MapaMetal_Plasticos

Plastics and Composites Engineering - M5036

Molecular Structure

chemical and physical aspects

Introduction

Rheology and mechanical properties

Plastics and composites

Chemical Characteristics

Linear Visco-Elasticity

Extrusion

Manufacturing processes

Injection

Extrusion

Definition: it is a manufacturing system in which raw polymer materials are melted, shaped, and formed into continous profiles or products.

Fiberspinning

Cast Film

Blown Film

Blow-up ratio

Bubble diameter vs. die diameter

Take-up speed

Film winding speed

Die gap

Die opening width

Die diameter

Size of the die

Injection

Definition: it is a manufacturing process where the polymer is metled, injected into a mold cavity, cooled, and solidified to form complex, precise shapes.

Blow Molding

Defects

Molecular Structure - Intimate nature of polymers

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Definition: Long molecules composed of repeated units called mers.

Use

Elastomer | Fiber | Plastics

Thermal Behavior

Mer

Polymer

Thermoplastics | Thermosets

Copolymer

Crystallinity

Classification

Semi-crystalline | Amorphus

Chemical Origin

Free Radical (chain growth | Condensation (byproduct release) | Catalyst (controlled growth) | Ionic (charged initiation)

Copolymers

Materials formed by a combination of differents monomers

Chemical Characteristics - Intimate nature of polymers

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Polymer

Polymerization is the chemical processes in which monomers form large repeating molecules known as polymers. The size of this polymer created directly influence the final product behavior. Analyzing the Molecular Weight Distribution of a sample can give you insights on the chemical and mechanical characteristics of a polymer.

Isotactic

Syndiotactic

Molecular Weight

Spatial Orientation

Atactic

Molecular Weight Distribution

Tg | Tm

Molecular Weight Distribution (MWD)

Molecular Weight Distribution of a given polymer describes how molecular weights are distributed within a sample. This distribution significantly influences the polymer's properties, including its mechanical behavior, processing characteristics, and application performance. Each molecular weight (Mn, Mw, Mz) provides specific insights into how the MWD affects a polymer's mechanical properties.

Mn

Mw

Mz

MWD

Weight Fraction

Molecular weight

Polydispersity measures the breadth of a polymer's MWD.

Broad Polydispersity (PDI>1): Mix of long and short chains. May enhance toughness and processing flexibility but reduce uniformity.

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Mw

PDI

Weight Fraction

PDI

Mn

Narrow Polydispersity (PDI<1): Uniform chain lenght. Results in more consistent physical and mechanical properties.

Molecular weight

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Mn

Mw

Mz

Mw takes into account the molecular weight of each chain in determining contributions to the molecular weight average.

Mz emphasizes the contribution of the longest chains in the polymer sample

Mn is the arithmetic mean molecular weight of all polymer chains in a sample. It gives equal weight to each chain, regardless of its size

NiMi

Mn

NiMi

Mn

NiMi

Ni

Mw

NiMi

NiMi

Mz takes into account the higher molecules weights, that is the ones at right tail of the MWD curve.

Mi is the molecular weight of a chain and Ni is the number of chains of that molecular weight

Mechanical Properties Affected

Mechanical Properties Affected

Mechanical Properties Affected

Tensile strength, impact resistance, elasticity; improves load-bearing capacity and energy absorption.

Ductility, brittleness, initial chain entanglement; affects toughness and elongation.

Melt strength, elastic recovery, stress relaxation; governs behavior in extreme mechanical stress scenarios.

What is it?

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Tg and Tm

Differential Scanning Calorimetry (DSC) is a thermal analysis technique used to measure the heat flow into or out of a polymer sample as it is heated, cooled, or held at a constant temperature. It provides valuable information about the thermal properties and transitions of polymers, which are critical for understanding their structure, processing, and performance.

• Tg: The temperature range where a polymer transitions from a rigid, glassy state to a rubbery, flexible state.
• Tm: Indicates the upper thermal limit for processing and application of the polymer

DSC

How does it works?

• A small sample of the polymer is placed in a pan alongside a reference pan.
• Both pans are subjected to the same temperature program (heating, cooling, or isothermal).
• The DSC measures the difference in heat flow between the sample and the reference as the temperature changes.

Introduction to Rheology

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Rheology looks for a quantitative relation between the force applied and the resulting deformation or flow (or between the deformation or flow applied and the induced force)

Flow Curve Viscosity Curve

Models

Newtonian Region

• In a purely Newtonian fluid, the dashpot is sufficient to describe the behavior. The viscosity ( 𝜂 η) is constant, meaning the stress is linearly proportional to the strain rate.
• This corresponds to the linear, steady shear viscosity observed in low to moderate shear rate regions for Newtonian fluids.

Non-newtonian Region

• For non-Newtonian materials, the dashpot alone is insufficient to capture behavior because the viscosity changes with shear rate.
• Shear-thinning (pseudoplastic): Viscosity decreases with increasing shear rate.
• Shear-thickening (dilatant): Viscosity increases with increasing shear rate.

Viscoelasticity

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Viscous

Viscoelastic

Elastic

Viscoelastic liquidsGlues | shampoos

Ideal-viscous liquidsWater | Oils

Viscoelastic solidsPaste | gels

Ideal-elastic solidsStone | Steel

Elasticity Law

Viscosity Law

Viscoelasticity

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Viscoelasticity is a material's property that explains and combines the viscous and elastic behavior.

Shear Viscosity

Constant Strain

Constant Stress

In reality, most materials are neither purely elastic nor purely viscous; they exhibit viscoelastic behavior, meaning they have properties of both solids and liquids.

Shear Viscosity

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Viscosity is a measure of fluid's resistance to flow. Shear Viscosity is the fluid's resistance to deformation under a shear force (a force applied parallel to the surface of the fluid).

Viscous modulus

Shear Stress

Shear rate

Creep and Recovery compliance

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Creep: describes the time-dependent deformation of a material under a constant stress. Recovery: is the time-dependent strain reduction observed after the stress is removed.

Creep and Recovery vs. Moduli: Creep and recovery tests provide time-domain insights into viscoelastic behavior, while the storage modulus (𝐺′) and loss modulus (𝐺′') represent frequency-domain properties. During creep, 𝐺′ and 𝐺'′ influence deformation, with 𝐺′ governing recovery and 𝐺′′ dictating the permanent viscous strain.

G' G''

G' G''

G* G

• Storage Modulus (G'):
• Represents the elastic (solid-like behavior of a material)
• Indicates how much energy is stored and recoverded per cycle of deformation
• Loss Modulus (G''):
• Represents the viscous (liquid-like) behavior of a material.
• Indicates how much energy is lost as heat per cycle of deformation.

• Complex Modulus (G*):
• Represents the total resistance to deformation
• Magnitud of the overall material stiffness.
• Shear Modulus (G):
• Describes the elastic response to small deformations
• Tan delta:
• Ratio of G'' to G', describing the balance of viscous and elastic behavior.

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Relaxation Modulus, DMA, and TTS

Relaxation modulus: A time-dependent modulus that describes how a material relaxes stress under a constant strain

DMA: A technique to measure a material's viscoelastic properties as a function of temperature, frequency, or time.

TTS: A principle used to extend viscoelastic data measured over a limited time or frequency range to predict material behavior over much longer times or wider frequencies.

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