Mechanical and damage tolerance behavior of short sisal fiber reinforced recycled polypropylene biocomposites

Abstract

Sisal fiber (SF) reinforced recycled polypropylene biocomposites were prepared by melt blending technique. Biocomposites prepared with the incorporation of 40 wt% untreated sisal fiber loading showed a marginal improvement in mechanical properties as compared with matrix recycled polypropylene. SF surface was mercerized and maleic anhydride grafted polypropylene was used as a coupling agent for better fiber matrix interfacial bonding. Mercerized sisal fiber reinforced biocomposites prepared with compatibilizer (maleic anhydride grafted polypropylene) shows significant improvement in tensile and flexural strength. Damage tolerance of recycled polypropylene matrix and its biocomposites were evaluated in monotonic and cyclic tensile test. Untreated sisal fiber reinforced biocomposites prepared with maleic anhydride grafted polypropylene shows improvement in damage tolerance compared with untreated sisal fiber biocomposites. Impact fractured morphology of biocomposites revealed better interfacial bonding between fiber, maleic anhydride grafted polypropylene, and recycled polypropylene matrix.

Keywords

Sisal fibers recycled polypropylene damage tolerance morphology

Introduction

Use of thermoplastic olefins (TPOs) is currently growing in the automotive industry as a front and back automotive car bumper.^1,2 Polypropylene is frequently blended with an elastomer such as ethylene propylene diene monomer (EPDM), ethylene propylene rubber (EPR), to obtain desired impact properties. The increasing application of TPOs in automotive bumper generates waste after end-of-life-vehicles (ELV) causing a major threat to our environment.³ Recycling of ELV is a waste prevention approach by reusing valuable resources rather than dumping or ends up in the landfill. Plastic recycling is a sustainable way of development, which provides an opportunity to conserve natural resources, saving energy and reducing landfill space.⁴ So there is a need for re-orient new technologies for recycling and value addition of ELV bumper to pursue environmentally sustainable development strategies for future growth.

In recent years, natural fiber became an ideal choice as a reinforced member of biocomposites,^5,6 because of its high specific strength, low density, light weight, and renewability.^7,8 Growing number of research in the relevant area signifies importance of natural fiber based biocomposites materials.^9–14 Lignocellulosic fiber shows better damage tolerance compared with the glass fiber.¹⁵ This is because the glass fiber is brittle, whereas the lignocellulosic fiber is flexible. Many researchers attempts to study fiber reinforced green composite and its relevant properties. However, few research only focus on study of fiber reinforced recycled polypropylene (rPP) and analysis of their damage tolerance.

Damage tolerance is the ability of a material to show resistance to damage and accommodates damage without being repaired. However, the term “damage” is included in a material during its manufacturing defect or/and in service use. Damage tolerance shows the ability of material to retain sufficient residual strength in damage condition. It includes the resistance of a structure for damage initiation and growth in the presence of a crack, notch, or another type of damage to prevent catastrophic failure.¹⁶ A material having good damage tolerance shows detectable damage at a higher tensile load or having much more potential to sustain damage. Whereas a material having low damage tolerance have less potential to resist damage. Damage tolerance evaluation enables the designer to know the safe mechanical stress for a material in damage condition during its service use. Liu et al.¹⁷ evaluated damage tolerance of silicon carbide matrix based composites reinforced with carbon fiber. The tensile test method was used and the tensile stress–strain curve is extracted for further analysis. The author concluded that damage tolerance up to 100 MPa of tensile stress is almost zero, thus, indicating a good damage tolerance at this stress. While above 100 MPa, the specimen tested at −40℃ is showing comparatively less damage tolerance compared with the specimen tested at −80℃. Mamiya et al.¹⁸ evaluated damage tolerance in plain woven fabric (Al₂O₃ F–ZrO₂)mc/Al₂O₃ composites. The tensile test of the composites sample was studied in two different fiber loading direction: (a) fiber axial direction and (b) 45° off axial direction. The author reported the undetectable damage in case of 0° fiber axial loading up to 10 MPa stress, due to good damage tolerance. While in the case of 45°, off axial fiber loading, a detectable damage after 5 MPa tensile load was observed indicating less damage tolerance.

Now a days, rPP is considered as a low cost material for the development of non critical structural components in various construction fields.¹⁹ Therefore, besides its strength and stiffness, a better understanding of damage tolerance is also essential. The damage tolerance of a material enables a designer to explore the application of the material in suitable fields. In the present investigation, our prime objective is to evaluate the damage tolerance and mechanical properties of biocomposite. The effect of fiber treatment along with the incorporation of compatibilizer on the damage tolerance of mechanically improved biocomposite was also studied.

Materials and methods

Material

rPP PP108MF10 with MFI 6–7 g/10 min (at 230℃, 2.16 kg) was obtained from a car dismantler from Delhi, India. OPTIM® maleic anhydride grafted polypropylene (MAPP) used as coupling agent with MFI of 100 g/10 min at 190℃, 2.16 kg, and 0.934 g/ml density with a high purity of MAH content was obtained from Pluss Polymers Pvt. Ltd, Delhi, India. LR grade of sodium hydroxide (NaOH) was purchased from M/s Specialities Pvt. Ltd, Mumbai, India.

Method

Surface treatment of sisal fiber

The fibers were initially scoured in warm detergent solution for 30 min to remove wax and other impurities. Then, the fibers were air dried followed by drying in a vacuum oven to remove moisture. The detergent washed fibers were soaked in 1 (N) NaOH at ambient temperature. A basic solution was prepared with demineralized water, followed with immersion in the solution for a duration of 18 h at room temperature. Treated fibers were washed with normal water containing a few drops of acetic acid, followed by washing under a continuous stream of water until complete removal of NaOH. Subsequently, the mercerized fibers were dried at room temperature for 24 h and then in a vacuum oven at 80℃ for 12 h.

Preparation of biocomposite

Biocomposites were prepared by using melt blending technique in two stages. In the first stage, chopped sisal fiber of length 5–10 cm was melt mixed at 190℃ in Haake Rheocord 9000 for 14 min. In the second stage, mixed masterbatch of fiber incorporated rPP is molded in a micro injection molding machine DSM, Netherlands, to prepare tensile and flexural test specimens.

Characterization

Mechanical test

Tensile strength was evaluated using universal testing machine (3382 Instron, UK) according to ASTM-D 638-03, with a gauge length of 50. Flexural properties were also evaluated using three point flexural bending test the same universal testing machine as per ASTM D790 at crosshead speed of 5 mm/min. The manufacturing arrangement and the machine used in this study demonstrates a good reproducibility of results with a low standard deviation (SD). In most of the experiment, the SD value comes near to or less than 1.

Damage tolerance in a monotonic tensile test

Four methods are generally used in literature to measure a damage behavior in a material. The first is measurement of physical parameter like a change in density. The second one is measurement in remaining life of material, for example, time to failure in creep. The third one is microstructural measurement such as measurement in a volume fraction and the last one is measurement in mechanical behavior, e.g. change in elastic modulus. In our study, last method was adopted for evaluation of damage tolerance. In this study, damage tolerance was evaluated using monotonic tensile test.^18,20 Damage tolerance ( $\tilde{D}$ ) presented in equation (1) is calculated for damage evaluation during the monotonic tensile fracture process and is systematically illustrated in Figure 1. $\tilde{D}$ is defined as

\tilde{D} = (1 - \frac{(d λ c / d ɛ c) i}{(d λ c / d ɛ c) 0}) (\frac{U p, i}{U_{p}^{*}})

(1)

where

(d λ c / d ɛ c) i

is the tangent modulus at i-th point in the stress–strain curve (stress

λ i

and strain ɛi), and

(d λ c / d ɛ c) 0

is the tangent modulus in the linear region of the tensile stress–strain curve.

U pi

is inelastic energy absorption at

σ i

during the tensile test, which has been evaluated using equation (2)

U pi = \int_{0}^{ɛ i} λ c d ɛ c - \frac{1}{2} λ i ɛ i

(2)

where

U_{p}^{*}

is the inelastic portion of the total energy absorbed during the tensile test up to the, ultimate tensile strength (UTS)

λ cm

, such that

U_{p}^{*} = \int_{0}^{ɛ cm} λ c d ɛ c - \frac{1}{2} λ cm ɛ cm

where

ɛ cm

is the strain at tensile stress

σ cm

(i.e. strain at fracture).

Damage tolerance in a cyclic tensile test

Cyclic tensile test was performed to evaluate the damage tolerance of biocomposites caused by a change in the material due to an application of stress. Cyclic test was performed in a strain-controlled way using the following steps. First, loading was applied on sample up to certain strain, then loading was retracted down at same crosshead speed up to minimum strain. In the next cycle, sample was subjected to a higher strain. During unloading, the deformed material is returned to the original position. In case of reversible deformation, the energy absorb during deformation is released during the recovery. However, reduction in area and size of curve as obtained from stress–strain which is subjected to cyclic loading shows some permanent deformation, which is characterized by a loss of stiffness of the composites. The cumulative damage, Dm, from cyclic tensile test is presented as follows:²¹

Dm = 1 - Ei / Eo

where Ei is Young's modulus after the ith cycle and Eo is the initial modulus.

Morphological analysis

The tensile fractured specimen of the biocomposites were taken for morphological study by using Scanning Electron Microscopy (EVO MA 15, Carlzeiss SMT Germany). The samples were gold sputtered (50-nm thickness) and dried for half an hour in vacuum at 70℃ prior to study.

Results and discussions

Mechanical properties

Tensile properties of rPP and its biocomposites

Tensile properties untreated sisal fiber (UTSF) reinforced rPP biocomposites as a function of fiber loading from 10 to 40 wt% is enumerated in Figure 2(a). It is obvious that tensile strength and modulus of rPP biocomposites steadily increases with increasing fiber loading. It can be seen that tensile strength and modulus of rPP biocomposites at 40 wt% of fiber loading increased to the tune of 12% and 280%, respectively, as compared with the rPP matrix. This increment is primarily attributed to better stress transfer from matrix to fiber, and good mixing of fiber in the rPP matrix. Uniform dispersion of the fiber in rPP matrix results in an enhanced tensile strength and modulus.²² Based on the optimal mechanical properties of 40 wt% fiber loading, rPP biocomposites is optimized.

To further increase the mechanical properties of biocomposites, surface of sisal fiber was mercerized and MAPP compatibilizer was used. The effect of compatibilizer on UTSF and mercerized sisal fiber (MSF) biocomposites is also shown in Figure 2(b). It can be seen that mercerization of sisal fiber further increased the interfacial bonding between fiber and matrix, because of good bonding with the matrix. The biocomposite prepared with MSF loading of 40 wt% and 5 wt% MAPP showed about 329% improvement in tensile modulus and 44% increase in tensile strength, respectively, as compared with rPP. Mercerization reduces the lignin content, waxy material, and ultimately opens the hydroxyl group of cellulose to make good bonding with thermoplastic matrix. Similarly, incorporation of coupling agent also increases the interfacial bonding between fiber and matrix by making esterlinkage at the fiber matrix interface. The presence of MAPP shows the best result for obtaining improved tensile strength and modulus in MSF reinforced biocomposites.^23–26 This is because of good stress transfer from fiber to the matrix at the interface due to ester linkage between anhydride group of fiber and hydroxyl group of cellulose.

Tensile stress–strain behavior of rPP and its biocomposites

Figure 3 shows the stress–strain curve of rPP and its biocomposites. The results revealed that the combined use of both MSF and compatibilizer results in the considerable effect on the tensile stress–strain behavior of rPP biocomposites, as compared with rPP and UTSF biocomposites. It is evident that rPP stretched rapidly under tensile load and area under the stress–strain curve is high. However, biocomposites showed a reduction in the stress–strain curve after incorporation of fiber in the rPP matrix.

It is evident that regardless of surface treatment and compatibilizer, the tensile stress–strain curve decrease in all biocomposites, therefore, shows a brittle like behavior, where a linear deformation is observed at the lower strain. But at higher strain, non-linear deformation characteristic up to fracture of the biocomposites was observed. The nonlinear deformation suggests microcrack initiation in the biocomposites at the fiber matrix interface, which grows up along with the fiber followed by plastic deformation of the matrix polymer. The ultimate result was microcrack opening and crack propagation through deformed matrix. The characteristic curve of untreated and MAPP incorporated biocomposites are same, but only differs in the maximum stress values that are higher in the case of MAPP incorporated MSF biocomposites. This is due to better fiber matrix interfacial bonding in the treated fiber and matrix.

Flexural properties of rPP and its biocomposites

Flexural properties of rPP and its biocomposites are represented in Figure 4(a). It can be seen from the figure that flexural strength of biocomposites increased up to 30% after incorporation of UTSF as a function of 40 wt% fiber loading. This result suggests mixing and better dispersion between fiber and rPP matrix. Furthermore, up to 55% increment in flexural strength was observed in the case of MSF and MAPP reinforced rPP biocomposites (Figure 4(b)).

This improvement is due to good interfacial bonding between treated fiber and matrix in the presence of MAPP. Flexural modulus for UTSF incorporated biocomposite matrix increase from 942 MPa (rPP) to 2965 MPa (rPP/40%UTSF). Also, flexural modulus of rPP/MSF (40 wt%)/MAPP (5 wt%) biocomposites have reported an increase to the tuen to 274% as compared with rPP matrix. This increment suggests better bonding between mercerized fiber and matrix in the presence of MAPP compatibilizer.

Flexural stress–strain behavior of rPP and its biocomposites

Figure 5 shows the flexural stress–strain curve of rPP, and its biocomposite reinforced with 40 wt% of UTSF and MSF along with 5 wt% of compatibilizer. A linear relationship can be seen between flexural stress and strain at a higher stress in biocomposites compared with rPP matrix. This behavior is due to increase in the stiffness in biocomposites. The non-linear pattern was observed after proportional limit stress (PLS) and is an equivalent stress causing debonding in between fiber and matrix. Post-peak stress–strain curve dip in compatibilizer incorporated biocomposites found a little decrease with increase in stress. This behavior is possibly because of when maximum stress are reached some fiber are not broken and they still withstand some stress.

Damage tolerance of rPP and its biocomposites using monotonic tensile test

The damage parameter $\tilde{D}$ for matrix rPP and its biocomposites is displayed in Figure 6. Damage tolerance for matrix rPP is almost zero up to 2 MPa stress, and no detectable damage is observed at this stress. However, in region of stress value above 2 MPa results in an increased $\tilde{D}$ value up to 0.4 indicates initiation of some detectable damage in the matrix. In case of biocomposites, “ $\tilde{D}$ ” value was observed near about 1 MPa of stress. Biocomposites prepared without MAPP compatibilizer showed a higher value of “ $\tilde{D}$ ” compared with MAPP incorporated biocomposites. This suggests untreated fiber shows detectable damage at lower stress. This may be because of a lower interfacial bonding between untreated fiber and matrix. However, incorporation of MAPP results in an improvement in the fiber matrix interfacial bonding. Damage tolerance properties of UTSF and MSF reinforced biocomposites prepared with compatibilizer shows a marginal difference in the value of “ $\tilde{D}$ ”. MSF reinforced biocomposites prepared along with MAPP shows better damage tolerance, compared with UTSF reinforced biocomposite. This behavior is probably because of compatibilizer, which makes a better interfacial bonding with mercerized fiber, and results in minorly low damage tolerance. It was reported that interfacial bond strength between fiber and matrix influences the composite properties in a number of ways. As the interfacial bonding is increased, the properties of composite changes such as, (a) static composite strength increased, (b) composite shows a fracture in the brittle mode, and (c) composite become notch sensitive.²⁷ From the above finding, it was observed that as the fiber matrix interfacial strength is increased, it results in an increase of damage tolerance. However, higher the interfacial strength up to a certain level results in a reduction in damage tolerance properties in the biocomposite.

Damage tolerance of rPP and its biocomposites using cyclic tensile test

Figure 7 and Table 1 show the effect of cyclic tension and compression loading on progressive accumulation of deformation on the specimen. The rPP matrix and its biocomposite shows a characteristic change in the shape and each hysteresis loops gives different dissipation area in each cycle. Results as depicted in Table 1 shows an increase in tensile strength in rPP and its both UTSF and MSF reinforced biocomposites, up to fourth cycle. The appearance of hysteresis loop discloses any damage inside the biocomposites subjected to cyclic loading. During cyclic loading and unloading, the interface of fiber and matrix suffered debonding and fiber matrix cracking. The energy dissipation during loading is due to various failure events related to load induced internal damage. In the sample, the elastic (store) energy and loss energy, which is dissipated in composite system depends upon the properties of the material, mostly interface properties. In the case of UTSF reinforced biocomposites, it is believed that the crack growth at the tensile side of the specimen is retarded by the fiber and composite seems to be unable to absorb as much energy and a fracture has occurred. This behavior is probably due to damage, which occurred in the previous cyclic loading, creating a permanent internal damage in the material, this suggests cause of instability of UTSF reinforced biocomposites for damage on increasing number of loading cycle. Cyclic tension creates an accumulation of damage during loading and retraction of tensile force, but with a less intensity due to diminishing effect at the interface in the biocomposites. It can be seen in the case of UTSF reinforced biocomposites prepared with compatibilizer, the loop area is less than MSF reinforced biocomposite. This behavior is because UTSF behaves like a elastic material due to suitable interface strength, whereas MSF reinforced biocomposite behaves as brittle material, due to more interfacial bonding at fiber matrix interface because of fiber surface mercerization.

Table 1.

Result of cyclic loading at constant strain.

	First cycle	Second cycle	Third cycle	Fourth cycle
	(MPa)	(MPa)	(MPa)	(MPa)
rPP	5.66	10.47	13.75	15.53
rPP/UTSF (40 wt%)	12	19.02	21.6	21.7
rPP/UTSF (40 wt%)/ MA-g-PP (5 wt%)	11.73	15.94	19.38	24.5
rPP/MSF (40 wt%)/ MA-g-PP (5 wt%)	16.51	20.74	23.15	27.3

In our case, the second cycle applies load more than previous one, therefore, extra load further caused an additional damage, in all biocomposites and reflected in all hysteresis loop. The energy absorbed by the reinforced biocomposite during deformation is not completely recovered during retraction and some energy was loss as an interface slip, between fiber and matrix and shows lose in stiffness at next cycle. In the case of compatibilizer incorporated biocomposites, during cyclic loading, the stored energy is more than dissipated energy due to better stress transfer at the fiber matrix interface, as reflected in the area of the loop. Based on experimental findings, it has been observed that better interfacial adhesion of fiber with matrix results in an increment in tensile strength during cyclic loading and accounts for damage in brittle mode.

Figure 8 shows the relationship between damage accumulation and maximum cyclic strain for rPP and its biocomposites prepared with 40 wt% of UTSF, and MSF with and without MAPP, respectively. rPP matrix shows D value about 0.15, due to high elastic effect of the rPP matrix. UTSF reinforced biocomposites without compatibilizer suffered a ruptured at D = 0.55. This behavior is probably due to the viscoelastic effect in biocomposites, which has been found to be reduced in the presence of fiber, as the natural fiber exhibits very low viscoelastic behavior. However, in case of MSF reinforced biocomposites, this rupture occurred around at D = 0.45, wherein a lower value of “D” suggest a better damage tolerance. This behavior is probably due to better interfacial bonding between MSF and matrix not require results as obtained enumerated the fact of enhanced interfacial bonding and adhesion with matrix in the presence of 40 wt% MSF in rPP matrix.

Scanning electron microscopy (SEM) study of rPP and its biocomposites

SEM image of fractured biocomposites surface is depicted in Figure 9(a) to (c) for UTSF, UTSF/MAPP, and MSF/MAPP reinforced biocomposites, respectively. It is evident from Figure 9(a) that incorporation of UTSF in the rPP matrix changes the material properties from elastic state to rigid state in biocomposites. Therefore, fracture properties like crack growth and stability in the biocomposites found to be broken without stable crack extension. It is evident from figure that the UTSF reinforced biocomposites shows poor interfacial bonding between the fiber and matrix. So, the fibers extensively pulled out from the matrix and fiber delamination was observed in the fractured surface, thereby plastic shear of the matrix occurs, which allows the transfer of stress in the composites. Short fiber are believed to be randomly oriented in the polymer matrix, as a result of batch mixture prior to injection molding thereby results better dispersion of fiber in the matrix accompanied by extensive fiber fibrillation in the rPP matrix. Furthermore, fibers are oriented in machine direction during injection molding results in an increase in tensile strength. A large area of visible delamination mark can be easily seen on the rPP matrix suggests substantial interfacial strength between UTSF and rPP.

SEM micrographs of UTSF reinforced biocomposites with compatibilizer as depicted in Figure 9(b) shows a reduction in the fiber delamination with a fiber breakage that suggests fiber have been dispersed in the matrix during processing. Fiber delamination was not observed but broken fiber on the surface suggests fiber matrix compatibility is fairly good in UTSF biocomposites prepared with 5 wt% of compatibilizer. Fiber matrix bonding is retained after fracture is an indication of some adhesion between them and the same was also corroborated in tensile test, where an improvement in tensile yield strength was observed compared with UTSF biocomposites without compatibilizer. Using MAPP as compatibilizer promotes an efficient fiber-matrix bonding with the formation of a chemical linkage at the interface. In the presence of compatibilizer, UTSF biocomposites required more energy compared with biocomposites in absence of compatibilizer, therefore, results in an increase in the yield strength, which corroborates well with the tensile results.

SEM image for MSF reinforced biocomposite with MAPP is enumerated in Figure 9(c). Mercerization of fiber induces rough surface, creates microspores on the fiber surface and thereby increase fiber matrix interfacial bonding. It can be seen that fiber were connected with rPP matrix and gap between fiber and matrix is reduced due to better interfacial bonding between MSF and rPP. Compatibilizer linked with hydroxyl group of fiber with a ester linkage. Micrograph also reveals an homogenously distribution of fiber in the matrix surface that results an equal stress transfer in all direction and therefore shows good mechanical strength. It can also be seen in SEM images that fiber exhibits very less pull-out length due to good adhesion between fiber and matrix, thus contributed to the improvement of mechanical properties.

Conclusion

This work demonstrate the effect of inclusion of different wt% chopped sisal fiber in recycled PP, their surface treatment and influence of compatibilizer on the damage tolerance and mechanical properties of biocomposites. Optimum mechanical properties was observed at 40 wt% fiber loading, the adhesion between fiber and rPP matrix has considerably improved in the presence of MAPP, and influenced the damage tolerance and mechanical properties of biocomposites. The experimental findings were revealed the following facts:

Tensile strength and modulus of rPP biocomposite steadily increases with increasing fiber loading. Biocomposites incorporated with 40 wt% of UTSF resulted in an increased in both tensile strength and modulus properties to the tune of 12% and 280%, respectively, whereas biocomposite prepared with 40 wt% of MSF loading along with 5 wt% MAPP as compatibilizer marked about 44% and 329% improvement as compared with rPP.

The tensile stress–strain behavior suggest that elongation at break is reduce in biocomposite relative to rPP matrix due to change in the rPP properties from ductile to brittle in the presence of fiber.

Flexural strength and modulus properties of biocomposite prepared with 40 wt% of MSF and 5 wt% compatibilizer increased 55% and 274%, respectively, compared with rPP matrix.

Post-peak curve dip in flexural stress–strain found a little decrease with increase in stress in compatibilizer incorporated biocomposites. This behavior is possibly because when maximum stress are reached, some fiber are not broken and they still withstand some stress.

Biocomposites prepared without MAPP compatibilizer showed a higher value of “ $\tilde{D}$ ” compared with biocomposites prepared with MAPP.

UTSF reinforced biocomposites prepared with compatibilizer shows the less loop area than MSF reinforced biocomposite. This behavior is because UTSF behaves like a elastic material due to suitable interface strength, whereas MSF reinforced biocomposite behaves as brittle material due to more interfacial bonding at fiber matrix interface because of fiber surface mercerization.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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