Method for forming reinforced thermoplastic composites

A method of improving the surface finish of a fibre-reinforced thermoplastic article in which the fibres are present as superimposed layers of continuous, aligned fibres comprising pressing the layers at a temperature above the softening point of the thermoplastic of the article characterized in that the pressure is transmitted to the superimposed layers through a resilient member and that a metal foil having a thickness of between 0.05 and 0.5 mm, preferably between 0.075 and 0.25 mm, is present between the resilient member and the reinforced article to be pressed.

This invention relates to fibre-reinforced thermoplastic composite articles 
of good surface finish and to methods for producing such articles. 
There is a strong interest in utilising fibre reinforced thermoplastic 
compositions in structural applications as replacements for metals because 
of advantages such as weight saving. This is particularly true in the 
aircraft and aerospace industries. To meet this need fibre reinforced 
polymer composites have been under development in which continuous, 
aligned fibres have been present in the composite. Because of the 
difficulty of preparing composites in which the fibres are well wetted by 
the polymer these composites, until recently, were composed of 
thermosetting polymers. The precursors for these thermosetting polymers 
are of low viscosity and can wet the reinforcing fibre thoroughly prior to 
being cured to the polymeric form. Recently, thermoplastic polymer 
composites having a high volume content of continuous, aligned fibres have 
been prepared in a manner which permits good fibre wetting. These are 
disclosed in European patent application No. 56703. 
The products described in European Patent Publication No. 56703 are 
normally obtained as a thin prepreg which for many applications needs to 
be converted to a thicker sheet. This is achieved by laying up layers of 
the prepreg with the fibres disposed in chosen directions, in planes 
through the layers, and pressing the assembly at elevated temperatures to 
form a consolidated sheet. The prepreg material may, if desired, be woven 
prior to consolidation. 
Other processes of producing thermoplastic polymer composite sheets having 
continuous, aligned fibres disposed therein are known. For example, 
British Patent Specification No. 1 570 000 discloses thermoplastic 
composites which are produced by stacking, alternately, layers of a 
thermoplastics material and layers of reinforcing fibres which have been 
impregnated with a second thermoplastic dissolved in a solvent. The stack 
is placed in a press and subjected to heat and pressure to consolidate the 
layers. 
The fact that the matrix polymer is a thermoplastic polymer gives such 
composites an advantage over thermoset composites because these 
thermoplastic composites are inherently thermoformable and can be 
repeatedly thermoformed if necessary. However, the nature of the 
continuous, aligned fibres in these composites makes them difficult to 
thermoform into articles having a good surface finish. This difficulty is 
increased to some extent when the thermoplastic matrix is an aromatic 
polymer of high softening point. 
For example, the products described in European Patent Publication No. 
56703 are normally obtained as a thin prepreg which for many applications 
needs to be converted to a thicker sheet. This is achieved by laying up 
layers of the prepreg with the fibres disposed in chosen directions, in 
planes through the layers, and pressing the assembly at elevated 
temperatures to form a consolidated sheet. The prepreg material may, if 
desired, be woven prior to consolidation. Although good consolidation of 
the layers can be achieved, articles shaped from these consolidated sheets 
leave something to be desired in terms of surface finish even though the 
sheets have been subjected to further heating and compression steps. Thus 
one of the best methods for thermoforming such products is the 
hydroforming process, used conventionally for forming metals. In this 
process forming pressure is transmitted through a resilient membrane by a 
fluid medium. Although the use of such a process enables sheet material to 
be formed the surface quality of the shaping still requires improvement, 
deficiencies being apparent as surface roughness or as area in which the 
fibre reinforcement is exposed at the surface of the shaping. A method has 
now been devised which enables the surface finish of articles thermoformed 
from highly reinforced thermoplastic composite materials to be improved. 
In particular, improvements can be obtained where the thermoplastic is an 
aromatic polymer of high softening point. 
According to the invention there is provided a method of improving the 
surface finish of a fibre-reinforced thermoplastic article in which the 
fibres are present as superimposed layers of continuous, aligned fibres, 
optionally woven, comprising pressing the layers at a temperature above 
the softening point of the thermoplastic of the article characterised in 
that the pressure is transmitted to the superimposed layers through a 
resilient member and that a metal foil having a thickness of between 0.05 
and 0.5 mm, preferably between 0.075 and 0.25 mm, is present between the 
resilient member and the reinforced article to be pressed. An improved 
surface finish is obtained at surprisingly low pressures of, for example, 
10 bar, with excellent results at 20 bar and above. Although pressures as 
high as 1000 bar may be used there is generally little additional 
advantage to be gained in using pressures in excess of 200 bar. 
Although the workpiece to be pressed can be heated to the required pressing 
temperature in the press it is more convenient to heat the workpiece to a 
required temperature externally of the press using an oven and then to 
transfer the heated workpiece to the press within a time in which the 
workpiece is still at or above the required pressing temperature. The 
metal foil interlayer may be at ambient temperature when located in the 
press or it may have been heated to an elevated temperature, preferably 
still below the softening point of the thermoplastic to be pressed. 
The process of the invention gives rise to an article having a surface 
which is improved in respect of smoothness and the avoidance of fibres 
which are proud of the surface. The surface is not necessarily glossy but 
usually has a dull gloss. It has been previously found to be difficult to 
prepare articles of high fibre content which are substantially free of 
surface blemishes such as dull patches caused by fibres being exposed at 
the surface of the article. The present invention allows the production of 
an article in which the fibres are not exposed at the surface and the 
surface appears to be uniformly smooth. The surfaces are visually 
attractive and in most cases do not require further surface finishing 
processes in order to be visually acceptable. 
Although it is not known why the presence of the metal foil results in such 
a marked improvement compared with the use of, for example the use of a 
resilient membrane in a hydroforming process it is believed that the 
thermal properties of the foil aid the crystallisation of the surface 
layers of the matrix polymer in those cases where a crystallisable polymer 
is used as the matrix polymer in the reinforced thermoplastic. 
The metal foil may be of any metal having a melting point in excess of the 
temperatures at which the pressing operation is performed. Suitable metals 
are aluminium, stainless steel or brass. Preferably the metal foil should 
have a polished surface to reduce adhesion between the foil and the 
composite as far as possible. The polished surface may be an impressed 
patterned surface. Such a surface could be decorative. Alternatively it 
could be functional, for example to provide a rough surface of the type 
which is desirable to improve the interface locking via an adhesive where 
surfaces are to be bonded together. For adhesive applications the rough 
surface should be clean, and this may be achieved by pressing an aluminium 
foil against the work piece without the commonly used surface release 
agent, and subsequently dissolving the foil with NaOH. 
The pressing operation may be performed using any of the processes known to 
those skilled in the art provided that the pressure is transmitted through 
a resilient member, such as is used in hydroforming. If the pressure is 
transmitted to the article using the metal foil but in the absence of the 
resilient member a much poorer surface quality is achieved. 
The process may be applied in any situation in which fibre-reinforced 
prepreg material is subjected to a pressing operation at a temperature in 
excess of the softening temperature of the thermoplastic matrix. Thus in a 
process in which thin prepreg material, such as produced according to the 
process of European Patent Publication No. 56703, is to be consolidated 
into a thicker sheet, layers of prepreg may be laid up with the 
uni-directional fibres of the prepreg disposed in any chosen direction and 
the process of the present invention applied as part of the consolidation 
process to produce flat sheet of better surface quality than that produced 
using conventional metal platens. The process may also be used to join 
several sheets of consolidated prepreg layers or may be used to joint a 
polymer film to a consolidated sheet. 
The process is not limited to the pressing of flat sheets and may be 
employed to advantage for producing shaped articles from prepreg or sheets 
using conventional shaping process adapted so that the pressure is 
transmitted through a resilient member and a metal foil contacting the 
fibre-reinforced material. 
The present invention may also be employed in the process described in 
copending British patent application No. 8329663, filed on even date 
herewith in which a heat-softened sheet of fibre-reinforced thermoplastic, 
in which the fibres are present as superimposed layers of continuous, 
aligned fibres, is shaped on a male tool by biassing the sheet to conform 
to the shape of the tool by progressively urging the sheet from the area 
contacting the tool towards the extremities of the sheet. The process of 
the present invention can be used in this shaping process by interposing a 
metal foil between the softened sheet and the biassing means and using 
resilient, rubber-coated rolls to roll the sheet onto the male tool.

The invention will now be described with reference to the following 
examples. 
EXAMPLE 1 
In this example sheets of continuous carbon fibre reinforced 
polyetheretherketone (PEEK), a high performance polymer obtainable from 
Imperial Chemical Industries PLC and having the repeat unit: 
EQU --Ph--O--Ph--CO--Ph--O-- 
where Ph is a phenylene unit, was used. The experiments were conducted on 
rectangular sheets of carbon fibre reinforced PEEK having a thickness of 1 
mm, and having a length and width of approximately 10 cm. The sheets were 
made up of consolidated superimposed layers of impregnated, aligned, 
continuous carbon fibres, the layers having been laid up so that the 
carbon fibres are disposed successively in directions at 0.degree., 
45.degree., 90.degree. and -45.degree. to a longitudinal direction of the 
sheet. A Saab `Fluid Form` hydroforming apparatus was used to press the 
samples with a brass foil interposed between the rubber diaphragm of the 
press and the sheet workpiece. The temperature of the brass foil was 
varied by preheating it to a desired temperature in an oven before 
assembly in the press. A stainless steel foil 0.25 mm thick was placed 
over the lower platen of the hydroforming press so that the workpiece was 
situated between the brass foil and the stainless steel foil. The sheets 
were heated in an infrared oven until a sheet temperature of about 
400.degree. C. was reached. 
The process of transferring the heated reinforced sheet to the press, 
inserting the metal foil and closing the press took, on average, 9 
seconds. The workpiece was then held under pressure for a further 10 
seconds before pressure was released. 
A series of experiments were carried out in which the following variables 
were evaluated: 
(1) thickness of foil, 
(2) temperature of foil before pressing, 
(3) hydroforming pressure. 
The surface quality of the samples below are tabulated below. In the table 
the following designations are used: 
GR glossy, but uneven, ridged surface, 
VG very glossy, 
S smooth, dull gloss surface, 
SD mainly smooth, dull gloss surface, but occasional dry patch with fibres 
exposed at the surface, 
SL mainly smooth, dull gloss surface, but more dry patches than SD, 
SU smooth, but unacceptable level of dry patches. 
TABLE 1 
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Temperature 
Hydrostatic 
of brass 
pressure 
Surface quality at a given foil thickness (mm) 
foil (.degree.C.) 
(bar) 0.025 
0.05 
0.0625 
0.0875 
0.125 
0.175 
0.25 
0.50 
1.2 
__________________________________________________________________________ 
20 20 GR GR VG VG S S SD SL SU 
20 60 GR GR VG VG -- S SD SL SU 
20 200 GR GR VG S -- S SD SL SU 
20 600 GR GR -- S S S SD SL SU 
20 1000 -- -- -- -- -- -- SD SD 
100 20 SD SL 
100 60 SD 
100 200 SD 
100 600 S S S SD 
150 20 
__________________________________________________________________________ 
In control runs in which the metal foil was not used, that is, a standard 
hydroforming process was used, the samples produced were less satisfactory 
showing evidence in most cases of surface undulation and scuffing, with 
some fibres being noticeably proud of the surface. 
The results above indicate the benefit of the process of the invention and, 
in particular, show that significant improvements can be obtained by 
moderate increases in hydrostatic pressure and pre-heat temperature of the 
metal foil. 
EXAMPLE 2 
Similar results were obtained using an aluminium alloy foil using 
thicknesses ranging from 0.025 mm to 0.5 mm. 
EXAMPLE 3 
A sheet of carbon fibre reinforced polyetheretherketone as used in Example 
1 but having dimensions of 75cm.times.37.5cm was formed into a double 
curvature leading edge having a paraboloid section using superplastically 
deformable aluminium alloy as a resilient diaphragm for transmitting 
pressure. Superplastic aluminium alloys are described in British Pat. Nos. 
1 387 586 and 1 445 181. The following procedure was used. The reinforced 
sheet was placed together with an aluminium foil of thickness 0.25 mm 
between two sheets of `Supral` superplastic aluminium alloy, the edges of 
the alloy sheets extending beyond the edges of the reinforced sheet so 
that the alloy sheets can be clamped in a forming tool around their edges 
without constraining the reinforced sheet. The sandwich was heated in an 
oven to 400.degree. C. and then clamped in a moulding tool having a female 
mould with the aluminium foil positioned so that it lies above the 
reinforced sheet with respect to the female mould. Air pressure is then 
applied to deform the sandwich against the female mould surface. On 
cooling the shaping was found to have a very glossy surface. The level of 
gloss was considerably higher than when the experiment was conducted 
without the aluminium foil.