TDI based prepolymer

Because of the high cross-link density of polyisocyanurates as prepared above, the resultant foams are brittle, so that there has been a move towards polyisocyanurate-polyurethane combinations. For example, isocyanurate-con-taining polyurethane foams have been prepared by trimerisation isocyanate-tipped TDI-based prepolymers. The isocyanurate trimerising reaction has also been carried out in the presence of polyols of molecular weight less than 300 to give foams by both one-shot and prepolymer methods. 

In a set of parallel studies, a series of polyethylene glycols were tested to confirm the effects of polyol molecular weight. The polymers including a TDI-based prepolymer... 

Monitoring must be carried out to check that the levels of isocyanate vapors do not exceed the local limits. These limits vary from area to area. In some areas of the world, only the isocyanate level is specified and is not controlled by the type of isocyanate. Care must be taken, as the vapor pressure of different isocyanates may vary by a factor of 100 or more. Plants that may comply with the atmospheric isocyanate levels when using MDI-based material may have too high a level when using TDI-based prepolymers. 

TDI-based prepolymer. This aromatic type of prepolymer is formed by the reaction of TDI with short-chain polyols. The resultant prepolymer will have an NCO group content of 14.2%, a viscosity of 1000 CP at 20°C and a solid content of 75%. This prepolymer may typically be used as a primer or as a base for one or two pack polyurethane coatings. The reaction is represented in Figure 2.23. 

Manufacturing Process. Both prepolymer and one-shot processes are available, but the polymeric isocyanate-based one-shot process is used in preference because of the easy processing due to the low viscosity of the system, relatively low toxicity of polymeric isocyanates, and fewer environmental problems. However, a disadvantage of the one-shot process is a possible risk of shrunken-foam formation due to its higher closed-cell content. In contrast, the TDI-based prepolymer process has advantages including better in-mold flowability and higher open-cell content. 

TDI Prepolymer Process. This process has the major advantage of better flowability in pour-in-place processes, e.g., household-refrigerator insulation, than crude TDI-based or polymeric MDl-based processes. An example of TDI-based prepolymers and a formulation for making rigid urethane foam on a small scale is shown below. 

Because of the high cross-Knk density of polyisocyanurates, the resultant foam tends to be brittle. Consequently, there has been a move toward making polyisocyanurate-polyurethance combinations. For example, the isocyanate trimerization reaction has been carried out with isocyanate end-capped TDI-based prepolymers to make isocyanurate-containing polyurethane foams. Isocyanate trimerization in the... 

An activated carbon (Nuchar RGC Powder, 879-R-02) was supplied by the Chemical Division of Westvaco, Covington, VA. A slurry of the carbon was made in water and emulsified with a TDI-based hydrophilic polyurethane prepolymer (Hypol 2002, Dow Chemical, Midland, MI). Immediately after mixing, the emulsion was grafted to a 30-ppi polyether polyurethane (Crest Foam T-30, Monachie, NJ.). The amount of carbon was determined gravimetrically to be 29% by weight. 

Perhaps the most interesting application of polyurethane foam as a substratum for cell growth was studied by Bailliez et al. While not specifically a remediation study, their work compared hydrophobic and hydrophilic polyurethanes, TDI- and MDI-based prepolymers, and entrapment and adsorption methods, and also investigated the production of hydrocarbons by Botryococcus braunii. An unfortunate feature of biotechnical research in the use of polyurethanes is that the chemistry is rarely explained. While Bailliez includes some detail, much of their work simply designates products without specific references to the polyols. It is, of course, part of the mission of this book to show that polyurethanes are specialty chemicals. It cannot be assumed... 

In the entrapment studies, a concentrated dispersion of the cells was mixed with an equal mass of prepolymer. For adsorption, the foams were prepared, washed, sterilized in an autoclave, then placed in suspensions of the algae for 14 days. The entrapped algae showed no respiration for the majority of the prepolymers including the TDI-based hydrophilic Hypol 2002. Only five of the prepolymers were thought to be acceptable they included the two MDI-based prepolymers (Hypol 4000 and 5000). 

This subject was studied in relation to the silicone breast implant controversy of the 1990s. Silicone-fiUed artificial breasts were coated with thin layers of a TDI-based polyurethane foam to encourage stabilization by cell ingrowth. Residual toluene diamine (TDA) was found in prepolymer-based polyurethanes at the part-per-biUion level and, more importantly, hydrolysis that would lead to the release of more TDA in vivo was suspected. 

The two main groups used as chain extenders are diamines and hydroxyl compounds. Triols are also used where some cross-linking is required. The choice of chain extender depends on the properties required and the process conditions. Diols are the most commonly used hydroxyl compound. In the normal course of events, diols provide good properties and processing speed with MDI-based prepolymers and diamines with TDI-terminated prepolymers. 

Amine curatives tend to give too fast a cure rate when used with MDI-based prepolymers and give the best results when used with castable systems based on TDI. 

Heat plays an important part in the curing of polyurethanes. The reaction itself gives out heat, so this must be taken into account in determining the temperature of the mold. The mold should be at the maximum temperature the curing prepolymer will reach. An MDI-based system will release heat more rapidly than a TDI-based system therefore the mold has to be hotter than when using a TDI-based material. 

Compression set is controlled by the cross-linking in the material. This may be carried out during the design of the prepolymers or by using some short-chain cross-linking agents. TDI-based polyurethanes have better compression set than the MDI-based materials. 

Amine-based curatives are a popular choice when working with TDI-terminated prepolymers. The two most used amine-based curatives are MOCA and Ethacure 300. Other amine curatives used include Versalink 740M, MCEDA, and Cyanacure. 

In principle, both the one-shot process and semi-prepolymer processes have been used for rigid-urethane-foam manufacturing. However, the monomeric TDI-based one-shot process was used only in the initial stage of the rigid-urethane-foam industry because of the toxicity problems of TDI and difficulties in controlling reactivity due to the high NCO percent. For these reasons TDI-prepolymers, blends of TDI prepolymers and polymeric isocyanates, and 100% polymeric isocyanate are most widely used. 

In the preparation of TDI-based semi-prepolymer, TDI-80/20 and a polyol, preferably sucrose- or sorbitol-based polyol, are reacted to obtain about 30% free NCO-containing semi-prepolymers. 

However it is also established that the commonly used diphenylmethane and naphthalene diisocyanate-based elastomers are frequently chain-extended with glycols. Table 3.12 contrasts the effects of diamine and glycol chain extenders with a toluene diisocyanate/polyether-based prepolymer, Adiprene LI00. With the unsymmetrical diisocyanates, such as TDI and MDI, polyol chain extenders give elastomers of poorer properties than diamines, and it is usually desirable to introduce additional crosslinking into diol-extender elastomers by the use of a proportion of a triol such as... 

The use of PMDI as a binder for foimdry cores, rubber waste products, and solid rocket fuel are also known. Isocyanate-terminated prepolymers, often prepared from TDI or MDI with polyether polyols are also used as binders for composite products that require elastomeric properties. Athletic surfaces are sometimes prepared from groimd rubber tire scrap bonded with isocyanate-based prepolymers. Similarly, flexible polyurethane foam scrap is bonded with isocyanate prepolymers to form rebonded foam usefiil as carpet imderlay. Solidification of incineration ashes with PMDI-based binder systems is another waste disposal application. In this manner hazardous waste materials imdergo chemical fixation and detoxification. 

The evolution of rigid foam technology has seen a change from TDI based foams, frequently made by a prepolymer method, to MDI based foams which can be manufactured in slabstock or moulded form, in continuous laminates (sandwich structure), or can be foamed Hn-situ without the hazards of toxicity associated with TDI. Urethane foams can also be sprayed, but positive air pressure hoods should be worn by operatives. 

Ionic polymers are also formulated from TDI and MDI (43). Poly(urethane urea) and polyurea ionomers are obtained from divalent metal salts of /)-aminohen2oic acid, MPA, dialkylene glycol, and 2,4-TDI (44). In the case of polyureas, the glycol extender is omitted. If TDI is used in coatings apphcations, it is usually converted to a derivative to lower the vapor pressure. A typical TDI prepolymer is the adduct of TDI with trimethyl olpropane (Desmodur L). Carbodiimide-modified MDI offers advantages in polyester-based systems because of improved hydrolytic stabihty (45). Moisture cure systems based on aromatic isocyanates are also available. 

Oil-Based SINs. The SINs produced were based on a castor oil polyester-urethane and styrene crosslinked with 1 mole percent of technical grade (55%) divinyl benzene (DVB) (7). This structure may be written poly[(castor oil, sebacic acid, TDI)-SIN-(Styfene, DVB)], poly[(CO,SA,TDI)-SIN-(S,DVB)]. Benzoyl peroxide (BP) (0.48%) was used as the free radical initiator for the styrene and 1,4-tolylene-diisocyanate (TDI) was used as the crosslinker for the polyester prepolymer. A 500 ml resin kettle equipped with a N inlet, condenser, thermometer, and high torque stirrer was used as the polymerization reactor. 

Important commercial isocyanates include the diisocyanate monomers toluene diisocyanate (TDI), methylene diphenyl diisocyanate (MDI), hexamethylene diisocyanate (HDI), and MDI-, TDI-, and HDI-based isocyanates (e.g., prepolymers and polyisocyanates). World-wide production volume is estimated at over 12 billion lb. Isocyanates (diisocyanates, polyisocyanates, and prepolymers) all cause similar health effects, most commonly asthma [32]. Isocyanates are reported to be the leading attributable cause of work-related asthma [16]. Isocyanates are potent sensitizers that can trigger a severe and potentially fatal asthma attack in sensitized persons at very low isocyanate exposure levels [16]. Toluene diisocyanate is reasonably anticipated to be a human carcinogen by National Toxicology Program. 

Rigid foams are based primarily on polyfunctional, low molecular weight alcohols and amines. Most global applications conventionally use polymeric isocyanates, TDI, or an undistilled grade of mixed TDI isomers. TDI prepolymers which have hydroxyl and isocyanate groups have been marketed as a low vapor pressure alternative to undistilled TDI. Density reduction is effected via the addition of chlorofluorocarbons, low molecular weight alkanes, or via the in situ generation of carbon dioxide. The resultant closed cell foams find applications as insulators in construction, appliance, transportation, pipeline, and tank end uses. 

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