Primers are used to improve adhesion between two dissimilar substrates by modifying the surface chemistry. Several types of surface modifications can be used, including a chemical modification, a mechanical modification to increase the surface area (such as sanding the surface to improve the bond) and electrical modification (such as corona treatment).
In this article, we’ll concentrate on chemical modification using water-based primers. Chemical treatment can be a cleaning of the surface, such as vapor phase cleaning of the surface. Other chemical treatments can include chemical etching of the surface to promote adhesion.
In addition to improving adhesion in laminations, water-based primers are also useful in improving adhesion of printing inks to various non-porous surfaces, such as film.
What's the task?The bonding of an adhesive to an object or a surface is the sum of a number of mechanical, physical and chemical forces that overlap and influence one another. As it is not possible to separate these forces from one another, we distinguish between (1) mechanical locking, caused by the mechanical anchoring of the adhesive in the pores and the uneven parts of the surface, (2) electrostatic forces, as regard to the difference in electro-negativities of adhering materials, and (3) the other adhesion mechanisms dealing with intermolecular and chemical bonding forces that occur at the interfaces of heterogeneous systems.
This chemical primer is explained in the case of the intermolecular forces by the adsorption theory and in the case of chemical interactions by the chemisorption theory. The processes that play a role in the bonding of similar types of thermoplastic high-polymer materials, such as homogeneous systems, can be determined with the diffusion theory. While the theories of this type are fascinating, the plain fact is that we need to find a way to stick polyethylene to polyester.
Why a 'green' alternativeWhile solvent-based adhesives and primers for use within the converting industries often use two-part systems, there may be a better answer, especially with so much attention on sustainability these days. Water-based primers, which are environmentally safe, can be used to manufacture technically complex combinations of laminations. These primers contain zero volatile organic compounds (VOCs) and require no disposal fees.
If you’ve bought gas recently or kept an eye on the price of oil, the advantage of water-based primers over solvent-based primers and adhesives is clear. Because solvent-based products emanate from oil feed stock, every gallon of solvent-based primer or adhesive that is replaced with water-based products means one more gallon which can be used for fuel to heat our homes or power our vehicles. It is good economic and ecological sense to use water-based products where ever and whenever possible.
Most solvent-based primers are reactive systems that rely on polyester and an isocyanate in a two-part package in a solvent vehicle. An example of an isocyanate is given in Figure 1. The solvent must then be removed from the film in a high-temperature drying process.
This reacts with a hydroxyl containing polyester within a few hours at room temperature as follows:
One type of water-based primer is a polyethylene imine polymer (see Figure 2). However, these primers have poor resistance to water and other chemicals found in packaging applications.
Other water-based primers include emulsified polyurethane polymers. However, these have limited or no clearance by the Food and Drug Administration.
Aqua Based Technologies makes a line of water-based primers using materials recognized by the Food and Drug Administration for use with a variety of packaging and converting structures. The products are used for extrusion laminating polyethylene or Surlyn to a variety of accepting substrates for film-to-film, foil and paper structures. Research at Aqua Based Technologies centers on using renewable resources for emulsion feed stocks. These raw materials include cellulose derivatives, corn products and soy bean products, among others.