Adhesion Improvement by UV Grafting onto Polyolefin Surfaces

High Density Poly(ethylene) (HDPE) and Poly(propylene) (PP) were subjected to several surface treatments, namely UV grafting of hydroxyethylmethacrylate (HEMA), plasma deposition of HEMA and oxygen plasma treatment. Treated surfaces were subjected to two post-treatment routines (extraction with ethanol and high temperature aging). The effect of these treatments on the adhesion of HDPE and PP to epoxy coated studs was evaluated by a pull test. No adhesion at all was recorded on untreated samples. On the other hand, all the treatments yield high bond strength in the case of HDPE: an average bond strength of about 290 kg/cm² and of about 200 kg/cm² was observed after UV grafting and plasma treatments. The treated samples were practically insensitive to post-treatments. As to PP, which undergoes chain scission in plasma, it is best treated by the comparatively milder conditions of UV grafting, which yields an average bond strength similar to that observed on HDPE. O₂ and HEMA-plasma-treated PP show a mean bond strength close to 50 kg/cm², and are deeply affected by the post-treatment routines.     

Development and characterization of an all‐olefin thermoplastic sandwich composite system

In this investigation an all‐olefin thermoplastic sandwich system 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. Single lap 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 interface strength. EPC tie layer provided higher bond strength (27.4 kg/cm²) to the all‐olefin sandwich system than HDPE/elastomer blend based one (19.7 kg/cm²). For EPC tie layer based sandwiches, a mixed mode a failure was observed in the failed lap shear samples; about 40% is cohesive failure through tie layer, and the rest of failure was adhesive either at PP composite or PE surfaces. Environmental scanning electron micrographs (ESEM) reveal that in the process of surface fusion bonding, PE foam cells in the vicinity of 0.80 mm interphase area were coalesced with high temperature and pressure. No macro level penetration of tie layer melt front into foam cells was observed. As the surface morphology of foam was altered on account of 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 surface.     

Peel strength improvement of silicone rubber film by plasma pretreatment followed by graft copolymerization

Here we discuss the improvement in the peel strength of silicone rubber film by O₂ plasma pretreatment followed by grafting with hydrophilic monomers: acrylamide (AAm) and acrylic acid (AA). The peroxides concentration after O₂ plasma treatment was determined by the 1,1-diphenyl-2-picrylhydrazyl (DPPH) method. ESCA analysis was carried out to confirm the existence of AAm. The peroxides concentration and hence the peel strength increased with increasing plasma treatment power and time, reached a maximum value, and then decreased with further increasing plasma treatment power and time. Peel strength of the silicone film with 3M-600 tape was observed to increase with grafting time; however, it was found to decrease with overgrafting. The maximum peel strength of 384.4 g/cm was found for the 20 W, 10 min plasma treated, AAm grafted film with maximum peroxides concentration of 4.86 x 10^-9 mol/cm² and also with maximum nitrogen-to-carbon ratio (N/C) of 0.247. Hydrolysis experiments show that -NH₂ provides higher contribution to adhesion than -COOH does and the grafting degree of AA is lower than that of AAm. The relationship between the degree of grafting and peel strength can be well explained by the mechanical interlocking theory of adhesion.     

Surface modification of polypropylene sheets by UV‐radiation grafting polymerization

1,6‐Hexanediol diacrylate (HDDA) was grafted onto polypropylene (PP) substrates in the presence of benzophenone (BP) and isopropylthioxanthone (ITX) photoinitiators, and then polyurethane acrylate formulations were coated onto the HDDA‐g‐PP substrates, using UV radiation. The amount grafted and the grafting efficiency of the polymerizations were determined gravimetrically. The effects of the photoinitiator concentration and the UV radiation intensity on the physicochemical surface properties and the grafting efficiency of the UV‐radiation grafting polymerizations were characterized in detail using contact‐angle measurements, Fourier transform infrared spectroscopy with attenuated total internal reflection, and scanning electron microscopy. The results showed that the amount grafted and the surface polarity of the HDDA‐g‐PP substrates both increased linearly with increasing BP photoinitiator concentration and UV radiation intensity, and that the addition of a small amount of ITX markedly enhanced both parameters, probably due to photosensitization. The adhesion of the UV‐cured coating onto the HDDA‐g‐PP substrates was evaluated using the crosshatch adhesion test. The results indicated that the amount of HDDA grafted onto the PP substrates should exceed about 1 mmol/cm² for satisfactory adhesion with the UV‐cured coating. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1446–1461, 2006     

Metallization of photosensitive epoxy

The adhesion strength of various photosensitive epoxies with copper or FR4 substrate was measured by the stud pull testing method. The adhesion strength was around 35–60 kg/cm² for photosensitive epoxy/copper laminate and 60–105 kg/cm² for photosensitive epoxy/FR-4 laminate. A high adhesion strength of metal/photosensitive epoxy can be obtained when the surface of photosensitive epoxy is subjected to 1 minute of microetching in the processes of metallization. Under such a processing condition, the adhesion strength of copper/photosensitive epoxy for Taiyo PVI-500/SA-50 can be as high as 145 kg/cm², and the adhesion strength remained higher than 145 kg/cm² after the solder float test. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1639–1645, 1998     

Plasma‐induced grafting of hydroxyethyl methacrylate (HEMA) onto chitosan membranes by a swelling method

Hydroxyethyl methacrylate (HEMA) was grafted onto chitosan membranes by plasma‐graft polymerization. Effects of monomer concentration, plasma power and plasma time on the amount of grafting were investigated. The results showed that there were two processes: grafting polymerization and etching of the membrane. The surface of the grafted membrane was evaluated by FTIR. Scanning electron microscopy indicated that the surface morphology of the grafted membrane could be adjusted through plasma power. Water contact angles of the chitosan surface decreased from 78.2° to 45.4° while the amount of grafting increased from 0 to 12.2%, indicating improved hydrophilicity of the membrane surface. Permeation coefficients through the original membrane, the membrane treated at 55 W for 15 min, and the membrane treated at 55 W for 30 min for creatinine were 9.12 × 10^?7, 10.6 × 10^?7 and 8.57 × 10^?7 cm² s^?1, respectively. Thermogravimetry and mechanical testing showed that there were no significant changes on the bulk property of chitosan membrane after modification. © 2003 Society of Chemical Industry     

Plasma surface treatment of poly(p-phenylene benzobisthiozol) fibers

Poly(p-phenylene benzobisthiozol) (PBZT) fibers were subjected to radio-frequency (RF)-induced, glow-discharge plasma treatments using argon and carbon dioxide gases in order to modify the adhesion of the fibers to bisphenol-A epoxy. The interfacial shear strength (IFSS) was used as a measure of the adhesion and was determined using the microbond technique. Scanning electron photomicrographs revealed no visible surface etching at magnifications of up to 10 000 x . Slight, but statistically significant, improvements in IFSS were noted with the CO₂ plasma-treated fibers as compared with control fibers, but Ar plasma-treated fibers showed no improvement.     

Surface modification of plasma‐pretreated high density polyethylene films by graft copolymerization for adhesion improvement with evaporated copper

Surface modification of Ar plasma‐pretreated high density polyethylene (HDPE) film via UV‐induced graft copolymerization with glycidyl methacrylate (GMA) and 2‐hydroxyethylacrylate (HEA) was carried out to improve the adhesion with evaporated copper. The surface compositions of the modified HDPE surfaces were characterized by X‐ray photoelectron spectroscopy (XPS). The adhesion strengths of evaporated copper with the graft‐copolymerized HDPE films were affected by the Ar plasma pretreatment time, the monomer concentration used for graft copolymerization, and the graft concentration. Post‐treatments, such as plasma post‐treatments after graft copolymerization and thermal treatment (curing) after metalization, further enhanced the adhesion strength of the Cu/HDPE laminates. The T‐type peel strengths of the laminates involving the graft‐modified and plasma posttreated HDPE films were greater than 15 N/cm. The enhanced adhesion strength resulted from the strong affinity of the graft chains for Cu and the fact that the graft chains were covalently tethered on the HDPE surface. XPS characterization of the delaminated surfaces of the Cu/HDPE laminates revealed that the failure mode of the laminates with T‐peel adhesion strengths greater than 5 N/cm was cohesive in nature.     

Characterization of acrylic acid‐grafted PP membranes prepared by plasma‐induced graft polymerization

Acrylic acid (AA)‐g‐polypropylene (PP) membranes were prepared by grafting AA on to a microporous PP membrane via plasma‐induced graft polymerization. The grafting of AA to the PP membrane was investigated using Fourier transform infrared spectroscopy (FTIR). Pore‐filling of the membranes was confirmed by field emission‐scanning electron microscopy (FESEM) and energy dispersing X‐ray (EDX). Ion exchange capacity (IEC), membrane electric resistance, transport number and water content were measured and analyzed as a function of grafting reaction time. The prepared AA‐g‐PP membranes showed moderate electrochemical properties as a cation‐exchange membrane. In particular, membranes with a degree of grafting of 155% showed good electrical properties, with an IEC of 2.77 mmol/g dry membrane, an electric resistance of 0.4 Ω cm² and a transport number of 0.96. Chronopotentiometric measurements indicated that AA‐g‐PP membranes, with a high IEC had a sufficient conducting region in the membrane. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Surface functionalization of polypropylene nonwovens by metallic deposition

In this study, melt‐blown polypropylene (PP) nonwovens were used as substrates for the metallic deposition of copper. The substrates were pretreated by O₂ plasma, followed by treatments such as sensitization, activation, and reduction. The effects of the copper sulfate concentration, reaction temperature, and plasma power on the conductivity and adhesion strength of the PP nonwovens were investigated after copper deposition. The morphology of the PP nonwovens after copper deposition, analyzed by scanning electron microscopy and atomic force microscopy, revealed that copper nanoclusters were deposited on the fiber surface with a smooth surface morphology and dense structure. X‐ray photoelectron spectroscopy indicated that the copper was present mainly in the form of the elementary substance, which coexisted with a little Cu²⁺. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Adhesion mechanisms of silica layers on plasma-treated polymers. Part I. Polycarbonate

We have studied the influence of plasma treatment in various gases (Ar, NH₃, N₂) on the surface modification of polycarbonate (PC) and on the adhesion of plasma-deposited silica layers to PC. Surface modification was investigated using in situ IR ellipsometry, X-ray photoelectron spectroscopy, and contact angle measurements interpreted in terms of electron-acceptor/ donor groups. In addition, in situ UV-visible ellipsometry enabled crosslinking analysis. Due to UV photon emission, Ar plasma treatment induced crosslinking as well as photo-Fries rearrangements and related reactions, thus creating phenolic groups. These groups are acidic sites and are likely to react with the electronegative oxygen atoms of silica. Adhesion of silica to PC, as measured by the micro-scratch test, was enhanced by Ar plasma treatment. The improvement is attributed to the crosslinking of PC, on the one hand, and to acid-base interactions and/or covalent bonding between PC and silica, on the other hand. A further improvement in adhesion was achieved using N₂ plasma treatments (pure or Ar-diluted). These treatments also induced crosslinking and phenolic group formation and, in addition, nitrogen grafting at the surface. Adhesion enhancement from Ar to N₂ treatments is thus attributed to nitrogen-containing groups, which are likely to promote covalent bonding between silica and the treated polymer surface. In contrast, Ar-diluted NH₃ plasma treatment following Ar treatment resulted in decreased adhesion, which is attributed to reduced acidity and low nitrogen grafting.     

Surface coating protective against oxidative plasma etching of polypropylene

Polypropylene (PP) sheets were coated with the ultrathin polymer layers by plasma polymerization of hexamethyldisiloxane and two other Si-containing monomers, and the protection effects from oxidative plasma etching were investigated. Etching was evaluated by the weight loss of PP sheets after the exposure to an oxidative plasma of O₂ or air. The effects of plasma polymer coating on the etching resistance were investigated with respect to the type of plasma polymer, thickness of a coating layer, oxidative plasma etching conditions, etc. Weight of the coated PP sheets was less changed and the substrates remained stable after a certain period of oxidative plasma treatments, during which time the original PP film had prominently lost weight. The importance of the crosslinked network with —Si— MPO components in plasma polymers on the etching resistance was suggested from the results. Infrared spectra were taken and analyzed with the plasma polymers after O₂-plasma treatments, and the increase in the Si—O structure was indicated by the increase in the peak intensity at 1023 cm⁻¹. Stabilization against oxidative etching was attributed to the crosslinked Si—O structure on the surface layer. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1049–1057, 1997     

Synthesis and characterization of poly(HEMA‐MAA) hydrogel carrier for oral delivery of insulin

Poly(HEMA‐MAA) hydrogel particles were synthesized by redox free‐radical polymerization using 2‐hydroxyethylmethacrylate, different concentration of methacrylic acid as monomer, ethyleneglycol dimethacrylate as crosslinking agent, and APS/TEMED as free‐radical initiator. Fourier transform infrared spectrum of poly(HEMA‐MAA) hydrogels showed intense absorption peak of carbonyl group at ～ 1700 cm^?1 due to carboxylic acid groups of MAA, peak at ～ 2960 cm^?1 due to CH stretching and vinylic peak at 1700 cm^?1 independent of MAA concentration. Highest swelling percentage 587% was observed in case of poly(HEMA‐MAA) hydrogel synthesized using 30% of MAA while lowest swelling percentage 413% was observed in hydrogel synthesized 10% of MAA at basic pH (8.0). Scanning electron micrograph of copolymeric particles showed the irregular shape of poly(HEMA‐MAA) particles with conglomeration with each due to ionization of carboxylic groups. Insulin was radiolabeled using technetium‐99m radionuclide and the radiolabeling efficiency was found to be 99%. Poly(HEMA‐MAA) hydrogel having 60% of MAA showed the highest insulin loading efficiency of 68% while lowest 37% was observed in case of 10% MAA hydrogel. Insulin release studies showed only 35–65% of insulin was released into the medium from particles at pH 2.5 in 60 min, while insulin release was significantly higher at pH 7.4. Hypoglycemic effect of the 60 and 80 I.U./kg insulin dose loaded in poly(HEMA‐MAA) copolymeric particles were carried out in fasted diabetic rats and highest decrease in blood glucose level from 506 mg/dL to 170 mg/dL was observed within first 3 h. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Photochemical surface modification of polypropylene for adhesion enhancement by using an excimer laser

Polypropylene (PP) surface in water was photochemically modified to render it hydrophilic using ArF excimer laser radiation. The chemical stability of PP is attributed to the C-H and C-H₃ bonds present. Thus, it is considered that H atoms are selectively pulled out from the area irradiated with ArF excimer laser light and are replaced with OH functional groups in the presence of water. In this treatment, the irradiated sample becomes hydrophilic with enhanced adhesion properties. The experimental conditions for this surface treatment were ArF laser fluence of 12.5 mJ/cm² and a shot number of 10000. The treated PP and stainless steel were bonded with epoxy adhesive and the tensile shear strength was 46 kg/cm².     

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.     

Electrochemical Pretreatment of Polymers with Dilute Nitric Acid Either Alone or in the Presence of Silver Ions

X-ray Photoelectron spectroscopy (XPS) was used to detect major changes to the surface chemistry of polypropylene (PP), high density polyethylene (HDPE) and styrene-butadiene block copolymer (SBS) caused by electrochemical treatment using dilute nitric acid as electrolyte. Large increases in adhesion levels were observed with PP and HDPE. The method has the potential to be a commercial pretreatment that is inexpensive and safe. Further treatments were carried out in which the complex ion (AgNO₃)⁺ was generated electrochemically. Effective pretreatment was achieved even with dilute solutions (0.001 M w.r.t. silver nitrate). Mechanisms are tentatively proposed for the electrochemical treatment when simply using an electrolyte of dilute nitric acid or where the anolyte consists of a solution of silver nitrate in dilute nitric acid.     

Structure and properties of polypropylene/high‐density polyethylene blends by solid equal channel angular extrusion

The solid equal channel angular extrusion (ECAE) process on polypropylene (PP)/high‐density polyethylene (HDPE) blends was carried out. Scanning electron microscopy (SEM) was used to observe the sample structures. Results showed that ECAE process could make PP/HDPE blends to produce orientation structure. Impact performance of ECAE‐PP/HDPE samples after ECAE process improved remarkably, especially for ECAE‐PP/HDPE (90/10)‐O whose impact strength reached 91.91 kJ/m², 18.1 times higher than that of pure PP and 11.2 times higher than that of PP/HDPE (90/10). The mechanism of enhancing between HDPE and PP was discussed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39759.     

Photografting of argon plasma-treated graphite/PEEK laminate to enhance its adhesion

Graphite/PEEK laminates were treated by argon plasma followed by air aging and then photografting of α- glycidyl ω- acrylate bisphenol A(GABA) to improve their adhesion characteristics. The effects of plasma time and power and photografting time on the epoxy bonded single lap shear joints between graphite/PEEK laminates were investigated. An optimum photografting time was found at which the single lap shear strength was optimized to 37 MPa compared to 28 MPa and 7 MPa obtained with air-aged argon plasma activated and pristine samples, respectively. Argon plasma treatment followed by air aging of graphite/PEEK laminate introduces surface peroxides and hydroperoxides and these when cleaved with ultraviolet (UV) light in the presence of the GABA monomer results in covalent grafting of the latter to PEEK/graphite laminate surface. The epoxy functionality of the GABA monomer then reacts with the epoxy adhesive. X-ray photoelectron spectroscopy (XPS) confirms the appearance of surface peroxides and hydroperoxides on air-aged argon plasma treated samples and disappearance of the same with UV irradiation. With UV irradiation of the air-aged argon plasma treated samples, XPS indicates the appearance of ester groups. Without the grafting monomer, UV irradiation in air cleaves the peroxide and causes oxidation resulting in the formation of surface esters. In the presence of the grafting monomer, UV irradiation results in covalent bonding of the monomer to the peroxide/hydroperoxide through the acrylate functionality resulting in increased concentration of ether linkages as confirmed by our XPS data; the ester functionality present in the grafted monomer caused the appearance of the ester peak in the XPS spectrum.     

Study of multi‐monomer melt‐grafting onto polypropylene in an extruder

Free‐radical melt‐grafting of the dual‐monomer systems glycidyl methacrylate–styrene (GMA‐St) and hydroxyethyl methacrylate–styrene (HEMA‐St) onto polypropylene (PP) has been studied using a single‐screw extruder. For single monomer grafting systems, degradation of PP was unavoidable and deterioration of the mechanical properties of the grafted PP subsequently occurred because of β‐scission of PP chains during the free‐radical melt‐grafting process. However, for the dual‐monomer systems, it is shown that the addition of styrene as a comonomer can significantly enhance the GMA or HEMA grafting levels on PP and reduce the extent of β‐scission of PP backbone. It has been found that the grafting degree of dual‐monomer melt‐grafted PP, such as PP‐g‐(GMA‐co‐St) or PP‐g‐(HEMA‐co‐St), is about quadruple that of single‐monomer grafted PP for the same monomer and dicumyl peroxide concentrations. Moreover, the melt flow rate of the dual‐monomer grafted PP is smaller than that of the unmodified PP. Hence, PP not only was endowed with higher polarity, but also kept its good mechanical properties. © 2000 Society of Chemical Industry     

Increase of gold–Teflon FEP joint strength by electron bombardment

The strength of joints between Teflon FEP (Type A) and 500- to 1000-Å gold layers deposited by evaporation can be greatly increased if the Teflon surface is subjected to electron-beam bombardment prior to the evaporation process. Typically, joint strengths of about 60 kg/cm², approaching the bulk strength of Teflon, are obtained for treatments with electron-beam energies in the range of 5 to 20 keV and intercepted charge densities of about 5 X 10^?6 C/cm². This compares with gold–Teflon joint strengths of about 10 kg/cm² for untreated material. The increase in joint strength is believed to be primarily due to crosslinking caused by the electron bombardment. Compared to the other known treatments to improve gold–Teflon joints, the present method has the advantage that the charge-storage properties of the Teflon are not irreversibly degraded. It is possible, for example, to store charge densities up to 3 X 10^?8 C/cm², on 25-μm films treated with this method, with the same favorable charge-retention properties and thermally stimulated current characteristics as obtained for untreated Teflon.