EFFECTS OF CROSS-LINKING ON THE HYDROSTATIC PRESSURE TESTING FOR HDPE PIPE MATERIAL USING ELECTRON BEAM MACHINE.

KESAN PENYILANGAN STRUKTUR RANTAI KEATAS UJIAN TEKANAN HIDROSTATIK PAIP HDPE MENGGUNAKAN MESIN ALUR ELEKTRON.

Mohd Jamil Bin Hashim1, Siti Noraini Binti Kawaeed2

1Bahagian Perkhidmatan Teknikal,Unit Pembangunan dan Penyenggaraan Awam,Pusat Pengurusan Sisa,Institut Penyelidikan Teknologi Nuklear Malaysia (MINT),Bangi, 43000

KAJANG, Selangor.2Kulliyah kejuruteraan (Bahan), Universiti Islam Antarabangsa Malaysia,53100 Gombak, Kuala

Lumpur.e-mail: Jamil@mint.gov.my

Abstract

One of the most inventive, sustainable strategies used in engineering field is to improve the quality of material and minimize production cost of material for example in this paper is HDPE material. This is because HDPE is an oil base material. This paper proposes to improve its hydrostatic pressure performance for HDPE pipe. The burst test is the most direct measurement of a pipe material’s resistance to hydrostatic pressure. Test will be conducted in accordance with ASTM standard for HDPE pipe that undergo electron beam irradiation cross-linking. Studies show the effect of electron beam irradiation will improve the mechanical properties of HDPE pipe. When cross-linking is induced, the mechanical properties such as tensile strength and young modulus is increase correspond to the radiation dose. This happen because the structure of HDPE, which is thermoplastic change to thermoset.This, will indicate the variability of irradiation dose which regard this pipe pressure rate. Hence, the thickness ratio of pipe will be re-examining in order to make the production of HDPE pipe become more economical but sustain its thickness. This research review the effects of electron beam on HDPE pipe, as well as to reduce the cost of its production to improve key properties of selected plastic pipe products.

Abstrak

Salah satu strategi yang paling inventif dan berterusan yang digunakan di dalam bidang kejuruteraan adalah berusaha meningkatkan kualiti bahan disamping mengurangkan kos produk bahan seperti penggunaan bahan HDPE.Ini kerana HDPE merupakan bahan berasaskan minyak. Oleh itu,kajian ini mencadangkan penambahbaikan prestasi ujian tekanan hidrostatik paip jenis HDPE. Ujian paip pecah merupakan satu ujian mengukur kekuatan rintangan paip keatas tekanan hidrostatik. Ujian akan dijalankan berpandukan kepada spesifikasi ASTM keatas paip yang telah di radiasi dengan mesin alur elektron. Kajian menunjukkan kesan mesin alur elektron meningkatkan sifat mekanikal paip HDPE.Apabila struktur mata rantai penyilangan berlaku terhadap struktur asal HDPE, sifat mekanikal seperti kekuatan tegangan dan “young modulus” paip meningkat.Ini terjadi kerana struktur asal HDPE iaitu “thermoplastic” telah bertukar kepada struktur “thermoset”. Kajian ini akan mengkaji semula kesan radiasi elektron ke atas paip HDPE untuk meningkatkan sifat mekanikal disamping mengkaji kesesuaian ketebalan diameter paip agar dapat mengurangkan kos produk yang telah dipilih.

KEYWORDS: Burst Test, Cross-Linking, Electron Beam, HDPE Pipe, Hydrostatic Pressure, Irradiation Dose, Pressure Rating. KATA KUNCI : Ujian Paip Pecah, Rantai Penyilangan, Alur Electron, Tekanan Hidrostatik, Jumlah Radiasi, Kadar Tekanan.

Introduction

Polyethylene has been used for water piping applications both domestically and internationally

since the 1960’s. In 1998 high performance PE materials were introduced in Europe. High-

density polyethylene (HDPE) or polyethylene high-density (PEHD) is a polyethylene

thermoplastic made from petroleum. It takes 1.75 kilograms of petroleum (in terms of energy and

raw materials) to make one kilogram of HDPE. HDPE is commonly recycled, and has the

number "2" as its recycling symbol.

High –density polyethylene (HDPE) (0.941 ≤ density < 0.965) is a thermoplastic material

composed of carbon and hydrogen atoms joined forming high molecular weight products1. In

HDPE and other thermoplastic materials, the molecular chains are not cross-linked and such

plastics will melt with the application of a sufficient amount of heat. With the application of heat,

thermoplastic resin may be shaped, formed, molded or extruded.

HDPE is resistant to many different solvents and has a wide variety of applications, including:

Arena Board (puck board),Bottles, suitable for use as refillable bottles,Chemical resistant piping

systems, Coax inner insulators (dielectric insulating spacer),Containers such as Fuel tanks for

vehicles, Laundry bottles,Milk jugs and cans,corrosion protection for steel pipelines, water, for

domestic water supply and others.

Carbon black was incorporated as reinforcement filler in HDPE compound. Carbon black has

unique ability to enhance the physical properties of elastometer. This well documented

phenomenon termed as reinforcement has a profound effect on today’s pipe industry. Electron

beam modification on filled polyethylene has been studied earlier 2. It is well known that

polyethylene belong to the class of cross-linkable due to the present of unsaturated polyisoprene

chain.

Literature Review

Polyethylene is one of the extensively used plastic in outdoor applications. For this reason, many

studies have been conducted over the years to investigate this polymer. A number have been

performed for improving the physical and mechanical properties of HDPE by combination of

different kinds of stabilizer.

On the other hand, cross-linking is a broadly used method for the modification of polymer

properties. Electron beam irradiation cross-linking high-density polyethylene (HDPE) is a kind

of ideal material suited for wire cable and water pipe. However, the stability of electron beam

irradiation processing of HDPE before modifying is not good enough. This research has studied

the effect of electron beam cross-linking HDPE 3. By comprehensive researching, we have

defined the formula of electron beam irradiation cross-linking HDPE.

High-energy radiation has unusual chemical effects on polymers in the solid state, inducting

cross linking and chains scissions by radical reaction. Cross-linking has been widely used for

improving physical properties of polymer materials. However, radiation, induced chain scissions

results in degradation of polymer materials. The carbonyl and double bonds concentration

increased with the increase in radiation dose when polyethylene was ᵞ-irradiated in air 4.

Irradiation produced further cross-linking in amorphous region plus extensive chain scissions of

taut tie molecules and led to increase the crystalline and crystal perfection.

All forms of ionizing radiation interact with matter by transferring energy to the electrons

orbiting the atomic nuclei of target materials. These electrons may then be either released from

the atoms, yielding positively charged ions and free electrons, or moved to a higher-energy

atomic orbital, yielding and excited atom or molecule (free radical). These ions, electrons, and

excited species are the precursors of any chemical changes observed in irradiated material 5.

Thus, by using ionizing radiation, it is possible to synthesize, modify, cross-link, and degrade

polymers.

High Density Polyethene (HDPE) is made at relatively low temperatures and pressures using

special catalysts. These are called Ziegler-Natta catalysts after the two men who were awarded

the Nobel Prize for their development and application to polymerisation of ethene and propene 6.

Figure 1: A typical reactive centre in Ziegler-Natta polymerisation

Use of these catalysts gives an unbranched product that is much harder and more crystalline than

LDPE.

Figure 2: The structure of High Density Polyethylene (HDPE)

ASTM Pressure Rating Method

The ASTM pressure rating method utilizes pipe samples tested at constant temperature with the

log stress vs log time regression line extrapolated to 100 000 hours. This extrapolated value is

the long-term hydrostatic strength (LTHS) of the material, and the categorized value of the

LTHS is called the hydrostatic design basis(HDB) in accordance with ASTM standard test

method D 2837 7. This HDB is reduced to maximum working stress by a design factor (F) to

establish the Hydrostatic Design Stress (HDS). The HDS is the product of the HDB and the

design factor for water.

HDS= HDB x F (for water)

HDB and HDS values for various thermoplastic materials used for piping applications are

published in PPI-TR4, which is available on the PPI website. These PPI listings of HDB and

HDS values are classified in accordance with the materials standard pipe material designation

code. In this designation system, the plastic pipe materials is identified by its standard

abbreviated terminology in accordance with ASTM D 1600. The water engineer then uses the

pipe design formula below to calculate the pressure rating (PR) for this PE pipe:

PR = [ 2 (HDS)/ (DR-1)]

Where: PR= pressure rating

HDS= hydrostatic design stress

DR= dimension ratio

Experimental

Materials

The materials used in this work were two kinds of commercial high-density polyethylene used as

water pipe. The base polyethylene material for both samples was petroleum. One type taken from

produced pipe and other one taken from the original compound without undergo any physical or

chemical treatment.This sample was supplied by polyolefins sdn bhd from Klang,Selangor.

Preparation

Pipe sample was prepaid by polyolefins factory based on standard ASTM D 638 type 1 with PN

12.5 SDR11 and S-5.Three different thickness was taken from 3mm, 6mm and 10mm. Besides

that, three other different sample was prepaid from compound given with thickness varies from

1mm,3mm and 5mm. these sample was cut using dumbbell according to ASTM D 638 type IV.

High Energy EB Irradiation

All the sample were exposed under high energy EB irradiation at different dose rate of 80, 100,

120 kGy at room temperature by using 3 MeV electron beam accelerator. The accelerator energy

beam current and dose rate were set to 2MeV, 2mA and 40,50,60 kGy/pass for 80,100,120kGy

respectively. These samples will undergo three different testing which are Izod impact test,

Tensile test and Scanning Electron Microscope (SEM).

Characterization Techniques

Tensile strength

The point at which a stress causes a material to deform beyond its elastic region (permanent

deformation) is called the tensile strength at yield. When stressed below the yield point, an

elastic material recovers all the energy that went into its deformation. Recovery is possible for

polyethylene when the crystals are subjected to low strain levels and maintains their integrity. A

formulation of greater density (higher fraction of crystals, lower melt index) is predictive of

greater tensile strength and increasing brittleness.The forced required to break the test sample is

called the ultimate strength or the tensile strength at break. The strength is calculated by dividing

the force by the original cross sectional area. Specimen were cut based on ASTM D 638 type IV,

using Instron 1115 machine with 5kN load. Crosshead speed and gauge lengths were set to

250mm/min and 25mm.

Impact test

Notched Izod impact test as specified by ASTM D 256 standard test method was applied

by using Ceast 6545/000 model. Specimens with 62 x 15 x 3 mm dimension were subjected to an

impact test with 2.54 mm depth of notch. Before the testing takes place, each sample is

immersed into liquid nitrogen for about 30 seconds. The energy absorbed by the specimen in the

breaking process is known as the breaking energy (J/m).

Scanning Electron Microscope (SEM)

SEM machine was used to visualize the surface morphology of the HDPE water pipe.

Surfaces included cut surfaces and failure surfaces. A cut surfaces was obtained by cutting a

specimen into a small dimension; a failure surface was obtained from a failed tensile testing

specimen. The surfaces were sputter coated with gold before SEM measurement.

Result and Discussion

Graph

80 100 1201440

1450

1460

1470

1480

1490

1500

1510

1520

18.2

18.3

18.4

18.5

18.6

18.7

18.8

18.9

19

19.1

HDPE Pipe material

Radiation dose (kGy)

Elon

gatio

n at

Bre

ak (%

)

Tens

ile S

tren

gth

(MPa

)

Figure 3

0 80 100 120920

940

960

980

1000

1020

1040

1060

1080

0

5

10

15

20

25

HDPE compressed

HDPESeries2

Radiation Dose (kGy)

Elon

gatio

n at

bre

ak(%

)

Tens

ile S

tren

gth

(Mpa

)

Figure 4

0 80 100 1200

10

20

30

40

50

60

70

80

69.33

56 5348

Hardness vs Radiation Dose

Series1

Radiation Dose(kGy)

Hard

ness

(J/m

)

Figure 5

Tensile properties are mainly a function of cross-link density, molecular imperfection and

polymer chain length. Figure 3 shows the graph of HDPE taken from original pipe. As we can

see, the elongation break decrease as radiation dose increase while the tensile strength increase

as radiation dose increase. Increasing of the tensile strength at higher doses is due to the cross-

linking joints among backbone chains in a net-like structure. This makes HDPE to attain higher

tensile strength after irradiation. This analysis same goes for HDPE compressed with same

thickness which is 3mm. Theoretically , the maximum tensile strength for HDPE is 24MPa.

According to figure 3, maximum tensile strength is 19MPa while in figure 4, maximum tensile

strength is 21MPa.

Figure 5 shows the relationship between hardness and radiation doses. Generally

increasing radiation doses will decrease the hardness of HDPE material. This happen because

cross-link causes structure to cross-link which reduces the ability of the polymer chain as plastic

material. Cross-link also can causes material to become brittle.

Figure 6: a) HDPE radiated 80 kGy, b)HDPE radiated 100 kGy, c) HDPE radiated 120

kGy, d) HDPE original pipe radiated 100 kGy

The change of chemical and physical properties after Irradiation can be seen clearly from

scanning Electron testing. As the radiation doses increases , more cross-link are produced in the

sample matrix, which prevent the structural reorganization during drawing. This SEM taken from

sample tensile strength. Generally the tensile strength increases with cross-link at lower cross-

link density. However, at higher cross-link density the network becomes dense due to chain

degradation and cross-links leading to inhomogeneity in the phase distribution.

Conclusion

Chain scission and cross-linking are two processes, which occur simultaneously during

irradiation of polyethylene pipe but cross-linking, is predominated reaction. Thus, EB radiation

causes partially cross-linking in both PE samples. These cross-linked polymers show higher

mechanical properties than unirradiated ones.

References

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2. Stress strain curves Department of materials sciences and Engineering, Massachusets Institute

of Technology Cambridge, 2001

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Bangi,Malaysia.

4. Polymer plastic Technology Engineering “ Irradiation modification of PVC/ENR blend:

effects of TBLS content” Chantara Thevy Ratnam, Malaysian Institue for Nuclear Technology

Research(MINT) Bangi, 2002

5. Scott G, “Mechanism of polymer Degradation and stabilization, Elsevier, England , Ch.6,169-

183(1990)

6.Min-Kang Seo, “ Electrical of fluorinated CB/HDPE polymer switch, Inha University,Incheon

korea (2009)

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Beijing