Adhesive Polymers as Efficient Binders for Additive Manufacturing

Dear Colleagues,

Adhesives are used to join two or more materials together. They are one of the oldest known applications of polymers. Depending on their molecular structure, adhesive polymers can be broadly classified as thermoplastic adhesives, or linear and long-chained polymers that become pliable at a specific temperature and solidify and harden upon further cooling. Acrylic or poly(methacrylate) (PMMA), nylon, polyvinyl acetate (PVA), etc. are some of the typical thermoplastic adhesives. Thermoset plastic adhesives form irreversible cross-links during the curing process and the cross-links, resulting in the increased strength of materials. Some of the thermoset plastic adhesives are phenolics, polyimides, epoxies, etc. Elastomers are polymeric adhesives with long lightly cross-linked molecules formed during the curing process; these have high elasticity. Some of the elastomeric adhesives are rubber, silicones, nitrile, etc. The adhesive polymers are cured by application of heat, UV radiation, solvent evaporation, pressure, moisture, etc.  

For several years, adhesive polymers have been widely used in feedstock development and processing of metals and ceramics using the injection moulding processes. Injection moulding consists of four consecutive processing steps: (1) feedstock preparation by mixing of metal or ceramic powders and binder materials to form a homogeneous powder-binder mix with a high metal/ceramic powder loading, (2) high pressure injection moulding of feedstock into a mould, (3) debinding, or the process of removing binder materials by retaining the shape of the part, and (4) sintering to high density and dimensional accuracy.

Adhesive polymers also play a significant role in the additive manufacturing (AM) of functional parts made of metals, ceramics, and sand using binder jetting and material extrusion-based AM technologies. Binder jetting is a high throughput powder bed additive manufacturing technology that uses a liquid bonding agent which is selectively dispensed onto the powder bed via a print head to physically bond the powder particles together. The binders can be in liquid form or solid powders mixed with the feedstock and activated by an ink jetted via the print head. The feedstock powders are spread layer-wise. The print head moves horizontally on the XY axis of the 3D printer and deposits the binder/ink onto the powder bed based on the cross section of the part to be 3D printed. Binder jetting is a support-free and a simple process that operates under room temperature conditions and enables fabrication of a variety of materials such as metals, ceramics, and sand. Some materials, like sand, do not require any additional processing. The other materials in as-printed green form are later cured, debinded, and sintered to achieve desired density and dimensional accuracy.

In the material extrusion-based additive manufacturing, the feedstock consists of metal or ceramic powders and adhesive polymers in pellet or slurry form. The feedstock is pushed through a nozzle and is selectively deposited layer-wise on the build platform based on the cross section of the part to be 3D printed. Material extrusion-based additive manufacturing is one of the most versatile and widely used techniques and enables processing of the exact same metal and ceramic feedstock as injection moulding. Several adhesive polymers, organic and phenolic based binders are used for the fabrication of metals, ceramics and sand parts via additive manufacturing.         

The main aim of this Special Issue on “Adhesive Polymers as Efficient Binders for Additive Manufacturing” is to present the state-of-the-art research that highlights the recent advancements made in the field of binder chemistry and new binder formulation, advanced binder characterization, role of adhesive polymers as sacrificial or functional binders for fabrication of metals, ceramics and sand parts using the ink jet, binder jetting, and material extrusion-based additive manufacturing techniques. Contributions related to injection moulding, feedstock modifications using polymeric binders, and other conventional processes are also invited.

Dr. Ganesh Kumar Meenashisundaram
Guest Editor

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