Gelling (also gelling or gelation ) refers to the process of forming a gel . First of all, isolated colloidal aggregates , which are present in suspension (sol), network to form a three-dimensional, viscoelastic network, the gel.
In addition to numerous technical application areas of sol-gel chemistry , e.g. B. in nanotechnology , the production of ceramics or inorganic and ceramic fibers , the process of gelling plays a role in cooking and in food technology, z. B. in the production of jellies .
In plastics technology, the entire process of curing a reaction mixture is often referred to as gelling, if this takes place via a gel-like intermediate state (e.g. in the case of polyurethanes , epoxides , polyester resins ). The manufacture of soft PVC also uses the process of gelling, in which plasticizers and PVC powder are processed into a homogeneous mass, from which products can then be manufactured.
The exact point in time of the transition to a gel is referred to as the gel point or gel point . However, there are different definitions for this.
With the increasing cross-linking of particles in the course of gelation or the sol-gel process, the gel point is usually defined by linking the clusters to form a macroscopic unit, i.e. a coherent chain at the expense of the sol. Another possible definition defines the gel point as the point at which the storage modulus and loss modulus are equal in terms of amount. A simple definition defines the gel point as the point in time at which a sample no longer deforms or flows. A definition via the sudden increase in viscosity is also possible. However, this can only be seen in a vague manner, as the gelation process often extends over very long periods of time.
The time until the gel point is reached, the gel time , is often identical to the pot life , as the mixture can then no longer be processed by pouring and brushing. It can easily be determined by measurement using the increase in viscosity with so-called gel timers . The sample goes through the gelling process in a possibly heated test tube . A metal helix moves continuously up and down in the sample. When the gel point is reached, the coil remains stuck in the sample and also lifts the test tube. This is recorded optically or mechanically and the gel time is determined.
- Karl-Heinz Lautenschläger, Werner Schröter, Andrea Wanninger von Deutsch: Taschenbuch der Chemie . Harri Verlag GmbH, 2007.
- Markus Meyer, Synthesis and Characterization of Novel Silica Gels , Herdecke 2004 - ISBN 3898631699 , ISBN 9783898631693
- DIN 16945: Reaction resins, reactants and reaction resin compounds ; Test procedure