Typically, aromatic thermosetting copolyester resins are produced by a two-part oligomerization process wherein branched aromatic crosslinkable copolyester oligomers are synthesized in a melt with average molecular weights between 1000 and 2000 g/mol with monomer feed ratios selected such that the oligomers preferentially are capped with either carboxylic acid or acetoxy functional groups. These are synthesized with an initial feed of TMA (trimesic acid), (4-acetoxybenzoic acid) ABA, IPA (isophthalic acid), and BPDA (biphenol diacetate). As an example, an oligomer structure designated “CB” (carboxylic acid-capped) oligomers can be synthesized by melt-condensation of TMA, ABA, IPA and BPDA (molar ratio 1:3:2:2, respectively). “AB” (acetoxy-capped) oligomers can be synthesized similarly with a molar ratio 1:3:0:3. These oligomers are typically brittle, glassy solids at room temperature which can have their particle size reduced to micron-scale powder by simple grinding with a laboratory blender. Following this, the oligomer powders can be loaded into a vibratory sieve with controlled mesh size, which after vibratory sieving, powders with constrained maximum diameters, for example less than 90 μm, can be achieved.
Prior methods to produce discontinuous ATSP-based composites have employed micron-scale fillers such as graphite powder, polytetraflurorethylene (PTFE) powder, milled carbon fiber, etc. When the carboxylic acid and acetoxy-capped oligomer powder mixture is heated above 260° C., acetic acid is released and the ester backbone is formed—advancing the molecular weight. During this process, what has previously been seen is that the neat resins will outgas and produce foam structures with significant porosity. To avoid this porosity, cured ATSP powders were produced by crosslinking the oligomers, such as CB and AB. CB and AB oligomer powders were mixed at a 1:1 weight ratio and cured via an imposed thermal cycle of 200° C. for 1 h, 270° C. for 2 h followed by 330° C. for 3 h wider vacuum. This produces a foam material with a typical density of 0.36-0.53 g/cm3. ATSP foamed structures were then ground to produce powders which pass through a 90 μm mesh sieve in a manner similar to above. However, substantially more grinding time was necessary due to the high mechanical properties of the ATSP resin and the reduction of the particle size becomes a significant rate-limiting step. Following this, fillers such as milled carbon fiber, PTFE powder, graphite, etc. can be mixed with cured ATSP powder. The mixture then is loaded in a mold and the loaded mold is compressed with 13.8 MPa (2000 psi) normal pressure in a vacuumed hot press machine with heating elements installed in the top and the bottom press plates. The temperature increased to 340° C. and held for 2 h for hot sintering of the mixture in to bulk ATSP based composites.
Prior method to produce ATSP-based continuous fiber composites used reinforcements such as carbon fiber or glass fiber fabrics. Blends of carboxylic acid and acetoxy-capped oligomer powders, with weight ratio of 1:1.18 were loaded into a container with polar aprotic solvents such as N-methylpyrrolidone added to the oligomer mixture at a ratio of 0.5 gr per mL of solvent. This mixture was blended well with a blade blender while heating the container to 80° C. and held for 4 hours to ensure dissolution of all the solid powder. The warm solution is then impregnated into the fabric preforms via processes such as hand layup or vacuum assisted resin transfer molding (VARTM). The temperature is then ramped to 220° C. and held for one hour, then the temperature is ramped to 270° C. and held for one hour, and followed by 340° C. and kept for 2 hour. For pressure, the pressure is increased to 0.1 MPa (15 psi) after 30 min hold at 220° C., following this the pressure is increased to 6.9 MPa (1000 psi) after 1 hour hold at 340° C. and held till the sample cools down. After this heat and pressure cycle, fully condensed ATSP based continuous fiber composite is formed. The using of solvent in this process is not preferred.
This patent describes a new, simple, and low-cost method to produce continuous and discontinuous polymer matrix composite from a pre-polymer system that generates a volatile by-product as a consequence of its cure reaction. For example, the crosslinkable aromatic polyester oligomers evolve acetic acid as their cure by-product, forming an ester bond and thereby advancing molecular weight. The enabling feature that allows this is an abundance of exchangeable bonds within the resin. At temperatures above the glass transition temperature, polymer resins that feature exchangeable bonds (that are active in a temperature regime well before the resin begins to thermally degrade) have an additional mechanism of stress relaxation. Porosity evolved from the generation of a volatile by-product can be collapsed to negligible values and densities that approach theoretical values can thereby be achieved. Examples of exchange reactions that enable this process include transesterification, transimidization, transamidization, urea exchanbe, hydrogen bonding exchange, and sulfone exchange. In this patent, we demonstrate this principle using aromatic thermosetting copolyester (ATSP) based composites. For this method, we eliminate the need to either pre-cure any of the ATSP powders or need to use solvent; instead, the ATSP-based composites are directly produced from the mixture of the ATSP oligomer powders with the composite fillers thereby eliminating rate-limiting steps. In the curing process, the acetic acid off-gassing escape from the system between empty spaces and microscopic flow-paths introduced by the fillers, with high pressure at high temperature, the cured mixture can be fully condensed using the additional stress relaxation mechanism as described above. The produced ATSP based composites have excellent thermal stability and tribological performance and the laminates of the composites can be bonded together ex-situ to form thicker multi-material composites.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fees.
A fuller understanding of the foregoing may be had by reference to the accompanying drawings, wherein:
Referring now to the figures, a process is described to produce fully dense materials of ATSP in spite of the off-gassing of the acetic acid. Once the oligomer powder mixture is blended with the right concentration of other fillers, such as milled carbon fiber, chopped carbon fiber, continuous fibers, and powders (ceramic powder, graphite, PTFE, etc.), the acetic acid produced at a high curing temperature can leak out through the free space among the fillers. Once the cure finishes, with compressive pressure at high temperature, the cured mixture/blend can consolidate and approach theoretical density. These composites can be used for variety of applications, such as for ablative material, bearing material, structure material, etc. Embodiments of the present invention provide a method for fabricating ATSP based composites and more generally a method for producing composites in cases where a by-product is evolved by the cure reaction but the resin features exchangeable bonds.
In an embodiment of the present invention, ATSP-chopped carbon fiber composite is produced directly from ATSP oligomers and chopped carbon fiber mixture. In accordance with such embodiments, ATSP oligomers and chopped carbon fiber are added together and then blended well in a blender. Then, the blended mixture is loaded in a mold, goes through a high temperature cure and compression cycle and forms the final ATSP-chopped carbon fiber composite. Different length and different material of chopped fibers are applicable for the same fabrication process.
In another embodiment of the present invention, ATSP-milled carbon fiber (and other small size fillers) composites are produced directly from ATSP oligomers and small size fillers mixture. In accordance with such embodiments, ATSP oligomers and small size fillers are added together with desired weight ration and then blended well in a blender. Then, the blended mixtures are loaded in a mold, and go through a high temperature cure and compression cycle and form the final ATSP-based composites. Various filler materials, combination of different fillers and different weight ratios are applicable for the same fabrication process with the significant requirement being that they do not melt or significantly thermally degrade as a consequence of the process cycle.
In another embodiment of the present invention, ATSP/milled carbon fiber composite is directly produced from ATSP oligomers and milled carbon fiber mixture; and the composite is attached on a metal surface at the same time. In accordance with such embodiments, ATSP oligomers and milled carbon fiber filler are added together with desired weight ration and then blended well in a blender. The mixture is loaded in a mold such that one side of the mold is mounted with the metal backing plate coated with ATS. Then, the blended mixture goes through a high temperature cure and compression cycle and form the final ATSP-milled carbon fiber composite attached on the coated metal. Various filler materials, combination of different fillers and different weight ratios are applicable for the same fabrication process. And the bonding between ATSP composites and metal can be also supplied by mechanical interlocking, such as dovetail grooves on metal substrate surface and sintered porous metal (e.g. bronze) powder on metal substrate surface.
In another embodiment of the present invention, ATSP-based multilayer composite was produced by bonding several plates of cured ATSP composites or ATSP-coated aluminum. Cured ATSP-milled carbon fiber and ATSP-chopped carbon fiber composite are directly produced from ATSP oligomers and milled/chopped carbon fiber mixtures. In accordance with such embodiments, ATSP oligomers and milled/chopped carbon fiber filler are added together with desired weight ration and then blended well in a blender. Load the mixtures in a mold that go through a high temperature cure and compression cycle and form the final ATSP-milled/chopped carbon fiber composites. Stack the cured ATSP composites and ATSP coated aluminum sheets in desired order and load the stack in hot press machine for a hot sintering process; and a multilayer composites can be formed. Various filler materials, combination of different fillers and different weight ratios are applicable for the same fabrication process. And the bonding between ATSP composites and metal can be also supplied by mechanical interlocking, such as dovetail grooves on metal substrate surface and sintered porous metal (e.g. bronze) powder on metal substrate surface.
In another embodiment of the present invention, ATSP based continuous fiber composites are produced directly from ATSP oligomers and continuous fiber weave mixture. In accordance with such embodiments, ATSP oligomers are deposited on the continuous fiber weave with three different ways: dry powder bath, dry powder spray and wet slurry method. Then, the blended mixtures are loaded in a hot press machine, and go through a high temperature cure and compression cycle and form the final ATSP-based continuous fiber composites. Various continuous fiber materials, with different formats (weave sheet, unidirectional fabric, tow, etc.), combination of different fillers and different weight ratios are applicable for the same fabrication process.
This example demonstrates the production of discontinuous ATSP composites filled with chopped carbon fiber (other chopped fibers such as glass fibers work with the same principle). First, mix the two oligomers (CB and AB) with desired weight percentage of chopped carbon fiber. Then the blended components are loaded into a mold and placed in a hot press. The hot press is ramped to 270° C. and held for one hour with no pressure applied to the mold. After one hour, the temperature is ramped to 360° C. At 360° C., the pressure is increased to 27.6 MPa (4000 psi) and the sample is held for 2 hours and then allowed to cool. An example of a fully dense discontinuous composites produced by this method (as seen in
Thermal degradation of the composites was performed by a TGA (TA-2950) from room temperature to 800° C. with heating rate of 10° C./min under nitrogen.
Table 1 and
52%
85%
68%
As for Tribological applications, polymers in pure form as unfilled polymers may have high COF, high wear rate and poor mechanical properties, so they typically do not satisfy the tribological application needs. Thus, it is of great interest in producing composites or blended polymers by adding different fillers and reinforcements in the polymers, improving significantly their mechanical, thermal or tribological properties. In this example, we produced ATSP bearing grade composites by mixing ATSP oligomers with different fillers such as milled carbon fiber (Zoltek PX35MF0150), graphite powder, PTFE powder, carbon black, carbon nanotubes, and graphene nanoplatelets with different weight percentages. Examples of mixing ratio between ATSP and fillers that have been tried is listed in
Table 2.
The parameter curve for curing of the mixture of ATSP oligomers and the fillers is shown in
The composites showed excellent machinability and they were made into ¼″ cylindrical pins for tribological performance evaluation by a pin-on-disk experimental configuration, as shown in
Tilting pad bearings (for use in e.g. electrical submersible pumps) were fabricated using a filled composition of ATSP. A 304 stainless steel base was roughened by grit blast and subsequently cleaned via ultrasonication in isopropanol and then dried at 70° C. A mixed layer of CB and AB ground oligomeric powders (50:50 mass ratio, mesh size <90 um) were deposited via electrostatic powder deposition. The deposited coating was melted and cured via convection oven at 270° C.
The coated 304 base was inserted into a cylindrical mold as shown in
To produce the ATSP based synthetic multilayer (5 layers) composite as shown in
Neat CBAB plate was produced as below: cured CBAB powders were produced by first crosslinking the CB and AB oligomers. CB and AB oligomers were mixed at a 1:1 weight ratio were mixed and cured via an imposed thermal cycle of 200° C. for 1 h, 270° C. for 2 h followed by 330° C. for 3 h under vacuum. This produces a foam material with a typical density of 0.36-0.53 g/cm3. ATSP foamed structures were then ground to produce powders which pass through a <90 μm mesh. The cured ATSP powder was then loaded into the mold was compressed with 6.7 MPa (1000 psi) normal pressure in a vacuumed hot press machine with heating elements installed in the top and the bottom press plates. The temperature increased to 340° C. and held for 2 h for hot sintering of ATSP powders in to bulk plate.
To produce the CBAB coating on aluminum, the aluminum substrate was roughened by grit blast and subsequently cleaned via ultrasonication in isopropanol and then dried at 70° C. A mixed layer of CB and AB ground oligomeric powders (50:50 mass ratio, mesh size <90 um) were deposited via electrostatic powder deposition. The deposited coating was melted and cured via convection oven at 270° C.
With the 5 plates as in
To produce the ATSP-continuous glass fiber (and carbon fiber, etc.) composites as shown in
While particular elements, embodiments, and applications of the present invention have been shown and described, it is understood that the invention is not limited thereto because modifications may be made by those skilled in the art, particularly in light of the foregoing teaching. It is therefore contemplated by the appended claims to cover such modifications and incorporate those features which come within the sprint and scope of the invention.
The present application claims priority to provisional application Ser. No. 62/627,337 filed Feb. 7, 2018 entitled Ablative Composites Based on Aromatic Thermosetting Copolyester, 62/659,844 filed Apr. 19, 2018 entitled Reversible Adhesion and Interchain Transesterification Composite Welding Mechanism, and 62/786,269 filed Dec. 28, 2018 entitled Adhesive and Processing Methods Utilizing Exchangeable Chemical Bonds, all of which are incorporated herein by reference in their entirety.
Number | Date | Country | |
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62627337 | Feb 2018 | US | |
62659844 | Apr 2018 | US | |
62786269 | Dec 2018 | US |