effect of coir content on mechanical and thermal

Effect of coir content on mechanical and thermal propertiesof LLDPE/coir blend processed by rotational molding

NIKITA GUPTA* and P L RAMKUMAR

Mechanical Engineering Department, Institute of Infrastructure Technology Research and Management,

Ahmedabad, India

e-mail: [email protected]

MS received 9 May 2020; revised 20 November 2020; accepted 1 January 2021

Abstract. Rotational Molding is a unique polymer processing technique that gives an advantage of having

stress free end product. Majorly, Linear Low Density Polyethylene (LLDPE) is preferred as a base resin for

rotomolding. However, incorporation of additives becomes necessary to improve mechanical properties and

viscoelasticity as incumbent for few applications. The present research makes an attempt to investigate the effect

of coir fiber on rotationally moldable LLDPE. Based on preliminary processing experiments, 10% or less Coir in

its powder form when mixed with LLDPE was found to account a better flow ability as a prerequisite for roto

moldability. Hence, the effect of 3%, 5%, 7% and 10% Coir mixed with LLDPE was experimented for the roto

molded product to determine the mechanical properties, viz. tensile, flexural, hardness and impact strength.

Also, the rate of heat transfer was evaluated subjected to the data recorded for mold, oven and internal air

temperature when processing LLDPE/Coir considering different weight percentages. Crystallinity of the blend

was examined based on DSC analysis for the distinguished percentage of coir incorporated in LLDPE. The

experimental results depicted an improvised mechanical properties by 7% or less fiber addition, and beyond that,

the properties were found to be regressed. The present work successfully engulfed Coir with LLDPE, producing

a uniform distribution along the sides of the mold without applying any external pressure. Therefore, fiber

integration in the rotomolding process is believed to be possible thus preserving homogeneous randomly

distributed fiber dispersion.

Keywords. Rotational molding; LLDPE; coir; mechanical characterization; DSC.

1. Introduction

Polymers and polymer composites have dominated the

world of materials in the present century due to their unique

properties deliberately found in the end product being light

in weight, corrosion resistant, etc. [1–4]. Majorly, all the

metallic goods have been replaced by the group of poly-

meric materials that are well known to the researcher in the

present century [5, 6]. Various manufacturing techniques

are into existence to frame a polymeric end product like

injection molding, blow molding, thermoforming, rota-

tional molding, etc. [7].

Rotational molding process has gained popularity in the

last few decades due to its idiosyncratic properties associ-

ated in producing the roto molded product. It is a polymer

processing technique which can manufacture hollow and

seamless structures having complex geometries. A major

benefit of rotationally molded product is it gives a stress

free end product [8]. It is used for the manufacturing of

tanks, containers, parts of cars, commercial aircraft

components, toys, kayaks, refrigerated panels, automotive

bumpers, etc. With rotational molding, multilayered parts

can also be fabricated [9, 10].

Numerous thermoplastics such as polyethylene,

polypropylene, acrylonitrile-butadiene-styrene, polylactic

acid, nylon, polyvinyl chloride can be used as a base resin

for rotational molding. However, polyethylene is accounted

for 85% of usage for this process because of its broader

processing window and excellent thermal stability [11–14].

Amongst polyethylene, LLDPE is of great interest for roto

molding due to its less shear sensitivity and unique melt

flow properties [15]. However, it frames a limitation in

delivering the adequate strength as needed in few applica-

tions like battle tanks, ships, etc. In order to enhance the

mechanical property of roto molded product, fibers, nano-

fillers, particulate reinforcements play a significant role

when blended with roto moldable LLDPE [16, 17]. A phase

has been on progressive mode in current decades wherein

the manufacturing of polymer and their composite is in a

developing era, for which natural and synthetic fibers or

fillers reinforced polymer matrix is gaining popularity

among the researchers around the globe [5].*For correspondence

Sådhanå (2021) 46:40 � Indian Academy of Sciences

https://doi.org/10.1007/s12046-021-01566-8Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

Recent year’s scenario facing to new environmental

legislations demand some promising materials based on

natural fibers reinforcement. Disparate traditional fillers

(glass fiber, talc, aramid fibers, carbon fibers, etc.), bio-

fillers offer countless advantages of being easily available,

low cost and short-term biodegradability. In this regards of

content, the usage of natural fiber composites industries are

developing rapidly [18]. Considering the issues related to

global warming and depletion of petroleum reserves, sci-

entists have become increasingly concerned on the use of

natural fibres such as bagasse, coir, sisal, jute, etc. This has

contributed more towards the research on materials which

are based on natural fiber composites. Due to low cost, easy

availability, low density, reasonable unique properties, easy

separation, biodegradability and recyclability in nature,

reinforcement with natural fibre in composites have

recently gained attention. In this regard, an investigation

was carried out in order to allow use of coir, a natural fibre

that is abundantly available in India. In this context and the

aim of improving mechanical property of roto molded

product, natural fibers have been considered to be rein-

forced with LLDPE for this particular study.

However, only few research studies have been assembled

till date regarding the incorporation of natural fiber rein-

forced matrix for rotational molding process [19–22]. The

reason mainly accuses towards the poor adhesion of fiber

with PE matrix [23]. A research study presented by Ortega

et al investigated the PE matrix composites based on abaca

and banana fiber at 5% by weight ratio, made in two and

three layer composites. An easily processible window was

observed by them for rotational molding process and even a

better aesthetic and elastic modulus property was sustained

in the product using the prepared composite [24]. In another

study pertained by Cisneros-Lopez et al have analysed the

effect of agave, pine and coir fibers on PE matrix when

surface treated with Maleated Polyethylene (MAPE) in the

weight ratio of 10, 20, 30 and 40%. Their research con-

cluded that the treated agave and coir fiber matrix gave

more successful results than pine due to their respective

chemical composition [25]. Wang et al used pre-treated flaxfibers with LLDPE and High Density Polyethylene (HDPE)

to prepare bio-composites using rotational molding.

Chemically treated fiber reinforced matrix showed increase

in strength compared to untreated fiber reinforced matrix

[22]. Szostak et al investigated the thermal and mechanical

properties of flax or hemp fiber reinforced in a rotational

molded polyethylene matrix. For 3% by weight fiber con-

tent showed an increase of 14% in tensile strength. How-

ever, the decrease in mechanical properties was observed

after 5% of fiber addition. Highest degree of crystallinity

was observed in composite having flame retardant [26].

Hanana et al used untreated and maleated polyethylene

(MAPE) treated maple wood fiber as a reinforcement with

LLDPE to produce composite using rotational molding.

Morphological and mechanical characterization showed

that the mechanical properties of MAPE treated fiber

composites were improved unlike untreated fiber compos-

ites [27]. Lopez et al produced composites from polylactic

acid and agave fibers by rotational molding and compres-

sion molding. Results showed that improvement in prop-

erties was more significant in compression molded

composites. Rotomolded parts had more porous structure

compare to compression molded parts [28].

Thus it can be concluded from literature survey that the

mechanical properties like tensile strength, impact, and

flexural strength can be improved to some extent when the

natural fillers are incorporated in low amounts in the

polyethylene (PE) matrix [29, 30]. Hence, as conferred by

the valuable articles, there is a large potential to produce PE

composites on the basis of reinforcing materials of natural

origin in rotational molding process. This inspired us to

progress our research based on reinforcing another natural

filler: Coir with LLDPE.

The main objective of this work is to determine the

feasibility of emulsifying coir fibers into linear low-density

polyethylene (LLDPE) to manufacture rotomolded com-

posite parts as coir fiber evinced to be the hardest fiber

among natural fibers in terms of processibility [31]. Besides

this, the properties of coir like low weight, inexpensive, not

abrasive, and biodegradable also marks a huge impact for

the incorporation of coir as part of research. Moreover, coir

as compared to other natural fibers has high resistivity to

water absorption [32]. Usage of this fiber also becomes

beneficial because once the coconut is processed to produce

its distinguished product, the fibers are disposed and

become a significant waste problem [33]. Coir is quite

durable as it possesses high amount of lignin. The fibres

also have high break elongation, which means that they can

also be highly elongated and stretched without rupture

beyond the elastic limit [34]. In the past, distinguished work

have been reported on creating various composite materials

made from coir for processing methods such as compres-

sion, extrusion, and injection molding [35, 36]. The addi-

tion of coir fiber, however has not been reported yet when

utilized in its powder form for rotational molding process.

The paper has been framed into majorly four sections: (1)

Procuring of materials and preparing the rotomoulded

composites utilizing coir in different concentrations with

LLDPE as base resin. (2) With the bio composites after

being roto molded, the mechanical properties will further

be then tested determining the effect of coir content on roto

molded product. (3) Section 3 deals with the analysis of

temperature during rotational molding process in order to

evaluate the heat transfer rate for distinguished percentage

of coir added in LLDPE. (4) The last part of the present

study investigates the crystallinity differences obtained for

different concentration of coir reinforced in LLDPE com-

pared to unreinforced LLDPE.

40 of 12 Sådhanå (2021) 46:40

1.1 Preliminary analysis

Certain relevant experiments were performed by Gupta

et al as a part of preliminary analysis in order to ensure the

requisite processibility for rotational molding process [13].

Satisfying material processibility for a particular process

becomes equally important as investigating the mechani-

cal properties of the end product. Thus, as a part of

prefatory analysis, the processibility tests were performed

in order to confirm the appropriate flow ability of the

material when fibers are added into base resin. The eval-

uation tests are as mentioned in table 1:

Thus, 10% or below coir added to LLDPE marked an

appropriate significance to be utilized yielding better

processibility for rotational molding process. Based on this

preliminary analysis, the mechanical properties were

checked for 10% or less coir reinforced with LLDPE

framing roto molded product.

2. Material and experimental details

The rotationally moldable LLDPE (Ge3645, Greenage

Industries, Ahmedabad, India) was utilized as the ther-

moplastic resin for the elaborated matrix and bio com-

posite. This polymer has a 4.2 g/10 min melt flow index at

190 �C, an average size particle of 300 lm and a density of

0.936 g/cc. Coir, which is used as filler in different weight

percentage, was obtained with density 1.4 grams/cc as a

part of residues of coconut husk from a local company

Greenman Garden Agro Center, Ahmedabad, India. Coir

was procured as a residue, so it has been chopped and

grinded into powder form from Varahi Industries,

Ahmedabad, India. Coir when converted to powder form,

was then sundried for 48 hours to ensure the removal of

moisture content after being washed with distilled water.

This was then sieved to obtain an average particle size of

125 lm. Here, the major focus is to study the effect of

adding coir on LLDPE for roto molded product in terms of

mechanical characterization. Hence, for different weight

ratios of coir being blended with LLDPE (0%, 3%, 5%,

7% and 10%), a total of 5 rotationally molded structures

were prepared. The distinguished roto molded product that

we prepared involves various steps as schematically

described in figure 1.

For each ratio, LLDPE and coir powders of measured

amounts were solid mixed in a Caliber Electromix T-10

S-blade mixer. The mold size utilized for the particular

experiment was a mild steel square dimension of 20 cm x

20 cm and the thickness was taken as 3 mm. PP-Steric acid

was applied on the inner side of the mold as a mold

releasing agent in order to ensure proper removal of the

roto molded product after complete processing. The

blended LLDPE/Coir was then fed into the mold. As the

mold is continued to be rotated biaxial, the major axis to

minor axis rotation was set on the ratio of 2:8. Both oven Table

1.

Preliminarytestsperform

edforLLDPE/Coirblendas

apartofRoto

moldability.

S.no.

Tests

Purpose

Results

1Fourier

Transform

Infrared

Spectroscopy(FTIR)

Tojustifytheusageofproperties

ofboth

-the

resinandadditivefrom

arangeof0%

to50%

LLDPE/Coirblend

3%

to15%

was

therangeobtained

wherethe

dominance

ofboth

LLDPEandCoirwas

found

tobesignificant

2MeltFlow

Index

(MFI)

Toaccountforproper

flow

abilityas

needed

for

rotational

moldingprocess

(3-8

grams/10

minutes)

[37]

Therangeof3%-12%

showed

thevalueofMFI

between3and8g/10min

asneeded

forroto

moldability

3Rheology

Tosatisfytheshearproperties

Thetestsconcluded

below

10%

asappropriate

deliveringneeded

storageandloss

modulus

4DSC

Tojustifythethermal

properties

Below

10%

coiradded

inLLDPEdonotmake

much

difference

inmeltingpointwhen

compared

tounreinforced

LLDPE

Sådhanå (2021) 46:40 of 12 40

heating and fan cooling cycle time was set as 30 minutes.

The oven temperature was adjusted to 300 �C. The differentweight ratios of coir being mixed with LLDPE is desig-

nated as shown in table 2.

2.1 Mechanical characterization

For the characterization to be experimented on the roto

molded product, the composites were cut into plates from

the square mold in order to obtain the relevant dimensions

as per ASTM standards. The tensile testing was done as per

ASTM D 638, for which a total of five sample for each

concentration of coir was analysed with LLDPE to get the

average value of tensile strength. The same Instron machine

with a crosshead speed of 2 mm/min was utilized to obtain

the result of flexural strength. ASTM D790 was used to get

the average value for which each samples were tested 5

times. Impact strength was another mechanical property

being investigated to study the effect of coir as per ASTM

D256. This method was followed in order to determine the

hardness of the bio-composite in the Shore D scale. The

universal testing and Izod impact testing machine used for

the experimental purpose are as shown in figures 2 and 3,

respectively. The specimens prepared for tensile, flexural

and impact are demonstrated in figures 4(a), (b) and (c),

respectively.

2.2 Temperature analysis to evaluate heat transferrate

Rotational molding process utilizes heat as a major

parameter to melt the material inside the mold. For pure

LLDPE, the melting process takes place around 120 �C.The powder starts to tumble around and sticks to mold at

115 �C to 140 �C [12, 38, 39]. But when the fibers are

reinforced in the matrix, the heat transfer analysis varies on

a major difference. This also makes a huge variation in the

heating cycle time and the cooling cycle time. As fiber

being added, reduces the bubble formation and hence the

heat transfer rate varies on a large scale. Peak internal air

temperature (PIAT) is another parameter that resembles the

highest temperature experienced by the internal surface of

the mold part. PIAT is often correlated to the amount of

cure experienced by the part. Higher PIAT demotes higher

mean failure energy impact values. Higher degree of cure

Grinding

+

(a) (b) (c)

(f) (e) (d)

Figure 1. Stages of elaboration of the roto moldable LLDPE/Coir bio composite: (a) Coir fiber, (b) Crushed Coir in powder form,

(c) LLDPE powder, (d) Mild Steel mold, (e) Rotational Molding Machine and (f) Roto Molded Products.

Table 2. Designation of rotationally molded composites.

Reinforcing

fiber

Concentration of Fiber (Weight

%) Nomenclature

Pure LLDPE 0% LLDPE

Coir 3% C1

Coir 5% C2

Coir 7% C3

Coir 10% C4

40 of 12 Sådhanå (2021) 46:40

results in better impact resistance [11, 40]. This made us

lead towards analyzing the internal air temperature and

evaluate the heat transfer rate for varying coir concentration

in LLDPE for rotational molding process.

2.3 Differential scanning calorimeter

DSC is an inspection technique that allows the user to

measure glass transition, melting temperature and temper-

ature of crystallization when heating or cooling a sample of

polymeric material. The polymer material being tried has a

particular melting and recrystallization point which is

expressed as an endothermic and exothermic peak sepa-

rately in the DSC thermographs. It is a thermal analysis

technique in which the heat flow in material is measured

with respect to temperature or time. During the process,

different mixtures were allowed to pass through a heating

stage, a cooling stage, and another heating stage. A TA

Instruments DSC (Model Q2000) uses, for example, a low-

mass aluminum test container and a weight of around 10

mg.

3. Results and discussion

3.1 Mechanical properties

Distinguished mechanical properties like tensile, hardness,

flexural and impact test were experimented on the prepared

roto molded products. The static tensile test results for the

prepared roto molded product of coir reinforced with LLDPE

are summarized in table 3. Figure 5 shows the tensile

strength graph of prepared bio composites when compared to

unfilled LLDPE. As depicted from the values mentioned in

table 3, the tensile strength and elastic modulus were found

to increase till 7% - (for 3% (1.31 MPa – 10.6%), 5% (0.73

Figure 2. Universal Testing Machine.

Figure 3. Izod Impact Testing Machine.

Sådhanå (2021) 46:40 of 12 40

MPa – 6%) and 7% (0.45 MPa – 3.7%). However, a

reduction of 25% in tensile strength was evident for 10%

coir added in LLDPE. The trend obtained shows an

agreement with the several other valuable research based

on natural fiber composites [21, 41, 42]. The elastic

modulus for unfilled LLDPE was 255.15 MPa. For all the

other prepared composites, when compared with pure

LLDPE, this modulus is higher (280.11 – 308.96 MPa)

except for 10% which is 206.13 MPa. The highest value of

elastic modulus was obtained for 5% coir filler when

added to LLDPE (308.96 MPa) which can be observed in

figure 6. This type of similarity in results is also been

Figure 4. (a) Tensile Specimen as per ASTM D 638. (b) Flexural Specimen as per ASTM D 790. (c) Impact Specimen as per ASTM D

256.

Table 3. Mechanical properties of LLDPE/Coir blend.

Tensile Strength (MPa) E Modulus (MPa) Strain at break (%)

LLDPE 12.36 255.15 14.17

C1 13.67 304.18 10.43

C2 13.09 308.96 9.57

C3 12.81 280.11 9.21

C4 9.28 206.13 8.09

12.36

13.6713.09 12.81

9.28

0

2

4

6

8

10

12

14

LLDPE C1 C2 C3 C4

Tens

ile S

tren

gth

(MPa

)

Pure LLDPE and its Blends

Figure 5. Tensile Strength of LLDPE/Coir.

255.15

304.18 308.96

280.11

206.13

0

50

100

150

200

250

300

LLDPE C1 C2 C3 C4

Mod

ulus

of E

las�

city

, (M

Pa)

Pure LLDPE and its blends

Figure 6. Elastic Modulus of LLDPE/Coir.

40 of 12 Sådhanå (2021) 46:40

reported in few remarkable literature using sisal, wood,

and agave for bio composites [27, 43, 44]. The decrease in

tensile strength and elastic modulus at higher filler con-

centration conveys that fiber depreciates the elastic prop-

erty of the material, thus making it fragile. When coir is

mixed with the base resin, it increases the stiffness of the

product and the rigid nature of bio composites is influ-

enced by the coir filler [45, 46]. Moreover, the powder

form of coir acts as particle reinforcement, which aids in

lowering the elasticity nature of LLDPE, making the

composite less ductile. The elongation at break was found

to decrease (8.09-10.43%) as the filler content increases.

This conveys that the maximum stress at break get

weakened with the addition of natural filler content. In

particular, the coir particles generate stress concentration

zones around them. This in turn reveals that the coir fillers

acts as the source of creating point of weakness and

defects which decreases the resistance of the bio com-

posite [18]. The increase in tensile strength and elastic

modulus for the composites being tested when compared

to unfilled LLDPE confirms the improvement of the

strength and stiffness of the LLDPE/Coir blend till 7% of

fiber addition, beyond which the properties are found to be

reverted.

Table 4 gives the data of flexural, impact and hardness

test for the various prepared LLDPE/Coir roto molded

composites. The experimental investigation for flexural test

reveals that the flexural properties obtained are satisfacto-

rily at par when compared with unfilled LLDPE for 3%

reinforcement of coir that increases the flexural strength by

26%, and that for 5% Coir filled LLDPE, an increase in

13% strength was evident. But a drop down to 48% is

observed under further coir addition. This behaviour in

Table 4. Mechanical Properties of LLDPE/Coir blend.

S. no. Nomenclature Flexural strength (MPa) Impact strength (J/m) Hardness test (Shore D)

1 LLDPE 14.65 21.48 58.69

2 C1 18.26 24.75 60.23

3 C2 16.55 24.82 65.93

4 C3 8.24 24.96 68.35

5 C4 7.65 20.24 72.12

14.65

18.2616.55

8.24 7.65

0

2

4

6

8

10

12

14

16

18

20

LLDPE C1 C2 C3 C4

Flex

ural

Str

engt

h (M

Pa)


Figure 7. Flexural Strength of LLDPE/Coir.

21.48

24.75 24.82 24.96

20.24

0

5

10

15

20

25

LLDPE C1 C2 C3 C4

Impa

ct S

tren

gth

(J/m

)


Figure 8. Impact Strength of LLDPE/Coir.

58.6960.23

65.93

68.35

72.12

50

55

60

65

70

LLDPE C1 C2 C3 C3

Hard

ness

(Sho

re D

)


Figure 9. Hardness value of LLDPE/Coir.

Sådhanå (2021) 46:40 of 12 40

flexural properties may be due to a reason that an increase

in filler content occupies a larger surface area, mainly

towards the inner surface of the mold. Moreover, higher

fiber concentrations gets agglomerated inside the mold and

thus the improper dispersion of fibers makes a significant

role in deteriorating the mechanical properties. This depicts

that the flexion initiates crack from that region [5]. And

hence, at higher coir concentration, the flexural properties

are found to be decreasing when compared to pure LLDPE.

Similar trends have been observed in the study that utilized

sisal and hemp as natural fibers [16, 47, 48]. Results of the

bio composites depicting flexural strength is reported in

figure 7.

A similarity in the trend was observed for impact spec-

imens also, that reported an improvement of 15.22% for 3%

coir, 15.6% for 5% and a maximum of 16% for 7% coir

reinforced in LLDPE. However, a negligible decrease of

5% was observed for 10% LLDPE/Coir blend as evident

from figure 8. Our results were found to be in agreement

with the research reported on other natural fillers [23, 49].

This sustains the properties without much depreciating the

roto moldability up to 7 wt% coir addition to LLDPE. This

also synchronizes with the tensile test results, wherein the

stiffness increases till 7%, after which the properties are of

significantly lesser usage in rotational molding process.

Figure 9 indicates the value of hardness obtained for

various concentration of coir in LLDPE. Hardness increases

significantly when compared with 0% Coir into base resin.

A maximum rise of 22.8% is descried for 10% coir from the

hardness experimented based on Shore D scale.

The data obtained from hardness test as mentioned in

table 4 reveals that the increasing percentage of coir in

LLDPE marks a significant rise in hardening the roto

molded product. This behaviour of increase in hardness can

be justified by the hard and brittle nature of coir being

incorporated in the soft polymer matrix. Modulus of elas-

ticity and hardness were found to be increasing significantly

whereas the tensile strength decreases with the higher

concentration of fiber in the base resin. As for tensile

testing, the adhesion between filler and base resin strongly

affects the tensile strength, whereas in the case of hardness

test, the stress is mainly applied to the bulk material [18].

Figure 10. Microstructure image of 5% LLDPE/Coir.

Figure 11. Microstructure image of 10% LLDPE/Coir.

Figure 12. The swollen surface observed on 10% LLDPE/Coir

Roto molded product.

0

50

100

150

200

250

0 10 20 30 40 50 60

Tem

pera

ture

(Deg

ree

Celc

ius)

Oven Residence Time (in Minutes)

3% Coir Fiber Composites

Internal Air Temperature Mould Temperature Oven Temperature

Figure 13. Variation of oven residence time in terms of

temperature for 3% LLDPE/Coir blend.

40 of 12 Sådhanå (2021) 46:40

Thus, the at room temperature, the mechanical properties

achieved for bio composites generated with our simplified

technique method are in good agreement with those of the

previously elaborated literature [50–52].

Figures 10 and 11 are the microstructure images obtained

from optical microscope for 5% and 10% coir reinforced in

LLDPE respectively. For 5%, a uniform distribution of

fiber is evident when observed from the inner surface of the

mold. 10% coir mixed with LLDPE is worst affected by the

hydrophilic nature of natural fiber on the polymer matrix.

Numerous pores were visible as depicted in figure 11. The

moisture content had affected the roto molded product

wherein a swell on the surface was visible as shown in

figure 12. This is one of the major reasons for the

mechanical properties being deteriorated for 10% (above

7%).

The experiments clearly state that an easy accommoda-

tion up to 7% of coir fiber without any property loss and a

gain in mechanical strength is possible in rotational mold-

ing process. But any further addition may signify to poor

mechanical properties which is thus not recommended. Any

further addition of fibers notifies a deficient impact on

properties. These results completely compliments to the

preliminary analysis performed to check the requisite pro-

cessibility for rotational molding process. The product

characterization reveals that the 7% coir concentrated with

LLDPE is suitable in terms of both material roto

moldability and mechanical characterization.

3.2 Heat transfer analysis

As temperature is an important parameter for the heating

cycle in rotational molding process, the heat transfer rate

becomes an integral part to be evaluated for each fiber

concentrated in the base resin. For this purpose, the tem-

perature was recorded at every 5 minutes interval for which

the data has been plotted in figure 13 for 3% coir fiber

composites whereas 10% LLDPE/Coir composites is rep-

resented in figure 14.

For heating purpose, the oven temperature has to be set

initially, through which the heat is transferred to the mold.

This temperature of the mold is then absorbed by the

polymer, through which with an appropriate viscosity, the

polymer starts to melt and gets stick to the inner mold

surface, thus forming the shape of the mold. Now when

fibers are added in this polymer resin, the temperature

variations are significant to be evaluated. Moreover, the

processing cycle time also varies with the addition of filler.

When the heat transfer investigation was performed on

distinguished LLDPE/Coir roto moldability, a remarkable

difference was observed in the internal air temperature. As

the fibers absorbs the heat more than the neat LLDPE, a

reduction in internal air temperature was noticeable as

natural fiber absorbs larger amount of heat. This revals a

reduction in bubble densification which acts as thermal

insulators. The phase change of polymer matrix (from

glassy state to rubbery state) was observed after 15 minutes

of processing time at 200 �C. At this stage, the needed melt

flow is achieved easily. At this particular temperature

parameter, a homogeneous composite can be obtained as

the fiber dispersion takes place uniformly in the LLDPE

matrix. A similar experimental analysis was observed in a

study carried by Abhilash et al [5]. Moreover, a reduction

in cycle time was also observed on higher concentration of

coir in LLDPE due to reduction in the cooling time. The

fiber content helps in improving the shrinkage effect, thus

helping the structure to be in lager contact with the mold

surface ensuring proper thermal transfer. This type of

similar results were also observed in the study conducted by

Lopez et al where they analysed the heat transfer effect on

LMDPE by the addition of agave fiber for rotational

molding process [43].

The temperature analysis compared to unfilled LLDPE

was also analysed with the aid of thermal thermographs

using Differential Scanning Calorimeter.

3.3 Thermal properties: DSC measurements

DSC characterization was done using setline DSC instru-

ment having aluminum 75 ll crucible. Experiments were

0

50

100

150

200

250

0 10 20 30 40 50 60Tem

pera

ture

(Deg

ree

Celc

ius)

Oven Residence Time (in Minutes)

10% Coir Fiber Composite

Internal Air Temperature Mould Temperature Oven Temperature

Figure 14. Variation of oven residence time in terms of

temperature for 10% LLDPE/Coir blend.

41.29

44.9315646945.6306808

46.32426878

47.78612059

36

38

40

42

44

46

48

LLDPE C1 C2 C3 C4

Degr

ee o

f Cry

stal

linity

, %


Figure 15. Degree of Crystallinity for various LLDPE/Coir

composites.

Sådhanå (2021) 46:40 of 12 40

performed under nitrogen atmosphere having flow rate of

20 ml/min. A sample mass of 5 to 10 mg was used for

characterization. DSC tests were performed in the range

from room temperature to 350 �C at heating rate of 10 �C/min. Sample was first heated up to 350 �C, then cooled

down up to room temperature and then heated again to 350

�C. DSC thermographs were analyzed using calisto soft-

ware. Based on the variation in the temperature, the crys-

tallinity of the structure changes for which the

%crystallinity can be determined using equation (1):

% Crystallinity ¼ DHm

DH0m

� 1

Wf� 100: ð1Þ

where, DHm is experimental melting enthalpy, DH0m ¼

293J=g is the melting enthalpy of 100% crystalline poly-

ethylene and Wf is weight ratio of polyethylene in com-

posite [53, 54].

Reinforcement of fiber/fillers in neat polyethylene make

changes in nucleating agents which subsequently changes

the melting and crystallization temperature and the % of

crystallization. Figure 15 determines the degree of crys-

tallinity (DoC) of the prepared composites based on the

DSC test.

In mostly all the composites, the DoC obtained was

found to be higher than the unfilled LLDPE. For pure

LLDPE, DoC is 41.29%, whereas for 10% LLDPE/Coir,

highest DoC was evident. Increase in melting temperature

may be accredited to the plasticization effect of the rein-

forcement diffused into LLDPE [22]. The percentage

crystallinity was increased due to reinforcement. The fillers

when added make a significant contribution in increasing

the nucleation arrangement as they make sure to remove the

bubble density from the inner surface mold. A similar trend

was observed in a research study done by Szostac et al,wherein they noticed a higher state of crystallinity in their

prepared LDPE/Flax and LDPE/Hemp composites when

compared to their unfilled base matrix LDPE [26]. From

DSC thermograph analysis, it can be said that Coir-rein-

forced LLDPE can be used for rotational molding processes

as it does not degrade the processing temperature range.

4. Conclusion

Rotational molding is a plastic processing technique in

which mainly LLDPE is preferred as a base resin. In order

to enhance mechanical properties targeting some applica-

tions demanding strength on major criteria, additives play a

vital role. The present study aimed in exploiting coir to be

mixed with LLDPE to achieve an improvised mechanical

property of the bio composite. The tensile strength and

elastic modulus were increased on a justifiable scale till 7%

coir reinforced in LLDPE as compared to unfilled LLDPE.

For 10% coir mixed with LLDPE, the value declined by

25% as related to pure resin. For all the tested bio

composites, flexural strength showed a decrease in value

beyond 7%. This may be possible due to the flexion crack

being initiated at the mold surface due to uneven fiber

distribution at higher concentrations. Even when the

microstructures were studied for 5% and 10% roto molded

product, a uniform fiber distribution was maintained in 5%

LLDPE/Coir blend. Also the impact strength and hardness

increased on increasing the coir content in LLDPE which

satisfactorily aids to the tensile test result. The heat at the

inner surface of the mold was found to be decreasing with

the increase in temperature, the reason being major heat

absorbed by the natural filler, and hence reveals the

decrease in bubble densification which acts as thermal

insulation. Degree of crystallinity was calculated for all the

composites, which signifies an increase in the crystallinity

for all the ratios of LLDPE/Coir blend. Thus the structure

crystallizes on a better scale with the addition of coir in

LLDPE. A homogeneous fiber distribution and a strong

filler interface with the resin is achievable at lower con-

centration of coir. However, higher percentage of coir

encourages the microstructural defect. The analysis con-

firms the prospects of incorporating natural fibers in the

polymeric material on a lesser scale content, considering

their usefulness in rotational molding process.

AbbreviationsASTM American Society for Testing and Materials

DSC Differential Scanning Calorimeter

FTIR Fourier Transform Infrared Spectroscopy

HDPE High Density Polyethylene

LLDPE Linear Low Density Polyethylene

MAPE Maleated Polyethylene

MFI Melt Flow Index

PE Polyethylene

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