Rubber rate impact resistance

The Role of Rubber Modification in Improving High Rate Impact Resistance... 

Analysis of these effects is difficult and time consuming. Much recent work has utilized two-dimensional, finite-difference computer codes which require as input extensive material properties, e.g., yield and failure criteria, and constitutive laws. These codes solve the equations of motion for boundary conditions corresponding to given impact geometry and velocities. They have been widely and successfully used to predict the response of metals to high rate impact (2), but extension of this technique to polymeric materials has not been totally successful, partly because of the necessity to incorporate rate effects into the material properties. In this work we examined the strain rate and temperature sensitivity of the yield and fracture behavior of a series of rubber-modified acrylic materials. These materials have commercial and military importance for impact protection since as much as a twofold improvement in high rate impact resistance can be achieved with the proper rubber content. The objective of the study was to develop rate-sensitive yield and failure criteria in a form which could be incorporated into the computer codes. Other material properties (such as the influence of a hydrostatic pressure component on yield and failure and the relaxation spectra necessary to define viscoelastic wave propagation) are necssary before the material description is complete, but these areas will be left for later papers. 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the rubber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene —acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

As one might intuitively expect, the incorporation of rubber particles within the matrix of brittle plastics enormously improves their impact resistance. Indeed, the impact resistance imparted by the rubber is the principal reason for its incorporation (Rosen, 1967) in rubber-plastic blends and grafts. Toughening in such polymers is also observed under other loading conditions, such as simple low-rate stress-strain deformation and fatigue. It is believed that several deformation mechanisms are important in all such cases, though their relative importance may depend on the polymer and on the nature of the loading. 

While the bulk of any cyanoacrylate formulation consists of monomer, a large number of modifiers have been used to impart desired properties to the composition. These include stabilizers, inhibitors, thickeners, plasticizers, dyes or colorants, adhesion promoters, and others. Each of these classes of modifier will be dealt with in subsequent parts of this chapter. Because of the variety of modifiers, and the variety of applications for cyanoacrylates, a bewildering number of cyanoacrylate adhesives are now commercially available. These can be generally divided into the following classifications adhesives of different viscosities and cure rates, adhesives based on different monomers, adhesives for the bonding of metal, plastic, rubber, or wood, various types of improved performance adhesives, i.e., heat, moisture, or impact resistant, and adhesives for bonding low surface... 

Polybutyleneterephthalate (PBT) is of growing interest as a material for injection molding. In fact, its rate of crystallization is faster than that of the other widely used linear polyesters such as polyethyleneterephtha-late (PET). However, PBT shows a low impact resistance, particularly at low temperatures. For this reason the uses of PBT are limited. The usual method to overcome this limitation is to add a second elastomeric phase to the PBT matrix. Rubber modification of PBT has been realized by melt blending with preformed rubbers such as poly(ethylene-co-vinylacetate) (EVA) and poly(ethylene co-vinylalcohol) (EVOH) [71], or by adding end-capped polymers to produce a second flexible component during PBT polymerization. 

The photo-oxidation rates of polystyrene and polyacrylonitrile are considerably accelerated when they are grafted into polybutadiene. The rate of photooxidation of polystyrene becomes linear after 250 h whereas high impact polystyrene (polystyrene-butadiene) reaches the same stage after 25 h (Fig. 3.63). The rubber segment thus behaves as a photopro-oxidant for the polystyrene matrix and is itself destroyed in the process with the disappearance of the impact resistance of the polymer [761, 1922]. Thermal processing of high impact polystyrene increases the initial rate of photo-oxidation of the polymer... 

It is clear from this work that most of the published values of and Gjc for rubber-tou ened plastics refer to plane-stress fracture. Measurements have been made on a variety of pdymers using SEN, double cantilever beam (DCB) and Charpy impact specimens. Figure 13 shows the results of one such study by Kobayashi and Broutman l, who used DCB specimens to measure C/c in AMBS polymers over a range of cr k speeds. The two most prominent features of the results are the rapid rise in Qc with rubber content, and the fall in Gx( at high crack eeds. Both effects are predicted by Eq. (8) the yield stress decreases with increas-in% rubber phase volume, so that the size of the plastic zone at the crack tip increases similarly, increating crack speed (and therefore strain rate at the crack tip) increases yield stress and reduces the size of the plastic zone. Thus the yield stress is die link between rubber phase volume and fracture resistance. 

In sulphur cured rubbers, accelerators are generally used to reduce the dependency on sulphur in order to achieve more efficient vulcanisation, to improve heat and flex resistance due to the presence of more monosulphidic crosslinks, and to increase the cure rate of the rubber and improve production capacity. Two accelerators which have been shown to enhance bondability of rubbers are 2-mercaptobenzothiazole (MBT) and mercaptobenzothiazole disulphide (MBTS). An accelerator which is known to negatively impact on adhesion is tetramethyl thiuram disulphide (TMTD). 

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