Stabilization of Adhesives

The stabilization of adhesives follows the general rules for polyurethane polymers which are now summarized with respect to adhesive applications. 

Urethanes are not high temperature polymers. Continuous service applications at more than about 100°C are not recommended. They are subject to hydrolysis other materials should be considered in applications which involve long-term continuous immersion in water. They undergo autoxidation on thermal or ultraviolet activation. Poly(ester-urethanes) are subject to microbiological degradation. 

The addition of carbodiimides, e.g. Staboxal PCD, which is supplied by Bayer, to poly(ester-urethanes) is one of the most effective ways to stabilize them against hydrolysis. Also the polyurethane structure can be tailored for better hydrolysis resistance through elimination or reduction of the ester groups present in the polyol. Hydrolysis resistance increases in the order of ether polycaprolactone polyester. Satrastab, developed by SATRA (Shoe and Allied Trades Research Association, Kettering, England), is also claimed to be an effective hydrolysis stabilizer for formulated polyurethanes in poromeric footwear materials. 

The response of polyurethanes to thermally activated autoxidation depends upon polymer structure. In general, polyurethane degradation by this mechanism is suppressed by the addition of antioxidant to the polymer. Ultraviolet initiated autoxidation is suppressed by a suitable screen (e.g. carbon black, titanium dioxide) or a combination of antioxidant and ultraviolet absorber. Irganox 1010 and Tinuvin P (Ciba-Geigy) are particularly suitable antioxidant and ultraviolet absorbers, respectively, for polyurethanes. Polyurethane structures with enhanced resistance to ultraviolet initiated autoxidation may be possible. 

Inclusion of an effective fungicide in poly(ester-urethanes) prevents their degradation by microbiological action. Effective agents of this type include copper-8-quinolinolate and A -(trichloromethylthio) phthalimide (Tungitrol 11 —Nuodex). Poly(ether-urethanes) resist microbiological degradation. 

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Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Allen, K.W., Greenwood, L. and Wake, W.C., The stability of adhesive bonding between silicone mbber and alumina for neural prostheses. J. Adhes., 16(1), 61-76 (1983). 

The performance and long-term stability of adhesive joints and coatings are strongly affected by the chemical and stractural properties of the interphase region situated between the uninfluenced adhesive polymer bulk and the adherend [1-16]. Unfortunately, this interphase is often hidden to the experimentalist, e.g., in closed joints. However, the interphase can be elucidated to a good approximation by studying thin polymer films [16-20] which are often referred to as open joints . Film thickness is varied in this approach to obtain information on microstructure gradients (e.g., in Refs. [1, 3, 4, 19, 20]). Bulk properties are expected to... 

As noted previously, the evidence for the contribution of microtubules to amoeboid cell motility and chemotaxis is mixed [242]. The microtubule organizing center has been reported to be localized to either the front or rear side of the nucleus, depending upon the cell type [167, 187, 188]. Alterations in microtubules can affect fibroblast lamellipod extension and motility [150], but in some assays, chemotactic responses may be unaffected [202]. Alterations in acetylation enhance chemotactic ability [98]. Microtubules have been proposed to alter the stability of adhesion sites, enhancing their disassembly [II9]. In sum, microtubules are likely to be permissive for amoeboid motility and chemotaxis, and respond to polarization of the cell generated by the actin system with polarization of the microtubule system. This may in turn stabilize cell polarity and enhance overall chemotactic efficiency. In the absence of a strong external stimulus, or in cases in which autocrine secretion influences cell polarization, the microtubule apparatus may provide critical signals for cell polarity [164]. 

Fleat or temperature has a considerable effect on curing, working life, and stability of adhesives. Several structural transitions can occur in adhesives during heating. Thermal analysis techniques can provide a detail road map of curing, properties, and stability of cured adhesives that has predictive utility. They also provide the ability to assess degree of cure and composition. 

As mentioned previously, hot-melt adhesives are primarily a blend of several hydrocarbon based components, each of which is susceptible to thermooxidative degration. The stabilization of adhesive compounds against thermooxidative degradation is complex. Typically used to inhibit or prevent degradation, antioxidants are added to each hydrocarbon component [6] as well as to the final adhesive formulation. To gain a better understanding of the stabilization of an adhesive formulation, it is beneficial to examine the degradation mechanisms of the individual components. 

Uses Antioxidant tor polymers for base stabilization of elastomers and the compd. stabilization of adhesives food pkg. adhesives, coatings,... 

Cationic Surfactants in Laundry 10.1.1 Heat stability of adhesion ... 

E. J. Hughes, J. Boutilier, and J. L. Rugherford, The effect of moisture on the dimensional stability of adhesively bonded joints, in Adhesive Joints—Formation, Characteristics, and Testing (K. L. Mittal, ed.), p. 137, Plenum Press, New York (1984). 

ISO 15908 2002 Adhesives for thermoplastic piping systems - Test method for the determination of thermal stability of adhesives... 

The discoloration was found to be caused by trace amounts of hydrochloric acid which form when Neoprene oxidizes on aging. This acid reacts with iron to form ferric chloride, which in turn reacts with the thiuram disulfide modifier to form a black iron sulfide compound. Acid acceptors such as magnesia and zinc oxide were found to be effective in preventing this discoloration. However, their addition tended to decrease the viscosity stability of adhesive cements even further. 

Temperature Service Range In Degrees F (a) Shear Strength Range at 70 Deg. F In Psi (b) Stability of Adhesive Types for Various Surface Conditions ... 

Application of the concepts of acceptor-donor interaction is an active area of re- search in adhesion science. In the last few years the foundations have been laid that will allow XPS, and to a lesser extent SSIMS. to play an important role in such investigations. Although information cotKeming the acid-base properties of polymer and inorganic surfaces is generally difficult to obtain, it is clear that the goal of achieving a predictive approach to adhesion and to the hydrolytic stability of adhesive joints and coated substrates is now much closer than a decade ago. 

Elevated temperature is often used to estimate the in-can stability of adhesives and sealant formulations. This is usually applied by a simple rule of thumb that says the reaction rate of the degradation mechanisms involved doubles for every 10°C increase in temperature, and therefore 1 week storage at 50°C approximates to 8 weeks at room temperature (20°C). Application of this rule will likely be conservative in the prediction of shelf life and must be verified by real-time validation testing conducted at room temperature for the targeted shelf life. 

Scott G (Winter 1998) Stabilisation of adhesives against environmental peroxidation - Society for Adhesion and Adhesives. One day meeting on Stability of Adhesives ... 

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