AN ATC REVIEW
of
COLLOIDAL DISPERSIONS AND NANOMATERIAL DEFINITION
ATC DOCUMENT 110
February 2013 (rev 1)
Overbased Additives
and Other Similar Lubricating Oil Additives
Summary
ATC do not consider overbased lubricant additives to be “nanomaterials” within the
Commission Recommendation of 18 October 2011 on the definition of a nanomaterial
(2011/696/EU) (i). This is because they are non-particular materials existing as a micellar
system wherein the metal carbonate core “particles” are part of and entirely dependent on
the micellar system and cannot be isolated as particles. They do not exist as unbound
particles from which the nano form can be released under any conditions of use.
Background
On 18 October 2011 the European Commission adopted the Recommendation on the
definition of a nanomaterial (i). According to this Recommendation a "Nanomaterial"
means:
“A natural, incidental or manufactured material containing particles, in an unbound state or
as an aggregate or as an agglomerate and where, for 50 % or more of the particles in the
number size distribution, one or more external dimensions is in the size range 1 nm - 100
nm.
In specific cases and where warranted by concerns for the environment, health, safety or
competitiveness the number size distribution threshold of 50 % may be replaced by a
threshold between 1 and 50 %.
By derogation from the above, fullerenes, graphene flakes and single wall carbon
nanotubes with one or more external dimensions below 1 nm should be considered as
nanomaterials.”
It is the view of the Commission that whilst nanomaterials are not intrinsically hazardous,
there is still uncertainty about the safety of nanomaterials and therefore the safety
assessment of these substances needs to be done on a case by case basis. It is the
stated intention of the Commission that the definition above will provide clear and
unambiguous criteria to identify such materials and that it will be used in subsequent
legislation. Further clarification of the definition is provided in a question and answer
paper contained in the following link.
http://ec.europa.eu/environment/chemicals/nanotech/index.htm (ii)
Based on the above background, this document is the ATC response considering whether
colloidal dispersions, in particular the overbased additives, should be considered to be
nanomaterials as defined by the Commission.
Use of Colloidal Dispersions in Lubricating Oils
The use of colloidal dispersions as additives in lubricating oils has a long history. Their
first use, as overbased sulphonates, is thought to have been in the 1940s with the first use
in commercial vehicles in the 1950s (iii)(iv).
These dispersions, often known within the lubricant additive industry as overbased
additives, have been characterised as having cores of Group I or Group II metal carbonate
and/or hydroxide stabilised by a layer of surfactant. The dispersion is formed in situ by
reaction of a metal hydroxide or oxide with carbon dioxide in the presence of the
stabilising surfactant. The resulting product is a clear, thermodynamically stable fluid
dispersion in a hydrocarbon medium. The diameter of several overbased additives has
been measured by small angle neutron scattering and is typically in the range 8 to 20 nm
(v).
Discussion
Colloids are typically defined as having at least one linear dimension approximately
between 1 nanometer and 1 micron. So if considering only size, colloidal dispersions
regardless of the composition of the phases (solid, liquid or gas) would fall within the
Commission definition provided the size fell at the lower end of the range (< 100nm).
In the Commission’s technical information to support the recommendation (i) it is stated
that:
“the Commission definition of “nanomaterial” is limited to materials consisting of particles
(excluding non-particular materials such as proteins or micelles as present for example in
mayonnaise)”. Further, it clarifies that the “the Recommendation only concerns particulate
nanomaterials”.
A particle is defined (i) as “a minute piece of matter with defined physical boundaries”.
Matter is commonly understood to be any substance which has mass and can exist in
various phases: solid, liquid, gas or plasma.
Mayonnaise is a classic example of a colloidal dispersion known as an emulsion or
micro-emulsion where two liquid phases are separated by a layer of surfactant. A micelle
or micro-emulsion has mass and defined physical boundaries, e.g. its size can be
measured. However, the guidance document states that such a system does not fall
within the definition.
Micelles are noted in the answer to Q3 of the Commission Questions and Answers
supporting document as an example of “non-particular materials” to which the definition
does not apply. Since, as noted by the European Commission-Joint Research Centre (v)
particles may be liquid it may be inferred that the exclusion of micelles from the definition
is based on the fact that micelles are a manifestation of the assembly of surfactant
molecules within a liquid dispersion medium and can exist only in this form; a micelle
cannot be isolated as a particle.
In the same way that micelles cannot be isolated, the metal carbonate core of an
overbased additive micelle cannot be isolated as a particle. The metal carbonate is
synthesised in situ by reaction of a metal hydroxide or oxide with carbon dioxide in the
presence of the stabilising surfactant. The metal carbonate exists in this form only within
the micelles of the colloid. Attempts to isolate the core material result in precipitation of
the metal carbonate as coarse sediment.
In summary, the structure of an overbased additive is a micellar system with a surfactant
layer dispersing the core material. Such systems are excluded from the scope of the
definition. The metal carbonate core of the overbased additive is part of and entirely
dependent on this micellar system. On this basis overbased additives do not fall within
the definition of a nanomaterial according to the EU Commission Recommendation.
(i) Commission Recommendation of 18 October 2011 on the definition of a nanomaterial
(2011/696/EU), http://eurlex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2011:275:0038:0040:EN:PDF
(ii) http://ec.europa.eu/environment/chemicals/nanotech/index.htm
(iii) Chemistry & Technology of Lubricants, Eds. R. M. Mortier & S. T. Orszulik Blackie,
(London), 1992
(iv) Automotive Lubricant Reference Book, A. Caines and R. Hancock, SAE, 1996
(v) EC-JRC (2010) Considerations on a Definition of Nanomaterial for Regulatory
Purposes. JRC Reference Reports
http://ihcp.jrc.ec.europa.eu/our_activities/nanotechnology/report-definition-nanomaterial
Further information:
Markovic, I.; Ottewill, R. H.; Cebula, D. J.; Field, I.; Marsh, J. F. Colloid and Polymer
Science (1984), 262(8), 648-56