Mechanical Test of LDPE and HDPE

Abstract

LDPE and HDPE were processed by extrusion, injection molding, and sheet extrusion. Their mechanical properties such as tensile strength and percent elongation were measured by tensile test and analyzed statistically (Table 2). During the extrusion process, both polymers underwent die swelling. The water cooled polymer cords have a higher tensile strength but lower % elongation compared to the air cooled cords. HDPE has a much higher strength than LDPE due to its high crystallinity.

LDPE and HDPE samples processed by injection molding and sheet extrusion show the same tendency in the extent of yield strength and elastic modulus. For sheet extrusion, the heat treated polymer sheet has a higher strength than the non-heat treated sheet because heating leads to an increase in both crystallinity and crystallite size. The specimen in rolling direction also has a higher strength than those in transverse direction due to the alignment of the polymer chains in rolling direction. UHMWPE (Ultra-high-molecular-weight polyethylene) saucer was processed by compression molding.

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The cross section of the saucer was examined by optical microscopy. Further, the melting temperature of PEO was determined to be 74. 0°C ~ 78. 9°C.

Introduction

A polymer is a chemical compound or mixture of compounds consisting of repeating structural units created through a process of polymerization.

The units composing polymers derive from molecules of low relative molecular mass. When all the repeating units along a chain are of the same type, the resulting polymer is called homopolymer. Chains composed of two or more different repeat units are termed copolymers.

The physical characteristics of a polymer depend both on its molecular weight and shape, and the structure of the molecular chains. The chain structures include linear polymer, branched polymer, crosslinked polymer and network polymer. The polymer synthesized in this experiment, LDPE and HDPE, have different chain structures (i. e. LDPE is a branched polymer and HDPE is a linear polymer. ). The polymer chain structure has a significant influence on polymer crystallinity, which is defined as the packing of molecular chains to produce an ordered atomic array. The mechanical properties that investigated in this paper, such as tensile strength, elastic modulus and percent elongation, greatly depend on the crystallinity of the polymer sample. Polymers play an essential and ubiquitous role in everyday life from those of familiar synthetic plastics and other materials of day-to-day work and home life, to the natural biopolymers that are fundamental to biological structure and function.

Quite a variety of different techniques are employed in the forming of polymeric materials. Molding is the most common method for forming plastic polymers.

The several molding techniques used include extrusion molding, compression molding, blow molding and injection molding.

During molding, crystal regions in polymer melts upon heating. The resulted polymer melts are non-Newtonian fluids, and their viscosity depends on the shear rate. Melt index (MI) could be used to indicate the viscosity of the fluid. It is defined as the mass of polymer flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied.

Polymer melts are formed into a continuous charge of viscous fluid. The viscous fluid then solidifies into polymer product with specific shapes. During the solidification process, polymer melts recrystallizes and forms spherulite structure consisted of both amorphous region and lamellar.

Experimental Procedure

Crystallization of PEO Crystallization of PEO (Sigma-Aldrich Co. , St. Louis, MD) was observed by optical microscopy. Sample of PEO powder was placed on glass slide and heated up using hot stage. Both the melting and the solidification processes were observed under optical microscope under crossed polarizer. The melting range was determined to be 74. 0°C ~ 78. 9°C.

Extrusion of LDPE and HDPE

Both LDPE (MI = 0. 25, Equistar ) and HDPE (MI = 9, Equistar) pellets were processed into cords using single-screw extruder (Model TL3215B, Extruders ). The die temperature was 350 °C, Zone temperatures were all 425 °C and the screw speed was about 3. 6 rpm. Water cooling and air cooling were carried out for mechanical properties evaluation. Tensile tests were performed on both LDPE and HDPE cords in order to evaluate their mechanical properties. Compression Molding UHMWPE (Ticona) powder was used to produce UHMWPE saucer by using a compression molding apparatus (Model 3725, Carver).

The starting temperature was 245 F, and the molding temperature was 310 F. The cross section of the saucer was examined by optical microscopy.

Injection Molding

Both LDPE (MI = 9, Equistar) and HDPE (MI = 7, Equistar) pellets were processed into dog bone specimens by injection molding. The barrel, nozzle and mold temperatures used are listed below. Tensile tests were performed on both LDPE and HDPE specimens in order to evaluate their mechanical properties.

Sheet Extrusion

Both LDPE (MI = 0. 25, Equistar) and HDPE (MI = 9, Equistar) pellets were processed into polymer sheets using sheet extruder (Model KLB125, Extruders). The die temperature was 375 °C. Zone temperatures were all 420 °C. Adapter temperature was also 420°C. The screw speed was about 20 rpm and the roller speed was set up to 2. 0 rpm. Heat treatment using glass furnace was performed on both polymers. Tensile tests were carried out in order to evaluate the mechanical properties of both LDPE and HDPE.

Result and Discussion

Crystallization of PEO The melting temperature of PEO was determined to be 74. 0°C ~ 78. 9°C, where the literature melting temperature of PEO is 65°C. The polymer melting behavior is a function of the rate of heating5. That the measured melting temperature is higher than the literature value results from the relative high heat rate (20°C/ min). Besides, the melting temperature for polymer is a range instead of a single point. The melting temperature depends on the molecular weight of the polymer chain, the thickness of the chain-folded lamellae, the heating rate and the impurity content.

Since the PEO samples used contain polymer chains with different size, and they were heat at an inconstant rate, the melting must take place over a range of temperatures. .

Spherulite structure of PEO and interspherulite boundary. The spherulite consisted of chain-folded crystallites (lamellae) and amorphous region start to grow from the nucleation site at 56. 7°C. Individual lamellae are separated by amorphous materials. The spherulite keeps growing and getting larger as the recrystallization process continues. When the crystallization of a spherulite structure nears completion, the extremities of adjacent spherulites begin to impinge on one another, forming planar boundaries. At 56. 0°C, the crystallization process of PEO sample reached completion.

Compression Molding

The mold is closed with a top force. Pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured. During that process, particles diffuse together and become one piece. The outer cut of the UHMWPE saucer has lower porosity, and the middle section of the saucer has a much higher porosity. The outer region of the saucer was cooled much faster than the middle section of the saucer.

Slowing cooling rate in the middle section led to the high porosity. Liquidus polymer shrinks as it solidify into solid leaving a large amount of pores in the middle section.

Extrusion and Sheet Extrusion

Die swell happened during extrusion. A flow stream has a constant rate before entering the die. It also occupies a spherical conformation and maximizes the entropy.

As it goes through the die, polymer loses its spherical shape and becomes less entangled. Therefore, the entropy is lowered. When polymer melt comes out of the nozzle, the remaining physical entanglements cause the polymer melt to relax (i. e. egain a portion of its former shape) and restore the entropy. It appears like the polymer is swelling at the nozzle.

Air cooled LDPE took 3 runs before fracture, and the water cooled LDPE took 2 runs before fracture. Both water cooled and air cooled LDPE cords have similar elastic modulus and yield strength. However, the air cooled cord has a higher tensile strength but lower percent elongation. Since the degree of crystallinity depends on cooling rate during solidification, the higher the cooling rate , the higher the degree of crystallinity.

Increasing in crystallinity increases the hardness but lower the ductility of the polymer cords. Thus, water cooled LDPE cord has higher tensile strength but lower % elongation compared to the air cooled one. Figure 5. Extrusion sample stress vs. strain plot. Red curve represents water cooled LDPE cord, and blue curve represents air cooled LDPE cord.

Blue curve represents heat treated LDPE specimen in rolling direction. Purple curve represents non-heat treated LDPE specimen in rolling direction. Red curve represents non-heat treated LDPE specimen in transverse direction. Green curve represents heat treated LDPE specimen in transverse direction.

Purple curve represents heat treated HDPE specimen in rolling direction. Red curve represents non-heat treated HDPE specimen in transverse direction. Green curve represents heat treated HDPE specimen in transverse direction.

HDPE specimens have higher yield strength, elastic modulus and % elongation. HDPE has a linear chain structure. It was highly packed. Therefore, it has a larger density and higher degree of crystallinity than LDPE. Molecular chains are closely packed in an ordered arrangement in crystalline region. The alignment of the packed chains in crystalline region makes the intermolecular secondary bonding much stronger than it is in amorphous region. Thus, HDPE with higher crystallinity has a much higher strength than LDPE. The heat treated specimen has higher yield strength and tensile strength than non-heat treated specimens.

It is also due to the increasing in the percent crystallinity, which makes the polymer harder but less ductile. Further, polymer specimen in rolling direction has higher yield strength and elastic modulus, but lower percent elongation compared to samples in transverse direction. It is because during drawing the molecular chains slip past on another and become highly oriented. This alignment once again enhances the tensile modulus in the direction of drawing (RD), while reduces the tensile strength in the direction (TD) perpendicular to the rolling direction.

Injection Molding

Stress vs. Strain plot of HDPE samples processed by injection molding.

HDPE specimen has a yield strength of 22. 61±3. 54 MPa, an elastic modulus of 0. 53±0. 27 GPa, and a percent elongation of ( 251. 50±301. 35 )% within 95% confidence interval. LDPE specimen has a yield strength of 5. 85±0. 94MPa, an elastic modulus of 0. 049±0. 012GPa, and a percent elongation of (142. 75±15. 72)% within 95% confidence interval. T-test was carried out to compare the difference between the literature value and the experimental value for both yield strength and elastic modulus. The result shows that the experimental yield strength and elastic modulus of both LDPE and HDPE differ from their literature values significantly. Both LDPE and HDPE have a wide range of % elongation, therefore, the difference between experimental %EL and literature %EL was not examined.

In addition, there are significant difference between the yield strength of LDPE and the yield strength of HDPE (T-test shown in Appendix) that HDPE has a much higher strength than LDPE. It is because HDPE has a highly packed linear chain structure, a larger density and a higher degree of crystallinity than LDPE.

Conclusion

Total three types of polymer, UHMWPE, LDPE and HDPE, were processed via four molding techniques. UHMWPE saucer was produced by compression molding. The cooling rate of the polymer melts has a great influence on the porosity of the polymer.

Die swelling happened during extrusion as a result of the relaxation of polymer melts. The melting temperature of PEO was determined to be 74. 0°C ~ 78. 9°C. The inconstant heat rate and the fact that PEO is composed of molecules having a variety of molecular weight make the temperature range well above the literature melting point. The air cooled extruded cord has a higher tensile strength but lower percent elongation due to higher crystallinity. HDPE is harder but more ductile than LDPE. It also results from its highly pack linear structure and higher degree of crystallinity.

T-test was performed on injection molded samples that HDPE specimen has a yield strength of 22. 61±3. 54 MPa, an elastic modulus of 0. 53±0. 27 GPa, and a percent elongation of (251. 50±301. 35 )% while LDPE specimen has much a lower yield strength of 5. 85±0. 94MPa, an elastic modulus of 0. 049±0. 012GPa, and a percent elongation of (142. 75±15. 72)% within 95% confidence interval.

References

1. Painter, Paul C. ; Coleman, Michael M. (1997). Fundamentals of polymer science : an introductory text. Lancaster, Pa. : Technomic Pub. Co. p. 1.
2. WIlliam, D. (2007). Material science and engineering an introduction. 7th ed. , pp. 489-568 John Wiley&Sons, Inc.
3. Todd, Robert H. ; Allen, Dell K. ; Alting, Leo (1994). Manufacturing Processes Reference Guide. Industrial Press, Inc.
4. A. V. Shenoy, D. R. Saini: Melt Flow Index: More Than Just a Quality Control Parameter. Part I. , Advances in Polymer Technology, vol 6, No. 1, pp 1 – 58.
5. Harrison, I. R. , & Runt, J. (2003). The heating rate dependence of polymer melting points. Journal of Polymer Science, 17(2), 321-328.

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