Magnesium and Plastics: Properties, Processes, and Environmental Impact
Magnesium
Properties and Processing
Magnesium is obtained from carnallite and dolomite. It is characterized by its bright white color, lightweight, malleability, and low ductility. It is corrosion-resistant when dry, but highly reactive in a liquid state or as flammable dust. Compared to aluminum, magnesium is slightly tougher but less effective at conducting heat and electricity.
Magnesium chloride (carnallite) yields magnesium through electrolysis, where the metal rises to the surface due to its lower density than the molten salt mixture. Magnesium carbonates (dolomite and magnesite) are processed through reduction in an electric oven with added flux to remove oxygen.
Magnesium is used in alloys for shaping, forging, and casting. In its pure state, it’s used in pyrotechnics and as a flux.
Environmental Impact
Magnesium production can lead to contamination during mineral extraction, processing, and recycling. Programs are in place to mitigate the impact of heavy metals, gases, dust, and hydrogen released during these processes. These include selecting appropriate factory locations, using masks, treating wastewater for metal and sludge removal (neutralization with chemical products, and disposal in landfills), and managing noise pollution from plants and equipment (exterior insulation and hearing protection).
Plastics
Components and Additives
Plastics are synthetic materials derived from chemical reactions. Their main components are polymers, which are large organic macromolecules formed by linking smaller units called monomers through a process called polymerization.
- Natural Polymers: Originate from animal and plant sources.
- Mineral Polymers: Derived from oil, coal, and natural gas.
- Synthetic Polymers: Manufactured in laboratories.
Additives enhance the properties of plastics. These include:
- Dyes and Pigments: Provide color. Pigments, unlike dyes, do not dissolve in plastic and make it opaque.
- Flame Retardants: Increase fire resistance.
- Antistatic Agents: Prevent electrical charging.
- Conductive Fillers: Make plastic electrically conductive.
- Stabilizers: Protect against degradation from ultraviolet light.
- Plasticizers: Increase flexibility and reduce hardness.
- Fillers: Improve properties and reduce cost.
- Catalysts: Control the speed of chemical reactions.
General Characteristics and Types
Plastics generally exhibit good resistance to oxidation and corrosion, provide thermal, acoustic, and electrical insulation, have high molecular mass, are impact resistant, can be molded under pressure or heat, and have low density.
Classification by Chain Length:
- Brittle: 20-30 monomers.
- Soft: 100-1000 monomers.
- Hard: Over 1000 monomers.
Classification by Heat Behavior:
- Thermoplastics: Can be melted and re-hardened, changing physical but not chemical properties. They are flexible and impact resistant. Examples include PVC, polyethylene, and polystyrene.
- Thermosets: Undergo irreversible chemical changes upon heating. They are hard but brittle. Examples include resins, Bakelite, and melamine.
- Elastomers: Highly elastic materials, often manufactured through vulcanization (treating with sulfur at 145°C and 5 ATM pressure). They are susceptible to degradation by oxygen, light, and heat. Examples include rubber used in underwear and pants.
Conformation Processes
- Compression Molding: Plastic with fillers and additives is placed in a mold, heated, and pressed until it fills the mold cavity. After cooling, the part is removed.
- Injection Molding: Heated and pressurized plastic is injected into a mold through nozzles. The mold opens after cooling to release the part.
- Thermoforming: A plastic film or sheet is placed over a mold, heated, and then vacuum or pressure is applied to conform it to the mold shape. The part is removed after cooling.
- Extrusion Blow Molding: Molten plastic is extruded into a tubular shape. Air pressure inflates the tube against the mold walls. The mold opens after cooling to release the part.
Composite Plastics
Composite plastics are reinforced with other materials to enhance properties like tensile strength.
- Kevlar-Reinforced Plastic: Highly resistant and difficult to machine, used in motorcycle helmets and bulletproof vests.
- Laminated Plastic: Layers of plastic are stacked but not mixed.
- Glass-Reinforced Plastic: Coated with a thin layer of glass for improved impact resistance.
- Metal-Coated Plastic: Metal products coated with a thin layer of plastic.
- Fabric-Reinforced Plastic: Used in tarpaulins and rainwear.
- Multi-Layer Plastic: Different plastic layers are combined for enhanced properties.