Processing Techniques For Plastic Films (Part 1)

Transcription

1 Processing Techniques For Plastic Films (Part 1) ABHAY MULAY Product Application & Research Centre Reliance Industries Limited Mumbai IPI,

2 Index Flexible packaging Overview Blown film extrusion Upward blown Downward blown Cast Film extrusion Uniaxial orientation Biaxial orientation Calendering

3 Flexible Packaging Overview

4 Flexible packaging : Overview Flexible packaging is the largest sector in plastics One of the fastest growing sectors, CARG ~ 20 % per capita consumption of plastics in packaging kg 3.7 kg A growing demand due to: Growing population Urbanization & increasing middle income group Booming rural marketing Better convenience, need for small size packs Increase in BRAND awareness Entry of MNC s

5 Flexible Packaging Sector : overview Total Film Market (02-03) ~1. 2 MMT 10% 20% 45% 25% LD/LLDPE PP HD/HMHDPE PVC PE films: Largest share in flexible packaging

6 Polypropylene film : overview PP Film Market (02-03) ~ 300KT 77% 3% 21% TQPP CPP BOPP...largest contribution from TQPP films

7 Processing Techniques

8 rocessing Techniques - Film The most commonly used processing techniques for the flexible packaging films are Blown film extrusion Upward blown Downward blown Cast Film extrusion Uniaxial orientation Biaxial orientation Calendering

9 Upward Blown Film

10 lown Film Extrusion : Upward Blown Technique predominantly used for PE films Process : Resin is plasticized in an extruder & pumped through the annular die Molten extrudate in form of tube is inflated by air into a bubble of desired diameter Cooling of bubble using air ring Haul off through collapsing boards & nip rolls Slitting & winding

11 lown Film Extrusion : Upward Blown

12 lown Film Extrusion : Upward Blown Machinery details : Extruders : Single screw extruders L/D 24 : 1 to 28 : 1 Compression ratio : 2.5 to 3:1 Dies Bottom fed spiral mandrel dies Single layer / multilayer (co-extrusion)

13 lown Film Extrusion : Upward Blown Machinery details : Corona treatment unit : Corona treatment is a high voltage discharge used to increase surface energy Treatment is measured using mixture of Ethyl cellosolve & formamide solution Surface tension requirements : For solvent based lamination : 44 dynes / cm For solvent based printing : 40 dynes / cm For water based inks & adhesives : dynes / cm Winding : Center winding or surface winding

14 ey Processing variables Melt temperature : Higher the melt temperature better is melt homogenization Typical melt temperatures LDPE : o C LLDPE : o C HDPE : o C HMHDPE : o C Higher melt temperatures also gives better optical properties & gloss

15 ey Processing variables Cooling air temperature : Lower temperatures ( higher cooling rate) leads to Higher crystallization rate Smaller crystal size Better opticals Better balance of properties for LDPE & LLDPE Internal bubble cooling (IBC) for higher outputs

16 ey Processing variables Die gap : LDPE & HDPE Broad MWD, Lower higher shear viscosity Narrow die gaps ( mm) for better properties LLDPE Narrow MWD, lower shear sensitivity Higher die pressures leads to shark skin effect Use of wider die gaps ( mm)

17 ey Processing variables Blow up ratio (BUR) Ratio of bubble diameter / die diameter To be optimized for balance of properties LDPE /LLDPE : 2.25 to 2.75 HDPE / HMHDPE : 4 to 5 Neck height : Lock-in position for branched polymers (LDPE / LLDPE) 12 to 15 inch for linear polymers (HDPE/HMHDPE) Draw Down ratio : Die gap Film gauge x BUR

18 ey Processing variables

19 pplications : PE Blown Films

20 Downward Blown Film (TQPP)

21 low Film Extrusion : Downward Blown Upward blown extrusion is difficult to be adopted for highly crystalline polymers like PP due to Slow crystallization rate Formation of bigger crystallites Leads to brittle & hazy film Downward blown extrusion process is conventionally used for PP. The process is popularly known as Tubular Quench Polypropylene (TQPP) process

22 QPP Film Process : The resin is extruded & the molten material in form of tube comes out of the circular die Hot bubble is initially cooled & stabilized by air ring Tubular film is then passed through water cooling / sizing ring Water flows downward through sizing ring keeping uniform & continuous contact with film Cooling water temperature depends upon film thickness & is between 8 20 o C Film is subsequently collapsed, passed through drying unit & finally rolled up

23 QPP Process

24 erformance of TQPP vs. Other films TQPP process is low capital intensive & economical compared to Cast PP & BOPP Compared to BOPP Tear strength is higher Lower shrinkage due to low orientation

25 erformance of TQPP vs. Other films Compared to PE film Higher yield Better clarity Higher heat resistance Superior tensile properties WVTR & OTR better compared to LDPE/LLDPE & inferior to HDPE

26 ffect of resin parameters Melt Flow Index (MFI) Higher MFI resin has poor melt strength & offers lower bubble stability Higher MFI resin gives better opticals, however gives poor mechanical properties Low MFI resin leads to difficulty in processing & poor opticals Optimum MFI range : 8 12 g/10 min

27 ffect of resin parameters Low molecular weight fraction (LMWF) These are usually olegomers & atactic fraction Determined by % xylene solubles Higher levels of LMWF leads to die drooling, smoking Low levels leads to difficulty in processing Optimum levels of LMWF %

28 ffect of resin parameters Type of resin Homopolymer & Terpolymer PP grades are commercially used Films made from Homo PP grades have Higher stiffness Higher SIT Films made from terpolymer PP grades have Higher softness Superior gloss Lower SIT

29 ffect of processing variables Desired film properties can be obtained by optimizing process variables Tensile strength of the film can be increased by Higher cooling rate Lower quenching temperatures Lower gap between die & cooling ring

30 ffect of processing variables Clarity improved by Lower quench water temperature Higher flow of quench water Higher die temperatures Gloss improved by Raise the die & barrel temperature Lower quench water temperature Higher flow of quench water

31 Advances in raw materials

32 esin for high throughputs significant improvement in throughputs 30 kg/hr 150 kg/hr over stretching leads to bubble breakage /punctures at higher shear rates, extensional viscosity decreases melts becomes thinner (stress thinning of PP) resin has to be designed for higher extensional viscosity resin has to be tailor made for high output lines

33 New grades of REPOL

34 New grades from RIL REPOL H 080 EY REPOL H 080EG

35 Repol H 080 EY PP homopolymer, with slip and antiblocking agents MFI : 8.0 g/10min The grade is specially designed to provide: Good processability at high throughputs Good opticals Better mechanicals Typical applications : packaging of textiles/food products, multi-layer TQPP films

36 Repol H 080 EG PP homopolymer, without slip and antiblocking agents MFI : 8.0 g/10min The grade is designed to provide: Good processability on high through put lines Excellent clarity and gloss Good stiffness Typical applications : packaging of textiles / food / other commodities

37 New applications for TQPP films Soft TQ textile bags & Liners Processed food packaging films Lamination films Soft Blister Stationery products Perforated films

38 Applications : TQPP films

39 ew Developments 2 layer co-extrusion TQPP plant developed by M/s. Rajoo Engg. Co-extrusion of Homo PP & Random PP Inner layer of random PP in co-extruded film offers superior sealing properties & low SIT Co-extruded film can be used for lamination application PET/Adhesive/Co-ex PP film

40 Cast Film Extrusion (Uniaxial Orientation)

41 ast Film Extrusion Process Polymer is plasticized & homogenised in the extruder Melt is passed through coat hanger die The extrudate comes out of a die as thin, wide curtain of the film Molten film is quenched in water tank or onto a chilled roll Finally film is corona treated, slit and rolled

42 ast Film Extrusion

43 ast Film Extrusion Advantages of Cast Film over blown film Higher output Better transparency, gloss due to quenching Better stiffness Better thickness control Better mechanical properties in MD due to unidirectional orientation Advantages of blown film Lower machinery cost Width flexibility Balance of mechanical properties

44 ast Film Extrusion Machinery details : Single screw extruders L/D : 28 to 32 : 1 Compression ratio : 3:1 or more Typical outputs in grooved feed extruders Screw dia (mm) Output (kg/hr)

45 ast Film Extrusion Die : Coat hanger die uniform pressure drop due to triangular flow path Uniform material flow, uniform thickness For multilayer co-extruded film Feedblock die Multimanifold die

46 ast Film Extrusion Chilled roll take-off Designed for maximum cooling efficiency Roll surface : Chrome plated, matt / mirror finished Roll width : 1200 mm to 3600 mm Roll diameter : 400 mm to 1200 mm Partially crystalline polymers : Cooling efficiency determines crystalline structure & opticals Lay on aids : Force film against surface of film roll ex. Air knifes & suction chambers

47 ast Film Extrusion : Key variables Melt temperature : Increasing melt temperature gives slight reductions in haze Caution : Increased volatilization of additives in raw material Odour issues Volatiles condensation on chill roll surface affects opticals Optimum melt temperatures LLDPE : o C PP : o C

48 ast Film Extrusion : Key variables Chill roll temperature : Increasing chill roll temperature Deteriorate film clarity Favours dimensional stability Increases slip properties Decreasing roll temperature Gives fine crystalline structure & excellent optical properties Due to build in stress, additional shrinkage on storage Optimum chill roll temperature : o C

49 olymer selection for Cast film LDPE / LLDPE / Homo PP / Co PP MFI : 3 10 g/10 min Broad MWD resins gives lower neck-in Higher the MFI, higher is clarity Random PP with 1.4 to 3% ethylene gives softer films with low haze Random PP also offer low SIT

50 pplications Cast Film Key applications of co-extruded cast film Structure Special feature Application LD + LLD / LD + LLD + HD / * One side cling * Good clarity Cling film, masking film LD + LLD + PIB PP / PP filled / * Low shrinkage Fast food trays PP * Smooth outer layer RCPP / PP * Sealability Industrial packaging /RCPP RCPP / TL / Nylon / TL / PP * Good sealability * Transparency * Thermoforming property Packaging of meat, Sausage & Cheese

51 Applications Cast Film

52 Cast Film Extrusion (Biaxial Orientation)

53 ast Film Extrusion : Biaxial rientation Process : Biaxial orientation is a process in which continuous cast film is heated to bring it to a stretchable temperature Subsequently it is stretched in machine & then in transverse direction Commonly used polymers are PP & PET. The films are termed as BOPP & BOPET films respectively

54 OPP Film Extrusion Machinery Details : Extruders : mm dia screws with L/D 26 to 30 : 1 Typical outputs : Kg/hr Dies : 600 mm to 2000 mm wide, coat hanger type Casting & Quenching : Molten polymer extruded from the die is placed against chilled roll or multiple chill rolls

55 OPP Film Extrusion Machinery Details : Casting & Quenching : Popular method is casting onto a chilled roll partially submerged in water bath Chill roll & water temperature : o C Chill roll diameter : mm Cooled web proceeds for MDO Orientation Biaxial orientation of the film is done by tenter frame process

56 OPP Film Extrusion Machine direction orientation (MDO) Cast film is heated by passing over heated rolls ( o C) When film reaches necessary stretching temperatures, it is passed over series of stretching rolls Stretch ratio is kept between 4 : 1 to 6 : 1 Subsequently film is passed over annealing rolls to prevent MD shrinkage

57 OPP Film Extrusion Transverse direction orientation (TDO) From MDO, film is guided onto the chain & is preheated at 160 o C As film leaves preheat section, chain diverges rapidly stretching film in ratio of ~ 8 : 1 Film is passed in annealing oven at 155 o C to reduce TDO shrinkage Winding : Edges of the film are trimmed off. The film is slit and wound on separate winders.

58 OPP Film : Double Bubble process Equipment is smaller compared to tenter frame process Difficult to produce thin film Output in terms of quantity & width is very low Extruders : 50 to 75 mm Dies : Circular dies of mm dia Quenching : Extruded tube from the die is directly taken into quenching o C, collapsed between nip rolls Film passes to set of 2 nd nip rolls through drying unit

59 OPP Film : Double Bubble process Preheating : Tube in flattened state is heated to o C using IR heaters Orientation : Heated tube is inflated to 5 6 times original diameter (TDO) With set of nip rollers, film is stretched in machine direction at ratio of 5 6:1 Cooling rings cools the inflated bubble before being collapsed

60 OPP Film : Double Bubble process

61 ffect of Processing variables BOPP film properties depends on different processing variables Tensile strength & modulus can be improved by Lowering the temperature of chill roll & quench water Increasing stretch roll temperature Increasing the line speed Increasing the stretch ratio Lower shrinkage achieved by Using higher stretching temperatures Using higher annealing temperature

62 ffect of Processing variables Haze can be reduced by Lowering temperature of chill roll & quench water Increasing the temperature of stretching & annealing Gloss can be improved by Using higher melt temperature Using chill roll, MD roll & die having clean surfaces

63 Applications : BOPP Film

64 Calendering

65 alendering Process of forming film/sheet by passing mass of molten material through successive pairs of counter rotating rolls Process is commonly used for PVC, Rubber, & Non-woven A typical PVC calender consist of 4 to 5 metal rolls arranged in various configuration Each roll has control for temperature, speed, roll gap For processing of PVC, 3 basic roll arrangements are used inclined Z, L & inverted L

66 alendering Roll Configurations Calendering line

67 alendering : Roll configuration L type calender can have 4 or 5 rolls of ~ 2 ft. dia & ~ 4 ft. length L type is commonly used for rigid PVC Inverted L type is commonly used for flexible PVC Relative separatory forces for Z type calender are more compared to L type calender. This leads to higher gauge variation in transverse direction

68 VC Calendering Process involves following steps : Blending Fluxing Calender feeding Calendering Take-Off

69 VC Calendering Blending : Resin, Plasticisers, Modifiers, Fillers, Stabilizers, etc. are mixed together in ribbon blender. High intensity mixer gives better distribution compared to ribbon blenders Fluxing : The output of the blender, typically well dispersed, dry free flowing powder is fed into one of the fluxing equipment like Banbury / Intermix batch mixers Farrel continuous mixer Ko-Kneaders / Twin screw extruders Planetary extruders ( Rigid PVC)

70 VC Calendering Calendering feeding : The output for fluxing is fed to either two roll mill or extruder strainer. Function of extruder strainer / two roll mill is To maintain stock temperature To act as a accumulator Degassing Straining of foreign particles

71 VC Calendering Calendering Output from two roll mill or extruder strainer is fed to calender unit by a conveyor system Compound passes through metal detector system Conveyor feeds the PVC compound into 1 st set of calender rolls. The sheet is formed as the material passes through the rolls Each nip in calender stack reform the sheets & makes it wider, thinner & refinished For easy & faster release, last calender roll is kept at lower temperature & higher speed compared to preceding roll

72 VC Calendering Calendering take off : Hot, unsupported sheet from the last calender rolls is removed using driven rolls called stripper rolls Rolls are normally teflon coated to prevent sticking Subsequently sheet passes over embossing rolls, chilling rolls & winding station

73 VC Resin characteristics Resin for flexible PVC Suspension PVC with relative viscosity of 2 to 2.80 (K value 60 to 77) High MW resins are used when higher physical properties are required or sheet is subjected to further heat (eg. Embossing) High MW resin have better UV resistance

74 VC Resin characteristics Resins for Rigid PVC Rigid PVC compounds are formulated with lower MW resins Relative viscosity 1.85 to 2.0 (k value 55 to 60) Resin can be homopolymer or copolymer of VC & VA

75 Applications :PVC Calendering R A IN W E A R

76 Thank You!