WO2015155040A1

WO2015155040A1 - Paints and gelcoats with high cnt content

Info

Publication number: WO2015155040A1
Application number: PCT/EP2015/056773
Authority: WO
Inventors: Mikael Johannes NORDENG; Paal A. SKYBAK; Stein Dietrichson; Joachim KARTHÄUSER
Original assignee: Re-Turn As
Priority date: 2014-04-09
Filing date: 2015-03-27
Publication date: 2015-10-15

Abstract

This invention concerns a novel method to produce paints or gelcoats highly loaded with fillers, in particular carbon nano tubes (CNT) which can be sprayed or which can be applied by rolling brushes. The core of the invention is to make use of a solvent such as water or organic solvent for dispersion of CNT. Subsequently, components of thermosets such as epoxy or polyurethane (PU) are added and integrated into the CNT dispersion until an appropriate viscosity is reached such that the product can be used for spraying (=paint) or rolling application (=gelcoat). Hardening times and evaporation speed of solvent, preferably water, are adjusted such that pore generation, sagging of paint etc is minimized. The products are useful for anti-radar coatings (RAM=radiation absorbing materials), deicing coatings, e.g. for microwave heating, general shielding from electromagnetic radiation purposes, areas where electrical conductivity of larger surfaces are required, such as antistatic coatings, and others. Preferred embodiments are described in detail.

Description

Paints and gelcoats with high CNT content Field of the invention:

This invention relates to paint and gelcoat production

technology, further to radiation-absorbing materials (RAM:s), deicing of e.g. wind turbine blades or aircraft wings by using microwave heating of coatings, electromagnetic shielding materials, and conductive coatings.

Background and prior art : The PCT document WO/2013/172762A1 describes deicing of e.g. wind power turbine blades by microwaves.

General background on paints and gelcoats as well as RAM:s etc can be found in the general literature. Related art can be found in the following documents: US 2011/028 1034 discloses a layer-by-layer fabrication method where nano-micro particles are suspended in a solvent and sprayed onto a porous

substrate. Application of vacuum allows production of e.g. nanopaper. WO 2010/144183 (Lockheed) discloses radar absorbing composites whereby CNT-infused fiber material (carbon or glass fiber) is part of the composite material. US 2010/0055450

(Xerox) discloses CNT- and PTFE-containing coatings for use in electrostatographic applications (copying etc) .

Other interesting documents providing related art are e.g. WO 2012/153 603 (Arkema) , describing highly CNT filled radar- absorbing coatings based on or comprising thermoplastic materials such as PVDF and solvents such as NMP, alternatively based on epoxy (see example 6) . Brief description of the invention:

In one aspect, the invention describes a method to produce paints and gelcoats with high loads, such as higher than 5% or even higher than 10% by weight, of fillers, especially fillers which after dispersion cause a significant increase of

viscosity, in particular CNT whereby CNT should be understood as carbon nano tubes, single wall and multi-wall,

functionalized CNT such as metallized CNT or any similar carbon based material including grafene, fullerene and certain carbon black materials and other materials essentially

consisting of carbon. Certain silica and microcellulose types also cause viscosity increases after dispersion, and they should also be understood as falling under the spirit of the invention. The production method includes dispersion of CNT in a liquid medium, preferably water or benign organic solvents such as lactates and acetates, by ultrasound or rapid

dissolving. Following the dispersion, epoxy or PU

(polyurethane) components, and optionally additives to

stabilize the dispersion, are added. Additives include water- soluble polymers, hyperbranched polymers, dispersants, foam controllers and the like, see under embodiments.

In another aspect of the invention, the products thus obtained are applied by spraying or as gelcoat. For sprays, viscosities of up to 3000 cps (1000 cps = 1 Pa*s, as measured according to Brookfield at about 50 rpm, can be used. Separate A- and Ef^¬ fractions containing CNT (A and B standing for epoxy and curing agents, respectively) can also be sprayed together whereby the A- and B-stream are mixed prior to entry into the spray nozzle. Gelcoats can show viscosities up to several thousand cps.

Gelcoats are usually applied by roll-up. It is important that sagging of paint is avoided. This is achieved by optimizing the paint thickness, adhesion to substrate, evaporation speed of solvent and hardening time. Equally, excessive porosity of paints is usually not desired. According to the invention, a system is provided which

exhibits a certain degree of paint contraction before the onset of hardening. Remaining pores can be covered by applying a top coat which often is desirable for reasons of colour choice, erosion stability requirements or other reasons. In comparison to prior art, the invention succeeds in

realizing paint and gelcoat formulations with extremely high contents of CNT, such as higher than 3%, 4%, 5%, 6%, 8%, 10%, 12%, 15%, 18%, 20%, 25% or higher than 30%. Such products are cheap to produce, easy to apply and highly functional in applications such as wind turbine deicing, aircraft deicing, especially leading edge of wings, and electromagnetic

shielding applications.

In other aspects, the invention describes useful embodiments for coating of wind turbines, aircrafts, civil engineering objects and off-shore constructions.

Embodiments of the invention:

Epoxy paint or gelcoat: the following raw materials were used:

CNT are provided in dispersed form, starting from a water- dispersable masterbatch a.o. containing CNT and CMC (carboxy- methylcellulose) , available from e.g. Arkema under the trade name "Graphistrength CW2-45", alternatively by dispersing pure CNT and CMC by conventional means, e.g. ultrasound. Water- bourne epoxy, e.g. available from Momentive (examples Epi-Rez 3510-W-60, 3515-W-60, 3520-WY-55, 3521-WY-53, 3522-W-60, 3540- WY-55, 3546-WY-55, 5003-W-55, 5520-W-60, 5522-WY-55, 6006-W- 68, 6520-WH-53, WD-510, or Epikote 828 in water), or as available from DOW (examples DOW DER 910, 911, 913, 914 or 915) is added to the aqueous CNT dispersion, as well as water- bourne curing agent, such as available from Air Products

(examples Epilink 360, 701, Anquamine 401, Anquawhite 100) or as available from Momentive (examples Epicure Curing agents 6870-W-53, 8290-Y-60, 8535-W-50, 8536-MY-60, 8537-WY-60, 8538- Y-68) . Optional additives are impact modifiers such as

available from Albidur (example Nanostrength) . For gelcoats, thickening agents such as micro- or nano-silica, such as

Aerosil ™ or Cab-O-Sil™ are added in concentrations of about 1-3% by weight.

PU paint or gelcoat: the procedure is very similar to the one useful for epoxy formulation. As raw materials, a range of water-dispersable candidates are available from companies such as Bayer, BASF and others.

In one general embodiment, two different epoxy or PU systems (thermoset systems) are used whereby the first system is curing rapidly, and the second system less rapidly. The purpose is to achieve a viscosity increase quickly in order to prevent sagging of paint and gelcoat layers. Useful raw materials are e.g. Bayhydrol® A 2695 / Desmodur® N 3900, Bayhydrol® A 2546 / Desmodur® N 3900, Bayhydrol® A 2227/1 / Desmodur® N 3900, Bayhydrol A 242 / Bayhydur 304, Bayhydrol A 2427 / Bayhydur 304, Bayhydrol A 2457/ Bayhydur 304, Bayhydrol 2546 / Bayhydur 304,Bayhydol A 2470 / Desmophen N 3900,

Bayhydol A 2542 / Desmophen 3900. It is also generally useful to add metal articles, such as aluminium flakes.

In one embodiment, it is desired to provide a paint

formulation containing a very high CNT content which in addition can be applied outside of a factory and under adverse climatic conditions, e.g. during rainy periods. For this purpose, the following general composition has been developed, based on a medium molecular weight epoxy (i.e. molecular weight 700-2500) diluted in xylene with typical

characteristics as follows:

Epoxy group content mmol/kg 2000 - 2220

Non- volatile content % m/m 74.0 - 76.0

Viscosity of solution at 25°C Pa*s 8.0 - 13.0

Typical trade names are Epikote 1001x75, Araldite GZ 601x75 The curing agent in the formulation is preferably a medium to low viscosity, reactive polyamide-based resin solution with typical values:

Amber liquid

Amine value 370 - 400 mg KOH/g Viscosity at 75°C 300 - 600 cps Typical trade name Versamid 140

A concentrate of CNT with an epoxy carrier- Graphistrength CSl-25, typically 25% CNT is mixed with the epoxy solvent cut and a combination of ketones, MIBK, MEK under high shear mixing until the CNT is evenly dispersed. The amount of solvents may vary from 100 to 500% calculated on the epoxy resin .

In order to obtain proper dispersion of the CNT in the polymer the dissolving process is followed by sonication, also called exposure to ultra sonic sound. A concentrate of CNT with a CMC carrier Graphistrength CW 2- 45, typically 45% CNT is mixed with the polyamide based curing agent and a combination of Xylene, MIBK and MEK under high shear mixing until the CNT concentrate is evenly dispersed. The amount of solvent may vary from 100 to 500% calculated on the polyamide curing agent.

In order to obtain proper dispersion of the CNT in the

polyamide, the dissolving process may be followed by

sonication / exposure to ultra sonic sound. In a formulation with 5% concentration of CNT in the ready cured film, the amount of CNT concentrate will be 10% CNT concentrate calculated on epoxy resin, combined with a curing agent with a content of 100% CNT concentrate calculated on polyaminoamide . In a formulation with 20% concentration (by weight) of CNT in the ready cured film, the amount of CNT concentrate will be 200% parts CNT concentrate calculated on epoxy resin, combined with a curing agent with a content of 75% CNT concentrate calculated on polyaminoamide. Added solvent may also contain CNT in dispersed form,

preferably after sonication. Useful solvents are e.g. xylene, MIBK, MEK, acetone, i.e. solvents of aromatic, ester, ketone, paraffin or (capped) glycol nature, or mixtures of those.

Water is useful as additive as the CNT/cellulose complexes are especially water-dispersable .

Useful products:

Wind power blade deicing coating: It was found that 500 micrometer of such a coating containing 11% CNT effectively absorb >90% of the microwave radiation emitted inside a wing. Deicing of a 1 m composite pipe used in cooled laboratory conditions (at minus 20 °C) could be achieved within 15 minutes, using < 0,1 kWh / m2. Higher or lower CNT

concentrations can be chosen. A very smooth surface is

preferred facing the inside of the wing as radiation will partly be reflected. A very rough coating may be preferred facing the outside of the wing to minimise radar interference, see also arguments below. In that case, a top coating is used which levels out the surface roughness of the CNT-containing coating. In general, a high quality top coating results in erosion stability and allows freedom to apply light colours to the final product. Also metal as surface, e.g. in the case of airplane wings, is acceptable.

Benefits of the invention : Wind power blades: the coatings according to the invention are preferably used as coatings beneath the top coat of wind power blades, and preferably above a thermally insulating coating on the composite material, to avoid unnecessary heat loss. It is not necessary for blade producers to repeat the stability calculations for blades as the invention provides a method to apply a light-weight coating (e.g. 50 kg coating for a 60 m blade weighing some 15 000 kg) . Production processes do not have to be changed, as opposed to a heatable material (Ohmic or resistance heating) integrated into the current base composite. It also turned out that the coating is partly absorbing and partly reflecting microwave radiation. The ratio of absorption / reflection depends on the conductivity of the coating whereby higher CNT content results in higher

conductivity (up to levels of very good semiconductors) and whereby reflectivity increases with conductivity. This can be used for dispersing radiation inside the hollow wind power blade, both for reaching distant or narrow regions such as the blade tip, and also for achieving a more even energy

distribution, i.e. more even heating. Further, as the coating can be embossed or structured, there are further technical options to control absorption and reflection. This can be done both at the surface facing the inside of the wing as well as the outside of the wing.

Radar absorbing and EMI shielding coatings: Based on the chemistries described above, radar absorbing and EMI shielding coatings can be formulated whereby it is desired that the coating shows minimum reflectivity. An optimum combination of high CNT- concentration, low conductivity and low layer thickness is chosen. In addition, the surface facing a radar source is preferably not smooth, but rather structured, e.g. embossed, such that radiation is reflected in an irregular manner. It can be suitable to apply a multilayer coating with increasing CNT concentration towards the inside of the object (and reverse in the case of EMI shielding coatings) . The task of the layer (s) with lower CNT concentration is to weaken incident and reflected radiation.

Claims

Claims :

1. A method for producing a coating highly loaded with fillers in the form of carbon nano tubes (CNT) , which coating can be applying on a product, whereby the method comprising the steps of:

using a solvent for dispersion of CNT,

adding and integrating components of a thermoset into the CNT dispersion until an appropriate viscosity of between 100 and 8000 cps is reached, characterized by adding at least 3% by weight CNT, preferably >4%, >5%, >6%, >8%, >10%, >12%, >15%, >18% or more than 20% by weight CNT to said coating.

2. The method according to claim 1, characterized by the term CNT encompasses carbon nano tubes, graphene, fullerene, functionalized carbon nano tubes, carbon black and other materials essentially consisting of carbon.

3. The method according to claim 1 or 2, characterized by selecting the thermoset from the group consisting of epoxy and polyurethane .

4. The method according to any one of claims 1 to 3,

characterized by applying said coating on a product by

spraying or rolling application.

5. The method according to any one of claims 1 to 4,

characterized by selecting the solvent from the group

consisting of water, xylene, MIBK, MEK, lactates, acetates and other other esters or ketones including acetone and MEK or paraffins, and mixtures thereof for adjustment of viscosity.

6. The method according to any one of claims 1 to 5,

characterized by said thermoset cotains CNT.

7. The method according to any one of claims 1 to 6,

characterized by providing the CNT as a water-dispersable masterbatch and preventing the dispersed CNT from re- agglomeration by adding at least one polymeric additive.

8. The method according to claim 7, characterized by said polymeric additive is a polar polymer added at a weight fraction of between 0.2 and 2 relative to CNT.

9. The method according to claim 8, characterized by said polar polymer being selected from the group consisting of cellulose and carboxymethylcellulose (CMC) .

10. The method according to any one of claims 7 to 9,

characterized by combining the water-dispersable CNT

masterbatch with epoxy and hardener and additives in order to formulate a one-component sprayable coating.

11. The method according to any one of claims 7 to 10, characterized by combining the water-dispersable CNT

masterbatch with di- or polyisocyanate and glycol or alcohol component and additives in order to formulate a one-component polyurethane based sprayable coating.

12. The method according to claim 10 or 11, characterized by said additives being selected from the group consisting of silica, impact and rheology modifiers.

13. The method according to one of the preceding claims, characterized by providing said epoxy or polyurethane coating in the form of reactive A- and B-components , and by separately mixing and combining epoxy/isocyanate and hardener during the applying process, in order to avoid premature hardening.

14. The method according to one of the preceding claims, characterized by said coating being selected from the group consisting of paints or gelcoats.

15. A coating produced by the method according to one of the preceding claims, characterized in that the coating is used for microwave-assisted heating of windpower wings, airplane wings or components, marine and off-shore objects, overhead power lines or other constructions.

16. The coating according to claim 15, characterized in the coating is used as radiation absorbing coating for camouflage purposes.