Plastics : Real life examples of Mechanical Property Challenges , solutions and Tradeoffs in the Plastics Industry

Mechanical property tradeoffs in plastics refer to the engineering compromises manufacturers must make, as improving one property (like strength) often comes at the expense of another (like flexibility). 

Finding the optimal balance depends on the specific application requirements.

Key mechanical properties:

Strength:

• Definition:  material's ability to resist deformation under load
• Measurement: tensile strength ( maximum stress a material can withstand while being stretched before breaking), compressive strength (maximum stress a material can withstand while being squeezed before failing), yield strength ( stress at which a material begins to deform permanently not returning to its original shape when the force is removed) 
• Molecular basis: depends on polymer chain orientation, crystallinity(degree to which atoms or molecules in a material are arranged in a regular, repeating pattern  rather than randomly), and molecular weight

Flexibility/Ductility:

• Definition:  material's ability to bend without breaking
• Measurement: elongation at break, flexural modulus 
• Molecular basis: related to polymer chain mobility and degree of crosslinking

Flexural modulus: measure of a material's resistance to bending under load, indicating its stiffness when bent
Chain Mobility: how easily polymer chains can move or rotate within a material.
Degree of Crosslinking: the extent to which polymer chains are connected to each other by chemical bonds, forming a network structure.

Impact resistance:

• Definition: material's ability to absorb energy during sudden impact
• Measurement: Izod impact strength, Charpy impact tests
• Molecular basis: Depends on molecular weight, crystallinity, and phase separation

Izod impact strength: measure of a material's resistance to sudden impact, tested by striking a notched sample with a pendulum.
Charpy impact test: similar to Izod, measures impact energy absorption using a pendulum to strike a notched specimen supported at both ends.
Molecular weight: sum of atomic weights of all atoms in a molecule, indicating its size.
Crystallinity: degree to which atoms or molecules in a material are arranged in a regular, repeating pattern rather than randomly.
Phase separation: process where a mixture separates into distinct regions with different chemical compositions or physical states.

Common tradeoffs:

Strength vs. Flexibility:

• Increasing crystallinity improves strength but reduces flexibility
• Higher molecular weight typically improves both properties to a point
• Crosslinking increases strength but decreases flexibility

Strength vs. Impact resistance:

• Highly crystalline polymers are stronger but more brittle
• Adding impact modifiers improves impact resistance but reduces tensile strength
• Processing conditions affect molecular orientation, creating strength/toughness tradeoffs

Impact modifiers: additives that improve a material's ability to absorb energy during impact without breaking.
Tensile strength: maximum stress a material can withstand while being stretched before breaking

Temperature Performance vs. Processability:

• High-temperature resistant polymers are harder to process
• Easier processing often correlates with lower mechanical performance

Real-Life examples:

Automotive bumpers:

• Challenge: Need impact absorption while maintaining structural integrity
• Solution: Thermoplastic polyolefin (TPO) with optimized rubber content
• Tradeoff: Higher rubber content improves impact resistance but reduces stiffness

Smartphone Cases:

• Challenge: Protection from drops while maintaining thin profile
• Solution: Polycarbonate (PC) for strength with Thermoplastic Polyurethane (TPU) for shock absorption
• Tradeoff: harder materials protect better from scratches but worse from impacts

Medical Catheters:

• Challenge: Flexibility for navigation with strength to maintain lumen
• Solution: Polyurethanes with varied hard/soft segment ratios
• Tradeoff: More flexibility compromises pushability and kink resistance

Lumen: hollow space inside a tube-like structure, such as a blood vessel or catheter.
Polyurethanes: versatile polymers used in foams, coatings, adhesives, and medical devices.
Pushability: ability of wire to transmit force from its proximal end to its distal end (part farthest from the point of attachment or origin) when pushed.

Kink Resistance: material's ability to resist folding or collapsing when bent at sharp angles.

Soda bottles (PET):

• Challenge: Lightweight while maintaining pressure resistance
• Solution: Biaxial orientation of PET molecules is the process of stretching PET (polyethylene terephthalate) in two perpendicular directions, aligning polymer chains to improve strength, clarity, and barrier properties.
• Tradeoff: optimized for strength in two directions at the expense of other properties

Construction piping:

• Challenge: Long-term durability vs. installation flexibility
• Solution: PVC for rigid applications, PEX for flexible needs
• Tradeoff: PVC offers strength but brittleness, PEX offers flexibility but lower pressure ratings

PVC (Polyvinyl chloride) rigid plastic polymer used in pipes, medical tubing, and construction materials.
PEX (Cross-linked polyethylene) a flexible plastic with improved temperature resistance and durability, commonly used in plumbing and radiant heating systems.

These examples demonstrate how manufacturers must carefully balance competing mechanical properties through polymer selection, additives, processing techniques, and design considerations to meet specific application requirements.