Yugang Duan

Curing Methods for Advanced Polymer Composites - A Review

Advanced polymer composites have obtained great application interest in a number of demanding aerospace, wind energy, automotive, infrastructure, and consumer applications. Great varieties of curing methods are investigated to develop low-cost and high-efficient fabrication of advanced polymer composites, which still remains as a great challenge and thorny issue. Especially, the autoclave curing process, which is widely used for curing of high performance advanced polymer composites, is labor- and capital-intensive, with costs increasing exponentially with part size and limiting increased use of advanced polymer composites. Researchers and industries have long desired to explore and develop low-cost and high-efficient curing methods for fabrication of advanced polymer composites and investigated different radiation and thermal curing alternatives. In this paper, current development status of the radiation curing (gamma ray, x-ray, ultraviolet, accelerated electron beams) and thermal curing (radiation heating (infrared, laser and microwave), convection and conduction heating (hot gas, flame, oven and hot shoe), induction heating, ultrasonic heating, resistance heating and thermal additives (magnetic particles, NIR absorbent particles) based heating methods applied for the curing of advanced polymer composites are reviewed. The curing mechanism and current application status of the different curing processes for fabrication of advanced polymer composites is discussed, and main advantages and disadvantages of these methods are comparatively analysed and evaluated according to the material, cost, feasibility and power criteria for successful curing application of advanced polymer composites.

Effects of exposure time on defects and demolding force in soft ultraviolet nanoimprint lithography

Based on a contact process, the ultraviolet (UV) nanoimprint process can overcome the linewidth limit of conventional photolithography caused by the diffraction effect. However, the contact between the mold and resist may introduce various defects to the two components. Some researchers have investigated the defects that emerge in the imprint process, such as particle contamination and curing shrinkage of the resist. However, the curing process of the resist itself should be studied because the curable resist is the carrier of the pattern transfer and the replication is accomplished by its shape remodeling. In this article, the authors focus on the effects of exposure time on the defects and demolding force in the soft UV imprint process. The replications are made by a soft polydimethylsiloxane mold under the same process parameters except that the curing time is different. Moreover, the curing degree of the resist and the value of the demolding force under various curing doses are measured. A mechanical ...

Imprint template fabrication based on glass wet etching using a soft etching mask

This paper presents a simple and reliable imprint template fabrication process for MEMS based on glass wet etching using a single-layer photoresist as the etching mask. In this work a novel vapour deposition process is adopted to cover a thin layer of silane coupler on the glass surface to improve the adhesion strength of the resist and hence attenuate the undercutting phenomenon, which reduces the extra undercut ratio to 0.6 and improves the sharpness of the etched pattern edges. Since the air bubbles in the photoresist and the particle contamination on the glass surface will cause some defects on the etched surface, the authors adopted a thick layer of resist to eliminate defects and reduce the stringent requirement for clean-room conditions. A smooth template surface is obtained by using hydrochloric acid as an etching agent additive, which can facilitate the template separation in the imprint lithography process. Imprint templates with a pattern feature size of 100 µm have been fabricated using the developed process and the imprint results are demonstrated.

Fabrication and mechanical properties of UV-curable glass fiber-reinforced polymer—matrix composite

A UV-curable glass fiber-reinforced composites was investigated in order to develop a cost-effective fabrication approach for manufacturing polymer—matrix composites. The effects of glass fiber-treating process and UV exposure power on the tensile strength were studied through scanning electron microscopy, contact angle determination, and curing degree test. The experimental results showed 11.5% increase in tensile strength for the UV-cured composite reinforced with heat-treated glass fiber, and 51.4% increase with glass fiber treated by coupling agent. The optimal exposure power was 0.94 J/mm2 to obtain the optimized mechanical properties. However, overexposure could seriously result in the loss of tensile strength. The maximum tensile strength and shear strength of composite cured by UV approach in this study were 902.09 and 25.04 MPa, respectively.A UV-curable glass fiber-reinforced composites was investigated in order to develop a cost-effective fabrication approach for manufacturing polymer—matrix composites. The effects of glass fiber-treating process and UV exposure power on the tensile strength were studied through scanning electron microscopy, contact angle determination, and curing degree test. The experimental results showed 11.5% increase in tensile strength for the UV-cured composite reinforced with heat-treated glass fiber, and 51.4% increase with glass fiber treated by coupling agent. The optimal exposure power was 0.94 J/mm2 to obtain the optimized mechanical properties. However, overexposure could seriously result in the loss of tensile strength. The maximum tensile strength and shear strength of composite cured by UV approach in this study were 902.09 and 25.04 MPa, respectively.

Room-temperature capillary-imprint lithography for making micro-/nanostructures in large areas

Compared to imprint techniques under the driving condition of external imprint pressure, imprint lithography under capillary force has the advantages of being suitable to produce large feature patterns, simple mold structure, very low imprint pressure, and conformal contact between the mold and the substrate. However, in existing capillary-force lithography, the necessary heating process will bring problems, such as separation between the mold and the substrate, long imprint time, shorter lifetime of the mold, etc. In this article, the authors propose a technique of room-temperature capillary-imprint lithography which is applied to making both micro- and nanostructures simultaneously in large areas under low temperature. In order to prevent the polydimethylsiloxane mold from swelling in the imprint process, a new ultraviolet curable resist is introduced in the imprint process. This resist contains no solvents and has a wide range of viscosity. Through the experiment, three-dimensional microstructures are ...

Investigation on LIGA-like process based on multilevel imprint lithography

A low-cost quasi-LIGA process is proposed, in which, instead of using thick resist technique, micro-structure with large structural height is achieved by multilevel imprinting and through-mask plating. To achieve precise alignment between individual layers, an alignment system based on computer micro-vision is developed and the experimental results show an average overlay accuracy within 1.5@mm with a standard deviation within 0.33@mm. Good adherence of resist on seed layer is achieved by substrate surface oxidation and using a coupling agent, which establishes a chemical bond between substrate surface and the resist layer. The deposit uniformity is improved by electroplating process optimization to enhance the cathode polarization. Through electrolyze etching on previous layer before next level of metal deposition, the fresh metal surface is obtained and the bonding strength between adjacent metal layers is enhanced. With the developed process, tri-layer photoresist and metallic structures with a pattern feature size of 20@mm were successfully fabricated.

Research on the Forming Mechanism of Micro/Nano Features during the Cast Molding Process

Cast molding process has provided a reliable, simple and cost-effective way to fabricate micro structures since decades ago. In order to obtain structures with fine, dense and deep nano-size features by cast molding, it is necessary to study the forming mechanism in the process. In this paper, based on major steps of cast molding, filling models of liquid are established and solved; and the forming mechanism of liquid is revealed. Moreover, the scale effect between the liquid and the cavity on the filling velocity of liquid is studied. It is also interesting to find out that the wettability of liquid on the cavity may be changed from wetting to dewetting depends on the pressure difference. Finally, we experimentally verify some of our modeling results on the flowing and filling state of the liquid during the cast molding process.

Making modified fluoropolymer films with low surface energy

A technique of using commercial polymers and additive to fabricate modified fluoropolymer films with low surface energy is proposed. A small amount of the additive can increase the oleophobic behavior of the fluoropolymer and so reduce its surface energy remarkably. The modified fluoropolymer films not only possess low surface energy and are solvent resistant but also have a high modulus. By a simple cast-molding process, both three-dimensional micropillars and complex letters with 100 nm linewidth can be replicated in the modified fluoropolymer films. Moreover, in the authors’ experiment, the modified fluoropolymer film has shown its replicating competence for replicating high-aspect-ratio sub-30 nm structures.

A new method for glass-fiber reinforced composites manufacturing: Automated fiber placement with in-situ UV curing

Automated fiber placement(AFP) is being widely used for producing large and complex composite structures, and ultraviolet(UV) curing has been proved as a good alternative to traditional thermal curing as a low-cost and environment-friendly curing method. A new manufacturing method combining AFP and UV in-situ curing was presented in this paper to manufacture glass fiber reinforced composites for large scale composite structures, especially with thick curing depth. A robot-based fiber placement machine integrating UV-LED for the in-situ curing was developed and a flat panel was fabricated with this method. UV transmission characteristic of single composite layer was obtained and preliminary researches about the effects of UV exposure on laminate performance have been conducted. The results showed that laminates fabricated by the in-situ curing process shows better interlaminar properties when the degree of polymerization is about 67.7% for the first cured layer with the incident UV irradiance. Finally, some key issues concerning the process are discussed.

Making high-fidelity imprint template by resist patterns over a flexible conductive polymer substrate

A technique to fabricate an imprint template is proposed, based on a direct replication of the electronic-beam lithography resist patterns over a flexible conductive polymer substrate. This approach will simplify the template-making process because no anisotropic dry etching is required to transfer the resist pattern into a template material as usual. The flexible polymer film used as the substrate will be helpful during the detaching step of the template obtained; moreover, the flexibility of the substrate can be used to fabricate templates with curved-surface patterns. Using the technique, both the polymer templates and metal templates can be fabricated, and the above advantages will allow the templates to obtain the high-fidelity patterns directly from the electronic-beam lithography.