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.     

Sulfonation of polymer surfaces - I. Improving adhesion of polypropylene and polystyrene to epoxy adhesives via gas phase sulfonation

A new method for surface treating polymers and polymer composites based on gas phase sulfonation has been shown to quickly and effectively increase wettability and adhesion to epoxy. A gas mixture containing a low concentration of sulfur trioxide in nitrogen (～ 1% v/v) was used to treat the surfaces of polypropylene and polystyrene films. The sulfonated surfaces were then neutralized with ammonium hydroxide. The effectiveness of sulfonation on the adhesion of these polymers to an epoxy adhesive was investigated using mechanical testing of sandwich lap-shear specimens. The lap-shear adhesive joint strength of epoxy to sulfonated polypropylene was compared with polypropylene treated with currently accepted surface treatments including chromic acid etching and flame treatment. Sulfonation greatly improves the adhesion of polypropylene to epoxy compared with other surface treatment techniques as measured by lap-shear strength. An optimum sulfonation treatment level was shown to exist for polypropylene. For polystyrene surfaces, it was shown that while sulfonation immediately increased wettability, it did not greatly improve its adhesion to epoxy; it did, however, significantly reduce the scatter in the ultimate strength values. Excess sulfonation treatment reduced the lap-shear strength for both polymers. X-ray photoelectron spectroscopic examination of the locus of failure of tested joints has shown that failure occurs in a weak boundary layer for these surface-treated polymers.     

聚丙烯熔体流动指数测试与分析

在不同负荷和不同温度下,利用熔体流动速率仪测试聚丙烯的熔体流动指数,通过熔体流动指数这种非常简单的方法计算得到剪切应力、剪切速率、非牛顿指数、零切粘黏度和流动活化能等流变参数。     

Influence of PEO and mechanical keying on the strength of AA 5052 alloy/polypropylene friction stir spot welded joints

A study has been carried out of the effect of plasma electrolytic oxidation (PEO) on the strength of AA 5052 alloy/polypropylene joints prepared using friction stir spot welding (FSSW). The joint strengths were determined using lap-shear tests and failure modes were investigated using scanning electron microscopy. Comparisons were made between control joints prepared with the alloy in the as-rolled condition or the as-rolled condition with a mechanical key and with PEO-treated alloy, with or without a mechanical key. Mechanical keying alone, provided by infiltration of polymer into holes of either 3 or 4.5 mm diameter drilled in the alloy, yielded enhancements of the joint strength by a factor of 1.8 and 3.8, respectively, compared with the as-rolled alloy. In contrast, PEO pre-treatment provided a much greater improvement in strength, by a factor of 21.3, with no significant influence of the presence of a mechanical key. The fracture of PEO-treated joints involved a mixture of de-bonding at the polymer/coating interface, cohesive failure within the coating and ductile failure of the polymer. The last resulted from formation of gas bubbles within the polypropylene due to thermal degradation of the polymer during FSSW. In contrast, in the absence of PEO, the fracture path of the as-rolled and also the as-rolled and mechanically keyed joints passed along the alloy/polypropylene interface and through gas bubbles within the polymer. The enhanced strength of the PEO-treated joints resulted from the strong polypropylene/coating bond resulting from flow of molten polymer into the coating porosity.     

Development of a new thermoplastic laminate system

In this investigation an all-olefin thermoplastic laminate was developed and characterized. Commingled glass-fiber polypropylene (PP) composite was used as skin and HDPE (PE) foam with closed cells as core. Infra-red heating was used for melting the surfaces of the substrates for surface fusion bonding with a cold press. Two tie-layer films, viz., ethylene-propylene copolymer (EPC) and HDPE/elastomer blend were used as hot-melt adhesives for bonding the substrates. Singlelap shear joints were prepared from PP composite and PE foam adherends with a bonding area of 25.4 mm × 25.4 mm to determine the bond strength. EPC tie-layer adhesive provided higher bond strength (2.68 × 10⁶ N/m²) to the all-olefin laminate than that based on HDPE/elastomer blend (1.93 × 106 N/m²). For EPC tie-layer-based laminates, a mixed mode of failure was observed in the failed lap shear samples: about 40% was cohesive failure through the tie-layer, and the rest of failure was interfacial, either at PP composite or PE foam surfaces. Environmental scanning electron micrographs (ESEM) revealed that in the process of surface fusion bonding, PE foam cells in the vicinity of interphase (800-μm-thick) were coalesced with high temperature and pressure. No macro-level penetration of the tie-layer melt front into the foam cells was observed. As the surface morphology of foam was altered due to IR surface heating and the PP composite bonding side had a resin-rich layer, the bonding situation was closer to that between two polymer film surfaces.     

Copolymer melt rheograms from melt flow index

Available literature data on the variation of melt viscosity with shear rate are shown to coalesce together to give unique curves for each copolymer type, independent of temperature and copolymer grade, based on a normalising technique using the Melt Flow Index. The unified curves obtained are useful for estimating the flow curves of the copolymers at the temperature of interest if the Melt Flow Index of the copolymer at the specific temperature is known. Coalesced curves are presented for a number of commonly known copolymers such as acrylonitrile-butadiene-styrene, styrene-butadiene-styrene. vinyl chloride-vinyl acetate, ethylene-vinyl-acetate, polyester elastomer and an olefinic-tyype thermoplastic elastomer.     

Evaluating Cure of a pMDI-wood Bondline Using Spectroscopic,Calorimetric and Mechanical Methods

The cure of polymeric diphenylmethane diisocyanate (pMDI)/wood bondline in a controlled saturated steam environment was monitored using micro-dielectric analysis (μDEA). Saturated steam environments were produced between 110°C and 140°C. The degree of cure calculated from μDEA was a basis for further spectroscopic, calorimetric, and mechanical evaluation. Interpretation of calorimetric and spectroscopic analysis revealed large consumption of isocyanate early in cure. However, mechanical strength, as revealed by lap-shear tests, did not develop until late in cure. Low lap-shear strengths and a plateau in conversion rates were detected for samples pressed at 110° and 120°C. Several components of the analysis suggest that low temperature cure may result in crystal formation, leading to diffusion controlled cure.     

Evaluating Cure of a pMDI-wood Bondline Using Spectroscopic, Calorimetric and Mechanical Methods

The cure of polymeric diphenylmethane diisocyanate (pMDI)/wood bondline in a controlled saturated steam environment was monitored using micro-dielectric analysis (μDEA). Saturated steam environments were produced between 110°C and 140°C. The degree of cure calculated from μDEA was a basis for further spectroscopic, calorimetric, and mechanical evaluation. Interpretation of calorimetric and spectroscopic analysis revealed large consumption of isocyanate early in cure. However, mechanical strength, as revealed by lap-shear tests, did not develop until late in cure. Low lap-shear strengths and a plateau in conversion rates were detected for samples pressed at 110° and 120°C. Several components of the analysis suggest that low temperature cure may result in crystal formation, leading to diffusion controlled cure.     

Study on effect of filler loading on the flow and swelling behaviors of polypropylene‐kaolin composites using single‐screw extruder

Melt flow and extrudate swelling behavior of polypropylene‐kaolin (PP‐Kaolin) composites were investigated using a single‐screw extruder. Kaolin was mixed with polypropylene (PP) using a heated two‐roll mill at 185°C and the filler loading were varied from 5 to 30 wt %. Subsequently, flow behavior of the compounded formulations were evaluated through Melt Flow Index (MFI) measurement at various temperatures ranging from 190 to 230°C. The extrudate swelling ratio was also measured by using an image analysis instrument and software. It was proven that the MFI decreased with increasing loading of kaolin for test temperatures of 190 and 200°C. However, for temperatures exceeding 200°C, the MFI value rose slightly at 5 wt % of kaolin content then seemed to reduce as more kaolin was added. This is also detected in rheological measurement where the apparent visosity, η_app, appear to be lowered at 5 wt % loading of kaolin. Further increase in kaolin loading resulted in increasing value of the composites η_app. The swelling ratio decrease with increasing filler loading for composites below 20 wt %. However, at 30 wt % of kaolin content, the extrudate swelling ratio increased and noticeable blistered surface texture was observed on the extrudate surface. Furthermore, at this level of filler loading, shrinkage occurence due to the existence thermal gradient between the surface and the inner core of the extrudate caused void formation in the middle section of the extrudate. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of neutral salt spray (NSS) exposure on the lap-shear strength of adhesive-bonded 5052 aluminum alloy (AA5052) joints

This work explores the mechanisms for the non-linear degradation of the lap-shear strength of adhesive-bonded 5052 aluminum alloy (AA5052) joints exposed to neutral salt spray (NSS) environment up to 1200?hours. The lap-shear testing results show that a mixed failure dominated by adhesive failure occurred in both unexposed and exposed adhesive-bonded AA5052 joints. The lap-shear strength of adhesive-bonded AA5052 joints declined sharply in the preliminary stage of NSS exposure (e.g. shorter than 240?hours), then it decreased moderately as the NSS exposure time increased. Differential scanning calorimetry (DSC), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and electrochemical noise measurement were applied to exploring the degradation mechanisms of the lap-shear strength of adhesive-bonded AA5052 joints. The adhesive properties remained nearly unchanged after exposure in the NSS environment. The decreased hydrogen bond at the AA5052/adhesive interface was responsible for the sharp degradation of the lap-shear strength of adhesive-bonded AA5052 joints in the preliminary stage of NSS exposure. In the later stage of NSS exposure (e.g. more than 240?hours), the surface oxidation of AA5052 substrates dominated the lap-shear strength degradation of adhesive-bonded AA5052 joints.     

Electropolymerised acrylic coatings for polymer-metal adhesion enhancement

Electrochemical polymerisation (ECP) of acrylic monomers produces thin coatings on metal substrates and is a potentially useful method for forming adhesion promoting tie-layers at a polymer-metal interface. Uniform, passive films of poly (methyl methacrylate) and poly (glycidyl acrylate) have been formed via a cathodic free radical mechanism on stainless steel electrodes from aqueous electrolytes. The thickness of these films was found to increase with electrolysis time and the passive nature has been demonstrated by cyclic voltammetry studies. Adhesion tests were performed to compare the adhesion strength and failure mechanisms of various adhesives to coated and uncoated stainless steel substrates. The results indicate that ECP tie-layers can significantly increase the adhesive bond strength and alter the failure mechanisms observed. Electropolymerised acrylic coatings on metal substrates are thus seen as a promising approach for pretreatment of metals for adhesion enhancement.     

Adhesion of statistical and blocky ethylene-octene copolymers to polypropylene

This study examined the effect of chain microstructure on the adhesion of ethylene-octene copolymers to polypropylene. The copolymers were candidates for compatibilization of polypropylene (PP) and high density polyethylene (HDPE) blends, and included a blocky copolymer, a statistical copolymer that had the same composition as the soft segment of the blocky copolymer, and a statistical copolymer that had the same comonomer content and crystallinity as the blocky copolymer. The compatibilized melt blend was modeled by a microlayered, one-dimensional structure consisting of alternating layers of PP and HDPE, each separated by a thin tie-layer. The microlayered structure made it possible to directly measure the adhesion using the T-peel test. Infrared analysis of matching peel fracture surfaces established that fracture occurred adhesively at the interface between PP and the tie-layer. Direct observation of the damage zone at the crack tip and microscopic examination of the fracture surfaces revealed that the tie-layer was highly deformed before final separation occurred at the interface. The substantially higher delamination toughness of the blocky copolymer compared to the statistical copolymers could be accounted for by considering both interspherulitic mechanically interlocking influxes and intraspherulitic entrapment of interdiffused tie-layer chains. The blocky copolymer also retained delamination toughness to a higher temperature due to the greater stability of lamellar crystals compared to fringed micellar crystals.     

Modeling Concepts for Studying Ultrasonic Wave Interaction with Adhesive Bonds

Recent research has shown that such adhesive bondline defects as chemical segregation, variation in cure, gas entrapment or inadequate surface preparation are responsible for many adhesively bonded structural failures. Analytical models have been developed in this work that can be used to relate these “flaws” to the manner in which they affect the reflection of an ultrasonic pulse from such a bondline. The results of this study provide a substantial resource base for extended research through which ultrasonic inspection can become a reliable NDT technique for bond strength determination.     

Multilayer films using polypropylene/polypropylene grafted with acrylic acid blends and polyamide 6

Blends of polypropylene (PP) with 0 to 100 wt% of polypropylene grafted with acrylic acid (AA-g-PP) were used to promote the adhesion to polyamide 6 (PA 6) in a three-layer coextruded film without using an additional adhesive or tie-layer. The effect of modified polymer content and its molecular weight on interfacial adhesion between PP and PA 6 was determined by T-peel strength measurements. The effect of melt temperature and bonding time on peel strength was determined. Oxygen and water vapor transmission rates of the films were measured. The peel strength of fusion bonded layers of PP/AA-g-PP blends with PA 6 strongly depends on bonding temperature and time, as well as on the molecular weight of the functionalized polymer. The peeled films surfaces were characterized using FTIR-ATR and scanning electron microscopy (SEM). Tensile properties of three-layer films, made up of PA 6 as the central layer and PP/AA-g-PP blends as the two external layers, are improved with increase in the acrylic acid (AA) content in the blend. The formation of an in situ copolymer between AA in the blend and the terminal amine groups of PA 6 was confirmed by the Moalu test.     

Dynamically Vulcanized EPDM/PP (40/60) Blends

Dynamically vulcanized thermoplastic elastomer (TPE) blends were first described by Fisher. In the 1960s, the usage of TPE was low. However, the consumption of TPE is increasing today. Due to hard and soft polymer phases in its structure, TPE replaced a lot of materials. TPE materials are preferred today due to their good thermal properties, oxidation resistance, transparency, adhesion, compatibility with other polymers etc. As a result of the studies that were done in 1975, TPV—vulcanized thermoplastic elastomers were developed. In this study, TPV elastomers were produced by forming crosslinks with peroxide from different ratios, of EPDM and PP. Mixing was done with twin screw extruder. After that yield and tensile strength, the modulus of elasticity, % elongation, Izod impact strength, hardness, Melt Flow Index (MFI), Vicat Softening Point, Heat Deflection Temperature (HDT), and density of crosslinks were determined. Thermal transition temperatures and microstructure were determined with DSC and SEM, respectively.     

聚丙烯纤维在聚3-羟基丁酸酯与木质素对甲酚复合膜中的改性作用研究

研究了聚3-羟基丁酸酯/聚丙烯纤维(PHB/PP)复合膜和PHB/PP/木质素对甲酚复合膜的力学和热力学性能的影响,分别探讨了PP纤维和木质素对甲酚的添加量对复合膜性能的影响。结果表明,PP纤维含量为8 %且木质素对甲酚含量为3 %时,PHB/PP/木质素对甲酚复合膜的综合性能最佳,适量的PP纤维改善了PHB的拉伸强度,并且木质素对甲酚一定程度上改善了复合膜的热降解性能。     

Crack propagation in biaxially oriented polypropylene at low temperature

The crack growth behavior of polypropylene biaxially oriented by cross-rolling was studied at low temperature. Single edge notch testing produced a stable tearing type of crack growth in both 50% and 80% biaxially oriented polypropylene at ?40°C, in contrast to the brittle fracture of unoriented polypropylene. The crack growth in the two oriented materials began slowly and accelerated to a constant rate that was higher in the 80% oriented material than in the 50% oriented material. The main difference between the crack growth behavior of the two was the longer period of initial slow growth in the case of 80% orientation. This period of slow growth corresponded to crack growth through the notch tip damage zone. Residual strength diagrams were used to present the crack growth data obtained when the stress state was intermediate between plane stress and plane strain. Fractography revealed large differences among the fracture surfaces of the three materials with the unoriented polypropylene showing a grainy appearance from the brittle fracture. The two oriented materials showed considerable ductility. The 50% oriented material showed many voids in the fracture surface, indicating that voiding during the fracture process contributed significantly to the toughness improvement. The 80% oriented polypropylene showed delamination crazing on the fracture surface with layered material and fibrils bridging the crazes.     

Adhesion between polymer substrates and plasma films from tetramethylsilane and tetramethyltin by glow discharge polymerization

Adhesion between various polymer substrates and plasma films, which had been prepared from either tetramethylsilane or tetramethyltin by glow discharge polymerization and deposited on the surface of the polymer, was evaluated by the Scotch tape test and by lap-shear strength. It was found that the plasma films exhibited fairly good adhesion to the polymer substrates (with the exception of polypropylene). The position where failure occurred was determined by X-ray fluorescence analysis, scanning electron microscopy and energy diffractive X-ray analysis. This position was at an inner layer of the plasma film (cohesive failure of plasma film), within the polymer substrate (material failure of polymer) or at the interface between polymer substrate and plasma film (adhesive failure) depending upon the polymer substrate. These results indicate an important aspect of durability of surface modification by glow discharge polymerization.     

POLYMER DIFFUSION AND THE EVOLUTION OF ADHESIVE BOND STRENGTH

An attempt has been made to establish the role of diffusion across the polymer interface as a contributing factor to lap-shear bond strength. The assemblies studied were polypropylene/linear low-density polyethylene (PP/LLDPE); polystyrene/polyvinyl chloride (PS/PVC); polystyrene/polymethyl methacrylate (PS/PMMA); and polyvinyl chloride/polyvinylidene fluoride (PVC/PVDF). Initial bond strength measurements were followed by measurements on joints that had been annealed for up to 72 h at temperatures in the range of 60-160°C. Following induction times of tens of minutes where bond characteristics remained constant, substantial increments in bond strength were observed for PP/LLDPE and for PS/PVC but not for PS/PMMA or PVC/PVDF. Results point to a significant contribution to bond strength arising from diffusion when dispersion forces and favorable acid/base interactions act at the interface. The times required to establish the apparent diffusion effects far exceed normal bonding times and may account for the failure of diffusion to be recognized as a significant mechanism in the formation and maintenance of adhesive bonds.     

Processing and characterization of multilayer films of poly(ethylene terephthalate) and surface-modified poly(tetrafluoroethylene)

Multilayer films were prepared from poly(tetrafluoroethylene) (PTFE) and poly(ethylene terephthalate) (PET) films together with using an adhesion promoting layer (tie-layer) consisting of ethylene-methyl acrylate-glycidyl methacrylate (E-MA-GMA) terpolymer and low density polyethylene (LDPE) blend. Na/naphthalene treatment and subsequent acrylic acid grafting were applied on the surfaces of PTFE for chemical modification. FT-IR spectroscopy, XPS analysis and surface energy measurements were performed to characterize the modified PTFE films. The analyses showed defluorination and oxidation of PTFE surface, and supported the acrylic acid grafting. The surface energy of modified surfaces enhanced with respect to unmodified one, which promoted adhesion. The multilayers were subjected to T-peel tests to measure the adhesion strength between PET and modified PTFE. Peel strength between the films increased with increasing E-MA-GMA amount in the tie-layer. A proportional dependence of peel strength on Na/naphthalene treatment time was observed for multilayers containing acrylic acid grafted or ungrafted PTFE. From SEM analysis, it was observed that the texture of the PTFE surface after modifications became rougher when compared to untreated PTFE. The peeled surfaces were also analyzed by SEM. The micrographs evidence that the energy absorbing mechanism is the plastic deformation of the tie-layer, which is responsible for obtaining high peel strengths.