The Root Knowledge of PU Coating Every Manufacturer Must Stay Acquainted With!
Reaction product of polyisocyanate with atleast one other species containing an active hydrogen result in urethane linkage and polymer are called as POLYURETHANES.
POLYURETHANES. Major application area of PU’s include: Industrial wood & Furniture finishing Plastic coatings, OEM finishes, Automobile coatings & refinished, Industrial coatings, Corrosion coatings, Concrete coatings, Sealants, Paper & textile, leather and printing inks. Polyurethanes are usually made by reacting isocyanates with polyols. Polyurethane is called an alternating copolymer because two types of monomers polymerize in succession. Both the isocyanates and polyols used to make polyurethanes contain two or more functional groups per molecule. Read More Business News on our website.
Isocyanates are an important raw material for it’s manufacturing. It is a functional groups with the formula R-N=C=O. Organic compounds containing isocyanate groups are called isocyanates. Diisocyanates are organic compounds with two isocyanate groups. Diisocyanates are used in the production of polyurethanes, a type of polymer. Isocyanates should not be confused with cyanates and isocyanides, which are very different families of compounds. The cyanic acid (cyanate ester) functional group (R-O-C≡N) is arranged differently than the isocyanate group (R-N=C=O). Isocyanides have the bonding R-N≡C and lack the oxygen of the cyanate group. Isocyanates are electrophiles, and as such they are reactive toward a variety of nucleophiles including alcohols, amines, and even water having a higher reactivity compared to structurally analogous isothiocyanates. Upon treatment with an alcohol, an isocyanate forms a urethane linkage where R and R’ are alkyl or aryl groups. If a diisocyanate is treated with a compound containing two or more hydroxyl groups, such as a diol or a polyol, polymer chains are formed, which are known as polyurethanes.
Manufacturing
Manufacturing polycarbonates or isocyanates by performing the following steps: supplying a stream of carbon monoxide and chlorine; combining the chlorine and carbon monoxide streams to create a phosgene stream; bringing the phosgene stream down to a temperature at which the phosgene in the phosgene stream is liquid, preferably 4°C below or more than 4°C below the phosgene’s boiling point, in order to create a gas stream and a liquid phosgene stream; separating the liquid phosgene stream from the gas stream; making an isocyanate or polycarbonate by reacting the chlorine-depleted phosgene stream with the residual chlorine that was removed from the liquid phosgene stream.
One must know, In dilution of Isocyanates, moisture scavenger plays an important role as most of the solvents contains some moisture content in it; which affects the life of thinned Isocyanate. Some moisture scavenger reacts with moisture where as just absorb the moisture with structural cavity.
Lastly, there are properties of Isocynates.
TDI (Tolulene Diisocyanate)
Properties imparted by TDI:
Good hardness & substantial physical curing, very good water & chemical resistance, good balance of elasticity, hardness and mechanical, poor weathering resistance due to aromaticity, substantial yellowing on UV exposure and relatively low cost than other diisocyanates.
MDI or PMDI
Properties of MDI: Major application are in PU foams,for construction, solvent free coatings, mastics and adhesives, its very low vapour pressure reduces its hazards during compared to the other major isocyanates (TDI, HDI) and MDI has a relatively low human toxicity compared to a cyanates.
HMDI (H12MDI)
Properties of HMDI (12 times hydrogenated MDI): colourless, no yellowing in exterior exposure, very expensive and used in PUD’s, excellent hydrolytic stability of PUDS and mechanical properties as superior TDI & MDI.