Surface tension hydrocarbon polymers

Release agents function by either lessening intermolecular interactions between the two surfaces in contact or preventing such close contact. Thus, they can be low surface-tension materials based on aUphatic hydrocarbon, fluorocarbon groups, or particulate soHds. The principal categories of material used are waxes, fatty acid metal soaps, other long-chain alkyl derivatives, polymers, and fluorinated compounds. 

In 1938, while attempting to prepare fluorocarbon derivatives, Roy J. Plunkett, at DuPont s Jackson Laboratory, discovered that he had prepared a new polymeric material. The discovery was somewhat serendipitous as the TFE that had been produced and stored in cylinders had polymerized into poly(tetra-fluoroethylene) (PTFE), as shown in Eigure 4.2. It did not take long to discover that PTFE possessed properties that were unusual and unlike those of similar hydrocarbon polymers. These properties include (1) low surface tension, (2) high Tm, (3) chemical inertness, and (4) low coefficient of friction. All of these properties have been exploited in the fabrication of engineering materials, wliich explains the huge commercial success of PTFE. 

With values in the range of about 10-18 mN m 1 perfluorinated liquids have the lowest surface tensions among the known organic liquids, and will completely wet any solid surface. Increasing amounts of hydrogen in the molecule increase the surface tension. Fluorinated solid surfaces, e.g. fluoropolymers, possess very low critical surface tensions yc, which relates to their antistick and low frictional properties, whereas hydrocarbon polymers have substantially higher values (PTFE yc = 16.0 mN m-1 PE yc = 31.0 mN m-1).7... 

The wetting properties of polyacetylene have been studied by Schonhom et al. 380) who measured a critical surface tension of 51 mN m 1, considerably higher than for other hydrocarbon polymers. This value was attributed to oxidation of the surface as no change was observed on further oxidation. Treatment of polyacetylene with aqueous potassium permanganate renders it hydrophilic, reduces the contact angle for water from 72° to 10° and renders the structure more water-permeable 381)... 

The dispersive and polar components of the surface tensions of the liquids were estimated to be 7 = 21.8 mN/m and 7 = 51.0 mN/m for water and 7 = 49.5 mN/m and 7 = 1.3 mN/m for methylene iodide. This estimation was done by measuring contact angles with various hydrocarbons and assuming that there are only nonpolar interactions. What are the surface energies, 7s, of the polymers ... 

Here, again, we start from compressible SCFT formalism described in Section 2.2 and consider a model system in which bulk polymer consists of "free" matrix chains (Ny= 300) and "active" one-sticker chains (Na= 100). Flory-Huggins interaction parameters between various species are summarized in Table 1. This corresponds to the scenario in which surfactants, matrix chains, and functionalized chains are all hydrocarbon molecules (e.g., surfactant is a C12 linear chain, matrix is a 100,000 Da molecular weight polyethylene, and functionalized chain is a shorter polyethylene molecule with one grafted maleic group). The nonzero interaction parameter between voids and hydrocarbon monomers reflects the nonzero surface tension of polyethylene. The interaction parameter between the clay surface and the hydrocarbon monomers, Xac= 10 (a = G, F, A), reflects a very strong incompatibility between the nonpolar polymers and... 

The predicted surface tensions of the remaining six polymers listed in Table 7.5 cannot be compared with experimental data due to the lack of such data. They do, however, follow trends which may be expected from basic physical considerations. They are predicted to increase with increasing fractions of (a) units of high cohesive energy density and (b) aromatic moieties in the hydrocarbon portions of the polymeric repeat units, and to decrease with increasing fraction of saturated aliphatic moieties. 

Finally, it is highly desirable to improve the ability to calculate the properties of surfaces and interfaces involving polymers by means of fully atomistic simulations. Such simulations can, potentially, account for much finer details of the chemical structure of a surface than can be expected from simulations on a coarser scale. It is, currently, difficult to obtain quantitatively accurate surface tensions and interfacial tensions for polymers (perhaps with the exception of flexible, saturated hydrocarbon polymers) from atomistic simulations, because of the limitations on the accessible time and length scales [49-51]. It is already possible, however, to obtain very useful qualitative insights as well as predictions of relative trends for problems as complex as the strength and the molecular mechanisms of adhesion of crosslinked epoxy resins [52], Gradual improvements towards quantitative accuracy can also be anticipated in the future. 

The extrusion aid must easily coat the resin yet be readily removable from the extrudate. It should also not leave a residue which could alter the color of the product. The volatilization temperature of the lubricant should be lower than the sintering temperature of the polymer. The other requirements of lubricants include high purity, low odor, low polar components, high auto-ignition temperature, low surface tension, and low skin irritation. Common lubricants are synthetic isoparaffmic hydrocarbons available in a wide boiling range. Some of the commercial lubricants include Isopar solvents (available from Exxon Corp.), mineral spirits, and VM P Naphtha (available from Shell Corp.). 

Complications such as these extend also to the case of polytetra-fluoroethylene. The large difference in estimated solid-vacuum tensions between this polymer and polyethylene is not imexpected, since a proportionately large difference exists for the liquid surface tensions of hydrocarbons and fluorocarbons having five to eight carbon atoms [58]. The underlying cause of this difference is, however, more obscure. The inter molecular forces for fluorocarbons apparently have features wuich lead to anomalous behavior, at least from the point of view of solubility parameter theory [59]. Thus, theoretical calculations of the surface tension for the bare solid in the case of polytetrafluoroethylene would face a number of difficulties not encountered with paraffin crystals. 

Four polymers with different surface compositions were used in this study—polystyrene (PS), poly(methyl methacrylate) (PMMA), polyacrylamide (PAM), and a poly(vinylidene chloride) (PVeC) copolymer (containing 20% polyacrylonitrile). Polystyrene has essentially a hydrocarbon surface, whereas the surfaces of poly (methyl methacrylate) and polyacrylamide contain ester and amide groups, respectively. The surface of the poly(vinylidene chloride) copolymer on the other hand will contain a relatively large number of chlorine atoms. The presence of acrylonitrile in the poly(vinylidene chloride) copolymer improved the solubility characteristics of the polymer for the purposes of this study, but did not appreciably alter, its critical surface tension of wetting. Values of y of these polymers ranged from 30 to 33 dynes per cm. for polystyrene to approximately 40 dynes per cm. for the poly(vinylidene chloride) copolymer. No attempt was made to determine e crystallinity of the polymer samples, or to correlate crystallinity with adsorption of the fluorocarbon additives. 

Structure, length of hydrocarbon chain, location [26] and amount of fluorine atoms in fluoroalkyl radical significantly affect properties of appropriate methacrylic polymer (see Table 8.9). Increasing the content of fluorine atoms in the polymer, increases the chemoresistance, thermo- and heat resistance and water absorption decreases. It should be noted that the fluorine atom in the ro-position in fluoroalkyl radical leads to a decrease of the surface tension, which is also the the reason for poor adhesion of the fluorine-containing polymer to the core and water absorption. In the a-position, the heat resistance and other properties of polymers obtained on this basis are presented in Table 8.10 [25]. 

SURFACE TENSION IN RELATION TO COHESIVE ENERGY WITH PARTICULAR REFERENCE TO HYDROCARBON POLYMERS. 

The great potential utility of this Equation lies in the fact that, since aU the variables are accessible through reasonably simple experiments, a sohd interacting by dispersion forces alone (e.g., a pure hydrocarbon wax or polymer) can be used to determine the dispersion component of the surface tension of a liquid. Alternatively, a standard nonpolar liquid can be used to characterize the surface tension of a solid, or if the clean solid has been previously characterized, the contact angle can be used to assist in determining the nature of a surface contaminant. 

On the basis of the already explained surface energy properties of fluorinated chemicals, it is understood that for instance a non-fluorine surface treatment, such as silicones on treated polymers, can provide rather good water repellency, but no oil repellency due to the fact that the oil has lower surface energy than that the silicone layer has. The surface energy obtained with a silicone surface cannot be lower than 22 mN/m, which is comparable to the surface tension of hydrocarbon, oily substances. This means that fluorotelomers are not always possible to replace with a non-fluorine surface treatment if oil or soil repellence is required. 

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