Blown film extmsion

Poly(vinyl chloride). To be converted into film, poly(viayl chloride) [9002-86-22] (PVC) must be modified with heat stabilizers and plasticizers, which increase costs. Plasticized PVC film is highly transparent and soft, with a very high gas-permeation rate. Water-vapor transmission rate is relatively low. At present, PVC film is produced by blown-film extmsion, although casting and calendering are employed for heavier gauges (see Vinyl POLYAffiRS). 

Fig. 3. Blown film extmsion procedure. Courtesy of Quantum Chemical Co.

Thermoplasticity. High molecular weight poly(ethylene oxide) can be molded, extmded, or calendered by means of conventional thermoplastic processing equipment (13). Films of poly(ethylene oxide) can be produced by the blown-film extmsion process and, in addition to complete water solubiUty, have the typical physical properties shown in Table 3. Films of poly(ethylene oxide) tend to orient under stress, resulting in high strength in the draw direction. The physical properties, melting behavior, and crystallinity of drawn films have been studied by several researchers (14—17). 

PE films are usually produced by the blown film extmsion method. The alternative method is sheet extrusion followed by passing through chilled rollers. Blown film is more economic biaxial orientation is introduced by the draw-off and blow ratios. Bubble stability is critical to the blown film process. Bubble stabihty is provided by the melt strength and rheological characteristics of the PE. Long... 

Figure 7-12. An example of how blown film-extmsion machine settings can affect the properties of plastics.

Several more recent variations of the film-to-fiber approach result in direct conversion of film to fabric. The film may be embossed in a controlled pattern and subsequently drawn uniaxiaHy or biaxiaHy to produce a variety of nonwoven products (47). Addition of chemical blowing agents to the film causes fibrillation upon extmsion. Nonwovens can be formed directly from blown film using a unique radial die and control of the biaxial draw ratio (48)... 

Melt Extrusion. By far the most important method for producing film and sheeting materials reties on one or another of the various melt extmsion techniques (5). The main variations of melt extmsion are the slot (or flat) die-cast film process, the blown films process, and the flat die sheeting-stack process. These may be combined with one or more steps such as coextmsion wherein multilayer film or sheet is formed, biaxial orientation, and in-line coating (6). 

Extmsion of polyethylene and some polypropylenes is usually through a circular die into a tubular form, which is cut and collapsed into flat film. Extmsion through a linear slot onto chilled rollers is called casting and is often used for polypropylene, polyester, and other resins. Cast, as well as some blown, films may be further heated and stretched in the machine or in transverse directions to orient the polymer within the film and improve physical properties such as tensile strength, stiffness, and low temperature resistance. 

Many polymers, including polyethylene, polypropylene, and nylons, do not dissolve in suitable casting solvents. In the laboratory, membranes can be made from such polymers by melt pressing, in which the polymer is sandwiched at high pressure between two heated plates. A pressure of 13.8—34.5 MPa (2000—5000 psi) is appHed for 0.5 to 5 minutes, at a plate temperature just above the melting point of the polymer. Melt forming is commonly used to make dense films for packaging appHcations, either by extmsion as a sheet from a die or as blown film. 

Knitted nets are produced from blown film or specialized extmsion lines with corotating dies. The film rolls are slit into small tapes of 2-5 mm width. Then the tapes are stretched on heated rolls (galettes) in several steps to achieve and maintain a high level of orientation. Typical stretching factors are three to five times. Thus, the materiel undergoes a strain-induced crystallization, which maximizes the crystallinity level in the tape. 

Extrusion. Sheet, film, and profiled articles ate made by extmsion (20). The resin is melted and forced through a die plate or head. Variations include multilayer and blown film applications. In multilayer coextmsions, different combinations of plastics are separately but concurtendy extmded to form layered sheet or film. In the packaging industry, specialty resins such as high battier ethjiene—vinyl alcohol copolymers are combined with heat- and impact-resistant thermoplastics for food packages. The properties of each resin layer ate additive, as opposed to the "averaging of property" in blends. Multilayers ate also used for blow-molded containers, films, and sheet products (see also Film AND sheeting materials). 

Commercially, the melt blending of an impact modifier in HOPE can be done simply during the extmsion process used for making the blown film or blow-molded rigid containers or in the extmsion of pipes and ducts. Ideally, such low-temperature... 

Typical processing methods - blown film, blow molding, coating, extmsion, injection molding, slot cast extrusion ... 

Later metallocene/single-site catalysts were developed, which drove the production of LCBs. These LCBs increased entanglement in the molten polymer, resulting in more stable blown film processing. They also reduced extmsion pressures and allowed higher throughput. 

Blends of biodegradable synthetic aUphatic polyesters and starch are used to produce sheets and films for packaging by film extmsion or blown film methods. Up to 50% of the synthetic polyester can be replaced with starch. A polyester synthesised from the poly-condensation of 1,4-butanediol and a mixture of adipic and succinic acids has been blended with wheat starch by Lim (1999) (Nolan-ITU Pty Ltd, 2002). The blends were found to have melting points near that of the polyester alone. Plasticisers were also added to the starch to improve flexibiUty and processability of the blend. The modified blends were found to retain a high tensile strength and elongation, even at high starch concentrations. 

Incorporation of silicate nanolayers in semi-crystalline polymers like polypropylene ean have a two-fold effect on the barrier properties, (1) well oriented large aspeet ratio platelets will increase the tortuosity of the diffusion path and (2) the nanolayers will affect the crystalline order (size and interlamellar spacing) and possibly affect the barrier properties. The extent of orientation is greater in blown film than in extmsion cast film and this leads to similar trends in barrier properties of polypropylene nanocomposites with 7wt.% 1.3 IPS (silated) clay as reported by Qian et al. With cast films, the nanoeomposite had a lower permeability to oxygen by a faetor of 1.5 compared to neat polypropylene. With blown films, the nanocomposite permeability to oxygen was lower by a factor of 2.5 compared to neat polypropylene. However, Ellis and D Angelo were able to prepare only intercalated polypropylene nanocomposites with the same 1.31 PS and obtained no improvement in permeability to a solvent over that for neat polypropylene. This underlines the greater sensitivity of barrier performanee to the level of dispersion and orientation. 

One of the key rheological properties in extmsion blown film is extensional viscosity. Indeed, it is well recognized that bubble stability, biaxial stretch and thickness uniformity are very sensitive to extensional viscosity. However, most of the extensional data reported is in uniaxial rather than biaxial deformation mode. 

Copolymer film is produced by extmsion blowing foUowed by water quenching. In-line, the film is blown, crystallized, and oriented. PVDC copolymer film is difficult to produce. 

Nip Rolls n (pinch rolls) In film blowing, a pair of rolls situated at the top of the tower that pinch shut the blown-fihn tube, seal air inside it, and regulate the rate at which the film is pulled away from the extmsion die. One roll is usually covered with a resilient material, the other being of metal and internally cooled. 

---