Study Notes on Polymer Composites
Chapter 24: Polymer Composites
Introduction to Polymer Composites
- Composites are materials composed of two or more constituents that combine to produce properties beyond those of the individual components.
- Commonly encountered in polymer composites.
Needs of Composites
- Combination of Properties: Composites are designed to meet specific performance criteria by integrating features from different materials.
- Copolymer: A type of polymer that consists of two or more different monomeric species.
- Matrix and Reinforcement: A soft flexible matrix is often reinforced by a stiffer fiber (e.g. fibers within a composite).
Rubber Toughened Polymer (Chapter 23)
- Rubber toughening enhances the processing and properties of polymers by improving their impact resistance and toughness.
Natural Examples
- Wood Composition: Wood is a natural composite composed of cellulose fibers embedded in a lignin matrix, which contributes to its light weight, stiffness, and strength.
Fillers for Composites
- Reinforcing Fillers: Rubber reinforcement is commonly achieved through the addition of mineral fillers and carbon black to enhance wear resistance, stiffness, and strength.
- Significance of Carbon Black: Carbon black is notable for enhancing the electrical properties and conductivity of the material.
- Cost Consideration: The use of fillers can impact the overall cost of composite materials, offering affordable enhancements.
Properties of Composite Materials
- Key properties of various materials that can be incorporated into composites include:
- Density: Measured in $p/Mg m^{-3}$
- Young's Modulus (E): Indicating stiffness, measured in $GPa$.
- Tensile Strength: Measured in $MPa$.
Table of Properties by Material Type:
| Material Type | Density (Mg/m³) | Young's Modulus (GPa) | Tensile Strength (MPa) |
|---|
| Rubber (Polyurethane) | 1.2 | 0.01 | 20 |
| Engineering Thermoplastic (Nylon) | 1.1 | 2.5 | 80 |
| Thermosetting Resin (Epoxy) | 1.25 | 3.5 | 50 |
| Metal (Mild Steel) | 7.8 | 208 | 400 |
| Advanced Polymer Composite (APC-2) | 1.6 | 200 | 1500 |
| Engineering Ceramic (Alumina) | 3.9 | 380 | 500 |
| Wood (Spruce) | 0.6 | 16 | 80 |
- Source for data: Hull, D. and Clyne, T.W., An Introduction to Composite Materials, 2nd edn., Cambridge University Press, 1996.
Types of Composites
- Composites can be classified into different types based on their structure and reinforcement:
- Particulate Composites
- Discontinuous Aligned Composites
- Discontinuous Random Composites
- Continuous Fiber-Reinforced Composites
Composite Material Applications
- Common applications include various structural and load-bearing elements like laminates, sandwich panels, pylons (made of aluminum, steel, titanium), and more.
- Examples: Carbon laminate, carbon sandwich, fiberglass composites.
Manufacturing Processes of Composites
- Injection Molding: A common manufacturing method where plastic granules are melted and injected into a mold.
- Injection Molding Equipment Components:
- Ejector Pins
- Cavity
- Cylinder (for melted plastic)
- Nozzle
Matrix Materials
- Thermosetting Network Polymers: Examples include polyester and epoxy resin, commonly used in glass-reinforced plastics and carbon fiber-reinforced resin composites.
- Thermoplastic Matrix: Common thermoplastics in composites include nylon, polypropylene, and polyetheretherketone (PEEK).
Types of Reinforcement
24.3.1 Particles
- Examples include:
- (a) Glass spheres in an epoxy resin.
- (b) Silica particles in an epoxy resin.
24.3.2 Fibers
- Types of Fibers Used in Composites:
- Glass fibers
- Aramid (e.g., Kevlar 49)
- PBO (Zylon HM)
- Carbon fibers (high strength and high modulus)
Mechanical Properties of Reinforcing Fibers (Table 24.2)
| Fiber | Density (Mg/m³) | Tensile Modulus (E₁, GPa) | Tensile Strength (GPa) |
|---|
| E-glass | 2.55 | 76 | 1.5 |
| Aramid (Kevlar 49) | 1.45 | 125 | 3.0 |
| PBO (Zylon HM) | 1.56 | 270 | 5.8 |
| Carbon (high strength) | 1.77 | 230 | 3.3 |
| Carbon (high modulus) | 1.90 | 360 | 2.5 |
Fiber Reinforcement
- Deformation Behavior:
- Elastic Deformation.
- Fracture mechanisms characterized by scanning electron micrographs display fracture surfaces and the role of glass particles in crack propagation.
Nanocomposites
- Introduction to why nanocomposites are preferred in modern manufacturing:
- Reduction in flaw sizes leads to stronger materials.
- Increased surface area improves stress transfer.
Applications of Nanoparticles
- Carbon Black: Enhances mechanical performance in elastomers and provides improved stiffness and wear resistance.
- Nanosilica: Better mechanical properties compared to larger particles; enhances epoxy resins with improved Young's modulus and fracture energy.
Nanoplatelets - Clays
- Montmorillonite: Known for its properties related to ion exchange, dispersion in polymers, and increased thermal and mechanical properties leading to flame retardance and barrier enhancement.
Graphene Properties
- One-atom thick sheets that exhibit exceptional mechanical properties, with a Young’s modulus of approximately 1000 GPa and tensile strength around 100 GPa. These properties make it innovative in advanced materials science.
Mechanical Properties of Epoxy Resin Nanocomposites (Table 24.4)
| Cured Epoxy Resin | Young's Modulus (GPa) | Tensile Strength (MPa) | Fracture Toughness (K, MPa m^1/2) | Fracture Energy (Gm²²) |
|---|
| No graphene | 2.85 | 55 | 1.0 | - |
| Graphene platelets | 3.74 | 78 | 1.5 | 260 |
Carbon Nanotubes
- Structure: Carbon nanotubes are defined as a single sheet of graphene rolled into a cylindrical shape. They are classified into single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT).
- Properties: They exhibit remarkable mechanical properties with a Young's modulus up to 1 TPa and a tensile strength of around 100 GPa, making them excellent candidates for reinforcing polymers.
- Challenges: Dispersion within a polymer matrix can be difficult due to weak interfacial bonding; tailored functionalization can help improve compatibility.
Conclusion
- The field of polymer composites is vast and continually evolving, with ongoing research focusing on optimizing the properties and functionalities of these materials for various applications.
Questions?