Blister detachment

As noted in the preceding section, IGC is an excellent tool for measurements of surface properties and of acid base interaction potentials of macromolecular solids (2,10,16). In this section the importance of acid-base interactions relative to the adhesion of PUs is considered in greater detail. IGC has been applied to a series of PU adhesives and to selected polymer substrates, allowing quantitative measurements to be made of the acid/base (electron donor-acceptor) interaction parameters applicable to the surfaces of these materials. Acid base pair-interaction parameters for substrate/PU combinations have been calculated. The bond characteristics of polymer/PU combinations have been measured, in part by conventional lap-shear procedures and in part, by the more recent constrained blister detachment method [11, 12]. Possible relationships between bond properties and acid base interactions have been considered, and a comparison of the two adhesion tests has been made. 

Figure 7.7 Schematic of arrangements for adhesion tests, (a) Specimen construction for lap-shear test, (b) Specimen construction and installation for blister detachment test.

A summary of lap-shear and blister detachment test data is presented in Table 7.8. The six adhesives perform differently with each of the substrates, suggesting a link with specific interactions at the adherent/adhesive interface. This is considered in the next section. 

Thus it is concluded that for assemblies of the type used here, lap-shear and blister detachment test methods produce sets of internally consistent data which correlate in a statistically acceptable manner. The choice of test procedure, therefore, may rest on convenience and on the proven relevance of test results to field performance. 

Figure 7.8 Correlation between lap-shear and blister detachment evaluations of bond strengths in Xenoy/PU and PVC/PU assemblies.

Fig. 3 Schematic representation of various types of failure associated with the mechanical instability of a film deposited on a substrate, (a) cracking of a thin film subjected to residual tensile stress, (b) plastic deformation of the substrate at the end of the crack, (c) deviation of the crack at the interface, (d) cracking of the substrate, (e) detachment and buckling (formation of a blister from an interface defect) of a film subjected to residual compressive stress, and (f) deviation of the crack through the thickness of the film (flaking).

Blistering Film becomes locally detached, forming a blister... 

The presence of mechanical stresses in the films is usually undesirable. With good adhesion of the film-substrate system, strong deformation of the substrate often takes place. If the stresses are very high, depending on sign, then because of transient effects in the case of overstressing of the elasticity, undesired crack formation or blistering can occur in the films. In the worst cases, the films can even become completely detached from the substrate. 

Although lap-shear evaluations are more familiar, they do not necessarily evalnate the strength of adherent/adhesive interfaces. Cohesive failnre of the weaker component of an assembly can be a complicating factor, tearing of the PU layer being a possibility in certain of the present measurements. In principle, the detachment of the adhesive in the blister test is a pure interfacial event and one that can be quantified provided parameters... 

The blister test has also been used to study mould adhesion. It requires a special mould, but could probably be used with any moulding machine. It is well suited to measuring the low levels of adhesion involved in mould release. Essentially a disc, like a beer mat, is moulded and the pressure is applied to displace it through a small hole ( penny-shaped crack ) at its centre. The detachment pressure can be related to the fracture energy. 

Soya polyurethane oil 5.2 isolated micro- blistering 60 microblistering over whole panel 9.4 no surface defects 0.0 microblistering along edges of scratch Film yellowed no surface defects About 10% weight loss after 30 hours film detached on washing... 

Flaking Detachment of pieces of material from the substrate or bulk, often preceded by cracking, crazing, or blistering. 

The first indication of deterioration of any paint film used for corrosion protection is the formation of a blister (10). Consequently a method was developed to investigate the behavior of such a coating material under conditions that may lead to film detachment in the form of a blister. 

There was a case of Sfevens-Johnson syndrome (SJS) associated with amoxicillin/clavulanic acid in an 18-month-old male child. Three days after initiation of amoxicillin/clavulanic acid (50 mg/kg/day) for a productive cough, he was brought to the hospital with fever (39 °C), a generalised erythematous rash (affecting his skin, eye, mouth, phar5mgeal and genital mucosa) with blisters filled with serous liquid and epidermal detachment on 5% of his body. Laboratory values were normal other than an elevated white blood cell count of 13.3 x 10 cells/L. After amoxicillin/ clavulanate was discontinued, the patient s lesions improved and he was discharged home [34 ]. 

Skin A 23-year-old woman developed ocular and vaginal itching. Later-on symmetrical erythematous macules appeared on her limbs, mainly with involvement of hands and feet. Within 7 days there was a significant spread of skin lesions, which evolved into blisters. She reported the use of lansoprazole 15 days prior to the appearance of skin lesions. On examination, the patient was pale, febrile and tachycardic. Epidermal detachment involved more than 90% of her body surface. There were also some areas of skin erosion and a mild involvement of oral mucosa and lips with crust formation. Lansoprazole was discontinued, and the adverse event was managed with intravenous immunoglobulin (2 g/kg for 3 days), skin debridement and daily dressings. A significant re-epithelialization was obtained in 15 days. After 2 months, the patient displayed only residual hyperchromic macules [48 ]. 

A third case, particularly important for thin films, is where the blister radius a is larger than the layer thickness t. Fig. 24. The principal mode of deformation of the layer is then biaxial stretching, and the relationship between inflating pressure II and height y of the blister before any further detachment occurs is given by ... 

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