Sustainable Approach for Mechanical Recycling of Poly(lactic acid

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Materials and Interfaces

A Sustainable Approach for Mechanical Recycling of Poly (lactic acid)/ Cellulose Nanocrystal Films: Investigations on Structure-Property Relationship and Underlying Mechanism Prodyut Dhar, Rajesh Kumar Murugan, Siddharth Mohan Bhasney, Purabi Bhagabati, Amit Kumar, and Vimal Katiyar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02658 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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Industrial & Engineering Chemistry Research

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A Sustainable Approach for Mechanical Recycling of Poly (lactic acid)/ Cellulose

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Nanocrystal Films: Investigations on Structure-Property Relationship and Underlying

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Mechanism

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Prodyut Dhar1, 2, Rajesh Kumar M3, Siddharth Mohan Bhasney1, Purabi Bhagabati1, Amit Kumar1 and Vimal Katiyar1*

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1

Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, 781039, Assam, India. 2 Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, P.O. Box 16300, 0076, Aalto, Finland. 3 Department of Nanoscience and Technology, Bharathiar University, Coimbatore, 638752, Tami Nadu, India

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* Corresponding Author, Email ID: [email protected]

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ABSTRACT

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This paper presents a green and sustainable route for mechanical recycling of poly(lactic

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acid)/cellulose nanocrystals based films for multiple times, which results enhanced thermal,

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rheological and structural properties along with improved processability. Recycling of reactively

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extruded PLA/CNC films in presence dicumyl peroxide (DCP) was carried out with sulphuric and

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hydrochloric acid hydrolysed CNCs (CNC-S and CNC-Cl)shows improved thermal

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stability(improved by 12°C), consistent Mw characteristics(180-150kDa) and enhanced melt-

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strength as evident from the thermal degradation studies and visco-elastic properties measured

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from rheological studies. The improved recyclability of PLA/CNC films was evident from

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enhanced complex viscosity and storage modulus of melt by ~4 and 10 times along with increased

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mechanical strength of~16-30% even after third recycling. Therefore, the present study provides

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a novel route to recycle PLA based CNC films after their service life into value-added

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biodegradable products with adequate properties competitive enough to replace the petroleum

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based conventional plastics for commodity applications.

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KEYWORDS: Poly(lactic acid) [PLA], Cellulose nanocrystals [CNC], Mechanical Recycling,

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Reactive extrusion.

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1. INTRODUCTION

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There is a growing interest for the replacement of petroleum based polymers with bio-based

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polymers as they are readily degradable, sustainable and eco-friendly in nature alongwith the

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structural and physico-chemical properties comparable to conventional polymers. However,

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higher production costs, poor processability and difficultly in recyclability using industrial scale

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machineries have hindered in large scale commercialization of such bio-based polymers[1].

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Among the various class of bio-based polymers, Poly (lactic acid) (PLA) is best suited for

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replacement of conventional petroleum based polymers for packaging applications, due to its

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improved mechanical, thermal and barrier properties [2]. Moreover, PLA is a readily compostable

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and biodegradable thermoplastic which belongs to a family of aliphatic polyesters commonly made

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from α-hydroxy acids derived from renewable resources such as corn and sugar beets. Due to its

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compostable nature, the challenges associated with conventional disposal methods of non-

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degradable synthetic polymers through incineration and secure landfills, can be easily overcomed.

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Further, PLA and its degraded products are not-toxic and non-carcinogenic hence it is approved

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by FDA for potential applications in biomedical engineering and healthcare related products such

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as sutures, tissue engineering scaffolds, drug delivery & fracture fixation implants[3].

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PLA based products are generally costly in nature and their degradation through composting after

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the service life, makes it economically costly process for production through industrial routes.


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With the rise in prices of fossil based fuels, the petroleum derived polymers are also becoming

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costlier day-by-day but they are known to be recycled for several times which results in their

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reduction of costs by several orders. Therefore, efficient routes for recycling of PLA for several

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number of times will provide a unique opportunity to reduce its costs by several magnitudes. In

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literature, PLA have been known to be recovered by several routes such as chemical recycling,

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mechanical recycling, energy recovery and composting. Among them the life cycle assessment

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studies indicate that mechanical recycling are eco-friendly solvent free processes with more energy

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efficient, reduced emission of toxic gases and low capital investment[4]. Skrifvars et al. 2016[5],

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studied that mechanical recycling of PLA reinforced wood composites shows decrement in the

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structural properties by ~23% after processing for five times. Recent reports by Torpanyacharn et

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al. 2018[6], shows that chemical recycling of PLA with poly(ethylene terephthalate)(PET) through

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curing process results in formation of thermoset (co)polyester but had relatively poor thermo-

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mechanical properties with non-degradability characteristics. Beltrán et al. 2017, studied the

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impact of two different recycling processes on physico-chemical properties of melt compounded

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PLA films through accelerated thermal and photochemical ageing followed by washing[7]. It was

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observed that the characteristic molecular weight of PLA decreased significantly which resulted

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in poor mechanical and rheological properties. Other approaches to chemically recycle PLA into

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lactic acid oligomers [8] or lactide (precursor for PLA synthesis)[9] have been developed through

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green-solvent based methods to develop value-added chemicals. Most of the recently reported

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studies on effective recycling of PLA, have focused on utilization of reinforcing agents (modified

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cellulose fibers/nanofibrils)[10] [11], additions of various dyeing agents to fabricate melt spun

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fibers[12] and using silica templates to develop PLA nanoparticles[13]. However, the reported

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studies were focused on utilization of recycled PLA for development of value-added



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chemicals/products with meagre improvement in some properties but doesn’t provides solutions

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to effectively recycle PLA based products through industrially viable extrusion process with

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adequate properties required for product development.

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In order to recycle PLA through large-scale industrial level methods such as extrusion, injection

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moulding, blow moulding and thermoforming the polymer must possess adequate thermal stability

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to prevent degradation and retain its inherent molecular weight properties. But PLA is highly

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sensitive to processing conditions such as high temperature, moisture and shearing force resulting

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in drastic molecular weight reduction which subsequently deteriorates its intrinsic properties[14]

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[15] (14, 15). The thermal processing of PLA through industrial scale equipment results in random

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chain scissions involving complex steps of oxidative degradation, hydrolytic degradation, cis-

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elimination and intra/inter-molecular transesterification, depending upon the processing

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conditions followed[16]. Moreover, for the PLA synthesized from chemical routes contains active

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chain end-groups and impurities such as unreacted monomers or catalysts in trace quantities which

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are known to enhance the degradation process[17]. This results in drastic reduction in molecular

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weight which leads to enhanced brittleness, lowered thermal stability and melt strength of PLA

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resulting in reduced processability and poor inherent properties. Several routes for chemical

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modifications, copolymerizations[18], blending with other polymers[19][20] and reactive

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compatibilization with other immiscible polymers using chain extenders[21] have been widely

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studied. Out of the abovementioned approaches, reactive extrusion based processing of PLA have

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proved to be most efficient route with formation of long and branched PLA extensions which

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prevents the degradation. This is due to the ability of chain extenders to chemically graft the low

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molecular weight PLA chains formed during thermal processing which leads to the improved melt

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strength and enhanced processability[22]. Till date several type of chain extenders have been


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widely used for processing of PLA such as 4,4-‐‑methylene diphenyl diisocyanate [23], pyromellitic

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dianhydride[24], polycarbodiimide [25], tris(nony1-phenyl) phosphite[26], triallyl isocyanurate

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[27], cardanol (bio-based phenolic derivatives) etc. It was observed that the reactively processed

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PLA were thermally stable, with higher melt strength resulting in improved processability and

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reduction in the PLA chain cleavages during thermal processing with provision for continuous

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production[28]. However, reactive extrusion of PLA in presence of different fillers and their

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potential for recyclability have been seldom studied, which are utmost required for improving the

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structural, oxygen and water vapour barrier properties of PLA, as discussed subsequently.

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Reinforcing PLA matrix with organic/inorganic micro/nanofillers and bio-fillers have been widely

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used to improve its thermal stability, heat distortion temperature, crystallization behaviour, barrier

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properties and broaden the melt processing window[14]. However, due to high functionality and

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surface-energy of such nanofillers, they tend to get agglomerate and acts as catalytically active

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sites to initiate the degradation of PLA backbone during extrusion process. Moreover, most of the

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studies reported in literature on recycling of PLA and its composites, have shown random chain

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scissions of PLA during thermal recycling with significant reduction in their molecular weight

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which subsequently leads to deteriorated structural and physico-chemical properties[29]. Several

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studies on utilization of different types of fillers such as starch [30], carbon nanotubes[31], lignin

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[32]. silk fibers [33] and other immiscible polymers such as polyamide11 [34], poly(acrylonitrile–

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butadiene–styrene)[35] [36], Poly(butylene adipate-co-terephthalate)[37], glycerol based

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polyesters[38] and cyclodextrin [39] have been recently reported through reactive extrusion

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process which result in their superior performance. Among the different class of nanofillers, CNCs

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have been widely used to reinforce the PLA matrix which leads to improved structural and barrier

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properties of the developed nanocomposite films along with enhanced biodegradability. However,



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CNCs due to their hydrophilic nature and presence of sulphate groups on their surface (adhered

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during sulphuric acid hydrolysis) undergoes agglomeration and initiates degradation of PLA

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backbone. In the literature, the problems related to dispersion of the CNCs in the PLA matrix have

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been overcome through three different approaches: (i) adsorption of the hydrophilic polymers onto

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CNC surfaces[40], (ii) strategic chemical modifications such as acetylation[41], epoxidation[42],

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esterification[43] and modification with hydrophobic polymers through grafting on and from

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approach[44] (iii) reactive processing with addition of the chain extenders[45]. Other routes such

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as modification of the extrusion process and liquid feeding of CNCs[46] [47]or through strategic

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electrospinning process results in improved dispersion of CNCs at higher volume fractions[48].

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The reactive extrusion of PLA with CNCs, have been recently reported with the addition of radical

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grafting agent glycidyl methacrylate in presence of polyvinyl alcohol(PVA) emulsion system [49]

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with initiator ammonium cerium nitrate, which results in improvement in structural strength by

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18% with improved biodegradability. Another study shows improved thermal stability and

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crystallization properties of Polyhydroxybutyrate (PHB)/cellulose fiber composites which is

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achieved through interfacial grafting of PHB onto cellulose fibers through insitu reactive extrusion.

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However, studies related to reactive extrusion of PLA with CNCs have been seldom reported in

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literature[50] [51] with the investigations on their recycling capabilities and evaluation of

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properties have been investigated in the current study.

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To overcome the limitations and challenges associated with processing and recycling of PLA/CNC

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nanocomposites, a novel single step and sustainable approach for reprocessing of biodegradable

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polymers for multiple cycles have been reported. In the current study, we utilize a biodegradable,

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non-toxic and green radical initiator, dicumyl peroxide (DCP) to covalently crosslink PLA/CNC

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through insitu reactive extrusion, which subsequently shows enhanced thermal and molecular


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weight stability when reprocessed for several cycles. A recent report by Arkema[52], shows that

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the cross-linking agent used in this study, DCP is inherently biodegradable, with no known toxicity

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and mutagenicity when examined through in vitro genetic studies. Moreover, quantitative risk

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assessment studies have shown it to be safe towards environment and due to its poor solubility in

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water doesn’t causes any contamination or aquatic pollution. With the addition, of trace quantities

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of such ´green´ and sustainable radical initiator during the recycling steps, it was observed that the

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structural and physico-chemical properties of the nanocomposites can be efficiently tailored

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depending upon targeted applications. The effect of recycling the reactively extruded PLA/CNC

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films on characteristic molecular weight and its distribution was studied through gel permeation

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chromatography (GPC). A detailed thermo-mechanical, crystallization and rheological

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investigations were carried out to determine the effect of recycling as well as covalent couplings

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during the insitu extrusion process and a basic mechanism undergoing during the phenomenon was

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proposed. To the best of our knowledge, recycling of thermally stable PLA/CNC films for multiple

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times through reactive extrusion process with or without addition of DCP have not been reported

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in literature and provides a unique industrially scalable, green and sustainable approach to

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reprocess PLA/CNC based biodegradable polymers into value-added products with adequate

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properties required for commodity applications.

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2. MATERIALS AND METHODS

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2.1 Materials

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Poly L-lactic Acid (PLA) granules (grade: PLA 4032D with weight average (Mw) and number

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average molecular weight (Mn) characteristics of ~200 and ~150kDa respectively) was procured

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from Nature Works® LLC, USA. CNCs, the reinforcing agents utilized in this study are fabricated

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from Whatmann® filter paper (Grade 1) as cellulosic source through sulphuric and hydrochloric



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acid (SISCO Research laboratories (SRL), India) based hydrolysis approach. Dicumyl peroxide

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(DCP), used as radical initiator in this study was purchased from Sigma Aldrich, India.

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2.2 Fabrication of the Cellulose Nanocrystals through Acid Hydrolysis.

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Two different type of CNCs were derived using sulphuric and hydrochloric acid based hydrolysis

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approach, using the filter paper as precursor which is disintegrated into cellulosic slurry through

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ultrasonic homogenization (at amplitude of ∼20% for ∼10 min using Biologics, 3000MP).

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Sulphuric acid hydrolysed CNCs (CNC-S), with cellulose slurry to acid solution ratio (1:10

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vol/vol) was hydrolysed for 2 hours at 1000 rpm using sulphuric acid (64 wt. %, 2 L) in a 5 L

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reaction vessel. For hydrochloric acid derived CNCs (CNC-Cl), the cellulose slurry was treated

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with hydrochloric acid (6 M, 2 L) at 120 °C for 6 hour (stirred at∼1000 rpm). The CNC suspension

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after hydrolysis was diluted with chilled ionized water (to stop the reaction), followed by dialysis

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(using cellulose acetate membranes with cut of molecular weight ∼14 kDa purchased from Sigma–

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Aldrich, India) to remove the excess acid until the pH reduced to ∼7. Finally, the neutralized CNC

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suspension was freeze dried into powdered form through lyophilisation process (using Scanvac,

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Denamrk).

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2.3 Recycling of reactively extruded PLA/CNC (PLA-g-CNC) nanocomposites.

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Recycling of the reactively extruded PLA/CNC nanocomposites were carried out for three times

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with and without addition of the DCP, as cross-linking agents. Firstly, PLA pellets were sprayed

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with DCP dissolved in acetone (at 1wt. %) followed by drying under vacuum for overnight and

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mixed with sulphuric and hydrochloric acid derived CNCs at various loading (~1 and 2wt. %

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respectively). Thereafter, the DCP coated PLA pellets and CNCs were processed initially in a twin-

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screw extruder to fabricate the reactively grafted PLA/CNC sheets. For the first recycle studies

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(R1), PLA-g-CNC films were cut into small sizes and feed into the twin-screw extruder (Haake


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Rheomix) maintained at a temperature of ~185°C, screw speed of ~50 rpm and residence time of

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~5 min and processed into film strips. The reprocessed films were hereafter named for first recycle

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as PLA/DCNC-S-1p-R1 and PLA/DCNC-S-2p-R1 (for 1 and 2wt. % CNC loadings) with CNC-S

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as nanofiller and with CNC-Cl as filler as PLA/DCNC-Cl-1p-R1 and PLA/DCNC-Cl-2p-R1 (for

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1 and 2wt. % CNC loadings respectively).

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For the subsequent recycling studies for second (R2) and third (R3) time, the PLA-g-CNC extruded

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strips were cut (0.5-2 cm) and reprocessed at similar operating conditions in the twin-screw

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extruder. The films after the second recycle were termed as PLA/DCNC-S-1p-R2 and

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PLA/DCNC-S-2p-R2 (for 1 and 2wt. % CNC loadings) with CNC-S as nanofiller and with CNC-

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Cl as filler as PLA/DCNC-Cl-1p-R2 and PLA/DCNC-Cl-2p-R2 (for 1 and 2wt. % CNC loadings

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respectively). Subsequently, after third cycle the films are designated as PLA/DCNC-S-1p-R3 and

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PLA/DCNC-S-2p-R3 (for 1 and 2wt. % CNC loadings) with CNC-S as nanofiller and with CNC-

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Cl as filler as PLA/DCNC-Cl-1p-R3 and PLA/DCNC-Cl-2p-R3 (for 1 and 2wt. % CNC loadings

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respectively).

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To understand the effect of DCP on recycling, chopped PLA-g-CNC extruded strips after the

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second recycle were spray coated with DCP (at 1wt. % loading) followed by reprocessing in the

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extruder under similar processing conditions. The reprocessed films with DCP were hereafter

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named as PLA/DCNC-S-2p-R3-DCP and PLA/DCNC-Cl-2p-R3-DCP with the incorporation of

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CNC-S and CNC-Cl as nanofillers (at 2wt. % loadings) respectively.

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3. ANALYTICAL INSTRUMENTATION AND CHARACTERIZATIONS

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3.1 Gel Permeation Chromatography (GPC)



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To determine the molecular weight, 15-20 mg of the reprocessed PLA/CNC samples were

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dissolved in HPLC chloroform (for 3 days) and filtered before analysis (0.2 µm filter). The samples

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were injected (5-10 µl) to precalibrated high performance liquid chromatography (HPLC),

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Shimadzu LC-20A system (Shimadzu, Japan) equipped with two PLgel 5 µm mixed D columns

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(Agilent, UK) in series.

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3.2 Field Emission Scanning Electron Micrograph (FESEM)

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The surface morphology of recycled PLA/CNC films (coated with gold for 180s in a sputtering

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unit) were observed with FESEM (ZEISS, USA) at an accelerating voltage of 2kV.

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3.3 Thermogravimetric Analysis (TGA)

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Recycled PLA/CNC films (~4-5 mg) were heated from 25-600ºC at a heating rate of 10ºC/min to

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determine the thermal degradation behaviour using TGA (Perkin Elmer STA) under inert nitrogen

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atmosphere.

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3.4 Differential Scanning Calorimetry (DSC)

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DSC (Netzsch, Germany) calibrated with Indium standards was used to determine the thermal

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properties of reprocessed films (~5 mg) with the following program: temperature ramped from 25

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to 200ºC at scanning rate of 10ºC/min for the first heating cycle followed by the cooling from 200

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to 25ºC at cooling rate of 10ºC/min and finally a second heating from 25 to 200ºC at heating rate

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of 10ºC/min.

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3.5 Rheology

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Rheological behaviour of reprocessed PLA/CNC films were studied with rheometer (Anton Paar,

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Physica MCR 301) fitted with parallel plate (PP50) at a constant temperature of 180ºC and plate

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gap of 0.1 mm. Initially, dynamic strain sweep test was carried to evaluate the linear viscoelastic


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region, thereafter, small amplitude oscillatory shear studies were measured at a constant 1% strain

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from frequency range of 0.1 to 100 rad/s.

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3.6 Universal Tensile Machine (UTM)

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To determine the mechanical behaviour of recycled PLA/CNC films, samples were cut at standard

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size of 50 mm х 5 mm (length х width) (following ATMD 882) and tensile tested with UTM

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(Kalpak Instruments, India), using load cell of 500N at a speed of 10 mm/min.

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3.7 Nuclear Magnetic Resonance (NMR) Spectroscopy

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The reactively extruded PLA/CNC films and after recycling were dissolved in the deuterated

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chloroform (CDCl3) by magnetically stirring for 3 days and thereafter filtered using a 0.25µm filter

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before the analysis. The dissolved samples were placed in NMR tubes and analysed with the 600

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MHz nuclear magnetic resonance (NMR) spectrometer (Bruker, Germany).

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3.8 Fourier Transform Infrared (FTIR) Spectroscopy

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The chemical structural analysis of the recycled films were confirmed through Fourier transform

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infrared spectroscopy, measured under transmission mode with the Perkin Elmer (Frontier)

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spectrometer, in attenuated total reflection (ATR) mode equipped with ZnSe crystal from the

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wavenumber range of 4000-500 cm-1 scanned for 128 times with a resolution of 4cm-1.

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4. RESULTS AND DISCUSSION

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4.1 Effect of Recycling on Molecular Weight, Grafting Parameters and Grafting Mechanism

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of the Reactively Extruded PLA/CNC films in presence of different CNCs

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Traditional route for the extrusion of PLA/CNC nanocomposite results in severe degradation of

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molecular weight, which makes it impossible to thermally recycle them into value-added products



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with adequate properties required for engineering applications. This is possibly because of the

252

presence of high hydroxyl functionality and trace amounts of sulphate/chloride groups present in

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the acid hydrolysed CNCs, which induces the degradation of polymeric backbone during extrusion

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at high temperature and shear forces[53] [54]. To overcome such challenges, reactively grafted

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PLA/CNC nanocomposites were developed and thermally recycled for three times to evaluated

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their effect on the molecular weight characteristics as shown in Table 1. Extrusion of the PLA/CNC

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films through traditional route followed by the recycling for first cycle shows a degradation in the

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Mw by~30% and Mn~35% respectively, in presence of both CNC-S and CNC-Cl nanofillers

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(compared to pristine PLA). Moreover, GPC chromatograms shows that fractions of high Mw PLA

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fragments decreased significantly (upto~63-69%) with increase in the low Mw chains in the

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reprocessed nanocomposites (upto~30-37%). This suggests that the nascent PLA chains undergoes

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severe degradation into small oligomeric fragments in presence of CNCs, during the thermal

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recycling process through traditional routes. Interestingly, it was observed that recycling of the

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reactively extruded PLA/CNC films (in presence of the DCP), shows improved stability towards

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Mw characteristics as evident from GPC chromatograms. On first recycle, the slight decrease in

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Mw by~8 and 4% (for 1 and 2 wt. % loadings respectively) was observed in case of CNC-S with

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Mn almost remaining unchanged. Similarly in case of CNC-Cl, the Mw remains unchanged with

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the slight decrease in Mn by~ 9 and 10% (for 1 and 2 wt. % respectively). It is noteworthy to

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mention that in comparison to traditional approaches, recycled films through reactive extrusion

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shows substantial improvement in Mw by~29 and 31% for nanofillers CNC-S and CNC-Cl

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respectively. The presence of major fractions of high Mw PLA chains in range of 80-85% for both

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the CNC-S and CNC-Cl with Mw of ~165-187 kDa, suggests that recycling of PLA/CNC films

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through reactive extrusion is a feasible process for development of value-added products. This is


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possibly because during the reactive extrusion process, PLA chains grafts onto the surface of CNCs

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through C-C linkages which are thermally more stable and prevents any further degradation in the

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developed nanocomposites. Moreover, the grafted PLA chains shields the sulphate/chloride

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functional moieties in CNCs through the formation of gel-like structure, which are possibly

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responsible for efficient processing with comparatively reduced degradation. To determine the

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effective number of reprocessing cycles, the reactively extruded PLA/CNC films could withstand

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before it have undergone significant Mw and Mn reduction, the samples were subsequently recycled

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for second and third time. On second recycle, the reduction in Mw was observed to be ~15 and

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12% for 1 and 2 wt. % CNC-S loadings and by ~ 21 and 18% for 1 and 2 wt. % CNC-Cl loadings

283

respectively. Moreover, from the GPC chromatograms it was observed that the major fractions of

284

high Mw PLA chains in the nanocomposite are still in range of ~80-84% which is comparable to

285

the first recycle, resulting it to be suitable for processing into daily degradable plastic-based

286

products. On further recycling for third time, the Mw was found to be reduced by ~16 and 19% for

287

1 and 2 wt. % CNC-S loadings and for CNC-Cl as nanofillers by ~28 and 24% for 1 and 2 wt. %

288

loadings respectively. However, the GPC chromatograms shows significant drop in the fractions

289

of high molecular weight PLA chains to ~67-75%, which was comparable to the PLA/CNC films

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(R1) recycled through traditional routes. In comparison to the previously reported studies, where

291

Mw and Mn reduces by 60-70% on recycling for 2-3 times, reprocessing through reactive extrusion

292

provides relatively lowered degradations (19-24%)[55] . Even though the Mw of recycled

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PLA/CNC films processed through reactive extrusion was quite stable at~150kDa, but it could be

294

concluded that the films could be effectively recycled for at least three times with Mw properties

295

suitable enough for fabrication of products for potential commodity applications.



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To determine the effect of cross-linking agents on the reprocessing of PLA/CNC films, radical

297

initiator, DCP was added during the third recycle step. Interestingly, it was observed that on

298

addition of only ~1wt. % DCP, the Mw of PLA chains shoots up by~ 29 and 30% for both CNC-S

299

and CNC-Cl at 2wt.% loadings respectively. It was observed that addition of DCP shows increase

300

in both Mw and Mn significantly with molecular weight distribution comparable with the

301

PLA/CNC films after first recycle. Further, the GPC chromatograms suggests presence of high Mw

302

PLA chains in the nanocomposites which comprises a significant fractions of ~87-89% (for both

303

CNC-S and CNC-Cl) with Mw higher than ~180kDa. The improvement in molecular weight

304

properties of PLA/CNC films during the recycling process with addition of DCP have been

305

investigated and illustrated schematically in Figure 1. During the first and second recycle process

306

of reactively extruded films, it was observed that due to thermal degradation of PLA backbone at

307

high shear forces during extrusion process, the fractions of low Mw PLA chains in nanocomposites

308

increases to ~25-33%. As discussed earlier, DCP are known to act as cross-linking agent through

309

the formation of C-C covalent linking’s between the PLA chains and CNCs. Therefore, it is

310

expected that during the recycling step in presence of DCP, the low Mw segments undergoes

311

covalent cross-linkages with the high Mw fragments forming its integral part of backbone. This

312

results in reduction of the low Mw oligomeric PLA segments in the nanocomposites, which are

313

otherwise known to induce or catalytically enhance the rate of polymer chain degradation during

314

the thermal recycling process[56]. As shown in Figure 1, the short PLA chains grafts with high

315

Mw PLA backbone forming a network of gel-like structure, which are thermally more stable and

316

enhances the dispersion of CNCs or grafted PLA fragments.

317

To confirm the abovementioned postulate (as shown in Figure 1), formation of C-C covalent

318

grafting between PLA and CNC during reactive processing and its stability during recycling steps,


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319

were investigated through FTIR and NMR spectroscopic analysis. The1H NMR spectra for the

320

PLA/CNC films processed through traditional approach (without DCP), shows peaks at 5.19 and

321

1.46 ppm representing the CH and CH3 protons of the PLA backbone. On comparison of the 1H

322

NMR spectrograms with reactively extruded PLA/CNC films, shows a distinct difference with

323

presence of a new characteristic peak at 3.43 ppm, which refers to the methine protons (represented

324

as ´a´ in Figure 2(a)) formed during the grafting phenomenon. It is expected that the radical

325

initiator DCP, at high temperature abstracts nascent hydrogen from the C-6 position of CNCs and

326

C-2 position of PLA backbone, providing sites for the formation of interfacial C-C cross-linkages

327

between two (as per Figure 2 (a)). Subsequently, FTIR spectroscopy analysis of the reactively

328

extruded PLA/CNC films after recycling shows the presence of a new distinct peak at 1020cm-1,

329

with the lowered intensity of the –CH stretching peaks. Moreover, the peak at 1750 cm-1

330

representing the carbonyl peak (C=O) of PLA, shows shift to 1739 cm-1 on reactive extrusion in

331

presence of DCP and its recycling. This potentially confirms the grafting of CNCs onto the PLA

332

backbone through the formation of C-C covalent couplings. On further recycling, the grafted

333

PLA/CNC films for the consecutive first and third recycles, it was observed that the peak at 3.43

334

ppm corresponding to the C-C cross-linkages remains prominent. This confirms that the C-C

335

covalent cross-linkages formed during the initial reactive extrusion of PLA/CNC are thermally

336

stable at subsequent recycle for three times. As discussed in previous section, the lowered

337

degradation in Mw of recycled and reactively extruded PLA/CNC films is possibly due to stable

338

chemical structure which was predominant for both CNC-S and CNC-Cl. Interestingly, 1H NMR

339

spectra for PLA/CNC films processed through reactive extrusion with the addition of DCP after

340

the third recycle shows a slight shift in the characteristic methine peak to 3.72 ppm. The shift in

341

peak is possibly due to the formation of new C-C covalent couplings between the short degraded



342

low Mw oligomeric PLA chains (formed during recycling steps) and CNC or PLA-grafted-CNC

343

backbone. Furthermore, increased intensity in the methine peaks suggests the presence of higher

344

fractions of C-C cross-linkages formed due to additions of DCP, during the recycling steps

345

resulting in the improved Mw characteristics. Similar observations were found in the FTIR

346

spectrograms of PLA/CNC films recycled with addition of DCP, which shows the presence of the

347

new peak at 1597 cm-1 corresponding to –CH bending, absence of the peak at 1360 cm-1

348

representing –CH stretching and shift (and significant reduction in intensity) of the carbonyl peak

349

of PLA from 1750 to 1742 cm-1. From both NMR and FTIR spectroscopic analysis, it could be

350

inferred that the reactive extrusion in presence of DCP results in formation of thermally stable

351

C- C covalent cross-linkages with low Mw PLA chains that are formed during several mechanical

352

recycling steps of PLA (as evident from GPC chromatograms). This results in the formation of

353

densely cross-linked PLA grafted CNC structures which are of high Mw~180kDa, present at

354

relatively higher weight fractions (>80%), required especially during efficient processing of

355

PLA/CNC films through industrially scalable extrusion process. Therefore, it could be concluded

356

that chemical structural analysis corroborates with the proposed mechanism (mentioned in Figure

357

1 and Figure 2(a)), confirming the changes in molecular conformations of PLA/CNC backbone

358

during reactive extrusion with the formation of C-C covalent linkages and their improved thermal

359

and structural stability during subsequent recycles. As per our knowledge such efficient routes for

360

reprocessing of PLA/CNC nanocomposites films with improved molecular weight properties after

361

recycling have not been reported yet, henceforth, were further used to investigate their physio-

362

chemical, thermal and structural properties as discussed in the subsequent sections.

363

4.2 Morphological Studies and Film Properties of the Recycled PLA/CNC films processed

364

through Reactive Extrusion


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365

Figure 3 (a) – (d) compares the effect of thermal recycling on the surface morphologies of

366

PLA/CNC films when processed with or without the presence of DCP as cross-linking agents.

367

Traditional route for processing of PLA/CNC films shows the presence of bunch of CNCs as

368

agglomerates (shown with black circles on Figure 2 (a)) which was accompanied with thermal

369

degradation resulting in the formation of films with black coloration. On further recycling of such

370

films for first recycle, PLA/DCNC-S-1p-R1 shows the presence of cracks propagating through the

371

film surface alongwith the agglomerates of CNCs in it. This is possibly because of the thermal

372

degradation of PLA backbone accompanied by the drastic reduction of Mw and increase in low Mw

373

fractions which easily leaches outs from films, resulting in formation of voids/cracks on surface.

374

Presence of such cracks are undesirable and are known to severely affect the structural properties

375

of the developed nanocomposites. On other hand, morphological investigations of the recycled

376

PLA/CNC films processed through reactive extrusion (after the third recycle) shows uniformly

377

dispersed CNCs (marked with black arrows) along the surface of the films with the presence of

378

trace amount of gel-like fractions (marked with black square). As discussed in earlier sections, it

379

could be confirmed from morphological studies that presence of PLA grafted CNCs segments in

380

nanocomposites are responsible for the enhanced thermal stability as well as helps in improving

381

the dispersion of rod-like CNCs in PLA matrix. Further, FESEM micrographs of PLA/CNC films

382

reprocessed with addition of DCP after third recycle shows interesting morphological

383

characteristics with the presence of both uniformly dispersed CNCs (marked with black arrows)

384

alongwith the large fractions of high Mw PLA/CNC grafted gel-like structures (marked with black

385

squares). The higher fractions of gel-like structures present in films is possibly due to formation

386

of the covalent cross-linkages between the low Mw oligomeric PLA chains along the backbone of

387

high Mw PLA (as described in Figure 1). It is noteworthy to mention that even after the third recycle



388

both PLA/DCNC-S-1p-R3 and PLA/DCNC-S-2p-R3-DCP shows complete absence of any voids

389

or cracks which are otherwise difficult to avoid in case of PLA/CNC films recycled through

390

traditional routes[57]. Therefore, it could be concluded that processing of PLA/CNC films through

391

reactive extrusion provides a unique approach to recycle bio-based thermoplastics with enhanced

392

dispersion of nanofillers and higher Mw stability which are in range ~180-185kDa, ideally suitable

393

for development of PLA based products for commercial use.

394

4.3 Effect of Recycling on Crystallization behaviour of reactively extruded PLA/CNC films

395

DSC studies were carried out at different heating and cooling cycles to determine the effect of

396

reprocessing the reactively extruded PLA/CNC films on their crystallization phenomenon and

397

grafting efficiency. Figure 4 (a) – (c’) shows the DSC second heating thermograms and cooling

398

thermograms of all the recycled samples and extracted data were tabulated in Table 2. Table 2,

399

shows the effect of recycling on the change in percentage crystallinity, crystallization temperature,

400

melting temperature, specific heat values at Tg and percentage of grafted PLA chains in the

401

nanocomposites after each reprocessing cycles. For neat PLA films extruded through traditional

402

routes shows glass transition and melting temperature of ~61.2 and 170.0ºC respectively, with a

403

crystallinity of ~32% similar to our previously reported studies (19). On the first recycle through

404

reactive extrusion process, it was observed that the melting temperature improves by~ 3.5 and 2.1

405

ºC on addition of both CNC-S and CNC-Cl as nanofiller respectively. Alongwith it, significant

406

enhancement in the overall crystallinity of reactively extruded PLA/CNC nanocomposites was

407

observed by ~9 and 15% for PLA/DCNC-S and PLA/DCNC-Cl at 1wt. % loadings respectively.

408

This is possibly due to the grafting of low Mw amorphous PLA chains (produced during thermal

409

recycling) with the CNCs or PLA backbone through C-C covalent linkages (as shown in Figure

410

1), which results in overall enhancement in percentage crystallinity. The g-PLA values measured


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411

from the specific heat capacity at glass transition temperature, shows increased fractions of grafted

412

PLA chains on recycling. For both CNC-S and CNC-Cl, the nanocomposites shows increased

413

grafting percentages of g-PLA to 52-72% after the first recycle (in comparison to g-PLA of 53-

414

66% on extrusion in presence of DCP). The slight enhancement in g-PLA values by 6-10% on

415

reprocessing, is due to the presence of some fractions of DCP which gets thermally activated in

416

films during recycle (at 180ºC) which could undergo formation of nascent radicals and initiate the

417

grafting or cross-linking reactions during insitu reactive extrusion process (as described in Figure

418

2). Therefore, from the crystallization studies it could be confirmed that the increased molecular

419

weight stability of reactively extruded PLA/CNC films (as shown in Table 1) on thermal recycling

420

is possible due to two reasons: (i) presence of radical initiator DCP which gets thermally active

421

during recycling steps resulting in insitu covalent coupling reactions between polymer and CNCs

422

and (ii) formation of high Mw grafted gel-like structures with degraded low Mw PLA oligomers

423

(produced during recycling steps) or CNCs through the formation of thermally stable C-C covalent

424

bonding’s, which could act synergistically in improving the overall processability and recyclability

425

of films. However, recycling of PLADCNC films didn´t showed any significant effect on the glass

426

transition and cold crystallization temperature, probably due to the presence of grafted structures

427

in nanocomposites which restricted the mobility of polymer chains. Subsequently, on further

428

reprocessing for second and third cycles, the reactively extruded PLA/CNC films shows

429

considerable changes to the crystallization temperature, degree of crystallinity and percentage

430

grafted PLA structures. After second recycle, increase in crystallinity a bit to 43-51% for the

431

recycled films was observed, possibly due to the formation of low Mw degraded PLA chains which

432

could easily crystallize. This was further confirmed from the slightly reduced g-PLA values by ~6-

433

8%, suggesting that during the recycling steps the degradation would occur from high Mw grafted



434

gels. However, the percentage degradation and formation of low Mw PLA chains was not

435

significant enough to have considerable effect on their crystallization properties. It was further

436

observed that due to the presence of generous fraction of high Mw PLA chains (~70-80%) in the

437

recycled films, no significant changes in the glass transition and melting point temperature was

438

observed. But, on third recycle percentage of g-PLA chains dropped down by ~6-19%, suggesting

439

that the low Mw oligomeric PLA chains formed during second recycle could acts as catalysts for

440

initiating the degradation process. This is also evident from the lowering of crystallization

441

temperature by 10-12ºC, suggesting that the high fractions of low Mw PLA chains (25-33%) could

442

easily crystallize resulting in their increased percentage crystallinity by 6-16%. Interestingly,

443

incorporation of 1wt. % of DCP during the recycling process, shows remarkable changes in the

444

crystallization behaviour and polymer chain dynamics. The degree of grafting in PLA, (as evident

445

from g-PLA values) was found to be significantly increased to ~70 and 87%, for both the

446

nanofillers CNC-S and CNC-Cl respectively. The increased g-PLA values is due to the cross-

447

linking of low Mw PLA segments (produced during second recycle present at relatively high

448

fractions 25-33%), with the high Mw PLA grafted gel-like structures (as shown in Figure 1). It is

449

expected that low Mw PLA chains would undergo covalent coupling with CNCs through C-C

450

cross-linkages which leads to increase in overall crystallinity by ~4 and 11% and crystallization

451

temperature by ~ 24ºC for CNC-S and CNC-Cl respectively. The recycled films with high PLA

452

grafting efficiency, reduced fraction of low Mw PLA and improved degree of crystallinity, are ideal

453

to process thermally stable films with adequate properties required for industrial viable

454

applications. Furthermore, it could be observed from this study that addition of the trace quantities

455

of cross-linking agent DCP, during the recycling steps will effectively tailor the crystallization

456

properties of the reactively extruded films. Therefore, it could be concluded that the recycling of


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457

reactively extruded PLA/CNC films with or without the crosslinking agents will result in

458

incorporation of requisite properties which can be strategically tuned by controlling the grafting

459

efficiency, degree of cross-linking and Mw characteristics.

460

4.4 Effect of Recycling on Thermal properties of the Reactively Extruded PLA/CNC films.

461

The thermal degradation phenomenon of reactively extruded PLA/CNC films after recycling were

462

investigated through TGA/DTG plots (as shown in Figure 5 (a) – (c’)) and from the parameters

463

such as T (10%) , T max (50%) and T endset (90%) (temperature profiles at which samples have undergone

464

10, 50 and 90% weight loss) (mentioned in Table 3). TGA profile for neat PLA shows single step

465

degradation profile with onset, T (10%) of 333 ºC and T max (50%) of 363 ºC respectively. Further from

466

TGA/DTG plots of the recycled PLA/CNC films, a single step degradation profile is observed

467

which suggests that the thermal degradation mechanism remains unchanged on the addition of

468

DCP as radical initiator. This is possibly because degradation process undergoes through the

469

hydrolysis and oxidative chain scissions of the PLA backbone even in cases of reactively extruded

470

samples which have high Mw gel-like fractions present in it. From our earlier reported studies, it

471

was observed that presence of the sulphate groups in CNCs hindered the thermal stability of

472

PLA/CNC films when processed through traditional approaches[58]. It was observed that the T

473

max (50%)

474

further recycling decreased by 8°C, respectively. On the other hand, reactive extrusion based

475

processing of PLA/CNCs helps in significant improvement of thermal stability which was found

476

to be predominant even after recycling for several times. After first recycle (R1), for case of CNC-

477

S as nanofiller improvement in T (10%) was observed by 11 and 8 ºC and T max (50%) increased by 4

478

and 3ºC (for 1 and 2wt.% loadings respectively) (compared to neat PLA). Similarly, for the case

479

of CNC-Cl, thermal stability was found to be further improved by T (10%) ~14 and 15 ºC and T max

decreased by 4°C on addition of the CNCs processed through traditional approach and on



480

(50%) by~

7 and 8 ºC at 1 and 2wt. % CNC-Cl loadings. On subsequent recycling for second time

481

(R2), it was observed that there was a slight drop in the thermal stability of reactively extruded

482

PLA/CNC films. After second recycle (R2), T (10%) was found to be increased by ~2 and 5 ºC and

483

T

484

incorporation of CNC-Cl (at 1 and 2wt. % CNC-Cl loadings), slight improvement in thermal

485

stability with T (10%) by~3-6 ºC and T max (50%) by~ 1ºC was observed (compared to neat PLA). The

486

improved thermal stability of recycled films is due to the formation of C-C covalent linkages due

487

to the grafting of CNCs onto PLA during the reactive extrusion process alongwith the formation

488

of micro-gel networks (as shown in Figure 2(a)). The presence of such covalent bonding’s are

489

thermodynamically quite stable at higher temperature and the formation of micro gel-domains

490

shields the sulphate/chloride groups present on CNCs preventing the degradation phenomenon.

491

On further recycling to third cycle, it was found that thermal properties of the recycled films are

492

comparable to neat PLA, which is possibly due to the enhanced degradation of PLA chains as also

493

confirmed from molecular weight investigations. The presence of low molecular weight PLA

494

oligomers (at relatively higher fractions of ~25-33%) in the nanocomposites reduces their thermal

495

stability, as it acts as sites for enhanced chain scissions or cleavage of PLA backbone at higher

496

temperature. Interestingly, recycling of PLA/CNC films on the additions of DCP (at 1wt. %

497

loading) shows significant improvement in the thermal characteristics of developed

498

nanocomposites. The onset degradation temperature T (10%) was found to be improved by~ 18 and

499

17 ºC and T

500

nanofillers respectively, at constant 2 wt.% loadings. As discussed in earlier section, the significant

501

improvement in thermal characteristics is due to the presence of uniformly dispersed high Mw

502

micro-gel networks which have the low Mw fragments (produced during thermal recycle at R1 and

max (50%)

by~ 2 ºC for CNC-S as nanofiller at 1 and 2 wt. % loadings respectively. With

max (50%)

enhanced by~12 and 7 ºC for the introduction of CNC-S and CNC-Cl as


Page 22 of 47

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503

R2) cross-linked to the PLA/CNC backbone at high grafting density of 70-87% ( as shown in

504

Figure 1). Moreover, due to covalent cross-linkages the oligomeric PLA fragments forms the

505

integral part of high Mw PLA backbone and inherits its intrinsic characteristics, as evident from

506

the single step thermal degradation profiles of TGA/DTG thermographs (as shown in Figure 5(a)

507

- (c’)). Improving the thermal stability of PLA/CNC films through the addition of trace amounts

508

of DCP during the recycle process provides us a unique approach to effectively recycle such

509

biodegradable polymers for several cycles which is rather difficult to obtain through traditional

510

routes of recycling polymers.

511

4.5 Effect of Recycling on Rheological Behaviour of the reactively extruded PLA/CNC films

512

The rheological behaviour of recycled PLA/CNC films processed through reactive extrusion was

513

studied under melt conditions and the effect of recycling times on complex viscosity and storage

514

modulus of films was investigated. Strain amplitude sweep was carried out to determine the linear-

515

viscoelastic region (LVE) and the Small amplitude oscillatory stress (SAOS) studies were carried

516

out at constant strain of 1%. Figure 6 (a) and (a’), shows the change in complex viscosity and

517

storage modulus measured at angular frequency of ~1 rad/s when PLA/CNC films processed

518

through reactive extrusion are recycled with and without addition of the DCP. It could be observed

519

that both PLA and PLA/CNC films when thermally recycled through traditional routes for three

520

times shows significantly lowered complex viscosity values. It is in line with the earlier reported

521

studies[59], in which the zero shear viscosity underwent a significant drop by ~87% after first

522

recycle and becomes almost negligible in subsequent injection cycles (after 3 cycles). The reduced

523

complex viscosity is due to the thermal degradation of the PLA chains into smaller oligomeric

524

fragments which was confirmed from the molecular weight analysis (Table 1). This is possibly

525

due to the presence of functional groups (sulphate and chloride) on the CNC surfaces alongwith



526

the existence of high temperature and shear forces during the extrusion process, which initiates the

527

thermal degradation phenomenon. However, recycling of the reactively extruded PLA/CNC films

528

shows comparatively improved melt strength, in line with the molecular weight and structural

529

characteristics of the nanocomposites, as discussed in the above section. After the first recycle,

530

complex viscosity was found to be improved by ~4 and ~7 times, when CNC-S and CNC-Cl is

531

used as reinforcing agent respectively. Improved complex viscosity for the PLADCNC-S

532

compared to PLADCNC-Cl, is probably due to the higher thermal stability of CNC-Cl, which

533

results in lower degradation and the presence of higher Mw PLA segments leads to improved

534

rheological properties. As in the Figure 6 (b), (b’) and (c), (c’), it can be understood that the

535

nanocomposite CNC-S, after the second and third recycle showed a gradual decreasing trend in

536

complex viscosity and storage modulus. However, the values of complex viscosity and storage

537

modulus after third recycle are still comparatively higher by orders of ~2 to 3 times in comparison

538

to the recycled PLA. This confirms that the presence of high Mw gel-like structures of PLA grafted

539

with CNCs formed during the reactive extrusion process are thermally quite stable after repetitive

540

recycles and could effectively act as a strong reinforcing agents. Moreover, CNCs are known to

541

form the percolation network-like structures in polymeric matrix which however depends upon

542

their aspect ratio and interfacial interaction. From the morphological analysis, it is observed that

543

the high Mw PLA grafted CNC gels undergoes improved dispersion, which is expected to form a

544

transient network in the nanocomposites, resulting in their higher reinforcing efficiency.

545

Moreover, with the presence of C-C cross-linkages between PLA and CNCs, the segmental motion

546

of PLA are restricted at higher temperature and shear-like conditions during extrusion process,

547

resulting in a more solid-like response during melt rheological studies. Interestingly, with the

548

addition of DCP during recycling process results in significant improvement in the complex


Page 24 of 47

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549

viscosity by ~4 times and storage modulus by~10 times in presence of both CNC-S and CNC-Cl

550

nanofillers. Such enhancement in rheological characteristics, is due to the grafting of the low Mw

551

oligomeric PLA chains with the CNCs or PLA backbone forming high Mw grafted gels of ~182-

552

188 kDa as evident from increased molecular weight (as shown in Figure 1). From Figure 6(b) and

553

(b’), it could be observed that on the addition of DCP during reprocessing cycles the complex

554

viscosity gradually increases with the decrease in angular frequency forming a constant at ɷ~0.1

555

rad/s, which was unlikely with the other recycled films. This is possibly due to the covalent

556

coupling of low Mw oligomeric PLA fragments (produced during recycle steps) with the PLA

557

grafted CNC gels (as shown in Figure 1), which formats a stable percolated network that restricts

558

the polymer chain motions in melt. Such phenomenon for formation of high Mw cross-linked gels

559

through generation of radical initiators, was however absent with the other recycled films

560

processed through reactive extrusion due to which the trend of complex viscosity with frequency

561

was almost constant. Therefore, form the rheological studies it could be concluded that the

562

recycled PLA/CNC films processed through reactive extrusion further with the addition of DCP,

563

provides a unique approach to develop thermally stable and recyclable films with improved

564

extrudability.

565

4.6 Effect of Recycling on Mechanical Properties of the reactively extruded PLA/CNC films

566

CNCs, due to their improved mechanical properties (with Young´s modulus of ∼140–220 GPa in

567

parallel and ∼2–50 GPa in transverse direction) are known to act as strong reinforcing agents when

568

dispersed in polymeric matrix which, however, depends upon the degree of dispersion, interfacial

569

interaction and aspect ratio of CNCs. Traditional routes for dispersion of CNCs in PLA matrix

570

shows the formation of black coloration due to its degradation at high temperature and shear forces,

571

during the extrusion process. Due to the thermal degradation, inherent characteristics of CNCs are



572

severely affected which results in poor mechanical properties of developed nanocomposites and it

573

becomes difficult to effectively recycle such materials[60] [61](25, 26). Moreover, morphological

574

analysis of recycled PLA/CNC films (Figure 3 (b)) shows the propagation of cracks and presence

575

of agglomerated CNCs, which results in poor structural properties. As discussed in previous

576

section, reactive extrusion based processing of PLA/CNC films, results in formation of C-C cross-

577

linking’s which are quite stable in presence of high shear and temperature during the recycle

578

process for several number of times. The effect of recycling the reactively extruded PLA/CNC

579

films on the ultimate tensile strength (UTS) and elongation behaviour are evaluated as shown in

580

Figure 7(a)-(d). It was observed that the reactively extruded PLA/CNC films in presence of 1 wt.

581

% loadings shows improvement in UTS by ~41% in comparison to ~14% when processed through

582

traditional routes of extrusion (compared to neat PLA). This is possibly due to the formation of C-

583

C based covalent bonding between the grafted PLA chains and CNCs which effectively transfers

584

the high modulus of CNCs to the developed nanocomposites. On the first recycle in presence of

585

CNC-S, it was observed that the UTS for reactively extruded PLA/CNC films were higher than

586

the traditionally reprocessed films by ~20 and 16% at 1 and 2wt. % loadings respectively

587

(compared to near PLA after first recycle). On further recycling for second time, the mechanical

588

properties slightly decreased but the UTS was found to be still higher by ~10 and 4% at 1 and 2wt.

589

% loadings respectively (compared to near PLA after second recycle). Similarly, for the case of

590

CNC-Cl as nanofiller, the UTS for first recycled films was found to be higher by ~21 and 12% and

591

subsequently in second recycle by ~8 and 18% for the 1 and 2wt. % loadings respectively

592

(compared to near PLA after first and second recycle). It was observed that recycled films with

593

CNC-Cl was structurally more stable compared to CNC-S, possible due to their higher thermal

594

stability which prevented its degradation during extrusion process. Alongwith it, the low aspect


Page 26 of 47

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595

ratio and improved interfacial interaction of CNC-Cl with PLA matrix may be responsible for the

596

improvement in UTS which makes it a better reinforcing agent compared to CNC-S. The decrease

597

in UTS after each recycle (from first to third cycles), is due to the degradation of high Mw PLA

598

chains into low Mw oligomeric fragments which are present at relatively high fractions, resulting

599

in its reduced strength as well as elongation behaviour. After the third cycle PLA was found to

600

undergo severe reduction in Mw, resulting in lower UTS which is possible due to the presence of

601

high fractions of low Mw PLA oligomers which are known to initiate the degradation of PLA

602

backbone. However, the reactively PLA/CNC films after the third recycle was found to be

603

comparatively more stable resulting in higher UTS by~ 16 and 10% for CNC-S and by~ 25 and

604

29% for CNC-Cl (compared to neat PLA after third recycle). The improved stability of the recycled

605

PLA/CNC films processed through reactive extrusion is possibly due to the presence of major

606

fractions of high Mw gel-like structures of PLA grafted CNCs in the nanocomposites, formed

607

through C-C cross-linking’s which are found to be tolerant towards heat and shear. Reprocessing

608

of PLA/CNC films with the addition of DCP after third recycle shows quite interesting effect on

609

the structural properties of the nanocomposites. Reprocessing of films after third recycle with

610

addition of DCP, results in significant improvement of UTS for the processed nanocomposites by

611

~25 and 30% with the addition of 2wt. % loadings of CNC-S and CNC-Cl respectively. As

612

discussed in earlier section, the improvement in UTS is possibly due to the grafting of the low Mw

613

segments (produced during the recycling steps due to thermal degradation) with the PLA backbone

614

resulting in the formation of high Mw gel-like structures. Further, from the morphological analysis

615

of PLA/DCNC-S-2p-R3-DCP, the improved dispersion of the high Mw gel-domains as well as

616

CNCs in the nanocomposites are possibly responsible for improved reinforcing effect of CNCs

617

resulting in enhanced structural properties (both strength as well as elongation). Therefore, it could



618

be concluded that PLA/CNC films could be effectively reprocessed for several times through the

619

reactive extrusion process and with the addition of DCP the structural behaviour can be finely

620

tuned, which is ultimately required for development of value-added products using recyclable

621

biodegradable polymers.

622

5. CONCLUSIONS

623

The bio-based polymers, such as PLA and its nanocomposites have been widely researched and

624

modified with fillers to enhance its properties comparable to petroleum based polymers, however,

625

their processing and recycling using industrial-scale extrusion based machineries have been really

626

challenging. The present study was addressed to overcome such challenges through a novel

627

reactive extrusion based processing of PLA/CNC films which could be effectively recycled with

628

the addition of ´sustainable´ and ´green´ cross-linking agent at smaller quantities (of 1wt. %) and

629

understanding its effect on reprocessing for several cycles. Reactive extrusion of PLA/CNC films

630

in presence of dicumyl peroxide (DCP), shows improved molecular weight stability (within the

631

range of 150-185 kDa) with higher melt strength when reprocessed for multiple times under high

632

shear forces and temperature during an extrusion process. The reactively extruded PLA/CNC films

633

even after the third recycle shows improvement in Mw by~6-10% (compared to first recycle films

634

through traditional routes) and with addition of DCP during the third recycle shoots up by Mw

635

by~30%, as evident from GPC chromatograms. Moreover, after the multiple recycles presence of

636

major fractions of high Mw PLA chains (~67-75%) in the reactively extruded nanocomposites with

637

Mw ~185kDa, are ideally suitable for their reprocessing into PLA based degradable products for

638

commercial use. Based on chemical structural analysis through NMR and FTIR spectroscopic

639

studies, the underlying reaction mechanism was investigated which suggests that the formation of

640

C-C covalent linkages between PLA and CNCs during reactive extrusion are thermally stable


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641

during mechanical recycling steps. The morphological investigations of reactively extruded

642

nanocomposites, shows improved dispersion of CNCs alongwith the presence of grafted high Mw

643

PLA gel-structures. The formation of such gel-like moieties formed through thermally stable C-C

644

cross linkages results in improved thermal, structural and rheological properties of films after

645

recycling. As evident from thermal analysis, the low Mw PLA chains (produced during recycling

646

steps) underwent C-C covalent crosslinking’s in presence of DCP which improves the g-PLA

647

values significantly. This also led to improved processability, enhanced recyclability, higher

648

thermal and melt stability of PLA/CNC nanocomposites, the underlying phenomenon of which

649

was investigated and discussed. The reinforcing effect of CNCs are also prevalent by the recycled

650

PLA/CNC films due to the presence of high fractions of grafted PLA/CNC gel-like structures,

651

which shows improved tensile strength by ~ 16-30% after third recycling. The thermally stable

652

C-C covalent couplings present between PLA and grafted CNCs, was predominant during

653

recycling which shows increase in maximum degradation temperature by ~7-12ºC with both CNC-

654

S and CNC-Cl as nanofillers. Furthermore, rheological investigations shows improvement in the

655

viscosity of polymer nanocomposite melts by~ 10 times and storage modulus by~ 50 times,

656

suggesting that the introduction of DCP as crosslinking agents significantly improved the

657

extrudability of PLA/CNC nanocomposites during the recycle process. Therefore, we envision that

658

the current study provides an environmentally-friendly and technologically scalable route to

659

efficiently recycle biodegradable PLA based CNC nanocomposites, with improved processability,

660

enhanced physico-chemical, thermal and structural properties essential for development of

661

products for high performance applications.

662 663

ACKNOWLEDGMENTS



664

Authors would like to recognize the research grant ´Sustain-Nano pack` received from Department

665

of Biotechnology, Ministry of Science and Technology, India (BT/345/NE/TBP/2012). Authors

666

expressed their sincere gratitude to the Centre of Excellence for Sustainable Polymers

667

(CoESuSPol) funded by the Department of Chemicals and Petrochemicals, Government of India

668

(Grant number: 15012/9/2012-PC.1) for the research facilities and the Central Instruments Facility,

669

Indian Institute of Technology, Guwahati, India for the analytical facility FESEM used in this

670

study.

671 672

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754 755 756 757



758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774


Page 34 of 47

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Figures and Tables

Figure 1: Schematic illustrations comparing the effect of recycling the PLA/CNC nanocomposite films for multiple cycles when processed through (i) traditional extrusion process, (ii) reactive extrusion and (iii) reactive extrusion in presence of cross-linking agents, DCP (mechanistic insight towards the molecular and structural changes of PLA chains and CNCs have been presented).



Figure 2: (a) Proposed reaction mechanism on the grafting of the PLA and CNCs during the reactive extrusion process and its recycling in presence of DCP, (b) NMR and (c) FTIR spectroscopy of the reactively grafted PLA/CNC films and its effect on the chemical structural analysis on further recycling steps without and with addition of DCP, and (d) comparison of the FTIR spectra marked with selected region in black dotted boxes in the wavenumber range of 2000-1000 cm-1and the characteristic peaks marked in black arrows.


Page 36 of 47

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Figure 3: FESEM micrographs of the (a) PLA/CNC-S nanocomposite and (b) PLA/CNC-S films after first recycle (R1) processed through traditional extrusion and reactively processed (c) PLA/DCNC-S-1p-R3 after the third recycle (R3) and (d) PLA/DCNC-S-2p-R3-DCP with addition of DCP after third recycle.



Figure 4: DSC plots showing the effect of recycling (a) second heating thermograms of samples after first recycle, (b) cooling thermograms of samples after first recycle; (c) second heating thermograms of samples after second recycle, (c’) cooling thermograms of samples after second recycle; (d) second heating thermograms of samples after third recycle, (c’) cooling thermograms of samples after third recycle.


Page 38 of 47

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Figure 5: (a) TGA and (a’) DTG plots of samples after first recycle; (b) TGA and (b’) DTG plots of samples after first recycle; (c) TGA and (c’) DTG plots of samples after first recycle.



Figure 6: Studies on the melt rheological behavior to understand the effect of recycling on the reactively extruded PLA/CNC films in presence of different compatibilizers (a) variation of complex viscosity of all samples after recycle 1, 2, 3 (a’) variation of storage modulus of all samples after recycle 1, 2, 3 (b) change in complex viscosity in frequency sweep of H2SO4 treated CNC based nanocomposites (b’) change in complex viscosity in frequency sweep of HCl treated CNC based nanocomposites; (c) change in storage modulus in frequency sweep of H2SO4 treated CNC based nanocomposites (b’) change in storage modulus in frequency sweep of HCl treated CNC based nanocomposites * Data of all nanocomposites are compared with respect to neat PLA after recycle 1, 2, and 3.


Page 40 of 47

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Figure 7: Evaluation of mechanical properties to understand the effect of recycling times on the reactively extruded PLA/CNC films in presence of various acid-derived CNCs processed with or without the presence of DCP; (a) change in ultimate tensile strength of H2SO4 treated CNC based nanocomposites after recycle 1, 2, and 3; (b) change in ultimate tensile strength of HCl treated CNC based nanocomposites after recycle 1, 2, and 3; (c) change in elongation (%) of H2SO4 treated CNC based nanocomposites after recycle 1, 2, and 3; (b) change in elongation (%) of HCl treated CNC based nanocomposites after recycle 1, 2, and 3. * Data of all nanocomposites are compared with respect to neat PLA after recycle 1, 2, and 3.



Page 42 of 47

Table 1: Effect of recycling PLA and PLA/CNC films on their molecular weight characteristics and distribution (weight average (Mw) and number average (Mn)) when processed through reactive extrusion and traditional approach. Samples Neat PLA extruded

Mw

Mn

High Mw (% Area) 72.9

181000 104000 First Recycle (R1) PLA/CNC-S-1p-R1 129157 67916 63.2 PLA/CNC-Cl-1p-R1 129234 69228 69.1 PLA/DCNC-S-1p-R1 165393 71702 79.5 PLA/DCNC-S-2p-R1 174194 76362 84.8 PLA/DCNC-Cl-1p-R1 181080 94127 80.6 PLA/DCNC-Cl-2p-R1 187045 95879 84.1 Second Recycle (R2) PLA/DCNC-S-1p-R2 154279 72238 77.8 PLA/DCNC-S-2p-R2 158360 71123 83.6 PLA/DCNC-Cl-1p-R2 142408 82547 78.1 PLA/DCNC-Cl-2p-R2 147936 88159 80.1 Third Recycle (R3) PLA/DCNC-S-1p-R3 151902 50556 68.6 PLA/DCNC-S-2p-R3 147487 49301 75.1 PLA/DCNC-Cl-1p-R3 130606 75835 67.0 PLA/DCNC-Cl-2p-R3 137331 77377 73.8 Third Recycle (R3 with 1% DCP addition) PLA/DCNC-S-2p-R3-DCP 182361 81254 87.3 PLA/DCNC-Cl-2p-R3-DCP 188021 95911 89.1


Low Mw (%Area) 27.1

PDI

36.8 30.9 20.5 15.2 19.4 15.9

1.90 1.86 2.30 2.28 1.92 1.95

22.2 16.4 21.9 19.9

2.13 2.22 1.72 1.67

31.4 24.9 33.0 26.2

3.00 2.99 1.72 1.77

12.7 10.9

2.24 1.96

1.74

Page 43 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Effect of recycling on the crystallization behaviour and grafting properties of the reactively extruded PLA/CNC films in presence of different acid derived CNCs. Samples

Tg (°C)

Cp, Tg

Tcc

Hcc

Tc

Tm

Hm

Xc

g-PLA (%)

(J/g.°C)

(°C)

(J/g)

(°C)

(°C)

(J/g)

(%)

105.5

36.2

-

170.0

-29.6

31.8

100

173.0 172.3

-32.6 -38.9

35.0 41.8

80.5 78.4

Neat PLA extruded

61.2

1.782

PLA/DCNC-S-1p PLA/DCNC-Cl-1p

63.5 63.6

1.435 1.398

PLA/DCNC-S-1p-R1

62.3

1.269

104.6

34.4

121.5

173.5

-37.6

40.4

71.2

PLA/DCNC-S-2p-R1

63.7

1.188

104.5

27.6

123.8

172.2

-36.6

39.3

66.6

PLA/DCNC-Cl-1p-R1

63.9

1.290

104.8

38.9

109.2

171.6

-43.5

46.7

72.3

PLA/DCNC-Cl-2p-R1

63.6

0.931

104.9

27.8

119.1

172.1

-41.4

44.5

52.2

118.9 35.1 124.5 121.3 36.9 110.3 First Recycle (R1)

Second Recycle (R2) PLA/DCNC-S-1p-R2

64.6

0.869

101.8

30.8

110.9

171.9

-43.0

46.2

48.7

PLA/DCNC-S-2p-R2

62.2

0.770

101.3

32.6

109.6

171.2

-51.2

41.0

43.2

PLA/DCNC-Cl-1p-R2

61.6

1.143

104.1

34.2

107.8

172.0

-45.0

41.7

64.1

PLA/DCNC-Cl-2p-R2

61.5

0.821

104.5

37.3

108.6

172.2

-46.2

46.0

46.0

Third Recycle (R3) PLA/DCNC-S-1p-R3

59.0

0.519

103.9

29.1

97.1

171.1

-44.2

47.5

29.1

PLA/DCNC-S-2p-R3

58.3

0.501

101.3

35.3

96.7

170.1

-48.8

52.4

28.1

PLA/DCNC-Cl-1p-R3

58.1

0.790

103.5

33.6

96.6

171.4

-53.6

57.6

44.3

PLA/DCNC-Cl-2p-R3

58.4

0.730

101.0

36.5

96.0

171.5

-49.0

52.6

40.9



Page 44 of 47

Third Recycle (R3 with 1% DCP addition) PLA/DCNC-S-2p-R3-DCP

56.4

1.251

96.0

40.7

121.7

167.4

-53.0

56.9

70.2

PLA/DCNC-Cl-2p-R3-DCP

56.5

1.554

96.0

44.5

120.2

167.0

-58.8

63.2

87.2

Where, Tg= glass transition temperature, ΔCp,Tg=specific heat corresponding to glass transition temperature calculated from first heating cycle, g-PLA=represents the % of PLA chains grafted onto CNC surface (measured as the ratio of ΔCp,Tg (for PLA-g-CNC gels)/ ΔCp,Tg (for PLA)), g-rPLA= represents the % of PLA chains grafted onto CNC surface after (measured as the ratio of ΔCp,Tg (for reprocessed PLA/CNC nanocomposites)/ ΔCp,Tg (for rPLA)), Tcc= cold crystallization temperature, ΔHcc= enthalpy for cold crystallization measured in first heating cycle, Tc= crystallization temperature (cooling cycle), Tm= melting temperature, ΔHam= melting enthalpy corresponding to 2nd heating cycle, ΔHom = melting enthalpy for 100% crystalline PLA was taken as 93 J/g [26] and Xc=degree of final crystallinity calculated from ratio of (ΔHam – ΔHcc) and ΔHom measured from second heating cycles.

Table 3: Effect of recycling on the thermal degradation behavior of the reactively extruded PLA/CNC films with or without presence of DCP. Samples Neat PLA extruded CNC-S CNC-Cl PLA/CNC-S-1p PLA/CNC-S-1p-R1 PLA/DCNC-S-1p PLA/DCNC-S-1p-R1

T (10%)

T max (50%)

T endset (90%)

333.2 223.1 271.9 336.8 330.4

363.0 317.7 346.6 359.4 352.3

379.0 367.4 363.5

First Recycle (R1) PLA/DCNC-S-1p-R1 344.4 367.3 382.1 PLA/DCNC-S-2p-R1 341.8 366.5 380.7 PLA/DCNC-Cl-1p-R1 347.6 370.8 384.6 PLA/DCNC-Cl-2p-R1 348.7 371.1 385.0 Second Recycle (R2) PLA/DCNC-S-1p-R2 335.7 365.1 376.4 PLA/DCNC-S-2p-R2 338.2 361.3 380.3 PLA/DCNC-Cl-1p-R2 339.1 363.3 377.0 PLA/DCNC-Cl-2p-R2 336.8 361.6 375.9 Third Recycle (R3) PLA/DCNC-S-1p-R3 334.7 363.3 377.5 PLA/DCNC-S-2p-R3 334.7 363.2 377.5 PLA/DCNC-Cl-1p-R3 325.7 354.8 370.0 PLA/DCNC-Cl-2p-R3 336.5 364.3 378.6 Third Recycle (R3 with 1% DCP addition) PLA/DCNC-S-2p-R3-DCP 352.1 375.5 390.3 PLA/DCNC-Cl-2p-R3-DCP 350.1 370.1 385.6


% Residue (at 400°C) 1.99 45.1 42.6 1.02 1.78

2.86 1.35 1.85 2.13 0.50 1.46 0.20 0.32 0.25 0.16 0.25 0.46 1.35 1.10

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Table of Content/Graphic




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Schematic illustration for a comparative understanding on the effect of multiple mechanical recycling of PLA/CNC nanocomposite films. 83x50mm (300 x 300 DPI)


Sustainable Approach for Mechanical Recycling of Poly(lactic acid

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