Page 347 - Carrahers_Polymer_Chemistry,_Eighth_Edition

310 Carraher’s Polymer Chemistry

Equation 9.44 may be transformed to Equation 9.45.
 
dT T  dl 
=−    
df  C p   dT  (9.45)
Unlike most solids, NR and other elastomers contract when heated.

9.11 RESINS

Shellac, which was used by Edison for molding his first photograph records and is still used as an

alcoholic solution (spirit varnish) for coating wood, is a cross-linked polymer consisting largely of
derivatives of aleuritic acid (9,10,16-triphydroxyhexadecanoic acid). Shellac is excreted by small
coccid insects (Coccus lacca) which feed on the twigs of trees in Southeast Asia. More than 2 mil-
lion insects must be dissolved in ethanol to produce 1 kg of shellac.
HOCH –(CH ) –CH(OH)–CH(OH)–(CH ) –COOH (9.46)
2 2 5 2 7
Rosin, the residue left in the manufacture of turpentine by distillation, is a mixture of the diter-
pene, abietic acid, and its anhydride. It is nonpolymeric but is used in the manufacture of synthetic
resins and varnishes.

H C
3
CH 3

CH 3 (9.47)
H C
3
HO
O
Abietic acid

Easter gum, a cross-linked ester, is obtained by the esterfication of glycerol or pentaerythritol
with rosin.
HO

HO

(9.48)
OH
HO

Pentaerythritol

Many natural resins are fossil resins exuded from trees thousands of years ago. Recent exudates
are called recent resins, and those obtained from dead trees are called semifossil resins. Humic

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