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Select Right Catalyst for Your Coatings

Catalysts are an essential component in many paints and coating formulations. But do not get confused with “catalyst accelerates cross-linking” statement. Indeed, some catalysts decelerate it. Keeping in mind that a catalyst modifies the energy required to proceed the cross-linking reaction will give you the basic ground to understand catalysts.

Learn more about role of catalysts in coatings and different catalysts used for urethane and amino-crosslinked systems. Also, get tips and ideas to test the catalyst you have chosen to ensure its the right one for your formulation.

Need for Catalyst & Selection Process


Catalysts for Coatings

  • Reducing the drying time and decreasing the curing temperature are possible ways to optimize productivity and costs
  • Enhancing the cross linking or polymerization of the resins may strengthen the dry film and offer better quality coatings

All formulators want to save time, energy and money while enhancing the quality of their coating. Some of these goals can be achieved by using a catalyst. But playing a crucial role in the polymerization, the catalyst will not only affect the reaction rate, it will also have an influence on many other properties such as:

  • Adhesion
  • Gloss
  • Chemical resistance or
  • Shelf life


Continue reading or click to go on specific section of the page:

But What Exactly are Catalysts in Coatings?


Nowadays, in many coatings, crosslinking reaction will not occur without the use of a catalyst. Or if it occurs, the kinetic is so low that the film forming reaction takes a long time.

A catalyst can be defined as a substance that initiates a chemical reaction making it able to proceed under modified rate. Unlike other reagents, catalyst is not consumed by this reaction. So, it may participate in multiple chemical reactions.

Due to its presence, the catalyzed reaction will have a lower rate limiting free energy of activation than the corresponding non-catalyzed reaction, resulting in a higher reaction rate under the same conditions.

Using a catalyst can help in accelerating the crosslinking reaction, or reducing the curing temperature


» View All Commercially Available Catalysts/Accelerators in Our Coatings Database

This coatings database is available to all, free of charge. You can filter down your options by ingredient type, suitable resin, trade name, application, supplier and regional availability.

For example: when the reaction between A and B requires a certain amount of energy, using a catalyst will reduce this amount.

Catalyst Reaction


  • Under the same curing conditions, it will accelerate the reaction, and as a consequence, increase the cross linking rate.
  • As the catalyzed reaction requires less energy, to obtain the same cross-linking rate as the non-catalyzed reaction, it is possible to reduce the curing time or curing temperature.

Curing and Crosslinking

Many coating systems such as high solids, specific waterborne, urethane, amino systems and other 2 components require a high reactivity, low viscosity resins and crosslinkers to achieve a perfect curing, especially in the fields of industrial coatings, automotive or also coil.

Catalysts can help to convert these systems into chemically resistant and high performance coatings at reduced cure temperatures and meet eco-friendly demands .

Polyurethanes, acrylics, alkyds, epoxies and polyesters with reactive functional groups, such as hydroxyl, carbamate or amide can be reacted with various crosslinkers. The selection of the proper catalyst will enhance the crosslinking reaction.

Catalyst can be used either in waterborne, solventborne or even powder coatings, as long as their delivery forms fit your paint system characteristics.


Catalysts for Amino Crosslinked Systems

Acid Catalysts for Coatings
While metallic catalysts are preferred for urethane systems, acid catalysts help the reaction between the hydroxyl binder and the amine crosslinker. As acidic conditions are required to proceed, acid catalysts are recommended. As a consequence, acid blocked catalysts (by an amine for instance) will not set the pH in the acidic range and allow the formulation of stable one component systems.

The market offers a broad range of catalysts based on sulfonic acid and its derivatives. Their relative strength is linked with their equivalent weights.

» Check Out Various Acid Catalysts Grades Available Today!

Relative Strength of Acid Catalyst


Catalyst for Amino systems

Equivalent weight (g/mol)

Relative Strength

p-TSA (para-Toluene Sulfonic Acid)

172

High


high strength of acid catalyst

Low

DNNDSA (Di Nonyl Naphtalene Di Sulfonic Acid)

270

DDBSA (Do Decyl Benzene Sulfonic Acid)

326

DNNSA (Di Nonyl Naphtalene mono Sulfonic Acid)

460

Phosphate Acid

Various

Carboxylic Acid

Various


The catalyst type strongly depends on the amino crosslinking resin. Read on to select catalysts on the basis of amino resin to be used...

Acid Catalyst Selection Based on Amino Resin


Amino Crosslinking Resin

Acid Catalyst Category

Acid Catalyst Type

Fully alkylated monomeric melamine / formaldehyde

Strong Acid

pKa <1

 Strong Sulfonic Acid:

  • p-TSA
  • DNNDSA
  • DDBSA
  • DNNSA

Urea formaldehyde

Benzoguanamine

Glycoluril

High imino melamine / formaldehyde

Weak Acid

pKa > 1

  • Phosphate acid
  • Carboxylic acid
  • Amine blocked sulfonic acid

Polymeric butylated melamine / formaldehyde



Blocked Acid Catalysts for Amino Crosslinked Systems


The crosslinking reaction proceeds under acidic condition. Block acid catalysts can be used to:

  1. Formulate one component systems, or
  2. Set a delay in the reaction after mixing (in case of two components systems)

Under specific conditions (usually temperature specific), the amine will separate from the acid and the reaction will start.

The market offers many amine-blocked acid catalysts. These side-effects of the presence of amine in these catalysts can be seen in the image below.


Amine-blocked Acid Catalysts

The boiling point of the blocking amine should be also taken in consideration as it can lead to film defects (wrinkling, pinholes, pendulum hardness reduction…)

Using covalent-blocked acid catalysts may have less negative side effects of the amine blocked type, but they require more energy to unblock. It means either a higher curing temperature or a longer curing time.

Factors influencing the reaction process
Functionality of the amino crosslinker and hydroxyl binder
Dosage and type of the acid catalyst
Temperature and curing time
Moisture (that deactivates the catalyst)


Factors influencing the choice of acid based catalyst
Type of formulation (1K/2K for blocked or non-blocked catalyst)
Type of amino crosslinker (relative strength of the catalyst)
Functionalities of binder and crosslinker (strength and dosage)


Catalysts for Urethane Coating Systems


Metallic Catalysts for Urethane Coating SystemsUnder ambient conditions, the reaction between the hydroxyl groups of urethane and isocyanates is relatively slow. Therefore, catalysts are used to improve the reaction rate especially with aliphatic isocyanates. Aromatic isocyanates being more reactive may require less or sometimes no catalyst at all.

To achieve this required reaction rate, metallic or amine catalysts may be used.

Metallic catalysts are widely used in urethane coatings. However, amine-based catalysts are less recommended due to the risk of color drift (yellowing) and moisture sensitivity. The well-known DBTL is toxic, but many alternatives exist.

New developments are focused on tin-free based catalysts. Most of catalyst types can be used either in waterborne or solventborne, depending on their commercial form.

Catalyst selection can be done by checking the decrease of free isocyanate (NCO groups) in time.

Metallic-based Catalyst Properties


Catalyst for Urethane

Advantage

Disadvantage


DBTL

(Di Butyl Tin Laurate)


Very well-known performances

Poor toxicological profile

Reprotoxic / Mutagenic


DOTL

(Di Octyl Tin Laurate)


Similar performances

as DBTL

Requires higher dosage

May be also classified in the near future


DBTO

(Di Butyl Tin Oxide)


Similar performances

as DBTL

Poor toxicological profile

Reprotoxic / Mutagenic


Bismuth based


Better toxicological profile

Versatile

Requires higher dosage


Zirconium based


Less gassing / foaming

Good pot life

Selective catalyst, less versatility


Metal Complex


May be good tin alternative, effective in waterborne

Need many trials to find the optimal product

Amine based

Better toxicological profile


May induce color drift

and water sensitivity




Tips for Catalyst Test and Selection


While testing a catalyst may be fastidious, but no complications are involved. Done with care, it may not only save time and energy in the long term production, but also improve your paint characteristics for high performance systems. Also, using a gradient oven can simplify the formulator’s work.

Tips for Catalyst Test and Selection

Test should be done at:

  • Different temperatures (example: from 100°C to 160°C, by step of 20°C)
  • Different catalyst dosages (example: from 0% to 2% by step of 0.5%)

Important Remark for Testing the Acid-based Catalysts

Testing Acid-based Catalysts for CoatingsWhen testing different catalysts, especially in the case of catalyst based on sulfonic acid, it is very important that the comparative tests should be done on similar equivalent weight and not on similar dosage.

For instance, p-TSA has an equivalent weight of 172 g/mol and DNNSA has an equivalent weight of 460g/mol. It means, to keep a perfect functionality comparison, we need 2.67g of DNNSA to have the equivalent functionality of 1g of p-TSA (460/172). If we just replace the p-TSA 1 by 1, we will have a lack of acid functionality in the system and the results would not be reasonably comparable.

When testing such catalyst, knowledge about their characteristics (acid type and content) is strongly required.

Important Characteristics to Test


  1. Viscosity - In 1K system stability or in 2K systems to determine the pot life, viscosity evaluation in time and under specific temperature is a key point. Even in 1K systems, due to the different interactions in the paint, a catalyst may have a latent action

  2. Effect of Catalyst on Viscosity

  3. Gloss - As the catalyst modifies the reaction rate, the film gloss may be influenced

  4. Whitening - As the catalyst modifies the reaction rate, whitening may occur especially at high curing temperature. Moreover when using an amine blocked acid catalyst can lead to yellowing.

  5. Hardness - As the catalyst modifies the reaction rate, the film hardness will also be influenced

  6. Testing Coatings Film Hardness

  7. Impact test - As the catalyst modifies the reaction rate, flexibility of the film may be influenced.

  8. Chemical resistance - Solvent, acid, alkali, water…And of course the general aspect of the film

Additionally, it is highly recommended to prevent moisture in order to avoid catalyst deactivation which may lead to abnormal results and unwanted negative side effects.


Tips for Testing


  • Tests should always be done using one fixed parameter: keep curing time constant, modify the curing temperature / catalyst dosage is a common test method
  • Tin based metal catalysts must be replaced for toxicological reasons
  • Bismuth based metal catalysts offers a good alternative
  • Zirconium based metal catalyst show less gassing
  • DDBSA based catalysts are better to improve the hardness
  • Stronger catalysts (such as p-TSA and DNNDSA) are more sensitive to inorganic material, and may lead to loss of gloss
  • Same acid may be blocked by different amine. If no change is possible regarding the acid type, try selecting another product with a similar acid but blocked with another amine.

Commercially Available Catalysts/Accelerators for Coatings





This coatings database is available to all, free of charge. You can filter down your options by ingredient type, suitable resin, trade name, application, supplier and regional availability.

1 Comments on "Select Right Catalyst for Your Coatings"
william T Jan 31, 2018
A good overview, question, What would be a good catalyst/catalysts to use to Precatalyized Polyvinyl Acetate to increase increase water resistance and become heat reactive.

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