Polyamide for Flexible Packaging Film

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    Polyamide for Flexible Packaging Film 2003 PLACE Conference12 – 14 May 2003   Rome, Italy   Dr. Walter Goetz, BASF AG, Ludwigshafen/Germany Abstract PA 6 is a widely used resin for the production of flexible packaging film, in most cases combined with polyolefins as a component of a multilayer structure. It has found numerous applications for its uniquecombination of properties like mechanical strength, transparency, thermoformability and barrier.This presentation summarizes the fundamental information about the chemistry and properties of nylon resins,their processing, and the properties and applications of packaging film produced utilizing nylon.Polyamides may be processed into film by blown or cast film process, oriented or non-oriented. Standardextrusion equipment may be used, however post-treatment by humdidification or annealing helps to postcrystallize PA film and to achieve dimensionally stable, ready-to-use film of high quality.  Polyamide for Flexible Packaging Film fig 1 1. Introduction Polyamide is widely used for the the production of flexible film for packaging of perishable food due to itsunique combination of properties: fig 2: z mechanical strength z high heat distortion temperature z high flexibility and toughness z good barrier to oxygen, chemicals and aroma substances z high transparency z thermoformability fig 3 In 2002, about 260 000 t of polyamides, mainly PA 6 and copolyamides PA6/66, were used for packaging filmworldwide. Multilayer films in combination with PE, PP or EVOH form the major share of the polyamide filmmarket. fig 4 The term “polyamide” describes a family of polymers which are characterized by the presence of amide groups.Since the first - and still most important - synthetic polyamides, PA66 (Nylon) and PA6 (Perlon), have beendeveloped in 1938/39, they have found wide applications as film, fibers, engineering resins and others. Today,DuPont”s srcinal brand name for PA66, “Nylon”, is often used as generic name for all synthetic polyamides.However, polyamides have been useful materials long before the development of these synthetic nylon resins asthe naturally occuring protein, the polymer of  α -aminoacid, the base material of wool and silk, is also to beconsidered a member of the polyamide group. 2. Chemistry and Properties of Polyamide Resins fig 5Polyamides are mostly aliphatic, linear polymers characterized by the presence of the amide group, thecondensation product of amine and carboxylic acid, as repeating unit in the polymer chain, separated byhydrocarbon unit.fig 6Polyamides may be synthesized either by(A) polycondensation of divalent carboxylic acid and divalent amines, or by(B) polycondensation of difunctional aminoacids containing both one amine and one carboxylic acidfunctionality in the same molecule (or their intramolecular ring-shaped condensation products lactams ).The many different types of polyamides are named according ISO 1874 by the using symbol PA (for  polyamide) and:fig 7- (A) the number of carbon atoms in the diamine monomer, followed by number of carbon atoms in thedicarboxylic acid (for linear aliphatic polyamides), e.g. PA 66 for the polyamide of hexene diamine andadipic acid or PA612 for the polyamide of hexene diamine and dodecanic diacid, or - (B) the number of carbon atoms in the aminoacid or lactam monomer (for linear aliphatic polyamides), e.g.PA 6 for the polyamide of caprolactam (aminocaproic acid) or PA12 for the polyamide of laurine lactam  Other monomers are indicated by letters defined in ISO 1874, e.g. I for isophthalic acid or T for terephthalicacid. Copolyamides are designated by listing their monomers separated by a slash, e.g. PA 6/66 for thecopolymer of caprolactam, hexene diamine and adipic acid.Many technically used synthetic polyamides are derived from monomers containing 6-12 carbon atoms; most prevalent are PA6 and PA66. The film sector is dominated by PA6 and the random copolymer PA6/66, mostlydue to their relatively easy processing that facilitates coextrusion with lower melting, thermally sensitive polyolefins.fig 8: important polyamidestype monomers meltingtemperaturewater absorption(in water, 23°C)PA66 A hexene diamine (C6)adipic acid (C6)260°C 8.5%PA 610 A hexene diamine (C6)sebacic acid (C10)220°C 3.3%PA 612 A hexene diamine (C6)dodecanic diacid (C12)215°C 2.8%PA 6 B caprolactam (C6) 220°C 9.5%PA 11 B aminoundecanic acid (C11) 185°C 1.8%PA 12 B aminododecanic acid (C12) 178°C 1.5%PA 6/66(85:15)A,B copolyamide of caprolactam (85%), hexenediamine and adipic acid (15%)195°C 10.5%PA 6I/6T A hexene diamine (C6)Terephthalic and Isophtalic acidTg = 132°C *) 1.3% *) glass transition temperatureThe amide groups in the mostly semicrystalline polyamides are capable of forming strong electrostatic forces between the -NH and the -CO - units (hydrogen bonds), producing high melting points, exceptional strengthand stiffness, high barrier properties and excellent chemical resistance. However, the amide units also formstrong interactions with water, causing the polyamides to absorb water in a range between 2 and 20%. Thesewater molecules are inserted into the hydrogen bonds, loosening the intermolecular attracting forces and actingas a plastisizer, resulting in the exceptional toughness and elasticity well known of polyamides. 3. Film Processing of Polyamide Resins3.1. Pre-Processing Polyamide, as a water absorbing polycondensation polymer, is sensitive to moisture. Processing nylon resinwith excessive moisture content will result in molecular weight degradation by hydrolysis and the formation of steam bubbles. To prevent this, the moisture levels should be maintained below 0.1 wt%. Most resin producerssupply nylon pre-dried and in moisture-proof packaging to ensure the resin stays dry and ready for use. Oncethis moisture proof packaging is opened, nylon will begin absorbing moisture from the atmosphere, thereforenylon exposure to air humidity after opening the bag must be minimized by rapidly using the resin or by blanketing the resin with dry air or nitrogen. This action is essential for bulk supply of polyamide resin which isof rapidly increasing importance.fig 9Only if a polyamide resin absorbs more than the recommended 0.1 wt.%, it should be dried at 80°C utilizing adry air dryer of dew point below –20°C. Simple hot-air dryers should be avoided as they may actually humidifynylon when ambient humidity levels are high. 3.2. Extrusion Polyamide can be processed as blown and cast film, and can be used for extrusion coating and the procution of  biaxially oriented film (BOPA).  Polyamide processes well in a wide variety of standard extrusion equipment. General recommendations for anextruder to process nylon are:fig 10: ã  barrier or 3-zone compression screw ã compression ratio between 3.1:1 and 4:1 ã length to diameter (L/D) ratio at least 24:1 or higher  ã 35% feed (preheating) ã 15% compression (melting) ã 50% metering (homogenizing)Polyamides melt at relatively high temperatures and need higher processing temperature than polyolefins. Atypical temperature profile for a screw with 5 heat zones for processing polyamide 6 would be (for copolyamide PA6/66, 20°C lower temperatures):fig 11: barreltypefilmtypethroat zone1[°C]zone2[°C]zone3[°C]zone4[°C]connecting pipe[°C]head[°C]melttemperature[°C]smooth blown 200 260 258 253 250 250 250 250gooved blown 200 240 243 247 250 250 250 250smooth cast 200 275 272 268 265 265 265 265grooved cast 200 260 255 260 265 265 265 265 3.3. Film Forming Polyamide is used in both cast and blown film equipment, predominantly for multilayer films. Since nylon is asemicrystalline material, processes that rapidly cool the polyamide film will result in lower crystallinity andtherefore provide more transparent, flexible, thermoformable film.fig 12For cast films, standard equipment as used for PE or PP is used for multilayer film with polamide layers.Factors to consider arefig 13: ã chill roll temperature 20-50°C for high thermoformability ã chill roll temperature 75-85°C for high dimensional stability ã  polyamide resins with viscosity number 200-230 cm³/g ã  polyethylene resins of MI = 3.0 – 4.0 cm³/10minStandard blown film equipment is used for producing multilayer films with nylon layers. In general, these topicsneed to be considered for blown film   crystallisation rate of PA6 0.000010.00010.0010.010.1150 150 250 crystallisation temperature [°C]   c  r  y  s   t  a   l   l   i  s  a   t   i  o  n   r  a   t  e   [   1   /  s  e  c   ]
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