Some noteworthy recent efforts have been dedicated to minimizing the use of isocyanates to synthesize polyurethanes, because the isocyanates raise severe polyurethane manufacturing process pdf issues. Polyurethanes neither contain nor are produced from ethyl carbamate.
Leverkusen, Germany, first made polyurethanes in 1937. Polyether polyols were cheaper, easier to handle and more water-resistant than polyester polyols, and became more popular. In 1960 more than 45,000 metric tons of flexible polyurethane foams were produced. Polyurethane foams are now used in high-temperature oil filter applications. North America and the EU, although chlorinated blowing agents remained in use in many developing countries. The properties of a polyurethane are greatly influenced by the types of isocyanates and polyols used to make it.
Long chains and low crosslinking give a polymer that is very stretchy, short chains with lots of crosslinks produce a hard polymer while long chains and intermediate crosslinking give a polymer useful for making foam. The crosslinking present in polyurethanes means that the polymer consists of a three-dimensional network and molecular weight is very high. In some respects a piece of polyurethane can be regarded as one giant molecule. The choices available for the isocyanates and polyols, in addition to other additives and processing conditions allow polyurethanes to have the very wide range of properties that make them such widely used polymers. Isocyanates are very reactive materials.
This makes them useful in making polymers but also requires special care in handling and use. Most of the isocyanates are difunctional, that is they have exactly two isocyanate groups per molecule. An important exception to this is polymeric diphenylmethane diisocyanate, which is a mixture of molecules with two, three, and four or more isocyanate groups. In cases like this the material has an average functionality greater than two, commonly 2. Polyols are polymers in their own right and have on average two or more hydroxyl groups per molecule. The polyols used to make polyurethanes are not “pure” compounds since they are often mixtures of similar molecules with different molecular weights and mixtures of molecules that contain different numbers of hydroxyl groups, which is why the “average functionality” is often mentioned. Despite them being complex mixtures, industrial grade polyols have their composition sufficiently well controlled to produce polyurethanes having consistent properties.
As mentioned earlier, it is the length of the polyol chain and the functionality that contribute much to the properties of the final polymer. Polyols used to make rigid polyurethanes have molecular weights in the hundreds, while those used to make flexible polyurethanes have molecular weights up to ten thousand or more. Alternatively, it can be promoted by ultraviolet light. This is often referred to as the gellation reaction or simply gelling. One of the most desirable attributes of polyurethanes is their ability to be turned into foam. The balance between gellation and blowing is sensitive to operating parameters including the concentrations of water and catalyst.
The amine reacts with more isocyanate to give a substituted urea. The urea is not very soluble in the reaction mixture and tends to form separate “hard segment” phases consisting mostly of polyurea. The concentration and organization of these polyurea phases can have a significant impact on the properties of the polyurethane foam. Rigid foam surfactants are designed to produce very fine cells and a very high closed cell content. Flexible foam surfactants are designed to stabilize the reaction mass while at the same time maximizing open cell content to prevent the foam from shrinking.
Such properties are desired in rigid foam products used in the construction sector. Foams can be either “closed-cell”, where most of the original bubbles or cells remain intact, or “open-cell”, where the bubbles have broken but the edges of the bubbles are stiff enough to retain their shape. Open-cell foams feel soft and allow air to flow through, so they are comfortable when used in seat cushions or mattresses. Other materials are added to aid processing the polymer or to modify the properties of the polymer.
Isocyanates used to make polyurethane have two or more isocyanate groups on each molecule. TDI and MDI are generally less expensive and more reactive than other isocyanates. Industrial grade TDI and MDI are mixtures of isomers and MDI often contains polymeric materials. Aliphatic and cycloaliphatic isocyanates are used in smaller quantities, most often in coatings and other applications where color and transparency are important since polyurethanes made with aromatic isocyanates tend to darken on exposure to light. They can be further classified according to their end use.
The order of addition and the amounts of each oxide affect many polyol properties, such as compatibility, water-solubility, and reactivity. Polyols made with only propylene oxide are terminated with secondary hydroxyl groups and are less reactive than polyols capped with ethylene oxide, which contain primary hydroxyl groups. Conventional polyester polyols are based on virgin raw materials and are manufactured by the direct polyesterification of high-purity diacids and glycols, such as adipic acid and 1,4-butanediol. Polyester polyols are usually more expensive and more viscous than polyether polyols, but they make polyurethanes with better solvent, abrasion, and cut resistance. Other polyester polyols are based on reclaimed raw materials.
The materials are used in elastomer, sealant, and adhesive applications that require superior weatherability, and resistance to chemical and environmental attack. Two-component fluorinated polyurethanes prepared by reacting FEVE fluorinated polyols with polyisocyanate have been used to make ambient cure paints and coatings. UV, acids, alkali, salts, chemicals, solvents, weathering, corrosion, fungi and microbial attack. These have been used for high performance coatings and paints. This covalent linkage prevents migration and leaching of the organophosphorus compound. Even polyols prepared from renewable sources like vegetable oils, derivatives of vegetable oil, sorbitol, cellulose, etc.
Many polyols are derived from renewable raw materials like vegetable oils. Vegetable oils are functionalized by different ways and modified to polyetheramide, polyethers, alkyds, etc. Various oils used in the preparation polyols for polyurethanes include soybean, cotton seed, neem seed, and castor. Recently neem oil acetylated polyester polyol have been prepared as a renewable alternative to petro based polyols. Moreover, the said polyol was used to formulate self-healing polyurethane coatings using encapsulated core materials. Some biobased and isocyanate-free polyurethanes exploit the reaction between polyamines and cyclic carbonates to produce polyhydroxurethanes.