Effect of tie-layer thickness on the adhesion of ethylene-octene copolymers to polypropylene

The adhesion of some ethylene-octene copolymers to polypropylene (PP) and high density polyethylene (HDPE) was studied in order to evaluate their suitability as compatibilizers for PP/HDPE blends. A one-dimensional model of the compatibilized blend was fabricated by layer-multiplying coextrusion. The microlayered tapes consisted of many alternating layers of PP and HDPE with a thin tie-layer inserted at each interface. The thickness of the tie-layer varied from 0.1 to 15 μm, which included thicknesses comparable to those of the interfacial layer in a compatibilized blend. The delamination toughness was measured in the T-peel test. Generally, delamination toughness decreased as the tie-layer became thinner with a stronger dependence for tie layers thinner than 2 μm. Inspection of the crack-tip damage zone revealed a change from a continuous yielded zone in thicker tie layers to a highly fibrillated zone in thinner tie layers. By treating the damage zone as an Irwin plastic zone, it was demonstrated that a critical stress controlled the delamination toughness. The temperature dependence of the delamination toughness was also measured. A blocky copolymer (OBC) consistently exhibited better adhesion to PP than statistical copolymers (EO). A one-to-one correlation between the delamination toughness and the reported performance of the copolymers as compatibilizers for PP/HDPE blends confirmed the key role of interfacial adhesion in blend compatibilization.     

Adhesion of propylene–ethylene copolymers to high‐density polyethylene

The adhesion of some propylene–ethylene (P/E) copolymers to polypropylene (PP) and high density polyethylene (HDPE) was studied in order to compare them with other olefin copolymers as compatibilizers for PP/HDPE blends. A one‐dimensional model of the compatibilized blends was fabricated by layer‐multiplying coextrusion. The microlayered tapes consisted of many alternating layers of PP and HDPE with a thin tie‐layer inserted at each interface. The thickness of the tie‐layer varied from 0.1 to 15 μm, which included thicknesses comparable to those of the interfacial layer in a compatibilized blend. In the T‐peel test, the P/E copolymers delaminated at the HDPE interface. An elastomeric P/E with higher ethylene content exhibited substantially higher delamination toughness than a more thermoplastic P/E with lower ethylene content. Inspection of the crack‐tip damage zone revealed that a change from deformation of the entire tie‐layer to formation of a localized yielded zone was responsible. By treating the damage zone as an Irwin plastic zone, it was demonstrated that a critical stress controlled the delamination toughness. The temperature dependence of the delamination toughness was also measured. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Comparison of olefin copolymers as compatibilizers for polypropylene and high‐density polyethylene

Some polyolefin elastomers were compared as compatibilizers for blends of polypropylene (PP) with 30 wt % high‐density polyethylene (HDPE). The compatibilizers included a multiblock ethylene–octene copolymer (OBC), two statistical ethylene–octene copolymers (EO), two propylene–ethylene copolymers (P/E), and a styrenic block copolymer (SBC). Examination of the blend morphology by AFM showed that the compatibilizer was preferentially located at the interface between the PP matrix and the dispersed HDPE particles. The brittle‐to‐ductile (BD) transition was determined from the temperature dependence of the blend toughness, which was taken as the area under the stress–strain curve. All the compatibilized blends had lower BD temperature than PP. However, the blend compatibilized with OBC had the best combination of low BD temperature and high toughness. Examination of the deformed blends by scanning electron microscopy revealed that in the best blends, the compatibilizer provided sufficient interfacial adhesion so that the HDPE domains were able to yield and draw along with the PP matrix. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Adhesion of olefin block copolymers to polypropylene and high density polyethylene and their effectiveness as compatibilizers in blends

Adhesion of four ethylene-octene block copolymers (OBCs) to polypropylene (PP) and high density polyethylene (HDPE) was studied by peeling PP/OBC/HDPE microlayered tapes. The four OBCs had different comonomer composition, mechanical properties and phase morphology. Through the Irwin damage zone analysis, it was found that the stress-strain behavior of OBC was the primary factor that determined the adhesion strength. Effectiveness of these OBCs as compatibilizers in PP/HDPE blends was also investigated. Toughness of all OBC-compatibilized blends was effectively improved. The OBCs having higher adhesion strength also resulted in better mechanical performance for the compatibilized blends. A quantitative correlation was established between the adhesion strength and mechanical performance of the blends.     

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     

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.     

Adhesion of polyethylene blends to polypropylene

The effect of chain microstructure on adhesion of ethylene copolymers to polypropylene (PP) was studied using coextruded microlayers. Adhesion was measured by delamination toughness G using the T-peel test, and interfacial morphology was examined by atomic force microscopy. Good adhesion to PP was achieved with homogeneous metallocene catalyzed copolymers (mPE) with density 0.90 g cm⁻³ or less. Good adhesion was attributed to entanglement bridges. In contrast, a heterogeneous Ziegler-Natta catalyzed copolymer (ZNPE) of density 0.925 g cm⁻³ exhibited poor adhesion to PP due to an amorphous interfacial layer of low molecular weight, highly branched fractions that prevented effective interaction of ZNPE bulk chains with PP. Blending mPE with ZNPE eliminated the amorphous interfacial layer and resulted in epitaxial crystallization of ZNPE bulk chains with some increase in G. Increasing the mPE content of the blend past the amount required to completely resolve the amorphous interfacial layer of ZNPE resulted in a steady, almost linear, increase in G. Phase separation of mPE and ZNPE during crystallization produced an interface with regions of epitaxially crystallized ZNPE bulk chains and other regions of entangled mPE chains. Entanglement bridges imparted much better adhesion than did epitaxially crystallized lamellae.     

Impact strength and fracture surfaces of interfaces between polyethylene and polypropylene and some ethylene-containing copolymers

The impact strength of annealed interfaces between high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP) and some ethylene-co-vinyl acetate (EVA) and ethylene-co-acrylic acid (EAA) copolymers was obtained using the Notched Izod test. The impact strengths of EVA-HDPE, EVA-LDPE, and EVA-PP interfaces using EVA copolymers with 9 to 27.5 wt % vinyl acetate (VA), and of EAA-PP interfaces using EAA with 3 to 20 wt % acrylic acid (AA), were all equal to or greater than those of the homopolymer used. However, the impact strengths of EAA-HDPE and EAA-LDPE interfaces were all lower than those of pure HDPE or LDPE, with the exception of 3EAA-LDPE. Scanning electron micrographs showed the presence of fibrils and/or voids, mostly on those copolymer-homopolymer fracture surfaces which had high impact strength. X-ray photoelectron spectroscopy of the fracture surfaces showed a greater calculated percentage of AA or VA on both the copolymer and homopolymer sides of the interface than in the bulk for most samples at 15 Å penetration. This greater calculated percentage of AA or VA is probably due to chain scission during sample preparation or fracture, which results in additional acid or alcohol groups at the surface that are calculated as increased VA or AA content. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2221–2235, 1997     

Effect of the addition of ethylene-propylene random copolymers on the properties of high-density polyethylene/isotactic polypropylene blends: Part 1—morphology and impact behavior of molded samples

Two random commercial ethylene-propylene copolymers (EPM) with different ethylene content have been added to binary isotactic polypropylene (iPP)/high density polyethylene (HDPE) blends by melt mixing in a Brabender-like apparatus. Impact Izod tests and a morphological analysis on the fractured surfaces of broken specimens have been performed and discussed, in order to improve the deficiency in toughness of the binary HDPE/iPP mixtures. The results show that the impact performance of both homopolymers and HDPE/iPP binary blends is strongly improved by the addition of the EPM copolymers. Such an effect is related to the fact that the overall morphology, as well as the mechanism and mode of fracture, are greatly modified by the presence of such additives. The extent is dependent on factors such as the nature of the matrix (HDPE or iPP), the composition, and the chemical structure and/or the molecular mass of the added copolymer.     

The morphology of fracture surfaces and mechanical properties of composites of polypropylene with glass fibers having different interface adhesion

For composites polypropylene–short glass fibers having different interface adhesion, correlation has been proved to exist between the morphology of fracture surfaces, the temperature dependence of impact strength, and the deformational and fracture behavior in tensile loading. The results are interpreted in terms of the mechanism of distortion plasticity for unfilled PP and for filled PP having weak interface adhesion, and on the basis of dilatation plasticity for filled polypropylene with a higher interface adhesion. The transition from the distortion to the dilatation mechanism can be seen in fracture surfaces after tensile destruction in composites possessing a higher interface adhesion.     

Structure and properties of PP/POE/HDPE blends

This work was aimed to counteract the effect of ethylene‐α‐olefin copolymers (POE) by reinforcing the polypropylene (PP)/POE blends with high density polyethylene (HDPE) particles and, thus, achieved a balance between toughness and strength for the PP/POE/HDPE blends. The results showed that addition of HDPE resulted in an increasing wide stress plateau and more ductile fracture behavior. With the increase of HDPE content, the elongation at break of the blends increased rapidly without obvious decrease of yield strength and Young's modulus, and the notched izod impact strength of the blends can reach as high as 63 kJ/m² at 20 wt % HDPE loading. The storage modulus of PP blends increased and the glass transition temperature of each component of the blends shifted close to each other when HDPE was added. The crystallization of HDPE phase led to an increase of the total crystallinity of the blend. With increasing HDPE content, the dispersed POE particle size was obviously decreased, and the interparticle distance was effectively reduced and the blend rearranged into much more and obvious core‐shell structure. The fracture surface also changed from irregular striation to the regularly distant striations, displaying much obvious character of tough fracture. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Morphology and mechanical properties of biaxially oriented films of polypropylene and HDPE blends

Biaxially oriented films of blends of high-density polyethylene (HDPE) with polypropylene (PP) homopolymer and PP copolymers prepared by twin-screw extrusion and lab-stretcher have been investigated by scanning electron microscopy (SEM), polarized microscopy, differential-scanning calorimeter, and universal testing machine. Three different kinds of PP copolymers were used: (i) ethylene–propylene (EP) random copolymer; (ii) ethylene–propylene (EP) block copolymer; (iii) ethylene–propylene–buttylene (EPB) terpolymer. In the SEM study of the morphology of films of HDPE with various PP blends, phase separation is observed between the PP phase and the HDPE phase for all blends and compositions. In all blends, HDPE serves to reduce the average spherulites size, probably acting as a nucleating agent for PP. The reduction of spherulite size appeared most significantly in the blend of EPB terpolymer and HDPE. A large increase of crystallization temperature was found in the blend of EPB terpolymer and HDPE compared with the unblended EPB terpolymer. For the blend of EPB terpolymer and HDPE, the improvement of tensile strength and modulus is observed with an increase of HDPE content, and this can be considered as a result of the role of HDPE in reducing average spherulite size. © 1994 John Wiley & Sons, Inc.     

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.     

Enhanced interfacial adhesion between polypropylene and nylon 6 by in situ reactive compatibilization

We used in situ reactive compatibilization to investigate the interfacial reinforcement between polypropylene (PP) and nylon 6 (Ny6). A certain amount of maleic anhydride grafted polypropylene (MAPP) was pre-blended with pure PP to form in situ a copolymer with Ny6. The fracture toughness was measured using an asymmetric double cantilever beam test (ADCB). An analysis of the locus of failure revealed that at constant bonding temperature, the fracture toughness between PP and Ny6 was influenced not only by the bonding temperature but also by the bonding time. The fracture toughness increased with the bonding temperature until 220 °C, and then decreased at higher bonding temperatures, which could be explained by the progressive occurrence of two different failure mechanisms, first adhesive failure at the interface and later cohesive failure between chains. X-ray diffraction measurements on specimens prepared at bonding temperatures of 210, 220, and 230 °C revealed no identifiably different crystalline PP phases. The fracture toughness increased with the annealing time, passed a peak, and then reached a plateau. The dependence of the fracture toughness on the bonding time could also be explained in terms of the two fracture mechanisms. X-ray photoelectron spectroscopy (XPS) results corroborated these explanations.     

Direct Correlation Between Adhesion Promotion and Coupling Reaction at Immiscible Polymer-Polymer Interfaces

Hugh Brown has shown that interfacial entanglements govern adhesion between two polymers. We demonstrate this for three systems by adding interfacial chains via chemical coupling. The adhesion between polypropylene (PP)/amorphous polyamide (aPA) was reinforced by the coupling reaction of maleic anhydride grafted PP (PP-g-MA) and the primary amine groups on aPA; huge increases in adhesion were observed. A good correlation between critical fracture toughness, G_c, and PP-g-MA concentration squared follows Brown's crazing mechanism. For a polystyrene (PS)/aPA interface reinforced by the coupling reaction of poly(styrene-r-maleic anhydride) (PS-r-MA)/aPA only modest adhesion increases in G_c were observed through the whole PS-r-MA concentration range. This different behavior of G_c vs. functional polymer concentration is believed to be caused by segregation of the formed graft copolymers at the interface. The relationship between G_c and the extent of coupling was studied quantitatively with a model PS/PMMA system. The interface was reinforced by the coupling reaction of 0-10% PS-NH₂/PMMA-anh. G_c was measured with the asymmetric dual cantilever beam test (ADCB) and the amount of copolymer formed at the interface was determined by a fluorescence labeling technique. G_c is low and is linear in block copolymer interfacial coverage (Σ), indicating a chain scission mechanism. Reasonable agreement was achieved between experiment and theoretical prediction based on the energy to break C-C bonds.     

Adhesion control of interface between cellulose and polypropylene by vapor‐phase assisted surface copolymerization

In order to achieve the effective interface bonding between biomass microfiller and commodity plastics, consecutive copolymerization of hydrophilic acrylic acid (AA) and hydrophobic butyl acrylate (BA) using vapor‐phase assisted surface polymerization (VASP) technology was applied to prepare microcomposites consisting of cellulose microcrystal (CμC) and polypropylene (PP). After the copolymerization by VASP, CμC surfaces were covered by accumulated polymers: P(AA‐co‐BA) including block‐type copolymer and homopolymers of 6.2–25.3 wt % versus CμC. Although structures of the products were unspecified, it was expected to be mixtures of block copolymers and homopolymers. Subsequently prepared P(AA‐co‐BA) on CμC/PP (5/95 wt/wt) composites expressed a superior mechanical toughness, which had increased threefold when compared to intact CμC/PP composite. This increase in toughness was mainly based on an increase in elongation rate, reflecting improvement of the adhesion strength at the interface between CμC surface and PP. The trace amounts: 0.31 wt % of accumulated P(AA‐co‐BA) on CμC surface must function as an effective adhesive/compatibilizer at the interface. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45647.     

Influence of a copolymer on the mechanical properties of a blend of PP and recycled and non-recycled HDPE

Summary The mechanical and morphological behavior of polypropylene (PP) and high-density polyethylene (HDPE), recycled and non-recycled, by the addition of an ethylene-propylene block copolymer, was studied. In the non-recycled samples, the effect of the copolymer on the mechanical and morphological properties is negligible. In the recycled samples, it was shown that with 5% copolymer, both particle size in the dispersed phase and interface thickness decreased, and that 5% copolymer composition is optimal in improving the adhesion and flexibility of the blend. Received: 23 May 1997/Revised version: 5 May 1998/Accepted: 14 May 1998     

Effect of the tie‐layer thickness on the delamination and tensile properties of polypropylene/tie‐layer/nylon 6 multilayers

This study examined the effect of the tie‐layer thickness on the delamination behavior of polypropylene/tie‐layer/nylon 6 multilayers. Various maleated polypropylene resins were compared for their effectiveness as tie‐layers. Delamination failure occurred cohesively in all the multilayer systems. Two adhesion regimes were defined according to the change in the slope of the linear relationship between the delamination toughness and the tie‐layer thickness. The measured delamination toughness of the various tie‐layers was quantitatively correlated to the length of the damage zone that formed at the crack tip. In addition, the effect of the tie‐layer thickness on the multilayer tensile properties was correlated with the delamination behavior. The fracture strain of the multilayers decreased with decreasing tie‐layer thickness. An examination of the prefracture damage mechanism of the stretched multilayers revealed a good correlation with the delamination toughness of the tie‐layers. In thick tie‐layers (>2 μm), the delamination toughness was great enough to prevent the delamination of the multilayers when they were stretched. In thin tie‐layers (<2 μm), the delamination toughness of all the tie‐layers was low; consequently, delamination led to premature fracture in the stretched multilayers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Phase morphology and impact toughness of impact polypropylene copolymer

Factors influencing the impact toughness of two impact polypropylene copolymers (IPC) with almost the same ethylene content, molecular weight and molecular weight distribution were studied by temperature gradient extraction fractionation (TGEF), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). The results indicate that poor interfacial adhesion between the disperse phase and the continuous matrix, larger dimensions and non-uniform distribution of disperse phases are main reasons for the low impact toughness of IPC B that possesses of a low content of ethylene-propylene segmented copolymer with long crystallizable PE and PP sequences as a compatibilizer between the disperse phase and the matrix.     

Puncture deformation and fracture mechanism of oriented polymers

In this study, we investigated the effect of orientation by solid‐state cross‐rolling on the morphology, puncture deformation, and fracture mechanism of an amorphous TROGAMID material and three semicrystalline polymers: high‐density polyethylene (HDPE), polypropylene (PP), and nylon 6/6. In amorphous TROGAMID, it was found that orientation preferentially aligned polymer chains along the rolling deformation direction and reduced the plastic deformation of TROGAMID in a low‐temperature puncture test. The decrease of ductility with orientation changed the fracture mechanism of TROGAMID from ductile hole enlargement failure in the unoriented control to a more brittle delamination failure in TROGAMID cross‐rolled to a 75% thickness reduction. For semicrystalline polymers HDPE, PP, and nylon 6/6, the randomly oriented crystalline lamellae in the controls were first oriented into an oblique angle to the rolling direction (RD) before the lamellae became fragmented and preferentially oriented with the chain axis parallel to the RD. The morphological change resulted in the decrease of ductility in HDPE in the low‐temperature puncture test. In PP and nylon 6/6, the brittle fracture of unoriented controls was changed into ductile failure when they were cross‐rolled to a 50% thickness reduction. This was attributed to the tilted crystal lamellae morphology, which permitted chain slip deformation of crystals with the chain axis parallel to the maximum shear stress direction. With further orientation of PP and nylon 6/6 to a 75% thickness reduction, the failure mechanism changed back to brittle fracture as the morphology transformed into a layered discoid structure with the chain axis of the fragmented crystal blocks parallel to the RD; this prevented chain slip deformation of the crystals. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012