Rubber crystallization

The principles were refined by Meyer in a second paper (70). In it he proposed that the micelles occurred at regular intervals. He also included an explanation of the elasticity of rubber based on the assumption that the molecular chains tended to roll together in knots in unstretched rubber, but line up when stretched. This explanation was especially elucidating since it agreed well with Katz s discovery (53) that amorphous rubber crystallizes when stretched. 

Beiistein Handbook Reference) AI3-08621 BRN 0507434 EINECS 203-674-6 1,3-Dibutylthiourea 1,3-Di-n-butyl-2-thiourea 1,3-Dibutyl-2-thiourea NSC 3735 Pennzone B Pennzone B 0685 Thiate U Thiourea, N,N -dibutyl- Urea, 1,3-dibutyl-2-thio- Urea, 1,3-di-N-butyl-2-thio- USAF EK-2138. Accelerator for mercaptan-modified chlotoprene rubber, an activator for ethylenepropylenediene terpolymers and natural nibber, an antidegradant for natural rubber-latex and thermoplastic styrene-butadiene rubber. Crystals mp = 63-65°. ElfAtochem N. Am. 

Beilstein Handbook Reference) AI3-14636 BRN 0773905 CCRIS 243 N,N -Diethyl-thiocarbamide 1,3-Diethylthiourea 1,3-Diethyl-2-thiourea N,N -Diethylthiourea N,N-Diethyl-2-thiourea EINECS 203-308-5 HSDB 4106 NCI-C03816 NSC 3507 Pennzone E Thiate H Thiourea, N,N -diethyl- U 15030 Urea, 1,3-diethyl-2-thio- USAF EK-1803. Accelerator for mercaptan-modified chloroprene rubber. Antidegradant for natural, nitrile-butadiene, styrene-butadiene, and chloroprene rubbers. Crystals mp = 78° bp dec. Xm = 234, 265 nm (c = 6310, 7244, MeOH) slightly soluble in CCI4, soluble in H2O (0.1 - 0.5 g/100 ml), EtOH, very soluble in Et20 LDso (rat orl) = 316 mg/kg. ElfAtochem N. Am. 

At low elongations, crystallization in natural rubber can be very slow. Natural rubber will crystallize slowly, often several months, at no elongation. This rubber crystallized at room temperature is termed stark rubber because it is stiff and rather rigid at room temperature in contrast to the flexible sheets of coagulated natural rubber latex usually found at room temperature. 

Unlike synthetic elastomers such as polybutadiene with a high level of cis configuration or polychloroprene, natural rubber crystallizes very slightly at cold temperature without strain. Kawahara et a/. suggested that the presence of free fatty acids in natural rubber might be conducive to cold crystallization. 

The measured dependence of a on A is shown in Figure 3.7 for natural rubber. The fit is good up to A = 1.2. The fit for compression (A < 1.0) is excellent. The lack of fit above A = 1.2 is due to several factors which include (i) rather simple assumptions in the model (ii) that the chains cannot be Gaussian at high extensions since they cannot extend further than their own contour length (see (3.N.6)) and (iii) at high extensions vulcanized natural rubber crystallizes. 

The melting range of rubber crystals as a function of the crystallisation temperature. 

Rapid decrease of E was observed with strain increase. Pure NR and composites with a low CNT content (1 wt%) showed a moderate increase of stress up to about 75% strain, a sort of plateau up to 300% strain and a final more evident increase above 300%. Said increase could be due to rubber crystallization and to CNT alignment. The stress-softening effect, known as Mullins effect, was observed at large strain and attributed to detachment of rubber molecules from the surface of filler particles. The presence of CNT bundles (at 5, 7, 10 wt%) was commented to bring about a decrease of the stress." ... 

Natural rubber is widely used in truck and aircraft tires, which require heavy duty. They are self-reinforcing because the rubber crystallizes when stretched. 

Explain in simple thermodynamic terms why a stretched band of natural rubber crystallizes. 

Dielectric loss data for different stretch ratios was fitted according to the HN equation (Eq. 1). The fit parameters (As, b, c and thn) obtained at T=-40°C can be visualized in Fig. 3.b-d). The dependence of the dielectric derived magnitudes can be separated into two regimes. In the first regime, at low strains (<300%), we are situated in the region before the emergence of rubber crystals and where only orientation of the amorphous phase takes place this regime is characterized... 

When stressed in a monoaxial direction, natural rubber crystallizes, thus causing a heating of the material since the enthalpy of crystallization is equal to 4.4kJ-mor The crystallization generates a helical structure close to a transplanar conformation with two monomeric units per helix turn. The stress relaxation causes the melting of crystalline zones that is an endothermic phenomenon easy to reveal. 

Natural rubbers crystallize to a greater extent than most other rubbers, and crystallization is limited by a number of factors, including molecular imperfections (branch points, cross links, chain ends) and chain entanglements. Butadiene copolymers such as GR-5, Hycar, or Buna 5 are considered as non-crystallizing due to the likelihood of a cis-trans isomer mixture in butadiene and the irregularities associated with the presence of a secondary constituent [21]. 

Fig. 10.40 Plot of half-time for crystallization against crystallization temperature for natural rubber crystallized with Tuads. Percent Tuads Di, 0.75 D2, 1.5 D3,...

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