Polarity acrylic

Energy Characteristics of SDDS Adsorption on Water-Alkyl Acrylate Interface Depending on Alkyl Acrylate Polarity... 

The Diels-Alder reaction provides us with a tool to probe its local reaction environment in the form of its endo-exo product ratio. Actually, even a solvent polarity parameter has been based on endo-exo ratios of Diels-Alder reactions of methyl acrylate with cyclopentadiene (see also section 1.2.3). Analogously we have determined the endo-exo ratio of the reaction between 5.1c and 5.2 in surfactant solution and in a mimber of different organic and acpieous media. These ratios are obtained from the H-NMR of the product mixtures, as has been described in Chapter 2. The results are summarised in Table 5.3, and clearly point towards a water-like environment for the Diels-Alder reaction in the presence of micelles, which is in line with literature observations. 

Resin and Polymer Solvent. Dimethylacetamide is an exceUent solvent for synthetic and natural resins. It readily dissolves vinyl polymers, acrylates, ceUulose derivatives, styrene polymers, and linear polyesters. Because of its high polarity, DMAC has been found particularly useful as a solvent for polyacrylonitrile, its copolymers, and interpolymers. Copolymers containing at least 85% acrylonitrile dissolve ia DMAC to form solutions suitable for the production of films and yams (9). DMAC is reportedly an exceUent solvent for the copolymers of acrylonitrile and vinyl formate (10), vinylpyridine (11), or aUyl glycidyl ether (12). 

Solution Properties. Typically, if a polymer is soluble ia a solvent, it is soluble ia all proportions. As solvent evaporates from the solution, no phase separation or precipitation occurs. The solution viscosity iacreases continually until a coherent film is formed. The film is held together by molecular entanglements and secondary bonding forces. The solubiUty of the acrylate polymers is affected by the nature of the side group. Polymers that contain short side chaias are relatively polar and are soluble ia polar solvents such as ketones, esters, or ether alcohols. As the side chaia iacreases ia length the polymers are less polar and dissolve ia relatively nonpolar solvents, such as aromatic or aUphatic hydrocarbons. 

The monomer pair, acrylonitrile—methyl acrylate, is close to being an ideal monomer pair. Both monomers are similar in resonance, polarity, and steric characteristics. The acrylonitrile radical shows approximately equal reactivity with both monomers, and the methyl acrylate radical shows only a slight preference for reacting with acrylonitrile monomer. Many acrylonitrile monomer pairs fall into the nonideal category, eg, acrylonitrile—vinyl acetate. This is an example of a nonideality sometimes referred to as kinetic incompatibiUty. A third type of monomer pair is that which shows an alternating tendency. 

A more polar comonomer, eg, an AN comonomer, increases the water-vapor transmission more than VC when other factors are constant. For the same reason, AN copolymers are more resistant to penetrants of low cohesive energy density. AH VDC copolymers, however, are very impermeable to ahphatic hydrocarbons. Comonomers that lower T and increase the free volume in the amorphous phase increase permeability more than the polar comonomers higher acrylates are an example. Plasticizers increase permeabiUty for similar reasons. 

Group-Transfer Polymerization. Living polymerization of acrylic monomers has been carried out using ketene silyl acetals as initiators. This chemistry can be used to make random, block, or graft copolymers of polar monomers. The following scheme demonstrates the synthesis of a methyl methacrylate—lauryl methacrylate (MMA—LMA) AB block copolymer (38). LMA is CH2=C(CH2)COO(CH2) CH2. 

To improve the performance of acryUc elastomers, side chains are required where the 5 value is higher than with alkyl groups. Thus the use of polar groups, for instance heteroatoms, is suggested. The general formula for these acrylate monomers may be portrayed as follows ... 

The process yields a random, completely soluble polymer that shows no evidence of crystallinity of the polyethylene type down to —60°C. The polymer backbone is fully saturated, making it highly resistant to ozone attack even in the absence of antiozonant additives. The fluid resistance and low temperature properties of ethylene—acryUc elastomers are largely a function of the methyl acrylate to ethylene ratio. At higher methyl acrylate levels, the increased polarity augments resistance to hydrocarbon oils. However, the decreased chain mobiUty associated with this change results in less fiexibihty at low temperatures. 

The humidity requirement is probably most significant for polar or hydrophilic adhesives, for example,. some of the acrylate adhesives. 

It is for this reason that the discovery by Ulrich was of significant importance to the successful development of acrylic PSAs. He found that by copolymerizing polar monomers, such as acrylic acid, one could greatly increase the cohesive strength of the polymer allowing PSA articles coated with this type of material to sustain a load without premature shear failure. These polar monomers commonly... 

An example of the contribution of polar interactions between an acrylic PSA and a substrate is shown in Fig. 6. By copolymerizing iso-octylacrylate and acrylic acid, using a monomer ratio of, respectively, 95/5 and 90/10, two otherwise identical PSAs were made. The PSAs were laminated to both sides of a foam core to make an attachment tape as used in the automotive industry for the application of body side moldings to a car. One side of the foam tape was laminated against an aluminum foil backing. The other side was laminated against an automotive paint-coated panel to make the final test sample. The test sample was allowed to... 

Fig. 7 shows the effect of an increasing amount of polar monomer on static shear holding tested at room temperature in a non-crosslinked iso-octylacrylate/ acrylic acid copolymer PSA. 

As the amount of acrylic acid in the polymer increases, the degree of hydrogen bonding between polymer chains also increases causing the cohesive strength to improve without the need for crosslinking. Very similar observations can be made for other polar monomers, such as acrylamide. 

The amount of polar monomer one would copolymerize with the alkyl acrylate monomer(s) very much depends on the type of polar monomer and the desired change in rheological properties one would like to achieve. Strong hydrogen bonding monomers, such as acrylic acid, methacrylic acid, acrylamide, or methacrylamide are typically used at levels of 12% or less of the total monomers. 

Weaker polar monomers like, vinylacetate, N-vinyl pyrrolidone, fV-vinyl caprolactam or MiV-dimethyl acrylamide may be used at levels as high as 30-40% of the total monomers. Combinations of polar monomers [65-67] can also be used to formulate acrylic PSAs with a good balance of properties. 

While polar monomers are usually beneficial in acrylic PSA formulations, there are times when their presence is deleterious. Examples of this may be the use of acrylic acid containing adhesives for electronic applications, for adhering to some metallic surfaces, or for application to paper used in books. Higher levels of acrylic acid not only increase the acidity of the PSA but they also increase the moisture uptake in the adhesive making dissociation of the acid easier. This can increase corrosion problems in the electronic or metal applications, or severe discoloration and degradation of paper with time. The latter is often a significant concern to librarians who deal with repair and archival restoration of books. In applications such as these, acid-free adhesives are more desirable, or at the very least the amount of acid has to be low and caution has to be taken to fully incorporate the monomer into the PSA. 

Polar monomers may also be introduced into the PSA because they provide a simple way to incorporate functional groups into the polymer backbone that are available for further chemical reaction. For example, monomers like 2-hydroxy ethyl acrylate or 3-hydroxypropylacrylamide will introduce hydroxy functionality in the polymer. These hydroxy groups can be used for chemical... 

As pointed out earlier, acrylics differ from the commonly used rubber precursors for PSA formulation in the fact that they often incorporate polar monomers, such as acrylic acid, A-vinyl pyrrolidone, vinyl acetate, or acrylamide. As a result, the solubility parameters of acrylic polymers are typically higher than those of rubbers, like polyisoprenes or polybutadienes. 

The increased polarity of the acrylic polymers puts more stringent requirements on the properties of the tackifiers or plasticizers that can be used. The very low polarity additives commonly found in rubber based PSAs are not useful in most acrylic PSA formulations. For example, materials like paraffin waxes, mineral oils, and synthetic hydrocarbon tackifiers have little or no value in most acrylic PSAs. 

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