HIGH-DENSITY COMPOSITE MATERIAL ESSENTIALLY BASED ON POLYISOCYANURATE

Abstract: A high-density composite material is disclosed which is essentially based on polyisocyanurate having a density of >/= 500 kg/m³ and containing reinforcing fillers, in which the polyisocyanurate prepolymer is derived from a repeating structural unit having the formula wherein at least one X represents the group -R₁-NCO and at least one X represents the group -R₁-NH-CO-OR₂-(-OCO-NH-R₁-NCO) n .

Description: Polymeric matrices, which are suitable for producing materials reinforced with glass fibres or with other inorganic or organic fillers and which have very high mechanical (structural) characteristics when they are utilized according to the SMC (Sheet Moulding Compounds) or analogous techniques (flow moulding, bulk moulding compounds, etc.) are known. Generally, polyester and polyepoxy matrices are utilized. So far, no analogous results could be obtained by using polymeric matrices derived from isocyanates (polyurethane matrices, polyureic matrices, etc.), the main reasons being, for example: a) Utmost difficulties in obtaining monocomponent fluid mixtures which are stable in the long run (as is required by the SMC technique), and which, simultaneously, contain isocyanates and compounds capable of reacting with the isocyanates; b) Difficulties in obtaining monocomponents of the above-mentioned type which are capable of reacting and completely polymerizing under the very restricted time and temperature conditions imposed by the application technologies; c) Difficulties in obtaining isocyanate-based mixtures of the desired type which are capable of impregnating high amounts of reinforcing agents (glass fibres or other agents) particularly, when they are longer than 1 cm, and in moulding the resulting mixture by means of known techniques (SMC and the like); d) Difficulties in obtaining, through the methods described above and starting from matrices of the conventional polyurethane or polyurea type, composite materials having the very good mechanical properties required for appliances generally coated with SMC-polyester or SMC-polyepoxide matrices. This is also due to the fact that only by using compact or nearly compact reinforcing agents, it is possible to reach the required levels of rigidity, thermal stability and impact strength. The prior art only describes examples of mean and low density reinforced rigid products (<500 kg/m³), predominantly reinforced with short fibres (≦1 cm) and substantially based on the formation of conventional polyurethane structures. Also known are techniques suitable for the production of foamed resins based on polyisocyanurate (PIR foams) having a very low density but mechanical properties which are far from being of the structural type, their importance residing mainly in their insulating capability along with a high stability to heat and to chemical or environmental agents. US-A-3,817,939 describes a method of making poly(isocyanurateurethane) comprising mixing polyisocyanate with polyol in the presence of an effective amount of a certain carbonate salt polymerization catalyst. The corresponding product is suitable for use in liquid coating compositions. DE-A-20 14 899 describes an isocyanurate polymer which is a product of the curing of a urethane prepolymer having terminal isocyanate groups and cyclic groups in the polymeric chain with an isocyanate trimerization catalyst. Said prepolymer is prepared by reaction of a diol with a molar excess of a diisocyanate, the ratio hydroxy groups/isocyanate groups ranging preferably from 1.3:1 to 2:1. A corresponding fiber-reinforced product is also disclosed. GB-A-1,323,312 describes a process for preparing a cross-linked urethane/isocyanurate polymer which process comprises reacting (1) a polyisocyanate compound comprising an isocyanate-capped prepolymer of a polyalkylene ether polyol or polyester polyol and (2) a polyol component comprising a polyalkylene ether polyol of molecular weight up to 5000 in the presence of a certain isocyanate polymerization catalyst. US-A-3,842,036 discloses a reaction product of (a) an alkylene oxide condensate of a novolak, (b) a stoichiometric excess of an organic polyisocyanate and (c) a catalytically effective quantity of a catalyst for promoting the formation of isocyanurate from isocyanate, said reaction product being subjected to heat and cooled before reaching the thermoset stage. Applicant has now succeeded in preparing high density composite materials reinforced with glass fibres or other agents, starting from specific polymeric matrices derived from isocyanates, in particular, essentially polyisocyanurate matrices having structural characteristics comparable with or better than those obtainable with the alternative matrices mentioned above, following the modalities and the typical characteristics of the SMC technology or of similar technologies. In the widest sense, the present invention relates to a high-density composite material essentially based on polyisocyanurate, having a density of ≧500 and, preferably ≧1000 kg/m³, containing reinforcing fillers, in which the polyisocyanurate matrix is derived from a repeating structural unit having the formula: wherein at least one group X represents the group -R₁-NCO and at least one group X represents the group -R₁-NH-CO-OR₂-(-OCO-NH-R₁-NCO) n , in which n is an integer of from 1 to 8, preferably of from 1 to 3, R₁ is an aliphatic, cycloaliphatic, aromatic or mixed group containing up to 20 carbon atoms and R₂, the same as or different from R₁, is an aliphatic, cycloaliphatic, aromatic or mixed group containing up to 20 carbon atoms or, also, a carbonic, siloxane, silane or corresponding mixed group; which composite material is obtainable by the process specified below. According to a preferred embodiment, in the repeating structural unit of formula (I), on the average two groups X represent the group -R₁NCO and the third group X represents the radical -R₁-NH-CO-OR₂-(-OCO-NH-R₁-NCO) n . The amount of reinforcing filler in the composite material usually ranges from 5 to 85% by weight, preferably from 30 to 70 % by weight. From a technological point of view, said composite material exhibits an exceptional validity and, in particular, possesses the following characteristics: Flexural elastic modulus (ASTM D 790) ≧ 4000 N/mm² preferably ≧ 5000 N/mm² and even more preferably ≧ 20000 N/mm² IZOD notch resilience at room temperature (ASTM D 256) ≧ 100 J/m preferably ≧ 200 J/m and even more preferably ≧ 500 J/m. The heat distortion temperature (HDT at 1.8 N/mm²; ASTM D 648) of said composite material is > 150 o C and, preferably ≧ 220 o C. Said composite materials exhibit an optimum surface appearance and a high glass transition temperature. They belong to the class of the thermosetting resins and can advantageously be utilized in a wide range of applications such as, for example, in the automobile industry, building industry, electrodomestic goods, rigid packings, etc. An essential feature of the present composite materials is the process by which they are obtainable, said process comprises the use of particular prepolymers, prepared by partial addition of polyisocyanates and polyols which remain fluid at room temperature, even in the presence of cross-linking catalysts, for at least a few days and, in any case, for a time more than sufficient to accomplish the impregnation of the glass fibres and/or of a different filler and until moulding is carried out. In particular, the process for preparing the above composite materials is carried out in three distinct steps as follows: In a first step, a partially polymerized urethane system (prepolymer) is prepared by condensing a polyisocyanate such as, for example, 4,4'-diphenylmethane-diisocyanate and/or its 2,4'-isomer with polyols having a functionality of at least 2 and having a wide range of molecular weights with a polyisocyanate/polyol equivalent ratio higher than 2. Thereby a stable and fluid mixture is obtained which contains free polyisocyanate and NCO-terminated urethane compound. In the preparation of thermosetting resins, polyols having a functionality equal to or higher than 3, in particular triols derived from glycerin or from trimethylolpropane, are usually utilized. In a second step, the resulting mixture is mixed with a cross-linking catalyst which is active at temperatures above 80 o C and which can be of organic, inorganic, metallorganic or metallic origin. The matrix thus prepared is utilized to impregnate a support consisting of chopped glass fibres or of glass fibre cloth or of any other reinforcing material (for example carbon fibres or aramide fibres). In this way, pastes, sheets or other shaped articles, which are usually called "pre-impregnated" articles or materials, are obtained. In the case of chopped fibres it is possible to first prepare a concentrate, for example, at 85% by weight of fibre. Then the concentrate is diluted with a pure prepolymer until the intended concentration (5 to 85%, preferably 30 to 70% by weight of reinforcing material, based on the total weight) is reached. Generally, it is preferred to use glass fibres whose surface is coated with substances capable of imparting compatibility with the matrix, said fibres having diameters of from 8 to 30 µm, preferably of from 10 to 18 µm, and lengths of from 1 to 100 mm, preferably of from 5 to 50 mm. In the third step said pre-impregnated materials are polymerized in a mould at 80 to 200 o C (preferably at 100 to 150 o C), in any case at temperatures which depend on the type of catalyst, on the type of isocyanate employed, on the polyols and on the pressure, which must be high enough to eliminate blemishes on the surface of the composite articles. Furthermore, it is necessary to reduce as far as possible the possibility of water absorption or of atmospheric humidity absorption by operating in a protective atmosphere. Prior to moulding, it is possible to add the usual additives, for example, pigments and/or flame-retarding agents. As regards the operative conditions of the first step, it is recommended that the operation be carried out at 50 to 90 o C, preferably under vacuum or in an inert gas such as N₂, for a time of from 5 minutes to 3 hours, using reaction mixtures in which a stoichiometric excess of polyisocyanate with respect to polyol is present. Under these conditions an A+B blend (prepolymer) is obtained in which: A = OCN-R₁-NCO (polyisocyanate in the free state) (IV) B = OCN-R₁-NH-CO-OR₂-(-OCN-NH-R₁-NCO) n (urethane compound) (V) Subsequently, under the act ion of heat and after addition of a cross-linking catalyst (activator), of the reinforcing fillers and/or of inert fillers and of other optional additives, the blends are converted to a compound having predominantly a polyisocyanurate structure, the structural unit of which can be represented as follows: wherein X can be either of the following groups: a) -R₁-NCO; b) -R₁-NH-CO-OR₂-(-OCO-NH-R₁-NCO) n ; in the reaction ratio. The final structure is a tridimensional lattice, obtained from the polyaddition of structural unit (I), according to mechanisms analogous to those described above, until a practically complete conversion of the residual -NCO groups is reached. In other words, in the first two process steps a pre-impregnated material is obtained which, in the moulding step, is cured at a high temperature within a very short time (0.5 to 4 minutes). An alternative process for obtaining the composite of the present invention comprises: a) Preparing a mixture which contains a polyisocyanate of formula (IV), a urethane compound of formula (V), and one or more reinforcing materials, optionally along with organic and/or inorganic fillers and with plastic and/or elastomeric materials; b) Heating the resulting mixture to a temperature of from 80 to 220 o C after having added a cross-linking catalyst which acts at temperatures above 80 o C. In this case as well the added amount of reinforcing material ranges from 5 to 85% by weight, preferably from 30 to 70% by weight. Various polyols may be used in the process according to the present invention, optionally in admixture with suitable antioxidants. In principle, any diol, triol or polyol with four or more hydroxyl end groups can be advantageously used. Excellent results have been obtained by using polyolpolyethers derived from the condensation of glycerin, of trimethylol-propane or of pentaerythritol with alkylene oxides such as ethylene oxides, propylene oxides, butylene oxides, etc., or with polyoxyalkylene chains (polyoxyethylene, polyoxypropylene, polyoxybutylene chains, mixed oxyethylene-oxypropylene chains etc.). According to a preferred but not limiting embodiment of the invention it is possible to use mixtures containing two triols having a mean molecular weight ranging from 250 to 10000, preferably from 250 to 7000, namely: i) a first triol, having a lower molecular weight than the second triol and composed of glycerin and/or trimethylolpropane condensed with propylene oxide and/ or ethylene oxide; ii) a second triol, having a higher molecular weight than the first triol, composed of glycerin and/or trimethylolpropane, etherified with mixed oxyethyleneoxypropylene chains, with propylene oxide/ethylene oxide weight ratios ranging from 95/5 to 20/80; the weight ratio being preferably in the range of from 70:30 to 10:90. As is known to those skilled in the art, the above-mentioned polyols can be substituted by analogous compounds in which the hydroxyl end groups are substituted by amine groups (NH₂). Furthermore, the above-mentioned polyol polyethers can be substituted by polyol polyesters, by polyol polythioethers and by all the other polyols which are cited, for example, in EP-A-12 417. There are also various polyisocyanates which may be employed in the process of the present invention. Examples of suitable polyisocyanates are 4,4'- and 2,4'-diphenylmethane-diisocyanate, toluene-diisocyanate (TDI), 1,4-tetramethylene-diisocyanate, 1,6-hexamethylene-diisocyanate, p- or m-phenylene-diisocyanate, 1-chlorophenylene-2,4-diisocyanate, 1,5-naphthalene-diisocyanate, diphenyl-4,4'-diisocyanate, 1,4- or 1,2-cyclohexylene-diisocyanate and/or polyisocyanates having a functionality higher than 2, such as polyphenylmethane polyisocyanates. A necessary condition for obtaining isocyanurate structures of formula (I) is that the equivalent ratio of polyisocyanate to polyol is higher than 2. Finally, there are various cross-linking activators (catalysts) of organic, inorganic, metallorganic or metallic nature which are utilizable for moulding the pre-impregnated materials. Preferred is the use of the lead and bismuth compounds described by J.W. Britain and P.G. Gemeinhardt in J. Appl. Polym. Sci., Volume 4, (1960), pages 207 to 211 or of particular metallorganic compounds based on Ti, V, Co, Ni, Sn, optionally in admixture with amines. Optimum results were obtained by using lead stearate, oleate and 2-ethyl hexanoate. Reinforcing materials to be used in the preparation of the pre-impregnated materials may, besides glass fibres and/or cloths, be other types of organic or inorganic fibres or cloths, for example, carbon fibres or polyamide fibres (in particular aromatic polyamide fibres), crystalline polyolefinic fibres, polyester, rock wool, etc. It is also possible to utilize inorganic or organic fillers such as talc, calcium carbonate or phosphate, silicas, aluminas, cements, zeolites, kieselguhrs, wood meals, cellulose, amides and the like. Finally, in order to improve mechanical characteristics, it may be useful to incorporate plastic and/or elastomeric materials of different nature. In particular, the incorporation of plastic materials or elastomers, optionally cross-linked, such as ABS, polycarbonates, polyesters, polyamides, elastomeric polyesters, butadiene rubber or SBR, polyurethane elastomers, nitrile rubbers, ethylene-propylene rubbers, acrylic rubbers etc., in the form of discrete particles (mean diameter: 0.01 to 10 µm) results in the final article being endowed with improved impact strength. The following examples illustrate the present invention without, however, limiting the scope thereof. EXAMPLE 1 70 g of 4,4'-diphenylmethane-diisocyanate (MDI) were condensed at 80°C for 2.5 hours with 20 g of a trifunctional polyether polyol having a molecular weight of about 5000, consisting of glycerin etherified with mixed oxyethylene-oxypropylene chains and with 10 g of a trifunctional polyether polyol based on oxypropylene having a molecular weight of about 300, prepared by condensing glycerin with propylene oxide. The product was then deaerated under vacuum at 70°C. Thus, a prepolymer having a viscosity of from 7000 to 8000 mPa.s (at 23°C) with a content of free NCO groups of about 18% by weight was obtained. 1.5% by weight of Pb stearate was then added as a catalyst and the mass was homogenized in a vessel equipped with a stirrer, taking care to carry out the operation under vacuum. This prepolymer was used to impregnate one continuous glass fibre cloth having a thickness varying from 0.1 to 0.2 mm. Further preimpregnated cloths were then superimposed in a number sufficient for the final composite to have the predetermined thickness and fibre content. The combination was introduced into a mould heated at 120°C and was then compressed at 45 bar. The article was withdrawn after 3.5 minutes and the compact composite obtained exhibited the following characteristics: Glass fibre content 80% by weight Thickness of the composite 1.3 mm Flexural elastic modulus 2.6×10⁴ N/mm² Bending strength 400 N/mm² Tensile strength 150-200 N/mm² Izod resilience (with notched bar, at room temperature) 900-1000 J/m Heat distortion temperature (HDT at 1.8 N/mm²) 220°C (Shore)hardness 90-92 D. EXAMPLE 2 To the prepolymer prepared according to example 1 were added glass fibres having a mean diameter of 12 um and a mean length of 6 mm, in a weight ratio of 40/60, based on the matrix, and 1.5% by weight of lead stearate. The operation took place in a mixer under vacuum. The paste thus obtained was moulded under conditions described in example 1 but employing a time of 2.5 minutes. An article having the following characteristics was obtained: Glass fibre content 40% by weight Flexural elastic modulus 5000 N/mm² Bending strength 100-150 N/mm² HDT (at 1.8 N/mm²) 200°C IZOD resilience 200 J/m (Shore)hardness 85-87 D EXAMPLE 3 To the paste obtained according to example 2 a finely-dispersed acrylic rubber (mean diameter of the particles = 0.5 µm) was added in a weight ratio of 10/90 with respect to the paste. The resulting blend was moulded as in example 2 (time = 2.5 minutes). An article having the following characteristics was obtained: Flexural elastic modulus 4700 N/mm² HDT (at 1.8 N/mm²) 190°C IZOD resilience 400 J/m (Shore)hardness 82-85 D. EXAMPLE 4 30 parts by weight of glass fibres having a mean diameter of 12 µm and a mean length of 25 mm were impregnated with 70 parts by weight of a blend prepared by mixing 100 parts of the prepolymer obtained according to example 1 with 50 parts of CaCO₃ as an inert filler and with 1 part of lead stearate as a catalyst. The mixture was then subjected to moulding by means of heat compression at 120°C and 100 bar, thus obtaining round articles having a thickness of 3 mm, a diameter of 25 mm and the following characteristics: Flexural elastic modulus 9600 N/mm², Bending strength 250 N/mm² IZOD resilience 600 J/m HDT (at 1.8 N/mm²) 200°C

Claims: Composite material based on polyisocyanurates and containing reinforcing fillers, in which the polyisocyanurate matrix is derived from a repeating structural unit having the formula: wherein, on the average, at least one group X represents the group -R₁-NCO and at least one group X represents the group -R₁-NH-CO-OR₂-(-OCO-NH-R₁-NCO) n , in which n is an integer of from 1 to 8, R₁ is an aliphatic, cycloaliphatic, aromatic or corresponding mixed group containing up to 20 carbon atoms and R₂, the same or different from R₁, is an aliphatic, cycloaliphatic, aromatic or corresponding mixed group containing up to 20 carbon atoms, or also a carbonic, siloxane, silane or corresponding mixed group; and is obtainable by a process comprising the steps of a) condensing a polyisocyanate with a polyol having a functionality of at least 2, in a polyisocyanate/ polyol equivalent ratio of higher than 2, whereby a stable and fluid mixture is obtained which contains free polyisocyanate and NCO-terminated urethane compound; b) mixing the mixture obtained in step a) with a cross-linking catalyst which is active at temperatures higher than 80°C and impregnating the resulting mixture with at least one reinforcing mate Composite material according to claim 1, wherein n ranges from 1 to 3. Composite material according to any one of claims 1 and 2, in which, on the average, two groups X represent the group -R₁-NCO and one group X represents the group -R₁-NH-CO-OR₂-(-OCO-NH-R₁-NCO) n in the repeating structural unit of formula (I). Composite material according to any one of claims 1 to 3, in which the amount of reinforcing fillers ranges from 5 to 85% by weight, based on the total weight of the composite material. Composite material according to claim 4, wherein the amount of reinforcing fillers ranges from 30 to 70% by weight. Composite material according to any one of the preceding claims, having the following characteristics: Flexural elastic modulus ≧ 4000 N/mm² IZOD resilience ≧ 100 J/m. Composite material according to claim 6, having the following characteristics: Flexural modulus ≧ 5000 N/mm² IZOD resilience ≧ 200 J/m. Composite material according to claim 6 or 7 in which the heat distortion temperature is > 150°C. Composite material according to any one of the preceding claims wherein the reinforcing fillers are selected from glass fibers and/or cloths, carbon fibers, polyamide fibers, in particular, fibers of aromatic polyamides, crystalline polyolefinic fibers, polyester fibers and rock wool. Composite material according to any one of the preceding claims wherein said reinforcing fillers consist of chopped glass fibers having a mean diameter of from 8 to 30 µm and a mean length of from 1 to 100 mm. Composite material according to claim 10 wherein said chopped glass fibers have a mean diameter of from 10 to 18 µm and/or a mean length of from 5 to 50 mm. Composite material according to any one of the preceding claims additionally containing organic and/or inorganic rial, whereby a pre-impregnated material is obtained; c) polymerizing the pre-impregnated material obtained in step b) in a mould, at 80 to 200°C. Process for the production of the composite material of claim 1, characterized by the following three steps: a) Condensing a polyisocyanate with a polyol having a functionality of at least 2, in a polyisocyanate/polyol equivalent ratio of higher than 2, whereby a stable and fluid mixture is obtained which contains free polyisocyanate and an NCO-terminated urethane compound; b) Mixing the mixture obtained in step a) with a cross-linking catalyst which is active at temperatures higher than 80°C and impregnating the resulting mixture with at least one reinforcing material, whereby a pre-impregnated material is obtained; c) Polymerizing the pre-impregnated material obtained in step b) in a mould, at 80 to 200°C. Process according to claim 13, wherein the polymerization temperature in step c) is from 100 to 150°C. fillers and/or plastic or elastomeric materials, optionally cross-linked. Process according to any one of claims 13 and 14, wherein the condensation of the polyisocyanate with the polyol according to step a) is conducted at a temperature ranging from 50 to 90°C for a period of time of from 5 minutes to 3 hours. Process according to claim 15 wherein the condensation is carried out under vacuum or in an inert atmosphere. Process according to any one of claims 13 to 16 wherein, in step a), a mixture of two triols having a mean molecular weight of from 250 to 10000 is utilized as polyol, in which mixture i) the first triol, having a lower mean molecular weight than the second triol, is composed of glycerin and/or trimethylolpropane condensed with propylene oxide and/or ethylene oxide; and ii) the second triol, having a higher mean molecular weight than the first triol, is composed of glycerin and/or trimethylolpropane etherified with mixed oxyethylene-oxypropylene chains, with weight ratios of propylene oxide to ethylene oxide varying from 95/5 to 20/80; the weight ratio of first to second triol ranging from 70:30 to 10:90. Process according to claim 17, wherein a mixture of two triols having a mean molecular weight of from 250 to 7000 is utilized as polyol. Process according to any one of claims 13 to 18 wherein, during step b), the prepolymer is also mixed with a plastic or elastomeric, optionally cross-linked material, preferably selected from ABS resins, polycarbonates, polyesters, elastomeric polyesters, polyamides, butadiene or isoprene rubbers, butyl rubbers, nitrile rubbers, ethylene/propylene rubbers and acrylic rubbers in the form of particles having a mean diameter of from 0.01 to 10 µm. Process for the production of the composite material according to claim 1, characterized in that it comprises: a) Preparing a mixture containing a polyisocyanate of formula (IV): OCN-R₁-NCO (IV) a urethane compound of formula (V): OCN-R₁-NH-CO-OR₂-(-OCO-NH-R₁-NCO) n (V) wherein R₁, R₂ and n are as defined in claim 1, and one or more reinforcing materials, optionally along with organic and/or inorganic fillers and with plastic and/or elastomeric materials; b) Adding a cross-linking catalyst which is active at a temperature above 80°C; and c) Heating the resulting mixture to a temperature ranging from 80 to 220°C. Process according to any one of claims 13 to 20, wherein the catalyst, active at temperatures above 80°C, is selected from compounds of lead or bismuth or from organic or metallorganic compounds of Ti, V, Co, Ni and Sn, optionally in admixture with amines. Process according to claim 21, wherein the catalyst is selected from lead stearate, lead oleate and lead 2-ethylhexanoate.

Inventors: BIANCHIN, EUGENIO, CREDALI, LINO, LONGO, GIOVANNI, LUNARDON, GIANFLAVIO, SERRA, VINCENZO
Applicants: MONTEDISON S.P.A.
Application year: 1986
Main classification: C08G18
IPC codes: C08K7, C08G18