Activity 3.4 - Preparation of polymers, influencing their properties

• On the basis of the experiment we will verify the fact that the conditions of preparation can influence the structure and consequently also the properties of the resulting polymer. It is therefore absolutely necessary to satisfy the conditions of preparation if our goal is that the polymer is expected to fulfil its function.

• Polyurethane foam (spray),
• two empty matchboxes,
• a lamp, and
• a watch

Procedure:

1. Take empty matchboxes and soak one of them in water. Both of them should be partially closed.
2. Try to fill them with the same amount of polyurethane foam, while the first to fill up will be a dry box.
3. Observe how fast the polyurethane foam increases.
4. Wait until the foam hardens and dries. Cut the box and compare the structure of PUR foam.

Discussion:

1. Why was one of the boxes wetted and what were the consequences effecting the formation of polymer.
2. According to the structure of polymer, for which purposes can be the polymer recommended.

Polyurethane (PUR) foams in a spray form can be one–component (prepolymer) or two-component (two monomers). In the polyaddition –NH– groups are formed, which can, in a smaller extent, convert into isocyanate groups. During this reaction, there occurs the initiation of these groups by water (air moisture), an intermediate product is formed – aryl carbamic acid, which leads to the elimination of carbon dioxide that will foam up the polyurethane.

…–R=N=C=O + H2O → …–R–NH–COOH…→ …–R–NH2 + CO2↑(gas)

isocyanate aryl carbamic acid amine

The formed amine reacts with isocyanate (monomer or also a newly formed group) and a polyurethane polymer is formed.

…–R=N=C=O + NH2–R–…→ …–R–NH–CO–NH–R–…

PUR foams are used as insulation; their advantage is that they are waterproof but on the other hand they absorb air moisture, ensuring thus “breathing“ of the insulated structure.

The reaction is initiated by water; the presence of water in the reaction mixture is therefore helpful for a full reaction throughout the entire volume and for a formation of quality reinforced polymer across the entire volume, which should be evident when the polymer was cut in half. Minimally the presence of water will fasten the reaction, i.e. the foam is formed in a shorter time.

From the experiment it is also evident that the structure and consequently also the properties of polymer can be affected by the conditions of preparation, which will further broaden the structure of polymer properties.

Prepared polyurethane foam, on the left PUR foam formed with a wetted box.

Cross section through PUR foam, on the left PUR foam formed with a wetted box.