Natural rubber crosslinked by dicumyl peroxide in amounts up to 25 parts per hundred of rubber (phr) showed a maximum in tensile strength near 1 phr, followed by a steep decrease to a minimum near 5 phr. The ultimate elongation decreased from 870% at 0.5 phr. to about 10% above 10 phr. The modulus increased linearly with increase of crosslinking. The creep rate decreased from 5.6% per decade at 0.5 phr to zero at 5 phr and higher values. Crystallization, with a resultant abrupt increase in creep, was noted in specimens held in the stretched condition for more than one day. Between 5 and 25 phr this system (when crystallization is avoided) appears to function as an ideal elastic network and can be recommended for studies of rubber elasticity since no variation of modulus with time is observed.

Natural rubber mix ed with varyin g a mounts of di c um yl pe roxid e was c ross-linked by heating 120 min . at 149 DC. T. he q uantitative measure of cross-linkIn g was take n as the amount fp of decom · posed dlc umy l peroxId e, th e 'product . of p , the numbe r of parts added per hundred of rubber and f the fracllon decomposed dunng the tIm e of c ure. The s hea r c reep modulus C was calculated from meas ure ments of th e indentation of a Aat rubbe r sheet by a ri gid sp here. Th e glass transition te m· pe rature Tg, was raised about 1.2 DC for e ac h part of de cumplJsed di c umyl perox ide . Above (7~+ 12) tl,,· nilldulu s-te mp c ratur~re latioll s we l'l' lin car with a slope tl, at ill c reased with ill t'reas in gc nJ ss-link in g. Th e cree p rate was neglIgibl e except near t.h e glass tran s ition and at low valu es of fp . Valu es of G, read from these plots at seven temperatures, we re plotted as a funct.ion of fp. h.ey word s: C ree p; cross-linkin g; di c um yl perox ide; glass tran s iti on; inden tati on measure me nts; rllbb er. natura l.

No abstract

The long-time creep of natural rubber cured with a conventional sulfur-accelerator recipe containing no filler can be conveniently shown near room temperature by a plot of ΔE/E1 with a double-abscissa scale, one marked in units of log t and the other in units of t. When experimental data from the present work and from previous studies reported in the literature are plotted in this manner it is noted that invariably the first scale yields a linear relation at short times and the second a linear relation at long times. The limiting linear relations just mentioned suggest the two-constant Equation (2), already proposed as a general creep equation for many materials. In the case of rubber the range of values of t investigated is from about 10 ms as studied by previous investigators to about 70 days in our work and other studies. Any significant deviations from the equation can be noted by inspection of the double-abscissa plot. We found that the equation furnished an excellent representation of almost all our experimental data up to the longest times. In one instance in our work and in a few other cases there was a prerupture increase of ΔE/E1 above the values given by the equation. This behavior can reduce somewhat the upper limit of validity of the general equation. The constants A and B can be evaluated from experimental observations of ΔE/E1 by solving two simultaneous equations obtained from the values at the longest time, at one minute, and at an intermediate time. In the present work, the constant A was essentially the same (about 2.4%/ (unit log t)) when the atmosphere surrounding the specimen was a vacuum, dry nitrogen, or dry air. The value was raised when the atmosphere was room air at 35% relative humidity and became about 4%/ (unit long t) when the air was saturated with water. The constant B was raised tenfold when the atmosphere was dry air instead of dry nitrogen. It was further increased by a factor of about 2, when the air was saturated. The value of B for the specimen in an atmosphere of stagnant room air was still greater than this by another factor of more than 2. It is possible that this atmosphere contained autocatalytic degradation products or other constituents which were removed when the air was bubbled through water or passed over CaCl2. The approximate boundaries of three different regions of time are determinable from the ratios A/B. In the first region, where t is less than 0.1(A/B), ΔE/E1 is approximately linear with log t. In the second region, where t is between 0.1(A/B) and 4.343 (A/B), ΔE/E1 is not linear with either log t or t. In the third region, where t is greater than 4.343 (A/B), ΔE/E1 is approximately linear with t. A fourth region of anomalous increase preceding rupture is sometimes found, especially when B is large.

Previous work on t he calculation of shear co mplia nce J (limit of t he ratio of s train to stress at zero deform ation) from observations of t he indenta t ion of a flat na tural r u bber surface by a rigid sphere h as been extended over greater ranges of time, temperature, a nd state of vulca nization. The extreme ranges of time wer e 5 sec to 23 .5 hr a nd of t e mperature -67 to + 100 DC. The curves of JT against log 1-60 (where I-GO is t he equivalent tim e at -60 DC a nd ' 1' is in O K ) were s igmoid wit h a r egion of m aximum slope and co nti Iluing t hrough a region of minimum slope extending over a bout 7 decades. In contras t w it h t he p redictions of t he Williams, Landel, a nd F erry eq uation, the empirically determined s hif t of abscissa necessary to give a single J T cur ve was found to be a linear funct ion of T -I with a slope corresponding to an "apparent activation encrgy" of 36.9 kcal (O K )-I (molc)-I (154.5 kJ (O K) -I (mole) -I).