8 Polyurethanes Zoran S. Petrovic ´ Pittsburg State University, Kansas Polymer Research Center, Pittsburg, Kansas I. INTRODUCTION The history of polyurethanes started in the 1930s in Germany when Otto Bayer proposed using diisocyanates and diols for preparation of macromolecules. The first commercial polyurethane, based on hexamethylene diisocyanate and butanediol, had similar proper- ties to polyamides and is still used to make fibers for brushes. However, fast growth of the production and expanded application range started in the 1950s with the building of toluene diisocyanate (TDI) and polyester polyol plants for flexible foams in Germany. However, the real jump in applications came with the introduction of polyether polyols in foam formulations. Further development and application of polyurethanes shifted from Europe to the USA and Japan. Today, polyurethanes are about the sixth largest polymer by consumption, right behind high volume thermoplastics, with about 6% of the market. The largest part of the urethane application is in the field of flexible foams (about 44%), rigid foams (about 28%), while 28% are coatings, adhesives, sealants and elastomers (CASE) applications. These data are taken at a certain moment in time (1996) and vary from year to year and region to region, but they illustrate the relative consumption in different categories. Consumption of polyurethanes in different industries is the following: about 40% of PU is used in the furniture industry, 16% in transport, 13% in construction, 7% in refrigeration and about the same in co atings, 6% in the textile industry, 4% in the footwear industry and 8% for other applications. Table 1 illustrates the consumption of urethanes in the United States in 1996. Polyurethanes are a broad class of very different polymers, which have only one thing in common – the presence of the urethane group: urethane group ð1Þ The number of these groups in a polymer can be relatively small compared with other groups in the chain (for example ester or ether groups in elastomers), but the polymer will still belong to the polyurethane group. Varying the structure of polyurethanes, one can vary the properties in a wide range. Polyurethanes are formed by reaction of polyis ocyanates with hydroxyl-containing compounds, most frequently Copyright 2005 by Marcel Dekker. All Rights Reserved. during processing. By selecting the type of isocyanate and polyols, or combination of isocyanates and combination of polyols, one can tailor the structure to obtain desired properties. For this, however, it is necessary to know the relationship between the structure and properties. The flexibility to tailor the structure during processing is one of the main advantages of polyurethanes over other types of polymers. Urethane groups form strong hydrogen bonds among themselves and with different substrates. Strong intermolecular bonds make them useful for diverse applications in adhesives and coatings, but also in elastomers and foams. One of the great advantages of polyurethanes arises from the high reactivity of isocyanates, which can react with a number of substances having different functional groups. This allows polymerization at relatively low temperatures and in short times (several minutes). One group of polymers, which is conditionally treated as urethanes, is polyurea, because urea is often formed during urethane production. Urea is formed in the reaction between isocyanates and amines. The urea group is similar to the urethane group, except that it has two –NH– groups, and can form more hydrogen bonds than the urethane group: urea group ð2Þ II. ISOCYANATE CHEMISTRY [1–8] A. Basic Reactions of Isocyanates The exceptionally high reactivity of the isocyanate group originates from its electronic structure, which can be represented by the following resonance structures: ð3Þ Table 1 US polyurethane 1996 market. a Consumption, Million of lb Flexible foam slab 1593 Rigid foam 1268 Molded flexible foam 451 Coatings 309 Binders and fillers 271 Adhesives 183 Cast elastomers 158 Molded thermoplastics 114 Automotive RIM 113 Sealants 70 Spandex fibers 45 Nonautomotive RIM 33 Total 4609 a Chem. Eng. News, August 4, 22 (1997). Copyright 2005 by Marcel Dekker. All Rights Reserved. It follows that the highest electron density is an oxygen (electronegative) and the least on the carbon (electropositive), while nitrogen is somewhat less electronegative than oxygen. Thus, NCO easily reacts with proton donors: ð4Þ Isocyanates are susceptible, however, to nucleophilic as well as electrophilic attacks. Typical nucleophilic reactions of isocyanates are urethane (carbamate) formation with alcohols: ð5Þ and formation of urea (carbamide) with amines: ð6Þ The reaction of isocyanate with alcohols is strongly exothermic (170–190 kJ/mol). One of the basic reactions in the urethane foam technology is the reaction of isocyanate with water with evolution of carbon dioxide and amine formation: ð7Þ Since the urethane group itself contains active hydrogen, it could react with isocyanate to produce allophanate: ð8Þ This reaction proceeds to a significant degree at about 120–140  C but it could occur also at lower temperatures at high excess of isocyanates. Similar is the reaction of biuret formation from isocyanate and urea groups: ð9Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. Biuret formation reaction proceeds to a considerable measure above 100  C. Both reactions (8) and (9) are utilized to introduce crosslinks with the excess of isocyanate. The previously given reactions are the most frequent ones in the polyurethanes chemistry. There are other important reactions such as the reaction of isocyanate with itself , which may occur during storage or are intentionally carried out to obtain new products. Isocyanates (particularly the reactive aromatic ones) easily form dimers (uretdiones): ð10Þ Dimers are formed in presence of mild based such as pyridine or isocyanates themselves. Dimerization can be prevented by adding acids or acid chlorides (e.g., benzoyl chloride). Dimers are thermally unstable, and upon heating they dissociate into starting components. Thus, they are sometimes used to form so called blocked isocyanates, which are quite stable at room temperature but react at elevated temperatures. Strong bases, however, favor the trimerization of isocyanate to form isocyanurate: ð11Þ Triisocyanurates possess exceptional thermal stability. The reaction (11) is used in industry to prepare thermally stable foams. Polymerization of isocyanate to polyisocyanates (polyamide 2) proceeds in presence of anionic polymerization catalysts, such as NaCN, triethylphosphine, butyllithium and strong bases, according to the following scheme: ð12Þ Polyisocyanates have no commercial application, and the conditions for their forma- tion should be avoided when planning other urethane chemical reactions. An important chemical reaction of isocyanates, which proceeds at high temperature without catalysts, is carbodiimide formation. CO 2 is generated in the process: RNCO þ OCNR! RN¼C¼NR þ CO 2 ðcarbodiimideÞ ð13Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. This reaction proceeds also at room temperature in the presence of special catalysts (e.g., 1-ethyl-3 methyl-3-phospholin-1-oxide). Carbodiimides are used as stabilizers against hydrolysis of polyester urethanes, since they react with acids produced by hydrolysis and thus slow down the process. Acids are catalysts for hydrolysis of polyesters. The carbodiimide reaction is used to modify isocyanates (e.g., Isonate 143 L from Dow Chemical is carbodiimide modified MDI). A number of self-reactions of isocyanates create a problem during storage. Acid inhibitors do not really slow down the isocyanate reactions but primarily react with bases, which are accelerators of these processes. B. Other Isocyanate Reactions Isocyanates react with organic acids forming unstable intermediaries, which decompose into an amide and carbon dioxide: RNCO þ R 0 COOH! RNHCOR 0 þ CO 2 ð14Þ Isocyanate reacts with HCl to form an adduct which decomposes at higher temperatures to starting components: RNCO þ HCl * ) R  NH  CO  Cl ð15Þ To avo id high sensitivity of isocyanates towards moisture and to increase their stability, blocked isocyanates are often used. They are obtained in reactions with some blocking agents, and decompose to isocyanates under certain conditions, most frequently at elevated temperatures. Isocyanates can react with activated methylene groups in the presence of sodium or sodium alcoholate to produce a blocked isocyanate, as in the case of a diester of malonic acid: ð16Þ A frequently used blocking agent is phenol: ð17Þ which produces an adduct that decomposes to the starting components at 160–180  Corat lower temperatures in the presence of catalysts. Isocyanates react with oximes to produce blocked (masked) isocyanates, which decompose at elevated tempe ratures to starting components: ð18Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. Isocyanates react with aromatic and aliphatic anhydrides to give imides: ð19Þ This reaction can be used to prepare polyimides (from dianhydrides and diisocyanates). Aldehydes and ketones may react with isocyanates to pr oduce unstable cyclic compounds, which decompose according to the scheme: ð20Þ Isocyanates may undergo addition to olefins (enamines, ketenketales) in the following way: ð21Þ Isocyanates also react with epoxides to produce cyclic compounds – oxazolidones: ð22Þ III. BASIC COMPONENTS IN URETHANE TECHNOLOGY A. Isocyanates Polyurethanes are formed in the reaction of isocyanates with polyols . The most important commercial aromatic isocya nates are toluenediisocyana te (TDI), diphenylmethane diisocyanate (MDI) and naphthalene diisocyanate (NDI), while the important aliphatic isocyanate is hexamethylene diisocyanate (HDI). Cycloalipha tic isocya nates of industrial importance are isophorone diisocyanate (IPDI) and hydrogenated MDI (HMDI). A number of triisocyanates, such as triphenylmethane triisocyanate, are used in coatings and adhesives. Chemistry and technology of a wide range of isocyanates is given in several books [9,10]. Toluene diisocyanate is usually supplied as the mixture of two isomers: 2,4-TD I and Copyright 2005 by Marcel Dekker. All Rights Reserved. 2,6-TDI with a ratio 80:20 (called TDI 80) or 65:35 (TDI 65). ð23Þ TDI is a liquid at room temperature, having density 1.22 g/cm 3 , boiling point 120  C at 1333.22 Pa (1 atm) and melting point 13.6  C (TDI 80) or 5  C (TDI 65). It is used primarily for flexible foams and different adducts-intermediaries for coatings. Pure MDI is a solid at room temperature, having melting point 39.5  C and density 1.18 g/cm 3 at 40  C. ð24Þ In the manufacture of distilled (pure) MDI, a residue is obtained, which contains a mixture of isomers, trimers and isocyanates with a higher degree of polymerization. Such a mixture is a dark brown liquid at room temperature and is called crude MDI or polymeric MDI (PAPI). The dominating species is a triisocyanate with the approximate structure: ð25Þ Pure MDI is used mainly for preparation of thermoplastic elastomers, while crude MDI is used for rigid and partly for flexible foams. Paraphenylene diisocyanate is another important isocyanate. It produces excellent elastomers but its use is limited due to a very high price. ð26Þ Aromatic diisocyanates are not suitable for products that are exposed to irradiation and external influences (such as coatings) because of yellowing. Those applications require aliphatic or cycloaliphatic isocyanates. One popular cycloaliphatic isocyanate is isophorone diisocyanate, a liquid at room temperature (melting point Copyright 2005 by Marcel Dekker. All Rights Reserved. is 60  C) having density 1.06 g/cm 3 , molecular weight 222 and boiling point 158  Cat 1333.22 Pa: ð27Þ The react ivity of an isocyanate group depends on the radical to which it is attached, as well as the position in the molecule. In principle aromatic isocyanates are more reactive than the aliphatic ones. The reactivity of an isocyanate group in symmetric diisocyanates decreases after the first group has reacted, which should be taken into account [4]. Reactivity also depends on temperature, and sometimes the difference in reactivity of two isocyanate groups may diminish with increa sing temperature. This effect is stronger in the cases with higher activation energies. Table 2 displays rate constants and activation energies for several diisocyanates in the reaction with hydroxyl groups from poly- ethyleneadipate diol. The constants and their relative ratios are different in reactions with alcohols, amines or water. The comparison of the reactivity of two groups in various diisocyanates is shown in Table 3. Rate con stants k 1 and k 2 show the relative rates for the first and second group (compared with a standard rate). The constant k 2 is obtained after the first group is reacted, and it should be half of k 1 if the reactivity is the same. Table 2 Rate constants, k, and activation energy, E, in the reaction of isocyanates with polyethyleneglycol adipate diol at 100  C. Diisocyanate k  10 4 , L mol s E, kJ/mol p-Phenylene 36.0 46 2,4-TDI 21.0 33.1 2,6-TDI 7.4 41.9 1,5-NDI 4.0 50.2 1,6-HDI 8.3 46.0 Table 3 Relative rate constants of isocyanate groups with a hydroxyl group. Isocyanate k 1 k 2 MDI 16 8.6 2,4-TDI 42.5 2 2,6-TDI 5 2 HDI 0.2 – Copyright 2005 by Marcel Dekker. All Rights Reserved. It follows from Table 3 that the first group in 2,4 TDI is much more reactive than the second one. The difference however, decreases with increasing temperature or in the presence of catalysts. Reactions of isocyanates can be accelerated either by increasing temperature or adding catalyst. Slowing down the reaction cannot be done by additives if the concentration of isocyanate and polyol is kept constant. Lowering the temperature or diluting the mixture polyol–isocyanate by adding a solvent or neutral diluents would, however, slow down the reaction. Act ivation energies of the reactions of isocyanates with polyols, as a rule, do not exceed 20–40 kJ/mol. The reaction rates increase with increasing polarity of the medium (e.g., solvent). The reactivity of different groups, proton donors, with isocyanates decreases in the order: aliphatic NH 2 > aromatic NH 2 > primary OH > water > secondary OH > tertiary OH > COOH. Urea group in R-NH-CO-NH-R is more reactive than amide group, R 0 CONHR, and amide is more reactive than the urethane group, R-NHCOO-R 0 . This sequence can be changed if the groups with different steric hindrances are attached. B. Polyols Second to isocyanate in the technology of polyurethane preparation is polyol. Polyether polyols (polypropylene glycols and triols) having molecular weights between 400 and 10,000 dominate in the foam technology. Foams are usually made with triols, which form crosslinked products with diisocyanates, whereas diols dominate in the elastomer technology. Polyether polyols have higher hydrolytic stability than the polyester polyols, but they are more sensitive to different kinds of irradiation and oxidation at elevated temperatures. Polypropylene oxide (PPO) polyols, also called polypropylene glycols (PPG), are cheaper than other polyols. PPG structure can be represented by the formula: ð28Þ Group R comes from the starter diol such as ethylene glycol (R ¼ –CH 2 –CH 2 ). If multifunctional starters, such as glycerin, trimethylol propane or sugars are used, the resulting polypropyleneoxide polyol would have the functionality of the starter component. Due to the weak intermolecular attractive forces (low polarity) and non-crystallizing nature, PPG polyols are liquid at room temperature even at very high molecular weight, unlike polyester polyols, which are often crystalline greases. Weaker interactions on the other hand cause lower strengths of the PPG based urethanes. Viscosity of polyether polyols is a function of the hydroxyl content (due to hydrogen bonding) and molecular weight. PPO diols have viscosities from 110 mPa s (cP) at 20  C for the molecular weight of 425 to 1720 mPa s for MW ¼ 4000. Glycerin for example has viscosity above 1000 mPa s at 20  C but when propoxylated to MW ¼ 1000 gives a triol with viscosity of about 400 mPa s. Polyether polyols based on polytetramethylene oxide (PTMO), sometimes called polytetrahydrofurane (PTHF), have better strengths than PPG polyols, mainly due Copyright 2005 by Marcel Dekker. All Rights Reserved. to their ability to crystallize under stress. Their structure is represented by structural formula (29): HO½ CH 2 CH 2 CH 2 CH 2 O n H ð29Þ Polyester polyols are an important class of urethane raw materials, with applications in elastomers, adhesives, etc. They are usually made from adipic acid and ethylene glycols (polyethylene adipate): ð30Þ or butane diol and adipic acid (polybutylene adipate). Both would crystallize above room temperature. In order to reduce their glass transition and de stroy crystallinity, copolyesters are prepared from the mixture of ethylene glycol and butane diol with ad ipic acid. Polycaprolactone diol is another crystallizable polyester diol: ð31Þ Polyols for coatings, rigid foams, and adhesives may co ntain aromatic rings in the structure in order to increase rigidity. These polyols may also crystallize, which is important in some applications, e.g. , adhesives. Special class of polyols are ‘polymer polyols’ containing usually copolymers of acrylonitrile and styrene or methylmetacrylate attached to the chains of polyether polyols, forming a dispersion. They are used for high modulus products such as froth and integral skin foams, RIM, shoe soles and one-shot elastomers. An important but less frequently used group of polyols, polybutadiene diols, are mainly used for elastomers: HO½ CH 2 CH¼CHCH 2  n OH ð32Þ Structural formula (32) shows poly-1,4-butadiene (BD), but 1,2-poly BD and the mixture of the two are also produced. Castor oil is a natural triol with a typical OH number 160 mg KOH/g (functionality ¼ 2.7). Although it has three ester groups, it is not considered a polyester type polyol. ð33Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. [...]... Dekker All Rights Reserved Table 5 IR absorption bands of polyether urethanes Wavelength, mm Frequency, cmÀ1 Relative intensity 3.06 3.40 3.50 3. 58 5.79 6.12 6. 28 6.35 6.53 6.61 6.71 6.91 7. 08 7.30 7.63 8. 12 32 68 2941 285 7 2793 1727 1634 1592 1575 1531 1513 1490 1447 1412 1370 1311 1232 m vs vs m m m m w s sh m m m s m sh 8. 22 8. 28 8.99 12.94 1216 12 08 1112 773 s sh vs Phase Urea, urethane PTHF Benzene... ð 48 Here, Mpol, MISO and MCE are molecular weights of the polyol, isocyanate and chain extender, respectively Setting the number of moles of the polyol, npol, to be 1, r becomes the number of moles of the chain extender Number of moles of the diisocyanate, nISO, at the stoichiometric ratio of NCO/OH groups is the sum of the moles of the polyol and the chain extender, i.e., nISO ¼ r þ 1 Thus, a prepolymer... where B is the number of ml of HCl used for titration of the blank, S is the number of ml of HCl used for titration of the sample, N is the molarity of HCl solution, and W is the weight of the sample in grams Other characteristics of isocyanates that are analyzed are total chlorine content, the content of hydrolyzable chlorine, acid content, freezing point, density and color B Analysis of Polyols The principal... number of milligrams of KOH (MKOH ¼ 56.11) used for titration of one gram of the sample OH number ðmg KOH=gÞ ¼ 56:1ðB À AÞN=W ð 38 where A is the number of mL NaOH, B is number of mL NaOH used for titration of blank, N is molarity of the NaOH solution, and W is the weight of the sample in grams Hydroxyl content in percent can be calculated from the proportion which takes into account that OH number of. .. Degree of phase separation (or phase mixing) affects the properties of the polymers, and it depends on the structure of the soft and hard segments and temperature Usually the hard phase is crystalline For example, the melting point of the hard segment consisting of MDI and butane diol is between 180 and 220  C, and it depends on the molecular weight of the hard segment The glass transition temperature of. .. and strength Optimal excess of NCO is about 2–5% Polyol (soft segment) molecular weight affects the modulus, E, of an elastomer The theory of rubber elasticity predicts that Young’s modulus of an elastomer is inversely proportional to the molecular weight of network chains, Mc: E¼ 3RT Mc ð51Þ This means that longer polyols produce softer polyurethane elastomers The Tg of the soft phase is also related... molecular weights of the hard and the soft segment must be equal When stress is applied, soft segments uncoil to give large deformation, while hard domains preventing slippage of the chain past each other, restrain plastic deformation Properties of a polyurethane elastomer depend on the selection of a diisocyanate, chain extender and polyol but also on the length and concentration of the soft and hard segments... and subsequent chain extension (reaction (47)) A prepolymer is prepared by reacting excess of isocyanate with a polyol (diol), typically of the molecular weight 2000 ð46Þ Figure 3 Schematic representation of the structure of the segmented polyurethane chain (a), association of hard segments into domains of globular morphology (b) and co-continuous soft and hard phase morphology (c) Copyright 2005 by... for a given SSC should be prepared from one mole of the polyol and (r þ 1) moles of diisocyanate and extended with r moles of the chain extender Number average molecular weight of the soft segment is determined by the selection of the polyol molecular weight The hard segment molecular weight, Mnhs, is determined by the soft segment molecular weight and soft segment concentration, according to the expression:... temperature of the linear long polymer, and K is a constant for the given system Glass transition temperature of the soft phase of an elastomer based on polytetramethyleneoxide is À 43  C when molecular weight of the polyol is 650, Tg ¼ À 60  C for the polyol with Mc ¼ 1000, or À 86  C for the Mc ¼ 2000 Thus, for semi-rigid elastomers and foams, polyol molecular weight should be below 1000 Modulus of polyurethane . is the number of ml of HCl used for titration of the blank, S is the number of ml of HCl used for titration of the sample, N is the molarity of HCl solution, and W is the weight of the sample. the number of milligrams of KOH (M KOH ¼ 56.11) used for titration of one gram of the sample. OH number ðmg KOH=gÞ¼56:1ðB  AÞN=W ð 38 where A is the number of mL NaOH, B is number of mL NaOH. [g/equiv]. Equivalent weight of hexamethylenediamine is 116/2 ¼ 58. From the above, 125 g of MDI should react with 500 g of the diol with M n ¼ 1000, or 9 g of water, or 58 g of hexamethylenediamine,