The SPE Library contains thousands of papers, presentations, journal briefs and recorded webinars from the best minds in the Plastics Industry. Spanning almost two decades, this collection of published research and development work in polymer science and plastics technology is a wealth of knowledge and information for anyone involved in plastics.
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Hygro-Thermal Effects on the Physical Aging Response of Glassy Polymers
Physical aging experiments are being performed in epoxy glasses subjected to relative humidity jumps. The premise of the study is that a change in moisture (plasticizer) content in the material is equivalent to a change in temperature because in both cases the distance from the glass transition is changed by either a constant relative humidity jump in temperature or an isothermal jump in relative humidity. As did Kovacs in his classic work on Poly (vinyl acetate) we are creating a data set of volume recovery for the epoxy. We have begun the determination of the intrinsic isopiestics--the RH-jump equivalent to the Kovacs' intrinsic isotherms. Similarly, we are determining the asymmetry of approach in up- and down-jump conditions as well as the memory response that results after in a two-step experiment. Finally, work is underway to model the material response by extending the KAHR model to the case of changing relative humidity.
The Impact Behavior of Injection Molded Plates with and without Weld Lines
A box-like part was injection molded in a polypropylene copolymer with systematic variations of the processing conditions (melt and mould temperature, injection flow rate and holding pressure), based on a design of experiments approach (L9 Taguchi orthogonal array). Due to the gating options (hot runner with two gate points), the major box surface shows a central weld line. This work studies the influence of the processing conditions on the impact behavior of the molded plates at two locations: in and away from the weld line. Both local processing thermomechanical environments were computed from mould filling simulations (thermal and stress levels), and characterized by thermomechanical indices (aiming at interpreting locally the microstructure development). The falling weight impact tests were performed at 2 m/s with a lubricated striker. The impact behavior was characterized by the peak force and energy. The experimental results are analyzed by ANOVA statistical tool. The dependences of the impact response upon the thermomechanical indices at the two locations are compared.
Impact Enhancement of High Melt Flow HDPE
The modification of impact properties of High Density Polyethylene (HDPE) having high flow to meet impact, stiffness and processability of heterophasic polypropylene copolymers (CPPs) was studied using a variety of impact modifiers like : high impact HDPEs, Low Density Polyethylenes (LDPEs), Linear LDPEs (LLDPEs) and Ethylene Vinyl Acetate (EVA) copolymers.Taking into account both the resultant properties (impact and stiffness), the flow properties and cost, it was shown that EVAs provide the most cost-effective solution to impact modification of HDPE with stiffness levels compared to CPPs. Experimental results showed that addition of up to 5.6% Vinyl Acetate (e.g.20% EVA with 28% VA content) yielded the optimum properties of the HDPE blends.Addition of LLDPE to HDPE though resulted in the sought mechanical properties, exhibited a major reduction in flow properties at the content level needed (40%) for properties modifications.
Impact Modification of Polypropylene
This paper compares the modification mechanism provided by ethylene-octene (EO) copolymer to that of ethylene propylene diene terpolymer (EPDM) rubber. Within the limits of this study, the highest impact strength was achieved at 30-40% rubber content, regardless of the rubber type. An increase in rubber melt viscosity resulted in overall greater impact strength. At the optimum concentration, the high viscosity (MFI = 1 to 5) EO rubber provided modification mainly via crazing mechanism, while the EPDM rubber by energy dissipation through the three-dimensional network structure formed with the polypropylene matrix. This paper also discusses the effect of the processing conditions on physical properties of PP/EPR copolymer. An increase in processing temperature and screw speed resulted in a reduced number of discreet rubber particles, nearly no or very slight increase in impact strength, but a very significant reduction in tensile strength and tensile modulus.
The Importance of Monitoring Mold Pressure during Rotational Molding
During the rotational molding of plastic parts, the pressure inside the mold can become positive or negative depending on a variety of factors such as the size of the vent, the quality of the mold, the heating rate, etc. In commercial molding, the pressure is likely to vary in an arbitrary manner, depending on particular combinations of key variables. This leads to conflicting reports about the causes and cures of problems such as warpage, residual stress and shrinkage. This paper reviews the effects of pressure variations on the quality of rotomoulded parts and using experimental data, demonstrates the importance of monitoring the pressure inside the mold throughout the cycle. Methods of doing this are illustrated and the benefits in terms of reduced cycle times and improved part quality and consistency are demonstrated.
Important Parameters for Operating and Selecting Dynamic Mechanical Thermal Analyzers
Dynamic mechanical thermal analysis (DMTA) is used to measure the stiffness (modulus) and mechanical damping (tan delta) of polymers rapidly over a range of temperatures. It is one of the best methods for examining the glass transition and other viscoelastic relaxations in materials. A sinusoidal mechanical perturbation (force or amplitude) is applied and the resulting sinusoidal response (amplitude or force) is measured. The three basic parameters of the measurement are force and displacement amplitudes, and time delay between the force and displacement signals. Several instrument and sample parameters must be considered when designing DMTA experiments including deformation mode and amplitude, sample strain target and ranges of stiffness and force that the instrument can measure. With good control of the operating parameters DMTA can provide good measurements of modulus. A common sense approach is given to setting experimental parameters for linear drive DMTA instruments.
Improved Screw Design for Maximum Conductive Melting
New advances in screw designs and mixing sections have allowed processors to take advantage of new resins, higher production rates, and improved product quality. Until now, the basic single screw geometry has changed little over the past 15 years. With the advances in new material formulations, additives and fillers, the screw design must be able to fully melt and disperse the additives in the polymer matrix without destroying the properties from excess shear. This paper will present data on the melting performance of a new screw design with a unique flight geometry that maximizes the conductive melting mechanism (low shear) in the screw channel.
Improvements in Design and Failure Prediction of Short Fiber Reinforced Plastic Parts
An improved prediction of a part's lifetime does not only require consideration of temperature, time dependent deformation and stress behavior of the polymer or composite, but also a precise estimation of ultimate stress and strain values for the particular material and application.Considering short fiber reinforced polymers, the dependencies of the characteristic mechanical properties on the boundary conditions of the application have to be regarded as well as their local anisotropy. A consistent simulation technique is presented, in which injection molding simulation and structural finite element simulation are coupled by suitable interfaces and subroutines, which take process dependent properties into consideration and adjust them to the according application condition.
Improving Polyethylene Performance - The Use of Nanocomposites in Ziegler-Natta Polyethylene for Rotational Moulding
A medium density conventional polyethylene (PE) with 1% maleic anhydride (MAH) was compounded with two different types of silicate organoclays. The modified PE was compounded using different weight loadings of each organoclay to investigate the effect of loading quantity on the materials properties. The results for the polyethylene nanocomposites show significant improvements in mechanical performance, especially at higher temperatures. Increases of over 80% in storage modulus for the PE organoclay composite resins over the conventional PE resins were recorded. D.S.C. analysis shows the PE resins with organoclay to be more crystalline in nature than the 1%MA polyethylenes.
Improving Vibration Weld Strength by Equipment Modification
Vibration welding glass reinforced nylon compounds is a commonly used joining technique. The butt joint strength achievable with these compounds is significantly lower than that of the bulk compound and is most often comparable to the strength of the polymer matrix. These lower properties are attributed to little glass fiber orientation perpendicular to the weld interface. This research examined introducing a secondary vibratory motion perpendicular to the weld plane during welding in an attempt to promote greater glass fiber orientation normal to the weld plane. The work was performed using 3.2 mm thick plates composed of 33 % glass filled nylon 6 and 66. These plates were butt welded on a lab scale linear vibration welder and then cut into strips before tensile testing. These process modifications resulted in strengths 20% higher than those of samples welded using the standard vibration welding process.
Increased Shear through Weldlines Using Controllable Compliance Accumulator
Weldlines, a common feature in injection molded parts, are known to be inherently weak. Vibration molding has been shown to increase shear within the mold thus increasing the strength of weldlines, but with high capital costs. The present experimental study focused on improving weldline strength through the use of a novel idea, the controllable compliance accumulator (CCA). This device works together with the vibration-assisted injection molding technique to increase the local shear at the weldline. Comparisons between the tensile strength of ASTM specimens made with and without the CCA were made and the details of the results will be discussed.
The Incredible World of Polymers: Tales of Innovation, Luck and Preserverance
The history of synthetic polymers, albeit a relatively short history only spanning some 100 years, is replete with many outstanding examples of innovation, luck and perseverance. Although science is perceived by most to be a strictly logical endeavor, where serendipity plays no role and well thought out hypotheses are tested by super-human nerds in a precise and systematic manner, this is not reality. We will attempt to show that we can learn something about the complex nature of innovation by drawing upon selected tales of the historical development of some of the most important polymers in use today.
Indentation of Some Plastic Materials by Indenters of Different Shapes
Indentation tests were performed on flat coupons of poly(formaldehyde), three types of reinforced nylon and reinforced poly(ethyl terephthalate). Spherical, conical and cylindrical indenters were selected for this study and the effects of indenter shape combined with the rate of indentation on the behaviour of these materials were examined. The load-depth relationship during the indentation process revealed the emergence of step-like transitions at high indentation rates. The effects of the assessed parameters on the indentation resistance were analysed. The modelling of the data was based on the approach developed by Johnson and the introduction of a modified parameter that took not only the geometrical characteristics of the indenter into account but also the indentation depth. The proposed method has proven to be useful since a steady state of the indentation pressure was observed for all materials.
Influence of Sample Preparation and Instrument Settings on Hot Tack Measurements of Thin Sealant Films
Sample preparation can greatly influence the results of laboratory testing of packaging sealant materials. Factors such as test strip width, edge uniformity, sealing die temperature and die load are recognizable variables to control.A design of experiments approach was used to explore the rank importance of these factors and any interactive effects that may occur. A generally well-behaved ethylene vinyl acetate (EVA) sealant on a high density polyethylene (HDPE) film was used for these tests. Experimental results indicate that sample width, die temperature and die pressure are the most influential factors. Unexpectedly, edge effects, namely a regular sine wave pattern and a generally random irregular edge had little influence on hot tack results.
Influence of Backing on Weathering Induced Color Change of Two Rigid Vinyl Building Materials
The vinyl industry has expressed concern over color changes of vinyl siding that correlate with underlying insulation patterns. Occasionally, homeowners observe visual differences in vinyl appearance when a single extrusion lies over different types of backing used in home construction. This paper reviews weathering-induced color change data from commercially available rigid vinyl exposures in Arizona and Florida. Results from weathering experiments show influences of backing on color change for dark brown and white vinyl during several years' exposure. This paper describes weathering behavior observed in vinyl color change as a function of backing and important considerations regarding effects of backing on vinyl building product weathering appearance.
The Influence of Operating Conditions on the Cooling Phase of the Extrusion Blow Molding Process
The cooling phase of the extrusion blow molding process has a large influence on the cycle time of the process as well as on the properties and quality of the molded products. A better understanding of the heat transfer mechanisms occurring during the cooling phase will help in the optimization of both mold cooling channels and operating conditions. A continuous extrusion blow molding machine and a rectangular bottle (motor oil type) mold were used to produce bottles. A high density polyethylene (HDPE) and a metallocene polyethylene (mPE) having different rheological properties were tested. Melt and mold temperatures, cooling time, inflating pressure and die gap were varied systematically. An infrared (IR) camera was used to measure the temperature distribution of the plastic part just after mold opening as well as after part ejection. The wall thickness and dimensions of the bottles of the finished parts were measured in order to determine the shrinkage and warpage. Finally, the temperature fingerprints were used to explain what happens during the cooling phase and correlated with the final part characteristics.
Influence of Semi-Crystalline Morphology on the Physical Aging Characteristics of Poly (Phenylene Sulfide)
In this study, we report on the influence of semi-crystalline morphology on the physical aging characteristics of poly (phenylene sulfide). The physical aging rates were observed to depend on the relative amounts of the mobile-amorphous and rigid-amorphous phases, with accelerated aging rates measured in specimens with higher rigid-amorphous phase fraction. We suggest that the rigid-amorphous phase, which includes chain segments that are more tightly packed relative to the mobile-amorphous phase, is able to accelerate physical aging due to its relative proximity to a state of lower configurational entropy.
The Influence of Small Amounts of LDPE on the Morphology and Resulting Haze of LLDPE Blown Films
Adding small amounts (0 to 5 wt.%) of a broad molecular weight distribution, free-radical polymerized low density polyethylene (LDPE) to a Ziegler-Natta catalyzed linear low density polyethylene (LLDPE) led to a reduction in haze of the resulting blown films. While the surface haze decreased with increasing LDPE content, the haze resulting from the internal or bulk structure remained fairly constant. The surface haze was caused by the spurious scattering of light from a rough film surface, which decreased linearly with decreasing r.m.s. roughness within the range of 0 to 5 wt.% LDPE. The presence of LDPE in the blend triggered a different morphological structure on the surface of the final films. The surface of the neat LLDPE blown film contained a bumpy morphological superstructure composed of small lamellar aggregates as observed by atomic force microscopy. Increasing the LDPE content in the blend (up to 5 wt.%) led to a transformation from this aggregate superstructure to the row nucleated structure when blown under the same processing conditions. The row nucleated structure was inherently smoother than the lamellar aggregate superstructure, yielding films with lower haze.
The Influence of Solid-State Morphology on the Impact Strength of Linear Low-Density Polyethylene Blown Films
In this work, we report on the influence of lamellar morphology and orientation on the dart impact strength of linear low-density polyethylene (LLDPE) blown films. Characterization of the solid-state morphology of a wide variety of LLDPE blown films reveals the importance of lamellar anisotropy on the impact strength performance of such films. Specifically, we find that lower degrees of lamellar orientation (more random lamellar orientation) favor higher impact strengths. Further, we also report on the solid-state deformations that take place as the films are stretched during the impact test and how these deformation modes discern blown films of high and low impact strength.
Influence of the Reactive Extrusion Conditions on the Free Radical Induced Carboxylation of Poly(L-Lactide)
The current research is focused on the carboxylation of poly(L-lactide) through a free radical grafting process utilizing an asymmetric functional peroxide within a co-rotating twin screw extruder. Similar experiments, as well as free radical maleation reactions, have been successfully carried out on polypropylene and PLLA. In order to maximize the grafting efficiency and minimize chain degradation, the effects of the extrusion conditions (screw design, screw speed and melt temperature) on the carboxylation reaction and molecular weight of the polymer are investigated. This report focuses on molecular weight and thermal analysis as well as titration to quantify the level of acid functionality grafted onto the PLLA chains.
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