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

Yes

 2. Experimental procedure has been modified

No

 3. A separate experimental procedure has been devised

No

 4. Video clip available

No

 5. Flash animation available

No

 6. Other materials: Worksheet 8, Worksheet 4, Worksheet 5, Slide DH7, Slide B18

Room Temperature Curing, One-Part Silicone Rubbers

TopDown 1 Materials, Chemicals, Time Needed
  • Scotch tape, pH indicator paper, spatula, water-color paints + brush
  • 3 U-shaped profiles or matchboxes
  • 3 thermometers
  • WACKER Silicon ELASTOSIL® E43, Xi, F, N
  • WACKER Silicon ELASTOSIL® N199, Xi, F, N
  • Calcium sulfate


Allow about 20 minutes to prepare the samples. The experiment will take around 40 minutes overall. The samples will need approx. 1 day to fully cure.

TopDown 2 Procedure and Observations

Cover the ends of two U-shaped profiles with adhesive tape. Fill one with ELASTOSIL® E43 and press the compound down with a wet finger. Do the same with the second profile, using ELASTOSIL® N199.

Fill the third profile with gypsum, and scrape off the excess with a spatula. If no profiles are available, use empty matchboxes.

Place a thermometer in each sealant, and place a moistened strip of pH indicator paper on the edge of the profile (see Photo 1).

Fig.: 1 Filling a U-profile with sealant
Photo 1: Experimental set-up

During the experiment, monitor the temperature and the change in odor of each sealant for about 1 hour and tabulate the results.
After final curing, check the samples for impact strength, consistency and ease of overpainting with water-colors (see Photo 2).

Photo 2: Gypsum and silicone sample after painting with water-color

Tabulate these results as well. The following observations were made when the experiment was repeated several times:
  ELASTOSIL® N199 ELASTOSIL® E43 Gypsum
Odor Indeterminate Like vinegar Odorless
pH paper Neutral Turns red (acid) Neutral
Consistency Soft, rubbery Soft, rubbery Firm
Impact strength Elastic Elastic Breaks into small pieces
Overpainting tolerance with water-color Water-color adheres poorly and can be washed off Water-color adheres poorly and can be washed off Water-color adheres well and cannot be washed off
Setting time  1 day  1 day  1 day

Table 1: Experimental observations

Measurements taken as the samples were curing revealed that the temperature of all three samples rose by around 1.5 °C.
The temperature changes plotted in the adjacent diagram were measured with the "All-Chem-Misst" classroom instrument and dedicated software (see Photo 3). The measurements are mainly intended to reflect the change in temperature. As gypsum cures, the temperature rises sharply at first and then drops continually until it remains almost constant (see diagram). In contrast, the temperatures of the silicones changed by different amounts and followed no particular pattern (see diagram).
Photo 3: Setup for "All-Chem-Misst" measurements    
Diagramm

TopDown 3 Discussion of Results

The test observations (see Table 1) reveal clear differences between gypsum and the two RTV-1 silicone rubbers. The two silicone rubbers cure by reacting in several stages with the moisture in the air to form elastomers (see Part 5 Supplementary Information). The cured material has a soft, rubbery consistency and is elastic.
The acrid odor and the acidity of ELASTOSILS ® E43 stem from the fact that this system contains an acetoxy-based crosslinker that releases acetic acid during the first hydrolytic reaction step.
ELASTOSIL® N199, in contrast, is a neutral system. Water-colors adhere poorly to silicones and can be easily washed off because silicones are highly water repellent and have a non-stick effect (see also the following experiments: "Water-repellent properties of silicone fluids", "Silicones in masonry protection" and "Silicone-coated paper").
When gypsum cures, water molecules are incorporated into the ionic lattice of the calcium sulfate. The calcium sulfate CaSO4 is thereby converted into calciumsulfate dihydrate (gypsum) CaSO4 · 2 H2O. Because gypsum is an ionic compound, it is brittle and breaks when subjected to mechanical force, such as a hammer blow.
The observed change in temperature accompanying the curing of gypsum may be explained as follows: The temperature rise is due to Coulombic forces between the water dipoles that penetrate into the ionic lattice and ions. Some of the heat released is needed for evaporating the excess water. The rest of the heat escapes to the surroundings. As a result, the gypsum cools down.
The differences observed in the temperature changes accompanying curing of the silicone rubber are the manifestation of several exothermic and endothermic processes. These vary in nature and extent with the materials employed and the method of preparation. Hydrolysis of the side groups (e.g. acetoxy groups) and condensation of the silanols are exothermic reactions whereas evaporation of the by-products (e.g. acetic acid) are endothermic reactions. This leads to totally different, non-uniform temperature changes.
The properties we have just observed indicate that gypsum would not make a good jointing material for joints subject to movement and stress due to the fact that it is not elastic. It is suitable, however, for purely decorative joints that involve no movement and are likely to be painted.
As for the two RTV-1 silicone rubbers, which make good "working joints," the choice of which one to use depends on the by-products.
For example, ELASTOSIL® E43 would not be suitable for joining two bits of marble as the acetic acid by-product would attack the marble and impair the joint quality.

TopDown 4 Tips and Comments

  • The experiment can be speeded up by omitting the “long-term” temperature measurements and simply measuring the rise in temperature at the start of curing. This can be used to demonstrate that exothermic reactions predominate during both curing of gypsum and curing of silicones.
  • The experiment could be extended by studying the influence of atmospheric moisture on curing. To do this, fill a further U-profile with an RTV-1 silicone in parallel to the experiment above, store in a desiccator over dry silica gel under a water-jet vacuum. Note that the time needed for curing increases. When this experiment was performed on ELASTOSIL® N199, three days were needed for curing, whereas only one day was needed when it was performed in air. Curing never fully failed to materialize. The reason is that the water-jet pump cannot remove all the atmospheric moisture, and curing only requires small amounts of water.
  • Another variant is to make an aquarium using RTV-1 silicone. Make the aquarium by using ELASTOSIL® E43 or N199 to join five grease-free glass plates (cleaned with acetone or ethanol) at right angles to each other (photo 4). When curing is complete, test that it is leakproof. A similar experiment was very successful, as seen in photo 4.
Photo 4: Glass plates bonded together with ELASTOSIL® silicone

TopDown  5 Supplementary Information

Room temperature curing, one-part silicone rubbers, also known as RTV-1 grades (1-part room-temperature vulcanizing) already contain all the ingredients necessary for curing, such as polyorganosiloxanes, crosslinkers and fillers. The curing process, which releases by-products, only takes place on exposure to atmospheric moisture. RTV-1 silicone rubber compounds are made by making hydroxy-terminated polysiloxane molecules react with crosslinkers to form curable products (see Fig. 2).

The standard base rubber compound is a polydimethylsiloxane (R = CH3).

Table 2 shows the most common types of crosslinkers (RSiX3) along with examples of the X group.

 Type of crosslinker  X group (Name)  X group (formula)
 Acidic  Acetoxy  
 Octoate  
 Neutral  Amide  
 Oxime  
 Alkoxy  
 Alkaline  Amine  
Curing proceeds in the presence of atmospheric moisture by the mechanism shown in Fig. 3. The by-products vary with the type of crosslinker employed. Crosslinking between silicone macromolecules occurs in subsequent reaction stages and involves residual X groups.

At the macroscopic level, curing starts on the surface of the silicone rubber with the formation of a skin and gradually works its way into the compound. Premature crosslinking is prevented by the use of sealed packaging, such as tubes and cartridges.
As shown in Fig. 4, the higher the content of moisture in the air, the higher the curing rate.

Fig. 4. Curing rate of ELASTOSIL® as a function of atmospheric moisture at room temperature (Source: Ref. [2]).

Like all silicones, RTV-1 silicone elastomers (the cured rubbers) are highly resistant to chemicals, ozone and UV radiation (see also experiments "Influence of ozone on silicone rubber compared with other types of rubber" and "Solubility and chemical resistance of silicone rubber"). They are also noted for their outstanding water repellency. However, the properties of silicone elastomers vary according to the crosslinking systems employed, and this determines their suitability for specific applications.

Alkaline systems Neutral systems Acidic systems
  No odor released during curing  
    Good adhesion to glass, ceramics, metals and plastics
Good adhesion to building materials, glass, ceramics No corrosion of metals or plastics  
Readily compounded (applies to amine systems)   Excellent transparency (applies to acetoxy system)
Good mechanical properties   Good mechanical properties
Environmental stress cracking of polycarbonates   Environmental stress cracking of polycarbonates
Table 3: Properties of various silicone rubber elastomers made with different crosslinkers (Source: Ref. [2]).
The aforementioned properties, simplicity of use and reliability, make RTV-1 silicone elastomers excellent sealant and jointing materials for avoiding stress cracking in masonry. This is caused by constant movement and stresses that result from thermal fluctuations, moisture-related causes, shrinkage of building materials, mechanical fluctuations or subsidence of the substrate. These deleterious effects can be avoided by bridging the movements with elastic joints. This is especially true for combinations of different materials, such as glass/metal or stone/metal, because each one has a different coefficient of expansion.
RTV-1 silicone rubbers are therefore mostly used in the construction industry and are probably the silicone products best known to the general public, especially the products sold in cartridges. Other application areas are to be found in the automotive, electrical, electronics and textile industries, where they serve as seals, bonded joints and coatings.
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. 50-51

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