Slip additives have a wide range of industrial uses, finding their way into everything from lubricants to healthcare products. Fatty acid amides have been used as slip additives since the 1960s, and erucamide is widely used in polymer manufacturing. Research into erucamide migration and distribution and its nanomechanical properties has shown that the assembly and performance of the slip-additive surface depend on concentration and application method, as well as the substrate surface chemistry. However, questions remain regarding the nanostructure of organised erucamide surface layers, including the molecular orientation of the outermost erucamide layer. In work recently published in the Journal of Colloid and Interface Science, a team of researchers from the University of Bristol and Procter & Gamble used a combination of techniques to investigate the erucamide nanostructure formed in a model system. Their findings will allow the use of rigorous scientific methods in real-world scenarios.
Manufacturers use slip additives to modify the surface structure of a wide range of materials, reducing friction without compromising the material's other properties (e.g. modulus). Slip additives are included in everything from food packaging and textiles, dyes and lubricants, to hygiene products such as nappies.
Erucamide (13-cis-docosenamide, C22H43NO) is a common fatty acid amide slip additive, widely used in the manufacture of polymers such as polyethylene and polypropylene. Erucamide has extraordinary slip properties, even at low concentrations, and is stable and transparent with low toxicity. It is used as an anti-fouling, anti-fogging, anti-viral and scratch-resistant agent. Erucamide can be applied as a surface coating. When it is included in polymers before extrusion, it naturally migrates to the surface of the finished object, a process known as 'blooming'. More additive molecules migrate to the surface over time, as the material ages, allowing the surface additive layer to "self-heal".
Manufacturers can control the properties of the additive surface layer (such as surface coverage, hydrophobicity, and adhesive response) by varying the erucamide concentration used. Some fundamental questions about erucamide persist, despite its widespread industrial use. For example, there has been uncertainty over the nanostructure of erucamide surface layers, particularly the outermost layer's molecular orientation.
A fuller understanding of the structure of erucamide surface layers is crucial to developing new and more environmentally friendly products. Therefore, Bristol Chemistry PhD student Dajana Gubała and a team of researchers used X-ray reflectivity (XRR) on beamline I07 to perform a quantitative characterisation of their structural parameters (e.g. molecular packing and thickness). They used a model system of erucamide layers prepared via spin-coating from its nonaqueous solution on hydrophilic bare silica.
Dr Wuge Briscoe, from the University of Bristol, explains:
We used Diamond comprehensively, and it provided the bulk of the data for this study. I have a long-standing relationship with beamline I07, and was part of the team that published the first results from the beamline in 2010. On every visit, the beamline staff are extraordinarily helpful. It starts before you arrive, as we communicate our requirements and check the feasibility of our experiments. And day or night, they are available to help align the beam, set up the experiment, resolve any issues, and extract the data. Visiting Diamond is also a wonderful learning experience for the new postdocs and PhD candidates in our lab.
The team used several complementary techniques to provide a comprehensive characterisation. Atomic Force Microscopy (AFM) imaging revealed localised in-plane structures, and contact angle measurements provided information on the wettability of erucamide-coated surfaces. Peak Force Quantitative Nanomechanical Mapping (QNM) showed a correlation between the erucamide nanostructure and the surface nanomechanical properties (i.e. adhesive response).
Together, these results offer us unprecedented insights into the nanostructure of organised erucamide surface layers. They show that the surface nanostructures change with erucamide concentration. In all cases, the results were consistent with the hydrophobic tails of the erucamide molecules pointed outwards.
A better fundamental understanding of these surface properties will be invaluable in tuning slip additive properties across a wide range of applications.
Francesco Carlà, Principal Beamline Scientist for I07, said:
Supporting the users activity is always very stimulating, meeting colleagues with different backgrounds and expertise continuously expose the beamline staff to new ideas and allows us to look at scientific problems from completely different perspectives.
Before and during beamtime everyone involved is very focused on making the experiment work and obtaining the best possible results. The beamline staff are part of this process and are there to help the users make this happen, thanks to a continuous exchange of information and discussions at any stage of the experiment, from the preparation to the data analysis.
This particular study is now largely concluded, with two more forthcoming papers - one looking at the effect of surface substrate chemistry on erucamide nanostructures and the other on the role of temperature and ageing.
Gubała D et al. Heads or tails: Nanostructure and molecular orientations in organised erucamide surface layers. Journal of Colloid and Interface Science 590:506-517 (2021). DOI:10.1016/j.jcis.2021.01.087.
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