U.S. patent application number 13/811054 was filed with the patent office on 2013-05-16 for composite materials.
This patent application is currently assigned to Hexcel Composites Limited. The applicant listed for this patent is John Ellis, Emilie Fisset. Invention is credited to John Ellis, Emilie Fisset.
Application Number | 20130122277 13/811054 |
Document ID | / |
Family ID | 42937866 |
Filed Date | 2013-05-16 |
United States Patent
Application |
20130122277 |
Kind Code |
A1 |
Ellis; John ; et
al. |
May 16, 2013 |
COMPOSITE MATERIALS
Abstract
A strip of curable prepreg comprising unidirectional fibres
aligned with the length of the strip, the fibres being at least
partially impregnated with curable thermosetting resin and
comprising a flexible polymeric sheet on an outer face of the
strip, wherein the strip has a substantially rectangular
cross-section defining a width and a thickness of the strip.
Inventors: |
Ellis; John; (Duxford,
GB) ; Fisset; Emilie; (London, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ellis; John
Fisset; Emilie |
Duxford
London |
|
GB
GB |
|
|
Assignee: |
Hexcel Composites Limited
Duxford
GB
|
Family ID: |
42937866 |
Appl. No.: |
13/811054 |
Filed: |
August 11, 2011 |
PCT Filed: |
August 11, 2011 |
PCT NO: |
PCT/EP2011/063889 |
371 Date: |
January 18, 2013 |
Current U.S.
Class: |
428/220 ;
156/192; 156/250; 414/800; 428/98 |
Current CPC
Class: |
B32B 2270/00 20130101;
B32B 2307/546 20130101; B26D 1/245 20130101; B32B 2260/021
20130101; B29K 2067/00 20130101; B29C 2793/0081 20130101; B32B 3/08
20130101; B32B 5/022 20130101; B32B 27/32 20130101; B29K 2995/0094
20130101; B32B 2260/046 20130101; B65H 2301/4148 20130101; B32B
2262/14 20130101; B29K 2105/243 20130101; Y10T 428/24 20150115;
B32B 7/06 20130101; B32B 2262/101 20130101; B32B 2605/18 20130101;
B29C 70/386 20130101; B32B 3/266 20130101; B32B 27/36 20130101;
B32B 2037/1253 20130101; B29C 2793/0036 20130101; B32B 5/00
20130101; B32B 2307/748 20130101; B32B 37/16 20130101; Y10T 156/10
20150115; B29K 2023/06 20130101; B29K 2023/12 20130101; B32B
2262/0269 20130101; B32B 5/08 20130101; B29C 70/086 20130101; B32B
2262/106 20130101; B32B 2307/58 20130101; B32B 27/12 20130101; B32B
37/12 20130101; Y10T 156/1052 20150115 |
Class at
Publication: |
428/220 ; 428/98;
156/250; 156/192; 414/800 |
International
Class: |
B32B 5/00 20060101
B32B005/00; B32B 3/08 20060101 B32B003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 12, 2010 |
GB |
1013524.2 |
Claims
1. A strip of curable prepreg comprising unidirectional fibres
aligned along the length of the strip, the fibres being at least
partially impregnated with curable thermosetting resin and
comprising a flexible polymeric sheet on an outer face of the
strip, Wherein the strip has a substantially rectangular
cross-section defining a width and a thickness of the strip, the
difference between the maximum width and the minimum width along
the length of the strip being less than 0.25 mm.
2. The strip of claim 1, wherein the strip following winding and/or
unwinding from a spool or bobbin has a difference between the
maximum width and the minimum width along the length of the strip
of less than 0.25 mm.
3. The strip of claim 1, wherein the strip has a length of at least
100 m.
4. The strip of claim 1, wherein the strip has a width of from 2.0
to 3000 mm.
5. The strip of claim 1, wherein the flexible polymeric sheet is a
polyalphaolefin film or a polyalphaolefin copolymer film.
6. The strip of claim 1, wherein the flexible polymeric sheet is
porous.
7. A process for forming a plurality of strips of prepreg from a
sheet of prepreg, wherein the sheet of prepreg comprises
unidirectional fibres at least partially impregnated with curable
thermosetting resin and comprising a flexible polymeric sheet on an
outer face of the sheet of prepreg, the process comprising the step
of slitting the prepreg into a plurality of strips, the slits being
substantially parallel to the direction of alignment of the
fibres.
8. The process according to claim 7, wherein the polymeric sheet is
applied under a compressive force before reaching the slitting
stage, of at least 0.1 MPa.
9. The process of claim 7, wherein the difference between the
maximum width and the minimum width along the length of the strip
is less than 0.25 mm.
10. The process of claim 7, wherein the strip following slitting
and winding and/or unwinding from a spool or bobbin has a
difference between the maximum width and the minimum width along
the length of the strip of less than 0.25 mm.
11. Use of a flexible polymeric sheet on an outer face of a prepreg
during slitting of the prepreg into a plurality of strips, said
prepreg comprising unidirectional fibres at least partially
impregnated with curable thermosetting resin, slitting taking place
in a longitudinal direction relative to the unidirectional fibers,
the difference between the maximum width and the minimum width
along the length of the strip being less than 0.25 mm.
12. Use of claim 11, wherein the strip following slitting and
winding and/or unwinding from a spool or bobbin has a difference
between the maximum width and the minimum width along the length of
the strip of less than 0.25 mm.
13. Use of claim 11, wherein the strip has a length from 10 to 5000
m.
14. A process of laying down a plurality of strips according to
claim 1, by means of an automated strip laying apparatus, the
apparatus being arranged to lay the strips down parallel to each
other and with a gap between the strips of less than 1.00 mm.
15. A process according to claim 14, wherein the strips are laid in
contact with one another.
Description
TECHNICAL FIELD
[0001] The present invention relates to strips of prepreg,
particularly for application by an automated lay-up apparatus.
BACKGROUND
[0002] Composite materials have well-documented advantages over
traditional construction materials, particularly in providing
excellent mechanical properties at very low material densities. As
a result, the use of such materials is becoming increasingly
widespread and their fields of application range from "industrial"
and "sports and leisure" to high performance aerospace
components.
[0003] Prepregs, comprising a fibre arrangement impregnated with
resin such as epoxy resin, are widely used in the generation of
such composite materials. Typically a number of plies of such
prepregs are "laid-up" as desired and the resulting laminate is
cured, typically by exposure to elevated temperatures, to produce a
cured composite laminate.
[0004] In a typical lay-up procedure, the prepreg is provided as a
roll of material to provide length (e.g. of the order of several
metres) of prepreg of a predefined width (e.g. of the order of
several centimetres) and thickness (e.g. of the order tenths of
millimetres up to several millimetres). Typically cure ply
thicknesses of prepregs for aerospace applications range from 0.125
mm to 0.25 mm and their fibre average weights range from 100 to 300
g/m.sup.2.
[0005] To facilitate such unrolling, the prepreg typically has an
outer layer of backing paper which prevents the adjacent layers of
prepreg from sticking together. Such backing paper is discarded
before lay-up.
[0006] A common method on laying up such prepreg is by means of an
automated lay-up apparatus. This is a much more efficient method of
laying up prepreg as compared to conventional hand lay-up. However,
it does impose additional constraints on the dimensions of the
prepreg, if it is to automatically lay down the prepreg at an
acceptable quality standard.
[0007] When laying prepreg to form a structure which has relatively
high curvature, it is known to lay down strips of prepreg which
have a much smaller width than conventional prepreg. This reduces
any wrinkling that a wider strip of prepreg may suffer from during
lay-up.
[0008] It is known in the art to produce so-called slit tape
prepreg, which is produced by passing a sheet of prepreg through a
slitting or cutting unit to produce a plurality of parallel strips
of prepreg. The width of the strips produced are very tightly
controlled and can be specified to within a fraction of a
millimetre.
[0009] Sheets of prepreg are typically manufactured with outer
sheets of backing paper which allows the high compressive forces
required for resin impregnation to be applied. As discussed above,
the backing paper also allows the prepreg to be rolled up onto
itself once produced without adjacent layers of prepreg adhering to
each other.
[0010] However, it is known that passing prepreg with its backing
paper through the slitting unit comprising slitting blades produces
paper debris which produces an unacceptable result. It is therefore
common practice to remove the backing paper before slitting.
[0011] Once the strips of prepreg are produced it is conventional
to apply a backing sheet (which is wider than the strip of prepreg)
and way-wind the strip onto a bobbin or spool. Such a bobbin is
usually capable of holding several thousands of metres of such
strip prepreg.
[0012] Such a bobbin or a plurality thereof, can be adapted for use
with an automated lay-up apparatus, which automatically unravels
the tape, removes the backing sheet and lays down the strips of
prepreg. Typically a plurality of strips of prepreg are laid down
parallel to each other.
[0013] Clearly it is desirable that any gaps or overlaps between
adjacent strips are minimised. This is all the more important when
a high quality result is essential, such as when constructing an
aerospace component.
[0014] However it has been found that an intentional gap must be
imposed if overlap of adjacent tapes is to be avoided. This is
because variation in the width of the strips of prepreg has been
observed in practice, and overlapping strips are more problematic
than a gap between strips.
[0015] The present invention aims to mitigate or at least obviate
the above described problems and/or to provide advantages
generally.
SUMMARY OF THE INVENTION
[0016] According to the invention, there is provided a strip, a
process and a use as defined in any one of the accompanying
claims.
[0017] In this way, any intentional gaps between strips could be
reduced without increasing the risk of overlaps between adjacent
strips by providing strips of prepreg with a tighter tolerance on
their width as they are laid down.
[0018] In a first aspect, the invention relates to a process for
forming a plurality of strips of prepreg from a sheet of prepreg,
wherein the sheet of prepreg comprises unidirectional fibres at
least partially impregnated with curable thermosetting resin and
comprising a flexible polymeric sheet on an outer face of the sheet
of prepreg, the process comprising the step of slitting the prepreg
into a plurality of strips, the slits being substantially parallel
to the direction of alignment of the fibres.
[0019] In a second aspect, the invention relates to a strip of
curable prepreg comprising unidirectional fibres aligned with the
length of the strip, the fibres being at least partially
impregnated with curable thermosetting resin and comprising a
flexible polymeric sheet on an outer face of the strip, wherein the
strip has a substantially rectangular cross-section defining a
width and a thickness of the strip.
[0020] The inventors have found that strips of prepreg immediately
following slitting have a very small variation in their width. It
has therefore been surmised that any distortion of the width of
conventional strips must be introduced in the process steps that
follow slitting. The inventors have now found that if the prepreg
sheet is provided with a polymeric sheet as its outer backing
sheet, then this can remain in place during slitting without
generating unacceptable debris as is found when paper is
employed.
[0021] Additionally, and more importantly, it has been found that
the variation in the width of the strips produced in this way is
significantly reduced, providing a tighter tolerance and allowing a
narrower gap to be required when automatically laying down a
plurality of such strips. It is believed that the polymeric sheet
assists in retaining the fibres in their initial position following
slitting, reducing any spreading of the fibres in subsequent
handling operations.
[0022] The strips produced are typically continuous in their
length, and can have lengths of several thousands of metres. Due to
processing limitations such lengths may involve a splice but this
is considered to be a continuation of the same strip. Thus, the
strips can have a length of at least 500 m, preferably at least
1,000 m, more preferably at least 2,000 m, most preferably of least
4,000 m.
[0023] The substantially rectangular cross-section of the strip is
typically well-defined with a clear width and a clear thickness. In
view of the fact that the polymeric sheet was present during
slitting there is no initial difference in width between the
polymeric sheet and the remainder of the strip. The width of the
strips is typically in the range of from 2.0 to 50 mm, preferably
from 3.0 to 25 mm. However depending on the applications the width
may also range from 10 mm to 3500 mm, or from 50 mm to 3000 mm, or
from 100 mm to 2000 mm, or from 150 mm to 2000 mm, or from 200 mm
to 2000 mm. The thickness is typically in the range of from 0.05 to
1.0 mm, primarily depending on the quantity of fibres per strip as
desired.
[0024] In one embodiment the sheet of prepreg comprises a second
polymeric sheet on the other outer face of the prepreg during the
slitting stage.
[0025] As discussed above, the strips of prepreg have a very tight
tolerance in their width. Thus, the difference between the maximum
width of the minimum width is typically less than 0.25 mm, or less
than 0.20 mm, or even less than 0.125 mm along the length of the
strip.
[0026] The polymeric sheet may take a variety of forms provided it
is sufficiently flexible. However it is preferably a film, being
non-porous and uniform across the sheet. Also, the polymeric sheet
may be porous or perforated to improve the release of the sheet
from the curable strip. The polymeric sheet may comprise holes or
apertures.
[0027] The thickness of the polymeric sheet can be selected as
desired according to the particular situation. However, thicknesses
in the range of from 10 to 150 micrometres, preferably from 10 to
100 micrometres, is a suitable range.
[0028] The polymeric sheet may comprise a polyolefin,
polyalphaolefin and/or combinations or copolymers thereof. The
sheet may be made from a wide variety of materials, for example
polyethylene, polyethylene terephthalate, polypropylene, and many
other suitable polymers and/or combinations or copolymers
thereof.
[0029] The unidirectional fibres may comprise cracked (i.e.
stretch-broken), selectively discontinuous or continuous fibres,
although continuous fibres are preferred. The term "unidirectional"
is a term of art and means that the fibres are aligned with a
common direction and each filament is free and not interwoven with
the other filaments.
[0030] The unidrectional fibres may be made from a wide variety of
materials such as carbon, graphite, glass, metallised polymers
aramid and mixtures thereof.
[0031] The fibres are preferably substantially completely
impregnated with resin, with all of the fibres being in contact
with the curable resin.
[0032] The curable thermosetting resin may be selected from epoxy,
isocyanate and acid anhydride, for example. Preferably the curable
resin is an epoxy resin.
[0033] Suitable epoxy resins may comprise monofunctional,
difunctional, trifunctional and/or tetrafunctional epoxy
resins.
[0034] Suitable difunctional epoxy resins, by way of example,
include those based on; diglycidyl ether of Bisphenol F, Bisphenol
A (optionally bromianted), phenol and cresol epoxy novolacs,
glycidyl ethers of phenol-aldelyde adducts, glycidyl ethers of
aliphatic diols, diglycidyl ether, diethylene glycol diglycidyl
ether, aromatic epoxy resins, aliphatic polyglycidyl ethers,
epoxidised olefins, brominated resins, aromatic glycidyl amines,
heterocyclic glycidyl imidines and amides, glycidyl ethers,
fluorinated epoxy resins, or any combination thereof.
[0035] Difunctional epoxy resins may be preferably selected from
diglycidyl ether of Bisphenol F, diglycidyl ether of Bisphenol A,
diglycidyl dihydroxy naphthalene, or any combination thereof.
[0036] Suitable trifunctional epoxy resins, by way of example, may
include those based upon phenol and cresol epoxy novolacs, glycidyl
ethers of phenol-aldehyde adducts, aromatic epoxy resins, aliphatic
triglycidyl ethers, dialiphatic triglycidyl ethers, aliphatic
polyglycidyl ethers, epoxidised olefins, brominated resins,
triglycidyl aminophenyls, aromatic glycidyl amines, heterocyclic
glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy
resins, or any combination thereof.
[0037] Suitable tetrafunctional epoxy resins include
N,N,N',N'-tetraglycidyl-m-xylenediamine (available commercially
from Mitsubishi Gas Chemical Company under the name Tetrad-X, and
as Erisys GA-240 from CVC Chemicals), and
N,N,N',N'-tetraglycidylmethylenedianiline (e.g. MY721 from Huntsman
Advanced Materials).
[0038] In view of the length of the strip according to the
invention, the strip is typically wound onto a bobbin or spool. A
particularly suitable winding involves the strip passing up and
down the bobbin as it is wound, like thread on a spool with
multiple windings being possible before the strip winds on top of
previous windings of strip. Such a method of winding is called
"way-wound".
[0039] Before being wound on the bobbin, the strip may be brought
into contact with a second backing sheet. Typically this will only
be required when there is only one polymeric sheet on one outer
face of the prepreg. This involves the face not covered in the
polymeric sheet coming into contact with the second backing sheet.
Unlike the polymeric sheet, the second backing sheet is preferably
wider than the resin and fibres in the strip. This helps to prevent
any adhesion of adjacent strips on the bobbin.
[0040] In an alternative embodiment, a second backing sheet may be
applied onto the polymeric sheet. Upon unwinding of the spool or
bobbin, the second backing sheet may be located on the outer
surface of the strip which is not covered by the polymeric sheet.
This promotes release of the backing sheet without distortion of
the fibres.
[0041] The backing sheet may be non-porous or may be porous to
facilitate removal of the backing sheet from the strip upon or
prior to its application in the lay up.
[0042] The process of manufacture of the strips according to the
invention is typically a continuous process.
[0043] In a typical process one or more rotary blades are
positioned as the sheet of prepreg is brought into contact with the
blade or blades. Generally it is desirable to produce strips of
prepreg of the same width from a single sheet of prepreg, thus
preferably any blades are evenly spaced apart.
[0044] Before slitting, the sheet of prepreg can be manufactured in
a conventional prepreg manufacturing process. As discussed above,
it is conventional for a backing paper to be applied during prepreg
manufacture. If this is the case then the paper must be removed
before the prepreg passes to the slitting stage. In this
embodiment, the polymeric sheet can be added before the sheet of
prepreg passes to the slitting stage.
[0045] Alternatively, the sheet of prepreg can be manufactured with
the polymeric sheet as the backing material instead of using paper.
As the resin impregnation stage of prepreg manufacture can involve
high temperatures, the polymeric sheet must be heat-tolerant in
this embodiment.
[0046] However the sheet of prepreg is manufactured, it is
generally the case that the polymeric sheet will have been pressed
onto the resin and fibres under high pressure.
[0047] This serves to form a stronger adhesive bond between the
polymeric sheet and the resin and fibres and is believed to
contribute to how the polymeric sheet acts to maintain the uniform
width of the strip.
[0048] Thus, preferably the polymeric sheet has been applied under
a compressive force before reaching the slitting stage, of at least
0.1 MPa, more preferably at least 0.2 MPa, most preferably at least
0.4 MPa.
[0049] As a result of the uniform width of the strip, it is
therefore possible to automatically lay down a plurality of
parallel strips with a reduced gap.
[0050] Thus, in a third aspect, the invention relates to a process
of laying down a plurality of strips by means of an automated strip
laying apparatus, the apparatus being arranged to lay the strips
down parallel to each other and with a gap between the strips of
less than 1.00 mm.
[0051] Preferably the gap is less than 0.80 mm, more preferably
less than 0.60 mm, or even less than 0.40 mm. Adjacent strips may
also be in contact with one another along at least part of their
length.
[0052] The invention will now be illustrated, by way of example,
and with reference to the following figures, in which:
[0053] FIG. 1 is a schematic representation of a process according
to the present invention;
[0054] FIG. 2 is a schematic representation of a process not
according to the present invention;
[0055] FIG. 3 is a perspective view of a portion of a strip of
prepreg according to the present invention; and
[0056] FIG. 4 is a chart showing the spread of slit tape width
compared to the slit width generated at the slitting stage for
embodiments according to the invention and comparative
embodiments.
EXAMPLES
[0057] Unidirectional carbon fibres were impregnated with an epoxy
resin to form a prepreg (M21E/34%/UD268/IMA GS 12K) and was
produced with process paper on one side. The paper on the prepreg
was removed and a low density polyethylene (LDPE) sheet (LDPE-Folie
from Huhtamaki) added to take its place. The polyethylene sheet was
pressed onto the prepreg with a pressure of from 0.5 to 3 MPa.
[0058] Slitting of the prepreg was carried out according to the
arrangement shown in FIG. 1. The prepreg 10 with backing sheet was
then passed to a series of parallel slitters 12, which are
precisely arranged to slit the prepreg into slit tapes of a
specified width with a +/-0.125 mm tolerance along the length of
the strips or tapes.
[0059] After slitting, the slit tapes are then passed over guide
and support rollers to their respective spool winding positions 16.
The slit tape is then way-wound onto a cardboard core holder
traversing the length of the core holder to produce a spool of slit
tape.
[0060] Samples of slit tape are then taken from the spool and their
width measured by using a BenchMike 283 series benchtop laser
micrometer. This is a non contact optical measurement involving a
transmitter and receiver, wherein light is projected from the
transmitter by a laser. The sample is placed between the
transmitter and the receiver and the light signal that the receiver
receives is used to calculate the dimensions of the sample.
[0061] A comparative example was carried out according to the
arrangement shown in FIG. 2. Prepreg 10 had its paper backing sheet
removed before slitting by paper rewind 14. The prepreg then passed
through slitters 12 with no backing material. In this case a
polyethylene backing strip, wider than that of the slit tape
produced is applied by polythene unwind 18 before winding onto the
core holder 16.
[0062] FIG. 3 shows a schematic representation of the slit prepreg
20 according to the invention produced by the arrangement shown in
FIG. 1. The strip of prepreg 20 comprises resin-impregnated carbon
fibres which are unidirectional and aligned with the long axis of
the prepreg. The strip 20 also comprises a backing layer of
polythene 24. It can be seen that the strip has a uniform width 26
and thickness 28, whilst the edge 22 has minimal deviations along
the length of the strip.
[0063] Numerous measurements of slit tape widths from slit tapes
according to the invention and the comparative examples were
generated to see if there was a difference in the variation from
the width established by the slitters.
[0064] To facilitate tape lay up in automated tape laying
equipment, slit widths at the slitters are generally set slightly
narrower than the intended application width of the strip or tape.
Following slitting, the width of the strip of tape generally
extends slightly to the desired width. In the embodiment
illustrated in FIG. 4, slit widths were set at 6.223 mm (0.245
inches), 6.274 mm (0.247 inches) and 6.35 mm (0.250 inches) for
material having polyethylene backing sheet whilst being slit and
material having no backing sheet while being slit. FIG. 4 shows the
variation in the slit widths measured with the widths shown in
inches.
[0065] The results are shown using conventional statistical
conventions, with the mean being shown as a horizontal bar and all
the data within 95% confidence limits being represented by the
rectangle around the mean. Outliers are represented by a *.
[0066] It can be clearly seen that a tighter size distribution of
widths can be achieved by the embodiments according to the
invention when a slit width of 6.35 mm (0.25 inches) is set.
[0067] However, although it is not clearly shown by this chart, the
widths are tighter for the 6.274 mm (0.247 inch) results as well.
Further statistical analysis was carried out looking at how wide
the distribution is and whether it corresponds to a normal
distribution. For this analysis, Cp and Cpk data was assessed.
[0068] Cp and Cpk
[0069] The distribution of data is assumed to be normally
distributed. Cp is "process capability" a measure of the spread of
the data within the specification (defined as specification
tolerance divided by six times the standard deviation). Cpk is
"process capability corrected" a measure of the skew of the data
from the mean to show how central the distribution is compared to
the specification (defined as (upper specification limit minus the
average) divided by three times the standard deviation).
[0070] Once the standard deviation of the data set is calculated it
is used to then calculate the Cp and Cpk. If standard deviation is
large then the deviation in data set is large and so process
capability is poor shown by a low Cp value. Conventionally a
Cp>1.33 is accepted as evidence that the process has good
control.
[0071] If the Cpk value is less than the Cp value then the data is
skewed to the left of the distribution and greater than Cp if the
data is skewed to the right of the distribution. Thus when Cp=Cpk
then there is no skew of data and distribution is centred on the
nominal specification target value.
[0072] For the data according to the invention the Cp value was
1.12 and Cpk was 0.83. For the comparative examples the Cp was 0.74
and the Cpk was 0.52. An ideal value of Cp is 1.33, derived from
the 4 sigma statistic.
[0073] The improved distribution in widths of the slit tape
therefore allow a narrower gap width to be set when laying down
such slit tapes without increasing the risk of overlap between
neighbouring slit tapes.
[0074] Alternative embodiments of the invention can be defined as
follows.
[0075] In embodiment 1, there is provided a strip of curable
prepreg comprising unidirectional fibres aligned with the length of
the strip, the fibres being at least partially impregnated with
curable thermosetting resin and comprising a flexible polymeric
sheet on an outer face of the strip, wherein the strip has a
substantially rectangular cross-section defining a width and a
thickness of the strip.
[0076] In embodiment 2, there is provided a strip according to
embodiment 1, having a length of at least 500 m, preferably at
least 1,000 m, more preferably at least 2,000 m, most preferably of
least 4,000 m.
[0077] In embodiment 3, there is provided a strip according to
embodiment 1 or 2, having a width of from 2.0 to 50 mm, preferably
from 3.0 to 25 mm.
[0078] In embodiment 4, there is provided a strip according to
embodiment 1 to 3, having a thickness in the range of from 0.05 to
1.0 mm.
[0079] In embodiment 5, there is provided a strip according to
embodiment 1 to 4, comprising a second polymeric sheet on the other
outer face of the prepreg.
[0080] In embodiment 6, there is provided a strip according to
embodiment 1 to 5, wherein the difference between the maximum width
of the minimum width is less than 0.25 mm.
[0081] In embodiment 7, there is provided a strip according to
embodiment 1 to 6, wherein the polymeric sheet is a film.
[0082] In embodiment 8, there is provided a strip of curable
prepreg according to embodiments 1 to 7, wherein the thickness of
the polymeric sheet is in the range of from 10 to 150 micrometres,
preferably from 10 to 100 micrometres.
[0083] In embodiment 9 there is provided a strip of curable prepreg
according to any one of embodiments 1 to 8, wherein this polymeric
sheet is made from polyethylene, polyethylene terephthalate,
polypropylene, or mixtures thereof.
[0084] In embodiment 10, there is provided a strip of curable
prepreg according to any one of embodiments 1 to 9 which is
way-wound onto a bobbin.
[0085] In embodiment 11, there is provided a process for forming a
plurality of strips of prepreg according to any one of claims 1 to
10 from a sheet of prepreg, wherein the sheet of prepreg comprises
unidirectional fibres at least partially impregnated with curable
thermosetting resin and comprising a flexible polymeric sheet on an
outer face of the sheet of prepreg, the process comprising the step
of slitting the prepreg into a plurality of strips, the slits being
substantially parallel to the direction of alignment of the
fibres.
[0086] In embodiment 12, there is provided a process according to
embodiment 11 which is a continuous process.
[0087] In embodiment 13, there is provided a process according to
embodiment 11 or 12, wherein one or more rotary blades are
positioned as the sheet of prepreg is brought into contact with the
blade or blades.
[0088] In embodiment 14, there is provided a process according to
any one of embodiments 11 to 13, wherein any blades are evenly
spaced apart.
[0089] In embodiment 15, there is provided a process according to
any one of embodiments 11 to 14, wherein the polymeric sheet has
been applied under a compressive force before reaching the slitting
stage, of at least 0.1 MPa, more preferably at least 0.2 MPa, most
preferably at least 0.4 MPa.
[0090] In embodiment 16, there is provided a process of laying down
a plurality of strips according to any one of embodiments 1 to 10
or obtainable by a process according to any one of embodiments 11
to 15, by means of an automated strip laying apparatus, the
apparatus being arranged to lay the strips down parallel to each
other and with a gap between the strips of less than 1.00 mm.
[0091] In embodiment 17, there is provided a process according to
embodiment 16, wherein the gap is less than 0.80 mm, preferably
less than 0.60 mm, more preferably less than 0.40 mm.
[0092] In embodiment 18, there is provided a process according to
embodiments 16 or 17, wherein the strips are laid down to construct
an aerospace vehicle body component.
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