Quality Control of Adhesives6th Installment

Quality Control of Adhesives6th Installment

Viscosity is an important indicator of the properties of a substance. Viscosity has from early on been actively utilized by the adhesives industry in its efforts to promote standardization in product development and in the quality control process. Progress on research of rheological properties has resulted in the introduction of a variety of adhesive products designed to meet diverse and high-level requirements - products which offer properties (e.g. non-Newtonian, thixotropic, yield value) appropriate to the conditions and requirements of the job. Rheological properties thus enable a means to control attributes of adhesives such as working properties, quality of finish, bonding strength, and leveling and sagging characteristics.
In this column we shall explore viscosity measurement as an adjunct to the evaluation of adhesive performance.

Working properties of adhesives and their assessment

The underlying motives regarding viscosity measurement of materials such as adhesives, paints, and coatings, which are vital in the manufacture of industrial products would, in general terms, involve the following rationales.

1) Flow behavior is an indirect indicator of uniformity and quality of the product.

2) Viscosity measurement is the most sensitive method of characterizing and differentiating substances (e.g. on the basis of molecular weight, molecular weight distribution, granularity, particle distribution, concentration, and structural level).

3) Viscosity measurement allows direct evaluation of a products working properties (e.g. application characteristics, pump delivery power, control of coating thickness, and sagging and leveling characteristics).

Obviously, the underlying assumption is that the adhesive can provide the required bonding function after hardening. But as described in point 3) above, the ease with which the adhesive can be worked is also a very important product attribute. Building construction - because of the numerous places adhesives are employed and the large quantities involved - provides a good illustration of this. At a job site, it is apparent that the construction schedule and work process is directly impacted by how easily the adhesives used can be worked and handled. Important factors in evaluating working properties of an adhesive are its sag and leveling characteristics. These particular characteristics however are contrary to one another and there is a tradeoff involved as adhesive manufacturers struggle to offer the best balance between the two in their products.
Let us look more closely at the aspect of the working properties of adhesives. Oft cited problems with adhesives at construction worksites include sagging or droop on wall surfaces, brush marks, brush drip, fatigue caused by poor spreadability of the adhesive, and poor cartridge flow. Such problems may arise because the associated properties conflict with other performance requirements of the adhesive such as method of application, purpose of the task, and the application involved. Therefore it is imperative to evaluate the products performance in order to be able to select the most suitable adhesive for the job.
There are various methods used to evaluate the application characteristics of adhesives but they fall short from the standpoint of scientifically sound quantitative determination, relying instead on experience based on trial and error. Thus the industry is also pursuing efforts to come up with a standard method of evaluation.

Plug Foamer method promoted for standardization

At present, the method of quality measurement for adhesives being supported under FDIS 14678 is a sag test which utilizes the Plug Foamer test apparatus. The recess in the test apparatus is filled with the adhesive to be tested. After filling, the surface of the adhesive is leveled smooth and flat. The Plug Foamer is then positioned vertically and the adhesive in the recess is extruded (the bottom of the recess has a movable section which is pushed to force the adhesive out of the recess). The distance that the adhesive sags from the recess and time is measured. In cases of samples which are highly flowable, the sample is not extruded and instead, the distance is measured as the material is allowed to droop from the recess.
The Plug Foamer method has the following features.

(1) Sag can be evaluated over a wide range of adhesives, from highly flowable test samples to building floor finishes and sealers.

(2) Sample setting (i.e., sample filling of plug recess) with this method does not vary greatly from individual to individual.

(3)It is simple and easy-to-use.

Based on these features, data is being referenced on this method in general evaluation of adhesive sag resistance.
However, as easy as sample setting may be with the Plug Foamer method, it is apparent that large variances in measurement may occur when measuring polymeric structures with their complex properties - general thixotropic adhesive substances, for example - depending upon how much the original state of the sample changes or is adjusted (e.g., through temperature, agitation stresses, etc.). Thus sag measurements with the Plug Foamer method, though simple, may vary widely according to the conditions of the test. Indeed, as evaluation results depend greatly on what changes were undergone by the sample and on the filling method of the tester, it may be difficult to refer to the Plug Foamer method as a quantitative measurement approach.
In evaluating the properties and quality of products, it has become important in measurement to minimize quantitative, scientific, and test personnel errors. One way to accomplish this is to evaluate an adhesives rheological properties using viscosity measurement instead of attempting to directly measure the products sag resistance.

So, what is the best test method ?

Most adhesives, as mentioned above, are non-Newtonian substances and the viscosities of such substances vary greatly according to shear rate. For this reason, when evaluating working properties (i.e., leveling and sagging) characteristics of an adhesive, it is important to measure rheological behavior at yield values and low shear rates. As mentioned above, there is difficulty in establishing with certainty the conditions of measurement with the albeit easy-to-use Plug Foamer method. With viscometer measurement on the other hand, it is quite simple to quantitatively apply shear load and perform measurement.
More specifically, a correlation between sagging measured with the Plug Foamer method and one of the viscosity characteristics - non-Newtonian coefficient μ (which corresponds to viscosity measured with a 3° cone rotor at 0.5 rpm) - has been proven from the results of various tests. This means that it is possible to determine the sag characteristics of a target substance from the results of a single point of measurement at 0.5 rpm using a general-purpose viscometer. Thus viscosity measurement provides a quantitative and simple means of evaluation which enables highly reliable appraisal of an adhesive’s working properties and quality, without having to resort to cumbersome testing involving a variety of methods.
At the same time, it should be noted that adhesives are compounds of intricate polymeric structures where rheological characteristics of their component raw materials are reflected in the product. Thus when evaluating thixotropic characteristics which are influenced greatly by temperatures and shear rates, the services of an experienced viscosity ‘pro’ with the requisite measurement know-how may be necessary. Toki Sangyo, with the wealth of data and experience we have accumulated over long years in the field , can provide such support as part of our effort to emphasize the advantages of viscosity measurement in the development and manufacturing quality control of adhesive products.

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