SYNOPSISThe 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.

Metallocene‐catalyzed, low‐density and linear low‐density polyethylenes with similar melt indexes were used to investigate how side‐chain structures influence the elongation viscosity and viscoelastic properties. The viscoelastic properties were determined with a rotation rheometer, while the elongation viscosities were acquired by using isothermal fiber spinning. The Phan‐Thien‐Tanner (PTT) model was also used to understand how the side‐chain structure affects the elongation behavior. Experimental results demonstrate that the log G′ vs. log G″ plot can qualitatively describe the effects of the side chain branch on the rheological properties of polyethylene melts. According to the results determined by the PTT model, low‐density polyethylene (LDPE) has low elongation viscosities at high strain rates. This low elongation viscosity can be attributed to the fact that LDPE has high shear thinning behavior. The long‐chain branching tends to increase entanglements, thereby enhancing the storage modulus, elongation viscosity and shear‐thinning behaviors. Uniform side‐chain distribution lowers the entanglements, which results in a low storage modulus, elongation viscosity and shear‐thinning behavior.

Metallocene polyethylene (mPE) fractions are recognized as being more homogeneous with respect to short-chain branch (SCB) distribution as compared with unfractionated mPEs. Differential scanning calorimetry and polarized optical microscopy (POM) were used to study the influences of SCB content on the crystallization kinetics, melting behavior, and crystal morphology of four butyl-branched mPE fractions. The parent mPE of the studied fractions was also investigated for comparative purposes. mPE fractions showed a much simpler crystallization behavior as compared with their parent mPE during the cooling experiments. The Ozawa equation was successfully used to analyze the nonisothermal crystallization kinetics of the fractions. The Ozawa exponent n decreased from about 3.5 to 2 as the temperature declined for each fraction, indicating the crystal-growth geometry changed from three-dimensional to two-dimensional. For isothermal crystallization, the fraction with a lesser SCB content exhibited a higher crystallization temperature (T c ) window. The results from the Avrami equation analysis showed the exponent n values were around 3 (with minor variation), which implied that the crystal-growth geometry is pseudo-three-dimensional. Both of the activation energies for nonisothermal and isothermal crystallization were determined for each fraction with Kissinger and Arrhenius-type equations, respectively. Double melting peaks were observed for both nonisothermally or isothermally crystallized specimens. The high-melting peak was confirmed induced via the annealing effect during heating scans. The Hoffman-Weeks plot was inapplicable in obtaining the equilibrium melting temperature (T m°) for each fraction. The relationship between T c and T m for the fractions is approximately T m ϭ T c (°C) ϩ 8.3. The POM results indicated that the crystals of parent or fractions formed under cooling conditions did not exhibit the typical spherulitic morphology as a result of the high SCB content.

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SYNOPSISA 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.