The Universal Selection Source: Adhesives Ingredients

Rheology Modifiers Selection for Adhesives

Rheology modifiers improve the viscosity and have a great impact on the application characteristics. Adding rheology modifiers to your adhesive formulation will prevent settling down of fillers during settling and transportation. The guidelines in this selection guide apply to rheology modifiers and thickeners which are used to adjust the flow properties of adhesive and sealants. Read on to select the right rheology modifier for your formulation and easily adjust the flow characteristics of your final adhesive.

Importance of Rheology


rheology modifiers for adhesivesRheology is the study of flow and deformation of matter. For many applications the formulation’s optimum viscosity will depend on the state of the product (i.e., during compounding, while setting on a shelf, during application, immediately after application, and once in a joint configuration). Although most product data sheets will define the viscosity by a single viscosity measurement, optimal application properties often require the viscosity to be measured and controlled at several shear rates.

During its life cycle, an adhesive must conform to several processes having different shear rates. Each of these processes may have different rheological demands. Different shear rates that are associated with various life-cycle processes are illustrated in the figure below. Various options exist to adjust the viscosity; however, rheology modifiers are used to control viscosity over specific shear rates that are important to both the formulator and end-user.

Characteristic shear rates during application of PSA dispersions

Figure: Characteristic shear rates during application of PSA dispersions1

The table below shows how liquid rheology affects different processes. Rheology is especially important for adhesives that must be applied as coatings (e.g., pressure sensitive adhesives, laminating adhesives) or adhesives that must not flow or sag before cure (e.g., adhesives that are applied to vertical surfaces, adhesives that must maintain their bond thickness during cure, and sealants that must resist sagging ).

Process

Rheological Effect

Mixing and pumping

Rheology affects the ease in which an adhesive is mixed during compounding or before application. Similarly it affects the ease in which it can be pumped or transferred. Ease of mixing and pumping reduces process time and energy costs.

Settling and storage

The adhesive’s rheology is a major factor determining its storage or shelf life. Rheology will affect the degree to which settling occurs during storage and transport.

Dispensing

Rheology determines how an adhesive can be applied (roll coating, extrusion, trowel, etc.). It also determines how the adhesive conforms to or “holds” a specific geometric pattern (lines, dots, vertical extrusion, etc.).

Application

Flow control is important in determining the application process and how fast the process runs.This is especially important in the production of tapes, labels, and laminates. Rheology is also important in controlling wetting and penetration into porous substrates and sag resistance.


Table: The Effect of Adhesive Rheology on Various Processes


Difference between Rheology Modifiers & Thickeners


Rheology modifiers and thickeners are used in both waterborne and solvent borne adhesives and sealants of all types. They are used extensively in waterborne formulations, and often two or more are used to achieve the required balance of properties for the particular application.

Rheology modifiers and thickeners are used to a lesser extent in solvent borne formulations where they are primarily used to control viscosity and provide non-sag properties.

There is a difference between rheology modifiers and thickeners, although both terms are considered interchangeable in commerce. They are often considered together since their primary function is to modify the viscosity profile.

The following are differences between a rheology modifier and a thickener. However, for the purpose of this guide, they will be considered together since their selection is usually done as a single step in the formulation process and in certain applications they may be used together.

Rheology modifiers Thickeners
Optimize the viscosity at a specific shear rate or range of shear rates
Increase the viscosity (resistance to flow) at relatively low concentrations and generally at low shear rates
They can be organic or inorganic These are typically inorganic
Organic rheology modifiers do not usually increase the solids content of the formulation Inorganic materials that will increase the solids content and provide a degree of thixotropy
More targeted type of thickener


1 Willenbacher, N., et. al., “Tailoring the Rheology of Aqueous Acrylic PSA Dispersions”, Pressure Sensitive Tape Council, 2003.



Role of Rheology Modifiers

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1 Comments on "Rheology Modifiers Selection for Adhesives"
Catherine B Feb 16, 2017
Great overview!

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