Técnica operatoria para la realización de Ensayos con el

Initially isothermal DSC measurements were used to investigate the relationship between degree of cure and time at isothermal temperature. In this case the specimens were loaded into the DSC at 30 °c, the temperature of DSC then ramped up to the specified isothermal dwell temperature at the fastest achievable rate and held for a specified period of time according to earlier results. A second identical run was then performed upon the same sample and the raw data subtracted to correct for calorimeter perturbation caused by the rapid heating process (100). The degree of conversion was then calculated according to the formula; 𝛼 =∆𝐻𝑗 _∆𝐻

𝑡𝑜𝑡,𝑇

⁄ × 100, where α = degree of conversion in % , ∆𝐻𝑗 the part

enthalpy of reaction calculated by the area between time = 0 and j in j/g. ∆𝐻𝑡𝑜𝑡,𝑇 is the total

enthalpy of reaction which was calculated by a dynamic DSC scan also in j/g. The dynamic DSC scan was performed from 30 °c until the end of the curing reaction at a rate of 10 °c/minute for a separate uncured sample of the same adhesive. If ∆𝐻𝑡𝑜𝑡,𝑇 was less than the total

enthalpy calculated at the end of the isothermal run this value was substituted by that from the isothermal technique. In all cases mass normalised heat flow values were used for ease of comparison between samples. The dynamic scan also provides important information on the cure onset and endset temperatures which is advantageous for cure cycle design. Dynamic DSC scan data for each candidate adhesive is presented in Figure 77.

Figure 77 highlights some of the striking differences in cure behaviours between the adhesive products. For example, Sika 7666/522 begins to cure at room temperature with a peak at approximately 55 °c explaining the rapid change in viscosity during application and variability noted in section 4.3.4. Contrastingly PU 1510, Lohmann DuploTEC® 10400 SBF and DuploTEC® 10600 SBF cure much more slowly until reaching an onset temperature of approximately 90, 115 and 145 °c respectably. DuploTEC® 10600 SBF being the 0.6 mm thickness version of DuploTEC® 10625 SBF. Generally only one peak was observed in the dynamic scan with the

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exception of 3M™ SA9816 which has two peaks at approximately 85 and 115 °c and two higher

peaks at 250 and 280 °c, these were believed to be due to a dual stage cure of the product which is identifiable with the distinct two steps observed in section 4.3.3 Figure 64.

40 60 80 100 120 140 160 180 200 220 240 260 280 300 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Hea t fl o w j /g Temperature / o c SA 9816 PU 1510 10400 SBF 10600 SBF 7666/522

Figure 77 Dynamic DSC scans at 10 °c/minute for candidate adhesives linear baseline subtracted

Degree of conversion vs isothermal temperature plots are shown in Figure 78. Upon first inspection it appeared that PU 1510 has by far the fastest rate of cure, followed by 10400 SBF, 10600 SBF and finally 7666/522. However, several concerns were identified with this method of data colelction. Firstly, for rapid curing products such as 7666/522 a substantial proportion of the reaction may have occurred in the time taken to prepare, weigh, load the sample and start the test. Thus, the loss of exotherm data during this time results in an apparent reduction in degree of cure. It was identified that the sampling rate of 1 sample/s may not be great enough to record all of the data with the fast reacting PU 1510 (101) as such data may be lost resulting in an apparent reduction in final degree of cure. Another consideration is that during the sample ramp to isothermal temperature cure will occur at non-isothermal temperature. For rapid curing products this could result in unreliable data. An alternative method is to load the specimen at the isothermal test temperature and start the analysis immediately after loading. However, this method introduces noise during loading especially using the available HP-DSC with a more complex chamber closing procedure, DSC systems with an automatic sample loader may be beneficial for this approach.

As a final conclusion of the isothermal DSC degree of conversion method, neglecting the considerations previously mentioned, the data is only relevant for the temperature profile

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investigated. For example, in a manufacturing environment where there may be a requirement to change the cure temperature or profile, it would be required to repeat the DSC data for an identical thermal profile to establish the exact relationship between degree of cure and the thermal profile. Due to these drawbacks of isothermal DSC data for evaluating degree of cure vs time it was determined than an alternative approach to degree of cure evaluation would be beneficial. As a result, it was identified that model fit approaches may be of much greater benefit to industry than isothermal methods, avoiding these limitations.

0 100 200 300 400 500 600 -20 0 20 40 60 80 100 Deg re e o f c o n v e rs io n / % Time / s PU 1510 Isothermal 150 °c 10400 SBF Isothermal 175o_c 10600 SBF Isothermal 175oc 7666-522 Isothermal 125oc SA9816 Isothermal 130o_c

Figure 78 Degree of conversion vs time at isothermal temperature, isothermal DSC method