copolymerization of silicone or polyether macromers with suitable monomers.
Authors: Dr. Guillaume Jaunky, Albert Frank, Dr. Jürgen Omeis
Macromer
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substrate wetting, slip, and oil and water repellency.
Compared to conventional silicone-based additives with 30-60% silicone, silicone-modified polyacrylates have a significantly lower silicone content, typically in the range of 2-15%. The significantly greater effect and orientation is due to the orientation of the silicone chain to the coating surface. With conventional silicone-based additives, the silicone chains are oriented towards surfaces in the form of loops. With silicone-modified polyacrylates, on the other hand, the silicone chains can be oriented optimally with their free ends, which gives them significantly greater surface activity (Fig. 5).
Utilization as an easy-to-clean additive:
BYK®-SILCLEAN® 3700
With high silicone content in combination with a slightly incompatible polyacrylate backbone, the polymer is strongly oriented towards the surface, and with relatively high frac-tions of silicone, it provides strong oil and water repellency and significantly enhanced slip.
This results in a significant increase in surface slip, which in turn enhances scratch resistance. The strong surface orien-tation of the silicone chains creates very strong repulsion effects against water and contaminants such as oil and dirt.
Moreover, this additive can also be used to equip coating systems with anti-graffiti properties (Fig. 6) and significantly enhance chemical resistance.
The reactive groups in the polymer can be used to anchor the additive in the coating system to guarantee that the effects are maintained over time. Moreover, the combination of silicone content and polyacrylate chain results in very good leveling of the coating surface.
Utilization as an anti-cratering additive in automotive coating systems: BYK®-3550
The silicone and acrylate-based additives typically used in a large number of modern automotive coating systems today are reaching their limits. Silicone additives with a moderate impact on surface tension provide good leveling, for example, but are not particularly well suited for achieving enhanced anti-cratering and substrate wetting characteris-tics. In order to obtain these properties in the coating, manufacturers mainly use active silicone additives. How-ever, these can have a negative impact on recoatability, intercoat adhesion, and leveling.
This often leads to problems, for example, when protective foils detach from new vehicles during transport. When this happens, they no longer perform their protective function and can even create hazardous conditions if they end up on the road or get caught in the railway system’s overhead power lines.
This led to the development of BYK®-3550, a silicone- modified polyacrylate characterized by low silicone content
Fig. 6: Easy-to-clean properties by using BYK®-SILCLEAN® 3700; two-component polyester coating, left panel: without additive, right panel: with additive.
The felt pen marks on the right panel can be easily wiped off with a dry cloth.
The right panel can be easily cleaned by simply flushing the surface with tap water.
Silicone-modified polyacrylates
Silicone and polyacrylate additives (Fig. 3) are used in coatings and paints for a variety of reasons. They can be used to help wet difficult substrates, i.e. non-polar or those that are contaminated, and to optimize a coating’s leveling or make coating surfaces smoother and more scratch- resistant.
The silicones rarely consist of pure, unmodified polydimeth-ylsiloxane, but rather they are almost always modified through the addition of coating-compatible groups such as polyether, polyester or alkyl structures.
Due to their very low surface tension, silicone additives exhibit a strong affinity to interfaces, i.e. to the substrate, to the coating surface, and between the different coating layers in a multi-layer coating system. The saturation at interfaces can influence both the surface tension of the wet coating and the dried coating film. For this reason, silicone additives are primarily used as substrate wetting and anti-crater additives.
Polyacrylates, on the other hand, have a relatively high surface tension and typically have little or no influence on the surface tension of coatings. Their effect has more to do with partial differences in surface tension at the coating surface, which results in improved leveling.
Silicone-modified polyacrylates are manufactured via radical polymerization of conventional acrylic monomers with so-called silicone macromers that essentially consist of a precisely defined chain of polydimethylsiloxane (Fig. 4).
The chemistry of silicone-modified polyacrylates is highly variable and can be adjusted as necessary to meet specific application requirements. The composition of the polyacryl-ate chain can be varied by incorporating different mono-mers. This can be used to control the polarity of the polya-crylate block and the compatibility of the additive with the coating system. The polyacrylate chain prevents cratering when the additive is used in coating systems and enhances leveling. Functional monomers can be used to incorporate additional reactive groups such as hydroxyl, epoxy, and carboxyl groups into the polymer. These reactive groups can then be used to anchor an additive in the binder matrix, and thus in the interface. By contrast, an additive without reactive groups remains mobile and can migrate to new surfaces during the recoating process. This can eliminate many problems regarding intercoat adhesion.
The silicone part of the silicone-modified polyacrylates can be varied both in terms of the length of the silicone chain and, of course, in the amount of silicone macromer. The strong incompatibility of the silicone chain reduces the surface tension of the coating and, depending on the amount of silicone, provides enhanced anti-cratering,
Fig. 5: Surface orientation of silicone-modified polyacrylates vs.
conventional silicone-based additives.
Polysiloxane macromonomer Siloxane chain
Polyacrylate chain Polyether chain Silicone content approx. 1-15%
Silicone chain maintains free mobility at one end for optimal orientation to the interface
Silicone content approx. 30 - 60%
Silicone chain is interrupted by polyether modification and cannot be optimally oriented
Silicone-modified polyacrylates Polyether-modified silicones
Conclusion
Polymeric additives based on macromer technology repre-sent a new class of surface-modifying additives. Versatile chem istry and modular molecular structure make it possible to adjust their properties for specific applications.
The macromer technology is an important innovative tech-nology that BYK is using on an industrial scale to develop customer-orientated, surface-active additives. The current research is focused on the design of surface additives specifically designated for solvent-based, waterborne, powder, and UV-curable formulations.
Fig. 7: BYK®-3550’s low influence on surface energy of the dry film improves adhesion of automotive protective foils and adhesives compared to conventional silicone additives.
and a polyacrylate chain with high coating compatibility.
BYK®-3550 dramatically reduces surface tension similar to conventional silicone-based additives but without signifi-cantly affecting slip or surface energy (Fig. 7).
Polyether-modified polyacrylates:
first-generation hydrophilic additives
The incorporation of polyethylene glycol-based macromono-mers into poly(meth)acrylate backbones led to the first gen-eration of hydrophilic additives that increased the surface tension of coatings, meaning rendering them hydrophilic.
This effect is required to obtain fogging-free surfaces, im-proved overcoatability with waterborne formulations or to realize the hydrophilic approach to self-cleaning surfaces.
The first additive, BYK®-3933 P, was especially developed for powder coating formulations. Its performance was optimized for leveling, transparency, gloss, and improved overcoatability of the resulting coatings (Fig. 8).
Innovative macromers
The first product developments were based on commer-cially available macromers. Because only a few macromer raw materials with limited molecular weight range are
available, BYK launched its own macromer research in or-der to use in-house developed products and processes to produce macromers on an industrial scale.
BYK also went one step further by developing innovative patented technologies based on hyperbranched hydrophilic macromers. This unique macromer structure (Fig. 9) is conducive to outstanding properties and crystallization behavior (Fig. 10), allowing the development of specific surface-active additives that are able to self-concentrate at the air-coating interface.
Fig. 8: Improved surface wetting using BYK®-3933 P (right side) v/s a standard polyacrylate (left side) in a powder coating base coat – overcoated with a waterborne blue paint.
Fig. 10: Crystallization behavior of copolymers made with standard monomers and a commercial linear polyether macromonomer (left) or with a hyperbranched polyether macromonomer (right) at different temperatures.
Fig. 9: Schematic representation of copolymers made with a unique hyperbranched macromonomer (left) and with a linear macromonomer (right).
Polyether chain
Polyacrylate chain
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