WO2017068159A1

WO2017068159A1 - Impregnation process using ultrasound energy

Info

Publication number: WO2017068159A1
Application number: PCT/EP2016/075460
Authority: WO
Inventors: Marco ARCIDIACONO; John Ellis; Javier MUNOZ; Herve Auduc; Matthieu BONNAFOUX
Original assignee: Hexcel Composites Limited; Hexcel Composites Sas
Priority date: 2015-10-21
Filing date: 2016-10-21
Publication date: 2017-04-27
Also published as: GB2559906A; GB201806391D0

Abstract

This invention relates to a process for the production of a fibrous material embedded in a matrix of liquid or semi solid curable resin. The process comprises the steps of applying the resin to a continuously moving web of the fibrous material and subjecting the moving web with the resin applied thereto to ultrasound.

Description

IMPROVEMENTS IN FIBRE REINFORCED COMPOSITES

The present invention relates to improvements in or relating to fibre composites and in particular to the impregnation of fibrous material with a resinous material and more particularly to the impregnation of a continuous moving web or fabric of fibres to form resin impregnated fibrous reinforcement.

Resin impregnated fibrous reinforcement as supplied to end-users for the moulding and curing of composite fibrous reinforced parts is commonly referred to as "prepreg". Prepreg is formed by the impregnation of a web of fiber tows or fabric with a liquid or semi liquid curable resin. Subsequently the impregnated web or fabric may be moulded and cured so that the resin hardens to produce a fibre reinforced article.

The desired degree of impregnation of the fibres by the resin may vary according to the nature of the article to be produced. Therefore, it is important to control the degree of impregnation when manufacturing the prepreg. For most applications, a high degree of impregnation is desired, leaving up to a maximum of 6%, preferably of from 2 to 4% by weight of the prepreg unimpregnated (as measured by the water pick up test). A high level of impregnation is not always achievable for certain combinations of resins and fibrous reinforcement materials.

Typically the resin is deposited whilst the fibrous reinforcement material is moving. The resin may be deposited in a variety of ways, for example the fibrous material may be passed through a bath of the resin. However in this process it is difficult to control the amount of resin that is deposited on the fibrous material. Alternatively the resin may be provided on a moving carrier such as paper or a plastic film which is brought into contact and pressed into the fibrous reinforcement. In all such systems the fibrous reinforcement provided with the resin is then passed through an impregnation system whereby the resin is pressed into the fibrous reinforcement comprising of a combination of one or more compaction rollers, optionally in combination with curved surfaces and/or heat sources to produce a uniform distribution of the resin within the fibrous reinforcement.

In each instance a backing sheet is usually provided on the free surface of the resin so that the resin does not contact and contaminate the impregnation equipment and other impregnation machine surfaces.

Typically, multiple compaction rollers and/or heated surfaces are required to achieve a good distribution of the resin within the fibrous web. Even so the degree of impregnation may not be as desired for certain combinations of resins and fibrous reinforcement. This cannot always be resolved by heating the resin upon impregnation as this can result in undesirable advancement of cure of the resin (pre-cure).

In the production of fibre reinforced materials from thermoset resins it is desirable that the resin uniformly impregnates the fibrous material for which a low viscosity is required and yet when the resin is cured it can be desirable that it has a high glass transition temperature (Tg). Although the resins employed are thermoset their viscosity can be reduced by heating to a temperature below their final cure. However the resins also have a cure cycle whereby they are cured by being subject to a certain temperature or range of temperatures for a specified period of time. Accordingly some cure (which is sometimes known as advancement) of the resin occurs when it is heated to reduce its viscosity to improve the impregnation process since considerable time may be required for resin application and impregnation. Such increase is detrimental as it limits material shelf life and handling properties of the uncured prepreg, including its tack level. Also, the glass transition temperature of the resin may be impacted by heating during the impregnation process. This in turn results in compromised mechanical and thermal properties in cured composite parts which may be manufactured from the preimpregnated fibrous reinforced materials.

In a typical prepreg process a resin is coated onto a silicone coated release paper or other process media in a coating machine, typically a reverse roll hot-melt coating machine known to those skilled in the art of composite material production. The resin film or films are then used in a prepreg machine to transfer the resin to the fibrous reinforcement causing it to contact and wet the filaments within the fibrous reinforcement and so impregnate the composite material into a preimpregnated product form termed a prepreg. In a typical prepreg machine heat and pressure are applied through a combination of heat plates and multiple sets of heated compaction rollers in order to reduce the viscosity of the resin such that it flows into the fibrous reinforcement impregnating the fibrous reinforcement to the required level. The product is then cooled down on a chill plate back to ambient conditions, process papers or other media are then in some part removed and replaced with new coverings and further processing is completed as necessary for width and winding for end use presentation. In this typical prepreg process the resin is advanced or partially cured through the exposure to heat.

The advancement or precure of the resin can be shown by differential scanning calorimeter (DSC) testing.

United States Patent 4689244 relates to a method of ultrasonically treating a fibrous strand for subsequent use in high speed filament winding operations. In this method a strand of a multiplicity of fibres such as glass or carbon is coated with a silane type binder which is dried. The coated strand is then passed at high speed through a bath of thermosetting resin and while submerged in the resin is subjected to ultrasonic energy which is said to increase the rate of interaction of the binder and the resin so increasing the wetting of the individual filaments. The technology is particular useful when the fibre speed through the bath is greater than 200 ft/minute and the contact time is less than 0.35 seconds. Ultrasonic energy of a frequency of 15 to 25 Kcycles per second is employed. The ultrasonic energy is said to activate the surface molecules of the binder which permits the resin to impregnate the strand at higher rates and full saturation of the strand bundle is said to occur in much shorter times. European Patent application 0416474 relates to the use of ultrasonic vibration to assist the take up of powder particles in fibrous reinforcement to produce prepregs containing woven fibre reinforcement. The powder particles may be thermoplastic or thermosetting polymers in the form of a slurry. The ultrasonic frequencies can range from 10 kHz to 100 kHz. The woven polymer is drawn through a bath of the polymer slurry with a residence time of from 20-40 seconds. Hot-melt woven prepregs are difficult to impregnate without excessive heat that advances (cures) the resin system and which can result in poor resin content control due to excessive resin flow either to from a resin bead behind a heated compaction roller in the nip gap formed between the compaction rollers or through cross web resin flow at the edges of the product.

The present invention aims to obviate or at least mitigate the above described problems and/or to provide solutions generally.

According to the invention there is provided a process and a use as defined in any one of the accompanying claims.

The invention therefore provides a process for the production of a fibrous material embedded in a matrix of liquid or semi-solid curable resin comprising applying the resin to a continuously moving web of the fibrous material and subjecting the moving web with the resin applied thereto to ultrasound . In this context, the term ultrasound is used for frequencies in the range of from 16 KHz to 1 GHz, particularly in the range of from 20 kHz to 70 kHz.

We have found that the treatment of the fibrous web with the resin applied thereto by a sonotrode either directly after traditional prepreg processing or as a post prepreg production process increases the level and/or speed of impregnation of the web by the resin while reducing the degree of cure of the resin compared to the degree of cure associated with prior methods of achieving the desired degree of impregnation. The sonotrode imparts energy to the resin causing the temperature of the resin to increase thereby increasing its flow properties. The energy is imparted to the resin in a limited resin volume for which increased flow is intended. In this way, a limited amount of energy is required and consequently, we have found that the exposure to ultrasound energy has no detrimental effect on the Tg or cure properties of the resin as confirmed by analysis using digital scanning calorimetry (DSC).

The present invention therefore provides the use of ultrasonic radiation or ultrasonic energy to improve the level and/or speed of impregnation of a continuously moving fibrous reinforcement with a thermo-curable resin that has been applied thereto. Thermo-curable in the context of theis application refers to a resin which is a thermoset resin. The cure of such a resin is enhanced with increases of its temperature.

Ultrasound energy is imparted to a material by mechanical vibrations with defined amplitude, force and duration. This results in intermolecular and surface friction heat in the material.

The core of an ultrasonic system is the stack. This is made up of the (piezoelectric) converter, the booster (amplitude transformer) and the sonotrode which imparts the ultrasound energy to the material. The stack expands and contracts at the selected ultrasound frequency which corresponds to the number of expansion and contraction cycles per second (measured in Hz). The travel of the stack, meaning the distance between the peak or maximum extension position and the zero position is referred to as the amplitude. The preferred amplitude is in the range of from 5 to 50 μιτι. The wave length is defined as the distance between two equal states along an expansion and contraction cycle and the wave length can also be calculated from the sound velocity c and the frequency f: wavelength = c/f.

In an embodiment of the invention, the ultrasonic system comprises an energy director for focussing the ultrasound. The director focusses the ultrasound energy to a locally defined area. The energy director comprises a sonotrode tip and an anvil. The sonotrode tip is adapted to concentrate the emitted ultrasound. The anvil is a passive metal element which is located below the material which is exposed to the ultrasound. The anvil reflects the emitted ultrasound back to the material.

In a preferred embodiment the anvil is shaped to concentrate the emitted ultrasound.

In this application we will refer to the sonotrode as the device that emits the ultrasound energy to the material. One of the basic parameters in ultrasonic engineering is power density which can be defined as the electrical power into the transducer divided by the surface area of the ultrasound emitting part of the transducer . Low intensity systems use a power density at the transducer face of the order 1 to 2 W.cm^"2 for a modern piezoelectric transducer. It is normal, therefore, to employ a number of transducers to output high power.

For high intensity systems, the power density at the radiating face of the sonotrode is in the order of from 1 to 10,000 W.cm^"2, preferably from 10 to 5,000 W.cm^"2, more preferably from 10 to 1,000 W.cm^"2 and even more preferably from 20 to 700 W.cm^"2 or from 30 to 300 W.cm^"2 , or from 40 to 120 W.cm^"2 or from 30 to 80 W.cm^"2 or from 40 to 60 W.cm^"2 and/or combinations of the aforesaid ranges. In this invention, the power output is similar to high intensity systems.

This can be of the order of several hundred W.cm^"2. In these systems, the operating frequencies may ranges from 20 kHz to 20 MHz, preferably from 20 kHz to 10 MHz, more preferably from 20 kHz to 2 MHz or from 30 kHz to 0.2 MHz and/or combinations of the aforesaid ranges, although lower frequencies of from 20-70 kHz may also be used.

Transducers used in modern power ultrasonic systems are almost without exception based upon the pre-stressed piezoelectric design. In this construction, a number of piezoelectric elements - normally two or four - are bolted between a pair of metal end masses. The piezo elements would be a pre- polarized lead titanate zirconate composition, which exhibit high activity coupled with both low loss and ageing characteristics. They are ideally suited to form the basis of an efficient and rugged transducer.

The vibrating motion generated by the transducer is normally too low for practical use and so it is necessary to magnify or amplify this motion. This is the function of the booster , which, like the transducer, is a resonant element in the compression mode. Normally, these are half a wavelength long, although, should the distance between the transducer and the sample being treated need to be increased, they can be designed in multiples of half wavelengths. This can also be achieved by screwing one booster into the other thereby building up the overall length. Boosters can be in different configurations:

(a) Linear taper: Simple to make but its potential magnification is limited to a factor of approximately four-fold.

(b) Exponential taper: This design offers higher magnification factors than the linear taper. Its shape makes it more difficult to manufacture but its length coupled with a small diameter at the working end makes this design particularly suited to micro applications. (c) Stepped: For this design the magnification factor is given by the ratio of the end areas. The potential magnification is limited only by the dynamic tensile strength of the booster material. This is a useful design and easy to manufacture. Gains of up to 16-fold are easily achieved in a practical booster . When choosing a material for sonotrodes , then, we look for the following characteristics: high dynamic fatigue strength; low acoustic loss; resistance to cavitation erosion; and chemical inertness.

In order of preference, suitable materials which fit the above, are: titanium alloy; aluminium alloy; aluminium bronze; and stainless steel.

Titanium alloys are way ahead of the other two in each of the four required characteristics.

Aluminium alloys are too soft for the irradiation of liquids and, compared with titanium alloys, losses in aluminium bronze and stainless steel will result in end amplitudes reduced by factors of 0.75 and 0.5 respectively, assuming a given power going into the transducer. This is because the latter two materials are acoustically lossier and this will show up as heat, i.e. the horn will become hot and transfer heat to the reaction - an undesirable side effect. The performance of transducers is measured by its operational frequency and the acoustic energy imparted to the treated sample.

There are three possible approaches to the determination of acoustic power:

Calorometric method: This is simply the calculation of power input by measuring the rate of temperature rise in the system, taking into account its thermal capacity. It is a rather cumbersome method and to be used properly it should be undertaken each time a sample is treated, in case there are system variations. Thus, it is not really a practical method.

Measurement of vibrational amplitude: This is the direct measurement of the amplitude at the working face of the sonotrode, and will give a parameter that is at least proportional to the acoustic power [Equation (1)].

Pac = ½ p.C.q² [1] where:

Pac = acoustic power ; p= density of load ; C = load sound velocity; ς = transducer amplitude

It does have the advantage that it can be continuously monitored but it cannot really be considered as an absolute method since r and c in a cavitating medium cannot easily be determined. Amplitude measurement offers a very sensitive measurement of acoustic change . It changes as the sonotrode is immersed further into the treatment sample , i.e. as it becomes more loaded.

A combination of the calorimetric method and measurement of vibrational amplitude is the most useful method of power monitoring and therefore control. In any event, by measuring amplitude we do have an indication of the acoustic power output rather than the electrical power into the transducer.

We can measure amplitude just by looking at the end of a free transducer with a microscope. A metallurgical microscope with a X15 calibrated eyepiece and a X10 objective will enable a measurement of down to 5 microns . Since most transducers will generate amplitudes of at least 10 microns and this value is magnified by the booster , quite accurate measurements can be made. The procedure is very simple: a small spot of aluminium paint is placed on the sonotrode and an individual metallic fleck is focussed in the graticule. On turning on the power the rapidly vibrating spot appears as a line. The amplitude of vibration, that is the peak to peak displacement, is the overall length of the line minus the spot diameter. Having the ability to measure amplitude with a microscope is clearly impractical during a sonochemical experiment. A method is required which provides continuous monitoring with a display. There are two possible approaches; electro-mechanical and purely electrical.

1. Electro-mechanical: The alternating stress in a resonant element is at a maximum in the centre If a strain gauge is bonded to the centre of such an element then the output from this will be proportional to the displacement or amplitude of vibration. This output signal can be rectified and displayed for example on a meter. The meter can then be calibrated by the use of a microscope (see the 'Directly by microscope' section above). It is also possible to derive a purely electrical signal that is proportional to transducer dis-placement and this possibly offers a more elegant solution since it eliminates the use of the strain gauge which can be a somewhat fragile element. Using this method it should only be necessary to calibrate the meter once since any subsequent change in transducer amplitude due to loading will be accompanied by a proportional change in strain in the transducer/acoustic system. The meter will thus follow any induced changes of amplitude which may occur either as a result of power input or load variation.

2. Purely electrical: Electrical methods of measurement can be contained within the ultrasonic generator. Essentially this is a power amplifier which converts energy at the mains frequency to energy at a chosen ultrasonic frequency. Because of the very narrow operating frequency band of the transducer, it is essential that the amplifier tracks any changes in resonant frequency of the system. This can be done by sensing with electrical means the transducer motion in a similar manner as that just examined.

The same electrical signal used to display amplitude can be fed back into the amplifier and this will enable the power generated to follow any frequency changes in the transducer/acoustic system. This is very important because the resonant frequency of the transducer decreases as it becomes warm and lengthens. Changes in the treatment sample can also affect the frequency. Both of these effects would be sufficient to shift the system off resonance with an accompanying performance loss were it not for the automatic tracking, normally referred to as automatic frequency control (AFC).

Another desirable feature of this method is that it can be used to limit the transducer amplitude and thus ensure that it does not damage either itself or the coupled resonant elements due to overstress.

In a preferred embodiment to produce prepregs the prepreg is passed underneath a sonotrode or transducer typically operating at 20,000 to 100,000 cycles per second, preferably from 25,000 to 70,000 cycles per second at typically 5 to 50 microns amplitude, preferably at 35,000 cycles per second at between 5 to 50 μιτι amplitude with a controlled gap between the sonotrode and the surface the prepreg is passing over. Preferably a reflective surface is located opposite the sonotrode for reflecting the ultrasound energy. The prepreg then passes in a gap between the sonotrode and the reflective surface.

Prepreg is exposed to the ultrasound energy emitted by the sonotrode for a residence time ranging from 0.1 s to 4 s, preferably from 0.3 s to 2 s, or from 0.5 s to 1.5 s, or from 0.9 s to 1.2 s, or from 0.95 s to 1.1 s and/or combinations of the aforesaid ranges.

Using this invention the impregnation speed of the prepreg is increased, entrapped air is removed and the composite product made in an out of autoclave process has lower porosity. The resin system after impregnation is also not advanced to the same degree as the product of the traditional prepreg process . The fibrous material to which the resin has been applied is processed such that it passes underneath a sonotrode with a controlled and preferential gap between the sonotrode and the surface that the material passes over. The resin system may be applied to the upper and lower surfaces of the fibrous reinforcement or to only one side of the fibrous reinforcement.

In a preferred embodiment the material is passed between a sonotrode and a reflective surface. The reflective surface reflects part of the emitted ultrasound energy to enhance the energy exposure of the material. The reflective surface may comprise an anvil. Preferably, the reflective surface comprises metal. Alternatively the resin system may be partially impregnated into the fibrous reinforcement such that there are still dry fibrous filaments through the thickness of the composite material.

In a preferred embodiment, the sonotrode is displaced from the resin applied fibrous material so that a controlled gap that is formed between the sonotrode working face and the surface that the fibrous reinforcement is passing over is close to the thickness of the composite product to be made. The vertical motion of the sonotrode at around 20,000 - 40,000 cycles per second will result in pressure waves being applied to the combined fibrous material and the resin. This results in heat being produced and the temperature of the composite material will rise rapidly as it passes under the sonotrode. It is possible that the temperature will rise to between 150 and 200°C when a 25mm wide sonotrode is used. This temperature rise results in a reduction of the resin viscosity which allows the resin to flow and wet or combine with the filaments of the fibrous material and so cause the fibrous material to be impregnated to the desired level. The composite material cools rapidly back to ambient temperature naturally and within 200mm distance from the sonotrode when moving at between 2 and 5 m/minute . Accordingly only a short time at elevated temperature is required to get the desired degree of impregnation, this in turn reduces the degree of advancement or cure of the resin caused during impregnation. It has been found that processing composite materials according to this invention can result in a lower day 1 sub-ambient Tg of the combined resin and fibrous composite material. In one example a resin has a day 1 sub-ambient Tg after mixing of +2.5°C, and a final impregnated composite product of this invention has a day 1 sub-ambient Tg of around 5°C.

A typical process for composite materials using known methods such as pre-coated resin films and a typical prepreg machine with heat and multiple pressure rollers with the same fibrous reinforcement and resin system resulted in a sub-ambient Tg of around 8°C on day 1. This indicates that the advancement or degree of cure during impregnation is considerably reduced when using the process of this invention.

In another embodiment the invention enables the impregnation of a carrier material with a thermocurable (curing) adhesive resin to provide tack on the carrier outer face with a lower level of resin advancement as shown by digital scanning calorimetry (DSC) testing.

An additional benefit of the process of this invention is that impregnation can be achieved using only fibrous reinforcement and resin as raw materials without the need for a resin carrier sheet. Hence an additional benefit of the process and method of this invention is that it can produce well impregnated fibrous material without the need for a coating machine, coating papers and a prepreg machine .

In a further embodiment this invention can be applied to the impregnation of a carrier material with an adhesive resin system or other resin systems. For a typical adhesive resin system the viscosity is very high and the reactivity of the resin system prevents the use of temperature to reduce the adhesive viscosity in order to apply it to a carrier material with sufficient impregnation that it does not remain on the surface of the adhesive as a dry outer layer. This reduces or eliminates the tack of the adhesive which is needed for tack of the product in certain end uses such as tack to mould surface for example in the case of surface finishing adhesive films where a carrier product is employed to yield the required surface finish properties. Here again the use of ultrasound enables the viscosity to be reduced more quickly so reducing the reaction of the resin and hence allow impregnation with little advancement or cure of the resin.

In this embodiment the carrier materials can be woven or non-woven material of polyester, polyamide, carbon, glass, and other reinforcements useful in the production of composite materials for aerospace and industrial applications.

In a further embodiment this invention can be applied to pre-impregnated composite materials to increase the level of impregnation, particularly in hot-melt woven composite materials. Use of the invention can also remove entrapped air which lowers the porosity of composite parts cured out of autoclave. The pre-impregnated prepreg material may be passed under the sonotrode such that the gap between the sonotrode and the surface the prepreg is passing over is controlled to be 0.0005" to 0.0002" smaller than the thickness of the product. If the protective covers are left on the product then the gap is controlled in the same way and set to be smaller than the product including its coverings.This invention is useful in the production of any fibrous material embedded in a matrix of resin. It is particularly useful in the production of prepregs or semipregs; the term prepreg or semipreg is used herein to describe a moulding material or structure in which the fibrous material has been impregnated with a liquid resin to the desired degree and the liquid resin is substantially uncured or partially cured. This invention allows good impregnation with reduced cure of the resin.

The degree of impregnation of the resin in the materials produced by this invention can be measured by the water pick up test. The water pick up test is conducted as follows. Six strips of impregnated materials are cut of size 100 {+1-2) mm x 100 {+1-2) mm. Any backing sheet material is removed. The samples are weighed to the nearest 0.001 g (Wl). The strips are then located between PTFE backed aluminium plates so that 15 mm of the strip protrudes from the assembly of PTFE backed plates on one end and the fibre orientation of the prepreg extends along the protruding part of the strip. A clamp is placed on the opposite end of the strip and 5 mm of the protruding part is immersed in water having a temperature of 23°C, relative air humidity of 50% +/- 35%, and at an ambient temperature of 23°C. After 5 minutes of immersion the sample is removed from the water and any exterior water is removed with blotting paper. The sample is then weighed again W2. The percentage of water uptake WPU(%) is then calculated by averaging the measured weights for the six samples as follows: WPU(%)=[(<W2>-<W1 >)/<Wl >)xl00. The WPU(%) is indicative of the Degree of Resin Impregnation (DRI).

The materials produced by this invention may have a resin concentration ranging from 20 to 50% by weight, preferably from 30 to 40% by weight and more preferably from 32 to 38% by weight of the material or structure and corresponding DRI in the range of from 0.1% to 10%, preferably from 2 to 6% and more preferably from 2 to 4% and/or combinations of the aforesaid ranges.

In addition to the water pick up the materials may be characterized by their overall resin content and/or its fibre volume. Resin and fibre content of the materials are determined in accordance with ISO 1 1667 (method A) for moulding materials or structures which contain fibrous material which does not comprise unidirectional carbon. Resin and fibre content of materials which contain unidirectional carbon fibrous material are determined in accordance with DIN EN 2559 A (code A). Resin and fibre content of moulding materials which contain carbon fibrous material are determined in accordance with DIN EN 2564 A. The fibre and resin volume % of a material can be determined from the weight% of fibre and resin by dividing the weight % by the respective density of the resin and fibre.

Typically, the values for the resin content by weight for the uncured material that can be produced according to this invention are in the ranges of from 15 to 70% by weight of the composite, from 18 to 68% by weight of the composite, from 20 to 65% by weight of the composite, from 25 to 60% by weight of the composite, from 25 to 55% by weight of the composite, from 25 to 50% by weight of the composite, from 25 to 45% by weight of the composite, from 25 to 40% by weight of the composite, from 25 to 35% by weight of the composite, from 25 to 30% by weight of the composite, from 30 to 55% by weight of the composite, from 35 to 50% by weight of the composite and/or combinations of the aforesaid ranges. Typically, the values for the resin content by volume for the material that can be produced according to this invention are in the ranges of from 15 to 70% by volume of the composite, from 18 to 68% by volume of the composite, from 20 to 65% by volume of the composite, from 25 to 60% by volume of the composite, from 25 to 55% by volume of the composite, from 25 to 50% by volume of the composite, from 25 to 45% by volume of the composite, from 25 to 40% by volume of the composite, from 25 to 35% by volume of the composite, from 25 to 30% by volume of the composite, from 30 to 55% by volume of the composite, from 35 to 50% by volume of the composite and/or combinations of the aforesaid ranges. Where tows are employed as the fibrous material in the present invention they may be made up of a plurality of individual filaments. There may be many thousands of individual filaments in a single tow. The tow and the filaments within the tow are generally unidirectional with the individual filaments aligned substantially parallel. In a preferred embodiment the tows within the moulding material or structure of the invention are substantially parallel to each other and extend along the direction of travel employed for the processing of the structure. Typically the number of filaments in a tow can range from 2,500 to 10,000 to 50,000 or greater. Tows of about 25,000 carbon filaments are available from Toray and tows of about 50,000 carbon filaments are available from Zoltek.

The resins used to impregnate the fibrous material in this invention may be any curable resin.

Examples of resins are epoxy resin, polyester resins and bismaleimide resins. Preferred resins are epoxy resins which may contain a hardener and optionally an accelerator. Dicyandiamide is a typical hardener which may be used together with a urea based accelerator. The relative amount of the curing agent and the epoxy resin that should be used will depend upon the reactivity of the resin and the nature and quantity of the fibrous reinforcement in the material. Typically from 0.5 to 10 wt % of the urea based or urea derived curing agent based on the weight of epoxy resin is used.

The viscosity of the resin and the conditions employed for impregnation of the fibrous material by the resin are selected to enable the desired degree of impregnation. It is preferred that during impregnation the resin has a viscosity of from 0.1 Pa.s to 100 Pa.s, preferably from 6 to 100 Pa.s, more preferably from 18 to 80 Pa.s and even more preferably from 20 to 50 Pa.s. It is preferred that the resin content is such that after curing the structure contains from 30 to 40 wt %, preferably 31 to 37 wt % more preferably 32 to 35 wt % of the resin.

The relative amount of resin and fibrous reinforcement, the impregnation line speed, the frequency of the applied ultrasound, the gap in the sonotrode through which the fibre and applied resin passes, the viscosity of the resin and the density of the fibrous reinforcement should be correlated to achieve the desired degree of impregnation of the fibrous material and if desired to leave spaces between the individual filaments which are unoccupied by the resin. The resins used in this invention are preferably epoxy resins and they preferably have an Epoxy Equivalent Weight (EEW) in the range from 150 to 1500 preferably a high reactivity such as an EEW in the range of from 200 to 500 and the resin composition comprises the resin and an accelerator or curing agent. Suitable epoxy resins may comprise blends of two or more epoxy resins selected from monofunctional, difunctional, trifunctional and/or tetrafunctional epoxy resins.

Suitable difunctional epoxy resins, by way of example, include those based on: diglycidyl ether of bisphenol F, diglycidyl ether of bisphenol A (optionally brominated), 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, glycidylesters or any combination thereof.

Difunctional epoxy resins may be selected from diglycidyl ether of bisphenol F, diglycidylether of bisphenol A, diglycidyl dihydroxy naphthalene, or any combination thereof. 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 polyglycidylamines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, or any combination thereof. Suitable trifunctional epoxy resins are available from Huntsman Advanced Materials (Monthey, Switzerland) under the tradenames MY0500 and MY0510 (triglycidyl para-aminophenol) and MY0600 and MY0610 (triglycidyl meta-aminophenol).

Triglycidyl meta-aminophenol is also available from Sumitomo Chemical Co. (Osaka, Japan) under the tradename ELM-120.

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 Ν,Ν,Ν',Ν'-tetraglycidylmethylenedianiline (e.g. MY0720 and MY0721 from Huntsman Advanced Materials). Other suitable multifunctional epoxy resins include DEN438 (from Dow Chemicals, Midland, Ml) DEN439 (from Dow Chemicals), Araldite ECN 1273 (from Huntsman Advanced Materials), and Araldite ECN 1299 (from Huntsman Advanced Materials).

The epoxy resin compositions used preferably also comprises one or more urea based curing agents and it is preferred to use from 0.5 to 10 wt % based on the weight of the epoxy resin of a curing agent, more preferably 1 to 8 wt %, more preferably 2 to 8 wt %. Preferred urea based materials are the range of materials available under the commercial name Urone^®. In addition to a curing agent, a suitable accelerator such as a latent amine-based curing agent, such as dicyanopolyamide (DICY).

The fibrous material to which this invention may be applied are multifilament tows which may comprise cracked (i.e. stretch-broken), selectively discontinuous or continuous filaments. The filaments may be made from a wide variety of materials, such as carbon, basaltic fibre, graphite, glass, metalized polymers, aramid and mixtures thereof. Glass and carbon fibres tows are preferred carbon fibre tows, being preferred for aerospace components and wind turbine shells of length above 40 metres such as from 50 to 60 metres. The structural fibres are individual tows made up of a multiplicity of unidirectional individual fibres. Typically the fibres will have a circular or almost circular cross-section with a diameter for carbon in the range of from 3 to 20 μιτι, preferably from 5 to 12 μιτι. For other fibres, including glass, the diameter may be in the range of from 3 to 600 μιτι, preferably from 10 to 100 μιτι. Different tows may be used in different layers of the material that are moulded according to this invention and different composites may be used together according to the properties required of the final cured article. Exemplary fibres include glass, carbon, graphite, boron, ceramic and aramid. Preferred fibres are carbon and glass fibres. Hybrid or mixed fibre systems may also be envisaged. The use of cracked (i.e. stretch-broken) or selectively discontinuous fibres may be advantageous to facilitate lay-up of the product according to the invention and improve its capability of being shaped. Although a unidirectional fiber alignment is preferable, other forms may also be used. Typical textile forms include simple textile fabrics, knit fabrics, twill fabrics and satin weaves. It is also possible to envisage using non-woven or non-crimped fiber layers. The surface mass of fibres within the fibrous reinforcement is generally 80-4000 g/m², preferably 100-2500 g/m², and especially preferably 150- 2000 g/m². The number of carbon filaments per tow can vary from 3000 to 320,000, again preferably from 6,000 to 160,000 and most preferably from 12,000 to 48,000. For fibreglass reinforcements, fibres of 600-2400 tex are particularly adapted.

Exemplary layers of unidirectional fibrous tows are made from HexTow^® carbon fibres, which are available from Hexcel Corporation. Suitable HexTow^® carbon fibres for use in making unidirectional fibre tows include: IM7 carbon fibres, which are available as tows that contain 6,000 or 12,000 filaments and weight 0.223 g/m and 0.446 g/m respectively; IM8-IM10 carbon fibres, which are available as tows that contain 12,000 filaments and weigh from 0.446 g/m to 0.324 g/m; and AS7 carbon fibres, which are available in tows that contain 12,000 filaments and weigh 0.800 g/m, tows containing up to 80,000 or 50,000 (50K) filaments may be used such as those containing about 25,000 filaments available from Toray and those containing about 50,000 filaments available from Zoltek. The tows typically have a width of from 3 to 7 mm and are fed for impregnation on equipment employing combs to hold the tows and keep them parallel and unidirectional.

Epoxy resins can become brittle upon curing and toughening materials can be included with the resin to impart durability. Where the additional toughening material is a polymer it should be insoluble in the matrix epoxy resin at room temperature and at the elevated temperatures at which the resin is cured. Depending upon the melting point of the thermoplastic polymer, it may melt or soften to varying degrees during curing of the resin at elevated temperatures and re-solidify as the cured laminate is cooled. Suitable thermoplastics should not dissolve in the resin, and include thermoplastics, such as polyamides (PAS), polyethersulfone (PES) and polyetherimide (PEI).

Polyamides such as nylon 6 (PA6) and nylon 12 (PA12) and mixtures thereof are preferred.

This invention can be used for the manufacture of hot-melt adhesives with carrier materials that require the outer surface of the carrier material to be of similar tack to the adhesive surface.

This invention can be used in-line as a prepreg is made or as a post process in order to increase the level of impregnation and remove entrapped air and so allow the use in out of autoclave process methods, either with unidirectional fibrous reinforcement or with woven fibrous material.

Additional embodiments are now briefly described below.

In an embodiment there is provided a process for the production of a fibrous material embedded in a matrix of liquid or semi solid curable resin comprising applying the resin to a continuously moving web of the fibrous material and subjecting the moving web with the resin applied thereto to ultrasound. The web may be subject to ultrasound after prepreg processing. The fibrous material and the resin may be the only materials employed.

In another embodiment, there is provided a process according to any of the preceding claims in which a prepreg is passed underneath a sonotrode operating at 20,000 cycles per second with a controlled gap between the sonotrode and the surface the prepreg is passing over. In a further embodiment there is provided a use of ultrasonic radiation to improve the speed of impregnation of a continuously moving fibrous web with a liquid or semi solid thermo-curable resin that has been applied thereto. The use may reduce the degree of cure of the resin effected during impregnation. Only fibrous reinforcement and resin may be used as raw materials. A carrier material may be impregnated with an adhesive resin system. The use may increase the level of impregnation of pre-impregnated hot-melt woven composite materials. The invention will now be illustrated by way of example only and with reference to the accompanying drawings in which:

Figure 1 shows an ultrasonic apparatus that may be used in the present invention; Figure 2 shows a diagrammatic view of impregnation equipment according to another embodiment of the invention; and

Figure 3 shows a diagrammatic view of an impregnation process which includes ultrasound equipment.

In Figure 1 ultrasound equipment is shown which consists of a converter (1) for converting electrical energy into mechanical energy by way of small amplitude vibrations in the range of from 5 to 50 μιτι at a frequency in the range of from 20 kHz to 70 kHz, a booster (2) for increasing the output of energy and a sonotrode (3) for directing the ultrasound energy to a material.

The invention is illustrated by reference to the following Examples as set out below. In the Examples, for resin advancement (curing) and other chemical data a digital scanning calorimeter (DSC) of the make Q100 V3.5 build 175 was used. For microscopy a Keyence VH80 microscope was used. For tack measurements a Medus tack test instrument was used. The equipment of this invention and methods described can be used with pre-coated or inline coated resin films if it is so desired.

Example 1 A liquid epoxy resin was applied to a 12K carbon tow and processed to obtain a prepreg, directly from resin and fibre with no other materials using the equipment shown in Figure 2 using a Telsonic ultrasound generator and Hermann sonotrode (21).

The spool of the sized 12k fibre (11) is shown to the left of the sonotrode (21) with the fibre passing under the sonotrode from left to right. The fibre contacts a resin bead before passing underneath the sonotrode (21) at 3 metres/minute and resin is exposed to the ultrasonic energy of the sonotrode such that temperature of the resin increase to between 150 and 200°C causing a viscosity drop such that the resin impregnates the carbon fibre tow as it passes under the sonotrode. The tow acquires 25% resin content by weight of the tow. The impregnated prepreg is then wound onto a cardboard core (31) with a polythene interleave layer. Testing of the product by DSC shows that the change in resin sub-ambient Tg after impregnation is around +2.3°C. When a comparable prepreg product is made from the same fibre and resin in the traditional prepreg machine with coating paper and impregnation rollers the change in sub-ambient Tg is typically + 6°C.

Hence the ultrasonic process of this invention results in less resin advancement than in the conventional impregnation process using compaction and heating.

Example 2

Example 1 was repeated using a liquid epoxy resin and unidirectional 24k carbon fibre web from Toray to produce ultrasonically assisted impregnation of the unidirectional prepreg with similar results to those of Example 1.

Example 3

A 5 harness satin weave carbon fabric was impregnated with an epoxy resin. Preimpregnated prepreg samples were ultrasound treated off-line which was found to increase the impregnation level and remove entrapped air such that out of autoclave processing of composite parts resulted in lower porosity than when the standard pre-impregnated product was used.

The resin content was nominally 48% by weight of the prepreg. The ultrasonic equipment used was from Telsonic UK, and consisted of: Telsonic Ultrasonics Generator Controller CU-1 Series SG-22, 20kHz Output; convertor - Telsonic Ultrasonics Type SE 50/50 - 4A and booster - Telsonic Ultrasonics 1.5:1 Gain.

Example 4

A process for producing an adhesive film impregnated carrier material is now exemplified. In this process a pre-coated film adhesive (4) which is located on a substrate is unwound from the substrate and is fed into a prepreg machine 100 from a lower unwind. The carrier material (5) is unwound into the prepreg machine 100 from the upper unwind onto the adhesive with a PET heat protective layer (6) on the outermost face. The compaction roll sets which are normally used in a prepreg machine remain open so they are not used in this process and therefore they are not shown. The 1st press roll (110) on the heat plate collimates the web materials together. The ultrasonic sonotrode (112) positioned over anvil (8) then causes the carrier material to impregnate into the adhesive material. Additional compaction occurs between compaction rollers (114) to improve the surface structure of the impregnated web material. Finally the impregnated web material is wound up on a roll (116).

Example 5 The following equipment is used: a sonotrode and anvil supplied by Telsonic having an output of 2000W at a frequency of 35 kHz. The anvil has a surface area of 25 x 150 mm.

A semi-preg which comprises a fibrous reinforcement is part impregnated with resin.

The semi-preg is passed between a sonotrode and an anvil similar to the equipment shown in Figure 3. The semi-preg is provided at a rate of 1.5 m/min. The anvil has a length of 2.54 cm (measured in the direction of travel of the semi-preg). Consequently the residence time for the semi-preg is 1 second.

Based on these measurements, the ultrasonic energy imparted to the semi-preg is 530 kW/m².

Claims

1. A process for the production of a fibrous material embedded in a matrix of resin comprising: a) applying the matrix resin to a continuously moving web of the fibrous material, and b) subjecting the moving web with the resin applied thereto to ultrasound energy.

2. A process according to Claim 1 in which prior to step b) and/or following step b) the web is subjected to a compression force without supplying additional heat.

3. A process according to Claim 1 or Claim 2 in which the fibrous material and the matrix resin are the only materials employed.

4. A process according to any of the preceding claims in which the ultrasound energy is provided to the moving web at a frequency in the range of from 20 kHz to 70 kHz, preferably from 25 kHz to 45 kHz and more preferably from 34 kHz to 40 kHz and/or combinations of the aforesaid ranges.

5. A process according to any of the preceding claims in which the ultrasound energy is provided to the moving web at an amplitude in the range of from 1 to ΙΟΟμιτι, preferably from 30 to 80 μιτι and more preferably from 5 to 50 μιτι and even more preferably from 30 to 60 μιτι or from 45 to 55 μιτι and/or combinations of the aforesaid ranges.

6. A process according to any of the preceding claims in which the ultrasound energy is provided to the moving web for a residence time ranging from 0.1 s to 4 s, preferably from 0.3 s to 2 s, or from 0.5 s to 1.5 s, or from 0.9 s to 1.2 s, or from 0.95 s to 1.1 s and/or combinations of the aforesaid ranges.

7. A process according to any of the preceding claims, in which the ultrasound energy is provided to the moving web at an energy in the range of from 20 to 90 W/cm², preferably from 40 to 120 W/cm².

8. The use of ultrasound energy to control the void fraction in impregnated fibrous materials as measured by the water pick up test in the range of from 0.5 to 2 % by weight of the impregnated fibrous material comprising: a) applying the matrix resin to a continuously moving web of the fibrous material, and b) subjecting the moving web with the resin applied thereto to ultrasound energy.

9. The use according to Claim 8 to reduce the degree of cure of the resin in comparison to a same impregnated fibrous material having the same void fraction as measured by the water pick up test as effected by impregnation without using ultrasound energy.

10. The use according to any of Claims 8 or 9 for the impregnation of a carrier material with an adhesive resin system.

11. The use according to any of Claims 8 to 10 to increase the level of impregnation of pre- impregnated hot-melt woven composite materials.