Mechanical, Degradation and Interfacial Properties of Chitosan Fiber-Reinforced Polypropylene Composites

Mechanical, Degradation, and InterfacialProperties of Synthetic Degradable FiberReinforced Polypropylene Composites

RUHUL A. KHAN* AND MUBARAK A. KHAN

Nuclear and Radiation Chemistry Division, Institute of Nuclear Science and TechnologyBangladesh Atomic Energy Commission, Dhaka 1000, Bangladesh

SABRINA SULTANA, M. NURUZZAMAN KHAN, QUAZI T. H. SHUBHRA

AND FARHANA G. NOOR

Department of Applied Chemistry and Chemical Technology, The University of DhakaDhaka 1000, Bangladesh

ABSTRACT: Polypropylene (PP) matrix synthetic phosphate based degradable fiber reinforcedunidirectional composites (10% fiber by weight) were fabricated by compression molding. Tensilestrength (TS), tensile modulus (TM), elongation at break (%), bending strength (BS), bendingmodulus (BM), and impact strength (IS) were found to be 38MPa, 1.5GPa, 12%, 44MPa, 4.9GPa,and 7.58 kJ/m2 respectively. Degradation tests of the fibers and composites were performed for sixmonths in aqueous medium at room temperature (258C). After six months, the mechanicalproperties of the composites retained almost 80% of their original properties. The interfacial shearstrength (IFSS) of the composites were also measured by single fiber fragmentation test (SFFT). TheIFSS of the composite system was found 5.9MPa that indicated good fiber matrix adhesion.

KEY WORDS: polypropylene, composites, degradable fibers, mechanical properties, compressionmolding, interfacial properties.

INTRODUCTION

COMPOSITE MATERIALS ARE widely using in many fields such as: civil, industrial,military, space craft, and biomedical applications mainly because of their excellent

thermo-mechanical properties. Synthetic fiber reinforced thermoplastic compositesattracted much attention due to its better durability and moisture resistance properties.Most of the composites are made of glass, carbon, or aramid fibers reinforced polymercomposites. Biodegradable and biocompatible composites are used mainly for orthopedicfields. Thermoplastic matrix materials are the most important part of a composite.Polypropylene (PP) is an amorphous thermoplastic polymer and are widely using asengineering thermoplastic because it possesses several vital and useful properties such astransparency, dimensional stability, flame resistance, high heat distortion temperature,

*Author to whom correspondence should be addressed. E-mail: [email protected] 5 appears in color online: http://jrp.sagepub.com

Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 00, No. 00/2009 1

0731-6844/09/00 0001–11 $10.00/0 DOI: 10.1177/0731684408100699� SAGE Publications 2009

Los Angeles, London, New Delhi and Singapore

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Journal of Reinforced Plastics and Composites OnlineFirst, published on February 23, 2009 as doi:10.1177/0731684408100699

and high impact strength. PP is also very suitable for filling, reinforcing, and blending.PP with fibrous natural polymers of biomass origin is one of the most promising routes tocreate natural-synthetic polymer composites [1–7].

Currently there are several synthetic degradable fibers such as bioglass, phosphate glass,and organic polymeric fibers being used as reinforcing agents for biomedical composites.The possibility of drawing heat softened glass into fibers was known to glassmakers inantiquity and is older than the technique of glass blowing. Many Egyptian vessels weremade with glass fibers. In the early nineteenth century some luxury brocades were madein France by interweaving glass fibers with silk of a deep color. Early in the 1930s, theOwens-Illinois Glass Company, Newark, USA made significant improvements to theprocess of glass fiber manufacture which made it commercially viable. The main productsmarketed by the glass fiber industry are mats, rovings, woven rovings, chopped and milledfibers, yarns, etc. In phosphate glasses, the primary network former is phosphorouspentaoxide (P2O5). The P2O5 provides the backbone of the glass structure formingtetrahedra composed of one phosphorus ion surrounded by four oxygen ions. Phosphateglasses have many unique properties, the most interesting of which is its ability to dissolvecompletely in aqueous media. Phosphate glasses can be synthesized to include ionsroutinely found in the body. Thus phosphate-based glass materials have potential for useas biomaterials, because their chemical composition is close to that of natural bone. In thelast two decades, phosphate glasses have been considered as potential materials for therepair and reconstruction of bone. Phosphate glass fibers are studied for use in polymercomposites intended as bone implants and dental materials. Phosphate fibers are also usedfor the insulation of pipes in submarines and similar naval uses. Since phosphate fibersabsorb moisture they have also been used as fillers for oil filters. Fibers have also beenused to add texture to water-based paints and it was noted that the paint dried morerapidly when it contained a few percent of phosphate fibers. Phosphate glass fiber papersare also used as asbestos substitutes for noise and fire control in air-conditioning ducts inhomes and offices, and in hair driers and similar domestic heating equipment [8–12].

Biodegradable polymer composites have attracted considerable attention due to theirpotential applications to orthopedic surgeries for hard tissue repair and reconstruction.These biodegradable composites may be used as fracture fixation devices. At presentmetallic implant devices are widely used for bone fracture fixation. Bioabsorbable fracturefixation devices offer advantages over conventional metallic implants. Now scientists aretrying to prepare biodegradable polymer composites which can replace the metal implantscurrently in use. It is reported in the literature that carbon/polyether ether ketone has asignificant history of invasive use. Bioglass� with polyethylene and polysulfone matriceshave been used for implant materials. Totally resorbable composites have been fabricatedusing polymers reinforced with higher organic polymer fibers of poly glycolic acid (PGA)or poly lactic acid (PLA). These glass fibers have potential for use as biomaterials, becausetheir chemical composition is close to that of natural bone [13–15]. Bone plates wereintroduced in the early 1900s to aid the fixation of fractures. Following the introduction ofstainless steels and cobalt alloys in the 1930s, greater success was achieved in fracturefixation, and the first joint replacement surgeries were performed. As for polymers,it was found that warplane pilots in World War II who were injured by fragments ofplastic (polymethyl methacrylate (PMMA)) aircraft canopy, did not suffer adversechronic reactions from the presence of fragments in the body. PMMA became widely usedafter that time for corneal replacement and for replacement of sections of damagedskulls [16].

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In the case of polymer–fiber systems, the quality of the interface is very importantas it controls the mechanical properties of the resulting composites especially the off-axisand compressive response. The interfacial properties of fiber reinforced compositescan be measured by different methods. The most common are: single fiber fragmentationtest (SFFT), fiber pull-out test, micro bond test, and micro indentation test. The SFFTmethod has several advantages over the other methods, such as: simple specimenhandling, replication of the stress transfer characteristics in real composites, convenientmonitoring of failure processes directly and the fact that critical length is sensitive to thelevel of fiber–matrix adhesion. Interface scientists now regard this test as a mature methodfor the determination of interfacial properties of fiber reinforced composites [17–21].

The aim of this study was to measure the mechanical properties of PP based degradablephosphate glass fiber reinforced composites. The degradation tests would like to performin aqueous media. The IFSS of phosphate glass fiber/PP composites were investigated.

EXPERIMENTAL

Materials

The granulated PP was purchased from Polyolefin Company Limited, Singapore.Synthetic degradable fiber was synthesized from the phosphate salts of iron, sodium,magnesium, and calcium. Salts were purchased from Merck, Germany. Appropriate ratiosof the salts were mixed then placed into a platinum crucible and heated at 3008C for 6 h todehydrate the salts. The crucible was then placed in a high temperature furnace at 10508Cfor 1 h. Fibers were generated using conventional hand drawing technique from themolten glass. The composition (mol%) of the degradable glass fiber are as follows:14Na2O–24MgO–16CaO–6Fe2O3–40P2O5. The diameter of the fibers varied from 30 to60 mm. The fibers were collected from the small bobbin (20 cm circumference) connectedto a motor and stored in a desiccator.

COMPOSITE FABRICATION

The PP matrix unidirectional composites were made by compression molding. Formaking PP sheets, granules of PP was placed into two steel plates and placed into the press(Carvar, USA). The press was operated at 2008C, steel plates were pressed at 5 barconsolidation pressure for 1min. The plates were then cooled for 1min in a separate pressunder 5 bar pressure at room temperature. The resulting PP sheets were cut into rectangles(60� 15� 2mm) for composite production. Composites were prepared by sandwichingthree layers of fibers between four sheets of PP. Fibers were placed unidirectionallybetween PP sheets. This was then placed against two steel plates and heated at 2008C for5min to soften the polymer prior to pressing 1 bar pressure for 1min. The fiber weightfraction of the composites was calculated to be 10%.

MECHANICAL PROPERTIES OF THE COMPOSITES

The bending and tensile properties of the composites were evaluated via three pointsbending on a Hounsfield series S testing machine (UK) with a cross-head speed of 1mms�1

at a span distance of 25mm. The dimensions of the test specimen were (ISO 14125):60� 15� 2mm. Composite samples were cut to the required dimension using a band saw.

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Impact strength (Charpy) of the composites was measured using Impact tester (MT-3016,Pendulum type, Germany). Hardness was determined by HPE Shore-A Hardness Tester(model 60578, Germany).

DEGRADATION TESTS OF THE FIBERS AND THE COMPOSITES

Degradation tests of the fibers and composites were performed in deionized waterat room temperature (258C). Up to six months of degradation tests were carried out. Thedegradation specimens were placed into glass vials containing 25mL of deionized water.At set time points, samples were taken out and dried overnight at 1008C before beingsubjected to mechanical testing.

INTERFACIAL PROPERTIES

Single fiber composite samples were prepared using one single filament of glass fibersbetween two sheets of PP. The sandwich was then hot pressed at 200C for 30 s betweentwo steel plates. The plates were cooled in a separate press at 5 bar pressure to roomtemperature. The thickness of the specimen was 0.40mm. The single fiber compositespecimens (25� 5� 0.40mm) were loaded on the tensile machine to bring about therepeated breakage of the fiber. A cross-head speed of 0.25mm/min was used. The gaugelength was 25mm. The experiment was monitored by a microscope (Hitachi) attached to amonitor. Digital images of the breakage of the glass fiber were taken from a video graphicprinter. Fiber fragment length at the saturation point was the key measurement for thisexperiment. To reach the saturation level, the number of fragments over the 25mm gaugelength at each load level (using 2N increments) was counted. Similarly the saturation pointwas also checked by the number of fragments against displacement. The critical length (lc)was then measured using the formula:

lc ¼4

3lf

where lf is the average fragment length. lf was calculated as the monitored length (25mm)divided by the number of breaks observed within that length of the experimental fragmentlength distribution. To find out the critical length, the number of fragments had to becounted. Fiber tensile properties were obtained by tensile testing filaments using theinternational standard BS ISO 11566. A single fiber was mounted on a paper frame with agauge length of 25mm. The fiber was fastened to the frame with epoxy adhesive. Onceprepared the sample was gripped in the tensile machine. Before starting the test the papersections were cut. A crosshead speed of 1mm/min was used. The IFSS of the compositeswas calculated from both the Kelly–Tyson and Drzal equations. Based on the forcebalance on a micro-mechanical model, Kelly and Tyson [22] showed that IFSS, is given by:

�i ¼ d��f2lc

where d is the fiber diameter, �f is the single fiber tensile strength at the critical fragmentlength lc. Drzal et al. [23] altered the above equation to reflect Weibull statistics to form:

�i ¼ �f ��

2�½1� 1=��

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where � and � are the scale and shape parameters in the Weibull distribution for theaspect ration and � is the Gamma function. Fiber strength can be calculated from theextrapolation gauge length using the Weibull weakest chart rule [24]. The fiber strengthat the critical fragment length is:

�f ¼ �f0lcl0

� �exp

1

�

� �

where �f 0 is the fiber strength at gauge length l0 and � is the shape parameter of theWeibull distribution for the fiber tensile strength.

RESULTS AND DISCUSSION

Degradation Tests of the Glass Fibers

Degradation tests (up to six months) of the synthetic degradable phosphate based glassfibers were carried out at room temperature (258C) in de-ionized water. The results arepresented in Figure 1. Fibers showed typical degradation kinetics, i.e., initially slighthigher degradation then almost similar degradation nature with time. After 7 days ofdegradation, weight loss was found to be 1.45%, but 3.6% of mass loss evidenced for 30days. After 60 and 180 days, mass loss of the fibers reached 5.2 and 16.3%, respectively.After 180 days, the fibers were still degrading in a similar manner. From this investigationthis is clearly found that the fibers are slowly degrading with time. It is reported [12,25]that phosphate based glasses dissolve in two stages. The first stage is controlled by the rateat which water diffuses into a volume of the bulk glass surface. These kinetics are obeyedonly until the inorganic polymer chains (glass) at the surface are entirely surrounded bywater. Totally hydrated chains can then disentangle from the partially hydrated chains still

Figure 1. Weight loss vs. degradation time graph for the degradable glass fibers.

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attached to the surface and float off into solution resulting in uniform glass dissolutionand kinetics which are linear with time. During the degradation of the phosphate glass,the chain ends can be interconnected by hydrogen bonding which reduces the degradationrate. It is also mentioned that this type of glass leach ions in the aqueous media. Since thisresearch is concerned with the synthetic degradable phosphate glass fibers, the degradationphenomena in aqueous medium is quite similar to that reported for phosphate basedglasses.

Mechanical Properties of the Matrix and the Composites

The mechanical properties (tensile, bending, impact and hardness) of the matrix PP andthe glass fiber reinforced PP based unidirectional composites are given in Tables 1 and 2.The fiber content of the compression molded composites was about 10wt%. It was foundthat fiber reinforcement occured and improved tensile strength (TS), bending strength(BS), tensile modulus (TM), bending modulus (BM), and impact strength (IS)significantly. On the other hand, percentage elongation at break (Eb%) reduceddrastically, this is because of low Eb% of the fibers compared to PP. Composites sawan 81% increase in TS and a 63% increase in BS over that of the matrix. It was also foundthat TM, BM, and IS was 187, 152 and 70%, respectively, that of the matrix material.Shore hardness of the composites did not show any significant improvement, but tensile,bending, and impact strengths of the composites were found to be superior compared toPP. The increased properties is attributed to the reinforcement of the fibers with the PP.

Degradation of the Mechanical Properties of the Composites in Aqueous Medium

Degradation tests of the glass fiber reinforced PP based composites were performed inde-ionized water at room temperature for up to 6 months. Tensile strength (TS) andbending strength (BS) values are plotted against degradation time and are shown inFigure 2. It was found that both TS and BS decreased slowly with time. After 180 days of

Table 1. Tensile and bending properties of the polypropylene sheet and the composite.

Tensile and bending properties

Tensile properties Bending properties

MaterialStrength

(MPa)Modulus

(MPa)Elongation

at break (%)Strength

(MPa)Modulus

(MPa)

Polypropylene sheet 21� 2 522� 14 378� 35 27� 1.4 1986� 66Composite (10 wt% fiber) 38� 2.4 1500�80 12� 4 44� 2.6 4997� 112

Table 2. Impact strength and hardness of the polypropylene sheetand the composite.

Impact strength and hardness

Material Impact strength (kJ/m2) Hardness (shore A)

Polypropylene sheet 4.47� 0.4 95� 1Composite (10 wt% fiber) 7.58� 0.3 95� 0.5

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degradation in aqueous media, composites lost almost 16 and 18% of TS and BS,respectively. Similarly tensile modulus (TM) and bending modulus (BM) also decreasedover degradation time and the results are depicted in Figure 3. It was found that TMand BM of the composites decreased 36 and 22%, respectively. From this investigationit is clearly showed that more than 80% of the strength of the composites retained aftersix month of degradation time in de-ionized water at room temperature. After six monthsof degradation of the composites, the mass loss was calculated and is shown in Figure 4.The mass loss of the composites increased slowly with the extent of degradation time. Aftertwo, four, and six months of degradation, the mass of the composites reduced to 0.20, 0.37,and 0.49%, respectively. The reduction of the mass from the composites is attributed fromthe loss of the degradable fibers. It is already reported that after 180 days, fibers degradedalmost 16% of its mass, but the composites lost just 0.49%. PP is strongly hydrophobicand the hydrophilic fibers are incorporated inside PP. As a result, hydrophilic nature of thefibers decreased drastically because of the coating of PP of the fibers inside the composite.During immersion of the composites in aqueous medium, water may enter from the edgesof the composites and thus degraded the fibers slowly inside the composite.

Inter Facial Shear Strength (IFSS) of the Composite

To find out the IFSS, a single fiber fragmentation test was carried out. Single glass fiberreinforced PP matrix composites were prepared by compression molding. A fragmentationtest was performed using universal testing machine and the number of fragments werecounted by microscope operated at transmission mode. The results are given in theTable 3. The PP matrix is quite transparent which facilitates the counting of the number offiber fragments in the specimen. Figure 5 shows the fragments of the fiber inside the single

Figure 2. Tensile and bending strength of the composites after degradation in deionized water at roomtemperature (258C). Fiber content was 10 wt%.

Properties of Synthetic Degradable FRPC 7

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Figure 3. Tensile and bending modulus of the composites after degradation in de-ionized water at roomtemperature (258C). Fiber content was 10 wt%.

Figure 4. Weight loss of the composites up to six months. Degradation tests were carried out at roomtemperature (258C) in de-ionized water.

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fiber composite sample. The total number of fragments reached 11 and the criticallength was found to be 3030 mm, calculated according to the equation mentioned in theExperimental section. Tensile strength of the fiber at the critical length was measuredfrom the Weibull weakest chart rule and it was found to be 765MPa. The diameter of thefibers used in this experiment was varied from 47� 3 mm. The IFSS was found to be5.93MPa. From this investigation it revealed that the composite has sufficient IFSS. It isreported [26] that the IFSS was found from 4 to 7MPa for PP/E-glass fiber system.The IFSS values found in this investigation is in the range of PP/E-glass fiber system whichis a good sign of the fiber matrix adhesion between PP and degradable phosphate glassfiber system.

CONCLUSIONS

Degradation tests (up to six months) of the synthetic degradable phosphate based glassfibers were carried out at room temperature (258C) in de-ionized water and it was foundthat the fibers were slowly degrading with time. After 60 and 180 days, mass loss of thefibers reached 5.2 and 16.3%, respectively. Polypropylene (PP) based degradable glassfiber reinforced composites were prepared by compression molding. The fiber content was10% by weight. It was found that the tensile strength (TS), bending strength (BS), tensilemodulus (TM), bending modulus (BM), and impact strength (IS) improved significantlythan that of the matrix PP. On the other hand, percentage elongation at break (Eb%)reduced drastically, this is because of low Eb% of the fibers compared to PP. Compositesfound 81% increase in TS and 63% increase in BS over that of the matrix. It was also

Table 3. The inter facial shear strength (IFSS) of PP/glass fiber system.

Specimen typeCritical fiberlength (mm) (rf) (MPa)

Fiberdiameter (mm)

IFSS(MPa)

Single fiber composite 3030� 200 765� 30 47�3 5.93�0.22

Figure 5. Fiber fragments in the single fiber composite.

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found that TM, BM, and IS increased to 187, 152, and 70%, respectively, than the matrixmaterial. From this investigation it showed that more than 80% of the strength of thecomposites retained after six months of degradation time in de-ionized water at roomtemperature. The IFSS of the composites were also measured by using the single fiberfragmentation tests and the interfacial shear strength was found to be 5.9MPa.

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