Polymer laminates permeability

A surface is that part of an object which is in direct contact with its environment and hence, is most affected by it. The surface properties of solid organic polymers have a strong impact on many, if not most, of their apphcations. The properties and structure of these surfaces are, therefore, of utmost importance. The chemical stmcture and thermodynamic state of polymer surfaces are important factors that determine many of their practical characteristics. Examples of properties affected by polymer surface stmcture include adhesion, wettability, friction, coatability, permeability, dyeabil-ity, gloss, corrosion, surface electrostatic charging, cellular recognition, and biocompatibility. Interfacial characteristics of polymer systems control the domain size and the stability of polymer-polymer dispersions, adhesive strength of laminates and composites, cohesive strength of polymer blends, mechanical properties of adhesive joints, etc. 

However, a laminate consisting of a layer of TPX, an intermediate layer of an adhesive resin composition and a layer of an olefin polymer has excellent peel resistance and can be easily controlled in the gas permeability (10). 

The book opens with a paper on the structure and composition of wood to define the material under discussion and then considers molds, permeability, wood preservation, thermal deterioration and fire retard-ance, dimensional stability, adhesion, reconstituted wood boards such as fiberboard and particleboard, plywood, laminated beams, wood finishes, wood-polymer composites, and wood softening and forming. A final paper treats the common theme of wastewater management. Only one of the papers presented at the meeting is not included in this volume, and its subject of conventional wood preservation methods is adequately treated in detail elsewhere (e.g., Nicholas, D. D., Ed Wood Deterioration and Its Prevention by Preservative Treatments, 2 vols., Syracuse University Press, 1973). 

The second system was developed by the Hereon Division of Health Chem Corporation and consists of a laminated plastic chip. Figure 2. The chip is composed of a pheromone saturated polymer reservoir with a semi-permeable plastic membrane on either side. The pheromone is thus released by diffusion from the reservoir through the membrane. The rate is controlled by the membrane composition and thickness(2,). The two systems are applied in a polybutene sticker to facilitate adhesion to the plant surface. 

Virtually the entire membrane manufacture today is based on laminate structures comprising a thin barrier layer deployed upon a much thicker, highly permeable support. Most are formed of compositionaUy homogeneous polysulfone, cellulose acetate, polyamides, and various fluoropolymers by phase inversion techniques in which ultrathin films of suitably permselective material are deposited on prefabricated porous support structures. Hydrophobic polymers as polyethylene, polypropylene, or polysulfone are often used as supports. A fairly comprehensive hst of microporous and ultrafiltration commercial membranes and produced companies are presented in Refs [107-109]. A review on inorganic membranes has been given in Ref. [110]. 

Layered Structures. Whenever a barrier polymer lacks the necessary mechanical properties for an application or the barrier would be adequate with only a small amount of the more expensive barrier polymer, a multilayer structure via coextmsion or lamination is appropriate. Whenever the barrier polymer is difficult to melt process or a particular traditional substrate such as paper or cellophane [9005-81-6] is necessary, a coating either from latex or a solvent is appropriate. A layered structure uses the barrier polymer most efficiently since permeation must occur through the barrier polymer and not around the barrier polymer. No short cuts are allowed for a permeant. The barrier properties of these structures are described by the permeance P which is described in equation 16 where Pt and L are the permeabilities and thicknesses of the layers. 

The immiscibility between both polymers is an important drawback that obliges us to look for a well-balanced set of properties, namely, mechanical, vapor, and gas permeability for barrier applications. Most applications are as multilayer, or sandwich sheet stmctures (polymer A/adhesive layer/polymer B), obtained by coextrusion, lamination or coating operations. The high operation costs associated with these processing techniques and the inability to obtain complex shapes that can expand... 

With the exception of packages for fresh produce, MAP trays or pouches need to be made of materials with low permeability to gases (CO2, Nj, O2). Laminates made of various combinations of polymer as PET, PVdC, PE, and polyamides such as nylons should have low oxygen permeability [175-178]. Respiration in fruits and vegetables leads to a build-up of carbon dioxide that may reduce the rate of respiration and help to prolong the shelf life of the product a reduction in the oxygen... 

Permeability. Many polymers are used in packaging and, in particular, for food. In this latter case the permeability to gases and vapors is of prime importance. The permeation or transmission of a gas or vapor is a function of the solubility of a gas or vapor in the polymer and the rate of diffusion through the matrix. The permeability coefficient, diffusion constant, and solubility coefficients can all be measured and are influenced by the chemical structure and morphology. In order to achieve the required permeability characteristics it is common to co-extrude a series of polymers to form a laminated structure. Such materials allow selective permeation of a specific species and enhance the life of the product (190,191) (see Transport Properties). 

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