Stable crack limiting length

FIGURE 13.9 The dependence of stable crack limiting length on ZD length for PASF film samples [1]. 

The choice of whether to use the CT or SENB test is influenced by several factors. First, the amount of material available CT specimens are larger than SENB specimens so if the quantity of adhesive is limited, this would favor the SENB test. Second, the type of crack propagation observed the tests are quasi-static and unstable crack initiation from the initial defect is intended in the test. If stable growth is obtained the CT specimen will usually permit a greater length of crack propagation to occur than the SENB test. O Figure 20.1a shows the CT and O Fig. 20.1b shows the SENB test specimens. For either, it is necessary to mold bulk adhesive samples with sufiScient dimensions to satisfy the validity criteria (see below) and to be sufficiently void free (see Chap. 19 for more details on how this can be achieved for common adhesive types). 

So, the ethylene production does correlate with coke presence, in particular with aromatics formation as far as the diffusion limitations are not significant. However, it seems that the majority of ethylene is not always formed directly from MeOH [115]. The aromatics and other coke species could be the products of the conversion of primary carbenium ions, which are mobile and could equilibrate each other [28]. This may explain the isotopic distribution in products and retained coke molecules and the coexistence of aromatics and carbenium ions [28], In addition to the coproduction of ethylene with aromatics in olefins interconversion cycle, formation of ethylene via alkylation-dealkylation of methyl aromatics with heavy olefins or with the equivalent carbenium ions like ethyP, propyP, and butyP could be an option. The alkyl aromatics with the side chain length of two carbons or longer are not stable in the pore and dealkylates on the acid sites due to too long residence time and steric hindrances. This may lead to formation of ethylene, other olefins, and alkylaromatics with different structure, namely PMBs [129]. In other words, the ethylene is formed via interaction of the carbenium ions like ethyP, propyP, and butyP formed from MeOH or heavy olefins with aromatics and other coke precursors followed by cracking and in a less extent by a direct alkylation of PMBs with methanol. The speculation is based properly on analysis of the prior arts and is not contradictory with the concept of the aromatic cycle for ethylene formation. 

---