Large Rubber Particles Research Articles

Formation and break-up of a bead-and-string structure during injection moulding of a polycarbonate/acrylonitrile-butadiene-styrene blend

The morphology gradient through the thickness of an injection-moulded blend of 10% by weight acrylonitrile-butadiene-styrene (ABS) and 90% by weight polycarbonate (PC) has been characterized. Brittle fracture surfaces were etched and examined both parallel and perpendicular to the injection direction in the scanning electron microscope. In the centre of the plaque, the morphology was isotropic with the ABS phase dispersed in the PC matrix as large composite rubber particles about 1 μm in diameter and smaller particles of free styrene-acrylonitrile (SAN) about 0.3 μm in diameter. About half the distance from the centre to the edge, the morphology of the free SAN phase changed from predominantly spherical to predominantly string-like. The SAN strings connected the rubber particles to form an oriented ABS bead-and-string structure that was essentially continuous in the injection direction. It was thought that the free SAN was highly extended under elongational or shear flow while remaining attached to the rubber particles because of miscibility with grafted SAN. The bead-and-string structure was retained near the edge where the melt solidified most rapidly. In the centre of the plaque, the low shear rate during mould filling and longer cooling time after mould filling favoured relaxation of the bead-and-string structure. The morphology gradient through the thickness was created by the competition between the relaxation rate and the cooling rate after mould filling. In this case, relaxation was thought to occur by interfacial-tension-driven break-up and end-pinching mechanisms to produce the dispersion of rubber and free SAN particles. Evidence to support the break-up mechanism was obtained when annealing above the glass transition temperature of PC for a matter of seconds, the timescale of mould cooling, caused the bead-and-string structure to convert to a dispersion of composite rubber particles and SAN particles.

Microstructure, mechanical properties, and fracture behavior of liquid rubber toughened thermosets

AbstractPhenolic epoxy resin was toughened by carboxyl‐randomized butadiene acrylonitrile copolymer (CRBN) for use as composite matrix. By adding different parts of butadiene acrylonitrile copolymer (BN‐26, without carboxyl contained) to CRBN, different sizes of rubber domains and different numbers of chemical bondings between the resin matrix and the rubber phase were obtained. It is found that small rubber particles (less than 0.1 μm) are cavitated during the crack development. The interaction between secondary crack zones caused by the cavitation makes the fracture toughness KIC of the materials high; by comparison, a local stress‐whitened zone is produced in the material with large rubber particles (more than 0.1 μm) when it is subjected to tensile stress. In this case, the flexure strength σf of the material is great. Using ultrasection and TEM techniques, the stress‐whitened zone was shown to be caused by the special multiple‐phase structure of the material, in which many caves and “macrocrazes” coexist.

Plastic zone in front of a mode I crack in acrylonitrile-butadiene-styrene polymers

The shape of the mode I plastic zone observed in the acrylonitrile-butadiene-styrene (ABS) polymeric material is different from that described by the Von Mises or Tresca criterion for yielding in metals. Two distinct, overlapping plastic zones are observed. One has cardioid shape whose through-thickness zone shape is completely different from the dog-bone shape in metals. The other consists of fingers radiating from the crack tip. The cardioid zone is associated with cavitation in the larger rubber particles. The fingers are branching crazes associated with the cavitation inside the small rubber particles. The stress criteria for the formation of these plastic zones are suggested. Also, the way the small rubber particles interact with the crazes to increase the fracture toughness is discussed.

Morphology and property control via phase separation or phase dissolution during cure in multiphase systems

AbstractThe morphology and properties of polymer alloys and blends can be controlled via phase separation or phase dissolution during cure. The morphology and properties control by phase separation was achieved by simultaneously manipulating miscibility, kinetics of phase separation, and cure reaction of polymer systems. Using phase diagrams consisting of binodal and spinodal curves, the morphology of epoxy/CTBN (carboxyl‐terminated butadiene acrylonitrile copolymer) systems can be controlled by the mechanism of nucleation and growth or by spinodal decomposition. We have found that the particle size of the rubber reinforcement in epoxies is affected by the mechanisms of phase separation. Phase separation by nucleation and growth gives larger rubber particles than the corresponding phase separation by spinodal decomposition. This contrast in the morphology development is the consequence of controlling phase separation through chemorheological behavior. Modification of the phase separation kinetics in epoxy/CTBN systems was extremely effective at altering both morphology and properties of these alloys. This technique offers a mean to shift the glass transition temperature of the rubber‐rich phase while leaving the glass transition temperature of the epoxy‐rich phase intact. Such control over morphology is the key to ultimately controlling material properties. The morphology and properties can also be controlled by phase dissolution during cure via invoking anion–cation interactions or intrachain repulsion forces. Phase dissolution in elastomer blending is the key for achieving optimal morphology having a synergistic effect in the polymer blends. The morphology and properties of NR (natural rubber)/EPDM blends were controlled through compatibilization and co‐vulcanization to promote phase dissolution during cure. A synergistic result in the blends was achieved by having an interconnected phase morphology. The domain sizes of the blends are 3–5 μm before cure and 1–2 μm after cure. These blends showed superior mechanical properties to those of the individual NR or EPDM, after a long term of high temperature aging.

Morphology and properties control on rubber‐epoxy alloy systems

AbstractMorphology and properties of polymer alloys can be controlled by thermody‐namlcally reversible (structure freeze‐in) or irreversible (structure lock‐in) processes by simultaneously manipulating miscibility, mechanisms of phase separation, glass transition temperature (structural relaxation), and cure kinetics of polymer systems. Using phase diagrams consisting of binodal and spinodal curves, the morphology of epoxy/CTBN (carboxyl‐terminated butadiene acryloni‐trile copolymer) systems can be controlled by the mechanism of nucleation and growth or by spinodal decomposition via simultaneously manipulating the kinetic processes of phase separation and curing reactions. We have found that the particle size of the rubber reinforcement in epoxies is affected by the mechanisms of phase separation. Phase separation by nucleation and growth gives larger rubber particles than the corresponding phase separation by spinodal decomposition. This contrast in the morphology development is the consequence of controlling phase separation through chemorheological behavior. Medication of the phase separation kinetics in epoxy/CTBN systems was extremely effective at altering both morphology and properties of these alloys. This technique offers a means to shift the glass transition temperature of the rubber‐rich phase drastically without reducing the glass transition temperature of the epoxy‐rich phase significantly. Such control over morphology is the key to ultimately controlling material properties. This morphology manipulation allows us to tailor the mechanical properties of alloy systems.

Effect of Particle Size on the Plastic Zone Formation in Front of Mode I Crack in ABS Polymers

AbstractTwo different sizes (0.4 and 0.1 μm) of rubber particles (polybutadiene) are usually used to toughen the glassy SAN (styrene acrylonitrile) matrix in ABS copolymers. Experiments on a batch of specimens without the large particles confirmedour previous contention that cavitation of large particles within a zone of cardioid shape is an important part of crack tip plasticity which protects the crack tip from the applied stress field. Large rubber particles require a less strong applied stress field than small rubber particles to initiate crazes so that the large rubber particles help avoid brittlefracture which occurs without crazes.

Rubber elongation factor from Hevea brasiliensis: Identification, characterization, and role in rubber biosynthesis.

The presence of a protein, rubber elongation factor (REF), which is tightly bound to serum-free rubber particles purified from Hevea brasiliensis latex, is necessary for prenyltransferases from a number of sources to add multiple cis-isoprene units to rubber molecules. These prenyltransferases show normal farnesyl pyrophosphate synthase activity (two trans additions of isopentenyl pyrophosphate to dimethylallyl pyrophosphate) in the absence of REF bound to rubber particles. REF bound to rubber molecules can be highly purified from all other proteins in whole latex by treatment of rubber particles with low concentrations of detergent. Treatment of rubber particles with trypsin which hydrolyzes bound REF, removal of REF with high concentrations of various detergents, or treatment of whole latex with polyclonal antibodies specific for REF all prevent prenyltransferase from adding [14C]isopentenyl pyrophosphate to rubber molecules. However, we have not been successful using detergent-solubilized REF in the reconstitution of in vitro rubber biosynthesis with either REF-depleted rubber particles or allylic pyrophosphate primers. REF has a molecular mass of 14,600 Da and is associated specifically with rubber particles in whole latex. It makes up between 10-60% of the total protein in whole latex but is absent in C-serum, the supernatant fluid obtained when rubber particles are removed by centrifugation. The amount of REF in whole latex is proportional to the rubber content. Based on a number average molecular mass of 500,000 Da for rubber and the content of rubber and REF in whole latex or serum-free rubber particles, the stoichiometry of REF molecules to rubber molecules is 1:1 in both cases. There is sufficient REF to form a monomolecular protein layer coating large rubber particles (700-1,000 nm). In the electron microscope, serum-free rubber particle preparations contain particles with diameters from 800 to as small as 10 nm. In the presence of 1% sodium dodecyl sulfate no particles smaller than 100 nm are observed. We suggest that the smaller particles may be mainly composed of REF molecules.

Plastic deformation mechanisms in poly(acrylonitrile-butadiene styrene) [ABS

Thin films of two poly (acrylonitrile-butadiene-styrene) [ABS] resins have been strained in tension, and the ensuing deformation has been characterized by transmission electron microscopy. To enhance contrast of the rubber particles, some of the specimens were stained with OsO4. Films containing only solid rubber particles 0.1 μm in diameter show little tendency for crazing. Instead, cavitation of the rubber particles occurs, together with localized shear deformation between the particles along a direction nearly normal to the tensile axis. For specimens containing a mixture of the same small particles plus larger (1.5μm diameter) particles containing glassy occlusions, some crazing does occur. Crazes tend to nucleate at the larger particles only. When crazes encounter the smaller particles these cavitate without appearing to impede or otherwise affect the craze growth. The occluded particles also show significant cavitation, with voids forming at their centres at sufficiently high levels of strain. These voids do not seem to lead to rapid craze break-down and crack propagation. In commercial ABS, which typically has both large and small rubber particles, both crazing, nucleated by the large particles, and shear deformation, encouraged by the cavitation of small rubber particles, can be expected to make important contributions to the toughness of the polymer.

The Polymerization of Vinyl Monomers in Natural-Rubber Latex

Abstract Practical methods are described for polymerizing methacrylic esters, styrene, and other vinyl monomers in natural-rubber latex. The larger rubber particles require an appreciable time to attain equilibrium with the monomer diffusing into them from a liquid monomer phase dispersed in the serum. Provided that substantial proportions of added surface-active substances are avoided, rubber-soluble monomers can be polymerized almost entirely within the rubber particles, and the modified latex then contains no separately emulsified free polymer. Such conditions favor combination of polymer with rubber. The addition of a sufficiently large amount of dispersing agent favors polymerization of emulsified monomer, with less involvement of the rubber. In this way there can be obtained mixtures of rubber and polymer from monomers whose polymerization is otherwise inhibited by the presence of polyisoprene hydrocarbons.