When a door or window is painted, a goal is often to return it to service quickly, without sticking to the jamb or a frame. In some cases, the door or window will adhere to the jamb or frame when closed. Upon opening, paint transfer or peeling will occur at the weakest point, with one of the films possibly tearing and ending up attached to the opposite part. This is a relatable example of paint blocking.
Blocking resistance is the ability of a paint, film, or wrap to avoid detrimental adhesion between two surfaces—to not stick to itself or another surface upon contact when pressure is applied.
In the manufacturing industry, the time needed to achieve blocking resistance can have a significant impact on a company’s throughput and production costs. This affects how fast a manufacturer can get products coated and then packaged for shipment. If the product is tacky and sticks to another surface, it can’t be stacked or packaged for shipment. For example, precoated exterior sheathing is stacked several feet high on pallets. This creates significant pressure within the stack and extreme contact of one product against another. If each piece of sheathing doesn’t have block resistance, the stack may be ruined.
There are manufacturing techniques, such as baking, that can be used to minimize time to recoat or packaging. Heating or curing allows the film to reach its final state faster. At a high level, if film is not coalesced properly or there isn’t proper film formation, the coating won’t deliver all expected properties, leaving it susceptible to damage. When it comes to blocking, that would mean that if the film is not dried/cured completely, the tackiness could make it more prone to adhere to another surface. So, if there’s a propensity for blocking when the film isn’t completely dry/cured, the bake can get to block resistance faster.
There are multiple standard ASTM test methods to test blocking resistance of paint film, plastic film and even leather that are performed in a lab during the development process to understand how the final product will perform during manufacturing or sale. These tests include Test Method D 4946, Test Method D 3003, Test Method D 2793 and more.
As an example, D 4946 tests face-to-face blocking resistance of architectural paints. In this test, a film is cast and left to dry for a set period of time. Small panels/coupons are cut out of the film and placed face-to-face in an oven with weight on them to apply pressure for a set amount of time. After cooling, the two panels are peeled apart. The degree of blocking is rated subjectively for tack or seal using a series of standard descriptive terms corresponding to numerical ASTM values of 10 to 0. If the panels are fully adhered, that is complete failure. If they can be pulled apart, but there’s paint delamination, that’s another level. If they can be removed but are a little tacky, that’s another level.
Formulating for Block Resistance
Formulations and polymers both influence blocking resistance. A polymer’s glass transition temperature (Tg)—the temperature at which material becomes brittle like glass- can influence this property. Higher Tgs mean harder polymers, which are typically less prone to blocking than those with a lower/softer Tgs. Polymer selection is only one aspect to consider when formulating a product. The end application may dictate the approximate Tg of the polymer needed. For instance, most metal and wood flooring applications use harder polymers for increased abrasion resistance. On the other hand, roofing applications need to have flexibility at a low temperature when field applied, so the polymer needs to be softer.
In instances where softer polymers or improved blocking is required, there are multiple formulation approaches that can be taken. Crosslinking the formulation will make it harder and tougher, usually imparting improved block resistance. Surface modifiers, like Lanco™ Surface Modifier additives or fillers can also be added. Either technique changes the topography of the coating’s surface, improving its tendency to block.
Contact your Lubrizol representative to learn more about Lubrizol polymers and surface modifiers and techniques to deliver blocking resistance.