TDI-terminated prepolymers

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-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. 

There are regulatory and handling problems in using methylene bis(2-chloroaniline) as a chain extender (curative or cross-linker) for toluene diisocyanate (TDI)-terminated prepolymers, to produce urethane elastomers (1,2). There is, therefore, a strong interest in achieving similar elastomer properties with other curatives and methylene diphenyl diisocyanate (MDI)-terminated prepolymers(3,4). 

TDI is also commonly used as a fast-reacting isocyanate that is reacted with the polyol. It Is very common for polyester-type polyols to be prereacted with TDI by the manufacturer and sold to fabricators as TDI-terminated prepolymers. 

Polyester polyols and sometimes polyether polyols are reacted with TDI to give better compression set and high-temperature properties than might be achieved with MDI. Better properties from a TDI-terminated prepolymer can be achieved more effectively when it is cured with a diamine. 

TDI-terminated prepolymers can impart unique properties to a PUR application, especially if a diamine curative is being used. MDI-terminated prepolymers have difficulty matching certain physical properties. 

MBCA (commonly referred to as MOCA) is a special diamine that has worked very well as a curative for TDI-terminated prepolymers. MBCA was first introduced in the mid-1950s for high-performance polyurethane applications. However, the use of MBCA has been restricted because this curative has been found to be a carcinogen requiring special handling to protect workers. 

The reaction between TDI and polyols has been simulated in a computer study [154], The objective was to prepare NCO-terminated prepolymers containing a minimum of monomeric TDI. Twelve simultaneous and/or consecutive reactions were treated mathematically. The calculated predictions were in good agreement with experimental results. 

The NCO-terminated prepolymer can be synthesized from a diisocyanate (e.g., TDI, MDI) and a polyol. The anilinosilane 1 can be prepared from the corresponding chloromethylsilane and aniline, i.e. from low cost components, too. 

The elastic textile fiber Spandex (Du Pont s Lycra) is an excellent example of a successful TPE material (U.S. Patent 2,692,873 issued in 1954, but not commercialized by Du Pont until 1962). Another example is the development of Du Pont s Adiprene PU rubber, which is formed by the reaction of 3 parts of PTMEG (an oligomer of 9—11 tetramethylene ether units) with 4.3 parts of TDI to give an isocyanate-terminated prepolymer that is subsequently treated with 0.3 part of TDI and some water to form an urea-linked end product. 

Nicholas and Gmitter (58) studied the relative effectiveness of a number of trimerization catalysts (see Table I). Isocyanurate coatings based on TDI and HDI have been reported by Kubitza (59). In addition, Sandler (60) prepared polyisocyanurate adhesives from isocyanate-terminated prepolymers employing calcium naphthenate as a trimerization catalyst. 

The silane-terminated prepolymers are prepared from polypropyleneglycol (M = 450), TDI, and (N-phenylaminomethyl)trimethoxysilane (1 Fig. 3). 

The prepolymer can be synthesized by a simple one-pot synthesis . During a first step an NCO-terminated prepolymer is prepared by a reaction of the polyol with an excess of TDI. The second step comprises the reaction of all remaining NCO groups with the silane 1. Phenyltrimethoxysilane 2 serves to reduce the viscosity of the prepolymer mixture. It has already been added during the prepolymer synthesis. 

Liquid-NCO terminated prepolymers, which are primarily based on low-viscosity polyoxypropylene glycols and TDI require only a small amount of solvent (5-10%). However, the solvent remains in the finished dispersion. In addition the reaction of the primary emulsion with water or diamines, which results in the formation of poly urea, is dependent upon diffusion and difficult to control. 

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. 

One-component urethane coatings are usually based on MDI, TDI or HDI terminated prepolymers containing free isocyanate groups. Pigmenting such systems is delicate due to moisture in the pigment, which can cause premature gelation. Therefore, these systems must contain additives for moisture removal such as zeolites (alumosilicates). Since the chain extension in such systems is carried out with water from air, one of the products of the reaction is CO2, which diffuses from the film without foaming. 

These isocyanate-terminated prepolymers can be terminated with either TDI or MDI (isocyanates). 

TDI is used to react with polyols to form isocyanate-terminated prepolymers for polyurethane elastomer applications. 

Ex. 2. Liquid, isocyanate-terminated prepolymers prepared from diisocyanates such as TDI (I) and Poly B-D glycol (XXVII), a polybutadiene glycol, may be placed on an adherend directly or from solution and permitted to cure via the action of moisture in the air. Useful as rubber-fabric adhesive. 

The most common polyisocyanate used in urethane sealant production is toluene diisocyanate (TDI). The TDI is usually prereacted to give an isocyanate-terminated prepolymer. Other aromatic polyisocyanates used in sealants include diphenylmethane diisocyanate (MDI) and polymethylene polyphenylene isocyanate (PAPI). All of the aromatic poly isocyanates contribute to yellowing of urethanes on exposure to light. 

The reaction rates of the various ingredients must be understood to follow the probable course of the reaction. Certain polyols may have secondary hydroxyl groups that are very much slower in reaction than the primary hydroxyl groups. The preparation of a prepolymer will allow the secondary hydroxyls to fully react. The reaction of a 100-percent 2,4 TDI is much faster than the standard 80 20 TDI isomer blend. The reaction rate between the terminal NCO and chain extender can be adjusted to allow the prepolymer mix to fill the mold and allow entrapped air to escape before the material gels. 

Some examples of modified polyisocyanates are isocyanate-terminated quasi-prepolymers (semi-prepolymers), urethane-modified MDI, carbodiimide-modified MDI, isocyanurate-modified TDI, and isocyanurate-modified isophorone diisocyanate. 

Buricus (201) and Nicholas and Gmitter (58) reported on TDI prepolymer-based isocyanurate foams prepared by trimerizing NCO-terminated jn-epolymers. TGA data (58) showed that there was no... 

A typical polyester could be poly (diethyleneglycol adipate) with a molecular weight of between 2000 and 3000. A typical polyether is obtained by polymerisation of propylene oxide, usually in the presence of a small proportion of glycerol or sorbitol to provide branched structures. Two of the most commonly used isocyanates are 2,4- and 2,6- tolylene di-isocyanate (TDI). These are chosen because of their reactivity, cheapness and relative low toxicity. The resulting prepolymer has -OH terminal groups when a deficiency of isocyanate is used, but -NCO terminal groups if an excess of isocyanate is employed. 

Other types of adducts are formed by reacting hydroxyl-terminated polyethers or polyesters with TDI. When these polyols are reacted at an NC0 0H ratio of 2 1, these adducts, also referred to as prepolymers, have the schematic structure, depending upon the functionality of the polyol, shown in Figure 1. 

Through the synthesis of poly(urethane-imide) films and their carbonization, carbon films were obtained whose macropore structure could be controlled by changing the molecular structure of polyurethane prepolymer [164-166]. Poly(urethane-imide) films were prepared by blending poly(amide acid), which was synthesized from pyromellitic dianhydride (PMDA) and 4,4 -oxydianiline (ODA), and phenol-terminated polyurethane pjrejwlymers, which were synthesized through the reaction of polyester polyol with either hexamethylene diisocyanate (HDI), tolylene-2,4-diisocyanate (TDI) or 4,4 -diphenyknethane-diisocyanate (MDI). The reaction schemes of two components, poly(imide) (PI) and poly(urethane) (PU), are shown in Fig. 46a). 

So-called anaerobic polyurethane adhesives also belong to this category. This system combines urethane poly-addition chemistry with free-radical initiated addition polymerization. A polymerizable alcohol such as j -hydroxyethyl methacrylate is reacted with an equivalent amount of a diisocyanate such as TDI or with an isocyanate-terminated urethane prepolymer. An organic hydroperoxide is then added to such intermediates... 

---