Polymer Reaction Types

Polymer Matrix Systems

• Polymer Matrix Systems are a key component in composite materials.
• This section covers motivation, basic chemistry, mechanical properties, and the thermoset/thermoplastic matrix systems.

Motivation Behind Using Polymer Matrix Systems

• Composites are classified by reinforcement structure, fiber type, and the matrix material used.
• The matrix serves to fix the fibers in place, transfer loads, and carry shear stresses.
• Matrix supports fibers against compressive loads, arrests cracks, and protects against hazards.
• Fiber's elastic modulus (Ef) should be higher than the matrix (Em).
• Fiber strength (RF) should be higher than matrix strength (Rm) to provide reinforcement.
• The fiber's failure strain (ef) should be less than that of the matrix (em).

Basic Chemistry of Polymers

• Polymers are molecules of repeating units
• Plastics are polymer compounds (or blends) with additives.
• Polyreactions are classified as step-growth or chain-growth based on reaction mechanisms.
• Step-growth reactions have all components reacting simultaneously.
• Chain-growth involves a growing chain reacting with a monomer only
• Molecular weight distribution affects the properties of polymers.
• Polymers do not all have chains of the same length
• Polymers are polydisperse which creates a distribution of molar mass within the polymer
• Long polymer chains increase mechanical properties.
• Short chains improve processing (reduced viscosity).
• The three main polymer types used depend on the degree of cross-linking between polymer chains.
• The three types are Thermosets, Elastomers, and Thermoplastics.
• Glass transition temperature (Tg) is when the material properties change the most.
• Service temperatures vary by polymer type.

Main Polymer Types

• Thermosets offer high modulus, low creep, and thermal/chemical stability but are brittle and hard to recycle besides grinding
• Thermosets are solid polymers formed by the chemical reaction of a resin a hardener
• Elastomers are weakly cross-linked, allowing for high strains and reversible deformations, but they are unsuitable for structural composites as they do not support loads
• Elastomers experiences service temperatures above the glass transition temperature
• Thermoplastics have secondary bonds like Van der Waals between molecules and become highly crystalline.
• Thermoplastics are easily meltable and recyclable, but they tend to creep, making them need to have partly high moisture absorption
• Common Thermosets include epoxy (EP), unsaturated polyester resins (UP), and phenolic resins.
• Common Elastomers include silicone and polyurethane (PU).
• Common Thermoplastics include polyethylene terephthalate (PET), polyamide (PA), and polyether ether ketone (PEEK).

Understanding the Glass Transition Temperature

• Transition occurs from the energy-elastic state to the entropy-elastic state
• It defines the glass transition state of polymers.
• Temperature that corresponds to the biggest material property change (elastic and shear modulus)
• Increases in thermal expansion coefficient, specific heat capacity, and failure strain.
• Decreases in elastic modulus, shear modulus, and strength occur.
• Service temperature of polymers depend on the polymer type.
• For Amorphous Thermoplastics the service temperature is less than the glass transition temp.
• For Semi-crystalline Thermoplastics the service temperature is less than the melting temp.
• For Elastomers the glass transition temp is less than the service temperature, which is, in turn, less than the decomposition temp.
• For Thermosets the service temperature is less than glass transition temp.

Additives in Polymer Matrix Systems

• Plasticizers increase the operating temp for shaping
• Fillers reduce volume
• Active chemicals include: Color pigments, anti-shrinking, self-healing etc.
• Thermal materials are Flame Retardants and Internal extinguishers
• Light materials are UV-Absorbers
• Stabilisers are fibers that use oxygen and anti-oxidants
• Atmospherics include Moisture and Hydro-phobics

Mechanical Properties of Polymers

• In stress-strain behavior, brittle polymers (thermosets) exhibit a sharp failure, elastic-plastic polymers have a yield point, and elastic polymers (elastomers) can withstand high strains.
• Thermoplastics have a linear elastic deformation, non-linear, necking and plastic deformation.
• Polymers don't respond to Hooke's law but Hooke's law is valid for approximation under glass transition temperatures.
• Creep is the Time-dependent deformation under constant load
• Stress Relaxation is Time-dependent stress at constant initial elongation.
• Rheology studies material deformations, including viscous behavior.
• This involves material response to mechanical loads, relations stress/strain/strain rate, and dependency with temperature/time.
• Rheology measures viscosity, and applies for non-cured thermosets or fused thermoplastics.
• Newtonian fluids have constant viscosity, while non-Newtonian fluids change depending on the applied shear rate.

Dynamic, Complex, and Kinematic Viscosity in Rheology

• Dynamic Viscosity involves having shear stress divided by shear rate.
• Complex Viscosity is equal to time dependent stress divided by time dependent rate
• Kinematic Viscosity is the dynamic viscosity divided by density
• Dynamic viscosity is the measure of resistance of a fluid to deformation under shear stress.
• Complex viscosity describes viscosity when shear stress varies w/ time.
• Kinematic viscosity considers both viscosity and density of a fluid.
• Shear-thickening is where there is increased cornstarch, and shear-thinning is when there are polymer melts with blood
• A Rheometer measures torques and angles to measure and observe rheology.

Rheology Models

• Hook Model is a spring that experiences elastic behavior that immediately deforms when the force/load is applied
• Newton Model is a damper that experiences viscous behavior where force is applied in sections but has delayed deformation
• Maxwell Model has both series connections between Hook Model and Newton Model for elastic plastic behavior
• Voigt-Kelvin Model has parallel connections of spring and damper; usable with creep, and is an ideal elastomer.
• 4-Parameter Burger Model is serial connections from both maxwell Model and Voigt-Kelvin Model for highly accurate real polymers.

Temperature and Time Equivalence in Polymers

Increasing deformation rate has a similar affect as reducing the temperature, and vice versa, where an increase leads to failure in strength.

• This is known as the Time-Temperature equivalence

Thermoset Matrix Systems

• Curing involves going from monomer stage, linear growth, gelled linked network, and the fully cured thermoset polymer.
• Gelled linked networks are incomplete which is why there needs to be a fully cured network.
• Pot life is when there's no apparent warning that curing is soon to occur so the resin can be used for infiltration.
• Rheological behavior determines infiltration/impregnation time.
• Rheology applies relevant behavior to the processing for viscosity which determines the time it takes to fill.

Common Thermoset and Epoxy Resin Systems

• These resins are unsaturated and are created by polycondensation which increases physical integrity
• Vinyl ester resins (VE) improves impact resistance
• Polyimides (PI) have polycondensation to perform in temperature ranges up to 240 C
• Epoxy Resins (EP) have polyaddition/copolymerisation reactions
• Epoxy Resins are the material the resin is used in and the hardener must have a stoichiometric ratio
• Post curing results in the influence of the mechanical properties on the resin system.
• These are used in highly stressed composite parts found in aerospace applications.

Characteristics of Epoxy Resin Systems

• Low shrinkage during curing results in dimension-ally accurate parts
• Excellent fiber matrix adhesion
• Good Fatigue strength
• Excellent isolating electrical properties.
• They are toxic than UP-resins which costs more.

Thermoplastic Polymers v Thermosets

• These are predominantly used in recycling applications
• However recycling reduces permanence.
• Thermoplastics offer qualities in flexibility/weldable components with fast processing and thermo forming.

Classifications, Structures, and Crystallinty in Themoplastic Polymers.

• Challenges are fibers that have been used previously for composite materials.
• This can occur with adhesion due to the composition of materials (PP, and Epoxy with pas)
• Other challenges include the high temperature stability that can result in creep and moisture absorption.
• High temperatures tools can cause molding issues for composites during temperature changes.
• These include PEEK and PA.
• Polymer structures are amorphous because they are exampled in materials that are transparent.

Impact of Polymers and the Influence of Temperature

• Polymers with high degree of crystallinity include metals, tensile strength, and resistance to solvent.
• In contrast are impact resistance, failure strain, mechanical dampening, and moisture absorption.
• It influences the solubility and properties for its material structure when connected by hydrogen bonding using secondary sources.
• Thermo plastics with high heat use a glass that makes it brittle. It forms a melted polymer shape that may degrade at any temperature. Most thermoplastic melts should be applied to rheometers when applied to low sheets to observe viscosity and properties.
• Polypropylene, polyamide, sulfied, ether ketone are great thermoplastic applications

Typical Thermoplastic Matrices for Composites

• Semi crystalline materials help reduce moisture absorption due to their high temperature -Polylactic usage as the Tg has minor moisture usage because it is used with fiber glass. Other common factors include bumpers
• The End