PVC plastisol is applied to a mould or surface as a fluid paste, but it has to go through heat treatment to become a usable product. This process is called gelation or fusion. Getting gelation right directly affects the flexibility, surface quality and durability of the final product. This article outlines how plastisol gels and what to watch in production.
What is gelation?
In plastisol, PVC particles start out dispersed in the plasticizer. When heat is applied, the plasticizer begins to penetrate the PVC particles, which swell and fuse together. The fluid paste turns into a homogeneous, flexible solid PVC. This transformation is thought of in two stages: pre-gelation and full gelation (fusion).
Pre-gelation
Pre-gelation takes place in the first stage of heating. The paste loses its fluidity and begins to hold its shape, but has not yet reached full strength; at this stage the product may be brittle or weak. In some processes pre-gelation is used as an intermediate step for layered applications or multi-colour products.
Full gelation (fusion)
As heating continues, the PVC particles and plasticizer combine completely and a homogeneous structure forms. At this stage the product reaches its flexibility and strength. The temperature and time needed for full gelation depend on the formulation, the product thickness and the heating method, and are set by trial for each formulation.
Balancing temperature and time
In gelation, temperature and time complement each other: a lower temperature may need longer, a higher temperature less time. Too high a temperature or too long a time, however, can cause degradation, colour change or surface defects. In thick parts heat takes time to get to the inside, so thickness is a decisive factor in choosing the time.
Effects of under-gelation
An under-gelled plastisol product may look normal from the outside but not show the expected flexibility and strength. It may tear or crack easily, or plasticizer may sweat out on its surface. That is why keeping gelation conditions under control and testing products regularly is important in production.
The effect of formulation
Plastisol’s gelation behaviour is directly affected by its formulation. Plasticizer type, resin type and additives can change the temperature at which gelation starts and how long it takes. Different plasticizers such as DOTP and DOA can show different gelation behaviour. For the formulation components, see our article PVC Plastisol Formulation.
Heating methods
- Mould heating: in micro-injection, the mould is heated to gel the plastisol.
- Oven: in coating applications the product passes through an oven.
- Infrared heating: used on some coating lines to heat the surface quickly.
Each method transfers heat at a different rate, and gelation conditions are adjusted accordingly.
In our own production
In our own production we use phthalate-free plastisol in the rubber labels, patches and home products we make by micro-injection, prepared with DOTP or DOA. We also prepare the same plastisol as a raw material for companies making their own products. If you are a producer using plastisol, share your processing method and product and we will assess a suitable plasticizer option together.
Process control and records
Recording heat treatment conditions for every batch makes it easier to find the source of problems. Noting oven temperature, time and product thickness allows comparison when an unexpected result appears. Periodically checking the temperature distribution in the oven also helps products in different zones be treated under similar conditions.
When a new product or mould is introduced, confirming heat treatment conditions with trial batches is recommended.
Frequently asked questions
At what temperature does plastisol gel?
Gelation temperature and time vary with formulation, product thickness and heating method; they should be determined by trial.
Can a pre-gelled product be used?
Pre-gelation is an intermediate stage; full gelation must be completed for the product to gain the expected properties.
Conclusion
Correct gelation is the basis for turning plastisol from a fluid paste into a flexible, durable product. See our PVC plastisol page for details, and send your needs via the quote form.