Prepolymer curatives

The casting operation will expose the personnel involved in close proximity to both chemical and physical hazards. Copies of MSDSs for all the various chemicals used must be available at all times. These documents must not be more than 3 years old. They include all prepolymers, curatives, solvents, plasticizers, adhesive paints, blast cleaning agents, solid and liquid additives, and any gasses used. 

In parts where amounts of entrapped air must be kept to an absolute minimum, the process may be carried out under full vacuum. The unit consists of two chambers, one for mixing the prepolymer, curative, and other ingredients, and the lower chamber for the mold. 

The fundamental calculations of the ratios of prepolymer, curative, and the rest of the system must be checked. The wrong EW or NCO levels may have been used. Alternatively, the weights for a different curative could have been substituted. The correct functioning of the scales used also must be confirmed. 

Amine stoichiometry was kept at 95% based on prepolymer content. Except where noted, prepolymers were preheated at 93°C and mixed with curatives at desired temperature. The prepolymer, curative and prepolymer-curative mixture were each vacuum degassed prior to pouring the mixture into a preheated mold. Degassing before and after mixing is especially important to achieve optimum cures. The mold was closed when gelation started. Demolding time was one hour or less. The sample was then post-cured for 16 hours at 120°C. The elastomer was conditioned for one week at 50% relative humidity at room temperature prior to physical properties measurement. 

Blocked isocyanate, for our purposes, will refer to the reaction product of a diisocyanate or isocyanate-terminated prepolymer in which the isocyanate functionality has been reacted with a blocking agent . Once blocked , the diisocyanate can be added to polyols or certain chain extenders, and these materials will not react at room temperature. The concept is shown in the sixth item of Fig. 1. An adhesive formulated with a blocked isocyanate is basically a two-component adhesive that does not react until heated to the activation temperature. When an adhesive is made with a blocked isocyanate together with hydroxyl-containing curatives, the adhesive has a good long shelf life at room temperature. However, once heated... 

Polyester polyurethanes are usually based on a blend of a quasi-prepolymer (polyester/MDl) and a diol/polyester suitable for spray-up operation. An alternative is to use a solvent-containing system using blocked curatives to give an extended pot-life of 2 to 3 hours enabling them to be brush, roller or spray applied. 

The reaction mixture is dehydrated at temperatures as high as 160°C (higher temperatures can be tolerated than with resoles). The prepolymer is cooled, crushed, blended with 5-15% hexamethylenetetramine, (CH2)6N4, and sold to the fabricator. Hexamethylenetetramine, referred to as hexa, is the product of the reaction of 6 mol of formaldehyde and 4 mol of ammonia. Curring occurs rapidly on heating with the formation of both methylene and ben-zylamine crosslinking bridges between benzene rings. The crosslinked network is pictured as... 

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. 

The reactivity of aliphatic diisocyanates is low in comparison to aromatic isocyanates. It is a problem in the manufacturing stage when using the prepolymer route. Quasiprepolymers and one-shot reactions require the correct choice of curative and catalyst for the system to work. 

Linear polyols of various molecular weights can be used in the chain extension of prepolymers. Their main use is to adjust the hardness of the final compound. They act as a reactive plasticizer, and their functionality and molecular weight must be taken into account in the curative calculations. Either polyether or polyester polyols can be used. The most important point is that they must be dry. 

The high exotherm and the number of feeds have led to the popularity of the quasiprepolymer variation of the one-shot technique. Quasiprepolymers are a version of the standard prepolymer method, where some of the polyol is reacted with an excess of diisocyanate. This is later reacted with the remainder of the polyols, including the chain extender and some catalysts. The advantage of this scheme is that the exotherm is spread between the preparation of the quasiprepolymer and the final cured part. In MD1 systems, BDO is often used as the curative. As the molecular weight of BDOs is low, the amount added is low. By dissolving it in the polyol, a more even mix ratio can be obtained. 

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. 

If a diamine curative is used, the amine groups (NH2) will react with an isocyanate group of the prepolymer to form a urea bond. The remaining amine group will react with a further isocyanate group to extend the chain, as shown in Figure 2.30. 

A polyurethane made from a 90 Shore A PTMEG prepolymer and the two curatives (DuPont) both had excellent physical properties, but the pot lives for the two were ... 

When triols are introduced into the prepolymer, they react in a slightly different manner when used as a curative. Prepolymers made containing some triols have a larger hard segment at the point where the triol reacts with the diisocyanate. The general reaction is given in Figure 2.38. 

Polyurethane systems can be produced to have near-equal or easy mixing component ratios (by either weight or volume). The prepolymers in this type of system are called quasiprepolymers. Quasiprepolymers are normally taken as having at least a fourfold molar excess of isocyanate in the isocyanate side of the system. The other side contains the remainder of the polyol, curative, and any catalysts. 

The prepolymer as received from the manufacturer has a simple chain that has been terminated with an isocyanate. The isocyanate ends with this magical NCO group. The NCO is the reactive part of it. The higher the percentage of NCO in the prepolymer, the harder the material will be. An 80 Shore A will have an NCO of approximately 3.1 to 3.2%, whereas a 75 Shore D will have an NCO content of about 11.2%. To obtain the chain extension, one must add an appropriate amount of an amine or diol curative. For every curative, there is a different amount that must be added. The manufacturers of the prepolymers and curatives will give the appropriate factors for mixing the polyurethane. The prepolymer must be heated before use. This is to reduce the viscosity of the material as well as to obtain the correct cure rate and complete cure time. 

Molds must be checked for damage prior to use and preheated to a temperature equal to the maximum that the exotherm will reach when the prepolymer and curative are reacted together. If required, a thin coating of mold release is applied to the mold to assist in the demolding process. It is often found that more that one release can be obtained from the mold that has previously been prepared. 

The calculation is to determine the amount of curative needed for 100 parts by weight of the prepolymer. 

Manufacturers of prepolymers will often provide the weight of curative per 100 units of prepolymer. This is based on the midpoint NCO for the grade and the normal index value for the curative. The calculations can readily be carried out using a calculator or spreadsheet. From a practical point of view, a set of shop scales with a discount function has been used to provide the weight required. 

Curatives are used to enlarge the prepolymer chain either by extension or by cross-linking. Materials with two active groups will extend the length of the chain by joining two chains together. Triols will provide the means to form permanent covalent bonds between different chains. 

Curatives can be either liquid or solid. The solid curatives such as MOCA and MCEDA need to be melted prior to use. The prepolymer and melted curative need to be at a temperature that will prevent any crystallization of the curative when added to the prepolymer. 

There are two main times during the processing of polyurethane where vacuum degassing may be required. The first is the prepolymer as supplied by the manufacturer. The second is after mixing the prepolymer with the rest of the system (curative, pigment, extenders, and fillers). 

Plasticizers and catalysts can be mixed into the liquid curative prior to addition to the prepolymer if required. They must be well blended so that there is a uniform mix. The correct weight of the curative or blend must be accurately weighed in a clean, dry container or with great care directly into the prepolymer. The material must be added in a thin, continuous stream that will form a single phase. The addition must be done rapidly. 

The time available for mixing is a function of the system and can vary from less than 30 seconds to 4-5 minutes. Once the prepolymer and curative are added, a chemical reaction commences. In very hard materials, the time available is very short. After mixing, the material must still be poured into the mold and any entrapped air allowed to escape. 

Foaming In solid materials Moisture in curative Very moist prepolymer... 

Physical properties Durometer hardness, tensile strength, tear strength all low Wrong curative ratios Prepolymer degraded Not fully cured Poor mixing... 

Off-ratio mixes can be a result of the NCO level in the prepolymer being too low. If the prepolymer is stored at too high a temperature for an extended period, the isocyanate at the end of the chains reacts with itself to form multifunctional chains. Thus when the curative is added, it will be in great excess as a result of the isocyanates having used themselves up. 

If the temperatures are too low, the viscosity of the prepolymer will make the mixing of the curative very much harder. The mixed prepolymer also will not flow easily into the mold and will tend to trap air against the mold surface. Another effect is that one can find a roll-type situation with the prepolymer s not joining on itself. 

Although not greatly influenced by the mixing ratio of prepolymer and curatives, hardness is one of the most frequently specified and carried-out tests. The two main handheld units are the Shore A and D range Durometers. 

Similar work can be carried out during the curing by studying the reaction rate between the curative and the prepolymer. The "-NCO" band at 2270 cm4 will decrease and the urethane or the urea bands will increase, depending on the type of curative used. 

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. 

Nitrogen is often used to blanket prepolymers and curatives after the container has been partially used. The blanketing system, if left in place permanently, must be so engineered that the container cannot build up a level of pressure in it that renders the situation unsafe. 

Additive A material combined with the prepolymer and curative to modify the final properties of the casting. This includes plasticizers, fillers, and stabilizers. 

Curative Materials that react with an isocyanate prepolymer to produce the final elastomer. 

Gel time For polyurethanes, the interval of time between mixing together the polyol and diisocyanate or prepolymer and curative and the formation of a nonflowing, semisolid, jelly-like system. 

Polyurethanes are made by extending chains of a prepolymer made from a macro diol and a diisocyanate. The prepolymer is further extended with a diol or an amine curative. The long chains form a solid which is relatively weak. When the part is given a longer heat treatment, the molecules align themselves and intermolecular bonds (hydrogen bonding) are formed. At this point the full mechanical properties are established and the material, if suitably formulated, has excellent mechanical and chemical properties. 

When the prepolymer chain is extended with a diol, the polymer formed has only urethane linkages. The polymer formed with the diamine chain extender is strictly a polyurethane polyurea. The first urethane component is from the initial chain extension when the prepolymer is prepared. A diamine curative will form urea linkages (Figure 2.3) between chains. 

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