Polymer weld strength predictions using a thermal and polymer chain diffusion analysis

A new simplified model for strength prediction of welded polymer-polymer interfaces is proposed. Two schools of thought exist for strength prediction of polymer-polymer interfaces: microscopic analysis of polymer chain behavior, and macroscopic analysis of bulk polymer properties during welding. The microscopic analysis is based on De Gennes' reptation theory for macromolecules (1), and has been described extensively by both analytical and empirical techniques. Reptation models are based on constant temperature interfaces, which are not found in any actual welding process. Conventional macroscopic analyses of welding empirically relate strength to important thermal process parameters, such as power density and heating time, but do not address the behavior of the polymer chains. Little interaction exists between these two schools of thought. This study seeks to combine these two areas, using reptation theory to explain the polymer chain interactions on a macromolecular level, and relate the interface thermal signature to strength prediction. The result of the new model is a method for strength prediction that takes into account fundamental materials properties as well as the engineering conditions imposed in a realistic welding process to predict weld strength. The model is adapted to run for similar conditions as two separate empirical endeavors. The results show that the model is effective in predicting overall trends, which emphasizes the importance of examining heat transfer effects in any polymer welding process.     

Mechanical properties of fusion welded ceramics in the SiC-ZrB2 and SiC-ZrB2-ZrC systems

Mechanical properties of welded SiC-ZrB₂ and SiC-ZrB₂-ZrC ceramics were measured up to 1700 °C. Commercial powders were hot pressed, machined into coupons, and preheated to 1600 °C before joining the ceramics using either tungsten inert gas welding or plasma arc welding. Toughness of the parent materials was 3–4 MPa*m^1/2 which decreased after welding to 2–2.5 MPa*m^1/2. Strength of the SiC-ZrB₂-ZrC parent material was ~700 MPa at 25 °C, ~300 MPa at 1700 °C, and retained 40–60% of this strength once welded. Strength of the SiC-ZrB₂ parent material was ~600 MPa at 25 °C and 1700 °C and retained 20–30% of this strength once welded. Griffith analysis indicated that the strength in the parent materials was controlled by the size of SiC clusters while strength of welds was controlled by the size of pores in fusion zones. Therefore, removal of pores in produced fusion zones should be investigated to improve strength of future ceramic welds.     

Thermoforming process of amorphous polymeric sheets: Modeling and finite element simulations

A temperature and strain rate dependent model for the thermoforming process of amorphous polymer materials is proposed. The polymeric sheet is heated at a temperature above the glass transition temperature then deformed to take the mold shape by the means of an applied pressure. The applied process temperature is taken uniform throughout the sheet and its variation is due only to the adiabatic heating. The behavior of the polymeric material is described by a micromechanically‐based elastic‐viscoplastic model. The simulations are conducted for the poly(methyl methacrylate) using the finite element method. The polymer sheet thickness and the orientation of the polymer molecular chains show an important dependence on the process temperature, the applied pressure profile, and the contact forces with the mold surface. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Relaxation behavior of conductive carbon black reinforced EPDM microcellular vulcanizates

Dynamic mechanical analysis and dielectric relaxation spectra of conductive carbon black reinforced microcellular EPDM vulcanizates were used to study the relaxation behavior as a function of temperature (−90 to +100°C) and frequency (0.01–10⁵ Hz). The effect of filler and blowing agent loadings on dynamic mechanical and dielectric relaxation characteristics has been investigated. The effect of filler and blowing agent loadings on glass transition temperature was marginal for all the composites (T_g value was in the range of −39 to −35°C), which has been explained on the basis of relaxation dynamics of polymer chains in the vicinity of fillers. Strain-dependent dynamical parameters were evaluated at dynamic strain amplitudes of 0.07–5%. The nonlinearity in storage modulus has been explained based on the concept of filler–polymer interaction and interaggregate attraction (filler networking) of carbon black. The variation in real and complex part of impedance with frequency has been studied as a function of filler and blowing agent loading. Additionally, the effect of crosslinking on the dielectric relaxation has also been reported. POLYM. ENG. SCI., 47:984–995, 2007. © 2007 Society of Plastics Engineers     

Characterization of a superabsorbent polymer

We studied an amorphous polymer superabsorbent, able to absorb until 1000 times its weight of water. It is consisted of macromolecular chains, dependent between them by chemical bonds. The swelling of the product in the presence of water gives rise to a polyelectrolytic gel. The chemical analysis of polymer by energy dispersive spectrometry and photoelectrons spectroscopy with a microsounder X showed that the product is homogeneous. It contains carbon, oxygen, and sodium. The measurements of specific surface of the product show that the polymer is nonporous and present a weak surface of about 2.1 m²/g. The thermal study of polymer showed that, under the effect of the temperature and with atmospheric pressure, the polymer is degraded only at temperatures higher than 200°C and becomes porous. When the material is heated at higher temperature of 200°C, its surface becomes increasingly porous with also an increase in the size of the pores. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 782–786, 2007     

Rheology studies of foam flow during injection mold filling

This work studies the flow behavior of a developing two‐phase gas‐polymer suspension during injection into the instrumented mold cavity of an injection molding machine. In the experiments, blowing agent type and concentration were varied along with processing conditions, to generate controlled cell structures in two different polymers, low density polyethylene and thermoplastic polyolefin. Experimental results indicate that the rheological properties of two phase gas‐polymer suspensions were sensitive to shear rate, blowing agent concentration, melt temperature, and mold temperature. The viscosity of all gas‐polymer suspensions revealed a reduction compared with neat polymer melt in the presence of gas bubbles, because of the reduced volume fraction of polymer matrix. A two‐phase rheological model has been used for fitting with our experimental results for estimating the shear viscosity of two‐phase flow in the mold cavity of the injection molding machine. POLYM. ENG. SCI., 47:522–529, 2007. © 2007 Society of Plastics Engineers.     

Hot pin welding of thin poly(vinyl chloride) sheet

This paper develops a method of welding two thin sheets of poly(vinyl chloride) (PVC) with a heated pin, thus allowing construction of a relationship between the weld temperature and weld strength. Constructing a relationship between weld strength and temperature is necessary for modeling many welding processes, including laser transmission welding. An experimental approach to establishing this relationship is required because of the complex melting behavior of PVC. The designed experimental device uses a single heated pin to weld samples by using varying pressure and temperature for one second dwell time. An electro‐mechanical loadframe pulled the welded samples until joint failure occurred, thereby allowing determination of the weld strength. An experiment varying welding pin temperature and joining pressure found the temperature to be a highly significant determiner of weld strength, while the pressure was found to be not significant. A transient numerical heat transfer model was used to calculate the weld interface temperature for each pin temperature. The relationship established in this paper can be used to predict the weld strength from the temperature output from models of alternative welding methods. J. VINYL ADDIT. TECHNOL., 13:110–115, 2007. © 2007 Society of Plastics Engineers.     

Gelation of poly(vinyl alcohol)/water/dimethylformamide solutions and properties of dried films

The properties of solutions for syndiotacticity-rich poly(vinyl alcohol) (s-PVA)/dimethylformamide (DMF)/water systems, the gelation of the s-PVA solutions, and the properties of the dried s-PVA gel films were examined. From the results of the dissolution temperature of the polymer, the gelation temperature of the solutions, the melting temperature of the gels, and the compressive modulus of the gels, the solubility of the polymer was the highest at DMF contents of 10–20 vol %. The maximum dynamic tensile modulus of the drawn (×18) films obtained from the dried gel films with a DMF content of 10 vol % was 54.9 GPa at 20°C. The orientation of the polymer chains in the amorphous regions was higher than that in the crystalline regions. The orientation of the polymer chains in the amorphous regions for the drawn films with a DMF content of 10 vol % was higher than that for the drawn films with a DMF content of 60 vol %. The orientation of the polymer chains in the amorphous regions was considered to play an important role in the properties of high strength and high modulus. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1661–1667, 1998     

Evaluation of the shear characteristics of steel–asphalt interface by a direct shear test method

Shear characteristics of steel–asphalt interface under the influences of temperature, normal stress level and tack coat material were investigated. The direct shear tests were conducted on composite specimens with epoxy asphalt (EA) and polymer modified asphalt (PMA) tack coat materials at temperatures of 25 and 60 °C and normal stress levels of 0, 0.2, 0.4, and 0.7 MPa for each temperature. Results show that the failure modes include adhesive failure at the primer-tack coat interface and material failure of asphalt concrete. Steel–asphalt interface shows strain softening behavior until it reaches the sliding state. The shear strength and the shear reaction modulus increase with decreasing temperature and increasing normal stress levels. The specimens with EA tack coat provides much higher interface shear strengths than those with PMA tack coat at 25 and 60 °C. In addition, the failure envelopes of the shear strength and residual shear strength were obtained for combinations of tack coat materials and temperature conditions based on the Coulomb failure law.     

Effect of the nanoclay types on the rheological response of unsaturated polyester–clay nanocomposites

The rheological behavior of styrene‐free unsaturated polyester resin–nanoclay (Cloisite 15A and 30B) nanocomposites at various nanoclay contents prepared by melt mixing was investigated. To investigate the effect of shear and diffusion induced phenomena as well as nanoclay surface modification on the rheological behavior, samples were prepared at two different temperatures of 40°C (cold mixed) and 150°C (hot mixed) and tested in a range of 40–120°C using dynamic rheometry. Viscosity–temperature curves for the cold mixed samples showed a decrease in viscosity with increasing temperature followed by the formation of a plateau with onset temperature which depends on the nanoclay type and content. The results of small angle X‐ray scattering and transmission electron microscopy analysis were in agreement with the formation of a physical network in which nanoclay particles act as the nodes, whereas polymer chains serve as the links. It is believed that for the cold‐mixed samples, higher shear forces break the nanoclay stacks associations efficiently, and hence more nanoparticles are available for polymer chains to form the network. In this context, the initial d‐spacing was much more effective than the nanoclay modification type. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers.     

Joining of Cf-SiC composites and ZrB2-SiC based composites for ultra high temperature applications

Gas tungsten arc welding (GTAW) of Cf–SiC composites to themselves and to ZrB₂-SiC based composites have been carried out with a filler material of (ZrB₂-SiC-B₄C-Y₂O₃-Al₂O₃) composite. The weld interfaces of joints of composites were clean and free from porosity and cracks. Penetration of filler material into voids and pores existing in the Cf-SiC composites was observed. An average shear strength of 25.7?MPa was achieved. The ZrB₂-SiC based composite joined to Cf-SiC (CVD) composite was exposed for 300?s to the oxy-propane flame at 2300?°C. The joint and interfaces between the filler material and parent composites were found to be unaffected by thermal cycling and oxidation during the exposure to the oxy-propane flame.     

Some aspects of the mechanical response of BMI 5250‐4 neat resin at 191°C: Experiment and modeling

The inelastic deformation behavior of BMI‐5250‐4 neat resin, a high‐temperature polymer, was investigated at 191°C. The effects of loading rate on monotonic stress–strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. Effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods during both loading and unloading. The strain‐time behavior was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 191°C. The model was employed to predict the response of the material under monotonic loading/unloading and multi‐step load histories. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The diffusion of nonionic penetrants in polyamide

The diffusion behavior of nonionic penetrants in aqueous solution into nylon 6 was examined in the temperature range 5°–95°C. The Arrhenius plot of the diffusion coefficients is linear and its slope changes at 30–40°C higher than the glass transition temperature in water, as determined by dilatometry and viscoelastic measurements. The results are discussed in relation to the molecular size of the penetrant and the segmental motion of polymer chains.     

Electrical properties of flexible graphene reinforced polyimide composites

The electrical properties of polyimide and the composite at different volumes fractions were studied in the frequency range 200–20 kHz and in the temperature range 30–200 °C. Increasing the volume fraction of graphene up to 10%, resulted in an extremely large increase in the dielectric constant, which indicates the composites remarkable ability to store electric potential energy under the effect of alternating electric field. An increase in dielectric constant was also observed with increasing temperature and decreasing frequency. The outstanding dielectric properties of polyimide graphene nanocomposites are attributed to the large volume fraction of interfaces in the bulk of the material. The measured increase in dielectric constant with increasing temperature was attributed to the segmental mobility of the polymer chains. The AC conductivity for polyimide and the composites was calculated from the loss factor and a remarkably high conductivity was obtained for the composites due to the formation of conducting paths in the matrix by the graphene sheets. Also this study showed that the thermal conductivity of the composites increased sharply with increasing graphene concentration. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45372.     

Ultrasonic welding using tie-layer materials. part I: Analysis of process operation

Ultrasonic welding of oriented polypropylene (OPP) using tie-layer materials has been examined. The thermal cycle at the joint interface was evaluated using a high speed data acquisition system, and concurrent changes in horn displacement (penetration) and the output power were monitored. The model explaining process operation involves four phases, i.e., I–where heating occurs because of the stresses generated in asperities on the contacting surfaces; II–where the whole tie-layer reaches the melting point; III–where the polymer melt is subjected to intense heating from viscous dissipation and is squeezed out; and IV–where the joint cools after welding. In the early stages of ultrasonic welding the heat generated at asperities on the contacting surfaces leads to melting of the tie-layer/oriented polypropylene interface within 50 ms. The tie-layer heats up because of a combination of viscoelastic dissipation and heat conduction from the oriented polypropylene/tie-layer interface, and the rate of temperature rise at the midline of the tie-layer is in the range 200°/s to 400°/s. The reduction in thickness of the test specimens (penetration) is negligible up to the time when the tie-layer melts completely, and then changes rapidly when the melted polymer at the joint interface is squeezed out. The influence of machine parameters (amplitude and contact pressure) and of tie-layer Melt Flow Index is also examined. The total time required for completion of the welding process decreases when the amplitude and applied pressure are increased. The use of low Melt Flow Index tie-layers produces peak temperature as high as 600° at the bondline, and little material is ejected during the ultrasonic welding operation.     

Interfaces in repair,recycling, joining and manufacturing of polymers and polymer composites

A basic set of 10 thermoset polymer–polymer interfaces has been identified to play a vital role in the technical and economic aspects of composite manufacturing (RIM/RTM, compression molding, autoclave lamination), recycling, repair, welding, and joining of polymer composites. Knowledge of the chemical interactions and molecular connectivity at these interfaces and their influence on processability and mechanical properties of the polymers and polymer composite is essential, and has been the focus of this research. Presented in this report are the results of an exploratory study performed to understand the interactions at the polymer–polymer interface and their influence on the interfacial fracture toughness of a thermoset vinyl ester, which is widely used in liquid molding applications. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 775–785, 1999     

Ionic conductivity,dielectric behavior,and HATR–FTIR analysis onto poly(methyl methacrylate)–poly(vinyl chloride) binary solid polymer blend electrolytes

Solid polymer electrolytes comprising blends of poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) as host polymers and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) as dopant salt were prepared by solution‐casting technique. The ionic conductivity and dielectric behavior were investigated by using AC‐impedance spectroscopy in the temperature range of 298–353 K. The highest ionic conductivity of (1.11 ± 0.09)×10^?6 S cm^?1 is obtained at room temperature. The temperature dependence of ionic conductivity plots showed that these polymer blend electrolytes obey Arrhenius behavior. Conductivity–frequency dependence, dielectric relaxation, and dielectric moduli formalism were also further discussed. Apart from that, the structural characteristic of the polymer blend electrolytes was characterized by means of horizontal attenuated total reflectance–Fourier transform infrared (HATR–FTIR) spectroscopy. HATR–FTIR spectra divulged the interaction between PMMA, PVC, and LiTFSI. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Uniaxially aligned poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐bithiophene] thin films characterized by the X‐ray diffraction pole figure technique

Aligned thin films of the liquid‐crystalline polymer poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐bithiophene] were prepared, and the correlation between the optical anisotropy and the structural properties was shown. A series of samples with different thicknesses were prepared via a spin‐casting process on rubbed polyimide surfaces. The alignment of the polymer chains was obtained by a temperature treatment just below the clearing temperature. The degree of alignment was investigated with ultraviolet–visible absorption spectroscopy and in‐plane X‐ray diffraction. Independently, each technique revealed Hermans orientation functions in the range of 0.75–0.8. Surprisingly, a layer‐thickness dependence was not observed. In addition, the X‐ray diffraction pole figure technique revealed that the polymer chains were uniaxially aligned along the rubbing direction. The aligned films were in the nematic state, with the director elongated along the rubbing direction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of the parent solution concentration on the flocculation performance of PAAm flocculants and the relation between the optimal parent solution concentration and critical concentrations

Flocculation performance of three kinds of polyacrylamide (PAAm), linear‐PAAm, Al(OH)₃–PAAm hybrid, and star–PAAm, in kaolin suspensions have been investigated by Spectrophotometer. It was found that the flocculation performance of the polymer flocculant is enhanced at the beginning and then impaired with increasing parent solution concentration (C_p) and an optimal parent solution concentration (C_op) exists, which is directly proportional to both critical concentrations of C* and C_s of the polymer in the dilute aqueous solution, and can be roughly expressed by an empirical formula, C_op = 3.1 × 10^?3 + 643.1C_s. The findings suggests that flocculation performance of a given polymer is dependent on both of the interchain association and the chain contact of the polymer chains in the parent solution and in the kaolin suspension, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1585–1592, 2007     

Peroxide crosslinking of a styrene‐free unsaturated polyester

Thermoset unsaturated polyesters are usually obtained by the crosslinking of unsaturated polyester chains dissolved in an unsaturated, reactive, monomeric diluent, which is usually styrene. This article describes a new approach in which styrene‐free unsaturated polyester chains are intrinsically cured into a crosslinked matrix. The gel time, gel content, swelling degree, glass‐transition temperature, dynamic mechanical properties, tensile properties, and molecular weight between crosslinks (calculated according to both the Flory–Rehner equation and the theory of rubber elasticity) of the crosslinked polymer are studied as a function of the peroxide concentration. All properties change considerably upon the addition of small amounts of peroxide (between 1 and 2 wt %) and change to a lesser extent with higher peroxide concentrations (up to 6 wt %). The thermal properties of the isolated gel fraction are studied as a function of the peroxide concentration. The sol fraction demonstrates a plasticizing effect on the crosslinked network, affecting the glass‐transition temperature and stress–strain behavior of the crosslinked polymer. In light of the crosslink densities derived from swelling experiments, a molecular structure and crosslinking mechanism are suggested for the gel fractions of 1 and 6 wt % peroxide crosslinked unsaturated polyester chains. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007