Claude Nootens – LuerChem LLC1
Introduction
Styrene allyl alcohol (SAA) copolymer is a hard, low molecular weight resinous polyol containing primary hydroxyl groups and aromatic structures. The SAA copolymer is produced by radical polymerisation of styrene and allyl alcohol monomers (Figure 1).
Fig. 1
Synthesis of SAA copolymer
This polyol copolymer was originally developed by Shell and patented in 19522. Several grades are commercialised depending on the amount of styrene and allyl alcohol used. The major grade (SAA-100™) has a styrene/allyl alcohol ratio of 70/30, expressed in mole %.
The added value of using this polyol copolymer is that it can facilitate the preparation of high solids formulations with a faster cure for coating/paint, ink or adhesive applications3.
The resulting formulations can demonstrate specific enhanced performance with respect to adhesion, gloss, hardness-flexibility balance, colour acceptance/development, water/chemical resistance and corrosion resistance of metallic substrates. The SAA copolymer can be used as such or as an intermediate in the preparation of polyol resin derivatives (SAA terpolymers).
Styrene allyl alcohol copolymer
Firstly, SAA copolymer can be used as a vehicle in ink applications, more specifically as a film former for water resistant inks in ballpoint pens, for printing inks having excellent adhesion to the polystyrene, for overprinting coating, or for conductive dry toners.
Secondly, it can also be used as a binder or co-binder in thermosetting coating/paint formulations which use amino-formaldehyde resin or polyisocyanate as a hardener.
In the case of amino-formaldehyde hardener, a cross-linked network will be formed by the condensation of the hydroxyl groups of the SAA copolymer with the methylol groups of the hardener resin.
SAA copolymer can be present as a binder in coating formulations based on melamine-formaldehyde resin hardener. It can also be used as a co-binder with other hydroxyl-based binders such as polyester, epoxy, acrylic or alkyd resins, thanks to its excellent compatibility with these types of resin. Interestingly, low temperature cure can be achieved when SAA copolymer is pre-mixed/pre-condensed with melamine-formaldehyde resin, and therefore heat sensitive substrates such as wood can be coated.
Moreover, SAA copolymer can also be pre-mixed/pre-condensed with etherified phenol-formaldehyde resin and added to a coil coating formulation based on epoxy resin or polyester resin and melamine-formaldehyde resin.
In case of polyurethane coatings, the cross-linking reaction will occur through the reaction of the hydroxyl groups of the SAA copolymer with the isocyanate group coming either from diisocyanates or from blocked diisocyanates.
In this case, SAA copolymer can also be present as a binder or co-binder with other hydroxyl-based resins such as acrylic polyol or polyester resins.
Due to SAA copolymer being a solid product, it can be used in thermosetting powder formulations for coating applications. In addition, polyurethane adhesives, based on different types of technology (hot melt, powder, solvent-borne and waterborne) can be formulated with SAA copolymer or SAA copolymer derivatives.
Derivatives of styrene allyl alcohol copolymer
As well as using SAA copolymer as a binder or co-binder, several of its derivatives (SAA terpolymer) have demonstrated their potential in coatings, inks or adhesives. These derivatives can be prepared via the reaction of a certain percentage of their primary hydroxyl groups. The main reactions involving these hydroxyl groups are esterification, addition reaction of isocyanate and alkoxylation (Figure 2).
Fig. 2
Main reactions of SAA copolymer
Ester derivatives of styrene allyl alcohol copolymer
Of special interest is the esterification reaction with unsaturated fatty acids4. This gives a terpolymer which can be used as a binder, or as an intermediate for the synthesis of some specific resins. For example, further reaction with an anhydride compound can give alkyd resins or polyol resins with carboxyl acid groups, depending on the type/amount of anhydride compound used and the reaction conditions.
When phthalic anhydride is used, high solids solvent-borne alkyd resins can be prepared, which have a practical handling viscosity, due to the reduction of the polydispersity of the molecular weights5.
When trimellitic anhydride or maleic anhydride are used, carboxylic acid functional resins can be obtained. Neutralisation of these resins will result in waterborne SAA-based resins. Moreover, additional carboxylic acid functionality can be incorporated via radical polymerisation of a monomers mixture containing (meth)acrylic acid monomer (Figure 3).
Fig. 3
SAA-based resins
The esterification reaction of SAA copolymer can be realised using carboxylic acids such as benzoic acid, ε-hydroxycaproic acid, or anhydrides (e.g. trimellitic anhydride); or by a transesterifi-cation reaction, using for example: ε-caprolactone, ethyl acetoacetate, methyl cyanoacetate. The resulting SAA-based ester terpolymers can then be formulated with a hardener to make thermosetting coatings.
UV curable coating compositions can be developed based on SAA ester terpolymer resulting from the esterification of SAA copolymer with (meth)acrylic acid or alkyl (meth)acrylate, or in case of solid UV curable composition with β-mercapto propionic acid.
Polyester resins can be also prepared using SAA copolymer and then be cured with melamine-formaldehyde resin hardener. Of special interest, are the phosphorous-based ester derivatives of SAA copolymer. Phosphate esters based on SAA copolymer/epoxy resin or SAA copolymer/alkyl phenol can be used as anti-corrosion primers and/or adhesion promotors (Figure 4). Additionally, the phosphate esters based on SAA copolymer/alkyl phenol can be neutralised and used as aluminium pigment passivators for waterborne coatings.
Phosphonate derivatives of SAA copolymer (Figure 5) can also be neutralised and used as aluminium pigment passivators.
Fig. 4
Phosphate derivatives of SAA copolymer
Carbamate derivatives of styrene allyl alcohol copolymer
Reactions of SAA copolymer with specific mono-isocyanates can be used to introduce new functionalities. For example, unsaturation, which allows the resulting resin to be cross-linked by radical curing technology; or alkoxysilane which allows the resulting resin to formulate superhydrophobic coatings. SAA copolymer can be reacted with diisocyanate (MDI) to give carbamate-based SAA resin which can then be used as a binder in polyurethane coatings.
Alkoxylate derivatives of styrene allyl alcohol copolymer
Reaction of SAA copolymer with ethylene oxide (or propylene oxide) gives ethoxylated (or propoxylated) SAA-based resin. This resulting resin can then be reacted with polyisocyanate to form polyurethane coatings; or be further esterified with acrylic acid to give a polyacrylate SAA-based resin which can be formulated in UV curable ink formulations.
Fig. 5
Phosphonate derivatives of SAA copolymer
Conclusions
SAA copolymer is an extremely powerful building block to make high performance coatings/paints, inks or adhesives based on diverse technologies (high solids solvent-borne, waterborne, UV or powder).
This resinous polyol is therefore relevant for the development of formulations which cope with the ongoing market requirements for VOC reduction. Moreover, it is versatile as it can be used as such or as an intermediate to prepare, via different types of reactions (esterification, addition reaction of isocyanate or alkoxylation), specific high value types of polyol resin.
References
1. LuerChem LLC is the exclusive distributor of SAA-100™ for EMEA.
2. U.S. Patent 2,588,890 (1952)
3. Lyondell brochure LCC-PC79-9904 (1999)
4. Official Digest: Journal of Paint Technology and Engineering; October 1965, p 1251
5. JCT Coatings Tech, February 2004, page 40
