15.7 How do I make Slime? (Chemistry)
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
15.7 How do I make Slime? (Chemistry)
" Slime " is a trademarked commodity obtained by cross-linking guar gum and
borax, and is marketed by the Mattel Toy Corporation. The slime produced for
demonstrations is usually made by cross-linking a poly vinyl alcohol (PVA)
product using borate. The normal method is to carefully prepare a 4%
mass/volume aqueous solution of a hydrolysed high molecular weight PVA
( >100,000 ) - available from Eastman Kodak. Commercial PVA-based adhesives
( such as Elmer's Glue ) will also produce a reasonable quality slime, as
will polymeric materials that have multiple hydroxyl groups and can form
highly-hydrated gels, such as guar gum - but some experimentation may be
required to ascertain optimum ratios.
High MW polymers are difficult to dissolve in solvents ( including water ),
and the best method is to carefully sprinkle the powder over a beaker of
water that is being gently stirred, and continue gentle stirring until a
uniform solution with no gelatinous lumps is obtained.
Any grade of borax ( Na2B4O7.10H2O ) can be used to prepare a 4% mass/mass
aqueous solution. The slime is made by vigorously mixing the two solutions
in the ratio of 1-2 parts of the borax solution to 10 parts of the PVA
solution using a paddle stirrer. Details of a suitable procedure for use in
classrooms have been published [10]. A firmer, less messy, slime can be
prepared from an 8% PVA solution - using equal ratios of the 4% borax
solution [11].
The properties of slime indicate that the cross-linking mechanism does not
consist of strong covalent bonds. Borax hydrolyses in water to form a boric
acid-borate buffer with an approximate pH of 9.
B(OH)3 + 2H2O <==> (B(OH)4)- + H3O+ pK = 9.2
The borate ion is tetrafunctional when interacting with the alcohol groups
of polyols, and thus builds the cross-linking structure. PVA has about 1-2%
of 1,2 diols amongst the remaining 98-99% of 1,3 diols. To obtain the desired
properties, the bonds between the borate and the PVA must be weak, and it is
believed they are hydrogen bonds ( shown as ... below ).
PVA Borate PVA
H H
| |
O-H...O O-H...O
\ / \ / \ /
H-C \ / C-H
/ B- \
CH2 / \ CH2
\ / \ /
H-C-O...H-O O...H-O-C-H
/ | | \
H H
Although individual hydrogen bonds are weak, the large number of available
OH groups in highly-hydrolysed PVA will result in a hydrated, 3-dimensional,
gel, rather than a borate precipitate. The continual breaking and reforming
of the bonds under low mechanical stress, and the large amount of water
incorporated into the gel, are responsible for the rheological properties of
the hydrated gel. Slime can be broken down by reducing the concentration of
borate by titration with a strong acid, and details of such a procedure have
been recently published [12].
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