4 DISCUSSION:

THE VALUE OF 300g/2.5cm has been offered as the minimum acceptable value for a lining by Phenix & Hedley. For the discussion which follows, 300g/2.5cm has been reduced to .115kg/cm for easier comparisons. Table 2 and Figure 2 include the results obtained by peel testing the Lascaux 360 H. V. and BEVA 371. By comparing average values, one sees that the Lascaux 360 H. V. exhibits higher peel strengths than BEVA 371 in all applications except PHU (heat activation of unsized canvas). One also sees that the xylene activation of Lascaux 360 H. V. (PXU, PXS) produces the highest peel values, almost 10 times the Phenix & Hedley parameter. When the xylene-activated samples (PXU, PXS) are compared to the values recorded for heat activation (PHS, PHU), one also sees that the qualitative results corroborate the intuitive and empirical tests which first instigated this experiment, namely, that xylene activation of Lascaux 360 H. V. produces a stronger bond than heat activation of the same adhesive.

When one looks at the effects of sizing the original canvas with 10% B-72, one sees that in all cases of heat activation, the sizing increases peel strength. In the case of the Lascaux 360 H. V. one sees almost a 10-fold increase in peel strength, from PHU 360 (.059kg/cm) to PHS 360 (.546kg/cm). In this case, sizing the original puts the value well over the Phenix & Hedley value of .115kg/cm. On the other hand, it would appear that heat activating a canvas not sized with B-72 would not produce a bond strong enough to hold down cupping or extreme distortions. In the case of the BEVA 371, sizing the original canvas with B-72 almost doubles the value for peel strength from PHU 371 (.100kg/cm) to PHS 371 (.180kg/cm). This again would put the adhesive into the Phenix & Hedley range.

Table 3 and Figure 3 compares the Lap/Shear tests. One sees on graph that the relative values coincide with the peel tests of each adhesive, respectively. However, it is very interesting to note that the values of Lap/Shear are consistently higher for BEVA 371 than Lascaux 360 H. V., except for LXU 371 and LXU 360. The data from Tables 1, 2, 3 would indicate that BEVA exhibits a relatively low peel strength and relatively high shear strength. It should also be noted that the values for the lap/shear test are a bit more consistent as seen in the standard deviations. If one compares BEVA 371 and Lascaux 360 H. V. in Table 1, one sees that standard deviations for the Lascaux 360 H. V. are lower, which may indicate a consistency in the adhesive's properties when used as in this experiment. The higher standard deviations in peel tests seen in the BEVA 371, especially with xylene activation, coincide with the inconsistencies and problems found in the use of xylene activated BEVA (see PROCEDURE).

Table 4 tests the relative amounts of adhesive left on the original canvas after mechanically peeling the lining canvas and original apart (as resulted from the experiment). It is clear that BEVA 371 consistently remains on the lining canvas, where it should be, and is not really affected by the B-72 sizing. On the other hand, one sees that especially in the xylene activation of Lascaux 360 H. V. residues can be a problem. One also sees that sizing with B-72 increases the probability of residue transfer. When one compares the values between PHS and PHU one sees a slight increase in residue transfer, as does one when comparing the values between PXU and PXS.

Table 4 Amount of Adhesive Remaining on Original Canvas (0 = No Residues … 5 = Uniform Amount of Total Surface)

What do these numbers really mean? First, it would appear that sizing an original canvas with B-72 increases peel strength and bonding when low temperature activation is used; in the case of BEVA 371, almost 2 times, in the case of Lascaux 360 H. V., 10 times. Second, xylene activation of Lascaux 360 H. V. produces the strongest bonds of any methods or materials tested. Third, mechanical reversibility may be determined by bond peel strength. One sees that the BEVA 371 remains consistently on the lining canvas as it should, but its peel strengths are relatively low. When one looks at the residue data for Lascaux 360, the lower peel strengths produce the same results as the Beva (no residues); but the higher values of Lascaux indicate a problem in mechanical reversibility. The data may also indicate that the B-72 sizing may increase the amount of residue when Lascaux 360 H. V. is xylene activated. In conclusion, the numbers give some insight into handling these two adhesives in the low heat level modes, or solvent modes, but a word of caution is needed. The data from this experiment gives relative strengths which Phenix & Hedley continue to establish by their ongoing research into peel and lap/shear testing. However, from a practical point of view, one has to test these techniques/applications personally to get a feel for the bonds produced. For example, it is my personal experience after peeling all the samples that even though BEVA 371 (PHU & PHS) have relatively low values and fall below Phenix & Hedley's .115kg/cm or hover around it, they still produce a strong enough bond for the ‘nap’ bond lining system for which they were intended. Of course it might not hold down cupping or severe distortion, but when used as intended by Lodge and Albano it appears to produce acceptable bonds. On the other hand, my experience with Lascaux 360 PHU indicates that the bond is slightly weaker than the ‘nap’ bond and an average figure of .059kg/cm should be considered a relatively weak bond strength. Furthermore, in speaking with colleagues, it appears that there are many variables at work affecting the strength of the bonds, including time between application and use of the adhesive, the using of a space blanket during lining, pressure used, and, in some cases, the amount of time between lining and testing. Some people report that the Lascaux 360 H. V. bond gets stronger with time.

These results are presented in the spirit of Phenix & Hedley to begin to quantify the empirical findings and give some broad insights into handling these materials.