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Vitrimers promise to impact composites

Strong and durable like thermosets, yet moldable and recyclable like thermoplastics, vitrimers are "malleable thermosets" which are challenging the status quo in the composites industry. Mallinda, a startup company founded by some of the pioneering inventors of the technology, is developing malleable CFRPs for rapid production cycle times.

Vitrimers promise to impact composites

For over 50 years, synthetic polymers have been divided into two general categories: thermosets which have excellent mechanical properties, but must be irreversibly cured; and thermoplastics, which can be melted down and reprocessed, but have inferior thermal and mechanical properties. This characteristic allows thermoplastics to be molded relatively quickly for high volume manufacturing, using techniques such as injection molding. Futhermore, these materials can be recycled by melt-processing. On the down side however, the  majority of thermoplastics are susceptible to mechanical deformations and creep at elevated temperatures; therefore, they don’t have a sufficiently robust dimensional stability at high temperature. In addition, commercial thermoplastics are typically of higher cost in comparison to analogous thermoset resins. Thermosets are the resins of choice for structural composites applications, such as aerospace composite part fabrication where stiffness and durability are critical.

While thermoset resins offer significant performance benefits compared to their thermoplastic counterparts, significant cure times for thermoset resins make them unsuitable for high volume   applications such as automotive. Furthermore, due to the inability to reprocess thermoset resins after they’ve been cured, these materials are refractory to repair and remolding.

Recently, this paradigm has been challenged by the arrival of malleable thermoset materials, also known as vitrimers. 

Chris Kaffer, CEO and Co-founder of Mallinda:

What is a Malleable Thermoset?

Vitrimers are covalently-bonded crosslinked network polymers, like thermosets, but with the distinguishing feature that the chemical bonds  in the network are  exchangeable. Typically, a catalyst is included in the resin formulation, which enables bond exchange reactions when the material is heated above the vitrimeric transition temperature

When vitrimers are heated to the malleable state, their total crosslink density remains constant, but the rate of bond exchange increases  with temperature, since all chemical reactions run faster at higher temperatures. This leads to a gradual decrease in viscosity with temperature, which differs from the  relatively abrupt drop in viscosity associated with the melting transition of thermoplastic materials. The reversible nature of vitrimers enables welding, molding, reshaping, and recycling of fully cured materials.

Figure 2: Temperature-dependent stress relaxation behavior of vitrimers shows that malleable flow of vitrimers correlates with reaction-rate of bond exchange.

Temperature-dependent stress relaxation behavior of vitrimers shows that malleable flow of vitrimers correlates with reaction-rate of bond exchange.

Mallinda co-founders invented the first known vitrimeric materials which do not require a catalyst, and behave like traditional thermosets  under ambient conditions. Mallinda’s catalyst-free vitrimers exhibit malleable behavior only when heated above the glass transition temperature (Tg) of a given formulation.This means that below the glass transition, the polymer network is frozen, and is thus  indistinguishable from a traditional thermoset. Mallinda’s vitrimeric polyimine platform is also quite tractable. Tg, for example, can be tuned with formulation from below room temperature to upwards of 200ºC.

What is a Vitrimer and how do they compare to thermosets?
Vitrimers represent a new class of polymers based on dynamically exchangeable imine-linked polymer networks. Like thermosets, vitrimers are highly crosslinked network polymers. However, unlike thermosets which are permanently in a fixed form after curing, vitrimer chemistry yields a product that can be remolded. When heated above the glass transition temperature, the fully cured network polymer undergoes rapid dynamic covalent bond exchange within the polymer network. This allows for facile processing of thermoset materials that after being fully cured can be heated and reshaped. Upon cooling, the materials retain thermoset-like mechanical performance.

What does this mean for composites?
The Mallinda team has shown that when structural composite materials, like carbon fiber reinforced plastics (CFRPs), are made using vitrimeric resins, several distinct advantages arise.

  • First, unlike traditional thermosets, vitrimeric composite materials can be reprocessed after being cured. Fully cured materials can be heated, remolded, and reshaped without losing their original strength.
  • Second, these composite materials can be heat-welded together to form monolithic cured multilayer laminates. 
  • Finally, CFRPs made with vitrimeric resins can be easily recycled in a solution-based process in which the fiber and resin can be separated, recovered and reused to their original strength

Mallinda is using vitrimer technology to develop a new class of “pre-cured pre-impregnated laminates (prepreg)” which will impact current prepreg users as well as high throughput producers who currently rely on Resin Transfer Molding (RTM) processes with quick-cure resins.

Vitrimer enabled high throughput FRC compression molding

Vitrimer enabled high throughput FRC compression molding

The envisioned pre-cured prepreg malleable composite materials can be cured upstream in the manufacturing process as individual laminates in a roll-to-roll process, and stored indefinitely at room temperature. To manufacture composite parts, any number of plies can  be heated to the material’s specifically-formulated Tg, laid up in a mold to the desired thickness, and both consolidated and formed into a final part through compression forming.

Pre-cured Vitrimer matrix prepreg: 3-minute in mold dwell part production

Pre-cured Vitrimer matrix prepreg: 3-minute in mold dwell part production

Within 1 to 3 minutes the formed part can be demolded and handled immediately, with full rigidity occurring upon cooling. Scrap material can be further repurposed to form other parts, or can be recycled in the solution-based process where in the fiber and resin materials are recovered and reused.

Vitrimers: intrinsically recyclable
Due to the reversible crosslink chemistry in vitrimers, cured vitrimer resins can be depolymerized using chemical precursors to the resin itself. This enables circular recovery and reuse of both resin and fiber. This process can be carried out at room temperature for energy-neutral recycling. Mild heat (30°C) can be applied for rapid depolymerization.

Vitrimer Matrix Composite Recycling

Mallinda revolutionary polymer recycling with vitrimer-based composites for circular economy.

Companies: Mallinda

Terms: Business, Innovations

This article has been edited by Basalt.Today
This article has been written on JEC Composites Magazine
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