Polyurethanes, isocyanate-free

Reactive Extrusion and Isocyanate-Free Polyurethane Chemistry... 

However, isocyanate reactants are very toxic and could entail adverse health effects such as asthma, dermatitis, conjunctivitis, and acute poisoning [313]. Hence, the synthesis of isocyanate-free polyurethanes is of great interest. Accordingly, an isocyanate-free method and new synthesis procedure has been developed to prepare... 

Polyurethane-Based Prepolymers Terminated with Silane-Compounds These prepolymers were based on the same chemistry and the same preparation procedure as all prepolymers mentioned above. But the isocyanate end group was reacted with a silane compound. These prepolymers are isocyanate free and have a different cross-linking mechanism. Adhesives based on these prepolymers are commonly used as sealants. They have good elongation properties, good adhesion to different kinds of materials and are usually more resistant against UV rays. They are not mixable with PU prepolymers of any kind. 

In the practice of this method the high molecular weight polyurethane gum or thermoplastic resin is dissolved in a suitable dry, isocyanate-inert solvent. In the case of the polyurethane gums free di- or polyisocyanate is added to the solution. This is also sometimes done in the case of the thermoplastic polyurethane resins but frequently it is unnecessary, in which case the use of dry, isocyanate-inert solvents is also unnecessary. 

These are two-pack cold-cure coatings and can be a blend of acrylic and epoxy resins for use with ferrous and non-ferrous metals as well as plastics. Designed to be applied by spraying to suitably pre-treated surfaces they are available as an isocyanate-free alternative to two-pack polyurethane systems. 

A wide range of polyurethane-type products has become available in recent years for coating applications. These include simple solutions of linear polyurethanes, two-pot alkyd-isocyanate and polyether-isocyanate systems and a variety of prepolymer and adduct systems. The coatings can vary considerably in hardness and flexibility and find use mainly because of their toughness, abrasion resistance and flexibility. Uses include metal finishes in chemical plant, wood finishes for boats and sports equipment, finishes for rubber goods and rain-erosion-resistant coatings for aircraft. One type of coating is potentially competitive with PVC leathercloth. Both alkyd-di-isocyanate and adduct-diisocyanate compositions may be coated on to fabrics from solutions of controlled viscosity and solids content. Such coated fabrics are soft, flexible and, unlike PVC leathercloth, free from plasticisers. 

The other foaming process is referred to as the prepolymer method. The monomers are reacted to form a low molecular-weight prepolymer. Later the prepolymer is mixed with small amounts of water and is heated. The water reacts with the free isocyanate groups to liberate carbon dioxide, which foams the polyurethane as cross-linking starts. 

In most cases the Ta values are related directly to the lignin content of the network. Soft segment-free networks show more scatter than those containing alkylether components, and this may be attributed to the crosslink density of the materials. The method of soft segment incorporation, simple addition or covalently bonded, has a minimal impact on Ts. A comparison of the HDI and aromatic isocyanate-crosslinked networks, at a given lignin content, shows that the HDI-crosslinked polyurethanes generally have a lower Tg than the aromatic isocyanate-crosslinked networks. 

The batch process equipment used for preparing the components is essentially a set of reactors equipped with heaters and agitators. They operate under vacuum or in an inert gas atmosphere. One of the main requirements of the chemical molding process is the production of pore- and defect-free articles. The volatile products and moisture must be thoroughly removed from the reactant mixture. Moisture imparts porosity to the final articles due to evaporation and the chemical interaction of water with the components of the reactant system, for example, with isocyanates in case of polyurethane formulations. In some cases, moisture can also inhibit the polymerization process, for example, anionic-activated polymerization of lactams. Many monomers, particularly acrylic compounds, require removal of die inhibitors to increase their shelf-life. 

The major problem is moisture that is absorbed into the polyurethane system or into the curative and auxiliary materials. Free water will liberate carbon dioxide when the chain extension is carried out. It is important to keep the reactants dry, as any moisture that may have come in contact with the prepolymer will react to give an amine and carbon dioxide. This amine reacts with more isocyanates to form a disubstituted diamine. The reaction is outlined in Figure 2.9. 

There are two major chemical areas of health concern in the polyurethane industry. The first is isocyanates, in their free form as a liquid or in the vapor form. The second is amines. These can be either curatives or catalysts. One curative, MOCA (CAS number 101-14-4), is classed as a suspect carcinogen in most parts of the world and is subject to many rules and regulations regarding its use. MOCA (MBOCA) is known by many different names, which are listed on various websites such as www.chemindustry.com/chemicals. 

Areas where there are hot drums and pipes must be clearly labeled with appropriate signage. Hot polyurethane prepolymer can cause severe bums, and the potential for free isocyanate vapors is very high. 

The best examples are polyurethanes containing water-labile free isocyanate groups these perform well on duties such as bonding polyurethane surfaces for sporting events. 

Linear polyurethane is used for coating paper. The abovementioned diols and polyethers based on ethylene oxide and propylene oxide with free hydroxyl groups, adipic acid, phthalic acid and the isocyanates indicated in c) may be used as starting materials. 

It is common in acrylic structural adhesives to use oligomers that have a desirable backbone and are terminated with free-radical-polymerizable bonds. A variety of isocyanate-terminated polyurethanes can be adapted to use in acrylic structural adhesives by reacting the terminal isocyanates with a hydroxy functional acrylic monomer such as 2-hydroxyethyl methacrylate.76,92 93... 

The laminating adhesive for Samples 1-3 was a 100 25 mix of an isocyanate terminated polyurethane prepolymer based upon polypropylene glycol and MDI (amount of free monomer approximately 25%) and a tri-functional polypropylene glycol with a molecular weight of approximately 450. 

One-component polyurethane sealants are based on a polyether or polyester isocyanate prepolymer end-capped with a room-temperature-curable group such as trialkoxy silane. Free isocyanate groups are not used in one-part systems because carbon dioxide is evolved on reaction with moisture from the atmosphere which results in bulging of the sealant in the joint. 

Presence of free isocyanate at levels above the minimum exposure limits may induce dermatitis, and inhalation may lead to respiratory problems. These symptoms, however, are cured by removing the cause. Polyurethanes do not involve any hazard as long as good standards of industrial and personal hygiene are practised. 

Polyurethane RIM (PU-RIM) was selected for the Initial RIM application of automotive fascia not only because 1t provides the required damage resistance and other performance requirements, but also because the reactivity of the urethane raw materials, Isocyanate and polyol, provide acceptable productivity. Because the urethane mnomers can be nucleated, high quality sink free surfaces that are suitable for automotive exteriors are produced at the time of demold. Little, 1f any, post mold surface repair 1s needed 1n preparation for painting the RIM part. 

Specifications for the analysis of phosgene will depend upon the end-use application, but a typical analysis of the liquid commercial product may be as given in Table 4.4 [ICI69,ICI78]. Military specifications for use in munitions (COClj, >98% free Cl, <1% acid (HCl), <0.5% residue, <0.5%) also exist [1394a], and the analysis of phosgene for isocyanate manufacture (for polyurethane production) has been described in detail [4S8a]. 

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