Influence of Processing Additives on Adhesive Properties of Surface‐Modified Low‐Density Polyethylene

Summary: The paper deals with the surface and adhesive properties of low‐density polyethylene modified by corona discharge, which appear during the long‐term hydrophobic recovery of the modified polymer. The study was aimed at the change in polarity during aging of low‐density polyethylene modified by corona discharge reducing the surface free energy, its polar component and the mechanical work of adhesion. During the long‐term hydrophobization of low‐density polyethylene the main decrease of the surface properties appeared within the first 30 d after modification. In the course of further aging the hydrophobic recovery of the polymer proceeded more slowly. It has been found that the value of the surface and adhesive properties of low‐density polyethylene after modification with corona discharge as well as the dynamics of their decrease during the aging is to a great extent dependent on the presence of the processing additives in the polymer.

Mechanical work of adhesion of LDPE modified by corona discharge to poly(vinyl acetate) during hydrophobic recovery: a) additive‐free LDPE, b) LDPE with additives.     

Surface modification of polyethylene by diffuse barrier discharge plasma

Low‐density polyethylene (LDPE) modified by atmospheric dielectric surface barrier discharge plasma in oxygen was investigated to improve surface properties and adhesion of LDPE to more polar polymers. The process of plasma modification was investigated using several methods—surface energy measurements, Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance (FTIR‐ATR), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM). The surface energy of LDPE increased significantly after activation by oxygen barrier plasma even at very short time of modification. The FTIR‐ATR spectra manifested the presence of carbonyl functional groups on the surface of polymer pre‐treated by oxygen barrier plasma. It was shown by SEM, and AFM, that the topography of modified LDPE was significantly changed and the surface of modified polymer exhibited higher roughness in comparison with unmodified polymer. The surface energy of treated LDPE diminished in the course of ageing especially during the first 10 days after modification by barrier plasma. Hydrophilicity of the modified LDPE surface was stabilized by photochemical post‐functionalization with 2,2,6,6‐tetramethylpiperidin‐4‐yl‐diazoacetate. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Surface modification of low‐density polyethylene films by UV‐induced graft copolymerization with a fluorescent monomer

Surface modification of argon plasma–pretreated low‐density polyethylene (LDPE) film via UV‐induced graft copolymerization with a fluorescent monomer, (pyrenyl)methyl methacrylate (Py)MMA, was carried out. The chemical composition and morphology of the (Py)MMA‐graft‐copolymerized LDPE [(Py)MMA‐g‐LDPE] surfaces were characterized, respectively, by X‐ray photoelectron spectroscopy (XPS) and by atomic force microscopy (AFM). The concentration of the surface‐grafted (Py)MMA polymer increased with Ar plasma pretreatment time and UV graft copolymerization time. The photophysical properties of the (Py)MMA‐g‐LDPE surfaces were measured by fluorescence spectroscopy. After graft copolymerization with the fluorescent monomer, the surface of the LDPE film was found to have incorporated new and unique functionalities. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1526–1534, 2001     

Evaluation of polymer hydrophobic recovery behavior following H2O plasma processing

Inductively coupled radio frequency (rf) H₂O vapor plasma was used to modify a range of polymers to better elucidate the dependence of hydrophobic recovery on polymer composition and structure. Freshly modified and aged samples were examined using scanning electron microscopy, (SEM), X‐ray photoelectron spectroscopy, (XPS), and water contact angle (wCA) goniometry. Initially, wettability was increased on each polymer surface due to the implantation of polar oxide groups, an effect exacerbated by increased surface roughness with plasma treatment. Polypropylene and polystyrene exhibit nearly complete hydrophobic recovery as polar groups are subsumed as samples age. High‐density polyethylene and polycarbonate exhibit minimal hydrophobic recovery, owing to plasma‐induced cross‐linking and intrinsic thermal stability, respectively. Overall, the hydrophobic performance following H₂O plasma modification is similar to other oxidizing plasmas, suggesting that recovery behavior is intrinsic to the polymer. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41978.     

Investigation of crystallinity effects on the surface of oxygen plasma treated low density polyethylene using X-ray photoelectron spectroscopy

Plasma treated polymer surface shows rapid hydrophobic recovery after plasma treatment. Surface rearrangement and the hydrophilicity change of low density polyethylene (LDPE) films with different degrees of crystallinity after oxygen plasma treatment were investigated using X-ray photoelectron spectroscopy (XPS) and water contact angle measurements. Since the mobility of polymer chain was restricted in the crystalline region, the chain movement of high crystalline polymer would be reduced. Water contact angles of quenched and annealed LDPE just after plasma treatment were almost same and oxygen contents of quenched LDPE was slightly higher than that of annealed LDPE. However, annealed LDPE showed slower contact angle raise and kept higher oxygen concentration than quenched LDPE with increasing aging time. By deconvolution of carbon XPS spectra, it was observed that major group contributed to decrease of oxygen contents was the hydroxyl groups and hydroxyl groups of quenched LDPE decayed faster than annealed LDPE while other oxygen species kept constant or decreased slightly. Increasing crystallinity reduce the surface rearrangement of polymer chain after plasma treatment.     

Simple route for the preparation of graphene/poly(styrene‐b‐butadiene‐b‐styrene) nanocomposite films with enhanced electrical conductivity and hydrophobicity

This paper reports a simple route for the preparation of graphene/poly(styrene‐b‐butadiene‐b‐styrene) (SBS) nanocomposite films employing a vacuum filtration method. Graphene is exfoliated well by an electrochemical procedure and homogeneously dispersed in the polymer matrix. The prepared nanocomposite films were characterized by XRD, Fourier transform IR (FTIR) spectroscopy, X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy, AFM and SEM. Morphological studies showed that graphene formed a smooth coating over the surface of SBS. The increase in graphene concentration induces the wrinkling of graphene sheets at the composite surface which causes a further increase in surface roughness. The FTIR, Raman and XPS spectra of graphene/SBS nanocomposite films indicate the strong interactions between graphene and the polymer matrix. According to the XRD patterns, introducing SBS into graphene did not modify the graphene structure additionally, i.e. the crystal lattice parameters do not depend on SBS content in graphene/SBS nanocomposite films. The graphene/SBS nanocomposite films also exhibited better hydrophobicity due to the increased surface roughness and lower sheet resistivity (reduced 10 times) compared to exfoliated graphene. © 2018 Society of Chemical Industry     

Surface degradation and hydrophobic recovery of polyolefins treated by air corona and nitrogen atmospheric pressure glow discharge

The surface degradation and production of low molecular weight oxidized materials (LMWOM) on biaxially oriented polypropylene (BOPP) and low‐density polyethylene (LDPE) films was investigated and compared for two different dielectric barrier discharge (DBD) treatment types, namely air corona and nitrogen atmospheric pressure glow discharge (N₂ APGD). Contact angle measurements, X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) analyses were performed in conjunction with rinsing the treated films in water. It is shown that N₂ APGD treatments of both polyolefins result in much less surface degradation, therefore, allowing for a significantly higher degree of functionalization and better wettability. Hydrophobic recovery of the treated films has also been studied by monitoring their surface energy (γ_s) over a period of time extending up to several months after treatment. Following both surface modification techniques, the treated polyolefin films were both found to undergo hydrophobic recovery; however, for N₂ APGD modified surfaces, γ_s ceases to decrease after a few days and attains a higher stable value than in the case of air corona treated films. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1291–1303, 2004     

Adhesion of extrusion‐coated polymer sealing layers to a fiber‐based packaging material with an atomic layer deposited aluminum oxide surface coating

Adhesion of extrusion‐coated polymer sealing layers on an atomic layer deposited (ALD) aluminum oxide (Al₂O₃) surface coating was investigated with a view to gain information on the applicability of ALD deposited barrier layers in fiber‐based packaging materials. The polymers used for the sealing layer were low density polyethylene (LDPE), polyethylene terephthalate (PET), and polylactide (PLA). They were extrusion‐coated onto the ceramic side of paper/PET/Al₂O₃ substrates, where the Al₂O₃ layer was a few tens of nanometers thick. According to the results, good adhesion was obtained for LDPE coating, whereas the other coatings showed a considerable lack of adhesion. Presumably, the oxidation faced by LDPE in the air gap of the extrusion‐coating process was able to create an extensive number of reactive sites that strongly bonded with the hydroxyl groups on the oxide surface of the substrate. With the PET and PLA coatings, such oxidation did not occur and the adhesion obtained remained at a relatively poor level. With all of the coatings, the adhesion levels were improved using corona discharge equipment as a pretreatment prior to the extrusion‐coating process. POLYM. ENG. SCI., 52:1985–1990, 2012. © 2012 Society of Plastics Engineers     

Surface modification of polysulfone membranes by low‐temperature plasma–graft poly(ethylene glycol) onto polysulfone membranes

A novel and general method of modifying hydrophobic polysulfone (PSF) to produce highly hydrophilic surfaces was developed. This method is the low‐temperature plasma technique. Graft polymer‐modified surfaces were characterized with the help of Fourier transform infrared attenuated total reflection (FTIR–ATR) and X‐ray photoelectron spectroscopy (XPS). Study results demonstrated that poly(ethylene glycol) (PEG) could be grafted onto the PSF membrane surface by low‐temperature plasma. The hydrophilic character of the modified surfaces was increased in comparison with that of the parent membrane. The contact angle for a modified PSF membrane was reduced apparently. We analyzed the effectiveness of this approach as a function of plasma operating variables including plasma treatment power and treatment time. Hence, plasma‐induced graft polymer modification of membranes can be used to adjust membrane performance by simultaneously controlling the surface hydrophilicity and hemocompatibility. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 979–985, 2000     

Effects of plasma treatment on greenhouse polyethylene cover aged under sub‐Saharan conditions

The effects of natural ageing, in sub‐Saharan regions, on treated low density polyethylene by cold plasma are presented in this work. The data analysis is performed by comparing the changes of physical, mechanical and structural effects on the material surface induced by plasma treatment with regard to the natural abrasion effect due to sand wind. Plasma treatments for short durations are applied to neutral LDPE (low density polyethylene) films. Few seconds suffice to observe chemical changes on the samples. The treated samples are then subjected to natural ageing in the region of Ouargla (south of Algeria), characterized by very frequent sand wind. The characterization of the effects of plasma treatment and its combination with sand wind is performed using XPS (X‐ray Photoelectron Spectroscopy), FTIR (Fourier Transform Infrared spectrometry), and mechanical tensile testing. The results led to conclude a synthesis of new materials on the surface that significantly change the material surface properties and physico‐chemical properties of material, more particularly, optical, and mechanical properties are thus strongly affected. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Effects of acid treatment at different temperatures on the surface dielectric properties of low‐density polyethylene

The surface dielectric properties of acid‐etched low‐density polyethylene (LDPE) were analysed in the frequency range from 20 Hz to 200 kHz. Samples were treated with various acids for a period of one hour, at temperatures ranging from 20 to 70 °C. After the treatment, the samples were analysed with Fourier transform infrared spectroscopy, revealing chemical and crystallinity changes on the surface, as a direct result of the treatment. The sample surfaces were analysed using atomic force microscopy. The micrographs show that the acid treatment increases the roughness of the samples. Compared to untreated LDPE, the etched samples may exhibit significantly different conductance values at low frequencies. It is also observed that an increase in the acid treatment temperature can result in lower values of conductance and susceptance compared to untreated samples. LDPE films with low value of surface AC conductivity after acid treatment are potentially useful substrates for high‐speed electro‐sensing applications. The presented results indicate that a suitable choice of acid treatment of LDPE can effect surface polarization while preserving low values of surface AC conductivity of the polymer. © 2014 Society of Chemical Industry     

Phase hold-up and critical fluidization velocity in a three-phase inverse fluidized bed

We studied the hydrodynamic characteristics of a three-phase inverse fluidized bed made of a transparent acrylic column of 0.115 m inner diameter and 2 m heights. Air, water and polyethylene particles were used as the gas, liquid and solid phase, respectively. We used both hydrophobic low density polyethylene (LDPE) and hydrophilic LDPE as solid phase, and distilled water as liquid phase, and filtered air as gas phase. The LDPE was chemically treated by chlorosulfonic acid to change the surface property from hydrophobic to hydrophilic. We tried to solely investigate the effect of the surface hydrophilicity of polymeric particles on the phase holdup and the critical fluidization velocity of three-phase inverse fluidization. Thus, we measured the static pressure and eventually observed critical fluidization velocity. Critical fluidization velocity became smaller in case of using MDPE hydrophobic particles than LDPE hydrophilic particles. This was thought to be due to the retardation of rising bubbles near hydrophobic particles and, subsequently, the increase of gas hold-up.     

Controlled gas‐phase sulfonation of low‐density polyethylene films

In the current research, a highly controllable system operating at low temperatures and for short reaction times is employed for the surface sulfonation of low‐density polyethylene. This system provides the advantages of short reaction times and low reaction temperatures, as compared with previous methods of surface sulfonation. Low‐density polyethylene films were sulfonated at 40°C for time periods ranging from 5 to 30 min. Subsequently, all films were analyzed by SEM, EDX, horizontal ATR–FTIR, surface roughness, and dynamic contact‐angle measurements. Sulfonation was effected at all reaction times. The degree of surface sulfonation increased through 10 min and reached a maximum between 10‐ and 30‐min reaction times with concomitant changes in the physicochemical properties of the material. At 30 min, the film topography changed substantially, indicating that sulfonation was no longer limited to a strictly surface reaction. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1865–1869, 2000     

Surface modification of LDPE with PE‐PEO graft copolymer

Films of LDPE containing 1–10 wt % of various polymeric additives were prepared by different techniques. Three poly(ethylene‐graft‐ethylene oxide)s synthesized by grafting poly(ethylene‐co‐acrylic acid) with poly(ethylene oxide) monomethyl ether (MPEO), and two pure MPEOs having molecular weights 750 and 2000 were used as additives. The additives were mixed with LDPE both by blending in a common solvent and by melt mixing. The blends were then solvent cast from xylene onto glass Petri dishes or compression molded between glass plates. The film surfaces were studied by water contact angle measurements and by X‐ray photoelectron spectroscopy (XPS), and melting points and heats of melting were recorded by differential scanning calorimetry (DSC). The blends had a two‐phase morphology, with enrichment of the graft copolymers at the glass–polymer interface, as shown by contact angle values and XPS spectra. Large differences in the interface accumulation between the different film samples were observed. Films prepared by compression molding of solution‐mixed blends exhibited much lower surface accumulation of graft copolymer at the glass–polymer interface than did the solvent cast or melt‐mixed/compression‐molded samples. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 316–326, 2000     

Surface Silanization of Polyethylene for Enhanced Adhesion

Low density polyethylene has been treated using a novel surface treatment process “SICOR” (“SIIane-on-CORona” treated polymer) in order to enhance adhesion with a range of adhesives including polyure-thane, methacrylate and cyanoacrylate. The process comprises two steps, i.e corona discharge followed by application of an organo-functional silane. The incorporation of surface hydroxyl groups onto the polymer surface enables organo-silane to create the hydrogen or covalent bonds with the oxidized polymer surface. The possibility of the creation of these bonds has been investigated using FTIR, XPS and wettability studies. The adhesion enhancement due to the new process is significant. Frequently, the strength increase exceeds 200% compared with the corona discharge treatment and more than 300% compared with LDPE priming using the “Loctite 770” polyolefin primer. The process is shown to be as good as, or better than, plasma treatment in terms of the strength increase following substrate treatment prior to adhesive bonding.     

Characterization of surface phenomena: probing early stage degradation of low‐density polyethylene films

Photo‐oxidative degradation of polyethylene triggers significant deterioration in the polymer properties. Much interest is aimed at characterizing and possibly predicting photo‐oxidative damages, at early stages, prior to the occurrence of profound changes in mechanical properties. Herein, we study the degradation of low‐density polyethylene (LDPE) films, focusing on surface deterioration processes. Thin films of various molecular weights are exposed to accelerated weathering while their chemical, mechanical and morphological characteristics are monitored throughout by apparent contact angle (CA) measurements, Fourier‐transform infrared spectroscopy (FTIR), tensile test, and electron microscopy. A significant decrease in the films' CA during degradation is observed. CA is highly sensitive to two simultaneous phenomena with opposing effect: nanoscale surface roughening and composition changes. We found the latter (specifically the evolution of polar groups) to be the dominant parameter affecting the CA behavior. Consequently, simple CA measurements coincide well with conventional FTIR analysis, and are more sensitive to changes occurring at early stages of degradation. The deterioration in the mechanical properties is also characterized and is found to present poor sensitivity and high variability at these early aging stages. Thus, simple CA measurements could potentially be used as a qualitative indicator for evaluating the aging impact on LDPE. POLYM. ENG. SCI., 59:E129–E137, 2019. © 2018 Society of Plastics Engineers     

Surface modification of ultra‐high‐molecular‐weight polyethylene. I. Characterization and sintering studies

We performed surface modification of ultra‐high‐molecular‐weight polyethylene (UHMWPE) through chromic acid etching with the aim of improving the performance of UHMWPE's composites with poly(ethylene terephthalate) fibers. In part I of this study, we evaluated the effects of chemical modification on the surface properties of UHMWPE with X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and contact angle measurements. The thermal properties, rheology, and sintering behavior of the modified UHMWPE were compared to those of the base material. XPS and FTIR analysis confirmed the presence of carboxyl and hydroxyl groups on the surface of the modified powders. The substitution of polar groups into the backbone of the polymer decreased its contact angles with water and hexadecane and increased its surface energy, as evidenced by contact angle measurements. The modified UHMWPE was more crystalline than the base resin and less prone to thermal degradation. Although the rheological properties were virtually identical, the modified powders sintered more readily, presumably due to their higher surface energy, which suggested enhanced processability by compression molding. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Formation and characterization of polyethylene blends for autoclave‐based expanded‐bead foams

Blends of linear‐low‐density polyethylene (LLDPE), low‐density polyethylene (LDPE), and high‐ density polyethylene (HDPE) were foamed and characterized in this research. The goal was to generate clear dual peaks from the expanded polyethylene (EPE) foam beads made from these blends in autoclave processing. Three blends were prepared in a twin‐screw mixing extruder at two rotational speeds of 5 and 50 rpm: Blend1 (LLDPE with 20 wt% HDPE), Blend 2 (LLDPE with 20 wt% LDPE), and Blend 3 (LLDPE with 10 wt% HDPE and 10 wt% LDPE). The differential scanning calorimetric (DSC) measurement was taken at two cooling rates: 5 and 50°C/min. Although no dual peaks were present, the results showed that blending with HDPE has a more noticeable effect on the DSC curve of LLDPE than blending with LDPE. Also, the rotational speed and cooling rate affected the shape of the DSC curves and the percentage area below the onset point. The DSC characterization of the batch foamed blends revealed multiple peaks at certain temperatures, which may be mainly due to the annealing effect during the gas saturation process. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Crystallization of low‐density polyethylene‐ and linear low‐density polyethylene‐rich blends

The crystallization of a series of low‐density polyethylene (LDPE)‐ and linear low‐density polyethylene (LLDPE)‐rich blends was examined using differential scanning calorimetry (DSC). DSC analysis after continuous slow cooling showed a broadening of the LLDPE melt peak and subsequent increase in the area of a second lower‐temperature peak with increasing concentration of LDPE. Melt endotherms following stepwise crystallization (thermal fractionation) detailed the effect of the addition of LDPE to LLDPE, showing a nonlinear broadening in the melting distribution of lamellae, across the temperature range 80–140°C, with increasing concentration of LDPE. An increase in the population of crystallites melting in the region between 110 and 120°C, a region where as a pure component LDPE does not melt, was observed. A decrease in the crystallite population over the temperature range where LDPE exhibits its primary melting peaks (90–110°C) was noted, indicating that a proportion of the lamellae in this temperature range (attributed to either LDPE or LLDPE) were shifted to a higher melt temperature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1009–1016, 2000     

The role of surface modification in digital printing on polymer‐coated packaging boards

Digital printing is increasingly being used for package printing. One of the major techniques of digital printing is dry‐toner electrophotography. This paper evaluates the printability of three different extrusion coatings used for packaging boards: low‐density polyethylene (PE‐LD), ethylene methyl acrylate (E/MA) and polyethylene terephthalate (PET). Extrusion coatings in general have an impervious, chemically inert, nonporous surface with low surface energies that cause them to be non‐receptive to bonding with toners. The most common methods used in improving the adhesion properties of polymer coatings are different surface treatments. These increase the surface energy and also provide the polar molecular groups necessary for good bonds between the toner and polymer molecules. The polymer coatings have been modified with electrical corona discharge treatment. The effects of corona on polymer surfaces and the correlation between surface modification and print quality have been evaluated. Results show that sufficiently high surface energy and surface‐charge uniformity are necessary for even print quality and toner adhesion. E/MA and PET have the required surface‐energy level without the corona treatment, but PE‐LD needs surface modification in order to succeed in the electrophotographic process. E/MA also has exceptional surface‐charge properties compared with PET and PE‐LD. Polym. Eng. Sci. 44:2052–2060, 2004. © 2004 Society of Plastics Engineers.