GB2462996A

GB2462996A - A moulding material

Info

Publication number: GB2462996A
Application number: GB0514764A
Authority: GB
Inventors: Graham Kemp
Original assignee: Hexcel Composites Ltd
Current assignee: Hexcel Composites Ltd
Priority date: 2005-07-19
Filing date: 2005-07-19
Publication date: 2010-03-03
Anticipated expiration: 2025-07-19
Also published as: DE102006032063B4; US20070036963A1; GB0514764D0; FR2888852B1; ES2289939A1; ITMI20061389A1; FR2888852A1; GB2462996B; DE102006032063A1

Abstract

A B-staged moulding material comprises discrete fibre pieces embedded in an epoxy resin matrix. The matrix comprises at least one epoxy resin material and at least one further resin material together with at least one B-staging agent. The matrix further comprises a curing agent and a cure catalyst and/or cure accelerator. For preference the B-staging agent is a primary or aromatic amine, whilst preferably the resin is diglycididylethers of bisphenol-A.

Description

A Moulding Material The present invention relates to a composite thermoset moulding material which finds utility in the formation of moulded parts, and in particular complex moulded parts.

Several types of moulding materials exist which are intended to be used for the formation of moulded parts by way of compression moulding. An example of such a system is described in EP 0916477.

A particularly pertinent example is produced by Hexcel Composites Ltd and is sold as HexMC�. HexMC comprises an epoxy resin matrix in combination with chopped carbon fibres and has a high fibre volume fraction (Vf) between 50 to 65% thus enabling it to be used in the manufacture of a wide range of moulded structural components. HexMC�is the subject of EP 1134314.

However, moulding short fibre systems such as HeXMC� successfully relies on the ability of the resin to carry the fibre as it flows in the mould during the curing process.

The flow of the material in the mould is clearly dependent on the rheological properties of the resin. Low viscosity epoxy resin matrix moulding compounds such as HexMC� are charactensed by having relatively high flow properties, which can V 2 sometimes lead to unwanted resin/fibre separation. Conversely, very high viscosity material has very low flow and the mould cannot be filled before gelling/curing has occurred.

Current products such as HexMC� / C / 2000 / R1A have flow characteristics such that the product can be conveniently used without problem for many applications.

However, they are not entirely suitable for moulding components having complex cross sections. The current solution offered to resolve this problem is to heat HexMC� at 100°C for 10-20 minutes immediately prior to moulding. A degree of advancement reaction takes place in this time leading to an increase in viscosity, thereby facilitating the moulding process. This process is known as resin staging or pre-staging.

Although this approach does provide a solution, it introduces an extra process step which the customer has to perform. Furthermore, the moulding process has to occur reasonably quickly after the resin staging as resin staging considerably diminishes the shelf-life of the product.

In addition, the resins utilised in many high volume fraction moulding systems are manufactured by way of a process requiring solvent removal. Solvent removal increases the cost of manufacture and any solvent residue can affect the quality of the ensuing product.

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Furthermore, when used to make flat components, HexMC has been known to blister, particularly towards the edges.

Therefore, it is desirable to produce a resin material which has a viscosity profile comparable to that of the resin staged}iexMC, but where such a viscosity is achieved without the need for high temperature resin staging.

It is also desirable to develop a method for the preparation of a resin system which does not require solvents in the production processes.

According to a first aspect of the present invention there is provided a B-staged moulding material comprising discrete fibre pieces embedded in an epoxy resin matrix, said matrix comprising at least one epoxy resin material at least one further resin material, at least one B-staging agent, at least one curing agent and at least one cure catalyst and/or cure accelerator.

According to a second aspect of the present invention there is provided a method for the preparation of a B-staged moulding material comprising the steps of a) preparing an epoxy resin matrix by mixing together components including at least one epoxy resin material, at least one further epoxy resin material, at least one B-staging agent, at least one curing agent, and at least one cure catalyst and/or cure accelerator; b) coating said resin matrix on to a carrier to form a resin matrix film;

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c) applying said film onto unidirectional fibres so as to form a unidirectional prepreg, dividing said prepreg into smaller discrete pieces and d) applying said pieces onto a release paper to form a moulding material wherein at any one of steps a to d a B-staging process occurs such that controlled advancement of the resin matrix is conducted.

Moulding materials such as those described herein are often referred to as chopped fibre moulding materials.

The resin matrix is such that it is preferably prepared by a hot melt process.

The precured moulding material is typically referred to as a moulding web or sheet.

The first generation of resin matrix suitable for conversion into a high fibre volume chopped moulding material is described in EP 1134314.

EP 1134314 also discloses suitable material forms and other physical characteristics of HexMC� which apply equally to the invention described herein, for example suitable fibre materials for use with the present invention.

Advantageously, the process of the present invention eliminates the need for solvent usage thereby increasing the efficiency of the manufacturing process. Furthermore, the process of the present invention improves the quality of the product as there are no issues with solvent residue in the product.

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All of the resin ingredients are included during the mixing stage.

B-staging is a term well known in the art for describing a controlled advancement in the physical state of the resin up to a predetermined level. The extent of B-staging is dependent on the selected B-staging agent and quantity used.

Herein B-staging refers to an intermediate stage in the curing reaction of certain thermosetting resin materials in which the material softens when heated and is plastic and fusible but may not entirely dissolve or fuse. A fuller technical description can be found in Principles of Polymerisation' by George Odian. Therefore, a B-staged material is one wherein the resin is plastic and fusible but not entirely dissolved or fused.

Advantageously, B-staging occurs at room temperature (i.e. 20-25°C) or slightly elevated temperatures (i.e. up to 3 0°C) and over a period of time not exceeding 7 days.

Advantageously, the B-staging agent of the present invention is consumed during the B-staging process such that the viscosity of the resin matrix increases and yet the cure facilitating components are unaffected and remain in-situ ready to bring about curing of the resin at the cure temperature.

B-staging may occur in any one of the steps a to d referred to above, for example B-staging can take place within the resin matrix, when it is in the form of the film and/or the prepreg roll and/or tIhe chopped fibre moulding material. However, B-staging preferably occurs in the resin matrix prior to commencement of step b. In fact, B-staging typically occurs as soon as the resin mixing process is complete.

Advantageously, B-staging enables the resin matrix to undergo a transition whereby the viscosity of the matrix increases. Therefore, prior to B-staging the resin matrix is sufficiently mobile such that it is easy to mix whilst following B-staging the material has a sufficiently high viscosity such that moulding of shapes can readily occur without unwanted separation of the resin matrix and the fibres.

Preferably, the B-staging agent is a reactive primary or aromatic diamine.

Suitable B-staging agents include any of the following either alone or in combination isophorone diamine (IJPDA), Laromin� C260, Jeffamine� T403, Jeffamine� C230 and Ancamine� 2264. Most preferably, the B-staging agent is IPDA.

The B-staging agent may be added to the resin matrix in an amount ranging from 2 to 5%wlw of the total resin composition.

The epoxy resin material of the present invention may be selected from any of the commercially available diglycidylethers of Bisphenol-A either alone or in combination; typical materials in this class include GY-6010 (Huntsman Advanced Materials, Duxford, UK), Epon 828 (Resolution Performance Products, Pernis, Netherlands) and DER 331 (Dow Chemical, Midland, Michigan).

The Bisphenol-A epoxy resin material preferably constitutes from 30 to 50%w/w of the total resin matrix.

The epoxy resin material of the present invention is a thermoset material and as such provides a means with which to manipulate further the viscosity and flow characteristics of the moulding material.

The further resin material may be a thermosetting resin material and/or a thermoplastic material.

The thermosetting resin material of the present invention preferably constitutes 7 to 1 Q0/ w/w of the total resin matrix.

Suitable thermoplastic materials for use with the present invention include any of the following either alone or in combination: phenoxy resins, polyethersulpbones, poly(vinylformal) resins, polyamides Preferably, the thermoplastic material of the present invention is SER 25, a Bisphenol-A epoxy resin modified with 25% phenoxy resin.

The thermoplastic material of the present invention preferably constitutes no more than 15% w/w of the total resin matrix.

Suitable curing agents for use with the present invention include any material that will achieve cure at the desired cure cycle and at the desired temperature, in this case 100°C to 120°C. There are many well known curing agents that can be used. Suitable curing agents, which may be used alone or in combination, include any of the following: anhydrides; Lewis acids, such as BF3; aromatic amines such as 3,3- diamino-diphenylsulfone (3,3-DDS), 4,4'-diaminodiphenyl sulfone (4,4'-DDS); 4,4'-methylenebis(2-isopropyl-6-methylaniline), e.g., Lonzacure M-MIIPA (Lonza S Corporation, Fair Lawn, NJ), 4,4'-methylenebis(2, 6-diisopropylaniline), e.g., Lonzacure M-DIPA (Lonza Corp., Fair Lawn, NJ); aliphatic amines such as dicyandiamide, amino or glycidyl-silanes; CuAcAc/Nonyiphenol (1/0.1);.

The curing agent preferably constitutes from 5 to 20 %w/w of the total resin matrix material.

For the avoidance of doubt, by curing agent it is meant a reactive material which, when added to a resin, causes polymerization.

In order to optimise the level of tack of a material of the present invention at least one tackifier may be added to the resin matrix.

For the avoidance of doubt, by tackifier it is meant a material that, when added to resin or reinforcement, provides a degree of stickiness or tack to the resin or reinforcement Tackifiers suitable for use with the present invention include any of the following either alone or in combination; Carboxyl-terminated Butadiene Nitrile (CTBN) rubber modified epoxy resins, Amine-terminated butadiene Nitrile (ATI3N) rubber modified epoxy resins and urethane modified epoxy resins.

Preferably, the tackifier of the present invention is Hypox� RA95 which is a liquid Bisphenol A epoxy resin modified with S to 7% of a butadiene-acrylonitrile rubber.

The tackifier may constitute from 5 to 20% w/w of the total resin composition.

The epoxy resin material and the further epoxy resin material may be introduced into the matrix mixture as individual components or as a blend. Where used, the tackifier may also be introduced into the matrix mixture as an individual component or as a blend with the epoxy resin andlor the further epoxy resin material.

Advantageously, the inclusion of a tackifier, in particular Hypox� RA95, also serves to further modify the viscosity of the resin.

Suitable accelerators for use with present invention include any of the following either alone or in combination; N, N-dimethyl, N'-3, 4-dichiorophenyl urea (Diuron), N, N-dimethyl, N' 3-chiorophenyl urea (Monuron), N, N-(4-methyl-m-phenylene bis [N' N'-dimethylurea] (UR500) and 1,1-dimetbyl 3-(3-chloro-4-methylphenyl) urea (Chiortoluron).

The accelerator preferably constitutes from 5-15 %w/w of the total resin matrix material.

For the avoidance of doubt, by accelerator it is meant a material which, when mixed with a catalyzed resin, will speed up the chemical reaction between the catalysts and the resin. The accelerator is present in small non-stoichiometric amounts that have been empirically determined to give the best properties.

Catalysts suitable for use with the present invention include any of the following imidazoles either alone or in combination: 2-methylimidazole (Curezol 2MZ), 2- ethylimidazole (Curiniid 2E1), 2-phenylimidazole (Curezol 2PZ), 2,4-diamino-6-(2- (2-methyl-i -imidazolyl)ethyl)-1,3, 5-triazine (Curezol 2MZ-Amine-S), 2-benzyl-4-methylimidazole (Curimid 2B4M1).

The catalyst preferably constitutes <1% wfw of the total resin matrix material.

For the avoidance of doubt, by catalyst it is meant a substance which markedly speeds up the cure of a compound when added in minor quantity as compared to the amounts of primary reactants.

In addition, it has been found that the use of a B-staging agent provides a polymeric backbone structure that delivers other advantages to the product web. For example, the resin matrix of the present invention offers improved melt flow and a higher Tg without the need for resin staging.

Therefore, the resin matrix of the present invention may have an isothermal viscosity in the range 2.0 -20.0 Pa.s at 120°C when measured using a Brookfield cone and plate viscometer Such a viscometer is well known to those skilled in the art and as such it is not necessary to provide details of how to use such a viscometer.

Preferably, the viscosity of the resin matrix is in the range from 2.0-17.0 Pa.s at 120°C and most preferably is in the range from 2.5 to 5.0 Pa.s at 120°C. V 11

Not only does the resin matrix of the moulding material of the present invention have an increased viscosity, but it also has a higher cured laminate glass transition temperature (Tg) than existing high fibre volume fraction epoxy matrix chopped fibre moulding systems. Therefore, the product web which comprises the resin matrix has a broader applicability such as providing cured products with higher service temperatures.

Thus, the resin matrix of the present invention has a cured Tg (IE') of at least 115°C The present invention has also been found to give rise to moulding materials, which once moulded and cured, have a gloss aspect greater than existing HexMC� type moulding systems.

Therefore, the resin matrix of the present invention may have a gloss aspect greater than 45.

This gloss aspect improvement was quantified from measurements obtained using a Tn-GLOSS master instrument which is available from Sheen Instruments Ltd, Kingston upon Thames, England.

The samples for the gloss aspect test were laminates prepared from 3 plies of HexMC� web that were press cured at 80 bar for 15 minutes at 120°C. The resulting laminates were 3.5mm thick. Laminate preparation is described in further detail in the product data sheets for HexMC� Moulding Compounds available from Hexcel Composites Ltd., Duxford, England. For comparison purposes, laminates from the current invention were compared with those from the C / 2000 I R1A material. The C / 2000 / R1A material had a reading of 42 units while the inventive material had a reading of 49 units. In both cases the quoted results were the average of six determinations.

An indication of the flow of chopped fibre type moulding materials can be obtained using several techniques one of which is a spiral flow test. This test is well known in the injection moulding and sheet moulding industries and is a method for determining the flow properties of a polymeric material based on the distance it will flow, under controlled conditions of temperature and pressure, along a special runner or flow channel of constant cross section. The test is performed in a moulding press using a mould into which the material is fed at the centre of a spiral cavity. This is described in further detail in ASTM Standard D3 13 3-98 (2004). The spiral flow mould used to measure material flow herein was a steel matching punch-cavity compression mould.

The cavity part on the mould is shown in Fig. 4 and has, as the critical dimensions, a central cavity of 152.4 mm diameter and a flow channel having a width of 38.1 mm.

and two radii of 101.6mm. The length of the flow channel was suf(icient to accommodate 710 mm of flow. A moulding material charge weight of 160 gm. was made from 90 mm x 90 mm. squares of sheet material, heated to 135 � 3°C, placed in the central cavity and held for about a 45 second dwell time. The flow tool mould was then closed and a pressure of 36,000 Newtons applied for about 10 minutes. The mould was cooled, the part removed and the flow length measured. The flow length measurement is taken at the point at the end of the flow path where the cured material begins to taper from full width into a tail.

The B-staged material (without resin staging) of the present invention, was found to have a flow length of 312mm compared to a flow length of 274mm for the existing heat staged Hex MC C / 2000 / R1A product.

Further minor ingredients may be included as performance enhancing or modifying agents in the matrix resin composition, such as any of the following: core shell rubbers; flame retardants; wetting agents; diluents; pigments/dyes; UV stabilizers; anti-fungal compounds; fillers and toughening particles.

The present invention will now be described further with reference to the following formulary example and experimental data in which: Fig. 1 is a graph showing the change in viscosity of the resin matrix of the present invention during B-staging and through its shelf-life at room temperature; Fig. 2 is a graph showing the change in un-cured Tg of the resin matrix of the present invention with time; Fig. 3 is a graph showing a comparison of the change in the viscosity of the resin matrix of the present invention (EMC 271-1) and the previous generation resin matrices (EMC 116 and EMC 172); and Fig. 4 is a diagrammatic representation of the cavity part of the spiral flow mould.

ExamDle 1 (EMC 271-1) Ingredient Function Source Amount (% __________ _____________ ___________ w/w) LY 1556 Epoxy Resin Huntsman 41 DEN 438 Epoxy Resin Dow 8.5 HyPox RA 95 Tackifier CVC Speciality 15.0 _______________ ___________________ Chemicals ________________ SER 25 Thermoplastic InChem Corp 10.0 ______________ Resin ________________ ________________ PAT 656/3 Release Agent Chemical 2.0 Release ______________ _________________ Company _______________ 1W 1571 Curiig Agent Huntsman 8.0 1ff 1524 Accelerator Huntsman 10.4 Curezol 2MZ Catalyst Air Products 0.6 Azine S _________________ _______________ ______________ IPDA B-staging agent BASF 4.5 Huntsman ingredients are obtainable from Huntsman Advanced Materials, Duxford, England Dow resin is available from Ilnivar Ltd, Cheadle, England CVC Speciality Chemicals, Moorestown, New Jersey,USA.

InChem Corp, Rock Hill, South Carolina, USA Chemical Release Company, Harrogate, North Yorkshire, England Air Products, Manchester, England BASF-Cheadle, England The present invention was borne out of a need to increase the viscosity of the matrix resin of a moulding compound, said moulding compound comprising a matrix resin having fibre pieces embedded therein. The modification was required to further guarantee that during cure, the resin matrix and fibres pieces are not separated.

Therefore, the following data is a comparison of the present invention (referred to as EMC 271-1 or 271-1) against the previous generation product HexMC (referred to as EMC 116 or 116). EMC 116 has been commercialized as HexMC� / C / 2000 / RIA.

EMC 271-1 differs from EMC 116 in that EMC 271-1 comprises IPDA combined with selected resins to give the desired viscosity. EMC 116 does not contain IPDA.

Table I shows the flow test results for EMC 27 1-1 as compared with EMC 116 both resin systems being without resin staging immediately prior to moulding..

_________ _______ __________ _____________________ Cookie Test Results Product Batch B-stage Thickness Diameter Resin: Surface ________ No. process (mm) (cm) Fibre Separation Aspect 271-1 1065 Prepreg 4.5 18 None No __________ ________ ____________ ____________ ____________ ___________________ Blisters 27 1-1 1091 Resin 4.5 19 None No ___________ ________ _____________-_____________ _____________ _____________________ Blisters 116 -None 6.8 15 Some separation Blisters ________ ______ _________ _________ _________ around the edge _________

Table 1

NB: All results are from product web using Fortafil� standard modulus 24,000 filament F503 carbon fibre The results shown in table 1 were obtained using the so-called cookie test.

The cookie test involves pressing a mass (ca. 220g) of chopped fibre moulding material into a flat circular/oval shape and measuring the thickness and diameter of the resulting cookie. The thinner and wider the cookie is, the better the results provided that the resin stays with the fibre at these dimensions. Therefore, any resin matrix/fibre separation is also noted. The mass for pressing is made from 10 plies of sq. cm. discs of moulding material that are placed on top of each other, weighed and the weight adjusted to 220g by adding or removing some material. The specimen is then pressed for 10 minutes at 13 5°C under 5OkN pressure.

Two B-staging options were investigated. One involved B-staging the 271-1 resin matrix prior to film formation, the other involved B-staging the prepreg prior to it being chopped into moulding compound flakes. It can be seen that irrespective of whether the B-staging process is conducted on the resin or the prepreg, there is little difference between the physical properties of cookie obtained. In any event, the test results show that the flow performance of EMC 271-1 is a significant improvement over that of EMC I 16.

Table 2 shows the spiral flow test results for EMC 27 1-1 and EMC 116 with two types of standard modulus carbon fibre. One is 12,000 filament AS4 from Hexcel Corporation, Salt Lake City, UT, USA and the other is 24,000 filament F503 from Toho Carbon Fibres FortafiI, Rockwood, TN, USA.

Set Fibre Resin Type Flow Surface Thermal Staging Type Length Quality ___ _______ (mm) _______ ____________ 1 F503 EMC 271-1 312 Good No resin staging 1 F503 EMC 116 274 Good 13 mm. @ 100°C 1 AS4 EMC 271-1 206 Bad No resin staging 1 AS4 EMC 116 127 Bad 13 mm. @ 100°C 2 F503 EMC 271-1 345 Good No resin staging 2 F503 EMC 116 351 Good 13 mm. @ 100°C 2 AS4 EMC 27 1-i 241 Good No resin staging 2 AS4 EMC 116 127 Bad 13 mm. @100°C

Table 2

The spiral flow tests in Set I were carried out with a 160 gram material charge having a 45 second dwell in the mould before closing the press and maintaining a temperature of 135°C for 10 minutes under a ram pressure of 40 tonS.

Set 2 tests were similar except that the dwell time was 90 seconds.

The spiral flow test results show that although the nature of the fibre type does affect the flow of the resin matrix, EMC 271-1 without resin staging, has, with one exception, better flow than resin staged EMC 116, all other parameters being equal.

Table 3 shows the mechanical test results and glass transition temperature (Tg -E') of cured laminates moulded from EMC 27 1-1 and EMC 116. The batches used and the B-stage process are identical to those ftom Table I. Dynamic mechanical thermal analysis (DMTA) was used to determine the Tg storage modulus (E') of cured samples. The value was determined from the onset of loss of the elastic modulus over the temperature range 50°C to 300°C using a 5°C per minute ramp rate at a frequency of 1Hz and a strain level of x 4 (peak to peak displacement of 64 microns). The equipment used was a Universal V3.9 analyser from TA Instruments.

Product Batch B-stage Tg-E' Flexural Fleura1 Tensile Tensile Inter-Number. Process (°C) Strength Modulus Strength Modulus Laminar (Mpa) (Gpa) (Mpa) (Gpa) Shear Strength ________ ________ _________ ______ ________ ________ _________ _________ (Mpa) 271-1 1065 Prepreg 120 440 36.1 288.8 478 ____ 49.2 271-1 1091 Resin 125 450 33.5 276.2 35.7 53.7 116 -None 100 450 40 280 45 45

Table 3

NB. All results are for a moulding web produced with Fortafil� F503-24K carbon fibre.

The mechanical properties of EMC 27 1-1 are generally in line with those for EMC 116 with the significant benefit of a 20-25°C increase in Tg. Clearly this broadens the applications for which the product can be used.

B-Staging The B-staging reaction is essentially completed in 3 to 4 days. In this period, for each batch of EMC 271-1 the viscosity at 120°C increases 5 fold from about 0.5 Pas to about 2.5 Pas. This initial increase is followed by a slow increase in viscosity over the next three weeks as is typical with all low temperature curing epoxy resin systems (see Fig. 1).

In the same 3 to 4 day period the uncured Tg changes from about -2.5°C to about 5°C (see Fig. 2), It can be concluded from Figs. 1 and 2 that a suitable period to allow for the B-stage process is 3 to 4 days.

It is worthy of note that the B-staging process must occur at temperatures up to 30°C and preferably at room temperature i.e. from 20 to 25°C. Attempts to accelerate the B-staging process by applying heat results in a less stable product. For example, heating freshly produced EMC 271-1 resin for 4 hours at 60°C achieves the same hot melt viscosity as that from room temperature ageing over 7 days. However, this system rapidly increases in viscosity with time and is completely unstable, Fig. 3 shows the average viscosity results from EMC 271-1, as shown in Fig. 1, but comparative data for EMC 116 and an additional system, EMC 172, have been added.

EMC 172 is a high viscosity product attained by increasing levels of solid Bisphenol A and phenoxy resins, but no B-staging takes place due to the absence of isophorone diamine. EMC 172 contains the same cure system as EMC 271-1 (curing agent, accelerator and catalyst).

In general the viscosity of EMC 271-1 resin after 4 to 5 days is 80 to 85% of the value after 30 days.

The EMC 116 graph appears almost as a straight line, but there is a gentle slope upwards and this is due to the fact that a change occurs towards the middle of the recommended shelf-life when the viscosity increases. Thus, in the example shown in Fig. 3, the viscosity value after 16 days is 85% of the result after 30 days.

The same trend is shown by the experimental resin matrix EMC 172. There is a period of reasonable stability for the first half of the 30 days and then there is a more upwards to higher viscosity, in this case the value after 8 days is 82% of the result after 30 days.

Figure 4 shows a spiral flow compression mould I having a central cavity 2 having a radius 4 of 76.2 mm and a flow or runner channel 3 of constant width 5 of 38.1 mm.

The mould has a radius 6 of 10 1.6mm.

In use, the material (non resin staged) under test is placed in the central cavity 2 and held for a 45 second dwell time. The compression mould 1 is then closed and a pressure of 36000 Newtons is applied for about 10 minutes. The mould is cooled, opened and the flow length measured.

Clearly, the flow rate is influenced by the radius 4 of the central cavity 2 and the width 5 of the flow channel 3. All references to spiral flow referred to herein have been determined under the conditions and using a spiral flow compression mould having the dimensions referred to in the paragraph herein abridging pages 12 and 13.

It is of course to be understood that the invention is not intended tO be restricted to the details of the above embodiments which are described by way of example only.