Gary R. Hamed

Peel adhesion and viscoelasticity of poly(ethylene-co-vinyl acetate)-based hot melt adhesives. I. The effect of tackifier compatibility

A series of poly(ethylene-co-vinyl acetate) (EVA)-based hot melt adhesives containing either a rosin or a hydrocarbon (C5–C9) tackifier have been prepared to investigate viscoelastic properties and peel adhesion. Fracture energies were determined by the use of a T-Peel geometry (two polypropylene films bonded with model EVA adhesives). The rosin has only one glass transition temperature, but the C5–C9 resin has two glass transition temperatures, indicating phase separation. The rosin has better compatibility with EVA than does the C5–C9 resin. The bond strength of tackified EVA to polypropylene depends not only on compatibility, but also on viscoelastic properties. A higher storage modulus results in a higher T-Peel strength. Under certain test conditions, glassy C5–C9-rich domains act as reinforcing filler, resulting in a higher storage modulus. Here, a C5–C9-tackified EVA adhesive has higher T-Peel strength than does one containing rosin.

Peel adhesion and viscoelasticity of poly(ethylene‐co‐vinyl acetate)‐based hot melt adhesives. II. The influence of wax

The influence of wax on the viscoelasticity and peel adhesion of poly(ethylene-co-vinyl acetate) (EVA)-based hot melt adhesives was evaluated. Wax does not affect the glass transition temperature of a homogeneous EVA/rosin blend. However, for a heterogeneous EVA/rosin blend, wax addition increases the EVA-rich phase portion, resulting a higher rubbery response. The T-Peel fracture energies of EVA/tackifier/wax blends bonded to polypropylene film are controlled by two factors: (1) a weak boundary layer of wax, which has a deleterious effect on bonding, and (2) on the other hand, an increased rubbery response in the stick-slip region, which tends to strengthen joints.

Comparison of the Effect of a Hydrocarbon Oil and a Terpene Tackifier on the Autohesion of Styrene-Butadiene Rubber

Abstract Blends of uncrosslinked styrene-butadiene rubber (SBR) with a terpene tackifier resin or a naphthenic oil have been characterized, and their autohesion and cohesion determined using a T-peel geometry. SBR/oil blends are homogeneous at all proportions, while SBR/resin blends, based on DSC and DMA analysis, undergo bulk phase separation at about 50% resin. However, migration of tackifier to the surface region is proposed at much lower resin contents. Compositions diluted with oil have autohesion similar to the neat SBR. This is attributed to compensating effects; although oil hastens self-bond formation by increasing chain mobility, this is nearly equally balanced by more facile chain separation during bond rupture. In short, oil-diluted compositions are soft and weak. On the other hand, SBR compositions containing small amounts of resin have high autohesion. Resin-diluted specimens deform easily at low strain, just as those containing oil, but intertwined chains of the former have greater resistan...

Adhesion of elastomer layers to an interposed layer of filler particles

Adhesion of elastomers to filler particles was studied by interposing a single layer of particles between two layers of a crosslinked elastomer and peeling the sandwich apart. Carbon black particles increased the peel strength by up to 100% compared with autohesion of the elastomer layers. Silica particles also increased the adhesion, but by a smaller factor, and there were significant differences using different elastomers. Also, the strength of adhesion depended on the degree of crosslinking of the elastomer layers: at higher levels of crosslinking, both self-adhesion and adhesion to particles were reduced. Nevertheless, this simple experiment gives an indication of the relative strength of adhesion for different combinations of elastomer and reinforcing filler.

Peel adhesion of natural rubber bonded to polyethylene terephthalate: Effect of crosslinking on rate/temperature response

Abstract Laminates consisting of natural rubber (NR) sandwiched between cloth fabric and polyester film were pulled apart at various rates and temperatures in a T-peel geometry. Peel energies for joints containing uncrosslinked or lightly-crosslinked NR did not obey simple time-temperature superposition. This behavior is attributed to strain-induced crystallization during peeling. However, when the rubber was highly crosslinked, strain crystallization seems to be absent, as peel energies now can be WLF shifted to form a mastercurve.

Adhesion of elastomers: Dwell time effects

The strength of adhesion of elastomers to rigid substrates generally increases with time of contact. This effect has been studied for samples of butyl and chlorobutyl rubber adhering to some rigid substrates. The peel strength increased continuously over long periods of contact until in some cases failure became cohesive within the elastomer layer. At higher temperatures the strength increased more rapidly, consistent with the WLF relation governing molecular motions. It is postulated that slow molecular rearrangements occur at the interface and increase the bond strength. A criterion for the observed transition from interfacial to cohesive failure is suggested.

Poly(Ethylene-co-Vinylacetate) Based Hot Melt Adhesives: I. Relating Adhesive Rheology to Peel Adhesion

Abstract The successful selection of bonding conditions for hot melt adhesives depends on melt morphology and rheological properties. Rheological properties were determined for model poly(ethylene-co-vinylacetate) hot melt adhesives. T-Peel fracture energies of joints consisting of two polypropylene films bonded with these adhesives were determined. Rheological data suggest that the EVA and its blends form homogeneous melts. Tackifiers and wax lower melt viscosity and elasticity but increase the activation energy for melt flow. Adhesives with higher melt activation energy, as well as lower elasticity and viscosity, show little dependence of peel strength on bonding temperature and require little time to reach the equilibrium bond strength. The equilibrium T-Peel strengths of adhesives containing wax are almost independent of bonding temperature. This may result from the existence of a weak boundary layer of wax or to their high flowability.

Adhesion and Failure Mechanisms of a Model Hot Melt Adhesive Bonded to Polypropylene

Abstract A model hot melt adhesive (HMA) based on an ethylene/vinyl acetate copolymer (EVA), an Escorez® hydrocarbon tackifier, and a wax has been used to bond together polypropylene (PP) films to give equilibrium bonding. Peel strengths were determined over a broad range of peel rates and test temperatures. Contrary to the peel behavior of joints with simple rubbery adhesives [1], peel strengths with this semi-crystalline adhesive are not rate-temperature superposable, and multiple transitions in failure locus occur. The semi-crystalline structure of the HMA also prevents rate-temperature superposition of its dynamic moduli. At different test temperatures, the dependence of peel strength on peel rate shows some resemblance to the dependence of the loss tangent of the bulk adhesive on frequency. This is consistent with a previous result [2] that the HMA debonding term. D, varies with the loss tangent of a HMA at the T-peel debonding frequency. This model HMA, similar to block copolymer/tackifier blends [3...

Stress Relaxation in Peel Adhesion

Abstract The peeling of an adhesive joint consisting of an SBS copolymer and two Mylar film substrates proceeds by cohesive rubber rupture, and the strength increases with test rate. Stress relaxation during peeling is shown to account for this behavior and relaxation data after peeling is used to predict the rate dependence of the peel force.

Peel strength of uncrosslinked styrene-butadiene rubber adhered to polyester film

Testpieces consisting of a fabric-backed styrene-butadiene rubber (SBR 1502) layer bonded directly to polyethylene terephthalate (PET) film were T-peel tested at various rates, R , and temperatures. Peel energies were superposed toformmastercurves using shift factors, a T , in accord with the universal WLF equation. When peeled at intermediate reduced rates, Ra T , failure was cohesive within the SBR 1502, while at sufficiently high or low Ra T interfacial separation between the rubber and PET occurred. These results markedly contrast with those found by Gent and Petrich using similar testpieces with another type of rubber, SBR 1513. They found cohesive failure at sufficiently low Ra T and interfacial failure when Ra T was high. The different behavior of the two elastomers is attributed to stronger interfacial attraction with SBR 1513 and its lower strength. General considerations governing the locus of failure during peel adhesion testing are discussed.