Originally published May1997
From natural rubber discovered in the hevea brasiliensis plants of the Amazon Valley to thermoplastic elastomers designed in modern research laboratories, elastomers have found an important niche in the materials arsenal of today's medical device designer. The growing number of design applications for elastomeric substrates has greatly increased the need for information on assembly techniques using these materials. Adhesives offer several benefits over mechanical fasteners for joining elastomeric materials. Unlike solvent bonds or ultrasonic welds, adhesives perform well with thermoset rubbers. In addition, adhesives distribute stress over a joint's entire bond area rather than in a single location, as do mechanical fasteners. Finally, a wide variety of adhesives have been developed that are compatible with high-speed automated manufacturing processes. Unfortunately, the wide selection of elastomers and adhesives can make it difficult to identify the optimal combination for a given design.
Elastomer-based devices, like these tracheal tubes, are assembled using various combinations of adhesives. Photo: Loctite Corp.
This article details a comprehensive study of the typical bond strengths achieved with a variety of adhesives. The study was planned in an effort to help designers identify the adhesive and elastomer combinations best suited for their design applications.
A matrix of 25 elastomers and 10 adhesives was chosen for this study. The adhesives and elastomers were selected from those most commonly used in medical device manufacture. Several different formulations of each elastomer type were tested with each of the adhesives.
From the total of 25 elastomers, 22 were selected in the following manner. The first formulation tested was a control of the pure elastomer gum stock (with a cure system if appropriate). To determine what other formulations of the elastomer would be used, the most common fillers and additives were identified. For each, a new formulation was compounded containing only that additive or filler added to the control. By comparing the bond-strength performance of the control with that of each of the other formulations, it was possible to isolate the effects of each of the additives and fillers on bond strength.
For the three remaining elastomers, comparative testing was performed on commercial grades chosen to represent the breadth of each product line. Generally, these materials are employed as received rather than compounded with fillers or additives by the end-user prior to use.
For thermoset rubbers, an additional processing variable was evaluated. When a thermoset rubber is heat-cured, the modulus of the rubber is maximized, at which point the cure is considered complete. If the heat-cure cycle ends when the modulus is 80% of the maximum value, it is designated as a "T80 cure." Such cures could potentially offer a benefit for bonding, since unreacted sites on the rubber backbone will be available to react with the adhesive. In addition, the lower cross-link density of the T80-cured rubber could allow the adhesive to diffuse into the rubber matrix, yielding improved bond strengths. Each of the thermoset rubbers was cured to 80% of its final modulus and compared with rubber cured to 100% of its maximum modulus to determine whether degree of cure would improve bond strength.
In this study, a broad range of adhesives represented the major families typically used to bond elastomers. A representative product was chosen from each adhesive family for the test program. A brief description of each adhesive family selected appears below.
Methyl Cyanoacrylates. Cyanoacrylates are one-part adhesives that cure rapidly to form a rigid thermoplastic when pressed in a thin film between two surfaces. They generally fixture in seconds and offer excellent adhesion to a wide variety of substrates. Methyl cyanoacrylates are based on the methyl cyanoacrylate ester and were the first cyanoacrylates commercialized.
Surface-Insensitive Ethyl Cyanoacrylates. The polymerization of cyanoacrylates is initiated when the acidic stabilizer they contain is neutralized by contact with basic species. For example, the moisture present on most surfaces at ambient conditions is basic enough to initiate polymerization. Acidic surfaces prohibit neutralization of the stabilizer and tend to retard the cure of cyanoacrylates. Surface-insensitive cyanoacrylates were developed to offer improved cure speed on acidic and dry surfaces.
Surface-Insensitive Ethyl Cyanoacrylates with a Primer. Some surfaces, such as polyolefins and fluorinated polymers, are very difficult to bond. To improve the performance of cyanoacrylates on these substrates, special primers were formulated that dramatically increase bond strength. These primers were applied, then bonded with surface-insensitive cyanoacrylates to evaluate which types of elastomers experienced an improvement in bond strength.
Rubber-Toughened Cyanoacrylates. The rigidity of cyanoacrylates can limit their ability to withstand peel and impact forces. Rubber-toughened cyanoacrylates were developed to address this limitation. The first generation of rubber-toughened cyanoacrylates were black and tended to have longer fixture times and lower bond strength than nontoughened cyanoacrylates. A second generation of clear rubber-toughened cyanoacrylates has been developed that offers faster fixture speeds, improved bond strength, and excellent thermal resistance to temperatures as high as 250°F (121°C). Both types of rubber-toughened cyanoacrylates were included in this test program.
Acetoxy-Cure Silicones. Acetoxy-cure silicones are one-part, solvent-free adhesives that react with moisture to form a thermoset elastomer with excellent flexibility at low temperatures and thermal resistance to 600°F (316°C). Their elastomeric qualities make them useful in applications for which rigid adhesives are unacceptable. Upon cure, these adhesives produce acetic acid as a by-product, which can be corrosive to some materials.
Oxime-Cure Silicones. Oxime-cure silicones are also one-part adhesives that react with moisture; however, they produce an oxime rather than acetic acid. Consequently, their by-products do not corrode ferric substrates, and generate a less pungent odor than does acetic acid.
Two-Part, No-Mix Acrylics. These adhesives comprise an activator and a resin. The activator is applied to one substrate, while the resin is applied to the other. When the parts are joined together, the activator catalyzes the polymerization of the resin, forming a tough, durable thermoset polymer with good temperature and chemical resistance. Because the activator is a catalyst rather than a reactant, it does not need to be mixed with the resin in any specific stoichiometric ratio.
Light-Cure Acrylics. Light-cure acrylics contain photoinitiators that absorb light energy and break down to form free radicals. The free radicals polymerize the adhesive via acrylate groups on the monomers, oligomers, and polymers in the adhesive. As a result, these systems can cure within seconds to form a tough thermoset polymer with excellent adhesion to many substrates. The ability to cure "on demand" offers significant processing benefits to manufacturers seeking to minimize work in progress and adhesive waste.
Moisture-Cure Polyurethanes. Moisture-cure polyurethanes are one-part systems that react with moisture to form a thermoset polymer matrix with good flexibility and cohesive strength. These systems were evaluated in conjunction with a primer to maximize adhesion to polymeric substrates.
A modified lap-shear method was used to evaluate the bond strength of the elastomers with various adhesives. For each assembly, two elastomer pieces measuring 1 * 1 * 0.125 in. were bonded together with a 0.25-in. overlap, yielding a bond area of 0.25 sq in. The elastomer pieces were bonded to steel lap shears to provide additional rigidity in testing. Samples were pulled apart at a rate of 2 in./min using an Instron 4206 mechanical properties tester.
For Table I, summarizing the bond-strength data of the elastomers tested (data are for elastomer formulations reinforced with carbon black), please refer to page 62 of Medical Plastics and Biomaterials May/June issue. When nonreinforced rubber formulations were tested for bond strength, the rubber substrate often failed before the adhesive/substrate bond, making it impossible to discern differences in bond strength between families of adhesives. The reinforced elastomer formulations offer superior mechanical properties compared with the nonreinforced formulations, and can handle higher forces before succumbing to substrate failure. Consequently, adhesive bond failure is more likely than substrate failure, making differences in bond strength easier to identify between adhesive types.
In Table II, those elastomers are highlighted that achieved a significant improvement in bond strength when the plastic primer was used.
Table II. Elastomers that showed an improvement in bond strength when a plastic primer was used along with a cyanoacrylate. A surface-insensitive ethyl cyanoacrylate was used in all testing. All bond strengths are in psi.
Table III lists thermoset rubbers that showed improved bond strength with various adhesives when the rubber was heat cured to 80% of its ultimate modulus.
Table III. Thermoset-rubber/adhesive combinations that increased in bond strength when the rubber was heat-cured to 80% of ultimate modulus.
Adding antistatic agents to plastic formulations has proven to significantly improve the bond strength of cyanoacrylates to many materials. In order to determine whether this effect could offer a similar benefit for elastomeric materials, antistatic agents were compounded with each of the elastomers studied. Table IV illustrates those elastomers that consistently showed improved bond strength when compounded with antistatic agents.
Table IV. Elastomers that benefited from the addition of an antistatic additive. Black rubber-toughened cyanoacrylate was used in all testing. All bond strengths are measured in psi.
The growing variety of elastomers and adhesives available to medical device manufacturers makes it increasingly difficult to identify the best combinations for specific applications. To help manufacturers with their selection, this article investigated the bond strength of 25 types of elastomers and 10 adhesives commonly used in the production of medical devices. The effect that each elastomer's composition and polymer structure had on bond strength was investigated for all adhesives.
The data from this study will help medical device manufacturers in identifying the key composition variables that affect the bond strength of elastomers. The general information on the 10 adhesive types studied will assist manufacturers in selecting adhesives that offer the necessary bond strength and compatible cure methods for a particular manufacturing process.
In some cases, the bond-strength improvement that results from varying the elastomer formulation proves insufficient for an application. Bond strength may be further improved by employing plastic primers or specific rubber-curing processes appropriate for different adhesive/elastomer combinations. The data provided in this article should help manufacturers to determine whether primers and/or special cure processes will enhance the adhesive/elastomer bond.
Harper CA (ed), Handbook of Plastics, Elastomers, and Composites, 2nd ed, New York, McGraw-Hill, 1992.
Ohm RF (ed), The Vanderbilt Rubber Handbook, 13th ed, Norwalk, CT, R.T. Vanderbilt Co., 1990.
Walker BM, and Rader CP (eds), Handbook of Thermoplastic Elastomers, 2nd ed, New York, Van Nostrand Reinhold, 1988.
Patrick J. Courtney is an engineering project manager at Loctite Corp. (Rocky Hill, CT), where he provides technical support to the company's customers in the Western United States. He holds a BS in chemical engineering from Worcester Polytechnic Institute.
James A. Serenson, whose responsibilities encompass the Southeastern region, is an application engineer at Loctite with a BS in chemical engineering from Northeastern University. Also an application engineer, with a BS in chemical engineering from the University of Connecticut, Chris Verosky works with Loctite clients in the Northeast.