Carbon blacks cure rate

Carbon black grades identified by four characters (ASTM D1765-67), ie, cure rate of normal (N) or slow (S), digit classifyiag typical particle size ia am, and two arbitrarily assigned characters. 

Fillers. Materials used as fillers (qv) in mbber can also be classified as acidic, basic, or neutral. Furnace blacks, ie, HAF, FEF, or SRF, are somewhat basic. As such, they can have an activating effect on sulfur cure rates. Furthermore, carbon blacks have been found to promote formation of mono/disulfide cross-links thereby helping minimize reversion and enhance aging properties. 

A typical super-rate burning of an HMX-GAP composite propellant is shown in Fig. 7.43. The lead catalyst is a mixture of lead citrate (LC PbCi), Pb3(C5H50y)2-x H20, and carbon black (CB). The composition of the catalyzed HMX-GAP propellant in terms of mass fractions is as follows gap(0.194), hmx(0-780), lg(0 020), and, q 0.00G). GAP is cured with 12.0% hexamethylene diisocyanate (HMDI) and then crossUnked with 3.2 % trimethylolpropane (TMP) to... 

On the other hand, the alkoxide system presented several problems in formulation. The system first chosen as a model consisted of a trimethoxymethyl silane crosslinker, 8000 centistoke HEB siloxane, and a catalyst. A number of catalysts were used and each exhibited different cure rates and electrical properties. DuPont tetraalkoxytitante-Tyzor appears to he one of the better catalysts used in this type of curing system. Fillers are usually incorporated into the silicone formulation to improve mechanical properties, promote adhesion, and to serve as light screening and pigment agents. Cab-o-sil, a form of fumed silica, carbon-black, titanium dioxide and calcium carbonate are then used as RTV fillers. 

Figure 7.23 Applications of the NMR-MOUSE to elastomer materials (a) T2 values for a curing series of carbon-black filled NR. Comparison of values obtained at high homogeneous field (DMX 300) and with the NMR-MOUSE, (b) cross-link series of unfilled SBR with different sulfur content, (c) T2 versus glass-transition temperature Tg of unfilled SBR by the CPMG and the steady-state CPMG methods, (d) normalized Hahn-echo decay curves for poly(butadiene) latex samples. Different decay rates are obtained for small medium and large cross-link densities...

The 13C NMR examination of CBS cured SBR was performed on unfilled, silica filled, and carbon black filled samples [41], Several different SBR samples with respect to styrene content and cis, trans, and vinyl BR content were used. The unfilled SBR samples gave 3 major peaks that appeared at 51.0, 50.2, and 49.3 in a spectrum similar to BR vulcanisation. Unfortunately, peaks below 45 ppm are obscured by the different main chain structural peaks of SBR. A difference was seen in the rate of formation of these peaks in filled samples with silica inhibit the vulcanisation rate compared to carbon black filled samples. 

For SBR modulus (Fig. 15), structure and either particle size or surface area were significant. We found no correlation when both particle size and surface area were used together. The results are similar between SBR and natural rubber. Modulus is the least predictable property in rubber from the three important properties of carbon blacks. Both modulus and cure-rate are affected by the slight surface oxidation which blacks receive during the drying of finished pellets. 

Curing formulation Polymer =100 HAF Carbon Black = 50 Naphthenic Oil = 5 ZnO = 5 S = 2 TMTD = 1 MBT = 0.5 Temp. = 145 °C. S. Table 1 ENB = 5-ethylidene-2-norbornene 1,4-HD = 1,4-hexadiene DCP = dicyclopentadiene MNB = 5-methylene-2-norbomene. Cross-linking reaction rate constant. 

Applications . NMR has found the following applications in filled systems carbon black adsorption of SBR, the effect of carbon black loading on cure rate of natural rubber, gel-like behavior of polybutadiene/carbon black mixtures, structure and dynamics of carbon black filled rubber vulcanizalcs, " interaction of... 

Special considerations zinc oxide is crosshnking agent carbon black is a more efficient UV stabilizer in sulfur-cured SBR than in radiation or peroxide cured " vulcanization conditions affects electric conductivity of carbon black filled SBR solvent diffusion and swelling rate decrease as concentration of carbon black increases ... 

Dutta and Ryan (1979) examined the effects of fillers (carbon black and silane-surface-treated silica) on the cure of DGEBA/MPDA epoxy-amine systems. They found that the rate constants of the cure reaction are affected by the presence of the fillers in an unusual fashion (a function of temperature and concentration) with respect to concentrations up to 10%. This was postulated to be due to the reactive surface groups on the fillers. The reaction order, however, is not affected. 

Carbon Blacks - Two classes of carbon blacks most frequently used in compounding liquid polysulfide polymers are the furnace and thermal blacks. SRF 3 and Sterling MT are fillers typical of these two classes of carbon blacks. Channel blacks are not used in polysulfide compounds. Their low pH considerably retards the rate of cure and the very large surface area greatly restricts volume loading. 

A simple qualitative visual method for rating the dispersion of fillers (50phr carbon black) In Blon elastomer was developed and Is Illustrated In Figure 2. A cross section of cured elastomer Is examined under a binocular microscope to check gross dispersion of fillers. The visual dispersion Is rated against a set of standard photographs of dispersions which had previously been ranked and correlated with certain Important physical properties. For example, In Figure 3, the flex life of a well dispersed elastomer was over 300 million cycles while that of a poorly dispersed one was below one million cycles. The correlation of physical properties to dispersion has been substantiated with other rubbers (14). 

For carbon-black fillers, structure, particle size, particle porosity, and overall physico-chemical nature of particle surface are important factors in deciding cure rate and degree of reinforcement attainable. The pH of the carbon black has a profound influence. Acidic blacks (channel blacks) tend to retard the curing process while alkaline blacks (furnace blacks) produce a rate-enhancing effect in relation to curing, and may even give rise to scorching. 

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