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Bonding plastics? Stuck for a solution?

Ewen J C Kellar
TWI Ltd, Granta Park, Great Abington, Cambridge, CB1 6AL, UK

Paper presented at Joining Plastics 2006, London, National Physical Laboratory (NPL), 25-26 April 2006


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Dr Ewen Kellar joined TWI in 1996 to work on adhesives technology and applications in the Advanced Materials and Processes Department. Over the subsequent years he has managed projects in a wide range of areas including, surface coatings for medical devices, durability and lifetime prediction of adhesively bonded joints, assessment of adhesives for the offshore industry and the recycling of bonded components in the F1 industry. His interests include finite element analysis (FEA) of bonded joints, the modification of surfaces to promote adhesion, application of adhesives/polymers in the medical sector and hybrid joining technology (where the joint consists of a combination of a fastener and an adhesive). Current work includes the development and evaluation of coatings for biopsy needles to enhance ultrasound visibility and the continuing development of the web based Adhesives Design Toolkit.

 

Abstract

In this day and age, adhesive bonding is becoming an ever more popular way to join plastics, in many instances the only way if differing materials are used. However, there are many types to choose from and selection of possible candidates requires considerable legwork, even for an expert in the field. The success of the bond depends upon many factors including, material type, joint geometry, operating environment, loading conditions, cure type, dispensing needs, volume, cost etc. When a selection is made, suppliers need to be located and trials need to be carried out with appropriate testing to verify performance and durability. These needs have been recognised by the UK Department of Trade and Industry (DTI), resulting in the formation and development of the Adhesives Design Toolkit web-site. The aim of the web-site is to create a focus for adhesives related information, to provide interactive software modules and to allow users to access useful links.

This paper looks at the challenges facing engineers when coming to select adhesives to bond plastics and other materials. It also introduces the Adhesives Design Toolkit, describing its development, content and plans for the future.

1. Introduction

In this day and age, the use of adhesive bonding as a primary joining technology is becoming an ever more popular way to address all manner of applications ranging from the trivial (cosmetic bows to fluffy slippers) to the critical(surgical devices). Since the arrival of plastics over a century ago, the need to combine these increasingly diverse materials in smarter, more effective ways has triggered an almost invisible revolution in the development of adhesive technology.

There is a common perception that the use of glue to mend a child's toy or to fill the gap between the skirting board and the wall, is a trivial, low tech approach to a temporary problem. The glue is blamed when the mended toy falls apart or the bookshelf detaches from the painted wall and the obvious solution is to avoid gluing and buy a new toy or to use screws because adhesives don't work. This domestic perception of adhesives still predominates with the majority of the population and creates a paradox in that adhesive manufacturers wish to increase their sales through sales to the public but at the same time are reinforcing the opinions of those consumers that although adhesives are simple to use, they don't often work very well. However, perhaps rather than blaming the process, the user needs to look more carefully at the materials being used and determine their suitability for the application in hand, the toy may be made from polypropylene which is notoriously difficult to bond without pre-treatment and bonding to a weak polymer such as paint, will always be structurally limited. There are a myriad of industrial applications where adhesives are being employed to great effect. Some examples include:

  • Brake and clutch components in cars and lorries - rivets were eliminated years ago and now all that holds the resin impregnated friction pads in place is a heat cure vinyl or nitrile phenolic adhesive.
  • Footwear fabrication - the upper to sole joint is crucial to the function of the shoe, although stitching is still used, the vast majority of shoes and virtually all sports footwear rely entirely upon adhesive bonding, the two main types being polychloroprene (neoprene) and polyurethane systems.
  • Medical devices (catheters, hypodermics etc) - probably one of the fastest growing industrial sectors but due to the stringent requirements of the national medical bodies such as the FDA, this is probably one of the most demanding areas for adhesive use. Despite this, acrylic, cyanoacrylate, epoxy and silicone based adhesives are critical for the fabrication of a wide range of medical devices.
  • Mobile phones - adhesives are used to enable different housing plastics to be combined, attach areas of flexible circuitry, to seal and locate display units and in some instances to directly replace soldering.
  • DVDs - these apparently simple disks are in fact combinations of two, three or even four coated polycarbonate (PC) disks which are laminated together using a extremely rapid UV light cure acrylic adhesive.
  • Domestic plumbing - it is becoming increasingly common for domestic piping, traditionally made from copper to be replaced by PVC piping. Permanent sealing/joining can only be achieved through the use of a solvent based adhesive cement.

With this in mind it is necessary to consider adhesive bonding technology as a serious process which requires expert skills and training grounded in core disciplines such as chemistry, physics, engineering and materials science.When successfully employed, adhesives technology offers the designer and the engineer a wealth of opportunities in terms of materials to choose from, cosmetic appearance and a tailoring of properties to the application in hand.

2. Making an adhesive bond stick

2.1. Overview of the bonding process

In common with most other production activities the adhesive bonding process can split into two distinct areas, design and manufacturing, which can subdivided as follows:

  • Design
  • Defining the function and operating requirements
  • Design of the joint
  • Selection of materials (adherends)
  • Selection of adhesives
  • Manufacturing
  • Surface pre-treatment
  • Assembly
  • Cure
  • Quality control and inspection

Each of these sub-areas or processes requires careful consideration often in an iterative manner, as particular choices can have significant impacts upon apparently unrelated factors which may require the original parameters elections to be revisited.

2.2. Definition of function and operating requirements

It is important that the first step in the process is to define the function and operating requirements of the overall structure/device. Clearly this is necessary in order to define the types of materials that can be considered but as adhesives are in the main polymeric, their needs/limitations must be taken into account too in order to make the correct selection. Ideally the adhesive should have similar if not superior, properties to the materials it is joining,especially if they are plastics. An example might be the use of a room temperature curing epoxy which could not be considered for a high temperature application (~200°C) where polyetheretherketone (PEEK) will be the primarypolymer as the adhesive will most likely suffer degradation. Conversely, the use of an adhesive that requires heat to cure would be of no use to join polymers that soften/melt at temperatures below the curing temperature.

Surface preparation can have a significant impact upon the long term performance of the joint, so clearly defining the operating requirements of the structure/device may influence the type of pre-treatment that needs to be considered.

2.3. Design of the joint

Adhesives are stress sensitive and despite the superior fatigue properties that they can offer a structure, the type and direction of loading can have a drastic impact on performance. As a rule of thumb adhesively bonded joints should be designed to maximise compression and shear and minimise/eliminate peel and cleavage. It has been shown that the load carrying ratio of compression:shear:peel can be approximated to 100:100:1.

A simple example of this is shown in Fig.1 where an 'L' bracket is attached in a vertical position. If attached in the conventional 'upside down' position and the bracket loaded in a vertical manner as indicated, then the forces acting on the joint will be focussed at the upper area in the form of cleavage/peel and some shear down the joint length. This will reduce the load carrying performance of the structure substantially. By simply reversing the orientation of the bracket the moment of the carrying arm causes the forces to be directed in compression to the base of the joint and as shear along the length of the joint. The bracket will be able to carry significantly more load as a result and all that was changed was the orientation.

Fig.1. Effect of bracket orientation on loading directions
Fig.1. Effect of bracket orientation on loading directions


2.4. Selection of materials (adherends)

The selection of the correct materials for the device/structure to be produced is core to the development of a successful product, however if adhesive bonding is to be employed then the decision process must take this into account. Adhesively bonding components produced from PC, polystyrene (PS) or polyamide (PA) may be relatively straightforward with a wide variety of systems to choose from, whereas polyolefins, fluoro-polymers and silicones may significantly reduce the choices available and may require considerable levels of pre-treatment. Ultimately however the base materials must meet the design specifications and should this exceed that of the adhesives available then alternative joining options may be required to be considered.

2.5. Selection of adhesives

It has been stated earlier that adhesive selection can present itself as a daunting task. There are many types to choose from in terms of chemistries (epoxies, acrylics, polyurethanes etc), physical form (liquid, paste, film etc)and cure mechanism (chemical reaction, heat, UV etc). As a rule of thumb, the more similar the adhesive chemistry is to the polymer to bond then the greater the likelihood of compatibility. However, the choice of potentially suitable adhesives is complicated by the fact that there are many possible alternatives commercially available. It has been estimated that world-wide there are over 250,000 adhesives to choose from. As a result, selection of a possible candidate shortlist for evaluation may require considerable effort even for an expert in the field. The success of the bond depends upon many factors including, material type, joint geometry, operating environment, loading conditions,cure type, dispensing needs, volume, cost etc. When a selection is made, suppliers need to be located and practical trials will need to be carried out with appropriate testing to verify performance and durability. Anyone who makes a selection of an adhesive on the basis of paper study alone does so at their peril.

2.6. Surface pre-treatment

An adhesive joint is only as strong as the weakest link and in many instances this can be the interface between the adhesive and the adherend. Polymers are no exception and it is important to consider this early in the design/manufacturing phase of work. There are many different options to consider, which can be put into the following groups as described in Table 1.


Table 1. Main pre-treatment options for plastics

Mechanical Chemical Energetic
Manual abrasion (paper or pad) Solvent cleaning Plasma
Automated abrasion or grit blast - cryo (solid CO2), soda, alumina Detergent wash Corona
Peel ply (used for composites) Acid/oxidising etch Flame
  Primer/coupling agent Laser (excimer)


In simple terms the mechanical processes are the most crude and are material non-specific other than the fact that some polymers do not provide a stable surface on which to adhesively bond after treatment, ie a layer of weakly attached material remains. In the case of crosslinked polymers (thermoset composites etc) and the higher modulus systems (PS, PC, PA), abrasion of the surface is very beneficial, providing additional surface area, enhanced wettability and the possibility of microscopic mechanical locking.

Chemical pre-treatments are most commonly used to treat plastic surfaces, through simple direct removal of surface contamination via solvents/detergents, by chemically activating the adherend surface by etching or chemical attack or, in some instances softening the polymer surface through local dissolution using a solvent. It should be stressed that care is needed when selecting the correct chemical pre-treatments as some plastics are much more susceptible than others and may be weakened or damaged as a result of chemical exposure. Significant examples include the effect of stress cracking that can occur when solvent-based adhesives are used with polymers such as PC and PS. A number of years ago there was a lot of publicity about the safety implications when customising motorcycle helmets made from PC based material. Although the main concern was with the use of solvent based paints, even the use of self adhesive decals was advised against, Fig.2.

Fig.2. Motorcycle helmet - early types were sensitive to solvent based stress cracking
Fig.2. Motorcycle helmet - early types were sensitive to solvent based stress cracking
 

In some instances, when the polymer is difficult to adhere to or when the bond area is very large (large structure or many small components) or when high levels of durability are required, the best types of pre-treatment are based upon energetic processes such as plasma, corona, flame or laser. These are non-contact processes where the surface is modified through the input of energy arising from radiation or hot gas particles. Flame processing is commonly used to activate the surface of polyolefin materials prior to bonding and painting. Although the process is effective, it can be very sensitive to conditions including the flame composition, duration of exposure etc.

For all types of pre-treatment, the length of time that the adherend surface is exposed to the atmosphere after pre-treatment is very important. Normally the longer the exposure time, the poorer the final bond quality becomes. It is very important that adhesive bonding be done as soon as possible after the pre-treatment process.

2.7. Joint assembly

The majority of adhesives are applied in a liquid or paste form which requires that the components be held together with some type of jigging. This can be either external, in the form of some type of rack or fixture plate or internal, via a design detail such as a snap fit, a lockable joint, fasteners etc. The type of assembly chosen will be highly dependent upon the volumes of product being produced, joint accuracy, type of cure and the type of adhesive applied.

2.8. Adhesive cure

It has been mentioned previously that adhesives can be cured (hardened) via a number of ways including heat, light, chemical reaction and pressure. Within the same chemistry, there can be a number of options to choose, for example epoxies are available that will cure after mixing two components at room temperature, heating a single component above a certain temperature or curing a single component using blue or UV light. All options may result in a similar type of cured adhesive but the different options gives designers flexibility depending upon other manufacturing requirements. Blue light (or UV) curing adhesives carry a price premium but they remain in a liquid state until the joint is required to be formed. Exposure to blue or UV light enables the structure to be 'snap-cured' in seconds, in contrast to a room temperature curing adhesive that starts to cure the moment the components are mixed (although the length of cure could vary from minutes to hours).

2.9. Quality control and inspection

Finally, once the joint is formed it may be required to be inspected. This can take the form of destructive testing or non-destructive testing (NDT). Clearly destructive testing is an extreme type of appraisal and cannot be applied to all structures! However an extensive set of destructive trials ought to be carried out prior to finalising materials and design in order to understand the limitations of the system that is being created.

Unfortunately the nature of adhesive bonding is such that NDT methods have only limited applicability when applied to determining the quality/integrity of bonded joints. Voids and defects down to ~0.5mm or so can be detected using ultrasound methods but the actual level of adhesion in relation to the strength of the joint cannot be determined. A so-called 'kissing bond', where the adhesive is in direct contact with the adherend but there is no adhesion (eg due to the presence of mould release agent), is invisible to all current NDT methods.

In view of this the best way to address bond integrity is to ensure that the whole bonding operation is very tightly controlled with strict protocols at every stage in the process. Spot destructive testing checks should be carried out at regular intervals, especially when the adhesive/adherend batches are being changed and at any point where other external changes occur. It has been shown that the adoption of such controls can be extremely effective in the delivery of a quality reliable product.

3. The Adhesives Design Toolkit

3.1. Overview

The successful implementation of adhesive bonding within a production environment requires significant effort and the benefits to those who succeed are considerable. However, the complexity of the area and the perceived difficulties surrounding adhesives has proved to be a barrier to many industries, an issue not lost to the UK Department of Trade and Industry (DTI). Such recognition has resulted in the formation and development of the Adhesives Design Toolkit web-site, www.adhesivestoolkit.com The aim of the web-site is to create a focus for adhesives related information, to provide interactive software modules and to allow users (both industrial and the general public) to access useful links relating to adhesives and related technologies. The initial phase of work ended in 2003 with a fully functioning web-site with the capacity for further expansion.

Within the web-site there are three primary areas of interest at present. These are:

  • Toolkits - currently contains six active modules to assist the end user with designing with, and using, adhesives.
  • Documents/data - contains a wealth of adhesives-based information generated within this project and previous DTI funded projects over the past decade.
  • Links - contains 80+ links to adhesives suppliers and related sites.

3.2. The toolkits

One area which deserves particular attention is the toolkit section where currently six working modules are available on-line. These modules are:

  • Adhesive suppliers - looking for who supplies what to where?
  • Case Histories - a searchable selection of case histories.
  • Stress analysis - a powerful tool to enable stress analyses to be carried out on a variety of co-axial joints. Change joint geometries, features, adherends, cure conditions (hot or cold) and see what effect it has on the stress profile.
  • Adhesive selection - want to find an adhesive for a particular application but don't know where to start? A simple but effective tool to assist with adhesive selection.
  • Design guidance - It is easy to bond badly with adhesives. This module is designed to assist with the process and enable the user to avoid 'obvious' mistakes.
  • Standards - the process of acquisition of test data requires that the associated test methods are consistent and reproducible, if the data is to be of real use. This module covers most, if not all, of the available standards (e.g. BSI, ISO, ASTM etc) that are relevant to adhesives and adhesive bonding. The information provided lists the standard reference number, the title and a short summary of each standard. The standards themselves are not present on the site, as they need to be obtained directly from the relevant bodies.

3.3. The future

A second phase of work addressing the development of additional modules in the areas of testing, durability and forensic analysis is currently underway. A beta version of the testing module is available at present with a final version scheduled for early April 2006. It is planned to generate more interest from the adhesives suppliers themselves in the hope that more and more data can be made available to users as and when it is generated. The ultimate aim would be for the web-site to become the primary resource for all adhesives users enabling everyone to get the most out of what is one of the most versatile joining technologies available.

4. Final thoughts

Adhesive bonding suffers from a schizophrenic image in that for many people it represents a low tech 'poor man's solution' to cheap assembly or quick repair. However, for those who have made the effort to understand the technology, it offers superior performance and flexibility to join and seal just about any available material, especially polymers. There are many thousands of products and applications that people use every day that depend upon adhesive bonding for their construction and performance. Adhesives are all around us and are increasingly here to stay. Resources such as the Adhesives Design Toolkit can only serve to increase awareness of such technologies and enhance the reputation that glue truly deserves.

5. Acknowledgements

The author wishes to thank Bill Broughton, Bruce Duncan and Greg Dean at NPL, John McCarthy at ESR Technology, and John Harris and Mike Samulak at MERL, for their help with the development of the Adhesives Design Toolkit.