Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: FOAMED ROTATIONALLY MOLDED ARTICLES
TECHNICAL FIELD
This invention relates generally to improved rotationally molded articles and
methods for producing foamed rotationaUy molded articles having illlplov~d ceU
structure. More specifically this invention relates to polymer micro-pellets t_at
10 incorporate a blowing agent into the micro-pellets to facilitate the manufacture of
rotationally molded articles having a foamed layer.
BACKGRO~JND
Rotationally molded articles c(~ p a foamed layer are known. Typically
15 such rotationally molded articles include at least one foam layer and at least one other
thermoplastic layer. GeneraUy, rotationaUy molded parts made utili7ing an outer skin
of a thermoplastic and at least one foamed layer, preferably on the interior of the first
thermoplastic layer, have found commercial acc~t~lce in such diverse articles as surf
boards, wind surfing boards, insulated tanks for chemicals and potable water; children's
20 toys, canoes, boats, m~t~.ri~l h~n(llin~ boxes (refrigerated and non refrigerated) and the
like. Such articles generally employ at least one foamed layer to improve insulation,
hll~rclv~; buoyancy, increase ~tiffn~s~, or a combination of these properties, or for other
purposes known to those of ordinary skill in the art.
A number of techniques have been suggested to achieve such foamed
25 rotationally molded articles. Among these suggested techniques are: utili~ing a well
stabilized outer-layer and a less or minim~lly stabilized inner-layer. Such a structure
may provide a stabilized outer-layer to resist ultraviolet light degradation, heat
degradation, oxygen degradation, and the like, and an inner-layer where the lower or
minimum stabilized layer may be oxidized during heating (roto-molding) to assist the
30 adhesion of a subsequently applied foam, for instance polyurethane or polystyrene. A
method known in the art for achieving a similar structure is roto-molding a feed of a
fine particle size and a coarse particle size where the fine particle size generally melts
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first and then the larger particle size melts second or next, providing again inner and
outer layers of diLr~lellLly stabilized polyolefins .
Another approach known in the art for producing rotationally molded articles
having a foamed layer, incl~ldes using relatively small polymer particles cnnt~ining no
S blowing agent and relatively large polymer particles cont~ining a blowing agent.
Another method known in the art, is to rotationally mold a first polyolefin charge and
after it has been substantially melted or soft,ened into the shape of the mold, a drop box
(an intern~l box or boxes cnnli.illi,~g a second or sequenti~l m~tt-ri~l which is inside the
mold cavity and substantially insulated from the heat of the mold cavity) is used to
drop a second charge. This second charge may, for in~t~n~e~ contain a polyolefin or
another thermoplastic with a blowing agent, optionally a second drop box may be used
for adding a third layer.
Problems associated with foamed rotationally molded articles are found in
substantially all of these suggested approaches. Such problems include, but are not
limited to, first; ~ min~tion of a polyurethane, poly~.Ly~ e, or thermoplastic foams
from a rotationally molded article interior (e.g. the inside of the outer-layer of the part)
causing either product failure or poor performance. Second; when such foamed
structures contain two or more polymers, substantial limitations on recyclability may
exist due to the dual polymer nature, especially where one of the polymers is, for
instance, a thermoset. Third; the large particle, small particle combinations, as well as
the drop box technology, present the rotational molder with complex and sometimes
lengthy molding operations. Fourth; long known in t'ne art, and a major industrial
concern of those skilled in the art is that foams usually display generally uneven or
wider margins of foam cell structure. Such varying cell structure, where for instance,
there are areas of fine micro-cells, and areas of medium andlor large cells, and areas of
large voids,can lead to poor performance. Such poor performance may be manifested
in poor appearance or in poor end use performance, or both. For instance causinglower insulation in some areas than others, areas of poor or differing physical
properties (such as impact), and the like.
Further problems known in t'ne art are evidenced with a
thermoplastic/polyuret'nane or t'nermoplastic/polystyrene foamed rotationally molded
article. These problems include environmental concerns wit'n blowing agents
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frequently used in polyurethane foam. ~ ition~11y, polyurethane is often not a closed
cell foam, once d~ ..;..AI;on occurs any holes in the skin layer can therefore render the
structure generally non-functional, for in~t~nr~, in applications ~Itili7ing buoyancy.
In the two or three charge, drop-box terhniq~le~ problems are based on, for
5 in~t~nr~, long cycle times required, because the primary, secondary, and further layers
must be molded subst~nti~lly sequentially. ~ lition~l haldwalt; is required for the
rotational molder to make and use the drop box technology. The process is generally
complex to operate, especially when one considers that a very low tolerance to
leakage from the drop boxes is generally nrcee~ry due to the fact that such le~k~ge
10 may likely lead to discontin--ities in one or more layers. The drop box process is
difficult to Oplillli ~e because the timing on opening of the drop box or boxes is critical.
Poor ullirc,llllily of cell structure may also result.
The third technique known in the art, wherein a powder or small particle size
polymer with little or subst~nti~lly no blowing agent, is charged to the mold at the
15 same time that a larger particle size polymer with a chemical blowing agent melt
compounded into the polymer. Such technology depends upon the polymer co.~ il-g
the blowing agent having characteristics which delay its melting or "laying down"
during the process. Characteristics known to those of ordinary skill in the art which
would permit this delay in melting or "laying down" are typically particle size and/or
20 density. Difficulties with this old method again, are poor cell structure and/or
ulliro~ y and poor surface appearance characterized by the lack of skin continuity.
There is a long felt commercial need, therefore, for a foamed thermoplastic
rotationally molded part and a method for producing such a part which will have
improved recyclability, improved process operability, lower levels of rotational2~ molding operational complexity, relatively consistent uniform closed cell structure and
size, and a miniml-m of del~min~tion between a foam layer and other layers.
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SUMMARY
What we have discuveled is that colllpounding and/or mixing a thermoplastic
with a blowing agent and r~,lllli"g micro-pellets of the colllbinalion~ can provide
advantages in the roto-molding process and articles made thelerlunl. The advantages
include PYc~ nt flowability of the micro-pellets in a rotomold, a less complicated
roto-molfling process, and a molded part having at least one foamed layer that has a
high percentage of relatively small, unir~Jllll, subst~nti~lly closed cells. This last
advantage leads to a generally lower range of variability of the physical properties of
the molded part, such as insulation value, and impact strength. We have thus found
that the above ~ c~lssed disadvantages associated with prior solutions to the problem
of obtaining a rotationally molded article having a foamed layer can be generaliy solved
by the articles and methods of various embodim~nt~ of our invention.
The term micro-pellets as used in the present speçifir~ti-~n and claims means a
pellet or particle of a thermoplastic which may have any shape, discs and cylinders are
plere.led. Such discs or cylinders will have rli~meters generally in the range of 250 to
750 llm. The plert;lled shapes may also include for in~t~nce oblong, spheroidal, and
the like. It will be understood by those of Ol dinaly skill in the art that generally any
shape will be acceptable with the underst~nt1ing that the volume of such particle shapes
be subst~nti~lly similar to the volume of a sphere described by the above ~ met~r
ranges. Further, the shape and size of the pellet or particle should generally be such
that its flowability in a rotational molding operation should be effective to permit flow
into subst~nti~lly all intricacies of a given mold to form a generally continuous
rotomolded surface.
The micro-pellets will optimally include a blowing agent, the blowing agent
may be partially decomposed leading to some cell formation in the micro-pellets. The
blowing agent will be present in the range of from 0.1 to 3 parts per hundred parts
resin (thermoplastic).
In variations of one of our embo-lim~nt~ a thermoplastic may be melt mixed
with the chemical blowing agent and then formed into micro-pellets.
In another variation the micro-pellets may be utilized in a process for producing
a molded part, preferably a rotationally molded part, where the process inçl~-dçs:
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s
a) charging a plurality of micro-pellets to a mold;
b) rotating the mold on at least one axis;
c) heating the micro-pellets to a temperature effective to produce a
molded part, the molded part will include at least one layer of thermoplastic foam
where the foam will have a density in the range of from 16 to 880 kg/m3.
The foamed layer will adv~nt~g~ously have a cell size that will be generally
small and the foam will have good cell ulfirollllily~ The cells will have an average size,
depending upon density, in the range of from 10 to 1400 ~m, and advantageously more
than 70% of the cells will have cell sizes in the described range, further the foamed part
will be ~ubs~ lly free from large cells, for example more than 50% larger than the
average described cell size. For a 30 Ib/ft3 (480 kg/m3) density foam the range of cell
sizes will be in the range of from 400 to 800,um, with 70 % of the cells in the foamed
part having a size in this range, preferably at least 75%, more preferably at least 80%,
even more preferably at least 85 %, most preferably at least 95%.
The thermoplastic may be selected from any suitable material, incllltling but not
limited to polymers of ethylene, propylene and other comonomers, in~ ing olefinic
col.lollomel~, as well as polyethylene terephth~l~te, polybutylene terephth~l~te, nylon,
polyvinyl chloride and the like.
In still another variation the molded part can include at least one ~ubs~ lly
non-foamed layer having a density in the range of from 900 to 1400 kg/m3.
These and other aspects and advantages of certain embo~lim~nt~ of the present
invention will become understood with reference to the following description andappended claims.
Description
Introduction
This invention concerns certain classes of thermoplastic resins, thermoplastic
micro-pellets made from these resins, and articles fabricated from these micro-pellets
and processes for producing the articles from the thermoplastic micro-pellets. These
thermoplastic micro-pellets have unique properties which make them particularly well
suited for use in producing certain classes of fabricated thermoplastic articles.
Rotationally molded articles made using the therrnoplastic blowing agent combination
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of various embollim~ont~ of our invention, will have co,llbillaLions of properties
r~n~1~rin~ them generally superior to articles previously available that used other
techniques of producing foamed rot~ti- n~lly molded articles. Additionally, these
thermoplastic micro-pellets show a surprising increase in their ability to be rotationally
molded and to provide at least one foam layer in a rotationally molded article.
Following is a detailed description of certain prer~ll ed thermoplastics, blowing
agents, micro-pellets made from the thermoplastic/blowing agent collll~in~lion~ and
methods for m~mlf~ctllring rotationally molded articles based on these micro-pellets.
Certain l~ler~;;ll~d applications of rot~tion~lly molded articles made according to the
10 disclosure embodied herein are also in~1.lde~ Those skilled in the art will appreciate
that numerous modifications to these pl~rt:lled embo~im~nt~ can be made without
departing from the scope of embo~ of our invention. For example, while the
properties of rotationally molded articles, thermoplastic/chemical blowing agentcollll)inalions from which they are made, and processes for using the thermoplastic
15 micro-pellet combinations to produce foamed rotationally molded article are
exemplified, those of ordinary skill in the art will appreciate that they have numerous
other uses. To the extent that the description is specific, this is solely for the purpose
of illustrating pler~lled emborlim~nt~ ofthis invention and should not be taken as
limiting this invention to these specific embo-limPnts
The use of subhe~-ling.c in the present application are intçntlecl to aid the reader,
and are in no way int~n~ed to limit our invention.
Various terms used in the specific~tion and claims have been determined and
are defined as follows:
Impact SL~ Lll, measured by Association of Rotational Molders (ARM) test
25 using a 15 Ib. (6.8 Kg) weight dropped at various heights to give an impact energy in ft
- Ib.F or Joules. Test done at -40~ C.
Part Thickness known as the average part thickness. millim~t.ors.
The Flexural Modulus, at 1% secant, in KPSI (MPa) measured using ASTM
D-790.
Rotational Molding Cure Time (minutes): Using a clam shell rotational
molding m~chine7 Model FSP M-60 available from FSP Machinery Co.. The time
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neceSS~y for a rotational mt)ltling form~ tiQn, typically in granular, micro-pellet, or
powder form, to fuse into a part at a given temperature.
Particle Size Distribution, measured by the amount retained on a screen, as
defined by ASTM D-1921 using a Rototap Model B, 100 gm sample, 10 minute shake.
Dry Flow of particles measured in seconds by a Funnel Flow Test, as defined
by ASTM D-i 895, Method A on a 100 gm sample.
Bulk Density in g/100 cc as defined by ASTM D-1895, Method A, using a
minimllm of a 200 gm sample.
Melt Index is defined by ASTM D-1238 using 2160 grams at 190~ C, units in
10 gm/10 min~ltçc, or decigrams/minute, dg/min.).
Foam De.,~ily; Molded part densities are measured in a clen~imPt~r. This
method uses a water displ~cl?m~nt technique in which the sample weight is measured in
air, and then the volume is measured by ~ plztc~m~nt of water. Provision is made for
air bubbles which may adhere to the surface of a part such that if any bubbles are
15 observed they should be removed, if they cannot be removed the sample is discarded.
Density is defined by ASTM D-1505.
Cell Size: Cell sizes will be described by an average ~liztmeter, however it will
be understood that such cells may be generally rounded or sphere-like, but shapes may
vary subst~nti~lly~ by describing average cell size or diztmet~r. It will be understood by
20 those of ordinary skill in the art that we intend that this size be descriptive of a
measurement made on a cross section of a cell or cells and the measurement will be of
the widest point of the cells.
Cell Uniformity: Cell ulliro,l,liLy may be determined by observing a cross-
sectional area ofthe part co~ it~ g a foamed layer. Using a magnifying glass or a
25 microscope the area is viewed and cell size measurements made. Cell uniro"llily will
be described by a percentage of the cells in a given cross-sectional area being within a
certain size range.
Heat Distortion Temperature: ASTM D 648-82
Aspect Ratio: ratio of pellet length to tli,tmet~r
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The Thermoplastic Material
The thermoplastic component may be a polyethylene, poly~roL,ylene,
polyethylene terephh~i~te, polybutylene terephh~l~tç, polyamide, polyvinylchloride,
and the like. Prc;r~l~ed are polyethylene and polyplol)ylene.
S
Polyl,lc,l ylene
Where the polyolefin is a poly~lopylene, various embo~lim~nte in~ d~, but are
not limited to, homopolymer polypropylene and copolymer polypropylene. The
copolymer poly~,opylenes may contain propylene and one or more monomer sç1ecte~110 from the group consisting of ethylene, butene-l, 4-methyl-1-pentene, hexene-l,
octene-l, and col,~inations thereo~ Copolymers of polypropylene will generally
contain in the range of from 0.2 to 10 mole percent comonomer or monomers, basedon the total moles of copolymer. The poly~ropylenes may be produced with either
conventional Ziegler-Natta catalysis or with metallocene catalysis. The density of such
polymer of propylene will generally range from 0.89 to 0.910 g/cc. The melt flow of
such propylene may range from 1 to 20 dg/min.
Polyethylene
Where the thermoplastic is a polyethylene, the polyethylene may be a
homopolymer or polymers of ethylene and one or more comonomers selected from thegroup consisting of propylene, butene-l, 4-methyl-1 -pentene, hexene-l, octene-l,
decene-l, and col"bin~lions thereof, plt;r~ d comonomers include butene-l, hexene-
1, and octene-l. Comonomers may also include ethylinically unsaturated acrylic acid
esters, acrylic acids, vinyl acetate and the like. Those of ordinary skill in the art will
appreciate, that such copolymers of polyethylene will generally contain in the range of
from 0.2 to 20 mole percent comonomer or comonomers, based on the total moles ofcopolymer. Such polyethylenes may include one or more of high density, medium
density, low density or linear density polyethylenes, generally having densities in the
range offrom 0.915 g/cc to 0.970 g/cc, preferably in the range offrom 0.915 to 0.950
g/cc, more preferably in the range of from 0.930 to 0.950 g/cc.
Polyethylene homopolymers or copolyrners suitable for use in embodiments of
the present invention may be made ~ltili7ing conventional Ziegler-Natta catalyst
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systems and processes, so called Phillips catalyst systems and processes, or
metallocene catalyzed polymers and processes.
Melt indexes of polyethylene for use in rotational molding and generally for usein foamable rot~tion~lly molded articles can range from 1 to 20 dg/min., preferably in
5 the range of from 2 to 10 dg/min.. The use of melt indexes higher than 20 dg/min.
polyethylenes is also contemplated, especially when combined with cross-linking
agents to improve the melt strength/cell structure balance in a foamed layer.
Also contPmpl~ted are blends ofthermoplastic materials such as poly~lv~,ylene,
low density polyethylene; high density polyethylene, linear low density polyethylene
10 and other co~ illalions of materials known to those of oldillaly skill in the art to
provide useful, functional, durable roto-molded objects. Such colllbillalions can be for
inct~nce in the micro-pellets co.~ g one or more thermoplastics and/or various
thermoplastics, generally in a physical form such as micro-pellets and or groundpowder to be conveniently roto-molded, can be used in conjunction with the foamable
15 micro-pellets. Such additions (conlbillalions) may be useful to incorporate diLrelelll
properties into or in addition to a foamable layer.
Blowing Agent
Blowing agents are known. The description which follows inrll~d~s exothermic
20 çhrmic~l blowing agents which are prer~lled, but is not limited thereto. Suchexothermic chemical blowing agent's include but are not limited to azodicarbon~mi~lr,
modified azodicarbon~mides~ p-toluene sulfonyl semi carbazide, p,p'-oxybis(benzene)-
sulfonyl hydrazide, p-toluene sulfonyl hydrazide. Preferred is azodicarbonamide.Chemical blowing agents may employ activators . In this context activators
25 will be understood by those of Ol dinal y skill in the art as materials that can alter, for
in.ct~nre, raise or lower the decomposition or gas evolution tempel~LLIre, tempel~ re
range and/or decomposition rate of the chemical blowing agents. Metal salts are
known activators. Other examples of strong activators are; surface treated urea, zinc
oxide, zinc stearate, dibasic lead phth~l~tç7 triethanolamine, and dibasic lead phosphite.
30 Other activators include dibutyl tin~ urate, calcium stearate, citric acid, and
barium/c~millm stearate combinations.
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Activators may be added, if used, at parts per hundred parts of thermoplastic
levels similar to the chemical blowing agent itself. Additionally, chemical blowing
agent decomposition te-l-pel~l~res, te."pe.~Lu-e ranges, or gas evolution rates may be
arre~i~ed due to the presence of other ~~.h~.mic~le either in the polyolefin itself and/or
S additives, such as stabilizers, ~ntioxifl~nte~ acids, metal catalyst residues and the like.
Endothermic blowing agents may also be used in various embollim~nte of our
invention. Endothermic agents maybe based on sodium bicarbonate/citric acid
mixtures. Such endothermic blowing agents can be blended with exothermic blowingagents to provide a mixture of properties as is known in the art.
Depending upon the amount of heat generated during compounding/melt
mixing and micro-pelletization, (incl~-ding one or more thermoplastics, other additives,
a rh~mic~l blowing agent and optionally an activator) and/or rotational molflinP~ of the
micro-pellet co.~ i"g a chemical blowing agent, and the rate of heat generation,those of o~dhla.y skill in the art will appreciate that adj~-stmtonte may be made to the
15 level of ~.h~miç~l blowing agent and activator, to o~li---i~e fo~minp~, foam density and
cell ..l ir~....i,y.
It will be understood by those of Ol dhla-y skill in the art that the level of one or
more blowing agents and optionally the level of an activator or activators will depend
on many factors in~ rling but not limited to: level of other additives in the polymer,
20 level of contained impurities in either a polymer or the arorc;---e ~lioned additives, the
thermal history of the blowing agent and/or blowing agent/thermoplastic combillalion,
the rate and level of heating and temperature ranges used in the rotational m~ldin
process, and the like.
Blowing agent decomposition te---~ LIlre should be taken into account during
25 melt mixing/compounding of the çhPmic~l blowing agent and thermoplastic, to
...i,~i...;,e decomposition in the compounding and/or pelletizing step. Some
decomposition of the blowing agent may take place during this step and is desirable,
but generally it is plt;f~..t;d that the largest part ofthe decomposition, leading generally
to gas evolution and foam cell formation, take place in the rotational molt1ing process.
30 Controlling such determinations will be the desired end product or foamed article.
Levels of inclusion of chemical blowing agents into a micro-pellet may
generally be in the range of from 0.1 to 3 parts per hundred parts of resin
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(thermoplastic). Preferably in the range of from 0.2 to 2 . More preferably in the
range of from 0.5 to 1.5 parts per hundred parts of resin (thermoplastic). If blowing
agent activators are incl~lded in the formulation, their presence will be at levels similar
to but not nece$~rily the same as the levels for the ~h~mic~l blowing agents.
S
Compounding and/or Microp~ ti7~tion of the Thermoplastic
Compounding and/or pelletization of the thermoplastic/ch~-mic~l blowing agent
combination or blend of various embod;~ of our invention, can be carried out by
any mixing/p~lleti7.ing s~h~me P.erel.ed are extruders commonly used to compoundor mix ingredients and pelletize the res-llting mixture or blend of thermoplastics,
blowing agent, and a wide variety of possible additive components.
In such extrusion operations, generally the thermoplastic or thermoplastics and
additives including but not limited to antioxidants, anti-static agents, ultraviolet
absorbers, ultra-violet blockers, colorants, acid neutralizers, blowing agents çh~mic~l
blowing agent blowing agent activators, cross-linking agents and the like are blended
with at least the thermoplastic in the melt phase, then extruded.
Micro-pellets may be produced in a manner similar to "standard" sized pellets,
in that the polymer (e.g. thermoplastic or polyolefin) is melted along with additives, in
a mixing device, such as an extruder. The molten polymer is then extruded through die
holes in the discharge end of an extruder and either "strand" cut, where strands exiting
from die holes are solidified/cooled then "chopped", or the micro-pellets may be "under
water cut". The "under water cut" generally allows a rapidly revolving blade to sweep
or cut offthe polymer extrudate as it comes through the die plate holes while the water
covers and "freezes" the molten polymer cut off, forming a pellet or micro-pellet.
Previous to our discovery, particles generally used in rotational molding were
the result of "standard" sized pellets being ground into powder. By "standard" sized
pellets, we intend that this terminology mean pellets that are commonly used in the
thermoplastic industry for storage and h~n~lling Whether strand cut or underwater
cut, such pellets generally have a range of size averages often from 2,000 to 5,000 ~lm.
These sizes offer several advantages which should fulfill a thermoplastic
m~nllf~cturer's need to have a pellet size that may be pnellm~tically conveyed, reduce
-- . :
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"hri~lging" in holding c~nt~inPrs, and will have a bulk density that permits economic
shipment of the thermoplastic.
"Standard" peliets, as disclosed above of 2,000 to 5,000 ~lm are generally
considered impractical for use in rotational mol~1in~ because such a relatively large
particle size inhibits the particle's ability to easily reach and fill all of an intricate mold's
features. Additionally sintering and fusing are made more ~lifficlllt due to the relatively
small surface to volume ratio (especially when co",pal ed to ground powders usedcommonly in roto-molding).
Accordingly, thermoplastics inten-led for use in rotational molding are generally
available in "standard" pellets. The "standard" pellets are ground, either cryogenically
or at ambient temperature, to a 200 - 300 ~m(average) particle size. It will be
understood by those of o,di,laly skill in the art that such grinding processes result in a
relatively wide particle size distribution, but a particle size and size distribution that has
proven succes~fill in flowing, si"Le,i,lg and fusing relatively well when used in a
rotational molding operation.
Attempts to compound a blowing agent into a "standard" sized pellet then
grind it, might lead to premature foaming and/or some fugitive escape of decomposed
chemical blowing agent (gas) leaving less gas/blowing agent available for fo~ming in
the rotomolding process. However, such an approach is not foreclosed.
Extruders used to produce micro-pellets can be any size, however the extruder
generally should be capable of extruding a wide range of polymer viscosities through a
die plate having numerous holes, at commercially viable rates. The number of holes
will range from 100 to 5000 relating to di~e,e"L capacities based on the melt index,
melt viscosity, extruder back-pressure, size of the extruder, and its die plate area. A
die hole size generally the size of the average ~i~mPter of the pellet desired is optimally
utili7e(1 In the examples which follow the die hole size of 500~1m is used however, a
micro-pellet or the die hole from which they em~n~e may range from 250~1m to
1 500~1m.
The size of the micro-pellets contemplated in certain embodiments of our
invention can have an average size in the range of 250 to 1500 ~m, preferably in the
range of from 300 to 1200 ~m, more preferably in the range of from 350 to 1000 ,um,
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even more preferably in the range offrom 400 800 ~m, and most p-~rel~bly in the
range of 400 to 600~m.
.
The Foamed Rotationally Molded Part
-- 5 Micro-pellets, due to their improved flowability may necç~ 1e a slower mold
rotation speed to take advantage of their improved flowability.
Parts made from the micro-pellets described above, (inrlll-ling a çhPmi<~l
blowing agent) display a relatively smooth outer surface. This outer part surface of
foamed parts made from micro-pellets (the surface generally defined by the inside of
10 the roto-mold) will generally have some surface roughnes.~, absent any material or
technique to render the surface subst~nti~lly smooth, but such surface smoothness is
not prerl~lde~ The inner surface of foamed parts made from the micro-pellets will
generally be smooth.
Additionally it is expected that a part or a part cross section of a foamed part15 will display a density in the range of from 1 to 55 Ib/ft3 (16 to 880 kg/m3), pr~rel~bly
in the range of from 2 to 35 Ib/ft3 (32-560 kg/m3), more preferably in the range of
from 5 to 30 Ib/ft3 (80-480 kg/m3), most preferably in the range offrom 5 to 25 lb/ft3
~80-400 kg/m3). However, the cross section may not display a uniform density and/or
cell unirc".. i~y across the cross section (e.g. from outside to inside surface) however,
20 such u--irc,~ iLy or general lack of gradient is desirable. There will be some
densification of the foam layer at these inside and outside surfaces. Those of ordinary
skill in the art will understand that this densification will depend on factors such as heat
transfer to and from the mold itself, the amount of blowing agent that escapes from the
region of either of these surfaces, and similar mech~nicm~
Further, the methods for obtaining a smooth or smoother inner and/or outer
surface or adding an inner or outer layer or layers are contemplated. Such methods
,, inrluclr, but are not limited to, spraying, dipping, p~in~ing using a ground powder of
smaller particle size in combination with the micro-pellets in a rotational molding
3 operation, and combinations thereof. Use of micro-pellets c~ g chemical
30 blowing agents in drop box techniques is also contemplated. The superior flowability
of micro-pellets may lead to an improvement in lay down, especially where micro-
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pellets are employed in a second or subsequent charge used in the roto-molding
process.
Foamed rot~tic)n~lly molded parts made accolding to pl~r~ d embo~ of
our invention will show a sul~lisingly small cell size variation, and the cells will be
S ~ubs~ ly closed cells. An average cell ~ met~r may be for inet~nre in the range of
from S0 to 1300 ~lm, preferably in the range offrom 100 to 1000 llm, more preferably
in the range of from 150 to 800 ~lm, most preferably in the range of from 400 to 800
m. The cell size variation may also be described as generally greater that 70% of the
cells have a di~meter in the above ranges, preferably 75%, more preferably 80%, even
10 more pl~rt;l~bly 85%, most preferably greater than 95%. While the term di~metçr is
used, it will be understood by those of o, dhlal y skill in the art that the cells will
generally have a rounded, but not n cece~rily spherical shape. The measurements
diec--esed above can be applied to the widest and or deepest dimension of a foam cell.
Cell size may also be dependent upon foam density, lower density foams generallyhaving larger cells. For example, the average cell size of a 30 Ib/ft3 (480 kg/m3)
density foam will be 600 ~m, with at least 70% of the cells being within ' 30% of the
average size. For a 10 Ib/ft3 (160kg/m3) density foam the average cell size will be 900
~m, with at least 70% of the cells being within ~ 30% of the average.
Foamed articles that may be made by various embodiments of our invention
20 in~ dç, but are not limited to surfboards, wind surfboards, inelll~ted tanks for
çh~mir.~le, potable water and similar liquids, children's toys, boats, material h~ntlling
boxes (refrigerated and non-refrigerated), playground eq--ipm~nt~ kayaks, sailboats,
canoes, power boats, boat seats, boat accessories, marine floats, buoys, marine
floatation devices, marine derl~ing picnic coolers, commercial display coolers,
25 structural cont~iners~ recycle boxes, newspaper boxes, fish boxes, p~c~ging, military
p~rlr~ging, and the like.
The micro-pellets of various embodiments of our invention may be
advantageously used in other low shear processes, such as pipe coating and otherLeling processes.
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E~amples
Example 1
Resins were chosen for micro-pellet screening analysis. All polyethylene resins
are available from Exxon Chemical Con,l)a--y. Escorene~) LL-8460.27, a 3.3 dg/min.
- S nomin~l melt index 0.938 g/cc nomin~l density material, Escorene HD-8660.26, a 2.2
dg/min melt index dg/min., density 0.942 g/cc. These materials were micro-pelletized
in a 2.5 inch (6.35 cm) Davis Standard, single screw pelletizing extruder.
TABLE 1
Cutter Speed Resin Used
LL8460.27 HD8660.26
~ 2600 rpm Longer cylinders* Longer cylinders
3300 rpm Shorter cylinders** Shorter cylinders
4000 rpm Discs*** Discs
* Pellet length 740 microns, pellet diameter soo microns, Aspect Ratio 1.45
** Pellet length 820 microns, pellet diameter 630 microns, Aspect Ratio 1.30
*** Pellet length 630 microns, pellet diameter 680 microns, Aspect Ratio 0.92
All the micro-pellets were extruded from die holes nominally 0.020"
(500,um) in tli~meter Variations will be seen in the size and shape of the pellets due
not only to the speed of the pellet cutter, but also to the fact that the pellet cutter speed
may not be constant across the cross section of the die, i.e. for all holes. Results are
shown in Table 1
The sieve analysis and dry flow results for the pellet shapes (described above)
in co...pa.ison to commercially available ground powders that are generally
commercially acceptable for roto-molding operations is shown below in Table 2.
The pellets produced had the following properties compared to a ground
powder (Escorene LL-8461.27, a nominal 300 ~lm average particle size polyethylene
25 having substantially the same melt index and density as the above described LL-
8460.27):
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TABLE 2
Micro-Pellets LL-84621.27 ~round power)
35 meshretention97.3% 0.5%
50 mesh retention 2.6% 50.0%
60 mesh retention 0.1% 20.0%
80 mesh retention 0% 15.0%
Pan 0% 14.5%
bulk density 0.46 g/cc 0.36 g/cc
dry flow 15 sec. 26 sec.
The micro-pellets can be seen to have excellent, smooth flow characteristics.
5 Dry flow values of 15 seconds are obtained coll-pal~d to 26 seconds for groundpowders, generally indicating an improved flowability and ~tten~l~nt improved mold
filling capabilities.
Table 3 sl ......... il. iGes the physical prope, lies of rotational molded parts using
ground powders and micro-pellets. As can be seen there is generally little difference in
10 the physical properties measured. Processing cycle time for a typically rotationally
molded object would also be sl~bst~nti~lly the same for both micro-pellets and ground
powders. Processing characteristics were dçmonstrated on a small scale roto-molder.
Other tests were run to determine processing characteristics on a large scale roto-
molder (Model FSP-60).
Table 4 sull-lll~ iGes range of angle of repose measurements for a ground
power and the micro-pellets. The micro-pellets generally have a lower angle of repose
than the typical ground powders usually indicating improved flowability. As can be
seen from Table 4 the angle of repose of micro-pellets is in the range of 15 to 25%
lower than ground powder made from the same polymer.
Example 2
The polyethylene of Example 1 (Escorene~) LL-8460.27 available from Exxon
Chemical Company) was compounded with 0.5 parts per hundred parts of resin of
azodicarbonamide (with a 2 micron particle size (Celogen(g) AZ 2990 available from
25 Uniroyal Chemical)).
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The col~lbinalion was micro-pelletized in the extruder of Example 1 at a melt
te-l,pel~ re of 375~F (190.5~C) and a die plate temperature of 500~F (260~C). The
pellet cutter speed was 3550 rpm, pellet cutter water was 180~F (82.2~C), and the die
hole ~ metPr was 0.020" (500 ~lm).
The pellets produced had the following properties:
35 mesh retention 99.4%
50 mesh retention 0.6%
bulk density 0.42 g/cc
dry flow 16 sec
The micro-pellets were placed in a rotation mold an FSP model 60 clam shell
rotational molding m~f~hin~ using a hexagonal shaped mold and cured at an oven set
point of 600~F (315.5~C) for 25 minl-tes The molded polymer is allowed to cool for 5
mimlteS with the top ofthe oven closed and then 5 mimltes with the top ofthe oven
open with ambient air circulated by a fan, followed by 11 mimltçs of water spray onto
15 the mold and part, then a 3 minute period of drying. The part thickness made was
1200 ,um Association of Roto-Molders Impact at -40~C was 42 ft.-lb (57 Joules).
Examples 3~ and 4
Three formulations (Example 2, 3, and 4) were roto-molded in the model FSP-
20 60 roto-molder. Example 2 utilizes micro-pellets and the chemical blowing agent.
Pellet size is as described above.
Example 3 is a dry blend of ground powder LL-8461 (described above) and
Celogen~) AZ 2990 at a nominal 2 ~lm particle size.
Example 4 is a physical blend of 20% of LL-8461, a commercial ground
25 powder and 80% of a pellet formed by melt mixing HD-6705 a 19 dg/min, 0.952 g/cc
polyethylene (available from Exxon Chemical Company) and 0.5 parts by weight
, Celogen AZ 2990 and pelletizing to a "standard" nominal 3000 ~lm pellet.
As can be seen from the results in Table 5, the part thicknesses of Examples 2,
3 and 4 are 1.27 cm, 1.27 cm and I cm, respectively. The Association of Roto-
30 molders impact ( 42 ft./lb. (57 joules) (~ -40~ C ) of Example 2 exceed those of dry
blended ground PE and blowing agent (Example 3 29 ft/lb (39.3 joules)) by over 40%,
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while the Association of Roto-molders impact of Example 2 exceeds the Association
of Roto-molders impact of Example 4 (12 ft/lb or 16.2 joules) by 350%.
Although the present invention has been described in considerable detail with
reference to certain prer~ d versions thereof, other versions are possible. For
5 example, other çh~mic~l blowing agent's, other micro-pellet sizes, and additional layer
formation are co..~ te~ Thelerc,l~;, the spirit and scope of the appended claimsshould not be limited to the description ofthe plt:rel~ed versions cont~in~l herein.
TABLE 3
SIIMMARY OF PHYSICAL PROPERTIES
MICROPELLETS VS. GROUND POWDER
Heat Distortion
Flexural Modulus Tensile ~ Yield Ultimate Flnn~ption T~ ,.l...e
Grade (psi) (p~ i) (O~) (o C)
GP MP GP MP GP M GP M
836072790 77890 2193 2252 1199 1216 55.0 58.5
2 846088100 89610 2656 2722 1035 1115 69.5 69.0
3 855576400 81300 2399 2416 788 437 56.5 61.0
4 866096880 96650 2786 2882 1141 1142 61.0 66.0
5 8760 121400 119400 3294 3415 1024 499 71.0 73.5GP=Ground Powder
MP Micro-pellets
15 l=Escorene 8360, 5 dg/n~in, 0.932 glcc
2=Escorene 8460, 3.3 dg/min, 0.938 g/cc
3=Escorene 8555, 6.7 dg/min, 0.936 g/cc
4=Escorene 8660, 2.2 dg/min, 0.942 g/cc
S=Escorene 8760, 5 dg/min, 0.948 g/cc
20 All available from Exxon Chrmir~l Co.
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TABLE 4
ANGLE OF REPOSE MEASUREMENTS
LL-8461 LL-8460
Ground Powder Micropellets
Run (~) (~)
2 35 31
3 40 30
4 38 30
Average 39 30
S
TABLE 5
Physical Properties
Example Appearance ThicknessARM Impact
(cm)(joules for -40~ C)
2 - ul~irOllll in cell structure 1.27 57
-absence of large voids smooth
surface
3 - broad cell size 1.27 39.3
4 - mottled surface 1.0 16.2
(large voids)
