Safety in the Rheology for Processing experiment

If you do not take care during this experiment you could be injured (mainly to hands or head). Therefore, do not start the experiment before a demonstrator has introduced the class.

1. Lab coats and safety goggles must be worn at all times.

2. The Rheometer is an extremely expensive and delicate machine, and it has to be used accordingly with all precaution and care. Make sure you have read and understood how to use it, or ask a GTA to demonstrate it for you. The user manual is located on the machine.

3. In the event of the rheometer not operating correctly, call the GTA or a technician and wait for their help. DO NOT attempt to fix it on your own.

4. Safety spectacles, lab coats and gloves MUST be worn when operating the machine and while working with the polymers provided. Once you have mixed the fluids to produce the “slime”, it is allowed to remove your gloves to work with the slime. However, if you do so, please ensure you wash your hands before touching anything else.

5. Please make sure you operate in the safety of others as the lab is a shared space and you clean everything after your experiment has been completed.

If you have read and understood all safety instructions, then please sign Risk identification and declaration sheet and hand it to your demonstrator before start of the lab.

Important note about lab etiquette:

If a group's work area is not left in the same state (or neater) as it was found, they will be penalised 10% of their total lab mark. If the general lab (floor, labstools, basins, etc) are not left in the state in which they

are found in the morning (or neater), the entire cohort present in the lab that day will be docked 10%. These mark penalties are additive.

Use of Rheology to Guide Polymer Processing

Aim

The aim of this experiment is to learn the use of the rheometer to perform studies on flow and oscillatory properties of various materials. This would strengthen the understanding that these properties play a fundamental  part in  processing  materials and  have often to  be  considered  before  production.  In this laboratory you will explore a range of materials to understand their rheological properties. You will then setup a matrix of samples with known composition to establish a set of viscoelastic properties. In the final part of the lab, you will be asked to use this information to predict the conditions needed to prepare a cross-linked polymeric material within given processing properties and windows and analyse the outcome of your experiment.

Intended learning outcomes

At the end of this experiment students will be able to:

•    Understand the key working principles of the Rheometer and the different types of tests.

•    Know how to measure the viscosity and modulus of very different materials, understanding how crucial the role of theology is in daily processing applications.

•    Recognise differences in materials by their rheological data, differentiating  between  viscous, viscoelastic andelastomeric and differentiate between Newtonian and non-Newtonian behaviour.

•    Synthesise “silly putty” with different concentrations and relate quantities to its cross-linking ratio.

•    Use pre-set data to fabricate a material with the required processing andrheological properties.

•    Use a laboratory book appropriately to record prelab work, experimental procedure and results.

Lab bookkeeping

You are training to be material scientist and engineer and therefore accurate record keeping is a critical skill. It is important that you use your lab book for prelab work, to record your procedure, key findings or observations,  and  make  notes  throughout  your  experiments.  These  may  include  calculations,  written comments or notes, sketches, or tabulated results. The quality of your (individual) record keeping will be monitored and evaluated during the experiment.

Safety and Good Laboratory Practices

Being a good materials scientist also involves working following the safety restrictions and learning to share spaces with other peers and members of staff respectfully. This involves learning to setup a lab space and cleaning/tidying after yourself together with your group. At the end of the experiment you will be expected to have cleared up your working space and the equipment in use. Any pieces of fractured samples will have to be disposed of according to the initial directions of the GTAs and instructions in the risk assessment associated with the experiment. Points will betaken off your final assignment if these practices as described are not followed.

1. Introduction:

Measuring materials properties is not always as straightforward as measurements of tensile or bending strength. Some materials possess way more complex mechanical behaviours that can only be described through complex analyses of their intrinsic properties.   For example, the description of the behaviour of vulcanised  rubber  needs  to  take  into  account  the  incredible  intrinsic  restorative  properties  that  this material uses due to its complex chemical structure. In its use in tyres, the material needs to be able to deform, but also to restore to its original shape without external forces being applied to it. At the same time, it cannot deform spontaneously, but needs to be rigid enough to hold its shape as prepared. Two questions  come  to  mind  immediately:  first,  what  allows  it  to  hold  the  shape  while  still  supporting distortion? Second, what causes it to restore its shape once deformed?

The answers to these questions are the viscoelastic properties of the material. Choosing these correctly gives an ideal behaviour for the selected application. Choosing these incorrectly will lead to a material that is either too stiff and suffers fracture, or too fluid and hence cannot hold its own shape. In this laboratory exercise, you will explore the use of rheology to assess the viscoelastic properties of a polymeric material, and hence learn to use these properties to form. a material with desired behaviour.

Cross-Linking of Polymers

Polymeric materials are useful in a variety of applications. The various processing methods applied to the same  precursor  materials  will yield  dramatically  different  resulting  materials  with  a  diverse  range  of applications. Common examples of cross-linking are observed in rubber tyres, tightly curled hair, contact adhesive or semi-solid gel materials. Each of these examples exhibits a different type of cross-linking, from rigid sulphur bonding in the case of tyres, to sulphide group linking in hair and from fixed covalent bonding in a contact adhesive to relatively weak hydrogen bonding ingels.

Taken individually, polymeric molecules will arrange in multiple geometric configurations. Some of these will have relatively high interaction cross-sections with the molecules around them. The level of interaction between  the  polymers  is  dependent  on  the  chemical   make-up  of  each  chain,  and  what  forms  of intermolecular interaction are therefore possible. The weakest of these, van der Waals interactions, are observed in unbranched chains of a polyethylene material. As branching is increased in the polymer chain, the side branches will act to increase the level of interaction and hence increase the resistance of the material to deformation.

The introduction of hydroxy, sulphur or nitrogen groups introduces the possibility of dipole or hydrogen-  bond  interactions,  which  are  several  orders  of  magnitude  stronger.  These  forms  of  interaction  will  therefore create materials that are more resistant to deformation, but can still be pulled apart relatively  easily. Due to the transient nature of the bonding, these bonds can also be used to create “self-healing” materials as the bonds can reform. on the edges of the material.

The strongest form. of cross-linking, achieved through chemical reaction, is the formation of covalent bonds between polymer molecules. The more covalent bonds are established between the molecules, the more rigid the material will become. However, unlike hydrogen bonded cross-linking, in the event of the bonds being broken, they will not spontaneously reform.

During the second worldwar James Wright was researching methods to create synthetic rubber as there was a high demand and shortage of rubber. In the process of these explorations, he found a method to create a cross  linked  polymer  material that exhibited some  unusual  properties.  His  reaction  involved boron-mediated chain breaking and hydroxy-boron bonding to link poly-methoxysilane molecules. The resulting material would bounce if dropped, and could be pulled or pushed into a rigid shape. However, if left to rest, the material would shift its shape to adopt the lowest energy shape for the environment, i.e. it would flow. If pulled slowly, the material could stretch out into along strand, but with a large rapid force applied, it would shear and break. Ultimately deemed useless at the time, it was a material that went on to be branded as “Silly Putty” (or slime), and ended up creating a multi-billion dollar company in the toy industry!

Cross-linking also has fundamental industrial applications and you should explore some of these in your own write-up.

Measurement of properties – Rheology Rheology

Rheology is a tool to study and measure the deformation (flow) of matter upon the application of stress (or strain), by analysing the internal response of materials to forces.

This study can be carried out by observing and recording the behaviour of a material when subjected to linear or oscillatory stresses, and it is useful to characterise many materials, from low viscosity Newtonian fluids (including water and air) to soft materials with high viscosity. It can also be applied to materials having  complex  structures  such  as  suspensions,   emulsions,  polymers,  gels,   bodily  fluids  and  other biological materials.  Some of these soft materials can exhibit a unique complex behaviour that cannot be characterised by standard mechanical testing. These materials in fact exhibit a “viscoelastic” behaviour, they respond in a viscous or elastic way, depending on the speed or the amplitude of the shear applied to them.

When defining viscoelastic properties of a material, it is important to introduce G defined as the shear modulus of a material. G is a complex number made by an imaginary and a real component as per below:

G = G ′ + iG"

The storage modulus G′ describes the elastic properties of a material and the loss modulus G" describes the viscous properties of a material. In other words, G′(storage modulus) could be related to the Young’s Modulus E, and G" (loss modulus) could be related to the shear modulus.

Read Appendix 1 to know more on how this equation can be derived.

Figure 1: Description of the broad range of Rheological properties and behaviours of different materials.

:Newtonian behaviour: fluids exhibiting such behaviour will have a stress response linearly proportional to the shear rate, and the proportionality constant will be the viscosity. Examples are Water, Air and some Organic Solvents.

Viscoleasticity: The  property  of  a  substance  of  exhibiting  both  elastic  and  viscous  behaviour  when deformed, therefore exhibiting a time-dependent strain. Their viscosity value will be straindependent.

•     Elasticity: A  material is called elastic if the deformation produced in the body is reversible (the object will return to its initial shape and size when these forces are removed)

•    Viscosity: Viscosity is a measure of a fluid's resistance to flow. A material will be purely viscous if all the shear stress is transformed in strain. (the object will not return to its initial shape once forces are removed)

These differences in viscous to elastic behaviour are due to a characteristic internal  material time of relaxation. If this time is very short, compared to the time of deformation applied, the material will behave in a viscous way. This time for water is, for example, 10-10s, meaning that any standard deformation will be always longer and the material will behave in a viscous way. If on the contrary, the material exhibits a very long intrinsic time compared to external stresses, it will mostly behave elastically. Time dependency is extremely crucial in Rheological measurements.

Polymers are viscous fluids. Silicone putty is a good example: It will behave like an elastic body if it is subjected  to  a rapid deformation such   as   bouncing  on  the  floor  (recoverable  deformation).   The stored energy makes the ball bounce back. On the other hand, if you hold the putty at rest for a long time, it will, by the gravitational force start flowing down. This flow is the viscous component of the behaviour of the silicone putty.

Non-Newtonian Behaviour: Some of the viscoelastic behaviour can be more complex, with G’ and G” varying depending on shear rate. Two examples of this behaviour are shear-thickening and shear thinning  substances. Most liquids, suspensions orgel materials will flow faster the harder they are pressed. This is  known as shear thinning (G’>G” at low shears and at high shears G”>G’ meaning that the material acts as  a solid at low shears and as a liquid at high shears). However in some cases the opposite effect is observed,  and the materials experience reduced flow with increased pressure, which is known as shear thickening  (G”>G’ at low shears and at high shears G’>G” meaning that the material acts as a liquid at low shears and  as a solid at high shears).

Shear-thickening examples: cornstarch + water, silly putty, quicksand Shear-thinning examples: blood, paints, toothpaste

This is due to Interactions between the molecules within the structure. Therefore, it is possible to tailor a material  to  select  between  shear-thinning  and  shear-thickening,  at  least  in  principle.  For  polymeric materials, especially those that are cross linked, this parameter can be controlled through consideration of the average molecular weight of the polymers, the extent of cross-linking, and the amount of solvent included in the matrix.

Tailoring rheological properties for processing and production

The longer a polymeric chain (i.e. the larger the molecular weight of the polymer), the more cross linking can occur between that polymeric chain and a neighbouring chain. This will lead to a more shear-thickening material. If the amount of cross-linking increases too much, then the material will become too stiff to ever exhibit any flow and hence will be prone to fracture. So there is a careful balance between increasing the cross linking to increase shear-thickening, and keeping it low enough to still allow flow of the material, if shear thickening is the desired principle.

It is important to realise that not all polymers that are cross-linked will flow. The conditions of flow are regulated as well by properties such as the glass transition temperature, melting point and decomposition or boiling point of the polymer. Some polymer materials are solid at room temperature and will undergo chain breaking at elevated temperatures before they reach a stage of flowing. Others will form crystalline solid structures that behave quite differently.

It is also important to recognise that in many cases, it is the presence of transient hydroxyl interactions that will lead to the properties such as shear-thickening. Therefore, the extent of water/hydroxy groups present in the material becomes an important consideration.

Also  worth  noting  is  that  polymeric  materials  are  not  alone  in  exhibiting  properties  such  as  shear- thickening. Suspensions in water can show the same properties.

Shear-thinning materials will follow exactly the opposite behaviour. If there is no force applied, they will hold their shape, while if a force is applied, they will become fluid and flow.

Many layered structures, such as hydrated clays, will follow the same behaviour. This can be a significant problem (such as the case of the Leaning Tower of Pisa), or a significant advantage (such as clays used in pressure transfer in synthetic diamond production).

Applications of Rheology in Processing:

When  in  processing  and  production,  the  rheological  behaviour  of  materials  can  affect  the  final applications. Examples of applications include injectability, extrusion moulding, dip coating, spreading (lotions and paints) and jetting (inks).

It is important to be able to properly tailor and characterise the rheological behaviour of viscous fluids and viscoelastic  materials  before  their  application  in  processing  techniques.  Improper  characterisation  of shear-thickening fluids could causestall or damage on equipment when not accounted for and the same is true for shear-thickening liquids that, if not accounted for, could lead to poor mixing, cavitation or fast flow extrusion. In Figure 2, the working windows for different processes are reported and compared with the working range of the rheometer you have available in the lab.

Figure 2: Rheological window of uses in shear rate for different processing techniques, compared with

Rheometer range.

Types of Rheological tests:

The Rheometer is an instrument essentially composed of 2 parts (although very expensive ones). The rotating  top  geometry  (can  be  interchanged  from  cone  to  flat,  depending  on  the  type  of  samples measured) and a bottom flat plate. The sample is placed very tightly in contact with the 2 surfaces and is subjected to a controlled strain or strain rate and the parameter recorded is the stress in the material in response to the applied strain.

Flow tests.

These tests are usually carried out on Newtonian fluids, where the viscosity is constant. They can however also be used to observe the change in viscosity with shear rate for non-Newtonian materials. Flow tests are based on measurements in controlled shear rate, the rheometer applies a rotational speed (w (rad/s)), translated into a shear rate (y(̇), s-1) and the measured data is a torque force that the material exerts in response to the plate rotating, recorded therefore as a shear stress (τ (Pa)). To obtain the viscosity η of a material the equation below (also referred to as Newton’s Law) is used:

Figure 3: Flow test mechanism and plots obtained using arheometer with relevant equations.

Oscillatory tests.

Oscillation tests are normally used to measure the viscoelastic properties of materials.  By imposing a sinusoidal wave (stress) in oscillation, varying with time, on the material we can measure both the viscous component and the elastic component by measuring the resulting sinusoidal strain on the material and the phase angle δ, the difference between the input and the output waves. When the behaviour of a material is purely elastic, δ will be 0°, as stress and strain will be in phase. When the behaviour of a material is purely viscous instead, with stress and strain out of phase, δ will be 90° .   Viscoelastic materials are somewhere in the middle between these 2 behaviours and will exhibit a phase angle comprised between these two ideal cases depending on the rate of deformation.

Figure 4: Oscillatory test  mechanism with associated  mechanical  sinusoidal response  using a  rheometer with relevant equations.

The   output   graph    of    a   rheometer    in   small    amplitude   oscillatory    measurements    (SAOS)   will show the storage  modulus (elastic part G’) and loss   modulus  (viscous part G") as  a  function  of  test frequency (“) (as in Fig 5). The phase shift is often expressed as loss tangent (tan δ) that is equal to the energy lost to energy stored (G"/G’), describing the relative degree of viscoelasticity. The modulus value at this point is called the cross-over modulus and this indicates the state of matter is crossing over from liquid to solid (onset of gelling) or viceversa. Different types test setups can be arranged which are strain or stress sweep, frequency sweep, temperature sweep and time sweep.  (Practical Food Rheology: An Interpretive Approach pp148-149).

We will be applying the frequency sweep test during this lab demonstration to investigate time-dependent shear behaviour of your polymeric material.  High frequency represents short term behaviour (remember the  bouncing  putty  example),  and  the  low  frequency  represents  the  long term  behaviour  (stretching putty).  In general elastic response is prominent for the short time and viscous behaviour for the longtime tests.  In frequency seep tests,a sinusoidal strain (or stress) of fixed amplitude is applied on the material, and the dynamic moduli are determined over a wide range of frequencies.

Figure 5: Example Plot of frequency dependence of G" and G’ .

2. Experimental procedure

Task 1 (spend no more than 1 hour on this task)

Materials needed: syringes (without needle - 5ml or 20ml), plastic beakers (10 per table), solutions as listed below, stirring sticks.

You have been provided with multiple materials in various forms. In this portion of the practical, you will need to experiment with each and predict whether each is shear-thinning or shear-thickening, Newtonian or viscoelastic.. In order to carryout this assessment, you should prepare the materials as described below and then explore the  behaviour of the  material when: stirred  rapidly,  poured from one  container to another, and when forced to flow out of a syringe. Using the theory as described in the introduction, you should then predict and discuss as a group what type of behaviour you are seeing.

•    Use distilled water as provided.

•    Use the tomato ketchup as provided.

•    Use the mayo as provided.

•    Use the oil as provided.

•    Use the golden syrup as provided.

•    Use the toothpaste as provided.

• Mix corn flour and water 2:1 volume ratio.

•    Use the hair gel as provided.

•    Use the rubber film as provided.

Observe, as shown by the GTA, how the rheometer works and, once you have explored and made your predictions  regarding  the  type  of   behaviour  of  each   material  (Newtonian,  shear  thickening,  shear thinning), consider the rheological data collected for a single material. You will be provided all the other sets of data but explore only one material at the rheometer. All materials can be disposed of down the drain, except the rubber.

A GTA will carryout 2 different tests: Flow tests and Oscillatory tests, depending on the material provided. The GTA will setup for both of the different tests. Compare and discuss with each other and your GTA regarding  your  predictions  and  what  was  observed  in  the  data.  Try  to  explain  using  molecular considerations,what is happening in each material to cause the observed rheological data.

Discuss in the lab with GTA: What is the difference  between the various materials? Can these materials show more than one behaviour at the sametime? E.g. Viscoelastic & shear-thickening?