Polymer composites: improving the environment, understanding scattering, and supporting enterprise

Why?

Plastic waste receives a lot of bad publicity but for some applications there are advantages for synthetic polymers over use of other materials.  Recycling is often readily practicable if the component materials are known, and so the balance of benefits and end-use costs needs to be carefully evaluated.  A recent study [1] came about from the use of 3d-printed boron carbide composite as neutron absorbing components for instruments and as part of sample holders.  In Sweden we are obliged to use environmentally favourable alternatives to metals like cadmium: composites have been demonstrated as effective in several applications.  Although simple calculations provide reasonable estimates of the absorption, it is important to verify their accuracy and, particularly, to determine the scattering of neutrons, even if it is low, that can arise as ‘albedo’ when the material is used in proximity to detectors.

Previous reports of materials that have been used for shielding have sometimes suggested that they did not attenuate as expected but often measurements were only performed with beams without determination of the neutron energy or wavelength and due to limits in fabrication, only relatively thick samples could be used.  Use of additive manufacturing allows thin layers with different thicknesses to be prepared and tested.

How?

Measurements of both transmission and scattering of the Addbor N25 composite (25% wt boron carbide in nylon) were made with the time-of-flight small-angle neutron scattering (SANS) instrument Larmor at the ISIS Neutron Facility as shown in the figure at wavelengths between 0.9 and 12.5 Å (about 0.5 to 100 meV).  Thicknesses between 0.07 mm and 0.6 mm were measured and showed the expected 1/velocity variation in transmission, with a slightly better absorption than calculated from tabulated cross-sections.  The scattering that was seen comprises both an upturn at small momentum transfer from the B4C particles in the polymer and an incoherent background from the nylon matrix material.  In normal use the scattering is strongly attenuated by the self-absorption but can be measured when samples are thin.

The clear wavelength dependence of the observed incoherent scattering is of interest not just for performance of absorbing composites but more generally can be important in many SANS experiments.  The effective cross-section for hydrogenous material is seen to increase by about a factor of 2 in the wavelength range investigated.  Usual data reduction that groups all data simply by assuming elastic scattering would convolute this effect with the incident spectral distribution and sample transmission giving rise to an apparent variation with momentum transfer.

Figure 1 Sample ready for measurement on the Larmor instrument

What is next?

The promising results have paved the way for the company involved in this research to produce focussing collimators and other components with established performance for neutron instrumentation that allows users, facilities and the nuclear industry to work with a safer, and environmentally friendly alternative to cadmium.  Materials that absorb X-rays and γ-rays are also available and can be combined to provide integrated shielding that protects not only against neutrons but also secondary radiation.  There is now a strong interest in development of shielding for other types of radiation such as medical X-rays where lead can be replaced.  The structure and performance of all these materials will be investigated.  Although the main activity of the company involved in this work involves polymer composites, a new study has found other aspects of neutron instrumentation that benefit from additive manufacturing.  It is now also exploiting an invention [2] of tough, high transparency, low background scattering glassy metal as windows for neutron beams or to make sample environment components.

The results also show how measurements initiated to verify technical performance of a commercial product can provide important insights into general aspects of scattering studies.

Who?

The study was initiated by the company Additive Composite Uppsala AB (www.additivecomposite.com) by Adam Engberg and Olle Eriksson in a collaboration with Uppsala University, Adrian Rennie.  The neutron measurements were carried out at the ISIS facility with Rob Dalgliesh.

References

[1] A. R. Rennie, A. Engberg, O. Eriksson, R. M. Dalgliesh ‘Understanding neutron absorption and scattering in a polymer composite material’ Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 984, (2020), 164613. https://doi.org/10.1016/j.nima.2020.164613

[2] M. Sahlberg, A. R. Rennie, J. J. Marattukalam, C. A. Ericsson, V. M. Pacheco ‘A beam path component for use in neutron scattering equipment and method of producing such’ Provisional Patent Applications 2020.

Contact:

Prof. Adrian Rennie, Uppsala University

Adrian.Rennie@physics.uu.se