Development of Flame-Retardant Polylactic Acid Formulations for Additive Manufacturing

Ci a ion: Agui esa obe, R.; Cala el, I.; Villanue a, S.; Sanchez, A.; Agi e, A.; Sukia, I.; Esnaola, A.; Sa alegi, A. De elopmen o Flame-Re a dan Polylac ic Acid Fo mula ions o Addi i e Manu ac u ing. Polyme s 2024,16, 1030. h ps://doi.o g/ 10.3390/polym16081030 Academic Edi o : Bob Howell Recei ed: 13 Ma ch 2024 Re ised: 6 Ap il 2024 Accep ed: 8 Ap il 2024 Published: 10 Ap il 2024 Copy igh : © 2024 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/). polyme s A icle De elopmen o Flame-Re a dan Polylac ic Acid Fo mula ions o Addi i e Manu ac u ing Robe Agui esa obe 1, I xaso Cala el 1, Sa a Villanue a 2, Albe o Sanchez 2,* , Amaia Agi e 1, I xa o Sukia 3, A i z Esnaola 3and Aina a Sa alegi 4,* 1POLYMAT and Depa men o Ad anced Polyme s and Ma e ials: Physics, Chemis y and Technology, Facul y o Chemis y, Uni e sidad del País Vasco/Euskal He iko Unibe si a ea, UPV/EHU, 20018 San Sebas ian, Spain; [email p o ec ed] (R.A.); [email p o ec ed] (I.C.); [email p o ec ed] (A.A.) 2TECNALIA, Basque Resea ch and Technology Alliance (BRTA), Pa que Tecnológico de San Sebas ián, 20009 San Sebas ian, Spain; [email p o ec ed] 3 Depa men o Mechanics and Indus ial P oduc ion, Mond agon Unibe si a ea, 20500 A asa e-Mond agon, Spain; [email p o ec ed] (I.S.); [email p o ec ed] (A.E.) 4G oup ‘Ma e ials + Technologies’, Depa men o Chemical and En i onmen al Enginee ing, Facul y o Enginee ing o Gipuzkoa, Uni e sidad del País Vasco/Euskal He iko Unibe si a ea, UPV/EHU, 20018 San Sebas ian, Spain *Co espondence: [email p o ec ed] (A.S.); [email p o ec ed] (A.S.) Abs ac : Polyme ic ma e ials, enowned o hei ligh weigh a ibu es and design adap abili y, play a pi o al ole in augmen ing uel e iciency and cos -e ec i eness in ailway ehicle de elop- men . The ailo ed o mula ion o compounds, speci ically designed o addi i e manu ac u ing, holds signi ican p omise in expanding he use o hese ma e ials. This s udy cen e s on poly(lac ic acid) (PLA), a na u al-based biodeg adable polyme ic ma e ial inco po a ing di e se halogen- ee lame e a dan s (FRs). Ou in es iga ion sc u inizes he p in abili y and i e pe o mance o hese o mula ions, aligning wi h he Eu opean ailway s anda d EN 45545-2. The indings unde sco e ha FR in he condensed phase, including ammonium polyphospha e (APP), expandable g aphi e (EG), and in umescen sys ems, exhibi supe io i e pe o mance. No ably, FR-inducing hyd oly ic deg a- da ion, such as aluminum hyd oxide (ATH) o EG, educes polyme molecula weigh , signi ican ly impac ing PLA’s mechanical pe o mance. Achie ing a delica e balance be ween i e esis ance and mechanical p ope ies, o mula ions wi h APP as he lame e a dan eme ge as op imal. This esea ch con ibu es o unde s anding he i e pe o mance and p in abili y o 3D-p in ed PLA compounds, o e ing i al insigh s o he ail indus y’s adop ion o polyme ic ma e ials. Keywo ds: poly(lac ic acid); lame e a dan s; addi i e manu ac u ing; ailway s anda d; mechanical p ope ies; p ocessabili y 1. In oduc ion Addi i e manu ac u ing (AM), a cu ing-edge p oduc ion echnology, exhibi s he po en ial o diminish s ockpiles by enabling on-demand pa ab ica ion, he eby o e ing signi ican p ospec s o op imizing ope a ional e iciency. This echnology has inc easingly cap i a ed, among o he s, he ailway indus y. Majo playe s like he F ench Na ional Rail- way Sys em (SNCF), Als om, MGA, Bomba die , CAF, and SIEMENS a e ac i ely explo ing and emb acing addi i e manu ac u ing echnologies o e olu ionize spa e polyme -based pa s p oduc ion [ 1 ]. Howe e , a pe sis en challenge hinde ing polyme -based AM appli- ca ions, pa icula ly in sa e y-c i ical ields such as he ailway indus y, is hei inhe en lammabili y. Applica ions such as in e io panels, sea ing, cable insula ion, and housing o elec onic equipmen demand no el s a egies o mi iga e hese isks e ec i ely [ 2 , 3 ]. This conce n is exace ba ed by he po ous na u e o 3D-p in ed componen s, necessi a ing inno a i e app oaches o mee s ingen i e sa e y s anda ds. Polyme s 2024,16, 1030. h ps://doi.o g/10.3390/polym16081030 h ps://www.mdpi.com/jou nal/polyme s Polyme s 2024,16, 1030 2 o 17 To add ess his challenge, lame e a dan addi i es ha e been commonly in oduced in o polyme s, demons a ing e ec i eness ac oss a wide ange o polyme s and applica- ions [ 4 , 5 ]. Howe e , polyme ho mel ex usion (HME)-based addi i e manu ac u ing (AM) demands p ecise ma e ial heology o success ul p in ing, posing challenges in in- co po a ing addi i es wi h speci ic unc ionali ies such as lame e a dan s [ 6 – 8 ]. The e o e, solu ions ha simul aneously mee ab ica ion equi emen s and ul ill i e e a dancy c i e ia emain la gely unexplo ed. This s udy add esses his gap by ocusing on he gold s anda d polyme ma ix, poly(lac ic acid) (PLA), and employing a a ie y o comme cially a ailable halogen- ee lame e a dan s. PLA exhibi s a o able p ope ies, such as biodeg adabili y and ease o p ocessing. Mo eo e , PLA is en i onmen ally iendly and de i ed om enewable esou ces, making i an a ac i e choice o a ious applica ions, including addi i e man- u ac u ing [ 9 , 10 ]. Howe e , i is impo an o acknowledge ha PLA also has se e al disad an ages, including weak esis ance o UV, low-glass ansi ion empe a u e, e c. These limi a ions may es ic i s sui abili y o ce ain applica ions and wa an ca e ul conside a ion when selec ing PLA as a ma e ial o speci ic p ojec s o indus ies. Fu - he mo e, PLA has a ela i ely low mel ing empe a u e, ypically a ound 150–160 ◦ C, which acili a es i s p ocessing in ex usion-based AM sys ems [ 11 ]. Thus, ho mel ex- usion has eme ged as a p ominen echnique o p ocessing PLA polyme s in addi i e manu ac u ing. The li e a u e epo s ha e highligh ed he e icacy o HME in achie ing a uni o m dis ibu ion o addi i es and enhancing he mechanical p ope ies o PLA-based ma e ials [ 12 – 14 ]. This p ocess in ol es mel ing he PLA polyme a ele a ed empe a u es, mixing i wi h addi i es, and ex uding i h ough a die o o m ilamen o pelle s sui able o 3D p in ing. PLA has ound widesp ead u iliza ion in he ields o d ug deli e y [ 15 ], ood packaging [ 16 ], he au omo i e indus y [ 8 ], ailway [ 17 ], e c. These applica ions highligh he e sa ili y o PLA in a ious indus ies, u he unde sco ing i s signi icance in addi i e manu ac u ing. P e ious esea ch has in es iga ed he pe o mance o lame e a dan s in PLA, encom- passing blends o con en ional lame e a dan s like ammonium polyphospha e (APP) and expandable g aphi e (EG) [ 18 ], he impac o nano ille s combined wi h lame e a dan s (such as nanoclays wi h APP [ 19 ], aluminum hyd oxide (ATH) o EG) [ 20 – 22 ], a ious in umescen lame e a dan sys ems in ol ing APP, cha ing agen s (e.g., pen ae y h i ol), blowing agen s like is (2-hyd oxye hyl) isocyanu a e (THEIC) [ 23 ] o melamine [ 24 ], and biobased addi i es, such as lignin [ 25 ]. Addi ionally, mo e complex s uc u es, includ- ing spi ocyclic pen ae y h i ol bisphospho a e disphospho yl melamine (SPDPM) [ 26 ], phospho us–ni ogen lame e a dan s (PNFRs) [ 27 ], chi osan-based o mula ions [ 28 ], and hype b anched cha ing agen s wi h APP [ 29 ], ha e been explo ed. These lame- e a dan sys ems enhance he i e pe o mance o PLA in di e se ways. Recen s udies ha e add essed he use o lame e a dan s on 3D-p in ed polyme ic composi es [ 30 ]. One s udy in es iga ed in umescen o mula ions o PLA composed o ammonium polyphospha e, lignin, and acidic-ac i a ed mon mo illoni es, which in luence i e pe o mance based on he acidi y a e [ 31 ]. Ano he s udy demons a ed ha a me e 2 w % o APP and 0.12 w % o eso cinol bis(diphenyl phospha e) (RDP) educe he lammabili y o PLA, enabling he composi e o easily a ain he UL94 V-0 a ing, wi h RDP ac ing as a compa ibilize o APP and PLA [ 32 ]. Addi ionally, esea ch is also ocused on he addi ion o melamine polyphospha e (MPP) as a lame e a dan o PLA and i s applica ion in 3D p in ing. Thei indings e ealed ha he addi ion o MPP ende s he PLA ma ix b i le; howe e , he p esence o nanoclays es o es he impac s eng h, wi h bo h addi i es a ec ing he heology o he polyme blend [ 33 ]. Despi e he signi ican in e es in his a ea, he e emains a pauci y o comp ehensi e s udies examining he impac o lame e a dan s on he p ocessabili y o PLA polyme s: a c ucial s ep owa d he de elopmen o p in able lame- e a dan PLA g ades. Hence, he main goal o his wo k was o es ablish a ma e ial selec ion p o ocol in e ms o i e pe o mance and 3D p in abili y, acili a ing he s eamlined iden i ica ion Polyme s 2024,16, 1030 3 o 17 o ma e ials o lame- e a dan 3D p in ing. A comp ehensi e analysis encompasses i e e a dancy, he in luence on he mal and mechanical p ope ies, p in abili y, and inal objec pe o mance. Mo eo e , igo ous i e es s adhe e o Eu opean ailway egula ions [ 34 ], ensu ing he p ac ical ele ance and compliance o he de eloped lame- e a dan PLA o mula ions in eal-wo ld applica ions, pa icula ly in indus ies like ailways whe e bo h sa e y and e iciency a e pa amoun . In addi ion o add essing he echnical aspec s o ou s udy, i is c ucial o p o ide a clea jus i ica ion o ou esea ch objec i es. The ailway indus y p esen s an ideal scena io o he applica ion o lame- e a dan PLA due o s ingen sa e y egula ions and he need o ma e ials ha can wi hs and high empe a u es and educe i e haza ds. By inco po a ing lame- e a dan p ope ies in o PLA, we aim o add ess sa e y conce ns associa ed wi h adi ional ma e ials while aking ad an age o PLA’s inhe en bene i s, such as i s be e en i onmen al p o ile, biodeg adabili y, and ease o p ocessing. This aligns wi h sus ainabili y ini ia i es aimed a educing en i onmen al impac and enhancing passenge sa e y. 2. Ma e ials and Me hods 2.1. Ma e ials Poly(lac ic acid) (PLA) Ingeo™ 3D450 Na u eWo ks (Minne onka, Minneapolis, MN, USA), wi h a molecula weigh o 150,300 g · mol −1 , is used as he polyme ma ix. Di e en comme cial halogen ee- lame e a dan s (FRs) we e inco po a ed as ollows: Budi ® 620, melamine-coa ed ammonium polyphospha e, APP, om Budenheim (Budenheim, Ge - many), expandable g aphi e Fi eca b TEG 345, EG, om LKAB Mine als (Luleå, Sweden), aluminum hyd oxide Apy al 120, ATH, om Nabal ec (Schwando , Ge many), c esyl diphenyl phospha e (CDP) Dis lamoll ® , DPK, om Lanxess (Cologne, Ge many) and an in umescen lame e a dan , IM, p epa ed in si u by he combina ion o he ollowing addi i es: ammonium polyphospha e, Exoli ® AP 422, om Cla ian (Mu enz, Swi ze - land), melamine, MEL, Mela ine ® om Oci Melamine (Ba celona, Spain) and a polyhyd ic alcohol, PA, Cha mo ™ PM40 om Pe s o p (Malmö, Sweden). Mo eo e , I ganox B215 was used as a he mal s abilize . 2.2. P epa a ion o Samples PLA and lame e a dan s we e d ied a 50 ◦ C o 8 h be o e e e y sample p epa a- ion. Then, se e al o mula ions based on he PLA ma ix and con aining di e en lame e a dan s we e p epa ed by mel blending in an in e nal mixe , Plas og aph ® EC om B abende . The mixing was pe o med h ough wo s eps a he same empe a u e (180 ◦ C) and o o speed (30 pm) condi ions: in he i s s ep, he PLA and he he mal s abilize , I ganox B215, we e added un il he polyme mel ed (6 min). In he second s ep, he lame e a dan was added o achie e a well-dispe sed mix u e. The o al mixing ime was 15 min. The ba ch was ex ac ed om he mixing chambe manually and hen cooled unde ai un il eaching oom empe a u e. The ob ained samples we e g ound in a cu ing mill SM 300 om Re sch a 700 pm using a 6 mm mesh. Specimen ype 1A (1 mm hickness) o en- sile cha ac e iza ion and specimens o lexion cha ac e iza ion ( 80 mm ×10 mm ×4 mm ) we e p epa ed by 3D p in ing and he injec ion p ocess. The composi ion pe cen ages o he ma e ials men ioned in Table 1we e selec ed based on a combina ion o ac o s, including compa ibili y wi h PLA, desi ed i e- esis ance p ope ies, and he conside a ion o mechanical pe o mance. Rega ding 3D-p in ed samples, hey we e p epa ed in a Del a Wasp 4070 3D imp es- sion machine wi h a pelle ex ude . The p in ing olume was 400 × 400 × 700 mm 3 , and a clima ized and closed chambe was used o allow hea all a ound. The p in e u ilized a nozzle wi h a diame e o 0.8 mm, acili a ing he p ecise deposi ion o ma e ial. The in ill pa e n was se e ically, enhancing he s uc u al in eg i y o he p in ed objec . Main aining a p in ing empe a u e o 185 ◦ C ensu ed he p ope low and adhesion o he ilamen , while he base empe a u e was se a 40 ◦ C o p omo e adhesion o he build Polyme s 2024,16, 1030 4 o 17 pla o m. A p in ing speed o 60 mm/s was employed o s ike a balance be ween e iciency and p in quali y. The laye heigh was se a 0.3 mm, con ibu ing o he o e all esolu ion and de ail o he p in ed laye s. Table 1. Composi ions o he s udied samples. Sample Flame Re a dan Type Composi ion (w %) PLA15APP APP 15 PLA30APP APP 30 PLA7.5EG EG 7.5 PLA15EG EG 15 PLA30EG EG 30 PLA15ATH ATH 15 PLA30ATH ATH 30 PLA15IM MEL/APP/PA 15 (3.75/7.5/3.75) PLA30IM MEL/APP/PA 30 (7.5/15/7.5) PLA15CDP CDP 15 PLA30CDP CDP 30 On he o he hand, in he case o injec ed samples, specimens we e injec ion-molded unde con olled condi ions using a cons an mold empe a u e o 60 ◦ C. The injec ion p ocess in ol ed a ixed injec ion ime o 10 s and a a iable injec ion olume anging om 20 o 50 cm 3 /s, wi h a consis en p essu e o 150 ba applied h oughou . Addi ion- ally, a a iable empe a u e p o ile was employed du ing he molding p ocess, including empe a u es abo e 160 ◦C. Finally, specimens o he mo-mechanical (25 mm × 5 mm × 1.5 mm) and cone calo ime y (100 mm × 100 mm × 3 mm) e alua ions we e ob ained by comp ession molding using an LP-S-50 (LabTech, So isole, I aly) hyd aulic p ess as well as 3D p in ing. Samples we e p ehea ed a 190–195 ◦ C wi hou p essu e o 3–4 min, and he ea e , a p essu e cycle o 70 kN was applied o 1 min. 2.3. The mal, Rheological, Mechanical and Fi e Beha io The he mal s abili y o he samples was analyzed by he mog a ime ic analysis (TGA) using a TGA Q500 he mal analyze om TA Ins umen s (New Cas le, DE, USA). Samples we e hea ed om oom empe a u e o 600 ◦ C a a hea ing a e o 10 ◦ C/min unde a ni ogen a mosphe e. The molecula weigh s and numbe o a e age mola masses, M w and M n , espec i ely, o he samples we e de e mined by gel pe mea ion ch oma og aphy (GPC) using a Wa e s 717 Au osample ch oma og aph (Mil o d, MA, USA), consis ing o a pump, a e ac i e index de ec o , and Wa e s S y agel (HR2, HR4, and HR6) columns. The analysis was ca ied ou a 35 ◦ C using e ahyd o u an (THF) as an eluen ( low a e o 1 mL/min). Measu ed dis ibu ions we e e e ed o polys y ene na ow s anda ds anging om 580 o 395 · 10 3 g/mole and co ec ed wi h he uni e sal calib a ion using he Ma k Houwink pa ame e s o polys y ene: K = 1.58 ×104mL/g, α= 0.704. The c ys alliza ion and mel ing empe a u es o he polyme s we e measu ed by Di e en ial Scanning Calo ime y (DSC) wi h a TA DSC25 equipped wi h an In acoole . Ul a-pu e ni ogen was used as a pu ge gas. Samples wi h 7–8 mg o app oxima e weigh we e encapsula ed in aluminum pans. Tin and indium s anda ds we e used as calib a es. Samples we e hea ed om 0 ◦ C o 100 ◦ C a a hea ing a e o 20 ◦ C/min; hen, hey we e held a 200 ◦ C o e ase he mal his o y. La e , hey we e cooled o 0 ◦ C a 20 ◦ C/min. A e 2 min o equilib a ion a 0 ◦ C, a second hea ing scan was eco ded be ween 0 and 200 ◦ C. The deg ee o c ys allini y, Xc, was calcula ed as ollows: Xc=∆Hm−∆Hcc ∆H◦ m·(1−α) ·100 (1) Polyme s 2024,16, 1030 5 o 17 whe e ∆ H m (J/g) is he expe imen ally ob ained mel ing en halpy o he sample, ∆ H cc (J/g) is he cold c ys alliza ion en halpy, ∆H◦ m is he equilib ium mel ing en halpy (a alue o ∆H◦ m = 93 J/g o nea PLA was employed, as epo ed in he li e a u e [ 35 ]) and α is he amoun o ille , in his case, FR. The dynamic mechanical beha io o he samples was analyzed by dynamic mechani- cal analysis (DMA). To his end, samples we e cu in o s ips o 25 mm × 5 mm × 1.5 mm (leng h × wid h × hickness) and we e es ed in ensile mode on an Eplexo Gabo 100N analyze om Ne zsch, using a s a ic s ain o 0.10%. The empe a u e a ied om − 100 o 150 ◦C a a scanning a e o 2 ◦C/min and a ixed ope a ion equency o 10 Hz. A i e pe o mance e alua ion was ca ied ou by cone calo ime y in FTT equipmen acco ding o ISO 5660 [ 36 ] unde a hea lux o 50 kW/m 2 o 1200 s. The es ed sample dimensions we e 100 mm × 100 mm × 3 mm. The ime o igni ion (TTI, s), peak Hea Re- lease Ra e (HRRpeak, kW/m 2 ), To al Hea Release (THR, MJ/m 2 ), Maximum A e age Ra e Hea Emission (MARHE, kW/m 2 ), To al Smoke P oduc ion (TSP, m 2 ), Smoke Ex inc ion A ea (SEA, m 2 /kg) and CO/CO 2 a io we e eco ded. Two epe i ions o each sample we e pe o med. The sample holde was co e ed wi h me al mesh o p e en he de o ma ion and loss o ma e ial when he in umescence o samples occu s. The mel iscosi y a p ocessing empe a u es o he samples was cha ac e ized by ex usion low expe imen s, which we e pe o med in a Gö e Rheog aph 25 heome e using a capilla y die wi h L/D = 30/1. Small ampli ude oscilla o y shea (SAOS) expe i- men s we e conduc ed in linea iscoelas ic condi ions in o de o ob ain he iscoelas ic beha io o he samples a di e en empe a u es. These expe imen s we e ca ied ou in an ARES heome e (TA Ins umen s), using a 25 mm pa allel pla e geome y. 2.4. Mechanical Cha ac e iza ion o he P in ed and Injec ion-Molded Samples Tensile and lexu al measu emen s we e pe o med using a uni e sal es ing machine. Tensile es s we e pe o med in acco dance wi h ISO 178 [ 37 ], and each specimen was es ed o ailu e a 23 ◦ C a a c osshead speed o 1 mm/min. Flexu al s eng h and modulus es s we e pe o med acco ding o ISO 527 [ 38 ]. The suppo span leng h was 5 cm. The head speed was 1 mm/min. Tes specimens o ensile and lexu al measu emen s we e p epa ed bo h by he injec ion and 3D p in ing p ocesses. 3. Resul s and Discussion This s udy p e ends o es ablish a c i e ion o be applied o he selec ion o 3D p in - able i e- e a dan PLA. These ma e ials should ul ill di e en equi emen s in e ms o unc ionali y, p in abili y and mechanical pe o mance. In Table 1, he composi ions o he s udied samples a e summa ized, speci ying he ype and amoun o lame e a dan added in each case. Selec ed composi ions ha e been de ined acco ding o he ange o e ec i eness o hese lame e a dan s in iew o he in o ma ion a ailable in he li e a u e. Flame e a dan s ac ing mainly in he condensed phase APP, EG, IM, and on he gas phases ATH and CDP we e selec ed. 3.1. Fi e Beha io The i e pe o mance o 3D-p in ed ma e ials is o main ele ance o ailway applica- ions. The e o e, all p epa ed mix u es we e es ed in he cone calo ime e , and he esul s a e shown in Table 2and Figu e 1. In his able, pa ame e s ela ed o hea elease (HRR, THR, MARHE) and smoke gene a ion (CO/CO 2 a io, SEA) a e collec ed, joined wi h ime o igni ion and inal esidue. Polyme s 2024,16, 1030 6 o 17 Table 2. Resul s ob ained o hea elease and smoke gene a ion. Sample igni ion (s) HRRpeak (kW/m2) THR (MJ/m2) MARHE (kW/m2) CO/CO2 (×103) Ra io SEA (m2/kg) Residue (%) PLA 55.5 ±1.5 575.7 ±28.3 70.5 ±0.1 310.8 ±5.3 9.2 ±0.8 3.3 ±2.7 10.5 ±0.9 PLA15APP 44.5 ±0.5 226.7 ±3.4 58.2 ±0.5 174.5 ±0.3 32.3 ±1.5 12.6 ±1.2 23.7 ±0.3 PLA30APP 48.0 ±0.1 102.8 ±0.1 15.5 ±1.5 49.3 ±0.5 157.7 ±42.2 56.6 ±4.4 53.7 ±0.2 PLA7.5EG 51.5 ±0.2 306.9 ±3.1 74.2 ±1.2 165.0 ±2.5 24.6 ±0.8 6.2 ±0.6 17.0 ±1.5 PLA15EG 59.0 ±1.0 129.3 ±1.2 37.2 ±0.1 73.5 ±0.3 59.8 ±2.5 2.5 ±2.5 44.4 ±0.1 PLA30EG 45.5 ±0.5 112.3 ±5.5 22.4 ±0.3 61.1 ±0.3 114.8 ±1.9 3.7 ±2.6 45.5 ±0.7 PLA15ATH 53.0 ±0.1 463.9 ±10.0 61.1 ±1.2 261.9 ±4.0 18.3 ±0.9 4.7 ±0.2 19.9 ±1.9 PLA30ATH 56.0 ±3.1 300.8 ±0.5 51.6 ±0.6 190.7 ±4.0 21.9 ±1.1 1.8 ±0.9 15.1 ±0.6 PLA15IM 55.0 ±0.2 348.9 ±1.9 62.3 ±0.2 226.7 ±3.3 33.0 ±4.6 19 ±0.3 15.1 ±0.6 PLA30IM 54.5 ±0.5 159.1 ±2.2 24.2 ±0.5 91.3 ±2.5 67.4 ±7.7 5.8 ±1.7 48.4 ±1.4 PLA15CDP 52.0 ±1.2 546.1 ±22.1 75.5 ±3.8 306.3 ±5.6 76.5 ±2.2 254.1 ±23.5 9.0 ±0.4 PLA30CDP 49.0 ±1.1 577.0 ±5.3 65.6 ±1.2 288.6 ±5.8 182.1 ±1.5 485.8 ±5.9 6.4 ±0.6 Polyme s 2024, 16, x FOR PEER REVIEW 6 o 18 Table 2. Resul s ob ained o hea elease and smoke gene a ion. Sample igni ion (s) HRRpeak (kW/m2) THR (MJ/m2) MARHE (kW/m2) CO/CO2 (×103) Ra io SEA (m2/kg) Residue (%) PLA 55.5 ± 1.5 575.7 ± 28.3 70.5 ± 0.1 310.8 ± 5.3 9.2 ± 0.8 3.3 ± 2.7 10.5 ± 0.9 PLA15APP 44.5 ± 0.5 226.7 ± 3.4 58.2 ± 0.5 174.5 ± 0.3 32.3 ± 1.5 12.6 ± 1.2 23.7 ± 0.3 PLA30APP 48.0 ± 0.1 102.8 ± 0.1 15.5 ± 1.5 49.3 ± 0.5 157.7 ± 42.2 56.6 ± 4.4 53.7 ± 0.2 PLA7.5EG 51.5 ± 0.2 306.9 ± 3.1 74.2 ± 1.2 165.0 ± 2.5 24.6 ± 0.8 6.2 ± 0.6 17.0 ± 1.5 PLA15EG 59.0 ± 1.0 129.3 ± 1.2 37.2 ± 0.1 73.5 ± 0.3 59.8 ± 2.5 2.5 ± 2.5 44.4 ± 0.1 PLA30EG 45.5 ± 0.5 112.3 ± 5.5 22.4 ± 0.3 61.1 ± 0.3 114.8 ± 1.9 3.7 ± 2.6 45.5 ± 0.7 PLA15ATH 53.0 ± 0.1 463.9 ± 10.0 61.1 ± 1.2 261.9 ± 4.0 18.3 ± 0.9 4.7 ± 0.2 19.9 ± 1.9 PLA30ATH 56.0 ± 3.1 300.8 ± 0.5 51.6 ± 0.6 190.7 ± 4.0 21.9 ± 1.1 1.8 ± 0.9 15.1 ± 0.6 PLA15IM 55.0 ± 0.2 348.9 ± 1.9 62.3 ± 0.2 226.7 ± 3.3 33.0 ± 4.6 19 ± 0.3 15.1 ± 0.6 PLA30IM 54.5 ± 0.5 159.1 ± 2.2 24.2 ± 0.5 91.3 ± 2.5 67.4 ± 7.7 5.8 ± 1.7 48.4 ± 1.4 PLA15CDP 52.0 ± 1.2 546.1 ± 22.1 75.5 ± 3.8 306.3 ± 5.6 76.5 ± 2.2 254.1 ± 23.5 9.0 ± 0.4 PLA30CDP 49.0 ± 1.1 577.0 ± 5.3 65.6 ±1.2 288.6 ± 5.8 182.1 ± 1.5 485.8 ± 5.9 6.4 ± 0.6 Figu e 1. ARHE s. ime plo s o diffe en PLA/FR o mula ions (A) and compa a i e i e beha io h ough Pe ella plo s (B). Time is a key ac o o gua an ee a sa e e acua ion in he e en o i e. ARHE (a e age a e o hea emission), Figu e 1, conside s he amoun o eleased hea along combus ion, bu i is modula ed in ela ion o he ime ha hea is gene a ed. The MARHE (maximum o ARHRE) (Table 2) is used as he c i e ia o classi ica ion in ailway Eu opean egula ion Figu e 1. ARHE s. ime plo s o di e en PLA/FR o mula ions (A) and compa a i e i e beha io h ough Pe ella plo s (B). Time is a key ac o o gua an ee a sa e e acua ion in he e en o i e. ARHE (a e age a e o hea emission), Figu e 1, conside s he amoun o eleased hea along combus ion, bu i is modula ed in ela ion o he ime ha hea is gene a ed. The MARHE (maximum o ARHRE) (Table 2) is used as he c i e ia o classi ica ion in ailway Eu opean egula ion EN45545 [ 34 ]. I is o no e ha , ega dless o he lame e a dan used, no sel -ex inguishable Polyme s 2024,16, 1030 7 o 17 mix u es we e ob ained wi hin his wo k. Howe e , in all he cases, MARHE alues dec eased in ela ion o he amoun o lame e a dan p esen ed in he FR-PLA composi ion, wi h ce ain solu ions (EG, APP, IM) mo e e ec i e han o he s (ATH, CDP). This obse ed end aligns wi h indings epo ed by o he esea che s in he scien i ic communi y [ 39 – 41 ]. This is clea ly shown in he Pe ella plo s, Figu e 1B. In his ep esen a ion, he o al hea elease (THR) as a unc ion o he HRR peak/igni ion ime a io is p esen ed. Thus, an imp o ed lame e a dancy is depic ed as a alue in he le -down pa o he plo . Rep esen ed da a o he di e en mix u es showed an imp o emen in i e pe o mance o pu e PLA, al hough wi h signi ican di e ences. Da a end o g oup in wo egions o he g aph, and a gene al end is obse ed. As expec ed, he highe he FR con en , he be e he i e beha io . Howe e , i is no ewo hy ha ATH and, especially, CDP- based ma e ials showed poo beha io agains i e, e en a a high FR con en . In con as , ma e ials con aining APP, EG as well as he in umescen mix u e showed mo e e ec i e i e pe o mance. Figu e 2shows pic u es o he ob ained esidues o cone calo ime ic- es ed samples. All ma e ials p esen ed a esidue, e en o e e ence PLA, co esponding o he ino ganic ille s al eady p esen in his comme cial g ade. Be ween he di e en samples, be e i e pe o mance was di ec ly ela ed o he highe esidue emaining a e combus ion [ 42 ]. I is ep esen ed in he pic u es and co ela es wi h he esul s p esen ed in Table 2. I can be concluded ha he mos e ec i e ac ion o lame e a dan s is p oduced in he condensed phase, wi h he special con ibu ion o he in umescen e ec o EG, APP, and IM blends. Flame e a dan s ha mainly wo k in he gas phase, such as CDP, showed mino lame- e a dan e ec s. ATH, ac ing bo h in he gas and he condensed phase, has an in e media e pe o mance. This conclusion is consis en wi h nume ous s udies epo ed in he li e a u e. Resea che s ha e consis en ly obse ed ha lame e a dan s wi h s ong ac ion in he condensed phase, such as in umescen sys ems con aining EG, APP, and IM, end o exhibi supe io lame- e a dan p ope ies. These sys ems wo k by o ming a p o ec i e cha laye upon exposu e o hea , e ec i ely insula ing he unde lying ma e ial om he lame [ 43 ]. Con e sely, phospho ous-based lame e a dan s p ima ily ope a ing in he gas phase, like CDP, ypically o e limi ed lame- e a dan e ec s as hey mainly ac by sca enging ee adicals and in e e ing wi h he combus ion p ocess [ 44 ]. ATH, known o i s abili y o elease wa e apo and cool he ma e ial du ing combus ion, occupies an in e media e posi ion due o i s dual-phase ac ion [ 45 ]. This collec i e body o e idence unde sco es he impo ance o conside ing bo h gas and condensed phase mechanisms when e alua ing he e icacy o non-halogena ed lame e a dan s in polyme ic ma e ials. Polyme s 2024, 16, x FOR PEER REVIEW 7 o 18 EN45545 [34]. I is o no e ha , ega dless o he lame e a dan used, no sel -ex inguish- able mix u es we e ob ained wi hin his wo k. Howe e , in all he cases, MARHE alues dec eased in ela ion o he amoun o lame e a dan p esen ed in he FR-PLA composi- ion, wi h ce ain solu ions (EG, APP, IM) mo e effec i e han o he s (ATH, CDP). This obse ed end aligns wi h indings epo ed by o he esea che s in he scien i ic com- muni y [39–41]. This is clea ly shown in he Pe ella plo s, Figu e 1B. In his ep esen a ion, he o al hea elease (THR) as a unc ion o he HRR peak/igni ion ime a io is p esen ed. Thus, an imp o ed lame e a dancy is depic ed as a alue in he le -down pa o he plo . Rep esen ed da a o he diffe en mix u es showed an imp o emen in i e pe o mance o pu e PLA, al hough wi h signi ican diffe ences. Da a end o g oup in wo egions o he g aph, and a gene al end is obse ed. As expec ed, he highe he FR con en , he be e he i e beha io . Howe e , i is no ewo hy ha ATH and, especially, CDP-based ma e ials showed poo beha io agains i e, e en a a high FR con en . In con as , ma e- ials con aining APP, EG as well as he in umescen mix u e showed mo e effec i e i e pe o mance. Figu e 2 shows pic u es o he ob ained esidues o cone calo ime ic- es ed samples. All ma e ials p esen ed a esidue, e en o e e ence PLA, co esponding o he ino ganic ille s al eady p esen in his comme cial g ade. Be ween he diffe en samples, be e i e pe o mance was di ec ly ela ed o he highe esidue emaining a e combus ion [42]. I is ep esen ed in he pic u es and co ela es wi h he esul s p esen ed in Table 2. I can be concluded ha he mos effec i e ac ion o lame e a dan s is p oduced in he con- densed phase, wi h he special con ibu ion o he in umescen effec o EG, APP, and IM blends. Flame e a dan s ha mainly wo k in he gas phase, such as CDP, showed mino lame- e a dan effec s. ATH, ac ing bo h in he gas and he condensed phase, has an in- e media e pe o mance. This conclusion is consis en wi h nume ous s udies epo ed in he li e a u e. Resea che s ha e consis en ly obse ed ha lame e a dan s wi h s ong ac ion in he condensed phase, such as in umescen sys ems con aining EG, APP, and IM, end o exhibi supe io lame- e a dan p ope ies. These sys ems wo k by o ming a p o- ec i e cha laye upon exposu e o hea , effec i ely insula ing he unde lying ma e ial om he lame [43]. Con e sely, phospho ous-based lame e a dan s p ima ily ope a ing in he gas phase, like CDP, ypically offe limi ed lame- e a dan effec s as hey mainly ac by sca enging ee adicals and in e e ing wi h he combus ion p ocess [44]. ATH, known o i s abili y o elease wa e apo and cool he ma e ial du ing combus ion, oc- cupies an in e media e posi ion due o i s dual-phase ac ion [45]. This collec i e body o e idence unde sco es he impo ance o conside ing bo h gas and condensed phase mech- anisms when e alua ing he efficacy o non-halogena ed lame e a dan s in polyme ic ma e ials. Figu e 2. Ob ained esidues o cone calo ime e - es ed samples wi h diffe en lame e a dan s. Figu e 2. Ob ained esidues o cone calo ime e - es ed samples wi h di e en lame e a dan s. P eselec ed samples (APP, EG, IM) p esen ed a MARHE alue below 90 kW/m 2 , which makes hem u u e candida es o use in ailway applica ions acco ding o he EN 45545-2 s anda d [ 34 ]. MARHE alues lowe han 90 kW/m 2 compliance wi h one o he Polyme s 2024,16, 1030 8 o 17 demands o R1 (in e io e ical su aces) and R7 (ex e nal su aces) equi emen s allowed he use o hese ma e ials o ce ain applica ions (Table S1, haza d le el 2 (HL2)). PLA wi h 30% o APP showed a MARHE below 60 KW/m 2 , making i sui able o he mos demanding haza d le el, HL3 (Table S1) [34]. The addi ion o lame e a dan s o he PLA ma ix inc eased he o al smoke p oduc- ion (TSP) as well as he da kness o his smoke (speci ic ex inc ion a ea, SEA), excep o he expandable g aphi e and ATH o mula ions. Released gases in e e ing in he combus ion eac ion in he gas phase could explain he highe smoke gene a ion o CDP, APP, and in umescen blend solu ions [ 46 ]. Addi ionally, he non-comple e combus ion induced by lame e a dan s esul ed in an inc ease in he CO/CO 2 a io. Flame e a dan s able o elease phospho us-based adicals, ac ing in he gas phase (CDP and in mino p opo ion APP), clea ly showed his e ec [47,48]. 3.2. E ec o FR Addi ion on he Physico-Chemical, The mal and Mechanical P ope ies I is well-known ha he inco po a ion o lame e a dan s migh in e e e wi h he he mal and he mo-mechanical p ope ies o he polyme ma ix, al e ing he pe o mance o he blend [ 49 ]. The empe a u e and he deg ada ion mechanism o lame e a dan s and, speci ically, he eleased e luen s du ing he combus ion p ocess migh in e e e wi h he deg ada ion pa h o he polyme . The TGA he mog ams shown in Figu e 3p esen he mass change in he samples as a unc ion o empe a u e. Fo compa ison, each g aphic shows he weigh loss s. empe a u e o e e ence o PLA, including each indi idual lame e a dan and heo e ical and eal cu e o lame e a dan PLA compounds (g aphics o he PLA pelle , e e ence PLA, and I ganox a e included in Figu e S1). Mo eo e , he ini ial mass loss (T i ) empe a u e da a om he TGA he mog ams we e ob ained conside ing he loss o 5% o he ini ial mass, he maximum mass loss a e empe a u e (T max ), which was de e mined by he maximum alue o he i s de i a i e; i he mass emained a 600 ◦ C, i was de ined as a esidue (Table 3). Polyme s 2024, 16, x FOR PEER REVIEW 8 o 18 P eselec ed samples (APP, EG, IM) p esen ed a MARHE alue below 90 kW/m2, which makes hem u u e candida es o use in ailway applica ions acco ding o he EN 45545-2 s anda d [34]. MARHE alues lowe han 90 kW/m2 compliance wi h one o he demands o R1 (in e io e ical su aces) and R7 (ex e nal su aces) equi emen s al- lowed he use o hese ma e ials o ce ain applica ions (Table S1, haza d le el 2 (HL2)). PLA wi h 30% o APP showed a MARHE below 60 KW/m2, making i sui able o he mos demanding haza d le el, HL3 (Table S1) [34]. The addi ion o lame e a dan s o he PLA ma ix inc eased he o al smoke p o- duc ion (TSP) as well as he da kness o his smoke (speci ic ex inc ion a ea, SEA), excep o he expandable g aphi e and ATH o mula ions. Released gases in e e ing in he com- bus ion eac ion in he gas phase could explain he highe smoke gene a ion o CDP, APP, and in umescen blend solu ions [46]. Addi ionally, he non-comple e combus ion in- duced by lame e a dan s esul ed in an inc ease in he CO/CO2 a io. Flame e a dan s able o elease phospho us-based adicals, ac ing in he gas phase (CDP and in mino p o- po ion APP), clea ly showed his effec [47,48]. 3.2. Effec o FR Addi ion on he Physico-Chemical, The mal and Mechanical P ope ies I is well-known ha he inco po a ion o lame e a dan s migh in e e e wi h he he mal and he mo-mechanical p ope ies o he polyme ma ix, al e ing he pe o - mance o he blend [49]. The empe a u e and he deg ada ion mechanism o lame e- a dan s and, speci ically, he eleased effluen s du ing he combus ion p ocess migh in- e e e wi h he deg ada ion pa h o he polyme . The TGA he mog ams shown in Figu e 3 p esen he mass change in he samples as a unc ion o empe a u e. Fo compa ison, each g aphic shows he weigh loss s. empe a u e o e e ence o PLA, including each indi idual lame e a dan and heo e ical and eal cu e o lame e a dan PLA com- pounds (g aphics o he PLA pelle , e e ence PLA, and I ganox a e included in Figu e S1). Mo eo e , he ini ial mass loss (Ti) empe a u e da a om he TGA he mog ams we e ob ained conside ing he loss o 5% o he ini ial mass, he maximum mass loss a e em- pe a u e (Tmax), which was de e mined by he maximum alue o he i s de i a i e; i he mass emained a 600 °C, i was de ined as a esidue (Table 3). Figu e 3. TGA o he selec ed o mula ions, PLA 30APP, PLA30EG, PLA30ATH, PLA30IM, and PLA30CDP. Figu e 3. TGA o he selec ed o mula ions, PLA 30APP, PLA30EG, PLA30ATH, PLA30IM, and PLA30CDP. Polyme s 2024,16, 1030 9 o 17 Table 3. Ti, Tmax and esidual mass alues ob ained om TGA and DTGA he mog ams. Sample Ti(◦C) Tmax (◦C) Residual Cha (%) PLA 330 360 11 PLA30APP 327 350 30 PLA30EG 133 175/360 32 PLA30ATH 273 305 28 PLA30IM 248 300 25 PLA30CDP 242 318 8.2 Re e ence PLA he mally deg ades in a single s ep, s a ing a a ound 330 ◦ C and lea ing a esidue o 11 w %. In gene al, PLA deg ada ion is a complex p ocess in ol ing andom chain scission in amolecula anses e i ica ions and selec i e depolyme iza ion s eps highly dependen on composi ion in e ms o s e eoisome ism, molecula weigh , and he p esence o mois u e, ca alys s, esidual monome s, and impu i ies [ 50 , 51 ]. Howe e , PLA does no lea e a esidual cha e en in an oxida i e a mosphe e. Thus, he measu ed esidue is ela ed o he p esence o ino ganic ille s on he comme cial-g ade PLA. The inco po a ion o lame e a dan s signi ican ly a ec s he he mal s abili y o he PLA/FR blend since he ini ial deg ada ion empe a u e and/o he deg ada ion s ages a e al e ed, as is e idenced by he di e ence be ween he heo e ical (no in e ac ion be ween PLA and lame e a dan s) and eal pe o mance. A ending o weigh loss, he de imen al e ec on PLA s abili y de i ed om he inco po a ion o ATH, EG, and IM as lame e a dan s was no iceable. In hese blends, he eal cu e shows highe weigh loss han he heo e ical cu e. In he case o EG, he signi ican deg ada ion o PLA/EG mix u es can be p oduced by he p omo ion o he hyd oly ic deg ada ion o PLA unde he ac ion o gases om he decomposi ion o expandable g aphi e. This gas, o an acid na u e (sul u ic acid) [ 52 ], can be eleased a PLA p ocessing empe a u es (170–180 ◦ C). This is an unexpec ed esul , as nume ous wo ks ha e epo ed he bene icial e ec o EG on i e pe o mance in di e en polyme s, such as polyole ins [ 53 ] and polyu e hanes [ 54 ]. In pa icula , p e ious s udies ha e demons a ed syne gis ic e ec s in lame- e a ded polylac ide wi h di e en lame e a dan s, such as blends o APP/EG [ 18 ], wi h he he mal s abili y o FR/PLA blends o e 300 ◦ C. Mo eo e , i was also obse ed in he li e a u e ha he blends o EG and clays show an inc ease in he he mal s abili y o PLA a he same ime, which imp o es he mechanical p ope ies and educes he lame p opaga ion a e by he o ma ion o a compac cha [ 21 ]. In ano he s udy, Mu a iu desc ibes how he addi ion o 6% o expanded g aphi e educes he molecula weigh o a PLA o a ound 50% [ 55 ], which is ela ed o he p esence o impu i ies. This a ec s mechanical p ope ies wi h lowe ensile s eng h, highe elas ic modulus, and be e i e pe o mance compa ed o plain PLA ( educ ion o 30% pHRR wi h 12% ille ). Howe e , i is known ha EGs could di e in p ope ies depending on di e en pa ame e s, such as he aw g aphi e used, he amoun and na u e o he in e cala ed species, and he p oduc ion p ocess. This ende s a ange o ma e ials ha di e in he se -up in umescence empe a u e and expansion a e. Di e en EG g ades could add ess di e en polyme blend pe o mances. In he p esen s udy, he na u e o he used EG migh ha e a ec ed he s abili y o he polyme , leading o a loss o he mal s abili y. Simila pe o mance has been ob ained o he PLA30IM o mula ion. In his case, he lowe deg ada ion empe a u e could be a ibu ed o he ea ly decomposi ion o he melamine-blowing agen and pen ae y h i ol, p omo ing he hyd oly ic deg ada ion o PLA. Simila esul s ha e been app ecia ed in PLA ATH blends, whe e he dehyd a ion o ATH p omo es he hyd olyza ion o es e bonds in PLA. Ins ead, he addi ion o APP o CDP has li le e ec on he he mal s abili y o he polyme . O e all, a highe inal esidue han heo e ical, ega dless o whe he hey a ec he s abili y o he polyme , p o es he condensed phase ac ion o mo e e ec i e lame e a dan s: APP, EG, and IM. 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