Exploring the chemistry behind silicone-based nonstick additives.
by Thorsten Schierle*
The need for certain non-stick properties is present in numerous applications of everyday life, as well as in industrial applications1. Depending on the nature of these applications, different levels of non-stick effect are desirable, and the requirements regarding the durability of the non-stick effect vary substantially. Therefore, it makes sense to carefully consider special additives, which produce this effect.
Image 1: Proof of the non-stick effect (cover photo)
In the first section of this article, the chemical origin of this class of additives is explained. The second section explains the mechanism of action in usual applications of non-stick additives, as well as the basic effects of different types of silicone-based non-stick additives. Finally, the basic principles of the formulation are set out.
Chemical origin
The simplest process for obtaining a silicone non-stick additive is to obtain polydimethylsiloxane (PDMS) from dichlorodimethylsilane (which can be easily obtained through the industrial-scale synthesis of Müller-Rochow 2 ) by means of a hydrolysis-condensationreaction 3 (see Figure 1 and 2).
Figure 1: Synthesis of dichloromethylsilane through Müller-Rochow synthesis
Figure 2: Synthesis of polydimethylsiloxane through a hydrolysis-condensation reaction
The PDMS obtained usually consists of 30-50 repetitive units. To further adjust the polymer, balancing processes are used. Under the influence of catalysts, PDMS polymers are forced into a condensation reaction (water removal), this produces a PDMS polymer with a greater number of repeating units4.
Figure 3: Condensation reaction of polydimethylsiloxanes
The condensation reaction concludes with the addition of "encapsulators at the ends", such as (H3C)3Si-O-Si(CH3)3. Thus, the number of repetitive units and the molecular weight of the PDMS are controlled (see Figure 3).
Figure 4: Completion of the condensation reaction of polydimethylsiloxanes through "encapsulators at the ends"
Likewise, the culmination of condensation can also be achieved with monofunctional chlor-alkylsilanes5 (see Figure 4).
Figure 5: Conclusion of the condensation reaction of polydimethylsiloxanes by monofunctional chloroalkylsilanes.
The PDMS obtained through these synthesis processes are non-crosslinkable. This generates a migration tendency influenced mainly by the molecular weight of the molecule, the compatibility of pdMS with the formulation of the coating and the formulation of the coating itself. The simplest way to modify molecular weight was described in the previous section. The compatibility of silicone non-stick additives can be modified by changing two parameters: the length of the siloxane chain and the degree of organic modification. As for the first parameter, the non-stick effect increases as the length of the siloxane chain increases. At the same time, compatibility decreases.
As for the second parameter, compatibility increases as the degree of organic modification increases. In addition, the better the coincidence of the organic modification with the polarity of the coating formulation, the greater the compatibility. To obtain cross-linked siloxanes, acrylate functionalities are introduced into polymer6. In principle, two main options for modification are possible:
- Modification in the fundamental chain of the polymer chain, which produces comb structures
- Modification at the ends of the polymer chain, which produces a modification α,ω (see Figure 6)
Figure 6: Polydimethylsiloxane with modification of α,ω (organic modification in blue, acrylate groups in purple).
The incidence of adequate organic modification in compatibility becomes very evident when comparing mixtures of modified PDMS/water and pure PDMS/water. The water mixture with pure PDMS has a cloudy appearance, while the water mixture with properly modified PDMS remains crystal clear. Figure 2 illustrates this effect.
Image 2: Water mixtures with pure PDMS (left) and with properly modified PDMS (right)
Mechanism of action
As explained in the first section, silicone nonstick additives can be divided into two groups: non-cross-link and cross-link non-stick additives.
Figure 7: Coating film with non-cross-link non-stick additive, covered by adhesive tape.
In a standard nonstick coating, the non-cross-linking non-stick additive is located, in part, on top of the coating surface. When, for example, a label is placed on this coating, the non-cross-linking non-stick additive (over time) migrates to the adhesive layer; therefore, the non-stick effect is adversely affected.
Figure 8: Coating film with non-cross-slip additive, covered by adhesive tape; migration of the non-stick additive to the adhesive layer.
If a cross-linking nonstick additive is used, most of the molecules of the additive will be permanently anchored due to cross-linking of the double bonds of the attached acrylate groups. When, for example, a label is placed on this coating, the molecules of the cross-linked non-stick additive remain in place and do not migrate to the adhesive layer; therefore, the non-stick effect maintains a high level of performance for a long time.
Figure 9: Anchored cross-link nonstick additive coating film, covered by adhesive tape
Strategy in formulation
Cross-linking nonstick additives are important when long-lasting nonstick effects are needed in printing applications. As discussed in the previous section, the possibility of incorporating the non-stick additive permanently into the coating or in the ink layer through the cross-linked acrylate groups offers an important advantage when durability is desired. The acrylate groups form a network with the binder in the radiation curing, thus minimizing the tendency of the additive to migrate. In this way, longer-lasting surface effects can be achieved.
Frequent applications of cross-linking non-stick additives:
- Applications where a surface is removed (for example, the section of a lottery ticket in which the surface is scraped)
- Applications where a surface is peeled (e.g. non-stick for easily opened labels or packaging)
- Applications to prevent jams (for example, prevent jams in stackable packaging)
In a formulation, the main goal is to level the nonstick properties and maintain a smooth surface. A common strategy is to use a combination of non-stick radiation curing additives.
Usually, additives that produce good leveling properties are combined with additives that produce excellent non-stick properties. Depending on the formulation and polarity of the system, different combinations and proportions of radiation curing additives are necessary. The most common method is to first choose the appropriate nonstick additive, and then choose the right combination to achieve the desired leveling.
Figure 10: Comparison of the non-stick effect of different TEGO® Rad additives
Figure 10 illustrates the non-stick effect of various additives. Therefore, if maximum anti-adhesion is desired, the Rad 2800 is the ideal product.
About radiation curing additives
Radiation curing additives are a range of modified silicone-based additives with organic modification. They have acrylate groups, so they are cross-linked, which gives them unique properties. Depending on the silicone character and the degree of organic modification, they improve sliding, substrate wetting, prevention of crater formation, scratch resistance and leveling. Moreover, some of them have non-stick and antifoaming properties.
Summary
Rad 2800 is the ideal non-stick additive for printing inks and varnishes. This product exhibits the greatest non-stick effect. The Rad 2800 has a pronounced silicone character, combining strong hydrophobic properties with optimal compatibility with the system.
Radiation curing additives are unique because they can be cross-linked in the coating; the resulting sliding and non-stick effects are particularly durable. With conventional additives, the non-stick effect is much weaker and less permanent because the additives are not cross-linked in the coating.
The use of the Rad 2200 N, 2250 and 2300 allows those responsible for the formulation to achieve a good level of slippage and flow in printing varnishes, which produces a defined surface smoothness and good haptic properties. Due to its excellent compatibility, its use does not impair transparency. The Rad 2500, Rad 2650, and Rad 2800 stand out mainly for their good non-stick effect, so the adhesive strips can easily detach without leaving residue.
Footer
1. See: Glöckner et. Al.: Radiation Curing Coatings and Printing Inks; Hannover: Vincentz Network, 2008 (1), p 142ff.
2. See: Koerner et. al. : Silicones; Essen: Vulkan Verlag, 1991 (1) p. 9-15
3. See: Brook: Silicon in Organic, Organometallic and Polymer Chemistry; New York: Wiley & Sons, 2000 (1), p. 258ff.
4. See: Brook: Silicon in Organic, Organometallic and Polymer Chemistry; New York: Wiley & Sons, 2000 (1), p. 261ff.
5. See: Klotzenburg; et al.: Polymere; Berlin: Springer, 2014 (1), p 204s
6. Glöckner et. Al.: Radiation Curing Coatings and Printing Inks; Hannover: Vincentz Network, 2008 (1), p 98s
* Thorsten Schierle is head of the inks division for applied technologies, digital applications and new applications at Evonik Resource Efficiency GmbH in Essen, Germany. Prior to joining Evonik in 2011, he worked for more than 11 years at Sun Chemical/DIC's liquid printing inks plant in Niederhausen, Germany, where he mainly dealt with inks for water-based packaging and inks for decorative applications (wallpaper, furniture, etc.). He studied chemistry in Mainz, Germany, and in Toronto, Canada.
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