THERMOFORMING OF PLA-BASED ARTICLES

Abstract

A method for producing a polylactic acid (PLA) shaped article by thermoforming and thermoformed PLA articles. The method for producing a shaped article includes: heating a sheet of crystallizable polylactic acid (PLA)-based resin having a ratio of cold crystallization over total melting enthalpy (ΔHcc/ΔHm) greater than 0.70 as determined by differential scanning calorimetry (DSC), wherein heating includes a heating step the sheet is heated from a surface temperature of at most 80° C. to a surface temperature of at least 90° C. to at most 150° C. at heating rate of 5° C. to 25° C. per second, to provide heated sheet having a ratio of cold crystallization over total melting enthalpy (ΔHcc/ΔHm) greater than 0.5 as determined by DSC; and immediately after heating, forming heated sheet to provide a shaped article by means of a mold having a temperature of at least 70° C. and at most 120° C.

Claims

1. A method for producing a shaped article, comprising: heating a sheet of crystallizable polylactic acid (PLA)-based resin having a ratio of cold crystallization over total melting enthalpy (ΔHcc/ΔHm) greater than 0.70 as determined by differential scanning calorimetry (DSC), wherein heating comprises a heating step wherein the sheet is heated from a surface temperature of at most 80° C. to a surface temperature of at least 90° C. to at most 150° C. at a heating rate of 5° C. to 25° C. per second, to provide a heated sheet having a ratio of cold crystallization over total melting enthalpy (ΔHcc/ΔHm) greater than 0.5 as determined by DSC; and immediately after heating forming the heated sheet to provide a shaped article by means of a mold, wherein the mold has a temperature of at least 70° C. and at most 120° C.

2. The method according to claim 1, wherein the crystallizable PLA-based resin comprises from 50 to 100 wt. % of a poly L-lactic acid (PLLA) polymer having an optical purity of at least 95%, based on the weight of L-lactoyl units over the total weight of lactoyl units in the PLLA; polymer; or from 50 to 100 wt. % of a poly D-lactic acid (PDLA) polymer having an optical purity of at least 95% based on the weight of D-lactoyl units over the total weight of lactoyl units in the PDLA polymer.

3. The method according to claim 1 wherein heating the sheet of crystallizable PLA-based resin comprises a preheating step prior to the heating step wherein the sheet is kept at a temperature from at least 30° C. to at most 80° C.

4. The method according to claim 1 wherein in the heating step the sheet is heated to a surface temperature of at least 100° C. to at most 145° C.

5. The method according to claim 1, wherein in the heating step the heating rate is of 6 to 25° C./s.

6. The method according to claim 1, wherein the heated sheet has a ratio of cold crystallization over total melting enthalpy (ΔHcc/ΔHm) greater than 0.6, or greater than 0.7, or greater than 0.75 as determined by DSC.

7. The method according to claim 1, wherein the heated sheet has a melting enthalpy (ΔHm,0) of less than 25 J/gram as determined by DSC.

8. The method according to claim 1, wherein the crystallizable PLA-based resin comprises a nucleation package and wherein if the crystallizable PLA-based resin comprises PLLA polymer, the nucleation package comprises PDLA as nucleating agent; or if the crystallizable PLA-based resin comprises PDLA polymer, the nucleation package comprises PLLA as nucleating agent, the PLLA or PDLA as nucleating agent being present in an amount of 0.1-10 wt. % based on the total amount of PLA present in the crystallizable PLA-based resin.

9. The method according to claim 8, wherein the nucleation package comprises minerals such as talc and/or kaolin as nucleating agent based on the amount of PLA present in the crystallizable PLA-based resin.

10. The method according to claim 1, wherein the PLA-based resin comprises a filler and/or an impact modifier.

11. The method according to claim 1, wherein the mold has a temperature of greater than 75° C. and at most 120° C.

12. The method according to claim 1, wherein the shaped article has a ratio of cold crystallization over total melting enthalpy (ΔHcc/ΔHm) of less than 0.5 as determined by DSC and has a melting enthalpy (ΔHm,0) greater than 15 J/gram as determined by DSC.

13. The method according to claim 1, wherein the shaped article is thermo-resistant and displays a shrinking of less than 2% when immersed in water at a temperature from 60 to 100° C. for 5 minutes.

14. A shaped article of a PLA-based resin obtainable by the method of claim 1.

15. The shaped article of claim 14 selected from a coffee cup and a food tray.

Description

EXAMPLES

Example 1

[0073] Example 1 shows the forming behavior of a 60 mm wide and 60 mm deep cup based on different PLA and nucleated PLA formulations, heated with different heat rates.

[0074] The materials tested are listed in table 1

Preparation PLA Formulations (Melt Compounding)

[0075] Samples 2-5 were prepared by melt compounding PLLA (Luminy® PLLA L175 from Total Corbion PLA) with an absolute weight average molecular weight between 110-130 kg/mol together with the additives described in Table 1. Prior compounding the PLLA and PDLA were dried for a minimum of 4 hours at 85° C. in a desiccant hot air dryer with a dew point lower than −40° C.

The compounding was performed on a Brabender 25 mm co-rotating twin screw extruder having a temperature profile of 170/180/190/200/200/200/200/200/200/190° C., hopper to die. The resulting strand was cooled down in a water bath and granulated.

Preparation of Extruded PLA Sheet

[0076] Commercial PLLA (Luminy® PLLA L175 from Total Corbion PLA) with an absolute weight average molecular weight between 110-130 kg/mol (sample 1) and samples 2-5 compounded as described above, were transformed into a 0.5 mm sheet on a Battenfeld single screw extruder having a temperature profile of 95/205/195/195/195/195° C., hopper to final zone. Prior sheet extrusion, the PLA formulations of samples 1-5 were dried for a minimum of 4 hours at 85° C. in a desiccant hot air dryer with a dew point lower than −40° C. The die temperature was set at 190-200° C. and the extruded sheet was cooled down on horizontal chill rolls operating at 25° C.

TABLE-US-00001 TABLE 1 PLA formulations and DSC properties (ΔH.sub.m,0 and ΔH.sub.cc/ΔH.sub.m) of the resulting crystallizable PLA sheets PLA formula- DSC properties extruded sheet Sample tion* (wt. %) ΔH.sub.cc ΔH.sub.m ΔH.sub.m,0 ΔH.sub.cc/ΔH.sub.m 1 PLLA (100) 49.7 55.6 5.9 0.89 2 PLLA (95) + 49.5 51.9 2.4 0.95 PDLA (5) 3 PLLA (94) + 48.4 52.0 3.6 0.93 PDLA (5) + talc (1) 4 PLLA (84) + 40.9 51.0 10.1 0.80 talc (1) + kaolin (15) 5 PLLA (79) + 34.1 42.2 8.1 0.81 PDLA (5) + talc (1) + kaolin (15) *In all formulations PLLA was Luminy .sup.® PLLA L175 and PDLA was Luminy .sup.® PDLA D070

[0077] The ratio of cold crystallization enthalpy over the melting enthalpy (ΔH.sub.cc/ΔH.sub.m) in the extruded PLA sheet was measured by DSC using a T.A. Instruments Q2000 DSC Apparatus equipped with a RCS (90) cooler, calibrated with certified Indium, running with a Nitrogen flow of 50 ml/minute.

[0078] A sample of the PLA-based sheet of 5.00±2.00 mg was weighted into a Tzero pan from T.A. Instruments and closed with an Tzero Hermetic lid from T.A, Instruments. The closed pan was placed in the oven of the DSC apparatus together with an empty reference pan. The pan with sample was equilibrated at 25° C. for 2 minutes followed by a heating step to 260° C. at a heat rate of 10° C./min.

[0079] The cold crystallization peak and melting peak were integrated using the TA Universal Analysis software, and the ratio of cold crystallization over total melting enthalpy (ΔH.sub.cc/ΔH.sub.m) and the melting enthalpy of the original sample (ΔH.sub.m,0) were calculated as described above.

The results for each extruded sheet are listed in Table 1.

Thermoforming Cups

[0080] For each of the extruded PLA sheets (samples 1-5) thermoforming (including both the heating and forming steps of a method as described herein) was performed on a 1989 Kiefel KD20/25 thermoformer. The thermoformer was equipped with 12 individually controllable top bottom ceramic heating elements, plug assist and a female mold of which the temperature was controlled by a water based temperature control unit operating to a maximum of 95° C.

[0081] The extruded PLA sheet was clamped in a frame and transferred to the heating station of the thermoformer. The sheet was heated from room temperature (25° C.) at different heat rates. The temperature of the sheet was measured immediately after leaving the heating station with a Testo 635-1 contact thermometer. The heating rates and the surface temperature of the heated sheets are listed in Table 2 and Table 4.

The heated sheet was either: [0082] 1. Thermoformed into a cup with a top diameter of 60 mm and height of 60 mm. In order to do so, the heated sheet was brought into the forming station in which the sheet was formed by pre-stretching the sheet by a plug, plug assisted thermoforming, followed by applying positive pressure of 5.5 bar forcing the sheet to shape into the mold. The mold temperature was set to 25° C. The thermoformed article was kept in the mold for 6 seconds before releasing manually. [0083] 2. Quenched immediately in cold water to be able to determine the ratio of cold crystallization enthalpy over the melting enthalpy of the heated PLA sheet.
The following analysis were performed: [0084] Differential Scanning calorimetry (DSC) measurement as detailed above for the PLA extruded sheet was performed on a sample of the heated sheet to obtain the melting enthalpy (ΔH.sub.m,0) and the ratio of cold crystallization enthalpy over the total melting enthalpy (ΔH.sub.cc/ΔH.sub.m) thereof (Table 2). The different PLA extruded sheets (samples 1-5) were heated from room temperature to the temperature indicated in Table 2 using different profiles as detailed. [0085] Visual rating of the thermoformed product. The thermoformed products obtained were rated according the rating listed in Table 3 the results are listed in Table 4, which shows the parameters of the thermoforming process and the resulting formability of the cups (rated according Table 3) for PLA extruded sheets of different formulations (samples 1-5 above).

[0086] The areas in grey in Tables 2 and 4 indicate materials which did not form well, independent of the cause.

TABLE-US-00002 TABLE 2 DSC properties of sheets of different PLA formulations heated using different heating profiles Heated sheet Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 T.sub.heated sheet heat rate ΔH.sub.cc/ ΔH.sub.cc/ ΔH.sub.cc/ ΔH.sub.cc/ ΔH.sub.cc/ (° C.) (° C./s) ΔH.sub.cc ΔH.sub.m,0 ΔH.sub.m* ΔH.sub.cc ΔH.sub.m,0 ΔH.sub.m* ΔH.sub.cc ΔH.sub.m,0 ΔH.sub.m* ΔH.sub.cc ΔH.sub.m,0 ΔH.sub.m* ΔH.sub.cc ΔH.sub.m,0 ΔH.sub.m* 68 4.1 40.3 4.5 0.90 39.3 2.0 0.95 38.0 3.6 0.91 30.9 10.7 0.74 30.7 10.9 0.74 84 2.9 37.0 8.0 0.82 38.3 4.6 0.89 26.7 15.4 0.63 12.4 27.2 0.31 0 35.3 0.00 99 2.1 36.6 8.8 0.81 33.5 8.6 0.80 10.1 31.2 0.24 5.7 37 0.13 0 41.4 0.00 115 1.5 37.6 6.4 0.85 29.4 13.3 0.69 7.6 37.5 0.17 3.0 43.1 0.07 0 42.2 0.00 68 8.3 41.6 4.6 0.90 40.2 0.4 0.99 39.3 2.5 0.94 32.2 10.3 0.76 33.4 8.6 0.80 84 6.3 40.1 6.4 0.86 37.5 4.2 0.90 35.2 6.3 0.85 20.0 20.6 0.49 17.3 22.2 0.44 99 5.1 41.0 4.7 0.90 36.6 6.1 0.86 28.5 16.1 0.64 12.0 30.1 0.29 6.6 34.7 0.16 115 4.3 38.5 8.1 0.83 34.3 7.1 0.83 27 15.7 0.63 11.1 34 0.25 7.5 37.5 0.17 146 4.1 43.0 4 0.91 35.1 8.7 0.80 32.2 9.7 0.77 32.1 9 0.78 22.2 19.4 0.53 68 16.5 39.4 4.9 0.89 41.9 −0.6 1.01 — — — 32.0 10.1 0.76 34.2 7.1 0.83 84 13.7 38.9 5.5 0.88 41.8 0 1.00 36.2 5.5 0.87 27.6 15.3 0.64 29.3 12.3 0.70 99 10.8 39.1 7.8 0.83 39.5 2.9 0.93 33.3 11.2 0.75 17.0 25.9 0.40 12.6 28 0.31 115 9.4 38.5 8.8 0.81 38.3 4.8 0.89 39.6 12.5 0.76 21.5 20.1 0.52 19.5 23.6 0.45 146 9.0 38.9 9.3 0.81 38.6 4.2 0.90 35.1 5.6 0.86 30.9 11.2 0.73 31.7 10.7 0.75 *Calculated from ΔH.sub.cc and ΔH.sub.m (which is derived from the values of the table as follows: ΔH.sub.m = ΔH.sub.cc + ΔH.sub.m,0)

TABLE-US-00003 TABLE 3 Visual rating of thermoformed cups Formability rating Description 1 The shaped article formed well, had excellent part detail and was easily removed from the mold 2 The shaped article formed well, sheet filled the mold the way to the bottom, but the part details were not sharp. 3 Shaped article contained stress whitening (due to the forming step being too cold). 4 The shaped article did not form well, sheet did not reach the bottom of the mold, holes were present in the sheet.

TABLE-US-00004 TABLE 4 formability of PLA sheets of different composition (materials 1-5) using different heat profiles heating Thermoformed material T.sub.heated sheet rate 1 2 3 4 5 (° C.) (° C./s) formability, 60 mm cup 68 4.1 3 3 3 2 4 84 2.9 2 2 2 2 4 99 2.1 1 1 2 4 4 115 1.5 1 1 4 4 4 68 8.3 3 3 4 2 4 84 6.3 2 2 2 2 2 99 5.1 1 1 2 2 4 115 4.3 1 1 2 2 4 146 4.1 1 1 1 1 1 68 16.5 4 4 4 4 4 84 13.7 2 3 2 2 2 99 10.8 1 1 2 2 2 115 9.4 1 1 1 1 2 146 9.0 1 1 1 1 1

[0087] It was found that for a good formability (e.g. a rating of 1) a combination of conditions were fulfilled. In particular: [0088] 1. The minimum temperature of the heated sheet was higher than 84° C., e.g. at least 90° C. [0089] 2. The addition of nucleating agents and fillers (as in samples 3-5) reduced the forming window (i.e. range of temperatures of the heated sheet resulting in good forming properties). [0090] 3. The ratio of cold crystallization enthalpy and melting enthalpy of the heated sheet was above 0.5.

[0091] It is noted all samples formed well when the temperature was near the melting point of the PLA sheets (regardless of the heating rate used to heat the sheet prior to thermoforming). However, when the PLA sheets are heated to a temperature near the melting point of PLA, they become very soft. This makes it difficult to (also) achieve a good material distribution in the product.

Example 2

[0092] Example 2 shows the thermoforming process of an extruded PLA sheet based on the formulation of sample 5 (as listed in Table 1).

Preparation of PLA-Based Resin Crystallizable Sheet (Sheet Extrusion)

[0093] Sheet extrusion was performed on a 100 mm single screw extruder with a screen pack, melt pump and flat die. the extruded sheet was cooled down on horizontal chill rolls operating at 25° C. and wound onto a core. The resulting sheet had a width of 450 mm and a thickness of 0.69 mm.

[0094] The melting enthalpy and the ratio of cold crystallization enthalpy over total melting enthalpy (ΔH.sub.cc/ΔH.sub.m) of the extruded sheet (before heating) was measured with DSC and is listed in Table 6.

[0095] Thermoforming

[0096] The resulting sheet was thermoformed on a Gabler M98 thermoforming machine with a single cavity prototype tool into cups of a diameter of 70.6 mm and a height of 90.6 mm.

[0097] The PLA roll stock was unwind and pre-heated with a roll heater up to 40° C. After the pre-heating station the sheet was transferred into the 3.6 meter long oven.

[0098] The temperature profiles used in this set of experiments are listed in Table 5.

[0099] After leaving the oven the sheet was formed and cut in place in an electrically heated mold using plug assist and positive pressure. The DSC properties of the heated sheet were not measured as the forming process was automated and was not possible to obtain a sample of the heated sheet after exiting the oven and prior to entering the heated mould.

[0100] The mold temperature was varied and can be found in Table 5.

[0101] Thermoformed cups were immediately visually checked for form detail and thickness distribution. The shrinkage was tested by filling the thermoformed cup immediately with water of 60, 70, 80, 90 and 100° C., obtained by a variable temperature kettle. The shrinkage of the thermoformed cup was calculated using the following equation:

[00003]$\mathrm{Cup}\mathrm{shrinkage}\left(\%\right)=\frac{\mathrm{Cup}\mathrm{height}\mathrm{after}\mathrm{hot}\mathrm{fill}\left(\mathrm{mm}\right)}{\mathrm{Original}\mathrm{cup}\mathrm{height}\left(\mathrm{mm}\right)}*100\%$

[0102] The results of the forming detail upon visual inspection and the shrinkage of the thermoformed products (cups) are shown in Table 7.

TABLE-US-00005 TABLE 5 Thermoforming conditions .sup.Theated sheet Heating rate Sample (° C.) (° C./s) .sup.Tmold (° C.) 5-A 85 1.3 80 5-B 95 1.5 80 5-C 100 1.6 80 5-D 105 1.7 80 5-E 93 3.6 60 5-F 100 7.1 95 5-G 100 7.1 85 5-H 100 7.1 75 5-I 100 7.1 30 5-J 130 11.3 75 5-K 130 11.3 85 5-L 140 9.0 85 5-M 120 14.6 85

TABLE-US-00006 TABLE 6 DSC properties Extruded sheet Cup Bottom Cup wall ΔH.sub.cc/ ΔH.sub.cc/ ΔH.sub.cc/ Sample ΔH.sub.cc ΔH.sub.m,0 ΔH.sub.m* ΔH.sub.cc ΔH.sub.m,0 ΔH.sub.m* ΔH.sub.cc ΔH.sub.m,0 ΔH.sub.m 5-A 29.8 0.7 0.98 31.4 0.5 0.99 5-B 29.8 0.7 0.98 20.6 12.3 0.62 5-C 29.8 0.7 0.98 19.9 12.3 0.62 5-D 29.8 0.7 0.98 14.5 15.7 0.48 5-E 34.0 5.3 0.86 1 38.1 0.02 1.2 35.4 0.03 5-F 34.0 5.3 0.86 2.2 34.1 0.06 5-G 34.0 5.3 0.86 4.9 29 0.14 9.7 27.6 0.26 5-H 34.0 5.3 0.86 9.9 31.2 0.24 8.9 28.5 0.24 5-I 34.0 5.3 0.86 5-J 34.0 5.3 0.86 5-K 34.0 5.3 0.86 5-L 34.0 5.3 0.86 5-M 34.0 5.3 0.86 1.2 31.8 0.04 2.4 34.7 0.07 *Calculated from ΔH.sub.cc and ΔH.sub.m (which is derived from the values of the table as follows: ΔH.sub.m = ΔH.sub.cc + ΔH.sub.m,0)

TABLE-US-00007 TABLE 7 form detail and heat resistance of thermoformed products Cup height shrinkage (mm) at form varying temperatures Sample detail* 60° C. 70° C. 80° C. 90° C. 100° C. 5-A nok 5-B nok 5-C nok 5-D nok 5-E nok 5-F ok 0 0 0 0 3 5-G ok 0 1 1 6 7 5-H ok 0 1 6 10 11 5-I ok 1 4 10 12 12 5-J ok 0 0.2 1.5 2.5 3 5-K ok 0 0 0.5 1 1.5 5-L nok 0 0 0 0.5 1 5-M ok 0 0 0 0 0 *ok: good form detail; nok: form detail which is not good

[0103] Samples 5-A to 5-F could not be formed into a well-formed product and, therefore, no cup height shrinkage could be measured. The shrinkage results are graphically presented in FIG. 1 for samples 5-F to 5-K, obtained with varying mold temperatures (30, 75, 85 or 95° C.) and varying heated sheet temperatures (100° C. for FIG. 1A and 130° C. for FIG. 1B). With regard to sample 5-L, it is noted the heated sheet was very soft before the forming process took place, resulting in an undesirable wall thickness distribution. The resulting product (cup) was, therefore, not good enough for commercial purposes. In all other aspects, the form detail of the product (cup) was “ok”.

[0104] It was found that cups with good product detail and/or a heat stability greater than 90° C. were obtained when: [0105] 1. The a heat rate was greater than 4° C./s, e.g. at least 5° C./s. Best results were achieved with heat rates of at least 7° C./s, or even at least 9° C./s. [0106] 2. The mold temperature was greater than 60° C., e.g. at least 70° C. Best results were achieved with a temperature greater than 80° C. [0107] 3. The PLA sheet was heated to a surface temperature greater than 85° C., e.g. of at least 90° C. Best results were achieved with mold temperatures greater than 100° C.