Re: What makes glue sticky

Dear Susan,

There is no common [or even uncommon] chemical characteristic shared by all 
glues. Tell your kids to stop looking; it isn't there.

The reason is not to be found in the adhesive but in the adherends [i.e., the 
things being glued together]. If all the world were made of paper, some common 
characteristics would emerge but it is the broad diversity of things that are 
glued that demands a diverse range of chemistries to bond them. There are about 
35 major chemical types of adhesives in use today, each with dozens [if not 
hundreds] of sub-types. Combine this with all the solid [and semi-solid] things 
that you can think of like wood, paper, glass, metal, rock, fiber, leather, 
skin, plastics, rubber etc., etc. Bonding glass to glass [to make 'safety 
glass'] requires a very different adhesive than one used for bonding glass to 
rubber [used to bond a 'safety glass' windshield to the rubber weather strip 
around the windshield on your car].

An excellent general reference on this subject is the Handbook of Adhesives, 
3rd Edition, Irving Skeist, editor, Van Nostrand Reinhold Co.

Your yucca experiment makes use of one of the most commonly used adhesives in 
the world today: starch. While corn starch is the most commonly used one, other 
commercial sources include tapioca, sago, wheat and potato starches.

Starch glues are widely used in the bonding of paper, wood and cotton. A 
general rule of thumb for adhesives is "Like bonds to Like" [the opposite of 
magnetism, in which like poles repel]. Starch is a carbohydrate that is very 
similar to the carbohydrate cellulose, which is the major component of paper, 
wood and cotton.

Starch is insoluble in cold water but dissolves readily in hot or boiling 
water. The process is exactly the same as making gravy -- mix some cornstarch 
in a little cold water and add it to the hot stock and stir. In a few minutes 
the starch dissolves and thickens the gravy. Starch glues are the same [but 
without the flavor].

Virtually all corrugated boxboard is made with starch glues as are all brown 
paper grocery bags, tubes [as in toilet paper cores], and such similar items. 
Exact formulations vary but these are usually about 12 to 15% starch with the 
balance being water and minor ingredients. White library paste is 45% starch, 
55% water.

The water contained in starch based glue is present only as a vehicle, or 
carrier, of the starch. For the bond to form, the water must either be 
evaporated or absorbed [dried].

When you take a dress shirt to the cleaners you'll be asked "How much starch do 
you want?" Yup, same stuff, but starch also played a major role in making the 
shirt in the first place.After the cotton fibers are spun into yarn, but before 
they are woven into cloth, the yarn is dipped into a starch solution and dried 
to give them enough extra strength to survive the rigors of the weaving 
process. After being woven, the cloth is boiled in water to re-dissolve the 
starch so that it doesn't feel too stiff in the store when you buy it [this is 
true for all cotton fabrics except denim, which still has the starch in it when 
sold which is why a new pair of blue jeans feel so stiff]. This type of starch 
glue is called a 'sizing'

Hope this helps!

Ken