“All-in-One” Thixotropic Polysiloxane Pastes for UV-Activated Room Temperature Hydrosilylation Cross-Linking in Additive Manufacturing

Abstract

In search of "inks" for additive manufacturing of silicone rubber parts, three different types of "all-in-one" pastes with suitable rheological flow properties, ultraviolet (UV)-activated fast cross-linking by hydrosilylation in air at room temperature (as quick as 20−30 s), and shelf lives of over six months in the dark and 6−9 days in laboratory daylight, depending on paste composition, have been developed. The pastes are based on vinylsiloxy-functionalized, completely amorphous, linear terpolysiloxanes containing predominantly dimethylsiloxy-, DiMeS, repeat units with small amounts (typically less than 5 mol %), of diphenylsiloxy-, DiPhS, methylphenylsiloxy-, MePhS, or diethylsiloxy-, DiEtS, crystallization disruptors. The base polymers were compounded with trimethylsilylated hydrophobic silica fillers, thixotropic flow agents, hydrosilyl-functionalized oligomeric crosslinkers, and catalytic systems containing either platinum(II) acetylacetonate, Pt(AcAc) 2 , or trimethyl(methylcyclopentadienyl)platinum(IV), MeCpPtMe 3 , and diethyl azodicarboxylate, DEAD, catalyst inhibitors. Paste preparation, steady and oscillatory shear rheology, and UV-activated cross-linking and shelf-life stabilities are described.

All-Silicone 3D Printing Technology: Toward Highly Elastic Dielectric Elastomers and Complex Structures

“…29,30 For example, according to manufacturer technical data, the commonly used silicone kit, Sylgard 184, based on thermally activated hydrosilylation, cures at 150 °C in 10 min, while curing at ambient temperature requires 48 h. 31 Unlike thermal hydrosilylation, the cross-linking process through photohydrosilylation enables silicones to cure in a matter of seconds, which lends itself better to 3D printing. 32 handling due to the risk of poisoning them with nitrogen-, sulfur-, and phosphorus-containing compounds. 30,33,34 Additionally, the hydride functional groups are sensitive to hydrolysis in the presence of moisture.…”

“…Most silicone 3D printing processes rely on hydrosilylation of polysiloxanes containing vinyl groups with hydride-functional polysiloxanes. − The photoinduced hydrosilylation has the benefit of fast curing without extra heating inputs, whereas the thermally activated hydrosilylation needs high temperatures to reach effective curing rates, typically above 100 °C. , For example, according to manufacturer technical data, the commonly used silicone kit, Sylgard 184, based on thermally activated hydrosilylation, cures at 150 °C in 10 min, while curing at ambient temperature requires 48 h . Unlike thermal hydrosilylation, the cross-linking process through photohydrosilylation enables silicones to cure in a matter of seconds, which lends itself better to 3D printing . However, both hydrosilylation mechanisms require platinum-based catalysts, which are generally expensive and need particular handling due to the risk of poisoning them with nitrogen-, sulfur-, and phosphorus-containing compounds. ,, Additionally, the hydride functional groups are sensitive to hydrolysis in the presence of moisture .…”

One-Component, Vinyl-Functional Fluorosilicone Pastes for Direct Ink Write Additive Manufacturing

“…where z was kept at less than 0.01 (1 mol %), y ranged from 0.03 to 0.07 (3–7 mol %), and x ranged from 0.92 to 0.96 (92–96 mol %). − The DiPhS (I), DiEtS (II), and MePhS (III) repeat units were introduced into these polymers to act as suppressors of crystallization, , which is a characteristic of pure PDMS, occurring between −50 and −40 °C. , Hence, when modified in this manner, the resulting terpolymers become precursors for completely amorphous elastomers that retain their characteristic elasticity to temperatures as low as −100 °C (i.e., just above their respective glass temperature, T g ) and eliminate any potential concerns on the cross-linking kinetics and desired mechanical properties of the resulting rubbers. , Vinylsiloxy-functional groups (both in –Si(CH 3 ) 2 –CHCH 2 chain ends and in –[Si(CH 3 )(CHCH 2 )–O]– repeat units) were introduced to serve as reactive sites for subsequent cross-linking by hydrosilylation. ,, …”

“…While the need for printable silicones in AM has been widely recognized, most reported formulations are based on traditional two-component systems − that often impose practical limitations and other difficulties . These include the need to mix together multiple reactive components (“Parts As and Bs”) and/or add catalysts immediately before printing, unavoidable covalent cross-linking of the resulting pastes in minutes to hours after mixing, and challenging printing operations, such as the pressures required to extrude the cross-linking pastes that gradually increase as the rheological properties of pastes evolve from viscous liquids to “solidification” when they can no longer be extruded. − To overcome these difficulties, we recently described several new types of terpolysiloxanes (see structures I–III), − specifically designed for one-component silicone printing inks , referred to as “all-in-one pastes” (A-1/Ps) for direct ink write (DIW) AM. ,, These terpolysiloxanes included diphenylsiloxy (DiPhS, –[Si­(C 6 H 5 ) 2 –O]–), diethylsiloxy (DiEtS, –[Si­(C 2 H 5 ) 2 –O]–), and methylphenylsiloxy (MePhS, –[Si­(CH 3 )­(C 6 H 5 )–O]–)-containing vinyl-functional derivatives of poly­(dimethylsiloxane) (PDMS) of the following general formulas I–III:…”

3D Printing of Transparent Silicone Elastomers

“…[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Thixotropic formulations that utilize hydrosilylation crosslinking to cure the printed part can be advantageous relative to UV curable formulations since the UV curable moieties are more susceptible to degradation over time, although recent efforts have focused on this challenge. [25,26] A notable advantage of UV curable formulations relative to thixotropic formulations is that UV-cured parts can be optically transparent [27,28] if the ink does not include other additives with a refractive index mismatched from the base. [8] Transparent polysiloxane inks that are processed by additive manufacturing have been previously formulated, for example, by using stereolithographic printing or by UV curing.…”