Selecting the proper adhesive for bonding PU.

Choosing the proper adhesive for urethane-to-metal bonding is crucial to the success of your products, but the decision need not be overwhelming. In fact, with the assistance of knowledgeable adhesive suppliers, selecting the appropriate adhesive can be fairly simple. If you can provide these experts with complete and accurate information such as your urethane and substrate type, proposed end-use and metal preparation procedures the end result will be superior bond performance.

This article explains how urethane processors, by providing the right information to their adhesive supplier, can meet their specific bonding needs. We will concentrate on the more complex castable urethane systems.

End-use application and environmental testing

It is impossible to formulate a single urethane compound or adhesive for every application. That is why adhesives are designed and developed for specific urethane technologies. Although urethane inherently bonds to a wide variety of metallic and non-metallic substrates, this bond does not offer the optimum quality performance obtained from an adhesive system designed specifically for your application.

Adhesive selection for castable urethanes should be based on the adhesive's bond strength, as tested before and after subjection to end-use environmental conditions. Testing to see if the urethane and/or adhesive will endure exposure or immersion is best done under even more severe conditions than will be encountered during the intended end use. This will ensure that they will hold up in less stressful conditions.

Destructive testing should be based on peel strengths or tensile strengths and percent of elastomer retention. (Initial bonding trials should be conducted with several adhesives if the supplier is not familiar with the specific formulation that will be used.)

You should work closely with your adhesive supplier in order to select the proper adhesives. Your adhesive supplier should be willing to send you prepared samples for casting at your facility. After casting, your supplier should conduct destructive testing and give a full report of the results, including all procedures used for later duplication at your facility. If your adhesive supplier cannot provide this service it could cost you thousands of dollars for testing elsewhere.

The following environmental test procedures for adhesive A were conducted by an adhesive supplier as a service to a customer:

* Test procedure - ASTM D-429, modified Method A test simulates tensile stress at 2" per minute. Initial bonds were pulled at ambient temperatures.

* Elastomer identification - Adiprene LW-570/MDA

* Cure parameters - Cured for 32 hours at 240[degrees]F

* Substrate Type - No. 1010 cold-rolled steel tensile buttons, 1" diameter

* Prepared surface area - .785 sq. in.

* Substrate preparation - Degreased by hand with solvent before and after blasting with No. 50 steel grit

* Method of application - Adhesives applied by brush

* Mold technique - Individually molded open-cast buttons

These results show that, even in this severe environment, this particular adhesive has strong bonding qualities (see table 1).

Samples were dropped to ambient temperatures to determine whether the adhesive or the urethane itself was degraded. This particular test indicates considerably higher PSI and R values were achieved when the immersed samples were allowed to recover for a 24-hour period. The ambient test's tensile values were more than 70% of those in the initial bonding trial; whereas the 180[degrees]F results dropped significantly. Because the oil results retained higher values, we can conclude that this urethane has a higher resistance to oil than water at the same temperature. Also, this particular adhesive is less affected by oil than water.

These environmental tests, if conducted properly, can provide extensive critical information to help aid in selecting the best adhesive for your application.

Substrate preparation

Substrate preparation can determine the success of a bond. Improper preparation or adhesive selection can lead to a poor initial bond, under-bond corrosion of the metal, or premature bond failure of the molded component.

Gritblasted metal commonly is used as the primary substrate for urethane processors and presents no major problems for bonding. This is a reliable procedure that consistently produces high-quality bonds. Vapor degreasing or solvent cleaning before gritblasting is necessary to prevent the grit from becoming contaminated. You can do more damage by using a contaminated grit than if you did not gritblast at all. So, degreasing before and after blasting is crucial for urethane-to-metal bonding if consistent, quality results are expected. Steel grits No. 25-50 are recommended because they cut metal deeply, remove heavy oxide scales and rust, and leave a good profile of peaks and valleys for the adhesive. Silica sand and aluminum oxide are not highly recommended because they usually leave a small dust layer on the surface. Unless this is removed, you can expect poor adhesive wetting and premature part failure.

Chemically cleaned substrates do not offer the distinct advantage of the increased surface area created by the gritblasting operation. And, installing equipment for chemical treatments can be costly, including waste water treatment.

Acid etching and zinc phosphatizing are suitable for preparing high volumes of small inserts that cannot be mechanically abraded. These surface treatments, if controlled, are receptive to adhesive application and subsequent bonding operations. The adhesive chosen must bond to these surface treatments without use of a high temperature prebake to set the adhesive.

Processing temperatures and oven capabilities

Polymer processing temperatures and oven prebaking capabilities

     Table 1 - adhesive A results
Initial bond results tested @ ambient temperature (4)
M (1)   C (2)   R (3)    PSI   Avg    Avg  % Bond
                                R     PSI  retention
5       10      85      4183
10      15      75      3525    80   3854
Bond results after two weeks salt water immersion tested
@ 180[degrees]F
65      35      -        379
95      5       -        502   -      441     11
Bond results after two weeks salt water immersion tested
@ ambient temperature
15      55      30       2924
15      50      35       2450  33    2687     70
Bond results after two weeks oil immersion tested @
180[degrees]F
10      80      10       1938
10      89      1        1615  6     1777     46
Bond results after two weeks oil immersion tested @ ambient
temperature
5       35      60       3822
5       60      35       2897  48    3360     87
(1) Percent M indicates the percentage of the test area in
which the failure occurred at the metal/adhesive interface.
(2) Percent C represents the percentage of failure at the
urethane/adhesive interface.
(3) Percent R indicates the percent failure that occurred
within the urethane compound itself.
(4) For ambient temperature results, samples were aged
for 24 hours, then tested.  180[degrees]F tests were conducted
immediately after immersion or exposure.

also are important considerations when choosing an adhesive. Some adhesives will withstand considerable prebaking prior to casting or molding operations without sacrificing quality. Others may cure prematurely during the prebake cycle. The weight, size and configuration of the insert or substrate also must be considered as these variables may influence the amount of time spent in the prebaking oven.

The oven itself must be equipped with fresh air make-up units or adequate ventilation. Without fans to recirculate the air supply, isolated hot spots may be encountered which can be detrimental to the adhesive film or the urethane elastomer. To avoid the possibility of precuring or contamination, ovens for prebaking an adhesive-coated insert should be used for that purpose only.

An adhesive's ability to cure properly at the polymer processing temperatures also is vital. Polymer processing temperatures may vary from ambient room temperatures to as high as the 300[degrees]F used in single-component urethane system processing. Each adhesive system has a minimum and maximum bond temperature range for preheating the adhesive-coated insert before the urethane is cast. In addition, each has a specific operating temperature based on the compound being bonded. This information should be stated on the product's technical data bulletin. Your adhesive supplier should work with you to choose a product that will work with your oven's capabilities.

Urethane selection

Many premature bond failures result from selecting the improper urethane and not the improper adhesive. Hence, the urethane selection itself may be the most important step of all. Because the urethane must be able to withstand end-use, it is crucial for the urethane processor and its customers to know the intended use of the parts. The adhesive and urethane may have been selected with all of the best intentions, but if the end user subjects the parts to more severe conditions than expected, the part may fail.

Conclusion

Rely on your adhesive supplier for technical expertise about everything from surface preparation and urethane selection, to destructive testing and curing procedures. With the proper combination of elements, procedures and adhesives, you can expect high-quality bond performance.