Document Qk2xnEdgNEwg8JqZgzN3OZze4

DFGoodrich Chemical Group RFGSrUS? Ingredients Used in Rigid PVC Compounding Although PVC is a very versatile polymer, proper compounding and good lubricant bafance are criticaf in obtaining good machine and end product properties. Resin PVC resin is available in a wide range of molecular weights. In the United States, the molecular weight is expressed as inherent viscosity (I.V.); in Europe it is expressed as K value. With both of these values, make sure the value was derived before comparing them. Commercial PVC resins available range from an I.V. of .50 to 1.16 (K value, 47 to 76). The higher the I.V., the higher the molecular weight and the greater the stiffness. Below is a table showing I.V. vs. equipment and end use. can be formed into a product before it degrades. The stabilizer does this job by absorption of hydrogen chloride, displacement of active chloride atoms, free radical scavenging, disruption of double bond formation, deactivation of degradation byproducts, peroxide decomposition and ultraviolet energy absorption. Stabilizers in pipe extrusion are used mainly to process the PVC through the hot hostile environment of the extruder. They do not significantly affect shelf life or weatherability. This is not true in weatherable compounds. In these materials, the amount of stabilizer is five to seven times the level in pipe and they do have a positive affect on weatherability. A given recipe, even with high levels of titanium dioxide, I.V. Range 0.50-0.65 0.65-0.85 0.85-0.98 0.98-1.16 Process Injection molding Injection molding and extrusion Extrusion Extrusion End Use Very complicated molded end use Pipe fittings - monofilaments sheet, film, profiles, cellular Pipe, siding, sheet, film profiles, flexible Complicated profiles (windows), profiles, flexible, wire and cable Stabilizers Polyvinyl chloride (PVC) is a heat and light sensitive material. It degrades by dehydrochlorination and oxidation. This can be seen by the development of color in PVC. In chemical terms, formation of conjugated double bonds causes the color change. PVC compounds experience heat history in mixing cycles, extrusion/molding, embossing, thermoforming, laminating and scrap rework. Oxidation products occur by exposure to weathering. The job of the stabilizer is to delay heat degradation so that the compound will have poor weatherability at half the normal level of stabilizer. Any changes in level or supplier of stabilizers should be checked for weatherability before complete changeover has been made. There are several types of stabilizers for PVC resin. Below is a partial list: t. Lead Salts -- These are mainly organic lead compounds like sulfates, silicates, phosphites, stearates, phthalates and maleates. They are inexpensive and provide good heat stability, excellent electrical properties and low water absorption. These stabilizers are used in wire, cable and pipe (in Europe). Potential toxicity and sulfur staining limit their use in pipe in the U.S. Lead stabilizers have some lubricating power and this must be taken into consideration. 2. Calcium-Zinc -- These are used in food contact applications like blow molded PVC bottles, film and sheet. PVC so stabilized can meet FDA requirements. The stabilization power of calcium- zinc in PVC is minimal and, therefore, scrap rework capability is poor. They can provide compounds with crystal clarity and low odor properties. 3. Barium-Cadmium -- This type of stabilizer provides good early color, light stability and heat stability. Plate-out tendencies, toxicity of cadmium and poor melt viscosity are characteristic of these stabilizers in PVC. 4. Tin Mercaptides -- These types of stabilizers are the most common ones used to stabilize rigid PVC in the U.S. They give good heat, light and color stability. Another good point is they promote fusion and reduce melt viscosity. The main type of tin mercaptides in use are methyl tins and butyl tins. These are used in pipe, pipe fittings, siding, profiles, cellular vinyl and some bottles. One type of tin mercaptide, ai-n-octyltin, is FDA acceptable for food contact applications with PVC. Some of the newer tin mercaptides have external lubrication power and this must be considered in compounding. 5. Stabilizer-Lubricant Package -- (a) Lead Based Systems -- In Europe, these systems have been used for many years in pipe. The advantage of this system is that only two or three ingredients must be mixed with the PVC resin instead of six to nine. The disadvantage is the lack of versatility. (b) Tin Based Systems -- Recently, US. tin stabilizer companies have introduced these systems to the U.S. market. They have not penetrated here to the extent of the lead systems in Europe. However, they offer the same advantages and disadvantages as lead systems. We may hear more about them in the future. Fillers There are many fillers which can be used with PVC including metal carbonates and silicates, gypsum, clay, alum, barytes and saw dust. The most common are metai carbonates, mainly calcium carbonate, which is usually in the form of ground limestone and coated with stearic acid. The coating reduces the abrasiveness of the calcium carbonate. It also reduces extruder barrel and screw wear. Calcium carbonate is available in ground and precipitated grades with a range of particle sizes. As a general rule, the finer the calcium carbonate, the better the impact strength of the finished product. Normally, increasing calcium carbonate content reduces tensile strength and increases tensile modulus. For pressure pipe, the level should never exceed five parts per hundred resin (phr) because higher levels of calcium carbonate could lower the stress design below 2000 ,2 psi which is normally required. Calcium carbonate can be used in v ,' PVC compounds to lower the raw * material cost of the finished ^3 compound. As the level of calcium C / carbonate increases, the material O cost decreases, but the weight per O foot of product increases. This is ^ 0 because the specific gravity of calcium carbonate is higher than the specific gravity of PVC. This relationship between compound cost reduction and increased weight per foot must be balanced to give an overall economic reduction. Today, this level is between three to four parts per hundred. Extruder barrel and screw wear are affected by the amount and the particle size of the calcium carbonate. As amount and coarseness increase, the wear will increase. At levels of five parts per hundred and under, wear is minimal, but higher levels increase wear drastically. Levels of 25 phr have been reported. This causes barrels and screws to wear out in six weeks. Therefore, this cost must be considered. Calcium carbonate particles larger caused when certain pigments (like than two microns often cause lead, cadmium, etc.) react chemically increased barrel wear and reduced with the atmosphere and the impact strength because they act like mercaptides of the stabilizer. Colored holes or stress concentration sites. reaction products are formed. This Increased levels can also cause a reaction can occur on processing or reduction in tensile strength. Other on weathering. Therefore, pigments than as a filler and enhancer of tensile must be carefully chosen in the modulus (at high part levels), calcium light of recipe components and the carbonate serves no other useful end use. purpose in PVC except to reduce Pinking is similar in appearance to compound cost. It does reduce staining because it forms grey or weatherability of compounds. I black streaks on the top three mils of Pigments j the profile. It seems to come from the j action of the compound on the There are three main reasons to use | extruder barrel. Changing the melting pigments in PVC compounding: to ! position in the extruder will often achieve opacity in non-weatherable I eliminate it. Coated rutile-containing compounds, for UV protection in ! compounds seldom show pinking. weatherable compounds and to make However, both anatase and rutile a given color. Titanium dioxide is the can show pinking. At the present major pigment used in PVC. Other time, we have no clear cut solution to pigments are used in small amounts eliminate pinking. to achieve the desired color in Plate-out is a buildup or deposit on combination with the titanium dioxide. the sizing sleeves, die, embossing roll In pipe and some profiles, opacity or screws. Plate-out has often been and color are the main purposes of analyzed to be titanium dioxide the pigments. For these uses, and/or calcium stearate, but other titanium dioxide levels are normally materials can cause this problem. between 0.5 phr to 5.0 phr. At these The amount of coating on the titanium levels, the effect on weathering is dioxide can aggravate piate-out,'as minor. Compounds with this level of can exceedingly high stock pigmentation should not be used temperatures. The type and amount outdoors. Both color and properties of calcium stearate can be another will be drastically affected by long cause. By changing types of term exposure to UV radiation. The lubricant, stabilizer, pigments and titanium dioxide used in these processing conditions, the amount of compounds is normally controlled plate-out can be eliminated or chalking rutiles. controlled. In siding, window and other weatherable applications, a high Processing Aids amount of UV screening is needed. The most common processing aids The purpose of this screening is two used with PVC are acrylic polymers. fold: to maintain color and properties. Alpha methylstyrene has been used. The major UV screener used in PVC A process aid reduces melt viscosity, is titanium dioxide. A15 phr level is increases frictional heat and reduces needed to achieve this purpose. In uneven die flow. In a compound, it colored PVC compounds, a non promotes fluxing and acts like an chalking rutile titanium dioxide is internal lubricant. Increasing levels of used along with another pigment to processing aid normally allow lower achieve the desired color. Because extruder/molding barrel temperatures. the titanium dioxide has very limited Also, it gives the melt hot strength for chalking, the color does not fade string-up and drawn-down. on weathering to the extent of Typical levels are between 0 and 5.0 chalking titanium dioxide. For white phr depending on the extrusion compounds, two titanium materials I process. Single screw high shear and are sometimes used, an anatase and i multi-screw extrusion require less a freely chalking rutile. The combination is used to achieve the i processing aid, while low shear, i single screw extrusion requires more right amount of chalking for i processing aid. it must be maintaining the desired whiteness remembered that the type of end and cleanliness on exposure to UV product made affects the amount of radiation. This ratio of rutile to processing aid used. For example, anatase is normally 2:1. All changes the more complicated the end in weatherable ingredients should be product, the higher the level of outdoor weathered two years before processing aid used. being commercialized. Some normal problems caused by Impact Modification pigments are staining, plate-out, poor Certain applications require higher dispersion and pinking. Staining is impact strength than PVC would demonstrate normally. Acrylate, chlorinated polyethylene (CPE), methacrylate-butadiene-styrene (MBS), and acrylonitrile-butadienestyrene (ABS) polymers are normally used to modify impact strength of PVC. These polymers provide a shock absorber when the PVC is heated to a high enough melt temperature 196C (3850F) to fuse it. At lower melt temperatures, the impact is lower than it would be without impact modification. At this lower melt temperature, the impact modifier acts like a hole or stress concentrator making the impact lower. Using the proper melt temperature will ensure that the full properties of the impact modifier will be realized. All impact modifiers reduce the chemical resistance, tensile strength and stress rupture of PVC compounds. If these properties are desired, the lowest possible level needed to achieve the desired impact should be used. Pipe generally has 0 to 2 phr of impact modifiers, while siding and profiles have 0 to 10 phr. High impact PVC compounds with lower chemical resistance and stress rupture have 6 to 14 phr of impact modifiers. Weatherability can be affected by both type and level of impact modification. Normally, CPE, EVA and acrylate polymers are used in weatherabie compounds. They allow good retention of properties, along with good color retention. Both MBS and ABS impact modifiers are poor in these characteristics, but are good for low temperature properties. Impact modifiers generally promote flux, with the exception of CPE. As flux promoters, they tend to reduce the temperature of the extruder barrel needed for a given melt temperature. This point needs to be considered when compounding PVC. Lubricants Lubricants are materials that control the fluxing (melting) point in the extruder/molder to achieve the best processing characteristics and physical properties. There are three types of lubricants. They are external, internal, and external/internal. They are defined by their effect on the melt in a plasticizing screw, as follows: External Lubricants -- Provide good release from metal surfaces and lubricate between the individual PVC particles. As the level of external lubricant is increased, it moves the melting point of the PVC in the direction of the die. Internal Lubricants -- Provide lubrication at the molecular level and reduce the melt viscosity. The internal lubricant moves the melting point of the PVC in the direction of the extruder hopper as it is increased in level. External/Internal Lubricants -- These materials provide both external and internal lubrication depending on the combination of chemical groups contained. External Lubricants External lubricants are normally non-polar molecules or alkanes. They are usually paraffin waxes, mineral oils or polyethylene. Some stabilizers have oils that carry active ingredients such as external lubricants. External lubricants are normally incompatible with PVC. They help the PVC slip over the hot melt surfaces of the dies, barrels and screws without sticking and contribute to the gloss on the end product surface. Extruder motor amperage is greatly affected by small changes of external lubricants. Common problems resulting from over-lubrication include: surging of the extruder, lumpiness of the extrudate, incomplete fusing and the necessity of high barrel temperature. In the end product, low impact strength and acetone failure are the common manifestations found. internal Lubricants Internal lubricants are normally polar molecules. They are normally fatty acids, fatty acid esters or metal esters of fatty acids and are very compatible with PVC. They lower melt viscosity, reduce internal friction and promote fusion. Common problems of under lubrication are rough extrudate, adhesion to metal surfaces, melt fracture, quick fusion and abnormally low barrel temperatures. In the end product, burning, plate-out, matte surfaces and poor impact strength are typical results of under lubrication. External/Internal Lubricants These are hard to define because they have chemical groups (polar and non-polar) of both lubricant types. In general, they have long hydrocarbon chains, along with amide, alcohol, acids and ester groups. Common types used in PVC are fatty acid amides and oxidized polyethylenes. Some of these materials will lubricate as an external lubricant before melting and as internal lubricants after melting. Others will do the reverse. Each of these lubricants should be characterized for its type of lubrication in a given compound. The purpose is to achieve the full properties of PVC and a melt that will extrude or mold without problems, at an economical cost. To achieve these goals, experiments must be performed. A bench extruder can be used for gross compounding. But, the final compound will need to be run on commercial size equipment. This is the only proof of the "goodness" of the compound. Avoid over-compounding for machine problems. After a compound has been developed, tested, and used successfully, usually very little change is necessary. Before any recipe changes are made, equipment should be thoroughly checked. Temperature controllers and thermocouples should be connected properly and the reading checked. Screw rpm, back pressure instruments and heat bands should be checked. All operating procedures should be compared with previous successful runs with the same compound. Cleanliness and freedom- ,; from obstruction of screen packs, nozzles, adaptors and dies should also be checked. If all are correct, establish clearly what is wrong with the melt before changing the recipe. An attempt should be made to correct a specific problem in the melt rather than a symptom in the end product which could have many causes. After all of these points have been checked and the problem is still present, the recipe should be changed. RFH504R4