The Basic Chemistry of Heat Seal Coatings

Design Inputs

The first design input to consider is the activation temperature. This is defined as the minimum temperature the heat seal coating must be exposed to before it can exhibit tack and adherence to the substrate. In addition to temperature, design inputs for seal pressure and dwell time are also important. Seal pressure is a measurement of pressure applied onto the coating during sealing, and dwell time accounts for temperature exposure time and how long pressure is applied during sealing. Collectively, these are known as sealing conditions. 

The next design inputs to consider are the substrates that the heat seal will be coated onto and the substrates that the coated film will be sealed to. The desired peel force is another design input to note. Peel force is usually measured in grams/inches and is a measurement of the force needed to open the seal applied to the package. The adhesive mode of failure required should also be considered.

The final two design inputs are specific to certain types of heat seals. First, it is important to note the requirement of hot tack, which is the actual strength of the heat seal when heated during the sealing process. This is critical for bottle applications where the shape may change during the cooling process. Second, it is prudent to know if the application or storage conditions create a specific blocking requirement. Blocking can occur when the heat seal coating begins to adhere to the top side of the film it is applied to in the roll form.

EVA Copolymers and Terpolymers

With the above design inputs in mind, basic resin systems used in heat seal coatings can be compared. The broadest chemistry used in heat seals is EVA (ethylene vinyl acetate) copolymers and terpolymers. These polymers are available in water-based and solvent-based carriers and are typically used in food, medical, and industrial packaging. Medical packaging applications usually require breathable EVA-based heat seal coatings that can facilitate EtO and gamma sterilization processes. EVA-based heat seal coatings are also widely used for in-mold labeling as well as shrink and heat transfer label applications. Solvent-based EVA heat seal coatings are especially useful in hot fill condiment packaging. In general, EVA-based heat seal products have broad compatibility with various apply-to and seal-to substrates, where there is often good adhesion to olefins, polyester, paper, polystyrene, PVC, HIPS, and many others.

If higher chemical resistance is needed, other chemistries may be considered. EAA (ethylene acrylic acid) polymers are available in a water-based carrier with higher grease and oil resistance, improved moisture resistance, and excellent hot tack properties. They are often used in aseptic packaging and as primers for foil. SURLYN™ ionomers may also be used in similar applications to the EAA products. Unlike EVA polymers, the seal-to and apply-to substrates for EAA and SURLYN™ ionomer-based coatings are narrower, typically limited to foil and paper. Nylon, PET, and PVDC-coated HIPS may be included as appropriate substrates. 

Vinyl, Modified Vinyl, PET, and Modified Polyester Polymers

Vinyl polymers are predominantly used as foil primers or wash coats to prevent corrosion. They can also be used for heat seals, offering good chemical, oil, and moisture resistance. Modified vinyl polymers are seen in pill blister packaging and provide good adhesion to foil, paper, and PVC blisters. PET and modified polyester polymers are used in heat seal coatings. These products will offer the highest heat and chemical resistance for aggressive food packaging, retortable packaging, and dual ovenable applications. PET and modified polyester coatings are more restrictive in their seal-to and apply-to substrate list, mainly compatible with PET, foil, PVC, APET, CPET, and ETP steel.

Conclusion

Many heat seal coating chemistries are available, from the carrier system to the types of backbones in coating polymers. Understanding these and knowing the design inputs of the heat seal’s final packaging can help guide a converter in the correct direction. A list of Bostik's heat seal coatings can be reviewed here. Working closely with the heat seal coating supplier for these choices is recommended. This will allow the converter to optimize their heat seal choice based on their applications and equipment assets. Click below to talk to a specialist at ChemPoint for Bostik's heat seal coatings.