One-step resin transfer molding of multifunctional composites consisting of multiple resins

Multiple resin transfer molding is the simultaneous injection of differing esins in fiber preforms with or without a separation layer. The flow of the resins is controlled by varying the permeabilities of the preforms and the separation layer. The method produces multifunctional hybrid composites made of multilayered preforms and multiple resins. A fundamental advantage of the invention is the simplification of the manufacturing process.

BACKGROUND OF THE INVENTION 
The present invention relates to manufacturing techniques to produce 
multifunctional hybrid composites requiring multi-layered preforms and 
inserts and multiple resins through the thickness of the composite part. 
Currently, multi-resin hybrid composite parts are produced through multiple 
process steps. Starting at the mold surface, each discrete resin/preform 
or prepreg combination is processed by hand lay-up, automated tow 
placement, Resin Transfer Molding (RTM), Vacuum Assisted Resin Transfer 
Molding (VARTM), Seemann Composites Resin Infusion Molding Process 
(SCRIMP), or other commonly used manufacturing processes. Layers are 
combined subsequently through co-cure or secondary bonding options. 
Various composites manufacturing processes are used to impregnate fiber 
preforms with resin. Particularly, RTM and VARTM are used to manufacture 
composite parts. The processes involve the layup of dry reinforcing fibers 
in fabric, tape or bulk form as a preform in a closed mold environment, 
subsequently impregnating the preform with liquid resin using positive 
pressure, as in RTM, or negative pressure (i.e., vacuum) as in VARTM or 
SCRIMP or a combined form of both. The resin is cured and the part 
demolded. However, these processes have been limited to a single resin 
system. 
Traditionally, multi-layered parts have been made using only plastics, 
using processing techniques such as injection molding, blow molding, and 
co-extrusion. However, these techniques have been limited to plastics 
without reinforcements. 
The method of the present invention, Co-Injection Resin Transfer Molding 
(CIRTM), offers the potential to reduce cost and improve part performance 
and quality by using a single-step process while still offering the 
possibility of producing hybrid parts. The procedure can be applied to 
several existing manufacturing processes such as RTM, VARTM, or SCRIMP, 
which have been limited to single resin systems prior to this invention, 
as further discussed below. 
A fundamental advantage of the invention is the ability to produce a 
multi-layer hybrid composite part in a single manufacturing step to 
improve performance, increase quality, and reduce costs. The CIRTM 
technique offers improved performance via co-cure of the materials, 
improving the toughness and strength of the interface and eliminating 
defects associated with secondary bonding. The CIRTM technique has several 
distinct advantages over the prior art: 
It offers considerable cost savings by: 
(1) reducing cycle times per part, allowing for higher volume production; 
(2) reducing manpower costs and increasing quality through a reduction in 
opportunities for defects to be introduced during the manufacturing 
process; 
(3) reducing the number of processing steps; 
(4) reducing the energy needed to run the machinery; 
(5) eliminating the need for adhesives and therefore eliminating the need 
for surface preparation to apply the adhesives and eliminating the set-up 
and tolerance problems and defects associated with secondary bonding. 
Second, it offers considerable environmental advantages by: 
(1) reducing emissions, due to the decreased number of steps; 
(2) reducing waste in general and allowing for a more efficient use of 
material; 
(3) completely eliminating the need for adhesives. 
Third, it offers a considerable performance advantage by: 
(1) reducing weight; 
(2) improving bonding through co-cure and therefore improving mechanical 
properties; 
(3) allowing for structural contribution from previously nonstructural 
layers. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of making a 
multiresin hybrid composite part which has fewer steps than prior methods. 
It is another object of the invention to provide a method of making a 
multiresin hybrid composite part which, compared to the prior art, has 
cost savings, environmental advantages and performance advantages. 
These and other objects of the invention are achieved by a method of making 
a composite part comprising providing first and second fiber preforms 
having first and second permeabilities; separating the first and second 
fiber preforms with a separation layer having a permeability lower than 
the fiber preform permeability; and simultaneously injecting a first resin 
in the first fiber preform and a second resin in the second fiber preform. 
In one aspect of the invention, the method further comprises providing 
additional fiber preforms having the fiber preform permeability; 
separating the additional fiber preforms with additional separation layers 
having the separation layer permeability; and concurrently with the first 
recited injecting step, simultaneously injecting the additional fiber 
preforms with additional resins. 
Another aspect of the invention is a method of making a composite part with 
a thin coating comprising providing a first fiber preform having a first 
thickness and permeability and a second fiber preform having a second 
thickness and permeability, the first thickness being less than the second 
thickness and the first permeability being greater than the second 
permeability; and injecting a first resin in the first fiber preform and a 
second resin in the second fiber preform. 
In a preferred embodiment, the separation layer is a prepeg impregnated 
with a third resin which is compatible with the first and second resins. 
In another embodiment, the separation layer is impermeable and compatible 
with the first and second resins. 
In yet another embodiment, the separation layer includes three layers 
comprising a thin thermoplastic polymer film sandwiched between two layers 
of thermoset film adhesive, the thin thermoplastic polymer film being 
compatible with and diffusing into the thermoset film adhesive by the 
method of diffusion enhanced adhesion, the thermoset film adhesive being 
compatible with and bonding to the first and second resins.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 schematically show multiple resin injection apparatuses. The 
apparatus for performing CIRTM techniques is very similiar to that for 
other RTM, VARTM, or SCRIMP processes. For example, in the case of the 
VARTM, the apparatus used for CIRTM applied to the VARTM process includes 
vacuum pumps, resin flow tubing, and resin mixing containers and 
equipment. The difference in the apparatus is that there may be multiple 
resin mixing containers and resin traps with multiple resin tubes in 
accordance with the number of separate resins or fill planes being 
incorporated in the part. Similiarly, in some cases, identical equipment 
may be used and a secondary or tertiary, etc. resin may be placed into the 
first or second, etc. resin bucket if a series-type resin flow method is 
being used. 
FIG. 1 shows the simplest case: two-resin injection. In FIG. 1, two 
different resins A, B are simultaneously injected by injectors 15, 17 into 
a mold 10 filled with fiber preforms 14, 16. A separation layer 12, which 
can be of various forms as discussed below, may or may not be present. 
FIG. 2 shows the general multiresin apparatus. In FIG. 2, a plurality of 
different resins are simultaneously injected by injectors 60, 62, 64, 66 
and 68 into a mold 40 filled with fiber preforms 42, 44, 45, 46 and 48. 
Separation layers 50, 52, 54 and 56, which can be of various forms as 
discussed below, may or may not be present. Fiber preforms 48 and 46 
represent the nth and n-1 preforms, respectively. Separation layers 56 and 
54 represent the nth and n-1 separation layers, respectively. Injectors 68 
and 66 represent the nth and n-1 injectors, respectively. 
The following techniques may be used to control the flow in the thickness 
direction (the direction indicated by the line 30 in FIGS. 1 and 2): 
A. LESS PERMEABLE LAYER AS SEATOR: Different permeability preforms can 
be used to control the flow of the resins. This technique is based on the 
fact that resins will follow the path of least resistance, and resin will 
therefore flow through the high-permeability material before impregnating 
the low-permeability material. In this case, the two fiber preforms 14, 16 
are made of higher permeability material than the separation layer 12. 
This will cause the resin to flow through the high-permeability material 
14, 16 and not through the low-permeability separation layer 12. 
B. HIGH PERMEABILITY LAYER ON SURFACE: Using the same method described in 
A. above, it is possible to manufacture a part with a thin coating. In 
this case, a thin preform layer 16 of high-permeability material is placed 
on a thicker preform layer 14 of low-permeability material, and no 
separation layer is used. Resin B will follow the path of least resistance 
and remain in the thin preform 16, while Resin A will fill the thicker 
preform 14. This allows for a thin coating to be placed on a structural 
component; however, the thin coating does have fiber reinforcement and 
contributes to the overall structural strength of the part. 
C. IMPERMEABLE LAYER AS SEATOR: An impermeable layer, such as a 
thermoplastic film or a rubber layer, can be used as a separation layer 
12. The layer 12 maintains the separation of the flow of the two resins. 
Additionally, the layer 12 which is compatible with Resins A and B should 
be chosen so that during the curing process the layer toughens the 
interface between the two resins. 
D. LOW PERMEABLE LAYER AS SEATOR: A very low-permeability preform can be 
used to stop the flow in the thickness direction. Returning to FIG. 1, the 
separation layer 12 can be a prepreg impregnated with resin that is 
compatible with Resins A and B. The prepreg serves a dual purpose: it 
controls the flow in the thickness direction and, additionally, allows for 
improved bonding because the resin on the preimpregnated material is 
partially cured and will therefore cure with the resin once the mold is 
filled. The preimpregnated layer 12 can be of two kinds: a commercially 
available prepreg or simply a layer of fabric that is wet out manually as 
the preforms are laid up. 
E. FILM ADHESIVE SEATION LAYER: A film adhesive can be used as a 
separation layer 12 to keep the resins separate. The specific kind of 
adhesive must be picked to be compatible with the resins being used in the 
process. 
F. CATALYST-RICH SEATION LAYER: A catalyst-rich separation layer 12 can 
be used to accelerate the kinetic reaction of the resin. The resin flowing 
through the preform will slow down when it comes in contact with the 
catalyst and a chemical reaction causes the viscosity to increase, rapidly 
slowing down the flow. 
G. SERIES FLOW METHOD: The techniques described above generally direct flow 
in the plane of the part, maintain separation of the resins through the 
thickness and allow for injection of the resins either simultaneously in a 
parallel flow fashion or one after the other in a series flow fashion. In 
another method, multiple resins can be injected in series without the use 
of a separation or any of the methods described above if flow can be 
generally achieved in the through-thickness direction. In this case, the 
first resin moves to the opposite surface of resin injection and the 
second resin follows behind the first filling in the next layer of the 
preform stack. Subsequent resins fill the next layers until the part is 
completely filled. 
H. COMBINATIONS OF METHODS A-G: In many cases, combinations of the above 
methods provide the most effective separation and, additionally, provide 
the best strength and fatigue attributes. An example is the combination of 
methods C and E and taking advantage of diffusion enhanced adhesion to 
obtain optimal bondline quality. Diffusion enhanced adhesion is discussed 
in U.S. Pat. No. 5,643,390 issued to Don et al and hereby expressly 
incorporated by reference into the present specification. 
All of the techniques explained above (A.-H.) were successfully reduced to 
practice between June 1996 and December 1996 at the University of 
Delaware's Center for Composite Materials (UD-CCM). Following are some 
examples of parts that were successfully manufactured using the various 
techniques. 
Among the parts successfully manufactured was a two-layered structure 20 
(see FIG. 3) comprised of a thin phenolic layer 22 and a thick SC-4 epoxy 
layer 24. The phenolic impregnated three layers 26 of E-glass random mat, 
while the epoxy impregnated 10 layers 28 of 24-oz. E-glass 4.times.5 
weave. The part 20 was fabricated by exploiting the permeabilities of the 
preforms, as explained in part B. above. The epoxy served as structural 
support, being particularly suited to support loads. The phenolic layer 22 
was used for its properties in protecting against fire, smoke, and 
toxicity. In this multi-layer, multi-resin structure, each resin served a 
specific purpose, while being integrated in a single structure. 
Additionally, no adhesives were necessary, and the whole part was 
fabricated in a single step and co-cured. 
Parts were successfully manufactured using a thin thermoplastic film as a 
separation layer 12. The film chosen was polysulfone because of its good 
compatibility with epoxy resins. For example, a part was constructed using 
two preforms, each made up of 10 layers of E-glass, 24-oz., 4.times.5 
weave and using polysulfone as a separation layer. The two resins used 
were SC-4 epoxy and Epon 826 epoxy, and the polysulfone successfully 
diffused in the resins. The same preforms were used to produce parts based 
on the method explained in part C. above. This was done successfully using 
a polyester prepreg as a separation layer 12. The two resins used were 
SC-4 epoxy and Derakane 411-350 vinyl ester resins. 
A combination of the separation techniques described above was also reduced 
to practice. A dual layered structure containing a layer of phenolic resin 
and another layer of vinyl ester resin was manufactured. The phenolic 
resin used was British Petroleum's J2027 with Phencat 381 curing agent 
mixed 5% by weight, this phenolic has a low viscosity, approximately 350 
centipoise at room temperature. The vinyl ester used was Dow Derakane 
411-350 with Cobalt Naphthenate as an accelerator and an organic peroxide 
as the curing agent. This vinyl ester has a similar viscosity to the 
phenolic used and a room temperature cure. The separation layer used was a 
combination of methods C and E described above. A layer of thin 
polysulfone film, approximately 1/1000 inch thick, was sandwiched between 
two layers of an epoxy based film adhesive. The film adhesive used was 
3M's AF-163-20ST. The adhesive was picked to have similar or compatible 
cure temperatures with the resins used and to be made with a resin which 
is compatible with both phenolic and vinyl ester. This adhesive is an 
amine cured epoxy which is compatible with both of the resins, has 
compatible cure cycles and additionally takes advantage of the Diffusion 
Enhanced Adhesion (See U.S. Pat. No. 5,643,390) of epoxy into polysulfone. 
This method offers a number of major advantages: toughened interphase, 
unlimited part size due to the presence of the polysufone film which acts 
as an impermeable barrier and a wide variety of layer thicknesses and 
resin viscosities. 
Some of the advantages of the invention are: 
Cost reduction and economic advantages: 
A. Reduced number of processing steps needed to manufacture a part. 
B. Reduced cycle times per part due to the reduction of processing steps. 
C. Reduced miscellaneous expenses due to fewer processing steps. 
D. Reduced manpower due to fewer lay-ups and fewer processing steps. 
E. Less manufacturing space required. 
F. Elimination of the need for adhesives. 
G. Elimination of the need for surface preparation for adhesives. 
H. Reduced energy consumption due to less manpower, fewer processing steps, 
and shorter cycle times. 
Environmental advantages: 
A. Reduced emissions. 
B. Reduced waste through better use of material and resources and reduction 
in processing steps. 
C. Offers the potential to eliminate completely the use of adhesives. 
Co-cure advantages: 
A. Improved interface toughness. 
B. No degradation of interface properties. 
C. Improved ballistic performance. 
Performance advantages: 
A. Reduction in weight due to the elimination of adhesives. 
B. Improved mechanical properties as the result of the co-cure advantages. 
C. Possibility of getting structural contribution from previously 
nonstructural layers. 
While the invention has been described with reference to certain preferred 
embodiments, numerous changes, alterations and modifications to the 
described embodiments are possible without departing from the spirit and 
scope of the invention as defined in the appended claims, and equivalents 
thereof.