Aqueous acrylic polymer emulsions are replacing solvent-based polymer systems thanks to environmental regulations limiting volatile organic compounds (VOCs) and major improvements in the performance characteristics of the emulsion-based products.
This substitution of emulsion systems for solvent-borne systems has been occurring in:
- Coatings and paints.
- Textile and paper treatments.
- Adhesives and sealants.
- Printing inks.
- Polishes and waxes.
Advancements in film mechanical properties; chemical, water, and abrasion resistance; durability; adhesive properties; and solvent resistance have further driven the growth of waterborne acrylic technologies. And a major contributor to these performance enhancements has been new polymer crosslinking chemistries.
In this article, we’re going to look at the best crosslinking technology—crosslinking chemistry based on diacetone acrylamide (DAAM) and adipic acid dihydrazide (ADH). This technology is known as keto-hydrazide crosslinking because it involves the direct reaction of the pendant ketone moiety on the DAAM segment with the hydrazide moiety of the ADH.
After examining the benefits of this system, we’ll then look at several coating applications of DAAM and ADH.
The Superiority of the Diacetone Acrylamide and Adipic Acid Dihydrazide Crosslinking System
The DAAM-ADH crosslinking system provides a number of benefits for consumers. From reducing safety concerns to improving performance properties, the benefits of these intermediates are numerous.
- DAAM and ADH are easy and safe to use.
- Both intermediates are formaldehyde free.
- Diacetone acrylamide uniformly copolymerizes within acrylic copolymers, creating well-dispersed pendant ketone crosslinking sites.
- Wet acrylic emulsions based on diacetone acrylamide with ADH in the aqueous phase, are initially non-reactive and afford emulsions with good long-term stability during shipping and storage in retail containers (i.e., “in-can” stability).
- After film coalescence, crosslinking becomes rapid at ambient temperatures, thanks to water evaporation in the drying process and a simultaneous reduction in pH arising from the loss of ammonia.
- Because crosslinking is post-coalescence, the resulting three-dimensional polymer network exhibits enhanced mechanical strength and durability as well as maximum film cohesive properties.
- Crosslinking with the keto-hydrazide chemistry enhances abrasion, scrub, stain, and blocking resistance; moisture and solvent resistance; and substrate adhesion.
With other crosslinking chemistries, premature crosslinking occurs within the emulsion particles prior to coalescence, thus retarding intermolecular diffusion between emulsion particles and resulting in weaker film products and coatings. This is particularly the case when diacrylates are used in the copolymerization recipe to produce crosslinks.
With the keto-hydrazide chemistry, crosslinking occurs post-coalescence, allowing good molecular inter-diffusion between emulsion particles as the film dries, which enhances film properties.
The Chemistry of Keto-Hydrazide Self-Crosslinking Monomers
The circled moieties in the chemical structures below highlight the business end of these two molecules.
The diacetone acrylamide monomer is soluble in water and most organic solvents, so it is readily incorporated into polymerization recipes.
DAAM has received much attention because it readily free-radically copolymerizes and can post-react with adipic acid dihydrazide (ADH) to form self-crosslinkable systems, as delineated below. (Copolymerization is effective with vinyl acetate monomer, MMA, acrylic esters, acrylamide, styrene, and others—whether in solution, bulk, or emulsion systems.)
A difunctional crosslinking agent, adipic acid dihydrazide (ADH) has a melting point of 177–184°C and is moderately soluble in water. Take a look at its structural formula below:
After adding ADH to an ammonia-neutralized acrylic emulsion containing DAAM, the emulsion remains stable when stored in one pot.
The Technology behind the Self-Crosslinking Process
Self-crosslinking chemistry between diacetone acrylamide and adipic acid dihydrazide begins with copolymerizing DAAM into an acrylic copolymer, using DAAM concentration at ~1-5 wt. % of the monomer mixture. This makes the emulsion copolymer crosslinkable via a pendant ketone carbonyl moiety.
Afterwards, the following steps will complete the process:
- The emulsion is neutralized with ammonia, and adipic acid dihydrazide (ADH) is then added to the emulsion as an aqueous solution. The ratio of DAAM to ADH is ~ 2.1 to 1.0.
- On drying off the water and evaporation of the ammonia, coalescence of the film occurs, and the pH decreases, becoming acidic. As the pH decreases, the crosslinking reaction rate begins to increase.
- The crosslinking process then takes place (acid catalyzed) with the formation of a chemical bond between the DAAM and the ADH, called a ketimine moiety. The reaction by-product is water.
See the depiction of the coalescence and the crosslinking mechanisms below.
Emulsion Film Formation and Keto-Hydrazide Crosslinking Mechanism
The Options for Diacetone Acrylamide’s Emulsion Copolymer Composition
When it comes to monomers for an emulsion, diacetone acrylamide boasts a wide range of options. Its co-monomer feeds can include the following:
- Butyl acrylate (BA)
- 2-Ethylhexyl acrylate (2-EHA)
- Methyl methacrylate (MMA)
- Vinyl acetate monomer (VAM)
- Glacial acrylic acid (GAA)
- Glacial methacrylic acid (GMA)
The monomer composition is often dictated by the desired Tg (glass transition temperature) of the polymer film, from -30 °C to > 30 °C. Please see the reference Tg values of several key monomers provided below.
To see examples of emulsion compositions you can use for copolymerization with DAAM, please consult the following recipe and tables. Key performance characteristics are shown to exemplify the attributes of DAAM-ADH crosslinked compositions.
|Deionized water||90.0 g|
|Monomer Feed System A||Weight|
|Deionized water||372.0 g|
|Rhodapex CO-436||8.2 g (1 weight%/polymer)|
|DAAM||9.4 g (1.9 weight%/polymer)|
|Acrylic acid (AA)||4.6 g (0.9% weight/polymer)|
|Styrene (St)||74.6g (15.1 weight%/polymer)|
|Methyl methacrylate (MMA)||80.3g (16.3 weight%/polymer)|
|Butyl acrylate (BA)||195.2g (39.5 weight%/polymer)|
|2-ethylhexyl acrylate (EHA)||129.6g (26.3 weight%/polymer)|
|Initiator Feed System B||Weight|
|Deionized water||34.6 g|
|Ammonium persulfate||1.46 g (0.3% weight/polymer)|
|Total Weight||1000.0 g|
|Comments on Emulsion Properties|
The Applications of Diacetone Acrylamide and Adipic Acid Dihydrazide
Self-crosslinking diacetone acrylamide self-crosslinking can be used in multiple applications, the most prominent ones being durable interior and exterior paints as well as coatings for architecture, wood, and concrete surfaces. Keep on reading to discover how to formulate a DAAM-based coating as well as its performance qualities.
To create a diacetone acrylamide based coating for architectural materials, you’ll want an emulsion composition with the following compositional characteristics:
- DAAM content is 2 to 5 wt. %.
- The polymer composition can be MA/MMA/BA/DAAM.
- Tg ~ 0 ˚C.
- ADH is about 0.8 N per DAAM.
- Drying conditions: 23 ˚C; 7 days.
Below is some valuable reference information on DAAM’s coating performance (paint contained TiO2):
|DAAM Content, wt. %||0||5|
|Solvent Resistance, Steel1 Xylene Scrub||20||>200|
|Stain Resistance, oak plywood||Poor||Superior|
|Impact Resistance, cm||<5||>50|
|Thermal Cooling Cycle Test2||Poor||Superior|
1 The steel substrate was treated with zinc phosphate
2 Substrate was a fir plank. Temperature conditions: 60˚C x 2 hrs. to 20˚C x 2 hrs.
For wood coatings, the DAAM emulsions can follow the guidelines below:
- DAAM content: 2 to 5 wt. %
- The polymer composition can be MA/MMA/BA/DAAM
- Tg ~ 30 °C
- ADH is 0.8 N per DAAM
- Drying conditions: 23 °C; 7 days
As you analyze diacetone acrylamide's performance in the table below, note how DAAM has a significant impact on stain resistance.
|DAAM Content, wt. %||0||2||5|
|Gel %, THF||0||95||95|
|Impact Resistance, Steel||<50||>50||>50|
|Ink (Red and Blue)||Poor||Excellent / Good||Excellent|
To evaluate your diacetone acrylamide coating, you’ll want to wipe the wood with a tissue. Then, 18 hours later, wash the wood surface with water.
To see how DAAM enhances the impact resistance of wood surfaces, take a look at the chart below. Notice the high-impact resistance of wood coatings with 5% DAAM versus those with no DAAM.
In addition, during a heating and cooling cycling test, coatings without DAAM can tend to crack. However, with a diacetone acrylamide coating, this is less likely. Check out these pictures to see the difference.
Concrete coatings are also another application of DAAM. To prepare the coating, here are some characteristics you’ll want your emulsion to have:
- DAAM content is 2 to 5 wt. %.
- Compositions such as AA/St/MMA/BA/EHA/DAAM.
- Tg ~ -30 ˚C.
- ADH is 0.8 N per DAAM.
- Drying conditions: 23 ˚C; 7 days.
The table below provides some key information on how diacetone acrylamide impacts concrete surfaces once applied as a coating.
|DAAM Content, wt. %||0||2||5|
|Hardness at High Temp.||Poor||Fair||Maintained|
As you can see, resin formulators use DAAM and ADH monomers in self-crosslinkable emulsion polymers for good reasons. In addition to both safe and convenient handling, this crosslinking system enhances mechanical properties, stain resistance, toughness, impact resistance, scrub resistance, adhesive properties, and more for multiple applications.
If you’re ready to leverage DAAM and ADH for your organization, Gantrade maintains good inventories of both intermediates that are high quality. In addition, we readily provide product information, technical support, and product sales information so you can make an informed decision.
Whether you want to know more about the capabilities of DAAM-ADH systems or you want to understand the inventory options we offer, contact Gantrade to speak with one of our team members.