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www.us-
tech.com
December, 2015 Production
Selecting the Right High Tech Adhesive
Barry Siroka, Business Development Manager, EpoxySet
ables to consider and numerous types of chemistries with different capabilities, and an understanding of the various parameters around adhesive technology can help to narrow down some of the many choices. When considering substrates, have a good knowledge of the material to be
C
bonded. Certain materials may adhere better to ceramic or glass. Note that “metal” and “plastic” are generic terms that encompass many different types of products. For example, ABS and Teflon are both plastics, but Teflon requires an acid etch to achieve a bond where ABS can usually be bonded with only an alcohol cleaning. Most adhesives require that the substrates be prepared properly. Whether it’s a simple cleaning, or if it’s necessary to do more complex operations (abrasion, chemical etch, plasma treatment, etc.), each will come into play in se- lecting an adhesive. The viscosity must also be considered. Low viscosity (thin) products are best
for thin bonds or for applications requiring wicking. Thixotropic materials, which will not run, are best for use in vertical applications. Also, the curing process must be taken into account. UV cured adhesives are
usually one part and can be cured quickly. They must be exposed to ultra-violet light to cure, which means at least one of the substrates being bonded must allow trans- mission of UV. Thermally cured materials can be cured at room temperature with or without more heat. Heat cured thermal adhesives can require high temperatures which might be detrimental to components, but in general will cure faster. How much working life the material has should be determined. Working life
(time before the material gels) is generally related to cure time. For thermal cure materials, longer working life usually means a longer cure time, at room temper- ature. Single-part UV cured materials can be cured quickly and the working life has no effect on the process. The dimensions of the parts and how they fit together are very important to
determine the best material. Larger parts and thicker bond gaps may require a slower cure to reduce shrinkage. Certain adhesives are designed to work best with thinner gaps.
Required Performance Be sure to consider whether or not the adhesive needs to form a structural
bond. Ideally, a structured bonding material will create a finished part that is as strong as the substrates themselves. Knowledge of the substrates’ composition helps in deciding the best adhesive and preparation to accomplish an ultimate bond. Also, understanding the configuration of the parts to be bonded will influ- ence the types of adhesives for use. With most adhesives, the larger the surface area of the mated parts, the stronger the bond. Bond gap is also critical. Various adhesive chemistries work best with cer-
tain gaps. In general, cyanoacrylates and anaerobics need the thinnest bond line and epoxies and urethanes are better with thicker gaps. However, be aware that each materials’ chemistry may vary depending on how they were formulated. Be aware also of the expected mechanical stress on the bond during use. It
may seem obvious, but the direction of the forces on the adhesive joint should af- fect the choice of material. Some chemistries may have excellent tensile strength (horizontal) and very little shear strength (vertical). Also consider if there are any compressive forces or torsion (twisting) on the joint. Adhesives may be formulated to be more flexible or with fillers to compensate for these stresses. Consider if stresses will be consistent or intermittent. Some adhesives can
take extreme stress for a short period of time, but cannot withstand it for a long period. Others may withstand consistent stress if it is not brought on suddenly. Other types of stress may also be of concern. For example, autoclaving is a stan- dard practice in many industries and many adhesives cannot withstand it. The temperature the material will be subjected to can be an extremely im-
portant factor. Materials with low temperature cures may not withstand high tem- peratures during use. Adhesives designed for high temperature uses might only cure with heat, which means the adhesive will have to be exposed to dry heat (an oven or heating element). The exposure temperatures during use are critical. Con- sider if the parts will be temperature cycled. Many materials that will withstand high temperatures can crack when cycled to low temperatures. Low temperature materials might lose their strength at high temperatures. Take into account the optical properties of the material, if any. Optical prop-
erties may be important for certain applications. If light transmission is impor- tant, the wavelength being transmitted and the duration must be known. The re- fractive index of the substrates should also be considered. Certain chemistries will not remain clear after long exposure to UV radiation. Finally, decide if the cosmetic appearance matters. Some adhesive
chemistries can be easily colored, while others cannot. Some materials will have a lot of variations which means the final appearance can change. In applications re- quiring specific colors (especially for consumer products) it may be necessary to en- sure that lots can be color-matched. Testing can often reveal parameters that had not been considered previous-
ly, proving once again that there is no substitute for sampling and evaluating a product. It is not always feasible to know all the required parameters before choos- ing the right adhesive, but it is important to have as much information as possi-
ble to make an educated decision. Contact: EpoxySet, 1 Industrial Circle, Lincoln, RI 02865 % 401-726-4500 fax: 401-726-4502 E-mail:
info@epoxyset.com Web:
www.epoxyset.com r
hoosing the best adhesive can be a complex process. Carefully selecting the correct adhesive for a job will reduce the number of products that need to be evaluated and increase the likeliness of its success. There are many vari-
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