Rheological methods to determine the predisposition of a polymer to form network or gel
A method is provided to determine the predisposition of a polymer to form network or gel in a lubricating oil which comprises obtaining a composition of a polymer in a diluent, measuring the shear stress as a function of shear rate, determining the yield stress using the Herschel-Bulkley equation and assessing the yield stress. A method is also provided to determine the predisposition of a polymer to form network or gel in a lubricating oil which comprises obtaining a composition of a polymer in a diluent, determining at least one of (i) the storage modulus G′ and loss modulus G″ of the composition by subjecting the composition to sinusoidal (oscillating) stress or strain of certain amplitude and frequency, (ii) the phase lag (angle) of the response strain or stress δ, or (iii) the tangent (tan) δ, and (c) assessing at least one of the G′, G″, δ, or tan(δ) determined.
1. A method to determine the predisposition of a polymer to form network or gel in a composition by polymer-polymer interaction, comprising the steps of:
(a) obtaining a composition consisting essentially of a polymer in a diluent, wherein the polymer is at least one alpha-olefin copolymer, and
(b) determining at least one of (i) the phase lag (angle) of the response strain or stress δ, or (ii) the tangent (tan) δ of the composition by subjecting the composition to sinusoidal (oscillating) stress or strain of certain amplitude and frequency, and
(c) assessing at least one of the (i) δ or (ii) tan(δ) determined, thereby determining the predisposition of the polymer to form a network or gel, whereby when at least one of δ is >60° or tan(δ) is >2, determining that said polymer has a reduced predisposition to form a network or gel by polymer-polymer interaction.
2. The method according to claim 1 , wherein the diluent comprises a lubricating oil and the polymer is an alpha-olefin copolymer.
3. The method according to claim 1 , wherein the diluent contains a Group I, II, III, IV or V base oil.
4. The method according to claim 1 , wherein the polymer is an ethylene copolymer having an ethylene content of at least about 50 wt. %, by weight of the ethylene copolymer.
5. The method according to claim 1 , wherein the polymer is a polymer composition that comprises at least 30 wt. %, based on the weight of the polymer composition, of a first copolymer of ethylene and an alpha-olefin comonomer, and 70 wt. % or less, based on the weight of the polymer composition, of a second copolymer of ethylene and an alpha-olefin comonomer, wherein the first copolymer has a lower ethylene content than the second copolymer.
6. The method of claim 1 , further comprising selecting the polymer for use in a lubricating oil composition, and forming a lubricating oil composition comprising the polymer.
7. A lubricating oil composition comprising a polymer selected according to the method of claim 6 .