Volume cost and important (filler mixing)

Volume Cost and its importance in Plastic applications and woven sacks The Volume Cost of a Raw Material input is the purchase cost of a unit volume of the material. It is extremely important to understand the Volume cost of Polymers and its additives as it plays a key role in their selection for a particular application. Compounding Costs and its implications: Mineral fillers are fine particle Inorganic powders. The particles agglomerate due to Vaan Der Waal forces, and it is essential to break up these agglomerates to disperse the filler particles uniformly in the Polymer Matrix. This requires energy and is additional to the energy required for melt formation and mixing. The energy requirements and compounding cost depend on various factors, like the physical form of the polymer, whether it is polar or non polar, the type of filler, whether the filler is untreated or treated and processing behavior. Physical Form     • If the Polymer is in liquid form, it is fairly easy to incorporate fillers. Examples are paint formulations, liquid adhesives and plastisols. A good quality stirrer is normally sufficient. However, as in the case of Leather Cloth Plastisols, where large quantities of low quality ground calcium carbonate is used, additional processes like triple roll milling are required to ensure adequate dispersion and homogenization. Each step increases compounding costs, but they are still comparatively low.     • If the Polymer is in Powder form, like PVC resin, fillers are quite easily incorporated in the dry blending step and high speed mixers are commonly used. All PVC has to be compounded with Stabilisers, lubricants, Plasticisers, if required and a host of other additives. The filler gets incorporated in the compounding process and there is hardly any additional filler dispersion cost. Many UPVC applications do away with the intermediate pelletising step (essential with plasticised compounds), hence filler addition cost in UPVC is negligible. Masterbatch manufacturers sometime pulverize Polyolefins so that large quantities of fillers can be added much more easily than granule feed. Of course this is an expensive step, taken only when filler loadings are high or the compounding equipment falls a bit short in dispersion.     • If the polymer is in granule form, the compounding cost is the highest. The primary compounding of the ex reactor resin has already been carried out by the polymer producer when antioxidants, stabilizers and other processing additives are added and the melt converted into pellets. The filling of mineral fillers are done by compounding companies which have the necessary equipment to melt the pellets, mix and disperse the fillers, homogenize the melt and convert them back again into granules. Intensive batch mixing processes like Banbury mixing have largely been replaced by modern high speed co-rotating multiported twin screw extruders, Buss Co-Kneaders and similar sophisticated equipment. Therefore for estimating the volume costs of a formulation, the compounding cost has to be added to the formulation costs before arriving at the true cost per kilo. This multiplied by the finished compound density gives the volume cost which is so essential in working out the economics. Mineral Filled Polyolefins With Polyolefins, the situation is different from PVC. Here fillers like talc and calcium carbonate are added to improve stiffness to PP, or desired properties like anti fibrillation in HDPE or PP Raffia Tape. Incorporation of fillers in polyolefins is an expensive process as explained above. Further, unlike PVC which is polar, POs are non polar, requiring more energy to encapsulate the polar fillers. Compounding costs for filling Polyolefins can be as high as 10-15/kg., with the higher figure more prevalent when state of the art compounding equipments are used. Filled Polyolefins (10-40%) are costlier per kg. than the base polymer because compounding costs outweigh the lower filler cost. The volume costs go up sharply with density increase, but requirements of better stiffness in auto components, moulded furniture and other technical parts is the driving force for filler addition. It is only at filler levels of over 50%, as in filler masterbatches, that the cost per kilo dips below polymer cost levels, but the volume cost will still be adverse. Thus normally filler addition does not automatically lead to cost savings with polyolefins as it does with PVC. Another important difference is that fillers are loaded in PVC in Parts per Hundred Resin (PHR). In Polyolefins, Filler loading is expressed as a percentage of the total compound weight. When somebody expresses amazement that his competitor is using 100% filler in his PVC Pipes, obviously you can�t extrude pipes out of 100% CaCO3!! What he means is 100 parts PVC resin, 100 parts filler plus the usual stabilisers, lubricants and pigment. Thus the actual filler Percentage would be 100/210 (say) = 48%. This is not an unusual loading, at least in polyolefins with Filler masterbatches exceeding these levels. I make this distinction because referring to PHR in PVC compounds as % is a common mistake. Let us have a closer look at CaCO3 filled PP & HDPE. Mineral Filled PPCo Applications • The high volume Filled PP applications are:        • Automotive Bumpers, Dashboards,and Components        • Talc is the main filler •  Moulded Furniture        • Precipitated or Ground Calcium Carbonate is the main filler. • Raffia Tape        • Precipitated or Ground Calcium Carbonate is the main filler. • Low Cost Films        • Talc is the preferred filler as it has least effect on translucency. •  The rationale for mineral filled PP and HDPE is more for improving physical properties rather than cost reduction (with an important exception which we will discuss later). • In PP, CaCO3, Talc, and other mineral fillers improve stiffness and improve paintability. • In HDPE & PP, CaCO3 is extensively used as an antifibrillating agent for raffia tapes. • LDPE/LLDPE is normally not filled as film blowing performance is badly affected. Effect of Fillers in PP-Copolymer (Compounding Costs ` 15/Kg.) Like in the previous article, calculations on the effect of Filler loading on Compound cost and Volume costs are done in Table 1. This is a theoretical exercise, as filler loadings of more than 60-70% are difficult to achieve. The filler loadings have been stretched to find out what is the effect on volume costs and draw conclusions. It is interesting to note that even though these are theoretical calculations, the predicted density is quite near the actually measured density with the difference being a few points in the third decimal place. Rarely do we find errors in the second decimal place. There is some density increase due to volatile loss, but this is quite low in Polyolefins, and I assume the Filler is not wet. Table -1 The graphical representation of the calculations are shown in Chart 2, Chart-2 Interpretation • Assuming that ` 15 a Kg is the Compounding cost, at a level of approx 25-26% filler does the cost of Compound dip below the raw PPCO price (70/Kg). When the business margins of the compounder is included, it is only at the 35-40% filler level that the purchase price of a filled PPCo compound will come below base polymer price. • The rate of decrease of Volume Cost is a lot slower, and even at 90% Filler levels, the Volume cost is higher than the base polymer. • This means that for all moulded (or Extruded PP Products which are sold per piece (i.e. by volume), purchased filled PP Compounds will not lower costs. They should only be used for value addition like better stiffness, paintability etc. There is a way to minimize the effect of the relatively high Compounding and conversion costs. Filler Masterbatches The route to reduce compounding costs is with filler masterbatches. A filler masterbatch is PP filled with 60-70% filler. These are blended with Unfilled PP just like Colour Masterbatches and then processed. I have assumed that the per kg. Compounding costs remain the same for filler masterbatches as for filled compounds. The compounding cost gets distributed by addition of unfilled PPCo. Mixing takes place in normal Blender and Injection Moulding machine. As an illustration let us assume that the cost of a Filler Masterbatch is as in Table 1 70% loading or around ` 40/Kg.: Cost Reduction with Filler Masterbatch. (70% Filled with CaCO3) • Cost of Bought in Filled PPCo • Cost of Blended PPCo/ Filler M.B. Benefits of Filler Masterbatch • At 20% Filler level, blend is ~ Rs 11/kg. cheaper (14.7% reduction) • At 30% Filler Level, blend is ~ Rs 9/kg. cheaper (12.8% reduction) • Though Density remains the same, Volume cost reduction vis a vis bought in Compound is higher, Rs 11.30 and 9.73 respectively, but is still higher than Unfilled PPCo (63/Ltr). • By blending, it is much easier to adjust the Filler level for the desired properties. Recommendations for Moulded Furniture (and other products sold by Volume) • While Filler Masterbatches is a good way to reduce costs, the volume costs stayhigher than unfilled PP. • Even at 90% Filler, Compound Volume cost does not go below Unfilled PPCo (Rs 63.5/Ltr.) • In Moulded Furniture, Cost per chair is unlikely to reduce by increasing Filler Loadings. • Resistance to deflection (Stiffness) improves with Filler Loading. • Reduction in Impact strength has to be balanced with required stiffness to arrive at optimum filler loading for a particular Moulded Furniture Model.      • Rigorous evaluation with blends of PPCo, PPHomo, and Filled Masterbatches with impact resistance, Stiffness and Weight tested for each blend should be done. Analysis of the results and trends will help establish the most cost effective solution for each mould. Mineral Filled HDPE applications: Raffia Tape This is one area of the Polyolefin processing scene that use of filler is widespread. In order to understand why it is so, it is important to understand Volume costs and the way raffia products are specified and sold. Antifibrillating Masterbatch Use of filler by the raffia tape industry started as an antifibrilating agent. This was started with HDPE tapes and then later with PP. The Industry soon realised that good cost savings could be achieved with filler loadings higher than that required for antifibrilation. Volume cost considerations can explain why Raffia manufacturers get cost savings by filler addition and not moulders or for that matter the large HDPE Pipe and other extruded products. What are the basic differences? In the moulding Industry, products are sold per unit volume. The volume is the mould volume. To reduce volume a new mould is required. This is an expensive proposition, but is the only way to reduce costs. Most moulded furniture manufacturers have built up a large stock of different moulds yielding similar design chairs but of different weights. These cater to different market segments. The mindless Filler loadings that have happened in the PVC Pipe industry has been mirrored by a mindless wall thickness reduction (Mr. M.P. Taparia�s words, not mine) in the moulded furniture industry. The results are as disastrous as outlined in my earlier paper. I give this as an illustration of the power of Volume cost. If Volume cost does not decrease on adding fillers even though the purchase cost of the filled compound goes down, other methods are required to reduce cost, which may run into several crores in capital costs. In the Raffia tape industry, however the Tape denier is the primary specification. Rafia tape is not sold by volume The Extrusion process allows easy modifications in tape dimensions without any added capital costs. We will now explore how Volume cost has made filler addition lucrative in the Raffia tape industry. Woven Sack Specifications The woven sacks or other end product woven from HDPE/PP Rafia are normally specified by: * Tape Denier. These are around 1000-1200 and is much higher than synthetic Yarn Deniers * Warp & Weft: No. of tapes per inch/cm. used in the ends and picks. * Weight of the bag. Denier & dtex- Definitions from Wikipedia * Denier is a unit of measure for the linear mass density of fibers. It is defined as the mass in grams per 9,000 meters. The denier has its standard based in nature, a single strand of silk is one denier. Therefore, a sampled 9,000 meters length of silk will weigh one gram. The term denier is a literal combination of the words linear and density. * dtex: In the International System of Units the tex is used instead Tex is a unit of measure for the linear mass density of fibers and is defined as the mass in grams per 1000 meters Tex is more likely to be used in Canada and Continental Europe, while denier remains more common in the United States and United Kingdom. The unit code is "tex". * The most commonly used unit is actually the decitex, abbreviated dtex, which is the mass in grams per 10,000 meters this comes fairly close to the denier definition. How Filler addition affects Tape Denier Let us start with unfilled HDPE Tape of 1000 Denier. At a Density of 0.96 gms/cc (Kg/Ltr.), 9000 mtrs of tape should weigh 1000 grams. When Mineral Filled HDPE is use, Density is higher, and Denier increases in proportion. Denier can be brought down to the specified 1000 by Downsizing: • Reducing Tape Thickness • Reducing Tape Width • Reducing both To maintain Denier, the volume per unit length of tape can be reduced. If the filled Compound has a density 15% higher, volume can be reduced by 15%. How Filler affects Woven products • If the ends and picks are kept the same after downguaging Rafia Tape, the bag weight should remain unaffected. However strength will go down due to lesser amount of polymer and the reduction in mechanicals due to the filler. Drop tests have to be carried out to assess suitability /failures. • If the Width is reduced too much to maintain Denier, the weave will become more open, and may not be acceptable in some applications. • While downsizing the tape, extra orientation can be applied to partially offset loss in strength. The stretch applied should be within the processing limit to avoid excessive tabe breakage. Limitations to Filler Addition in Raffia Tape • Physical properties like Tensile Strength, Elongation at Break, Tensile Modulus etc. reduce with filler. Loss in strength is dramatic at higher filler levels (above 15% Filler) • Stiffness increase is not much of an issue in Raffia Tape as thickness is much lower than mouldings. • For Woven Raffia products where high performance is demanded, like Jumbo bags, Filler loadings should not increase over 6-8%. • For less demanding applications like 25 Kg. Bags or lightweight Tarpaulins, higher filler levels can be tolerated. Filler Masterbatch addition In this example we will consider what happens when Filler Masterbatch is added to HDPE. The Filler Masterbatch should have a suitable Carrier. This is important as HDPE and PP are not very compatible, thus a PP based Filler Masterbatch should not be used with HDPE and Vice Versa High MFI LLDPE Masterbatches are compatible with both HDPE & PP. High MFI LLDPE allows large filler levels. However, high dosages of High MFI based Filler masterbatches can adversely affect the resultant MFI an reduce physical properties further. In Pigment Masterbatches, this is not an issue as addition levels are 1-4 % unlike 20% to as much as 50% and more resorted to with Filler Masterbatches. Assumptions: A 60% Filler loading is selected for this study. Compounding costs are the same as the PP example. • While Volume costs are still higher than unfilled HDPE (64 `/Kg.), costs are saved as less Volume of material per bag is needed. • As costs keep on decreasing with addition of filler, caution is advised not to overdo the loading. What is the Optimum Filler Loading in Raffia • Unlike in many other PP/HDPE applications, costs can be reduced by filler loading in Raffia Tape. There is a temptation to go on increasing filler loadings as markets become more and more competitive. • Care should be taken not to disturb the MFI chosen by high levels of Filler Masterbatch. • Filler addition levels for a particular application should be self regulatory. Self Regulatory considerations • The performance requirements of the end use should be clearly understood. Tests like Loaded Drop Test, Bursts strength, Stitch ability should be set up to reflect the performance requirements. • Filler Masterbatch levels should be carefully experimented with to find the optimum %. • Ash tests similar to BIS-4985 could be formulated for critical applications like Bulk Sacks/ Jumbo Bags. Summary for Raffia • The previous calculations and findings are equally true for PP Raffia. Similar considerations are valid when Talc or combinations of Talc & CaCO3 are used. • These studies are theoretical and follow the reasoning of Volume Costs. It would be interesting to know how close these findings compare to actual Raffia industry experience. Some words of caution I understand that in the last few years, there have been concerted steps taken to reduce the Filler addition costs in Polyolefins. It is quite clear that as compared to the PVC Industry, the high compounding costs is a major barrier to filled Polyolefins from finding wider applications and market share. The route taken is quite worrying. It is well known that Plasticised PVC Compounding is successfully done on single screw extruders, some of which are quite unsophisticated, and therefore very cheap. The capital costs are a fraction of co-rotating Twin Screw extruders and compounding `/kg cost for Plasticised PVC is in the low single digits. It seems a similar route is now being used for filling Polyolefins, mainly HDPE for the Raffia tape and Blown film industry. One must understand that in SPVC, the filler is already well dispersed in the High Speed Mixer/Cooler Mixer before being fed to the Single Screw extruder. The extruder is essentially for melting and pumping the PVC through the die for pelletising. The induced mixing action of the single screw is enough to complete the homogenization. With HDPE and relatively high filler loadings as required in a filler masterbatch, a single screw extruder, even with mixing zones can never come even close to the intensive mixing capabilities of Co- rotating multi-segmented Twin Screw Compounders or Buss Ko-Kneaders. As the single screw extruder is so much cheaper, quite a few have been pressed into service to compound Polyolefins with high filler levels, while still keeping the compounding costs down to ` 6- ` 7/kg. If dispersion is not proper, multiple passes are resorted to compensate for the improper mixing. This is self defeating as repeated heat history eats into the Stabiliser and Antioxidant levels incorporated by the polymer producer. There is every chance that the filler masterbatch will reduce the life of the finished product it is used for. Filler masterbatches are used at much higher levels than Colour Masterbatches, and presence of degraded polymer in the masterbatch will adversely affect product quality. I would urge those who are compounding HDPE with filler on single screw extruders in multiple passes to add additional Antioxidants and stabilizers to compensate for degradation. There is also a trend for using Talc filled filler masterbatches for HDPE and Even LLDPE Blown film. I am convinced that if the Film or the bags made thereof are sold by Volume, filler addition would not reduce costs. Assuming that the film rolls are sold in meters of a specified gauge, it is being sold by Volume. Bags sold per piece of a fixed gauge (thickness); again it is sold by Volume. We have seen that in Polyolefins, Volume cost does not go below the unfilled Polymer levels even at high filler loadings. Thus the processor may be lulled by the fact that the filled compound he is extruding is of a lower cost in `/kg. Terms, his product weight will go up for the fixed volume units he sells. The additional material cost will outweigh whatever savings he was expecting over unfilled product. If the film is being sold by weight basis, it is another matter. Here the customer suffers. He gets fewer meters for the same gauge film as density goes up with filler. The meterage reduction % will be more than the price discount offered with Talc filled films. I would request the industry leaders to nip this trend in the bud and educate their customers on the Volume Cost concept so that they are not exploited by unscrupulous competition.