Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONVEYOR MODULE, SMALL FRAGMENTS OF WHICH
ARE MAGNETICALLY AND X-RAY DETECTABLE
TECHNICAL FIELD OF THE INVENTION
[0001] The invention relates generally to a conveyor system, and, more
particularly,
to a conveyor system in which conveyor modules are manufactured from a mixture
of a
thermoplastic polymer, stainless steel powder, and, optionally, barium sulfate
powder, small
fragments of which module are X-Ray and/or magnetically detectable.
BACKGROUND OF THE INVENTION
[0002] Low friction, wear resistant polymeric materials are used in modular
plastic
conveyor belt modules in numerous industries. In the meat and food product
packaging
industry, most conventional modular plastic conveyor belt modules are molded
using
polypropylene ("PP"), polyethylene ("PE"), or polyoxymethylene ("POM" aka
acetal). The
environment and use conditions of the conveyor dictate which polymer is best
suited for a
given conveyor. Environmental conditions include ambient temperature,
temperature swings
such as hot to cold, humidity, immersion in liquid treatment baths, and
chemical cleaning
solutions. Use conditions can be described as speed of the conveyor, direction
of travel and
contact pressure of the conveyor belt module against contact surfaces.
Conveyor belt
modules are exemplified in U.S. Pat. No. 7,134,545 B 1, issued on November 14,
2006, to
Chris Smith, and U.S. Pat. No. 10,773,896 Bl, issued on September 15, 2020, to
Chris Smith,
both of which patents are incorporated herein by reference in their entirety.
[0003] Selecting the best polymer for a conveyor weighs the pros and cons of
the
interaction of that polymer with the conditions to which it will be subjected.
For example, it
is known to those skilled in the art that:
= Polyamides, polyacetal and polyester have various coefficient of friction
ratings
which are ideal in sliding or rubbing applications like conveyors depending on
product being conveyed.
= Polyamides absorb 4-8% moisture and will swell in physical size with
moisture.
= Polyacetal is chemically degraded when exposed to low pH (or acidic)
solutions.
= PBT polyester chemically degrades in the presence of moisture above 80 C.
[0004] Among the many end use markets where polymeric conveyors are used is
the food processing segment for both human and animal foodstuffs, referred to
herein as the
"protein market". The protein market includes processing plants for the
conversion of
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chickens, hogs, cattle, and fish into consumer products. Food safety to
prevent foodborne
illness and to prevent foreign material contamination is of utmost importance.
[0005] Recent advances in sanitizing techniques and sanitizing chemical agents
have positively affected food safety but have had an adverse effect on the
integrity and
longevity of plastic conveyors. What has occurred is that the newer sanitizers
now in use are
lower in pH, and contain chemical oxidizers like hydrogen peroxide and peroxy
acetic acid.
Where before polyacetal was widely used as the polymer of choice in protein
market
conveyors, the rise of acidic oxidizers has rendered polyacetal nearly
unusable because of its
chemical incompatibility with both acidic agents and its high susceptibility
to oxidative
attack.
[0006] When polymers are chemically attacked, they lose their mechanical
integrity, including tensile strength and impact resistance. Material science
has described the
loss of tensile properties and/or loss of impact resistance as embrittlement.
A brittle polymer
will fracture or shatter, generating small pieces of plastic debris, when
external stresses are
placed on it.
[0007] Conveyor modules manufactured from such polymers may, over time,
through normal wear, cutting directly on modules, neglect, and/or by
incidental impact,
degenerate such that small fragments and particulates from the conveyor
modules become
integrated into the food products. These contaminants can be dangerous as
choking hazards.
If a piece of belt breaks off and gets into the food chain, the costs to the
food processor can
be in the 10's of millions of dollars. All the product from a particular
production run must be
recalled and disposed at the processor's expense. Recently, the USDA issued
new guidelines
for "foreign body contamination" recalls and the steps necessary to comply.
The example the
USDA used was what would happen if a piece of a modular plastic conveyor belt
broke off
and got into the food chain. This is a huge issue that is costing food
companies billions of
dollars each year.
[0008] Additionally, in industries such as pharmaceutical processing, the
plastics
may contain organometallic catalysts and plasticizers that can degrade the
pharmaceutical
product. Food contaminates such as wood and cloth and conveyor contaminates
can be
harmful to humans and/or animals that consume the meat or other food products.
[0009] Because it has been proven to be extremely difficult and inadequate to
detect, by visual inspection alone, conveyor contaminate in meat and food
being processed,
food and drug regulations have been enacted to require metal and X-ray
detection of
conveyor fragments and other contaminants.
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[0010] With the best available technology for magnetic and X-ray contaminate
detection systems fully employed, there remains a need to improve the
detectability of
predictable process contaminates, such as conveyor systems fragments.
[0011] It is known to use magnetic metal detection for the identification of
magnetic metal contaminates in food processing. However, many contaminates to
processed
food are non-magnetic. Conventional composite and plastic conveyor belt
systems are non-
magnetic. It is known to add magnetic steel powder with polypropylene and
polyethylene
resin to render the molded plastic conveyor fragments magnetically detectable.
[0012] Another way to detect conveyor fragments and particulate in meat and
food
being processed is by X-ray. However, X-ray is only effective if the X-ray
image of the
conveyor particulate is distinguishable from the meat or food product being
conveyed.
Therefore, it is necessary to include an X-ray opaque substance in sufficient
proportions into
the plastic conveyor resin to render a fragment of the conveyor X-ray
detectable. Barium
sulfate is known as an additive for use with polypropylene (PP) and
polyethylene (PE) resin
to render the molded plastic conveyor fragments detectable by X-rays.
[0013] It has recently been introduced to mix both powdered metal and barium
sulfate as additives for use with polypropylene (PP) and polyethylene (PE)
resin to render the
molded plastic conveyor fragments both magnetically and X-ray detectable.
[0014] Each of these modified products, though magnetically and/or X-ray
detectable, suffer from having significantly reduced performance
characteristics that result
from the combination of the barium sulfate and metal particles with the resin.
In particular,
these modified products are substantially more brittle. As a result, the
detectable conveyor
materials break easier and shed greater amounts of contaminant, and fail
sooner than previous
conveyor modules did.
[0015] In view of the foregoing, there continues to be a need for a plastic
conveyor
module that is both magnetically and X-ray detectable, and that has superior
toughness
SUMMARY OF THE INVENTION
[0016] The present invention, accordingly, provides a novel thermoplastic
polymer
that overcomes the serious drawbacks described above in the protein market
conveyors. This
new thermoplastic polymer, aliphatic polyketone resin, referred to herein as
polyketone resin,
does not swell with moisture, is unaffected by aqueous low pH acids, and
withstands
exposure to peroxy acids with early immeasurable effect. In addition to an
ideal chemical
resistance profile of polyketone resin, this polymer has frictional properties
that are superior
to polyacetal, polyamides and polyester in protein market conveyors. Finally,
the physical
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properties of polyketone resin including melting point, molecular weight,
percent mold
shrinkage, and degree of crystallinity enable polyketone resin to be used in
existing injection
molds, avoiding the need for expensive capital investment for new injection
molds.
[0017] As is typical with many polymers, polyketone resin is produced in high,
medium, and low molecular weight ranges. In the protein market, it has been
shown that
high molecular weight polyketone resin provides the most desirable performance
in friction,
wear resistance, toughness retention, and high impact resistance. It is known
to those skilled
in the art that the melt flow rate of a polymer is inversely proportional to
its molecular
weight. Specifically, polyketone resin with a melt flow of less than 2
grams/10 minutes,
measured at 240 C, performs well in protein conveyors, and a polyketone resin
polymer with
a melt flow rate of 2 ¨4 grams/10 minutes is the most optimal flow and
molecular weight.
[0018] Further, polyketone resin does not become brittle after repeated
exposure to
the acidic peroxy sanitizers now used in the protein market. Therefore,
polyketone resin
conveyors do not generate small pieces of plastic when they break, which
inherently
contributes to higher confidence in preventing foreign matter contamination in
food.
[0019] In one preferred embodiment of the invention, a relatively high
concentration of stainless steel powder, without barium, when added to the
polyketone resin
makes the belt modules both X-ray and magnetically (also referred to as
"metal") detectable.
This "single additive" also reduces the issue of increased brittleness of the
belt module. The
single additive also reduces cost to produce. A "1.5 mm ferrous equivalent"
may be obtained
for belt modules. This means that if a piece of belt breaks off, the detection
equipment can
detect a piece that is approximately as small as a 1.5 mm metal sphere.
[0020] In accordance with principles of the invention, in the molding process,
the
polyketone resin is dried prior to molding to properly mold the parts. The
resin is preferably
"compounded" prior to molding instead of being "batch mixed" with the
stainless steel
powder and possibly colorant in the molding machine. "Compounding" entails
properly
mixing the polyketone resin and the stainless steel powder into homogeneous
pellets.
Thousands of such pellets are then melted in the injection process to form one
or more belt
modules. The mold pressure, mold temperature, water temperature, and cycle
times are
adjusted to properly mold the parts.
[0021] An advantage of the various embodiments of the disclosed invention is
that
the modules of a conveyor system are both X-ray and magnetically detectable.
Another
advantage of the disclosed invention is that it is less expensive to
manufacture than other
products with this capability. Another advantage of the disclosed invention is
that it provides
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a conveyor with a higher modulus of elasticity than other X-ray and
magnetically detectable
conveyor products.
[0022] Another advantage of the disclosed invention is that it provides a
conveyor
with a higher impact resistance than other X-ray and magnetically detectable
conveyor
products, and will therefore resist breaking and spalling on incidental
contact. Another
advantage of the disclosed invention is that it provides a conveyor with a
higher chemical
resistance than other X-ray and magnetically detectable conveyor products, as
such conveyor
products are exposed to harsh chemicals during cleaning operations.
[0023] Another advantage of the disclosed invention is that it provides a
conveyor
with a higher abrasion resistance than other X-ray and magnetically detectable
conveyor
products, and will therefore wear longer. Another advantage of the disclosed
invention is that
it provides a conveyor that requires fewer USDA approvals for food grade
application
component ingredients.
[0024] Another advantage of the disclosed invention is that it provides a
conveyor
with a wide operating temperature range, from 32 F ¨ 305 F. Another advantage
of the
disclosed invention is that it provides a conveyor with a low adhesion factor
to protein fat,
fatty meat, and animal oils, which results in the conveyor remaining cleaner
longer and being
easier to clean than other plastics.
[0025] In a further embodiment of the invention, barium sulfate is added to
the
polyketone resin with the stainless steel powder to enhance X-ray
detectability and,
surprisingly, to significantly reduce the quantity of stainless steel required
to render small
fragments of a conveyor module to be magnetically and X-ray detectable. Unlike
a
combination of barium sulfate as an additive to polypropylene (PP) and/or
polyethylene (PE)
resin, which rendered a module brittle with reduced magnetic and X-ray
detectability, as
discussed above, adding barium sulfate to a polyketone resin has been found to
enhance
magnetic and X-ray detectability of a module fragment without rendering the
module brittle.
[0026] The foregoing has outlined rather broadly the features and technical
advantages of the present invention in order that the detailed description of
the invention that
follows may be better understood. Additional features and advantages of the
invention will
be described hereinafter which form the subject of the claims of the
invention. It should be
appreciated by those skilled in the art that the conception and the specific
embodiment
disclosed may be readily utilized as a basis for modifying or designing other
structures for
carrying out the same purposes of the present invention. It should also be
realized by those
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skilled in the art that such equivalent constructions do not depart from the
spirit and scope of
the invention as set forth in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The objects and features of the invention will become more readily
understood from the following detailed description and appended claims when
read in
conjunction with the accompanying drawings in which like numerals represent
like elements.
[0028] FIGURE 1 is a flow chart depicting steps at a high level for producing
material for forming conveyor modules in accordance with principles of the
invention.
[0029] FIGURE 2 is a flow chart depicting, in greater detail, one step of the
flow
chart of FIG. 1 in accordance with principles of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] The following description is presented to enable any person skilled in
the art
to make and use the invention, and is provided in the context of a particular
application and
its requirements. Various modifications to the disclosed embodiments will be
readily
apparent to those skilled in the art, and the general principles defined
herein may be applied
to other embodiments and applications without departing from the spirit and
scope of the
present invention. Thus, the present invention is not intended to be limited
to the
embodiments shown, but is to be accorded the widest scope consistent with the
principles and
features disclosed herein.
[0031] Unless indicated otherwise, ratios and percentages of elements
constituting a
compound, composition, or mixture are given with reference to the total weight
of the
compound, composition, or mixture. The acronym ASTM refers to the American
Society for
Testing and Materials, an international standards organization that develops
and publishes
voluntary consensus technical standards for a wide range of materials,
products, systems, and
services. As used herein, the term "polyketone resin" includes the compounds
"polyketone",
P0KET0NE , "POK", and "POK resin".
[0032] It has been determined through extensive experimentation that a
conveyor
module can be produced that is both X-ray and magnetically detectable and that
retains
superior performance characteristics over conventionally known modules
designed for this
purpose. Such a conveyer module can be produced by forming the module using a
thermoplastic polymer, namely, a polyketone resin, such as produced by Hyosung
Chemical
in Seoul, South Korea, under the tradename of POKETONE , also referred to as
"POK". A
terpolymer polyketone resin is preferred, or, alternatively, an aliphatic
polyketone resin may
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be used. A terpolymer polyketone resin is preferred, comprising ethylene,
carbon monoxide,
and propylene in an approximate ratio of 47.5:47.5:5, respectively, in the
polymer backbone.
The propylene preferably constitutes about 2% to 12% of the terpolymer
polyketone resin,
with the ratio of carbon monoxide to ethylene preferably being approximately
1:1.
[0033] The preferred melt flow rate for the polyketone resin is about 2.5 g/10
minutes measured at 240 C, per ASTM D1238. Such a melt flow rate imparts an
optimal
balance of processability and mechanical toughness of the final article.
Alternatively, the
melt flow rate may vary in an operable range of 2.5 ¨ 70 g/10 minutes,
measured at 240 C,
per ASTM D1238.
[0034] In a further embodiment of the invention, the magnetic and/or the X-ray
susceptibility and detectability of a small fragment of a conveyor module
formed from
polyketone resin may be enhanced by compounding a mixture of the polyketone
resin with a
ferrous metal powder, such as iron powder, iron alloys, any 400 series
stainless steel powder
(preferably 409 or 430 stainless steel), any high nickel content stainless
steel powder, such as
a 300 series stainless steel (e.g., 304, 316, or 320), and the like.
Polyketone resin accepts a
higher weight percent of stainless steel additive compared to other plastics,
and it still retains
a higher percentage of mechanical properties with the stainless steel added.
The amount of
ferrous metal powder should be small enough so as not to materially affect
properties
associated with the function of the polyketone resin, but be large enough to
enhance the
magnetic and/or X-ray susceptibility and detectability of the conveyor module.
Accordingly,
in one preferred embodiment of the invention, the amount of 400 series
stainless steel powder
effective for enhancing both magnetic and X-ray detectability, by weight of
the mixture with
polyketone resin, is from about 8% to about 60%, typically, from about 12% to
about 45%,
and preferably, from about 15% to about 30%:
[0035] In a further embodiment of the invention, the X-ray detectability of
small
fragments of a conveyor module formed from polyketone resin may also be
enhanced by
compounding a mixture of the polyketone resin with barium sulfate powder,
preferably
comprising barium sulfate particles having a size from about 0.5 to about 500
microns and,
typically, from about 1 to about 100 microns and, preferably, about 1 micron
in diameter.
Barium sulfate may be added to the polyketone resin without rendering the
polyketone resin
brittle, which is surprising since barium sulfate renders polypropylene (PP)
resin and
polyethylene (PE) resin brittle. The amount of barium sulfate powder should be
small
enough so as not to materially affect properties associated with the function
of the polyketone
resin, but be large enough to enhance the X-ray detectability of the conveyor
module.
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Accordingly, the amount of barium sulfate powder effective to enhance X-ray
detectability,
by weight of the mixture with polyketone resin, is from about 2% to about 50%,
and
typically, from about 10% to about 40%, and preferably, from about 20% to
about 26%.
[0036] In a still further embodiment of the invention, both the magnetic and X-
ray
detectability of small fragments of a conveyor module formed from polyketone
resin may be
further enhanced by compounding a mixture of the polyketone resin with both a
ferrous metal
powder (e.g., 400 series stainless steel powder) and barium sulfate powder.
The amount of
stainless steel powder and barium sulfate powder should be small enough so as
not to
materially affect properties associated with the function of the polyketone
resin, but be large
enough to enhance the magnetic susceptibility and X-ray detectability of the
conveyor
module. Accordingly, with barium sulfate added to the mixture for X-ray
detectability, the
amount of 400 series stainless steel powder needed for enhancing magnetic
detectability, by
weight of the mixture with polyketone resin, would be from about 4% to about
40%, and
typically, from about 6% to about 30%, and preferably, from about 8% to about
20%.
[0037] The 400 series stainless steel powder is preferably 409 stainless steel
powder
or 430 stainless steel powder. The 409 and 430 stainless steel powders are
preferred as they
allow for the best balance of magnetic detection at the lowest weight percent
in the polymer,
while providing very good oxidation resistance. The 300 series stainless steel
powder, which
is traditionally not attracted to a magnet, could be used, but the loading
(weight percent) for
metal detectability would need to be increased to an amount ranging from about
15% to about
60% by weight of the mixture and, typically, from about 20% to about 50% by
weight of the
mixture and, preferably, from about 24% to about 40% by weight of the mixture.
To match,
for example, 18% by weight of 400 series metal detection, 300 series would
need to be added
at 26% by weight. However, at 26% loading, both cost and mechanical
performance are
adversely affected.
[0038] The amount of 300 series stainless steel powder effective for enhancing
magnetic and X-ray detectability, by weight of the mixture with polyketone
resin, would be
from about 18% to about 60%, and typically, from about 23% to about 43%, and
preferably,
from about 26% to about 35%.
[0039] Iron powder works extremely well for magnetic detection, but is highly
prone to oxidation (rusting) in use and can stain food on a conveyor. Iron
oxide black (Fe+3)
provides magnetic and X-ray detection action, and doesn't stain food, but it
renders the
polyketone resin black which is not acceptable by the USDA in food plants.
Amounts of iron
powder effective to enhance magnetic detectability, by weight of the mixture
with polyketone
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resin, are from about 0.3% to about 50%, and typically, from about 0.4% to
about 40%, and
preferably, from about 0.5% to about 30%.
[0040] The stainless steel powder preferably has a particle size of about 100
mesh
or smaller, or, alternatively, in the range of 100 mesh to 325 mesh. Larger
particle size
powders, e.g., in the range of 60-80 mesh (170-250 microns), will decrease
mechanical
impact incrementally compared to 100-325 mesh powders, while still imparting
useful
detectability qualities in both X-ray and metal detection devices.
Alternatively, ultra-fine
particle sizes, less than 325 mesh, pose dust explosion and fire hazards for
the compounder,
as well as higher cost than larger size particles.
[0041] Polyketone resin having a melt flow rate in the range of about 4 ¨ 90
g/10
minutes measured at 240 C, per ASTM D1238, or preferably about 6 g/10 minutes,
works
better for compounding with stainless steel powder.
[0042] The various combinations of stainless steel powder and barium sulfate
powder will be referred to herein collectively as an "additive".
[0043] FIGURES 1 and 2 are flow charts 100 and 102 setting forth steps in a
method for making a mixture of polyketone resin with an additive for use in
forming
conveyor modules. Accordingly, in FIG. 1, step 102, a given amount of an
additive powder
is preferably extrusion compounded into the polyketone resin to form
homogeneous,
cylindrical pellets, or the like. Extrusion compounding is preferred over
injection molding
because injection molding machines do not provide the same high degree of
homogeneity in
distributive mixing of additives into polymer. Also, phase separation readily
occurs when
trying to blend plastic resin and the considerably more dense additive.
Further, injection
molding machines do not allow for gravimetric addition of additives, like an
extrusion
compounder. Step 102 is described in further detail below with respect to FIG.
2.
[0044] In step 104, the resin pellets are dried prior to molding. Drying the
resin, in
a manner well-known to those skilled in the art, prior to molding is necessary
for creating a
blemish free exterior surface of the molded conveyor module.
[0045] The initial samples using polyketone resin having a melt flow rate of
2.5
g/10 minutes were molded into test coupons and exhibited exceptional strength
and impact.
But when conveyor modules were attempted to be molded, the compositions were
so viscous
that complete parts could not be formed, or the surface quality was too rough
or the
combination of heat pressure of the molding process caused the composition to
chemically
degrade.
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[0046] Only when a high melt flow rate (i.e., greater than 2.5 g/10 minutes
flow)
polyketone resin was selected was it possible to make acceptable parts. The
finished articles
exhibited surprisingly high impact resistance and strength almost comparable
to the
polyketone resin without the additive.
[0047] In step 106, a number of pellets, sufficient to form a conveyor belt
module,
are melted in an injection process to form the conveyor belt module. The mold
pressure,
molding temperature, water temperature, cycle times, and other such parameters
to perform
this step are considered to be well-known to those skilled in the art, and so
will not be
described in further detail herein.
[0048] With reference to FIG. 2, flow chart 102 sets forth details of step 102
depicted above with respect to FIG. 1 to compound additive powder with
polyketone resin to
form pellets. Accordingly, in step 202, a twin screw, or optionally single
screw, continuous
compounding extruder is preferably used to melt polyketone resin into a molten
polymer. It
may be appreciated that other forms of melt mixing, such as batch mixing, may
be used in
step 202, as known to those skilled in the art. In step 204, stainless steel
powder, in quantities
discussed above, is added precisely and gravimetrically, or alternatively,
volumetrically, to
the molten polymer. In step 206, barium sulfate powder, in quantities
discussed above, is
optionally added precisely and gravimetrically, or alternatively,
volumetrically, to the molten
polymer. Alternatively, prior to step 204, stainless steel powder and barium
sulfate powder
may be mixed and added together in step 204, rendering step 206 moot. In step
208, colorant
is optionally added to the molten polymer. In step 210, the molten polymer is
preferably
extruded as strands, which may, for example, be diced into pellets, or
directly die-face cut
into pellets. In step 212, the strands are cooled and preferably cut (e.g.,
diced, chopped) into
homogeneous pellets, which pellets are preferably cylindrical pellets.
Execution then
proceeds to step 104 (FIG. 1).
[0049] By use of the method described above with respect to FIGS. 1 and 2,
conveyor modules may be formed, small fragments of which are detectable by X-
ray and by
magnetic sensors (e.g., Hall effect sensor, magnetometer, and the like),
meeting a 1.5 mm
ferrous calibration standard. Further, compared to the prior art, such modules
have been
shown to have higher impact resistance, higher abrasion resistance, higher
chemical
resistance, and a lower coefficient of product release.
[0050] It will be readily apparent to those skilled in the art that the
general
principles defined herein may be applied to other embodiments and applications
without
departing from the spirit and scope of the present invention. By way of
example but not
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limitation, if magnetic detection is not needed, the additive in steps 204 and
206 may consist
of barium sulfate powder (with no stainless steel powder) to thereby enable X-
ray detection
only. Or, alternatively, if X-ray detection is not needed, the additive in
steps 204 and 206
may consist of stainless steel powder (with no barium sulfate powder) to
thereby enable
magnetic detection only. Other paramagnetic metals may be used in place of
stainless steel
and other ferrous metals, such as Group 8 metals, including ruthenium and
osmium, and
Group 10 metals, including the triad of nickel, palladium and platinum. While
such other
paramagnetic metals are technically susceptible to X-ray and magnetic
detection, they are
costly and/or pose health issues.
[0051] Having thus described the present invention by reference to certain of
its
preferred embodiments, it is noted that the embodiments disclosed are
illustrative rather than
limiting in nature and that a wide range of variations, modifications,
changes, and
substitutions are contemplated in the foregoing disclosure and, in some
instances, some
features of the present invention may be employed without a corresponding use
of the other
features. Many such variations and modifications may be considered desirable
by those
skilled in the art based upon a review of the foregoing description of
preferred embodiments.
Accordingly, it is appropriate that the appended claims be construed broadly
and in a manner
consistent with the scope of the invention.
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