Application to surface treatments.
In a previous paper (1) the theoretical background on polymer adhesion has been presented. It is interesting to examine how this knowledge can be applied in the course of an assembly process.
In practice, each assembly case is specific. However, the principal steps prior to the realization of a bonded joint can be presented: i) choice of an appropriate adhesive; ii) treatment of the surfaces to be joined; iii) stress distribution analysis in the joint and design of the assembly; iv) measurement of the adherence level of the formed bond. We shall focus rather on the joint formation than on the failure. Step i) includes both aspects and steps iii) and iv) the strength of the bonded joint alone.
In order to illustrate the practical importance of the knowledge of the surface energy of materials, we present the step concerning the preparation of the surfaces to bring together (step ii)), which is often decisive in adhesion. The relation between the surface state of materials and their ability to join together will be examined.
The various systems studied in the present work will exemplify some adhesion mechanisms; the adsorption mechanism will generally be applicable, but mechanical keying and chemical bonding will sometimes play an important role.
Considering the technological interest of polymer-metal assemblies as far as coatings, paints, varnish, packaging are concerned, and for specific high technology applications such as aeronautics, special attention will be paid to metallic and plastic substrates.
SURFACE AND SURFACE STATES
Neither the geometric limit nor the chemical composition of a real solid surface are well-defined. When two surfaces are placed in contact, one can sometimes observe a macrogeometric undulation to which a microgeometric undulation is superimposed [ILLUSTRATION FOR FIGURE 1 OMITTED]. From the outside to the inside of the matter, one usually finds the layers pictured as in Fig. 2. An adhesive bonding is strong when it takes place on a compact and cohesive substance, namely on the treated surface layer or possibly on a reaction products layer. But in the latter case, a layer generated by treatment is preferable in order to eliminate the risks of more or less cohesive and adhesive natural layers. Consequently, the natural surface layers definitively need to be removed. These operations, as a whole, are called surface treatment.
The modern techniques of surface analysis such as, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), as well as contact angle measurements, allow the study, after treatment, of the results obtained regarding the topography (SEM), the nature and chemical composition (XPS or SIMS), as well as the energy of the surface layer (contact angle measurements). It should be observed that the information provided by XPS and/or SIMS is complementary to that supplied by contact angle measurements because the sensitivity (2) of these various techniques to structure and surface composition is about one molecular layer.
MAIN SURFACE TREATMENTS
The purpose of surface treatments is to generate a well-defined and active surface with an affinity for the coating considered. The surface energy [Gamma] of the new surface must also be sufficient so that the applied adhesive or coating can wet and spread out. This condition is fulfilled when [[Gamma].sub.solid] [greater than or equal to] [[Gamma].sub.liquid]. It is known that the surface energies of polymers, about 15 to 40 mJ.[m.sup.-2], are smaller than the surface energies of most usual organic solvents, about 20 to 70 mJ.[m.sup.-2]. The wettability and the corresponding work of adhesion will be low. The surface energy of polymers must then be increased by preliminary treatments, as indicated Table 1.
These abrasive treatments are used with all materials and can be divided as follows:
* abrasion: performed with an emery paper of known granulometry, either manually or with a machine. The abrasion can also be obtained with a water jet containing an abrasive powder. A washing is necessary.
* sand blasting: can be a dry-blast cleaning with sand or silicon carbide powder, aluminum oxide, micro balls of glass or granite. Very large industrial facilities are needed.
The major effects of these methods are an increased roughness of the object surface, its oxidation, and an enhanced surface energy. The mechanical treatments are followed by a solvent cleaning. The solvent must be inert with respect to the substrate and must dissolve greases and various settlings. To facilitate the drying, the use of low-boiling point solvents, such as acetone, methylic or isopropylic alcohols, methylethylketone, chloroform, and fluorinated or chlorinated solvents as well as detergent aqueous solutions, is recommended. In the chemical degreasing, frequently performed with alkaline solutions, the quality of the baths (temperature, concentration) and of the washings must be carefully controlled. These treatments are sometimes sufficient, considering the results expected. But in some cases, other treatments listed, Table 1, must be employed.
Table 1. Main Surface Treatments.
Kind of Treatment and Examples Materials to be Treated
Mechanical All materials: metals, plastics, Abrasion, sand blasting wood
Solvent Degreasing Bare supports, except some (preferably in vapor phase) plastics
Alkaline Chemical Degreasing Metals, sheet molding (quality control of baths compounds and washings)
Chromic or sulphochromic Light alloys, plastics, oxidation elastomers Anodization Light alloys Phosphatization Ferrous metals, light alloys Special treatments Some fluorinated derivatives
Flame Synthetic materials, Corona discharge particularly polyolefins Cold plasma (polypropylene, polyethylene) Application of Adhesion Silicates, ceramics, plastics, Promoters (or Primers) some metals
The preparation of metallic surfaces or plastics for adhesive bonding necessitates a preliminary surface oxidation. This is operated by immersion of the substrate in salt solutions of sulphuric and/or chromic acids, to which phosphoric acid is occasionally added. Various formulations are proposed (3) for metals and plastics, such as, in the case of polyolefins: [H.sub.2]S[O.sub.4]/[K.sub.2][Cr.sub.2][O.sub.7], 2[H.sub.2]O (88.5/4.5/7% in weight), and for aluminum alloys: [H.sub.2]S[O.sub.4]/Cr[O.sub.3]/[H.sub.2]O (27.3/5.2/67.5% in weight). The time and temperature of the etching procedure depends on the nature of the substrate. Sulphochromic oxidation attacks and pickles the substrate surface.
The morphology and chemical structure of the surface of an aluminum alloy, after sulphochromic etching in a standard and optimized baths, have been studied (4) by SEM. This technique reveals the oxide layer's cellular structure. For the standard bath, the oxide layer is thin ([less than]5 nm) and wide-meshed, whereas for an optimized bath, the oxide layer is three times thicker, with a very tight mesh, as presented Fig. 3. The wedge adherence tests carried out (4) on this aluminum alloy show that the porous structure of the oxide layer originates the good adhesive bonding results obtained with this substrate. Other things being equal, the adhesion is all the more efficient that the surface porosity is enhanced.
In the case of polyolefins, a microscopic observation points out that the sulphochromic etching creates or improves the substrate roughness, the weakest parts being preferentially attacked. The polymers are modified in the order below:
polypropylene [greater than] low-density polyethylene
[greater than] high-density polyethylene
The wettabillty rapidly increases. Infra-red and XPS allow, in certain cases, the detection (5, 6) of the following oxidation groups: C-OH, C=O, COO and S[O.sub.3]H. The attacked thickness is estimated by means of XPS, up to about 10 nm. It is observed (6) that adhesion on polyolefins of an epoxy adhesive increases with the oxidation degree of the polymer surface. The failure (6) is adhesive, for poorly oxidized substrates, and cohesive, for strongly oxidized substrates.
Anodization is an electro-chemical process of formation and growth of an oxide film on the metal surface. It is therefore an oxidation. It can occur either with chromic acid or with phosphoric acid or in sulphuric medium. The latter is the most widely used on aluminum and its alloys.
When an aluminum surface plays the role of the anode of a working electrolysis cell, the nascent oxygen reacts with the metal according to an ionic exchange process which produces an almost anhydrous [Al.sub.2][O.sub.3] oxide film. The layer produced this way is partially dissolved by the electrolyte (sulphuric acid is currently the most common electrolyte). This dissolution results in the formation of numerous pores at the bottom of which remains an homogeneous oxide layer, whose thickness is a function of the voltage between the electrodes. Then one obtains a porous layer (7) composed of a compact system of hexagonal cells, each containing one pore, as pictured in Fig. 4. About [10.sup.10] pores per [cm.sup.2] are formed, with a diameter (7) ranging from 12 to 33 nm. The latter depends on experimental conditions (electrolyte composition, concentration, temperature, current density.
The thickness of the oxide layer, a function of the treatment time, is generally about 15 to 20 [[micro]meter]. The technological properties of the coatings produced by anodic oxidation are interesting:
* a protective character like natural films,
* a porous structure in which various substances such as colorings, corrosion inhibitors, varnish may be interlocked.
The surface obtained by anodic oxidation is porous and formed by anhydrous alumina. These pores can be clogged by a subsequent sealing in boiling water, in presence of acetates, the transformation of the anhydrous alumina into hydrated alumina ([Al.sub.2][O.sub.3], [H.sub.2]O) being accompanied by an increase in volume. Sealed alumina loses its adsorption ability with respect to colorings or other molecules.
Phosphatization is a chemical treatment, in aqueous phase, of a metal surface. The phosphatizing solution contains phosphoric acid, dihydrogenophosphates of divalent metals (Me[([H.sub.2]P[O.sub.4]).sub.2]) and organic or mineral oxidizing substances. After a variable time depending on the composition of the treatment bath, a phosphates layer appears on the metal.
Zinc phosphatization is well-known for the interlocking of paint coatings on automotive steel sheets. In that case, the phosphated layer is essentially composed of hopeite ([Zn.sub.3][(P[O.sub.4]).sub.2], 4[H.sub.2]O) and of phosphophyllite ([Zn.sub.2]Fe[(P[O.sub.4]).sub.2], 4[H.sub.2]O). Its thickness is generally of about 1 to 2 [[micro]meter]. Chromic phosphatization is used in the case of aluminum. After degreasing or alkaline etching, aluminum is treated with a solution containing phosphoric acid, chromic acid, and fluorides in order to catalyze the reaction. The blue-green film formed on the surface is assumed to have the following overall chemical composition: [Al.sub.2][O.sub.3], 2CrP[O.sub.4], 8[H.sub.2]O. The coating, when dry, is essentially composed of [Cr.sup.III] and aluminum phosphates (8). Those treatments constitute a particularly efficient and economical surface preparation.
Carre and Schultz (9, 10) have studied the relation between the surface properties of aluminum and adhesion for aluminum/styrene-butadiene rubber (SBR) and aluminum/nitril-butadiene rubber (NBR) assemblies. It is clearly observed that the surface energy, [[Gamma].sub.S], of hexane-degreased aluminum, equal to 49.5 mJ.[m.sup.-2], becomes 169 mJ.[m.sup.-2] for anodized aluminum, 56 mJ.[m.sup.-2] for sealed anodized aluminum, and 151 mJ.[m.sup.-2] for phosphatized aluminum. The ability of aluminum to adhere against SBR and NBR elastomers is as follows:
anodized A1 [greater than] sealed anodized A1 [greater than] phosphatized A1
We note that surface energies alone do not predict the adhesive capability of aluminum in regard to its surface treatment. SEM reveals (11) that the surface of sealed anodized aluminum exhibits fissures of about 0.2 [[micro]meter] wide. These fissures favor the interlocking of polymer in the oxide layer. Carre (11) has pointed out that the morphology of interfaces must also be taken into account because it determines the stress distribution and, consequently, the geometry of fracture surfaces. To a large extent, the porosity and roughness contribute to the increase of the contact area and to the strength of adhesion accordingly. This is exemplified by anodized aluminum, as already noted (1).
The substrates are exposed to the flame of a burner with a fixed air/gas mixture in order to obtain a stable and oxidizing flame. The gases used are methane, propane or butane. An excess of oxygen of about 10%, with respect to that required for a complete combustion, is needed. In these conditions, the flame temperature approximates 1200 [degrees] C. The optimum exposure time usually ranges from 0.02 to 0.2 s. The most important variables in the process are the air/gas ratio, the air/gas flow rate, the gas nature, the flame-substance distance, and the exposure time.
Surface structures studies (12) indicate that oxidation reactions occur. However, only a small proportion of oxygen is fixed, due to the short treatment times. Some nitrogen atoms are also bonded (13) (amino, [NH.sub.2], nitrile, CN, and occasionally amide, CONH groups).
In the case of polyethylene (13, 14) double bonds and methyl groups appear, following upon chain scissions by radicalar mechanisms (14). The modified layer is below 10 nm deep. The polyethylene surface energy increases (14), depending on the treatment time, from 30 to 50 mJ.[m.sup.-2], as reported in Fig. 5. Its adherence with respect to certain adhesives can be multiplied by thirteen (13).
Martz et al. (15) systematically studied how flame treatment improved the adhesion of polypropylene and its copolymers on either polar substrates, e.g. polyurethane, or on nearly apolar substrates, e.g. a SBR elastomer. Before treatment, the ranges of the dispersive, [Mathematical Expression Omitted] and polar, [Mathematical Expression Omitted], components of the surface energy of polypropylene and its copolymers are respectively: 30 to 32 mJ.[m.sup.-2] and 0.4 to 0.9 mJ.[m.sup.-2]. These polymers have a low surface energy and are almost apolar which explains their poor adhesive capability. After a flame treatment, one notices a slight increase (+2 mJ.[m.sup.-2]) of the dispersive component, [Mathematical Expression Omitted], which does not vary with the severity of the treatment. In contrast, the polar component, [Mathematical Expression Omitted], rapidly increases with the exposure time and can rise to 10 or 20 mJ.[m.sup.-2], depending on the polymer nature. These authors observe that the flame treatment increases the polypropylene adhesion strength, not only against a polar substrate, such as polyurethane, but also against an almost apolar elastomer, such as SBR. Garbassi et al. (16, 17) have also studied the effects of flame treatments on polypropylene. The surface energy of untreated polypropylene samples is 28 mJ.[m.sup.-2] and can rise to 40.3 or 43.8 mJ.[m.sup.-2] following a number of flame treatments. These authors have observed by XPS and SIMS the formation of hydroxyl groups and, in smaller amounts, carbonyl and carboxyl groups. These groups favor chemical interactions with polyurethane. The adhesion tests have been carried out on polypropylene coated with polyurethane paints. The results obtained for flamed polypropylene in comparison to the untreated sample show that the flame treatment increases considerally the adhesion of the polymer to the coating. There is at least a factor of thirty in favor of the treated samples. A good correlation (17) between the increased surface energy and the improved resistance of polypropylene-polyurethane assemblies has also been established.
The corona discharges have been used industrially for many years to treat polymeric films, fibers, and flat objects. This procedure is used for polyolefins (polyethylene and polypropylene) and some polyesters (polyethylene terephtalate). The electrical discharge between two electrodes is obtained under atmospheric pressure from a high voltage alternative current. The electrical field generated excites the gas molecules, i.e. air, and dissociates some of them. The charged active species may react with polymer molecules which are activated accordingly.
Machine configurations vary with the electrode geometries, the electrical characteristics, the film speed. An example is given in Fig. 6. One of the important parameters is the electrode-film surface distance which should not usually exceed 2 or 3 mm. Most frequently, 20 kV electrical currents are applied, with frequencies ranging from 10 to 20 kHz.
Numerous fundamental works describe the reactional mechanisms of this type of treatment (18-21). Depending on experimental conditions, ozone and nitrogen oxides are formed in air discharges. Polymers are oxidized. Carbonyl groups appear.
With polyethylene, an autoadhesion phenomenon is observed (21, 22); it could be caused by hydrogen bonds and depends directly on the oxygen/carbon ratio, as measured by XPS. Peroxides could be formed (23) and their decomposition (14) could result in the formation of carbonyls (22), carboxylic acids (24), ethers (22), hydroxyls (25), and ozonides (22). The treatment of polyolefins (23) by corona discharge in air develops their polar character, as indicated in Fig. 7, and their aptitude to adhesion.
A study (26) devoted to the corona discharge treatment of polypropylene sheets, under atmospheric pressure, shows that the best results are obtained with an oxygen-nitrogen mixture, with over 20% oxygen. The C-OH, C = O, and COOH groups have been identified. The surface energy of treated polypropylene is clearly higher than that of the reference. The adherence level of a polypropylene-polyurethane assembly is significantly improved after treatment and depends on the surface oxidation degree.
A "plasma" is a dilute state of matter, similar to an ionized gas, in which charged particles are in such proportions that the medium is globally neutral. In the case of polymers, cold plasmas are interesting. They correspond to ionization ratios below [10.sup.-4] and to electronic densities of about [10.sup.9] to [10.sup.12] electrons.[cm.sup.-3], with energies ranging from 1 to 10 eV. They are created under partial vacuum ([less than] 600 Pa), by electrical discharges supplied by high frequency generators, which vary from kHz to GHz. The electrons and ions interact with neutral gas molecules. Free radicals and excited species are formed. Upon impact, these strongly reactive species modify the chemical composition of the polymer surfaces. The initial bonds are replaced by new functions which alter the surface energy of the material.
Depending on the nature of the plasmagen gas, the surface energy will be either increased or decreased. Thus, in the case of oxygen, nitrogen, ammoniac, argon, and helium, a decreased contact angle on water, i.e. an increased surface energy, has been observed (27-29). In contrast, treatment by fluorinated gases (27), such as carbontetrafluoride ([CF.sub.4]) or sulfurhexafluoride ([SF.sub.6]), leads to a greater contact angle on water and, therefore, to a lower surface energy. Results regarding observations performed with NBR and ethylene-propylene-diene (EPDM) elastomers, high-density polyethylene (HDPE), and polymethyl-methacrylate (PMMA) are included in Table 2.
For polymers treated with either oxygen or rare gas plasmas, the chemical analysis indicates linked oxygen atoms. The latter are fixed either directly within the oxygen plasma or in the ambient atmosphere, for treatments carried out with rare gases. The electronic paramagnetic resonance allows the detection of radicals for polymers subjected to oxygen plasmas. The following structures have been observed (30): [CH.sub.2] - [CH.sub.2] - CH - [CH.sub.2] for polyethylene and [Mathematical Expression Omitted] as well as [Mathematical Expression Omitted] for polytetrafluoroethylene. The treatments by oxygen plasmas are also employed in carbon fiber reinforced epoxy composites to improve the fibers adhesion. Sun [TABULAR DATA FOR TABLE 2 OMITTED] et al. (29) note an increased concentration of COOH, C-OH, and C = O groups at the fiber surfaces. A lowered contact angle of water on carbon fiber, from 75 to 61 [degrees], is noted and consequently, an increased surface energy, from 36.8 to 45 mJ.[m.sup.-2]. This treatment significantly strengthens the fiber-matrix adhesion.
The enhancement of the adhesive properties of the so-called inert polytetrafluoroethylene (PTFE) can be obtained by means of inert gas plasmas, such as argon or helium, following a CASING (Crosslinking of Activated Species of Inert Gases) process (31), or by means of ammonia gas ([NH.sub.3]) plasma (28). In the latter case (28), the PTFE surface treatment by [NH.sub.3] plasma renders the surface hydrophilic, with formation of carbonyl and amide groups. The water-PTFE angle decreases from 118 to 16 [degrees] and the surface energy rises from 11.8 to 62 or 63 mJ.[m.sup.-2]. PTFE adhesion strength against a nitril rubber, via a phenolic-type adhesive, is extended.
The surface treatments by plasmas of fluorinated gases, such as [CF.sub.4] or [SF.sub.6], leads to a lowered surface energy, as already mentioned. Therefore, Table 2 shows that the PMMA surface energy varies from 43 to 37 mJ.[m.sup.-2]. The chemical surface analysis by XPS of polymers treated by plasmas of fluorinated gases reveals the presence (32) of fluor atoms linked by covalent bonds CF; likewise [CF.sub.2] and [CF.sub.3] groups are observed (32). The thickness treated is about 10 nm. This type of treatment leads to a passivation of the surfaces.
In conclusion, the use of cold plasmas to change the surface state of materials seems promising because the final state results from the equilibrium, which can be modified, between the surface functionalization and degradation processes. These treatments are, however, technologically complex (33).
Primers or Adhesion Promoters
In order to facilitate the adhesion of a deposit on a surface, various preparation products called primers or adhesion promoters can be spread out on it. A primer is a dilute solution, either of an adhesive in an organic solvent or of an organometallic product (an organo silane, for instance) in a solvent of alcohol type. This solution is applied onto the surfaces to be joined, obtaining a film from 2 to 50 [[micro]meter] thick. The primer chemical types are usually the same as those of the adhesive for which they are used. A primer applied in ultra-thin film on the substrate surface generally meets the following requirements:
* enhanced wettability with respect to subsequent coating,
* improved adherence by promoting chemical and physical links at the substrate-primer and primer-coating interfaces,
* protected adherend surface against an excessive oxidation in air. For example, the maximum time available between the surface preparation of aluminum and its overlaying with an adhesive is 12 h. By means of a primer, this time can be extended up to 30 days and sometimes even longer.
Currently, the most popular types of coupling agents are of silane type (34) X-[([CH.sub.2]).sub.n]-Si[(R[prime]).sub.3] where n = 0-3. These molecules possess a dual reactivity, as indicated (1). Therefore, R[prime] being an alkoxy, chloro or acetoxy group, the R[prime]-Si bond gives, after hydrolysis, silanols (SiOH) which are able to condense with similar groups on glass or other silicated substrates. The organo-functional sites, X, are designed to match either the adhesive or, in the case of composite materials, the organic resin matrix. Among these are the amino and epoxy groups for bonding to epoxy resins and other polymers, and the vinyl group to react with unsaturated polyesters. It has been observed that some silanes promote the water-resistance of metal-epoxy bonds. This has been interpreted as resulting from the formation of interfacial bonds between the silanes and the metal. Direct evidence of this interpretation has been obtained by Gettings and Kinloch (35, 36) who have used SIMS to study the interaction between [Gamma]-glycidoxypropyltrimethoxysilane and mild or stainless steel. The existence of FeSi[O.sup.+] radicals for a mild steel primed surface, and FeSi[O.sup.+] and CrO[Si.sup.+] radicals for a stainless steel primed surface provided direct evidence of chemical bonds between the metal oxide and the primer. For other silane primers which did not improve the durability the FeSi[O.sup.+] radicals were not detected.
In the same context, an electrochemical formation process of a polymeric film on stainless steel or on a titanium alloy Ti-6Al-4V (6% Al, 4% V) is studied (37, 38). This electropolymerization process allows deposit of a polyaniline film by anodic oxidation of an aqueous solution of the monomer. The coating obtained may act as a primer, providing an adhesive or a painting with a mechanical interlocking base (37, 38).
In the case of polymers, primers can be used to bring together materials difficult to bond. Therefore, when manufacturing adhesive tape on polyvinylchloride substrate, the autoadhesive deposited on one of the faces should strongly adhere to it. Thus, the polyvinylchloride is overlaid with a primer of acrylic, polyurethane or nitril robber types. Then the autoadhesive coating is applied onto the primer. The chemical nature of the coating must be the same as that of the primer.
Polyolefins, such as polyethylene and polypropylene, are not usually activated by primers. However, a recent work (39) has pointed out that these materials, after applications of organic or organometallie primers, become easily bondable with cyanoacrylate adhesives. Besides, it has been observed (40) that primers, such as cobalt acetoacetonate or triphenylphosphine, clearly accelerate the polymerization of cyanoacrylate adhesives against polyolefins.
Before the formation of an adhesive bonding, a surface treatment of one of the materials to be joined is frequently required. The objectives of this treatment, whatever the nature of the adhesives, are:
* the elimination of impurities and weak boundary layers,
* the modification of the chemical composition and surface morphology.
We have seen that, as a result of those operations, the substrate surface energy is generally increased and its surface is provided with an appropriate roughness. We have frequently noted a correlation between the increased surface energy and the assembly adhesive qualities. The importance of surface energies in adhesion received support from Levine et al. (41) who measured joint strengths for various polymers bonded with an epoxy adhesive. They found, with one or two exceptions, a good relationship between joint strengths and the polymer's surface energy. However, the substrate surface energy is not the only important factor in adhesion. The surface topography plays an essential role. The cases of sealed anodized aluminum and sulphochromic etched aluminum alloy, discussed previously, are good examples. Therefore, the knowledge of the surface state of the materials regarding the topography, the nature and the chemical composition of the surface layer is very useful in the study of adhesion mechanisms. The modern techniques of surface analysis (SEM, XPS, SIMS) are bound to play an important role for both the surface characterization and the verification of proposed adhesion mechanisms.
The author wishes to thank Prof. R. M. Leblanc for critically reading the manuscript and making helpful suggestions.
SEM = Scanning electron microscopy.
XPS = X-ray photoelectron spectroscopy.
SIMS = Secondary ion mass spectroscopy.
[[Gamma].sub.S] = Surface energy of solid.
[Mathematical Expression Omitted] = Dispersive component of [[Gamma].sub.S].
[Mathematical Expression Omitted] = Polar component of [[Gamma].sub.S].
[CF.sub.4] = Carbontetrafluoride.
[SF.sub.6] = Sulfurhexafluoride.
SBR = Styrene-butadiene rubber.
NBR = Nitril-butadiene rubber.
EPDM = Ethylene-propylene-diene.
HDPE = High-density polyethylene.
PMMA = Poly(methyl methacrylate).
PTFE = Polytetrafluoroethylene.
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|Title Annotation:||An Overview of the Basic Aspects of Polymer Adhesion, part 2|
|Publication:||Polymer Engineering and Science|
|Date:||Jun 1, 1995|
|Next Article:||Mechanical properties of polyurethane-unsaturated polyester interpenetrating polymer networks.|