Patent Application
20170028698
This application claims priority to European patent application No. EP 15400036.8 filed on Jul. 31, 2015, the disclosure of which is incorporated in its entirety by reference herein.
(1) Field of the Invention
The present invention relates to the field of methods for joining parts made of thermoset material by using a thermoplastic material.
(2) Description of Related Art
Many structures being at least partially made of thermoset polymers are in turn formed by various substructures that need to be joined. Several techniques for attaching these substructures are well-known in the art.
One of these techniques is the mechanical fastening by means of screws, rivets, bolts or the like. Several drawbacks are usually associated to joints of this kind—excess of weight, problems with stress concentrations and fiber breakage resulting from hole drilling, expensive maintenance and difficulty to control the state of the joints, among others.
Likewise, adhesion bonding is disadvantageous in that it may entail a thorough surface preparation and long curing times. Moreover, adhesive-based processes are not certified for aerospace structural joints yet, mostly due to the risk of undetected weak bonds between the parts.
A further alternative, suitable for thermoset composite materials, consists in co-curing the thermoset substructures in a one-shot process. However, co-curing normally requires dedicated, expensive tooling and the quality of the final structure depends on each substructure—should one of the substructures present defaults, the whole structure cannot be used. Therefore, the method risks being insufficient in terms of quality assurance and cost and efficiency of manufacturing.
Welding is another alternative to join thermoset polymers or thermoset polymer composites. This method necessitates one or more thermoplastic films interposed between the thermoset components so as to give rise to interfaces between the thermoplastic and the thermoset materials—or between two thermoplastic materials—which eventually bond these thermoset components.
Welding joints are beneficial in comparison with the above-explained procedures—since the bond results from the entanglement of chemical species, the quality problems are less frequent than in an adhesive bonding, based on interactions like the Van der Waals force. Moreover, as the thermoplastic material can be reversibly melted, welding may allow for an easy replacement of the joined components.
Several prior art documents describe welding processes in which a thermoplastic component and a thermoset component, or two thermoplastic components, are adhered through a network formed between them when the process is carried out.
The document XP DENG.CAMT 2014 by S. Deng et al., published on Oct. 13, 2014 in the magazine Composites: Part A, edited by Elsevier, offers a review on this matter by explaining different ways to perform the mentioned attachments. The article focuses on fusion bonding methods, according to which thermoplastic materials are joined under the application of a sufficient amount of heat and pressure, which provokes the interdiffusion of molecular chains across the interfaces of the material. Particularly, it discloses a process for joining two thermoset composite components in which each one of these components is first joined to a thermoplastic layer during its curing by means of fusion bonding, so that a thermoplastic surface is formed on each thermoset component. Then, the two thermoplastic surfaces are brought into contact and adhered with a further fusion bonding joint, thus attaching the thermoset components. Accordingly, this joint consists of three interfaces—two between a thermoplastic and a thermoset material and another between the thermoplastic layers.
Document US2009/0246548 describes a process for joining a semi-crystalline or crystalline thermoplastic polymer to a thermoset component, which can be a pure thermoset polymer or a thermoset composite. The resulting pieces can then be bonded by their filmed surfaces, which are placed in intimate contact with respect to one another and heated above the melting temperatures of the semi-crystalline thermoplastic surfaces to allow for a fusion bonding. Therefore, as in the process of the previous paragraph, the final component depends on a joint between thermoplastic materials. Besides, in order to make the fusion welding of the thermoplastics possible, a heat platen or another heating element is necessary for permitting heat to be focused on the welding line of the pieces to be bonded.
Document WO2014/088704 discloses a process for joining two stacks of uncured thermoset composite components by providing an amorphous thermoplastic layer between the stacks and by curing the stacks at a temperature above the glass transition temperature of the amorphous thermoplastic layer so that the rubbery thermoplastic layer fuses with the viscous thermoset resin. In order to assist in this fusion process, consolidating pressure should be applied. Since both components are uncured before being brought into contact with the thermoplastic layer and the curing takes place in only one shot, the manufacture of the final component presents the difficulty of dealing with the instability of the thermoset components before curing, which may lead to problems of tolerance and quality assurance.
Document DE102010007824 also proposes a method for joining thermoset components. These components are coated in one of their surfaces with a thermoplastic layer with which they form an interpenetrating network. After that, an additional thermoplastic component is interposed between the filmed surfaces and heat is applied to give rise to a fusion welding joint between the thermoplastic materials.
The document EP2784106 describes a composite structure comprising thermoset resin containing elements, thermoplastic elements and an interface between the thermoset resin containing elements and the thermoplastic elements. These elements comprise functional groups at the interface which bond to each other when the composite structure is cured. The functional groups are independently selected from amines, carboxylic acids, acid anhydrides, oxiranes, and derivatives thereof in their non-bonded condition. A blade comprises such a composite structure.
Other documents were considered, i.e.: the document XP TLP 2015, entitled “Measurement of Tg” and published online at http://www.doitpoms.ac.uk/tlplib/glass-transition/measurement.php on May 18, 2015 from the University of Cambridge DoITPoMS—TLP Library The Glass Transition in Polymers, the document XP AGEORGES 2000, entitled “Advances in fusion bonding techniques for joining thermoplastic matrix” and published on Oct. 25, 2000, the documents WO2013/60890, WO2006/89534, WO93/19926, US2004/0231790, U.S. Pat. No. 6,040,563, U.S. Pat. No. 5,667,881, U.S. Pat. No. 5,643,390, EP2433780 and EP1423256.
The present inventions aims at providing a method of joining thermoset components wherein the strength of the final assembly is given by the resistance of the interpenetrating network formed between a thermoplastic and a thermoset component, wherein only one element is cured at a time in order to have a better control of the quality of the process, and wherein the process may take place at a wide range of temperatures, including those under the glass transition temperature of the thermoplastic material. To achieve so, this method comprises the steps of:
The terms “thermoset component” or “thermoplastic component” are construed as including both pure polymer thermoset or thermoplastic materials and composite materials having as matrix a thermoset or thermoplastic material. Therefore, then the term “curing a thermoset component” refers to the curing of the thermoset polymer (either pure or forming part of the composite material).
During the first step of this process, by means of which the at least one first-type thermoset component is obtained, an interpenetrating network, referred to as first-type interpenetrating network, is created between the thermoset polymer of the starting thermoset component and the thermoplastic material of the thermoplastic material layer when the thermoset polymer is cured. One or more first-type thermoset components can be manufactured this way. Although, in a preferred embodiment, a complete curing of the starting thermoset component is achieved, the process is also plausible with a partial curing during this first step, as long as such partial curing already gives enough structural stability to the at least one first-type thermoset component before performing the subsequent steps.
The second-type thermoset component to which the one or more first-type thermoset components will be joined is uncured. Hence, in the final step, only one component is cured, which has the following advantages relative to the other alternatives. If all the elements are uncured, the process is considerably more complicated and it is prone to suffer manufacturing deficiencies; if all the elements are cured, the final step bonding cannot be the result of the interpenetrating network forming between a thermoset and a thermoplastic component when the former cures—it is instead a fusion welding joint between thermoplastic materials, which would additionally require that the second-type thermoset component is also filmed with a thermoplastic layer before such final step. Furthermore, the fact that one of the components—the second-type thermoset component—is uncured before taking the final step reduces the tolerance requirements of the cured first-type thermoset components, as possible superficial irregularities or deformations in such cured components can be levelled during the formation of the strong interface with the second-type thermoset component.
In order to carry out the final step, the at least one first-type thermoset component is placed on the second-type thermoset component with the filmed surface, coated with the thermoplastic material layer, on a surface of the uncured second-type thermoset component. When this second-type thermoset component is cured, a further interpenetrating network, referred to as second-type interpenetrating network, is formed between the thermoset polymer of the second-type thermoset component and the thermoplastic material of the thermoplastic material layer. Since, as explained above, this layer is bonded to the at least one first-type thermoset component by a first-type interpenetrating network, the thermoset components are attached strongly enough to withstand, among others, the loads typical of aircraft structures. Besides, the process is advantageously simple and faster, because the second-type thermoset component need not be coated with thermoplastic and only one thermoset component is cured at the same time. This also implies that the process is more reliable in terms of quality control.
In an embodiment, the first-type curing temperature is, during the manufacturing of each at least one first-type thermoset component, less than the thermoplastic glass transition temperature of the thermoplastic material layer, and the second-type curing temperature is also less than the thermoplastic glass transition temperature of each at least one thermoplastic material layer.
The values of the glass transition temperatures are measured, in the present invention, by differential scanning calorimetry (DSC) according to the standardization ASD-STAN prEN 6041. In any case, the limitation of this embodiment can similarly be expressed by stating that the first-type curing temperature is such that the corresponding thermoplastic material layer is always in a glassy phase during the curing process of each at least one first-type thermoset component, and the second-type curing temperature is such that each at least one thermoplastic material layer is always in a glassy phase during the curing process of the second-type thermoset component. In other words, the thermoplastic materials do not reach the liquid or rubbery phase. Therefore, the formation of the first-type and second-type interpenetrating networks of this embodiment occurs through a process of diffusion bonding across the solid surfaces of the at least one thermoplastic material layer.
In other embodiments, the glass transition temperature of one or more of the at least one thermoplastic material layer is surpassed.
A composite material can be defined as an arrangement formed by a matrix and a reinforcement, which when combined have properties superior to those of the individual components. The at least one first-type thermoset component and/or the second-type thermoset component may be composite materials formed by a matrix of thermoset polymer resin and a reinforcement such as fibers. In an example, the at least one first-type thermoset component and/or the second-type thermoset component are composite materials having carbon fiber as reinforcement.
In a further example, the starting thermoset component of the at least one first-type thermoset component and/or the uncured second-type thermoset component can be composite materials provided in form of uncured pre-preg. A pre-preg is made of pre-impregnated fibers embedded in a matrix material to be cured during the final manufacture of the structure.
In an alternative embodiment to that of the previous paragraph, the starting thermoset component of the at least one first-type thermoset component and/or the uncured second-type thermoset component are composite materials manufactured by using liquid composite molding, which consists in the resin infiltration -in the case of the present invention, the resin is a thermoset polymer—of a textile preform by either positive or negative pressure. Some well-known examples of this technique are resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), vacuum assisted resin infusion (VARI) and vacuum assisted processing (VAP).
The thermoplastic material, in turn, can be an amorphous thermoplastic, e.g. polyetherimide (PEI), polysulfone (PSU), polyether sulfone (PES), Poly (methyl methacrylate) (PMMA) or polycarbonate (PC). The use of amorphous thermoplastic material is particularly convenient when the joint is obtained by way of diffusion welding. In fusion welding, the glass transition temperature of the amorphous thermoplastic material must normally be exceeded by at least 50° C., and such temperatures can also exceed the glass transition temperature of the thermoset components, even when completely cured, which leads to reduced stiffness, dimensional instability or degradation of the components. Such inconveniencies are avoided in a diffusion welding process, where the glass transition temperatures of the thermoplastic layers need not be reached.
The thermoplastic material layer can be comprised in a multi-layered film. This multi-layered film may have a disposable protective layer to cover the thermoplastic material layer that will take part in the joining process. Likewise, several thermoplastic material layers can be included in the film, each layer being intended for its attachment to a different thermoset component whose polymeric system is compatible with the thermoplastic material of the layer.
As explained above, the present invention covers the possibility of providing multiple first-type thermoset components which, once cured and filmed with a thermoplastic material layer by means of first-type interpenetrating networks, are placed on the second-type thermoset component. These first-type thermoset components are then strongly joined to the second-type thermoset component when the latter cures and second-type interpenetrating networks are created between the second-type thermoset component and the thermoplastic material layers.
In an example of such embodiment, an upper and a lower first-type thermoset component are provided, the filmed surface of the upper first-type thermoset component being placed on an upper surface of the second-type thermoset component and the filmed surface of the lower first-type thermoset component being placed on a lower surface of the second-type thermoset component opposite the upper surface. Thus, the final structure comprises three thermoset components, resulting from the second-type thermoset component being in between the two first-type thermoset components, and two thermoplastic material layers, each one being attached with interpenetrating networks to one of the two first-type thermoset components and to the second-type thermoset component.
An advantageous application of this embodiment is the attachment of thermoset components presenting a complex geometry, which makes the one-shot curing of the final piece difficult. The claimed method enables the individual curing and coating with the thermoplastic material layer of each of these components, which are in consequence first-type thermoset components.
Subsequently, in order to join these first-type thermoset components with an interpenetrating network between a thermoplastic and a thermoset material, and not by the thermoplastic/thermoplastic joint that would be achieved by bringing the filmed surfaces into contact, a second-type thermoset component is taken as a joining means between the first-type thermoset components. Accordingly, a pure thermoset resin is a suitable option for such second-type thermoset component playing the role of a joining means. Once the second-type thermoset component is cured, the first-type and the second-type interpenetrating networks make for a strong joint between the thermoset components.
The strength of the interfaces between the components that the methods of the present invention yields make the final products obtained by these methods adequate for the requirements of many aircraft structures.
In an example, the at least one first-type thermoset component is an aircraft stiffener and the second-type thermoset component is an aircraft skin.
The attachment between skins and stiffeners can be performed with different types of joints, such as mechanical joints, adhesives or one-shot curing of the whole assembly. All of these types of joints raise the problems explained above. With the method of the present invention, one or more aircraft stiffeners are formed by providing a starting thermoset component on which a thermoplastic material layer is placed and by curing them so as to form a first-type interpenetrating network between the first-type thermoset component, that is, the aircraft stiffener, and the thermoplastic material layer. As can be read above, this separate curing and filming of the stiffeners is a simple process which does not require expensive tooling specifically designed for it.
The filmed surface of each stiffener, coated with a thermoplastic material layer, is then placed on the uncured skin, and a second-type interpenetrating network will form when the skin is cured. Such final step can also be performed with conventional curing equipment. The assembly of the skin and the one or more stiffeners is therefore strong and achievable with conventional inexpensive equipment.
In an embodiment, a plurality of aircraft stiffeners is provided to form a grid on the aircraft skin. Such grid is formed by conveniently placing the cured stiffeners with the filmed surfaces in the desired grid-like arrangement before the curing of the skin.
T-stringers are an example of stiffener that can be manufactured this way. The uncured T-shaped stiffener constitutes the starting thermoset component on which base the thermoplastic material layer is placed. Once the T-stringers have been cured and the first-type interpenetrating network has formed between the thermoplastic material layer and the base, which constitutes the filmed surface of the cured T-stringer, this base is positioned on the uncured skin before performing the final step of the process, wherein the second-type interpenetrating network is formed during the curing of the skin.
In a further aeronautical application of the method of the invention, the at least one first-type thermoset component is an aircraft flap core and the second-type thermoset component is an aircraft flap. The structural requirements of aircraft flaps demand the addition of cores between the spars in the inner part of the flap. There are several alternatives to provide these cores—foam cores are disadvantageous because they suppose an additional weight; aluminum cores are not releasable when the structure is cured; composite cores are a beneficial option if the claimed method is followed. The composite cores are manufactured by being independently cured and filmed as any first-type thermoset component; later, the cured cores are placed on the right locations at the inside of the uncured flap and, finally, the flap is cured and the cores are strongly bonded to the flap through the interpenetrating networks that the thermoplastic material layer forms with both thermoset components. The strength of the interfaces enables the core to act as structural load bearing material. This embodiment is equally applicable to rotor blade cores as first-type thermoset components and rotor blades as second-type thermoset component.
Another application of the method permits the manufacturing of structures having thickness variations. Such structures aim at reducing weight by adapting the thickness of their different parts to the forces the structure withstands. However, since the overall geometry is more complicated than that of constant thickness structures, the manufacture of the structures is normally difficult and requires dedicated tools.
The present invention overcomes these inconveniencies by providing at least two first-type thermoset components in form of at least two thermoplastic stacks of different thickness. As in all of the above examples, these components are cured and coated with the thermoplastic material layer independently of one another.
The filmed surface of each of the at least two stacks of different thickness is then placed on a flat surface of the second-type thermoset component such that, when this second-type thermoset component is cured, the intended structure of varying thickness is obtained thanks to the appropriate location of the stacks of different thickness on the flat surface of the second-type thermoset component and to the first-type and second-type interpenetrating networks.
These and other features and advantages of the invention will become more evident from the following detailed description of preferred embodiments, given only by way of illustrative and non-limiting example, in reference to the attached figures:
Figures from 9 to 12 illustrate the steps of reinforcing an aircraft skin with T-stringers using the method of the invention.
In the embodiment of
In the present example, the filmed surface of the upper first-type thermoset component 1′ is placed, before the step of curing the second-type thermoset component 2, opposite the filmed surface of the lower first-type thermoset component 1″ relative to the second-type thermoset component 2, as shown in
In the embodiment of
Before joining the stacks of different thickness 101, 101′, 101″ to the flat surface of the second-type thermoset component 102, these stacks 101, 101′, 101″ undergo the same making process of any first-type thermoset component—each of the stacks 101, 101′, 101″ is first brought into contact with a thermoplastic material layer 103, 103′, 103″ and then cured so that a first-type interpenetrating network 121, 121′, 121″ forms between the thermoset polymer of the stacks 101, 101′, 101″ and the thermoplastic material of the layers 103, 103′, 103″.
Likewise, the step of joining of the stacks 101, 101′, 101″ to the second-type thermoset component 102 is carried out by placing the filmed surface of each stack 101, 101′, 101″ on the flat surface of the second-type thermoset component 102 and by curing this second-type thermoset component 102, thus forming second-type interpenetrating networks 122, 122′, 122″ between the thermoset polymer of the second-type thermoset component 102 and the thermoplastic material of the layers 103, 103′, 103″.
The inventive method can also be used for manufacturing aircraft parts having a skin 202 and one or more stiffeners 201 for reinforcing the skin 202, as depicted in
This embodiment is detailed in figures from 9 to 12 for the example wherein the stiffeners are T-stringers 202. A starting thermoset T-stringer 211, represented in
In the next step, shown in
In the step of
As illustrated in
In
In all the depicted embodiments, the thermoplastic material layers 3, 3′, 3″, 103, 103′, 103″, 203 maintain their original solid shape -that is, the thermoplastic materials are kept in their glassy phase—when the curing steps take place, since the first-type curing temperature or temperatures and the second-type curing temperature are inferior to the thermoplastic glass transition temperature of the thermoplastic material layer or layers 3, 3′, 103, 103′, 103″, 203.
1. —First-type thermoset component
1′. —Upper first-type thermoset component
1″.—Lower first-type thermoset component
2, 102.—Second-type thermoset component
3, 103, 103′, 103″, 203.—Thermoplastic material layer
3′.—Upper thermoplastic material layer
3″.—Lower thermoplastic material layer
11.—Starting thermoset component
21, 121, 121′, 121″, 221.—First-type interpenetrating network
21′.—Upper first-type interpenetrating network
21″.—Lower first-type interpenetrating network
22, 122, 122′, 122″, 222.—Second-type interpenetrating network
22′.—Upper second-type interpenetrating network
22″.—Lower second-type interpenetrating network
101, 101′, 101″.—Thermoset stacks of different thickness
201.—Aircraft stiffener
202.—Aircraft skin
211.—Starting thermoset T-stringer
301.—Aircraft flap core
302.—Aircraft flap
305.—Aircraft flap spar
| Number | Date | Country | Kind |
|---|---|---|---|
| 15400036.8 | Jul 2015 | EP | regional |
