TMA Investigation of a Hot Melt Adhesive

Hot melt adhesives, also known as hot glues, are very popular because they are solvent-free and very convenient and easy to use. They are often supplied in cylindrical sticks of different diameters, designed to be melted in a hot gun. Chemically, they are based on thermoplastic polymer raw materials such as polyamides or polyurethane and polyester elastomers. Their melting behavior or gel point is of great importance depending on the particular application. This can be determined by means of TMA measurements and is illustrated in the following example.

Introduction

A TMA measurement records the dimensional changes of a sample as a function of time or temperature while it is subjected to a defined force [1]. The force is exerted by a probe (a sensor) that usually rests on the surface of the sample during the measurement. Small forces are used in an experiment to determine the expansion or shrinkage of a sample and appreciably larger forces to record the softening of the sample [2].

In addition, so-called Dynamic Load TMA (DLTMA) measurements can be performed in which the value of the force changes periodically, that is, the force oscillates between a small and large force [3]. The smaller the amplitude of the resulting measurement curve, the lower the elasticity of a sample. 

In this article, TMA experiments were performed to characterize a hot melt adhesive using a large force in order to detect the glass transition and the softening and melting ranges. In addition, the DLTMA technique was used to measure the gel point of the sample. Here the gel point is defined as the point at which the sample softens and the probe can finally be pulled out of the previously hard sample to which it had adhered. 

Nowadays, many different types of adhesive are in use, for example acrylate adhesives, rubber adhesives and hot melt adhesives to name a few. Hot melt adhesives were originally based on nonadhesive synthetic resins that can be heated and melted at temperatures of up to 180 °C. The glue is soft and tacky when hot but hardens on cooling, thus providing good adhesive properties at room temperature. The melting of the resin at high temperatures led to the name “hot melt”. 

Hot melt adhesives have very varied applications. They are used as packaging adhesives, for example in the food industry, as adhesives in the clothing industry, or even for the manufacture of diapers (nappies). Besides being used in stick form, they can also be applied by spraying or dipping

Experimental Details

The yellowish hot melt sample was first investigated by TMA using a TMA/ SDTA841e equipped with a ball-point probe. The results were confirmed by DSC using a DSC 1 equipped with an IntraCooler and an FRS5 sensor. The measurements were evaluated with the STAR^e software.

For the TMA experiments, small samples with a height of about 0.7 mm were cut from the bulk material and placed between two quartz glass disks. The disks were 6 mm in diameter and 0.5 mm thick. The probe rested in contact with the upper quartz glass disk.

The TMA measurements were recorded at a heating rate of 10 K/min using a force of 0.1 N. The DLTMA experiments were performed at a heating rate of 2 K/ min using a force that varied periodically every six seconds between +0.1 and –0.1 N; the TMA probe was in direct contact with the sample.

The DSC measurements were performed in 40-μL aluminum crucibles with a hole in the lid from –90 °C to +200 °C at a heating rate of 10 K/min. The sample mass was approximately 7 mg.

Results

Figure 1 compares the TMA and DSC measurement curves of the hot melt adhesive. The TMA curve (black) first exhibits a glass transition that begins at about –37 °C and is observed as a change in the rate of expansion of the sample. This is shown on an expanded scale in Figure 2 in the lower curve (black).

Summary

The glass transition and the melting behavior of a hot melt adhesive can be characterized by means of a TMA measurement using a constant force. In addition, the gel point can easily be determined using the DLTMA technique. To do this, the TMA probe is placed in direct contact with the sample and an alternating force applied. 

The gel point is reached when the probe no longer sticks to the sample and can be raised and lowered from its surface. This gel point is characteristic for adhesive performance and allows different products to be compared. It is also particularly useful for quality control. The TMA results were confirmed by a DSC measurement.

_{Investigation of a Hot Melt Adhesive by TMA | Thermal Analysis Application No. UC 337 | Application published in METTLER TOLEDO Thermal Analysis UserCom 33}