Improved stabiliser for Biodegradable polyester processing

Abstract: This invention relates to a stabilised biodegradable polyester composition, comprising a biodegradable polyester of lactic acid and/or glycolic acid, and a stabiliser which comprises (a) a phosphite- or phosphate-containing compound and/or a hindered phenolic compound, and (b) an acrylate.

Description: Biodegradable polymers are finding increased use as an environmentally friendly option to non-degradable polymers such as PP or PS or PET in many applications. In the class of biodegradable polyesters, poly(lactic acid) or polylactide polymers, known hereafter as PLA, are of commercial interest. Poly(glycolic acid) and lactic acid/glycolic acid copolymers are also useful. However, PLA suffers from stability problems at the high temperatures used during production and processing. This can result in deficiencies of PLA polymer and products such as discolouration or molecular weight (mw) reduction or lowering of crystallinity levels. Cargill Inc (WO 94/07949) have made an attempt to overcome the high temperature depolymerisation problems associated with PLA. This depolymerisation is cited in terms of lactide monomer regeneration and mw reduction. The use of antioxidants or water scavengers or mixtures of antioxidants and water scavengers for this purpose is disclosed. The range of antioxidants includes alkyl and aryl phosphites, phosphonites, hindered phenolic compounds, aromatic amines, thioethers, quinones and mixtures of these. However. Cargill only demonstrate the effects of each of these stabiliser types singly through the Examples quoted in WO 94/07949 and do not evaluate mixed antioxidant packages. The stabiliser addition could be either before or after PLA polymer production. Cargill failed to prevent discolouration of the PLA polymer through the use of antioxidants unless conversion of lactide to polymer was restricted to <85% and mw<40,000. This low mw and level of conversion to PLA is not sufficient to develop the full properties of PLA polymer. This requires the PLA mw to be >70,000 according to Cargill. In commercial grade PLA, where good and consistent levels of properties are necessary, Cargill failed to demonstrate an effective anti-yellowing stabiliser. The stabilisers used in WO 94/07949 generally lead to deterioration in the level of crystallinity in PLA polymers. This results in deleterious consequences for a semi-crystalline polymer such as PLA. We have demonstrated that these problems with crystallinity and yellowing exist for the single antioxidants cited by Cargill (see Comparative Examples 1-27). This problem with molecular weight reduction is also highlighted by Neste OY (WO 95/18169). In this document, it is noted that conventional stabilisers which can be used with aromatic polyesters are not effective with lactic acid polymers. Neste OY made use of peroxides to limit PLA mw reduction during processing by extrusion. Peroxides are limited to use after PLA production and cannot be added to the PLA production process (see also Comparative Examples 26-27). Shell (GB 1,161,932) disclose the use of amine-based stabilisers for reducing the thermo-oxidative decomposition of aliphatic polyesters, especially cyclic lactone derived polymers. The stabiliser can be used singly, or in combination with a wide range of other species. The stabiliser(s) can be added either to the lactone precursor or the polymer product as alternative routes for incorporation of the stabiliser(s) into the polymer. However, although the intrinsic viscosity of the polymer may be maintained by addition of stabiliser, it is clear that discolouration of PLA cannot be prevented by amine-based stabilisers (see Comparative Examples 21-23). Thus, there exists a need for an improved stabiliser for use in PLA which prevents discolouration and maintains mw and crystalline melt temperature (Tm) of commercial-type PLA polymer. We have found that a mixture of stabilisers is required for this purpose. EP 0 677 561 and EP 0 587 069 disclose a degradable resin composition suitable for use as an adhesive tape containing 100 parts by weight of a lactic acid base polymer and 0.001-5 parts by weight of one or more additives. The additives include UV absorbers and light stabilisers as well as plasticizers, antioxidants, heat stabilisers, lubricants, pigments and other additives. We have provided a stabilising package for aliphatic polyesters, especially PLA polymers. The stabiliser package serves several purposes; prevents the discolouration or yellowing of the polymer during production and moulded articles manufactured from it prevents any reduction in Tm (often observed with standard stabilisers) limits the reduction in crystallinity of the polymer (observed with standard stabilisers). The present invention provides a stabilised biodegradable polyester composition, comprising a biodegradable polyester of lactic acid and/or glycolic acid, and a stabiliser which comprises a phosphite- or phosphate-containing compound, a hindered phenolic compound, and an acrylate. The polyester is preferably obtained by polymerisation of lactide and/or glycolide. The phosphite- or phosphate-containing compound is preferably of the general formula (I), (II) or (III): P(O) x (OAr) 3 (ArO) 2 P―R 2 ―P(OAr) 2 in which x is 0 or 1; Ar each independently is an optionally substituted aromatic group; and R 1 and R 2 are aliphatic or aromatic groups. Ar is preferably phenyl or naphthyl, and is preferably substituted by C 1 -C 10 alkyl or C 1 -C 10 alkoxy. R 1 is preferably a C 4 -C 10 aliphatic hydrocarbon group (most preferably C(CH 2 -) 4 ) or an aromatic group of the formula: where L is a direct bond or a linking group such as S, SO, SO 2 , CO, O or C 1 -C 6 alkylene. R 2 is preferably a C 1 -C 10 aliphatic hydrocarbon group (most preferably an alkylene, alkyleneoxy or oxyalkyleneoxy group, where the alkylene moiety is optionally interrupted by one or more O atoms) or an aromatic group of the formula: where L is as defined above. The hindered phenolic compound is preferably of the general formula (IV): where M each independently is an aliphatic group; Y is an aliphatic group; m is an integer from 1 to 4; and Z is an aliphatic or aromatic group or H if m is 1. M is preferably C 1 -C 10 alkyl or C 1 -C 10 alkoxy. Y is preferably C 1 -C 10 alkylene or C 1 -C 10 alkylenecarboxy, where the alkylene moiety is optionally interrupted by one or more O atoms. Z is preferably a C 1 -C 20 aliphatic hydrocarbon group, or a 6-membered carbocyclic or heterocyclic aromatic ring. The acrylate is preferably of the general formula (V): where Ar each independently is an optionally substituted aromatic group; R 3 is an aliphatic group; and X is an electron-withdrawing group. Ar is preferably phenyl or naphthyl, and may be substituted by C 1 -C 10 alkyl or C 1 -C 10 alkoxy. R 3 is preferably C 1 -C 10 alkyl optionally interrupted by one or more O atoms. X is preferably cyano or halogen. The total content of stabiliser is preferably up to 2 wt%, more preferably from 0.1 to 0.5 wt%, of the weight of biodegradable polyester. The invention also relates to use of a stabiliser composition comprising a phosphite-containing compound, a hindered phenolic compound, and an acrylate, for stabilising a biodegradable polyester of lactic acid and/or glycolic acid. The invention further provides a process for preparing a stabilised polyester composition, which comprises polymerisation of lactide and/or glycolide in the presence of a stabiliser comprising a phosphite-containing compound, a hindered phenolic compound, and an acrylate. The invention additionally provides a shaped product prepared from a stabilised biodegradable polyester composition as defined above. The stabiliser package is a combination of different chemical types taken from the list; phosphite or phosphate, hindered phenolic, acrylate. Multifunctional and closely related chemical types such as phosphonite and fluorophosphonite perform in a similar manner as simple phosphite, phenolic and acrylate compounds. Materials which are found to destroy the desired action and outcome of using our stabiliser package include aromatic amines, peroxides, organo-metallic complexes, thioethers or other organic sulphur compounds and are specifically excluded at addition levels of >0.5 wt%. Any member of the phosphite or phosphate and phenolic and acrylate classes may be used, but the preferred materials are alkyltriphenyl phosphites, alkyldiphenylphosphates, bis[alkylphenyl]pentaerythritol diphosphites (phosphite or phosphate class), alkyl-[di- t -butyl hydroxyphenyl]propionates, butylated hydroxytoluenes (hindered phenolic class), alkyl esters of 2-cyano-3,3-diphenylacrylic acid (acrylate class). The combination of stabilisers can be present when the lactide monomer is converted to PLA without interfering with the polymerisation catalyst. The PLA polymer produced is of high mw (>150,000), high Tm (>170°C), high crystallinity levels (>40%) and very low colour (essentially colourless). This combination of PLA properties has never before been achieved together. The presence of the stabilisers in the "as produced" PLA also protects the PLA in any further fabrication steps and eliminates the need to compound the PLA further before moulding to final product form. The following Examples illustrate the invention. Comparative Examples 1-27. Vial polymerisations of pure L-lactide were carried out at 180°C for 20 hours. The catalyst used was tin octoate at a concentration level of 50ppm with respect to the lactide monomer. All stabilisers were added at 0.25 wt%. Analysis of mw, Tm, crystallinity level (%cr) and yellowness index (YI) of PLA product was carried out by standard methods and the results are given in Table 1. The method of analysis were: mw - GPC (gel permeation chromatography) Tm, crystallinity - DSC (differential scanning calorimetry) YI - spectrometry. Product designations in Table 1 are tradenames or registered trademarks. Example Stabiliser PLA mw PLA Tm %cr of PLA PLA YI 1 None (control sample) 278,100 177 49 15 2 None (control sample) 256,900 178 47 15 3 None (control sample) 264,000 177 49 15 Phosphorus-based. 4 Irgafos 168 279,300 173 27 10 5 Santicizer 148 265,000 98 8 11 6 Triphenyl phosphite 292,300 159 21 13 7 Alkanox P24 312,500 175 26 13 8 Alkanox 240 235,700 169 32 12 9 Lowinox TNPP 210,200 167 32 12 10 Ethanox 398 66,800 144 22 13 Hindered phenolics. 11 Butylated hydroxytoluene 228,600 176 40 17 12 Irganox 1010 154,500 175 44 15 13 Irganox 1076 190,900 175 38 16 14 Alvinox FB 105,400 177 48 17 15 Alvinox 100 155,400 178 50 15 16 Ronotec 201 112,900 157 28 14 17 Uvasorb 20H 132,100 170 29 14 18 Uvasorb MET 155,600 173 40 16 Acrylates. 19 Uvinol 3035 185,500 179 45 12 20 Uvinol 3039 118,800 173 35 14 Amines. 21 Anox ODS 189,700 177 43 31 22 Chimassorb 944 No conversion to PLA - 23 Tinuvin 770 No conversion to PLA - Sulphur-based. 24 Lankromark DLTDP 157,900 174 39 17 25 Robac ZDBC 65,700 163 31 45 Peroxides. 26 Dibenzoyl peroxide 67,700 169 18 14 27 Cumylperoxide 59,200 147 10 17 Comparative Examples 28-45 and Examples 46-49. Vial polymerisations of pure L-lactide were carried out at 180° C for 20 hours. The catalyst used was tin octoate at a concentration level of 50ppm with respect to the lactide monomer. Stabilisers used and addition levels (in wt%) are given in Tables 2-4. Analysis of mw, Tm, crystallinity level (%cr) and yellowness index (YI) of PLA product was carried out by standard methods and the results are given in Table 5. Product designations in Tables 2-4 are tradenames or registered trademarks. Example 28 29 30 31 32 33 34 35 Hindered phenolics. Butylated hydroxytoluene 0.2 0.2 0.2 0.2 0.2 - - - Irganox 1076 - - - - - 0.2 0.2 0.2 Phosphorus-based. Santicizer 148 0.1 - - - - 0.1 - - Triphenyl phosphite - 0.1 - - - - 0.1 - Irgafos 168 - - 0.1 - - - - 0.1 Alkanox 240 - - - 0.1 - - - - Alkanox P24 - - - - 0.1 - - - Example 36 37 38 39 40 41 42 43 44 45 Acrylates. Uvinol 3035 0.2 0.2 0.2 0.2 0.2 0.2 - - - 0.2 Uvinol 3039 - - - - - - 0.2 0.2 0.2 - Hindered phenolics. Irganox 1076 - - - - - - - - - 0.1 Phosphorus-based. Ethanox 398 0.1 - - - - - - - - - Santicizer 148 - 0.1 - - - - 0.1 - - - Triphenyl - - 0.1 - - - - 0.1 - - phosphite - - - 0.1 - - - - 0.1 - Irgafos 168 - - - - 0.1 - - - - - Alkanox 240 - - - - - 0.1 - - - - Alkanox P24 Example 46 47 48 49 Acrylates. Uvinol 3035 0.15 0.15 0.15 0.15 Hindered phenolics. Irganox 1076 0.1 0.1 0.1 0.1 Phosphorus-based. Santicizer 148 0.05 - - - Irgafos 168 - 0.05 - - Alkanox 240 - - 0.05 - Alkanox P24 - - - 0.05 Example PLA mw PLA Tm %cr of PLA PLA YI 28 (Comp) 366,300 150 19 11 29 (Comp) 363,400 144 18 12 30 (Comp) 283,900 172 2 9 31 (Comp) 272,400 170 6 9 32 (Comp) 202,900 170 30 13 33 (Comp) 341,100 169 14 10 34 (Comp) 311,100 147 15 9 35 (Comp) 225,000 174 39 13 36 (Comp) 186,500 148 24 5 37 (Comp) 327,800 169 33 9 38 (Comp) 276,400 162 19 8 39 (Comp) 251,500 151 17 7 40 (Comp) 244,100 170 28 7 41 (Comp) 243,000 156 12 8 42 (Comp) 363,400 153 18 6 43 (Comp) 304,000 160 19 5 44 (Comp) 177,900 164 28 7 45 (Comp) 247,800 172 38 10 46 (Inv) 176,600 163 31 3 47 (Inv) 176,200 177 41 2 48 (Inv) 169,300 176 47 3 49 (Inv) 99,400 175 47 2 The stabilisers used in the Examples are manufactured or supplied by the following: Ciba Irgafos 168 Irganox 1010 Irganox 1076 Chimassorb 944 Tinuvin 770 3v Alkanox P24 Alkanox 240 Alvinox FB Alvinox 100 Uvasorb 20H Uvasorb MET Monsanto - Santicizer 148 Aldrich - Triphenylphosphite Lowe - Lowinox TNPP Ethyl - Ethanox 398 ICI - Butylated hydroxytoluene Roche - Ronotec 201 BASF - Uvinol 3035 - Uvinol 3039 Great Lakes Chemicals - Anox ODS Akros - Lankromark DL TDP Robinson Brothers - Robac ZDBC Akzo - Dibenzoyl peroxide - Cumyl peroxide Comp = Comparative Example Inv = Invention

Claims: A stabilised biodegradable polyester composition, comprising a biodegradable polyester of lactic acid and/or glycolic acid, and a stabiliser which comprises a phosphite- or phosphate-containing compound, a hindered phenolic compound, and an acrylate. A composition according to claim 1, in which the polyester is obtained by polymerisation of lactide and/or glycolide. A composition according to claims 1 or 2, in which the phosphite- or phosphate-containing compound is of the general formula (I), (II) or (III): P(O) x (OAr) 3 (ArO) 2 P―R 2 ―P(OAr) 2 in which x is 0 or 1; Ar each independently is an optionally substituted aromatic group; and R 1 and R 2 are aliphatic or aromatic groups. A composition according to any of claims 1 to 3, in which the hindered phenolic compound is of the general formula (IV): where M each independently is an aliphatic group; Y is an aliphatic group; m is an integer from 1 to 4; and Z is an aliphatic or aromatic group or H if m is 1. A composition according to any of claims 1 to 4, in which the acrylate is of the general formula (V): where Ar each independently is an optionally substituted aromatic group; R 3 is an aliphatic group; and X is an electron-withdrawing group. A composition according to any of claims 1 to 5, in which the total content of stabiliser is up to 2 wt%, preferably from 0.1 to 0.5 wt%, of the weight of biodegradable polyester. Use of a stabiliser composition comprising a phosphite- or phosphate-containing compound, a hindered phenolic compound, and an acrylate, for stabilising a biodegradable polyester of lactic acid and/or glycolic acid. A process for preparing a stabilised polyester composition, which comprises polymerisation of lactide and/or glycolide in the presence of a stabiliser comprising a phosphite- or phosphate-containing compound, a hindered phenolic compound, and an acrylate. A shaped product prepared from a stabilised biodegradable polyester composition according to any of claims 1 to 6.

Inventors: QURESHI NASEER MOHAMMED, WOODFINE BARRY, QURESHI, NASEER MOHAMMED, WOODFINE, BARRY, Woodfine, Barry, c/o Kobe Steel Europe Ltd.
Applicants: KOBE STEEL LTD, KABUSHIKI KAISHA KOBE SEIKO SHO ALSO KNOWN AS KOBE STEEL LTD.
Application year: 1999
Main classification: C08K5
IPC codes: C08L101, C08K5, C08L67