A sugar-beet waste based thermoplastic agro-composite as substitute for raw materials

Journal Pre-proof A sugar-beet waste based thermoplastic agro-composite as substitute for raw materials M. Suffo, M. de la Mata, S.I. Molina PII:

S0959-6526(20)30429-7

DOI:

https://doi.org/10.1016/j.jclepro.2020.120382

Reference:

JCLP 120382

To appear in:

Journal of Cleaner Production

Received Date: 4 May 2019 Revised Date:

25 December 2019

Accepted Date: 1 February 2020

Please cite this article as: Suffo M, Mata Mdl, Molina SI, A sugar-beet waste based thermoplastic agro-composite as substitute for raw materials, Journal of Cleaner Production (2020), doi: https:// doi.org/10.1016/j.jclepro.2020.120382. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

A sugar-beet waste based thermoplastic agro-composite as substitute for raw materials

M. Suffo1 ;M. de la Mata2 ; S. I. Molina2 1Department

of Mechanical Engineering and Industrial Design, High Engineering School, Universidad de Cádiz, Campus Río San Pedro s/n 11510, Puerto Real

(Cádiz), Spain. 2Departamento

de Ciencia de los Materiales e Ing. Met. y Q. I. IMEYMAT. Universidad de Cádiz. Campus Río San Pedro, 11510 Puerto Real, Cádiz, Spain

[email protected]

ABSTRACT: The creation of plastic composites allows tuning the polymer properties in order to reach the desired performance for many different applications. Within this context, agricultural wastes and biopolymers offer suitable alternatives to be used as additive or reinforcement agents of polymer matrices. There is a wide variety of biocomposites or agro-composites containing natural fibers or solid residues of agricultural products as an additive, but there are not reported studies on agro-composites formed by the combination of synthetic petroleum plastics, and solid wastes from sugar beet working as a reinforcement of the polymer matrix. This work presents a new agro-composite synthesized by combining linear low-density polyethylene, one of the most used and versatile thermoplastic materials, and “Carbocal®”, an organic-inorganic residue from the sugar beet processing industry. The synthesis of the composite is direct and does not require the use of grafting, adhesion polymers or coupling agents. The tests carried out showed the improved mechanical performance of the composite compared with the neat polymer, proving the reinforcement role played by the waste within the polymer matrix. Therefore, this new agro-composite has potential applications replacing some highly demanded synthetic petroleum plastics. Additionally, the composite can be processed by extrusion and injection, according to its thermo-mechanical properties. Indeed, the composite was successfully injected with packaging purposes. The use of this new agro-composite contributes to recycle agricultural wastes otherwise hardly recovered and valorized, reducing the C02 footprint and favoring the use of high quality secondary-raw materials, well aligned with the circular economy.

KEY

WORDS:

“Sugar-Beet

Agro-composite

(SBA)”;

“Biocomposite”;

“Polymer-matrix

composites

(PMCs)”;“LLDPE/PE”;“Carbocal”

List of abbreviations / Nomenclature Ag-coated Silver cover BBB biomass based bioplastics bycow by-products / co-products / waste CaCO3 calcium carbonate CB Carbocal

1

CBXPEY Agro-composite, X% Carbocal weight in grams and Y% LLDPE weight in grams CPCs conductive polymer composites CY cyperusodoratus DBTDL Dibutyltindilaurate DCP dicumylperoxide DSC differential scaning calorimetry EDX X-ray dispersive energy Et Young´s modulus, MPa EMI electromagnetic interference FBP fossil based polymers Hc enthalpy of crystallinity, J/g HDT Heat Deflection Temperature, ºC Jc fracture energy per unit area, kJ/m2 LLDPE low density polyethylene linear MFI Melt flow indicator, g/10min PE polyethylene PET polyethylene terephthalate PMCs polymer-matrix composites PP polypropylene PS polystyrene PVA acetato de polivinilo PVC polyvinyl chloride SBA sugar-beet agro-composite SEM

2

Scanning Electron Microscopy t time, min T Temperature, ºC Tm the polymer melt temperature V rotation speed, r.p.m. WCB Carbocal weight in grams WPE LLDPE weight in grams XRD X-ray Diffraction ∆H enthalpy rate, J/g ε Stress, % εb Stress at break, % σ Strain, MPa σb Strain at break, % χc degree of crystallization, %

1. INTRODUCTION

Nowadays, reducing the consumption of synthetic plastics or fossil based polymers (FBP) has turned into an essential issue for environmental reasons and sustainability (La Mantia and Morreale, 2011; Ojeda et al., 2009). Some of the sectors with the highest plastic consumption are packaging, construction and automotive. Consequently, the top five most used thermoplastic materials in the industry are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET) and polystyrene (PS). These thermoplastic materials are mainly applied in conventional manufacturing procedures, such as injection or extrusion, although they are also becoming popular in additive manufacturing processes.

One milestone to target in by the materials engineering sector is to strength the use of "green materials" in the plastic industry (Akil et al., 2011; Dicker et al., 2014; Gurunathan et al., 2015), which either contain different amounts of biomass based bioplastics (BBB) (Arasaretnam and

3

Kirudchayini, 2019) or agricultural fibers based polymeric matrices. These compositions turn them into "agro-composites", with a better performance and ease tuning during the manufacture process. The additive to be used may work as an additive agent, filling the polymer matrix. Thereby, they would replace the environmentally harmful synthetic materials while contributing to control pollution problems (Sanjay et al., 2018). In addition, agro-composites are cheaper and display better mechanical and rheological properties, resulting in the most suitable alternative to synthetic plastics (Sathishkumar et al., 2014; Yusriah et al., 2014).

Additionally, the Circular Economy is driving the valorization of by-products / co-products / waste ("bycow" hereinafter) from different agro-industrial activities (Sanyang et al., 2016), by reusing them and get new biodegradable or completely reusable products.

Some currently explored solutions cover the use of composite materials, based on mixtures of different amounts of synthetic thermoplastic and a wide variety of reinforcement agents (Mazouz and Merbouh, 2019). The resulting composites can be then oriented as specific products for certain sectors (Cui et al., 2019; Dahy, 2017)(HOEDL, Herbert, 1991; KlonerEcotec_SP, 2012; LANKHORST_TOUWFAB BV, 1980; Prusinski, 1984; SignodeSystem_GMBH, 1990). The related bibliography on this topic mainly focuses on the description of well-known physical-chemical processes to properly mix the thermoplastic materials with the reinforcement agent. In most of the cases, the blending requires the use of adhesion polymers or coupling agents (La Mantia and Morreale, 2011) without exceeding 50% of the bycow content. Furthermore, several studies validate the use of a large number of agricultural residues as additives or reinforcements for synthetic thermoplastics in various industrial sectors (Ramesh et al., 2017; Saba et al., 2016; Sepe et al., 2018; Väisänen et al., 2016). For instance, (Li et al., 2018) show the development of conductive polymer composites (CPCs) for electromagnetic interference (EMI) shielding. Among the most used fillers, calcium carbonate (CaCO3), glass fibers, talc, kaolin, mica, wollastonite, silica, graphite, synthetic fillers (e. g. PET- or PVA-based fibers)(Mermerdaş et al., 2017), high performance fibers (carbon, aramid, etc.) (Özen and Şimşek, 2015), and reinforcing fibers (hemp, jute, sisal, kenaf, bamboo, etc.) (Gurunathan et al., 2015; Väisänen et al., 2016) can be mentioned. However, up to our knowledge, there are not reported works or registered patents on the use of bycows from the processing of sugar beet. The bycow can be used to guarantee a homogeneous mixture with the synthetic thermoplastic material, for instance, in the food industry. (Keller, Bohacek_GmhH, 1986) described a process for obtaining enzymes from beet peels, although no other works report the valorization of the bycow, or its use in the aforementioned mixtures of composites. On the other hand, calcite is a commonly used additive within polymer composites, rendering expensive thermoplastic materials (Özen and Şimşek, 2015; R.N. Rothon,

4

2003). (Kashyap and Datta, 2017) described the feasibility of a CaCO3-rich waste from the paper industry to work as a thermoplastic filling, improving the properties of the polymer matrix and pointing to promising properties expected for calcite-based wastes working as additive agent. This is the case of the sugar beet bycow Carbocal® (Carbocal hereafter).

The sugar beet industry produces large amounts of bycow that may reach 80% of the raw material (Nanou and Fuentes, 2002) regardless of the plant variety (Mahmoud et al., 2018). One ton of beet produces approximately 140 g of sugar, with the remaining products being recyclable bycow (Canales et al., 2004). A small fraction of the bycow is derived to the animal feeding industry and concrete production, but most of the material is discharged as waste. Carbocal, as dried pulp or “rabihojas” (tails and leaves), is a massively produced (20,000 tons / year on average) solid byproduct from the beet sugar manufacturing process. It corresponds to the trademark of the carbonation foams from the sugar beet refinement that is used in sucrose extraction. The addition of slaked lime and carbon dioxide to eliminate the macromolecules and particles in suspension in the sugar juice leads to the formation of carbonation foams (Carbocal). Therefore, Carbocal is mainly composed by CaCO3, a known additive material that is combined with polymeric matrices to act as filler or reinforcement. Thus, the strategy proposed here is aimed at opening an alternative way to develop agricultural based composites.

In particular, this work describes the manufacturing process of a new agro-composite material obtained from the blending of conventional LLDPE and Carbocal. The newly formed material meets the mechanical and rheological requirements to be implemented, for instance, in the food packaging industry, the main user of synthetic plastics. Moreover, this product can be processed by injection molding or by extrusion-based 3D printing machines, by simply balancing the relative amount of both constituents (Carbocal and polymer). A variety of SBA with different bycow/polymer ratios have been obtained and characterized. For example, SBA containing 50 wt% of additive was directly processed successfully by means of a twin-screw extruder.

2. MATERIAL AND METHODS

The approach followed in this study is summarized in Figure1 where pure materials, composites and processed materials are displayed in blue, green and reddish respectively.

5

Figure 1. Flow chart of the research methodology

2.1. Equipments and materials

Carbocal can be considered a suitable bycow due to its nature, volume of annual production and its classification as waste with no-guaranteed valorization. Chemical properties and generation process of the Carbocal used in this work have been indicated in Table 1, which was acquired in 2017 from a sugar beet factory located in the province of Cádiz (Andalusia, Spain).

Table 1. Characteristics of Carbocal (CB). Morphology

Powder

Genesis

Purification process of juice sweetened with lime hydroxide and CO2

Chemical composition

>80% CaCO3; 7% organic matter; oligo-elements (N, K2O, P2O5 and Mg);

6

assimilable organic acids Humidity

<35%

Volume

20,000 average annual tons

In the case of the LLDPE used in the performed assays, properties are summarized in Table 2 and it was provided by a local company known as the main commercial representative of well-known thermoplastics processing companies.

Table 2. Characteristics of the LLDPE used in this study. Izod Melt flow Supplier

indicator [g/10min]

LLDPE

SABIC M500026

50*

Density [kg/m3]

926

Stress at yield [MPa]

13

Tensile

Strain at break

impact

modulus

1% elongation

notched at

[MPa]

[%]

23 °C

354

120

Vicat softening temperature

Hardness

at 10 N (VST/A)

shore D

[KJ/m2]

[ºC]

0.45

88

55

* for 2.16 kg at 190ºC

Figure 2 shows the appearance of both materials, Carbocal and LLDPE, before the mixing process. The Carbocal was dried for 24 hours in an oven at 80°C before mixing (Ling et al., 2016). The materials underwent a melt mixing process using a Scamex Rehoscam (France) "internal mixer" machine-type, working with a small amount of input material to set up the later extrusion and injection processes of the LLDPE based agro-composites. This mixing process ensures a homogeneous blending of the constituents.

(a)

(b)

Figure 2. a) Dry Carbocal before mixing. b) Weighing container for different Carbocal percentages + LLDPE (CBx-PEy).

The agro-composite was subsequently chopped using a WSGM-250 equipment (J. Purchades, Spain), consisting on a stainless steel blade mill designed for plastic materials cutting and designed to get a particle size suitable for the injection process. Finally, the material was sieved until reaching a size not smaller than 2 mm, which can be unfavorable for the injection process.

7

The prototype tests were carried out following the characteristics for operation and distribution of the CBXPEY mass and using a maximum product quantity of 150g.

2.2 Methodology

2.2.1. Agro-composite manufacture process

The experimental parameters ensuring a proper mixing process were selected according to previous studies conducted with similar composites. The constituents were blended by a commonly reported melt mixing process used for similar materials and based on prior results obtained with other thermoplastic matrices in the same equipment. For instance, when using PP as polymer matrix mixing temperatures higher than 220ºC area required, which must be applied for more than 15 min, whereas other tested additives from the same sugar industry but with a stronger fiber character have been observed to blend successfully at slower spin speeds and lower temperatures (190ºC) during 10 min. Since the new material had not been used previously, similar examples available in the literature and summarized in Table 3 were consulted and used as starting point for process optimization.

Table 3. References used to set up the parameters for the mixing process. Material

Pre-Process

Internal Mixer process

Dry time Matrix

Composite

Oven dry

Reference

Temperature

Rotor speed

Time

[ºC]

[r.p.m]

[min]

150

50

10

160

120

8

Weight Ratio [H] LLDPE/PVOH

LLDPE

PVOH/Kenaf

Kenaf at 80 ºC

24

60:40 with Kenaf

(Ling et al., 2016)

10/20/30/40 LLDPE

Ciperus Odoratus (CY)

LLDPE

Goldenbirch

LLDPE

starch/Glycerol/

60ºC

24

5/10/15/20

(Faris et al., 2017) (Bravo et al.,

60ºC

24

10/20/30/40

170

80ºC

24

10/20/30/40

150

50

70ºC

20

10/20/30/40

130/170

33/40

2015)

Sago

(Sarifuddin et al., 2015)

Kenaf fiber Chitosan, closite, dicumylperoxid LLDPE

e (DCP), dibutyltindilaura

3/6

(Sadullah et al., 2016)

te (DBTDL), and stearic acid

8

According to Table 3, the temperature range to process LLDPE composites must be 130 ºC - 170 ºC. However, many factors play a role on the processing temperature of the polymer composite, which need to be considered:

- The melting temperature of the polymer matrix must be overpassed (in this particular case, 120 - 130 ºC, according to Differential Scanning Calorimetry “DSC” curves). - The material degradation temperature imposes the upper working temperature limit (typically, around 180 - 230 ºC). - The temperature degradation of the material used as filler is also important, although it is negligible when using inorganic fillers that degrade at temperatures much higher than that of the polymer matrix (e.g., >350ºC for CaCO3). - The melt flow index of the mixture depends on the melt flow index of the polymer matrix and the percentage of filler in the composite (the higher the percentage, the lower the melt flow index of the mixture). - The mixing time is also a relevant parameter, since the more easily the filler is integrated into the polymer matrix, the less mixing time will be necessary. - The geometry and rotation speed (rpm) of the screws.

The parameters finally used during the melt mixing process have been indicated in Table 4. Note that the use of an unknown bycow as Carbocal requires a good miscibility between the materials to optimize the rotor speed and temperature. As mentioned above, the success of the melt mixing process depends on both the materials involved and the final composition of the composite (i.e., the final Carbocal content). As the composites produced may contain different content of the additive and the final material exploitation depends on the composite properties, which is directly related with the composition, the optimization process was carried out

with each material

individually to obtain the most adequate parameters for the melt mixing process. The raw polymer was also processed in the internal mixer in order to guarantee the same preparation conditions for every sample. Table 4. Parameters tested to allow a successful mixing process. CB [%]

LLDPE [%]

WCB* [g]

WPE** [g]

V [r.p.m]

t [min]

T [ºC]

CB20PE80

20

80

30

120

55

10

190

CB30PE70

30

70

45

105

55

10

190

CB40PE60

40

60

60

90

55

10

190

CB50PE50

50

50

75

75

55

10

190

*WCB: Carbocal weight in grams **WPE: LLDPE weight in gram

9

Therefore, test series to optimize the mixing process of LLDPE composites loaded with 20, 30, 40 and 50 wt.% of Carbocal were conducted, resulting in the following optimum parameters: 190 °C of temperature; 55 rpm of screw angular velocity and 10 min of mixing time. The selection of a mixing temperature exceeding the upper limit indicated in Table 4 was due to the specific LLDPE used in this work, which resists such heating temperature safely without suffering thermal degradation. Moreover, the heavily loaded composites (filler content up to 50%) processed had significant lower melt flow indexes, hindering the blending that is favored at higher temperatures.

Figure 3 displays the obtained agro-composites and evidences the homogenous mixing, which results in solid masses before chopping in the blade mill. It is worthy to note that Carbocal always exhibited the same morphology, composition and granulometry and the only different parameter was the material humidity, which varied between 20 and 35 %. The chopping process provided a pellet material with controlled size, suitable for further processing in an injection machine.

(a)

(b)

(c)

(d)

Figure 3. a) Physical aspect of the agro-composite formed by: a) CB20PE80; b) CB30PE70; c) CB40PE60; d) CB50PE50.

The chopped agro-compositeis shown in Figure 4. The mechanical and rheological properties of the materials were also characterized, as well as their microstructure and chemical composition to assess the reinforcement behavior of the additive within the composite material.

Figure 4. Chopped agro-composite with a size <2mm.

Hence, composites were subsequently injected in an Babyplast 6/10P (Cronoplast, Spain) to obtain the test samples (Figure 5) for a mechanical characterization.

10

2.2.2. Description of materials and test methods

A. Structural and Chemical Characterization To ensure the suitability of the mixtures, the resulting materials were analyzed by X-ray diffraction (XRD) using an X-ray powder diffractometer equipment (BRUKER, model D8ADVANCEA25, LINXEYE rapid detector). The materials are analyzed by means of Scanning Electron Microscopy (SEM) and related techniques in order to investigate their morphology at the micro-/nano-scale and the distribution of the phases forming the agro-composites, along with their composition.

The chemical composition of the samples was studied by X-ray dispersive energy (EDX) analyses performed in the SEM. The samples were partially Ag-coated to avoid electrical charging during the EDX experiments. In particular, the powdered Carbocal, the pure LLDPE and two agrocomposites resulting from their combination - (with 20 and 50 wt.% of Carbocal content) were studied using a FEI Quanta 200 microscope equipped with an X-ray detector. The chemical composition has been further measured by X-ray diffraction.

B. Mechanical Properties The mechanical properties of the materials were analyzed through specimens built specifically for each of the composites and mixed LLDPE. Figure 5 shows test sample materials obtained following the ISO 527-2:2012 standard (Figure 5(d)) for the tensile testing.

Figure 5. Specimens used to carry out the tensile tests for the following mixing ratios: CB:PE. a) 20:80; b) 0:100; c) Specimens built for impact and Heat Deflection Temperature “HDT” tests; d) dimensions of the type 1BA specimens (mm).

Figure 5 also shows the Charpy impact, Vicat and HDT test specimens got according to the corresponding standard (75 mm x10 mm x 2 mm, length, width, thickness). Charpy impact tests were conducted according to the ISO 179 standard to determine the Charpy impact strength of the materials. The equipment used for tests was a Charpy-Izod IMPATS 15 (Metrotec, Spain). A total

11

of 5-10 specimens were tested for each material by using a pendulum nominal potential energy of 2J-5J and an impact speed of 2.9 m/s. HDT and Vicat test were performed according to ISO 75 and ISO 306, respectively by using a HDT/Vicat MP3 equipment (Astfaar, Italy) and 2-3 specimens of each material for every test. The test conditions were 23ºC and 50%RH.

A Tinius Olsen H10KS Universal Testing Machine was used following the UNE-EN ISO 527-1 and ISO 527-2 standards in order to address the tensile strength properties of the materials. The equipment allows using two types of load cells, 100N and 10kN, depending on the range of forces required by the test. In addition, it has 1100 mm of maximum displacement between claws, a sample range of force of 200 Hz (nominal) and an extension reading resolution of 0.001 mm.

C. Thermo-mechanical and Rheological Properties Thermo-mechanical tests lead to determine the HDT and the Vicat temperatures according to the UNE-EN ISO 75 standards (Astfaar MP-3HDT/Vicat equipment). In the tests carried out with 3 standardized tubes, conditioning and annealing parameters were established as 23ºC/hour up to 88 hours and at 50HR of hardness (Wong, 2003).

Measuring the fluidity allows checking the influence of the residue on the rheological properties of the polymer composite. The melt flow indicator (MFI) was determined in an MP600 extrusion plastometer (Tinius Olsen, Norway) according to the ISO 1133-1: 2011 standard by introducing 5 and 7 g of the chopped material under 2.16 kg, at an inner cylinder temperature of 190°C and with a cutting time interval of 5 s.

DSC measurements were carried out to the agro-composites and the pure materials separately (processed LLDPE and powdered Carbocal) in a Mettler Toledo (U.S.A.) DSC 1/200 equipment connected to a cryogenic device working between -40ºC and 400ºC. It also permits the entry of a gas stream into the chamber where the sample is located. Every test requires approximately 10 mg of sample under the following experimental conditions: 10K/min ramp at a temperature range of 30° C - 350° C and a N2 stream of 50 ml/min. The tests followed the UNE-EN-ISO 11357 standard. 40 µL aluminum crucible were tested applying a three phases thermal program, as described in the following:

Phase 1: 25-190 ºC (10 ºC/min); Phase 2: 190-35 ºC (-10 ºC/min); Phase 3: -35-356 ºC (10 ºC/min.

12

D. injection molded plastic A direct injection tests were performed in a Krauss Maffei Model CX80Tn fuel injection pump, equipped with a double spindle and owned by a packaging company located in the vicinity of the sugar company.

3. RESULTS AND DISCUSSION 3.1. X-Ray Diffraction (XRD) and SEM Analyses Results indicate the presence of calcite crystals as the main constituent of Carbocal. The performed X-rays diffraction measurements allowed the identification of the crystallographic phases involved in the agro-composites. Figure 6 shows the resulting X-ray diffraction patterns obtained for agro-composites containing 20, 30, 40 and 50 wt% of Carbocal, along with the pattern of pure Carbocal. The chemical composition of Carbocal was further investigated by energy dispersive X-ray analysis (EDX) in a scanning electron microscope (SEM). The analyses performed in the Carbocal alone confirmed that the bycow was mainly constituted by calcite (CaCO3), as it is evident in the indexed X-ray pattern and X-ray element maps displayed in Figure 6 ((a) and (b), respectively).

Figure 6. (a) XRD patterns of the agro-composites (CB20PE80, CB30PE70, CB40PE60, CB50PE50) and the pure Carbocal. Characteristic peaks of PE and CaCO3 are indexed. (b) EDX-SEM Ca, O and C maps and corresponding SEM image obtained from pure Carbocal.

All the agro-composites analyzed showed characteristic peaks of both, PE (Zhu et al., 2017) and

13

calcite (Kashyap and Datta, 2017; Ritika Gupta, 2004). Therefore, Carbocal acted as a source of calcite, which will be integrated in the resulting composite.

SEM studies were performed on pure LLDPE and composites with two different Carbocal content to investigate the morphology and distribution of the phases involved. Figure 7 (a) depicts the topography of the LLDPE used to obtain the composite. The analyses conducted on the composites (Fig. 7 (b,c)) allowed visualization of the filler particles resulting from blending the LLDPE with the bycow. These particles have micrometric dimensions and are homogeneously distributed within the plastic matrix without the need to incorporate coupling agents such as maleic anhydride (Liu et al., 2002; Pérez et al., 2013) or stearic acid (Özen and Şimşek, 2015).

Figure 7. SEM analyses performed on (a) pure LLDPE and derived composites with a Carbocal content of (b) 20% and (c) 50%. EDX results obtained in the composites (b,c) are also displayed showing the distribution of Ca (blue), C (cyan) and O (purple).

As shown in Figure 8, even though most of the Carbocal particles were smaller than 1µm, they tended to agglomerate into larger clusters (2-3µm) containing a few particles, which result in an average particle size of 1.5 µm (± 0.7). Regarding the microstructure of the materials, the surface morphology of the composite showed plain differences compared to the pure polymer. While the neat PE exhibited smooth surfaces with few dimples, the PE at the composite showed interconnected fibrillated structures creating a net where the particle fillers were trapped. Such configuration of constituents within the composite leads to the formation of voids and/or cavities in the plastic matrix, with relevant consequences on the mechanical performance. Moreover, further tuning of the composites to achieve improved properties might be possible (i.e., by using coupling agents), resulting, however, in more expensive materials. The additional EDX measurements carried out on the composites lead to the evaluation of the chemical composition of the filler particles, mainly composed by Ca, C and O, as displayed in Figure. 7 (b,c), in good agreement with the presence calcite phase (CaCO3) provided by the Carbocal.

14

Figure 8. Carbocal particle-size (µm) frequency distribution.

3.2.

Thermo – Mechanical properties tests

Figure 9 plots the results of the tensile tests, evidencing the different mechanical behavior of the composites depending on their composition (i.e., on the filler/polymer ratio).

15

Specimen 1 Specimen 4

10 9

Specimen 2 Specimen 5

(a)

Specimen 3

Specimen 2 Specimen 5

(b)

Specimen 3

10

8 7

8

Stress,σ Strain (MPa)

Stress,σ Strain (MPa)

Specimen 1 Specimen 4

12

6 5 4 3

6 4

2

2

1 0

0 0

100

200

300

400

500

600

0

10

20

Strain, ε (%) Strain Specimen 1 Specimen 4

12

Specimen 2 Specimen 5

(c)

Specimen 3

12 10

8

8

Stress,σ Strain (MPa)

10

Stress,σ Strain (MPa)

30

40

50

Strain, ε (%) Strain

6

4

2

Specimen 1

Specimen 2

Specimen 4

Specimen 5

Specimen 3

(d)

6

4

2

0

0 0

5

10

15

20

25

30

35

40

45

0

2

Strain, ε (%) Strain Specimen 1 Specimen 4

12

4

6

8

10

12

14

Strain, ε (%) Strain

Specimen 2 Specimen 5

(e)

Specimen 3

10

Stress,σ Strain (MPa)

8

6

4

2

0 0

1

2

3

4

5

6

7

8

9

Strain, ε (%) Strain

Figure 9. Agro-composites stress-strain curves for: a) pure LLDPE (processed material); b) CB20PE80; c) CB30PE70; d) CB40PE60; and e) CB50PE50.

As the Carbocal content increased, the elastic modulus (Young's modulus) rose (Figure 10(a)). Note that the measured value for the pure LLDPE differs from the one shown in Table 2, as a

εb (%)

Et(MPa)

consequence of the temperature treatments applied in the mixing procedure (see Table 4).

16

Figure 10. Mechanical properties of the agro-composites. (a) Young´s modulus, Et; (b) Stress at break, εb.

The increase of the elastic modulus is a consequence of the particle insertion within the plastic matrix (see Fig. 7(b,c)), as the toughness of the calcite, the main constituent of the Carbocal, provides a filling character to the additive (Greco et al., 2014; Yan et al., 2006). Such result may be explained attending to the microstructural features observed in the composites. As earlier mentioned, SEM observations revealed interconnected fibrillated polymer formations upon Carbocal addition, along with the development of tiny voids/cavities. Those voids/cavities and fibrils resist strong deformation before failure (at the microscale), enhancing the ductility of the material and, thus, hindering the crack propagation. Consequently, the increase of the elastic modulus of with the Carbocal content indicates the reinforcement role played by solid waste within the polymeric matrix, as reported elsewhere (Kwon et al., 2002).

Data were evaluated by means of parametric ANOVA tests in each data set (n = 22), resulting a Snedecor F for significant (p <0.05, F = 26.58). Thus, the time groups produced an effect on the response variable (Young’s Modulus).

Conversely, the stress significantly decreased with the addition of the residue(Kwon et al., 2002). It is apparent from Figure 10(b) that the inclusion of Carbocal changed strongly the stress, dropping dramatically for composites with 20 wt.%Carbocal, as compared to the pure polymer. From there, the strain at break kept on falling smoothly for increased Carbocal contents.

The ANOVA test in each data set (n = 22) result in Snedecor F for significance (p <0.05, F = 493.72), which indicates that the time groups had an effect on the strain at break.

The impact fracture energy per unit area, Jc, has been evaluated by Charpy impact tests (Figure 11). According to the results, there are two differentiated compositional ranges with different behaviors, going from 0 to 30% of Carbocal content and beyond 30%.

17

Figure 11. Impact test (Charpy) results showing the fracture energy per unit area (Jc) for different Carbocal concentrations.

For low Carbocal concentrations (up to 30%), Jc decreases slowly with the Carbocal content, while in composites containing more than 30% of Carbocal, Jc drops substantially by increasing the particle concentration. It is important to note that many factors arising from the combination of different phases within a uniform material, such as stress concentration, debonding, shear bands, etc., may influence the mechanical performance of the composites. Small particle amounts within the polymeric matrix will induce the formation of stress concentrating points around the particles, rising the impact fracture energy per unit area. However, increasing the number of stress concentrators above a certain threshold may result in particle agglomeration and, thus, the material becomes gradually brittle leading to fast crack propagation and failure of the material by fracture, when the Carbocal content exceeds 30%. In a similar way, the results of the ANOVA tests in each data set (n = 43) indicated a Snedecor F for significant (p <0.05, F = 24.9). Thus, the time groups produced an effect on the response variable (Charpy).

Figure 12(a) depicts the results of HDT tests carried out for each agro-composite and for the processed LLDPE. It is evident that the progressive addition of Carbocal up to 40% increased the thermo-mechanical resistance of the material subjected to a constant load of 1.86 MPa for at least 88 hours and when the mold temperature was kept at 23 62 ºC.

Figure 12. Thermo - mechanical and rheological properties of the agro-composites. a) HDT test; b) MFI test.

18

However, for Carbocal contents higher than 40%, the measured HDT dropped. These tests demonstrated that they had a strong linear relationship with the blend ratio, as reported by Wong et al.(Wong, 2003).

Finally, the statistical analyses of the results evaluated by means of a parametric ANOVA test in each data set (n = 14) showed a Snedecor F for significant (p <0.05, F = 11.4), confirming that the time groups produced an effect on the response variable (HDT).

According to the melt flow index (MFI)values, the fluidity of the composite decreased as function of the Carbocal content (Figure 12b). This effect is due to the higher viscosity of the loaded material. The mineral percentage was inversely proportional to the flow index because the processing temperature of 195 ºC causes the melting of the polymeric matrix (the PE), whereas the Carbocal remains as a solid in suspension, increasing the viscosity of the mixture and the inertia of the molten compound, finally resulting in a decrease of the flow index. This result is in agreement with the observations reported by (Djellali et al., 2015) by mixing PE with nanoclays.

The thermograms included in Figure 13 show the DSC results on 10 mg samples. The striped areas corresponding to the pure LLDPE and the agro-composites were very similar. The first phase corresponds to the material homogenization, whereas the next phase change observed indicates the polymer melt (Tm). The subsequent line present in all the graphs corresponds to the pulse line or solidification. Both, the enthalpy (∆H) of the process and the subsequent crystallization remained unchanged in all cases. The Tm of the pure LLDPE processed was around 134.55 ºC whereas Tm in the agro-composites (CBX+PEY) varied between 123.85 and 126.37 ºC, indicating that it did not change significantly with the Carbocal addition.

19

Figure 13. DSC thermograms corresponding to the agro-composites: a) Pure LLDPE; b) CB20PE80; c) CB30PE70; d) CB40PE60; e) CB50PE50.

20

The degree of crystallization, χc, was calculated according to the equation (1) (Luyt et al., 2006; Marcovich and Villar, 2003):

=

(1)

∙

where: HcLLDPE is the enthalpy of crystallinity of the pure polymer at 100% crystallization (Özen and Şimşek, 2015) HcAgro is the enthalpy of crystallinity of the agro-composite wLLDPE is the percentage of mass polymer in the agro-composite (1, 0.8, 0.7, 0.6 and 0.5)

Table 5 summarizes the crystallization rates of the agro-composites calculated by equation (1). It can be observed that the crystallization rate, χc, is not significantly affected by the increase of the additive content. These results demonstrate the suitability of Carbocal as an additive without distorting the crystalline properties of the polymer matrix, in agreement with the XRD measurements (Fig. 6).

Table 5. Crystallization rate of different polymers. Polymers

Tm[ºC]

Hc [J/g] 293

100

LLDPE pure

123.85

134.55

45.92

CB20PE80

123,42

118,13

50,40

CB30PE70

123.09

94.95

46.29

CB40PE60

123.19

83.93

47.74

CB50PE50

123.25

66.99

45.73

LLDPE pure [21]

χc [%]

Based on these results, in the injection tests, 15 Kg of agro-composite containing 50% of additive and manufactured by direct extrusion in filament were used, without intermediate stages of drying, chopping or mixing. The compound obtained was successfully used in the packaging injector (Figure 14).

(a)

(b)

(c) 21

Figure 14. Results obtained by the direct injection of the agro-composite in 50:50 ratio: a) LLDPE original element; front (b) and back (c) of the injected element using 50:50 agro-composite.

4. CONCLUSIONS.

This work valorizes an abundant bycow from the sugar beet industry, the so-called Carbocal, extracted from local sugar beet factories, economically important in the area and with relevant impact throughout Spain. This material was mixed with conventional thermoplastic matrices, such as LLDPE, resulting in a new composite material that can be classified as agro-composite due to its agricultural origin. The residue-polymer miscibility covers the full compositional range, with the micrometric filler being well integrated within the polymeric matrix without significant defects. Increasing the Carbocal concentration results in more rigid and less fluid composites, with higher thermo-mechanical resistance while keeping the polymer structure. According to the thermomechanical properties, Carbocal works as a reinforcement of the polymer but not as a load agent, increasing the Young's modulus up to 175% with the addition of 50% of Carbocal. Additionally, the incorporation of the additive keeps the crystallization rate almost invariant as inferred form the calorimetric measurements. On the other hand, the rheological properties of the agro-composite depended on the additive amount since the MFI decreased up to 50% with the addition of 50% of Carbocal. This dependence resembles the incorporation of pure CaCO3 in the polymer, eliminating the associated economic cost. The study also shows the feasibility to obtain elements and mechanical assemblies from industrial byproducts through different manufacturing, conventional (extrusion, injection, etc.) and emerging (3D printing) techniques, reducing the costs of the raw materials. Moreover, the injection tests demonstrated the application of the new agro-composite to reduce the amount of conventional thermoplastic materials, valorizing an abundant waste driven, for instance, to manufacture packaging products. Altogether, the presented approach contributes to save material costs while providing an agro-compound that meets the Circular Economy criteria.

Acknowledgments 22

This work was funded by the Junta de Andalucía (Research group INNANOMAT, ref. TEP946). Co-funding from UE is also acknowledged. The authors wishes to thank “AZUCARERA IBERIA S.L.U.” and“ NUEVA COMERCIAL AZUCARERA S.A.” group for products supply, and “TORRENT INNOVA, S.A.” for allowing the use of its facilities for injection testing. MdlM acknowledges postdoctoral contract from Ministry of Education (Juan de la Cierva).

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Declaration of interests    ☒ The authors declare that they have no known competing financial interests or personal relationships  that could have appeared to influence the work reported in this paper.    ☐The authors declare the following financial interests/personal relationships which may be considered  as potential competing interests: