Crystalline vs Amorphous Solids and Polymer Classification
Crystalline vs Amorphous Solids
Crystalline and amorphous describe the structure of elements and compounds in the solid state. A single element can exist in both forms.
Crystalline Solids
Crystalline solids have a regular repeating pattern, or long-range order. The repeating unit is called the unit cell. Examples include diamond, table salt, and minerals.
Amorphous Solids
Amorphous solids lack long-range order, with no repeating unit. Examples include glass, wax, and plastics.
Silicon can be found in both forms. Crystalline silicon is used in computer chips, while amorphous silicon is used in solar panels.
Polymer Classification
Thermoplastics
Thermoplastics are linear or branched polymers that melt with heat and are soluble. Examples include polyethylene (PE), polypropylene (PP), and polymethyl methacrylate (PMMA).
Thermosets
Thermosets do not melt and decompose at high temperatures. They are used for adhesives, paints, and primers. Examples include Bakelite, melamine, and epoxy.
Elastomers
Elastomers have excellent elastic properties and high elongation. They are used for foams, rubber components, gaskets, and sealants. Examples include natural rubber (NR), butadiene rubber (BR), and silicone.
Additives
Additives enhance polymer properties. Examples include plasticizers, fillers, stabilizers, flame retardants, and colorants.
Processing
Thermoplastics are processed by injection molding, extrusion molding, and blow molding. Thermosets are processed by compression molding and transfer molding.
Characteristics of Highly Crystalline Thermoplastics
- Higher mechanical properties
- Higher density
- Defined melting temperature
- Chemical resistance
- Better creep behavior
- Less gas permeability
- Lower water absorption
- Lower transparency
Polymerization
Polymerization is the joining of monomers to form polymers.
Chain Growth (Addition Polymerization)
A rapid chain reaction of chemically activated monomers.
Step Growth (Condensation Polymerization)
Individual chemical reactions between pairs of reactive monomers.
Cross-Linking
Formation of a network molecular structure by primary bonds between adjacent linear molecules.
Elastic and Plastic Deformation
Elastic Deformation
Reversible deformation where the object returns to its original shape after the force is removed. Governed by Hooke’s Law: σ = Eε
Plastic Deformation
Irreversible deformation. Characterized by strain hardening, necking, and fracture.
Metal Processing and Strengthening
Metals are processed by various methods, including casting, joining, powder metallurgy, and machining. Strengthening methods include cold working, alloying, and phase transformation.
Ferrous Alloys (Fe-Based)
Steels
High-alloy steels with non-carbon alloy additions greater than 5%. Types include stainless steel, tool steel, and superalloys.
Cast Irons
Contain greater than 2% carbon. Types include white iron, gray iron, malleable iron, and ductile iron.
Non-Ferrous Alloys
Alloys based on metals other than iron, such as copper, nickel, aluminum, zinc, and titanium. Each type has unique properties and applications.