WO1999041059A1

WO1999041059A1 - Joining method using a ptc polymer

Info

Publication number: WO1999041059A1
Application number: PCT/FI1999/000105
Authority: WO
Inventors: Ralf Strümpler; Vishal Mallick; Joergen Skindhoj; Joachim Glatz-Reichenbach
Original assignee: ABB Fläkt Oy
Priority date: 1998-02-12
Filing date: 1999-02-11
Publication date: 1999-08-19
Also published as: AU2426399A; DE19805650A1

Abstract

The invention concerns an improved joining method for joining two parts (1 and 2) by means of a joining material layer (3). By means of contacts (4 and 5), a voltage is applied to a PTC polymer material layer in the joining material (3) from an energy supply (6) via an external electric circuit. The PTC effect leads to automatic uniform distribution of the Joulean heat loss and to uniform two-dimensional melting when the joint is being produced.

Description

Joining method using a PTC polymer

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method in the area of joining technology. In joining technology, two or more parts are joined to one another in a mechanically secure way, so that the joint between them is at least no longer easily and unintentionally releasable .

Discussion of Background Known for this is of course the use of joining elements of a specific physical form, for example screws, nails, rivets and clamps. Also known are many methods of welding technology (for example laser welding), in which the joint is produced without using an additional joining material, in other words from the material of the parts to be joined themselves. In the area of plastics technology, examples of such methods are ultrasonic and vibration welding.

This invention relates, however, to a joining method in which an additional joining material is used (which does not, however, necessarily have to differ in its composition from the materials of the parts to be joined) . In the area of metal working, the soldering techniques and welding techniques which use an additional welding wire may be cited in this respect. Furthermore, all adhesive bonding techniques belong in this area.

In the area of plastics technology, furthermore, there are known plastics welding methods in which the parts to be joined are melted and fused with one another, at least in the vicinity of one joining surface, by heating elements made of metal or carbon fibers embedded in the joint and ultimately remaining there : - 2 -

"A Review of Methods for Fusion Bonding Thermoplastic Composites", A. Benatar, T.G. Gutowski, SAMPE Journal, January/February 1987, p. 33 et seq.;

"Thermoplastic Aromatic Polymer Composites", F.N. Cogswell, Butterworth-Heinemann Ltd. 1992, section 5.3.4;

"Resistance Welding of Graphite Polyetheretherketone Composites: An Experimental Investigation" E.C. Eveno, J.W. Gillespie, jr., Journal of Thermoplastic Composite Materials, vol. 1, 1988, p. 322 et seq.;

"Joining Thermoplastic Composites", D.M. Maguire, SAMPE Journal, vol. 25, no. 1, January/February 1989, p. 11 et seq.

Since, although in these techniques the embedded heating element helps to produce the joint, it tends to restrict rather than enhance its durability, there can be no question of it being an additional joining material. However, the cited sources also mention additional films of insulating polymer material which are intended to bring about an improvement in the distribution of the heat of the heater in the joining surface by thermal insulation (A. Benatar, T.G. Gutowski) or electrical insulation (F.N. Cogswell) .

The cited sources state that a major difficulty of the welding techniques presented there is the uniformity of the temperature distribution along the joining surface. In particular, the insulating layers mentioned are intended to prevent excessive introduction of heat or current from the heater into the parts to be joined at the outer border of the joining surface. Nevertheless, considerable non- uniformities cannot be avoided.

It should further be stated with respect to the prior art that there are known current-limiting elements made of polymer materials which have a PTC effect (greater positive temperature coefficient of the electrical resistance) : - 3 -

"Polymer Composite Thermistors for Temperature and Current Sensors", R. Strumpler, J. Appl . Phys . 80 (11) , December 1996, p. 6091 et seq.;

"Polyethylene Current Limiters for Short-Circuit Protection", T. Hansson, ABB Review 4/92;

"PolySwitch PTC Devices - A New Low-Resistance Conductive Polymer-Based PTC Device for Overcurrent Protection", F.A. Doljack, IEEE Transactions on Components, Hybrids and Manufacturing Technology, vol. CHMT-4, no. 4, December 1981, p. 372 et seq.; US Patent 5,313,184.

Elements with PTC polymers are also known as part of self-regulating heaters for bicycle handlebars, car batteries, lines and tanks, antennas, electrical components etc. In such cases, a PTC polymer layer forms a heating element with metal electrodes for supplying current: US Patent 4,761,541.

SUMMARY OF THE INVENTION

Accordingly, one object of the invention is to provide a joining method using a joining material for producing a secure joint between two parts. It proceeds from a known method of joining at least two parts by a joining material, the joining material containing a polymer material and the joint being produced by the action of Joulean heat loss of electric currents m the joining material.

The technical problem on which the invention is based is that of specifying a novel and improved joining method.

This problem is solved according to the invention by the polymer material being a PTC polymer material and by the electric currents producing the Joulean heat loss through the resistance in the PTC polymer material produced by the PTC effect.

The invention is based on the idea that m many joining techniques the joint is produced by the action of heat. In many cases and for various reasons, the - 4 -

generation of this heat by electric currents in the joining material is advantageous. Such reasons may be that the parts to be joined are not to be subjected to thermal energy, at least not in their entirety, that the joint is to be produced as quickly and specifically as possible, that it is difficult to access the joining surface between the two parts to be joined for an external introduction of heat, or other reasons.

The Joulean heat of the electric currents may in this case induce or enhance various processes which produce the joint. This may be a melting of a material in a way corresponding to a welding or soldering process, the intensifying of an interdiffusion, the activation of a chemical reaction, the improvement of an adhesive action etc. In any case, it is important that the heat is distributed as uniformly as possible.

Instead of improving the heat distribution by insulating layers, as in the prior art, the invention sets out on the basis that the generation of heat itself should take place as uniformly as possible. For this purpose, the joining material contains a PTC polymer, the PTC effect of which produces the electrical resistance responsible for the major part of the Joulean heat loss used. The term PTC polymer refers here to electrically conductive polymers which, in the proximity of a transition temperature, switch from a readily conductive state into a less readily conducting or insulating state. In the case of semi- crystalline polymers, this transition temperature is close to the melting temperature.

A delimitation is possible on the basis of an increase in resistance in a temperature interval of 14 K by at least a factor of 2.5 or in a temperature interval of 30 K by a factor of 6 or in a temperature interval of 100 K by a factor of 10. (Meeting this criterion defines the material as a PTC polymer) . Preferred is a delimitation with an increase in the resistance in the temperature interval of 14 K by a factor of 5 or in the - 5 -

temperature interval of 30 K by a factor of 20 or m the temperature interval of 100 K by a factor of 100.

Particularly preferred is a delimitation with a factor of 7.5 m the interval of 14 K, a factor of 100 m the interval of 30 K or a factor of 1000 in the interval of 100 K.

Such PTC polymer materials may comprise a polymer matrix with a filler distributed therein and enhancing the electrical conductivity, as described m more detail in the cited prior art. The filler may be metal particles, carbides, borides, nitrides, chopped carbon fibers, conductive polymer particles or else carbon black.

If when producing the joint the electric currents flow through the PTC polymer material and the

PTC effect with increasing heating is responsible for the major part of the voltage drop, and consequently the major part of the Joulean heat loss, a uniform generation of heat is automatically obtained. (Ma_jor part is intended here to mean at least 50%, preferably

80, 90 or 95% of the total voltage drop m the joining material.) The strong increase m resistance m the temperature range of the PTC effect thereby brings about a distribution of the electric currents to the slightly colder parts of the PTC polymer material and their restriction m ranges above a certain temperature .

Consequently, overheating effects of the joining material or of the parts to be joined can be easily avoided. By suitable choice of the PTC temperature range, a temperature which is adequate for the specific application process and is compatible for all the materials involved can be chosen and set here.

The strong increase m resistance m this temperature range provides an automatic setting of the total supply of power to this temperature range if there is a suitable external system setup for supplying the electric currents . On the other hand, PTC polymer materials can be produced from a very large selection - 6 -

of polymers for the polymer matrix, so that the joining material can be optimized not only with regard to the PTC temperature range but also in consideration of other physical or chemical aspects with a view to the application.

According to a preferred development of the invention, the electric currents are generated by applying a voltage to the joining material by connecting to an external electric circuit. In this way, currents of virtually any magnitude can be generated in a simple manner. In particular, the geometrical restrictions existing in the case of inductive methods of introduction, also conceivable within the scope of the invention, are obviated. For supplying the current or for applying the voltage, use is preferably made of contact areas which enclose between them regions of comparatively thin PTC polymer material, i.e. the current should flow through relatively short paths in the PTC polymer material. The reason for this is that, with relatively long current paths, planes of higher temperature which run transversely to the direction of the current and in which the PTC effect brings about the major drop in resistance can form through the PTC polymer material. In that case, pronounced gradients of the temperature and the specific resistance thus exist in the direction of the current, the temperature and the specific resistance being relatively homogeneous two- dimensionally transversely to the direction of the current. Depending on the intensity of the PTC effect, i.e. the degree to which resistance is derived from temperature, and on the thermal conductivity of the material, corresponding layer thicknesses of the "PTC plane" form. These layer thicknesses are technically relevant and should, as far as possible, not be less than the aforementioned regions of thin PTC polymer material between the contact areas .

Favorable contact area geometries are obtained, for example, from comb-like serrated finger structures - 7 -

for the power supply conductors in the joining plane or by flat (not necessarily continuous) contact areas running on and under a relatively thin layer of the joining material. In the last-mentioned case, power supply conductors in the form of networks or fingers, for example, may lie in the contact areas. It is also possible, however, to supply the current through the parts to be joined themselves, if they have a certain electrical conductivity, so that the parts to be joined are not excessively heated. This is of course particularly appropriate in the case of metals; readily conductive plastics are also conceivable, however.

Apart from metal structures, carbon fibers may also be used for the power supply conductors. These are highly compatible with many polymer materials and can consequently remain in the joining surface or in the joining material particularly well after producing the joint. Apart from the function as a power supply conductor - on one side or two sides - their use may also give rise to mechanical reinforcing properties. By contrast with the prior art, these carbon fiber structures are not, however, to serve themselves as a heating element by passing the current through substantially insulating polymers. In particular, carbon fiber structures give rise to good possibilities of shortening the length of the current paths in the joining material. The currents can in this case be divided and distributed very well through the multiplicity of carbon fibers. As already indicated above, it is also conceivable, however, within the scope of the invention to generate the electric currents inductively as eddy currents. For this purpose, an electromagnetic wave must be introduced into the joining material, as is known from the conventional induction welding of plastics .

However, here too the already described essential effect of the invention is obtained. If a non-uniform heating of the joining material takes place - 8 -

due to varying degrees of absorption or for other reasons, this is automatically compensated in the temperature range of the PTC effect in that greater eddy currents can flow in the somewhat colder regions. In comparison with direct connection into a heating circuit, the inductive method has the advantage that no power supply conductors are necessary and, in particular, the problem of contact devices or power supply conductors remaining in the joining surface, or at least at the border thereof, after producing the joint does not arise. On the other hand, the device for introducing the electromagnetic wave, for example a coil, must be geometrically adapted precisely to the joint to be produced, in order that excessive power losses do not occur. This may also lead to geometrical restrictions on the parts to be joined. From the aspect of apparatus, the inductive method is altogether more demanding than the method described above.

The invention is suitable in principle for an extremely wide variety of materials of parts to be joined. Frequently, however, it will only result in an adhesion effect similar to an adhesive bond, whether imparted by the heated PTC polymer material or by additional adhesion layers contained in the joining material. A preferred application area is, however, that in which at least one of the parts to be joined likewise consists of a polymer material. Then in particular, a very much more secure joint may be accomplished for this part, in a way similar to the conventional plastics welding technology, which is particularly stable if at least one of the polymer materials (begins to) melt(s) when a joint is being produced. However, very stable joints can also be created by a certain softening and intensified interdiffusion at the boundary layers. Experience shows that thermally induced joints between polymer materials are particularly good if the opposing materials correspond to one another as much as possible. For this purpose, the polymer material of - 9 -

the part or parts to be joined should be identical, or at least closely related, to the material of the polymer matrix of the PTC polymer material .

To the benefit of joints which are also reliable at a somewhat higher temperature, the invention is aimed particularly at high-temperature PTC polymer materials which have their PTC effect and melting point above 140°C, preferably above 200°C. In this case, the joining material may contain not only the electrically conducting fillers of the PTC polymer material but also further constituents for mechanical stabilization. A preferred material is, for example, a composite material of carbon fibers and a PEEK

(polyetheretherketone) as the polymer (melting point at 334°C) , it being possible for TiC, TιB₂ or TiN, for example, to be used as fillers. Further preferred polymers are PPS (polyphenylene sulfide, melting point 288°C) and s-PS ( syndiotactic polystyrene, melting point 263°C) . It has already been stated that, owing to the formation of "PTC effect planes" transversely to the direction of the current, relatively long current paths through the PTC polymer material may be disadvantageous. An alternative to the formation of the already mentioned power supply conductor structures extending m a two-dimensional form or engaging m one another in a comb-like manner is to provide on an outer side of the joining material a conductive polymer contact layer of better electrical conductivity than that of the PTC polymer material in the volume of the _joining material. This contact layer may then distribute two-dimensionally the current supplied by a metallic or carbon power supply conductor provided only at the border of the outer side, so that short current paths are m turn obtained in the actual PTC polymer material. An example of a material combination is PEEK as the polymer matrix with carbon black as the conducting filler for the PTC polymer material and PEEK with a metal powαer, for example Ni or Ag, as a readily - 10 -

conductive filler for the contact material. What is essential here is that the metallic filler produces a much higher conductivity of the contact material than that of the carbon-black-filled PEEK in the volume of the joining material.

The same applies correspondingly with the same objective to the conductivities in the range of the PTC effect. This is so because, if a conductive polymer which either has no PTC effect or a comparatively weaker PTC effect or a PTC effect of higher temperature in comparison with the PTC polymer material in the volume of the joining material is used for the contact layer, ultimately the same effect is obtained as in the case of the previous alternative when the resistance increases in the volume of the joining material on account of the PTC effect. Conversely, the ratio of the conductivities in the above example must of course not be reversed by a PTC effect.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description of two exemplary embodiments when considered in connection with the accompanying drawings, wherein individual features disclosed may be essential for the invention either individually or in combinations other than those represented and wherein: Figure 1 shows a first exemplary embodiment of the joining method according to the invention in a schematic view;

Figure 2 shows a joining material used in the method from Figure 1 in a schematic cross-sectional view;

Figure 3 and Figure 4 show schematic plan views of a joining material with two power-supplying structures and a PTC polymer material layer according to a second exemplary embodiment in a separated state; - 1 1 -

Figure 4 shows the elements from Figure 3 m a joined together state;

Figure 5 shows an alternative power-supplying structure for the second exemplary embodiment from Figures 3 and 4;

Figure 6 shows a perspective view of a floor panel produced by the joining method according to the invention according to the second exemplary embodiment; and Figure 7 shows a detail from a cross-sectional representation through the floor panel from Figure 6.

Figure 1 shows a simple schematic example of the joining method according to the invention. Two parts 1 and 2 to be joined, made of PEEK, are pressed onto a joining-material intermediate layer 3 and against one another, as indicated by the arrows. The joining-material intermediate layer 3 is further described with reference to Figure 2. It is connected via two electrical contacts 4 and 5 into an external electric circuit and is provided with current by a power supply 6.

The Joulean heat loss heats the joining material 3 up to the temperature range of its PTC effect and makes it melt there in the joining surface (i.e. vertically m the figure and perpendicularly with respect to the plane of the drawing) . As a result, a welded joint is obtained with the likewise melting outermost border layers of the parts 1 and 2 to be joined. The slight pressure on the parts is maintained during cooling down to a temperature which is far enough below the melting point of PEEK that the parts 1 and 2 are joined in a stable manner.

Figure 2 schematically shows the structure of the joining material 3. It comprises two thin outer contact layers 7 and 8 of Ag-filled PEEK and in between a PTC polymer material layer 9 of carbon-black-fllled

PEEK.

The contact layers 7 and 8 are respectively joined only at tne border to the power-supplying - 12 -

contacts 4 and 5 for the external electric circuit. On account of the Ag-powder fill, however, even at room temperature they have a much lower specific resistance than the PTC polymer material 9. Therefore, the electric voltage provided by the external electric circuit is applied in a substantially planar manner to the large limiting surfaces of the PTC polymer material layer 9 and the current paths through the PTC polymer material layer 9 correspond to the direct paths (horizontal in the figure) between these two large limiting surfaces.

In addition, Ag is a "soft" filler and therefore has virtually no appreciable PTC effect (cf. R. Strumpler, loc. cit.) . The difference between the specific resistances of the layers 7 and 8, on the one hand, and 9, on the other hand, increases accordingly extremely if the PTC polymer material layer 9 enters the range of the PTC effect under increasing heating. The contact layers 7 and 8 then act virtually as a short circuit for the large surfaces.

Since they have a PEEK matrix, these contact layers 7 and 8 do not disturb in any way the fusing of the two PEEK parts 1 and 2 to be joined. As a result, they have a distinct advantage over alternative contact possibilities, for example metal networks or foils, metal pastes or conductive polymers of another kind, as long as the filler presents no problems and the conductivity is adequate with regard to the dimensions of the joining surface. Suitable PTC materials for the PTC polymer material layer 9 are to be found in the variously cited publication by R. Strumpler (loc. cit.), the disclosure of which is included here by reference.

In the case of the second exemplary embodiment, represented in Figures 3, 4, 6 and 7, use is made on the other hand of contact structures 10 and 11 of network form, which are represented in Figure 3 together with a PTC polymer material layer 12 lying in between. Figure 3 shows the layers in the separated - 13 -

state in a schematic plan view. The contact structures 10 and 11 of network form consist of carbon fibers and have at one border a soldered-on metal contact 13 and 14, respectively. These connections 13 and 14 are depicted in Figure 3 on the narrow sides of the rectangular joining surface, but may be positioned at any point of the border of the rectangle on the basis of practical considerations .

The PTC polymer material layer 12 consists of PEEK with a carbon black fill. Carbon black is advantageous here as a filler because it has adequately high resistance to prevent the greatest losses in the contacts .

The structure from Figure 3 can be seen in a pressed together state in Figure 4. The carbon-fiber contact structures 10 and 11 are simply pressed into the polymer layer 12, it having to be ensured that no short circuits occur at the border. The PTC polymer material layer 12 should thus protrude somewhat. Figure 4 shows overall a prefabricated joining material layer 15.

Figure 5 shows an alternative power-supplying structure by comparison with the structure shown in Figures 3 and 4. In this case, two comb-like conductor tracks are arranged intermeshed in a plane, so that in each case short paths between the conductor tracks are obtained over a relatively large rectangular surface. This structure is mechanically less stable than that from Figures 3 and 4, but has the advantage that it only has to be applied to one side of a thin PTC polymer material layer. It could likewise be used in the case of the second exemplary embodiment.

The joining material layer 15 from Figure 4 is used for a joining problem represented in Figures 6 and 7. Figure 6 shows a floor panel of fiber-reinforced plastic, which is represented in section A-A in Figure 7. It comprises a carbon-fiber-reinforced upper panel 16 and a similar lower panel 17, which are joined by I- beam-like intermediate sections 18. - 14 -

Such floor panels are used as prefabricated standard structural members in industrial construction, at airports and m other areas of the construction industry and have considerable linear dimensions of several metres in length and width. In their production, the I-sections 18 must be joined at the joining points 19 to the upper side of the upper T crossmember and at the joining points 20 to the lower side of the lower T cross member respectively to the upper panel 16 and lower panel 17. The great length of the joining surfaces 19 and 20 in the direction of the I-beam sections 18 give rise to great difficulties here with the conventional methods .

In particular, the floor panels are so large that induction welding methods necessitate very large induction coils and consequently considerable costs. Conventional plastics welding methods with resistance heating have not been able to produce joints which are uniformly satisfactory over the entire surface area of the joining surfaces 19 and 20. Other techniques, for instance ultrasonic or vibration welding, are likewise ruled out for reasons of the size of the structures.

According to the invention, the rectangular _joining material layers 15 represented in Figure 4 (comparatively shortened somewhat) are now interposed at the joining surfaces 19 and 20 and are heated via the connections 13 and 14, which m this example are advantageously arranged on the shorter sides of the joining surfaces 19 and 20 and are consequently easily accessible.

In the range of the PTC effect, i.e. m the case of PEEK at 334°C, the PTC polymer material layer 12 in the joining material layer 15 melts, the directly adjoining region of the upper panel 16 and of the lower panel 17 beginning to melt along with it. This produces an intimate welded joint between the I-beam sections 18 and the panels 16 and 17, the carbon-fiber contact networks 10 and 11 being cast m as it were. In this way, they virtually do not affect the quality - 15 -

of the joint produced. On the contrary, in the region of the joining surfaces 19 and 20 they offer an additional reinforcement to the fiber reinforcement of the panels 16 and 17. In particular, in spite of the length of the joining surfaces 19 and 20, they provide a low resistance.

Altogether, a quick, simple and reliable joining method which is quite suitable for very large geometrical dimensions of the joining surfaces is obtained. The improved strength and reliability of the joints at the joining surfaces 19 and 20 significantly improve the stability of the overall floor panel shown in Figure 6.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

- 1 6 -WHAT IS CLAIMED AS NEW AND DESIRED TO BE SECURED BY LETTERS PATENT OF THE UNITED STATES IS:

1. A method of joining at least two parts (1, 2, 16, 17) by a joining material (3, 15) arranged between the parts and in the form of a layer, the joining material (3, 15) containing a polymer material (9, 12) and the joint being produced by the action of Joulean heat loss of electric currents in the joining material (3, 15), wherein the polymer material (9, 12) is a PTC polymer material, wherein the electric currents produce the Joulean heat loss through the resistance in the PTC polymer material (9, 12) produced by the PTC effect, and wherein the parts (1, 2, 16, 17) to be joined are electrically conductive and are used for supplying the electric currents.

2. A method of joining at least two parts (1, 2, 16, 17) by a joining material (3, 15) arranged between the parts and in the form of a layer, the joining material (3, 15) containing a polymer material (9, 12) and the joint being produced by the action of Joulean heat loss of electric currents in the joining material

(3, 15), wherein the polymer material (9, 12) is a PTC polymer material, wherein the electric currents produce the Joulean heat loss through the resistance in the PTC polymer material (9, 12) produced by the PTC effect, and wherein the joining material (3, 15) contains at least one layer with carbon fibers (10, 11), which is used for conducting the electric currents.

3. The method as claimed in claim 1 or 2, in which the thickness of the PTC polymer layer (9, 12) is small in comparison with the dimensions of the power- supplying contact areas (7, 8, 10, 11).

4. The method as claimed in one of the preceding claims, in which at least one of the parts (1, 2, 16, 17) to be joined consists of a polymer material.

5. The method as claimed in claim 4, in which at least the polymer material of this part (1, 2, 16, 17) or the polymer material (9, 12) in the joining material - 17 -

(3, 15) at least partially melts when the joint is being produced.

6. The method as claimed in claim 4 or 5, in which the polymer material of this part (1, 2, 16, 17) is substantially the same as the polymer material (9, 12) in the joining material (3, 15) .

7. The method as claimed in one of the preceding claims, in which the PTC polymer material (9, 12) is a high-temperature polymer with a temperature of the PTC effect of over 200┬░C.

8. The method as claimed in one of the preceding claims in conjunction with claim 2, in which the joining material (3) has on an outer side a contact layer (7, 8) of a conductive polymer of higher electrical conductivity than the PTC polymer in the volume of the joining material (3) .

9. The method as claimed in one of the preceding claims in conjunction with claim 2, in which the joining material (3) has on an outer side a contact layer (7, 8) of a conductive polymer which has in the joining method no PTC effect or a weaker PTC effect in comparison with the volume of the joining material (3) .