Tape Adhesion with Tedlar


The following was taken from Ultraflight Lazair Newsletter #8:

“In the past five months we have had four reports of loosening of Tedlar covering, apparently due to poor  tape adhesion.   As  the reports are all quite different, there is no indication of any one particular problem, and therefore determining the cause (and remedy)  is  not  easy.   However,  based  on  the  information  we  have available, we can make the following suggestions. 

(a)     Avoid overheating the tape (and  the  Tedlar)  when  heat  shrinking.   As  stated  in  the  manual, overheating  the tape will cause it to shrink excessively and will lift it at the edges.  It is also probable that excessive heat will have an adverse effect on the adhesive.   Overheating  the  Tedlar will cause it to shrink excessively and could tend to pull it away from the tape. 

(b)     When cleaning the aluminum prior to the application of the tape (whether on a new aircraft  or  when recovering)  use only lacquer thinner as suggested in the assembly manual.  There is some indication (but no proof) that the use of acetone for cleaning the aluminum may effect the acrylic adhesive  on the  tape.   Do  not  use  metal cleaners (such as Met-All, Nev-R-Dull, Flitz etc.) as many of these are designed to apply a protective coating as well as clearnthe metal.  These  coatings  (especially the ones which contain silicones) can severely impede tape adhesion. 

(c)     Make sure there Is sufficient overlap of  tape  on  the  aluminum  (as  described  in  the  assembly manual),  especially  along  the D-cell and along the root rib.  If in doubt, additional tape should be applied with at least 3/4 of an inch in contact with the aluminum. 

(d)     If there is any indication of Inadequate adhesion around the perimeter of the Tedlar,  some  of  the wide single face tape could be removed and replaced, or additional tape could be applied as  In  Cc) above.

(e)    Lack of adhesion of the foam tape on the ribs, while not a common problem, could be a bit  more difficult  to  fix.  We have only seen this problem once, and the effected area was so small, it was just left (though watched closely) and the condition has  not  worsened.    If   you   should   ever encounter  this  situation  (and  assuming  you  don’t wish to recover the wing), you could rivet an additional aluminum capstrip to the effected ribs on the outside of the covering (similar to  C4  on the  wingtip’).  However, if you do this, be sure to put at least one layer of 1 1/2″ or 2″ tape over the Tedlar before the capstrip Is put on and use double face tape under the capstrip.   Be  sure  to file  or  sand  the edges of the capstrip so they do not cut Into the covering.  In any case, do not (as one customer suggested) attempt to rib stitch the  Tedlar.   Rib  stitching  a  non  rip-stopped material  could  potentially  create  many  more  problems than it could cure.  While the additional capstrip suggested above does necessitate drilling rivet holes through the covering, the  stress  on the  covering  is  distributed  by the relatively large area of the capstrip.  If rib stitching were used, the stress would be much higher due to the small diameter of the rib cord.

(f)        If you paint your Tedlar and/or tape, use a light colour.  There is some indication that painted  a  dark colour. and left in direct sunlight for a prolonged period, the covering creep under the tape due to the extremely high temperature developed.

It  is may tend to

(g)       To check for overshrinkage on your wings, put a straightedge on the trailing edge  and  measure  the deflection of the T25 trailing edge tub& between each pair of ribs.  A deflection of one sixteenth of  an inch is about right.  An eighth of an inch is excessive but acceptable.  A quarter of an inch deflection indicates that the particular panel has been overshrunk.  The covering and tape  on  that particular panel should be inspected and watched very carefully or replaced.  

(h)       An inspection of the covering and tape should be included in every preflight.  

The following two paragraphs have been added to the assembly manual, and should be  observed  if  you  recover your Lazair’·:  

“As with most acrylic adhesives, the initial tack with this tape is only moderate, but the  adhesion  improves as  it  ages.  For this reason, it is essential that the tape be firmly pressed down to make sure there is 100 percent initial contact.  Then, as the adhesive cures, a proper bond will develop.”  

“Unlike Mylar and most other heat shrinkable covering materials, Teldar will continue to shrink  significantly after  the heat source has been removed.  Therefore, to avoid overheating the Tedlar, apply the heat for a few seconds, then remove it and check for signs of shrinkage.  If there is no indication, heat it  a  bit  longer, then  remove  the  heat  and check again for shrinkage.  As the heating period is Increased, you will find the correct exposure so most of the shrinkage will occur after the heat source has  been  removed.   If  the  heat is  maintained  on  the  Tedlar  for a significant period of time after it begins to shrink, it is possible to overheat the material and reduce the adhesion of the tape.” ”

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