GB2200590A - Bonding thermoplastic materials - Google Patents

Abstract

To bond thermoplastic materials a pair of thermoplastic members 3, 5 are placed adjacent one another to define a joint region. A tape 4 of carbon fibres embedded in thermosplastic materials 6, 7 (e.g. PEEK material) is laid adjacent the joint region with the fibres extending transverse to the joint region. Current is caused to pass between electrodes 1, 2 through the carbon fibres such that heat generated by the current causes thermoplastic material in the joint region to bond the members together. To lap-weld sheets, the electrodes and exposed carbon fibres are on opposite sides of the sheets. A parallel array of tapes may provide separate current paths. The electrodes may be mounted on caterpillar tracks (Fig. 7). Electrode contact is improved using soft conducting pads 17, 18. <IMAGE>

Description

BONDING THERMOPLASTIC MATERIALS This invention relates to methods and apparatus for bonding or welding thermoplastics and in particular the so-called advanced thermoplastics which are reinforced with conducting fibres.

An example of these advanced thermoplastics is known as APC2, which is an aromatic polymer composite comprising layers of unidirectional carbon fibres laid in poly-ether-ether- ketone (PEEK) which is a thermoplastic. Such materials may be joined by heating the interface between them by means of a heating element or hot plate such that the local material is raised to a temperature in excess of 3500C so that the two components may be forged together immediately the heating plate is removed. Similarly interfaces can be heated radiantly and typical heating densities quoted are 75kW per metre square. This technique has the disadvantage of requiring appreciable time (such as one minute) to allow sufficient heat soak into the material and results in appreciable set-down when the components are forged together.

Alternative heating techniques include the application of ultrasonics at high frequency (in excess of 15 kHz), or vibrating one component with respect to the other at low frequency (usually at less than 200 Hz).

The former method is suited to small components but is impractical for large areas of bonding and, while vibration methods have been applied to some thermoplastics, they are not suited to the advanced thermoplastic composites and joint strengths are poor.

A third method relies on heating an interface conductor such as a thin metallic tape or mesh, by direct resistance heating. This method, although reasonably satisfactory, has the disadvantage of introducing a foreign material into the thermoplastic joint.

Particularly in hostile environments, corrosion and seepage in association with the metallic material can lead to deterioration of the plastic joint.

Recently it has been proposed to use a suitable implant material, known as pre-preg tape, which comprises unidirectional continuous carbon fibres set in PEEK to act as a compatible intermediary between the thermoplastics composites, such as APC-2, to be joined.

Resistance heating of the carbon fibre has been employed and for example currents of up to 15A at 30V d.c. per 25mm width of pre-preg tape have been suggested to provide sufficient heating in a minute or so.

Experience has established that this technique cannot be used satisfactorily since at these current levels there tends to be very uneven current distribution such that only a group of fibres become heated and the remainder are not sufficiently heated for a satisfactory weld. Also higher currents are precluded since, with lengths greater than about 100mm, the total voltage required (at the increased resistance) is excessive and leads to breakdown between fibres or between fibres and pressurising shoes and the like.

In accordance with one aspect of the present invention, a method of bonding thermoplastic materials comprises placing a pair of thermoplastic members adjacent one another to define a joint region; positioning a plurality of electrical conductors adjacent the joint region and defining an effective current path extending across the joint region; and causing electric current to pass along the effective current path whereby heat generated by the current passing along the effective current path causes thermoplastic material in the joint region to bond the members together.

A method is proposed which is both fast in operation and suitable for long seams, as in the lap joining of large sheets or adding stiffeners or ribs to sheet material.

Preferably, the electrical conductors extend substantially parallel with the effective current path.

Alternatively the conductors could be in- the form of a mesh.

In some cases, the thermoplastic members may be butted together but preferably the members overlap to define the joint region.

In some examples, the electrical conductors, such as carbon fibres, could directly contact the thermoplastic members and for example may be formed into a mat-like tape. Preferably, however, the conductors are embedded in a material which is compatible with the thermoplastic members, and which is most preferably the same as the material of at least one of the thermoplastic members.

Preferably, each thermoplastic member comprises a thermoplastic composite including fibres of the same composition as the electrical conductors. This is particularly advantageous since no foreign (incompatible) material will be added into the joint.

Conveniently, the conductors are formed from carbon fibres and in the preferred arrangement, the carbon fibres are embedded in PEEK in the form of a pre-preg tape.

Typically, the conductors will be arranged in a number of groups positioned side-by-side, the method comprising causing the current to pass through each group or a number of adjacent groups in turn.

A new approach is proposed in which the current is introduced into the conductors over shorter lengths such as not more than about 50mm.

In the case of pre-preg tape, it has been found beneficial to use much higher currents than hitherto, more than double, and even in excess of 50A per 25mm width of pre-preg tape. A particularly useful condition is the application from a constant current source of some 60A per 25mm width of tape with an open circuit voltage of not more than 40V at contact points not more than 50mm apart. Under these conditions the heating amounts to some -lkW per square inch (650mum2) of tape area. This is more than an order of magnitude greater than that recommended for radiant heating and results in adequate temperatures being reached in times of the order of a second.This technique can be used for joining long seams by laying the pre-preg tape with its fibres transverse to the seam direction, or at least at an angle greater than about 300 to the longitudinal seam so that the fibre ends are exposed for current contact at regions not more than about 50mm apart. It is also beneficial to add a thin layer of PEEK material to the pre-preg interlayer to assist in electrical insulation and to enrich the thermoplastic in the region of the joint.

This additional thin layer is preferably placed on either side of the pre-preg and may be integral with it.

In accordance with a second aspect of the present invention, apparatus for bonding thermoplastic materials comprises at least one electrode assembly positioned, in use, adjacent an elongate joint region defined by a pair of thermoplastic members to be bonded, the contact assembly comprising a plurality of mutually insulated contact segments, whereby each contact segment is connected in turn by respective current paths spaced along the joint region with a conductor opposite the electrode assembly through contact of the segment with electrical conductors positioned adjacent the joint region and defining an effective current path extending across the joint region.

Some examples of methods in accordance with the present invention and apparatus for carrying out those methods will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-section illustrating the attachment of thermoplastic reinforcing ribs or stiffeners to a thermoplastic sheet; Figure 2 is a schematic cross-section illustrating the bonding of a pair of overlapping thermoplastic sheets; Figure 3 illustrates part of apparatus for sequentially heating groups of carbon fibres; Figure 4 is a graph illustrating the variation of failure force against weld time for resistance heating pre-preg tape inserts in l00#m pre-consolidated PEEK coated APC-2 (manufactured by ICI) with surface fibres perpendicular to the specimen axis; Figure 5 illustrates a system for preparing the exposed ends of the carbon fibres;; Figure 6 is a schematic, perspective view of ? apparatus for bonding two members using segmented electrodes; and, Figure 7 is a schematic circuit diagram of the power supply arrangement for one pair of segments in the Figure 6 arrangement.

In the arrangement of Figure 1 conducting electrodes 1,2 are applied under pressure on one side of a common sheet or web 3 of thermoplastic material such as APC2.

A tape 4 of carbon fibres embedded in thermoplastic such as PEEK is positioned between two layers 6,7 of thermoplastic such as PEEK each having a thickness of about O.lmm. The layers 4, 6, 7 are positioned between the web 3 and electrodes 1, 2 with the carbon fibres extending in a transverse direction, typically orthogonally, to the longitudinal axis of the joint between the web 3 and a pair of thermoplastic stiffeners 5.

Holding and consolidating pressure (typically about 1 N/mm2) is applied to the stiffeners 5 as shown in Figure 1 and a voltage is then applied across the tape 4 via the electrodes 1,2 which contact exposed ends of the conductors in the tape 4 causing the PEEK material 6,7 on either side of the carbon fibres and adjacent portions of the web 3 and stiffeners 5 to soften or melt due to resistance heating following current flow through the conductors. On removal of the current, this softened or melted material will harden and bond the elements together.

Alternatively, for a lap weld between thermoplastic sheets 36,37, (Figure 2), the fibres of the pre-preg tape 4 (again laid transversely to the line of the joint between PEEK layers 6,7) have electrodes 1,2 on either side of the pair of sheets 36,37 to make contact with exposed ends of the fibres at regions not more than about 50mm apart.

It has been found that the electrical contact can be further improved by providing a sufficiently soft malleable electrical conducting pressure pad. For example, a soft 30% tin-lead solder 17,18 or pure lead insert (about 0.1-0.2mm thick) may be used as an intermediary between the electrode segments 1,2 and the carbon fibres 4.

The sides of the fibres opposite the electrodes 1,2 are usually supported by extensions of the PEEK material.

Alternatively, as shown in Figures 1 and 2, further soft conducting material 20,19 may be used to support the fibres from the non-current contact side opposite the electrodes 1, 2 respectively to assist in maintaining sufficient interconnection. For the latter a suitable material is thin lead sheet (e.g. 0.15mm thick) mounted on thin card, as is commonly used for enhancing X-ray radiographs.

The single sided approach, Figure 1, may also be used for fixing a reinforcing strip or patch of suitable thermoplastic composite onto a given sheet of thermoplastic composite, where in one dimension the additional strip or patch does not exceed about 50mm, so that suitable current connections can be made to the exposed extremities of pre-preg tape used to join the additional strip or patch to the sheet.

For seams of moderate length, high current low voltage supplies can be used with fixed electrodes which are placed in turn in pairs along the seam direction.

Yet again current may be applied to segmental electrodes dispersed along the length of the seam, such that a limited length of seam (eg. not more than about lOOmm) is heated at one time.

Figure 7 illustrates a simple circuit for use with the apparatus of Figures 1 and 2. The electrodes 1, 2 are connected via respective resistors 30 to a power supply 31.

It should be noted that, although the power supply 31 (Figure 7) coupled to the electrodes will be current controlled, ie. a constant current source, it appears virtually as a constant voltage system with respect to the pre-preg width. Thus the local current distribution is directly governed by the resistance of the individual carbon fibres (lying transverse to the joint line) at the mean voltage caused by the preset total current.

Therefore it is necessary to ensure that the electrode segments or contact bars are both equidistant and contact the fibres at the same points right across the full width of the pre-preg. That is, making contact close to the weld zone in every place and not further away at random places elsewhere. To ensure better evenness of heating, it is advisable to split up the parallel copper bar electrodes into segments, each supplied by a separate current path, and to mechanically load the segments individually. The latter is to avoid, with some variation in pre-preg or APC-2 thicknesses, the applied load being reacted more in one zone than another.

Figure 6 illustrates a modified form of apparatus in which a series of electrode pairs are provided by two assemblies of segments 25. To minimise dead spots along the assemblies, the gaps between the copper segments 25 of each assembly should be as thin as possible, such as provided by strips of mica 26 (with insulated tie bars 27 extending through the segments 25 to hold the assembly together) as shown in Figure 6 which illustrates the bonding of two thermo-plastic members 33, 34.

Compression discs or leaf springs 28 can be used on each segment to distribute the applied load from a load bar 29. Also a separate current path is required for each segment 25, which preferably is resistive to help maintain current balance. As the resistance of the carbon fibres, at the current loading used, causes a potential drop of about 1 OV for 20-25 mm length of current path, then it would be necessary to add a further drop which is not insignificant in comparison. If the segmented arrangement leads to a reasonably even current distribution on its own, then only a small external voltage drop of say IV would be adequate. Otherwise this external drop should be increased to some 5V.

(Higher voltages would impair the efficiency of the setup, and ultimately could lead to breakdown between the pre-preg and the surrounding APC-2).

Thus an ideal setup, with 25mm segments at 60A, would need resistances 30 of about 1/40Q on each side (Figure 7).

These resistances 30 could readily comprise separate lengths of stainless steel welding rod, say 200mm by about 3mm diameter.

Also if it transpires that the outer end segments require a slightly different current level than the rest, then this could conveniently be obtained by changing the external resistance feed to those segments. Moreover such outer segments may not need to be full size, but could be limited to say 15 mm or less to correct any end effects on the heating.

In addition it may be beneficial to have an overall preweld current burst to help condition the contacts and reduce the effect of any irregularities. This could be either a low current - say 1/3 welding current (one tenth heating) - for a similar duration (about 1 second), or a short duration (say 0.1 sec) burst of higher current.

The limitations of the latter are the maximum output of the power supply and the possibility of spurious breakdown from the fibres at the higher voltages concerned.

An example of typical welding parameters is given in Table 1 below for a component having 127mm x 25 mm weld area; with APC-2 pre-preg (implant fibres in the 25mm direction).

TABLE 1 Weld time 1.5 s Cool time under pressure 5.0 s Weld pressure 1.0 N/mm2 Current 300A Voltage 15V falling to llV during the weld In this example, a 600A D.C. constant current power source was used with 50V maximum output at full load.

In a further example, a moving electrode system is used, such as illustrated in Figure 3, using a pair of caterpillar track arrangements 8 with mutually insulated and segmented pads 9 hinged together to form the track, the arrangements being positioned on opposite side of the joint. Alternatively, a single caterpillar track arrangement could be used on one side with a common earth line on the other. One track arrangement 8 is shown in Figure 3. Here, the current is introduced via the caterpillar rolls 10,11 mounted on a carriage 9' such that a limited number of segments, preferably one (for example segment 90) or at the most two are used to pass current into the exposed ends of the carbon fibres in a tape 12 comprising the fibres embedded in PEEK, which is laid on the web 13 with the fibres transverse to the direction of motion of the caterpillar.The caterpillar track arrangement may contain further pressurising rolls 14 to assist in consolidation of the joint and for controlling the cooling of the previously electrically heated material. Equally a further roll or rolls may be used to introduce further heating current. For example, the leading roll 11 may be considered as a preheating stage and the following roll 10 as a welding stage and so forth.

A soft malleable conducting layer adjacent to the contacting electrodes may be provided as a tape 21 which is wound out, from a spool 22 mounted on the carriage 9', along the seam with the progression of the welding tractor head. For convenience the roll of soft metallic material is mounted on the tractor and taken around the caterpillar track to form a capstan type feed in. This is particularly suitable as the material is too soft to be pushed into the closing gap and as a thin tape is not capable of supporting itself over a significant length.

A soft metal backing sheet is also provided prior to the welding operation by laying the sheet 23 on the web 13 prior to placement of the tape 12.

Typical strengths per 25mm length of seam are given in Figure 4 for the high current technique (60A per 25mm) and short welding time (about one to two seconds) at applied loads of 0.9 and l.5N/mm2.

In order to make electrical contact to the carbon fibres, these may be prepared as exposed ends on the pre-preg tape, and may be plated such as with nickel to improve the current contact and current distribution.

Alternatively it has been suggested that liquid metal (Woods alloy) contact baths are used. (See 18th International SAMPE Technical Conference, October 1986, paper by Benatar and Gutowski from Massachusetts Institute of Technology, USA in a Review of Methods for Fusion Bonding Thermoplastic Composites). The carbon fibres are normally coated with a thin layer of thermoplastic for consoilidation of pre-preg tape. It has been found that the thermoplastic such as PEEK can be removed from the carbon## fibres by melting/burning using a small high intensity oxyacetylene flame or similar heating source 15 (Figure 5), or even a preheated metal contact plate. It has been found necessary to remove the carbonised deposits from the carbon fibres, and other sporadic remains of the melting/burning operation, by combing the fibres.In one method a blade 16 can be passed along the array of fibres 4 deflecting them sharply through a right-angle angle to break off the remnant deposits and hence expose the carbon fibre for subsequent electrical connection.

Alternatively a two pass melting/burning operation can be used. In general, a large torch with a large diameter gas nozzle- is needed such as a BOC Saffire FN blowpipe size 13 nozzle (orifice 1.4mm dia). During this process, areas not be exposed to the torch are masked with an aluminium mask. The other parameters neded for a typical two pass cleaning operation are given in Table 2.

TABLE 2 Gas - A neutral flame is used.

Oxygen at 13 ft3/hr, 4 psi.

Acetylene at 13 ft3/hr, 4 psi The above parameters give a very wide, hot flame so that during cleaning the carbon fibres glow white hot.

Position - The torch is positioned 20-30mm above the fibres.

Speed - Two passes are required. The first, at 10-15 mm/s, burns the majority of the PEEK. The second, at 8-l0mm/s, removes any remaining sooty deposits to leave -a fine brush-like finish. The only post-treatment that may be required is a wash in water.

Weave - With large touch no weave pattern is used.

Note however that it is important to heat the pre-preg right up to the aluminium mask for best results.

In one arrangement to enable continuous seams to be joined the burning and combing operation is carried out sequentially in conjunction with the welding or joining operation by a suitable cleaning stage mounted ahead of the welding tractor. This avoids the use of either the molten metal baths for electric contact or the necessity to have pre-cleaned and prepared sides to a tape with transverse oriented fibres.

Where a backing strip is provided (eg. the sheet 23), this may be inserted (on the remote non-contact side of the fibres) in between the cleaning stage and the electric resistance heating and welding stage.

After the welding operation the exposed open fibres may be trimmed off and if required the joint sealed with further application of PEEK material using a suitable heating source such as a hot air stream, radiant heaters or an infra red C02 laser.

Claims (16)

1. A method of bonding thermoplastic materials, the method comprising placing a pair of thermoplastic members adjacent one another to define a joint region; positioning a plurality of electrical conductors adjacent the joint region and defining an effective current path extending across the joint region; and causing electric current to pass along the effective current path whereby heat generated by the current passing along the effective current path causes thermoplastic material in the joint region to bond the members together.

2. A method according to claim 1, wherein the electrical conductors extend substantially parallel with the effective current path.

3. A method according to claim 1 or claim 2, wherein the electrical conductors are embedded in a material which is compatible with the thermoplastic members.

4. A method according to any of the preceding claims, wherein the conductors comprise carbon fibres.

5. A method according to claim 4, wherein the carbon fibres are embedded in PEEK.

6. A method according to any of the preceding claims, wherein electric current is caused to pass in sequence through respective groups of current paths positioned side-by-side.

7. A method according to any of the preceding claims, wherein current paths through the conductors have lengths of not more than about 50mm.

8. A method according to any of the preceding claims, wherein the current passing through an effective current path with a width of 25mm exceeds 50A.

9. A method according to claim 8, wherein the current exceeds 60A.

10. A method according to any of the preceding claims, wherein the joint region is elongate and the effective current path extends at an angle greater than 300 to the direction of the elongate joint region.

11. A method according to any of claims 1 to 10, further comprising positioning electrically -conducting pressure pads between the electrical conductors and electrical contacts which supply current to the conductors in use.

12. A method according to any of the preceding claims, wherein each thermoplastic member comprises athermoplastic composite including fibres of -the same composition as the electrical conductors.

13. Apparatus for bonding thermoplastic materials, the apparatus comprising at least one electrode assembly positioned, in use, adjacent an elongate joint region defined byXa pair of thermoplastic members to be bonded, the contact assembly comprising a plurality of mutually insulated contact segments, whereby each contact segment is connected in turn by respective current paths spaced along the joint region with a conductor opposite the electrode assembly through contact of the segment with electrical conductors positioned adjacent the joint region and defining an effective current path extending across the joint region.

14. Apparatus according to claim 13, wherein the contact assembly is provided in the form of a caterpillar track mounted about respective rolls for movement alongside the joint region.

15. A method of bonding thermoplastic materials, substantially as hereinbefore described with reference to the accompanying drawings.

16. Apparatus for bonding thermoplastic materials, substantially as hereinbefore described with reference to the accompanying drawings.