Everyone is familiar with elastomers. From party balloons and rubber bands to shoe soles, we are surrounded by these stretchy and tough polymers. The term “elastomer” is in fact a portmanteau of “elastic polymer,” and as such, elastomers share much of the same physical characteristics as rigid polymers. Typical elastomers we are familiar with: butyl rubber, latex, nitrile, neoprene, and silicone are chemically crosslinked and thus are in the thermoset family (think epoxy). By definition elastomeric polymers must be above their glass transition temperature, should have a low degree of crystallinity, and be lightly crosslinked.
That is where physical cross-links come in. Most thermoplastic elastomers are block copolymers with one block forming an amorphous rubbery phase (above the glass transition temperature), and the other block which crystallizes or forms reversible bonds upon cooling. These so-called physical crosslinks are entirely reversible upon heating and therefore give this polymer class their thermoplastic behavior.
It consists of alternating blocks of “hard” and “soft” polymer segments. The hard segments in TPU separate and crystallize upon cooling to form the physical crosslinks necessary to give them their structure. By altering the ratio of hard and soft segments, the mechanical properties of TPUs may be fine-tuned for a particular application. Additionally, the chemistry of the soft phase polyols (typically polyether or polyester) may be selected to impart desired properties to the TPU.
One of the most common measures of an elastomers flexibility (ranging from hard to soft) is the Shore hardness. Shore hardness is the measure of a materials ability to resist indentation when an indenter of a certain geometry is pressed into the material with a given amount of force. The greater the resistance to indentation, the higher the Shore hardness number. Testing specifications and procedures for Shore hardness are outlined in ASTM D2240 and DIN 53505 (ISO 868). Shore hardness values can be confusing since there are multiple scales with crossover in values; however conversion charts are widely available. Generally, materials suitable for FDM 3D printing are in the A and D range.
Generally speaking, the harder the TPU, the easier it is to print. Softer TPUs typically begin to have problems feeding with Bowden type extrusion systems and smaller 1.75mm diameter filament machines. This is generally attributed to Euler buckling throughout the feed section, and the Poisson effect in the liquefier section. Additionally, softer TPU’s tend to have higher surface friction and thus more resistance in the feed section. Because of this, flexible filament materials for 3D printing have generally been limited to about 85A Shore hardness.
During the extrusion process, we coextrude a thin layer of harder TPU shell on a softer TPU core. The result is a filament that prints like a 95A TPU, and yet finished parts that feel like an 80A TPU. It is the most flexible filament currently available for 3D printing that is able to print on any open platform. Check out the TDS for TPU 80A to learn more, or buy now.
By C. Brandon Sweeney, Ph.D. - Head of R&D for Materials and Co-founder
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