MDI Prepolymers

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30—50% of the prepolymer the remainder consists of pigments, fiUers, plastici2ers, adhesion promoters, and other additives. The curing of the sealant is conducted with atmospheric moisture. One-component windshield sealants utili2e diethyl malonate-blocked MDI prepolymers (46). Several polyurethane hybrid systems, containing epoxies, siUcones, or polysulfide, are also used. 

Liquid MDI prepolymer — 2.0 functionality, equivalent wt = 182 Polyol component ... 

Liquid polyester MDI prepolymer, 9% NCO Liquid polymeric MDI, 2.2 functionality Polyol component ... 

Synthesis MDI prepolymer with 1000 MW polyester polyol, NCO/OH = 2.0, chain-extended with 1,4-butandiol, acid number of polyester 0.6. 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30-50% of the prepolymer the remainder consists of pigments, tillers, plasticizers, adhesion promoters, and other additives. 

Using a straight polyurethane system, for example, a PTMEG/MDI prepolymer, there will initially be an absorption band at approximately 1733 to 1725 cm1 (Seymour et al., 1970). As the cure starts, this band will initially... 

This area is expected to see further growth for RIM, part of which will come at the expense of cast systems (4). Many new formulations with inproved properties will become possible through the use of "HOT RIM" equipment which will allow low melting solid isocyanate MDI prepolymers to be used along with low melting solid polyols in the "B" side. 

Preparation of the polymers was carried out by a two step method. First, MDI prepolymers were prepared in a 5 liter round bottom flask equipped with a stirrer, thermometer, addition funnel, and nitroggn inlet. The dried polyols were added dropwise tg MDI at 75°C and the reaction temperature was maintained at 85°C. The mole ratios of MDI to polyol listed in Table II gave prepolymers containing 18.67% free NCO groups by weight or an isocyanate equivalent weight of 225. In the case of prepolymers 2 and 4, the DPG was added first and then the 2000 molecular weight polyol was added. 

The materials used are listed in Table I. All curatives were dried overnight at 80 C in vacuum. MDI prepolymer were used as received. 

TABLE II - Properties of Elastomers Prepared from Polyester-MDI Prepolymer (Vibrathane 6020) Cured with B.D, EG, HQEE Polacure No. 740M, Polyol 50-1180 and Vibracure 3095. 

Uniroyal Chemical, MDI-Prepolymer Systems - Viable Alternatives to MOCA Cured Urethanes. April 18, 1978. 

Uses PU curing agent producing high-performance R.T. cast and cured elastomers, cast prototypes and elastomers, coatings, adhesives, sealants, and spray systems epoxy flexibilizer Features Eor use with TDI or MDI prepolymers Properties Liq. equiv. wt. 225 Versalink P-650 [Air Prods./Polyurethanes]... 

Adhesives, based on MDI-prepolymers and/or prepolymers comprising aliphatic isocyanates with a low content of isocyanate (2-5%). 

MDI Prepolymers with a High Content of Isocyanate This type of adhesive was formulated on the basis of MDI-prepolymer with a relatively high concentration of isocyanate in the uncured adhesive (around 15-16%). This also means a low molecular weight (around 500-600 g/mol). This adhesive type is often used to bond wood. In standard practice, the adhesive is applied under pressure so that most of it flows out between the wood members the thickness of the adhesive layer is virtually zero. 

MDI Prepolymers with a Low Content of Isocyanate The adhesives were formulated on the basis of prepolymers with a low content of isocyanate (2-5%). The molecular weights of these prepolymers were between 1500 and 4500 g/mol. With these types, flexible adhesives and sealants can be formulated. Such adhesives are usually applied in thicker adhesive layers (0.5-5 mm). Due to the relatively low isocyanate content, the tendency to foam is very low. Soft, bubble-free adhesive layers were obtained. 

Polyether and polyester-based MDI prepolymers obtained from commercial sources were used. The details are given in Table 8.2... 

The importance of monol content in 4,T-MDI/BDO cured elastomer systems was determined by comparing 4000-MW PPG diols prepared via ultra-low monol technology, DMC and potassium hydroxide. They are designated as ultra-low monol, low monol and conventional and have monol contents of 0.005,0.016 and 0.085 meq/g, respectively. This corresponds to functionalities of 1.98, 1.94 and 1.71. We prepared 6% NCO 4,4"-MDI prepolymers and chain extended with BDO at an isocyanate to hydroxyl ratio (NCO OH) of 1.03 [15]. Table 9.3 summarises the monol effect on elastomer processing characteristics (pot life and demould time), and physical properties. 

Table 9.3 Comparison of polyurethane elastomers based on ultra-low monol, low monol and conventional 4000-MW diols (6% NCO MDI prepolymers BDO cured) ...

Ultra-low monol PPG diols are commercially available in molecular weights from 2000 to 12,000. A product summary is given in Table 9.4. To determine the formulating range of the various diols in MDI/BDO cured elastomers, we prepared MDI prepolymers based on the 2000, 4000 and 8000 MW diols at a wide range of isocyanate contents. These prepolymers were chain extended with BDO to produce elastomers with a broad range of properties. Tables 9.5 to 9.7 summarise the prepolymer compositions and viscosities, processing characteristics and physical properties of these elastomers. 

Table 9.6 Polyurethane elastomers based on 4000-MW, ultra-low monol PPG (MDI prepolymers EDO cured) ...

Table 9.21 Elastomer properties of MDI prepolymers cured with EDO ...

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