Plastisols, sometimes referred to as vinyl pastes, are liquid dispersions of polyvinyl chloride (PVC) resins in plasticizer. They are generally 100 percent solid pastes or viscous liquids which gel on heating to relatively high temperatures and solidify on cooling to form plastigels.1 They can be flexible or rigid depending on the type and amount of plasticizer added and give good adhesion to most types of (oiled) metals, and plastics.2 They are often the preferred material for applications where low-flammability at a low cost is required or advantageous. They are also easy to apply, require no meter mixing, and allow for fast processing. A major disadvantage of plastisols are the relative high cure temperatures of about 150 - 210 °C which limits their use on heat-sensitive substrates. Exposure to heat during cure and service can also lead to release of hydrochloric acid (hydrogen chloride, HCl) which can cause corrosion on hot spots on metal substrates. To reduce degradation, corrosion and loss of adhesion, heat stabilizers and acid scavengers are generally added to all plastisol formulations. Common stabilizers and acid scavengers are CaZn, BaZn and conventional liquid epoxy resin (DGEBA) which reacts with acid that may be released during cure or service life.
The main ingredients of a
plastisol are fine powdered polyvinyl chloride or copolymers of vinyl
chloride and, more recently, methacrylate or styrene copolymers
which are dispersed in plasticizer.3 For a long time, phthalates such
as di(2-ethylhexyl) phthalate (DEHP), diisononyl phthalate (DINP),
and diisodecyl phthalate (DIDP) have been extensively used as plasticizers in
plastisols. Because of health concerns,
however, phthalates have been gradually replaced with
none-phthalte plasticizers in the European Union, United States, Japan, and several other countries. Common none-phthalate plasticizers include benzoates, phosphates, adipates, sebacates
and alkylsulfonic acid esters.
Some of the most common non-phthalate plasticizers are benzoates such as dipropylene glycol benzoate (DPGDB) and
triethylene glycol benzoate which offer improved
performance.
The plastic powder and the plasticizers make up
about 60 to 70 percent of the plastisol formulation. To meet the
application and performance requirements, many other ingredients are
added to the plastisol formulations. For example, ground fillers such as calcium
oxide and calcium carbonate are added to achieve non-sag properties.
An additional benefit of these ingredients is their ability to bind and neutralize HCL formed by partial decomposition of the plastisol during bake and service.
Other ingredients commonly added include heat stabilizers, acid
scavengers, cross-linkers and secondary plasticizers (hydrocarbon oils).
For some applications, lower viscosities and a reduced plasticizer content are required or preferred. This can be achieved by adding volatile solvents to the plastisol. These products are frequently called organosols. The solvent reduces the viscosity before gelation and flashes of during heating, leaving a more rigid material behind. Organosols are often prefered when better gap filling, penetration of porous substrates or more rigid and stronger bonds are needed.
Plastisols are generally considered very stable at room temperature but fuse or gel when heated and turn into a homogenous solution at sufficiently high temperatures. The first step is gelation which typically occurs around 50 to 60°C4 which converts the plastisol into semisolid material. As the temperature is further increased, more and more plasticizer penetrates and swells the particles until a point is reached where phase inversion occurs, that is, the dispersed solid resin particles convert to plasticizer dispersed in resin. Further heating results in fusion of the particles to a homogenous material.
Plastisol sealants and adhesives are extensively used in the building, textile, appliance and automotive industries. They are relative inexpensive, are easy to apply, and offer many performance advantages. In the automotive industry, they can be used to seal around the crimped panel edges, to bond steel sheets to stiffener panels, and to seal the underbody of cars. An additional advantage of plastisols is their ability to provide sound deadening and vibration/flutter reduction, particularly when foamed.
Vinyl plastisols have typically been considered as 100 percent solids. In reality, they will release small amounts of gas during fusion due to degradation or volatility of the plasticizers.
To improve adhesion to metals, adhesion promoters are frequently employed or the parts are primed prior bonding.
In many applications, PVC based plastisols have been replaced with acrylic plastisols or with urethanes. These materials have improved adhesion to many substrates including fiberglass reinforced composites and heat sensitive substrates, but are also more expensive.
J. Verdu, A. Zoller and A. Marcilla, Applied Polymer, Vol. 129, Is. 5, pp. 2840-2847 (2013)