Structural Methacrylates

September 1, 2010
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Figure 1. Higher stress test (0.34-6.89 N/mm2).

The term structural adhesive typically defines an adhesive that can withstand high pressures under a variety of stresses for long periods of time. Structural adhesives usually contain molecules of a high molecular weight, which are required to withstand and distribute the loads within a joint.

The reasons for using an adhesive instead of a mechanical fastener are twofold. Mechanical fastenings can cause stress around the fixing, resulting in stress cracking and, sometimes, failure of the substrate. Adhesives can remove this stress. The bond line between the two materials produces lower stresses because the load is applied over a larger surface area. In addition, the adhesive provides a layer between the materials that can absorb energy in impact, allow for thermal expansion, and act as a layer to resist fatigue. The development of methacrylate technology for structural adhesives has enabled manufacturing improvements in many markets.



Figure 2. Lower stress test (0.34-3.44 N/mm2).

Methacrylate Technology

Methacrylate adhesives have been developed to perform under a range of conditions through the improvement of curing, bonding, adhesion and toughening mechanisms. The free-radical cure mechanism of methacrylates is catalytic in nature. When the adhesive and activators are mixed, amines in the adhesive cause the peroxide to decompose into free radicals; these initiate the process of chain polymerization of the methyl methacrylate (MMA) to poly(methyl methacrylate). These reactions are exothermic and take place at room temperature, removing the need for heat application in order to initiate the cure.

The improved adhesion mechanism of methacrylate adhesives derives from the process of the low-viscosity monomer solvating the surface of most thermoplastics prior to the start of the curing mechanism. The monomer can solvate most surface contaminants, which removes the need for any surface preparation prior to applying the adhesive.

Plexus methacrylate adhesives have a unique combination of polymers and impact modifiers that enable them to be strong yet flexible. The resulting high tensile strength, along with impressive elongation (over 100% at 30°C), gives them the processing advantages and convenience of epoxies and polyurethanes without the associated increased cost. The total combination of all these factors, along with good rheology, thixotropic properties, ease of use, and lack of volatile organic compounds (VOCs) and solvents, results in a range of structural methacrylate adhesives that can be successfully used in the marine, construction, and automotive markets on a variety of substrates.



Adhesive Selection

Selecting an adhesive for a bonding application should be done during the design procedure while parameters are being outlined and determined. Before the substrate and adhesive are selected, the user must first determine the process constraints and the conditions to which the final bonded assembly will be exposed. Involving the adhesive manufacturer at this early stage can omit any later problems associated with incorrect selection (such as delayed production and field failures).

In addition, the main application factors that need to be defined prior to adhesive selection include:

  • What is the function of the bonded assembly?

  • What stresses will the adhesive experience (e.g., shear, peel, etc.)?

  • To what environmental/durability conditions will the final bonded assembly be exposed?

  • What is the required bond-ine thickness?

  • To what temperatures will the adhesive be subjected?

  • What are the plant environments where the adhesive will be used?

  • What are the required working/fixture times?

  • How will the chosen adhesive be applied?

     

Once these factors have been determined, testing can be carried out to ensure that the correct adhesive is chosen for the specific application.

 



Applications

Plexus adhesives are used successfully by major domestic and foreign automotive and truck manufacturers. The ability of the adhesives to bond most thermoplastic and composite materials allows manufacturers to simplify assembly and generally reduce the weight of the final bonded assembly. Certain automotive parts that use Plexus adhesives can include bumpers, instrument panels, spoilers, cosmetic surface repairs, glove boxes, door panels, strengthening and structural body parts, grill components, side and roof panels, and sleeper cabins.

Composite is a general term that refers to the assembly of dissimilar materials used together to perform a job that the individual materials cannot accomplish by themselves. Plexus adhesives have provided an alternative to traditional methods of bonding composites and also allow the bonding of dissimilar materials (e.g., SMC to E-coated steel, and fiberglass to aluminum).

In the marine market, where large quantities of fiberglass products are used, traditional methods of bonding involve the formation of bond joints by building up layers of fiberglass scrim and resin or by using marine putties. Conditions such as temperature, UV radiation, moisture, impact, chemicals, and general wear and tear can weaken joints so dramatically that they fail and cause leaks that hinder boat performance.

In response to these challenges, Plexus developed Fiberglass Fusion adhesives, which, when applied, dissolve a thin layer of each mating surface and create a chemically crosslinked bond. These adhesives actually fuse two fiberglass surfaces into one; the adhesion is so strong that the fiberglass itself will delaminate or break before the bond fails.

When bonding the deck to the hull, and when stringer bonding, methacrylate adhesives save time and reduce labor costs while providing a strong, durable, flexible, energy-absorbing bond line that is not susceptible to cracking or brittleness. Major considerations when choosing an adhesive for this type of application include fatigue resistance, flexibility (to absorb vibrations and stresses caused by pounding wind and water), and high speed on performance boats. When tested for fatigue resistance against samples bonded with a marine urethane adhesive and parts traditionally glassed together, Plexus adhesives outperformed the urethane and equaled the glassed parts (see Figures 1 and 2).

Methacrylate adhesives’ beneficial qualities are also important in the construction industry, where many bonded assemblies are positioned outside or exposed to different chemicals (e.g., acids, bases, diesel and petrol). Plexus adhesives’ durability is a result of its adhesion mechanism, which ensures that the bond strength is higher than either the substrate strength or the cohesive strength of the adhesive. This is particularly important with bonding applications such as external signs or exterior doors. Both of these bonded applications are submitted to all types of environmental conditions and often prior to use, such as in paint bake cycles. Methacrylate adhesives have been proven to perform as well as or better than control values when exposed to extreme hot/cold cycles, humidity and salt conditions.



Summary

Methacrylate adhesives offer an alternative to many traditional methods of joining two substrates. Due to the unique combination of polymers and impact modifiers, they possess the high strength of an epoxy combined with the flexibility of a polyurethane. The free radical chain-polymerization cure of methacrylate adhesives means they cure at room temperature and are not as susceptible to off-ratio mixing.

Many thermoplastics, composites and metals can be bonded with Plexus adhesives with little or no surface preparation. The bonding of dissimilar materials can be achieved successfully, and the use of an adhesive over other methods can reduce labor costs and increase productivity while providing a strong, flexible, tough, durable bond that can withstand fatigue, vibration, and impact.

The combination of all these factors, combined with good rheology, thixotropic properties, ease of use, recyclability, and lack of VOCs and solvents, results in a range of structural methacrylate adhesives that can be used to bond many substrates in a variety of different industries.



About ITW Plexus

ITW Plexus specializes in the design and manufacture of sophisticated structural adhesives for the bonding of materials used in diverse markets such as transportation, marine, automotive engineering and construction. The company is a division of Performance Polymers Europe (PPE), headquartered in Rushden, England.



For more information, contact ITW Plexus at (44) 870458-7588 or visit www.itwplexus.co.uk.

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