ORGANIC POLYMER STABILISATION

Abstract: Reactive stabilising compounds, able to stabilise organic polymers, contain in their molecule a sterically hindered amino group and a hydrolysable silyl function. In stabilising organic polymers, said reactive stabilising compounds can be hydrolysed at the silyl function with the formation of silanol groups, which are made to interact in order to form complex resinous stabilising structures. These latter are added in stabilising quantities to the polymer to be stabilised. According to one particular embodiment, the hydrolysis at the silyl function and the formation of the resinous struc­ ture take place spontaneously within the polymer to be sta­ bilised. According to a further embodiment, the reactive stabi­ lising compound is added to the polymer after being stably supported on a solid support by reaction with an inorganic solid having surface hydroxyl groups. According to a further embodiment, the reactive stabi­ lising compound is made to interact with the polymer so that the stabilising compound becomes chemically bonded to the polymer chains. In all cases, stabilised polymers are obtained containing the stabilising compound in a form which is not removable from the polymer. The process for preparing the reactive stabilising com­ pounds and for preparing the stabilised polymer composi­ tions are also described.

Description: This invention relates to the stabilization of organic polymers, and, more particularly, to a process for stabilizing organic polymers, more particularly the organic polymers, such as polyolefins, polydiolefins and polyethers, which are among the most sensitive to degradative actions due to oxidation and exposure to actinic light rays. Such a degradation, as is well known to those skilled in the art, unfolds itself, as a rule, with a decay of the physico-mechanical properties of the polymer concerned, such as the reduction of both the ultimate tensile stress and flexibility, accompanied by changes in the viscosity index. The use of antioxidants to be incorporated in the polymer mass is a common expedient to counteract the degradative actions referred to above. Thus, for example, reactive antioxidant compounds which contain the sterically hindered phenol group and carrying a silyl function which is hydrolyzable to silanol and is connected to the phenol ring via an Si-C bond are described in C.A., Vol . 95 (1981), N° 133880v, and also in FR-A-2 433 533. However, the incompatibility between such antioxidants and the polymer concerned is a problem, and, moreover, another serious problem is the release of the antioxidant from the polymer mass. There was, consequently, a long felt want of an oxidation-combating approach capable of providing polymer-compatible antioxidants which, moreover, might have a satisfactory persistence within the polymerizates. To offset the drawbacks enumerated above, the present invention provides a process for stabilizing an organic polymer, characterized by comprising the steps of: a) Preparing an antioxidant by causing the silyl function of a silylated phenolic compound selected from the group consisting of: wherein R₁ and R₂ are linear or branched alkyl radicals contaning from 1 to 10 carbon atoms; R is a linear or a branched alkylene radical contaning from 1 to 10 carbon atoms, or is wherein R₃, R₄ and R₅ are linear or branched alkylene radicals containing from 3 to 10 carbon atoms; X is a linear or a branched alkyl radical contaning from 1 to 5 carbon atoms; Y is a hydrolyzable grouping selected from the group consisting of: hydrogen, halogen, C₁-C₄ acyloxy, C₁-C₄ alkyloxy, amino, amino-oxy, or silyloxy; m is zero or 1, and n i 1 , 2 or 3, to react at room temperature with water to hydrolyze said silyated phenolic compound at its silyl function, and b) Incorporating in the organic polymer mass to be stabilized an amount of from 0,01% to 5% on a weight basis relative to the organic polymer mass, of said antioxidant. According to an embodiment of said process, the antioxidant is prepared by incorporating, in the organic polymer mass to be stabilized, such an amount of non-hydrolyzed silylated phenolic compound as to have a final concentration of said antioxidant of from 0,01% to 5% on a weight basis relative to the organic polymer mass, and causing the reaction with water to take place in the mixture so prepared. Specific examples of reactive antioxidant compounds usable with the process of this invention are: The reactive antioxidant compounds (III), (IV) and (V) can be obtained from the compound: by silylation with methyldiethoxysilane, dimethylchlorosilane and γ-mercaptopropyltrimethoxysilane respectively. A further specific example of a reactive antioxidant compound usable with the process of the present invention is: The reactive antioxidant compound (VII) can be obtained by silylation of the compound: with triethoxysilane. A further example of a reactive antioxdant compound which can be used in the process under discussion is: The reactive antioxidant compound (VIIIa) can be obtained by silylation with trimethoxysilane of the compound: In general the reactive antioxidant compounds of the present invention can be prepared by silylating a sterically hindered phenol carrying on its ring a preferably terminal ethylenically unsaturated group. One class of silylation agents suitable for this purpose is definable by the formula: A further class of silylation agents suitable for the purpose is definable by the general formula: where R₅, X, Y and n have the aforesaid meanings. Specific examples of silylation agents falling within general formula (IX) are: HSi(CH₃)₂Cl; HSi(CH₃)Cl₂; HSiCl₃; HSi(O-CH₃)₂(CH₃); HSi(CH₃)(O-C₂H₅)₂; HSi(O-C₂H₅)₃; H₂Si(C₂H₅)₂; HSi(OCH₃)₃; HSi(CH₃)₂-O-Si(CH₃)₂H; HSi(CH₃)₂-O-Si(CH₃)(OCH₃)₂; HSi(CH₃)₂ONC(CH₃)₂; HSi(CH₃)[ONC(CH₃)₂]₂ Specific examples of silylation agents which fall within general formula (X) are γ-mercaptopropyltrialkoxysilanes and in particular γ-mercaptopropyltrimethoxysilane. The silylation reaction is conveniently conducted at a temperature of between 0° and 200°C, and preferably between ambient temperature (20°-25°C) and 120°C, with a reagent quantity varying from stoichiometric to an excess of the silylation reagent. Said excess usually reaches up to 20% on a molar basis. However, if disilanes are used it is convenient to use a large excess of the silylation agent, for example up to about 10 times the stoichiometric quantity. The silylation reaction is catalysed by metal catalysts, by ultraviolet radiation and by radical intiators. The preferred catalysts are platinum compounds and complexes of platinum with olefins, in particular chloroplatinic acid. In the case of platinum catalysts, the catalyst concentration, evaluated as metal, can vary from 1 to 200 parts per million and preferably from 5 to 50 parts per million in the reaction medium. The silylation reaction can be conducted in an inert (unreactive) organic solvent, normally chosen from aliphatic, cycloalpihatic and aromatic hydrocarbons and ethers, which are liquid under the operating conditions. Specific examples of solvents suitable for this purpose are heptane, cyclohexane, toluene, tetrahydrofuran, dioxane and dimethoxyethane. The reaction times depend on the particular reagents used and the reaction temperature, and vary normally from 0.5 to 10 hours. On termination of the silylation reaction, any solvent used and any excess silylation agent are removed by stripping, and the reactive stabilising compound is recovered from the residue of said stripping by normal methods such as crystallisation and distillation under vacuum. However, generally the high yield and selectivity of the silylation reaction make any separation or purification of the final required product unnecessary. If silylating compounds falling within formula (X) are used, the reaction is conveniently conducted under the aforesaid general silylation conditions with catalysts in the form of azo compounds such as azobisisobutyronitrile, which are used in a quantity of between 0.1% and 10% and preferably between 0.5% and 2% in the reaction environment. The reactive antioxidant compounds of the present invention hydrolyse at the silyl function under mild conditions, to generate silanol groups which can be condensed together to form complex resinous stabilsing structures. These resinous structures, of silicone resin type, preserve the inherent stabilizing characteristics of sterically hindered phenols, and have a high level of compatibility with organic polymers, and practically no extractability from said polymers. Hydrolysis at the silyl function takes place simply by contact with water or with environmental moisture at ambient temperature (20°-25°C) or lower than ambient. Condensation between the silanol groups to give the complex resinous structures can be facilitated by acid or basic agents, soaps or metal esters, and organometal compounds, especially of lead and tin. Preferred catalysts for this purpose are lead naphthenate and tin dibutyl-laurate. Conveniently, the catalyst quantity can vary from 0.1% to 10% by weight and preferably from 0.2% to 3% by weight with respect to the reactive antioxidant compound subjected to resinification. Said resinification reaction can be conducted at ambient temperature (20°-25°C) or at higher or lower than ambient. The complex resinous structure thus obtained can be introduced into the organic polymer to be stabilized by the usual methods used for this purpose. According to a further embodiment of the present invention, the reactive antioxidant compounds are introduced directly into the organic polymer, within which the hydrolysis reaction at the silyl function and the interaction between the silanol groups take place spontaneously, to thus give the stabilized polymer composition. According to a further embodiment, hydrolysis at the silyl function of the reactive antioxidant compounds takes place externally to the polymer, together with partial resinification of the hydrolysis products thus obtained. The product of the partial resinification is then introduced into the organic polymer to be stabilised, within which complete resinification takes place. According to a further embodiment, the reactive antioxidant compounds of the present invention are co-resinified with conventional silicone varnishes. Generally, the co-resinification requires baking at relatively high temperature. The materials thus obtained are complex resins in which the antioxidant is one of the constituent elements. These materials are generally hard, transparent glasses which can be ground to give a powder with an average particle size of 1 µm or less. In this form, the antioxidant can be distributed and homogenised in the polymer to be stabilized, in pigment form. The structure of these resinification products depends essentially on the number of groups hydrolysable at the silyl function in the reactive antioxidant compounds. For example in the case of compound (IV), which contains only one hydrolysable group, the hydrolysis and resinification reactions proceed until a dimer is produced, which in the case in question can be defined by the following general formula: In the case of reactive antioxidant compounds which contain more than one hydrolysable group in the silyl function, more complex resinification products are obtained. The reactive antioxidant compounds of the present invention can be fixed to a solid support containing surface hydroxyl groups. Supports suitable for this purpose are siliceous materials, of either natural or synthetic origin, such as diatomaceous earth, celite, silica gel, cement, glass, glass fibres and silco-aluminates in general. The preferred of all these supports is that type of silica commonly known as fumed silica which, together with good optical characteristics, has low apparent density, a high specific surface (generally exceeding 200 m²/g) and a high surface concentration of hydroxyl groups. The bond to the support is produced by reacting the reactive anti oxidant compound in its hydrolysed form with the surface hydroxyl groups of the support. In practice, the support, in the form of powder or granules, is brought into contact with a solution of the reactive antioxidant compound in an inert solvent, such as an aliphatic, cylcoaliphatic or aromatic hydrocarbon or an ether. The procedure is carried out in the liquid phase at a temperature of between ambient (20°-25°C) and about 100°C. The reactive antioxidant compound becomes hydrolysed and bonded to the support within a time of the order of between 0.5 and 10 hours. The stabilising compound thus supported is added to the organic polymer to be stabilised, by normal methods. This embodiment has the further advantage of excellent distribution of the antioxidant in the polymer. According to a further embodiment, the reactive antioxidant compounds of the present invention are bonded chemically to the organic polymer to be stabilized. This method is particularly effective in the case of diolefinic polymers and copolymers of low molecular weight. The reaction between the reactive antioxidant compound and the polymer generally takes place at a temperature of between ambient (20°-25°C) and about 100°C, in the presence of an inert (unreactive) diluent, in a time of between 0.5 and 10 hours. The reactive antioxidant compounds of the present invention are able to stabilise organic polymers in general, such as polyolefins, including polyethylene, polypropylene, polyisobutene and ethylene-propylene copolymers, polydiolefins including polybutadiene and polyisoprene, and polyethers, including polyethyleneoxide and polypropyleneoxide. The stabilised polymer compositions of the present invention contain a stabilizing quantity of the antioxidant compound, the term "stabilizing quantity" signifying that quantity of 0.01% to 5% by weight of phenolic antioxidant. The following experimental examples are given for illustrative purposes, and do not limit the range of the invention. EXAMPLE 1 Preparation of the compound (III): The compound (VI) which is a slightly yellow solid with a melting point of 36°-37°C and obtained by esterifying the corresponding acid with allyl alcohol (5.0 g; 18.0 mmoles), is reacted in toluene (5 ml) with methyldiethoxysilane (4.1 ml; 3.4 g; 25.0 mmoles) in the presence of traces of chloroplatinic acid dissolved in isopropanol (10µl of a 2 weight% solution of H₂PtCl₆.6H₂O), operating firstly at ambient temperature (the reaction is slightly exothermic) and then at 80°C for 4 hours. The completeness of the reaction is indicated by the disappearance of the IR band at 1650 cm⁻¹ (allyl unsaturation). The reaction product thus obtained is subjected to fractional distillation under reduced pressure (1 torr) in a bulb still to give 5.9 g of a slightly straw-coloured oil [yield 73% with respect to compound [VI)], which distils at a boiler temperature of 240°C (1 torr) and is constituted by the compound (III). The mass spectrum (M + 452) and IR and ¹Hnmr analysis confirm the aforesaid structure. EXAMPLE 2 Preparation of the compound (IV): The compound (VI) (3.5 g; 11.0 mmoles) is reacted with dimethylchlorosilane (1.45 ml; 1.2 g; 13.0 mmoles) under the conditions of Example 1. The reaction product is stripped, and a residue remains consisting of 4.5 g of the compound (IV) (yield practically total). The mass spectrum (M + 412) and IR and ¹Hnmr analysis confirm the aforesaid structure. EXAMPLE 3 Preparation of the compound (XI): by hydrolysis and resinification of the compound (IV). The compound (IV) (2.5 g; 6.1 mmoles) of the preceding Example 2 is dissolved in 15 ml of diethyl ether and hydrolyzed by adding about 5 g of ice, which makes contact therewith. After liquefaction of the ice, the aqueous and organic phases are agitated for 2 hours at ambient temperature. The ether layer is then separated, washed with aqueous sodium bicarbonate and water and dried under vacuum, with the elimination of the diethyl ether. 2.15 g (yield practically total) are thus obtained of a viscous oil constituting the compound (XI). The mass spectrum does not produce the parent ion. IR and ¹Hnmr analysis confirm the aforesaid structure. EXAMPLE 4 Preparation of the compound (V): The compound (VI) (4.1 g; 14.8 mmoles) is reacted with γ-mercaptopropyltrimethoxysilane (3.7 g; 3.5 ml; 19.0 mmoles) and with azobisisobutyronitrile (130 mg) dissolved in 4 ml of toluene, operating in a flask fitted with a magnetic agitator. The solution is agitated for 4 hours at 85°C and is then heated to 110°C and kept at this temperature for one hour. The reaction product thus obtained is a slightly straw-coloured oil which after stripping under reduced pressure cannot be further distilled with a boiler temperature of 240°C. The stripping residue [4.8 g; yield 63% with respect to the compound (VI)] is constituted by the required compound (V). The mass spectrum (M + 514) and IR and ¹Hnmr analysis confirm the aforesaid structure. EXAMPLE 5 Grafting of the compound (IV) onto metal-treated polybutadiene. A low-molecular weight liquid polybutadiene is prepared by polymerising 1,3 butadiene (10.8 g; 200 mmoles) dissolved in cyclohexane (150 ml), with n-butyllithium (hydrocarbon solution; 10 mmoles) in a bottle. The reaction is conducted at 50°C until complete conversion of the butadiene to polybutadiene (average molecular weight 1080) is obtained, with Li termination. The bottle is cooled to ambient temperature, and the compound (IV) (2.1 g; 5 mmoles) is cautiously added to the bottle, which is occasionally shaken. After 2 hours at ambient temperature, 5 ml of methanol are added, the organic phase is washed with aqueous hydrochloric acid and then with water, and the washed organic phase is stripped, to obtain a residue or 12 g of polybutadiene to which the compound (IV) is bonded. The stabilizing action of the antioxidants of the present invention is verified in a laboratory test under conditions suitable for accelerating the thermal degradation of the polypropylene. Polypropylene films containing the stabilizer are subjected to prolonged heat treatment in an air circulation oven at 130°C. The progress of the degradation is verified by the formation of carbonyl compounds, which are demonstrated by IR spectra. Specificaily, the carbonyl index (I CO ) is calculated from the relationship [100 x (intensity of the band at 1720 cm⁻¹ - base intensity at 1850 cm⁻¹)/ film thickness in µm]. The longer the induction time for the formation of the carbonyl compounds, the more effective is the stabilizing effect. The film containing the stabilizer is prepared by dissolving the antioxidant in benzene and mixing the solution thus obtained with powdered polypropylene. - The polypropylene is free from any other additive. The solvent is evaporated under reduced pressure, and the film is then prepared with a thickness of about 100 µm, by pressing at a temperature of 150°C under a pressure of 900 bar for a time of 2 minutes. The film is extracted from the press and rapidly cooled under running water. For the stabilization tests, the compound (XI) is used prepared in accordance with Example 3, in a quantity of 0.7% by weight with respect to the polypropylene. For comparison purposes, the polypropylene is also stabilized with the commercial antioxidants IRGANOX 1010 and IRGANOX 1076 used in quantities such as to add the same quantity of phenolic hydroxyls as the compound (XI). The results of these tests are given in the following Table 1.

Claims: Process for stabilizing an organic polymer, characterized by comprising the steps of: a) Preparing an antioxidant by causing the silyl function of a silylated phenolic compound selected from the group consisting of: wherein: R₁ and R₂ are linear or branched alkyl radicals containing from 1 to 10 carbon atoms; R is a linear or a branched alkylene radical containing from 1 to 10 carbon atoms, or is: wherein R₃, R₄ and R₅ are linear or branched alkylene radicals contaning from 3 to 10 carbon atoms; X is a linear or a branched alkyl radical containing from 1 to 5 carbon atoms; Y is a hydrolyzable grouping selected from the group consisting of: hydrogen, halogen, C₁-C₄ acyloxy, C₁-C₄ alxyloxy, amino, amino-oxy, or silyloxy; m is zero or 1 , and n is 1, 2 or 3, to react at room temperature with water to hydrolyze said silylated phenolic compound at its silyl function, and b) Incorporating in the organic polymer mass to be stabilized an amount of from 0,01% to 5% on a weight basis relative to the organic polymer mass, of said antioxidant. Process for stabilizing an organic polymer, characterized by comprising the steps of: a) Incorporating in the organic polymer mass to be stabilized such an amount of a silylated phenolic compound selected from the group consisting of: where,n R₁ and R₂ are linear or branched alkyl radicals containing from 1 to 10 carbon atoms; R is a linear or a branched alkylene radical containing from 1 to 10 carbon atoms, or is wherein R₃, R₄ and R₅ are linear or branched alkylene radicals containing from 3 to 10 carbon atoms; X is a linear or a branched alkyl radical containing from 1 to 5 carbon atoms; Y is a hydrolyzable grouping selected from the group consisting of: hydrogen, halogen, C₁-C₄ acyloxy, C₁-C₄ alkyloxy, amino, amino-oxy, or silyloxy; m is zero or 1, and n is 1 , 2 or 3, as to have a final concentration of hydrolyzed silylated phenolic compound of from 0,01% to 5% on a weight basis relative to to the organic polymer mass, and b) causing the reaction with water to take place, at room temperature, in the mixture so prepared, to hydrolyze said silylated phenolic compound at its silyl function. Process according to claim 1 or 2, wherein the organic polymer to be stabilized is a polyolefin, a polydiolefin, or a polyether. Process according to claim 1 or 2, wherein said reaction with water is carried out in the presence of a catalyst selected from the group consisting of acidic or basic agents, metal soaps, metal esters, or organic metallic compounds. Process according to claim 4, wherein said catalyst is lead naphthenate, or tin dibutyl laurate.

Inventors: CASSAR, LUIGI, COSTANZI, SILVESTRO, GRECO, ALBERTO, NERI, CARLO
Applicants: ENICHEM SYNTHESIS S.P.A.
Application year: 1985
Main classification: C08K5
IPC codes: C07F7, C09K15, C08L23, C08K5, C08L7, C08L101, C08L21