GB2519444A

GB2519444A - Method of coating a composite material and a coated edge of a composite structure

Info

Publication number: GB2519444A
Application number: GB1418477.4A
Authority: GB
Inventors: William Alan Thompson; Michael Ware; Neha Borkar; Marc Froning
Original assignee: Boeing Co
Current assignee: Boeing Co
Priority date: 2013-10-21
Filing date: 2014-10-17
Publication date: 2015-04-22
Anticipated expiration: 2034-10-17
Also published as: GB2519444B; GB201418477D0; US20150111058A1

Abstract

A method of coating a composite 20 comprises applying an adhesion promotion layer 22 containing metal particles 26 in a binder paint 24 to the composite and applying a thermal spray coating 28 to the adhesion promotion layer. Typically, the metal particles are irregularly-shaped and may be produced by water atomisation. The particles may be steel particles having a size of 5-90 microns. In one embodiment, at least some of the metal particles are exposed through the binder paint so as to engage the thermal spray coating when applied. The adhesion promotion layer and the overlying thermal spray coating may be applied to an edge 20a of the composite. Preferably, the edge has a rounded or bevelled profile. The thermal spray coating may comprise a high velocity oxygen fuel spray coating. Alternatively, a twin wire arc bond coating may be applied to the adhesion promotion layer before application of the high velocity oxygen fuel spray coating.

Description

METHOD OF COATTNG A COMPOSTTE MATERTAL AND A COATED EDGE OF A

COMPOSITE STRUCTURE

TECHNOLOGTCAL FILED

An example aspect of the present disclosure relates generally to a method of coating a composite material and an associated coated composite structure and, more particularly, a method that incorporates a technique for promoting adhesion of a thermal spray coating on a composite material as well as the resulting coated composite structure,

BACKGROUND

Composite materials, such as carbon-fiber reinforced composite materials, are utilized in a wide variety of applications as a result of the weight savings provided by composite parts relative to comparable metal components in combination with the improved fatigue resistance, strength and corrosion resistance relative to comparable metal components. For example, composite materials are utilized by a variety of vehicles, including aircraft, watercraft and the like, as well as buildings and other structures, to name but a few applications.

Some applications that would otherwise utilize composite materials require more wear resistance or surface protection, such as erosion resistance, than conventionally provided by composite materials. As such, metal foils have been bonded to the surface of composite materials in order to provide improved wear resistance and surface protection for the composite material. However, the application of metal foils to composite materials may be somewhat difficult and may consequently require substantial experience and skill on the part of a technician in order to properly install the metal foil. Additionally, in an instance in which the metal foil is improperly applied, the composite material may have to be reworked, thereby potentially incurring significant time and expense.

Some composite materials may be formed of a plurality of layers, plies or laminates that are stacked upon one another and bonded together to form an integral structure. In some instances, edges of the plurality of layers are exposed along an edge of the composite material, Once deployed and subjected to various forces, the exposed edges of the plurality of layers of the composite material may structurally deteriorate more quickly than other portions of the composite material, thereby potentially limiting the applications in which the composite material may be deployed.

I

Therefore, a need exists to develop a composite edge that provides increased wear resistance and surface protection.

BRIEF SUMMARY

A method of coating a composite material and an associated coated composite structure are therefore provided in accordance with an example aspect with the coating being configured to provide increased wear resistance and surface protection, such as erosion resistance. In one aspect, the coating provides protection for the edge of the composite structure, such as the edges of a plurality of layers of the composite structure. As such, the resulting composite structure may be utilized in a broad range of applications including those that may require more wear resistance or surface protection and/or those that may subject the edge of the composite structure to forces that might otherwise cause the edges of the layers of the composite structure to structurally deteriorate.

In one aspect, a method of coating a composite material is provided that includes applying an adhesion promotion layer to the composite materiaL The adhesion promotion layer includes a binder paint with a plurality of metal partides, such as steel particles, within the binder paint. The plurality of metal particles may be irregularly shaped. As such, the method of one aspect may also include producing the irregularly shaped metal particles by water atomization, The method also includes applying a thermal spray coating to the adhesion promotion layer. In this regard, the thermal spray coating may be applied by applying a high velocity oxygen fuel spray coating. Tn one aspect, the thermal spray coating may be applied by applying a twin wire arc bond coating to the adhesion promotion layer prior to applying the high velocity oxygen fuel spray coating to the twin wire arc bond coating.

The application of the adhesion promotion layer may include, in one aspect, applying the adhesion promotion layer to at least one edge of the composite material with the thermal spray coating then being applied to the adhesion promotion layer on at least the edge of the composite material. In this aspect, the edge of the composite material may be processing prior to the application of the adhesion promotion layer so as to have a rounded or beveled profile. The method of one aspect may also include removing some of the binder paint prior to app'ying the thermal spray coating so as to expose at least a portion of the metal particles. The plurality of metal particles of one aspect have a size between 5 j.tm and 90 p.m.

Tn another aspect, a method of coating a composite material is provided that includes applying an adhesion promotion layer to the composite material including, for example, to at least an edge of the composite material. The adhesion promotion layer includes a binder paint and a plurality of irregularly shaped metal particles within the binder paint. In one aspect, the method may also include producing the irregularly shaped metal particles by water atomization.

The method also includes applying a twin wire arc bond coating to the adhesion promotion layer and applying a high velocity oxygen fuel spray coating to the twin wire arc bond coating. In one aspect, the method may also include removing some of the binder paint prior to applying the twin wire arc bond coating so as to expose at least a portion of the metal particles.

In a further aspect, a coated composite structure is provided that includes a composite material, an adhesion promotion layer on the composite material and a thermal spray coating on the adhesion promotion layer, The adhesion promotion layer includes a binder paint and a plurality of metal particles, such as steel particles, within the binder paint. In this regard, the plurality of metal particles may be irregularly shaped. At least some of the metal particles may be exposed through the binder paint so as to engage the thermal spray coating, The thermal spray coating of one aspect may include a high velocity oxygen fuel spray coating. Tn this aspect, the thermal spray coating may also include a twin wire arc bond coating on the adhesion promotion layer with a high velocity oxygen fuel spray coating on the twin wire arch bond coating. The composite structure of one aspect includes an edge and the adhesion promotion layer is on at least the edge of the composite structure so as to provide protection thereto,

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Raving thus described aspects of the present disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: Figure 1 is a flowchart illustrating operations performed in accordance with an example

aspect of the present disclosure;

Figure 2 is a cross-sectional view of a composite materia' having an adhesion promotion layer applied thereto in accordance with an example aspect of the present disclosure; Figure 3 is a cross-sectional view of a composite material and an adhesion promotion layer following removal of at least some of the binder paint in accordance with an example

aspect of the present disclosure;

Figure 4 is a cross-sectional view of a composite material and an adhesion promotion layer having a twin wire arc bond coating applied thereto in accordance with an example aspect

of the present disclosure; and

Figure 5 is a cross-sectional view of a composite material, an adhesion promotion layer and a twin wire arc bond coating having a high velocity oxygen fuel spray coating applied thereto in accordance with an example aspect of the present disclosure.

DETAILED DESCRWI'ION The present disclosure now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all aspects are shown, Indeed, the disclosure may be embodied in mary different forms and should not be construed as limited to the aspects set forth herein; rather, these aspects are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

A method is provided of coating a composite material to form a coated composite structure, such as to provide increased wear resistance and/or surface protection, e.g., erosion protection. In one aspect, the edge of the composite material may be coated so as to reduce structural degradation of the edge of the composite structure following deployment. The composite structure may be designed for a wide variety of different applications including vehicular applications in which the coated composite structure forms various parts of an aircraft, water craft, automobile or the like. Alternatively, the coated composite structure may be designed to be used in a building or other structure, With respect to an aircraft, for example, the coated composite structure may have improved erosion resistance and, as a result, may form the leading edge of a wing or rotor or the forward edge of a fuselage section. The coated composite structure may also provide protection from electromagnetic effects (ETvffls) so as to consequently form portions of the wing, fuselage or other aerodynamic surfaces, As a result of its improved wear resistance, the coated composite structure may form an actuator, In other aspects, the coated composite structure may serve as a thermal barrier by providing heat protection so as to form a heat shield, may serve as a durable surface for parts such as a high speed leading edge for aircraft designed to travel at Mach 1,5 or an engine nacelle, may serve as a gas diffusion barrier so as to form a hydrogen storage tank, may serve as a liquid barrier so as to replace fuel tank paint for jet fuel storage applications and/or may provide anti-skid properties so as to form a stow bin torque tube cover.

Regardless of the intended application, the composite material of one aspect may include a plurality of layers, laminates or plies stacked one upon another and bonded to form an integral structure. The composite material may therefore include a major surface formed by an outermost composite layer and an edge portion at which the edges of a plurality of composite layers, plies or laminates are exposed. The composite material may be formed of a variety of composite materials. In one aspect, however, each composite layer is formed of a plurality of carbon fibers disposed within a resin or matrix material, such as a plastic material.

In order to provide wear resistance and/or surface protection for the composite material 20, a thermal spray coating may be applied on the composite material. In one aspect, the thermal spray coating is applied not only on the major surface, but also on the edge of the composite, such as the exposed edges of the layers of the composite material. However, in some instances, the thermal spray coating may not readily adhere directly to the composite material. As such, an adhesion promotion layer 22 may be applied to the composite material with the thermal spray coating then applied to the adhesion promotion layer to facilitate reliable adhesion of the thermal spray coating to the adhesion promotion layer and, in turn, to the composite material.

As shown in block 10 of Figure 1 and also in Figure 2, an adhesion promotion layer 22 is initially applied to the composite material 20 and, in one aspect, to both the edge 20a of the composite material and a major surface of the composite material, such as by spraying. Tn order to facilitate adhesion of the adhesion promotion layer to the composite material, the surface of the composite material, including the major surface and/or the edge surface of the composite material, may be roughened, such as by being sanded or grit blasted, and then wiped with a cloth that carries a solvent. Additionally, in an instance in which the edge initially has sharp corners that could serve to concentrate stress, the edge may be processed so as to cause the edge to have a rounded or beveled profile. For example, the edge may be processed by sanding, routing or other mechanical material processing techniques. Thereafter, the adhesion promotion layer may be applied to the composite material including, in one aspect, to an edge of the composite material.

The adhesion promotion layer 22 may include a binder paint 24 and a plurality of metal particles 26 within the binder paint. The binder paint may be an organic binder, a spray applied adhesive, a sealant or the like. Tn one aspect, the binder paint may be a polyurethane paint having a base component, an activator and a hardener, While the binder paint may have various formulations, the binder paint of one embodiment includes PPG RW-7042-94A, CA9000B and CA9000C, as the base component, the activator and the hardener, respectively; all of which are available from PPG industries. In an aspect in which the binder paint is formed of a polyurethane base paint, the polyurethane base paint may not include pigments so as to improve its capability for holding the metal particles. For example, PPG RW-7042-94A is a base component from which the pigments have been removed. The mix ratio by volume of the binder paint 24 to the metal particles 26 and, more particularly, the mix ratio of the base, activator, hardener and metal particles may be varied, but, in one aspect, the mix ratio by volume of the base, activator, hardener and metal particles range from about 4:4:1:1,6 to about 4:4: 1:2.8, The binder paint of one aspect has a service temperature range with an upper end temperature that is sufficiently high so as to withstand the transient high temperatures to which the binder paint may be subjected during a thermal spray process, In one aspect, for example, the binder paint service temperature ranges from about -65°F to about +350°F, Tnstead of spherical particles which may not provide sufficient adhesion for the thermal spray coating or metal flakes that may lie fiat and may protrude little, if any, from the binder paint 24, the metal particles 26 advantageously have an irregular shape so as to facilitate subsequent adhesion with the thermal spray coating. Tn one aspect, the method may also include producing the irregularly shaped metal particles by water atomization, Although the metal particles may be formed of various materials, the metal particles of one aspect are formed of steel, such as stainless steel and, more particularly, water atomized 316 stainless steel particles.

The adhesion promotion layer 22 may be of various thicknesses and the metal particles 26 may have various sizes. Tn one aspect, for example, the metal particles have a size ranging from about 5 pm to about 90 pm and, more particularly, ranging from about 20 I.Lm to about 53 jim, In this aspect, the adhesion promotion layer may have a thickness ranging from about 25 m to about 50 jim. As described below, the adhesion promotion layer may be roughened, such as by sanding and/or grit blasting, the metal particles may have a smaller size, that is, smaller than about 20 rn, and the adhesion promotion layer may have a thickness ranging from about 40 m to about 60 jim, such as about 50 p.m. As such, at least some of the metal particles of one aspect may be larger in size than the thickness of the adhesion promotion layer to ensure that at least some of the metal particles protrude beyond the adhesion promotion layer in order to facilitate engagement with the thermal spray coating.

Although a single coat of the adhesion promotion layer 22 maybe applied to the composite material 20, a plurality of coats of the binder paint 24 with the plurality of metal particles 26, such as a two or three coats of the binder paint with a plurality of metal particles, may be applied in some aspects in order to form the adhesion promotion layer. Following application, the adhesion promotion layer may be cured. While the adhesion promotion layer may be cured in various manners, the adhesion promotion layer of one aspect is cured by placement of the composite material including the adhesion promotion layer in an oven at an elevated temperature for at least a minimum length of time, such as placement in an oven maintained at temperature of between 100°F and 200°F, such as about 160°F, for a time of between 1 hour and 4 hours, such as for two hours.

The thermal spray coating may then be applied to the adhesion promotion layer 22, In order to facilitate adherence of the thermal spray coating to the adhesion promotion sayer, at least some of the binder paint 24 may be removed prior to applying the thermal spray coating so as to expose (including further exposure of) as least some of the metal particles 26. See Figure 3. At least some of the binder paint may be removed in various manners including sanding of the surface of the adhesion promotion layer, thereby sanding the surface of the binder paint.

Thereafter, the sanded surface of the adhesion promotion layer may be wiped with a cloth that carries a solvent, such as acetone or isopropyl alcohol. In one aspect, the resulting surface of the adhesion promotion layer may then be roughened, such as by grit blasting the surface at a relatively low pressure, such as ranging between about 20 psi and about 40 psi, and at a relatively high angle, such as ranging between about 30° and about 60°. The resulting grit residue may then be removed, such as by subjecting the surface to a blast of compressed air.

Thereafter, the thermal spray coating may be applied to the adhesion promotion layer 22.

In one aspect, the thermal spray coating is applied to that portion of the adhesion promotion layer that covers a major surface of the composite material 20 and/or to that portion of the adhesion promotion layer that covers an edge 20a of the composite material, As such, the thermal spray coating may be applied to the major surface of the composite material and/or to the edge portions of the plurality of layers that comprise the composite material, In one aspect, the thermal spray coating includes a single coating, such as a twin wire arc coating or an air plasma coating. In this aspect, the single coating is configured to provide the desired functionality. For example, the single coating may be formed of or otherwise include a metal, such as copper, such that the resulting copper provides protection from electromagnetic effects (EME's), such as protection from lightning strikes.

As described below, however, the thermal spray coating of other aspects may be comprised of a plurality of coatings, such as two or more coatings, In this regard, the thermal spray coating may include a relatively thick functional coating to provide the desired wear resistance and/or surface protection, such as erosion protection, as well as an intermediate coating between the adhesion promotion layer 22 and the thicker functional coating to further facilitate the adherence of the thicker functional coating to the adhesion promotion layer and, in turn, to the underlying composite material 20, In the illustrated aspect, the thermal spray coating may include first coating 28 that facilitates the adherence of a second functional coating 30, different than the first coating, to the adhesion promotion layer 22. See block 12 of Figure 1 as well as Figure 4. Although the first coating will be described hereinafter by way of example as a twin wire arc bond coating that is applied to the adhesion promotion layer, the first coating may be another type of thermal spray coating, such as an air plasma coating, such that subsequent reference to a twin wire arc bond coating is by way of example, but not of limitation. For example, the twin wire arc bond coating may be applied to the adhesion promotion layer that covers the major surface of the composite material 20 and, in one aspect, also to the adhesion promotion layer that covers the edge 20a of the composite material, The twin wire arc bond coating may be applied at a relatively low deposition rate such that the resulting twin wire arc bond coating has a relatively low residual stress which facilitates the adherence of the twin wire arc bond coating to the adhesion promotion layer and reduces any tendency for the twin wire arc bond coating to peel away from the adhesion promotion layer, particularly at the edges of the twin wire arc bond coating. The twin wire arc bond coating may be rough so as to facilitate the subsequent adhesion of the thicker functional coating to the twin wire arc bond coating. Further, the twin wire arc bond coating is generally thinner than the subsequent functional layer, but should be sufficiently thick so as to withstand the heat incurred during the application of the subsequent functional layer and, in one aspect, may have a thickness ranging from about 50 m to about 150 jim. The twin wire arc bond coating may be formed of a variety of materials, but, in one aspect, may be formed of metals including, but not limited to, Ni, Al, NiAl, e.g., the combination of 95% Ni and 5% Al, NiCr, Tnconel® alloy 625, nickel alloy 718, Fe, Co, Mo, Cr, Ti, Ta, Cu, Zn, Sn, Ag, Au, Pt and alloys thereof. Oxides, such as A1204, Ti07, Cr03 or the like; carbides, such as CrC, WC, TiC or the like; and/or suicides, such as TiSi2, SiC, MoSi2 or the like could alternatively be incorporated using a cored wire process. Among other functions, the twin wire arc bond coating may provide thermal protection to the composite material during the subsequent application of the functional layer so as to permit higher thermal spray temperatures during the application of the functional layer without materia' degradation, such as without damaging the composite material.

Thereafter, a second coating 30 that may serve as a functional layer to provide the desired wear resistance and/or surface protection may be formed upon the twin wire arc bond coating 28 including that portion of the twin wire arc bond coating that covers the major surface of the composite material 20 and/or the edge 20a of the composite material. Although the second coating will be described hereinafter by way of example as a high velocity oxygen fuel spray coating that is applied to the twin wire arc bond coating, the second coating may be another type of thermal spray coating, different than the first coating, that provides the desired functionality, such as wear resistance, erosion protection or the like, such as a plasma applied coating, a cold sprayed coating or the like, As such, subsequent reference to a high velocity oxygen fuel spray coating is by way of example, but not of limitation.

Tn one example, the functional layer may be a high velocity oxygen fuel spray coating 30 that is applied to, e.g., deposited upon, and that adheres to the twin wire arc bond coating 28.

See block 14 of Figure I as well as Figure 5. Tn order to facilitate the adherence of the high velocity oxygen fuel spray coating to the twin wire arc bond coating, the surface of the twin wire arc bond coating may be prepared prior to the application of the high velocity oxygen fuel spray coating. Tn one aspect, the surface of the twin wire arc bond coating may be sanded so as to increase the planarity of the surface of the twin wire arc bond coating, such as by reducing the high spots, such that the surface of the twin wire arc bond coating varies in height by no more than a predetermined amount, such as no more than about 1,000 j.tin. Following the sanding, the surface of the twin wire arc bond coating may be wiped with a cloth that carries a solvent, In one aspect, the surface of the twin wire arc bond coating, including, for example, the surface of the twin wire arc bond coating on the edge 20a of the composite material, may then be roughened to facilitate the adherence of the high velocity oxygen fuel spray coating thereto, The surface of the twin wire arc bond coating may be roughened, for example, by grit blasting the surface of the twin wire arc bond coating, such as at a relatively low pressure ranging, for example, from about 20 psi to about 40 psi and at a relatively high angle ranging, for example, from about 300 to about 600. Thereafter, the grit residue may be removed, such as by blasting the surface of the twin wire arc bond coating with compressed air.

Once the surface of the twin wire arc bond coating 28 has been prepared, the high velocity oxygen fuel spray coating 30 may be applied, as shown in block 14 of Figure 1. The high v&ocity oxygen fuel spray coating may be thicker than the twin wire arc bond coating and, in one aspect, may have a thickness ranging from about 150 pm to about 400 pm. Additionally, the high velocity oxygen fuel spray coating may be denser than the twin wire arc bond coating so as to provide the desired wear resistance and/or surface protection. During the application of the high v&ocity action fuel spray coating, the temperature of the composite material and the coatings thereon, such as the adhesion promotion layer, is generally maintained at a temperature of no more than a predefined threshold temperature, such as at a temperature of no more than 130°F as measured from the backside of the composite material, such as the side of the composite material opposite the major surface that is being coated.

The high velocity oxygen fuel spray coating 30 may be formed of various materials including various metals, such as, for example, Ni, Al, NiAI, NiCr or an austenitic nickel-base superalloy, such as Ni625, e.g., INCONEL® alloy 625, consisting of 61.0% nickel, 21.5% chromium, 9.0% molybdenum, 3.6% niobium and 2.5% iron; oxides, such as Al,03, Ti07, Cr03 or the like; carbides, such as CrC, WC, TiC or the like; and/or silicides, such as TiSi7, SiC, MoSi, or the like, The high velocity oxygen fuel coating generally includes a plurality of metal particles, such as nickel alloy 625 particles, that are fed to a high velocity oxygen fuel gun that directs the metal particles to the surface to be coated while melting the metal particles. In one aspect, the size of the metal particles may range from, for example, about 5 pm to about 90 I.tni.

In order to increase the number of metal particles that are melted prior to contact with the twin wire arc bond coating 28, the metal particles may have a smaller size distribution ranging from, for example, about 20 pm to about 45 rim.

A variety of different types of high velocity oxygen fuel guns may be utilized in order to apply the high velocity oxygen fuel coating 30 including a high velocity oxygen fuel gun that utilizes fuel, e.g., kerosene or hydrogen fuel. Tn addition, while the metal particles may be fed to the high velocity oxygen fuel gun in a radial manner, the metal particles of one aspect may be fed to the high velocity oxygen fuel gun in an axial manner. Regardless, the high velocity of such a high velocity oxygen fuel gun is established by the jet velocity at the exit of the barrel of the high velocity oxygen fuel gun being generally greater than the speed of sound and typically exceeding 1000 rn/s prior to the introduction of the metal particles. In one aspect, the composite material 20 may be cooled during the application of the high velocity oxygen fuel coating, such as by liquid nitrogen or liquid carbon dioxide, so as to avoid or reduce material degradation attributable to elevated temperatures.

In one aspect, the surface of the high velocity oxygen fuel coating 30 may also be finished following its application. For example, the surface of the high velocity oxygen fuel coating of one aspect may be processed, such as by being sanded and/or ground, so as to flatten the surface of the high velocity oxygen fuel coating. In one aspect, the surface of the high velocity oxygen fuel coating may be sanded or ground in a progressive manner with decreasing grit sizes, such as beginning with a relatively low grit paper, such as an 80 grit paper, and moving progressively to smaller grit sizes, In order to avoid localized heating during the sanding or grinding process, a wet sanding of the surface of the high velocity oxygen fuel coating may be employed so as to avoid any softening or warping of localized portions of the high velocity oxygen fuel coating that may occur in instances in which the sanding or grinding belt contacts only a few high spots of the high velocity oxygen fuel coating and causes localized heat build-up. Although the surface of the high velocity oxygen fuel coating may be finished to within various tolerances, the surface of the high velocity oxygen fuel coating of one aspect may be finished such that the average surface roughness Ra is within a predefined range, such as by having an average surface roughness Ra that ranges from about 8 in to about 24 pin.

In one aspect, the surface of the high velocity oxygen fuel coating 30 may thereafter be sealed. Additionally or alternatively, the surface of the high velocity oxygen fuel coating may optionally be surface treated and/or painted.

By coating the composite material 20 with a thermal spray coating, such as the high velocity oxygen fuel coating 30, the resulting coated composite structure may have a relatively high hardness and low porosity so as to provide increased wear resistance and surface protection, such as erosion resistance, thereby permitting the resulting coated composite structure to be utilized in a broad range of applications. The thermal spray coating of one aspect may also advantageously provide EME protection for a resulting coated composite structure, In one aspect in which the edge 20a of the composite material is also coated, such as with a metal, including the edges of the layers that comprise the composite material, the edge of the composite material is no longer exposed and, as such, is protected from structural degradation by the thermal spray coating, Many modifications and other aspects of the disclosure set forth herein will come to mind to one skilled in the art to which this disclosure pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. For example, the first and second coatings may be formed of other types of thermal spray coatings if so desired, Therefore, it is to be understood that the disclosure is not to be limited to the specific aspects disdosed and that modifications and other aspects are intended to be included within the scope of the appended claims, Although specific terms are employed herein, they are used in a generic and descriptive sense on'y and not for purposes of limitation,