Currently in two-component cathodic epoxy electrophoretic paint, the most commonly used catalyst is DBTO (dibutyl tin oxide), followed by bismuth hydroxide, bismuth oxide and our water-based bismuth. First of all, their common mechanism of action is to reduce the activation energy required for the cross-linking reaction of -NCO and -OH during the baking process, thereby reducing the baking temperature by 10-15 degrees Celsius to save the energy consumption of downstream plants.
DBTO has been used for many years, with high catalytic activity and low density, and it is not easy to settle or agglomerate in the electrolyzer. However, in recent years, European and American countries have listed tin as a toxic and harmful contraband, and many industries have gradually released the document of tin restrictions. As a result, metal bismuth catalysts ushered in the spring, and bismuth oxide and bismuth hydroxide emerged. Considering the natural disadvantage of metal bismuth relative to tin: the catalytic activity is low, and the amount of use can only be increased to make up for it.Later, it was found that the relative atomic mass of bismuth element is relatively large, resulting in too high density of bismuth oxide. Adding bismuth oxide to the color paste and grinding together, eventually bismuth oxide will settle at the bottom of the electrolyzer due to gravity and will not adhere to the metal, the subsequent catalytic effect will not be achieved.In addition, the bismuth oxide powder is also prone to agglomeration, which blocks the ultrafiltration membrane and hinders the continuation of the electrophoresis process.
The content of bismuth in bismuth hydroxide is about 80%, and the added amount will be larger than that of bismuth oxide, and it is further processed from bismuth oxide, so the price will be higher and the overall cost will rise. Another big disadvantage is that bismuth hydroxide is a strong alkaline substance, which easily affects the pH value of the overall electrophoretic paint, thereby affecting the ability of the forming membrane.
Based on the above situation, our water-based bismuth catalyst came into being. The advantage is to abandon the traditional tin that is unfriendly to the environment and people, and secondly to abandon the traditional powder catalysts. The water-based bismuth is a colorless and transparent liquid at room temperature, and is generally built into the water-based emulsion, or mixed with the emulsion and color paste in a certain proportion into the electrolyzer before electrophoresis. The catalysis at the molecular level is achieved during the baking process, and the catalysis efficiency is much higher than that of the powder. The only disadvantage is that the water-based bismuth contains 1% free acid, which can be solved by adding a little bit less when adding lactic acid or acetic acid to adjust the pH, so as not to lower the overall pH of the electrophoretic paint due to the addition of water-based bismuth.
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.
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:
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.
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.
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.
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.
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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 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 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 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 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.
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 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 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.
Table 1: Advantages and disadvantages of amine and metallic catalysts for polyurethane production.
Depending on the final application, polyurethane formulators will also include other additives in the formulation including, but not limited to:
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