Basics of Polymer Rheology in Adhesives and Sealants

This article was originally published in 2005 and updated in 2020.

TAGS:  Rheology Modifiers      Rheology Testing    

Rheology is the science of deformation and flow of matter. It is as important as any other discipline when studying adhesive systems. To ensure that adhesives and sealants function well during their application and end-use, the formulator must be able to control the flow properties of the product. The main challenge that the formulator faces is that the adhesive or sealant may need different flow characteristics at different times.

(Rheology can be elusive for many adhesive formulators and end-users because it uses complex models and theories that are not familiar except, perhaps, to the polymer scientist)

When we talk about adhesives, rheology plays a vital role in all the adhesion processes which include:

• Application
• Surface flow and wetting
• Penetration into surface topology, pores, and narrow joint gaps
• Diffusion over interfaces
• Solidification
• Resistance to internal and externally applied stress

Certain adhesives and sealants must also be capable of easy flow application by trowel or extrusion, but they must also exhibit sag and slump resistance, even at elevated temperatures. Therefore, the flow properties, or rheology, of the material must fit the desired method of application.

Moving forward, you will get to know the basics of rheology as it is applied to adhesive systems. You will learn about a few of the rheology principles (and not complex rheological expressions and equations) applied at the two levels:

1. Formation of the joint and
2. Physical properties of the joint once it is made.

Let’s begin by understanding rheology and polymer properties.

Basics of Rheology

Rheology in the adhesives industry deals primarily with material characteristics that are caused by the change of force. This change is time-dependent and can be expressed as rate or velocity.

For example, the loading of a tensile-shear specimen in an Instron tester can be expressed as the movement of the crosshead in cm/min. If fatigue is encountered in service, this change can be expressed by frequency. Note that no loading of an adhesive joint is possible unless there is a change in force that can be measured as a function of time. So, intuitively the study of rheology as it applies to adhesion is important.

There are three categories of polymer properties that control rheological response:

1. Chemical composition (all properties derived from the monomeric unit), 
2. Molecular structure (all properties derived from the organization of monomeric units in a polymeric chain), and 
3. Molecular free volume (all properties resulting from the general physical state of the polymer). 

Table below describes the specific polymer properties that fall under each of these categories.

Chemical	Molecular Structure	Molecular Free Volume
Monomeric units	Tacticity	Degree of crystallinity
Side groups	Main chain symmetry	Temperature
Secondary bonding forces	Molecular weight distribution	Pressure
Cohesive-energy density	Chain branching	Time
	Entanglements
	Crosslinking

Polymer Properties that Control Rheological Response¹

Rheological Behavior of Polymers

The rheological behavior of polymers involves several widely time-dependent phenomena, which can be related to the different polymer property categories above. These phenomena and their major mechanisms are as follows:

• Viscous flow – It is the irreversible bulk deformation of the polymeric material, associated with irreversible slippage of molecular chains past one another.
• Rubberlike elasticity – It involves local freedom of motion that is associated with small scale movement of chain segments, but large-scale movement (flow) is prevented by the restraint of the diffuse polymer network structure.
• Viscoelasticity – The deformation of the polymer specimen is reversible but time dependent and is associated (as in rubber elasticity) with the distortion of polymer chains from their equilibrium configurations.
• Hookean elasticity – It involves the motion of chain segments is drastically restricted and involves only bond stretching and bond angle deformation (i.e., the material behaves as a glass).

All these mechanisms come into play when describing adhesion. However, the two that are, perhaps, the most important are viscous flow and viscoelasticity. 

Much rheological work is concerned directly with polymers because they exhibit such interesting, unusual, and difficult to describe properties. Not only are the elastic and viscous properties of polymer usually nonlinear, but they also exhibit a combination of viscous and elastic responses and the relative magnitudes of such responses are dependent on the time scale.

A toy - "silly putty" which is a silicone polymer, dramatically illustrates this viscoelastic response. When bounced (stressed rapidly), the silly putty is highly elastic, recovering most of the potential energy it had before it was dropped. Thus, it responds like a rubber ball. However, if stuck on a wall and left alone (stressed over a long period of time), it will slowly flow down the wall, and show little tendency to recover any deformation.

Another analogy i.e. bowl of spaghetti, perhaps better describes the chemical structure of polymers, molecular entanglement, and their relation to rheology. If one slowly pulls apart spaghetti, the noodles will separate without much resistance. The noodles will separate from one another even though the original mixture was highly entangled. However, if the spaghetti were separated quickly, the entanglements of the noodles would provide resistance, (there is no time for orderly separation), and fracture of the noodle strands is a likely result.

Let us discuss viscous flow and viscoelasticity in more details.

Viscous Flow

If the force per unit area ‘S’ caused a layer of liquid at a distance ‘x’ from a fixed boundary wall to move with a velocity ‘v’, the viscosity is defined as the ratio between the shear stress S and the velocity gradient dv/dx or rate of shear g:


If ɳ is independent of the rate of shear, the liquid is said to be Newtonian or to exhibit ideal flow behavior.

Newtonian or Non-Newtonian Fluids - Newtonian liquids are those in which the viscosity remains constant as the shear rate is varied. While, Non-Newtonian fluids, however, have a viscosity that depends on shear rate.

There are two types of deviation for Newtonian flow commonly observed in polymeric systems. These are compared to Newtonian flow in the figure below.


One deviation is shear thinning, a reversible decrease in viscosity with increasing shear rate. Shear thinning results from the tendency of the applied force to disturb the long chains from their favored equilibrium conformation, causing elongation in the direction of shear (i.e., the slow-moving spaghetti above). An opposite effect, shear thickening, in which viscosity increases with increasing shear rate, is rarely observed in polymers.


Yield Value/Yield Stress in Polymeric Systems

Another deviation from ideal or Newtonian flow, is the occurrence of a yield value in polymeric systems. This is the critical stress below which no flow occurs. Above the yield value the flow may be either Newtonian or non-Newtonian. This is the tendency called thixotropy that is observed in many sealants and paste adhesives.

Thixotropy is caused by the occurrence of a yield value.

• It prevents the sag of the sealant or adhesive under its own weight when placed on a vertical surface. (ASTM C920)
• Whereas, when placed under a higher stress (such as when pumped or extruded), the sealant flows easily.

The thixotropic effect is shown in the figure below where the curves are for a material that is first exposed to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as well as shear rate, the up and down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop.


Thixotropic effects are often noticed in dispersed systems (a solid or liquid in a liquid). The viscosity of a dispersed system depends on:

• Hydrodynamic interactions between particles or droplets and the liquid,
• Particle to particle interaction, and
• Interparticle attractions that promote the formation of aggregates or networks.

Thixotropy results from the ability of the dispersed particles to come together and form network structures when at rest or under low shearing forces. Viscosity decrease occurs when this structure breaks down due to shearing stress and the resistance to flow decreases. (See Figure Below)


The thixotropic materials required for non-sag adhesives and sealants are Non-Newtonian, plastic materials with a defined yield point. These materials are generally mastic adhesives, caulks, and sealants that are applied by trowel or extrusion. Additives to achieve these effects are generally referred to as thixotropes.

Viscoelasticity

Several common models are used to describe the viscoelastic strain - time behavior of an actual polymer. The basic elements for these models are the dashpot and spring as shown in the figure below.


An ideal elastic element is represented by a spring, which obeys Hooke's law where the elastic deformation is instantaneous and independent of time. A completely viscous response is that of a Newtonian fluid, whose deformation is linear with time while the stress is applied and is completely irrecoverable. A simple analogy to a Newtonian fluid is a dashpot.

These elements can be placed together in various configurations to represent different polymer systems. The spring and dashpot elements can be combined in different ways to represent a Maxwell and Voigt element.

• The Maxwell element exhibits flow plus elasticity on the application of stress.
• The Voigt element shows a retarded elastic or viscoelastic response.

The dashpot acts as a damping resistance to the spring. This response is the most indicative of a polymeric material.

Although these models can be used to represent the chief characteristics of viscoelastic behavior of polymers (figure below), they are nevertheless very much oversimplified. However, they will give the reader some idea of the possible complexities in modeling the time-dependent behavior of polymeric materials.



Role of Temperature on Viscoelasticity - Time-Temperature-Superposition (TTS) Theory

One important factor that can be added to this interpretation is temperature. An increase in temperature accelerates molecular and segmental motion, bringing the system more rapidly to equilibrium or apparent equilibrium and accelerating all types of viscoelastic effects.

Thus, there is a relationship between time and temperature in viscoelastic materials.

• At higher temperatures, the molecules can slip by one another more easily such as observed with slower rates of stress.
• At lower temperatures, the molecules are more frozen in place and provide a greater resistance to stress, much like when stressed at higher shear rates.

This relationship is known as time-temperature-superposition (TTS) theory and can be quantitatively determined by what is called the Willams-Landel-Ferry (WLF) equation. This is a very useful relationship when it comes to extrapolating test data as will be described below.

The underlying basis for TTS are that the processes involved in molecular relaxation or rearrangements in viscoelastic materials occur at accelerated rates at higher temperatures and that there is a direct equivalency between the time over which the stress is applied and the temperature. Thus, with TTS, measurements are made over a range of times and temperatures.

The time over which these processes occur can then be translated to different rates by shifting the temperature data. The result of this shifting is a "master curve" where the material property of interest at a specific end-use temperature can be predicted over a broad time scale.

Rheological Processes and Adhesive Systems

There are several processes that illustrate the importance of rheological properties to adhesive systems, such as:

• Those occurring during adhesive application
• Those occurring during adhesive setting
• Those occurring during adhesive deformation in a joint

Let’s discuss these processes using several examples. They are certainly not the only processes where rheology is important.

Adhesive Application

The wetting of a surface by an adhesive can be a rheological or time-dependent process.

Once the adhesive is applied to a substrate surface, Van der Waal's forces act to create a strong bond between the adhesive and the substrate. The degree of wetting, in theory, can be determined from the equilibrium contact angle. But this assumes equilibrium, and equilibrium will not normally be reached as the adhesive will be advancing and probably under pressure during the setting process.

Also, surfaces are never perfectly smooth. They will have many microscopic peaks and valleys of various topology and size. Because of its viscous flow, the adhesive will need time to penetrate these surface irregularities to maximize the contact area and to eliminate any gas pockets that could be trapped in the surface irregularities.

There are several common examples of this time-dependent characteristic. Some of them are described below.

• Very fast curing adhesive generally have less bond strength than slower curing adhesives of the same type. Fast setting epoxies, such as those cured with a mercaptan, have lower tensile shear strength than slower polyamide cure epoxies. This is due partly to the fact that the adhesive gels before it has time to completely wet the substrate.
• Hot melt adhesives do not bond well to cold metal because the adhesive gels before it has time to completely wet the substrate surface.
• Pressure sensitive tapes provide another example. Generally, their bond strength will increase with time. An acrylic pressure sensitive adhesive, for example, will continue to gain strength for several days after it is applied. This is primarily due to more efficient wetting of the substrate that occurs when allowed to occur over a greater time.

Adhesive Setting

Adhesives undergo a change in rheological properties on setting or gelation.

Whether it is a thermosetting epoxy adhesive or a thermoplastic hot melt, the adhesive proceeds from a liquid state to a solid state with corresponding change in viscosity and other rheological properties. Therefore, rheological measurements are often used to measure and characterize the rate at which an adhesive forms a permanent bond.

With epoxy resin adhesives, for example, the thermosetting cure process involves the passage of the system from a viscous liquid to a gelled rubber and finally to a hard glass-like material. The dynamic mechanical cure spectrum can be measured over this range as shown in figure - Stain-time relationship.

The analytical process for doing this is called dynamic mechanical analysis. It is of particular interest that the damping curve goes through a broad maximum, whereas the modulus goes through an inflection. It appears that this region represents a condition in which molecular motions become restricted.

Another common use of rheometric testing is the measurement of mechanical properties after cure. Dynamic mechanical analysis can provide information on thermally induced relaxation transitions and corresponding mechanical stiffness.

• Glass transition temperature, Tg, can be measured using dynamic mechanical techniques.
• Tg can also be measured by other analytical techniques such as differential scanning calorimetry (DSC) and thermal mechanical analysis (TMA). However, many times the transition of interest is not well pronounced. The Tg of crosslinked and semi-crystalline materials is particularly difficult to measure by DSC or TMA.

Adhesive Deformation in a Joint

Rheological properties are especially important for pressure sensitive adhesives.

The properties (shear resistance, tack, and peel strength) are directly related to the PSA's response to the application of stress, and this may be measured by rheology. For example, tack is the ability to spontaneously bond to another material under light pressures within a short application time. As the contact time increases, higher shear resistance and peel strength properties (related to the material's flow behavior) are found.

Mazzeo² has related several rheological properties to several viscoelastic properties of the adhesive: 

• G, the dynamic modulus or the ratio of shear stress to shear strain (independent of shear amplitude)
• G', elastic (storage) modulus
• G'', viscous (loss) modulus, and
• Tan delta, the ratio of G'' to G'

These properties can be measured using a variety of rheological instruments. Mazzeo used an AR2000 Advance Rheometer (TA Instruments). These viscoelastic properties are related to adhesive characteristics are detailed in the table below.

High tack	Low tan delta peak and low G' Low crosslinks (G'' > G') at 1 Hz
High shear resistance	High G' modulus at low frequencies (< 0.1 Hz) High viscosity at low shear rates
High peel strength	High G'' at higher frequencies (> 100 Hz)
High cohesive strength	High G' and low tan delta
High adhesive strength	High G'' and high tan delta

Viscoelastic Properties Related to Pressure Sensitive Adhesives Characteristics³
Modulus values - Modulus values in the frequency range of 0.1 to 100 Hz were found to describe the wetting and creep behavior of PSAs. When the modulus is too large, the ability to wet the substrate is reduced. For tack enhancement, tan delta should be greater than unity. That is G'' is greater than G' indicating that the polymer dissipates energy through its own deformation. This allows the material to adhere and easily form good contact to the substrate. Rheological information at higher frequencies is related to the peel strength of the material. Materials exhibiting a higher G'' at frequencies above 100 Hz provide high peel strength.

Several common examples can be offered to illustrate the rheological properties of the adhesive in an assembled joint. These are presented here to provide an indication of the importance of viscoelasticity.

Peel Strength - It is well known that the peel strength of an adhesive is dependent on the rate at which the peeling occurs. Thus, ASTM and other standards require that the crosshead speed of the testing machine be measured and recorded. The faster the rate of peel the more the adhesive acts like a glassy film and the lower the peel strength.

Impact Strength - Similarly, the impact strength of an adhesive is dependent on the rate the impact force is applied. Most common laboratory impact testers cannot apply impact forces with the speed at which certain (e.g., ballistic) forces are observed in service.

An example of this is the shock pads that are used to mount equipment to the floors of naval ships. When tested in the laboratory using drop impact or pendulum type impact testers, an epoxy-polyamide bonded shock pad showed no indication of failure. However, in practice the shock pads are exposed to very fast rates of stress - the type that can be produced by the ship's collision with a mine or torpedo. Proof testing places actual equipment on barges, and mines are detonated at various depths below the barge. Failure can occur during this test where no failure is evident during laboratory testing.

Instances such as those illustrated in the above two examples can be handled using the time-temperature-superposition (TTS) theory. Adhesives, being viscoelastic materials, exhibit temperature and time dependent behaviors during deformation. The TTS treatment overcomes the difficulty in extrapolating limited laboratory tests to real world conditions. Basically, the laboratory tests can be conducted at the longer time scales provided by laboratory equipment but at lower temperatures. These data then can be translated into shorter time ranges and actual service temperatures using the TTS.

Controlling the Flow Properties of Adhesives

Adhesive and sealant manufacturers employ rheological additives for thickening, control flow properties and prevent sag of their products. In practice, rheological additives may provide benefits in addition to viscosity or flow control. When properly formulated into adhesives and sealants, rheological additives can:

• Lower energy needed to pump, mix, extrude, spray, etc.
• Prevent phase separation
• Improve suspension characteristics
• Increase cohesive strength through reinforcement of the base polymer
• Control bondline thickness
• Improve surface texture
• Reduce materials cost

Table below summarizes the primary types of rheological additives which contribute to the desired viscosity behavior in adhesives and sealants.

Common Additives	Rheology Property
Fibers	Commonly used as thixotropes in both adhesive and sealant formulations.
Silica/Fumed Silica	An amorphous silicon dioxide, is a versatile, efficient additive used in adhesive and sealant formulations for flow control and thixotropy.
Precipitated calcium carbonate (CaCO3)	Functions as a thixotrope in sealant and adhesive formulations as well as being a low-cost extender (often used in the 40-50% by weight range) and reinforcement.
Mineral Additives	Kaolin controls viscosity to prevent drip or sag. Bentonite is a colloïdal clay that is both hydrophilic and organophilic.  Bentonite produces thixotropic-like dispersions.  Talc is also often used as an extender in adhesives in sealants but has flow control properties.  Attapulgite is a very cost effective thixotrope.
Organic Additives	Xanthan gum (an anionic polysaccharide) - Pseudoplastic with excellent yield values. Sodium carboxymethylcellulose - Most solutions are pseudoplastic with little or no yield value. All solutions show a reversible decrease in viscosity at elevated temperatures.  Methylcellulose, hydroxypropylmethylcellulose - Solutions are pseudoplastic and have characteristic gelation temperatures between 50 and 85°C. The gels are reversible and return to fluidity on cooling. Non-gelled solutions lack yield value.  Hydroxyethylcellulose - Solutions are pseudoplastic with no yield value, and they show reversible decrease in viscosity at elevated temperatures.
Other Additives	Many modern sealants use plastisols as their primary thixotrope.  Microcellular fillers such as glass and plastic are also used to provide non-sag properties to adhesives and sealants.  Polyamide waxes are also used in decorative and industrial coatings as well as adhesives and sealants. They are especially useful for high solids coatings. Modified urea is a liquid thixotrope used in some polar sealant systems. These products exhibit high resistance to sag and settlement.

Explore the importance and role of rheology modifiers along with their chemistries, selection process, formulation and testing guidelines in order to easily adjust the flow characteristics of your final adhesive formulation.