Product Bulletin Prepolymers
Krasol Prepolymers Polyurethanes based on hydroxyl terminated polybutadiene resins are known for outstanding hydrolytic stability, very low moisture vapor transmission rates, low temperature flexibility and electric insulation properties. These materials can be extended with asphalt, hydrocarbon oils and effectively loaded and reinforced with fillers. The preparation of polyurethanes is typically classified by the sequence of addition of the reactants (one-step, two-step process) or by the number of components, which the user has to mix together (a one-component or a two-component system). In the one-step (or one-shot) process, the components are mixed together and the resulting mixture is then reacted. In contrast, the two-step process is carried out in two discreet reaction steps. In the first step, the polyol and isocyanate are reacted to form a prepolymer with terminal isocyanate groups. In the second step, the prepolymer is reacted with the chain extender and/or with a crosslinker to form the final product. The two-step synthesis of elastomers from the basic polyurethane components is often employed to reduce the potential exposure to free isocyanate. However, this procedure is time consuming and requires special equipment. It is therefore often simpler to use prepolymers supplied by a manufacturer. Until recently, TDI and MDI prepolymers based on hydroxyl terminated polybutadiene resins that have good storage stability were not readily available. Kaucuk developed hydroxyl terminated polybutadiene resins, Krasol LBH polyols, based on anionic polymerization technology. These products have a very narrow molecular weight distribution with polydispersity indices (Mw/Mn) of about 1.2 and functionalities close to 2. These polybutadiene resins have now been used as the basis for the manufacture of relatively low viscous prepolymers with good storage stability by reaction with toluene diisocyanate (TDI), Krasol LBD prepolymers, or diphenylmethane diisocyanate (MDI), Krasol NN series prepolymers. Sartomer is now the exclusive worldwide distributor of these prepolymers in addition to the remainder of the Krasol product line. The following table summarizes the properties of the primary commercial Krasol prepolymers. Table 1 Typical Properties of Krasol Prepolymers Prepolymer LBD2000 LBD3000 NN-22 NN-23 NN-25 Isocyanate type TDI TDI MDI MDI MDI NCO content, wt % 2.7-4.2 2.1-3.2 8.0-9.7 4.0-5.0 10.9-12.1 Viscosity, mpa.s @25 C 12-22 0.5-3.0 0.7-1.7 70-90 Free TDI, wt % 0.8-1.5 0.8-1.5 0 0 0 Paraffin Oil wt % 0 0 0 50 20 For the preparation of these prepolymers, only a slight excess of TDI is necessary to obtain a product with viscosity below 100,000 mpa.s at 25 C. To achieve the same viscosity with the liquid, modified MDI type isocyanate used here, the NCO/OH ratio must be 3:1 or higher. In the synthesis of polyurethane elastomers based on these prepolymers the stoichiometric ratio of the reacting groups, the NCO of the prepolymer and the OH or NH group of the chain extender and/or crosslinker is a critical factor. Optimum physical properties of the elastomers are obtained with the NCO/OH or NH ratio between 1.0 and 1.1. For best results, chain extenders or crosslinkers that are compatible with the polybutadiene structure of the prepolymer are suggested. Low molecular weight diols such as N, N-diisopropanol amine and 2-ethyl-1,3-hexanediol are recommended. Commonly used diols 1,4-butanediol and diethylene glycol have limited miscibility with the polybutadiene prepolymers. Low molecular weight 2
diamines, for example, Lonzacure M-CDEA, can be used to cure polybutadiene-isocyanate prepolymers. Catalysts commonly used with polyurethane systems can also be employed to increase the rate of reaction. For example, the organometallic catalyst dibutyltin dilaurate and the amine catalyst triethylene diamine are often used for this purpose. An important feature of polybutadiene elastomers is the ability to incorporate relatively high amounts of hydrocarbon oils without a substantial deterioration in the mechanical properties of the final product. Elastomers, prepared by the reaction of TDI prepolymer KRASOL LBD 3000, with diamine Lonzacure M-CDEA, were extended with a naphthenic oil, Enerthene 2368, in amounts from 0 to 17 weight percent. The addition of the oil reduces the viscosity of the prepolymer thus improving processability. With increasing amounts of oil essentially no change in mechanical properties is observed. Table 2 Polyurethane elastomers prepared by the reaction of Krasol LBD 3000 and Lonzacure M-CDEA (NCO/NH 2 =1.00) extended with Enerthene 2368 Naphthenic Oil Formulation (parts by weight) 1 2 3 4 5 Krasol LBD 3000 100 100 100 100 100 Enerthene 2368 0 6 12 18 24 Lonzacure M-CDEA 13.6 13.6 13.6 13.6 13.6 Amount of oil in the elastomer (%) 0 5 10 14 17 Viscosity of the oil extended prepolymer At 25 C (mpa.s) 75,000 46,000 40,000 30,000 21,000 At 40 C (mpa.s) 22,000 12,000 10,500 6,200 6,000 At 80 C (mpa.s) 1,500 1,000 900 750 600 Processing characteristics Pot life at 80 C (min) 5 5 5 5 6 Demolding time at 100 C (minutes) 45 50 50 60 60 Properties of the elastomer Hardness (Shore A) 82 80 80 78 76 Tensile strength (MPa) 18.1 17-19 18.1 14-16 16,2 Elongation at break (%) 400 400-450 490 380-420 440 Rebound resilience (%) 47 48 49 51 54 Abrasion resistance (ARI %) 86 84 80 96 104 Thermal characteristics a Glass transition temperature ( C) -40-40 -50-40 -50 Softening temperature ( C) 270 280 270 240 230 a dilatometric measurement by thermomechanical analyzer TMA-CX04R (Intertec Ltd., Czech Republic) The hydrolytic and chemical resistance of polybutadiene-urethanes in aqueous media is much higher than that of conventional polyurethanes based on polyesters and polyethers. For example, elastomer samples based on Krasol LBD 3000 as shown in formulation 1 of Table 2 were subjected to a variety of aqueous systems for 28 days including immersion in acid and base. Although the stress-strain properties were affected, the samples retained their integrity. In contrast, a similar polyurethane elastomer based on polytetramethylene glycol (PTMG), under these conditions was completely destroyed after only 3 days. 3
Table 3 Resistance of a KRASOL LBD 3000 / Lonzacure M-CDEA elastomer to aqueous media Conditions Change of properties (%) Hardness Shore A Tensile strength Elongation at break Volume Water vapor, 100 C, 28 days +1.0-37.2-43.1-0.39 60 % sulfuric acid, 23 C, 28 days +1.2-12.8-18.6 +1.26 40 % nitric acid, 23 C, 28 days +3.8-54.4-68.3 +2.40 50 % sodium hydroxide, 23 C, 28 days -0.8-2.8-9.0 +0.58 It is also possible to modify polyurethane elastomers based on Krasol with various types of asphalt and a large selection of solid fillers such as calcium carbonate, clay, carbon black, silica, etc. Such modification provides the user with a wide range of possible formulations for sealants, potting compounds, coatings, mastics and adhesives. As previously mentioned, one distinct advantage of using prepolymers in the preparation of polyurethane elastomers is the reduction in the potential exposure to free isocyanates. Some differences in the physical properties of the polyurethanes prepared by a one-step versus a two-step process are often observed. One-step polyurethanes are typically softer in comparison with two-step products and have lower tensile strength and higher elongation at break. Table 4 Elastomers based on Krasol NN-22 or Krasol NN-23 prepolymer and Krasol LBH 3000 and glycerol as curatives Formulation (parts by weight) 1 2 3 Part A Krasol LBH 3000 92.9 46.5 57.5 Glycerol 7.1 3.5 4.0 1,4-Butanediol - - 4.0 Asphalt AP-80 - - 23.0 Paraffinic oil - - 11.5 Part B Krasol NN-22 100.0-100.0 Krasol NN-23-100.0 - Mixing ratio A/B 1.00 0.50 1.00 DBTL 1% in paraffinic oil, g/100g of prepolprepolymer 0.60 0.65 0.90 Processing characteristics Pot life at 23 C (min.) 30 50 45 Demold time at 23 C (hours) 24 48 24 Elastomer properties Hardness (Shore A) 47-55 35-40 47-52 Tensile Strength (MPa) 6.6-7.6 2.0-3.0 6.3-6.7 Elongation at break (%) 300-400 100-200 270-330 Rebound resilience (%) 38-42 44-48 38-42 Abrasion resistance (ARI%) 50-55 45-50 28 Glass trans temp. ( C) -40-55 -55 Softening temp. ( C) 170 175 180 Compression set 72h/25 C (%) - - - Volume resistivity (Ω.m) 1.0 10 14 6.0 10 13 - Surface resistivity (Ω) 5.0 10 15 5.0 10 15-4
The Krasol prepolymers based on liquid MDI can be used as high quality binders for the production of composite materials from ground rubber scrap, polyurethane foam and cork. Crosslinking of these one-component polyurethane binders by ambient moisture may occur at normal or elevated temperature. The mode of curing affects the final properties of the composite material. The processing characteristics can be adjusted by the addition of the catalyst. Depending on the nature and particulate size of the ground material, it is possible to obtain a large number of products for varied applications. The binder content is usually 7.5-20 weight percent depending on the ground material and the application of the final product. Polybutadiene-urethane binders based on these prepolymers exhibit outstanding adhesion to the ground cured rubber material and other nonpolar materials. Composite materials prepared using these binders have excellent physical and mechanical properties, good elasticity at low temperatures and outstanding resistance to aqueous environments. However, they exhibit lower resistance to solvents, oils, gasoline and diesel oil. In contact with these materials the binder itself swells but after evaporation of the liquid the composite regains its original properties. These polybutadiene prepolymer binders contain an aromatic diisocyanate. As a result, they tend to turn yellow and degrade when exposed to UV radiation. However, this degradation of the composite materials is only on the surface and therefore does not affect the physical and mechanical properties and utility of the product. If necessary, the products from composite materials can be coated with protective layers or coatings. Additional information on the use of these prepolymers as binders for the production of composite materials is contained in a separate bulletin. One-component polyurethane coatings are used in a great variety of applications, such as for protection of steel structures and poles against corrosion. Binders for these coatings are isocyanate prepolymers based on the reaction of a polyol with an aromatic or an aliphatic diisocyanate. The Krasol NN-22 prepolymer offers exceptional properties as a special polyurethane binder for coatings. These polyurethane/polyurea coatings exhibit very good resistance to water and aqueous solutions of acids, bases and salts. However, materials prepared from the prepolymer based on MDI exhibit a low resistance to UV radiation. Therefore these prepolymers are predominantly recommended for base anticorrosion coatings. Additional information on the use of these prepolymers in coatings is contained in a separate bulletin. 5
Corporate Headquarters Cray Valley USA, LLC 468 Thomas Jones Way, Suite 100 Exton, PA 19341 Tel: 877-US1-CRAY (877-871-2729) E-mail: TechSupport@crayvalleyus.com For contact information worldwide, please refer to Cray Valley's web site at: http://www.crayvalley.com 3140 01/10 The information in this bulletin is believed to be accurate, but all recommendations are made without warranty since the conditions of use are beyond Cray Valley Company's control. The listed properties are illustrative only, and not product specifications. Cray Valley Company disclaims any liability in connection with the use of the information, and does not warrant against infringement by reason of the use of its products in combination with other material or in any process.