GB2408005A

GB2408005A - Coating a resin infused article

Info

Publication number: GB2408005A
Application number: GB0326485A
Authority: GB
Inventors: Robert Charles Backhouse
Original assignee: Trysome Ltd
Current assignee: Trysome Ltd
Priority date: 2003-11-13
Filing date: 2003-11-13
Publication date: 2005-05-18
Anticipated expiration: 2023-11-13
Also published as: GB0326485D0; GB2408005B

Abstract

A method of manufacturing a composite article coated with a coating that softens at elevated temperature and re-hardens on cooling, from an absorptive structure infiltrated with a resin. The method comprises the steps of: at least partially curing the infiltrated absorptive structure with the relative spatial arrangement of the resin and the structure being substantially that at a temperature near the hardening temperature of the coating; and applying the coating to the structure. The resin has different thermal expansion characteristics to the fibres it impregnates.

Description

RESIN INFUSED ARTICLE

The invention relates to a composite article and a method of manufacturing a composite article, in particular but not exclusively for composites comprising carbon fibre for use in automotive exterior body components.

There are various known materials for and methods of forming composite structural panels, particularly panels intended for use in the highperformance automotive field.

For such panels one significant advantage is their low mass compared with conventional steel or aluminium panels manufactured by pressing or superplastic forming (SPF).

Such composite materials comprise two or more materials integrated together.

Typically the composites comprise high-strength fibres (such as carbon fibre, glass fibre or aramid fibre) in a resin matrix. Other composites have a laminate structure.

The composite components may be painted to improve their appearance.

A composite material has an "equilibrium temperature" at which the two components have no differential thermal stresses or relative displacement. Known composites are equilibrated at this temperature and subsequently set such that the resin gels and then vitrifies, thereby setting the structure at its equilibrium state. In this way, the two materials together can form a smooth outer surface when appropriately moulded. If subsequently heated/cooled to a temperature above or below this equilibrium temperature, one of the materials will expand or contract more than the other, thus creating unevenness in the outer surface. For a fibre-resin composite, the equilibrium temperature is typically over 1 00 C, depending on the curing characteristics of the resin system.

Such articles tend to be subjected to a wide variation in temperature in use. For example, the in-use temperature of the exterior of a car can vary from around -40 to 120 C depending on the climate in which it is used. Most paints begin to soften at around 80 C, which may be lower or higher than the equilibrium temperature of the composite material. When the paint softens, the underlying composite structure becomes visible to the eye and since the surface of the composite material has already become uneven due to its being below its setting temperature, this unevenness will be visible through the softened paint. This particular problem does not occur with a metal panel for example, since a metal panel effectively only contains one material. If plastic deformation occurs in the composite material, this "fabric print-through" will be permanent and result in a visually deficient product.

It would be desirable to provide a thermally stable composite article that may be considered to yield what is termed a class-A surface finish component, most preferably over a wide temperature range. This high quality finish would ideally include properties such as an ability to be painted, impact resistance and durability.

According to a first aspect of the present invention, there is provided a method of manufacturing a composite article coated with a coating that softens at elevated temperature and re-hardens on cooling, from an absorptive structure infiltrated with a resin, the method comprising the steps of: at least partially curing the infiltrated absorptive structure with the relative spatial arrangement of the resin and the structure being substantially that at a temperature near the hardening temperature of the coating; and applying the coating to the structure.

Suitably the step of at least partially curing the infiltrated structure is performed at a temperature at or close to the hardening temperature of the coating. Such a temperature is preferably within 20 C, more preferably 10 C, most preferably 5 C of the hardening temperature of the coating. Such a temperature is preferably above the hardening temperature of the coating.

According to a second aspect of the present invention, there is provided a composite panel that includes at least a matrix containing fibres and a resin, which matrix and resin have different thermal expansion characteristics such that a surface relief of the panel changes in response to temperature, wherein the panel has a substantially smooth surface finish at a temperature of about 80 C.

According to a third aspect of the present invention, there is provided a method of coating a composite panel that includes at least a matrix containing fibres and a resin, the matrix and the resin having different thermal expansion characteristics, whereby the surface relief of the panel changes in response to temperature, the method comprising the step of coating the panel with a coating that hardens on reducing temperature, the hardening point of the coating being a temperature at which the surface of the panel is substantially smooth.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows an example of a laminate construction to which the method of the invention can be applied; Figure 2 shows a schematic representation of the preforming and resin infusion process of an automotive body panel; Figure 3 shows a curing profile of a composite article in accordance with the invention; and Figure 4 shows a finished body panel made of the construction of figure 1 and cured in accordance with figure 3.

In the figures, like reference numerals indicate like parts.

Figure 1 shows an example of a laminate structure suitable for carrying out embodiments of the invention, labelled generally with reference numeral 1. The structure has three layers: layer 2 is a i45 warp knitted carbon reinforcement fabric, layer 3 is a layer of warp knitted polymer fabric which in this example is a polyester flow media incorporating thermoplastic binder threads, and layer 4 is another +45 warp knitted carbon reinforcement fabric. Layers 2 and 4 are either woven or preferably stitched multi-axial fabric. It can be seen that in this case layer 2 is arranged with the stitching vertically in the figure whereas layer 4 is arranged with the stitching horizontally, for reasons of panel balance and symmetry. The multidirectional reinforcement improves the mechanical properties of the resulting composite. The structure 1 is a resin permeable construction and the layer 3 is preferentially permeable over layers 2 and 4.

As mentioned above, the layer 3 of warp knitted polymer fabric contains a proportion (typically 0.5 to 5%) of low-melting point binder threads or powder. In this embodiment there are provided binder threads of low-melting point thermoplastic polyester or polyamide which serve to bind the surrounding reinforcement fabric in layers 2 and 4 under the application of applied heat and pressure.

The structure of figure 1 is manufactured by laminating the pre-woven or pre-knitted sheets together.

A general process for turning the structure of figure 1 into an automotive body panel will now be described with reference to figure 2. The various steps are shown in the seven sub-figures as follows: (a) A portion of the structure 1 of figure 1 is taken and cut using Computer Numerical Control (CNC) to roughly the correct size and shape for the body panel to be made.

The structure is substantially flat at this stage.

(b) The cut portion of the structure 1 is located over a male preform tooling 6. This mould tool 6 represents the impression of the desired body panel surface.

(c) The structure 1 is formed to shape over the tooling 6 using a hot vacuum technique. The structure is covered by an air-tight membrane 12 and forced against the tooling 10 by drawing a vacuum between the tooling and the membrane.

(d) The shaped structure 1 is then placed in a female preform tooling 10, which has the equivalent concave shape to the convex shape of the male tooling 6.

(e) The structure 1 is encapsulated by a flexible membrane 12 laminated onto the female mould tool 10 and infiltrated by liquid resin using a Resin Infusion technique that involves applying a vacuum across the encapsulated assembly such that resin is infused into the structure 1. Thus a resin-infused panel 14 is manufactured.

(f) The encapsulated structure 14 is cured in an oven whilst still under a vacuum.

(g) The flexible membrane 12 is removed and the cured structure 14 is cut to its final shape using, for example, a CNC technique. The panel is then removed from the female mould 10.

The resulting panel is suitable for subsequent application of a protective coating with minimal surface preparation. The most commonly- used protective coating is paint or a coating comprising paint. The need for minimal surface preparation is due to the curing process applied in figure (f), which will be described in more detail below.

The liquid resin used in this embodiment is a two-part epoxy resin, high purity Bisphenol-F Epoxy resin cured with Isophoronediamine, a cycloaliphatic amine. The resin is preferably mixed in the correct stoichiometric ratio. Any suitable thermosetting resin could be used instead.

The curing process of figure 2(f) will now be described with reference to figure 3.

Figure 3 shows a graph of curing time in minutes plotted against structure temperature in degrees Centigrade. The various stages of the procedure will now be described: (i) (reference numeral 16) - following infiltration of liquid resin into the structure 1, the resulting structure 14 is brought up to a temperature of about 45 C and is held there for about an hour to attain a uniform temperature distribution. At this temperature the resin is better able to flow through the fibres since its viscosity is lower than at room temperature, and tends to distribute evenly within the structure 14. This temperature is lower than the temperature at which a subsequent intended paint coating softens and hardens. For typical paints this Will be the paint's glass transition temperature (Tg), which is typically around 80 C. Thus the resin uniformly surrounds the fibre infill.

(ii) (reference numeral 18) - the temperature is ramped up steeply to about 90 C.

The increase in temperature is rapid so as to ensure that the gelation and subsequent vitrification of the resin occurs isothermally at the desired equilibrium temperature rather than during a period of transition between temperatures where undesirable internal stresses or variations-on equilibrium temperature may result.

Instead, the relative spatial arrangement of the resin and fibres will be retained in or close to the state achieved after step (i). There will be some relative thermal expansion between the resin and the fibres as the temperature is ramped-up in this step but this is kept to a minimum by the rapid temperature increase.

(iii) (reference numeral 20) - the temperature is held at about 90 C for about 15 minutes. This is a first curing step and partially hardens the resin to a partially cured state in which there is sufficient crosslinking within the resin that further plastic flow between the resin and the fibres is prevented. In this state the resin may be termed as vitrified. By means of the rapid heating rate employed in step (ii) and the temperature selected for step (iii), the structure of the composite is fixed with a relative spatial arrangement of the resin and the fibres that is equilibrated to a temperature as close as possible to the soffening/hardening temperature of the intended coating. During the hardening step the resin is coating the smooth mould surface.

The required smoothness of the panel at the hardening point of the paint can be quantified using optical interferometry methods or by contact stylus methods, or can be qualitatively characterized by skilled visual examination. The hardening point of the paint can be measured quantitatively by Differential Scanning Calorimetry (DSC) or Dynamic Mechanical Thermal Analysis (DMTA).

(iv) (reference numeral 22) - the temperature is brought slowly up to around 120 C over around 50 minutes. This rise in temperature is gradual so as to avoid re- softening the resin and to avoid getting local hot spots.

(v) (reference numeral 24) - the temperature is maintained at around 120 C for about 20 minutes in order to fully harden the resin. Thus much more cross-linking occurs so that the relative spatial positions of the resin and fibre are fixed in the same state as they were partially fixed in step (iii). The temperature for this step is chosen to give a reasonable rate of cross-linking for an acceptable manufacturing process.

For example, a lower temperature may result in the cross-linking process taking too long.

After cooling and cutting to final shape in accordance with figure 2 (9), the resulting panel is coated with primer and/or paint in the usual way as a metal panel would be coated. This achieves a high quality surface finish, as shown in figure 4.

Subsequently in use, if the panel heats up to, say, 1 05 C, the paint will soften as the temperature passes through around 80 C. Furthermore, the resin and the fibres in the panel will expand differentially, the resin expanding more than the fibres. Any change in appearance of the structure due to this differential expansion e.g. surface bumps caused by expanded areas of resin, may at least partially be accommodated by the softened paint. As the panel cools back down in temperature, the thermal expansion will be reversed. When the panel has cooled to 80 C, the paint will reharden. As the fibre and resin were set with the resin in contact with the smooth mould surface and with the resin and the fibres equilibrated to a temperature at or close to the hardening temperature of the paint, the surface of the panel is relatively smooth at the hardening temperature of the paint. Therefore as the paint hardens it does so over the smooth equilibrium state of the underlying structure and thus surface finish quality is maintained once the paint has hardened and the panel cooled to lower temperatures. This is in contrast to prior art systems in which the resin is cured with the mechanical structure of the fables and resin of the panel in equilibrium at much higher temperatures and therefore when cooled to 80 C the resin will have shrunk more than the fibres causing dimples in the composite structure. In this prior art situation, this uneven surface finish of the panel will be retained by the paint as the panel subsequently cools to below the paint's hardening temperature.

For refinishing systems typically employed for the painting of composite components the softening temperature or glass transition temperature (Tg) of the primer, colour coat, and clear coat is typically in the range 6080 C depending on formulations selected. Differential Scanning Calorimetry (DSC) is the industry standard method for characterizing the Tg, which is essentially a material property/physical characteristic.

It can be seen that in the above examples the softening temperature of the coating (around 80 C) to be applied to the structure 1 is somewhat below the curing temperature of the resin (90 C) in steps (iii) and (v). Furthermore, the equilibrating temperature of 45 C has been chosen as providing a good environment for the necessary equilibration. The combination of the two temperatures of 45 C and 90 C coupled with the rate of increase in temperature between the two in step (ii) allows setting of the resin at 90 C, such that the structure functions in use as described above. A long equilibrating stage is not necessarily required providing that the vitrification of the resin occurs at a fixed temperature at, or close to, the softening temperature of the coating, in this case selected to be 90 C.

The above-described embodiment can be varied within the scope of the invention.

For example, instead of using an Rl system to infiltrate the structure with resin, other prior-art techniques can be used such as Resin Transfer Moulding (RTM), VARTM (Vacuum Assisted RTM) or VARI (Vacuum Assisted Resin Injection). Instead of the flexible membrane used in figure 2 (e) another rigid or semi-rigid tool could be used to encapsulate the structure 1.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A method of manufacturing a composite article coated with a coating that softens at elevated temperature and re-hardens on cooling, from an absorptive structure infiltrated with a resin, the method comprising the steps of: at least partially curing the infiltrated absorptive structure with the relative spatial arrangement of the resin and the structure being substantially that at a temperature near the hardening temperature of the coating; and applying the coating to the structure.

2. A method according to claim 1, wherein the step of at least partially curing the infiltrated structure is performed at a temperature at or close to the hardening temperature of the coating.

3. A method according to claim 1 or claim 2, comprising the further step of, prior to the step of at least partially curing the infiltrated structure, placing the structure in a mould.

4. A method according to claim 3, wherein during the step of at least partially curing the infiltrated structure, the resin is coating the smooth mould surface.

5. A method according to any preceding claim, wherein the step of at least partially curing the resin has a duration of about 15 minutes.

6. A method according to any preceding claim, comprising the further step of thermally equilibrating the absorptive structure and the resin such that they acquire substantially the relative spatial arrangement as that at a temperature near the hardening temperature of the coating.

7. A method according to claim 6, wherein the step of thermally equilibrating the absorptive structure and the resin is performed at a temperature which is lower than the hardening temperature of the coating.

8. A method according to claim 6 or claim 7, wherein the step of thermally equilibrating the absorptive structure and the resin has a duration of approximately an hour.

9. A method according to claim 7 or claim 8, as dependent on claim 2, comprising the further step of heating the infiltrated absorptive structure from the temperature at which the step of thermally equilibrating the absorptive structure and the resin is performed to the temperature at which the step of at least partially curing the infiltrated structure is performed, at a rate that substantially minimises disruption of the thermally equilibrated structure.

10. A method according to any preceding claim, comprising the further step of substantially fully curing the resin to substantially set the infiltrated absorptive structure and the resin with the relative spatial arrangement of the resin and the structure being substantially that at a temperature near the hardening temperature of the coating.

11. A method according to claim 10, wherein the step of substantially fully curing the resin is performed at a temperature which is higher than the temperature at which the step of at least partially curing the resin is performed.

12. A method according to claim 11, comprising the further step of increasing the temperature of the infiltrated absorptive structure from the temperature at which the step of at least partially curing the resin is performed to the temperature at which the step of substantially fully curing the resin is performed, at a rate which minimises un- setting of the resin.

13. A method according to any preceding claim, of manufacturing a composite article intended to be used within a temperature range from below to above the hardening temperature of the coating.

14. A method according to any preceding claim, wherein the resin comprises an epoxy resin.

15. A method according to any preceding claim, wherein the resin comprises a two- part epoxy resin.

16. A method according to any preceding claim, wherein the coating comprises paint.

17. A method according to any preceding claim, wherein the structure comprises fibres.

18. A method according to any preceding claim, wherein the structure further comprises a hardener system arranged to promote curing of the resin.

19. A method substantially as herein described with reference to the accompanying drawings.

20. A composite panel that includes at least a matrix containing fibres and a resin, which matrix and resin have different thermal expansion characteristics such that a surface relief of the panel changes in response to temperature, wherein the panel has a substantially smooth surface finish at a temperature of about 60 C to about 90 C.

21. A composite panel according to claim 20, wherein the resin is an epoxy resin.

22. A composite panel according to claim 20 or claim 21, wherein the resin is a two- part epoxy resin.

23. A composite panel according to any of claims 20 to 22, wherein the resin comprises a cycloaliphatic amine.

24. A composite panel according to any of claims 20 to 23, wherein the fables comprise warp knitted carbon reinforced fabric.

25. A composite panel according to any of claims 20 to 24, wherein the fibres comprise a polyester flow media incorporating thermoplastic binder threads.

26. A composite panel according to any of claims 20 to 25, further comprising a coating, which softens at elevated temperature and rehardens on cooling and has a hardening temperature in the range from about 60 C to about 90 C.

27. A composite panel according to claim 26, which has been at least partially cured with the relative spatial arrangement of the resin and the matrix being substantially that at a temperature near the hardening temperature of the coating.

28. A composite panel according to claim 27, which has been at least partially cured at a temperature which is at or close to the hardening temperature of the coating.

29. A composite panel according to claim 27 or claim 28, which has been thermally equilibrated such that the matrix and the resin have acquired a relative spatial arrangement substantially as that at a temperature near the hardening temperature of the coating.

30. A composite panel according to claim 29, which has been thermally equilibrated at a temperature which is lower than the hardening temperature of the coating.

31. A composite panel according to claim 30, wherein the temperature of the article has been raised from the equilibrating temperature to the temperature at which the article is at least partially cured at a rate which minimises disruption of the equilibrated structure.

32. A composite panel according to any of claims 27 to 31, which has been substantially fully cured at a temperature higher than the temperature at which the article was at least partially cured, to substantially set the article with a relative spatial arrangement of the resin and the matrix substantially as that at a temperature near the hardening temperature of the coating.

33. A composite article according to any of claims 26 to 32, for use at temperatures above and/or below the hardening temperature of the coating.

34. A composite article according to any of claims 26 to 33, wherein the coating comprises paint.

35. A composite article according to any of claims 20 to 34, for use as a vehicle body component.

36. A composite article substantially as herein described with reference to the accompanying drawings.

37. A method of coating a composite panel that includes at least a matrix containing fibres and a resin, the matrix and the resin having different thermal expansion characteristics, whereby the surface relief of the panel changes in response to temperature, the method comprising the step of coating the panel with a coating that hardens on reducing temperature, the hardening point of the coating being a temperature at which the surface of the panel is substantially smooth.