FLUORINATED POLYURETHANES CONTAINING POLYOXYPERFLUOROALKYLENE BLOCKS

Abstract: Fluorinated polyurethanes, having improved mechanical characteristics and a glass transition temperature lower than -80°C, containing polyoxyperfluoroalkylene blocks deriving from perfluoropolyethers having hydroxylated end groups and/or from perfluoropolyethers having isocyanate end groups and furthermore units deriving from an aromatic or a cycloaliphatic diol.

Description: The object of the present invention is to provide fluorinated polyurethanes having a glass transition temperature lower than -80°C and endowed with improved mechanical characteristics with respect to the ones of the fluorinated polyurethanes of the prior art. BACKGROUND OF THE INVENTION There are known polyurethanes characterized by the presence, in their structure, of polyoxyperfluoroalkylene blocks deriving from the use of perfluoropolyethers with hydroxylated end groups. Products of such type are described in particular in Italian patent No. 903446: they are characterized by a glass transition temperature (Tg) lower than -78°C and therefore they retain flexibility and elastic properties even at very low temperatures. The fluorinated polyurethanes known so far, however, do not possess in the whole optimum mechanical characteristics as regards hardness, tensile strength, elastic modulus, elongation. This state of the art is represented also by US-A- 3,972,856, relating to fluorine-containing elastomeric polyurethane prepared by reaction of hydroxyterminated poly(perfluoroalkylene oxide)prepolymers with polyisocyanate. It is known from US-A- 3,852,222 to prepare fluorinated polyurethanes by cross-linking of polyisocyanates prepolymers obtained by reactions of two fluorinated diols with epichlorohydrin. Fluorinated polyurethanes, endowed with a low glass temperature and a high resistance to solvents can be prepared according to US-A- 4,085,137 by reaction of hydroxy-terminated poly(perfluoroalkylene oxides) with polyisocyanates. Fluorinated polymers, suited for the use in the manufacture of highly hydrophobic paints, are obtained according to US-A-3,872,045 by reaction of one or more fluorinated diols with epichlorohydrin. THE PRESENT INVENTION The fluorinated polyurethanes according to the present invention are characterized by a block structure, comprising: A) Sequences of fluorooxyalkylene units selected from the following classes: I (C₂F₄O), (CF₂O) said units being randomly placed along the fluoropolyoxyalkylene chain; II (C₃F₆O), (C₂F₄O), (CFXO) wherein X = F or CF₃, said units being randomly placed along the fluoropolyoxyalkylene chain; III (CH₂CF₂CF₂O) said units in the fluoropolyoxyalkylene chain being linked between them as follows: -(OCF₂CF₂CH₂) p - O-R ' / f-O-(CH₂CF₂CF₂O) q - wherein R ' / f is a fluoroalkylene group and p and q are integer, p+q being higher than 2; IV said units being linked between them in the fluoropolyoxyalkylene chain as follows: wherein R f is a fluoroalkylene group, x is 0 or 1, a and b are integer and a+b higher than 2. The single blocks of fluorooxyalkylene units have an average molecular weight between 500 and 7000. The fluorinated polyurethanes according to the present invention are furthermore characterized by the presence of: B) units deriving from fluorinated or non fluorinated diols or triols, containing one or more aromatic or cycloaliphatic or polycyclic rings. Such diols and triols are selected from: From 5 to 100 type B units are present for 100 polyoxyfluoroalkylene blocks of type A. The polyurethanes according to the invention may advantageously include also perfluoropolyethereal-chain blocks, deriving from the use of starting diols, having an average molecular weight from 500 to 7000, and included in the following general formula: HO-(CH₂CH₂O) z -CH₂-CF₂O-(C₂F₄O) m -(CF₂O) n - -CF₂-CH₂(OCH₂CH₂) z OH (1) wherein z ranges from 0 to 3 and m/n ratio ranges from 0.2 to 2, preferably from 0.5 to 1.2. It is possible also to use, as perfluorinated diols, the compounds having a pefluoropolyethereal chain of the type indicated hereinbefore and end groups of the type: -CONHR'(CH₂) q OH where R' = H or alkyl, q = 1-6. The perfluoropolyethereal diols are obtainable according to known methods, for example as is described in Italian patent No. 903446. The ones having z = 0 are obtained by reacting the hydroxymethyl end groups with 1-3 moles of ethylene oxide. The fluorinated polyurethanes according to the present invention can be of linear structure when the starting hydroxylated or isocyanate components are merely bifunctional and of cross-linked structure when tri- or tetra-functional components are also used. For example it is possible to use tetrahydroxy or tri-hydroxy polyoxyperfluoroalkylenes having an average molecular weight from 500 to 7000 and having a perfluoropolyethereal chain of the type mentioned above and di-hydroxylated end groups at one or both ends, of the type: -CH₂O-CH₂CH(OH)-CH₂OH, -CO-N(CH₂CH₂OH)₂, the other end group being optionally a mono-hydroxylated group of the type as indicated under c). They are obtainable according to known methods for example the ones cited in Italian patent No. 903446. In preparing the fluorinated polyurethanes of the present invention the following polyisocyanate compounds can be used: a) the diisocyanate having sequences of fluorooxyalkylene units of class I comprised in the general formula: OCN-R-(Z) d -CF₂O(C₂F₄O) m -(CF₂O) n -CF₂-(Z) d -R-NCO wherein: d = 0 or 1, m, n = integer number such as the average molecular weight is comprised in the above mentioned range, Z is a bivalent linking radical and can be of the type: -CONH-, -CH₂-, CH₂O-, -CH₂OCH₂, CH₂OSO₂-, R is a bivalent aliphatic radical C₁-C₁₂, or a cycloaliphatic or aromatic bivalent radical, in particular can be: and wherein (CF₂O) and (C₂F₄O) units are randomly distributed along the chain. The above mentioned fluoropolyoxyalkylene isocyanates can be obtained according to prior art processes, for instance those disclosed by USP 3,810,874; b) fluorodiisocyanates having sequences of fluorooxyalkylene units of class II are in particular chosen from amongst the compounds of the general formula: OCN-R-(Z) d -(C₃F₆O) r (C₂F₄O) s (CFXO) t -(Z) d -R-NCO wherein X = -F or -CF₃; d, Z, R have the above defined meaning and indexes r, s and t are integer numbers such as the average molecular weight of the chain is in the above defined range, and (C₃F₆O), (C₂F₄O), (CFXO) units are randomly distributed along the chain. These compounds can be obtained through photooxidation of mixtures of C₃F₆ and C₂F₄ as described in USP 3,665,041 and subsequent conversion of the terminal -COF groups into groups containing the radical -NCO, carried out according to prior art processes described by USP 3,847,978 and 3,810,874. c) fluroodiisocyanates having sequences of fluorooxyalkylene units of class III are in particular chosen from amongst the compounds of the formula: OCN-CF₂CH₂(OCF₂CF₂CH₂) p -OR' f O-(CH₂CF₂CF₂O) q CH₂CF₂-NCO wherein p, q and R ' / f have the above defined meaning; these compounds can be obtained from the corresponding compounds having end groups -COF described by European Patent 148,482, through the conversion of the end groups -COF into -NCO groups as described by USP 3,847,978 and 3,810,874; d) fluorodiisocyanates having sequences of fluorooxyalkylene units of class IV are chosen in particular from amongst the compounds of the following general formula: wherein A is a group containing the -NCO function, for instance -CH₂NCO; a, b and R f have the above meaning and u is an integer equal or higher than 1. These compounds are described by European Patent 151,877; e) non fluorinated diisocyanates, for instance those comprised in the general formula: OCN - R - NCO wherein R is the organic bivalent radical described in preceding item a) As starting diols, besides the ones non containing fluorooxyalkylene units and comprising aromatic or cycloaliphatic or polycyclic rings as above defined, the following compounds can be used: a) diols having sequences of fluorooxyalkylene units of class I and comprised in general formula (1); b) diols having sequences of fluorooxyalkylene units of class II and comprised in general formula: HO-CH₂CF₂-(C₃F₆O) r (C₂F₄O) s (CFXO) t -CF₂-CH₂-OH wherein X is F or CF₃, indexes r, s, t are integers such as the molecular weight is comprised in the above defined range. These compounds can be obtained through photooxidation of mixtures C₃F₆ + C₂F₄ as described in USP 3,665,041 and subsequent conversion of end groups -COF into end groups containing -OH according to the methods described by USP 3,847,978, 3,810,874; c) diols having sequences of fluorooxyalkylene units of class III are in particular those obtained from the compounds of general formula: FOC-CF₂CH₂(OCF₂CF₂CH₂) p -OR' f O-(CH₂CF₂CF₂O) q CH₂CF₂-COF wherein p, q, R ' / f have the above defined meaning, and described in European Patent 148,482, the conversion of -COF groups into -CH₂OH groups heing carried out according to the methods described by USP 3,847,978 and 3,810,874. d) diols having sequences of fluorooxyalkylene units of class IV are in particular chosen from amongst the compounds of general formula: where in A is a group containing -OH, for instance -CH₂OH, and a, b, n, x, R f have the above said meanings. These compounds are described in European Patent 151,877. The polyurethanes of the present invention are prepared starting from isocyanate and hydroxylated components in such amounts as to be included in the ranges indicated hereinafter. For 100 equivalents of isocyanate group are used: from 5 to 100 equivalents of -OH of type B), the balance being made up from equivalents of -OH of polyoxyfluoroalkylene diols defined in items a, b, c and d; and/or from equivalents of -OH of trihydroxy or tetrahydroxy components. Furthermore, for 100 equivalents of isocyanate are used from 50 to 100 equivalents of total hydroxyls and preferably from 70 to 100. The use of polyols of type B) in the fluorinated polyurethanes synthesis according to the invention permits to obtain polymeric materials with a hardness, which is high enough for most of appliances, and having improved mechanical properties as compared with the perfluoropolyurethanes of the art: reference should be made to the above-cited Italian patent No. 903,446 in particular as regards tensile strength, elongation and modulus of elasticity, the hardness being equal. The introduction of rigid, sterically hindering monomeric units, deriving from component B), into the perfluoropolyethereal chain did not lead, surprisingly, to an increase in the Tg typical of the perfluoropolyethereal chains. In fact, the products according to the invention exhibit, analogously with the perfluoropolyurethanes of the art, a very low Tg and by consequence a good flexibility even at extremely low temperatures. The proper cross-linking degree in the final polymer is obtained through the use of tri- or tetrafunctional polyols, or also by the use of an excess of isocyanate and subsequent formation of isocyanurate cyclic groups. The polyurethanes of this invention are preparable by a one-step process, by mixing together the various components and allowing them to react, for example in a mold at temperatures from 20°C to 100°C, or, more suitably, by a two-step process, mixing at first the diisocyanate component with the diol of type B and optionally also with polyoxyfluoroalkylene diols and reacting them at 20° to 100°C in order to lengthen the macromolecular chain till a certain molecular value is obtained. In this way it is possible to obtain a sufficiently stable prepolymer, which can be successively converted into the final polymeric product by addition of polyfunctional trihydroxy or tetrahydroxy component which acts as a cross-linking agent, by employing the modalities and apparatuses which are most suitable for the type of application required. The addition of an efficient catalytic system permits to operate at low temperature (20-60°C) and in short times. Furthermore, a proper dosing of the catalyst enables to optimize the pot life, i.e. the time during which the reaction mixture remains sufficiently fluid. As catalysts it is possible to utilize derivatives of tin, such as dibutyltin dilaurate, dibutyltin acetate, dibutyltin oxide, iron derivatives such as ferroacetylacetonate, titanium alcoholates such as titanium tetraisopropylate, tertiary amines such as triethylamine, in amounts ranging from 0.001 to 2 % by weight referred to the total weight, preferably from 0.01 to 0.5 %. According to another process, a difunctional hydroxylated prepolymer is reacted with an excess of isocyanate in order to obtain a prepolymer with isocyanate end groups, the latter being then subjected to an isocyanic group, trimerization step, in the presence of trimerization catalysts of the tertiary amine type (triethylamine or N-methyl-morpholine) or of the alkaline metal alkoxide type, or the like. The polyurethanes obtained according to the present invention are elastomers free from residual tackiness and, in the case of a sufficient cross-linking, are insoluble in the common organic solvents and in particular fluorinated solvents, such as 1,1,2-trichloro-trifluoroethane or in mixtures thereof. They exhibit an excellent flexibility at very low temperatures as their glass transition temperature is below -80°C and in most cases below -100°C. Furthermore, they are resistant to the commonest chemical agents and are stable towards hydrolytic degradation. For example, the mechanical properties of the materials prepared according to this invention remain unaltered after a 4-day treatment with steam or after a 14-day immersion in water containing 3 % of NaCl. Appliances The fluorinated polyurethanes of the present invention can be usefully employed for obtaining protective coatings, characterized by low wetabiiity and high resistance to atmospherical agents. Another important utilization is chat as high-density binder for rocket propellants. In such case, the starting products for the preparation of polyurethane or the polymerization intermediate (prepolymer) are mixed with the oxididant (for example perchlorate) and with the solid fuel (for example aluminium powder), and the resulting mixture is poured into the proper container and is caused to cure by polymerization. The polymeric products according to the invention offer the advantage of making possible to reach a sufficient cross-linking degree and, thus, a sufficient degree of hardness and of mechanical properties even by making the curing to occur at a relatively low temperature (not higher than 100°C). A further utilization of the polymeric products of the invention is in the field of the adhesives. The extremely low Tg value and the consequent good flexibility at low temperatures is an important characteristic for the utilization in apparatuses subjected to very low temperatures. The following examples are given to illustrate the present invention. EXAMPLE A In a three-neck flask there were mixed, under a nitrogen atmosphere, 80 g (26.7 m.moles) of α,ω-bis(tolylisocyanate)-polyoxyperfluoroalkylene having a molecular weight equal to 2000 (formula II), 0.45 g (1.35 m.moles) of 2.2-hexafluoropropylidene-bisphenol and 0.25 g of a 0.25 M solution of dibutyltin dilaurate in acetone. The reaction mixture was stirred at 60°C during 2 hours. Subsequently, 14.3 g (6.675 m.moles) of a tetrahydroxypolyoxyperfluoroalkylene having a molecular weight = 2148 (formula III) were added and, after mixing for a few minutes, the mixture was stripped and poured into a mold, were it was kept at 50°C during 48 hours. Obtained was an elastomeric transparent, non-tacky polyurethane, insoluble in 1,1,2-trichlorotrifluoroethane (Freon 113), characterized by a glass transition temperature equal to -124°C (determined by differential thermal analysis) and by the following mechanical properties: formula I HOC₂-CF₂O(C₂F₄O) m -(CF₂O) n CF₂CH₂OH formula II HOCH₂-(OH)CH-CH₂O-CH₂CF₂O-(C₂F₄O) m -(CF₂O) n -CF₂-CH₂- OCH₂CH(OH)-CH₂OH formula III In the above formulas the m/n ratio is of about 0.7. The average molecular weights are the ones indicated hereinabove. EXAMPLES B - D Examples B - D show the influence exerted by the equivalents of tetrafunctional polyoxyperfluoroalkylene on the polymers' final characteristics. According to the modalities of example A, a series of elastomeric polyurethanes was prepared, maintaining unaltered the ratio: equivalents of isocyanate/equivalents of total OH = 1 and varying the tetrahydroxypolyoxyperfluoroalkylene content. The characteristics of polymers obtained are reported in Table 1. EXAMPLE E 80 g (26.7 m.moles) of α,ω-bis(tolylisocyanate)-polyoxyperfluoroalkylene having a molecular weight = 3000 and 0.25 g of a 0.25 M solution of dibutyltin acetate in acetone were additioned, in a nitrogen atmosphere, with a solution of 4x5 g (13.35 m.moles) of 2,2-hexafluoroisopropylidenebisphenol (Bisphenol AF) in a 80/20 by volume mixture of 1,1,2-trichlorotrifluoroethane (Freon 113®)/acetone. It was allowed to react at 60°C during 2 hours, while slowly removing the solvents, thus obtaining a homogeneous mixture. To the resulting prepolymer there were added 14.3 g (6.675 m.moles) of tetrahydroxypolyoxyperfluoroalkylene having a molecular weight = 2148 and, after having rendered the mixture homogeneous, the gases were stripped off from the mixture, which was poured into a mold and left at 50°C during 48 hours. Obtained was a transparent, consistent and nontacky elastomeric polyurethane, insoluble in Freon 113®, characterized by a glass transition temperature = -123°C. EXAMPLE F Example E was repeated, except that the Freon 113®/acetone solution contained equimolar amounts of bisphenol AF and α,ω-bis(hydroxymethyl)-polyoxyperfluoroalkylene having a molecular weight = 2000, for a total amount of 13.35 m.moles. Table 2 shows the effect of the starting molar ratio diol 2000/bisphenol AF. EXAMPLE G Example A was repeated by using an α,ω-bis(hydroxymethyl)-polyoxyperfluoroalkylene with a molecular weight = 400 instead of 2000. This monomer imparts a higher-rigidity to the polymeric structure. Obtained was an elastomeric polyurethane, insoluble in Freon 113®, characterized by an excellent flexibility to very low temperatures and by the following mechanical properties: - hardness (Shore A/3") = 24 - T.S. (tensile strength) = (15.6 kg/cm²) 1,53 N/mm² - E.B. (elongation at break) = 533 % - modulus (100 %) = (3.4 kg/cm²) 0,33 N/mm² EXAMPLE H A solution of 9 g (26.7 m.moles) of bisphenol AF in a 80/20 by volume mixture of Freon 113/acetone was added, in a nitrogen atmosphere, to 80 g (26.7 m.moles) of α,ω-bis(tolylisocyanate)-polyoxyperfluoroalkylene having a molecular weight = 3000, and 0.25 g of a 0.25 M solution of dibutyltin acetate in acetone. It was allowed to react at 50°C during 3 hours, while gradually removing the solvents. A homogeneous and viscous mixture was obtained which, after stripping, was poured into a mold and left at 50°C during 24 hours and at 100°C during 6 hours. A thermoelastomeric, transparent polyurethane was obtained, which was soluble in Freon 113®, exhibited a Tg = -120°C and was characterized by the following mechanical properties: - hardness (Shore A/3") = 73 - yield strength = (21.8 kg/cm²) 2.14 N/mm² - E.B. (elongation at break) = 128 %. EXAMPLE I (comparative test) In a nitrogen atmosphere there were mixed 33.3 g (10.9 m.moles) of α,ω-bis(isoforondiisocyanate)-polyoxyperfluoroalkylene of formula: having a molecular weight = 3048, 10.9 g (5.45 m.moles) of bis-(hydroxymethyl)-polyoxyperfluoroalkylene having a molecular weight = 2000 and 0.8 g of a 0.20 M solution of dibutyltin dilaurate in acetone. The whole was reacted at 50°C during 3 hours; during this time, a progressive increase in the viscosity was observed. At last, 5.9 g (2.75 m.moles) of tetrahydroxy-derivative (molecular weight = 2148) were added and, after mixing and stripping off of the gases, the whole was poured into a mold. After 48 hours at 50°C and 24 hours at 100°C the mass remained tacky and non-consistent. EXAMPLE L Example I was repeated, adding to the component mixture, as a substitution for the only dihydroxy-derivate, a mixture of 9.8 g (4.9 m.moles) of the same dihydroxy-derivative and 0.19 g (0.55 m.moles) of bisphenol AF. In such case the resulting polyurethane was consistent and non-tacky, exhibited a Tg = -122°C and the following mechanical properties: - hardness (Shore A/3") = 12 - T.S. (tensile strength) = (8 kg/cm²) 0.79 N/mm² - E.B. (elongation at break) = 716 %. From this example, by comparison with the preceding one, it is apparent the efficaciousness of the addition of di- or polyhydroxy-derivatives containing hindering groups, which, even if they are additioned in small amounts, are sufficient to impart rigidity and consistency to the polymeric structure. EXAMPLE M According to the modalities described in example F, elastomeric fluorinated polyurethanes were prepared by synthesis, using, instead of bisphenol AF, 1,4-cyclohexane dimethanol (cis + trans). The resulting polymer, which was insoluble in Freon 113, in organic solvents or in mixtures, was characterized by flexibility to low temperatures (Tg = -119°C) and by an excellent resistance to hydrolysis. The mechanical properties were as follows: - hardness (Shore A/3") = 27 - T.S. (tensile strength) = (22 kg/cm²) 2,17 N/mm² - E.B. (elongation at break) = 600 % - starting modulus = (16 kg/cm²) 1,57 N/mm² EXAMPLE N Into a 3-neck flask containing 50 g (25 m.moles) of α,ω-bis-(tolylisocyanate)-polyoxyperfluoroalkylene having an average molecular weight = 2000, and 2.25 g of a solution at 2.5 % by weight of N-methylmorpholine in acetone, there was introduced, in a nitrogen atmosphere, a solution of 6.5g (19.2 m.moles) of bisphenol AF in a 80/20 mixture of Freon®/acetone. The ratio between isocyanate equivalents and hydroxyl resulted to be equal to 1.3. After having stirred the mixture during 1 hour at 40°C, it was poured into a mold and the cross-linking reaction was started, leaving the mixture during 24 hours at 70°C and during further 24 hours at 130°C. The resulting polyurethane was a non-tacky elastomeric material, insoluble in Freon®, and exhibiting the following characteristics: - Tg = -122°C - hardness (Shore A/3") = 42 - T.S. (tensile strength) = (28.7 kg/cm²) 2,81 N/mm² - E.B. (elongation at break) = 580 % - modulus (100 %) = (5.6 kg/cm²) 0,55 N/mm²

Claims: Fluorinated polyurethanes having a glass transition temperature lower than -80°C and possessing a block structure comprising: A) sequences of average molecular weight 500-7000 of fluorooxyalkylene units selected from the following classes: I - (C₂F₄O), (CF₂O) said units being randomly placed along the fluoropolyoxyalkylene chain; II - (C₃F₆O), (C₂F₄O), (CFXO) wherein X = F or CF₃, said units being randomly placed along the fluoropolyoxyalkylene chain; III - (CH₂CF₂CF₂O) said units in the fluoropolyoxyalkylene chain being linked between them as follows: -(OCF₂CF₂CH₂) p - O-R ' / f-O-(CH₂CF₂CF₂O) q - wherein R ' / f is a fluoroalkylene group and p and q are integers, p+q being higher than 2; IV - said units being linked between them in the fluoropolyoxyalkylene chain as follows: wherein R f is a fluoroalkylene group, x is 0 or 1, a and b are integers and a+b is higher than 2; B) units deriving from fluorinated or non fluorinated diols or triols, containing one or more aromatic or cycloaliphatic or polycyclic rings, such diols and triols being selected from: wherein for each type B unit there are present 1-20 blocks of fluoropolyoxyalkylene of type A obtainable by reacting a) one or more diisocyanates containing sequences of fluorooxyalkylene units of class from I to IV, and having average molecular weight 500-7000, with: b) polyols of the type defined in item B and optionally with c) one or more fluoropolyoxyalkylene diols having sequences of fluorooxyalkylene units of class I to IV, having average molecular weight 500-7000, and/or with, d) trihydroxy- or tetrahydroxypolyoxyperfluoroalkylenes having average molecular weight 500-7000, by using per 100 equivalents of isocyanate a total amount of OH equivalents equal to 50-100. Polyurethanes according to claim 1 obtainable by using per 100 equivalents isocyanate 70-100 equivalents of total hydroxyls. Polyurethanes according to claim 1 obtainable by reacting in a first step the diisocyanate component (a) with a diol (b) or with both a diol (b) and a polyoxyfluoroalkylene diol (c), and by subsequently reacting the thus obtained prepolymer with a polyfunctional tri- or tetrahydroxy component. Polyurethanes according to claim 1 obtainable by reacting in a first step a difunctional hydroxylated prepolymer with an excess of isocyanate and by subsequently subjecting the thus-obtained product to an isocyanic group trimerization in the presence of a trimerization catalyst of the tertiary amine tpye. Polyurethanes according to anyone of claims 1 to 4 obtainable by using additionally a non fluorinated diisocyante. Use of the fluorinated polyurethanes according to anyone of claims 1 to 5 for producing protective coatings characterized by a low wettability and by a high resistance to atmoshperical agents. Use of the fluorinated polyurethanes according to anyone of claims 1 to 5 as binders of propellant mixtures for rockets. Use of the fluorinated polyurethanes according to anyone of claims 1 to 5 for preparing adhesives which retain a sufficient elasticity and flexibility even at very low temperatures.

Inventors: GAVEZOTTI, PIERO, RE, ALBERTO, STREPPAROLA, EZIO
Applicants: AUSIMONT S.P.A.
Application year: 1986
Main classification: C08G18
IPC codes: C09J175, C06B45, C08G18