GCC in PVC, More than a Pipe dream


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Siddhartha Roy explores calcium carbonate’s (CaCO3) role
in filler applications, with a look at processes around the
world with regard to PVC pipes in India and China.

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GCC in PVC, More than a Pipe dream

  1. 1. Calcium Carbonate GCC in PVC: More than a pipe dream A s with anything, the way a plastic is processed and formulated varies from country to country. Overleaf is a table outlining the different forms of plastics observed in various parts of the world. There has been considerable convergence, especially in the last two decades of globalisation, but the broad trends are still discernible. Pipe colours in India In PVC pipes details are important. Colour, pressure and models vary. The uses of colour in August 2012 Siddhartha Roy explores calcium carbonate’s (CaCO3) role in filler applications, with a look at processes around the world with regard to PVC pipes in India and China. different regions of the world is intriguing. PVC pipes are mainly installed underground, and apart from Drain Waste Vent (DWV) applications, are rarely used exposed. Even in DWV lines the pipes are often painted over. So why is the colour of pipe important when it gets hidden anyway? Until recently, in India, PVC pipes were made in white like the US system, or black, similar to PVC conduits for electrical installation. Wavin India, the world’s largest manufacturers of plastic pipe systems, stuck to the German grey. White pipes needed 3-4 PHR (parts per hundred resin) TiO2, a more expensive pigment. Carbon black is much cheaper, and all High-density polyethylene (HDPE) pipes were industrial minerals 45
  2. 2. Calcium Carbonate CaCO3 usage in Indian plastics Pipes and fitting 73% Footwear 2% Others 4% Wires & Films & cables sheets 6% 5% Profiles Calendering 3% 7% black until a problem was detected with these pipes. It was noticed that the pipes were deforming in storage during the Indian summer. Investigations showed that the sun radiation temperature was the cause. This is the temperature that a black body attains under direct exposure to sunlight. At the height of summer in many parts of India, temperatures can hit the 45oC mark (115 o F), which leaves sun radiation temperature to soar to 79-80 o C. This is higher than the Vicat Softening point of UPVC which is 72oC. The black pipes were deforming under their own weight in the sun or even in closed metal containers, like railway wagons. Injection moulded fittings which have lower Vicats were very badly affected. It was noticed that under similar conditions, white pipes were unaffected. The sun radiation temperatures were a good 10-12oC lower than black pipes and, thus, below the softening point. However the industry was reluctant to adopt white as the pigment cost was high. A compromise was reached with about 1 PHR TiO2 and a pinch of carbon black to give a light grey colour. The colour lowered the sun radiation temperatures to a manageable level and was not too expensive. It was necessary for the industry to standardise the colour as it would allow long runs during production without the need to change colours frequently. The light grey colour standardised in India had a remarkable and positive side effect. HDPE pipe was the main competition, and it had to be coloured black as carbon black is the preferred UV Protection agent in polyole46 industrial minerals fins. Unscrupulous manufacturers started adding recycle and scrap which went undetected by the customer because of the black colour. On the other hand, the light grey of PVC did not allow excessive scrap addition to go undetected. HDPE pipe quality plummeted; there were large scale failures in the field and the HDPE pipe market imploded. PVC pipes took off in this phase and have left HDPE pipes far behind ever since. For specialist waste recycling (SWR) pipes, Indian PVC Pipe has opted to use a dark grey colour. In Europe, and in most parts of the world, a reddish brown colour is prevalent. China, for some reason, has stuck to white pipes. Their colour is not as white as the American pipes, perhaps because TiO2 levels are lower. Perhaps in China, the drive to use more and more fillers has something to do with enhancing the white colour. The major highly filled applications in Thermoplastics are in the fields detailed in the pie chart above. These applications alone account for between 80-85% of CaCO3 usage in plastics. PVC pipes along with wires and cables and calendering is 85% of the 2 MTA PVC market in India, CaCO3 in PVC is a significant market, and of considerable interest to the minerals community. The development of filler masterbatches as a tool to spread out the compounding costs over a larger quantity of polyolefins has been described in IM previously (Calcium Carbonates Polymer Promise, December 2011) The trends are similar worldwide — Automotive, white goods and custom mouldings use filled compounds, while the more competitive moulded furniture market exclusively use filler masterbatches. There is not much difference in the pattern of CaCO3 usage in wires and cables, leathercloth, calendered films and sheets worldwide. Individual processors have their preference for precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC) and the micron sizes as per their equipment and processes. Extruded floor tiles seem to be exclusively on GCC and cost considerations have forced use of coarser particle sizes (10-20 microns). Extruded floor tiles are very well developed in China and the Far East, while calendared floorings are well entrenched in West, US and India. Cushion vinyl floorings has seen amazing effects being offered and is a major paste grade PVC resin outlet. It is in the area of PVC pipes that there has been a divergence of approach, below is an example of the path followed in India and China. PVC piping systems can be broadly classified under two groups: pressure pipes and non-pressure pipes. Pressure pipes Pressure pipes are designed to convey many different fluids under pressure. Ratings are according to the pressures to be withstood and range from 2.5 bar right up to 16 and even 20 bar. The wall thickness progressively increases as the pressure ratings increase. The stringent specifications restrict CaCO3 levels to 8-10 PHR. Good quality, fine particle size GCC and PCC actually improve impact strength and processability at these levels. Top of the line GCCs like the offerings from Omya, or the ultrafine PCCs offered by firms like Solvay actually improve the impact strength at even higher levels (15-20%), but burst pressures could be affected adversely over 10 PHR. As pressure pipes are a critical application, mature producers worldwide self control the filler levels to 8-10 PHR. Some specifications, like the Indian BIS 4985 for pressure pipes, specify a sulphated ash content limits which CaCO3 usage in plastics Application Area Product Typical Filler Loading HDPE and PP Raffia Tape Blow Mouldings HMHDPE Shopping bags Filler Masterbatch Up to 70% CaCo3. PVC Wires and Cables Insulation Inner Sheating Outer Seathing, PHR 10 50 100 % 6% 25% 35% Top coat Base Coat PHR 10 150 % 5% 45% Soft Hard PHR 300 400 % 65% 70% Pressure DWV Highly Filled-1 Highly filled 2 8 40 100 200 7% 27% 48% 65% PVC Leathercloth Extruded Floor Tiles PVC Pipes August 2012
  3. 3. Calcium Carbonate would not permit higher dosages of mineral fillers. Worldwide, PVC pressure pipes form approximately 30% of the total PVC market. In India it is a bit different. There is a huge irrigation sector using many thousands of kilometers of PVC pipes annually. These are normally the lower pressure rating pipes like 2.5 bar and four bar and some six bar rated pipes. This skews the pressure pipe market share right up to about 70% in the Indian market. In India, it is in the agri pipe area that competitive pressures have pushed up filler loadings. 1. Impact Modifiers: These introduce a rubbery element in the brittle filled PVC matrix. They act as shock absorbers and improve the impact strength of PVC, sometimes dramatically. a. Acrylic, MBS and ABS types b. Chlorinated Polyethylene (CPE) Non pressure pipes: In India, such impact modifiers and flow promoters are extensively used in critical PVC applications like pharmaceutical blister film, blown bottles, industrial sheeting, and in the niche market of column pipes. Impact modifiers like the ones developed by Rohm & Haas (KM 323B) and flow promoters like K-120ND & K-175 are very well known and are manufactured locally. Products from Kane Ace are also popular. However these are very costly additives and their use is restricted to critical applications. The Indian PVC pipe industry has tended to steer clear of these additives because costs will The bulk of PVC pipes are used in construction related applications like DWV, SWR (sewage, waste, rainwater) conduits and tube well casings, rain water harvesting systems. These represent nearly 70% of the PVC pipe market and are a major contributor to filler tonnage consumed in pipes. These pipes competed with asbestos cement pipes and have rapidly replaced them, especially after the awareness of asbestos’s link with cancer grew worldwide. PVC pipes offered much lighter and safer systems, which were easy to install. PVC pipes need to be protected against UV degradation as many of them are exposed to sunlight, unlike pressure pipes which are normally buried or concealed. However, because of competitive pressures an unhealthy trend of getting rid of expensive UV stabilisers seems to have permeated the Indian SWR industry. In many buildings the exposed PVC pipes are painted to match the building exterior. The paint layer offers some protection from UV attack. The success of PVC pipes in the SWR and related DWV sectors owes a lot to development of a wide range of injection moulded fittings. These fittings either have a rubber ring in groove jointing systems or are threaded (as in the case of casing pipes and plumbing). It is self evident that it is unwise to use more that 10 PHR filler for pressure pipes. In non-pressure pipes a balance has to be made between the reduction in cost and the deterioration in impact properties which directly translates to losses due to pipe breakage during transportation and installation. In order to to improve the performance of highly filled PVC pipes, the impact properties need to be modified. It is in this area that the practices followed in India are widely different from China. Unplasticised PVC There are two important additives which enhance unplasticised PVC (UPVC) processing and performance. These need to be studied in some detail. August 2012 2. Flow Promoters: They smoothen the flow of the UPVC melt allowing it to encapsulate and homogenise the filler particles. The extrusion of the filled PVC melt improves and the pipe surface finish is enhanced. a. Predominantly Acrylic (ACR) increase. In fact many producers take pride in the fact that they are able to produce saleable pipe without the use of these expensive additives. It should be noted that smaller producers who employ single screw extruders for PVC are forced to use at least the flow promoter to make acceptable pipes. They were forced to use single screw extruders as they could not afford the costlier twin screw technology. Hence they compromise with a costlier formulation. Column pipes which have to support heavy submersible pumps in deep tubewells, is about the only application where impact modification is resorted to, with nominal filler levels < 10PHR. The starting torque of the submersible pumps has to be resisted by a tough pipe, especially as they are attached by threading, which weakens the joint as PVC is inherently notch sensitive. Highly filled PVC pipes are processed in India without any impact modifiers and flow promoters. Even the agri pipe, which is a low pressure application, has seen high filler loadings. However it becomes increasingly difficult to make acceptable pipe at filler loadings higher than 40PHR, which seems to be the norm in non branded SWR pipes. Examples of PVC pipe technology Technology Style Approach Examples in PVC Pipe technology Countries following German High precision Automation High cost Rugged and robust machinery Pioneered twin screw extrusion Automation Precision in extrusion Long life machinery Switching from lead stabilisation to lead free systems Grey is the preferred colour European countries, UK Australia South Africa India Gulf Countries US Strong in manufacturing, Lags Germany in technology but catches up when newer innovations are accepted Persisted with single screw extruders well into the 1980s while the world had embraced twin screws Tin stabilised white pipe is the norm Conservative with filler loading Most countries in North and South America Japanese Precision instrumentation and control systems compensate for technological gap with German technology Strong in Automation. Embraced twin screw technology early. Parallel screw systems with excellent controls competed with German conical screw technology Comparable capital costs and machinery life Lead based stabilisation, but shifting to non lead Grey preferred colour Conservative with filler loading Japan Taiwan South Korea Chinese Low cost Manufacturing Machinery derived from German and Japanese technology, but of lower cost and shorter life. Conical twin screw extrusion seems to be the norm, though parallel screws also used Lead based systems Very high filler loading in non pressure pipes White is the preferred colour Most aggressive in filler loadings China Vietnam Thailand Malaysia Sri Lanka industrial minerals 47
  4. 4. Calcium Carbonate Main effects on PVC pipe properties as CaCO3 loadings increase Property 0- 8 PHR 10-50 PHR 50-100 PHR Density Gms/cc 1.38-1.44 1.45-1.66 1.66-1.84 Formulation Cost, Rs/Kg `50.08 to `47.19 `46.53 to `37.39 `37.39 to `30.67 Volume Cost Rs/Kg `69.99 to `68.33 `67.94 to `62.05 `62.05 to `56.21              Impact Strength, Falling Dart. Breakage in Transportation/ Unaffected Installation Tensile Modulus. Unaffected Burst Pressure (Hoop Stress_ Unaffected Rigidity (Flexural Modulus) Unaffected Processing Ease Unaffected (The dangers of such “mindless filler loadings” were explained in ‘Volume Cost in Plastics Applications’ July 2011). In China, the approach has been quite different and has spawned its own industrial sector. PVC processors in China realised the problems of using expensive impact modifiers and flow promoters which were the norm in Western and Japanese Industries. They started investigating CPE and results were good. At 3-4 PHR CPE levels, filler loadings could be boosted substantially and still yield a workable pipe. The CPE grades available from DuPont and Dow were found to be too expensive, and CPE from local technology gradually found good acceptance with the pipe manufacturers in China. Concurrently, flow modifiers were developed to replace the more expensive Western counterparts. The locally developed acrylic flow modifiers were generically known as ACR. It was quite effective at 1-1.5 PHR Level. The Chinese PVC pipe industry has progressively increased the filler levels from 40PHR to 75 PHR using 3-4 PHR of CPE and 1-1.5PHR of ACR. The results were acceptable to the non pressure pipe consumers, and filler levels were boosted to 100PHR, and there are reports of even 200PHR being used. What is lesser known is that it is CPE and ACR which are making such staggeringly high filler loadings possible. A lot of research has gone into this activity and, while this is not in the public domain yet, use of impact modifiers and flow promoters seem to be the cornerstone of the success. The Chinese PVC pipe industry is huge. Added to this is the profile and ceiling tile markets, meaning the CPE and ACR consumption must be impressive. Many PVC resin manufacturers now state that they produce CPE and sometimes ACR also. Chlorinated PVC uses chlorine as a feedstock — a major raw material in PVC 48 industrial minerals plants. So it makes sense for such plants to put up an auxiliary polyethylene chlorination plant to support PVC sales. Non pressure PVC pipes are not only solid wall — corrugated, two layer corrugated, foam core PVC pipes and other variants have been developed where the pipe is lighter than the equivalent solid walled pipes. The key feature in the design of such lightweight pipes is resistance to crushing from external loads rather than any internal pressure resistance. Filler levels are not very high in such applications. PVC resin Suspension grade PVC resin can be manufactured from the petrochemicals or non petrochemical route. Ethylene obtained by cracking of petroleum fractions like naphtha, heavy ends and light ends like c4 gases is the key input. The process is: PVC made by this route is termed ePVC (Ethylene or Petrobased PVC). PVC is a major plastic which can be and is manufactured from non petrochemical sources. Key inputs are two minerals — coal and limestone — and cheap electricity. The carbon in coal is reacted with the calcium carbonate in limestone in an electric furnace. The calcium carbide then formed is treated with water to release acetylene, which is then converted to vinyl chloride by combining with HCl and polymerised to PVC. PVC made by this route is known as carbide based PVC. The availability of cheap electricity and abundant coal and limestone is crucial to the economic viability of carbide based PVC. China has these inputs in abundance — about 80% of the PVC capacity in China is carbide based. This translates to about 9m tpa of the 11m tpa of China’s installed PVC capacity. Thus huge quantities of coal and limestone are consumed. There are many such producers which are located close to major chlor-alkali projects and also near coal, limestone and hydel power units in the interior of China. Many such units have their captive coal fed power units if hydel power is not conveniently available. The electricity generated is used for the carbide furnaces as well as the chlor-alkali plants which produce caustic soda and the hydrochloric acid gas needed for converting acetylene to vinyl chloride. India V China on carbide process In India, out of the six PVC plants in operation, only one is based on the carbide process. Here also there is a major difference. The Chinese carbide passed PVC is considered inferior in quality to ePVC and fetches lower prices. ePVC is found suitable for all PVC applications including stringent applications like clear film, medical tubing and highly plasticised applications. The Chinese carbide route PVC is deficient for such applications. Fortunately carbide based PVC works for opaque low plasticiser applications, like PVC pipes and profiles, which accounts for 70-80% of the PVC market. In India, the scene is quite different. The carbide based plant produces a quality of PVC resin universally accepted in India as the best quality PVC, and in many cases fetches a premium. The contradiction is puzzling. Perhaps the Indian plant uses superior technology in purifying the vinyl chloride and world-class polymerisation technology to convert it to PVC? Perhaps, then, the Chinese carbide PVC plants could follow the technologies implemented by the Indian unit to upgrade their quality. Even with their huge PVC capacity, China is a net importer of ePVC for critical applications. With the slowing down of construction and infrastructure projects in China, which were huge consumers of PVC products, there is a currently a severe PVC overcapacity in China. If China was able to upgrade its carbidebased PVC quality, not only can ePVC imports stop, but also they would be able to export to wider PVC markets overseas to tide over the — hopefully temporary — overcapacity situation. There is no reason why this cannot be done. If an Indian plant can produce PVC from carbide which is superior to ePVC, the Chinese PVC plants should be able to do the same if they employ similar technologies. August 2012