Thin film barrier coating for CFRP

Abstract

An article comprises a carbon fibre reinforced plastic (CFRP) substrate, a buffer layer disposed adjacent the substrate, the buffer layer comprising a poly(para-xylylene) polymer; and a moisture barrier coating disposed adjacent the buffer layer.

Claims

1. An article comprising: (i) a carbon fibre reinforced plastic (CFRP) substrate, wherein the CFRP substrate comprises a resin matrix system with a reinforcement comprising carbon fibres, aligned with each other in substantially one direction, disposed therein, wherein the reinforcement does not comprise a particulate dispersion, (ii) a buffer layer disposed adjacent the CFRP substrate, and comprising a poly(para-xylylene) polymer; and (iii) a moisture barrier coating comprising diamond-like carbon disposed adjacent the buffer layer.

2. The article according to claim 1, wherein the poly(para-xylylene) polymer is represented by the formula I, as follows: ##STR00002## wherein R1, R2 and R3 are independently selected from the group consisting of hydrogen and a halogen, and wherein n is greater than 2.

3. The article according to claim 1, wherein the article comprises more than one buffer layer comprising poly(para-xylylene) polymer.

4. The article according to claim 3, wherein the thickness of each buffer layer is about 0.1-1000 m.

5. The article according to claim 1, wherein the article comprises a plurality of buffer layers and moisture barrier coatings alternatively disposed, such that the plurality of the plurality of buffer layers and moisture barrier coatings can be identified by general formula (I):
[BL:MBC].sub.n (I) wherein BL corresponds to a buffer layer, MBC corresponds to a moisture barrier coating, and n is greater than 1.

6. The article according to claim 1, wherein the article comprises silsesquioxane, or a derivative or analogue thereof.

7. The article according to claim 6, wherein the silsesquioxane, or a derivative or analogue thereof is represented by a general formula (RSiO.sub.1.5).sub.n, wherein n is an even number, and R is hydrogen, or a hydroxyl or alkoxy group, or an optionally substituted straight or branched alkyl, alkylene, aryl or arylene group.

8. The article according to claim 6, wherein the silsesquioxane comprises a polyhedral oligomeric silsesquioxane.

9. The article according to claim 6, wherein the silsesquioxane comprises aminopropylisobutyl polyhedral oligomeric silsesquioxane, or a derivative or analogue thereof.

10. The article according to claim 6, wherein the silsesquioxane comprises epoxycyclohexylisobutyl polyhedral oligomeric silsesquioxane, or a derivative or analogue thereof.

11. The article according to claim 6, wherein the CFRP substrate comprises between about 0.5 and 5 wt % silsesquioxane, or a derivative or analogue thereof.

12. The article according to claim 1 comprising carbon nanotubes.

13. A method of manufacturing an article comprising a carbon fibre reinforced plastic (CFRP), the method comprising: (i) providing a CFRP substrate comprising a resin matrix system with a reinforcement comprising carbon fibres, aligned with each other in substantially one direction, wherein the reinforcement does not comprise a particulate dispersion; (ii) depositing a buffer layer comprising a poly(para-xylylene) polymer on the CFRP substrate; and (iii) depositing a moisture barrier coating comprising diamond-like carbon on the buffer layer.

Description

(1) All of the features described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

(2) For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figures, in which:

(3) FIG. 1a is a photo of some CFRP objects, and FIG. 1b shows the microstructure of a CFRP resin matrix showing fibres embedded therein;

(4) FIG. 2 illustrates the mechanism for mechanical failure of known moisture barrier coatings (MBC's) on CFRP when no buffer layer is present to accommodate mechanical stress deformations;

(5) FIG. 3 is a schematic showing the parylene deposition process;

(6) FIG. 4 is a graph showing the performance of moisture barrier coated (MBC) on CFRP (.square-solid. and .diamond-solid.) exposed to 70 C. and 85% relative humidity (RH), compared to CFRP that is non-coated (.box-tangle-solidup., .circle-solid. and );

(7) FIG. 5 shows various molecular structures of polyhedral oligomeric silsesquioxane (POSS) compounds with varying number of silicon atoms, i.e. (a) T.sub.8, (b) T.sub.7, (c) T.sub.4, (d) T.sub.6, (e) T.sub.12, (f) aminopropylisobutyl (POSS), and (g) epoxycyclohexyisobutyl (POSS);

(8) FIG. 6 is a graph showing water absorption at an accelerated rate (70 C., 85% RH relative humidity at room temperature);

(9) FIG. 7 illustrates carbon nanotubes (CNT's) incorporated in the resin matrix. Right: schematic of CNTs bridging electricity and heat flow across adjacent fibres; and

(10) FIG. 8 is a graph showing the weight gain of dried samples placed in an environmental (humidity) chamber, set at 70 C. and 70% relative humidity, as a function of time.

EXAMPLES

(11) With reference to FIG. 1a, carbon fibre reinforced polymer (CFRP) materials are used in many applications, for example the aerospace and space industry, but suffer the problem that it is sensitive to the absorption of moisture from the environment, and subsequent release of moisture (e.g. during space flight), and this gives rise to dimensional and weight instabilities. Furthermore, CFRP also experiences the problems of low out-of-plane electrical and thermal conductivities. The inventors have therefore investigated ways in which these problems can be solved either independently or simultaneously.

(12) The invention described herein relates to the use of several approaches, which can be used either in combination or independently of one another, to reduce the rate of water absorption into the CFRP, and thereby reduce the amount of moisture that can transport in and out of the CFRP, and/or enhance the electrical and thermal conductivity. The first technology is the use of a moisture barrier coating (referred to herein as MBC) around the CFRP component, and the second approach is the incorporation of a small amount of polyhedral oligomeric silsesquioxane (POSS) or a derivative or analogue thereof, into the resin that is used for producing the CFRP material.

Example 1

Increasing Dimensional and Weight Stabilities of CFRP

(13) CFRP is known to suffer from dimensional instabilities caused by the effect that moisture has on CFRP components and articles. Moisture from the environment can diffuse deep into the CFRP resin matrix and locate at particular sites in the resin that can accommodate water molecules, and this can cause the CFRP to swell up. If the CFRP component is transported to an area with lower humidity (for example, into space or high altitude), the absorbed water can then diffuse away from the CFRP into the environment, for example into space. The release of this water causes a dimensional change to the CFRP component that may render the component unusable.

(14) The objective of this example was to develop technologies to mitigate this problem, by reducing the extent of this transport of water into and out of the CFRP resin matrix. The inventors have addressed this problem with two solutions, as described in Examples 2 and 3.

Example 2

Encapsulate the CFRP Component in a Moisture-Barrier Coating (MBC)

(15) The CFRP component is encapsulated in a thin-film coating that is a barrier to moisture. This reduces the speed of water transport, and therefore, the amount of water ingress into the CFRP during production and storage (as it is stored for a limited amount of time). The MBC also limits the speed at which water is released (e.g. during spaceflight), which would keep the component within the dimensional tolerance during the lifespan of the system (for example, a satellite in space). The CFRP material or component is intended to be coated with this MBC layer as soon as possible after its manufacture.

(16) Despite the fact that there are many materials that are known to have good moisture barrier properties (for example, those used in the food packaging industry, aluminium foil, alumina, etc.), coating CFRP is significantly more complex, and it presents a unique challenge in itself, involving a specific methodology for the coating process. Indeed, this problem has been researched for a number of years, but numerous researchers have been unable to solve this problem. After a significant amount of inventive endeavour, the inventors have at last solved the problem, and the solution is non-trivial.

(17) The reason for the complexity of coating CFRP with a MBC is the complex nature of its upper or outer surfaces, where the carbon fibres are aligned with each other in substantially one direction (FIG. 1b). Furthermore, the fibres and the resin matrix have very strong mechanical properties, which are in opposition with each other. For example, the carbon fibres have 0 coefficient of thermal expansion (CTE), and sometimes a negative CTE, whereas the resin has some of the highest CTE values, i.e. 60 parts per million (ppm). This severe mismatch of CTE's between the carbon fibres and the resin causes a highly dynamic surface on the CFRP component. In addition, since the fibres are aligned along substantially one single direction, the displacements also take place along one direction. The CTE values of commonly used MBC materials are in the range about 4-10 ppm. All of these mismatches cause the typical MBC materials to fail due to mechanical reasons, including stretching and tearing. Referring to FIG. 2, the failure mechanism of CFRP is illustrated. The surface of CFRP is generally anisotropic (i.e. directionally dependent), and so has very different mechanical properties in different directions.

(18) In order to solve this problem, the inventors have developed a novel process where a 20 micrometer thick coating of parylene-D (acting as a buffer) is pre-deposited onto the CFRP component prior to the deposition of the moisture barrier coating consisting of diamond-like carbon (DLC). Parylene-D is a known molecule (http://www.paratechcoating.co.uk/engineering-properties-of-parylene.asp), but it has never been used for coating CFRP for moisture barrier applications in conjunction with a barrier layer. It is a flexible layer that can be vacuum-deposited onto a substrate, and can accommodate the mechanical motions of the surface of the CFRP.

(19) Parylene-D Coating

(20) The process steps are as follow:

(21) 1) The CFRP component is first cleaned with isopropanol (IPA) alcohol. This cleans any grease, dust and dirt etc. off the component;

(22) 2) The CFRP component is then inserted into a vacuum chamber and pumped to 1e-3 Torr;

(23) 3) Bis(Trimethylsilyl)amine (also known as hexamethyldisilazane, or HMDS) vapour is then injected into the chamber. HDMS has the molecular formula: C.sub.6H.sub.19NSi.sub.2. It is an adhesion promoter and forms a monolayer on the surface of the CFRP. The process for application is in the manual of HMDS processes. In brief, the HMDS is fed into the chamber as a vapour and forms a monolayer on the surface by condensation (this process is proprietary information of the parylene deposition company);

(24) 4) Once the CFRP component has been contacted with HMDS, the chamber is then evacuated;

(25) 5) Parylene-D monomer is then introduced into the chamber. This is typically achieved by heating Parylene-D dimer in a separate chamber, and then subjecting it to pyrolysis in a furnace, and then injecting the resultant Parylene-D monomers into the coating vacuum chamber where the CFRP component is disposed, as shown in FIG. 3. The parylene monomer is carried along the tube in hydrogen or nitrogen or any other inert gas (argon, helium, etc); and the monomer polymerises on the surface forming parylene; and

(26) 6) After coating, the chamber is vented, and the parylene-coated component is then removed.

(27) Diamond-Like Carbon (DLC) Coating

(28) After deposition of parylene-D, a barrier film of diamond-like carbon (DLC) is then deposited over the parylene-D, which forms an MBC. The DLC that is deposited over the parylene-D is described in Properties of Amorphous Carbon, Issue 29 of EMIS Datareviews Series, Editor S. R. P. Silva, 2003, ISBN 0852969619, 9780852969618. DLC is a vacuum-deposited form of carbon which is very hard, and has many carbon-carbon sp3 bonds, like those that are found in diamond. There are many different methods that can be used to deposit DLC, for example by a sputtering method, which is also known as physical vapour deposition (PVD). For example, see (http://www.diamonex.com/products/dlccoatings/?gclid=CPv4nt2nsLMCFcrItAodKw 4ATA). In this method, the carbon source for the DLC is a solid form, and the coating is deposited in a way similar to a spray process, which requires line-of-sight. However, this method may not be preferred for coating complex structures or inner bores of tubes with DLC.

(29) The inventors deposited DLC onto the parylene-D coating by a plasma-enhanced chemical vapour deposition (PECVD) process, where the carbon source is a hydrocarbon gas, which is able to come into contact with all the surfaces of the component to be coated. The DLC coating can be summarised as follows:

(30) 1) The parylene-coated component is inserted in a vacuum chamber;

(31) 2) The chamber is pumped down to pressures around 1e-3 Torr or lower;

(32) 3) A mixture of hydrogen and a hydrocarbon gas (e.g. methane, acetylene, etc) is bled into the chamber through a mass flow control (MFC) device. Typically, the hydrocarbon is diluted in the hydrogen to a concentration of around 1-20% (v/v), but it can be higher. Also, other gases such as argon, helium, nitrogen can be used.

(33) 4) The pressure of the chamber is regulated automatically by a computer-controlled valve to pressures in the rage of 50-1000 mTorr. Typically, pressures around 100-200 mTorr are used;

(34) 5) A plasma is then created in the chamber by applying a radio-frequency (RF) electric signal to the CFRP component. The frequency of the signal is typically 13.56 MHz, although other frequencies can be used. A DC signal can also be used. The RF signal is applied to the CFRP component via an RF matching unit;

(35) 6) The plasma has the effect of growing a DLC layer on top of the parylene-D. After the desired thickness is reached (about 10 nm-50 m), the RF signal is switched off, the chamber purged and then vented to remove the coated CFRP component.

(36) The unique aspect and advantage of this process is that the parylene-D/DLC coatings both need a vacuum chamber, and therefore, both layers can be deposited in a single chamber, without breaking the vacuum, which has not been carried out before. The inventors have demonstrated surprisingly efficient moisture barrier performance from CFRP components that have been coated with parylene/DLC structures. It is also possible to multiply the performance of the barrier by depositing many parylene/DLC multilayers, and this can be achieved in a single process run, which is also a novel finding.

(37) The performance of the MBC is measured by placing coated and non-coated CFRP samples in an environmental chamber that has been set at high humidity conditions. The samples are weighed at time intervals after exposure to the moist environment, and the weight gain (by moisture ingression) after exposure to moisture is calculated registered as a percentage of the sample's weight before the exposure measurement. The data are shown in FIG. 4. As can be seen, the figure shows an initial gain of weight (as water penetrates into the material), then the weight levels out after exposure of several hours, as is typical from a diffusion process. However, the figure shows that the coated samples gain weight more slowly than the un-coated (reference) samples, which demonstrates that the coatings clearly act as retardants (or barriers) for the ingression of water into the CFRP.

Example 3

Addition of POSS to CFRP Resin

(38) The inventors have developed another way to reduce the extent of water or moisture transport into and out of the CFRP resin matrix, and that is by adding polyhedral oligomeric silsesquioxane (POSS) or a derivative or analogue thereof, into the resin. The POSS molecules blend into the resin, and occupy the sites where the water molecules would normally reside in the resin. The addition of POSS therefore leaves fewer vacant sites for the water molecules, and the result is that the resin can absorb less water. This solution can be used either on its own, or in conjunction with the moisture barrier coating (MBC) to obtain enhanced performance.

(39) Referring to FIG. 5, there is shown a number of examples of cage-like POSS compounds that can be used. Materials derived from POSS molecules exhibit enhanced properties by bridging the property space between organic substituent and inorganic silsesquioxane materials. For example, the specific functionalities on a POSS molecule impart unique thermal, mechanical, electrical, rheological, solubility, and diffusivity properties to materials, rendering them as very useful candidates for high-temperature spacecraft coatings, in electronics packaging and low dielectric constant materials. The POSS compound that is used is different for different types of CFRP resin, and this allows tailoring the chemistry of the POSS to suit the chemistry of the resin system. The inventors have exemplified two resin systems, which are amongst the most common types of resins used in CFRP manufacture. Below is the process for mixing the POSS into these resins.

(40) Resin System 1:

(41) 1) Bisphenol A Diglycidyl ether (BADGE), purchased from Haas Group International, UK;

(42) 2) 2,2-dimethyl-4,4-methylene bis(cyclohexylamine) purchased from Sigma-Aldrich; and

(43) 3) Aminopropylisobutyl POSS (AM0265) purchased from Hybrid Plastics, USA.

(44) Mixing ratio (parts by weight):

(45) BADGE: 100

(46) Amine: 34

(47) AM0265: Varies depending on percentage weight incorporation.

(48) AM0265 was first dissolved in minimal amounts of THF before adding BADGE. This was then heated to 50 C. on a heating plate and stirred with a magnetic stirrer for 1 hour. To this, the amine curing agent was then added and stirred for another 30 minutes or until all the solvent had evaporated. The resulting mixture was then cured in the oven for 3 hours at 60 C. and 4 hours at 130 C.

(49) Resis System 2:

(50) 1) Bisphenol A Diglycidyl ether (BADGE) purchased from Haas Group International, UK;

(51) 2) Methyl Nadic anhydride purchased from Haas Group International, UK;

(52) 3) i-methyl imidazole purchased from Sigma-Aldrich; and

(53) 4) Epoxycyclohexyisobutyl POSS (EP0402) purchased from Hybrid Plastics, USA.

(54) Mixing ratio (parts by weight):

(55) BADGE: 100

(56) Anhydride: 95

(57) Imidazole: 0.5-2

(58) EP0402: Varies depending on percentage weight incorporation

(59) EP0402 was first dissolved in minimal amounts of THF before adding the anhydride and imidazole. This was then heated at 80 C. on a heating plate and stirred with a magnetic stirrer for 2 hours. To this, the BADGE was then added and stirred for another 5 minutes until a homogenous mixture is formed. The resulting mixture was then cured in the oven for 2 hours at 120 C. and 8 hours at 160 C.

(60) In order to determine the optimum concentration of POSS to be added to the resin, a range of different POSS concentrations were tested (i.e. 0%, 0.5%, 1%, 2%, and 4% w/v). With reference to FIG. 6, testing the resin/POSS mixtures for moisture uptake (by measuring the % weight uptake in a 75% humidity environment) clearly showed that the POSS-containing samples absorbed less water than the neat resin samples (i.e. 0% (w/v) POSS). When 0.5%, 1% or 4% (w/v) POSS was added to the resin, water absorption performance was improved. However, the inventors were very surprised to observe that there is a sweet-spot at about 2% (w/v) POSS incorporation at which there is only minimal water ingress. The reason for this 2% (w/v) sweet-spot is not understood as was an unexpected result.

(61) Furthermore, it was observed that as well as resulting in a lower moisture uptake, the addition of POSS to the resin also lead to an increase in the thermal performance of the resins. Differential scanning calorimetry (DSC) experiments were carried out, and the data are summarised in Table 1, which show the benefits of adding POSS into the epoxy matrix system.

(62) TABLE-US-00001 TABLE 1 Glass transition temperatures for L20 resin with AM0265 (i.e. POSS compound) POSS concentration in resin Glass transition (T.sub.g) 0 wt % (i.e. Neat diamine resin) 123 0.5 wt % 146 1 wt % 148 2 wt-% 170 4 wt-% 164

(63) The DSC data show the varying glass transition temperatures of the individual resins with the different percentages of POSS incorporation. Even at low POSS concentrations (0.5% wt), there is a significant improvement in T.sub.g ranging from 23-47 C. when compared with the neat resin (0% wt POSS). This was also an unexpected result.

Example 4

Increasing Electrical and Thermal Conductivity in CFRP

(64) Experiments to increase the out-of-plane thermal and electrical conductivity of the CFRP were focused at increasing the thermal and electrical conductivity of the resin. In order to achieve this, carbon nanotubes (CNT's) were added to the resin as an additive. CNTs have high levels of thermal conduction (better than diamond) and electrical conductivity (better than silver), and do not form surface oxides, or rust. This makes them ideal for combining with CFRP materials and their resins. Although people have added CNTs into the resin in CFRP, to date, no-one has previously done this in a manner that forms a good, uniform dispersion, and does not interfere with the operation of the resin.

(65) Obtaining a good dispersion of CNT's in a CFRP resin is important to the usability of the CNT mixture, especially when making mixtures in large amounts. In order to improve their dispersion, the inventors have functionalised the nanotubes using COOH acid (carboxylic acid) functionalisation, which allows the CNT's to disperse well into acidic resin. The usual method for producing acid functionalised CNT's is by treating them with acid prior mixing with the resin. However, this has been found to cause damage to the nanotubes. Therefore, in order to overcome this problem, the inventors have used plasma-functionalised CNT's. These are carboxylic acid (COOH) functionalised nanotubes. They are purchased from a company, but this type of functionalisation (carboxylic acid) is known e.g. http://www.sigmaaldrich.com/catalog/product/aldrich/652490?lang=en&region=GB)

(66) Referring to FIG. 7, there is shown 1-5 wt % CNT's dispersed in the resin. The CNTs create bridges for electricity and heat flow across adjacent resin fibres. A crack in the resin reveals the thin nanotubes (left). The inventors have obtained CFRP that is electrically conducting out-of-plane (200 Ohmcm). Also, they have achieved an increase in the thermal conductivity of the neat resin by a factor of 2.5.

Example 5

Measuring the Weight Gain of a Set of CFRP Samples Placed in a Humidity Chamber as a Function of Time

(67) An experiment was devised to measure the affect of the MBC in reducing the extent of water or moisture transport into and out of the CFRP resin matrix. A set of CFRP samples were placed in an environmental chamber (also known as a humidity chamber) set at 70 C. and 70% relative humidity and the weight gain was measured as a function of time. The weight gain corresponds to the moisture absorption of the samples.

(68) The test conditions were selected to accelerate the rate of moisture transport into and out of the CFRP resin matrix as opposed to leaving the samples in normal ambient conditions. This meant the samples could be analysed over 2250 hours of testing in the selected conditions rather than having to be analysed over a year or more which would have been necessary if the samples had been left in ambient conditions.

(69) FIG. 8 compares the weight gain of an uncoated CFRP sample compared to a number of MBC samples with various barrier layers when placed in the environment chamber for 2250 hours. It will be understood that the two barrier layer composition is CFRP/ParyleneD/DLC/ParyleneD/DLC/ParyleneD where the two barrier layers are the DLC layers, sandwiched between the parylene-D layers which provide mechanical coupling. The MBC samples range from two barrier DLC layers to five barrier DLC layers.

(70) The results clearly show that all the coated samples absorb less moisture and have a lower rate of moisture transport into the CFRP resin matrix than the uncoated reference sample. Additionally, the samples with a larger number of DLC layers absorb less moisture and have a lower rate of moisture transport into the CFRP resin matrix than the samples with fewer DLC layers. This shows that the performing characteristics of the MBC increase with the number of DLC layers.

SUMMARY

(71) The inventors have successfully produced a CFRP material, which exhibits dimensional stability. This has been achieved by producing a CFRP material, which has been coated with a parylene-D buffer and then a moisture-barrier coating (diamond-like carbon) and a 2% wt POSS additive in the resin. The observation was that moisture ingression/release from the CFRP has been eliminated, thereby increasing the dimensional stability. Furthermore, the inventors have added carbon nanotubes in combination with the POSS (and in some embodiments parylene-D and DLC) into the CFRP, and this significantly enhanced the electrical conductivity, thermal conductivity, onset of thermal degradation (thermal stability), and dimensional stability performance of the CFRP. The inventors have shown that the combined use of a moisture barrier coating, carbon nanotubes additive and POSS additive into CFRP results in the enhancement of the electrical conductivity, thermal conductivity, onset of thermal degradation (thermal stability), and dimensional stability performance of CFRP.