Page 58 - Handbook of Adhesion Promoters

3.6 Peeling 51

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erties of the polymer?). The answers show the complexity of the system. Figure 3.5
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shows the fracture energies of all polymers as a function of RH. Three distinct regions of
crack growth behavior may be identified for PMMA and PEMA, which are denoted as
regions I, II and III. In the region I (below 60% RH), the adhesion was relatively insensi-
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tive to moisture. The failure was cohesive in the polymer layer. As the RH was raised
from 60 to 70% (region II), the fracture energies dropped dramatically (nearly two orders
of magnitude) over a small change in RH. The failure was a mixture of adhesive and cohe-
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sive failure. In region III (above 70% RH), the fracture energies of PMMA and PEMA
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joints were extremely low. The joints in the region III failed predominantly at the poly-
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mer/substrate interface.
As for the third among the systems examined, the PBMA joints exhibited the worst
adhesion to glass in dry conditions, but showed the best adhesion strength of all polymers
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examined in wet conditions (above 70% RH). The fracture energies in dry state were
only slightly higher than those in the wet conditions for the PBMA joints, suggesting that
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moisture has little impact on the adhesion of this system. It is quite interesting to notice
that a small change in the chemistry of the polymer (ethyl to butyl) can dramatically
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change performance.
The above observations suggest that the origin of the criticality in adhesion loss
might be attributed to the combination of interface weakening by ingress of water mole-
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cules assisted by moisture-induced swelling stresses. PBMA has a much lower modulus
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and exhibits significant creep. The creep may reduce stresses that arise from swelling
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induced by water. This creep behavior is not observed for the much stiffer PMMA or
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PEMA.
Hot water immersion is a broadly approved method of testing of epoxy coated sub-
strates. The recommended methods of testing based on hot water immersion were issued
by German and Swiss authorities, ASTM standards, Ontario Ministry of Transportation,
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and Texas Department of Transportation.
Parylenes are used in a wide range of applications in microelectromechanical sys-
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tems (MEMS) devices. Their poor adhesion in a harsh liquid environment (potassium
hydroxide wet etching) limit the fabrication processes of complex MEMS and bioMEMS
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devices. The delamination of Parylene C layers in a KOH wet etching bath was caused
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by the penetration of the KOH into the substrate-Parylene interface. Silanization (3-
methacryloxypropyltrimethoxysilane) in combination with thermal treatment (air plasma)
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improved the adhesion of Parylene C to Si, Si N , and SiO wafers.
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3.6 PEELING
Pressure sensitive adhesives are removed from the adhering surface by peeling. They per-
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form if they have a tacky surface of rather high viscosity. They adhere spontaneously to
a solid within a split second and after only a light pressure, and they reach higher peel and
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shear resistances after succeeding slow flow processes. The behavior of pressure sensi-
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tive adhesives is strongly influenced by rheological properties. Figure 3.6 shows the
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relationship between peel force and peel rate. Two observations are apparent from this
graph. Peel force increases with the peel rate increasing, and water presence has only lim-
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ited effect on adhesive properties of pressure sensitive adhesives.