Emulsification and dispersion is accomplished by building
hydrophilicity into the polymer backbone with either
cationic or anionic groups or long hydrophilic polyol segments or, less frequently, through the use of external emulsifiers. There are several approaches to manufacturing polyurethane dispersions
1,2.
One-Component Systems
One-component systems are generally derived from fully reacted polyurethane pre-polymer dispersions or similar dispersions with blocked isocyanate end groups. A blocked isocyanate is an isocyanate which has been reacted with a material which will prevent its reaction at room temperature will permit that reaction to occur at higher temperatures.
The chemical blocking mechanism provides the isocyanate with protection from the water carriers and results in excellent shelf life. This type of adhesive is generally used for bonding non-porous substrates such as unlike metals and composites.
Two-Component Systems
Two-component systems also utilize fully reacted polyurethane pre-polymers along with a water dispersible isocyanate as a second component in the formulation. The isocyanate is added by the end-user and additional crosslinking is achieved after application, two component polyurethane dispersions are claimed to provide properties similar to solvent borne polyurethane adhesives.
Selecting Right Polyurethane Dispersion
The properties of a polyurethane dispersion are determined mainly by:
Choice of Raw Materials
The principal raw materials for polyurethane dispersions are:
- Polyols (linear polyether, polyester, and other polyols)
-
Isocyanates and their derivatives which are generally classified as either aliphatic or aromatic
The primary selection choice of polyurethane dispersion will be based on the type of polyol and this is usually specified in the
supplier’s PDS. Other properties of the dispersion are often not provided in the PDS. A wide range of linear polyether, polyester, and other polyols can be used.
Polyols are simply materials that contain two or more hydroxyl groups. Typically,
low molecular weight polyols (mw <2000) provide the
best adhesive properties. The most common polyols are polyether or polyester based polyols. The major differences between polyether and polyester based polyols are shown in Table below:
Property
|
Polyester
|
Polyether
|
Wear resistance
|
+
|
-
|
Load bearing, compression set, etc.
|
+
|
-
|
Low temperature flexibility
|
-
|
+
|
Hydrolytic stability
|
-
|
+
|
Heat aging
|
+
|
-
|
Swelling in oil, grease, solvents
|
+
|
-
|
O2, O3, and UV stability
|
+
|
-
|
Stability to radiation
|
+
|
-
|
Microbe and fungus resistance
|
-
|
+
|
Reactivity
|
+
|
-
|
Processing (low viscosity)
|
-
|
+
|
Low cost
|
-
|
+
|
Major Differences between Polyether and Polyester Based Polyols
Polyether Polyols
The polyether polyols are used in polyurethane adhesives because of performance and economics. Their Tg is about
-60°C.
Therefore, the resulting adhesives have very good properties like:
- Low temperature performance
- Elongation
- And impact properties
Polyether polyols are also more
resistant to hydrolysis than the polyester based types.
Polyester Polyols
Polyester polyols are often used in formulating
polyurethane dispersions and
hot melt polyurethane adhesives for the
shoe industry because of their
high crystallinity resulting in
high green (immediate) strength.
Polyester based polyols are available in many molecular structures ranging from
linear to highly branched. With more branching, more hydroxyl functionality is available for
crosslinking. Polyester polyols have higher tensile strength and greater heat resistance than polyether polyols, but they have poorer hydrolytic resistance, low temperature properties, and chemical resistance.
Isocyanate
The number of available isocyanates and their derivatives is very large, but are generally classified as either aliphatic or aromatic. Aliphatic isocyanates are preferred for polyurethane dispersions because of the lower reactivity of their isocyanate group with water. The most commonly used are
4,4'-Dicyclohexylmethane diisocyanate (H12MDI) because of
finer dispersion and better mechanical properties of the final product.
Aromatic isocyanates can also be used if a suitable preparation process is followed. These are sometimes used because of their
reactivity and lower cost. However, the aromatic isocyanates are not as light stable as the aliphatics nor are they as resistant to oxidation.
Crystallinity
Polyurethane dispersions can be manufactured with crystalline as well as amorphous molecular backbones.
- A crystalline (polyester) polyol is used and provides very open time when heat activated.
- Amorphous and blends of crystalline and amorphous polyols are used to adjust performance properties.
Products also differ in heat resistance.
The effects of amorphous versus crystalline content on heat activation temperatures, heat resistance, and other properties are shown in table below. It also indicates the suitability of each product for specific industry applications.
Property
|
Polyurethane Dispersion Product
|
A
|
B
|
C
|
D
|
E
|
F
|
Heat activation |
-
|
-
|
-
|
+
|
+
|
++
|
Crystallization |
-
|
-
|
-
|
0
|
+
|
++
|
Initial peel strength |
-
|
+
|
+
|
++
|
++
|
-
|
Tack at room temperature |
+
|
+
|
-
|
-
|
-
|
-
|
Tack at 50°C |
+
|
+
|
+
|
++
|
++
|
+
|
Hot tack life |
|
+
|
0
|
++
|
+
|
-
|
Heat resistance (1K system) |
++
|
+
|
++
|
-
|
-
|
-
|
Applications |
Furniture |
|
+
|
++
|
++
|
++
|
+
|
Automobile |
|
|
|
++
|
++
|
++
|
Shoe |
|
|
|
++
|
++
|
++
|
Textile |
++
|
++
|
+
|
+
|
+
|
|
Packaging |
+
|
++
|
|
|
|
|
A: Clear, soft, amorphous polymer for wet bonding applications (e.g. textile) using at least one porous substrate
B: Amorphous polymer for flexible bonding applications
C: Fast crystallizing polymer optimized for higher application temperatures
D: Higher solids level (50%) dispersion with a fast crystallization rate and a longer open time after heat activation than Product E
E: Standard product with 40 % solids content with a fast crystallization rate used for a wide range of applications (furniture, automotive)
F: Fast crystallizing product optimized for bonding operations with a low temperature requirement; crosslinker needed for heat resistance
++: Very high
+: High
0: Medium
-: Low
--: Very low |
Properties and Applications of Selected Polyurethane Dispersion with Properties Noted3
Ionic Content
Incorporating ionic groups into the polyurethane structure help in hydrophilic modification for dispersion in aqueous media. Therefore, polyurethane dispersions can also be classified by their ionic charge.
Ionic Charge
|
Stable pH Range
|
Characteristics
|
Anionic
|
> 7
|
Most common form of stabilization for commercial products.
Dispersing agent is normally a bishydroxy carboxylic acid
|
Cationic
|
< 7
|
Normally based on alkylated or protonated tertiary amines
|
Non-ionic
|
Wide pH range
|
Have no polarity and are stable over a very wide pH range
|
Particle Size & Viscosity
The particle size of polyurethane dispersion can be varied from about 0.01 to 5 µm. Particle size also affects the
appearance of the final film, from opaque (smaller particles) to milky white (larger particles).
Also, unlike solvent based systems, the viscosity of polyurethane dispersions is independent on
molecular weight.
Dispersion Properties
The particle size of polyurethane dispersion can be varied from about 0.01 to 5 µm. It
affects the properties of dispersions as:
-
Dispersions with relatively large particle size (>1 µm) are generally unstable with respect to sedimentation.
- Dispersions with smaller average particle size are more useful as they are more storage stable and have a high surface energy.
- Dispersions with smaller particle size have a stronger driving force for film formation.
Viscosity
Unlike solvent based systems, the viscosity of polyurethane dispersions is independent on molecular weight. Aqueous polyurethane dispersions have relatively
low viscosity at room temperature (50-1000 cps) with a
typical solid content of 30-50%.
The low viscosity and high solids content provides adhesive systems that can be
easily sprayed or rolled onto a substrate and give formulators a high level of freedom in designing their formulation. The adhesive dispersion viscosity must:
-
Ensure good wetting
- Should penetrate into the substrate, but not dive too deeply
Attention must also be paid to the relationship between viscosity and shear rate. For the formulator to adjust viscosity, thickeners are employed. Their effectiveness differs according to the
type of thickener and the amount added.
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 formulations.
Crosslinkable Systems: One-Component or Two-Component
Crosslinkable polyurethane dispersion adhesives can provide properties very similar to
solvent borne polyurethane adhesives. They have very
good moisture resistance and
heat resistance compared to non-crosslinked dispersions.
Crosslinkable polyurethane dispersion adhesives can be formulated as either one-component (1K) systems or two-component (2K) systems where:
- The one-component system is a polyurethane dispersion already containing a deactivated latent-reactive isocyanate.
- The two-component system is made with a polyurethane dispersion which can be crosslinked with a second component, such as a water emulsifiable polyisocyanate.
Both systems will provide the
same level of heat resistance, good initial and final
bond strength as well as
long-term durability. The choice between these two systems will be primarily determined by the manufacturing process parameters used to assemble the parts to be bonded as shown in table below:
2K PUD with Emulsifiable
Polyisocyanate
|
1K PUD with Deactivated
Isocyanate
|
- Limited pot life once the polyisocyanate is emulsified
- Error in the mixing ratio is possible when the polyisocyanate is added
- Processing step must take place at the end-user
|
- Stable if stored < 30°C
- Ready to use
- Long open time when applying the adhesive (if < 30°C)
- Parts to be bonded can be pre-coated and sold as such to the end-user. Bonding process is simplified as there is no adhesive handling at the end-user
|
Manufacturing Process Parameters with Crosslinkable Polyurethane Dispersions
Blending with Other Polymers
Polyurethane dispersions are generally more
expensive than other waterborne systems. Therefore, the addition of less expensive resins will
optimize the price/performance ratio of the final adhesive formulation. In some circumstances blending with other resins will also improve certain properties of the formulation for particular applications.
Acrylic latex resins are most commonly used to upgrade performance with respect to toughness, flexibility, abrasion resistance and film forming properties. Water dispersed rosin esters and modified terpene phenolics have also been used to improve creep resistance and tack. These
tackifiers are especially useful in formulating polyurethane dispersion contact adhesives.