Difficult-to-bond substrate

Type AD-G is used in an entirely different sort of formulation. The polymer is designed for graft polymerisation with methyl methacrylate. Typically, equal amounts of AD-G and methyl methacrylate are dissolved together in toluene, and the reaction driven to completion with a free-radical catalyst, such as bensoyl peroxide. The graft polymer is usually mixed with an isocyanate just prior to use. It is not normally compounded with resin. The resulting adhesive has very good adhesion to plasticised vinyl, EVA sponge, thermoplastic mbber, and other difficult to bond substrates, and is of particular importance to the shoe industry (42,43). 

During World War II, several new synthetic elastomers were produced and new types of adhesives (mainly styrene-butadiene and acrylonitrile copolymers) were manufactured to produce adequate performance in joints produced with new difficult-to-bond substrates. Furthermore, formulations to work under extreme environmental conditions (high temperature, resistance to chemicals, improved resistance to ageing) were obtained using polychloroprene (Neoprene) adhesives. Most of those adhesives need vulcanization to perform properly. 

Polychloroprene elastomer. Neoprene AC and AD are the most commonly used, mainly Neoprene AD because of its superior viscosity stability. For difficult-to-bond substrates, graft polymers Neoprene AD-G or AF) show better performance. For sprayable adhesives or high-viscosity mastics, the Neoprene AG offers excellent results. When specific properties (e.g. increase tack, improve wetting, increase peel strength) need to be met, blends of Neoprene AC or AD with Neoprene AG provide adequate performance. 

Flexibilized epoxy adhesives have moderate strength on flame and corona treated polyolefin substrates. Elevated cure temperature results in better adhesion because of more efficient wetting of the substrate surface. Table 16.13 shows a starting formulation for an epoxy adhesive that develops high peel strength to many difficult-to-bond substrates such as polyethylene, thermoplastic rubber, and polyester film. 

Metal activators are formulated in aerosol solvents so that they can be applied to substrates by spray action causing the solvent to evaporate leaving the active metal(s) on the surface. The active metal enhances the anaerobic cure particularly on substrates that are slow to cure with anaerobic products. They are formulated at concentrations to suit application on difficult-to-bond substrates and for a particular adhesive. 

Classification Modified propylene copolymer Uses Prod, of ionic/nonionic/cationic wax disps. primer for fiber-reinforced plastics, difficult-to-bond substrates antislip agent in floor care prods. release agent for aluminum diecasting handle modifier for textile nonslip applies. 

Styrene butadiene copolymers (SBCs) Available as both a pressure-sensitive and nonpressure-sensitive adhesive, good flexibdity and toughness, resistant to both hot and cold temperatures. Used to join difficult-to-bond substrates (e.g., polyolefin fibre). Medium... 

Polyvinyl acetate emulsions can be further modified by the incorporation of functional groups. These groups permit the design of polymers which have the ability to bond a wider spectrum of surfaces, including difficult-to-bond substrates. The addition of functional groups also permits crosslinkability of the polymer in order to achieve a high degree of water, solvent, or heat resistance. 

Considering difficult-to-bond substrates, the adhesion of screen printing inks is most problematic with plastics and non-porous materials. The surface energy differences, for example, between metals, glass, and different plastics are extremely wide-ranging, as can be seen in Table 8.3. 

Uses Tackifier for pressure-sensitive adhesives, construction adhesives, pick-up gums for labeling adhesion promoter for difficult-to-bond substrates food-pkg. adhesives, coatings, paper defoamer in food-contact paper/paperboard Regulatory FDA 21CFR 175.105,175.300,176.170,176.180,176.210 Properties Gardner 11 color soften, pt. (R B) 139 C acid no. 133-147 Sylvatac 295 [Arizona]... 

Epoxy resins are also used in special appHcations, such as an overlaying procedure requiring a durable, heat-resistant bond of a difficult-to-bond overlay on a wood-base panel substrate. Metal sheets used as overlays, for example, often require an epoxy adhesive. 

These results demonstrate some interesting chemical principles of the use of acrylic adhesives. They stick to a broad range of substrates, with some notable exceptions. One of these is galvanized steel, a chemically active substrate which can interact with the adhesive and inhibit cure. Another is Noryl , a blend of polystyrene and polyphenylene oxide. It contains phenol groups that are known polymerization inhibitors. Highly non-polar substrates such as polyolefins and silicones are difficult to bond with any technology, but as we shall see, the initiator can play a big role in acrylic adhesion to polyolefins. 

Low surface energy substrates, such as polyethylene or polypropylene, are generally difficult to bond with adhesives. However, cyanoacrylate-based adhesives can be effectively utilized to bond polyolefins with the use of the proper primer/activa-tor on the surface. Primer materials include tertiary aliphatic and aromatic amines, trialkyl ammonium carboxylate salts, tetraalkyl ammonium salts, phosphines, and organometallic compounds, which are initiators for alkyl cyanoacrylate polymerization [33-36]. The primer is applied as a dilute solution to the polyolefin surface, solvent is allowed to evaporate, and the specimens are assembled with a small amount of the adhesive. With the use of primers, adhesive strength can be so strong that substrate failure occurs during the course of the shear tests, as shown in Fig. 11. 

Ethylene Copolymers. Ethylene copolymers probably are the most important materials in hot-melt formulations. Ethylene-vinyl acetate and ethylene-ethyl acrylate polymers are very versatile and available in a wide range of grades offering different co-monomer contents and viscosities. The melts are stable and compatible with various modifying resins, waxes, extenders, and fillers. Adhesion to many substrates is good—including the polyolefin plastics, which are difficult to bond with most other types of adhesive unless the surfaces are pre-treated. 

Copper is used in three basic forms pure, alloyed with zinc (brass), and alloyed with tin (bronze). Copper and copper alloys are difficult to bond satisfactorily, especially if high shear and peel strengths are desired. The primary reason for this difficulty is that the oxide that forms on copper develops rapidly (although not as fast as the rate of oxide development on aluminum). The copper oxide layer is weakly attached to the base metal under usual conditions. Thus, if clean, bare copper substrates were bonded, the initial strength of the joint would be relatively high, but on environmental exposure an oxide layer could develop which will reduce the durability of the joint. 

Plastics are usually more difficult to bond with adhesives than are metal substrates. Plastic surfaces can be unstable and thermodynamically incompatible with the adhesive. The actual bonding surface may be far different from the expected substrate surface. The plastic part can possess physical properties that will cause excessive stress in the joint. The operating environment can change the adhesive-plastic interface, the base plastic, the adhesive, or all three. 

Polyphenylene sulfide parts are commonly bonded together with adhesives. A suggested surface preparation method is to solvent-degrease the substrate in acetone, sandblast, and then repeat the degreasing step with fresh solvent. The polyphenylene sulfide surface that forms next to a mold surface is more difficult to bond than a freshly abraided surface. This is possibly due to a different chemical surface structure that forms at high temperature when the resin is in contact with the metal mold surface. 

Pyruvate-dependent aldolases catalyze the breaking of a carbon-carbon bond in nature. This reaction can, however, be reversed if an excess of pyruvate is used, establishing one new stereocenter in the course of it. The natural function of phosphoenolpyruvate (PEP)-dependent aldolases on the other hand is to catalyze the synthesis of a-keto acids. Since PEP is a very reactive, unstable and difficult to prepare substrate, they are not commonly used in synthesis. 

Thermosetting Foam Substrates Most thermosetting plastics are not particularly difficult to bond. Obviously, solvent cementing is not suitable for bonding thermosets to themselves, since they are not soluble. In some cases solvent solutions can be used to join thermoplastics to thermosets. In general, adhesive bonding is the only practical method of joining a thermoset to itself or to a non-plastic material. Epoxies or modified epoxies are the most widely used adhesives for thermosets (1). 

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. 

N-(n-Butyl)-3-aminopropyltrimethoxysilane 3-Glycidyloxypropyltriethoxysilane primer, substrates difficult-to-bond Polypropylene wax, maleic anhydride-modified primer, surface repeated use food-contact Polymethylmethacrylate/poly (trimethoxysilylpropyl) methacrylate copolymer... 

Adhesives manufacturers are continually trying to develop adhesives to meet the needs of industry. One group of plastics that have been difficult to bond are polyolefins and related low-energy substrates (see Surface energy). They could not be bonded without elaborate surface preparation such as Flame treatment or Plasma pre-treatment, Corona discharge treatment or oxidative chemical methods. 

Careful surface preparation before adhesive application is essential for consistent and successful bonding to most substrates (see Pre-treatment of metals prior to bonding), and this certainly applies to copper, which has a reputation for being difficult to bond. Part of the difficulty is the friability of the black copper(ll) oxide, which forms on the surface in air at temperatures in the range 200-500 °C. At ambient temperatures, a thin layer of copper(I) oxide is present. 

Sprayed ceramic coatings can be made chemically active by selection of the spray parameters, which result in metastable phases within the coating. Ceramic bond coats are useful for difficult to bond materials such as ceramic components, including carbide-containing parts and refractory metals. These materials may be used in combination with a metallic bond coat on metallic substrates to mitigate stress differentials between the metallic substrate and the ceramic bond coat due to thermal or mechanical stress. 

Plastics that are readily bonded with induction methods include aU grades of ABS, nylon, polyester, polyethylene, polypropylene, and polystyrene, as well as those materials often considered more difficult to bond such as acetals, modified polyphenylene oxide, and polycarbonate. Reinforced thermoplastics with filler levels up to 65 percent have been joined successfully. Many combinations of dissimilar materials can be bonded with induction welding processes. Table 8.5 shows compatible plastic combinations for electromagnetic adhesives. Thermoset and other nonmetallic substrates can also be elec-tromagnetically bonded. In these applications the bonding agent acts as a hot-melt adhesive. 

Stainless steel is well known as being difficult to bond, especially when long-term environmental resistance is required. The degree of difficulty increases with the increasing chromium content of the steel. The oxides associated with low carbon steels are very stable but are thin and very smooth they have none of the micro-roughness which is typical for other substrates. 

However, some polymeric substrates, such as polytetrafluoroethylene, polyethylene, polypropylene and cured butyl rubber, are particularly difficult to bond and some form of pretreatment is nearly always necessary. Also, even for... 

---