The Universal Selection Source: Adhesives Ingredients

Polyurethane Dispersions (PUD) Selection

This guide will endeavor to direct the formulator through the process of selecting polyurethane dispersions for a specific adhesive formulation and end-use requirement. The chemistry of the polyurethane dispersion will dictate the performance properties. Therefore, the formulator needs to know what properties are affected by certain characteristics of the dispersion.

Versatility of Polyurethane



Polyurethane is a versatile polymeric material that can be tailored to meet the demands of a number of adhesive systems such as shown in the figure below:

Types of Poleurethane Adhesives

Figure: Various types of polyurethane adhesives


Waterborne Polyurethanes Over Solvent Borne Polyurethanes


The use of waterborne polyurethane dispersions has grown in recent years as they have replaced solvent borne polyurethanes in a number of application areas.

Characteristics of Waterborne Polyurethanes

  • Fully reacted, linear polymers that are emulsified and dispersed in water
  • Have very low or no VOC (volatile organic content) emissions
  • Do not contain residual isocyanate
  • Non-flammable

Table below compares the properties of solvent based and waterborne polyurethane adhesives. One of the most important differences is the combination of high solids content and low viscosity in the aqueous dispersion.

 
Property
Solvent Based Polyurethane
Aqueous Polyurethane
Physical state
Solution
Particles dispersed in water
% Solids
15-18%
30-65%
Viscosity
50-1000
1000-5000
Polar monomeric groups
No
Yes
Toxicity, flammability, and explosion hazard
Yes
No
Coalescing solvent (cosolvent)
Unnecessary
Often necessary
Presence of foam
Sometimes
Yes; use of defoaming agents is usually necessary
Drying mechanism
Simple evaporation of solvent
Evaporation of water followed by interdiffusion of polymer particles
Drying time
Short (little energy required)
Long (more energy required)
Stability at low temperatures
Yes (freezing can be reversed)
No (Irreversible freezing)
Stability of the adhesive once applied (e.g., moisture and heat resistance)
High
Medium (unless crosslinked)
Adhesive properties
Excellent
Excellent

Property Comparison of Solvent-Based and Aqueous Polyurethane Adhesives 


Advantages & Disadvantages of Aqueous Polyurethane


In addition to their environmental benefits, the popularity of these systems also originates from their excellent combination of performance properties. The versatility of the polyurethane molecular backbone offers the adhesive formulator a wide range of properties.

Advantages and disadvantages of using polyurethane dispersions in adhesive and sealant formulations are listed in Table below:

Advantages
Disadvantages
  • Early green strength and high ultimate bond strength
  • High molecular weight and low viscosity
  • Easy processing via spray, rollers, etc.
  • Good open time
  • Good adhesion to a variety of difficult to bond substrates (e.g., PVC)
  • Flexible bonds, resistant to low temperatures
  • High heat resistance (especially if crosslinked)
  • Low heat application temperature
  • Ability to be blended with other waterborne polymers and additives
  • Availability of single or two-component systems
  • Minimal need for solvent or other additives
  • Relatively high cost for an aqueous system
  • Susceptibility to hydrolytic breakdown before and after application unless crosslinked
  • Optimal properties (equivalent to solvent based systems) generally require crosslinking
  • Susceptible to freezing
  • Long drying times (high energy consumption)
  • Wetting and tack generally inferior to other waterborne systems


As mentioned earlier, chemistry of polyurethane dispersion decides the performance properties. Therefore, properties that are affected by certain characteristics of the dispersion are a must to know…


Chemistry Polyurethane Dispersion

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