Elastomer (Rubber) Bonding Principles

Elastomers

Natural rubber and many synthetic elastomers make up the range of rubber polymers that are available for fabrication. Factors to consider when selecting the elastomer are performance requirements of the part, ease of mixing, processing and molding.

The largest percentage of vulcanization bonded assemblies makes use of:

• Natural rubber (NR)
• Styrene-butadiene copolymers (SBR)
• Polychloroprene (CR)
• Acrylonitrile-butadiene copolymers (NBR)

Other commonly used synthetic elastomers include:

• Butyl rubber (IIR)
• Isoprene, synthetic (IR)
• Polybutadiene (BR)
• Chlorosulfonated polyethylene (CSM)
• Polyacrylate (ACM)
• Ethylene-acrylate ester types (AEM)
• Various castable polyurethanes (AU or EU)

High and ultra-high performance elastomers are specified where durability and extreme service conditions are mandated. These include various fluoroelastomer (FKM) and silicone (MQ) types, and hydrogenated NBR (HNBR).

Part designers are beginning to use melt-processable or thermoplastic elastomers for assemblies whose main function is cushioning or shock control. These elastomers include various polyolefins (TPO), styrene-butadiene block copolymers, and thermoplastic polyurethanes. These materials are atypical for bonded assemblies as they do not require vulcanization, but they are easy to process, and waste can be recycled. End-uses generally require service at ambient temperatures.

Many of the above mentioned elastomers have features which satisfy specific end-use requirements: oil and organic fluid resistance, heat resistance, resistance to chemical attack, high strength, superior dynamic properties, and/or ease of processing.

Compounding Effects

Data generated by LORD technical service laboratories, combined with customer input, provides the information needed for understanding compounding variables and bonding. These formulation guidelines pertain mainly to the non-polar diene elastomers: EPDM, IIR, and NR, and to a lesser extent, the easier to bond and more polar types, such as CR and NBR.

The following compounding ingredients, cure system, fillers, extender oils/plasticizers, and antidegradants all affect “bondability” to varying degrees. The effects of these ingredients are listed below:

• Sulfur Levels – The amount of sulfur in the compound has a significant role: sulfur levels of one p.h.r. or higher have a favorable effect on bondability. Little or no sulfur results in a compound that is more difficult to bond.
• Accelerators – Of the more commonly used accelerators, MBT generally allows good bondability. ZDMC and the ultra-accelerators such as TMTD detract from bond-ability, particularly in “EV” or “semi-EV” cure systems. The prevulcanization inhibitor (PVI) is often added to fast-vulcanizing stocks to increase processing safety. However, when using ultra-accelerators. High levels of PVI in NR formulations are detrimental to bonding. Amounts of PVI below 0.15 p.h.r. usually allow satisfactory bonding.
• Fillers – The type and amount of filler is critical. Compounds with 40 to 80 p.h.r. of carbon black are easier to bond than those with lower black levels. Non-black fillers, such as clays and silicas, also facilitate bonding.
• Waxes and Oils – Waxy or oily compounding ingredients that migrate to the vulcanizing elastomer surface cause bonding difficulties. These include low molecular-weight polyolefin auxiliaries, (i.e., low-melting polyethylene and polypropylene processing aids/lubricants), aromatic oils and fatty acid esters (i.e., ricinoleates). Naphthenic or paraffinic oils are less problematic.
• Phthalate Ester Plasticizers – Although phthalate esters such as dioctyl phthalate are often recommended for maintaining the mechanical properties of polyolefinic elastomers (EPDM and IIR) in low-temperature, end-use applications, they are detrimental for bonding. Using phthalate esters can compromise the bondability of NBR stocks. However, incorporating high surface area, inorganic fillers, such as silicas, can sometimes neutralize the negative effects of phthalate ester plasticizers.
• Anti-ozonants – High levels of anti-ozonants and certain antioxidants, particularly the p-phenylene diamine type, may detract from bondability.
• Non-diene Elastomers – Elastomers not cured with sulfur and accelerators are easier to bond through inclusion of high surface area fillers. They become more difficult to bond when compounded with certain oils, plasticizers and waxes.

Elastomer Blends

Blends of two or more gumstocks (e.g., NR-SBR mixtures, NBR mill-mixed with IR) are chosen so the most desirable features or properties of each component are available. Blends are also selected in an effort to improve raw material economics, without compromising finished part quality.

Elastomer blends are almost always heterophase systems, i.e., dispersions of one type of elastomer in a continuum or matrix of the other. This heterogeneity is because most elastomer pairs are not mutually soluble. Blending results in less-than-uniform distribution of the compounding ingredients, which often causes one of the elastomers to be preferentially vulcanized by the sulfur and accelerators.

The overall effects of elastomer blending can impact bondability and adhesive selection. For example, blends of NBR and NR will be more difficult to bond than compounds comprised entirely of nitrile elastomer. For more details, review the elastomer property evaluation grid.

Additional Considerations

The finer points of adhesive selection include considerations regarding the design of the part, the molding method and the compound formulation.

• Part Design – The design or geometry of the assembly will influence bonding and how well that part will withstand service environments. Fluid engine mounts or bushings (i.e., those with contained fluid) may place atypical demands on the environmental resistance of the cured adhesive. If the elastomer-metal interface is exposed to a confined fluid such as hot glycol/water mixture, the adhesive system will need to withstand this particular service exposure (i.e., Chemlok 259 adhesive in conjunction with Chemlok 207 primer).
• Molding Method – The molding method will affect the tendency for undesirable wiping or sweeping of the adhesive. This phenomenon sometimes results when a molten elastomer compound moves across the adhesive-coated metal part surfaces prior to vulcanization. Under these conditions, some adhesives can be swept away from the interfaces where they are needed. Chemlok 220 adhesive has good resistance to sweeping or wiping. Chemlok 234B adhesive, by contrast, should not be selected if sweeping is apt to be problematic.
• Prebaking – Prebake tolerance is the adhesive’s ability to withstand high-temperature exposure before it contacts the vulcanizing elastomer. Bonding is dependent on chemical reactions that occur at elevated temperatures between the adhesive and the vulcanizing elastomer compound (i.e., across the adhesive-elastomer interface). If chemical reactions begin in the adhesive before elastomer contact, a significant amount of the adhesive’s bonding capability can be lost. Reaction of key adhesive ingredients from the adhesive-coated metal surface can cause the adhesive to lose some of its bonding activity. Reaction may result in migration of highly reactive species to the compound surface. Pre-reaction of key ingredients can then occur at the outer layer of the elastomer, before adhesive elastomer contact.

Bonding Process

There are four stages to the bonding process:

1. Substrate Preparation
2. Primer and Adhesive Application
3. Elastomer Preparation
4. Molding, Curing, and Finishing

For more details, review the elastomer bonding flowchart.

Substrate Preparation

Proper surface preparation is essential to achieving maximum bond strength. Use the below Surface Preparation Chart to determine the appropriate surface-cleaning procedure and recommendations for metallic and non-metallic surfaces.

During the surface preparation, there are certain control parameters which need to be taken into account. These are listed in the Process Control Checkpoints Chart.

• Removal of Oily Contaminants – Remove cutting oils, die lubricants, and particle contaminants by alkaline degreasing. The alkaline bath must have temperature and concentration controls and have an overflow system. Cold and hot water rinse tanks are required to ensure removal of alkali and detergent traces.
• Removal of Insoluble Materials – Remove scale, rust or other oxide coatings by mechanical or chemical treatments. Mechanical Treatments include blasting, abrading, machining or grinding. Clean grit or abrasives must be used. Alkaline cleaning before and after blasting is preferred. These methods remove dry soil and corro-sion, increase the surface area and provide an active surface for bonding. Chemical Treatments include phosphatizing or conversion coating to provide a clean surface. Treat-ment solutions must be controlled. Rinse water and drying air must remain pure.
• Maintaining Prepared Substrates – Prevent exposure to dust, moisture, chemical fumes, mold sprays and other contaminants. Apply the primer as soon as possible after surface preparation.

Adhesive Application

Thoroughly mix pigmented adhesives prior to and during application. Evenly apply the primer and allow to completely dry before topcoating. A thin coat of primer is preferred, as heavy coats can lead to solvent entrapment and subsequent bond failure during molding. Maintain film uniformity by controlling the temperature and viscosity of the wet adhesive or primer. When applying more than one coat of adhesive, allow adequate time and temperature between coats to ensure complete solvent evaporation.

Apply the primer or adhesive by dipping, spraying, brushing, roll coating or tumbling. The choice of application method depends on size, shape of parts and the number of pieces being coated. Listed below are the features of the five application methods:

1. Dipping – Used for solvent and aqueous adhesives. Dipping accommodates both large and small production runs, depending on the level of automation.
2. Spraying – Provides the highest level of bond perfor-mance and the most rapid evaporation rate of carrier solvent.
3. Brushing – Tecommended only for solvent adhesives. Useful for small runs or production which is not continuous.
4. Roll Coating – Provides an excellent method of coating large flat areas as well as cylindrical objects.
5. Tumbling – Economically coats the parts in a revolving barrel. The adhesive can be dried by discharging the parts into drying trays, by circulating warm air through the tumbling drum, or by drying in an oven.

Precise guidelines for control of the application processes can be found in the Chemlok Adhesives application guide.

Molding and Finishing Operations

Molding is the most important step in the bonding procedure; any variation in the individual molding parameters can result in bond failures or a high scrap rate. When designing the mold, make provisions for easy loading of the adhesive-coated metals as well as for easy removal of the vulcanized part.

Place the adhesive-coated metal and rubber compound in the mold cavity. Use the correct time, temperature and pressure to form a quality bonded assembly. Periodically check the mold cavity temperatures by using thermocouples, pyrometers, Tempilsticks^® or selective melting point wax pencils. Leaky molds, temperature variations, lack of curing or overcuring will adversely affect the bond integrity.

The ideal bonding environment occurs when the elastomer is under maximum pressure and at a minimum viscosity during vulcanization and curing. To obtain these conditions, follow the specified time and temperature requirements of the elastomer being cured. The below chart lists the process control checkpoints for molding and finishing operations.

Tempilsticks^® is a trademark of Tempil, Inc.

Molding Methods – There are three techniques of molding: transfer molding, injection molding and compression molding. Transfer and injection molding comprise the majority of all manufactured rubber-to-metal parts. Listed in the below chart are typical conditions imposed for satisfactory vulcanization bonding.

Finishing Operations – It is often necessary to perform additional treatments to bonded parts. Common bond failures associated with these additional treatments:

• Deflashing with dry ice or nitrogen – Failures between the metal and rubber materials when large loads are in the tumbler at too low of a temperature for an extended period of time.
• Wire brushing, grinding or machining – Failure of the bonded part due to heat build-up.
• Post-painting – Failure when the adhesive does not resist the solvents in the post-paint operation.

Troubleshooting

ASTM International provides a set of detailed symptom descriptions for bond failures. These descriptions can be used to assess the problem and effect swift, corrective action. In this document, the terms “elastomer” and “adhesive” should be interpreted as “rubber” and “cement,” respectively.

Covering approximately 80% of all bond failures, the four basic ASTM designations are:

• R – Failure in the rubber. This classification can be further broken down into additional subclassifications as described below.
• RC – Failure at the rubber-cement interface.
• CM – Failure at the cover cement-metal interface; or at the primer-metal interface.
• CP – Failure at the cover cement-primer interface.

Rubber (R) Failures – Commonly used industry designations for types of rubber failure include:

• SR (Spotty Rubber) – Appears on the metal surface looking like splattered rubber. Often caused by a metal surface contaminated with dust or other foreign deposits prior to bonding. An SR break can also be caused by ultra-fast drying of adhesive as it leaves the spray nozzle (cobwebbing).
• TR (Thin Rubber) – An even, but very thin rubber residue on the metal surface. Usually occurs with butyl or rubber stocks that are very highly oil-extended. When oils migrate to the RC interface, they create a bond layer that is part adhesive, part oil and part rubber. This weak layer easily fails when the part is stressed.
• HR (Heavy Rubber) – A thick or heavy layer of rubber remaining on the metal surface indicates an excellent bond. The stock fails because it is stressed beyond its cohesive strength.
• SB (Stock Break) – A failure of the rubber that makes the elastomer appear to have been folded back on itself and then broken off. The break is jagged and at a sharp angle to the metal surface.

Rubber-Cement (RC) Failures – Separation between rubber and cement is usually characterized by a relatively glossy, hard surface on the metal with little or no rubber visible. Common causes of RC failure are: precuring of the adhesive or rubber before the rubber comes in contact with the adhesive; inadequate cement film thickness; low molding pressure or temperature; inadequate cure; and migration of plasticizers, oils and other incompatible compounding ingredients.

Cement-Metal and Primer-Metal (CM) Failures – A clean separation between metal and primer or adhesive indicates that no adhesion has occurred. This may be due to several factors. Oil, dirt, dust or other foreign matter on the metal surface may have prevented adhesion from taking place. Environmental factors affecting the metal surface may have caused under-bond separation.When adhesive solvents evaporate too quickly, ultra-fast drying of the adhesive as it leaves the spray nozzle (cobwebbing) may occur. Flow of the elastomer stock during bonding may cause displacement of the adhesive from the metal (sweeping).

Cement-Primer (CP) Failures – Separation at the cover cement-primer interface is easily detected if primer cement and cover cement are of different colors. Such a failure is invariably due to contamination of the primer, plasticizer migration from the elastomer, or inadequate primer/adhesive mixing or drying.

Combination Failure – Combination failures can occur when cement-metal, rubber-cement and rubber failures are found on the same part. Consult the below charts for remedies to combination failures.