Melting Point

Melting point is defined as the temperature at which the solid phase exists in equilibrium with its liquid phase. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid ordering of molecular entities in the solid breaks down to a less-ordered state and the solid liquefies. An object that has melted completely is molten. Substances in the molten state generally have reduced viscosity with elevated temperature.

Melting point is a wax property that is of interest to most wax consumers. It can be an indication of the performance properties of the wax.

Methods

The open capillary tube measures the temperature at which a wax rinses in an open capillary tube when heated, and the drop point method is the temperature at which first drop falls from the thermometer. The open capillary tube method is frequently used for natural waxes as Candelilla wax, Beeswax, etc. and drop point is used primarily for petrolatums and other microcrystalline wax.

Open Capillary tube Method. USP741 Class II

Carefully melt the material to be tested at as low a temperature as possible, and draw it into a capillary tube, which is left open both ends, to a depth of about 10 mm. Cool the charged tube at 10°C, or lower, for 24 hours, or in contact with ice for at least 2 hours. Fill a flask with approximately 200 mL of water at room temperature and insert the thermometer into a rubber stopper. Stick together the capillaries to the thermometer bulb with adhesive tape (the capillaries must not be linked together). Check that the sample is level with the bulb of the thermometer and then introduce the thermometer bulb with the capillaries in a test tube containing water; make sure the sample in the capillaries is approximately 10 mm below the water level in the test tube (this is taken from the top of the sample in the capillary to the water level within the test tube). Regulate the rate of rise of temperature to 0.5° to 1.0° per minute. The temperature at which the material is observed to rise in the capillary tube is the melting temperature.

Closed Capillary tube Method

Seal the end of a capillary tube by heat and push the open end of it into the sample of solid powdered wax, then move the power to the closed end of the capillary tube by tapping it on the table. Repeat until the powered wax occupies 1-2 mm of the capillary tube end. With rubber bands, attach the capillary tube to a thermometer and align the bulb of the thermometer with the closed end of the capillary tube. Make a water bath by half filling a 100mL beaker with warm tap water and place the thermometer/capillary tube assembly in it so that the surface level of the powered wax is beneath the surface level of the water bath. Place the beaker on the burner stand and, stirring frequently to insure even heating, carefully heat the water bath with your heat source. The procedure is repeat two more times and averages the results.

Drop Melting point Method. ASTM D127

Drop melting point of petrolatum wax is the temperature at which the material becomes sufficiently fluid to drop from the thermometer used making the determination under definite prescribed conditions. This test method covers the determination of the drop melting point of petroleum wax. It is used primarily for petrolatum and other microcrystalline wax.

Secure a sample of sufficient size that is representative of the material under inspections. Melt the sample slowly until the temperature reaches 93°C, or about 11°C above the expected drop melting point, whichever is higher. Place sufficient sample in a flat bottom container to give a sample depth of 12 ± 1 mm and adjust the temperature of the sample to 6 to 11°C above its drop melting point using any general laboratory thermometer for measurement. Chill one of the test thermometer bulbs to 4°C. Wipe dry, and, quickly but carefully, immerse the chilled bulb vertically into the heated sample until it touches the bottom of the container (12 mm submerged) and withdraw it immediately. Hold the thermometer vertically away from the heat until the surface dulls and the place it for 5 minutes in the water having a temperature of 16°C. The next thing to do is prepare another specimen from the same sample using the procedure, securely fix the thermometer in the test tubes by means of corks so that the tip of each thermometer is 15 mm above the bottom of its test tube. Insert the test tubes in the water bath which is at 16°C and adjust the height f the test tubes so that the immersion marks on the thermometers are level with the top surface of the water, then raise the temperature of the bath at a rate of 107°C/min to 38°C then at a rate of 10°C/min until the first drop of material leaves each thermometer. Record in each case the temperature at which the first drop falls from the thermometer and report the average of the two determinations as the drop melting point of the samples under test.

Melting Point-Cooling curve. ASTM D87

Melting point (cooling curve) is the temperature at which melted petroleum first shows a minimum rate of temperature change when allowed to cool under prescribed conditions. This method covers the determination of the melting point (cooling curve) of petroleum wax. It is unsuitable for waxes of the petrolatum group, microcrystalline, or blends of such waxes with paraffin wax or scale wax. Support the air bath in its proper position in the water bath. Fill the water bath within 13mm of the top with water at the temperature of 16 to 28°C. The bath temperature is kept within these limits throughout the test. Heat the wax sample to at least 8°C above its expected melting point. To heat the wax sample use a suitable container in an oven or water bath which is held at a temperature not exceeding 93°C. Avoid the use of direct heat such as flame or hot plate. Do not keep the sample in the molten state longer than 1 h. Fill the test tube to height of 51 mm with the melted sample. Insert the melting point thermometer trough the center of a cork so that the 79-mm immersion line at the lower surface of the cork. Insert the cork into the test tube so that the bottom of the thermometer bulb is 10 mm from the bottom of the test tube. Support the test tube assembly in the air bath while the temperature of the molten wax is still at least 8°C above its expected melting point. Read the melting point thermometer every 15 s. Record each reading to the nearest estimated 0.05°C. Observe the progress of the sequential readings to determine the appearance of the plateau. Identify the plateau as the first five consecutive readings all of which agree within 0.1°C. Discontinue the test after obtaining these five plateau readings. Average the first five consecutive thermometer readings of the identified plateau which agree within 0.1°C.

Melting Point Devices

Fisher – Johns Melting Point Block

The Fisher-Johns melting point apparatus consist of a rheostatically controlled heated block, a thermometer, and a viewing magnifier. A meniscus magnifier makes it possible to see the actual points at which the wax (a) starts to soften, (b) to spread, and (c): to fully melt.

A small piece of wax to be tested is placed on the electrically heated block along with a few drops of silicone oil. A cover glass is placed over the material and the heat is gradually increased until the sample material melts or softens enough to deform. The meniscus formed by the silicon oil is viewed through the magnifier. The temperature at which the meniscus moves is considered the melting point.

At the present time the Fisher-Johns apparatus has been discontinued and is most commonly used for melting point determination for polymers.