TPi April 2008

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60 Tube Products International 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.
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.
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.
▲ Slow crack propagation caused by external point load
▼ Table 2: Summary of test results for Borsafe™ HE3490-LS-H
Test Standard Test conditions Result Requirement
NPT (a) ISO 13479 80°C / 9.2 bar > 18,000 h
165 h1
500h2
FNCT (b) ISO DIS 16770-3 80°C / 4 MPa / Arkopa ca 6,000 h >3,500 h3
ACT (c) 90°C / NM-5 ca 4,000 h 330 h4
PENT (d) ASTM F-1473 80°C / 4.4/ MPa5 >10,000 h 50 h
(a) NPT = notched pipe test 1 = EN 1555 and EN 12201 requirement
(b) FNCT = full notch creep test 2 = PE100+ association requirement
(c) ACT= accelerated creep test 3 = DVGW requirement for sandless bedding
(d) PENT = Pennsylvania University notch test 4 = Provisional DVGW requirement for sandless bedding