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Rheological investigation of adhesives and sealants

Adhesives are used to join surfaces, typically by changing their phase from liquid to solid. Sometimes this is triggered by a temperature change (hotmelt adhesives), in other cases the glue hardens at ambient conditions (contact adhesives) e.g. due to solvent evaporation or due to the humidity of the surrounding air. There are also reactive and multi-component adhesives, which harden by mixing two or more components that chemically react with each other, and one-part adhesives, which harden via a chemical reaction when an external energy source, such as radiation or heat, is applied. Adhesives are also found in nature, like pitch (from wood), bitumen, beeswax, or starch (from plants).

Sealants can be used to protect surfaces by impregnation; and also to fill small openings and thus prevent the penetration of gas or liquid.

Typically measured adhesives and sealants

Hotmelts

There are numerous applications for hotmelts in a variety of industries, such as the automotive, electronics, hygiene/medicine (drug patches, swaddling clothes, etc.), and packaging (box and carton sealing, adhesive films, labeling, etc.) industries as well as for clothes and shoes (adhesives for soles, coating, etc.) and household items (hot glue cartridges, adhesive films, etc.). Hotmelt adhesives make up one of the largest volume segments of the adhesive industry. Easy to apply and relatively low in cost, hotmelts can help to increase throughput in production and reduce costs due to time saving during applications. In addition, since hotmelts contain little or no solvent, environmental issues and associated costs are avoided. What distinguishes hotmelts from other adhesives is that they are applied in their molten state and are then rapidly cooled to form a tough, adherent solid at room temperature. Their fast setting time coupled with their relatively high viscosity makes them ideal for bonding porous materials. 

Rheological tests on hotmelts

Hotmelt adhesives are thermoplastic polymers of relatively high molecular weight (MW), giving them a high stiffness. However, high MW polymers generally do not have sufficient adhesive power (tack) on their own, so these polymers are blended with a variety of additives, which can include plasticizers, tackifiers, and stabilizers, to increase adhesive performance. Hotmelt adhesives are applied in a molten state and must flow smoothly onto surfaces to ensure both wetting and adhesion. For this reason, testing viscosity as a function of temperature with a rheometer is essential for ensuring proper hotmelt performance. Furthermore, hotmelts must be stable over time in order to form a tough bond. By knowing the rheological characteristics of a given hotmelt, its suitability for a given task can be determined, or its formulation can be modified to customize it for a specific application. One of the best ways to study the rheological behavior of hotmelts is temperature-dependent oscillatory measurement under constant dynamic mechanical conditions (meaning constant strain as well as constant frequency).

This test requires a rheometer equipped with a Peltier temperature control system.

This is just one of the rheological investigations typically used in the automotive industry.

Plastisol pastes

PVC plastisol pastes are used for injection, coating, dipping, molding, or extrusion processes. Such pastes can be applied, for example, as spray-coating plastisols on the underbody of vehicles, as coating for corrosion prevention in the chemical industry, as flooring in the building industry, for the production of artificial leather, protective clothing or sealing profiles. Usually, coating and shaping processes are carried out at room temperature. Subsequent heating should cause gelling and hardening of the plastisol.

Rheological tests on plastisol pastes

A factor of great significance for the operator is the temperature of the onset of gelation. The best way to describe the temperature-dependent thickening behavior of a plastisol paste is an oscillatory test at constant amplitude and frequency, performed as a temperature sweep. The processing temperature is usually room temperature. Due to particle friction occurring in the course of the process, however, temperatures of up to +30 °C and higher are to be expected, e.g. when using a spray gun or a blade coating. Furthermore, the temperature at which the viscosity is at a minimum is important. If this minimum viscosity is too low, the paste may run off or drip down the surface after application. Producers need to know the maximum temperature at which the material can be stored before being processed, especially in summer. The onset temperature of the gelation process is reached when the viscosity and stiffness of the paste increase (typically above +60 °C). Finally, the plastisol paste usually reaches its maximum stiffness at oven temperatures (typically above +120 °C).

This test requires a rheometer equipped with a Peltier temperature control system.

Silicone sealants

The need to protect something from external influences by sealing it is almost as old as mankind itself. Even in the new stone age, humans protected their houses from wind and weather by sealing gaps with natural sealants like grass, mud or wax. Starting with putty in the 18th century, numerous synthetic sealants have been invented. Silicone or acrylic sealants for interior fittings, for example in the bathroom, are among the most common.

Rheological tests on silicone sealants

One important test for the rheological characterization of silicone sealants is the determination of the thixotropic behavior. This describes the regeneration of the material after being pressed out of the cartridge. With a so-called step test, or three-interval thixotropy test, the material is measured under variable applied deformation – from small-amplitude to large-amplitude oscillation and back to small-amplitude oscillation. Oscillatory tests provide information about the visoelastic behavior of the material, so the test results can be used to characterize the viscous (loss modulus G'') as well as the elastic (storage modulus G') properties of a sample. A three-interval thixotropy test in oscillation provides valuable information on whether a sample is still flowing (with G’’ > G’) or already in a solid state (with G’ > G’’) after the application. In this way, it is possible to clearly distinguish between a good and a poor sealant, depending on the desired behavior during and after the application.

This test requires a rheometer equipped with a Peltier temperature control system.

UV-cured adhesives

Adhesives that cure under irradiation with UV light have a number of advantages over conventionally cured adhesives. They generally contain no solvents, and the curing process is fast – often only a fraction of a second is required – so that high processing speeds can be achieved. Furthermore, the thermal stresses on the materials are usually very small, as no external heating is required.

Rheological tests on UV-cured adhesives

The mechanical properties of the cured material depend on a number of factors, such as the amount and type of reactive agent, the intensity and wavelength of the UV source, and the irradiation time. Rheological measurements can provide a great deal of information about these factors. UV curing is best monitored by using oscillatory measurements at constant frequency and strain. For example, the figure below shows how the curing time of a UV-curing glue, as well as its final stiffness, are affected by the intensity of the UV irradiation.

This test requires a rheometer equipped with a UV curing system (a Peltier temperature control system with a UV light source and exchangeable glass plates to allow irradiation of the samples from underneath).