Revolve XL-400
Crosslinkable PE
For Rotomoulding
General Guide
Introduction
Revolve XL400 is a rotomoulding crosslinkable PE grade which provides the right balance of
performance between high flow, outstanding impact and excellent ESCR properties. While
XL400 is globally approved by the leaders of the automotive industry, it has only been
available in natural and black until today. However thanks to a sophisticated colouring
system which ensures pigment compatibility with the crosslinking chemistry, Matrix now
produces XL400 in a range of specific colours. The pictures shown in this guide are examples
of rotomoulded products made from Revolve XL400.
What is Crosslink?
Crosslinkable PE (sometimes referred to as X-PE) is standard polyethylene material which
contains additives (usually peroxide based) which, when the material reaches an activation
temperature, cause a chemical reaction to take place. This reaction causes very strong
chemical bonds (crosslinks) to form between polymer chains and significantly alters the
nature of the material.
Crosslinking is a very common process in the plastics industry and has been used for many
years in the rubber industry where it is called vulcanisation.
Why choose Crosslink?
Prior to the onset of the crosslinking reaction XL400 has a MFI (Melt Flow Index) greater
than 20 g/10min. This means that it can be used for mouldings which contain complex
shapes such as threads and tight radii, which would be difficult to fill with a standard grade.
Conventional rotomoulding grades have generally got a MFI of between 3 and 9 which is
much lower than that of XL400. Higher MFI grades are not generally rotomoulded because
the low molecular weight required from the material gives poor physical properties and
ESCR values.
However, the high flow characteristics of XL400 do not reduce the physical properties of the
material since once the article has moulded the crosslinking reaction takes place and
reduces the MFI to essentially Zero.
This confers a number of properties on the finished part, including improved physical
properties (see below) excellent chemical resistance properties and ESCR values well in
excess of 1000hrs, even for the more aggressive 10% Igepal solution.
How does crosslinking work?
Polymer Chains
Figure 1 Amorphous (Molten) PE
Figure 1 shows a schematic of a PE material in its molten state. The polymer chains are
randomly arranged and show signs of entanglement. It is the length of the polymer chains
and the degree of entanglement that determine the Melt Flow characteristics of a PE. The
shorter the length of the chains is, the lower the degree of entanglement and therefore the
higher the MFI are. Both standard linear PE and XL400 (prior to crosslinking) resins display
this type of configuration when they melt.
Polymer Chains
Figure 2 Crystalline Linear PE
Figure 2 shows a schematic of linear PE on cooling. The polymer chains arrange themselves
in an ordered crystalline fashion.
Crosslink
Polymer Chains
Figure 3 Crosslinked PE
Figure 3 shows the structure of XL400 after the crosslinking reaction has taken place.
Chemical bonds (crosslinks) have formed bridges between the polymer chains, effectively
increasing the length of the chains dramatically, and therefore reducing the MFI
considerably, and at the same time increasing the physical and chemical resistance
properties.
Revolve XL400 - Impact Performance
Unlike a standard Linear PE XL400 exhibits two very distinct molecular configurations.
Before crosslinking occurs the material is essentially a very high melt flow HDPE material. As
a result moulding which are removed from the oven before the internal temperature of the
mould has reached the point at which crosslinking occurs (> 180ºC) the part will exhibit
properties typical of this type of material: very poor impact strength and a high degree of
brittleness.
Figure 4 compares the impact strength of XL400 with a standard rotomoulding LLDPE. At
the beginning of the cycle the XL400 is very weak and brittle.
ARM Impact - 3 mm rotomoulded samples - Ferry RS 1.90
LLDPE
XL400
140
Impact Strength (J)
120
100
80
60
40
20
0
5
10
Cook time (min)
15
Figure 4 ARM Impact comparison between Revolve XL400 and a LLDPE
20
Figure 4 shows a comparison of Linear PE and XL400 for the whole cycle. There are several
points to notice about the graph, most importantly once the crosslinking reaction takes
place the impact strength of the XL400 increases dramatically, to a value almost double that
of a standard LMDPE. Also whilst the linear material will eventually overcook and degrade,
once crosslinking occurs the XL400 continues to display superb impact strength regardless
of how long it is in the oven, therefore it will not become brittle.
XL400 - Processing Guidelines
 Mould type – Although both
Aluminium and Steel moulds can be
used for XL400 the crosslinking
reaction produces some aggressive
agents that can corrode steel tools
over time and may slightly reduce tool
life expectancy, therefore Aluminium
tooling is recommended.
 Processing Conditions – as can bee seen from the information above it is important to
ensure that crosslinking occurs, and this generally means a slightly increased cook
time (15-30%) over a standard material. Additionally the temperature of the oven
must not be too high; otherwise the material may crosslink before it has had time to
fully flow around the mould. Too high an oven temperature can also result in
“pockmarks” on the surface of the part. As a guide a processing oven temperature of
250-280ºC is recommended.
 Part Thickness – Because of
the
increased
impact
performance and Flexural
properties exhibited by XL400
the thickness of the part wall
can often be reduced,
reducing in turn the need for
an increased cycle time.
 Rotation Ratio – Rotation
speeds and ratios should not
require alteration for the
running of XL400 compared to
standard materials.
 Quality Control – Degree of crosslinking can be measured by xylene extraction and is
the most reliable method for determining the degree of cure of a product, but there
are other tests (described below) that can be performed more easily and less
expensively which can determine if crosslinking has occurred.
1. Dart Impact – Since the crosslinking reaction progresses at a rapid rate it
can be fairly easily determined if this has occurred, especially since the
difference in impact performance is so big – see figure 4.
2. Bent Strip Test –Bend through 180º a strip of moulded article – a fully cured
part should not fracture or crack. This test does not determine the degree
of crosslinking that has occurred. For further information about this test
please contact Matrix.
Revolve XL400 – features
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Outstanding cold impact properties
High flow characteristics
Excellent ESCR
Chemical resistant
Excellent long term properties
TÜV approved
Available in specific colours
Further information on Revolve XL400 is available on request or simply log on
www.matrixpolymers.com
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