Development and Characterization ofPlastic Composites

Shannon Culbert Cathleen Luoshanculbert@gmail.com catluo27@gmail.com

Alison Park Branden Spitzeralisonrebeccapark@gmail.com branden.spitzer@yahoo.com

Dr. Jennifer Lynch*jklynch@soe.rutgers.edu

Elizabeth Chang*lizchang718@gmail.com

New Jersey’s Governor’s School of Engineering and TechnologyJuly 27, 2018

*Corresponding Authors

Abstract—Plastic composites, materials that are madeup of two different constituents, are relatively affordable,lightweight, and durable. They are currently being studiedfor future applications as a replacement to traditionalmaterials, such as aluminum, concrete, glass, and steel.The purpose of this work was to characterize plasticcomposite blends of varying percent weight of polystyreneand high-density polyethylene using a unique single screwextrusion melt-processing method. Mechanical, chemical,and thermal properties of each composite blend weretested to characterize the samples. Flexural tests wereperformed on bars to characterize the amount of loadthe bars can withstand. Impact data also revealed thatthe mid level blends absorbed the least amount of energy,reflecting a low impact resistance. The differential scanningcalorimetry (DSC) analysis portrayed that with an increasein polystyrene content, the melting and crystallizationtemperatures remained constant. This data allows fora detailed characterization of the various compositionsof polystyrene-high density polyethylene. The determinedproperties will be beneficial for future applications inmaterials science.

I. BACKGROUND

Materials science and engineering is a field that aimsto design and develop particle combinations with noveland useful properties. Polymers have been developed foradvanced engineering in materials science such as plastic

electronics from recycled materials and biomedical ap-plications. Studying and characterizing plastic polymersis important research for pursuing various and newfoundpotential applications for composite materials. Com-posites are valuable for their attempts to combine therespective, beneficial properties of two unique materials.Compared to traditional materials, plastic compositeshave enhanced mechanical and chemical properties, aswell as reduced costs. For instance, in the commercialsector, wood-plastic composites are being used for theirdurability in furniture, floors, and other building uses [1].These composites are of growing interest because theyare inexpensive and low maintenance. Similarly, plasticcomposites are also ideal as they can be utilized as analternative materials in the construction and aerospaceindustries [2].

It is important to note that the blending of polystyreneand polyethylene is a new development in the fieldof materials science as they are highly incompatiblewith each other. Few polymer pairs mix well and thus,continuous optimization of the processes to combinethem is done industrially. Often, mixtures of polymersare phase-separated and are not homogeneous. Thesemixtures are known as immiscible blends, meaning theyonly mix on a physical level, rather than chemical.

New composites allow two different plastics to be

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combined while preserving beneficial properties from thetwo individual plastics. Producing these combinationspromote scientists to pursue beyond the normalized usesof a material to visualize greater, revolutionary purposes.In the modern world of manufacturing, plastics, specif-ically Polystyrene (PS) and high density Polyethylene(HDPE), have evolved into a staple substance [3].

II. METHODSA. Materials

Polystyrene (PS) is a long chain hydrocarbon wherealternating carbon centers are attached to a phenyl group,C6H5 (Fig. 1). The weak intermolecular forces result ina brittle structure with poor strength and elongation. Itis a commonly used thermoplastic, most seen in dispos-able plastic utensils and Styrofoam. It is a clear, solidplastic that is also utilized in many commercial sectors,from automobile parts to laboratory ware. PS is non-biodegradable, which contributes to ocean pollution andincreasing volume of landfills [4]. However, polystyreneis of interest because of its affordable, resourceful, andlong lasting qualities.

Figure 1. A polystyrene molecule. [5]

High-density polyethylene (HDPE) is a thermoplasticmade from petroleum. HDPE is a linear polymer madeup of closely packed CH2 molecules (Fig. 2). Thisstrong plastic has excellent impact performance, tensilestrength, and is very lightweight. Due to its high strengthto density ratio, HDPE is often found in householditems such as fresh produce bags and bottle caps [6].Thus, high-density polyethylene can be used in a varietyof applications due to its strong and flexible chemicalproperties, affordability, and recyclable properties.

Figure 2. A high-density polyethylene molecule [7].

B. Sample Preparation

Polystyrene and high density polyethylene are im-miscible polymers, meaning that they do not mix toform a chemical mixture. Until recently, polystyrene andpolyethylene had not been combined into a polymerblend (IMPB). PS and HDPE were combined usinga novel, uniform shear single screw extrusion melt-processing method developed by Randcastle ExtrusionSystems at 0, 20, 35, 50, 65, 80, and 100 wt. % PSin HDPE. Each concentration was pelletized, followedby using a mini molder to fabricate testing specimensfor flexural and impact mechanical property testing.This was done to combine the two different plasticsin specific concentrations, all while preserving some oftheir respective, original characteristics. Polymer blendsof PS and HDPE were tested at different percentages ofcomposites (Table I).

Table IPERCENT COMPOSITION OF COMPOSITES

Polystyrene % High Density Polyethylene % Sample Label

0 100 HDPE

20 80 20PS-HDPE

35 65 35PS-HDPE

50 50 50PS-HDPE

65 35 65PS-HDPE

80 20 80PS-HDPE

100 0 PS

C. Injection Molding

The tensile and impact bars were molded using asix ton heated, pneumatic and pressurized bench shoppress injection molder (Fig. 3). The funnel was filledwith plastic pellets and melted into the mold, which wasscrewed tightly in place with the vice.

Bars had to be discarded if the following occurred: ifthe plastic was flashing or squeezing through the mold, ifthe plastic was deformed, or not reaching the bottom ofthe mold, and if the plastic melted too long and started tospeck or degrade and turn brown. Samples of successfulimpact and tensile bars are shown in Figures 4 and 5respectively.

Often, the mixture had to be packed beforehand,meaning that the mixture was compressed to push outthe air, preparing it for the molding procedure. Thesample was ready to be injected when the pellets melted.The injector was held down until the mold overflowed.

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Figure 3. Six-ton pneumatic bench shop press injection molder usedto mold impact bars and tensile bars.

Figure 4. A successfully molded impact bar.

The processing temperatures and pressures varied foreach sample (Table II, Table III). Eight tensile bars andeight impact bars were made for each composite. Afterprocessing, the samples were stored at room temperatureprior to mechanical and thermal testing.

D. Flexural Testing

Flexural tests were conducted to measure the strengthand stiffness of the material. Samples were measuredfor thickness, width, and length and recorded beforetesting. Using a three-point Mechanical Testing SystemQTest/25 (Fig. 6), impact bars were placed betweenthe supports at the span length which was calculatedfrom the average thickness of the specimens multipliedby 10. The bar was loaded on the loading nose half

Figure 5. A successfully molded tensile bar.

Table IITENSILE BAR INJECTION MOLDING DATA

Sample Temperature Pressure Heat Time

Label (°C) (psi) (min)

HDPE 220 105 3

20 PS-HDPE 240 110 3.5

35 PS-HDPE 240 120 4

50 PS-HDPE 240 110 4

65 PS-HDPE 245 120 5

80 PS-HDPE 250 135 8

way between the supports, perpendicular to the supports.Due to a specimen overhang, the support span-to-depthratio was changed from the standard 16:1 to 10:1 asstated in the American Society for Testing and Materials(ASTM) D790. The load was lowered at the crossheadrate, calculated from the equation:

R =ZL2

6d

R = rate of crosshead motion, mm/min,Z = .01L = support span, mmd = depth of beam, mm

10 Newtons of force was applied to the specimen andthen zeroed in order to preload the specimen. The loadwas then zeroed again and the stress and strain weregraphed. The slope was the modulus or stress/strain ratio.Five samples were tested for the following compositions:HDPE, 20PS-HDPE, 35PS-HDPE, 80PS-HDPE.

E. Impact Testing

Impact testing is a destructive mechanical test methodthat measures a samples ability to resist high-rate load-ing, revealing how much energy is absorbed during a

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Table IIIIMPACT BAR INJECTION MOLDING DATA

Sample Temperature Pressure Heat Time

Label (°C) (psi) (min)

HDPE 220 105 2

20PS-HDPE 220 110 3

35PS-HDPE 240 110 4

50PS-HDPE 240 110 4.5

65PS-HDPE 245 120 5.5

80PS-HDPE 250 135 6

Figure 6. Three-point Mechanical Testing System QTest/25 flexuralmachine used.

dynamic impact. Notched Izod impact resistance wasdetermined using a pendulum Instron Dynatup POE 2000(Fig. 7). The specimen is impacted with a pendulumenergy of 21.7 J and impact velocity of 3.46 m/s.

Eight impact bars at the following compositions:HDPE, 20PS-HDPE, 35PS-HDPE, and 80PS-HDPEwere prepared by cutting off from the runners and hada notch drilled. In accordance to the Izod method, thebar is held vertically when struck by the pendulum.After the impact, the net energy absorbed by the barsare calculated by subtracting energy left over from thestarting energy of the pendulum. This test is significantas it will measure the physical capabilities of our com-posites. To ensure accuracy, ASTM D256 was followedand the test was performed eight times per compositeas impact testing has many sources for error; theseincludes specimen geometry, strike plate positioning, and

Figure 7. The pendulum Instron Dynatup POE 2000 used.

excessive notch indentation.

F. Differential Scanning Calorimetry

Thermal properties were determined using a TAQ1000 Differential Scanning Calorimeter (DSC). Tensilebars for each composite were cut using the SKIL 3386Band Saw in preparation for DSC. DSC is used forthermal analysis and the plastic composite bars werecut into thin squares 3.5mm wide and 1mm thin. Thesamples were measured using a Mettler AE 240 HighPrecision Scale to a mass 5mg and 10mg (average 8mg).The specimens were placed in standard aluminum pansand encapsulated using the T-Zero Press (Fig. 8). Afterthe species were encapsulated they were transferredinto the Differential Scanning Calorimeter Autosampler(Fig.9).

The heat/cool/reheat (HCH) method was applied overa temperature range of 0°C to 200°C at a rate of10°C/min. The curves produced show the glass tran-sition, melting temperature, crystallization temperature,and heat of fusion and crystallization. Two samples weretested per composite.

G. Fourier Transform Infrared Spectroscopy

Structural properties were analyzed using FourierTransform Infrared Spectroscopy (FTIR). During FTIR,a broadband light source is utilized with an interferome-ter to identify the characteristic wavelengths at which the

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Figure 8. The T-Zero Press used to encapsulate pans for DSCsamples. [8]

Figure 9. TA Q1000 Differential Scanning Calorimeter [9].

sample absorbs the light [10]. The instrument exposesthe sample to different wavelengths of infrared lightand measures which wavelengths are absorbed. Thecomputer uses the raw absorption data for a math processcalled Fourier Transform. Fourier transform processesthe data from the light absorption associated with mirrorpositions to generate a readable absorbance spectrum[11]. The infrared absorption bands are useful as theyidentify molecular components and structures.

III. RESULTS

A. Mechanical Properties

Mechanical properties are key components in char-acterizing a material. The range of usefulness and theservice life of materials can be determined throughphysical tests such as impact and flexural.

1) Impact Test: The impact test provided signifi-cant data for determining mechanical properties. Animportant quality determined through the test is impactstrength. Impact strength is found by dividing the bar'stotal energy absorbed by the quantity of bar's thickness

times the depth under notch. The impact strength fol-lows a U-curve, originally decreasing in value, but thenincreasing for 80PS-HDPE (Fig. 10). These values aresupposed to represent the material's ability to absorbenergy upon impact. However, the curve is inconsistentwith previous research that supports polystyrene havinga low impact strength [12]. While the impact tests hada standard deviation of less than 10 percent, furtherresearch should be done to evaluate why the compositesdid not experience a downward trend.

Figure 10. Impact strength results.

Another valuable information that was acquired fromthe impact test was that all the different compositions ofPS-HDPE withstood complete breakage. This is impor-tant as it shows how the materials are able to withstandthe pendulum's force of 21.7 N. Although all the bars hadthe same hinging fracture, as the percentage of PS varied,so did the neatness of the bars'breaks. For example, the20PS-HDPE and 35PS-HDPE bars had a much smootherhinge line than the breaks from the HDPE and 80PS-HDPE bars. This is indicative of how the HDPE and80PS-HDPE bars had a much higher impact resistanceand strength.

2) Flexural Test: Flexural testing also revealed fur-ther mechanical properties of the plastic composites. Themodulus, or stress/strain, was recorded (Fig. 11). It wasfound that the modulus increased as the concentration ofPS increased, indicating that 80PS-HDPE has the higheststiffness at 1.64GPa (Fig. 12). As per the ASTM D790,the test stops at break or five percent strain, so none ofthe samples reached breaking point.

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Figure 11. Flexural testing stress-strain data.

Figure 12. Flexural testing modulus.

B. Thermal Properties

Thermal properties also provided crucial informationin characterizing a material as it describes the glasstransition temperature, melting and crystallization tem-perature, and the heat of fusion and crystallization. Thesequalities were able to be analyzed through the datafrom the Differential Scanning Calorimeter (DSC). Thefirst heating, cooling, and second heating (or reheating)curves for each PS-HDPE sample composite show theaverage melting temperature and crystallization temper-ature, and the area under each curve revealed the heatof fusion and heat of crystallization (Fig. 13). In thefirst heating curve, the melting temperature remainedconstant at 129°C (Fig. 14). However, melting heat offusion decreased with increasing PS content. In thecooling curve, the crystallization temperature remained

constant at 118°C (Fig. 15). Heat of crystallizationdecreased with increasing PS content. In the secondheating curve (reheat curve), the melting temperatureremained constant at 131°C (Fig. 16). Similarly to thefirst heating curve, the melting heat of fusion decreasedwith increasing PS content. Glass transition temperaturewas only observed in the cooling curve and was notdetectable in the heating curves. The glass transitiontemperature decreased slightly as PS content increased.There was a significant drop in temperature betweenthe 35PS-HDPE and 50PS-HDPE (Fig. 17). Due toinstrumental error, the PS was not analyzed.

Figure 13. Top left: first and reheat melting temperature. Top right:cooling crystallization temperature. Bottom left: first and reheat heatof fusion. Bottom right: cooling heat of crystallization. First heatingis represented in red, cooling is represented in blue, and reheating isrepresented in orange.

Figure 14. First heating curve showing melting temperature for eachcomposite.

This information is relevant as melting temperatureremained constant, showing that HDPE properties were

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Figure 15. Cooling curve showing glass transition(left) and crystal-lization temperature (right) for each composite.

Figure 16. Second heating curve/ reheat curve showing meltingtemperature for each composite.

still maintained with the addition of PS. This is usefulfor future applications and determining what tempera-tures a material can withstand without compromisingmechanical properties. The glass transition temperaturealso remained constant and remained close to the glasstransition temperature of HDPE, which is approximately85°C. The qualities of HDPE were still maintained withthe addition of PS. The results also show that withincreased PS, there is a decrease in latent heat. Thisinformation is crucial for determining the specific heatof the material and what environments the material canbe applied to.

C. Structural Properties

The structural properties of the PS/HDPE compos-ites were analyzed through FTIR. The data producedfrom the FTIR analysis reveals variations in chemicalstructures and polymer compositions between the sevensamples tested. Structural properties such as molecularstructure, bonding, and deformation type are revealed inthe graphs the Agilent 4100 FTIR system returns [13].The two peaks that develop around a wavenumber of2850 and 2920 are due to bending of CH2 moleculesand characterize polyethylene polymers [14]. The inten-sity of these two peaks is directly correlated with the

Figure 17. Glass transition temperature recorded only for coolingcurve.

percentage composition of HDPE in a sample relativeto the other composites. A similar correlation is true ofPS but the large peaks occur at wavenumbers of 1160,1190, and 1225 with residual yet noticeable intensitiesaround 1015, 1080, 1500, 1750, and 825 wavenumbers.These high absorbance intensities appear due to bendingbetween double bonded hydrocarbons, CH and CH2, andbending between C and H bonds. The trends in theFTIR data change in accordance with the percentageof PS and HDPE, which confirms the percent weightcompositions of the composites analyzed (Fig. 18). Theabsorbance falls as the samples contain a greater ratioof PS to HDPE (Fig. 19). There is a steady increasein the absorbance percentage at the wavenumbers thatcharacterize the chemical characteristics of PS (Fig. 20and 21). An exception to both absorbance trends occursat the 50PS-HDPE composite data; the 50PS-HDPE datais similar to that of the 35PS-HDPE composite.

The FTIR data shows that there are no significantextraneous materials or contaminants in the compositestested as absorbance of any degree occurs at the samewavenumber for the samples. Presence of a contaminateor polymer other than PS or HDPE would have shownan additional peak or one of a drastically differentabsorbance in some of the samples. The data does notseem to show any variations when comparing all sevensamples and it can be concluded that all of the compositeblends are purely composed of PS and HDPE polymers.

IV. CONCLUSIONS

Immiscible PS/HDPE composites have been preparedthrough single screw extrusion melt processing. Thedifferent composites were successfully characterized

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Figure 18. FTIR results for all wavenumbers.

Figure 19. FTIR results for wavenumbers 3000 to 2800 in HDPEcharacteristic region.

through impact and flexural testing, differential scan-ning calorimetry, and Fourier Transform infrared spec-troscopy. These tests were necessary to properly analyzethe mechanical, thermal, and structural properties of eachcomposite.

The impact tests reflected that the HDPE and 80 PS-HDPE blends should be utilized for intensive structuralapplications. The low energy absorbed by the mid levelblends showed that toughness is not an additive property.Flexural testing revealed that the modulus increased asthe concentration of PS increased, indicating that com-posites with greater PS content had a higher stiffness.From the DSC analysis, it was recognized that theaddition of PS did not affect the melting and crystal-lization temperatures and glass transition temperature ofHDPE, but did cause a decrease in heat of fusion andcrystallization. This reflects the temperatures the materialcan withstand without compromising its mechanical andchemical properties. These results will be useful forfuture applications in the manufacturing industry. Forexample, these materials can be used in the construction

Figure 20. FTIR results for wavenumbers 1850 to 650 in PScharacteristic region.

Figure 21. FTIR results for wavenumbers 1350 to 950 in PScharacteristic region.

and aerospace industries due to their strength and abilityto withstand high temperatures.

The data received from FTIR confirms that the plasticpolymer composites contain purely PS and HDPE andare absent of contaminant material. By comparing trendson the graph the PS and HDPE composition of theHDPE, 20PS-HDPE, 35PS-HDPE, 65PS-HDPE, 80PS-HDPE, and PS are verified to be the percentage masscomposition we name them by. Thus, the results reflecthow the properties of each composite has the potentialto vary greatly, despite being composed of the samematerial.

In conclusion, PS/HDPE composites sourced fromrecycled plastics can be manufactured and be used forapplications in various industries. Applications includerailroad ties, pilings, I-beams, and high load capacitybridges [15]. Plastic composites will replace traditionalmaterials such as concrete, steel, and timber due totheir strength, affordability, and other benefits. However,further research is needed to find a better processing

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method that would enhance these products and reduceproduct variability.

ACKNOWLEDGMENTS

We would like to express our gratitude towards ourproject mentors: Dr. Jennifer Lynch for her expertise,Elizabeth Chang for her invaluable guidance, and ourRTA liaison, Apoorva Agarwal, for her support. We alsothank our Research Coordinator, Brian Lai, and HeadRTA, Nick Ferraro, for their integral leadership towardsthe NJ Governors School of Engineering and Technology(GSET) program; Dean Ilene Rosen, the Director ofGSET and Dean Jean Patrick Antoine, the AssociateDirector of GSET; Rutgers University, Rutgers School ofEngineering, the State of New Jersey, and our corporatesponsors: Lockheed Martin, Silver Line, Rubiks, etc.for affording us the opportunity to pursue our research;lastly, we thank NJ GSET Alumni for their ongoingparticipation and assistance.

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