4 Advice to Choose a pu paint catalyst

Polyurethanes are incredibly versatile (Figure 1); they are flexible, have high impact and abrasion resistance, strong bonding properties, are electrically insulating and are relatively low cost compared to other thermoplastics. 

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Figure 1. Polyurethanes are versatile materials and can be used to make hard and rigid materials through to soft flexible foams.  Common applications for polyurethane include automotive seats, shoes, floor coatings and furniture.

Furniture foams are the dominant application (Figure 2) however uses of polyurethane also include:

• rigid foam (construction and insulation)
• moulded foam (furniture and automotive)
• flexible foam (slabstock for furniture, mattresses and cushions)
• elastomers (footwear, synthetic leather)
• adhesives and sealants (construction, packaging, textiles)
• coatings (automotive refinish and OEM, flooring)

Figure 2. Polyurethane consumption worldwide ().  Flexible foams for furniture and automotive account for the largest share of polyurethane usage followed by rigid foams for construction and insulation applications. 

The polyurethane reaction

Polyurethane and its related chemistries were first discovered in by Otto Bayer however it wasn’t until the ’s that they became commercially available.  The basic synthesis involves the exothermic condensation reaction of an isocyanate (R’-(N=C=O)n) and a hydroxyl-containing compound, typically a polyol (R-(OH)n) (Figure 3).  

The reaction proceeds readily at room temperature, regardless of a catalyst, and is typically completed in a few seconds to several minutes depending on the formulation, in particular the choice of isocyanate.  Therefore compared to other polymers such as polyethene or polypropene which are produced then heated and moulded at a later stage, polyurethanes are made directly into the final product via reaction injection moulding (RIM), or applied onto the substrate in the case of adhesives and coatings.

Figure 3. The condensation polymerisation of an isocyanate (R’-(N=C=O)n) and a polyol (R-(OH)n) to form polyurethane.

An important side reaction involves the isocyanate component and water.  If moisture is present in the mixture (Figure 4), then the isocyanate will react with this water to form an unstable carbamic acid which then decomposes to form urea and carbon dioxide gas thus resulting in foaming.  The selection of an appropriate catalyst can either suppress this reaction or can promote this reaction if foam formation is desired.

Figure 4. Isocyanates are highly reactive with hydroxyl (-OH) groups.  When in contact with water, isocyanates react to form carbamic acid which then decays to form an amine and carbon dioxide gas.  This gas is responsible for foaming and is often used in the production of PU foams for furniture or construction applications. 

Polyurethanes are typically supplied as two-component formulations; a part A containing the polyol, catalyst, and any additives, and a part B compromising of the isocyanate.

Raw materials used to produce polyurethane

Part A: Polyol

The majority of polyols used in polyurethane production are hydroxyl-terminated polyethers though hydroxyl-terminated polyesters are also used.  The choice of polyol ultimately controls the degree of cross-linking and therefore the flexibility so formulators must consider not only the size of the molecule, the degree of branching but also the number of reactive hydroxyl groups present.

If a polyol containing two hydroxyl groups (a diol) is reacted with TDI or MDI, then a linear polymer is produced.  Polyols with a greater number of reactive hydroxyls result in a higher level of crosslinking and a more rigid final product.

Part B: Isocyanate

The most commonly used isocyanates for polyurethane production are the aromatic diisocyanates toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI) which form the basis for >90% of all polyurethanes (Figure 5).

TDI is a mixture of two isomers and is primarily used in the production of low-density flexible foams whereas MDI is a more complex mixture of three isomers and is used to make rigid foams and adhesives.

Figure 5. Chemical structures of the aromatic isocyanates toluene diisocyanate (TDI) and methylene diphenyl isocyanate (MDI).  TDI and MDI account for 90% of all isocyanate usage globally and are mostly used to produce flexible and rigid foams.

Less reactive are the aliphatic isocyanates (Figure 6) however these are important for coatings applications due to their excellent UV and colour stability.  Aliphatic isocyanates account for <5% of isocyanate usage worldwide and include hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI).

Figure 6. Chemical structures of the aliphatic isocyanates hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI).  HDI and IPDI mostly find use in coatings applications and account for <5% of isocyanate usage.

Blocked isocyanates

Blocked isocyanates are a relatively new development whereby the reactive NCO- groups are further reacted with groups such as dimethyl malonate (DEM), dimethyl pyrazole (DMP) or methylethyl ketoxime (MEKO) to produce inert and non-hazardous materials.  These materials can be selectively unblocked at elevated temperatures (+100°C) thus opening up a greater variety of applications such as usage in 1K or waterbased formulations, or for lower free isocyanate levels.

Catalysts

Catalysts play an important role in the production of polyurethane as not only do they increase the reaction rate and control gelling time, they also assist with balancing the side reactions including the water reaction and therefore control gas-formation and foaming. 

Broadly speaking, the catalysts used for polyurethane manufacture fall into two categories: amines or organometallic catalysts including organotin, bismuth and zinc.

Amine catalysts

Amine catalysts are derived from ammonia (NH3) by substituting one (primary) or two (secondary) or three (tertiary) of the hydrogen atoms with an alkyl group.  Their catalytic activity is determined by both the structure and the bascity with increased steric hinderance of the nitrogen atom resulting in decreased activity and increased bascity increasing activity.  Tertiary amines are predominantly used in the manufacture of foam as whilst they drive urethane formation, they also promote the water reaction leading to CO2 gas generation.

Mercury catalysts

Mercury catalysts such as phenylmercuric acetate, propionate, and neodecanoate are highly efficient at driving urethane formation and characteristically result in a long pot life in combination with rapid back-end cure. However despite their excellent performance, mercury catalysts are less common due to their poor toxicological status.

Tin catalysts

Outside of amine catalysts, organotin catalysts are the most widely used in polyurethane production with grades such as TIB KAT® 218 (dibutyltin dilaurate DBTL), TIB KAT® 216 (dioctyltin dilaurate DOTL), and TIB KAT® 318 (dioctyltin carboxylate) widely used in CASE applications (coatings, adhesives, sealants, and elastomers).

TIB KAT® 218 (DBTL) is the workhorse grade (Figure 7) and strongly drives the urethane reaction however in some instances longer ligand dioctyltins such as TIB KAT® 216 (DOTL) or TIB KAT® 318 are preferred due to more favourable labelling. 

Other grades such as TIB KAT® 223 or TIB KAT® 214 can provide varying curing profiles such as a rapid cure in the case of TIB KAT® 223 or a “mercury-like” curing profile with TIB KAT® 214.

Figure 7.  Mechanism of polyurethane catalysis using TIB KAT® 218 (dibutyltin dilaurate DBTL).  DBTL acts as a Lewis acid and accepts the non-bonding electrons from the oxygen on the isocyanate molecule to initiate the reaction. 

Bismuth and zinc catalysts

Bismuth and zinc catalysts are growing in popularity due to their low toxicity and both TIB KAT® 716 (bismuth) and TIB KAT® 616 (zinc) are used in CASE applications as they are strongly selective towards the urethane reaction. 

Bismuth, in particular, can mimic DBTL performance and in some instances offers a shorter pot life than organotins. However, bismuth typically requires higher dosage levels than organotins and is sensitive to hydrolysis; even low moisture levels can have a detrimental effect on activity. 

Zinc on the other hand results in increased pot life with a good through cure and is especially useful when curing at elevated temperatures (>60 °C).

Other metallic catalysts

Other catalysts such as aluminium, titanium and zirconium complexes are being used in some instances though are not widespread as have lower activity and can require much higher dosages.  They can also be more selective towards primary alcohols in a polyol mixture leading to poorer and breakable polyurethane material.

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Table 1: Advantages and disadvantages of amine and metallic catalysts for polyurethane production.

Other components

Depending on the final application, polyurethane formulators will also include other additives in the formulation including, but not limited to:

• Pigments to create coloured polyurethanes
• Fillers for mechanical reinforcement and to reduce overall costs such as those from Quarzwerke, from KaMin or from 3M
• Plasticisers to increase flexibility
• Cross-linkers and chain-extenders to improve physical properties
• Blowing agents and foam stabilisers to control bubble formation and cell structure in foams
• Moisture scavengers and zeolites to remove moisture and suppress foam formation such as those from Zeochem
• Flame retardants and smoke suppressants to reduce flammability and reduce smoke generation if burnt
• UV absorbers and antioxidants to minimize degradation such as those from Chitec

Summary

Polyurethane paint catalyst

Catalyst for two-component polyurethane paint

In the case of two-component polyurethane paint, which is made by mixing polyol and polyisocyanate, which have been combined with fillers, pigments, catalysts, etc., and then painting and curing, the curing speed is faster when a metal catalyst such as a tin-based catalyst is used, but the usable time is shorter, and the mixed paint may cure before it has been used up. This kind of problem can be avoided by using a thermosensitive catalyst.
The “U-CAT SA Series” is a salt of DBU [Figure 1], and is a thermosensitive catalyst that has low catalytic activity at room temperature, but rapidly demonstrates its activity as a urethane catalyst at high temperatures. When these thermosensitive catalysts are used in two-component polyurethane coatings, the pot life at room temperature is extended, solving the problems mentioned above. Furthermore, the special amine-based thermosensitive catalyst U-CAT has a pot life that is 2 to 3 times longer than that of the DBU salt U-CAT SA® 102 [Table 1].

(Formula 1) Main chemical reactions during polyurethane foam production
〈Resinification reaction〉
OCN-R-NCO + HO-R'-OH → ~CONH-R-NHCOO-R'-O~
Polyisocyanate Polyol Polyurethane
〈Foaming reaction〉
OCN-R-NCO + H2O → ~R-NHCONH~ + CO2↑
Polyisocyanate + Water + Polyurea + Carbon dioxide

Table 1: Curing time and usable time of two-component polyurethane

Can be scrolled horizontally

Catalyst Amount used(parts) Curing time(min) Pot life(min) None ー 24.0 355 U-CAT 0.2
0.4 10.2
5.0 158
110 U-CAT SA102 0.2
0.4 10.3
5.2 64
40 Dibutyltin
Dilaurate 0.2
0.6 10.8
5.0 33
17

●Usable time
The time it takes for the viscosity to double at 25℃
●Curing time
The time it takes for the oscillation cycle to begin to decrease in a pendulum-type viscoelasticity
●Temperature increase rate
Room temperature → 80℃ 25℃／min, 80℃ or above 5℃／min
●Polyol 　Polyester polyol 18.2parts ／Solvent 48.0parts
●isocyanates
　Modified TOI 28.8 parts ／ Toluene 5.0 parts

Table 2: Curing time of aliphatic isocyanates (120°C)

Catalyst Curing time (min) DBU 5.5 U-CAT SA1 21 U-CAT SA102 50 Triethylenediamine >180 Dibutyltin dilaurate 3.0 Lead octanoate 26

●Curing time
The time it takes for the gel to harden and be removed from the hot plate of the gelation tester
●PCL 200 88.1 parts ／ 1,4-BG 11.9 parts ／ IPDI 39.1 parts ／ Catalyst 0.5 parts

As shown in Figure 2, the temperature at which the salt in DBU exhibits catalytic activity differs depending on the strength of the acid, so it is possible to select a catalyst that is suitable for the curing temperature of the polyurethane paint.

〈Catalyst for non-yellowing polyurethane paints〉

The above is based on the use of highly reactive aromatic polyisocyanates, but non-yellowing aliphatic polyisocyanates are sometimes used in polyurethane paints. In this case, amine catalysts are said to be less active than metal catalysts and are not normally used. However, DBU and DBU salts (U-CAT SA series) show relatively high catalytic activity even with aliphatic polyisocyanates [Table 2], and are also used in type 2 polyurethane paints.

〈Catalyst for liquid heat-curing polyurethane paints of the block isocyanate type〉

Block isocyanates are isocyanate groups to which volatile active hydrogen compounds (blocking agents, e.g. phenol, MEK oxime, e-caprolactam, etc.) have been added to render them inactive at room temperature. When these block isocyanates are heated, they dissociate and the original isocyanate groups are regenerated [Formula 2]. Using this principle, it is possible to make one-component heat-curing polyurethane paints that are stable at room temperature and cure when heated by mixing blocked isocyanate and polyol in advance [Formula 2], and these are put to practical use in fields such as paints for electrical wires and cationic electrodeposition coatings.

When a suitable urethane catalyst is added to a one-component heat-curing polyurethane coating of the block isocyanate type, the dissociation temperature (curing temperature) can be lowered. Metal catalysts such as lead octoate and dibutyltin dilaurate are generally used as block isocyanate dissociation catalysts, but due to toxicity and environmental issues, there has been a growing need for alternatives to these. If the blocking agent is phenol, the curing temperature can be lowered more When the blocking agent is MEK oxime, the DBU salt has little catalytic effect, but the special amine-based catalyst 'U-CAT 18X' shows a reduction in curing temperature comparable to that of a metal catalyst, making it useful as a substitute for metal catalysts ［Table 3］

Formula 2) Chemical reaction of block isocyanate
〈Generation of block isocyanate〉 R-NCO + HB → R-NHCO-B Isocyanate Blocking agent Block isocyanate 〈Dissociation and Urethane Formation of Block Isocyanate〉 Heating R-NHCO-B + HO-R' → R-NCO + HB↑ (scattering) + HO-R' Block Isocyanate Isocyanate → R-NHCOO-R' Urethane

Table 3: Curing time of aliphatic isocyanates (120°C)

Catalyst Curing time (min) Phenol MEK oxime None 212 179 U-CAT 18X 147 162 U-CAT SA603 142 176 U-CAT SA102 155 176 Dibutyltin dilaurate 158 167 Lead octanoate 162 162

●Curing time
Thed temperature at which the oscillation cycle begins to decrease in a penulum-type viscoelasticity measuring device
●Temperature rise rate
Phenol block
Room temperature → 106℃ 15℃/min, 106℃ or above 6℃/min
MEK oxime block
Room temperature → 80℃ 18℃/min, 80℃ or above 5℃/min
●Phenol block TDI base material 20 parts / curing agent 10 parts / solvent 70 parts / catalyst 0.3 parts
●MEK oxime block
Hydrogenated MDI base material 100 parts / curing agent 25 parts / solvent 20 parts / catalyst 0.15 parts

〈Catalyst for liquid moisture-curing polyurethane paints〉

Liquid moisture-curing polyurethane paints are a type of paint that hardens when exposed to moisture in the air, using NCO-terminated urethane prepolymers made by reacting polyol with an excess of poly。
Liquid moisture-curing polyurethane coatings take a long time to cure without a catalyst, but curing is accelerated when a catalyst is used. However, metal catalysts have the problem of poor paint storage stability. In contrast, U-CAT 660M is a special amine catalyst developed for two-component moisture-curing polyurethane, and shows excellent storage stability even when blended with urethane prepolymers. They also have excellent low-temperature curing properties and are used in two-component moisture curing adhesives and sealants (Table 4).

Table 3: Curing temperature (°C) of block isocyanate

Can be scrolled horizontally

Catalyst Amount used (parts) Tack-free time Viscosity ratio U-CAT 660M 0.4 22 1.8 Dibutyltin dilaurate 1.2 24 Gelation (2 weeks)

●NCO-terminated polyurethane prepolymer (NCO% 12.4%)
●Tack-free time min (25°C, 50% R.H.)
●Viscosity ratio Viscosity after 20 months at room temperature / initial viscosity

Formula 3) Main chemical reactions of one-component moisture-curing polyurethane paint
〈Synthesis of NCO-terminated polyurethane prepolymer〉
HO-R1 -OH + OCN-R2-NCO → OCN~~~NCO

Polyol Polyisocyanate NCO-terminated polyurethane prepolymer


OCN~~~NCO + H2O → H2N~~~NH2 + CO2↑
NCO-terminated urethane prepolymer
H2N~~~NH2 + OCN~~~NCO → NHCONH~~~

Catalyst for polyurethane foam

〈Catalyst for water foaming〉

There are two types of foaming agents used to make polyurethane foam: (1) water foaming, which uses carbon dioxide produced when water reacts with NCO groups to create uniform bubbles in the polyurethane, and (2) CFC foaming and pentane foaming, which use low-temperature volatile CFC compounds or pentane to create bubbles using the heat generated by the urethane reaction. However, HCFC-141b (CH3-CFCI2), which is currently the main CFC used as a foaming agent, contains chlorine atoms and destroys the ozone layer in the atmosphere, so it was decided to phase it out completely by the end of . HFC-245fa, HFC-365mfc and pentane are being actively considered as alternatives to HCFC-141b. However, there are problems with these next-generation CFCs, such as the fact that they have a non-zero global warming potential, and the fact that pentane is a fire hazard.
In this respect, water has none of these problems and is also inexpensive, so the use of water alone (total water foaming) or a combination of water and other foaming agents is also being widely investigated.
In the case of water-based rigid mold foam that does not use CFCs, the reaction between the water added as a foaming agent and the isocyanate group forms urea bonds, and as the number of urea bonds increases, the polyurethane becomes brittle, which can easily lead to problems such as the peeling of the foam surface skin layer.
In order to solve the above problem of total water foaming, our company has developed a new catalyst, “U-CAT 420A”, as a result of our investigations. The characteristic of this catalyst is that it has very good skin-forming properties, and the surface of rigid polyurethane foam foamed using “U-CAT 420A” is much smoother than that of conventional catalysts. U-CAT 420A has also attracted attention as a catalyst that can improve the surface of foam even when using a combination of next-generation CFC foaming agents and water. Please note that U-CAT 420A is no longer being introduced.

〈Trimerization catalyst〉

Polyurethane foam is also widely used as thermal insulation in houses and buildings. There are two main methods of manufacturing polyurethane thermal insulation: a factory production method, in which the foam is injected between paper or other surface materials or panels to form boards or panels, and a construction site foaming method, in which the polyol and polyisocyanate components are mixed and sprayed on site. In both cases, flame retardancy is required for polyurethane for fire prevention reasons. However, in June , the Building Standards Law was revised to tighten the fire retardancy standards (enforced in June ), and measures were taken to improve the fire retardancy of building materials that use polyurethane foam. One such measure is to increase the proportion of isocyanurate rings in the polyurethane foam [Formula 4]. This method makes use of the fact that isocyanurate rings, which are trimers of isocyanate, are extremely stable in relation to heat. With normal amine catalysts, trimerization (isocyanurate formation) does not progress, and potassium octanoate and potassium acetate, etc., were used as catalysts. However, when using these metal catalysts, the tri-addition reaction is difficult to progress at the edges of the foam where the reaction temperature is difficult to raise, so the flame retardancy of the foam is likely to be insufficient. In this respect, when our special amine-based catalyst “U-CAT 18X” is used, the tri-addition rate is high even at the edges of the foam where the reaction temperature is difficult to raise, and foam with excellent flame retardancy can be obtained [Table 5]. U-CAT 18X is used in thermal insulation boards and panels for houses and buildings, as well as for on-site foaming, and is popular for its ability to produce highly flame-resistant foam even in winter.

〈Reactive polyurethane catalyst〉

Ordinary amine-based catalysts remain in the polyurethane foam even after it has been manufactured, and they gradually diffuse out of the foam, causing problems such as vinyl staining (catalysts that have migrated to the polyvinyl chloride sheet promote dehydrochlorination reactions, causing the polyvinyl chloride sheet to discolor) and fogging (catalysts gradually evaporate from the interior materials of a car and adhere to the window glass, causing it to become foggy). In this respect, the urethane catalyst 'U-CAT ', which has a reactive group within its molecule, reacts with the isocyanate group during foam production and is incorporated into the polyurethane framework, so it does not remain in a free state within the polyurethane product. For this reason, U-CAT is mainly used in the automotive industry as a reactive catalyst for manufacturing non-vinyl stain-resistant or anti-fogging polyurethane foam.

Table 5: Isocyanurate Conversion Rate of Flame-Retardant Foam

Can be scrolled horizontally

Catalyst/th> Isocyanurate Conversion Rate Type Quantity used (parts) Lower edge of
foam Center of
foam Upper edge of
foam U-CAT 18X 4
8 69
94 95
104 76
94 Potassium octanoate 4
8 57
69 97
105 75
71 Potassium acetate 4
8 54
73 95
100 65
67

●Amount of catalyst used: Parts per 100 parts of polyol
●Isocyanurate ratio: Calculated using the following formula based on the IR chart of the foamed flame-retardant foam
(height of the isocyanurate peak / height of the urethane peak) X 100

Table 6: Our company's polyurethane resin catalysts

Can be scrolled horizontally

Product name Composition Main features and uses U-CAT SA1 Phenol salt of DBU A typical grade of thermosensitive catalyst that has low catalytic activity
at low temperatures and rapidly increases its catalytic activity at high temperatures.
The stronger the acid, the higher the temperature at which catalytic activity is expressed.
For two-component polyurethane coatings, elastomers and adhesives,
it has a long pot life and rapidly hardens when heated.
It is used as a catalyst to promote demolding in polyurethane foam. U-CAT SA102 2-Ethylhexanoate salt of DBU U-CAT SA810 o-Phthalic acid salt of DBU U-CAT SA506 p-Toluenesulfonic acid salt of DBU U-CAT SA603 DBU formate A thermosensitive catalyst.
There is a large difference in activity between low and high temperatures.
It also has excellent performance in dissociating blocked isocyanates,
and is particularly suitable for use in electrical wire coatings. U-CAT 2-Ethylhexanoic acid A thermosensitive catalyst. It has a long pot life in two-component polyurethane paints.
It is used as a catalyst to promote demolding in polyurethane foam. U-CAT 660M Bis(2-morpholinoethyl) ether Catalyst for moisture curing. Even when added to urethane prepolymers, it shows excellent storage stability.
It also shows excellent moisture curing properties even at low temperatures of around 5°C. U-CAT 18X Triethylmethylammonium 2-ethylhexane salt Triethylmethylammonium 2-ethylhexane salt A trimerization catalyst. It excels in polyurethane foam with high flame retardance,
and is excellent in the dissociation performance of block isocyanate. U-CAT 1,1’-｛｛3-(Dimethylamino)propyl｝imino｝
bis(2-propanol) (80% content) A catalyst with a reactive group in the molecule.
Used as a non-pinyl stain or anti-fogging reactive urethane catalyst.

The above is an overview of urethane catalysts, focusing on our catalysts.

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