TPi April 2008

Optimising the raw material: the polymerisation process In the pipe business the flexibility of the bimodal (or multimodal) process for producing polyethylene materials has provided the greatest scope for producing ‘tailor-made’ materials. The choice of catalyst, co monomer type, content, and selective distribution thereof in the polymer chains, and the selectionofprocessparameters in each reactor all affect the development of the polymer structure and the properties of the end product. Changing these variables enables the properties to be optimised for a manufacturing process or end use application.

▲ Slow crack propagation caused by external point load

Borsafe ™ HE3490-LS-H With the help of the Borstar process and according to the description in the previous section, it has been possible to create a material which enables the pipe producer to make any dimension from 20mm to up to more than 2,000mm and wall thicknesses of >100mm. Not only does this material provide the possibility to make any pipe dimension, it also possesses an excellent resistance to impact failures eg rapid crack propagation, and excellent resistance to internal pressure manifested by the PE100 designation according to ISO 9080 standard extrapolation method.

The bimodal process consists of two polymerisation reactors in series. The Borstar ® low pressure slurry loop and gas phase reactor process is illustrated. The catalyst is fed into the first reactor, where polymer is formed as powder particles through polymerisation of the ethylene monomer and suitable amounts of co monomer, continuing in a series mode into the second reactor. The process offers great flexibility with regard to the type of co monomer that can be incorporated into the correct regions of the polymer. For example, the use of the co monomer hexene in the bimodal Borstar process results in polymers having extremely high resistance to slow crack growth.

▼ Table 2 : Summary of test results for Borsafe ™ HE3490-LS-H

Test

Standard

Test conditions

Result

Requirement

165 h 1 500h 2

NPT (a)

ISO 13479

80°C / 9.2 bar

> 18,000 h

FNCT (b)

ISO DIS 16770-3

80°C / 4 MPa / Arkopa

ca 6,000 h

>3,500 h 3

ACT (c)

90°C / NM-5

ca 4,000 h

330 h 4

PENT (d)

ASTM F-1473

80°C / 4.4/ MPa5

>10,000 h

50 h

(a) NPT = notched pipe test (b) FNCT = full notch creep test (c) ACT= accelerated creep test

1 = EN 1555 and EN 12201 requirement 2 = PE100+ association requirement 3 = DVGW requirement for sandless bedding

(d) PENT = Pennsylvania University notch test

4 = Provisional DVGW requirement for sandless bedding

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Tube Products International April 2008

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