Sodium alginate as a mould release agent

Sodium alginate as a mould release agent

Sodiuma&hate as a mouldrelease agent AG. Andreopoulos Laboratory of Special Chemical Technology, N. T.U., Athens, fReceived 27 June 1984; accepted 2...

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Sodiuma&hate as a mouldrelease agent AG.


Laboratory of Special Chemical Technology, N. T.U., Athens, fReceived 27 June 1984; accepted 20 August 7984)


The use of sodium alginata as a mould release agent during the casting of epoxy or polyester msins in plaster moulds is discussed. The optimal concantmtion of a mtarder is determined, taking into considrmtion the sat&g behaviour and tbe adhesive bond stmngth. The above chamcteristics are critical for the application and efficiency of the coating.



sodium alginate,


mould release

The film formed by salts of alginic acid can be used as a release agent during the moulding of polymers, because of a reduced chemical affinity which results from their structure’. On the other hand, the reactivity of the water soluble salts of alginic acid (such as sodium or potassium alginate) with calcium ions can lead to the formation of a gel layer when a solution of the above salts is applied to a plaster surface’* ‘* 3. This behaviour, i.e. the adherence to plaster substrates combined with the incompatibility with many polymers, dictates the use of soluble alginates as mould coatings for the formation of a separating film. The requirements of such a separating film are subject to its thickness, setting time and to the adhesive bond. The thickness must be limited to avoid dimensional changes of the moulded article and the corresponding setting time must permit proper application. On the other hand the film must be adhered sufficiently to the mould surface to prevent the formation of gaps or other moulding imperfections. The above requirements can all be controlled by the concentration of a retarder added to the alginate solution 5*‘* ’ . The retarder has to precipitate calcium ions, forming an insoluble salt or a complex. Such compounds can be phosphates, oxalates, citrates etc.2’4. As their concentration increases the setting time for a given thickness increases. EXPERIMENTAL




ness. For determination of the gel thickness, the solution was wiped at different time intervals and the gel measured using a micrometer. The thickness of the dried gel was measured in the same manner. The detemination of the adhesive bond strength, was based on the ASTM D 1002-72 designation*, using small plaster plates. The coating solution had a thickness of about 200 ,um. RESULTS

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The thickness measurements for different time intervals and retarder concentrations are shown in Tables 1 and 2. Table 1 Hydrogel concentrations



with time, for various retailer



Time (min)





0.5 1.0 2.0 3.0 4.0 5.0 6.0

80 130 190 220 250 270 290

50 90 150 190 230 250 -



90 130 160 210 240 200

90 130 170 180 220 -

Table 2

An alginate solution was prepared by dissolving 20 g of sodium alginate (SATIALGINE S-l 100, SATIA S.A., France) in a litre of deionized water. A citrate retarder (10% aqueous solution) was added to 250 ml fractions of this solution in concentrations of 1, 2 and 3%. Thus, 4 solutions were available containing the retarder from &3%. A calcium sulphate dihydrate (CaS04.2H20) plate was prepared, using a dental plaster (Moldano, Bayer A.G., W. Germany). This plate was coated on one side with the solution to form a layer of some millimetres [email protected] 1985



of dried gel for verious retarder




Time (min)





0.5 1 .o 2.0 3.0 4.0 5.0 6.0

21 34 50 57 65 70 75

13 24 39 50 60 65 -

24 34 47 55 62 73

24 34 44 47 57 -


Table 3 trations.

The adhesive


bond developed




The thickness

bond (kg/cm2)


20 h


63 81 91 94

80 92 101 106

X lo3


of layer formed as a function of time.

The data are plotted in Figure 1 and the curves indicate, there is a decrease in gelling rate as the retarderconcentration increases, for setting periods greater than 4 min. The gel thickness is greater in the absence of retarder but essentially the same for concentrations of l-3%, at least for the first 3 min. For longer periods the thickness decreases with increasing retarder concentration. It is also evident that a thickness of 55-65 pm (dry gel) is reached within 5-6 min for a retarder concentration of 2-3%. This can be explained if the gel formation is considered as a two stage process. The first stage (gel thickness about 200 p) consisted of the immediate reaction of sodium alginate with the dissolved calcium cations. When retarder is present an inhibition is observed independent of the retarder concentration. This is due to an excess concentration of calcium ions (resulting from dissolution) compared with those required for precipitation of the retarder in concentrations of l-3%. In the second stage (above 200 p) the gel thickness is controlled by the diffusion rate of calcium ions through the layer formed in the first stage. The film growth rate is therefore decreased because of the reduction of calcium ion concentration. For the same reason, the film growth rate is diversified (in the second stage) bythe retarderconcentration, being lower with increasing concentration. The data referring to the adhesive bond on the plaster surfaces are shown in Table 3 and in Figure 2. As the measurements indicate, there is an increase in adhesive bond with retarder concentration after 30 s and 20 h and this increase seems to reach a limiting value for concentrations greater than 2%. It cannot be attributed to the adherence of the first layer to the plaster surface, because the first setting stage is almost the same in all cases. This behaviour is a consequence of the film strength variation with setting time, i.e. the rate of its formation.


with varying retarder concen-

Retarder (%)

1 2 3


agent: A.G. Andreopoulos

An alginate solution, suitable for use as a mould release agent must contain 2-3% retarder. This con-


60 I






Retarder Figure 2


The adhesive


bond changes


with the reterder


centration leads to the formation of a film of 55-65 p and enough time (5 to 6 min) is allowed forthe coating of the mould. The adhesive bond (although increasing up to concentrations of 3%) is sufficiently strong when the retarder concentration is 2%.

Whistler, R.L. Polysaccharides in Encyclopedic of Polymer Science end Technology, Vol. 11, John Wiley and Sons, New York, 1969. pp 409-10 Freemen, F.H., Dental materials in Encyclopedic of Chemical Technology, Vol. 6. John Wiley and Sons, New York, 1965. pp 801-2 US Pat. 2 249 694 (Sept. 1939), S.W. Wilding (to the Amalgamated Dental Co.) US Pat 2 434 005 (Jan. 1948). SE. Noyes Fisher, F.E. and Dijrtel, H., Z. Physiol. Chem. 1955, 302, 185-203 Hang, A., Composition and production of alginates, Report No. 30, 1964, Norwegian Ind. of Seaweed Research, Trondheim McDowell, R.H., Properries of Alginetes, (2nd Edn). Walker House, London, 1961, p 24 7977Annual book of ASTM Standards, (Part 22). American Sot. for testing Materials, Easton, 1977. pp 248-52



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