Fluorinated Oxetane Oligomers as Versatile Intermediates for Polymer Modification
March 1, 2008
Fluorinated materials in many forms are used in coatings formulations. For example, fluorosurfactants are used as wetting, flow and leveling aids due to their low surface tension among other attributes. Fluoropolymers are legendary for their robustness to most forms of assault in addition to providing very low coefficients of friction and dielectric constants/refractive indices.
Fluorinated intermediates that can be used to modify polymers have had less commercial penetration. The single largest use is textile repellents. Part of this is due to limited structural diversity and, more recently, international concern regarding the use and environmental fate of PFOS-, PFOA- and telomer- ((-CF)nF n- » 8) type materials and their possible degradation products. This concern clouds the future of fluorosurfactants, fluorochemicals and intermediates currently on the market.
In the 1990s, OMNOVA Solutions developed a line of fluorochemicals based on poly(oxetane) chemistry and short (£ C4F9) perfluoroalkyl (Rf) groups. These fluorochemicals, marketed under the PolyFox brand name, are differentiated by the use of (a) oligomeric/polymeric architectures and (b) short Rf groups. The combination of these two factors has several technical and environmental advantages. Typically, small molecules with short Rf groups do not have the low surface tensions and rheological properties to effectively or efficiently provide attributes such as wetting, flow and leveling when added to a coating. Attaching short Rf groups to a polymer imparts significant entropic freedom to pack effectively and efficiently at a coating surface and, therefore, reduce surface tension beyond what a single short Rf group could achieve alone. Secondly, sufficient data exists to show that persistence, bioaccumulation and toxicity (PBT) concerns decrease significantly and are close to nil when Rf £ C4F9.
IntroductionThe basic poly(oxetane) chemistry platform is shown in Figure 1. Synthesis involves cationic ring-opening polymerization of a fluorinated oxetane monomer using a hydroxyl-functionalized monomer and a Lewis acid catalyst. Shown in Figure 1 are mono- and difunctional intermediates. Excellent control of molecular weight and polydispersity are easy to achieve. The synthetic scheme from Figure 1 can be extended readily to other hydroxyl-containing materials including polyhydric alcohols, as shown in Figure 2.
These intermediates can be used alone or in combination with other types of chemistry as basic building blocks for dendrimers or hyperbranched polymers to raise molecular weight without the concomitant increase in viscosity observed with linear polymers.
Terminal hydroxyl groups on the poly(fluorooxetanes) can be transformed into a host of other functionalities using routine synthetic techniques. These modifications include carboxylic acids, amines and esters. The diversity of poly(fluorooxetane) terminal group chemistry is such that intermediates that are able to be used to produce polyurethanes, polyesters and polyamides, etc., can be readily designed.
Additional modification steps can be used to produce reactive monomers such as acrylics. Examples are shown in Figure 3, where a poly(fluorooxetane) terminated on each end with hydroxyl groups, is coupled to a hydroxyacrylate using isophorone diiso-cyanate.
The preparation of block copolymers using monomers such as acrylates is difficult and expensive, requiring rather exotic synthetic schemes. A marked advantage of cationic, ring-opening polymerization is the ability to prepare block copolymers as readily as random copolymers. Like dendrimers or hyperbranched monomers, poly(fluoroxetane) block copolymer intermediates can be used to provide novel attributes to other polymer systems. An example of a poly(fluorooxetane) block copolymer intermediate is shown in Figure 4.
ApplicationsOMNOVA Solutions uses poly(fluorooxetane) intermediates in several commercial products. Memerase® dry erasable wallcoverings employ a coating that provides a surface that accepts dry erase ink, yet is able to have the dried ink removed easily with standard erasers. The coating is based on a poly(fluorooxetane) intermediate esterified with adipic acid. The hybrid intermediate is then used in a polyester synthesis to form the base resin. Not only did the surface prove to be dry erasable, other valuable attributes of the poly(fluorooxetane)-modified polyester coating were noted. In addition to lowering the coating surface tension necessary to formulate a dry erasable surface, the modified polymer became a wetting, flow and leveling agent in situ and allowed for much higher gloss coatings. The performance of the poly(fluorooxetane) modified coating was such that manufacturing could increase solids (lower VOC), reduce emissions and still prepare a superb, high-gloss product.
Surf(x)® films are used in the production of laminates for furniture. Those films employ a poly(fluorooxetane)-modified polyester that can be reacted with an amine resin in a two-stage technology. Product attributes desired of the poly(fluorooxetane) intermediate include a high-gloss product, less staining and easier cleanability.
Figure 5 shows an example of a hyperbranched poly(fluorooxetane) intermediate. The propenyl ether-modified poly(fluorooxetane) was used as a monomer in a cationic, photopolymerization reaction. The polymerization reaction proceeded with high yield and produced a coating with a markedly reduced surface tension.
In another example, poly(fluorooxetane) monofunctional hydroxyl intermediates were used in cationic photopolymerization of epoxy resins. In a glassy resin, the addition of the poly(fluorooxetane) intermediate lowered the glass transition temperature while maintaining viscoelastic moduli and desired mechanical properties. Furthermore, the presence of the poly(fluorooxetane) intermediate resulted in a substantial lowering of the resin surface tension as estimated by contact angle measurements.
ConclusionsThe OMNOVA poly(fluorooxetane) platform has been demonstrated to produce a wide variety of functional intermediates that can be used alone or in conjunction with other chemistries to modify polymers. Attributes observed include increased wetting, flow and leveling of the coating, lower dielectric constant/refractive index and reduced surface tension. Poly(fluorooxetane) intermediates can be modified readily to introduce a host of other reactive functionality including carboxylic acids, amines and acrylates. These intermediates can then be incorporated onto most commercially viable polymers of the condensation or radiation-curable type.
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