Both the formulating of adhesives and the 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.
Although there are sophisticated testing and analysis equipment available, many of these processes do not require advanced equipment. Simple equipment, visual examination, and common sense are the main tools in most quality control departments. They are supported by strong specifications and proper training.
By having strong specifications, one is placing much of the quality control process at the front door. Receiving inspection is the first step in the total control program. This generally consists of comparison of what is received to the specification used in developing the purchase order.
Acceptance tests on raw materials should be directed toward assurance that the incoming materials are correct and identical from lot to lot.
A receiving inspection program consists of comparison of the purchase order with what is received. If the vendor's test reports are required with the shipment, it is verified that they were received and that the test values are acceptable as defined by the original material specification.
Because of cost and time required for extensive in-house testing, the trend today is to have most of the quality control tests done by the supplier. The supplier then provides a certified test report to the customer with shipment of the product. The test program used by the supplier, his internal controls, etc. are usually verified and approved by the customer on a periodic basis, such as once per year. The approval tests are used to determine the adequacy of the particular materials in question and the production methods that will be employed. The tests that are done with every lot and submitted on delivery usually are to ascertain the consistency of the product from lot to lot.
Specifications are, perhaps, the cornerstone of any good quality control system. Specifications are intended to define what is needed for all parties involved. 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 in order 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. Bonding or sealing specifications should not only account for the adhesive or sealant, but they should also define the adherends and the accessory processes for preparing the adherends and the joint assembly.
The specification writer must try to put into the specification the requirements that, if met, will provide the greatest likelihood of success. These requirements should be standard tests that are agreed upon by both the supplier and the user. The tests should be indicative of how the adhesive is used in production and how the finished joint is to be used in service. Tests that are not directly applicable to the specific application should not be included. Tests should not be used simply because they are standard test methods or have been used in the past.
The language of the specification is extremely important. The specification must be unambiguous and use direct and simple words and phases. For example the words "must" and "should" have very different meanings and connotations in a specification. Proprietary or trade names should be avoided, if possible, since they may be changed with a company's merger or acquisition. The typical specification has the following format 1:
- general requirements
- performance requirements
- test methods
- reference documents
- approved source list
Specifications from other sources (e.g., ASTM, ISO, etc.) may be used if they are applicable and often test methods from ASTM, etc. are used within the specification. The trend is to use well established ASTM or ISO test methods where possible.
Specifications may require different categories of testing. For example, there may be extensive series of tests that are required for initial verification or qualification of the adhesive and supplier. These tests would be used to approve a certain product at the onset. Other receiving tests may be used to verify the consistency of the product from lot to lot.
DeFrayne, G.O., "Adhesive Specifications and Quality Control", in Adhesives in Manufacturing, G.L. Schneberger, ed., Marcel Dekker, New York, 1983.