Polyurethane handling isocyanate

A number of "crude" isocyanates (polymeric isocyanates), undistilled grades of MDI and TDI, are available in the market. Some of these products, such as various forms of crude MDI, have a functionality varying between 2 and 3. They have a lower reactivity and a lower vapor pressure than the corresponding pure isocyanate. They have found extensive use in one-shot rigid foams. However, they are also employed in coating, sealant, and adhesive applications. The Upjohn Chemical Division has published an excellent bulletin on the use and precautions in handling isocyanates, polyurethanes, and related materials (13A). 

Some of the chemicals used in the production of polyurethanes, such as the highly reactive isocyanates and tertiary amine catalysts, must be handled with caution. The other polyurethane ingredients, polyols and surfactants, are relatively inert materials having low toxicity. 

The principal additive shrink-resist treatment uses the polymer Synthappret BAP (Bayer AG) which is a polypropylene oxide polyurethane containing reactive carbamoyl sulfonates (or isocyanate bisulfite adduct groups, —NHCOSO —Na" ). An aqueous solution of this polymer is padded onto woven fabrics, which are immediately dried. Other polymers may be appHed at the same time to modify the handle. 

There is persisting interest in nylon-RIM materials as alternatives to polyurethane-RIM. Advantages of the nylon materials are the better shelf life and lower viscosity of the reaction components, ability to mould thick-walled articles, absence of a need for mould lubrication and the ability to avoid using isocyanates with their associated hazards. The main disadvantages of nylon-RIM are the need to have heated storage tanks and elevated temperature reactions, difficulties in catalyst handling and the high water absorption of the product. Possible markets include exterior car body components and appliance and business machine components. 

This handbook provides detailed working procedures for those exposed to isocyanates and other chemicals used in flexible foam manufacture, supplying hazard data for individual chemicals and recommending handling practices. This second edition provides the most up-to date information regarding topics covered in the first edition and also includes a code of practice for the manufacture of polyurethane elastomers using MbOCA. 

Fully cured polyurethanes present no health hazard they are chemically inert and insoluble in water and most organic solvents. Dust can be generated in fabrication, and inhalation of the dust should be avoided. Polyether-based polyurethanes are not degraded in the human body, and are therefore used in biomedical applications. Some of the chemicals used in the production of polyurethanes, such as the highly reactive isocyanates and tertiary amine catalysts, must be handled with caution. The other polyurethane ingredients, polyols and surfactants, are relatively inert materials having low toxicity. 

The production of polyurethane involves the controlled polymerization of an isocyanate, a long-chain-backbone polyol and a shorter-chain extender or cross-linker. The reaction rates can be controlled through the use of specific catalyst compounds, well known in the industry, to provide sufficient time to pour or otherwise transfer the mix and to cure the polymer sufficiently to allow handling of the freshly demolded part. The use of blowing agents allows the formation of a definite cellular core (thus the term microcellular elastomer ) as well as a non-porous skin, producing an integral sandwich-type cross section. 

Cast polyurethanes. Polyurethanes are reaction products of an isocyanate, a polyol, and a curing agent. Because of the hazards involved in handling free isocyanate, prepolymers of the isocyanate and the polyol are generally used. 

Sensitization may last for many years or for life it usually lessens with time and most, but not all, people slowly become desensitized. A sensitized person must avoid further work with isocyanates and also asthmatic individuals should not work with isocyanates. Many employers who use isocyanates require their employees to be medically certified as free from respiratory problems and will systematically monitor their employees for maintenance of lung function through forced respiratory volume (FRV) monitoring. Further information is provided in the BRMA code of practice entitled Toxicity and Safe Handling of Diisocyanates and Ancilliary Chemicals a code of practice for polyurethane flexible foam manufacture. 

After curing, the fully reacted polyurethane resin formed is no longer chemically active and is not a hazard to health unless heated to the point of decomposition. Handling materials containing prepolymers is not likely to give rise to an inhalation hazard from the isocyanate unless they are heated, sprayed or distributed into the air as dust before the curing process has had time to go to completion. This could happen in some printing processes where the characteristics of the ink and the process can lead to the production of spray. In addition, solvents are used in some ink formulations and these are dried by the application of heat, which would drive off any residual isocyanate as vapour. 

Raw materials for polyurethane preparation are isocyanates, polyols (sometimes polyamines) and catalysts. Polyols are fairly harmless substances, but the polyol component may contain hazardous additives and catalysts. Amine chain extenders and catalysts present usual hazard like all amines, i.e., they are often carcinogenic. This is especially true of MOCA. Metal catalysts are also very toxic and careful handling is necessary. 

Initial capital investment requirements in the polyurethane product industry are not high compared to some other traditional rubber fabricating operations. Therefore, the economic barriers to entry are often relatively low compared to other segments of rubber product manufacture. Not only is PU cast molding less expensive, it is in many ways easier. On the other hand, worker safety and health issues can be of great concern, especially with the use of isocyanates and especially MBCA-type raw materials. Safe handling methods are very important. 

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