kind and performance of plastic
1. The category of plastic
In order to facilitate the research and use of plastics, it is necessary to classify plastics from different angles. There are two common classification methods: one is based on the performance characteristics of plastics after heating, and the plastics can be divided into two categories: thermoplastics and thermosets.
The molecular structure of a polymer in a thermoplastic is linear or branched, often referred to as a linear polymer. It can be plasticized into a shaped plastic part when heated, and retains its shape after cooling. If it is heated again, it can be softened and melted, and a plastic piece of a certain shape can be made again, which can be repeated many times and has reversibility.
1.2 thermosetting plastic
Thermosetting plastics also have a chain or dendritic structure at the beginning of heating, and are also plastic and fusible, and can be molded into a certain shape of plastic. When heating is continued, these chain or dendrimer chains form a chemical bond and gradually become a network structure. When the temperature rises to a certain value, the crosslinking reaction proceeds further, and the molecule eventually becomes a bulk structure, and becomes a substance which neither melts nor dissolves, and is called solidification. When heated again, the shape of the plastic part is fixed and does not change due to chemical changes between the chains of the molecules. Plastics no longer have plasticity.
Commonly used phenolic, melamine-formaldehyde, unsaturated polyester, etc. are thermosetting plastics
Second, according to the specific use occasions and characteristics of plastics, plastics can be generally divided into general plastics, engineering plastics and characteristic plastics.
a) General purpose plastic
General-purpose plastics generally refer to a class of plastics that have large output, wide application, relatively low performance, and low price. such as:
Polyethylene, polypropylene, polyvinyl chloride, polystyrene, phenolic plastics, aminoplasts, etc. They account for about 60% of the total plastic production.
b) Engineering plastics
Engineering plastics refer to plastics that can be used as structural materials. They have no obvious boundaries with general-purpose plastics. The strength, impact resistance, heat resistance, hardness and anti-aging properties of engineering plastics are relatively good. They can replace some metal materials. Engineering materials such as nylon, polycarbonate, polyoxymethylene, ABS, etc.
c) special plastics
Refers to those plastics with special functions suitable for a special occasion, mainly medical plastics, photosensitive plastics, magnetic conductive plastics, super conductive plastics, radiation-resistant plastics, high temperature resistant plastics, etc. The main components are resins, some are specially synthesized resins, and some are specially treated or modified with the above-mentioned general plastics and engineering plastics to obtain special properties. This type of plastic has a small yield, excellent performance and high price.
2. Forming properties of thermoplastics
Plastics and molding processes, various properties related to molding quality, collectively referred to as the process properties of plastics.
In addition to thermodynamic properties, crystallinity and orientation, thermoplastic molding process properties include shrinkage, fluidity, heat sensitivity, water sensitivity, moisture absorption, and compatibility.