Production of high-quality bonded parts begins with two choices: the elastomer and the adhesive system.
1. Elastomer- The first choice is the elastomer. The gumstock type and the details of its formulation will be predicated by the intended function of the bonded assembly. The rubber for a highly engineered automobile engine mount will be selected for its dynamic performance in controlling vibrations and for its ability to endure under-the-hood operating conditions. Conversely, the elastomer for an engine seal must provide superb resistance to attack by engine fluids.
2. Adhesive System- The second choice is the adhesive system. The adhesive system must provide an excellent bond under the specified vulcanization conditions, as well as maintain its bond under service conditions.
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:
Other commonly used synthetic elastomers include:
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.
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:
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.
The finer points of adhesive selection include considerations regarding the design of the part, the molding method and the compound formulation.
There are four stages to the bonding process:
For more details, review the elastomer bonding flowchart.
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.
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:
Precise guidelines for control of the application processes can be found in the Chemlok Adhesives application guide.
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:
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:
Rubber (R) Failures – Commonly used industry designations for types of rubber failure include:
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.