Characterising Structural Sealant
A structural sealant is a special type of adhesive that is used in the manufacturing of mechanical parts to increase their strength and durability. Structural sealants come in various types and are designed to resist abrasion and shear forces. They also have the ability to dissipate energy that is lost during the processing of the material.
Mechanical parameters
There are a number of mechanical parameters that are used to characterize structural sealant. These include the modulus, cohesive strength and adhesive strength. Each of these properties must be balanced with the design and configuration of the joint.
The modulus of elasticity is a critical property of a sealant. It describes the dynamic mechanical behavior of structural sealants at varying temperatures. Sealant moduli can be significantly reduced in comparison to the reference samples when exposed to combined mechanical and climatic loading. This is related to the melting of the crystalline phase.
Sealant adhesion is another property that is of importance. It is the ability of the sealant to adhere to a substrate. Sealants that do not have adequate adhesive strength will fail under tensile stress.
Joint movement is also a key property that is needed for sealing. Sealant movement capability is the amount of movement from the original width of the cured joint. To measure the degree of movement, it is necessary to conduct a test of the joint. Various testing procedures have been developed.
Differential scanning calorimetry, small-angle X-ray scattering and Fourier transform infrared spectroscopy have been employed. The former three methods have been shown to provide useful insights into the differences in the physical properties of sealants.
In addition to a range of characterization tests, the performance of structural sealants is subject to aging and fatigue studies. Those results should be compared with data from other characterization tests to determine if the sealant is a suitable candidate for a certain application.
The most important property of a sealant is the modulus. Moduli can range from a few hundred microns Structural Sealant to a few millimeters. A low modulus may cause substrate failure while a high modulus can facilitate the movement of a joint.
The modulus of elasticity can be measured by ASTM C1735. It is the most reliable test method for determining the actual modulus of a sealant. However, it is not a part of the C920 specification.
Other factors to consider are the makeup of the polymer. There are several types of polymers that have varied performance characteristics.
Resistance to abrasion and shear forces
Abrasion and shear forces are important factors that affect the mechanical performance of structural sealant materials. For example, the skin of an automobile instrument panel needs to have a high friction loss resistance. This can be achieved by using lubricants and masterbatches to decrease frictional force, abrasion heat, and whitening. In addition, macromolecular organic additives can help improve the surface adhesion and avoid sticky or white patches.
Polytetrafluoroethylene (PTFE) can also increase the abrasion and wear resistance of material. It is a self-lubricating material that reduces the friction of the sample’s surface. The material is also used for abrasion and wear resistance in composite materials. Common lubricants include polyethylene wax, siloxanes, and amides. These lubricants can help reduce frictional force, abrasion heat, wear, and staining.
High-flow TPE prepared by a star-shaped SEBS has the best abrasion and wear resistance. The material’s elasticity is strong, its flow rate is low, its VOC is relatively low, and it has good compatibility with PP. Moreover, it is suitable for automotive interiors.
Besides, the abrasion resistance of the material can be improved by combining the use of high and low molecular weight silicone. Although, the difference between these two types of masterbatches is small, the combination is beneficial to decrease the frictional coefficient of the material. Moreover, the interface between PP and SEBS is prone to delamination during the abrasion test.
The abrasion and wear resistance of a material can also be improved by combining the use of siloxane-based masterbatches. Siloxane can decrease frictional force, abrasion wear, and abrasion heat. M-H/LSi is a low-molecular-weight silicone masterbatch that exhibits a better feel, smoothness, and heat generation.
To study the abrasion and wear resistance of TPE, cross-scratch samples were prepared with Taber abrasion. During the test, a cylindrical standard eraser was used with a 5 mm cross section. After abrasion, the resulting layer of AR was measured.
Abrasion and wear resistance are the properties that influence the practical application of composite materials. Most of the practical applications of composite materials involve several types of abrasion. The abrasion resistance of TPE can be improved by increasing the molecular weight of the material.
Residual strength test
A new test method has been developed in partnership with the Federal Institute for Materials Research Berlin/Germany to Structural Sealant explore the durability of structural silicone sealant (SSG) systems. Based on a performance-based approach, this method exposes system test specimens to artificial weathering, multiaxial mechanical loadings and temperature and humidity.
The new methodology was developed to complement existing mechanical characterization methods. It consists of a set of test procedures that simulate one year of service exposure. This period of time is required to simulate the natural aging behavior of a SSG. These cycles exposed test specimens for 24 hours to simultaneous climatic and multiaxial mechanical loadings.
One of the more interesting tests is the peel test. In this test, a flexible adherend is clamped in the jaws of a tensile tester. Each of the two arms of the specimen is then pulled apart at a constant rate. During this process, the amount of deflection of each specimen arm is recorded.
Another useful test is the DCB test, which is a quasi-static test that measures the strength of a bonded joint. In this test, a test specimen is pushed out from a fixed position and a crack propagates in the mid-plane of the specimen. Stresses are measured as pounds per linear inch. Table 10 gives conversion factors for commonly used units.
There are several other types of tests that can be conducted to determine the mechanical properties of a structural sealant. They include peel and beam tests. While these methods provide a variety of information, they do not necessarily yield a standardized residual flexural strength value. Therefore, correlation testing is needed to verify these values.
In addition to the durability test, a round panel test was conducted. This test only provides an indication of toughness. Depending on the mix design and type of fiber used, a structural lining can have different performance requirements.
Finally, there was a study of the maximum sealant deformations. This included a 60-minute test assembly that consisted of burnt samples, heat distribution measurements and midspan load-displacement behavior. During this test, the adhesive bond between the CLT-wall headers and the Glulam-beam was subjected to partial exposure to a non-standard fire.
Dissipated energy
Dissipated energy is a valuable parameter in characterising the performance of a joint, during the exposure period. It represents the amount of energy which is lost in the joint during loading cycles. This is dependent on the temperature, strain level and type of mechanical load. The dissipation of a joint depends on the capacity of the structural sealant to absorb the energy.
The energy dissipated by a joint decreases with increasing strain. In addition, the dissipation of a joint decreases with an increase in temperature. These characteristics are important for the analysis of the response of structural sealants to fatigue and ageing. However, the mechanism by which dissipated energy is induced is not fully understood.
To understand the behaviour of dissipated energy, a number of experiments were carried out on two different structural sealants. These were designed to emulate mechanical loading and climatic loads. A total of nine repeated measurements were performed, with an indentation time of 3 s. For each test, the maximum stress was evaluated, and the yield strain was calculated. All the results were then compared with those of a reference structure. Results are presented in tabular and graphical format. Moreover, the performance of both structural sealants is compared in terms of strength, tensile and shear capacity.
Compared to the reference, the system in series B exhibits higher dissipated energy. These results may be attributed to the increased susceptibility of sealant b to fatigue and ageing. Another difference between the two systems is that the damping capacities of sealant b are lower than those of sealant a. Although there is little variation in the overall response of the two systems, there are differences in the responses of the specimens.
The results show that the dissipated energy course of specimen 1 of series A is comparable to that of the dynamic moduli of a joint. This suggests that the degradation of the structural sealant may be attributed to the cyclic weathering and mechanical loading. Similarly, the dissipated energy course of the specimen in series B is not compatible with the common behavior of a mechanical parameter during a strain-controlled fatigue test.