Formulations chemical peels

Bridenstine JB, Dolezal JF (1994) Standardizing chemical peel solution formulations to avoid mishaps. Great fluctuations in actual concentrations of trichloroacetic acid. J Dermatol Surg Oncol 20 813-816... 

The book provides information about each single chemical peel, fully explained in terms of its properties, formulations, indications, performing technique, advantages and disadvantages. 

Despite some general predictable outcomes, even superficial chemical peeling procedures can cause hyperpigmentation and undesired results. Popular standard salicylic acid peeling techniques involve the use of 20% and 30% salicylic acid in an ethanol formulation. Salicylic acid peels are performed at 2-4 week intervals. Maximal results are achieved with a series of 3-6 peels. 

Physical agents include zinc oxide and titanium dioxide. These are the most effective sunscreens because they reflect UVA and UVB. When applied to the skin, they induce a white or ashen color, which many patients find cosmetically unacceptable. Micronized formulations of these agents are available which enhance cosmetic acceptability. Sunscreens should be worn regularly prior to chemical peeling procedures. 

A brief study of the chemistry of the molecules and solutions used in chemical peels immediately questions the hypothesis that acidity is the only basis for the action of peeling solutions. In fact, with the exception of trichloroacetic add (TCA) and non-neutralized glycolic acid solutions, the most commonly used peeling solutions are only weakly acidic, and phenol and resorcinol mixtures may not be acidic at all, having a pH greater than 7 in some formulations. 

The usual classification of chemical peels comprises superficial, medium and deep peels. For superficial peels, AHA, Jessner s solution, tretinoin, TCA in concentrations of 10-30% and most recently hpo-hydroxy add are used to induce an exfoliation of the epidermis. Medium-depth agents such as TCA (30-50%) cause an epidermal to papillary dermal peel with subsequent regeneration. Deep peels using TCA (>50%) or phenol-based formulations penetrate the reticular dermis to induce dermal regeneration. The success of peeling in darker skin is crudally dependent on the physician s understanding of the chemical and biological processes, as well as of indications, clinical effectiveness and side effects of the procedure (see Box 9.1). 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

The major problem is that the attainment of properties such as peel, flexibility, and toughness is generally accompanied by the reduction in properties such as heat resistance, chemical resistance, and shear strength. Future chapters discuss how the epoxy adhesive formulator can merge these properties. 

Aliphatic amines Convenience Room temperature cure, fast elevated-temperature cure Low viscosity Low formulation cost Moderate chemical resistance Critical mix ratios Strong skin irritant High vapor pressure Short working life, exothermic Poor bond strength above 80°C Rigid, poor peel and impact properties Adhesives and sealants Casting and encapsulation Coatings... 

Polysulfide resins combine with epoxy resins to provide adhesives and sealants with excellent flexibility and chemical resistance. These adhesives bond well to many different substrates. Tensile shear strength and elevated-temperature properties are low. However, resistance to peel forces and low temperatures is very good. Epoxy polysulfides have good adhesive properties down to -100°C, and they stay flexible to -65°C. The maximum service temperature is about 50 to 85°C depending on the epoxy concentration in the formulation. Temperature resistance increases with the epoxy content of the system. Resistance to solvents, oil and grease, and exterior weathering and aging is superior to that of most thermoplastic elastomers. 

Polymerized epoxy adhesives are amorphous and highly crosslinked materials. This microstructure results in many useful properties such as high modulus and failure strength, low creep, and good chemical and heat resistance. However, the structure of epoxy resins also leads to one undesirable property—they are relatively brittle materials. As such, epoxy adhesives tend to have poor resistance to crack initiation and growth, which results in poor impact and peel properties. In sealant formulations, epoxy resins do not often provide the degree of elongation or movement that is required for many applications. 

Silyl-terminated polyether can be blended with epoxy resins to form elastic adhesives at room temperature.9 In this system, small rigid epoxy particles are chemically linked by an elastic poly ether phase. A typical formulation is shown in Table 8.4. Elongation and peel... 

The primary mode of bonding for a pressure-sensitive adhesive is not chemical or mechanical but rather a polar attraction to the substrate. This always requires pressure to achieve sufficient wet-out onto the surface thereby providing adequate adhesion. The four main varieties of pressure-sensitive adhesives are derived from rubber-based, acrylic, modified acrylic and silicone formulations. Release liners are used to carry the sticky label and enable it to be printed. The release liners are normally paper, treated with a very thin silicone coating to allow the label to be peeled away easily without tearing. Some transfer of the silicone into the adhesive is inevitable. 

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