Viscoelasticity

Viscoelasticity

KEY POINTS: After reviewing the Viscoelasticity presentation, students should: • Understand entanglement and viscoelasticity • Understand how changes in temperature and strain rate affect thermoplastic products • Understand how other polymer factors such as molecular weight, fillers, branching, crystallinity, and structure affect a material’s properties.

Overview Because of the entanglement of the molecules thermoplastic materials have different properties compared to other materials like metals. The polymer chains can slide past each other because they do not share chemical bonds with the other chains around them. Polymer materials have viscoelastic properties They stretch (elastic) and they flow (viscous)

Viscoelasticity If you put a load on a metal rod, it will stretch to a certain length. As long as the load does not exceed the yield strength of the rod, when the weight is removed, the rod will return to its original length. This is elastic behavior If you put a load on a plastic rod, it too will stretch. If you remove the load quickly the rod may return to its original length, but if you leave the load on the rod for a length of time, the polymer chains will slide past each other and flow to make the rod’s length longer (an increase in strain) The longer the load is applied, the more the rod will lengthen (the more the chains will have flowed) until the bar ruptures.

Viscoelasticity This cold flow exhibits the viscoelastic nature of thermoplastic materials. An increase in length with a constant load over time is known as creep. Think of a plastic plant hanger stretching over time A decrease in stress with a constant strain is known as stress relaxation. Think of a plastic cap that loosens as it sits on a shelf. If you roll silly putty into a ball, and set it on the desk, it flattens out by itself. This is viscoelastic behavior. Gravity causes the molecules to flow to relieve the stress.

Viscoelasticity There are two main things that will affect the viscoelasticity of a plastic part. Temperature and Strain Rate As you increase the temperature, the polymer chains are further apart, there is more free volume, and can slide past one another more easily. As you increase the strain rate, the polymer chains don’t have enough time to flow past one another and they will break sooner. They get tangled with each other.

Viscoelasticity It is important to remember that Viscoelasticity only deals with solidified plastic materials. When the material is molten, viscosity refers to how the chains move in relation to each other.

Viscoelasticity Why is viscoelasticity important? • Thermoplastic materials have both long-term and short term properties – they flow due to stress over time • Most mechanical testing of plastic materials is actually testing the material’s viscoelasticity – how the plastic flows when different stresses are applied • Plastics are rate sensitive • It is important to understand the viscoelasticity of plastic materials so you know how they will behave in their intended application Viscoelasticity is a fundamental concept of plastic behavior that you need to understand

Viscoelasticity A good way to understand how material factors affect the viscoelasticity of a polymer is to look at how they are commonly tested. Some common mechanical tests performed on most plastic materials are: • Tensile testing • Impact testing

Testing - Tensile In a tensile test, a test bar commonly called a dog-bone is pulled from one end while the other end is fixed. Fixed

Testing – Tensile The test machine causes the moveable grip to move away from the fixed grip at a constant rate. While the grip is moving away, the equipment measures the force necessary to maintain the constant speed and creates a curve to evaluate the results.

Testing – Tensile Some important properties gained from the stress/strain graph are: • Modulus • Yield stress • Ultimate elongation • Toughness The stress is the load divided by the cross sectional area of the test bar and the strain is the change in length divided by the initial length.

The Modulus is the slope of the curve in its most linear (initial) portion. It is the change in stress/change in strain. It is a measure of the stiffness of the material. Testing – Tensile

Testing – Tensile The yield point is taken when the slope of the line is zero and is sometimes called the zero slope yield.

Testing – Tensile The ultimate elongation is how far the bar stretched prior to failing

Testing – Tensile The toughness is the area under the curve, it is a measure of the amount of energy absorbed by the sample before failure.

Testing – Tensile These figures shows how the curve indicates the material properties.

Properties – Tensile How would the tensile properties change: • if the temperature were increased? • if the strain rate (test speed) were increased? • If the molecular weight was higher? • If a filler were added to the material? • If there were a higher level of crystallinity in the material? • If the material had a higher level of branching?

More free volume allows the chains to flow past each other easier Properties – Tensile How would the tensile properties change if the temperature were increased?

The polymer chains don’t have time to slide past each other. Like running Your fingers quickly through long hair Properties – Tensile How would the tensile properties change if the strain rate (test speed) were increased?

Bar run at 1 in/min strain rate Bar run at 20 in/min strain rate Properties – Tensile How would the tensile properties change if the strain rate (test speed) were increased?

Run at 1 in/min Run at 20 in/min Properties – Tensile How would the tensile properties change if the strain rate (test speed) were increased?

Properties – Tensile Changing the temperature in one direction is comparable to changing the test speed in the other direction in terms of the effect on the material’s properties.

Properties – Tensile How would a higher molecular weight affect the tensile properties? • A higher molecular weight will affect some of the properties, but typically the modulus does not change much. • A higher molecular weight increases the entanglement of the polymer chains, which will increase the elongation, toughness, and yield strength (slightly). • The modulus is more dependent on the structure of the molecule and less on entanglement forces, so it is relatively unaffected by longer chain lengths.

Properties – Tensile How would adding a filler affect the tensile properties? • Fillers hinder molecular movement, they prevent the molecules from sliding past one another • Fillers make the material stiffer • Increase modulus and yield strength • Decrease elongation and toughness • The amount of the effect of the filler on the properties will depend on: • Amount of filler • Type of filler • Shape and size of the filler particle

Properties – Tensile How does the amount of crystallinity affect the tensile properties? • In a semi-crystalline material, a higher level of crystallinity will provide: • Higher modulus and yield stress • Lower elongation and toughness • The densely packed crystalline regions prevent the chains from sliding past one another • This does not necessarily mean that a semi-crystalline material will be stiffer than an amorphous material. It is more a matter of the individual material’s chain structure and where the material is in relation to its Tg.

Properties – Tensile How would a higher level of branching affect the tensile properties? • More shorter branches would cause a higher degree of entanglement and the material would behave similarly to if you increased the molecular weight. • Longer branches provide more space between the molecules and reduce the entanglement similarly to if you increased the temperature

Testing – Impact The most common impact test performed on plastic materials is probably the notched Izod impact test. In this test a 2.5 inch long test bar is fixed with the notch held level with the fixture.

The machine compares the height that the hammer swings after passing through the bar with the height that the hammer swings when there is no bar present and calculates the energy absorbed by the bar. One problem with this method is that plastic materials are rate sensitive and although you can change the size of the hammer, the speed that it hits the sample is the same every time. Testing – Impact A weighted hammer is released and swings through the test bar.

ABS sample -note the hinged break Polypropylene sample -note the complete break Testing – Impact A weighted hammer is released and swings through the test bar.

Properties – Impact How would the impact properties change: • if the temperature were increased? • if the strain rate (test speed) were increased? • If the molecular weight was higher? • If a filler were added to the material? • If there were a higher level of crystallinity in the material? • If the material had a higher level of branching?

Properties – Impact How would the impact properties change if the temperature were increased? Impact properties generally increase with a rise in temperature – to a point. The polymer chains being able to move more easily allows them to absorb more energy before failure, but as you approach the melting temperature the material cannot absorb the energy and fails sooner.

Properties – Impact How would the impact properties change if the strain rate (test speed) were increased? An increase in the rate of impact will reduce the impact strength The chains cannot slide fast enough to absorb the energy.

Properties – Impact How would the impact properties change if the molecular weight was higher? An increase in molecular weight would provide more entanglement and definitely allow the material to absorb more energy before failure?

Properties – Impact How would the impact properties change if a filler were added to the material? The impact properties would change depending on the same factors that would affect a material’s tensile properties • Amount of filler • Type of filler • Shape and size of the filler particle • Fillers can either increase or decrease the impact strength, but they do always make the break more brittle

Properties – Impact How would the impact properties change if there were a higher level of crystallinity in the material? A higher level of crystallinity generally lowers impact strength in polymers. The dense crystal structure does not allow the chains to slide past each other to absorb as much energy as when the material is in a random amorphous state.

BRITTLE DUCTILE Properties – Impact How would the impact properties change if the material had a higher level of branching? Small short branches would increase the impact strength by improving the entanglement, but large branches could allow the chains to slide too easily making the material too ductile.

Conclusion Having a firm grasp on the concept of viscoelasticity and how it affects part properties will provide an understanding of how plastic materials will behave in different applications and under different conditions. As was mentioned previously, this is one of the fundamental concepts dealing with polymeric material behavior and you need to understand it if you plan on working in the plastics industry.

Viscoelasticity Questions?

Viscoelasticity