Thermosetting plastics , adhesives epoxies

Thermoset plastics are cross-linked by chemical reaction so that their molecules cannot slip by one anotber. They are rigid when cool and cannot be softened by the action of heat. If excessive heat is applied, thermoset plastics will degrade. Consequently, they are not weldable. Because of their chemical resistance, they cannot be solvent cemented. Thermoset plastics are usually joined by either adhesive bonding or mechanical fastening. Typical thermosetting plastics are epoxy, urethane, phenolic, and melamine formaldehyde. 

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). 

Plasticizers today are used in commercially important amounts in about 35 different polymers with poly(vinyl chloride) (PVC) taking the largest share. Beyond the starting point in cellulosics, plasticizers have become highly important in poly(vinyl butyral), acrylic polymers (especially for surface coatings) and poly(vinyl acetate) (coatings and adhesives) and even in thermosets such as epoxies. Plasticizers are also used in crystalline polymers such as nylon, polypropylene, and poly(vinylidene chloride). Traditionally, extenders for rubber and synthetic elastomers were considered outside the field of plasticizers yet many such... 

Polyester resins (alkyds) were commercialized for coatings use in 1926, and unsaturated polyesters were used as thermoset fiberglass composite matrix resins in the 1940s, but the early resins made poor adhesives. When flexibilized resins appeared in the 1950s, they were used as adhesives. Today, unsaturated polyesters are widely used as adhesives for thermoset plastics bonding, and even for metal bonding in most countries, but are seldom used as adhesives in the United States, where the more expensive epoxy adhesives are used in similar applications. The saturated polyesters, used as thermoplastic hot-melt adhesives, seem to have appeared in the literature first in the 1954—1957 period. 

Of all the thermosetting plastics, epoxies are more widely used than any other plastic, in a variety of applications. There are resin/hardener systems (two-part) that cure at room temperature, as well as one-part systems that require extreme heat cures to develop optimum properties (e.g., 121°C and 177°C). Proper selection of various hardeners, resins, modifiers, and fillers allows the development of desired properties for a particular application. Because of the wide versatility and basic adhesive qualities, epoxies make excellent structural adhesives that can be engineered to widely different specifications. Essentially no shrinkage occurs during polymerization because epoxies are completely reactive producing no volatiles during cure. Epoxy adhesives can be formulated to meet a wide variety of bonding... 

Thermosetting plastics (e.g., epoxies diaUyl phthalate polyesters melamine, phenol and urea formaldehyde and polyurethanes) are joined either mechanically or by adhesives. Their thermosetting nature prohibits the use of solvent or thermal welding processes however, they are easily bonded with many adhesives. 

Thermosetting plastics Epoxy resin High strength when reinforced, good chemical and wear resistance. Adhesives, encapsulation of electronic components Moulding, boat and car bodies... 

Thermoset plastics are undamaged by the liquid adhesive, and these include Glass fibre polyester composite. Epoxies, Polyurethanes, and Bakelite materials based on phenolic resins. 

A number of plastic, adhesive, and coating materials are utilized in a densely cross-linked form known as thermosets. Some of the more important of these include the phenol-formaldehyde resins, urea-formaldehydes, polyimides, epoxies, amino resins, and the alkyds, among others (8). Because these... 

Epoxies generally have excellent adhesion to metals, ceramics and glass, although on most amorphous thermoplastics epoxies will usually he outperformed by MMA, UV adhesives or cyanoacrylates. Epoxies will bond well to thermoset plastics and are widely used for bonding sheet moulding compound door and body panels in the transportation industries. Epoxies do not adhere well to elastomers, fluoropolymers or polyolefin plastics. 

There are now so many thermoset plastics available that it can be difficult to differentiate between the various types. Thermoset resins are used for the manufacture of a wide range of parts from canoes to wind turbine blades and from polymer concrete to epoxy adhesives and so there is only a brief discussion of thermoset plastics included in this guide as it is essentially aimed at the bonding of small parts. 

The Araldite series of epoxy resins, manufactured by Huntsman, are perhaps the most well-known, typically two-part systems and these epoxy adhesives wonld generally be expected to bond well to epoxy thermoset plastics. Cyanoacrylates showed good results in these trials [3], but would not be expected to offer the same durability as epoxy-based adhesives. Note that there are so many different grades of epoxy materials that it is impossible to give specific data (Table 3.2) is intended as a guideline and tests would always be necessary to confirm adhesion performance. 

Structural adhesives for thermoset plastics are more often than not epoxies, two-part acrylics and sometimes cyanoacrylates [4], UV-cure adhesives are not widely used as many thermoset plastics are not clear. 

Epoxies will bond to most thermoset plastics but as mentioned in Section 1.4 there are many different epoxy adhesive grades available and some epoxies are far more suited for bonding metals than plastics. 

Two-part acrylics (or MMA) bond well to many thermoset plastics and these products are often used for the assembly of larger thermoset plastic components. They generally cure faster than epoxies and the mix ratio of two-part MMA acrylics is not as critical as it can be for epoxy adhesives. However, many MMA have a high odour and have a tendency to string and many grades are flammable. 

Epoxies will show quite good adhesion (and hence durability) to thermoset plastics but many thermoplastics may require careful surface preparation if this type of adhesive is to be used. 

---