Put simply, crosslinking involves a chemical reaction between polymer chains to link them together. Picture a bowl of spaghetti. Each strand is separate and can be pulled out of the bowl (if it doesn’t slip out of your fingers). With crosslinking, there are chemical reactions that take place among the pieces of “spaghetti” to bond them, so you can no longer pull out an individual strand.
Crosslinking can influence several end properties across most applications, including:
- Coating chemical resistance
- Polymer flow properties – block and print resistance
- Coating toughness
- Coating flexibility
- Coating abrasion resistance
Why does crosslinking affect these properties? There are several reasons. Polymer chains that are un-crosslinked are freer to move around than polymers that are linked together through crosslinking (like in the spaghetti analogy, where you can no longer pull out one strand without pulling out the entire bowl of spaghetti). Crosslinked or “bound together” chains will behave quite differently. They will not flow as much under stresses like pulling or heating. They will not swell as much in a solvent as unbound chains, which improves chemical/ solvent resistance. And, they will make a stronger/ tougher coating or binder.
The Importance of Understanding Formulation Requirements
Knowing processing, formulation and application requirements is important —and there’s always a tradeoff and a balance. For example, if abrasion resistance is achieved through a high degree of crosslinking, elongation or flexibility might be lost. You can’t go all in on one property without affecting the others. What are the cost targets for the coating? Are VOCs an issue? Can coalescents be used in the formulation? What other properties are important?
It is also critical to define what is trying to be achieved, such as will the end product be used inside or outside? Will it be exposed to sunlight or water? One coat or two coats? Direct to the substrate (wood, for example) or will there be a primer? How is it applied—roller, sprayer? Why does it need to be tough? A thorough understanding of these factors will help formulate the best crosslinking method for the application.
The Three Types of Crosslinking
Self-Crosslinking—1K vs. 2K
Crosslinking can be designed with 1K or 2K approaches. 1K systems self-crosslink, without any additional additives. 2K systems require an additive being included close to the point of application to make it crosslink. For example, this could be an isocyanate crosslinker added to a formulation right before application to get the correct type of reaction.
While many customers prefer 1K coatings that are ready to go, there are reasons to use 2K approaches, such as with wood floor coatings for example. There’s a precedent in the wood flooring market for contractors to put an additive into floor coatings at the time of application.
Another example is a filter paper binder application, where there might be multiple grades of filter paper that need different levels of crosslinking for various application uses. The polymer can be bought in bulk, then the customized level of crosslinker can be added for each of filter paper grades. It can be more convenient and cost effective because the level of crosslink can be customized while still only using one base polymer.
Polymer Reaction with Substrate
Going back to the spaghetti analogy, we can also make the “spaghetti” react with the substrate, not just one strand to the others. The polymer can be crosslinked directly with the cellulose if it’s a paper application or cotton in a textile application. The polymer isn’t necessarily only crosslinking with itself, it’s crosslinking with the substrate. This strong covalent chemical bond between the substrate and polymer is sometimes needed for different reasons, such as wash resistance (textiles), strong mechanical properties, high tensile resistance or chemical resistance.
Pre- vs. Post-Crosslinking
Polymers can be partially crosslinked as they are made or can be crosslinked post-application to the substrate. Some Lubrizol acrylic emulsions and PUDs have pre-crosslinking designed into them. But, our preference is to have the polymer mostly crosslink after application to a substrate. Using paper as an example again, the polymer would be applied to the paper and fused together after application. In many cases, the best performance occurs when most of the crosslinking happens after the product is applied to the paper.
As mentioned previously, the more information that is gathered up front, the better is our ability to recommend products with appropriate crosslinking. Choosing the right method of crosslinking is based on balancing several factors, including:
- Use conditions (for example, how it will be applied)
- Substrate (eg. paper, metal, plastic)
- Performance requirements (based on a discussion with the customer)
- Regulatory constraints
- Safety precautions
- Cost targets
- Cosolvents and film formation aspects (especially in paint and printing applications)
Formaldehyde or Formaldehyde-Free
In some forms of crosslinking, formaldehyde is released, which can create major challenges in terms of regulatory requirements and health safety issues. Lubrizol offers options for formaldehyde-free systems that can be tailored based on formulator needs.
It bears repeating—having a deep dialogue about many factors is essential to get to the crosslinking product recommendations that would be most suitable for a variety of conditions, substrates, uses, regulatory issues and more.
Contact your Lubrizol account manager with additional crosslinking questions or for help choosing the most appropriate crosslinking method.