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Adhesives Ingredients
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Adhesives Ingredients
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Basics of Polymer Rheology in Adhesives and Sealants

Edward M. Petrie – Jul 15, 2020

Basics of Polymer Rheology in Adhesives and Sealants 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

Adhesives

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


 »  Continue reading to learn more about how to control the rheology behavior of polymers with different rheological processes and the common types of additives which contribute to the desired viscosity behavior in adhesives and sealants.

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