Patent Application
20130089733
This invention pertains to thin, light-weight structural panels stiffened by the magnetically-assisted application of relatively thick polymer coatings. Magnetizable particles are dispersed in a liquid thermosetting prepolymer composition and one or more magnetic fields are used to deposit a single layer of increased viscosity, curable polymer coating material in a predetermined stiffening thickness pattern on predetermined surfaces of a suitably shaped structural panel member. In preferred embodiments, the practice is used, for example, to selectively stiffen thin, high strength steel alloy sheet members for automotive vehicle structures to satisfy vibration or noise requirements, or like structural requirements.
The bodies of motor vehicles must satisfy numerous requirements, to meet customer expectations and regulatory requirements, including managing the loads to which the body is subjected and minimizing the transmission of road and engine noise and vibration to vehicle occupants. A body is usually assembled from a number of simple structural elements which may include generally planar sheets, or “C”- or “U”-shaped sections. Often such “C”- and “U”-shaped members are ‘closed out’ by addition of another element to form a combination element with a closed or “box” section.
In seeking to reduce vehicle mass and improve fuel economy, motor vehicle manufacturers continue to substitute thinner-gage sheet steels or lighter-weight materials, such as aluminum, magnesium and polymer composites in vehicle structures. Downgaging of steel sheets is enabled by the development of advanced high strength steels that provide higher strength to weight ratios. Less dense materials such as aluminum or magnesium alloys may be of comparable thickness but such materials exhibit a lower elastic modulus than steel; low-density polymer panels, often reinforced with glass or carbon fiber, also exhibit a lower elastic modulus than steel.
Such materials may be selected and applied to achieve a suitable strength if the body deforms homogeneously. But the substitution of such thinner gage or lower elastic modulus materials may render such vehicle bodies more prone to unacceptable deflection and/or vibration, resulting in the perception of lower stiffness or more properly, flexural rigidity which is defined as the product of elastic modulus and moment of inertia. Also, such thinner gage bodies may be less effective in damping or suppressing noise and vibration.
There is therefore a need for materials which, when formed and assembled into a vehicle body, enable mass reduction without loss of noise and vibration management functionality, and with high perceived flexural rigidity.
Certain high strength steel alloys, including, but not limited to, DP 1000, a Dual Phase Steel exhibiting a Tensile Strength of 1000 MPa, after hot and cold rolling to sheet thicknesses of, for example, about 0.5 mm (500 micrometers), are found to have suitable tensile strength values for many automotive vehicle structural panel applications. And body structures of such reduced thickness would provide weight reduction in the vehicle for improved fuel economy. But in many vehicle structure applications the narrow thickness of the steel member may result in unacceptable structural deflections. In accordance with embodiments of this invention, the shape of the potential reduced thickness steel member is analyzed to determine if the perceived flexural rigidity of such steel member may be suitably improved with an applied coating thickness of a thermoset polymer precursor composition, such as an epoxy-based polymer material. Such a coating would be evaluated for application over the entire surface of one or both sides of the formed or un-formed steel sheet material, or in predetermined selected surface regions of the sheet material. It is understood that such stiffening polymer coatings, having thicknesses in significant proportion to the thickness of the steel member, could be applied without increasing the weight of the coated steel member as much as by using a heavier grade of the steel material. Of course, such polymer coating stiffening-potential could be considered when using thin sheets of other metal alloys, such as suitable aluminum and magnesium alloys. And the polymer coating stiffening-potential could be considered on thin nonmetallic workpieces, but, often, with less advantage to increasing the thickness of the original part.
The thickness of the coating deposited in a single application will depend upon the viscosity of the coating since excess coating precursor will not be retained on the part but will simply flow or drip freely off the part. Thus a suitable coating thickness may require multiple applications of coating precursor, and intervening curing steps to progressively build the coating thickness to a suitable extent.
In a practice of this invention suited to magnetizable substrates such as steels, magnetizable particles, such as pure iron particles, are dispersed in a substantially solvent-free mixture of precursor materials for a thermosetting resin. A magnetic field applied to the magnetizable substrate that is to be coated with the polymerizable mixture. The particles dispersed in the polymerizable mixture increase the viscosity of the mixture, and any magnetic field acting on the dispersed particles magnetizes them to further increase the viscosity of the coating material. It is preferred that such magnetic viscosity enhancement occur at or near the substrate surface to enable a less flowable and more drip-resistant coating so that a thicker coating may be applied to the substrate, and at least fewer coating applications are required to achieve a suitable overall coating thickness. The surface or surfaces of the thin workpiece to be stiffened are brought into contact with the dispersed particle-containing, polymer precursor material and the magnetic field is established so as to direct the polymer material against surfaces of the workpiece coating them with a magnetically-induced, desirably thick coating layer of the viscous, uncured material. The workpiece is removed from contact with the coating material while still under the influence of the magnetic field. The polymerization of the reacting precursors is completed using any suitable polymerization reaction-inducing energy producing means. After curing of the polymer coating matrix, the dispersed particles are demagnetized, as well as the workpiece if it was magnetized by the applied magnetic field.
A reinforced polymer coating thickness of 400 micrometers is readily obtained with addition of only 5% by weight of iron particles (or other suitable ferromagnetic particles. This thickness is sufficient to appreciably increase the flexural stiffness of a 500-650 micrometer thick high strength steel panel or of an 800-1000 micrometer high strength aluminum or magnesium alloy panel.
The liquid polymer precursor material may include an initiator or catalyst to promote curing subsequent to application as well as a suitable surface agent or surfactant for improving the dispersion of the particles with the polymer precursor. Suitable thermoset resin chemistries include, but are not restricted to, epoxy-amine resins, epoxy-isocyanates, epoxy-polyester resins, vinyl-ester resins, and polyurethane resins. The magnetizable particles may be of any convenient shape, generally spherical, acicular or needle-like, or flake-like to achieve the desired thickness of polymer coating in the applied magnetic field. As stated, the particles increase the viscosity of the precursor polymer materials and, when exposed to a predetermined magnetic field, the particles induce the flow of the resin to desired surface regions of the workpiece.
The coating may be applied to any of, both sides of a panel or other workpiece, only one side of a panel or selected regions of one or both sides of a panel. Application may be by dipping in a bath or spraying or other convenient method. The method may, in different embodiments, be adapted to ferromagnetic steel panels or to non-magnetic aluminum alloy, magnesium alloy and polymer panels.
As stated, by incorporating initially un-magnetized ferromagnetic particles into the resin, in suitable concentration, the primary benefit is that the particle-containing resin, if subjected to a magnetic field, will exhibit a magnetorheological-driven graded viscosity governed by the field. The cooperative interaction between the external magnetic field and the ferromagnetic particles in developing a higher viscosity in predetermined locations and adjacent to the workpiece surface that is to be coated may enable application of an appreciably thicker resin layer in a single operation. Once cured, the magnetic particles may serve as reinforcements, enhancing both the strength and elastic modulus of the coating as well as creating polymer-particle interfaces for enhanced damping of noise and vibration.
The interaction between the particles and the magnetic field may, because of the induced higher viscosity, result in a gradually increasing resin layer with time. Thus, development of a preferred coating thickness may require that the panel remain exposed to the resin for a pre-determined period of time.
Magnetically soft materials are preferred since, in general it is preferred that the panel, once formed, be substantially non-magnetic. The materials, as incorporated into the resin, should not be magnetized so that they may remain substantially dispersed in the resin without clumping or otherwise aggregating. Suitable magnetic materials may include metals such as iron, suitably carbonyl iron powder, nickel or cobalt or the soft ferrites, for example, manganese-zinc ferrite, Mna Zn(1-a)Fe2O4 or nickel-zinc ferrite, Nia Zn(1-a)Fe2O4 or iron oxide Fe2O3.
Concentrations of magnetic material of between 1 and 25 percent by mass or between about 0.2 and 7 percent by volume may be effective in building coating layers of suitable thickness. The magnetic particles should be generally uniformly dispersed in the resin and may range in size from about 1 to 500 micrometers in longest dimension. Those particles of more compact, generally spherical, morphology may range from about 1 to 50 micrometers while more acicular or more flake-like particles may range from about 10 to 500 micrometers in longest dimension.
A thick coating is desirable since the stiffness-enhancing character of the coating varies with its thickness, so, a thicker layer will be more effective. Similarly the damping characteristics of the polymer layer resulting from curing of the resin will be improved by addition of more material. In addition, the plurality of resin-particle interfaces resulting from the incorporated particles may further enhance the damping characteristics of the coating. Because of their large surface to volume ratio, flake-like particles may be most effective in enhancing damping.
Magnetic structural materials, such as steels, may be magnetized by exposure to an external magnetic field using an electromagnet or by passage of an electric current. After removal of the magnetic field or current, some remnant magnetism will remain, which, on exposure to a resin charged with ferromagnetic particles, may increase its viscosity locally through magnetorheological effect and enable deposition of a thicker coating than would be achieved with resin alone. If a sheet metal structural element were so magnetized, a flowable resin-hardener-magnetic particle blend or slurry may be applied to one surface of a structural element by spraying or otherwise coating only that surface or to both surfaces by dipping or spraying or other convenient method. If selective magnetization is desired, the current density in different regions of the element may be varied appropriately or a magnetic field may be applied to only select parts of the element. Such selective application of coating may be required if the element is to be assembled by welding, for example electric resistance spot welding.
The resin may then be cured to form, on the structural element, a magnetic particle-reinforced, polymer layer. It will be appreciated that, if cured by heating, the viscosity of the resin will first decrease as it is heated above ambient temperature before increasing again as cross-linking proceeds. During this low viscosity transient state, the resin, even in the presence of the magnetized structural element, may flow to produce a thinner layer than was first applied. This thinning may be counteracted by applying a supplementary current or an external magnetic field during curing. But thermal thinning is most readily dealt with by avoiding any necessity for thermal curing by using a resin-hardener system suitable for ambient temperature cure or by using a non-thermal cure process such as an electron beam or ultraviolet (UV) light with appropriate additions of initiator to the resin if required.
Non-magnetic materials may also be coated by positioning groups of magnets, permanent magnets or electromagnets, on one side of the sheet-based reinforcing element in regions where a coating is desired and applying the resin on the opposing side. One preferred approach is to magnetize a steel panel and sandwich the magnetized steel sheet between two non-magnetic panels. The three-layer sandwich may then be dipped into a magnetic particle containing resin bath where the remnant magnetism of the steel sheet will promote resin deposit of suitable thickness on the non-magnetic panels. Once the resin is deposited it may be cured, as before, to develop the desired thick coating comprised of the magnetic particle-containing polymer-composite.
Arrays of electromagnets or permanent magnets in face-to-face contact with a non-magnetic panel may also be employed as sources of a suitable magnetic field for resin deposition. In this approach the thickness of the resin will depend on the placement of the magnets, being generally thicker directly over a magnetic pole and thinner elsewhere. The behavior of the coating when two or more magnetic poles are placed adjacent to one another will depend on their relative polarity.
All of the approaches for non-magnetic panels, in contrast to the method for magnetic panels, produce a panel with polymer reinforcement on one side only. This, depending on the coating thickness, may be adequate. If a two-sided coating is desired, the process must be repeated with the panel reversed to present its uncoated side to the resin.
Other objects and advantages of the invention will be apparent from a detailed description of various embodiments of the invention.
The bodies of motor vehicles must manage the loads applied both during normal vehicle service and under extraordinary conditions such as a collision. Increasingly, vehicle bodies are constructed using materials such as high strength steel, aluminum alloys, magnesium alloys and polymer-based composites, all of which offer higher strength to weight ratios than the low strength, low carbon steel used in older vehicles and enable mass reduction. Such materials are sized and selected to ensure that plastic or permanent deformation does not occur in normal use and that under abnormal operating conditions, such as a collision, deformation occurs in a predictable and controlled manner.
But, such strength-perfected designs may be stiffness limited. The two most important properties relating to the stiffness of a structure are the elastic modulus of the material and the moment of inertia of the structure, also known as material stiffness and geometric stiffness, respectively. Reductions in panel thickness may reduce the perceived geometric stiffness and substitution of lower modulus aluminum and magnesium alloys or polymer-based composites for steel involve substitution of conventional steel with lower stiffness materials. Hence any mass-reduction strategy must be tailored to retain the body stiffness as contributed by the material and the part geometry, including its gage.
An approach to increasing the body stiffness is to increase the flexural rigidity or geometric stiffness of structural panels by: applying a polymer precursor to at least a portion of a surface of such a panel; and curing the precursor to form a polymer coating on the panel surface; and repeating the process as required to develop a suitable coating thickness.
In this invention, a magnetic particle reinforced polymer resin blend or precursor, optionally incorporating surfactants, initiators and/or catalysts, may be deposited, in the presence of a magnetic field, on a structural panel. The thickness of the deposit may increase over time requiring that the panel and resin remain in contact for some pre-determined period of time and that the magnetic field continue to be applied during this time. The introduction of the magnetic particles and the interaction of the magnetic particles with the magnetic field, will increase the viscosity of the resin blend-magnetic particle mixture relative to the resin blend alone. And, as a consequence of the increased viscosity a greater thickness of the magnetic particle-reinforced resin may be deposited in a single application.
The deposit may then be cured, forming a particle-reinforced polymer composite of sufficient thickness to increase the moment of inertia of the panel and hence its geometric stiffness. Even modest concentrations of magnetic particles, say 5% by weight or about 1% by volume, may, in the presence of readily-achievable magnetic fields, enable polymer layer thicknesses of 200 micrometers. Such a polymer layer thickness is readily capable of imparting enhanced geometric stiffness and noise and vibration damping to a high strength steel panel with a thickness of between 500 and 650 micrometers or to a high strength aluminum or magnesium alloy panel of 800 to 1000 micrometers in thickness.
In many applications it may be preferred to deposit a continuous layer of near-uniform thickness on both panel surfaces, but in other situations, a deposit of varying thickness on one or both panel surfaces may be adequate. In yet other applications it may be preferred to deposit resin as one or more discrete stiffening ribs or ridges in only the most highly flexed regions of the panel and leave the remainder of the panel uncoated.
The addition of the magnetic particle reinforcements which may be of any suitable geometric form including generally spherical, acicular or flake-like increases the viscosity of the resin blend. Also the particle reinforcements are intended to be ferromagnetic so that, in interacting with an imposed magnetic field, they will yet further increase the viscosity of the resin. Hence the magnetic field and the magnetic particles cooperate to deposit a thick, adherent resin layer on the surface. Because of this higher viscosity, a thicker resin layer may be deposited in a single application. And by maintaining the magnetic field during curing, the resulting polymer layer will retain the thickness of the resin deposit. Preferably curing occurs at room temperature to avoid any thermally-driven viscosity reduction, and thinning of the resin layer, during curing, but alternatively, the magnetic field may be tuned to offset the viscosity reduction by a suitable extent during high-temperature cure. Once cured and polymerized, the magnetic field may be removed.
The magnetic particles should be ferromagnetic, but, as blended with the resin, should be substantially demagnetized to minimize clumping or aggregation of the particles and achieve a generally uniform particle dispersion in the resin. Any of the classes of ferromagnetic materials may be used, including: metals such as iron, nickel or cobalt; alloys and compounds such as those based on neodymium and iron and samarium and cobalt as well as Alnico; and oxides including iron oxide, Fe2O3 and ferrites. Where it may be required to minimize any remnant magnetism, magnetically soft materials, such as substantially pure iron, iron oxide, Fe2O3 and soft ferrites such as, for example, manganese-zinc ferrite, MnaZn(1-a)Fe2O4 or nickel-zinc ferrite, NiaZn(1-a)Fe2O4 may be employed. Particles of several such magnetically soft materials may be employed in combination.
While a sufficient thickness of polymer should be applied to increase the geometric stiffness of the panel or structure, the added thickness should not be such as to negate the mass reduction which initially prompted the addition of the stiffening layer. In this regard the low specific gravity of the polymer, ranging from about 1.1 to about 1.4, lower even than that of magnesium, is beneficial. However the density advantage enjoyed by the polymer will be offset by the higher specific gravity of the magnetic material which may range from about 5 for ferrites to about 7 for iron.
Pulverized iron particles or flakes may be preferred. Such plate-like particles may be formed by ball milling substantially spherical iron particles dispersed in a fluid to prevent welding of the particles to one another. At a specified volume fraction, such flake-like particles may be more effective in increasing the modulus of the resulting polymer-magnetic particle coating and enhancing the stiffness of the coating as well as the geometric stiffness of the substrate. Also the large surface area of such flake-like particles enables a large polymer-particle interface which may be effective in damping noise and vibration.
In a first embodiment, applicable to ferromagnetic structural panels fabricated, for example, of high strength steel, the panel itself or, more commonly, a portion of the panel may first be magnetized, either by passage of electric current through the panel or by exposure to a magnetic field. It will be appreciated that vehicle bodies are often joined using resistance spot welding so that having some portion of the panel remain uncoated by polymer, and electrically conductive, may be required to facilitate subsequent assembly and joining of panels. If mechanical joining methods such as clinching or riveting are contemplated however, complete coverage of the panel may be acceptable.
As shown in
After the panel is magnetized, in whole or part, it may be coated with polymer resin into which magnetic particles have been incorporated. A procedure is shown in
The magnetic particle-containing resin 34 deposited on and attached to the coated panel 11 may then be cured, either at ambient temperature or at elevated temperature or by radiation 38, for example in chamber 36 (
An alternative approach to magnetizing large panels is shown in
In operation, a common current may be passed through each of the plurality of electromagnets shown in
For non-magnetic panels, such as those fabricated of aluminum or magnesium alloy or of polymer composites, a different practice may be followed, best illustrated in
As shown in
The resulting non-magnetic panel, with its applied magnetic particle reinforced polymer pattern 58″ is shown at
If multiple similar parts are to be processed, magnetized panel 60′ may be re-used and returned to the stage in the process (shown at
When a new pattern is required, magnetized panel 60′ may be demagnetized as previously described and then re-enter the process as ferromagnetic panel 60 at
Greater layer thickness may be achieved by applying a higher intensity magnetic field such as by applying the generated magnetic field of magnetic frame 50 directly to the non-magnetic panel. This would require placing a face of non-magnetic panel 58, oriented horizontally, in direct contact with the magnetic poles of magnetic frame 50′. A dip process would be challenging and it would be more suitable to apply magnetic particle-containing resin using a spray or roller. However, since the panel is required to remain under the influence of the magnetic field during cure the productivity of such a procedure will be less than is achievable through using a readily-replicated, low cost, magnetized panel to apply the magnetic field.
Replacing the electromagnetic array with permanent magnets would also enable processing of two non-magnetic panels simultaneously and facilitate dipping. A suitable arrangement is shown at
Mounting block 72 may be a polymer or a non-magnetic metal such as copper, brass or bronze among others. Block 72 may be reuseable and fabricated with an array of openings each sized to accept and retain a magnet so that any desired magnet array may be developed by selectively placing magnets in some locations but not others. Alternatively block 72 may be uniquely designed for each magnet array, for example by casting a polymer support block around pre-positioned magnets.
In an exemplary practice, two-part, room-temperature-curing, epoxy-amine resin was prepared containing dispersed, generally spherical, 10 micrometer diameter iron particles in a mass fraction of about 5%. Two thin, mild steel sheets, 0.8 millimeters thick, one magnetized at one end using a permanent magnet and the other un-magnetized, were placed in the bath for a few (1 to 10) seconds to develop a resin coating and then withdrawn. On curing the resin coating the thickness of the polymer layer on the magnetized sample was about 400 micrometers or about 24% thicker than the polymer coating on the unmagnetized sample.
Practices of the invention have been described using certain illustrative examples, but the scope of the invention is not limited to such illustrative examples.
