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Novel Processing Additives for Rotational Molding of Polyethylene

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					           Novel Processing Additives for Rotational Molding of Polyethylene

Introduction
     Rotational molding, also referred to as rotomolding or rotational casting, is a plastics
  processing technology for producing hollow seamless articles and it comprises four steps:
  charging of polymer as solids (free-flowing powder or pellets) or liquids (melt) into the mold;
  heating of the mold in an oven to processing temperatures with simultaneous rotation in two
  axes, see Bruins (1971), Beall (1998), Crawford and Throne (2002), Crawford and Kearns
  (2003), Dodge (2004). While heated, the polymer adheres to the mold and gets fused
  completely. Next steps are cooling of the mold by air or a water spray, and unloading of a
  produced item from the mold. In contrast to other polymer forming processes like extrusion
  or injection molding, fusing of the particles occurs at conditions of nearly "zero-shear" stress,
  because only forces of surface tension govern the flow of the melt. Advantages of the
  rotational molding in comparison with other techniques are the following: larger size of
  molded parts; lower cost of tooling and easer prototyping; thicker outer corners of the parts
  that impart higher strength and structural stability to the molded items; an opportunity to
  produce large and small parts of complicated shapes as one piece. Examples of the items
  produced by rotational molding include road barriers and buffers; traffic signal cones; plastic
  containers, tanks and barrels; automotive parts; hobby and sporting equipment; playground
  equipment and toys; furniture.
     One of the problems in rotomolding is bubbles of gasses trapped during sintering of the
  PE powders. Pick and Harkin-Jones (2004) report a correlation between the number of voids
  (bubbles) in a rotomolded article and its impact performance, with a higher number of voids
  resulting in lower impact performance. According to Spence and Crawford (1996) the particle
  size distribution affects the porosity of the product while overheating of the polymer melt for
  bubble removal results in lowering of mechanical properties of the product. Greco and
  Maffezzoli (2004) predicted existence of an optimum sintering temperature at which the
  density of the sintered product gets to its maximum for a good quality rotationally-molded
  product having limited number of voids. Kontopoulou et al. (1999) as well as Kontopoulou
  and Vlachopoulos (1999) investigated the formation and dissolution of bubbles during
  sintering of PE powders to evaluate how it is affected by powder properties, chemical
  structure, thermal properties and rheology. Bellehumeur and Tiang (2000) conclude that the
  polymer rheological properties seem to dominate the initial size of the bubbles formed. Many
  efforts have been done to decrease the number of bubbles by changing the internal mold
  pressure. While application of vacuum has shown some improvement in decreasing the
  bubbles content, see Evans (1998), the use of positive pressures appears to be more efficient
  according to Xu and Crawford (1993). Gogos (1999) investigated bubble dissolution in typical
  polymer melts encountered in rotational molding. He writes that a sharp pressure increase
  leads to a steep concentration gradient in the vicinity of the bubble/melt interface that can
  lead to extremely fast bubble shrinkage due to increased rate of the gas dissolution. The
  presence of water has also been recognized as a possible cause of bubbles, see Foster
  (1970) and Conway (1973), but according to Spence and Crawford (1996) preheating of PE
  powders to remove moisture have shown no effect on their content.
     In production of the PE powders for rotomolding with particle sizes less than 0.5 mm (35
  mesh) a raw resin powder from the polymerization reactor is blended with pigments and
  additives in an extruder, mixed and extruded through a set of dies and pelletized in
  conditions of water cooling. The produced pellets are ground and classified to proper particle
  size. The pelletizing and grinding operations are costly because of high energy consumption.



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An attempt was made to produce PE powders for rotomoding without pelletizing and
grinding, see Kallio et al. (2001) in U.S. Patent Appl. 20010025092, but from a practical view
point it is more convenient to manufacture micro-pellets for rotomolding. With the use of
micro-pellets the number of post-processing steps is reduced thereby reducing the cost of the
material, see Nugent (2006). Opposite to micro-pellets for rotomolding with regular oval
shapes and averaged sizes from about 0.5 to 0.6 mm, PE powders for rotomolding have
smaller averaged size of the particles (from 0.3 to 0.35 mm) and wide particle size
distribution as well as irregular shapes. Potentially, powders can be compacted so that fine
particles would fill the voids between the larger ones. Wang and Kontopoulou (2007)
observed fewer bubbles in the melt and better mechanical properties of the rotomolded
articles when powders were used in comparison to the use of micro-pellets. Nevertheless,
last ten years the use of micro-pellets is considered as a step forward for the rotomolding
technology with following benefits: uniform melting; improved handling; elimination of
dusting; faster sintering times (10-12% reduction); more uniform wall thickness; and
improved surface finish.
    Free-flow behavior of the PE particulates is important to get even thickness of the walls of
the rotomolded articles. The fluidity of PE powder is influenced by various properties, such as
particle sizes, shapes and surface roughness of the particles. As a general tendency fine
powders show less fluidity, see Yokoyama (1997). Fine powders of PE often demonstrate low
fluidity because of elongated shapes of the particles, sharp edges as well as such features
like "fibers" and "tails" at the surface of the particles. With low fluidity of the particulate
matter, an inner surface of the rotomolded parts gets wavy appearance with high variations
of the wall thickness. With PE powders a bridging often occurs and air gets trapped in the
corners of the mold so that the produced parts are not suitable for use, see Beall (1998).
Large pellets (> 2 mm) have too high fluidity, they are bouncing inside the mold and do not
stick easily to flat surfaces but accumulate in the corners of the mold. Also sintering of such
pellets creates a lot of bubbles that do not dissolve in polymeric melt. Optimization of flow
and fewer voids can be achieved by downgaugeing of the pellets to sizes from 0.3 to 0.8 mm
(micro-pellets).
    Fine mineral powders, e.g. talcum, silica fume, micronized silica and fly ash, are often
used in industry as free-flow agents. For example, micronized silica was used as a flow agent
for polymeric powders by Umiński and Saija (1998). However a temptation to improve flow
behavior of fine PE powders by mixing them with fine mineral powders is dangerous because
these hydrophilic powders impact mechanical properties of the rotomolded articles. The use
of dry mixing of mineral pigments (usually <0.4% by weight) with powder of PE is used
widely in such countries like India as a most economical and flexible way of coloring of the
rotomolded articles. However, it reduces the toughness and impact strength of the
rotomolded articles by well over 50 % in most cases, see Nagy and White (2004) as well as
Henwood (2006). The drop of mechanical performances can be tentatively explained by
following mechanism: The mineral particles separate PE grains and delay a sintering of them.
Thermal degradation of the polymer caused by longer exposures to heat reduces its
mechanical properties. Additionally, the particles with hydrophilic surface are not wetted by
the hydrophobic polymer melt and therefore such particles create local discontinuities of
adhesion of one grain of the polymer to another. Small particles with hydrophilic surfaces are
prone to agglomeration because of hydrogen forces between them. The hydrogen forces are
weak and if such agglomerates are embedded into polymer they create weak zones inside
the polymer matrix that concentrate mechanical stresses.




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   Polyolefin composition containing wax and metal salts of long-chain fat acids were
proposed long ago to improve processability of PE resins, e.g. Happoldt et al. (1945).
Chaudhary et al. (2001) used low Molecular Weight (MW) additives like wax, mineral oil and
glycerol monostearate as sintering enhancers in rotational molding. With this, melt viscosity
and elasticity were reduced while much faster densification and bubble removal were
observed. Suitable sintering enhancers include aromatic or aliphatic hydrocarbon oils, esters,
amides, alcohols, acids, and their organic or inorganic salts as well as silicone oils, polyether
polyols such as PEG, glycerol monostearate (GMS), pentaerytritol monooleate, erucamide,
stearamides, adipic acid, sebacic acid, styrene-alpha-methyl-styrene, calcium stearate, zinc
stearate, phthalates and blends thereof. The sintering enhancers allow a reduction in
sintering time, cycle time and/or maximum mold temperature but problems in the use of the
additives are following: The sintering enhancers that are dissolvable in polyolefins work as
plasticizers and reduce toughness of the rotomolded articles. The low-viscous sintering
enhancers that are immiscible with polyolefins, e.g. silicone oils and PEG, coalesce inside the
melt into large beads. The macroscopic discontinuities in the polymer matrix concentrate
mechanical stresses and certainly reduce impact strength of the rotomolded parts.
   Our proposal is related in part to fumed silica with sizes of particles about 10 nm. For
manufacturing of fumed silica, industry is commonly using a continuous flame hydrolysis
technique. It involves conversion of silicon tetra chloride (SiCl4) to the gas phase using an
oxy-hydrogen flame. It then reacts with water to yield silica (SiO2) and hydrochloric acid
(HCl). The hydrochloric acid is easily separated as it remains in the gas phase, while the
fumed silica is solid. The primary silicone oxide particles with hydrophilic surface form
aggregates with sizes which much exceed the optimum size desired for the purpose of our
proposal. A method for reducing the aggregate sizes is proposed by Kratel et al. (1976) in
U.S. Patent 3 953 487. It includes reaction of pyrogenically obtained silica with organo-silicon
or ornano-silane compounds and water vapors in a ball mill. Such treatment of silica fume
makes surface of the nano-particles hydrophobic. An organo-silane cross-linking process is
used widely in industry in manufacturing of cable isolations and pipes of small diameter from
polyolefins, e.g. Penfold et al. (2000) in U.S. Patent 6 048 935. We can expect therefore that
with proper selection of organo-silanes, e.g. vinyltrimethoxysilane (VTMOS) and
vinyltriethoxysilane (VTEOS), the silica particles with modified surface would do not
agglomerate and can reinforce the polymer matrix by chemical bonding between the silica
surface and PE molecules.
   A considerable number of patents are dedicated to removing of bubbles and reinforcement
of the molded shapes. We can categorize them roughly into three groups:
- Sizing of a particulates to improve densification of melt
     Maziers (2005) mentioned in U.S. Patent Appl. 20050255264 that micro-pellets exhibit a
     better bubble removal as a function of temperature than do powders. Swain (2004)
     proposed in U.S. Patent 6 682 685 to use resin pellets and from 20 to 40% by weight of
     ground resin powder to yield rotomolded objects having enhanced impact strength,
     Environmental Stress Crack Resistance (ESCR) and other strength characteristics.
- Processing and nucleating additives
     Rotational molding compositions normally contain heat and/or light and/or UV stabilizers
     for the olefin polymers. Whereas an unstabilized resin-based system provides a molded
     object having inferior physical properties but which is free of surface pinholes, the
     stabilized system provides a molded object which has vastly improved physical
     properties, but which has surface pinholes and volume voids. It was proposed by Sowa
     (1976) in U.S. Patent 3 974 114 to use about from 0.01 to 0.04 per cent by weight of the



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       fatty acid salt such as calcium stearate and lithium stearate per 100 parts by weight of
       olefin polymer-based rotational molding compositions to provide rotational moldings with
       pinhole-free surfaces and void-free product. Needham (1987) proposed in U.S. Patent 4
       668 461 addition of zinc stearate into rotational molding compositions containing cross-
       linking agents to prevent the formation of voids in the final rotationally molded article.
       He also proposed in U.S. Patent 5 530 055 (1996) to use nucleating agents, like calcium
       carbonate; zinc oxide; pigments such as carbon black and titanium dioxide to improve
       resistance of rotomolded articles such as rotomolded containers to puncture. The
       supposed nucleating agents should have a particle size in the range of from about 0.05
       to 20 microns, preferably from about 0.1 to 10 microns and they have to be
       homogeneously distributed in the melt of PE, e.g. by a compounding extruder.
  - Densification aids
       Densification aids were proposed recently by Maziers (2008) in U.S. Patent Appl.
       20080018019 and U.S. Patent Appl. 20080103264. One densification aid comprises
       fluoropolymer as a major component and a minor component selected from the group
       consisting of a polyether-block copolymide, a thermoplastic polyurethane, a
       polyetherester and Polyethylene Glycol. Other densification aid comprises a
       polyetherester, optionally consisting essentially of a mixture of a polyetherester as major
       component with a minor component selected from the group consisting of polyether-
       block co-polyamide, thermoplastic polyurethane, Polyethylene Glycol (PEG) and
       fluoropolymer. As for the polyetheresters, these are copolymers having polyester blocks
       and polyether blocks. They generally made of soft polyether blocks, which are the
       residues of polyetherdiols, and of hard segments (polyester blocks), which usually result
       from the reaction of at least one dicarboxylic acid with at least one chain-extending short
       diol unit. The PEGs in two referred inventions are preferably selected with an average
       MW from 150 to 700 Da.
     We propose here novel formulations of a hydrophilic densification aid, blending of the
  densification aid with PE resins in an extruder and manufacturing of micro-pellets for
  rotomolding. Underwater pelletizing is used in industry nowadays to produce micro-pellets.
  Instead of underwater pelletizing we propose extrusion at reduced temperatures and
  pelletizing of strangle in air or gas atmosphere. Amazingly, the same formulations of the
  densification aid work as Polymer Processing Additives (PPA) for extrusion. Micro-pellets
  produced with novel PPA manifest better flow than industrial grades of powders for
  rotomolding. The idea of such improvement did not come to mind at once and the
  experiments described below show steps in development of the proposal.

Experiments
      In series of experiments on sintering of PE particulates we used coarse powder of LLDPE
  (LL6301RQ, melting point 125 °C, MI=5.0 g/10 min) from ExxonMobil Chemical and a grade
  of LMDPE (M9001RW, MI=3.2 g/10 min) for rotomolding from the SCG Chemicals. A fraction
  of fine particles with sizes below 350 µm was separated from the LL6301 grade for some
  experiments by sieving the coarse powder of LLDPE. To mix PE powders with additives we
  used as a blender a meat grinder equipped with a heater and a thermostat to keep the metal
  body of the grinder at temperatures in range from 80 to 90 ºC. We used as additives vinyl-
  silane "Geniosil X10" (VTMOS) from Wacker Chemie, glycerol mono-stearate (GMS),
  polyethylene glycols (PEG): PEG 200, PEG 400, PEG 2000, PEG 6000. We also used citric acid
  from Aestar and fumed silica Aerosil 300 from Degussa as components of the additive batch.
  Fumed silica Aerosil 300 is characterized by extremely small sizes of silica particles (10 nm)



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while the particles are agglomerated in larger clusters. To facilitate mixing of the additives
with the PE powders we added some amount of methanol to the blender. In the process of
mixing the methanol was evaporated. To sinter particulates we used a simple mold that was
assembled from a 3 mm steel plate and a thick (about 15 mm) steel ring at the plate with an
inner diameter of 50 mm. Samples in the amount of 5 g were loaded to the mold and gently
compacted by a disk from PMMA to ensure equal thickness of the polymer material in the
mold. A digital camera equipped with a magnifying objective lens was arranged at the top
side of the mold to record process of sintering of PE particulates under magnification. Some
of the recorded photos are presented in figures from FIG 1 to FIG 4 and arranged in vertical
rows for comparison. Width of the photos corresponds to about 6.8 mm of the mold inside in
its central area.
    In experiments on extrusion of PE resins to demonstrate an opportunity to produce micro-
pellets with novel Polymer Processing Additives (PPA) we used a screw-extruder from
Extrudex. A barrel of the extruder is made with 4 shallow grooves in its feeding zone. The
grooves are 8 mm width and the depth of them is made with gradual slope to zero
thicknesses in the direction of a conveying of polymer material. It is well known that
extruders with axial grooves in the feeding zone of the barrel produce stable conveying of
pellets and powders, see Gruenschloss (2003) and Rauwendaal (1986). A feeding zone of the
extruder is cooled by water to room temperature. All parts of the extruder that are in contact
with molten polymer are produced from a steel alloy (34CrAlNi7) and nitrided, that is
saturated by Nitrogen to harden a thin (about 0.1 mm) surface layer of the parts. The die for
extrusion is made with diameter 2 mm and length 60 mm also from this nitrided steel. The 2
mm hole of the die is conjugated with a 50º cone having 8 mm diameter at the entrance of
the die. To produce some amount of micro-pellets we used the same screw extruder but the
die was replaced.
    Narrow MW distribution of PE resin is important for rotomolding and therefore we used for
our extrusion experiments the grades of LLDPE from ExxonMobil Chemicals with such MW
distribution: LL1201 XV (density 0.925 g per cc, melting point 123 °C and MI=0.7 g/10 min)
and LL 1001 XV (0.918 g per cc, melting point 120 °C and MI=1 g/10 min). We also used for
comparison LDPE with wide MW distribution from the ExxonMobil Chemicals: LL166 BA
(0.923 g per cc, melting point 110 °C and MI=0.2 g/10 min).
    An induction oven with maximum electrical power of 1.8 kW was used to heat the steel
mold to controlled temperature in open air. Heating was going in two stages: first, preheating
of the mold to 70 ºC for 5 min and, second, a main sintering period with the temperature of
the bottom plate of the mold rising up to 230 ºC. The steel ring was coated by a thin layer of
silicone rubber to ensure easy separation of polymer from the mold. The same ring was used
after cooling to room temperature in next experiments. The bottom plate in our experiments
was not coated and therefore we replaced the used plate by an identical one for every
experiment. The used plates were cleaned from molten polyethylene and heated overnight in
an electrical oven to temperatures up to 400 ºC to burn out organic residues.
    First, we made experiments on sintering of LLDPE (LL6301RQ) powders from ExxonMobil
Chemicals. In sintering of coarse LLDPE powder a lot of bubbles of various sizes are trapped
inside the melt. While small bubbles dissolve and disappear, a number of larger bubbles stay
inside the melt after 20 min of heating and longer. In contrast to the coarse powder, in
melting of fine powders of LLDPE with particles of sizes below 350 µm even a greater
number of bubbles is present soon after merging of the PE particles but the bubbles mostly
dissolve and disappear in about 15 min of heating to high temperatures, see FIG 1A. In
further experiments we used only fine powders.



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    Glycerol monostearate (GMS) is known as a densification and processing aid to accelerate
fusion of LLDPE powder. In our experiments, additives of GMS indeed show some
improvement in sintering of the blend of LLDPE powders with 0.2 weight % of GMS. In 9 min
of heating of the LLDPE particulate we observe approximately two times fewer bubbles
trapped inside the melt, see FIG 1B. Surprisingly, additives of polyethylene glycol (PEG 400)
at the same concentration 0.2 % show a sharp reduction in the number of bubbles as well as
a shortening in a sintering time about 30%, compare photos in FIG 1A and FIG 1C. The
observed reduction of amount of bubbles in the melt can be explained by very high
permeability of PEG to water vapors and Oxygen. So, these gasses may escape from the melt
along the boundaries coated by PEG. Sintering of PE powder is accelerated further at higher
concentrations of PEG. The optical refraction index of PEG is higher in comparison with
LLDPE and therefore we can distinguish in photos of high resolution macroscopic beads of
PEG inside the melt, see FIG 1C. It seems so that the PEG evenly distributed among the
LLDPE powder coalesces under heating to a few large beads inside the melt. In FIG 1C a
white circle shows location of one large inclusion of PEG inside the melt. Macroscopic
inclusions of foreign material inside the polymer matrix are not desirable. It was also
detected that PEG seriously impedes flowability of the PE powder if it is used in
concentrations above 0.2 %.
    A sintering of fine particles of polyethylene with additives of hydrophilic fumed silica in
concentrations from 100 to 1000 ppm occurs in a distinctively different manner from the
sintering of pure LLDPE-powder, see FIG 2A. In addition to bubbles of trapped gasses we
observe a dense network of channels (gaps between the PE grains) inside the melt. Both the
bubbles and channels disappear in time and leave macroscopically large clusters of silica
fume inside the melt. In general, additives of hydrophilic silica fume delay sintering of the PE
powder and the higher the concentrations of silica fume the further is the delay. If mineral
particles of submicron and micron sizes, e.g. a fly ash or bentonite, are blended with PE
powder we observe long delays in fusion of the PE-grains. The powder of silica fume can be
used together with PEG to improve free-flow of the blend. With the use of such combination
sintering times are short and we do not observe large beads of PEG inside the melt. Yet,
silica fume is present inside the melt as large clusters, see also about the use of combination
of PEG and silica fume below.
    It is known that nano-particles of hydrophilic silica fume attract each other by hydrogen
forces and agglomerate to macroscopic clusters but if fumed silica is treated by organo-
silanes it gets a hydrophobic surface and the particles are less prone to agglomeration. We
can expect that with the use of the treated silica fume the gaps between merging grains of
LLDPE would be narrower, while nano-particles themselves are better embedded into the
molten polyethylene. In FIG 2B we see a time sequence of sintering of the LLDPE powder
with additives of silica fume and vinyl-silane (VTMOS, GenioSil XL10). The additives were
blended with powder of LLDPE at about 100 ºC in a ball mill to reduce the aggregate sizes of
silica fume. In this case, we observe that after 9 min of heating the number of bubbles of
trapped gasses in the melt is considerably fewer in comparison with reference experiments
without additives. So, we can speculate that the fumed silica separates PE grains and delays
fusion of them so that trapped gasses can escape from the melt through the gaps between
the PE grains. With the use of silica fume treated by VTMOS the gaps are thinner than with
not treated silica fume and the delay times are negligible. In fact, we observe acceleration in
sintering of PE grains if they are blended in a ball mill with vinyl-silane and silica fume.
Combination of three components: fumed silica, VTMOS and PEG shows short sintering times
as well as a quick removal of trapped gasses from the melt. In contrast to GMS and PEG, a



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sole addition of organo-silane (VTMOS) at the same concentration shows neither fewer
bubbles nor shorter sintering times, see FIG 2C.
    In experiments on sintering of LMDPE grade (M9001RW) for rotomolding from SCG
Chemicals we observe a large number of bubbles trapped inside the melt, see FIG 3A. These
bubbles are larger in size in comparison with the case of fine powders of LLDPE powder from
ExxonMobil and they do not disappear after long heating of the melt. Fine powders of
amorphous silica are known in industry as a flow-improving agent. Flow of LMDPE powders
can be improved by additives of fumed silica: The angle of free repose of the LMDPE powder
without any additives is 45° and it is reduced to 42.5° by the additives of silica fume (800
ppm). The angle of repose was measured by pouring hot powders onto a horizontal surface
so that they formed a conical pile. The angle between the surface of the pile and the
horizontal surface is the angle of repose. In our experiments it was measured from photos of
the piles. When hydrophilic silica fume is added to the LMDPE powder it visually reduces an
amount of bubbles in the melt after 7 min of heating but the gaps between PE grains
collapse slowly. So, after 10 min of heating the amount of the trapped gasses seems to be
unchanged in comparison with the reference experiment, see FIG 3B. We can clearly see in
FIG 3C that additives of PEG 6000 to the LMDPE powder in the concentration 0.4 % shorten
duration of sintering and nearly eliminate bubbles inside the melt similar to additives of PEG
to fine LLDPE powders. Similar to the experiments with LLDPE we can distinguish at photos
in FIG 3C macroscopic beads of PEG trapped inside the melt. White circles in FIG 3C show
positions some of them. Viscosity of PEG 6000 is higher and these inclusions are of smaller
size in comparison with the use of PEG 400.
    Particles of LMDPE are characterized by elongated shape and jagged surface and therefore
they resist to flow. Additives of PEG impede flow of LMDPE powders further. Flow of LMDPE
powder with additives of PEG can be improved by silica fume. For example, the angle of free
repose of the LMDPE powder with 0.4 % of PEG is reduced from 72° to 60° by the additives
of silica fume (800 ppm). Such amount of silica fume delays slightly sintering of PE powder
as we can see from comparison of FIG 3C and FIG 4A but opposite to the use of PEG alone
we do not observe macroscopic inclusions of PEG inside the melt. PEG reacts with citric acid
with release of water under heating above 140 °C. If PEG with MW below 4000 is used and
weights of citric acid are close to or above stoichiometry the produced ester-condensate at
room temperature is a soft and elastic substance of an amber color. When placed to water it
swells but does not loose integrity for days. We used a mixture of PEG 6000 with citric acid
(ca, 0.64%) as densification aid. This amount of citric acid is below stoichiometry. We see
from FIG 4B that sintering of the LMDPE powder with additives of the reacting mixture of
PEG + ca goes virtually without bubbles. Opposite to the use of PEG 6000 alone we cannot
distinguish macroscopic beads of PEG inside the melt. Silica fume can be used as a
component of the additive package to improve flowability of the LMDPE powder with the
additives of the mixture PEG + ca. As it is mentioned above, silica particles delay merging of
PE grains but, surprisingly, in FIG 4C we can see that for the powder with the reacting
mixture PEG + ca the delay is negligible even at relatively high concentrations of fumed silica
(800 pm).
    In extrusion experiments at first we compared extrusion of LLDPE with and without PPA at
various temperatures: 130, 135, 145, 165, 185, 205, 225, 235 ºC. Characteristic curves that
are the curves of Pressure at the extrusion die versus Extrusion Rate (linear velocity of the
extrudate derived from volumetric extrusion rate) for extrusion trials of LL1201 XV without
PPA are presented in FIG 5A for two temperatures: 145 (a solid line) and 225 ºC (a dashed
line). The best-fit curves connect experimental data: symbols of open circles (145 ºC) and



                                                                                            7
open squares (225 ºC). At the characteristic curve for extrusion at 145 ºC an onset of
sharkskin (at 2 mm/s) and stick-slip (at 27 mm/s) instabilities are marked by arrows. For
extrusion at higher temperatures onsets of surface instabilities are delayed. However, for
LLDPE with narrow MW distributions the sharkskin instability appears at low rate of extrusion
(below 6 mm/s for LL1201 XV) even at temperatures as high as 225 ºC. The sharkskin
instability greatly disturbs surface of the extrudate and therefore manufacturing of pellets for
rotomolding with smooth surface from LLDPE with narrow MW distribution is hardly possible.
The instabilities can be eliminated by the use of recently proposed PPAs made from a blend
of PEG with silica fume, see Kulikov et al. (2009). For extrusion experiments with such PPA
we mixed under heating PEG 2000 with silica fume (1 %) and added 5 g of this PPA per 1 kg
of the LLDPE pellets. Extrusion trials were made at various temperatures, see above. In the
range of extrusion rates from 2 to 45 mm/s and for temperatures below 185 ºC the
measured pressures manifested little variations versus temperature. Surprisingly, we
observed an increase of the extrusion pressures at temperatures from 185 to 235 ºC.
Characteristic curves of extrusion of LLDPE with PPAs for two temperatures: 145 (a solid line)
and 225 ºC (a dashed line) are presented in FIG 5A. The best-fit curves connect experimental
data: symbols of solid circles (145 ºC) and solid squares (225 ºC). We can see from
comparison of the curves that pressures at the extrusion die can be considerably less for
reduced temperatures of extrusion than for elevated temperatures if LLDPE is blended with
PPA. This result is very opposite to extrusion of neat LLDPE where apparent viscosity drops at
elevated temperatures.
   To compare an impact of PPA on extrusion of PE with wide and narrow MW distribution
we used a blend of PEG 2000 with 1 % of silica fume as PPA for extrusion of LDPE (LD
166BA) and LLDPE (LL 1001 XV) in concentration 0.5 % at the temperature 165 ºC and for
extrusion rates from about 4 to 100 mm/s. For comparison we extruded these PE resins also
without PPA. Characteristic flow curves that are the curves of Pressure at the extrusion die
versus Extrusion Rate are presented in FIG 5B. The best-fit curves connect experimental
points for extrusion of PE resins without PPA: a solid line with symbols of open circles for
LLDPE and a dashed line with open squares for LDPE. For extrusion of PE resins with PPA the
best fit curves connect experimental points: a solid line with symbols of solid circles for
LLDPE and a dashed line with solid squares for LDPE. At the characteristic curve for extrusion
of LLDPE without PPA an onset of sharkskin (at 4 mm/s) and stick-slip (at 52 mm/s)
instabilities are marked by arrows. Opposite to extrusion of neat LLDPE, extrusion of LLDPE
with PPA goes stable at extrusion rates up to 80 mm/s where so called elastic instability or
"gross melt fracture" occurs. While extrusion of LLDPE is greatly improved by the PPA and
pressures at the extrusion die are reduced 4-5 times in comparison with extrusion of LLDPE
without PPA there is only marginal pressure reduction for extrusion of LDPE with PPA.
   In one set of experiments we used a reacting mixture of PEG 2000 with 2 % of citric acid
as PPA in the concentration 0.2 % for extrusion of LLDPE (LL1201 XV) and compared it with
other processing additives. For comparison we used also PEG 6000, PEG 2000, and Viton that
is a standard Processing Additive in industry to improve extrusion of LLDPE with narrow MW
distribution. First, we extruded 1 kg of PE pellets with every one of the additives and then
purged the extruder to see how quick it can be cleaned from the additives. The curves of
Pressure Reduction vs. Extrusion Time are presented in FIG 5C for extrusion at 165 ºC and at
Extrusion Rate (averaged extrudate velocity) about 40 mm/s. We start the time counting at
the moment when LLDPE with PPA is loaded to the hopper of the extruder. A vertical line
separates a stage of conditioning of the die and a stage of purging by neat LLDPE when neat
LLDPE is loaded to the extruder. We observed suppression of sharkskin when Pressure



                                                                                             8
  Reduction gets above 20%. Dead time in FIG 5C corresponds to the time to convey the
  polymer material through the extruder. The conditioning time by definition is the time
  duration that is necessary to suppress sharkskin since the PE resin with PPA starts to flow
  through the die. In FIG 5C we can see that a composition of the additive comprising PEG
  2000 and citric acid (PEG2K+ca) shows best pressure reduction at the extrusion die as well
  as a shortest conditioning time in comparison with other additives. It seems so that catalytic
  properties of the extrusion die are important and we observed much longer conditioning
  times for extrusion with the die made from stainless steel. It appears also that silica fume in
  amount about 1 % in the composition of PPA helps to shorten conditioning times and to get
  good lubrication at the die. We observed a reduction in these performances if no silica fume
  or concentrations of silica fume different from 1 % were used to prepare the PPA. Without
  silica fume in the composition of PPA curling of the extrudate of small diameters, e.g. 0.5
  mm, may happen so that it leaves the die shaped like a helix. Details of our extrusion
  experiments and manufacturing of micro-pellets we are going to publish elsewhere.
      Pelletizing of LLDPE material in air without PEG additives is hardly possible at low
  temperature of extrudate, e.g. 138 ºC, not only because of high pressures at the die but also
  because of pronounced die drool and accumulation of molten PE resin at the die exit. PE melt
  does not stick to the metal surface wetted by PEG while the low viscous PEG lubricates a
  rotating knife. If pellets are cut by the rotating knife at higher temperatures, e.g. 165 ºC,
  they stick together. Therefore pelletizing with air cooling would be not possible at such
  temperatures. Extrusion of LLDPE with additives of low viscous PEG, e.g. PEG 2000, at
  temperatures below 130 ºC cannot be recommended too as the extrudate surface gets
  rough, friction losses at the die oscillate in time while the averaged pressure is increasing.
  With the use of PEG 8000 extrusion goes stable down to temperatures as low as 128 ºC but
  at higher pressures at the die as compared to the use of PEG 2000.
      To demonstrate an opportunity to produce micro-pellets by extrusion at reduced
  temperatures and pelletizing in open air we used as a die set a pack of 7 injection needles
  with inner diameter about 0.5 mm and 20 mm length soldered by silver alloy inside a steel
  housing. We used no additional cooling in the process but convection of air. LLDPE (LL1201
  XV) was blended inside a screw extruder with additives (0.5 wt. %) of a reacting blend of
  PEG 8000 and 1.6 wt. % of citric acid. We started extrusion at 165 ºC and after a
  conditioning time about one hour that is necessary to get stable lubrication at the die we
  reduced gradually a temperature of the die. We used a rotating knife from a meat grinder to
  cut strangle to pieces at averaged melt velocity about 22 mm/s. The produced micro-pellets
  do not stick to each other at temperatures from 130 to 135 ºC. With this simple appliance it
  is possible to cut micro-pellets as cylinders or disk-like pellets of oval shape with thickness
  about half of the diameter by varying the rotation speeds of the knife. Appearance of the
  micro-pellets is presented in FIG 5D. The produced micro-pellets demonstrate good flow
  properties (an angle of free repose is about 40°) and obviously can be used for rotomolding
  or flood feeding of an extruder.

Discussion
     PEGs are slightly dissolvable in PE resins at high temperatures. They reduce surface
  tension and viscosity of molten PE and the additives of PEG are already known in industry as
  an agent to improve cohesive strength of a weld joint in hot sealing of PE films. So, it is no
  wonder that additives of PEG accelerate sintering of the PE powders but mechanism of the
  PE melt densification with additives of PEG is not yet understood. We can tentatively explain
  the observed improvements by high permeability of PEG to Oxygen and water vapors. To our



                                                                                              9
knowledge a systematic research on gas permeability of PEG and PEG-based esters is missing
but from the data in literature, e.g. Metz (2003), Massey (2003), Lin and Freeman (2004), we
can derive that diffusion of water vapors and Oxygen through PEG is much better in
comparison to PE resins. Zhang and Cloud (2006) proposed a qualitative mechanism of gas
permeability of silicon rubber: thermal motion of long and flexible siloxane molecules
provides "openings" in silicones for the gas molecules which permit diffusion of gasses.
Chemical structure of PEG molecules resembles molecules of siloxanes and gasses can diffuse
through the PEG and PEG-based esters in a similar way. In fact, hydrophilic properties of PEG
make diffusion of water vapors through the bulk of PEG and PEG-based esters much easier in
comparison with silicone rubbers.
   PEGs with molecular weights from 200 to 10 000 Dalton can react under heating with
carboxylic acids and anhydrides of the carboxylic acids, e.g.: stearic acid, oxalic acid, adipic
acid, citric acid, maleic anhydride, phthalic anhydride, etc., see Bauer et al. (2000) in U.S.
Patent 6 048 937. Reactivity is higher with lower MW PEGs, stronger acids and with
anhydrides. If weights of the curing agents are selected close to stoichiometry and above it
and the mixture has in average more than two reacting groups per molecule, the ester-
condensate can be a thermoset. It is clear that such reacting mixtures of PEG with carboxylic
acids (CA) and anhydrides gain higher viscosity in time of heating and if they are used as an
additive package for rotomolding the additive is less prone to coalescence to large beads
inside the melt in comparison with neat PEG. In our experiments the amount of citric acid
(MW=192.1) was below stoichiometry and we cannot count on sharp increase of viscosity.
So, another tentative explanation can be proposed that flow of the reacting mixtures under
forces of surface tension is disturbed by tiny bubbles of water vapors that appear in the
reaction of condensation and decomposition of citric acid at temperatures above 175 ºC. We
may think that foamed hydrophilic substance at the surface of PE grains keeps open channels
between the PE grains during heating so that gasses can escape from the voids.
   From our experimental observations we can conclude that blends of PEG with silica fume
as well as reacting mixtures PEG + CA do not coalesce into a few large beads like neat PEG
do but stay trapped inside the melt as small-size inclusions. We used citric acid (ca) as a
reactant but special mixtures can be used to facilitate formation of the inclusions of smaller
sizes and with core-shell structure. For example, stearic acid and stearates, e.g. glycerol
monostearate, can be added to the reacting mixture of PEG and citric acid as a surface-active
reagent to facilitate formation of the inclusions with core-shell structure where the core is
from hydrophilic visco-elastic material with low modulus while the shell is hydrophobic and
embedded into PE. Nano-particles with core-shell structures are well known for the use as
impact modifiers. So, opposite to large beads of PEG in PE matrix the small-size inclusions of
PEG + CA can even enlarge toughness and impact resistance of the rotomolded articles.
   Polyethylene is exposed during grinding to extreme shear stresses and to temperatures
close to its melting point in presence of Oxygen. If PE powders with high surface area are
stored in an air atmosphere for prolonged periods, oxidative degradation of PE molecules
would continue. Additionally, surface of grains of the PE powder is enriched by chemical
components of low molecular weights (MW) that migrate from the bulk of PE grains.
Presence of low MW PE and waxes significantly reduces entanglement of long PE molecules
at the boundaries of the sintered PE grains and causes local weakness of cohesive strength
of the polymer. We believe that the cohesive strength can be enlarged by cross-linking of PE
molecules across the grain boundaries and by anchoring nano-particles of silica fume in both
contacting grains of PE during the fusion process. In our experiments we added vinyl-silane
to the blend of the PE powder with silica fume in hope to get reinforcement of the



                                                                                             10
boundaries between the merging grains of PE. The additives of vinyl-silane do not improve
sintering of the PE grains if they are used alone. Therefore we believe that shorter time of
sintering of such blend in comparison with PE powder without any additives can be an
indication that the nano-particles of silica indeed get embedded into molten PE. To ensure
embedment of the silica particles in PE after rotomolding some amount of vinyl-silanes can
be used in the additive package for extrusion. The vinyl-silanes VTMOS and VTEOS dissolve
in PE resins and they would migrate in time to the surface of PE grains and react with silica
and water vapors. During rotomolding process the vinyl-silanes molecules that are grafted to
the surface of silica particles may create chemical bounding between the modified silica
particles and molecules of PE as well as cross-linking of the PE molecules in a boundary layer
of the merged grains of PE. Yet, direct measurements of impact strength to confirm
integration of silica particles to polymer matrix are missing and have to be done in future.
   Flow performances of the PE particulates are important to get uniform thickness of the
walls and to avoid "bridging" inside the mold between opposite walls and in the corners of
the rotomoded article. The use of PEG and reacting mixtures of PEG with carboxylic acids as
PPA impede flow of PE powders. Additionally, blending of the PE powders with additives is
costly. We propose that blending of PPA with PE resins has to be done inside an extruder for
production of micro-pellets. Micro-pellets of oval shape with sizes from 0.3 to 0.8 mm provide
superior handling and processing performances in rotomolding, see Beall (1998), Whatcott
(2008) as well as Crawford and Kearns (2003). Our experiments demonstrate that micro-
pellets can be produced by extrusion of PE resins at reduced temperatures. High fluidity of
micro-pellets is considered sometimes as a disadvantage of the material while flowability of
PE powders is below optimum. The proposed PPAs adjust flowability of micro-pellets in
proper direction that is reduce it. Yet micro-pellets with PPA show better flowability than PE
powder without PPAs.
   It is amazing that the same composition and concentration of additives used as a
densification aid in sintering of PE particulates helps to improve extrusion of the PE melt: to
suppress sharkskin and to reduce extrusion pressure. As a tentative mechanism of the
observed lubrication we can propose following: Commercially available LLDPE grades
comprise low MW organo-phosphites as antioxidant additives. The antioxidants deplete
Oxygen from the PE melt and get converted to organo-phosphates that are low MW esters of
alcohols and phosphoric acid. PEG reacts with these esters at the surface of the die so that
high MW esters appear in the reaction of trans-esterification. Reaction rates depend on
presence of catalysts. We used in our experiments the die that is made from a steel alloy
comprising Aluminum and Vanadium. These metals are good catalysts for a poly-
condensation and trans-esterification reactions. The high MW esters of PEG and phosphoric
acid have high affinity to metal surface and therefore they accumulate at the die surface and
form a layer of a plastic lubricant. With the use of the catalytic die we observe relatively short
conditioning times while with the dies having low catalytic properties, e.g. a Chromium
coated die, the conditioning times is much longer.
   The use of carboxylic acids in the composition of PPA for extrusion of PE resins with
narrow MW distribution allows to shorten conditioning times and to get better lubrication at
reduced temperatures of extrusion. Actually, inside an extruder the temperatures can be
higher (185 – 195 °C in our experiments) while the die is at the reduced temperatures.
Extrusion can go at temperatures of the die so close to the melting point of the PE resin that
the extrudate with cold surface can be pelletized with a rotating knife at the die exit in
conditions of air cooling. We observe that at reduced temperatures of extrusion the additives
are ejected from the extrudate to its surface. We have some indirect indications from our



                                                                                               11
experimental observations that if silica fume is suspended in PEG it moves to the extrudate
surface together with PEG. If pelletizing goes in air or gas atmosphere, these additives would
stay at the surface of the micro-pellets and therefore they can be reused as densification and
sintering enhancers for rotomolding.

Conclusions
    We present here first experimental results on the use of reacting mixtures of PEG with
citric acid (ca) as sintering and densification enhancers for rotational molding of PE
particulates. Sintering of PE grains with these additives goes virtually without bubbles of
trapped gasses and duration of heating of the mold in rotomolding can be 20-30 % shorter.
Therefore, higher productivity and better mechanical properties of the rotomolded articles
can be achieved. From our observations we can conclude that reacting mixtures PEG + ca do
not coalesce into a few large beads like neat PEG does but stay trapped inside the melt as
small-size inclusions.
    We demonstrated here that reacting mixtures of PEG with citric acid improve sintering of
PE powders but impede flow of them. So, it is not advisable to use them with PE powders.
Surprisingly, the same mixtures that are helpful to reduce sintering time of PE particulates do
work as Polymer Processing Additive (PPA) for extrusion of PE resins with narrow Molecular
Weight (MW) distributions. These PPAs suppress extrusion instabilities, reduce extrusion
pressure 2-5 times and allow extrusion at temperatures close to solidification points of the PE
resins with narrow MW distributions. Extrusion at so low temperatures allows manufacturing
of micro-pellets from the PE resins with sizes less than 0.8 mm by cutting the extrudate with
a rotary knife at the die exit in conditions of air-cooling. Flow of micro-pellets produced with
novel PPAs is better than flow of the powder from the same PE resin without the additives.
Rotomolding experiments with such micro-pellets we reserve for next publications.
    The use of micro-pellets for rotational molding is already cost efficient in comparison to
the use of PE powders. It seems so that cost of micro-pellets can be reduced by pelletizing in
air with proposed PPAs while the rotomolded articles can be produced in shorter times and of
better quality. More experimental and analytical investigations are required to prove better
Impact Strength and Environmental Stress Crack Resistance of the rotomolded articles made
from micro-pellets and novel PPA as well as to get deeper insight in the mechanisms of the
observed improvement of sintering and extrusion of PE resins. However, we believe that
manufacturing of micro-pellets with proposed PPAs from PE resins in conditions of air cooling
and the use of these micro-pellets for rotomolding can be a quantum leap in technology of
rotomolding. Patent pending.

Acknowledgements
   We would like to thank Dr. Winyu Tanthapanichakoon and Dr. Chris Rauwendaal for
stimulating discussions. We thank gratefully ExxonMobil Chemicals and SCG Chemicals for
presenting us Polyethylene resins for our experiments. Financial support by the German
Science Foundation (Deutsche Forschungsgemeinschaft) is gratefully acknowledged.

Figure captures
FIG 1. A: Sequence of photos showing sintering of fine LLDPE-powder. B: Sequence of
       photos showing sintering of fine LLDPE-powder with additives of Glycerol Mono-
       Stearate (0.2%). C: Sequence of photos showing sintering of fine LLDPE-powder with
       additives of PEG 400 (0.2%).




                                                                                             12
FIG 2. A: Sequence of photos showing sintering of fine LLDPE-powder with additives of silica
       fume (400 ppm). B: Sequence of photos showing sintering of fine LLDPE-powder with
       additives of silica fume (132 ppm) and of VTMOS GenioSil XL10 (250 ppm). C:
       Sequence of photos showing sintering of fine LLDPE-powder with additives of VTMOS
       GenioSil XL10 (0.2%).

FIG 3: A: Sequence of photos showing sintering of fine MDPE-powder without any additives.
       B: Sequence of photos showing sintering of fine MDPE-powder with additives of silica
       fume (800 ppm). C: Sequence of photos showing sintering of fine MDPE-powder with
       additives of PEG 6000 (0.4 %).

FIG 4. A: Sequence of photos showing sintering of fine MDPE-powder with additives of PEG
       6000 (0.4 %) and silica fume (800 ppm). B: Sequence of photos showing sintering of
       fine MDPE-powder with additives of PEG 6000 (0.4 %) and Citric Acid (26 ppm). C:
       Sequence of photos showing sintering of fine MDPE-powder with additives of PEG
       6000 (0.4 %), silica fume (800 ppm) and Citric Acid (26 ppm).

FIG 5. A: Characteristic Flow Curves (Pressure vs. Extrusion Rate that is averaged extrudate
       velocity) for extrusion of LLDPE (LL1201 XV) with (0.5 %) and without Polymer
       Processing Additives (PPA, PEG 2000 + 1% of Silica Fume) at two temperatures: 145
       and 225 °C. Onsets of Sharkskin and Stick-slip instabilities are marked by arrows. B:
       Characteristic Flow Curves for extrusion of LLDPE (LL1001 XV) and LDPE (LD166 BA)
       with (0.5 %) and without PPA (PEG 2000 + 1% of Silica Fume) at 165 °C. Onsets of
       Sharkskin and Stick-slip instabilities are marked by arrows. C:. Curves of Pressure
       Reduction vs. Extrusion Time for extrusion of LLDPE (LL1201 XV) at 165 °C and
       Extrusion Rate about 40 mm/s with various PPAs: Viton, PEG 6000, PEG 2000, PEG
       2000 + 1% of citric acid. A vertical line separates a stage of conditioning of the die
       and a stage of purging by neat LLDPE. D: Micro-pellets produced by extrusion at 132
       °C of LLDPE with melting point 125 °C in conditions of air-cooling.




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