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It is a well-known fact that weldlines are unavoidable in most injection-molded products of even moderate complexity. While there are many situations where they are barely perceptible, weldlines represent a potential source of weakness in molded parts. In injection molding weldlines are generated when two separate melt streams join either in multigated molds or as a consequence of flow around obstacles. The development of many interesting materials has been hampered by poor weldline strength. Among such materials are plastics reinforced with fibers or platelets, liquid crystal polymers, and a number of multiphase polymer blends. Weldlines have ever been called the "Achilles' heel" of these multiphase materials. This article is a review of the literature published on weldlines in injected parts. It deals primarily with the aspects related to the mechanical behavior of weldline-containing parts. It begins with a brief description of the phenomena important for the part formation in the mold, including those leading to weldlines, in addition to the techniques used to characterize weldline-containing parts. The following three sections consider the structure and properties of weldlines in neat amorphous and semicrystalline polymers, filled and reinforced plastics, and finally in polymer blends and alloys. In the last section methods developed for increasing the weldline strength are discussed.

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Epoxy resins are increasingly finding applications in the field of structural engineering. A wide variety of epoxy resins are available, and some of them are characterized by relatively low toughness. Several approaches to improve epoxy resin toughness include the addition of fillers, rubber particles, thermoplastics, or their hybrids, as well as interpenetrating networks and flexibilizers, such as polyols. It seems that this last approach did not receive much attention. So in an attempt to fill this gap, the present work deals with the use of hydroxyl-terminated polyester resins as toughening agents for epoxy resin. For this purpose, the modifier, that is, a hydroxylterminated polyester resin (commercially referred to as Desmophen), which is a polyol, has been used at different concentrations. The prepared modified structure has been characterized using Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) prior to mechanical testing in terms of impact strength and toughness. Two types of Desmophen (800 and 1200) have been used as modifiers. The obtained results showed that hydroxyl-terminated polyester improves the epoxy toughness. In fact, the impact strength increases with Desmophen content and reaches a maximum value of 7.65 J/m at 10 phr for Desmophen 800 and 9.36 J/m at 7.5 phr for Desmophen 1200, respectively. At a critical concentration (7.5 phr), Desmophen 1200 (with higher molecular weight, longer chains, and lower branching) leads to better results. Concerning K c , the effect of Desmophen 800 is almost negligible; whereas a drastic effect is observed with Desmophen 1200 as K c reaches a maximum of 2.41 MPa m 1/2 , compared to 0.9 MPa m 1/2 of the unmodified epoxy prior to decreasing. This is attributed to the intensive hydrogen bonding between epoxy and Desmophen 1200, as revealed by FTIR spectroscopy. Finally, the SEM analysis results suggested that the possible toughening mechanism for the epoxy resin being considered, which might prevail, is through localized plastic shear yielding induced by the presence of the Desmophen particles.

Polypyrrole (PPy) thin films were prepared electrochemically at a constant potential. Gas-sensing behaviors, including reproducibility, sensitivity, and response time to various benzene, toluene, ethylbenzene, and xylene (BTEX) compound concentrations, were investigated. BTEX compounds were found to be able to compensate for the doping level of PPy and, hence, decrease the conductivity of PPy on exposure to them. A reasonable reproducibility of the resistance change (⌬R) was obtained. The sensitivity for each compound was 2.3 m⍀/ppm (benzene), 0.4 m⍀/ppm (toluene), 8.3 m⍀/ppm (ethylbenzene), and 2.9 m⍀/ppm (xylene). An adsorption model correlated well with the experimental results and was used to interpret the sensing behaviors. The parameters of this model, including the adsorption equilibrium constant and the ⌬R caused by a pseudomonolayer of the detecting layer {[m(r 1 Ϫ r 0 )]/n, where m is the number of active sites on the pseudomonolayer; r 1 and r 0 are the site resistances when the site is vacant and occupied, respectively; and n is the thickness of the film}, were determined. According to the parameters, toluene vapor had the most prominent effect in undoping PPy film but the poorest affinity to the active sites of the film. On the other hand, ethylbenzene showed the highest affinity to PPy film compared to the other BTEX compounds and consequently led to the highest sensitivity for such a sensor.

Epoxy resins are increasingly finding applications in the field of structural engineering. A wide variety of epoxy resins are available, and some of them are characterized by a relatively low toughness. One approach to improve epoxy resin toughness includes the addition of either a rigid phase or a rubbery phase. A more recent approach to toughen brittle polymers is through interpenetrating network (IPN) grafting. It has been found that the mechanical properties of polymer materials with an IPN structure are fairly superior to those of ordinary polymers. Therefore, the present work deals with epoxy resin toughening using a polyurethane (PU) prepolymer as modifier via IPN grafting. For this purpose, a PU prepolymer based on hydroxyl-terminated polyester has been synthesized and used as a modifier at different concentrations. First, the PU-based hydroxyl-terminated polyester has been characterized. Next, an IPN (Epoxy-PU) has been prepared and characterized using Fourier transform infrared (FTIR) spectroscopy, thin-layer chromatography (TLC), and scanning electron microscopy (SEM) prior to mechanical testing in terms of impact strength and toughness. In this study, a Desmophen 1200-based PU prepolymer was used as a modifier at different concentrations within the epoxy resin. The results also showed that, further to the IPN formation, the epoxy and the PU prepolymer reacted chemically (via grafting). Compared to virgin resin, the effect on the mechanical properties was minor. The impact strength varies from 3-9 J/m and K c from 0.9 -1.2 MPa m 1/2 . Furthermore, the incorporation of a chain extender with the PU prepolymer as a modifier into the mixture caused a drastic improvement in toughness. The impact strength increases continuously and reaches a maximum value (seven-fold that of virgin resin) at a modifier critical concentration (40 phr). K c reaches 2.5 MPa m 1/2 compared to 0.9 MPa m 1/2 of the virgin resin. Finally, the SEM analysis results suggested that internal cavitation of the modifier particles followed by localized plastics shear yielding is probably the prevailing toughening mechanism for the epoxy resin considered in the present study.

Polymer blends based on polyolefins are of a great interest owing to their broad spectrum of properties and practical applications. However, because of poor compatibilities of components, most of these systems generally exhibit high interfacial tension, a low degree of dispersion and poor mechanical properties. It is generally accepted that polypropylene (PP) and nylon 6 (N6) are not compatible and that their blending results in poor materials. The compatibility can be improved by the addition of a compatibilizer, and in this study PP was functionalized by maleic anhydride (MAH) in the presence of an optimized amount of dicumyl peroxide (DCP). The reaction was carried out in the molten state using an internal mixer. Then, once the compatibilizer polypropylene‐graft‐maleic anhydride (PP‐g‐MAH) was prepared, it was added at various concentrations (2.5–10 wt%) to 30/70 glass fibre reinforced N6 (GFRN6) PP, and the mechanical properties were evaluated. It was found that the incorporation of the compatibilizer enhanced the tensile properties (tensile strength and modulus) as well as the Izod impact properties of the notched samples. This was attributed to better interfacial adhesion as evidenced by scanning electron microscopy (SEM). The optimum in these properties was achieved at a critical PP‐g‐MAH concentration. Copyright © 2005 Society of Chemical Industry

A polymeric coupling agent acrylic acid grafted polypropylene (AAgPP) was synthesized and its efficiency in CaCO3/PP composite was investigated. The grafting of acrylic acid monomer (AA) onto polypropylene was performed using an internal mixer. The effect of peroxyde, acrylic acid monomer content, temperature and RPM was studied. A grafting reaction between the polypropylene and the acrylic acid was evidenced through FTIR, UV, DSC and MFI testing. The highest grafting yield was obtained at 0.85 phr peroxide and 5 phr acrylic acid. The selected mixing temperature was 200°C, the rotor speed 150 rpm and the residence time 5 min. The obtained coupling agent (AAgPP) was used with 30 wt% CaCO3 filled polypropylene. Strong interactions with the composite were observed. The effect of increasing the coupling agent content on Izod impact and tensile properties was investigated. A maximum in the above properties is attained at 15 wt% AAgPP. The most important effect is clearly shown in the Izod test. In fact, a threefold increase has been observed for either notched and untoched specimen. The 15 wt% AAgPP is considered to be a critical concentration for the composite considered. This corresonds to maximum interactions occurring between the matrix and the filler. SEM analysis clearly shows strong interactions between the filler and the matrix in the presence of acrylic acid grafted polypropylene. This is another proof of the efficiency of the synthesized AAgPP as a potential coupling agent for CaCO3 filled PP.