Penetration of preservatives

Liquid Contamination. As treating solutions are continuously reused, they become contaminated with particulate matter from chemical reactions and extraneous sources. It has been shown that this particulate matter can significantly reduce the penetration of preservative solutions into wood. Consequently, it appears that methods for continuously removing particulate matter could result in improved penetration of preservative solutions. 

Thermal Treatment. Thermal treatment, or hot and cold dipping, is similar to dipping, except the member is first heated in the preservative in an open tank and is then submerged in cold preservative. This procedure provides deeper penetration of preservative than dipping and steeping. 

Other. 2-Nitro-1-butanol is an excellent solvent for many polyamide resins, cellulose acetate butyrate, and ethylceUulose. It can be utilized in paint removers for epoxy-based coatings. 2-Hydroxymethyl-2-nitro-l,3-propanediol is usebil for control of odors in chemical toilets. Its slow release of formaldehyde ensures prolonged action to control odor, and there is no reodorant problem which sometimes is associated with the use of free formaldehyde. 2-Hydroxymethyl-2-nitro-l,3-propanediol solutions are effective preservative and embalming fluids. The slow Uberation of formaldehyde permits thorough penetration of the tissues before hardening. 

The sapwood of all timber species is susceptible to attack, so the use of a durable species usually requires the removal of the sapwood. However, since sapwood can usually be fully penetrated with preservative by an impregnation treatment, an overall durable product can be obtained from the timber of small trees of a durable species containing a high proportion of sapwood. 

The conventional concentration of benzalkonium chloride in eyedrops is 0.01%, with a range of 0.004-0.02% [111]. While uptake of benzalkonium chloride itself into ocular tissues is limited [113], even lower concentrations of benzalkonium chloride have been reported to enhance corneal penetration of other compounds including therapeutic agents [93,112,114]. The differential effect of this preservative on the cornea compared to the conjunctiva can be exploited to target a drug for corneal absorption and delivery to the posterior segment of the eye [115]. Its use has been proposed as a means of delivering systemic doses by an ocular route of administration [116]. 

This preservative is comparatively new to ophthalmic preparations and is a polymeric quaternary ammonium germicide. Its advantage over other quaternary ammonium seems to be its inability to penetrate ocular tissues, especially the cornea. It has been used at concentrations of 0.001-0.01% in contact lens solutions as well as dry eye products. At clinically effective levels of preservative, POLYQUAD is approximately 10 times less toxic than benzalkonium chloride [87,137], Various in vitro tests and in vivo evaluations substantiate the safety of this compound [137,141,142], This preservative has been extremely useful for soft contact lens solutions because it has the least propensity to adsorb onto or absorb into these lenses, and it has a practically nonexistent potential for sensitization. Its ad-sorption/absorption with high water and high ionic lenses can be resolved by carefully balancing formulation components [143],... 

The preservative must be sufficiently soluble in water to achieve adequate concentrations in the aqueous phase of a two or more phase system. The proportion of preservative remaining undissociated at the pH of the preparation must be capable of penetrating the microorganisms and destroying its integrity. 

Rapid freezing is the best preservation tool. The use of liquid nitrogen and cooled isopentane are for the purposes of cooling quickly. The tissue should be relatively small in size to allow for rapid and thorough penetration of the cold-temperature chemicals. 

Percutaneous penetration of 7V-nitrosodiethanolamine was measured using cryo-preserved human trunk skin and three vehicle formulations (isopropyl myristate, sunscreen cream or a 10% shampoo) containing 7V-nitroso[ C]diethanolamine. The absorption rate of a low dermal dose (10 ixg/cm ) of 7V-nitrosodiethanolamine was a linear function of the concentration (0.06, 0.2 or 0.6 Xg/ xL) applied to the skin. The peak rates for the isopropyl m uistate and shampoo vehicles were seen within five hours and for the sunscreen somewhat later. Total 48-h absorption ranged from 35 to 65% of the dose and was formulation-dependent (isopropyl m uistate > shampoo > sunscreen). A total absorption of 4-6 x JcaE was estimated to equate to an applied N-nitrosodiethanolamine dose of 10 x%lcaE. When applied as a large infinite dose (0.5 mg/cm ), total 7V-nitrosodiethanolamine absorption (4-35% of the applied dose) followed a different rank order (shampoo > isopropyl m uistate > sunscreen), probably due to the barrier-damaging properties of the vehicles. The permeability coefficient for isopropyl myristate was 3.5 X 10 cm/h (Franz etal., 1993). 

H. Kumatsu, K. Higazi, H. Okamoto, K. Miyakwa, M. Hashida, and H. Sezaki, Preservative activity and in vivo percutaneous penetration of butylparaben entrapped in liposomes, Chem. Pharm. Bull. 34 3415-3422 (1986). 

In the past, it has been well documented that the relative permeability of wood has a significant influence on the effectiveness of preservative treatments. Without adequate penetration, even the best preservatives will not provide sufficient protection and the wood will fail prematurely. Consequently, any developments in methods which increase the treatability of wood which are difficult to treat could have a significant impact on the wood preserving industry. 

Improved penetration, higher preservative retention, and more uniform treatments were the general rule in the above studies. Furthermore, the results were similar among the species of fungi tested. 

Weather affects the performance of wood finishes adversely. Both types of finish the film-forming (paints, varnishes) and the penetrating (stains, preservatives) are adversely affected by a combination of the following factors ... 

There are two types of pressure treatment, the full-cell and the empty-cell. The full-cell process seeks to fill the cell lumens of the wood with the preservative liquid, giving retention of a maximum quantity of preservative. The empty-cell process seeks deep penetration with a relatively low net retention of preservative by forcing out the bulk liquid in the wood cells, leaving the internal capillary structure coated with preservative. 

In the empty-cell process, the preservative liquid is forced under pressure into the wood, containing either its normal air content (Lowry process) or an excess of air, by first subjecting the wood to air pressure before applying the preservative under pressure (Rueping process). In the former case, the preservative is put in the cylinder containing the wood at atmospheric pressure, and, in the latter case, under air pressure of 25-100 psi. After the wood has been subjected to the hot preservative (about 190-200°F) underpressure (100-200 psi in the Lowry process and 150-200 psi in the Rueping process) and the pressure has been released, the back pressure of the compressed air in the wood forces out the free liquid from the wood. As much as 20-60 percent of the injected preservative may be recovered, yet good depth of penetration of the preservative is achieved. 

---