Synthetic Fibers, Elastomers, and Plastics: A Comprehensive Guide
Synthetic Fibers
Definition
Synthetic fibers are produced through chemical processes involving the polymerization of low molecular weight substances. These fibers are typically linear or slightly branched and belong to the thermoplastic group.
Procurement Process
Several methods are used to create synthetic fibers:
- Polymerization: Used to obtain acrylic and olefin fibers.
- Polycondensation: Used to obtain polyester fibers.
- Polyaddition: Used to obtain fibers from polyoxymethylene, polyoxyethylene, and polyurethane.
Features
Due to the stretching and tension applied during manufacturing, synthetic fibers exhibit both amorphous and crystalline regions. These regions are formed by the grouping of parallel linear macromolecules. Key features include:
- Elasticity
- Lightweight
- High resistance to wear, acids, and fading
- Ability to be dyed without setting operations
Elastomers
Definition
Elastomers are composed of long chains with few chemical bonds, allowing for significant intermolecular movement and flexibility. They possess memory, returning to their original shape after being stretched. Elastomers are typically amorphous and cannot be reprocessed. They serve as replacements for natural rubber.
Elastomers can be either thermoset or thermoplastic. Thermoset elastomers are more common and retain their shape after curing. Their elasticity stems from the ability of chains to rearrange themselves under stress and return to their original form due to covalent bonds. They can be stretched from 5% to 700% depending on the material.
Classification by Chemical Composition
Elastomers are classified into groups based on their chemical composition:
- Group R: Main chain consists of carbon and hydrogen with double bonds (e.g., natural rubber, neoprene).
- Group M: Main chain contains only carbon and hydrogen atoms and is saturated (no double bonds).
- Group N: Contains nitrogen atoms in the main chain (e.g., polyester).
- Group O: Contains oxygen atoms in the main chain (e.g., epichlorohydrin rubber).
- Group Q: Contains siloxane groups in the main chain (e.g., silicone rubber).
- Group U: Contains oxygen, nitrogen, and carbon atoms in the main chain (e.g., polyurethane elastomer).
- Group T: Contains sulfur atoms in the main chain (e.g., polysulfide rubber).
Thermoset Elastomers
Thermoset elastomers maintain their strength and shape upon heating but degrade above a certain temperature. They are prepared from low molecular weight, semi-fluid substances that form hard, insoluble materials with high degrees of molecular crosslinking.
Thermoplastic Elastomers
Thermoplastic elastomers become soft and pliable when heated and can be melted and shaped multiple times without altering their properties. Examples include styrenic, olefinic, and vulcanized thermoplastic polyurethane rubbers. These materials are typically copolymers or physical mixtures of polymers (rubber + plastic).
Differences and Characteristics
The primary difference between thermoset and thermoplastic elastomers lies in the crosslinking of their structures. Thermoplastic elastomers exhibit the following characteristics:
- Ability to be stretched
- Suitability for casting processes at high temperatures
- Absence of significant creep
Plastics Processing Techniques
Molding
Molding involves shaping plastic using a mold and pressure. It can be categorized based on pressure levels:
- High-Pressure Molding: Compression, injection, extrusion
- Low-Pressure Molding: Casting, foaming, calendering
Additives are often used to enhance specific properties, such as antioxidants, UV stabilizers, plasticizers, lubricants, and pigments.
Blowing
Blowing creates hollow products by expanding hot plastic tubes against the inner surface of a mold. The process involves two steps: extrusion of the polymer melt and inflation of the tube within a mold.
Compression
Compression molding is primarily used for thermoset polymers. The material is placed in an open mold, and a heated press applies heat and pressure to soften and shape the polymer. The plastic hardens rapidly within the mold.
Injection
In injection molding, the raw material is heated separately and injected into a cold mold. The plastic cools and hardens upon contact with the mold walls, allowing for quick removal of the molded part.
Vacuum Forming
Vacuum forming is used to create large sheets of plastic with surfaces that cannot withstand other forming processes.
Extrusion
Extrusion involves forcing molten plastic through a die to create continuous products like pipes, hoses, and wire coatings. It is suitable for thermoplastics and thermoset elastomers.
Printing
Printing, or laminating, is a forming process for thermoplastics and thermosets. The key difference lies in the application of heat for thermoplastics and pressure for thermosets.
Rolling
Rolling reduces the thickness of the material by passing it between rotating rolls. It can be performed at room temperature or with heat.
Layered Compression
Layered compression reduces the thickness of plastic walls by applying pressure to the polymer’s elastic limit.
Polymerization Techniques
Bulk Polymerization
Bulk polymerization involves only the monomer and initiator, with no solvent or emulsifier. The reaction occurs at room temperature and results in high-purity polymers. However, increasing viscosity during the reaction can be a challenge.
Solution Polymerization
Solution polymerization involves dissolving the monomer in an organic solvent, allowing for temperature and viscosity control. The resulting polymer solution can be used directly, but solvent extraction can be difficult.
Suspension Polymerization
Suspension polymerization disperses the monomer as droplets in a liquid (typically water) using mechanical agitation. The final polymer is obtained as pure beads. This method offers easy temperature control but may require constant stirring.
Emulsion Polymerization
Emulsion polymerization is widely used due to its speed and ease of temperature control. It is similar to suspension polymerization but uses smaller droplets and an emulsifying agent. However, the polymer may be contaminated with emulsifiers and water.
Interfacial Polymerization
Interfacial polymerization occurs at the interface between two immiscible liquids. The polymer is extracted directly from the interface, making it suitable for heat-sensitive polymers.
Thermoset Polymers
Definition and Classification
Thermoset polymers become soft and pliable only upon initial heating and cannot be reprocessed after cooling. This is due to their three-dimensional network structure with cross-links. The formation of these links is influenced by heat, catalysts, and the proportion of formaldehyde in the preparation.
Thermoset materials include:
- Phenolic resins (e.g., Bakelite)
- Urea resins
- Melamine resins
- Polyester resins
- Epoxy resins
Phenolic Resins (Bakelite)
Phenolic resins are formed through the polycondensation of phenol and formaldehyde. The proportion of formaldehyde determines whether the final material is thermosetting or thermoplastic.
Urea Resins
Urea resins are obtained by the polycondensation of urea with formaldehyde. They share similarities with Bakelite but can be colored. Advantages include high resistance to surface leakage currents, while disadvantages include lower moisture resistance.
Melamine Resins
Melamine resins are formed by the polycondensation of melamine and formaldehyde. They exhibit a reddish color, high softening point, and resistance to alkalis, solvents, and electric arcs.
Polyester Resins
Polyester resins are obtained by the polyesterification of polyacids with polyols. They offer rigidity and resistance to moisture, solvents, electric arcs, and surface leakage currents.
Epoxy Resins
Epoxy resins are obtained by reacting diphenylolpropane and epichlorohydrin. They can be solid, viscous, or liquid and are chemically hardened thermoplastics. Epoxy resins do not emit gases during hardening, do not shrink after curing, and adhere well to most surfaces.
Advantages and Disadvantages of Thermosets vs. Thermoplastics
Advantages of Thermosets
- Better impact resistance
- Resistance to solvents and gas permeation
- Tolerance to extreme temperatures
Disadvantages of Thermosets
- Complex processing
- Requirement for curing
- Brittleness
- Inability to be strengthened under tension
Cooling in Polymer Industrialization
Thermoplastics and Thermosets
Thermoplastics can be repeatedly heated and cooled for manipulation and casting, while thermosets retain their shape once formed and degrade upon further heating.
Cooling Process
Cooling is essential for restoring strength and toughness to polymers after casting. It can be the final step in product transformation or repeated multiple times with reheating.
Common cooling methods include immersion cooling, spray cooling, and bath cooling.