Need for Catalyst and Selection Process
Need for Catalyst and Selection Process
- 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:
What are Catalysts?
What are Catalysts?
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
For example: when the reaction between A and B requires a certain amount of energy, using a catalyst will reduce this amount.
- 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.
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
Catalysts for Amino Crosslinked Systems
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.
Relative Strength of Acid Catalyst
|
Catalyst for Amino systems
|
Equivalent weight
(g/mol)
|
Relative Strength
|
|
p-TSA (para-Toluene Sulfonic Acid)
|
172
|
High
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:
|
|
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:
- Formulate one component systems, or
- 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.
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
Catalysts for Urethane Coating Systems
Under 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
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.
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
When 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
- 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
- Gloss - As the catalyst modifies the reaction rate, the film gloss may be influenced
- 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.
- Hardness - As the catalyst modifies the reaction rate, the film hardness will also be influenced
- Impact test - As the catalyst modifies the reaction rate, flexibility of the film may be influenced.
- 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.
Catalysts/Accelerators/Initiators for Paints & Coatings
View a wide range of catalysts/accelerators/initiators available in the market today, analyze technical data of each product, get technical assistance or request samples.