HANDBOOK OF POLYOLEFINS PDF
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Handbook of Plastic Films - Free ebook download as PDF File .pdf), Text File . txt) or read book online for free. The handbook of polyolefins brings in the attention of the researches the latest data from the science and technology of polyolefins. The book was co-edited by. itly in this book, several of the methods explained herein can be easily adopted to . EPDM processes. Polyolefin Reaction Engineering, First Edition. Jo˜ao B. P.
Further, sodium benzoate has been found to be insoluble and immiscible in polyolefins. Therefore, the performance of sodium benzoate as a nucleating agent is dependant upon its dispersion in the polymer melt in as fine a form as possible; in the range of 1 to 10 microns. Conversely, the nucleation effects of di-acetals of sorbitol and xylitol appear to be largely independent of their physical characteristics prior to compounding, given the requirement that they are dispersed and recrystallized in the polyolefin resin.
The present invention provides a technique to process sorbitol and xylitol acetal clarifiers so they can be compounded with polyolefin resins to produce fabricated parts without "white points" or bubbles without the use of excessive compounding temperatures which can cause discoloration and odor. All of the photomicrographs were made at a magnification of X.
Of particular interest are clarifying agents where p is 1 and R is selected from C alkyl, chlorine, bromine, thioether and a 4-membered alkyl group forming a carbocyclic ring with adjacent carbon atoms of the unsaturated parent ring. Examples of specific clarifiers having utility herein include: Dibenzylidene sorbitol, di p-methylbenzylidene sorbitol, di o-methylbenzylidene sorbitol, di p-ethylbenzylidene sorbitol, bis 3,4-dimethylbenzylidene sorbitol, bis 3,4-diethylbenzylidene sorbitol, bis 5',6',7',8'-tetrahydronaphthylidene sorbitol, bis trimethylbenzylidene xylitol, and bis trimethylbenzylidene sorbitol.
Also within the scope of the present invention are compounds made with a mixture of aldehydes, including substituted and unsubstituted benzaldehydes, such as Kobayashi, et al. The di-acetals of the present invention may be conveniently prepared by a variety of techniques known in the art. Generally, such procedures employ the reaction of 1 mole of D-sorbitol or D-xylitol with about 2 moles of an aldehyde in the presence of an acid catalyst.
The temperature employed in the reaction will vary widely depending on the characteristics, such as melting point, of the aldehyde or aldehydes employed as the starting material in the reaction.
Examples of suitable reaction medium are cyclohexane, or a combination of cyclohexane and methanol. Water produced by the condensation reaction is distilled off.
Typically, the mixture is allowed to react for several hours, after which the reaction is cooled, neutralized, filtered, washed, for example, with water or an alcohol, and then dried. The cited background references, which have been incorporated herein, provide additional details of the synthesis of clarifying agents of the present invention. The di-acetals of sorbitol and xylitol of the present invention prepared by the above techniques may contain byproducts of mono-acetal and tri-acetal as impurities.
Although it may not always be necessary to remove these impurities prior to incorporation of the di-acetal into a polyolefin resin, it may be desirable to do so, and purification may serve to enhance the transparency of the resin produced thereby. Purification of the di-acetal may be accomplished, for instance, by removal of tri-acetal impurities by extraction with a relatively non-polar solvent prior to filtration.
In commercial manufacturing operations the product is dried by using heat or heat and vacuum. The product is reduced in size by using mechanical delumping devices followed by milling in a pin or stud mill. The milled produce is usually classified using screening devices to remove oversized particles.
Particle size measurement is not an exact science. By convention, the 97th percentile or d97 is typically used as a measure of maximum particle size. Screens finer than 80 mesh are not used because they tend to blind or plug up very quickly. Correlation of mesh screen and opening size is as follows: Notes: A U.
The dibenzylidene sorbitol product of Example 1 was examined under a scanning electron microscope SEM at a magnification of X. Referring to FIG. It is believed that these sintered particles trap gas or volatile liquids which are released when the particle softens prior to melting. When this phenomena occurs during fabrication operations, then white points or bubbles are created. Additionally, it is also believed that gases trapped within the sintered particles provide insulation and otherwise interfere with efficient heat transfer which is required to melt and dissolve the clarifier in the polymer melt.
Surprisingly, an analysis of other commerical sorbitol acetal clarifiers which included the products of several different manufacturers revealed that all of the products were agglomerates of tiny fibers or "primary particles" with surfaces that seem to be sintered. Table 1 below indicates the figure number, clarifying agent, trade name, and manufacturer for these commercial products.
The mill was equipped with a deflector-wheel type classifier. The sample was intensely milled and classified to produce a particle characterized by a d97 of less than 8 microns and a mean particle diameter of less than 4 microns as measured by laser light scattering. These measurements were confirmed by microscopic image analysis. Also, the packed bulk density of the powdered sample was reduced from 0. The term "dissolved" is used herein to describe the phenomenon of the clarifying agent diffusing into the molten resin, even at temperatures below the melting point of the clarifying agent.
However, considering the viscosity of the resin melt, the clarifying agent is not necessarily homogeneously distributed throughout the resin. Nevertheless, the clarifying agent is observed to recrystallize from the polymer melt, after it has dissolved.
In addition to the particle size reduction performed in Example 2, the clarifying agents set forth in Table 1 were also similarly milled and classified. As a result of the analysis of these comminuted materials, the following general observations may be made. The diacetals of sorbitol and xylitol of interest herein may be characterized by a "fibrous, crystalline primary particle" having a length of about 5 to 10 microns and a diameter of about 0.
It has been found that these primary particles and small agglomerates of these primary particles containing up to several individual particles, do not exhibit the tendency to trap gasses, which has been found to result in the formation of bubbles in the clarified resin and insulation of the clarifying agent during the compounding step.
Thus, clarifying agents, in the form of a powder having a d97 of less than 30 microns and a mean particle size of less than 15 microns have been found to be useful in the practice of the present invention.
Preferably, the clarifying agent has a particle size characterized by a d97 of less than 20 microns and a mean particle size of less than 10 microns, more preferably a d97 of less than 10 microns and a mean particle size of less than 6 microns. In addition to particle size reduction using an opposed jet fluidized bed, there are other methods which can be used to produce a sorbitol acetal clarifier with the primary particles exposed and un-sintered.
Fluidized bed spray drying is one viable option. Applications of Plastic Films in Agriculture Ageing Resistance of Greenhouse Films Recycling of Plastic Films in Agriculture Selection of the Properties of Tested Burn Dressings Determination of Air Penetrability of Burn Dressings Results and Discussion Testing of Plastic Films List of Requirements Some Properties of Plastic Films Mechanical Tests Tear Resistance Some Physical, Chemical and Physicochemical Tests Recycling of Plastic Waste Collection and Sorting Recycling of Separated PE Waste Recycling of HDPE Recycling Using Radiation Technology The plastic industry continues to grow very rapidly and plays an important role in many fields such as engineering, medical, agriculture and domestic.
It is now very difficult to find the point at which plastic cannot be considered as an essential component. The understanding of the nature of plastic films, their production techniques, applications and their characterisation is essential for producing new types of plastic films. This handbook has been written to discuss the production and main uses of plastic films. Chapter 1 deals with the various types of polyolefins and their suitability for film manufacture.
The rheology, structure and properties of the polymers are discussed in relation to the type of film manufacturing processes that are most applicable to the types of polymer. Post-extrusion modifications of the films such as orientation, surface chemistry and additives are discussed.
Characterisation methods used to measure film mechanical properties; structure and additives are described, as well as other more specific properties.
Finally some particularly important applications that require special structures or modifications are given. In Chapter 2, the main parameters influencing resin basic properties are described. The methods of processing of polyethylene films such as cast film extrusion, blow extrusion of tubular films are discussed. Effects of extrusion variables on film characteristics and effect of blow ratio on film properties are considered.
Chapter 3 details the structure, synthesis and film processing of polypropylene. The effects of some additives and UV stabilisers are discussed. The solubility of additives plays an important role in determining the efficiency and the properties of the films as well. For this reason Chapter 4 deals with different aspects of additives solubility in polymers in relation to the polymer degradation and stabilisation.
The topic covered in Chapter 5 is the stability of polyvinyl chloride PVC films during procesing and service.
The possibility of increasing the intrinsic stability of PVC during processing with the minimal contents or in total absence of stabilisers and other additives is discussed. Handbook of Plastic Films Chapter 6 discusses flame retardants, which as special additives have an important role in saving lives.
These flame retardant system basically inhibit or even suppress the combustion process by chemical or physical action in the gas or condensed phase. Conventional flame retardants have a number of negative attributes and the ecological issues surrounding their applications are driving the search for new polymer flame retardant systems forward. Chapter 7 covers thermal and photochemical oxidation of polymers under the influence of the aggressive, polluting atmospheric gases.
Among pollutants, sulfur dioxide, ozone, nitrogen oxides stand out as the most deleterious impurities of atmosphere. Thus, this chapter is devoted to consideration of the results obtained in studies of interactions of nitrogen oxides with polymers. Chapter 8 discusses the modifications of plastic films to improve their mechanical or physical properties to meet the requirements of certain applications.
This can be achieved by subjecting the films to mechanical or chemical treatments. A number of surface modification techniques such as plasma, corona discharge and chemical treatments have been used. Chapter 9 deals with applications of plastic films in packaging. Chapter 10 deals with the application of plastic films in agriculture.
The mechanical properties suitable that make these films suitable for use in agriculture are discussed. Stability of these plastic films under the effect of different environmental conditions is reported. Types of UV stabilisers and their compatibility with plastic are given.
Also, recycling of plastic films used in agriculture is of great importance and finally, a case study of their reuse as agriculture films is given.
Chapter 11 deals with the principal medical treatment of burns using dressings made with a polymeric layer or layers. It is difficult to estimate the effectiveness of the new burn dressings, as their physicochemical properties are not usually presented in literature. Thus, chapter 11 discusses this subject for the first time. The physicochemical criteria for estimating the efficiency of burn dressings and the possibility of using plastic films is given.
Chapter 12 covers the most common test methods generally used for plastic films. The requirements necessary for the test methods are summarised. Preface The problem of plastic films recycling is touched on in Chapter Non-polyethylene resins constitute the remainder of the plastic film. Different types of recycling are given and recycling of some selected types of films are discussed.
This handbook represents the efforts of many experts in different aspects of plastic films. Their efforts in preparing contributions to the volume are to be noted and I take the opportunity to express my heartfelt gratitude for their time and effort. My gratitude extends also to many colleagues for their kind comments in many aspects. A special thanks is extended to the staff of Rapra Technology, for the fine production of this Handbook, particularly Claire Griffiths, Editorial Assistant, Steve Barnfield who typeset the book and designed the cover and Frances Powers who commissioned the book and oversaw the whole project.
Elsayed M. Abdel-Bary January A film is typified by a large surface area to volume ratio. Films are required to exhibit barrier properties to any contaminating substances that may try to enter, or any desirable substances that may try to leave, across the film. This property is resistance to diffusion. Since a film is very thin, it must have high mechanical properties such as tensile strength, impact resistance and tear strength.
The mechanical properties usually depend on molecular structure, molar mass and molar mass distribution. Visibility through a film is often important, so low haze will be required. These are the bulk properties of the film . The film will often be required to improve the appearance of an item contained within it, so surface properties such as gloss and printability are important. The latter property, printability, is related to a relatively high surface energy to achieve wetting and good work of adhesion.
Suitable surface energy may be achieved through modification. Protection may also be improved if the friction is low; this property is called slip.
When a film is used to enclose and protect items, it may need to provide adhesion to itself or to the contents. The immediate form of adhesion is called tack. Subsequently the polymer must flow to provide complete adhesion. Manufacture of a film will usually be through an extrusion of the melt, so the melt rheology must be suited to the manufacturing process. Rheology is controlled by chemical structure, molar mass and long branches. The way in which the film is extruded, extended and solidified by cooling will control the microstructure and hence many of the properties.
A summary of the various polyolefins used in film manufacture is provided in Table 1. In this chapter, polyolefin films are reviewed. First, the various types of polyolefins and their suitability for film manufacture are considered.
Post-extrusion modifications of the films, such as orientation, surface chemistry and additives, are discussed. Characterisation methods used to measure film mechanical properties, structure and additives are described, as well as other more specific properties.
Finally, some important particular applications that require special structures or modifications are described.
Table 1. Mechanical properties High tensile strength, low impact strength Non-Newtonian melt rheology, good impact strength Intermediate strength with elasticity, melt rheology more Newtonian than LDPE Tough elastic, moderate strength Tough elastic, moderate strength, nonNewtonian melt rheology Elastic, low tensile strength, low modulus High tensile strength, brittle, temperature resistance Tough with high melting temperature block or softer with lower melting temperature random Narrow molar mass distribution, random comonomer distribution and high isotacticity.
Comments Brittle film, with good gas barrier properties Good blown extrusion characteristics for flexible films High-clarity, glossy film, difficult to extrude High-clarity, very glossy film, very thin films possible Easy to process, improved melt strength Thermoplastic elastomer, narrow low temperature melting, good for heat seal Transparent, high-strength and temperature-resistant glossy films. LDPE has rheological properties that are suitable for production of film by the blown film process .
LDPE has some long branches and many short branches. Typically, there may be three long branches and 30 short branches per molecule. The molar mass is relatively low, and it has a broad molar mass distribution. The melt strength, or zero-shear viscosity, and the shear-thinning nature of LDPE enhance processing. The film has relatively low tensile strength but good impact strength. LDPE films show good clarity i. The good clarity and gloss result from relatively low crystallinity.
LDPE is polymerised by the high-pressure radical process. There are two main reactor types, the autoclave and the tubular reactor. The autoclave tends to provide more branching and broader molar mass distribution. LDPE has a broad melting range, with a peak melting temperature of C. The density may vary from 0. Figure 1. Each process involves relatively low pressure and is catalysed by an organometallic complex with a transition metal. Polymerisation is usually performed in slurry with a liquid such as heptane, or in the gas phase with the catalyst in a fluidised bed form.
HDPE has higher crystallinity and therefore shows higher tensile strength than LDPE, though its impact strength is deficient for many applications. UHMWPE provides increased tensile strength due to the longer molecules providing more tie molecules between crystals.
MDPE provides better impact strength because of its reduced crystallinity. These polymers have densities in the range 0. They are polymerised using multisite catalysts such as Ziegler-Natta with either a gas-phase or slurry process. Since the boiling temperature of 1-octene is too high for the gas-phase process, the slurry process must be used. The comonomer composition has a broad distribution, so that some molecules, or segments of molecules, have few branches while others have many branches.
They have short branches but not long branches, so that crystallisation-dependent mechanical properties are improved, but processing rheological properties are inferior to those of LDPE .
These polyethylenes have recently been commercialised as a result of the new metallocene catalyst technology that allows higher comonomer levels and provides a narrower distribution of comonomer composition as. Technology of Polyolefin Film Production well as of molar mass. These polymers have lower melting temperatures, less crystallinity, greater toughness and elasticity, but lower tensile strength than other polyethylenes.
They mainly only have short branches, but some varieties also have some long branches . The rheological properties are not ideal for the blown film process, but such processing is used in a two-stage extrusion and blowing process. Syndiotactic PP sPP is now becoming available commercially as a result of metallocene catalyst polymerisations. It is particularly suitable for more durable products . Random copolymers show the greatest property changes, such as increased elasticity and a decrease in melting temperature.
Copolymers with more block-like structure, where the ethylene is distributed in some of the molecules or molecular segments, provide a good compromise in properties between toughness and strength. The crystallinity provides the tensile strength but reduces the transparency.
Larger crystals scatter transmitted light, producing an opalescent appearance, known as haze. Crystals on the surface reduce the surface smoothness and cause surface scattering of incident light and reduce the gloss. An example of the morphology of a polypropylene film is provided in the optical microscope picture in Figure 1. Processing conditions can modify the natural tendency of each polyolefin to provide these crystallisation-dependent properties. Rapid cooling will give smaller crystals.
So the use of cold rollers in the cast film process usually gives smaller crystals and in particular greater surface smoothness. In the blown film process, the use of a refrigerated air stream increases the crystallisation rate. Crystallisation is evident as a fogging of the film a short distance from the extrusion die; this is called the frost-line height.
The copolymer is a dispersed phase shown by the dark regions mainly at the edges of the polypropylene spherulites.
Handbook of Plastic Films
Orientation of crystals will direct the axes of the crystals, and correspondingly the crystaldependent properties, along the orientation or draw direction. Usually films are oriented, or drawn, in two orthogonal directions, called biaxial drawing, first parallel to the extrusion direction, then laterally.
Drawing in the extrusion direction involves cooling the melt until crystallisation takes place, then passing the film between rollers with increasing differential speed. The lateral drawing depends on the method of manufacture.
Plastics Technology Handbook
When the blown film process is used, orientation is provided during the blowing process. The cast film process requires a lateral drawing frame called a tenter. The edges of the film are grasped and the frame moves apart as the film moves forward. Orientation provides enhanced physical properties in the drawn directions.
When the film is biaxially drawn, the properties are greater in the direction that was drawn last . The shear stress versus shear rate curve will be approximately linear except for very high molar mass.
The linear relationship is Newtonian. This means that at high shear rates, as experienced in processing, the viscosity. Technology of Polyolefin Film Production is high and so the force required for extrusion will be high.
Another problem is that the viscosity at low shear rates is not increased. This zero-shear viscosity is related to the melt strength of the polymer. If the melt strength is low, the molten film may rupture as it emerges from the extruder as a tube that is then rapidly expanded by a gas pressure. High melt strength is required to resist rupture and create a dimensionally stable bubble. The melt strength is less critical in the cast film process, although the film must remain stable until it reaches the cooling rollers.
The force required for extrusion will still be a problem, since more energy will be needed to extrude a particular mass of polymer, and this will require more electricity and a more powerful extruder motor.
HDPE has high tensile strength, but low impact and tear strengths, so damage during processing by tearing is a potential problem. They have improved toughness compared with HDPE. Though they have short branching comparable with LDPE, they do not have long branches.
The lack of long branches decreases their shear-thinning rheological characteristics compared with LDPE and so processing is not as efficient. They are often blended with LDPE since the long branches enhance processing.
They have greater tensile strength than LDPE, but, with their higher crystallinity, they are less transparent. Short branches are not important in the rheology.
These polymers will have essentially Newtonian behaviour. Polymers with very high molar mass have more pronounced shear thinning, though entanglements between only the main chains are not as effective as between several long chains such as when long branches are present.
LDPE is shear thinning, so that the power required for extrusion at typical high shear rates is less than proportional to the shear rate.
This makes extrusion of LDPE more. At low shear rates, the viscosity rises significantly, so the zero-shear viscosity, or melt strength, is high. LDPE has better bubble strength in the blown film process, so that resistance to bursting and bubble stability are greater prior to solidification. In the cast film process, the film will be stable in the molten state between the extrusion die and the cold rollers. The long branches provide more intermolecular entanglements when the shear rate is low.
As the shear rate increases, the long branches break free of entanglements and the viscosity decreases markedly. These rheological characteristics are of prime importance during processing .
This introduces tacticity, and the isotactic form of polypropylene is the only one that is suitable for film formation. Polymerisation is performed using ZieglerNatta and several other newer proprietary catalysts. The catalysts have been developed. High catalytic activity is desired so that residual catalyst does not need to be extracted from the polymer.
Polypropylene has a lower density than most of the polyethylenes 0. Its melting temperature at C is significantly higher than that of HDPE, making it suitable to form retortable and microwave-resistant products. The glass transition temperature is high, e. Impact resistance is improved by copolymerisation with ethylene. The toughness is increased without decreasing the overall melting temperature significantly.
Other random copolymers provide increased toughness and elasticity with decrease in tensile strength and melting temperature. Single-site or metallocene-catalysed polypropylenes have narrower molar mass distribution, though the isotacticity may not be greater.
The copolymers with ethylene have a more even distribution of ethylene, and so a very small proportion of ethylene will provide a large decrease in melting temperature compared with the traditional polypropylene copolymers.
The extrusion process involves a series of events that each affect the stability and consistency of the extrudate and hence the film. The processes in the extruder include feed, melting, mixing, metering and filtration. The die is an annular shape that produces a tube of polymer. The tube is inflated by air pressure injected inside at the die. Inflation of the tube makes the film dimensions greater and provides orientation of the polymer. The tube passes through zones of cooled air, which solidifies the polymer and controls the crystallisation .
A diagram showing the essential features of the blown film process is shown in Figure 1. In the formation of polypropylene, a two-step tubular orientation process is required. This is because of the poor melt strength of polypropylene.
The film must first be cooled to enable crystallisation. The film is reheated to be just at the melting temperature and the tube is blown again before passing through a cooling ring. A comparison of film orientations in the transverse direction TD and machine direction MD shows the properties to be similar if the stretching occurs simultaneously in each direction. In sequential stretching, the last stretching step predominates, so TD is usually stronger,.
Shrink films can be prepared from LLDPE and copolymers of ethylene and propylene, but radiation modification is necessary to partially crosslink the polymer.
Less powerful extruders may only be suitable for LDPE production, since its shearthinning characteristics assist high throughput at lower power. Additives such as antioxidants, ultraviolet stabilisers, lubricants, slip agents and tackifiers may need to be included at the extrusion stage, so a facility for separate injection or dry blending of these additives may be required.
The extruder must provide the means to melt and convey the molten polymer through a die that will produce the film. Typically, a singlescrew extruder will be suitable. There are many types of single-screw extruders, but, generally, they are best suited to distributive mixing. Distributive mixing is where the components only need sufficient mixing to provide a uniform melt. Twin-screw extruders provide more intensive mixing, and so are used when dispersive mixing is required.
Dispersive mixing is where high shear is needed to subdivide a dispersed phase into smaller particles, where the dispersed phase may be another polymer or a filler with aggregated particles . The feed zone conveys the polymer pellets, filler and additives from the hopper into the main part of the extruder. In the compression zone, the polymer is melted, mixed with any other components and compressed into a continuous stream of molten polymer compound.
The metering zone provides a uniform flow rate to convey the polymer to the die. The melting or compression zones of the screw must be broad. This is the region where the depth of flight is decreased to provide the compression. Polyethylenes have a higher molar mass than other polymers used for extrusion, so the melt viscosity is reasonably high. Polyolefins have weak intermolecular forces, so the mechanical properties are derived from a high molar mass and regularity of the chains for close packing.
In addition to the force required for extrusion, the strength of the molten films is important in successful film formation. Of the polyolefins, polypropylene is the most difficult for film production because it has relatively low melt strength. Very high molar mass will improve the film formation, but make the extrusion part of the process more energy-consuming .
The tube is sealed at the top as it passes between pinch rollers. The tube is expanded using air pressure. The tube will only expand significantly when the polymer is molten. The rate at which the polymer exits from the die, the air pressure and the impingement of external chilled air determine the blow ratio. The blow ratio is the ratio of the final tube diameter to the diameter of the annulus in the die.
This ratio, together with the width of the slot in the die, determines the film thickness and the transverse orientation of the film. The film is also oriented in the direction parallel to the die by a differential between the speed of the polymer exiting the die and the speed of the pinch rollers pressing the tube flat and feeding it to the auxiliary equipment. The transverse orientation occurs up until the polymer solidifies and is often the dominant orientation for properties.
The blow ratio able to be used is limited by the melt strength of the polymer. Linear polymers are more likely to exhibit film rupture in the melted region of the tube. Polymers with long branches have higher melt strength and so are much better for production of blown film. The rheology of the polymer is important for other aspects, since an unstable bubble may be formed .
The bubble should be symmetrical about the centre-line of the die to the pinch rollers. If the film is uneven in thickness or in solidification, then symmetry will be difficult to control. A thicker portion of the film will be stronger and will resist blowing and so remain thicker. A thinner portion of the. Handbook of Plastic Films film will expand easier and will become thinner, so this part of the film will bulge outwards even more. The thinner part of the bubble may even rupture.
Differential heater bands can be placed around the film near the exit from the extruder to provide fine adjustment of the film temperature as the film is expanded.
The frost-line is the point at which the polymer solidifies by crystallisation. The transparency of the polymer is decreased on crystallisation, and this is observed as a sharp transition in the film not very far above the die. The frost-line depends on the extrusion speed and the temperature, as well as on the cooling air that is directed on to the polymer tube from the outside.
The cooling air is usually refrigerated, and its temperature, velocity and angle of impingement on to the film may all be varied. Rapid crystallisation will provide smaller crystals, and so the film will be clearer, apparent in a low haze, and have a smoother surface, apparent in a high gloss . The film is drawn from the extruder by calender rolls. This process does not expand the width nor decrease the thickness of the film, though the calendering occurs immediately after extrusion.
The extrusion process is the same as for other extrusions. The melted polymer must be distributed evenly along a slit die, usually using channels in the die. The die is referred to as a coat hanger or fish tail die.
The calender rolls are chilled so that they provide a melt quenching, giving smaller crystals than the blown film process. The film has a very smooth surface due to the calendering process . The smooth surface can cause self-adhesion of the film, called blocking.
An antiblocking agent may be added to reduce the blocking. Cast films will usually have superior gloss and low haze compared with blown films. Orientation of polypropylene flat film uses a tenter frame chain with clamps in the transverse direction, a quench roll, then reheating rolls followed by tenter and wind-up roll for the machine direction.
The tenter frame is enclosed in an oven that is used to heat and relax the film . Coextrusion is used to make multilayer films by extruding several polymers at the one time through a single complex die. Each individual polymer will have its own extruder feeding into a central die. An individual polymer may be included into more than one layer, yet it only need come from one extruder. Multilayer films are common despite the complexity of the equipment required for their manufacture.
Each layer has a special. The requirements for mechanical protection, diffusion barrier properties, substrate and interlayer adhesion, and heat shrinkage cannot all be met through a single polymer. The most suitable polymer for each purpose can be chosen and assembled into the multilayer structure. The rollers will be of highly polished steel to provide a smooth glossy surface to the film.
Rapid cooling also assists formation of a glossy surface on the film, since the crystals will be kept small and crystallisation may be minimised. A series of rollers may be used to provide orientation by stretching the film in a longitudinal direction. The existing film will be another polymer, metallic foil or paper. Multiple layers may be formed by extrusion coating both sides of the primary film or building a multilayer structure by introducing several extrusioncoated layers.
Coextrusion can only be used for polymers with similar processing conditions. Where the processing conditions are different, particularly in the case of substrates that cannot be melted with the polymer, such as metallic foils and paper, then extrusion coating is the only choice . During blowing, the diameter of the extruded tube is increased, and this causes the structure of the film to be oriented perpendicular to the extrusion direction. Orientation should take place below the melting temperature when the polymer is crystalline so that the crystals are oriented.
The expansion of the extruded tube will take place while the polymer is entirely melted, so that the effect of blowing will not provide a level of orientation equivalent to the diameter expansion . At the same time as the film is being expanded by blowing, it is being drawn by pulling along the axis of extrusion. This provides a parallel orientation. Again, most of the parallel drawing occurs on the polymer melt between when it leaves the extruder and when it crystallises.
The crystallisation region is called the frost-line. At the frost-line the film will have its maximum diameter and resist further expansion or drawing compared with the region immediately before the frost-line.
At the frost-line the completely transparent melt becomes foggy due to crystallisation. The change in opacity depends on the crystallinity of the particular polymer.
Sometimes additional orientation is imparted on the film after the blowing process. This is the case if the film is to be a shrinkable film. Shrinkable films will contract upon heating.
This is useful for providing tightly fitting wrapping. Polyolefins, particularly polypropylene, are normally moderately heated. The enhancement of tensile properties is directly related to the draw ratio. After drawing, the film should be further heated to relax or set the structure. This will provide dimensional stability. If the film is to be heat-shrinkable, then the relaxation is not performed. Some crosslinking from radiation treatment prior to drawing may be used to increase the plastic memory effect in the film.
Excessive drawing can cause strain hardening because of the introduction of extended chain crystals at the expense of chain-folded crystals. The strain-hardened film will have a stiffer or more leathery feel; it will lose elasticity and may have a rougher surface. The parallel orientation can be provided by a draw-off faster than the extrusion speed.
The perpendicular direction has been described for blown film production. In extruded sheet, a frame that attaches to the edges of the film and moves apart must provide the perpendicular orientation. This is called a tenter frame. The film is often drawn to about three times its original width. As well as adding strength to the film, a thinner gauge of film can be produced. The drawing process overcomes the effect of die swell that occurs as the film leaves the die.
Orientation of cast film is illustrated in Figure 1. The nature and origin of gloss and haze are illustrated in Figure 1. A high gloss requires a smooth surface. Surface imperfections may be introduced by the processing. Excessive drawing into the strain-hardening region will usually reduce the gloss.
Blown film usually has a lower gloss, since crystallisation of the film at the frost-line introduces surface roughness due to the crystals.
Rapid crystallisation of the film by the use of chilled air impinging on the bubble reduces the size of crystals and improves the gloss. Extrusion cast film passes through chilled rollers after leaving the. The rapid cooling and the polished surface of the rollers provide a high-gloss surface. Extrusion cast films have the higher gloss, but the extrusion blown process produces film at a lower cost.
The rheology of the polymer will contribute to the surface of the film. Shark skin is the term applied to a rheological problem in the processing . Crystallinity, optical defects, fish eyes, phase separation of blends, contaminants, gel particles and dispersion of pigments carbon black are structures that increase haze.
Haze is the internal scattering of light.
Editor: Elsayed M. Abdel-Bary
Haze makes it difficult to clearly see an object through a film as a result of the interference from randomly scattered light reaching the viewer in addition to light coming straight from the object. Smaller crystals provided by a nucleating agent will decrease haze.
The other phenomena described above can also be reduced by nucleating agents, better formulation and processing. It is difficult to find other substances that will adhere to polyolefins.
Suitable adhesion can be obtained by melt adhesion of polyolefins to each other, but only when the polyolefins are very similar. For instance, polyethylenes have good mutual adhesion. The branched polyethylenes with lower melting temperature are most used because they can be melted more rapidly and they have suitable.
Technology of Polyolefin Film Production rheology to flow on to the adherend. Melt adhesion of films will require higher-melting layers other than the surface layer, since if more than the surface layer melts then the structural integrity of the film will be destroyed. Copolymers of ethylene with vinyl acetate, methyl acrylate, acrylic acid, maleic acid and many other polar monomers are used to increase the surface energy of polyethylenes to make them more readily wettable.
Similarly, polypropylene can be grafted with maleic anhydride to increase the adhesion of other substances to it. The surface energy of polyolefins is also increased through corona discharge treatment. They have a low surface energy and so frictional forces are low. Relative to their strength, the frictional forces can cause damage to the films. Slip additives can decrease the frictional forces.
The factors that cause poor slip are often desired for other attributes such as adhesion of printing and adhesion to other surfaces in packaging. Additives that increase self-adhesion of packaging films will decrease the slip, so that desirable properties are not universal they depend on the intended application of the film.
This self-adhesion is called blocking. The polyolefins can flow, or creep, under load, and so mutual adhesion can occur if the pressure is sufficient or the time of contact is long. This is a significant problem in large film rolls or when film is stacked in large quantity.
Blocking is reduced when the surface is less smooth, such as when crystallisation or processing conditions cause microtopographies.
A smooth surface with high gloss and clarity is generally preferred, so that blocking will be a serious problem. Surface adhesion can be increased by oxidative treatments such as corona, flame, priming or subcoating. Corona discharge is most widely used, and surface oxidation of the film.
A corona discharge treatment facility is shown in Figure 1. Increased polarity will increase the surface energy and enable wetting by inks or adhesives. Research is ongoing in the development of fiberboards from various sources of fiber1 and also in studying the plasticization of lignin to obtain a deeper understating of board properties2.
There has also been research into the use of powder kenaf core in binderless boards3,4. The use of natural fibers and wood fibers as fillers and reinforcements in thermoplastics has increased recently, with considerable growth in the automotive and building materials sector. In the United States the building and construction industry is the predominant consumer of these materials, and this market is growing in Europe5,6.
In Brazil, fibers such as sisal have great potential7,8, and are used in the automotive sector and have great potential in the building and construction industry.
There have also been studies of using non-wood natural fibers as reinforcements in polyester and other thermoset polymers9. A recent literature review covers a variety of aspects of cellulose based composites This technique involved the use of glycerine during the compounding of the composite which permits high fiber loading in the polyolefins, without thermal degradation of the fiber.
Initially we modeled the system as a typical natural fiber composites in which the matrix is the continuous phase and reinforced with kenaf fibers However, additional experiments and a recent review of the work along with additional scanning electron microscopy have given us new insights into this material. It appears that linear polyolefin chains act more as a binder and adhesive between the kenaf fibers, similar to traditional wood composites, such as particle and fiber boards with urea or phenol formaldehyde adhesives.
In these materials, typical thermoplastic polymers have been used as binding agents in natural fiber boards. In these systems, these materials are true fiber reinforced polymers since the polymer is in a continuous phase with fibers or fillers dispersed in the material.
There are applications where higher lignocellulosic content thermoplastics are used commercially. However, their process is exclusive to extrusion, where higher viscosities are permissible. Also, to achieve higher performance, the process must also include thermosets unsaturated binding agents- e. The aspect ratio of the kenaf fibers we have used can be higher than The inherent polar and hydrophilic nature of the lignocellulosic fibers and the non-polar characteristics of the polyolefins create difficulties in compounding and result in composites with low stress transfer efficiency.
Proper selection of coupling agents and other additives are necessary in improving the interaction, adhesion and stress transfer The amount of glycerine used to make these boards was optimized for the best properties. Differential scanning calorimeter DSC studies were conducted to determine if glycerine had any effect on the crystallinity of the PP based binder system. This MAPP had about 6 wt. Kenaf bast fibers were cut into about 1 cm in length.
Glycerine was added to water at about a ratio. The mixture was stirred and put into a typical garden spray bottle. The fibers were spread out and the mixture sprayed over the fibers, while constantly turning and moving the fibers to get uniform distribution of the glycerine.
The samples were air dried for at least one week at ambient conditions prior to the next stage of preparing the boards. The precise time to discharge depended on the amount of fiber and polymer, and their ratios and how hot the mixer was when the blending started.
Three boards were made for each set of experiments. Four flexural specimens were cut from each of the three boards. Six specimens two from each board were tested for each data point. Specimen dimensions were according to the respective ASTM standards. Flexural testing was done using the ASTM standard.
The fracture surfaces of some of the tested specimen were observed using a JEOL scanning electron microscope. The sample sizes were kept between 9 and 11 mg and all tests were run under nitrogen gas.
The samples were cut to size from the center of the boards. A single cantilever test was used for the testing with a span of 14 mm, and sample width and depth were about 5 mm and 1 mm, respectively. Results and Discussion The effect of glycerine on board preparation was evaluated.
It appears that the glycerine broadens the processing window of compounding the fibers and polymer Figure 1. The processing window is the temperature range over which the polymer has melted and is well dispersed and at the same time there is no fiber thermal degradation.
There could be several reasons why the use of glycerine helped in preparing boards without kenaf fiber degradation. The processing window could be very narrow when there is no glycerine and our control over the compounding stage was not good enough to make sample boards. The use of glycerine expands the processing window, Figure 1. The use of glycerine explains the shift of the upper end of the processing window to higher temperatures since its presence prevents the burning of the fibers.
This is probably due to the plasticization and lubricant effect of the glycerine on the polymer and also the plasticization of the lignin and amorphous polymers present in kenaf. The flow of thermoplastic melts cannot be increased as desired beyond practical limits by raising the temperature: polymer molecular weight loss or crosslinking along with discoloration can occur due to the chemical instability of the macromolecule Furthermore, in the case of lignocellulosic composites, fiber thermal degradation can take place at elevated temperatures.
Higher processing temperatures require additional energy and could also lead to lower molding outputs through longer cool times. Furthermore, lignocellulosic fibers thermally degrade at higher temperatures. Polymer rheology is affected by several factors including stress, strain, time, temperature and additives such as plasticizers and lubricants.
It is well known that sorption of vapors or gases by polymers can cause significant plasticization resulting in a substantial decrease in the glass transition temperature For example, solvent crazing of glassy polymers e. In the case of wood, moisture or other plasticizers drastically affect the glass transitions of the amorphous parts lignin and hemicellulose. Kelley, Rials and Glasser28 have studied the transitions for wood. They also found that strong H-bonding solvents, such as formamide, could be effective plasticizers for the lignin and hemicellulose than weaker H-bonding solvents.
Recent work has studied the plasticizing effect of various molecules on lignin2- unfortunately the effect of glycerine was not studied. Furthermore, the structure of the lignin can also alter the molecular transitions of the lignin Ralph30 mentioned that kenaf has an unusual type of lignin, with extensive side chains that are acetylated, and this will change the transition of lignin compared to wood lignins.
The transition of hemicelluloses are also affected by the type and amount of the plasticizer. Olsson and Salmen31 have studied the transition behavior with respect to the amount of moisture. An interesting point to note is that the glycerine plasticizes the amorphous components lignin and hemicellulose of the kenaf fibers by breaking some of the H-bonds present.
A highly filled kenaf-PP board can thus have a significant amount of amorphous polymers in the composite. Since the boiling point of glycerine is very high, the plasticization can occur well above the boiling point of water, and during the compounding of the kenaf boards.
The presence of glycerine may permit the kenaf fibers to deform without extensive breaking during the high intensity of mixing also during the hot press cycle.The process of claim 5 wherein p is 1 and R is selected from C alkyl, chlorine, bromine, thioether and a 4 membered alkyl group forming a carbocyclic ring with adjacent carbon atoms of the unsaturated parent ring.
Melting will destroy the original crystal structure and any orientation in the film, so the mechanical properties will be changed.
Mathot, Carl Hanser, Munich, Germany, , Some estimates of extruder size and die sizes based on throughput and cooling capacity are shown in Table 2. Three boards were made for each set of experiments. The particles must be small enough not to introduce haze when in the interior or decrease gloss when at the surface.
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