Creating a region of heat sealed Tyvek where that sealed region has become “transparent” or “Transparentizing Tyvek” is characterized as an oversealed area or region of Tyvek.  Excerpted from ASTM F 1886-98:

“X1.4. Oversealed Areas–A combination of too much heat, pressure, or dwell time may cause oversealing to the point that it may cause packaging componenets to melt and become brittle and susceptible to cracking. When Tyvek sppunbonded polyolefin or a similar porous material is one of the substrates, this condition often is seen as a transparent or translucent seal where the material fibers are fused. This may become evident through an evaluation for pinholes or cracks along the seal. Another result of oversealing may be a spotty or incomplete looking seal, where, in this case, the heat seal adhesive is overheated an flows into the porous fiber structure of the material.”

The functional effect of oversealing Tyvek is to create potentially weakened transitional regions within the Tyvek structure.  That is, a potentially weakened region is formed in the transition area between the sealed and non-sealed areas of the Tyvek structure.  The transition area between sealed and unsealed region can be weakened, brittle and more prone to physical integrity compromises resulting from the fusing and embrittlement of the Tyvek structure.  Of course, any physically compromised area of a sterile barrier package increases the risk level for loss of physical package integrity which compromises sterile barrier integrity.

Oversealing is not a positive attribute and should be avoided by means of good sealing process development and sealing process control.  A Design of Experiments is a useful tool in developing sealing parameters which avoid oversealing your Tyvek structure.