Effect of chain extending cross-linkers on the disintegration behavior of composted PBAT/PLA blown films

Ci a ion: Aze edo, J.V.C.; Hausne o a, B.; Möginge , B.; Sopik, T. E ec o Chain Ex ending C oss-Linke s on he Disin eg a ion Beha io o Compos ed PBAT/PLA Blown Films. In . J. Mol. Sci. 2023,24, 4525. h ps://doi.o g/10.3390/ ijms24054525 Academic Edi o s: Te ence L. Ma sh and Ra ael Au as Recei ed: 10 Oc obe 2022 Re ised: 21 Feb ua y 2023 Accep ed: 22 Feb ua y 2023 Published: 24 Feb ua y 2023 Copy igh : © 2023 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). In e na ional Jou nal o Molecula Sciences A icle E ec o Chain Ex ending C oss-Linke s on he Disin eg a ion Beha io o Compos ed PBAT/PLA Blown Films Juliana V. C. Aze edo 1,2,3 , Be enika Hausne o a 1,4,* , Be nha d Möginge 2and Tomas Sopik 4 1Facul y o Technology, Tomas Ba a Uni e si y in Zlín, Va ecko a 275, 76001 Zlín, Czech Republic 2Depa men o Na u al Sciences, Uni e si y o Applied Sciences Bonn-Rhein-Sieg, on Liebig S . 20, 53359 Rheinbach, Ge many 3 BIO-FED, B anch o AKRO-PLASTIC GmbH, BioCampus Cologne, Na e mannallee 1, 50829 Köln, Ge many 4Cen e o Polyme Sys ems, Uni e si y Ins i u e, Tomas Ba a Uni e si y in Zlín, Nam. T.G. Masa yka 5555, 76001 Zlín, Czech Republic *Co espondence: hausne [email p o ec ed] Abs ac : A biodeg adable blend o PBAT—poly(bu ylene adipa e-co- e eph hala e)—and PLA— poly(lac ic acid)— o blown ilm ex usion was modi ied wi h ou mul i- unc ional chain ex end- ing c oss-linke s (CECL). The aniso opic mo phology in oduced du ing ilm blowing a ec s he deg ada ion p ocesses. Gi en ha wo CECL inc eased he mel low a e (MFR) o is(2,4-di- e -bu ylphenyl)phosphi e (V1) and 1,3-phenylenebisoxazoline (V2) and he o he wo educed i (a oma ic polyca bodiimide (V3) and poly(4,4-dicyclohexylme haneca bodiimide) (V4)), hei com- pos (bio-)disin eg a ion beha io was in es iga ed. I was signi ican ly al e ed wi h espec o he unmodi ied e e ence blend (REF). The disin eg a ion beha io a 30 and 60 ◦ C was in es iga ed by de e mining changes in mass, Young’s moduli, ensile s eng hs, elonga ions a b eak and he mal p ope ies. In o de o quan i y he disin eg a ion beha io , he hole a eas o blown ilms we e e alua ed a e compos s o age a 60 ◦ C o calcula e he kine ics o he ime dependen deg ees o disin eg a ion. The kine ic model o disin eg a ion p o ides wo pa ame e s: ini ia ion ime and disin eg a ion ime. They quan i y he e ec s o he CECL on he disin eg a ion beha io o he PBAT/PLA compound. Di e en ial scanning calo ime y (DSC) e ealed a p onounced annealing e ec du ing s o age in compos a 30 ◦ C, as well as he occu ence o an addi ional s ep-like inc ease in he hea low a 75 ◦ C a e s o age a 60 ◦ C. The disin eg a ion consis s o p ocesses which a - ec amo phous and c ys alline phase o PBAT in di e en manne ha canno be unde s ood by a hyd oly ic chain deg ada ion only. Fu he mo e, gel pe mea ion ch oma og aphy (GPC) e ealed molecula deg ada ion only a 60 ◦ C o he REF and V1 a e 7 days o compos s o age. The ob- se ed losses o mass and c oss-sec ional a ea seem o be a ibu ed mo e o mechanical decay han o molecula deg ada ion o he gi en compos s o age imes. Keywo ds: poly(bu ylene adipa e e eph hala e); poly(lac ic acid); blown ilm; chain ex ending c oss-linke ; deg ee o disin eg a ion; disin eg a ion kine ics; molecula mass deg ada ion 1. In oduc ion The ad ances in ma e ials sciences and echnology allow o de eloping en i onmen- ally sui able packaging p oduc s [ 1 – 3 ]. As a esponse o inc easing plas ic use and he ela ed en i onmen al pollu ion, bioplas ics ha e become highly in e es ing in he las decade [ 4 – 6 ]. The de elopmen o biodeg adable polyme s seems o be an e ec i e way o pa ly sol e he plas ics was e p oblem. In his espec , comme cially a ailable PBAT— poly(bu ylene adipa e-co- e eph hala e)—and PLA—poly(lac ic acid)—a e p edominan ly selec ed by bioplas ics manu ac u e s [ 7 – 11 ] and used o he p oduc ion o blown ilms mainly o shoppe s bags, ui and ege able bags, and was e bags (e.g., Eco io ® by BASF). PBAT is a andom copolyme o bu ylene adipa e and e eph hala e ha owes i s biodeg adabili y o he bu ylene adipa e g oups and i s s abili y and mechanical p ope ies In . J. Mol. Sci. 2023,24, 4525. h ps://doi.o g/10.3390/ijms24054525 h ps://www.mdpi.com/jou nal/ijms In . J. Mol. Sci. 2023,24, 4525 2 o 21 o he e eph hala e g oups [ 11 – 14 ]. I is a lexible polyme wi h high elonga ion a b eak and good p ocessing p ope ies. PLA is en i ely enewable i o igina ing om a s a ch [ 15 ]. Due o he b i le beha io o PLA, i is inapp op ia e o applica ions equi ing high de o ma ion s ains [ 16 , 17 ]. The e o e, i is o en modi ied by plas icize s and chain ex en- de s [18–20]. The lexibili y and oughness o PBAT may compensa e o he b i leness o PLA in hei blends. The s udies conduc ed on he p ocessing o PBAT and PLA blends ocus on balancing he p ope ies equi ed o speci ic applica ions [21–23]. An impo an ac o in ol es deno ing he expe imen al condi ions unde which biodeg ada ion akes place in o de o in es iga e i s e ec s on he disin eg a ion o PLA/PBAT blends. Weng e al. [ 24 ] in es iga ed a 40/60 PBAT/PLA blend cas o ilms a e g anula ion, which we e hen compos ed in a soil en i onmen . DSC analysis o he pu e polyme s showed ha he mel ing empe a u e o he PBAT was sligh ly dec eased a e deg ada ion, while he mel ing empe a u e o he PLA was inc eased. The changes in he mel ing empe a u es o he blend be o e and a e deg ada ion we e iden ical o hose o he co esponding polyme s. Howe e , combined he mog a ime ic and elemen al analyses e ealed ha he deg ada ion a es o he componen s in he PBAT/PLA blend di e ed om hose o he indi idual polyme s. Ne e heless, all samples o PBAT, PLA and PBAT/PLA we e agmen ed a e ou mon hs o deg ada ion in soil. Scanning Elec on Mic oscope (SEM) e ealed an inc eased oughness on he su aces, bu he blend did no show oids. Gi en ha he in oduc ion o ille and addi i es also a ec s he biodeg ada ion beha io and kine ics o PBAT/PLA blends, due o inc eased di usion pa hs and he possi- ble abio ic p ope ies o he ille sys ems, Tolga e al. [ 25 ] in es iga ed he disin eg a ion p ocess o a PLA con aining mine al ille s blended wi h o he biodeg adable polyme s. Fo 2 mm hick pla es o he 30/70 PBAT/PLA blend, hey ound ha he disin eg a ion p ocess s a ed a e an ini ia ion ime o 4 weeks in a compos a 60 ◦ C and 70% ela i e humidi y. A e 12 weeks, he deg ee o disin eg a ion exceeded 25%. Fu he , om he beha io o PLA/PBAT blends, one can conclude ha he disin eg a- ion a es dec ease wi h inc easing ille con en s du ing he i s s age o disin eg a ion [ 25 ]. F ei as e al. [ 26 ] s udied he e ec o mon mo illoni e clay (MMT) ille on PLA/PBAT blends ha we e modi ied wi h chain ex ende s. Fo un illed PLA/PBAT blends, hey ound he highes emission a e o CO 2 , which indica ed ha he deg ada ion a e de- pended on he a ailable polyme su ace. Fille pa icles may also hinde mic oo ganisms om pene a ing he polyme o ha e pa ly an ibio ic e ec s. In each case, MMT e- duced he a ailable su ace, which led o lowe emission a es o CO 2 , and, hus, lowe biodeg ada ion a es. Touchaleaume e al. [ 27 ] in es iga ed 40 µ m hick ilms o a 70/30 PBAT/PLA blend in ineya d soil o 24 mon hs a ambien empe a u es wi h espec o he amoun o deg aded su ace a ea. They obse ed no deg ada ion o e 24 mon hs, which indica ed ha he PBAT ma ix does no allow signi ican deg ada ion a ambien empe a u es, al hough an inc eased su ace oughness was ound. This esul is suppo ed by deg ada ion es s o a 55/45 PBAT/PLA blend o 180 days a 30 ◦ C, in which Lampa elli e al. [ 28 ] ound weigh gains o 0.7% in soil and 1.4% in an aqueous medium. On he o he hand, compos s o age o 70 days a 60 ◦ C [ 27 ] e ealed ini ia ion imes o he deg ada ion p ocess o 4 o 5 days o an 18 mon h aged blend and o 6 days o he esh blend. They success ully i ed hei measu emen s (R 2 > 0.99) using he Hill model [ 29 ], bu hey did no p o ide any i pa ame e s o deg ada ion kine ics. F om he abo e-men ioned in es iga ions, one can conclude ha biodeg ada ion can occu only a e ce ain ini ia ion imes in which he polyme chains a e ac u ed by hyd olysis, pho o-oxida ion, and he mal oxida ion in o pieces ha can be me abolized by mic o-o ganisms [25,27,30,31]. Deepe insigh s in o deg ada ion modes o PBAT ilms a e gi en in a e iew a icle by Liu e al. [32]. A p esen , an e ec o mul i- unc ional chain ex ending c oss-linke s (CECL) on disin eg a ion, as well as mechanical and he mal p ope ies, ha e been in es iga ed o he In . J. Mol. Sci. 2023,24, 4525 3 o 21 ex usion o blown ilms o PBAT/PLA blends e y sca cely. Mos s udies we e ca ied ou on samples manu ac u ed by o he p ocessing echniques, e.g., injec ion o comp ession molding ha did no ake in o accoun he p onounced aniso opic mo phology in oduced by blown ilm ex usion. Ou p e ious s udy [ 33 ] showed ha he chemical eac ions caused by CECL in PBAT/PLA blends we e incomple e a e compounding, and ha he s e ching du ing blown ilm ex usion b ough he app op ia e molecula g oups in o each by p omo ing u he c oss-linking, chain scission, o o he eac ions. The objec i e o his s udy is o in es iga e in de ail he e ec s o ou CECLs on he disin eg a ion beha io and kine ics o a PBAT/PLA blend unde compos s o age. 2. Resul s The isual inspec ion o he ilms shows ha empe a u e had a signi ican e ec on he disin eg a ion beha io o he PBAT/PLA compounds modi ied by ou CECLs ( o de ails see Sec ion 4.1). Figu e 1shows ha , a 30 ◦ C, he samples o REF (unmod- i ied blend), V1 (CECL = is(2,4-di- e -bu ylphenyl) phosphi e), and V2 (CECL = 1,3- phenylenebisoxazoline) exhibi ed small s ains on he su ace, which indica ed a s a ing disin eg a ion a e 2 mon hs. A 30 ◦ C, PLA is in he glassy s a e (glass empe a u e o 60 ◦C), which p e en s i s deg ada ion. Figu e 1. Compos s o age o 2 mon hs a 30 ◦ C ( a ) and 7 days a 60 ◦ C ( b ) on 25 µ m- hick ilms o PBAT/PLA compounds; scaling in mm. A 60 ◦ C, he REF exhibi ed a mean disin eg a ion a e 7 days, and V1 and V2 exhibi ed se e e disin eg a ions, whe eas V3 (CECL = a oma ic polyca bodiimide) showed a s a ing disin eg a ion, and V4 (CECL = poly(4,4-dicyclohexylme hane ca bodiimide) s ill seemed o be una ec ed. The s a es o disin eg a ion we e in line wi h he inc eased mel low a ios o V1 and V2 when compa ed o REF, as well as wi h he dec eased mel low a ios o V3 and V4 [ 33 ]. This showed ha disin eg a ion was eased in he case o sho e and mo e mobile polyme chains. In . J. Mol. Sci. 2023,24, 4525 4 o 21 2.1. E ec s o Compos S o age on Mass Change A 30 ◦ C, he ela i e masses o he samples o REF and V1 o V4 emained mo e o less unchanged wi hin he ange o he sca e o he mass de e mina ion a e compos s o age o 8 weeks. Figu e 2indica es ha he disin eg a ion p ocesses did no lead o mass dec eases ye . The mass a ia ions can be a ibu ed o p ocesses which occu wi hin he ini ia ion ime, e.g., mass inc ease due o chemical eac ions, humidi y up ake, and emaining compos pa icles, o mass dec ease due o eleases o deg ada ion eac ions o loss o mic o-plas ic pa icles du ing cleaning. Figu e 2. Rela i e mass change o 25 µ m- hick ilms o REF and V1 o V4 o e 8 weeks a 30 ◦ C ( a ) and 7 days a 60 ◦ C ( b ) o disin eg a ion in compos ; s anda d de ia ions (STD) a e be ween 0.03 and 0.07, he lines se e o be e isualiza ion. A 60 ◦ C, he ela i e masses emained unchanged only o 4 days. Figu e 2shows ha , o V1 and V2, a p onounced mass dec ease was consequen ly obse ed. The REF exhibi ed a less p onounced mass dec ease be ween day 5 and 6. The V3 showed a endency o dec easing mass, whe eas he V4 seemed o no be disin eg a ed, e en a e 7 days. This shows ha he ini ia ion imes o disin eg a ion a e signi ican ly a ec ed by he chosen CECL and he induced chemical changes. Fu he mo e, signi ican disin eg a ion was obse ed o he REF, V1, and V2 a e 7 days a 60 ◦ C. This suppo s he in e p e a ion ha he s ains on he REF, V1, and V2 in Figu e 1 ep esen ed a beginning disin eg a ion. Because o he sca e o he mass measu emen s, he disin eg a ion was con i med only i he masses had dec eased o less han 90% o he ini ial alue. Thus, he ime 80% was he de e mined ime a which he masses o he ilms eached 80% o hei ini ial masses, as is shown in Figu e 2, and i ep esen s he uppe limi o he ini ia ion ime. The da a indica ed ha he e was a mass inc ease a he beginning o he compos s o age due o he ini ial disin eg a ion p ocesses. To es ima e he mass sca e due o adso bed and biodeg ada ion-gene a ed low molecula weigh componen s, he ini ial masses o he DSC expe imen s we e compa ed o he masses a e he second hea ing uns. The mass losses we e be ween 2 and 12%, which showed ha he e was a ema kable con en o low molecula componen s in he ilms, as is shown in Table 1. In . J. Mol. Sci. 2023,24, 4525 5 o 21 Table 1. E ec s o condi ions o compos s o age on mass loss and ela i e mass loss a e second un, and on en halpies om 70 o 90 ◦C o i s and second un. S o age 8 Weeks a 30 ◦C 7 Days a 60 ◦C ∆H be ween 70 o 90 ◦C Mass Loss Rela i e Mass Loss ∆H be ween 70 o 90 ◦C Mass Loss Rela i e Mass Loss Uni mJ mg - mJ mg - REF 15 0.25 0.04 34 0.43 0.07 V1 21 0.44 0.07 46 0.67 0.11 V2 23 0.45 0.08 45 0.74 0.12 V3 26 0.24 0.04 28 0.25 0.04 V4 27 0.20 0.03 25 0.13 0.02 A e 8 weeks a 30 ◦ C, he REF had los 4% o i s mass, and 7% o i s mass a e 7 days a 60 ◦ C. The dec ease in he V1 and V2 exceeded ha o he REF, which showed ha he biodeg ada ion o he V1 and V2 was mo e p onounced. Fo he V3 and V4, he dec eases we e less han o he REF, which showed ha c oss-linking hinde ed biodeg ada ion and decele a ed disin eg a ion. Howe e , he dec eases we e s ill wi hin he sca e o he ela i e masses du ing he ini ia ion imes. This is a u he indica ion o a mass inc ease due o he up ake and gene a ion o low molecula weigh componen s du ing he ini ia ion imes o compos s o age, in addi ion o he con ibu ions o adhe en compos pa icles. Fo all compounds, he DSC aces o he i s un di e ed signi ican ly om he second un, as is shown in Figu e 3, and he en halpies also di e ed be ween 20 and 170 ◦C . These di e ences can be a ibu ed o he p ocessing his o y (which was simila o all ilms) and he eached s a e o biodeg ada ion. As polyamides a e ypically d ied a 80 ◦ C (a empe a u e which is also abo e he T g o PLA), he en halpies be ween 70 and 90 ◦ C we e de e mined, as is shown in Table 1. I can be seen ha hese en halpies co ela ed quali a i ely wi h he mass loss o he REF, V1, and V2 and ha hey we e almos equal o he V3 and V4. Gi en ha he con en o CECLs was low, one can assume ha he wa e con en s o all ilms we e iden ical. Thus, he di e ences in he mass losses in Table 1 ha e o ha e been gene a ed by ola ile low molecula linked o he ini ial p ocesses o biodeg ada ion. Figu e 3. Fi s and second hea ing un o REF a e compos s o age o 7 days a 60 ◦ C showing he en halpies be ween 20 and 170 ◦ C as ha ched a eas. The dashed lines ma k he ange 70 o 90 ◦ C in which mos o he wa e deso p ion is expec ed o ha e occu ed. 2.2. E ec s o Compos S o age on Disin eg a ion Kine ics A e compos s o age a 60 ◦ C, he ilms we e u he in es iga ed wi h espec o hei disin eg a ion kine ics by de e mining he c oss-sec ional a eas o holes o elucida e he kine ic pa ame e s o ini ia ion ime ini and disin eg a ion ime τdisin o he REF and o V1 o V4. The expe imen al da a can sa is ac o ily be i ed by Equa ion (8) wi h a easonable R 2 , as a e shown in Figu e 4and Table 2. In he model, a single domina ing disin eg a ion p ocess was assumed using an app oach ha was used o desc ibe he In . J. Mol. Sci. 2023,24, 4525 6 o 21 hyd oly ic deg ada ion o nea PLA, PLA modi ied wi h a ca bodiimide, and PLA wood lou composi es [ 34 , 35 ]. The smalle measu ed ime dependen deg ees o disin eg a ion x disin o he REF, V1, and V2 when compa ed o he p edic ed alues can be explained by he ac ha he disin eg a ion had al eady happened p io o he i s isible holes. Figu e 4. Time dependen deg ee o disin eg a ion a 60 ◦ C o REF and V1 o V4; compa ison o expe imen al da a (symbols) o i s acco ding o Equa ion (8); STD om 0.03 o 0.05. Table 2. Kine ic pa ame e s o REF and V1 o V3 compounds o e 7 days a 60 ◦ C o disin eg a ion in compos . Kine ic Pa ame e s Compound Uni REF V1 V2 V3 Time 80% o 80% o ini ial mass h 132.7 105.6 105.8 174.1 Ini ia ion ime ini h 32.8 34.1 23.7 45.8 Disin eg a ion ime τdisin h 385.1 147.3 94.8 1179.5 R2o i s 0.96 0.95 0.97 0.83 The imes 80% ( o he mass dec ease o 80%) exceeded he ini ia ion imes ini by h ee o ou ac o s, as is shown in Table 2. This shows ha he ini ia ion o disin eg a ion depends on he conside ed p ope y—mass loss o hole gene a ion. Ob iously, some deg ee o disin eg a ion has o happen p io o he appea ance o he i s holes. Thus, he de e mined ini ia ion imes ini ha e o depend on he ilm hickness. I one assumes ha he i s appea ance o holes depends linea ly on he ilm hickness, he ini ia ion ime o V1 (30 µ m ins ead o 25 µ m) would be oughly 20% oo long—leading o a co ec ed ini ≈ 27 h. The V4 did no show any holes wi hin 7 days, which indica ed ha he In . J. Mol. Sci. 2023,24, 4525 7 o 21 molecula s uc u e gene a ed by he CECL poly(4,4-dicyclohexylme hane ca bodiimide) p e en ed signi ican disin eg a ion. The disin eg a ion imes τdisin show ha he CECLs signi ican ly a ec ed he disin- eg a ion p ocesses. The disin eg a ion a es o he V1 and V2 exceeded ha o he REF by 2.5 imes and 4 imes, espec i ely, whe eas i was dec eased o a ac o 0.25 o he V3. Gi en ha he V4 did no show any holes wi hin 7 days, i was no e alua ed. 2.3. E ec s o Compos S o age on Mechanical P ope ies The Young’s moduli in he ex usion di ec ion (ED) we e oughly double o hose in ans e se di ec ion (TD), which e lec ed he ilm aniso opy due o p ocessing. Compos s o age o he REF and V1 o V4 o 8 weeks a 30 ◦ C ini ially led o an inc ease in he Young’s moduli in he o de o 20% be o e hey we e slowly dec eased again, as is shown in Figu e 5. This inc ease can be explained by he annealing e ec s in he amo phous phases o PLA [ 36 ] and by he pos -c ys alliza ion o PBAT. The PLA in i s dispe se phase was p o ec ed by he PBAT ma ix agains any kind o deg ada ion and disin eg a ion a he beginning o he compos s o age a 30 ◦ C, and annealing could happen undis u bed. Jian e al. [8] epo ed ha PBAT s a s o c ys allize a 60 ◦ C i hea ed o 10 ◦ C/min du ing DSC. The e o e, a slow pos -c ys alliza ion o amo phous PBAT can al eady be expec ed a 30 ◦ C i humidi y p o ides mo e mobili y o he polyme chains. Bo h p ocesses may inc ease s i ness. In e es ingly, he maximum Young’s moduli we e de e mined a e 2 weeks in he ED, while 4 weeks we e needed in he TD, which indica ed ha mechanical deg ada ion happened di e en ly in he ED and TD du ing ini ia ion ime. This can be unde s ood by he ac ha he ilms exhibi signi ican ly di e en mo phologies on hei ac u e su aces in he ED and TD [ 37 ]. Thus, du ing he ini ia ion phase, one di ec ion can be mo e a ec ed by biodeg ada ion p ocesses han he o he . Figu e 5. Time dependen e ec s o disin eg a ion on mechanical p ope ies (Young’s modulus E, ensile s eng h σmax , elonga ion a b eak εb eak ) in ex usion di ec ion (ED) and ans e se di ec ion (TD) o ilms a e s o age in compos a 30 ◦C. In . J. Mol. Sci. 2023,24, 4525 8 o 21 Compos s o age o 8 weeks a 30 ◦ C led o a con inuous dec ease in he ensile s eng hs o he REF, V1, and V2 in he o de o 50 o 70% in he ED and TD. The V3 and V4 exhibi ed a pla eau o ensile s eng h o 4 weeks in he ED be o e a dec ease o 60 o 70% occu ed. In he TD, he V3 exhibi ed a pla eau o ensile s eng h o 4 weeks, and he V4 exhibi ed a pla eau o ensile s eng h o 2 weeks. The elonga ions a b eak exhibi ed a pla eau o 2 weeks o he REF, V1, and V2 in he ED, wi h subsequen dec eases o 35% (REF) and almos 100% (V1 and V2). Fo he V3, he pla eau las ed 4 weeks, ollowed by a dec ease o 20%, whe eas no dec ease was obse ed o he V4, e en a e 8 weeks. In he TD, he elonga ions a b eak o he REF and V2 exhibi ed a pla eau o 2 weeks be o e hey dec eased, whe eas he V1 showed a con inuous dec ease. Thei elonga ions a b eak a e 8 weeks d opped o 5 o 30% o he ini ial alues. The V3 and V4 exhibi ed pla eaus o elonga ions a b eak in he TD ha las ed 4 weeks and 2 weeks, espec i ely, wi h subsequen dec eases o 30 and 70%, espec i ely. Compos s o age a 60 ◦ C, as is depic ed in Figu e 6, had p onounced impac on he mechanical p ope ies o he ilms. Annealing e ec s in he amo phous phase o he PLA did no con inue o occu , as he PLA was now abo e he glass ansi ion empe a u e. The e o e, no inc eases in Young’s moduli we e u he obse ed. A e one week, i was ha dly possible o pe o m ensile es s wi h ilms o he REF, V1, and V2, due o se e e mechanical disin eg a ion. In he ED and TD, he Young’s moduli o he REF emained on a pla eau o he i s 3 days. Then, hey con inuously dec eased close o ze o wi hin a week. The Young’s moduli o he V1 and V2 con inuously dec eased a e 7 days o 70% and 50%, espec i ely, in he ED and TD. Fo he V3, i s Young’s moduli in he ED and TD emained on a pla eau o 14 days be o e hey dec eased by a leas 50%. The Young’s moduli o he V4 emained on a pla eau o almos 28 days. Only he modulus in he ED was 15% lowe a e 28 days. Figu e 6. Time dependen e ec s o disin eg a ion on mechanical p ope ies (Young’s modulus E, ensile s eng h σmax , elonga ion a b eak εb eak ) in ex usion di ec ion (ED) and ans e se di ec ion (TD) o ilms a e s o age in compos a 60 ◦C. In . J. Mol. Sci. 2023,24, 4525 9 o 21 In he ED and TD, he ensile s eng hs o he REF, V1, and V2 dec eased con inuously o app oxima ely 20% o hei ini ial alues a e 7 days. Fo he V3, i s Young’s moduli in he ED dec eased con inuously o less han 20% a e 28 days, whe eas in he TD, hey dec eased o 30% du ing he i s 2 days and emained on ha pla eau o 2 weeks be o e hey u he dec eased o less han 20%. Fo he V4, he ensile s eng h in he ED dec eased con inuously o app oxima ely 40% a e 28 days, whe eas in he TD, i emained on a pla eau o 3 days be o e i dec eased o 30%. The elonga ions a b eak in he ED and TD o he REF emained on a pla eau o 1 day and hen dec eased o app oxima ely 10% o hei ini ial alues a e 3 days. Fo he V1, a pla eau was obse ed in he ED o 1 day wi h a subsequen dec ease ha was almos close o ze o a e 7 days. In he TD, he dec ease close o ze o was al eady eached a e 3 days. The V2 exhibi ed a con inuous dec ease and eached elonga ions a b eak o a ew pe cen age poin s a e 7 days in he ED and a e 3 days in he TD. The V3 exhibi ed a pla eau o 1 day and a slow dec ease o a ew pe cen age poin s a e 14 days in bo h he ED and TD. Fo he V4, a pla eau was obse ed o 3 days, ollowed by a dec ease o 50% in he ED and 10% in he TD. Tensile s eng hs and elonga ions a b eak indica e a high sensi i i y wi h espec o s uc u al changes o he ilms, and hey showed ha he ilms o he REF and V1 o V4 had also been mechanically deg aded du ing compos s o age a 30 ◦ C. Ob iously, hese p ocesses we e s ill in he s a e o ini ia ion wi h negligible e ec s on ilm masses. A 60 ◦ C, he deg ada ion p ocesses we e signi ican ly accele a ed, as he REF, V1, and V2 we e se e ely disin eg a ed a e 7 days (see Figu e 1), and bo h hei ensile s eng hs and elonga ions a b eak d opped o low alues. 2.4. E ec s o Compos S o age on The mal P ope ies The biodeg ada ion caused by compos s o age a ec ed he he mal p ope ies in a signi ican manne , as he DSC cu es o he i s hea ing un a e 8 weeks a 30 ◦ C di e ed signi ican ly om hose a e 7 days a 60 ◦ C (see Figu e 7), which also occu ed o he co esponding ansi ion empe a u es and hea s o usions, as is shown in Table 3. Figu e 7. DSC aces o he i s hea ing un o REF and V1 o V4 a e compos s o age o 8 weeks a 30 ◦C (a) and 7 days a 60 ◦C (b). In . J. Mol. Sci. 2023,24, 4525 16 o 21 I is ob ious ha he disin eg a ion p ocesses occu ing in he PBAT/PLA ilms a e complex and ac on di e en ime scales wi h espec o p ope ies such as mass loss, ensile s eng h, elonga ion a b eak, ansi ion empe a u es, and hea s o usion. I he blown ilms a e subjec ed o compos s o age, he i s p ocesses a e mainly wa e di usion in he amo phous phases o he PBAT and PLA and he se ling o mic o-o ganisms on he ilm su aces. As a compos consis s o a mani old o mic o-o ganisms, o he subs ances o low molecula weigh s—being ye unknown—can di use in he a ailable ee olume o wa e -so ened amo phous phases, whe e hey unde go u he chemical eac ions and ini ia e s ess c acking. This p opaga es mechanical disin eg a ion by mic o- oid o ma ion and ac u ing. As ac u ing is a de ec -con olled p ocess, elonga ions a b eak and ensile s eng hs can al eady dec ease i he e a e small oid concen a ions close o he su ace a he ea ly s ages o biodeg ada ion. In e nal s ess ields in he ilms (wha is p obable due o he aniso opic s uc u e gene a ed by he ilm blowing p ocess), p opaga e hese oids and ease he ac u ing o he ilms o mic o-plas ic pa icles. Wi h espec o comple e deg ada ion, his ac u ing owa ds mic o-plas ic pa icles is bene icial. Fi s ly, he di usion pa hs o low molecula weigh componen s (such as wa e ) dec ease, and hyd oly ic molecula chain deg ada ion equi es sho e ini ia ion imes. Secondly, he ac u ing gene a es a new su ace a ea whe e mic o-o ganisms can se le and me abolize he polyme . 4. Ma e ials and Me hods 4.1. Ma e ials and Sample P epa a ion Fou chain ex ending c oss-linke s (1 w .%) we e compounded o he e e ence PBAT/PLA blend (REF) M · VERA ® B5029 [ 40 ] om BIO-FED, a b anch o AKRO-PLASTIC GmbH, Cologne, Ge many. M · VERA ® B5029 consis s o 65% PBAT (ma ix), and 11% PLA (dispe se phase) and 24% o CaCO 3 ille pa icles. I is mainly used o packaging and ag icul u al applica ions. The ollowing CECLs we e employed: • V1— is(2,4-di- e -bu ylphenyl)phosphi e, Songnox 1680 (Songwon Indus ial Co, Ulsan, Sou h Ko ea) [41], •V2—1,3-phenylenebisoxazoline, 1,3-PBO powde (E onik, Essen, Ge many) [42], • V3—a oma ic polyca bodiimide, S abaxol P110 (Lanxess, Cologne, Ge many) [43], and • V4—poly(4,4-dicyclohexylme hane ca bodiimide), Ca bodili e HMV-15CA (Nisshinbo, Tokyo, Japan) [44]. All ing edien s we e e enly mixed using a Mixaco CM 150-D (Mixaco Maschinenbau, Neuen ade, Ge many) and compounded by a win-sc ew ex ude (FEL 26 MTS, Fed- dem GmbH, Sinzig, Ge many) wi h 26 L/D, sc ew speed o 260 pm, and ou pu a e o 20 kg h−1 . The ilms we e manu ac u ed using he blow molding machine LF-400, (Lab ech Enginee ing Company Co., L d., Samu P akan, Thailand) wi h an ex usion empe a u e o 165 ◦ C and a blow-up a io (BUR) o 1:2.5 o 25 µ m- hick ilms. This was o a i m almos simila mo phology de elopmen s in all in es iga ed compounds. CECLs in luence he mel low a io and, hus, a ec he mel iscosi ies o he compounds [ 33 ]. Fo he gi en blowing condi ion, his may a ec ilm hicknesses and d aw a ios (DR), as is shown in Table 6. The ilm hicknesses (Table 6) exhibi ed ela i ely small s anda d de ia ions, because hey we e de e mined on s ipes be o e compos s o age om a ela i ely sho sec ion o blown ilms. A de ice o inline-moni o ing o he hickness was no a ailable. The ilm hicknesses du ing blown ilm ex usion ypically may a y by up o 15% [45,46] o bo h he ED and TD due o p ocess luc ua ions such as changes in mel iscosi y due o a ia ions in die empe a u e o die gap con ol. Since he o e all mean hickness in Table 6 was (28 ±2) µm, he a ia ion ange was wi hin he limi . The samples we e s abilized a 23 ◦ C o 24 h a e ex usion due o cus ome s’ equi e- men s enabling u he packaging p ocessing. Films we e cu o ensile es s ipes wi h he dimensions 170 × 15 mm and weighed. T ays wi h a leas 3 L o olume we e illed wi h a compos (Plan i lo P o Na u o ei, Do mund, Ge many) wi h 3 subsequen laye s: a bo om laye o compos ( hickness 5 cm), a laye o indi idualized es specimens, In . J. Mol. Sci. 2023,24, 4525 17 o 21 and a op laye o compos ( hickness > 5 cm). To p e en c oss-con amina ion, di e en ly colo ed ays we e used o he REF and V1 o V4 and s o ed in an o en a 30 ◦ C o 60 ◦ C, espec i ely, wi h 60% ela i e humidi y o be close o indus ial compos ing condi ions desc ibed by ISO 16926:2018. The s o age imes a 30 ◦ C we e 168, 336, 672 and 1344 h; a 60 ◦C hey we e 24, 48, 72, 96, 120, 144 and 168 h. Since he V3 and V4 ha dly showed isual disin eg a ion, hey we e also subjec ed o compos s o age a 60 ◦ C o 336 and 672 h. To ollow ISO 16926:2018, pH alues we e de e mined when he samples we e emo ed and cleaned. Table 6. Mean hicknesses o ilms o he e e ence (REF) and CECL-modi ied (V1 o V4) PBAT/PLA blends and co esponding d aw a io. Thickness in µm D aw Ra io REF 25.8 ±0.7 12.4 V1 30.5 ±0.5 10.5 V2 26.3 ±0.5 12.2 V3 27.3 ±0.6 11.7 V4 29.9 ±0.8 10.7 No e: Thicknesses we e de e mined on s ipes be o e compos s o age. 4.2. Me hods 4.2.1. De e mina ion o Mass Change a e Disin eg a ion in Compos The ilm masses a e compos exposu e we e measu ed using he Sa o ius BP221S balance (Sa o ius AG, Gö ingen, Ge many) wi h a measu ing accu acy o 0.1 mg. The de e mined masses showed sca e s in he ange o 3 o 7% due o s icking compos pa icles, abso bed disin eg a ion subs ances, and loss o mic o-plas ic pa icles du ing cleaning in cases o se e e ilm disin eg a ion. Fo be e compa ison o mass changes o he REF and V1 o V4, he da a we e con e ed o ela i e masses: m el( )=m( ) m0 (1) wi h mass m( ) a e he gi en s o age ime in compos and ini ial mass m0. 4.2.2. Kine ics o he P ocess o Disin eg a ion in Compos Due o he Dec ease in Film C oss-Sec ion A e he gi en s o age imes, he samples we e aken ou o he compos , cleaned wi h a so pain b ush o emo e compos pa icles, and subsequen ly s o ed a 23 ◦C/50% .h. Then pic u es o he PBAT/PLA ilms we e aken wi h a Sony Cybe -sho DSC- HX60V came a. The sample ilms appea ed in g eyish colo s, whe eas holes appea ed in black. Then, he pic u es we e disc imina ed o black and whi e pixels, and he co esponding pixel a eas we e calcula ed using he public domain image analysis so wa e Fiji [ 47 ]. The deg ee o disin eg a ion xdisin was de e mined by xdisin ( )=Ablack( ) Awhi e( )+Ablack( )(2) wi h he black pixel a ea ep esen ed as Ablack and whi e pixel a ea ep esen ed as Awhi e. The quan i a i e kine ic app oach assumes ha he mic o-o ganisms om he compos a ack he ilm a he con ac a eas and s a disin eg a ion p ocesses om he su ace. Since he compos has a coa se and c umbly s uc u e, he disin eg a ion does no happen uni o mly on he su ace bu locally a ce ain poin s. A e an ini ia ion ime ini , i s , holes appea in he ilms, which g ow in size and numbe , and, hus, dec ease he emaining In . J. Mol. Sci. 2023,24, 4525 18 o 21 ilm c oss-sec ion. I one assumes ha he ime dependen change o he ilm c oss-sec ion dA ilm d depends on he a ailable ilm c oss-sec ion A ilm, one ge s dA ilm d ∼A ilm( )=A0−Ahole( )(3) wi h hole a ea Ahole. A e di iding he ini ial c oss-sec ion A 0 , one ge s he no malized a e equa ion wi h he empe a u e dependen disin eg a ion cons an kdisin : dA ilm el d =−kdisin (T)A ilm el ( )(4) Sol ing by sepa a ion o a iables and subsequen in eg a ion o e equi alen limi s gi es: ln A ilm el ( )=−kdisin (T) +C(5) wi h he in eg a ion cons an C= 0. Sol ing o A ilm el ( )and subs i u ion by xdisin ( )yields: xdisin ( )=1−e−kdisin (T) (6) To ake in o accoun he ini ia ion ime ini o he disin eg a ion p ocess, one has o modi y Equa ion (6) in o: xdisin ( )=1−e−kdisin (T) ( − ini )(7) o xdisin ( )=1−e−( − ini ) τdisin (8) wi h he disin eg a ion ime τdisin being a measu e o how as he disin eg a ion p o- cess p oceeds. A leas squa e i ing p ocedu e o Equa ion (8) wi h he excel sol e was used o de e mine ini and τdisin o quan i y he disin eg a ion kine ics. 4.2.3. Tensile Tes s and Mechanical P ope ies Young’s modulus, ensile s eng h, and ac u e s ain we e de e mined acco ding o ISO 527-3 using a ensile es ing machine (2.5 kN Zwicki, Zwick Roell, Ulm, Ge many) a 23 ◦ C/50% .h. and a c osshead speed o 200 mm min −1 . Tensile es s we e pe o med in he ex usion di ec ion (ED) and ans e sal di ec ion (TD) wi h n = 5 as long as he sample disin eg a ion allowed ha . All samples ha ing ini ial dimensions o 170 × 15 mm we e condi ioned a 23 ◦ C and 50% .h. o 24 h be o e es ing o adjus he same es ing condi ions unde which he ini ial p ope ies we e de e mined in [33]. 4.2.4. Di e en ial Scanning Calo ime y (DSC) and The mal P ope ies DSC expe imen s we e pe o med using a DSC Diamond (Me le Toledo, G ei ensee, Swi ze land) o g anules and a DSC 214 Polyma (Ne zsch Ge ä ebau GmbH, Selb, Ge - many) o ilms in s anda d Al pans wi h pinholed lids in h ee s eps—1s hea ing, cooling, 2nd hea ing. The measu ing condi ions we e: •Sample weigh ms: (6 ±1) mg •S a ing empe a u e Ts a : 0 ◦C •End empe a u e Tend: 200 ◦C •Hea ing/cooling a e: 10 K min−1 •Repe i ion: n ≥2 In . J. Mol. Sci. 2023,24, 4525 19 o 21 The DSC aces acco ding o ISO 11357-3:2018 we e e alua ed wi h espec o glass ansi ion empe a u es o he ha d segmen T g,hs , mel ing empe a u es o PBAT T m1 and PLA T m2 and he co esponding hea s o usion ∆ H m1 , ∆ H m2 (1s un). C ys alliza ion empe a u es T c and hea s o c ys alliza ion ∆ H c we e de e mined om he cooling un. All samples we e also condi ioned a 23 ◦C and 50% .h. o 24 h be o e es ing. To es ima e he con en o humidi y, he en halpies be ween 70 and 90 ◦ C we e de e - mined om he 1s and 2nd un. Finally, he ilm masses a e he 2nd un we e measu ed o de e mine he mass loss due o all low molecula weigh componen s. 4.2.5. Gel Pe mea ion Ch oma og aphy (GPC) The g anules o pu e PBAT and PLA as well as ilm samples o he REF and V1 o V4 we e dissol ed in THF and il e ed using PTFE il e s wi h a mesh o 0.45 µ m in o ials o 2 mL. All samples showed a u bidi y due o a ille con en o 24%. The sample concen a ion in each ial was 2.5 g/L. The GPC measu emen s we e conduc ed by a Wa e s HPLC sys em equipped wi h a Wa e s model e2695 and Wa e s model 2414 di e en ial e ac ome e (Wa e s Co po- a ion, Massachuse s Uni ed S a es o Ame ica). The GPC sys em was calib a ed wi h polys y ene s anda ds ha ing molecula masses o 580; 10,440; 38,640; 132,900; 492,500; and 990,500 g/mol. The measu ing condi ions we e: • Se ies o gel-mixed bed columns: PL gel MIXED-A (300 × 7.5 mm, 20 µ m) + PL gel MIXED-B (300 ×7.5 mm, 10 µm) + PL gel MIXED-D (300 ×75 mm, 5 µm); •Mobile phase: e ahyd o u an (THF) s abilized wi h bu yla ed hyd oxy oluene; •Tempe a u e: 40 ◦C; •Injec ion olume: 100 µL; •Flow a e: 1 mL/min; •De ec o : e ac i e index de ec o (RI). All da a p ocessing was ca ied ou using Empowe 3 so wa e (Ve sion FR 3). Au ho Con ibu ions: Concep ualiza ion, B.M.; me hodology, B.M.; so wa e, J.V.C.A.; alida ion, J.V.C.A.; o mal analysis, J.V.C.A.; in es iga ion, J.V.C.A. and T.S.; esou ces, J.V.C.A.; da a cu a ion, J.V.C.A., T.S.; w i ing—o iginal d a p epa a ion, J.V.C.A. and B.M.; w i ing— e iew and edi ing, B.H.; isualiza ion, J.V.C.A.; supe ision, B.H. and B.M.; unding acquisi ion, B.H. All au ho s ha e ead and ag eed o he published e sion o he manusc ip . Funding: The au ho s B.H. and T.S. acknowledge he Minis y o Educa ion, You h, and Spo s o he Czech Republic—DKRVO (RP/CPS/2022/003) and (RP/CPS/2022/002), espec i ely. Ins i u ional Re iew Boa d S a emen : No applicable. In o med Consen S a emen : No applicable. Da a A ailabili y S a emen : Da ase s analyzed and gene a ed du ing he s udy will be p o ided upon eques . Acknowledgmen s: BIO-FED, a b anch o AKRO-PLASTIC GmbH, is acknowledged o sponso ing he samples and wo king place o he p esen ed wo k. Con lic s o In e es : The au ho s decla e no con lic o in e es . Re e ences 1. Chen, W.; Qi, C.; Li, Y.; Tao, H. The deg ada ion in es iga ion o biodeg adable PLA/PBAT blend: The mal s abili y, mechanical p ope ies and PALS analysis. Radia . Phys. Chem. 2021,180, 109239. [C ossRe ] 2. Shen, M.; Song, B.; Zeng, G.; Zhang, Y.; Huang, W.; Wen, X.; Tang, W. 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