Substrate bonded plastics

A wide range of substrates can be bonded. The inherent tackiness of natural rubber enables it to coat most non-polar substrates (mainly plastics and rubbers). 

This section focuses on adhesives that are used for the assembly of medical devices. In medical device assembly, the primary substrates are plastics, elastomers, and metals. The total medical adhesive market is much larger since it encompasses a broader definition of products. For example, medical adhesives can be used for bonding human tissue, transdermal drug delivery systems, dental restoration, and wound care in addition to medical device assembly. 

Because of their excellent flexibility, epoxy-polysulfide adhesives are commonly used for bonding plastic substrates where differences in thermal expansion are a concern. 

The plastic surface, at the time of bonding, may be well suited to the adhesive process. However, after aging, undesirable surface conditions may present themselves at the interface, displace the adhesive, and result in bond failure. These weak boundary layers could come from the environment or from within the plastic substrate itself. Plasticizer migration and degradation of the interface through uv radiation are common examples of weak boundary layers that can develop with time at the interface. 

Starting formulations for several epoxy adhesives recommended for bonding plastic substrates... 

The use of primers is also advisable if the bonded parts are to be subjected to temperature changes, damp conditions, tension compression, shear forces, etc. Some porous substrates and plastic materials also require primers. Primers can be applied to a thoroughly prepared surface by brushing, spraying or dipping to form a thin film. All primers contain flammable solvents and therefore safety precautions are essential. 

Broad applicability to a wide variety of substrates metal, plastic) with minimal surface preparation Tolerance for oil-contaminated surfaces Rapid bonding at room temperature Excellent bond flexibility... 

In addition to the benefits of low odor and redueed fogging, these adhesives form stronger bonds to low-energy substrates sueh as EPDM rubber, natural rubber, and other difficult-to-bond plastics. This property seems to be a funetion of the solvent action of the uncured adhesive, so care must be taken to avoid stress eraeking when the adhesive is used on sensitive substrates sueh as polyearbonates and polyaerylates. 

Uses Bonding agent for coating of polyester and polyamide substrates with plasticized PVC Features Good thermostability Properties Liq. vise. 12,500 mP-s-Bonding Agent TN/N [Bayer AG]... 

Uses Adhesion promoter for plasticized vinyl substrates bonding agent for bonding thin-film decorative vinyl to particleboard, bonding vinyl underlayment for artificial turf, encapsulation of vinyl insulated elec, components, bonding of vinyl floor tiles, base boards, and pipe Features Solv.-free... 

There are many types and forms of adhesives that can be used with plastics to provide strong structural bonds. Plastics generally have lower tensile strength than other materials, such as metals, and an adhesive strength can often be achieved that is greater than the strength of the substrate itself. 

Flexible adhesives are often chosen for bonding plastics and elastomers. Lower-modulus adhesives generally have the flexibility to bond well to plastic substrates. However, these are generally weaker in shear than more rigid adhesives. Fortunately, exceptionally high shear strength is often not required for an adhesive for plastic, since the plastic substrate itself is relatively weak. 

However, often the temperature required to cure the adhesive will adversely affect heat-sensitive plastic parts. Also, heat-curing adhesives are generally more rigid than those that cure at room temperature, and the resulting modulus is too high for many plasticbonding applications. As a result, most adhesives recommended for bonding plastic substrates cure at room temperature. 

Examples of these are the bonding of decorative glass figures, jeweUeiy, sporting equipment, toys, loudspeakers (both ferrite/metal and paper and plastic cones) and small motors. Because the acrylic adhesives bond a wide range of substrates (metals, plastics, wood, glass), they can be used to bond different components and have found wide use in many industries. 

Apart from their use as tie layers in coextension, the modified polymers can find other adhesive applications such as improving adhesion of extrusion coatings, thermal lamination interplies, and as dispersions or powder coatings. As coatings, the modified polymers applied to metal surfaces can act as an adhesive layer to bond plastic components. A particularly useful version is to prepare a coextended film of a non-modified base polymer such as polyethylene or polypropylene, forming the bnlk of the structure, and apply a thin layer of the modified polymer to one or both external surfaces. This steucture may now be used to thermally bond to a metal substrate. 

Weak boundary layer theory. According to the weak boundary layer theory, when bond failure seems to be at the interface, usually a cohesive break of a weak boundary layer is the real event. Weak boundary layers can originate from the adhesive, the adherend, the environment, or a combination of any of the three. When bond failure occurs, it is the weak boundary layer that fails, although failure seems to occur at the adhesive-adherend interface. Figure 9.16 shows examples of certain possible weak boundary layers for a metallic substrate. For plastic substrates there are many more opportunities for weak boundary layers, such as mold release, plasticizer migration, and moisture migrating to the interface. Certain weak boundary layers can he removed or strengthened by various surface treatments. 

Polyurethane adhesives bond well to many substrates, including hard-to-bond plastics. Since they are very flexible, pol5nirethane adhesives are often used to bond films, foils, and elastomers. Moisture curing one-part nrethanes are also available. These adhesives utilize the humidity in the air to activate their curing mechanism. 

Table 4.1 Tensile strength of various plastic substrates bonded with an epoxypolyamide adhesive [7]...

Figure 8.7 Effect of immersion in water at 40 °C on the tensile lap-shear strength of glass fibre reinforced plastic (GRP) substrates bonded with a two-part acrylic adhesive [40]. The surface pretreatment was simply an abrasion/solvent-cleaning process.

While the polyvinyl acetate-based emulsions have multiple applications (office glues, wood glues, packaging industry, etc.), the acrylic emulsions are used mainly for the production of autoadhesive films and tapes or for other labels (see PSAs part) their main value therefore lies in the large variety of formulations accessible (using variable composition copolymers) and in the good tolerance they demonstrate toward the plasticizers present in certain substrates (bonding of polyvinylchloride films for example). 

The cure reaction for many silicone sealants is initiated by acid added at low levels to the sealant formulation. Acids are chemically incompatible with concrete, marble, and limestone. When acid-containing silicone sealants are used in joints with these substrate materials, the acid reacts with the substrate bond surfaces, creating salts at the bond interface. These salts destroy the sealant/substrate adhesion and cause debonding and loss of the seal. In order to use a silicone sealant with these substrates, a silicone formulated without acid is required. Other known chemical incompatibilities are silicone and polychloroprene. Use of these two materials together in a sealant joint is to be avoided. Solvated sealant use in joints containing plastic or rubber materials should be undertaken only after chemical compatibility studies of the sealant with these materials is performed. Typical incompatibility will manifest itself over time by causing the sealant or substrate to soften, harden, crack, and/or craze. A standard test method for determining chemical compatibility is ASTM D-471. 

The lap shear joint has been the subject of much academic study, particularly with metal substrates but due to the wide variety of plastics available there seems to be little published data when it comes to bonding plastics. 

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