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Table9.9Property Comparison of Products Obtained by Free-Foaming vs.Celuka Foaming[4] Properties Free-foam Celuka
Average density(g/cm3)0.60.5
Surface hardness(Shore“D”)4269
COTE(in./in./ºC) 5.6×10–5 4.5×10–5
Impact resistance(kJ/m)4581148
Tensile strength(kPa)10,04521,726
Flexural strength(kPa)20,30525,097
familyradioCOTE=Coefficient Of Thermal Expansion
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(a)(b)
Figure9.12Comparison of foam density distribution.(a)Free foam;(b)Celuka process
9.4.1.2Effect of Process Parameters on Foam Morphology
Foam formation is the critical step in the process of foam extrusion.Most rigid PVC foams are made with chemical blowing agents.Regardless whether the gas are chemical or physical blowing agents,the foaming mechanisms during extrusion foaming involve veral stages,as was first propod by Hann[51]and extended by Reichert [52].
3249PVC Foams
Upon merging with the polymer melt,the gas dissolve in the polymer melt under high pressure.When the melt exits the die,a sudden pressure drop caus a thermodynamic instability of the gas/polymer solution.This leads to the formation of a parate gas pha.At this stage,a large number of bubbles are formed in the polymer matrix.The are the nuclei of the cells.The bubbles grow until the gas pressure inside the bubbles and the surface tension of the bubble walls are in equilibrium.The foam must harden and solidify as soon as this equilibrium is reached,otherwi the cells will collap.The foam structure can be stabilized by rapid solidification of the melt during the cooling process. The cell nucleation,cell growth,and cell morphology are thus affected by:
•The type and amount of gas available in the system,
•The temperature,pressure,viscosity,and elasticity of the melt before exiting the die,•The geometry of the die,and
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•The cooling rate outside of the die.
For a given formulation and equipment,the conditions boil down to control of melt temperature and shear effect control during extrusion.The following ca will demonstrate how the adjustable parameters influence the cell structure of rigid PVC foams prepared with the free-foaming process.
9.4.1.2.1Foam Density and Cell Size
The temperature of the polymer melt affects the foam density and cell size,thus affecting the melt viscosity,the degree of fusion,and gas generation from the blowing agent.For a successful foaming process,the temperature of the melt must lie in an interval between values that are giving a sufficiently low viscosity of the melt to allow cell formation and expansion and a sufficiently high viscosity of the melt to prevent cell collapsing[49]. Figure9.13[51]indicates that the foam density gradually decreas with an increa of the temperature of the melt becau it affects the decomposition of the CBA,until it reaches a minimum.A further increa in melt temperature,however,caus an increa in density becau of too low a melt viscosity and too high
a gas diffusivity.Other rearchers working not only with PVC[14],but with other polymers[49],also obrved this U-shaped curve.
As shown in Fig.9.14,the cell structure also changed significantly becau of the variation of the melt temperature.In particular,when the temperature is low,the cells are either non-existent in some spots,or they are very small.As the temperature increas,the cells grow and become more uniform becau of more decomposition of CBA.At very high tempera-tures,however,large coalesced cells result in a non-uniform cellular morphology[53].
The other relevant variables in the extrusion-bad foaming process are a variation of the screw rotation speed(RPM)and stretching of the melt after exiting the die with respect to mi-finished foams.Experimental results[53]revealed that as the screw RPM incread, the density decread,reaching a minimum value before it incread again.This V-shaped relationship between the density and the screw RPM was considered similar to the one obtained for the density vs.the melt temperature becau an increa in the screw RPM would cau greater shear heating,resulting in a temperature increa.The temperature changes,rather than the screw speed alone,became a dominating factor governing the PVC foam density.However,when the melt temperature was held constant by adjusting the
9.4Processing Technology
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0.70.8Melt temperature,°C
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Figure 9.13Effect of melt temperature on density of a rigid PVC foam
[53]
Figure 9.14Influence of melt temperature on the cell structure of a PVC foam [53](Courtesy of Society of Plastics Engineers)
barrel and die temperatures,the density incread with increa in the screw speed becau of the l
ower residence time and the high back pressure,thereby preventing the blowing agent from realizing its full potential and thus generating a greater density.可爱的英文名字
市场营销研究生The effect of stretching downstream of the extrusion line also possibly influences the density and the cell structure.The same study [53]indicated that under identical processing conditions,the foam density was nearly independent of the downstream stretching.This was attributed to the fast expansion and foaming taking place at the time
权力的游戏第一季103269PVC Foams
when the melt exited the die.The cell shape,however,changed from fairly spherical to more elongate,and the cell morphology became less uniform.
9.4.1.2.2Cell Size Distribution(Formation of Large Voids)
The formation of large voids in the core of the profile cross ction is one of the major problems in the extrusion of PVC foams.The voids are usually formed as the melt exits the die and they grow as the distance from the die increas.
Various studies[50,53]identified a strong correlation between the tendency to form voids and the temtodou
perature of the melt,die pressure,and cooling.It was found that as the melt temperature incread and the die pressure decread,surprisingly,the tendency to form large voids decread.In fact,for a given screw speed,there appeared to be a critical temperature below which large voids were formed[53].
Rabinovitch[53]attributed this to the fact that when PVC is premixed with additives it fus at a higher temperature and its behavior then rembles that of thermoplastics.It is thus believed that the incread melt temperature improved the fusion,which,in turn, initially incread the melt strength.It was generally obrved that fusion was improved and thus cell opening was prevented.On the other hand,due to the further temperature increa,the melt strength decread.If the pressure were higher,the pressure drop would be higher as well.However,incread temperatures and reduced melt viscosity resulted in lower die pressure and less pressure drop at the die exit.This reduced the rate of gas expansion inside the cells,which are cooled and solidified,thus preventing cell rupture.This explanation does not simply imply,however,that a higher melt temperature would be beneficial for the foaming process.In fact,the thermal nsitivity of PVC is always a limiting factor in the extrusion of PVC foams.Too high a melt temperature can also cau collap of over-grown cells,which results in large voids.
The screw speed also has a significant effect on the formation of large voids.It was obrved[53]that as the screw speed incread,the amount of fusion incread becau of the incread shear heating,but when the screw speed was too high,the melt was too soft,and therefore,cells collapd inside the extruded foam profile while the skin of the extruded PVC foam was solidified by cooling.Conquently,voids were easily formed, and the resulting foam density was higher.
Another factor affecting void formation is the cooling after the melt exits the die[50]. Due to the low thermal conductivity of PVC,a broad temperature profile exists in the extrudate.Insufficient cooling will leave the center of the extrudate hot,which results in weak cell walls prone to collap under the pressure of hot gas inside the cells.This is why the voids are more visible for foams made by the free-foaming process than for tho made by the Celuka process.
9.4.1.2.3Surface Smoothness
The surface appearance of PVC foams is greatly influenced by the extrusion parameters, especially when using the free-foaming process.Extrudates of rigid PVC foams are usually rough at low temperatures,become smooth at intermediate temperatures,and rough again at high temperatures.The roughness appearing at low temperatures is mainly associated with insufficient fusion and melt fracture,whereas the roughness appearing at high temperatures
9.4Processing Technology327 is due to sticking and cell rupture on the surface caud by shear at the die wall[53].With the exception of fusion at the limit of an extruder heating system,a higher screw speed results in incread shear rates,which usually caus an incread roughness.
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9.4.1.3Influence of Extrusion Equipment
Although single-and twin-screw extruders intended for processing solid PVC can be ud for processing of foamed PVC products,specially designed extruders for PVC foams are also in u.The screw configuration of the extruder should allow plasticization of the compound to occur as rapidly as possible and at a relatively low melt temperature so that the melt prevents the escape of already evolved gas through the throat of the hopper[1].As the PVC melt containing gas are conveyed under high pressure through the screw channels,the decompression zones must be avoided to prevent bubble formation[2],or the decomposition temperature of the CBA must be higher than the melt temperature in the decomposition zones.
The design of the dies derves special attention.The cross-ctions of the die should decrea uniformly down to the die outlet.This helps to keep the melt pressure above the gas pressure or vapor pressure of the blowing agent almost to the end of the die.The die length should be kept as sh
ort as possible to ensure that the melt pressure in the die is maintained above the gas pressure as long as possible.Otherwi,premature foaming of the melt in the die will occur.The premature foaming is undesirable becau it will destroy the cells at the extrudate surface.
Typically,the calibration unit and cooling bath must be longer for foamed products than for solid ones becau of the lower heat conductivity of cellular products versus solid products.Many new devices promoting the cooling process have recently become available on the market,in respon to demands for high output foam extrusion.
9.4.1.4Recently Developed Extrusion Equipment for Rigid PVC Foams Significant developments have been made in the area of extrusion equipment for rigid PVC foams during the last decade.The general trends are aimed towards increasing the flexibility and efficiency of the process.To meet the increasing demands on the equipment,multiple improvements along the extrusion line–both upstream and downstream–have been introduced.
A trend in the core-foamed pipe industry is to u generic formulations for foamed and non-foamed products,thanks to the improvement of upstream feeding systems.The new system can precily add chemical blowing agents on-line[54],thereby avoiding the need for making a compound exclusiv
ely for each foamed product.Usually,this approach results in a slightly heavier pipe becau the PVC formulation is not optimized for greater density reduction.
At the heart of an extrusion line is an extruder(s).Currently,most of the vinyl foam sheet and pipe products u twin-screw extruders,whereas in the production of rigid foam profiles,the single-screw extruder is most frequently found[4].Although single-screw extruders are still much more inexpensive and simpler,one of the recent trends is to extend the u of counter-rotating twin-screw extruders becau of the following benefits[55]:
