FLAME RETARDANT THERMOPLASTIC POLYURETHANE RESIN COMPOSITION

Abstract: This invention discloses a flame retardant ther­ moplastic polyurethane resin composition containing a halogen flame retardant and a flame-retardant, water-­ resistant thin insulated wire prepared by applying the flame retardant thermoplastic polyurethane resin com­ position to a conductor (21) to form an insulating layer (22) having a thickness of 0.3 mm or less. The flame retardant thermoplastic polyurethane resin composition is prepared such that 0.2 to 20 parts by weight of a cycloparaffinic compound having an epoxy group is added to 100 parts by weight of a thermoplastic polyurethane resin alone or contained as a major constituent in a thermoplastic resin.

Description: The present invention relates to a flame retardant resin composition comprising a thermoplastic polyurethane resin, which is excellent in formability and water resistance. A thermoplastic polyurethane resin has the advantages of both rubber and plastic materials and is used in a variety of applications such as tubes, electric wire insulation, and sheets. According to a method of molding such a resin, resin pellets are heated, plasticized, and melted, and the molten resin is molded by conventional thermoplastic resin molding equipment according to injection molding, extrusion molding, or the like. In recent years, strict flame retardancy is required for the electric wires and the like in order to prevent accidents. From this point of view, a halogen flame retardant such as a halogen-containing phosphoric ester flame retardant is often mixed in the thermoplastic polyurethane resin. A thermoplastic polyurethane resin having low hardness (e.g., Shore hardness A80 complying with test method ASTM 2240-75) is melted at about 180°C to mold it. As the hardness of the resin is increased, it becomes harder to plasticize it. Thus, a thermoplastic polyurethane resin having high hardness (e.g., Shore hardness D75) is melted at about 240°C. When molding continues for a long time under such temperature conditions, surface roughing or scorching occurs due to deterioration or thermal decomposition of the polyurethane resin left in the molding machine. As a result, the product often has poor appearance when molding continues. For this reason, the molding machine must be stopped, the interior of the machine must be cleaned and washed, and molding must be started again, thus greatly degrading productivity. In particular, in extrusion molding, a molded body preferably has a uniform thickness along its longitudinal direction. However, since viscosity changes caused by a chemical reaction such as the above-mentioned thermal decomposition occur in the extruder, the amount of delivery changes and a molded body having a uniform thickness cannot often be obtained. Continuous long-time formability of the thermoplastic polyurethane resin containing a halogen flame retardant is inferior to that of other thermoplastic resins such as polyethylene or polyvinyl chloride resin for the following reasons: (i) An urethane bond is thermally weak, and a free NCO group generated at a molding temperature of 180°C or higher tends to react with the urethane group to form a three-dimensional bond (crosslinking bond) such as an allophanate bond in the molding machine; and (ii) A halogen compound such as halogenated hydrogen generated by halogen elimination from the above-mentioned halogen flame retardant accelerates deterioration of the resin. The water resistance of the resultant molded body tends to be undesirably degraded by hydrolysis caused by halogenated hydrogen produced by decomposition of the flame retardant. The present inventors made extensive studies to eliminate problems concerning molding of a thermoplastic polyurethane resin composition containing a halogen flame retardant and improve its water resistance and found that mixing of a cycloparaffinic compound having an epoxy group into the thermoplastic polyurethane resin was effective in solving the above problems. Therefore, the present inventors achieved the present invention. According to the present invention, there is provided a flame retardant resin composition comprising: (A) a thermoplastic polyurethane resin, (B) optionally up to 50% by weight - based on the combined weight of (A) and (B) - of a further thermoplastic resin compatible with (A), (C) a halogen containing flame retardant, and (D) 0,2 to 20 parts by weight per 100 parts by weight of the combined weight of (A) and (B) of a cycloparaffinic compound having an epoxy group. Furthermore, according to the present invention, there is provided a thin insulated wire excellent in flame retardancy and water resistance wherein the above-mentioned flame retardant thermoplastic polyurethane resin composition is extrusion-coated on a conductor to form an insulating layer having a thickness of 0.3 mm or less. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view of a flame retardant polyurethane insulated wire according to the present invention; Fig. 2 is a schematic side view of a scrape tester used for measurement of abrasion resistance of the flame retardant polyurethane insulated wire of the present invention; and Fig. 3 is a schematic view of a device for testing flame retardancy of the flame retardant polyurethane insulated wire of the present invention. A thermoplastic polyurethane resin used in the present invention is prepared by reacting a bifunctional polyol with a diisocyanate. More specifically, a bifunctional polyol, e.g., a polyester diol [e.g., poly(ethylene adipate), poly(1,4-butyl adipate), poly(1,6-hexane adipate), poly-ε-caprolactone, or poly(hexamethylene carbonate)] or a polyether diol (e.g., polyoxytetramethylene glycol), a glycol (e.g., 1,4-butylene glycol, 1,6-hexane diol, or ethylene glycol) as a chain coupling agent, and a diisocyanate (e.g., 4,4'-diphenylmethane diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, or isophorone diisocyanate) are reacted with each other to prepare a polymer comprising a soft block of the polyol and diisocyanate and a hard block of the glycol and diisocyanate. This polymer is thermoplastic. The hardness of the resin can be arbitrarily adjusted by changing a ratio of the hard segment to the soft segment. In particular, a thermoplastic urethane resin having Shore hardness A95 or more which requires a molding temperature of 200°C or higher is very effective to achieve the present invention. The present invention is also applicable when a thermoplastic polymer (e.g., an ethylene-vinyl acetate copolymer, an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, polystyrene resin, an acrylonitrile-butadiene-styrene copolymer, methacrylic resin, nylon, or polyvinylchloride) compatible with the above polyurethane resin and to be physically blendable therewith is mixed with the polyurethane resin in an amount of 50 wt% or less. If the polyurethane resin is a polyester polyurethane resin or a blended resin, the most typical effect can be obtained. Examples of the halogen flame retardant are a chlorine-containing phosphate [e.g., tris(2-chloroethyl)phosphate or tris(dichloropropyl)phosphate], a bromine-containing phosphate [e.g., tris(2,3-dibromopropyl)phosphate], an aromatic chlorine compound (e.g., dodecachlorododecahydrodimetanodibenzocyclooctene), an aromatic bromine compound (e.g., decabromodiphenyloxide), and a flame retardant system using one of the above flame retardants and another flame retardant such as antimony trioxide. 5 to 20 parts by weights of such a flame retardant are preferably added to 100 parts by weight of the thermoplastic polyurethane resin alone or a thermoplastic resin containing the thermoplastic polyurethane resin. In order to mix such a flame retardant in a resin with good dispersibility, a liquid halogen-containing phosphate is preferably added during its synthesis. How ever, the flame retardant may be sprinkled in the resin during molding or masterbatched if a flame retardant powder is used. A cycloparaffinic compound having an epoxy group comprises compounds having the following groups: where X is a radical such as an ester, ether, acetal, imide, amide, or hydrocarbon and is exemplified by compounds having cyclic epoxy groups, such as Chissonox 221 (*1), Chissonox 201 (*2), Chissonox 289 (*3), and Chissonox 206 (*4), all of which are tradenames of products which are commercially available from CHISSO CORPORATION, and Celloxide 2000 (*5), Celloxide 3000 (*6), and Celloxide 4000 (*7), all of which are tradenames of products which are commercially available from DAICEL CHEMICAL INDUSTRIES, LTD. These compounds are commercially available as alicyclic epoxy resins. 0.2 to 20 parts by weight and preferably 1 to 15 parts by weight of the cycloparaffinic compound is added to 100 parts by weight of the thermoplastic polyurethane resin alone or contained as a major constituent in a thermoplastic resin. If the content of the cycloparaffinic compound is less than 0.2 part by weight, the effect of the present invention cannot be obtained. However, if the content exceeds 20 parts by weight, the effect cannot be enhanced any longer. For this purpose, other epoxy resins may be used together with the cycloparaffinic compound having an epoxy group so as not to degrade the effect of the present invention. In order to mix an epoxy compound or the like in a resin such as a thermoplastic polyurethane resin, in molding by an extruder or an injection molding machine, any method can be utilized: a method of sprinkling the compound on the resin, a method of supplying the compound under pressure by utilizing a quantitative pump in the molding machine and of melting and kneading the compound and the resin, or a method of reacting the polyol component with the diisocyanate compound and mixing the compound in the synthesis of the polyurethane resin. In the last method, no problem occurs when synthesis conditions (temperature and time) are determined so as not to allow the progress of a reaction between the NCO group of the diisocyanate component and the OH group of the polyol component and the epoxy group. An alkaline earth metal carbonate or oxide or a compound containing the carbonate or oxide can further be contained in the flame retardant resin composition of the present invention to greatly improve water resistance. Examples of the alkaline earth metal oxide are calcium oxide and magnesium oxide, which are obtained by substituting X in formula XO with alkaline earth metal elements. Examples of the alkaline earth metal carbonate are calcium carbonate, barium carbonate, and magnesium carbonate, which are obtained by substituting H in HCO₃ with alkaline earth metal elements. An example of the compounds containing alkaline earth metal carbonate and oxide is magnesium hydroxycarbonate [MgCO₃·Mg(OH)₂·2H₂O]. The content of such a compound falls within the range of 0.01 to 15.0 parts by weight with respect to 100 parts by weight of the thermoplastic polyurethane resin alone or contained as a major constituent in a thermoplastic resin. If the content of the compound is less than 0.01 part by weight, the effect of the present invention cannot be achieved. However, if the content exceeds 15.0 parts by weight, the molded product has poor appearance due to surface roughing. The alkaline earth metal carbonate or oxide is mixed by sprinkling in the resin during molding by means of an extruder, an injection molding machine, or the like or is masterbatched. Alternatively, the carbonate or oxide may be mixed in the resin during synthesis of the polyurethane resin prepared by reacting the polyol component with the diisocyanate component. Assistant agents such as carbon black, a blowing agent, an antioxidant, a mildew proofing agent, and a coloring agent can be added within the range which does not change the properties of the resultant composition. These assistant agents can be added any time if they do not interfere with the synthesis reaction between the polyol and diisocyanate. However, if the synthesis reaction is interfered, these agents are preferably added to the resin during molding. The actions of the present invention do not clarify all unknown factors. A cyclohexene oxide as an example of the above epoxy compound is represented by the following formula: In the thermoplastic polyurethane resin composition containing the halogen flame retardant, a free halogen compound and a free NCO group are produced by heat applied in the molding machine. The compound having an epoxy group reacts with the free halogen compound and the free NCO group. However, in the present invention, the epoxy group of the epoxy compound is located in a six-membered ring. Therefore, reactivity between the epoxy group and the NCO group is greatly weakened, while that between the epoxy group and the halogen compound is accelerated. When a bisphenol A type epoxy resin as a typical example of a halogen trapping agent of a general-purpose vinyl chloride resin is used, the epoxy group serves as a molecular terminal group, the reactivity between the epoxy group and the NCO group is enhanced to eliminate the epoxy group, and the halogen trapping effect inherent to this resin cannot be achieved. The different reactivities between the epoxy group and the NCO group according to different structures of the epoxy groups can be assumed as follows. Since the NCO group has a resonance structure as in the following formula and electrons between the oxygen and carbon atoms are polarized in the epoxy group as in the following formula, an ionic reaction is assumed to occur between the NCO group and the epoxy group. In cyclohexene oxide as an example of the epoxy compound in the present invention, four atoms C₆, C₁, C₂, and C₃ are located on a single plane, atoms C₄ and C₅ are located behind the epoxy group and extend vertically from the plane defined by atoms C₆, C₁, C₂, and C₃. For this reason, very large steric hindrance is assumed in order to allow the nitrogen atom of the NCO group to react with the carbon atom of the epoxy group: The halogen trapping reaction is given such that a halogen compound is added to an oxygen atom of the epoxy group, and therefore the reactivity tends not to be weakened due to the steric hindrance. When the cycloparaffinic compound having an epoxy group is mixed in the thermoplastic polyurethane resin containing a halogen flame retardant, scorching and surface roughing can be prevented. In order to perfectly prevent a crosslinking reaction for bonding the molecules, a cycloalkane compound preferably contains one epoxy group in one molecule. The improvement of water resistance of the composition of the present invention can be explained as follows. In the polyurethane resin containing the halogen flame retardant, since halogenated hydrogen produced by the decomposition of the flame retardant is acidic, it serves as a catalyst in hydrolysis, thus undesirably increasing the water-resistance degradation rate. When cyclohexene oxide as an example of the cycloparaffinic compound having an epoxy group is added to the polyurethane resin containing the halogen flame retardant, cyclohexene oxide can be easily reacted with halogenated hydrogen for the above reason, thereby inactivating halogenated hydrogen. In order to further improve the water resistance of the resin, if calcium oxide or carbonate as an example of the alkaline earth metal oxide or carbonate of the present invention is added to the resin, the following reaction occurs and halogenated hydrogen is inactivated. The present invention is effectively applied to a polyester-polyurethane resin of the polyurethane resins for the following reason. A molecular chain having an ester bond in polyester-based polyurethane has a structure represented by the following formula and is subjected to hydrolysis with water. The fatty acid produced by the decomposition is acidic and serves as a hydrolytic catalyst in the same manner as halogenated hydrogen, thereby increasing the water-resistance degradation rate. When cyclohexene oxide and calcium carbonate as components of the composition of the present invention are added, the fatty acid is inactivated due to the same mechanism as in halogenated hydrogen, thereby improving the water resistance of the composition. The present invention will be described in detail by way of Examples 1 to 26 and Comparative Examples 1 to 4. Comparative Example 1 1,000 g of dehydrated chain caprolactone having a molecular weight of 2,000 and 56 hydroxyl number, 320 g of 1,4-butane diol, and 1,100 g of 4,4′-diphenylmethane diisocyanate were stirred at 82°C for about 2 minutes, and the resultant mixture was poured on a heated molding plate (115 to 120°C) and was left to stand for about 10 minutes. Thereafter, the mixture was further left to stand on the molding plate at room temperature for 48 hours. The mixture was then granulated (stage I). 20 parts of tris(2-chloroethyl)phosphate was added to 100 parts of the resultant thermoplastic polyurethane resin (Shore hardness D68) and uniformly blended by a blender. The resultant mixture was extruded by a 210°C extruder to obtain an insulated wire having an outer diameter of 2.0 mm (outer diameter of conductor: 1.60 mm) for about 3 hours (stage II). Changes in outer diameter of the insulated wire and its outer appearance were observed in detail. The extruder was stopped and an extrusion screw was removed. The scorched resin on the screw was checked. Results are summarized in Table 1. Flame retardancy, tensile properties, and water resistance of the insulated harness wire of Fig. 1 and the resultant insulated wire were evaluated. The insulated harness wire shown in Fig. 1 is obtained by compression-molding a stranded wire consisting of a large number of strands, thereby obtaining compressed conductor 21 having a circular cross section. Insulating layer 22 having a thickness of 0.2 mm or less is formed on compressed conductor 21. In order to form compressed conductor 21, the stranded wire is squeezed through a die or a molding roll and the outer diameter of the stranded wire is thus reduced. Strands 23 are compressed and deformed in a noncircular manner so as to fill the gaps in the stranded wire. Therefore, the strands are not separated from each other or not projected from the stranded wire, so that the cross section of the stranded wire becomes substantially circular. Even if the thickness of insulating layer 22 is as small as 0.2 mm or less, variations in thickness of the thin layer are reduced. The insulated wire was tested according to the following method. Abrasion resistance of the insulated harness wire is represented by the number of scrape test cycles. The scrape test is performed using a scrape tester shown in Fig. 2. Insulated wire sample 32 is fixed on firing base 31 with fixing clamps 33 in the horizontal position. Scrape blade 34 having a 0.125-mm R edge is placed on sample 32. Scrape blade 34 is reciprocally moved by mechanism 36 with a stroke of about 10 mm while weight (outer diameter of conductor 1.6 mm; 1.2 kg) 35 corresponding to a wire size is placed on blade 34. In this manner, sample 32 is rubbed with scrape blade 34. When the insulating layer is cut and scrape blade 34 is brought into contact with the conductor, the operation of mechanism 36 is stopped, and the number of reciprocal cycles is displayed on a counter. A count of 12 or more is required for the insulated harness wire. The flame retardancy test is performed as follows, as shown in Fig. 3. Ten wires 41 each having a length ℓ of about 600 mm are bundled with a 0.32-mm diameter copper wire at five positions at intervals of about 100 mm to prepare a sample. The sample is vertically fixed by support members 42 and 43 such that distance I between members 42 and 43 is set at 550 ± 25 mm. Flame 45 from Bunsen burner 44 is continuously brought into contact with the sample for 1 minute at an angle ϑ of 45° at a position having distance D of 475 mm from upper support member 42 in such a manner that 10-mm long reducing flame portion E is located at a sample portion corresponding to the above-mentioned flame position. The evaluation of flame retardancy is given such that a sample having length L exceeding 50 mm or more (Fig. 3) is accepted as a good product. The tensile test was performed complying with JIS (Japanese Industrial Standard) C3005 (item 16). The water resistance test was performed in the following manner. A core wire was pulled from the insulated wire. The insulating layer was dipped in hot water at 100°C and its tensile properties with the passage of time were measured. The number of days corresponding to an absolute elongation of 50% was measured. Test results are summarized in Table 1. Comparative Example 2 An insulated wire was prepared following the same procedures as in Comparative Example 1 except that a stabilizer EPICOAT 828® having the following formula: was added in stage II of Comparative Example 1. Results are summarized in Table 1. Examples 1 - 6 Samples were prepared following the same procedures as in Comparative Example 1 except that stabilizers shown in Table 1 were added in stage II of Comparative Example 1. Results are summarized in Table 1. Examples 7 - 9 In Example 3, long-run properties of samples, i.e., every 10 hours (Example 7), every 20 hours (Example 8), and every 30 hours (Example 9) were measured. Test results are summarized in Table 1. Examples 10 - 12 Samples were prepared following the same procedures as in Comparative Example 1 except that stabilizers and inorganic materials were added in stage II of Comparative Example 1. Results are summarized in Table 1. Example 13 A sample was prepared following the same procedures as in Example 2 except that the same composition as in Example 2 was added in stage I of Comparative Example 1. Results are summarized in Table 1. Example 14 A sample was prepared following the same procedures as in Example 10 except that the same composition as in Example 3 was used. Examples 15 - 17 In Example 11, long-run properties of samples, i.e., every 10 hours (Example 15), every 20 hours (Example 16), and every 30 hours (Example 17) were tested, and results are summarized in Table 1. Examples 18 - 21 Samples were prepared following the same procedures as in Comparative Example 1 except that other polymers are mixed with a polyurethane resin obtained in stage I of Comparative Example 1 in stage II thereof at mixing ratios of Table 1, and that stabilizers and inorganic materials in Table 1 were added. Results are summarized in Table 1. Comparative Example 3 20 parts by weight of tris(2-chloroethyl)phosphate was added to 100 parts by weight of polyether type polyurethane resin Miractran® E-395PNAT (Shore hardness: A95), which is not flame retardant, available from NIPPON MIRACTRAN COMPANY LIMITED, and they were mixed by a blender. The resultant mixture was extruded by a 180°C extruder to obtain an insulated wire having an outer diameter of 2.0 mm (outer diameter of conductor: 1.60 mm) for 3 hours. The sample was evaluated following the same procedures as in Comparative Example 1. Results are summarized in Table 1. Comparative Example 4 A sample was prepared following the same procedures as in Comparative Example 3 except that a stabilizer (EPICOAT® 828) as in Comparative Example 2 was added to a composition as in Comparative Example 3, and the evaluation of the sample was performed following the same procedures as in Comparative Example 1. Results are summarized in Table 1. Example 22 A sample was prepared following the same procedures as in Comparative Example 3 except that the stabilizer in Table 1 was added to a composition as in Comparative Example 3, and the evaluation of the sample was performed following the same procedures as in Comparative Example 1. Results are summarized in Table 1. Example 23 A sample was prepared following the same procedures as in Comparative Example 3 except that the stabilizer and the inorganic material in Table 1 were added to a composition as in Comparative Example 3, and the evaluation of the sample was performed following the same procedures as in Comparative Example 1. Results are summarized in Table 1. Example 24 Tris(2-chloroethyl)phosphate, the stabilizer and the inorganic material in Table 1 were added to 100 parts by weight of polycaprolactone type polyurethane resin Miractran® E-574PNAT (Shore hardness: D74), which is not flame retardant, available from NIPPON MIRACTRAN COMPANY LIMITED, and they were mixed by a blender. The resultant mixture was extruded by a 230°C extruder to obtain an insulated wire having an outer diameter of 2.0 mm (outer diameter of conductor: 1.60 mm) for 3 hours. The sample was evaluated following the same procedures as in Comparative Example 1. Results are summarized in Table 1. Example 25 A sample was prepared following the same procedures as in Example 24 except that polycaprolactone type polyurethane resin Miractran® E-595PNAT (Shore hardness: A95), which is not flame retardant, available from NIPPON MIRACTRAN COMPANY LIMITED was used as a polyurethane resin, and the evaluation of the sample was performed following the same procedures as in Comparative Example 1. Results are summarized in Table 1. Example 26 A sample was prepared following the same procedures as in Example 24 except that polycarbonic acid ester type polyurethane resin Miractran® E-968PNAT (Shore hardness: D68), which is not flame retardant, available from NIPPON MIRACTRAN COMPANY LIMITED was used as a polyurethane resin, and the evaluation of the sample was performed following the same procedures as in Comparative Example 1. Results are summarized in Table 1. As is apparent from the above description and examples, the secondary reaction in the molding machine can be restricted by the presence of the cycloparaffinic compound containing an epoxy group and the inorganic compound in the composition of a thermoplastic polyurethane resin containing a halogen flame retardant or the composition, which contains a halogen flame retardant, of the thermoplastic resin containing the thermoplastic polyurethane resin as a major constituent. Therefore, formability can be stabilized, and water resistance of the products can be greatly improved. The resin composition of the present invention is particularly effective for a thin insulated harness wire having Shore hardness of D 68 and more and provides good industrial advantages.

Claims: A flame retardant resin composition comprising (A) a thermoplastic polyurethane resin, (B) optionally up to 50% by weight - based on the combined weight of (A) and (B) - of a further thermoplastic resin compatible with (A), (C) a halogen containing flame retardant, and (D) 0.2 to 20 parts by weight per 100 parts by weight of the combined weight of (A) and (B) of a cycloparaffinic compound having an epoxy group. A composition according to claim 1, characterized in that it comprises further 0.01 to 15.0 parts by weight of an oxide or carbonate of an alkaline earth metal or a compound containing the oxide or carbonate thereof per 100 parts by weight of the combined weight of (A) and (B). A composition according to claim 1 or 2, characterized in that the content of polyurethane resin having ester bonds in the thermoplastic polyurethane resin is not less than 50 wt%. A composition according to anyone of the claims 1 to 3, characterized in that component (D) is cyclohexene oxide. A composition according to anyone of the claims 1 to 4, characterized in that the halogen flame retardant is at least one compound selected from chlorine-containing phosphate, bromine-containing phosphate, aromatic bromine compound, or an aromatic chlorine compound. A composition according to anyone of the claims 1 to 5, characterized in that it comprises 5 to 20 parts by weight of component (C) per 100 parts by weight of the combined weight of (A) and (B). A composition according to anyone of the claims 1 to 6, characterized in that component (B) is at least one material selected from ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer, ethylene-methyl acrylate copolymer, methacrylic resin, acrylonitrile-butadiene-styrene copolymer, nylon, or polyvinyl chloride. A composition according to anyone of the claims 1 to 7, characterized in that hardness of component (A) is not less than Shore A95. A flame retardant polyurethane insulated wire having good water resistance, characterized in that it comprises an insulating layer (22) having a thickness of not more than 0,3 mm extruded on a conductor (21), said insulating layer being constituted by a composition according to any one of the claims 1 to 8.

Inventors: NISHIYAMA, HIDEMI, OHTANI, KENICHI
Applicants: THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1987
Main classification: C08L75
IPC codes: C08K3, C08K5, C08L75