U.S. patent application number 12/596013 was filed with the patent office on 2011-06-30 for method of producing a curved product comprising drawn polymer reinforcing elements and product obtained thereby.
Invention is credited to Hen H. Hoefnagels, Sotiris S. Koussios, Roelof R. Marissen, Lucas L. Van Den Akker.
Application Number | 20110159233 12/596013 |
Document ID | / |
Family ID | 39618882 |
Filed Date | 2011-06-30 |
United States Patent
Application |
20110159233 |
Kind Code |
A1 |
Marissen; Roelof R. ; et
al. |
June 30, 2011 |
METHOD OF PRODUCING A CURVED PRODUCT COMPRISING DRAWN POLYMER
REINFORCING ELEMENTS AND PRODUCT OBTAINED THEREBY
Abstract
The invention relates to a process for manufacturing a curved
product, wherein the process comprises positioning a plurality of
drawn polymeric reinforcing elements onto a mandrel, locally
adhering at least a part of the elements to each other, and
removing the product from the mandrel. The invention also relates
to a process for manufacturing a curved article, preferably an
armour article, from the curved product, the process comprising
positioning the curved product in a mould and compressing said
product at an elevated temperature and pressure. The curved armour
article has good antiballistic properties and is substantially free
of wrinkles.
Inventors: |
Marissen; Roelof R.; ( Born,
NL) ; Van Den Akker; Lucas L.; (Vught, BE) ;
Koussios; Sotiris S.; (Delft, NL) ; Hoefnagels; Hen
H.; (Hulsberg, NL) |
Family ID: |
39618882 |
Appl. No.: |
12/596013 |
Filed: |
April 17, 2008 |
PCT Filed: |
April 17, 2008 |
PCT NO: |
PCT/EP2008/003119 |
371 Date: |
March 29, 2010 |
Current U.S.
Class: |
428/98 ;
156/196 |
Current CPC
Class: |
B29C 70/04 20130101;
B29C 48/05 20190201; H01Q 1/42 20130101; B29C 70/543 20130101; B29C
48/07 20190201; B29K 2223/0683 20130101; Y10T 428/24736 20150115;
B29C 48/00 20190201; B29L 2031/4821 20130101; B29K 2105/06
20130101; Y10T 156/1002 20150115; Y10T 428/24 20150115; B29C 70/347
20130101; B29C 53/564 20130101; Y10T 428/1369 20150115; F41H 1/08
20130101; B29L 2031/3456 20130101; B29K 2105/256 20130101; Y10T
428/24628 20150115; B29C 53/8066 20130101; B29D 25/00 20130101 |
Class at
Publication: |
428/98 ;
156/196 |
International
Class: |
B32B 3/00 20060101
B32B003/00; B29C 70/30 20060101 B29C070/30 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 18, 2007 |
EP |
07007850.6 |
Apr 2, 2008 |
EP |
08006711.9 |
Claims
1. Process for manufacturing a curved product, wherein the process
comprises positioning a plurality of drawn polymeric reinforcing
elements onto a mandrel, locally adhering at least a part of the
elements to each other, and removing the product from the
mandrel.
2. Process according to claim 1, wherein the step of locally
adhering at least part of the elements to each other comprises
locally bringing the drawn polymeric reinforcing elements to above
their melting temperature and fusing them together.
3. Process according to claim 1, wherein the step of locally
adhering at least part of the elements to each other comprises
locally applying a binder to the drawn polymeric reinforcing
elements and solidifying the binder.
4. Process according to claim 1, wherein the process further
comprises providing the curved product with a polymer film layer,
which layer has a lower melting point than the reinforcing
elements.
5. Process according to claim 4, wherein the curved product is
enclosed in a polymer film envelope, in contact with the outer
and/or inner surface of the product, to which envelope a vacuum is
applied.
6. Process according to claim 1, wherein the drawn polymeric
reinforcing elements comprise fibers and/or films and/or tapes,
having a strength of at least 1.6 GPa.
7. Process according to claim 6, wherein the drawn polymeric
reinforcing elements comprise fibers and/or films and/or tapes of
ultrahigh molecular weight polyethylene.
8. Process according to claim 1, wherein a plurality of reinforcing
elements is simultaneously positioned onto the mandrel by supply
means.
9. Process according to claim 8, wherein the supply means control
the tension of a reinforcing element between the mandrel surface
and the supply means.
10. Process according to claim 1, wherein part of the reinforcing
elements is positioned in a pattern that is at least partly
non-geodetic.
11. Process for manufacturing a curved article, the process
comprising positioning a curved product obtained by a process
according to claim 1, in a mould and compressing said preform at an
elevated temperature and pressure.
12. Process according to claim 11, wherein the elevated temperature
is chosen between the melting point of the reinforcing elements and
40.degree. C. below the melting point.
13. Process according to claim 12, wherein the elevated temperature
is chosen between the melting point of the reinforcing elements and
20.degree. C. below the melting point.
14. Curved article, preferably a combat helmet or a radome,
obtainable by a process according to claim 11.
Description
[0001] The invention relates to a process for manufacturing a
curved product, comprising drawn polymer reinforcing elements. The
invention also relates to a process for making a curved armour
article. The invention further relates to a helmet, preferably a
combat helmet, and to a radome obtained by the process.
[0002] Curved articles, and in particular curved armour articles,
are generally made of composites of high strength fibers embedded
in a polymer matrix. Such armour articles combine good high-speed
projectile ballistic protection with low mass. To ensure the
required high quality of the armour articles, fiber-polymer armour
prepregs have been developed in particular for anti-ballistic
performance. An example of a particularly suitable armour prepreg
includes Dyneema.RTM.HB25 from DSM. To form an armour article, such
prepregs are stacked and compressed at elevated temperature and
pressure.
[0003] It has been proposed in WO 2005/065910 to manufacture curved
articles from a relatively flat package of stacked plies of
polymeric fibers by deforming the stack into its three-dimensional
shape at elevated temperature. It is suggested in WO 2005/065910 to
solve the problem of wrinkling of plies when deforming a flat stack
into a three-dimensionally shaped article by imposing a tensile
stress on the fibers at a temperature high enough for the fibers to
be drawn.
[0004] Although the known process yields suitable curved articles,
their performance, and in particular their anti-ballistic
performance can be further improved. The object of the present
invention is to provide a process for manufacturing a curved
product without substantial wrinkling. Another object is to provide
a process for manufacturing a curved armour article with at least
similar anti-ballistic properties as the curved article described
in WO 2005/065910.
[0005] This object is achieved according to the invention by a
process according to claim 1. According to the invention a process
for manufacturing a curved product is provided, the process
comprising positioning a plurality of drawn polymeric reinforcing
elements onto a mandrel, locally adhering at least a part of the
elements to each other, and removing the product from the mandrel.
The process of the invention provides a curved product that is
substantially form stable, due to the local adherence of the
reinforcing elements, without however having to embed the majority
of the reinforcing elements in a polymer matrix. With majority of
the reinforcing elements is meant at least 80% of the total amount
of reinforcingelements, preferably at least 90% of the total amount
of reinforcing elements, more preferably at least 95% of the total
amount of reinforcing elements. The curved product may be used to
advantage as an intermediate product, also referred to as a
preform, to manufacture a curved armour article. According to the
invention such a process for manufacturing a curved armour article
comprises positioning the curved preform in a mould and compressing
said preform at an elevated temperature and pressure. In the
process, the curved preform substantially has the curved shape
required in the final end article, prior to compressing. This
largely prevents wrinkles from occurring, which is beneficial for
the (anti-ballistic) properties of the curved article.
[0006] The invented process allows products and articles, curved in
one or more directions, to be produced from reinforcing elements
directly without appreciable wrinkling. With a curved product is
meant in the context of this application, a product which, when
positioned on a flat surface, has a ratio of maximum elevation with
respect to said surface to the largest linear dimension within the
projected surface of the product on the flat surface of at least
0.20. With a highly curved product is meant in the context of this
application a product which, when positioned on a flat surface has
a ratio of maximum elevation with respect to said surface to the
largest linear dimension within the projected surface of the
product on the flat surface of at least 1.00. By positioning a
plurality of drawn polymeric reinforcing elements onto a mandrel in
the shape required by the final product, the reinforcement elements
are built up into a preform, which preform does not need to be
deformed to a large extent during compressing thereof. It becomes
even possible to built up the preform with low internal
deformability. By positioning the reinforcement elements in the
required shape prior to compressing, deformability of the preform
is no longer a limiting factor and virtually any shape can be
obtained.
[0007] According to the invention a plurality of drawn polymeric
reinforcing elements is positioned onto a mandrel. A suitable
process comprises winding the reinforcing elements onto a
preferably rotating mandrel. Curved and highly curved
three-dimensional articles like helmets may hereby be obtained by
direct placement of reinforcing elements, and in particular by
filament winding. Filament winding itself is known per se.
GB2158471 for instance describes a method for making armour
articles, wherein fibers, impregnated with a conventional
thermosetting resin are positioned according to a pattern onto a
mandrel using winding techniques. In contract to the method
disclosed in GB2158471, the process according to the invention does
not need the presence of a polymer matrix. It has never been
reported that curved articles, such as armour articles, and helmets
in particular, may be obtained by positioning reinforcing elements
onto a mandrel substantially free from polymer matrix.
Surprisingly, the quality of the filament wound curved armour
article according to the invention is superior despite the fact
that the article is made from reinforcement elements, substantially
free from polymer matrix, instead of from the optimized armour
prepregs known in the art, which use a polymer matrix to embed the
reinforcing elements. Moreover filament wound articles generally
comprise various locations having more than two fiber directions.
This is normally disadvantageous for the ballistic performance
thereof. This does not apply to the curved armour article of the
present invention, due to the fact that the preform and article are
substantially free from polymer matrix.
[0008] A further advantage of the process according to the
invention is that the reinforcing elements may be oriented in any
desired directions across the surface of the mandrel, whereby a
wrinkle-free object with more homogeneous properties may be
obtained.
[0009] The process according to the invention is particularly
useful for the manufacture of articles, curved in one or more
directions. Examples thereof include radomes, helmets, ballistic
protection panels for shoulders or other means of protection for,
for instance, soldiers and armour panels for automobiles or for
military helicopters.
[0010] The invention in particular also relates to a radome,
obtained by the process of the invention. The invention further
relates to a radome for enclosing and protecting a radar antenna,
particularly the type carried by aircrafts, said radome comprising
the article of the invention. By radome is herein understood any
structure used to protect electromagnetic radiation equipment, e.g.
radar equipment, for e.g. aircraft, ground or ship based. In case
that the radome is aircraft based, the radome can be shaped and
positioned as the nose of the aircraft, a portion of the wing or
fuselage or the tail of the aircraft. The advantage of the radome
of the invention is that is has an improved distribution of
stiffness while having also an improved E-field distribution.
[0011] A further important advantage of the inventive radome is
that said radome has a lighter weight, especially when gel spun
fibers of ultrahigh molecular weight polyethylene are used thereof,
than known radomes with similar constructions while having improved
structural and electromagnetic functions. It was surprisingly
discovered that the inventive radome it is not tuned to a narrow
frequency band as compared to known radomes. Yet a further
important advantage of the inventive radome is that it has an
increased resistance against projectiles, e.g. in case of military
aircrafts, as well as against bird strikes, hail and the like.
[0012] Local adhering of reinforcing elements to each other may be
accomplished in a number of ways. With locally adhering is meant
that at most 20% of the reinforcing elements are adhered to each
other in any direction, i.e. across the surface of the curved
preform or over the thickness thereof. A preferred embodiment of
the process according to the invention comprises the step of
locally adhering at least part of the reinforcing elements to each
other by locally bringing the drawn polymeric reinforcing elements
to above their melting temperature and fusing them together.
Bringing the reinforcing elements to above their melting or
softening temperature and fusing them together may for instance be
accomplished by local heating means such as IR sources, heated
cutting elements and the like.
[0013] In another embodiment of the process, the step of locally
adhering at least part of the elements to each other comprises
locally applying a binder to the drawn polymeric reinforcing
elements and solidifying the binder. The binder may be any material
having adhesive properties, and may be applied by spraying,
dipping, and the like.
[0014] In still another embodiment of the process, the process
further comprises providing the curved product with a polymer film
layer, which layer has a lower melting point than the reinforcing
elements. Such a polymer film layer further stabilizes the curved
preform. The polymer film layer may extend over part of the
preforms surface, or alternatively over substantially the entire
surface of the preform, including its inner, outer and side
surfaces.
[0015] In still another preferred embodiment of the process, the
process further comprises enclosing the curved preform in a polymer
film envelope, to which a vacuum is applied. Preferably the polymer
film envelope is in virtually full contact with the surfaces of the
preform, prior to applying the vacuum inside the envelope. This
prevents the preform from deforming or even collapsing when
applying the vacuum pressure. Such an embodiment of the process
maintains the quality during transport of the preform product and
increases its coherence. Moreover, when the curved preform product
is used to manufacture a curved article by compressing it in a
mould at an elevated temperature and pressure, the polymer film
envelope does not need to be removed. Instead it may at least
partly melt and form an integral part of the compression moulded
armour article, or be wasted.
[0016] Reinforcing elements may comprise drawn polymer films and/or
fibers. Such drawn polymer films are preferably slitted to form
tapes. Films may be prepared by feeding a polymeric powder between
a combination of endless belts, compression-moulding the polymeric
powder at a temperature below the melting point thereof and rolling
the resultant compression-moulded polymer thereby forming a film.
Another preferred process for the formation of films comprises
feeding a polymer to an extruder, extruding a film at a temperature
above the melting point thereof and drawing the extruded polymer
film. If desired, prior to feeding the polymer to the extruder, the
polymer may be mixed with a suitable liquid organic compound, for
instance to form a gel, such as is preferably the case when using
ultra high molecular weight polyethylene. Drawing, preferably
uniaxial drawing, of the films to produce tapes may be carried out
by means known in the art. Such means comprise extrusion stretching
and tensile stretching on suitable drawing units. To attain
increased mechanical strength and stiffness, drawing may be carried
out in multiple steps. The resulting drawn tapes may be used as
such for filament winding the curved preform product, or they may
be cut to their desired width, or split along the direction of
drawing. The width of suitable unidirectional tapes usually depends
on the width of the film from which they are produced. In the
product and method according to the invention, the width of the
tapes preferably is at least 3 mm, more preferably at least 5 mm.
The width of the tapes preferably is less than 30 mm, more
preferably less than 15 mm, and most preferably less than 10 mm, to
further prevent wrinkling during the winding process. The areal
density of the tapes can be varied over a large range, for instance
between 5 and 200 g/m.sup.2. Preferred areal density is between 10
and 120 g/m.sup.2, more preferred between 15 and 80 g/m.sup.2 and
most preferred between 20 and 60 g/m.sup.2.
[0017] Various forms of fiber can be employed in the process
according to the invention. "Fiber" includes a body whose length is
far greater than the transverse dimensions, and comprises a
monofilament, a multifilament yarn, a strip, ribbon or tape and the
like. Suitable fibres include multifilament yarns, the thickness
and number of filaments not being critical. Suitable yarns have a
titer of for example 100 to 4000 dtex. The filament thickness from
which the yarns are made may vary from for example 1 to 20 dpf. It
is also possible to use a yarn spun from short filaments or staple
fibers. Preferably, however, multifilament yarns are used.
[0018] Suitable polymeric reinforcing elements, such as fibers and
tapes are prepared from a polymeric material whose macromolecules
exhibit a certain degree of chain slip at a temperature below the
melting point, i.e. in the solid phase, under the influence of an
imposed stress. Examples hereof include various polyolefins, such
as for example polyethylene, polypropylene, and copolymers thereof,
optionally with other monomers, polyvinyl alcohol, and polyamides
and polyesters, especially polyamides and polyester that contain at
least one aliphatic monomeric unit. Preferably, a polyolefin, and
in particular a polyethylene is applied. A polyethylene fiber or
tape is preferably made from a linear polyethylene, i.e. a
polyethylene with less than 1 side chain, containing at least 10
carbon atoms per 100 C atoms, more preferably less than 1 side
chain per 300 C atoms.
[0019] The reinforcing elements preferably have a tensile strength
of at least 1.6 GPa, more preferably at least 1.8 GPa, and most
preferably at least 2 GPa, and a tensile modulus of at least 50
GPa. More preferably, the tensile strength is at least 2.5 and even
3 GPa and the modulus at least 70 and even 90 GPa. The tensile
properties of the reinforcing elements are determined by a method
as specified in ASTM D885M. The use of reinforcing elements with
such a high modulus and tensile strength allows objects to be
manufactured with good flexural stiffness and ballistic properties
and high resistance to extraneous forces, such as helmets.
[0020] Particularly preferred fibers to be used in the process
according to the invention comprise fibers prepared from ultra-high
molecular weight polyethylene (UHPE). UHPE is understood to be a
preferably linear polyethylene with an intrinsic viscosity (IV, as
determined on solutions in decalin at 135.degree. C.) of at least 4
dl/g, preferably at least 8 dl/g. The preparation and properties of
these fibers are described in numerous publications including GB
204214 A and WO 01/73171 A1 and such fibers are commercially
available, for instance with the trade name Dyneema.RTM. of DSM
(NL).
[0021] The reinforcing elements may also comprise a combination of
different fibers and/or tapes, in addition to the reinforcing
elements already mentioned above. By combining different fibers
and/or tapes the anti-ballistic performance of the article can be
even better adjusted to the expected ballistic threat. Other fibers
and/or tapes to be suitably applied in the product according to the
invention include glass fibers, aramid fibers, carbon fibers, as
well as other drawn thermoplastic polymer fibers, comprising
poly(p-phenylene-2, 6-benzobisoxazole) fibers (PBO, Zylon.RTM.),
and
poly(2,6-diimidazo-(4,5b-4',5'e)pyridinylene-1,4(2,5-dihydroxy)phenylene)
fibers (better known as M5.RTM. fibers).
[0022] According to the process of the invention, a curved product
is made by positioning a plurality of reinforcing elements, such as
fibers or tapes onto a mandrel. The process preferably comprises
unwinding bobbins of reinforcing elements under tension. In a
preferred embodiment the curved product is manufactured by
simultaneously positioning a plurality of reinforcing elements such
as fiber bundles onto the mandrel using supply means during the
filament winding process. Suitable supply means comprise a creel
provided with bobbins, and eventually guiding means, for instance
in the form of dispensing tubes, to guide the reinforcement
elements over the surface of the mandrel. This measure allows for
filament winding with reduced production times, while keeping the
device simple. It is especially advantageous in case of producing
large series of products. A plurality of reinforcing elements is
preferably comprised between 10 and 60 reinforcing elements, and
more preferably between 24 and 48. A compromise between
manageability and production time is hereby obtained.
[0023] During the filament winding process the supply means
preferably move relative to the mandrel. In still another preferred
embodiment the supply means adjust the length of a reinforcing
element spanning the distance between the mandrel surface and the
supply means. This measure leads to a more constant tension on the
reinforcing elements and therefore to a product with better
quality. Adjusting the free length of the reinforcing elements
between supply means and mandrel surface may for instance be
carried out by providing supply means in the form of fiber
dispensing tubes, provided with electro motors that act on the
fiber bobbins. According to a preferred embodiment the supply means
control the tension of a reinforcing element between the mandrel
surface and the supply means. Control of the tension in the
reinforcing elements is favorable for the mechanical properties of
the product or article, and in particular for its stiffness and
strength.
[0024] Filament winding patterns are preferably restricted to
"about geodetic patterns", although this is not necessary for the
invention. A winding trajectory is geodetic when it spans the
shortest distance between two points on the curved surface of the
product. The design of winding patterns requires special care
regarding the choice of a pattern that fulfils the "about geodetic
pattern" and prevents local accumulation of fibers at certain
locations. Optimal curved products are preferably wound in such a
way that local accumulations are sufficiently "diluted" or spread
over the product surface. Modern software, known per se, allows
filament winding specialists to design winding patterns with
sufficient "dilution" of local fiber accumulations. Also, trial and
error methods may be employed to obtain adequate winding
patterns.
[0025] In a preferred embodiment the curved product is produced by
filament winding reinforcing elements onto a mandrel with polar
surfaces, which mandrel rotates around a central shaft, the polar
surfaces being that part of the mandrel where the central shaft
enters or exits. Reinforcing elements are positioned onto such a
mandrel substantially over its surface and its polar surfaces. In
this way a substantially closed product is obtained which is
subsequently partitioned in two halves, thus producing two similar
curved products in one time. Due to the presence of the shaft
during winding, the apex of the curved products will generally have
an opening when the shaft is removed after the winding process.
Such an opening, if present after winding and/or pressing, may for
instance be closed by inserting in it a fitting plug. This process
may be further enhanced by adopting a non-geodetic pattern on the
polar surfaces. Because of the presence of the shaft, the
reinforcing elements may be wound around this shaft under tension
and in a non-geodetic pattern (in the end article). When removing
the shaft, the non-geodetic reinforcing elements under tension
reposition from their non-geodetic pattern to a pattern that is
closer to a geodetic pattern, thereby reducing the opening and/or
clamping, and thereby better fixing the plug.
[0026] The curved product is readily removed from the mandrel
without there being any need to dry and/or cure any polymer matrix,
and can therefore be readily finished by edge trimming for
instance, e.g. by using the heated cutting device that was
mentioned earlier.
[0027] The invention also relates to a process for manufacturing a
curved article, the process comprising positioning a curved preform
obtained by a process according to the invention in a mould and
compressing said preform at an elevated temperature and pressure.
Since the curved preform has substantially the shape of the curved
article, the step of compressing said preform at an elevated
temperature and pressure does not appreciably cause any wrinkles or
other distortions in the produced article, preferably the produced
armour article. This improves the anti-ballistic properties of the
armour article. Since the curved product of the invention does not
need the presence of a polymer matrix, an armour article with a low
amount or no polymer matrix at all can be obtained which also
benefits the anti-ballistic properties of the armour article.
[0028] According to the invention the filament wound curved preform
contains reinforcement elements, in particular fibers, only. In the
process of manufacturing a curved armour article according to the
invention, the reinforcing elements by a combination of elevated
temperature, pressure and time, partly soften and fuse, also known
as sintering, at the surface. Since the reinforcement elements were
already positioned in their desired number and direction in the
filament wound curved preform, this product is given the desired
properties and shape by only slightly deforming it at elevated
temperature and pressure in a mould. Such deformation may be
effected using suitable techniques known per se such as forming
with the aid of a heated die and mould. Prior to compression
moulding the intermediate product may be brought to a temperature,
for example in an oven, which is preferably below the melting point
of the reinforcing elements. Preferably, this temperature is about
equal to the temperature of the mould. Preferably also, an envelope
is used to which vacuum is applied, as described above.
[0029] In the process of manufacturing a curved article from a
filament wound preform by compression moulding, the person skilled
in the art will generally be able to choose a suitable combination
of elevated temperature, pressure as well as time to adequately
consolidate the product. The desired shaping will generally take
place in about 1 to 60 minutes, preferably about 5 to 45 minutes,
for a preform containing fibers made of ultra-high molecular
polyethylene. Elevated pressures applied to the preform in
producing a curved article, such as an armour article or radome,
may vary widely, but are preferably higher than about 7 MPa, more
preferably higher than about 10 MPa, even more preferably higher
than about 15 MPa, a higher pressure yielding the better results.
Due to the applied elevated pressure, the reinforcement elements
are substantially kept tensioned, thus preventing the good
mechanical properties from being lost or significantly diminished
as a result of molecular relaxation. The elevated temperature is
preferably selected between 80.degree. C. and 40.degree. C. below
the melting or softening temperature of the reinforcing elements,
and more preferably between 80.degree. C. and 20.degree. C. below
the melting or softening temperature of the reinforcing elements,
which range for most practical applications is between 80.degree.
C. and 145.degree. C. After forming at elevated temperature and
pressure, the curved article is preferably cooled until the article
has reached a temperature lower than about 80.degree. C., in order
to avoid any undesired relaxation processes in the reinforcing
elements.
[0030] The number of reinforcing elements positioned on the mandrel
during filament winding may be varied and is generally such that
the desired thickness is reached. In the process according to the
invention, many different winding patterns for the reinforcing
elements are in principle possible. The process according to the
invention allows producing preforms and end articles having a wide
variety of wall thicknesses without substantial wrinkling, which is
beneficial for anti-ballistic performance. The winding process is
preferably carried out such that the produced article has a
constant thickness over substantially its entire surface. From a
standpoint of antiballistic performance, the wall thickness of the
article according to the invention is preferably above 3 mm, more
preferably above 5 mm, even more preferably above 9 mm, and most
preferably above 12 mm.
[0031] In a preferred embodiment of the product according to the
invention the mandrel used in producing the product by the filament
winding process is a similar product but made with another
technology. More in particular, a conventional helmet with a thin
shell is used as a mould for filament winding. In this way, only
the outer part of the helmet consists of filament wound material.
The advantage is that a thin helmet is more easily produced with
conventional technology than a thick helmet, yet the advantage of
filament winding is present to a large extent. Moreover, adaptation
of the helmet to heavier threats is easy, just by applying
additional windings.
[0032] The present invention will now be further elucidated by the
following example and comparative experiment, without however being
limited thereto.
EXAMPLE I
[0033] Antiballistic Dyneema.RTM. UHMWPE fibers of type SK76 1760
dtex were wound onto a cone-shaped mandrel, fixed by a shaft having
a diameter of 20 mm. The fibers were held under tension during
winding. The last windings of the UHWMPE fibers were lightly
sprayed with 0.1% by weight with respect to the total weight of the
fibers of an adhesive binder to partly adhere these to each other.
The wound product was partitioned in two halves using a hot knife
and then removed from the mandrel, thereby obtaining two similar
preforms in the shape of a combat helmet, each weighing 1140 grams.
An opening with a diameter of about 20 mm was present at the apex
after removal of the shaft. Both preforms were then enclosed in a
tightly fitting polyethylene film envelope, to which a vacuum was
applied. The preforms with envelope were then placed between the
two mould halves of a compression moulding press, leaving a free
space in the closed position between the two mould halves of 7 mm.
The mould halves were heated to an elevated temperature of
140.degree. C. An elevated pressure of 165 bar was applied to the
preforms for a period of about 60 minutes. After cooling to
80.degree. C. under pressure, the compression moulded helmet was
removed from the mould halves and underwent edge finishing. The
final weight of the helmet was 0.98 kg and the diameter of the
cranial opening was during compression moulding decreased to 10 mm.
The opening was closed by inserting a fitting plug.
COMPARATIVE EXPERIMENT A
[0034] A flat stack of 38 plies of Dyneema.RTM. HB25 sheets were
compression moulded between two mould halves at a temperature of
140.degree. C., and an elevated pressure of 165 bar for a period of
about 60 minutes. During compression moulding the flat stack was
deformed into a three-dimensionally shaped helmet. After cooling to
80.degree. C. under pressure the helmet was removed from the mould
halves and underwent edge finishing.
Results
[0035] The characteristics of the helmets produced according to the
invention are summarized in Table 1.
TABLE-US-00001 TABLE 1 characteristics of produced helmets
Comparative Example I Experiment A Weight [kg] 0.98 0.83 Forming
method Compression of filament Compression moulding wound
intermediate product of flat stack
[0036] The helmets were shot with 1.1 gram (17 grain) Fragment
Simulating Projectiles (FSP). The obtained anti-ballistic test
results V.sub.50 and absorbed energy E.sub.abs are given in Table
2. The helmets were also loaded by a so-called `ear-to-ear`
compression load. In such a test, a compression force is applied to
the sidewalls of the helmets at the area of the ears of a wearer,
whereby the total inner displacement of the sidewalls is measured.
The results of this test are summarized in Table 3.
TABLE-US-00002 TABLE 2 anti-ballistic results E.sub.abs
[J/kg/m.sup.2] Example I 41 Comparative Experiment A 31
TABLE-US-00003 TABLE 3 `ear-to-ear` compression test results
Applied load [N] Displacement [mm] Example I 1500 17 Comparative
500 >40 Experiment A
[0037] Based on the amount of energy a helmet can absorb, a
filament wound helmet according to the invention performs better
than a conventionally pressed helmet. The typical value for the
absorbed energy of a conventional helmet is around 30 J/kg/m.sup.2.
The absorbed energy of a helmet according to the invention is more
than 40 J/kg/m.sup.2, a performance increase of at least 30%.
[0038] As to the "ear to ear" compression test results, a filament
wound helmet according to the invention performs considerably
better than a conventionally pressed helmet. The measured "ear to
ear" displacement is 17 mm only, whereby the conventional helmet
shows a higher displacement at a lower load.
[0039] Adding to the performance increase the filament winding
process in combination with the preferred absence of a polymer
matrix offers other advantages, such as the absence of wrinkles in
the final article, cheaper starting materials, obviating the use of
expensive cross-ply prepregs, freedom of combining different
materials, a low amount of waste and the opportunity for
far-reaching automation.
* * * * *