Abstract

This tutorial report focus on the choices of adhesives. The most common type of adhesives, characteristics of each type are introduced. The basic procedure for polymer adhesives as well as the corresponded mechanical properties and test methods are introduced.

Introduction

As we all know, the basic function of adhesives is joining parts (different material, different pieces). Compared to mechanical joints, like fasteners, adhesives own their own advantages, like lighter weight, eliminate or minimize corrosion, eliminate joint fatigue, distribute lodes over wider surface area and so on. Compare to metal like solder or glass like inorganic glass powder, adhesives is relatively quick and simple to operate.

Table1. basic adhesive way

Adhesive types do not follow any common nomenclature, however, we can describe them in our own way to avoid confusion. Optical adhesives are usually glass-to-glass adhesives, which imply the transmission quality. Structure adhesives are more for mechanical parts of a system. Elastomers use rubbery properties for sealing and to provide compliance. In this tutorial, instead of cement optical components together (doublets and achromats) we focus on the situation of optics-to-mechanics or structural adhesives.

properties

There is no intermediate state for adhesives. ”best” when they work correctly, “worst” when they fail. To make it effective, it must be able to wet the substrate, It must harden and it must be able to transmit load between the two surfaces/substrates being adhered well. Thousands of adhesives are available now, and each of them owe their own characteristics, which affect their performances.

Wetting

An ability of an uncured adhesive to make intimate contact with the substrate in order to facilitate a bond. Surface tension 30~35 dynes/cm3

Viscosity

A fluid’s resistance to flow. The “thickness” of the fluid

Cure

multi-part adhesives- harden by mixing two or more components, individual components of a multi-component adhesive are not adhesive by nature, they react with each other after being mixed and show full adhesion only on curing.

one part adhesives- cured by chemical reaction with external energy source,for example, UV light, heat and moisture.

Stiffness

Stiffness of adhesive which can be defined for shear and axial stiffness is an important parameter. For stiffness analysis, we have to use the proper modulus for stiffness analysis, which depend on the bonded thickness and area.

Shear stiffness —thermal effect, out gassing process

axial stiffness

figure1. tensile loading(left) no loading(middle) compressive loading(right)

tensile elongation
when axial length>> width young’s modulus

when thickness << width bulk modulus

compression

while shape factor

From which we now that thinner it is, larger young’s modulus it is, stiffer it is.

Strength

The strength of an adhesive indicates the maximum amount of stress that can be applied in a particular geometry (i.e. tensile, compressive, shear, peel, or cleavage) without failure of the adhesive. For permanent bonds, this should be high; for temporary bonds, it should be low. Structural adhesives typically have a tensile or shear strength in excess of 1,000 psi.

figure 2. examples of out of adhesive strength

Hardness

Will change corresponded with time, good compliance need higher hardness.

Thermal Properties

Temperature can strongly affect the properties of adhesives. In fact, this is one of the major tradeoffs when considering the use of adhesives vs. mechanical fasteners. The following are several important temperature related properties of adhesives.

Maximum Continuous Operating Temperature

This is the highest temperature that the cured adhesive can endure for an extended time without deteriorating. For epoxies, this is usually 125 degrees C or higher. Adhesive suppliers normally report both the maximum and minimum recommended extended exposure temperatures for their formulations and it is important to consider both extremes in the context of the proposed application.

Coefficient of Thermal Expansion (CTE)

Materials expand linearly with an increase in temperature. The coefficient of thermal expansion is the constant of proportionality between the temperature change and the induced thermal strain:Coefficient of Thermal Expansion (CTE) Materials expand linearly with an increase in temperature. The coefficient of thermal expansion is the constant of proportionality between the temperature change and the induced thermal strain:

∆𝐿 = 𝐿𝛼∆T

The CTEs of adhesives are typically much higher than the structural materials that they are joining (e.g. 30-300 ppm/deg C for adhesives compared to 23 ppm/deg C for aluminum and 7ppm/deg C for BK7). Typically, the designer attempts to match the CTEs of the two substrates as closely as possible and then find an adhesive with a CTE near that value. If there is a large mismatch between the CTE of the adhesive and substrate(s), large stresses can result upon temperature variations. If the adhesive has a low Young’s modulus, the stresses resulting from thermal expansion can be lessened. But, in general, low CTE adhesives are desired.

Creep rate

This could perhaps be best described as a long term shape change and is critical for applications where dimensional stability is important. Keep in mind that creep rate is exponentially proportional to temperature and is also influenced by stress and humidity.

Common categories

Epoxies: available in one and two-part liquids and pastes, these provide the highest strength and elevated temperature resistance.

figure4. two-part epoxy

Acrylics: are two-part liquids and pastes to bond the widest variety of substrates including hard-to-bond plastics and oily metals. The distinction is high strength bonding without the surface preparation needed for epoxies and urethanes.

Cyanoacrylate: are high strength liquid formulations known as instant adhesives. On rigid plastic, glass, metal, rubber, and other low porosity substrates, they harden in seconds through reaction with surface moisture.

Urethane adhesives: are generally lower cost two-part liquids and pastes that cure quickly to an elastic bond in applications requiring flexibility between dissimilar materials. Impact resistance is a distinctive characteristic.

figure 7. Urethane

UV curable adhesives

When exposed to a suitable UV light source, UV curable compounds usually cure in a matter of seconds. Upon cure, they exhibit minimal shrinkage. Excellent bond strength to metals, ceramics, many plastics and most rubbers.

Table 2. list of adhesives

Choice guideline

You should know

Do not use adhesive if your requirement is to joint part for a long time(>10 years), unless past experience prove it. Always use pedigreed engineering adhesive.

Preparation

performance

How long you want to use it, which is about bond longevity. Material, which material you want to use.

Function, compliance or joint two different part or change stuff out look for any other reason

Stress analysis

Exact bond geometry, especially stress near corners and fillets.

Operating environment

Temperature humidity chemical species present

Substrate particulars

Metal: heat treatment or acid situation during your application

Glass: do not affect the optical properties

Apart from that, make sure no critical damage or dust on the surface you want to use, datasheet is always a good reference for you to look at.

Conclusion

In this tutorial, a general introduction to structure adhesives is given, important properties, types of material, as well as a choice guideline included.

Reference:

1. J. Burge, Course notes, OPTI-521 Introductory Optomechanical Engineering, University of Arizona,2016

2. B. Olbert, Presentation: “Adhesive selection & characterization – What you don’t know can kill you,” UASO Engineering Seminar, Aug 2004

3. C. Oh, Course notes, OPTI-521L Introductory Optomechanical Engineering, University of Arizona

4. K. Schwertz, Useful Estimations and Rules of Thumbs for Optomechanics, University of Arizona, May 2010

5. N. Langer, Where adhesives beat mechanical fasteners , Aug 2005

6. John G. Daly, Damien J. Daly, structural adhesives for bonding optics to metals: a study of optomechanical stability, Proc. SPIE Vol.4444

7. Alson E. Hatheway, Analysis of adhesive bonds in optics,Proc. SPIE Vol.1998

8. J. Burge K. Schwertz, field guid to optomechanics