Generally, two-component films provide higher hardness with enhanced resistance to moisture permeation, soluble salts, and chemicals. In the context of the chemistry detailed in this article, a hardener is defined as a separate ingredient that reacts with the first component of a two-component paint. The pigmented functional resin portion is normally in Part A and the hardener in Part B.
Crosslinked films are not soluble in most solvents; however, solvents swell most crosslinked films. As crosslink density (XLD) increases, solvent swelling decreases. A polyurethane is formed from the reaction of a polyol with an isocyanate prepolymer. The number and type of functional groups and structure dramatically impacts the reaction speed and the cured performance
As Table I illustrates, an amine functional resin can also react with an isocyanate to form a polyurea. The reaction of an amine with an isocyanate is extremely fast and requires mixing at the point of application. For ambient cured urethane paints, normally a slight excess of isocyanate functional groups is used as moisture, which also reacts with isocyanate.
In general, aromatic isocyanates react with hydroxyl groups faster than aliphatic isocyanates and primary functional isocyanates react faster than secondary isocyanates. Also, primary hydroxyl groups react faster than secondary hydroxyl groups.
Table I – Common reactions of isocyanate prepolymers
Aromatic isocyanate prepolymers are used primarily in primers as well as for interior applications where light stability is less of an issue. Aliphatic isocyanate prepolymers are used primarily in applications where exterior weathering and light stability is of primary importance.
Another common ambient cure two-component paint chemistry involves the reaction of an epoxy hardener with that of an amine functional resin. Due to their tenacious adhesion to a variety of surfaces including metals and concrete, epoxy two-component compositions are used in a variety of applications including primers for exterior and interior application and interior coatings.
As Table II illustrates, epoxy groups react with primary amines at ambient temperatures to form secondary amines that in turn react to form tertiary amines. Reactivity of amines increases with the strength of the base and decreases with steric hindrance. The general order of amine reactivity is: primary > secondary > tertiary amines. Aliphatic amines are more reactive than aromatic amines as the former are more basic.
Acrylics containing glycidyl methacrylate or cycloaliphatic epoxies react more rapidly than do aromatic epoxies such as those based on bisphenol A. With the correct catalyst, aliphatic epoxy resins can react with carboxyl functionality even at room temperature. Cycloaliphatic epoxy-based systems also provide improved light stability for exterior applications.
Table II – Example of Reactions of epoxy with amine
When formulating a stoichiometric reaction, it is desirable to discuss reactants in terms of equivalents. Molecular weights of common functional groups are listed in Table III.
Table III – Molecular Weight of Common Functional Groups
Functional Group | Molecular Weight of Functional Group |
Isocyanate | 42 g/mole |
Hydroxy | 17 g/mole |
Terminal Epoxy | 43 g/mole |
For example, using the equation above, if the equivalent weight of an isocyanate functional hardener is 200, to have a 1:1 stoichiometric reaction for a polyol with an equivalent weight of 400, a weight ratio of 2:1 polyol:isocyanate is required as the polyol has half the functionality on a weight basis. Other common hardeners and their reactions are listed in Table IV.
Table IV – Other Common Hardeners and Reactions
Hardener
Cross-linker Functional Group |
Resin
Cross-linkable Group |
Cross-linked group |
Polyaziridine
|
R-COOH
(carboxyl) |
Acetyl urea |
Silane
Triethoxy silane and aliphatic epoxy
|
Dual or self cure mechanism | Siloxane & epoxy ester |
Carbodiimide
R-N=C=N-R |
R-COOH | N-Acyl Urea |
Hydrazide
|
R-C=O
Ketone |
Hydrazone |
A search of Prospector’s search engine provides a number of hardeners, and resins for formulating two-component coatings.
Further Reading:
- Patently innovative: latest polyurethane technologies
- Polyisocyanates deep dive
- Reactive silanes for enhancement of coating performance
Resources include:
- UL Prospector
- Covestro (formerly Bayer Materials Science)
- Journal of Macromolecular Science
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Thank you Ron for sharing the information. Helps in gaining a lot of knowledge and insights.
Regards,
Vishal
Informative Ron, thank you!
We are manufacturer of 2K PU Paint. We are facing issue in paint like too fine bubles in coating after curing. We can also saw these bubles in hardener before mixing. This is major issue. Please Help……
Dir. Many thanks for your information.would you tell me which hardner polyurthane and acrylic resin is siutble for hot stamping foil lacquer?best regards
I am not familiar with the requirements needed for the Hot Stamping Process. If further consultation is needed please have respond directly on our contact request at http://www.chemicaldynamics.net
Dear Ron
I am looking for help to formulate a epoxy resin system for non yellowing composite fiberglass lamination. I want to use only Cycloaliphatic materials. So the adduct will be a Cycloaliphatic amine/ Cycloaliphatic epoxy resin benzyl alcohol free. The resin side will be a low viscosity Cycloaliphatic resin ( Dyglycidyl ester of hexahydrophthalic anhydride or Hydrogenated Bisphenol A resin. Please let me know how you can help me
I would appreciate your help
Best Regards
Dear Jose,
Please let me know if you would like us to have a consulting agreement with your company as I anticipate that your request will require considerable time on our part to adequately answer your question.
Thank you,
I am using Alkyd and Urea Formaldehyde Combination,PTSA hardener is used for curing purpose,
But this hardener is taking time for curing .Is there any other chemical need o be mixed for faster curing.
Any one confirm on above,
Thanks for your question,
You are curing by two mechanisms, one is through auto-oxidation of the unsaturation (conjugated unsaturation works best) in the alkyd. The auto-oxidation requires the addition of appropriate metal driers. Secondly depending on the UF you are using and it’s functionality, a weaker acid catalyst (for example an alkyl phosphate or catalyst with carboxyl functionality) can work better than a strong acid catalyst like pTSA. The reason for the latter is that depending on the level and type of alkylation of the UF, pTSA can be neutralized by the basicity of the amido groups on the UF and thus loose effectiveness whereas a weaker acid catalyst in this case may be more effective.
Ron Lewarchik
I have a facing problem for filling stage of isocynate in MS tin is various packing for gas formation on storage condition and the tin is blinging from top and bottom. Please give me a solution. My isocynate formulation is 40% isocynate and 60% Butyl Acetate.
I need to make hardner for polyurethane laquer can you help me please..how to make hardner
Hello Manoj,
To provide an answer I would need to have more specifics on the resin you are using, what is the hardness value you currently have and what hardness would you like to have. Feel free to contact me through the contact form on our web site: http://www.chemicaldynamics.net.
Thank you,