High temperature adhesives comparisons

High-temperature adhesives S J SHAW Comparison of different types recent developments... 

The large volume applieations for moisture-euring polyurethanes are diseussed briefly in the applieations seetion below. These adhesive offer superior low temperature, high temperature, and speeifie adhesion performanee in comparison to conventional hot melts. These attributes have allowed them to excel in various product assembly applications, including structures such as windows, doors, furniture, and automotive headlamps and trim. Newer applications include bookbinding, fabric laminating, and assembly of athletic shoes. 

FIGURE 15.5 Comparison of (a) heat resistance and (b) thermal aging of several high-temperature structural adhesives.11... 

A comparison of the various surface treatments is presented in Table V. The resin used was methylenedianiline cured at high temperature. All of the joints were polished with 600 grit emery cloth and treated with ammonium citrate, with the exception of the control and sand blasted joints. Sandblasting the surface increases the surface area and provides a means of mechanical interlocking of the adhesive with the substrate surface. This does improve the water durability to some extent, but the effect is small when compared to the results achieved with the coupling agents. Ammonium citrate, in addition to activating the surface, has a beneficial effect on durability. 

The modification of bitumen with SBS copolymer powder was done using the mechanochemically devuicanized GRT (Zhu et al., 2009). The penetration index, softening point, 5°C ductility, aging behavior, and rheological properties of bitumens modified by 8,10, and 12 wt.% GRT/SBS mixtures were measured. In comparison with the bitumen modified by incorporation of 5.5 wt.% SBS alone, the majority of properties of the blends were improved, except their penetration. Rheological properties indicated that at high temperatures 10 wt.% GRT/SBS-modified bitumen was better than SBS-modifled bitumen. The SEM observation of the fractured surfaces showed that bitumens mixed with the GRT/SBS powder had a better interfacial adhesion with matrix than with SBS alone. 

Continuous scans of modulus versus temperature utilizing the DuPont Dynamic Mechanical Analyzer (DMA) has provided a comparison of the high temperature service capabilities of radiation-cured experimental formulations of a vinyl-modified epoxy resin. Shell Epocryl-12. These scans were compared to data obtained when the same materials were applied as adhesives on aluminum test panels, radiation-cured with an electron beam, and lap shear strength tested at discrete temperatures. The DMA instrument utilizes a thin rectangular specimen for the analysis, so specimens can be cut from blocks or from flat sheets. In this case the specimens were cured as sheets of resin-saturated graphite-fibers. The same order of high temperature stability was obtained by each method. However, the DMA method provided a more complete characterization of temperature performance in a much shorter test time and thus, it can be very useful for quick analyses of formulation and processing variables in many types of materials optimization studies. The paper will present details of this study with illustrations of the comparisons. 

If an adhesive failure occurs under high-temperature conditions, its Tg should be checked using one of the thermal analysis techniques discussed above. Even at room or low temperatures, anomalies in Tg can be indicative of overcure and potential microcracking. For multiphase adhesives - e.g., toughened epoxy - deviations of Tg could indicate a problem with the composition of the modifier, the adhesive preparation, the formulation, or the cure cycle. A control specimen for comparison is always advisable, rather than use literature values that can vary significantly. Another influence on Tg can be the exposure to moisture or humidity, which lowers the Tg considerably. Increases in Tg can be attributed to the incorrect cure or port-cure, or even by exposure to temperatures above Tg during operation. 

Tensile testing machine operated in a controlled temperature and humidity environment. Many applications involve bonding paper, which exhibits properties that are highly dependent on humidity. In addition, adhesive Tg values are often close to room temperature, and thus properties change substantially over even a few degrees. Therefore, for reproducible comparisons, temperature and humidity control is necessary. 

In this study, in order to improve the adhesion properties of aramid fibers to rubber as matrix, nylon thin films were securely formed on the surfaces of the aramid fibers by a radio frequency ion-plating (RFIP) method which represents an application of low temperature plasma treatment. These fibers were coated with a RFL (resorcinol-formaldehyde-latcx) adhesive which has high affinity to both nylon and rubber The adhesion properties of the fibers to rubber were evaluated, and the effects of the RFIP method were confirmed by a comparison of the RFIP treated fibers with those subjected to the low temperature plasma treatment. The usefulness of such surface modification methods will be discussed. 

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