Quality control adhesives

Laminated tapes In more general use now than pressure sensitive tapes are tapes consisting of polyvinyl chloride or polyethylene films in conjunction with butyl rubber. These tapes are applied with an adhesive butyl rubber primer. Thicknesses of up to 0-75 mm are in use and loose protective outer wraps of p.v.c. or polyethylene sheet are commonly applied. Tape quality control is exercised with reference to ASTM standard test methods and may include water vapour transmission rate and elongation. 

Advanced adhesives are composite liquids that can be used, for example, to join aircraft parts, thus avoiding the use of some 30,000 rivets that are heavy, are labor-intensive to install, and pose quality-control problems. Adhesives research has not involved many chemical engineers, but the generic problems include surface science, polymer rheology and thermodynamics, and molecular modeling of materials... 

Detecting known substances, and determining their quantity, is also important. In synthetic research, it is essential to know the relative proportions of various reaction products. In manufacturing, it is important to detect any impurities in the product and to determine whether they are present in a significant amount. Analytical characterization is critical in pharmaceutical products, for instance. Products for practical uses—paint or adhesives, for example—will typically consist of several components. For proper and reliable performance it is important to measure the amounts of each of the components as part of a manufacturing quality control system. Manufacturers also commonly need to analyze the raw materials they receive, measuring the amounts of various substances in them to be sure that the material meets their requirements. Before it can be correctly processed into steel, iron ore must be analyzed to determine how much of other components need to be added to produce a metal alloy of the desired composition and properties. 

The original applications of NIR were in the food and agricultural industries where the routine determination of the moisture content of foodstuffs, the protein content of grain and the fat content of edible oils and meats at the 1% level and above are typical examples. The range of industries now using the technique is much wider and includes pharmaceutical, polymer, adhesives and textile companies. The first in particular are employing NIR spectrometry for the quality control of raw materials and intermediates and to check on actives and excipients in formulated products. Figure 9.26(b) demonstrates that even subtle differences between the NIR spectra of enantiomers can be detected. 

The copper compounds include Bordeaux mixture and the so-called fixed coppers including the copper oxides, copper oxychloride, copper oxychloride sulfate, and related materials. Bordeaux mixture, since its discovery about 70 years ago, has been the classic fungicide for the control of late blight and still is more widely used than any other single material. Certain of its qualities, particularly adhesiveness, remain unequaled by its commercial competitors, and it is cheap and effective. However, the inconvenience of preparing Bordeaux mixture, and the unreliable quality of hydrated lime available in most places, have tempted many growers to switch to other copper compounds or organics. 

Packaging Paper, cardboard, plastics, films, adhesives Raw materials screening Physical properties, such as crystallinity - polymers Quality control... 

Adhesion tests can be broken into two categories qualitative and quantitative. They vary from a simple Scotch tape test to a complicated flyer tape test, which requires precision-machined specimens and a very expensive testing facility. Quantitative (such as peeling) tests have been developed for coatings on plastics (12), but not to the same extent for metal-to-metal systems. The quantitative testing systems in limited use, mainly in the electronics industry, are not commonly present in production plants but have been used to aid in process development. For quality control purposes, qualitative tests for metal-to-metal adhesion (13) are usually adequate. The adhesion of some plated metal parts is improved with baking for 1 to 4 h at relatively low (120 to 320°C) temperatures. 

A number of instruments are based on the extmsion principle, including slit flow and normal capillary flow (Table 6). These instruments are useful when large numbers of quality control or other melt viscosity test measurements are needed for batches of a single material or similar materials. When melt viscosities of a wide range of materials must be measured, rotational viscometers are preferable. Extmsion rheometers have been applied to other materials with some success with adhesives and coatings (10,161). 

Adhesive remains unchanged until it is cured No multiroll coating needed No complex tension controls needed Adhesive bond established almost instantly Real-time quality control... 

The adhesive bond is established almost instantly. Real-time quality control. 

Dipping method for adhesive coating of the metal discs is the best method for moulded rubber products manufacture while the quality control required is frequent checks on viscosity and solid content of the dipping bath. The later will be maintained by slow and constant agitation both vertically and horizontally in the bath. 

Adhesive Disadvantages. There are some limitations in using adhesives to form assemblies. The major limitation is that the adhesive joint is formed by means of surface attachment and is, therefore, sensitive to the substrate surface condition. Another limitation of adhesive bonding is the lack of a nondestructive quality control procedure. Finally, adhesive joining is still somewhat limited because most designers of assemblies are simply not familiar with the engineering characteristics of adhesives. 

The application of a primer is an additional step in the bonding process, and it comes with associated costs and quality control requirements. Therefore, primers should be used only when justified. The most likely occasions for a primer to be used are when (1) the adhesive or sealant cannot be applied immediately after surface preparation, (2) the substrate surface is weak or porous, or (3) the adhesive-adherend interface requires additional protection from service environments such as moisture. 

The following section looks at the processes and equipment commonly used in the compounding of epoxy adhesive systems. Since they can be utilized by either the formulator or the end user, there will be no strict division of the discussion by user. Hazards and safety issues related to these materials and processes are discussed somewhat in this chapter, but a more thorough discussion is found in Chap. 18. Test methods, quality control processes, and standards that are commonly used at the formulation level are addressed in Chaps. 19 and 20. 

This chapter discusses quality control and the writing and use of specifications that apply to epoxy adhesive systems. These subjects are of interest to both the formulator and the end user. 

Quality control testing processes that are commonly used with epoxy adhesives are discussed in Chap. 20. Tests included are those used by the formulator as well as those employed by the end user of the adhesive system. 

Both the formulating of adhesives and bonding with adhesives are complex, multiple-part processes, complete with interacting and sometimes unexpected parameters that may contribute to success or failure of the final product. Thus, it is important that the quality control process consider the entire operation from receipt of materials to final product testing. 

A generalized flowchart for the quality control process in formulating epoxy adhesives is shown in Fig. 19.1. A flowchart for controlling the quality of the adhesive bonding process is shown in Fig. 19.2. It must be realized that in both cases, the decisions made in one phase of the process may affect the subsequent phases. Therefore, all the individual phases must be carefully coordinated and controlled. 

Quality control encompasses all the processes and activities that ensure adequate quality in the final product from receipt of materials, through manufacture, to final product test. Quality control is very important when one is using adhesives because once a product is fully... 

FIGURE 19.1 Generalized flowchart of quality control process in manufacturing epoxy adhesives. 

FIGURE 19.2 Flowchart of a quality control system for adhesive bonding. 

As with the quality control plan for the formulator described above, acceptance tests on adhesives should be directed toward assurance that incoming materials are identical from lot to lot. However, this plan should also extend to the receipt and control of incoming adherends and other materials used in the bonding process. 

In addition to tests performed on the incoming materials, test specimens may be made to verify the strength of the adhesive joint. The quality control tests should be those that can quickly and accurately detect deficiencies in the adhesive s physical or chemical properties. ASTM lists various test methods that are commonly used for adhesive acceptance. 

Specifications are a necessary part of a quality control program. A specification simply is a statement of the requirements that the adhesives or sealant must meet to be accepted for use. A specification is an agreement between supplier and user. Conformance to a specification does not mean that the adhesive or sealant will perform perfectly in service. It only means that the product conforms to the specification. Specifications not only account for the adhesive or sealant, but also define the adherends and the accessory processes for preparing the adherends and the joint assembly. 

DeFrayne, G. O., Adhesive Specification and Quality Control, in Adhesives in Manufacturing, G. L. Schneberger, ed., Marcel Dekker, New York, 1983. 

Hardness. Hardness of the adhesive or sealant may be used as an indication of cure. It may also be used as a quality control check on certain substrates. Hardness can be determined in several ways (1) resistance to indentations, (2) rebound efficiency, and (3) resistance to scratching or abrasion. The first method is the most commonly used technique. 

Cure Rate as Measured by Strength Development of Prototype Joints. Cure rate is an important factor when the expense of jigs and fixturing equipment is high or fast production rates are critical. It is also used as a quality control test to determine if the curing mechanism within the adhesive has changed from lot to lot or if it may have been spoiled by storage, moisture contamination, etc. 

There are several ways that actual joints can be tested. For quality control purposes, a proof test is commonly used, as described above. This test imposes a stress on the specimen but limits it to a point well below where any destruction of the joint can occur. This type of test only looks for serious flaws in the bonding processes such as interface contamination, air entrapment in the joint, or undercured adhesive. The prototype joints can also be tested to destruction by using similar test methods described above for standardized testing. This, however, is generally not done at great frequency because of the cost. Generally, most actual joint tests are performed to ascertain the cause of failure (i.e., forensic analysis of the failed joint) or for nondestructive determination of the adequacy of the bond. 

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