Materials can soften without melting, and a softening point can be determined for quality control. The glass transition temperature is when a material becomes more flexible. Tests like the vicat and ring and ball can determine softening points. Softening point tests are used to determine optimum handling temperatures for adhesives. Synthetic polymers have a wider range of softening temperatures due to different molecular chains and additives.
When a material like ice melts, it changes from a solid to a liquid at a single temperature known as its melting point. Many materials do not melt as the temperature increases, but do soften without turning into liquid. For product formulation and quality control, a temperature called the softening point can be determined for these non-melting materials. There are several tests to determine these temperatures which vary based on the material or intended use.
A related value is the glass transition temperature, which can be determined using similar tests. Materials that do not melt at one temperature will contain what is called a crystalline structure at low temperatures. As the temperature rises, the molecules can start to move and become more like a soft rubber or flexible plastic rather than a rigid material. This is the glass transition temperature, which gets its name from the fact that glass changes from a rigid sheet to a flexible plastic-like material at its transition point.
Asphalt used for roads and roofing is a material that gets softer as the temperature rises. Producers and fabricators need to know the temperatures at which asphalt will become soft enough to be used in construction. The softening point test will provide temperatures needed for quality control and working temperatures during construction.
A vicat softening point test uses a needle that presses down on a sample with a specified weight on the needle. Samples are placed in an oil bath and heated slowly at specified rates until the needle is lowered a certain distance into the test material. The softening point is the temperature at which the needle penetration reaches the specified distance.
Another type of softening test is the ring and ball test. A ball with a known weight is placed on a sample which is heated at a specific rate. The softening point temperature is reached when the ball has pressed a specified distance into the sample.
Some adhesives, called hot-melts, are heated in a dispensing gun to a point where they flow and can be used. These products don’t work well as liquids, so heating beyond the softening temperature can create a weaker adhesive bond. Formulators will use softening point tests to determine optimum handling temperatures.
Synthetic or man-made polymers may have a wider range of softening temperatures due to the different molecular chains available to formulators. A crosslinked polymer can have a high softening point due to the rigid structure provided by the crosslinked chemical bonds. Polymers can contain additives that allow them to flow at lower temperatures, which can be useful for injection molding or adhesive applications.
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