Pigment & Resin Technology
Developments in raw materials part 5: Convertible media
J Boxall
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To cite this document:
J Boxall, (1986),"Developments in raw materials part 5: Convertible media", Pigment & Resin Technology, Vol. 15 Iss 11 pp.
16 - 18
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16 PIGMENT AND RESIN TECHNOLOGY November 1986
Developments in raw materials part 5:
Convertible media
by J Boxall
The binder or media component of surface coatings can lied films. Environmentally, high-solids coatings offer a
be subdivided into two broad groups, non-convertible route towards lower atmospheric pollution and conser-
and convertible, which differ in their mode of film forma- vation of natural resources.
tion. Binders of the non-convertible type do not undergo Media for conventional coatings use have high
chemical conversion reactions whenthey cure, and film molecular weights, and hence high viscosities, and
formation here involves loss or evaporation of a volatile require the addition of possibly substantial amounts of
solvent and concomitant deposition of the solid binder. solvents prior to application. The high-solids resins differ
Important members of this group are the rubber deriva- in being prepared w i t h lower molecular weights thus
tives and the many examples of vinyl copolymers; some enabling coatings to be formulated at application viscosi-
recent developments in the technology of these poly- ties. The low molecular weights can bring formulation
mers were described in the previous article in this series. problems however, and Ariss 5 of B and N Chemicals
This article will consider the convertible media used in describes some of those that can be experienced with
coatings, where film formation involves some form of high-solids polyester stoving resins.
chemical reaction. Commencement of this reaction is These problems largely relate to differences in curing
dependent upon either the presence of a suitable initia- rate and rheology. Experience with high-solids stoving
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tor, or by exposure to some form of radiant energy, which polyesters has shown that applied solids are often lower
causes the binder (in the form of a monomer or partially- than would be expected, this being due to loss of low
polymerised component) to rapidly polymerise. This molecular weight fractions of the base resin and/or
polymerisation reaction can take one of several forms volatilisation of low molecular weight by-products of the
though the result is the same; the binder and hence the crosslinking reaction. This type of problem can be
liquid paint is converted into a solid material which is reduced by use of highly reactive curing agents which
substantially insoluble in the original carrier solvent. ensures a more rapid reaction, but it is normally
This review will consider some of the recent literature expected that these stoving high-solids systems will
concerned with three important members of the convert- require more catalyst than conventional solids coatings
ible group of media; alkyd, epoxy and polyurethane in order to achieve equal cure rates.
resins. Differences in rheological behaviour can result in the
high-solids polyesters wetting the substrate less well, a
Alkyd resins problem which can be overcome by careful selection of
Certainly in volume terms alkyd resins must rank as the available solvent and flow control additives, in excessive
most important member of the convertible media. flow which can result in sagging during stoving, and in
Expressed simply alkyd resins are complex esters pigment settlement during storage. These latter two prob-
formed from the reaction between polycarboxylic acids, lems can be reduced by the use of suitable thixotropes.
polyhydric alcohols, and a fatty acid or oil. There is much Ariss clearly shows that these problems are not insur-
scope in alkyd technology for tailoring the resin to suit a mountable and that high-solids industrial stoving coat-
wide variety of end-use requirements, and also for the ings can be successfully formulated at up to 9 0 per cent
inclusion of novel raw materials. volume solids. Most importantly, user experience has
This latter aspect of alkyd technology is being actively shown that final film properties are very similar to those
explored by researchers in countries such as India of coatings formulated along conventional routes.
where indigenous alternatives to the traditional feed- Developments in oxidative curing (air-drying) high-
stocks are available. Recent publications have primarily solids alkyd resins have been described by Cornet 6 of
been concerned with oils and fatty acids, and described Synres International and Newton 7 of Cray Valley
the preparation and properties of both air-drying and Products.
stoving alkyds modified with materials such as epoxi- Cornet 6 outlines the properties of several recent high-
dised rubberseed oil 1 , epoxidised maroti oil 2 and blown solids additions to the Synresate range of alkyds. Exam-
karanja oil 3 . A recent paper 4 has also considered the ples from this range enable the formulation of brushing
preparation and properties of alkyds based on maleopi- viscosity retail paints with solids contents in excess of 75
maric acid derived from resin. per cent (by weight). The resins are slightly thixotropic
A n area of alkyd technology also currently receiving and this rheology results in excellent brushability, long
considerable attention is the production of resins for use open time, and good flow with minimal sagging. Film
in high-solids coatings formulations. This interest is in properties in respect of gloss and weather resistance of
response to economic and environmental pressures; in paints based on these resin types are stated to be excel-
economic terms the reduced solvent content of the lent, though some impairment of the rate of through
higher solids formulations results in savings in materials drying is possible.
ordering and storage, and, for industrial coatings, lower The problem of obtaining speed of drying in these low
energy requirements to evaporate solvents from the app- molecular weight high-solids alkyds can be overcome by
November 1986 PIGMENT AND RESIN TECHNOLOGY 17
the incorporation of aluminium complexes during resin media, which can be solvent-borne, water-borne, or
manufacture, and this approach has been outlined by solvent-free. Pigmentation can be of the conventional
Newton 7 . These complexes differ from conventional type though modification can be of value in situations
drier additives, which react at unsaturated sites in the where improved levels of water resistance and flexibility
resin molecule, and rely on reaction with terminal func- are required. For example, glass fibre variants, particu-
tional groups within the alkyd to promote crosslinking. larly of solvent-free epoxies, have been used where
As a result curing efficiency is improved and drying enhanced resistance to impact and hydrostatic pressure
times, particularly through drying, as well as other prop- was needed.
erties such as gloss retention and durability are all Moss 11 of Ciba-Geigy has also provided a useful
enhanced and match the performance level of paints summary of recent developments in the field of epoxy
based on conventional alkyds. Cray Valley Products have resins, and considers the various types which are used
two high-solids resins in their Synolac range of alkyds in civil engineering as well as those finding industrial
which are based on this technology, and Newton pro- applications in powder coatings, electro-deposition pri-
vides comprehensive data on formulation procedures, mers and ultra-violet curable coatings. Details are pro-
and on the properties of decorative white gloss paints vided in this paper of examples of epoxy resins and
based on these media. curing agents from the Ciba-Geigy range which can be
A n alternative approach to the preparation of high- used in many of these applications.
solids alkyds, as yet at the research stage, has been Many types of curing agent can be used with epoxies
described by Larson and Emmons 8 . This involves the and amongst the most important of these are the ali-
inclusion into the alkyd of a reactive vinyl monomer phatic and aromatic amines and polyamides, and their
which functions initially as a solvent but which converts adducts. A wide range of other materials also crosslink
to a binder during curing. with epoxies however, including cycloaliphatic amines,
Certain requirements for successful candidate reac- anhydrides, polysulphides, polyisocyanates and amino
tive diluents have been identified, and these include low resins. Two recent articles by Dunn 12 and Sinclair 13 of
volatility, toxicity and odour together with good compat- Anchor Chemicals have considered the types of curing
ability and stability. Dicyclopentenyloxyethyl methacry- agents that can be used with the ambient-temperature
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late has been found to satisfy thee requirements; levels curing epoxies.
in the range of 15-30 per cent have been found to be Formulations based on solvent-based epoxies (pre-
appropriate in an alkyd resin which itself must have pared from the resin in solid form) are generally cured
properties that render it satisfactory for a high-solids with aliphatic amines or polyamides, or their adducts.
resin system. The authors discuss in detail the chemistry Solvent-free and high-solis systems (based on liquid
of these novel resin systems, and present results from epoxy resins) are generally cured with aliphatic amines
testing programmes of trial formulations which show and adducts, modified cycloaliphatic amines and adducts,
enhanced levels of through drying, alkali resistance and aromatic amines, polyamides, and several speciality
petrol resistance, when compared with conventionall materials. Both authors offer very comprehensive advice
air-drying gloss paints. on selection of curing agents from the Anchor range,
together with much useful information of a general
Epoxy resins nature to assist coatings formaulators in this field of
Epoxy resins are normally made from epichlorhydrin and technology.
diphenylol propane and can be prepared in forms rang-
ing from very low viscosity liquids to high melting point Polyurethane resins
solids. The epoxies are capable of wide diversification Polyurethane resins are available in several forms nota-
and as well as possible variations in these basic raw bly single-pack moisture-curing, and two-pack types
materials, it is possible to modify the resin during manu- which can be ambient-temperature or stoving cured. As
facture with materials such as diluents and flexibilisers. for the epoxy media the polyurethanes resins are typi-
Furthermore, film formation of epoxy media is effected cally characterised by good resistance to chemicals and
by the use of curing agents and again there is wide scope a wide range of advantageous film properties, and tend
for modifying the properties of the cured system. to find their main applications in speciality high-
A n indication of the scope for creating new epoxy performance coatings.
resin can be seen in a paper by two researchers, Sawent Polyurethane resins are a class of convertible media
and Potnis 9 , who have developed resins which appear to produced by the reaction of materials containing an
have potential in industrial coatings. These workers isocyanate group and hydroxy-containing compounds
have reported the preparation of novel epoxy resins in such as polyesters, polyethers, on even water. The iso-
which the diphenylol propane constituent has been cyanate containing materials used in polyurethane resin
replaced by the lower cost isophthalic acid. The resins technology can be either aromatic or aliphatic in nature.
produced in this way were solid in character, and when Examples of aromatic types are toluene diisocyanate
blended with amino resins and stoving cured, possessed (TDI),diphenyl methane diisscyanate(MDI),dicyclohexyl
film properties which in respect of hardness, adhesion methane diisocyanate (HMDI) and isophorone diisocya-
and flexibility, were comparable to those produced from nate (IPDI), whilst hexamethylene diisocyanate (HDI) and
a diphenylol propane containing epoxy system. trimethyl hexamethylene diisocyanate (TMDI) are
A n indication of the several categories of epoxy sys- examples of aliphatic types. The aliphatic isocyanates
tems in general commercial use, and their properties, is tend to be of lower reactivity than aromatic types though
given by Cortellezzi and McManus 1 0 , with special possess superior colour stability and weathering proper-
emphasis on those types that are used by the civil engi- ties.
neering industry. The epoxies most favoured here are The properties of the isocyanate resins have been
based on the two-pack ambient-temperature curing described in a recent paper by Gaglioppa 14 of SAPICI
18 PIGMENT AND RESIN TECHNOLOGY November 1986
SpA, who also presents details of a new range of from ultra-violet degradation has limited the colour
polyurethanes tailored for the wooden furniture finish- availability to black or reflective aluminium. Such resins
ing industry. One of the key requirements here is a are also finding completely new applications as high
degree of film flexibility in order to accommodate sub- quality chip resistant underseals for vehicles, and
strate movement, and whilst polyurethane coatings though they tend to be more expensive than existing
based on aliphatic isocyanates such as HDI derivatives underseal materials their physical properties are far
ca be used in this application they are very expensive. superior.
Furthermore their low reactivity does not suit factory Hughes provides several starting point formulations for
finishing where speed of throughput is important. these various types of high-performance coatings, and
Gaglioppa describes the properties ofthree new iso- discusses the special procedures required for manufac-
cyanate resins from the Polyurene range. These are a turing paints based on moisture sensitive resins. This
mixed TDI-MDI-isocyanurate (an autopolymerised iso- paper will be of interest to all technologists concerned
cyanate), a polyol-MDI prepolymer, and a trimethylol with exploring the potential of moisture-curing polyure-
propane—TDI-MID prepolymer. All are stated to be able thane coatings.
to form the basis for low cost, fast curing, flexible, two-
pack polyurethane coatings for furniture, though due to References
the presence of the aromatic component in the resins 1 Dabhade, S B and Gogte, B B. Paint and Resin, 1983,
some yellowing discoloration of white and pastel fin- 5, 24-26.
ishes is a possibility. 2 Dabhade, S B and Gogte, B B. Paint and Resin, 1984,
There has been considerable interest in the recent 3, 48-49.
literature on the properties and uses of the single-pack 3 Dhake, P M and Gogte, B B. Paint and Resin, 1986, 2,
moisture-curing polyurethanes. In this system the resin 40-41.
is prepared as a prepolymer by reacting an excess of the 4 Shukla, M C and Vasishtha, A K. Journal of the Oil
isocyanate with a polyol material. As their name sug- and Colour Chemists Association, 1986, 3, 71-77.
gests film formation occurs by reaction of the unreacted 5 Ariss, A. Paint and Resin, 1983, 5, 38-39.
isocyanate in the polymer w t h atmospheric moisture to 6 Cornet, P. Polymers Paint Colour Journal, 1984,
Downloaded by RMIT University At 00:52 22 April 2016 (PT)
form the crosslinked polyurethane structure. It is this 4114,157-158.
unique featue which makes this media type so attractive 7 Newton, L Paint and Resin, 1983, 5, 33-36.
for applications where the use of other types of high- 8 Larson, D B and Emmons, W D. Journal of Coatings
performance coating might be restricted due to moisture Technology, 1983, 702, 49-56.
sensitivity during curing. Further advantageous features 9 Sawant, V D and Potnis, S P. Paint and Resin, 1984, 4,
of the moisture-curing reaction are the ability to proceed 32-34.
at sub-zero temperatures, and tolerance for damp 10 Cortellezzi, G and McManus, L D. Polymers Paint
substrates. Colour Journal, 1983,4104, 652-654, 660.
Jolly 15 has described in some detail the chemistry of 11 Moss, N S. Ploymers Paint Colour Journal, 1984,
the moisture-curing polyurethanes and considers the 4110, 7 - 8 , 1 2 .
health and safety implications of using coatings contain- 12 Dunn, P A. Polymers Paint Colour Journal, 1984,
ing isocyanates. The general properties and uses of 4125,559-566.
these resins are also described here, and also in other 13 Sinclair, J H. Polymers Paint Colour Journal, 1985,
articles 16 , 17 . 4157, 900-905.
Hughes 18 of Baxenden Chemicals has provided a 14 Gaglioppa, G. Polymers Paint Colour Journal, 1983,
comprehensive paper describing the techniques of for- 4086, 12-20.
mulating paints based on moisture-curing isocyana- 15 Jolly, A C. Polymers Paint Colour Journal, 1984,4125
tes. He categorises these media into two broad groups, 569-572.
" r i g i d " and "flexible"; rigid types have a highly branched 16 Hughes, A H. Journal of the Oil and Colour Chemists
molecular structure and produce hard films, whilst the Association, 1984, 9, 242-243.
flexible types are normally linear and have higher elon- 17 Anon. Polymers Paint Colour Journal, 1984, 4129,
gations producing softer films. Hughes stresses though, Euro Suppl, 60-64.
that these terms are relative and even the " r i g i d " grades 18 Hughes, A H. Journal of the Oil and Colour Chemists
can possess considerable flexibility. Association, 1986, 1, 10-14.
One of the most important applications for the "rigid"
grades of moisture-curing polyurethanes is in anti-
corrosion coatings, and here, for example, it is possible
to formulate primers based on red iron oxide, zinc dust or
micacous iron oxide. For topcoats darker colours are
preferred since the resins currently in use are usually
based on aromatic isocyanates which tend to yellow.
Heavy duty one-coat anti-corrosion paints have also
been produced using pitch as an additive. The rigid
polyurethanes also find applications in floor paints and
screeds and as concrete coatings.
The main application for the "flexible" polyurethanes
is in the formulation of moisture-curng roof membranes
where a seamless coating covering the whole roof can
easily be applied. Again the current trend is to use aro-
matic isocyanates and the need to protect the base resin
