Foam films multilayers

The production of foamed films and sheets from polypropylene and polystyrene is discussed, with particular reference to packaging applications. Advantages of foamed materials for this application are examined, and the chemical and physical foaming processes are described. Extrusion technology for film and sheet by chemical and physical foaming processes is discussed, and recent developments in the coextrusion of multilayer packaging trays for the food industry are considered. 

Chicago, II., 29th Aug-2nd Sept.l993,p.403-6. 6A MULTILAYER EXTRUSION DIE GEOMETRY FOR THERMOFORMED POLYPROPYLENE FOAM SHEETS AND BIAXIALLY ORIENTED FOAM FILMS Raukola J Savolainen A Tampere,University of Technology (TAPPI)... 

It is also interesting to study the role of surface forces in formation of multilayer structures from amphiphiles in the foam film as well as the comparison with the lamellar mesomorphic phases, produced in aqueous dispersions of amphiphile molecules. 

Exerowa and Lalchev [21] have reported a n(7iw) isotherm of multilayer foam films from AS sample solutions containing 47% ethyl alcohol and 710 2 mol dm 3 NaCl, which provides very important information. It is shown in Fig. 11.10. This isotherm has a stepwise... 

In order to develop the model, a first approximation of the multi-voided complex stmcture of the foamed film is defined as presented in a cross-section view in Fig. 1 a. This simplified design was rearranged in a multilayer system composed by solid polymer films spaced by air layers as shown in Fig. lb. From this, a further simplification of the stmcture can be performed by grouping all of the solid layers and the air layers into a three-layer system as shown in Fig. 2a. 

Multilayered films can be formed by various processes to create a structure with specific mechanical and physical properties. Microlayers of foam/film were formed by microlayer coextrusion technology [176], resulting in unique mechanical properties studied by optical microscopy. Cohen et al. [177] studied... 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

Tween 20 was considerably more effective at reducing the stability of foams of a-la than was the case with /3-lg. There was a significant decrease in a-la foam stability in the presence of Tween, at R values as low as 0.05. Minimal foam stability was observed at R = 0.15. There was no observed change in film drainage behavior or onset of surface diffusion in the adsorbed protein layer up to this R value. The only observed change was a progressive decrease in film thickness. Therefore, it is likely that disruption of adsorbed multilayers is responsible for a reduction in the structural integrity of the adsorbed protein layer and that this increases the probability of film rupture. 

Another example of a lack of correlation is the behaviour of a foam consisting of multilayer films, obtained from concentrated surfactant solution with a solubilised oil [59]. The viscosity of such foaming solutions is much higher compared to that of the solutions of the individual components and the foam drainage rate is considerably lower. However, in gravitational field as well as under applied pressure drop, the stability of such foams is strongly reduced. 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

Of particular note to date are developments using ESI-based materials in foam applications. Novel foam structures offer attractive properties and characteristics including softness, esthetics and drape for a wide range of thermoplastic and crosslinked foam applications. Other product technologies of interest are as injection molded structural foams, as foamed layers in multilayer structures and as foamed blends of interpolymers with styrenic and olefinic polymers [77-79], Interpolymers also have potential for co-extruded film and sheet applications. 

A number of manufactured products comprising thermoplastics (polyethylene, polyvinylchloride, polyamides,. ..) or elastomers (natural rubber, nitrile rubber, ethylene propylene rubber, silicones) are processed by cross-linking to improve their performances in insulated electric wires and cables, multilayered films for cooking pouches, shape memory tubes, pressure resistant water pipes, expandable foams, automotive parts exposed to motor... 

A complicated many-layer structure of foam cells is formed when gas bubbles are sparged into solutions of surfactants. According to [429], each bubble has a two-sided envelope which is a layer of the solvent containing hydrophilic polar parts of surfactant molecules (see Figure 7.2). Nonpolar hydrophobic parts of molecules on the inner surface of the envelope are oriented toward the bubble, and on the outer surface, outward the envelope. Between two cells, each of which is a capsule with envelope, there is a lamella, that is, an interlayer of a complicated structure. In the middle of the lamella, there is a liquid layer that is a continuous phase. On each of two surfaces of this layer, there is a monolayer of the surfactant. The hydrophobic parts ( tails ) of surfactants molecules in each monolayer and the tails of the envelope form two direct plate micellae [413], which separate the envelope and the continuous liquid film at the center. Thus, gas bubbles in foam are separated at least by five distinct layers. The multilayer structure of a foam lamella is well seen in photographs (e.g., see [429], p. 54). This fact is also confirmed by the ladder-type shape of the disjoining pressure... 

Since the boundary of a foam cell is a multilayer film [429], the elasticity of this shell is many times larger than the elasticity of the adsorption layer. 

The point is that the lamella [429] has a complex multilayer structure (see Figure 7.2). It consists of two boundaries of foam cells, two direct plate micellae [413], and a liquid film, which is a part of the continuous phase of the foam. All in all, the lamella has six parallel adsorption layers. Hence, its modulus of elasticity must be many times larger than that of a simple adsorption layer. 

PS is limited in its use and is mostly applied as a film-forming material because of its brittleness, insufEciently high heat resistance and instability in organic solvents. To improve its impact resistance and elasticity PS is often modified by rubbers [20,22]. Foamed impact-resistant PS is rather seldom used as a layer-vessel incorporating Cl in multilayered anticorrosion films. 

The simplest modifications of named materials are multilayered and combined films in which either solid [41] or porous polymer layers (foamed, fibrous, etc.) are filled with the inhibiting liquid [4,18,23,24,30,39]. In addition, film materials where the adhesive layer is filled by a liquid Cl are used in packaging techniques [4,23,37]. 

Apart from this, combined and multilayered films containing a damping layer foamed by Cl vapors are known [3,24]. 

All that we need to do is to optimize the ceramics and, perhaps the more challenging step, optimize the geometry of the sample to allow repeated cycling (1) without degradation and (2) while allowing easy extraction of the hydrogen. So the usual process is to improve the design (tailor) of both the material (e.g., powder versus multilayer films or foams) and of the reactor itself. 

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