1. Experiment is contained in the WACKER's Experimental Kit.


 2. Experimental procedure has been modified


 3. A separate experimental procedure has been devised


 4. Video clip available


 5. Flash animation available


Silicone-Coated Paper

TopDown 1 Materials, Chemicals, Time Needed

  • Wooden board
  • Paper
  • Glass rod
  • Drying cupboard or hot plate
  • Scotch tape (or other adhesive tape)
  • Glass beaker, 100 ml
  • DEHESIVE® 920, Xi, F, N
  • WACKER Crosslinker V24, Xi, F, N
  • WACKER Catalyst OL, Xi

(See Part 5 Supplementary Information and References for details about abbreviations)
Allow around 45 minutes to prepare the DEHESIVE® mixture and to coat the paper. The curing time depends on the temperature and the coating thickness. However, it should be around 30 minutes max. at the temperature of the experiment. The experiment can therefore be performed during a double lesson.

TopDown 2 Procedure and Observations

Add 1.3 g Crosslinker V24 (clear liquid) to 50 g DEHESIVE® (clear liquid) and stir both liquids with a glass rod. Add 0.5 g of Catalyst OL (yellowish liquid) and then stir again vigorously. Coat the papers as follows:

  1. Tape the paper to the wooden board or other flat, solid washable surface.
  2. Apply a bead of the reaction mixture to one end of the sheet.
  3. Use the glass rod to spread the mixture over the paper. Do this by pressing the rod firmly onto the paper and drawing it downward (see photo, right).
  4. Then place the coated paper in the drying cupboard at approx. 130 °C and measure how much time elapses until curing is finished. When curing is finished, the paper will no longer be tacky. Repeat the experiment to determine the curing time at 50 °C and 100 °C.
  5. Finally, test the adhesive strength of Scotch tape on the coated and uncoated paper.

When the reaction mixture is being prepared, a clear, readily flowing liquid is formed (right) that is easy to spread over the paper to be coated.
It cures in the drying cupboard to form a clear, smooth, glaze-like coating on the paper.

Foto 1

Foto 2
DEHESIVE® mixture
The following observations may be made when comparing the adhesion of strips of tape to coated and uncoated paper:
When the tape is removed from the untreated paper, the paper is damaged. The tape has lost its tacky properties and has become useless.
In contrast, the tape attached to the coated paper can, just like sticky labels, be removed easily and without damage, and can be reused (see photos below).
Foto 3
  Paper coated with DEHESIVE®
Foto 4 Foto 5
Strips of tape attached to siliconized (a) and non-siliconized (b) paper   Siliconized (a) and non-siliconized (b) paper after the strips have been removed

The experiment to investigate the rate of curing as a function of temperature shows that the curing time depends heavily on the coating thickness and decreases with increasing temperature. In several reference experiments, mean curing times were measured as a function of the thickness of the silicone coating and the temperature (see Table 1).

 Temperature in °C   Curing time in minutes 
130 Approx. 1
100 Approx. 3
50 Approx. 20
Table 1: Curing times at different temperatueres

TopDown 3 Discussion of Results

The DEHESIVE® 920 silicone rubber used in the experiment consists of polydimethylsiloxane molecules that have crosslinkable vinyl groups. These undergo addition curing (see Part 5 Supplementary Information) to form a strongly adhering layer of silicone rubber consisting of DEHESIVE® 920 on the treated paper. The methyl groups of the silicone molecules align themselves such that they are pointing away from the surface. Adhesion between the strip of tape and the siliconized paper surface is relatively weak because the intermolecular forces bonding the aligned silicone molecules on the paper to the molecules of the adhesive are fairly weak. The strip of tape can be readily removed from the surface, without loss of adhesion.
In contrast, strong interactions occur between the cellulose molecules of the untreated paper surface and the molecules of adhesive. Once the strip of tape has been removed, it cannot be reused and the paper itself is damaged.
The rate of the addition-curing reaction is increased by raising the temperature. The curing time therefore falls as the temperature rises.

TopDown 4 Tips and Comments

  • It is best to weigh out and prepare the mixture in a glass beaker because the catalyst and crosslinking agent are viscous. If they were transferred separately to the beaker, the amount transferred would not be the same as the amount weighed out.
  • Before heating, remove the paper from the wooden surface as otherwise the curing time will be much longer. The reason is that the wood does not heat up as quickly as the paper and tends to have a cooling effect.
  • The curing time varies with the layer thickness. Therefore, ensure that layers of similar thickness are applied to the paper. The temperature-dependence of the curing rate can be readily observed despite different layer thicknesses. However, the curing times are less reproducible.
  • If a drying cupboard is not available, heat the pieces of paper on standard hot plates. The results will be less accurate because the temperature cannot be set so precisely. The curing times will also be different from those in the drying cupboard. Nevertheless, the dependence of the curing time on the temperature is readily observed. Several reference trials conducted at 50 °C and 100 °C yielded the curing times shown in Table 2.

 Temperature in °C 

 Curing time in minutes 

 Approx. 100  Approx. 1
 Approx. 50  Approx. 15
Table 2: Curing times as measured with an electric hot plate

TopDown  5 Supplementary Information

DEHESIVE® silicone release papers are widely used to cover and protect the adhesive on sticky labels so that they will stick where they are supposed to. In the experiment described, just as in practice, both solvent-based and solventless addition-curing or condensation-curing polydimethylsiloxanes (silicone rubber) are used for coating purposes. These cure very quickly at temperatures of 100 – 200 °C. Release papers generally are constructed as follows:

Abb. 1
Fig. 1. Structure of a pressure-sensitive adhesive with a silicone coating (Source: Ref. [11], p. 40)
The application areas of silicone release papers are shown below:
Abb. 2
Fig. 2. Use of silicone release papers (Source: Ref. [11], p. 40)

To produce a very flexible, individual consistency for different coatings, the DEHESIVE®- silicone coating used in the experiment is generally processed in the form of a 3-component material in practice.

These components are:

  • Polymer (DEHESIVE®): Solvent-based and solventless polydimethylsiloxanes whose end groups consist of crosslinkable hydroxyl or vinyl groups.
  • Crosslinker: hydrogen polysiloxanes with a high content of reactive Si-H groups for thermal hardening of addition-curing systems. The nature and quantity of the crosslinker determines the rub-off resistance and adhesive strength of the silicone film.
  • Catalyst: platinum complexes (0.1 %, 1,000 ppm) for thermal curing of silicones. The quantity of catalyst employed determines the production rate, the curing temperature and the pot life (the maximum length of time during which the mixture can still be used).
The following reactions occur according to the type of curing reaction:
Abb. 3
Fig. 3. Addition curing of DEHESIVE® silicone rubber (Source: Ref. [11], p. 41)
Abb. 4
Fig. 4. Condensation curing of DEHESIVE® silicone rubber (Source: Ref. [11], p. 41)

In addition curing, the hydrogen atoms from the Si-H groups add across the vinyl groups of the polymers. Condensation curing also involves the active Si-H groups from the crosslinking agent, but in this case hydrogen is split off. In both cases, the silicone macromolecules are linked together to form a network. The resultant material adheres well to cellulose or glass, the polar sections of the silicone molecules interacting (dipole-dipole and hydrogen bonds) with the substrate. The methyl groups of the silicone molecules project outwards (see also the experiment "Hydrophobic properties of silicone fluids").
As a result, the silicone on the paper has a non-stick effect or release effect on common adhesives, i.e. the adhesive either bonds weakly or not at all to the silicone layer. This behavior is due to the weak forces of interaction (Van der Waals) between the molecules of the adhesive and the methyl groups of the silicone.
The unusually weak intermolecular bonding forces are also found when the surface tension (surface energy) of silicones is compared with that of other organic polymers, all of which have higher surface energies (Table 3).

 Polymertyp   Surface tension in [mNm-1
 Polydimethylsiloxane  21-22
 Polyphenylmethylsiloxane  26
 Polyvinylchloride  40
 Polyethylene  30
 Starch  40
 Wool  45
Table 3: Surface tension of various polymers (Source: Ref. [2], p. 59)
TopBottom 6 References

W. Held et al., Learning by Doing – School Experiments with WACKER Products (handbook accompanying WACKER's Experimental Kit), Wacker Chemie AG, Munich, 2007, p. 40 - 42