Polystyrene adhesive stress

Since ejection involves overcoming the forces of adhesion between the mold and the plastic, the area provided for the knockout (KO) is an important factor. If the area is too small, the KO force will be concentrated, resulting in severe stresses on the part. As a result, the part may fail immediately or in later service. In materials such as ABS and high-impact polystyrene, the severe stresses can also discolor the plastic. 

The presence of stresses in the adhesive layer alters the polymer s physical and chemical properties. The glass-transition temperature depends on the internal stresses that appear in the polymer. For example, the glass-transition temperature is known to decrease with increase in internal stresses in films based on polystyrene [169] and other pol5rmers. Note that restriction of the macromolecular mobility... 

Scanning electron microscope studies were performed on polystyrene spheres sitting on polished silicon surfaces by Rimai, Demejo and Bowen. The bulk polymer had a Young s modulus of 2.55 GPa and a yield stress of 10.8 MPa when measured on a testing machine. With such a low yield point it was estimated that the particles should be plastically deformed under the adhesion forces. Therefore they applied the plastic deformation theory of Maugis and Pollock to fit the results, as shown in Fig. 9.28. This gave the expression for contact diameter d in terms of sphere diameter D... 

Because it is more corrosive than fresh water, salt water not only attacks metals but also degrades even the so-called inert polymers polyethylene, polypropylene, polystyrene, polymethyl methacrylate, etc. (26). Indeed, salt water is very corrosive to adhesive joints. For example, it was reported (27) that exposure to a 5% salt spray for three months had a more severe effect on aluminum joints (stressed DCB) than exposure to a semi-tropical environment for three years. In Table 1, McMillan (27) compares the severity of salt spray with a natural, semi-tropical environment and various in-service conditions. 

Solvents, or solvents containing small amounts of hodying resin, are used for bonding thermoplastic resins and film adhesives. An example is toluol, which can he used to soften and dissolve polystyrene molded articles to allow joining the softened pieces. Ketones can be used to bond PVC films in a similar manner. A small amount of resin can be used to thicken the solvent so that a sufficient amount would stay in place to dissolve the substrate. It should be noted, however, that solvent welding of molded plastics can cause stress cracking and weakening of the structure as the parts age. ... 

The modulus of OPF-polystyrene composites increased with fiber loading up to 30%, however the maximum strain and flexural strength decreased [7]. The drop in strain was suspected due to irregular shape of the fibers, which caused inabflity of transferring stress from the matrix. The flexural properties viz., maximum stress, maximum strain, modulus of elasticity and Yoxmg s modulus of OPF-polystyrene composites at 10% fiber content (300-500pm size fibers) were reported as 46.43 MPa, 0.027 mm, 1665.35 MPa and 2685.84 MPa, respectively. The flexural properties were not affected by fiber size when the size was below 300 pm. The flexural properties of polystyrene composites improved upon benzoylation due to better interfacial adhesion and hydro-phobicity of the fibers. 

The modifier forms a particulate dispersion in the polymer phase, and under stress, crazing develops around modifier particles. Moreover, the addition of a compatibilizer improves particle dispersion and interfacial adhesion, resulting in engaged toughness. Rubber modified polystyreue, high-impact polystyrene (HIPS), or polyphenylene oxide can be used as a modifier. The compatibilizer can be a triblock copolymer of styrene-ethylene-propylene. The copolymer can even be random. 

The molecular weight of the reactive polymers, thus of the constitutive blocks of the compatibilizer, is also critical for efficient entanglements with the phases to be compa-tibilized. Indeed, good interfacial adhesion is essential for stress transfer from one phase to the other one to be efficient and for cracks initiated at the interface to be prevented from growth until catastrophic failure occurs. Kramer et al. studied the fracture mechanism of the polystyrene/poly(2-vinylpyridine) interface modified by the parent di-block copolymer. They found that the minimum degree of polymerization of PVP for entanglement (Npvp) was 255, below which the PVP block was pulled out in slow crack opening experiment [93, 94]. 

Crystal polystyrene and HIPS can usually be bonded with most engineering adhesives although polystyrene can be susceptible to stress cracking by some activators, primers and accelerators. 

---