Plastics : Testing plastics mechanical properties - Stretch, tensile strength, ... -

Mechanical properties of plastic refer to how plastics behave under different types of forces and loads, while thermal properties describe how they respond to temperature changes and heat exposure.

 
In-Depth Breakdown:

 
Mechanical Properties:

 
Tensile properties 

Material's behavior when pulled apart 

Measurements:

• Tensile strength (resistance to pulling)
• Elongation (stretching ability)
• Modulus (stiffness)

Examples:

• PET bottles need high tensile strength
• Rubber bands require high elongation
• Building materials need specific modulus values

PET (Polyethylene Terephthalate) is a strong, lightweight, clear plastic commonly used in beverage bottles, food containers, and synthetic fibers (polyester). It's highly recyclable and provides excellent moisture and gas barrier properties.

Impact Strength: 

Ability to absorb shock without breaking 

Types:

• Notched impact strength measures a material's ability to resist breaking when hit with sudden force at a specific point where there's a pre-cut notch, simulating the worst-case scenario of impact at a stress concentration point.
• Unnotched impact strength measures a material's resistance to impact without any pre-cut notch, showing how the material performs under sudden impact in its normal state.
• Drop impact resistance measures how well a material or product can withstand being dropped from various heights, testing its ability to absorb and resist impact energy in real-world conditions.

Examples:

• Car bumpers (high impact PP)
• Safety helmets (ABS)
• Tool housings (modified PC)

High impact PP (Polypropylene) is a modified version of standard PP plastic that's engineered with additives or rubber particles to increase its impact resistance and toughness while maintaining its good chemical resistance and processing properties.
ABS (Acrylonitrile Butadiene Styrene) is a tough, durable plastic that combines the strength and rigidity of acrylonitrile and styrene with the toughness of butadiene rubber, making it ideal for both cosmetic and structural applications.
Modified PC (Polycarbonate) is an enhanced version of standard polycarbonate plastic that's been chemically or physically altered with additives to improve specific properties like impact resistance, UV stability, or chemical resistance while maintaining its optical clarity and strength.

Flexural Properties: 

Plastic behavior when bent 

Measurements:

• Flexural strength is the ability of a material to resist bending deformation before breaking or failing - essentially, how much force it can withstand before breaking when bent.
• Flexural modulus is a measure of a material's stiffness when bent. Specifically, it indicates how much the material resists bending deformation under load.
• Bend recovery is the ability of a material to return to its original shape after being bent or flexed, measuring its elastic recovery properties after deformation.

Examples:

• Living hinges in bottle caps (PP)
• Plastic furniture (HDPE)
• Building panels (PVC)

PP (Polypropylene) is a versatile thermoplastic polymer known for its good chemical resistance, heat resistance, and flexibility, commonly used in food containers, car parts, and packaging.
HDPE (High-Density Polyethylene) is a strong, durable plastic with high tensile strength and chemical resistance, widely used in bottles, pipes, and containers. 
PVC (Polyvinyl Chloride) is a durable plastic that comes in rigid or flexible forms, known for its strength and fire resistance. It's commonly used in construction (pipes, window frames) and medical applications (tubing, blood bags).

Hardness:

Resistance to indentation 

Types:

• Shore hardness
• Rockwell hardness
• Barcol hardness

Shore hardness is a measure of a material's resistance to indentation using a durometer, commonly used for soft plastics, elastomers, and rubbers. Different scales (A, D) are used for different material hardnesses.
Rockwell hardness is a testing method that measures the hardness of materials by measuring the depth of penetration of an indenter under a specific load, commonly used for harder plastics and metals.
Barcol hardness is a hardness testing method specifically designed for rigid materials like hard plastics and composites, using a spring-loaded needle-point indenter to measure surface hardness.

Examples:

• Shoe soles (varying hardness TPU)
• Machine parts (PA66)
• Display screens (PC)

TPU (Thermoplastic Polyurethane) is a flexible, elastic plastic that combines rubber-like softness with durability, commonly used in shoe soles, cable covers, and flexible medical devices.
PA66 (Polyamide 66/Nylon 66) is a strong, heat-resistant engineering plastic known for its high mechanical strength and wear resistance, often used in automotive parts, electrical connectors, and industrial components.
PC (Polycarbonate) is a tough, transparent thermoplastic known for its high impact strength and optical clarity, commonly used in eyewear, safety shields, electronic housings, and automotive parts.

Thermal Properties:

 
Heat Deflection Temperature (HDT): 

Temperature at which plastic deforms under load

Applications:

• Hot-fill containers
• Automotive parts
• Appliance housings

Hot-fill containers are packaging vessels (usually plastic bottles or containers) specifically designed to withstand the high temperatures used during hot-fill processing, where products are filled while still hot (typically 180-185°F) without deforming or failing.

Examples:

• PP for microwave containers
• PET for hot-fill bottles
• PA66 for under-hood components

PET (Polyethylene Terephthalate) is a strong, lightweight, clear plastic known for its excellent moisture barrier and recyclability, commonly used in beverage bottles, food containers, and synthetic fibers (polyester clothing).

Glass Transition Temperature (Tg): 

Temperature where material transitions from rigid to rubbery 

Importance:

• Processing conditions
• Service temperature range
• Material selection

Examples:

• PS (high Tg for rigidity)
• PVC (variable Tg with plasticizers)
• PMMA (high Tg for optical clarity)

PS (Polystyrene) is a clear, rigid plastic that can be either brittle in its standard form or impact-resistant when modified, commonly used in disposable cutlery, packaging, and insulation foam.
PVC (Polyvinyl Chloride) is a durable, versatile plastic available in both rigid and flexible forms, widely used in construction (pipes, window frames), medical devices, and cable insulation.
PMMA (Polymethyl Methacrylate/Acrylic) is a transparent thermoplastic known for its excellent optical clarity and weather resistance, commonly used in windows, signs, and light fixtures. Also known by the brand name Plexiglas.

Melting Temperature (Tm):

Temperature at which crystalline regions melt. Crystalline regions are areas within a plastic material where polymer chains are arranged in an organized, orderly pattern (like a crystal structure), giving the material specific properties like strength and opacity. These regions contrast with amorphous regions where chains are randomly arranged. 

Applications:

• Processing temperature selection
• Maximum use temperature
• Welding parameters 

Examples:

• PE for rotomolding
• PP for injection molding
• PET for bottle blowing

PE for rotomolding (Polyethylene) is a plastic powder specifically formulated for the rotational molding process, used to create large hollow products with uniform wall thickness like tanks and containers.
PP for injection molding (Polypropylene) is a versatile thermoplastic designed for the injection molding process, offering good flow properties and quick cooling to create precise, detailed parts efficiently.
PET (Polyethylene Terephthalate) is a clear, strong plastic known for its excellent barrier properties and recyclability, commonly used in beverage bottles and food packaging, particularly suitable for blow molding processes.

Thermal conductivity:

Ability to conduct heat 

Applications:

• Heat sinks
• Insulation
• Thermal management

Examples:

• Thermally conductive compounds for LED housings
• Insulating foams for construction
• Heat-dissipating electronic enclosures

 
Industrial applications:

 
Automotive:

• High HDT materials for engine compartments
• Impact resistant materials for bumpers
• Weather resistant materials for exterior parts

HDT (Heat Deflection Temperature) is the temperature at which a plastic material will deform under a specific load, indicating its heat resistance and maximum service temperature for practical use.

Packaging:

• High tensile strength for bottles
• Flex crack resistance for films
• Thermal stability for hot-fill containers

High tensile strength refers to a material's ability to resist breaking or deforming when pulled apart under tension. It measures the maximum stress a material can withstand while being stretched before failing.

Flex crack resistance is a material's ability to withstand repeated bending or flexing without developing cracks or failing, particularly important in applications where the material undergoes frequent bending cycles.

Construction:

• UV resistant materials for outdoor use
• High modulus for structural components
• Thermal insulation properties

High modulus refers to a material's high resistance to elastic deformation under stress, meaning it remains stiff and maintains its shape when force is applied. Materials with high modulus require more force to bend or stretch them.

Electronics:

• Flame retardant properties
• Heat dissipation
• Dimensional stability

Dimensional stability is a material's ability to maintain its original size and shape when exposed to various environmental conditions like temperature changes, moisture, or stress. A material with good dimensional stability will resist warping, shrinking, or expanding.

Testing methods:

 
Mechanical Testing:

• Tensile testing machines
• Impact testing equipment
• Flexural testing apparatus
• Hardness testers

Tensile testing machines are devices that pull a material sample from both ends to measure its strength, elongation, and behavior under stretching forces until it breaks.
Impact testing equipment measures how much energy a material can absorb before breaking when hit suddenly, helping determine its toughness and resistance to sudden forces.
Flexural testing measures a material's behavior when bent, determining how much force it can resist before breaking or permanently deforming under bending loads.

Thermal Testing:

• DSC (Differential Scanning Calorimetry) is a testing method that measures how a material's physical properties change with temperature by comparing the heat required to increase the temperature of a sample versus a reference, used to determine melting points and glass transition temperatures.
• TGA (Thermogravimetric Analysis) is a testing method that measures changes in a material's weight as temperature increases, used to analyze decomposition temperatures, material composition, and filler content in plastics.
• HDT testing equipment is specialized apparatus that measures the temperature at which a plastic sample deflects under a specific load, determining the maximum temperature at which the material can be used structurally.
• Thermal conductivity analyzers

Design considerations:

 
Operating environment:

• Temperature range
• Load conditions
• Chemical exposure
• UV exposure

Processing requirements:

• Molding temperatures
• Cooling rates
• Mold shrinkage
• Flow properties

Performance criteria:

• Strength requirements
• Stiffness needs
• Impact resistance
• Heat resistance

This comprehensive understanding of mechanical and thermal properties is crucial for proper material selection and successful plastic product design.