = T_{DP expected} – 20 °C if DP <= 220 °C
• T_start = 200 °C if DP > 220 °C
• No waiting time after T_start
• 1 °C/min T-ramp to DP

The METTLER TOLEDO instruments-based ASTM D6090 specifies the requirements for the automatic softening point determination of resins. The test procedure and detection principle is almost identical to that described in the previous ASTM D3461 standard. One important difference is that a steel ball instead of a lead ball is used to weight the sample in the cup. The ASTM D6493 specifies the softening point test of resins according to the ring-and-ball method, which is almost identical to the ASTM D36 standard. The AOCS Cc 18-80 standard specifies the automatic dropping point detection of edible fats & oils using a dropping point instrument from METTLER TOLEDO. This standard also covers the test of samples with dropping points below ambient temperature. The ASTM D3954 standard is also specified on a METTLER TOLEDO Dropping Point instrument and is used for automatic dropping point tests of waxes including paraffin, microcrystalline polyethylene, and natural waxes. The European Pharmacopeia 2.2.17 B standard is analogous to this, however, the repeatability specifications are tighter. Lastly, the ASTM D127 describes the manual procedure for dropping point testing of waxes.

Edible oils, fats and organic solvents can be solidified and measured at low temperatures. Substances often need to be cooled down or frozen for a sufficient amount of time prior to the dropping point measurement. The METTLER TOLEDO Excellence Dropping Point system DP90 performs dropping point measurements down to -20 °C. Place the separate DP90 measurement cell, the furnace and the optical detection system, in a refrigerator or a freezer until the required temperature is attained.

The following workflow is recommended for dropping point measurements below ambient temperature. Note that "cooling device" (CD) refers to a freezer or refrigerator.

1. Initial installation: Ensure that the DP90 measurement cell and the CD are at ambient temperature.
2. Cooling the DP90: Place the DP90 measurement cell in the CD and start cooling. Cool overnight if possible.
3. Cooling the test equipment: Cool all test equipment (sample cups, collecting glasses and lids) and the sample preparation tool (sample carrier) for at least for 1 hour in the CD.
4. Sample transfer: Place the sample cup in the sample preparation tool. Slowly transfer the liquid with a pipette into the precooled sample cup until it is completely full.
5. Sample solidification: Allow the sample to solidify for at least 1 hour in the CD. Some samples may need to be cooled for longer. The CD should only be opened when necessary (e.g. sample preparation, sample carrier exchange). A top-loading CD is found to support the workflow best.
6. Brightness factor: Reduce the brightness factor in the method to 30%, in order to cope with any reflection from the sample.
7. Handling the sample carrier: Perform all manipulations e.g. sample carrier (dis)assembly, inside the CD to avoid thawing and condensation. We recommend that disposable gloves be used.

Along with proper sample preparation, the settings on the instrument are just as essential for the exact determination of the dropping point and softening. Correct selection of the start temperature, the end stop temperature and the heating ramp rate are necessary to prevent inaccuracies due to a heat increase in the sample that is uneven or too fast:

a) Start Temperature

Dropping point and softening point determination starts at a predefined temperature close to the expected drop point or softening point.
Up to the start temperature, the heating stand is rapidly preheated. At the start temperature the samples are introduced into the furnace, and the temperature starts to rise at the defined heating ramp rate.
The common formula used to calculate the start temperature is: Start Temperature = expected DP – (3 min * heating rate)

b) Heating Ramp Rate

The heating ramp rate is the fixed rate of temperature rise between the start and stop temperatures for the heating ramp.
Typical heating rate (as stated by norms): 2 °C/min

c) Stop Temperature

The maximum temperature to be reached in the determination.
The common formula used to calculate the stop temperature:
Stop Temperature = expected DP + (3 min * heating rate)
Or activate the "Stop at event" condition. In this manner, the measurement is automatically self-terminated as soon as the dropping point is detected.

Before the unit is put into operation, it is recommended to verify its measurement accuracy. In order to check the temperature accuracy, the instrument is calibrated using reference substances with certified temperature values. Thus, the nominal values including tolerances can be compared with actual measured values. The basis for comparison is the thermodynamic melting point of the calibration substance that is detailed on the corresponding certificate. The thermodynamic melting point is the physically correct melting point temperature of the actual sample, not the furnace temperature, and is independent of the experimental setup. The dropping point of a reference substance is not equal to the thermodynamic melting point as the measured temperature is actually the furnace temperature and not the sample temperature. Therefore, the furnace temperature value needs to be adjusted using a correction value, which is stored for each reference substance in METTLER TOLEDO's Dropping Point instrument.