Production of thermoplastic starch and poly (butylene adipate-co-terephthalate) films assisted by solid-state shear pulverization

A novel processing technique involving Solid-State Shear Pulverization (SSSP) was used to produce thermoplastic starch (TPS) and Poly (Butylene Adipate-co-Terephthalate) (PBAT) films to improve processability and produce well-dispersed blends.

Available online 30 January 2021

0144-8617/© 2021 Elsevier Ltd This article is made available under the Elsevier license (http://www.elsevier.com/open-access/userlicense/1.0/)

Production of thermoplastic starch and poly (butylene

adipate-co-terephthalate) films assisted by solid-state shear pulverization

H.S.M Lopesa,* , G.H.M Oliveiraa, S.I Talabia,b, A.A Lucasa

aFederal University of Sao Carlos, Graduate Program in Materials Science and Engineering, CEP 13565-905, Sao Carlos, SP, Brazil

bUniversity of Ilorin, Materials and Metallurgical Engineering Department (MME), PMB 1515, Ilorin, Nigeria

A R T I C L E I N F O

Keywords:

TPS

PBAT

Films

Biodegradable

Blends

SSSP

A B S T R A C T

A novel processing technique involving Solid-State Shear Pulverization (SSSP) was used to produce thermoplastic starch (TPS) and Poly (Butylene Adipate-co-Terephthalate) (PBAT) films to improve processability and produce well-dispersed blends Four different compositions (50− 80 wt% TPS content) were processed using two different production routes In one instance, the compositions were pre-treated by SSSP before melt extrusion (SSSPE) Secondly, starch was initially plasticized and thereafter blended with PBAT by melt extrusion (EXT) method Flat films were produced using both routes and processability, visual and tactical aspects, mechanical and optical properties, crystallinity, and water absorption behavior were evaluated High starch content films (70 and 80 wt

%) prepared based on SSSP incorporation showed easier processability, and better visual aspect and mechanical integrity than EXT ones However, EXT films with 50 and 60 wt% of starch presented higher elongation at break and lower water absorption due to finer dispersion of TPS and better starch plasticization

1 Introduction

Government policies with focus on reducing non-biodegradable

plastics have been implemented over the last years Nevertheless, the

high cost of biopolymers is one of the main obstacles to a wide

com-mercial application Starch is one of the most promising biopolymers to

produce biodegradable materials, especially for packaging and food

applications, due to its low cost, abundance and renewable sources

(Av´erous & Halley, 2009; Fakhouri et al., 2013; Halley & Av´erous,

2014) It is mainly composed of two macromolecules, namely, linear

amylose and branched amylopectin, and their quantity affects its

crys-tallinity and mechanical properties (Averous, 2004; Hoover, 2001;

Jenkins & Donald, 1995) Depending on the area of utilization, starch

can be processed by many techniques to obtain different properties

(Fakhouri et al., 2013; Halley & Av´erous, 2014)

In its native structure, starch does not flow or melt, and plasticization

under high temperature and shear conditions is required to transform it

to a thermoplastic material (Moad, 2011; Olivato et al., 2017; Yu &

Christie, 2005) The procedure disrupts starch granules in the presence

of water and a plasticizer additive After plasticization, thermoplastic

starch (TPS) has a metastable amorphous structure that causes its

recrystallization over time via a process called retrogradation During this process, amylose and amylopectin recrystallize, leading to proper-ties change, which limits its application to a large extent (Fu, Wang, Li, Zhou, & Adhikari, 2013; Hoover, 2001; Wang, Li, Copeland, Niu, & Wang, 2015) Due to additional limitations such as low mechanical properties and high-water absorption, starch is commonly blended with synthetic polymers for the development of commercially attractive films with good visual aspects and improved mechanical properties (Raquez, Nabar, Narayan, & Dubois, 2008; Brandelero, Yamashita & Grossmann, 2010; Olivato, Grossmann, Bilck, Yamashita, & Oliveira, 2013; Silva, 2013)

Blending thermoplastic starch, a hydrophilic polymer, with non- polar synthetic polymers, like polyesters, produces immiscible blends Hence, the employed processing route and quantity of the blend com-ponents are important parameters that can affect the developed material properties (Paul & Bucknall, 2000; Utracki, 1990) A novel technique called Solid-State Shear Pulverization (SSSP) has been employed to produce finely dispersed immiscible blends in the solid-state Previous works used this technology to produce polymer blends that have unique physical characteristics and better mechanical properties compared to other processing routes (Furgiuele, Lebovitz, Khait, & Torkelson, 2000;

* Corresponding author

E-mail addresses: henrique.lopes01@fatec.sp.gov.br (H.S.M Lopes), marcatto@ppgcem.ufscar.br (G.H.M Oliveira), talabi.si@unilorin.edu.ng (S.I Talabi), alucas@ufscar.br (A.A Lucas)

Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

https://doi.org/10.1016/j.carbpol.2021.117732

Received 5 October 2020; Received in revised form 12 January 2021; Accepted 27 January 2021

Carbohydrate Polymers 258 (2021) 117732

Furgiuele, Lebovitz, Khait, & Torkelson, 2000; Furgiuele, Khait, &

Torkelson, 1998; Khait, Carr, & Mack, 2001) It involved processing the

material using a twin-screw co rotational extruder at temperatures

below its melting/softening point under high shear and compression

forces, which causes particle size reduction and intense mixing This

technique can facilitate the mixing of PBAT with starch before its

plasticization

Some researchers have manufactured polymer blends by

incorpo-rating SSSP technique in the preparation and processing routes Walker,

Tao, and Torkelson (2007), attributed improved mechanical properties

of HDPE/starch blends to intense fragmentation of starch granules

caused by incorporating SSSP technique in the material production The

stabilization of dispersed phase in PS/PMMA and PS/HDPE blends has

been observed (Lebovitz, Khait, & Torkelson, 2002; Lebovitz, Khait, &

Torkelson, 2002) The improved morphology and in-situ

compatibili-zation of these blends were attributed to the utilicompatibili-zation of pulvericompatibili-zation

technique, which resulted in a fine filler dispersion in the immiscible

blends Regarding these blends, similar results were obtained with the

incorporation of SSSP technique during processing (Lebovitz, Khait, &

Torkelson, 2003; Tao, Kim, & Torkelson, 2006) In this instance, the

attained properties were attributed to intense and repeated

fragmenta-tion, which led to higher compatibility of the blend components

Based on such previous studies, this work investigated the

incorpo-ration of SSSP in the processing of TPS/PBAT blends for film production

The proposed processing routes involving this innovative technique was

used to produce finely dispersed biodegradable PBAT blends/films with

high thermoplastic starch content (≥50 wt%) using conventional

equipment Processing parameters like ease of manipulation, feeding

and granulation were observed as an indication of better processability

during preparation Based on the surveyed literature, this is the first

report involving SSSP incorporation in the manufacturing of these

materials

2 Materials and methods

2.1 Materials

Cassava starch (23 % amylose content) was purchased from

Fecu-laria Pantanal® (Mato Grosso do Sul, Brazil) Its molecular weight was

estimated using intrinsic viscosity method, described by Millard,

Dint-zis, Willett, and Klavons (1997) and found to be 422 × 106 g mol− 1 The

detail about the experimental procedure used for this measurement can

be found in a supplementary information file Poly (butylene

adipate-co-terephthalate) (PBAT) was purchased from BASF® (Sao

Paulo, Brazil), under the commercial name Ecoflex F Blend C1200

Glycerol P.A was obtained from Neon® Commercial Ltda (Sao Paulo,

Brazil) and Sodium Chloride, Citric and Stearic Acid from Synth® (Sao

Paulo, Brazil)

2.2 Methods

2.2.1 Cassava starch modification

The cassava starch was oven-dried for 12 h at 60 ◦C before

me-chanically mixing it with glycerol and water solution (plasticizers),

following the composition 250:100 g 1000g− 1 of starch, to obtain a

homogeneous formulation The resulting mix was then oven-dried for 12

h at 90 ◦C (Olivato et al., 2017)

2.2.2 Preparation of TPS/PBAT films

Two different processing techniques were used to prepare the TPS/ PBAT blends A batch was prepared using both SSSP and melting extrusion techniques (designated as SSSPE), while another batch was prepared using only the latter method (designated as EXT) The pro-duction was done in a Baker & Perkins co-rotational twin-screw extruder, with a screw diameter of 19 mm and L/D of 25

Samples prepared using the SSSPE route followed this procedure: (i) pulverization of starch/plasticizers mix in the presence of PBAT (SSSP); (ii) melting extrusion to plasticize starch and homogenize the blend components Samples prepared using EXT route followed this sequence: (i) starch/glycerol/water mix was plasticized through conventional melting extrusion to obtain TPS; (ii) TPS and PBAT were subsequently blended using the same technique

Thereafter, flat films were produced from the blends in a single- screw extruder AX Pl´asticos equipment with a screw diameter of 40

mm and L/D of 25 Due to the unique behaviour of each formulation during processing, in terms of feeding, flow and pulling, the film thickness varied between 0.110 to 0.450 mm The film thickness was measured by a micrometre

The temperature profiles and screw rotation used during each pro-cessing stage are provided in Table 1

Furthermore, the samples were designated as shown in Table 2

2.2.3 Water absorption

Water absorption measurement was done following ASTM E104-02 standard (American Society for Testing and Materials, 2012) All the film samples were oven-dried at 60 ◦C until their mass stabilized and were then kept in a desiccator containing saturated sodium chloride solution (NaCl, 75 % relative humidity) at 23 ◦C ± 1 ◦C After that, each sample was weighed at different intervals for a total period of 200 h and the weight gain was plotted as a function of time Each formulation was assayed in duplicate

2.2.4 Mechanical properties

Mechanical properties were determined with Instron 5569 equip-ment according to ASTM D882-00 standard (American Society for Testing and Materials, 2010) An initial grip separation of 125 mm and 12.5 mm/min crosshead speed at 23 ◦C were used for the test The specimens, 50 mm wide and 175 mm long, were maintained under two

Table 1

Processing parameters

◦ C)

Screw rotation (rpm)

EXT ii i Melting extrusion Melting extrusion 142 130 142 132 147 137 137 132 130 127 120 80

Table 2

Samples designations

H.S.M Lopes et al

different conditions before measurements Some samples set were kept

in a desiccator containing saturated sodium chloride solution, at a

relative humidity (RH) of 75 % and 23 ◦C (designated as high RH) and

the other groups were kept in a ventilated oven at 60 ◦C for 12 h

(designated as low RH) The analysis provided information about tensile

strength at yield, percentage of elongation at break and Young’s

modulus under the various storage conditions The measurements were

performed in ten replicates

2.2.5 X-ray diffraction (XRD)

For the measurements, 1 cm2 film samples were kept in a desiccator

containing saturated sodium chloride solution at 75 % relative humidity

and 23 ◦C The examination was done 20 and 50 days after production

The XRD patterns were obtained using a Rigaku Geiger-Flex

diffrac-tometer, with voltage and current of 40 kV and 30 mA, respectively,

from 5 to 50◦at 2◦/min scan speed Crystallinity index was subsequently

calculated based on the crystalline and amorphous peaks areas using Eq

(1) (Canevarolo, 2004)

X c= I c

I c+I a

Where:

Xc =crystallinity index;

Ic =crystalline area;

Ia =amorphous area

2.2.6 Scanning Electron Microscopy (SEM)

The morphology of selected film samples was observed under a FEI Magellan 400R scanning electron microscope, operated at 5 kV The samples were cryogenically fractured and covered with a thin gold layer before the examination

2.2.7 Polarized light microscopy (POM)

The films were also observed under polarized light using a Leica DMRXP optical microscope at room temperature, 180 days after production

3 Results and discussion

3.1 Processing of thermoplastic starch and blends

The incorporation of solid-state shear pulverization during produc-tion resulted in easier manipulaproduc-tion of the materials in terms of feeding, mixing and granulation compared to when only melting extrusion method was employed This observation was made relative to the pro-cessing conditions and equipment used in this work Due to high water absorption of TPS, the samples’ surfaces become sticky and brittle, which made manipulation and granulation more difficult during EXT steps such as feeding and granulation This brittle and sticky behaviour

Fig 1 Visual aspect of the films Fig 1(a) EXT films (i-iv) and Fig 1(b) SSSPE films (i-iv)

Carbohydrate Polymers 258 (2021) 117732

led to a decrease in the films’ mechanical integrity However, this

challenge was reduced with the adoption of SSSPE since the starch has

already been mixed with PBAT before plasticization Similar drawbacks

relating to sticky surfaces and brittleness are found in the literature and

are mostly attributed to TPS water absorption characteristic,

crystal-linity and low plasticization degree during processing (Fakhouri et al.,

2013; Thunwall, Kuthanova, Boldizar, & Rigdahl, 2008; Thuwall,

Bol-dizar, & Rigdahl, 2006)

3.2 Visual and tactile aspects

Samples of the produced films are shown in Fig 1 EXT films with

high starch content (70 and 80 % of the total film mass) are brittle and

have high adhesivity The adhesiveness and brittleness of these films

increased with increasing TPS content due to glycerol exudation from

the starch However, films produced using the SSSPE route did not

exhibit this behaviour

Furthermore, low starch content films (50 and 60 % of total mass)

prepared by SSSPE presented high adhesiveness and poor mechanical

integrity, compared to the ones produced by EXT method Generally,

SSSPE films have more flexibility, ease of handling and mechanical

integrity, especially for films with high TPS content, compared to the

EXT ones

3.3 Microstructure of the TPS/PBAT films

Morphological differences were observed as a function of the films’

compositions (Fig 2) Films with low TPS content (50 wt%) have higher

rough surfaces (Fig 2ai and bi), irrespective of the adopted production

route This observation suggests that the sample matrix is less brittle

Residual granular structure attributed to incomplete plasticization of

starch granules was observed in all the films at high TPS content

However, this structure was conspicuous in samples prepared by SSSPE

method Films with 50 and 60 wt% of starch content processed by EXT

route presented well-dispersed, homogeneous surfaces with less

pres-ence of the starch residual granular structure, which indicates higher

plasticization compared to those produced by SSSPE method The

presence of this structure hinders interfacial adhesion, which negatively affects their mechanical properties More so, as the starch content in-creases, the surface of the film became less rough and more brittle, howbeit with a similar distribution level of the second phase These observations agree well with literature findings regarding similar blends (Fourati, Tarr´es, Mutj´e, & Boufi, 2018; Garcia et al., 2018; Li, Luo, Lin, & Zhou, 2013)

3.4 Mechanical properties

The mechanical properties of the TPS/PBAT films were investigated after subjecting them to different relative humidity conditions Firstly, the elongation at break and tensile strength at yield were found to decrease with an increase in TPS content due to its brittle nature This result agrees with an earlier observation regarding surface characteristic revealed through SEM examination of the samples For low starch con-tent (50 and 60 wt%), the EXT method produces finely dispersed blends with significantly higher tensile strength, elongation at break and Young’s modulus, especially for those subjected to a high RH condition For example, at low RH, EXT films with 50 wt% of starch have about 150

% elongation at break and the SSSPE ones have approximately 10 %, while at high RH, it was about 320 % and 35 %, respectively More so, at high RH, EXT and SSSPE samples containing 60 wt% starch have elon-gation at break values of 250 % and 25 %, respectively The attained result can be attributed to the higher plasticization of the TPS phase and better interfacial adhesion, which were promoted by adopting the former procedure Nevertheless, at a low RH condition, these films (60 wt% starch), have similar values, irrespective of the employed pro-cessing routes

The sample containing high starch content (80 wt%) exhibited similar mechanical properties despite the difference in the production procedure Hence, it can be inferred that SSSPE provides a good alter-native for the preparation of TPS/PBAT blend containing a high quantity

of TPS

Generally, there is a clear difference in the mechanical properties of films conditioned under high and low relative humidity environment (Fig 3) Low RH conditioned films presented significant higher Young’s

Fig 2 SEM images of the films: (a) EXT films prepared with 50, 60, 70 and 80 wt.% TPS, respectively (i-iv), and (b) SSSPE films prepared with 50, 60, 70 and 80 wt

% TPS, respectively (i-iv)

H.S.M Lopes et al

modulus and lower elongation at break, except for the 50 wt% starch

content films produced by EXT method This behaviour can be

attrib-uted to plasticization and increased chain mobility of starch by water

molecules (Mali, Sakanaka, Yamashita, & Grossmann, 2005; Brandelero

et al., 2010; Li et al., 2013) The results agree well with literature

findings (Teixeira, R´oz, Luzia, de Carvalho, & da Silva Curvelo, 2005;

Brandelero, Yamashita & Grossmann, 2010; Fakhouri et al., 2013;

Fakhouri, Martelli, Caon, Velasco, & Mei, 2015; Gonz´alez-Seligra, Guz,

Yepes, Goyanes, & Fam´a, 2017; Moraes et al., 2017)

3.5 Water absorption

The water absorption property of the films was directly proportional

to the starch content and exposure time (Fig 4) This behaviour was due

to starch hydrophilic nature associated with a high quantity of hydroxyl groups that are present within its chemical structure and the use of

Fig 3 Elongation at break (a and b), tensile strength at yield (c and d) and Young’s modulus (e and f) of TPS/PBAT films under low (left column) and high (right

column) relative humidity conditions

Carbohydrate Polymers 258 (2021) 117732

glycerol as plasticizer (Pelissari, Grossmann, Yamashita, & Pineda,

2009; Silva et al., 2015; Van Soest & Knooren, 1996) SSSPE and EXT

samples exhibit a similar pattern of water absorption, although the films

produced by the former procedure show a slightly higher water intake

ability, especially for the formulation containing 50 wt% TPS The EXT

films containing 50 wt% TPS have the lowest level of water absorption

due to a finer dispersion of TPS phase (see Fig 2), which allows them to

have a more compact structure in agreement with Li et al (2013)

observation Regarding high starch content blends (70 and 80 wt%), the

weight gain was almost the same after 200 h, irrespective of the

employed preparation techniques Generally, the results suggest that the

blends morphology (at low starch content) and composition play a role

on the water absorption characteristic of the samples

3.6 Phase composition of the TPS/PBAT films

The XRD patterns and crystallinity index (Xc) of PBAT and the TPS/

PBAT films prepared by SSSPE and EXT methods are shown in Fig 5

PBAT film presented peaks at 17.2◦, 20.6◦, 23.0◦and 24.7◦, as seen in

Fig 5(a) The calculated crystallinity level and the polymer

diffracto-gram peaks agree with literature findings (John, Mani, & Bhattacharya,

2002; Raquez et al., 2008; Silva, 2013; Silva et al., 2015)

Compared to films produced by EXT method, the intensity of the

peak at 17.0◦(belonging to the B-type crystalline phase) was observed to

be higher in SSSPE films (Fig 5b and c) This peak is due to residual cassava starch granules with double-helical crystal lattices from native amylopectin The increased intensity of this peak in SSSPE films suggests lower plasticization of the samples Additionally, retrogradation could

be occurring since chain mobility could promote recrystallization of amylopectin molecules after 20 days in high-moisture storage (Van Soest

& Knooren, 1997; Van Soest, Hulleman, De Wit, & Vliegenthart, 1996)

As observed by (Raquez et al., 2008), TPS/PBAT blends may not be completely plasticized after processing and this can be revealed by XRD measurement A slight increase in the intensity of the B-type crystalline phase after 50 days of storage, further confirms retrogradation This observation applied to all the samples, irrespective of the adopted pro-cessing route The increase in chain mobility by water molecules and/or the plasticizer content contributed to the faster retrogradation of the films Furthermore, the presence of this phase can reduce the blends/-films mechanical properties such as their elongation at break (Van Soest

& Knooren, 1997) Consequently, the result agrees with an earlier observation regarding the detrimental effect of starch residual granular structure

The intensity of the peaks at 13.0◦ and 19.8◦ assigned to V-types structures of TPS crystallinity becomes higher as the samples’ starch content increases The presence of this crystalline phase can be attrib-uted to fast amylose recrystallization in single-helical crystal lattice (Li

et al., 2013; Raquez et al., 2008; Van Soest et al., 1996) The orientation

Fig 4 Water absorption of the TPS/PBAT films: 50 (a), 60 (b), 70 (c) and 80 (d) wt% TPS, produced by EXT (squares) and SSSPE (circles) routes

H.S.M Lopes et al

Fig 5 XRD diffraction patterns and crystallinity index (Xc) of PBAT film (a) and TPS/PBAT films processed by EXT and SSSPE after 20 days (b) and 50 days (c)

Carbohydrate Polymers 258 (2021) 117732

of V-types structures is processing-induced in the presence of water

molecules, generating several V- type structures (Van Soest & Knooren,

1997; Van Soest et al., 1996)

The calculated crystallinity index value of each film was presented

(Fig 5) The observed changes in the Xc values could be related to

structural rearrangement during ageing as proposed by (Van Soest &

Knooren, 1997)

Irrespective of the processing route, TPS/PBAT blends presented

similar characteristics and retrogradation pattern However, a higher

amount of B-type crystalline phase, which could be due to residual

granular structure, was observed in SSSPE films

3.7 Optical property of the TPS/PBAT films

Fig 6 shows the presence of granular birefringent structures in the

films As observed earlier, the presence of residual starch granules

negatively affects the samples mechanical properties The effect was

more pronounced in films with high starch content and more residual

granular structure, irrespective of the difference in their preparation

procedure Regarding the low starch content films, this structure was

obvious in films produced by SSSPE method This was expected since the

micrography of EXT films revealed less presence of residual granular

structure Generally, the results obtained by mechanical properties,

water absorption, SEM and XRD examination followed a similar pattern

However, it is still not clear how mixing of starch with larger

quantities of PBAT (blends containing 50 and 60 wt% of starch) by

SSSPE technique hinders starch plasticization during melting extrusion

Consequently, future efforts would be focused on improving starch

plasticization when SSSPE method is used for the preparation of TPS/

PBAT blends for films production

4 Conclusions

The incorporation of SSSP technology provides benefits such as better processability and easier manipulation of TPS, which resulted in the production of films with improved visual aspects and good me-chanical integrity by conventional equipment However, low starch content blends prepared by SSSPE have relatively lower mechanical properties and water absorption due to low starch plasticization compared to when EXT route was employed Also, the low starch con-tent films produced by EXT method have finely dispersed composition as

a result of better plasticization Irrespective of the processing and preparation procedure, the samples containing high amount of starch showed no significant difference in mechanical properties Furthermore, these blends have similar morphology, crystallinity level and water absorption Generally, the results suggest that SSSPE can be a promising technique to produce high-TPS/PBAT blends using typical equipment and procedures Consequently, there is need for future studies to pro-mote better starch plasticization when SSSP is incorporated in the preparation of TPS/PBAT blends This can be achieved by optimizing production conditions and processing parameters such as the equipment screw design and temperature or incorporating methods that can pro-mote starch plasticization

CRediT authorship contribution statement H.S.M Lopes: Conceptualization, Data curation, Investigation, Methodology, Writing - original draft G.H.M Oliveira:

Conceptuali-zation, Data curation, Investigation, Methodology, Writing - review &

editing S.I Talabi: Conceptualization, Writing - review & editing A.A Lucas: Supervision

Fig 6 POM images of the films and native starch granules: (a) EXT films prepared with 50, 60, 70 and 80 wt.% TPS, respectively (i-iv); (b) SSSPE films prepared

with 50, 60, 70 and 80 wt.% TPS, respectively (i-iv), and (c) native starch granules

H.S.M Lopes et al

Acknowledgements

This study was financed in part by the Coordenaç˜ao de

Aperfeiçoa-mento de Pessoal de Nível Superior - Brasil (Capes) – Finance Code 001

The authors would like to thank Nidustec/Tecbio and LCE/UFSCar for

financial and technical support

Appendix A Supplementary data

Supplementary material related to this article can be found, in the

online version, at doi:https://doi.org/10.1016/j.carbpol.2021.117732

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