The term polyurethane is attributed to the wide range of polymers obtained by the polymerization reaction between the polyol compounds with hydroxyl groups and the compounds with isocyanate groups. By varying the chemical nature of these two compounds and the relationship between them, different products can be obtained to cover a wide range of applications in different industrial sectors, such as flexible foam for the automotive sector or for the manufacture of sofas and mattresses, rigid foam for insulating sandwich panels, integral foam for shoe soles or even in applications such as adhesives or elastomers.
At Concentrol we have a wide range of silicone stabilizers that cover the particular needs of each manufacturer of polyurethane systems. In the following study we evaluated different silicone stabilizers in a rigid polyurethane system used in the manufacture of continuous panels. In this case we will focus on two of the most important characteristics of this type of foam, its low thermal conductivity (lambda value) and its dimensional stability, calculated as the ratio between the compression forces in the vertical and horizontal directions of expansion of the foam.
The foaming method consists of a manual mixture of polyol formulation with isocyanate, the most important parameters being:
- Preparation of the mixture of polyol, a silicone stabilizer, catalysts and a foaming agent with a special agitator for 15 seconds at 2500 rpm.
- The different reactants are kept at a temperature of 25ºC.
- Addition of MDI and mix for 8 seconds at 2500 rpm.
- Pouring the reaction mixture into a mold of dimensions 30x30x20 cm, where the foam freely expands. The foaming starts at 25 °C.
First test: density of the foam
The foam density was measured 24 hours after foaming. The result is as follows:
Second test: thermal conductivity
The thermal conductivity was analyzed using a system of heat flow meter NETZSCH model HFM 436 Lambda. The sample size is 300 mm X 300 mm base length and 30 mm thickness.
The system operates according to ASTM C 518, ISO 8301, DIN EN 12667, DIN EN 13163 and JIS A1412.
Third test: dimensional stability
In this process, the effect of compressing the foam 10% is measured vertically and horizontally.
Fourth test: affectation of aging in conductivity
The foams have been re-analyzed after an aging process. The foams were kept for one month, tempered at 20 °C, stored without the contact of natural light, and the value of the thermal conductivity (lambda) was analyzed again.
Fifth test: Fluidity of the foam
Once the foams with the best thermal conductivity values have been selected, they have also been evaluated in a model test that simulates the flowability of the molten polyurethane. Silicone stabilizers affect this property, and this test enables us to analyze this effect. In this case, the free expansion of the foam develops inside a plastic tube 125 mm in diameter. The height reached by the foam is directly related to fluidity.
Conclusions
In the first series of experiments, 3 stabilizers were identified, CONCENTROL STB PU-2242, STB PU-2285 and STB PU-2245B whose lambda values are the lowest of all the candidates.
In terms of dimensional stability, CONCENTROL STB PU-2242 obtains the best value. All the candidates are far from the ideal coefficient (1) due to the large differences between the vertical and horizontal compression forces.
Regarding the value of lambda aged, the best result is obtained with the stabilizer CONCENTROL STB PU-2285. In all cases the aged values are worse than the initial values, which coincides with the usual behaviour in the sector.
We conclude that 3 of our silicone stabilizers, CONCENTROL STB PU-2242, STB PU-2285 and STB PU-2245B are the best candidates.
At Concentrol we consider that this type of studies and tests are key to know in depth the products we offer and to be able to inform our clients as best as possible about the possibilities of our products and help them to choose the most suitable ones in each case.
To review Concentrol’s STB PU range of polyurethane stabilizers and all its possibilities consult here.
