Uses of Waste Starch - The Economic Utilisation of Food Co-Products - page 69

5.3.1.2 Use of Additives in Starch/Dextrin Adhesives
Starch and dextrin adhesives are usually combined with a number of modifiers
that impart additional desired characteristics to the adhesive.
Tackifiers such as sodium tetraborate (borax), NaOH and sodium meta-
borate increase the speed at which the adhesive can stick to a substrate and can
also increase bond strength. 29 They also impart increased viscosity and
viscosity stability, giving better fluid properties. 30,31 Excessive use of additives
such as borax, however, can result

d n 9 r 3 n g | 0

in extreme cohesiveness leading to

processing problems.
Plasticisers are added to improve the finished film characteristics, increase
film flexibility and the ease of processing. There are three main types: chemicals
that solidify with the material, humectants such as glycerol that regulate the
moisture content of the film and fatty compounds that lubricate the material.
They include soaps, polyglycols, fatty alcohols, etc., and have the advantage of
imparting permanent film flexibility, but can only be added at low levels as they
reduce bond strength and cohesiveness of the adhesive. 30
Inert fillers such as clays and bentonite reduce the cost of the adhesive
substantially but can also reduce penetration of the adhesive in the substrate
and, therefore selection must be carefully made taking particle size and
dispersibility into consideration.
If colouration is important, certain bleaches can be added such as sodium
bisulfite, hydrogen peroxide and sodium perborate. However, such additives
can affect the chemical stability of the adhesive.
Preservatives are added to control microbial growth in starch-based
adhesives. This is particularly important as any deterioration can dramatically
affect service life. In the past, formaldehyde and chlorinated hydrocarbons have
been used, but now the focus is very much into using natural substances to
impart this effect.
Modification of starch alters the properties of the material. Modification can
be classed as degradative as in the case for dextrinisation (as mentioned
previously), or nondegradative when acetylating etc. Degradative methods such
as acid hydrolysis, oxidation and thermal processes are usually carried out with
the purpose of reducing viscosity of the product for higher paste concentration
with good flow and dispersion properties. 32 Nondegradative methods that
chemically modify the material through ester or ether linkages change the final
properties by increasing stability. Modification of the hydroxyl groups
decreases the tendency of the starch fragments to retrograde or age. They can
also increase the hydrophobicity of the material, broadening their use while
increasing the resistance to shear. However, such treatments usually reduce
tack and increase the price per kilogram of the material.

3 .

5.3.1.3 General Starch Adhesives
Starch- and dextrin-based adhesives are commonly water based and used to
bond various paper and cardboard products. 33

These include envelope