The Basic Chemistry Of Heat Seal Coatings
Heatseal coatings are supplied in a wide variety of chemistries and carriers. It is essential to have a basic understanding how each chemistry performs then match this to the various design inputs. This will rapidly help narrow down the various options and enable the converter to choose the best option for their needs. By first understanding and gathering all the design inputs allows the job of choosing much easier and makes for a more successful choice.
Let’s begin with the various design inputs since they will be mentioned as strengths and weaknesses as we talk about the various chemistry choices. All of them are important to understand. First input is activation temperature. This is defined as the minimum temperature the heatseal must see to begin to get tacky and adhere to the substrate its being sealed to. In addition to temperature the seal pressure and dwell time is also important to know. Collectively these are known as the sealing conditions. Next are the “apply to substrates” (the substrates the heatseal will be coated on) and the “seal to” substrates(the substrates that the coated film will be sealed to). In both cases list, all that apply. The next input would be the desired peel force(usually in grams/inch width)needed to open the package and the mode of failure required(IE coating split, clean removal off substrate or coated film etc). The last two design inputs are specific to certain types of heatseals. Hot tack is the actual strength of the heatseal when in the heated form during the sealing process. This is critical on bottle applications that may change shape during cooling process. And lastly if the application has a specific blocking test its important to know. Blocking can occur when the heatseal begins to adhere to the top side of the film its applied to in the roll form.
Next let’s begin to review, compare and contrast the basic resin systems used in heatseal coatings. By far the broadest chemistry used in heatseals are products based on EVA(Ethylene Vinyl Acetate) co and terpolymers. These polymers are available in waterbased and some solventbased carriers. These products are used in a wide variety of end uses from food and medical packaging to some Industrial packaging. The medical packaging applications are usually for breathable coatings used in EtO and gamma sterilization packages. Products are also widely used in IML(In Mold Labeling), shrink and heat transfer labeling applications. These products have a very broad apply to and seal to substrate list(see above). Good adhesion is often seen to olefins, polyester, paper, polystyrene, PVC, HIPS and many others. If higher chemical resistance is needed over the EVA products other chemistries may be looked at. EAA (Ethylene Acrylic Acid)polymers are available in waterbased carrier. These show 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 are often used in similar applications to the EAA products. Unlike EVA polymers the seal to and adhesive to substrate list is narrower and is often limited to foil and paper. Nylon, pet and PVDC coated HIPS may also be included.
When moving over to strictly solventbased carriers the EVA polymers have already been discussed above. The primary uses in those cases are labeling and food packaging especially hot fill condiments. Vinyl polymers are often seen and used predominantly as foil primer or wash coat to prevent corrosion. They can also be used as a heatseal offering chemical, oil and moisture resistance. Modified vinyl polymers are also seen in pill blister packaging offering good adhesion to foil, paper and PVC blisters. The last common chemistry are PET(Polyester) and modified polyester polymers. These are more restrictive in their seal to and apply to substrate list focusing on pet, foil, PVC ,APET and CPET surfaces and ETP steel. These products will offer the highest heat and chemical resistance for aggressive food packaging, retortable packaging and dual ovenable applications.
As one can see that there are many chemistry options available. From the carrier system to the types of backbones available. Having a very basic understanding of these will help guide the converter in the correct direction. Working closely with the adhesive manufacturer on these choices is essential. This will allow the converter to best optimize their heatseal choice for the range of applications and the equipment assets they have available.