Biobased auxetic foams: an assessment of manufacturing and multifunctional properties

Auxetics are a class of mechanical metamaterials exhibiting a negative Poisson's ratio; that is, they expand laterally when extended longitudinally. Foams with auxetic characteristics also possess other desirable features in terms of indentation resistance to shear, energy absorption under cyclic quasi-static and dynamic loading, resistance to low kinetic energy impact, the enhanced dynamic modulus under relative humidity conditions, and tailorable shape memory properties. Over the years, multiple manufacturing processes have been developed to transform conventional foams into auxetics, ranging from thermo-mechanical (involving volumetric compression, annealing, and cooling) to chemical treatments (exploiting acetone and carbon dioxide to soften foam cell walls). No artificial auxetic foam metamaterial has, however, so far been knowingly produced.

Recent developments in bio-based polyurethanes (and related foams) make producing bio-based negative Poisson's ratio foam materials a distinct possibility. Vegetable oil-based polyols (such as castor oil and soybean oil) have been recently used in different blends to produce biobased open- and closed-cell foams via free-rising both at the laboratory scale and commercially, showing improved compression and thermal properties compared to fossil-based counterparts. An auxetic foam made of bio-based substrates would further increase the appeal of using this class of metamaterials in a wide range of applications because of the enhanced life cycle properties and global warming power reduction compared to their fossil counterparts. Moreover, auxetic biobased foams would also be among the first mechanical metamaterial products designed to be sustainable and eco-friendly from the onset. Potential applications of such materials range from the aerospace industry, where enhanced impact resistance and lightweight would contribute to improved efficiency, to sports equipment in running shoes, helmets, and padding, which would benefit from the unique properties of this class of materials.

In this context, the objectives of this project are:
- Characterise available castor and soy oil-based PU foams from the chemical, thermal, and morphological aspects, combined with mechanical (quasi-static, dynamic, and viscoelastic), acoustic and vibration testing.
- Manufacture biobased auxetic open- and closed-cell metamaterial foams through different conversion routes.
- Assess the viability of the auxetic foams obtained through the different processes by investigating the morphology and the mechanical, acoustic and vibration responses following a Design of Experiment procedure.
- Adapt existing constitutive models available for foams' mechanics, vibration, and acoustics to describe the corresponding properties of these new classes of biobased metamaterials.

There are seven principal groups of beneficiaries for our new EPSRC Centre for Doctoral Training in Composites Science, Engineering, and Manufacturing.

1. Collaborating companies and organisations, who will gain privileged access to the unique concentration of research training and skills available within the CDT, through active participation in doctoral research projects. In the Centre we will explore innovative ideas, in conjunction with industrial partners, international partners, and other associated groups (CLF, Catapults). Showcase events, such as our annual conference, will offer opportunities to a much broader spectrum of potentially collaborating companies and other organisations. The supporting companies will benefit from cross-sector learning opportunities and

- specific innovations within their sponsored project that make a significant impact on the company;
- increased collaboration with academia;
- the development of blue-skies and long-term research at a lowered risk.

2. Early-stage investors, who will gain access to commercial opportunities that have been validated through proof-of-concept, through our NCC-led technology pull-through programme.

3. Academics within Bristol, across a diverse range of disciplines, and at other universities associated with Bristol through the Manufacturing Hub, will benefit from collaborative research and exploitation opportunities in our CDT. International visits made possible by the Centre will undoubtedly lead to a wider spectrum of research training and exploitation collaborations.

4. Research students will establish their reputations as part of the CDT. Training and experiences within the Centre will increase their awareness of wider and contextually important issues, such as IP identification, commercialisation opportunities, and engagement with the public.

5. Students at the partner universities (SFI - Limerick) and other institutions, who will benefit from the collaborative training environment through the technologically relevant feedback from commercial stakeholder organisations.

6. The University of Bristol will enhance their international profile in composites. In addition to the immediate gains such as high quality academic publications and conference presentations during the course of the Centre, the University gains from the collaboration with industry that will continue long after the participants graduate. This is shown by the

a) Follow-on research activities in related areas.
b) Willingness of past graduates to:

i) Act as advocates for the CDT through our alumni association;
ii) Participate in the Advisory Board of our proposed CDT;
iii) Act as mentors to current doctoral students.

7. Citizens of the UK. We have identified key fields in composites science, engineering and manufacturing technology which are of current strategic importance to the country and will demonstrate the route by which these fields will impact our lives. Our current CDTs have shown considerable impact on industry (e.g. Rolls Royce). Our proposed centre will continue to give this benefit. We have built activities into the CDT programme to develop wider competences of the students in:

a) Communication - presentations, videos, journal paper, workshops;
b) Exploitation - business plans and exploitation routes for research;
c) Public Understanding - science ambassador, schools events, website.