Gel formation during extrusion of LDPE and
LLDPE
Henk Lourens, Senior Scientist, Polythene Business, Sasol Polymers
1) Introduction
Gel formation during the extrusion of LDPE and LLDPE can be a vexing problem. To the
casual observer it often seems to be a random phenomenon, with a strategy that reduces
the problem in one instance seemingly making it worse in another.
The purpose of this article is firstly to identify the types of gels one may come across and
secondly to look at the polymer chemistry involved in the process of gel formation.
Wrongly identifying the type of gel will cause our problem solving strategy to be a hit or
miss affair because the causes for gel formation differ according to the gel type. At Sasol
Polymers our experience shows the gel of oxidised type to be involved in the majority of
gel problems. The article will thus focus on this type of gel.
Once we understand the chemistry involved in oxidised gel formation we will be in a
position to formulate strategies to deal with particular instances of gel formation
experienced in industry. This aspect comprises the third part of the article and will be
dealt with in the final pages.
One definition of gel formation states that “ A gel is any visible imperfection in a
polyethylene film.” (1) This means that gels are not only polymeric in nature but can also
be inorganic. The inorganic gels would consist of undispersed pigments or contamination
such as by metal particles. It is only slightly comforting to know that gel formation is still
a worldwide problem. The following quote is from a presentation during 1998 in the
United States. “Gel formation in polyolefin film is a phenomenon that is difficult to
predict, reproduce and solve” (2)
Why are gels such an important issue? This is definitely a rhetorical question, but as most
converters have found it leads to scrapping of product as a result of unacceptable
aesthetic properties and or problems with the printing and lamination processes. An
operational issue is the (seemingly) unpredictable manifestation of gels which can upset
the best production planning, usually when it is too late to adjust the schedule.
2) Types of gels
One gel classification used widely in industry classifies gels into four main groups (1).
These groups are the Crosslinked/Oxidised gels, Cross Contamination gels,
Unmixed/Non-homogenous gels and finally Fibers/Contamination gels.
Crosslinked/Oxidised gels are the result of drastic molecular growth. This growth results
in molecules of such a large size that they have much higher viscosity/elasticity
compared to the rest of the melt. As a result the flow pattern of the melt is disrupted,
especially after the melt has left the die, resulting in a protrusion from the film which we
1
call a gel. The actual molecule responsible constitutes less than 1/100th of the volume of
protrusion. Crosslinked gels are primarily formed in the absence of oxygen and would
therefore form inside the reactors of the polymer producer. Oxidised gels tend to form
during extrusion, in the presence of oxygen.
Cross Contamination gels are the result of unintended mixing between polymers of
different density, molecular weight or comonomer type. The polymer mix is then
extruded under conditions which do not allow adequate melting and mixing to occur. An
example of this gel type would occur when using an extruder set up for LDPE film and
running polymer (LDPE) contaminated by a small amount of HDPE. The HDPE would
not fully melt under these conditions and would create gels.
Unmixed/Non-homogenous gels are the result of polymer blends extruded under
conditions of inadequate melting and mixing. Sources of these gels include poorly
dispersed additives, polymer blends with very different melting points. An example of the
latter would occur during the extrusion of a PP/HDPE blend.
Fibres/Contamination gels result from floss (conveying of polymer using air), fibers from
bagging material or contaminants picked up as a result of transport of the material.
The experience at the Polymer Technology Centre (PTC) has been that the majority of
gel complaints result from oxidised gels formed during extrusion. Because of this it is
useful to evaluate this class of gel in more detail.
3) The polymer chemistry of oxidised gel formation
3.1) Mechanism of gel formation
For a LDPE or LLDPE molecule to become large enough to be visible as a gel we need a
mechanism through which the molecules can grow larger, until such time as they have
reached the critical size where they become visible as a gel.
For this process of molecular growth we need free radicals. A free radical is a molecule
possessing an unpaired electron, and therefore can react with another free radical to form
a new bond. The moment free radicals are created the degradation process can start. The
degradation can comprise scission(breaking of molecules), oxidation (if oxygen is
present), molecular growth, branching etc. The reaction conditions and molecular
structure will determine which of these processes will dominate. With LDPE and LLDPE
unfortunately, it is the molecular growth reaction that dominates.
It has been reported that two mechanisms are involved in the process of molecular
growth. The first is molecular growth through the formation of ether groups (figure 1). In
this process a radical on a oxygen atom connected to a polymer molecule reacts with a
radical on the carbon atom of another molecule. Another radical can form on the newly
created larger molecule and repeat the process just described. In this manner the molecule
can grow until the critical size for gel formation is reached. This mechanism was first
2
proposed by a group from Melbourne University in Australia to explain the high
concentration of ether groups found in gels that had been stuck inside an extruder. (3)
Primary reactions
responsible for gel formation
during extrusion
1) Molecular growth as a result of ether group
formation
R1-O-R2
R1 +  O-R2 
polymer molecules)
(R1 and R2 are
2) Molecular growth as the result of polymerisation
of vinyl groups (unsaturation)
Figure 1
A second mechanism, crosslinking through vinyl groups (unsaturation), have been shown
to be the most important process for crosslinking reactions (figure 2). Vinyl groups are
particularly capable of gel formation since they have high mobility and low steric
hindering compared to other types of unsaturation. (4,5 ) The free radical remains available
after the it has attacked the vinyl group and molecular growth will continue until such
time as it meets another and is terminated. This mechanism is supported by work
published by CIBA and GE. (4,5)
3
Polymerisation of vinyl groups
(unsaturation)
R1 + R2 –CH=CH2
(R1,,R2,R3 and R4 are polymer molecules)
R1-CH2-CH-R2 + R3 –CH=CH2
CH2-CH-R3
R1-CH2-CH-R2 + R4 –CH=CH2
CH2-CH-R4
CH2-CH-R3
etc.
R1-CH2-CH-R2
Figure 2
In figure 2 we demonstrate the process of molecular growth through polymerisation of
vinyl groups. In the upper left hand side we have a free radical on polymer molecule R 1.
This radical attacks the unsaturated group on molecule R2 and forms a new bond, while
leaving the free radical available to repeat the process. This sequence will repeat until the
free radical meets and terminates with another.
Since free radicals are critical for the crosslinking process it is nescessary to understand
the factors responsible for radical formation. Free radical formation will be enhanced by
the following changes.




Increasing the temperature to which the polymer is exposed,
Increasing the unsaturation and branch content of the polymer,
Subjecting the polymer melt to more mechanical stress,
Increasing the oxygen content of the melt.
4
The effect of the thermo-oxidative
degradation cycle on radical formation
Figure 3
Why is oxygen such an important factor in gel formation? The answer can be found in the
well known thermo oxidative degradation cycle (figure 3). The problem is that
hydroperoxides form in the presence of oxygen. These molecules decompose into
hydroxy and alkoxy radicals, adding 2 more radicals to the 1 we started with. Each time
we move around on this cycle we thus have a threefold increase in radical concentration.
The level of free radicals in the polymer melt will increase as a result of this cycle until
an equilibrium level is reached. This equilibrium value will depend upon the oxygen
concentration, antioxidant availability, temperature, residence time distribution of the
polymer, molecular structure of the polymer and shear inside the extruder.
5
Molecular growth
Precursor Gels
Critical size
(visible gel)
Time
Figure 4
We have seen that for molecules to reach the critical size we need free radicals as well as
a mechanism through which these radicals can enable the growth of molecules. What we
have not yet specifically referred to is the need for the molecules to spend sufficient time
inside the extruder (figure 4). Only if the molecules stay inside the extruder long enough
will they be able to grow to the critical size where they are able to disrupt the flow of the
melt once the polymer leaves the die.
Under a given set of extrusion conditions the time required for reaching the critical size
will depend on the presence of precursor gel material. Precursor gels are molecules which
are larger than the average molecule size in the polymer being extruded. They are not
large enough to be visible as gels, but can grow to the critical size in less time than the
average molecule. The more precursor gels , and the larger they are in size, the more
likely that we will have gel formation. All polymers contain precursor gels, however they
differ in number and size of these.
6
Deformation in flow pattern
resulting in gel
Protrusion due to
flow disruption
nucleus
film
Figure 5
3.2) Why does the oxidised gel become visible?
If we look at the protrusion we call a gel it is important to note that the actual
crosslinked/oxidised molecule makes up a very small part of the gel. As we explained
before, the disproportionate size of the gel compared to the molecule responsible results
from the disruption in the flow pattern (difference in elastic behaviour) as the melt leaves
the die (figure 5). The oxidised molecule makes up between 1/100th to 1/1000th of the
volume of the gel. In the picture at the bottom of figure 5 we can identify the nucleous as
the black center of the gel. The colour of the center depends on the time the molecule
forming the nucleous has spent inside the extruder and thus reacted with oxygen. With
short residence time it will be clear, as time increases it will become amber and
eventually become black.
It is important to remember that using a smaller screenpack size will not be able to
prevent oxidised or crosslinked gels from passing through. The gel nucleus is flexible,
can break or squeeze through easily. A screenpack of different mesh size may be
beneficial however in that it will change the melt flow profile and residence time
distribution of polymer in the extruder as a result of change in back pressure.
7
3.3) Summary of chemistry of oxidised gel formation
We have dealt with a large amount of information up till now. Let us summarise what we
have discussed until now. For the molecules to reach the critical size to become visible as
a gel we need to meet 4 requirements.
a) sufficient free radicals
b) a mechanism for molecular growth
c) precursor gel material and
d) sufficient time to grow molecules to the critical size.
Free radical concentration can be influenced by the polymer type, polymer manufacturing
technology, antioxidant type used for stabilisation, the amount of antioxidant added
It is important to note that during extrusion a very large number of combinations of the
levels(value or size ) of these factors are able to produce gels. If, in a situation where we
are experiencing gel formation, we are able to reduce the severity (value) of some of
these factors, we will have a workable strategy. This is the focus of the next section.
8
4) Oxidised gel formation in practice, causes and
strategies to deal with this.
Effect of temperature and time
on gel count
Unacceptable gel level
Melt Temperature=220+2y
Gel count
Melt Temperature=220+y
Melt Temperature=220
0hr
x hrs
3x hrs
Time
Figure 6
In reality, unless a major precursor gel problem exists, it will take time for the oxidised
gels to become so large and numerous that an unacceptable gel level is reached. In this
slide we assume that we are running the identical batch of polymer on the same extruder,
at the same output, but using different melt temperatures.
The graph in figure 6 shows the level of gels over time given different melt temperatures.
In case 1 we start the extruder with a melt temperature of 220 degrees and observe that
after a slight increase in gel level the curve levels off.
In case 2 we increase the melt temperature by a few degrees and find that the gel level
slowly increases until the unacceptable gel level is reached.
In case 3, with further melt temperature increase, the unacceptable gel level is reached
even sooner.
The point on the curve marked 3x hrs can represent anything from 1 to 9hrs. Thus x hrs
may be anything from 20 minutes to 3 hrs. The actual time taken to reach the
unacceptable gel level depends on not only the melt temperature but also equipment and
9
equipment setting considerations which, as we will see later, influences amongst other
aspects the residence time distribution and shear heating inside the extruder.
We will now attempt to discuss some of the reasons for the actual shape of these curves.
The oxidised molecules, as they grow larger, will often (preferentialy in stagnant or low
shear areas) attach themselves to the metal surfaces of the extruder, and grow further as
long as they are able to stick to the metal. These molecules will slowly grow in size until
such time as they are large enough to be pushed out by the stream of melt flowing past.
This process of molecular growth on the metal surfaces and being pushed out of the
extruder takes place continuously. When the extruder is started most of these molecules
are too small too be removed from the metal areas and the gel levels will be low. As they
grow more of them will end up in the melt (according to a random statistical) and the gel
level increases. At some point in time an equilibrium is reached where the gel level
fluctuates around a fixed level. If this level is above the acceptable point we have
problems.
It is frequently observed that the gels are present in the film, scattered around a line
pointing in the machine direction. This is often referred to as a splurge of gels. Frequently
this can be ascribed to very large molecules which have been broken into smaller pieces
as a result of shear in the die and screenpack area.
Before discussing the factors which may induce gels to form during extrusion we may
summarise them as follows.
1)The screw design
2)Wear on screw
3)Dead spots in die/extruder
(These 3 factors relate mainly to the time the polymer molecules will spend inside the
extruder, called the residence time)
4)Shutdown and startup procedures
5)High shear
6)Use of recycle/regrind in a formulation
7)Temperature control/profile on extruder
8)Order of production runs
9)Precursor gels from supplier
10
Residence Time and RTD
gram/min
2 minutes
Figure 7
To discuss the first 3 factors mentioned we need to understand what is meant by Average
Residence Time and Residence Time Distribution. If we take an extruder running only
polymer we will see a clear melt exiting at the die. In order to obtain an idea of how long
the average polymer molecule spends inside the extruder (Average Residence Time) we
may perform a simple experiment using a handful of pigment. The pigment is dropped
into the extruder and by monitoring the intensity(or weight) of blue pigment in the
extrudate we may form an idea of the distribution in time taken for different molecules in
moving through the extruder. In this slide we see that at just under two minutes some
blue is observed. This increases in intensity, goes through a maximum and finally reduces
again. The whole of the curve shaded blue is the Residence Time Distribution, while the
line indicates the Average Residence Time of molecules. The greater the time difference
between the moment the first blue becomes visible and the time when the last blue
disappears, the greater the “width” of the Residence Time Distribution is said to be. We
will see shortly that the wider this distribution, the more problems we may expect with
gel formation.
4.1) Screw design
When LDPE was introduced into the South African market the Archimedean screw was a
very popular design. This design consists of a constantly increasing root diameter as one
moves from the feeding section to the melting section and finally the metering section.
The pitch stays constant for this screw. The major drawback of this design is the high
shear it exerts in the part where the root diameter is a maximum. LDPE can often handle
11
the shear and temperature increases in this area (to a large extent as a result of its ability
to shear thin), but LLDPE cannot and will tend to degrade and form gels.
As a result of this, designs such as the GPDP screw (optimized at PTC) have become
commonplace where the ability to run both LDPE and LLDPE is required. The GPDP
screw imparts much less shear to the polymer since it has a constant root diameter and
compression occurs as a result of decreasing the pitch towards the metering section.
Barrier screws represent very advanced design technology. They are typically designed
for particular blends and blend ratio’s of polymers. Typically these blends consists of
LLDPE and HDPE. When using significant amounts of LDPE in the blend degradation
and gel formation can result in some of barrier screw designs.
The golden rule in extrusion is thus to know what type of screw you have in your
machine and what blend combination/blend ratio it was designed for.
12
Residence time
distribution differences due to
screw wear (same output).
Proportion
of
molecules
Extrusion on extruder with
new screw
Extrusion on extruder
with worn screw
Residence Time
Sufficient time to
reach critical size
Figure 8
4.2) Wear on the screw
Let us assume that we have an extruder in good condition and that we are running at a
constant output. The black curve would represent the residence time distribution of
polymer molecules for this example. Over time, as a result of friction, the screws will
start to wear. This means that the gap between the screw flights and the barrel will
increase. This change will result in more backflow of polymer melt. To keep the output
constant the operator will gradually , over time, increase the screw speed. A higher screw
speed imparts more shear thinning (lower polymer viscosity) to the melt in the high shear
areas and even more back flow will result over the screw flights. Some polymer
molecules can now stay inside the extruder significantly longer than before. (The higher
screw speed means that some of the molecules that do not take part in backflow will be
pushed out of the die faster than before. This results in the outward shift of the curve on
the left side.) We now end up with the red curve which has a significantly wider
residence time distribution compared to the previous curve. This right hand side of this
curve is the source of our gel problem.
For this machine, with the particular polymer and under the extrusion conditions used the
vertical line represents the critical time needed for gel formation. We see that a part of the
polymer molecules will now be inside the extruder for long enough to form gels.
13
Residence time
distribution with dead spots
Proportion
of
molecules
Extruder with dead spots
Extruder with no dead
spots
Residence Time
Sufficient time
to reach critical
size
Figure 9
4.3) Dead spots in the die and extruder
Dead spots are areas inside the extruder where stagnant or slow moving polymer melt is
found. These areas may result from extruder design, or areas with polymer sticking to the
sides of the extruder which creates stagnant areas behind them. Die design is frequently a
cause of this problem.
The dead spots influence the residence time distribution by adding a tail on the right hand
side of it. Although the amount of polymer subjected to longer time inside the extruder is
less than in our previous slide, the time factor is much greater. The colour of the nucleus
of the oxidised gel can be used as an indication of how long the molecule spent inside the
extruder. With increasing time the colour will change from clear to brown to black.
How can we deal with the problem of polymer staying inside the extruder for too long?



By reducing the temperature profile settings we may retard radical formation and
increase the critical time required for gels to form.
By replacing or refurbishing the screw (within certain limits) the flowback of
polymer over the screw flights is reduced and the residence time distribution
narrowed.
By optimising the screw design for the polymer or blend being run temperature
overruns as a result of high shear areas can be minimized. For instance by
replacing a constant pitch, increasing root screw (baseball bat design) with a
14
decreasing pitch, constant root design (GPDP screw) when extruding LLDPE a
substantial improvement in gel levels will be observed.

Processing aid works by coating the metal interior of an extruder with a thin layer
of polar polymer. This layer changes the flow profile of the melt by reducing the
friction at the barrel/melt interface. The flow pattern changes to the plug flow type
and greater shear occurs at the metal polymer interface. The combination of more
polar surface and higher shear at the barrel prevents hangup of polymer and
subsequent gel formation.
4.4) Shutdown/Startup procedures
When the power to a extruder is shut off immediately after the last film has been
produced the polymer inside is exposed to high temperature for a long period. These
conditions are very beneficial to gel formation. The remaining antioxidant is used up
first, after which the free radical concentration will increase. Since time is not the
constraint even very slow growing molecules will have enough opportunity to grow to
critical size. These molecules can become charred and some will stick to the barrel. On
startup the agglomeration of burnt polymer on the barrel and die will disrupt the flow of
polymer melt and may create stagnant areas where more gels will form. Purging of the
extruder with processing aids or purge masterbatches is an option, but in severe cases a
strip and clean operation may be required.
Incorrect startup would result from waiting too long from the time of switching on the
heating zones until starting the screw. The formation of gels will be similar to the
situation described before.
The best way to deal with these eventualities is to put in place a procedure for starting up
and shutting down of extruders.
4.5) High Shear
The existence of high shear zones in extruders generally create problems as far as gels are
concerned. High shear zones exist between the screw flights and barrel of an extruder, in
the large root diameter area of archimedean screws, at the screenpack and in some die
systems.
The problem with these zones are twofold. In the first place the high shear may cause
local overheating. Because only a small proportion of the polymer is subjected to this it
will usually not manifest as a drastic increase in the zone or melt temperatures and may
be difficult to detect. Secondly, the mechanical stress experienced by the polymer in
these zones may cause the breakage of molecules with the formation of two radicals for
every bond broken. More free radicals enhances gel formation.
High shear zones may be beneficial in that stagnant areas cannot exist close to it. The
precursor gels cannot stick for long enough to grow to gel size.
15
4.6) Regrind
By constantly increasing the amount of regrind material in the blend it is almost
guaranteed that gels will appear at some point, irrespective of the age of the extruder and
screw design used.
The problem with regrind material is that it has already been through an extruder before
and as a result contains oxygenated molecules. These will help to kick off the auto
degradation cycle almost immediately. The equilibrium free radical concentration
reached will be higher than without regrind material present. Finally, the regrind
containing polymer will contain more precursor gels, because the molecules in the
regrind portion had time to grow during the previous extrusion run.
The preferred strategy to deal with this problem would be to remove the regrind from the
formulation, or to make sure that the percentage used is low enough not to be close to the
critical amount for the particular extruder and polymer being extruded. Where possible
the temperature profile should be lowered.
The use of extra antioxidant and processing aids will help but may introduce
unacceptable cost as well as other complications. Increasing the antioxidant level may
cause migration to the surface with resultant problems with sealing and organoleptics.
Yellowing may also result.
4.7) Temperature control
Problems with zone temperature control often lie at the heart of gel problems. This may
be as a result of faulty thermocouples, giving too low readings, or heater bands that do
not switch off when they should. The result is increased free radical formation and gel
problems.
The best and most obvious solution is to fix the thermocouple/heater bands at the earliest
opportunity. Lowering of the temperature profile and melt temperature will help, but may
affect other film properties or output. Antioxidant masterbatch and processing aids may
be used as stopgap measures.
4.8) Production cycle
When observing gels one should always take cognisance of the production cycle which
has been involved on the extruder in question. If polymer of higher viscosity had been
run before one with lower viscosity the possibility exists that the second polymer was
unable to displace all of the previous polymer from the extruder/die system. In some
areas the first polymer will stay stuck to the barrel and cause stagnant areas to form
behind it. Gel growth can occur in these areas. Typical examples of this problem would
be when running a 1MFI PP film grade and then switching over to a 1MFI LDPE grade.
Although the MFI’s are seemingly similar the viscosities of these two polymers under the
actual conditions of temperature and shear inside the extruder would be very different.
Apart from different shear thinning behaviour the difference in viscosity (between these
16
two grades) within the extruder is due to PP’s MFI being measured for data sheet
purposes at 40 degrees higher temperature than that of LDPE.
Where possible polymers of very different viscosity and type should be run on different
extruders. If, after changing from a high viscosity polymer to one of lower viscosity, gels
are observed, the best strategy is to reintroduce the higher viscosity polymer to the
extruder. One could now try to match the viscosity of the next polymer by changing the
melt temperature setting, taking care not to burn the polymer. When reintroducing the
previously lower viscosity polymer the viscosities should now be more closely matched
and once the temperature settings are optimised for the new polymer , no more gels
should be observed. Alternatively in this case one could use polymers of intermediate
viscosity such as a 0.3 MFI LDPE or purge masterbatches to reduce the magnitude of
viscosity change to the final product.
4.9) Precursor gels
Earlier in this article it was mentioned that very large molecules or precursor gels require
less time to reach the critical size where they become visible as a gel. All polymer
received from polymer suppliers contain some amount of precursor gels. The amount and
size of these gels are influenced by the production technology used, operational
procedures, antioxidant type and addition level of antioxidant.
Extruders with worn screws or with certain screw and die designs are more sensitive to
precursor gels. The best strategy would be to refurbish or replace worn screws and to
ensure that the screw design is optimum for the polymer used.
Sasol Polymers have realised the importance of precursor gels in causing gel problems on
some extrusion equipment. As a result a number of initiatives have been launched in the
past 3 years.
To reduce precursor gels in LLDPE the F resins were removed from the grade range. F
resins, because of their high density, low MFI and broad molecular weight distribution
compared to standard LLDPE were often responsible for crosslinked and cross
contamination gels. By removing the butene resins from the grade range the run length of
the remaining grades were increased, giving rise to more stable production runs.
In the case of LDPE polymers new screws were designed and fitted to the production
units at Sasolburg. These impart much less shear and heat to the polymer and thus reduce
the amount of precursor gels in the polymer.
To conclude we may list the strategies to deal with gel formation in the following order.
17
Summary of strategies to deal with gel formation
Primary Strategies
Replace worn screws
Use a screw design optimised for the polymer being extruded.
Reduce the temperature profile and melt temperature where possible.
Simplify the production cycle so that polymer of similar viscosity is being extruded head
to head.
Perform preventative strip and clean operations every 6 months.
Secondary strategies
Use processing aids where the cost can be recovered.
Use antioxidant masterbatch, provided these do not affect downstream operations.
References
1) Characterisation of gels in polyethylene film; T.J.Obijeski, D.W.Dixon; 1992
Polymers, Lamination &Coatings Conference; p 61-69
2) The use of polymer processing AIDS to reduce gel formation in polyolefin
plastomer extrusion.; Woods, Susan S.; Amos, Stephen E.; (Dyneon LLC,
Oakdale, MN 55128, USA). Polym., Laminations Coat. Conf.; Volume 2; 1998;
675-685
3) Efficiency of processing stabilisers using a micro-oxygen uptake technique; J.
Scheirs, S.W. Bigger, O. Delatycki; Polymer; 1989; Vol 30; p 2080-2087
4) Degradation and Stabilisation of High Density Polyethylene during Multiple
Extrusions; S. Moss, H. Zweifel; Polymer Degradation and Stability; 25; (1989);
217-245
5)
Thermal scission and crosslinking during polyethylene melt processing.
Johnston, R. T.; Morrison, E. J. (Dow Plastics, Freeport, TX 77541, USA). Adv.
Chem. Ser., 249(Polymer Durability), 651-82 (English)
18