Reducing Brittleness of Hard 3D Resin Prints

Brittleness is a common problem for photopolymer 3D resins. Making a formula that is hard is relatively easy. Making one that is hard, and durable takes extra effort and thought. Here we are defining durable as materials that show little or no change in properties over time. How the materials in a formula react, and the ultimate crosslink density are two keys to understanding how to improve durability.

In general, hard 3D photopolymer resins have limited conversion. This can be due to several factors including vitrification and the low intensity/exotherm of 3D printers.

As they age, these materials can tend to crack with or without exposure to light. While some additives can be used to alter the crosslink network to improve the durability, that route will also alter final print properties. Here we will discuss the roles of total (meth)acrylate functionality, and degree of post-curing in reducing brittleness.

First, we will examine the impact of post-cure. Dynamic Mechanical Analysis (DMA) can be used to quantitatively evaluate the effect of post-cure.

DMA of UV curable resins for 3D printing can highlight the differences between green properties (after simply washing the part and drying) and final properties (after wash/dry and post cure step). Tα,, the alpha transition, occurs when the network's large segments start moving. Post curing will shift the curves to higher temperatures for a given 3D resin, and there will be no change in properties after the final conversion is reached.

Incomplete curing leads to lower crosslinking. This may fake a higher flexibility, but it will also result in tackiness. It will also lead to lower HDT, strength and hardness. For a hard thermoset, lower crosslinking means higher chain length which leads to higher elongation.

A less measurable, but very influential, byproduct of incomplete cure is "dark polymerization", or a gradual change in properties over time. This can lead to a cured part with good initial properties to turn brittle over time.

In our webinar Improving Toughness of Photopolymer 3D prints, we went into some detail on the role of cure as it relates to toughness. We also spoke on the behavior of monofunctional monomers and how they can add a degree of thermoplasticity to a 3D photopolymer resin.

The figure below visually shows those two concepts at work. Here we examine how modifying materials can affect brittleness of a hard Oligomer. Even with a higher degree of acrylate conversion (with aging), the monofunctional materials promote a lower overall crosslink density within the part, and thus less internal stress, leading to less cracking and brittleness.