What is the melting range and the melting point comparison table of commonly used plastics

Plastic products are organic substances, which are non-crystalline and are not suitable for the definition of melting point, so there is no melting point. The melting point referred to here is the temperature at which a solid transforms its state from a solid state (melt) to a liquid state. The temperature at which the opposite action (that is, from liquid to solid) takes place is called the freezing point. Unlike the boiling point, the melting point is little affected by pressure. Substances are crystalline and amorphous. Crystals have melting points, while non-crystals have no melting points. Crystals have different melting points depending on their type.

Generally speaking, the melting point of crystals is from high to low, atomic crystals>ionic crystals>metal crystals>molecular crystals. In molecular crystals, there are more special ones, such as water, ammonia, etc.. Their molecules are only due to hydrogen bonds. The regular melting range that conforms to the "regular change in the melting point of hydrides of the same main group of elements" means that the melting point of a substance is not a point, but a temperature range, called the melting range. The two limits are called the initial melting temperature and Final melting temperature, the initial melting temperature is the temperature at which the substance begins to melt, and the final melting temperature is the temperature at which the substance completely melts.

Basic principles of melting point determination

Melting point definition: Under a certain pressure, the temperature at which the solid phase and the liquid phase of a solid compound are in equilibrium.

At this time, the vapor pressures of the solid and liquid phases are equal. Each pure solid organic compound generally has a fixed melting point, that is, under a certain pressure, from initial melting to full melting (this range is called melting range), the temperature does not exceed 0.5 to 1 ℃. Melting point is an important physical constant for identifying solid organic compounds, and it is also a criterion for compound purity. When the compound is mixed with impurities, the melting range is longer and the melting point is lowered.

The melting point and freezing point of a pure substance are the same.

Melting point: The general definition is the temperature at which a crystalline substance changes from a solid state to a liquid state when heated. The strict definition should be the temperature of a crystalline substance in solid-liquid equilibrium under a certain atmospheric pressure. At this time, solid and liquid coexist and vapor pressures are equal. The melting point of the compound can be measured using the Thiele tube method or melting point apparatus. Secondly, the microscope method is also good, but the operation requirements are too high.

Melting range: the difference between the two temperatures of full melting and initial melting.

Initial melting: the temperature at which the sharp corners and edges of the crystal become rounded (or the temperature at which a small amount of liquid is observed).

Full melting: The temperature at which the crystals just melt.

The purity of organic matter can be tested according to the length of the melting range. The shorter the melting range, the purer the compound! When the compound is mixed with impurities, the melting range is longer and the melting point is lowered. Usually, a pure substance has only one melting point under one-atmosphere pressure, which is not a range, but some polymer compounds are more special. When heated, they soften and then melt, and there is a melting range. If the melting point or melting range of the substance you extract is consistent with the data, it means that the substance you extract is relatively pure, otherwise, the purity is not high.