PKG 323 Lecture 3.5: Barrier and Thermal Properties

What are barrier properties and product interactions?

The ways plastics interact with products through permeability, sorption, and migration

What is permeability?

The movement of gases or vapors such as O2, CO2, and water vapor through plastic

What is sorption?

The uptake of compounds like aromas or colorants from the product into the plastic

What is migration?

The transfer of molecules such as additives or monomers from plastic into the product

What is permeation?

A mass transfer process that affects shelf life and includes sorption, diffusion, and desorption

What are the three steps of permeation?

Sorption, diffusion, and desorption

What is sorption in permeation?

Uptake of permeant molecules at the surface of the plastic

What is diffusion in permeation?

Movement of molecules through the plastic film

What is desorption in permeation?

Release of molecules from the opposite side of the plastic

What is the permeability equation?

P = D × S

What does the diffusion coefficient (D) represent?

How fast a penetrant moves through a polymer; measured in cm^2/s

What does the solubility coefficient (S) represent?

Amount of penetrant absorbed by a polymer; higher solubility means lower barrier

What does the permeability coefficient (P) represent?

Steady-state rate of transport through plastic for a given thickness and driving force

How does chemical structure affect permeability?

Polar permeants pass more easily through polar polymers and nonpolar permeants through nonpolar polymers

How does temperature affect permeability?

Permeability increases with temperature and often doubles for every 10°C increase

How does relative humidity (RH) affect barrier properties?

High RH reduces barrier in hydrogen-bonding polymers by increasing free volume

How does crystallinity affect barrier properties?

Higher crystallinity improves barrier since diffusion occurs in amorphous regions

How do plasticizers affect barrier properties?

Plasticizers increase void space and reduce barrier properties

What are thermal properties of polymers?

They describe how polymers respond to temperature changes

What is the glass transition temperature (Tg)?

Temperature where amorphous regions soften and become rubbery

How does a polymer behave below Tg?

It is stiff, hard, and brittle

How does a polymer behave above Tg?

It is soft, flexible, and has segmental mobility

What is the melting temperature (Tm)?

Temperature where crystalline regions melt into a liquid

Do amorphous polymers have a melting temperature?

No, amorphous polymers do not have a Tm

Why is polystyrene brittle at room temperature?

Its Tg is above room temperature

Why is polypropylene flexible at room temperature?

Its Tg is below room temperature

Why does polyethylene stay flexible at very low temperatures?

It has a very low Tg

What factors increase Tg?

Anything that restricts polymer chain movement

How do bulky side groups affect Tg?

They restrict chain rotation and increase Tg

How do rigid groups affect Tg?

They reduce segmental mobility and raise Tg

How do polar or hydrogen bonding groups affect Tg?

They increase intermolecular forces and raise Tg

What is heat capacity (Cp)?

Energy required to raise the temperature of a polymer by one degree

Which polymers usually have higher heat capacity?

Semi-crystalline polymers

What is heat of fusion?

Energy required to melt crystalline regions and is proportional to crystallinity

What is thermal conductivity of plastics like?

It is very low compared to metals

Why are plastics good thermal insulators?

Because they have low thermal conductivity

What is the thermal expansion coefficient?

A measure of how much a material expands with temperature

How does polymer thermal expansion compare to metals and glass?

Polymers expand more

What is Differential Scanning Calorimetry (DSC)?

A technique used to measure Tg, Tm, and Tc

What is the principle of DSC?

It measures heat flow difference between a sample and a reference pan

How is Tg shown on a DSC curve?

As an endothermic step change or downward shift

How is crystallization temperature (Tc) shown on a DSC curve?

As an exothermic upward peak

How is melting temperature (Tm) shown on a DSC curve?

As an endothermic downward peak

How can you identify an amorphous polymer on a DSC curve?

By the absence of a melting peak