FI99124C - Method of stabilizing polyhydroxy acids - Google Patents

Description

99124

The invention relates to the stabilization of lactic acid polymers by means of peroxides. In particular, the invention relates to the stabilization of the molecular weight of polylactic acids during processing by adding peroxide compounds to the melt or mixture.

Biodegradable polymers, biopolymers, are a group of materials that are subject to continuous development. Their applications are all kinds of applications from packaging products to hygiene products and also medical applications. Polyhydroxy acids are polymers whose monomers contain both a carboxylic acid group and a hydroxyl group. Examples of such polymers are polylactic acid (polylactide, PLA), poly (hydroxybutyrate), polyglycolide and poly (ε-caprolactone).

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Polylactic acid has been used for many years for medical purposes, for example in the manufacture of surgical sutures, disintegrating bone nails and in the controlled delivery of drugs. In this case, the polymerization is carried out at a relatively low temperature and using long reaction times. The molecular weight of the polymer is. 20 is typically very high and the polymer is purified by dissolving and precipitating prior to processing, resulting in less thermal decomposition. The use of polymers as packaging material and in other bulk products has so far been limited by the high cost of polymers and, in part, their susceptibility to degradation in engineering. For bulk products: · 'it is not economically viable to manufacture and process the polymer in the same way as for medical applications.

Absorbable polyesters (e.g. polylactic acids) can be prepared from polycondensate. thio reactions typical of the preparation of polyesters. However, the highest molecular weights • · · '·! · * Are achieved by ring-opening polymerization of cyclic dimers «· ·; / 30 (lactones, e.g. lactide). Polylactic acid is a thermoplastic polyester that resembles many conventional polymers. The problem, however, is that the polymers degrade during processing and the molecular weight decreases significantly. As a result, the lifespan and, in part, the mechanical properties of the final products are deteriorated. With conventional polymers, these problems can be overcome by using stabilizers.

The aim of the stabilizers is to keep the molecular weight as constant as possible after polymerization 2 99124 and, in particular, also during processing. The change in molecular weight can be monitored, for example, by means of the melt viscosity.

Conventional stabilizers that can be used with aromatic polyesters 5 are not effective on lactic acid polymers. Boric acid, which is used to stabilize poly (hydroxybutyrate) according to German patent application DE4102170, also does not work with lactic acid polymers.

Some experiments on different stabilizers have been published. In Japanese Patent Publication No. JP68008614, polylactic acids are stabilized with lactone compounds such as γ-butyrolactone or α-acetal-γ-butyrolactone. Japanese Patent Publication JP68002949 describes the addition of an isocyanate to improve the heat resistance of a lactic acid polymer. These methods have been tried, but the results have remained poor.

U.S. Pat. No. 3,523,920 discloses crosslinked polylactones, mainly c-caprolactone and 7-valerolactone, in which crosslinking is effected by a reaction between polylactones and free radical generating compounds. Peroxides can also be used for crosslinking.

* * *:: '; 20 According to the present invention, it has now surprisingly been found that polylactic acids can be: ·'; stabilize during processing by adding various peroxides to the mixture. Peroxide addition «*. ! ’· Reduces chain breakage, i.e. slows down the decrease in molecular weight.

The stabilizing effect of the peroxides can also be seen from the melt viscosity value, which decreases considerably more slowly during the treatment with peroxide addition than without the peroxide.

»·« I · • · ·

Many commercially available organic peroxide compounds can be used in the stabilization according to the invention. Peroxide compounds which form acids as a decomposition product are particularly suitable. It is found that this acid radical stabilizes the hydroxyl end group of the polymer. In addition, it has been found that that the peroxides acting as stabilizers are characterized by a short half-life, preferably less than 10 s, but most preferably less than 5 s. Examples of suitable peroxides are dilauroyl peroxide (half-life at 200 ° C 0.057 s), tert-butyl peroxy-diethyl acetate ( 0.452 s), t-butyl peroxy-2-ethylhexanoate (0.278 s), tert-butyl peroxyisobutyrate (0.463 s) and tert-butyl peroxyacetate (3.9 s), tert-butyl peroxybenzoate (4.47 s) 5 and dibenzoyl peroxide (0.742 s) The latter two have proved to work particularly well only a few examples of active peroxides and that other similarly active peroxides are also within the scope of the invention.

The amount of peroxide used is preferably 0.01 to 3% by weight. The most suitable amount depends on the peroxide compound.

The invention is further illustrated by the following examples.

Two different poly (L-lactic acids) (PLLA N30 and PLLA N35) were used in the experiments, with weight average molecular weights Mw of 160,000 and 140,000 g / mol, respectively, and number average molecular weights Mn of about 70,000 and 60,000 g / mol, respectively. Thus, the molecular weight ratio Mw / Mn was about 2.3. The polymers were manufactured by Neste Oy. and were used in all experiments as such. The following were used for stabilization in the experiments: 20 peroxides: •; A. dibenzoyl peroxide (Fluka), half-life 0.742 s B. di-tert-butyl peroxide (AKZO), half-life 20.5 s C. dicumyl peroxide (AKZO), half-life 14.7 s D. tert-butyl peroxybenzoate (AKZO), half-life 4, 47 s:. ·. 25 •: ·. All peroxides were commercial products and were used as such.

Molecular weights were determined on a GPC (Gel Permeation Chromatography) apparatus using a polystyrene standard. DSC (Differential Scanning Calorimetry) measurements were performed on 30 Perkin Elmer DSC 7 and Mettler DSC-30S instruments. The weight of the samples in these ‘·’ · measurements was 10 mg. The heating and cooling rate was 10 ° C / min and the measurements were 4,99124 two cycles. Thermogravimetric analyzes (TGA) were performed on a Mettler TG50 instrument. The sample size in the measurements was 10 mg, the sample was heated to 500 ° C under a nitrogen atmosphere and the heating rate was 10 ° C / min. The gel content of the polymer was determined according to the method of ASTM D 2765 using chloroform as a solvent.

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Example 1. Films

The stabilization of polylactic acid was studied using a Brabender mixer (Plasti-corder®PL 2000), which also made it possible to directly monitor the melt viscosity of the pulp from the 10 torques of the machine. 40 g of polylactide PLLA N30 were mixed with 0-3% with dibenzoyl peroxide (A) for 5 min at 180 ° C. All peroxide was assumed to react and no additional peak was observed in the DSC measurements. The polymer samples were pressed between PET films for 15 s at a pressure of 200 bar and a temperature of 180 ° C. After molding, the films were rapidly cooled to room temperature.

15 The film thickness was 250-300 μπι. The film dissolved in chloroform and no gel could be optically detected.

The amount of peroxide added affected the melt viscosity as can be seen from Figure 1. The melt viscosity increases when peroxide is added from 0 to 0.5% by weight, but the difference is small between the addition of 0.5% by weight and the addition of 3.0% by weight. If peroxide is not used: ·.,; ' the melt viscosity decreases sharply during machining.

The 5 99124 extruder had about 1.5 min.

The molecular weight Mw of the unstabilized control sample of polylactide decreased to at least one-third of its original value during processing. Mw / M „decreased from an initial value of 5 2.3 to about 1.9. According to DSC analysis, the control sample had a glass transition point Te at about 50 ° C, a melting point Tm at about 170 ° C, and a crystallinity Cr of about 50%. According to thermogravimetric analysis, the decomposition temperature (TD) was about 303 ° C. Table 1 shows the molecular weights using different peroxides and Table 2 the DSC / TGA measurement results of the corresponding experiments. From the results in Table 1, it can be seen that the desired stabilizing effect is not achieved with peroxides B and C, but the molecular weight decreases only slightly with peroxides A and D.

• · · ♦ · · * 1 v »·« »« · 6 99124

Table 1. GPC measurements: PLLA N35 + peroxide A-D.

Peroxide M „(g / mol) Mw (g / mol) Mv (g / mol) Mw / Mn 5 comparison 24930 48490 45190 1.95 A 0.1 wt% 44250 98800 90600 2.25 A 0.5 p- % 52650 107000 98050 2.05 A 0.7 wt% 56200 109500 100250 1.95 A 1.0 wt% 56600 115000 105000 2.00 10 B 0.1 wt% 17100 32850 30650 1.90 B 0, 5 wt% 14750 27550 25750 1.85 C 0.1 wt% 16950 32400 30250 1.90 C 0.5 wt% 15450 31750 29500 2.10 D 0.1 wt% 38200 80300 73850 2.10 15 D 0.2 wt% 48450 115500 105000 2.40 D 0.5 wt% 43900 92650 84300 2.05 D 0.7 wt% 55250 127000 114500 2.30 D 1.0 wt% 41600 95950 85300 2 .30 D 3.0 p-% 30200 121000 97900 4.00 •• '20 - • · »• · •« · • · «« · · 1 • <1 · • 1 1 * · h · 7 99124

Table 2. DSC / TGA measurements: PLLA N35 + peroxide A-D.

Peroxide TE (° C) Tm (° C) * Hm (J / g) __ C, (%) TD (° C) __I__II__I II I II aH100% = 93.6 J / g 5 comparison 42.6 49.6 168 , 1,169.8 41.5 46.9 50.1 303 A 0.1 wt% 50.1 50.2 165.5 172.7 32.3 33.5 35.8 315 A 0.5 p- % - 47.9 160.4 171.8 33.9 36.2 38.7 313 A 0.7 wt% - 49.8 155.6 172.7 30.7 37.9 40.5 313 A 1 .0 wt% - 41.0 162.4 168.4 34.7 39.4 42.1 313 10 B 0.1 wt% 33.6 45.1 164.4 166.9 41.4 46, 3 49.5 305 B 0.5 wt% 33.6 46.3 163.2 166.7 41.8 45.6 48.7 303 C 0.1 wt% 34.0 47.8 164.4 168.5 41.7 46.2 49.4 307 C 0.5 wt% 34.3 46.5 163.1 166.6 41.0 43.8 46.8 309 D 0.1 wt% 35 , 2 48.2 165.3 170.2 36.0 37.5 40.1 313 15 D 0.2 wt% 39.5 49.5 165.3 171.8 33.2 33.9 36.2 313 D 0.5 wt% 33.9 46.5 164.3 168.6 34.1 36.1 38.6 315 D 0.7 wt% 30.6 44.5 164.4 170.3 34 , 3 40.7 43.5 317 D 1.0 wt% 32.4 45.6 162.3 165.1 33.8 35.6 38.0 319,. 1 D 3.0 wt% 34.1 47.6 155.3 161.8 28.5 31.2 33.3 321; :: 20 - 1

: I = first heating at 200 ° C (10 ° C / min); II = second heating at 200 ° C

{- * · (10 ° C / min).

• · • * »« · t m · «• · * • ·« · 8 99124

Example 3

The possible crosslinking of polylactide was investigated by determining the gel concentrations of the polymer after peroxide treatment. The polylactide was mixed according to Example 1 5 using different amounts of dibenzoyl peroxide. The gel content was determined according to ASTM D 2765, but using chloroform instead of xylene. No gel formation was observed at all, when the amount of peroxide was less than 0.25% by weight and still at a peroxide content of 3% by weight, gel formation was very low. When the results were further examined with a Malvem 2600c particle size analyzer, it was found 10 that the size of the particles formed with small amounts of peroxide was very small. The proportion of large particles increases as the amount of peroxide increases. The results are summarized in Table 3.

Table 3 peroxide gel-particle-particle-particle-particle weight% pit. size size size size% 1.9-10 / xm 10-50 / xm 50-100 / xm 100-190 / xm '': '%%%% 0 0 0 oooo, io oo ooo • · · • · * _ __ 20 0.25 0.9 86.0 13.0 0.2 0.8 o, 5o i, 8 5o, o 31.0 3.7 15.5 3.00 5.1 0.2 1.3 5.1 93.4 • ««

• · t _ I ___________1_ 1 1 I

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Claims (6)

99124 A process for stabilizing lactic acid polymers, characterized in that 0.05 to 3% by weight of an organic peroxide compound which decomposes to form one or more acid radicals is added to the polymer 5 during the melt-processing step. Process according to Claim 1, characterized in that the lactic acid polymer is poly-L-lactic acid (poly-L-lactide). 10 Process according to Claim 1 or 2, characterized in that the amount of peroxide to be added is from 0.05 to 0.8% by weight. Process according to Claims 1 to 3, characterized in that the half-life of the peroxide used at a temperature of 200 ° C is less than 10 s. Process according to Claims 1 to 4, characterized in that the peroxide is tert-butyl peroxybenzoate or dibenzoyl peroxide. Products made from a stabilized polyhydroxy acid prepared by a process according to any one of the preceding claims. * 99124