Interlayer adhesion of coextruded films: Effect of biaxial stretching

In order to understand the effect of thermoforming on the interlayer adhesion of coextruded films, a peel test was performed for coextruded films after being stretched. To simulate the non‐uniaxial stretching nature of actual thermoforming processes, planar stretching and biaxial stretching were applied to the coextruded films prior to the peel test. Both the planar stretching and biaxial stretching were performed at an optimum thermoforming temperature under well‐controlled stretch rates to a predetermined stretch ratio. It was found that there was a significant amount of reduction of interlayer adhesion due to stretching. Furthermore, the loss of interlayer adhesion at the optimum thermoforming temperature was linearly related to thickness drawdown as a result of stretching regardless of stretching modes. Therefore, it is suggested that the effect of thermoforming on interlayer adhesion of coextruded films can be easily estimated from the thickness distribution of thermoformed parts. Polym. Eng. Sci. 44:948–954, 2004. © 2004 Society of Plastics Engineers.     

Adhesion and delamination failure mechanisms in alternating layered polyamide and polyethylene with compatibilizer

The effects of compatibilizer and the number of layers on the interfacial adhesion and delamination model of coextruded microlayer samples consisting of alternating layers of high‐density polyethylene (HDPE) and polyamide 6 (PA6) were studied with T‐peel test. When more maleic anhydride‐grafted HDPE was incorporated into HDPE layer, the interfacial delamination model changed from adhesive to cohesive failure in the case of bilayer samples. For high‐layer samples, the results of X‐ray photoelectron spectroscopy showed that the areal density of copolymers at the interfaces increased with increasing number of layers due to strong and durable shearing forces during microlayer coextrusion. Scanning electron microscopy observation revealed that the interfacial delamination model changed from single‐ to multiple‐interface delamination when the number of layers increased from 16 to 32. The crack propagation included a large number of layer–layer jumps. The peel strength of microlayer samples was found to be greatly influenced by the interfacial delamination mechanisms. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Stress relaxation in low‐shrink‐force polyolefin films

The morphology and stress relaxation of coextruded five‐layer LLDPE (linear low‐density polyethylene)/EVA (ethylene‐vinyl‐acetate) copolymer films were studied. Increasing VA (vinyl acetate) content in EVA causes a decrease of shrink tension in the films, which can be explained by a decrease in amount of crystallinity. The relaxation time spectrum of the coextruded crosslinked LLDPE/EVA films is similar to the relaxation time spectrum of crosslinked LLDPE film at room temperature. However, at elevated temperatures, an additional peak appears on the spectrum of coextruded film. The cause of this peak is temperature‐ and stress‐induced recrystallization of EVA during the relaxation test. This recrystallization was confirmed with DSC and wide angle X‐ray analysis. Polym. Eng. Sci. 44:1716–1720, 2004. © 2004 Society of Plastics Engineers.     

Numerical analysis of the peel test for characterisation of interfacial debonding in laminated glass

Laminated safety glass is widely used in construction and as automotive windshield. When the glass plies break under dynamic loading, the adhesion between the glass plies and the interlayer is key to achieving the required safety performance. However, direct measurement of the interfacial adhesive properties is not possible with the existing test methods. In corresponding calculations, material behaviour is often simplified, which leads to inaccurate results. In this article, a finite element model for the 90° peel testing of laminated glass is studied. Hydrogen bonding at the interface between poly-vinyl butyral (PVB) interlayer and glass is represented in the model by a cohesive zone. It is seen that the experimentally measured peel force can successfully be matched by the simulations, but several combinations of variables can give the same result. Therefore, a parameter study is performed to establish the influence of each variable. It is found that the peel arm, consisting of the PVB and an aluminium backing foil, cannot be regarded as a thin film. Furthermore, the exact shape of the traction-separation law governing the cohesive zone has negligible influence on the simulation results, whereas the combination of interfacial strength and fracture energy fully characterises the delamination. The simulation results show that small-strain material behaviour can no longer be assumed for the PVB material in the vicinity of the crack tip.     

Analysis of the Symmetrical 90°-peel Test with Extensive Plastic Deformation

A numerical 2-D study of the symmetrical 90°-peel test (a similar geometry to the T-peel test) in which extensive plastic deformation occurs in the adherends is presented in this paper. A traction-separation relation is used to simulate failure of the interface, and the conditions for both crack initiation and steady-state crack growth are investigated. The numerical predictions for the steady-state peel force are compared with those based on elementary beam theory. It is shown that two competing effects dominate the mechanics of the peel test to such an extent that the results of beam-bending analyses cannot be used to predict the peel force. At one extreme range of parameters, delamination is driven by shear rather than by bending, resulting in a lower peel force than would be predicted by beam-bending analyses. At the other extreme, where delamination is bending-dominated, the constraint induced by the interfacial tractions cause an increase in the peel force. The numerical results are compared with the results of experiments in which adhesively-bonded specimens are tested in the symmetrical 90°-peel configuration. Excellent agreement between the numerical and experimental results validates the numerical approach.     

Effect of a tie layer on the delamination toughness of polypropylene and polyamide‐66 microlayers

The effect of a thin tie layer on the adhesion of polypropylene (PP) and polyamide‐66 (PA) was studied by delamination of microlayers. The microlayers consisted of many alternating layers of PP and PA separated by a thin layer of a maleated PP. The peel toughness and delamination failure mode were determined using the T‐peel test. Without a tie layer, there was no adhesion between PP and PA. A tie layer with 0.2% MA provided some adhesion; however, delamination occurred by interfacial failure. Increasing the maleic anhydride (MA) content of the tie layer increased the interfacial toughness. With 0.5% MA, the interfacial toughness exceeded the craze condition of PP, and a transition from interfacial delamination to craze delamination occurred. Crazing ahead of the crack tip effectively reduced the stress concentration at the interface and dramatically increased the delamination toughness. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1461–1467, 1999     

Interfacial crosslinking and diffusion via extensional rheometry

When a coextruded multilayer sheet is pulled in a uniaxial extensional rheometer the interfacial area per unit volume increases, amplifying interfacial effects. To investigate interfacial reaction, a pair of functional and miscible polyethylenes were co‐extruded into multilayer sheets (256 layers). One polyethylene contained 8 wt% glycidyl methacrylate comonomer the other 4 wt% anhydride grafts. The extensional force to pull the molten multilayer sheet was measured via a rotating clamp extensional rheometer (RME, Rheometric Scientific). Owing to the reaction in the interfacial region before and during the extension, the measured extensional force increased dramatically. The interfacial stress was extracted and correlated to the extent of interfacial crosslinking. The rate of interfacial crosslinking was found to exceed the rate of area generation at extension rates < 0.01 s^?1. By annealing samples and then stretching them, the growth rate of the interlayer was determined, and a diffusion coefficient estimated.     

Modifying adhesion of linear low‐density polyethylene to polypropylene by blending with a homogeneous ethylene copolymer

Adhesion of a Ziegler–Natta catalyzed ethylene copolymer (ZNPE) to polypropylene (PP) was studied by measuring the delamination toughness G of coextruded microlayers by using the T‐peel test. Low values of G compared to a homogeneous copolymer with approximately the same short chain branch (SCB) content were attributed to an amorphous interfacial layer of low molecular weight, highly branched ZNPE fractions. Blending ZNPE with a homogeneous metallocene catalyzed copolymer (mPE) increased G. In this regard, mPE with higher SCB content was more effective than mPE with slightly lower SCB content. The ZNPE interface was mimicked by microlayering ZNPE and ZNPE blends with polystyrene from which the ZNPE layers were easily separated without damage to the surface. Examination with atomic force microscopy revealed a soft coating about 8 nm thick on the surface of the ZNPE layer. Blending with mPE reduced or eliminated the amorphous interfacial layer. It was proposed that mPE increased miscibility of low molecular weight, highly branched fractions of ZNPE and prevented their segregation at the interface. After blending with mPE eliminated the interfacial layer, G increased to a value comparable to that of a homogeneous copolymer with about the same SCB content as ZNPE bulk chains. The increase in G was attributed to epitaxial crystallization of the ethylene copolymer in the absence of an amorphous interfacial layer. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 109–115, 2004     

The effect of molding conditions on mechanical and morphological properties at the interface of film insert injection molded polypropylene‐film/polypropylene matrix

An extensive study on the peel strength between a polypropylene (PP) film and PP substrate fabricated using film insert injection molding technique was carried out through a 180° peel test. Injection molding conditions such as barrel temperature, injection speed and holding pressure were varied to gauge their effects on the mechanical and morphological properties. Morphological observations were made at the film‐substrate interfacial regions by means of transmission electron microscopy (TEM). The injection molded products, with the films still attached, were subjected to bending and impact tests to determine if there is any relationship between film‐substrate adhesion and bulk properties. Observation of the load‐displacement curves during the peel test revealed three unique and interesting curves, corresponding to different peeling and film fracture mechanisms. Increases in injection speed, barrel temperature and holding pressure lead to increased bonding between the film and substrate surfaces. The enhancement of bonding between these two polymer surfaces could be attributed to polymer‐polymer interdiffusion. Substantiating evidence from TEM, which shows the fading of the interface as the bond strengthens, further boosts the accuracy of this assumption. The hope that the films could contribute to enhancing bulk properties has been diminished since the bending properties appeared to be similar with or without the film attached. Polym. Eng. Sci. 44:2327–2334, 2004. © 2004 Society of Plastics Engineers.     

The effect of residual stresses on adhesion measurements

The adhesion of films and coatings is often measured by determining the load required to separate them from their substrate. If there are residual stresses that are relaxed upon delamination, then an additional contribution to the energy-release rate will affect the measurements. These residual stresses may also cause a shift in the mode-mixedness of the interface crack which, in turn, can affect the interfacial toughness. To ensure an accurate interpretation of adhesion measurements, therefore, the effects of these stresses must be considered. These effects are discussed with particular reference to two commonly used test geometries: the blister test and the peel test.     

Delamination failure mechanisms in microlayers of polycarbonate and poly(styrene-co-acrylonitrile)

The peel strength and delamination failure mode of coextruded microlayer sheets consisting of alternating layers of polycarbonate (PC) and poly(styrene-co-acrylonitrile) (SAN) were studied with the T-peel test. Four delamination modes were observed: two modes where the crack propagated along the PC–SAN interface and two other modes where the crack propagated through crazes in the SAN. The SAN layer thickness determined whether crack propagation was interfacial or through crazes. Crazing and crack propagation through crazes were observed only if the SAN layer was thicker than 1.5 μm. As the thickness of the SAN layer increased, the amount of crazing in front of the crack tip and the amount of craze fracture gradually increased; the peel strength increased accordingly. If the SAN layers were thinner than 1.5 μm and the PC layers were relatively thick, the crack propagated along a single interface. The peel strength for this delamination mode was the lowest and equal to about 90 J/m², independent of layer thicknesses. This delamination mode came closest to providing a ”real” measure of the adhesive toughness of PC to SAN. With both interfacial and craze delamination, the crack could move from layer to layer if the PC was thin enough. Tearing of the relatively thin PC layers increased the peel strength of the multiple-layer delamination modes. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:793–805, 1998     

Analysis of the Symmetrical 90°-peel Test with Extensive Plastic Deformation

A numerical 2-D study of the symmetrical 90°-peel test (a similar geometry to the T-peel test) in which extensive plastic deformation occurs in the adherends is presented in this paper. A traction-separation relation is used to simulate failure of the interface, and the conditions for both crack initiation and steady-state crack growth are investigated. The numerical predictions for the steady-state peel force are compared with those based on elementary beam theory. It is shown that two competing effects dominate the mechanics of the peel test to such an extent that the results of beam-bending analyses cannot be used to predict the peel force. At one extreme range of parameters, delamination is driven by shear rather than by bending, resulting in a lower peel force than would be predicted by beam-bending analyses. At the other extreme, where delamination is bending-dominated, the constraint induced by the interfacial tractions cause an increase in the peel force. The numerical results are compared with the results of experiments in which adhesively-bonded specimens are tested in the symmetrical 90°-peel configuration. Excellent agreement between the numerical and experimental results validates the numerical approach.     

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.     

Peeling of heterogeneous thin films: Effect of bending stiffness,adhesion energy,and level of heterogeneity

The peeling behaviour of a heterogeneous thin film bonded to a rigid substrate was investigated by using both experiments and finite element modeling. The enhancement in peel force was studied specifically for heterogeneous thin films with periodic stiff and compliant portions along the length. Peel tests with homogeneous thin films (uniform film thickness) showed that the maximum peel force can be observed before the onset of steady state peeling process. Moreover, this maximum peel force was observed to be a function of the bending stiffness of the film and adhesion energy at the film-substrate interface. For the heterogeneous thin films, maximum peel force can be observed either before the onset of steady state or when the peel front traverses from compliant to the stiff portion of the film. The three-dimensional finite element model, based on cohesive zone technique was developed, which provided further insight into the enhancement in peel force. The maximum force was shown to be dependent on the level of heterogeneity in addition to adhesion energy and bending stiffness as was observed with homogeneous films. The improvement in peel force was found to be prevalent at relatively low adhesion energy. This study may be helpful for the better design of homogeneous and heterogeneous thin film-substrate systems having improved bonding strength.     

Analysis of surface structure with regard to interfacial delamination in polyester films with incompatible polymers

Poly(ethylene terephthalate) (PET) films containing incompatible polymer particles were analyzed, with particular reference to the relationship between the PET particle interfacial tension and the microvoids, or the protrusion that were formed when the composite material was stretched at 90°C. A model was developed to simulate void formation and surface protrusion due to interfacial delamination between PET and three types of dispersed incompatible polymers, poly(4‐methyl‐1‐pentene), polypropylene, and polystyrene. The numerical results, obtained with the finite element method, were compared with experimental data of the blends for both the internal and subsurface regions. The experimental measurements showed that the increase in the difference in the surface tension between PET and the added incompatible polymer was associated with the formation of larger voids. The protrusions were also generated in the stretching and delamination between PET and the incompatible polymers, but a decrease in the interfacial tension agreed with the formation of a larger protrusion. Modeling studies showed that increasing the interfacial tension between the two components in a blend causes a decrease in the critical stress for delamination. Interfacial tension values related qualitatively to the critical stress for void formation and protrusion calculated with the numerical analysis. A concavity was also necessary for understanding the surface structure of the films, along with protrusion. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1243–1251, 2004     

Comprehensive experimental study of a starch/polyesteramide coextrusion

A comprehensive study of the three‐layer film coextrusion was performed. Plasticized wheat starch (PWS) was chosen as the film central layer, and poly(ester amide) (PEA) was used as the surface outer layers. Single‐screw extruders and a standard feedblock attached to a flat coat‐hanger die were used to prepare the three‐layer films. The layer deformation and interfacial instability phenomena, inherent to multilayer flows, were thoroughly investigated. The effect of process variables, such as viscosity ratio, extrusion rate, layer thickness, and die geometry, were studied. Encapsulation of the central layer by the skin layers readily occurred at the edges of coextruded films. The stability of PEA/PWS/PEA coextrusion flows was closely related to the shear stress at the interface. Increasing global volumetric flow rates and the die gap geometry seemed to promote instabilities. Finally, the existence of instabilities at the interface increased the adhesion strength of multilayered products, due to mechanical interlocking between adjacent layers. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2586–2600, 2002     

LLDPE‐based mono‐ and multilayer blown films: Property enhancement through coextrusion

In this study we investigated the performance of multilayer coextruded linear low‐density polyethylene (LLDPE) blown films. Five‐layer films were compared with monolayer dry‐blended films, and the effects of layer composition and layout on the end‐use properties of the coextruded films were highlighted. Three different LLDPEs were used: a conventional Ziegler‐Natta LLDPE gas phase butene copolymer, an advanced Ziegler‐Natta LLDPE solution octene copolymer, and a single‐site LLDPE solution octene copolymer. Numerous five‐layer coextruded structures comprising the single‐site resin and the other two Ziegler‐Natta resins were produced. The coextruded structures composed of the LLDPE butene and the single‐site resin yielded improved end‐use properties relative to the monolayer‐blended films. This result was ascribed to the presence of interfacial transcrystalline layers. Also, blends of the single‐site LLDPE and the advanced Ziegler‐Natta LLDPE octene resins within selected layers of coextruded films showed slightly enhanced tear resistance. Finally, it was found that haze was significantly reduced when the outside layers were composed of the single‐site resin. POLYM. ENG. SCI., 45:1222–1230, 2005. © 2005 Society of Plastics Engineers     

Microporous membranes of polyoxymethylene from a melt‐extrusion process: (II) Effects of thermal annealing and stretching on porosity

A two‐part study utilizing polyoxymethylene (POM) was undertaken to investigate a three‐stage process (melt‐extrusion/annealing/uniaxial‐stretching) utilized to produce microporous films. In this report, the thermal annealing (second stage) and subsequent uniaxial‐stretching (third stage) results of selected POM films from two commercial resins, labeled D & F, are discussed. Specifically, the annealing and uniaxial stretching effects on film morphology, orientation, and other pertinent film properties are addressed. Additionally, sequential analysis was performed regarding the influence each stage had on the resulting microporosity. It was found that the melt‐extruded precursor morphology and orientation, as a consequence of the first stage extrusion parameters and resin characteristics, are crucial to controlling the membrane permeability. The annealing parameters were also deemed critical, where a temperature of 145°C applied for 20 min under no tension was the optimum annealing condition for producing a highly microporous film upon stretching. For the conditions studied, the stretching parameters that were found to be optimum for producing the desired characteristics in the final film were a cold temperature of 50°C and hot stretch temperature of 100°C. The optimum extension levels were concluded to be 90% for both the cold and hot stretch steps, and thus a total overall extension level of 180%. However, these results were only with respect to resin F films. Because the resin D melt‐extruded precursors possessed twisted lamellar morphologies and relatively low crystal orientation, their samples could not be produced into microporous films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1762–1780, 2002; DOI 10.1002/app.10587     

The morphological properties of PP coextruded tape fabrics

In the field of self‐reinforced composites many researchers have focused their attention on the coextruded tapes composed of polypropylene core and PP/PE copolymer skin. Two similar commercial fabrics (P and T) have been compared in respect of their peel resistance. For both materials, peel resistance has a periodic trend that regularly follows fabric weave style. T has demonstrated an average peel resistance and a well‐bonded area slightly greater than P. Skin/core interfacial properties have been investigated and a crosscheck between differential scanning calorimetry (DSC) and Raman spectroscopy has been adopted to understand the influence of skin structure on consolidated laminate. DSC curves exhibit three melting peaks during first heating for both fabrics, corresponding to copolymer, skin/core interface, and core melting. After consolidation at 140°C stretching‐induced superstructure and PP crystallinity degree are preserved. The presence of PP/PE copolymer + PE blend only in fabric P has been pointed out and PE content has been calculated. POLYM. ENG. SCI., 56:727–734, 2016. © 2016 Society of Plastics Engineers     

Adhesion promotion of thick polyphosphate-poly(allylamine) films onto polyolefin substrates by plasma polymers

The adhesion of thick poly(allylamine)-polyphosphate layers (1 μm) deposited by the wet-chemical layer-by-layer (LbL) technique onto polyethylene or polystyrene (each 100 μm) was very low. To promote the adhesion of these LbL deposited layers, the polyolefin substrates were oxidized at the surface by short exposure to the oxygen plasma (2 or 5 s) and subsequently coated with an interlayer of plasma-deposited poly(allylamine) or poly(allyl alcohol) (100 nm). The plasma polymer interlayers have improved strongly the adhesion between polyolefin substrates and polyphosphate coatings. Such phosphate coatings are interesting for life sciences (nucleotide formation) but also for fire retardancy in combination with N-rich compounds such as melamine. The intention was to prefer chemical hydrogen bonds for adhesion promoting because of their high binding energy. Therefore the introduced oxygen-containing groups at the polyolefin surface could interact with the OH or NH₂ groups of the adhesion-promoting plasma polymer interlayer. These groups were also able to interact strongly with the poly(allylamine)-polyphosphate topcoating. The coated polyolefins were investigated using Fourier transform infrared spectroscopy in attenuated total reflectance mode (FTIR-ATR), X-ray photoelectron spectroscopy, thermogravimetric analyses and atomic force spectroscopy, and 90° peel test.