Differential Scanning Calo~imetry Study of the Cross-linking of Styrene and an Unsatura :ed Polyester The Chemistry of Canoe Manufacture Joel ~ e b r e l ' lnstitut Universitaire de Technologic, 25000 Besanpn, France Yves Grohens and Abderazak Kadmlrl Universite de Franche-Comte. 25000 B e s a n ~ nFrance ,
owl in^^
Eric W. The Polytechnic of Huddersfield, Huddersfield, UK, HD1 3DH to investigate both the variation with concentration of accelerator and the percentage cross-linking.
In the chemistry department of the IUT at Besan~onone group of students undertakes a specialized course in materials science. Course experiments are designed to investigate the preparation of materials, the properties essential to understanding their formation, and the optimization of their application.
Student Preparation As part of the overall assignment students are encouraged to research the reaction (3). For example they are asked to do the following.
The Extent of Cmss-Linking The experiment described here, using differential scanning calorimetry (DSC), allows the investigation of the extent of cmss-linking between styrene (ST) and a n unsaturated polyester (UPE) as they react to give the polymer STUPE. The cross-linked polymer STUPE is used with glass fiber in the fabrication of canoes a s part of the students' practical applications program.
Supply both the IUPAC name and the common names for the reagents. Provide an assessment with respect to the safe operation of the proeess. Research the mechanism of the reaction to explain why the STUPE is not a one-dimensional ribbon-like polymer. Suggest methods for determining the relative molecular masses of UPE polymer. Describe the purpme of the accelerator, and tell why its wncentration should be varied with ambient temperature.
Its PracticalApplication
Because the canoes are made outdoors, in all types of weather, the students must know the percentages of initiator and accelerator required to sustain the reaction in a controlled manner as ambient temperatures vary from 10 to 30 'C. I t is also important to know over what ~ e r i o dthe cross-linkine - mate- ~olvmerizinp - rial 411 be workable. I n other words, it is helpful, both practically and economically, to anticipate the time lapse between the initiation of the cross-linking reaction and the material becoming too viscous to pour easily. This is governed by the time needed for the reaction to reach a c e r t s n percentage of crosslinking. The Reaction
Here we study the cross-linking reaction between ST and the UPE derived by condensation from maleic anhydride and propylene glycol to give STUPE polymer as shown in Figure 1. The reaction is initiated by free-radical organic peroxides, and cobalt salts are used as accelerators. The double bonds are activated by the freeradical initiator. Bonds can then be formed between ST a n d t h e UPE. eivine .. t h e three-dimensional matrix STPE. The exothermic reaction may he followed conveniently by DSC ( 1 , Z ) . The magnitude of the exotherm is proportional to the advancement of reaction. Thus, this thermal technique may be used
Experimental
Caution: Organic peroxides are extremely flammable and corrosiue. Skin contact should absolutely be avoided.
'-
3 3
H
0
+
2rn
II I ((cH2)3e-c=c-c-o) II UF€
ST
HI
0
"
I
-
-+-+(cy),uE-6-
I
H
I
STUPE
13-4 0
I
rn
'Author to whom correspondence should be addressed. F~gure1. Scheme,formation of STUPE. Z ~ a tyevisting professorat IUT (Cnim e), Besanqon. Volume 70 Number 6 June 1993
501
Gloves and protective eyewear should be used when conducting procedures with organic peroxides. 'Caution:If heated or in a mixture with a potential accelerator or other impurities, the organic peroxides may spontaneously and uiolently decompose. Peroxide-accelerator mixtures should alwavs be made uo immediatelv before use in the unsaturated polyester-styrene system. Caution: Cobalt salts are misonous even bv contact with s k ~ n Gloves . and protective eyewear should be used when conducting procedures with cobalt salts. Reagents The unsaturated polyester ia ahtained by the polycundensation of maleic anhydride with propylene glycol (Aldnch) aRer the usual method (1).The ~alvmerUPE is diluted ta 60% bv mass with stvrene (~ldri&): .The initiator is methyl ethyl ketone peroxide (MEW) (Naury,New York). 'The accelerator is cobalt(I1)oetanoate diluted to 6% by mass in dibutylphthalate (Noury). Apparatus The differential s c a ~ i ncalorimeter e is a Mettler TC 11 mupled with a DSC 20 oven. The oven is placed in a freezer to obtain the lower range of temperatures. Procedure Experiment 1. Investigation of the Initiator-Accelemtor System The students determine the concentration of accelerator required to sustain the cross-linking reaction by varying the temperature of the surroundings (4,5). STlCTPE solution (20 e) was weighed into an aluminum dish that was immers2 in an ice-sodium chloride bath a t approximately -5 'C. The initiator-accelerator system. which was methodically varied (see the table), b a s then added. After thorough mixing and the minimum delay (less than 3 mid, a 10-mg sample of the reaction mixture was transferred to an aluminum thimble (capacity: 40 pL). The thimble was then capped with a pierced lid to allow contact between the reactants and the air. The thimble and contents were immediately placed in the oven of the calorimeter, and the temperature was raised from -10 'C to 200 'C a t a rate of 10 'C m i d .
Experiment 2. Inuestiation of the Rate of Cross-Linking
In this experiment the initiator system used must initiate and sustain the reaction a t room temperature (20 'C) (6).In the first experiment, this was determined to be 0.3% cobalt salt. The reaction mixture, as described above, was kept a t 20 OC, and 10-mgsamples were taken after 20,60,120, and 240 min. These samples were subjected to investigation by DSC as above from 20 "C to 200 'C with the temperature increasing at a rate of 10 'C m i d . Results Experiment 1 The thennograms plot the enthalpy evolved per unit time idHldt~alrainst temoerature T (Fie. 2). In R m r e 3 initiation temperature is plotted against accel&ator concentration with initiator concentration held constant. (See the table.) Thus, the percentage accelerator required to sustain the reaction a t various temperatures can be determined.
-
