U.S. patent application number 12/532877 was filed with the patent office on 2010-06-03 for fibre-reinforced composite moulding and manufacture thereof.
This patent application is currently assigned to GURIT (UK) Ltd.. Invention is credited to Christopher William Bunce, Kevin Steven Cadd.
Application Number | 20100136278 12/532877 |
Document ID | / |
Family ID | 38050511 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100136278 |
Kind Code |
A1 |
Cadd; Kevin Steven ; et
al. |
June 3, 2010 |
FIBRE-REINFORCED COMPOSITE MOULDING AND MANUFACTURE THEREOF
Abstract
Moulding material comprising at least one dry fibrous
reinforcement layer having a surface resin material conjoined to a
first surface thereof and a structural resin material conjoined to
a second surface thereof wherein the structural resin material is
provided with a recess which is free from resin and is located at
the edge of the structural resin layer and wherein the dry fibrous
reinforcement layer provides a venting structure to allow entrapped
air to pass out during processing. The material of the present
invention can be used to form a composite which can be used as a
surface coating for large components such as wind turbines.
Inventors: |
Cadd; Kevin Steven; ( Isle
of Wight, GB) ; Bunce; Christopher William; (Hants,
GB) |
Correspondence
Address: |
RISSMAN HENDRICKS & OLIVERIO, LLP
100 Cambridge Street, Suite 2101
BOSTON
MA
02114
US
|
Assignee: |
GURIT (UK) Ltd.
Isle of Wight
GB
|
Family ID: |
38050511 |
Appl. No.: |
12/532877 |
Filed: |
March 26, 2008 |
PCT Filed: |
March 26, 2008 |
PCT NO: |
PCT/GB2008/001030 |
371 Date: |
January 27, 2010 |
Current U.S.
Class: |
428/60 ; 264/257;
428/157; 428/58 |
Current CPC
Class: |
B29C 66/1286 20130101;
B29C 66/72326 20130101; B32B 2250/03 20130101; B32B 3/06 20130101;
B29C 66/727 20130101; B29C 66/7392 20130101; B29C 66/721 20130101;
B29C 63/0021 20130101; Y10T 428/2476 20150115; B29C 66/128
20130101; B29C 66/71 20130101; B29C 66/73122 20130101; B29C 70/021
20130101; Y10T 428/197 20150115; B32B 2250/04 20130101; B29C 66/723
20130101; B29C 66/73941 20130101; B29C 70/36 20130101; B32B 5/12
20130101; B32B 5/022 20130101; Y10T 428/195 20150115; B29K 2067/00
20130101; B32B 2603/00 20130101; Y02P 70/523 20151101; Y10T
428/24132 20150115; B29C 65/02 20130101; B29C 66/472 20130101; Y02P
70/50 20151101; B29C 66/43 20130101; Y10T 428/24752 20150115; B29L
2009/00 20130101; B32B 2307/724 20130101; F01D 5/282 20130101; B29C
65/18 20130101; Y10T 428/249942 20150401; B29C 66/1122 20130101;
B32B 3/02 20130101; B32B 27/12 20130101; B32B 2262/101 20130101;
Y10T 428/192 20150115; B32B 2419/00 20130101; B29K 2105/246
20130101; B32B 5/028 20130101; Y10T 428/24488 20150115; Y10T
428/24942 20150115; B29C 66/72141 20130101; Y10T 428/187 20150115;
B29C 70/30 20130101; B32B 2262/0269 20130101; B29C 70/086 20130101;
B29C 70/342 20130101; B29L 2031/08 20130101; B32B 7/02 20130101;
B29C 66/73756 20130101; B29C 70/44 20130101; B29D 99/0025 20130101;
B29C 66/12821 20130101; B29C 66/7212 20130101; B29C 70/465
20130101; B29K 2063/00 20130101; B32B 3/04 20130101; B32B 2605/12
20130101; B29C 66/1122 20130101; B29C 65/00 20130101; B29C 66/128
20130101; B29C 65/00 20130101; B29C 66/43 20130101; B29C 65/00
20130101; B29C 66/7212 20130101; B29K 2307/04 20130101; B29C
66/7212 20130101; B29K 2309/08 20130101; B29C 66/7212 20130101;
B29K 2277/10 20130101; B29C 66/71 20130101; B29K 2067/00 20130101;
B29C 66/71 20130101; B29K 2063/00 20130101 |
Class at
Publication: |
428/60 ; 428/157;
428/58; 264/257 |
International
Class: |
B32B 3/06 20060101
B32B003/06; B32B 7/04 20060101 B32B007/04; B32B 27/12 20060101
B32B027/12; B29C 70/06 20060101 B29C070/06; B29C 70/28 20060101
B29C070/28; B29C 65/00 20060101 B29C065/00; B29B 11/16 20060101
B29B011/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2007 |
GB |
0706198.9 |
Oct 19, 2007 |
GB |
0720583.4 |
Claims
1. A moulding material comprising at least one dry fibrous
reinforcement layer having a surface resin material layer conjoined
to a first surface thereof and a structural resin material layer
conjoined to a second surface thereof, wherein an elongate edge of
the structural resin material layer is located inwardly of an
adjacent elongate edge of at least the surface resin material layer
to provide a stepped configuration in which the structural resin
material is provided with a stepped recess which is free from resin
and is located at the edge of the structural resin layer.
2. (canceled)
3. (canceled)
4. (canceled)
5. A moulding material as claimed in claim 1 wherein the dry
fibrous reinforcement layer adjacent to the structural resin
material layer has an elongate edge aligned with the elongate edge
of the structural resin material whereby the stepped configuration
is provided with a stepped recess of the said dry fibrous
reinforcement layer having the same dimensions as the stepped
recess of the structural resin material.
6. A moulding material as claimed in claim 1 wherein the structural
resin material and the surface resin material have a different
viscosity.
7. A moulding material as claimed in claim 6 wherein the viscosity
of the structural resin material is selected to be higher than that
of the surface resin material at room temperature.
8. (canceled)
9. A moulding material as claimed in claim 1 wherein the surface
resin material layer is air permeable.
10. A moulding material as claimed in claim 9 wherein the thickness
of the surface resin material layer is 100-400 microns.
11. A moulding material as claimed in claim 10 wherein the
thickness of the surface resin material layer is 170-270
microns.
12. A moulding material as claimed in claim 1 wherein the surface
resin material layer is provided with a layer of scrim
material.
13. A moulding material as claimed in claim 12 wherein the scrim
layer is a polyester material.
14. A moulding material according to claim 13 wherein the scrim
layer is a polyester spun bonded scrim material.
15. A moulding material according to claim 1 wherein the layer of
scrim material is located at or proximal to a first face of the
surface resin material layer that is conjoined to the at least one
dry fibrous reinforcement layer so that a majority of the surface
resin material is between the layer of scrim material and a second,
free, surface of the surface resin material.
16. (canceled)
17. (canceled)
18. (canceled)
19. (canceled)
20. (canceled)
21. A moulding material as claimed in claim 1 wherein the width of
the stepped recess is 10-75 mm.
22. A moulding material as claimed in claim 21 wherein the width of
the stepped recess is 20-40 mm.
23. A moulding material as claimed in claim 1 wherein the stepped
recess extends along the length of an elongate edge of the moulding
material.
24. A composite material which comprises an assembly of a plurality
of moulding materials according to claim 1, the assembly comprising
a continuous array of the moulding materials, the peripheral edge
of each moulding material contacting an adjacent moulding material
of the array and the surface resin material layers being arranged
on a common face of the assembly, wherein the composite material is
assembled by overlapping the plurality of moulding materials such
that an elongate edge of one moulding material is received in the
stepped recess of an adjacent moulding material whereby the surface
resin material layer of the one moulding material is conjoined to
the dry fibrous reinforcement layer of the adjacent moulding
material.
25. (canceled)
26. A method of manufacturing a fibre-reinforced composite
moulding, the method comprising the steps of: (a) disposing a
plurality of moulding material segments on a mould surface to form
a continuous array of segments on the mould surface, each segment
comprising a surface resin material layer and a dry fibrous
reinforcement layer over the surface resin material layer, wherein
the continuous array is assembled by overlapping the plurality of
segments such that at least one elongate edge of each segment
overlaps an adjacent segment whereby the surface resin material
layer of each segment overlies an elongate edge of the dry fibrous
reinforcement layer of the adjacent segment; (b) providing a
structural resin material layer over the dry fibrous reinforcement
layer, (c) heating the assembly to cause the structural resin to
flow into and impregnate the dry fibrous reinforcement; and (d)
curing the surface and structural resin materials to form the
fibre-reinforced composite moulding which comprises a surface
portion formed from the surface resin material layer laminated to a
structural portion formed from the at least one layer of fibrous
reinforcing material and the structural resin.
27. A method according to claim 26, wherein each segment comprises
at least one dry fibrous reinforcement layer having a surface resin
material layer conjoined to a first surface thereof and a
structural resin material layer conjoined to a second surface
thereof, wherein an elongate edge of the structural resin material
layer is located inwardly of an adjacent elongate edge of at least
the surface resin material layer to provide a stepped configuration
in which the structural resin material is provided with a stepped
recess which is free from resin and is located at the edge of the
structural resin layer; and wherein steps (a) and (b) are carried
out by assembling the plurality of the moulding materials in the
mould, the assembly comprising a continuous array of the moulding
materials, the peripheral edge of each moulding material contacting
an adjacent moulding material of the array and the surface resin
material layers being arranged on a common face of the assembly
adjacent to a mould surface, wherein the continuous array is
assembled by overlapping the plurality of moulding materials such
that an elongate edge of one moulding material is received in the
stepped recess of an adjacent moulding material whereby the surface
resin material layer of the one moulding material is conjoined to
the dry fibrous reinforcement layer of the adjacent moulding
material.
28. A method according to claim 27 wherein the overlap between
adjacent segments has a width of from 10 to 75 mm.
29. A method according to claim 28 wherein the overlap between
adjacent segments has a width of from 20 to 40 mm.
30. A method according to claim 27 wherein the segments overlap on
opposing edges.
31. A method according to claim 27 wherein the surface resin
material layer has a thickness of from 100 to 400 microns.
32. A method according to claim 27 wherein the surface resin
material layer has an applied weight thickness of from 100 to 400
grams pre square metre (gsm).
33. A method according to claim 27 wherein the surface resin
material layer is supported on a carrier of a sheet material.
34. A method according to claim 33 wherein the sheet material of
the surface resin material layer has a weight of from 10 to 90
gsm.
35. (canceled)
36. (canceled)
37. (canceled)
38. A method according to claim 33 wherein the sheet material of
the surface resin material layer comprises a polyester spun bonded
scrim material.
39. A method according to claim 33 wherein the sheet material of
the surface resin material layer is located at or proximal to a
first face of the surface resin material layer.
40. A method according to claim 39 wherein first face of the
surface resin material layer is remote from the mould surface in
disposing step (a) so that a majority of the surface resin material
is between the sheet material and the mould surface.
41. (canceled)
42. (canceled)
43. (canceled)
44. A method according to claim 26, wherein the surface resin
material and the structural resin material cure at least partially
simultaneously in the curing step (d).
45. (canceled)
46. A method according to claim 26, wherein the structural resin
material has a higher viscosity than that of the surface resin
material at room temperature (20 degrees Centigrade).
47. A method according to claim 46, wherein the ratio of the
viscosity, measured at 20.degree. C. ambient temperature, of the
structural resin material and of the surface resin material is from
2 to 14/1.
48. A method according to claim 45, wherein the surface resin
material has a higher viscosity than that of the structural resin
material during the heating step (c).
49. A method according to claim 48, wherein the ratio of the
viscosity, during the heating step (c), of the surface resin
material and of the structural resin material is from 5 to
25/1.
50. A method according to claim 26, wherein in the curing step (d)
the curing reaction of the structural resin material is exothermic
which generates heat which accelerates the curing of the surface
resin material.
51. (canceled)
52. A method according to claim 26, wherein the surface resin
material has a viscosity of from 0.1.times.10.sup.5 to
5.times.10.sup.5 Pas measured at 20.degree. C.
53. A method according to claim 26, wherein the structural resin
material has a viscosity of from 0.75.times.10.sup.5 to
5.times.10.sup.6 Pas measured at 20.degree. C.
54. A fibre-reinforced composite moulding comprising a surface
portion laminated to a structural portion, the surface portion
being formed of a surfacing layer comprising a plurality of
surfacing layer segments moulded together to form a continuous
surfacing layer, the surfacing layer comprising a first cured resin
material supported on a carrier of a sheet material, and the
structural portion being formed from at least one layer of fibrous
reinforcing material and a cured second resin material, at least
one layer of the fibrous reinforcing material being formed of a
plurality of segments each of which overlies a respective surfacing
layer segment, and each surfacing layer segment overlapping an
adjacent segment of the fibrous reinforcing material.
55. A fibre-reinforced composite moulding according to claim 54,
wherein the overlap between adjacent surfacing layer segments has a
width of from 10 to 75 mm.
56. (canceled)
57. A fibre-reinforced composite moulding according to claim 54,
wherein the surfacing layer segments overlap on opposing edges.
58. A fibre-reinforced composite moulding according to claim 54,
wherein the first resin material of the surfacing layer has a
thickness of from 100 to 300 microns.
59. A fibre-reinforced composite moulding according to claim 54,
wherein the sheet material of the surfacing layer is located nearer
to an interface between the surface portion and the structural
portion than to an opposite exposed surface of the surface
portion.
60. A fibre-reinforced composite moulding according to claim 54,
wherein the sheet material of the surfacing layer has a weight of
from 10 to 90 gsm.
61. (canceled)
62. (canceled)
63. (canceled)
64. A fibre-reinforced composite moulding according to claim 54
wherein the sheet material of the surfacing layer comprises a
polyester spun bonded scrim material.
65. (canceled)
66. (canceled)
67. (canceled)
68. (canceled)
Description
[0001] The present invention relates to a moulding material, to a
method of manufacturing a fibre-reinforced composite moulding and
to a fibre-reinforced composite moulding. In particular, the
present invention relates to a fibre-reinforced composite moulding
suitable for manufacturing large composite structures, such as
turbine blades, bridges and boat hulls.
[0002] Most fibre reinforced composite components require an outer
surface coating to provide an aesthetic and protective finish to
the component. Traditionally such components are either painted
after moulding or a liquid in-mould coating (gelcoat) with
sufficient environmental resistance is used. In some applications
painting is preferred, especially when multiple component parts
need to be assembled together and any misalignment or joint lines
can thereafter be hidden by filling and fairing steps to give a
more seamless finish. Painting can also be useful when the final
colour has not been defined at the start of the build and the parts
can be supplied in a ready to paint format.
[0003] A key problem in painting a fibre composite part can be that
of preventing the fibre reinforcement pattern appearing in the
final surface. This is more of a problem when heavier weight, lower
cost reinforcement fibres and fabrics are used to reduce the
material cost and the time taken to build up the thickness of the
laminate. It is common to use a more expensive lower weight glass
fibre layer or a non-structural surfacing tissue in addition to a
gelcoat layer to buffer the paint from the fibre reinforcement. It
is usual practice to first apply a liquid gelcoat into the mould,
which in this case is designed to be easy to sand and repair any
defects prior to painting. The gelcoat provides a resin barrier
layer between the paint and the first fibre layers by providing a
sufficient thickness to stop the fibre pattern showing in the final
surface. If the laminate is applied into the mould without the
gelcoat barrier coat it is common for the final surface to have
pin-hole like defects. Pin-holes are a particular problem when
painting as they can be hard to spot on the initial moulding, but
when the part is painted, the paint then reticulates to form a
larger defect around the pin-hole, requiring rework.
[0004] Even when using the gelcoat, it is also the case that
sometimes a few pin-holes are present. It would be desirable to
have a manufacturing process that substantially completely
eliminated the problem of pin-holes.
[0005] To apply a gelcoat to larger parts, such as wind turbines,
marine craft, architectural mouldings, and bridges additional
equipment, such as gelcoat spraying machines and extraction
equipment, or mixing equipment used in combination with manual
brushing or rolling, is needed to reduce defects and achieve
reasonable deposition rates of the gelcoat. A time delay then
occurs while waiting for the gelcoat to partially cure to build
sufficient strength for the remaining laminate to be added on the
mould.
[0006] The three main thermoset composite processing methods
currently used for manufacturing wind turbine blades are:
[0007] 1. wet-laminating (also known as open moulding)--in this
method, the thermoset resin can cure in ambient conditions, but the
tools are usually heated to elevated temperature, 50-90.degree. C.,
to speed up the resin curing process;
[0008] 2. the use of pre-preg materials, and the Applicant's own
& pre-impregnated dry touch composite material sold under the
product name SPRINT.RTM.--such materials are typically cured at an
elevated temperature between 85.degree. C. to 120.degree. C.;
and
[0009] 3. vacuum assisted resin transfer moulding (also known as
VARTM, resin infusion, or vacuum infusion)--in this method liquid
resin is infused under a vacuum into a dry fibre composite, and
then can cure in ambient conditions, although the tools (i.e. the
moulds) are usually heated to an elevated temperature between
50-90.degree. C. to speed up the curing process.
[0010] The surface finish quality plays an important role in the
aerodynamic efficiency. Some blade manufacturers apply a weather
resistant in-mould gelcoat to be the final surface layer, others
manufacture spray-paint the blades afterwards. In either case the
surface needs to be smooth and defect free. The blade manufacturers
currently spend a considerable amount of time filling and fixing
the blade surfaces and with the increase demand of wind turbine
blades, a solution to decrease the amount of time each blade spends
in the finishing production area would save time, reduce cost and
increase the production capacity.
[0011] One solution for pre-preg parts is detailed in WO 02/094564
which discloses a pre-preg surface film material which is designed
to provide a resin layer which is easy to prepare for painting. The
material of WO 02/094564 gives a very low defect finish as it
contains an air venting structure to remove any air trapped between
the tool and surface film during a pre-preg vacuum bag curing
process. The resin viscosity is formulated to first allow the resin
to wet the tool surface and then control the resin viscosity
through the cure. The resin viscosity and cure profile in
combination with a resin retention fabric prevents the pin-holing
defects normally found when curing fibre-prepreg materials direct
against a mould surface using a vacuum bag process.
[0012] This solution avoids the need to have separate equipment to
handle, mix, and apply the gelcoat. This also adds a health and
safety advantage as liquid gelcoats are generally more
hazardous.
[0013] The surface film material of WO 02/094564 is most suitable
for use with pre-preg materials such as those disclosed in WO
00/072632. This surface film material is suitable for building
smaller components such as automotive door wings and bonnets and is
successfully marketed by Gurit as SF95 and SF86 pre-preg system.
The separate surface film is particularly suitable for detailed
lamination work as it can be cut and tailored into tight corners
before the main structural laminate is added into the mould. On
demoulding the surface has a high quality without pinholes or other
defects and is easy to sand in preparation for painting.
[0014] The surface film material described in WO 02/094564 uses a
partially impregnated polyester scrim at the surface of the
moulding material which is in contact with the tool surface. The
level of impregnation of the scrim controls the material tack. This
can be a drawback if the scrim is dry and un-impregnated as it will
sit on the surface of the resin. The material then lacks surface
tack and it cannot be positioned to stay in place on the tool
surface. If the material is too impregnated then the scrim, which
forms part of the air venting pathway, loses its air permeability
and surface defects are formed. As this is a fine and light weave
material it is very easily impregnated and this makes the handling
and room temperature storage of the material critical. The material
can be difficult to work with and un-reliable when used in
industrial applications, e.g. as part of a large composite
component.
[0015] The surface film material of WO 02/094564 is less suitable
for larger parts, that generally have a more consistent curvature.
These large parts require multiple widths of the material and the
ability to then walk on the mould to easily laminate the remaining
fibre layers. If the fibrous reinforcement in the surface film
described in WO 02/094564 becomes too impregnated from handling
pressure causing the resin flow, this reduces the air venting
properties of the material. With reduced venting trapped air can be
left between the mould surface and surface film, which then forms
defects requiring repair prior to painting. In a small component a
reduced venting air path can be accommodated, as the distance to
the vacuum source is small. In larger parts the air has to flow
from one overlapped fabric to another and then out to the vacuum
source. This can lead to a significant pressure drop from the
combination of the increased distance to the vacuum source and more
torturous venting path.
[0016] Using the material from WO 02/094564 on larger parts
requires additional process steps. For example a motor boat hull
was successfully made using the SF95 material defined in WO
02/094564 only when carefully controlling the handling by providing
scaffolding in the mould to avoid walking on the material, ensuring
the material was kept frozen prior to use to avoid it
self-impregnating in a warm workshop, and putting in an additional
40.degree. C. vacuum de-bulking step after applying the surface
film layer to the tool before starting the main laminate. These
additional steps were required for larger parts to ensure a defect
free surface. The de-bulking step and scaffolding added a
significant time and cost penalty. As a result the material
described in WO 02/094564 lost its main time advantage over using a
gelcoat. The production had to be tightly scheduled to avoid
leaving the surface film out in the workshop, and to keep the
material in as a fresh state as possible to maintain the full
venting structure.
[0017] The present invention overcomes this problem by integrating
a surface resin layer into the first structural fibre reinforcement
layer to form a novel moulding material which gives a moulded
finish that is easy to prepare for painting.
[0018] According to a first aspect of the present invention there
is provided a moulding material comprising at least one dry fibrous
reinforcement layer having a surface resin material layer conjoined
to a first surface thereof and a structural resin material layer
conjoined to a second surface thereof, wherein an elongate edge of
the structural resin material layer is located inwardly of an
adjacent elongate edge of at least the surface resin material layer
to provide a stepped configuration in which the structural resin
material is provided with a stepped recess which is free from resin
and is located at the edge of the structural resin layer.
[0019] Typically, the moulding material is provided with more than
one dry fibrous reinforcement layer. The separate dry fibrous
reinforcement layers can be made of the same or different
materials. Preferably, if the moulding material is provided with
more than one dry fibrous layer, the reinforcement layer adjacent
to the structural layer is provided with a recess having the same
dimensions as that of the structural resin material.
[0020] Typically the structural resin and surface resin materials
have a different viscosity. The viscosity of the structural resin
is usually selected to be higher than that of the surface resin at
room temperature. The surface resin typically has a higher
viscosity than the structural resin when heated to keep the surface
resin close to the mould surface during processing. Preferably the
structural resin impregnates the dry fibrous layer during
processing.
[0021] The surface resin is selected such that it is air permeable
to provide an additional pathway for the removal of air. The
thickness of the surface layer is preferably selected to be 100-400
microns. The thickness of the surface layer is preferably 170-270
microns.
[0022] The surface resin layer of the moulding material may be
provided with a layer of scrim material to assist resin retention
of the mould surface. Typically the scrim layer is a polyester
material.
[0023] The surface resin is preferably selected from the group
consisting of thermoset resins such as epoxy, cyanate ester and
phenolic resins. Suitable epoxy resins include diglycidyl ethers of
bisphenol A, diglycidyl ethers of bisphenol F, epoxy novolac resins
and N-glycidyl ethers, glycidyl esters, aliphatic and
cycloaliphatic glycidyl ethers, glycidyl ethers of aminophenols,
glycidyl ethers of any substituted phenols and blends thereof. Also
included are modified blends of the aforementioned thermosetting
polymers. These polymers are typically modified by rubber or
thermoplastic addition. Any suitable catalyst may be used. The
catalyst will be selected to correspond to the resin used. One
suitable catalyst for use with an epoxy resin is a dicyandiamide
curing agent. The catalyst may be accelerated. Where a
dicyandiamide catalyst is used, a substituted urea may be used as
an accelerator. Suitable accelerators include Diuron, Monuron,
Fenuron, Chlortoluron, bis-urea of toluenediisocyanate and other
substituted homologues. The epoxy curing agent may be selected from
Dapsone (DDS), Diamino-diphenyl methane (DDM), BF3-amine complex,
substituted imidazoles, accelerated anhydrides, metaphenylene
diamine, diaminodiphenylether, aromatic polyetheramines, aliphatic
amine adducts, aliphatic amine salts, aromatic amine adducts and
aromatic amine salts.
[0024] The surface material can be provided with a toughening
agent. Suitable toughening agents can be selected from liquid
rubber (such as acrylate rubbers, or carboxyl-terminated
acrylonitrile rubber), solid rubber (such as solid nitrite rubber,
or core-shell rubbers), thermoplastics (such as poly
(EtherSulphone), poly (Imide)), block copolymers (such as
styrene-butadiene-methacrylate triblocks), or blends thereof.
[0025] The structural resin is preferably selected from the group
consisting of thermoset resins such as epoxy, cyanate ester and
phenolic systems. Suitable epoxy resins include diglycidyl ethers
of bisphenol A, diglycidyl ethers of bisphenol F, epoxy novolac
resins and N-glycidyl ethers, glycidyl esters, aliphatic and
cycloaliphatic glycidyl ethers, glycidyl ethers of aminophenols,
glycidyl ethers of any substituted phenols and blends thereof. Also
included are modified blends of the aforementioned thermosetting
polymers. These polymers are typically modified by rubber or
thermoplastic addition. Any suitable catalyst may be used. The
catalyst will be selected to correspond to the resin used. One
suitable catalyst for use with an epoxy resin is a dicyandiamide
curing agent. The catalyst may be accelerated. Where a
dicyandiamide catalyst is used, a substituted urea may be used as
an accelerator. Suitable accelerators include Diuron, Monuron,
Fenuron, Chlortoluron, his-urea of toluenediisocyanate and other
substituted homologues. The epoxy curing agent may be selected from
Dapsone (DDS), Diamino-diphenyl methane (DDM), BF3-amine complex,
substituted imidazoles, accelerated anhydrides, metaphenylene
diamine, diaminodiphenylether, aromatic polyetheramines, aliphatic
amine adducts, aliphatic amine salts, aromatic amine adducts and
aromatic amine salts.
[0026] The dry fibrous layer is fibrous material such as glass
fibre, aramid, PAN or pitch based carbon fibre.
[0027] The length of the recess is preferably selected to be 10-75
mm. More preferably the length of the recess is preferably selected
to be 20-40 mm.
[0028] According to a second aspect of the present invention there
is provided a composite material which comprises an assembly of a
plurality of moulding materials according to the first aspect of
the present invention, the assembly comprising a continuous array
of the moulding materials, the peripheral edge of each moulding
material contacting an adjacent moulding material of the array and
the surface resin material layers being arranged on a common face
of the assembly, wherein the composite material is assembled by
overlapping the plurality of moulding materials such that an
elongate edge of one moulding material is received in the stepped
recess of an adjacent moulding material whereby the surface resin
material layer of the one moulding material is conjoined to the dry
fibrous reinforcement layer of the adjacent moulding material.
[0029] According to a third aspect of the present invention there
is provided the use of the composite material of the second aspect
as the surface coating for a large component, in particular a wind
turbine, marine craft or architectural moulding.
[0030] According to a fourth aspect of the present invention there
is provided a method of manufacturing a fibre-reinforced composite
moulding, the method comprising the steps of (a) disposing a
plurality of moulding material segments on a mould surface to form
a continuous array of segments on the mould surface, each segment
comprising a surface resin material layer and a dry fibrous
reinforcement layer over the surface resin material layer, wherein
the continuous array is assembled by overlapping the plurality of
segments such that at least one elongate edge of each segment
overlaps an adjacent segment whereby the surface resin material
layer of each segment overlies an elongate edge of the dry fibrous
reinforcement layer of the adjacent segment; (b) providing a
structural resin material layer over the dry fibrous reinforcement
layer, (c) heating the assembly to cause the structural resin to
flow into and impregnate the dry fibrous reinforcement; and (d)
curing the surface and structural resin materials to form the
fibre-reinforced composite moulding which comprises a surface
portion formed from the surface resin material layer laminated to a
structural portion formed from the at least one layer of fibrous
reinforcing material and the structural resin.
[0031] Preferably, each segment comprises at least one dry fibrous
reinforcement layer having a surface resin material layer conjoined
to a first surface thereof and a structural resin material layer
conjoined to a second surface thereof, wherein an elongate edge of
the structural resin material layer is located inwardly of an
adjacent elongate edge of at least the surface resin material layer
to provide a stepped configuration in which the structural resin
material is provided with a stepped recess which is free from resin
and is located at the edge of the structural resin layer; and steps
(a) and b) are carried out by assembling the plurality of the
moulding materials in the mould, the assembly comprising a
continuous array of the moulding materials, the peripheral edge of
each moulding material contacting an adjacent moulding material of
the array and the surface resin material layers being arranged on a
common face of the assembly adjacent to a mould surface, wherein
the continuous array is assembled by overlapping the plurality of
moulding materials such that an elongate edge of one moulding
material is received in the stepped recess of an adjacent moulding
material whereby the surface resin material layer of the one
moulding material is conjoined to the dry fibrous reinforcement
layer of the adjacent moulding material.
[0032] Preferably, the overlap between adjacent segments has a
width of from 10 to 75 mm, more preferably from 20 to 40 mm. The
segments preferably overlap on opposing edges. The surface resin
material layer typically has a thickness of from 100 to 400
microns, more preferably from 100 to 300 microns, yet more
preferably from 170 to 270 microns, and/or an applied weight
thickness of from 100 to 400 grams pre square metre (gsm). Most
preferably, the surface resin material layer is supported on a
carrier of a sheet material, which may preferably have a weight of
from 10 to 90 gsm, more preferably from 20 to 50 gsm. The sheet
material of the surface resin material layer may comprise a polymer
or glass scrim material, more preferably a polyester scrim
material, yet more preferably a polyester spun bonded scrim
material. Preferably, the sheet material of the surface resin
material layer is located at or proximal to a first face of the
surface resin material layer. The first face of the surface resin
material layer is preferably remote from the mould surface in
disposing step (a) so that a majority of the surface resin material
is between the sheet material and the mould surface.
[0033] At least one dry fibrous reinforcement layer of fibrous
reinforcing material may comprise a plurality of stacked dry
fibrous reinforcement layers, at least a lower layer being
coextensive with the surface resin material layer and at least an
upper layer being coextensive with the structural resin material
layer. Preferably, at least one of the plurality of dry fibrous
reinforcement layers comprises oriented fibres. More preferably, at
least one of the plurality of stacked dry fibrous reinforcement
layers comprises oriented fibres having a first orientation and at
least one of the plurality of stacked dry fibrous reinforcement
layers comprises oriented fibres having a second orientation.
[0034] Preferably, the surface resin material and the structural
resin material cure at least partially simultaneously in the curing
step (d).
[0035] The surface resin material and the structural resin material
may have different viscosities. Preferably, the structural resin
material has a higher viscosity than that of the surface resin
material at room temperature (20.degree. C.). The ratio of the
viscosity, measured at 20.degree. C. ambient temperature, of the
structural resin material and of the surface resin material is
typically from 2 to 14/1, more preferably from 4 to 12/1. The
surface resin material preferably has a higher viscosity than that
of the structural resin material during the heating step (c). The
ratio of the viscosity, during the heating step (c), of the surface
resin material and of the structural resin material may be from 5
to 25/1, more preferably from 10 to 15/1.
[0036] Preferably, in the curing step (d) the curing reaction of
the structural resin material is exothermic which generates heat
which accelerates the curing of the surface resin material.
[0037] The surface resin material and the structural resin material
may be thermosetting epoxy resins. The surface resin material
typically has a viscosity of from 0.1.times.10.sup.5 to
5.times.10.sup.5 Pas measured at 20.degree. C. and/or the
structural resin material typically has a viscosity of from
0.75.times.10.sup.5 to 5.times.10.sup.6 Pas measured at 20.degree.
C.
[0038] In this specification the resin viscosity is measured using
a TA Instruments AR2000 rheometer with a 20 mm diameter steel plate
and a Peltier cooling system. The experiment was carried out under
the following conditions: oscillation experiment from 40.degree. C.
down to 0.degree. C. at 2.degree. C./min with a controlled
displacement of 1.times.10.sup.-4 rads at a frequency of 1 Hz and a
gap of 1000 .mu.m.
[0039] The present invention also provides in a further aspect a
fibre-reinforced composite moulding comprising a surface portion
laminated to a structural portion, the surface portion being formed
of a surfacing layer comprising a plurality of surfacing layer
segments moulded together to form a continuous surfacing layer, the
surfacing layer comprising a first cured resin material supported
on a carrier of a sheet material, and the structural portion being
formed from at least one layer of fibrous reinforcing material and
a cured second resin material, at least one layer of the fibrous
reinforcing material being formed of a plurality of segments each
of which overlies a respective surfacing layer segment, and each
surfacing layer segment overlapping an adjacent segment of the
fibrous reinforcing material.
[0040] In a preferred embodiment of the present invention there is
provided a thermoset fibre reinforced structural moulding material
containing an integrated surface primer resin layer with air
venting properties that enables faster production of large painted
moulded composite parts. It gives a defect free surface form vacuum
bag processing that is easy to prepare for painting. The material
itself contains a layer of thermosetting surface resin, dry
reinforcement fibre, and thermosetting structural resin. It
contains a recess at one edge which is free of structural resin in
order to increase the total air venting properties of the
overlapped pieces of the material to achieve a defect free surface
finish when a large component is to be formed from many overlapping
pieces of material to cover the full surface of the mould. This
recess can be in the range 10-75 mm with a preferable size being
20-40 mm.
[0041] This preferred embodiment is tolerant to handling pressure
and is stable enough to be provided on large rolls without losing
its air venting properties. It provides a high quality surface
direct from the mould, which only requires minimal sanding prior to
painting. Considerable time and cost is saved in the production
process as this material replaces the first structural layer and
gelcoat layer. The material requires no de-bulking operations to
remove trapped air and is cured at the same time as the main
laminate saving further time in the manufacturing process.
[0042] In the material of the present invention a good surface
finish is obtained without the need for additional tissues and high
cost fine weave fabrics enabling lower cost heavier weight
reinforcement to be used as the first ply into the mould. This
makes the material particularly suitable for the production of
wind-turbine aerofoil sections and any other large components with
simpler curvature such as marine craft, ray-domes, architectural
mouldings and bridges. The heavier weight version (600 gsm fibre
layer and above) is not suitable for more complex parts, like
automotive door wings, which require the material to be cut and
draped around tight detailed features.
[0043] The term `conjoin` is used herein to mean that when two
layers are brought together, e.g. a fibrous layer and a resin
layer, there is essentially no impregnation between said
layers.
[0044] Example embodiments of the invention will now be described,
by way of example only, with reference to the accompanying Figures,
in which;
[0045] FIG. 1 illustrates a cross-sectional view of a moulding
material in accordance with the present invention;
[0046] FIG. 2 illustrates a cross-sectional view of an alternative
embodiment of the moulding material in accordance with the present
invention;
[0047] FIG. 3 illustrates two pieces of the material of the present
invention overlapping each other;
[0048] FIG. 4 illustrates a number of the possible pathways for
removing air from the material of FIG. 3; and
[0049] FIG. 5 illustrates the material of the present invention in
the form in which it is manufactured;
[0050] FIG. 6 illustrates a sample of the material of the present
invention having the improved surface quality;
[0051] FIG. 7 illustrates the defects which result from using a
surface film having a high viscosity;
[0052] FIG. 8 illustrates the relative difference between a working
system (SPX13734) and a non working system (SF2); and
[0053] FIG. 9 illustrates the cold flow resistance improvement of
resins versus temperature.
[0054] Referring to FIG. 1, there is provided a layer of material
100 which contains surfacing resin 106, reinforcing fibres 102 and
103, and the structural resin 101 for impregnating the reinforcing
fibres 102 and 103.
[0055] Additional layers of pre-prep or SPRINT.RTM. material and
other materials such as foam core can then be added to complete the
laminate stack.
[0056] The surfacing layer 106 contains surfacing resin 105 and a
polyester veil 104. During manufacture of the material the
polyester veil 104 is first applied to the top of the surface resin
104. Some pressure is the applied to push the polyester veil 104
into the top of the surface resin 105 and also some of the surface
resin 105 into the fibre layer 103 to ensure the surface material
106 stays fixed to the fibre layer.
[0057] Sufficient structural resin 101 is applied to the top of the
fibre layers 102 and 103 to fully impregnate the fibre when the
material is later cured using heat and a vacuum bag process. This
structural resin has a very high viscosity at room temperature to
prevent it impregnating the fibre layers 102, 103. A suitable
material is the Gurit WT93 resin system which as a viscosity of
around 1.times.10.sup.6 Pas at 20.degree. C. On applying heat the
resin 101 drops sufficiently in viscosity, typically to 3 Pas, to
then fully impregnate the fibre layers 102, 103 during a 85.degree.
C. or 120.degree. C. cure cycle.
[0058] It is also possible to use a lower viscosity pre-preg system
such as WE92 which has a viscosity of around 1.times.10.sup.5 Pa.
at 20.degree. C. This resin system impregnates the dry fibre layers
approximately 10.times. faster than the higher viscosity WT93
system. This reduces the time the material can be left unfrozen in
the workshop prior to use without the venting structure starting to
become blocked due to the resin migration and impregnation of the
fibrous venting layers.
[0059] The material 100 contains a region 107, which has no
structural resin 101 and in the example shown also contains reduced
thickness of fibre reinforcement. In the material 100 only fibre
layer 103 and surfacing layer 106 continue to the edge of the
material beneath the region 107.
[0060] FIG. 2 shows an alternative material 400 where all the fibre
reinforcement continues to the edge.
[0061] When assembling the material onto a mould each piece of
material 100 is overlapped so that the new surface material 206
overlaps onto the dry fibre layer 103. This is a particularly
advantageous format if the fibre layer is a stitched tri-axial
fabric such as Gurit YE900 or Y1200. In example shown in FIG. 3 the
material layers 103 and 203 are constructed from +/-45.degree.
fibres, and layers 102 and 202 are uni-directional fibres all
stitched together to form a multi-axial fabric. To achieve a
structural load transfer between the two pieces of reinforcement it
is necessary to overlap only the +/-45.degree. fibres. The
0.degree. fibres can be butt jointed together. In this format the
narrower width of the 0.degree. uni-directional fibre gives a less
pronounced increase in thickness and the advantage of a smoother
overlap.
[0062] The same method of overlapping the material is also used
with the alternative configuration of material 400 which does not
contain the reduction in fibre layer, such as woven cloth, or chop
strand matt fabric. This is because, in general, in fibre
composites an overlap is used for load transfer unless the material
contains uni-directional) (0.degree.) fibre which runs in same
direction as the overlap. The present invention is not limited to
glass fibre reinforcements. Other fibres such as aramid, carbon, or
natural fibres can be used.
[0063] The dry reinforcement layers 102, 103, 202 and 203 provide
one or more highly permeable air venting paths to remove air when a
vacuum is applied to the laminate stack. As the pieces of material
are overlapped the surface layer 206 is now in connection with the
highly air permeable dry fibre layer 103 allowing a more direct and
effective air path to the vacuum source.
[0064] The zone 107 which is free from structural resin allows more
effective connection of the dry reinforcement to give a highly
permeable venting structure. The continuous surface resin 106
prevents defects occurring at the point of overlap of the
material.
[0065] The zone 107 is an important feature of the present
invention, and is necessary for heavier weight fabrics above 600
gsm, so that as the resin film 101 becomes thicker and less air
permeable there is sufficient resin to impregnate the structural
fibre 102,103.
[0066] Without zone 107 the air permeability across the overlapped
fabric is reduced leading to defects in larger components.
[0067] In the present invention it is preferred that the thickness
of the surface film resin 106 is between 100 and 400 microns, more
preferably from 100 to 300 microns, yet more preferably from 170 to
270 microns. Within this thickness range it has been found the
resin can be made partially air permeable. FIG. 4 shows that the
air 301 between the mould surface 306 and the surface layer 106 can
pass through the surface layer 106 and into the more highly air
permeable dry fibre layers 102 and 103, to then be drawn away into
the vacuum source 307. In the material of the present invention it
is not essential that an air breathing scrim is provided at the
tool surface. This means the tack of the material is more
consistent and dependent only on the resin formulation of the
surface resin 105 which allows it to be formulated to give the
desired and consistent tack level. In addition, the material is
more tolerant to handling pressure as the venting channels are not
closed by handling pressure, or the pressure generated when the
product is wound onto a roll. As a result the material of the
present invention has extended room temperature storage prior to
use.
[0068] Air 302 can also escape by moving between the tool surface
306 and the surface of the surface resin layer 106, 206 towards and
into the dry fibre 102 and 103. This air path 302, 303, 304 is made
more permeable by the absence of the structural resin 101 in the
zones 107 and 207. The air flows directly into the highly permeable
dry fibre 102, 103 and does not have to pass through the thick
resin layer 101 which would greatly reduce the flow and stop
effective air transport on larger parts containing multiple
overlaps of material.
[0069] The viscosity and the reactivity of the surface resin 105 in
the surface layer 106 are higher than the structural resin 101.
This ensures the surface resin 105 stays closer to the tool surface
306 to maintain the thickness of the surface layer 106 in the final
component.
[0070] Both resins are preferably formulated to be a thermosetting
epoxy resin with a latent curing agent, which is activated by heat.
Other thermosetting resins may be used; such as those derived from
cyanate ester and phenolic systems. Suitable epoxy resins include
diglycidyl ethers of bisphenol A, diglycidyl ethers of bisphenol F,
epoxy novolac resins and N-glycidyl ethers, glycidyl esters,
aliphatic and cycloaliphatic glycidyl ethers, glycidyl ethers of
aminophenols, glycidyl ethers of any substituted phenols and blends
thereof. Also included are modified blends of the aforementioned
thermosetting polymers. These polymers are typically modified by
rubber or thermoplastic addition. Any suitable catalyst may be
used. The catalyst will be selected to correspond to the resin
used. One suitable catalyst for use with an epoxy resin is a
dicyandiamide curing agent. The catalyst may be accelerated. Where
a dicyandiamide catalyst is used, a substituted urea may be used as
an accelerator. Suitable accelerators include Diuron, Monuron,
Fenuron, Chlortoluron, bis-urea of toluenediisocyanate and other
substituted homologues. The epoxy curing agent may be selected from
Dapsone (DDS), Diamino-diphenyl methane (DDM), BF3-amine complex,
substituted imidazoles, accelerated anhydrides, metaphenylene
diamine, diaminodiphenylether, aromatic polyetheramines, aliphatic
amine adducts, aliphatic amine salts, aromatic amine adducts and
aromatic amine salts.
[0071] If the resin layer 105 is too thin then a sufficient
thickness barrier is not obtained between the fibre reinforcement
and the paint causing a print pattern on the surface. If the layer
is too thin, this can lead to dry fibre close to the surface which
can cause problems when sanding the surface prior to painting. The
resulting dry glass fibre particles get trapped on the abrading
disc and are very abrasive leading to scratch marks requiring
repeated abrasive disc changes and additional filling and fairing
repair steps prior to painting.
[0072] The fine polyester scrim 104 within the surface film layer
106 serves two purposes. It helps prevent fibres from the
reinforcement entering the surface resin layer 106. The fine weave
layer also helps prevent the resin 105 in the surface film layer
reticulating off the tool surface giving a better quality of
finish. The polyester scrim itself is easy to sand and does not
result in abrasive particles damaging the surface.
[0073] As well as providing a thickness buffer to avoid fibre print
the surface resin layer 106 provides a protective barrier for
reducing moisture ingress into the laminate. Glass fibre strands
close to the surface can accelerate moisture ingress by a wicking
mechanism. The surface resin 105 is toughened and the modulus
reduced which is a particular advantage as this helps to prevent
cracks from the mismatch in thermal expansion between the paint and
the laminate. The tailored surface resin helps improve paint
chipping that occurs in impact situations. Due to the venting
structure the trapped air is removed by the application of vacuum
to the material and the cured surface layer is virtually free of
voids, which also have been found to reduce the rate of coating
erosion.
[0074] For easier handling during fitting into a mould it can be
desirable to have narrow strips of this material. FIG. 5 shows an
economic method of producing 4 narrow strips of the material 100
from an industry standard 1270 mm width of multi-axial fibre cloth
502 to maximise the output of a resin coater designed for coating
resin onto this width of fabric. The structural resin 501 was cast
onto the fabric with various drop out zones 507, 508. The distance
represented by 508 is twice the distance of 507. After manufacture
the material 500 was then cut at positions 509 to give 4 pieces of
the material 100. The present invention is not restricted to
producing a material having 4 evenly sized strips, as it is
possible to use the method to produce many different combinations
of fabric widths by adding the appropriate the resin free zones to
give the desired width pieces.
EXAMPLE 1
[0075] A modified film coater was used to apply a Gurit 707 gsm
WT93 structural epoxy pre-preg resin film (101) to a 1260 mm wide
Gurit YE900 tri-axial fabric containing 450 gsm of +/45 glass fibre
(103) and 450 gsm of 0 glass fibre (102). Areas free of structural
resin, zones 507 and 508, were created by placing dams in the
coating machine to prevent depositing resin in these areas. Zone
507 was 35 mm wide, zone 508 was 70 mm wide. The coating machine
also applied a continuous 220 micron layer of Gurit toughened epoxy
resin SPX13734 (105) containing a Gurit RP35, a 35 gsm spunbonded
polyester scrim to the reverse side of the glass fibre. 100 micron
MDPE backers where then applied to encapsulate the product and
protect it from dirt and the product was wound up onto a 322 mm
outer diameter tube.
[0076] The material was then tested as manufactured and material
tested after being left for 4 weeks on the roll original product
roll at 20.degree. C. to check if any pre-impregnation had occurred
reducing its air venting performance.
[0077] The material was then cut into 4 strips of 315 mm width as
detailed FIG. 5, with one edge 35 mm free of structural resin, and
applied to an 8.5 m marine RIB deck composite mould. The material
was overlapped as described in FIG. 3 allowing for error (+/-10 mm)
to test the tolerance of the system to slightly misaligned
overlaps. Three of the four edges were sealed to simulate an even
larger part forcing the air to have to travel a longer path back to
a single edge. On this edge a Gurit WT93/XE 600 35% SS glass biax
was cloth and a peel ply were used to provide a good venting air
path to the vacuum source.
[0078] A layer of Gurit WE92/YE1200/TEA50/1260/43 +/-3 prepreg was
then applied, and several pieces of K500 25 mm saw cut Corecell
core with chamfered edges applied on top of this layer. To increase
the severity of the test several core pieces were used to increase
the number of core edges to test these transition points at the
edges of the core panel were known to promote the formation of
defects. Core is also known to increase the likely-hood of defects
at overlap. The laminate was competed using 2 more layers of Gurit.
WE92/YE1200/TEA50/1260/43 +/-3. A standard vacuum consumable pack
consisting of P3 perforated release film, breather cloth, and a
vacuum bag was applied. The material was then cured for 85.degree.
C. for 90 minutes followed by a further cure at 120.degree. C. for
90 mins.
[0079] The composite was free of surface defects on
de-moulding.
[0080] The test was successfully repeated for the material that had
been allowed to stand for 4 weeks on the roll at 20.degree. C. Due
to the resin formulation and construction the material retained its
venting structure and again gave a successful moulding again free
of surface defects.
EXAMPLE 2
[0081] To simulate the production of a wind turbine shell a test
tool was used where a 3 m wide section was laminated using 11
strips of the 315 mm wide material described in example 2. Three of
the four edges were again sealed so that any trapped air would have
to travel across the overlaps to escape the laminate. This test was
developed to better simulate a wind turbine section, which is many
times greater in length than width. As such significant drop in
vacuum can occur along the length and in order to account for
limited air-venting along the length, and wider parts, air was only
allowed to flow to escape across the width of the material in one
direction.
[0082] FIG. 6 shows the excellent surface quality even at the
furthest point of from the vacuum source.
[0083] The panel was then prepared for painting with the following
surface preparation; [0084] Degrease panels [0085] Sand P280 [0086]
Degrease
[0087] A DuPont Primer: PercoTex LA-Haft-Grund 040 10/1 (wt) with
hardener 4060 was applied followed by a Topcoat: Percotex Rotorlack
680 4/1 (wt) with hardener 3880. This was then tested and passed
the following DuPont Standards
TABLE-US-00001 Humidity Temperature resistance: DIN shock 50017:
240 hrs at test: VW- Du Pont Standard Initial Tests 40.degree. C.
and 100% P1200: Adhesion test: Gt3 Gt2 Gt2-3 gitterschnitt ISO 2409
Adhesion: scratch KO KO KO test: DBL 7399 - 5.1 Pull off test
according 1.9 N/mm2 2.2 N/mm2 1.1 N/mm2 ISO 24624 2.6 N/mm2 Stone
chip test: DBL K1.5 Not tested Not tested 7399 - 5.3.2:
EXAMPLE 3
[0088] During development various resin systems were experimented
for use as the surface layer. FIG. 7 shows the results of the test
defined in example 2 when using the SF2 system for the surface
layer 106. Voids were formed around the overlap region as the resin
was too viscous to flow into the gaps. When the WT93 resin was used
for the surface layer pin-holes were formed on the surface.
[0089] Resin viscosity and the difference in the viscosity between
the surface film resin (105) and structural resin (106) is an
important feature of the present invention. FIG. 8 shows the
relative difference between a working system (SPX13734) and non
working SF2 for the cure cycle defined above. A working system has
a high viscosity at the beginning of the cure (or at storage or
conditioning temperature) and a very low minimum viscosity during
the cure which improves the impregnation of the fibre. SPX13734 and
SF2 have a high viscosity at the beginning of the cure and SPX13734
has a lower minimum viscosity than SF2 during the cure: this
explains why SPX13734 is a working system and SF2 is a non-working
system. Materials with different viscosity profiles can be made to
work by adjusting the cure cycle provided a differential viscosity
exists between the surface resin layer (105) and structural resin
(101).
* * * * *