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Epoxy Adhesives, Part 3

Posted on 9/11/2013 3:44:12 PM By Sandra Niks
  

The epoxy resins, curing agents, fillers and other additives used in formulating epoxy adhesives have been reviewed in the first two parts of this discussion. Now for a few points regarding manufacturing and applying the adhesive.


Temperature must be carefully controlled. Some warming may be useful to lower viscosity during compounding, but not enough to start a chemical reaction that might prematurely cure the adhesive. Mixing is generally achieved under vacuum to minimize any air and moisture entrapment, neither of which would benefit an adhesive bond.


When making a two-part adhesive, obviously the curing agent must be separate from the resin, but the other additives need to be divided to achieve similar viscosities and a mix ratio of 1:1 if at all possible. Also, if the two parts are not sufficiently visually distinct, add a colorant to one or both parts, such as a little carbon black to one side and/or adding titanium dioxide to the other. This helps when visually inspecting the mixed adhesive bead as applied to the substrate to see that there are no striations, the color is uniform, and the adhesive is thoroughly mixed. Meter/mix equipment is certainly available to handle ratios other than 1:1, even for relatively low tech consumer applications, but a ratio of 1:1 is generally more forgiving if the ratio is slightly off.


Thixotropy and viscosity need to be tailored to the application – should the adhesive as applied be able to stay put and maintain the bead shape until force is applied to fill the bondline, or should the adhesive have less thixotropy and lower viscosity to flow into a bondline and fill any unevenness in the surface of the substrate? 


Weldability, which also affect thixotropy and viscosity, include requirements that the adhesive will not interfere with the formation of the weld nugget created in the spot-welding process. The adhesive must be fluid enough to flow out of the weld area without leaving significant residue that would weaken the weld, but not so fluid that it escapes the joint area or contaminates the weld equipment.  The adhesive must not catch fire, smoke excessively, or produce any toxic gases.


Bondline thickness is generally maintained through the tolerances of the manufacturing equipment, such as folding the sheet metal over to make a hem flange bond or molding features into the bond area of a plastic panel. When manually preparing test samples, a technique using glass beads or wire of the appropriate diameter can sometimes be employed.


Before the adhesive is applied to the substrate, surface preparation may be required, including one or more cleaning steps, surface activation (such as plasma treatment), and/or the application of a primer.

Cure is most often achieved with an oven cure, usually concurrent with the paint bake cycles in automotive applications.


Many additional details related to epoxy adhesive, particularly for specific applications, have already been described in previous blogs. A list is included below.


References:


Adhesive and Sealant Compound Formulations, Second Edition, Ernest W, Flick, Noyes Publications, 1984, ISBN: 0-8155-0966-9.


Adhesives and the Engineer, edited by W. A Lees, Mechanical Engineering Publications Limited, 1989, pp 15-17.


Adhesives Technology Handbook, Arthur H. Landrock, Noyes Publications, 1985, ISBN: 0-8155-1040-3, pp 144, 148-149.


Engineered Materials Handbook®, Volume 3, Adhesives and Sealants, Hal F. Brinson, Technical Chairman ASM International Handbook Committee, ASM International, 1990, ISBN: 0-87170-281-9, pp 94-102, “Epoxies”,  by Dean T. Behm  and John Gannon.

Handbook of Adhesive Bonding, edited by Charles V. Cagle, McGraw-Hill Book Company, 1973, ISBN:  0-07-009588-4; 1982 reissue; Chapter 3, “Epoxy Adhesives”, by Charles E. Chastain and Charles V. Cagle.


Handbook of Adhesive Technology, edited by A. Pizzi, K. L. Mittal, Marcel Dekker, Inc., 1994, ISBN:  0-8247-8947-1, Chapter 33, “Epoxy Resin Adhesives”, by T. M. Goulding.


Handbook of Adhesives, Third Edition, edited by Irving Skeist, Ph.D., Van Nostrand Reinhold, 1990, ISBN:  0-442-28013-0, Chapter 19 “Epoxy Resin Adhesives”, by Allan R. Meath.


Handbook of Epoxy Resins, Henry Lee, Kris Neville, McGraw-Hill Book Company, 1967; 1982 reissue.


Joining Technologies for the 1990s, edited by John D. Buckley, Bland A. Stein, Noyes Data Corporation 1986, ISBN:  0-8155-1095-0.



Wikipedia - epoxy

More Information:

Resins, diluents, curing agents, etc.:

Air Products

Dow

Henkel

Huntsman

Momentive

Reichhold

Royce International

Toray

 

Fillers:

Bentonite Direct

Cabot

Sigma-Aldritch


Previous Blog Posts

Date

Title

September 2012

Adhesives & Sealants in Transportation

September 2012

Automotive Body Panels

October 2012

Standards

November 2012

Hem flange bonding, Part 1

November 2012

Hem flange bonding, Part 2

December 2012

Material Specification for Pumpable Adhesives

January 2013

Structural Adhesive Bonding in Aerospace

February 2013

Introduction to Vehicle Interior Bonding

June 2013

Marine Applications



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